Handbook of Visual Display Technology [1] 3540795669, 9783540795667

Table of contents :

Preface v
Foreword vii
Editors-in-Chief xi
Editorial Board xv
Advisory Panel xxi
List of Contributors xxii
Volume 1
Section 1 – Fundamentals of Optics for Displays
1.1 Properties of Light
1.1.1 Properties of Light 5
Timothy D. Wilkinson
1.2 Geometric Optics
1.2.1 Geometric Optics 23
Timothy D. Wilkinson
1.3 Optical Modulation
1.3.1 Optical Modulation 47
Timothy D. Wilkinson
Section 2 – Human Vision and Photometry
2.1 Vision and Perception
2.1.1 Anatomy of the Eye 73
Christine Garhart Lakshminarayanan
2.1.2 Light Detection and Sensitivity 85
Lakshminarayanan
2.1.3 Visual Acuity 93
Lakshminarayanan
2.1.4 Flicker Sensitivity 101
Lakshminarayanan
2.1.5 Spatial Vision and Pattern Perception 109
L. Srinivasa Varadharajan
2.1.6 Binocular Vision and Depth Perception 121
Robert Earl Patterson
2.2 Color Science
2.2.1 Color Communication 131
Stephen Westland
2.2.2 The CIE System 139
Stephen Westland
2.2.3 RGB Systems 147
Stephen Westland Vien Cheung
2.2.4 CMYK Systems 155
Stephen Westland Vien Cheung
2.2.5 Uniform Color Spaces 161
Vien Cheung
2.2.6 Color Perception 171
Marina Bloj Monika Hedrich
2.2.7 Colour Vision Deficiencies 179
Lakshminarayanan
2.3 Visual Ergonomics
2.3.1 Displays in the Workplace 191
Sarah Sharples
2.3.2 Display Screen Equipment: Standards and Regulation 203
Sarah Atkinson
2.4 Photometry
2.4.1 Light Emission and Photometry 217
Teresa Goodman
xxiv Table of Contents
2.4.2 Measurement Instrumentation and Calibration Standards 229
Teresa Goodman
2.4.3 Overview of the Photometric Characterisation of Visual Displays 243
Teresa Goodman
Section 3 – Image Storage and Processing
3.1 Introduction to Electronic Imaging
3.1.1 Introduction to Electronic Imaging 261
Jon Peddie
3.2 Image Storage and Compression
3.2.1 Digital Image Storage and Compression 277
Tom Coughlin
3.2.2 Video Compression 287
Scott Janus
3.2.3 Fundamentals of Image Color Management 301
Matthew C. Forman Karlheinz Blankenbach
3.3 Image Manipulation
3.3.1 Digital Image Operations 313
Matthew C. Forman
3.3.2 Signal Filtering: Noise Reduction and Detail Enhancement 325
Karl G. Baum
3.3.3 TV and Video Processing 345
Scott Janus
3.4 Case Study: Medical Imaging and Display
3.4.1 Reliability and Fidelity of Medical Imaging Data 363
Alfred Poor
3.4.2 Ultrasound Imaging 373
Robert M. Nally
Table of Contents xxv
3.5 Case Study: Security Imaging and Display
3.5.1 Data Hiding and Digital Watermarking 387
Daniel Taranovsky
3.5.2 Biometrics and Recognition Technology 401
Daniel Taranovsky
Section 4 – Driving Displays
4.1 Direct Drive, Multiplex and Passive Matrix
4.1.1 Direct Drive, Multiplex and Passive Matrix 417
Karlheinz Blankenbach Andreas Hudak Michael Jentsch
4.2 Active Matrix Driving
4.2.1 Active Matrix Driving 441
Karlheinz Blankenbach
4.3 Panel Interfaces
4.3.1 Panel Interfaces: Fundamentals 461
Karlheinz Blankenbach
4.3.2 Serial Display Interfaces 471
Thomas Wirschem
4.3.3 High Definition Multimedia Interface (HDMI®) 481
Jim Chase
4.4 Embedded Systems
4.4.1 Embedded Systems: Fundamentals 493
Karlheinz Blankenbach
4.4.2 Graphics Controllers 503
Andreas Grimm
xxvi Table of Contents
4.4.3 FPGA IP Cores for Displays 511
Davor Kovacˇec
4.4.4 APIX: High Speed Automotive Pixel Link 531
Markus Ro¨mer
4.5 Signal Processing Tasks
4.5.1 Video Processing Tasks 549
Markus Schu
4.5.2 Dimming of LED LCD Backlights 567
Chihao Xu Marc Albrecht Tobias Jung
4.6 Power Supply
4.6.1 Power Supply Fundamentals 577
Oliver Nachbaur
4.6.2 Power Supply Sequencing 591
Oliver Nachbaur
Volume 2
Section 5 – TFTs and Materials for Displays and
Touchscreens
5.1 Display Glass
5.1.1 Glass Substrates for AMLCD, OLED and Emerging Display
Platforms 599
Peter L. Bocko
5.2 Inorganic Semiconductor TFT Technology
5.2.1 Hydrogenated Amorphous Silicon Thin Film Transistors
(a Si:H TFTs) 627
A. J. Flewitt
5.2.2 Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs) 647
S. D. Brotherton
Table of Contents xxvii
5.3 Emerging TFT Technologies
5.3.1 Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors 677
Hagen Klauk
5.3.2 Organic TFTs: Solution-Processable Small-Molecule
Semiconductors 697
David Redinger Marcia Payne
5.3.3 Organic TFTs: Polymers 709
Feng Liu Sunzida Ferdous Alejandro L. Briseno
5.3.4 Oxide TFTs 729
Hideo Hosono
5.3.5 Carbon Nanotube TFTs 751
Axel Schindler
5.4 Transparent Conductors: ITO and ITO Replacements
5.4.1 Indium Tin Oxide (ITO): Sputter Deposition Processes 779
Paul Lippens Uwe Muehlfeld
5.4.2 ITO Replacements: Carbon Nanotubes 795
Axel Schindler
5.4.3 ITO Replacements: Polymers 809
Wilfried Lo¨venich Andreas Elschner
5.4.4 ITO Replacements: Insulator-Metal-Insulator Layers 819
Bernd Szyszka
5.5 Patterning Processes
5.5.1 Photolithography for Thin-Film-Transistor Liquid
Crystal Displays 835
Wen-yi Lin W. B. Wu K. C. Cheng Hsin Hung Li
5.5.2 Wet Etching 861
Hua-Chi Cheng
5.5.3 Dry Etching 871
Eugen Stamate Geun Young Yeom
xxviii Table of Contents
5.6 Flexible Displays
5.6.1 Flexible Displays: Attributes, Technologies Compatible with Flexible
Substrates and Applications 885
Kalluri R. Sarma
5.6.2 Flexible Displays: TFT Technology: Substrate Options and TFT
Processing Strategies 897
Kalluri R. Sarma
5.7 Touchscreen Technologies
5.7.1 Introduction to Touchscreen Technologies 935
Robert Phares Mark Fihn
5.7.2 Transparent Conducting Coatings on Polymer Substrates for
Touchscreens and Displays 975
Charles A. Bishop
5.7.3 Anisotropic Conductive Adhesives 989
Peter J. Opdahl
5.7.4 Touchscreen Computer Interfaces: Electronics 997
Lance Lamont Carol Crawford
Section 6 – Emissive Displays
6.1 Inorganic Phosphors
6.1.1 Luminescence of Phosphors 1013
Robert Withnall Jack Silver
6.1.2 Physics of Light Emission from Rare-Earth Doped Phosphors 1019
Robert Withnall Jack Silver
6.1.3 Chemistry and Synthesis of Inorganic Light Emitting Phosphors 1029
Jack Silver Robert Withnall
6.2 Cathodoluminescent Displays
6.2.1 Cathode Ray Tubes (CRTs) 1043
Gerhard Gassler
Table of Contents xxix
6.2.2 Vacuum Fluorescent Displays (VFDs) 1055
Andrew Stubbings
6.2.3 Field Emission Displays (FEDs) 1071
Yongchang Fan Mervyn Rose
6.2.4 New Field Emission Technologies 1105
Mervyn Rose Yongchang Fan
6.3 Plasma Display Panels
6.3.1 Plasma Display Panels 1139
David N. Liu
6.4 Light Emitting Diode (LED) Displays
6.4.1 Light Emitting Diodes: Fundamentals 1155
M. R. Krames
6.4.2 LED Display Applications and Design Considerations 1169
Robbie Thielemans
6.5 Inorganic Electroluminescent Displays
6.5.1 Thin Film Electroluminescence (TFEL) 1183
Adrian H. Kitai Feng Chen
6.5.2 AC Powder Electroluminescence (ACPEL) and Devices 1193
Feng Chen Adrian H. Kitai
6.6 Organic Electroluminescent Displays
6.6.1 Organic Light Emitting Diodes (OLEDS) 1209
Ruiqing Ma
6.6.2 Active Matrix for OLED Displays 1223
Ruiqing Ma
xxx Table of Contents
Volume 3
Section 7 – Liquid Crystal Displays
7.1 Liquid Crystal Fundamentals and Materials
7.1.1 Materials and Phase Structures of Calamitic and Discotic Liquid
Crystals 1243
J. W. Goodby
7.1.2 Introduction to Defect Textures in Liquid Crystals 1289
J. W. Goodby
7.1.3 Liquid Crystal Materials for Devices 1315
Melanie Klasen-Memmer Harald Hirschmann
7.1.4 Physical Properties of Nematic Liquid Crystals 1343
Carl V. Brown
7.2 Liquid Crystal Material Physics
7.2.1 Optics of Liquid Crystals and Liquid Crystal Displays 1365
Philip W. Benzie Steve J. Elston
7.2.2 Alignment Properties of Liquid Crystals 1387
Lesley Parry Jones
7.2.3 Liquid Crystal Theory and Modelling 1403
N. J. Mottram C. J. P. Newton
7.3 LCD Device Technology
7.3.1 Twisted Nematic and Supertwisted Nematic LCDs 1433
Peter Raynes
7.3.2 Smectic LCD Modes 1445
Per Rudquist
7.3.3 In-Plane Switching (IPS) Technology 1469
Hyungki Hong
7.3.4 Vertically Aligned Nematic (VAN) LCD Technology 1485
Hidefumi Yoshida
Table of Contents xxxi
7.3.5 Bistable Liquid Crystal Displays 1507
Cliff Jones
7.3.6 Cholesteric Reflective Displays 1545
David Coates
7.3.7 Polymer Dispersed LCDs 1565
Francesco Bloisi Luciano Rosario Maria Vicari
7.4 LCD Addressing
7.4.1 Active Matrix Liquid Crystal Displays (AMLCDs) 1589
Mervyn Rose
7.5 LCD Backlights and Films
7.5.1 LCD Backlights 1609
Gary Boyd
7.5.2 Optical Enhancement Films 1625
Gary Boyd
7.6 LCD Production
7.6.1 LCD Processing and Testing 1649
Yoshitaka Yamamoto
7.7 Emerging Technologies
7.7.1 The p-Cell 1675
Philip Bos
7.7.2 Flexoelectro-Optic Liquid Crystal Displays 1681
Harry J. Coles Stephen M. Morris
Section 8 – Paper-Like and Low Power Displays
8.1 Colorant Transposition Displays
8.1.1 Electrophoretic Displays 1699
Karl Amundson
xxxii Table of Contents
8.1.2 In-Plane Electrophoretic Displays 1715
Kars-Michiel H. Lenssen
8.1.3 Video-Speed Electrowetting Display Technology 1731
Johan Feenstra
8.1.4 Droplet-Driven Electrowetting Displays 1747
Frank Bartels
8.1.5 Electrofluidic Displays 1761
Kaichang Zhou Jason Heikenfeld
8.2 MEMS-based Displays
8.2.1 Mirasol® – MEMS-based Direct View Reflective Display
Technology 1777
Ion Bita Alok Govil Evgeni Gusev
8.2.2 Time Multiplexed Optical Shutter Displays 1787
Daniel K. Van Ostrand Ram Ramakrishnan
Section 9 – 3D Displays
9.1 3D Display Fundamentals
9.1.1 Introduction to 3D Displays 1807
Mark Fihn
9.1.2 Human Factors of 3D Displays 1815
Robert Earl Patterson
9.2 Stereoscopic 3D Display Technology
9.2.1 Introduction to Projected Stereoscopic Displays 1825
Lenny Lipton
9.2.2 Addressing Stereoscopic 3D Displays 1831
Matthew C. Forman
9.2.3 3D Cinema Technology 1843
Bernard Mendiburu
Table of Contents xxxiii
9.3 Autostereoscopic 3D Display Technology
9.3.1 Autostereoscopic Displays 1861
Adrian Travis
9.3.2 Head- and Eye-Tracking Solutions for Autostereoscopic
and Holographic 3D Displays 1875
Enrico Zschau Stephan Reichelt
9.3.3 Emerging Autostereoscopic Displays 1899
Phil Surman Ian Sexton
9.4 Volumetric & Pseudo-Volumetric 3D Display Technologies
9.4.1 Volumetric 3D Displays 1917
Barry G. Blundell
9.4.2 Pseudo-Volumetric 3D Display Solutions 1933
Ismo Rakkolainen
9.5 Holographic 3D Displays
9.5.1 Principles of Display Holography 1945
Graham Saxby
9.5.2 Electronic Holographic Displays – 20 Years of Interactive Spatial
Imaging 1963
Mark E. Lucente
Volume 4
Section 10 – Mobile Displays, Microdisplays, Projection and Headworn Displays
10.1 Mobile Displays
10.1.1 Introduction to Mobile Displays 1983
Jyrki Kimmel
10.1.2 Transflective Displays for Mobile Devices 1993
Jyrki Kimmel
10.1.3 Alternative Technologies for Mobile Direct-View Displays 2003
Jyrki Kimmel
xxxiv Table of Contents
10.1.4 Liquid Crystal Optics for Mobile Displays 2013
Jyrki Kimmel
10.1.5 Energy Aspects of Mobile Display Technology 2023
Jyrki Kimmel
10.2 Microdisplay Technologies
10.2.1 Introduction to Microdisplays 2033
Ian Underwood
10.2.2 Liquid Crystal on Silicon Reflective Microdisplays 2043
Ian Underwood
10.2.3 Transmissive Liquid Crystal Microdisplays 2057
Ian Underwood
10.2.4 MEMS Microdisplays 2067
Hakan Urey Sid Madhavan Margaret Brown
10.2.5 DLP® Projection Technology 2081
David W. Monk
10.2.6 OLED and Other Emissive Microdisplays 2095
Ian Underwood
10.3 Microdisplay Applications: Projection Systems
10.3.1 Methods of 2-D Image Formation in Microdisplay-based and
Related Systems 2111
Ian Underwood
10.3.2 Digital Cinema Projection 2125
David W. Monk
10.3.3 Data Projectors 2135
Patrick Vandenberghe
10.4 Microdisplay Applications: Head-Worn Displays (HWDs)
10.4.1 See-Through Head Worn Display (HWD) Architectures 2145
Jannick P. Rolland Kevin P. Thompson Hakan Urey Mason Thomas
Table of Contents xxxv
10.4.2 Human Interface Factors Associated with HWDs 2171
Robert Earl Patterson
10.4.3 Optical Components for Head-Worn Displays 2183
Ozan Cakmakci Michael J. Hayford
10.4.4 Examples of HWD Architectures: Low-, Mid- and Wide-Field of
View Designs 2195
Ozan Cakmakci Jannick P. Rolland
10.5 Electronic Viewfinders
10.5.1 Electronic Viewfinders 2215
Ian Underwood David Steven
10.6 Emerging Technologies
10.6.1 Multifocus Displays 2229
Brian T. Schowengerdt Eric J. Seibel
10.6.2 Occlusion Displays 2251
Kiyoshi Kiyokawa
10.6.3 Cognitive Engineering and Information Displays 2259
Robert Earl Patterson Jannick P. Rolland
Section 11 – Display Metrology
11.1 Introduction to Display Metrology
11.1.1 Introduction to Display Metrology 2275
Karlheinz Blankenbach
11.2 Standard Measurement Procedures
11.2.1 Luminance, Contrast Ratio and Grey Scale 2289
Karlheinz Blankenbach
11.2.2 Color 2307
Karlheinz Blankenbach
xxxvi Table of Contents
11.3 Advanced Measurement Procedures
11.3.1 Spatial Effects 2331
Karlheinz Blankenbach
11.3.2 Temporal Effects 2345
Karlheinz Blankenbach
11.3.3 Viewing Angle 2367
Karlheinz Blankenbach
11.3.4 Ambient Light 2383
Karlheinz Blankenbach
11.4 Display Technology-Dependent Issues
11.4.1 Display Technology-Dependent Issues 2417
Karlheinz Blankenbach
11.5 Standards and Test Patterns
11.5.1 Standards and Test Patterns 2429
Karlheinz Blankenbach
11.6 Measurement Devices
11.6.1 Measurement Devices 2447
Karlheinz Blankenbach
Section 12 – Display Markets and Economics
12.1 Introduction to Markets and Economics
12.1.1 The Problem with Forecasts 2471
Mark Fihn
12.1.2 Display Market Forecasting 2483
Ross Young
Table of Contents xxxvii
12.2 Economic Considerations
12.2.1 The Crystal Cycle 2493
David Barnes
12.2.2 Opportunities for Alternative Display Technologies: Touchscreens,
E-Paper Displays and OLED Displays 2499
Jennifer Colegrove
12.3 Computer Graphics
12.3.1 The History of Graphics: Software’s Sway Over Silicon 2511
Adam Kerin
12.3.2 Design Tools: Imaging, Vector Graphics and Design Evolution 2519
Kathleen Maher
12.4 High-Resolution Displays
12.4.1 Introduction to High-Resolution Displays 2541
Mark Fihn
12.4.2 Eye Resolution Displays 2551
Norman Bardsley
12.4.3 The IBM T221 9.2M Pixel Display 2563
Alan D. Jones
12.4.4 Multi-Display Desktops and the Case for More Pixels 2571
Andrew Garrard
12.5 High-Definition TV
12.5.1 LCDs, Growth and Market Coverage 2583
Bruce Berkoff
12.5.2 Quadrupling HD and Beyond 2589
Mark Fihn
12.6 3D Displays
12.6.1 Trends in the 3D TV Market 2599
Chris Chinnock
xxxviii Table of Contents
12.7 Green Technologies
12.7.1 The Display Industry in a Green World 2609
Keith J. Baker
12.7.2 Sustainability in LCD Manufacturing, Recycling and Reuse 2621
Avtar Singh Matharu
Glossary 2641
Index 2663

Citation preview

Handbook of Visual Display Technology

Janglin Chen, Wayne Cranton, Mark Fihn (Eds.)

Handbook of Visual Display Technology

With 1419 Figures and 146 Tables

Editors Janglin Chen Industrial Technology Research Institute Taiwan Wayne Cranton Nottingham Trent University UK Mark Fihn Veritas et Visus USA

ISBN 978-3-540-79566-7 DOI 10.1007/978-3-540-79567-4 This publication is also available as: Electronic publication under ISBN 978-3-540-79567-4 Print and electronic bundle under ISBN 978-3-540-79568-1 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2011942896 Published by Springer Heidelberg Dordrecht London New York In association with Canopus Academic Publishing Limited, 15 Nelson Parade, Bristol BS3 4HY, UK www.springer.com and www.canopusbooks.com © Springer-Verlag Berlin Heidelberg 2012 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Printed on acid-free paper Springer is part of Springer ScienceþBusiness Media (www.springer.com)

Preface This handbook grew out of an international graduate training program in visual displays – a new approach to postgraduate training for the display technologists of the future. Leading edge international researchers and academics were invited to contribute, representing the knowledge supply chain from blue skies academic research through industrial design and development, to market analysis and creative content developers. Over a series of intensive one-week taught modules, these innovators helped deliver a stimulating and effective program which became known as DisplayMasters. It soon became clear that, in addition to providing a novel educational program for new researchers in visual displays, this convergence of minds was providing a catalyst for networking and new collaborations throughout the professional community that was rapidly building around the delivery of the course modules. It was through this community that discussions began around how best to capture the diversity of science and technology, engineering, human factors, ergonomics, market analysis, and economics of choice required to cover the scope of the subject comprehensively. While there were excellent texts available dealing with separate aspects of the field, including several focused on important display technologies, there was no core reference text that brought them all together. Hence, the project to develop this Handbook of Visual Display Technology was launched. The result is a major reference that covers the full range of subjects underpinning the field of display technology, a field which has become pivotal in our daily lives and in the impact of technology development and interaction. The electronic display is our primary interface to the technological world. Through the screens of our desktop computers, interactive televisions, e-books, portable phones, and tablet devices, we are increasingly dependent upon the performance of the display interface to undertake our daily tasks, communicate with each other, and to consume our entertainment. This handbook presents the science behind the screen. With over 150 expert contributors from around the globe, this four-volume reference is a comprehensive and robust platform of knowledge for anyone involved in the research, design, development, marketing, and utilization of display systems. A handbook of this scale is very much the result of the commitment and collaboration of a large number of people, and as Editors-in-Chief we would like to extend our gratitude to all of the contributors for their excellent work that constitutes the book, and in particular to the Editorial Board and Advisory Panel for their tireless input to the project. We would like to thank Dr. Anandan, President of the Society for Information Display, for his support and for kindly providing a Foreword which also serves as a comprehensive introduction to the handbook. Finally, we are indebted to Tom Spicer and Robin Rees of Canopus Academic Publishing for the excellent guidance, support, professionalism, and enthusiastic motivation

vi

Preface

they have provided throughout the project, without which it would not have been possible. We look forward to a continuing relationship with Canopus and Springer, through regular online updates of the handbook to ensure that it remains a live and comprehensive resource of the science behind the screen. Janglin Chen, Wayne Cranton, and Mark Fihn November 2011

Foreword I am honored to have been invited to write a Foreword to the Handbook of Visual Display Technology, a monumental work indeed! Display devices have changed the way we live, and the current generation is especially fortunate to be experiencing substantial changes in display technology. This book, to the best of my knowledge, is the first of its kind. The authors of the chapters in the Handbook are world experts, some of whom I have acquaintance with, engaged in R&D and the business of displays. The Editors have successfully met the challenge of selecting the best authors for the book. From the stage of capturing the source image, through a CCD camera as an example, until the image is reproduced truthfully on a display device, all the steps along the way including image storage, image processing, image communication, video processing, display driving and finally the display itself are all vividly described by experts in their fields. As is required of a handbook, this book gives a clear account of the historical perspective of all display devices regardless of their obsolescence and commercial success. Directions for future research and suggestions for further reading are given along with exhaustive references at the end of every chapter. All major display technologies and display systems are discussed from microdisplaysize through to large TV-size and cinema-scale displays. The king of displays is the Liquid Crystal Display (LCD) and the book does full justice to its dominance. The reader will notice obvious overlap in some areas and this is unavoidable due to the nature of the scope of applications of displays. As the title of the book contains the words ‘‘Visual Display,’’ it is most appropriate that Sect. 1 starts with the physics of light. Light is fundamental to display devices both for emissive and non-emissive displays. Definition and description of terms such as ‘‘coherence,’’ ‘‘interference,’’ ‘‘polarization,’’ ‘‘double refraction’’ and the like have direct relevance to visual displays. Section 2 follows in a natural sequence, dwelling on vision and perception. The neural network is well described with signals from 126 million photoreceptors reaching one million nerve fibers and finally leaving the eye to the brain. ‘‘Color vision’’ and ‘‘color blindness’’ have been described well. Section 3 deals with image storage and manipulation. A good introduction to electronic imaging is outlined, tracing its historical development and illustrating the flow of image processing beginning with the capture of the source image. Video compression with various coding and encoding techniques is described and different video compression standards are outlined. The goal of digital color management, to maintain the color of the captured original image and render it truthfully on display device, is clearly emphasized. The perceptual rendering method that enables the colors in the source gamut to be scaled in to the destination gamut is interesting to read. Finally, the reliability and fidelity of image data as applied to medical imaging is well brought out. Currently, it is shown that six megapixel color resolution monitors are better than films. Applications in security such as facial recognition and finger print recognition are nicely illustrated. Section 4 traces the development of display drivers starting from direct drive and multiplex drive through to active matrix drive both for LCDs and OLEDs (Organic Light Emitting Diodes). Pixel drive circuits for LCDs and OLEDs are compared. For high content display

viii

Foreword

driving the recent ‘‘panel electronic system’’ is explained. The integration scheme for display controllers is emphasized. For driving large displays of 10800 diagonal, the challenge in terms of line resistance and capacitance is explained with special reference to gate pulse distortion. The problem of image sticking as a result of the pulse distortion is outlined. System-on-glass technology for mobile display driving is covered, and display interfaces and display controllers for graphics are described. Under video processing, different de-interlacing methods employed for converting the interlaced fields received from the broadcast into the progressive fields necessary for operation of the display are described. The problem of ‘‘Motion Blur’’ is explained and the solution to this problem through the driving scheme is described. Addressing schemes starting from the Alt-Pleshko scheme to the active matrix driving scheme are sequentially described with distinct waveforms. Examples for TN (Twisted Nematic)-LCD, STN (Super Twisted Nematic)-LCD and TFT (Thin Film Transistor)-LCD driving are illustrated. With regard to LED backlight driving, the PWM (Pulse Width Modulation) driving of LEDs and local dimming of LEDs are also explained. This section on display driving is carefully structured and written well. Section 5 focuses on display materials, TFTs and touch screen. The thermal expansion of glass, its stability against chemicals used in manufacturing and flatness requirements under high temperature processes are all well described. The use of Jade glass for OLEDs, and Gorilla glass for increased durability applications are emphasized. ‘‘Flexible glass’’ for ePaper applications is another interesting write-up under this section. Flatness, thinness and largeness are explained in the context of Gen 10 glass size. The subsection on a-Si:H and poly-silicon for TFTs for driving active matrix (AM) displays is written well. The device physics and fabrication of TFTs are well explained. Laser annealing for fabricating poly-silicon TFTs is clearly described. Drain field interference in TFTs and the solution to eliminate this through doping of adjacent regions is well illustrated. The display related characteristics of a-Si:H TFTs, polysilicon TFTs and organic TFTs are well elucidated and compared. The most recent and attractive oxide TFT technology is traced through its historic development and it is interesting to read that for 30 years, work on oxide TFTs was abandoned. Stability of oxide TFTs and charge mobility are reported. This section also comprises touch screen technology and this is exhaustively covered starting from sensor to controller to software driver to the computer. Resistive touch, capacitive touch, projected capacitive touch, in-cell touch, optical touch and camera based touch are all well explained, including the touch electronics. Section 6 is exclusively devoted to Emissive Displays. Most emissive displays employ phosphors of either an inorganic or organic type. The book structures appropriately the topic on phosphors, before embarking on the actual emissive displays. Synthesis of phosphors and their application in CRTs, Vacuum Fluorescent Displays (VFDs), Field Emission Displays (FEDs), Plasma Display panels (PDPs) and inorganic electro-luminescent displays are all well described with emphasis on the demand placed by each display technology on the phosphor. AMOLEDs are described in terms of light emission from organic phosphors with the description of TFT technology dominating this section. Inclusion of LED as a main display completes the list of emissive displays. Appropriately Sect. 7 is completely devoted to LCDs. This section is superbly done. The authors of this section have covered the subject matter in the greatest possible detail, tracing the history of liquid crystals (LCs), going through the physics and electro-optical properties of LCs, describing the exploitation of various modes of LCs in different type of LCD, explaining all display modes [namely, TN, STN, VAN (vertically aligned nematic), Pi-cell, PVA (patterned vertical alignment), MVA (multi-domain vertically aligned), PDLC (polymer

Foreword

dispersed LC), FLC (Ferro-electric LC), blue-phase LC], outlining the fabrication steps, illustrating the driving methods, placing emphasis on Active Matrix TFT driving and finally LED backlighting. Applications for all the modes of LC have been well illustrated including the application of PDLC for ‘‘smart windows.’’ One observes after reading this section that there are still plenty of opportunities left for LCDs, in the improvement of optical transmission and simplification of the manufacturing process. For those who want to know the art of current mass manufacturing of LCDs, an excellent process flow is elucidated with the methods employed for each process. One interesting feature that the reader will be able to find is the correlation of LC display modes with the companies that use them in the mass manufacturing line. Under ‘‘device processing and testing of large scale TFTs’’ the illustrations are outstanding. Overall the past, present and future of LCDs are vividly described. One of the ‘‘hot topics’’ of today is ePaper-displays for eReaders. Section 8 deals with this subject, in addition to MEMS (Micro Electro-Mechanical System). Electro-phoretic displays dominate the market in eBooks and hence the focus on these display technologies is natural. Both vertical and in-plane electrophoretic displays along with ‘‘hybrid electrophoretic’’ displays are detailed. In addition, the prospects and potential of electro-wetting displays, electro-fluidic displays and reflective LCDs for application in ePaper displays, emphasizing reflectivity and response time, are well described. MEMS-based displays comprise DLP®, IMOD (Interferometric Modulator), Micro-shutter and TMOS (Time Multiplexed Optical Shutter) based displays. DLP technology is covered in the next section and the potential of all other MEMSbased displays are made clear in Sect. 8. Another current hot topic is 3D, the subject of Sect. 9. As noted above, DLP is described in relation to 3D Cinema technology and this chapter is well written. 3D displays incorporating both active shutter glasses and passive shutter glasses are well described. Human factors, such as nausea and headaches, seem to be limiting the penetration of 3D TV and hence a sub-section is devoted to the human factors associated with viewing 3D images. Because of this limitation, the consumer trend is to go for auto-stereoscopic displays that do not require glasses. This part is well elucidated under ‘‘lenticular lens’’ and ‘‘parallax barrier.’’ ‘‘Displays on the move’’ is an area dominated by many small-area displays involving various display technologies. A smooth presentation of the subject is given in Sect. 10, tracing the display technologies involved in mobile applications and the stringent demands satisfied by these technologies. The potential of OLEDs is well illustrated along with the current dominant LCD technology that has undergone custom designs for mobile applications. Power efficiency, which is critical for mobile displays is presented well, including the non-traditional approaches to power saving. Section 10 also deals with microdisplays, projection displays and headmounted displays. The common element is the microdisplay which underpins these display systems. This section is very exhaustive and describes well the high resolution challenges relating to pixel sizes as small as 3–5 mm. The technologies covered under this section include LCoS (Liquid Crystal on Silicon), DLP, Polymer-OLEDs, Small molecule-OLEDs, and LCDs. For cinema scale projection systems, DLP cinema is stated to be the dominant technology with almost 30,000 screens in operation during 2010, and the chapter on DLP projection technology is well written. The applications of microdisplays in viewfinders, digital cameras and headmounted systems are outlined. There is a proliferation of display technologies and intense competition for many applications, making it difficult for consumers to select the display device that is suitable for a specific situation. There is a need to evaluate displays and a book of this type needs to dwell on the methods of measurement that truly capture the characteristics of displays. Section 11 of the

ix

x

Foreword

Handbook does precisely this. Various display parameters including luminance, contrast ratio, grayscale, viewing angle, color gamut, response time, spatial uniformity, etc., are all defined and the methods of measurement are described for various types of displays. It is interesting to read in this section that one of the shortcomings of display metrology is that judgment by human vision cannot be completely replaced by measurement. Another unique and admirable aspect of the Handbook, seldom seen in works of this type, is the discussion of ‘‘market forecasts’’ and ‘‘preservation of our environment.’’ The final Sect. 12 deals with these aspects. Readers will be interested to see a question on the credibility of market forecasts and the narration of the author’s personal experience through several lessons. There is also a conclusion that the market forecast becomes simple because there is only one display (i.e., the LCD) that dominates all applications. The ‘‘Crystal Cycle,’’ and the ‘‘wave theory’’ of the 3D market forecast and 3D displays attempting for the past 60 years to come in to our lives will be of interest to readers. Finally the display industry is viewed in the context of a ‘‘green world’’ and suggestions are made for the recycling of the electronic waste emerging from various display technologies. After scanning through the Handbook of Visual Display Technology, I feel that no engineering or science library can be without this book. It will be an asset for all companies engaged in display and display-related business. For researchers this book provides substantial guidelines for the future of display technology. Dr. M. Anandan President, Society for Information Display Austin, TX USA

Editors-in-Chief Janglin Chen Industrial Technology Research Institute Taiwan

Janglin (John) Chen is a Vice President of Industrial Technology Research Institute (ITRI) in Taiwan, and the General Director of ITRI’s Display Technology Center. Prior to Joining ITRI, Dr. Chen was a Research Fellow of Eastman Kodak Company in Rochester, New York, where he held many R&D managerial positions from 1982 to 2006, and is the author of sixty technical articles, and 33 issued US Patents. A native of Taiwan, Dr. Chen holds a Ph.D. degree from Polytechnic University in Brooklyn, New York (1982), and is a graduate of Senior Executive Program, Stanford University, CA. In ITRI, Dr. Chen and his staff focus on new display and advanced technology research, including flexible displays, substrates, metal oxide TFTs and electrowetting displays. Dr. Chen, an Associate Editor of IEEE/OSA Journal of Display Technology, is presently the Vice Chairman of Taiwan Display Union Association.

xii

Editors-in-Chief

Wayne Cranton Nottingham Trent University UK

Wayne Cranton is Professor of Visual Technology at Nottingham Trent University, and Director of the Physical Sciences, Engineering and Computing Research Centre within the School of Science and Technology. Wayne obtained his Ph.D. in Electrical and Electronic Engineering from the University of Bradford in 1995, following an investigation into the growth and characterisation of thin films for electroluminescent devices. He then moved to Nottingham Trent University to continue the work on materials for electroluminescent displays with the Thin Film and Displays Research Group. His research is concerned with the study of thin film materials for electronic displays, sensors, and light emitting devices. This has involved a number of collaborative applied research and development programmes on the deposition and processing of phosphors, dielectrics, and metal oxide semiconductors, with recent emphasis on the localised photonic processing of materials for low temperature fabrication of flexible electronics and displays. In 2001, Wayne was a founding partner of the DisplayMasters Inter-University Programme in the UK, which brought together display experts and students from around the globe and which became the catalyst for the collaborations resulting in the Handbook of Visual Display Technology.

Editors-in-Chief

Mark Fihn Veritas et Visus USA

Mark Fihn currently heads his own consulting company called VeritaVis, where he supports the flat panel display industry based on his expertise related to notebook PCs, Tablet PCs, touch technologies, the LCD TV market, and display related human factors, including high resolution and wide aspect ratios. Veritas et Visus is part of this consultancy, enabling Mark to reach a broader audience in association with his research activities. Prior to VeritaVis, Mark worked for 3 years at the market research firm DisplaySearch. He additionally participated for 15 years in computer system and LCD-related procurement at Texas Instruments and Dell Computer while living in the United States and Taiwan. He has been active in many display-related areas, most specifically in publicly championing industry-wide adoption of high resolution displays, notebook LCD standardization, and video sub-system integration. Mark was educated at St. Olaf College (Northfield, Minnesota), the American Graduate School of International Management, (Phoenix, Arizona); St. Edward’s University, (Austin, Texas), and in the University of Texas at Austin’s doctoral program in International Business. Most recently, Mark has been an active supporter and lecturer at the DisplayMasters degree program in the UK, contributing course lectureships at Cambridge University, Dundee University, and Nottingham Trent University.

xiii

Editorial Board Karlheinz Blankenbach Pforzheim University Germany

Karlheinz Blankenbach has been with Pforzheim University since 1995 where he was one of the first professors of the newly established Technical Department. In 1998 he founded the Display Lab (http://www.displaylabor.de) which focuses on applied R&D on display systems, display driving and display metrology. In 2007 he was given an award by his university for outstanding and long-lasting research. The activities of Karlheinz and his team resulted in numerous projects funded by government and industry as well publications, talks and workshops. Karlheinz started his industrial career in 1990 at AEG (a subsidiary of DAIMLER), Ulm, Germany where he developed display electronics and LCDs for public information systems as used in airports and railway stations. He holds an M.Sc. (Diplom) in Physics and a Ph.D. degree, both from the University of Ulm, Germany. He is head of the advisory board for the conference ‘‘ELECTRONIC DISPLAYS’’ (http://www.electronic-displays.de/), chairman of German Flat Panel Forum (http://www.displayforum.de/), speaker of the technical committee for Displays of VDE/ITG and chairman of the Steering Committee ADRIA (advanced displays research initiative, www.adria-network.org).

xvi

Editorial Board

Stan Brotherton TFT Consultant UK

Stan Brotherton started his research career at the GEC, Hirst Research Laboratory, England, before taking a post-doctoral Research Fellowship at Southampton University in 1971. From there he moved to the Philips Research Laboratory, Redhill, England, where he was a Senior Principal Scientist, and he now works as an independent TFT Consultant. He has led a wide range of research projects investigating semiconductor devices, and related materials issues, where the devices have included MOSFETs and CCDs, power devices, and IR imaging devices. His most recent field of activity has been thin film transistors, within which he initiated the Philips research programme on poly-Si TFTs. Activity within this field has continued with consultancy contracts from a number of organisations. He has published 120 papers on the physics and technology of silicon devices, and in 1989 he was awarded a DSc by London University for published work on deep level defects in silicon. He has presented numerous invited and contributed papers at major international conferences, and has been a regular contributor of specialist lectures for Masters degree courses at Southampton and Dundee Universities.

Editorial Board

Jason Heikenfeld University of Cincinnati USA

Jason Heikenfeld received the B.S. and Ph.D. degrees from the University of Cincinnati in 1998 and 2001, respectively. During 2001–2005 Dr. Heikenfeld co-founded and served as principal scientist at Extreme Photonix Corp. In 2005 he returned to the University of Cincinnati as a Professor of Electrical Engineering. Dr. Heikenfeld’s university laboratory, The Novel Devices Laboratory www.secs.uc.edu/devices, is currently engaged in electrofluidic device research for lab-on-chip, optics, and electronic paper. Dr. Heikenfeld has now launched his second company, Gamma Dynamics, which is pursuing commercialization of electrofluidic displays. Dr. Heikenfeld is a Senior member of the Institute for Electrical and Electronics Engineers, a Senior member of the Society for Information Display, and a member of SPIE. Dr. Heikenfeld is an associate editor of IEEE Journal of Display Technology, and an IEEE National SPAC Speaker on the topic of entrepreneurship.

xvii

xviii

Editorial Board

Jon Peddie Jon Peddie Research USA

Jon Peddie is a pioneer of the graphics industry, starting his career in computer graphics in 1962. After the successful launch of several graphics manufacturing companies, Peddie began Jon Peddie Associates in 1984 to provide comprehensive data, information and management expertise to the computer graphics industry. With those same goals in mind, he left JPA to form Jon Peddie Research in 2001 to provide a more customer intimate environment for clients, and to further explore the business of multimedia. Peddie lectures at numerous conferences on topics pertaining to graphics technology and the emerging trends in digital media technology. He is frequently quoted in trade and business publications, and contributes articles to numerous publications as well as appearing on CNN and TechTV. Peddie is also the author of several books and a contributor to Advances in Modeling, Animation, and Rendering. Jon Peddie is recognized as one of the leading analysts in the USA by AdWeek Magazine.

Editorial Board

Peter Raynes University of York UK

Peter Raynes joined the Royal Signals and Radar Establishment at Malvern in 1971, and moved to the Sharp Laboratories of Europe Ltd at Oxford in 1992, where he became Director of Research. He took up the Chair of Optoelectronics in the Department of Engineering Science at Oxford University in 1998, and in 2010 joined the Department of Chemistry, University of York, as a Leverhulme Emeritus Research Fellow. He has played a key role in developing liquid crystal displays to the pre-eminent position they hold today, with over 130 published papers and over 60 filed patent applications. He was responsible for two key device inventions (Supertwist LCD and defect-free Twisted Nematic LCD) which were both licensed to the world’s major manufacturers, are widely used in products and resulted in considerable royalties to QinetiQ. He has also contributed to several highly successful ranges of liquid crystal materials that resulted in Queen’s Awards for Technological Achievement in 1979 and 1992. He was awarded the Rank Prize for Opto-electronics in 1980, the Paterson Medal of the Institute of Physics in 1986, the 2009 Jan Rajchman Prize and a Special Recognition Award in 1987 of the Society for Information Display, and the G W Gray Medal of the British Liquid Crystal Society in 2001. He is a Fellow of the Royal Society, the Institute of Physics and the Society for Information Display.

xix

xx

Editorial Board

Jannick Rolland University of Rochester USA

Jannick Rolland earned a Diploma from the Institut D’Optique in 1984, and MS (1985) and Ph.D. (1990) degrees in Optical Science from the University of Arizona. As a postdoctoral fellow in the Department of Computer Science at the University of North Carolina at Chapel Hill (UNC-CH), she started her work in head-worn display design and fabrication. Today her work focuses on freeform optics and compact eyeglass formats. With a strong interest in human perception, Dr. Rolland headed the UNC-CH Vision Research Group for Medical Displays (1992–1996). In 1996, she joined the College of Optics and Photonics at the University of Central Florida (1996–2008) where she built the Optical Diagnostics and Applications Laboratory (www.ODALab-spectrum.org). In 2009, she joined the Institute of Optics at the University of Rochester as the Brian J. Thompson (endowed) Professor of Optical Engineering and as Associate Director of the R.E. Hopkins Center for Optical Design and Engineering, together with joint appointments in the Department of Biomedical Engineering and in the Center for Visual Science. She serves the Institut d’Optique in Paris as invited Professor for their Pole Aquitaine in the Bordeaux region in an effort to help develop a new focus area in 3D scientific visualization and Augmented Reality. Professor Rolland served on the editorial board of the Journal Presence (MIT Press) (1996–2006), and as Associate Editor of Optical Engineering (1999–2004). She is a Fellow of the Optical Society of America and SPIE, a senior member of IEEE, and a member of SID. She is a Director at Large on the board of the Optical Society of America (2010–2012).

Advisory Panel Thomas Coughlin Coughlin Associates USA

Robert Phares Consultant USA

Matthew Forman Create 3D UK

David Rodley University of Dundee UK

Teresa Goodman National Physical Laboratory UK

Mervyn Rose University of Dundee UK

Hideo Hosono Tokyo Institute of Technology Japan

Kalluri Sarma Honeywell International USA

Jyrki Kimmel Nokia Finland

Graham Saxby Consultant UK

Vasudevan Lakshminarayanan University of Waterloo Canada

Ian Underwood University of Edinburgh UK

Paul Lippens Umicore Belgium

Tim Wilkinson University of Cambridge UK

Lesley Parry-Jones Sharp Laboratories of Europe UK

Chris Williams Logystyx UK

Table of Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Editors-in-Chief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi Editorial Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv Advisory Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi List of Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xli

Volume 1 Section 1 – Fundamentals of Optics for Displays 1.1 1.1.1

Properties of Light Properties of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Timothy D. Wilkinson

1.2 1.2.1

Geometric Optics Geometric Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Timothy D. Wilkinson

1.3 1.3.1

Optical Modulation Optical Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Timothy D. Wilkinson

Section 2 – Human Vision and Photometry 2.1 2.1.1

Vision and Perception Anatomy of the Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Christine Garhart . Vasudevan Lakshminarayanan

2.1.2

Light Detection and Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Vasudevan Lakshminarayanan

xxiv

Table of Contents

2.1.3

Visual Acuity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Vasudevan Lakshminarayanan

2.1.4

Flicker Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Vasudevan Lakshminarayanan

2.1.5

Spatial Vision and Pattern Perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 L. Srinivasa Varadharajan

2.1.6

Binocular Vision and Depth Perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Robert Earl Patterson

2.2 2.2.1

Color Science Color Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Stephen Westland

2.2.2

The CIE System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Stephen Westland

2.2.3

RGB Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Stephen Westland . Vien Cheung

2.2.4

CMYK Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Stephen Westland . Vien Cheung

2.2.5

Uniform Color Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Vien Cheung

2.2.6

Color Perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Marina Bloj . Monika Hedrich

2.2.7

Colour Vision Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Vasudevan Lakshminarayanan

2.3 2.3.1

Visual Ergonomics Displays in the Workplace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Sarah Sharples

2.3.2

Display Screen Equipment: Standards and Regulation . . . . . . . . . . . . . . . . . . 203 Sarah Atkinson

2.4 2.4.1

Photometry Light Emission and Photometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 Teresa Goodman

Table of Contents

2.4.2

Measurement Instrumentation and Calibration Standards . . . . . . . . . . . . . . 229 Teresa Goodman

2.4.3

Overview of the Photometric Characterisation of Visual Displays . . . . . . 243 Teresa Goodman

Section 3 – Image Storage and Processing 3.1 3.1.1

Introduction to Electronic Imaging Introduction to Electronic Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 Jon Peddie

3.2 3.2.1

Image Storage and Compression Digital Image Storage and Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Tom Coughlin

3.2.2

Video Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 Scott Janus

3.2.3

Fundamentals of Image Color Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Matthew C. Forman . Karlheinz Blankenbach

3.3 3.3.1

Image Manipulation Digital Image Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Matthew C. Forman

3.3.2

Signal Filtering: Noise Reduction and Detail Enhancement . . . . . . . . . . . . . 325 Karl G. Baum

3.3.3

TV and Video Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 Scott Janus

3.4 3.4.1

Case Study: Medical Imaging and Display Reliability and Fidelity of Medical Imaging Data . . . . . . . . . . . . . . . . . . . . . . . . . 363 Alfred Poor

3.4.2

Ultrasound Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 Robert M. Nally

xxv

xxvi

Table of Contents

3.5 3.5.1

Case Study: Security Imaging and Display Data Hiding and Digital Watermarking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 Daniel Taranovsky

3.5.2

Biometrics and Recognition Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 Daniel Taranovsky

Section 4 – Driving Displays 4.1 4.1.1

Direct Drive, Multiplex and Passive Matrix Direct Drive, Multiplex and Passive Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 Karlheinz Blankenbach . Andreas Hudak . Michael Jentsch

4.2 4.2.1

Active Matrix Driving Active Matrix Driving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441 Karlheinz Blankenbach

4.3 4.3.1

Panel Interfaces Panel Interfaces: Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 Karlheinz Blankenbach

4.3.2

Serial Display Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471 Thomas Wirschem

4.3.3

High Definition Multimedia Interface (HDMI®) . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 Jim Chase

4.4 4.4.1

Embedded Systems Embedded Systems: Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 Karlheinz Blankenbach

4.4.2

Graphics Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503 Andreas Grimm

Table of Contents

4.4.3

FPGA IP Cores for Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511 Davor Kovacˇec

4.4.4

APIX: High Speed Automotive Pixel Link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531 Markus Ro¨mer

4.5 4.5.1

Signal Processing Tasks Video Processing Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549 Markus Schu

4.5.2

Dimming of LED LCD Backlights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567 Chihao Xu . Marc Albrecht . Tobias Jung

4.6 4.6.1

Power Supply Power Supply Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 Oliver Nachbaur

4.6.2

Power Supply Sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591 Oliver Nachbaur

Volume 2 Section 5 – TFTs and Materials for Displays and Touchscreens 5.1 5.1.1

Display Glass Glass Substrates for AMLCD, OLED and Emerging Display Platforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 599 Peter L. Bocko

5.2 5.2.1

Inorganic Semiconductor TFT Technology Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627 A. J. Flewitt

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs) . . . . . . . . . . . . . . 647 S. D. Brotherton

xxvii

xxviii

Table of Contents

5.3 5.3.1

Emerging TFT Technologies Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors . . . . 677 Hagen Klauk

5.3.2

Organic TFTs: Solution-Processable Small-Molecule Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 697 David Redinger . Marcia Payne

5.3.3

Organic TFTs: Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709 Feng Liu . Sunzida Ferdous . Alejandro L. Briseno

5.3.4

Oxide TFTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729 Hideo Hosono

5.3.5

Carbon Nanotube TFTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751 Axel Schindler

5.4 5.4.1

Transparent Conductors: ITO and ITO Replacements Indium Tin Oxide (ITO): Sputter Deposition Processes . . . . . . . . . . . . . . . . . . . 779 Paul Lippens . Uwe Muehlfeld

5.4.2

ITO Replacements: Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795 Axel Schindler

5.4.3

ITO Replacements: Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 809 Wilfried Lo¨venich . Andreas Elschner

5.4.4

ITO Replacements: Insulator-Metal-Insulator Layers . . . . . . . . . . . . . . . . . . . . . 819 Bernd Szyszka

5.5 5.5.1

Patterning Processes Photolithography for Thin-Film-Transistor Liquid Crystal Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835 Wen-yi Lin . W. B. Wu . K. C. Cheng . Hsin Hung Li

5.5.2

Wet Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 861 Hua-Chi Cheng

5.5.3

Dry Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 871 Eugen Stamate . Geun Young Yeom

Table of Contents

5.6 5.6.1

Flexible Displays Flexible Displays: Attributes, Technologies Compatible with Flexible Substrates and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885 Kalluri R. Sarma

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 897 Kalluri R. Sarma

5.7 5.7.1

Touchscreen Technologies Introduction to Touchscreen Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935 Robert Phares . Mark Fihn

5.7.2

Transparent Conducting Coatings on Polymer Substrates for Touchscreens and Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975 Charles A. Bishop

5.7.3

Anisotropic Conductive Adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 989 Peter J. Opdahl

5.7.4

Touchscreen Computer Interfaces: Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997 Lance Lamont . Carol Crawford

Section 6 – Emissive Displays 6.1 6.1.1

Inorganic Phosphors Luminescence of Phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1013 Robert Withnall . Jack Silver

6.1.2

Physics of Light Emission from Rare-Earth Doped Phosphors . . . . . . . . . 1019 Robert Withnall . Jack Silver

6.1.3

Chemistry and Synthesis of Inorganic Light Emitting Phosphors . . . . . 1029 Jack Silver . Robert Withnall

6.2 6.2.1

Cathodoluminescent Displays Cathode Ray Tubes (CRTs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043 Gerhard Gassler

xxix

xxx

Table of Contents

6.2.2

Vacuum Fluorescent Displays (VFDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1055 Andrew Stubbings

6.2.3

Field Emission Displays (FEDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1071 Yongchang Fan . Mervyn Rose

6.2.4

New Field Emission Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1105 Mervyn Rose . Yongchang Fan

6.3 6.3.1

Plasma Display Panels Plasma Display Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1139 David N. Liu

6.4 6.4.1

Light Emitting Diode (LED) Displays Light Emitting Diodes: Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 M. R. Krames

6.4.2

LED Display Applications and Design Considerations . . . . . . . . . . . . . . . . . . 1169 Robbie Thielemans

6.5 6.5.1

Inorganic Electroluminescent Displays Thin Film Electroluminescence (TFEL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1183 Adrian H. Kitai . Feng Chen

6.5.2

AC Powder Electroluminescence (ACPEL) and Devices . . . . . . . . . . . . . . . . . 1193 Feng Chen . Adrian H. Kitai

6.6 6.6.1

Organic Electroluminescent Displays Organic Light Emitting Diodes (OLEDS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209 Ruiqing Ma

6.6.2

Active Matrix for OLED Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1223 Ruiqing Ma

Table of Contents

Volume 3 Section 7 – Liquid Crystal Displays 7.1 7.1.1

Liquid Crystal Fundamentals and Materials Materials and Phase Structures of Calamitic and Discotic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1243 J. W. Goodby

7.1.2

Introduction to Defect Textures in Liquid Crystals . . . . . . . . . . . . . . . . . . . . . 1289 J. W. Goodby

7.1.3

Liquid Crystal Materials for Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1315 Melanie Klasen-Memmer . Harald Hirschmann

7.1.4

Physical Properties of Nematic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . 1343 Carl V. Brown

7.2 7.2.1

Liquid Crystal Material Physics Optics of Liquid Crystals and Liquid Crystal Displays . . . . . . . . . . . . . . . . . . 1365 Philip W. Benzie . Steve J. Elston

7.2.2

Alignment Properties of Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387 Lesley Parry Jones

7.2.3

Liquid Crystal Theory and Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1403 N. J. Mottram . C. J. P. Newton

7.3 7.3.1

LCD Device Technology Twisted Nematic and Supertwisted Nematic LCDs . . . . . . . . . . . . . . . . . . . . . 1433 Peter Raynes

7.3.2

Smectic LCD Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445 Per Rudquist

7.3.3

In-Plane Switching (IPS) Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469 Hyungki Hong

7.3.4

Vertically Aligned Nematic (VAN) LCD Technology . . . . . . . . . . . . . . . . . . . . . 1485 Hidefumi Yoshida

xxxi

xxxii

Table of Contents

7.3.5

Bistable Liquid Crystal Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1507 Cliff Jones

7.3.6

Cholesteric Reflective Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1545 David Coates

7.3.7

Polymer Dispersed LCDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1565 Francesco Bloisi . Luciano Rosario Maria Vicari

7.4 7.4.1

LCD Addressing Active Matrix Liquid Crystal Displays (AMLCDs) . . . . . . . . . . . . . . . . . . . . . . . . 1589 Mervyn Rose

7.5 7.5.1

LCD Backlights and Films LCD Backlights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1609 Gary Boyd

7.5.2

Optical Enhancement Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1625 Gary Boyd

7.6 7.6.1

LCD Production LCD Processing and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1649 Yoshitaka Yamamoto

7.7 7.7.1

Emerging Technologies The p-Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1675 Philip Bos

7.7.2

Flexoelectro-Optic Liquid Crystal Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1681 Harry J. Coles . Stephen M. Morris

Section 8 – Paper-Like and Low Power Displays 8.1 8.1.1

Colorant Transposition Displays Electrophoretic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1699 Karl Amundson

Table of Contents

8.1.2

In-Plane Electrophoretic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1715 Kars-Michiel H. Lenssen

8.1.3

Video-Speed Electrowetting Display Technology . . . . . . . . . . . . . . . . . . . . . . . 1731 Johan Feenstra

8.1.4

Droplet-Driven Electrowetting Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1747 Frank Bartels

8.1.5

Electrofluidic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1761 Kaichang Zhou . Jason Heikenfeld

8.2 8.2.1

MEMS-based Displays Mirasol® – MEMS-based Direct View Reflective Display Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1777 Ion Bita . Alok Govil . Evgeni Gusev

8.2.2

Time Multiplexed Optical Shutter Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1787 Daniel K. Van Ostrand . Ram Ramakrishnan

Section 9 – 3D Displays 9.1 9.1.1

3D Display Fundamentals Introduction to 3D Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1807 Mark Fihn

9.1.2

Human Factors of 3D Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1815 Robert Earl Patterson

9.2 9.2.1

Stereoscopic 3D Display Technology Introduction to Projected Stereoscopic Displays . . . . . . . . . . . . . . . . . . . . . . . 1825 Lenny Lipton

9.2.2

Addressing Stereoscopic 3D Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1831 Matthew C. Forman

9.2.3

3D Cinema Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1843 Bernard Mendiburu

xxxiii

xxxiv

Table of Contents

9.3 9.3.1

Autostereoscopic 3D Display Technology Autostereoscopic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1861 Adrian Travis

9.3.2

Head- and Eye-Tracking Solutions for Autostereoscopic and Holographic 3D Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1875 Enrico Zschau . Stephan Reichelt

9.3.3

Emerging Autostereoscopic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1899 Phil Surman . Ian Sexton

9.4 9.4.1

Volumetric & Pseudo-Volumetric 3D Display Technologies Volumetric 3D Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1917 Barry G. Blundell

9.4.2

Pseudo-Volumetric 3D Display Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1933 Ismo Rakkolainen

9.5 9.5.1

Holographic 3D Displays Principles of Display Holography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1945 Graham Saxby

9.5.2

Electronic Holographic Displays – 20 Years of Interactive Spatial Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1963 Mark E. Lucente

Volume 4 Section 10 – Mobile Displays, Microdisplays, Projection and Headworn Displays 10.1 10.1.1

Mobile Displays Introduction to Mobile Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1983 Jyrki Kimmel

10.1.2

Transflective Displays for Mobile Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1993 Jyrki Kimmel

10.1.3

Alternative Technologies for Mobile Direct-View Displays . . . . . . . . . . . . 2003 Jyrki Kimmel

Table of Contents

10.1.4

Liquid Crystal Optics for Mobile Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2013 Jyrki Kimmel

10.1.5

Energy Aspects of Mobile Display Technology . . . . . . . . . . . . . . . . . . . . . . . . . . 2023 Jyrki Kimmel

10.2 10.2.1

Microdisplay Technologies Introduction to Microdisplays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2033 Ian Underwood

10.2.2

Liquid Crystal on Silicon Reflective Microdisplays . . . . . . . . . . . . . . . . . . . . . . 2043 Ian Underwood

10.2.3

Transmissive Liquid Crystal Microdisplays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2057 Ian Underwood

10.2.4

MEMS Microdisplays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2067 Hakan Urey . Sid Madhavan . Margaret Brown

10.2.5

DLP® Projection Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2081 David W. Monk

10.2.6

OLED and Other Emissive Microdisplays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2095 Ian Underwood

10.3

Microdisplay Applications: Projection Systems

10.3.1

Methods of 2-D Image Formation in Microdisplay-based and Related Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2111 Ian Underwood

10.3.2

Digital Cinema Projection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2125 David W. Monk

10.3.3

Data Projectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2135 Patrick Vandenberghe

10.4 10.4.1

Microdisplay Applications: Head-Worn Displays (HWDs) See-Through Head Worn Display (HWD) Architectures . . . . . . . . . . . . . . . . 2145 Jannick P. Rolland . Kevin P. Thompson . Hakan Urey . Mason Thomas

xxxv

xxxvi

Table of Contents

10.4.2

Human Interface Factors Associated with HWDs . . . . . . . . . . . . . . . . . . . . . . . 2171 Robert Earl Patterson

10.4.3

Optical Components for Head-Worn Displays . . . . . . . . . . . . . . . . . . . . . . . . . . 2183 Ozan Cakmakci . Michael J. Hayford

10.4.4

Examples of HWD Architectures: Low-, Mid- and Wide-Field of View Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2195 Ozan Cakmakci . Jannick P. Rolland

10.5 10.5.1

Electronic Viewfinders Electronic Viewfinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2215 Ian Underwood . David Steven

10.6 10.6.1

Emerging Technologies Multifocus Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2229 Brian T. Schowengerdt . Eric J. Seibel

10.6.2

Occlusion Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2251 Kiyoshi Kiyokawa

10.6.3

Cognitive Engineering and Information Displays . . . . . . . . . . . . . . . . . . . . . . . 2259 Robert Earl Patterson . Jannick P. Rolland

Section 11 – Display Metrology 11.1 11.1.1

Introduction to Display Metrology Introduction to Display Metrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2275 Karlheinz Blankenbach

11.2 11.2.1

Standard Measurement Procedures Luminance, Contrast Ratio and Grey Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2289 Karlheinz Blankenbach

11.2.2

Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2307 Karlheinz Blankenbach

Table of Contents

11.3 11.3.1

Advanced Measurement Procedures Spatial Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2331 Karlheinz Blankenbach

11.3.2

Temporal Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2345 Karlheinz Blankenbach

11.3.3

Viewing Angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2367 Karlheinz Blankenbach

11.3.4

Ambient Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2383 Karlheinz Blankenbach

11.4 11.4.1

Display Technology-Dependent Issues Display Technology-Dependent Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2417 Karlheinz Blankenbach

11.5 11.5.1

Standards and Test Patterns Standards and Test Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2429 Karlheinz Blankenbach

11.6 11.6.1

Measurement Devices Measurement Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2447 Karlheinz Blankenbach

Section 12 – Display Markets and Economics 12.1 12.1.1

Introduction to Markets and Economics The Problem with Forecasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2471 Mark Fihn

12.1.2

Display Market Forecasting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2483 Ross Young

xxxvii

xxxviii

Table of Contents

12.2 12.2.1

Economic Considerations The Crystal Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2493 David Barnes

12.2.2

Opportunities for Alternative Display Technologies: Touchscreens, E-Paper Displays and OLED Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2499 Jennifer Colegrove

12.3 12.3.1

Computer Graphics The History of Graphics: Software’s Sway Over Silicon . . . . . . . . . . . . . . . . 2511 Adam Kerin

12.3.2

Design Tools: Imaging, Vector Graphics and Design Evolution . . . . . . . 2519 Kathleen Maher

12.4 12.4.1

High-Resolution Displays Introduction to High-Resolution Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2541 Mark Fihn

12.4.2

Eye Resolution Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2551 Norman Bardsley

12.4.3

The IBM T221 9.2M Pixel Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2563 Alan D. Jones

12.4.4

Multi-Display Desktops and the Case for More Pixels . . . . . . . . . . . . . . . . . . 2571 Andrew Garrard

12.5 12.5.1

High-Definition TV LCDs, Growth and Market Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2583 Bruce Berkoff

12.5.2

Quadrupling HD and Beyond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2589 Mark Fihn

12.6 12.6.1

3D Displays Trends in the 3D TV Market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2599 Chris Chinnock

Table of Contents

12.7 12.7.1

Green Technologies The Display Industry in a Green World . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2609 Keith J. Baker

12.7.2

Sustainability in LCD Manufacturing, Recycling and Reuse . . . . . . . . . . . 2621 Avtar Singh Matharu

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2641 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2663

xxxix

List of Contributors Marc Albrecht Institute of Microelectronics Saarland University Saarbruecken Germany Karl Amundson E Ink Corporation Cambridge, MA USA Sarah Atkinson Human Factors Research Group Department of Mechanical, Materials and Manufacturing Engineering University of Nottingham Nottingham UK Keith J. Baker Glasgow Caledonian University Glasgow, Scotland UK Norman Bardsley Bardsley Consulting Danville, CA USA David Barnes BizWitz LLC Georgetown, TX USA Frank Bartels Bartels Mikrotechnik GmbH Dortmund Germany

Karl G. Baum Qmetrics Technologies Rochester, NY USA Philip W. Benzie Department of Engineering Science University of Oxford Oxford UK Bruce Berkoff LCD TV Association Beaverton, OR USA Charles A. Bishop C.A.Bishop Consulting Ltd Shepshed, Nr. Loughborough Leicestershire UK Ion Bita Qualcomm MEMS Technologies San Jose, CA USA Karlheinz Blankenbach Display Lab Pforzheim University Pforzheim Germany Francesco Bloisi CNR-SPIN and Dipartimento di Scienze Fisiche Universita` di Napoli ‘‘Federico II’’ Naples Italy

xlii

List of Contributors

Marina Bloj Bradford School of Optometry and Vision Sciences University of Bradford Bradford UK Barry G. Blundell School of Computing and Mathematical Sciences Auckland University of Technology Auckland New Zealand Peter L. Bocko Glass Technologies Group Corning Incorporated Corning, NY USA Philip Bos Liquid Crystal Institute & Chemical Physics Program Kent State University Kent, OH USA Gary Boyd 3M Optical Systems Division 3M Center St. Paul, MN USA Alejandro L. Briseno Polymer Science and Engineering Conte Research Center University of Massachusetts Amherst, MA USA S. D. Brotherton TFT Consultant Forest Row UK

Margaret Brown Microvision Inc. NE Redmond, WA USA Carl V. Brown School of Science and Technology Nottingham Trent University Nottingham UK Ozan Cakmakci Optical Research Associates Optical Solutions Group of Synopsys, Inc. Pasadena, CA USA Jim Chase HDMI Licensing, LLC Sunnyvale, CA USA Feng Chen University of Waterloo Waterloo, ON Canada K. C. Cheng Manufacturing Technology Center AU Optronics Corporation Taichung, Taiwan Hua-Chi Cheng Display Technology Center Industrial Technology Research Institute (ITRI) Hsinchu, Taiwan PRC Vien Cheung School of Design University of Leeds Leeds UK

List of Contributors

Chris Chinnock Insight Media Norwalk, CT USA David Coates R2Tek Culham Innovation Centre Abingdon UK Jennifer Colegrove Emerging Display Technologies DisplaySearch Santa Clara, CA USA Harry J. Coles Centre of Molecular Materials for Photonics and Electronics Department of Engineering University of Cambridge Cambridge UK Tom Coughlin Coughlin Associates Atascadero, CA USA Carol Crawford Microchip Technology Inc Chandler, AZ USA Andreas Elschner Heraeus Clevios GmbH Leverkusen Germany Steve J. Elston Department of Engineering Science University of Oxford Oxford UK

Yongchang Fan Division of Electronic Engineering and Physics University of Dundee Nethergate, Dundee UK Johan Feenstra Liquavista BV Eindhoven, AG The Netherlands Sunzida Ferdous Polymer Science and Engineering Conte Research Center University of Massachusetts Amherst, MA USA Mark Fihn Veritas et Visus Temple, TX USA A. J. Flewitt Electrical Engineering Division Cambridge University Cambridge UK Matthew C. Forman Create 3D Sheffield UK Christine Garhart College of Optometry University of Missouri St. Louis, MO USA Andrew Garrard Consultant UK

xliii

xliv

List of Contributors

Gerhard Gassler Samtel Electron Devices GmbH Ulm Germany J. W. Goodby Department of Chemistry University of York York UK Teresa Goodman National Physical Laboratory Teddington, Middlesex UK Alok Govil Qualcomm MEMS Technologies San Jose, CA USA Andreas Grimm Fujitsu Semiconductor Europe GmbH Graphic Competence Center Neuried Germany Evgeni Gusev Qualcomm MEMS Technologies San Jose, CA USA Michael J. Hayford Optical Research Associates Optical Solutions Group of Synopsys, Inc. Pasadena, CA USA Monika Hedrich Bradford School of Optometry and Vision Sciences University of Bradford Bradford UK

Jason Heikenfeld Novel Devices Laboratory School of Electronics and Computing Systems University of Cincinnati Cincinnati, OH USA Harald Hirschmann Merck KGaA Performance Materials Business Unit Liquid Crystals Darmstadt Germany Hyungki Hong Department of Visual Optics Seoul National University of Science and Technology Nowong-gu, Seoul South Korea Hideo Hosono Frontier Research Center & Materials and Structures Laboratory Tokyo Institute of Technology Japan Andreas Hudak Display Lab Pforzheim University Pforzheim Germany Hsin Hung Li Manufacturing Technology Center AU Optronics Corporation Taichung, Taiwan Scott Janus Visual and Parallel Computing Group Intel Corporation Folsom, CA USA

List of Contributors

Michael Jentsch Display Lab Pforzheim University Pforzheim Germany Alan D. Jones Consultant Salisbury, Wilts UK Lesley Parry Jones Sharp Laboratories of Europe Limited Oxford UK Cliff Jones ZBD Displays Ltd Malvern, Worcestershire UK Tobias Jung Institute of Microelectronics Saarland University Saarbruecken Germany Adam Kerin Intel Corp Santa Clara, CA USA Jyrki Kimmel Nokia Research Center Tampere Finland

Melanie Klasen-Memmer Merck KGaA Performance Materials Business Unit Liquid Crystals Darmstadt Germany Hagen Klauk Max Planck Institute for Solid State Research Stuttgart Germany Davor Kovacˇec Xylon d.o.o. Zagreb Croatia M. R. Krames Soraa, Inc. Fremont, CA USA Vasudevan Lakshminarayanan School of Optometry and Departments of Physics and Electrical Engineering University of Waterloo Waterloo, Ontario Canada and Michigan Center for Theoretical Physics University of Michigan Ann Arbor, MI USA

Adrian H. Kitai McMaster University Hamilton, ON Canada

Lance Lamont Microchip Technology Inc Chandler, AZ USA

Kiyoshi Kiyokawa Cybermedia Center Osaka University Toyonaka, Osaka Japan

Kars-Michiel H. Lenssen Philips Research HTC34-51 Eindhoven, AE The Netherlands

xlv

xlvi

List of Contributors

Wen-yi Lin Manufacturing Technology Center AU Optronics Corporation Taichung, Taiwan

Sid Madhavan Microvision Inc. NE Redmond, WA USA

Paul Lippens UMICORE – Thin Film Products Olen Belgium

Kathleen Maher Jon Peddie Research Tiburon, CA USA

David N. Liu Display Technology Center Industrial Technology Research Institute (ITRI) Hsinchu, Taiwan ROC

Avtar Singh Matharu Department of Chemistry Green Chemistry Centre of Excellence University of York UK

Lenny Lipton Lipton IP Los Angeles, CA USA

Bernard Mendiburu VP Innovation Volfoni Los Angeles, CA USA

Feng Liu Polymer Science and Engineering Conte Research Center University of Massachusetts Amherst, MA USA Mark E. Lucente Consultant Austin, TX USA Wilfried Lo¨venich Heraeus Clevios GmbH Leverkusen Germany Ruiqing Ma Universal Display Corporation Ewing, NJ USA

David W. Monk Engineering and Mathematical Sciences City University London and European Digital Cinema Forum London UK

Stephen M. Morris Centre of Molecular Materials for Photonics and Electronics Department of Engineering University of Cambridge Cambridge UK

N. J. Mottram Department of Mathematics and Statistics University of Strathclyde Glasgow UK

List of Contributors

Uwe Muehlfeld AKT Display Products Applied Materials GmbH & Co. KG Alzenau Germany Oliver Nachbaur Advanced Low Power Solutions Display Power Texas Instruments Deutschland GmbH Freising Germany Robert M. Nally CRO, Innovative Card Scanning, Inc Plano, TX USA C. J. P. Newton Hewlett-Packard Laboratories Bristol UK Peter J. Opdahl Ito Corporation Chuo-ku, Tokyo Japan Daniel K. Van Ostrand Research and Development Uni-Pixel Displays, Inc. The Woodlands, TX USA Robert Earl Patterson Air Force Research Laboratory Wright-Patterson AFB, OH USA Marcia Payne Outrider Technologies, LLC Lexington, KY USA

Jon Peddie Jon Peddie Research Tiburon, CA USA Robert Phares Display Sourcing & Service LLC Knoxville, TN USA Alfred Poor HDTV Almanac Perkasie, PA USA Ismo Rakkolainen School of Information Sciences University of Tampere Tampere Finland Ram Ramakrishnan Uni-Pixel Displays, Inc. The Woodlands, TX USA Peter Raynes Department of Chemistry University of York York UK David Redinger 3M Company St. Paul, MN USA Stephan Reichelt SeeReal Technologies GmbH Dresden Germany Markus Ro¨mer Inova Semiconductors GmbH Munich Germany

xlvii

xlviii

List of Contributors

Jannick P. Rolland Institute of Optics University of Rochester Rochester, NY USA

Markus Schu 3D Impact Media R&D Technology Center Munich Germany

Mervyn Rose Division of Electronic Engineering and Physics University of Dundee Nethergate, Dundee UK

Eric J. Seibel Department of Mechanical Engineering Human Photonics Laboratory, and Human Interface Technology Laboratory University of Washington Seattle, WA USA

Per Rudquist Department of Microtechnology and Nanoscience Chalmers University of Technology Go¨teborg Sweden Kalluri R. Sarma Honeywell Phoenix, AZ USA Graham Saxby University of Wolverhampton (retired) West Midlands UK Axel Schindler Institute for System Theory and Display Technology University of Stuttgart Stuttgart Germany Brian T. Schowengerdt Department of Mechanical Engineering Human Photonics Laboratory, and Human Interface Technology Laboratory University of Washington Seattle, WA USA

Ian Sexton Imaging and Displays Research Group De Montfort University Leicester UK Sarah Sharples Human Factors Research Group University of Nottingham Nottingham UK Jack Silver School of Engineering and Design Wolfson Centre for Materials Processing Brunel University Uxbridge, Middlesex UK Eugen Stamate Risø DTU Technical University of Denmark Roskilde Denmark David Steven Optovise Ltd Roslin, Edinburgh UK

List of Contributors

Andrew Stubbings Itron UK Limited Great Yarmouth, Norfolk UK Phil Surman Imaging and Displays Research Group De Montfort University Leicester UK Bernd Szyszka Department of Large Area Coating Fraunhofer Institute for Surface Engineering and Thin Films (IST) Braunschweig Germany Daniel Taranovsky Advanced Micro Devices Markham, ON Canada

Robbie Thielemans Inverto NV Evergem Belgium

Mason Thomas Microvision Inc. Redmond, WA USA

Kevin P. Thompson Synopsys, Inc., Rochester Rochester, NY USA Adrian Travis Applied Sciences Group Microsoft Corporation Redmond, WA USA

Ian Underwood School of Engineering University of Edinburgh Edinburgh UK

Hakan Urey Department of Electrical Engineering Koc¸ University Istanbul Turkey Patrick Vandenberghe Consultant Harelbeke Belgium L. Srinivasa Varadharajan Srimathi Sundari Subramanian Department of Visual Psychophysics Elite School of Optometry Unit of Medical Research Foundation Chennai, Tamil Nadu India Luciano Rosario Maria Vicari CNR-SPIN and Dipartimento di Scienze Fisiche Universita` di Napoli ‘‘Federico II’’ Naples Italy Stephen Westland School of Design University of Leeds Leeds UK Timothy D. Wilkinson Centre of Molecular Materials for Photonics and Electronics (CMMPE) Electrical Engineering Division University of Cambridge Cambridge UK

xlix

l

List of Contributors

Thomas Wirschem High Speed Data Path Division National Semiconductor Santa Clara, CA USA

Robert Withnall School of Engineering and Design and Wolfson Centre for Materials Processing Brunel University Uxbridge, Middlesex UK

W. B. Wu Manufacturing Technology Center AU Optronics Corporation Taichung, Taiwan

Chihao Xu Institute of Microelectronics Saarland University Saarbruecken Germany

Yoshitaka Yamamoto Display Technology Laboratories Corporate R & D group Sharp Corporation Tenri, Nara Japan

Geun Young Yeom Advanced Materials Science and Engineering Sungkyunkwan University Suwon South Korea

Hidefumi Yoshida Display Engineering Laboratories Corporate R & D Group Sharp Corporation Tenri, Nara Japan

Ross Young SVP Displays LEDs and Lighting IMS Research Austin, TX USA

Kaichang Zhou Gamma Dynamics Cincinnati, OH USA

Enrico Zschau SeeReal Technologies GmbH Dresden Germany

Section 1

Fundamentals of Optics for Displays

Part 1.1

Properties of Light

1.1.1 Properties of Light Timothy D. Wilkinson 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2

Coherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3

Irradiance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4

Material Properties for Optical Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5

Optical Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6 Polarized Light and Birefringence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.1 Retardation Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_1.1.1, # Springer-Verlag Berlin Heidelberg 2012

6

1.1.1

Properties of Light

Abstract: In order to understand the physical properties, design, and fabrication of any display technology, it is essential to have a good appreciation of the basic physics involved in the light sources and materials used in their construction. This does not mean that to be a display engineer you need an in-depth knowledge of Maxwell’s equations, but rather you need to understand the properties of light that lead to its control, propagation, and modulation. Key to this is an understanding of the basic wave properties of light and how this leads to an energy transfer, and properties such as polarization which allow light to be controlled and manipulated. From these properties also stems the concept of optical coherence which dictates which set of rules needs to be applied to understand how light interacts with the display technology and its environment. The wave properties of light also dictate many aspects of the display performance from wavelength and color control through to optical efficiency and dispersive effect. These can all be explained and analyzed using often simple properties and models to build a picture of how well a display works. This section is designed to introduce some of the fundamental concepts behind light and its propagation without getting buried in the heavy mathematics or physics which often lies just beneath the surface. By using simple analogies, a very powerful analysis can be performed as long as the correct assumptions are made about the fundamental properties of light, its sources and propagation.

1

Introduction

Light is a very complex notion which stems from the general properties of electromagnetic radiation. However, to use light in applications such as displays, it is often only the simplest concepts of propagation that are required. The basic theory of light propagation stems from complicated mathematical theories such as Maxwell’s equations [1] which form the fundamental basic theory; however, from these relationships emerge a few basic concepts such as rays, waves, and polarization which can be used to solve the majority of display problems. The properties of light have been postulated for many centuries and there are still many phenomena which are not fully understood. The first significant steps were made by Newton, in his book Opticks, who described light as a stream of particles or corpuscles [2]. The wave properties of light were first published by Grimaldi [3] and then further developed by Huygens [4]. The debate about the properties of light has continued right through to modern quantum theory, with major contributions from Maxwell and Schro¨dinger [5]. The modern concept of light has shown that it has both wave and particle (now referred to as photon) properties and is the heart of duality theory. The simplest way to understand the properties of light is to apply the correct theoretical mechanism to the problem that is to be solved. In the case of displays, most problems can be solved using either photons, through ray or geometrical optics, or by wave propagation or wave optics. Hence we will discuss these two techniques in simple detail. The ability to solve problems with simple theory quickly becomes limited in complex optical systems and modern optical software can take a large amount of the pain out of solving these problems, however it is important to understand the fundamentals on which these packages operate. A good mechanism for visualizing the propagation of light is as rays or wavefronts as shown in > Fig. 1. This is an idealized representation as it does not take into account what happens to small features or edges in the optical path; however, it is a very powerful tool for solving optical problems. The more complex properties of light at edges and boundaries are covered by diffraction theory. Different levels of difficulty can be included in ray theory from simple geometric rays to paraxial rays, skew rays, and aberrations.

Properties of Light

1.1.1

Direction of propagation

Ray

a

b

. Fig. 1 Light propagation as (a) rays and (b) wavefronts

Ray and wavefront propagation are the same representations using different geometric properties, hence problems can be solved with either technique. A ray can be drawn as if extending from a wavefront as shown in > Fig. 1b and a wavefront can be generated from a group of rays using Huygens principle of wavelet propagation. In most display applications, geometric ray theory will be used, but wavefronts could equally be applied to solve the problem. The wave properties of light can used to generate wavefronts which propagate like ripples on the surface of a pond. Light propagation can be expressed in both space and time; however, the two dimensions are separable and can be solved independently. A typical propagating optical wave can be expressed as:
z t  c ¼ Acos 2p  l t where A is the amplitude of the wave propagating in space z and time t, l is the wavelength of the light, and n ¼ 1=t is the velocity of the wave. A more common representation is: c ¼ Acos ðkz  ot Þ where k ¼ 2p=l is the wave number and o is the angular frequency of the wave. The cosine function represents the oscillatory nature of light and will often be presented is the harmonic form using a complex exponential. Above is the scalar form of the wave, but it is also possible to express it in vector form by adding suitable unit vector to the wave number k. James Clerk Maxwell forged the connection between light and electromagnetism discovered by Michael Faraday. He formulated four important equations describing Faraday’s discoveries in mathematical terms. Using the terminology of vector calculus, these are: rH¼ rB¼0

@D þJ @t

rE¼ rD¼r

@B @t

where H is the magnetic field strength, D is the electric flux density, t is time, J is the current density, B is the magnetic flux density, E is the electric field strength, and r is the volume density of charge. These equations show that each field vector obeys a wave equation; that a varying electric field is accompanied by a varying orthogonal magnetic field and vice versa; and that the two form an electromagnetic field that can propagate as a transverse wave.

7

1.1.1

Properties of Light

320 nm

1024 1021

1015 1012 109

Gamma rays 1pm

400 nm X-rays

1nm

Solar spectrum

1018 Frequency (Hz)

8

UV Visible

1μ m IR 1m

760 nm 1m Radio

106

1km

3,500 nm

103 Wavelength 100

. Fig. 2 The electromagnetic spectrum

The speed of light had been established in 1849. He was able to relate it to purely electrical and magnetic constants by the equation 1 c ¼ pffiffiffiffiffiffiffiffiffi e0 m0 where e0 and m0 are, respectively, the permittivity and permeability of free space. A key property of light is its wavelength l, as this represents its spatial frequency which can be related to the speed of light c, as c ¼ f l. The wavelength and frequency of light have similar properties to those more commonly associated with radio and microwaves. Maxwell’s insight paved the way for Heinrich Hertz to discover and identify radio waves and to show that they behaved in a similar manner to light. The later discovery of further ranges of electromagnetic radiation led to the construction of the electromagnetic spectrum (> Fig. 2), running from the lowest usable radio waves to the highest frequency gamma radiation. The visible part of this enormous spectrum covers barely an octave, a wavelength range of approximately 380–750 nanometers (nm). This, however, matches closely the power spectrum of sunlight at the Earth’s surface. > Figure 2 also shows the different frequency bands as well as the visible spectrum of wavelengths critical to the field of displays.

2

Coherence

Optical coherence is difficult to define as there are many types and unusual definitions. If we assume that light has a basic wave property as described above, then we are assuming that light is fully coherent. Unfortunately such light sources are very rare and even the highest quality

Properties of Light

1.1.1

laser light source will have some degree of unpredictability. The measure of this unpredictability is referred to as its coherence properties and is often expressed in term of a source’s coherence length. The problem is that all light sources will have some degree of predictability; hence, coherence is often a widely miss-quoted term. For example, why is it possible to see diffraction patterns such as the replay from a hologram or the diffraction colors from a compact disk (CD) even when viewed with a thermally random light source such as a tungsten filament light bulb? The answer lies in the relative size of the features on the CD or hologram with respect to the coherence length of the light source. As long as the feature size is of the order of or less than the coherence length of the source, then diffraction effects will be seen. Even the most random of light sources will have some degree of predictability in its propagation, even if it is only over a few microns distance. Hence, there are two basic definitions that can be applied to coherence, which depend on the relative feature sizes and light source predictability and statistics of emission. The first definition is the degree of axial coherence within the source. This means that there is some sort of relationship (not random) between the temporal and spatial emission of optical energy in the same direction as the propagation of the light energy. This is shown in > Fig. 3 and the coherence length is defined as the distance the light propagates before the ability to predict where you are on the wave is lost. We can also define the source’s spatial coherence. This is when a light source has a spatial distribution or physical area and there is some form of relationship (not random) between energy emitted from different positions across the area of the light source. This is shown in > Fig. 4. Different light sources will have different degrees of either axial or spatial coherence based on the physics of the process used to generate light. Thermal sources such as tungsten filament light bulbs tend to be very random and so have a very short (a few microns) coherence lengths, whereas sources based on stimulated emission trapped within a defined optical cavity (such as found in a laser) tend to have long (often meters) coherence lengths. Light sources such as arc

z

Source Coherence length

. Fig. 3 Definition of coherence length for axial propagation of light

. Fig. 4 Definition of spatial coherence in the propagation of light

9

10

1.1.1

Properties of Light

lamps and light emitting diodes (LEDs) often are classified as partially coherent as they exist somewhere between the two extremes. The most important reason for defining coherence properties of the light source is that it dictates which propagation theory and principles can be used to understand its properties within a given application. For instance, coherent sources such as lasers which have a high degree of predictability are subject to the rules of diffraction where constructive and destructive interference can occur between waves as seen in > Fig. 5. On the other hand, incoherent light sources can be modelled using very simple principles based on the average energy and direction of propagation as defined in > Fig. 6. The vast

Constructive

Destructive

. Fig. 5 Constructive and destructive interference of waves

. Fig. 6 Incoherent interference of waves

Properties of Light

1.1.1

majority of optical design that is done in displays is based on either harnessing or suppressing the effects that can be due to the coherence properties of the light source [6, 7]. As the majority of light sources are relatively incoherent, simple geometric rules as used by ray tracing systems apply. More recent developments into bright arc sources and LEDs along with laser projection (both holographic and direct) have meant that it is increasingly important to understand coherent diffraction–based properties of light within a given display system [8].

3

Irradiance

Another significant property of electromagnetic radiation is that it transports energy; hence, it is logical to represent this as an energy density, u, or radiant energy per unit volume. This is dealt with in more detail in the section on radiometry and photometry (see > Part 2.4). In terms of electric field E or magnetic flux density B, this can be expressed as: u ¼ e0 E 2 ¼

1 2 B m0

where e0 is the electric permittivity or dielectric constant of free space (8:85  1012 C2 N1 m2 ) and m0 is the permeability of free space (4p  107 Ns2 C2 ). To represent the flow of electromagnetic energy, let S represent the transport of energy per unit time (the power) across a unit area. If we assume that the energy is flowing in the same direction as that of the propagation, then we can express S as a vector S such that 1 S¼ EH 2 The magnitude of S is the power per unit area crossing a surface whose normal is parallel to S and is often referred to as the Poynting vector. E represents the electric field and H represents the magnetic field (B ¼ mH) which are always orthogonal to one another. The term E  H will vary from maxima to minima, and S will vary in accordance to this. At optical frequencies, this is very fast indeed, hence it is often more logical to talk about the time averages of such quantities. The time averaged value of the magnitude of the Poynting vector is symbolized by hS i and is a measure of the important optical quantity known as the irradiance   c  2 B : I  hS i ¼ e0 c E 2 ¼ m0

4

Material Properties for Optical Components

The job of the optical component design engineer is to marry together many different specifications along with the packaging and environmental specifications in order to create a commercially cost-effective component. Two very important considerations are the choice of materials used to make the waveguide and the fabrication methods employed.

11

12

1.1.1 Radio

Properties of Light

Visible

Microwave Infrared

Applied electric fields

Ultraviolet

Optical comm.s

Field induced polarization Dielectric loss Breakdown Relaxation Phonons, Ionic drift Vibrations Librational modes Relaxation

X-ray

Various processes and ambient conditions Electronic absorption spectra

Inner shell and nuclear absorption spectra

. Fig. 7 Optical properties often found in display materials

One of the key aspects to understanding the material properties is to recognize the phenomena which may cause effects at certain wavelengths. > Figure 7 describes some of the more common phenomena, including those that may exist outside the visible light band, but have a strong physical or chemical effect on a material’s optical properties: High frequency modulation (MHz, Microwave) ● Excitation of librational and vibrational modes in extended structures leads to dielectric loss (heating) and field-induced structural relaxation. ● Distinct spectral features lead to fluctuations in the dielectric function (with respect to frequency) and cause difficulties in predicting performance (and design) of travelling wave electrodes. ● Some structural moieties (and trapped species) can absorb so strongly that damage results. Infrared ● Fundamental vibrational spectra of chemical species, atom–atom bonds, and their overtones. ● In intense fields, the sub-resonant absorption can lead to multiphoton events; this can be the precursor to optical damage. ● Some specific photoinduced chemical reactions are driven by near infrared absorption, e.g., triplet to singlet oxygen conversion leading to photo-oxidative decay of polymers. Visible ● Electronic absorption spectra (color vision), causing optical loss. ● Strong absorption features underlie most nonlinear phenomena, charge transfer complexes, excitons, and other molecular orbital/band edge phenomena. ● Optical damage driven by multiple photon absorption leading to defect formation in semiconductors and dielectrics, photolysis in many systems. The problem is in finding a unified theory which links dielectric properties to optical properties such as absorption, dispersion, and nonlinearities. A causal relationship linking

Properties of Light

1.1.1

absorption and the (complex) dielectric properties of all materials has been postulated and is known as the relation [9]: pffiffiffiffi n ¼ e1 ; e ¼ e1  ie2 ; a ¼ 2p  106  e2  logðeÞ=l; Z 0 2 u :e2 ðu0 Þ 0 du e1 ðuÞ  e1 ¼  p u02  u2 Z 2 e1 ðu0 Þ  e1 0 e2 ðuÞ ¼  :u  du p u02  u2 where n is refractive index; e’s are dielectric constants, 1 is Real, 2 is Imaginary, 1 at infinite frequency; a is the absorption; n is frequency; l is wavelength. This theory can be combined with the material’s response to applied fields to create a theory which describes the complex interaction of the material to applied fields via the concept of polarizability n2 ¼ e0 þ P=E; P ¼ e0  ðw  E þ w2  E 2 þ w3  E 3 :::Þ; n2 ¼ e0  ð1 þ w þ w2  E þ w3  E 2 :::Þ where P represents the interaction between the material and the electric field within the materials and the coefficients represent different nonlinear interactions and effects within the materials. They are known as the susceptibilities or hyper-polarizabilities and are directly related to fundamental materials constants wi /

miþ1 ðEg   hoÞi

:

Here the m are the dipole moments, the denominator is the energy difference between the band gap (for electronic absorption) and the energy of a photon at the observation frequency.

5

Optical Dispersion

When electromagnetic radiation enters a different medium, such as water or a piece of glass, it interacts with the medium in a very complex manner. The effect of this interaction can be expressed through either reflection or refraction as expressed in the next section of this document. A simple way of expressing certain interactions between the radiation and the medium is the medium’s refractive index. This is the ratio of the speed of light c to the speed of light in the medium v. n¼

c v

This interaction is not as simple as it looks, as it is a function of wavelength as well. The change in refractive index with wavelength is called dispersion and is often difficult to derive as it depends on the chemical composition of the medium. Dispersion can arise from many chemical and physical interactions; however, a common form of dispersion is due to

13

1.1.1

Properties of Light

Dense flint glass

1.7

Index of refraction n

14

1.6

Light flint glass

Crystal quartz 1.5

Bor osilicate crown glass Acrylic plastic Vitreous quartz

1.4

0

200

400

600

800

1,000

Wavelength λ (nm)

. Fig. 8 Dispersion for typical materials (refractive index versus wavelength) (Reprinted from [7])

resonance with the chemical components of the medium which can be expressed in the dispersion equation:   Nqe2 1 n2 ðoÞ ¼ 1 þ e0 me o20  o2 where qe is the electron charge, N is the number of electrons per unit volume, me is the electron mass and o0 is the resonant frequency. In fact, there will be a whole range of different interactions between atoms and components within the medium, which can be represented as a summation of the above equation for different resonant frequencies. This is made even more complicated when considering other electronic interactions such as fixed atomic boundaries. Hence the dispersion is a very complex concept, with many different contributing effects. Typical dispersion plots for some common optical materials are shown in > Fig. 8.

6

Polarized Light and Birefringence

Up to this point, it has been assumed that refractive index is a real property; however, it can also be complex. For almost all isotropic materials such as soda-lime glass or water, refractive index is a real property. For anisotropic materials such as Calcite and liquid crystals, the case is not so simple, as the interaction between the radiation and the medium depends on the direction of propagation. This is especially the case with particular crystal lattices and structures. This complex interaction is simplest to analyze using polarized light. Monochromatic, coherent light sources such as lasers can be represented in terms of an orthogonal set of propagating eigenwaves which are usually aligned to the x- and y-axis in a coordinate system with the direction of propagation along the z-axis as depicted in > Fig. 9.

1.1.1

Properties of Light

y

y

z

x

a

z

x

b

. Fig. 9 Vertically (a) and horizontally (b) polarized light

These eigenwaves can be used to describe the propagation of light through complex media. The Jones calculus invented by RC. Jones in 1940 [10] allows us to describe these waves and their propagation. There are also situations where the polarization is scrambled in a random manner leading to unpolarized light, but we are only interested in purely polarized light. If we have an electromagnetic wave propagating in the z direction along the x-axis then the light is classified as linearly polarized in the x direction or horizontally polarized. This wave can be represented as a Jones matrix, assuming an amplitude Vx   Vx V¼ 0 If the light is polarized in the direction of the y-axis, then we have linearly polarized light in the y direction of amplitude Vy or vertically polarized light   0 V ¼ Vy We can now combine these two eigenwaves to make any linear state of polarization we require. If the polarization of the light were to bisect the x- and y-axis by 45 , we could represent this as Vx = Vy and use the two states combined into a single Jones matrix:     Vx 1 ¼ Vx V ¼ Vy 1 We can also represent more complex states of polarization such as circular states. So far we have assumed that the eigenwaves are phase (i.e., they start at the same point). We can also introduce a phase difference f between the two eigenwaves, which leads to circularly polarized light. In these examples, the phase difference f is positive in the direction of the z-axis and is always measured with reference to the vertically polarized eigenwave (parallel to the y-axis); hence, we can write the Jones matrix:   Vx V ¼ Vy ejf There are two states which express circularly polarized light. If f is positive, then the horizontal component leads the vertical and the resultant director appears to rotate to the right around the z-axis in a clockwise manner and is right circularly polarized light. Conversely, if the

15

16

1.1.1

Properties of Light

horizontal lags the vertical, then the rotation is counterclockwise and the light is left circularly polarized. In the case of pure circularly polarized light, f = p/2 for right circular and f = p/2 for left circular:     1 1 Left circular V ¼ Vx Right circular V ¼ Vx j j Other values of f lead to elliptical polarization states which are more complex to analyze. If f = 0, then we have linearly polarized light at 45 to the y-axis and if f = p, then we have linearly polarized light at 135 to the y-axis.

6.1

Retardation Plates

Some crystals such as sodium chloride have a cubic molecular structure. When light passes through these structures it sees no preferred direction and is relatively unaffected. If the crystal has a structure such as hexagonal or triagonal, different directions of light will see very different crystalline structures. This effect is called birefringence, a property which is exploited in retarders. In a birefringent material, each eigenwave sees a different refractive index and will propagate at a different speed as in > Fig. 10. This leads to phase retardation between the two eigenwaves which is dependent on the thickness of the birefringent material and the wavelength of the light. The preferred directions of propagation within the crystal are defined as the fast (or extraordinary) axis and the slow (or ordinary) axis. An eigenwave that passes in the same direction as the fast axis sees a refractive index nf and the eigenwave that passes along the slow axis sees ns. For light of wavelength l passing through a birefringent crystal of thickness t, we define the retardation G as G¼

2pt ðnf  ns Þ l

The effect of this retardation can be expressed as a Jones matrix, assuming that the fast axis is in the same direction as the y-axis:   0 ejG=2 W0 ¼ 0 ejG=2

Vertical sees ne

. Fig. 10 Refractive index in anisotropic materials

Horizontal sees no

Properties of Light

1.1.1

It is more useful to be able to express the retardation from an arbitrary rotation of the fast axis by an angle c about the y-axis. Rotation with Jones matrices can be done as with normal matrix rotation. If we define a counterclockwise rotation of angle c about the axis y as positive then the rotation matrix is   cos c sin c R ðcÞ ¼ sin c cos c Hence the general form of the retardation plate is W ¼ R ðcÞW0 R ðcÞ which can be expanded right to left (in normal matrix fashion) to give  jG=2 2  cos c þ ejG=2 sin2 c jsin G2 sin ð2cÞ e W ¼ ejG=2 cos2 c þ ejG=2 sin2 c jsin G2 sin ð2cÞ This matrix now allows us to place an arbitrary waveplate in an optical system and illuminate it with polarized light. Half waveplate: A halfwave plate is a special example of the generalized retarder. In this case, the thickness of the plate has been chosen to give a phase retardation of exactly G = p. Hence, the Jones matrix for a halfwave plate at an angle c will be   cos 2c sin 2c j sin 2c cos 2c If the fast axis is aligned with the y-axis then c = 0, and the Jones matrix is   j 0 0 j A half waveplate can be used to rotate the direction of linearly polarized light from one linear state to another, which is a very useful property in an optical system. The quarter waveplate: In a similar fashion, we can tailor the thickness to give a quarter wave retardation of G = p/2. Such a waveplate is useful for converting to and from circularly polarized light. For a quarter wave plate with its fast axis aligned to the y-axis (c = 0), the Jones matrix will be   1 1j 0 pffiffiffi 0 1þj 2 Linear polarizers: An important function in an optical system is to be able to filter out unwanted polarization states while passing desired states. This can be done using polarizers which pass a single linear state whilst blocking all others (crossed) as shown in > Fig. 11. The polarizer can be written as a Jones matrix. If the direction of the polarizer is such that is passes vertically polarized light:  Py ¼

0 0

0 1



Similarly, the polarizer can be rotated about the z-axis by an angle c such that   0 0 P ¼ R ðcÞ R ðcÞ 0 1

17

18

1.1.1

Properties of Light

Polarizer

Polarizer

. Fig. 11 Principle of a linear polarizer

Halfwave plate 45⬚

H Polarizer

V Polarizer

. Fig. 12 Jones algebra example

Giving a generalized polarizer P¼



sin2 c  12 sin 2c 1  2 sin 2c cos2 c



Thus if we rotate the polarizer by 90 , we get a horizontal polarizer. In a similar manner we can make a right circular polarizer which passes right circularly polarized light, but blocks left circularly polarized light. We can now use Jones calculus to solve the propagation of light through optical systems. A combination of optical elements starting from left to right can be expressed as a series of matrix multiplications. For example, if we have a pair of crossed polarizers (vertical and horizontal), then there will be no light propagated through the system. If we place a halfwave plate with its fast axis at 45 to the y-axis between the two crossed polarizers, and illuminate with vertically polarized light, then the resultant will be as in > Fig. 12.          V 0x 0 1 0 jVy 0 j 0 0 ¼ ¼ V 0y Vy 0 0 0 j 0 0 1

Properties of Light

7

1.1.1

Summary

A display engineer does not need an in-depth knowledge of Maxwell’s equations, but rather to understand the fundamental physical properties of light that lead to its control, propagation, and modulation. Key to this is an understanding of the basic wave properties of light and how this leads to an energy transfer, and properties such as polarization which allow light to be controlled and manipulated. From these properties also stems the concept of optical coherence which dictates which set of rules needs to be applied to understand how light interacts with the display technology and its environment. The wave properties of light also dictate many aspects of the display performance from wavelength and color control through to optical efficiency and dispersive effects. These can all be explained and analyzed using often simple properties and models to build a picture of how well a display works. By using simple analogies, a very powerful analysis can be made as long as the correct assumptions about the fundamental properties of the light sources and its propagation are well defined.

References 1. Maxwell JC (1865) A dynamical theory of the electromagnetic field. Philos Trans R Soc Lond 155:459– 512 2. Newton I (1730) Opticks, 4th edn. Willian Innys, London (Dover, 1952) 3. Grimaldi FM (1665) Physico mathesis de lumine, coloribus, et iride, aliisque annexis libri duo (Bologna [‘‘Bonomia’’]). Vittorio Bonati, Italy, pp 1–11 4. Huygens C (1690) Traite´ de la lumiere, Chap 1. Pieter van der Aa, Leiden (Note: In the preface to his Traite´, Huygens states that in 1678 he first communicated his book to the French Royal Academy of Sciences)

5. Schro¨dinger E (1926) An undulatory theory of the mechanics of atoms and molecules. Phys Rev 28(6):1049–1070. doi:10.1103/PhysRev.28.1049 6. Smith FG, King TA (2000) Optics and photonics – an introduction. Wiley, New York 7. Hecht E (1987) Optics, 2nd edn. Addison Wesley, USA 8. Goodman JW (2005) Introduction to Fourier optics, 3rd edn. Roberts and Co, Englewood 9. Kramers HA (1927) La diffusion de la lumiere par les atomes. Atti Cong Intern Fisica (Trans Volta Centenary Congr) Como 2:545–557 10. Jones RC (1941) New calculus for the treatment of optical systems. J Opt Soc Am 31:488–493

19

Part 1.2

Geometric Optics

1.2.1 Geometric Optics Timothy D. Wilkinson 1 1.1 1.2 1.3 1.4 1.5

Ray Propagation of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Total Reflection from a Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Snell’s Law of Refraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 The Thin Prism and the Thin Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Aberrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Aperture Stops, Entrance and Exit Pupils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2

Reflections at a Boundary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3

Interference Films and AR Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4

Multiple Reflections and Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5

Display Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

6

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_1.2.1, # Springer-Verlag Berlin Heidelberg 2012

24

1.2.1

Geometric Optics

Abstract: The ability to model the propagation of light is a vital element of understanding any display technology. Whether it is a liquid crystal display backlight or a digital cinema projector, the principles of optical propagation still apply. For the majority of applications, a simple geometric optical model will suffice and allow the system to be carefully defined. Ray tracing is the most fundamental property of geometric optics. By defining a ray and its direction, its path can be traced through the system using fundamental physical rules. This chapter covers the concept of ray tracing for simple lenses including the derivation of the lensmaker’s equation and the paraxial approximation. This is then analyzed further to define the basic set of aberrations which characterize imperfections in an optical system. Rays can then be traced through boundaries to build up the theory of reflections and transmission. This leads to the concept of total internal reflection which is a powerful technique often used in displays to control the flow of optical energy within a system. The basic principles are explored from Fresnel reflection through to total internal reflection and a simple display application is identified to illustrate the power of these applications. Finally, the properties of reflections are also used to define the function of antireflection coatings and optical enhancement cavity structures. A simple on-axis theory is presented to analyze the basic function of these optical strictures based on the principles of geometric optics.

1

Ray Propagation of Light

The simplest way to visualize the propagation of light is as rays or wavefronts as shown in > Fig. 1. This is an idealized representation as it does not take into account what happens to small features or edges in the optical path; however, it is a very powerful tool for solving optical problems [1, 2]. Different levels of difficulty can be included in ray theory from simple geometric rays to paraxial rays, skew rays, and aberrations. Ray and wavefront propagation are the same representations using different geometric properties, hence problems can be solved with either technique. A ray can be drawn as if extending from a wavefront as shown in > Fig. 1b and a wavefront can be generated from a group of rays using Huygens principle of wavelet propagation as explained in > Sect. 7 of Chap. 1.3.1 on diffraction. In the next few examples, the ray theory will be used, but wavefronts could equally be applied to solve the problem.

Direction of propagation

Ray

a . Fig. 1 Light propagation as (a) rays and (b) wavefronts

b

Geometric Optics

1.1

1.2.1

Total Reflection from a Surface

> Figure 2 shows a ray from a point A incident on a reflecting surface (such as a mirror) as an angle of incidence i, intersecting at the point P on the surface. The reflected ray passes through point B and is reflected at an angle r. The obvious answer would be that r must equal i, but this was not proven fully until Fermat postulated that the ray must take minimum time to get from point A to B. If we consider a mirror point A0 underneath the mirror, then the shortest path will be from A0 to B. Any other path such as the one shown through point P 0 will be longer and will take more time. Hence the angle of incidence must be equal to the angle of reflection [2].

1.2

Snell’s Law of Refraction

A ray propagating through a vacuum (which is approximately the same for air) will travel at the speed of light c; however, a ray passing through any other medium will travel at a slower speed such that c2 ¼ c=n2 where n2 is referred to as the refractive index of that medium. A ray travelling from one medium will be refracted as demonstrated in > Fig. 3. The proof comes

A

B i

P⬘

r

P

A⬘

. Fig. 2 Reflection from a surface using ray notation

Q n1

q1 P q2

n2 S

. Fig. 3 A ray refracted from one medium to another

25

26

1.2.1

Geometric Optics

from Fermat’s principle which states the ray should transit through the system in the shortest possible time [2]. Snell’s law tells us that a ray travelling from a medium n1 to another medium n2 will be refracted (assuming n1 > n2) such that n1 sin y1 ¼ n2 sin y2 This is one of the fundamental properties used to describe ray propagation through optical systems as the interaction between media such as glass and plastic surfaces can be used to control and focus the direction of the light in displays.

1.3

The Thin Prism and the Thin Lens

The principle of Snell’s law can be used to solve the optical problem of light propagation through a thin wedge-shaped prism such as the one shown in > Fig. 4. The refraction of the rays at each surface dictates how light will pass through the thin prism. From Snell’s law we have n sin b2 ¼ sin b1 and n sin b3 ¼ sin b4 , and the total deflection of the ray through the prism is y such that y ¼ b1  b2  b3 þ b4 ¼ b1 þ b4  a If we have a thin prism such that a is small and a small angle of incidence such that sin b  b(often referred to as the paraxial approximation) then the total deflected angle can be approximated to: y ¼ ðn  1Þa This is the basic principle used in most geometric ray problems. Deviation from small values of a and b lead to aberration in the optical system, hence these values form a solid basis for good lens design and minimization or potential aberrations. They do, however limit what can be done in an optical system, especially if size is a constraint. A good example of how this property can be used is shown in > Fig. 5, where a thin lens is made from a series of thin prism sections [2]. Each prism section in > Fig. 5 is at a height y from the optical axis of the thin lens. Hence as they are thin lenses, the apex angle can be expressed as a ¼ 2y=r where r is the radius of

α

b1

b4 b2

b3

n

. Fig. 4 Light through a thin prism

θ

Geometric Optics

1.2.1

curvature of both surfaces, hence the deflected angle of a ray passing through each prism section will be: y ¼ ðn  1Þ

2y r

This deviation of each ray means that if parallel rays are incident on the lens, then they will all converge to the same point called the focal point or focal length of the lens. We can represent this as f ¼ y=y ¼ r=2ðn  1Þ. Moreover, parallel rays incident at an angle to the lens will converge to a different point on the optical axis. From this we can define the classical optical system in > Fig. 6, assuming that the thin lens is circular symmetric about the optical axis. From the dimensions shown in > Fig. 6, we define object distance as u and the image distance as v and we can then use classical geometrical optic theory to give the relationship: 1 1 1 ¼ þ f u v This relationship forms the basis of most geometrical optical systems and is one of the fundamental relationships which are exploited on a regular basis in optical design procedures. It is important, however to have a convention for signs when describing optical systems, as

Focal point

y r

. Fig. 5 A thin lens made from prism sections

Object plane

u

. Fig. 6 Classical thin lens optical system

Image plane

f

v

27

28

1.2.1

Geometric Optics

some image planes will be real and others will be virtual, depending on the lens system described. The convention we will be assuming is based on the Cartesian system as shown in > Fig. 7: ● ● ● ● ● ● ● ●

Object distance u is +ve when to the left of the lens. Object focal length f0 is +ve when to the left of the lens. Object distance x0 is +ve when to the left of the object focal plane F0. Image distance v is +ve when to the right of the lens. Image focal length fi is +ve when to the right of the lens. Object distance xi is +ve when to the right of the image focal plane Fi. Radius of curvature R is +ve if its center C is to the right of the center of the lens. Object and image height y0 and yi are +ve above the optical axis.

A thin lens can be made from two spherical surfaces using the above equation and adding them for each radii R1 and R2. There are many different types of this lens depending on their radii and sign of curvature, some of which are shown in > Fig. 8. The thin lens equation from

R

Object plane

u

f0

Image plane

fi

x0

v

C

xi

. Fig. 7 Definition of sign convention

R1 > 0 R2 < 0 Bi-convex

R1 < 0 R2 > 0 Bi-concave

R1 = ∞ R2 < 0 Planar convex

. Fig. 8 Different thin lenses, surface on left is R1

R1 = ∞ R2 > 0 Planar concave

R1 > 0 R2 > 0

R1 > 0 R2 > 0 Meniscus convex

Meniscus concave

Geometric Optics

1.2.1

the prism approximation can also be expressed in the form known as the lensmaker’s equation, which incorporates the two surface radii, R1 and R2, as well as the refractive index of the lens material, nl
 1 1 1 1 1 þ ¼ ¼ ðnl  1Þ  u v f R 1 R2 From this equation, it is possible to see two special situations which may occur and are shown in > Fig. 9. ● If the object distance u equals the focal length f, then the image is at infinity and the rays on the image side will be collimated. ● Similarly, if the image distance v is equal to the focal length f, then the object must be at infinity. There is also a third special case, which allows us to set up the basic structure of an imaging system through any thin lens. If a ray passes through the center of a thin lens (i.e., at the same point where the optical axis passes through the thin lens) then that ray will not be deviated on its course [2, 3]. Hence, we have the three cases we need to find the image of an object placed in front of a thin lens as shown in > Fig. 10.

Object plane f0 u

fi v=∞

Image plane fi v

f0 u=∞

. Fig. 9 Two special collimated cases

Object plane

Image plane

3 1

u

f0

v fi

2

. Fig. 10 Three rays used to locate the image of an object

29

30

1.2.1

Geometric Optics

A ray from the top of the object through the center of the lens will be undeviated. 1. A ray from the top of the object through the object focal point will leave the lens parallel to the optical axis on the image side. 2. A ray from the top of the object parallel to the optical axis will pass through focal point in the image plane. 3. This can be repeated for the bottom of an object, if it is not on the optical axis. The majority of optical systems designed are used to perform some sort of imaging operation; hence, it is logical to define a series of magnifications to define the type and function of an imaging optical system. The longitudinal magnification is different to the transverse magnification, which means that an object with depth will suffer from distortions due to the differing magnifications. This can be compensated by multiple surface lens design and ray tracing. The imaging properties of a thin lens are summarized in > Table 1. An important point in geometric optics is the concept of apertures and stops. They form the fundamental limits of the rays and also effectively define the aberrations and quality of the images generated. The aperture stop of any optical system will restrict the cone of angles allowed through the optical system. Another feature of the aperture stop is that it defines the f-number of a lens with a given aperture D and focal length f such that: f =# ¼

f D

For instance, a 50-mm focal length lens with an aperture size of 25 mm will have an f-number of 2, which is traditionally (and somewhat confusingly) written as f/2. If the aperture on the lens was reduced to 12 mm, then the f-number would be 4 (or f/4). A smaller f-number clearly allows more light to pass through the lens. Camera lenses are normally specified with their focal length and f-number, as the photographic exposure time is proportional to the f-number squared. Hence, the lower the f-number the ‘‘faster’’ the lens. A typical camera lens will have a range of f-numbers from f/1 (fastest), 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 to f/22 (slowest).

. Table 1 Definitions of the object and image planes of a thin lens Convex Object

Image

Location

Type

Location

Orientation

Relative size

1 > u > 2f

Real

f < v < 2f

Inverted

Minified

u = 2f

Real

v < 2f

Inverted

Same size

f < u < 2f

Real

1 > v > 2f

Inverted

Magnified

u=f

1

u u

Erect

Magnified

Concave Object

Image

Location

Type

Location

Orientation

Relative size

Anywhere

Virtual

|v| > |f |, u > |v|

Erect

Magnified

Geometric Optics

1.2.1

Another way of expressing this relationship is by the numerical aperture, often shortened to NA. This is expressed as the index of refraction times the sine of the cone half-angle of illumination: NA ¼ n1 sin

y 2

NA and f-number are similar ways of expressing the same concept in different optical systems. NA is more suited to conjugated systems where illumination angle is a useful measure of performance such as in microscope objectives [2].

1.4

Aberrations

The analysis made so far has been using the paraxial ray approximation and that the lens will only operate at one wavelength; however, this is rarely the case in real optical applications. Most lenses operate over range of wavelengths, which means that dispersion will have an effect, as each glass element will have a different refractive index at each wavelength. This is usually referred to as chromatic aberration. This aberration can be corrected for by using multiple glass surfaces, with air gaps between, or by combining two different glass surfaces with different dispersions together to form an achromatic doublet. The power of a single thin lens (reciprocal focal length) is given by P = (n – 1)/R, a small change in refractive index dn will change the power: dP ¼ P

dn n1

Varieties of optical glass differ quite widely in the way in which n varies with wavelength; hence, it is possible to combine two lenses with power P1 and P2 in such a way that dP1 + dP2 = 0 without making P1 + P2 = 0 at the same time. Since dn(n – 1) has the same sign for all glasses, P1 and P2 must have different sign and the combined pair will have a lower power than the two components. Hence for a given wavelength range, we must satisfy the equation: P1

dn1 dn2 þ P2 ¼0 n1  1 n2  1

The two surfaces may be in contact if they have the same radius of curvature, which is advantageous as it reduces the reflections from each air-glass interface. Such touching doublets are normally cemented together with transparent glue which has the same refractive index as the glass [1]. The paraxial approximation is very useful for setting up basic systems, but it will also lead to aberrations. Hence any optical system must undergo a series of optimizations after the initial approximation to minimize the desired aberrations. No lens system will be perfect, so compromises must be made during the optimization procedure, based on which aberrations will most affect the image quality of the lens system. This is usually done through a series of trial-and-error simulations using ray tracing software such as ZEMAX and CODE V. The main types of aberration are either point-based aberrations such as spherical, coma, and astigmatism; or image-based aberrations such as Petzval field curvature and distortion. In the paraxial analysis of the spherical boundary problem, we generated the lensmaker’s equation using the approximation that sin x  x; however, if we use the third-order approximation of sin x  x  x 3 =3! then a more accurate estimate of the ray trajectories can be calculated.

31

32

1.2.1

Geometric Optics

Most modern software packages perform both calculations and then compare the two results. The differences are expressed in a variety of plots to express the aberrations within the optical system. The more difficult aberrations to avoid and compensate for are those that occur off-axis when the ray angles begin to get larger. A common technique for expressing aberrations is to consider a cone of rays which strike the boundary of an optical element in an off-axis circular region. The cone is specified in polar coordinates r and y and it is possible to use the thirdorder approximation to calculate the way in which the original ray cone is aberrated by the boundary transition. As a result of these aberrations, the lens will generate an ellipse of rays and it is possible to calculate the associated minor and major axes using the form [2]: bx ¼ Br3 sin y  Fy sin 2y þ Dy 2 r sin y   by ¼ Br3 cos y  Fyr2 1 þ 2cos2 y þ ð2C þ DÞy 2 r cos y  Ey 3 The structure of this ellipse of rays is a powerful and simple method of presenting the different aberrations within the system. The aberrations associated with the coefficients B, C, D, E, F are known as the Seidel aberrations and are often listed in tabular form so the lens designer can see which element has which associated aberration. The first term B is the same for both axes; hence, it will affect on-axis rays, and is the spherical aberration. > Figure 11 shows three different types of Seidel aberrations. Coma is an aberration in addition to spherical aberration which only appears for object points off the axis and is associated with term F. Rays intersect the image plane in a comet like fashion. Rays along a line y = 90º do not contribute to coma, but rays at y = 0º are significant. Astigmatism, related to coefficients C and D, is a result of a cylindrical wavefront aberration, which increases as the square of the distance off axis. The focus consists of two groups of rays referred to as the focal lines, with a blurred circular region in between. The Petzval field curvature comes from terms C and D also, in which the wavefront has an added curvature proportional to y2. This shows that the focal length of the lens changes for off-axis points. A flat object plane will give a curved image plane. It is usual to find Petzval curvature even after a lens has been corrected for astigmatism. Distortion is related to term E and represents an angular deviation of the wavefront increasing as y3. This spreads or contracts the image, destroying the linear relationship between dimension in the object and the image. Common distortions are referred to as the pin-cushion or the barrel effects [2].

Coma

Spherical aberration

Focal lines Astigmatism

. Fig. 11 Different types of Seidel aberrations

Geometric Optics

1.5

1.2.1

Aperture Stops, Entrance and Exit Pupils

A very important tool in ray tracing an optical system are stops and pupils, as they define the limiting paths and angles that a ray can take when traversing the system. As a result of these limits, it is possible to estimate the angular limits within the system and hence estimate the aberrations that might occur due to those limits. An aperture stop (AS) is placed within a system to limit the possible ray paths. It also allows the limiting angles and rays through the system to be calculated, which is often very important when calculating imperfections and aberrations. Once such rays and paths have been defined, the performance of the optical system can then be optimized using commercial software algorithms. Another common stop used is the field stop (FS) which defines the useful or operating region in the output of image plane of the optical system [1, 2]. From the aperture stop it is possible to derive the entrance and exit pupils, which are very useful tools in understanding the operation of an optical system. A pupil is simply a projection of the aperture stop through the optical system, and they can be used to find the maximum permissible cone of rays at both the object and image portions of the optical system. The entrance pupil of a system is the image of the aperture stop seen from an axial point on the object through those elements that precede (i.e., are before) the aperture stop as shown in > Fig. 12. If there are no elements before the AS, then the stop itself forms the entrance pupil. In contrast, to the entrance pupil, the exit pupil is the image of the aperture stop as seen from an axial point on the image plane through the lenses between the image and the aperture stop as shown in > Fig. 13. Once again, if there are no lenses to the right of the aperture stop, then the aperture stop will be the exit pupil. The diagrams in > Figs. 12 and > 13 both include a ray labelled as the chief ray. This is defined as any ray from an off-axis object point that passes through the center of the aperture stop. The chief ray enters the optical system along a line directed toward the midpoint of the entrance pupil, Enp and leaves the system along a line passing through the center of the exit pupil Exp. The chief ray is associated with a conical bundle from a point on the object, effectively behaves as the central ray of the bundle and is representative of it. Chief rays are particularly important when considering aberrations in an optical system.

Entrance pupil

Exp Enp Chief ray

A.S

. Fig. 12 Example of an entrance pupil

33

34

1.2.1

Geometric Optics

> Figure 14 shows how the entrance and exit pupils can be used to investigate an optical system. The aperture stop defines the entrance and exit pupils and then they can be used to find the chief rays as well as the marginal ray, which goes from the axial object point to the rim or margin of the aperture stop. In most optical systems, one of the elements will be acting as an effective aperture stop as each element will have defined a finite aperture diameter. In the situation when it is not clear which element is effectively acting as the aperture, each element must be imaged in turn and the image that subtends the smallest angle at the axial object point is the entrance pupil.

Exit pupil

Exp Enp Chief ray

A.S

. Fig. 13 Example of an exit pupil

Exit pupil

Entrance pupil

Marginal ray Enp Exp Chief ray A.S

. Fig. 14 Example three lens system with pupils and key rays

Geometric Optics

2

1.2.1

Reflections at a Boundary

The basic principle of Snell’s law describes the process of refraction through a medium of different refractive index; however, this process does not account for the combination of both reflection and refraction at an interface. Unless a surface is antireflection (AR) coated, there will be a reflection due to the change in refractive index. Unfortunately, this is a rather complex area to analyze as we have to understand Fresnel’s equations for reflections at a boundary [1]. The reflection and transmission at an interface or boundary between materials with different refractive indices can be derived by breaking the radiation into its electrical (E) and magnetic (H) field components and then solving the relevant conditions which exist at the boundaries. This is broken into two sets of conditions which depend on the orientation of the electric field E relative to the plane of incidence for the interaction (the plane in which the incident, reflected, and transmitted rays all lie) (> Fig. 15). The relationships can be summarized into two cases. The first is when the electric field (E field) is perpendicular to the plane of incidence and the second case is when the E field is parallel to the plane of incidence. The derivation of these relationships is based on the boundary conditions at the point of reflection. Here the materials interaction dictates that certain properties of the E and H components must be continuous or inverted, depending on its orientation with the materials themselves. From this analysis we can define the reflected (r) and transmitted (t) components by using the Fresnel equations combined with Snell’s law to give the following relationships for the perpendicular and parallel components of the reflected and transmitted ray [1].

Plan

e of

incid

ence

Ei θi

θr

lane

Et

. Fig. 15 Irradiance incident at a material boundary

ry p

ni

nt

Hr

nda

Er

Bou

Hi

Ht

θt

35

1.2.1

Geometric Optics

sinðyi  yt Þ sinðyi þ yt Þ tanðyi  yt Þ ¼ tanðyi þ yt Þ 2 sin yt cos yi ¼ sinðyi þ yt Þ 2 sin yt cos yi ¼ sin ðyi þ yt Þ cos ðyi  yt Þ

rperp ¼  rpara tperp tpara

We can use the relationships to analyze the complex interaction between the E and H fields at the point of interaction. > Figure 16 shows a plot of all four coefficients versus angle of incidence for an air glass (nt = 1.5) boundary. As can be seen from this plot there is an angle yp which is referred to as the polarization (or Brewster) angle, where the sign of the parallel component is inverted. As the angle of incidence approaches 90º (glancing angles) we see that the reflection becomes stronger. This is the technique used to reflect X-rays in telescopes. If we consider the conservation of energy in the system of > Fig. 15, then the total energy flowing into the surface area (A) must be the same as the total energy flowing out of it: Ii A cos yi ¼ Ir A cos yr þ It A cos yt This can be expressed in terms of the electric field and broken into perpendicular and parallel reflectance (R) and transmittance (T) components such that R + T = 1:

1.0 t|| t^

0.5 Amplitude coefficients

36

r|| 0 qp r^ –0.5

56.3° –1.0 0

30

qi (degrees)

60

90

. Fig. 16 Plot of the amplitude reflection (r) and transmission (t) coefficients versus angle of incidence

1.2.1

Geometric Optics

R¼

Ir cos yr Ir ¼ Ii cos yi Ii

T¼

It cos yt Ii cos yi

We can also express R and T in terms of the reflected and transmitted field components: Rperp ¼ rperp 2 nt cos yt Tperp ¼ tperp 2 ni cos yi

Rpara ¼ rpara 2 nt cos yt 2 Tpara ¼ t ni cos yi para

Hence we can say that Rperp þ Rpara ¼ 1; Tperp þ Tpara ¼ 1. > Figure 17 shows the components of the reflectance and transmittance for an air glass boundary. As can be seen from the figure above there is an angle at which the parallel reflected component is zero. This is the polarization angle shown before and is also referred to as the Brewster angle. This can be shown as being the angle where yi = yt. This effect is exploited in Brewster prisms where two orthogonal polarization states can be separated with a very high extinction ratio. We can also define both T and R at normal incidence (yi = 0):
 nt  ni 2 4nt ni R ¼ Rperp ¼ Rpara ¼ ; T ¼ Tperp ¼ Tpara ¼ nt  ni ðnt þ ni Þ2 From the above relationships we can see that something interesting will happen in the case of internal reflection, when ni > nt. In this situation, we have a critical angle yc, which leads to total internal reflection. If we consider a boundary between a dense medium such as soda lime glass and air, with rays hitting the boundary at an angle of incidence yi as in > Fig. 18, and return to Snell’s law we see that as yi increases, the refracted ray approaches the tangent to the point P in the system of > Fig. 18. This continues until yt = 90º when we have reached the critical angle: nt yi ¼ yc and sin yc ¼ ni For any incident angle greater than yc the ray will be totally internally reflected. Note that there is no discontinuity at the point of total internal reflection. At angles less than yc, there is a gradual decrease in the refracted ray up till yc when all to the light will be reflected as described earlier with the Fresnel equations. The equations can be rearranged (using nti = nt/ni):

1.0

0.5

nti = 1.5 R^

0

a

T||

T^

Reflectance and transmittance

Reflectance and transmittance

1.0

30 60 q i (degrees)

0.5

nti = 1.5

R|| 0

90

b

. Fig. 17 Plots of the reflection and transmission coefficients versus angle

qp

30

60 q i (degrees)

90

37

38

1.2.1

Geometric Optics

qt

nt

nt

P

P

ni

ni

a

q i qr

qt

b

qi q r

nt P

nt

qt = 90°

P ni

c

qi = qc qr = qc

ni

d

qi > qc qr > qc

. Fig. 18 Internal reflection (ni > nt) and the critical angle

rperp

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n2ti  sin2 yi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ cos yi þ n2ti  sin2 yi cos yi 

rpara

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n2ti  sin2 yi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ n2ti cos yi þ n2ti  sin2 yi n2ti cos yi 

As can be seen from these coefficients when TIR occurs at yi > yc the reflection coefficients of the E field components become complex and alter the phase of the light in a complex fashion. At this point we have that R = 1, and the entire wave is reflected. The process of total internal reflection (TIR) is used in many optical components such as wedge and corner prisms which can make reflectors very close to 100% efficient. There is a further interesting point when yi = yc, as yt will be 90º giving TIR; however, there is in fact an evanescent wave in the direction of the refracted wave (which is now parallel to the optical boundary surface). This evanescent wave will couple back into the surface to generate the internally reflected ray; however, at the boundary the evanescent wave extends a small distance beyond the surface of the boundary. If another medium with the same refractive index (ni) as the first medium is placed on top, then the evanescent wave will couple into the second medium and the ray will continue without being reflected at the point P. This evanescent wave can be used to partially couple between two different media in a process called frustrated TIR or FTIR. A thin layer of material is placed on top of the boundary so that a percentage of the evanescent wave will pass through and couple into a second medium with the same refractive index. Hence a percentage of the light will be reflected while the rest will go straight through. This is the technique used in optical beam splitters and the separation optics in microscopes. The process of refraction and TIR can be used to control a whole range of operations in an optical system rather than just for deviating the path of a beam. One of the commonest is the dispersive glass prism originally demonstrated by Newton and shown in > Fig. 4, where the dispersive qualities of the glass can be harnessed. By analyzing the ray path through the prism, it is possible to derive an equation for the total deviation of the ray, d:

Geometric Optics

d ¼ b1 þ sin

1



1.2.1

 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðsin aÞ n  sin b1  sin b1 cos a  a

The angle d is a function of n which in turn is a function of wavelength through dispersion, hence the deviation angle will be different for different wavelengths. This prism is a simple means of splitting up white light into wavelengths which was commonly used in early spectrometers. The process can also be reversed by finding the minimum angle of deviation for a given wavelength and then using this to calculate the refractive index at the wavelength. This is one of the most accurate ways of measuring refractive index.

3

Interference Films and AR Coatings

Any flat surface where there is a difference in refractive index will generate a reflection which is proportional to the difference in refractive indices. For a ray hitting such a boundary at normal incidence, the reflected R and transmitted T components are given previously in > Sect. 2. If we have parallel sided slab of material such as glass in air, then there will be a reflection from the top surface and a reflection from the bottom surface of the ray transmitted from the top surface. The second reflection will then reach the top surface again and it is possible to see interference between these two reflections if the path difference between them is correct. This interference is more pronounced if the slab of material is in fact a thin film or layer of material, as the path difference is simpler to control. This is the effect seen when oil or petrol form a thin layer on water. The colors come from the fact that the interference is a function of both thickness and wavelength [1, 2]. The analysis of reflections at a boundary is very complex and requires the solution of Maxwell’s equations for light hitting a surface at an angle leading to a complex series of phase differences. This can be a very useful property of a multilayer stack of different thickness layers and refractive indices as this allows the reflection or transmission to be angularly selective which is a very desirable element in flat panel displays such as LCDs. The design of these multilayer stacks requires complex software and often results in over 20 layers being required, which can be difficult to make. Here we will consider the simpler case of light at normal incidence to a thin optical coating. For a lot of simple coatings such as antireflection (AR) coatings, the angular sensitivity is quite broad and it is only above angles of 20–30º that the normal case is no longer accurate. The case of a single layer coating is shown in > Fig. 19. The rays are in fact normal to the surface and have only been drawn with a slight angle to demonstrate the principle. The reflectance (subscript denotes layer number) of a single layer with wave number k = 2p/l can be derived and then simplified further when kd = p/2. This is equivalent to a layer thickness of d = l/4 which means that the ray reflected from the lower boundary will deconstructively interfere with the ray reflected from the upper boundary as shown in > Fig. 19  2 n0 ns  n21 R1 ¼ 2 ðn0 ns þ n21 Þ The normal angle reflectance will in fact be zero if n21 ¼ n0 ns . Generally, for a visible AR coating, l is set in the visible yellow-green region, which sets d as the quarter wavelength thickness and allows a suitable refractive index for the layer to be chosen to cancel the reflection. Cryolite (n = 1.35), a sodium aluminum fluoride compound and magnesium

39

40

1.2.1

Geometric Optics

These two normal reflections will interfere (and cancel) n0 n1

d

ns

. Fig. 19 Reflections off a layer at normal incidence

fluoride (MgF2, n = 1.38) are common low-index AR films for an air glass boundary at visible wavelengths. For a typical glass (n = 1.5), MgF2 is a little low, but is still used as it is a tough layer and reduces reflections from 4% to below 1%. For a double layer coating, the reflections at each surface will cascade, giving a more complicated reflectance; however, for quarter wave thickness layers, the reflectance simplifies to:  2 2 n2 n0  nn n21 R2 ¼ 2 n2 n0 þ nn n21 Which equals zero at a particular wavelength when
2 n2 ns ¼ n1 n0 This sort of film is referred to as a double-quarter, single-minimum coating. When n1 and n2 are as low as possible, the reflectance will have its single broadest minimum equal to zero at the chosen frequency. It should be clear, however, that n2 > n1, and it is common practice to designate a (glass)-(high index)-(low index)-(air) as a gHLa. Zirconium dioxide (n = 2.1), titanium dioxide (n = 2.4), and zinc sulfide (n = 2.6) are commonly used for H and MgF2 used for the L layer. Other double- and triple-layer stacks can be used to create different spectral and angular responses. A common approach is to make a multilayer periodic stack such as a g(HL)3a. The maximum reflectance can be increased further by adding a final H layer to give a stack of g(HL)mHa. A very-high-quality mirror can be produced this way. The small peak on the short wavelength (UV) side can be reduced by adding an eighth wave low index film at each end of the stack forming a high pass filter g (0.5L) (HL)m H (0.5L) a.

4

Multiple Reflections and Cavities

Another interesting case can be seen in > Fig. 20, where multiple reflections between two surfaces occur. This will only occur when special measures are taken to increase the reflections, usually by partially silvering the surfaces of the slab or coating with dielectric mirrors. This increased reflection means that the reflections do not die out as they would with just an air glass boundary [2].

1.2.1

Geometric Optics

Atr'4t'

Atr'2t'

Att'

T2

T3

T1

θ

R3

Atr'5t'

Atr'3t'

R2

Atr't'

R1

A

Ar

. Fig. 20 Multiple reflections off a layer

If r and t are the reflection and transmission coefficients from the surrounding medium to the slab and r 0 and t 0 are the reflection and transmission coefficients from the slab to the surrounding medium, we can write the amplitude of the reflected rays as rA, tt 0 r 0 A, tt 0 r 0 3A. . . and the transmitted rays tt 0 A, tt 0 r 0 2A, tt 0 r 0 4A. . .. With the exception of the first large reflected ray, the amplitudes go in a geometric progression and the closer to unity that r0 is made, the more slowly the reflections will die away. The phase of the reflections depends on y the angle of refraction inside the slab, its thickness h, and the refractive index n. The relative phase on a plane on the outside of the slab perpendicular to the ray depends on y as: c¼

4pnh cos y l

If we use a lens to combine the reflected rays, then they will interfere and constructive interference will occur when 2nhcos y = Nl, for each integer N. This will lead to a series of interference fringe rings. What is interesting is when you consider the gaps between each interference fringe. The brightest point on each fringe corresponds to a c value of 2p. Any value of c other than this creates a poorly defined phasor, which means that the gaps between the fringes will be dark and the fringes very thin. We can prove this by summing all of the reflected rays to get a ratio of the transmitted irradiance to the incident irradiance of: It ðtt 0 Þ2 1 ¼ ¼ 2 Ii ð1  r 02 Þ þ 4r 02 sin2 ðc=2Þ 1 þ Fsin2 ðc=2Þ

where F ¼

ð2r 0 Þ2 ð1  r 02 Þ2

Note that the F parameter becomes very large as r 0 2 approaches unity. For example if r 0 = 0.8 then F = 80. This has the effect of keeping the transmitted irradiance very small, except when c is close enough to a multiple of 2p for sin2(c/2) to be less than 1/F. Hence for a large value of F, the ratio shoots to unity rapidly as c approaches multiples of 2p. The sharp fringes of multiple interference are known as Fabry–Perot fringes. The sharpness of the Fabry–Perot fringes is often referred to as the finesse. pffiffiffi p F Finesse ¼ 2

41

42

1.2.1

Geometric Optics

A Fabry–Perot cavity is capable of producing very-high-quality resonant peaks, given a suitably high finesse. This can be used as a wavelength filter or as an angular filtering system. More importantly, the cavity can be made tunable by placing a variable refractive index in the cavity with either a liquid crystal material or possibly a nonlinear optically active material such as a chromaphore polymer. A more subtle effect which can be exploited in a multiple path interference device is that by operating a modulator on the very edge of a resonance peak, it is possible to create a large optical effect from a very small electro-optical effect, which may only have a weak response in its nonresonant state.

5

Display Applications

The process or TIR can be harnessed in many ways. The process of waveguiding through TIR has revolutionized the telecommunications industry, with the birth of the silica optical fiber. There are also applications for waveguides in displays, especially as backlights. Plastic optical fibers (POFs) are already being used to guide light by TIR as light pipes in displays such as motorway signs and traffic lights. They offer a cheap means of controlling the direction of light over a few tens of centimeters without significant loss or cost and they can transmit high power without interference or crosstalk. Another use of waveguides is in the backlights of displays such as LCDs and mobile phone displays. > Figure 21 shows a series of rays launched into a plastic slab at angles which are less than the critical angle for the plastic air boundaries. The rays are bounced by TIR along the waveguide [4]. The two lines in > Fig. 21 show two different angles often referred to as modes within the waveguide. There is a limit as to how far they will propagate through the waveguide, as there is a loss associated with the absorption of the plastic material. Hence there is a limited distance of waveguiding before efficiency and uniformity become an issue. Even so, this is a good means of distributing light across an area as would be required in an LCD backlight. The thinness of the modern-day laptop display is due mostly to the plastic waveguides at the back which are edge illuminated by thin, low-pressure tubes. The waveguide in > Fig. 21 would be no use as a backlight as there is no light emitted by the top surface. Light is allowed to escape from the top surface in a controlled manner by either tapering the waveguide or by putting a series of shaped features on the top surface to allow some of the light to escape. This is shown in > Fig. 22, and most commercial backlights would employ both techniques in some form. Features and reflectors are also added on the bottom surface to aid emission. The backlights shown in > Fig. 22 are greatly simplified, as a great deal of effort has been made over the last 10 years perfecting the waveguide structures. The lamps along the edges are partially mirrored and are often embedded into the waveguide edge for maximum transmission of light. More modern designs now include light emitting diodes (LEDs) as sources and the waveguides are further optimized to cater for the nonuniform illumination characteristics [5]. Some use edge illumination LED sources and some are now including LED sources

. Fig. 21 Waveguided modes in a plastic slab

Geometric Optics

Lamp

Lamp

1.2.1

Lamp

. Fig. 22 Double-ended and tapered backlights

. Fig. 23 The wedge waveguide display system

launched into the back surface of the backlight waveguide itself. The features on the surface of the waveguide are designed for both a bright backlight and for uniformity of the brightness. The surface is designed to counter for the nonuniformity of the lamps as well as the absorption of the waveguide material. The waveguiding function of a slab waveguide has also been used to form a new large area display as shown in > Fig. 23. With careful design, a wedge-shaped piece of plastic can be used to propagate an image from an engine such as a microdisplay [6].

6

Summary

For the majority of display-based optical applications a simple geometric optical model is more than sufficient and allows the system to be carefully designed. Ray tracing is the most

43

44

1.2.1

Geometric Optics

fundamental property of geometric optics and forms the basic tool of any optical engineer. By defining a ray and its direction, its path can be traced through the system using fundamental physical rules. Simple lenses result from this basic concept along with Snell’s law, leading on to the derivation of the lensmaker’s equation and the paraxial approximation. The purpose and power of this approximation is then analyzed further to define the basic set of aberrations which characterize imperfections in an optical system. Rays can then be traced through boundaries to build up the theory of apertures, aberrations, reflections, and transmission. This then leads to the concept of total internal reflection which is a powerful technique often used in displays to control the flow of optical energy within a system. The basic principles are explored from Fresnel reflection through to total internal reflection and a simple display application is identified to illustrate the power of these applications. Finally, the properties of reflections are also used to define the function of antireflection coatings and optical enhancement cavity structures. A simple on-axis theory is presented to analyze the basic function of these optical strictures based on the principles of geometric optics.

References 1. 2. 3. 4.

Hecht E, Zajac A (1987) Optics, 2nd edn. Addison Wesley, Reading Smith FG, King TA (2000) Optics and photonics. Wiley, Chichester Smith WJ (2000) Modern optical engineering, 3rd edn. SPIE/McGraw-Hill, New York Liu J-M (2005) Photonic devices. CUP, New York

5. 6.

Mottier P (2009) LEDs for lighting applications. Wiley, Hoboken Travis A, Payne F, Zhong J, Moore J (2000) Flat panel display using projection within a wedge-shaped waveguide. Proceedings of Conference, SID, Palm Beach, pp 292–295

Part 1.3

Optical Modulation

1.3.1 Optical Modulation Timothy D. Wilkinson 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

2

The Point Spread Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3

Modulation Transfer Function and Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4

The Classical MTF Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5

Experimental Measurement of the MTF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6 Liquid Crystal Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 6.1 Out-of-Plane Optical Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 6.2 In-Plane Optical Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 7

Diffraction of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

8

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_1.3.1, # Springer-Verlag Berlin Heidelberg 2012

48

1.3.1

Optical Modulation

Abstract: This chapter covers the issues that lead to developing and understanding a successful display technology. Optical modulation is a very important aspect of any display and a fundamental principle is that of resolution and its definition in the context of the display technology. Whether the limiting factors are pixels, optics, or materials, the theory of modulation transfer and point spread functions is essential in assessing the resolution properties of any display technology. Optical modulation characteristics are especially important in liquid crystal displays, so the latter part of the chapter develops some of the relevant theory required. The analysis stems from the basic birefringence of these materials, and describes their optical properties using Jones matrices. Finally, a brief description of optical diffraction theory is given, as this forms a basis for most modulation and resolution analysis.

1

Introduction

The concept of resolution is key to understanding and analyzing modern display technologies. The majority of displays in use today use a finite number of picture elements or ‘‘pixels’’ in an array to form images. These pixels are then combined with optics to illuminate, project, or reorient in order to form the final image. The concept of optical modulation ties all of these elements together and can be used to analyze the performance of any display system and define parameters such as resolution and image reproduction quality.

2

The Point Spread Function

It is impossible to create an optical system that has perfect resolution properties. Even an infinitely small point source imaged through a perfect aberration free lens will not form a perfect spot due to diffraction, as described in > Sect. 7, Liquid Crystal Displays. The wave properties of light which lead to the beam spreading at the edges of apertures, thus there will always be some form of diffraction effect which will cause the spot to spread and develop sidelobes. If we shine collimated light through a circular aperture and focus it through a lens, the distribution of the light in the focal plane of the lens will have effects due to the diffraction of the light at the edge of the aperture, as shown in > Fig. 1. It is possible to calculate the pattern of light generated from a given aperture via the Fourier transform relationship discussed in > Sect. 7, Liquid Crystal Displays on Fourier optics and

fi

. Fig. 1 Focused light through an aperture

Airy disk

Optical Modulation

1.3.1

diffraction. If we have a circular aperture, then the resulting distribution in the focal plane will be a first-order Bessel function, with light and dark circular rings. The central bright spot is called the Airy disk and it contains up to 84% of the total energy [1]. This pattern, based on a finite aperture, can be used to define the resolution limit of an optical system [2, 3]. It is also useful to define the numerical aperture (NA) of the optical system in > Fig. 1 as NA = n1siny/2, where n1 is the refractive index of the lens material and y is the angle subtended by the lens diameter at the principle focal point. Let us consider a system which uses a lens to image two bright sources of light of wavelength l, and each source is imaged to an Airy disk and associated rings. If the sources are close together, then the disks will overlap. When the separation is such that it is just possible to resolve two separate disks rather than one, then the points are said to be resolved. This is shown in > Fig. 2 for various amounts of separation. When the image points are closer than 0.5l/NA, the central maxima of both patterns blend into one and the combined patterns appear to be from a single source. When the image separation reaches 0.61l/NA, the maximum of one pattern coincides with the first dark ring of the other and there is a clear indication of two separate maxima in the combined pattern. This is referred to as Rayleigh’s criterion for an optical system: Z¼

0:61l ¼ 1:22lð f =noÞ NA

This expression is widely used in defining the resolution of telescopes. For the imaging system, the image NA is used, and for the object, the object NA is used. The equivalent separation for a pair of line images is simply l f/no. The way in which an optical system alters the structure of the aperture or illumination source is called the point spread function (PSF) or in the one-dimensional case the line spread function (LSF). The effects of this can be seen in the next section, where the PSF and LSF are used to describe the resolution limits of a display. The PSF, LSF, and modulation transfer function (MTF) are all interrelated and can be used to interpret the limits of an optical system or display. Furthermore, the PSF can also be used to detect or indicate any defects in the optical system such as aberrations. By looking at the distortion in the PSF we can spot sources of aberration. For example, spherical aberration will lead to a broadening and flattening of the PSF, whereas coma and astigmatism lead to asymmetric shapes in the PSF and its side-lobes.

0.5λ/NA Not resolved

Barely resolved

0.61λ/NA Resolved

. Fig. 2 Definition of the separations of resolvable points

Fully resolved

49

50

1.3.1

Optical Modulation

In a commercial software package such as CODE V or ZEMAX, the PSF is calculated by wavefront propagation rather than simple ray tracing. Hence it not only provides a different perspective, it may also indicate different distortions such as diffractive effects which would have not shown up in a purely geometric analysis.

3

Modulation Transfer Function and Resolution

One of the most important considerations in the performance of a display is its resolution, i.e., its ability to display high-resolution images. There are several techniques that can be used to judge the performance of a display in terms of resolution. The choice of technique depends on the type of display and the way in which it operates. These techniques have one thing in common: they generate the MTF for the display, which allows the performance of the display to be evaluated either analytically or graphically [4]. Another feature these techniques have in common is that the effects of color are ignored. The MTF is commonly used to evaluate the resolution performance in optical systems, especially as it can often be represented with mathematical analysis for these systems. One of the commonest uses of the MTF is in the design of lens systems for cameras, as it provides a means of combining the effects of several optical aberrations to judge the quality of the lens design [2]. With this technique it is possible to follow the propagation of light through the optical path and then take into account the MTF to show the effect of the aberrations. This is not so easy in displays as there is a finite resolution limit set by the pixelation and grayscale of the display. It is possible, however, to use the MTF to show how the display performs, taking into account the effects of the optics surrounding the display used for illumination or projection. Probably the simplest test for a display is to use a resolution chart such as the one in > Fig 3. This chart is the standard US Air Force (USAF) chart. The chart contains an array of bars of decreasing separation, which indicate different spatial. The chart is displayed and the resolution limit measured from the limit of the bars that can be visibly resolved on the image. There is a fundamental limit on the resolution based on the number of pixels, but there will also be other more influential limits due to the optics used to illuminate or project the image from the display. There is a resolution limit that can be seen in the image > Fig. 3 that is set by the performance of the printer, compression of the original bitmap and further limits due to the photocopier used to repeat it. The resolution chart gives us a visual evaluation of the performance, but does not describe the source of the limitation. The only way this can be obtained with a resolution chart is to eliminate the possible sources one by one, with a carefully chosen sequence of chart tests. It is possible to replace the display with a high-resolution slide of the chart, which allows us to evaluate the quality of the optics as a separate system. Other charts also exist to test other features of the display system, including diffraction effects and grayscale quality.

4

The Classical MTF Approach

If we consider an image as a luminance distribution, then we can mathematically evaluate the MTF as the effect that the display system has on reproducing an ideal image [5]. In order to understand this, we shall look at the case in one dimension, which is equivalent to looking at the shortest aspect of a display. The analysis can be directly converted into a two-dimensional case.

1.3.1

Optical Modulation

–2

–1 1

2

2 0

3

2 3 4 5 6

4

3

1 1 2 3 4 5 6

0

1

4 5 6

–2

5

1

6 . Fig. 3 The standard USAF resolution chart

If we have a one dimensional luminance function f(x), we can use Fourier theory to express this as a sum of an infinite number of sinusoids with a spectrum of spatial frequencies. We can express the sum as an integral: f ðxÞ ¼

Z1 FðuÞei2pux du 1

where u is the frequency and F(u) is the complex amplitude of the spatial frequency components. Conversely, we can calculate F(u): Z1 f ðxÞei2pux dx FðuÞ ¼ 1

The function F(u) is the Fourier transform of f (x). It describes the spatial frequency spectrum of the luminance pattern. If the original (undisplayed) luminance pattern is f(x) and its reproduction on a display is f 0 (x), then for f 0 (x) we can calculate the Fourier transform F 0 (u). The image rendition of a display system is characterized by the way in which the various spatial frequency components of an image are reproduced. A lack of rendition of high spatial frequency components causes unsharpness of the image. The MTF M(u) describes the ability of a system to reproduce the modulation of the various spatial frequency components and is defined as the output modulation of a system F 0 (u) divided by the original input modulation F(u), as a function of spatial frequency:  0  F ðuÞ  MðuÞ ¼  FðuÞ  In the MTF, the phase relation between the input and output modulation is not taken into account. For cases where this could play a role, the optical transfer function (OTF) should be

51

52

1.3.1

Optical Modulation

used. The definition of the OTF is the same as for the MTF, but without the absolute value signs. If the MTF becomes negative, this indicates a phase reversal, i.e., a black bar is imaged as white or vice versa. This is often common in defocused optical systems. The MTF is a very important quantity for the characterization of the resolution capability of a display system. It is analogous with a filter function in electronic theory. > Figure 4 shows a typical MTF for a cathode ray tube (CRT) display. At low spatial frequencies, the MTF is 1. At higher frequencies it decreases to 0. However, the following precautions must be accounted for: 1. The concept of MTF (and also OTF) strictly applies only to linear systems. In a nonlinear system, higher frequency harmonics appear after the Fourier transform, leading to erroneous MTF values. 2. The average luminance of a display system may include effects such as ambient or stray illumination. This can also lead to errors in the MTF as the display system is being influenced by external factors. In many cases the displayed luminance pattern results from an image forming process where each point of the original pattern is spread over a certain area in the displayed image. Examples are the image forming by a non-perfect optical lens and the scanning process of an electron spot in a CRT. Such systems can be characterized by a line spread function (LSF) l(x) by which the original f(x) is scanned (or convolved) to form the displayed f 0 (x)[5]: Z1 f ðxÞ ¼ f ðxÞl ðx  xÞdx 0

1

where x is a variable used for the integration. If an image is formed by a convolution process, the MTF for this process can be determined from the LSF. From Fourier transform convolution identities, we can directly state the relationship between the displayed image F 0 (u) and the Fourier transform of the LSF, L(u): F 0 ðuÞ ¼ LðuÞFðuÞ

1

MTF

0.5

0

0

1 2 3 Spatial frequency [cycles/mm]

. Fig. 4 The MTF of a CRT tube with a Gaussian spot

4

Optical Modulation

1.3.1

This means that MTF can be directly gained from the absolute value of the Fourier transform of the LSF:  1  Z    MðuÞ ¼ LðuÞ ¼  lðxÞei2pux dx    1

> Figure 5 shows a LSF and its corresponding MTF. It is interesting to note from this example that the number of sample points required in the LSF is low. In this example, three different numbers of sample points are shown. The resulting MTFs are very similar, with the 8 sample point one being slightly less accurate, but the 16 and 32 point cases being virtually the same.

Luminance 32 points 16 points 8 points

0

a 1

0.8

1.6 Distance (mm)

2.4

3.2

MTF Calculated from: 32 points 16 points 8 points

0.5

0 0

b

0.5 1 Spatial frequency (cycles/mm)

. Fig. 5 A typical LSF for 8, 16, and 32 sample points with its corresponding MTF

1.5

53

54

1.3.1

Optical Modulation

If the LSF is symmetrical about some central axis, then the sine terms in the Fourier transform disappear, leaving:  1  Z    MðuÞ ¼  lðxÞ cosð2pux Þdx    1

Instead of the LSF, we can also use the PSF p(x, y). In this case, the LSF must first be calculated from the PSF: Z1 lðxÞ ¼ pðx; y Þdy 1

For a case of a rotationally symmetric PSF, the MTF can be directly calculated:   1  Z   MðuÞ ¼  pðrÞJ0 ð2pur Þ2pr dr   1

where p(r) is the radial version of the PSF and J0 is the zero order Bessel function. This operation is known as the Hankel transform [5]. We can also use the convolution rule to cascade the MTFs from component effects in the total MTF of the display. Hence, we can calculate the MTF components M1(u), M2(u), etc., separately and then obtain the final overall MTF: MðuÞ ¼ M1 ðuÞM2 ðuÞM3 ðuÞ . . . The cascade rule applies only when each element is incoherently detached from the others. In the case of a coherent optical system, each element would have to be separated by a diffuser. In a multi-element coherent system, the MTF is defined for the entire system and not as a cascade of elements. This is because it is possible to correct for the deficiencies of one element with another within a coherent group of optical elements such as lenses. The LSF needed for the calculation of the MTF can sometimes simply be obtained by measuring the luminance transition at a sharp luminance step (edge). This is called the Foucault method or knife-edge test. A small razor blade is used in the object plane to realize the step. If we assume that the image f(x) is a step function (0 to 1 transition), then we have: Z1 f ðxÞ ¼ 1:l ðx  xÞdx 0

0

Differentiating this expression gives: lðxÞ ¼

df 0 ðxÞ dx

Hence by differentiating the step response of the display system, we can obtain the LSF and hence the MTF. The image forming process in a matrix display such as a liquid crystal display, and electroluminescent display or a plasma display, differs from a convolution process. The image on a matrix display consists of luminance samples arranged in a regular array. If the distance between samples is Dx, the sampling frequency is 1/Dx. From a Fourier analysis

Optical Modulation

1.3.1

of the sampled signal, it appears that the frequency spectrum of this function is equal to that of the original continuous spectrum with the addition of repetitions of the original spectrum at distances 1/Dx. Overlapping these spectra will cause interference visible as aliasing or a moire´ effect on the display. To avoid this, the original signal must be pre-filtered to remove components above 0.5/Dx. If the filtering is correct, then the components below the cutoff frequency of 0.5/Dx will be completely reconstructed. In information theory this is known as Shannon’s theorem and the limit of 0.5/Dx is known as the Nyquist frequency. In practice, the above mentioned reconstruction of the original signal is only possible with electronic circuits. Even in this case, the reconstruction will not be perfect, because of the finite limits of the filters. On a sampled display, the only post-processing that occurs is the spreading of the luminance over the finite size of each pixel. This is equivalent to the convolution of the sampled function with a block shaped LSF. The MTF of this pixel convolution process is given by the absolute value of a sinc (sin(x)/x) function, which is the FT of the block function:   sinðpDxuÞ   MðuÞ ¼  pDxu  assuming that the width of the pixel elements is the same as the sampling distance (Dx). The zero points of this function occur at frequency multiples of 1/Dx, so that the maximum suppression of the repetition spectra is obtained. According to the sampling theorem, no frequency above the Nyquist frequency can be transferred, so the MTF will drop to zero at the Nyquist frequency. The effects of sampling are also seen in the CRT tube in the vertical direction because of the line structure. In order to avoid possible aliasing caused by this sampling, television systems are designed such that an over-sampling factor (known as the Kell factor) is used. For most CRTs, a Kell factor of 0.7 is used.

5

Experimental Measurement of the MTF

The above techniques are all for the mathematical analysis of the resolution of the display system. In practice, it is often easier to measure the MTF before interpreting its effects. One of the most direct ways of measuring the MTF is by the FT of progressively increasing sinusoids displayed in the system. This directly relates to the above theory as we are essentially scanning different spatial frequencies through the display and then measuring the modulation. This is an especially useful technique for nonlinear display systems or for where the performance is nonstandard. By scanning through the sinusoidal frequencies we are evaluating each point on the MTF. The effect of the display on the scanned frequency is done by the use of a Fourier transform (focal plane of a lens). This is shown in > Fig. 6, where a sinusoid and its FT are shown. By scanning the frequency of the sinusoid it is possible to shift the position of the diffracted spot in the Fourier plane. Even in the case of a binary display, it is possible to measure the MTF by looking at the diffracted first-order spot. The MTF shown in > Fig. 7 is an example of a scanned MTFs for a 128 pixel matrix display. As expected, the shape of the MTF is a truncated sinc (sin x/x) function. The truncation is due to the fact that it is a matrix display and the frequency scanning is stopped at the Nyquist frequency to avoid aliasing. From these types of MTF it is possible to measure nonlinear binary devices and displays which have nonlinear grayscale responses.

55

56

1.3.1

Optical Modulation

x

u

FT

. Fig. 6 Fourier transform of a single frequency sinusoid

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

100

200

300

400

500

. Fig. 7 MTF for a 128 pixel matrix display

6

Liquid Crystal Modulation

Liquid crystals (LCs) are unique compounds with the properties of both the liquid and crystalline phases. They exist in mesophases, which have diffuse molecular order and orientation. Phase changes are often initiated by temperature (thermotropics) or solvent concentration (lyotropics). Different liquid crystal compounds have different mesophase combinations. Some are subtle and microscopic changes in order whilst others are much more dramatic. One of the commonest liquid crystal mesophases is the nematic phase. This is the least ordered mesophase before the isotropic. Here the molecules have only long range order and no longitudinal order. This means that the molecules retain a low viscosity, like a liquid, and are prone to flow. This can greatly effect the speed at which nematic LCs can modulate the light. The order in the nematic phase arises from the climatic molecular shape, which means that on average the molecules spend slightly less time spinning about their long axis than they do about their short axis. The molecular shape leads to an optical anisotropy or LC birefringence, with the two axes of the molecule appearing as the refractive index as shown in the right of > Fig. 8. The refractive index along the long axis of the molecules is often referred to as the extraordinary ne (or fast ns) and along the short axis as the ordinary no (or slow ns) axis. The difference between the two is the birefringence: Dn ¼ ne  no . Along with the optical anisotropy there is also often a charge imbalance due to the molecular shape leading to a dielectric anisotropy. The dielectric anisotropy is also linked to the elastic properties of the liquid crystal molecule, which means that we can move

1.3.1

Optical Modulation

Molecular director

ne

no q

Optical indicatrix

. Fig. 8 The optical indicatrix of a typical liquid crystal molecule

Light

Light

E

E

. Fig. 9 Out-of-plane bulk reorientation of a liquid crystal with an applied electric field

the molecules around by applying an electric field across them. This combined with the flow properties means that a liquid crystal molecule can be oriented in any direction with the use of an applied external electric field. This is a very desirable feature as it leads to their ability to perform grayscale modulation of the light.

6.1

Out-of-Plane Optical Modulation

The ability to manipulate the liquid crystal position leads to two basic types of optical modulation: out-of-plane and in-plane rotation. The example shown in > Fig. 8 shows a liquid crystal molecule at an angle y to the plane of the cell (i.e., the upper and lower horizontal walls of the device in question). Hence, for a positive dielectric anisotropy material, when an electric field is applied to the upper and lower electrodes, the molecule will rotate about its central point and the angle y will vary as is shown in the bulk case in > Fig. 9. This is an example of out-of-plane rotation and the refractive indices seen by the light will rotate accordingly. It is important to note that these parameters are bulk parameters which are based on a statistical average across billions of individual molecules. The bulk ordering of liquid crystals such as nematics is not always inherent in the material beyond domains of molecules a few micrometers in size. The electric field used to reorient the molecules is normally applied by either an active backplane, such as a thin film transistor (TFT) or CMOS array, or a passive backplane using indium tin oxide (ITO) electrodes (> Fig. 9). The combination of the flow allowing the molecules to move when an electric field is applied and the optical anisotropy means that we can effectively rotate the axes of the indicatrix as the molecules move, creating a movable wave plate or optical retarder. This, along with polarizing optics, forms the basis of

57

1.3.1

Optical Modulation

103

Γ

6 I 4

101

Γ (rad)

102 arb. units

58

2 100

6

8

10

20 Voltage

40

60

80

. Fig. 10 Typical optical modulation of an out-of-plane liquid crystal with applied voltage

most liquid crystal intensity and phase modulation characteristics. If we have an optical indicatrix oriented at an angle of y to the plane of the cell as in > Fig. 8, then we can calculate the refractive index seen by light passing perpendicular to the cell walls: no ne nðyÞ ¼  1=2 2 2 ne sin y þ no 2 cos2 y We can then calculate the retardance G of the liquid crystal layer for a given cell thickness d and wavelength l. The retardance is the phase difference between a wave passing through the short axis and the wave passing through the material oriented at an angle y: G¼

2pd ðnðyÞ  no Þ l

We can now use this expression in a Jones matrix representation of the optical LC retarder to get the optical characteristics of the LC material as in > Fig. 10. In the case of nematic LC materials, there is little restriction on the flow properties of the material; hence it is possible to continuously vary G through several rotations of p (> Fig. 10), provided a thick enough cell. The main drawback of these materials is that the speed of the modulation is often very slow (tens of milliseconds to seconds). The other main limitation of nematic LCs is that they are inherently polarization insensitive; so multiple passes through a cell with waveplate optics are required to make them insensitive [4]. The effects discussed with nematic LCs so far have all been planar effects, where there is just the simple angle y with respect to the cell walls. Because the nematic LC molecules are free to rotate in any position, it is possible to make more complex geometries such as twisted structures. In these devices, the molecules follow a twisted or helical path, often rotating through an angle of 90 as is the case with twisted nematic or TN displays. The twist can be extended by adding suitable chiral dopants to create ‘‘supertwist’’ structures with twists of the order of 270 . The analysis of twisted nematic LC structures is far more complicated and is usually accomplished, using commercial software packages or a series of thin slice approximations across the cell.

Optical Modulation

6.2

1.3.1

In-Plane Optical Modulation

Some liquid crystals have a higher degree of order than nematics and one such class is smectic materials. Within this class is a group of materials known as chiral smectic C or ferroelectric liquid crystals (FLCs). If these materials are restricted to a cell thickness of 2–5 mm, then motion of the molecules within the cell is restricted by the surfaces, and the molecules are bounded into two stable states either side of a cone of possible molecular positions. The angle between these two states is defined as the switching angle y. This is referred to as a surface stabilized FLC geometry and creates a high degree of ferroelectricity and a large birefringent in-plane electro-optical effect. The penalty is that the molecules are only stable in the two states and therefore the modulation will be only binary. The advantage of this binary modulation is that it can be very fast (10 ms) and that the stability can lead to the molecules remaining in the two states in what is known as bistable switching. > Figure 11 shows the two stable positions of the molecules. The plane that the molecules occupy is the same as the one forms the physical boundary of the cell itself. The liquid crystal molecules switch through the angle y in the plane of the cell, as shown in > Fig. 11, when the electric field (E) is applied across the cell. The motion of the molecule is very fast, which is attractive for spatial light modulators (SLMs). The motion of the molecule is only stable in two states, which limits the modulation to binary. The interaction between the light and the liquid crystal molecules is dependent on the polarization of the light and the orientation of the molecules. The director of the molecules acts like the extraordinary axis of a retardation plate, and the normal to the direction of the molecules represents the ordinary axis. Hence the liquid crystal displays birefringence aligned in the direction of the molecules across the cell. This means that the liquid crystal acts like a switchable waveplate whose fast and slow axes can be in two possible states separated by the angle y and whose retardation depends on the thickness and birefringence of the material. Using Jones matrix notation, a pixel with retardation G at an angle y can be represented as:  jG=2 2  cos y þ ejG=2 sin2 y j sin G2 sinð2yÞ e ejG=2 cos2 y þ ejG=2 sin2 y j sin G2 sinð2yÞ Binary Intensity Modulation

If the light is polarized so that it passes through a liquid crystal pixel parallel to the fast axis in one state, then there is no change due to the birefringence and the light will pass through a polarizer which is also parallel to the fast axis. If the pixel is then switched into state 2,

Alignment direction p θ

E

θ P

. Fig. 11 In-plane switching of a liquid crystal molecule

E

59

60

1.3.1

Optical Modulation

Polarizer

Polarizer

SLM Pixel

SLM Pixel

Fast

Fast

θ

Slow Slow

Γ

State 1

Γ

State 2

. Fig. 12 In-plane binary modulation states

as shown in > Fig. 12, the fast axis is rotated by y, and the light now undergoes some birefringent action. We can use Jones matrices to represent the optical components: State 1



V 0x



 ¼

V 0y

 ¼

0 0

0 1



0

!

ejG=2

0

0

ejG=2



0



Vy

Vy ejG=2

State 2



V 0x V 0y



 ¼ ¼

0 0

0 1 

Vy e



jG=2

ejG=2 cos2 y þ ejG=2 sin2 y

j sin G2 sinð2yÞ

j sin G2 sinð2yÞ

ejG=2 cos2 y þ ejG=2 sin2 y

0  cos y þ ejG=2 sin2 y

!

!

0 Vy



2

If the thickness of the liquid crystal cell d, is set so that G = p, then the light in the direction of the slow axis will be rotated by 180 . This leads to a rotation of the polarization after the pixel, which is partially blocked by the following polarizer. Maximum contrast ratio will be achieved when state 2 is at 90 to the polarizer and the resulting horizontal polarization is blocked out. This will occur when     Vy ejG=2 cos2 y þ ejG=2 sin2 y ¼ Vy jcos2 y  jsin2 y ¼ 0 and the optimum switching angle will be when y = 45 . Binary Phase Modulation

If the light is polarized so that its direction bisects the switching angle and an analyzer (polarisor) is placed after the pixel at 90 to the input light, then phase modulation is possible.

Optical Modulation

Polarizer

Fast

Fast

θ/2

1.3.1

Polarizer

−θ/2 Slow Slow

State 1

State 2

. Fig. 13 In-plane binary modulation states

If we start with vertically polarized light, then the liquid crystal pixel extraordinary axis positions must bisect the vertical axis and will be oriented at angles of y/2 and y/2 respectively for each state, as shown in > Fig. 13. Once again we can use Jones matrices to express the system: State 1



V 0x



 ¼

V 0y

 ¼

1 0

0 0

1

0

0

0



ejG=2 cos2 y2 þ ejG=2 sin2 y2

j sin G2 sinðyÞ  Vy j sin G2 sinðyÞ 0

j sin G2 sinðyÞ ejG=2 cos2 y2 þ ejG=2 sin2 y2

State 2



V 0x V 0y



 ¼  ¼



ejG=2 cos2 y2 þ ejG=2 sin2 y2

j sin G2 sinðyÞ

j sin G2 sinðyÞ 

ejG=2 cos2 y2 þ ejG=2 sin2 y2

Vy j sin G2 sinðyÞ

!

0



Vy

!

0 Vy



0

From these two expressions we can see that the difference between the two states is just the minus sign, which means that the light has been modulated by 180 (p phase modulation). Moreover, the phase modulation is independent of the switching angle y and the retardation G. These parameters only affect the loss in transmission through the pixel which can be gained by squaring the above expressions:   G T ¼ Vy2 sin2 ðyÞsin2 2 Hence maximum transmission (and therefore minimum loss) occurs when G = p and y = p/2.

7

Diffraction of Light

Diffraction is the principle that we use to solve what happens when the limits of geometrical optics is exceeded and is often a critical part of understanding the modulating properties

61

62

1.3.1

Optical Modulation

of an optical system. The theory that governs resolution as defined in > Sects. 2, > 3, > 4, and > 5 all stems from diffraction analysis of the optical system. The effect most noticeable is when a series of rays (or a wavefront) is incident upon an obstruction, aperture, or edge. Geometrical optics tells us that once the rays have passed the edge, those rays that were blocked stop propagating and those that passed continue. If we look further down the optical path, a perfectly clear shadow of the edge would be maintained by the rays still propagating under this model. This is not the case in reality as the light is seen to bend outward around the edge, eventually forming a series of rings or fringes. The process of forming these fringes is known as diffraction theory and is based on the light propagating as waves (like ripples on a pond). Let us assume we have an arbitrary aperture function, A(x,y) in the plane S, with coordinates [x,y] as shown in > Fig. 14. The light passing through this aperture will be diffracted at its edges and the exact form of this pattern can be calculated [6]. We want to calculate the field distribution at an arbitrary position at a point P, a distance R from the aperture. If we consider an arbitrarily small quantity of the aperture, dS, we can model this as a point source of light emitting spherical ‘‘Huygens’’ wavelets with an amplitude of A(x,y) dS. The wavelet acts as a radiating point source, so we can calculate its field at the point P, a distance r from dS. The point source dS can be considered to radiate a spherical wavefront of frequency o. The total field distribution at P is evaluated by superposition (summation) of all the wavelets across the aperture. The process of interference of these spherical wavelets is then the fundamental principle behind diffraction and is based on the Huygens–Fresnel approximation. In order to analyze the propagating wavelets, a series of approximations and assumptions must be made. If we consider only the part of the wavelets which are propagating in the forward (+z) direction and are contained in a cone of small angles away from the z axis, then we can evaluate the change in field dE at the point P due to dS. As the wavelet dS acts as a point source, we can say that the power radiated is proportional to 1/r 2 (spherical wavefront), hence the field

y es

av

w ne

Pla

x β dS

P

r

α

R

z S

. Fig. 14 Aperture and diffraction coordinate system

Optical Modulation

1.3.1

dE will be proportional to 1/r. Thus for a propagating wave of frequency o and wave number k, defined as k = 2p/l, we have the complex wave: dE ¼

Aðx; y Þ iwt ikr e e dS r

Now, we need to change coordinates to the plane containing the point P, which are defined as [a,b] using the relationship from Pythagoras: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ax þ 2by x 2 þ y 2 þ r ¼R 1 R2 R2 The final full expression in terms of x and y (dS = dxdy) for dE will now be: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi jkR

1

2axþ2by 2

þ

R Aðx; y Þe jot e qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dE ¼ 2 2 R 1  2ax Rþ2 2by þ x Rþy2

x 2 þy 2 R2

dxdy

Such an expression can only be solved directly for a few specific aperture functions. To account for an arbitrary aperture, we must restrict the regions in which we evaluate the diffracted pattern. If the point P is reasonably close to the z axis, relative to the distance R and the aperture A(x,y) is small compared with the distance R, then the lower section of the equation for dE can be assumed to be almost constant, and to all intents and purposes, r = R: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi x 2 þy 2 þ R2 Aðx; y Þ jot jkR 12axþ2by R2 e e dxdy dE ¼ R The similar expression in the exponential term in the top line of the original equation is not so simple. It cannot be considered constant as small variations are amplified owing to the presence of the exponential. To simplify this section we must consider only the far field or Fresnel region where: R 2 >> x 2 þ y 2 and so the final term in the exponential ((x2 + y2)/R2) can be neglected. To further simplify, we use the binomial expansion to give the Fraunhofer region: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d d2 ð1  d Þ ¼ 1      2 8 and keep the first two terms only, as this expansion converges rapidly. The simplified version of the field dE can now be expressed as: dE ¼

Aðx; y Þ iðotkr Þ ik ðaxþby e e R Þ dxdy R

The acceptability of the approximation depends on a sufficiently large value of R to obtain the far field diffraction pattern of the aperture. One useful guideline proposed by Goodman [7] is to assume that the far field pattern occurs when:  2  2 k xmax þ ymax R >> 2 where xmax and ymax are the maximum dimensions of the aperture A(x,y). The regions of the approximation are defined such that in the far field or Fraunhofer region, the approximations

63

64

1.3.1

Optical Modulation

z

Fraunhofer region Fresnel region

. Fig. 15 Diffraction regions for a square aperture

are accurate, hence the field distribution E(x,y) only changes in size with increasing z, rather than changes in structure as shown in > Fig. 15. In the case where the approximation is bearably accurate, we are in the Fresnel region. Before the Fresnel region, the evaluation of E is extremely difficult and is called the near field region. The exact boundary of the Fresnel region will depend on the acceptable accuracy. The total effect of the dS wavelets can be integrated across dE to get an expression for the far field or Fraunhofer diffraction pattern: ZZ 1 Aðx; y Þeikðaxþby Þ=R dxdy Eða; bÞ ¼ eiðotkRÞ R Aperture i(ot  kR)

refers the wave to an origin at t = 0, but we are only The initial exponential term e interested in relative points at P with respect to each other, so it is safe to set this term to 1. Thus, our final expression for the far field diffraction pattern becomes: ZZ Eða; bÞ ¼ Aðx; y Þeik ðaxþby Þ=R dxdy A

which is recognizable as the two-dimensional Fourier transform of the aperture function A(x,y). Fraunhofer region ¼ Far field pattern ¼ FTfAperture functiong The final step is to remove the scaling effect of R in the equation, as it does not effect its structure, only its size. The coordinates [a,b] are absolute and are scaled by the factor R. For this reason, we normalize the coordinates and define the Fourier transform of the aperture in terms of its spatial frequency components [u,v] (not to be confused with u and v in geometrical optics): ka 2pR kb v¼ 2pR

u¼

Optical Modulation

Hence the final relationship is:

ZZ

Eðu; vÞ ¼

1.3.1

Aðx; y Þe2piðuxþvy Þ dxdy

A

And inversely, we can calculate the aperture from the far field pattern: ZZ Aðx; yÞ ¼ E ðu; y Þe2piðuxþvy Þ dudv A

We define the Fourier transform pair as: ZZ Fðu; vÞ ¼ f ðx; y Þe2piðuxþvy Þ dxdy ZZ f ðx; yÞ ¼

1

F ðu; v Þe2piðuxþvy Þ dudv

1

The coordinates [u,v] in the Fourier plane are now defined as the spatial frequencies. The far field pattern of a square aperture of width 1 mm, at a wavelength of 633 nm, can only be accurately measured 10 m away from the aperture. Such a far field distance is clearly difficult to achieve in practical terms, so a means of shortening the distance is needed. If a positive lens is positioned directly after the aperture, the far field pattern appears in the focal plane of the lens as shown in > Fig. 16. The positive lens performs a Fourier transform of the aperture placed behind it. If we consider the aperture A(x,y) placed just before a positive lens of focal length f, then we can calculate the field just after the lens, as described by Goodman [7], by the paraxial approximation: Aðx; y Þ0 ¼ Aðx; y Þei2f ðx k

2

þy 2 Þ

The application of Snell’s law at the spherical lens/air boundaries shows that the lens converts plane waves incident upon it into spherical waves converging to the focal plane. For this reason, the diffraction to the far field pattern now occurs at in the focal plane of the lens, as the Fresnel approximation only holds true in this plane. The effect of this is to shift the far field or Fraunhofer region away from R to the principle focal length f. The principles of Fresnel diffraction can be applied to A(x,y)0 as if it were the aperture. The exp(x2+y2) which was

f Aperture

. Fig. 16 Diffraction through a positive lens

65

1.3.1

Optical Modulation

originally ignored due to the large R is now included with the term from A(x,y)0 and cannot be removed as the focal distance from the aperture is not large enough. The expression can now be analyzed by a combination of Fresnel diffraction and paraxial ray approximations and is outlined in Goodman [5]. The final result for the diffracted aperture A(x,y) through the lens is: ZZ 2 ik 2 ik Aðx; y Þe f ðaxþby Þ dxdy Eða; bÞ ¼ e f ða þb Þ A

Once again we can translate the equation into spatial frequency (u,v) coordinates creating the Fourier transform relationship shown above for the far field region, with an added phase distortion due to the compression of R down to the focal plane of a positive lens. It looks as if we are getting something for nothing, but this is not the case, as the lens introduces the quadratic phase distortion term in front of the transform. This can lead to a smearing of the Fourier transform and must be corrected for in compressed optical systems. There are several methods used to correct the phase, such as adding further lenses close to the focal plane or a compensating hologram to counter the phase distortion. If the aperture is placed a distance d behind the lens, then there will be a corresponding change in the phase distortion term of the Fourier transform: ZZ ik ik 1df Þða2 þb2 Þ ð 2f Aðx; y Þe f ðaxþby Þ dxdy Eða; bÞ ¼ e A

From this equation we can see another way of removing the phase distortion. If the distance is set so that d = f, then the phase distortion is unity and we have the full Fourier transform scaled by the factor of the focal length, f. This is a very important feature used in the design of optical systems and is the principle behind the 4f system shown in > Fig. 17. In a 4f system, there are two identical lenses separated by a distance 2f. This forms the basis of a low distortion optical system. In both of the examples in > Fig. 17, the distortions are minimized. In the top system, the aperture is transformed in the focal plane of the first lens and then re-imaged by the inverse

Aperture or Image

Aperture or Image

GIVE WAY

WAY

GIVE

66

f

f

. Fig. 17 Two possible 4f optical systems

f

f

f

f

f

f

Optical Modulation

1.3.1

transform of the second lens. The image shown at the aperture (give way sign) appears at the output rotated by 180 . The reproduced image would be perfect if the two lenses were ideal, however there are lens imperfections such as chromatic and spherical aberrations. Some of these distortions are reversible and cancel in the system, but some are not, leading to a slightly distorted image. The lower 4f system is in fact the same configuration for a hypothetical point on the axis. This model demonstrates how the wavefront from the input can be conserved and translated to the to the output and is an essential concept in optical systems such as free-space optical interconnects and telecommunications [6, 7].

8

Summary

In this chapter we have covered the most relevant issues concerning the design, development, and understanding of a successful display technology. Optical modulation is a very important aspect of any display and a fundamental principle is that of resolution and its definition in the context of the display technology. Whether the limiting factors are pixels, optics, or materials, the theory of modulation transfer and point spread functions is essential in assessing the resolution properties of any display technology. Optical modulation characteristics are especially important in birefringence-based technologies such as liquid crystal displays, so a basic theory for analyzing such optics is described. The analysis stems from the basic birefringence of these materials, and describes their optical properties using Jones matrices. Finally, as a vital element in any optical analysis of modulation and its effects, a brief description of optical diffraction theory is given as this forms a basis for most modulation and resolution analysis. Diffraction is often a limitation but is now becoming a powerful technique for display generation in its own right [8].

References 1. 2. 3. 4.

Hecht E, Zajac A (1987) Optics, 2nd edn. Addison Wesley, Reading Smith FG, King TA (2000) Optics and photonics. Wiley, New York Smith WJ (2000) Modern optical engineering, 3rd edn. SPIE/McGraw-Hill, New York Barten PJG (1995) Display Image Quality Evaluation. SID Applications Seminars, Florida, 23 May, 1995, pp A-5/1-40

5.

6.

7. 8.

Williams CS, Becklund OA (1989) Introduction to the optical transfer function. Wiley-Interscience, New York Wilson RG (1995) Fourier series and optical transform techniques in contemporary optics. Wiley, New york Goodman JW (1996) Introduction to Fourier optics, 2nd edn. McGraw-Hill, New York See http://www.lightblueoptics.com

67

Section 2

Human Vision and Photometry

Part 2.1

Vision and Perception

2.1.1 Anatomy of the Eye Christine Garhart . Vasudevan Lakshminarayanan 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2

Basic Dimensions of the Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

3

Eye Formation and Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4

Layers of the Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5

Accessory Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

6

The Humors of the Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

7

The Cornea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

8

The Crystalline Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

9

The Retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.1.1, # Springer-Verlag Berlin Heidelberg 2012

74

2.1.1

Anatomy of the Eye

Abstract: This chapter gives a basic introduction to the anatomy of the eye. This background is critical to understanding the physiology of the eye, and in particular aspects of visual perception which are discussed elsewhere in this handbook. List of Abbreviations: GRIN, Gradient Index; ILM, Inner Limiting Membrane; INL, Inner Nuclear Layer; IPL, Inner Plexiform Layer; OCT, Optical Coherence Tomography; OLM, Outer Limiting Membrane; ONL, Outer Nuclear Layer; PEs, Pigment Epitheliums

1

Introduction

The human eye is the basic organ of sight. The mechanism of sight and visual perception involves a set of structures (each of which has a definite function). The eye is housed in a protective framework of bones and connective tissue and this is called the orbit. The eyelids contain glands that produce the tear film layer over the anterior (front surface) of the eye, as well as protecting the anterior surface. The muscles that are attached to the eyeball control the movement of the eyes are called extraocular muscles. In addition, the muscles are coordinated between the two eyes, a necessary condition for binocular vision. A complex network of blood vessels and neurons provide nutrients as well as sensory and motor innervations to the eye. The crystalline lens of the eye plays a major role in focusing the light rays through a process called accommodation controlled via the ciliary muscles. The retina, the innermost of various layers, contains the light-absorbing rod and cone photoreceptors, as well as a neural network to process and transmit the electrical signals via the optic nerve to the visual cortex in the brain via the lateral geniculate body. > Figure 1 illustrates the main features of the human eye. It also shows the eye in its bony orbit. Additional details on the optical elements and functional and physiological properties of the eye can be found, for example, in the books by Grosvenor [3], Oyster [4], Remington [5], and Hart [6]. Detailed description of the optics of the eye can be found in the book by Atchison Smith [7].

2

Basic Dimensions of the Eye

The eye is a spheroid structure that rests in the orbit on the frontal surface of the skull. The dimensions of the human eye are reasonably constant in adults, varying by only about a millimeter or so. The sagittal diameter (the vertical) is about 24 mm and is usually less than the transverse diameter which is about 24.5–25 mm. The adult human eye weighs approximately 7.5 g.

3

Eye Formation and Growth

Eye formation begins during the end of the third week of development when outgrowths of brain neural tissue, called the optic vesicles, form at the sides of the forebrain region. The major structures of the eye are initially formed by the fifth month of fetal development. The eye structures enlarge, mature, and form increasingly complex neural networks prenatally. At birth, infant eyes are about two-third the size of an adult eye. Until the first month of life infants lack complete retinal development, with consequent effects on development of visual functions (i.e., visual acuity, contrast sensitivity, motion, etc.). From the second year until puberty, eye growth progressively slows. It should be noted that infants are born hyperopic (too much positive power), and a process of emmetropization occurs. The mechanism and development of emmetropia (as well as myopia development in children) is a major area of current research (e.g., [8, 9]).

2.1.1

Anatomy of the Eye

Eyelid

Sclera

Eyelashes Orbital Muscles

lris Cornea

Optic Nerve

Pupil

a Eye Socket tear/film posterior chamber lirnbal zone

cornea iris conjunctiva canal of Schlemm ciliary muscle

ci

lia

ry

bo

dy

anterior chamber

rectus lendon

lens ciliary process retrolental space

optic axis

zonule fibers ciliary epithelium orc terminalis

visual axis

vitreous retina sclera chorioid

dis

fovea

c

lamina cribrosa

b

macula

5 mm

. Fig. 1 (a) The human eye in its bony orbit, showing the extraocular muscles (From [1]. (b) Anatomic cross section of the eye (From [2])

4

Layers of the Eye

The thick wall of the eye contains three layers: the sclera, the choroid, and the retina. The sclera, the white part of the eye, is the outermost layer. This is a thick layer and gives structural stability

75

76

2.1.1

Anatomy of the Eye

and shape to the eye. A delicate membrane called the conjuctiva covers the visible portion of the eye. A recent study [9] of 50 eye-bank eyes found that the mean scleral thickness  SD was 0.53  0.14 mm at the corneoscleral limbus (where the cornea and sclera meet), significantly decreasing to 0.39  0.17 mm near the equator, and increasing to 0.9–1.0 mm near the optic nerve. The mean total scleral surface area by surface area was found to be about 17.0 cm2. The sclera is optically opaque and the transparent cornea allows light rays to enter the eye. Underneath the sclera is the second layer of tissue, the choroid, composed of a dense pigment and blood vessels that nourish the tissues. The vascular layer is also called the uvea. The choroid is a dense vascular network and supplies nutrients to the retinal layers. The choroid is thought to play a role in eye growth. Recent studies show that the choroid is about 426 mm and the thickness undergoes diurnal fluctuations [10]. Optical coherence tomography (OCT) results reveal that thickness decreases with increasing axial length of the eye [11]. Near the center of the visible portion of the eye, the choroid layer forms the ciliary body, which contains the muscles used to change the shape of the lens (Accommodation; see > Sect. 8). The ciliary body in turn merges with the iris, a diaphragm that regulates the size of the pupil. The iris is the area of the eye where the pigmentation of the choroid layer, usually brown or blue, is visible because it is not covered by the sclera. The pupil is the round opening in the center of the iris; it is dilated and contracted by muscular action of the iris, thus regulating the amount of light that enters the eye and is therefore like the aperture stop of the optical system of the eye. The change in pupil size not only controls the amount of light incident on the retina, but can also affect the retinal image quality that is due to diffraction and depth of focus. The diameter of the eye can change because of factors such as illumination, age, accommodation, and drugs. Behind the iris is the lens, a transparent, elastic, but solid ellipsoid body that focuses the light on the retina, the third and innermost layer of tissue. Accommodation is the process by which the eye lens changes its shape (and increases its power) to bring a near object to focus. The mechanism of accommodation is discussed for example in references [12–14]. The retina is a network of nerve cells, notably the rods and cones, and nerve fibers that fan out over the choroid from the optic nerve as it enters the rear of the eyeball from the brain. Unlike the two outer layers of the eye, the retina does not extend to the front of the eyeball. The retina will be discussed in greater detail later in this chapter.

5

Accessory Structures

These include the lachrymal gland and its ducts in the upper lid, which bathe the eye with tears. The tear film layer is about a quarter of a wavelength thick and keeps the cornea moist and clean. The drainage ducts carry the excess moisture to the interior of the nose. The eye is protected from dust and dirt by the eyelashes, eyelid, and eyebrows. In addition, there are six muscles which extend from the eye socket to the eyeball, enabling it to move in various directions. The extraocular muscles and their actions are shown in > Fig. 2. There are two types of movement; conjugate (both eyes move in the same direction) and disjunctive (the eyes move in opposite direction). There is an antagonist–agonist reciprocal innervation for eye muscles. More details on oculomotor mechanisms and characteristics can be found in the book by Ciuffreda and Tannen [16].

Anatomy of the Eye

Lateral rectus Abduction

Medial rectus Adduction

Superior rectus Elevation

Inferior rectus Depression

Inferior oblique

Superior oblique

Excycloduction

Incycloduction

2.1.1

. Fig. 2 The extraocular muscles and their actions (Reprinted from [15])

6

The Humors of the Eye

The space between the cornea and iris known as the anterior chamber is filled with a thin watery liquid, which is optically clear and slightly alkaline, called the aqueous humor. The aqueous humor is secreted into the chamber by the ciliary body, and is drained by the trabecular meshwork. The aqueous humor inflates the globe of the eye, maintains the intraocular pressure and provides nutrients to the avascular structures of the eye, namely the posterior cornea, lens, etc. The refractive index of the aqueous humor is taken to be about 1.336 (for sodium D line) and the anterior chamber has a volume of about 151 mm3 [17]. The space between the back of the lens and retina, called the posterior chamber, is filled with vitreous humor, a jellylike substance. The vitreous is colorless, transparent, and gelatinous. The viscosity of the material is about two to four times that of pure water. It too has a refractive index of about 1.336. The vitreous in contact with the retina helps to keep it in place by pressing against the choroid. In addition, unlike the aqueous, the vitreous is stagnant – it does not get replaced. Therefore, if blood, cells, or other debris get into the vitreous, they will cast shadows, diffraction effects, etc., on the retina, and produce floaters. The depth of the posterior chamber is approximately 16.03 mm.

77

78

2.1.1 7

Anatomy of the Eye

The Cornea

The cornea is the major structure that optically aids in retinal image formation. If the unaccommodated eye has a power of about 60 D (i.e., the eye is looking at an object at optical infinity), then the cornea contributes roughly 42 D of this effective power. The cornea is the first refractive surface element that light comes in contact with. The cornea has an anterior radius of curvature of about 7.8 mm. The cornea is roughly 11.5 mm in diameter, a thickness of about 0.5–0.6 mm in the center and about 0.8 mm in the periphery and takes about one-sixth of the globe. It is slightly raised from the sclera at the limbus. Even though a single anterior radius of curvature is given, in reality the cornea is spherical only in the central 1–3 mm zone. In the paracentral zone, approximately 3–4 to 7–8 mm, it is approximately a prolate spheroid; in the periphery, with the outer zone diameter of about 11 mm, there is greatest asphericity and flattening. At the limbus, the cornea steepens. The front surface of the cornea can be roughly modeled as an ellipsoid, with an eccentricity factor of about 0.6–0.8 [18]. Because of the asphericity, the cornea minimizes spherical aberration and coma in the retinal image. The Young’s modulus of elasticity of the cornea is 0.45–1.0 MPa and the Poisson ratio is about 0.49. There are five layers of the human cornea. The cornea is completely transparent and hence has no blood vessels. There is also a systematic arrangement of collagen fibrils in a lattice formation. It gets its oxygen through direct diffusion from the air. The refractive index of the cornea is approximately 1.376. Because of the change in refractive index between the cornea (posterior) and the aqueous is not very big, it contributes about 6 D to the overall refraction.

8

The Crystalline Lens

The lens, which is part of the anterior segment of the eye, is behind the iris of the eye. The lens is suspended in place by the zonular fibers which attach to the lens near its equatorial line and connect the lens to ciliary body. Posterior to the lens is the vitreous. The lens contributes about 18 D to the overall effective power of the eye. The lens has an ellipsoid, biconvex shape. The anterior surface is less curved than the posterior. In the adult, the lens is typically 10 mm in diameter and has an axial length of about 4 mm, though it is important to note that the size and shape can change due to accommodation and because the lens continues to grow throughout a person’s lifetime. During accommodation, for near objects, the ciliary muscle contracts, zonule fibers loosen and relax, and the lens thickens, resulting in a rounder shape and thus increasing refractive power. Changing focus to an object at a greater distance requires the relaxation of the ciliary muscle, which in turn increases the tension on the zonules, thereby flattening the lens and thus decreasing the lens. It should be noted that with age, this ability to accommodate decreases, resulting in a condition called presbyopia which affects people typically in their forties. This decrease of accommodation with age is called the Donder’s curve [19]. The refractive index of the lens varies from approximately 1.406 in the central layers down to 1.386 in less dense cortex of the lens. The lens has a gradient index nature [20], which helps to reduce various optical aberrations. The lens grows throughout life; as a result, the gradient index (GRIN) structure is formed with the highest index in the lens nucleus and the lowest index in the capsule of the lens. Even though the lens is completely transparent, it is made up of three layers: the lens capsule, the lens epithelium, and lens fibers. The lens capsule completely covers the lens, and contributes to a higher curvature on the anterior side than the posterior side. The lens fibers

Anatomy of the Eye

2.1.1

form the majority of the lens material. Opacities in the lens are called cataracts and if they are large enough to interfere with vision, they are surgically removed and an artificial intraocular lens is substituted.

9

The Retina

The retina is the fundamental sensory layer of the eye. The cross section of the retina is shown in > Fig. 3. Note that light comes in from below in the diagram. It contains about 200 million photoreceptors, both rods and cones. These photoreceptors absorb visible light and convert the absorbed light into nerve impulses that are sent out to the brain via the optic nerve. The central retina contains the macula, which a specialized area for perceiving fine detail and color. The center of the macula is called the fovea. The fovea contains only cone photoreceptors and is completely devoid of rods. The density of cones decreases as we go toward the periphery of the retina (> Fig. 4; [22, 23]). When a photoreceptor absorbs a photon, there is a series of photochemical events that result in a hyperpolarization of the photoreceptor cell. This electrical signal is transmitted down through the cell layers till they reach the optic nerve head/ optic disk, a region devoid of photoreceptors. The region of the optic nerve head is called the blind spot. The optic nerve is made up of millions of nerve fibers that collect information from the eye and send it to the visual cortex of the brain [24, 25]. The human retina is approximately 0.2 mm thick, and has an area of approximately 1100 mm2 (about the size of a silver dollar). The various layers of the retina are as follows: 1. Inner limiting membrane (ILM) is the boundary between the vitreous humor in the posterior chamber and the retina itself. 2. Ganglion cell layer comprises the cell bodies and axons of ganglion cells. 3. Inner plexiform layer (IPL) contains the synapses made between bipolar, amacrine, and ganglion cells. The thickness of this layer varies considerably across species, where

Sclera Choroid Pigment Layer Rod

Direction of Light

Cone

Horizontal Cell Bipolar Cell Amacrine Cell Ganglion Cell Optic Nerve Fibres

. Fig. 3 The structure of the retina (Reprinted from [21])

79

2.1.1

Anatomy of the Eye

cone peak

180 160

rod peak

140 120 100

OPTIC DISK

RECEPTOR DENSITY (mm–2 x 103)

80

80 60

rods

40 20 0

cones 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 fovea TEMPORAL NASAL ECCENTRICITY in degrees

Osterberg,1935

. Fig. 4 Distribution of rods and cones in the retina (Data from [22])

4. 5. 6. 7. 8.

9.

‘‘simpler’’ organisms (such as frogs, pigeons, and squirrels) possess thicker IPLs than ‘‘higher’’ organisms like primates. The thicker IPL indicates that these retinas perform more peripheral and specialized image processing. Inner nuclear layer (INL) contains bipolar cell, horizontal and amacrine cell bodies. Outer plexiform layer (OPL) contains bipolar cell, horizontal cell and receptor synapses. Outer nuclear layer (ONL) contains the nuclei of photoreceptors. Outer limiting membrane (OLM) interfaces with the base of inner segments of photoreceptors. Photoreceptor layer contains the inner and outer segments of rod and cone photoreceptors. The photoreceptors contain the opsins (the various proteins) and the chromophores (the photon-catching molecules; consisting of retinal, an aldehyde of vitamin A). The absorption of a photon by the chromophore results in a conformational change of the chromophore, which in turn results in a photochemical cascade. The isomerization of 11-cis retinal to all-trans begins the process of phototransduction. The exact chain of events is: isomerization of photopigment breaks apart a molecule called transducin, which activates an enzyme called phosphodiesterase. Phosphodisterase, in turn, breaks cGMP into its inactive form, which causes Na+ channels (which are open in the resting state) to close. Closing Na+ channels hyperpolarizes the neuron (unlike in other neuronal systems). Light stimulation thus causes fewer transmitters to be released at the synapse. The hyperpolarization of the outer segment spreads to the inner segment by electrotonic conduction. Since receptors are small, the receptor potential is still large at the axon terminal in the inner segment. Thus, most retinal neurons transmit information using only graded potentials. Some amacrine cells and all ganglion cells use action potentials. It is of interest to note that photoreceptors act as classical fiber optic devices and guide light to sites of absorption [26]. Photoreceptors are found to be oriented toward the center of the exit pupil of the eye [27]. Pigment epitheliums (PEs) are darkly pigmented cells which absorb light not captured by photoreceptors, thus reducing scattering; they also play a role in ‘‘trimming’’

Anatomy of the Eye

2.1.1

photoreceptors which follows a diurnal cycle. Diurnal species (active in bright-light environments) typically possess dark PEs; nocturnal species (active in dim-light environments) possess an adaptation called a tapetum. The tapetum is a mirrorlike layer behind the photoreceptors which reflects photons not captured by the photoreceptors back out the eye, thus giving the receptors a ‘‘second chance’’ to capture them. Sensitivity to light in these animals is thus increased by approximately twofold. The dominant wavelength of light reflected by the tapetum is usually close to the absorbance peak of rhodopsin (the photopigment contained by rods). Thus, the red eye seen in flash photos. There are three types of cones, thus giving rise to trichromatic vision. The first responds maximally to light of long wavelengths, peaking in the yellow region (564–580 nm); this type is designated L for long wavelength-sensitive cones or red cones. The second type responds most to light of medium-wavelength, peaking at green (534–545 nm), and are known as M cones or green cones. The third type responds most to short-wavelength light, of a violet color, the S-cones (or Blue cones), and have a peak of absorption at 420–440 nm (see > Fig. 5). The packing arrangements of these three cone types were recently studied using adaptive optics techniques [28]. The packing arrangements are important because of sampling issues and aliasing that can occur in human vision [29]. The absence of one (or more) of these cone types leads to various color deficiencies (see > Chap. 2.2.7 on color vision deficiencies). The rods on the other hand are more sensitive for low-light levels and peak around 420 nm. The sensitivity curves for rods and cones are shown in > Fig. 5. As noted previously, when light falls on a receptor it sends a proportional response synaptically to bipolar cells which in turn signal the retinal ganglion cells. The receptors are also ‘‘cross-linked’’ by horizontal cells and amacrine cells, which modify the synaptic signal

420

495 530 560

1.0

relative absorptance

0.8

0.6

0.4

0.2

0.0

370 400 S

. Fig. 5 Rod and cone sensitivity curves

V⬘

5

L 600 500 M wavelength (nanometers)

39

58 45

670

81

82

2.1.1

Anatomy of the Eye

before the ganglion cells. Rod and cone signals are intermixed and combine. Despite the fact that all are nerve cells, only the retinal ganglion cells and few amacrine cells create action potentials. Although there are more than 120 million photoreceptors, there are only about 1.2 million fibers in the optic nerve; a large amount of preprocessing is performed within the retina. The retina spatially encodes (compresses) the image to fit the limited capacity of the optic nerve. The retina does so by encoding the incoming images in a suitable manner. These operations are carried out by the center-surround receptive field structures as implemented by the bipolar and ganglion cells. These center-surround structures are functional and not anatomical. These center-surround structures of the ganglion and bipolar cells encode the information by performing edge detection and other tasks. The fovea produces the most accurate information. Despite occupying about 0.01% of the visual field (less than 2 of visual angle), about 10% of axons in the optic nerve are devoted to the fovea. That is, there is an almost 1:1 connection between each cone cell in the fovea and a ganglion cell. As a result, there is great spatial resolution in the fovea. However, in the periphery there is a multiplexing of approximately ten rod cells to one ganglion cell, and hence resolution falls off considerably (see > Chap. 2.1.3). The information capacity of the retina is estimated at 500,000 bits per second without color or around 600,000 bits per second with color coding.

10

Summary

In this chapter, a brief summary of ocular anatomy is given. Emphasis has been given to topics dealing with visual optics and retinal function.

References 1. http://www.99main.com/charlief/Blindness.htm 2. Walls GL (1942) The vertebrate eye and its adaptive radiations. Cranbook Institute of Science, Bloomfield Hills 3. Grosvenor TP (1989) Primary care optometry. Professional Press, New York 4. Oyster CW (1990) The human eye. Sinauer, Sunderland 5. Remington LA (2005) Clinical anatomy of the visual system, 2nd edn. Butterwoth Heinemann, St. Louis 6. Hart Wm (1992) Adler’s physiology of the eye, 9th edn. Mosby, St. Louis 7. Atchison DA, Smith G (2000) Optics of the human eye. Butterworth Heinemann, St. Louis 8. Charman WN, Radhakrishnan H (2010) Peripheral refraction and development of refractive error: a review. Ophthalmic Physiol Opt 30:321–338 9. Pang Y, Maino DM, Zhang G, Lu F (2006) Myopia: can its progression be controlled? Optom Vis Dev 37:75–79 10. Brown JS, Flitcroft D, Ying G, Frances EL et al (2009) In vivo human choroidal thickness measurements:

11.

12.

13. 14.

15.

16.

evidence for diurnal fluctuations. Invest Opthalmol Vis Sci 50:5–12 Esmaeelpour M, Povazay B, Hermann B, Hofer B et al (2010) Three dimensional 1060 nm OCT: choroidal thickness maps in normal subjects and improved posterior segment visualization in cataract patients. Invest Ophthalmol Vis Sci 51:5260–5266 Muller M, Strobel J (2007) Mechanisms of accommodation in human eye – some new aspects. Klin Monbl Augenheilkd 224:653–658 Charman WN (2008) The eye in focus: accommodation and presbyopia. Clin Exp Optom 91:207–225 Koretz JF (2000) Development and aging of human visual focusing mechanisms. In: Lakshminarayanan V (ed) Vision science and its applications, vol 35, Trends in optics and photonics series. Optical Society of America, Washington DC, pp 246–258 Simon JW, Calhoun JH (1998) A child’s eyes: a guide to pediatric primary care. Triad Publishing, Gainesville Ciuffreda K, Tannen B (1995) Eye movement basics for the clinician. C.V.Mosby, St. Louis

Anatomy of the Eye 17. Behndig A, Markstrom K (2007) Determination of aqueous humor volume by 3-D mapping of the anterior chamber. Ophthalmic Res 37:13–16 18. Applegate RA, Howland HC (1995) Non-invasive measurement of corneal topography. IEEE Eng Med Biol 14:30–42 19. Stark L, Sun F, Lakshminarayanan V, Wong J, Nguyen A, Muller E (1985) Presbyopia in the light of accommodation. Work reports on the 3rd international symposium on presbyopia, vol 2, Essilor International, Paris, pp 340–352 20. Pierscionek B (2010) Gradient index optics in the eye. In: Bass M, De Cusatis C, Enoch JM, Lakshminarayanan V et al (eds) Handbook of optics, 3rd edn. McGraw Hill, New York, Chapter 18 21. http://www.catalase.com/retina.gif 22. Osterberg G (1935) Topography of the layers of the rods and cones in the human retina. Acta Ophthalmol Suppl 13:1–102 23. Curcio CA, Sloan KR, Kalina RE, Hendrickson AE (1990) Human photoreceptor topography. J Comp Neurol 292:497–523

2.1.1

24. Rodieck RW (1998) The first steps in seeing. Sinauer, Sunderland 25. Dowling J (1987) The retina: an approachable part of the brain. Belknap Press, Cambridge, MA 26. Lakshminarayanan V, Enoch JM (2010) Biological waveguides. In: Bass M, DeCusatis C, Enoch JM, Lakshminarayanan V et al (eds) Handbook of optics, vol III. McGraw Hill, New York, Chapter 8 27. Enoch JM, Lakshminarayann V (1991) Retinal fiber optics. In: Charman N (ed) Vision and visual dysfunction, vol I. McMillan Press, London, pp 280–309 28. Roorda A, Williams DW (1999) The arrangement of the three cone classes in the living eye. Nature 397:520–522 29. Lakshminarayanan V, Nygaard RW (1992) Aliasing in the human visual system. Concepts Neurosci 3:201–212

Further Reading Chalupa L, Werner J (2000) The visual neurosciences, vol 2. MIT Press, Cambridge, MA Davson H (1972) The physiology of the eye, 3rd edn. Academic, New York Fatt I, Weissman BA (1992) Physiology of the eye: an introduction to vegetative functions. ButterworthHeinemann, St. Louis

Palmer SE (1999) Vision science. Photons to Phenomenology. MIT Press, Cambridge, MA Rolls ET, Deco G (2004) The computational neuroscience of vision. Oxford University Press, Oxford Toyoda J, Murakami M, Kaneko A, saito T (1999) The retinal basis of vision. Elsevier, Amsterdam

83

84

2.1.1

Anatomy of the Eye

2.1.2 Light Detection and Sensitivity Vasudevan Lakshminarayanan 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 2 Nature of Vision at (or Near) Absolute Threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3 Behavioral Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 4 The Effect of Noise on Detection of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5 Intensity Discrimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 6 Visual Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.1.2, # Springer-Verlag Berlin Heidelberg 2012

86

2.1.2

Light Detection and Sensitivity

Abstract: This chapter deals with the absolute threshold of vision, namely, the minimum amount of light necessary to elicit a visual response. The effect of intrinsic retinal noise which affects light detection is considered; this is followed by a discussion on intensity discrimination by both rods and cones. List of Abbreviations: tvi, Threshold Versus Intensity

1

Introduction

Of all our sensory systems, the visual sense dominates – about 60% of all nerve fibers from a sensory organ to the brain come from the eyes. The visual cortex contains about 500 million nerve cells (the corresponding number for the auditory cortex is about 800,000 nerve cells; from the ear 30,000 nerve fibers convey acoustic information to the brain, while from the eye 1–3 million nerve fibers convey visual information to the brain). The eye operates over an amazing range of light levels, covering an intensity range of approximately 12 log units and possesses exquisite sensitivity. A good review is given by Rodieck [1].

2

Nature of Vision at (or Near) Absolute Threshold

Absolute threshold implies that the rod photoreceptors signal the absorption of single photons. The question of the sensitivity of the eye is not simply one of physics but also of the criterion used by the observer, thus bringing in the behavioral response of the observer. Hence, the question of the absolute threshold of human vision can only be answered in terms of a response probability. Lorentz in 1901 hypothesized that a just detectable flash of light delivered approximately 100 photons to the cornea (quoted in [2]). Identifying the minimum number of photons required for seeing from this number is difficult because of uncertainties in determining the number of photons absorbed by the retinal photoreceptors. This quantum efficiency has been estimated to be in the range of about 0.1–0.3. The best values of thresholds are obtained after prolonged dark adaptation of about 30–45 min. It is found that completely dark adapted rod photoreceptors approach the sensitivity limit imposed by the quantization of light and the Poisson fluctuations of photon absorption. In fact, isolated single photoreceptors signal the absorption of a single photon [2]. The number of photons required to give a psychophysical response (behavioral) was established by Van der Velden [3] and by Hecht, Schlaer, and Pirenne [4].

3

Behavioral Results

In their classic experiment, Hecht et al. measured the fraction of trials in which a flash was seen as a function of the number of photons incident on the cornea. This curve, a psychometric function, showed a broad transition from flashes that were rarely seen to those frequently seen, and from this curve the threshold and quantum efficiency can be derived based on the assumption that the variability in a subject’s response was due to Poisson statistics of the photon absorption. Implicit in this analysis are two other basic ideas: only those instances wherein the number of photons which exceed a threshold number were seen and that the average number of photons contributing to ‘‘seeing’’ was directly proportional to the number of photons incident on the cornea and the constant of proportionality being the quantum

Light Detection and Sensitivity

2.1.2

efficiency. From these beautifully conducted and analyzed experiments, they concluded a quantum efficiency of about 0.06, which is much lower than that derived from light scattering and ocular media properties. The authors conclude that in order for a visual effect to be produced, one quantum must be absorbed by each of 5–8 or so rods in the retina (at a 65% probability of correct response). To give the reader a sense of how sensitive human observers can be, Pirenne [5] used a white light test stimulus spread out over many degrees on the retina, and the threshold was determined to be 0.75
106 Cd/m2. This is of the order of 5–30% of the luminance of the darkest night sky measured by the National Physical Laboratory. There is a big discrepancy between the in vitro result that individual photoreceptors can detect and signal a single photon while psychophysically 5–8 photons absorptions are required. The reason for this discrepancy is the presence of biological noise in the visual system. However, it should be emphasized that the human eye sensitivity approaches the limit set by quantization of light, Poisson fluctuations in absorption, and the internal noise.

4

The Effect of Noise on Detection of Light

Visual sensitivity is hampered by background noise occurring along the visual retina–cortex pathway. There have been many studies to describe and show physiologically and mathematically the events occurring in the retina in response to stimuli and when no stimuli are present (see for example [6, 7]). The physiology and biochemistry is discussed in detail by Rodieck [1] and Wickehart [8]. What is retinal background noise? Spectral absorptions that occur in the photoreceptors cause a neural cascade which is encoded as sight in the occipital cortex, however, there are ever-present visual stimulations occurring randomly without photon absorption by the photoreceptors. Barlow and others [9, 10] advocated the concept of ‘‘dark light’’ – a name given to internal events such as spontaneous decomposition of photopigment. It was shown by these and other researchers that additive Poisson noise can account for the discrepancy. In fact, it has been estimated based on spatial and temporal summation characteristics of the rod array, rod density, assumed quantum efficiency, etc., that the equivalent rate of photon-like noise events in rod photoreceptors, the dark light ranges from 0.002 to 0.03 per second [11]. Spontaneous background noise in the retina is usually described as having two components. Baylor et al. described discrete thermal activations in photoreceptors, which accounts partly for background noise [12, 13]. The rest of the noise is thought to be due to fluctuations of the concentrations of certain chemicals in the process of phototransduction. Background retinal noise is generated among both the rod and the cone photoreceptors. Cones have been shown to be noisier than rods, possibly hampering our vision when we fixate on important stimuli in our environment, but presumably have less of an effect on our sensitivity at absolute visual threshold levels. This would make sense because we know rods function as low light detectors, and would therefore necessitate quieter biological conditions. In addition, it is also found that the power spectrum of dark noise had the same shape as the spectrum of a dim flash of light, evidence that retinal noise consists of random events with an average shape of the single photon response. There are a number of unanswered questions such as the effect of continuous noise, false-positives due to the noise in setting threshold, etc., and these are beyond the scope of this chapter.

87

2.1.2

Light Detection and Sensitivity

In summary, physiological noise is indistinguishable from signals generated by light stimuli, and it is hypothesized to be the main neural limit to our visual acuity. A more detailed description of the problem of noise in photoreceptor can be found in the articles by Lakshminarayanan [14], Reike and Baylor [15], and Field et al. [16].

5

Intensity Discrimination

Up to this point, we have been discussing the detection of light at (or near) absolute threshold values. Here, we discuss the important issue of intensity discrimination – that is, how does the visual system determine if one luminous stimulus differs in intensity from another. It should be obvious from the above discussion that the quantum fluctuations provide a theoretical lower limit for intensity discrimination by an ideal observer. How about at values of stimuli well above threshold? In an increment threshold measurement, a test stimulus of luminance Lt is compared to an adjacent stimulus of luminance L, which is a reference stimulus. The psychophysical task is to determine how different Lt must be from L (above or below) for it to be seen as different (at some preassigned probability value, say 50% of the time). Let this difference be given by DL. If we determine DL for a number of different values of the reference L, we get a curve called a threshold versus intensity function (tvi function). A typical tvi curve is shown in > Fig. 1; typically there are two branches showing the duplex nature of the retina.

3 4 Saturation 2

3

Cones

5

6

Log increment threshold

5° PARAFOVEA λ = 580 nm μ = 500 nm

4 Log Nλ

88

Rods

7

3 Weber’s law

0 –1 –2

1 Dark light 2 Square root law

–3 –4 –8

8 (ZERO) 9

a

1

8

7 6 5 Log Mμ

4

3

2

1

b

–4

–2

0

2

4

Log field intensity

. Fig. 1 (a) Light adaptation curve plotted as increment threshold versus background luminance (or a threshold versus intensity: tvi curve). The above plot shows increment threshold (Nl) and background luminance (Mm). Light of two different wavelengths are used in this case (580 nm for the test and 500 nm for the background) (Adapted from Stiles’ data from [17]). (b) Schematic of the increment threshold curve (Adapted from Aguilar and Stiles’ data from [17])

Light Detection and Sensitivity

2.1.2

If we start from near absolute threshold, the threshold in the flat horizontal portion of the curve is determined by the dark light level or the internal noise (portion 1); Increases in luminance does not change DL very much; this implies that an imposer would not be able to always detect an intensity difference between two flashed stimuli that on average differ in intensity only be a few quanta. As the background luminance is further increased, we pass on to region 2 of the curve, the ‘‘square root’’ law region. Quantum fluctuations increase with number of quanta in the stimulus, and as stimulus luminance increases, the minimum discriminable threshold increases in proportion to the square root of the intensity level. This is known as the deVries Rose law or the square root law and is expressed as: DL pffiffiffi ¼ K L In this portion, the slope of the curve is ½. For the rod pathway, a slope of 0.6 is often found. At low reference luminance levels, humans behave as ideal detectors and follow the deVries Rose law. As we further increase background levels, Weber’s law holds and the intensity discrimination threshold is higher than expected from an ideal detector. In this region, also called Weber’s law region, we get a straight line portion of the curve, where the slope is a constant. The constant proportional relationship between increment threshold (DL) and reference luminance (L) is called Weber’s law, DL/L = constant. This proportional change in threshold DL with L implies that the visual system is not detecting luminance differences at the theoretical limit. It should be noted that the Weber constant is affected by stimulus size, duration, wavelength, and retinal location [18–21]. Weber’s law implies that there is a limitation on intensity discrimination due to loss of information. At higher luminance levels, the Weber fraction becomes large – that is DL increases faster than L and the visual system saturates.

6

Visual Adaptation

As noted before, the visual system operates over a huge range of retinal illuminances. One of the reasons that the visual system can move its ‘‘operating curve’’ over such a wide range of illuminances is the fact that we have different photoreceptor subsystems – the rods and cones (see > Chap. 2.1.1). This adjustment of the operating level to the existent light level is known as adaptation. In this section, we will discuss the dark and light adaptation characteristics of the visual system. Light adaptation refers to the process that increases/decreases threshold luminance/sensitivity (recall that threshold is the inverse of sensitivity) in response to an increased level of illumination. Dark adaptation is the reverse of this change in sensitivity. Dark adaptation can be easily measured in the laboratory (and routinely tested in the clinic) using standard psychophysical methodology. The eyes are exposed to a suprathreshold adapting light (monochromatic or multichromatic) of large spatial dimension. Then, the adaptation light is turned off (time 0) and a test flash of a certain wavelength(s) and size is presented at a specific retinal location, and the DL is measured at specific times. The classic dark adaptation curve is shown in > Fig. 2.

89

2.1.2

Light Detection and Sensitivity

8 Cones 7 Log intensity (μμL)

90

6 Rods 5 4 3 2 0

5

10 15 20 Time in dark (min)

25

30

. Fig. 2 Dark adaptation curve. The shaded area represents 80% of the group of subjects (Adapted from Hecht and Mandelbaum’s data from [22])

The dark adaptation curve has certain specific features. There are two main branches – the cone branch and the rod branch. Initially, there is a rather large decrease in threshold luminance (about 3 min mark or so), followed by a rather slow change until about 8–12 min or so. Here, the cones approach their lowest threshold level and are fully dark adapted. The curve is pretty much flat during this portion. After about this time period, the cones saturate, and the rod system takes over, and the curve drops again as the rods become more sensitive. This point of transition is called the cone–rod break. Within about 30 min or so, the rod portion becomes flat and rods have adapted. Rods begin to dark adapt at the same time as the cones, but initially they are less sensitive than cones and have a larger time constant. The reader is referred to the chapter by Birch [23] for a detailed description of the physiology and mechanism of dark adaptation.

7

Summary

In this chapter, we have discussed the fact that luminance detection under certain conditions is limited only by internal noise in the visual system. We have also examined intensity discrimination by the eye and the basic laws of psychophysics, namely, the DeVries Rose square root law and Weber’s law. Finally, we have examined the response of the photoreceptors to dark adaptation.

References 1. Rodieck RW (1998) The first steps in seeing. Sinauer, Sunderland 2. Bialek W (1987) Physical limits to sensation and perception. Ann Rev Biophys Biophys Chem 16:455–478

3. van der Velden HA (1946) The number of quanta necessary for the perception of light of the human eye. Ophthalmologica 111:321–331 (originally published in Dutch in Physica)

Light Detection and Sensitivity 4. Hecht S, Schlaer S, Pirenne MH (1942) Energy, quanta and vision. J Gen Physiol 25:819–840 5. Pirenne MH (1962) Absolute thresholds and quantum effects. In: Davson H (ed) The eye. Academic, New York 6. Hamer RD, Nicholas SC, Tranchina D, Liebman PA, Lamb TD (2003) Multiple steps of phosphorylation of activated rhodopsin can account for the reproducibility of vertebrate rod single photon responses. J Gen Physiol 122:419–444 7. Hamer RD, Nicholas SC, Tranchina D, Lamb TD, Jarvinen JLP (2005) Toward a unified model of vertebrate rod phototransduction. Vis Neurosci 22:417–436 8. Wickehart DR (2003) Biochemistry of the eye. Butterworth-Heinemann, Philadelphia 9. Barlow H (1956) Retinal noise and absolute threshold. J Opt Soc Am 46:634–639 10. Sakitt B (1972) Counting every quantum. J Physiol 223:131–150 11. Donner K (1992) Noise and the absolute thresholds of cones and rod vision. Vis Res 32:853–866 12. Baylor DA, Lamb TK, Yau KW (1979) Responses of retinal rods to single photons. J Physiol 288:613–634 13. Baylor DA, Nunn B, Schnapf JL (1984) The photocurrent, noise and spectral sensitivity of the rods of the monkey Macaca fascicularis. J Physiol 357:575–607

2.1.2

14. Lakshminarayanan V (2005) Vision and the single photon. Proc SPIE 5836:332–337 15. Reike F, Baylor DA (1998) Single photon detection by rod cells of the retina. Rev Mod Phys 70:1027–1036 16. Field GD, Sampath AP, Rieke F (2005) Retinal processing near absolute threshold: from behavior to mechanism. Annu Rev Physiol 67:491–514 17. Davson H (1990) Physiology of the eye, 5th edn. Macmillan Academic and Professional Ltd., London 18. Blackwell HR (1946) Contrast thresholds of the human eye. J Opt Soc Am 36:624–643 19. Lynn JR, Felman RL, Starita RJ (1996) Principles of perimetry. In: Riitch R, Shields MB, Krupin T (eds) The glaucomas. Mosby, St. Louis, pp 491–521 20. Harwerth RS, Smith EL, DeSantis L (1993) Mechanisms mediating visual detection in static perimetry. Invest Ophthalmol Vis Sci 34:3011–3023 21. Gescheider G (1997) Psychophysics: the fundamentals. Psychology Press, Philadelphia 22. Pirenne MH (1962) Chapter 5: Dark adaptation and night vision. In: Davson H (ed) The eye, vol 2. Academic, London 23. Birc DG (2003) Chapter 24: Visual adaptation. In: Kaufman PL, Alm A (eds) Adler’s physiology of the eye, 10th edn. Mosby, St. Louis

Further Reading Cornsweet T (1970) Visual perception. Academic, New York Palmer SE (1999) Vision science. Photons to perception. MIT Press, Cambridge

Wandell BA (1995) Foundations of vision. Sinauer, Sunderland

91

2.1.3 Visual Acuity Vasudevan Lakshminarayanan 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 2 Representation of VA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 3 Factors Affecting VA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.1.3, # Springer-Verlag Berlin Heidelberg 2012

94

2.1.3

Visual Acuity

Abstract: Visual Acuity (VA) is a measure of the visual system’s ability to see distinctly the details of an object. This chapter will discuss the commonly used VA measure and its measurement.

List of Abbreviations: MAR, Minimum Angle of Resolution; VA, Visual Acuity 1

Introduction

Visual acuity (VA) is a measure of the spatial vision of observers. Eye doctors (ophthalmologists and optometrists) routinely use VA measures to assess spatial vision using high-contrast stimuli – namely, the finest spatial detail that an observer can discern. Some common examples of acuity measures are: (1) Snellen eye chart where the observer names the Snellen letters, and which is a form of recognition acuity (see > Fig. 1a), (2) Landolt rings (or tumbling E) where the observers report where the gap is the letter C (or where the prongs of the letter E are pointing to) is located (resolution acuity; > Fig. 1b), and (3) parallel lines where the observer reports the orientation of the bars (resolution acuity). The VA that is measured is related to the highest spatial frequency grating (usually sine-wave grating; finest detail) that can be detected (see > Chap. 2.1.5). In this chapter, we will only discuss the clinically measured VA and not other specialized forms of acuity such as detection acuity and hyperacuity.

2

Representation of VA

Visual acuity is quantitatively represented in one of two ways: (1) as the reciprocal of the minimum angle of resolution (MAR measured in minutes of arc) or (2) as a Snellen fraction. This is measured, using as noted above, letters or Landolt rings. VA can also be expressed as a decimal, i.e., the Snellen fraction is reduced or is given by the reciprocal of the minimum angle of resolution. The Snellen fraction represents the visual acuity in the form of a fraction (e.g., 20/20, 20/80, 20/200 if measured in feet or equivalently 6/6, 6/24, 6/60 if measured in meters). Here the numerator is the testing distance (6 m or 20 ft in a clinician’s office) and the denominator is the distance at which the smallest Snellen letter has an angular size of 5 min of arc and each detail in the letter (for example, the gap between the ends of letter C subtends 1 min of arc or the width of a horizontal single bar in the letter E subtends 1 min of arc. Each letter subtends a total of 5 arc min). The minimum angle of resolution, MAR, is calculated simply by looking at the minimum details on a target that can be seen by an eye and can be calculated for example using the Rayleigh criterion 1:22l ymin ¼ D where D represents the pupil diameter of the eye. Under photopic levels D for the human eye is about 2–4 mm, and ymin works out to be about 1 min of arc in the mid-visible range. However, under optimal viewing conditions, the eye’s optics and spacing of foveal cones (center to center spacing of about 20–40 s) could allow some to resolve points that are approximately 0.5 min apart [1]. However, even with optimum correction of refractive errors, aberrations, and cone sizes (which vary somewhat), there will be variations in MAR.

Visual Acuity

a

2.1.3

b

c . Fig. 1 (a) Snellen acuity chart. (b) Landolt ‘‘c’’ chart in log MAR form. (c) Bailey–Lovie log MAR chart

95

96

2.1.3

Visual Acuity

. Table 1 Various measures of VA for different performance levels MAR Snellen Performance (min denominator criterion of arc) (m) 6  MAR Normal adult

0.82

Unrestricted driving

2.00

Moderate visual impairment

3.50

Snellen denominator (ft); 20  MAR

Snell–Stirling visual Decimal efficiency (%) Log VA 0.836(MAR-1) MAR

16.4

1.22

103.3

0.09

12

40

0.50

83.6

0.30

21

70

0.29

64

0.54

20

1

4.9

Legal blindness

10

60

200

0.10

Profound visual impairment

25

150

500

0.04

1.4

1.40

Other scales are used for representing VA – these include log MAR, decimal VA, and the Snell–Stirling visual efficiency scale. > Table 1 gives the relationship between various measures of VA for different levels of visual performance It should be noted that the log MAR is the preferred scale for representing VA in the research (and increasingly in the clinical) literature. This is especially true if the VA is better than 1 min or much worse. The reason for using this is because the inverse slope of the visual acuity psychometric function (between log MAR and percent correct) remains more or less independent of the acuity value; this means that the measurement error remains approximately constant [2]. As a result, log MAR charts such as the Bailey–Lovie charts are commonly used ([3]; see > Fig. 1b and c). In these log MAR charts, the letters on each line are approximately 26% smaller than the size of the letters on the line above it.

3

Factors Affecting VA

Measurements of letter acuity are affected by a number of factors. These include: (a) choice of letters, (b) letter spacing, (c) retinal illumination, (d) target contrast, (e) retinal eccentricity, (f) target motion and duration of target presentation, (g) neural defocus, and (h) age. We shall briefly discuss each of these factors below. (a) Choice of letters: even if letters are constructed using a standard font and of the same size, relative legibility can be a factor. For example, upper case S and B (or C and O) are more difficult to identify when compared to L and T. The relative legibility of letters has been quantified by specifying the difference in size that is required for each letter to reach its identification threshold [4, 5]. In addition to these confusion letters, observers with uncorrected astigmatism will experience defocus of specific contours (e.g., a myopic axis 180 astigmat will see vertical contours defocused making letters like V and Y difficult to distinguish).

2.1.3

Visual Acuity

(b) Letter spacing: Because of the so-called contour interaction [6] due to lateral inhibition, the ability to identify letters depends not only on size, but also on how close the letters are to each other. In addition, other factors such as inaccurate eye movements can contribute to reduced VA [7]. These are all called ‘‘crowding effects.’’ It is found that the maximum degradation of legibility is produced when a letter and its neighbor are separated by approximately half a letter width. It should be noted that contour interaction is of greater magnitude in peripheral vision (see > Chap. 2.1.5). (c) Retinal illumination: at high levels of retinal illumination, cones mediate VA. Normally VA is measured using a standard test chart at approximately 100 Cd/m2. At low luminance levels, the maximum VA is reached by normal observers is about 0.7–1.0 log MAR (see > Fig. 2). Best scotopic vision occurs at a location smaller than 20 eccentricity on the retina, at which rod density is highest. The variation in VA with retinal illumination strongly correlates with the high-frequency cutoff of the contrast sensitivity function as the illumination is lowered. (d) Contrast: VA is high for high-contrast letters. Even for well focused foveal targets, letter acuity falls as contrast is lowered. Standard clinical tests use high-contrast targets. However, low-contrast targets have been used in acuity charts and are used clinically (e.g., Pelli–Robson charts, Regan charts; [8, 9]). (e) Eccentricity: VA falls rapidly with increasing eccentricity on the retina (correlating with the shift toward lower spatial frequency cut off on the contrast sensitivity function). In fact,

1.8 1.6 Cone 1.4

Visual acuity

1.2 1.0 0.8 0.6 Rod 0.4 0.2 0 –5

–4

–3

–2 –1 0 1 log L (millilamberts)

2

3

4

. Fig. 2 Variation of VA with illumination. Red lines show the maximum of photopic and scotopic VA

97

98

2.1.3

Visual Acuity

it has been shown that letter acuities worsen to twice the foveal value at eccentricities of approximately 2.0 . In addition, the rate at which resolution changes with eccentricity is not the same along the different retinal meridians – the rate of change of acuity is smaller along the horizontal meridian than along the vertical and is better in the temporal than in the nasal visual field [10]. > Figure 3 shows a radial eye chart developed by Prof. Stuart Anstis at the University of California at San Diego. Although letter sizes increase dramatically from center outward, the letters are scaled to be equally legible at different eccentricities. (f) Motion: for high-contrast targets, VA is not affected by low velocities of retinal image motion (to approximately 2 deg/s), but becomes progressively worse at higher image velocities [11]. VA measured when target or observer is in motion is called dynamic VA. If the target is intermittently flashed, VA becomes worse as flash duration decreases, but improves as flash duration increases to about 500 ms (temporal summation period; see > Sect. 6 of Chap. 2.1.4 and [12]). (g) Neural defocus: the convergence of photoreceptor signals at ganglion cells is more extensive for peripheral rods than cones (almost 10:1 multiplexing, compared to 1:1 for foveal cones). As the signal goes up the neural visual system, there is greater chance of neural ‘‘defocus’’ which can be thought of as the blending together of the gradations of responses of neighboring cells. For example, VA goes down when target moves so quickly that small receptive fields become insensitive. (h) Age: VA starts out relatively poor in a new born infant. It has been shown that using a forced choice preferential looking psychophysical procedure VA develops from approximately 1.5 log MAR at 1 month of age to about 0.7 log MAR by age 6 months. Adult levels

. Fig. 3 Peripheral VA chart. See text for details

Visual Acuity

2.1.3

of VA are reached (0 log MAR) at approximately 3 years of age [13]. At the other end of the age spectrum, high-contrast VA remains relatively invariant with age, but there is a significant decrease in VA measured with low-contrast targets [14].

4

Conclusion

In this chapter, we have briefly discussed visual acuity of the human eye. The measurement of VA as a psychophysical test and a number of factors that affect the VA have been discussed.

References 1. Geisler WS (1989) Sequential ideal observer analysis of visual discriminations. Psychol Rev 96:26–319 2. Horner DG, Paul AD, Katz B, Bedell HE (1985) Variations in the slope of the psychometric acuity function with acuity thresholds and scale. Am J Optom Physiol Opt 62:895–900 3. Bailey IL, Lovie JE (1976) New design principles for visual acuity letter charts. Am J Optom Physiol Opt 53:740–745 4. Hedin A, Olsson K (1984) Letter legibility and construction of new visual acuity chart. Ophthalmologica 189:147–156 5. Gervais MJ, Harvey LO, Roberts JO (1984) Identification confusions among letters of the alphabet. J Exp Psychol Hum Percept Perform 10:655–666 6. Flom MC (1966) New concepts on visual acuity. Optom Wkly 57:63–68 7. Flom MC (1991) Contour interaction and the crowding effect. Probl Optom 3:237–257 8. Pelli DG, Robson JG, Wilkins AJ (1988) The design of a new letter chart for measuring contrast sensitivity. Clin Vis Sci 2:187–199

9. Regan D, Neima D (1983) Low contrast letter charts as a test of visual function. Ophthalmology 90:1192–1200 10. Wertheim T (1980) Peripheral visual acuity. Am J Optom Physiol Opt 57:915–924 11. Baron W, Westheimer G (1973) Visual acuity as a function of exposure duration. J Opt Soc Am 63:212–219 12. Demer JL, Amjadi F (1993) Dynamic visual acuity of normal subjects during vertical optotype and head motion. Invest Ophthalmol Vis Sci 34:1894–1906 13. Mayer MJ, Beiser AS, Warner AF et al (1996) Moncular acuity norms for the Teller acuity cards between ages one month and four years. Invest Ophthalmol Vis Sci 36:671–685 14. Enoch JM, Werner G, Hagerstrom Portnoy G, Lakshminarayanan V, Rynders M (1999) Forever young: visual functions not affected or minimally affected by aging. J Gerentol Biol Sci 54A:B336–B352

Further Reading Westheimer G (1979) The spatial sense of the eye. Invest Ophthalmol Vis Sci 18:893–912 Westheimer G (2002) Visual acuity. In: Kaufman P, Alm A (eds) Adler’s physiology of the eye, Chap. 17. CV Mosby, St. Louis Bedell HE (2002) Spatial acuity. In: Norton TT, Corliss DA, Bailey JE (eds) The psychophysical

measurement of visual function, Chap. 5. Butterworth-Heinemann, Woburn Norton TT, Lakshminarayanan V, Bassi CJ (2002) Spatial vision. In: Norton TT, Corliss DA, Bailey JE (eds) The psychophysical measurement of visual function, Chap. 6. Butterworth-Heinemann, Woburn, MA

99

2.1.4 Flicker Sensitivity Vasudevan Lakshminarayanan 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

2

Temporal Resolution Acuity and Critical Flicker Fusion Frequency . . . . . . . . . . . . . . 102

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

Some Properties of the CFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 CFF and Stimulus Luminance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 CFF and Area of Stimulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 CFF and Retinal Eccentricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 CFF and Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 CFF and Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 CFF and Ambient Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 CFF and Refresh Rates of Monitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 CFF and Phosphor Persistence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

4

Brightness Enhancement Effects Because of the Temporal Properties of Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

5

The Temporal Contrast Sensitivity Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

6

Temporal Summation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

7

Methods to Predict Flicker in Monitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

8

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.1.4, # Springer-Verlag Berlin Heidelberg 2012

102

2.1.4

Flicker Sensitivity

Abstract: The visual system is sensitive to temporal changes in stimuli. The image appears to be continuous as the visual system integrates the responses with respect to time. A crucial factor is the CFF – critical flicker fusion frequency – and refers to the temporal frequency beyond which flicker is no longer perceived. CFF is a measure of the minimum temporal interval that can be resolved by the visual system. This chapter discusses the CFF and the effect of various parameters such as luminance, refresh rate of the monitor, wavelength, and retinal eccentricity on flicker perception. List of Abbreviations: CFF, Critical Flicker Fusion Frequency; CSF, Contrast Sensitivity Function

1

Introduction

As is well known, images on a monitor are not continuous and the images are continuously refreshed. The rate at which the image is refreshed plays a crucial role in making the image appear continuous, even though the actual luminance of a point on the screen is intermittent. Because the visual system is sensitive to temporal changes, it integrates the responses with respect to time. Flicker arises when the display images are not repeated quickly enough. Flicker perception can be studied using grating stimuli whose luminance varies sinusoidally with time. Flicker perception depends upon stimulus size, luminance, retinal location, and temporal modulation amongst other factors. It has been found that chromaticity has little or no effect on CFF if the luminance is held constant [1].

2

Temporal Resolution Acuity and Critical Flicker Fusion Frequency

If we present to an observer alternating repetitive cycle of low and high luminance of a temporal square (or sine) wave stimulus, the light will appear to flicker (be intermittent) when the temporal frequency (in Hertz) is low. If we increase the temporal frequency, it will appear to be steady beyond a certain frequency. Psychophysically, we define the CFF (critical flicker fusion frequency) as the frequency at which the stimulus is seen flickering 50% of the time and as steady or fused 50% of the time. The CFF is a measure of the temporal resolving power of the visual system. This minimum interval of resolution is analogous to the minimum angle of resolution in spatial vision (See > Chaps. 2.1.5 and > 2.1.3). In this sense, temporal acuity is analogous to grating acuity in spatial vision. The neural basis of CFF is the modulation of firing rates of retinal neurons [2, 3]. When a light is flickering above the CFF, it will appear steady, and the time-averaged luminance of a flickering light determines its brightness above the CFF. This timeaveraged luminance is called the Talbot brightness. The Talbot brightness can be easily calculated using: Talbot Brightness ¼ Lmin þ ð½Lmax  Lmin   f Þ Here Lmax and Lmin refers to the maximum and minimum luminance of the grating, and f is the fraction of time that Lmax is present during the total period.

3

Some Properties of the CFF

The CFF depends upon a number of factors. In this section, some of the basic laws governing the behavior of CFF and flicker issues will be discussed.

2.1.4

Flicker Sensitivity

50

Critical frequency (Hertz)

0° 40 10° 30

15°

20

10

0 –4

–3

–1 0 –2 1 Retinal illumination - log L - photons

2

3

. Fig. 1 Critical flicker frequency as a function of retinal illuminance and retinal position (From [5])

3.1

CFF and Stimulus Luminance

The CFF increases linearly with the log of the stimulus luminance (> Fig. 1). This is known as the Ferry–Porter law and is expressed as: CFF ¼ klogL þ b Experimental results hold over a range of approximately 4 log units of luminance. k is the slope of the line and b is a constant; L is the stimulus luminance [4]. This law implies that the temporal resolution acuity improves as the flickering stimulus luminance increases. This law holds at many stimulus eccentricities.

3.2

CFF and Area of Stimulus

CFF increases linearly with the logarithm of the stimulus area (> Fig. 2). This is known as the Granit–Harper law and is written as: CFF ¼ klogA þ b Here, k and b are constants (different from those in the equation for Ferry–Porter law). A is the area of the flickering stimulus. The Granit–Harper law holds for approximately a 3 log unit range of luminance for stimuli presented in the fovea and up to about 10 eccentricity.

3.3

CFF and Retinal Eccentricity

The slope of the CFF luminance function changes being steeper outside the fovea, changing from a k of about 10 Hz/log unit at the fovea to about 20 Hz/log unit at about 10 eccentricity

103

2.1.4

Flicker Sensitivity

60 19° 50 Critical frequency (cps)

104

6° 2°

40

0.3° 30 20 10 0 –3

–2

–1 0 1 2 3 Log retinal illuminance (trolands)

4

5

6

. Fig. 2 Critical flicker frequency as a function of retinal illuminance and stimulus size (From [6])

(> Fig. 1). This implies that the peripheral retinal neurons respond with greater temporal acuity than central neurons. Tyler [7] speculates that this might be due to the larger peripheral cone diameters. The maximum CFF measured at approximately 35 is about 90 Hz. The CFF is higher in the periphery at high luminance levels. At low luminance levels, there is not much change in CFF with eccentricity.

3.4

CFF and Wavelength

In general, flickering lights with equal photopic luminance, but different wavelengths have equal CFFs. For intensities in the photopic range greater than 1 log Troland, CFFs are independent of the wavelength. At low stimulus intensities, CFFs are highest for shorter wavelengths [8].

3.5

CFF and Adaptation

Given the relative densities of rods and cones in the different retinal areas, their different retinal sensitivities and their neural interactions, the CFF will depend in a complex manner on the adaptation state of the eye. It has been found that the CFF is the highest when the eye is completely light adapted or when a uniform background of high luminance surrounds the flickering stimulus. As a result, a flickering stimulus at a fixed intensity can appear to be flickering when the observer is light adapted and steady when the observer is dark adapted [9, 10]. It is found that the adaptation effects on flicker are evident at frequencies above 15 Hz.

3.6

CFF and Ambient Illumination

At high levels of display illumination, ambient illumination has a small or negligible effect on perception of flicker perception [11]. The ANSI standards for room illumination

Flicker Sensitivity

2.1.4

are 200–500 lx. Illumination affects flicker perception on a CRT screen only to a small extent under these conditions.

3.7

CFF and Refresh Rates of Monitors

The refresh rate (regeneration rate) is one of the most important factors in the perception of flicker on a display. If the rate at which the screen is refreshed is greater than the CFF of the observer, the observer will not perceive flicker. Refresh rates need to be higher for displays with higher luminance and wider field of view in order to make the display flicker free. Refresh rates can be decreased by an increase in phosphor persistence.

3.8

CFF and Phosphor Persistence

It has been found that the medium-short phosphors P20 and P4 cause maximum flicker effect [12]. Many modern CRT phosphors contain a blend of two or more phosphors, and hence the ripple ratio is considered to be a better index to describe the persistence effect in a CRT. The modulation index of the fundamental frequency gives the ripple ratio [1]. Pearson [13] gives a list of ripple ratios for various refresh frequencies. In general, the smaller the ripple ratio, the less susceptible the monitor is to flicker. It should be emphasized that the ripple ratio alone does not give much information on perceived flicker; other factors such as luminance and angle of viewing also play a major role.

4

Brightness Enhancement Effects Because of the Temporal Properties of Vision

Two brightness enhancement effects that are known: the Brucke–Bartley Phenomenon and the Broca–Sulzer effect. If one views a flickering light and the flicker rate is varied without changing the timeaveraged luminance, the brightness of the flickering light appears to be enhanced at certain frequencies. This is the Brucke–Bartley phenomenon. The maximum brightness enhancement appears for flicker rates at around 5–20 Hz [14]. The second brightness enhancement effect, the Broca–Sulzer effect is one wherein the brightness of a suprathreshold flash depends upon its duration (when compared to the brightness of a steady light of the same luminance). Here, it is found that flash durations of a test light shorter and longer than 50–100 msec produce less brightness effect and the effect becomes stronger, and the peak brightness occurs at short durations with increasing luminance levels [14, 15].

5

The Temporal Contrast Sensitivity Function

A complete description of the temporal responsiveness of the human visual system is given by the temporal contrast sensitivity function (temporal CSF). Like its counterpart

105

2.1.4

200

0.01

100

0.02

50

0.05

20

0.1

10

0.2

2 9300 trolands 850 trolands 77 trolands 7.1 trolands 0.65 trolands 0.06 trolands

0.5

Amplification, m–1

Flicker Sensitivity

0.005

Threshold modulation ratio, m–1

106

2

1

1.0 2

5

10 Frequency

20

50

. Fig. 3 The Human contrast sensitivity function for several mean luminance levels (From [16])

the spatial CSF (See > Chap. 2.1.5), the temporal CSF has a band-pass shape, with a peak, a high temporal frequency cutoff (the CFF), and a low temporal frequency roll-off (> Fig. 1). In > Fig. 3, the Amplification scale on the right is nothing but the contrast sensitivity and the threshold modulation is the threshold contrast. The peak contrast occurs at an intermediate flicker frequency; the cutoff high temporal frequency is the limit of temporal acuity above which flicker cannot be resolved (even if contrast is 1) and a reduction in sensitivity at low temporal frequencies. The temporal peak frequency shifts from approximately 20 to 5 Hz as the mean luminance decreases. The cutoff high temporal frequency goes from 60 Hz to about 15 Hz as the luminance decreases. Because the visual system has a low CSF at low luminances, we can only see low temporal frequencies of medium to high contrast. Kelly [16], in his classic experiments on flicker, used a large flickering field with blurred edges to measure the temporal CSF functions (see [1]). If a sharp-edged field is used, the visibility of low frequency flicker is enhanced. The presence of spatial detail improves the visibility of flicker at low temporal frequencies.

Flicker Sensitivity

6

2.1.4

Temporal Summation

The visual system does not distinguish the temporal shape of light flashes shorter than a critical duration. This is inferred from Bloch’s law. The law essentially states that the visual system summates visual inputs over a brief time period and is given by: Lt¼C Here, L is the threshold luminance of the flash, t is the duration of the flash, and C is a constant. Bloch’s law has a spatial analogy, namely, the Ricco’s law in spatial vision (See > Chap. 2.1.5). Bloch’s law holds for flashes that are shorter than a critical duration tc for approximately 30–100 msec. During this time period, the visual system adds together the effects of the absorbed quanta regardless of the temporal pattern in which they arrive. Bloch’s law is a consequence of the temporal filtering properties of the visual system. For more detailed description, see [17].

7

Methods to Predict Flicker in Monitors

Various empirical methods have been proposed to predict whether a particular VDT will appear to flicker in a given environment [18]. Many of these methods are cumbersome and time consuming. Farrell [19, 20] has developed analytic methods for predicting whether a given VDT will flicker given screen phosphor persistence, refresh frequency, distance to VDT from the observer, etc.

8

Summary

The detection of temporal changes is important for the organism. In this chapter, we have discussed the factor most important in use with VDTs, namely, the perception of flicker. The crucial factor is the critical flicker fusion frequency (CFF). The CFF depends upon a number of parameters, and these were discussed, along with the temporal CSF (the temporal analog of the spatial CSF). The CFF is the temporal analog of the minimum angle of resolution.

References 1. De Lange HD (1958) Research into the dynamic nature of the human fovea-cortex systems with intermittent and modulated light: attenuation characteristics with white and colored lights. J Opt Soc Am 48:777–784 2. Tyler CW, Hamer RD (1990) Analysis of visual modulation sensitivity IV. Validity of the Ferry Porter law. J Opt Soc Am A 7:743–758 3. Lee BB, Martin PR, Valberg A (1989) Sensitivity of macaque retinal ganglion cells to chromatic and luminance flicker. J Physiol 414:223–243 4. Tyler CW, Hamer RD (1993) Eccentricity and the Ferry Porter law. J Opt Soc Am A 10:2084–2087

5. Hecht S, Verrijp CD (1933) The influence of intensity, color and retinal location on the fusion frequency of intermittent illumination. Proc Natl Acad Sci USA 19:522–535 6. Hecht S, Smith EL (1936) Intermittent stimulation by light. VI. Area and the relation between critical frequency and intensity. J Gen Physiol 19:979–991 7. Tyler CW (1985) Analysis of visual modulation sensitivity II. Peripheral retina and the role of photoreceptor dimensions. J Opt Soc Am A 2:393–398 8. Hecht S, Shlaer S (1935) Intermittent stimulation by light V. The relation between intensity and critical

107

108

2.1.4 9. 10.

11.

12. 13. 14.

Flicker Sensitivity

frequency for different parts of the spectrum. J Gen Physiol 19:321–337 Coletta NJ, Adams AJ (1984) Rod cone interaction in flicker detection. Vision Res 24:1333–1340 Goldberg SH, Frumkes TE, Nygaard RW (1983) Inhibitory influence of unstimulated rods in the human retina: evidence provided by examining cone flicker. Science 221:180–182 Isenseee SH, Bennett CA (1983) The perception of flicker and glare on computer CRT displays. Hum Factors 30:689 Turnage RE (1966) The perception of flicker in CRT displays. Inf Display 3:38–42 Pearson RA (1991) Predicting VDT flicker. Inf Display 7&8:22 Wu S, Burns SA, Reeves A, Elsner AE (1996) Flicker brightness enhancement and visual nonlinearity. Vision Res 36:1573–1583

15. Aiba TS, Stevens SS (1964) Relation of brightness to duration and luminance under light and dark adapted conditions. Vision Res 4:391–401 16. Kelly DH (1961) Visual responses to time dependent stimuli I. Amplitude sensitivity. J Opt Soc Am 51:422–429 17. Roufs JA (1972) Dyanmic properties of vision, I. Experimental relationships between flicker and flash thresholds. Vision Res 12:261–278 18. Rogowitz B (1986) A practical guide to flicker measurement. Behav Inform Technol 5:359–378 19. Farrell JE (1986) An analytic method for predicting perceived flicker. Behav Inform Technol 5:349–358 20. Farrell JE (1987) Predicting flicker thresholds in video display terminals. Proc Soc Inform Display 28:18

Further Reading Coletta N (2002) Temporal factors in vision. In: Norton TT, Corliss DA, Bailey JE (eds) The psychophysical measurement of visual function. Butterworth/ Heinemann, Woburn, Chapter 7 Cornsweet TN (1970) Visual perception. Academic, NT

Harwood K, Foley O (1987) Temporal resolution: an insight into the video display terminal problem. Hum Factors 29:447 Kelly DH (1972) Flicker. In: Jameson D, Hurvich L (eds) Visual psychophysics, vol VIII/4, Handbook of sensory physiology series. Springer, Heidelberg

2.1.5 Spatial Vision and Pattern Perception L. Srinivasa Varadharajan 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 2 Ricco’s Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 3 Contrast Sensitivity Function (CSF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 4 Factors Affecting CSF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 5 Contrast Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 6 Contrast Discrimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 7 Superimposed and Lateral Masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.1.5, # Springer-Verlag Berlin Heidelberg 2012

110

2.1.5

Spatial Vision and Pattern Perception

Abstract: This chapter deals with the visual system’s sensitivity to contrast and the various ways by which it is affected. Specifically, contrast detection and discrimination thresholds, and superimposed and lateral masking effects will be discussed. Effects of pattern adaptation and the consequent modification of the contrast sensitivity function will also be discussed. List of Abbreviations: CSF, Contrast Sensitivity Function; MTF, Modulation Transfer Function; FACT, Functional Acuity Contrast Test; cpd, Cycles Per Degree

1

Introduction

The term spatial vision encompasses all things related to seeing the space around us. This definition is so broad that it includes everything related to vision; however, it is usually restricted to visual perception of non-moving two-dimensional luminance patterns. With this restriction the terms spatial vision, pattern vision, and pattern perception become interchangeable. Human pattern vision measurements can be grouped under two broad categories, namely, threshold measurements and stimulus matching. In the first category, the value of a particular dimension of interest, such as the contrast, at which the stimulus is detected (or discriminated from another stimulus with a slightly different value) with a given probability is measured. This stimulus value is called the threshold; sensitivity is usually defined as the reciprocal of this threshold value. Most of our understanding of the visual system comes from some sort of threshold measurements. In the second category, the value mentally assigned to the particular dimension of interest is measured. When the stimulus is presented in isolation in a uniform field or background of some sort, we have a simple detection, discrimination, or a matching measurement. When other stimuli are presented, either superimposed on or spatially separated from the stimulus of interest, we have a masking measurement. Unless stated otherwise, all information given below pertains to foveal vision and when viewing is done with one eye only.

2

Ricco’s Law

The simplest stimulus to display, and detect, is a spot of light in a uniform background. The earlier chapter on light detection and sensitivity (> Chap. 2.1.2) dealt with the temporal aspects of this detection task. When the target is small, the luminance detection threshold is inversely proportional to the area of the target. This is known as Ricco’s law and it is usually written as: logðLt Þ ¼ K  logðAÞ where Lt denotes the threshold luminance for detecting a target of area A (given in deg2). The product of the threshold luminance, area, and the stimulus duration gives the threshold energy. Therefore Ricco’s law implies that for small targets, the visual system essentially integrates the energy over this entire target and sees it when the total energy just matches or exceeds the threshold value. The maximum area over which this integration happens is called the Ricco’s area. In other words, Ricco’s area is the area of the target beyond which the relationship given above does not hold true. Estimates of Ricco’s area are affected by various stimuli and observer parameters. In general, the Ricco’s area varies from 2 to 14 min2 in the fovea [1]. This area increases enormously as the target is presented in regions away from the fovea [2, 3], or when the background illumination is increased or the stimulus duration is decreased [4], and with age [5]. These studies

Spatial Vision and Pattern Perception

2.1.5

also showed that spatial summation of stimuli with areas smaller than the Ricco’s area happens at the retinal level. When the stimulus is larger than the Ricco’s area, the threshold luminance decreases as the square root of the area of the stimulus. This relationship is called the Piper’s law. A plot of the log(threshold) versus log(area) will have a slope of 1/2 for such stimuli. However, this relationship is found to breakdown quite quickly beyond the Ricco’s area [2, 4]. As one increases the size of the target beyond the Ricco’s area, the threshold luminance keeps decreasing albeit by smaller amounts. Such continued summation could be explained only through the cortical area of the brain.

3

Contrast Sensitivity Function (CSF)

Our ability to perceive luminance variation across space is denoted by the contrast sensitivity function (CSF). In general, sine wave gratings are used as the stimuli for obtaining the CSF. These sine wave gratings are shown on a background that has the same luminance as the average luminance of the grating. The threshold Michelson contrast at which a sine wave grating with a particular spatial frequency is detected is determined for various spatial frequencies. Contrast sensitivity at any given spatial frequency, as mentioned earlier, is the inverse of the threshold contrast. The dependence of this sensitivity on the spatial frequency is called the contrast sensitivity function (> Fig. 1). What the modulation transfer function (MTF) is to an optical system is what CSF is to our optical-neural system. The image formed on the retina is solely dictated by the MTF (or more correctly, the optical transfer function) of the optics that precedes the retina. The neural system then operates on this retinal image to form the final mental picture. Unlike MTF, CSF is a bandpass filter. Typically, normal adults have maximum sensitivity between 2 and 6 cycles per degree of visual angle. There is a steep reduction in sensitivity at lower spatial frequencies. On the higher spatial frequency side, sensitivity reduces gradually reaching a value of 1 (since the maximum contrast possible is 1, and sensitivity is the inverse of the threshold contrast, the minimum possible sensitivity is 1) at about 60 cycles per degree (CPD). This high spatial frequency cutoff determines the resolution limit of the visual system (see also > Chap. 2.1.3). This is called the grating acuity. It is also interesting to note that the photoreceptor size and arrangement in the fovea results in the Nyquist frequency which is close to the grating acuity. The threshold contrast for human subjects could go as low as 0.3%. On a linearized display driven by the usual 8-bit DAC, such a low contrast cannot be realized. Techniques like dithering, superimposition with a high uniform luminance, bit expansion circuits like Pelli and Zhang’s attenuator [6] or BITS++TM (Cambridge Research Systems Ltd, UK) are routinely used to overcome this problem. Clinically, CSF is commonly measured using the functional acuity contrast test (FACT) (Vision Science Research Corporation, CA, USA) chart – or the Pelli-Robson Chart (Precision Vision, IL, USA). The FACT chart contains five rows of sine wave gratings. Each row has gratings of fixed spatial frequency; contrast decreases from left to right. Each grating is oriented vertical or tilted 15 to the left or right from the vertical (> Fig. 2). The subject is usually seated 1 m from the chart and asked to say the orientation of each grating. For each row, the contrast at which the subject makes the first error is taken as the threshold at that spatial frequency. The Pelli-Robson chart [7] contains triplets of letters arranged as two triplets per line. The letters in each triplet are at the same contrast and the contrast decreases by a factor of 1/√2 from

111

2.1.5

Spatial Vision and Pattern Perception

1000

Contrast sensitivity (threshold contrast)−1

112

100

10

1

0

30 40 20 Spatial frequency (c/deg)

10

50

60

. Fig. 1 The CSF of two subjects (Adapted from [20])

1

2

3

4

A

B

C

D

E

. Fig. 2 The functional acuity contrast test (FACT) chart

5

6

7

8

Spatial Vision and Pattern Perception

2.1.5

. Fig. 3 Pelli-Robson chart (Adapted from [7])

one triplet to the next (> Fig. 3). The letters subtend an angle of 0.5 at 3 m, i.e., the PelliRobson chart measures the contrast sensitivity at 5 cpd. The lowest contrast at which the subject reads at least two letters correctly is taken as the threshold contrast. The Pelli-Robson chart has been shown to have good reproducibility and reliability [8] and is now increasingly used in clinics throughout the world.

4

Factors Affecting CSF

Various factors are known to affect the human CSF in predictable ways. In general, alterations in the optics of the eye, the retinal/neural structure, the stimulus, or the adaptation state of the observer would alter the CSF.

113

114

2.1.5

Spatial Vision and Pattern Perception

Optical Factors: Any degradation in the optics of the system would show its effect on the MTF and consequently on the CSF. Therefore, one would expect no or little effect on the low spatial frequencies and increasing amounts of reduction in sensitivity as the spatial frequency is increased. Therefore, the high spatial frequency cutoff would be moved toward lower spatial frequencies, and in some cases the peak sensitivity would also be shifted toward lower spatial frequencies. Some of the typical optical effects are defocus found in persons with uncorrected refractive errors or with cataract or when the pupil size is increased [9–11]. Retinal and Neural Factors: The photoreceptor arrangement in the retina dictates the high spatial frequency cutoff of the CSF. The receptor arrangement could be altered due to various reasons including age [12], disease [13], or the increased overall dimension of the eyeball as found in people with high refractive errors. The effect of this retinal factor is similar to the optical factor in that sensitivity is unaffected at low spatial frequencies and that losses in sensitivity are seen at the high spatial frequencies. The term ‘‘neural factors’’ include all cells upstream from the retina. Depending upon the type of affect, which is usually due to some disease, sensitivity losses could be seen at all spatial frequencies. Another of the retinal effects is the position of the retina where the stimulus is presented. The photoreceptor packing becomes less dense and the photoreceptors themselves become larger in size as we move away from the fovea. This results in the reduction in the sampling rate. The effect of this is a reduction in the high-frequency cutoff value and reduction in the sensitivity to high spatial frequencies; the low spatial frequencies are not much affected [12]. Stimulus Factors: There are many stimulus parameters that could affect the CSF but the mean luminance and the size of the target are the two most important parameters. At any given spatial frequency the threshold decreases as the square root of the average luminance (de Vries – Rose law) and then remains constant for high luminance values (> Fig. 4). Also, as the luminance is increased, the peak sensitivity moves toward higher spatial frequencies [14, 15]. Similarly, increase in the size of the target results in profound decrease in the

M in % 100 4 cpd 0.5 10

48 cpd 40 32 24 16 8

1

0.1

10−3

10−2

10−1

1

10

102

103

. Fig. 4 Percentage threshold contrast (M) as a function of retinal illumination B0 in Trolands (Adapted from [14])

104 B0 in td

2.1.5

Contrast sensitivity (arbitrary units)

Spatial Vision and Pattern Perception

100

10

1.0 1.0

10

100

Spatial frequency (cycles/degree, cpd)

. Fig. 5 Effect of adaptation to a sine wave grating of 7.1 cpd (for observer F.W.C.). The dots with the error bars show the contrast sensitivities at various spatial frequencies after adaptation to a 7.1 cpd grating indicated by the thick arrow (Adapted from [16])

threshold initially; this decrease becomes less pronounced with large sizes appearing to approach an asymptote. In general, the rapid reduction in thresholds appears to take place for sizes up to 5 cycles of the target. Adaptation: Adaptation is the process by which sensitivity to a particular stimulus parameter could be altered by prolonged exposure to a carefully chosen stimulus. Adaptation to a sine wave grating of a particular spatial frequency reduces the sensitivity around that frequency (> Fig. 5) with a bandwidth of about 1 octave [16].

5

Contrast Detection

The presence of such a notch effect is used to explain the CSF as an envelope of various band limited sensitivity functions centered at various spatial frequencies (> Fig. 6). Detection of a grating of a given spatial frequency is then explained using the concepts of linear filters. In general, an image is processed by such a set of filters, and when the output of a filter exceeds a certain value – a threshold response – the image or that component of the image with the specified spatial frequency content will be detected. In the spatial vision parlance, such filterresponse systems are called mechanisms or channels. The response of such a mechanism is usually modeled as the product of the spatial sensitivity profile of the mechanism and the luminance profile of the stimulus. This spatial sensitivity profile is called the receptive field of the mechanism. The visual system consists of vast number of such mechanisms. Besides the central spatial frequency, these mechanisms could be differentiated based on the center of their spatial profile (i.e., the point where the receptive field is centered), the orientation of the grating they are

115

2.1.5

Spatial Vision and Pattern Perception

CSF

Contrast sensitivity

116

Spatial frequency (cycles/degree)

. Fig. 6 The overall contrast sensitivity as an envelope of different band limited sensitivity functions centered at various spatial frequencies

responsive to, the phase of the grating, the size of stimulus, the direction of motion of the stimulus, etc. The values of these various parameters define the ‘‘tuning’’ of the mechanism. The presence of such mechanisms could be used to explain many of the factors that have been described above. A thorough treatment of these mechanisms can be found in the excellent book by Graham [17].

6

Contrast Discrimination

Threshold for detecting a change in the contrast in a stimulus is dependent on the initial contrast value. The stimulus over which the change in contrast is detected is called the pedestal. Typically the threshold decreases as a function of the pedestal contrast, reaches a minimum, and then increases monotonically. At large values of the pedestal contrast, the threshold increases almost linearly. It is important to note that the discrimination threshold is smaller than the detection threshold when the pedestal contrast is low, i.e., very faint stimuli enhance our discrimination ability.

7

Superimposed and Lateral Masks

The pedestal, as mentioned above, can be thought of as a superimposed mask; the task then becomes a detection task instead of a discrimination one. With this change of perspective, we can then manipulate the parameters of the pedestal and study the detection of a grating in the presence of superimposed masks. The parameters that could be manipulated are the spatial

2.1.5

Spatial Vision and Pattern Perception

−10 −15

KMF 0 deg 11.25 deg 22 deg

−20 −25 −30 −35

−40

a

Target Contrast Threshold (dB re 1)

Target Contrast Threshold (dB re 1)

frequency, orientation, phase, and size of the pedestal. In general, when the parameters are close to the values of the target and when the pedestal contrast is low, the detection task mimics the discrimination task mentioned above. When the values of the parameters are very different from that of the target and when the pedestal contrast is low, the detection of the target becomes more difficult and hence the thresholds are increased. The opposite is true when the pedestal contrast is high. In this case, when the values of the parameters of the pedestals become more and more different from that of the target, the target becomes easy to detect and hence the thresholds are reduced from the discrimination thresholds. > Figure 7 shows the effect of phase and orientation of the pedestal on the detection threshold of the target. Instead of a pedestal, if we have other stimuli that are spatially displaced from the target, we have a lateral masking measurement. Usually, to maintain symmetry, masks are presented either completely surrounding the target or in pairs on opposite sides of the target. Lateral

−∞

−50

−40

−30

−20

−10

0

−10

KMF 0 deg 45 deg × 90 deg

−15 −20

× ×

−25

× ×

−30

×

×

−35 −40 −∞

−50

b

Masker Contrast (dB re1)

×

−40

−30

−20

−10

0

Masker Contrast (dB re1)

Target Contrast Threshold (dB re 1)

−10 Phase −15 −20 −25 −30 −35 −40 −45

c

AHS

0 180 90

−∞

−50

−40

−30

−20

−10

0

Masker Contrast (dB re1)

. Fig. 7 Effect of the changing the pedestal parameters on the detection thresholds of the target. All contrasts are given in dB re 1, i.e., in units of 20log(contrast). In all the three graphs, the graph corresponding to the value ‘‘0’’ corresponds to discrimination threshold; (a) and (b) show the effect of changing the orientation of the pedestal with respect to the target [19]; (c) shows the effect of the pedestal phase with respect to the target on target detection (Adapted from [21])

117

118

2.1.5

Spatial Vision and Pattern Perception

masks produce similar effects on the detection of a target as superimposed masks. The question of greatest importance is the effect the distance between the mask and the target has on the detection threshold because this would give us the information about the interactions of the underlying mechanism. In general, low contrast flankers that are close to the target seem to reduce detection thresholds and the effect of this integration extends to quite a separation, sometimes up to eight times the target size [18]. Such measurements clearly indicate that mechanisms with receptive fields that are widely separated interact. As mentioned earlier, each mechanism’s response would be a product of its spatial sensitivity profile and the luminance distribution within its receptive field. These responses would then be combined to produce various interesting effects [19]. This lateral interaction of receptive fields is used to explain a lot of interesting phenomena including Hermann’s grid, Mach band, grating induction, etc. Letter charts using the log MAR progression are designed to provide relatively equal amounts of lateral interactions for any letter in the chart except the ones in the extremities.

8

Summary

Spatial vision refers to the process in detecting and discriminating simple sinusoidal patterns. Our spatial vision ability is described by our contrast sensitivity function. This sensitivity is affected by various factors such as the optical, retinal, neural, and adaptation state of the observer. The detection of these simple patterns is described invoking a set of linear filters that have various tuning properties. These mechanisms interact to produce a variety of effects.

References 1. Davila KD, Geisler WS (1991) The relative contributions of pre-neural and neural factors to areal summation in the fovea. Vis Res 31:1369–1380 2. Westheimer G (1965) Spatial interaction in the human retina. J Physiol 181:881–894 3. Vassilev A, Ivanov I, Zlatkova MB, Anderson RS (2005) Human S-cone vision: relationship between perceptive field and ganglion cell dendritic field. J Vis 5:823–833 4. Barlow HB (1958) Temporal and spatial summation in human vision at different background intensities. J Physiol 141:337–350 5. Schefrin BE, Bieber ML, McLean R, Werner JS (1998) The area of complete scotopic spatial summation enlarges with age. J Opt Soc Am A 15:340–348 6. Pelli DG, Zhang L (1991) Accurate control of contrast on microcomputer displays. Vis Res 31: 1337–1350 7. Pelli DG, Robson JG, Wilkins AJ (1988) The design of a new letter chart for measuring contrast sensitivity. Clin Vis Sci 2:187–199 8. Elliott DB, Sanderson K, Conkey A (1990) The reliability of the Pelli-Robson contrast sensitivity chart. Ophthalmic Physiol Opt 10:21–24

9. Atchison DA, Woods RL, Bradley A (1998) Predicting the effects of optical defocus on human contrast sensitivity. J Opt Soc Am A 15:2536–2544 10. Hess R, Woo G (1978) Vision through cataracts. Invest Ophthalmol Vis Sci 17:428–435 11. Strang NC, Atchison DA, Woods RL (1999) Effects of defocus and pupil size on human contrast sensitivity. Ophthalmic Physiol Opt 19:415–426 12. Crassini B, Brown B, Bowman K (1988) Age-related changes in contrast sensitivity in central and peripheral retina. Perception 17:315–332 13. Bour LJ, Apkarian P (1996) Selective broad-band spatial frequency loss in contrast sensitivity functions. Invest Ophthalmol Vis Sci 37:2475–2482 14. Van Nes FL, Bouman M (1967) Spatial modulation transfer in the human eye. J Opt Soc Am 57:401–406 15. De Valois RL, Morgan H, Snodderly DM (1974) Psychophysical studies of monkey vision-III. Spatial luminance contrast sensitivity tests of macaque and human observers. Vis Res 14:75–81 16. Blakemore C, Campbell FW (1969) On the existence of neurones in the human visual system selectively sensitive to the orientation and size of retinal images. J Physiol 203:237–260

Spatial Vision and Pattern Perception 17. Graham N (1992) Visual pattern analyzers. Oxford University Press, New York 18. Adini Y, Sagi D, Tsodykes M (1997) Excitatory – inhibitory networks in the visual cortex: psychophysical evidence. Proc Natl Acad Sci USA 94: 10246–10231 19. Foley JM (1994) Human luminance pattern-vision mechanisms: masking experiments require a new model. J Opt Soc Am A 11:1710–1719

2.1.5

20. Campbell FW, Green DG (1965) Optical and retinal factors affecting visual resolution. J Physiol 181: 576–593 21. Foley JM, Chen C-C (1999) Patten detection in the presence of maskers that differ in spatial phase and temporal offset: threshold measurements and a model. Vis Res 39:3855–3872

Further Reading De Valois R, De Valois K (1988) Spatial vision. Oxford Science Publication, New York

Shapley R, Man-Kit Lam D (1992) Contrast sensitivity. MIT Press, Cambridge, MA

119

2.1.6 Binocular Vision and Depth Perception Robert Earl Patterson 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 2 Horopter and Binocular Disparity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 3 Binocular Rivalry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 4 Spatio-Temporal Frequency Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 5 Distance Scaling of Disparity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 6 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 7 Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.1.6, # Springer-Verlag Berlin Heidelberg 2012

122

2.1.6

Binocular Vision and Depth Perception

Abstract: This chapter covers several topics that are important for a basic understanding of binocular vision and depth perception. These topics include the horopter, binocular disparity, binocular rivalry, spatio-temporal frequency effects, and distance scaling of disparity.

1

Introduction

Stereopsis refers to the perception of depth based on binocular disparity, a cue that derives from the existence of horizontally separated eyes. Wheatstone [1] was the first to report that disparity is the cue for stereopsis, which he called ‘‘seeing in solid.’’ Since his original observations, the phenomenon of binocular depth perception has attracted much interest from the basic and applied scientific and engineering communities. For a recent review of stereopsis, see Howard [2] and Howard and Rogers [3]. For a review of stereopsis as applied to stereo displays, see Patterson and Martin [4] and Patterson [5]. This chapter reviews the basics of human stereopsis, which includes the horopter, binocular disparity, and binocular rivalry. It also covers related topics such as spatio-temporal frequency effects and disparity scaling.

2

Horopter and Binocular Disparity

The basics of binocular vision begin with the concept called corresponding retinal areas, or corresponding retinal points. Corresponding retinal areas in the two eyes are stimulated when fixation is directed toward an object in the visual field. The images from the fixated object stimulate the two foveae, which are considered to be corresponding. When a pair of images stimulates corresponding retinal areas, those images are said to possess zero binocular disparity. There is an imaginary arc passing through the fixation point called the ‘‘horopter.’’ The horopter is also an important concept in binocular vision because all points or objects along its length define the locations in space which will project images that also strike corresponding retinal areas in the two eyes and therefore, possess zero disparity (see > Fig. 1). One way to consider corresponding retinal areas is that they give rise to a common visual direction. Thus, an object that is positioned at the horopter will give rise to images that will stimulate each eye in such a way that the images will appear to come from the same direction out in the visual field. Because the horopter defines the locations in space that project zero binocular disparity, the horopter is considered a reference or baseline depth plane from which the depth of other objects are judged, that is the depth of objects that lie in front of or behind the horopter. The region of depth surrounding the horopter is subdivided into a front region and a back region, which correspond to the direction of the disparity information projected by objects in those regions. An object located in a depth plane in front of the horopter (and therefore in front of fixation) will project images with ‘‘crossed’’ disparity (> Fig. 1), whereas an object located in a depth plane behind the horopter (fixation) will project images with ‘‘uncrossed disparity.’’ Moreover, there is a zone of space surrounding the horopter that defines the region of binocular fusion and single vision called Panum’s fusional area (> Fig. 1). Objects situated within Panum’s area are seen as fused, and depth perception is generally reliable within this region. Objects situated outside Panum’s area project images that cannot be fused (i.e., the images are diplopic or seen as double) and depth perception becomes unreliable.

2.1.6

Binocular Vision and Depth Perception

F X

Horopter

Panum ’s Fusion al area

Y

Crossed disparity region

Uncrossed disparity region

Y X FX LE

Y

F

RE

. Fig. 1 Drawing depicting the basics of stereoscopic viewing. The drawing shows two circles which represent a top-down view of the two eyes, fixation point F, the horopter passing through the fixation point, Panum’s fusional area, crossed and uncrossed disparity regions, and object X and object Y. When point F is fixated, the images from F stimulate corresponding retinal points (foveae) in the two eyes and are fused. Object X is positioned in front of the horopter and thus carries a crossed disparity, but the images from X which stimulate non-corresponding (disparate) retinal points in the two eyes are fused because X is located within Panum’s fusional area. Object Y is positioned farther in front of the horopter and also carries a crossed disparity, but the images from Y which also stimulate disparate retinal points in the two eyes are seen as diplopic (double) because Y is located outside Panum’s fusional area. This figure was reproduced from Patterson [36] with permissions by The Society for Information Display

It has been established that these two directions of disparity, crossed versus uncrossed disparity, are processed differently by different sets of cortical neurons in the visual brain [6–8]. Specifically, there are cortical neurons that are excited by crossed disparity and inhibited by uncrossed disparity, and different neurons excited by uncrossed disparities and inhibited by crossed disparities. There also are neurons which are excited or inhibited by zero or near-zero disparities. The conscious perception of depth is thought to be derived from the pooled signals from these various sets of neurons, which form a distributed-channel network for stereoscopic processing: depth in front of the horopter is perceived if neural responding is greatest in the neurons activated by crossed disparity, whereas depth behind the horopter is perceived if neural responding is greatest in the neurons activated by uncrossed disparity; depth in the plane of the horopter is perceived if neural responding is greatest in the neurons activated by zero disparity. An object that is positioned in front of the horopter and therefore projects crossed disparity to the visual system may end up projecting uncrossed disparity, or vice versa, when an observer executes vergence eye movements by shifting fixation to different objects in the visual field. In this case, the relative disparity between stationary objects in the visual field remains constant but their absolute disparity, which is the relevant cue for stereopsis [9], will change.

123

124

2.1.6

Binocular Vision and Depth Perception

Note that there are concepts associated with accommodation, such as the depth of field of the human eye, which are related to stereoscopic depth perception. These concepts are discussed in > Chaps. 9.1.2 and > 10.4.2.

3

Binocular Rivalry

When an object is positioned outside Panum’s fusional area and its two monocular images cannot be fused (i.e., the images are diplopic), one of the two images may be perceptually suppressed and inhibited via a process called ‘‘binocular rivalry.’’ Binocular rivalry [10–13] refers to a situation in which one eye inhibits the visual processing of the partner eye, causing the visibility of the two monocular images to fluctuate over time. When viewing in the real world, the binocular rivalry may go unnoticed. However, when viewing a stereo display with a very large disparity that cannot be fused, the binocular rivalry may be noticed. The neural inhibition provoked by binocular rivalry occurs at many levels of the visual system [13] which make visual processing unstable and unpredictable; the inhibition is known, for example, to impair the ability of observers to visually guide and direct attention to targets in the visual field [14].

4

Spatio-Temporal Frequency Effects

Panum’s fusional area, the zone of binocular fusion and single vision shown in > Fig. 1, is not static but rather it varies with the luminance spatio-temporal frequency content of the images impinging upon the retinae [15–18]. Here, the term ‘‘spatio-temporal frequency’’ refers to the rate of modulation of luminance information over space and time in an image. High spatial frequency refers to fine luminance details in an image, and low spatial frequency refers to coarse details. Similarly, high temporal frequency refers to a high rate of temporal modulation or to sudden changes in luminance information in time (e.g., brief stimulus exposures), and low temporal frequency refers to a low rate of temporal modulation or gradual changes. Panum’s fusional area varies with the luminance spatio-temporal frequency content of imagery because the spatio-temporal frequency differentially engages various types of visual channels, and many of those channels feed into the neural substrate for disparity processing in the brain. Specifically, high spatial frequencies engage visual channels that process relatively small disparities and which subserve fine depth discrimination (i.e., fine stereoacuity) within a narrow Panum’s area. For example, stereoacuity is about 20 arcsec in the spatial frequency range of about 2–20 cycles per degree of visual angle. The maximum disparity that can be reliably discriminated is a little over 40 arcmin within this spatial frequency range (these values apply equally to both the crossed and uncrossed directions from the horopter) [15]. Conversely, low spatial frequencies engage visual channels that process relatively large disparities and therefore support a relatively large Panum’s area but which lack the capacity for fine depth discrimination [15]. Below a spatial frequency of about 2 cycles per degree, both stereoacuity and the maximum disparity that can be discriminated increase with decreasing spatial frequency such that at a spatial frequency of about 0.1 cycles per degree, stereoacuity is about 5 arcmin and the maximum discriminable disparity is about 4 arc degrees (again, these values apply equally to both the crossed and uncrossed directions from the horopter). However, low spatial frequencies combined with moderate to high temporal frequencies can support fine stereoacuity [16, 17]. In general, fine stereoacuity occurs near the horopter and fixation point [18], which translates into stimulation near the fovea.

Binocular Vision and Depth Perception

2.1.6

F

Y

Short viewing distance

Y Y F RE

F

a

LE

F

Y Long viewing distance

Y F

b

LE

Y F RE

. Fig. 2 Drawing depicting the change in disparity magnitude with variation in viewing distance. Diagram in the top panel of the figure shows a top down view of two eyes fixating point F at a short distance, whereas the diagram in the bottom panel depicts two eyes fixating the same point F at a long distance. In both diagrams, the depth between F and Y is the same magnitude. Increasing the viewing distance causes disparity magnitude to decrease. For depth to be perceived reliably, viewing distance as well as disparity must be registered by the visual system. This figure was reproduced from Patterson [36] with permissions by The Society for Information Display

5

Distance Scaling of Disparity

Stereoscopic processing does not simply require binocular disparity information in order to generate a perception of depth. Rather, stereoscopic processing is a complicated phenomenon

125

126

2.1.6

Binocular Vision and Depth Perception

that entails synergistic processing of a number of visual cues in addition to disparity information, namely, cues as to the viewing distance established by an observer. Thus, a critical distinction needs to be made between binocular disparity, which is relative depth information (i.e., relative to the horopter), versus egocentric viewing distance information which refers to the distance between an observer and the point of fixation. Stereoscopic vision requires a synergistic processing of binocular disparity coupled with viewing distance information because the magnitude of binocular disparity that is projected to the two eyes will vary as viewing distances change. Generally, the magnitude of binocular disparity varies approximately inversely with the square of the viewing distance in the real world (see > Fig. 2). If viewing distance to a constant interval of depth between two objects in the visual field is halved, then disparity will be approximately four times its initial value, and if viewing distance is doubled, disparity will be approximately one-fourth its original value. This relation can be seen in the expression for computing the magnitude of disparity for real-world viewing [19]: with a relatively large viewing distance and symmetrical convergence, disparity magnitude is computed as: r (in radians) = (I ∗ d)/D2, where r is disparity, I is interpupillary distance, d is the depth interval, and D is viewing distance. Note that the relationship between disparity and depth is different for stereo displays [20], a topic covered in > Chap. 10.4.2. Thus, a given amount of binocular disparity will be ambiguous for determining how much depth exists out in the visual field unless viewing distance is taken into account. It is believed that the synergistic processing involves having the visual system recalibrate the relationship between disparity and depth for different viewing distances, a process called ‘‘distance scaling of disparity,’’ or disparity scaling for short [4, 5, 21–24]. A number of viewing distance cues have been suggested as playing a role in the disparity scaling operation, such as accommodation and vergence [25–28], and vertical disparity [29–31], but these cues would do so only for short viewing distances. There is also limited evidence that field cues, such as linear or texture perspective, may provide distance information for disparity scaling [32].

6

Summary and Conclusions

The ability to perceive depth with binocular vision arises from the existence of horizontally separated eyes. This horizontal separation between the eyes produces lateral shifts in the location of corresponding monocular images, called binocular disparity, which the visual system processes as relative depth information (i.e., relative to the horopter). This visual translation of disparity into relative depth entails a synergistic operation that recalibrates the relationship between disparity and depth for different viewing distances. The existence of this recalibration process, called disparity scaling, means that the reliability of stereoscopic depth perception is vulnerable to those factors that affect the visual registration of viewing distance.

7

Directions for Future Research

Future research should investigate further the factors that affect the disparity scaling process, especially the relative strength of the various distance cues thought to play a role in such scaling. The visual combination of various cues in the perception of depth is a recognized issue in the basic vision literature [33–35]. Nonetheless, more could be known about the process, particularly how it affects depth perception in synthetic stereo displays.

Binocular Vision and Depth Perception

2.1.6

References 1. Wheatstone C (1838) Contributions to the physiology of vision: 1. On some remarkable and hitherto unobserved phenomena of binocular vision. Philos Trans R Soc Lond 128:371 2. Howard I (2002) Seeing in depth, vol 1, Basic mechanisms. Porteous, New York 3. Howard I, Rogers B (2002) Seeing in depth, vol 2, Depth perception. Porteous, New York 4. Patterson R, Martin W (1992) Human stereopsis. Hum Factors 34:669–692 5. Patterson R (2009) Human factors of stereo displays: an update. J Soc Inf Disp 17:987–996 6. Cumming B, DeAngelis G (2001) The physiology of stereopsis. Annu Rev Neurosci 24:203–238 7. Poggio G (1995) Mechanisms of stereopsis in monkey visual cortex. Cereb Cortex 5:193–204 8. Poggio G, Motter B, Squatrito S, Trotter Y (1985) Responses of neurons in visual cortex (V1 and V2) of the alert macaque to dynamic random dot stereograms. Vis Res 25:397–406 9. Cumming B, Parker A (1999) Binocular neurons in V1 of awake monkeys are selective for absolute, not relative, disparity. J Neurosci 19:5602–5618 10. Blake R (1989) A neural theory of binocular rivalry. Psychol Rev 96:145–167 11. Breese B (1899) On inhibition. Psychol Monogr 3:1–65 12. Levelt W (1965) On binocular rivalry. Institute for Perception RVO-TNO, Soesterberg 13. Blake R (2001) A primer on binocular rivalry, including current controversies. Brain Mind 2:5–38 14. Schall J, Nawrot M, Blake R, Yu K (1993) Visual guided attention is neutralized when informative cues are visible but unperceived. Vis Res 33:2057–2064 15. Schor C, Wood I (1983) Disparity range for local stereopsis as a function of luminance spatial frequency. Vis Res 23:1649 16. Schor C, Wood I, Ogawa J (1984) Spatial tuning of static and dynamic local stereopsis. Vis Res 24:573–578 17. Patterson R (1990) Spatio-temporal properties of stereoacuity. Optom Vis Sci 67:123–125 18. Blakemore C (1970) The range and scope of binocular depth discrimination in man. J Physiol 211:599–622 19. Cormack R, Fox R (1985) The computation of retinal disparity. Percept Psychophys 37:176 20. Cormack R, Fox R (1985) The computation of disparity and depth in stereograms. Percept Psychophys 38:375

21. Ono H, Comerford T (1977) Stereoscopic depth constancy. In: Epstein W (ed) Stability and constancy in visual perception: mechanisms and processes. Wiley, New York 22. Wallach H, Zuckerman C (1963) The constancy of stereoscopic depth. Am J Psychol 76:404 23. Ritter M (1977) Effect of disparity and viewing distance on perceived depth. Percept Psychophys 22:400–407 24. Patterson R, Moe L, Hewitt T (1992) Factors that affect depth perception in stereoscopic displays. Hum Factors 34:655–667 25. Foley J (1980) Binocular distance perception. Psychol Rev 87:411–434 26. Owens D, Leibowitz H (1976) Oculomotor adjustments in darkness and the specific distance tendency. Percept Psychophys 20:2–9 27. Owens D, Leibowitz H (1980) Accommodation, convergence, and distance perception in low illumination. Am J Optom Physiol Opt 57:540–550 28. von Hofsten C (1976) The role of convergence in visual space perception. Vis Res 16:193–198 29. Gillam B, Lawergren B (1983) The induced effect, vertical disparity, and stereoscpic theory. Percept Psychophys 34:121–130 30. Gillam B, Chambers D, Lawergren B (1988) The role of vertical disparity in the scaling of stereoscopic depth perception: an empirical and theoretical study. Percept Psychophys 44:473–483 31. Rogers B, Bradshaw M (1993) Vertical disparities, differential perspective and binocular stereopsis. Nature 361:253–255 32. Cormak R (1984) Stereoscopic depth perception at far viewing distances. Percept Psychophys 35:423 33. Hillis J, Watt S, Landy M, Banks M (2004) Slant from texture and disparity cues: optimal cue combination. J Vis 4:967–992 34. Jacobs R (1999) Optimal integration of texture and motion cues to depth. Vis Res 39:3621–3629 35. Knill D, Saunders J (2003) Do humans optimally integrate stereo and texture information for judgments of surface slant? Vis Res 43:2539–2558 36. Patterson R (2009) Human factors of stereoscopic displays. In: Society for information display international symposium digest of technical papers, pp 805–807

127

Part 2.2

Color Science

2.2.1 Color Communication Stephen Westland 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 2 Color and Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 3 Perceptual Color Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 4 The Munsell System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 5 Other Color-Order Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 6 Numerical Color Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.2.1, # Springer-Verlag Berlin Heidelberg 2012

132

2.2.1

Color Communication

Abstract: The use of language to describe color is natural and intuitive and there seems to be some evidence that different languages refer to color in a consistent way. It is clear, nonetheless, that the reliance of language to communicate color is limited not least by the number of color names but also by the lack of precision that language affords. As an alternative to natural language, color-order systems have found widespread use; these usually consist of physical books of patches or swatches each of which carries a notation. Three representatives of such systems are referred to in this chapter: the Munsell system, the Pantone system, and the NCS system. Although physical-color order systems can be effective they are also limited by, for example, consisting of relatively few physical samples. The last couple of decades have seen increased use of numerical color communication and specification based upon the CIE system. List of Abbreviations: CIE, Commission Internationale de l’Eclairage; CMM, Color Matching Module; ICC, International Color Consortium; NCS, Natural Color System; PMS, Pantone Matching System

1

Introduction

A desire to communicate color is natural in the arts and in our everyday lives. However, in the last 50 years color communication has become essential as part of the design and specification of products in industrialized societies. The use of language to describe color is natural and intuitive and there seems to be some evidence that different languages refer to color in a consistent way. It is clear, nonetheless, that the reliance of language to communicate color is limited not least by the number of color names but also by the lack of precision that language affords. There is no clear answer to the question of how many different colors we can distinguish between; however, estimates range from three million to about ten million. It is clear that even if we use color names in a consistent and reliable way then there are limitations in the use of natural language for color communication. As an alternative to natural language, color-order systems have found widespread use; these usually consist of physical books of patches or swatches each of which carries a notation. Color-order systems are, however, themselves limited as tools for color communication. Numerical color communication is increasingly becoming the preferred method for color communication by professionals working in the field.

2

Color and Language

Previous research has proposed that in the English language black, white, red, green, yellow, blue, purple, orange, pink and gray are basic color terms or universal categories. The classic study in this area was conducted by Berlin and Kay who proposed that not only are there 11 basic color terms but that cultures evolve the use of these terms in way that is predictable and almost universal [1]. As languages evolve, they acquire new basic color terms in a strict chronological sequence; if a basic color term is found in a language, then the colors of all earlier stages should also be present. The sequence is as follows: Stage I: Dark-cool and light-warm (this covers a larger set of colors than English ‘‘black’’ and ‘‘white’’) Stage II: Red Stage III: Either green or yellow

Color Communication

2.2.1

Stage IV: Both green and yellow Stage V: Blue Stage VI: Brown Stage VII: Purple, pink, orange, or gray The Berlin and Kay study contested the Sapir-Whorf hypothesis; the idea that the varying cultural concepts and categories inherent in different languages affect the cognitive classification of the experienced world in such a way that speakers of different languages think and behave differently because of it. The study achieved widespread influence but has recently been criticized [2] and the notion of universality that has endured for the last 30 or so years is under attack from cultural relativists. Critics note that the language sample from which Berlin and Kay collected data was strongly biased in favor of written languages from industrialized societies. However, the Berlin and Kay study has not been refuted entirely and a current study underway at U.C. Berkeley and the University of Chicago is statistically testing comprehensive color-naming data, collected from 110 unwritten languages from nonindustrialized societies, through the World Color Survey [3]. More recently, there is evidence that possession of linguistic categories facilitates recognition and influences perceptual judgments [4]. There is therefore doubt about the universality of color perceptions and, more crucially, color naming. A further limitation in the use of natural language to communicate color is that the number of colors that we can differentiate between is extremely large. The number of colors that are discernable is difficult to quantify but is certainly measured in the millions. Judd and Wyszecki [5] estimated that there were ten million discernable colors but a more recent estimate [6] was more conservative and placed the number somewhere between two and three million.

3

Perceptual Color Attributes

Since the number of colors that can be observed is extremely large it is natural to consider systematic ways of organizing and describing colors that could lead to a more efficient and meaningful representation. One of the first people to arrange colors in a circle appears to have been Aron Sigfrid Forsius (1550–1637) although his work was not discovered until the twentieth century [7]. Forsius’s color circle included white and black. The first hue circle is credited to Newton who considered the spectral hues and presented them in a circular diagram along with the non-spectral hues (which Newton realized were required to complete the circle). Hue is that attribute of a visual sensation according to which an area appears to be similar to one of the perceived colors: red, yellow, green, and blue, or to a combination of two of them [8]. Although hue is perhaps the most distinguishable attribute of a color, it is now established that color vision is based on three perceptual attributes: brightness, colorfulness, and hue (or correlates of three such attributes). Brightness is that attribute of visual sensation according to which an area appears to emit more or less light whereas colorfulness is that attribute according to which the perceived color of an area appears to be more or less chromatic [8]. Relative color terms are frequently used. For example, lightness is a relative brightness (normalized for changes in illumination and viewing conditions) and chroma, saturation, and purity are all distinct from each other but describe various aspects of relative colorfulness. In this chapter a full explanation of these terms is not given but readers are directed to Fairchild [8] for authoritative definitions. In this chapter, the terms lightness, chroma and hue will be

133

134

2.2.1

Color Communication

. Fig. 1 The three attributes of color vision: lightness (upper), chroma (middle), and hue (lower)

used in a general way to describe the three perceptual aspects of color perception. > Figure 1 illustrates the three attributes. The significance of these perceptual attributes is that it becomes possible to describe a color using three attributes in a semi-systematic way; thus, we might describe a light, saturated orange or a dark, desaturated blue. It also becomes possible to describe differences between two similar colors in a meaningful way; thus, we may say that one color is darker, stronger and bluer, for example, than another. This method of color communication avoids the use of arbitrary color names but is still limited as a method for precise and accurate color communication. However, a systematic understanding of color ontology led to the development of sophisticated tools for color communication such as the Munsell system.

4

The Munsell System

The idea of using a three-dimensional color solid to represent all colors was developed during the eighteenth and nineteenth centuries. For example, in 1810 Philipp Otto Runge developed a system based upon a sphere. However, although these systems became progressively more sophisticated, before Munsell none was based on any rigorous scientific understanding of human color vision. Prior to Munsell’s contribution, the relationship between hue, lightness, and chroma was not well understood. Albert Munsell, an artist and educator, wanted to create a rational way to describe color that would use an alphanumeric notation instead of color names which he could use to teach his students about color. In 1905 he published A Color Notation, a description of his system with the first atlas being produced in 1907 [9]. In 1918, shortly before Munsell’s death, the Munsell Color Company was formed and the first Munsell Book of Color was published in 1929. An extensive series of experiments carried out by the Optical Society of America in the 1940s resulted in an improvement to the system known as the Munsell Renotations [10]. Munsell was the first to separate lightness, chroma and hue into perceptually uniform and independent dimensions. The system consists of three independent dimensions which can be represented cylindrically as an irregular color solid: hue, measured by degrees around horizontal circles; chroma, measured radially outward from the neutral (gray) vertical axis; and value, measured vertically from 0 (black) to 10 (white). Munsell’s value scale can be interpreted as a lightness scale. Munsell determined the spacing of colors along these dimensions by taking measurements of human visual responses.

Color Communication

Value

2.2.1

Hue

Chroma

. Fig. 2 The Munsell system arranged color in three dimensions. This idea remains in modern numerical color-communication tools such as CIELAB

Munsell based his hue notation on five principal hues: red, yellow, green, blue, and purple, along with five intermediate hues halfway between adjacent principal hues. Each of these ten steps is then broken into ten sub-steps, so that 100 hues are given integer values. Value, or lightness, varies vertically from black (value 0) at the bottom, to white (value 10) at the top. Neutral grays lie along the vertical axis between black and white. Chroma, measured radially from the center of each slice, represents the ‘‘purity’’ of a color (see > Fig. 2). Note that there is no intrinsic upper limit to chroma. Different areas of the color space have different maximal chroma coordinates. For instance light yellow colors have considerably more potential chroma than light purples. A color is fully specified in the Munsell system by listing the three numbers for hue, value, and chroma. For instance, a fairly saturated red of medium lightness would be R 5/10 with R indicating the hue, 5/10 indicating lightness and chroma respectively. The Munsell system is an example of a physical color-order system. By reference to the physical samples of color it is possible to provide reasonably accurate and precise color communication. The merits in the system are evident in the observation that the system is still in use today more than 100 years after it was developed. The current Munsell atlas is published in two parts, glossy (1,488 samples) and matt (1,277 samples) [11]. However, as a method of communication the system is not without limitations. One problem of relying upon physical samples is that over time they may fade or become soiled. Perhaps more critically the system is limited to a couple of thousand color samples and invariably the color that one wishes to communicate lies somewhere between the colors of two adjacent samples in the system.

5

Other Color-Order Systems

The Munsell system was important for the influence that it had upon ideas about color spaces and representations [9]. Although the Munsell system is still in use today, other color-order systems have been developed and have found widespread use. The Pantone system is particularly popular in the graphic arts and printing industries. Pantone Guides consist of a large number of cardboard sheets, printed on one side with a series of related color swatches and then bound into a small flipbook. For instance, a particular sheet might contain a number of

135

136

2.2.1

Color Communication

yellows varying in luminance from light to dark. There are several Pantone systems; the Pantone solid color system, for example, consists of over 1,100 unique, numbered colors (e.g., Pantone 198). The Pantone samples, unlike those of the Munsell system, are not organized in a way that is consistent with the human visual system nor spaced uniformly with respect to perception. However, Pantone systems have found widespread use because their use can assist printers to match colors (Pantone samples typically include information about which inks can be used to match that color). The Pantone Matching System (PMS), for example, provides information on how a printer should obtain the solid colors and there are also guides that provide the closest process color (CMYK) equivalent. Another color-order system that has been successful is the Natural Color System (NCS) [12]. The NCS system is perhaps more like the Munsell system than the Pantone system. The color samples are logically arranged in a three-dimensional space. However, there are some important differences between Munsell and NCS. For example, the Munsell system is based upon five primary hues whereas the NCS system is based upon four hues. In fact, the NCS system is based upon three pairs of elementary color percepts: white-black, red-green, and yellow-blue. NCS colors are defined by the amount of blackness, chromaticness, and a percentage value between two hues, red, yellow, green or blue. For example, the NCS color NCS 0580-Y10R refers to a color with 5% darkness, 80% saturation, and whose hue is 90% yellow and 10% red.

6

Numerical Color Communication

That color-order systems such as Pantone and Munsell contain relatively few samples is problematic for modern color communication. Physical color-order systems are also subject to the limitation that even if they are manufactured to a very close tolerance the samples inevitably fade over time or change in color because they become soiled. The use of color-order systems also requires so-called normal color vision, whereas approximately 5% of the population is estimated to suffer from some type of color-vision defect. The Commission Internationale de l’Eclairage (CIE) developed a system for the specification of color stimuli that was recommended for widespread use in 1931 [13]. This system allows measurements of spectral reflectance factors of spectral radiance to be converted to CIE XYZ tristimulus values or, in turn, to other (more uniform) color spaces such as CIE (1976) L∗a∗b∗ or CIELAB. The CIE system is described in more detail in > Chaps. 2.2.2 and > 2.2.5. The second half of the twentieth century saw color measurement using the CIE system become ubiquitous in many industries including textiles, paints, and plastics. The advent of affordable color-imaging devices in the last couple of decades, however, has led to an explosion in digital color communication. Color management is a process that aims to allow color to be transferred across various technologies (printers, cameras, displays etc.) without loss of fidelity. Color management systems are now embedded as part of most popular operating systems that make use of imaging device profiles that allow a device’s color space to be transformed into a standard device-independent color space. Several device-independent color spaces are used including the CIE system but also sRGB (a standard RGB color space) [14]. The International Color Consortium (ICC) is an industry consortium which has defined an open standard for a Color Matching Module (CMM) at the operating system level, and color profiles for the devices and working space (the color space the user edits in).

Color Communication

7

2.2.1

Summary

There are numerous ways to specify and communicate color. The use of language is natural and intuitive but lacks precision for all but crude descriptions of color. The use of reference to physical samples (arranged in a color-order system) has become widespread. The Munsell system was revolutionary in its approach and is still in use today but other systems (such as Pantone and NCS) have developed specialist appeal and offer advantages for certain applications. The main advantage of a color-order system is that it is easy to use. However, the number of different colors that we would like to communicate is certainly in the millions and yet even the largest color-order systems contain only a few thousand samples. Arguably, the notation systems of some of these color-order systems allow colors that are between colors in the systems to be notated but with some loss of precision. There is also the argument that to use a color-order system effectively one should possess normal color vision. For this reason, and others, numerical color specification is widely used in industry and is based upon a system for specifying color that was introduced in 1931.

References 1. Berlin B, Kay P (1969) Basic color terms: their universality and evolution. University of California Press, Berkeley 2. Saunders B (2000) Basic color terms. J Roy Anthropol Inst 6:81–89 3. http://www.icsi.berkeley.edu/wcs/. Last accessed 11 Aug 2010 4. Roberson D, Davies I, Davidoff J (2000) Colour categories are not universal: Replications and new evidence from a stone-age culture. J Experiment Psychol 129:369–398 5. Judd DB, Wyszecki G (1975) Color in business, science and industry, 3rd edn. Wiley, New York 6. Pointer MR, Attridge GG (1998) The number of discernible colours. Color Res Appl 23(1):52–54 7. Koenig B (2003) Color workbook. Pearson Education, Harlow 8. Fairchild MD (2005) Color appearance models. Wiley, New York

9. Kuehni RG (2003) Color space and its divisions. Wiley, New Jersey 10. Newhall SM, Nickerson D, Judd DB (1943) Final report of the OSA subcommittee on the spacing of the Munsell colors. J Opt Soc Amer 33:385 11. McLaren K (1987) Colour space, colour scales and colour difference. In: McDonald R (ed) Colour physics for industry. Society of Dyers and Colourists, Bradford, UK 12. Hesselgren S (2007) Why colour order systems? Color Res Appl 9(4):220–228 13. Publication CIE No. 15.2 (1986) Colorimetry, 2nd edn. Bureau of the Commission Internationale de l’Eclairage, Vienna, Austria 14. Stokes M, Anderson M (1996) http://www.w3.org/ Graphics/Color/sRGB.html. Last accessed 11 Aug 2010

Further Reading Fairchild MD (2005) Color appearance models. Wiley, New York

Berns RS (2000) Billmeyer and Saltzman’s principles of color technology, 3rd edn. Wiley-Interscience, New York

137

2.2.2 The CIE System Stephen Westland 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 2 Additive Color Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 3 Trichromatic Color Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 4 The CIE 1931 System of Colorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 5 Chromaticity Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.2.2, # Springer-Verlag Berlin Heidelberg 2012

140

2.2.2

The CIE System

Abstract: Colorimetry is a branch of color science concerned with numerically specifying the color of physically defined stimuli such that two stimuli that look the same (under certain criteria) have identical specifications. The Commission Internationale de l’Eclairage (CIE) developed a system for the specification of color stimuli that was recommended for widespread use in 1931 and that has formed the basis of colorimetry for the last 80 years. This chapter briefly describes the development of the CIE system and explains key principles (such as additive color mixing and Grassman’s laws) upon which the system is based. Specification of color by tristimulus values is described and the importance of chromaticity diagrams is discussed.

1

Introduction

Colorimetry is a branch of color science concerned with numerically specifying the color of physically defined stimuli such that two stimuli that look the same (under certain criteria) have identical specifications [1]. The Commission Internationale de l’Eclairage (CIE) developed a system for the specification of color stimuli that was recommended for widespread use in 1931. The CIE system has been the cornerstone of modern colorimetry for most of the twentieth century, and although today there are other color spaces and color metrics that are frequently used, these are invariably derived from the original work that was carried out in the 1920s and 1930s. This chapter therefore outlines the development of the CIE system. The key to understanding the CIE system is to understand the principles of additive color mixing; the associated experimental laws that were derived and clarified around the beginning of the twentieth century are referred to as Grassman’s Laws and these are described. The notion of a color space is introduced and some properties of chromaticity diagrams are described.

2

Additive Color Mixing

There are two main types of color mixing: additive color mixing and subtractive color mixing. Subtractive color mixing occurs when colorants (inks, paints, dyes etc.) are mixed together; additive color mixing refers to how lights of different wavelengths add together to form different colors. Perhaps the most important feature of additive color mixing is that we can mix together three colors – which we will call color primaries – and create a surprising range of colors. A common misapprehension is that it is possible to define three color primaries that could create any color by mixture. Unfortunately, the range of reproducible colors (or gamut) for a trichromatic additive (or subtractive) system is limited and is always smaller than the gamut of all the colors possible in the world. However, the gamut is smaller or larger depending upon the choice of primaries. Pragmatically, for additive color mixing the largest gamut is achieved when the primaries are red, green, and blue. > Figure 1 illustrates additive color mixing with red, green, and blue primaries. The situation described by > Fig. 1 can be realized using three projection lamps each emitting a circular beam of one of the three additive primaries that are then superimposed onto a projection screen so that they partially overlap. Mixtures of red and green, red and blue, and blue and green can be seen to result in yellow, magenta, and cyan, respectively. Mixing all three primaries can result in a white. The colors (cyan, magenta, and yellow) that result from mixing any two of the primaries in equal amounts are sometimes referred to as secondary colors. It is

The CIE System

2.2.2

. Fig. 1 Additive color mixing using RGB primaries

important to be aware that there is no clear answer to the question of exactly which red, green, and blue would make the ‘‘best’’ additive primaries. Many image-display devices use different sets of primaries and even different sets of primaries have been used, at various times, within the CIE system of colorimetry.

3

Trichromatic Color Specification

The results of additive color matching are assumed to obey certain laws known as Grassman’s laws of additive color mixture [1]. Let us represent the three additive primaries red, green, and blue by the symbols [R], [G], and [B] and the amounts or intensities of these primaries by R, G, and B respectively. Grassman’s first law states that we can match the color of an arbitrary color stimulus [C] with an additive mixture of the primaries, thus ½C  R½R þ G½G þ B½B; where  denotes ‘‘matches’’ or ‘‘is equivalent to.’’ The amounts of the primaries needed to affect the match are referred to as tristimulus values and constitute a trichromatic or colorimetric specification of the color stimulus [C] according to the color-matching equation. At first, such a specification may appear somewhat arbitrary since it is clear that the actual tristimulus values RGB that specify a particular color stimulus [C] will depend upon the precise nature of the primaries [R], [G], and [B] that were selected. However, we need to consider Grassman’s remaining laws. Grassman’s second law states that an additive mixture of two stimuli [C1] and [C2] can be matched by linearly adding together the mixture of the primaries that individually match the two stimuli, thus if ½C1   R1 ½R þ G1 ½G and B1 ½B; and ½C2   R2 ½R þ G2 ½G and B2 ½B;

141

142

2.2.2

The CIE System

then ½C1  þ ½C2   ðR1 þ R2 Þ½R þ ðG1 þ G2 Þ½G þ ðB1 þ B2 Þ½B: Grassman’s third law states that color matching is invariant to changes in intensity. Thus, a½C  aR½R þ aG½G þ aB½B; where a is the intensity of the unit stimulus [C]. The implication of Grassman’s laws is that although the tristimulus values that specify a color stimulus will depend upon the choice of color primaries, the matching condition will not be affected by this choice. Therefore, if two color stimuli are deemed a match by virtue of having the same tristimulus values under one system of primaries, then they would also be deemed a match under any other set of primaries.

4

The CIE 1931 System of Colorimetry

Trichromatic additive color mixing, described in the previous section, provides a basis for colorimetry since the tristimulus values required to match a color stimulus for a given set of primaries form a colorimetric specification. One possible practical implementation of this idea would be to construct a visual colorimeter. With such a system the user would view a screen showing (on, say, the left-hand side) the color stimulus to be specified and (on the right-hand side) an additive mixture of the three primary lights. The user would adjust the intensities of the three primaries to achieve a match and the tristimulus values selected by the user could be communicated to other users, who could then replicate the color stimulus using other identically constructed colorimeters. However, colorimetry based upon visual colorimeters would face several challenges, not least the problem that there is variation in terms of the trichromatic matches made by observers as a result of individual variation in color vision. In addition, it would only be possible to construct the colorimeters to be identical within a certain tolerance and maintaining that tolerance over time would not be trivial. However, considering the full implications of Grassman’s laws, if the tristimulus values needed to match a color stimulus at each wavelength (or wavelength interval) are known separately, the tristimulus values for the color stimulus can be calculated by summing across all wavelengths in the visible spectrum. This is the basis of the system of colorimetry recommended in 1931 by the CIE. The tristimulus values required to match 1 unit of energy at each wavelength in the visible spectrum were determined experimentally by two sets of workers (Guild working at the National Physical Laboratory used 7 observers and Wright working at Imperial College London used 10 observers) [1]. The two sets of experiments used different additive primaries. It is a relatively trivial matter to convert the tristimulus values obtained under one set of primaries to those for a second set of primaries. The data from both experiments were pooled and transformed to a single set of primaries different to either of those used in the actual experiments and the results are illustrated in > Fig. 2. > Figure 2 is based upon three monochromatic primaries at standardized wavelengths of 700, 546.1, and 435.8 nm. The latter two wavelengths were chosen because they were easily reproduced from the discharge spectrum of mercury vapor. Guild, in particular, maintained that the primaries had to be reproducible with national-standardizing-laboratory accuracy [2]. It was reasoned that by positioning the long-wavelength primary at 700 nm where the visual system is not very sensitive, small errors in its wavelength would have relatively little impact. Radiometrically, an equi-energy white light

The CIE System

2.2.2

1.8 1.6

Tristimulus values

1.4 1.2 1 0.8 0.6 0.4 0.2 0

400

450

500

600 650 550 Wavelength (nm)

700

750

800

. Fig. 2 CIE 1931 RGB color-matching functions for the red (red line), green (green line), and blue (blue line) RGB color primaries

could be matched using 1.0000, 4.5907, and 0.0600 lumens of red, green, and blue primaries respectively. The units of the RGB primaries were then defined such that one unit each of the primaries would result in an equi-energy white . The curves in > Fig. 2 are known as the CIE 1931 RGB color-matching functions and they are available in CIE Publication 15.2 [3]. The reader may notice that the red color-matching function is negative at certain wavelengths. The implication of this is that it was not possible to match these wavelengths using all-positive amounts of the three primaries. In order to obtain a match for these wavelengths one of the primaries (the red) was additively mixed with the stimulus, which could then be matched using a mixture of the other two primaries. The color-matching equation is then represented by ½C þ R½R  G½G þ B½B; which, assuming that Grassman’s laws apply, can be transformed into the following form ½C  R½R þ G½G þ B½B: Thus, this situation can be represented as using a negative amount of one of the primaries (in this case, the red). When tristimulus values had to be calculated manually (the 1930s), the presence of both negative and positive values made the complications complicated and prone to error [1]. Therefore, the CIE introduced a transformation that would allow the use of three so-called ‘‘imaginary’’ primaries that are referred to as [X], [Y], and [Z]. One of the conditions of this transformation was that the XYZ tristimulus values would be all-positive for all real color stimuli. Another condition was such that the Y tristimulus value would represent the luminance of the stimulus (the luminance values of the other two primaries are correspondingly equal to zero). The additional conditions are widely available in the literature [2],

143

2.2.2

The CIE System

0.35 0.30 0.25 Tristimulus values

144

0.20 0.15 0.10 0.05 0

−0.05 −0.10

400

450

500

550 600 650 Wavelength (nm)

700

750

800

. Fig. 3 CIE 1931 XYZ color-matching functions for the X (red line), Y (green line), and Z (blue line) XYZ color primaries

but it is interesting to note that it is unlikely that any of these conditions would be adopted if the CIE system was formulated today. However, the CIE 1931 XYZ color-matching functions are firmly established and are the basis on which many of the developments in colorimetry since 1931 have been based. Note that what is important is the matching condition (that two stimuli are a visual match if they have the same tristimulus values) and this is independent of which primaries we choose to base the system on (assuming that Grassman’s laws hold true). The color-matching functions x(l), y (l) and z (l) were defined by the CIE at intervals of 1 nm at wavelengths l between 360 and 830 nm and are shown in > Fig. 3. The CIE 1931 standard also specified CIE illuminants A, B, and C although illuminant C was subsequently supplemented by the CIE D (daylight) illuminants in 1964 of which D65 and D50 are perhaps the most important today. The introduction of tables of illuminants allowed the computation of tristimulus values for surface colors as well as for self-luminous colors. Practical formulae for computing the CIE 1931 tristimulus values for a surface with spectral reflectance P(l) under an illuminant of relative spectral power E(l) were provided by the CIE in 1986 [4]; thus, X 830 EðlÞ x ðlÞPðlÞ; X¼k 360 Y ¼k Z ¼k

X 360

X 360

830

EðlÞy ðlÞPðlÞ;

830

EðlÞz ðlÞPðlÞ;

where k is a normalizing factor. The significance of this normalizing factor is that the Y tristimulus value (which corresponds to the luminance) would be equal to 100 for

The CIE System

2.2.2

a perfectly white surface irrespective of which illuminant is used for the calculation. Alternative methods for the calculation of tristimulus values using tables of weights (which pre-calculate the illuminant and the color-matching functions at each wavelength) are available [5] and software implementations can also be found [6]. The 1931 CIE system was derived using a stimulus field size of 2 of visual angle. In 1964, an additional set of color-matching functions were derived based on 10 of visual angle and are preferred for many practical applications. This additional set of color-matching functions is known as the 1964 or 10 standard observer. If the primaries [X], [Y], and [Z] are considered as vector components, the three-dimensional space thus constructed can be used for the geometric expression of colors and is called a color space [1]. The tristimulus values for a particular color locate that color in the color space.

5

Chromaticity Diagrams

The concept of color can be divided into two parts: luminance and chromaticity. The CIE 1931 XYZ system was deliberately designed so that the Y tristimulus value was a measure of luminance. The chromaticity of a color can then be specified by the two remaining values and the calculation of chromaticity coordinates x and y; thus, x ¼ X=ðX þ Y þ ZÞ; y ¼ Y=ðX þ Y þ ZÞ allows chromaticities to be plotted in a chromaticity diagram (see > Fig. 4). The chromaticity diagram reveals the characteristic horseshoe shape of the spectral locus. It is sometimes convenient to refer to the dominant wavelength and purity of a color. The former is obtained by extending a line from the white point through the color stimulus to the spectral locus and

520 nm 0.8

CIE y

0.6

0.4 700 nm 0.2

0.0

400 nm 0.0

. Fig. 4 The 1931 CIE chromaticity diagram

0.2

0.4 CIE x

0.6

0.8

145

146

2.2.2

The CIE System

noting the wavelength of intersection and the latter is the proportional distance of the color stimulus from the white point to this intersection point on the spectral locus. If Grassman’s Laws hold true then the color that results from additively mixing two lights will fall on the straight line in the chromaticity diagram that joins the two points that represent the two lights. Although the CIE 1931 (and 1964) systems have proved very effective for color specification, the corresponding color spaces and chromaticity diagrams are not visually uniform. That is, the distance between two points in the space does not properly correspond to the color difference between two color stimuli represented by those two points. This property restricted the usefulness of the system for certain practical problems such as predicting color difference. > Chapter 2.2.5 describes the development of uniform color spaces.

6

Summary

The CIE system is the basis of modern colorimetry. The system can be confusing at first because of the use of so-called imaginary primaries (XYZ). However, it is important to realize that the most important properties of the CIE system are independent of the actual primaries that are used. It is also important to note that the system was developed to address the problem of color specification. The original 1931 CIE system was not concerned with color appearance, but rather focused on whether two color stimuli would be a visual match when viewed under identical and standardized viewing and illumination conditions. The matching condition is satisfied if two stimuli have identical tristimulus values and this is invariant to the actual primaries that the tristimulus values refer to. Despite the success of the CIE system, a huge effort was made between 1960 and 2000 to address the lack of visual uniformity in the system. Some of these developments are described in > Chap. 2.2.5. Also, during the last quarter of a century the issue of color appearance has started to become more and more important. There have been considerable advances in this area and readers are directed to Fairchild’s book for a summary of these [7].

References 1. Ohta N, Robertson AR (2005) Colorimetry fundamentals and applications. Wiley, New York 2. Fairman HS, Brill MH, Hemmendinger H (1997) How the CIE 1931 color-matching functions were derived from Wright-Guild data. Color research and application 22(1):11–23. 3. Publication CIE No. 15.2 (1986) Colorimetry, 2nd edn. Bureau of the Commission Internationale de l’Eclairage, Vienna, Austria

4. Publication CIE No. S2 (1986) Standard colorimetric observers. Bureau of the Commission Internationale de l’Eclairage, Vienna, Austria 5. ASTM E308-01 (2001) Standard practice for computing the colors of objects using the CIE system, ASTM 6. Westland S, Ripamonti C (2004) Computational colour science using MATLAB. Wiley, London 7. Fairchild MD (2005) Color appearance models. Wiley, New York

Further Reading Hunt RWG (1998) Measuring colour, 3rd edn. Fountain Press, London Kuehni RG (2003) Color space and its divisions. Wiley, New Jersey

Wyszecki G, Stiles WS (1982) Color science – Concepts and methods, quantitative data and formulae, 2nd edn. Wiley, New York

2.2.3 RGB Systems Stephen Westland . Vien Cheung 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 2 Trichromatic Color Reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 3 RGB Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 4 Color Management and RGB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 5 Recent Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.2.3, # Springer-Verlag Berlin Heidelberg 2012

148

2.2.3

RGB Systems

Abstract: The additive primaries are red, green, and blue or RGB. Unfortunately, there is no single set of RGB primaries that has achieved universal acceptance. Rather, RGB primaries have evolved over time in response to consumer demand and technological advancement. Three important sets of primaries, however, are known as SMPTE-C, ITU-R BT.601, and ITU-R BT.709-3. Many display systems currently use SMPTE-C and ITU-R BT.601 and, as highdefinition television develops, ITU-R BT.709-3 is becoming more prevalent. Two important issues for RGB color-reproduction system are the color gamut and device dependency. These topics are briefly described and some recent developments are introduced.

1

Introduction

Using additive color mixing and just three primaries, it is possible to create color-reproduction systems that can generate a wide range of colors. Modern examples of such systems include television, LED displays, plasma displays, and cinematography. The additive primaries are red, green, and blue or RGB. Unfortunately, there is no single set of RGB primaries that has achieved universal acceptance. Rather, RGB primaries have evolved over time in response to consumer demand and technological advancement. Three important sets of primaries, however, are known as SMPTE-C, ITU-R BT.601, and ITU-R BT.709-3. Many display systems currently use SMPTE-C and ITU-R BT.601 and, as high-definition television develops, ITU-R BT.709-3 is becoming more prevalent. Two important issues for RGB color-reproduction system are the color gamut and device dependency.

2

Trichromatic Color Reproduction

Human color vision is trichromatic. In most humans, our color vision systems are based upon the responses of three classes of cones in the retina, each of which has broadband sensitivity but maximum sensitivity at different wavelengths. A consequence of trichromacy is that color reproduction is trichromatic – the use of three color primaries allows a wide range of colors to be reproduced. The gamut of reproducible colors for a trichromatic additive system is limited and is always smaller than the gamut of all the colors possible in the world. However, the gamut is smaller or larger depending upon the choice of primaries. Pragmatically, the largest gamut is achieved when the additive primaries are red, green, and blue. It is sometimes useful to represent the RGB color space as a cube, with black (corresponding to zero intensity for R, G, and B) in one corner and white (corresponding to maximum intensity for R, G, and B) in the opposite corner. Such an illustration (see > Fig. 1) makes explicit that the secondary colors of the RGB color solid are cyan (blue + green), magenta (red + blue), and yellow (red + green). The origins of the RGB color model can be traced back to the Young–Helmholtz theory of trichromatic color vision in the early nineteenth century. Early experiments by Maxwell (1861) used three separate filters (red, green, and blue) to capture three black-and-white images. These could be combined using three projectors and appropriate filters to generate color photographs in what became known as the color-separation method [1]. In modern imaging, the RGB color model is ubiquitous in digital photography, cinematography and, most notably, in image-display devices such as LCD and plasma displays.

RGB Systems

Blue [001]

2.2.3

Magenta [101]

White [111] Cyan [011]

Black [000] Red [100]

Green [010]

Yellow [110]

. Fig. 1 The RGB color cube

3

RGB Standards

There is no single RGB color space that has achieved universal acceptance. Rather, many RGB standards and RGB primaries have evolved over time in response to consumer demand, professional interests, and technological advances. In the 1950s, the National Television System Committee (NTSC) specified a set of primaries that were representative of phosphors used in CRTs of that era in the USA [2]. The NTSC primaries were more saturated than those now found in many modern displays but as a consequence were not very bright. Meanwhile, the European Broadcast Union (EBU) established a different set of primaries for use in European countries (also in Japan and Canada) known now as ITU-R BT.601 or simply Rec. 601. The NTSC primaries were eventually replaced by SMPTE-C primaries that are slightly smaller in gamut but can achieve greater brightness. In 1990, a new set of primaries was agreed for highdefinition television (HDTV) known as ITU-R BT.709-3 or simply Rec. 709. > Table 1 lists the CIE 1931 chromaticity coordinates of the SMPTE-C, Rec. 601, and Rec. 709 primaries. It is currently possible to find displays that correspond to each of these standards and, indeed, to several others. > Figure 2 shows the gamut of colors that can be achieved using the Rec. 709 primaries. Note that, for a display system based on Rec. 709, colors that lie outside of the triangle in > Fig. 2 cannot be reproduced by the display and are said to be out of gamut. However, even within the triangle many chromaticities would be out of gamut at certain luminance levels because gamuts are, of course, three dimensional [3]. The use of more saturated primaries would, in principle, allow a greater gamut of reproducible colors; however, in practice – when the 3-D nature of the gamut is considered – the range of colors may even be reduced. Furthermore, in digital RGB systems it is normal to allocate 8 bits per color channel resulting in 256 values for each of R, G, and B (such 24-bit color systems can reproduce approximately 16 million colors though not all may be discriminable [4]). Using a wider RGB gamut would mean that digital steps would be more widely spaced and this may not be desirable. Consequently,

149

2.2.3

RGB Systems

. Table 1 The CIE chromaticities of the SMPTE-C, Rec. 601, and Rec. 709 primaries SMPTE-C

ITU-R BT.601

ITU-R BT.709-3

R

G

B

R

G

B

R

G

B

x

0.6300

0.3100

0.1550

0.6400

0.2900

0.1550

0.6400

0.3000

0.1550

y

0.3400

0.5950

0.0700

0.3300

0.6000

0.0600

0.3300

0.6000

0.0600

z

0.0300

0.1050

0.7750

0.0300

0.1100

0.7900

0.0300

0.1000

0.7900

520 nm 0.8

0.6 CIE y

150

0.4 700 nm 0.2

400 nm 0.0 0.0 0.2

0.4 CIE x

0.6

0.8

. Fig. 2 CIE chromaticity diagram showing sRGB / Rec. 709 (solid lines) and Adobe (1998) RGB (dashed lines) gamuts

there is growing demand for high-resolution (in terms of bit depth) systems that allocate color using more than 24 bits per pixel [5]. It is important to note that the standards in current use for RGB systems have evolved for practical use subject to a number of considerations. Nevertheless, the range of colors that many printing systems can generate (using a subtractive mixing system based on cyan, magenta, and yellow inks) exceeds the RGB gamut for certain colors. Thus, bright yellows and magentas that are outside the gamut of RGB display systems can often be obtained using a CMY printing system; correspondingly it is often not possible to obtain in print the bright greens and reds that can be obtained with display systems. The use of image-editing software on computers, in particular, has led to the introduction of some additional RGB standards, some of which have very large gamuts. In 1996, Hewlett–Packard and Microsoft proposed a standard color space, sRGB, intended for widespread use but particularly within the Microsoft operating systems, HP products, and

RGB Systems

2.2.3

. Table 2 The CIE chromaticities of the Adobe RGB (1998) and sRGB primaries Adobe RGB (1998)

sRGB

R

G

B

R

G

B

x

0.6400

0.2100

0.1500

0.6400

0.3000

0.1550

y

0.3300

0.7100

0.0600

0.3300

0.6000

0.0600

z

0.0300

0.0800

0.7900

0.0300

0.1000

0.7900

the Internet [6]. sRGB was designed to be compatible with the Rec. 709 standard and therefore the chromaticities of the primaries are the same as those in > Table 1 for Rec. 709. The full specification – which includes a transfer function (gamma curve) that was typical of most CRTs – allows images encoded as sRGB to be directly displayed on typical CRT monitors and this greatly aided its acceptance. However, sRGB is sometimes avoided by high-end print publishing professionals because its color gamut is not big enough, especially in the bluegreen colors, to include all the colors that can be reproduced in printing. Adobe RGB (1998) was established by the Adobe software company (SMPTE-240M) and designed to encompass most of the colors achievable on CMYK color printers. As can be seen in > Fig. 2 the Adobe (1998) RGB space improves upon the gamut of the sRGB color space primarily in cyan-greens (> Table 2).

4

Color Management and RGB

RGB color values are often said to be device dependent. Imagine a camera system was used to capture a scene so that the RGB values are recorded at each pixel. Now imagine that the image is displayed on two displays, one that is based on Rec. 709 primaries and one that is based on Adobe RGB (1998) primaries. Without adjustment for the differences in the two sets of primaries, it is likely that the colors will look different in the two displayed images. The RGB values captured by the camera relate to the camera’s spectral sensitivities; in other words they are dependent upon that device and cannot be relied upon to produce reasonable color accuracy on a display device unless the RGB values are adjusted to account for the differences in the capture device and the display device (e.g., Rec. 709 or Adobe RGB). Fortunately such adjustment regularly takes place and is the key process of color management (see > Chap. 3.2.3). For color to be reproduced in a predictable manner across different devices and materials, it has to be described in a way that is independent of the specific behavior of the mechanisms and materials used to produce it. To address this issue, current methods require that color be described using device-independent color coordinates, which are translated into device dependent color coordinates for each device [6, 7]. Originally, operating systems supported color for a particular color space but since even RGB varies between devices, color was not reliably reproduced across different devices. The high-end publishing market could not meet its needs with the traditional means of color support and the work of the International Color Consortium (ICC) was critical in terms of increasing color fidelity across a wide range of imaging devices [7]. The purpose of the ICC is to

151

152

2.2.3

RGB Systems

‘‘promote the use and adoption of open, vendor-neutral, cross-platform color management systems.’’ The ICC process converts input color values to output color values via a profile connection space (PCS). For this system to be effective, each device should be associated with a device profile that describes the relationship between the device’s color space and the deviceindependent color space (PCS). However, the ICC process involves the overhead of transporting the input device’s profile with the image and running the image through the transform. There is also the problem of what to do if an image file does not have a profile associated with it. As described earlier, the sRGB color space was proposed as an alternative means of managing color that is optimized to meet the needs of most users without the overhead of carrying an ICC profile with the image [6]. Most color management systems now assume that the default color space is sRGB if confronted with a digital color image that does not have a profile. For this reason, sRGB is often the color space of choice for creating images for display over the internet on a variety of platforms.

5

Recent Developments

There are a great number and variety of technologies that implement RGB image reproduction. These include phosphors, light-emitted diodes (LED), liquid crystal displays (LCD), plasma, organic LEDs [8]. The color properties of the RGB primaries vary from one device and manufacturer to another, as does the spatial arrangement of the RGB primaries. However, until recently all of these systems have been based on three primaries. A recent development by Sharp, known as QuadPixel technology has introduced a fourth primary (yellow) in addition to the RGB primaries in some LED display devices [9].

6

Summary

Color reproduction is fundamentally based on trichromacy. A wide range of colors can be generated using three color primaries and for additive systems (such as TVs, computer screens, and mobile phone displays), the optimum primaries (giving the greatest color gamut) are based on RGB. However, there are several standards for the design of the RGB primaries. Partly for this reason, color management is critical to enable good color fidelity as images are communicated between different imaging devices. Fortunately, color management is built-in to almost all modern operating systems and, in most cases, makes appropriate adjustments with the user being aware of the underlying process or being required to provide any information or intervention. However, very accurate manipulation and communication of color requires a high level of knowledge about color mixing, color primaries, and color management. Although the underlying idea of additive color reproduction has remained largely unchanged for at least 50 years, advances in technology have led to better implementations of the RGB with wider gamuts, better resolution, and increased image quality. A recent development, however, has seen the introduction of a quadchromatic additive color system that is claimed to provide further advances in the color gamut that is achievable. It is likely that the next decade will see similar advances, possible with more than four primaries, that move toward a spectral reproduction method rather than a colorimetric one.

RGB Systems

2.2.3

References 1. Hirsch R (2004) Exploring colour photography: a complete guide. Lawrence King Publishing, London 2. Poynton C (2009) http://www.poynton.com/PDFs/ ColorFAQ.pdf. Last accessed 12 Aug 2010 3. Morovicˇ J (2008) Color gamut mapping. Wiley, Chichester, UK 4. Pointer MR, Attridge GG (1998) Color Research and Application 23(1):52–54 5. Garcia-Suarez L, Ruppertsberg AI (2010) Why higher resolution graphics cards are needed in colour vision research. Color Research and Application 36(2):118–126

6. Stokes M, Anderson M (1996) http://www.w3.org/ Graphics/Color/sRGB.html. Last accessed 12 Aug 2010 7. International Color Consortium (2010) http://www. color.org/. Last accessed 12 Aug 2010 8. Jackson R, MacDonald L, Freeman K (1994) Computer generated color. Wiley, New York 9. http://www.sharp.co.uk/cps/rde/xchg/gb/hs.xsl/-/ html/lcd-tv.htm?FACET5_1=true. Last accessed 19 Aug 2010

Further Reading Hunt RWG (2006) The reproduction of colour, 6th edn. John Wiley, Chichester, UK

Sharma G (2002) Digital color imaging handbook. CRC Press, Boca Raton, FL

153

2.2.4 CMYK Systems Stephen Westland . Vien Cheung 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 2 Additive and Subtractive Color Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 3 Ideal and Realistic Subtractive Primaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 4 Process Colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 5 Beyond CMYK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.2.4, # Springer-Verlag Berlin Heidelberg 2012

156

2.2.4

CMYK Systems

Abstract: Color vision is based upon the responses of three classes of cones in the retina, each of which has broadband sensitivity but maximum sensitivity at different wavelengths. A consequence of this is that color reproduction is trichromatic – the use of three primaries allows a wide range of colors to be reproduced. Color-mixing behavior can be broadly classified as either additive or subtractive. The optimum primaries of the subtractive color system are cyan, magenta, and yellow. The use of cyan, magenta, and yellow subtractive primaries allows a surprisingly large – albeit limited – gamut of colors to be reproduced. In practical printing systems black is also used so that CMYK is ubiquitous in printing. Extended gamuts can be achieved using more than three or four primaries and systems based on six or more primaries are becoming quite common.

1

Introduction

Color reproduction is trichromatic – the use of three primaries allows a wide range of colors to be reproduced. The notion that there was something of a triple nature in color emerged in the seventeenth century and by 1722 LeBlon was creating color images using three separations. The first trichromatic color photograph was produced by Maxwell in 1861 and Maxwell’s method remains fundamental to modern processes of color reproduction [1]. The optimum primaries of the subtractive color system are cyan, magenta, and yellow. The use of cyan, magenta, and yellow subtractive primaries allows a surprisingly large – albeit limited – gamut of colors to be reproduced.

2

Additive and Subtractive Color Mixing

The gamut of reproducible colors for a trichromatic system is limited and is always smaller than the gamut of all the possible colors in the world. Moreover, the gamut is smaller or larger depending upon the choice of primaries. In order to fully appreciate this it is necessary to differentiate between two classes of color reproduction: additive and subtractive color mixing. Additive color mixing describes the color mixing of lights and is normally realized in the spatial superposition of light from three primaries; pragmatically, the largest gamut is achieved when the additive primaries are red (R), green (G) and blue (B) (see > Chap. 2.2.3). However, many color-image reproduction technologies (most notably printing) are based on primaries that absorb light rather than emit light and these are characterized by subtractive mixing theory. Subtractive systems involve colored dyes, pigments or filters (generically referred to as colorants) that absorb radiant power from selected regions of the electromagnetic spectrum. The dyes used in inks and paints selectively absorb certain wavelengths more than others; the light that is reflected (e.g., from the paper or textile to which the dye is applied) is that light that is not absorbed. The color physics of pigments (also used in inks and paints) is a little more complex since they typically also scatter light. However, in the case of both dyes and pigments since the optimum gamut of an additive trichromatic color-mixing system results from red, green, and blue primaries, it follows that the optimum subtractive gamut would result from a dye (cyan) that primarily absorbs in the red region of the spectrum, a dye (magenta) that primarily absorbs in the green region of the spectrum, and a dye (yellow) that primarily absorbs in the blue region of the spectrum. So, for example, by controlling the amount of cyan used in a subtractive system the amount of red light reflected is modulated. > Figure 1 provides a schematic representation of subtractive color mixing with cyan, magenta, and yellow primaries.

CMYK Systems

2.2.4

. Fig. 1 Subtractive color mixing with cyan, magenta, and yellow primaries. The combination of two primaries can produce red, green, and blue

3

Ideal and Realistic Subtractive Primaries

Since the purpose of the cyan (C), magenta (M), and yellow (Y) colorants is to absorb red, green, and blue light, respectively, one could argue that ideally they should have block spectral transmission curves. Such ideal transmission curves, also referred to as block dyes, are such that at every wavelength two of the colorants have 100% transmission and the third is absorbing. Hunt [1] notes that the optimum positions of the transition wavelengths are somewhat indeterminate but cites a study that gives values of around 490 and 580 nm [2]. The gamut of a subtractive color-mixing system comprising three such block dyes would be very similar to the gamuts of many additive color-mixing systems based on RGB. In practice, however, it is not possible to achieve dyes or pigments with ‘‘block’’ spectral properties. The reason for this is that the spectral properties of colorants are constrained (by the mechanisms by which they interact with light) to vary smoothly with the wavelength of light [3]. > Figure 2 shows the spectral transmission properties of a set of physically realizable CMY dyes. The sloping sides of most available colorants’ spectral curves result in so-called unwanted absorptions. So, for example, the cyan dye illustrated in the upper pane of > Fig. 2 absorbs at wavelengths greater than 580 nm but also exhibits some (unwanted) absorption at lower wavelengths. These unwanted absorptions result in some colors, especially blues and greens, being reproduced too dark. However, they also allow some colors to be reproduced that lie outside the triangle formed by the primaries in a chromaticity diagram [1]. > Figure 3 shows the gamut of the three dyes described in > Fig. 2. We can therefore see that whereas additive trichromatic systems have strictly triangular gamuts, subtractive trichromatic systems in practice typically have convex or concave gamuts (when plotted in the 2-D CIE chromaticity space) whose shape can be quite complex. For a good choice of primaries (such as CMY) the gamut is large and convex; for a poor choice of primaries (such as RGB) the subtractive gamut is small and

157

2.2.4

CMYK Systems

1

0.5

0 400 Reflectance factor

450

500

550

600

650

700

450

500

550

600

650

700

450

500

550 Wavelength (nm)

600

650

700

1

0.5

0 400 1

0.5

0 400

. Fig. 2 Spectral reflectance factors of physically realizable cyan (upper), magenta (middle), and yellow (lower) dyes each shown at four concentrations

0.8

0.6 CIE y

158

0.4

0.2

0.0 0.0

0.2

0.4 CIE x

0.6

0.8

. Fig. 3 Schematic representation of the gamut of the dyes illustrated by > Fig. 2

CMYK Systems

2.2.4

concave. However, when comparing gamuts of color-reproduction systems we need to be aware that the gamuts are 3-D and that looking at 2-D projections of these can be misleading [4]. The literature on computer graphics presents relatively simple formula for the relationship between the additive primaries (RGB) and the subtractive primaries. For example, Foley et al. [5] postulate the following: C¼1R M¼1G Y ¼ 1  B: These relationships would only be true, of course, if the subtractive primaries were ideal (block) colorants. Due to the spectral overlap among the colorants, converting CMYusing the ‘‘one-minusRGB’’ method works for applications such as business graphics where accurate color need not be preserved, but the method fails to produce acceptable color images for color-critical applications. The true relationship between RGB and CMY in the real-world is complex and nonlinear [6].

4

Process Colors

Since printing systems using cyan, magenta, and yellow primaries can generate relatively large color gamuts, such systems are ubiquitous in commercial color printing systems. However, printing black by overlaying cyan, yellow, and magenta ink suffers from major problems. Colored ink is expensive and printing three ink layers can result in the printed paper becoming excessively wet (which can reduce printing press speeds). In addition, small errors in registration of CMY layers could result in the black having colored edges and the black produced from a mixture of three colors is not always a very good black. For these reasons, and since black is a very important and common color when printing, a fourth color is often incorporated into CMY systems. Black ink – which can be manufactured inexpensively from carbon black pigment – is denoted by the letter K; the black printing plate in offset printing processes is historically referred to as the key plate which is where the initial K derives [7]. The CMYK fourcolor printing model is sometimes referred to as the process color model. The use of CMY inks of different color properties would affect the range of colors that can be produced. Some standardization of the colors of inks is clearly desirable [1]. Consequently, the Comite´ Europe´en d’Imprimerie (CEI) have standardized the colors that should be produced, under CIE illuminant D65 and for certain print conditions (e.g., 1 mm ink thickness on coated

. Table 1 Standard colors produced by CEI inks Inks

CIE x

CIE y

CIE Y

Yellow

0.437

0.494

Magenta

0.464

0.232

77.8 17.1

Cyan

0.153

0.196

21.9

Magenta over yellow

0.613

0.324

16.3

Cyan over yellow

0.194

0.526

16.5

Cyan over magenta

0.179

0.101

2.8

159

160

2.2.4

CMYK Systems

paper with no optical bleaching agent), for the CMY inks printed individually and in pairs. The CEI standard colors are shown in > Table 1 [1]. Note that the specification of the inks used in pairs better constrains the spectral absorption curves than if the inks were only specified in use individually.

5

Beyond CMYK

Four-color printing is used in many applications; however, the gamut of CMYK printing is limited. For example, it has been estimated that the CMYK process can generate only about 60% of the Pantone color formula guide [8], which is itself a limited color gamut. Consequently, for high-quality color prints a spot-color process can be used where individually colored inks are employed to create specific colors rather than rely upon CMY subtractive mixing. An alternative approach is to use more than three or four colorants. For example, Pantone’s proprietary six-color process using CMYKOG (orange and green being added to the process colors) and other so-called hexachrome systems (often based on CMYK with a light cyan and light magenta ink added) are frequently found even in relatively low-cost desktop inkjet printing systems. These systems can provide quite large color gamuts.

6

Summary

The optimum primaries of the subtractive color system are cyan, magenta, and yellow. The use of cyan, magenta, and yellow subtractive primaries allows a large – albeit limited – gamut of colors to be reproduced. Primarily for practical reasons, a separate black ink is usually used so that CMYK is the basis of many color-reproduction systems. Extended gamuts can be achieved using more than three or four primaries and systems based on six or more primaries are becoming quite common.

References 1. Hunt RWG (2006) The reproduction of colour, 6th edn. Wiley, Chichester, UK 2. Clarkson ME, Vickerstaff T (1948) Brightness and hue of present-day dyes in relation to colour photography. Phot J 88b:26 3. Maloney LT (1986) Evaluation of linear models of surface spectral reflectance with small numbers of parameters. J Opt Soc Am A 3(10):1673–1683 4. Morovic J (2008) Color gamut mapping. Wiley, Chichester, UK

5. Foley JD, van Dam A, Feiner SK, Hughes JF, Phillips RL (1997) Introduction to computer graphics. Addison-Wesley, Reading, MA 6. Westland S, Ripamonti C (2004) Computational colour science using MATLAB. Wiley, Chichester, UK 7. Gatter M (2004) Getting it right in print: digital prepress for graphic designers. Laurence-King Publishing, London 8. Drew JT, Meyer SA (2008) Color management: a comprehensive guide for graphic designers. RotoVision SA, Switzerland

Further Reading Hunt RWG (2006) The reproduction of colour, 6th edn. Wiley, Chichester, UK

Sharma G (2002) Digital color imaging handbook. CRC Press, Boca Raton, FL

2.2.5 Uniform Color Spaces Vien Cheung 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

2

The CIELAB and CIELUV Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

3

Applications to Colorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

4 4.1 4.2 4.3 4.4

Optimized Color-Difference Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 JPC79 and CMC(l:c) color-difference equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 BFD(l:c) color-difference equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 CIE94 color-difference equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 CIEDE2000 color-difference equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.2.5, # Springer-Verlag Berlin Heidelberg 2012

162

2.2.5

Uniform Color Spaces

Abstract: In 1976, the CIE specified CIELAB and CIELUV, two perceptually uniform color spaces to estimate the magnitude of the difference between two color stimuli. These color spaces were also designed to provide color-difference equations and to interpret color difference in terms of dimensions in lightness, hue, and chroma. The CIELAB and CIELUV color-difference equations have been widely used in industry. However, they do not accurately quantify small- to medium-size color difference. More advanced equations based upon the modification of the CIELAB color-difference equation were developed. The CIEDE2000 colordifference equation is the current CIE recommendation for computing small color differences. Future research in the area of the color difference may be based upon a more uniform color space, based on color-vision theory and be capable of accounting for different viewing parameters.

1

Introduction

The CIE (Commission internationale de l’e´clairage or International Commission on Illumination) 1931 XYZ system has been effective to measure the luminance and chrominance of a color (see > Chap. 2.2.2). However, the distribution of colors in its x, y chromaticity diagram is very non-uniform. This leads to the problem of change in chromaticity or luminosity and visual perception are not linearly related. Equal changes in x, y or Y do not correspond to perceived differences of equal magnitude. In 1976, the CIE suggested two color spaces, CIELAB and CIELUV, as a way to overcome the limitations of the CIE XYZ system [1]. The visual magnitudes of color differences are intended to be approximately proportional to the distance in these spaces.

2

The CIELAB and CIELUV Spaces

CIELAB and CIELUV were primarily suggested to provide color-difference equations but they were also designed to express color differences correlated with perception attributes: hue, lightness, and chroma. These spaces are represented by plotting the three attributes along axes at right angles to one another. Both CIELAB (concerned with subtractive mixture, e.g., surface colorant) and CIELUV (for additive mixture of colored light, e.g., television) color spaces have the same lightness scales L
, which is defined in terms of the ratio of the Y tristimulus value of the color considered to that of the reference white Yn as follows: L
¼ 116ðY =Yn Þ1=3  16


L ¼ 903:3ðY =Yn Þ

for Y =Yn > 0:008856 for Y =Yn  0:008856

ð1Þ

The opponent color axes, approximately red-green (a
) versus yellow-blue (b
) for the
) and hue (hab) CIELAB color space (as shown in > Fig. 1) are defined in > Eq. 2. Chroma (Cab

> > are calculated using a and b as Eqs. 3 and 4 respectively. a
¼ 500½ f ðX=Xn Þ  f ðY =Yn Þ b
¼ 200½ f ðY =Yn Þ  f ðZ=Zn Þ where f ðIÞ ¼ I 1=3 f ðIÞ ¼ 7:787I þ 16=116

for I > 0:008856 for I  0:008856

ð2Þ

Uniform Color Spaces

2.2.5

100 White

+b* Yellow

L*

C*ab

–a*

hab Red

Green

+a*

–b* Blue

Black 0

. Fig. 1 A schematic representation of the CIELAB color space


Cab ¼ ða
2 þ b
2 Þ1=2

ð3Þ

hab ¼ arctanðb
=a
Þ

ð4Þ

Similarly, the CIELUV color space contains the opponent color axes, approximately redgreen (u
) versus yellow-blue (v
), which are defined in > Eq. 5. u
¼ 13L
ðu0  u0 n Þ v
¼ 13L
ðv 0  v 0 n Þ

ð5Þ


), which can also be correlated for saturation in the CIELUV Saturation (suv), Chroma (Cuv space and hue (huv) are calculated respectively in > Eqs. 6–8.

suv ¼ 13½ðu0  un0 Þ2 þ ðv 0  vn0 Þ2 1=2

3

ð6Þ


1=2
Cuv ¼ u
2 þ v
2 ¼ L
suv

ð7Þ

huv ¼ arctanðv
=u
Þ

ð8Þ

Applications to Colorimetry

The CIE system offers a precise means of specifying a color stimulus under a set of viewing conditions and suggested CIELAB and CIELUV uniform color spaces as useful representations of colors that correlate with perceptual attributes [1]. Another important use of the CIE system is the evaluation of perceived color difference. Color-difference equations are designed to provide quantitative representations of the perceived color differences between pairs of colored

163

164

2.2.5

Uniform Color Spaces

samples. Color differences specified by CIELAB (> Eq. 9) and CIELUV (> Eq. 10) spaces are measured by the Euclidean distance between the co-ordinates for two stimuli. One unit represents approximately one just-noticeable difference (JND) for a pair of samples viewed side by side [2].

2
2 1=2 ¼ ðDL
2 þ Da
2 þ Db
2 Þ1=2 or ðDL
2 þ DHab þ DCab Þ DEab

ð9Þ

where

2
2 1=2 DHab ¼ ½ðDEab Þ  ðDL
Þ2  ðDCab Þ

2
2 1=2 DEuv ¼ ðDL
2 þ Du
2 þ Dv
2 Þ1=2 or ðDL
2 þ DHuv þ DCuv Þ

ð10Þ

where

2
2 1=2 DHuv ¼ ½ðDEuv Þ  ðDL
Þ2  ðDCuv Þ

4

Optimized Color-Difference Equations

CIELAB and CIELUV were derived from non-linear transformations of the CIE XYZ system and have been widely used in color industries. The two CIE recommended equations, however, do not accurately quantify small- to medium-size color difference [3]. Many attempts have been made to modify the CIELAB color-difference equation to develop more advanced equations including JPC79 [4], CMC(l:c) [5], BFD(l:c) [6, 7], CIE94 [8], and CIEDE2000 [9].

4.1

JPC79 and CMC(l:c) color-difference equations

McDonald accumulated a large number of data involving polyester thread pairs and carried out visual pass/fail color-matching assessments [4]. The visual results were used to derive the JPC79 equation. At a later stage the JPC79 equation was modified, due to the problem that some anomalies were found for colors close to neutral and black [10], and renamed as the CMC(l:c) equation. h
2 i1=2 2

2


=ðlSL Þ þ DCab =ðcSC Þ þ DHab =SH ð11Þ DECMCðl:c Þ ¼ DLab where

=ð1 þ 0:01765Lab;std Þ if LS
 16 SL ¼ 0:040975Lab;std

otherwise SL ¼ 0:511 and

=ð1 þ 0:0131Cab;std Þ þ 0:638; SC ¼ 0:0638Cab;std

SH ¼ SC ðTf þ 1  f Þ:

Uniform Color Spaces

2.2.5

The terms T and f are given by

Þ4 =ððCab;std Þ4 þ 1900Þ1=2 f ¼ ½ðCab;std

and T ¼ 0:36 þ j0:4 cosðhab;std þ 35Þj if hab;std  164 or hab;std  345 Otherwise T ¼ 0:56 þ j0:2 cosðhab;std þ 168Þj:
The CMC(l:c) equation is based upon the CIELAB color space and the terms L
, Cab , and
are corresponded to the CIELAB lightness, chroma, and hue respectively. The terms SL, SC, Hab and SH define the lengths of the semi-axes of the tolerance ellipsoid at the position of the standard in CIELAB space in each of the lightness (SL), chroma (SC), and hue (SH) directions. The ellipsoids were fitted to visual tolerances determined from psychophysical experiments and the dimension of the ellipsoid is a function of the position of the standard in the color space. The parametric terms l and c allow the ratio between lightness and chroma components to be adjusted. It is considered that there is greater acceptance for shifts in lightness dimension than in chromatic (chroma and hue) dimension. For predicting the perceptibility of color differences, it was recommended that both l and c equal to 1 whereas for predicting acceptability of color differences it was recommended that l and c equals to 2 and 1 respectively. The subscript std refers to the standard of a pair of samples. The CMC(l:c) equation has been widely used in a number of industries and became an ISO standard for the textile industry in 1995 [9]. It was also adopted as a British standard (BS 6923) and an AATCC test method (AATCC 173).

4.2

BFD(l:c) color-difference equation

Luo and Rigg [6, 7, 11] accumulated a large set of experimental data relating to small and medium color differences between pairs of surface colors and developed the BFD(l:c) colordifference equation. The structure of the BFD(l:c) equation is similar to that of the CMC(l:c) equation. However, it was found that an additional term in the BFD(I:c) equation is considered which take into account the fact that the chromaticity ellipses do not all point toward the neutral point as assumed in the CMC(l:c) equation. The effect is most significant in the blue region.



=ðcDC ÞÞ2 þ ðDHab =DH Þ2 þ RT ðDCab DHab =DC DH Þ1=2 DEBFD ¼ ½ðDLBFD =lÞ2 þ ðDCab

where LBFD ¼ 54:6 logðY þ 1:5Þ  9:6
=ð1 þ 0:00365 C
Þ þ 0:521 DC ¼ 0:035 Cab ab

DH ¼ DC ðGT 0 þ 1  GÞ

ð12Þ

165

166

2.2.5 4

Uniform Color Spaces

4


=ðC
þ 14000Þ1=2 G ¼ ½Cab ab

T 0 ¼ 0:627 þ 0:055 cosðhab  254 Þ  0:040 cosð2hab  136 Þ þ 0:070 cosð3hab  32 Þ þ 0:049 cosð4hab  114 Þ  0:015 cosð5hab  103 ÞRT ¼ RC RH RH ¼  0:260 þ 0:055 cosðhab  308 Þ  0:379 cosð2hab  160 Þ  0:636 cosð3hab  254 Þ þ 0:226 cosð4hab  140 Þ  0:194 cosð5hab  280 Þ 6

6


=ðC
þ 7  107 Þ1=2 RC ¼ ½Cab ab
and h The terms Cab ab refer to the arithmetic mean values of chroma and hue angle respectively. Both l and c equal to 1 for predicting perceptibility of color differences and l and c, respectively, equals to 1.5 and 1 for predicting acceptability of color differences.

4.3

CIE94 color-difference equation

Berns suggested a color-difference equation derived also by modifying the CIELAB equation [12]. The equation was later recommended by the CIE in 1994 [8] and is named as CIE94 colordifference equation. It has a similar structure to that of the CMC(l:c) equation but with simpler weighting functions. The CIE94 formula is given by


¼ ½ðDL
=ðKL SL ÞÞ2 þ ðDCab =ðKC SC ÞÞ2 þ ðDHab =KC SH Þ2 1=2 DE94

ð13Þ

where SL ¼ 1;
SC ¼ 1 þ 0:045Cab;std ;
SH ¼ 1 þ 0:015Cab;std :

The parametric factors KL, KC, and KH are included to correct for the variation in experimental conditions. For all applications except for textile industry, a value of 1 is recommended for all the parametric factors. For the textile industry CIE94(2:1:1) is recommended where KL equals to 2, and KC and KH equal to 1.

4.4

CIEDE2000 color-difference equation

It has been realized that both the CMC(l:c) and CIE94 color-difference equations are standardized but by different organizations: CMC(l:c) by ISO [13] and CIE94 by CIE [8]. However, there are large discrepancies between the two equations in predicting lightness differences, and have problems predicting grayish and bluish colors [3]. The value of the function in the CMC(l:c) equation increases markedly as L
increases, implying that for equal differences in L
the visual difference should be largest for the smaller L
values. However, the lightness correction in the CIE94 equation implied that equal differences in L
would yield equal visual differences for all values of L
.

Uniform Color Spaces

2.2.5

The CIE subsequently formed a Technical Committee (TC) 1-47 to develop a generalized color-difference equation. The CIEDE2000 equation was agreed by the CIE [14] and includes not only lightness, chroma, and hue weighting functions, but also an interactive term between chroma and hue differences for improving the performance for blue colors and a scaling factor for the CIELAB a
scale for improving the performance for colors close to the achromatic axis. The equation is given by: Step 1: calculate the CIELAB L
, a
and b
values 0 0 and hab Step 2: calculate a0 , Cab L0 ¼ L
;

a0 ¼ ð1 þ GÞa
; b0 ¼ b
; 0 ¼ ða02 þ b02 Þ1=2 ; Cab

and 0 ¼ arctanðb0 =a0 Þ: hab

where 7

7


=ðC
þ 257 Þ1=2 G ¼ 0:5  0:5½Cab ab

Step 3: calculate DL0 , DC 0 , and DH 0 0 0 DL0 ¼ Lbatch  Lstd 0 0 0 DCab ¼ Cab;batch  Cab;std 0 0 0 0 DHab ¼ 2ðCab;batch Cab;std Þ0:5 sinðDhab =2Þ

where 0 0 0 ¼ hab;batch  hab;std ; Dhab

Step 4: calculate CIEDE2000 DE00 0 0 DE00 ¼ ½ðDL0 =ðkL SL ÞÞ2 þ ðDCab =ðkC SC ÞÞ2 þ ðDHab =ðkH SH ÞÞ2 0 0 þ RT ðDCab =ðkC SC ÞÞðDHab =ðkH SH ÞÞ1=2

ð14Þ

where SL ¼ 1 þ ½0:015ðL0  50Þ2 =½20 þ ðL0  50Þ2 1=2 ;
; SC ¼ 1 þ 0:045 Cab

and
T: SH ¼ 1 þ 0:015 Cab

where T ¼ 1  0:17 cosðh0 ab  30 Þ þ 0:24 cosð2h0 ab Þ þ 0:32 cosð3h0 ab þ 6 Þ  0:20 cosð4h0 ab  63 Þ:

167

168

2.2.5

Uniform Color Spaces

and RT ¼  sinð2DyÞRC where Dy ¼ 30 exp f½ðh0 ab  275 Þ=252 g and
=ðC
þ 257 ÞÞ1=2 RC ¼ 2ðCab ab
, and h0 are the arithmetic mean of the L0 , C 0 , and H 0 values of a pair of Note that L0 , Cab ab ab ab samples. Caution needs to be taken for neutral colors having hue angles in different quadrants. If the difference is less than 180 , arithmetic mean of the samples should be used; 360 should be subtracted from the larger angle, followed by calculating the arithmetic mean, if otherwise. The CIEDE2000 equation has been shown to perform better than the CMC(l:c) and CIE94 equations [9, 15, 16] and it is the current CIE recommendation for computing small color differences.

5

Summary

The need for a uniform color space resulted in the specification of the CIELAB and CIELUV color spaces, as a way to represent colors that correlate with perceptual attributes. Another important use of the CIE system is the evaluation of perceived color difference between pairs of color stimuli. Color differences specified by CIELAB and CIELUV spaces are widely adopted by industry. More advanced color-difference equations were developed based upon the modification of the CIELAB color-difference equation to better quantify small- to medium-size color difference. Note, however, that all these color-difference equations were derived based on the perception of spatially uniform color patches. CIE colorimetry only considers color matching between two stimuli under identical conditions including surround, background, size, shape, texture, and illuminating/viewing geometry. Color matches defined by CIE may no longer be valid if any of these constraints is violated. Future research in the area of color difference may be based upon a more uniform color space rather than modifications of CIELAB. Instead of the empirical approach, it is expected that formulas may be based on color-vision theory and be capable of accounting for different viewing parameters such as sample size, size of color difference, spatial separation, background, and luminance level [17].

References 1. CIE (1978) Recommendations on uniform color spaces, color difference equations, psychometric color terms, Supplement 2 to CIE publication 15 (E1.3.1) 1971/(TC1.3). Central Bureau of the Commission Internationale de l’E´clairage (Vienna, Austria) 2. Hunt RWG (1998) Measuring colour, 3rd edn. Fountain Press, Kingston-upon-Thames, UK

3. Luo MR (1999) Colour science: past, present and future. In: MacDonald LW, Luo MR (eds) Colour imaging, vision and technology. Wiley, New York 4. McDonald R (1980) Industrial pass/fail colour matching – Part 1: Preparation of visual colourmatching data. J Soc Dyers Colourists 96:372–376

Uniform Color Spaces 5. Clarke FJJ, McDonald R, Rigg B (1984) Modification to the JPC79 colour difference formula. J Soc Dyers Colourists 100:128–132 6. Luo MR, Rigg B (1987) BFD(l:c) colour-difference formula – Part I – Development of the formula. J Soc Dyers Colourists 103:86–94 7. Luo MR, Rigg B (1987) BFD(l:c) colour-difference formula – Part II – Performance of the formula. J Soc Dyers Colourists 103:126–132 8. CIE (1995) Industrial colour-difference evaluation, CIE publication 116. Central Bureau of the Commission Internationale de l’E´clairage, Vienna, Austria 9. Luo MR, Cui G, Rigg B (2001) The development of the CIE 2000 colour difference formula: CIEDE2000. Color Res Appl 26(5):340–350 10. Smith KJ (1997) Colour-order systems, colour spaces, colour difference and colour scales. In R MacDonald (ed) Colour physics for industry. Society of Dyers and Colourists, Bradford 11. Luo MR, Rigg B (1986) Chromaticitydiscrimination ellipses for surface colours. Color Res Appl 11:25–42

2.2.5

12. Berns (1993) The mathematical development of CIE TC 1-29 proposed colour difference equation: CIELCH. Proc Seventh Cong Inter Colour Assoc B, C19.1–C19.4 13. ISO (1995) ISO 105-J03:1995 Textiles – Test for colour fastness – Part 3: Calculation of colour differences, International Organization for Standardization, Geneva, Switzerland 14. CIE (2001) Improvement to Industrial ColourDifference Evaluation, CIE publication 142. Central Bureau of the Commission Internationale de l’E´clairage, Vienna, Austria 15. Cui G, Luo MR, Rigg B, Li W (2001) Colourdifference evaluation using CRT colours. Part I: Data gathering and testing colour-difference formulae. Color Res Appl 26(5):394–402 16. Luo MR (2002) Development of colour-difference formulae. Rev Prog Color 32:28–39 17. CIE (1993) Parametric effects in colour difference evaluation, CIE publication 101. Central Bureau of the Commission Internationale de l’E´clairage, Vienna, Austria

Further Reading Kuehni RG (2003) Color space and its divisions. Wiley New York

Ohta N, Robertson A (2003) Colorimetry: fundamentals and applications. Wiley Chichester

169

2.2.6 Color Perception Marina Bloj . Monika Hedrich 1 1.1 1.2 1.2.1 1.2.2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Our Visual System is More than Trichromatic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Our Visual System is Color Constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Why Should the Visual System Be Color Constant? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 Mechanisms and Cues Contributing to Color Constancy . . . . . . . . . . . . . . . . . . . . . . . . . 175

2

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.2.6, # Springer-Verlag Berlin Heidelberg 2012

172

2.2.6

Color Perception

Abstract: In this chapter we present an overview of how we perceive color. We start with an outline of the physiology of the human visual system and discuss both the trichromatic and color-opponent theories of color vision. We conclude with a description of the phenomenon of color constancy and the factors that contribute to it. List of Abbreviations: L, Long; M, Medium; nm, Nanometers; S, Short

1

Introduction

The human visual system is sensitive to electromagnetic radiation with wavelengths between approximately 380 and 780 nm. The process of vision starts when radiation of this wavelength range is absorbed by the photoreceptors in the retina. The human retina contains two types of photoreceptors, called rods and cones. There are approximately 120 million rods and 6 million cones, which are unevenly distributed throughout the human retina [1, 2]. Rods are completely absent from the center of the fovea. Moving away from this area the number of rods increases and reaches its maximum at about 15–20 eccentricity before the density gradually declines again. Rods are responsive to low light levels, that is, in scotopic conditions. Cones have their highest concentration in the fovea. Outside the fovea, their density decreases rapidly and reaches at approximately 10–15 eccentricity, a constant level. Cones are responsive to high light levels, that is, in photopic conditions, and are fundamental for visual acuity and color vision. Within the cones three different photopigments have been identified. Each photopigment absorbs light over a broad range of wavelengths but has its maximum sensitivity at a different wavelength. The cones are named according to the wavelength range of their peak sensitivity: S-cones have their maximum sensitivity in the short wavelength range at approximately 420 nm, M-cones in the mid wavelength range at 534 nm and L-cones in the long wavelength range at 563 nm (see > Chap. 2.1.1 and [3]). The sensitivity curve of a cone class is defined by the probability of a photon to be absorbed as a function of wavelength (> Fig. 1). After a photon of a certain wavelength is absorbed, this energy is transduced into an electrical signal by a complex photochemical reaction. This signal does no longer carry separate information about intensity and wavelength but only about the number of photons that have been absorbed by the cone (Principle of Univariance) [5]. The signals released by cones and rods are sent via bipolar cells to the ganglion cells. Most ganglion cells receive their input, however, not from a single photoreceptor but from many. These neural networks allow condensing the signals of 126 million photoreceptors onto approximately one million nerve fibers through which the information leaves the eye. Ganglion cells are crucial for color vision but carry also temporal and spatial information. Regarding color vision the ganglion cells can be described as chromatically selective as their responses depend on differential inputs from the three cone classes. > Figure 2 shows a schematic of the contribution of the three cone classes to color opponency. While neural processing of visual information is often described as being rather complex within the early stages of the visual system, it becomes much more complex at the higher (cortical) stages. Around 30 regions have been identified in the brain cortex to respond to color and it has been shown that complex interactions take place between them. This indicates that

Color Perception

2.2.6

1

Relative sensitivity

S

L M

0.1

0.01

400

450

500 550 600 Wavelength (nm)

650

700

. Fig. 1 The relative sensitivities of the S-, M-, and L-cones as a function of wavelength (derived from [4]). Each curve is normalized to its maximum. Note that the peak sensitivities of the cones are shifted with respect to the ones mentioned above. The present data was obtained using psychophysical measurements in contrast to microspectrophotometry as used by [3]. The shifts are due to the transmission characteristics of the media of the eye

S

M

L

+

S

M

L

S

−

M

L

++ −

Luminance channel

Red-green channel

Blue-yellow channel

. Fig. 2 The color-opponent process. The luminance channel is formed from the sum of L- and M-cone signals (L + M); the red/green channel from the comparison of L- and M-cone signals (L-M); the blue/ yellow channel from the comparison of S-, M- and L-cone signals (S-(L + M)). (Adapted from [6])

there is no single color center in the cortex; however, it remains unclear where ultimately the perception of color is formed.

1.1

Our Visual System is More than Trichromatic

Because we have three types of cones, our visual system is referred to as trichromatic. A color signal excites each of the cone types in different degrees. Each cone mechanism on its own

173

174

2.2.6

Color Perception

cannot distinguish between a change in excitation due to an alteration in intensity of the color signal and one due to a change in the spectral composition of the signal. It is through the comparison of the output of the three classes of cones that we discriminate color signals. The trichromacy of our visual system becomes apparent in the fact that we can use the three colored guns (red, green, and blue) of a television set to display more than a million of colors (see also > Chap. 2.2.3). Maxwell and Helmholtz in the nineteenth century independently demonstrated that any colored light can be matched by a mixture of three suitably chosen spectral lights (Any three lights can be used as primaries as long as one of them cannot be matched by a mixture of the other two. The more spaced the primaries are in the spectrum, the more lights that can be matched by direct mixture of the three primaries. To match some lights it is necessary to mix one of the primaries with the ‘‘test’’ light and obtain the ‘‘matching’’ light by combination of the other two primaries). The resulting ‘‘matching’’ light has the same appearance as the ‘‘test’’ light but might have a different spectral distribution. These lights are known as metameric. The fact that metameric stimuli exist for our visual system indicates that the cones fail to produce a different set of outputs for every color signal. The trichromatic theory of color vision was developed solely based on observations from light mixing experiments rather than on anatomical studies of the eye. Thomas Young, Helmholtz, and others inferred from their observations that there were three different types of receptors with overlapping spectral sensitivities. This pioneering idea was later confirmed by the discovery of the three cone classes (e.g., [7, 8]) and the trichromatic theory became also known as the Young–Helmholtz theory. The opponent-process theory was proposed firstly by the physiologist Ewald Hering in 1892 and is based on the appearance of color. He suggested, after observing the effect of afterimages, that color is processed by the visual system in terms of opponent color pairs. When we stare, for instance, at a red circle for a while and then look at a white piece of paper, we see green, the opponent color of red. Hering specified originally two pairs of opponent colors, red and green, and blue and yellow. However, a third opponent pair, black and white, has been included as brightness is transmitted in a similar way as color. Hering also noted that the perception of opponent colors is mutually exclusive; we never experience yellowish blue or greenish red. Physiological evidence that supports Hering’s opponent-process theory was first found by [9]. Since then extensive research has been carried out to investigate the opponent color processing in the visual system (e.g., [10–12]). Nowadays it is generally accepted that both theories of color vision are partially correct and that color processing occurs in two stages [13]. In the first stage, on the receptor level, the Young–Helmholtz theory is correct with regard to the existence of the three cone classes and the subsequent transmission of the signal to the cortex, the second stage, can be explained by the opponent-process theory.

1.2

Our Visual System is Color Constant

In everyday life, we refer to color as a constant property of an object and we are unaware that most objects only reflect light. A red balloon looks red because it reflects mainly the long wavelengths of the spectrum when illuminated by white light. When the same balloon is illuminated by a specific green light it appears black because no light will be reflected. This is a rather extreme example but it demonstrates clearly that objects’ color appearance depends likewise on the reflectance properties of the object and the light it is illuminated with. When the

Color Perception

2.2.6

same object we have seen before is illuminated by a different light source then the spectral composition of the reflected light changes and should theoretically lead to a changed color appearance of the object. However, we know from experience that a banana looks yellow no matter which light it is under; in other words, the change in the reflected light stays unnoticed. The fact that object colors appear unchanged despite a change in illumination is known as color constancy (e.g., [6,14]), and in order to recognize a surface as unchanged the visual system must access information about the illuminant and the surface reflectance separately. In a simplified world the light that reaches our eyes, commonly referred to as color signal, is the product of the spectral power distribution of the illuminant and the spectral reflectance function of the surface (> Fig. 3). Physically the reflectance spectra can be determined by measuring first the color signal (with a spectroradiometer) wavelength by wavelength and dividing it then by the spectral power distribution of the illuminant. However, in the human eye the color signal is detected by the three classes of cones on the retina, which are unable to transmit full spectral information. They rather provide the visual system with information about their individual photon catch. This information changes when the color signal changes but with only this information, the visual system cannot differentiate whether the alteration is due to an illumination or surface change. If there is no possibility to access separate information about the illumination and the surface reflectance from the color signal, how does the visual system achieve color constancy? In the paragraph above, a simplified world is assumed, though the real world is more complex. Usually objects are surrounded by other objects and there is often more than just one light source. Therefore, the light reflected from an observed object not only depends on the reflectance properties of the object’s surface but also on the illumination, other objects, their location, and position with respect to the illuminant (or illuminants) and each other. Thus, to be color constant the visual system has to compensate for these various changes.

1.2.1

Why Should the Visual System Be Color Constant?

Natural daylight is not constant during a day but changes considerably from dawn until dusk. Much larger changes in illumination can even be experienced when coming from outside into a room with artificial illumination. However, the visual system compensates for these permanent changes and allows the stable perception of color in our environment. Color is a reliable cue to object identification and it has also been shown to enhance scene recognition [15, 16]. As long as objects are under a constant illumination and context, it is sufficient to simply remember the color to recognize it as the same, though such situations are rare. It is much more common to encounter colored objects in changed illumination and context conditions, and to recognize colors they must firstly be remembered and secondly the illuminant and context change must be accounted for. Therefore, color constancy can be considered as a sophisticated form of color memory. In everyday life, it is not always necessary to perceive colors as perfectly constant but to make confident judgments about color categories. For example, the ability to decide whether a banana is yellow or green, and thus, is ripe or unripe.

1.2.2

Mechanisms and Cues Contributing to Color Constancy

It has been shown that color constancy is not achieved by a single, but by the interaction of several, mechanisms and cues. It is the variety and combination of several cues that support

175

490 590 690 Wavelength (nm)

Illumination

0 390

0.005

0.01

0.015

0.02

0.025

490 590 690 Wavelength (nm)

Surface reflectance

0 390

0.2

0.4

0.6

0.8

1

Relative sensitivity

Relative radiance

Cone sensitivities 1.0 0.8 0.6 0.4 0.2 0.0 390 490 590 690 Wavelength (nm)

490 590 690 Wavelength (nm)

Colour signal

0 390

0.002

0.004

0.006

0.008

0.01

. Fig. 3 The emitted light hits a surface and is reflected; the reflected light reaches the eye of an observer who experiences color. Each light source emits an illumination with a characteristic spectral power distribution. Together with the surface reflectance of an object, they create the color signal, which reaches an observer’s eye. The incident color signal is absorbed by the cones, dependent on the cone sensitivities

Radiance

2.2.6

Radiance

176 Color Perception

Color Perception

2.2.6

color constancy (e.g., [17, 18]) and it has been suggested that the importance of particular cues varies depending on their availability and that the weighting of them is a dynamic process. The phenomenon of color constancy has been known for more than two centuries. Monge (1789) and Hering (circa 1880) produced early demonstrations of the phenomenon. Helmholtz, in 1868, proposed that the perceived color of the object was the result of mechanisms that took into account the color of the illumination. Since then it has been the goal of different studies to try to quantify the strength of color constancy, determine how the viewing context influences it, and identify the mechanisms involved. Chromatic adaptation has been identified as a powerful mechanism contributing to color constancy. Pioneering research in this area was conducted by von Kries, who postulated that the mechanisms of chromatic adaptation operate as gain controls on the cone signals. Since then an enormous number of studies have been dedicated to the investigation of chromatic adaptation (for review see [6, 19]). It was shown that von Kries’ hypothesis can explain the phenomenon of color constancy only partially (for review see [14]) and that chromatic adaptation involves not a single but several mechanisms at low-level as well as high-level stages (e.g., [20, 21]). Different mechanisms within chromatic adaptation have been identified to determine color appearance. One of these is chromatic adaptation to the spatial average of a scene. This process occurs over a large spatial area and is rather slow, taking approximately 2 min to stabilize, that is, approaching an asymptotic steady state at 90–95%, for example, [22]. Another mechanism is chromatic adaptation to local color contrast [23,24]. Color contrast occurs through the interaction of adjacent surfaces. This process is almost instantaneous [15] and determines color appearance radically [25]. The extent to which adaptation to the spatial mean of a scene and adaptation to local color contrast mediate color constancy was studied by Kraft and Brainard [17] using an achromatic setting task. They also investigated the effect of adaptation to the most intense scene region, which has been suggested to be crucial for color constancy, for example, [26]. The setup used by Kraft and Brainard allowed isolating cues that activate these mechanisms and study of their effects on color constancy separately. Under each condition, observers showed a rather moderate level of color constancy indicating that more than a single mechanism is necessary to achieve high levels of color constancy.

2

Conclusion

The physiology that underpins human color perception is well documented. However, it does not fully explain our everyday experience of colors including our ability to remember and recognize them; for this we need to look at the combination of several retinal and cortical mechanisms that are not yet fully understood.

References 1. Curcio CA, Sloan KR, Packer O, Hendrickson AE, Kalina RE (1987) Distribution of cones in human and monkey retina: individual variability and radial asymmetry. Science 236(4801):579–582 2. Osterberg G (1935) Topography of the layer of rods and cones in the human retina. Acta Ophthalmol Suppl. 6(1):11–97

3. Bowmaker JK, Dartnall HJ (1980) Visual pigments of rods and cones in a human retina. J Physiol 298:501–511 4. Naka KI, Rushton WA (1966) S-potentials from colour units in the retina of fish (Cyprinidae). J Physiol 185(3):536–555

177

178

2.2.6

Color Perception

5. Stockman A, Sharpe LT (2000) The spectral sensitivities of the middle- and long-wavelength-sensitive cones derived from measurements in observers of known genotype. Vis Res 40(13):1711–1737 6. Kaiser PK, Boynton RM (1996) Human color vision, 2nd edn. Optical Society of America, Washington, DC 7. Marks WB, Dobelle WH, MacNichol EF Jr (1964) Visual pigments of single primate cones. Science 143:1181–1183 8. Nathans J, Darcy T, Hogness DS (1986) Molecular genetics of human color vision: The genes encoding blue, green, and red pigments. Science 232(4747):193–202 9. Hurvich LM, Jameson D (1957) An opponentprocess theory of color vision. Psychol Rev 64(6):384–404 10. De Valois RL, Abramov I, Jacobs GH (1966) Analysis of response patterns of LGN cells. J Opt Soc Am 56(7):966–977 11. De Valois RL, De Valois KK, Switkes E, Mahon L (1997) Hue scaling of isoluminant and conespecific lights. Vis Res 37(7):885–897 12. Derrington A, Krauskopf J, Lennie P (1984) Chromatic mechanisms in lateral geniculate nubleus of macaque. J Physiol 357:241–265 13. De Valois RL, De Valois KK (1993) A multi-stage color model. Vis Res 33(8):1053–1065 14. Jameson D, Hurvich LM (1989) Essay concerning color constancy. Annu Rev Psychol 40:1–22 15. Rinner O, Gegenfurtner KR (2000) Time course of chromatic adaptation for color appearance and discrimination. Vis Res 40(14):1813–1826 16. Wichmann FA, Sharpe LT, Gegenfurtner KR (2002) The contributions of color to recognition memory

17.

18.

19.

20. 21.

22.

23.

24.

25.

26.

for natural scenes. J Exp Psychol Learn Mem Cogn 28(3):509–520 Kraft JM, Brainard DH (1999) Mechanisms of color constancy under nearly natural viewing. Proc Natl Acad Sci USA 96:307–312 Kraft JM, Maloney SI, Brainard DH (2002) Surfaceilluminant ambiguity and color constancy: Effects of scene complexity and depth cues. Perception 31(2):247–263 Webster MA (1996) Human colour perception and its adaptation. Netw-Comput Neural Syst 7(4): 587–634 Albright TD, Stoner GR (2002) Contextual influences on visual processing. Annu Rev Neurosci 25:339–379 Zaidi Q, Spehar B, DeBonet J (1997) Color constancy in variegated scenes: role of low-level mechanisms in discounting illumination changes. J Opt Soc Am A 14:2608–2621 Fairchild MD, Lennie P (1992) Chromatic adaptation to natural and incandescent illuminants. Vis Res 32(11):2077–2085 Hurlbert A, Wolf K (2004) Color contrast: a contributory mechanism to color constancy. Prog Brain Res 144:147–160 Webster MA, Mollon JD (1995) Colour constancy influenced by contrast adaptation. Nature 373(6516):694–698 Webster MA, Webster SM, Malkoc G, Bilson AC (2002) Color contrast and contextual influences on color appearance. J Vis 2(6):505–519 McCann JJ, McKee SP, Taylor TH (1976) Quantitative studies in retinex theory a comparison between theoretical predictions and observer responses to the ‘‘color mondrian’’ experiments. Vis Res 16(5): 445–458

Further Reading Kaiser PK, Boynton RM (1996) Human color vision, 2nd edn. Optical Society of America, Washington, DC Ebner Marc (2007) Color constancy. Wiley, West Sussex: England Stockman A, Brainard DH (2010) Color vision mechanisms. In: Bass M (ed) The OSA handbook of optics, 3rd edn. McGraw-Hill, New York

Brainard DH (2004) Color constancy. In: Chalupa L, Werner J (eds) The visual neurosciences. MIT Press, Massachusetts Wandell BA (1995) Foundations of vision. Sinauer Associates, Inc, Massachusetts

2.2.7 Colour Vision Deficiencies Vasudevan Lakshminarayanan 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 2 Types of Color Vision Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 3 Physiological/Genetic Basis of Color Vision Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 4 Color Vision Deficiencies and the CIE Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 5 Acquired Color Vision Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 6 Tests of Color Vision Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.2.7, # Springer-Verlag Berlin Heidelberg 2012

180

2.2.7

Colour Vision Deficiencies

Abstract: We will discuss basic color vision deficiencies, namely, protanomaly, deuteranomaly, dichromacy, and tritanomaly. The effect of these deficiencies on the Commission Internationale de l’Eclairage (CIE) diagram will be discussed. We also discuss the effect of eye diseases on color vision and briefly describe some color vision tests.

1

Introduction

John Dalton presented to the Manchester Literary and Philosophical Society in 1794, (the first of 116 lectures he would give to that body), a talk entitled Extraordinary Facts Relating to the Vision of Colours. In this talk, he discussed his own variant color vision. Thomas Young discussed Dalton’s observations in his own Lectures on Natural Philosophy (1807). Twenty years later, the term Daltonism was coined to describe red-green color deficiency. If a video display is to be used by those with color deficiencies, then it is essential to use color that effectively conveys information to them. In this chapter, we will discuss some aspects of color vision variations or abnormalities.

2

Types of Color Vision Deficiencies

It should be noted that the term ‘‘colorblind’’ is often used to describe all abnormalities in color vision. However, this term is not quite correct since there are only two classes of people who cannot see color and can properly be called colorblind: the rod monochromats or achromats and the cone monochromats. The former refers to people who only have the rod photoreceptors and the latter refers to people who only have rods and one class of cones. Recall that there are three classes of cones: the short wavelength sensitive (S-cones or blue cones), middle wavelength sensitive (M-cones or green cones), and long wavelength sensitive (L-cones or red cones) (see > Chap. 2.1.1). These people are not colorblind, but have color vision that is different from the ‘‘color normal observer.’’ In general, color variant (or deficient) people see a much smaller number of spectral hues than normal observers. In addition the relative luminous efficiency of the eye (i.e., the CIE Vl curve; see > Chap. 2.2.2) is altered and the color matching functions are abnormal [1–3]. Because of this abnormal color matching, there is color confusion – some colors that look different to color normal observers will look identical to people with color deficiencies. People with congenital color deficiencies are born with the defect from birth and this imperfection remains stable throughout life. There are also acquired color vision defects due to pathology. The most common types of congenital color vision defects are based on whether individuals will have difficulty discriminating colors along the red-green axis of the color circle or the blue-yellow axis, with the red-green axis confusion being the most common problem. This kind of deficit occurs in about 8% of males and 0.4% of females in the human population. However, the second type of deficiency, confusion along the blue-yellow axis is rather rare, with a prevalence of about 0.005% of the population [4]. Other facts about the prevalence of color deficiencies are (see below for explanations of the disorders): 1. 2. 3. 4.

0.38% of women are deuteranomalous (around 95% of all color-deficient women). 0.005% of the population are totally color blind. 0.003% of the population have tritanopia. Protanomaly occurs in about 1% of males.

Colour Vision Deficiencies

2.2.7

5. Deuteranomaly occurs in about 5% of males. 6. Protanopia occurs in about 1% of males. 7. Deuteranopia occurs in about 1% of males.

3

Physiological/Genetic Basis of Color Vision Deficiencies

There are significant differences in the cone photopigments of the color-deficient population compared to the color normals. In particular, the red-green color vision defects occur because the photopigments in the L- or M-cones are different or one of the photopigments is not present. These defects have an X-linked recessive inheritance pattern [5], and the color variant population can be further divided into two groups: (a) Protans, whose L-cone photopigment is either missing or the absorption curve is shifted to shorter wavelengths relative to the normal L-cone photopigment and (b) Deutans, whose M-cone photopigment is either missing or the absorption curve is shifted to longer wavelengths. People with a congenital blue-green defect are called tritans, and have S-cones with a nonfunctional or abnormal photopigment. Tritans have an autosomal dominant pattern of inheritance but with variable penetrance (individuals with the same genotype will have variable degrees of severity) [5, 6]. In addition, within each group, we can have dichromats or anomalous trichromats. That is, there are people who have only two classes of functioning cones (dichromats) and people who do have the three cone types, but do not see the world as color normals – the anomalous trichromats. It should be emphasized that the dichromats have the same number of cones as color normals. Dichromatism is the most severe form of the congenital defects. Anomalous trichromats comprise the majority of red-green color defectives. The tritanope appears to have a nonfunctional S-cone and behaves as though he or she only had M- and L-cones in his or her retina. Anomalous trichromats when asked to make a color match, use three primaries (just like color normals); however, the proportions of the primaries in the mixture are outside the normal range. The majority of anomalous trichromats also have reduced color discrimination [7]. Just like dichromats, anomalous trichromats can be classified into two categories: (1) deuteranomalous or ‘‘green weak’’ and (2) protanomalous or ‘‘red weak.’’

. Table 1 Classification of congenital color deficiencies (Adapted from [2]) Number of cone pigments

Type

Denomination

Hue discrimination

None One

Monochromat Monochromat

Rod Monochromat Atypical, incomplete (cone) Monochromat

None

Two

Dichromat

(a) protonope

Severely impaired

Limited ability in mesopic viewing conditions

(b) deuteranope (c) tritanope Three

Anomalous Trichromat

(a) protanomalous (b) deuteranomalous (c) tritanomalous

Continuous range from slight to severe impairment

181

2.2.7

Colour Vision Deficiencies

People who are deuteranomalous require more of the green primary when mixing with a red to form a standard yellow. These individuals have three distinct cone classes, but the M-cone photopigment has an absorption function that is shifted to longer wavelengths relative to the M-cones found in color normals. It has been speculated that this photopigment could actually be a hybrid L-cone photopigment that has an absorption spectrum shifted to a shorter wavelength compared to the other L-cone pigment they have in the retina [6–8]. In contrast, protanomalous individuals require more red when making a standard yellow match. Similar to protanopes they have a decreased sensitivity to red. The L-cone photopigment in these individuals has an absorption curve that is shifted to shorter wavelengths relative to the color normal L-cone absorption. Analogous to the deuteranomalous case, the anomalous protanomalous pigment is actually a hybrid M-cone photopigment that absorbs light at slightly longer wavelengths than the other M-cone [6–8]. The classification of congenital color vision deficiency is given in > Table 1.

0.9 520

Protan colour confusion lines

0.8 540

510 0.7

Green 0.6

570 500

Yellow

0.5 y

182

Orange 600 Red

0.4 Grey White

Blue green 0.3 490

700 0.2 Purple 480

Blue

0.1

400 0.0 0.0

0.1

0.2

0.3

0.4 x

0.5

0.6

0.7

0.8

. Fig. 1 Color confusion lines for a Protan, based on Pitts’s data and plotted on the 1931 CIE chromaticity diagram

2.2.7

Colour Vision Deficiencies

4

Color Vision Deficiencies and the CIE Diagram

Color discrimination by color variant observers can be analyzed easily using a CIE chromaticity diagram (see > Chap. 2.2.2). The ‘‘color confusion lines’’ are shown in > Figs. 1 and > 2 for protanopes and for deuteranopes. The tritanopic color confusion line is shown is > Fig. 3. These lines are based on extensive psychophysical color matching and hue discrimination experiments [see, e.g., reference 9]. The orientation and spacing of the color confusion lines represents the averages of data obtained from several dichromats using a 2 field of view, and of moderate exposure duration. If the field size or the duration is different from that used by Pitts [10], from which the figures are derived, individual dichromat performance might vary from predictions based on color confusion lines. Colors lying on the same line will appear identical if the luminances are equal. Each line in the figures corresponds to the colors that require the same ratio of the two primaries that the observer uses to match colors. Different lines represent different ratios and therefore the colors on different lines should appear different even when

0.9 520

Deutan colour confusion lines

0.8 540

510 0.7

Green 0.6

570 500

Yellow

y

0.5

Orange

0.4

Grey

600 Red

White

Blue green 0.3 490

700 0.2 Purple 480 0.1

Blue

400 0.0 0.0

0.1

0.2

0.3

0.4 x

0.5

0.6

0.7

0.8

. Fig. 2 Color confusion lines for a Deutan, based on Pitts’s data and plotted on the 1931 CIE chromaticity diagram

183

2.2.7

Colour Vision Deficiencies

0.9 520 Tritan colour confusion lines 0.8 540

510 0.7

Green 0.6

570 500

Yellow

0.5 y

184

Orange 600 Red

0.4 Grey White

Blue green 0.3 490

700

0.2 Purple 480

Blue

0.1

400 0.0 0.0

0.1

0.2

0.3

0.4 x

0.5

0.6

0.7

0.8

. Fig. 3 Color confusion lines for a Tritan, based on Pitts’s data and plotted on the 1931 CIE chromaticity diagram

they are equal in luminance. The distance between any two lines represents a just noticeable difference (JND) in color. The area is referred to as the zone of confusion because any colors that fall within a given zone will appear identical to a person with dichromatic defect. For example, the red, green, and yellow colors all fall near the same solid line and these colors will appear identical to the protanope if they are equal in luminance. The number of confusion lines indicates that protanopes and deuteranopes can only distinguish 21 and 31 distinct wavelengths respectively. However, color normals can distinguish 150 distinct wavelengths [10]. Protans and deutans will have difficulty in discriminating between greens, yellows, oranges, and reds. The tritans will have difficulty in discriminating between gray and white, gray and yellow, gray and green, green and dark green, and blue and blue green. They can distinguish only 44 distinct wavelengths. The color discrimination performance of anomalous trichromats will fall in between the color normals and the dichromats. Their confusion zones will appear as a series of ellipses (> Fig. 4) with the major axes of each ellipse along the corresponding dichromatic confusion

2.2.7

Colour Vision Deficiencies

0.9 520 0.8 540

510 0.7

0.6

570 500

y

0.5

0.4

600 White

0.3 490 700 Normal 0.2 480 0.1 400 0.0 0.0

0.1

0.2

0.3

0.4 x

0.5

0.6

0.7

0.8

. Fig. 4 Color confusions obtained for deuteranomalous observers. The length of the major axis indicates the severity. The gray lines are the deuteranope’s color confusion lines

zone, but shorter than that of dichromats as they do not include complete range of confusions as in the case of dichromats. It should be noted that the length of the major axes of the ellipse varies with the severity of the defect [12].

5

Acquired Color Vision Diseases

Injury or diseases of the eye or the neural pathways can result in color vision changes. These are called acquired color vision defects and Kollner (1912) proposed one of the first classifications of acquired color vision deficiency to the location of the pathology. Kollner’s law as it is known, says that blue deficiencies develop from diseases of the retina while red-green deficiencies develop from diseases of the pathways from the inner layers of the retina to the visual cortex. Pokorny and Smith [3] state that this rule is valid even though there are occasional contradictions [13]. In general, acquired color vision deficiencies can occur in diseases such as cone degeneration, optic nerve disorders, vascular disorders, and glaucoma. The most common

185

186

2.2.7

Colour Vision Deficiencies

a

Normal vision

Protanope vision

Deuteranope vision

Tritanope vision

b . Fig. 5 a Pseudo Isochromatic Plates: Ishihara plates. b Appearance of pseudoisochromatic plates to normal and color-deficient observers

form of acquired color vision defect occurs as a natural part of aging – with age, the crystalline lens of the eye absorbs progressively more light at the shorter wavelengths. Therefore, there is a loss of color discrimination.

6

Tests of Color Vision Deficiencies

There are many techniques to detect color vision deficiencies in the clinic and in the laboratory. The ‘‘gold standard’’ for color vision testing is the anomaloscope, which uses the Rayleigh

Colour Vision Deficiencies

2.2.7

. Fig. 6 The Farnsworth Munsell (FM) 100-hue test

match procedure for diagnosing the four major X-linked color defects. Here the observer adjusts the relative energies of 670 nm and 546 nm primaries to do an additive match of a 589 nm reference (see > Chap. 2.2.2). Most color vision testing is based on a knowledge of the color confusion lines. The most common are test books/plates. Here the colors of the figures and background are chosen to lie close to the confusion lines. The most common test is the Ishihara color test plate (See > Fig. 5a and b). A similar test is the Hardy–Rand–Rittler color test plates. Clinically the most commonly used test is the Farnsworth D-15 test (and its desaturated versions). The test colors are the so-called Munsell colors. The Farnsworth Munsell (FM) 100-hue test is the most complicated of the lot – consisting of 85 caps from 100 possible hues of the Munsell color ordering system (> Fig. 6). There are also color tests such as the Cambridge color test and the City University color test that are monitor based. However, caution should be used since the monitors have to be calibrated in order to get reliable results. The reader is referred to the book by Birch [2] for detailed discussions. In terms of video displays, it is essential that color be varied in terms of intensity and saturation and not in terms of hue for color-deficient observers. Single color distinction should be avoided in such displays. Color contrast can be achieved by varying both chromaticity and luminance. Varying luminance alone can also benefit the color-deficient person. Finally, Kovacs et al. [14] have designed a filter that can be used along with a video display and can compensate for the color deficiency. In order to use this method, a thorough knowledge of the exact shape of the response function of abnormal and anomalous cones is necessary.

7

Summary

In this chapter, we have described the characteristics, epidemiology, and genetics of common color vision deficiencies. The effect of these deficiencies as analyzed on a CIE diagram was also presented. In addition to genetic color vision defects, it is also possible to acquire color vision deficiencies as a result of disease. Finally, we described briefly some color vision tests.

187

188

2.2.7

Colour Vision Deficiencies

References 1. Kaiser PK, Boynton RM (1996) Human color vision, 2nd edn. Optical Society of America, Washington, DC 2. Birch J (2001) Diagnosis of defective color vision, 2nd edn. Butterworth-Heinemann, Oxford 3. Pokorny J, Smith VC, Verriest G, Pinckers AJLJ (1979) Congenital and acquired color vision defects. Grune and Stratton, New York 4. Delpro WT, O’Neill H, Casson E, Hovis J (2005) Aviation relevant epidemiology of color vision deficiency. Aviat Space Environ Med 76:127–133 5. Wissinger B, Sharpe LT (1998) New aspects of an old theme: the genetic basis of human color vision. Am J Hum Genet 63:1257–1262 6. Neitz M, Neitz J (2000) Molecular genetics of color vision and color vision defects. Arch Ophthalmol 118:691–700 7. He JC, Shevell SK (1995) Variation in color matching and discrimination among deutranomalous trichromats: theoretical implications of small differences in photopigments. Vis Res 35:2579–2588 8. Shevell SK, He JC, Kainz P, Neitz J, Neitz M (1998) Relating color discrimination to photopigment genes in deutan observers. Vis Res 38:3371–3376

9. Wyszecki G, Stiles WS (1982) Color Science, 2nd edn. Wiley, New York 10. Pitt FHG (1935) Characteristics of dichromatic vision, Report of the Committee on the Physiology of Vision: XIV Special report series. Medical Research Council of Britain, His Majesty’s Stationary Office, London 11. Sharpe LT, Stockman A, Jagle HJN (1999) Opsin genes, cone photopigments, color vision and color blindness. In: Gegenfurtner KR, Sharpe LT (eds) Color vision, from genes to perception. Cambridge University Press, Cambridge, pp 3–51 12. Birch-Cox J (1974) Isochromatic lines and the design of color vision tests. Mod Probl Ophthalmol 13:8–13 13. Schnek ME, Hagerstrom-Portnoy G (2002) Color vision defect type and spatial vision in the optic neuritis trial. Invest Ophthalmol Vis Sci 42:29–39 14. Kovacs G, Abraham G, Kucsera I, Wenzel K (2000) Improving color vision for color deficient patients on video displays. In: Lakshminarayanan V (ed) Vision science and its applications, vol 35, Trends in optics and photonics. Optical Society of America, Washington DC, pp 333–337

Further Reading Gegenfurtner K, Sharpe LT (1999) Color vision. Cambridge University Press, Cambridge Norton TT, Corliss DA, Bailey JE (2002) The psychophysical measurement of visual function. ButterworthHeinemann, Woburn, Massachusetts, pp 217–288, Chap 8

Shevell SK (2003) The science of color, 2nd edn. Elsevier, Oxford Valberg A (2005) Light vision and color. Wiley, New York

Part 2.3

Visual Ergonomics

2.3.1 Displays in the Workplace Sarah Sharples 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 2 Challenge 1: Continuing to Consider ‘‘Traditional’’ Personal Computing . . . . . . . . . 193 3 Challenge 2: The Changing Nature of Displays in the Workplace . . . . . . . . . . . . . . . . . . 194 4 Challenge 3: The Diverse Context of Use of Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 5 Challenge 4: Predicting How Displays Will Be Used in the Future . . . . . . . . . . . . . . . . 198 6 Challenge 5: Displays as Part of an Interactive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.3.1, # Springer-Verlag Berlin Heidelberg 2012

192

2.3.1

Displays in the Workplace

Abstract: The way in which we use displays in the workplace is changing, with increasing diversity in display type, tasks, and context of use. This chapter outlines five challenges that we need to consider when designing and implementing displays in the workplace: (1) The need to continue to consider ‘‘traditional’’ personal computing; (2) The changing nature of displays in the workplace; (3) The diverse context of use of displays; (4) The need to predict how displays will be used in the future; and (5) Considering displays as part of an interactive system. The chapter presents a set of display/human factors considerations that should be remembered when designing and evaluating displays in the workplace.

1

Introduction

We typically spend between 30 and 45 h per week at work, in addition to time spent traveling to and from our workplace. The displays in our workplace are therefore probably the ones that we encounter and interact with more than any others. However, the nature of work, work environments, and the displays we use is continually changing, and developments such as wireless and mobile communication technologies, large screen displays, and portable computing mean that where 10 years ago most work users had a single dedicated workplace with one cathode ray tube computer screen, the displays with which we interact on a daily basis in conducting our work are of a wide range of types and forms. This chapter considers the changing nature of displays in the workplace. It then presents five key challenges that designers of displays in the workplace must address if we are to ensure that future workplaces are as safe, effective, usable, and comfortable as possible in the future. The range and type of displays that we might encounter, along with the tasks that we complete using these displays, during a typical day, varies considerably. > Table 1 presents a selection of these examples and demonstrates that we encounter a wide diversity of display size and form, uses of displays including preparation of documents, analysis, review or monitoring, and a range of roles of displays and associated technologies in communication and collaboration.

. Table 1 A selection of typical displays and tasks encountered by everyday users Example display

Example tasks

Smartphone/mobile device

Email, phone call, application, web page viewing

Stand-alone desktop LCD/TFT display

Email, word processing, CAD, shared viewing of collaborative work

Large screen projection or flat screen display

Lecture/presentation, entertainment, shared decision making viewing (2D or 3D)

In car technologies

Satellite navigation, control of entertainment/communications system

Handheld device

Signature for online shopping, collection of inventory/asset management data

Integrated device display (e.g., laptop)

Email, graphics/word processing, video viewing

Displays in the Workplace

2.3.1

Perry and O’Hara [1] identified three key requirements for display-based activity in the workplace: 1. Ready access to information 2. Social orientation 3. Coordination and planning This chapter considers these three requirements in the context of different types of workplace tasks and activities. If we are to design display technologies in order to effectively consider human factors requirements, we need to ensure that any assessment methods or design techniques applied are appropriate to the different types of work display use that we may encounter now and in the future. The following sections of the chapter articulate five key challenges that we need to take into account in designing workplace displays. The conclusion then draws upon these challenges to identify the priorities for consideration in display design for the display user, task, and environment as well as the display technology itself.

2

Challenge 1: Continuing to Consider ‘‘Traditional’’ Personal Computing

It would be tempting to consider that, as we embrace a wide range of novel technologies, we can move our focus away from the standard office computer. However, the vast majority of workers still have either a dedicated space or ‘‘hot desk’’ setup containing the typical components of a screen, keyboard, mouse, and processor. Call-center work is prevalent (in 2007 it was estimated that there were 647,000 people in the UK working in call centers (BBC news website February 14, 2007)). In a report for the UK Health and Safety Executive Sprigg, Smith, and Jackson [2] found that on average workers spent 2 h 30 min continually using a display screen without a break, as part of a typical shift pattern (7–9 h shifts); for the majority of respondents over 75% of the day was spent using display screen equipment. Of the 1,141 call-center employees surveyed, 53% used hot desks and 42% reported that they were moderately or very dissatisfied with glare or reflection on their display screen. Pheasant and Haslegrave [3] (p. 161) specify the typical office workstation as containing a desk, chair, and computer at which a user will undertake paper- and screen-based tasks. They note that the ‘‘paperless’’ office has not become as prevalent as was predicted in the 1980s and remind us that in many ways the standard desk, chair, computer setup has become even more prevalent, with screenbased displays replacing physical or manual control panels in many contexts. In addition to this, many of us, either formally or informally, work at home on desktop computers. The setup of a traditional workstation may vary for individuals; individuals may vary in their preferred viewing distance [4] and screen height [5], for example. In addition, many workers may use a laptop or notebook portable computer for a significant part of their working day. Typically, the height of a laptop screen will be lower than a fixed setup of a separate screen on a desk – primarily due to the context in which the laptop is used (e.g., train, home living room, airport lounge). Pheasant and Haslegrave also note that a laptop screen is typically often only legible when viewed at a narrower range of angles. This property, anisotropy, is encountered when there is a deviation of luminance of more than 10% depending on target location or viewing angle [6, 7] and studies have demonstrated that if a viewing angle is between 10 and 50 off axis, performance will deteriorate [8, 9].

193

194

2.3.1

Displays in the Workplace

Whilst Heasman et al. [10] found no major difference between discomfort experienced by users of standard and portable PC setups, it still remains important to acknowledge that these types of displays are likely to be the primary form of viewing and interacting with information for most office-based workers. Although regulations and advice regarding the setup of the traditional personal computer has been available for several decades, it appears that users do still experience discomfort or dissatisfaction when using such displays.

3

Challenge 2: The Changing Nature of Displays in the Workplace

For over a decade, researchers have been debating the way in which our workplaces will change in their nature and form (e.g., [11, 12]). The human desire for collaboration and communication is acknowledged. Ironically, the increased prevalence of open-plan offices does not necessarily encourage collaboration due to the impact of noise disturbance; as a consequence, open-plan workplaces can tend to be quiet and not conducive to collaboration with physically colocated colleagues. This fits with the category of social orientation proposed by Perry and O’Hara and potentially increases the likelihood of displays being used to support either textbased communication (both email and instant messaging) as well as voice-based systems such as Skype. Displays are getting both larger and smaller in a number of contexts. For example, in rail control, Wilson and Morrisroe [13] note that traditional hard-wired panel displays are increasingly being replaced by banks of small screens, usually in combination with larger overview displays. > Figure 1 shows an example of a hard-wired display which comprises a combination of fixed static elements printed on the panel, accompanied by a combination of screens and LED indicators.

. Fig. 1 Example of hard-wired panel display in rail

Displays in the Workplace

2.3.1

> Figure 2 shows an example of the type of control environment now more typically seen – taken from a Finnish power control environment. It can be seen that within this environment there is a combination of large screen, small screen, and of course paper displays. This combination of displays presents challenges in terms of viewing distances and lighting levels as well as consistency of interactive method and information presentation method – for example, typically the lighting level requirements for a light emitting or projected display will be lower than that ideally used for a paper display, due partly to the luminance of the display itself contributing to the overall light levels. A typical situation that we encounter when viewing a presentation in a meeting or conference is that the lighting that results in the most clearly viewed display are lower than those needed for making personal notes on paper. As well as large screen displays, we are more frequently using small screens – both as a primary work tool and a communication tool that is used in our work and home environment. Asset management has been revolutionized by the use of handheld technologies – we see handheld displays being used by couriers and those delivering goods to our work and home, and increasingly handheld technologies are being used for quality monitoring and recording in a manufacturing environment as well as in the context of engineering and maintenance. The challenge of presenting information on small screens has been acknowledged [14, 15]; yet we maintain the desire to view increasingly large and complex data sets on increasingly small screens. These complex and detailed small screen displays present ergonomic challenges regarding viewing distance, brightness, and resolution. It is often the case that the size of the text or displays on such screens is small, and the postures used when viewing such technologies may be less comfortable and controlled than a dedicated workstation; however, this may not be of a concern if such small devices are only used for small periods of the day. Therefore, it is important for those involved in the design and development of such devices to maintain an

. Fig. 2 Example of typical multiple monitor display with overview screen (Image: Fingrid Oyj/Juhani Eskelinen)

195

196

2.3.1

Displays in the Workplace

. Fig. 3 Mixed reality architecture display [16]

understanding of how such displays are being used, and, if they do become the primary tools for work, ensure that good work design guidelines are implemented. Displays are also getting larger, and we are increasingly using displays to support collaboration. Large shared screens facilitate colocated collaboration, and technologies such as Internet-mediated video messaging and dedicated collaborative meeting applications allow us to conduct ‘‘face to face’’ meetings (distributed collaboration) via technology. One such type of system is the Mixed Reality Architecture (see > Fig. 3). This is an ‘‘always on’’ technology that supports collaboration between multiple users via a display that presents a different view of the shared environment depending on the position of the user’s ‘‘pod’’ within the virtual space. However, the use of a display for collaboration does not automatically imply a large screen – we may use technologies such as instant messaging via a standard PC or text messaging (SMS) via a mobile device. In addition, we are increasingly taking advantage of 3D displays. VR technologies have increased their capacity for high resolution and real-time rendering, and such technologies are now being used in a range of applications including the medical, automotive, and design fields. These display types present additional challenges relating to visual strain, sickness [17], and comfort (see > Sect. 9, 3D Displays). Finally, our notion of the workplace has extended. Mobile technologies extend communications to allow work to be completed at home and whilst traveling; the concept of the ‘‘office’’ being our normal or regular space where we complete work may be limiting.

4

Challenge 3: The Diverse Context of Use of Displays

Displays in the workplace are not always used as originally predicted, intended, or designed. An obvious example of this is found in the control contexts where multiple standard CRT/flat screen displays are used by a single operator. The use of these multiple displays means that the standard guidance regarding font size, viewing angle, and viewing distance are not directly

Displays in the Workplace

2.3.1

. Fig. 4 Envisionment of future collaboration in visualization of prototypes (from VIEW of the Future)

transferable – a viewer of multiple displays will typically be seated further away from an individual screen and will not be viewing the screen from directly in front of the display. As mentioned before, the context of use of laptops is also unpredictable – as well as the impact of this on user comfort, display angle, and brightness may also vary. In addition, users may adjust settings on portable computers for reasons other than ease of use – for example, a user may reduce the brightness of a portable display to preserve battery life, regardless of the impact this may have on viewing comfort. Oetjen and Ziefle [6] note that in fact, whilst LCDTFTscreens, such as seen in laptops are typically intended for a single user, in fact they are often used in collaborative contexts, such as radiology or patient monitoring and in schools. An alternative scenario to this is the typical view of a number of people gathered around a small screen, perhaps looking over the main ‘‘workspace’’ user’s shoulder. With flat screen displays in particular, this can lead to difficulties, as the angle in front of the screen from which it is possible to clearly see the data may be limited. As new technologies such as e-paper emerge, it is critical that we apply methods to enable us to understand and predict not only the type of display that will be used, but also the context in which that display will be implemented. In the EU-funded project VIEW of the future, a scenario was envisaged where multiple users may in fact interact with the same display via a number of input devices, including handheld screens with displays of their own. > Figure 4 shows an example of one such envisionment – one user, either colocated or in a different location, can interact with the system via a handheld mobile device, whilst other physically present users can use devices such as 3D joysticks to move the viewpoint or interact with the object (e.g., opening and closing car doors, changing the color of the car). This illustrates that the context of display use can even vary for multiple users of a single system.

197

198

2.3.1 5

Displays in the Workplace

Challenge 4: Predicting How Displays Will Be Used in the Future

Displays have changed considerably in the past 20 years. We have moved to larger and smaller displays, technologies that allow flat screens will in the future extend into e-paper amongst other display types. Projected image quality has improved and the weight, size, and lag of 3D headsets/glasses have decreased, accompanied by an increase in resolution. However, if we are to design effective and useful displays, we need to anticipate the workspace of the future. In 1998, Dalton et al. [12] proposed the concept of a Smartspace, a personal workspace that enabled information visualization, manipulation, and exchange high bandwidth connectivity and a combination of a large, curved ‘‘immersive’’ screen with a flat touch screen and sophisticated sound (> Fig. 5). Whilst this concept has not manifested in the exact form proposed, elements can be seen in the Apple iPad and other types of high spec displays. The EU project SATIN [18] identified a potential future application of technologies in multimodal interfaces for visualization of virtual prototypes. This interface (> Fig. 6 ) combined visual, haptic, and auditory information to convey the shape and form of an object in design stage. A user visually sees the representation of a virtual object whilst the haptic strip (in the figure on the left) takes the form of the section of the visual object as highlighted, and the accompanying sound varies in pitch to represent the curvature value of any point on the curve being explored. This prototype technology is an illustration of the way in which future

. Fig. 5 Demonstration of the Smartspace concept (image provided with permission of BT (A. Gower))

Displays in the Workplace

2.3.1

. Fig. 6 SATIN (Sound and Tangible Interface for Product Design) prototype

technologies may facilitate the integration of multiple modalities of display – whilst the development of visual displays technology is of course critical, we also need to understand how the visual elements complement or contradict the information presented by other modalities. It is critical that future technologies are anticipated to support good ergonomic design. Whilst this is difficult, there are some methods available to support this. A road map exercise was conducted as part of an activity to anticipate the types of technology that might be used in future workplaces to support activities related to design.

6

Challenge 5: Displays as Part of an Interactive System

The final challenge facing the design of current and future displays in the workplace is that increasingly displays are not separate devices or monitors, but inherently integrated into the device itself. We have long moved past the assumption that the display is a separate device such as a monitor, and in fact arguably the situation in which the display is still a separate device, apart from on a desktop PC, is in shared projected or large screen displays. For a laptop or similar device, the display is part of the same physical device as the keyboard, and increasingly the display essentially is the device – as Weiss [15] pointed out increasingly many phonepersonal digital assistant combination devices do not have a separate keyboard and instead include an on-screen keyboard operated by touch technologies utilising a stylus or finger based system, see > Chap. 5.7.1. The implication of this is that even when the device is separate, increasingly it is only meaningful to evaluate the impact, effectiveness, usability, and appeal of a display in combination with other peripheral devices or interaction metaphors used. Work completed as part of a project called ‘‘VIEW of the Future’’ (see > Fig. 7 for image of sample interaction device, menu, and display) demonstrated that it was not appropriate to evaluate the display and its contents alone, but that perceived usability and performance with the display resulted from an interaction between the device and menu designs. In this example, a device creates a wand effect of a ‘‘laser beam’’ that is used to select part of the display with the required level of accuracy.

199

200

2.3.1

Displays in the Workplace

. Fig. 7 Example of interaction metaphors designed in VIEW of the Future

Current smartphones also feature integrated displays and interaction devices, and increasingly work is focusing on the use of gestures and movement to enhance interaction – for example, you may rotate your screen through 90 to select either a portrait or landscape view of the screen. It is likely that this integration of displays and interaction mechanisms will increase as innovation continues.

7

Conclusion

This chapter has outlined five key challenges facing design of displays in the workplace as follows: 1. Despite the increasing variety in display types, we must not forget to continue to develop the quality and form of conventional computer monitors and displays. 2. The nature of displays in the workplace is changing, and increasingly includes collaborative displays and multiple devices per individual user. 3. We can no longer assume that a worker will have a single dedicated workplace – many people transit between work, home, and a mobile location when completing their work, and the increasing mobility of devices means that displays are often used in a divers set of contexts. 4. We need to keep ahead of technological developments, and it is important to develop techniques that allow us to envision the way in which devices and displays will be used in the future. 5. We can no longer assume that the display is a separate device – it may be physically integrated with other peripherals or interaction devices or may be integrated into the main device itself as in a smartphone or PDA. We therefore need to ensure that we design the display and interaction techniques as a whole, rather than considering the separate components in isolation.

Displays in the Workplace

2.3.1

It is useful to articulate these challenges in terms of characteristics associated with the user, task, technology, and context (see > Table 2) to highlight the diversity of areas that should be considered when design and evaluating displays in the workplace. Displays in the workplace are changing, diversifying, and developing. Whilst display quality and power is increasingly improving, developers are building new device types and forms, and users are continually finding new work-related tasks and contexts of use in which to employ them. Device designers and evaluators must ensure that we track these developments carefully and acknowledge the impact of device design and type on user use and interaction. . Table 2 Examples of influential factors affecting displays in the workplace and impact of factors on display/human factors considerations

Influential factor User

Examples of impact on display/human factors consideration

User experience

Extent to which display has been customized; ability of user to select appropriate display settings (e.g., scale, brightness)

User visual characteristics

Eyesight (e.g., scale, resolution), color blindness

User linguistic experience/ Type of text displayed (e.g., character vs letter based), ability use of icons vs. text Task

User preference

Color, content (e.g., text vs. icons) selected

Office-based tasks (e.g., word processing, email)

Design of stand-alone peripheral display

Short tasks (e.g., email, SMS)

Use of mobile devices to complete typing and reading task

Reading/individual viewing

Desire to sit/stand in range of locations (e.g., whilst traveling, on sofa)

Shared viewing

Size and resolution of large screen or projected display

Viewing objects or images Resolution and color display requirements for CAD, video, film, or text displays Technology Conventional PC monitor

Context

Continue to consider standard display screen equipment design considerations (e.g., height of screen, angle of screen)

Laptop

Screen angle, battery use, resolution, contrast

Mobile device

Environmental conditions (rain, brightness of ambient environment), use of display as interaction device

E-paper

Requirements for lighting in ambient environment, resolution, contrast

Conventional office setting Standard display screen equipment design considerations Mobile

Varying requirements depending on environmental conditions, postural impact of use of mobile display for extended period of time

Home setting

Use of displays in comfort, prolonged use of ‘‘occasional’’ work settings

201

202

2.3.1

Displays in the Workplace

Acknowledgements The author of this chapter is partially funded by Horizon Digital Economy Research, through the support of RCUK grant EP/G065802/1.

References 1. Perry M, O’Hara K (2003) Display-based activity in the workplace. In: Proceedings of human-computer interaction conference, INTERACT ’03. IOS, Amsterdam 2. Sprigg CA, Smith PR, Jackson PR (2003) Psychosocial risk factors in call centres: An evaluation of work design and well-being. HSE Research Report 3. Pheasant S, Haslegrave CM (2006) Bodyspace: anthropometry, ergonomics and the design of work. CRC, London 4. Jackinsi W, Heuer H, Kylian H (1999) A procedure to determine the individually comfortable position of visual displays relative to the eyes. Ergonomics 42(4):535–549 5. Jackinsi W, Heuer H (2004) Vision and eyes. In: Delleman NJ, Haslegrave CM, Chaffin DB (eds) Working postures and movements. CRC, Boca Raton, pp 73–86 6. Oetjen S, Ziefle M (2009) A visual ergonomic evaluation of different screen types and screen technologies with respect to discrimination performance. Appl Ergon 40:69–81 7. International Organisation for Standardization (2001) ISO 13406-2: Ergonomic requirements for work with visual displays based on flat panels – Part 2: Ergonomic requirements for flat panel displays. ISO, Geneva 8. Oetjen S, Ziefle M, Groger T (2005) Work with visually suboptimal displays: in what ways is the visual performance influenced when CRT and TFT displays are compared? In: Proceedings of the HCI International. Vol 4: Theories, Models and Processes in Human Computer Interaction. Mira Digital Publishing. CD-ROM

9. Oetjen S, Ziefle M (2007) The effects of LCDs’ anisotropy on the visual performance of users of different ages. Hum Factors 49(4):619–627 10. Heasman T, Brooks A, Stewart T (2000) Health and safety of portable display screen equipment. HSE Books, Sudbury 11. Tanaka R (2002) Future workplace design. Displays 23(1):41–48 12. Dalton G et al (1998) The design of SmartSpace: a personal working environment. Pers Technol 2(1):37–42 13. Wilson JR, Morrisroe G (2005) Systems analysis and design. In: Wilson JR, Corlett EN (eds) Evaluation of human work, 3rd edn. Taylor & Francis, London 14. Jones M, Marsden G (2006) Mobile interaction design. Wiley, Chichester 15. Weiss S (2002) Handheld usability. Wiley, Chichester 16. Schna¨delbach H, Penn A, Steadman P (2007) Mixed reality architecture: a dynamic architectural topology. In: Space syntax symposium. Technical University Istanbul, Istanbul 17. Sharples S, Stedmon AW, D’Cruz M, Patel H, Cobb S, Yates T, Saikayasit R, Wilson JR (2007) Human factors of virtual reality – where are we now? In: Pikaar RN, Koningsveld EAP, Settels PJM (eds) Meeting diversity in ergonomics. Elsevier, Amsterdam. 18. Sharples S, Hollowood J, Lawson G, Pettitt M, Stedmon A, Cobb S, Coloso C, Bordegoni M (2008) Evaluation of a multimodal interaction design tool. in Create 2008. In: Proceedings of the conference on creative inventions, innovations and everyday designs in HCI. London

2.3.2 Display Screen Equipment: Standards and Regulation Sarah Atkinson 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

2

International Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

3

BS EN ISO 9241 The Ergonomics of Human-System Interaction . . . . . . . . . . . . . . . . . 204

4

ANSI/HFES 100–2007 Human Factors Engineering of Computer Workstations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 4.1 Other Relevant Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 5

Case Study: Implementation of Health and Safety Law in the UK . . . . . . . . . . . . . . 209

6

The Health and Safety (Display Screen Equipment) Regulations . . . . . . . . . . . . . . . . . 210

7

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.3.2, # Springer-Verlag Berlin Heidelberg 2012

204

2.3.2

Visual Ergonomics, Standards and Display Screen Equipment

Abstract: The increasing amount of time spent in front of display screens, involving, sometimes, prolonged and daily use has led to an expanse of legislation and recommendations to encourage employers to identify and adopt design features and work practices which minimize any risk to computer users and others using Display Screen Equipment (DSE). As technologies change, the risks and the measures needed to address them change too. This chapter outlines the existing international legal framework for DSE, the standards and guidance, which exists and considers its application to both current and future technologies. An example of the implementation of DSE standards, regulations and guidance is discussed. Lists of Abbreviations: ANSI, American National Standards Institute; DSE, Display Screen Equipment; HFES, Human Factors and Ergonomics Society; ISO, International Organization for Standardization; VDU, Visual Display Unit

1

Introduction

Visual displays can be found in offices, in shops, on the factory floor, in laboratories and in control rooms. A very high proportion of the workforce now uses a computer or computercontrolled equipment as an integral part of their work. Display screen equipment (DSE) is any work equipment which has a screen that displays information. The most prolific type is the computer, whether a desktop PC with a visual display unit (VDU) or a laptop with an integral screen. However, DSE can also refer to other types of equipment and some of ‘‘nonstandard’’ design such as those found in control rooms for process monitoring or for communication and travel. This chapter will outline the legislation and standards relevant to DSE use.

2

International Standards

The International Organisation for Standardisation (ISO) is the largest developer and publisher of International standards with members from over 150 countries. Standards are developed under the auspices of technical committees and subcommittees with technical development undertaken by working groups. The objective of a Standard is to define clear and unambiguous provisions in order to facilitate international trade and communication. To achieve this objective, the Standard shall be as complete as necessary; consistent, clear and concise; and comprehensible to qualified persons who have not participated in its preparation. Such standards are regarded as advisory, but not mandatory.

3

BS EN ISO 9241 The Ergonomics of Human-System Interaction

ISO 9241 originated as a multipart standard entitled Ergonomics requirements for office work with visual display terminals. It has been an influential standard, referenced in many countries’ regulations. The interest in ISO 9241 encouraged the standard subcommittees to broaden its scope, to incorporate other relevant standards, and to make it more usable, now entitled The Ergonomics of Human-system interaction [1]. The structure and overview of each part is shown in > Table 1.

Visual Ergonomics, Standards and Display Screen Equipment

2.3.2

. Table 1 Overview of ISO 9241 Relevant part of ISO 9241

Overview

ISO 9241-1:1997/ Amd 1:2001

General Introduction This part introduces the multipart (supersedes ISO 9241-1:1993) standard ISO 9241 for the ergonomic requirements for the use of visual display terminals for office tasks and explains some of the basic underlying principles. It provides some guidance on how to use the standard and describes how conformance to parts of ISO 9241 should be reported

ISO 9241-11:1998

Guidance on usability

Defines usability as ‘‘Extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use’’ and provides guidance on how to address usability in design projects

ISO 9241-20:2008

Accessibility guidelines for information/communication technology (ICT) equipment and services

A high-level overview standard covering both hardware and software. It covers the design and selection of equipment and services for people with a wide range of sensory, physical, and cognitive abilities, including those who are temporarily disabled and the elderly

ISO 9241 110:2006

Dialogue principles (supersedes ISO 9241 10:1996)

Sets out seven dialogue principles and gives examples. The dialogue should be suitable for the task (including the user’s task and skill level); self-descriptive (it should be obvious what to do next); controllable (especially in pace and sequence); conform to user expectations (i.e., consistent); error tolerant and forgiving; suitable for individualization and customizable; and should support learning

ISO 9241-14:1997

Menu dialogues

Recommends best practice for designing menus (pop-up, pull-down, and textbased menus). Topics include menu structure, navigation, option selection, and menu presentation (including placement and use of icons). One of the annexes contains a ten-page checklist for determining compliance with the standard

205

206

2.3.2

Visual Ergonomics, Standards and Display Screen Equipment

. Table 1 (Continued) Relevant part of ISO 9241

Overview

ISO 9241-15:1998

Command dialogues

This part provides recommendations for the ergonomic design of command languages used in user-computer dialogues. The recommendations cover command language structure and syntax, command representations, input and output considerations, and feedback and help. Part 15 is intended to be used by both designers and evaluators of command dialogues, but the focus is primarily towards the designer

ISO 9241-16:1999

Direct manipulation dialogues

This part provides recommendations for the ergonomic design of direct manipulation dialogues, and includes the manipulation of objects and the design of metaphors, objects, and attributes. It covers those aspects of ‘‘Graphical User Interfaces’’ which are directly manipulated, and not covered by other parts of ISO 9241. Part 16 is intended to be used by both designers and evaluators of command dialogues, but the focus is primarily toward the designer

ISO 9241-17:1998

Form-filling dialogues

This part provides recommendations for the ergonomic design of form-filling dialogues. The recommendations cover form structure and output considerations, input considerations, and form navigation. Part 17 is intended to be used by both designers and evaluators of command dialogues, but the focus is primarily toward the designer

ISO 9241-151:2008

Guidance on World Wide Web user interfaces

Sets out detailed design principles for designing usable web sites – these cover: high-level design decisions and design strategy; content design; navigation; and content presentation

ISO 9241-171:2008

Guidance on software accessibility

Aimed at software designers and provides guidance on the design of software to achieve as high a level of accessibility as possible. Replaces the earlier Technical Specification ISO TS 16071:2003 and follows the same definition of accessibility – ‘‘usability of a product, service, environment or facility by people with the widest range of capabilities.’’Applies to all software, not just web interfaces

Visual Ergonomics, Standards and Display Screen Equipment

2.3.2

. Table 1 (Continued) Relevant part of ISO 9241

Overview

ISO 9241-300:2008

Introduction to electronic visual display requirements (The ISO 9241-300 series supersedes ISO 9241 parts 3, 7 and 8)

A very short (4 pages) introduction to the ISO 9241-300 series which explains what the other parts contain

ISO 9241-302:2008

Terminology for electronic visual displays

Definitions, terms, and equations that are used throughout ISO 9241:300 series

ISO 9241-303:2008

Requirements for electronic visual displays

Sets general image quality requirements for electronic visual displays. The requirements are intended to apply to any kind of display technology

ISO 9241-304:2008

User performance test Unlike the other parts in the subseries methods for electronic visual which focus on optical and electronic displays measurements, this part sets out methods which involve testing how people perform when using the display. The method can be used with any display technology

ISO 9241-305:2008

Optical laboratory test Defines optical test methods and expert methods for electronic visual observation techniques for evaluating displays a visual display against the requirements in ISO 9241-303. Very detailed instructions on taking display measurements

ISO 9241-306:2008

Field assessment methods for Provides guidance on how to evaluate electronic visual displays visual displays in real life workplaces

ISO 9241-307:2008

Analysis and compliance test Supports ISO 9241-305 with very detailed methods for electronic visual instructions on assessing whether a display meets the ergonomics requirements set displays out in part 303

ISO/TR 9241-308:2008 Surface-conduction electron- Technical report on a new eco-friendly emitter displays (SED) display technology, called "SurfaceConduction Electron-Emitter Displays" (SED) ISO/TR 9241-309:2008 Organic light-emitting diode (OLED) displays

Technical report on another new display technology called "Organic Light-Emitting Diode Displays" (OLED), which are better for fast moving images than LCDs

ISO 9241-12:1998

This part contains specific recommendations for presenting and representing information on visual displays. It includes guidance on ways of representing complex information using alphanumeric and graphical/symbolic codes, screen layout, and design, as well as the use of windows

Presentation of information

207

208

2.3.2

Visual Ergonomics, Standards and Display Screen Equipment

. Table 1 (Continued) Relevant part of ISO 9241

Overview

ISO 9241-4:1998

Keyboard requirements. (some clauses in this standard have been superseded by ISO 9241-400 and ISO 9241-410

This part specifies the ergonomics design characteristics of an alphanumeric keyboard which may be used comfortably, safely, and efficiently to perform office tasks

ISO 9241-400:2007

Principles and requirements for physical input devices

Sets out the general ergonomics principles and requirements which should be taken into account when designing or selecting physical input devices

ISO 9241-410:2008

Design criteria for physical input devices (supersedes ISO 9241-9:1998)

Describes ergonomics characteristics for input devices, including keyboards, mice, pucks, joysticks, trackballs, touchpads, tablets, styli and touch sensitive screens. The standard is aimed at those who are designing such devices and is very detailed

ISO 9241-5:1998

Workstation layout and postural requirements

This part specifies the ergonomics requirements for a Visual Display Terminal workplace which will allow the user to adopt a comfortable and efficient posture

ISO 9241-6:1998

Guidance on the work environment

This part specifies the ergonomics requirements for the Visual Display Terminal working environment which will provide the user with comfortable, safe, and productive working conditions

ISO 9241-13:1998

User guidance

This part provides recommendations for the design and evaluation of user guidance attributes of software user interfaces, including Prompts, Feedback, Status, Online Help, and Error Management

ISO 9241-2:1992

Guidance on task requirements

Deals with the design of tasks and jobs involving work with visual display terminals. It provides guidance on how task requirements may be identified and specified within individual organizations and how task requirements can be incorporated into the system design and implementation process

4

ANSI/HFES 100–2007 Human Factors Engineering of Computer Workstations

The American National Standards Institute approved ANSI/HFES 100–2007, Human Factors Engineering of Computer Workstations [2], as an American National Standard which was developed by the Human Factors and Ergonomics Society (HFES), it provides specific guidance for the design and installation of computer workstations, including displays, input devices, and furniture that will accommodate a wide variety of users. ANSI/HFES 100–2007

Visual Ergonomics, Standards and Display Screen Equipment

2.3.2

includes computer mice and other pointing devices in its inputs chapter, and the displays chapter has been expanded to cover colour devices.

4.1

Other Relevant Standards

Medical Electrical equipment – Medical image display systems – Part 1: Evaluation methods (IEC 62563-1:2010) [3] provides evaluation methods for testing medical image display systems. It is directed to practical tests that can be visually evaluated or measured using basic test equipment. IEC 62563-1:2010 applies to medical image display systems, which can display monochrome image information in the form of gray scale values on colour and gray scale image display systems (e.g., cathode ray tube (CRT) monitors, flat panel displays, projection system). This standard applies to medical image display systems used for diagnostic (interpretation of medical images toward rendering clinical diagnosis) or viewing (viewing medical images for medical purposes other than for providing a medical interpretation) purposes and therefore having specific requirements in terms of image quality. Head-mounted image display systems and image display systems used for confirming positioning and for operation of the system are not covered by this standard. ISO 11064-5:2008, Ergonomic design of control centres Part 5: Displays and controls [4] is part of a seven part standard under the general title Ergonomic design of control centers. This part includes principles for selection, design, and implementation of displays for control room operation and supervision. ISO 13406, Ergonomic requirements for work with visual displays based on flat panels, consists of two parts, Part 1: Introduction and Part 2: Ergonomic requirements for flat panel displays [5]. This standard is a companion standard for ISO 9241 to account for the significant differences in ergonomic trade-offs present when flat panels are used. It is intended for evaluators and users of this technology. The legibility of flat panels is the principle concern. ISO 13406-2 includes requirements and recommendations that are based on legibility, comfort, and acceptability that arise when multicolour displays are used, based on the visual ergonomics research described in ISO 9421-8, but modified and extended to consider the unique trade-offs of flat panels. It specifically addresses viewing direction, viewing distance, and the use of area luminance. BS IEC 61772:2009 Nuclear power plants – Control rooms – Application of visual display units (VDUs) [6] presents design requirements for the application of VDUs in main control rooms of nuclear power plants. It is designed to assist the designer in specifying VDU applications, including displays on individual workstations and larger displays for groupworking or distant viewing. It covers the use of large-screen displays, provides improved recommendations on the use of colour, and improves the coverage of back-fit or upgrade applications, as well as presenting examples of good practice.

5

Case Study: Implementation of Health and Safety Law in the UK

In recent years, much of Britain’s health and safety law has originated in Europe. Proposals from the European Commission may be agreed by Member States, who are then responsible for making them part of their domestic law. Modern health and safety law in this country, including much of that from Europe, is based on the principle of risk assessment.

209

210

2.3.2

Visual Ergonomics, Standards and Display Screen Equipment

The basis of British health and safety law is the Health and Safety at Work etc. Act 1974 [7]. The Act sets out the general duties which employers have toward employees and members of the public, and employees have to themselves and to each other. These duties are qualified in the Act by the principle of ‘‘so far as is reasonably practicable.’’ In other words, an employer does not have to take measures to avoid or reduce the risk if they are technically impossible, or if the time, effort, or cost of the measures would be grossly disproportionate to the risk. The Management of Health and Safety at Work Regulations 1999 (the Management Regulations) [8] generally make more explicit what employers are required to do to manage health and safety under the Health and Safety at Work Act. Like the Act, they apply to every work activity.

6

The Health and Safety (Display Screen Equipment) Regulations

Britain has implemented the European Directive 90/270/EEC [9] by the Health and Safety (Display Screen Equipment) Regulations 1992. The Health and Safety (Display Screen Equipment) Regulations came into force on January 1, 1993, and have been subsequently updated with amendments (2002). The majority of the regulations apply to self-employed workers and those people working from home but paid by an employer, in addition to employees at company workplaces. The aim of this Directive and the Regulation is to reduce the risks of ill health associated with display screen equipment (DSE) work, notably musculoskeletal disorders (MSDs), stress, and visual fatigue. The Health and Safety Executive, (HSE), published guidance on the regulations, notably booklet L26,Work with display screen equipment (2002) [10]. This gives detailed and comprehensive guidance about work with display screen equipment covering both office work and other environments where display screen equipment (DSE) may be used. Further guidance is also available in publications by the HSE (HSG90) [11] and INDG36 [12]. The legislation consists of nine Regulations, and a Schedule of ‘‘minimum requirements.’’ The Regulations cover aspects of the display screen workstation – including minimum specifications for workstation, work equipment and accessories, the work environment, and work organization. With a few exceptions, the definition of DSE covers both conventional (cathode-ray tube) display screens and other types such as liquid crystal or plasma displays used in flat panel screens, touch screens, and other emerging technologies. Display screens mainly used to display line drawings, graphs, charts, or computer-generated graphics are included, as are screens used in work with television or film pictures. The definition is not limited to typical office situations or computer screens but also covers, for example, nonelectronic display systems such as microfiche. DSE used in factories and other non-office workplaces is included, although in some situations, such as screens used for process control or closed-circuit television (CCTV), certain requirements may not apply. The regulations place a number of obligations on employers as follows: ● ● ● ● ●

Analyze workstations and assess health and safety risks Reduce any risks to the lowest reasonably practicable level Provide eye and eyesight tests for employees Provide training and information to employees Plan work to allow breaks and changes of work activity

Visual Ergonomics, Standards and Display Screen Equipment

2.3.2

The regulations apply to people who habitually use display screen equipment as a significant part of their normal work and those classed as ‘‘users.’’ DSE users: ● Use display screen equipment for continuous or near-continuous spells of an hour or more at a time ● Use DSE in this way more or less daily ● Have to transfer information quickly to or from the screen ● Have limited control over the time spent working on DSE The legal distinction made between users and non-users of DSE reflects the nature of the potential risks in the work and the likely causative factors. The degree of exposure and the factors linked to the health problems reported are strongly influenced by the duration and frequency of periods spent working with DSE and by the intensity of the work. The DSE regulations also define a ‘‘DSE workstation’’ as an assembly comprising: ● Display screen equipment (whether provided with software determining the interface between the equipment and its operator or user, a keyboard, or any other input device) ● Any optional accessories to the display screen equipment ● Any disk drive, telephone, modem, printer, document holder, work chair, work desk, work surface, or other item peripheral to the display screen equipment and ● The immediate work environment around the display screen equipment. The following are excluded from the specific requirements of the DSE regulations, although their users will still be covered by general health and safety legislation. ● ● ● ● ●

Drivers’ cabs or control cabs for vehicles or machinery Display screen equipment on board a means of transport Display screen equipment mainly intended for public operation Portable systems not in prolonged use Calculators, cash registers, or any equipment having a small data or measurement display required for direct use of the equipment ● Window typewriters The demands of work undertaken using DSE may vary but the basic unit of chair, desk, and computer are similar in these jobs [13]. Increasingly, workers may use a laptop or notebook portable computer for a significant part of their working day. The way in which a laptop is used varies mainly due to the environment in which the laptop is used (e.g., train, home living room, home working (see > Fig. 1). The DSE Regulations apply to portable DSE in prolonged use – which can include laptop and handheld computers, personal digital assistant devices, and some portable communication devices. While there are no hard-and-fast rules on what constitutes ‘‘prolonged’’ use, portable equipment that is habitually in use by a DSE user for a significant part of his or her normal work is to be regarded as covered by the DSE Regulations. While some of the specific minimum requirements in the Schedule may not be applicable to portables in prolonged use, employers are required to ensure that such work is assessed and measures taken to control risks. Portable DSE, such as laptop and notebook computers, is subject to the DSE Regulations if it is in prolonged use. Increasing numbers of people are using portable DSE as part of their work. While research suggests that some aspects of using portable DSE are no worse than using full-sized equipment [14], that is not true of every aspect. The design of portable DSE can

211

212

2.3.2

Visual Ergonomics, Standards and Display Screen Equipment

. Fig. 1 Non-standard use of portable DSE for home working

include features (such as smaller keyboards or a lack of keyboard/screen separation) which may make it more difficult to achieve a comfortable working posture. Portable DSE is also used in a wider range of environments, some of which may be poorly suited to DSE work.

7

Conclusion

The recent revision of ISO 9241 sets out clear guidance related to DSE design and use, and the Health and Safety Display screen regulations in the UK have attempted to address changes in technology, for example, the increased use of portable equipment. Neither international standards nor UK Regulations explicitly consider the increasing use of in-car technology, handheld devices, and smart phones. Neither is able to consider the complexity of some environments, for example, where multiple displays are used, the location of working, for example, working across multiple work sites. When assessing risk or evaluating a workplace, this diversity will need to be considered in the development of new guidance or revision of existing. International Standards are developed slowly, by consensus, using consultation and development processes. This can be a disadvantage in a field where technology is rapidly advancing; however, having consensus results in standards that represent good practice.

References 1. ISO 9241 Ergonomics of human-system interaction. International Standards Organisation, Geneva 2. ANSI/HFS (2007) 100 American National Standards Institute, human factors engineering of

computer workstations. Human Factors Society, Santa Monica 3. IEC 62563-1:2010 Medical electrical equipment – medical image display systems – part 1: evaluation

Visual Ergonomics, Standards and Display Screen Equipment

4.

5.

6.

7. 8.

9.

methods. International Standards Organisation, Geneva ISO 11064-5:2008 Ergonomic design of control centres part 5: displays and controls. International Standards Organisation, Geneva ISO 13406 Ergonomic requirements for work with visual displays based on flat panels, part 1: introduction and part 2: ergonomic requirements for flat panel displays. International Standards Organisation, Geneva BS IEC 6177:2009 Nuclear power plants – control rooms – application of visual display units (VDUs). International Standards Organisation, Geneva The Health and Safety at work etc. Act (HSWA) (1974) HMSO, London Management of health and safety at work (2000) Management of health and safety at work regulations 1999. Approved code of practice and guidance L21, 2nd edn. HSE Books, London. ISBN 0 7176 2488 9 Council Directive 90/270/EEC of 29 May 1990 on the minimum safety and health requirements for work with display screen equipment (fifth individual

Further Reading Pheasant S, Haslegrave CM (2006) Bodyspace: anthropometry, ergonomics and the design of work. CRC Press, London

10.

11.

12.

13.

14.

2.3.2

Directive within the meaning of Article 16(1) of Directive 89/391/EEC) [Official Journal L 156 of 21.06.1990] HSE (Health and Safety Executive) (2003) Work with display screen equipment – health and safety (display screen equipment) regulations 1992 as amended by the health and safety (miscellaneous amendments regulations 2002. Guidance on regulations, 2nd edn. HSE L26. HMSO, London HSG90 (2000) The law on VDUs: an easy guide: making sure your office complies with the health and safety (display screen equipment) regulations 1992 (as amended in 2002). HSE Books, London. ISBN 0 7 76 2602 4 Leaflet INDG36(rev3) (2005) Working with VDUs. HSE Books, London. ISBN 978 0 7176 2222 www. hse.gov.uk/pubns/indg36.pdf Pheasant S, Haslegrave CM (2006) Bodyspace: anthropometry, ergonomics and the design of work. CRC Press, London Heasman T, Brooks A, Stewart T (2000) Health and safety of portable display screen equipment. Health and Safety Executive, Sudbury

213

Part 2.4

Photometry

2.4.1 Light Emission and Photometry Teresa Goodman 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

2

Spectral Luminous Efficiency for Human Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

3

Luminous Efficacy of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

4 4.1 4.2 4.3 4.4 4.5 4.6

Photometric and Radiometric Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 Solid Angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 Radiant Intensity, Luminous Intensity, and the SI Unit of Light . . . . . . . . . . . . . . . . . . . 221 Radiant Flux, Spectral Radiant Flux, and Luminous Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Irradiance and Illuminance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Radiance and Luminance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 Radiant Exitance and Luminous Exitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

5

Non-SI Photometric Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

6

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.4.1, # Springer-Verlag Berlin Heidelberg 2012

218

2.4.1

Light Emission and Photometry

Abstract: Displays, obviously, are intended to be seen. It is therefore important to characterize their optical performance using measurements that relate to their ability to stimulate a human visual response – so-called photometric measurements. In this chapter, we explore the various measures that are used in photometry, which provide an internationally agreed measurement framework for quantifying the visual effectiveness of displays (and all other sources of optical radiation). List of Abbreviations: CIE, Commission International de l’E´clairage; SI, Syste`me International d’Unite´s, the International System of Units

1

Introduction

Measurement is at the heart of our modern technological world, supporting trade, industry, and science and underpinning the regulatory framework that helps maintain and improve our quality of life, in areas as diverse as medicine, transportation, and sport and leisure. Unlike a large proportion of the millions of measurements made each day, those relating to things that we can ‘‘see’’ (i.e., that emit optical radiation in the visible portion of the electromagnetic spectrum) are not based only on physical parameters, but must also take account of the ‘‘visual effectiveness’’ of the radiation produced, or in other words the ability to stimulate a visual response and facilitate vision. The science of the measurement of light in terms of its ability to stimulate human vision is termed photometry. It is distinct from the purely physical measurement of optical radiation in terms of absolute power or energy at each wavelength, which is termed radiometry.

2

Spectral Luminous Efficiency for Human Vision

The sensitivity or spectral response of the human eye is not constant over the visible spectrum but varies with wavelength; the relative ability of optical radiation at different wavelengths to produce a visual response is termed its spectral luminous efficiency. However, the spectral response of the human eye varies not just with wavelength, but also according to the level of illumination, the position in the visual field, the visual task being performed, with age, and even from one individual to another. In order to make photometric measurements on a consistent basis, it is therefore necessary to define the spectral luminous efficiency curve that is being used. To this end two internationally agreed curves have been defined [1], which are accepted as representing the relative spectral luminous efficiency function of a typical human observer in light-adapted conditions (the ‘‘photopic’’ or V(l) function) or in darkadapted conditions (the ‘‘scotopic’’ or V 0 (l) function). These have been adopted as part of the SI system [2] and are shown graphically in > Fig. 1. When a scene is viewed in good lighting conditions (e.g., ‘‘normal’’ indoor lighting levels), the spectral response of the eye is not affected by the actual level of illumination. These are the conditions for photopic vision, in which the visual process is entirely governed by cone receptors. The photopic condition is the most common one, and almost all measurements are made using the photopic spectral luminous efficiency function, V(l). Under very dim lighting conditions (e.g., outdoors under starlight) only the rods are sensitive and the eye

Light Emission and Photometry

1

l⬘m = 507 nm

2.4.1

l⬘m = 555 nm

Relative spectral luminous efficiency

0.9 0.8 0.7

V⬘ (l)

V (l)

0.6 0.5 0.4 0.3 0.2 0.1

0 350

400

450

500

550

600

650

700

750

Wavelength (nm)

. Fig. 1 The CIE photopic and scotopic luminous efficiency functions V(l) and V0 (l)

operates in quite a different mode. This is the region of scotopic vision and extends down to the visual threshold, which corresponds to an illuminance in the region of a few microlux. At illuminance levels between photopic and scotopic vision (e.g., twilight), the eye is said to operate in the mesopic range. Under these conditions the eye is in a state somewhere between the stable photopic and scotopic states, with the precise spectral response characteristics depending on the actual level of illumination and the field of view. Because of this nonlinear behavior, it has proved extremely difficult to reach international agreement for visual response functions for the mesopic range, and although this situation has now been resolved for applications such as nighttime road lighting [3], debate continues for applications involving brightness evaluations [4].

3

Luminous Efficacy of Radiation

The photopic and scotopic spectral luminous efficiency functions describe the relative visual effectiveness of optical radiation at different wavelengths, but in order to determine absolute values for ‘‘light output’’ it is necessary to know the luminous effect produced for each watt of optical power entering the eye. This is termed the spectral luminous efficacy and is measured in lumen per watt (lm W1), where the lumen is the unit of luminous flux (see > Sect. 4.3). Through the definition of the candela, the SI system of units defines the spectral luminous efficacy at a frequency of 540  1012 Hz (which in standard air corresponds to a wavelength of 555.016 nm) as being 683 lm W1. This applies regardless of the spectral luminous efficiency function that is used. At this wavelength, V(l) has a value of 0.999998, whereas V 0 (l) is

219

2.4.1

Light Emission and Photometry

1,800 1,600 Spectral luminous efficacy (lm W–1)

220

K⬘ (l)

1,400 1,200 1,000 800 600 K (l)

400 200 0 350

400

450

500

550

600

650

700

750

Wavelength (nm)

. Fig. 2 Spectral luminous efficacy functions for photopic vision, K(l), and scotopic vision, K0 (l)

0.401750. The photopic or scotopic spectral efficacy at any other wavelength, denoted K(l) and K 0 (l), respectively, can be calculated as follows: K ðlÞ ¼

683  V ðlÞ 683  V 0 ðlÞ or K 0 ðlÞ ¼ 0:999998 0:401750

The photopic and scotopic spectral luminous efficacy curves are shown in > Fig. 2; K(l) peaks at 555 nm and has a maximum value of 683.002 lm W1, whereas K 0 (l) peaks at 507 nm and has a maximum value of 1,700.06 lm W1 (these are usually rounded to 683 lm W1 and 1,700 lm W1, respectively).

4

Photometric and Radiometric Units

There are many different units used in radiometry and photometry, some of which are now purely historical but still cause considerable confusion. Those given here are as set down in the International Lighting Vocabulary [5]; other units that may be encountered are summarized in the Tables given later (see > Sect. 5). 3Each radiometric quantity has a photometric equivalent, obtained by weighting the radiometric values by the photopic spectral luminous efficiency function, V(l). The same symbols are used for both types of quantities, with subscripts to distinguish between them (the subscript v denotes a photometric quantity while e or no subscript indicates a radiometric quantity). In the case of flux, for example, which is usually given the symbol F, the symbol for radiant flux is Fe (or just F) and that for luminous flux is Fv.

Light Emission and Photometry

2.4.1

Subscripts are also used to denote whether the radiometric quantity relates to a specific wavelength. For example, if F is to be referenced to a wavelength l, then the symbol used becomes Fl, meaning: Fl ¼

dF dl

We can also define the quantity Fl over a range of wavelengths as though it were a function of wavelength: Fl(l).

4.1

Solid Angle

Many of the measurement quantities listed below refer to solid angle. The unit of solid angle is the steradian (sr). The solid angle is defined by a closed curve and a point in space. Its magnitude is the area of a closed curve projected onto a sphere of unit radius, as shown in > Fig. 3. Equivalently, the solid angle can be defined as the quotient of the area of the projected curve onto a sphere of radius R and the radius squared. In other words an area A at a distance R from a point subtends a solid angle O at that point given by: O¼

4.2

A R2

Radiant Intensity, Luminous Intensity, and the SI Unit of Light

> Figure

4 represents a point source S emitting radiant flux in various directions. The radiant intensity Ie of the source in any given direction is defined as the quotient of the radiant flux dFe

Curve C in space

Solid angle W defined by curve C and point P

R P

. Fig. 3 Definition of solid angle, V

Projection of curve C on sphere of radius R

221

222

2.4.1

Light Emission and Photometry

dA

dW r S

. Fig. 4 Radiant intensity

leaving the source and propagated in the element of solid angle dO containing the given direction, by the element of solid angle: Ie ¼

d Fe dO

The unit of radiant intensity is the watt per steradian (W sr1) and the corresponding spectroradiometric unit is spectral radiant intensity, Ie,l, usually measured in watts per steradian per nanometre (W sr1 nm1). The photometric quantity analogous to radiant intensity is termed luminous intensity Iv and the unit of measurement is the candela (cd). A luminous intensity of one candela is equivalent to a luminous flux of one lumen emitted within unit solid angle: 1 cd ¼ 1 lm sr1 The candela is in fact the SI unit of light. Since 1979 this has been defined as follows: "

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5401012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

In other words, this definition states that at a specific frequency of green light, a monochromatic radiant intensity of 1/683 W sr1 will produce a luminous intensity of 1 cd. Therefore, at this specific frequency of green light, 1 W of radiant flux is directly equal to 683 lm. The specific frequency of the radiation was chosen because it corresponds to a wavelength of 555 nm, the peak of the photopic spectral luminous efficiency curve V(l).

Light Emission and Photometry

2.4.1

Thus the definition of the candela clearly recognizes the fact that light is a form of energy by basing the candela directly upon the watt. Luminous intensity Iv can be calculated from the spectral radiant intensity, Ie,l(l) by weighting with the photopic spectral luminous efficiency function, V(l): Z1 Iv ¼ 683

Ie;l ðlÞV ðlÞdl 0

4.3

Radiant Flux, Spectral Radiant Flux, and Luminous Flux

Although the SI unit for photometry is the candela, the most fundamental measure of optical radiation is that of radiant flux, denoted Fe. This is simply the total power in watts (W) of optical radiation emitted, transmitted, or received. It may include visible, ultraviolet, and infrared radiation and can be measured for any stated solid angle. The spectroradiometric equivalent of radiant flux is spectral radiant flux, denoted Fe,l(l) or Fe,l, and is usually expressed in watts per nanometre wavelength interval, W nm1. To find the total radiant flux Fe we integrate the spectral radiant flux Fe,l(l) over the entire spectrum: Z1 Fe ¼

Fe;l ðlÞdl 0

Luminous flux, Fv, is measured in lumen. It is calculated from the spectral radiant flux, Fe,l(l) by weighting with the photopic spectral luminous efficiency function, V(l): Z1 Fv ¼ 683

Fe;l ðlÞV ðlÞdl 0

The most common use of radiant flux, and the photometric and spectroradiometric equivalents, is that of the geometrically total radiant flux emitted by a source, in all directions, into a solid angle of 4p sr (see > Fig. 4). In particular, when we talk of the ‘‘luminous flux’’ of a source, it is generally the geometrically total luminous flux that is meant (> Fig. 5).

4.4

Irradiance and Illuminance

The irradiance Ee at a point on a surface is defined as the quotient of the radiant flux dFe incident on an element of the surface containing that point, by the area dA of that element. Ee ¼

d Fe dA

The unit of irradiance is watt per metre squared (W m2) and so the unit of spectral irradiance Ee,l is W m2 nm1. The corresponding photometric quantity is termed the illuminance, denoted Ev. The unit of illuminance is lumen per metre squared, lm m2, usually termed lux (lx).

223

224

2.4.1

Light Emission and Photometry

S

. Fig. 5 Integrating the flux emitted by a source over the full solid angle gives the total radiant (or luminous) flux

An illuminance of one lux is equivalent to a luminous flux of one lumen falling on an area of one square metre: 1 lx ¼ 1 lm m2 Illuminance Ev can be calculated from the spectral irradiance, Ee,l(l) by weighting with the photopic spectral luminous efficiency function, V(l): Z1 Ev ¼ 683 Ee;l ðlÞV ðlÞdl 0 > Figure 6 represents a point source S irradiating an element of area dA, at a distance r from the source. The normal to the element of area forms an angle y with the radius vector r. The projection of the element of area dA onto a plane that is normal to the radius vector r will be given by: Projected area ¼ dA  cos y

Therefore the solid angle subtended at the source by the element of area dA is: dO ¼

ðdA  cos yÞ r2

and so the radiant intensity of the source in the given direction will be given by: d Ie ¼

d Fe d Fe  r 2 ¼ dO dA  cos y

2.4.1

Light Emission and Photometry

S

Normal to dA θ

r

Solid angled dW

dA

. Fig. 6 Point source S irradiating surface dA

From above, the irradiance produced by the source at dA is given by: Ee ¼

d Fe dA

If we substitute the expression for dFe from the equation given above for the radiant intensity, we can rewrite the irradiance as: Ee ¼ Ie

cos y r2

Thus the irradiance produced by a point source is inversely proportional to the square of the distance; this is known as the inverse square law. Furthermore, the irradiance is proportional to the cosine of the angle between the direction of irradiation and the normal to the surface; this is known as the cosine law.

4.5

Radiance and Luminance

> Figure

7 represents a radiant surface acting as a source or an irradiated surface acting as a secondary source. The concept of radiant intensity cannot be applied to such a source, and so we consider it as a collection of small radiant surfaces of area dA, to which the concept of radiant intensity is applicable. The quotient of the radiant intensity dIe of one of these small surface elements, when viewed in a particular direction by the projected area of the source under study, is known as the radiance of the surface Le: Le ¼

dIe dA  cos y

Equivalently, the radiance in a given direction, at a given point on a real or imaginary surface, can be defined by the formula: Le ¼

d Fe d Ie  dA  cos y  dO dA  cos y

where dFe is the radiant flux transmitted by an elementary beam passing through the given point and propagating in the solid angle dO containing the given direction, dA is the area of a section of that beam containing the given point, and y is the angle between the normal to that section and the direction of the beam.

225

226

2.4.1

Light Emission and Photometry

Projected area dA cosq Element of solid angle dW

Element of flux dFe q Normal to dA

Source element of area dA

. Fig. 7 Radiance of an extended source

Radiance is measured in watts per steradian per metre squared (W sr1 m2), and the unit of spectral radiance, Le,l, is therefore W sr1 m2 nm1. The corresponding photometric quantity is termed luminance Lv and the unit is cd m2. Luminance can be calculated from the spectral radiance, Le,l(l) by weighting with the photopic spectral luminous efficiency function, V(l): Z1 Lv ¼ 683 Le;l ðlÞV ðlÞdl 0

Radiance and luminance are generally the most relevant and useful quantities for characterizing the amount of light emitted from the surface of a visual display.

4.6

Radiant Exitance and Luminous Exitance

The radiant exitance Me at a point of a surface is defined as the quotient of the radiant flux dFe leaving an element of the surface containing that point, by the area dA of that element: Me ¼

d Fe dA

It is measured in watts per metre squared, W m2. For a uniform diffuse source the relationship between radiant exitance and radiance is: Me ¼ p  Le The spectroradiometric quantity is known as spectral radiant exitance Me,l and is usually measured in W m2 nm1. The analogous photometric quantity is luminous exitance Mv measured in lumen per metre squared, lm m2.

5

Non-SI Photometric Units

There are a number of non-SI units which may sometimes be encountered in the measurement of illuminance and luminance. The most common of these are summarized in the > Tables 1 and > 2 below.

Light Emission and Photometry

2.4.1

. Table 1 Non-SI units for illuminance Unit

Abbreviation

Description

Multiplication factor to convert to lux

2

Phot

ph

lm cm

Milliphot

mph

103 lm cm2

10

Footcandle

fcd

lm ft2

10.76

10 [4]

. Table 2 Non-SI units for luminance

Unit

Abbreviation Description

Nit

nt

cd m2

Stilb

sb

cd cm2

Multiplication factor to convert to cd m2 1 10 [4]

2

asb

(1/p) cd m

0.3183

Lambert (Equivalent phot)

L

(1/p) cd cm2

3,183

Millilambert

mL

103 (1/p) cd cm2

3.183

(1/p) cd ft2

3.426 10.76

Apostilb (Blondel) (Equivalent lux)

Footlambert fL (Equivalent footcandle) Candela/foot2

cd ft2

cd ft2

2

2

2

Candela/inch

6

cd in

cd in

1,550

Summary

The SI system is an internationally agreed system of units, which is used throughout the world to provide a consistent basis for measurement. The importance of vision in our daily lives is recognized within this system by the fact that one of the seven SI base units relates to the visual effectiveness of optical radiation: this is the unit of luminous intensity, the candela. The candela relates the visual effectiveness of a light source to its spectral radiant intensity through defined spectral luminous efficiency functions, the most important being the photopic function, V(l). Other parameters that are of importance for quantifying the performance of sources of optical radiation can be related to luminous intensity by considering the different geometrical configurations used for measurement. They are therefore expressed in terms of derived units (some of which have special names) that are related to the candela through these geometrical considerations, for example, luminous flux is measured in lumen (equivalent to candela steradian) and luminance is measured in candela per metre squared. The use of these internationally agreed units allows us to measure, express,

227

228

2.4.1

Light Emission and Photometry

and compare the visual performance of displays on a reliable basis and is therefore essential not only for trade and specification purposes, but also for ensuring that regulatory requirements are met.

Acknowledgments This work was funded by the National Measurement Office of the UK Department for Business, Innovation and Skills.

References 1.

2.

3.

Commission International de l’E´clairage (2004) ISO 23539:2005(E)/CIE S 010/E:2004 joint ISO/CIE standard: photometry - the CIE system of physical photometry. Commission International de l’E´clairage, Vienna. www.cie.co.at/ Organisation Intergouvernementale de la Convention du Me`tre (2006) The international system of units (SI), 8th edn. Bureau International des Poids et Mesures, Paris. ISBN 92-822-2213-6. www.bipm.org Commission International de l’E´clairage (2010) CIE 191:2010 recommended system for mesopic

4.

5.

photometry based on visual performance. Commission International de l’E´clairage, Vienna. www.cie. co.at/ Commission International de l’E´clairage (1989) CIE 81:1989 mesopic photometry: history, special problems and practical solutions. Commission International de l’E´clairage, Vienna. www.cie.co.at/ Commission International de l’E´clairage (2009) CIE DS 017.2/E:2009 ILV: international lighting vocabulary. Commission International de l’E´clairage, Vienna. www.cie.co.at/

Further Reading Commission International de l’E´clairage (1983) CIE 18.21983: the basis of physical photometry. Commission International de l’E´clairage, Vienna. www.cie.co.at/ DeCusatis C (1998) Handbook of applied photometry. Springer, New York

Grum F, Becherer RJ (1979) Optical radiation measurements volume 1: radiometry. Academic, New York McCluney R (1994) Introduction to radiometry and photometry. Artech House, Boston

2.4.2 Measurement Instrumentation and Calibration Standards Teresa Goodman 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 2 Measurements Using a Photometer or Other Broadband Meter . . . . . . . . . . . . . . . . . . . . 230 3 Measurements Using a Luminance Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 4 Measurements Using a Spectroradiometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 5 Calibration Reference Artifacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 7 Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.4.2, # Springer-Verlag Berlin Heidelberg 2012

230

2.4.2

Measurement Instrumentation and Calibration Standards

Abstract: Correct and reliable measurement requires not only the selection and use of appropriate measurement instrumentation, but also: (a) the use of approved or validated measurement procedures; (b) the use of traceable calibration reference artifacts or standards; and (c) the identification, correction, and/or allowance for potential measurement errors and uncertainties. This chapter will provide an overview of all of these considerations and highlight some fundamental issues that affect the key (generic) measurement approaches used when characterizing the optical properties of displays. List of Abbreviations: CIE, Commission International de l’E´clairage; CIE x(l), y(l), z(l) functions, CIE Color-Matching Functions that Define the Internationally Agreed Standard Colorimetric Observer; LED, Light Emitting Diode

1

Introduction

The starting point for any measurement of a display is to decide on what is the appropriate quantity to measure, which is usually based on what the measurement will be used for. This will determine: (a) the required measurement geometry, (b) whether the measurements need to be made spectrally or in terms of the photometric value, and (c) the most appropriate measurement instrumentation to use. For example, one of the key characteristics of a display is its ‘‘brightness,’’ which is usually specified in terms of its average luminance. In this case, the measurements would generally be made using a luminance meter that is able to average over a large area of the display. If, however, it was required to measure the spectral characteristics of the optical radiation falling on the surface of the display from other light sources in the environment, it would be more appropriate to use a spectroradiometer system and measure the spectral irradiance at the position of the display. Once these basic questions have been decided, consideration can be given to the other factors that are necessary in order to ensure that measurements are both valid and reliable, namely: ● Clear linkage to national standards via an unbroken traceability chain (i.e., the use of measurement instrumentation or reference artifacts whose calibration can be traced directly to national standards) ● Recalibration of measurement instrumentation or reference artifacts at appropriate intervals, which may be determined by time or (as is commonly the case where the reference artifact is a lamp) by usage ● Consideration of the impact of environmental and other influences on the measurement results (i.e., an uncertainty evaluation) ● Use of approved or validated measurement techniques More detail for specific measurement parameters, including information on recommended techniques for minimizing measurement errors and uncertainties, is given in > Sect. 11, Display Metrology.

2

Measurements Using a Photometer or Other Broadband Meter

Most photometric measurements are made using a photometer, which typically consists of a photosensitive element (usually a silicon photodiode) combined with a filter that is designed to

2.4.2

Measurement Instrumentation and Calibration Standards

modify the spectral responsivity to provide an approximation to the V(l) function. Other elements may also be included depending on the geometry required for the quantity being measured, e.g., a diffuser is usually used for measurements of the amount of light falling on a working surface (illuminance), and a lens is often incorporated for measurements of the light emitted from a defined area on a source (luminance). The photometer is generally calibrated by comparison with a source of known photometric output, and this source is usually a tungsten lamp operating at a correlated color temperature of 2,856 K (CIE standard illuminant A) – see > Sect. 5, TFTs and Materials for Displays and Touchscreens. The relative spectral responsivity of a photometer should, ideally, exactly match the V(l) function. In practice, such an ideal is impossible to achieve and all photometers show some departure from V(l), as shown in the example in > Fig. 1. This is termed the spectral mismatch error of the photometer, and its effect on the results of measurements depends not only on the degree of departure from the V(l) function, but also, critically, on the spectral characteristics of the source being measured (see also > Chap. 11.6.1). Consider as an example a photometer that has a perfect match to V(l) across most of the spectral range, but with a significant departure from V(l) in the region between about 450 nm and 500 nm, as shown in > Fig. 2. In this case, there will be no error due to spectral mismatch when measuring a source with emission only at longer wavelengths, such as a red LED, but significant error when measuring a source with emission at shorter wavelengths, such as a blue LED. The impact of any spectral mismatch errors can be minimized by calibrating the photometer using a reference source with spectral characteristics that are identical those of the sources to be measured. Large errors can arise if sources with different spectral characteristics are compared, even if these sources have the same color appearance [1]. Alternatively, if the relative

1.0 0.9 Ideal V(l)-corrected detector

Relative spectral responsivity

0.8 0.7

Practical highquality photometer

0.6 0.5 0.4 0.3 0.2 0.1 0.0 400

450

500

550 600 Wavelength/nm

650

. Fig. 1 Example of the spectral responsivity of a high-quality photometer

700

750

231

2.4.2

Measurement Instrumentation and Calibration Standards

1

Spectral luminous efficiency, photometer responsivity or LED radiance (relative values)

232

0.9 0.8 0.7 0.6 0.5 0.4

Photopic spectral luminous efficiency

0.3

Photometer with spectral mismatch in blue region Blue LED

0.2 Red LED

0.1 0 400

500

600 Wavelength/nm

700

. Fig. 2 Example of the impact of spectral mismatch errors for measurements on different sources

spectral responsivity of the photometer and the spectral power distributions of the standard and test sources are all known, then a correction can be calculated to allow for departures from V(l). This ‘‘spectral mismatch correction factor,’’ F, is given by: R R St ðlÞV ðlÞdl  Sr ðlÞsðlÞdl R F¼R St ðlÞsðlÞdl  Sr ðlÞV ðlÞdl where St(l) and Sr(l) are the spectral power distributions of the test and reference sources, respectively, and s(l) is the spectral responsivity of the photometer. Similar corrections can be calculated for other instruments that are designed to match a defined spectral responsivity function, such as tristimulus colorimeters that are intended to match the CIE x(l), y(l), z(l) functions. The correction factors for even well-corrected photometers can be large, particularly for colored sources such as displays (even a ‘‘white’’ display). It should also be remembered, as mentioned previously, that most equipment which gives a direct read-out in photometric units (e.g., illuminance meters calibrated in lux) will have been calibrated with a source approximating to CIE standard illuminant A (i.e., a tungsten lamp at a correlated color temperature of 2,856 K). Large errors may be introduced when displays or other non-tungsten sources are measured with such instruments. Thus, it will almost always be necessary to calculate and apply a spectral mismatch correction factor when measuring the photometric characteristics of a display using a photometer or, similarly, when measuring the colorimetric characteristics using a tristimulus colorimeter. A diffuser is often incorporated into a photometer to avoid problems associated with nonuniformity of the detector and/or the filter. Many diffusers have an appreciable coloration,

Measurement Instrumentation and Calibration Standards

2.4.2

and this must obviously be allowed for in the calculation of spectral correction factors. They also result in a very large field of view (approximately 180 ), which means that photometers fitted with diffusers are generally very susceptible to stray light. Careful screening is therefore essential, and stray light checks especially important. A baffle is often attached to the front of the instrument in order to reduce the field of view and thus minimize stray light. Another common use of a diffuser is in an illuminance meter, where it is intended to provide a cosine-correction, such that sðyÞ ¼ sð0 Þ cos y where s(y) is the responsivity at angle y to the normal. Such a correction is an essential requirement in situations where light is incident in all directions (e.g., when measuring the illumination falling on a desk in an office), but it cannot be perfectly achieved in practice and can lead to measurement errors (see > C7hap. 11.6.1). Illuminance meters are calibrated using luminous intensity standards (luminous intensity Iv) operating at a known distance d meters from the front surface of the diffuser, the illuminance, Ev, being given by: Ev ¼

Iv d2

Hence, the calibration does not relate to the conditions of use and departures from ideal cosine law behavior are consequently often overlooked. It is possible to check the cosine correction of a meter fairly easily using a highly directional source. If the angle between the detector and the source is varied (keeping the distance constant), the signal should, ideally, vary as the cosine of this angle. All photometers have a ‘‘limiting aperture’’ of some kind, be it the sensitive area of the detector itself, the front surface of the diffuser (if there is one) or some other aperture. There are two cases where it is essential to know where this limiting aperture is located. The first is in the calibration of an illuminance meter using standards of luminous intensity. As has already been mentioned, the illuminance is given by the luminous intensity/distance2, and it is therefore necessary to know from where the distance should be measured. If the illuminance meter is fitted with a plane diffuser, there is usually no problem; the front surface of the diffuser is the effective stop of the system. If no diffuser is present, it is necessary to determine exactly where the limiting aperture lies, and to measure the distance from this point. The second situation where the location (and in this case, the size as well) of the limiting aperture must be known is when calibrating a highly directional source for luminous intensity. Here, the measured intensity depends critically on the solid angle over which it is measured and any statement of the measured luminous intensity should therefore also give the solid angle used. This means that the distance between the source and the photometer and the size of the aperture must be correctly determined.

3

Measurements Using a Luminance Meter

Luminance meters usually incorporate some form of imaging system to focus the area being measured onto the detector. The optics are generally designed to allow the area being measured to be viewed and identified through an eyepiece. As in the case of other photometric measurements, spectral mismatch between the luminance meter responsivity and the V(l) function means that spectral differences between the reference and test sources can lead

233

234

2.4.2

Measurement Instrumentation and Calibration Standards

to significant error and spectral correction factors may have to be applied. The spectral transmittance of the imaging system must be allowed for when determining the spectral responsivity of the meter and calculating spectral correction factors. Some luminance meters also have an additional ‘‘close-up’’ lens that can be affixed to enable very small areas to be measured. These are frequently antireflection coated and therefore have a non-neutral spectral transmittance. It may be necessary to apply an additional color correction factor to allow for this coloration. Even a spectrally neutral lens will change the overall responsivity of the instrument, so that unless it has been calibrated with the lens in position, an appropriate correction should be applied. When making measurements on a display using a luminance meter, it is important to minimize the effect of stray light, either through the use of a stray light elimination tube (on the measurement instrument) or a mask (on the surface of the display) – see > Chap. 11.2.1 and > Chap. 11.6.1.

4

Measurements Using a Spectroradiometer

The use of a spectroradiometer is essential if spectral values are required. However spectroradiometers are also often used to avoid the problems due to spectral mismatch errors associated with photometers and other detectors that are designed to match a defined responsivity function, such as tristimulus colorimeters. In this case, the required photometric value is obtained by calculation as follows, where Qv is the photometric quantity, Ql(l) is the corresponding radiometric quantity, and Dl is the step interval for the measurements (similar calculations can be performed to obtain other integral values, e.g., X, Y, Z tristimulus values): X Qv ¼ 683 Ql ðlÞV ðlÞDl Spectroradiometric measurements require the radiation produced by a source to be isolated into discrete bands of energy of known wavelength and bandwidth. There are various methods by which to do this, but the most common is to use a monochromator. A basic monochromator system consists of an entrance slit, a dispersing element, imaging optics, and an exit slit. The dispersing element serves to convert the homogeneous radiation from the source into a spectrum and is typically either a prism or a diffraction grating. The entrance slit is imaged by the monochromator optics onto the exit slit, and these two slits together determine the ‘‘slit function’’ and bandwidth of the monochromator. There are several different geometrical arrangements used in monochromators, and > Fig. 3 shows one common version. Mirrors are used to collimate light from the entrance slit, which then falls onto a diffraction grating, is dispersed by the grating, and finally refocused onto the exit slit. The dispersed spectrum appears in the plane of the exit slit, so that any desired narrow wavelength band can be isolated by adjusting the angle of the grating. Adjusting the width of the exit slit changes the bandwidth of the emergent radiation. The slit function (i.e., the relative spectral transmittance of the monochromator) depends on the relationship between the sizes of the entrance and exit slits. It is triangular if the image of the entrance slit exactly fills the exit slit, and trapezoidal in other cases. It is important to remember that all the radiation at ‘‘unwanted’’ wavelengths is reflected or refracted within the monochromator, i.e., it is not absorbed by the dispersing element. This unwanted radiation is scattered within the instrument, often in such a way that it can leave through the exit slit – this is termed ‘‘stray light.’’ The problem is reduced if two

Measurement Instrumentation and Calibration Standards

2.4.2

Diffraction gratings

Polychromatic light

Monochromatic light

Focussing mirrors

. Fig. 3 The optical layout of a double Czerny Turner monochromator

monochromators are used in series (as shown in > Fig. 3); a coupled monochromator of this form is called a double monochromator. Increasingly, scanning type spectroradiometers of the type described above are being replaced by array-based systems, which use a fixed monochromator with an array of detectors in the position normally occupied by the exit slit. The spectrum is distributed across the detector array and each of the individual detector elements (pixels) effectively acts as a separate ‘‘exit slit’’ for the monochromator. An example of a typical system is illustrated in > Fig. 4. Array-based systems can offer a number of advantages over scanning systems, e.g.,: ● They sample all wavelengths in the spectral range simultaneously, making them well suited to measurements on pulsed or time-varying sources. ● The absence of moving parts means they can be more stable, reproducible and rugged. ● They are usually smaller and therefore more portable. ● Simultaneous wavelength sampling is also an advantage in situations such as process control, where rapid data collection is necessary. ● It is usually possible to integrate the signal over a period of time, thus reducing the effect of noise on the measured output. The cost of array-based spectroradiometers has fallen significantly in recent years, and their reliability has also improved, making their use increasingly widespread. It is important to note, however, that although in many respects array-based spectroradiometers are very similar to conventional systems which use discrete sampling, some of the features of array-based systems mean that special calibration techniques and precautions are needed if correct results are to be obtained [2]. In particular, the problem of in-system stray light is significantly more acute with array systems than with more traditional systems, for the following reasons: ● Array systems use a single monochromator to disperse the radiation across the array, whereas the majority of high-quality traditional spectrometers use a double monochromator (which has much better stray light performance) and mechanically scan through the spectrum by rotating the monochromator gratings.

235

236

2.4.2

Measurement Instrumentation and Calibration Standards

Source

Integrating sphere Baffle Array detector

Slit

Monochromator

Diffraction grating . Fig. 4 Schematic of an array-based spectroradiometer for irradiance measurements

● Radiation can be reflected (or inter-reflected) off the array, onto the walls and/or other components within the monochromator and then back onto the array. Reflections from detectors placed after an exit slit (as in scanning systems) are much less likely to re-enter the monochromator and be re-reflected onto the detector. ● The attraction of many array systems lies in their small size and portability; it is much more difficult to make effective use of baffles in a physically small system. ● Radiation must be spread across the whole array, making the placement of baffles within the monochromator more difficult than in the situation where only the narrow exit slit is to be irradiated. ● Most array spectrometers use silicon detectors, which have high responsivity in the red and near infrared (to  1,100 nm) and much lower response in the blue and ultraviolet spectral regions. This makes them highly sensitive to any stray radiation in the red and near infrared, which is also the region of highest emission from many commonly used sources, particularly tungsten-based sources typically used as calibration reference artifacts (see > Sect. 5, TFTs and Materials for Displays and Touchscreens). As a result of these problems, in-system stray light is a major factor influencing the performance of array systems for spectrometry and, in the blue region in particular, can dominate all other sources of measurement uncertainty. Other major factors to be considered are wavelength calibration, bandwidth, linearity, noise, dark current, and external stray light [2]. Because of the problems of stray light, the use of array spectrometers for measurements of displays is best restricted to situations where it is possible to make a direct comparison between the display under

Measurement Instrumentation and Calibration Standards

2.4.2

test and a reference display with known characteristics; use of a traditional calibration reference artifact, of the type described in > Sect. 5, TFTs and Materials for Displays and Touchscreens, is likely to lead to large measurement errors.

5

Calibration Reference Artifacts

In the vast majority of cases, the measurement instrumentation used in photometry and spectroradiometry is calibrated using a reference source, which is most commonly a tungsten lamp operating at a correlated color temperature of 2,856 K. This type of reference source often bears little resemblance to the types of source being measured, and this, in turn, leads to many potential sources of error. Thus, if a specific calibration problem arises, the best approach may well be, first of all, to see whether it is possible to obtain a standard with the same characteristics as the test sources. If this can be done, it may be possible to achieve a given level of accuracy with a less complex and, therefore, less expensive measuring system. Tungsten lamps used as calibration standards for luminous intensity, radiant intensity, illuminance, or spectral irradiance are usually operated cap down and calibrated in a specified horizontal direction. They are generally designed so that it is possible to set up the lamp on each occasion of operation in exactly the same position relative to the photometer or irradiated target. Although it is not essential for a source of illuminance or irradiance to obey the inverse square law if it is always used at the distance at which it was calibrated, it must be constructed so that the calibration distance can be measured precisely from a reference point on the lamp, the lamp enclosure, or the lamp mount. It is often useful, however, if a lamp used for this purpose does obey the inverse square law, at least over a limited range of distances, so that it can be used to provide a range of illuminance/irradiance levels. Another requirement of a source to be used as a standard of intensity, illuminance, or irradiance is that its field should be uniform, preferably to better than 0.25% over the irradiated area or angle. Coiled filaments, especially single coils arranged in a regular pattern, can show rapid changes of intensity with angle of view as the front turns mask the rear turns to a greater or lesser extent. Uniformity is improved if the lamp envelope is diffusing, but if clear, it should be of good optical quality. Lamps are often grit blasted to achieve more uniform irradiance, but the surface is then vulnerable to contamination and consequent discoloration. Furthermore, where a lamp has a diffusing envelope, it is almost impossible to define the position of the light center, so the inverse square law is unlikely to be obeyed. Ribbon filament lamps can be used as luminance or spectral radiance standards when high power levels are required. These are normally operated cap down, with the ribbon vertical, and have a plane window of glass or silica to permit good optical imaging. The calibration applies to radiation about an axis, normally horizontal, from a specific area of the ribbon. The calibrated area must be readily identifiable and for this reason a pointer is often provided adjacent to the ribbon, or a small notch may be cut into the ribbon itself. Because the ribbon can move relative to the lamp base during warm-up, the alignment should be checked and adjusted if necessary once the lamp has reached its final operating temperature. In the more common situation where lower power levels are to be involved, such as for the measurement of a display, a standard of lower radiance is generally used. The most fundamental is a plane surface of known luminance (or spectral radiance) factor, such as a barium sulfate or magnesium oxide plaque or a calibrated white opal, which is illuminated using an

237

238

2.4.2

Measurement Instrumentation and Calibration Standards

Intensity standard

Diffuse reflectance standard

Diffuser viewed at 45⬚

. Fig. 5 Lower level radiance or luminance standard

intensity standard (see > Fig. 5). The diffuser acts as a secondary source and is usually illuminated normally and viewed at 45 . The radiance or luminance, L, is given by: L¼

Is p d2

where I is the intensity, d is the distance between the intensity standard and the diffuser and s is the radiance factor of the diffuser for illumination at 0 and viewing at 45 . In the more general case, where a source of intensity, I, irradiates a diffusing surface at an angle yA to the normal, which is then viewed at angle yB to the normal, the radiance or luminance is given by: L¼

I sðyA ; yB Þ cos yA p d2

where s(yA,yB)is the radiance factor of the diffuser under these conditions of irradiance and view. In the case of a perfect diffuser (one for which s(yA,yB) is independent of yA and yB – often referred to as a cosine or Lambertian diffuser), the luminance varies as the cosine of the viewing angle; this ideal is rarely achieved, however, and significant errors can arise if the conditions of use do not replicate closely the conditions under which the diffuser was calibrated. A less direct, but widely employed, standard for luminance or radiance is a ‘‘luminance gauge.’’ This often consists of a small integrating sphere coated internally with a white diffusing material of high reflectance, such as barium sulfate paint, with an illuminating source located either inside or outside the sphere (see > Fig. 6). The device is frequently provided with some means of varying the luminance/radiance over a range of values (e.g., by use of an adjustable diaphragm or aperture). Whatever type of luminance/radiance source is chosen, the reference direction and alignment method, the location and size of the calibrated area, and the solid angle subtended by the optical system of the detector, must all be specified. When using the integrating sphere–type source as a standard of spectral radiance, it is also necessary to ensure it has been calibrated against another standard of known spectral radiance. It is not sufficient to calculate the spectral distribution from the color temperature, since the sphere coating is not always neutral and this can lead to very large errors, particularly in the blue region (see > Fig. 7).

Measurement Instrumentation and Calibration Standards

Entrance port with adjustable aperture

2.4.2

Integrating sphere

Lamp in enclosure

Exit port giving uniform Lambertian field

. Fig. 6 Integrating sphere–based luminance standard

1.2 1.1

Ratio of Power

1.0 0.9 0.8 0.7 0.6 0.5 0.4 400

450

500

650 550 600 Wavelength/nm

700

750

800

. Fig. 7 Spectral power distribution of a range of luminance gauges relative to a Planckian radiator at the same temperature

6

Summary

For any measurement, it is important not only to select the most appropriate measurement equipment, but also to ensure that it is correctly calibrated using stable reference standards that

239

240

2.4.2

Measurement Instrumentation and Calibration Standards

are directly traceable to national measurement standards. In addition, it is essential to consider and evaluate all potential sources of error or measurement uncertainty; these can arise from the measurement instrumentation used, the device being measured, the calibration reference artifact, and/or the environmental conditions, and are typically exacerbated by any differences between the characteristics of the calibration reference and the device being measured (spectral properties etc.). The calibration reference artifacts used to calibrate instrumentation for measuring the optical emission characteristics of displays (and other sources of optical radiation) are usually based on tungsten lamps, which provide relatively stable output across the whole of the visible spectral region. Typically some sort of diffuser is also incorporated, to provide a spatially uniform luminance or radiance source. The majority of measurements on visual displays are performed using broadband detectors, such as photometers or illuminance meters (also called luxmeters), luminance meters (sometimes referred to as spot photometers), and tristimulus colorimeters. For these devices, the main source of measurement error is usually the degree of spectral mismatch between the actual detector spectral responsivity and the desired spectral response function(s). Other major sources of potential error are stray light, which can affect luminance, illuminance, and colorimetric measurements, and departures from ideal cosine response, which is important mainly for illuminance measurements on large sources. Further factors such as nonlinearity, temperature coefficient, noise, dark current, polarization sensitivity, drift, etc., also need to be considered, but are generally (but not always) less significant. Measurements of the optical characteristics of displays may also be made using spectroradiometers, which provide measures of spectral radiance or spectral irradiance from which photometric and colorimetric quantities can be calculated. Spectroradiometer systems may be based on a scanning monochromator, but increasingly these are being superseded by systems using array spectrometers. In-system stray light is the major source of error for this type of instrument and can lead to very large measurement uncertainties; for displays measurement their use is currently restricted primarily to direct comparisons between the device under test and a calibrated reference display.

7

Directions for Future Research

Research into improved instrumentation for measurements on displays is focussed on two main areas: (1) the reduction of measurement error, particularly through improved spectral matching for broadband detectors (photometers and tristimulus colorimeters) and reduced stray light for array-based spectroradiometers; and (2) the capture of more information in a single measurement, e.g., camera-based luminance meters, which can provide information on luminance nonuniformities across the entire surface of a display in a single measurement (see > Sect. 6, Emissive Displays), and conoscope systems, which provide information on angular variations in the display output (see > Chap. 11.6.1).

Acknowledgments This work was funded by the National Measurement Office of the UK Department for Business, Innovation and Skills.

Measurement Instrumentation and Calibration Standards

2.4.2

References 1.

Lambe R (1995) The role of measurements and of a national standards laboratory in energy efficient lighting. In: Proceedings of 3rd European conference on energy efficient lighting, pp 271–278

2.

Hopkinson GR, Goodman TM, Prince SR (2004) A guide to the use and calibration of detector array equipment. SPIE Press, Bellingham

Further Reading DeCusatis C (1998) Handbook of applied photometry. Springer, New York Grum F, Becherer RJ (1979) Radiometry. In: Optical radiation measurements, vol 1. Academic, New York

McCluney R (1994) Introduction to radiometry and photometry. Artech House, Norwood

241

Section 2

Human Vision and Photometry

2.4.3 Overview of the Photometric Characterisation of Visual Displays Teresa Goodman 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

2

Luminance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

3

Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

4

Gray Level Step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

5

Contrast Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

6

Spatial Luminance and Color Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

7

Angular Variations in Luminance and Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

8 8.1 8.2 8.3

Reflectance Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Diffuse Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Specular Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 Angular Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

9

Temporal Performance (Motion Blur) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

10

Basic Measurement Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

11

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_2.4.3, # Springer-Verlag Berlin Heidelberg 2012

244

2.4.3

Overview of the Photometric Characterisation of Visual Displays

Abstract: A wide range of measurements may be required in order to fully characterize the properties of a display, but these are generally based on a relatively small number of fundamental measurement parameters: luminance, color, spatial uniformity, angular distribution, reflectance, and temporal characteristics. This chapter provides an overview of the methods and instrumentation used for these underpinning measurements, and outlines the major associated sources of potential measurement error and uncertainty. List of Abbreviations: BRDF, Bidirectional Reflectance Distribution Function; CIE, Commission International de l’E´clairage; CRT, Cathode Ray Tube Display; LCD, Liquid Crystal Display; LED, Light Emitting Diode; PC, Personal Computer

1

Introduction

Measurements of the photometric characteristics of displays are important for many reasons, such as assessing performance for product development, manufacture and quality control purposes, enabling purchasers to compare products on a consistent basis, and as inputs to software used to predict display legibility in specific installations. A range of measurements may be required, as summarized in > Table 1. This chapter will provide a brief overview of these measurements, the types of instrumentation that are available, and the major potential sources of measurement error.

2

Luminance

Luminance is the most basic measurement for a display. Not only is it the foundation of many of the other measurements (as detailed in > Part 11.3), but it is also often used within the display industry as a general rule of thumb regarding the suitability of the display for a particular application. For example, a display with a luminance of 500 cd m
2 will be on the borderline of being acceptable (‘‘bright enough’’) for use in daylight or other high illumination conditions, whereas a luminance of 1,500 cd m
2 would be considered to be very likely to be acceptable. Luminance is usually measured for both a black and white screen using a luminance meter (sometimes called a spot photometer) or a telespectroradiometer (see > Part 11.2 and > Chap. 11.5.1 for more details). The measurements are usually performed with the measuring instrument perpendicular to the surface of the display, or at the angle at which the user would normally view the display (if this is not perpendicular to the surface). The size and location on the display of the measured area should be stated, since most displays show some nonuniformity in luminance over the surface. It is also important not to measure too small an area, since this can result in large differences in the measured results with small changes in size or position (at the extreme, if the measurement area is the same size as a single pixel, small movements can mean that the area is either located precisely over a pixel, giving a ‘‘high’’ reading, or only partially over a pixel, giving a ‘‘low’’ reading; neither reading will adequately represent the true display luminance). Precautions must also be taken to minimize the effect of stray light from areas of the screen other than the defined measurement area, and this generally involves the use of either a stray light tube on the measurement instrument or

Overview of the Photometric Characterisation of Visual Displays

2.4.3

. Table 1 Measurements for characterizing the optical performance of a visual display Measurement

Used for

Luminance

Key element for basic specification and comparison of display performance

Color

Key element for basic specification and comparison of color display performance

Gray level step

Key element for basic specification and comparison of display performance

Contrast ratio

Key element for basic specification and comparison of display performance

Spatial luminance uniformity

Assessing whether luminance variations across the surface of the display will result in unacceptable disturbance to the end user

Spatial color uniformity

Assessing whether luminance variations across the surface of the display will result in unacceptable disturbance to the end user

Angular luminance distribution

Assessing the range of angles over which the display can be viewed

Angular color distribution

Assessing the range of angles over which the display can be viewed

Diffuse reflectance

Assessing legibility of the display under specific ambient illumination conditions

Specular reflectance

Assessing legibility of the display under specific ambient illumination conditions

Angular reflectance

Assessing legibility of the display under specific ambient illumination conditions

Temporal characteristics

Assessing display susceptibility to motion blur for moving images

a mask on the surface of the display. Other major sources of error are the performance of the measurement instrument (see > Sect. 10, Mobile Displays, Microdisplays, Projection and Headworn Displays and > Chap. 11.6.1, Measurement Devices) and external influences on the output of the display (e.g., CRTs are susceptible to external magnetic fields and LED displays can be affected by changes in ambient temperature). Results are expressed in terms of candela per metre squared (cd m
2).

3

Color

The range of colors that can be displayed (the ‘‘color gamut’’) is evaluated by measuring the chromaticities of red (R), green (G), blue (B), and white (R = G = B) screens. The approach, precautions, and sources of error are similar to those for measurements of luminance, but in this case the instrumentation used is a colorimeter (which is typically placed directly on the

245

2.4.3

Overview of the Photometric Characterisation of Visual Displays

600

500

Luminance (cd m−2)

246

400

300

200

100

0 0

4

8

12 16 20 24 28 32 36 40 44 48 52 56 60 64 R, G, B demand level

. Fig. 1 Example of results from gray level step measurements

surface of the screen) or a telespectroradiometer (which is imaged onto the screen and provides measurements of the spectral radiance). Results are expressed in terms of CIE (x,y) or (u0 ,v 0 ) chromaticity coordinates, or using another specified color system. More details are given in > Part 11.2 and > Chap. 11.6.1, Measurement Devices.

4

Gray Level Step

These measurements are also performed in a similar manner to luminance measurements, but in this case the white screen is varied over the full range of RGB drive levels (i.e., from 0 to 256) and the luminance is determined for each step (see > Parts 11.2, Standard Measurement Procedures and > 11.3, Advanced Measurement Procedures). A test pattern generator is usually used for this purpose (see > Chap. 11.5.1, Standards and Test Patterns) to avoid problems that may arise if a personal computer (PC) is used to set the gray levels (a PC provides the means for altering brightness, contrast, and gray level step in a way that is often hidden from the operator). Results are usually expressed graphically, as shown in > Fig. 1.

5

Contrast Ratio

Contrast ratio is defined as the ratio of the highest luminance to the lowest luminance that the display system is capable of producing. The larger the contrast ratio, the greater is the difference between the brightest whites and the darkest blacks that can be displayed. A high contrast ratio

Overview of the Photometric Characterisation of Visual Displays

2.4.3

is a desirable aspect of any display, but it is not always possible to make a direct comparison between the contrast ratio values provided by different display manufacturers, due to differences in the measurement methodologies used. The most representative measure of contrast ratio for assessing overall display performance is static contrast ratio, which refers to the ratio between the luminances of the brightest white and the darkest black that can be displayed simultaneously. Dynamic contrast, on the other hand, refers to the ratio between the deepest blacks and the brightest whites that a display can display, but not at the same time, and generally results in higher contrast values. This is particularly true in the case of displays employing backlights (e.g., LCDs), where light can bleed through from the backlight into black areas when an image containing both white and black areas is being viewed (thus reducing contrast ratio), whereas the backlight can be reduced or even turned off if a fully black image is displayed. A further complication is that, regardless of whether static or dynamic contrast is measured, the results obtained can depend significantly on the ambient lighting conditions. Contrast ratio is often quoted for dark room conditions, that is, with no ambient illumination present and minimal reflections from the surroundings. In most instances, this is not the environment in which displays are used, and different values will be obtained if ambient illumination is present and/or the surrounding walls, floor, and ceiling can reflect light from the display back onto the screen. There are two commonly used methods of measuring contrast ratio. The ‘‘full on/off ’’ method compares the luminance of a white screen (R = G = B = max) with that of a black screen (R = G = B = 0) and has the advantage that it largely cancels out the effect of the external environment (equal proportions of light are reflected from the display to the room and back for both the ‘‘black’’ and ‘‘white’’ measurements, as long as the room stays the same). This method is generally suited only to dynamic contrast measurements, unless it is possible to control the display such that the backlight is fully on even when displaying a black image. The second method is to use a checkerboard pattern, in which the luminance values of all the white squares (or rectangles) are measured and averaged, and similarly the luminance values of the black squares (or rectangles) are measured and averaged. The ratio of the averaged white readings to the averaged black readings is the contrast ratio. This method provides static contrast values. However, accurate measurement of contrast using this checkerboard approach requires the use of a well-controlled dark room, with all walls, floors, ceilings, etc., totally black and nonreflective; this can be difficult and expensive to achieve. Whatever method is used, and regardless of whether static or dynamic contrast is being measured, results are expressed as the ratio between the luminance of the ‘‘white’’ to the ‘‘black’’ condition, for example, 1,000:1. More details of contrast ratio definitions and measurement methods and are given in > Chaps. 11.2.1 and > 11.5.1, Standards and Test Patterns.

6

Spatial Luminance and Color Uniformity

The visual appearance and effectiveness of a display can be significantly degraded if there are perceptible variations in the luminance and/or color over the active area of the screen. Such nonuniformities may appear as a gradual variation from one part of the screen to another or as localized variations due, for example, to the structure within the backlight. Prior to the development of imaging photometers/colorimeters, display nonuniformity was assessed by

247

248

2.4.3

Overview of the Photometric Characterisation of Visual Displays

Contrast ratio 0.00–54.7 54.7–109.4 109.4–164.1 164.1–218.8 218.8–273.5 273.5–328 328–383 383–438

. Fig. 2 Example of results from an imaging colorimeter, showing contrast ratio values in false color

making measurements at a large number of discrete points across the full area of the screen using a spot photometer, colorimeter, or telespectroradiometer (see > Chap. 11.3.1). The problem with this approach lies in achieving sufficient spatial resolution and conducting a sufficient number of measurements to characterize fully the performance of the display. In practice, the scientist/engineer performing the measurement generally identifies the brightest and dimmest locations on the display by visual inspection and then performs several measurements around these areas. The nonuniformity can be calculated as a contrast ratio between the areas of highest and lowest luminance. The main problem with this approach is that it does not fully represent the overall nonuniformity of the display, but gives only two specific worst case points. The low spatial resolution of the measurements can also mask small area nonuniformities. More recently, therefore, this point-by-point approach to measurements of display nonuniformity has been largely superseded by the use of imaging photometers and colorimeters, which produce two-dimensional maps of the variations in luminance (or chromaticity) over the full screen surface, as illustrated in > Fig. 2.

7

Angular Variations in Luminance and Color

Measurements of the angular variation in luminance and color provide the characteristics of the display over the whole forward hemisphere, and can be used not only to determine the angular field of view of a display, but also to provide a more comprehensive understanding of display legibility under a range of conditions (see > Chap. 11.3.3). The highest angular resolution and measurement sensitivity is achieved through goniometric methods, in which the luminance or color distribution is mapped as a function of angle. However, these methods require long setup and measurement times and are consequently often too expensive to fulfill

Overview of the Photometric Characterisation of Visual Displays

2.4.3

the needs of the display industry. Other methods have therefore been developed, based on the use of the latest imaging technologies [1], but a detailed description of these is beyond the scope of this chapter (see > Chap. 11.6.1, Measurement Devices for more information).

8

Reflectance Measurements

Light reflected from the display surface into the user’s line of sight is superimposed on the displayed image and results in a degradation of the legibility of the displayed image. Conventionally, two types of reflection have been considered within the display community: specular reflection and diffuse reflection. However, with the increased use of antiglare and touch screen coatings a third type of reflection, termed ‘‘haze,’’ is now also considered and can be a significant contributor to degraded visual performance.

8.1

Diffuse Reflectance

Diffuse reflectance measurements (see > Chap. 11.3.4 for more details) are intended to quantify the amount of reflected light that will be superimposed on the displayed image from a uniformly distributed diffuse light source. This diffuse light source provides a reasonable approximation of the illumination environments in which displays are often used. For example, the illumination from the sky is diffuse in nature, so this is an appropriate condition to use for displays used out-of-doors, and although indoor environments generally have a somewhat complicated illumination distribution, even these are often adequately represented by a diffuse source to a first degree (e.g., office lighting is often designed to produce good uniformity across the working plane with no visible ‘‘bright spots’’). The baseline measurement technique for diffuse reflectance is to place the display inside a large integrating sphere, with a lamp (with baffle) placed behind the display such that this

Light source Baffle

Light trap Display or reflectance standard

Exit port

. Fig. 3 Schematic of sampling sphere measurement of diffuse reflectance

Luminance meter

249

250

2.4.3

Overview of the Photometric Characterisation of Visual Displays

provides diffuse illumination onto the display. Measurements are made of the screen luminance for a measured level of diffuse illuminance and compared with the measured luminance under the same conditions for a calibrated reflectance standard [2]. However, this method requires access to an integrating sphere that is large enough to accommodate the display (the diameter of the sphere should be at least ten times the diagonal of the display) and it is therefore not widely used. An alternative approach is the ‘‘sampling sphere method’’ [2], in which the display is placed against the sample port of an integrating sphere, rather than inside it. A lamp is placed inside the sphere, close to the wall, and baffled to prevent direct illumination of the display. The luminance of the display surface is measured using a luminance meter and compared with that measured under identical conditions but with the display replaced by a calibrated diffuse reflectance standard (see > Fig. 3). The diffuse reflectance of the display, rdis, is given by: rdis ¼

rstd Ldis Lstd

where rstd is the reflectance of the calibrated standard, Ldis is the luminance measured when the display is in position, and Lstd is the luminance measured with the reflectance standard in position. If the display emits light, then the luminance of the display must be subtracted from the luminance measured under reflection to obtain the net reflected luminance. Measurements are usually made for a range of display conditions (white and black as a minimum) since the reflectance may vary depending on the display settings.

8.2

Specular Reflectance

Measurements of specular reflectance are made to determine the degree of ‘‘mirrorlike’’ reflection from a display (see > Chap. 11.3.4 for further details). Specular reflections are generally several orders of magnitude larger than diffuse reflections and can be a major source of discomfort if a display is incorrectly positioned within a lit environment. Measurements are usually made by comparing the luminance of display when viewed at angle y to the normal and illuminated by a point source at
y to the normal with that for a calibrated specular reflectance standard (typically a piece of black glass) that is illuminated and viewed under identical conditions.

8.3

Angular Reflectance

Measurements of angular reflectance provide the reflectance characteristics of the display over the whole forward hemisphere and can therefore be used to determine the details of how the display reflectance will impact on its legibility under any given conditions (see > Chap. 11.3.4 for further details). The importance of these measurements has grown in recent years due to the increasing use of display screens with antiglare or touch screen coatings, both of which introduce nontrivial ‘‘haze’’ reflections, that is, reflections that are intermediate between diffuse and specular in nature. The most serious effect of haze is to ‘‘broaden out’’ the specular reflection, making it less easy for an observer to avoid the reflection by changing their viewing

Overview of the Photometric Characterisation of Visual Displays

2.4.3

location and thus resulting in a display that is less legible than would be the case if only specular reflection were present. As in the case of measurements of angular luminance and color variations, the most comprehensive and accurate measurements of the angular reflectance properties of a display are obtained using goniometric methods, yielding the bidirectional reflectance distribution function (BRDF) [3]. As for other angular measurements, simplified approaches are under development based on imaging technologies, but these have not yet gained international acceptance and are outside the scope of this chapter – see > Chap. 11.6.1, Measurement Devices for further information.

9

Temporal Performance (Motion Blur)

Measurements of the temporal performance of a display relate to the degree of image persistence for a dynamic image (see > Chaps. 11.3.2 and > 11.5.1, Standards and Test Patterns). These measurements are particularly important for video images containing fast-moving, high-contrast targets, such as television coverage of football and tennis (where motion blur can cause the fast-moving ball to be hard to distinguish). The issue of motion blur has become more important with the advent of new displays, such as LCD screens. Unlike CRT displays, where the image is displayed only for a short time during each refresh cycle and is blank between each image, in an LCD display the image is held on the screen during the entire refresh period. This means that for a fast-moving object in the image, the object position is correct for only a fraction of the time, and the eye interprets this as the object being blurred. In practice, there are two contributors to motion blur: the rise and decay time of the pixels and the hold time. The former can be measured using a fast photodiode and the latter is dictated by the display drive electronics.

10

Basic Measurement Instrumentation

As described in > Chaps. 2.4.2 and > 11.6.1, Measurement Devices, most measurements of a display are made using either a spectroradiometer [4], which provides measurement results as a function of wavelength, or a filtered broadband detector [5], which is designed to give an approximation to one or more of the CIE standard observer functions. The different characteristics of these instruments can lead to different, but in each case significant, measurement errors, as summarized in > Table 2. Both types of instrument are typically calibrated using a stable and reproducible reference source, such as a luminance gauge. The reference source may be calibrated in terms of its luminance, its chromaticity, or its spectral output (usually absolute spectral radiance) as a function of wavelength, by a laboratory that is traceable to national standards. The instrument is calibrated by comparing the measured values for the reference light source with the calibration data, yielding a correction factor or factors. All instruments will show some drift in calibration with time, so it is important to check the calibration at regular intervals. Furthermore, reference sources also drift, both with time and with usage, so it is important that these are recalibrated at regular intervals by a laboratory providing measurements that are traceable to national standards.

251

SR

BB

Stray light

Spectral mismatch

The match between the spectral response of a broadband meter and the target CIE standard observer function is never perfect and residual mismatch errors lead to errors when comparing sources with different spectral characteristics. Errors of many tens of percent are common with colored sources such as displays.

Radiation scattered within the spectrometer is measured at a wavelength that does not correspond to its true wavelength, leading to errors in the measured spectral power distribution and in any calculated results, for example, tristimulus values.

St ðlÞsðlÞdl

Sr ðlÞRðlÞdl

Correction: spectral mismatch errors can be minimized by calibrating the meter using a source with spectral characteristics close to those of the sources to be measured. A correction factor F can be applied if the target spectral function (R(l)), the spectral responsivity of the meter (s(l)), and the spectral power distributions of the reference and test sources (Sr(l)) and St(l) respectively, are known: R R St ðlÞRðlÞdl Sr ðlÞsðlÞdl R F¼R

Evaluation: measure the spectral responsivity of the meter as a function of wavelength and compare with the desired function.

Correction: this is possible using evaluation method (a) but is difficult and complex. A better approach is to select a spectrometer with low levels of stray light – typically this means using a scanning double monochromator.

Evaluation: (a) measure the output as a function of wavelength for a large number of monochromatic inputs or (b) use cut-on or cutoff filters to investigate levels of stray light arising from particular spectral regions (e.g., no signal should be observed at wavelengths below the filter cut-on wavelength when the monochromator is set to wavelengths above the cut-on).

Evaluation/correction methods

2.4.3

Type of Source of error instrument Description

. Table 2 Major potential sources of error with spectroradiometer (SR) and filtered broadband detector (BB) systems

252 Overview of the Photometric Characterisation of Visual Displays

SR

SR and BB

SR and BB

Wavelength scale

Polarization

Linearity

The output signal does not vary in proportion with the input quantity.

The optical radiation from some displays (e.g., LCDs) is highly polarized and this can lead to significant errors if the responsivity of the detector system is polarization sensitive. Polarization effects can be wavelength dependent and can lead to errors in measured color or luminance values.

Wavelength errors result in the measured irradiance or radiance values being assigned to an incorrect wavelength. This also impacts on quantities derived from spectral measurements, such as tristimulus values.

Correction: corrections can be applied, based on the evaluation measurements, but it is usually preferable to restrict the range of input values to those over which the instrument is linear.

Evaluation: can be assessed by measuring the output value for a number of known input values that span the range of inputs over which the instrument will be used. Often checked using calibrated neutral density filters, lamps of different intensities or superposition techniques [6].

Correction: if the detector system does show polarization sensitivity, corrected results should be calculated from the mean of two measurements made at orthogonal polarizations.

Evaluation: make measurements of a uniform, non-polarized light source (such as a luminance gauge) through a polarizer that is rotated to several different positions in turn. Any variation in the measurements is due to polarization sensitivity of the detector.

Note that wavelength errors can vary significantly across the spectral region of interest and even relatively small shifts (less than 1 nm) can result in a change of several DE*ab units for some display colors.

Correction: calibrate the wavelength scale of the spectrometer at several wavelengths covering the wavelength range of interest, for example, by using monochromatic emission lines from a lowpressure discharge lamp or several laser lines.

Overview of the Photometric Characterisation of Visual Displays

2.4.3 253

The color of a display is rarely uniform over a refresh cycle, so correct results will only be obtained if the exposure time is an exact integer multiple of the display refresh period. Any partial cycles captured will distort the measurement result.

The choice of spectral bandwidth for a measurement is a compromise between signal level and spectral resolution. A wide bandwidth gives a high signal level and improved signal to noise ratio, but at the cost of the resolution of narrow peaks, which may lead to errors, for example, in the calculation of tristimulus values. The step interval should be an integer multiple of the bandwidth, and to avoid significant errors in calculated tristimulus values, intervals of no greater than 5 nm should be used.

Synchronization SR and BB

Bandwidth and step interval

Evaluation/correction methods

Correction: it is not easy to correct for bandwidth or step interval. Instruments with poor slit profiles (e.g., highly nonsymmetrical) or where the step interval is not an integer multiple of the bandwidth are best avoided.

Evaluation: bandwidth can be measured by scanning through a monochromatic line at very fine wavelength intervals and measuring the full width at half maximum. Note that the band-pass function may not remain the same size and shape over the entire wavelength range.

Correction: if possible, the instrument should be synchronized with the refresh cycle of the display and set so that the measurement exposure time captures an integer number of cycles (here ‘‘synchronization’’ means that the sampling time of the measuring instrument is directly related to the refresh rate of the display, not that the measurement is initiated at a particular time in the display cycle). If this is not possible, a long measurement time should be used, so that the number of whole refresh cycles is large compared to the number of part cycles captured.

Correction: calibrate and use the system with a neutral density filter than provides sufficient attenuation to ensure that no saturation occurs at the peak of the pulse, while also ensuring that signal levels are high enough to avoid problems due to noise.

Evaluation: calibrate the system both with and without a neutral density filter in place and compare measurements of the display made under both conditions. Any difference in results indicates a probable problem due to saturation at the peak of each pulse.

2.4.3

SR

The output of many displays (e.g., CRTs) can show large variations over the course of each refresh cycle. The dynamic range of the measurement instrument must be sufficiently large to avoid saturation at the peak of each pulse and to minimize the effect of noise on the signal at the low point of each cycle. This is important even for instruments in which the readings are averaged over several cycles, since saturation and noise effects can cause errors in the averaged signal.

SR and BB

Dynamic range (saturation and noise)

Type of Source of error instrument Description

. Table 2 (Continued)

254 Overview of the Photometric Characterisation of Visual Displays

Overview of the Photometric Characterisation of Visual Displays

11

2.4.3

Summary

The key measurements required in order to characterize the performance of a display are luminance, color, spatial uniformity, angular distribution, reflectance, and temporal characteristics. These provide basic, underpinning information relating to the quality, usability, and legibility of the display under different conditions of use. This section has provided an overview of the methods, instrumentation, and major sources of potential error and uncertainty associated with these measurements; more details on all these aspects are provided in > Sect. 11, Display Metrology.

Acknowledgments This work was funded by the National Measurement Office of the UK Department for Business, Innovation and Skills.

References 1. Rykowski R, Kreysar D, Wadman S (2006) The use of an imaging sphere for high-throughput measurements of display performance - technical challenges and mathematical solutions. SID international symposium digest of technical papers 37, pp 101–104 2. Kelley EF (2006) Diffuse reflectance and ambient contrast measurements using a sampling sphere. Proceedings of the 3rd Americas display engineering and applications conference, Society for Information Display, Atlanta, pp 1–5. Available from National Institute of Standards and Technology. ftp://ftp. fpdl.nist.gov/pub/reflection/ADEAC06_Sampling_ Sphere_2-1.pdf

3. Kelley EF, Jones GR, Germer TA (1998) Display reflectance model based on the BRDF. Displays 19:27–34 4. Commission International de l’E´clairage (1984) CIE .63: the spectroradiometric measurement of light sources. Commission International de l’E´clairage, Vienna. www.cie.co.at/ 5. Commission International de l’E´clairage (1982) CIE 53: methods of characterising the performance of radiometers and photometers. Commission International de l’E´clairage, Vienna. www.cie.co.at/ 6. Hopkinson GR, Goodman TM, Prince SR (2004) A guide to the use and calibration of detector array equipment. SPIE Press Book. SPIE Press, Bellingham

Further Reading DeCusatis (1998) Handbook of applied photometry. Springer, New York Grum F, Becherer RJ (1979) Optical radiation measurements volume 1: radiometry. Academic, New York

Keller PA (1997) Electronic display measurement: concepts, techniques and instrumentation. Wiley, New York McCluney WR (1994) Introduction to radiometry and photometry. Artech House, Boston

255

Section 3

Image Storage and Processing

Part 3.1

Introduction to Electronic Imaging

3.1.1 Introduction to Electronic Imaging Jon Peddie 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 2 Historical Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 3 Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 4 Solid-State Devices: The CCD and CMOS Image Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 264 5 Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 6 In Between the Sensors and Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 7 Computer Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 8 Vector and Raster Image Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.1.1, # Springer-Verlag Berlin Heidelberg 2012

262

3.1.1

Introduction to Electronic Imaging

Abstract: This chapter presents a brief and partly historical overview of electronic imaging, image processing, and display. It traces the development of imaging devices from photomultipliers through to CCD and CMOS sensors, and outlines their fundamental operating principles. The evolution of early electronic display systems is then described, and the chapter concludes with a review of image processing and the basic principles of computer graphics. List of Abbreviations: CCD, Charge-Coupled Device; CMOS, Complementary Metal Oxide Semiconductor; NURBS, Nonuniform Rational Basis Spline; PMT, Photomultiplier

1

Introduction

Electronic imaging is often thought to mean the use of computers and/or specialized hardware/ software to capture, store, process, manipulate, and distribute information such as documents, photographs, paintings, drawings, and three-dimensional objects, through digitization and scanning. Electronic imaging is basically the conversion of light to electrons and vice versa. Photosensors in cameras and scanners convert light received directly or reflected from images (photos, printed pages, etc.) into a digital stream for processing. Displays and printers convert the digital stream from a TV or computer to light so we humans can see it (emitted light from a display, reflected light from the print.) In between are marvelous processes of data management commonly referred to as image processing (> Fig. 1), and it involves clever software and high-speed digital circuits that are amazingly inexpensive today.

Sensor

Filters and frontend processor

Operating System User Interface (if Applicable)

Decoders and processors

Translators and display drivers

Display

. Fig. 1 Image processing pipeline

Where the light (reflected or direct) is detected

Filters discriminate wanted from unwanted (noise) signals, and do first order signal enhancement

Is needed, decoding of the signals is done here, and then image manipulation and analysis is done The final results are translated from raw digital to display standards, and converted into signals for the display standard

Introduction to Electronic Imaging

2

3.1.1

Historical Overview

Most historians trace the development of electronic imaging to the early facsimile machine [1] or FAX as it is commonly known. Scotsman Alexander Bain [2] is credited with the first patent on such technology in 1842, and it was demonstrated by 1847, by the Englishman Frederick Bakewell [3] and called a copying telegraph machine (> Fig. 2). In 1861, Giovanni Caselli [4], an Italian living in France, produced the first commercial fax machine for use between Paris and Lyon, and it had the capability to transmit illustrations which caught many people’s imagination – and scared others. Those pioneering FAX machines were electrometrical. It was not for another 50 years that electronic imaging devices were developed. The first electronic display, called the ‘‘Braun tube,’’ and known as the cathode ray tube (CRT) is generally agreed to have been developed by Karl Braun [5] in 1897 in Germany. In 1911, Boris Rosing [6] claimed he received an elementary image on the screen of his primitive TV set. Then in 1922 Jenkins [7] sent a still picture by radio waves and in 1925 Kenjiro Takayanagi [8] began research on television in Japan after reading about the new technology in a French magazine. He developed a system similar to that of John Logie Baird. Two years later in 1927 Philo T. Farnsworth adapted Braun tube for TV [9] in the United States, turning it into both a sensor (camera) and display. There is some debate as to whether Farnsworth was first or if Vladimir Kosma Zworykin [10] invented the cathode-ray tube called the kinescope in 1929 [11] and the iconoscope, an early television camera (other names mentioned of that era are Kenjiro Tahayangi in Japan [12] and Manfred Von Ardenne [13] (Albrecht) in Germany). Nonetheless, it was the TV industry that can be credited with providing the economy of scale needed to bring down the cost of electronic displays and cameras. The displays (CRTs) were adopted for RADAR scopes, electronic test equipment (oscilloscopes), medical diagnostic equipment, and ultimately PCs. Because television was the catalyst that inspired so much technology and catapulted the fledging electronics industry, many countries lay claim to being first. It will probably never be answered to everyone’s satisfaction.

. Fig. 2 One of the first working FAX machines, ca. 1847, called the Copying Telegraph Machine

263

264

3.1.1 3

Introduction to Electronic Imaging

Sensors

In addition to the iconoscope, early image scanners using a light-sensitive vacuum tube known as a photomultiplier (PMT) and rotating drum were developed to scan in an image from a drawing or photograph. The first image scanner was developed by a team led by Russel Kirsch in 1957 at the US National Bureau of Standards, and in fact heralded the era of computer based image processing [14]. The PMT was developed by Malter, Morton, and Zworykin [15] at RCA in 1936. Like the TV sensor, the PMT relied on a compound that efficiently converted photons to electrons (or vice versa in the case of the CRT.) The front of the PMT has a photocathode that converts light (photons) into electrons, and then the rest of tube consists of a series of multipliers that increase the quantity of electrons along the way to the output. For decades, RCA was the leader in the field. PMTs were used extensively and exclusively as light-sensing amplifiers for all types of medical instrumentation, nuclear radiation detectors, telescopes, and anything that required sensing low-level light and converting it into electrons. The PMTs held sway until early 1970s. They were slowly replaced by a solid-state device known as an avalanche diode. The first paper dealing with avalanche transistors was Ebers and Miller in 1955 [16]. It was known that shining light on a transistor’s junction would cause it to switch on and let current flow. This was applied to avalanche diodes and originally used for particle-physics experiments.

4

Solid-State Devices: The CCD and CMOS Image Sensors

The introduction of solid-state image sensors spelled the end for the vacuum tube PMTs except in specialized very low light situations. The first solid-state image sensors, charge-coupled device (CCD), were developed by Willard Boyle [17] and George E. Smith at Bell Labs in 1969 (Regan 1999)[18], which became the image sensor that was in most of the first digital cameras, and the first flatbed scanners. Willis Adcock, an engineer at Texas Instruments, designed a filmless camera and applied for a patent in 1972 [19]. The first recorded attempt at building a digital camera was in 1975 by Steven Sasson, an engineer at Eastman Kodak [20]. The complementary metal–oxide–semiconductor (CMOS) image sensor was first conceived in 1968 (described by Noble in 1968 [21], by Chamberlain in 1969 [22], and by Weimer et al. in 1969 [23]), but it was not until the 1990s that fabrication became a practical proposition, and their eventual introduction drastically lowered the cost of cameras, FAX, and scanner sensors and possibly enabled the explosive growth in camera phones, cameras, PCs, security, and automobiles. CCD and CMOS image sensors (also called ‘‘imagers’’) are two different technologies for capturing images digitally and each has unique strengths and weaknesses. Both types of imagers convert light into electric charge and process it into electronic signals. In a CCD sensor (> Fig. 3), every pixel’s charge is transferred through a very limited number of output nodes to be converted to voltage, buffered, and sent off-chip as an analog signal. In a CMOS sensor (> Fig. 4), each pixel has its own charge-to-voltage conversion, and the sensor often also includes amplifiers, noise correction, and digitization circuits, so that chip outputs are digital bits. Because a CMOS imager converts charge to voltage at the pixel, most functions are integrated into the chip. This makes imager functions less flexible but, for applications in rugged environments, a CMOS camera can be more reliable.

Introduction to Electronic Imaging

Charge-Coupled Device Image Sensor

Camera (Printed Circuit Board)

Bias Generation

Clock and Timing Generation

Oscillator

Clock Drivers

Line Driver

3.1.1

Gain

Photon-to-Electron Conversion Electron-to-Voltage Conversion

Analog-to-Digital Conversion

To Frame Grabber

. Fig. 3 Diagram of a CCD imaging chip (Reproduced from [24] with permission from DALSA)

Oscillator Column Amps Line Driver

Gain

Column Mux

Electron-to-Voltage Photon-to-Electron Conversion Conversion

Row Access

Clock and Timing Generation

Row Drivers

Complementary Metal Oxide Semiconductor Image Sensor

Bias Generation

Bias Decoupling

Connector

Camera (Printed Circuit Board)

To frame Analog-to-Digital Conversion Grabber

. Fig. 4 Diagram of a CMOS imaging chip (Reproduced from [24] with permission from DALSA)

CCD sensors were the most popular solid-state devices, and often compared to PMTs. However, as semiconductor manufacturing processes improved, the CMOS sensor with its lower cost of manufacturing and natural digital signal gradually replaced CCD in all but the most demanding applications.

265

266

3.1.1

Introduction to Electronic Imaging

. Table 1 Comparison between CCD and CMOS sensors Displays Feature

CCD

CMOS

Signal out of pixel

Electron packet

Voltage

Signal out of chip

Voltage (analog)

Bits (digital)

Fill factor

High

Moderate

System noise

Low

Moderate

Sensor complexity

Low

High

Responsivity

Moderate

Slightly better

Dynamic range

High

Moderate

Uniformity

High

Low to moderate

Uniform shuttering

Fast, common

Poor

Speed

Moderate to high

Higher

Antiblooming

High to none

High

Some of the feature differences are shown in > Table 1. Today there is no clear line dividing the types of applications each can serve. CCD and CMOS technologies are used interchangeably. As a result, you can find CMOS sensors in highperformance professional and industrial cameras and CCDs in low-cost, low-power cell phone cameras. For the moment, CCDs and CMOS remain complementary technologies – one can do things uniquely the other cannot.

5

Displays

As mentioned, the first electronic display was the ‘‘Braun tube,’’ (1897) and now commonly known as the cathode ray tube (CRT) and it used what is known as a cold cathode, that is, the electrons that struck the phosphor making it light up were not thermionically emitted from the cathode but merely attracted to a higher voltage field near the phosphor (> Fig. 5). It was not until 1922, 25 years later, that the first adaptation to use a hot cathode was developed at Western Electric by John B. Johnson and Harry Weiner Weinhart, both types used electrostatic deflection. Western Electric commercialized the device and was used initially in test equipment for oscilloscopes, and they were known as direct write displays (unlike raster displays which will be discussed a little later). Because the eye is most sensitive to green, and the early CRTs had no storage, green-emitting phosphors were chosen so we could see the traces. The CRT was adapted to the first TV experiments in 1927 and successfully demonstrated by Philo Fransworth. There were earlier electromechanical examples of TV using spinning disks but Fransworth’s system seems to the first totally electronic implementation. Germany may be the first country to sell TV receivers with the Telefunken FE-III in 1934. TVs went on sale in 1936 (Baird T5) and Baird is believed to be the first shop to sell television receivers, in London in late 1936. In the USA, the DuMont offered the model 180 in 1938. Almost all of the prewar American sets were used in New York City, where electronic broadcasting began in 1938. A few were sold in Chicago, Boston, Philadelphia, Washington, and Los Angeles, where there were also stations.

Introduction to Electronic Imaging

3.1.1

. Fig. 5 The Braun CRT (Source: O’Neill’s Electronic Museum)

The CRT was pressed into military service as a RADAR display. Dr. Robert Watson-Watt is credited for discovering the theory of radar in the 1920s in Britain while trying to find a way to detect thunderstorms. However, German engineer Christian Hu¨lsmeyer demonstrated the concept in 1904 with his ‘‘telemobiloscope’’ [25]. In August 1917, the unappreciated genius Nikola Tesla first established principles regarding frequency and power level for the first primitive radar units [26]. The term RADAR was coined in 1941 as an acronym for Radio Detection and Ranging. RADAR CRTs were made with a longer persistence phosphor and grew to 10 in. in diameter, and by the end of World War II were being made up to 20 in. in diameter with a flat face. Perhaps the best known example of the large screen vector scopes as they were known are the units used in the 1957 SAGE (Semi-Automatic Ground Environment) early warning system (> Fig. 6). These systems used a light pen to mark items of interest. The light pen was basically a small PMT and the timing of the trigger pull to where the beam was gave the computer the necessary coordinate information. The SAGE system actually drew on The 1950s Whirlwind computer developed at MIT and was the first computer that operated in real time, used video displays for output, and the first that was not simply an electronic replacement of older mechanical systems. The first adaption of a display for use in a game is often attributed to William Higinbotham who used an oscilloscope at Brookhaven National labs in 1958 to create a tennis game [27]. However, others believe the first interactive electronic game, a missile simulator inspired by radar displays from World War II, was created by Thomas T. Goldsmith Jr. and Estle Ray Mann [28] on a cathode ray tube in 1947. As computers became more popular and costs kept diminishing, experiments were made in adapting the lower cost electromagnetic scanning TV displays. Unlike oscilloscopes, RADAR displays, and early CAD vector scopes which position a dot on the screen by delivering x–y coordinate data to the display, a raster scan starts the dot at the upper left corner and scans it across to the right, then flies back (turned off) and scans a second line. The number of lines determines the display’s resolution, and standard TV designed in the 1930s and still in use today has between 230 and 275 scan lines. Introduced to computers in the 1960s, the poor resolution of the early raster CRTs relegated them to alphanumeric computer displays. They were first used on minicomputers and large time-share computers in the early 1960s. Also, the display had to be backed by memory, and a simple screen of 80 columns and 24 lines of characters that were 7  9 dots required 128 k bits of memory, which in the 1960s was expensive. However, as memory costs dropped, and CRT scanning resolution and bandwidth increased, computer displays began using bitmapped displays which then became graphics

267

268

3.1.1

Introduction to Electronic Imaging

. Fig. 6 Air Surveillance Officer checking Tracker Initiator Consoles where operators use light guns to pinpoint tracks. When a blip appears on the scope, the light beam causes the computer to assign a track number and to relay speed, direction, and altitude information to various consoles. (Picture used with the permission of The MITRE Corporation)

displays. The first such displays were similar to today’s mobile phones, with a resolution of 320  240 and four bits deep. They quickly evolved to 640  48 eight bits deep, which became the IBM VGA ubiquitous industry standard.

6

In Between the Sensors and Displays

While sensor and output devices were gaining in resolution and dropping in price, advances were also being made on mainframes and minicomputers and the development of image processing software. Work at Bell Labs, Jet Propulsion Labs, MIT and elsewhere for the manipulation of satellite imagery was improving rapidly as better algorithms were developed and computers were getting faster with more memory. The first satellite photographs of Earth were made on August 14, 1959 by the US satellite Explorer 6 [29]. During WWII, advancements were made in aircraft and reconnaissance balloons [30] and were deployed with high-resolution cameras. As the cold war began to form in the early 1950s, high-altitude aircraft like the U2 were developed to overfly and photograph military activities in the Soviet Union and other communist nations [31]. In the early 1960s, the USA launched the Corona and Argon reconnaissance satellites [32], and the Soviet Union launched the Zenit series satellites (> Fig. 7) [33]. These platforms were first equipped with high-resolution cameras and film packs that could be parachuted. The photo cameras were replaced with high-resolution image scanners in the satellites. With the improved and subsequent increase in quantity of images being generated there was

Introduction to Electronic Imaging

3.1.1

. Fig. 7 Soviet Union’s Zenit photo reconnaissance satellite ca. 1964 (Source: astronautix.com)

the need for more automated image processing. And coincidentally in the 1960s, the lower cost and powerful minicomputer was introduced. Soon, power and clever imaging processing software became available. Satellites were deployed in the early 1970s for peaceful uses. They were tested to determine their value in estimating crop acreage and production. That was something that had been done using aircraft and film till then. Around that same time, many commercial geographical image system (GIS) image processing vendors were just getting started. LANDSAT 1 (then the Earth Resources Technology Satellite (ERTS) 1) was launched on July 23, 1972. More than three million images from the Multispectral Scanner System (MSS) on LANDSATs 1–5 have been acquired and stored at the National Satellite Land Remote Sensing Data Archive. Image processing algorithms were applied to an increasing role in the analysis of medical images. Applications ranged from motion compensation to automatic detection, labeling and quantification of organs and lesions. There has been significant research in this field, and major advances have been made in the development of image segmentation and registration algorithms. Image processing software is used to manipulate the data held in digital images of stars. In astronomy, this is typically applied to digital camera images, CCD-images, and (less common) scanned photographs and slides. There are many different software packages available. In addition to general image processing software like Adobe PhotoShop and Paint Shop Pro, there is dedicated astronomical image processing software. Most CCD cameras come with some image processing software. While military, scientific, and medical image processing had moved to the digital domain by the late 1950s, TV stayed in the analog domain until the mid-1970s; and then it was a hybrid with analog-to-digital converters on the front (sensor) end and digital-to-analog converters (DACs) on the back (display) end. Scottish inventor, John Logie Baird, was the first to record television in 1927 [34]. This was only a year after being lauded as the first person to demonstrate television in England. With only 30 lines per picture (television frame), the highest frequency present was low enough to be audible. The video signal could therefore be recorded as an audio signal onto disc, like a music record. It was not until 1956 that Ampex that achieved high-speed recording and playback using magnetic tape [35].

269

270

3.1.1

Introduction to Electronic Imaging

The displays for computers evolved from large circular stroke writers that were developed for early warning RADAR systems in the late 1950s to raster-scan displays (like a TV) in the late 1970s. And in 1981, IBM popularized the personal computer and 3 years later introduced the ubiquitous VGA display. Now all the pieces were in place for the explosion of computer imaging – low-cost solidstate scanners with 150 DPI resolution, low-cost computers, and low-cost raster scan displays with reasonable resolutions. Matching this pace of hardware development was new low-cost image processing software, most pronounced with the introduction of Photoshop in 1985.

7

Computer Graphics

In the early 1980s, work was being done on algorithms for image processing in real time for computer graphics applications. Computer graphics (CG) is the generation of images from computer data. An image can be anything from a highly stylized calligraphic font such as Japanese or Arabic, to a complex model of a protein, automobile, or building, to the fantastic imaginations of science fiction movies. The basic mechanics of CG are not difficult to understand. We start the process with drawing lines. In 2D that means connecting two points, x1, y1 and x2, y2. You extend that by adding two more lines to form a triangle and you have got the basis for all CG modeling. Add a third z value to each point and you have moved to 3D. The lines can be curved applying specialized formula to them (like Coons nonuniform rational basis spline – NURBS) and you can describe almost any complex surface by building up hundreds of triangles to form what is known as a mesh (> Fig. 8). Next you color the surfaces, shine lights on them, and maybe add some textures. Creating a realistic image, which may be part of a movie or computer game, requires an understanding of the environment that is being simulated. For more details of the processes involved, see > Chaps. 3.3.1 and > 3.2.3, and > Chap. 12.3.2 for a review of the development of design software.

. Fig. 8 A dolphin portrayed in a mesh of triangles

Introduction to Electronic Imaging

1 A

B

X

3.1.1 20

G

F Y

C

E Vector image

D

H 20 Raster image

. Fig. 9 Comparison of a vector to raster image (Source: Arts-Humanities.net)

. Fig. 10 A bitmap image, original size in the upper right corner, and zoomed in. Notice the individual pixels that make up the image

8

Vector and Raster Image Generation

In the creation of a graphics image, there are two techniques: raster (composed of pixels) and vector (composed of line). Vector images are curves and lines derived by mathematical formulas. They allow easy and accurate scaling of the image, larger or smaller while maintaining image quality. Adobe Illustrator and Macromedia Freehand are two applications that use vector images. Flash is a vector-based video technology. Raster images are made up entirely of pixels or bitmaps (> Fig. 9). Bitmaps do not scale as well due to their discrete or quantum nature and so computer graphics techniques like antialiasing and anisotropic filtering are used to trick the eye into seeing smooth edges. Photo imaging software like Adobe Photoshop is an example of an application used for editing raster

271

272

3.1.1

Introduction to Electronic Imaging

. Fig. 11 Scaling a vector image maintains the integrity of the lines, a bitmap loses its definition when scaled (Source: LogoDesign Works)

images. Raster-based images are best when working with continuous tone images, like photographs, and in solid modeling designs. Raster or bitmap images are made up of pixels. Most computer monitors display approximately 70–100 pixels/in. – the actual number depends on your monitor and screen resolution settings. As an example consider a typical desktop icon, as shown in > Fig. 10. Vector images as mentioned scale well and maintain their image integrity. Raster images when scaled will reveal the pixels used to make the image (> Fig. 11). Image sensors produce bitmapped images. Photographs from a digital camera are bitmaps consisting of millions of pixels. Raster images can get quite large and so it is necessary to compress them as much as possible without losing any information (see > Chap. 3.2.1). It is possible with specialized software to convert a raster or bitmap image into a vector image for better scaling and manipulation.

9

Summary

This chapter has provided a brief overview of the historical development of electronic imaging and image processing. The following chapters build on these topics and present detailed reviews of specific topics in the fields of image storage, compression, processing, and manipulation.

References 1. Fink DG (1996) Facsimile. Colliers Encyclopedia CDROM, vol 9, Colliers, New York, 28 February 1996 2. Hattiangadi JN, Bain A (1970) Dictionary of scientific biography 1. Charles Scribners, New York, pp 403–404. ISBN 0684101149 3. Roberts S (2008) Distant writing: a history of the telegraph companies in Britain between 1838 and 1868, an excellent website detailing the early history of Britain’s telegraph industry before the takeover

by the British post office – maintained by Steven Roberts. http://distantwriting.co.uk/default.aspx 4. Huurdeman AA (2003) The worldwide history of telecommunications: two centuries of progress from semaphore to multimedia. Wiley-IEEE. ISBN 0471205052, 9780471205050 5. Keller PA (1991) The cathode-ray tube: technology, history, and applications. Palisades, New York. ISBN 0-9631559-0-3

Introduction to Electronic Imaging 6. History of modern television started in Russia in 1900. http://english.pravda.ru/science/19/94/377/16010_ television.html 7. Jenkins CF (2010) Encyclopædia Britannica. Encyclopædia Britannica Online, 11 July 2010. http://www.britannica.com/EBchecked/topic/875332/ Charles-Francis-Jenkins 8. The invention of television: television timeline 1923–1931. http://www.teletronic.co.uk/televisiontimeline2.htm 9. Farnsworth R, Farnsworth PT (2002) The life of television’s forgotten inventor. Mitchell Lane, Hockessin. 10-ISBN 1-584-15176-5; ISBN 13-ISBN 978-1-584-15176-0 (cloth) 10. Abramson A (1987) The history of television 1880 to 1941. McFarland, Jefferson 11. Abramson A (1995) Zworykin, pioneer of television. University of Illinois Press, Champaign 12. Takayanagi K The Father of Japanese Television – A tribute to Kenjiro Takayanagi at the NHK website. http://en.wikipedia.org/wiki/Kenjiro_Takayanagi 13. Ulrich A, Heinemann-Gru¨der A, Wellmann A (Dietz, 1992, 2001) Die Spezialisten: Deutsche Naturwissenschaftler und Techniker in der Sowjetunion nach 1945. ISBN 3320017888 14. Kirsch RA (1998) SEAC and the start of image processing at the National Bureau of Standards. IEEE Ann Hist Comput 20(2):7–13 15. Zworykin VK, Morton GA, Malter L (1936) The secondary-emission multiplier-a new electronic device. Proc IRE 24:351–375 16. Ebers JJ, Miller SL (1955) Alloyed Junction Avalanche Transistors (abstract here). Bell Syst Tech J 34:883. The first paper analyzing the use of bipolar junction transistors in the avalanche region 17. In 1969, Boyle and George E. Smith invented the Charge-coupled device (CCD), for which they have been joint recipients of the Franklin Institute’s Stuart Ballantine Medal in 1973, the 1974 IEEE Morris N. Liebmann Memorial Award, and the 2006 Charles Stark Draper Prize 18. Regan P (1999) George Smith and Willard Boyle win C&C prize for charge-coupled device. Bell Labs, Murry Hill. http://www.bell-labs.com/news/1999/ september/20/1.html

3.1.1

19. US patents 4057830 and 4163256 were filed in 1972 but were only later awarded in 1976 and 1977 20. Digital photography milestones from Kodak. http:// www.womeninphotography.org/Events-Exhibits/ Kodak/EasyShare_3.html 21. Noble PJW (1968) Self-scanned silicon image detector arrays. IEEE Trans Electron Devices ED15(4):202–209 22. Chamberlain SG (1969) Photosensitivity and scanning of silicon image detector arrays. IEEE J SolidState Circuits SC-4(6):333–342 23. Weimer PK, Pike WS, Sadasiv G, Shallcross FV, Meray-Horvath L (March 1969) Multielement selfscanned mosaic sensors. IEEE Spectrum 6(3):52–65 24. Litwiller D (2001) CCD vs. CMOS: Facts and Fiction. Photonics Spectra January 2001, Laurin 25. Christian Hu¨ lsmeyer by Radar World. http:// en.wikipedia.org/wiki/Radar; http://en.wikipedia. org/wiki/History_of_radar; http://www.designtechnology.info/resourcedocuments/Huelsmeyer_ EUSAR2002_english.pdf 26. The Electrical Experimenter (1917) http:// einhornpress.com/inventor.aspx 27. ‘‘Who really invented the video game?’’ Creative Computing Magazine, October 1982. http://www. atarimagazines.com/creative/index/ 28. http://www.pong-story.com/2455992.pd 29. Space Exploration, Encyclopedia Article, MSN Encarta. http://encarta.msn.com/encyclopedia_761556756_2/ space_exploration.html 30. Balloon Reconnaissance, History, JUDSON KNIGHT. http://www.espionageinfo.com/Ba-Bl/Balloon-Reconnaissance-History.html 31. National Museum of the USAF. http://www. nationalmuseum.af.mil/factsheets/factsheet.asp?id= 9166 32. National Reconnaissance Office. http://www.nro. gov/corona/facts.html 33. Anatoly Zak. http://www.russianspaceweb.com/ spacecraft_military.html 34. Hills A (1996) Eye of the world: John Logie Baird and Television: Part I. Kinema (5):5 35. Ampex Corp background. http://www.ampex.com/ 03corp/03corp.html

273

Part 3.2

Image Storage and Compression

3.2.1 Digital Image Storage and Compression Tom Coughlin 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 2 Image Capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 3 Still Image, Image Formats, and Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 4 Video Image Storage Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 5 Image Storage Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 6 Storage Requirements for Image Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 7 Directions for Further Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.2.1, # Springer-Verlag Berlin Heidelberg 2012

278

3.2.1

Digital Image Storage and Compression

Abstract: There are many still and video image formats in current use. These vary by resolution and compression technique used. Higher-resolution formats are generally used in content capture while content distribution uses compressed formats to reduce bandwidth demand. This chapter exhibits examples of digital image (still and video) storage requirements for various common formats and resolutions. It also includes discussion of the types of storage devices used in capture of digital still and video images, as well as storage used for field editing and archiving. Future content will require even greater storage and bandwidth as demand for higher-resolution and 3D content drive developments and changes in the required technology. List of Abbreviations: CCD, Charge-Coupled Device; GIF, Graphics Interchange Format; HDTV, High-Definition TV; JPG/JPEG, Joint Photographics Expert Group; LZW, Lempel Ziv Welch; PNG, Portable Network Graphics; SDTV, Standard Definition TV; TIF, Tagged Image File

1

Introduction

Key factors influencing the size of image files are (1) increased sensitivity and lower cost of image sensors, (2) availability of faster and more integrated electronics allowing rapid processing of images and content compression, and (3) the decreasing price and increasing availability of digital storage. All of these factors have enabled the increasing resolution of digital still and video cameras.

2

Image Capture

Various types of storage devices have been used in image capture. Analog silver halide film produces still and video images of great resolution, which is only now being matched by the highest-resolution digital images. Today’s digital still and video cameras use charge-coupled devices (CCDs) or active pixel CMOS sensors. There are many factors in comparing the visual quality of digital images vs. analog images on, e.g., 35-mm silver halide film, and many of the differences will only show up if the image is blown up to look at details. It is estimated that there are about 20 ‘‘quality’’ megapixels in a high-end film camera with a good lens system, with the finest grained film in good quality light. If the light level is less or a lower quality lens system is used, the number of ‘‘quality’’ megapixels may be as low as 4 (or even fewer) [1].

3

Still Image, Image Formats, and Compression

Still digital imaging using electronic cameras produces data files of many different formats as well as ‘‘Raw’’ formats. The ‘‘Raw’’ data captured by a camera’s sensors is often processed by the camera electronics to produce a lossy or lossless image. Lossy compressed still image formats lose some of the original image detail, while lossless compressed image formats allow the details of the original image to be reconstructed. The maximum lossless compression possible in still images is about 50% [2]. While lossless compression can save storage space processing, the compression algorithm makes the time to open or save files longer.

Digital Image Storage and Compression

3.2.1

If a captured raw image is 3,000
2,000 pixels in size (or 6 megapixels) and it has 24 bits 3 bytes) of color information per pixel, then the total size of this image is 6 megapixels
3 bytes = 18 megabytes. Compression can be used to make the required storage capacity smaller than this. Some of the more popular still image formats include TIF, PNG, JPG, and GIF. There are also several uncompressed ‘‘Raw’’ formats. > Table 1 compares some information about these formats [3]. Note that the popular JPG digital still images use a lossy compressed format. Also image compression (pixels only) may be separate from the number of colors that are used in the image (essentially a color compression). TIF or TIFF is Tagged Image File Format (compressed or uncompressed) [4]. The TIF format is supported by several image processing applications and is often the archiving format of choice for still image content. TIF can be compressed or uncompressed. The header section of a TIF file contains color depth, color encoding, and compression type information. TIF images can use LZW lossless compression. The LZW compression uses a table-based lookup algorithm invested by Abraham Lempel, Jacob Ziv, and Terry Welch. The LZW algorithm looks for reoccurrences of byte sequences in its input. The table maps input strings to their associated output codes. The table initially contains mappings for all possible strings of length one. Input is taken one byte at a time to find the longest initial string present in the table. The code for that string is output, and then the string is extended with one more input byte, b. A new entry is added to the table mapping the extended string to the next unused code (obtained by incrementing a counter). The process repeats, starting from byte b. The number of bits in an output code, and hence the maximum number of entries in the table is usually fixed. and once this limit is reached, no more entries are added [5, 6]. PNG is Portable Network Graphics (standardized compression). This is a lossless compressed image format [7]. This image format is sometimes used for web content, although GIF images are more common. PNG supports truecolor (up to 48 bits), grayscale (up to 16 bits) and palette-based (8-bit) color. This format was designed to replace the older GIF format. PNG has three main advantages over GIF for web content: alpha channels (variable transparency), gamma correction (cross-platform control of image brightness), and two-dimensional interlacing (a method of progressive display). PNG also compresses better than GIF by around 5–25%. PNG is a still image only format, while GIF supports animation as well. JPG or JPEG is Joint Photographic Experts Group (variable compressed format [8]. This is the most popular image format produced by many digital still cameras. JPG is generally a lossy

. Table 1 Some characteristics of common still image file formats Still image format

Color depth

Compression

Loss of detail on saves?

TIF

Variable

Lossless

No

PNG

Variable

Lossless

No

JPG

24

Lossy

Yes

GIF

8

Lossless

No

279

280

3.2.1

Digital Image Storage and Compression

. Table 2 Some characteristics of common still image file formats Image file type

Size of file in KB

TIFF, uncompressed

901

JPG, high quality

319

JPG, medium quality

188

JPG, low quality

50

PNG, lossless compression

741

GIF, lossless compression (only 256 colors)

286

compressed image format, and it is generally used to limit the size of the image files so more images can be recording on a given storage capacity. JPG files may be compressed to 1/10 of the size of the original data. Every time JPG images are processed or edited, more resolution is lost. It is much better to save images in a lossless format and only generate JPG images when a smaller format is needed for email or on a web site. GIF is Graphics Interchange Format (compressed format) [9]. This format was created in the 1980s by CompuServe Information Service for transmitting images across data networks. When the World Wide Web was created in the 1990s, GIF was adopted as the primary image format for web sites. GIF files use a lossless compression scheme to keep file sizes at a minimum, but they are limited to 8-bit (256 or fewer colors) color palettes. The GIF file format uses a LZW (Lempel Zev Welch) file compression that squeezes out inefficiencies in the data storage without losing data or distorting the image. The LZW compression scheme is best when compressing images having large fields of homogeneous color. It is less efficient compressing complicated pictures having many colors and complex textures. As an example of the relative size of various still images with variable image compression, see > Table 2 [10].

4

Video Image Storage Formats

> Table 3 compares digital storage capacity, pixels/frame, frame rate, and streaming bandwidth requirements for various uncompressed and compressed video formats [11–14]. Compression of this content can change these numbers considerably and is very common in content distribution. Note that MPEG-4, DVD MPEG-2, and Blu-ray are all compressed formats. It is clear that richer digital content requires much greater storage capacity and bandwidth. Ultra-HD is over 16 times larger than today’s HDTV [15] even though it is assumed compressed as much as HD. A few simple equations can be used to calculate the overall digital storage requirements for an hour of video content with a given level of pixel surface density. > Equations 1 and > 2 show how the data rate can be calculated for a 3-color RGB video.

Width
Height
Bytes=pixel
Colors ¼ Frame size

ð1Þ

Digital Image Storage and Compression

3.2.1

. Table 3 Storage and streaming bandwidth requirements for various formats of video content

Format

Pixels in a frame Frame (width
height) rate (fps)

Data rate (MBps)

Storage capacity/h (GB)

MPEG-4 (compressed)

Varies

Varies

0.750

0.337

DVD MPEG 2 (NTSC, compressed)

720
480

29.97

1.22

4.39

SDTV (NTSC, 4:2:2, 8-bit)

720
480

29.97

21

75.6

Blu-ray disc (compressed)

1,920
1,080

24

4.56

16.4

HDTV (1080p, 4:2:2, 8-bit)

1,920
1,080

24

149

536

Digital cinema 2K (4:2:2, 10-bit) RGB 2,048
1,080

24

199

716

Digital cinema 4K (4:4:4, 16-bit) RGB 4,096
2,160

24

1,274

4,586

7,680
4,320

60

3,233

11,640

Ultra-HDTV

NTSC DVD (720 ⫻ 480) HDTV 720p (1280 ⫻ 720)

HDTV 1080p (1920 ⫻ 1080) Digital cinema - 2K (2048 ⫻ 1080)

Digital cinema - 4K (4096 ⫻ 2160) RED digital cinema - 2540p (4520 ⫻ 2540p)

Super hi-vision / ultra high definition video (7680 ⫻ 4320)

. Fig. 1 Comparison of the pixel size (width
height) of video resolutions (Reproduced from [16])

(For a three color 2K image, this gives 2,048
1,080
10 bits/pixel
3 colors = 66.3 Mb/ frame = 8.29 MB/frame. Note that this is a 10-bit deep file.) Size
frames per second ¼ Data rate

ð2Þ

8:29 MB=frame
24fps ¼ 199 MB=s > Figure

1 graphically compares the resolution of various video formats [16]. Note that the pictured resolution of the The Red One camera may be somewhat exaggerated. The reason is that a single large image sensor cannot deliver the true resolution possible with multiple large,

281

282

3.2.1

Digital Image Storage and Compression

high-quality color sensors. While the Red One can deliver 4K or higher resolution using interpolation; the actual native resolution may be more like 3K quality. This makes it somewhat better than HD, but less than 4K. This should be good enough for broadcast and independent film use. Thus, the true resolution of the Red One camera in > Fig. 1 should be less than indicated. Compressed video images, like still video images can be compressed by a lossy or lossless compression algorithm. Like still images, lossless compression is not possible for compression greater than about 50%. Further details of video compression techniques are given in > Chap. 3.2.2.

5

Image Storage Devices

In digital still imaging, floppy disks were one of the first media used (Sony’s Mavica Camera) [17]. Very small form factor hard disk drives (so called 1-in. HDDs) were introduced in the late 1990s to provide very high storage capacity for professional photographers [18]. The storage capacity of 1-in. HDDs achieved over 10 GB, but today production of 1-in. HDDs has almost ceased. NAND-based flash memory is today the most popular recording media for still images. In video content capture, magnetic tape is still the most popular medium, but there are also video camcorders available that use recordable optical discs as well as hard disk drives. The hard disk drive products can be plugged into a computer so the content can be accessed and copied just like any other external hard disk drive. This is an advantage to users since they don’t have to play back the content as with most magnetic-tape-based cameras. Optical discs can be inserted into a DVD player for instant playback access and can also be inserted into and copied on to computers. In the last few years NAND flash storage capacities have increased and prices have gone down, making possible flash-based video cameras with a reasonable capture time on a single flash card. Because card-based flash camcorders do not have the moving parts of a tape or optical media camera and because they do not have to have the additional cost of an internal hard disk drive, the original purchase price of consumer flash-based camcorders is generally the lowest in the market. This has enabled a whole new low-cost market for camcorders and thus increased the overall market for these products. It should also be noted that many mobile phones have still and video camera capabilities that store their images on flash memory cards. Note that the professional video market follows the same trends in storage media discussed here for the consumer market. Non-tape based professional video cameras are most popular with younger professionals and higher-end facilities.

6

Storage Requirements for Image Distribution

> Figure 2 shows a projection for the required data rate and storage capacity to contain various sizes of multimedia objects. As can be seen from this chart, blue laser optical discs will be large enough to contain compressed HDTV quality movies, but higher-resolution compressed

3.2.1

Digital Image Storage and Compression

Virtual reality, 3D movie 1,000 Ultra HD movie

100

HD movie 10 Data rate (Mbps)

CD quality stereo audio

DVD movie (MPEG-2)

1

0.1 One page ASCII text 0.01 1 KB 10 KB 100 KB 1 MB 10 MB 100 MB 1 GB 10 GB 100 GB 1 TB Multimedia object size

. Fig. 2 Comparison of storage capacity requirements and data rates for different size multimedia objects [19]

content will require hundreds of gigabytes, maybe beyond the 200-GB maximum projected with multilayer blue laser products. It is likely that in the early part of this decade resolutions as high as those currently used in digital cinema (4K
2K pixel; HDTV is 2K
1K pixel) could appear in some high-end households and could be commonplace by about 2020. In addition, humans want ever more realistic video experiences which require even higher resolution and depth. Imagine, for instance, the storage requirements for a 3D movie displayed in full depth at 4K resolution in some sort of home projection system viewable from every direction. Such products could well become common household products within the first 30 years of this century. The result would be demanded for optical physical distribution media with storage capacities of 1 TB or higher. One-terabyte discs or larger is a possible role for mass produced holographic discs, large number of optical layers on a disc, or for even higher-frequency optical disc products. Even with the rise of content distribution through the internet, as long as the resolution requirements increase faster than the bandwidth available to most consumers, there will continue to be demand for physical media. The mass production cost of optical media tends to be a few cents per disc. As the storage capacity increases, the cost per unit of storage becomes very low. As we move from DVD to blue laser optical discs, the price per GB consequently decreases. The $/GB of optical media will continue to drop if holographic recording or even higher-frequency optical recording moves into consumer products.

283

284

3.2.1 7

Digital Image Storage and Compression

Directions for Further Research

As transport and storage technologies improve, it is likely that even higher video resolutions than UHD will be developed, especially for larger format display. In addition to higher resolution, adding true stereoscopic depth to images, especially moving images at very high resolution will require even faster data transfer rates and storage capacities. Captured data with multiple camera angles would require immense storage by today’s standards, and even a commercial 2-h movie distribution media with very-high-resolution stereoscopic content could require 1 TB (1012 bytes) of information or greater [7]. We must develop storage systems capable of finding and using this content effectively as we work with today’s usually smaller content files. As the ways to use content increases with many different size display devices, transcoding of one content format to another becomes important. Transcoding processes are needed that can be done real-time enabled by faster electronic signal processing. This will ease management issues associated with multiple formats of a single piece of content.

8

Conclusions

Rich content requires lots of storage and with growing resolution storage requirements are increasing. Professional content continues to increase in resolution and richness, stereoscopic moving images being an example of this. Making higher-resolution content useful will require faster processing and delivery systems operating at much higher data rates. Compression of content will help in making the most use of the bandwidth available, but the amount and type of compression is a function of how the content is used. Lossy compression is generally only appropriate for content delivery. Even with the use of compression, the digital storage requirements for the professional media and entertainment markets will continue to increase.

References 1. How many pixels are there in a frame of 35 mm film, Brad Templeton’s Photo Pages. http://pic. templetons.com/brad/photo/pixels.html 2. Rabbani M, Jones PW (1991) Digital image compression techniques. SPIE Press, Bellingham 3. Digital photography: photo file formats. www. cywarp.com/faq_digital_photo_formats.htm 4. Tagged Image File Format. http://www.scantips. com/basics9t.html 5. Welch TA (1984) A technique for high performance data compression. IEEE Comput 17(6):8–19 6. Ziv J, Lempel A (1977) A universal algorithm for sequential data compression. IEEE Trans Inf Theory IT-23(3):337–343 7. A basic introduction to PNG features. http://www. libpng.org/pub/png/pngintro.html 8. JPEG – Joint Photographic Expert Group. http:// www.scantips.com/basics9j.html 9. GIF files. http://webstyleguide.com/graphics/gifs.html

10. Digital image file types explained. http://www.wfu. edu/matthews/misc/graphics/formats/formats.html 11. Coughlin TM (2008) Digital storage in consumer electronics. Newnes Press, Burlington, MA and Oxford, UK 12. Common video data rates. Integrity Data Systems. www.integritydatasystems.net 13. The Inquirer (2006) Industry plays with ultra high definition 14. Video data specifications. www.mpeg.org 15. NHK Broadcast Ultra HD Web site. http://www.nhk. or.jp 16. What is ultra HD? http://www.ultrahdtv.net/ 17. Business Wire (2000) From film to floppy to CD: new Sony Mavica first to store images on CD-R, using Cirrus Logic optical storage chip, August 7, 2000 18. EE Times (1999) One inch no cinch for IBM storage gurus, July 7, 1999 19. Coughlin TM (2008) Storing your life, distinguished lecture for IEEE consumer electronics society

Digital Image Storage and Compression

3.2.1

Further Reading Miano J (1999) Compressed image file formats. ACM Press, New York Panasonic (2006) The video compression book. www. roadcastpapers.com Symes P (2001) Video compression demystified. McGraw-Hill, New York

Taubman D, Marcellin M (eds) (2002) JPEG2000: image compression fundamentals, standards and practice, The international series in engineering and computer science. Kluwer Academic, Norwell

285

3.2.2 Video Compression Scott Janus 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

2

Chroma Subsampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

3

Color Difference Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

4 Predictive Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 4.1 Coded Versus Frame Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 5

Transform Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292

6

Quantization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

7

Entropy Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

8

Deblocking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

9

Decoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

10

Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

11

MPEG-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

12

AVC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

13

VC-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

14

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

15

Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.2.2, # Springer-Verlag Berlin Heidelberg 2012

288

3.2.2

Video Compression

Abstract: This chapter provides an introduction as to why video compression is necessary (namely due to technological transmission and storage constraints). After a brief historical overview which sets the context that video compression has been with us since the first television broadcasts, it describes the fundamental compression stages used in contemporary codecs (i.e., entropy coding, domain transforms, and temporal coherence). This chapter also describes how several of these algorithms were chosen due to various limitations and quirks of the human psychovisual system. The chapter concludes with a deeper look at the details of the three main video compression algorithms in use today (MPEG-2, AVC, and VC-1). List of Abbreviations: AVC, Advanced Video Coding; MPEG, Moving Picture Experts Group; RGB, Red, Green, Blue; VC-1, Video Codec 1

1

Introduction

Video refers to the electronic representation of moving images. Whereas film can be directly perceived by the human eye, video requires processing to be converted into a viewable image. As is the case with film, each individual picture does not actually move: the illusion of movement is created by rapidly presenting a series of still images. Unlike film, video is almost always compressed. Film images are uncompressed in the sense that the entire image is immediately available within a single frame. Storing uncompressed images electronically takes a large amount of space. Correspondingly, it takes a large amount of bandwidth to transfer uncompressed images from one place to another. Historically, video compression has been employed to make video systems affordable. Video compression is also used to overcome technological or physical limitations as well.

2

Chroma Subsampling

There certainly are venues that use uncompressed video. For instance, an increasing number of Hollywood movies are digitally edited and manipulated (adding special effects, for instance) using uncompressed ultra high definition (such as 4096  4096) video. This approach requires extremely large amounts of disk space and specialized high-speed networks to transform the video from one work area to another. The expense of these real-time systems is well outside the range of affordability of the average consumer, although PC-based systems that can operate on uncompressed HD video in real time are becoming more affordable. Still, even on these systems it is typically impractical to store and transfer a large catalog of content in the uncompressed domain. There are many different video compression techniques. In this section, we will examine some of the most commonly used approaches. Like all compression schemes, video encoding relies on detecting redundant information and replacing it with a more efficient representation. Video compression also takes into consideration the human psychovisual system by discarding information that is difficult or impossible for people to see. Almost all video is therefore lossy.

Video Compression

3

3.2.2

Color Difference Spaces

As discussed elsewhere in this handbook, it is very efficient to create image sensors and display devices that operate in an RGB color space (> Chap. 2.2.3). These systems sample the real world using a tristimulus approach: taking a picture of the scene by conceptually using a red filter, a green filter, and a blue filter. The resulting images can be recreated by using red, green, and blue sub-pixels. Although RGB is great for cameras and monitors, it is a very inefficient color space when it comes to human perception of reality. Many RGB spaces allocate equal bandwidth to the red, green, and blue channels, but in fact humans do not have equal sensitivity to these channels. People are most sensitive to green and least sensitive to blue. Furthermore, RGB allocates equal bandwidth to brightness and hue, even though humans can perceive changes in luminance much better than they can perceive changes in chrominance. To address these issues, video is typically stored in a family of color spaces that is colloquially known as YUV. This terminology is not formally correct; there is no color space officially named YUV. Actual color difference spaces include Y’PrPb, Y’(B’-Y’)(R’-Y’) , Y’CrCb, and xvYCC. This family of spaces breaks each color into a luminance and chrominance components. YUV color spaces offer advantages for video. For instance, many types of video processing such as brightness and contrast adjustment (see > Chap. 3.3.3 on TV and Video Processing) can be readily accomplished by operating on only the luma or chroma component. It is also possible to maintain independent resolution for luminance and chrominance information. A commonly used digital YUV format is known as YCrCb. For today’s consumer applications, 8 bits are assigned for each Y, Cr, and Cb sample. 10-bit and 12-bit versions are also used in certain professional applications. The highest fidelity version of YCrCb uses one Y, Cr, and Cb sample for every pixel, as shown in > Fig. 1. This approach is known as 4:4:4 and is very intuitive. A 4:4:4 video signal can be compressed by downsampling the resolution of the chrominance. Although this is a lossy technique, it is not perceptually intrusive because humans do not notice color differences as readily as they do intensity differences. Almost all consumer video is stored and transmitted in a 4:2:0 format (> Fig. 2) wherein there is only one chroma

= Y sample = Cr, Cb samples

. Fig. 1 4:4:4 sampling

289

290

3.2.2

Video Compression

= Y sample = Cr, Cb samples

. Fig. 2 4:2:0 sampling

pair sample for every 2  2 grid of pixels. By reducing the chroma resolution to 25% of 4:4:4, the 4:2:0 space offers substantial bandwidth savings with relatively minimal artifacts. Specifically, 8-bit 4:2:0 has 12 bits per pixel, compared to 8-bit 4:4:4 that has 24 bits per pixel. Although chroma-subsampled video is compressed compared to 4:4:4, such content is generally labeled as uncompressed in the vernacular of the video industry. In this domain, video compression refers to a set of well known techniques for converting YUV data into a compressed bitstream. The most commonly used compression standards for widespread consumer consumption are MPEG-2, H.264, and VC-1. These will be discussed in more detail later in the chapter. However, the underlying concepts of all these formats, as well as other formats such as MPEG-1 and H.263 are fundamentally the same. As such, we will first review these basic techniques and later describe how each of the formats uses them differently.

4

Predictive Coding

There is a lot of redundancy within video. For a typical video stream, any two adjacent frames are likely to be quite similar. A great amount of compression can be had by taking advantage of temporal coherence between adjacent frames. For example, rather than completely storing a frame, we can instead take the approach of just storing the differences between the current frame and one or more reference frames. Conceptually, in the case of a camera pan of static scene, picture 1 could be stored as ‘‘picture 0 offset by two pixels to the left and one pixel downward.’’ The offset between one image and another is stored as a two-dimensional motion vector. This temporal compression technique is an example of predictive coding, so-called because a compressed sample is estimated from a previous decoded sample. When the prediction is done from a different frame, it is known as inter-coding. There is also intra-frame predictive coding, wherein a particular sample is estimated from adjacent samples. In the case of inter-coding, the processing of estimating the vectors during encoding is known as motion estimation. The decoding equivalent is known as motion compensation. Merely storing motion vectors is not sufficient for general video. Motion information will not capture changes in brightness or color, handle the introduction of new elements to the scene (such as someone walking on screen), or handle changes in perspective due to camera

Video Compression

3.2.2

movement. In practice, therefore, error correction data is also stored. This difference between the block being coded and the reference block is known as the error prediction signal or residual. Inter-coding consists of creating a predicted picture using motion vectors applied to one or more reference frames, and then adding the residual terms to the resulting prediction. Inter-coding only offers compression if the storage, or coding, of the picture as motion vectors and error terms is smaller than describing the picture in a completely self-contained manner. If inter-coding a particular region of the image did not create good compression – as might be the case during a scene change – then that region can instead be intra-coded. Intuitively, it makes sense to apply motion vectors only to regions of the scene that are moving. For instance, if somebody was waving their hand, you would ideally want to define the outline of the hand and specify a motion vector that applied just to that region. Although precisely this type of coding has been defined [1], it is rarely used in practice. It is simply too expensive to perform this type of detailed image preprocessing to detect oddly shaped regions of movement. Also, it requires a relatively large number of bits to describe arbitrarily shaped areas. The more practical approach used by modern codecs is to decompose the scene into blocks of variable size. These blocks crudely define regions of motion, but are efficient to code. One or more motion vectors are associated with each of these blocks. Specifically, the image is typically broken into 16  16 pixel regions known as macroblocks. These macroblocks may contain a single motion vector, or it may be broken into smaller units known as blocks, each of which has its own motion vector. These blocks can be small as 4  4 samples, although 8  8 blocks are quite common. These motion vectors are derived mathematically, and the algorithms used to derive them have no true understanding of the scene. As such, the derived motion vectors may not actually correspond to true motion on the screen. The simplest form of motion compensation uses a single reference picture. These types of frames are known as predicted or P frames. P frames usually reference a self-contained or intracoded (I) frame. Within a P picture, individual blocks can be coded as intra or inter blocks. Intra blocks are known as I blocks, and in this case the inter-coded blocks are P blocks. When dealing with P frames, the reference I picture comes chronologically before the P frame. In this case, the reference frame is known as a backward reference. Bi-directionally predicted (B) frames use forward and backward references. Within a B-frame, each block can be an I-block, P-block, or a B-block. The B blocks have at least two motion vectors: one for the forward reference and one for the backward reference. Some formats use an equal weighting between the forward and backward reference. More complex codecs allow for the forward and backward references to be weighted, so that one can count more heavily toward the predicted value. Classical B-frames use I and P pictures as references, never other B pictures. The bidirectional prediction allows for B-frames to be more highly compressed than P or I frames. As such, most streams consist primarily of B-frames. However, I frames must still be used (typically once every second or so) to make sure the picture quality does not degrade too far. It is hypothetically possible to encode an entire movie using the first frame as an I picture, the last frame as a P picture, and every one of the frames in between as a B-frame, but obviously the prediction of all those frames would be horribly inaccurate and have massive error terms. I frames must also be inserted frequently to the handle any corrupted bitstreams or to handle the scenario of beginning playback in the middle of stream, as may be the case when changing television channels or jumping to a particular scene in a movie. Any such random

291

292

3.2.2

Video Compression

access must begin on an I frame, otherwise the P and B pictures will use a reference that is not available. As such, the frame cannot be reconstructed. The precision of the motion vectors varies amongst formats. The simplest ones use a halfpixel resolution (often referred to as half-pel), while more sophisticated ones use quarter-pel or even eighth-pel. Generally speaking, the greater the motion vector precision, the better compression the compression ratio that can be achieved. Further compression can be achieved by taking advantage of the motion coherence. In most scenes, the motion vector of a particular block is likely to be very similar to the motion vectors of neighboring blocks. As such, compression efficiency can be further improved by predictively coding the motion vectors themselves. As an example, the motion vector of a block might be coded as the difference or error from the motion vector to the block immediately to its left.

4.1

Coded Versus Frame Order

The fact that predicted pictures may refer to forward references has a serious implication: the order of the frames in the bitstream, also known as the coded order, must be different than the display order. For instance, in a simple four picture sequence consisting of an I picture, a P picture, and two B pictures, the display order would be I, B, B, P. To help identify the frames, we can assign them numbers based on their display order. To wit: I0, B1, B2, P3. The B pictures cannot be decoded until the P picture has been decoded, therefore P3 must come in the bitstream before the B-frames. As such, the order of the pictures in the bitstream (also known as the coded order ) is I0, P3, B1, B2. These dependencies are illustrated in > Fig. 3. After a decoder has applied the motion compensation to decode the frames, it must reorder them to place them back into display order.

5

Transform Coding

Both intra-coded blocks and residuals tend to have spatial redundancy. By transforming from the spatial domain into a different domain, the redundancy can be reduced and the resulting signal more efficiently coded. A well chosen transform can decorellate the spatial redundancies and result in a set of coefficients that can be efficiently encoded. A good transform also allows for perceptual coded that takes advantage of the fact that humans are more sensitive to some spatial frequencies than others. The transform itself does not actually provide any compression,

I0

. Fig. 3 Picture dependencies

B1

B2

P3

Video Compression

3.2.2

but converts the data into a format that is more amenable to compression. The 8  8 Discrete Cosine Transform (DCT) was a commonly used transform for video coding, although it has been replaced by simpler 4  4 transforms in modern codecs.

6

Quantization

The outputs of the transform operation are quantized to provide further compression. Unlike preceding stages, the quantization is a lossy operation. The quantization is done in the transform domain so as to provide perceptual coding: higher precision is given to the coefficients that are most noticeable to the human eye. Typically, low frequency components are quantized with finer granularity, while high frequency components are quantized more coarsely. Motion vectors are also quantized. It is worth noting that quantization is the only irreversible stage of encoding. Information is destroyed and cannot be recovered. It is this stage that makes video compression algorithms lossy. Therefore, picking the correct quantization parameters is of great importance, in order for the reconstructed signal to be acceptable. The quantization stage increases the number of zero-value coefficients in a block, as many low-value components get rounded down to zero. For example, a typical 4  4 block in H.264 only has four non-zero samples. This outcome will prove helpful in the next stage of encoding.

7

Entropy Coding

Entropy coding refers to the technique of efficiently coding symbols using variable length codes. Variable length coding works by using short codes for common symbols and longer codes for infrequent symbols. In the case of English, for instance, we’d want to code the common letter E with a short symbol, and the less common letter Z with a longer symbol. Consider a simple coding scheme where every letter is coded as five bits. A is coded as 00000, B as 00001, up to Z coded as 11001. Regardless of the text being coded, it will always average out to five bits per letter. Now consider a variable length coding scheme tuned to English letter frequency, say E is 011, T is 000, A is 1100, and Z is 11111101010. At first glance, it seems unlikely that this is an efficient scheme, since some letters are coded at much more than five bits. However, for large passages of text, compression does indeed happen. For instance, a word like TEATIME can be encoded at about 3.7 bits per letter. Effective variable length encoding requires a coding scheme that is tuned to data be coded. A code table designed for standard English text would not fare as well when applied to acronymriddled instant messages (i.e., TTYL, LOL, CYA). The code would also be very inefficient when applied to a stream of random letters. There needs to be some symbols that consistently appear more frequently than others in order for variable length coding to be effective. Luckily, this condition is true for natural video content that has been predicted, transformed, and quantized. As such, video compression schemes use variable length encoding to great effect. The two fundamental approaches are using a fixed coding table for all video content and using adaptive coding. Fixed code tables require careful analysis of a wide variety of video streams during the creation of the compression standard. Once this a prior work is complete, implementation of

293

294

3.2.2

Video Compression

a encoder or decoder is relatively inexpensive as the table can be hard-coded into place. However, since this approach provides a coding scheme that is effective on average to every possible video stream, there are certainly going to be specific video streams that are not coded as efficiently as others. The converse approach of context adaptive entropy coding allows for the encoding-time creation of a coding table that is well-tuned for the current video stream. However, this efficiency comes at the cost of great complexity and cost in the encoder and decoder.

8

Deblocking

The block-based transform can sometimes lead to noticeable visual artifacts, such as obvious discontinuities between neighboring blocks. This blockiness most notably manifests when insufficient bits are allocated to compress the picture, which can happen commonly in lowbitrate video or in scenes of high complexity, such as one of waterfall. Deblocking filters can be applied as part of the decoding process to alleviate these artifacts. The deblocking operation does not provide any compression in and of itself, but for some types of video it does allow a lower bitrate to be used without a noticeable degradation in visual quality. Deblocking can be applied to reconstructed pictures before they are used as references for other pictures. This approach is known as in-loop deblocking. Filtering can also be applied out of loop, where the deblocked pictures are not used for any predictions, but are just intended to be displayed.

9

Decoding

It is worth noting that video compression specifications typically specify the behavior of the decode operation and bitstream syntax, but not how an encoder operates. Given a valid compressed bitstream, there will only be one valid decompressed set of frames. It is usually relatively straightforward to determine if a decoder is compliant to the spec by seeing if the output of a candidate decoder matches that of a reference decoder. By comparison, there is a many-to-one mapping of uncompressed frames to valid compressed bitstreams. Two different encoders can come up with completely different – yet equally valid – compressed bitstreams. Even the output of a single encoder can vary wildly depending on how parameters such as target bitrate are adjusted. This approach allows for encoder developers to differentiate based on quality and performance while maintaining industry-wide playback compatibility. Much of the value of a particular encoder stems from the peculiarities of its underlying algorithms. There are professional grade encoders that offer radically better quality compared to more inexpensive applications. Let us now put all of the aforementioned compression tools together and see how they interoperate. > Figure 4 is a block diagram of a generalized decoder. It is a simplified schematic showing the operations common to classical compression algorithms such as MPEG-2. Beginning with the compressed bitstream, entropy decoding is applied. The primary outputs of this stage are quantized coefficient data and motion vectors. On a block by block basis, the quantized coefficient data is scaled in a process known as inverse quantization (which is simply a multiplication). The results are moved back into the spatial domain via

Video Compression

Compressed bitstream

Entropy decoding

Inverse quantization

Motion vectors

Inverse transform

Motion compensated prediction

+

3.2.2 Deblocking

Intra prediction

Uncompressed pictures

Memory

. Fig. 4 Generalized decoder block diagram

an inverse transform. At this point, the block either contains actual pixel samples or error prediction terms. In the later case, the block is added to a predicted block to generate pixel samples. The predicted block is created either using motion vectors (inter-prediction) or by predicting values within the block itself (intra prediction). The final stage is the application of a deblocking filter, resulting in the final uncompressed picture. The resulting picture may now also be used as a reference for decoding other pictures.

10

Encoding

Encoding is a more complicated procedure than decoding. This reality is not accidental. In most applications, including broadcasts, webcasts, and optical disk storage, the ecosystem has many more decoders than encoders. It thus makes a great deal of economic sense to have relatively inexpensive decoders available to consumers and limit more costly encoders to the smaller number of content providers. There are also scenarios such as teleconferencing where there are a 1:1 ratio of encoders and decoders. The encoder contains most – if not all – of a decoder, as can be seen in careful inspection of > Figure 5. Specifically, the inverse quantization, inverse transform, deblocking, and motion compensation stages of the decoder are used in encoding. The entropy decoder is not necessarily needed, although it is often included in more sophisticated multi-pass implementations that are optimized for high quality compression. When an uncompressed block is fed into the encoder, the first thing that must happen is the encoder must decide if the block will be inter-coded or intra-coded. If block is part of an I picture, it must naturally be intra-coded. However, for most blocks in P and B pictures, each block can be either inter-coded or intra-coded. Generally, the encoder must compare the coding efficiency of each case and then choose which one provides the most efficient representation. This mode decision is typically made by performing motion compensation on the block and comparing the results with intra-coding it. The mode decision also has to determine a block shape, motion vector accuracy, motion vector prediction mode, as well as other coding details. It is a complex operation. The mode decision is a complex operation. Just considering motion vector selection, several different vectors have to be examined. The motion compensation stage ultimately

295

296

3.2.2 Uncompressed picture

+

Video Compression

Entropy coding

Quantization

Transform

Compressed bitstream

Inverse quantization

Motion estimation

Inverse transform

Motion vectors

+

Motion compensated prediction

Intra prediction

Deblocking

Memory

. Fig. 5 Generalized encoder block diagram

picks the one(s) that offer the best result given a finite search area or search time. There are various algorithms for selecting motion vectors. Many of these techniques are proprietary, as the choice of motion vectors has a major impact on the final quality of the compression. The optimal result can always be found by exhaustively searching all possible candidates, but this is computationally impractical. For quarter-pel precision, there are over 30 million candidates for a single high definition reference frame! Once a final motion vector has been chosen, it is used to form a predicted block. This block is subtracted from the original block to form the prediction error coefficients. These coefficients are transformed and quantized, with the result being entropy coded along with the motion vector. With this generalized concept of video encoding and decoding in mind, we will now embark on a brief survey of popular compression standards.

11

MPEG-2

MPEG-2 is one of the most widely used video compression formats in the world today. It is the basis of many terrestrial and satellite television systems, is used in DVD-Video disks, and is one of the three formats supported in Blu-ray. It was created by the Moving Picture Experts Group (MPEG) in 1994 to replace MPEG-1. MPEG-1, designed to store VHS-quality video at

Video Compression

3.2.2

CD bitrates had very limited applicability. MPEG-2 was designed to support a wide range of applications, including high definition studio grade video. MPEG-2 has support for 4:2:0, 4:2:2, and 4:4:4 video. The MPEG-2 standard includes video and audio compression schemes, as well as a multiplexed stream formats. MPEG-2 video is specified in ISO/IEC 13818-2 [2]. It is also specified as ITU-T Recommendation H.262, but the MPEG-2 moniker is far more common. MPEG-2 uses half-pel precision motion vectors applied to blocks of size 16  16, 16  8, or 8  8. The format does not offer any sophisticated intra prediction. Its P pictures use a single reference, and the B pictures use a forward and a backward reference. The B pictures use an equal weighting between the two references. The entropy coding uses a fixed variable length coding table. The only supported transform is an 8  8 Discrete Cosine Transform (DCT). MPEG-2 is a very successful codec and has been widely adopted throughout the industry. It will continue to be used for quite some time, even though more recent formats – namely, the next two formats we will discuss – offer better compression quality.

12

AVC

A follow-on to MPEG-2 was standardized by MPEG and ITU in ISO/IEC 14496-10 [3] and ITU-T Recommendation H.264. It is colloquially referred to by many different names. AVC and H.264 are the two most common labels; other variations include MPEG-4/AVC and MPEG-4 Part 10. It is occasionally referred to as JVT, because it was developed by the Joint Video Team. Sometimes the term MPEG-4 is used, but this term is ambiguous, as there is also a video standard known as MPEG-4 Part 2. Part 2 defines a standard that is incompatible with Part 10. AVC has found much wider adoption in the industry than MPEG-4 Part 2. AVC was developed to provide greater compression than that offered by MPEG-2. It is targeted at a broad range of usage models including broadcasting, digital storage, and webcasting. As is the case with MPEG-2, it has support for a wide span of resolutions and bitrates. Likewise, it supports 4:2:0, 4:2:2, and 4:4:4 video. AVC supports 8 to 12 bits per sample. AVC can achieve about 2X bit rate savings compared to MPEG-2[4]. Naturally the exact savings depends on many variables, including target bitrate and image resolution. However, AVC consistently outperforms MPEG-2. AVC achieves its high compression through the use of more complex tools not available in MPEG-2. Unsurprisingly, AVC encoding and decoding is computationally more intense than MPEG-2. In particular, CABAC (discussed below) is quite a challenge. The increased efficiency comes at the cost of more expensive components. AVC decoders have proven affordable on the commercial scale, and the format is used for Blu-ray disks, television broadcasting, and Internet streaming, most notably by YouTube. AVC supports quarter-pel motion vectors. For motion prediction, the macroblocks can be decomposed into 16  16, 16  8, 8  16, 8  8, 8  4, 4  8, and 4  4 blocks. Unlike MPEG-2, P pictures can use multiple backward references, as shown in > Fig. 6. As such, each motion vector is associated with an index number indicating which reference frame it uses. The standard B-picture concept is likewise extended for AVC. Entities known as B-slices can use multiple previously decoded pictures as references. These references can be forward only, backward only, or backward and forward. Any given block within the slice can only use two references at a time, but the references used can vary from block to block within the slice. The two references can also be arbitrarily weighted, as compared to the equal weighting used in

297

298

3.2.2

Video Compression

Previously decoded pictures

Current picture

. Fig. 6 Multiple backward references

MPEG-2’s B pictures. The arbitrary weighting can be very useful in situations like a crossfade between two scenes. Blocks in B-slices can also use direct mode, wherein the motion vector is not explicitly coded, but recovered from previously decoded information. Unlike many other standards, B pictures can also be used as references. In addition to inter-prediction, AVC supports intra-block prediction. Samples in a block are predicted from already decoded blocks. > Figure 7. shows some examples of the different types of predictions. The coding of this simple intra prediction mode and resulting residuals typically results in a smaller compressed bitstream than non-predicted intra-coding. Whereas MPEG-2 used fixed 8  8 DCT transforms, AVC primarily uses a 4  4 transform. This smaller size allows better mapping of blocks to the boundaries of moving objects in the scene. The AVC transforms exclusively use integers, making them cheaper to implement and free of any floating point precision issues endemic to DCT. AVC supports two types of entropy coding: Context-based Adaptive Variable Length Coding (CAVLC) and Context-Based Adaptive Binary Arithmetic Coding (CABAC). Being adaptive, both of these schemes offer greater compression efficiency compared to static variable length coding schemes, such as used in MPEG-2. Of course, this increased efficiency comes at the price of increased encoder/decoder complexity. CAVLC is the simpler of the two schemes. It uses several different variable length coding tables. A particular code set is chosen as the input stream is parsed, with the most efficient set being chosen depending on the content. CABAC is substantially more complex than CAVLC, but offers greater compression efficiency. CABAC has three major steps: binarization, context modeling, and binary arithmetic coding. In binarization, elements such as motion vectors and transform coefficients are converted into a binary code. This operation is similar to assigning a VLC to symbols, but it is only first stage of CABAC. Next, binarized symbols are analyzed to select a probability model that is well suited to code the stream. Finally, the probability model is used to arithmetically code the symbols. Other AVC tools include motion vector prediction and an in-loop deblocking filter.

13

VC-1

VC-1 is a video compression format that is a contemporary of AVC, and offers roughly the same level of compression compared to AVC. It was derived from Microsoft’s WMV9 video

3.2.2

Video Compression

M

A

B

C

D

E

F

G

M

H

I

I

J

J

K

K

L

L Mode 0: Intra_4⫻4_vertical M

A

B

A

B

C

D

E

F

G

H

Mode 1: Intra_4⫻4_horizontal C

D

E

F

G

H

I J K L Mode 3: Intra_4⫻4_diagonal _down_left

. Fig. 7 Intra prediction examples

format, and standardized by SMPTE as VC-1 in SMPTE 421M [5]. VC-1 has numerous minor changes compared to WMV9, perhaps the most notably being the addition of interlaced content support. There is a lively debate as to whether VC-1 or AVC is better. The codec that offers the best results depends on the content being encoded, the resolution and bitrates involved, and personal preference with regards to the subtle artifacts of each of the formats. On the whole, VC-1 is less complex than AVC. Keeping VC-1 simple while maintaining high quality was a conscious design goal of VC-1(5). Some of the simplicity was achieved by limiting the scope of the format. For instance, it only supports only supports 8-bit per sample 4:2:0 video. This limitation is acceptable for delivering video to consumers, but makes the format unacceptable for high end studio grade use. VC-1’s most common usage is on Blu-ray disks. It is also used in Microsoft’s Silverlight framework for distributing video across the Internet. VC-1 supports quarter-pel motion vectors with classical P and B pictures: P pictures refer to one backward reference. B pictures use bilinear prediction between one backward and one forward reference. One unique element of VC-1 is its intensity compensation feature. Designed to efficiently code crossfades between scenes, it allows scaling of the luma and chroma values of the reference pictures prior to motion compensation. VC-1 has an integer-based adaptive transform scheme, where 4  4, 8  4, 4  8, 8  8 blocks can be used. The flexible block sizes allow for precision mapping of moving objects in the scene. VC-1’s intra prediction is done in the frequency domain, unlike AVC’s spatial domain intra prediction. VC-1 uses a frame-adaptive VLC for entropy coding. Similar to AVC, VC-1 also includes a deblocking support. VC-1’s deblocking filter is applied to a narrower region around block boundaries compared to AVC.

299

300

3.2.2 14

Video Compression

Conclusions

Video must be compressed for practical and affordable storage and transmission. There are several different video compression standards in use in the industry today. Some are open standards, while others are proprietary. In general, they all take advantage of spatial and temporal coherence in the video, as well as coherence in the intermediate codes. Pictures are divided into regular blocks. These blocks are intra-predicted or inter-predicted, transformed, quantized, and entropy coded. The most prevalent codecs today are MPEG-2, AVC, and VC-1, with the later two offering roughly two times the coding efficiency of the former.

15

Directions for Future Research

Video compression is by no means a completed technology. Research continues into new algorithms that endeavor to greatly improve upon the efficiency offered by AVC and VC-1. One example is the ITU’s efforts on H.265.

References 1. ISO/IEC 14496-2 Coding of audio-visual objects – Part 2: Video 2. ISO/IEC 13818-2 Generic coding of moving pictures and associated audio information, Part 2: Video 3. ISO/IEC 14496-10 Coding of audio-visual objects – Part 10: Advanced Video Coding

4. Wiegand T et al (2003) Rate-constrained coder control and comparison of video coding standards. IEEE Trans Circuits Syst Video Technol, s.l., July 2003, vol. 13, pp. 688–703 5. SMPTE 421M VC-1 Compressed Video Bitstream Format and Decoding Process

Further Reading Ostermann J et al (2004) Video coding with H.264/AVC: tools, performance, and complexity, vol 1. IEEE Circuits Syst Mag 4:7–28 Wiegand T et al (2003)Overview of the H.264/AVC Video Coding Standard. 7, July 2003. IEEE Trans Circuits Syst Video Technol 13:560–576

Janus S (2002) Video in the 21st Century. Intel Press, s.l. Keith J (2007) Video demystified: a handbook for the digital engineer, 5th ed. Newnes, s.l Poynton C (1996) A technical introduction to digital video. Wiley, s.l.

3.2.3 Fundamentals of Image Color Management Matthew C. Forman . Karlheinz Blankenbach 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

2 2.1 2.1.1 2.1.2 2.2 2.2.1

Practical Color Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 Color Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 Specification and Transport of Color Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 Generating Color Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 The Color Management Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 Gamut Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

3

Color Management System Implementations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

4

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.2.3, # Springer-Verlag Berlin Heidelberg 2012

302

3.2.3

Fundamentals of Image Color Management

Abstract: Ideally in modern multimedia systems, exact colors and tones should be maintained throughout the processing chain from input device (e.g., scanner, camera) to output device (e.g., display monitor, printer). This ensures that all viewers will be presented with practically the same final display which is also consistent with the image originally captured. Each realworld capture or display device, however, interprets color values in its own specific manner, even if devices use ostensibly the same representations (e.g., RGB levels). Tonal (gray level) responses also vary widely. Specific steps must therefore be taken to ensure consistency of output on different devices; color management systems provide frameworks for this. This chapter presents the fundamental concepts of image color management and investigates some of the practicalities of their application in color management systems. List of Abbreviations: CIE, Commission Internationale de l’E´clairage (International Commission on Illumination); CMM, Color Management Module; CMY[K], Cyan, Magenta, Yellow, [Black] (Color Coordinates); CRT, Cathode Ray Tube; ICC, International Color Consortium; LC, Liquid Crystal; LCD, Liquid Crystal Display; LED, Light Emitting Diode; PCS, Profile Connection Space; RGB, Red, Green, Blue (Color Coordinates); UCS, Uniform Chromaticity Scale

1

Introduction

Color displays and print systems should be able to reproduce images with color and luminance content that are as close as possible in an absolute sense to those in an original, real-world scene. However, maintaining accurate real-world image colors through complex digital processing chains and a wide variety of input and output hardware devices requires significant efforts. Although image sensors in cameras and scanners generate RGB signals and display systems are also RGB-addressed, these red, green, and blue intensity values are not related in any consistent, deterministic way. Such RGB representations are examples of device-dependent color spaces. In a simple everyday example to illustrate the issues involved (> Fig. 1), a digital camera captures a picture. This is transferred to a computer where it is displayed on an LCD monitor as a preview and possibly manipulated, and is also printed on paper using cyan,

Output devices

Input device Computer processing

RGB (monitor)

RGB (camera)

CMY[K] (printer)

. Fig. 1 A typical color image processing chain, involving three separate device-dependent color spaces

Fundamentals of Image Color Management

3.2.3

magenta, yellow, and black inks. There are many differences between the spectral responses of the camera sensor and monitor pixels, and many variables involved in converting to a chemical-based ink/paper process. Unless account is taken of these issues, differences between the original scene and the displayed and printed outputs in terms of gray level and color representation are highly likely to occur. Color management systems take steps to minimize these variations so that displayed and printed colors more closely match those originally captured [1–3]. This is important to ensure consistency of output both between individual jobs running at different times and between different installations. Accurate characterization and control of color in a digital imaging process also means that useful techniques such as soft proofing become possible. This is the previewing of an image intended to be output on a certain device (for example a printing press), by displaying it on a different device (e.g., a monitor) in a representative manner.

2

Practical Color Management

The overall goal of digital color management is to maintain understanding and control of colors, and the relationships between colors, in a digital image processing chain. This is usually achieved by defining a device-independent color representation to serve as an absolute, neutral color reference space. Device-dependent source colors as produced by an input device are mapped into this reference space, followed by onward conversion to the device-dependent destination color space of the display or print process [4]. The International Color Consortium (ICC) [5] has defined a standard layout for color management, and has also specified flexible processes for conversion between device color spaces [6]. An image is captured in a device-dependent color space, such as camera or scanner RGB. The colors in the source image are converted internally by the Color Management Module (CMM) software into a device-independent Profile Connection Space (PCS). The PCS represents chromaticities directly, and is thus completely independent of the specifics of particular input or output device color characteristics.

2.1

Color Profiles

The conversions between input and output device color spaces that take place in the CMM are specified by color profiles. These are collections of data structures which characterize the source and target device color spaces to the CMM. When fed to the CMM along with colors from the source image data, a pair of color profiles takes care of the mapping from source to destination device colors via the intermediate PCS. A color profile describes a color space in terms of a set of colorimetric parameters, defining the mapping to/from the PCS. These are chiefly: grayscale gamma value or curve (tone response), white point chromaticity, reference luminance, and primary color chromaticity coordinates. The profile defining the sRGB color space (see > Table 1 for basic parameters) is a commonly available example. sRGB was developed as a standard color space for computer processing and communication of images via the Internet, with a response that approximates most CRT computer monitors for reasonable cross compatibility with end-user computer systems that do not include color management [7].

303

304

3.2.3

Fundamentals of Image Color Management

. Table 1 Basic sRGB color space definitions Parameter

Value(s)

Luminance

80 cdm

White point

D65 (x = 0.3127, y = 0.3291)

Primaries (CIE 1931)

Red: x = 0.64, y = 0.33; Green: x = 0.30, y = 0.60; Blue: x = 0.15, y = 0.06

Gamma

2.4

Ambient illuminance

64 lx

Ambient white point

D50 (x = 0.3457, y = 0.3585)

Veiling Glare

1%

2.1.1

2

for 100% white

Specification and Transport of Color Profiles

For color management to be a practical proposition, color profiles must be self-contained, portable entities that can be made readily available alongside image data that is in the color space they describe, ready to be used by a CMM in any system that needs to convert such an image to its destination device space. Color profiles are generally provided in ICC (or ICM) files. These use a standard file format defined by the ICC for the purpose. Manufacturers of input devices (cameras, scanners) and output devices (monitors, printers) often provide ICC profile files with their products, though these often need tweaking and regenerating to suit local conditions where color workflows need to be particularly well calibrated. The calibration data stored in ICC files can also be embedded in various common image files using special extensions. TIFF, JPEG, PDF, SVG, and PNG are well-known formats with this facility. Embedding of a profile, directly describing the color space to which image pixel values relate, greatly aids the color management process by removing any ambiguity about the source color space for a profile-aware application opening the image. Digital cameras can often be set to embed a profile describing their source color space in this way automatically in every image captured.

2.1.2

Generating Color Profiles

Many input and output devices are not supplied with ICC profile files, and a calibration process is needed to generate a suitable one to match the actual response of a device [8]. Even if a profile is supplied as a result of a manufacturer’s own calibration procedures, it is also often necessary for an end user to repeat it at some stage because the color response of a device can shift for many reasons. In printing, ink and media variations will do this, and display screen characteristics will change over the lifetime of the unit, not to mention the effects of fluctuations in ambient light and other external conditions. In environments such as commercial printing, where output consistency and accuracy is a very important consideration, calibration is normally carried out at regular intervals as an element of an organization’s quality assurance procedures. In general, calibration to generate profiles is performed using a standard color test chart containing a number of known chromaticity samples uniformly distributed in the device color

Fundamentals of Image Color Management

3.2.3

space – see > Fig. 2 for a representative example using output device RGB coordinates (though note that the chart as reproduced here is not suitable for use in any calibration procedure). Test targets with patches of known gray and primary color tone densities are also used to set the gamma (tone curve) parameters in the profile. Such charts are sometimes provided, together with calibration software, with mid-range image scanners. They are more generally available as part of aftermarket color profiling kits intended to generate profiles for several types of device in environments where recalibration is often needed [9, 10]. Calibrating an RGB input device such as a scanner is a fairly straightforward procedure, since scanners have their own enclosed light sources with known properties. Various test charts are captured, and the resulting images are loaded into supplied calibration software. This software reads the scanned color and tone patches. Using a priori information on the absolute chromaticity and tone values of the test charts, it then produces a profile that transforms the device RGB values of the scanned test files to the PCS with errors minimized over the whole representable color space and all luminance (tone) levels. Calibrating a camera uses a similar process, though great care must be taken to use appropriate lighting of the test targets when they are captured. To calibrate an RGB output device (e.g., a monitor), the same process is effectively used in reverse. Rather than measuring known chromaticities and tones and then measuring the device’s response, a series of known device RGB colors and tone values are displayed (under well-controlled lighting conditions and monitor setup) and the absolute chromaticity and luminance values are then measured using a colorimetric probe temporarily attached over the display. Calibration software controls the entire process in a closed loop: it displays the sample patches, directs the colorimeter to take measurements, and then processes the results to produce the final ICC profile file. Simple calibration processes can often be performed by computer users without specialized equipment using only operating system software utilities, though these are no substitute for dedicated calibration systems in terms of accuracy of the final profile. Some high-end LCD monitors are available with built-in color and/or luminance calibration functions. These are particularly useful in environments where consistent, accurate reproduction is essential and ambient luminance variations must be compensated for, such 94 28 13

241 149 108

97 119 171

90 103 39

164 131 196

140 253 153

255 116 21

7 47 122

222 29 42

69 0 68

187 255 19

255 142 0

0 0 142

64 173 38

203 0 0

255 217 0

207 3 124

0 148 189

255 255 255

249 249 249

180 180 180

117 117 117

53 53 53

0 0 0

. Fig. 2 Color test chart adapted from MAZeT, Jena, Germany, with corresponding device stimulus RGB values (8 bit, R top, G center, B bottom)

305

306

3.2.3

Fundamentals of Image Color Management

as in clinical medical imaging and prepress applications [11, 12]. In an LCD monitor, the color characteristic of the backlight is of great importance in ensuring well-calibrated output colors, so these systems use sensors to measure the luminance and chromatic output of the backlight either before or after it passes through the LC matrix layer. They then make automatic compensating adjustments to the backlight chromaticity, typically by modulation of separate red, green, and blue LED intensities. Calibration of printers, usually CMYK devices, is carried out using a conceptually similar process as is used for monitors. Test prints are made, comprising patches of known device color and tone on media with a certain white point. The absolute chromaticity and density of each patch is then read using a colorimeter with a self-contained light source. Finally the calibration software generates a profile for this particular combination of printer settings and media. Typically, many profiles are generated for various different combinations of print media and printer resolution and output settings, and possibly even different inks.

2.2

The Color Management Module

The Color Management Module contains the core processing functionality in the ICC color management system. It is a piece of software which, given source and destination color profiles, handles the conversion of image colors from the input to the output device color spaces, via the Profile Connection Space. The role of the CMM in the color management system is shown in > Fig. 3 and is now described in detail. An image taken by a camera or scanner is in a device-dependent space, specified by its color profile. The CMM extracts a set of calibration information from this profile, and uses it to map colors into the PCS. This is a device-independent space which represents colors as chromaticity values that map well to human color perception. In practice, either the CIE L∗a∗b∗ or CIE XYZ color spaces are usually used as the PCS. The input mapping takes place in two stages: firstly gamma adaptation to account for the tonal response of the device, and secondly a matrix-based transformation of color coordinates. A look-up tablebased transformation can be used in place of the matrix transformation if faster processing is needed (at the expense of a small reduction in color accuracy). At this point, image colors are in the device-independent PCS. The reverse of the above input transformation is now applied, using calibration data from the destination device profile, From camera, scanner, etc.

To display, printer, etc.

Input device space Gamma adaptation

Matrix/LUT transform

Input profile (calibration data)

Profile Connection Space Gamut mapping

Rendering intent

Output device space Matrix/LUT transform

Gamma adaptation

Output profile (calibration data)

. Fig. 3 Typical color data path in the ICC color management framework

3.2.3

Fundamentals of Image Color Management

to produce image colors in the output device color space. Note that the matrix or look-up table color coordinate transformation also takes account of the source or destination color coordinate system (RGB, CMY, CMYK, or even 6- or 8-ink subtractive systems in some print applications). When the image is displayed or printed, the chromaticities and tones of points captured by the input device will be reproduced as accurately as possible to the viewer.

2.2.1

Gamut Mapping

There is one significant caveat in the color transformation process as described. In practically all cases, the color gamuts of the input and output devices (the ranges of chromaticities that the devices can represent) will not match, and a further gamut mapping stage is needed to map input to output chromaticity coordinates within the PCS [13, 14]. > Figure 4 illustrates the problem by plotting the color gamut of a typical CMYK printer and RGB monitor in deviceindependent coordinates. The gamut of the printer is much smaller than that of the monitor, and either device can only represent a small subset of the total CIE 1976 color range. Only colors represented by the area of overlap between the printer and monitor can be reproduced accurately by both devices. The situation is similar when comparing typical input and output device gamuts, and algorithms must be used to map from one to the other according to a specific set of rules. This color rendering procedure defines the way in which source colors in the PCS are treated if the input and output device gamuts do not match, for example, if input chromaticity values fall outside the smaller gamut of the destination device in the PCS. Very broadly, there are two distinct approaches to the problem; > Fig. 5 represents a situation where the gamut of the output device is far smaller than that of the input device, as will typically be the case when transforming a scanned image for print. The input device in the figure has a larger span (represented here as blue to green) than the output device, so the problem is of determining what should be done with input colors that are outside the span of the output device. If colors

0.6 0.5

Printed CMYK

v′

0.4

Monitor RGB

0.3 0.2

CIE 1976 UCS 0.1 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

u′

. Fig. 4 Illustrative comparison of the color gamut of a typical RGB emissive display and a typical CMYK printer

307

308

3.2.3

Fundamentals of Image Color Management

Source gamut

Destination gamut

a

Colorimetric rendering

b

Perceptual rendering

. Fig. 5 Basic color rendering approaches from a larger to a smaller gamut. (a) Colorimetric. (b) Perceptual

must be maintained at their exact chromaticity values, then colorimetric rendering is used (> Fig. 5a): chromaticities are mapped exactly from source to destination. Unfortunately however, this results in the complete loss of color information which falls outside the destination gamut; out of gamut colors are typically saturated to the chromaticity value at the top or bottom of the range. This is generally unacceptable for real-world (e.g., photographic) images, and a perceptual rendering is more appropriate (> Fig. 5b). Here, all colors in the source gamut are scaled into the destination gamut, and thus no color information at all is actually lost, although the absolute chromaticity values of all points in an image (except at the absolute center of the range) will shift from their original values. The perceptual scaling operation is typically not linear; it attempts to transform colors with greater perceptual relevance in the final display with greater accuracy than others. In the ICC color management framework, four color rendering schemes are encapsulated as specific rendering intents. Each of these has a different aim and makes specific assumptions to suit different applications. The perceptual intent compresses or expands the full gamut of the input device to fill the full gamut of the destination device. Tonal balance is therefore preserved, but colorimetric accuracy is not. The mapping is nonlinear, with more perceptually relevant colors in the mid-tones being mapped more accurately than those in the lower or upper ranges. This rendering intent was designed for use with real-world (photographic) images, to give a pleasing (though not colorimetrically accurate) display. Notwithstanding numerical precision issues in device color vs. PCS representations and in the calculations made by the CMM, a transformation made using the perceptual intent is generally reversible. The saturation rendering intent preserves heavily saturated colors in an image, and is designed for use with bold computer graphics output such as business chart presentations. It ensures that saturated source colors remain vivid even in a larger destination gamut, though it does not attempt to maintain colorimetric accuracy and is therefore not suitable for transforming photographic image colors. It is also not generally reversible. When colorimetric accuracy must be preserved from source to destination, a colorimetric rendering method must be used, as already outlined. The relative colorimetric intent maintains the chromaticity values of the source device. If the destination device gamut is a superset of the source gamut there are no losses and the mapping is exact, but when the destination device gamut is smaller than that of the source, this intent clips unrepresentable colors leading to a loss of color information. Also, the mapping moves the white point if it is different for the source and destination devices; this typically occurs when comparing RGB monitor and CMYK printer color spaces. Note, however, that this white point correction will result in a shift in the

Fundamentals of Image Color Management

3.2.3

chromaticity values of the colors in the image. The relative colorimetric rendering intent is well suited for applications such as soft proofing of printer colors using a display monitor. The transformation is only reversible if the destination gamut is a superset of the source gamut. Finally, the absolute colorimetric rendering intent makes a direct mapping from source to destination gamut as for the relative colorimetric intent, but without remapping the white point if it has different coordinates in the two spaces. The results will therefore theoretically be the most colorimetrically accurate of all intents overall with respect to the source, but if the white points are in fact different then there will be a noticeable, possibly undesirable shift of all of the colors in the image as displayed or printed. It can be seen that every rendering intent represents a compromise, and it is therefore important that an appropriate one is chosen for the task at hand.

3

Color Management System Implementations

Color management can be implemented on computer systems either at the operating system or application level, or a mixture of both. The advantage of operating system color management is that input devices and output devices (via functions in device driver modules) and applications that use operating system-supplied APIs are then fully integrated. The color workflow for image data throughout the system can then be controlled from capture, through application processing, right to display or print output. Certain high-end applications, however, make use of their own CMMs to maintain full control of the workflow. This is typically done to offer more control over transformations as is needed in a production environment, to be able to address more exotic input and output devices, and to be able to offer higher quality conversions than operating system CMMs. In Microsoft Windows, the integrated Image Color Management subsystem has recently been superseded by the new Windows Color System architecture [15]. These systems allow profiles to be loaded and rendering intents to be set for scanners, cameras, displays, and printers, and transformations can be made automatically or under application control. Windows names the four ICC rendering intents described in the previous section according to the typical application of each – see > Table 2. Apple Mac OS contains the ColorSync subsystem for system color management [16], which is also integrated with camera, scanner, printer, and display drivers. It also provides a set of utilities that allow inspection and manipulation of profiles and visualization of their color spaces. Linux and UNIX operating systems do not generally have such mature support, and color and profile management is usually left to individual imaging applications to perform. . Table 2 ICC rendering intents and names used in Microsoft Windows ICC Rendering Intent

Windows name

Perceptual

‘‘Picture’’

Saturation

‘‘Graphic’’

Relative colorimetric

‘‘Proof’’

Absolute colorimetric

‘‘Match’’

309

310

3.2.3

Fundamentals of Image Color Management

Many image processing applications have color management functionality, supporting image-embedded profiles and the concepts of working and output color spaces. However most Web browsers in use, despite being the means via which a large proportion of final images are viewed, do not support color management consistently, and many do not yet support it at all.

4

Summary

Representations of colors and tones in real-world imaging and output devices are generally not compatible even if they appear to use the same variables (e.g., red, green, and blue levels). This is the case partly because they have completely different physical origins, and is also due to the influence of external variables such as ambient illumination and print media and ink differences. Environmental and component aging factors also cause variations. Color management systems attempt to correct for as many of these differences as possible by defining deviceneutral representations of color values and the means to transform image data between the device’s own color spaces and this device-independent color space. This chapter has presented these fundamental issues, introduced the ICC specification as a standardized framework for color management, and addressed some of the practicalities of its use.

References 1. Roy S (2000) Berns. Billmeyer and Saltzman’s principles of color technology, Wiley 2. Giorgianni EJ, Madden TE (2008) Digital color management: encoding solutions. Wiley, Chichester 3. Kohler T (2000) The next generation of color management system, Eighth Color Imaging Conference, Scottsdale, pp 61–64, Nov 2000 4. Green P, Macdonald L (eds) (2002) Colour engineering: achieving device independent colour. Wiley, Chichester 5. International Color Consortium (2011) International color consortium. http://www.color.org/. Accessed 11 March 2011 6. International Color Consortium, ISO 15076–1:2010: Image technology colour management – Architecture, profile format and data structure, 2010 7. Stokes M, Anderson M, Chandrasekar S, Motta R (1996) A standard default color space for the internet – sRGB, Nov 1996. http://www.w3.org/ Graphics/Color/sRGB.html 8. Green PJ, Johnson T (2000) Issues of measurement and assessment in hard copy color reproduction. Proc SPIE 3963:281–292

9. Datacolor Inc., Datacolor (2011). http://www. datacolor.com/ 10. X-Rite Inc., X-Rite, (2011). http://www.xrite.com/ 11. Abileah A (2005) DICOM Calibration for medical displays: a comparison of two methods, Planar systems. http://medicaldisplaysforless.com/Dome/WP/ DICOM_Calibration_for_Medical_Displays.pdf 12. EIZO, EIZO ColorEdge CG245W – The first selfcalibrating monitor for graphics, 2010. http://www. eizo.com/global/products/coloredge/cg245w/ 13. Stone MC, Cowan WB, Beatty JC (1988) Color gamut mapping and the printing of digital color images. ACM Transactions on Graphics 7(3):249–292 14. Morovicˇ J (2008) Color Gamut Mapping. Wiley, New York 15. Microsoft, Windows color system, 2011. http://msdn. microsoft.com/en-us/windows/hardware/gg487409 16. Apple Inc., Technical note TN2035: ColorSync on Mac OS X, 2010, http://developer.apple.com/ library/mac/#technotes/tn/tn2035.html

Part 3.3

Image Manipulation

3.3.1 Digital Image Operations Matthew C. Forman 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 1.1 Raster Image Processing Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 1.2 Color Image Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 2 2.1 2.2 2.3

Global Pixel Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 Intensity Transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 Color Saturation Adjustment and Matrix Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 Application Example: White Point Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

3 3.1 3.2 3.3 3.4

Geometric Image Operations and Resampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Raster Image Resampling and Scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Image Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 Other Geometric Image Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 Implementation of Fast Geometric Transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322

4

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.3.1, # Springer-Verlag Berlin Heidelberg 2012

314

3.3.1

Digital Image Operations

Abstract: In applications that deal with digitally represented visual images, various forms of processing are generally required before the results are ready to be displayed. Although many of the methods used are complex, all have their roots in a small number of core concepts and techniques. This chapter looks at these common core spatial domain operations, firstly reviewing those that rely on applying transformations of brightness and color in place within digital images. It then moves on to consider geometric manipulation of image data and resampling issues. List of Abbreviations: RGB, Red, Green, Blue (Color Space/Color Storage Method); HSV, Hue, Saturation, Value (Color Space); CPU, Central Processing Unit; GPU, Graphics Processing Unit; API, Application Programming Interface; Y 0CbCr , Luma, Blue-Difference Chroma, Red-Difference Chroma (Color Space/Color Storage Method)

1

Introduction

The rapid growth of digital storage of visual images has been driven by several factors. Digital representations inherently have far better robustness and noise immunity than direct analog recordings. This is extremely advantageous where both long-term storage and communication are concerned. One of the most significant advantages of digital representation, however, is the ease with which useful and complex processing operations can be implemented. The precise details of a particular implementation of a digital image storage scheme depend on the application: the manner of source content creation, storage or transmission system requirements, and the final destination of the image. At a low level, however, an image is stored in either a raster (also commonly known as bitmap or pixmap) or vector representation. A vector representation consists of instructions and parameters for drawing the final image, element by element, from geometric primitives such as lines, curves, polygons, and text. A raster format represents a lower level of abstraction of image data. It contains a sampled representation of any captured or synthesised image, and thus offers a more general means of storage. Since display systems themselves are addressed in this manner, the final destination for all image representations is effectively raster; an image in a vector format is rasterized for display by executing the appropriate drawing instructions and sampling the result. This article therefore concentrates on processing that can be achieved when an image is stored in a raster format.

1.1

Raster Image Processing Format

In the most general sense a raster image is comprised of a rectangular array of pixels (‘‘picture elements’’) [1, 2]. Each pixel is a sample of the information in a finite area of a spatially continuous image source, centered on a particular geometric location in the plane. The sample value may simply be the scalar irradiance arriving at an image sensor pixel, or equivalently the emittance of a display pixel; an array of these represents a grayscale image. Alternatively a pixel may carry color information, typically by encoding irradiance/emittance proportions of red, green, and blue light; the array of such pixels can represent a full color image. > Figure 1 illustrates the layout of a general raster image (note that the origin is also commonly at bottom left).

Digital Image Operations

Origin

3.3.1

x

y Pixel sample value at (x,y): f (x,y) H

W

. Fig. 1 General raster (bitmap) image layout

1.2

Color Image Processing

Although color images are usually RGB-encoded in capture and display devices, such a color representation is not necessarily best suited for supplying full color image data to image processing operations. Hence, for processing, images are often transformed into alternative color spaces that are more compatible with the operation(s) to be carried out, or simply for ease of implementation. For example, it is often desired to separate the luma (brightness) from the chroma (color) information and process them separately – the Y 0 Cb Cr (luma, chroma-blue and chroma-red) space is commonly used in these cases. In other operations the HSV space may be appropriate. Many image processing operations may be carried out directly in the pixel spatial domain – some of which may require resampling – though others are more easily applied in a spatial frequency domain such as Fourier space. This chapter continues by looking at spatial domain pixel operations, a group of common global processing operations that rely on applying transformations of brightness and color within digital images.

2

Global Pixel Operations

A fundamental class of image processing techniques, global pixel operations apply a single operation identically to each pixel. This section introduces a number of intensity-only and color-specific pixel operations. A second class of local pixel operations, where a number of values in a local neighborhood of the operation pixel are considered, is often used in filtering and enhancement applications (see > Chap. 3.3.2).

2.1

Intensity Transformations

An intensity transformation uses a linear transfer function to remap input pixel intensity values [1]. If f ðx; y Þ represents an intensity raster image and T ðiÞ is an intensity transfer function, then the processed image is: g ðx; y Þ ¼ T ð f ðx; y ÞÞ:

315

316

3.3.1

Digital Image Operations

A general remapping facility such as this allows a number of practical enhancement operations to be carried out. It is convenient to visualize the transfer function as a line plot relating output to corresponding input values, and software with intensity transformation features often allows transformations to be defined graphically in this way, generally with reference to the intensity histogram of the image. Some common intensity transformations are illustrated by > Fig. 2, which also shows their effects on sample images. Contrast stretching (> Fig. 2a) expands the intensity range of an image in parts of intensity space (typically the center) while compressing or clamping the intensity dynamic range in other parts. The goal is to improve the utilization of dynamic range for the most important parts of the image. The results can be seen in the example shown as improved contrast. Here, limiting low- and high-intensity ranges have been clamped to black and white. Normalization is a related process that determines stretch limits automatically from the image brightness histogram, so that the image’s existing intensity range is mapped exactly on to the maximum range available. This is particularly useful to compensate for photographic under-exposure. An offset can be added to the transfer function to increase or decrease overall image brightness; however, this generally results in saturation at the white or black level. As an alternative, nonlinear brightness adjustment (> Fig. 2b) applies a smooth curve to increase or decrease overall image brightness in such a way that saturation at maximum or minimum intensity cannot occur. A power function, such as that used for display gamma correction (see > Chap. 11.2.1), is often used.

Original image & histogram

Transfer function

Processed image & histogram

T(i)

i

a

T(i)

i

Log

Log

b . Fig. 2 Intensity transformation operations on sample images. (a) Contrast stretching. (b) Nonlinear brightness adjustment

Digital Image Operations

3.3.1

As in the examples shown, any intensity transformation can be applied to a color image by first transforming the image data into a color space which represents intensity information separately from color information, applying the transformation to the intensity component and then transforming back to the original color space. Using the Y 0 Cb Cr space, for example, the luma (Y 0 ) component would be subject to transformation, while the chroma components (Cb and Cr ) would be passed unchanged.

2.2

Color Saturation Adjustment and Matrix Methods

It is often desired to make adjustments to color saturation in an image in RGB space. One way to accomplish this is to convert the image data into HSV space and make the appropriate modification to the S (saturation) component before transforming the data back to RGB for display. However, this requires several separate operations and hence may not result in a particularly efficient implementation. It also carries the risk of introducing precision, rounding, and overflow issues. A convenient way to implement global pixel processing operations directly in RGB space uses a general matrix-based framework [3]. The matrix multiplication of the input color pixel value (in the form of a column vector) with an operation matrix yields the output pixel value. We define the operation as a 4  4 matrix to result in a general linear transformation, and several operations can then be concatenated into one just by multiplying operation matrices. The input pixel vector, F ðx; y Þ ¼ ½ fR

fG

fB

1T

with fR , fG , and fB being the source red, green, and blue component values, and the output pixel vector, G ðx; y Þ ¼ ½gR

gG

gB

gw T

with gR , gG , and gB being the destination red, green, and blue component values, and gw is not generally computed. If T is a 4  4 matrix defining the desired pixel operation, then the overall operation is represented as: G ðx; y Þ

¼

T :F ðx; y Þ:

The operation matrix for saturation adjustment, 2

aþs 6 a 6 T sat ðsÞ ¼ 4 a 0

b bþs b 0

g g gþs 0

3 0 a ¼ 0:3086ð1  sÞ 07 7 with b ¼ 0:6094ð1  sÞ : 05 g ¼ 0:0820ð1  sÞ 1

Here, a, b and g are factors derived according to the contributions of red, green and blue components, and s is the saturation adjustment value. If s ¼ 0, all color is removed leaving only brightness information. If s ¼ 1, there is no change, and values 0 < s < 1 result in various levels of desaturation. For values s > 1, saturation is enhanced. > Figure 3 demonstrates the effects of applying saturation enhancement (example value s ¼ 1:35) and desaturation (example value s ¼ 0:65) to a sample image, using this process.

317

318

3.3.1

Digital Image Operations

Source image

De-saturated (s = 0.65)

Saturation enhanced (s = 1.35)

. Fig. 3 Image color saturation modifications using matrix methods in RGB space

Intensity transformations for contrast and brightness changes, as well as many colorspecific transformations – for example hue rotation – can be specified concisely and applied using the matrix framework. Combined operations can also be computed efficiently, concatenating individual operations by multiplying together the appropriate matrices before applying the result. Matrix pixel transformations can be implemented efficiently using precomputed lookup tables. The fast matrix arithmetic facilities of GPUs and some CPUs are also useful for this.

2.3

Application Example: White Point Correction

When source material is shot, the color and luminance responses of the scanner or camera used are ideally calibrated and known. Color management techniques (see > Chap. 3.2.3) can then be used to ensure that the image that is ultimately displayed is perceived to be as close as possible to the original scene, with neutral shades being reproduced as accurately as possible at the display. The response of a camera is not always known a priori, however, or material may have been shot with an incorrect white point setting in force. Correction can be achieved by remapping the white point using a simple global color pixel operation, with the capture device having been used to record a physical reference white point in the scene. This operation is also useful for deliberate manipulation of the white point of an image as for special effect purposes. There are a number of options when defining this operation, particularly with regard to color space [4]. Here we assume operation in the RGB space of the camera itself. If the measured color pixel value of the white point reference, W ¼ ½wR wG wB ; then assuming the full-scale color component value is 255 (i.e., 8 bits per component color), the white point correction operation can be defined in the matrix framework above, as 2

T wpt

255=wR 6 0 ¼6 4 0 0

0 255=wG 0 0

0 0 255=wB 0

3 0 07 7: 05 1

Digital Image Operations

3.3.1

Note that if any color component is zero, the result of the corresponding computation above would be undefined. Although this is very unlikely in practice, it must be considered in an implementation. Also, because of the likelihood of saturation of at least one color component of a white reference to the full-scale representable value, it is often more reliable to measure at least one known gray physical reference instead. The white point reference, W, can then be computed from these values.

3

Geometric Image Operations and Resampling

A further important set of processing techniques commonly applied to image data is the class of geometric image operations. Image resizing (scaling), rotation, and morphing are some very common practical applications. In the case of vector images, any required geometric transformations are generally applied directly in the vector representation, before rasterization for display takes place. Indeed this point encapsulates the chief advantage of using a vector rather than a raster representation whenever possible: although images destined for digital display inevitably must be sampled into raster form eventually, transformations applied before sampling effectively take place in a continuous domain and are therefore scale independent. Geometric transformations on images in raster form must take account of the fact that these images have already been sampled and discretized. While the pixel processing methods outlined in the previous section deal with modifications only to color or intensity sample values in raster digital images, geometric operations involve changes to the positions of image samples.

3.1

Raster Image Resampling and Scaling

Scaling is a very common requirement in image processing, for example, when zooming at the display to inspect detail closely, or preparing an image optimally for a display device with a certain resolution. Consider for example that an image must be scaled up so that every six pixel rows and columns are instead represented by seven in the destination image. > Figure 4a illustrates a section of a single row of pixels of the source image, and also the corresponding section of pixels in the destination, scaled image. The problem now is of determining appropriate values for the new pixels. Many of the destination pixel centers are not aligned with source pixel centers, so the source pixel values must be mapped onto the destination pixel grid and resampled. A naı¨ve approach simply selects the closest source pixel value to the center of each destination pixel; however, this results in significant image distortion with portions of the image information being deleted completely or replicated. An approach to scaling without introducing such distortion involves approximating the original continuous intensity/color surface and sampling it at locations corresponding to the new pixel centers [5]. This approximation is made by convolving the source image pixels with a filter impulse response function and setting destination pixel values at locations in the convolved signal that correspond to their centers. The theoretically ideal reconstruction filter is defined by the sinc function; it corresponds to an ideal low-pass cutoff characteristic in the spatial frequency domain. However, the sinc function contains negative values and has infinite support, and thus is not practical to use directly.

319

320

3.3.1

Digital Image Operations

Original pixel samples

a

New pixel samples (magnification)

Tent

New pixel samples

Original pixel samples

Reconstructed signal

Cubic

b

Filter function

Reconstruction by convolution

. Fig. 4 Image scaling and resampling. (a) New pixel centers are not aligned with original ones. (b) Filtered resampling using tent and cubic filter functions

Increasing the scale of an image as above (magnification) requires resampling to a higher pixel rate. This corresponds to interpolation in signal processing terms. Reducing image scale, sometimes known as minification, requires resampling to a lower pixel rate – decimation in signal processing terms. Note that in this case care must be taken that the new, lower sampling rate is still adequate for the spatial frequency content of the image so as not to introduce aliasing distortion. This is generally achieved by scaling the filter function before convolution. The reconstruction process is illustrated in > Fig. 4b using two typical filter functions: the ‘‘tent’’ (corresponding to linear interpolation between samples) and cubic filters (describing the shapes of their impulse responses). The cubic function approximates the ideal sinc function with better precision resulting in higher fidelity results, but is slower to compute than the ‘‘tent.’’ Others such as the Lanczos filter, offer still better reconstruction accuracy [6]. In implementation, the filter is generally applied one dimensionally through all rows and all columns separately.

3.2

Image Rotation

A second extremely useful geometric operation is the rotation of an image. For rotation through multiples of 90 , resampling is not required unless pixels are nonsquare; a simple transfer of pixel values directly from one location to another is sufficient. However, rotation of an image through an arbitrary angle is often needed, and this does require resampling (see > Fig. 5). A general, direct implementation would use filter functions for reconstructing the continuous

Digital Image Operations

3.3.1

Original pixel grid

Rotated pixel grid

. Fig. 5 Arbitrary rotation of a raster image requires resampling due to the complex overlay of source and destination pixels

intensity surface, and then resample this according to new, rotated pixel centers. Such an approach is, however, computationally inefficient and cumbersome to implement. A more practical algorithm is due to Paeth [7]. Here, the rotation operation to be applied anticlockwise through angle y is decomposed into three simple shear operations.  IfTsource and and destination pixel coordinates are represented as column vectors S ¼ sx sy T D ¼ dx dy respectively, a general transformation from source to destination coordinates according to matrix M is: D ¼ M:S: The rotation matrix can be considered as the product of three shear matrices:       cosy siny 1 tanðy=2Þ 1 0 1 tanðy=2Þ ¼ : M rot ¼ siny cosy 0 1 siny 1 0 1 Implementation of shear operations is straightforward, requiring only a shift of pixel data along one axis (effectively, resampling of translated pixels using a simple tent function) proportional to the distance along the second axis. > Figure 6 illustrates the process for a clockwise rotation through 10 . Note that as a consequence of rotation, the rectangular pixel array area required to hold the image is increased, though cropping is often used to retain a rectangular sub-region that does not include the rotated image boundary. For improved accuracy, rotations by angles of 90 or more are implemented by transfer of pixels for right angle portions followed by the three-shear algorithm for the remaining portion.

3.3

Other Geometric Image Operations

In addition to scaling and rotation, a number of other geometric operations requiring resampling are possible on raster image data. Considering straightforward affine

321

322

3.3.1

Source image

Digital Image Operations

Shear 1

Shear 2

Shear 3

(y axis)

(x axis)

(y axis)

[Partial 1]

[Partial 2]

Final rotated result

. Fig. 6 Rotation of a raster image using the three-shear method

transformations, the simple shear has already been outlined in its application to rotation using the Paeth method. Translation is also sometimes useful – this is effectively a phase shift of pixel data by an amount which is not necessarily an integer number of pixels. More general remapping and warping techniques are often required in certain higher level applications [1]. An image may be mapped to a regular or irregular mesh, and mesh nodes manipulated to apply modifications to the image structure in a local sense. A common application of such a technique is perspective transformation, often used (with knowledge of camera parameters) to correct for perspective distortion in an image captured from a camera. Warping methods are also used in morphing: creating a smooth transition from one image to another, driven by relationships between the nodes of the mesh in both images, defined by the user. A closely related application to morphing is the synthesis of new viewpoints of a scene, given at least two known viewpoint images and a set of correspondences between mesh nodes in the source images. This is useful in three-dimensional imaging and modeling.

3.4

Implementation of Fast Geometric Transformations

Modern commodity GPUs have evolved chiefly to accelerate three-dimensional object transformations and rendering for computer entertainment and visualization applications. The parallel processing facilities that make this possible, however, also greatly simplify implementation of very fast geometric operations on raster images – both affine transformations and more complex warps [8]. This can be achieved through common APIs such as OpenGL [9] and Microsoft DirectX [10]. A general technique for implementing 2D affine transformations of raster images is as follows (see > Fig. 7): 1. Define a virtual camera, usually with an orthogonal projection and a viewport mapping to a destination pixel buffer. 2. Create a geometric entity in object space. For affine transformations, a simple quadrilateral surface is suitable. 3. Using the texture handling facilities of the API, map the source image to the geometric entity just created.

V3

V1

Example B: Perspective warp Vertices translated individually in (x,y) plane

Source image texture–mapped to our geometry

Vertex transformation

Example A: Rotation All 4 vertices transformed by rotation in z axis

V4

V2

Destination projection and viewport (Mapped to destination pixel buffer)

Geometry to carry source image Quadrilateral in (x,y) with vertices: V1, V2, V3, V4

. Fig. 7 Fast-geometric-transforms

y

[z into paper] x

Digital Image Operations

3.3.1 323

324

3.3.1

Digital Image Operations

4. Transform the vertices of the geometric surface, either directly or using the vertex affine transformation facilities of the API. Since the geometric surface is ‘‘carrying’’ the source image, the final rendered result will be a destination image transformed accordingly. The GPU’s texture lookup filtering functionality ensures that appropriate resampling takes place automatically. General mesh warps can also be achieved directly, simply by using more complex geometry to define a suitable mesh containing internal vertices, and then transforming those vertices as necessary to apply the desired warp.

4

Summary

All practical image and video processing applications are built on a core set of low-level operations on digital representations of images. Some of these are applied in place at the pixel level, but others involving geometric transformations result in changes to the inherent structure of the image representation, and therefore must take into account sampling issues. It is relatively straightforward to use modern graphics hardware and APIs to implement extremely fast fundamental image processing operations.

References 1. Watt A, Policarpo F (1998) The computer image. Addison-Wesley, Reading 2. Gonzalez RC, Woods RE (2008) Digital image processing, 3rd edn. Pearson Prentice Hall, New York 3. Haeberli P (1993) Matrix operations for image processing. November 1993, http://www. graficaobscura.com/matrix/index.html 4. Viggiano JAS (2004) Comparison of the accuracy of different white balancing options as quantified by their color constancy. Proceedings of the SPIE, vol 5301. Bellingham, WA 5. Schumacher D (1995) General filtered image rescaling. In: Kirk D (ed) Graphics gems III. Academic Press

6. Turkowski K (1995) Filters for common resampling tasks. In: Glassner AS (ed) Graphics gems. Academic Press 7. Paeth AW (1995) A fast algorithm for general raster rotation. In: Kirk D (ed) Graphics gems. Academic Press 8. Qureshi S (2001) Image rotation using OpenGL texture maps, C/C++ users Journal, pp 10–17, September 2001 9. Silicon Graphics, Inc., (1992) The OpenGL graphics system: a specification. Version 1.1 10. Akenine-Mo¨ller T, Haines E, Hoffman N (2008) Real-time rendering, 3rd edn. A. K Peters, Natick

3.3.2 Signal Filtering: Noise Reduction and Detail Enhancement Karl G. Baum 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 2 Linear Shift-Invariant Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 3 Smoothing Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 4 Sharpening Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 5 Band Pass Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 6 Order Statistic Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 7 Adaptive Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.3.2, # Springer-Verlag Berlin Heidelberg 2012

326

3.3.2

Signal Filtering: Noise Reduction and Detail Enhancement

Abstract: In most display systems and imaging systems some form of image manipulation is necessary. This section discusses the use of operators that can be used to remove noise or provide detail enhancement. The techniques discussed here can be used both to correct the information to be displayed and to compensate for deficiencies in the display system. First linear shift-invariant operators will be presented from a mathematical point of view. Their application using convolution and Fourier techniques will be discussed. Several examples of linear shift-invariant operators, specifically those that can be used to smooth or enhance edges in images, will be presented. The section concludes with a presentation of order statistic filters and an example of an adaptive filter.

1

Introduction

Filtering is the application of signal processing techniques in order to remove unwanted components within images. Often this unwanted information can be noise or speckle hindering the observation of an object. In other cases it may be the suppression of certain features in order to see others more clearly. For example, interior regions may be suppressed in order to better differentiate a boundary (i.e., enhance the edge of an object). The material in this section provides the foundation on which most current filtering techniques are built. Readers mastering this material will have gained the mathematical basis necessary to pursue other advanced techniques.

2

Linear Shift-Invariant Operators

A special class of filters is those that can be implemented with linear shift-invariant operators. When dealing with a linear shift-invariant filter an impulse response provides a description of the filter’s effect on a signal, allowing an understanding, and straightforward implementation of complex image processing operators. Linear shift-invariant operators are a simple yet widely used tool for image manipulation. The action of a linear operator upon a weighted sum of inputs is identical to the weighted sum of the individual outputs. This can mathematically be represented as: ( ) X X !    a f x ¼ a F f ! x F n n

n

n

n

n

  x is the nth input signal, a function of position within the image, F is the operator, Where fn ! and an is the scale factor of the nth input signal. Linearity ensures that if we can represent a signal by a sum of simpler signals, we can apply the operator to the simpler signals, then sum the results to obtain the result of applying the operator to the original complex signal. The derivative and differentiation are important linear operators. A shift-invariant operator is one whose response is independent of the location within the input signal. Mathematically, this is represented as: n  o        ! ! If F f ! x ¼g ! x then F f ! x þ t ¼g ! x þ t ! Where t is the translation of the input signal. This ensures that the only effect of translating the input signal by some distance is the identical translation of the output signal.

Signal Filtering: Noise Reduction and Detail Enhancement

3.3.2

If an operator is both linear and shift-invariant, we can calculate its effect on a complex signal by decomposing it into simpler functions. The operator output can then be found by translating, scaling, and summing the result of applying the operator to those simpler functions. The simplest function that we can decompose our signal into is the Dirac delta function, also known as the impulse function. The Dirac delta function, symbolized by d, has finite support and unit area. In other words it has zero width and an infinite height, giving it unit area. For a Dirac delta function located at ! xo these properties are represented as: ! ! d x  xo ¼ 0 for ! x 6¼ ! xo Z1   d ! x ! x o d! x ¼1 1

We can decompose signals into a series of shifted and scaled Dirac delta functions:   f ! x ¼

Z1   ! ! ! f ! a d x  a da 1

Now since we are applying a linear shift-invariant operator, we only need to know the filter’s response to the Dirac delta function. This response is known as the impulse response.      F d ! x ¼h ! x We are able to write the effect of the operator on our signal as: 8 1 9 Fig. 1. Convolution is used to perform image filtering. The impulse response is known as the filter kernel and describes the effect of the filter on the image. For each pixel in the image, examine a region around the pixel and determine a new value for that pixel based on the values of the image within that neighboring region. It is important to understand the effect convolution has on the frequency spectrum of a signal. The spectrum of a signal is found by taking the Fourier Transform (FT). The spectrum represents the magnitude of each frequency within the signal. Consider this, any real world signal can be created by summing together sinusoidal waves of different frequencies and phase. Examine > Fig. 2. As additional sinusoidal waves are added we get closer to representing a rectangular wave. The low frequency sinusoidal waves determine the general shape, while the high frequency ones make the sharper corners possible. The frequency spectrum of a signal identifies the amplitude corresponding to each frequency. The DC component or constant component is located at the origin, and frequency increases linearly with distance from the origin. > Figure 3a and > b show two sinusoidal waves. The summation of the waves is shown as > Fig. 3c and the magnitude of the Fourier Transform as > Fig. 3d. Notice the four brighter points in the spectrum. The top and bottom points represent the lower frequency vertical wave

Convolution Problem:

0 0 1 0 0 ∗ 1 2 3

Rotate Filter: 3 2 1 Slide reversed impulse response accross signal to find each point in convolved signal. Overlap 0 0 1 0 0 3 2 1

Multiply and Sum 0∗3+0∗2+0∗1=0

0

0 0 1 0 0 3 2 1

0∗3+0∗2+1∗1=1

0 1

0 0 1 0 0 3 2 1

0∗3+1∗2+0∗1=2

0 1 2

0 0 1 0 0 3 2 1

1∗3+0∗2+0∗1=3

0 1 2 3

0 0 1 0 0 3 2 1

0∗3+0∗2+0∗1=0

0 1 2 3 0

. Fig. 1 The convolution process

Result

Signal Filtering: Noise Reduction and Detail Enhancement

3.3.2

2 1.5 1 0.5

a –10

–5

5

10

5

10

5

10

2 1.5 1 0.5

b –10

–5 2 1.5 1 0.5

c –10

–5

. Fig. 2 Summation of sinusoidal waves to form a square wave. (a) One sinusoidal, (b) sum of two sinusoidal waves, and (c) sum of five sinusoidal waves

shown in > Fig. 3a, while the right and left points are the result of the higher frequency horizontal wave shown in > Fig. 3b. Every sinusoidal contributing to the image appears as a pair of points on opposite sides of the origin of the spectrum, with higher frequencies further from the origin. This is intuitive since the Fourier Transform of a single sinusoidal is a pair of Dirac delta functions. Performing the convolution of two signals is equivalent to multiplying their spectrum. By taking the Fourier Transform of the signals and multiplying the results we obtain the filtered spectrum. By performing the Inverse Fourier Transform (FT1) on this filtered spectrum we obtain our convolved signal. Spectrums will be represented with capital letters, and are of course a function of frequency rather than spatial location.    !        !  f ! x  hð! x FT h x n H n x Þ ¼ FT 1 FT f ! ¼ FT 1 F !

329

330

3.3.2

Signal Filtering: Noise Reduction and Detail Enhancement

a

b

c

d

. Fig. 3 (a) Lower frequency vertical wave. (b) Higher frequency horizontal wave. (c) Summation of (a) and (b). (d) Magnitude of Fourier transform of (c)

When we are working with the images we say we are working in the spatial domain. Similarly, when we are working with the spectrum of an image we say we are working in the frequency domain. Highly optimized Fourier Transform software packages exist. Enhancements in speed can often be found by using the Fourier Transform instead of computing the convolution integral. As a result linear shift-invariant filters can have highly efficient and even real time implementations.

3

Smoothing Operators

Smoothing is one of the most common filter operations. Smoothing, also known as spatial averaging, is used primarily to reduce noise and speckle in the image. The impulse response of the standard smoothing filter is uniform. A standard smoothing filter kernel is shown as > Fig. 4, along with the affect of applying the filter. The value of the kernel is constant causing each pixel in the filtered image to be the local average of the surrounding pixels of the unfiltered image. The coefficients of the kernel sum to unity, which helps prevent invalid pixel values in the filtered image (i.e., values greater than 255 for an 8-bit image). The kernel can have any

Signal Filtering: Noise Reduction and Detail Enhancement

1 ∗ 25

b

a

3.3.2

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

c . Fig. 4 (a) Original Image. (b) Smoothing kernel. (c) Smoothed image resulting from convolving (b) with (a)

dimension; however, typically an odd size is chosen so that it has a well-defined center. The larger the kernel the greater the smoothing effect. Smoothing can of course be implemented in the frequency domain, and is typically referred to as low-pass filtering. The name is derived from the fact that smoothing is achieved by removing the high frequencies necessary to define sharp edges while leaving the low frequencies which define general structure intact. The ideal low-pass filter and the result of its application are shown in > Fig. 5. It completely passes all frequencies below a certain threshold, and removes those above this threshold. The filter is simply a cylinder centered at the origin (remember the DC component is located at the origin and frequency increases linearly with distance from the origin). Taking the Fourier Transform of > Fig. 4a provides the frequency spectrum (> Fig. 5a). Multiplication of the spectrum by the ideal low-pass filter (> Fig. 5b) provides the filtered spectrum (> Fig. 5c). Returning to the spatial domain via the inverse Fourier Transform recovers the filtered image (> Fig. 5d). Notice the ringing present around the edges in > Fig. 5d. This ringing is due to the sharp cutoff frequency and can be explained by looking at the spatial representation of the filter

331

332

3.3.2

Signal Filtering: Noise Reduction and Detail Enhancement

a

b

c

d

. Fig. 5 (a) Magnitude of frequency spectrum of > Fig. 4a. (b) Ideal low-pass filter. (c) Application of filter (b) to (a). (d) Inverse Fourier transform of (c) to recover low-pass filtered image

(> Fig. 6). It is often desirable to prevent or reduce this ringing and so filters with a gradual cutoff frequency may be preferred. A filter defined by the Gaussian function meets this requirement.  2 p! n !  a Gaus n ¼ e The Gaussian function has a height of 1 and a volume of a. A two-dimensional Gaussian function is plotted in > Fig. 7. The width of the Gaussian function is controlled by varying the volume. The Fourier transform of a Gaussian function is another Gaussian function, so it is smooth in both the frequency and spatial domains with weights that decrease radially. This helps to prevent the ringing that was observed when applying the ideal low-pass filter from manifesting. The Gaussian filter can be applied either by convolution in the spatial domain or multiplication in the frequency domain. An example Gaussian filter and the result of its application is shown in > Fig. 8. The image is clearly smoothed (blurred) and the noise present in the four boxes on the left of the image is greatly reduced.

Signal Filtering: Noise Reduction and Detail Enhancement

3.3.2

. Fig. 6 Spatial domain representation of filter shown in > Fig. 5b, created by taking the Inverse Fourier Transform of the filter. Ringing is clearly present in the filter

. Fig. 7 Two-dimensional Gaussian function

4

Sharpening Operators

Another common image manipulation procedure is to sharpen or enhance features in the image that may be blurry or difficult to see. The first example we will examine is the use of the derivative operator to enhance vertical and horizontal edges in the image. The derivative is defined as the difference between neighboring points of a function and can be implemented with the kernels shown in > Fig. 9. It is desirable to keep the size of the kernels odd so that they have a well-defined center. The kernel for finding the vertical edges in the

333

334

3.3.2 1 ∗ 330

a

Signal Filtering: Noise Reduction and Detail Enhancement

1

4

7

4

1

4

20

33

20

4

7

33

54

33

7

4

20

33

20

4

1

4

7

4

1

b

. Fig. 8 (a) Sample Gaussian kernel. (b) Result of applying the kernel to > Fig. 4a

–1 0

a

–1

0

1

b

1

. Fig. 9 (a) Vertical edge detection kernel. (b) Horizontal edge detection kernel

image (> Fig. 9a) takes horizontal neighbors and subtracts them from each other. If this happens on a uniform area of the image, the subtraction will yield a value of zero. On the other hand, if there is a vertical edge, there will be a large difference between the neighboring pixels and the kernel will give a large response. The result of applying the vertical and horizontal edge operators to > Fig. 4a is shown as > Fig. 10. The vertical edge detection kernel clearly enhances the vertical edges while suppressing uniform areas and horizontal edges. In many situations it is desirable to have an edge enhancement tool that works independently of the edge orientation. The second derivative or Laplacian operator can achieve this goal. The Laplacian operator (r2 ) is represented as: r2 f ¼

@2f @2f þ @x 2 @y 2

A discrete Laplacian kernel and its application to > Fig. 4a is shown in > Fig. 11. The change of sign between the center and borders of the kernel enhances edges of any direction by giving them stronger positive and negative responses in the enhanced image, while uniform areas will have a response of zero. Both the derivative and Laplacian kernels have a drastic effect on the intensities of the images which can make them difficult to interpret. A technique called high boosting can be

Signal Filtering: Noise Reduction and Detail Enhancement

a

3.3.2

b

. Fig. 10 (a) > Figure 4a after application of the vertical edge detection kernel (> Fig. 9a). (b) > Figure 4a after application of the horizontal edge detection kernel (> Fig. 9b)

a

–1

–1

–1

–1

8

–1

–1

–1

–1

b

. Fig. 11 (a) Laplacian kernel. (b) > Figure 4a after application of the Laplacian kernel

used in situations where it is desirable to enhance the edges of the image, while preserving the other areas. When performing high-boost image enhancement the edge enhanced image is added to the original image. This is done by incrementing the center coefficient in the kernel as shown in > Fig. 12. Every time we increment the center coefficient we are adding in another copy of the original image and decreasing the impact of the edge enhancement. When the center coefficient is only incremented once this procedure is known as unsharp masking, a procedure named after a similar process used for enhancing film by photographers. > Figure 13 demonstrates the application of the Laplacian and high-boost filters, as well as color image processing. There are several ways grayscale image processing techniques can be extended to color. In this example it is done by applying the filters to each of the red, green, and

335

336

3.3.2

Signal Filtering: Noise Reduction and Detail Enhancement

–1

–1

–1

–1

8+1

–1

–1

–1

–1

. Fig. 12 High-boost kernel

a

b

c

d

. Fig. 13 (a) Original image. (b) After application of Laplacian kernel. (c) After application of high-boost kernel with center coefficient of 9. (d) After application of high-boost kernel with center coefficient of 12

blue color channels separately. > Figure 13a contains the original slightly blurry image and > Fig. 13b is the image after application of the Laplacian. By using a high-boost kernel instead of the Laplacian we are able to retain the general look and feel of the image only with enhanced details (> Fig. 13c and > d). The enhancement is particularly noticeable in areas with lots of small detail such as the hair.

Signal Filtering: Noise Reduction and Detail Enhancement

3.3.2

As with any linear shift-invariant operator, the sharpening operations discussed here can be implemented in the frequency domain. The ideal high pass filter completely removes low frequencies while leaving the high frequencies untouched. It is the inverse of the ideal low-pass filter shown as > Fig. 5b. Just as the Gaussian was used to provide a low-pass filter with a smooth transition between frequencies passed and removed, its inverse (1-Gaussian function) can be used as a high pass filter with a smooth transition.

5

Band Pass Filters

An additional advantage of filtering in the frequency domain is the ability to remove a specific frequency or range of frequencies from the image. Consider > Fig. 14a, which has been corrupted by a low frequency sinusoidal. Recall that a sinusoidal wave shows up in frequency spectrum as a pair of points on opposite sides of the origin (> Fig. 14b). By taking the Fourier Transform of the corrupted image (> Fig. 14c) and removing the peaks due to the corrupting sinusoidal (> Fig. 14d), we can do a decent job of recovering the original image (> Fig. 14e). It is difficult to completely remove the corruption for a number of reasons. First, the corrupted sinusoidal actually consists of many frequencies. This is because the image is 8-bit and so, due to quantization, a sinusoidal wave can only be approximated, and also because the wave is truncated at the edge of the image. This truncation means we have many high frequency sinusoidal waves running parallel with our main sinusoidal. Another difficulty is due to the fact that the image we are trying to recover includes signal from the sinusoidal we are trying to remove. By completely removing the sinusoidal we end up also removing one of the waves that makes up the original image. In this situation only two points were removed from the frequency spectrum. This was because the corrupting signal consisted of a wave running in a single direction. If it is desired to completely remove a given frequency from the image, regardless of the orientation of that frequency, it would have been necessary to remove a band (i.e., all points a specific distance from the origin) of the frequency spectrum. This would have the desired effect of removing all sinusoidal waves of a given frequency regardless of orientation.

6

Order Statistic Filters

With order statistic filters the value of a pixel in the filtered image is determined by examining the pixels within the surrounding neighborhood of the original image. The pixels in the neighborhood are sorted (usually from minimum to maximum) and the intensity at a particular position in the list is assigned to that pixel in the filtered image. With a minimum filter the lowest intensity within the neighborhood is assigned to a pixel within the filtered image. Similarly, with a maximum intensity filter the greatest intensity value within the neighborhood is used to determine the intensity in the filtered image. Order statistic filters can be designed to select the intensity at any position within the sorted list, for example, the third highest intensity, or the middle intensity. This process is demonstrated by > Fig. 15 where a 3  3 minimum filter is being applied. > Figure 15a shows the pixel values of a small sub-region of an image. The minimum filter is being evaluated at the upper left corner of the sub-region. > Figure 15b shows the sorted pixel

337

338

3.3.2

Signal Filtering: Noise Reduction and Detail Enhancement

a

b

c

d

e . Fig. 14 (a) Corrupted version of > Fig. 4a. (b) Frequency spectrum of corruption. (c) Frequency spectrum of corrupted image. (d) Frequency spectrum after the main peak associated with the corruption has been removed. (e) Image recovered by taking the Inverse Fourier Transform of (d)

values within the 3  3 region. The smallest value is selected and assigned to the filtered image as shown in > Fig. 15c. The most widely used order statistic filter is the one that takes the intensity at the middle of the sorted list, in other words the median value of the neighborhood. The median filter is an

Signal Filtering: Noise Reduction and Detail Enhancement

a

9

7

3

2

1

8

4

6

3

1

9

2

4

1

3

5

4

1

7

6

3

1

1

8

5

3.3.2

b 2, 3, 4, 4, 6, 7, 8, 9, 9

2

c . Fig. 15 Demonstration of the use of a minimum filter. (a) Sub-region of an image. The minimum filter is being evaluated at the upper left corner. (b) Sorted list of pixel values within the region being evaluated. (c) The minimum value is assigned to the filtered image

a

b

. Fig 16 (a) > Figure 4a after application of a 5  5 median filter. (b) > Figure 4a after application of a 3  3 median filter

339

340

3.3.2

Signal Filtering: Noise Reduction and Detail Enhancement

a

b

. Fig. 17 (a) Photograph that has been corrupted by salt and pepper noise. (b) Figure (a) after application of a 3  3 median filter

effective choice for removing impulse noise (including salt and pepper). The median filter tends to be a better choice for removing impulse noise than the local average smoothing filter (uniform filter composed of all ones) because it introduces significantly less blurring. > Figure 16a demonstrates the use of a 5  5 median filter on > Fig. 4a. Notice that the noise is reduced in the four boxes at the middle of the figure, while the sharp edges of the grayscale in the bottom right remain intact. Features smaller than the filter are removed without introducing a significant amount of distortion. > Figure 16b applies a smaller 3  3 median filter. Examine the row of As where more of the smaller features remain. Compare > Fig. 16a and > b with > Figs. 4c and > 8b; blurring is significantly reduced. Median filters provide an effective means for removing salt and pepper noise. > Figure 17a is a photograph that was corrupted. > Figure 17b shows the image after the application of a 3  3 median filter. The impulse noise points get replaced generating an image with no noticeable salt and pepper noise. Order statistic filters are nonlinear and cannot be implemented using the convolution operator. They are however easily implemented in a very similar manner. A neighborhood is moved from pixel to pixel in the image to be filtered and the filtered value calculated for each location.

7

Adaptive Filters

Often better results can be accomplished by using a different filter on each region of the image. The filter to apply at a particular location can be selected by examining the characteristics of the local neighborhood. This procedure is known as adaptive filtering and can often achieve better results than applying one filter to the entire image. The Nagao-Matsuyama filter is an example of an adaptive smoothing filter designed to maintain fine detail [1]. The filter is composed of the nine different 5  5 masks shown in > Fig. 18. For each pixel in the image the variance is calculated for the pixels under each mask when centered on the pixel. The mask that has the lowest variance is selected for application.

Signal Filtering: Noise Reduction and Detail Enhancement

3.3.2

The output pixel is created by averaging the pixels covered by the mask. The idea being that the variance of a mask that covers an edge will be higher than that of a uniform region (even with noise). Thus, when applying the filter, the edge is not used when calculating the output value and so is prevented from influencing the value of neighboring pixels.

a

b

c

d

e

f

g

h

i

. Fig. 18 The nine 5  5 masks used when performing Nagao-Matsuyama filtering

341

342

3.3.2

Signal Filtering: Noise Reduction and Detail Enhancement

. Fig. 19 > Figure 4a after application of the Nagao-Matsuyama filter

> Figure 19 shows the result of filtering > Fig. 4a with the Nagao-Matsuyama filter. Compare the results with those from the application of the local average smoothing filter, the Gaussian filter, and the median filter. Noise is reduced in the four boxes, while preserving more of the details than the other filtering techniques presented in this section.

8

Conclusion

While we have only touched on a few image filtering techniques in this section, the application to display systems is apparent. Filtering techniques can be used to process images for display. For example, noise from a digital sensor can be reduced or the impact of a compression algorithm on image quality can be compensated for. In addition, deficiencies in the display system can be corrected. For example, an image enlarged for print out on a poster or by projection on a wall with a digital light projector will experience a certain amount of blurring. By applying a high-boost filter prior to image display the crispness of the original image can be retained. Entire textbooks have been written on image filtering and manipulation. The mathematical basis presented in this section provides an essential background, enabling the interested reader to understand additional available literature.

References 1.

Nagao M, Matsuyama T (1979) Edge preserving smoothing. Comput Graph Image Process 9(4):394–407

Signal Filtering: Noise Reduction and Detail Enhancement

3.3.2

Further Reading Brigham EO (1988) The fast Fourier transform and its applications. Prentice Hall, Upper Saddle River Gaskill JD (1978) Linear systems, Fourier transformations, and optics. Wiley, New York

Zonst AE (1995) Understanding the FFT, 2nd edn., Revised. Citrus Press, Titusville, FL

For Additional Image Manipulation Procedures Please See Gonzalez RC, Woods RE (2008) Digital image processing, 3rd edn. Prentice Hall, Upper Saddle River

Russ JC (2002) The image processing handbook, 4th edn. CRC Press, Boca Raton

343

3.3.3 TV and Video Processing Scott Janus 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346

2

Scan Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

3 Film Mode Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 3.1 Frame Rate Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 4

Aspect Ratio Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

5

Basic Adjustments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354

6

Brightness and Contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

7

Saturation and Hue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

8

Advanced Adjustments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

9

Adaptive Brightness and Contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

10

Video Artifact Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

11

Advanced Color Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

12

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

13

Directions of Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.3.3, # Springer-Verlag Berlin Heidelberg 2012

346

3.3.3

TV and Video Processing

Abstract: This chapter describes video processing algorithms commonly in use today. In addition to describing how they work at a generic level, this chapter will also explain why such processing is necessary, even on today’s high definition content. Sample pictures are included that visually demonstrate the key principles of the various algorithms. The processing categories that are covered include: deinterlacing, film mode detection, scaling, anamorphic scaling, nonlinear anamorphic scaling, hue, saturation, brightness, contrast, de-noise, detail enhancement, and frame rate conversion. List of Abbreviations: CCD, Charge-Coupled Device; FPS, Frames Per Second; i, Interlaced, as in 1080i, Referring to 1920
1080 Interlaced Content; p, Progressive, as in 720p, Referring to 1280
720 Progressive Content

1

Introduction

The quality of video in the past few decades has improved remarkably, to the point where watching a nostalgic show from your childhood can be a surprisingly low-fidelity experience. Some of the quality improvements have come from an increase in resolution; standard definition broadcasts are slated to cease in 2009 in the US, and DVD-Video is slowly but surely being supplanted by Blu-ray. Part of the increase in quality comes from new video compression codecs: H.264 and VC-1 offers substantial improvements in efficiency over MPEG-2 (as discussed in > Chap. 3.2.2). Yet despite these advances in the baseline video itself, many of the quality enhancements have come from the application of algorithms applied to the decoded, uncompressed video. These techniques are sometimes referred to as post processing algorithms, in that they are applied after decoding. Few enhancements were needed in the early days of video, because the content almost always precisely matched the available displays. The format of the live broadcasts was guaranteed to match the characteristics of all the televisions capable of receiving the transmissions. Today, however, the world is a much different place. Even constrained to the traditional consumer electronics (CE) ecosystem of the early years of the twenty first century, there was still a wide range of different video formats with different resolutions, scan types, and aspect ratios. PCs make the situation more complex, in that the content can be practically any size and displayed via an arbitrary sized window. PC resolutions and refresh rates are much more varied than that supported by traditional CE interfaces. The much discussed convergence of PCs and dedicated CE video systems has steadily – yet asymptotically – progressed. Many engineers spent the previous decade working to ensure that CE video could be displayed well on PCs. Today, many engineers are working to ensure that PC/ Internet video can be displayed well on CE devices. At the same time, video on mobile devices such as cell phones and mobile Internet devices (MIDs), as well as console game stations further adds to the combinatorial complexity of the video ecosystem. This entry will review many of the enhancement algorithms in place today. For many of the algorithms, there is no universal implementation, and indeed, there is a great deal of intellectual property and company differentiation tied up in the nuances of a particular implementation. However, the basic concepts are discussed in a generalized form herein. These algorithms can be classified into three main categories. The first category contains those algorithms that are needed to address a fundamental difference between the video

TV and Video Processing

3.3.3

content and the display device. These differing parameters include the scanning type, aspect ratio, resolution, and frame rate. Without some form of conversion between these mismatched parameters, the video will basically be unwatchable. The second group consists of those algorithms needed to ensure the presentation of the video on a particular display in a particular environment accurately reproduces a visual experience consistent with a reference implementation. These adjustments ensure that all the dark and bright areas are correctly visible and the colors are properly represented. The final category consists of those algorithms that strive to somehow improve upon the baseline implementation or otherwise differentiate a given product from its competitors.

2

Scan Conversion

There are two fundamentally different scan types: interlaced and progressive. Progressive scanning records every line of scene for a particular instance of time. 1080p Blu-ray content and 720p HDTV broadcasts are examples of progressive content. Film content is also inherently frame-based. Being an analog physical medium, there is no real concept of rows or pixels. Still, the entire scene is captured at full spatial resolution with every opening and closing of the shutter. Progressive scanning is illustrated in > Fig. 1. Interlaced scanning records or displays every other line of the scene for a given time stamp. The half frame is known as a field. The fields corresponding to the even-numbered lines of the scene are unsurprisingly known as even fields, and the odd-lines compose the odd fields. Legacy analog television and HDTV’s 1080i format are examples of interlaced scanning. Interlaced scanning has historically been used to address technological bandwidth constraints of the time. It is illustrated in > Fig. 2.

Frame 0

Frame 1

Frame 2

Frame 3

Field 1

Field 2

Field 3

. Fig. 1 Progressive scanning

Field 0

. Fig. 2 Interlaced scanning

347

348

3.3.3

TV and Video Processing

At first glance, it may seem as though having every other line of the video missing would result in noticeably visible results. After all, half the content is missing! However, when interlaced content is displayed on an interlaced monitor (which includes almost every standard definition television made in the twentieth century), the human psychovisual system’s perception of the TV’s phosphor decay results in an acceptable presentation of the content. The missing lines are not obvious. The fact that there are two fundamentally different scanning techniques means that some type of conversion must take place if one type of content is displayed on a different type of display. This problem cropped up in the early days of television as engineers considered how to transmit progressive film content over the interlaced-only television broadcast system. In geographies using a 60 Hz interlaced scan rate, the 24 FPS film content is converted using a technique known as 3:2 pulldown. The 3:2 comes from the fact that the first frame is sampled three times and the second frame is sampled two times to create fields at the necessary cadence. This procedure is shown in > Fig. 3. A similar technique known as 2:2 pulldown is used to convert 24 FPS content to 50 fields per second for other geographies. Telecine [1] is the generic term for converting film content to interlaced video. With the advent of progressive displays – most notably computer monitors – in the closing years of the twentieth century, the problem of deinterlacing content became widespread. Merely natively displaying the interlaced content is inadequate, as the human eye can easily see that half of the data is missing. The missing rows of data must be filled in with something. It is impossible to exactly recreate the missing fields, as that data was never captured. Instead, we must approximate the missing information. One simple approach is to estimate the missing field by interpolating from the pixel directly above and below the absent pixel [2]. This approach is shown as bobbing, and is illustrated in > Fig. 4. It is easy to implement, but has noticeable artifacts around high contrast, static images such as subtitles. Such regions tend to flicker, due in part to the fact that the odd and even fields are sampled at different spatial locations. Another simple deinterlacing algorithm is to simply combine two adjacent odd and even fields into a single frame, as show in > Fig. 5. Weaving, as it is known, works great with static images. However, regions of large movement result in an artifact known as combing or feathering. In these regions, the fact that the two fields – which are samples of different instances in times – are presented at the same time. Note that whereas bobbing converted 60 fields per second into 60 frames per second, weaving creates 30 frames per second.

24P source content

60i 3:2 pulldown content

. Fig. 3 3:2 pulldown

TV and Video Processing

Field 0

Field 1

Frame 0

Frame 1

Field 0

Field 1

3.3.3

. Fig. 4 Bobbing

Frame 0

. Fig. 5 Weaving

A more sophisticated approach is to apply bobbing to regions of the screen with high motion and weaving to regions of the screen with little or no movement. This adaptive deinterlacing can be quite effective, as it discriminately applies bob or weave to minimize the artifacts of each algorithm, as demonstrated in > Fig. 6. The size of the regions varies based on the implementation; they can be as small as a pixel, in which case the algorithm is known as pixel-adaptive

349

350

3.3.3

TV and Video Processing

deinterlacing. Generally speaking, the smaller the region, the more expensive it is to implement the algorithm. Similarly, expense also increases with the range of motion the algorithm can detect. An even more advanced approach is to use the motion data of adjacent fields to reconstruct a completely new frame. The position of moving objects in the missing field is calculated from the references and placed appropriately. This motion-compensated approach is quite complex to implement.

3

Film Mode Detection

It is possible that the interlaced content harbors progressive content, such as a telecined movie. Applying the aforementioned deinterlacing algorithms to such content will result in suboptimal results, an example of which is shown in > Fig. 7. The sample illustrates the problem when applying weaving, but the problem applies to all the algorithms. The best possible result is to monitor the content and compare adjacent fields to see if they originate from the same progressive frame. If done properly, the original progressive

Field 0

. Fig. 6 Advanced deinterlacing

60i 3:2 pulldown content

30P woven content

. Fig. 7 Incorrect deinterlacing

Field 1

Field 2

Frame 1

Frame 2

Field 3

TV and Video Processing

3.3.3

frames can be completely recovered from the interlaced content and optimally displayed, as shown in > Fig. 8. This process is known as film mode detection (and correction) or inverse telecine.

3.1

Frame Rate Conversion

It is possible for the frame rate of the video content to be different than the frame rate of the display device that is showing the content. For instance, a Blu-ray movie may be recorded at 24 FPS and viewed on a PC that has an 85 Hz refresh rate active. Obviously, some sort of conversion between the frame rates must occur in order for the video to be viewed in an acceptable fashion. The simplest approach is simply to replicate the frames, as shown in > Fig. 9. This approach provides reasonable quality, although an alert eye can detect the fact that each unique film frame is being presented for a different duration of time. The variable duration is due to the fact that the display refresh rate may not be an even multiple of the content’s frame rate. In the case of 24 FPS content and a 60 Hz display, the video frames are alternately presented for three and two display refreshes. This cadence results in each video frame being presented for an average of 60/24 = 2.5 video frames per display refresh. In the cast of 24 FPS content and an 85 Hz display, the cadence is quite complex, as 85/24 is an irrational number. A more sophisticated approach is to not merely replicate frames, but to create new intermediate frames. This is a complex process; merely performing a linear interpolation of the intermediate frames will only result in blurred content, as conceptualized in > Fig. 10. Instead, some motion-compensated approach must be used. This can be done on a global (picture-wide) basis, or at a finer grain.

60i 3:2 pulldown content

Recovered 24P content

. Fig. 8 Correctly recovering progressive content

24 FPS 85 Hz

0 0

. Fig. 9 Frame replication

0

1 0

0

1

1

2 1

1

2

3 2

2

3

3

3

3

351

352

3.3.3 24 FPS 85 Hz

TV and Video Processing

0 0

1 0

1 1 0

1 0

1

1 2

2 1 2

1 2

2

3 3 2

3 2

3

3 4

3 4

3 4

. Fig. 10 Frame interpolation

Typically, these more advanced frame rate conversion algorithms are used in systems where the content and display ratios are a multiple of one another. For instance, many of the newer HDTVs take in a 24 FPS signal from Blu-ray movies, and internally frame rate convert it to the display’s native 120 Hz refresh rate. The increased number of frames can decrease judder artifacts inherent in the cinematic content’s low frame rate and help mitigate motion blur issues experienced by some display technologies.

4

Aspect Ratio Conversion

Often, the aspect ratio of the content differs from the aspect ratio of the display. Most retail video has an aspect ratio of 4:3 or 16:9. Older TVs have a 4:3 aspect ratio; most newer ones have a 16:9 ratio. Playing 4:3 content on a 16:9 display or 16:9 content on a 4:3 display requires some form of conversion. The aspect ratio topic is further complicated by the wide availability of Internet video that can have practically any aspect ratio and the fact that video is often played on PCs in a window of arbitrary size. Nevertheless, the techniques used below can be used to span any mismatch between content and display aspect ratios. Although I will use the term display aspect ratio in the following entries to refer to the properties of a monitor’s entire screen, the concepts can be generalized to smaller target regions of video, such as might be seen in picture-in-picture scenarios or when watching windowed video on a PC. One simple approach is to simply stretch the video to completely fill the screen, as demonstrated in > Fig. 11. This approach is popular with many consumers, as the entire screen is active. Video purists decry the technique, however, because the aspect ratio of the content is distorted. For instance, circles are deformed into ovals. Another approach is to cut out a section of the content that matches the video’s aspect ratio. This cropping technique (> Fig. 12) makes use of the display’s entire real estate while preserving the content’s aspect ratio. However, portions of the video are lost. This artifact is most notable in scenes containing two people facing at each other at the extreme edges of the screen. Cropping can remove both of the actors from the viewable picture. The cropping of key elements can be mitigated to some extent by moving the cropping window back and forth across the content to the areas of interest. This technique is known as pan and scan. Note that pan and scan is only practical with human control. It is only suitable for a prior processing, such as preparing widescreen content to be broadcast in a 4:3 format. Letterboxing is a technique that displays the entire scene on the screen while preserving the original aspect ratio. This feat is accomplished by uniformly scaling the content to match the most constraining dimension and filling the remainder of the screen with black, as illustrated in > Fig. 13. This approach is the preferred one for videophiles. Some people do not like it because it leaves regions of the screen unused. Letterboxing refers to insertion of horizontal black bars above and below the content when adapting content to a display with a wider aspect

TV and Video Processing

3.3.3

. Fig. 11 Full screen stretching

. Fig. 12 Cropping

. Fig. 13 Letterboxing

ratio. When adapting content to a narrower aspect ratio, black columns are inserted on either side; this technique is known as pillarboxing. People are sometimes surprised to find letterboxes appearing on their widescreen TVs when watching movies. This scenario occurs because cinematic aspect ratios are completely disconnected from video aspect ratios. The most common film aspect ratios (such as 1.85:1 and 2.35:1) are wider than 16:9, so letterboxing is still necessary. A special case of cropping can be used when dealing with letterboxed content. If widescreen content is letterboxed into a narrowscreen transmission or storage format and subsequently displayed on a widescreen TV, the cropping window can be set to the display aspect ratio. The resulting image completely fills the screen while preserving aspect ratio (> Fig. 14). In all the aspect ratio conversion schemes discussed so far, some scaling or resampling of the content is typically required. All of the scaling for the above techniques uses linear scaling, by which, I mean that the scale factor is consistent across the picture. Stretching content to fill the screen uses

353

354

3.3.3

TV and Video Processing

. Fig. 14 Widescreen cropping

. Fig. 15 Nonlinear anamorphic scaling

anamorphic scaling: the scale factor in the horizontal direction is different than the scale factor in the vertical domain. Still, it is linear in that the scale factor remains constant in each direction. Anamorphic scaling is common in the cinema. It is also used in DVD-Video playback, because the video is sampled using non-square pixels. Typically, DVD-Video content is stored as 720
480 samples for both 4:3 and 16:9 contents. For each aspect ratio, a different anamorphic scaling must be used to properly scale the 720
480 content to the square pixels of today’s displays. Scaling always results in a degradation of the image quality. The topic is worthy of a book itself. For this brief overview, let us just note that unnecessary scaling should be avoided. Competitive solutions today use sophisticated algorithms involving many source samples (or taps) per single destination sample. Simple bilinear scaling is usually inadequate. The most sophisticated algorithms use temporally adjacent frames to increase the effective resolution of the current frame. This brief segue on scaling leads us to the final aspect ratio conversion technique. It has many different brand names, but no common generic name. I refer to it as nonlinear anamorphic scaling. In this technique, the horizontal scale factor is constant, but the vertical scale factor varies across the picture [3]. This allows the content aspect ratio to be correctly reproduced in the center of the picture at the expense of exaggerated distortions at the edges of the screen, as seen in > Fig. 15. This approach utilizes the entire real estate of the screen. As such, it is seen by some as an optimized compromise between cropping and stretching. However, it has very noticeable artifacts. For instance, a car driving across the frame will start with wildly distorted tires that morph into proper circles as it reaches the center region, then stretch back out as it reaches the far edge of the screen.

5

Basic Adjustments

Even in the early days of a primarily homogenous video environment, some basic video processing controls were necessary to account for variations in products from different manufacturers and to adjust for ambient lighting conditions. These adjustments were

TV and Video Processing

3.3.3

necessary to get a perceptually uniform behavior across the different products and installations. In other words, by properly calibrating the TV using these basic adjustments, a person would ideally perceive the same image regardless of the living room or showroom he happened to be in. Of course, in practice, this precise level of calibration rarely happens, as the average consumer does not understand how to properly adjust all the settings to match the reference behavior.

6

Brightness and Contrast

Brightness and contrast are two commonly used but commonly misunderstood controls [4]. Both of these operate solely on luma components. Brightness is more technically known as black level adjustment. Contrast refers to the gain of a picture. Adjusting the brightness of a picture adjusts the entire range of output values. In other words, decreasing the brightness will lower both the black level and the white level. Increasing the brightness will increase both the black and white levels. An example of brightness adjustment is shown in > Fig. 16. Contrast, however, actually alters the distance between the black and white levels. Just manipulating the contrast itself will not alter the black level, but will raise and lower the white level. All intermediate values will be stretched accordingly. An example is shown in > Fig. 17.

. Fig. 16 Adjusting brightness

. Fig. 17 Adjusting contrast

355

356

3.3.3

TV and Video Processing

Loosely speaking, the function of brightness and contrast can be calculated by the following function: Output value ¼ ðcontrast  input valueÞ þ brightness

7

Saturation and Hue

Hue and saturation manipulate the color. They operate solely on the chroma components and have no impact on luma. Saturation refers to the intensity of the chroma components. Saturation is accomplished by multiplying the chroma values by a constant. Saturation adjustment examples are shown in > Fig. 18. Hue adjustment basically rotates and shifts all the colors. It can be thought of as a rotation of the hue ring, as demonstrated by examples in > Fig. 19.

8

Advanced Adjustments

For quite some time now, television and display manufacturers have been adding features to their products that are designed to make video appear better than a reference display. In this context, better is subjective. Basically, the intent is to differentiate from competing products. If

. Fig. 18 Adjusting saturation

. Fig. 19 Adjusting hue

TV and Video Processing

3.3.3

all displays exactly matched the reference display, then consumers presumably would just buy the cheapest unit. However, if one display presents the video in a fashion that somehow looks more pleasing to the eye than the others, then the consumer might be willing to pay more money for it. Even when dealing with the latest high definition video formats, there is still a lot of room for improvement. As discussed in > Chap. 3.2.2, consumer video is compressed in a lossy fashion. Most of the content is stored as 8-bit per sample 4:2:0, which has limited dynamic range and color fidelity. There is a distinguishable gap between this content and the limits of human perception. Work is underway to narrow this gap by increasing the bit depth and color gamut of the compressed video, but in the meantime, advanced video enhancement algorithms strive to improve today’s content

9

Adaptive Brightness and Contrast

One such class of algorithm is adaptive contrast enhancement. This feature generates a histogram of the image and automatically adjusts the contrast to provide an ideal presentation for the current scene. Some hysteresis is needed to ensure the contrast does not change widely with every single frame, and care must be taken to make sure that intentionally dark scenes remain dark. Advanced versions of the algorithm can adaptively adjust the contrast in various regions of the screen. Another technique is to monitor the ambient lighting conditions and adjust the brightness (and possibly contrast) of the image to match the characteristics of the human perceptual system. For instance, in a dark room, the brightness should be lower than when the same content is being viewed in a bright room. This approach does not necessarily monitor the video content, but instead modulates parameters based on external conditions.

10

Video Artifact Removal

As previously noted, video compression is lossy and introduces artifacts. These artifacts can be mitigated by applying clean up algorithms to the decoded images. Deblocking attempts to smooth the boundaries between block-shaped regions with different average values. The human eye is surprisingly good at detecting these regions. Newer video codecs [5, 6] include deblocking filters as part of the decoding algorithm, but not all video streams encoded in these formats take full advantage of the deblocking capabilities. Also, older codecs such as MPEG-2 do not inherently support deblocking, so applying it as a post processing stage can noticeably improve the picture quality. An example of deblocking is shown in > Fig. 20. High frequency information is often discarded as part of the compression process. Detail or sharpness algorithms can reproduce some of these characteristics, as demonstrated in > Fig. 21. Poorly implemented sharpness filters can create new artifacts, such as creating ringing around high frequency details. Good filters recreate original detail without introducing new problems. High frequency noise is always a problem with analog content, and even with most digital content. The CCDs that actually digitize the real world introduce noise, as do the compression algorithms. Noise removal algorithms can remove these artifacts, as seen in > Fig. 22, and

357

358

3.3.3

. Fig. 20 Deblocking

. Fig. 21 Sharpening

. Fig. 22 Noise reduction

TV and Video Processing

TV and Video Processing

3.3.3

create a cleaner picture. Care must be taken to only remove the unwanted noise; just applying a general blurring filter will mitigate the noise but will also lose fine details that are part of the desired picture.

11

Advanced Color Processing

There are many different techniques in contemporary products that strive to improve the color response of the system. These basically strive to create more intense or vibrant colors than the reference implementation. Some of these algorithms try to boost colors that have limited bandwidth in a particular domain. Some take advantage of the broader range of colors that the new wide gamut displays are capable of displaying. Often, these color enhancements are not technically correct, in that they produce colors that are different than a defined spec, such as BT.709. However, many viewers prefer the enhanced colors to the baseline implementation. One interesting class of algorithms is skin tone detection. These algorithms attempt to detect regions of the screen that contain samples of human skin and adjust the color of the skin independent of the rest of the picture. These can arguably make the depicted people look better. Again, this is straying from an accurate reproduction of the original scene. Presumably, a properly calibrated system would accurately depict any flesh tones just as accurately as the rest of the scene. There are a wide range of different flesh tones out there in the real world, so it is not always clear if there is a universal advantage to this type of processing.

12

Conclusion

We are in the middle of an amazing period of video quality improvement. Even as the transition from standard definition to high definition continues, a widespread proliferation of high quality HDTVs and high resolution PC monitors has sparked an arms race of competitive video enhancement features. As long as video storage techniques remain restricted to reproducing a significant subset of what humans can perceive – which seems likely to be the case for decades to come – there will always be a market to enhance that video to create a perceptually improved experience. It is also worth noting the recent influx of relatively low bit-rate content, such as Internet-streamed video content a` la YouTube, is a particularly fertile ground for new video processing techniques to emerge.

13

Directions of Future Research

The field of video processing research remains active, with new capabilities being introduced with the annual release of new televisions by major manufacturers. These features include more sophisticated implementations of the basic techniques described in this paper, as well as adaptive mechanisms that attempt to automatically improve subjective video quality. The increasing availability of displays with wider gamuts and higher refresh rates has also prompted research into mapping existing video standards into these new domains. Also of interest are techniques for improving the display of low bit-rate and low resolution usergenerated content, which has become widely available due to the proliferation of video-capable phones and Internet sites such as youtube.com.

359

360

3.3.3

TV and Video Processing

References 1. Matchell R (1982) Digital techniques in film scanning. IEE Proc: Sci Meas Technol 129:445–453, 7 Sept 1982 2. De Haan G, Bellers EB (1998) Deinterlacingan overview. Proc IEEE 86:1839–1857, 9 Sept 1998 3. Intel Corporation. Intel® Clear Video Technology controls questions and answers. intel.com. [Online]

http://www.intel.com/support/graphics/sb/CS-029863. htm 4. Keith J (2007) Video demystified: a handbook for the digital engineer, 2007th edn. Newnes, New York 5. ISO/IEC 14496-10 coding of audio-visual objects – Part 10: advanced Video Coding 6. SMPTE 421M VC-1 compressed video bitstream format and decoding process

Further Reading Keith J (2007) Video demystified: a handbook for the digital engineer, 2007th edn. Newnes, New York Janus S (2002) Video in the 21st century. Intel Press, Hillsboro Poynton CA (1996) Technical introduction to digital video. Wiley, New York

Robin M, Poulin M (2000) Digital television fundamentals. McGraw-Hill, New York List P et al (2003) Adaptive deblocking filter. IEEE Trans Circuits Syst Video Technol 13(7):614–619, 7 July 2003

Part 3.4

Case Study: Medical Imaging and Display

3.4.1 Reliability and Fidelity of Medical Imaging Data Alfred Poor 1 The Power of Medical Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364 2 The Digital Advantage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 3 The Challenges of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 4 DICOM Grayscale Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366 5 Falling Short of Theoretical Perfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 6 Controller Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 7 Physical Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 8 Monitor Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 9 The Next Step: 3D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.4.1, # Springer-Verlag Berlin Heidelberg 2012

364

3.4.1

Reliability and Fidelity of Medical Imaging Data

Abstract: Few areas of human endeavor can match the advances achieved in modern medical technology, and the key to many of these has been the ability to display digital data. Diagnostic procedures that use x-ray, ultrasound, and magnetic resonance imaging give medical professionals two-dimensional (2D) and three-dimensional (3D) access to regions inside the human body, on a scale ranging from the cellular level to the whole body. Real-time imaging guides doctors during procedures, making possible new techniques for minimally invasive surgery. Key to all of these is the reliability and fidelity of the imaging data, and the subsequent display of that information. Not long ago, many doctors held film to be the highest standard for imaging. Now, flat panel LCD monitors with up to 6 megapixel (MP) resolution and 1,024 gray-level response let doctors and diagnosticians zoom in on digital imagery to a degree not possible with traditional film. The combination of such displays with a picture archiving and communication system (PACS) frees medical personnel from the physical limitations of film, making it possible for experts on opposite sides of the globe to consult on patient data at the same time. This section explores the issues in processing and delivering the image information, as well as the display systems required to portray this information with the necessary precision and accuracy. List of Abbreviations: ACR, American College of Radiology; AIP, Advanced Image Processing; AR, Anti-reflective; BLOS, Backlight Output Stabilization; CAT, Computed Axial Tomography; COTS, Commercial Off-the-Shelf; CT, Computed Tomography; DDL, Digital Driving Levels; DICOM, Digital Imaging and Communications in Medicine; DIMSE DICOM, Message Service Element; DSP, Digital Signal Processing; E-R, Entity-Relationship; GSDF, Grayscale Standard Display Function; HAI, Healthcare Associated Infection; HIS, Hospital Information System; IOD, Information Object Definition (DICOM Reference); IPS, In-Plane Switching; ISO-OSI, International Standards Organization Open Systems Interconnection; JND, Just-Noticeable Difference; LUT, Lookup Table; MIP, Maximum Intensity Projection; MIS, Minimally Invasive Surgery; MITA, Medical Imaging and Technology Alliance (Part of NEMA); MPR, Multi-planar Reconstruction; MRI, Magnetic Resonance Imaging; MVA, Multi-domain Vertical Alignment; NEMA, National Electrical Manufacturers Association; NMRI, Nuclear Magnetic Resonance Imaging; OR, Operating Room; PACS, Picture Archiving and Communications Systems; PET, Positron Emission Tomography; RLE, Run Length Encoding; RIS, Radiology Information System; SNOMED, Systematized Nomenclature for Medicine; SOP, Service-Object Pair; TN, Twisted Nematic; UID, Unique Identifier; VR, Value Representation/Volume Representation

1

The Power of Medical Imaging

It is difficult to overstate the impact of medical digital imagery. It was chosen as one of 11 ‘‘developments that changed the face of clinical medicine’’ during the last millennium by the editors of the New England Journal of Medicine (NEJM) [1]. From computed axial tomography (CAT) to nuclear magnetic resonance imaging (NMRI or MRI), to ultrasound, medical imagery has made possible a wide range of advances in the diagnosis and treatment of medical conditions. Real-time imaging allows physicians to see inside patients through a variety of methods, allowing the development of new minimally invasive surgery (MIS) procedures that reduce risk and speed recovery. In addition to the benefits of new ways to ‘‘see’’ through living tissues, the digital nature of modern medical imaging yields significant benefits in its own right. Traditional imaging

Reliability and Fidelity of Medical Imaging Data

3.4.1

technologies either relied on ephemeral real-time displays – such as early fluoroscopes that displayed x-ray images on a fluorescent screen – or on sheets of photographic film. Real-time displays did not provide a means of recording the results. Film creates a static image that provides a physical record of the image, but this has limitations. First, the silver-based film is expensive, and extensive systems must be developed and maintained to manage the storage and tracking of the films. In order for a physician to review the results, he or she must obtain the film result and view it. This makes it difficult for multiple physicians to review the same image.

2

The Digital Advantage

Digital imaging has revolutionized the process. Instead of relying on expensive physical films that can be damaged or misplaced, the data that represents those images can be stored in a database. The data can then be made available on a network, so that any physician with access to the network (and the proper permissions) can review the images from a display terminal. They do not have to be located in the radiology department of the hospital, or even within the hospital at all. They do not even have to be on the same continent; this makes it possible for physicians on the other side of the world to provide nighttime analysis of radiology images when a local radiologist might not be available. A computerized medical imagery database is known as a picture archiving and communications systems (PACS). While these systems are still developing, they hold the promise to make all of a patient’s medical imagery – from still images to motion video of diagnostic and clinical procedures – available at workstation screens anywhere at any time. The digital nature of medical imaging data also makes it possible to create new views. For example, MRI and CAT imagery can create ‘‘slices’’ through the patient’s body. By mapping these images in space, it is possible to create a three-dimensional (3D) image of the data, allowing the images to be ‘‘sliced’’ along other planes, providing other two-dimensional (2D) views. The data can also be used to create 3D views that can be manipulated so that it can be viewed from various angles. The data can also be mathematically manipulated in a variety of ways to alter the appearance of the image – such as remapping values to different colors to produce ‘‘false coloring’’ – that can highlight portions of particular interest. These alternate views profoundly improve a physician’s understanding of the image data, as they provide additional perspectives on the information.

3

The Challenges of Data

As might be expected, managing this data can be complicated. Different imaging processes produce different types of data sets. Some create monochrome images, with only one value per pixel. Others create color images, with separate values for each of the red, green, and blue color channels. In either case, the amount of data stored per channel per pixel can range from 8 bits to 32 bits or more. To complicate matters further, a given study might consist of a single static image, or a series of images representing different views of the same portion of the patient’s body. It could also be a consecutive series of ‘‘slices’’ such as from an MRI or CAT session. In addition to the actual image data, the management system needs to make sure that the images are associated with the correct patient’s records. In response to this evolving

365

366

3.4.1

Reliability and Fidelity of Medical Imaging Data

demand for a complex yet robust system to manage medical imagery, the Medical Imaging & Technology Alliance (MITA) – part of the National Electrical Manufacturers Association (NEMA) – manages an industry-wide specification that covers the storage of medical imaging data. This standard is known as Digital Imaging and Communications in Medicine (DICOM) [2]. The standard consists of 16 independent parts that cover every technical detail of how this data is to be stored, accessed, and managed. For example, the data records include a header that starts with 128 bytes of data for use by the PACS. The text string ‘‘DICM’’ then follows to signal the start of DICOM-standard data. The header can include essential identifying data for the patient, including name, ID, date of birth, and sex. The record header can even include provisions for the patient’s primary language, military rank, country of residence, and medical alerts [3]. The result is that many key elements of the patient’s history can be directly associated with the imaging data, reducing the chances for misidentification, which could have serious negative consequences. The DICOM specification covers imaging data details, such as data formats and data compression details. For example, JPEG images are supported, using either lossless or lossy compression schemes [4]. It also specifies lossless run length encoding (RLE) and MPEG2 MP@ML compression for motion images. (Note that the specification permits lossy compression, which can result in visible artifacts and loss of detail; DICOM leaves it to the medical specialty organizations to set their own standards for what constitutes acceptable fidelity of the recorded imagery.)

4

DICOM Grayscale Function

One of the most critical aspects of the DICOM standard is its specifications for performance of medical displays [5]. In order to be of value, the stored image data must be recreated with sufficient fidelity for the medical experts to be able to rely on it to make a diagnosis or perform a procedure. While ‘‘shades of gray’’ is a phrase that may infer a lack of detail definition in the general society, these shades are the tools of the trade for radiologists and other medical professionals. As a result, when a pixel or a region of a medical display distorts the shades of the data represented on the screen, it could result in a misdiagnosis. The DICOM specification addresses this problem by defining performance standards for medical displays. Part 14, Grayscale Standard Display Function, provides a mathematical definition for the Grayscale Standard Display Function of Standardized Display Systems. This standard is designed to cover the performance of both hard copy (film) and soft copy (CRT or LCD monitor) representations of the medical imagery. The basic concept is that the same set of presentation values (P-Values) will result in the same luminance output regardless of the display system used. The grayscale function is based on the human visual system’s ability to perceive contrast. Sensitivity to visual contrast is nonlinear, as humans are less sensitive to the darker parts of an image than to the brighter areas. As a result, just-noticeable differences (JND) will be greater for dimmer parts of an image than for brighter parts. The Grayscale Standard Display Function is defined to cover a luminance range of 0.05–4,000 cd/m2. This is intended to include the lowest grayscale level that can be produced by a CRT to the brightest level available in very bright light-boxes used for x-ray film interpretation. This range includes 1,023 JNDs [6].

Reliability and Fidelity of Medical Imaging Data

3.4.1

The formula that defines this function is as follows [7]: log10 Lð j Þ ¼

a þ c Lnð j Þ þ e ðLnð jÞÞ2 þ g ðLnð jÞÞ3 þ m ðLnð jÞÞ4 1 þ b Lnð jÞ þ d ðLnð jÞÞ2 þ f ðLnð jÞÞ3 þ h ðLnð jÞÞ4 þ k ðLnð jÞÞ5

where Ln refers to the natural logarithm, j is the index (1 – 1023) of the Luminance levels Lj of the JNDs, and the ten constants are a = 1.3011877, b = 2.5840191E-2, c = 8.0242636E-2, d = 1.0320229E-1, e = 1.3646699E-1, f = 2.8745620E-2, g = 2.5468404E-2, h = 3.1978977E-3, k = 1.2992634E-4, m = 1.3635334E-3. The mathematical definition results in the curve shown in > Fig. 1.

5

Falling Short of Theoretical Perfection

Unlike analog information that is susceptible to noise and loss during transmission and processing, digital data is simple; it consists of a series of ‘‘on’’ and ‘‘off’’ – or ‘‘1’’ and ‘‘0’’ – bits of information. It should be a simple matter to collect the data, store it, and deliver it without any errors. In theory, you should get a precisely accurate image of the data every time. The problem is that defining this performance is one thing, but achieving it in real-world practice is another matter entirely. It is essential to understand the possible shortcomings of the displays, and how these can be minimized. The displays may not be able to achieve the full range of theoretical JNDs for a variety of reasons. First and foremost is the fact that a given display may not have the full luminance range described in the DICOM standard. As a result, it will be limited in its ability to create all 1,023 JNDs. Knowing the minimum and maximum luminance values for a display will allow one to predict the subset of JNDs that the display can theoretically present. The ambient light conditions where the medical display is used can also impact its performance. Glare and other reflected light can degrade the number of JNDs delivered by the display. Internal reflections of ambient light as well as light from the presented image itself can also affect the luminance of areas of the image, distorting the grayscale response of the image.

Luminance (CD/M2)

1,000

100

10

1

0.1 0

200

400 600 JND INDEX

. Fig. 1 Grayscale standard display function (Reprinted from [8])

800

1000

367

368

3.4.1

Reliability and Fidelity of Medical Imaging Data

In addition to the ambient lighting and physical characteristics of the display itself, the actual presentation data can limit performance. For example, a data stream may be limited to 8 bits of data per pixel. This means that the resulting image will be limited to just 256 different values per pixel. Even though these values can be mapped into a larger space using lookup tables (LUT), it does not increase the number of JNDs that they can present. It is important to note that data stream is limited by the most constricting step in its processing. For example, > Fig. 2a provides a schematic illustration of a medical imaging system. In this example, the data acquisition system captures 12 bits of data for the input image. The LUT also is based on 12 bits per pixel, as is the display signal processing component in the system. The problem is that the processing application that receives the data from the image scanner only processes 8 bits per pixel, as does the graphic processing board and the DVI data connection. As the data steps down from 12 to 8 bits, one third of the original information is lost, and cannot be recovered even though the data is eventually restored to a 12-bit format. This type of a system is characterized as an ‘‘8 to 12 bit’’ topology. In contrast, the following schematic represents a system that maintains 10-bit processing throughout > Fig. 2b.

12 bit

Acquisition

a

application

8 bit

8 bit

Graphic board video memory and processing

DVI transport

12 bit

LUT

12 bit

Display signal processing

Input image

Rendered image

12 bit

Acquisition

b

8 bit

Input image

10 bit

Application

10 bit

Graphic board video memory and processing

10 bit

LUT

10 bit

DVI transport

10 bit

Display signal processing

Rendered image

. Fig. 2 (a) The image pipeline of an 8–12 bit topology. (b) The image pipeline of a 10–10 bit topology. (Reprinted from [9])

Reliability and Fidelity of Medical Imaging Data

3.4.1

Starting with the same 12 bits per pixel from the scanning device, this is processed by an application as 10 bits per pixel. As a result, the amount of information lost at this step is one quarter (22 vs. 24) of that lost in the prior example. By matching the 10-bit application with a graphics board, LUT, DVI data connection, and display signal processing that can also maintain the 10-bit precision, the resulting image will contain four times as much information as the image that was reduced to 8 bits during processing. As a result, ‘‘10–10 bit’’ topology should yield more information, which in turn should lead to more information displayed on the screen.

6

Controller Limitations

Another factor is the display’s ability to create different luminance levels, and how well those levels are controlled. The digital driving levels (DDLs) must be calibrated to match the DICOM grayscale standard display function (GSDF). This is done by mapping the DDLs to the JNDs defined in the GSDF. One problem is that some displays do not have the bandwidth required to accurately map all the required JNDs. For example, if an LCD monitor is capable of producing luminance levels ranging from 0.8 to 500 cd/m2, it is theoretically capable of displaying 720 JNDs as defined by the DICOM GSDF. Some monitor controllers are limited to 8 bits per pixel, however. This constrains it to just 256 levels per pixel, which clearly is not adequate to map the 720 JNDs. A monitor with 10 bits per pixel – 1,024 possible values – will have sufficient control to map the JNDs more accurately.

7

Physical Limitations

Another problem is that the performance of all displays changes with time. CRT phosphors age, resulting in reduced luminance. LCDs that use fluorescent backlights also get dimmer with use. This means that it is not sufficient to simply calibrate a display system as it leaves the factory and expect it to perform up to standard over its useful life. One solution is to have technicians routinely recalibrate the displays. This approach can be expensive, as it requires management of the process as well as the time to locate the display and perform the calibration. In addition, the display could be out of calibration most of the time using this approach. Some display manufacturers have addressed this issue by providing monitors that have a closed-loop calibration system built in. Some simply monitor the light output of the backlight so that the total available light is known at all times. Others use a sensor embedded on the front panel of the display that monitors the actual performance of the display and its ability to present various gray levels. This uses a tiny portion of the display, and is able to take readings and make adjustments multiple times per second. As a result, the display’s performance will remain compliant with the DICOM standards much better than would be the case with human calibration procedures. Some monitors are also equipped to notify administrators using a local network connection in the event that the display is not performing within specifications. Some monitors also have the ability to automatically adjust the luminance levels in response to changing ambient lighting conditions in the room.

369

370

3.4.1

Reliability and Fidelity of Medical Imaging Data

In addition to changing luminance levels over time, the combination of backlight and LCD panel may not produce uniform luminance at all points on the screen when set to display a single level. This lack of uniformity can be identified at the factory, and some monitors are calibrated in order to cancel out this inherent lack of uniformity. Another physical limitation that is peculiar to LCD monitors is that the image characteristics can be altered when viewed off-axis (see also > Chap. 11.3.3). This can be manifested by reduced luminance, which translates to reduced contrast. More problematic, however, is that the gray level of a specific pixel can shift to a noticeable degree well before the loss of contrast becomes noticeable. This poses a significant challenge for diagnostic applications, especially when two or more people are trying to view the same image at the same time. They can actually see different shades in portions of the image, which could affect their interpretation of the image. Viewing angle also becomes a more significant problem even for single viewers as the screen size gets larger. When viewing an image on a 3000 monitor from a viewpoint perpendicular to the center of the screen, the viewing angle of the image in the corner of the screen is smaller than it would be when viewing the same image on a 2000 monitor from the same distance The actual physical structure of the LCD cells can have an impact on viewing angle effects. In general, in-plane switching (IPS) has better horizontal viewing angle performance than other structures, such as the standard twisted nematic (TN) that is often used in general purpose monitors, or multi-domain vertical alignment (MVA) panels.

8

Monitor Resolution

The whole purpose of the digital display of medical imagery is to convey information to the healthcare professionals involved with the patients case. The need for this information can range from simply monitoring the patient’s records, to diagnosis, to guiding a surgical procedure. In the different instances, more or less information may be required. There are two main ways that monitors can convey more information to the viewer. One attribute is monochrome versus color. All other specifications being equal, a color monitor is capable of conveying much more information than monochrome. A 10-bit monochrome monitor can convey 1,024 different values per pixel. A 10-bit-per-channel color monitor can convey 1,024 different values for each red, green, and blue sub-pixel. This works out to more than 1 billion different values per pixel. So one way to convey more information is to switch from monochrome to color. This is not a practical option for many medical imaging sources, as they generate monochrome images that are representative on non-visual data such as an x-ray, and there is no more information available that could be shown on a color screen. (Note that false color can be applied to monochrome images in order to make differences more pronounced and easier to see. Unless there are the additional bits of data in the source data, however, no additional information is actually being displayed; the existing information is simply mapped to contrasting colors.) The other way to convey more information is to increase the number of pixels displayed on the monitor. Again, if the scanning source of the imagery does not have equivalent resolution, then having additional pixels available on the screen will serve no positive purposes. (In fact, having more pixels can result in visible scaling artifacts that could introduce errors into the display of the source imagery.) While the computer industry in general uses acronyms such as XGA and SXGA to represent different monitor resolutions – or a simple horizontal by vertical pixel count like

Reliability and Fidelity of Medical Imaging Data

3.4.1

1,280  1,024 – the medical display industry has moved to classify monitors in general by the total pixel count. As a result, instead of referring to a monitor as having 1,600 by 1,200 pixels, the industry now classifies it as a 2 MP monitor. The total pixel count is 1.92 million, so this is rounded to the nearest million. This practice makes it much easier to compare monitor resolution. For simple tasks such as clinical review, monitors do not need a high resolution. One megapixel color resolution is often adequate, though 2 MP color monitors are used. Color displays used in surgery typically range from 1 to 2 MP (in a widescreen format); the resolution is typically limited by the fiber optic cameras used for minimally invasive surgery (MIS) procedures and their associated imaging and processing systems. Radiology diagnostic displays have the most demanding requirements, with monochrome and color monitors ranging from 2 to 5 MP. The highest resolutions are used for digital mammogram analysis. The newest class of medical displays offers 6 MP color resolution. These can provide extremely detailed display of medical imagery. They also are the equivalent of two 3 MP monitors side by side, making it easier to compare two images at a time.

9

The Next Step: 3D

The additional resolution available in medical imagery and displays can also be applied for another purpose. By providing different images to the left and right eye of the viewer, the system can create a stereoscopic view that appears in three dimensions (see > Chap. 9.2.1). This can be especially helpful in multi-planar reconstruction (MPR) imagery, such as from MRI and CAT studies that can generate far more images than can be analyzed effectively. By creating a three-dimensional model that medical professionals can observe and manipulate, the information can be far more useful. 3D medical imagery systems are still in the early stage of development, but their ability to take enormous amounts of image data and make it readily accessible will lead to more efficient and effective diagnosis and treatments.

References 1. (2000) Looking back on the millennium in medicine. N Engl J Med 342(1):42–49 2. DICOM (Digital Imaging and Communications in Medicine) (2008) NEMA. http://medical.nema.org. Accessed 6 Mar 2011 3. DICOM (Digital Imaging and Communications in Medicine) (2008) NEMA. Part 3: information object definitions, p 259. http://medical.nema.org. Accessed 6 Mar 2011 4. DICOM (Digital Imaging and Communications in Medicine) (2008) NEMA. Part 1: introduction and overview, p 15. http://medical.nema.org. Accessed 6 Mar 2011 5. DICOM (Digital Imaging and Communications in Medicine) (2008) NEMA. Part 14: grayscale standard display function http://medical.nema.org. Accessed 6 Mar 2011

6. DICOM (Digital Imaging and Communications in Medicine) (2008) NEMA. Part 14: grayscale standard display function, p 11. http://medical.nema.org. Accessed 6 Mar 2011 7. DICOM (Digital Imaging and Communications in Medicine) (2008) NEMA. Part 14: grayscale standard display function, p 12. http://medical.nema.org. Accessed 6 Mar 2011 8. DICOM (Digital Imaging and Communications in Medicine) (2008) NEMA. Part 14: grayscale standard display function, p 15. http://medical.nema.org. Accessed 6 Mar 2011 9. Matthijs P (2003) Grayscale resolution: how much is enough? Barco, Belgium, p 20

371

372

3.4.1

Reliability and Fidelity of Medical Imaging Data

Further Reading Digital Imaging and Communications in Medicine (DICOM), published by Springer, Berlin Heidelberg, 2008, ISBN: 978-3-540-74570-9 (Print) 978-3-54074571-6 (Online)

Changing the landscape: How medical imaging has transformed health care in the U.S. (Dec 2006) NEMA Sherrow V (2006) Medical imaging. Marshall Cavendish, New York

3.4.2 Ultrasound Imaging Robert M. Nally 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374

2 The Ultrasound Imaging System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 2.1 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 3

Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

4

Data Conversion and Rendering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

5

Surface Identification and Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

6

Speckle Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380

7

Rendering and Displaying the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380

8

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.4.2, # Springer-Verlag Berlin Heidelberg 2012

374

3.4.2

Ultrasound Imaging

Abstract: This chapter provides a broad overview of developments in ultrasound imaging. It discusses the barriers to this progress and why they are coming down, offers a high-level view of the major components in an ultrasound system and how new technologies are redefining these components, and what the future directions of ultrasound imaging in the medical industry may be. List of Abbreviations: DSP, Digital Signal Processor; FPGA, Field Programmable Gate Array; GPU, Graphics Processing Unit

1

Introduction

Despite its long history, ultrasound imaging is closer to the beginning of its evolution than to the end. Barriers to its progress have been somewhat unique to this industry, but events are changing rapidly and you could say that ultrasound imaging is entering into a reinventive era. To understand why, we first need to understand how ultrasound imaging systems work (> Fig. 1). Ultrasound is high-energy sound waves, above the frequency audible to the human ear (about 22 kHz). As sound waves travel through a media, such as the human body, they will reflect off surfaces of discontinuity in density. The different organs in the human body are made of different tissues, giving each organ a different density. A device called a transducer is used to both generate the energy (i.e., source sound waves) and to detect the reflected energy (i.e., the reflected waves echoing off the organs in the body). A technique called beam forming is used to concentrate the energy along a straight line from the head of the transducer. The head is the metal plate on the transducer that comes into contact with the body. There is always a time delay between when the sound is generated and when the reflected sound wave returns to the transducer. By timing when the source energy is generated and when the reflected energy is detected, the distance from the transducer head and the surface that generated the reflection can be calculated. Inside the transducer are many sources of sound, each one very similar to a very small speaker. The difference being, the drivers are not driving a cone to generate sound, but rather beating against the head of the transducer like it was a drum. Each one of these little drums or speakers is an energy source. By timing these energy sources so that each one is generated at a different time will form a shaped wave of energy. As this energy gets reflected off tissue discontinuities in the body, it is reflected in a scattered pattern back toward the transducer. By measuring both the flight times (the time between when a source wave left the transducer and when a reflected wave returns to the transducer) and degradation of each reflected wave and summing them all together according to their flight times, all the events or tissue discontinuities along an imaginary line can be realized. This process of controlling and measuring the flight times of all the energy sources and energy reflects to realize events along a straight line is known as beam forming. The ultrasound system captures hundreds of these lines of data, each line being a beam directed at a different angle into the body from the transducer. From all these beam lines, it will generate just one image frame. Thirty frames are generated every second. In order to generate just one frame, these beam lines have to be transposed from a radian format to a two dimensional Cartesian format. Then, a series of imaging filters are applied to identify, enhance, and display the reflective surfaces (organs) inside the body. In the past, microprocessors were too slow to handle this much data and all this processing was done with hardware (i.e., logic devices), but today the microprocessors do have the power, and it is the power of this and future generations of microprocessors that are driving the changes in this industry.

Ultrasound Imaging

3.4.2

. Fig. 1 In vivo ultrasound image of a human kidney

2

The Ultrasound Imaging System

Every ultrasound imaging system has three major components: a data acquisition unit, a data processing unit, and a data rendering and display unit. The data acquisition unit collects time domain data on a very thin cross-sectional slice of a three-dimensional subject matter. The data processing unit transforms the collected time domain data into spatial domain data, assigning sample values to a number of sample points in the thin cross-sectional area of the subject matter on which the data was collected. The data rendering unit then transforms the processed data into a viewable format and renders it into an image, which is what the doctor and patient see when they look at the systems display monitor (> Fig. 2).

2.1

Data Acquisition

The data acquisition unit is composed of the head, the handheld device the technician holds over the subject area, the controls within either or both the head and the main system box used to collect the data, and the cabling and communications between the head and the main system. The head contains an array of transducers. The transducers are arranged in a straight line on the head, but there are new concepts coming out where the transducers are arranged in two-dimensional arrays. There are even transducer arrays designed to fix on the end of an endoscope so that the ultrasound scan can get even closer to the target area for greater clarity. Each transducer is both an energy source and a data collector. Each transducer produces a short burst of energy in the form of ultrahigh-frequency sound waves at a different time. The timing controls determine when each transducer fires so that the energy from all the transducers is directed to a common point along a straight line inside the three-dimensional subject matter. Hundreds of beams are generated from the head in a radian pattern, as shown in > Fig. 3. Each beam line is a vector and each sample point along that line is a scalar value.

375

376

3.4.2 Data Acquisition unit

Ultrasound Imaging

Data Processing unit

Data Rendering unit

Imaging PipeLine

Transducer

Data Alignment and Scan Conversion

Note: More and more of the data acquisition controls are migrating into the Transducer. In older systems there is still a lot of this functionality embedded in the data processing unit

Image Filtering PipeLine

System Controls

Graphical Overlay (Doppler Effects, Text, etc)

Display

Storage

. Fig. 2 Schematic of an ultrasound imaging system

Transducer Components and Functionality

Energy Transfer Plate Coupling Energy Transfer Plate

Electronics devices for Beam forming, Electronics transmitter drivers, assemble board and acquisitioning controls Connector to connect to rest of system

Transmitter-Receiver Array Individual Transmitter-Receiver

Directional lines beam form is shaped in

2-diminsional acquisition plane

. Fig. 3 Artistic rendering of the electronic assembly components and functionality in an ultrasound transducer head

The position of each sample point is determined by the time it takes for the energy to make a round trip from the transducer to the sample point and back to the transducer. All the beam lines radiate outward from the center point of the transducer array. The data from each transducer is sent by cable back to the main system as a continuous data stream. For a greater understanding of beam-forming concepts and practices, see [1].

3.4.2

Ultrasound Imaging

3

Data Processing

The data processing component receives a continuous data stream from each transducer. The data processing unit is receiving a sample value for each sample point from each transducer. It has to merge all these data streams together to yield one sample value for each sample point. The data stream from each transducer has to be aligned with the data streams of all the other transducers. The acquired data streams are time aligned when they are received by the data processing unit and the timing of each stream is different from each vector acquired. After the data is properly aligned, each sample point is then either summed or averaged to produce the resultant sample data. The data is formatted into an array of vectors with each vector containing the same number of sample points. The data field is composed of hundreds of vectors, each with hundreds of data sample points. The coordinate form of each sample point in the data field is S(y, t), where y is the angle between the vector the sample point is in and the vector that is normal to the transducer array and t is the sample point along that vector. The data field is in the same plane as the transducer array. After processing the data, the processing unit then presents the data in this format to the data conversion and rendering unit.

4

Data Conversion and Rendering

In the data conversion and rendering unit, the data is converted to a viewable format, filtered, and displayed. The data input into the data conversion and rendering unit is in a S(y, t) format, but it has to be displayed in a P(x, y) format. Converting from the S(y, t) format to the P(x, y) format is known as scan conversion. > Figure 4 is a graphical representation of a scan

P(x,y) S(θu,tv)

P(x+1,y+1)

S(θu,tv+1) θu+1 θu Drawing 1: Graphical Representation of Scan Conversion

. Fig. 4 Graphical representation of scan conversion

377

378

3.4.2

Ultrasound Imaging

conversion operation. Each sample point S(y, t) of the vector data is mapped to the closest pixel position P(x, y) of the display frame buffer according to the formula: Sðy; tÞ ¼ arctanðx; y Þ

ð1Þ

where y is the angle between the vector that is the normal to the transducer array and the vector sample point S(y, t) is in and t is the magnitude in terms of time along vector y. The values x and y are the two-dimensional coordinates (i.e., pixel position) of the displayed data. As can be seen, this is not a straight one-to-one mapping. Moreover, each display pixel P(x, y) is a bilinear interpolation of the values of the four S(y, t) sample points that map closest to it. P ðx;y Þ ¼ ðaÞ½ðbÞfSðyu ;tv Þg þ ð1  bÞfS ðyuþ1 ;tv Þg þ ð1  aÞ½ðbÞfS ðyu ;tvþ1 Þg þ ð1  bÞfS ðyuþ1 ;tvþ1 Þg

ð2Þ

For any given pixel P(x, y), the nearest sample point S(y, t) can be calculated using > Eq. 1. The values (a) and (b) in > Eq. 2 are the fractional values from pixel P(x + b, y + a) which is located between pixels P(x, y) and P(x + 1, y) and pixels P(x, y) and P(x, y + 1) that the sample point S(y, t) actual maps to. After the data has been converted to a display format, it then goes through a series of filters. Every manufacturer filters the data a little differently and each considers this information to be proprietary, so as a result there is not a lot of information about the pipeline filter in public domain. The information on the filter pipeline in this chapter is generic in nature and avoids getting into proprietary areas. The filter pipeline is designed to accomplish only two goals: (1) to identify and enhance surface areas in the image and (2) to remove artifacts like speckle from the image.

5

Surface Identification and Enhancement

Surface identification and enhancement is usually a two-, three-, or four-step process. The first step would be to detect the edges of the surfaces in the image. A couple of filtering techniques may be used to achieve edge detection. One technique would be to create a gradient direct mask field by identifying the direction of greatest gradient for each pixel in the display field. The theory behind this technique is based on the fact that human tissue absorbs sound energy. When the energy reflects off an internal organ, some of the energy penetrates the organ and gets absorbed and some of the energy is reflected back. Where the energy is absorbed, the pixel data is darker and where it is reflected back it is brighter. Therefore, at the surface of the organ, the gradient between dark and light should be the greatest. Another way of stating it would be to say that the gradient is greatest in the direction perpendicular to the surface. If the gradient direction is defined as the direction from darker to lighter intensity, then most of the direction vectors will be pointing up toward the top of the image. A horizontal surface in the viewing field would generate direction vectors pointing downward for those pixels on the edge of that surface. Of course, the process is not as simple as that. Sometimes, as many as three or four additional passes have to be made at the data and mask fields with reiterative averaging algorithms in order to remove errors introduced to the mask field due to noise in the data field. Another technique would be to identify pixels of similar brightness and then string them together using an active contouring or ‘‘snaking’’ technique. This technique is similar to the snaking technique used to track roads in satellite images, but a lot harder because of a number

Ultrasound Imaging

3.4.2

of reasons. The main challenge being noise introduced in both the data acquisition and scan conversion operations. In ultrasound imaging, the snakes are usually fragmented and additional passes at the data must be made to tie the fragments together and to eliminate false fragments. Once again, a mask is used to identify the pixels in the snakes (better known as ‘‘snaxels’’) making the mask a snaxel mask field. The section on ‘‘Boundary Lines’’ in Chap. 7 of [2] is a very good source for learning more about snaxeling and other methods for determining boundary lines. Both techniques have their drawbacks. The gradient direction technique can yield some unusual patterns when the subject area has a lot of air in it. For example, the lungs are large air sacks in the body and trying to take any ultrasound images of them would be difficult. If the ultrasound system used the gradient direction technique to close surface contours, what would be generated would not be contours. The contours would be degraded to patterns, making it impossible to render the surface from contours. One problem with the active contouring technique is that it cannot always identify and eliminate false snake fragments. As a result, these fragments will show up on the display. Whether it is false patterns or false snake fragments, displaying false information generated by the filter pipeline could result in faulty interpretations by the doctor or imaging specialist; therefore, great effort has been made by ultrasound equipment manufacturers to prevent these anomalies from occurring. Many man-hours have gone into developing filtering algorithms that minimize these artifacts, but they cannot be eliminated 100%. The next step in the filter pipeline would be to enhance the edges of the surfaces that were identified in the first step. In order to enhance an edge, the pixels on one side of the edge are reduced in color intensity while the ones on the other side are increased in intensity. The modification to the color value is very small, just enough to make the object defined by the edge easier to see. The technique used for the edge-enhancement operation will depend on the technique used to identify the edge in the first step. If the gradient direction technique was used for the edge detection algorithm, then the edge enhancement algorithms would exaggerate the differences between adjacent pixels in the data field according to the information in the mask field. There are a number of variations that depend on the information in the mask field. For example, if the mask field used a value from one to eight to identify the closest neighbor of greatest gradient difference from the pixel being addressed, then that neighboring pixel identified by the mask value (1–8) would be reduced in intensity and the pixel being addressed would be increased in intensity. For a line of pixels where the mask value for all the pixels in the line were the same, the effect would be a sharper contrast between the pixels in the line and its closest neighbor identified by the mask value. If the active contour technique was used for the edge detection, the data in the snaxel mask field may or may not carry directional information. By the nature of the data in the data field, an assumption can be made that the side of the snaxel to enhance is the side closest to the top of the image. For the best image though, the snaxel should carry some directional information with it much like the directional information in the gradient-directional mask field. The price to be paid for carrying directional data in the snaxel field is that extra passes at the data field have to be made to extract this information. The third operation in the filter pipe line is a de-convolution or image-smoothing operation. Most ultrasound systems use some form of a modified Laplacian filter to smooth out the enhanced edges. A one-dimensional Laplacian perpendicular to the edge or an adaptive Laplacian could be used along with a host of other variations of the Laplacian.

379

380

3.4.2 6

Ultrasound Imaging

Speckle Removal

The last stage of the filter pipeline is called ‘‘speckle removal.’’ Speckle is a noise artifact generated during data acquisition. Speckle is the product of wave interference patterns. Each transducer generates a sound wave burst. All these waves are echoing around in the patient’s body. As they bounce around back and forth, they superimpose on each other creating salt and pepper noise in the image data. As much of this noise as possible must be removed with minimal degradation of the image. About the only way to remove this kind of noise is with a weighted averaging kernel. A 3  3 or 5  5 kernel that averages the pixel in the center of the kernel with different negatively weighted values of the 8 or 24 other pixels in the kernel will average out most of this salt and pepper noise. It is impossible to filter out noise without some loss of image quality; therefore, the size and weighting of the elements of the kernel depend on the nature and severity of the noise and the acceptable degradation of the image quality. See [3] for a more detailed review of any of the image-filtering techniques explored in > Sects. 5 and > 6. Chap. 3 does an excellent job of comparing the results of the different techniques and presents a number of examples.

7

Rendering and Displaying the Data

Finally, the data is ready for display, but the user interface and color planes have to be overlaid on top of the data plane. Color data overlaid on top of the data plane is usually used to show fluid velocities. The velocity data is calculated using Doppler theory. Just as meteorologist can use data from radars to calculate wind directions and velocities, the data collected in the acquisition unit can be used to calculate the velocity of fluids like blood flowing through arteries. Other overlay planes are the graphical user interface (GUI) and measuring control planes. Doctors sometimes need to acquire very accurate measurements of objects in the viewing area. These measuring controls need to be accurately calibrated as the surface area is zoomed in on as well. The overlay planes are rendered to the desired display resolution using graphic primitives while the ultrasound image plane is scaled to the desired display resolution using bilinear interpolation. When upscaling (zooming in), a single pixel location in the un-zoomed image covers an area of four, eight, or even more pixels, depending on the zoom ratio, in the zoomed image. Sub-pixel mapping to the zoomed resolution of the two independent operations (interpolation and rendering) has to be coordinated. The one thing you should always keep in mind when dealing with ultrasound imaging is that the image seen on the display device is not an image of the subject being monitored, but rather a computer-constructed model of that subject. A model derived by bouncing sound waves off that subject. This data is not captured by a camera, it is data generated mathematically from waves of sound energy reflecting off the surfaces of organs in the body. Different densities in body fluids that these sound waves pass through can and will affect the quality of the image being displayed. Now that we have a basic understanding as to how an ultrasound imaging system works we will review its evolution and speculate on its future. The evolution rate of ultrasound imaging has and always will be limited by barriers unique to this industry. In the beginning, the barrier was technology. No processors at that time had the horsepower to process 30–60 frames per

Ultrasound Imaging

3.4.2

second in vivo. Equipment manufacturers had to implement the data acquisition unit and data rendering and display unit using only hardware components and depended mostly on field-programmable gate array (FPGA) technology. The transducer energy generation, timing controls, and data sampling was all done with FPGAs. The scan conversion and image filtering pipeline were also all done using FPGAs. These solutions were very expensive and were limited to small market areas. Today, digital signal processors (DSPs) and graphics processing units (GPUs) do have the power to do most of the work, but new barriers have surfaced to impede their adoption. One barrier is the rendered image itself. Recall the statement made about the image being viewed is a computer-constructed model of the subject matter. Well, doctors and technicians know how to interpret these constructed models and they want the constructed images of these models to be consistent going forward. Their knowledge is based on use. They have been using the old equipment for years, and they feel comfortable in interpreting these images. What makes this an issue is the fact that the algorithms used to generate these images are optimized for FPGA designs. For a pipelined FPGA designs, each FPGA had an image buffer to work out of. When the bandwidth or processing limits of the FPGA and its local image buffer were exceeded, a new FPGA and image buffer was added to the design. The algorithm in each FPGA was designed without any intension of one day being merged with the algorithms in the other FPGAs, which is what would have to happen if ported to a DSP or GPU. When the equipment manufacturers tried making the jump, they discovered the processors did not have the bandwidth to absorb the combined bandwidth requirements of all these local image buffers. There is currently an effort underway to modify these algorithms to fix into the processor paradigm. The more practical solution would be to develop new and better algorithms more optimal for the processors. Besides, most of the algorithms developed for the FPGAs were not the best adaptations of the theoretical models they were based on. Because the algorithms were executed in hardware-only solutions, there were some constraints like ‘‘gate count’’ limitations, memory or data storage limitations, and pin count of FPGAs. With the FPGAs, the designers had to take some liberties with the theories and apply some close approximations to the theory at times. Adoption is slow, but yes it will happen. The established equipment manufacturers will see competition from new startups that do not have to maintain a legacy image expectation. With the power of the processors, the startups will add new features that will attract the older users. The new users will accept the changes without question. In the end, the older manufacturers will either have to adapt or get pushed out of this market space. As a result, the future of ultrasound imaging is wide open. The Doppler technology borrowed from the radar industry is becoming more and more important. More information is delivered to the doctor or technician via the graphical user interface. Color is used to indicate fluid velocities and other important information the doctor wants to see as he or she looks at the ultrasound image. Image quality is making quantum leaps as the processors displace the FPGA devices in the ultrasound system. The power of the processors will allow the designers to use better filtering. Scan rates and display resolution of the scanned image will both go up. New features like real-time zooming and bird’s eye pop-up images will become standard features. There are also efforts being made to develop three-dimensional transducers and threedimensional ultrasound solutions will soon be available. If ultrasound systems follow the evolution path other high-tech systems have, within a decade you will see doctors toting portable ipad-sized scanners with them on their rounds

381

382

3.4.2

Ultrasound Imaging

. Fig. 5 The new GE VScan portable ultrasound system (transducer head not shown)

in the hospitals and the ultrasound scanner could render the stethoscope obsolete. The GE VScan portable ultrasound system (> Fig. 5) is just a glimpse of what the future holds for ultrasound imaging applications in the medical industry.

8

Conclusion

The high-tech advances in electronics have finally begun to affect the definition and design of ultrasound imaging systems in the medical industry. One can expect to see many new applications for ultrasound imaging to appear in the operating room, the doctor’s office, in patient care, and many other areas. In developing countries, one can even expect to see ultrasound imaging equipment where the closest doctors could be hundreds of kilometers away, and these technician-operated systems will allow the doctors to see and diagnose patients living in these remote areas. This will be an exciting industry to be in over the next few years.

References 1.

Park S, Aglyamov SR, Emelianov SY (2007) Beamforming for photoacoustic imaging using linear array transducer. In: Ultrasonics symposium, 2007, New York. IEEE, Volume, Issue 28–31 pp 856–859. doi:10.1109/ULTSYM.2007.219

2. 3.

Russ JC (2007) The image processing handbook, 5th edn. Taylor & Francis, London, ISBN 0-8493-7254-2 O’Gorman L, Sammon MJ, Seul M (2008) Practical algorithms for image analysis. Cambridge University Press, Cambridge, ISBN 978-0-521-88411-2

Ultrasound Imaging

3.4.2

Further Reading Committee on the Mathematics and Physics of Emerging Dynamic Biomedical Imaging, National Research Council & National Academy of Sciences (1996) Mathematics and physics of emerging biomedical imaging. National Academy Press, Washington, DC Lee C, Sohn H-Y, Han D-H, Song T-K (2008) Real-time implementation of the echo signal processing and digital scan conversion for medical ultrasound

imaging with a single TMS320C6416 DSP (Proceedings Paper). In: McAleavey SA, D’hooge J (eds) Medical imaging 2008: ultrasonic imaging and signal processing, Proc. SPIE, vol. 6920, 692004 (2008). doi:10.1117/12.770136 Szabo T (2004) Diagnostic ultrasound imaging. Academic Press, Burlington

383

Part 3.5

Case Study: Security Imaging and Display

3.5.1 Data Hiding and Digital Watermarking Daniel Taranovsky 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388

2 2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 2.2 2.2.1 2.2.2 2.2.3

Principles of Steganography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 Characteristics of Steganography Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389 Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389 Perceptual Transparency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 Robustness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 Complexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 Applications of Steganography Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 Digital Watermarking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 Covert Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 Photo Annotations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393

3 3.1 3.2

Steganography Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 Hiding in the Spatial Domain: Least-Significant-Bit Modification . . . . . . . . . . . . . . . 393 Hiding in the Frequency Domain: DCT Coefficient Modification . . . . . . . . . . . . . . . . 394

4

Steganalysis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396

5

Summary/Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397

6

Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.5.1, # Springer-Verlag Berlin Heidelberg 2012

388

3.5.1

Data Hiding and Digital Watermarking

Abstract: Securely embedding data in digital images has important applications for covert communication and copywrite protection. The specific algorithm used to embed information depends on the characteristics of the application. Two prominent techniques for embedding information are least-significant-bit modification, and discrete-cosine-transform modification. Detecting modifications to a high bit-rate signal such as a digital image can be difficult, and is the subject of cross-disciplinary research in statistics and signal processing. This chapter gives an overview of the important issues associated with security imaging with specific examples of techniques to embed and detect hidden information. List of Abbreviations: DCT, Discrete Cosine Transform; LSB, Least-Significant-Bit of a Binary Number; YCrCb, A Representation of an RGB Color in Terms of Its Luminance (Y) and Chrominance (Cr and Cb) Components.

1

Introduction

Digital photography, social networking, and ubiquitous Internet access has made the distribution of digital media common place. This trend creates new problems and opportunities, and warrants the development of a technology for ownership control and secure embedding of information to media assets. Traditional computer networks can offer a brute-force security approach that limits access privileges, but is only effective as long as there is no breach of the network. An alternative is to implement embedded security that ‘‘travels with the content’’ in a sense. Data hiding technology, such as digital watermarking, can protect content without explicitly fire-walling access. Covert communications channels can be established with data hiding in digital images. The hiding of this data in images shares similar theoretical foundations as digital rights management. Digital images are so common that it is impossible to have an inventory of all images shared, posted, stored, and otherwise in existence. This provides ample opportunities for sharing images containing hidden data without arousing suspicion. Images are emailed, available on servers for download, and posted on websites everyday. The act of sharing or viewing a digital image is part of the way people communicate with each other on a regular basis. Even if one were inclined to be suspicious of digital image sharing activity, the volume of content available makes it impractical to effectively search for covert communication. Digital images are high bit-rate signals making them excellent media for data hiding. The human eye also has limitations in detail it can perceive, making it possible to alter the digital representation of an image with imperceptible difference to a viewer. Data hiding refers to the process of securely embedding information to some cover medium, and is the focus of this section. Different applications have unique security requirements that reflect the usage model of the embedded information, while there are overarching concepts that apply to all cases. Steganography traditionally refers to data hiding for the purpose of covert communication, and digital watermarking applies data hiding for digital rights management. In terms of problem definition we refer to steganography as being the general data hiding problem, and digital watermarking as a specific instance of steganography.

2

Principles of Steganography

We define a steganography embedding algorithm as a function: f ðM; C; kÞ ! Z

ð1Þ

Data Hiding and Digital Watermarking

3.5.1

C is the original digital image (referred to as the cover object or cover image) used to embed data M. M is the message being embedded, referred to as the stego-message. Z is the resulting image after embedding message M in cover object C, referred to as the stego-object or stego-image. Z is intended to be indistinguishable from C. k is some digital key (referred to as the stego-key) to extract and decipher the message M from the stego-image. k should only be known by the originator and intended recipient of M. Cryptography is the study of encrypting data, and steganography techniques attempt to hide data. Cryptography and steganography are often used together. Rather than extracting a legible message from the stego-object, the message is encrypted prior to embedding. The mere detection of a covert message would be considered a breach of the steganographic algorithm, but encryption would serve as additional protection for the message in the event of a successful attack on the stego-object. > Figure 1a is an example of a stego-object with an encrypted paragraph of text embedded.

2.1

Characteristics of Steganography Algorithms

> Figure

2 illustrates the steganography process. There are different steganography functions f , all varying depending on target application criteria. Improving one criteria is typically achieved by compromising another [1, 2].

2.1.1

Capacity

Capacity refers to the size of the message that can be embedded in the cover object. A message can be compressed to fit in a smaller cover object at the expense of computational complexity.

a

b

. Fig. 1 Image with paragraph of text secretly embedded (a), and the original image (b)

389

C : Cover Image

f : Stego-Function

. Fig. 2 Steganography in practice

M : Stego-Message

Z : Stego-Image

Transmission

Z¢ : Received Stego-Image

Noise, manipulation, attack, etc...

f -1 : Inverse Stego-Function M¢ : Received Stego-Message

101010111001101. . .

k : Stego-Key

3.5.1

1010 101 1 100 1 101 ...

k : Stego-Key

390 Data Hiding and Digital Watermarking

Data Hiding and Digital Watermarking

3.5.1

Algorithms that offer higher capacity reduce the bandwidth and time required to transmit the stego-object. Low capacity algorithms require larger distortions to the cover image as the message size increases, resulting in compromised perceptual transparency [3].

2.1.2

Perceptual Transparency

Perceptual transparency is the degree that the algorithm distorts the original cover object. As a greater proportion of the stego-image signal is the message, higher distortion and higher capacity is observed. Steganographic algorithms should apply distortions characteristic of signal noise. A typical expectation of perceptual transparency is that the stego-object is visually indistinguishable from the cover object. An algorithm that offers low perceptual transparency is not useful for any application. A higher order requirement is that statistical analysis of the stego-image signal should not reveal any evidence of an embedded message. While an image may appear to be identical, statistical analysis of the color data distribution can reveal anomalies when compared to everyday images. > Section 4 describes these cases in more detail.

2.1.3

Security

Security refers to the difficulty an attacker has in determining whether a message is embedded in the cover object. A digital watermark, for example, may not alter the cover image and be impossible to remove but is clearly embedded in the object. In this case the perceptual transparency and robustness is high but the security of the communication channel is low. Security in this sense is defined as the level of ‘‘invisibility’’ of the message. An algorithm that is not perceptually transparent is not secure, but the inverse is not necessarily true. Some digital watermarking applications may be secure, but all covert communication must be secure to be effective.

2.1.4

Robustness

Robustness is the ease with which one can destroy, modify, or remove the stego-message without damaging the cover medium. Compressing, resizing, or cropping images are transformations typically applied to digital images. Some applications require hidden information to remain intact and extractable even after such geometric modifications. If an attacker must damage the image beyond recognition to remove the stego-message then the algorithm is robust. If the quality of the cover object after an attack is not important, then the problem becomes trivial – simply deleting the stego-image will destroy both the cover and message object. Steganography algorithms are designed to force attackers to severely damage the image while removing the message. In the case of covert communication, a good steganography algorithm will make determining the presence of a message no more successful than a random guess. The highest level of security breach in covert communication is if the attacker is able to read the stego-message and replace it with an alternate message. In this case, not only has the presence of covert communication been identified (a security breach on its own), but the robustness of the algorithm has also been compromised.

391

392

3.5.1 2.1.5

Data Hiding and Digital Watermarking

Complexity

Complexity refers to the computational complexity of the algorithm. Long messages embedded in small images may require a lot of computational resources to encode and decode. Depending on the application, this may or may not be a concern. For example, an application that embeds precise time stamp information or requires real-time copywrite verification may be sensitive to the computation complexity of the algorithm.

2.2 > Table

2.2.1

Applications of Steganography Algorithms 1 characterizes the principle characteristics of data hiding applications.

Digital Watermarking

A proprietary image or logo can be protected by having ownership information bound to it. If the distribution rights are violated, then the offending image would have information on who violated the usage agreement bound to it. Other security features can include download information and time stamping to track the origin of ownership violations. Users typically know (or are told) a digital watermark is embedded in the image to deter copywrite infringement. Reading the watermark message may not necessarily be considered a security breach. The content is expected to remain perceptibly unaltered after the digital watermark is applied, and it should be impossible to alter the watermark even after cropping, resizing, and other geometric image processing.

2.2.2

Covert Communication

Two parties communicating covertly with one another will want to hide the existence of messages. Depending on the communication channel and the expected mode of attack, robustness may not be an issue. If a message is embedded in an image on the World Wide Web, then the recipient will download the image without any transformation or processing applied to it. However, if the image is sent over a protected server that may selectively attack suspecting image attachments, then robustness may become a concern. High capacity and high security are the most important algorithm considerations to avoid detection.

. Table 1 Characterizing data hiding applications Image data hiding application

Prime algorithm considerations

Digital watermarking

Robustness

Covert communication

Security

Photo annotations

Capacity

Data Hiding and Digital Watermarking

2.2.3

3.5.1

Photo Annotations

Information about archived photos may be most conveniently stored as embedded information. By embedding the information in the image with steganography techniques, attachments do not require separate storage maintenance, will not be lost or destroyed on their own, and access to annotations is controlled by those who have the stego-key. Information about the contents of the image, its contextual significance, or other information may be useful for search engines and people with need-to-know privileges.

3

Steganography Techniques

This section discusses data hiding principles and techniques. A comprehensive survey of steganographic algorithms is beyond the scope of this text. We focus on LSB (least-significant-bit) and transformation-based techniques, which are the basis for many application specific implementations [1, 2].

3.1

Hiding in the Spatial Domain: Least-Significant-Bit Modification

LSB techniques involve selectively modifying the least-significant-bits of a pixel’s color. The bits modified can be a single least-significant-bit in a single channel, or multiple bits in one or more RGB color channels. As more bits are modified, the perceptual transparency decreases while capacity increases. An example of least-significant-bit insertion is shown in > Fig. 3. Some file formats (such as .GIF) use an indexed palette to color pixels. For example, each pixel is an 8-bit index referencing one entry in a palette of 28 24-bit colors. Storing the stego-message in the least-significant-bit of the pixel’s index can result in high degradation of perceptual transparency since adjacent colors in the palette are not guaranteed to be contiguous in color space. Embedding the message in the palette (rather than the palette index) retains more perceptual transparency, but the size of the embedded message is limited to the size of the palette. Another consideration is what pixels in the image to modify. Cover images with many ‘‘color pairs’’ that differ by a single bit, preferably in the same spatial locality, are best suited for

. Fig. 3 Insertion of 011 in least-significant-bit of Pixel A, B, C’s red (‘‘R’’) channel

393

394

3.5.1

Data Hiding and Digital Watermarking

least-significant-bit modification. Regions of flat color are prone to show discrepancies in neighboring pixels. Some algorithms may select random pixel locations, intelligently identify favorable image regions, or have hard-coded locations. The principle deficiency with least-significant-bit algorithms tends to be robustness. Resizing, cropping, signal noise, and other basic modifications usually destroy the stegomessage. Stego-messages can also be removed by simply overwriting all least-significant-bits in the image.

3.2

Hiding in the Frequency Domain: DCT Coefficient Modification

A digital image can be considered a multidimensional signal. Every pixel is represented as a three channel (RGB or YCbCr) value on a two-dimensional image plane. Each channel can be mapped over the surface of the plane and treated as a distinct two-dimensional signal [4–6]. Signals can be represented as a sum of oscillating functions of varying frequencies. The DCT (discrete cosine transform), for example, represents a signal as a sum of cosine functions. The JPEG image encoding algorithm divides an image into 8  8 blocks of pixels and encodes each block as its DCT coefficients. This is particularly effective for images since there is a high degree of correlation among neighboring pixels. If one pixel is red, there is high likelihood the next pixel is close to red. Therefore, in an 8  8 block of pixels, the color variations can usually be represented in the low-frequency bands, while high frequency variations (large color changes from one pixel to the next) are rare, imperceptible to the human eye, and can be discarded without changing the principle content of the image. Data hiding in an image’s frequency domain can be accomplished by modifying the lowfrequency DCT coefficients (the upper left values in > Fig. 4e) [4]. Let us assume cover image C is represented by a series of 8  8 DCT coefficient matrices, and we intend to embed data in one coefficient denoted ci . Message M is composed of a series of values m0, m1 ,. . ., mn and the resulting value of the coefficient ci after embedding mi is zi . One simple algorithm for hiding mi is: zi ¼ ci þ a  mi

ð2Þ

a is a scaler value to ensure a  mi is significant enough to ensure robustness. The message should be hidden in a number of different coefficients ci throughout the image, and only those with the stego-key are able to identify which coefficients were modified and by how much. Consider the example illustrated in > Fig. 5. An 8  8 pixel block has a portion of stegomessage M embedded in its ð1; 1Þ coefficient. a  mi ¼ 63, and is added to the original DCT coefficient ci ¼ 63 to give zi ¼ 126. Comparing the decoded stego-object (> Fig. 5b) with the original image (> Fig. 5a), we see that modifying ci ¼ 63 to zi ¼ 126 results in minor degradation. Some data is lost in the upper left corner (> Fig. 5c), but otherwise the color structure remains intact. This satisfies the perceptual transparency condition. The algorithm for embedding and extracting a message is application specific. There are overriding principles that are shared among applications that implement data hiding in the frequency domain. We consider a naı¨ve example of a stego-key that is comprised of the message M and the original DCT coefficients C. A watermark application checks for the presence of mi at coefficient location ci . By comparing the original ci ¼ 63 to the discovered value zi ¼ 126 in the stego-image, one can apply statistical analysis to determine the likelihood zi  ci ¼ 63 is the result of random noise. Assuming one concludes it is statistically unlikely for the coefficient

Data Hiding and Digital Watermarking

a

3.5.1

b

d

e

f

c

. Fig. 4 An image and one 8  8 pixel block (a, d), its compressed discrete-cosine-transform representation (b, e), and the resulting decoded image (c, f)

a

b

d

e

c

. Fig. 5 An 8  8 pixel block. Cover image (a), stego-image (b), cover difference image (c), attacked image (d), and stego-difference image (e)

395

396

3.5.1

Data Hiding and Digital Watermarking

ci to change from 63 to 126 due to random noise while mi ¼ 63, then we determine zi exhibits evidence of a watermark or some other embedded message mi . In this simple example the recipient of the cover image knows mi ¼ 63 and ci ¼ 63, and the stego-key will look for a value close to zi ¼ 126 at location i. If the algorithm discovers, for example, zi ¼ 100 at location i, then it is less likely that mi is present at location i, and more likely that the discrepancy between zi ¼ 100 and ci ¼ 63 is due to noise or other unrelated modifications. The interpretation of the presence of mi is a matter of implementation. This simple example is referenced for illustrative purposes. In practice more rigorous methods are used to select coefficients for embedding data. Some algorithms embed data in the middle frequency coefficients, or calculate a perceptual mask to determine the most suitable coefficients to modify. The perceptual mask computes how much each coefficient can change without perceptually modifying the image. Coefficients that can be modified by a larger amount are better candidates for hiding data. Embedded data can employ multiple coefficients within an 8  8 block. For example, a DCT block can represent mi ¼ 1 if zi > zj , and mi ¼ 0 otherwise. The interpretation of these bits depends on the application. With covert communication and photo annotation one does not have any knowledge of message M. Digital watermarking applications, however, may strictly test for the presence or absence of a prescribed watermark M. In this circumstance statistical correlation is performed on extracted M΄ and known M to determine whether the image is marked. The risk of false positive or false negative determinations is related to the size of a, and the size of a is inversely proportional to the watermark’s perceptual transparency. The larger the value of a, the more the coefficients will change when mi is embedded. If a is large, naively trying to removing mi without the stego-key (which can identify location i) would require modifying many low-frequency coefficients by a larger amount. Such an attack would materially alter the quality of the image. The strength of embedding messages in the frequency domain is robustness. Resizing the image, additive noise, and other pixel-based modifications will not alter the underlying structure of the image in its frequency domain. Using statistical methods to extract message mi (as in the above example) means the message is protected from minor modifications to the stego-object Z. Changing zi from, for example, 126 to 123 will not significantly change the probability mi was added to ci . Alternatively, large modifications to Z (from a naı¨ve attack) will destroy the integrity of the object. Other modifications such as cropping may remove portions of the watermark, but assuming the watermark is distributed in the perceptually relevant portions of the image one will still detect the presence of a partial watermark. For some applications detecting the presence of a watermark anywhere on the image is sufficient. In > Fig. 5d we illustrate a naı¨ve attack on the stego-object in > Fig. 5b. Assume we attempt to remove the watermark by adding noise to all low-frequency bands. In this example we simply add 100 to all coefficients in the upper left quadrant of the DCT matrix. The result is significant color deviation from the original image (illustrated in > Fig. 5e). Applying this across all 8  8 pixel blocks in the image will show serious degradation in image quality. This satisfies the robustness condition.

4

Steganalysis Techniques

Steganalysis is the study of techniques to attack data hiding algorithms. For covert communication applications, simply detecting the presence of a message is considered a security breach.

Data Hiding and Digital Watermarking

3.5.1

A successful digital watermarking attack would constitute removing or altering the watermark without damaging the media object. Successful techniques for attacking specific algorithms will always be considered on a case by case basis. However, there are statistical characteristics of images that can be applied to form a general framework for identifying hidden data in images. The general techniques that can be applied to any data hiding algorithm are most interesting, since one can rarely assume one knows all available algorithms being employed. These techniques also assume no knowledge of the hidden message or original cover image, yet can provide insight as to where data may be hidden in the image [1]. An encrypted stego-message will be indistinguishable from a random sequence of bits. However, the distribution of bits in an image is not random. Colors in images are correlated, and specific colors tend to be prominent and clustered in ways that would not be characterized as random. When a message is written to the LSB of pixels in an image, the number of even and odd values in the image converges. If the stego-message is a random sequence of bits, and the message bits overwrite the LSB, then the pixels with hidden data will have even and odd parity (approximately equal number of 0 and 1 LSB values). Images generally do not share this property. Most images are skewed to being even or odd due to prominent, repetitive colors in the image. When an LSB technique is used on an image, the number of color pairs that differ by one bit increases. A higher proportion of pixels will be adjacent to colors that differ by a single bit in a single color channel, and the total number of colors in the image will increase beyond what is statistically typical of a natural image. This is particularly evident in palette based images where colors that differ by negligible amounts may be discarded to reduce the size of the palette. Stego-messages that are embedded in the LSB of palette indices run the risk of severely degrading image quality since there is no guarantee neighboring colors in the palette share locality in the color spectrum. To alleviate this, the palette may be ordered so index LSB modifications will reference similar colors, but an ordered palette itself may arouse suspicion [7–9]. DCT coefficients in the frequency domain are similarly distributed as pixel colors. Stego-message insertions tend to even the frequency of odd and even coefficient values, and increase the number of distinct coefficient values. Methods similar to statistics based LSB steganalysis can be performed on DCT encoded images by analyzing DCT coefficients rather than pixel colors.

5

Summary/Conclusion

Securely embedding data in an image provides a new means of protecting digital content and communicating covertly. Digital security has traditionally been associated with monitoring strict access privileges with a secret password. However, this does not address several key usage models of digital content. In some circumstances one may want others to access the digital content but limit their usage. For example, they may have the right to view, store, and modify a collection of images, but does not have the right to distribute to others. In this circumstance a strict ‘‘access’’ or ‘‘no access’’ policy does not apply, and researchers need to be creative in addressing this growing usage model. Digital watermarking serves as the image’s ‘‘passport’’ in a sense; the origin and usage rights information is uniquely bound to the content.

397

398

3.5.1

Data Hiding and Digital Watermarking

Similar to digital rights management, covert communication conducted with a strict ‘‘access’’ or ‘‘no access’’ policy may have limitations. For example, assume the message is unencrypted and access is restricted by stealth, physical barriers, or other means. Such a scheme works as long as access is not breached. Covert communication embedded in digital images redefines the problem to one of trying to determine where to look, and the computational complexity associated with scanning an insurmountable pile of images.

6

Directions for Future Research

Digital steganography is a relatively new subject with research opportunities spanning many directions. The robustness of hiding information in the frequency domain and the prevalence of JPEG images makes DCT-based algorithms more popular from a research point of view than LSB implementations. While there does exist algorithms that hide information in images, it remains difficult to assess how well the data is hidden. Quantifying the security of an algorithm and developing tools to systematically test the performance of information hiding implementations would be valuable [10]. This technology would allow researchers to assess how well one algorithm performs relative to another and expose specific weaknesses. Improving steganography algorithms is another prominent area of research. A smarter heuristic for selecting candidate pixels, for example, would increase the security, robustness, and capacity of the system. Determining the portions of an image that are perceptually relevant (such as faces and foreground objects) helps identify areas that cannot be modified without altering the image’s primary subject. Finding pixels and coefficients that can be substantially modified without degrading perceptual transparency would help increase algorithm robustness. Further research in the statistical characterization of colors in digital photographs would identify common and unlikely patterns in natural images. With such information one can employ preventative measures to ensure hidden information does not appear as a statistical anomaly, which could arouse suspicion [11].

References 1. Chandramouli R, Kharrazi M, Memon N (2004) Image steganography and steganalysis: concepts and practice. Lecture Notes in Computer Science, Springer, Heidelberg, vol 2939 2. Provos N, Honeyman P (2003) Hide and seek: an introduction to steganography. IEEE Security and Privacy 1:32–44 3. Moulin P, Mihcak M (2002) A framework for evaluating the data-hiding capacity of image sources. IEEE Trans Image Process 11(9):1029–1042 4. Cox I, Kilian J, Leighton T, Shamoon T (1997) Secure spread spectrum watermarking for multimedia. IEEE Trans Image Process 6(12): 1673–1687 5. Mohanty S, Ramakrishnan K, Kankanhalli M (2000) A DCT domain visible watermarking technique for images. IEEE International Conference on Multimedia 2:1029–1032

6. Westerfeld A (2001) F5 – a steganographic algorithm: high capacity despite better steganalysis. Lecture Notes in Computer Science, Springer, Heidelberg, vol 2137, pp 289–302 7. Dumitrescu S, Wu X, Wang Z (2003) Detection of LSB steganography via sample pair analysis. Lecture Notes in Computer Science, Springer, Heidelberg, vol 2578, pp 355–372 8. Ker A (2005) Improved detection of LSB steganography in grayscale images. Lecture Notes in Computer Science, Springer, Heidelberg, vol 3200, pp 97–115 9. Lee K, Westfeld A, Lee S (2006) Category attack for LSB steganalysis of JPEG images. Lecture Notes in Computer Science, Springer, Heidelberg, vol 4283, pp 35–48 10. Ming C, Ru Z, Xinxin N, Yixian Y (2006) Analysis of current steganography tools: classification and

Data Hiding and Digital Watermarking features, Intelligent Information Hiding and Multimedia Signal Processing 2006, pp 384–387 11. Provos N (2001) Defending against statistical steganalysis. In: Proceedings of the 10th USENIX

3.5.1

Security Symposium, Washington DC

vol

10,

pp

323–335,

Further Reading Cox I, Miller M, Bloom J, Fridrich J, Kalker T (2008) Digital watermarking and steganography, 2nd edn. Morgan Kaufman, Burlington Katzenbeisser S (2000) Information hiding techniques for steganography and digital watermarking. Artech House, Norwood

Salomon D (2003) Data privacy and security. Springer, New York Wayner P (2009) Information hiding: steganography and watermarking, 3rd edn. Morgan Kaufman, Burlington

399

3.5.2 Biometrics and Recognition Technology Daniel Taranovsky 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402 2 Principles of Biometric Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 3 Facial Recognition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404 4 Fingerprint Recognition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 5 Summary/Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 6 Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_3.5.2, # Springer-Verlag Berlin Heidelberg 2012

402

3.5.2

Biometrics and Recognition Technology

Abstract: The technology of accurately identifying people has improved with the advent of digitizing biometric data. Cross-disciplinary advancements in medical and computer sciences have allowed anatomical characteristics to be digitized so traditional pattern recognition algorithms can be employed to reconcile a single instance with potentially millions of database entries. Two important biometric applications are facial and fingerprint recognition. One facial recognition technique expresses face images in terms of eigenfaces using principal component analysis. This chapter provides an overview of the biometric recognition problem with examples of solution methods for fingerprint and facial recognition.

1

Introduction

The problem of uniquely identifying people is often encountered in forensics and security applications. Biometric recognition has evolved considerably with display, image capturing, and computer vision being applied to develop cross-disciplined solutions to the most difficult identification problems. Biometric applications are broadly positioned along two axes: identification versus verification and involuntary versus voluntary. Identification systems match measured biometric data with instances in a database. An ideal system matches measured data with a single database entry with 100% accuracy. In reality, the system may return false positives, false negatives, or multiple entries that fall within some acceptable threshold of confidence. The quality of the measured data is often worse with involuntary systems, so the identification process is even more challenging. Verification systems confirm the person is who she/he claims to be, and only needs to match measured data with a single database entry. A Boolean result (with some confidence factor) is output. Standardized photo and signature identification systems have been in existence for some time, and still serve as a cost effective way of deploying secure verification processes in many circumstances. Fingerprint scanners and digital photography are used around the world to accompany passport control’s document inspection. > Figure 1 shows the application space of biometric systems. Involuntary identification systems are designed to measure and match biometric data without requiring explicit participation from the sampled individual. Cameras that photograph subjects in a target area and the analysis of fingerprints left behind in a crime scene are examples of involuntary identification scenarios. Acquiring biometric data covertly is typical of involuntary biometric identification applications. The principal benefit of covert biometric acquisition is it becomes more difficult to circumvent the security system. Voluntary systems can be more intrusive and overt with the benefit of acquiring better measurements and more accurate results. Retinal scans, fingerprint scans, and signatures are three examples of biometrics typically employed in voluntary identification and verification systems. Examples of voluntary participation in identity verification are passport control areas and providing a signature during credit card transactions. A biometric measurement is a quantitative or qualitative characterization of the human body. Height, hair color, DNA, fingerprints, eye vascular structures, and voice patterns are all examples of biometrics [1, 2]. A suitable biometric for forensic and security applications should be universal, unique, permanent, and collectable. > Table 1 describes the four key biometric characteristics. A person’s height is universal, somewhat permanent, and easily collectible but far from distinct. Every person has a distinct and permanent personality or sense of humor, but this would prove difficult to collect. Vascular retinal scans are said to be among the most secure biometrics since it

Biometrics and Recognition Technology

3.5.2

Involuntary (Harder problem) • Crime scene investigations • Video surveillance

• Tracking missing persons

Verification

Identification

(Easier problem) • Passport control • Credit card transaction

Voluntary

. Fig. 1 Biometric application space

. Table 1 Biometric data characteristics Characteristic Description Universal

Every person should have an instance of the biometric

Distinct

No two people should share the same biometric measurement or representation

Permanent

The biometrics should be tamper-proof and remain unchanged over time

Collectable

It should be recordable, measurable, and have a suitable digital representation

is universal, distinct, and impossible to tamper with without seriously compromising one’s health. It is, however, difficult to collect retinal scans in involuntary collection environments. The person must be willing to subject themselves to a retinal scan in order to collect the data. Applications in the lower right quadrant (voluntary identification) are not typical since the voluntary participation of the subject serves to narrow the search problem to one of verification, rather than a broader identification problem.

2

Principles of Biometric Identification

Facial and fingerprint biometric data is universal, distinct, permanent (relatively), and collectable. Fingerprint matching has a long history in forensic investigations, and is one of the few nonintrusive biometrics that have voluntary and involuntary applications. A person may volunteer samples of her/his DNA or fingerprints, but is also likely to leave DNA and fingerprint samples where they visit. Facial recognition is particularly interesting for security applications since the biometrics can be collected through the lens of a covert camera. Video footage collected anywhere in the world can be a source of biometric data, and concealing one’s face in all circumstances is not practical in most parts of the world. Subsequent sections focus on fingerprint and facial identification algorithms.

403

404

3.5.2

Biometrics and Recognition Technology

A formal description of the biometric identification problem is presented below. Assume 0 0 one is attempting to match biometric data S with entries in a database. S features are extracted 0  and represented as P . Let P denote the entry in the database which correctly corresponds to 0 0 the same person from whom S was extracted. The identification system will compare P with 0 0 entry Pi, and calculate a score f ðP ; Pi Þ. Without loss of generality we assume 0  f ðP ; Pi Þ  1. 0 Higher f ðP ; Pi Þ scores implies the system identified a higher likelihood that Pi ¼ P  . Sometimes the identification system may not compute f for every Pi in the system due to resource constraints. In this case, unlikely candidates may be pruned very early in the search algorithm to avoid computing f for entries that will probably not generate a high score. For example, if the database is partitioned by sex, race, or other identifiable characteristic, and this character0 istic can be reliably determined from the biometric data S , then a large portion of the database can be disregarded prior to computing the expensive evaluation function f for the remaining 0 entries. After f ðP ; Pi Þ is computed it is compared with some threshold t. If f ðP 0 ; Pi Þ  t then it is determined that Pi is a candidate match for P . Setting threshold t depends on the application. Higher t results in higher probability of false-negative errors and lower t results in higher probability of false positive errors. Forensic applications consider false-negative errors objectionable since falsely identified entries may be disregarded in subsequent investigation. Secure access verification systems seek to minimize false-positive errors. Errors occur due to the design of the underlying function f , the quality of 0 the biometric data S , the quality of the entry representations Pi , and the threshold t. A formal characterization of the system’s output is presented in > Table 2. > Figure 2 illustrates a system flow diagram for biometric identification.

3

Facial Recognition

Human brains have evolved a remarkable capability to recognize faces. Variable lighting conditions, partial feature occlusion, years of aging, and other seemingly material alterations do not prevent humans from quickly recognizing a friend’s familiar face. Given a suitable face representation and an effective matching algorithm, computer technology can be used to mine through large databases of face data. An example of a reliable, high performance facial representation and matching algorithm is the focus of this section. While there are many research topics in the area of facial recognition, we broadly characterize two approaches. Statistical methods attempt to find closest-fit approximations in image space. Geometric methods attempt to model and match key features of the face. Psychological research on how the human brain recognizes faces suggest both a holistic and feature-based method is used. Some facial recognition systems employ a hybrid statistical and geometric method [3–5]. . Table 2 Biometric system output probability expressions Probability expression 0

Pðf ðP ; Pi Þ  tjPi 6¼ PÞ 0

Pðf ðP ; Pi Þ  tjPi ¼ PÞ 0

Pðf ðP ; Pi Þ < tjPi ¼ P Þ 0

Pðf ðP ; Pi Þ < tjPi 6¼ P Þ

Natural language interpretation Probability of false positive error Probability of a correct positive match Probability of false negative error Probability of a correct negative match

Feature extraction

Feature extraction

. Fig. 2 System flow diagram for biometric identification

S′ : Biometric sample

Si : Biometric sample

P′ : Data representation

Pi : Data representation Database

f : Matching function

If the system outputs P* then a positive match has occurred. All other entries in the output set are false positive errors.

P* is the database entry representing the same person from whom biometric sample S′ was collected.

{Pa, Pb, Pc, ...} : Set of potential matches.

Biometrics and Recognition Technology

3.5.2 405

406

3.5.2

Biometrics and Recognition Technology

One notable method for facial recognition involves adapting the concept of principal component analysis to images of faces [1]. Principal component analysis is a method to transform a set of data to a coordinate space that aligns itself along trends in the data. For example, assume we have a set of data {(1,1), (2,2), (3,3)}. The point (3,3) represents a single sample, but does not provide insight in how its position relates to the other samples. If one modifies the coordinate system so the axes are along the lines y ¼ x and y ¼ x, then we see a more descriptive coordinate system in the new coordinate pffiffiffi pffiffiffi appear. The dataset coordinates pffiffiffi 0 0 space is {( 2,0)0 , (2 2,0)0 , (3 2,0)0 }. All samples have Y ¼ 0, so Y does not provide 0 descriptive information on the dataset. Y may be discarded altogether to condense the dataset 0 representation without loss of information. X is the most descriptive dimension since we know the sample data is expressed along the dominant trend line rather than an arbitrary 0 coordinate system. In this case we refer to X as being the principal component of the dataset. > Figure 3 illustrates this example. Now consider N  N dimensional data representing images of size N  N . Every pixel is an axis in N  N dimensional space. We are interested in finding the principal components of this data set. An ðN  N Þ  R matrix is constructed with every row corresponding to an image; N  N is the number of pixels in each image, and R is the number of images in the dataset. The covariance matrix of the dataset is calculated and the eigenvectors of the covariance matrix are determined. The covariance matrix indicates how pixel colors change relative to one another. The eigenvectors of the covariance matrix will orient the axes along prominent trend lines much like the example in > Fig. 3. In the context of facial recognition and image data, the eigenvectors of the covariance matrix are referred to as eigenfaces [6]. One can imagine these eigenfaces as representing the

Y

X′ Y′

(3,3) → (3 √2)′ (2,2) → (2 √2)′ (1,1) → (√2)′ X

. Fig. 3 Sample set mapped on alternate axes

Biometrics and Recognition Technology

3.5.2

‘‘direction’’ in which the images differ from one another. They identify important relationships about how pixel colors change relative to one another. Consider a naı¨ve coordinate system where each axes define one pixel’s color. In this example, a pixel color is assumed to have no correlation with other pixel colors. We know this not to be true of human faces. The eigenfaces orient the coordinate system to represent statistical correlation among pixel colors. Principal component analysis normalizes the data around its mean (the average face given all the faces in the dataset). The average face becomes the new origin in the eigenface coordinate system. The eigenfaces will indicate the prominent directions that best characterize how faces tend to differ from the average face. > Figure 4 shows three example eigenfaces. Once the series of eigenfaces is generated, the system orders the eigenfaces in order of their eigenvalues. If some eigenvalues are close to zero these eigenfaces may be discarded altogether without much 0 consequence. In > Fig. 3, we saw that Y did not contribute to the characterization of the 0 dataset since no samples differed along the Y axis. The same concept applies with eigenfaces, and the system will usually be able to accurately reconstruct all the faces in the dataset using a small subset of prominent eigenfaces. Faces are composed of a linear combination of the mean face in the dataset and the eigenfaces (> Fig. 5). d0 is the weight associated with the most prominent eigenface (eig0), d1 is the weight associated with the second most prominent eigenface (eig1), and so on. Typically, not all dNN eigenfaces are required to get a close approximation to the target face image. Since each image is represented by a small set of weights di, a high level of compression can be achieved. The process is illustrated in > Fig. 5. 2 3 I0;0 ; I0;1 ; :::; I0;M 4 5 ::: ð1Þ IR;0 ; IR;1 ; :::; IR;M The dataset of R images composed of M=N  N pixels are arranged with one image per row. The mean value per dimension is subtracted. Each dimension (column) is a pixel location (> Eq. 1). 2 3 covð0; 0Þ; covð0; 1Þ; :::; covð0; MÞ 4 5 ::: ð2Þ covðM; 0Þ; covðM; 1Þ; :::; covðM; MÞ

a . Fig. 4 Example eigenfaces [6]

b

c

407

408

3.5.2

Biometrics and Recognition Technology

Mean face

=

eig0

+ δ0 ·

eig1

+ δ1 ·

eig2

+ δ2 ·

eig3

+ δ3 ·

+ ···

. Fig. 5 Approximating face images as a linear combination of eigenfaces [6]

A covariance matrix is computed from the image data matrix (> Eq. 2). 2 3 2 3 2 3 eig10 eigM0 eig00 4 ::: 5; 4 ::: 5; :::; 4 ::: 5 eig0M eig1M eigMM

ð3Þ

Eigenvectors are computed from the covariance matrix, and arranged according to their eigenvalues. eig 0 is the most significant eigenvector, and eigM is the least significant. These vectors correspond to eigenfaces. The most significant eigenvector has the largest eigenvalue (> Eq. 3). 2

3 2 3 2 0 3 I 0;0 ; I 0 1;0 ; :::; I 0 R;0 I0;0 ; I1;0 ; :::; IR;0 eig00 ; eig01 ; :::; eig0M 4 54 5¼4 5 ::: ::: ::: I0;M ; I1;M ; :::; IR;M I 0 0;M ; I 0 1;M ; :::; I 0 R;M eigM0 ; eigM1 ; :::; eigMM

ð4Þ

Eigenvectors are arranged per row with the most significant at the top. The original image data matrix is transposed and transformed by the eigenvector matrix. This will express the image dataset in eigenface space. Transformed images are arranged in columns (> Eq. 4). The column entries in the resulting matrix of > Eq. 4 correspond to a weight factor associated with an eigenface. For example, I 0 0,0 corresponds to eigenface 0’s weight when constructing database image entry 0. I 0 3,2 corresponds to eigenface 2’s weighted contribution to face image 3’s representation. A face image’s representation in eigenspace is its collection of eigenface weights. This results in a very compact and manageable digital representation since many of the eigenfaces will be discarded as in > Fig. 3. Given an image one wishes to match with entries in the database, the system begins by representing the identification candidate image as a linear combination of eigenfaces. This is achieved by transforming the pixel data into eigenface space. For example, (I 0 0,0 , I 0 0,1 , . . . , I 0 0,M ) represents face image 0 in eigenspace, and every component of this M-tuple corresponds to an eigenface weight. The matching criteria can then be as trivial as finding the Euclidean distance between the candidate image and the images in the database. The eigenface weights di serve as coordinates in eigenspace, and are used to compute closest neighbor likely candidates. The faces in the dataset that are deemed possible matches are those with tolerable distances to the candidate image. Tolerable distances in this case are analogous to the system’s threshold described in > Sect. 2. This technique can be applied to normalized frontal face images. However, in an arbitrary photograph or video sequence there are a host of problems to be solved prior to the identification process. Faces in the image must be located, features extracted, and subsequent compensation for different lighting conditions and face orientation [7]. One method used to identify faces in

Biometrics and Recognition Technology

3.5.2

a scene is to search for symmetry since the face is inherently symmetric [8]. Edge detection methods can also be useful to locate faces and features. Eyes, mouth, and nose are then located on the face to determine the face’s position and orientation. Once this information is known, methods can be employed to normalize the data and initiate the identification process [9, 10]. Another class of facial recognition techniques is based on geometric reconstruction of facial features, and making comparisons with three-dimensional models in the database. Range image data is extracted from the face and curvatures of the face surfaces are analyzed. Surface minima and maxima, and convex and concave gradients are used to locate the nose, eyes, and mouth. Given the location of these facial features the data is normalized and correlations are calculated with the dataset.

4

Fingerprint Recognition

Fingerprint recognition methods are broadly characterized as correlation or minutiae-based techniques. Correlation-based techniques attempt to characterize the ridge patterns over the fingerprint and calculate a match in frequency space. Minutiae-based techniques identify distinctive points on the fingerprint, then perform a position and orientation best-fit computation to score the match. Current minutiae systems achieve more reliable results than correlation techniques, although systems adopting a hybrid approach claim to achieve lower false-positive matches than minutiae techniques alone. Some correlation techniques use 2D Gabor filters to represent a fingerprint’s local orientation and fingerprint ridge frequency. The ridges on a fingerprint are represented as a sinusoidal function with inter-ridge distances corresponding to the function’s frequency. The global pattern representing a fingerprint is best characterized as ridges wrapped around control points, called loops and deltas. The loop is the innermost cul-de-sac of ridges, and deltas are flat areas arising when ridges change orientation. Correlation techniques aim to describe the global ridge pattern about these control points [11].

. Fig. 6 Examples of ridge bifurcation and ridge termination minutiae

409

410

3.5.2

Biometrics and Recognition Technology

Minutiae techniques scan the fingerprint for key markers, such as ridge bifurcation and ridge termination (referring to instances of forking into two separate ridges and terminating its path, respectfully). A vector of minutiae fm0 ; m1 ; m2 ; :::; mn g can be used to digitally represent a fingerprint, with each minutiae being a three-tuple fa; ’; cg. The three-tuple represents location (a), orientation (’), and type of minutiae marker (c) on a normalized image of a fingerprint. When attempting to match a fingerprint with records in the database, the system attempts to minimize logical distances between corresponding minutiae points. This is not an easy task, since one must first normalize both datasets to compensate for scaling, rotations, and other discrepancies likely to occur when a finger is placed on a sensor. Logical distances between corresponding minutiae nodes increase as the orientation or location discrepancies increase. A minutiae substitution or insertion would contribute substantially to the cumulative logical distance between the two fingerprints. > Figure 6 shows an example fingerprint with identified minutiae.

5

Summary/Conclusion

The key challenge with biometric recognition systems is establishing a method of digitally representing the face, fingerprint, or other biometric signature. The digital representation must be concise enough to allow fast comparisons and storage without discarding important differentiating features. In the case of fingerprints, minutiae location and orientation can be used to digitally represent a fingerprint. A scoring function is used to determine closest matches with candidate samples. Using eigenfaces, images of faces can be digitally represented as a simple series of weights di. These weights correspond to the contribution of eigenfaces in the ‘‘reconstruction’’ of the candidate sample image. The eigenface weights define a new coordinate system, and two images with similar weights will have similar appearance. The effectiveness of representing images as eigenfaces is most apparent when trying to find a match between a candidate sample image and the images in the database. Since the weights can be visualized as a coordinate system, the problem of facial recognition is reduced to calculating Euclidean distances.

6

Directions for Future Research

The problem of bridging the natural analog and computer digital worlds to enable biometric recognition required advancement in a number of areas. Every step of the recognition pipeline in > Fig. 2 is the object of potential improvement. Identifying faces and normalizing the image prior to computing a face’s digital representation is a challenge for automatic facial recognition systems. This step takes place prior to any recognition tasks, and can still be improved. Developing face image representations that are invariant to environmental factors is a difficult problem. Different lighting conditions and orientations of the face can provide a serious challenge for facial recognition systems, and is the subject of ongoing research. Some research considers 3D modeling of the face to relight the image based on environmental conditions. Compensating for variations from facial hair, aging, and emotional expressions is also an open research topic.

Biometrics and Recognition Technology

3.5.2

References 1. Zhao W, Chellappa R, Rosenfeld A, Phillips PJ (2003) Face recognition: literature survey. ACM Comput Surv 35(4):399–458 2. Delac K, Grgic M (2004) A survey of biometric recognition methods. In: 46th International Symposium Electronics in Marine, ELMAR-2004, Zadar, pp 184–193 3. Lu X, Jain A (2006) Automatic feature extraction for multiview 3D face recognition. In: Proceedings of the 7th International Conference on Automatic Face and Gesture Recognition, Southampton, pp 585–590 4. Manjunath B (1992) A feature based approach to face recognition. In: Proceedings of Computer Vision and Pattern Recognition, Champaign, pp 373–378 5. Gross R, Shi J, Cohn J (2001) Quo vadis face recognition? – The current state of the art in Face Recognition. Technical Report, Robotics Institute, Carnegie Mellon University, Pittsburgh, Pennsylvania

6. Busey T (2009) The face machine. http:// cognitrn.psych.indiana.edu/nsfgrant/FaceMachine/ faceMachine.html 7. Brunelli R, Poggio T (1993) Face recognition: features versus templates. IEEE Trans Pattern Anal Mach Intell 15(10):1042–1052 8. Sirovich L, Meytlis M (2009) Symmetry, probability, and recognition in face space. PNAS P Natl Acad Sci 106(17):6895–6899 9. Kirby M, Sirovich L (1990) Application of the Karhunen-Loeve procedure for the characterization of human faces. IEEE Trans Pattern Anal Mach Intell 12(1):103–108 10. Sirovich L, Kirby M (1987) Low-dimensional procedure for the characterization of human faces. J Opt Soc Am 4(3):519–524 11. Ross A, Reisman J, Jain A (2002) Fingerprint matching using feature space correlation. In: Proceedings of Post-EECV Workshop on Biometric Authentication, Springer, Heidelberg, vol 2356, pp 48–57

Further Reading Li S, Jain A (2005) Handbook of face recognition. Springer Science+Business Media, New York Maltoni D, Maio D, Jain A, Prabhakar S (2009) Handbook of fingerprint recognition. Springer Science+Business Media, London

Wechsler H (2007) Reliable face recognition methods: system design, implementation and evaluation (International Series on Biometrics). Springer Science+Business Media, New York

411

Section 4

Driving Displays

Part 4.1

Direct Drive, Multiplex and Passive Matrix

4.1.1 Direct Drive, Multiplex and Passive Matrix Karlheinz Blankenbach . Andreas Hudak . Michael Jentsch 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418

2 2.1 2.2

Direct Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Direct-Driven Liquid Crystal Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Direct-Driven OLED Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420

3

Multiplex Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

4 4.1 4.2 4.3 4.3.1

Matrix Driving Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 Fundamentals of Passive Matrix LCDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 Fundamentals of Passive Matrix OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 Fundamentals of Active Matrix Driving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 Adjustment of Electro-optical Characteristics to Gamma Curve . . . . . . . . . . . . . . . . . . 431

5 5.1 5.1.1 5.1.2 5.2

Passive Matrix LCD Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432 Character Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432 Driver Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 Interfacing a Character Display Driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 Low-Resolution Passive Matrix Graphic Displays (up to QVGA) . . . . . . . . . . . . . . . . . 436

6

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.1.1, # Springer-Verlag Berlin Heidelberg 2012

418

4.1.1

Direct Drive, Multiplex and Passive Matrix

Abstract: This chapter is dedicated to the driving of low-content displays. It gives an overview of these methods starting from the simplest, direct drive, and segmented low-content displays through multiplex passive and active matrix drives for displays with low- to mid-size resolutions. The introduction explains some of the electrical principals involved in driving a display. Later on passive matrix addressing schemes will be described in more detail before the active matrix addressing scheme is presented. Finally the handling of two typical lowresolution passive matrix LCD modules will be explained. List of Abbreviations: AM, Active Matrix; ASCII, American Standard Code for Information Interchange; CGRAM, Character Generator Random Access Memory; CGROM, Character Generator Read Only Memory; DC, Direct Current; DC/DC, Direct Current Converter; DDRAM, Display Data Random Access Memory; DR, Data Register; E, Enable; EOTF, Electro-Optic Transfer Function; IF, Interface; IR, Instruction Register; ITO, Indium Tin Oxide; LC, Liquid Crystal; LCD, Liquid Crystal Display; LED, Light Emitting Diode; mC, Microcontroller; MPU, Micro Processor Unit; MOSFET, Metal Oxide Semiconductor FieldEffect Transistor; OLED, Organic Light Emitting Diode; OTP, One Time Programmable; PM, Passive Matrix; PWM, Pulse Width Modulation; QVGA, Quarter Video Graphics Array; RAM, Random Access Memory; RGB, Red, Green, Blue; RMS, Root Mean Square; RS, Register Select; R/W, Read/Write; TCON, Timing Controller; TFT, Thin Film Transistor; VGA, Video Graphics Array; XOR, Exclusive OR

1

Introduction

Besides the physical properties of a display such as viewing angle, black state, response time, and so on, which are mainly dependent on the display technology being used, the electrical driving of a display also plays an important role. Simply considered, an LCD is a valve that controls the transmission of light. Of course this approach is a little weak for emissive displays (see > Sect. 6, Emissive Displays): they do not just control the light output of a backlight because the light is generated by the pixel itself. From the electrical point of view a single pixel can be considered as a parallel plate capacitor for a voltage-driven technology like LCD and for current driven technology like organic light emitting diode (OLED) as a simple LED. So, a

% Light Output I

100

U

Driving Voltage Driving Current

. Fig. 1 Visualization of voltage-driven (left) and current-driven (center) pixels and their typical light output characteristics (right)

Direct Drive, Multiplex and Passive Matrix

Driving electrodes

On Off On Off

4.1.1

5ⴛ7 Electrode matrix

On On On

. Fig. 2 Direct-driven eight-segment digit (left) and a 5
7 electrode matrix (right), both showing a ‘‘3’’

voltage (U) or current (I) across the pixel controls the light output as is shown in > Fig. 1. The simplest way of driving displays are direct-driven displays, which means every single pixel has its own connection line (> Fig. 2 left). But with an increasing amount of pixels that would be impossible. Nowadays modern televisions have a resolution of 1,920
1,080 pixels, which means for direct driving 6 million lines are needed (horizontal pixels
vertical pixels
RGB). This is technically impossible. To overcome this issue in high-resolution displays, the pixels are addressed by a matrix where every interconnection belongs to a pixel as is shown in, for example, a 5
7 matrix in > Fig. 2 on the right. This reduces the number of lines needed for driving to 5,760 columns (1,920
RGB) plus 1,080 rows. The next step is the differentiation between active and passive matrix driven displays. Active matrix driving enables most modern display technologies to have a higher resolution (more pixels), higher contrast ratio, and more gray levels and colors when compared to passive matrix addressing. However, passive matrix displays have competitive advantages in terms of price, especially for character and lowresolution graphic displays for LCDs and OLEDs [1].

2

Direct Drive

Today direct-driven displays are used in low information applications like simple digital watches, status indicators, or simple instrumentation for home appliance or industrial equipment. These displays are normally used for monochromatic applications, sometimes they are combined with multicolor backlights, or they are used without a backlight in reflective applications. The addressable pixels of direct-driven displays are called segments and often differ in shape and size. Each segment is driven by one dedicated signal line, which is related to a common ground electrode. The resolution of the displays is limited by the number of signal lines that can be integrated on the display glass. Also the driving circuit has to address every segment which leads to a large number of connection pins.

2.1

Direct-Driven Liquid Crystal Displays

The most important direct-driven liquid crystal display is the eight-segment display, which is also available as starburst or multiburst displays. These displays are made as a stack of two glass sheets

419

420

4.1.1

Direct Drive, Multiplex and Passive Matrix

On-state S

DU

C

Segment electrode Common electrode

Off-state

+V 0 +V 0

+V Segment – 0 Common voltage (DU) −V 50–70 Hz

t

. Fig. 3 Waveform for direct-driven liquid crystal displays

each patterned with transparent electrodes. The gap between the sheets is filled with liquid crystal material. To reduce the number of lines typically the rear glass is structured with one common electrode. The overlap of a top electrode with the bottom electrode forms an addressable segment. Thus for a segmented display with n segments, the number of connection lines is n + 1. The electrical driving for liquid crystal displays needs to be DC free. Otherwise the lifetime of the material will be reduced by ionization of the LC molecules. This leads to a pulsemodulated driving scheme. A typical waveform which is used for the static driving of liquid crystal displays is shown in > Fig. 3. The figure shows a square wave voltage with a duty cycle of 50% for the common electrode. To drive a segment into the off state, the same voltage pulses are applied to the segment line, so the voltage across this segment is zero. To drive a segment into the on state, the square wave voltage applied to the segment line must be out of phase. This leads to a voltage across the segment which is two times the driving voltage. For good contrast the voltage across the on segment should be about three times the threshold voltage of the liquid crystal material. To drive different segments with one common electrode a simple driving circuit with logical XOR gates can be used. > Figure 4 shows the driving circuit using a clock signal which is applied to the common electrode to control the waveforms of the segment signals. Often the single segments of an eight-segment digit are labeled with letters from a to g. For graphical displays which need to show more information the direct driving scheme is not suitable due to the increased number of connections, which also limits the active display area, because the signal lines must be integrated within the display glass.

2.2

Direct-Driven OLED Displays

As the lifetime of organic light emitting diode (OLED) material is suitable for display applications, many segmented OLED displays are now available on the market. The driving scheme for direct-driven OLED displays differs completely from that of liquid crystal displays. The organic material is a self-emitting material that leads to a current-controlled driving approach instead of the voltage-controlled driving which is used for liquid crystal devices.

Direct Drive, Multiplex and Passive Matrix

4.1.1

LCD

1

1

0

1

1

1

1

1

1

0

Common electrode (square wave)

. Fig. 4 Electronic driving circuit for direct-driven liquid crystal displays

OLED displays are also encapsulated between two glass sheets. The front sheet is coated with transparent ITO electrodes and the rear sheet is usually coated with a common metallic electrode. The segments are formed between the front and the rear electrodes, so for a segmented display with n segments the number of connection lines is n + 1. To drive a segment in the ON state, a current must be applied from the top electrode to the rear electrode. The emitted light of a segment is proportional to the current driven through the segment. Without applying a current source to a segment, the segment is in the off state. The brightness of the segments depends on their active area. To achieve brightness uniformity for the whole display, segments with different sizes must be driven with different currents. > Figure 5 shows an electrical driving circuit for direct-driven OLED displays. Each segment is driven by one dedicated current source. Usually the maximum current of the current source is limited by the driver IC itself. To address larger segments it is necessary to connect several current sources together. To reduce cost direct-driven OLED displays are usually built up with a driver IC mounted on the display glass or on the flex cable. This reduces the number of signal lines between display and electronics. For OLED displays the driver ICs usually offer a dimming pin, which allows general dimming by applying a pulse width modulated (PWM) dimming signal. The frequency of the PWM dimming signal should be above 120 Hz to avoid flicker effects [2].

3

Multiplex Drive

The main issue for direct-driven displays is the high number of connections that also leads to an electrical driver which offers many output pins. To address a higher number of segments usually a multiplex or duty-cycle driving approach is used. Therefore the bottom electrode is divided into m independent parts, as shown in > Fig. 6. On the other side up to m top electrodes are joined together. So for a display with n segments

421

422

4.1.1

Direct Drive, Multiplex and Passive Matrix

Segment electrodes

I

Current source 0 Current source 1

Current source n OLED Data Interface for microcontroller Dimming

Common electrode

. Fig. 5 Electronic driving circuit for direct-driven OLED display

Common electrode I 1

2

3

4

Segment electrodes

Common electrode II

5 MUX

. Fig. 6 Multiplex-driven display showing ‘‘7 C’’

the number of connections is n/m + m. For example, a display with 64 segments needs 65 connections at direct drive and 34 connections at multiplex drive with m = 2. There are several limitations, like the switching time of liquid crystals, display contrast, etc., so usually m is limited to m  4 for multiplex-driven segment displays.

Direct Drive, Multiplex and Passive Matrix

4.1.1

T +V Common electrode I 0 +V Common electrode II 0

+V Segment electrode 1 0 +V Segment electrode 2 0 +V Segment electrode 3 0 +V Segment electrode 4 0 +V Segment electrode 5 0

. Fig. 7 Driving waveform for a multiplexed LCD (see > Fig. 6) showing ‘‘7 C’’ on the display

For driving, the m bottom electrodes (grid electrodes) are separated by a duty cycle of 1/m. This means that the segments are not driven all the time, so the achievable contrast is reduced compared to direct-driven segments. Furthermore the transmission of the display is also reduced due to the multiplex drive and leads to the need of a brighter backlight. > Figure 7 shows the driving waveform to picture ‘‘7 C’’ on an LCD. What has to be taken into account is that an LCD responds to the RMS value applied to the electrodes. That means that to activate a segment a voltage has to be applied to the grid and the segment must be driven with zero volts [3].

4

Matrix Driving Fundamentals

Compared to active matrix (AM) driven displays, the passive matrix (PM) drive uses no active elements. In this case the active part in an AM display would be one or more thin film transistors (TFT) per pixel. Through the ALT & Pleshko effect (see next section), the resolution of PM displays is restricted to QVGA (320
240). Due to their poor performance in terms of

423

424

4.1.1

Direct Drive, Multiplex and Passive Matrix

image quality and resolution, they are mainly used in cheap devices which do not need a high perceived quality. For example, a normal pocket calculator does not need a high-resolution color display, and nobody would pay the price for a high-end panel in such a device. A clear benefit of PM displays is their relatively low-cost manufacture.

4.1

Fundamentals of Passive Matrix LCDs

The driving of passive matrix displays is done by dedicated integrated circuits (IC), which are connected to the display glass and the data source, usually a microcontroller, further details are presented in > Sect. 5. Passive matrix displays replace the segments with pixels that are arranged in a matrix with row and column electrodes, as simplified and shown in > Fig. 8 left for 2
2 pixels. A passive matrix LCD is made up of two glass substrates sandwiching the liquid crystal material, with ITO electrodes structured on their surface. For example, on the top substrate there are N row electrodes and on the bottom substrate M column electrodes. This results in a matrix with N + M electrodes. Every intersection of the matrix equates to a pixel. So we can address N
M pixels. Assuming a resolution of 100
100 pixels, only 200 electrodes will be needed. A direct-driven display would require 10,000 electrodes, so clearly there is a massive saving of electrodes. In a passive matrix display each row is sequentially scanned by a pulse. For the period of a frame T the pulse lasts for the duration of T/N (TON) at one row. During this pulse the data for a row is applied to the columns. This time is the so-called refresh rate or frame period and is usually set above 50 Hz to prevent flicker. In > Fig. 8, this is demonstrated for the upper right pixel. Note that the signal between row and column must have a 90 phase shift to activate a pixel. > Figure 9 shows the sequential scanning of rows. Assuming a frame period of 60 Hz (16.67 ms) and a display with 128 lines, we get a pulse width of around 130 ms [4]. A passive matrix display is sensitive to the RMS voltage that is applied to a pixel cell. So pixels that are on the same line or column are also addressed by the half voltage U as shown in > Fig. 8. This effect is called ghosting and can be overcome by selecting a voltage that is Passive Matrix Driving voltage

C1

C2

0

-U

Waveform for pixel ‘A’ Column (data) (frontplane)

Row

R1

C2 R1 + U

ITO

Row (scan) (backplane) R2

Data

A |U|

|2U|

=

Ghosting

DU

0 1 Pixel

0

|U| Pixel voltage

V t

. Fig. 8 Principle of PM addressing of an LCD. Pattern of row and column electrodes where the black square is a switched-on pixel, left, and the driving waveform, right

Direct Drive, Multiplex and Passive Matrix

4.1.1

Frame period T

Row

Pulse width (Ton)

1 2 3

N

. Fig. 9 Sequential scanning pulses for row addressing in a passive matrix display

% Transmission Threshold

Saturation

Uoff

Uon

100 90

10 Driving Voltage

. Fig. 10 Example of an optoelectronic response curve of a liquid crystal cell

above the threshold of the optoelectronic response curve, also known as the electro-optic transfer function (EOTF). For the unselected pixels a voltage below this threshold has to be applied. > Figure 10 shows an example of an EOTF for a typical liquid crystal cell. The threshold and saturation values are defined as 10% and 90% of the transmission [5]. Another effect of passive matrix addressed displays is that the voltage between a selected and a nonselected pixel becomes smaller when the number of multiplexed lines increases. The result is a smaller contrast. Alt and Pleshko formulated the limits of multiplexing in RMS responding displays. They calculated the voltage ratio between the on and off state depending on the number of multiplexed lines N: pffiffiffiffi 1 N þ1 2 Uon ¼ pffiffiffiffi Uoff N 1

425

426

4.1.1

Direct Drive, Multiplex and Passive Matrix

μC IF TCON Column driver

Column driver

Row driver

Display Row driver

. Fig. 11 Typical assembly of the interface between display and driver

So the voltage gap between the on and off state is decreased by increasing the number of lines. This results in a reduced contrast ratio unless a liquid crystal with a very steep optoelectronic response curve is used [6, 7]. > Figure 11 illustrates a typical assembly of a low-resolution passive matrix display and the interfacing to a microcontroller, which provides the data to be displayed. For displays more complex than the segmented layout, the column and row drivers are separated from the Timing Controller (TCON) and are often directly assembled on the glass substrate of the display. The TCON is configured via a microcontroller interface (mC IF), and the data transmission is also done via this interface. The TCON itself contains a lot of functional blocks. These blocks are the Host Interface, LUT (Look-up Table), Display Data RAM, Color Processing, Gray Generation, Control Logic, Timing Controller, OTP (One Time Programmable) Memory, Oscillator, Temperature Sensor, Temperature Compensation, DC/DC Converter, and finally the outputs for the column and row Drivers. The important blocks are described in > Sect. 5 and [1].

4.2

Fundamentals of Passive Matrix OLEDs

The mechanical setup for passive matrix OLEDs (see also > Part 6.6) is similar to that of the liquid crystal versions. It consists of organic material instead of liquid crystals, which is sandwiched between with glass sheets. The sheets are coated with ITO electrodes crossing each other as shown in > Fig. 12. The organic layer depends on the OLED technology and production technique [2]. The driving of passive matrix OLED displays differs significantly from the driving of liquid crystal displays. Unlike LCDs, OLEDs are driven by current sources. For the passive matrix approach the display driver needs one current source per display row. To address one display line, the row driver switches the line to logical ground, whilst all unaddressed lines are switched to a voltage similar to the anode voltage of the display driver. This is shown in > Fig. 13 for line two. To switch on the dedicated pixels, the current source from the column driver has to drive the OLED pixel. For passive matrix OLEDs crosstalk is not a problem, because light has to be generated by a current flow, and that is not possible for non-driven pixels. But the resolution for passive matrix OLEDs is limited to something less than 200 lines. This depends mostly on the

4.1.1

Direct Drive, Multiplex and Passive Matrix

Column electrodes Row electrodes

Organic layers

. Fig. 12 Setup for passive matrix OLED display

Column driver signal: 1 0 1 0 I

I

I 0

1

I 1

0

Row driver signal: 1 0 U VKH 1 U VKH 0

. Fig. 13 Passive matrix driving for OLED displays

lifetime of the OLED material at high brightness levels. For the passive matrix line addressing scheme the display luminance Ldisplay of a display with n lines depends on the pixel luminance Lpixel as followed: Ldisplay ¼

Lpixel n

This means that if a display should have a luminance of 100 cd/m2 and 100 lines, the luminance of a pixel must be 10,000 cd/m2. So typically a compromise between resolution, lifetime, and display brightness has to be found. To use the passive matrix approach for displays with more lines, there are some additional driving techniques like dual line addressing or multiline addressing [8].

427

428

4.1.1

Direct Drive, Multiplex and Passive Matrix

As already considered, the lifetime of OLED displays depends on the display luminance. But there are also some lifetime-optimized passive matrix driving techniques known from Eisenbrand [8]. As OLED displays are self-emitting displays, the refresh rate of these displays should be above 100 Hz. For lower refresh rates some flicker effects like flashing lines can occur for a moving observer.

4.3

Fundamentals of Active Matrix Driving

For better understanding, the typical characteristics and in consequence the limitations of passive and active matrix driving are visualized in > Fig. 14 for LCDs and summarized in > Table 1. As a passive matrix pixel is formed by the crossing of two ITO (indium tin oxide, transparent semiconductor) electrodes, it is obvious that a voltage which is applied to a row

Active Matrix Passive Matrix Column (data) (frontplane)

C1

C2

Column (data) Row (scan) Address TFT

ITO R1

ITO

CLC pixel

Storage capacitor

Row (scan) (backplane) R2

Front plane 1 Pixel

Row and column on backplane

. Fig. 14 Basic principle of passive (left) and active (right) matrix for LCDs

. Table 1 Fundamental characteristics of active and passive matrix driving

Principle

Passive matrix

Active matrix

Crossings of two orthogonal ITO line arrangements (one on frontplane, the other one on the backplane) form the pixel

Row and column electrodes are on the backplane. A thin film transistor (TFT) as a MOSFET acts as nonlinear switching element for each pixel

Time No storage of gray level (voltage or characteristics current, if no bistable technology)

A pixel capacitor stores gray level (voltage or current)

Pixel voltage or current

Dedicated values for each pixel selected via gate of TFT (row, line)

Set up by row and column values for the whole row or column which vary over frame time

Direct Drive, Multiplex and Passive Matrix

4.1.1

(line) affects all pixels on this row; the same applies analogously for columns. The consequence of such a matrix arrangement is that the voltage of each passive matrix pixel is affected during the frame time by the (gray level) voltages for other pixels of the same column. The left side of > Fig. 14 demonstrates this for a simplified 2
2 matrix: The top row (line) is selected by ‘‘+U’’ and the column contains the gray levels ‘‘0’’ for black for the top left pixel and ‘‘-U’’ for the top right pixel. The top left pixel voltage (difference column  row) is then +U which results in a certain gray level but not black (for a normally white LCD). In contrast, the top right pixel is black as the voltage difference is 2U. In the figure, the amount of voltage is given as this is relevant for the LC transmission and the voltage sign changes from frame to frame to avoid a DC voltage. Due to the passive matrix principle the bottom pixels are also affected by the column voltages of the top lines. Therefore the bottom right pixel is set to the same voltage as the top left one. This simplified example shows that during a frame scan all pixels are affected in their gray-level behavior. It is obvious that this limits the useful resolution and gray-scale capability for passive matrix LCDs. By introducing a nonlinear switching element – usually a MOSFET (metal oxide semiconductor produced in thin film transistor (TFT) technology, see also > Chap. 7.4.1 and > Part 5.2) – the ghosting of a passive matrix drive is suppressed (> Fig. 14 right): The (address) TFT only transfers column voltage (data) to the LC pixel if its gate (connected to the row line) is set to an appropriate positive voltage (e.g., +20 V, for details see > Chaps. 4.2.1 and > 7.4.1). Therefore the column voltage for other rows (lines) does not affect this pixel. In the next step we will now have a look at the differences between passive and active matrix driving from the point of view of scanning the rows (> Fig. 15). All the rows (lines) of a matrix display are scanned subsequentially (here from top to bottom); when the last row is reached, the next frame starts with the first row (line). As there is no ‘‘memory’’ in a passive matrix LCD pixel (the same for PM OLEDs), the intended gray level (here black) vanishes over time (depending on the response time which is negligible for PM OLEDs). This results in a low contrast ratio for PM LCDs. Active matrix pixels however ‘‘store’’ the gray level (voltage) and therefore all pixels show their intended gray level. This results in a higher contrast ratio but this ‘‘hold-type’’ approach causes motion blur (see > Chap. 4.5.1).

Passive Matrix Leftover from actual scan

Scan

Data

Active Matrix Stored from actual scan

Data

Scan

No leftover from last scan (frame) Stored from last scan (frame)

. Fig. 15 Visualization of passive (left) and active (right) scan and pixel (gray level) representation for LCDs

429

430

4.1.1

Direct Drive, Multiplex and Passive Matrix

LCD

OLED

Column (data)

Column (data)

Power

Row (scan)

Row (scan)

Add Address TFT

Address TFT

CLC pixel

Storage capacitor

Drive TFT

Storage capacitor

OLED Frontplane

Frontplane

. Fig. 16 Basic circuitry of active matrix drives of an LCD (left) and an OLED (right) pixel

. Table 2 Comparison of active matrix parameters for LCDs and OLEDs LCD

OLED

Driving principle

Voltage

Current

Number of TFT per pixel

1

2

Aperture

70%

30–70%a

a

30% for bottom emission (normal lateral stack), 70% for top emission (inverted stack)

> Figure

16 and > Table 2 demonstrate the differences between AM LCDs (left) and AM OLEDs (right, see also > Chap. 6.6.2): As LCDs are voltage driven, only a capacitor is necessary to store (hold) the (gray level) voltage between two scans (frames). In contrast, OLEDs are current driven and need therefore permanent current to emit light. Therefore a power (column) line has to be implemented as well as a (additional) drive TFT to transfer the current to the OLED layer structure. It is clear that this secondary TFT and the power line reduce the aperture ratio (useful pixel area/total pixel area) significantly. An approach to overcome this is the top emission structure (see > Chap. 6.6.2) at the price of higher manufacturing complexity. Another difference is that voltage-driven display technologies like LCDs need only one TFT per pixel while emissive displays like OLEDs require at least two of them. Actual designs of AM OLEDs have four TFTs per pixel due to uniformity issues. As an example, an XGA display has 1,024
768
3  2.4 million subpixels resulting in the same number of TFTs for LCDs but up to 10 million TFTs for an AM OLED. This reduces, in consequence, the production yield. After discussing the differences in pixel circuitries, the next step toward the requirements of panel electronics (e.g., row and column driver) is to look at the waveforms required for active matrix pixels. > Figure 17 visualized the row and column signals for a simplified 2
2 AM LCD in normally white mode (see bottom right, electro-optical curve, see > Chap. 7.3.1). On the left side, the waveforms plotted for two subsequent frames and the right side visualized the pixel voltages of the pixels labeled with ‘‘a’’ (black) and ‘‘b’’ (white, transparent).

4.1.1

Direct Drive, Multiplex and Passive Matrix

Waveform for pixel ‘a' and ‘b'

Column (data)

1 2 1 2

VD

b

0 VD

a

0

R1

Row

R2

C2

Data FP

C1

1

Frontplane (VCom) VFP 0

2

1

2

1

C1 C2 VG 0 VG 0

On

1 2 1 2 R2

b

Row (scan, gate)

TFrame

t (simplified example)

1

2

=

a

R1

2

Off

DU

V

V t

t

Rel. L ΔV

. Fig. 17 Visualization of waveforms of a simplified 2
2 active matrix LCD (left) and resulting pixel voltage for two pixels (a, b, right)

First we will start with the waveforms for scanning each row sequentially (left): Row 1 (red) applies a pulse to the connected gates of this line and therefore the corresponding column voltage (data) is transferred to the pixel. In the next step, row 2 (green) is activated and the columns have to provide the gray-level data for this row. This process is repeated typically 60 times a second (frame rate or frame time). The column (data) voltage and the front plane voltage set the gray level via their difference. On the right side the time diagram of the voltages of the pixels ‘‘a’’ and ‘‘b’’ are shown: Pixel ‘‘a’’ receives its gray-level voltage at the first row and this voltage is stored (dotted line) while the second (or, in case of real panels, all other rows) is activated. A similar procedure, but on the second row, is applied to pixel ‘‘b’’. As pixel ‘‘a’’ is intended to show black, the voltage difference DU (data voltage – front plane voltage) must be maximized while for pixel ‘‘b’’ this difference should be zero. Comparing this to the passive matrix LCD in > Fig. 14, it is clear that the image quality of AM LCDs is superior as no voltage limitations happen (like Alt & Pleshko) and the electro-optical curve can be used in its total span. From those waveforms, the requirements for the panel electronics are: delivering the waveforms for row and columns at the right time with the right value.

4.3.1

Adjustment of Electro-optical Characteristics to Gamma Curve

As all examples up to now refer practically to black and white content, we have to discuss how gray levels are reproduced. The electro-optic characteristics of nearly all displays except CRTs are far from being similar to the ideal luminance (LIdeal)gray level (DInput) relationship L  Dg (> Fig. 18 left, see > Chap. 11.2.1). DInput stands here for the digital gray-level data (usually 6 or 8 bit per color). In order to achieve a luminance–gray level relationship as plotted in, some adjustments within the driving electronics system have to be performed. This will be discussed for a positive mode AMLCD for which the electro-optic characteristic is shown in > Fig. 18

431

432

4.1.1

Direct Drive, Multiplex and Passive Matrix

Lideal 1.0

DLCD

LLCD

1.0

1.0

Ideal gamma curve

Positive mode

0.5

Transfer function 0.5

0.5

0.0

0.0

0.0 0.5

1.0

0.5

DInput

1.0 DLCD

0.5

1.0 DInput

. Fig. 18 Visualization of adaptation of an LCD electro-optic curve (center) to the required luminance (L)–gray level (D) relationship (left) via transfer function (right)

center; DLCD can be interpreted as driving voltage as shown, for example, in > Fig. 17. This basic electro-optic dependency is practically opposite to the ideal gamma curve (left). Therefore, the gray-level voltages have to be modified according to the properties of the display by a transfer function (> Fig. 18 right). This transfer function is realized for AMLCDs by gamma reference voltages and the column digital-to-analog converters (see > Chaps. 4.2.1 and > 7.4.1). The following example will illustrate the issue and the task (see dashed lines in > Fig. 18): If a normalized gray level from the source of 0.6 (= DInput) (left) is set, the corresponding luminance LIdeal represents 30% of the maximum level. If no further precautions are made, this would lead only to 10% of LLCD for the LCD (center). The right gray shade for achieving 30% of the maximum luminance for this LCD is, however, 0.45. This task is done by a transfer function (right) so that the source gray levels (DInput) are modified to appropriate display gray levels (DLCD). The transfer function is usually represented by ten or more interpolation points (gamma voltages for LCDs).

5

Passive Matrix LCD Modules

At least two low-resolution LCD modules with matrix driving will be described: starting from the display driver IC, going over the connection between microcontroller, driver, and the display panel, and giving an example of how to program such an IC. In the field of character displays the HD44780 by Hitachi is a very famous one. But it is only able to address two lines each with 16 characters. Going to high resolutions we have the T6963 by Toshiba. This IC is able to drive panels with a resolution of 240
128 pixels and is also able to generate graphics.

5.1

Character Displays

Character displays are often used in devices in which a high-resolution display is not needed for presenting information. A benefit is that they are very cheap to produce and easily accessible by

Direct Drive, Multiplex and Passive Matrix

4.1.1

HD 44780 Interfacing: Controller ´ Display glass SEG 40

Data SEG 1

COM 1

COM 8

Scan COM 9

COM 16

. Fig. 19 Interfacing between a character display controller and the electrodes of a character display

a 4- or 8-bit microcontroller. > Figure 19 shows a typical character display that is connected to the common and segment driver for a character display. Character displays are available in sizes from 1
8 up to 4
40 characters where one character is represented by 5
10 dots or 5
8 dots. The last configuration is more commonly used. The standard for driving such kinds of displays is the HD44780 controller by Hitachi. Most variants of character displays have more or less the same architecture, which is why this device is used for an introduction to driving a character display.

5.1.1

Driver Architecture

Let us start with some features of the HD44780. It is possible to drive displays with one 8-character line or two 8-character lines. As mentioned before one character can consist of 5
8 or 5
10 dots. It has a low power operation support from 2.7 to 5.5 V and a wide range of liquid crystal display driver power from 3 to 11 V. For accessing the driver a high speed 4- or 8-bit MPU (micro processor unit), bus interface with up to 2 MHz is provided. > Figure 20 shows the simplified block diagram for the HD44780 with the core features. The MPU interface consists of three control lines (RS, R/W, and E) and eight data lines (DB0 to DB7). This interface is used to transmit or receive data and commands to and from the display driver. The control lines are RS (Register Select), R/W (Read/Write) and E (Enable). RS selects if the data are written to the instruction register (IR) or written/read from the data register (DR). The IR is used to store instructions like display clear or cursor shift. The DR stores data that will be written or read from the DDRAM (display data RAM) or CGRAM (character generator

433

434

4.1.1

Direct Drive, Multiplex and Passive Matrix

HD 44780 Instruction Register (IR)

RS

SEG 1

Timing generation

SEG 40

R/W E

mC

DB 7 DB 4 DB 3

MPU interface

Display Data RAM (DDRAM)

Control logic

Segment & Common driver

LCD COM 1 COM 16

Data Register (DR)

DB 0

Character Generator RAM (CGRAM)

Character Generator ROM (CGROM)

. Fig. 20 Simplified block diagram of a HD44780-Architecture

Upper 4 Bits

0001

0010

0011

0100

0101

Lower 4 Bits

0000

0001

0010

0011

. Fig. 21 Extract of character generator ROM table

RAM). The R/W selects if a read or a write operation will be performed and finally with the enable signal the interface starts to read or write. When only a 4-bit data interface is used, DB0 to DB3 are not connected and the data will be transferred in two cycles over DB4 to DB7. The HD44780 has three different types of memory. First there is the display data RAM (DDRAM). The data being displayed is stored into this memory. Every pixel or dot on the display is mapped into this memory. So, if there is no content change on the display, the data does not have to be sent again to the driver. The second is the character generator ROM (CGROM). This memory contains 8-bit character codes and generates 5
8 or 5
10 dot character patterns. > Figure 21 shows an extract of the content from the CGROM of a HD44780. The code mapping is very close to the

Direct Drive, Multiplex and Passive Matrix

4.1.1

American Standard Code for Information Interchange (ASCII). Instead of addressing every dot for a character the user can use these predefined signs for writing letters and signs on the display. Depending on the manufacturer and country, the content of the CGROM can differ a little bit. Finally we have a character generator RAM (CGRAM). If the user needs a sign that is not implemented in the CGROM he can create his own and load it into the CGRAM. As for the DDRAM, the addresses of the memory are mapped to dots on the display. The mapping is usually described in detail in the datasheet of a driver. The timing generator generates a clock signal for the operations of internal circuits like the memory and synchronizes the column and row operations for writing data to the display. Finally the column and row signal drivers will write the data onto the display [1].

5.1.2

Interfacing a Character Display Driver

In the following a short introduction of what has to be done to write content on a character display is given. As for most display drivers an initialization routine is necessary. In the case of a character display this is relatively simple compared to a color display with high resolution. Only a few parameters have to be set. As an example, this is done for the HD44780 character display driver. It is assumed that a microcontroller is connected to the MPU interface and an 8-bit interface is used for data transmission. The display connected to the driver has two lines and a character size of 5
8 dots. The initialization routine is listed in > Table 3. The first step is to configure which interface is used. For this case it is an 8-bit interface. Because of the availability of a 4-bit interface the last four bits are don’t care for this command. The command has to be sent three times. Some drivers require a minimum wait time for the three commands. If this is the case, this is marked in the datasheet of the driver. Afterward the number of display lines and the character font is set. The display is turned on by setting data bit 2. Bit 1 determines if the cursor is turned on or off and bit 0 can be used to turn the blinking of the cursor on and off. Finally a display clear command is transmitted. The entry mode is used to set the cursor movement direction (increment or

. Table 3 Initialization routine for a HD4470 display controller Instruction Initialization 8-bit interface, 3
instruction

RS RW D7 D6 D5 D4 D3 D2 D1 D0 0

0

0

0

1

1

0

0

0

0

0

0

0

0

1

1

0

0

0

0

0

0

0

0

1

1

0

0

0

0

2 lines, 5
7

0

0

0

0

1

1

1

0

0

0

Display on

0

0

0

0

0

0

1

1

0

0

Display clear

0

0

0

0

0

0

0

0

0

1

Entry mode

0

0

0

0

0

0

0

1

0

0

1

0

0

1

0

0

0

1

1

0

Write character to LCD RAM ‘‘F’’ Note: ‘‘ENABLE’’ pulse for valid data required

435

436

4.1.1

Direct Drive, Multiplex and Passive Matrix

decrement) by bit 1 and the display shift on or off by bit 0. After that the controller is ready to write content into the DDRAM. In the example it is the letter ‘‘F’’. By setting the enable pin to high the content of the DDRAM will be written onto the display.

5.2

Low-Resolution Passive Matrix Graphic Displays (up to QVGA)

The next step up from character displays are low-resolution graphic displays. They start from a size of 32
64 pixels going up to 240
320 pixels, which is equivalent to a resolution of QVGA (quarter VGA, VGA = 640
480). Displays with resolutions above QVGA can be considered as high-resolution displays and nearly all of them use active matrix driving because of the better performance it delivers. The main use of low-resolution graphic displays are applications such as portable MP3-players, mid-end mobile phones (high-end mobile phones or smart phones nowadays are equipped with VGA active matrix displays), and so on. Compared to a simple character display they have the ability to show the user more information in terms of text as well as graphics and they are able to produce gray scale and colors. As for character displays the HD44780 is widely known as the driver IC, for low-resolution passive matrix displays it is the T6963 by Toshiba. This driver is able to drive displays with resolutions up to 240
128 pixels. The handling of the device is very similar to the handling of a character display driver, so we will not go into too much detail about the programming of a T6963. It has an 8-bit data/command microcontroller unit interface with four control lines. The configuration of the driver is a little bit more extensive because more parameters have to be addressed. In contrast to a character display driver, the row and column drivers are not included in the driver because of the higher pin count. The memory for storing and displaying content is also separated from the driver. > Figure 22 illustrates a typical assembly between microcontroller, driver with external components, and display.

External memory RS R/W

Column driver

Column driver

E

μC

CS

T6963

Row driver

DB 7

DB 0

Display Row driver

. Fig. 22 Interfacing between microcontroller and a low-resolution graphic display

Direct Drive, Multiplex and Passive Matrix

6

4.1.1

Summary

This chapter dealt with the driving of low-content displays, starting from the simplest, direct drive, and going through to passive matrix driven displays. The matrix addressing of pixel cells allows the reduction of number of lines that is required to drive a display. In terms of cost efficiency PM are still the technology of choice because of their simple fabrication process and the fact that the electronics can be relatively simply realized. Of course there are some drawbacks, like less optical performance when compared to AM-driven displays. This is caused by the effect of crosstalk and the response of gray scales to the electro-optical curve. Although PM driving technology is well established, there is still a lot of progress going on in the improvement of drivers related to intelligent driving algorithms and energy saving.

References 1.

2. 3.

4. 5.

Cristaldi DJR, Pennisi S, Pulvirenti F (2009) Liquid crystal display drivers. Springer, Berlin, pp. 75–78. ISBN 978-90-481-2254-7 Shinar J (2003) Organic light-emitting devices: a survey, Springer, New York. ISBN 0-387-95343-4 Scheffer TJ, Nehring J (1984) A new highly multiplexable liquid crystal display. Appl Phys Lett 48(10): 1021–1023 Gulick P, Mills T (1994) Active addressing(TM) of passive matrix displays. Inf Disp 10:14–17 Lueder E (2005) Liquid crystal displays addressing schemes and electro-optical effects. Wiley, Chichester, pp 161–166. ISBN 0-471-49029-6

6.

7.

8.

Alt PM, Pleshko P (1974) Scanning limitations of liquid-crystal displays. IEEE Trans Electron Devices 21(2):146–155 Nehring J, Kmetz AK (1979) Ultimate limits for matrix addressing of RMS responding liquid crystal displays. IEEE Trans Electron Devices 26(5): 795–802 Eisenbrand F, Karrenbauer A, Xu C (2007) Algorithm for longer OLED lifetime, experimental algorithms. In: Proceedings of the 6th international workshop, WEA 2007, Rome, June 2007, pp 338–351

Further Reading Cristaldi DJR, Pennisi S, Pulvirenti F (2009) Liquid crystal display drivers. Springer, Berlin. ISBN 978-90481-2254-7 Mitescu M, Susnea I (2005) Microcontrollers in practice. Springer, Berlin, Heidelberg. ISBN 978-3-54028308-9 Nauth P (2005) Embedded intelligent systems. Oldenburg, Germany. ISBN 978-3-486-27522-3 Kleitz W (1998) Microprocessor and microcontroller fundamentals: the 8085 and 8051 hardware and software. Prentice Hall, Upper Saddle River. ISBN 0-13262825-2 Virenda Kumar (1995) Digital technology: principles and practice. New Age International, New Delhi. ISBN 81-224-0788-9

Brody TP (1984) The thin film transistor - a late flowering bloom. IEEE Trans Electron Devices 31(11):1614–1628 IEC 61966–4 (1998) Colour measurement and management in multimedia systems and equipment – Part 4: equipment using liquid crystal display panel Kristiansen H, Liu J (1999) Overview of conductive adhesive technologies for display applications. In: Liu J (ed) Conductive adhesives for electronics packaging. Electrochemical Publications, Port Erin, pp 376–399 Kawakami H (1976) Method of driving liquid crystal matrix display device. US Patent #3 976 362, issued 1976 Kuijk KE (2000) Minimum-voltage driving of STN LCDs by optimized multiple-row addressing. J SID 8(2):147–153

437

Part 4.2

Active Matrix Driving

4.2.1 Active Matrix Driving Karlheinz Blankenbach 1

Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442

2

Timing Controller and Intra-panel Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444

3

Row and Column Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446

4

Gamma- and VCom- Supply for LCDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450

5

All-In-One Display Driving Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452

6 Dynamic Performance of AMLCDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454 6.1 Inversion to Prevent Flicker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456 6.2 Summary and Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.2.1, # Springer-Verlag Berlin Heidelberg 2012

442

4.2.1

Active Matrix Driving

Abstract: Multimedia systems require electronic displays with high resolution and video performance. The solution in terms of display technologies such as LCDs and OLEDs is active matrix (AM) driving. This chapter describes the signal path from digital panel input to row and column signals in terms of ICs (integrated circuits); the so-called panel electronics system. List of Abbreviations: AM, Active Matrix; AMLCD, Active Matrix Liquid Crystal Display; AMOLED, Active Matrix Organic Light Emitting Display; DAC, Digital to Analog Converter; EEPROM, Electronic Erasable Programmable Read Only Memory; EOC, Electro-Optic Curve; IC, Integrated Circuits; LVDS, Low-Voltage Differential Signaling; MOSFET, Metal Oxide Semiconductor Field Effect Transistor; PCB, Printed Circuit Board; PPDS, Point to Point Differential Signalling Interfacing; QVGA, Quarter Video Graphics Array; RSDS, Reduced Swing Differential Signalling Interfacing; SVGA, Super Video Graphics Array; TCON, Timing Controller; TCP, Tape Carrier Package; TTL, Transistor-Transistor Logic; XGA, Extended Graphics Array

1

Fundamentals

As described in > Chap. 4.1.1, matrix displays are addressed via row and column waveforms. In this chapter, the tasks and electronics (the so-called panel electronics) required to generate the row and column driving signals from the (digital) input data are discussed. Most professional displays up to (S)VGA have a digital parallel RGB TTL input (Transistor-Transistor Logic, see > Chap. 4.3.1), while for higher resolutions, serial interfaces (see > Chap. 4.3.2) are used. However, the fundamentals and principles of the panel electronics are independent of the interface type; for ease of understanding, the digital TTL RGB parallel interface is employed here. Details on Active Matrix pixels can be found in the corresponding chapters of the specific display technologies and in > Chap. 4.1.1. Most of the examples refer to AMLCDs (often misleadingly named TFTs). Their basic driving method can be easily understood (an excellent reference for all LCD display driving is [1]) and applied with some modifications to other electro-optical principles like AMOLEDs (see > Chap. 6.6.2 and Chap. 14, Advances in AMOLED Technologies in Ref [2]) and AM e-paper displays (see > Chap. 8.1.1 and [3]). Plasma Display Panels (PDPs) are Passive Matrix driven by special waveforms, as discussed in > Chap. 6.3.1. > Figure 1 shows a typical AM LCD panel block diagram (see also [1], Chap. 6.1, AMLCD Driver Architectures, [4, 5]) with a single AM pixel magnified for reference. The digital RGB TTL input, synchronization, and control data (left) are captured by an input IC (integrated circuit). These data are then transferred to the Timing Controller (often abbreviated as TCON). The TCON rearranges the input data, so they are then in the right format for the row and column drivers. The column drivers are supported by the gamma buffer which provides the appropriate voltage for the digital-to-analog conversion within the column driver ICs to supply the grey level LCD pixel voltage with correction for electro-optic curve (EOC) (or the OLED voltage for the driver TFT). The VCom driver delivers the front plane voltage. The following subchapters follow the data path from Timing Controller input to pixel voltage and row (gate) pulses. The additional built-in electronics of an LCD panel are often the power supply (see > Chap. 4.6.1) and the backlight driver (mainly for LEDs, see > Chap. 4.5.2). > Figure 2 shows the typical waveforms of a parallel digital RGB data interface (detailed interface timing data and parameters can be found in the specifications of panel manufacturers) for one frame: The vertical synchronization signal VSync sets the pulse for a new frame.

4.2.1

Active Matrix Driving

Gamma buffer for EOC

driver 1

R Digital input signals

Timing controller

G B

Column driver driver 2 3

driver R 1 o w driver 2

Sync Backlight Controls

VCom Backlight driver

Power supply Sync

. Fig. 1 Block diagram of a digital parallel input LCD module

Example for XGA 1 frame: 16.7 ms(60 Hz) VSync

Data enable

VPorch 21.5 μs

HPorch HSync

Data

invalid

1

2

3

768 (XGA: 768 rows)

# of data per line : 1024 x RGB x bpp

Clocks and other Controls omitted

. Fig. 2 Typical example of parallel panel interface timing which is practically the same for Timing Controller data input (for details see > Sect. 2)

There are no pixel data during and around this sync pulse which is visualized by VPorch and ‘‘Data enable’’ input. The same applies for the horizontal synchronization HSync for every row (line). Between two HSync pulses, the grey level RGB color data for one row (line) have to be transmitted. For an XGA color panel, 1,024 (24-bit) grey level data have to be provided and

443

444

4.2.1

Active Matrix Driving

Row (line), column (data) Example for XGA 1, 2

1,1024

2,1 2,2

2,1024

1,1

R G B

768,1

768,1024

. Fig. 3 Usual nomenclature of pixels (matrix notation, values for XGA resolution) and RGB subpixel (inset). The dedicated arrangement is provided in display specifications

processed within less than 21 ms. The correspondence between input and pixel data is drawn in > Fig. 3 for an XGA display. Rows (lines) and columns start in the top left corner of the panel in the notation (row, column) for the pixel. A color pixel itself consists of a red, green, and blue subpixel (inset). The panel input data are usually in the right order to reduce the data processing required within the Timing Controller.

2

Timing Controller and Intra-panel Interface

From the block diagram of an LCD module (> Fig. 1), the tasks of a Timing Controller (TCON) become clear – it must deliver control and data signals for both row and column drivers. The most relevant input and output signals (see also > Fig. 2) are listed in > Table 1 including some common abbreviations. The TCON usually receives the data to be displayed in the panel resolution; therefore, no scaling is required (see > Chap. 4.5.1). > Figure 4 shows a typical block diagram of an AMLCD Timing Controller (see also [1] Chap. 6.1, AMLCD Driver Architectures, [6, 7]). The input RGB grey level data are separated within the input logic in terms of data and synchronization (frame and line). The grey level data have to be formatted and analyzed for the column drivers. The timing control block controls via horizontal and vertical reference (panel resolution) the row driver output and synchronizes that with the column data output. As the column data have a high data rate, dedicated interfaces (see below) have to be implemented; furthermore, the Timing Controller must be able to process these data in real time. The Timing Controller is often not designed for a dedicated display resolution and driver ICs, those data have to be set via control registers via the programming interface and have to be stored in an EEPROM (electronic erasable programmable read only memory). As mentioned before and calculated in > Chap. 4.1.1, the data rate of the grey level data is relatively high (MHz to GHz range); for example, in XGA panels 1,024
768
60 Hz
3
8 bit  1.2 Gb/s. The intra-panel data lines (see > Fig. 5) must be able to transmit this

Active Matrix Driving

4.2.1

. Table 1 Signals for row and column driver delivered by the Timing Controller (TCON)

(Most relevant) Signals from TCON

Main output signal of driver

Row driver

Column driver

VSync (vertical start)

HSync (horizontal start)

Scan direction (DIR)

Polarity (inversion)

Output Enable (OE)

Digital grey level data (video data)

Row (TFT gate) pulse

Analog voltage acc. to grey levels

Data to be displayed

Input

Data alignment

Input logic Vertical and horizontal reference External programming interface

Output formatting

Column RGB, polarity

Timing control

Programmable registers

Row

EEPROM with panel parameters

. Fig. 4 Simplified block diagram of an LCD Timing Controller with its inputs and outputs

grey level color stream, and the physical interface has to be ‘‘small’’ as this increases the size of the display module. There exist basically two approaches for the intra-panel data interface: parallel and serial transmission from the Timing Controller (TCON) to the column drivers (grey levels). The most prominent implementations including the main features and limitations are described in the following overview and summarized in > Table 2 (for further information see, e.g., [5, 8–10]). ● Transistor-Transistor-Logic (TTL) Interfacing. For 6-bit grey scale, 18 parallel wires (lines) only for data (grey levels) are required as a minimum. Dual banking cuts the data rate in half by providing two banks of data busses but doubles the number of data wires to 36. This approach is only good for low-resolution and small-sized panels (but with a relatively large PCB stripe). Some information can be found in [11] as this intra-panel interface technology is somewhat outdated. ● Reduced Swing Differential Signalling (RSDS) Interfacing. RSDS (see [6]) halves the number of data lines, e.g., to 18 compared to 36 for dual data TTL banking. Midresolution and mid-sized panels are suitable for RSDS.

445

446

4.2.1

Active Matrix Driving

DAC voltages

Gamma voltages

Data lines Data bus clock Start pulse

Column driver 1

Start pulse

N

1 Display input data

Row driver 1

Timing controller

Column driver 2 N

1

N

1 N LCD glass

Gate clock Panel power supply

1

Column driver y

Row driver x

1 N

. Fig. 5 Block diagram of the panel electronics (module see > Fig. 1). Depending on the intra-panel interface, the number of data lines from the Timing Controller to the column drivers varies significantly

. Table 2 Comparison of intra-panel interfaces

Physical layer

TTL

RSDS

PPDS

TTL

LVDS

High-speed serial

a

Number of lines for 6-bit grey scale

36

18

2b

Recommended resolution

QVGA

SVGA

SVGA

a

For dual data bus which is necessary to keep data rate and EMI low Number is independent of grey scale resolution

b

● Point to Point Differential Signalling (PPDS) Interfacing. Only two wires are required independent of grey resolution for PPDS (see, e.g., [8, 12]) compared to 18 for RSDS and 36 for TTL (dual data banks). This method is mainly suitable for high-resolution and largesized panels; for lower resolution, the price is not competitive. PPDS can deliver up to 600 Mb/s which is fast enough for WUXGA panels. As only a few lines are required, the failure rate is significantly lowered and the tiny number of lines enables a small-sized PCB stick and reduced electro-magnetic interference (EMI) effects.

3

Row and Column Drivers

This section provides details about the tasks, characteristics, features, and implementation of drivers for rows and columns. The examples refer mostly to LCDs but also provide the basis for other display technologies. Their input signals are provided by the Timing Controller (TCON)

Active Matrix Driving

4.2.1

in such a way that no further data handling, manipulation, or calculation is required. The fundamental task of these drivers is to output the right (grey level) data (column) at the right time (selected by the row driver) to all pixels of the (active) matrix. The major task for the row driver (often also named the gate driver) is to ‘‘activate’’ a row (also called a line). For AM LCDs (see > Chap. 7.4.1, and [1] Chap. 6.4, Gate Drivers), a positive pulse opens the gate of the pixel MOSFETs to transfer the column voltage (grey level data) to the pixel and storage capacitor. For a certain resolution in terms of rows, the activation time of a row signal can be calculated via TON ¼

TFrame 1 ¼ N  f Frame N

where N is the number of rows (e.g., 768 for XGA); TFrame = frame time (e.g., 16.67 ms for 60 Hz); fFrame = frame frequency (e.g., 60 Hz) At 60 Hz frame frequency, the Ton duration for every row (line) equals about 35 ms for VGA and 21 ms for XGA. During that time, all column data for a line have to be transferred to the pixels of this row and in parallel the grey level data for the next line have to be transferred from the Timing Controller to the column controller. This is described more in detail below. For addressing a whole frame, the rows are activated sequentially until the last row is reached (> Fig. 6). After that, the new frame starts with a VSync pulse from the Timing Controller (see > Chap. 4.5.1). A more detailed view into the operation of a row driver and its I/O signals is provided in > Fig. 7 (see also [1] Chap. 6.4, Gate Drivers): The basic functionality is a bi-directional shift register with typical input signals from the Timing Controller (TCON) like clock, Vsync (vertical start), scan direction (DIR, for projection or row driver IC on left or right side of panel), and Output Enable (OE) to cut the gate pulse length in order to prevent row-to-row cross talk (see > Sect. 6). The clock frequency is calculated via frame rate multiplied by the number of lines; for XGA, we obtain 60 Hz
768 = 46 kHz. Row drivers are cascadable and have typically 256 outputs (low impedance buffer) switching between two voltages (typical values): +20 V for Gate High (row is ‘‘activated,’’ grey level data via TFT to pixel) and 5 V at

Frame time Row #

TFrame = 16.67 ms(60 Hz)

Pulse for TFT gate

1 2 3

768 Pulse width TON (≈21 μs for XGA)

. Fig. 6 Sequential activation of rows (lines) for one frame (values for XGA). An XGA with 768 lines usually has three gate driver ICs (3
256 outputs)

447

448

4.2.1

Active Matrix Driving

Gate Low (row is not selected). The typical package of a row driver IC is a TCP (Tape Carrier Package) with a die size pad of about 50 mm which has to be ‘‘expanded’’ to the pixel size (PC monitor 300 mm). > Figure 8 visualizes all typical and necessary timing signals of an AM LCD row driver. It is essential for good image quality that no delay occurs between subsequent row drivers (here row driver 1 and 2). But as the dynamic performance of a row with a certain resistance and with many MOSFET gates having a certain capacity the waveform at a TFT gate is not ideal, this will be discussed in > Sect. 6. Summarizing, row drivers are relatively simple ICs with low clock speed and digital output compared to Timing Controllers and column drivers. As the row driver delivers the gate signal for the MOSFET (TFT) of an AM pixel, the task of the column (or source) driver is to provide the appropriate grey level voltage for the (LC) pixel.

VSync Clock Dir

Output enable

VGate High VGate Low 1

256-bit shift register

Output control logic

Output buffer

to TFT gates

256 Sync for next driver

. Fig. 7 Block diagram with TCON input signals (top left and center) and TFT-output signals (right) of a typical AMLCD row driver

Row driver output number : 256 1 1 2 ...

2

Clock VSync Row 1 U

+20V –5V

Row 2

0V

Row 256

Out enable (OE) to prevent crosstalk

Trigger next row driver

Row driver 1

. Fig. 8 Typical timing diagram of an AMLCD row driver

Row driver 2

Active Matrix Driving

4.2.1

Therefore, each output consists of a digital-to-analog converter (DAC) for the grey level data. The fundamental input signals of a column driver from Timing Controller (TCON) are: ● RGB grey level data (PC domain: 8-bit, automotive 6-bit; mobile phones 4- to 6-bit) delivered via intra-panel interface (see > Sect. 2) ● HSync (horizontal start) for next row (line) ● Polarity for inversion (LCDs only) ● Load for transferring grey level data to DAC ● Clock for data synchronization A typical column driver (block diagram see > Fig. 9, for further information see [1] Chap. 6.3, Source Drivers, [7, 8, 11] and [12]) has 384 outputs which is equivalent to 128 RGB pixels. The 6- to 8-bit DACs provide a typical span from 0 to 20 V being at a first approach symmetrical to VCom (see > Fig. 11) for large panels. An output buffer reduces the impedance for fast rise of the output DAC voltage (e.g., load storage capacitor). In order to avoid electrophoresis of the liquid crystals and image sticking, the effective pixel voltage must be DC free; therefore, the output voltage swings around VCom. Like row drivers, column drivers are designed for cascading. However, their operating frequency is significantly higher: 384 grey level data have to be transferred by clock pulses from the data input to the ‘‘last’’ data latch. This results for XGA typically in a clock frequency of about 50 MHz (the data have to be processed 60 times per second for 768 rows). A column driver has a voltage supply for the logic (3.3 or 5 V) and reference voltages for the DAC (VGamma, see > Sect. 4). The package of a column driver is typically a TCP with 50-mm die size pad pitch which has to be ‘‘expanded’’ for PC-monitors to about 100 mm (subpixel size). A typical timing diagram of an AMLCD column driver is plotted in > Fig. 10 for an XGA display: The top part of this figure shows an excerpt of a row duration (see > Fig. 2) at the time of an HSync pulse which starts the transfer of the grey level data for a new row. As a typical column driver has 384 outputs, 128 RGB grey level data are stored in the data latch. The load pulse transfers these grey level data to the DACs. This means that the DACs output is set for one

Sync for the next driver

128-bit shift register

HSync

RGB Data latch

Load Polarity VGamma

DA converter 10 Output buffer

1

Analogue grey level voltage to TFTs

. Fig. 9 Block diagram of a typical AMLCD column driver

384

449

450

4.2.1

Active Matrix Driving

HSync

Data (RGB)

126

127

128

invalid

1

2

3

HPorch Load

Load Polarity Output acc. to grey level

negative

positive

VCom

. Fig. 10 Typical timing diagram (clock and some control signals omitted) of an XGA AMLCD column driver

row while the grey level data of the next row are loaded into the data latch in parallel. The signals at the bottom demonstrate how the polarity signal changes the output level of the DACs (row inversion is plotted, see > Fig. 19 and [13]) relative to Vcom (constant for large panels). The rising and falling curves of the output waveform are not of perfect rectangular shape because of RC limitations (see > Fig. 16).

4

Gamma- and VCom- Supply for LCDs

To provide the appropriate voltage supply for the column driver and the frontplane (VCom), dedicated buffer circuitries have been introduced. Beside cost, the motivation for highly integrated ICs was low footprint, thus reducing the non-active area of a display module. At first, the buffer for the gamma voltage supply will be presented. The fundamental task of the column drivers is to set the pixel voltage to the corresponding grey scale value with respect to the (mostly) non-linear electro-optic curve of the display. The final luminance output has to fulfil the ‘‘gamma curve’’ L  D g (see > Chap. 4.1.1, > Sect. 3 of Chap. 11.2.1 and [14]). An example of such a curve is visualized in > Fig. 11 where the output voltage of the column driver is plotted over the 6-bit digital grey level (6-bit is used because it makes the figure simpler and more readily understandable, it can be easily transferred to 8-bit). The gamma supply buffer IC delivers to voltages V1 to V10, as reference voltage to the column driver’s DACs; in the ‘‘middle’’ (here between V5 and V6) is the VCom voltage for the frontplane (see [1] Chap. 6.3.4, Analog Buffers). This approach is typically used for larger sized AMLCD panels (>500 ). VCom modulation and a single branch grey-level-to-voltage curve are an optimized method for smaller sized AMLCD panels (see [2] Chap. 9, Advances in Display Driver Electronics and [15]). Due to the dynamic effects of AMLCD driving, a kick-back voltage VKB (see > Sect. 6) lowers the pixel voltage in the same direction. This is visualized on the right side of the figure for the grey level voltage rising behavior when the gate voltage ON-pulse (row selection) is finished. Therefore – to avoid a residual DC offset (electrophoresis, image

Active Matrix Driving

4.2.1

sticking, see > Sect. 3 of Chap. 11.3.2) – a non-linear and asymmetric correction for the (grey-level-dependant) kick-back voltage VKB has to be introduced. > Figure 12 shows a fundamental block diagram of the gamma voltage buffer ([16, 17]) supplying the DACs of column drivers and the connection to the column drivers ICs. The typical number of outputs is ten voltages (V1 . . . V10), which should be independently adjustable, e.g., by digital potentiometers for good grey level display performance. The

Output voltage (to TFT) VDD V1 Positive polarity

VCom (≡ FP) Negative polarity

Asymmetric (CLC corrected)

V2 V3 V4

ΔVKB

Symmetric (uncorrected)

V5 V6 V7 V8 V9 V10 0

ΔVKB 0

15 31 47 Input grey level (6-bit data)

63

. Fig. 11 Example of 6 bit digital grey-level-to-voltage curve of an AMLCD without (symmetric) and with (asymmetric) kick-back voltage (DVKB) correction. V1 to V10 are the gamma buffer reference voltages

VDD

DAC reference voltages V1

V5 6-bit system V6

V10 Column driver Gamma buffer IC

Column driver

AM LCD panel

. Fig. 12 Example of a 6-bit gamma voltage supply for AMLCD column drivers

451

452

4.2.1

Active Matrix Driving

VDD R1 R2

Vcom = VDD /2 R3 R4 Feedback from panel

. Fig. 13 Basic buffer circuitry for the AMLCD frontplane voltage Vcom

gamma buffer IC is normally fed by a single voltage supply of typically 12 V DC, and the outputs are capable of delivering 10 mA at 10 V. Particularly for vertical aligned (xPA) LCs, the electro-optic curves for the RGB primaries differ slightly. Therefore, e.g., the white point will shift while stepping through different grey levels (Color Tracking, see > Sect. 2 of Chap. 11.3.1). Implementing a gamma reference buffer for each primary allows precise tuning of each color or grey level without color shifts, resulting in constant color temperature. The frontplane VCOM (> Fig. 13) is adjusted by the resistors R1 and R2 (digital and programmable potentiometers are recommended) so that an overall compensation of the kick-back voltage is done by gamma voltages and frontplane voltage. The basic function should be a low-frequency transconductance amplifier (usually an Operational Amplifier) with highspeed active feedback (see, e.g., [18]) compensating deviations during operation. All necessary functionality for driving AMLCDs is now presented. However, some dynamic effects happen which are discussed in > Sect. 6 after a brief introduction to highly integrated panel electronic ICs.

5

All-In-One Display Driving Controllers

Especially for high-volume electronic systems with display resolutions up to QVGA or even VGA, there is a competitive advantage in integrating various panel electronics ICs (see > Fig. 1) into a single one (for an overview and further details see [1] Chap. 6.1, AMLCD Driver Architectures and [2] Chap. 9, Advances in Display Driver Electronics). This results also in benefits like space reduction (also in terms of Printed Circuit Board) and improved reliability. The first approach (> Fig. 14) is similar to low-resolution Passive Matrix display modules with built-in character or graphics controller (see > Sect. 5 of Chap. 4.1.1): The microcontroller is connected via a standard memory interface to the display module, and the Timing Controller (TCON) is equipped with the display frame memory. This keeps the processor load low because only the data of pixels which have to be changed have to be transmitted. The Timing Controller reads the data to be displayed for every frame from the memory and transfers the data to row and column drivers. It could be considered that the display module is in a ‘‘free-running’’ mode if there is no change in display content. The TCON can either be mounted on a separate PCB or foil as well as on the display glass. A higher integration can be achieved by integrating TCON, display data RAM (frame memory), and driver ICs (row and column) into a single IC; > Fig. 15 shows an example; more details are provided by the specification of such ICs. The RGB data and system interface

4.2.1

Active Matrix Driving

mC

Address

A

Data

D C

Control

Data

Timing controller with display memory

Memory interface

HSync VSync

LCD with row and column drivers

Display module

. Fig. 14 Typical block diagram of an embedded graphics system with microcontroller (mC) and a display module with Timing Controller including built-in frame memory and display (only row and column drivers). The state-of-the-art interface for this configuration is a memory interface type

1

N

1

N

Row driver Column driver

Power supply

Gamma buffer for EOC & Digital voltage supply

Display data latch Row address

Display data RAM (frame memory)

Line address

Display timing generator

Column address

Control logic RGB interface

Data

System interface

Parameters

. Fig. 15 Typical block diagram of an ‘‘all-in-one’’ display controller with display memory, row and column drivers

(bottom) is normally dedicated to the micro controller or processor used. Those input data are transferred to the built-in display memory (Random Access Memory type) by the row and column address. Each pixel corresponds to 3 bytes for 8-bit grey level color displays. The dual ported display RAM is read out via line address and display data latch block. The whole memory access is steered by the timing generator. This block is also responsible for the real time data transfer to row and column driver via display address and data latch. Via power and gamma (g) voltage inputs, all necessary supply is provided for the all-in-one display driving IC.

453

454

4.2.1 6

Active Matrix Driving

Dynamic Performance of AMLCDs

As Active Matrix display can be manufactured to a relatively large size (e.g., up to 10800 diagonal for AMLCDs), it is obvious that some dynamic effects will occur due to high-frequency driving (50 kHz for XGA row clocking) and signal propagation over long distance via thin film lines and transistors (TFT). A typical equivalent circuitry of an AMLCD is shown in > Fig. 16 (see also [1] Chap. 5.2, Structure of AMLCD Panel and [19] Chap. 2.3, Design Analysis): The row and column lines have a non-negligible resistance (from pixel to pixel) and capacitors like pixel (CLC) and storage capacitor and parasitic Gate-Source capacitor CGS from the TFT MOSFET (metal oxide semiconductor manufactured in Thin Film Technology, see Chap. 5.2.1 and [19] Chap. 3, Thin-Film Transistors) are present. The gate pulse (see > Fig. 8) will be distorted from left to right in the sense of the figure by the resistance of the row line and the parasitic GateSource capacitor CGS in a way that the rise and fall time is increasing ([1] Chap. 5.3, General Considerations, [19] Chap. 2.3, Design Analysis). The effect is an RC low pass filter degradation of the pulse waveform; details will be discussed later on. Similar effects occur for the column drivers with column line resistance and mainly the storage capacitors (visualized by horizontal arrows) from top to bottom. The consequences of these pulse smearings are plotted in > Fig. 17: The center shows the above discussed distortion of the rectangular gate pulse by RC low pass filters near (green line) and far (red line) from the row driver IC. The difference is caused by the propagation of the pulse along the row line with resistors and capacitors (see > Fig. 16). At the rising edge, a delay of the start of the (distorted) pulse will occur which is named as ‘‘Gate delay’’ ([1] Chap. 5.3, General Considerations, [20]). This means that TFT gates far from the row driver IC are reaching the TFT threshold ‘‘later’’ than TFT gates near the row driver (additional to smeared edges). This causes less available charges from the column drivers to reach the storage and pixel capacitor. A consequence is a DC voltage component which can reach values of 100 mV being critical in terms of image sticking (see > Chap. 11.3.2). At the trailing edge, the fall time will increase from left to right (far from the row driver). Therefore, the TFT OFF threshold will be Column drivers TFT

CGS

Row drivers

CLC

Storage capacitor

Frontplane

. Fig. 16 Typical equivalent circuit diagram of an AMLCD. More details on pixel circuitry can be found in > Fig. 18

Active Matrix Driving

4.2.1

Row driver output enable signal (OE) Near row driver

Far from row driver

TFT OFF threshold

“Gate delay” Column data

Actual row

Next row

Time of 1 row (line)

. Fig. 17 Waveform of an AMLCD TFT gate signal near and far from the row (gate) driver. The ideal pulse from row driver is distorted via RC low pass characteristics of row (resistance) and gate-source capacity. Without the Output Enable (OE) pulse, the TFT of a row will let pass the grey level data of the subsequent row (dashed line)

reached later than for ideal conditions. This will allow the column grey level voltage dedicated to the next row to influence the pixel voltage of the current row, leading to blurry images through crosstalk. This effect is avoided by the Output Enable (OE) pulse ([20], see also > Fig. 8): The trailing edge of the gate pulse is cut off by the OE pulse. In a well-designed AMLCD panel, the OE pulse is so long that the furthest TFT is closed when the next row’s column grey level data are transferred. However, the OE pulse will also shorten the time to load the grey level voltage from the column driver DACs into the pixel including a storage capacitor as this signal (pulse shape) is also distorted by RC effects along the column line. The parasitic Gate-Source capacitor CGS as shown in > Fig. 16 has another effect beside acting as capacitor for the RC low pass row filter – a falling gate pulse (‘‘Gate OFF’’ edge) will therefore diminish the pixel’s grey level voltage. This voltage drop is in the range of 1 V and is called kick-back voltage DVKB ([1] Chap. 5.3, General Considerations). The consequence will be (potential) grey level shift, flicker, and image sticking if not properly compensated! To explain this effect and the consequences, all necessary things are drawn in > Fig. 18: On the left side, the gate pulse (VG), the column driver grey level data voltage VD and the frontplane voltage VCom are plotted over time. These waveforms are applied to the (equivalent) circuitry in the center of the figure. The parasitic Gate-Source capacitor CGS (dotted) couples the row (gate) line with the pixel capacitor. Therefore, the switch-off of the row signal (via OE pulse) lowers the grey level voltage of the pixel as shown on the right side. This drop is in the same direction for positive and negative output (polarity, see > Fig. 10). In consequence – as mentioned before in > Sect. 4 and plotted in > Fig. 11 – the grey level output voltage from the column drivers must be asymmetrical to avoid any DC component in the resulting column driving signal ([1] Chap. 5.5, Kickback Compensation Methods). The kick-back voltage DVKB can be calculated via parameters from both pixel and MOSFET ([1] Chap. 5.3.1, General Considerations, [21] and [19] Chap. 2.3.1, Design Analysis): CGS DVKB ¼ DVGate CGS þ CLC þ CSt

455

456

4.2.1 Driver waveforms

Active Matrix Driving

LC waveforms

Pixel Column (data)

VD VCom 0

Address TFT

Row (gate)

VG VD VCom

CGS VLC

1 frame

VG CLC

Storage capacitor

0

ΔVKB

t (simplified example)

VLC

pixel

Frontplane

Row (gate)

t

. Fig. 18 Input waveforms for the pixel transistor (left), AMLCD pixel circuitry (center) and resulting pixel voltage VLC (right)

with ΔVGate as gate pulse voltage, CGS as parasitic Gate-Source capacity, CLC as pixel capacity, and CSt as capacity of the storage capacitor. With typical values for those parameters (ΔVGate = +20 V, CGS = 10 fF, CLC = 100 fF, CSt = 50 fF), we obtain a kick-back voltage ΔVKB of about 1.2 V. However, this value is not constant as the pixel capacity CLC depends on pixel voltage (grey level, polarity). Therefore, a compensation ΔVKB for various grey levels and the two polarities has to be made via the gamma voltage buffer as described above (see > Fig. 11). Furthermore, the gate waveform at a TFT (see > Fig. 17) has an additional effect but is minor compared to the ‘‘Gate OFF’’ one. The longer fall time of the gate pulse far from the row driver allows more charge injection through distant TFTs than through near gate TFTs. Both effects have to be properly handled and adjusted in order to prevent noticeable flicker (see below) and residual DC (will cause image sticking).

6.1

Inversion to Prevent Flicker

Slight differences in the two gamma curves for positive and negative polarity (see > Fig. 11), kick-back voltage dependencies, and rise and fall time discrepancies lead to a slight modulation of the luminance output of a pixel showing a permanent grey level. In order to compensate for this effect, neighboring ‘‘regions’’ of an AMLCD are driven with opposite polarity (inversion) so that potential flicker will be largely reduced in visibility. There are several methods (see > Fig. 19 [1] Chap. 5.4.1, Crosstalk Reduction and Polarity Inversion Techniques, [13, 18]) for implementation like row-, column-, pixel-, and dot-inversion. Advantages and drawbacks of these approaches are discussed in [1] Chap. 5.4.1, Crosstalk Reduction and Polarity Inversion Techniques and [14]. The frame inversion is ‘‘automatically’’ implemented via the polarity signal. But with some signal processing, it can be used to increase the grey scale resolution by ‘‘Frame Rate Control’’ (FRC): For even frames, the original grey level, let us say ‘‘i,’’ is used; for the subsequent odd frame, the grey level is increased by one digit to ‘‘i + 1.’’ For 60 Hz frame rate, vision will integrate to an intermediate grey level as the luminance difference is too small to be detected (no flicker). Therefore, a 6-bit column driver can provide a 7-bit grey scale level resolution. Applying 4-frame FRC will push a 6-bit DAC to ‘‘8-bit resolution.’’

Active Matrix Driving

Row inversion

Column inversion

4.2.1 Pixel inversion

Even frame

Odd frame

. Fig. 19 Examples of AMLCD polarity inversion methods

6.2

Summary and Directions for Future Research

In this chapter, we have described the path from digital input data to TFT waveforms generated by panel electronics consisting mainly of the Timing Controller (TCON) as well as row and column drivers. Additionaly power supply circuitry, which is not described here, is integrated in the panel electronics such as gamma buffer IC for providing the grey level references for the digital-to-analog converters (DAC) of the column ICs. The examples refer to AMLCDs ([1] is an excellent reference) but can be easily adapted, e.g., to AMOLEDs as described in the fundamentals (> Chap. 4.1.1, for further details see e.g., in [2] Chap. 14, Advances in AMOLED Technologies). As cost and space is always an issue, R&D activities focus on higher integration of functions like all-in-one ICs (see > Fig. 15) and high speed and low wire interfaces from TCON to column drivers. Further challenges for panel electronics are high frame rates to reduce motion blur (see > Chap. 4.5.1). Another approach, especially for mobile displays, is to integrate electronic functions as much as possible onto the display (glass) substrate (details see [2] Chap. 13, Recent SOG Development Based on LTPS-Technology, [22] and [23]).

References 1. Cristaldi DJR, Pennisi S, Pulvirenti F (2009) Liquid crystal display drivers. Techniques and circuits. Springer, New York 2. Bhowmik A, Li Z, Bos PJ (2008) Mobile displays: technology and applications. Wiley, Hoboken 3. Henzen A, van de Kamer J, Nakamura T, Tsuji T, Yasui M, Pitt M, Duthaler G, Amundson K, Gates H, Zehner R (2004) Development of active-matrix electronic-ink displays for handheld devices. J SID 12(1):17–22 4. Kim SS, Kim ND, Berkeley BH, You BH, Nam H, Park JH, Lee J (2007) Novel TFT-LCD technology for

5. 6.

7.

8.

motion blur reduction using 120 Hz driving with McFi. SID Dig 38:1003–1006 Lee AY (2002) TFT-LCD module architecture for notebook computers. Inf Display 1:14–17 Lee A, Lee DW (2000) Integrated TFT-LCD timing controllers with RSDS column driver interface. SID Dig 31:43–45 Kim EG, Martin R (2000) A compact LCD driver and timing controller system. SID Dig 31:46–49 McCartney RI, Bell MJ (2005) A third-generation timing controller and column-driver architecture

457

458

4.2.1 9.

10.

11. 12.

13.

14. 15.

16.

Active Matrix Driving

using point-to-point differential signalling. J SID 13(2):91–97 McCartney R, Kozisek J, Bell M (2001) WhisperBus™: an advanced interconnect link for TFT column driver data. SID Dig 32:106–109 Koh H (2009) pLVDS: a new intra-panel interface for the future flat-panel displays with higher resolution and larger size. SID Dig 40:1237–1240 Connor B, Velamuri S, Mank D (1994) Low power 6-bit column driver for AMLCDs. SID Dig 25:351–354 McCartney RI, Bell MJ, Poniatowski SR (2005) Evaluation results of LCD panels using the PPDS™ architecture. SID Dig 36:1692–1695 Meinstein K, Ludden C, Hagge M, Bily S (1996) A low-voltage source driver for column inversion applications. SID Dig 27:1–6 Lueder E (2010) Crystal displays. Addressing schemes and electro-optical effects. Wiley, New York Kudo Y, Akai A, Furuhashi T, Matsudo T, Yokota Y (2003) Low-power and high-integration driver IC for small-sized TFT-LCDs. SID Dig 34(2):1244–1247 Lee B, Kim KD, Jeon YJ, Lee SW, Jeon JY, Jung SC, Yang JH, Park KS, Cho GH (2009) A buffer amplifier with embodied 4-bit interpolation for 10-bit AMLCD column drivers. SID Dig 40:371–374

17. Blyth T, Orlando R (2005) A programmable analog reference memory for adaptive gamma correction. SID Dig 36:1094–1097 18. Lee JB, Won T (2007) Feed-back control for the reduction of flicker and gray scale errors for large panel TFT-LCD. IDMC 2007:536–539 19. Tsukada T (1996) TFT/LCD. Liquid-crystal displays addressed by thin-film transistors. Gordon & Breach, Amsterdam 20. Kim SH, Park H, Kim S, McCartney R (2006) A new driving method to compensate for row-line signalpropagation delays in an AMLCD. J SID 14(4): 379–386 21. Park Y, Lee E, Kim S (2007) An analysis of common reference voltage architecture in wide TFT-LCD. IDMC 2007:259–260 22. Nakatogawa H, Tsunashima T, Aoki Y, Motai T, Tada M, Ishida A, Nakamura H (2009) 3.5 inch VGA TFT LCD with system-on-glass technology for automotive applications. SID Dig 40:387–390 23. Kim CH, Kim CM, Moon KC, Park KC, Kim IG, Joo SY, Park TH, Huk Maeng HS, Jung EJ, Kim CW (2004) Development of 200 ppi SOG-LCD. IMID’04 Dig:1–4

Further Reading Den Boer W (2005) Active matrix liquid crystal displays. Fundamentals and applications. Newnes, Amsterdam Maini AK (2007) Digital electronics: principles, devices and applications. Wiley, Chichester

Myers RL (2002) Display interfaces. Fundamentals and standards. Wiley, Chichester Wobschall D (1987) Circuit design for electronic instrumentation: analog and digital devices from sensor to display. McGraw-Hill, New York

Part 4.3

Panel Interfaces

4.3.1 Panel Interfaces: Fundamentals Karlheinz Blankenbach 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462 2 Analog Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 3 Parallel Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466 4 Summary and Directions of Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.3.1, # Springer-Verlag Berlin Heidelberg 2012

462

4.3.1

Panel Interfaces: Fundamentals

Abstract: This chapter describes the fundamentals of interfaces between graphics adapters (or graphics controllers) as signal sources and the input of display modules. We distinguish between analog and digital transmission while the latter divides into parallel and serial methods. In this chapter, analog and digital parallel interfaces are described; serialized data transmission standards will be presented in the chapters that follow. List of Abbreviations: ADC, Analog-to-Digital Converter; AM, Active Matrix; APIX, Automotive Pixel Link; bpp, Bit Per Pixel; BW, Bandwidth; CRT, - Cathode Ray Tube; DAC, Digitalto-Analog Converter; DP, Displayport; DVD, Digital Versatile Disk or Digital Video Disk; DVI, Digital Visual Interface; EMI, Electromagnetic Interference; HDMI, High Definition Multimedia Interface; HDTV, High Definition Television; LVDS, Low Voltage Differential Signaling; MDDI, Mobile Display Digital Interface; MIPI, Mobile Industry Processor Interface; NTSC, National Television Systems Committee (US); PAL, Phase Alternating Line (Analog Color Television); PC, Personal Computer; QVGA, Quarter Video Graphics Array; RGB, Red, green, blue; RS-170, US Standard Black and White Video Format; SDTV, Standard-Definition Television; TCON, Timing Controller; TTL, Transistor-Transistor-Logic; TV, Television; USB, Universal Serial Bus; VESA, Video Electronics Standards Association; VGA, Video Graphics Array; XGA, Extended Graphics Array

1

Introduction

The interface between the video data source (e.g., tuner, video player, PC) and the display panel has the task of sending all image data in real time. This can be done in an analog or digital way; a good overview and introduction to display interfaces can be found in, e.g., [1–3]. Depending on the display resolution, the pixel data rate to transmit the data information (gray level and color) from the graphics adapter to the display can reach relatively high values. This interface data rate determines the very basic interface characteristics and can be calculated by the following formula (> 1); its basic unit is bit/s (baud); however, practically Mbit/s or Gbit/s is used: Data rate ¼ horizontal resolution  vertical resolution  frame rate  gray scale  RGB ð1Þ ‘‘gray scale’’ describes the numbers of gray levels (also known as grayscale resolution) in bits such as 8 bit for 256 gray levels or 6 bit for 64 gray levels. ‘‘RGB’’ is three for standard color displays with RGB primaries and RGB = 1 for monochrome displays. Assuming the widespread color XGA display, the data rate results in 1,024  768  60 Hz  8 bit  3 = 1,132,462,080 bit/s  1.1 Gbit/s. This is a relatively high amount for long (copper) cables; therefore, PC projectors, until today (2010), rely mostly on analog signals often known as VGA (do not confuse this with resolution). The most prominent analog interfaces such as PC VGA and TV are presented in > Sect. 2. On the other hand, digital transmission is less sensitive to external EMI issues especially for twisted pair cables (serial interfaces). A basic visualization of those fundamental interface methods is provided in > Fig. 1 including typical voltage levels; a comparison will follow below and is summarized in > Table 1. The waveform on top visualizes an analog interface (RS-170; composite: synchronization and gray levels on one line) starting with a synchronization pulse (here horizontal sync) followed by a blank time. The linear ramp will be shown as increasing luminance (from left to right) on the display. The center and bottom waveforms of > Fig. 1 represent typical digital interface waveforms: the center timing diagram describes the basic features of a parallel interface with

Panel Interfaces: Fundamentals

4.3.1

. Fig. 1 Simplified visualization for analog (RS-170), parallel, and serial interfaces showing the first part of a line signal (start of line)

. Table 1 Fundamental characteristics of display panel interfaces Interface

Analog

Parallel

Serial

Typical applications

SDTV, PC, laptop (standard IF 2010)

Panels up to VGA including intra-panel interfaces

DVI, LVDS, DISPLAYPORT, HDMI, intra-panel interfaces

Typical 767  575 . . . 1,280  96  64 . . . 640  480 resolution range 1,024 (H  V) Merits

Hassle free (standardized and widespread)

No DAC – ADC needed

Good quality

High quality

640  480 . . . 1,920  1,080

Large cable length Few lines High quality

Cable length up to 20 m Shortcomings

DAC – ADC necessary Limited length of cable (< 0.5 m) No timing standards

Costs for transmitterreceiver

463

464

4.3.1

Panel Interfaces: Fundamentals

synchronization pulse, clock, and several lines with gray level data (including color). The clock signal synchronizes the gray level waveforms to pixels etc. The data rates calculated above have to be transmitted in parallel for parallel interfaces. Thus the factors ‘‘gray scale RGB’’ are represented by parallel lines (e.g., 24 for standard 8-bit color plus control and supply lines); on each line the data rate is given by formula (> 2): Data rateðparallelÞ ¼ horizontal resolution  vertical resolution  frame rate

ð2Þ

This results for a 60 Hz color XGA display in about 47 Mbit/s. The large amount of parallel lines limits in total the useful cable length to about 50 cm due to electromagnetic interference (EMI) issues. More details for the parallel interface are presented in > Sect. 3. The bottom waveform in > Fig. 1 shows a serial interface as a high frequency signal compared to a parallel interface as the lines of the latter are ‘‘compressed’’ in terms of serialization to a single twisted pair line as visualized here (more twisted pairs in parallel are feasible as for DVI). For serial interfaces the highest data rate is set by the maximum allowance of the twisted pair cable(s). The data can be calculated via formula (> 1) for a single twisted pair; for more, divide the result by the number of lines; the fewer lines in parallel, the higher the data rate – for a single line 1.1 GBit/s have to be transmitted serially for color XGA panel. Serial interfaces such as DVI and HDMI are described in detail in the following chapters: > Chap. 4.3.2, LVDS, DVI, DP; > Chap. 4.4.4, APIX; and > Chap. 4.3.3, HDMI. As mobile phones have special requirements (like cost and multimedia capability), dedicated display interfaces such as MDDI and MIPI were developed, which, however, go beyond the scope of this handbook; details can be found in, e.g., Bhowmik et al. [4]. The overview and comparison of analog, parallel, and serial interfaces is provided in > Table 1 pinpointing applications, merits, and shortcomings. The biggest advantage of analog interfaces is the huge number of TV and PC devices in use and their high degree of standardization (also valid for serial interfaces) compared to parallel digital transmission. Analog interfaces combine good image quality with long cable length. The historical reason for parallel interfaces was to get rid of costly digital-to-analog converters (DAC) for the signal source and analog-to-digital converters (ADC) for the display panel input. However, the parallel interface is limited to short cable length due to EMI and to resolutions up to VGA; its cable is relatively bulky with typically 30–60 lines. If the system works well, the image quality is excellent as no analog processing has to be carried out. Serial interfaces became feasible with the development of high-speed digital electronics. They are able to transmit high resolution display data up to 20 m like analog interfaces but without their visible bandwidth limitations and analog conversion artifacts. The cost for a serial interface is higher as serializer and deserializer are needed compared to a parallel interface but its maximum resolution is significantly higher. Parallel (low resolution) and serial (high resolution) data transmission is used for intra-panel-interfaces; this is described more in detail in > Chap. 4.3.2.

2

Analog Interfaces

The history of displays started with monochrome Cathode Ray Tubes (see > Chap. 6.2.1) in the early twentieth century, which convert an analog gray level input voltage directly via cathode amplifier to luminance (light) generated by electrons hitting the phosphor layer. The voltage level determines the light output (the higher the brighter); in order to ‘‘complete’’ the analog interface, synchronization signals (frame and line pulse) are added. Color TV was introduced around 1950;

Panel Interfaces: Fundamentals

4.3.1

the requirement was that the color analog waveform and interface had to be made compatible with monochrome TV sets, so color bursts were implemented. The most prominent analog TV standards are NTSC and PAL with waveforms as shown in > Fig. 1 (top); details on these TV waveforms can be found in [1] Chap. 8, and [3]. The signal consists of synchronization pulses for frame (not shown here) and line. After the line pulse, the color burst, which contains the color data, follows. The luminance signal represents the (monochrome) gray image data, which are combined with the color burst to create a colored image. The signal voltage levels may vary, but most standards use a peak-to-peak voltage VPP of 1.0 V. Standards refer to a sync level of 0.3 V, while for systems with digital-to-analog converters (DAC) the sync level equals the ground. The most widespread analog TV standards are NTSC [5] and PAL [6]. For PCs further analog interfaces were introduced; however, today (2010), only the analog RGB VGA standard ([1] Chap. 9, VESA [7]) is widespread. It consists of a single line for each RGB color (instead of a color burst). Despite all of the progress in digital transmission, analog interfaces are still widespread due to their hassle-free compatibility and large achievable cable length. The typical waveforms for such an analog PC RGB interface are plotted in > Fig. 2 for a 60 Hz monitor. For better visualization, a single frame triggered by VSync (top line) is shown; it consists of many (768 for XGA) lines controlled by the horizontal synchronization HSync. The synchronization signals have Transistor–Transistor Logic (TTL) level (5 V) and can have positive or negative (shown) polarity. The video signals of the (example) single line is plotted for some analog gray levels for red (R), green (G), and blue (B); PC monitors use the additive RGB color mixture as described in > Chap. 2.2.3. The example line in > Fig. 2 starts with a monochrome ramp from black to white (as R, G, and B have the same signal over time) followed by black, red, green, and blue

. Fig. 2 Typical waveforms of analog RGB VGA PC interface for 60 Hz

465

466

4.3.1

Panel Interfaces: Fundamentals

neighboring pixels. After that a green pixel is followed by a black one, repeated five times (not to scale). The end of the line symbolizes various colors mixed with different RGB gray levels; their width is a few pixels. It is obvious that low pass bandwidth limitations will lead to blurry edges of, e.g., a pulselike gray level waveform. The recommended minimum video bandwidth BWVideo of the complete analog system should be higher than BWVideo  2  horizontal resolution  vertical resolution  frame frequency However, this approximation formula neglects horizontal and vertical flyback durations of the electron beam; this would lead to slightly higher bandwidth values (about 5–10%). For an XGA CRT PC monitor running with 100 Hz frame frequency (to avoid flicker, see > Chap. 11.3.2), we obtain a minimum bandwidth of 2  1,024  768  60 Hz = 95 MHz. When comparing this with typical specifications of analog input PC monitors and projection systems, we must state that this value is only reached for relatively expensive analog input monitors. Modern PC-based analog interfaces between graphics adapters and monitors share monitor information (Display Data Channel, Extended Display Identification Data) via digital transmission. Despite all of the effort and progress in digital interfaces, analog output is still (2010) common, from PC devices through digital cameras to DVD players, and all modern TV sets have analog inputs; some of them even have an analog PC VGA connector.

3

Parallel Interfaces

Compared to analog interfaces the gray level (color) data are transmitted only ‘‘digitally’’ as ‘‘0’’ or ‘‘1’’ by TTL voltage levels (e.g., 3.3 Vor 5 V) completed by control signals. For parallel digital interfaces all signals each have a dedicated line, an overview of the fundamentals of parallel interfacing could be found in [8] and [9]. The most relevant signals, which can be regarded basically as pulses are: – – – –

Pixel clock (to synchronize gray level data) Line clock (HSync, next line or row) Frame clock (VSync, a new frame is starting at the first line) Gray level data (every bit has a separate line)

The line and frame synchronization signal has the same function as for analog interfaces. The parallel interface is less standardized than its analog or serial equivalents; examples of pinning and signal names are provided in > Fig. 3. The number of gray levels and colors often determines the abbreviation for parallel interfaces: 12 bpp stands for 4-bit gray scale for three colors (3  4 = 12) per pixel (bit per pixel), 18 bpp and 24 bpp are often used in industrial panels up to VGA (640  480) resolution. The advantage of a 16 bpp interface from the digital point of view is due to 2  8 bit, which can be implemented more efficiently – also in terms of cost – than 18 bpp. However, as 16 is not a multiple of the three RGB primaries, the following scheme applies: 5 bits are dedicated for red, 6 bits for green, and 5 bits for blue. This is reasonable as vision is more sensitive to luminance rather than to color (green ‘‘delivers’’ typically 60% of the white luminance). The lack of common standards concerns mostly the control signals like positive or negative pulses as well as rise and fall times. In order to reduce the data rate, the RGB data are transmitted sometimes in parallel for two pixels or lines (odd and even data). However, this doubles the number of lines to, e.g., about 60 for a 24-bit dual data parallel

Panel Interfaces: Fundamentals

Pin No.

GND CK HSync VSync GND R0

Function Ground Clock signal for each data signal (pixel clock) Horizontal synchronous signal (line clock) Vertical synchronous signal (frame clock) Ground Red data signal (LSB)

•••

•••

•••

11 12 13

R5 GND G1

Red data signal (MSB) Ground Green data signal (LSB)

•••

•••

•••

18 19 20

G5 GND B0

Green data signal (MSB) Ground Blue data signal (LSB)

•••

•••

•••

25 26 27

B5 GND VCC

Blue data signal (MSB) Ground Positive power supply (+ 5 VDC)

•••

•••

•••

1 2 3 4 5 6

Symbol

4.3.1

. Fig. 3 TTL-interface pinning (excerpt of an example digital TTL-interface specification for 18 bpp)

. Fig. 4 Typical TTL-interface timing diagram

467

468

4.3.1

Panel Interfaces: Fundamentals

interface: 2  24 lines for gray level data with additional control land power lines. As the parallel data are generated by the graphics controller, which is also not standardized, the Timing Controller (TCON, see > Chap. 4.2.1) in parallel input panels features a programmable input logic so it can be adapted to various graphics controllers and timing standards. In the next step, we will look into some details of the timing of an example digital parallel display interface as visualized in > Fig. 4 over time for slightly longer than one frame. The uppermost signal is the vertical synchronization VSync (frame synchronization, trigger here falling edge, high ! low), which sets the start of a new frame, normally every 16.7 ms for 60 Hz frame rate. The length of the VSync pulse can be extracted from the specification of the panel used. The data enable signal indicates valid gray level data as being ‘‘1’’ (high). For certain times the level is ‘‘0’’ for no valid data; this happens around the VSync pulse (VPorch) and HPorch for the line sync pulse (HSync). The pulse for a new line is provided by the HSync (falling edge); these triggers happen every 21.5 ms for the 60 Hz XGA example (16.7 ms divided by 768 lines). Between two line pulses, the gray level data (bottom signal) for a whole line (here 1,024 pixels) have to be transmitted. This is controlled by the pixel clock pulses, which are omitted in the figure due to scaling reasons; their length is the line time divided by the number of horizontal pixels (for XGA about 10 ns). Some selected parameters in terms of width (duration) and time stamp (start, end) are listed in > Table 2 (QVGA is used here for easier understanding) as an example of a specification. Such timing data can also be set by an embedded operating system via the graphics controller. As some of these data allow a certain range of values like VPorch we must tune all these values in order to obtain a failure-free image on the display panel. The only way to achieve this is by trial and error within the limits of the specification by changing the corresponding register values of the graphics controller. Further parameters in parallel interface specifications are pixel clock frequency, pulse shape data like pulse width, rise and fall time as well as phase differences between signals. Summarizing, the specification of a digital parallel interface contains a lot of data and parameters, which must be carefully implemented to avoid, e.g., image degradations like black lines (mainly for frame start) and blinking pixels. . Table 2 Typical interface timing diagram for a QVGA display in portrait mode (320 lines, 240 rows) Signal

Minimum value

Vertical pixel number

Typical value

Maximum value

Unit

326

HSync pulse

1

HSync pulse

VSync start

322

HSync pulse

VSync end

323

HSync pulse

6

HSync pulse

320

HSync pulse

VSync width

VPorch

1

4

Vertical display end Horizontal pixel number

256

272

492

Pixel clock pulse

4

6

10

Pixel clock pulse

HSync start

246

250

308

Pixel clock pulse

HSync end

250

256

318

Pixel clock pulse

16

32

268

Pixel clock pulse

HSync width

HPorch Horizontal display end

240

Pixel clock pulse

Panel Interfaces: Fundamentals

4

4.3.1

Summary and Directions of Future Research

The data rates for transmitting megapixel content over an interface are relatively high reaching Gbits/s for digital transmission and require over 100 MHz bandwidth for analog interfaces. However, analog interfaces are increasingly being replaced by serial digital interfaces like HDMI for modern TV sets and DVI as well as DISPLAYPORT in the PC domain; further information about these interfaces can be found in > Chaps. 4.3.2 and > 4.3.3. Parallel digital interfaces are limited by both cable length and resolution (up to VGA). This results in the fact that future research focuses on serial digital interfaces. However, more and more serial-to-parallel converter ICs are available, such as DVI-to-RGB. Another trend is the USB interface for monitors delivering both display data and power.

References 1. Myers RL (2002) Chapter 6: Basics of analog and digital display interfaces, Chapter 8: Standards for analog video – Part I: Television, Chapter 9: Standards for analog video – Part II: The personal computer, Chapter 10: Digital video interface standards. In: Display interfaces. Wiley, Chichester 2. Kim T, Nam H (2006) Interface technologies for flat panel display. IDW’06: 1969–1972 3. Weise M, Weynand D (2007) How video works: from analog to high definition, 2nd edn. Focal, Burlington 4. Bhowmik A, Li Z, Bos PJ (eds) (2008) Chapter 10: Mobile display digital interface (MDDI), Chapter 11: MIPI high speed serial interface standard for mobile

5. 6. 7. 8.

9.

displays. In: Mobile displays: technology and applications. Wiley, Chichester NTSC: Recommendation ITU-R BT.470-7, conventional analog television systems. www.ITU.int PAL: Recommendation ITU-R BT.470-6, conventional television systems. www.ITU.int Analog VGA (PC): VESA generalized timing formula, 1999-9. www.VESA.org Watkinson J, Rumsey F (2003) Chapter 3: Digital transmission, Chapter 7: Digital video interfaces. In: Digital interface handbook. Focal, Burlington Noergaard T (2007) Chapter 4: Embedded board buses and I/O. In: Ganssle J et al (eds) Embedded hardware. Newnes, Burlington

Further Reading Hong D, Cheng KT (2010) Efficient test methodologies for high-speed serial links. Springer, Dordrecht Pease R (2008) Analog circuits. Newnes, Burlington

Poynton C (2002) Digital video and HDTV: algorithms and interfaces. Morgan Kaufmann, San Francisco

469

4.3.2 Serial Display Interfaces Thomas Wirschem 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472

2 Electrical Signaling on the Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 2.1 Low Voltage Differential Signaling (LVDS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 2.2 Current Mode Logic (CML) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 3 3.1 3.2 3.3 3.4

Serialization Architectures and Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 Transition-Minimized Differential Signaling (TMDS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 Flat Panel Display Link (FPD-Link) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 Embedded Clock Serialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475 8B10B Coded Serialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475

4 4.1 4.2 4.3

Serial Display Interface Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 DVI (Digital Visual Interface) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 HDMI (High-Definition Multimedia Interface) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 DisplayPort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

5

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.3.2, # Springer-Verlag Berlin Heidelberg 2012

472

4.3.2

Serial Display Interfaces

Abstract: The optimal choice of serial display interface technology depends on the use case of the application in terms of data throughput needed, cable length reach to be supported amongst other aspects. An overview of the underlying signaling technologies and serialization/deserialization concepts is provided to facilitate this assessment. Interfacing of systems supplied by different vendors more than often requires full compliance to comprehensive standards including detailed protocols and pin assignment for corresponding connectors and cable assemblies. For internal, embedded display interfaces proprietary solutions can offer advantages in terms of reduced coding overhead and simplified link synchronization. Serialization can not only yield slim and flexible interconnections but also lower power, lower EMI, and more robust interface solutions. List of Abbreviations: 1080p, Full High-Definition Progressive Scan (1920  1080 Display Resolution); CEC, Consumer Electronic Control; CML, Current Mode Logic; DDC, Display Data Channel; DDWG, Digital Display Working Group; DVI, Digital Visual Interface; EDID, Extended Display Identification Data; EMI, Electromagnetic Interference; FPD-Link, Flat Panel Display Link; Gbps, Gigabits per Second; HDCP, High-Bandwidth Digital Content Protection; HDMI, High-Definition Multimedia Interface; IEEE, Institute of Electrical and Electronics Engineers; JEDEC, Joint Electron Device Engineering Council; LVDS, Low Voltage Differential Signaling; Mbps, Megabits per Second; SerDes, Serializer/Deserializer; SPWG, Standard Panels Working Group; TIA, Telecommunications Industry Association; TMDS, TransitionMinimized Differential Signaling; VESA, Video Electronics Standards Association

1

Introduction

More than often, the display unit needs to be placed remote from the graphics host. Instead of routing a broad, parallel graphics bus to the presentation device, it is more practical to timemultiplex the graphics and control data onto serial interfaces with less than a handful of differential transmission pairs. These concepts not only reduce the copper wiring required, but also reduce the radiated emissions and interference (EMI) while providing excellent noise immunity.

2

Electrical Signaling on the Physical Layer

There are many differential signaling technologies available today. Some are defined by industry standard committees, such as the TIA, IEEE, or JEDEC groups. Others are more ‘‘vendor specific’’ flavors and may have unique electrical characteristics all their own. Both types were developed for different reasons, some focus on ultra speed such as CML, while others focused on two or more attributes. LVDS, for example, focused on both high-speed and low-power operation. A common advantage to differential signaling technologies is their excellent common mode noise rejection. Differential drivers broadcast the ‘‘true’’ signal on one terminal while sending a ‘‘negative’’ copy of the signal on the ‘‘inverted’’ output terminal. The receiver compares these two signals at its input terminals rather than referring to a potentially noisy ground level, as is the case with single-ended signaling. As long as the transmission lines are treated equally (kept close together, same electrical length), they exhibit an astonishing resistance to pick up (differential) noise from the environment. As an example of popular differential signaling choices, the I/O schemes of LVDS and CML are compared in > Fig. 1.

4.3.2

Serial Display Interfaces

Coupled fields Driver Current source

= 3.5 mA

–

Fringing fields

+ Cross section of differential pair

= 350 mV 100Ω

+ Receiver – –

+

VCC 50

50

100

Differential Z0 = 100Ω

. Fig. 1 Comparison of LVDS (top) and CML (bottom) I/O schemes

2.1

Low Voltage Differential Signaling (LVDS)

LVDS – Low Voltage Differential Signaling – is a high-speed and low-power differential interface for generic applications. It dates back to the early 1990 s and was pioneered by National Semiconductor. LVDS is standardized as an electrical layer standard by the TIA and is published as ANSI/TIA/EIA-644-A [1]. The driver provides a typical 350 mV differential output voltage centered at about +1.25 V. The receiver is specified with a 100 mV threshold over the receiver’s input range of ground to +2.4 V. This allows for the nominal active signal to shift up or down 1 V in common mode due to ground potential differences or coupled noise. The driver is intended to be used with 100-Ohm differential impedance lines terminated in 100 Ohms. The data rate achievable is device and application specific, but it tends to be in the DC to 2.5 Gbps range. Power is minimized in three ways. The load current is limited to 3.5 mA, the

473

474

4.3.2

Serial Display Interfaces

current mode driver tends to limit dynamic power dissipation, and static current is minimized by the use of submicron CMOS processes. LVDS provides true odd mode transmission and equal and opposite currents flow within the pair. This and the small output current (3.5 mA) tend to make LVDS low in radiated emissions and interference (EMI).

2.2

Current Mode Logic (CML)

CML – Current Mode Logic – is a very high-speed point-to-point interface. Its implementation details are vendor specific rather than referring to an official standard. Hence, careful review of interfacing schemes is recommended to determine interoperation of devices from different suppliers. The CML drive currents and signal amplitudes tend to be higher in comparison to the LVDS signaling standard. Today, CML has become very popular due to its simplicity and speed especially for multi-gigabit Serializer/Deserializer (SerDes) applications. A unique feature of CML is that it typically does not require any external resistors as termination is provided internally by both the driver and the receiver devices. CML uses a passive pull up to the supply rail, which is typically 50 Ohms. Due to its relatively simple output stage, the rise and fall times of output transitions can be well balanced to obtain symmetrical and open eye diagrams. CML supports data rates above 10 Gbps depending upon the silicon fabrication process for the drivers and receivers.

3

Serialization Architectures and Protocols

3.1

Transition-Minimized Differential Signaling (TMDS)

Transition-Minimized Differential Signaling is a transmission technology to transfer uncompressed multimedia data to high-resolution displays. TMDS was developed by Silicon Image Inc. as a member of the Digital Display Working Group (DDWG). It is employed with the DVI and HDMI standards. The transmitter introduces a proprietary data encoding with the goal to reduce the number of transitions in the serial data stream for lower electromagnetic interference over twisted pair copper cables, while enabling reliable clock recovery at the receiver end. The coding is similar to 8B10B coding, but uses a nonstandard code set. The 10-bit TMDS symbol can represent either an 8-bit data value during normal data transmission or 2 bits of control signals during screen blanking. When transmitting video data, TMDS uses four twisted pairs of cabling. Three pairs are assigned to the video data for the Red, Green, and Blue color components each. An additional channel carries the clock signal. The physical layer for TMDS is Current Mode Logic (CML).

3.2

Flat Panel Display Link (FPD-Link)

Flat Panel Display Link (FPD-Link) is a high-speed interface connecting the output of a video controller to the display panel. Most laptops, LCD computer monitors, and LCD TVs use this interface internally. FPD-Link bases on the LVDS physical layer, which is ideal for high-speed, low-power data transfer. It uses one twisted pair to transmit a clock signal. The color bits and display timing signals (HSYNC, VSYNC, and DE) are mapped onto 3 data channels (for 18-bit

Serial Display Interfaces

4.3.2

color displays) or 4 data channels (for 24-bit color displays). In 24-bit color mode, the least significant bits are mapped onto the 4th data channel to allow interoperability between graphics host and displays with 18- or 24-bit color depth. The open mapping scheme was developed by National Semiconductor and is referred to by the Standard Panels Working Group (SPWG) in their notebook panel specification [2]. The video and control data are transferred at a bit rate that is seven times the frequency of the pixel clock. With the use of MUX/DEMUX circuitry the overall size of the bus is reduced. This enables high-resolution panels to be supported over a smaller interface, simplifying notebook hinge design while supporting larger classes of panel resolutions. The FPD-Link receiver deserializes the LVDS data streams and provides the pixel data and control signals to the TCON (Timing Controller) on the panel board.

3.3

Embedded Clock Serialization

A disadvantage of serialization schemes that employ a parallel clock channel is the signal delay – skew – between the data channels and the clock channel. This skew is often the limiting factor for cable reach, as the signal delay between the transmission pairs of a cable might lead to sampling incorrect (prior or later transmitted) data on the receiver side. One way to embed the clock information is by framing the data stream with a Start (‘‘High’’) bit at the beginning and a Stop (‘‘Low’’) bit at the end of a pixel word. The consistent Low–High transition within the data stream enables the deserializer to synchronize onto the data stream and to de-multiplex the data towards the parallel output bus. Because the clock bits are traveling along with the data bits, the skew issue is eliminated. In comparison to the popular 8B/10B coding scheme, the requirements for a pre-synchronization of the deserializer can often be relaxed. While 8B/10B often requires a frequency accuracy of some +/ 50 to +/ 100 ppm to the embedded clock signal in order to guarantee link synchronization, the embedded clock SerDes chipsets often with just some +/ 5% (+/ 50,000 ppm) accuracy of reference clock. Some of the newer implementations even eliminate the need for a reference clock altogether and lock to random data inputs within a guaranteed lock time [3]. There are several, often proprietary implementations of this concept for internal, embedded display interfaces in the automotive and industrial market segments.

3.4

8B10B Coded Serialization

With the 8B10B line coding, an 8-bit data word is mapped into a 10-bit symbol. The coding is defined in a way to provide a DC balanced data stream as well as enabling reliable clock recovery on the receiver side. Within two symbols (20 bits), there is a maximum disparity of two between binary high and low bits. Moreover, the maximum run length for bits of same polarity is five, i.e., there is never more than five 1 s or 0 s in a row without a transition. This facilitates the clock recovery out of the serialized bit stream. The coding was introduced and published in [4] by Al Widmer and Peter Franaszek in the IBM Journal of Research and Development [4]. After expiration of the respective IBM patent, the scheme has become even more popular as reference for a DC-free line code. Not at last the DC balancing of data allows straightforward interfacing to optical modules in order to drive glass or plastic fiber media. The conceptional differences between the serialization schemes discussed are summarized in > Fig. 2.

475

476

4.3.2

Serial Display Interfaces

Parallel clock

Embedded clock 8b/10b Coding

. Fig. 2 Comparison of common SerDes concepts

4

Serial Display Interface Standards

4.1

DVI (Digital Visual Interface)

The Digital Visual Interface (DVI) is a digital video interface standard designed for carrying uncompressed digital video data to a display. It was developed by the Digital Display Working Group (DDWG) industry consortium lead by Intel, Compaq, Fujitsu, Hewlett Packard, IBM, NEC, and Silicon Image. The DVI standard also supports the Display Data Channel (DDC) and the Extended Display Identification Data (EDID), which allows computers to communicate with different monitor extensions [5]. The data format was introduced by Silicon Image Inc. in their PanelLink chipset family. DVI is using TMDS as its physical layer. In a single link configuration the interface consumes four twisted pairs of copper cabling (red, green, blue, and clock) to transmit 24 color bits per pixel. The pixel clock rate for a single link can be as high as 165 MHz. The largest resolution achievable at 60 Hz frame rate is 2.75 megapixels (or 1600  200 pixels resolution). A second link may be employed in a dual link configuration to increase the data throughput even further. In this case a respective cable assembly and connector set is needed with additional pins and wires (24 + 1 or 24 + 5). Alternate pixels are transferred as ‘‘even’’ and ‘‘odd’’ pixels on either link, which increases the resolution support to 4 megapixels at 60 Hz (or 2560  1920 pixels resolution). When more than 24 bits of color depth per pixel is required, the second link can also be used to carry the least significant bits. The data pairs carry binary data at ten times the pixel clock reference frequency, for a maximum data rate of 1.65 Gbps  3 data pairs for a single DVI link. The DVI connector includes pins for the display data channel (DDC). DDC allows the graphics adapter to read the monitor’s extended display identification data (EDID). The maximum cable length to the display is rated with 5 m. For longer distances, either cable equalizers can be employed on the display side or active DVI repeaters used along the connection path.

4.2

HDMI (High-Definition Multimedia Interface)

The High-Definition Multimedia Interface (HDMI) is a compact audio/video interface for transmitting uncompressed digital data of copy-protected, high-resolution video and audio between a graphics host (set-top boxes, Blu-ray Disk players, personal computers, video game

Serial Display Interfaces

4.3.2

. Fig. 3 Connection scheme between HDMI source and sink

consoles, and AV receivers) and a presentation device (digital audio devices, computer monitors, digital televisions, loud speaker, projector, etc.) [6]. Founders of the HDMI licensing organization were Hitachi, Panasonic, Philips, Silicon Image, Sony, Thomson, and Toshiba. HDMI specifies a 19-pin connector for video data, audio data with sampling frequencies up to 192 kHz and CEC (Consumer Electronics Control) for universal remote controls. This enables devices connected through HDMI to be controlled by just one remote control and infrared connection. The connection scheme between HDMI source and sink is shown in > Fig. 3 (with the HDMI equalizer function being only optional for extended cable reach). HDMI is downward compatible to DVI. This means that DVI signals can be transmitted over HDMI via an adapter. With the HDMI 1.3, a mini-connector version was introduced, which is employed in mobile devices, for instance, HD-camcorders. HDMI supports any TVor PC video format, including standard, enhanced, and high-definition video along with up to eight channels of digital audio with up to 192 kHz sampling rate. HDMI encodes the video data using TMDS. There are currently different versions of the specification as 1.0, 1.2, or 1.3a revisions. With HDMI 1.2, the maximum pixel clock rate of HDMI interface was 165 MHz which addresses resolutions of 1080p and WUXGA (1920  1200) at 60 Hz frame rate each. HDMI 1.3 increases the pixel clock even further to 340 MHz which allows for resolutions as high as WQXGA (2560  1600). An HDMI connection can also be expanded into a dual link and pixel clocks up to 680 MHz. In March [6], HDMI Licensing, LLC, on behalf of the HDMI Founders, released the HDMI Specification Version 1.4a, featuring key enhancements for 3D applications, including the addition of mandatory 3D formats for broadcast content as well as the addition of the 3D format referred to as Top-and-Bottom. The HDMI 1.4 specification adds support for extremely high video resolutions that go up to 4,000 lines wide by 2,000 lines high, or roughly four times the resolution of a 1080p display. Not at last, an HDMI Micro Connector, a new, smaller connector for phones and other portable devices, supporting video resolutions up to 1080p, was introduced. The HDCP (High-bandwidth Digital Content Protection) scheme is used to provide copy-protection to high-definition content through authentication and encryption processes. Each HDCP enabled device holds 40 keys of 56 bits each. Before sending HDCPprotected data, the transmitting device initiates an authentication process to confirm that the receiver is authorized to receive the data. Once the receiver has been authenticated, the transmitter encrypts the data stream before broadcasting to the receiver. In order to avoid certification

477

478

4.3.2

Serial Display Interfaces

for each and every HDCP-enabled device, the licensing authority (DCP LLC) defines rules for mass production [7]. For instance, chip vendors must only sell respective decoder chips to trustworthy business partners. Attempts to record full quality video and audio must be effectively frustrated by the licensed product supplier. Additionally, the players can verify revocation lists of compromised keys and reject to transmit content to invalid decoder IDs.

4.3

DisplayPort

DisplayPort is a VESA (Video Electronics Standard Association) standard of a digital audio/ video display interface, including respective connector and cable assembly specifications [8]. It contains guidelines for adapter design to HDMI and DVI standards. It defines a license-free, royalty-free, digital interconnect, intended to be used primarily between a computer and its display monitor, or a computer and a home-theater system. Driving forces behind the standardization were amongst others: AMD (ATI), Dell, Genesis, HP, Intel, Lenovo, nVidia and Samsung. DisplayPort is based on the coding of digital data without additional clock channel. It is a serial point-to-point connection, scalable up to four channels. The DisplayPort connector supports one, two, or four data pairs in a Main Link that also carries clock and optional audio signals, each with a symbol rate of 1.62 or 2.7 Gbps per channel. The video signal path supports 6 to 16 bits of pixel color depth. An additional AUX channel contains not only the Display Data Channel (DDC), management and device control data for the Main Link, but also offers 1 Mbps of bidirectional bandwidth. Possible use cases for this AUX channel include monitor web-cams, microphones, or loud speakers. With poor transmission media characteristics, the transmitter can scale back to the lower speed grade. As an example a connection with four channels at 2.7 Gbps each translates into 10.8 Gbps of aggregate throughput. This yields a net data payload of 9.71 Gbps. DisplayPort cable assemblies for such bandwidth are available for maximum length of 2 m. This is sufficient for a WQXGA display with 2,560  1,600 pixels and 30-bit color depth. A more cost-effective DisplayPort cable assembly with just one channel suffices for shorter haul connections between a computer and monitor with resolution up to 1,280  1,024. The maximum cable length is about 7 to 10 m. In order to enable longer cable connections, DisplayPort is feeding a power supply of 3.3 V with 500 mA (1.5 W) to active DisplayPort repeater stations. Starting with version 1.1, DisplayPort supports the HDCP (High-Bandwidth Digital Content Protection) encryption scheme. This is for instance required to transfer digital video data from a Blu-ray player to a display. Additionally, DisplayPort integrates an encryption scheme of its own – known as DisplayPort Content Protection (DPCP). The maximum bandwidth is 10.8 Gbps – even higher than with HDMI 1.3 at 10.2 Gbps. While DVI and HDMI require separate clock signals, DisplayPort uses 8B10B coding to embed the clock in the data signal. The data transmission protocol is based on micro packets rather than data aligned to a pixel clock. DisplayPort supports both external (box-to-box) and internal (laptop LCD panel) display connections. Newly featured in version 1.1 is support for fiber-optic cables as an alternative to copper, allowing a much longer reach between source and display entity.

5

Conclusion

The need for slim, high-bandwidth, power efficient, and EMI-friendly display interfaces keeps up the requirement list for recent and upcoming standards for both internal and box-to-box

Serial Display Interfaces

4.3.2

high-bandwidth video and audio transfer solutions. Further advances in terms of sophisticated signal conditioning through the supporting silicon chipsets are likely to happen. This may lead to either more cost-effective copper cable assemblies or longer cable reach even at the seemingly excessive speed grades beyond the 10 Gbps range. The future directions will be most interesting to monitor. Will the end-customer demand drive toward even higher display resolutions, deeper color, 3-D experience, or else?

References 1. TIA (2001) Electrical characteristics of low voltage differential signaling (LVDS) interface circuits. Telecommunications Industry Association, Washington, DC (February 1, 2001) 2. SPWG Notebook Panel Specification, version 3.8 (March 14, 2007) 3. Lewis D (2004) SerDes architectures and applications. In: DesignCon, 2004 4. Widmer AX, Franaszek PA (1983) A DC-balanced, partitioned-block, 8B/10B transmission code. IBM J Res Develop 27(5):440

5. Digital Display Working Group, Digital visual interface, revision 1.0 6. HDMI Licensing LLC (2010) HDMI Specification 1.4a, (March 2, 2010) 7. Digital Content Protection LLC (2006) HDCP v 1.3 Specification (December 21, 2006) 8. Video Electronics Standards Association (VESA), DisplayPort 1.2 Standard (December 22, 2009)

Further Reading Brian Weatherhead (2002) The digital link: understanding the digital home video controversy on commercial content (copy) protection. Home Theater High Fidelity (February, 2002)

Jack K (2007) Video demystified: a handbook for the digital engineer, 5th edn. Newnes, UK Kruegle H (2006) CCTV surveillance: video practices and technology, 2nd edn. Butterworth-Heinemann, Newton

479

4.3.3 High Definition Multimedia Interface (HDMI®) Jim Chase 1

Introduction and Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482

2

Applications and Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

3 3.1 3.2 3.3 3.4 3.5

Capabilities and Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 Power Up and Device Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 Content Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 Content Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 Downstream Device Control via Consumer Electronics Channel (CEC) . . . . . . . . . . . 487 Optional Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487

4 4.1 4.2 4.3 4.4 4.5 4.6 4.7

Version History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 HDMI Specification Version 1.0 (December 2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 HDMI Specification Version 1.1 (May 2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 HDMI Specification Version 1.2 (August 2005) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 HDMI Specification Version 1.2a (December 2005) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 HDMI Specification Version 1.3 (June 2006) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 HDMI Specification Version 1.3a (November 2006) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 HDMI Specification Versions 1.3b (March 2007), 1.3b1 (November 2007), and 1.3c (August 2008) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 4.8 HDMI Specification Version 1.4 (June 2009) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 4.9 HDMI Specification Version 1.4a (March 2010) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 5

Licensing and Testing Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489

6

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.3.3, # Springer-Verlag Berlin Heidelberg 2012

482

4.3.3

High Definition Multimedia Interface (HDMI®)

Abstract: HDMI® technology has become ubiquitous in connecting high-definition digital devices in consumers’ living rooms. With its robust performance, broad feature support, ease of use, low cost, and wide adoption in the industry, HDMI-enabled devices now extend beyond the original consumer electronics applications to include portable, automotive, and commercial applications. The specification development history, an overview of the technology features and capabilities, its applications, and licensing requirements for HDMIcompliant products are covered in this chapter. Terms: 1080p, 1920  1080 progressive scan display resolution; 3DTV, 3D digital televisions; 4 K  2K, High-definition resolutions four times beyond the resolution of 1080p; 4KTV, TV capable of displaying 4 K resolution; ARC, Audio Return Channel; AV, Audio/video; CEC, Consumer electronics control; DDC, Display Data Channel; DVI, Digital Visual Interface; EDID, Extended display identification data; Gbps, Gigabits per second; HDCP, Highbandwidth digital content protection; HDMI, High-Definition Multimedia Interface; Mbps, Megabits per second; sRGB, Standard RGB color space (used by PC monitors); TMDS, Transition Minimized Differential Signaling; x.v.Color, xvYCC colorimetry

1

Introduction and Overview

HDMI, or High-Definition Multimedia Interface, technology was introduced publicly at the 2003 Consumer Electronics Show (CES) as a single cable solution to allow devices to share uncompressed high-definition video along with multichannel audio. Built on the same Transition Minimized Differential Signaling (TMDS) technology as its predecessor, DVI (see > Chap. 4.3.2), HDMI technology has quickly grown to become the de facto standard of audio-video (AV) connectivity in the home. The over 1,000 licensed HDMI Adopters are expected to ship more than 600 million HDMI-compliant products in 2011 according to market research firm In-Stat. This translates into an installed base of over 2.2 billion HDMI products deployed worldwide, an increase of 20% and 46% respectively over year 2010 [1]. The HDMI founders – Hitachi, Panasonic, Philips, Silicon Image, Sony, Technicolor (formerly Thomson), and Toshiba – began work on HDMI in 2002 in an effort to improve on the DVI interface standard, specifically for consumer applications. In addition to native support for uncompressed high-definition video, additional capabilities including up to eight channels of digital audio, standard device control (CEC), and optional copy protection (via HDCP) were added to the protocol and small, consumer friendly 19-pin connectors were defined. Support for interfacing to existing DVI-enabled devices via a passive adapter was also specified. The Specification Version 1.0 was released in December 2002, with several updates announced since. As of this writing, HDMI Specification Version 1.4a, which added support for broadcast 3D formats, is the most current release. The HDMI Specification is available for licensing to HDMI Adopters. The Adopter Agreement also gives access to the testing requirements and allows potential products to be tested at HDMI Authorized Test Centers (ATCs). Adopter fees include an annual license fee and product per unit royalty, both of which are based on the volume of product shipped. Per unit royalty discounts are also offered for compliant products that implement copy protection and follow HDMI logo guidelines. Licensing of the HDMI Specification, which is confidential and restricted to adopters, is managed through HDMI Licensing, LLC.

High Definition Multimedia Interface (HDMI®)

2

4.3.3

Applications and Benefits

As a high-performance alternative to analog interfaces such as composite video, S-video, coaxial cable, SCART, and component video, HDMI technology has found its way into the vast majority of home entertainment products. The combination of reduced cable complexity and enhanced device control for consumers, lower cost for device makers, and high-quality content distribution and protection for content owners have all contributed to the high adoption rate of HDMI technology. HDMI technology currently has 100% penetration in HDTVs and Blu-ray Disc™ players and is expected to reach that milestone in DVD players by 2012 [1]. The latest generation of game consoles include HDMI outputs to support up to 1080p display, and Sony PS3 has recently released a firmware update to support 3D Blu-ray Disc playback in compliance with HDMI Specification Version 1.4a. In addition, support for audio, both multichannel transmission and the two-channel digital Audio Return Channel (ARC), makes HDMI technology the ideal cabling solution for home theater systems with integrated receivers supporting 5.1 or 7.1 surround sound speaker systems. The success of HDMI technology in the home entertainment ecosystem has impacted related markets where the convenience of single-cable connectivity brings ease of use to consumers. PC makers are increasingly adding HDMI connections to media-center PCs to allow content to be viewed on larger cinema-style displays. In addition, notebook PC adoption of HDMI outputs continues to grow to allow connections to the increasing number of HDMIenabled displays. According to the annual Residential Technology Survey (RTS) conducted in February 2010 by market research firm In-Stat, 22% of respondents connected their notebook PCs to HDTVs via HDMI technology [1]. More recently, Over-the-Top video devices, or IPTV boxes, such as the Roku Player, Apple TV, and D-Link’s Boxee Box, which are essentially embedded PC systems enabling streaming media services, have come to market with HDMI connectors. Smaller form factor Type C and Type D HDMI connectors have specifically been introduced by the HDMI Consortium to support low power portable devices such as HD digital camcorders, digital still cameras, and mobile phones. The HDMI Automotive Connection System was introduced in HDMI Specification Version 1.4 and is currently available in the 2011 Honda Odyssey minivan to allow external HDMI source devices to be connected to the Odyssey’s entertainment system and watched and heard on its integrated displays/speaker system. Content owners benefit from HDMI connectivity by providing a robust, uncompressed connection for high-definition content to be delivered from broadcast, locally stored, and streaming sources. Support for HDCP offers improved copy protection over those offered in legacy analog distribution techniques. Commercial and industrial applications for HDMI technology have grown recently as a result of a variety of adapter products introduced for application-specific uses. Long-distance connections between HDMI-enabled devices have been made possible by adapter products than allow HDMI signals to be sent over a variety of media, including fiber optic, wireless, and category 5/6 networking cable. The success of HDMI technology, within the home entertainment market and beyond to related segments, is due to its robustness and ease of use. > Figure 1 shows HDMI adoption by product type from 2008–2014, with strong growth through most of the categories.

483

484

4.3.3

High Definition Multimedia Interface (HDMI®)

Units Shipped (thousands) 1,200,000

1,000,000

Automotive* (New Category) Mobile Phones PC & Peripherals

800,000

PMPs Digital Still Camera Digital Camcorder

600,000

Game Console AV Receiver 400,000

Blu-Ray (Player/Recorder) DVD (Player/Recorder) STB (All)

200,000

DTV

2008

2009

2010

2011

2012

2013

2014

Year

. Fig. 1 HDMI device growth: 2008–2014 (Source: In-Stat [1])

3

Capabilities and Specifications

The HDMI Specification defines a set of capabilities and performance specifications for four types of licensed products: source, sinks, repeaters, and cables. Source devices are designed to output video (and audio); examples include Blu-ray Disc players, desktop PCs, and set-top boxes. Sink devices such as HDTV displays and PC monitors are capable of rendering compliant HDMI signals to decode and display audio and video, as well as respond to any CEC commands. Repeaters must act as both source and sink, passing through commands and signals as required by the upstream and downstream devices. The HDMI system diagram is shown in > Fig. 2. The HDMI interface includes four differential pairs (with shields) that carry the TMDS clock and data channels. These pairs carry audio, video, and supplemental data. The DDC channels manages configuration and control, while the CEC line enables optional consumer electronics control between devices connected in the same cluster. The optional HDMI Ethernet and Audio Return Channel (HEAC) lines enable bidirectional Ethernet compatible networking and two-channel audio in the opposite direction from the TMDS lines. The hot plug detect line validates the presence of two directly connected devices. There are a total of 19 pins on each HDMI connector. While pin number conventions vary among the five connector types, common functional names and descriptions are as follows: ● TMDS Data2+ ● TMDS Data2 Shield ● TMDS Data2

High Definition Multimedia Interface (HDMI®)

TMDS Channel 0

Video

4.3.3 Video

TMDS Channel 1 Audio

Audio

TMDS Channel 2 HDMI Receiver

HDMI Transmitter

Control / Status

TMDS Clock Channel

Display Data Channel (DDC)

CEC Line

CEC

Control/Status

EDID ROM

CEC

Utility Line HEAC

HEAC

High/Low

detect

HPD Line

. Fig. 2 HDMI system architecture overview [2]

● TMDS Data1+ ● TMDS Data1 Shield ● TMDS Data1 ● TMDS Data0+ ● TMDS Data0 Shield ● TMDS Data0 ● TMDS Clock+ ● TMDS Clock Shield ● TMDS Clock ● ● ● ● ● ● ●

CEC SCL SDA DDC/CEC/HEC Ground HEC Data Hot Plug Detect/HEC Data+ +5 V Power (max 50 mA)

Consumer Electronics Control I2C Serial Clock for DDC I2C Serial Data Line for DDC HEC optionally used from HDMI 1.4 with Ethernet HEC optionally used from HDMI 1.4 with Ethernet Used with Hot Plug Detect

Cables must be able to transmit all signals between the other device types; cable construction must be controlled to meet the strict timing requirements for transmitting high-speed uncompressed video over three pairs of differential TMDS lines and the TMDS clock lines. Since HDMI Specification Version 1.4a, there have been five different cable types; each supports all mandatory features while higher video resolutions and the optional HDMI

485

486

4.3.3

High Definition Multimedia Interface (HDMI®)

Ethernet Channel are only supported in two. The new system identifies cables by features in an effort to make the capabilities of each cable more clear; the cable type convention is no longer used. The new cable types are: ● HDMI standard cable (formerly known as HDMI 1.3 Type 1) supports video resolutions up to 1080i at 60 frames per second (fps) with a 75 MHz TMDS clock frequency (up to 2.25 Gbps data rate for all three TMDS channels combined). ● HDMI standard with Ethernet cable, which supports up to 1080i/60 as above plus the optional HDMI Ethernet Channel. ● HDMI high-speed cable (formerly known as HDMI 1.3 Type 2) supports 2D video resolutions up to 1080p at 120fps, with a 340 MHz TMDS clock frequency (up to 10.2 Gbps data rate for all TMDS channels combined). All HDMI 1.4a 3D and 4KTV formats are also supported. ● HDMI high speed with Ethernet cable support the same feature set as HDMI high-speed cables plus the optional HDMI Ethernet Channel. ● HDMI Standard Automotive cable and Type E connector supports the enhanced environmental requirements of the HDMI Automotive Connection System. Each of the four HDMI licensed product types has general capabilities specified that allow it to interoperate with other devices in a compliant way. Sources are connected to sinks or repeaters (which must act like a sink at one end) through a cable, and vice versa. HDMI functionality can be broken down into several broad categories. Each will be covered in subsequent subsections.

3.1

Power Up and Device Discovery

When two active devices are connected and powered on, the hot plug detect signal is asserted in the sink and the sink responds with Extended Display Identification Data (EDID), which is sent over the DDC lines, to allow the source to discover the sink’s capabilities. This in turn enables the source to supply content matching the configuration of the sink.

3.2

Content Transmission

Video, audio, and auxiliary data are transmitted across the three TMDS data channels. The TMDS clock, which runs at the video pixel rate, is transmitted across the TMDS clock line. Data is encoded as an equalized 10 bit sequence with data rate correspondingly equal to 10 the clock rate. A variety of video formats are supported with color depths ranging from 24,048 bits per pixel. Video encoding schemes include sRGB, YCbCr 4:4:4, or YCbCr 4:2:2. Stereo LPCM Audio at 32, 44.1, or 48 kHz is the minimum requirement for HDMI compliance; optional audio formats up to eight channels of 192 kHz sample rates are specified. Support for compressed surround sound and One-Bit-Audio formats is also specified.

3.3

Content Protection

The HDMI Specification itself does not mandate, though it recommends, copy protection of protected content. The only requirement is that if HDCP is used, it shall adhere to HDCP Specification Version 1.4. Other content protection schemes are not referenced.

High Definition Multimedia Interface (HDMI®)

3.4

4.3.3

Downstream Device Control via Consumer Electronics Channel (CEC)

The Consumer Electronics Control (CEC) line is a bidirectional serial line using the AV.link protocol to perform remote control functions. CEC allows the remote of one HDMI product to control functions of other HDMI-compliant products connected to it. While physical CEC wiring is required in all HDMI licensed products, the use of CEC in a product is optional. If CEC is implemented, the two functions ‘‘one touch play’’ and ‘‘system standby’’ must be implemented; others are defined in the specification and are optional.

3.5

Optional Features

Since the introduction of HDMI Specification Version 1.3, many new features have been added to address new product categories, features, and performance capabilities. Examples of this include extended color depths, 3D video display, Audio Return Channel (ARC), HDMI Ethernet Channel (HEC), and new color spaces for digital cameras. It should be noted that implementing any or all of these features is optional on a per product basis. If implemented, these features must be clearly documented when the device is certified and must meet the performance requirements detailed in the HDMI Specification.

4

Version History

The HDMI Specification has undergone several enhancements since its first release as Rev 1.0 in December 2002. Subsequent releases have targeted higher data rates for higher resolution video (up to and including 4K  2K), enhanced color fidelity, higher quality audio, support for expanded device types, additional audio and video formats, and more. Below is a brief chronology of each key version release, with a description of the main enhancements introduced.

4.1

HDMI Specification Version 1.0 (December 2002)

The initial release of the HDMI Specification, prior to CES 2003, supported a maximum clock rate of 165 MHz, which correlates to a total TMDS bandwidth (three TMDS data channels combined) of 4.95 Gbps. Maximum video resolution support was 1080p at 60 Hz at 24 bit color depth, which utilizes 3.96 Gbps of throughput. Audio was specified at eight channels of LPCM at up to 192 kHz, 24 bit resolution.

4.2

HDMI Specification Version 1.1 (May 2004)

The primary change to HDMI Specification Version1.1 was the addition of support for DVDAudio and its multichannel encrypted content.

487

488

4.3.3 4.3

High Definition Multimedia Interface (HDMI®)

HDMI Specification Version 1.2 (August 2005)

This version, in addition to adding support for one-bit audio formats used by Super Audio CD (SACD) – the primary alternative to DVD-Audio – featured a series of enhancements designed to support PC applications as an improvement to DVI. These improvements included: (a) the allowed use of Type A connectors in PC sources and displays with full support of PC video resolutions, (b) the option to use native PC sRGB color spaces in addition to the YCbCr color space defined for CE applications, and (c) support for low-voltage/AC coupled sourced in new displays to allow notebook PCs to connect to larger screen displays.

4.4

HDMI Specification Version 1.2a (December 2005)

CEC features and command sets were fully specified in HDMI Specification Version 1.2a. CEC compliance tests were also established.

4.5

HDMI Specification Version 1.3 (June 2006)

HDMI Specification Version 1.3 marked a major improvement in performance in many areas of the feature set. Most notably, the clock rate was increased to 340 MHz, which corresponds to a combined channel throughput of 10.2 Gbps. Color depth was increased as ‘‘deep color’’ was introduced allowing for an increase in the 8-bit color-bit depth to 10, 12, and up to 16 bits per color. Deep color provides an increase in the shades of colors for more precise image representation. x.v.Color was also added, which allows for the representation of more colors in the color pallet than are defined in the RGB color space. Audio enhancements included support for output of the Dolby TrueHD and DTS-HD Master Audio lossless formats, as well as an automatic AV synchronization protocol to allow for different processing delays in decoding audio and video data streams inside ICs and devices. Two classes of HDMI cables were defined; Category 1 cables were specified to work up to 75 MHz clock rate while Category 2 cables supported the full 340 MHz clock rate. In addition, the HDMI Type C ‘‘mini’’ connector was defined for use in portable devices.

4.6

HDMI Specification Version 1.3a (November 2006)

Several modifications for type C connections and source termination recommendations were made in HDMI Specification Version 1.3a. It also modified CEC capacitance limits, provided quantization details for sRGB color space, and broadened SACD audio format support. CEC commands were further enhanced.

4.7

HDMI Specification Versions 1.3b (March 2007), 1.3b1 (November 2007), and 1.3c (August 2008)

Each of these versions provides enhancements to the testing requirements for compliance. No new features or performance specifications were introduced.

High Definition Multimedia Interface (HDMI®)

4.8

4.3.3

HDMI Specification Version 1.4 (June 2009)

HDMI Specification Version 1.4 added support for several substantial new features. The higher bandwidth of HDMI Specification Version 1.3 was leveraged by adding definition of new video display formats for 3D resolutions for cinema and gaming applications as well as Super High Definition/4KTV (3840  2160p at 24/25/30 Hz and 4096  2160p at 24 Hz). To support the emergence of internet-accessible content in CE devices, support for 100 Mbps Ethernet was added, and two new cable types were defined to support this feature. Audio Return Channel (ARC) allows display devices to send audio information back to a receiver for decoding. New color spaces were defined to support for digital still cameras. Content Type was defined, which provided the capacity for source devices to define the type of content in real time so that a display can be configured based on the content type to provide an optimized viewing experience. Finally, two new connection options, the Micro-connector (Type D) and the Type E connection system for embedded automotive distribution were defined.

4.9

HDMI Specification Version 1.4a (March 2010)

3D broadcast formats were defined in the HDMI Specification Version1.4a. Further details were provided on mandatory, optional, and informative 3D formats for all media types.

5

Licensing and Testing Requirements

All manufacturers interested in developing HDMI-compliant products (which include cables, sources, sinks, or repeaters) must sign the HDMI Adopter Agreement. This agreement requires Adopters to develop products in accordance with the specification, certify the product in accordance with the latest released specification version and Certified Test Specification (CTS), and adhere to the Approved Trademark and Logo Usage Guidelines (ATLUG). The specification and CTS are developed to enable manufacturers to develop HDMI-enabled products. When an Adopter develops a new HDMI-enabled product in one of the four licensed product categories, it must be tested for compliance at an HDMI Authorized Test Center. (As of this writing, there are ten ATCs worldwide.) Once an adopter successfully completes ATC testing for a licensed product, subsequent products in the same category may be self-tested. In addition, there are third-party companies that offer advanced interoperability testing of licensed HDMI products to further evaluate a specific device’s performance with other products in the HDMI ecosystem. In all cases, the adopter is ultimately responsible to ensure their products are compliant. HDMI Adopters are required to pay annual license and royalty fees. The HDMI royalty structure is as follows: ● High volume manufacturers (more than 5,000 total units per year) – $0.15 per unit from the end-product manufacturer – $0.05 per unit if the product uses the HDMI logo in accordance with the ATLUG – $0.01 per unit discount if the end product implements HDCP ● Low volume manufacturers (less than 5,000 total units per year) – Same per-product royalty schedule as the high volume manufacturer above – Additional $1.00 per unit from the end-product manufacturer administrative fee

489

490

4.3.3

High Definition Multimedia Interface (HDMI®)

In addition to per unit royalties, HDMI Licensing, LLC, the licensing agent for the HDMI Specification, charges $5,000 (for low volume manufacturers) or $10,000 (for high volume manufacturers) as an annual membership fee. More details on becoming an adopter can be obtained from the HDMI Consortium web site www.hdmi.org.

6

Conclusion

The transition from analog to digital video and the availability of high-definition video content have generated unprecedented demand for low cost, easy-to-use consumer electronics devices. Consumer electronics makers continue to develop innovative products that offer increasing capabilities and an improved viewing experience, even in mainstream products. The HDMI interface has helped to usher in this concept of ‘‘anytime, anywhere’’ content through its easyto-implement, easy-to-use solution. HDMI technology continues to evolve to meet market needs for robust, feature-rich connections between digital media products, and is expanding to address new markets and applications where high-definition content can be shared easily and securely.

References 1.

Brian O’Rourke, In-Stat (2010) DVI and HDMI: DVI Won’t Die and HDMI Rolls On, SKU: IN1004688MI

2.

HDMI Licensing, LLC (2010) HDMI Specification 1.4a

Further Reading Bob O’Donnell, IDC (2006) HDMI™: The Digital Display Link

DDWG (1999) Digital Visual Interface. Revision 1.03. Digital Content Protection LLC, High-Bandwidth Digital Content Protection System, Rev. 1.2

Part 4.4

Embedded Systems

4.4.1 Embedded Systems: Fundamentals Karlheinz Blankenbach 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 2 Microcontrollers with Built-In Segment Driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496 3 Character and Graphics Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 4 Software for Graphics Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 5 Summary and Directions of Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.4.1, # Springer-Verlag Berlin Heidelberg 2012

494

4.4.1

Embedded Systems: Fundamentals

Abstract: Microcontrollers and microprocessors are the central part of an embedded system. However the display determines the system’s design, as it has demanding requirements in terms of (video) memory and data rate. In this chapter, the fundamentals of implementing displays into embedded systems are presented, including software approaches for the (graphical) user interface. List of Abbreviations: CPU, Central Processing Unit; FPGA, Field Programmable Gate Array; GPIO, General Purpose (Digital) Inputs and Output; LCD, Liquid Crystal Display; LVDS, Low Voltage Differential Signalling; PC, Personal computer; QVGA, Quarter Video Graphics Array; R&D, Research and Development; RAM, Random Access Memory; TTL, Transistor-Transistor Logic; VGA, Video Graphics Array; VRAM, Video RAM; XGA, Extended Graphics Array; mC, Microcontroller

1

Introduction

The development of modern embedded systems can be divided into hardware (e.g., [1]) and software (e.g., [2]) tasks; both domains are presented in this section with focus on systems equipped with a display. The hardware of an embedded system consists of many subassemblies (> Fig. 1) including microcontroller, memory, GPIOs (general purpose (digital) inputs and outputs), display controller and display. Microcontrollers range from low-cost 4-bit devices to high-performance 32-bit microprocessors. However as there is no exact classification or differentiation between a microcontroller and a microprocessor, we will use microprocessors often as a synonym for 32-bit devices. Generally speaking, an embedded system has less computing power, less memory, less storage capacity, and lower display resolution than a modern PC. Such an embedded system is mostly optimized for a single task like controlling a machine. The software development is mostly done by dedicated development tools for the microcontroller or microprocessor used. More and more 32-bit systems are equipped with an embedded operating system that simplifies the software development. We will now discuss the approaches of microcontrollers, display controllers, and displays for an embedded system. The display interfaces between display controller and display panel input are presented in > Part. 4.3. A good overview with many useful details of microcontroller – display topics can be found in, e.g., [3]. PC systems will not be discussed here as they are mostly standardized and built with commercial plug-and-play subassemblies. We divide electronic displays in terms of pixel layout, resolution, and embedded systems implementation into mainly three categories; a summary is provided in > Table 1: ● Segmented displays like 8-segment displays for visualizing, e.g., time (clock) or temperature with typically 32–96 segments and 4 commons (in total nearly 128–384 pixels). The display driver is usually integrated into the microcontroller.

IO, ADC, DAC, power supply, keypad, ...

μC or μP (4–32 Bit)

Display controller

Display

. Fig. 1 Fundamental embedded systems (several functions can be integrated into a single IC)

Embedded Systems: Fundamentals

4.4.1

. Table 1 Overview of typical display categories in terms of resolution and interfacing Low-resolution graphics displays

High-resolution graphics displays

96
64 to 240
128, 320
240 (QVGA)

320
240 to 1280
768

Display

Segment type

Resolution

32
4 to 96
4

Pixel shape

‘‘Free’’ as defined by Square (Matrix) electrode layout, often segmented numbers

Gray levels and color

Black/white

Black/white, some with 6 to 8-bit gray scale and color gray levels and color

Panel interface (see > Part. 4.3)

Multiplexed LCD voltage

TTL parallel

TTL ( VGA), LVDS ( VGA)

Display controller In microcontroller (see > Fig. 3)

In panel

Graphics controller (can be integrated in microcontroller)

Data rate

Low

Low

Real-time data to display

Yes

Applications

Clock, temperature meters

Square (Matrix), color

High Yes

Status visualization, Man–machine interfaces, small-sized (mobile) multimedia devices with user input

● Low-resolution displays like character and graphics displays are typically monochrome. Most character displays have 2 lines with 20 characters with 5
7 font size and cursor lines resulting in a resolution of 100
16 = 1,600 pixels. Low-resolution graphics displays have typically a built-in display controller with 8-bit parallel interface and resolutions from 96
64 through 240
128 to 320
240 (QVGA, some with color). ● High-resolution graphics displays start at a screen size of about 3.500 and QVGA resolution. These panels rarely have a display controller integrated into the panel electronics, so that the data to be displayed must be delivered under real-time conditions (for data rate see > Chap. 4.3.1 formula (1)) to the display input. The critical point when designing embedded systems equipped with a display is the requirement for the display interface: Character- and low-resolution graphics displays usually have a display controller integrated into the panel electronics (see > Chap. 4.1.1). The display controller manages the visualization autonomously without any further interface inputs. This means that the data rate from microcontroller to the display controller can be quite low (or zero). This design approach is limited to slow screen updates but there is no need for the microcontroller to deliver the display data in real time. In contrast, displays without built-in controller-like segmented displays and highresolution displays require a permanent and real time stream of the data to be displayed. The display controller is then either built-in to the microcontroller (the standard for segmented displays; some high-end embedded microprocessors as well). To drive a high-resolution display direct from a microcontroller is practically impossible due the high data rate – an XGA color panel requires 47 Mbit/s for the interface (see > Chap. 4.3.1). As the clock speed of

495

496

4.4.1

Embedded Systems: Fundamentals

. Table 2 Overview of microcontrollers and typical display features Segment controller Microcontroller built-in

Low-resolution graphics displays ( QVGA)

4–8 bit

X

X

8–16 bit

X

X

16–32 bit

X

High-resolution graphics displays ( QVGA)

Graphic Operating libraries system

X X

X

X

microcontrollers is within the same range, it is clear that transferring data from the memory to the output can not be managed in this time. As the corresponding tasks consume over ten assembler instructions, it is clear that even high-clocked microprocessors can not manage this task. Fundamentals of microcontroller interfaces are described, e.g., in [4]. After discussing the interface, the two other important factors in designing embedded systems are the microcontroller bus width (in bits) and the software to program the image on the screen (see > Sect. 4). > Table 2 provides a matrix of microcontroller types vs. display category and software. Four-bit microcontrollers are practically limited to devices requiring only low computing power. Microcontrollers with 8 bits are the professional standard for many control applications. In terms of the memory which can be addressed by an 8-bit microcontroller, this can be at most 64 Kbit. It is clear that this is too low even for a black/white QVGA display: 320
240
1 bit = 75 kbit. On the other hand, it makes little sense to produce a 32-bit microprocessor with huge computing power and to visualize the data on a segmented display. Low-resolution graphics displays (which include in this discussion also character displays) with a built-in display controller require less software effort as at least one font for displaying text is integrated. High-resolution display controllers have usually no ‘‘intelligence’’ built-in, so the content has to be programmed pixel-by-pixel even for characters. As this would be very time consuming and costly in the development phase of an embedded system, one uses graphic libraries or operating systems. Both deliver a software interface for the application software (see > Sect. 4). Summarizing, designing non-PC-based (embedded) systems equipped with a display requires a lot of conceptual work to achieve the optimum approach. However, the design process should start with establishing what data is to be displayed. This results in a display resolution, which then determines the display controller and microcontroller.

2

Microcontrollers with Built-In Segment Driver

For low-cost systems or systems that need only to display simple status information, segmented displays are a good solution. This is the reason that there is practically no 8-bit microcontroller family without a built-in display driver option. Most of these drivers are dedicated to segmented LCDs; however, there are some types that can drive LEDs. > Figure 2 visualizes such a microcontroller driving a segmented clock and meter LCD. The built-in LCD controller is programmed via the microcontroller (mC) and is mostly self-sustaining (it displays data even when the microcontroller is in energy saving mode). More details, including software can be found in > Chap 4.1.1.

Embedded Systems: Fundamentals

4.4.1

COM IO

μC LCD controller Flash memory SEG

. Fig. 2 Typical microcontroller system with built-in LCD outputs for segmented displays

Display (≤ QVGA) with built-in Display controller

μC (8–32 Bit)

Display (≥ QVGA)

Display controller

μC (32 Bit)

Display (≥ QVGA)

Display controller μC (32 Bit) or FPGA

. Fig. 3 Fundamental approaches to implement a display controller: within display (left), separated IC (center) and integrated into a microcontroller (right)

3

Character and Graphics Systems

The next step in user interface and data visualization is character displays, which are able to show numbers, characters, and some simple 5
7 graphic icons. But this is more in terms of the capabilities of the display driver. From the displays point of view, these devices are lowresolution (passive) matrix displays – just another word for graphics displays. Figures, animation, charts, etc., require a further step to higher-resolution graphics displays. However, depending on the display resolution, three fundamental approaches apply for embedded systems as shown in > Fig. 3; some explanations have already been given in > Sect. 2. So we will focus here on new topics: The common basis for all designs is that a display controller is part of the system in order to free the processor from the display interfacing load. But there are three possibilities for implementation of the display controller: in the display itself (left, character and low-resolution graphics displays), a separated version (center) and a microcontroller with built-in high-resolution display driver (right). Displays with built-in controllers like HD44780 for characters and T6963 for lowresolution graphics including software aspects are presented in > Chap. 4.1.1. The separate display (graphics) controller will be described more in detail in > Chap. 4.4.2. The topics mentioned there are also relevant for the all-in-one solution (> Fig. 3, right), which is described below and in > Sect. 4 in terms of software.

497

498

4.4.1

Embedded Systems: Fundamentals

> Figure 3 provides also a visualization of embedded PC-based systems: The approach with a separated display controller (center) equals a traditional PC system with processor and graphics adapter card. The right side symbolizes the latest (2010) approach to integrate the display adapter into an embedded-optimized X86-processor such as the INTEL ATOM in order to save space and cost. The block diagram of a microprocessor with built-in display (graphics) controller is visualized in > Fig. 4: The microcontroller (here Central Processing Unit because this is established in such diagrams) is the heart of the system and processes inputs and outputs. The data to be displayed are written by the CPU via the memory controller into the dual ported video RAM (Random Access Memory). The Display controller reads the data from video RAM (abbreviation: VRAM) and sends the data in the right interface format in real time to the display; this frees the CPU from display driving (except for video-like content). For industrial applications, the CPU must only update those ‘‘pixels’’ in the video RAM that have to be changed. An example is a visualization mask with text and numbers that represents measurement data. Only the latter have to be ‘‘redrawn’’, not the mask, etc. The features of the connected display-like resolution have to be set by the CPU software via the system bus to the display controller registers. With FPGAs (field programmable gate arrays, [5]) one can realize a display controller (> Fig. 3, center) as well as a whole system-on-chip (> Fig. 3, right) by ‘‘software cores’’; details are presented in > Chap. 4.4.3. Furthermore, there are many cases where a microcontroller system exists but it needs to be enhanced with a higher-resolution display. However, the system’s microcontroller is often not able to drive such a display. The solution is an ‘‘intelligent display’’ that eases such an upgrade or the development of a new system. Such a device combines a display with resolutions in the XGA range, a microprocessor and a (built-in) graphics controller. Examples are in > Fig. 3 center and right. This device is interfaced typically by an 8-bit parallel interface or a serial interface; both with relatively low data rates. The software of this device has commands like ‘‘printtext’’ or ‘‘drawrectangle’’ (this is also common

Basic functions (RTC, timer, key port, UART, …)

Advanced functions (SD IF, ADC, touch, camera IF, …)

Serial interfaces (I2C, SPI, USB , …)

Display Display controller S y s t e m

Memory

Video RAM

controller

SRAM

b u s CPU incl. cache

. Fig. 4 Microcontroller with built-in display controller connected to display

Embedded Systems: Fundamentals

4.4.1

for graphics libraries, see > Sect. 4) implemented. This ‘‘intelligent display’’ is connected to the system’s microcontroller, which delivers the data to be displayed via these easy-to-handle commands. The advantage is that virtually no knowledge is needed to integrate a high(er) resolution display into a system. However, due to cost, this method is mainly attractive for relatively low volumes.

4

Software for Graphics Displays

Beside hardware, software is an important element – also in terms of R&D cost – of an embedded system. Medium- to high-resolution displays and controllers usually do not support any high-level graphics function; one can just set the gray levels and color of a pixel. As it needs a lot of programming effort with such a basic command set to draw a line or to output a character, graphics libraries or operating systems are mostly used. An overview of both approaches is provided in > Fig. 5: The application software is the program that implements the task or function of the embedded system, normally written in C or C++; an overview of embedded systems programming is provided in, e.g., [6]. Here we will focus on graphics libraries rather than reviewing operating systems (OS, see e.g., [8] and [9]) support for graphics programming. Both approaches assist the application programmer with high-level commands and functions (see below). The main difference between both approaches is that one has to take care of the device drivers for the system including the display driver when using a graphics library (left). So the developer has to deal with many hardware- and software-related topics. In contrast, an operating system (right) handles all of these driver operations, which makes the application programming much easier and faster (if the operating system is properly implemented). However there are two main reasons for using a graphics library instead of an operating system: One is that an operating system needs a more powerful processor for achieving the same computing performance for a given application than the graphics library method due to the operating systems overhead. However, one can regard a graphics library as a rudimentary small-scale operating system (see > Fig. 7). The second is cost, as an operating system is often not free of license fees while the graphics library is mostly free of charge as it is provided by the graphics controller or microprocessor manufacturer in order to promote the product. > Figure 6 provides a block diagram-like visualization of the software structure of a graphics library (center). It is embraced by device drivers to control the hardware of the

Application software

Various device drivers

Application software

Graphics library Operating system Display driver

Hardware

Hardware

. Fig. 5 Software block diagram for graphics library (left) and operating system (right)

499

500

4.4.1

Embedded Systems: Fundamentals

embedded system. As is common in such a software environment, we use the name ‘‘layer’’ in this structure. The application layer software uses an API software interface (Application Programming Interface) for inputs and outputs (including the display). From the point of view of the application software, one can address all layers directly (↕ arrows) as well as each layer addressing its neighbor on a top-down basis (↓ arrows). A graphics library normally consists of a graphic object layer and a graphics primitive layer, so the higher-level (layer) functions call subroutines from the primitive layer; this is described in the following. Typical basic graphics features consist of two different types of layers: ● Graphics primitive layers that draw simple objects like points, lines, circles, text, cursors, rectangles, and polygons by setting pixels on the basis of the display device driver. ● Graphics object layers that display advanced objects such as buttons, checkboxes, sliders, meters, progress bars, windows, and bitmap images, which mainly provide the objects for the user interface [7]. Those objects are based on elements of the graphics primitive layer. The programmer can add self-written objects by writing and calling subroutines. The graphics object layer software is sometimes open source, so one can implement, for example, a 3D-like shape of objects by playing with surrounding gray levels.

Application layer

User message interface (touch screen, keyboard, ...)

Input and output device drivers

Graphics object layer (button, textbox, ...) Graphics library Graphics primitive layer (line, circle, box, ...)

Display device driver layer (draw pixel)

Display driver

Graphics display interface

. Fig. 6 Software block diagram for direct display driving with graphics library

Initialization (boot)

Initialize system

Initialize device drivers

Application software (loop)

Initialize graphics library

Draw and update objects

Get (user) inputs

Process inputs to output

. Fig. 7 Software block diagram for graphics library initialization and usage including user input handling by application software

Embedded Systems: Fundamentals

4.4.1

The last topic presented here is the boot sequence and application software structure of an embedded system equipped with a graphics library (see > Fig. 7). First the most basic system, and then all device drivers (including display driver) as well as the graphics library are initialized (left) Then the main task – the application (right) – is started. As most industrial and professional software controls, for example, a machine or a process continuously, it runs through an ‘‘infinite’’ loop (arrow with direction to the left): Inputs are captured and processed including hardware outputs (e.g., switch a relay) and then the display content is updated. The loop processing can be event based (runs through the loop at maximum speed and checks inputs), forward structured (execution waits for user inputs), or a combination of both. Summarizing, a graphics library acts like a rudimentary operating system, the block ‘‘process user inputs’’ sets and read the microcontroller peripherals other than display and user input devices such as keypad or touch screen.

5

Summary and Directions of Future Research

When designing and optimizing embedded systems, one must consider both hardware and software. Low-resolution displays have dedicated display controllers built-in with functions like character output. High-resolution displays require graphics controllers that are fed by the system’s microprocessor. The only software support at a low-level basis is the setting of a pixel. Therefore, graphics libraries or operating systems are usually used in order to keep the development effort at a reasonable level. Improvements in the future will deal with the integration of advanced graphics controllers into the microprocessor’s silicon. The software support will be improved in terms of easier integration and the addition of more functionality like 3D objects and video (camera and playback); details are described in > Chap. 4.4.2.

References 1. 2. 3. 4.

5.

Catsoulis J (2005) Designing embedded hardware. O’Reilly Media, Sebastopol, CA Labrosse JJ et al (2007) Embedded software. Newnes, Burlington Grodzik R (2008) Universal display book for PIC microcontrollers. Elektor Electronics Publishing, UK Gupta S, Mukhopadhyay G, Chandra S (2010) Embedded microcontroller interfacing. Designing integrated projects. Springer, Berlin Sass R (2010) Embedded systems design with platform FPGAs: principles and practices. Morgan Kaufmann, Burlington

6.

7.

8. 9.

Barr M (2006) Programming embedded systems: with C and GNU development tools. O’Reilly Media, Sebastopol Stone D, Jarrett C, Woodroffe M, Minocha S (2005) User interface design and evaluation (interactive technologies). Morgan Kaufmann, Burlington Sally G (2009) Pro linux embedded systems. Apress, Berkeley Pavlov S, Belevsky P (2008) Windows embedded CE 6.0 fundamentals. Microsoft Press, Redmond

Further Reading Gajski DD, Abdi S, Gerstlauer A, Schirner G (2009) Embedded system design. Modeling, synthesis and verification. Springer, New York

Zurawski R (2009) Embedded systems handbook, CRC Press, Boca Raton

501

4.4.2 Graphics Controllers Andreas Grimm 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504 2 GPU Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504 3 Computer Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505 4 Video Capturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508 5 Display Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.4.2, # Springer-Verlag Berlin Heidelberg 2012

504

4.4.2

Graphics Controllers

Abstract: Graphics operations require quite high data bandwidth and processor performance. As display sizes have increased, it has become more and more necessary to unload graphic operations from the CPU to a coprocessor. A GPU is a dedicated processor which is instructed by the main CPU to perform those graphics operations which would otherwise have been performed on the main CPU. List of Abbreviations: BitBlt, Memory to Memory Graphic Block Transfer; CPU, Controlling Processing Unit; CRT, Cathode Ray Tube; GPU, Graphic Processing Unit; QVGA, Display Resolution, 320  240 Pixel; ITU656/601, Interlaced Video Standard; NTSC, Analogue Television Standard; OpenGL, Open Graphic Library; OpenVG, Open Vector Graphics Acceleration; OpenCL, Open Computing Language; PAL, Analogue Television Standard, 625 Lines, 50 Hz; RGB888, Digital Pixel Format, 24/32 Bit per Pixel; SMPTE, Digital HD Video Standard; SOC, System on Chip; VBO, Vertex Buffer Objects; VGA, Display Resolution, 640  480 Pixel; XGA, Display Resolution, 1024  768 Pixel

1

Introduction

In embedded applications, there is typically one host processor controlling the system. In graphics applications, this brings a big data load on the CPU. Beside the network connection and sensors, the video data are processed, the graphics are rendered, and the display is controlled. A typical embedded graphic system is shown in > Fig. 1. The load on the CPU can be reduced if its workload is split and the graphics part is outsourced to a GPU (Graphics Processing Unit), which is an optimized processor with dedicated algorithms and memory arbitration specifically for graphic operations. In addition, a GPU is practically indispensable for high-end graphics, as these would require a higher clock frequency and performance from the standard CPU if the same algorithms were to run on it in software and achieve the same performance. Originally built as external coprocessor, CPU and GPU became meanwhile integrated into one chip. With the progressing development of the technologies, the CPU, GPU, and the peripheral interfaces have been merged together into one package. As there are many different applications with a wide range of different needs on performance and functionality, there are equally as many various embedded GPUs available on the market. The complexity of embedded graphics applications is steadily increasing, and with increasing display sizes, ambitious animations require a huge amount of processor performance as well as high data bandwidth to the graphics memory. The feature set of an embedded GPU is driven by the requirements of according applications. There are many applications which combine computer rendered graphics with video processing functions. Think of camera or DVD applications in the automotive or industrial segments, where video sequences are overlaid or blended with all types of HMI (HumanMachine Interface) or menu layers.

2

GPU Architecture

Looking under the hood of the package, the basic GPU has a modularized block structure. Here are dedicated blocks for host access, video input, display output and rendering, which have access to the graphics memory as arbitrated by an internal memory controller.

Graphics Controllers

4.4.2

Graphic Application Node 1

Sensors

Node 2

Network

Sound

Multimedia Sources

Display V-IN Power Touch screen

CAM 1

CAM 2

Graphic SoC

Audio Video CD, HDD, SD, USB

V-IN Power

Main Display

2nd Display

Touch screen

Memory

. Fig. 1 Typical embedded graphics application

The graphics memory is the central working space for the GPU and contains various kinds of data – such as instruction lists, VBOs (Vertex Buffer Objects), textures, mask arrays – all of which are involved in different stages of the rendering process and require high bandwidth on the memory interface with effective access control and data management (> Fig. 2). The module that actually does the pixel processing is the rendering unit, which is controlled by instructions that generate and compute the images using operations executed on a raster base or a vector base.

3

Computer Graphics

Raster graphics are data structures that represent a (generally) rectangular grid of pixels, transferred by BitBlt operations from a source to a destination, whereas vector graphics are constituted of vertex-based primitives which are the geometrical representation of an image. In the world of computers, graphics are typically described using a mathematical model which consists of 2D/3D vector data, colors, material textures, and surface normal information [1, 2]. Virtually every complex 3D geometrical object can be described using basic primitives, such as points, lines, and polygons. The rendering process maps the said model to an image on the physical screen using several coordinate systems. One transformation medium is a 4  4 matrix, which translates one coordinate system into another (> Fig. 3). A multilevel transformation sequence converts the vertices from the model view, via the world and camera view to the final device coordinates. The model view is the local view of each independent object. By performing scaling, rotation, and translation actions, the model view of each object is transformed to the world view and sized and moved to its place in the

505

506

4.4.2 Graphic Memory

Graphics Controllers

SOC

CPU

Memory Controller

GPU

Instructions, VBO’s.. Mask (3D, alpha, stencil…) Texture

3D Geometry and Rendering Unit

Display Controller

Layer

LCD

Video

Video Input Controller

. Fig. 2 Internal GPU structure and data flow

Triangles, Lines, Polygons

API

Vertices Primitive Transform Processing & Lighting

Vertex Buffer Object

Primitive Assembly

Rasterizer

Texturing

Colour Sum

Alpha Test

Depth, Stencil

Fog

Colour Buffer Blend

Dither

Frame Buffer

. Fig. 3 Fixed rendering pipeline

virtual 2D/3D world. Camera view is a dedicated perspective built into the model which is adjacently transformed to the device coordinates. During this step, ‘‘culling’’ removes back facing primitives and ‘‘clipping’’ trims primitives to the screen size in order to reduce the amount of data that needs to be processed [1–3]. Following these steps, the rasterization process transfers the final vector data to a raster image before the color is set depending on the attributes. The simplest form of processing is the flat shading mode which fills the primitive using a solid color.

Graphics Controllers

4.4.2

. Fig. 4 3D mesh, flat and gouraud shading

The smooth shading mode inserts a color gradient on the primitive surface which is interpolated between the colors of the different vertices (> Fig. 4). More detailed and realistic graphics can be generated using texture mapping, a method which uses a separate texture definition held in graphics memory which is then mapped to the primitive surface. A texture is usually thought of as being two dimensional and can be a rendered graphic, a photo, or the frame of a video stream captured by the video input. Video textures can be used in real time without any recognizable latency. Any of several possible functions can be chosen for computing the final RGB value of a fragment. One possibility is simply to use the texture color as the final color (‘‘replace mode’’) whereby the texture is simply painted over the primitive. Another way is to use the texture to modulate, or scale the fragment’s color, which is useful for combining lighting effects with texturing. Finally, a constant color can be blended with that of the fragment, based on the texture value. Artifacts and distortions caused by aliasing processing are reduced with dedicated algorithms for filtering and perspective correction. An enhanced type of texture mapping is mip-mapping. This was introduced to reduce the load of required data. As there are so many possibilities, texture mapping is a fairly large and complex subject, and because much data is read from, and written to memory, it exerts a heavy load on the memory interface. After the source color has been defined by shading or texturing as described above, the source color can be combined with the destination color. Furthermore, the rendering operations can be linked with blending or logical operations for special transparency and color effects. Blending – the OpenGL term for transparency calculations – uses the equation C = CS∗S + ∗ CD D for defining the final color, whereby CS and CD are the source and destination colors, respectively, and S and D are determinable weighting factors. A typical use case for transparency is anti-aliased font rendering with any type of background layer. The complete geometry and rendering process is controlled by the display lists. Such lists consist of the binary code for rendering commands, rendering attributes, configuration data, vertices, and VBOs, and they can be located in CPU memory or even better in the graphics memory.

507

508

4.4.2

Graphics Controllers Triangles, Lines, Polygons

API

Vertices Primitive Vertex Processing Shader

Vertex Buffer Object

Primitive Assembly

Depth, Stencil

Rasterizer

Colour Buffer Blend

Fragment Shader

Dither

Frame Buffer

. Fig. 5 Programmable rendering pipeline

Today’s high-end GPUs have a programmable rendering pipeline and support the standard interfaces like OpenGL ES 2.0 (> Fig. 5). The OpenGL standard (which has its origins in PCbased applications) has been introduced into the embedded field too and is an indispensable requirement for such applications [4, 5]. A programmable pipeline makes it possible to implement use-case specific operations on the vertices and fragments. The pipeline structure can be configured in the integrated shader unit to enable high-level geometry and color algorithms.

4

Video Capturing

There are two ways to transfer frame to the graphics memory: either by computing the graphics locally using the GPU or by reading them in from external devices. Many GPU incorporate dedicated interfaces which are for capturing video streams in ITU 601/656, RGB888, and SMTPE 296 M formats. In many video applications, the captured video frame cannot be used directly in further processing, such as placing it on the display as a dedicated layer or using it as source data for further rendering operations. Further pretreatment of the data (such as de-interlacing, scaling, converting the frame rate or colors) might be necessary and is conducted by the internal capture module when a video frame is written to memory. For historical reasons, many video devices output in interlaced video formats (usually in NTSC or PAL) even today. Interlacing a frame is a form of lossy data reduction done by splitting a frame into half frames on an odd/even line basis. This method was introduced at a time when a technical solution was required for TV applications for CRT (Cathode Ray Tube) monitors which had limited bandwidth. De-interlacing became necessary because modern TFT-LCD (Thin Film Transistor-Liquid Crystal Display) and plasma displays can only be driven with progressive scan data (i.e., each line is input from the top to the bottom of a frame with no regard to odd/even lines). Basic de-interlacing methods named ‘‘BOB’’ and ‘‘WEAVE’’ are implemented in a GPU, each with their own advantages for different video scenes. BOB mode de-interlacing works using vertical interpolation, whereas WEAVE mode de-interlacing merges two half frames but does not provide sufficient quality in every case.

Graphics Controllers

4.4.2

Better quality provides motion detection by blending and extending subsequent frames but with requiring enormous higher bandwidth and performance. Scaling is used to change the size of a captured input video frame. Scaling operations are necessary in situations where the sizes of the source frames and those of the target device are different, e.g., a video stream supplied from a DVD in PAL format (720  576 at 25 Hz) is processed and finally shown in full screen mode on a VGA display with a 50 Hz frame rate. Another example is PIP (picture-in-picture) applications, where the video is displayed in a smaller dedicated window on the target screen. Filter algorithms are applied that generate the scaled image with the right quality. Refer to > Chaps. 4.5.1 and > 3.3.3 for more details of scaling and de-interlacing techniques. During frame-rate conversion, the video input is synchronized on a per-frame basis with the display controller. The video buffer address management is synchronized in a specific ratio between the input and output. The upscaling of a video frame is also done using the display controller with the advantage of lower memory consumption and required interface bandwidth. Memory is only used for the original size, and the upscaling is done in the next step when the video frame is transferred from the memory to the display. Besides interacting in these ways with a video input, the main task of a display controller is to generate the complete output signaling for a display device. Via two individual multiplexed outputs it is possible to drive up to four display panels using a maximum of two separate timing parameter sets for dual view features.

5

Display Output

The display controller needs to be flexible as far as driving different kinds of display panels is concerned. An output timing is a complex combination of many parameters which include the definition of the size of the visible area, the total size, and the size and position of the vertical/ horizontal sync pulses. As each parameter can be set individually, the display controller can output timings to match the requirements of different display panels. The layer handling is a further function of the display controller. This technology allows the allocation of separate memory areas for different kinds of graphic information. A layer is a defined space in memory which contains pixel color or alpha element information which can be handled by the display controller. The implementation of layer technology has a huge advantage for increasing performance and reducing the required bandwidth of a system, because information updates then do not require the complete screen to be redrawn but just those layers and areas affected. The logical and physical size of a layer can be different. Typically, embedded GPUs have a maximum logical layer size of 4096  4096 pixels, but for today’s HD format, the physical layer size reaches their maximum with 1980  1080 pixels. Up to eight layers, each of which is individually programmable in size and position, can be used for overlay and blending operations. A simplified method for transparency effects is used at the display output which calculates the final pixel color using the equation C = Ci ∗A + (1-A)∗Ci+1, whereby Ci+1 is the color of layer (i) which is overlaid with the color of layer (i + 1).

509

510

4.4.2

Graphics Controllers

At the end of the pipeline, the final result of eight overlaid and blended layers can even be fed back into memory. Of course, the screen size, number of layers, the drawing, and display frame rates need to be handled within the restrictions of the available memory bandwidth.

6

Summary

There is a strong trend in the embedded electronics to more and more standardization. Where in low end fields proprietary architectures are still widespread, there is strong demand for standardized interfaces and architectures at the high-end level. Having standardized interfaces means for the user a higher flexibility in using devices from different vendors and reusing application software which obviously reduces development costs and shortens the time to market. The integration process will continue in the semiconductor industry. The time of standalone processors is over in embedded applications. Future devices will have more performance and features which are provided by scalable SoC with single- or multi-core processing units. OpenGL, OpenVG, and OpenCL which are the graphic standards of today will be continued, enhanced, and extended [4, 5].

References 1.

2.

3.

Shirley P, Marschner S (2009) Fundamentals of computer graphics, 3rd edn. A K Peters/CRC Press, Natick/Boca Raton Foley JD, van Dam A, Feiner SK, Hughes JF (1990) Computer graphics: principles and practice. Addison Wesley, Reading, 2nd revised edn Watt A (1999) 3D computer graphics, 3rd edn. Addison Wesley, Reading

4. 5.

Hearn D, Baker MP (2003) Computer graphics with openGL, 3rd edn. Prentice Hall, Englewood Orlamuender D, Mascolous P (2004) Computergrafik und OpenGL: Eine systematische Einfu¨hrung. Fachbuchverlag Leipzig

Further Reading Akenine-Mo¨ller T, Haines E, Hoffman N (2008) Realtime rendering, 3rd edn. A K Peters/CRC Press, Natick/Boca Raton Encarnac¸a˜o J, Straßer W, Klein R (1996) Graphische datenverarbeitung I. R Oldenbourg Verlag, Mu¨nchen

Rogers DF (1997) Procedural elements for computer graphics, 2nd edn. McGraw-Hill, USA

4.4.3 FPGA IP Cores for Displays Davor Kovacˇec 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512

2 2.1 2.2 2.2.1 2.2.2 2.3 2.3.1

FPGA Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 What is FPGA? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 FPGA Market Landscape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 Low-Cost FPGA Devices for Display/Video Applications . . . . . . . . . . . . . . . . . . . . . . . . . 515 High-End FPGA Devices for Special Display/Video Applications . . . . . . . . . . . . . . . . . 515 FPGA – Gaining Semiconductor Market Share . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516 GDC Implementations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517

3 3.1 3.1.1 3.1.2 3.1.3 3.2 3.2.1 3.2.2 3.3 3.4 3.4.1

FPGA GDC IP Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517 Multilayer Video Controller IP Core (Frame Buffer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 Display Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519 Multi-layering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519 Layer Content Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 520 Frame Grabber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 520 Video Stream Formats and Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 Frame Grabber Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 Multiport Memory Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 Graphic Accelerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524 3D Animation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524

4

How to Build the Best FPGA GDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526

5 FPGA GDC Design Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 528 5.1 Design Flow for Low- and High-Volume Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 528 5.1.1 Configurable FPGA GDC and Impact on SW Development . . . . . . . . . . . . . . . . . . . . . . 529 6

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.4.3, # Springer-Verlag Berlin Heidelberg 2012

512

4.4.3

FPGA IP Cores for Displays

Abstract: During the last decade the advance in a Field Programmable Gate Arrays (FPGAs) reached the point where complex computer interfaces such as FPGA-based Graphic Display Controllers (GDCs) is feasible and competitive. The inherent programmability of FPGAs allows for design of GDCs which provide unprecedented flexibility through integration of optimal GDC’s feature sets. The feature sets can be set up at the hardware level by IP cores that support certain GDC features. The FPGA silicon, the IP cores, and user friendly FPGA implementation design tools are key ingredients for FPGA GDC development. Diverse graphic applications demand different FPGA GDC configurations, and system tradeoffs are illustrated by several examples. The tradeoffs are mainly driven by GDC requested memory bandwidths and feature sets. FPGA design flows for low- and high-volume applications are proposed. Configurable FPGA GDCs also impose new design challenges on Embedded GDC SW drivers’ development. List of Abbreviations: ASIC, Application Specific Integrated Circuits; ASSP, Application Specific Standard Products; CVBS, Composite Video Broadcast Standard; DDR DRAM, Dual Data Rate Dynamic Random Access Memory; DLL, Digital Locked Loop Clock Generator; FPGA, Field Programmable Gate Array; GDC, Graphic Display Controller; HDL, Hardware Description Language; HMI, Human Machine Interface; IP, Intellectual Property; LVDS, Low Voltage Differential Signals IO Standard; MAC, Multiple Add Accumulate; PCIe, Peripheral Component Interconnect Express; PLL, Phase Locked Loop Clock Generator; SoPC, Systemon-Programmable-Chip; SRAM, Static Random Access Memory; UMA, Unified Memory Architecture; USB, Universal Serial Bus

1

Introduction

During the last decade the advance in a Field Programmable Gate Arrays (FPGAs) reached the point where complex computer interfaces such as FPGA-based Graphic Display Controllers (GDCs) were feasible and competitive. The inherent programmability of FPGAs allows for design of GDCs which provide unprecedented flexibility through integration of optimal GDC’s feature sets. The GDC’s feature sets can be set up at the hardware level by combining various IP cores that support certain GDC features. The IP cores are predesigned and pre-verified functional blocks which can be combined within the Systemon-Programmable-Chip (SoPC) FPGAs. A number of used IP cores for a single GDC SoPC design directly influences the consumption of available silicon resources within the FPGA device, and consequently, the cost of the final FPGA based GDC. Precisely tuned GDC’s feature set requires less silicon resources and allows for cost reductions. Besides the FPGA silicon and the IP cores, user friendly FPGA implementation design tools are key ingredients for a successful and productive FPGA GDC development. Diverse graphic applications demand different FPGA GDC configurations, and system tradeoffs are illustrated by several examples. The following paragraphs explain the design tradeoffs, which are mainly driven by GDC requested memory bandwidths and feature sets, and FPGA design flows for lowand high-volume applications. The configurable FPGA GDCs impose new design challenges on Embedded GDC SW drivers’ development that play a crucial role in development of graphic applications.

FPGA IP Cores for Displays

2

FPGA Technology

2.1

What is FPGA?

4.4.3

FPGAs (Field Programmable Gate Arrays) are semiconductor devices that contain programmable logic gates and interconnections. The FPGAs are generic silicon devices with undefined functionality and, as such, the same devices can be used in versatile applications. The programmed functionality can be defined by IP cores. Today the most FPGAs are volatile devices, i.e., the interconnection configuration and logic gate equations are stored in the internal FPGA SRAM cells. Non-powered SRAM–based FPGA does not contain configuration data which must be stored in a non-volatile memory device within a system and loaded into the FPGA at the power up (> Figs. 1, > 2, and > 3). Modern FPGAs are powerful silicon devices which can implement complex designs. Complex GDCs can run at 100 MHz clock speed in low cost FPGAs, and up to 300 MHz clock speed in high-end FPGAs. The newest FPGAs are much more than an array of gates. They include advanced architecture blocks (silicon hardened IP cores), such as internal SRAM memory blocks, hardened MAC (Multiple Add Accumulate) DSP blocks, multipliers, PLL or DLL Clock generators, versatile IO cells supporting almost any IO logic standard and high-speed serial I/Os (LVDS serializers/de-serializers, Gigabit transceivers, etc.). In addition to that, the FPGAs include hardened super macros like the microprocessor cores, PCI Express endpoints, Ethernet MACs, DDR controllers, etc. The advanced architecture blocks achieve similar or better performances than the equivalent blocks in ASSP or ASIC devices, and additionally benefit all the flexibility of the FPGA.

Logic block

Interconnection resources

I/O cell

Logic block

Logic block

Programmable switch matrix

Logic block

. Fig. 1 Interconnection

Logic block

513

514

4.4.3

FPGA IP Cores for Displays

H1

DIN

S/R CE S/R control

G4 G3

Logic function of G1-G4

SD G'

G2

M2

G1

Logic function of F',G' and H1

M3

CE

M4

H'

RD

1

G

F4 F3

Q2

D

M1

Logic function of F1-F4

S/R control F'

SD

M5

D

Q1

F2 M8

F1

M6 M7

K (Clock)

CE RD

1

F

. Fig. 2 Logic gate configuration

D

Q 0 1

D

Q 0 1

Q

. Fig. 3 IO block

D

PAD

FPGA IP Cores for Displays

2.2

4.4.3

FPGA Market Landscape

The FPGA market is dominated by Xilinx and Altera as the biggest FPGA vendors. Xilinx [1] holds more than 50% of the market share, Altera [2] 35%, Lattice 7%, Microsemi 6%, while the other FPGA vendors share the remaining few percentages of the market. All FPGA vendors offer multiple FPGA families, which can be roughly divided to low-cost and high-end families.

2.2.1

Low-Cost FPGA Devices for Display/Video Applications

Xilinx low-cost FPGA families are marketed under the name Spartan [1]. The Spartan-6 is the latest and the most advanced Xilinx’s low-cost FPGA family. The Spartan-3E and the Spartan3A families are still recommended as cost-effective solutions for new design starts. Xilinx has announced 7 Series families named Artix, Kintex, and Virtex, respectively low-, mid-, and high-end families, which will have unified architecture (> Table 1). Altera’s low-cost FPGA families are Cyclone III and IV [2].

2.2.2

High-End FPGA Devices for Special Display/Video Applications

Xilinx high-end FPGA families are marketed under the name Virtex. The Virtex-6 is the topend family with subfamilies focused on high-speed logic, high performance DSP blocks, and high speed and high bandwidth serial links. ZYNQ is entirely new FPGA family on the market, combining dual core ARM Cortex A9 processor subsystem tightly integrated with FPGA fabrics using high bandwidth buses [1] (> Table 2). Older Xilinx’s Virtex-5 family, in addition to logic, DSP and high speed serial links, provides hardened PowerPC processors running at 550 MHz. Altera’s high-end FPGA families are Stratix V and IV [2].

. Table 1 The following table shows Spartan-6 products and resources Configurable logic blocks (CLBs) Device

Logic cells

Slices

Max distributed RAM (Kb)

DSP48A1 slices

Block RAM blocks (18 Kb)

Max user I/O

XC6SLX4

3,366

526

32

4

8

120

XC6SLX9

9,101

1,422

90

16

32

200

XC6SLX16

14,579

2,278

136

32

32

232

XC6SLX25

23,770

3,714

228

38

52

264

XC6SLX45

43,456

6,790

401

58

116

370

XC6SLX100

101,005

15,782

975

182

268

498

XC6SLX150

147,456

23,040

1,358

182

268

498

515

516

4.4.3

FPGA IP Cores for Displays

. Table 2 The following table shows Virtex-6 products and resources Configurable logic blocks (CLBs) Logic cells

Device

Slices

Max distributed RAM (Kb)

DSP48E1 slices

Block RAM blocks (18 Kb)

Max user I/O

XC6VLX75T

74,496

11,640

1,045

288

312

360

XC6VLX130T

128,000

20,000

1,740

480

528

600

XC6VLX195T

199,680

31,200

3,040

640

688

600

XC6VLX240T

241,152

37,680

3,650

768

832

720

XC6VLX365T

364,032

56,880

4,130

576

832

720

XC6VLX550T

549,888

85,920

6,200

864

1,264

1200

XC6VLX760

758,784

118,560

8,280

864

1,440

1200

XC6VSX315T

314,880

49,200

5,090

1,344

1,408

720

XC6VSX475T

476,160

74,400

7,640

2,016

2,128

840

2.3

FPGA – Gaining Semiconductor Market Share

FPGA designs are gaining market share in favor of ASIC design starts and make a viable alternative to ASSP-based designs. There are multiple reasons which dictate such reshape of the semiconductor market. FPGAs make very competitive silicon solutions for low- and mid- to high-volume products due to lower costs of silicon devices, and shorter and lower-cost design cycles. The predefined and validated FPGA modules, such as clock generators, clock trees, RAMS, and IO cells significantly shorten the design cycle. ASIC design flows require full validation of such blocks and much more thorough verification of the whole design. Incurred costs due to longer design cycles and complex and expensive prototyping significantly raise development costs. Longer developments result by delayed market hits, which may have serious impact on overall product’s success and profitability. FPGAs are used in low-, mid- and mid- to high- volume products. At the production scale’s end are the FPGA designs manufactured in volumes of million pieces. For an example, the production volumes of hundreds thousands per year, over the life time span of 5–7 years, are pretty common in the automotive market. The ASIC advantages over FPGA are higher system clock speeds, mixed analog/digital logic, low-power, and lower cost per device for high volume production. Even though the FPGAs successfully overcome their limitations, such as slower system clock speeds in comparison to ASIC clock speeds, in order to be comparable with the high-end embedded processors running at hundreds of MHz or GHz, the FPGAs must integrate siliconhardened microprocessors. Inherent FPGA parallelism increases performances, diminishes slower clock speed impacts, and may provide advantages over sequentially based ASIC and ASSP devices (processors). Some FPGA vendors attempt to integrate analog circuitry and low power process for FPGAs, but today’s mainstream applications usually utilize external analog front-ends and external microprocessors to manage FPGA power. The FPGAs are the preferred choice in applications where single ASSP designs are falling behind.

FPGA IP Cores for Displays

2.3.1

4.4.3

GDC Implementations

Application Specific Standard Product (ASSP) Graphic Display Controllers (GDC) implementations are the most common today (see > Sect. 3 of Chap. 4.4.1). However, FPGA GDC implementations are gaining ground. The ASSP/ASIC silicon process provides higher clock speeds and analog interfaces, while FPGA technology provides higher flexibility. ASSP GDCs interface to the host processor is usually through relatively slow parallel bus. The FPGAs due to the HW re-programmability assures more efficient interconnection and more interconnection choices, such as PCIe, USB, or different flavors of parallel or serial buses. Additionally, FPGA GDCs commonly use unified memory architecture (UMA), i.e., shared memory between host processor and GDC. UMA architecture assures better performance since there isn’t CPU to GDC data copied. ASSP GDC supports usually only one external memory device. Limited memory device support results in lower memory bandwidth. Since the memory bandwidth is crucial to the graphic display controller operation, lower memory bandwidth is clearly a disadvantage. The FPGAs’ external memory device selection is flexible, so newest memory families can be supported quickly and highest memory bandwidth can be achieved. Due to the shorter FPGA development cycle, new graphic acceleration and processing algorithms, as well new interfacing standards can be supported quicker than within ASSPs. FPGA flexibility and shorted development cycle is an advantage in new and non-standard applications where ASSP would not be economically or technically feasible. GDCs with more than two display outputs or more than one video inputs are the perfect candidates for FPGA designs. Underutilized ASSPs silicon devices are very good candidates for replacement by the FPGA silicon. The cost of the ASSP silicon is fixed, and designers pay for all ASSP features including the unwanted ones. The FPGA designers can integrate only the features (IP cores) needed for the targeted application, and consequently use smaller and lower-cost FPGA silicon. Today’s low-cost FPGAs, such as the Xilinx Spartan-6, are ideal for implementation of wide range of GDCs. The cost of the Spartan-6 silicon implementing a simple frame buffer display controller and 2D graphic acceleration controllers or a video input display controller is in the range of $3–15 in 50 K + volumes.

3

FPGA GDC IP Cores

The Graphic Display Controllers [3] commonly require multiple IP cores for the following functions: – – – –

Frame buffer or Multi-layer Video controller Frame Grabber IP core Multi-port Memory controller Graphic accelerators, such as 2D, 2.5D, 3D

In addition to these most common functions, digital image processing functions like video codecs, such as the MPEG, H264, MJPEG or others, may be included as parts of the GDC (> Fig. 4). FPGA IP core market is very fragmented. Relatively small number of IP cores is really optimized and extensively validated for use in FPGAs. A selection of FPGA-optimized IP cores can be found within FPGA designing tools provided by FPGA vendors, while the other IP cores come from third-party IP vendors. Non-optimized IP cores do not utilize the FPGA

517

4.4.3

FPGA IP Cores for Displays

Xilinx spartan 6 FPGA logiWIN versatile video input

logiCVC-ML LCD controller

logiCVC-ML LCD controller

logiWIN logiWIN versatile video versatile input video input

LVDS/SDVO receiver

LCD touch

LCD touch LCD touch

LCD controller

XPS_IIC

logi3D 3D accelerator 2,5D accelerator 2D accelerator

logiMEM flexible SDR/DDR memory controller

DVI/CVBS decoder

SDR/DDR2/3

DDR2

ONboard IC

XPS_TIMER XPS_INTC

RS232

XPS_UARTLITE

XPS_SPI

Multi I/O SPI flash

Audio codec

logiI2S

SD_HOST

SD card

MicroBlaze RISC CPU

User/ customer inerface

AXI

518

Digital

Memory

Mixed signal

CPU

FPGA

XILINX IP

logic BRICKS IP

3RD party IP

. Fig. 4 FPGA GDC block schema

silicon in ways that assure the highest possible performances and require bigger and more expensive FPGA devices. Optimized FPGA IP cores allow for an additional configurability that decreases silicon costs and makes them easy to use. The GDC IP cores are available from FPGA vendors, but a few third-party IP core vendors provide better IP cores. The third-party vendors are usually more focused on specific IP cores than the FPGA vendors, and their GDC IP cores have richer feature sets and use less FPGA silicon resources. The third-party IPs are exhaustively tested and delivered with SW drivers for popular operating systems (OSs). Popular IP portals such as Design and Reuse [4] or ChipEstimate.com [5] list several GDC IP core vendors who optimize the IP cores for use in FPGAs.

3.1

Multilayer Video Controller IP Core (Frame Buffer)

FPGAs usually do not provide enough memory for on-chip frame buffer that stores the rendered graphic for a display, and must use external memories for its implementation.

FPGA IP Cores for Displays

4.4.3

HSYNC DE VSYNC R0-7 G0-7 B0-7 VFP

VS

VBP

Vertical resolution

VFP

. Fig. 5 Clk, VSync, DE, HSync, R0-R7, G0-G7, B0-B7 timing

The core of the display controller is DMA engine that reads image stored in the external memory and sends it to the display (see > Sect. 3 of Chap. 4.4.2). In addition to data transfers, the display controller generates necessary Vertical and Horizontal synch and other display timing control signals, i.e., the Display Enable signal that marks active (visible) display data. The image may be stored in different formats, such as the most common RGB format and YUV (YCrCB). The most popular display data interface is the digital RGB interface (see > Sect. 3 of Chap. 4.3.1) (> Fig. 5).

3.1.1

Display Interface

Well-designed and configurable IP cores for display control can support different display interfaces besides the digital RGB interface. Larger or remote displays may use a single or multiple serial LVDS interfaces (see > Chap. 4.3.2). The LVDS interfaces serialize parallel RGB digital bus and send it through low-voltage swing LVDS wire pairs. FPGAs support the majority of open standard LVDS serializers, which include different LVDS serialization types and data coding. Lower cost displays may require Timing Controller Circuit (TCON) to be integrated in the GDC. Display monitors use the DVI/HDMI and CVBS (PAL/NTSC/SECAM) interfaces. There are also new and emerging interfaces such as the SDVO or the DisplayPort interface (see > Sect. 4.3 of Chap. 4.3.2). The DVi/HDMI, SDVO and DisplayPort are serial interfaces which serialize and code digital RGB data stream. CVBS interfaces are analog standards and require external CVBS (D/A) encoder device sourced by the FPGA GDC’s digital interlaced output coded in the ITU656 format.

3.1.2

Multi-layering

Multi-layer GDC grabs images (layers) from multiple positions in the frame buffer memory, blends them together, and sends the resulting image to the display (> Fig. 6). There are two most common usage models of the multi-layering: – Video display on one layer with the overlaying graphic content on the second layer – Graphic animation by blending of multiple layers usually used in low cost embedded display systems

519

4.4.3

FPGA IP Cores for Displays

Layer 0 width Hres

Background layer Layer 2 Layer 1

0h

eig

ht

Vre

s

Lay er 0

pos y

Layer 0 pos x

Lay er

520

Layer 0

. Fig. 6 Multilayering

Each FPGA GDC layer may be configured differently. The configurable parameters are the color depth (8bpp with CLUT, 16 bpp, 24 bpp, 32 bpp, etc.), data format (RGB, ARGB, YUV, etc.), blending method, size, and position. The blending method may be set to CLUT alpha, pixel alpha, and layer alpha. The pixel alpha blending is the most advanced blending method which requires a unique alpha value for each displayed pixel. The layer alpha blending method used the global alpha value for all layer pixels, which makes it easier for implementation. Some GDCs may have programmable blending layer order. The resulting pixel of blending operation between two layers is: Display_Pix[x,y] = L0_Pix[x,y] ∗ alpha + L1_Pix[x,y] ∗ (1-alpha) The alpha value in a range from 0 to 1, determines weight factors of L0 and L1 pixels in the resulting output image. If three layers are used, the intermediate result of Layer2 and Layer3 blending is blended with Layer0. Multiple layer blending follows the same pattern (> Fig. 7).

3.1.3

Layer Content Synchronization

Video streaming layer must be synchronized with the display’s refresh rate to avoid flickering and image tearing artifacts. The synchronization is accomplished by dual or triple frame buffering per layer. Dual buffering is used when video streaming and display have the same frame rate, while triple buffer is used for different frame rates. Multiple frame buffers separate write and read operations from GDC layers, i.e., while the Video Frame Grabber or the 2D graphic accelerator writes data to one layer’s buffer, the display controller reads the image from another buffer. The buffer switch is triggered by the Vsync signal (> Fig. 8).

3.2

Frame Grabber

Frame grabber IP core processes incoming video stream and stores it to frame buffers implemented in video memory. The display controller reads the captured video from frame buffers and sends it to the display (> Fig. 9).

4.4.3

FPGA IP Cores for Displays

1. Layer alpha, Pixel = xRGB

Layer 0 a = 50%

Layer 1

2. Pixel alpha, Pixel = αRGB α = 100%

α = 0% Layer 0

Layer 1

3. CLUT indexed, Pixel = Index Pix index 0 Pix index 134 Pix index 2

Layer 1

Layer 0

aRGB (a = 100%) aRGB aRGB (a = 0%)

Index 0 Index 1 Index 2 : Index 134 : Index 254 Index 255

:

aRGB (a = 50%) :

aRGB aRGB

. Fig. 7 Layer alpha, pixel alpha, Color LUT alpha

logiCVC-ML triple buffer cycle

CVC frame period

logiCVC-ML

1

0

SW_VBUFF CURR_VBUFF

Video input

0

1

0

1

1

2

0

1

1

2

NEXT_VBUFF

0 Video input frame period

. Fig. 8 Triple buffering

2

0

1

2

Video input triple buffer cycle

0

1

2

0

1

521

MEMORY

MIXED SIGNAL CPU

FPGA

User/Customer Inerface

MicroBlaze RISC CPU

. Fig. 9 Frame grabber FPGA GDC block schematics

DIGITAL

XPS_SPI

logiCVC-ML LCD Controller

XPS_UARTLITE

XPS_TIMER XPS_INTC AXI

XILINX IP

logicBRICKS IP

Xilinx Spartan 6 FPGA

3RD PARTY IP

Multi I/O SPI flash

SDR/DDR2

LCD TOUCH

4.4.3

RS232

DVI/CVBS Decoder

logiWIN Versatile Video Input logiMEM Flexible SDR/DDR Memory Controller

522 FPGA IP Cores for Displays

FPGA IP Cores for Displays

3.2.1

4.4.3

Video Stream Formats and Converters

The video input formats are similar to display formats: digital RGB, LVDS, DVI/HDMI, CVBS, or others. Most of the digital formats are progressive and use fast serial links. External converters or FPGA IP cores convert input video to the digital RGB. The CVBS video streams can be converted by A/D CVBS decoders to digital ITU656 interlaced streams formatted in the YCrCb format. Frame grabbers usually de-interlace stream, i.e., converts it to the progressive video stream. The Bob or the Weave de-interlacers are commonly used in FPGA IP cores. Other motion-adaptive de-interlacers may provide better results, but require more FPGA silicon, higher memory bandwidths and introduce longer frame latencies.

3.2.2

Frame Grabber Processing

Video stream’s resolution can be scaled up and down in a real time. Video scaling without advanced interpolations decreases video quality. Good FPGA GDCs use single or cascaded bi-linear or cubic interpolations to preserve all input image details. Poly-phase or other advanced interpolations may be used for better image quality, but induce additional costs in terms of the FPGA size, memory bandwidth, and frame latencies. In addition to the scaling, the video input can be cropped or stencil masked before buffering.

3.3

Multiport Memory Controller

The GDC’s frame buffers are usually implemented in external DDR memories. The storage capacity is an important design parameter, but the optimal use of the DDR memory bandwidth is a key of high GDC performances. Today’s DDR2 and DDR3 memories combined with low cost FPGA families provide up to 800 Mbytes/s when used with the 8-bit data interface. Wider DDR data interfaces provide more memory bandwidth, i.e., 32-bit memories provide up to 3.2 GB/s. Use of the DDR memory is efficient when data are accessed sequentially and in blocks (data bursts). Common burst access consists of memory bank opening, data access at every clock edge, and optionally, memory bank closing. The memory bank should not be opened if consequent data are accessed from the same bank, which results by shorter and faster memory cycles. The DDR2 and DDR3 memories are designed for maximum efficiency and support multiple simultaneously opened memory banks (four or more). High memory bandwidth, or the data throughput between the FPGA and the memory, can be shared by different video IP cores, CPUs, and other IP cores. Multi-port memory controller IP Cores, which are able to interface different DDR types, assure efficient memory bandwidth sharing. Different GDC IP cores, such as e (multilayer) Frame buffer IP core, Frame grabber and 2D–3D graphic accelerators, share common memory by means of the multi-port memory controller. Priority, Round-robin or other arbitration policies, implemented in the memory controller, assure seamless dataflow without stalls or starvation, and a flicker-free image display.

523

524

4.4.3 3.4

FPGA IP Cores for Displays

Graphic Accelerators

Graphic accelerators speed up and off-load CPUs creating graphic animations. The graphic accelerators span from the basic 2D to high-end 3D accelerators. They are used for rendering of graphics user interfaces (GUIs) of different complexities, from simple animations on coffee machines to 3D-like iPhone applications. Basic 2D graphic operation examples: – Bitmap move/copy from the off-screen memory to display on-screen layers, including font color expansion – Multi-layer video controller compose blended layers in the display output image Advanced 2D/2.5D graphic operation examples: – Alpha channeled bitmap move/copy (Porter-Duff compositing rules) including semitransparency and composing of multiple bitmaps – 2D texturing, i.e., scaling, rotating, translating, stretching of the BMP to the defined polygon, perspective, and other effects – Anti-aliased line drawing, i.e., line drawing with edge smoothing to avoid jagged line edges due to a discrete and finite pixel display resolution. Drawing anti-aliased line of arbitrary thickness; round, square, or other line end types, and line fill by the color or the bitmap content – Vector font rendering, i.e., rasterizing and fill of the letter body with ink, while the font’s edges are smoothed by anti-aliasing – Usually 2D accelerators modify graphical content created in the previous animation cycle, so-called restore of the dirty rectangle, and create new animation content over that screen area – rectangle. 2D graphic accelerators must be supported by different software tools, such as the popular OS graphic drivers (Windows CE, Windows, Linux, QNX, etc.), graphic libraries (OpenVG, DirectFB, QT, emWIN, PEG, etc.), or automatic HMI creation tools for embedded GDC (Altia-Designer, EB-Guide, TAT-Cascade, Tilcon, WAPS, etc.). Some IP core vendors provide generic drivers, i.e., init/control IP core examples in the source code, which can be used as OS/Lib driver template. Available SW drivers increase the value of the IP core and shorten development time (> Fig. 10).

3.4.1

3D Animation

3D animations present an ultimate graphic animation complexity in the embedded world. There are two de facto standards for the embedded 3D graphics, OpenGL ES (ES stands for Embedded Systems), and DirectX (Microsoft graphic engine). OpenGL ES1.1 and OpenGL ES 2.0 open standards are gaining popularity in the high-end embedded applications and all 3D graphic accelerators support OpenGL ES standard. The OpenGL ES 1.1 version uses fixed graphic pipeline, while 2.0 versions uses programmable pipeline. Advanced 2.0 feature set and its programmable pipeline add SW complexity and results in high penalties regarding FPGA’s size and performance. Consequently, the OpenGL ES 2.0 FPGA implementation is rare today. It can be found in special applications where the costs of silicon and the level of 3D performances are not crucial.

FPGA IP Cores for Displays

ON SCREEN MEMORY

4.4.3

OFF SCREEN MEMORY

1.

logiBITBLT move

2.

logiBMP rotation

logiBITBLT BMP composing

3.

. Fig. 10 Dirty rectangle animation

FPGA 3D graphic accelerator operation includes (see > Sect. 3 of Chap. 4.4.2). – 3D object models drawing with different surfaces rendering – Rotation, Translation, Scaling, Point of view, frustum culling, view port, lightening – Single or Multiple perspective correct Texturing, flat, Gouraud shading, blending pixel color logical operations, multi-texture composing, different Texture formats – Minification filters, magnification filters: nearest, bilinear, trilinear, MIP-MAP – Line and point drawing with or without anti-aliasing – Occlusion culling – Z buffering – Stencil buffer, clipping planes, visibility tests – Post-filtering: FAA, Fog, Motion blur

525

526

4.4.3 4

FPGA IP Cores for Displays

How to Build the Best FPGA GDC

The first step in designing of the FPGA GDC should be a set up of its features set. The main features include: – Display resolution, color depth, number of layers and displays – Graphic animation effects for selection of 2D/2.5D, or 3D graphic features. Designers should use worst case scenarios to determine the requested system parameters (i.e., memory bandwidth) and system performances (i.e., 60 Hz frame rate) for rendering of graphic objects of selected size (i.e., 400  400 pixels) – Number of video inputs and their resolutions and color depths The requested memory bandwidth should be calculated for the determined features set. The FPGA GDCs provide an ultimate flexibility since the memory controller IP cores can be configured to support external memory banks implemented by a great variety of memory devices, from the 8-bit SDRAMs to multiple 32-bit DDR2 or DDR3 devices (i.e., 128-bit or 256-bit wide memory banks). The GDC design adapted to the calculated worst case graphic animation scenarios assure GDC seamless processing of multiple video inputs, display of multiple video outputs, and accomplishment of the desired graphic animation’s speed (> Tables 3, > 4, and > 5; > Fig. 11).

. Table 3 2.5D FPGA GDC Memory bandwidth calculation logiMEM - Multiport SDR Memory Ctrl.

logiCVC - Multilayer Video Ctrl.

Memory Type Frequency Data width - bits Timing tRCD Timing tRP Timing tDPL/tWR Period (ns) Overhead (cycles) Burst length (N) Efficiency Memory type coef.

Width 800 600 Height 60 FPS 0 Layer0 bits 24 Layer1 bits 24 Layer2 bits 0 Layer3 bits 0 Layer4 bits 0.9230769 Active/Total Vsync 0.8 Bandwidth correction coef.

Total Bandwidth (MB/s) Total Required BW (MB/s)

DDR2 333 16 15 15 15 3.003003 16 64 0.8 2 1332.00

312

1212.672

logiBITBLT - 2D Graphic Acc. by Fill Area 400 Width 400 Height Fill area (Mpixels) 0.16 FPS 60 Frequency 333 Fill rate (Mpixels/s) 9.6 BPP 4 Bandwidth correction coef. 0.8 Operation factor 2 Required Bandwidth(MB/s)

Bandwidth (MB/s)

logiBMP - 2.5D Graphic Acc. by Fill Area Width 400 400 Height 0.16 Fill area (Mpixels) 60 FPS 333 Frequency 66.6 Fill rate (Mpixels/s) Improved core 1 9.6 Required Fill (Mpixels/s) 9.6 Actual Fill (Mpixels/s) 1 Pixel fetch coef. Bandwidth correction coef. 0.333333333 Cache hit 0.8 Required Bandwidth(MB/s) 120

96

Microblaze - 32bit RISC

logiWIN - Versatile Video Input

MIPS Cache coef. Cache line Cache corr. Coef. Cache hit

40 1 8 0.3333333 0.8

load/store access Load/Store corr. Coef. Load/Store-Instr ratio

1 0.0588235 0

Bandwidth (MB/s)

1. Width Height FPS Active/Total Vsync Bits per pixel 2. Width Height FPS Active/Total Vsync Bits per pixel

720 486 30 0.923076923 24 720 486 30 0.923076923 0

96 Bandwidth correction coef. Bandwidth (MB/s)

0.8 57

FPGA’s flexibility and configurability of IP cores allow for exploring of different system architectures in a search after the best system performances at lowest system’s cost [3].

4.4.3

FPGA IP Cores for Displays

. Table 4 2.5D FPGA GDC FPGA resource table Function

IP Core

IP Provider

No

REGs

LUTs

BRAMs

DSP

Display controller - 1 x 16 bpp, logiCVC-ML 1 x 32 bpp

Xylon

1

1084

653

2

3

SDR/DDR Mem. controller

logiMEM

Xylon

1

1520

2624

0

0

Versatile video input

logiWin

Xylon

1

915

907

3

6

Bitmap 2.5D graphics accelerator

logiBMP

Xylon

1

1939

2593

3

12

Bit block transfer 2D graphics accelerator

logiBITBLT

Xylon

1

1061

1015

4

0

32-Bit RISC CPU

microBlaze

Xilinx

1

1263

1339

14

3

UART interface

xps_u artite

Xilinx

1

349

321

0

0

Time/counter

xps_timer

Xilinx

1

298

251

0

0

Interrupt controller

xps_into

Xilinx

1

79

82

0

0

SPI interface

xps_spi

Xilinx

1

70

123

1

0

8878

9908

27

24

Total

XC6SLX25 => 30064 REGs, 15032 LUTs, 52 BRAMs, 38 DSPs => Target design will fit the device

*

. Table 5 2.5D FPGA GDC FPGA input/output pins Function

IP core

Display controller RGB

logiCVC-ML D0-23,,HS, VS, DE, DSP_CLK

1

28

SDR/DDR mem. Controller

logiMEM

CLK+/ , CS, CKE, RAS, CAS, WE, A0-12, B0-1, DQS0-1+/ , DQM0-1, D0-15,

1

47

SPI interface

xps_spi

SCS, SDI. SDO, SCK

1

4

Versatile video input

logiWIN

Data[7:0], CLK, FLD_VS

1

10

Bitmap 2.5D graphics accelerator

logiBMP

None

1

0

Bit block transfer 2D graphics accelerator

logiBITBLT

None

1

0

UART interface

xps_uartlite None

1

0

Timer/counter

xps_timer

None

1

0

Interrupt controller

xps_intc

None

1

0

32-bit RISC CPU

MicroBlaze

None

1

Total

PIN names

No. IOs

0 79

527

4.4.3

FPGA IP Cores for Displays

Xilinx Spartan 6 FPGA

XPS_TIMER XPS_INTC

RS232

LCD touch logiMEM flexible SDR/DDR memory controller

logiCVC-ML LCD controller

SDR/DDR2

Multi I/O SPI flash

XPS_SPI

XPS_UARTLITE

AXI

528

Microblaze RISC CPU

User/ customer inerface

logiBMP

logiWIN

Camera

logiBITBLT

Digital

Memory

Mixed signal

CPU

FPGA

XILINX IP

logic BRICKS IP

3RD party IP

. Fig. 11 2.5D FPGA GDC FPGA block scheme

5

FPGA GDC Design Flow

5.1

Design Flow for Low- and High-Volume Products

FPGA GDC designs heavily depend on GUI–based FPGA implementation tools provided by FPGA vendors, such as the Xilinx Platform Studio, and third-party providers. IP cores can be used in the drag and drop fashion and easily connected to on-chip system busses. The IP features can be configured through GUI dialog boxes. This design flow does not require HDL coding and produces pretty optimized FPGA implementations for low- and mid-volume production programs. In high-volume programs savings in a range of sub-dollar to few dollars can influence product profitability. It requires adapted design flow that includes manual optimizations of IP cores at the HDL code level (> Fig. 12). The both design flows benefit from configurable IP cores that shorten time to market, and diminish a need for designing from the scratch.

FPGA IP Cores for Displays

4.4.3

. Fig. 12 Xilinx platform studio – IP integration and FPGA implementation tool

5.1.1

Configurable FPGA GDC and Impact on SW Development

Configurability and great flexibility of FPGA GDCs calls for more complex, and in some cases, more expensive SW driver support in comparison to the software support for ASIC GDCs. The ASIC GDCs have fixed feature sets that make software drivers developments much easier. Some GDC IP vendors offer optimal SW drivers that can be used in flexible ways like their IP core counter parts.

6

Summary

FPGA Graphic Display Controller provides unprecedented flexibility. Configuring the IP cores for required feature set, performance, and the smaller silicon size leads to optimal graphic display controller. Today’s FPGA silicon offers a large number of gates at low cost and following the trend we will have more gates at lower silicon cost. The availability of graphic display controller FPGA optimized IP cores will continue to grow as well. These two trends may position the FPGA GDC to market winner in the near future.

References 1. 2. 3.

www.xilinx.com www.altera.com www.logicBRICKS.com

4. 5.

www.design-reuse.com www.chipestimate.com

529

530

4.4.3

FPGA IP Cores for Displays

Further Reading Akenine-Moeller T (2002) Real-time rendering. A.K. Peters, Natick. ISBN 1-56881-182-9 Ashenden PJ (1996) The designer’s guide to VHDL. Morgan Kaufmann, San Francisco Bergeron J (2003) Writing testbenches: functional verification of HDL models. Springer Science+Business Media Inc., New York. ISBN 1-4020-7401-8 Bovik AC (2005) Handbook on image and video processing. Elsevier Academic Press, USA. ISBN 0-12-119792-1 Chapman K. Xilinx White paper, Get your priorities right – make your design up to 50% smaller. http://

www.xilinx.com/support/documentation/white_ papers/wp275.pdf Foley J (1995) Computer graphics principles and practice, System programming series. Addison-Wesley, Reading Graphic GEMS Series I-V. Academic. ISBN 0-201-84840-6 Jack K (2005) Video demystified. Elsevier Inc., Newnes. ISBN 0-7506-7822-4 Keating M, Bricaud P (2002) Reuse methodology manual for system-on-a-chip designs. Kluwer Academic Publishers, USA. ISBN 0-7923-8558-6

Section 4

Driving Displays

4.4.4 APIX: High Speed Automotive Pixel Link Markus Ro¨mer 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532

2 2.1 2.2 2.3

Automotive Design Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 Ground Offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 Electromagnetic Emissions and Immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 Cable Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534

3 3.1 3.2 3.3 3.4 3.5 3.5.1 3.5.2 3.5.3 3.6

APIX Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535 CML Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 Optimized Chip Design for EMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539 Nominal Current and Preemphasis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539 Lowering Emissions and Transmission Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539 Connectors and Cabling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542 Pixel Clock Jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543 Power over APIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544

4

Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.4.4, # Springer-Verlag Berlin Heidelberg 2012

532

4.4.4

APIX: High Speed Automotive Pixel Link

Abstract: With the increasing demand on driver information, multimedia content, and even Internet connectivity, displays and video signaling are receiving increasing attention in the automotive industry. The requirement of transmitting video signals includes applications like infotainment displays, dashboard and head-up displays, and also driver assistance systems that require real-time video streams. A car environment has specific challenges and requirements that need to be considered for video transport in terms of system design. This chapter provides an overview of common challenges the designer of automotive display and camera applications needs to deal with. On the basis of the architecture of the automotive pixel link (APIX) technology, the article first explains the basic concepts of high-speed video transmissions and then focuses on considerations and mechanisms to overcome issues involved in these. List of Abbreviations: APIX, Automotive Pixel Link, High-Speed Serial Interface Standard Developed by Inova Semiconductors GmbH; AWG, American Wire Gauge, Standardized Wire Gauge System Used for the Diameter of Wires; CID, Central Information Display; CML, Current Mode Logic; CMOS, Complementary Metal Oxide Semiconductor; DAB, Digital Audio Broadcasting; DPI, Direct RF Power Injection Method; DVB, Digital Video Broadcasting; DVI, Digital Video Interface; EMC, Electromagnetic Compatibility; EMI, Electromagnetic Interference; Gbit/Mbit, Gigabit/Megabit (Transmission Speed); GPS, Global Positioning System; GSM, Groupe Spe´cial Mobile; IC, Integrated Circuit; LVDS, Low-Voltage Differential Signaling; PCB, Printed Circuit Board; PLL, Phase Locked Loop; RF, Radio Frequency; STP, Shielded Twisted Pair; TEM, Transverse Electromagnetic Cell; VDA, Verband Der Automobilindustrie; VIA, Vertical Interconnect Access, Used on PCBs to Create Through-Connections

1

Introduction

Since the last 20 years, the added value through electronics in cars has increased to around 25% and is forecasted by the Verband der Automobilindustrie (VDA) to further increase to around 40% in 2015. The main innovation steps have, of course, been in safety features like air bags, traction control, and braking control. Further, the driver is surrounded by sensors and cameras, monitoring the status of the car and the environment in all kinds of situations, and assisting in parking and as navigations systems or even managing the car in critical situations like lane departures. The automotive pixel link (APIX) technology has been specifically designed to address the different requirements for video transmission in automotive applications. Because the APIX technology is optimized for low EMI, it offers the ability to combine high-speed video data, full-duplex communication channels, and power supply over a single cable. Additional features like configurable preemphasis and nominal current control provide high flexibility for many different applications.

2

Automotive Design Challenges

In comparison with video transmission standards used in consumer products (e.g., DVI), the video link used in car environments has to meet additional or more stringent requirements. The technology needs to offer high speed transmission over a distance of up to 10 m but also the ability to be used at just 50 cm as on a dashboard; it needs to be robust against

APIX: High Speed Automotive Pixel Link

4.4.4

electromagnetic emissions from mobile phones or radios, and it needs to be designed for low emissions so as not to disturb the surrounding environment. The APIX technology incorporates features and mechanisms to address the challenges of ground offset, electromagnetic compatibilty (EMC), and varying cable lengths, which are described in more detail in the following chapters.

2.1

Ground Offset

A critical challenge for electronic design in cars is the common ground. Since a car is a ‘‘nongrounded’’ system, a typical approach is to use the car chassis as the common ground for all electronic equipment. Therefore, only positive supply is brought to the equipment; the ground connection is done locally to the chassis. However, with the long ground distance between different devices and the devices to the battery, the ground voltage level for the different components may show a significant difference of up to several volts. The differences can be caused through different resistive circumstances for the equipment to the battery path as well as local, high dynamic currents, for example, caused by control units or by electric motors. This ground offset may have a significant impact on systems with analog to digital conversion like sensors or camera systems, requiring a stable reference for the conversion. In the case of high-speed video interfaces, the ground offset may have an impact on the clock and data recovery after the transmission.

2.2

Electromagnetic Emissions and Immunity

The area of EMC is one of the most challenging aspects in systems designs for the automotive environment. The growing number of electronic or electromechanical devices also increases the risk of electromagnetic interference (EMI). Modern cars include a number of devices, each requiring highly sensitive receivers for proper functionality. These include navigation systems (global positioning system, GPS), digital radios and televisions (DAB, DVB), or mobile phone units (GSM). Due to high sensitivity, the level of acceptable emissions for the automotive environment is well below the requirements specified for consumer electronic devices. > Table 1 illustrates the level of typical emission limits in the automotive environment compared with the limits defined by the CE regulations [1]. Regulations valid for the automotive environment are, for example, CISPR25 or EN55025, defining requirements for systems that are used in cars. For example, CISPR25b defines a strip-line test, which verifies the emission of the transmission line. Critical sources of EMI are devices that require very high currents and, therefore, generate electromagnetic fields, for example, starter motors, comfort systems like electric window lifts, electrical seat adjustment mechanisms, or seat heating elements. Another source of EMI is the board design, which may cause EMI by parallel bus switching at the same clock rate and, therefore, adding up noise for one or multiple specific frequencies (switching noise). Long traces, ground loops, or oscillation circuits like PLLs, either on the board layout or even within the chip itself, may also cause radiations. Therefore, in addition to the above system tests, semiconductors need to be tested at the device level to measure emissions and the immunity of the chip itself. > Figure 1 shows an example of the 150-Ω test setup as defined by IEC61967-4 [2], specifically testing the signal at

533

534

4.4.4

APIX: High Speed Automotive Pixel Link

. Table 1 Comparison of consumer and typical automotive emission limits RF noise level (dB mV)

Automotive limits

CE emission limits

RF noise voltage (mV)

0

0.0010

3

0.0014

6

0.0020

10

0.0032

15

0.0056

20

0.0100

30

0.0316

35

0.0562

40

0.1000

45

0.1778

50

0.3162

60

1.0000

Items in bold represent the typical emission levels

a device output. The setup uses a 150-Ω antenna, which represents the emission characteristics of a typical cabling network. The test procedure measures the emissions from the antenna on a spectrum analyzer. Another common test is the TEM cell test defined by IEC61967-2, which verifies the emission of the chip in an isolated chamber [3]. The TEM cell is also used to measure the immunity of the device. In terms of immunity, the geometries of chip architectures typically are too small to act as antennas for the reception of radio energy. Geometries more likely to be affected are the traces or wires connected to the pins. Therefore, immunity tests as described by the IEC62132 verify the immunity of the IC against RF energy, which is brought in through the pins. The test as described in the IEC62132-4 (also known as DPI, direct RF power injection test) induces a frequency at a probe point on the PCB, which is directly connected to the pin [4]. Especially the immunity tests show that EMI is not just a chip or a system problem; it needs to be considered for all parts of a design, as every component, trace, or even mechanical part may act as an antenna or as part of an oscillating circuit.

2.3

Cable Length

Cable length as such is not a specific design challenge in automotive applications; however, it is discussed in this article, as the automotive environment requires high flexibility with regard to this. Centralized systems like head units should be able to send or receive video data at various distances. If the head unit is placed somewhere behind the instrument cluster, then the interface technology should be able to serve the central information display (CID), which displays the radio menu or the navigation screen right above the head unit, as well as provides the content to the rear-seat displays. The difference, therefore, can be anywhere from 30 cm to 10 m. The length of the cable influences the required nominal swing, which is required to receive a stable eye at the end of the line. The longer the cable, the higher the signal swing to be

APIX: High Speed Automotive Pixel Link Impedance matching network

C3

4.4.4 ZL = 150 Ω

ZL = 150 Ω

IC Gnd

Vsupply

C2 IC Gnd

R3 C4 120 Ω 6,8 nf R2 R1* 51 Ω

51 Ω

I/O

Q

VRF

C1 Measuring equipment

RF current probe C5 Gnd 0V

+5 V

Power supply * Pull up/pull down may be required depending on application

. Fig. 1 150-Ω emissions test as defined in the IEC 61967-4 ed.1.1. Copyright ß 2006 IEC, Geneva, Switzerland. www.iec.ch

supplied to the cable. However, the higher the swing, the higher the energy supplied to the cable. Therefore, the physical layer of the transmitter needs to be adjusted to the requirements of the cable length.

3

APIX Technology

3.1

Architecture

The APIX architecture is designed to act as a single interface to a display or to a remote digital camera solution. In order to support high transmission speeds at distances of up to +15 m (1 Gbit/s mode) and up to +40 m (500 Mbit/s mode), the data are serialized and transmitted via the current mode logic (CML) technology (see > Sect. 3.2). The APIX link provides three independent channels for data transfer, which allow full duplex transmission in both directions:

 High-speed downstream pixel channel up to 1 Gbit/s  Downstream sideband channel up to 26 Mbit/s  Upstream sideband channel up to 20 Mbit/s The pixel channel and the downstream sideband channel are multiplexed and are commonly transmitted over the downstream link. The upstream sideband channel can either be established over the same pair of cables as that of the downstream link (embedded return channel, see > Fig. 2) or alternatively over a separate pair of cables (see > Fig. 3) [6].

535

INAP125 T12/24

Upstream sideband channel (embedded as common mode signaling)

. Fig. 2 Automotive pixel link (APIX) channel transmission on a single pair of wires

SBUP_DATA[..]

SBUP_CLK

SBDOWN_DATA[..]

Downstream sideband channel

Pixel channel

INAP125 R12/24

10 MHz

SBUP_DATA[..]

SBDOWN_CLK

SBDOWN_DATA[..]

PX_CLK PX_DATA[..] PX_CTRL

4.4.4

PX_CLK PX_DATA[..] PX_CTRL

10 MHz

536 APIX: High Speed Automotive Pixel Link

INAP125 T12/24

Pixel channel

Upstream sideband channel (as dedicated CML signal)

Downstream sideband channel

. Fig. 3 APIX channel transmission on two pairs of wires for optimized EMI

SBUP_DATA[..]

SBUP_CLK

SBDOWN_DATA[..]

PX_CLK PX_DATA[..] PX_CTRL

10 MHz

INAP125 R12/24

10 MHz

SBUP_DATA[..]

SBDOWN_CLK

SBDOWN_DATA[..]

PX_CLK PX_DATA[..] PX_CTRL

APIX: High Speed Automotive Pixel Link

4.4.4 537

538

4.4.4 3.2

APIX: High Speed Automotive Pixel Link

CML Technology

High-speed data transmission over long distances requires the use of differential signaling technologies like low-voltage differential signaling (LVDS) or CML. In comparison with single-ended and parallel interfaces, these technologies offer high immunity against environmental noise and lower emissions, with the benefits of lower voltage swings and low power consumption. The reason behind these features lies in the fact that the data are transmitted via a pair of twisted cables, on which the digital bit is transmitted as +VSwing on cable 1 and VSwing on cable 2 or vice versa (logic ‘‘0’’ or ‘‘1’’). Since the information is transmitted differentially, the noise induced on the line would affect both lines, therefore just influencing the DC level of the VSwing, but not the relation of cable 1 to cable 2. In addition to immunity, the electromagnetic emissions are kept at a minimum. In order to ensure an efficient operation of differential signals, the layout, the cable connectors, and the cable itself need to be designed to ensure a constant distance between the differential cables and a constant impedance, to avoid reflections and, along with these, errors on the differential signal. Please see also > Chap. 4.3.2 for more details on this subject. The APIX technology uses CML, as it operates with a constant current source, which, when compared with LVDS, switches between the logical stages without generating spikes on the power supply, which in turn generates high dynamic currents, thus generating EMI. In addition, CML allows faster switching times. Due to the architecture of CML, the twisted cables carry the same current in the opposite direction, which compensates the electromagnetic field of the cable (> Fig. 4).

DPO brightness: 60% Tek Run: 100 GS/s ET sample DPO [.................................................... ] T

+VSwing 3 –VSwing

Ch3 100 mVΩ

Ch4 100 mVΩ

. Fig. 4 Current mode logic (CML) eye pattern

M 500 ps

Ch3

0 V October 4, 2007 13:10:40

APIX: High Speed Automotive Pixel Link

3.3

4.4.4

Optimized Chip Design for EMI

The principle and the benefits of CML, using a constant current source driving a differential pair of wires to eliminate emissions, may also be used for chip design. Especially at high switching frequencies and due to the high density of transistors, semiconductor devices need to deal with supply and ground noises that influence all components of the chip design. Traces within the chip may generate electromagnetic fields, similar to any line on a PCB design. The analog frontends of the APIX transmitter and the receiver ICs have been designed using CML techniques. Chip signals are integrated using two wires for each connection with the current mode switching for the signal, compensating the electric field of the lines and reducing the switching noise to a minimum (> Fig. 5).

3.4

Nominal Current and Preemphasis

Different cable lengths require that the physical layer of the line driver be adjusted, to compensate the difference in cable attenuation and to minimize emissions and reflections. The APIX transmitters are, therefore, designed to allow the configuration of the nominal swing. The nominal swing (VSwing) should be reduced for short cable lengths and needs to be increased for long cable lengths or if additional filters or connectors are placed in the transmission line. Please see also > Chap. 4.3.2 for more details on the nominal swing. Preemphasis allows to reduce the current driven into the line, and with this reduction, the level of reflections too. > Figure 6 compares the signal on a 20-m cable with and without preemphasis enabled. By using preemphasis, the signal is cleared from reflections, which can disturb the pattern recognition.

3.5

Lowering Emissions and Transmission Errors

3.5.1

Connectors and Cabling

Because differential signaling has already been in use since many years and used by various standards like Ethernet or digital video interface (DVI) [4], the selection of cables and connectors available is quite large. However, the final decision on the cable and the connector depends on the application requirements for EMI, distance, reliability, and cost. In order to obtain optimum performance for EMI and transmission length, the differential link needs to be optimized from the transmitter to the receiver. This includes 1. 2. 3. 4. 5.

Routing and layout of the signal from the pin to the connector (see > Chap. 4.3.2) Quality of the plug in terms of EMI (shielding) Connection of the signals inside the plug and the connector (same length, matched) No change in impedance from the plug to the connector A 100-Ω impedance cable

The main requirement for the line is to have a continuous impedance of 100 Ω. Each ‘‘conversion’’ from the board to the connector or to the plug may induce an impedance mismatch, generating reflections and, therefore, causing emissions.

539

+ −

0.2 µs

0.4 µs

Vin_p

0.6 µs

1.0 µs

1.2 µs

CMOS logic

0.8 µs

output

V[out_P]

input

V[in_P]

IVDD

I[Vdd]

nmos_1p8

. Fig. 5 Difference between standard CMOS and CML designs

0.0 V 2.0 V 1.8 V 1.6 V 1.4 V 1.2 V 1.0 V 0.8 V 0.6 V 0.4 V 0.2 V 0.0 V 0.0 µs

0.2 V

0.4 V

0.6 V

0.8 V

1.0 V

1.2 V

1.4 V

1.6 V

2 mA 0 mA −2 mA −4 mA −6 mA −8 mA −10 mA −12 mA −14 mA −16 mA −18 mA −20 mA −22 mA −24 mA 1.8 V

1.8

VDD

M1

1.4 µs

1.6 µs

1.8 µs

out_p

1.2 V 0 ns

1.4 V

1.6 V

1.8 V

1.2 V 0 ns 2.0 V

1.4 V

1.6 V

–10.5 mA 0 ns 1.8 V

–10.0 mA

−9.5 mA

1.8

+ −

20 ns

20 ns

20 ns

40 ns

+ −

V[in_p]

60 ns

60 ns

60 ns

Vin_p Vin_n

V[out_p]

40 ns

40 ns

.tran 0 250ns 50ns

+ −

VDD

100 ns

output

100 ns

input

100 ns

IVDD

I[Vdd]

nmos_1p8

M1

120 ns

120 ns

120 ns

0.01

I1

R2 50

V[in_n]

140 ns

V[out_n]

140 ns

140 ns

nmos_1p8

M2

Current mode logic

80 ns

80 ns

80 ns

in_n

in_p

R1 50

160 ns

160 ns

160 ns

180 ns

180 ns

180 ns

out_p out_n

4.4.4

.tran 0 2500ns 500ns

+ −

in_p

M2 pmos_1p8

540 APIX: High Speed Automotive Pixel Link

Ch3 50.0 mVΩ

M 200 ps Ch2

No preemphasis

Ch4 50.0 mVΩ

1.52 V

. Fig. 6 Signal quality on a 20-m cable with and without preemphasis

3

[............................................ ] T

Run: 250 GS/s ET sample DPO Δ: 316 mV @: 157 mV

3

DPO

Ch3 50.0 mVΩ

M 200 ps Ch2

With preemphasis

Ch4 50.0 mVΩ

1.52 V

Δ: 102 mV @: –54 mV

DPO brightness: 90%

T [............................................ ]

Run: 250 GS/s ET sample

APIX: High Speed Automotive Pixel Link

4.4.4 541

542

4.4.4

APIX: High Speed Automotive Pixel Link

Several cable and connector providers offer highly optimized solutions, which fulfill these requirements for the cable and the connector. The APIX technology has been tested with different connectors and cables like standard RJ45 connectors and Cat5 shielded twisted pair (STP) cables, which are typically used in Ethernet applications and also with specific automotive cables and connectors. An example for a robust connector is the RosenbergerHSD1 connector, optimized for a two-pair connection. The connector is based on the star-quad principle for minimized interference, has a controlled impedance of 100 Ω across several interconnections, and includes a shield for high EMI performance (> Figs. 7 and > 8).

3.5.2

Layout

Since the high-speed interface acts at a frequency of 500 MHz, the design needs to be treated as a high-frequency design. The main problems that generate noise or radiations are caused by high current spikes, which are generated by strong output drivers or by switching multiple outputs. Also important is to avoid ground loops or long traces, which could resonate at undefined frequencies. The following recommendations can help reduce noise and avoid performance issues. Of course, the list can just be seen as a simple starting point. 1. Power supply filtering The chip supplies should be filtered with block capacitors, which help to reduce the influence of high current requirements on the remaining system. The capacitor values

. Fig. 7 Star-quad principle for optimized differential signaling

APIX: High Speed Automotive Pixel Link

4.4.4

. Fig. 8 RosenbergerHSD® connector

depend on the requirements of the chip and should be calculated based on the magnitude of the voltage ripple and the frequencies present. The components need to be placed as close as possible to the devices for maximum effect. Typical values can be found in the datasheets of the transmitter and the receiver devices. 2. Loop filter design The loop filter components of the APIX devices should be laid out as close as possible to the input and output pins. The loop filter design should not include any vertical interconnect access (VIA) to avoid the influence of the additional inductance and capacitance. 3. Series resistors The video data are sampled into the serializer devices using a 27-bit parallel bus, synchronous to a pixel clock signal. Graphics controllers with strong output stages might generate radiations through very fast rise times. Series resistors in the pixel data lines and the pixel clock limit the rise time and, therefore, act as a filter for high frequencies. In addition, these resistors reduce the current flowing into the chip.

3.5.3

Pixel Clock Jitter

A key advantage of the APIX architecture is that the high-speed clock for the serial interface is generated from an internal system clock, independent from the pixel clock. As described in > Sect. 3 of Chap. 4.3.1, the parallel pixel interface between the graphics controller and the APIX transmitter is a synchronous parallel interface, which may cause a significant peak in the emission spectrum and disturb surrounding devices or receivers. Using series resistors can help filtering the high-frequency components of the emission, but since all lines of the parallel bus switch simultaneously, additional mechanisms are necessary. In case the graphics controller hardware supports it, the problem can be addressed using staggered outputs. With this, the output drivers are not switched simultaneously, but instead are driven with small time offsets. This method reduces the switching currents and, therefore, the radiated power. Another way is to spread the spectrum of the radiated emissions and, therefore, flatten the EMI spectrum. This can be achieved by jittering the pixel clock, that is, instead of using a

543

544

4.4.4

APIX: High Speed Automotive Pixel Link

constant frequency, for example, 40 MHz, the pixel clock is provided with a continuously changing frequency around this central frequency. The influence of the jitter to the APIX link has been tested in various configurations by varying the pixel clock, the frequency deviation, for example, 39 MHz instead of 40 MHz, and the modulation frequency, for example, the offset being supplied at a frequency of 1 kHz. The results have shown that the jitter added to the pixel clock can be up to 10% of the pixel clock with a modulation of 50 kHz, without causing transmission errors on the video data [5]. This immunity of the APIX link architecture against a wide range of jitter provides the system designer a flexible tool to optimize the EMI performance of his design.

3.6

Power over APIX

The APIX interface allows implementing the complete communication of video and controlling information onto a single cable, using the sideband channels for the control information. This saves the need for an additional cable and, therefore, saves cost and weight. Besides the communication, the APIX link can also be used to power the remote system. Due to the robust characteristics of the APIX link, one or both cable pairs can be used for the power supply transmission. The power signal should be coupled in through ferrites as shown in > Fig. 7, where a display application completely controlled and powered over an APIX link is shown. The amount of current and voltage possible with this kind of setup depends on the cables used. A single wire supply as illustrated in > Fig. 9 has been successfully tested at 12 V with a current of up to 1.5 A, using wires with a nominal cross-section of 0.14 mm2 (American wire gauge, AWG 26). The ferrites/inductors in the setup act as a lowpass filter between the link and the DC domain. The graph also shows common mode chokes, which can help reduce the impact on the chips in case of high current transients (e.g., during power up). The values of these components depend on the specific application requirements. As shown in the example, power over APIX also addresses the problem of ground offsets, also described in > Sect. 2.1. In case the remote system is also supplied through the APIX link, the remote system uses the same ground reference as the supplying unit and, therefore, eliminates the problem of the ground offset between these two devices. The possibility of routing video, control data, and the power supply over a single cable provides significant savings on cost and wiring.

4

Summary and Outlook

The APIX high-speed display link offers a number of features and concepts that addresses the different requirements for the car environment. As the APIX technology is optimized for low EMI, it can act as a single interface for a display or a camera application to reduce cabling and system integration costs and to overcome ground offset issues. Even though the technology is well defined, the designer still needs to pay attention to the high-frequency component of such a design in terms of the layout and in the selection of cables and connectors. Looking into the future, the call for more bandwidth will also require higher frequencies, and, with this, continued optimizations for EMI. Large, high-resolution displays as well as cameras with megapixel resolutions will require bandwidths of up to 3 Gbit/s. The growing

GND

VCC

28

APIX TX

Coupling caps

–

–

Ferrites/ inductors

+

+

VREG

APIX RX

Local GND

VCC

Coupling caps

28 Graphics and timing controller

Display

. Fig. 9 Example of powering a remote display system, completely supplied and controlled over the APIX link, eliminating ground offset

Power supply

VREG

Main processor

Head unit

Common mode chokes

APIX: High Speed Automotive Pixel Link

4.4.4 545

546

4.4.4

APIX: High Speed Automotive Pixel Link

demand for more bandwidth combined with lowest EMI and cost reduction requirements define the need for efficient, highly optimized solutions. The APIX architecture will continuously be enhanced in order to serve the requirements of the automotive market.

Acknowledgments The author thanks the International Electrotechnical Commission (IEC) for permission to reproduce Information from its International Standard IEC 61967-4 ed.1.1 (2006). All such extracts are copyright of IEC, Geneva, Switzerland. All rights reserved. Further information on the IEC is available from www.iec.ch. IEC has no responsibility for the placement and context in which the extracts and contents are reproduced by the author, nor is IEC in any way responsible for the other content or accuracy therein.

References 1. 2.

3.

Schwab AJ (1996) Electromangnetische Vertra¨glichkeit. Springer, Berlin IEC 61967-4 (2006-07) Integrated circuits – measurement of electromagnetic emissions 150 kHz to 1 GHz – part 4: measurement of conducted emissions – 1 Ω/150 Ω direct coupling method, 1.1 edn. International Electrotechnical Commission, Geneva IEC 61967-2 (2005-09) Integrated circuits – measurement of electromagnetic emissions 150 kHz to 1 GHz – part 2: measurement of radiated emissions, TEM-cell method and wideband TEM cell

4.

5.

6.

method, 1st edn. International Electrotechnical Commission, Geneva IEC 62132-4 (2006-02) Integrated circuits – measurement of electromagnetic immunity 150 kHz to 1 GHz – part 4: direct RF power injection method, 1st edn. International Electrotechnical Commission, Geneva Inova Semiconductors GmbH (2008) AN105 APIX video interface application note, Inova Semiconductors GmbH, Munich Inova Semiconductors GmbH (2008) INAP125 datasheet

Further Reading Johnson H, Graham M (1993) High speed digital design: a handbook of black magic. Prentice Hall, Upper Saddle River

Wadell BC (1991) Transmission line design handbook. Artech House, Norwood

Part 4.5

Signal Processing Tasks

4.5.1 Video Processing Tasks Markus Schu 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 550

2

Deinterlacing and Frame Rate Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552

3

Scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555

4 4.1 4.2 4.3 4.3.1 4.3.2 4.4

Motion Blur Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557 Motion Blur Originated by Slow Response Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 558 Motion Blur Originated by Eye Tracking of Moving Objects . . . . . . . . . . . . . . . . . . . . . 559 Techniques for Motion Blur Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561 Overdrive or Response Time Compensation Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 561 Impulse Driving Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 564

5

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565

6

Directions of Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.5.1, # Springer-Verlag Berlin Heidelberg 2012

550

4.5.1

Video Processing Tasks

Abstract: New display technologies including Liquid Crystal Display (LCD) systems offer large screens and impressive picture quality. However, all displays require sophisticated picture processing to allow these panels to perform at their optimum levels. This chapter focuses on the major signal processing techniques for optimal picture quality. At first, the state-of-the-art system architecture of the display electronics of a modern LCD displays is given. After explaining the latest deinterlacer technologies, the focus is set to scaling technologies. Then, techniques to reduce motion blur are presented. Finally, a conclusion of the display electronics of modern displays is given, which is followed by an outlook of future developments. List of Abbreviations: AMLCD, Active-Matrix Liquid Crystal Display; BLB, Back Light Blinking; Back Light Blinking, Backlight Is Switched On and Off Synchronously; BLS, Back Light Scanning; Back Light Scanning, Backlight Is Switched On and Off Synchronously Row by Row; BFI, Black Frame Insertion; Black Frame Insertion, Black Frames are Inserted After Doubling the Picture Frame Rate; CRT, Cathode Ray Tube, Monitor or TV; Deinterlacing, Process of Converting Interlaced Video into an Noninterlaced Format; DLP, Digital Light Processing, Display Technique from Texas Instruments for Projectors; DRAM, Dynamic Random Access Memory; Eye tracking, Synonym for Smooth Pursuit; FPD, Flat-Panel Display; Full-HD, Complete High Definition, Means Resolution of 1920 x 1080; HD, High Definition, Minimum Resolution of 1280 x 720 Required; Hz, Hertz = 1 per Second; Motion blur, Apparent Streaking of Rapidly Moving Object in Movies; MC, Motion Compensation; Motion compensation, Process for Interpolation of Additional Images with the Help of Motion Vectors Calculated by Motion Estimation; ME, Motion Estimation; Motion Estimation, Process of Determining Motion Vectors that Describe the Displacement of One 2D Image to Another; ms, Milliseconds = 1 per One Thousand Seconds; LCD, Liquid Crystal Display; Overdrive, Synonym for RTC; PDP, Plasma Display; QID, Quasi-Impulse Driving; Quasi-Impulse Driving, Varying the Luminance from One Image to Another Image, Approximation of a Back Light Blinking; RTC, Response Time Compensation, Accelerates the LCD’s Response Time; Smooth pursuit, Ability of the Eye to Follow a Moving Object; Scaler, Module for Conversion of Video Signals Between Different Resolutions; TCON, Timing Controller; Timing Controller, Device for Addressing and Controlling a LCD Panel; 2-2/3-2 pull-down, Method to Transmit Movie Sources Over Interlaced Transmission Channels

1

Introduction

Displaying video on Flat-Panel Displays (FPDs) requires a lot of signal processing as the video source is often not adequate for the panel resolution and frame frequency. Furthermore, some video content is interlaced; FPDs operate in noninterlaced (progressive mode). When this is ‘‘done,’’ the video stream has to be fitted to panel-specific driving like Active-matrix liquid crystal displays (AMLCDs) of laptops (used for DVD or Blue-Ray video watching) as well as subframes for Plasma Displays (PDPs) and Digital Light Processing (DLP)-systems. Even in TV sets, some additional tasks can be made like 24-Hz movie detection for optimized motion compensation. In consequence, we can distinguish between a ‘‘common’’ input module (HW & SW), which is at first glance independent of the panel technology used and a panel-specific module, which arrange the output from the input block in a way that they are compatible with row (lines) and column

Video Processing Tasks

4.5.1

. Table 1 Fundamental signal processing tasks for displaying video on Flat-Panel Displays (FPDs) with typical examples

Task

To do

AMLCD in laptop, monitor, or projection AMLCD TV Set

PDP TV set, DLP projection

Example video input

SD TV (PAL, NTSC)

HD Ready (720p)

Full HD (1080i)

Example panel resolution

1280
800

1920
1080

1280 x 720

Deinterlacing Interlaced (I) ! Noninterlaced (progressive P)

I!P

None

I!P

None or 24 Hz ! 120/240 Hz

None or 50 Hz ! 60 Hz

Frame rate conversion (FRC)

Video frame rate PAL: 50 Hz ! ! panel frame rate 60 Hz

Scaling

Video resolution ! NTSC: 480 ! 800 720 ! 1080 panel resolution (upscaling) (upscaling)

1080 ! 720 (downscaling)

Motion blur reduction

Frequency doubling (often done in FRC)

Often none

Frequency doubling

Frequency doubling

Panel specific Adapt to row and column signals

Nothing special

Overdrive

Subframe driving

Comment

Typically done by Less tasks when video equals panel optical disk parameters player (SW)

Less tasks when video equals panel parameters

drivers of the panel. For hold-type displays like AMLCDs and PDPs, the reduction of motion blur is an issue – this could also be achieved by hard- and software in the panel-specific module. These tasks are summarized in > Table 1 and discussed in detail in this chapter. The order there gives also the order of sequential processing of these tasks. In a first summary, one can state that the image quality of video on FPDs strongly depends on the quality of the algorithms for video signal processing. In > Fig. 1, the video system architecture of a modern FPD-TV display is shown. Besides the panel, some major video processing steps can be identified. The last mile before the LCD panel and its driver is occupied by the so-called TCON (Timing Controller). Before that, the display-specific signal processing technology is located. For example, the overdrive function needs an external Dynamic Random Access Memory (DRAM). In the case of Plasma, the display processing contains the subfield generation processing. The front-end signal processing is directly located before the display-specific processing. These technologies need external memory too. The first step of processing is the so-called video front end. Here the video signal is captured by different input interfaces and prepared for further processing. The main technologies of the video front end are the deinterlacer, frame rate converter, and scaler.

551

552

4.5.1

Video Processing Tasks

Analogue and digital inputs

Decoder (Colour decoder, Digital decoder, Colour conversion,..)

De-interlacing (interlaced to progressive)

Display specific signal procesing (Motion Blur, Overdrive, Subframes,...)

Frame Rate Convertion (frame rate adaption)

Scaling (adaption of resolution)

Panel electronics (TCON, Drivers,...)

. Fig. 1 Typical video signal processing chain

2

Deinterlacing and Frame Rate Conversion

In TV broadcasting, each picture, known as a frame, consists of two consecutive fields. Each field contains half of the overall picture. The first field carries all the odd-numbered lines from the original image, the second field holds all the even-numbered lines. Broadcasters call this method of splitting the odd- and even-numbered lines into two fields ‘‘interlacing’’ or ‘‘interlaced scanning.’’ It is different from progressive scanning, which shows all the lines of the image in sequence. When conventional TV cameras generate interlaced images, there is a 1/25-s or 1/30-s gap between the two frames. As a result, the same moving object will appear in slightly different places within the two frames. Flat-panel displays work best with progressively scanned images. Therefore, deinterlacing is necessary (> Fig. 2). Some source material is sampled at a lower rate, on 2-2 pull-down at 25 Hz for 50 Hz or at 30 Hz for 60 Hz displays, on 3-2 pull-down at 24 Hz for 60 Hz displays. Then, each sampled picture is multiply displayed to fit the video frame rates of the target display. > Figure 3 shows the generation process for a film source by 3-2 pull-down technique in more detail. The origin is a movie shot at 24 frames per second (24p Hz, p – progressive). To broadcast this signal with 60 fields per second (60i Hz, i – interlaced), 3 fields and 2 fields are alternatively shown. The generation process for a film source by 2-2 pull-down technique works accordingly. There are several methods for deinterlacing [1, 2]. The simplest method involves line doubling, which causes the system to show each line of the present field twice. With this technique, the horizontal edges remain sharp, but the diagonal edges appear jagged. Additionally, the fact that each field in a frame starts in a slightly different place means that the vertical position is not fixed precisely for horizontal lines. This leads to line flicker, causing jagged lines to appear on the horizontal and diagonal edges of objects, especially moving objects. One way to improve on the line-doubling technique is to interpolate the missing line (> Fig. 4). This involves averaging the two original lines. Such interpolation decreases the jagged edges on moving objects, but does not eliminate them. Additionally, the horizontal edges lose their sharpness. Line flicker still exists, too. In general, however, the display of moving objects is better with interpolation techniques than with simple line doubling. To overcome the loss of resolution and to avoid jagged lines, it is possible to perform the interpolation in two dimensions, horizontally as well as vertically. Additional analysis looks for

Video Processing Tasks

4.5.1

. Fig. 2 Basic principle of deinterlacing

24 Hz Pull down

60 Hz Same image is shown twice or thrice -> no movement of ball

. Fig. 3 Basic principle of frame rate conversion

edges in local areas of the picture. Two-dimensional interpolation then takes place along the direction of any such edges that the analysis finds. Besides avoiding a jagged appearance in the edges, this method preserves resolution. However, the image still exhibits the line flicker that occurs from incorrect vertical positioning information. The only solution that maintains the full vertical resolution and avoids line flicker is one that merges two consecutive fields into a single frame (> Fig. 4). Indeed, this solution offers an optimum reconstruction for still objects, but it fails completely for moving objects. Recording the two fields 1/25-s or 1/30-s apart means that moving objects have shifted slightly between the first and second fields. Merging these two fields produces an image with a saw-toothed appearance that is more annoying than jagged lines. Additional temporal mixing uses two frames in sequence in a way that diminishes the saw-toothed effect and improves the display of moving objects. However, it weakens the vertical resolution. Therefore, each of the conventional deinterlacing methods has weaknesses as well as strengths. It is possible, however, to combine the strengths of two different techniques into a single technique known as motion-adaptive deinterlacing [3, 4] (> Fig. 5). Based on an analysis of the image content the motion-adaptive deinterlacing method switches between field-merging and intra-field deinterlacing. It conducts switchovers on a per-pixel basis. In this way, each frame can combine optimum processing for still areas and for moving areas. Even so, vertical resolution for moving areas suffers. Motion-adaptive methods distinguish only between areas with motion, and areas without it. They neither generate nor use information about the direction and speed of motion. In contrast, true motion compensation determines this information and uses it to preserve

553

554

4.5.1

Video Processing Tasks

Vertical interpolation

Field merging

Line doubling

. Fig. 4 Simple deinterlacing techniques

Motion compensated interpolation

Motion adaptive interpolation

. Fig. 5 Motion-adaptive/compensated deinterlacing techniques for video sources

vertical resolution, even in moving objects. Thus, it gives the best overall performance for deinterlacing. Nevertheless small details cannot always be restored in both cases. > Figures 4 and > 5 show two consecutive fields of an interlaced video signal. The red object on the left side is not moving and the red object on the right side is moving. Only parts of the object are visible on the interlaced signal. Field merging gives the best result for the still object. The details are just put together and the noninterlaced signal results in a ‘‘red cross.’’ In case of line doubling or vertical interpolation, only parts of the detailed object are reconstructed and this will lead to flicker. If the motion compensation works properly, it can also reconstruct correctly the moving object on the right side. In case of motion-adaptive deinterlacing, this object shows blurred edges. Just doing field merging results in jagged lines. Motion compensation also gives the best result when film is the original image source, for a number of reasons. The slow frame rate allows a long shutter exposure. As a result, moving images are a bit blurry in most film sources. Engineers refer to this as the film judder effect. The slow frame rate and the slight blurring lead to special problems for deinterlacers. Although this is not possible with video images, engineers can merge the interlaced images in film sources without generating artifacts, if they use the correct sequence. The correct sequence must be the exact opposite of the pull-down sequence. The term for this is field jam or reverse pull-down (> Fig. 6).

Video Processing Tasks

4.5.1

Movie in 25 frames/s

25 Hz

2-2 pull down conversion before broadcasting to generate 50 fields/s

50 Hz

50 Hz

Reverse 2-2 - pull down merges fields that are from the same movie frame

50 Hz

Motion compensation

. Fig. 6 Motion-adaptive/compensated deinterlacing techniques for film sources

However, broadcasters do not want to transmit this information, so they use a film-mode detector. This detector must be very accurate: if the technicians merge the fields in the wrong sequence, they will generate a saw-toothed structure that is worse than no merging. Conventional deinterlacing methods resolve some, but not all the problems of working with interlaced images. People may often refer to these methods as motion compensation techniques, but many really are merely motion-adaptive processes. A true motion-estimation technique and a correct film pull-down can generate new images to give true frame rate conversion, yielding 60 unique frames per second.

3

Scaling

Video signals for TV or PC applications differ in resolution and/or frame rate as well as in scan format. All these different inputs formats have to be scaled to a fixed display resolution. Therefore, interpolation techniques are used. Interpolation is the process of determining the values of a video signal at positions lying between its samples [5]. The following are the most common fundamental methods for interpolation: ● ● ● ●

Dropping (downscaling) or doubling (upscaling) Nearest neighbor (up- and downscaling) Linear interpolation (up- and downscaling) Bilinear interpolation (up- and downscaling)

555

556

4.5.1

Video Processing Tasks

● Bicubic (up- and downscaling) ● Lanczos (up- and downscaling) The interpolation techniques mainly reconstruct new samples from the original ones. Interpolation reduces the bandwidth of the video signal by applying a low-pass filter. Thereby the interpolation can be done either independently on horizontal and vertical direction or using directly a two-dimensional approach. Upscaling is the process of generating a higher resolution and downscaling the process of generating a lower resolution. Deinterlacing is a kind of upscaling by factor 2, because the vertical resolution is increased by a factor 2. In many video processing chains, interpolation is often done at multiple places. The selection of the right interpolation method can influence the overall sharpness of the video signal significantly. Therefore, it is an essential task to select and implement the right interpolation method. The simplest upscaling method is just a repetition of identical values (doubling is just repeating once, see > Fig. 7). The simplest downscaling method is just deleting of existing values. Both methods work simple for integer factors. Fractionally scaled video signals can also be obtained by varying the number of copies made of each value. Another simple upscaling interpolation algorithm is nearest-neighbor interpolation (> Fig. 8). The nearest neighbor algorithm simply selects the value of the nearest point, and does not consider the values of other neighboring points at all, yielding a piecewise-constant

Upscaling: Pixel doubling 1 2 3 4 5 1

Original 5⫻7

Upscaled 10 ⫻ 14

2 3 4 5 6 7

Downscaling: Pixel dropping 1 2 3 4 5 Original 5⫻7

1 2 3 4 5 6 7

. Fig. 7 Example for basic up- and downscaling (doubling, dropping)

Downscaled 5⫻4

Video Processing Tasks

4.5.1

1 2 3 4 5 Original 1 5⫻7 2 3

Repetition Nearest neighbour Linear

4 5 6 7 Bilinear

. Fig. 8 Example for basic up- and downscaling (doubling, nearest neighbor, (bi-)linear)

interpolation like repetition. Because of the inexactness of the spatial correspondence between two original values, this process can lead to spatial distortions in the scaled video signal. An interpolation technique that reduces the visual distortion caused by the non-integer interpolation is the linear- or bilinear interpolation algorithm, where the fractional part of the value position is used to compute a weighted average of values over a small neighborhood of surrounding values in the original video signal. Bilinear interpolation is an extension of linear interpolation for interpolating functions of two variables on a regular grid. The key idea is to perform linear interpolation first in one direction, and then again in the other direction. Although each step is linear in the sampled values and in the position, the interpolation as a whole is not linear but rather squared in the sample location. More advanced interpolation algorithms try to approximate the theoretically optimum sinc function, e.g., Bicubic interpolation or windowed sinc functions (e.g., Lanczos interpolation filter) [5]. The quality of the popular interpolation methods is ranked in ascending order as follows: repetition/dropping, nearest neighbor, linear, cubic, and sinc methods.

4

Motion Blur Reduction

Several methods for motion blur reduction have been developed in recent years. This progress shows that the reduction of motion blur is becoming more and more important, due to the increasing size of LCDs and the corresponding larger visible motion artifacts. The LCD is a hold-type display that always emits light. Therefore, even if the response time speed of the liquid crystal is improved, motion blur will remain due to afterimage in the observer’s retina. On the other hand, motion blur is not generated from the impulse type display Cathode Ray Tube (CRT) due to cancellation of the retina afterimage influence within the 50/60 Hz refresh time. In the case of film sources with 24/25/30 Hz refresh rate, this assumption is not still valid, even for CRTs. The reduction of motion blur and film judder is therefore essential for large screen LCDs. One solution for motion blur reduction is using high refresh rate LCD panels, which are available now also for Full-HD resolution. Another is the video processing IC for converting

557

4.5.1

Video Processing Tasks

video content with 24/25/30/50/60 Hz to 100/120 Hz or in the future maybe even higher frame rates [2, 6, 7]. There are two major reasons for the occurrence of motion blur: on the one hand, the slow response time of the Liquid Crystal, and on the other hand, the hold-type nature of the display. This chapter explains why these two topics lead to motion blur. For the principle understanding of technical problems, models are very well suited. Therefore, a moving object is used in order to explain later the problem of motion blur and its solution. The object is a white bar that is moving with a constant speed for a black background. Some figure will later refer to this model.

4.1

Motion Blur Originated by Slow Response Time

LCDs can be seen as a matrix of independent controllable turning valves, which control the flow of light. The light source is a backlight with polarization filters. The valve is able to influence the luminance of the light source. Of primary interest are two states of the valve: on state (the valve is complete open and the light is visible) and off state (the valve is closed and the light is not visible). But the valve cannot instantaneously switch between off and on states. The time needed to change from one state to another state is called the response time. The on response time is mainly influenced by the square of an applied electrical field and the off response time is mainly given by an elasticity constant, which defines the restoring torque. Further parameters are the rotation viscosity of the liquid and the thickness of the liquid, which are intrinsic to the LC material and its environment. Assuming a white bar that is moving from the left to the right with a certain speed in front of a black background, > Fig. 9 shows for one position the response time of the input signal. The input signal changes the level in zero time from black-to-white (step function) where the output signal needs several frame periods to complete the transition (step function response). This slow response time leads to a significant motion blur artifact. To avoid this motion blur artifact, the response time must be faster than the input frame period (e.g., for 60 Hz input signal frequency, the response time must be significantly smaller as approximately 16 ms). In practice, the response time differs from transitions from ‘‘off ’’ to ‘‘on’’ or vice versa (black-to-white transition) or from one on state to another on state (gray-to-gray transition), respectively. Techniques to reduce this artifact are explained later.

100% Input signal Luminance

558

Response signal

0%

Position of interest n

n+1

n+2

n+3

. Fig. 9 Liquid Crystal Display (LCD) optical response time

Time [frames]

Video Processing Tasks

4.2

4.5.1

Motion Blur Originated by Eye Tracking of Moving Objects

The slow response time of LCDs was long seen as major cause of motion blur. But in fact it contributes only to the motion blur, which is received by human beings. In the early twentyfirst century, it was found that the major cause of motion blur is the hold-type characteristic of the electrooptical response of the display and the integration of the human visual system while smoothly following the movement of the target (i.e., smooth pursuit or eye tracking) [8]. In > Fig. 10, a moving object is used to explain the problem of motion blur [8–10] and its solution. Furthermore, we assume that the response time is zero, meaning that the final luminance level is reached in one time frame. The object (white bar, black background) is moving from right to left with a constant speed. The human eye follows this movement. Due to the eye’s integration function, the brightness is integrated over time. > Figure 10 illustrates the

CRT Display 60 Hz “Impulse”

Position

Snapshot time

1

/60 s

Position

LCD Display 60 Hz “Sample-and-Hold”

Time

Resulting image due to eye integration function at snapshot time

Field/Frame distance Time

. Fig. 10 Comparison between Cathode Ray Tube (CRT) display and LCD, video source

559

4.5.1

Video Processing Tasks

human perception watching a CRTand an LCD at a video source. Due to the short impulse, the CRT motion is perceived as sharp. In contrast, the LCD holds the picture. The brightness is integrated and the object seems to be blurring. The example above deals with a video source. Even more critical is a movie or film source [10]. The human eye tracks the object. Due to the noncontinuous motion the CRT display shows some artifacts. Indeed the impulses are sent at 60 Hz, but on 2-2 pull-down the motion changes only at 30 Hz. This causes a motion judder, potentially shown as doubled contour or slight blur. However, the picture is perceived sharply. In contrast to the CRT, the LCD holds a value over time. But the 2-2 pull-down causes a doubled hold time compared to video sources. Due to the integration function of the human eye the brightness is integrated over the doubled time, as illustrated in > Fig. 11. Techniques to reduce this artifact are explained in the following section.

CRT Display 60 Hz “Impulse”

Position

1

/60 s

Position

LCD Display 60 Hz “Sample-and-Hold”

Snapshot time

560

Time

Resulting image due to eye integration function at snapshot time

Field/Frame distance Time

. Fig. 11 Comparison between CRT display and LCD, film source

Video Processing Tasks

4.3

4.5.1

Techniques for Motion Blur Reduction

There are several approaches to address these particular problems. A possible classification of these may be [11]: 1. Improve the response time by faster liquid, overdriving techniques, optimization of liquid cells, or other novel materials 2. Change the driving of the panel from sample and hold to impulse driving (CRT approximation) 3. Increase the frame rate for high-speed driving Some selected solutions will be discussed now.

4.3.1

Overdrive or Response Time Compensation Techniques

The key to resolving the slow response time is to accelerate the switching time, which must be significantly smaller than the period of the input signal. The first simple technique used for this problem was called ‘‘Overdrive’’ or ‘‘Response Time Compensation’’ technique. The basic idea is to apply a higher electrical field than is originally required, in order to reach the new luminance value more quickly. > Figure 12 shows the basic idea of the overdrive compensation [12, 13]. The block diagram of such a method is sketched on the left-hand side of > Fig. 13. If the desired luminance level cannot be reached in one frame, the proposed method will introduce systematically wrong compensation values [13]. If the response time from, e.g., grayto-gray transition is slower than one time frame the Look-Up-Table (LUT) value may not be correct for the next frame, because the information of the attained level versus the assumed level is missing. > Figure 13 shows on the right-hand side a more advanced overdrive block diagram. Here the input of the frame memory is not the input signal, but a corrected input signal. For some gray-to-gray transitions, the second LUT models the response time to be slower than one time frame and stores the expected value for the next frame. The LUT values can be based on proper models or measurement values [13]. Once the response time has been shortened, to overcome hold-type effects, there are two broad categories of techniques, which will be described in more detail in the next sections.

Input signal 100% Luminance

Overdrive level Response signal without Overdrive Response signal with Overdrive

0% Position of interest n

n+1

n+2

. Fig. 12 Basic idea of the overdrive compensation

n+3

Time [frames]

561

562

4.5.1

Video Processing Tasks

Frame memory Frame memory

LUT

Input signal

LUT1 Output signal

Output signal LUT2

. Fig. 13 Block diagram of a conventional and advanced overdrive

4.3.2

Impulse Driving Techniques

Impulsive driving involves insertion of black or gray levels between or within frames to shorten the active data time, thereby reducing the eye tracking integration time. Examples of impulsive driving techniques include backlight blinking, backlight scanning, and black or gray data or frame insertion [2, 6, 11]. Black or Gray Frame Insertion (BFI, GDI)

A simple approach is to double the frame rate of the picture, e.g., from 60 to 120 Hz and to insert black or gray pictures. The motion blur will therefore be reduced due to less integration time during the increased black phase. Furthermore, the overall brightness and contrast will be reduced. To keep the same overall brightness and contrast, the intensity of the backlight has to be increased for the same ratio. This may be possible in the future with LED backlights. But the picture frame rate remains at nearly 60 Hz and with increased brightness the large area flicker will be more visible. Quasi-Impulse Driving (QID)

More advanced techniques use motion-adaptive processing methods to generate 120 frames out of 60 frames. > Figure 14 shows also the Quasi-Impulse Driving method (also called alternate frame driving or flexible data insertion) [11]. The same data is presented twice in each input interval (e.g., 60 Hz), once at bright gamma or luminance and again at dark gamma or luminance, mixing each to achieve the target gamma or luminance. Its main advantage is that it does not require a source of motion interpolated frames. Its disadvantage is an inability to eliminate motion blur in cases where there is significant high luminance content. Some publications mention a decreasing factor of motion blur of 26% [14]. Back Light Blinking, Scanning, or Cycling (BLS, BLC)

For black data insertion, the refresh rate of the panel has to be increased [11]. To overcome this problem, the backlight can be synchronously switched on and off to approximate the performance of a CRT. This method is very simple, but introduces ghost images, loss of luminance, and large area flicker. In the case of backlight scanning or cycling [11, 15], the horizontally oriented backlight lamps are sequentially turned on and off, row by row in order, and synchronized with each frame. Not only is the ghost image effect reduced, but also the luminance is lost. Furthermore, the costs are increased due to expensive individual inverters for each lamp.

Video Processing Tasks

4.5.1

BFI

QID

. Fig. 14 Principles of Black Frame Insertion (BFI) and Quasi-Impulse Driving (QID) techniques

All techniques explained above work more or less for video sources. If film sources are investigated, which may be transmitted by using pull-down techniques (3-2 for 24 Hz sources to 60 Hz or 2-2 for 25/30 Hz to 50/60 Hz), the above techniques will be less useful. But High Frame Rate techniques combined with proper video signal processing can solve this problem. High Frame Rate Techniques

High Frame Rate or high-speed driving involves more frequent updates of active data in order to reduce the eye tracking integration. Often this technique is combined with impulse driving techniques for further improvement of the motion blur artifact. Motion-compensated frame rate techniques are well known from 100-Hz interlaced Television systems [3, 4]. This technique was developed for double scan CRTs to remove the motion blur and double contour effect resulting from the frame rate conversion from 50 to 100 fields. Here the motion blur occurred due to showing the same field at the same position twice (simple conversion from 50 to 100 fields by repeating). The motion compensation system can generate new images to give true frame rate conversion, yielding, e.g., 100/120 unique frames per second. With these unique frames, the motion blur can be removed for CRT displays; for hold-type displays, the same can be done by also minimizing the spatiotemporal integration time. Applying motion compensation to a conventional display when displaying a video source has no effect on the motion blur. No new image positions must be created. But if the frame rate is doubled, the hold time of the images is halved using frame rate conversion (120 frames are generated out of 60). Furthermore, the generated images show true motion due to the motion compensation technique, which reduces the eye integration effect. This altogether reduces the motion blur and improves the overall sharpness of the display. Even more visible is the effect for film sources, as shown in > Fig. 15. Already when applying the motion compensation to a conventional 60-Hz LCD display the integration

563

4.5.1

Video Processing Tasks

LCD Display 60 Hz

Position

LCD Display 60 Hz “Motion Compensated”

LCD Display 120 Hz

LCD Display 240 Hz

“Motion Compensated”

“Motion Compensated”

Position

Position

Position

Snapshot time 1/60 s

564

Time Resulting image due to eye integration function at snapshot time

. Fig. 15 Motion compensation on conventional 60 Hz and advanced 120 Hz LCDs, film source

time can be halved. The 30 motion steps per second are doubled to full 60 motion steps per second. The motion blur is reduced to the same level as found for video sources. Using advanced 120-Hz LCD displays, the motion compensation converts the available 30 frames per second to full 120 frames per second. So the eye integration time is reduced dramatically by a factor of 4 when compared to conventional display techniques. The latest LCD panel technologies available on the market use now 200/240 Hz refresh rate, which again introduces theoretically a factor of 2 in motion blur reduction capabilities. In practice the improvement can be seen, but is not a yet major step compared to the mature 100/120-Hz displays. Further approaches are based on combinations of previous methods. These ideas will be covered in the next section. Combinations of High Frame Rate and Impulse Driving Techniques

Combination of High Frame Rate and back light blinking or scanning [15] is used to improve the motion blur artifact. Beside the high cost of fast switching backlights, the ghosting effect is still visible. Through the introduction of LED backlights, this ghosting effect could be eliminated. Nevertheless, the LED backlight has still multiple higher costs as a conventional backlight. Some companies call the combination of High Frame Rate and back light blinking for a 200/240-Hz LCD 400/480-Hz LCD TV. By combining the motion-compensated 100/120-Hz signal with the quasi-impulse driving approach [7], the result of the motion-compensated algorithm can again be improved and finally it also works for film sources.

4.4

Comparison

Refer to > Chap. 11.3.2 for a comparison of the different methods described above.

Video Processing Tasks

5

4.5.1

Conclusion

Flat-panel displays today challenge the superiority of CRTs on many levels, and some of the latest and largest models deliver eye-popping high-definition images. Getting the best picture from these panels requires the very best upstream electronics. Overdrive and Motion compensation together with frame rate conversion technology seeks to provide a sophisticated methodology to reduce the motion blur, still a major drawback compared to CRT displays. The film judder effect is also a disadvantage, especially for large displays. With the same approach used to reduce the motion blur effect, it is also possible to remove the judder.

6

Directions of Future Research

The optimization of the latest 200/240-Hz panels will lead to further improvements of the motion blur artifact. Furthermore, the video signal processing, which is needed for the proper interpolation of the 200/240-Hz video signal will be improved, especially concerning existing artifacts. Finally, the combination of all new methods will lead to best picture quality with less blur. Cost reduction will be mainly driven by integration of different processing steps. In addition, the numbers of external memories can be reduced. Currently the research activities focus on higher resolution and optimization for displaying stereo and auto-stereo 3D signals.

References 1. Schu M, Beintken H (2005) Micronas unravels secrets for improving resolution in FPD TVs. Disp Devices Spring 2005:18–19 2. Schu M, Rieder P, Tuschen C, Scheffler G, Bonnenberg H (2005) System-on-Silicon solution for high quality HDTV video de-interlacing and frame rate conversion for flat panel displays. ADEAC 05 Proceedings, pp 146–149 3. de Haan G, Kettenis J, Lo¨ning A, De Loore B (1996) IC for motion-compensated 100 Hz TV with natural-motion movie mode. IEEE Trans Consum Electron 42(2):165–174 4. Schu M, Scheffler G, Tuschen C, Stolze A (1999) System-on-silicon for motion compensated scan rate conversion, picture-in-picture processing, split screen applications and display processing. IEEE Trans Consum Electron 45:842–850 5. Wolberg G (1990) Digital image warping. IEEE Computer Society Press, Los Alamitos, CA. pp 124–161. Monograph 6. Schu M, Rieder P (2007) Frame rate conversion IC for 120 Hz flat panel displays. Electronic Displays Conference 2007 Digest 7. Schu M, Hahn M, Rieder P (2007) Frame rate conversion IC for full HD 120 Hz LCD flat panel displays. IMID 07 Digest pp 1089–1092

8. Sekiya K, Nakamura H (2002) Eye-trace integration effect on the perception of moving pictures and a new possibility for reducing blur on hold-type displays. SID 02 Digest pp 930–933 9. Boher P, Glinel D, Leroux T, Bignon T, Curt JN (2007) Relationship between LCD Response Time and MPRT. SID ’07, Symposium Digest 38:1134–1137 10. Poynton C (1996, 1998) Motion portrayal, eye tracking, and emerging display technology. Proceedings of the 30th SMPTE advanced motion imaging conference. New York, pp 192–202 11. Kim SS, Berkeley BH, Kim T (2006) Advancements for highest-performance LCD-TV. SID 06 Digest pp 1938–1941 12. McCartney RI (2003) A liquid crystal display response time compensation feature integrated into an LCD panel timing controller. SID 03 Digest pp 1350–1353 13. Gatti G (2007) Motion artifacts in medical applications. SID 07 Digest pp 117–119 14. Furuhashi T, Yoshioka H, Miyazawa T, Ono K (2007) Display system for high performance TFT-LCD. Electronic Displays Conference 2007 Digest 15. Sluyterman AAS, van der Poel WAJA (2007) Motion-fidelity improvement at a frame rate of 120 Hz via the use of a scanning backlight. SID 07 Digest 38(1):127–130

565

566

4.5.1

Video Processing Tasks

Further Reading Autronic-Melchers GmbH, Greschbachstrasse 29, 76229 Karlsruhe. (Rev 02/Nov 07) MOTION Artifacts and display Response Time analysis system Becker ME (2007) Motion blur measurement and evaluation: from theory to the laboratory. SID 2007 Digest pp 1122–1125 Bellers EB, de Haan G (2000) De-interlacing: A Key Technology for Scan Rate Conversion. Elsevier Hartmann S, Schu M (2008) Comparison of various motion blur reduction methods. Electronic Displays Conference 2008 Digest Kim SS, Kim ND, Berkeley BH, You BH, Nam H, Park J-H, Lee H (2007) Distinguished Paper: Novel TFT-LCD Technology for Motion Blur Reduction Using 120Hz Driving with McFi. SID 2007 Digest pp 1003–1006 Laur J, Becker ME (2007) Motion Blur Measurement with a High-Speed Camera. IMID 07 Digest pp 1135–1138

Lee B-w, Yang Y-c, Park D-j, Park P-y, Jeon B, Hong S, Kim T, Moon S, Hong M, Chung K (2006) Spatio-temporal edge enhancement for reducing motion blur. SID 2006 Digest pp 1801–1803 Michael EB (2007) Motion blur measurement and evaluation: from theory to the laboratory. SID ’07, Symposium Digest of Technical Papers 38(1): 1122–1125 Pan H, Feng XF, Daly S (2006) LCD motion blur analysis and modeling based on temporal PSF. SID 2006 Digest pp 1704–1707 Poynton C (2003) Digital Video and HDTV – Algorithms and Interfaces. Morgan Kaufmann, San Francisco, http://www.poynton.com Sluyterman AAS, Boonekamp EP (2005) Architectural choices in a scanning backlight for large LCD TVs. SID 2005 Digest pp 996–999

4.5.2 Dimming of LED LCD Backlights Chihao Xu . Marc Albrecht . Tobias Jung 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568

2 2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.2

Principles of Backlight Dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568 Determination of the LED Duty Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 570 0D-Dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 570 1D-Dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 570 2D-Dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 571 3D-Dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 572 Post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 572

3 3.1 3.2 3.3 3.4 3.5

Effects on Visual Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573 Contrast Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573 Color Gamut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573 Clipping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573 Flicker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574 Halo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574

4

Summary and Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.5.2, # Springer-Verlag Berlin Heidelberg 2012

568

4.5.2

Dimming of LED LCD Backlights

Abstract: This chapter focuses on dimming of LED LCD backlight. The main goal is the reduction of the backlight’s power consumption by adapting the backlight luminance according to the image properties without degradation of the image quality. First of all, the principles of backlight dimming are explained. The focus is on the most worthwhile method called local dimming. The post-processing, where the TFT values are adapted is shown afterward. Then the effects on the visual quality of backlight dimming are explained. List of Abbreviations: Backlight Dimming, Method to Dim the Backlight According to Image Properties; CCFL, Cold Cathode Fluorescent Lamp; LCD, Liquid Crystal Display; LED, Light Emitting Diode; PWM, Pulse Width Modulation; TCON, Timing Controller; TFT, Thin Film Transistor

1

Introduction

The reduction of power consumption is one of today’s most important topics of the LCD industry. With about 80% of LCDs’ total power consumption, the backlight is the main consumer. In conventional displays, the backlight acts as a constant light source and consumes the same power for dark as well as for bright images. Nowadays the cold cathode fluorescent lamp (CCFL) light sources of LCD’s backlight are more and more replaced by LEDs. Beside other advantages such as higher efficiency and smaller dimensions, the LEDs can easily and rapidly be dimmed with PWM. So the backlight luminance can be adapted according to the properties of the image. The challenge is to find a proper, hardware-efficient solution for the LED duty cycles so that image quality is conserved and the sum of duty cycles, which is proportional to the power consumption, is minimized.

2

Principles of Backlight Dimming

As seen in > Chap. 7.5.1, the function of a standard backlight is to produce a preferably homogeneous luminance over all pixels. The light sources are white LEDs (also rare backlights with RGB LEDs are available) which are either placed on the side of the display (edge-lit) or behind the pixel plane (direct-lit). > Figure 1 shows the relation between backlight type, LED type, and dimming method. As it can be seen, four dimming methods are possible. In 0D-dimming (also known as global dimming) all LEDs are dimmed by the same amount, in 1D-dimming the LEDs in each column or in each row are dimmed by the same amount. The idea of local dimming (2D-dimming) is to adapt the (2D-distributed) backlight according to the image content. 3D-dimming (colored local dimming) is local dimming with the red, green, and blue LED colors as an additional dimension which can be varied. It is obvious that the higher the number of independently controllable LEDs, the more complex is the calculation of the LED duty cycles, but also the higher is the possible power saving. As a comparison, > Fig. 2 shows an example image and the backlight luminance of the different dimming types.

Dimming of LED LCD Backlights

Direct-lit

3Ddimming

2Ddimming: local dimming

RGB LED backlight

4.5.2 Backlight

Edge-lit

1Ddimming

0Ddimming: global dimming

White LED backlight

Dimming

LED color

. Fig. 1 Relation between backlight type, LED type, and dimming type

Original image

No dimming

0D-dimming

1D-dimming

2D-dimming

3D-dimming

. Fig. 2 Overview dimming types (bright = high luminance, dark = low luminance)

The following table compares the average power saving of the different dimming types. Dimming

0D

1D

2D

3D

Power saving

15%

30%

50%

60%

In general, a local dimming approach consists of the following two steps, which are described in the following sections: 1. Determination of LED duty cycles (see > Sect. 2.1) 2. Adaption of the TFT values/post-processing (see > Sect. 2.2)

569

570

4.5.2 HDMI/ YUV/ CVBS/ ...

Video processor

Dimming of LED LCD Backlights

RGB/ TTL/ LVDS

Dimming hardware 1. Calculation of LED duty cycles 2. Adaption of TFT transmission

RGB/ TTL/ LVDS

TFT LCD module

LED values

Backlight module

. Fig. 3 Integration in image data flow

The adaption of the TFT values is very similar in the different approaches. In contrast, there exist principally different approaches for the determination of the LED duty cycles. The dimming hardware can easily be integrated in an existing LCD (see > Fig. 3). The dimming hardware is plugged between the video processor and the TFT LCD module. The data stream is buffered for at least the time which is necessary for the adaption of the TFT values (see > Sect. 2.2). The dimming hardware delivers the calculated LED duty cycles and the adapted TFT transmission to the TFT LCD module and the backlight module, respectively.

2.1

Determination of the LED Duty Cycles

As already mentioned, the challenge of backlight dimming is to find a proper, hardwareefficient solution for the computation of the LED duty cycles in time. In this section, a rough explanation of the differing approaches is given.

2.1.1

0D-Dimming

The easiest dimming type is 0D-dimming which is useful especially for dark images. The most common method is to spread the gray-level histogram of the input image. A critical gray-level a must be determined. Above this gray-level, everything is to be considered as noise. Then the backlight luminance is reduced by factor a/max, where max is the maximum adjustable graylevel. At the same time the TFT values are enhanced by factor max/a. The brighter the image, the higher is a thus the lower is the power saving. With the choice of a low critical gray level for bright images, clipping is expected (see > Sect. 3). > Figure 4 visualizes the above-explained procedure.

2.1.2

1D-Dimming

1D-dimming is the only feasible way if several CCFLs are placed behind the LCD. For LED backlight, the procedure can be considered as a simplification of 2D-dimming (see the following section).

Dimming of LED LCD Backlights

Original histogram

TFT original

Spreaded histogram

All-on backlight 100%

Noise

TFT adapted

4.5.2 Original image

=

Dimmed backlight 70%

Adapted image

=

. Fig. 4 0D-dimming example

2.1.3

2D-Dimming

Image Processing Method

The most common approach for the calculation of local dimming backlight is based on image processing methods. In general, the approach consists of two phases. In the first phase, the original image passes through a low-pass filter to reduce the high resolution image to the backlight resolution. In the second phase, the duty cycles of all LED are calculated based on the low-resolution image’s gray-level histogram of those pixels that are assigned to this LED. The viewing direction – from pixel to LED – leads to a restriction of the LED arrangement. It is, for example, not applicable for an edge-lit backlight with LEDs on each side. For a detailed description of the presented method we refer to [1–3]. Specific Local Dimming Algorithm

For a better consideration of the physical characteristics of the display, a specific algorithm solely for local dimming has been developed. It is based on the physical viewing direction – from LED to pixel – because the backlight power consumption is a physical quantity as well. The basis is a mathematical model (1) for local dimming backlight, which is analyzed as follows: 2 3 2 3 2 3 2 3 a1;1 a1;1 ::: a1;L x1 b1 i1 6 a2;1 a2;2 7 6 x2 7 6 b2 7 6 i2 7 6 7
6 7¼6 76 7 ð1Þ 4 ::: 5 4 ::: 5 4 ::: 5 4 ::: 5 ::: aP;L aP;1 xL bP iP A is the so-called influence matrix and describes the influence of each LED on each pixel. The vector x stands for the LED’s duty cycles. The product of both results in the achieved luminance B in each pixel, which has to be higher than the required luminance I of the image. The goal is to minimize the sum of the LED duty cycles, which is proportional to the backlight power consumption. So the physical characteristics of the backlight unit are considered right from the beginning. The key observation to this approach is that the influence matrix A is sparse, because an LED of a local dimming backlight mainly influences its neighbored pixels. In other words, every pixel is mainly influenced by only a few LEDs. This fact allows the use of an approximation algorithm. This algorithm consists two phases. First of all, the initial image condensing phase reduces the image resolution. Several condensing functions are possible to control the power saving

571

572

4.5.2

Dimming of LED LCD Backlights

and clipping ratio, respectively. The condensed image still has a much higher resolution than the LED resolution. The determination of the LED duty cycles has a fixed number of iterations in which a predefined number of pixels are processed in a predefined sequence. In each iteration, the LED duty cycles are successively increased to cover the desired luminance of each pixel. During the calculation of the LEDs, the interactions (crosstalk) between the LEDs are fully considered. This method assures clipping-free as well as several clipping-affected results close to the theoretical optimum solution. Due to the physical viewing direction, this method is universal and can be applied to any kind of LED arrangement. For an edge-lit backlight with six LEDs, a power saving of 50% is possible. For a detailed description of this approach we refer to [4, 5].

2.1.4

3D-Dimming

Simplified, the LED duty cycles for 3D-dimming can be determined by separation of the three color channels. In this case, the hardware effort is three times higher than that of 2D-dimming. However a real 3D-dimming is by far more complex than that. The color crosstalk of the LEDs must be considered during the calculation of the LED duty cycles and the post-processing. For example, a green LED also has a significant contribution to the red- and blue-colored subpixels. This contribution must be considered during the determination of the LED duty cycles. At present, there are some publications in the field of 3D-dimming such as [1], but none of them takes account of color crosstalk during LED duty cycles determination. Beside conventional LCDs, another application for local dimming is Field Sequential Color (FSC). An FSC’s luminous efficacy is three times higher than that of conventional LCDs, because FSC LCDs do not have color filters. The color is generated with red, green, and blue LEDs. Each frame is divided in at least three subframes, one for each color. That’s why very quick LC cells are needed. For a detailed description of FSC we refer to [6, 7].

2.2

Post-processing

As the backlight luminance is reduced, the TFT values must be adapted to meet the image properties. This can be seen in > Fig. 4. The aim of the post-processing step (> Fig. 5) is to adapt and increase the LC transmission to the dimmed (usually darker) backlight and achieve the same visual appearance like in the undimmed mode. First of all, the achieved luminance in each pixel has to be calculated (see > Eq. 1). Hence the following set of equations must be fulfilled: 0 0 ¼ bp0 tc;p ; bp0 tc;p

8p ¼ 1 . . . P and c 2 fred; green; blueg

ð2Þ

with bp being the backlight luminance behind the pixel p in undimmed mode and bp0 for the 0 are the corresponding TFT ratios. In other words, the dimmed mode, respectively, tc;p and tc;p light output for each subpixel of the display matrix should remain unchanged. As the original value ðbp  tc;p Þ and the adapted backlight luminance bp0 are fixed now, the 0 must be manipulated to satisfy the equations. The linear correction can be easily TFT ratios tc;p 0 : computed by solving > Eq. 3 for tc;p

Dimming of LED LCD Backlights

Standard backlight

Dimmed backlight

Original TFT values

4.5.2 Adapted TFT values

. Fig. 5 An example of post-processing

0 tc;p ¼

bp  tc;p ; bp0

8p ¼ 1 . . . P and c 2 fred; green; blueg

ð3Þ

Of course, also nonlinear post-processing can be adapted in the gamma correction. As the post-processing step usually includes an increase of TFT transmission, there is always the risk of getting out of the valid gray-level range. This effect is called clipping, which is explained and discussed in > Sect. 3.

3

Effects on Visual Quality

The impact of local dimming to the visual quality is treated in this section. Beside the advantages, there are also some parasitic effects that can occur.

3.1

Contrast Enhancement

Beside the reduction of power consumption, local dimming is also a valid help to enhance the visual quality of the image. The static contrast can be increased, so a higher number of gray values can be realized. For further information, we refer to [8, 9].

3.2

Color Gamut

The use of red, green, and blue LEDs (3D-dimming) allows a high color gamut [10].

3.3

Clipping

When a backlight is dimmed, it is possible that the achieved luminance Β of a pixel is smaller than the required luminance I (see > Eq. 1). This leads to a loss of image information which sometimes is perceivable for the human eye. A proper processing should assure that clipping does not degrade the image quality.

573

574

4.5.2 3.4

Dimming of LED LCD Backlights

Flicker

When some parts of the content of single frames in a video quickly and frequently change while other parts remain static, several LED values may also change quickly and frequently. The more the light generated by the quickly and frequently changing LEDs spreads into static image regions, the more the flicker is visible. This problem can be solved with temporal filters, for example, as seen in [1].

3.5

Halo

Halo is caused by light leakage [11] of the LCD panel around a bright object. It occurs especially with locally dimmed direct-lit backlights. Some publications such as [12] show how halo can be limited.

4

Summary and Directions for Future Research

With the specific local dimming algorithm, the power consumption of an LCD is substantially reduced. The logic complexity/HW cost is low while the processing speed is high. Also higher visual quality – for example, more contrast – can be achieved. This physical approach should further extend to 3D dimming and field sequential color.

References 1. Chen H, Sung J, Ha T, Park Y (2007) Locally pixelcompensated backlight dimming on LED-backlit LCD TV. J SID 15(12):981–988 2. Yeo D-M et al (2008) Smart algorithms for local dimming LED backlight. SID ‘08 Dig 39(2):986–989 3. Groot Hulze H, de Greef P (2009) Power savings by local dimming on a LCD panel with side lit backlight. Proceedings of SID ‘09, 749–752 4. Albrecht M, Karrenbauer A, Jung T, Xu C (2010) Sorted sector covering combined with image condensation - an efficient method for local dimming of direct-lit and edge-lit LCDs. IEICE Trans Electron E93-C(11):1556–1563 5. Albrecht M, Karrenbauer A, Xu C (2009) A videocapable algorithm for local dimming RGB backlight. Proceedings of SID ‘09, 587–590 6. Yamada F, Nakamura H, Sakaguchi Y, Taira Y (2002) Sequential-color LCD based on OCB with an LED backlight. J SID 10(1):81–85

Further Reading Kobayashi S, Mikoshiba S, Lim S (2009) LCD backlights. Society for Information Display - Wiely, Hoboken, NJ; ISBN 9780470744826

7. Lin F-C, Huang Y-P, Wei C-M, Shieh H-P (2009) Color-breakup suppression and low-power consumption by using the Stencil-FSC method in field-sequential LCDs. J SID 17(3):221–228 8. Seetzen H et al (2004) High dynamic range display systems. ACM Trans Graph 23(3):760–768 (SIGGRAPH 2004) 9. Langendijk E, Muijs R, van Beek W (2008) Contrast gain and power savings using local dimming backlights. J SID 16(12):1237–1242 10. Schwedler W, Nguyen F (2010) LED backlighting for LCD TVs. Proceedings of SID ‘10, 1091–1096 11. Kim S-E, An J-Y, Hong J-J, Lee T-W, Kim C, Song W-J (2009) How to reduce light leakage and clipping in local-dimming liquid-crystal displays. J SID 17(12):1051–1057 12. Zhang W, Chen M, Niu W, Huang D (2009) LED control signal extraction by using multiple representative values. In: Proceedings of IDW 2009, 1519–1522

Part 4.6

Power Supply

4.6.1 Power Supply Fundamentals Oliver Nachbaur 1 Basic Operation of a Liquid Crystal Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578 2 Active Matrix LC Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580 3 Video Driver Supply Voltage Requirements (Vs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584 4 Gate Driver Supply Voltage Requirements (VGH and VGL ) . . . . . . . . . . . . . . . . . . . . . . . . . . 584 5 VCOM Voltage Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584 6 Compensation for Parasitic Gate Line Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587 7 Gamma Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588 8 Fully Integrated LCD Bias Power Supply IC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590 9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.6.1, # Springer-Verlag Berlin Heidelberg 2012

578

4.6.1

Power Supply Fundamentals

Abstract: Liquid Crystal Displays require dedicated power supply circuits to support their specific requirements. Many different display technologies coexist in the market and compete for their market share. While the passive matrix Twisted Nematic (TN) or Super Twisted Nematic (STN) LCDs require a fairly simple bias supply circuit, the active matrix Liquid Crystal (LC) Displays require a more sophisticated bias supply. The active matrix usually requires several power rails with power up and down sequencing. In this chapter, the power supply requirements and implementation for the standard passive matrix LC Displays are discussed. The passive matrix LC Display usually requires a single supply rail. The active matrix LC Display uses Thin Film Transistors (TFT) as an active element turning each pixel on and off. To control the TFTs two additional supply rails are required. The active matrix display also requires a common voltage rail to establish an alternate current (AC) signal across the pixel. This is implemented by using a dynamic or static common voltage drive architecture. The LCD power supply not only provides several voltage rails but also generates the voltages for the gamma reference, and compensates voltage shifts due to gate signal pulses and parasitic gate line capacitance and resistance. This chapter discusses the LCD requirements and how they are supported and influenced by the LCD bias supply and control circuits.

1

Basic Operation of a Liquid Crystal Cell

> Figure

1 shows a simplified liquid crystal cell. Applying a voltage Vp across the cell changes the twist of the liquid crystal structure. Depending on the crystal twist and polarizer filter the light passes through the liquid crystal cell or is blocked as the voltage Vp is applied. The liquid crystal does not allow a constant Direct Current (DC) voltage being applied across the cell. A DC voltage damages the display. An Alternate Current (AC) voltage is applied across the cell and a simple implementation is shown in > Fig. 1. Vc is the common voltage and Vs is the source voltage. The pixel voltage Vp is then calculated. Vp ¼ Vc  Vs

Liquid Crystal

Vs

Vc

. Fig. 1 Simplified liquid crystal cell

ð1Þ

Vp

Power Supply Fundamentals

4.6.1

> Figure 2 shows the implementation of the voltage waveform Vs and Vc yielding an AC voltage Vp across the liquid crystal cell. This type of addressing scheme is known as direct addressing and is used for numeric displays. A direct addressing scheme is limited to displays with only a few segments or pixel. When more pixels and higher display resolution is required, a passive matrix addressing scheme is used. In a passive matrix addressing scheme, one row after the other is selected by applying the select voltage. The signal or video voltage is applied to each column simultaneously. This addressing scheme applies not only a voltage to the selected pixel but also part of the voltage to its neighborhood pixels. This limits the display contrast depending on the voltages applied and number of rows. Using the Alt&Pleshko formula allows calculating the row select and video voltage required for a typical passive matrix LC Display. Depending

Common Signal Vc V

0V Off Segment Signal Vs Voff

0V On Segment Signal Vs Von

0V

Off Segment Pixel Voltage Vp 0V

On Segment Pixel Voltage Vp V+

V–

. Fig. 2 Voltage waveforms for displays using direct addressing

579

4.6.1

Power Supply Fundamentals

Vmain(V5) S+F(V4) R Column Driver

S(V3) R S–F(V2) (S / F–3)R

Row Driver

580

TN / STN Display

2F(V1) R F(V0)

S = Row select voltage F = Video signal voltage

R

. Fig. 3 Voltage generation for passive matrix addressing scheme

on the display a practical supply voltage ranges between 17 and 20 V. From this voltage rail all the other voltage rails are generated as shown in > Fig. 3. The load current requirement for the voltage rail generating Vmain in > Fig. 3 is typically in the 20 mA range, depending on display size and resolution. Most of the systems using this type of display operate from a rechargeable battery with a voltage range between 1.8 and 5.0 V. Alternatively a standard supply rail of 5 or 3.3 V is already available in the system. To generate the voltage rail Vmain the most common implementation uses a boost converter. An example of using the TPS61040 is shown in > Fig. 4. The brightness and contrast of the display is set by changing the output voltage of the boost converter. One solution to implement this is by connecting a resistor to the feedback pin (FB). By applying a control voltage to this resistor sets the output voltage of the boost converter thus regulating the LCD brightness. There are also devices on the market allowing to set the output voltage of the boost by applying a digital signal as supported by the TPS61040. For detailed application circuits please refer to Texas Instruments datasheets TPS61040 and TPS61045.

2

Active Matrix LC Displays

The active matrix Liquid Crystal (LC) Display uses a Thin Film Transistor (TFT) deposited on an LCD backplane using an amorphous silicon process typically for LCDs larger than 700 . A simplified typical matrix array is shown in > Fig. 5. Compared to the passive matrix each pixel is controlled by a thin film transistor. When the image is written to the display then one line after the other is turned on starting with the first line on the top of the screen and ending with the last line at the bottom of the screen. The gate line is turned on by applying a high voltage to the entire line turning on all the TFTs. When the TFTs are turned on then the video

4.6.1

Power Supply Fundamentals

L1 10uH

D1 Vout 18V

Vin 1.8V to 6.0V Cff 22pF

R1 2.2M Vin

Co 1uF

SW FB

Cin 4.7uF

EN

GND

R2 162K

. Fig. 4 Generating the voltage rail for passive matrix displays

Video Signal m

Video Signal m+1

Gate Line n−1

CLC

CS

Vcom Gate Line n−2

CLC Vcom

CS Gate Line n−2

Gate Line n

CLC

CS

Vcom Gate Line n−1

CLC

CS

Vcom Gate Line n−1

. Fig. 5 Active matrix array using TFT transistors

signal is applied to each pixel in the line, charging the pixel cell represented by CLC and its storage capacitor Cs. The storage capacitor Cs is connected to the previous gate line to save an additional ground line. Then the line is deselected by applying a negative voltage to the gate line and applying a high voltage signal to the next line being addressed. The liquid crystal cell stores the charge until it is being addressed again. For this purpose a storage capacitor is used that is connected to the previous gate line (n  1). Basically two control topologies are used to establish an AC signal across the pixel. One is the dynamic VCOM control and the other is the static VCOM control. The dynamic VCOM

581

582

4.6.1

Power Supply Fundamentals

control is used for medium to small size displays. Dynamic VCOM control has basically a 5 V video signal driver. Large size displays, usually >700 use a static VCOM control. With a static VCOM control, the VCOM voltage is held constant and usually half of the maximum video driver voltage. The AC voltage across the cell is generated by a VCOM using an OpAmp. This chapter focuses on the operation and voltage rails for a static VCOM control. Due to the relative low electron mobility of the TFT a high turn on voltage between 24 and 35 V is required to achieve a sufficiently low drain source on resistance of the TFT. To minimize the discharge of the cell when not being addressed a negative voltage is applied to the TFT gate. This minimizes the cell discharge current for non-addressed rows. Depending on the panel manufacturing process a voltage between 5 and 7 V is used. For the video driver voltage a voltage between 7 and 18 V is used, depending on the LC threshold voltage and panel technology. For example, a notebook panel usually operates with a 7 V video driver voltage and a TV panel uses around 15–18 V. To have an AC signal across the liquid crystal cell a common voltage, Vcom, is used. The liquid crystal is connected to the common ITO driven by the Vcom voltage. > Figure 6 shows a simplified block diagram generating the main LCD supply voltages. Vs is the power supply for the source driver providing the video signal. The common voltage is generated by the VCOM buffer typically set slightly below half of the source driver voltage, Vs. This allows a simple implementation of an AC signal across the liquid crystal cell with a voltage swing of the source driver above or below the common reference voltage. In order to allow a stable VCOM voltage, this voltage rail needs to be generated using an OpAmp or unity gain buffer. This voltage rail sinks and sources current. > Figure 6 shows the main voltage rails required for driving an active matrix liquid crystal panel. The complete block diagram of an LCD system is shown in > Fig. 7.

Gate Driver

Row select signal

Source Driver

TFT

VLC = Vs-Vcom

Video Signal

Vcom

– +

VGL

+ –

VGH

+ –

∼Vs/2 Vs

. Fig. 6 Active matrix pixel driving circuit using static VCOM control

CLC

Cs ITO Common Electrode

Graphic Controller (Scaler)

TPA3005

MCU

3.3V 2.5V etc.

System Power

3.3V/2.5V

LCD Bias Supply and LCD Support IC TPS65162

LVDS/ RSDS

Timing Controller TCON I2C Bus

VGL

VGH

3.3V

Vs

Vs

Inverter supply (12V/18V/24V)

3.3V/5V/12V

Audio

OSD

. Fig. 7 Block diagram of LCD system and panel board

3.3V 2.5V

VGA/DVI HDMI

12V

5V

Temp Sensor TMP100

12V/5V

AC/DC Wall Adapter and inverter supply

LCD Control Driver Control

220V 110V

Single or multi-channel VCOM Buffer

Video Driver IC

TFT LCD Panel

MEMORY

CCFL Backlight TPS68000 RGB Driver/White LED Driver TLC5940

Gamma Correction Buffer BUF16821

Panel Board

Gate Driver IC

System Board

Power Supply Fundamentals

4.6.1 583

584

4.6.1

Power Supply Fundamentals

The right side of > Fig. 7 shows the components usually placed on the panel board. The LCD bias supply provides the TFT voltages, VGH and VGL, as well as the video driver voltage rail Vs. It also provides the logic voltage rails for the timing controller, gate and source driver IC and memory. > Figure 7 includes the reference voltage generator IC for the gamma correction. The VCOM buffer can be a stand-alone IC or integrated into the LCD bias supply or gamma correction IC. The left side shows the components of the system board. The system board is designed by the set maker designing and manufacturing the entire system, for example, an LCD TV. The main components are the AC/DC converter and the system power supply providing all the logic rails. The graphic controller processes the video signal and scales it to the required display resolution. Then the data is sent to the timing controller using a LVDS or RSDS interface. To achieve a superior picture quality the LCD bias supply, gamma correction, and VCOM voltage generation have to be optimized. These requirements and operation principles are discussed in the next sections.

3

Video Driver Supply Voltage Requirements (Vs)

The supply voltage for the video driver needs to be very stable to achieve an output voltage accuracy of 1%. Good output voltage accuracy is especially important when the gamma reference voltages are being generated from this rail. Any change in output voltage would directly influence the gamma curve. As a result the gamma curve differs from panel to panel depending on the output voltage of the source driver supply, Vs. The other important parameter is the load transient regulation that should be kept below 100 mV during a load transient. This minimizes cross coupling effects between display areas when one line is driven with a high load current and the next line is driven with low load current.

4

Gate Driver Supply Voltage Requirements (VGH and VGL )

The gate driver supply is required turning the TFT on and off. The load seen by the gate driver supply is effectively a series resistor with the gate capacitance of the TFTs. The overall capacitance is the sum of the TFT gate capacitance and the gate line resistance. Display size and technology determines the load current for VGH and VGL. For small displays < 700 a 2 mA average load current is sufficient whereas large LCD TV panels around 4000 require up to 100 mA average current. The output voltage accuracy and load transient regulation for these voltage rails is more relaxed compared to the source driver voltage supply rail. If the output voltage ripple during load transients becomes much larger than 100 mV, then the modulation of the resistance of the TFT shows visible image artifacts.

5

VCOM Voltage Regulation

The video signal is shown in > Fig. 8 with a positive voltage swing above and below the VCOM reference voltage. Since the LC cell voltage is reference to the VCOM voltage an AC signal appears across the cell. With this implementation no negative voltage rail is needed simplifying driver and power supply circuits.

4.6.1

Power Supply Fundamentals

Gate Line Signal VGH

VGL

Video Signal Row select time VVideo

Charge hold up time

ΔV VVCOM ΔV

. Fig. 8 Gate line signal block diagram of LCD system and panel board

In an ideal case the VCOM voltage needs to be adjusted to half of the video drive voltage to have a DC free voltage across the cell. Because each TFT has its associated parasitic capacitance the falling edge of the gate signal couples into the liquid crystal cell. This causes a voltage shift DV. The voltage shift would cause a constant DC voltage across the pixel causing image sticking and display flicker. This is avoided by setting the VCOM voltage slightly lower compensating the voltage shift. The voltage shift depends on the parasitic TFT capacitance CGS, liquid crystal capacitance CLC, storage Capacitance CS, and gate line voltage difference VGH to VGL. DV ¼ VGH  VGL

CGS CGS þCLC þCS

ð2Þ

In practise, the LCD flicker is minimized by either programming the VCOM voltage manually or using an optical measurement system. To measure the LCD flicker a special test pattern needs to be used according to the LCD driving method of odd and even frames. The storage capacitance and liquid crystal capacitance depends on the applied video voltage. Therefore the test has to be performed with additional test patterns going through the entire grey levels of

585

4.6.1

Power Supply Fundamentals

the display to find the optimum VCOM voltage. Another parameter changing the compensation voltage, DV, is the ambient temperature. The parasitic capacitance changes over temperature and therefore the coupling into the LCD pixel changes over temperature as well. To compensate for this effect the new LCD bias devices have a temperature compensated VCOM buffer adjusting its output voltage according to the ambient temperature. For a manual adjustment of the VCOM voltage a simple potentiometer is used. The automatic or half-manual setup allows programming the VCOM voltage using an I2C interface. For the VCOM voltage generator itself two basic VCOM circuits are used depending on panel size and performance requirements. The simple unity gain buffer of > Fig. 9 drives the ITO connected to the LC cell. A typical output voltage waveform for such a unity gain buffer is shown. The load seen by the buffer is a combination of series resistance and load capacitance. In fact, the LC pixel and storage capacitance does not represent the dominant load impedance. The load impedance is rather dominated by the parasitic capacitances of the panel formed by gate and source line capacitance. A single unity gain buffer is a good solution for panels smaller than 1700 . As the panel gets larger a single unity gain buffer is not able to regulate the VCOM voltage across the entire panel. For instance, when the buffer is connected to the left side of the panel then the regulation is only good at this side of the panel. On the right side of the panel the regulation gets gradually worse, due to parasitic series resistance and load capacitance. In such a case the panel shows cross talk and increased flicker toward the right side of the panel. To improve the regulation of the VCOM reference the VCOM line is driven by multiple unity gain buffer, connected to each side of the panel. Another solution is to use active feedback from the panel to overdrive the VCOM buffer as shown in > Fig. 10. The circuit of > Fig. 10 uses active feedback from the panel regulating the VCOM voltage. Compared to the unity gain buffer of > Fig. 9 the output voltage over and undershoot is much larger. The advantage is the fast settling time of the VCOM reference voltage within the addressing time of the gate line. For such an application the OpAmp has to provide output currents up to 200 mA for a large size LCD TV panel. The gain of the feedback signal from the panel and thus the OpAmp overdrive is adjusted using R3 and R4. Gfeedback ¼ 

Vs

R1 Vcom Fig. 12. The control signal (CTRL) for the gate shaping circuit is generated by the timing controller TCON.

587

4.6.1

Power Supply Fundamentals

CTRL 5V/div

4

VGH 10V/div

1

4us/div

. Fig. 12 Gate voltage shaping reduces flicker across the panel

1,0 0,9 Output signal luminance

588

0,8

g = 2.2

0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 0,0

0,1

0,2

0,3

0,4 0,5 0,6 Input signal luminance

0,7

0,8

0,9

1,0

. Fig. 13 Input to output signal luminance transfer function

7

Gamma Correction

The gamma correction is the transfer function of the video input signal luminance versus the output signal luminance of the LCD or CRT. Traditionally the CRT has an inherent gamma characteristic of g = 2.2 to g = 2.8. > Figure 13 shows a typical gamma curve derived from its exponential function: y ¼xg where y is the output luminance, x the input luminance.

ð4Þ

R4 150k

R3 620k

D2

D3

1

D1

2

C5 1uF

Step Down Converter

SWI

GND

DLY2

DLY1

FBB

SWB

SWB

CBOOT

VDD

RE

FBP VGHM

VGH

DRVP

FB

C6 1uF

PGND1 PGND2 PGND3

Gate Voltage Shaping

D S

Boost Converter

C4 22uF

Negative VIN Charge Pump Driver

OGND

NEG1

POS1

AGND

REF

FBN

DRVN

VDPM VFLK

CE

VC

SS

VIN

FREQ

. Fig. 14 Fully integrated LCD bias IC, TPS65162 from Texas Instruments

C7 470u

Enable Gate Voltage shaping VFLK VCL –5V/50mA

EN1 EN2

PVIN

C3 22uF

PVIN SW OUT1

C1 C2 22uF 22uF

SW NEG2

SWO AVIN

L1 10uH

POS2

SUP

Vin 8V to 14V

OUT2

R1 825k

R2 75k

C7 22p

L2 10uH

R6 33k

R5 715k

R6 33k

R5 715k

Vs

C8 C9 C10 22uF 22uF 22uF

Vlogic 3.3V/ 1.5A

VGH 23V/ 50mA

Vs 18V/1.3A

Power Supply Fundamentals

4.6.1 589

590

4.6.1

Power Supply Fundamentals

The objective of gamma correction is to achieve a linear brightness curve being perceived by the human eye. This is achieved by compensating for the human visual system (eye), nonlinear brightness functions sent to the liquid crystal panel, and by compensating nonlinearity of the LCD itself. To achieve compatibility with the CRT the LCD has to have a gamma close to the CRT plus some modifications compensating for the LCD characteristic. The simplest generation of the gamma reference voltages is a simple resistor ladder connected to the source driver voltage Vs. This is the most cost effective solution. Since these reference voltages are load current depended and noise sensitive the gamma curve slightly changes during operation. The more precise solution uses gamma correction buffers providing a stable and accurate voltage reference. Due to the voltage shift, DV, discussed in the previous chapter it is also of advantage having slightly different gamma curves for odd and even frames. The latest gamma correction ICs allow programming the gamma reference voltages using an I2C interface. This allows a simple implementation of different gamma curves for odd and even frames and allows varying the gamma curve according to the display image. For example, such a solution can be implemented using the BUF16821 and BUF08821 from Texas Instruments. Standard LCD panels currently use 8 bit grey scale levels. The new LCD TV panels use already 10 bit further increasing grey level resolution.

8

Fully Integrated LCD Bias Power Supply IC

Optimizing performance, component count and overall system cost drives higher integration. On the LCD panel board only a few ICs are required to build the entire system. Main components are timing controller, memory, LCD bias IC, and gamma correction buffer. > Figure 14 shows a fully integrated LCD bias IC providing the voltage rails Vs, VGH, VGL, VCOM Buffer, and gate voltage shaping.

9

Conclusion

The fast improvement of liquid crystal displays in terms of image quality, cost, and power efficiency will continue in the future. The backlight will use RGB and white LEDs providing features like local dimming. The CCFL backlight will improve in terms of color gamut and also allow ‘‘local dimming’’ by using scanning backlight. The display will integrate the gate drive on the amorphous silicon backplane and reduce the numbers of source drivers as much as possible. This will require new power supply features like temperature compensation for the gate driver voltage rails VGH, VGL, and also VCOM reference voltage.

Further Reading den Boer W (2005) Active matrix liquid crystal displays: fundamentals and applications. Newnes, Burlington Lueder E (2010) Liquid crystal displays: addressing schemes and electro-optical effects, 2nd edn. Wiley-Blackwell, Chichester

Texas instruments datasheets: TPS65120, TPS65165, TPS65161, TPS65162, TPS65167 (Active matrix) Texas instruments datasheets: TPS61040, TPS61045 (Passive matrix LCD bias)

4.6.2 Power Supply Sequencing Oliver Nachbaur 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 2 Power Supply Sequencing Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 3 Turn on Sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 4 Turn off Sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 5 Passive Matrix Sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593 6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_4.6.2, # Springer-Verlag Berlin Heidelberg 2012

592

4.6.2

Power Supply Sequencing

Abstract: Passive matrix and active matrix liquid crystal displays require sequencing of their power supply rails. This is required to avoid image artifacts and possible malfunctions of the integrated circuits, like row and source driver as well as timing controller. While the sequencing for the passive matrix displays is simple and straightforward, the active matrix TFT LCD requires a more dedicated sequencing. This sequencing depends mainly on the panel and is slightly different for each panel manufacturer. In this short chapter the main sequencing requirements are discussed.

1

Introduction

Displays smaller than 7‫ ״‬are usually shipped with all power, control circuit, and timing controller assembled on the panel board. Alternatively they are shipped without LCD bias. The advantage of purchasing a display with all circuits on the panel board is the simple system integration without having to care about power sequencing requirements. Usually they only require a single 3.3 V supply rail or operate directly from a Li-ion battery. The disadvantage is the overall increased thickness of the panel. In some cases it is also of advantage to place the power supply circuit away from the display in an area of the system board where electromagnetic interference (EMI) can be better controlled and shielded. When purchasing a display without power supply circuit, sequencing between the different power rails is required.

2

Power Supply Sequencing Requirements

Power supply sequencing is required avoiding image artifacts when the display is turned on or turned off. All displays require a logic supply rail for timing controller, gate and source driver, interface and, in some cases, memory. They also require the LCD driving voltages like the high video voltage supply rail, TFT turn on and off voltages, VCOM supply rail, etc. For the block diagram please refer to > Chap. 4.6.1.

3

Turn on Sequencing

For a reliable start-up all the logic and control devices in the system need to be enabled first by applying the voltage supply rail. Then the LCD voltages are applied. The first voltage rail to come up is the negative TFTsupply rail. This avoids possible malfunction of the gate driver and assures that all the TFTs are turned off during start-up. Then the video voltage rail, TFT turn on voltage, and VCOM buffer are enabled. A typical start-up sequence is shown in > Fig. 1. If the logic voltage rail does not come up first, then there is no control of the gate and source driver. This could cause image artifacts during start-up.

4

Turn off Sequencing

As the system is turned off the LCD voltages should be turned off first and the logic supply rail last. Depending on the system architecture turn off sequencing can be quite challenging. The simplest implementation for turn off sequencing is the use of different load resistors connected on each output rail. However, especially for the notebook, monitor, and TV panels the supply

Power Supply Sequencing

4.6.2

Vin = 12V Vlogic

Vaux

1

2 Vs

3

VGH

4 R1

VGL

Ch1 Ch3 Ref1

2.0V 10.0V 10.0V

2.0ms

Ch2 Ch4

1.0V 10.0V

M 2.0ms 500kS/s 2.0µs/pt A Ch1 / 2.44V

2.0ms/div

. Fig. 1 Start-up sequencing for a typical active matrix LCD system

voltage to the panel is turned off and then a controlled shutdown needs to be assured. The challenge here is, as the supply voltage is turned off, there is no power supply available for a controlled sequencing. One solution to overcome this is to turn off the LCD backlight first and then shut down the LCD panel. Thus the control over gate and source driver is lost but possible image artifacts do not show up because the backlight is turned off first. Such a solution would require sequencing control of the LCD backlight. Depending on the system it is not always possible to control the sequencing of the backlight. In such a case a different approach is required using a reset signal generator. The reset IC detects the falling input voltage and sends a rest signal to the LCD bias IC, timing controller, and gate driver. Then the LCD pixels are all turned off and the LCD bias IC holds the TFT turn on voltage as long as possible high. This assures that all the LCD pixels are being turned off during shutdown having a black LCD image independent of backlight sequencing. In order to support this function dedicated LCD bias ICs are required holding the TFT gate voltage, VGH, high during shutdown.

5

Passive Matrix Sequencing

Usually passive matrix displays require only a 3.3 V rail for the logic and a high voltage rail around 18 V. Similar to the active matrix, the logic supply rail needs to be applied first and then the LCD voltage.

593

594

4.6.2 6

Power Supply Sequencing

Conclusion

Power supply sequencing is required by all LCD panels. Since they differ from display to display the majority of LCD bias supply ICs support flexible and adjustable sequencing.

Reference 1.

Texas Instruments datasheets: TPS65120, TPS65165, TPS65161, TPS65162, TPS65167, etc.

Section 5

TFTs and Materials for Displays and Touchscreens

Part 5.1

Display Glass

5.1.1 Glass Substrates for AMLCD, OLED and Emerging Display Platforms Peter L. Bocko 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 600

2 2.1 2.2 2.3 2.4 2.5

Substrate Innovation in Response to Display Applications . . . . . . . . . . . . . . . . . . . . . . 600 Notebook Computer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 600 Desktop Monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601 Television . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601 Portable Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601 e-Paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602

3 3.1 3.2 3.3 3.4

Manufacturing Platforms Employed to Meet the Requirements of Display Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602 Amorphous Silicon LCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602 Polysilicon LCD: p-Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603 Organic Light-Emitting Diode (OLED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603 Reflective Bi-stable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603

4 4.1 4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.1.6 4.1.7 4.1.8 4.2 4.2.1 4.3

Glass Substrate Requirements for AMLCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604 Amorphous Silicon (a-Si) LCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604 Glass Composition Requirements for a-Si LCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604 Surface Quality Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 607 Advanced Physical Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 608 Substrate Manufacturing Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 610 A Brief History of AMLCD Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 611 Movement to Larger Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 612 Trend to Thin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 612 Environmental Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615 Polysilicon (p-Si) and OLED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616 High-Temperature Substrates for p-Si and OLED Backplanes . . . . . . . . . . . . . . . . . . . . 617 High-Strength Glass Used in AMLCD Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617

5

Summary and Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621

6

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.1.1, # Springer-Verlag Berlin Heidelberg 2012

600

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

Abstract: The properties of today’s glass substrates for AMLCDs, OLEDs, and emerging display platforms have resulted from decades of collaboration between display manufacturers and the glass industry. The development of flat panel applications led to display manufacturing platform innovations which, in turn, have driven glass composition and substrate design. This article discusses basic glass technology and substrate innovation as influenced by the evolution of the display industry. It begins with a description of the principal display applications and an overview of the active platforms employed to meet the requirements of those applications – amorphous silicon, polysilicon, organic light-emitting diodes, and reflective bi-stable – including brief statements of the key challenges to substrate manufacturers engendered by each platform. Substrate requirements – including glass composition, surface quality, advanced physical attributes, and other factors – for each of the active matrix platforms are discussed. Because glass is increasingly employed as a structural and design element of the display device, the article closes with a discussion of glass reliability within the context of display-centric information appliances. List of Abbreviations: AMLCD, Active Matrix Liquid Crystal Display; a-Si, Amorphous Silicon; CF, Color Filter; CTE, Coefficient of Thermal Expansion; DOL, Depth of Layer; Gen, Generation (of Glass Substrate); LCD, Liquid Crystal Display; LTPS, Low-Temperature Polysilicon; OLED, Organic Light-Emitting Diode; PMMA, Poly(Methyl Methacrylate); p-Si, Polysilicon; TFT, Thin-Film Transistor

1

Introduction

In considering the development of glass substrates for active matrix liquid crystal displays (AMLCDs), the primary factor driving substrate design was the emerging needs of the application and the manufacturing processes developed to meet the device performance. Essentially, the glass design process must be considered as an intrinsic part of the display platform development. Thus, substrate properties have been and continue to be defined through a true collaboration between display manufacturers and the glass industry as the liquid crystal display (LCD) platform has evolved through successive waves of applications.

2

Substrate Innovation in Response to Display Applications

Innovation in display substrates has been driven largely by the requirements of four applications: notebook computers, desktop monitors, televisions, and high-performance portable devices. An emerging application, e-Paper, is likely to bring its own requirements, as well.

2.1

Notebook Computer

As the first high-volume application for AMLCD, notebook computers demanded portability and operation from battery power; therefore, LCD’s low-voltage operation made it the only practical choice for this application. The key requirements for glass substrates used in the development of the notebook computer application were the ability to withstand elevated temperatures (as high as 450
C) without deformation, the ability to withstand the harsh and

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

5.1.1

diverse chemical environment of the early AMLCD process, and a surface quality (freedom from particulate contamination and scratches) consistent with achieving acceptable yields in the manufacturing process. During the early development of the notebook computer, screen dimensions were relatively small (8.5–10.400 diagonal measure). With glass substrates correspondingly small (approximately 300  400 mm), panel size was not a focus in AMLCD glass substrate development.

2.2

Desktop Monitor

The next wave of application growth was the displacement of the cathode ray tube (CRT) in the desktop computer monitor application. Adapting the LCD platform for this use required a major scale-up of the panel size (up to approximately 2000 ). Unlike notebooks, where the uniqueness of a thin and light form factor brought value, the desktop monitor had to be economical enough to replace a CRT while simultaneously delivering compelling on-screen performance. The need for large screens manufactured at lower cost drove innovation in glass substrate size. Trends to higher resolution and implementation of wide viewing-angle technologies required continuous improvement of dimensional attributes, such as substrate stress and thickness control.

2.3

Television

Television serves as a social congregator, bringing people together for entertainment and information. This fact has driven further innovation in AMLCD viewing performance, addressing the so-called LCD legacy issues of viewing angle and pixel response speed that were of lesser importance in Information Technology applications. The neutralization of legacy issues through advanced liquid crystal modes and novel array design drove base substrate sheet characteristics (such as thickness tolerance and surface quality) to even higher levels of performance. Critical, as well, was the scalability of the substrate to dimensions as large as 3 m and the substrate’s compatibility with large-size manufacturing processes. The trend to new content will continue to drive demand for improved LCD TV performance. Applications such as interactive gaming and digital workspaces bring users closer to the display, creating a need for higher resolution, larger displays, and new forms of interaction, including multi-touch capability. As movie studios, sports networks, and live concert venues realize the potential for stereoscopic 3-D, more high-value content will be available, creating a demand for 3-D in the home. Generally, a television capable of a compelling 3-D experience must be larger, higher resolution, and brighter than a television used only for 2-D content. The multiplicity of approaches to adapt LCD TV for 3-D will make the implications to the substrate, beyond simply sustaining existing attribute trends, difficult to predict.

2.4

Portable Device

This category, which comprises a broad range of devices used for personal entertainment and information by an individual rather than a group, includes cell phones, camera phones, navigation systems, mobile TV, video phones, and personal music or video players.

601

602

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

These displays must be robust, vivid, and rugged. Therefore, for the first time, the strength of the substrate became a critical design criterion as the panel dimensions increased (even while becoming thinner) and touch functionality was incorporated. Portable devices achieve a premium based upon low power consumption (long battery life), better picture quality, and thinner form factor. Glass attributes for portable devices include thinness, hightemperature capability for advanced array processing, high strength, and very precise thickness control to enable smaller design rules versus larger displays.

2.5

e-Paper

e-Paper is an emerging application (see > Sect. 8, Paper-Like and Low Power Displays) for which performance trends in glass are yet to develop. At a minimum, e-Paper will require extremely low power consumption, thin and light form factor, and a paper-like reading experience. Flexibility is also expected as a requirement for high-value applications. Thus far, e-Paper has been limited to e-books, which are essentially hybrid devices leveraging existing LCD capabilities for the backplane (amorphous silicon [a-Si] on glass substrate) and electrooptic front plane polymer composites. Although it is too early to say what the role of glass will be for this application as new technologies such as organic semiconductors are adapted, substrates will most likely need to be thin, light, strong, and compatible with low-cost manufacturing and therefore possibly flexible. Options for low-cost manufacture for flexible e-Paper include roll-to-roll processing [1]. Materials that are routinely processed in roll form, such as polymer or metal [2] would seem to be more practical choices for flexible display. Yet thin glass brings a number of advantages over other flexible materials in the dimensions of transparency, high dimensional stability, and superior surface [3], and thin flexible glass is also a feasible option for roll-to-roll processing [4].

3

Manufacturing Platforms Employed to Meet the Requirements of Display Applications

The manufacturing platforms employed in the aforementioned display applications include amorphous silicon, polysilicon, organic light-emitting diodes, and bi-stable reflective. Amorphous and polysilicon each brought specific performance challenges to the substrate – challenges that have created an emphasis upon a subset of possible substrate attributes as the backplane technology and substrate evolved to the highly optimized platform. Understanding the optimization of the glass substrate therefore has a strong historical component. In the emerging platforms of organic light-emitting diode (OLED) and bi-stable reflective displays, it is reasonable to expect the same dynamic.

3.1

Amorphous Silicon LCD

The versatility and extraordinary scalability of the amorphous silicon LCD (a-Si) platform have made it the dominant technology in notebook computer, desktop monitor, and TV applications. This, despite the fact that in the early days of LCD development a-Si was considered, at best, a temporary solution until polycrystalline silicon with its higher semiconductor

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

5.1.1

performance could be industrialized. Additional details on the amorphous silicon platform may be found elsewhere in this encyclopedia (> Chap. 5.2.1). The first thin-film layer deposited on the substrate in the preferred embodiment of a-Si (known as the bottom-gate thin-film transistor [TFT]) is the gate line. The earliest industrialized TFTs employed refractory metal gate lines, including chromium, tantalum, molybdenum, and a variety of alloys. This choice drove the optimization of two attributes of a-Si glass substrates: durability to corrosive chemicals (to etch the gate line without unduly damaging the glass surface) and mechanical strength of the surface (required because of the impact of high levels of intrinsic stress in the refractory gate metal and their impact on glass surface reliability). Also, the processing temperatures of a-Si, which originally exceeded 450
C, as required by the deposition of a high-quality gate insulator, drove the adoption of glasses with hightemperature capability. Although aluminum alloys have largely replaced the problematic refractory metals, and channel processing temperatures have been reduced to approximately 300
C, highly durable and high-temperature glasses have remained the standard because of their benefits for a robust processing window.

3.2

Polysilicon LCD: p-Si

After a protracted development, p-Si is now a leading platform in the high-performance portable device application. In p-Si’s most common backplane design, the top gate design, the channel of the TFT device is in more intimate contact with the substrate surface (usually with a thin-film oxide buffer layer interposed) than in most a-Si backplanes. Thus, the substrate surface can have a more profound impact on the transistor characteristic. Substrates for p-Si must deliver the highest level of surface quality among display substrates. For example, surface contamination must be monitored and controlled in the sub-micron regime. In addition, the p-Si process often requires steps that exceed 600–700
C (such as activation following ion implantation) and potentially even higher; therefore, the core challenge for the substrate industry is in achieving dimensional stability at these high temperatures. Additional details on the polysilicon platform may be found elsewhere in this encyclopedia (> Chap. 5.2.2).

3.3

Organic Light-Emitting Diode (OLED)

OLED devices share backplane technologies adapted from LCD, especially polysilicon, because OLEDs’ current-driven operation is effectively served by p-Si. The substrate properties for OLED have not yet significantly diverged from those of polysilicon LCD because OLEDs are primarily a high-performance option for the portable application. However, this may change because OLED (see > Part 6.6) is seen as having some potential in the TV application, where the processes required to scale to large size will drive substrate innovation in the dimensions of thermal and surface quality dimensions.

3.4

Reflective Bi-stable

The low-energy consumption requirement of e-Paper has stimulated a variety of candidate systems in which an electro-optic mechanism, including certain modes of liquid crystal, results

603

604

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

in a change of reflective state. At the early stage of development, the electrophoretic display, which uses electrophoresis to switch pixels between a light and dark state, appears to have an early lead. However, as is the case for e-Paper, it is too early to predict the impact on substrates of the future for this segment of displays.

4

Glass Substrate Requirements for AMLCD

4.1

Amorphous Silicon (a-Si) LCD

In this subsection, glass composition criteria and substrate dimensional performance factors are discussed.

4.1.1

Glass Composition Requirements for a-Si LCD

Virtually all of the a-Si LCD substrates today belong to the old and extremely versatile glass composition family of alkaline earth aluminosilicate glass systems. The chemistry of this glass family has delivered the key performance criteria of control of alkali contamination, low density, low thermal expansion, and thermo-mechanical reliability [5, 6]. Control of Alkali Contamination

Although glassmakers recognized that sodium could be detrimental to thin-film electronic functionality, early experiments indicated that perhaps a low level of alkali could be tolerated in the LCD substrate, provided the glass were of sufficient chemical durability [7]. However, during the active matrix technology development, even a small amount of sodium was seen as problematic given the experience of sodium as a contaminant in the semiconductor industry. This motivated the reduction of sodium to trace levels in glass melting processes to prevent the numerous performance and reliability issues that were feared. However, in a conventionally melted glass it is impractical to drive sodium to the part-per-billion level normally associated with semiconductor devices. Why have not the low but measureable tramp levels of sodium caused issues in LCD? Part of the reason is the adoption of silicon nitride as the gate dielectric in a-Si TFT as well as barrier films in p-Si. However, other factors may have contributed to this. Sodium out-diffusion from the glass into the liquid crystal device can result from interdiffusion (i.e., exchange) of sodium in the glass with hydrogen ions that may be present as water in the device or environment. Hydrogen exchange maintains charge neutrality when the sodium moves, and one would expect that there would be a driving force for sodium to move from the high concentration of the glass (which may be of the order 102–103 ppm) to the lower concentration in the LCD device. However, it has been observed that for LCD aluminosilicate glass, the presence of boron and aluminum may suppress the out-diffusion of sodium. The aluminum and boron ions in the glass can change from three to four coordinates in the presence of charge compensating cations, thus acting as a potential sink for alkali [8]. Today’s LCD glass still has sodium levels of about 100 ppm, but because of the stabilizing effect of the aluminosilicate glass structure of LCD substrates, sodium contamination has not been an issue.

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

5.1.1

Low Density

The adoption of low-density glass contributed to achieving low device weight targets for portable applications. It also resulted in more facile robotic handling during high-speed automation because of reduced sag of low-density glass. Sag is the deflection of the glass substrate due to gravity. Sag of an elastic material is proportional to
 S / L4 rg 1  n2 t 2 E where S is the sag, L is the unsupported span, r is the density, g is the gravitational constant, n is Poisson’s ratio, t is the thickness, and E is Young’s modulus. Gravimetric sag was a special challenge in the adoption of large sizes because equipment manufacturers had not yet learned how to support the substrate from the back side of the glass (thereby reducing the unsupported span) without inducing damage. The introduction of lower density glasses therefore helped facilitate the evolution to larger substrate sizes. Low Thermal Expansion

Low coefficient of thermal expansion (CTE) brought several benefits, including reduced breakage due to thermal shock or gradients and minimizing dimensional distortion during transient thermal steps [9]. One manifestation of the beneficial impact of low thermal expansion is the impact of thermal gradients within the plane of the substrate during process steps. The lowest expansion substrate glass, when exposed to a thermal gradient, will have the lowest nonlinearity of the relative positions of features on the glass. It was this effect that caused early notebook innovators to adapt designs that had a high tolerance to these dimensional distortions. The adoption of low-expansion glass therefore facilitated the development of brighter displays and longer battery life by increasing the aperture ratio of the displays. In fact, the 80% aperture ratio milestone for notebook computers was first achieved by improved TFT design and employing a glass with a CTE of less than 4 ppm/K. Thermal Stability

One of the defining criteria that distinguish glass from crystalline material is glass’s lack of a discontinuity of physical properties (such as density and viscosity) between the solid and liquid phases. Instead, the physical properties of glass are highly affected by relaxation of its structure in its transition region – very roughly defined by the temperature range in which stress is relaxed in the glass from a timescale of minutes to several hours. Therefore, the structure of the glass and its key physical properties are highly dependent upon its thermal history in addition to its composition. A glass article cooled rapidly from the liquid state (i.e., quenched) results in a lower density than one cooled more slowly because of the lack of time for structural relaxation to the denser and lower energy structure. Moreover, when a glass substrate is subjected to higher temperatures, such as those employed in a TFT process, relaxation can cause significant dimensional changes in high-temperature processes that need to be managed or compensated for, namely, thermal shrinkage and thermal sag. Thermal shrinkage is caused by viscous relaxation within the plane of the substrate, causing permanent dimensional change. The TFT process can be simply characterized as several cycles of film deposition or other steps at elevated temperature followed by photolithographic patterning. Shrinkage on the order of 1–3 mm, significant enough to cause a pattern error between successive photolithographic exposures in a TFT process, can be observed in

605

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

120 12 h 100 Shrinkage (PPM)

606

80 5h 60 40 1h 20 0 350

400

450

500

550

600

Temperature (°C)

. Fig. 1 Shrinkage of a typical a-Si substrate in the 400–500
C range. The transition range of this glass is above 650–800
C (Courtesy of Corning Inc.)

a substrate when process steps occur even several hundred degrees below the transition region of glass. > Figure 1 shows shrinkage of a typical a-Si glass as a function of temperature and time at temperatures above that of a typical a-Si process but well below the transition region of the glass [10]. While the TFT process can employ magnification during photolithographic exposure to compensate for the movement of thin-film features due to thermal shrinkage, this has practical limitations. For production lithographic equipment, the limit for compensation of absolute thermal shrinkage should generally be in the range of 10–20 ppm. Also, the thermal shrinkage must be both isotropic and uniform within the sheet. Thermal sag is a permanent deformation of the substrate due to the effects of gravity combined with viscous relaxation of the glass as it experiences a thermal treatment while held horizontally. Thermal sag can result in difficulty in robotic handling of the sheet. One particular handling issue that can be an outcome of thermal sag is the inability of the sheet to be vacuum-chucked into flatness on a photolithography stage. The glassmaker has two options for reducing thermal shrinkage and sag in order to meet the requirements of the TFT process. The first is to control the tendency of the glass to thermal relaxation at elevated temperature, in effect stabilizing or ‘‘pre-shrinking’’ the glass by adding an additional high-temperature pre-treatment step. The other is to design the glass composition for intrinsic thermal durability by increasing the glass high-temperature viscosity. The aluminosilicate glass system is particularly suited for design of high-thermal-stability glasses for AMLCD applications; however, a practical consequence and challenge for the glass manufacturer is correspondingly higher temperatures required in glass manufacture. Strength and Fatigue Resistance

Strength is that value of mechanical stress causing glass to break. If the glass is of high quality, that is, pristine and without defects, the strength of glass will approach 14 GPa, which is higher

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

5.1.1

than any other material. Unfortunately, AMLCD substrates’ glass surfaces are exposed to imperfections – during manufacturing, finishing, handling, and in the TFT process – which induce flaws and reduce their strength. Defining in absolute terms the mechanical reliability of an AMLCD glass substrate in the application is problematic; strength of a substrate is not an intrinsic property of its composition alone but is highly dependent upon the flaw size distribution and the testing method. However, an intrinsic property of the glass composition is its fatigue resistance, which has a profound effect upon the long-term reliability of the glass under stress [11]. The glass fatigue mechanism impacts several aspects of the AMLCD application. The strength of the panel under long-term stress (whether structural mechanical stress in the device or the small-scale stress exerted on the glass surface by adherent films) is enhanced by the use of fatigue-resistant compositions. Aluminosilicate AMLCD substrates of today have fatigue resistance 40–60% higher than that of soda-lime glass [12]. Glasses with high fatigue resistance can be cut more reliably in back end processes of panel manufacture [13].

4.1.2

Surface Quality Requirements

Substrate Flatness

Deviation of the substrate surface from a perfect plane on a range of scale from a few microns to the full dimension of the substrate (several meters) has potential impact on a variety of aspects in AMLCD processing and device performance. Collectively, the deviation of the substrate surface is imprecisely termed ‘‘flatness’’ with contributions arising from variation of the substrate thickness at all scales as well as bending (‘‘warp’’ or ‘‘shape’’) of the sheet. As process and device design have evolved in the AMLCD platform to achieve increased productivity and display performance, substrate quality attributes in the flatness regime likewise have become more tightly controlled. > Table 1 illustrates the trend in glass sheet quality attribute at two points in AMLCD evolution. Commonly used terminology for deviation of a substrate from a plane is used in this table; admittedly these terms, non-standardized in the glass industry, are imprecise. When characterizing a substrate requirement in an aspect of flatness, it is important to define the dimensional scale at which the deviation is manifested.

. Table 1 AMLCD substrate flatness performance trend in sheet attributes. Attributes are defined by the typical required deviation as well as the dimensional scale at which the deviation occurs Year

1995

2010

AMLCD generation

Gen 2

Gen 10

Substrate size

37  47 cm (approx)

288  313 cm

Nominal thickness

1.1 mm

0.7 mm

Surfaces roughness

1 nm/10s mm

100 nm/10s mm

Fig. 3, respectively, tabulate and illustrate the increase of substrate size in the LCD era as a function of time and application. Today’s state-of-the-art Gen 10 platform is unique in that it efficiently produces a wide range of panel sizes, including 3200 , 4200 , and 6500 . > Figure 4 shows a simulated Gen 10 substrate with these sizes illustrated. Note that the substrate thickness is only 0.7 mm. Because the glass is so large and so thin, the substrates are handled exclusively by robots during substrate and panel manufacture. Sizes beyond Gen 10 may be introduced if panel makers require them, but at present 3 m2 seems to fulfill the needs for large screens; the need for a platform capability beyond the 60-in. range is unclear.

4.1.7

Trend to Thin

The benefits of using as thin a substrate as practical in LCD has been evident since the early days of LCD, primarily to eliminate weight and thickness in the notebook computer application.

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

5.1.1

. Table 3 Approximate AMLCD substrate sizes listed by generation. Since the actual size deployed differed between panel manufacturers, typical or representative dimensions have been chosen for the generation Size (L  W, cm)

Generation

Date introduced

1

30  40

1990

2

37  47

1993

3

55  65

1995

3.5

60  72

4

68  88

2000

73  92

4.5 5

110  130

5.5

130  150

6

150  180

2003

7

187  220

2004

7.5

195  225

2005

8

220  250

2006

10

288  313

2009

2002

Gen 10

2009 Monitors Gen 8

2006 2005

Gen 7.5 Gen 5

TV

2004 Gen 6

2003 Gen 5

Monitors

2002 2000 1995

Gen 4 Gen 3

Information technology

Gen 2

1993

. Fig. 3 Evolution of substrate sizes (Courtesy of Corning Inc.)

Notebook

613

614

5.1.1

a

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

Twenty-four 32-in. panels

b

c

Six 65-in. panels

Fifteen 42-in. panels

. Fig. 4 Simulated Gen 10 substrate illustrating to scale the high utilization of the manufacture of popular 16:9 format LCD-TVs (Courtesy of Corning Inc.)

The use of thinner glass also brought some process advantages such as faster heating ramps for thermal process. Today, design is a contributing criterion for the use of thin glass as the thinnest possible panel is a source for product differentiation in both portable and stationary applications. The fusion process among other sheet manufacturing processes has demonstrated the potential for making very thin glass, even to 0.1 mm and below. However, thin glass capability at the substrate manufacturer has outpaced the ability of AMLCD manufacturers to employ thin glass in their process. The primary challenges have been glass handling and breakage in TFT and color filter process automation. In the early 1990s, most AMLCD processes were designed around the 1.1 mm standard. By the advent of Gen 3 in 1995, 0.7 mm had almost entirely become the standard through mechanical engineering innovations in the AMLCD process equipment. Today 0.5 mm is a commonly used substrate thickness for most portable device panels manufactured on Gen 4 and Gen 5 lines, while even larger Gen sizes may have installed capability for thin glass in order to have ‘‘flexible manufacture’’; that is, the capability to produce both portable and larger stationary panels on the same line. Currently, many portable panel manufacturers employ an acid thinning process on finished displays to create the thinnest possible panel while avoiding the difficulties in handling thin glass in their front-end processes. This approach has worked well for the manufacture of low volumes of panels, but concerns with the impact upon strength and the added cost of this

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

5.1.1

process have motivated investment into 0.3–0.4 mm handling as thin devices have become mainstream and destined for premium devices. It is likely that even thinner glass will be targeted for display substrates or other components. The advent of touch capability into many display devices has also stimulated increased emphasis on thin glass as device designers do not want to increase panel thickness with the additional pieces of glass required for touch sensor and a cover glass. It is not unreasonable to expect that glass as thin as 0.2 mm may be required for touch components and perhaps even within the panel. At this point, the glass is thin enough to allow it to be conformable or even flexible if suitably packaged in the device. At 0.1 mm thickness and below, the possibility of truly flexible glass delivered to the panel manufacturer on a spool for roll-to-roll manufacture of displays can be discussed. e-Paper seems to be a target for this approach given the interest in ultra-low-cost printing processes for e-Paper applications and the emphasis on true flexibility of the display. At the present time, glass manufacturers have demonstrated spooled glass and it appears that its commercialization is primarily a matter of solving difficult but tractable mechanical engineering problems. However, roll-to-roll processing for display manufacture is still in early research and development. > Figure 5 summarizes the status of thin glass capability as a function of substrate size and intended application.

4.1.8

Environmental Trends

Increasingly, new technologies and products are evaluated through the lens of their environmental impact across the entire life cycle, including materials manufacturing, product

Substrate thickness (mm) 0.1

0.2

1 2

Gen size

3 4 5 6 7

0.3

0.4

0.5

0.6

0.7+

= Future anticipated need e-Paper

Portable LCD panels and new touch sensor plates

= Emerging capability = Commercial availability LCD panels and touch sensor plates for notebooks, net-books and mobile internet Flexible devices manufacture for NB, TV and desktop monitor and monitors. Typical current TV thickness for cover glass and sensor plates

8+

. Fig. 5 Substrate thickness as a function of Gen size: current commercial availability, emerging capability (limited commercial availability or in development), future anticipated need requiring substantial platform innovation (Courtesy of Corning Inc.)

615

616

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

fabrication, device use, and end of life. While AMLCDs are a substantial improvement over the CRT in this regard, the life-cycle trend still poses a challenge for the LCD manufacturer, since there are technical barriers to removing all materials of environmental concern from the display. This also includes the glass composition and its manufacturing process. One of the main quality requirements for LCD substrates has been that they must be completely free of even microscopic bubbles. Eliminating them has traditionally meant using ‘‘fining’’ agents such as arsenic and antimony to prevent bubbles from forming. But the use of these heavy metals can result in potentially harmful byproducts in panel manufacturing. Removing them was the challenge in a ‘‘green’’ LCD substrate. The elimination of heavy metals from the glass composition was seen as necessary in the long term because of emerging environmental regulations worldwide. While in the glass, these heavy metals constitute no hazard to the user. However, concerns arise about what happens when the display reaches the end of its useful life. Impending environmental regulations require manufacturers to establish plans for recycling electronic products and their components. For electronics manufacturers, then, there are two choices: recycle the glass from discarded equipment, or use a glass that does not contain heavy metals in the first place. In CRT technology, heavy metals such as lead are used in the glass composition to protect the viewer from X-rays. The CRT industry’s approach to the environmental challenge has been to recycle televisions at the end of their life and reuse the glass. Because of the sheer amount of glass in a CRT television (over 25 kg glass in a 3200 television), the simplicity of disassembling a CRT and the less demanding quality requirements of CRT, this approach made economic sense. Yet the CRT approach of recycling glass is not adaptable to LCD, for several reasons. First, LCD televisions contain less glass than CRTs, so there is less benefit to recycling. Second, recycling is much more difficult due to the complex construction of the LCD device itself. Finally, whereas the quality of CRT glass does not affect the viewing experience, LCD glass has demanding quality requirements, which call for a pristine glass. For all of these reasons, it is impractical to recycle LCD glass back into substrate manufacture. The only logical solution for minimizing the environmental impact of LCD glass was to eliminate heavy metals up front. In 2006, Corning commercialized the industry’s first composition to contain no added heavy metals or halides: EAGLE XG1. In addition to removing arsenic, antimony, and barium from the glass melting process, EAGLE XG also eliminates the use of halides, such as chlorine, fluorine, and bromine, which can create corrosive acid byproducts during glass manufacturing. The adoption of this glass and similar green glass compositions from other AMLCD substrate manufacturers that have eliminated heavy metal content has been rapid.

4.2

Polysilicon (p-Si) and OLED

Over the last few years, p-Si has clearly differentiated itself from a-Si in various applications, especially where display manufacturers needed fine-quality visual performance in a compact form factor with optimum power efficiency. Polysilicon semiconductor performance is superior to that of a-Si in certain uses, allowing manufacturers to place more complex circuitry on the glass during fabrication of the display array. These functions include much of the display driver circuitry, which would otherwise require off-panel discrete chips with complex packaging and interconnection.

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

5.1.1

Compared to the a-Si process, the low-temperature polysilicon (LTPS) process is much more demanding in terms of temperature, surface, and dimensional requirements. The LTPS process requires the glass to be stable, without changing its shape or dimensions at 600
C and higher. Previously, p-Si manufacturers had to heat-treat a-Si glass to stabilize it to high temperature in what is effectively a pre-shrinkage process. This pre-treatment makes the entire process more complex, and it creates the potential for the glass to change shape under high temperature through thermal sag. The extra handling can also degrade the surface quality of the glass, possibly requiring (for some substrate manufacturers) special polishing to ensure the extraordinary surface quality that the polysilicon backplane demands. In OLEDs, the limitations of p-Si backplane processes have comprised a substantial technical challenge that has slowed the growth of the OLED platform by limiting its adoption to small displays. Because OLEDs are current-driven devices, they require a backplane with much better performance than a conventional AMLCD. Laser annealing technology used for crystallization of the silicon layer in p-Si AMLCD meets OLED backplane current requirements but is not economical or technically feasible for scaling to large size that would be required for OLEDs to enter the TV market. One possibility of a scalable OLED backplane technology is microcrystalline silicon technology, which, although not yet commercialized, may incorporate high-temperature processes such as solid phase crystallization and rapid thermal annealing in the 600–700
C range [19–21], which are beyond the capabilities of a thermally stabilized conventional a-Si substrate.

4.2.1

High-Temperature Substrates for p-Si and OLED Backplanes

As an alternative to a stabilized a-Si glass, glasses with an intrinsic stability at high temperatures are accessible in the aluminosilicate glass family [22]. One such composition, Jade®, a fusionformed glass, requires no secondary heat treatment or polishing to meet the surface and thermal stability requirements of the LTPS and OLED processes [23]. The high intrinsic viscosity of Jade substrates enhances thermal durability, providing a potentially wider process window for TFT fabrication. A decrease in thermal sag at elevated temperatures reduces losses due to handling and vacuum-chucking errors. > Figure 6 contrasts the relative (accelerated) deformation of the Jade substrate and an a-Si substrate at elevated temperature. The aforementioned option for eximer laser crystallization p-Si process, microcrystalline silicon process, features a solid phase crystallization step at temperatures above 600
C. > Figure 7 illustrates compaction performance of Jade in this thermal regime compared to that of a conventional a-Si glass.

4.3

High-Strength Glass Used in AMLCD Panels

Although not necessarily a contributor to the functionality of the display, high-strength glass is increasingly incorporated in AMLCD panels. Many portable devices, especially handsets, require a protective cover for shock and abrasion. As previously mentioned, the addition of touch functionality has added thin glass to the device while increasing the need for mechanical protection. Finally, design esthetics is stimulating the employment of high-strength glass

617

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

2.0 1.8

T = 650°C

1.6 Deflection (mm)

1.4 a-Si glass

1.2 1.0 0.8 0.6

Jade®

0.4 0.2 0.0 0

500

1,000

1,500

2,000

2,500

3,000

Time (min)

. Fig. 6 Elastic deformation of Jade glass compared to conventional a-Si glass (EAGLE XG®) at 650
C (Courtesy of Corning Inc.)

600 = Jade 500 Compaction (PPM)

618

= a-Si glass

400 300 200 100 0 600/1 h

600/2 h

625/1 h 625/2 h 650/1 h Simulated crystallization cycle

650/2 h

. Fig. 7 Compaction of Jade compared to a-Si glass (EAGLE XG®) in the solid phase crystallization thermal range (Courtesy of Corning Inc.)

as a cover in so-called borderless design, that is, with no protective bezel surrounding the display panel. Portable devices from cell phones to notebook computers require more durability against mechanical shock, stress, and abrasion than stationary display-centric devices. In order to protect the display panel, manufacturers have employed a cover of tempered soda-lime glass or

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

scratch-resistant polymer, which is counter the trend toward minimal device thickness. Increasingly, thin chemically tempered glass is being used as a protective cover. Corning1 Gorilla Glass is one example of a new class of materials used in AMLCD and OLED displays that have been engineered for optimum mechanical performance in cover glass applications [24]. Glass strength can be enhanced through a chemical tempering process that exchanges small mobile alkali ions for larger ones in a high-temperature salt bath, inducing a compressive layer on the glass surface. This compressive layer increases the amount of force required to either scratch the surface or cause failure by exerting tensile force. The Gorilla Glass composition is also of the aluminosilicate family; however, unlike aluminosilicate AMLCD substrates, it contains alkali constituents of sodium and potassium to allow ion exchange. While any glass containing sodium can be strengthened by chemical tempering, the aluminosilicate glass structure optimizes the effect. > Table 4 is a summary of key properties of Gorilla Glass compared to other cover materials commonly used in AMLCD. Ultimately, the functional strength of a device will be determined not by the design strength of its components in their pristine state, but by the ability of the various components to maintain that strength after use in the intended application. This is how employing chemically tempered aluminosilicate glass as a cover material brings value to strong and thin portable devices. > Figure 8 shows that the reduction of strength of Gorilla Glass is far less than that of chemically tempered soda-lime glass after abrasion damage. The difference in retention of strength in use is due, in part, to the greater depth of the strengthening compressive layer in Gorilla Glass compared to that of soda lime. The Gorilla

. Table 4 Corning Gorilla Glass properties compared to two other commonly used AMLCD cover materials, PMMA (polymer) and ion-exchanged soda-lime glass

PMMA

Ion-exchanged soda lime

Ion-exchanged Gorilla® Glass

Process

Extrusion, injection

Float, slot draw

Fusion

Tg (Range) (
C)

105
C

514–546
C

553–602
C

Strain point (
C)

NA

514
C

553
C

CTE  10 / C (0–300
C)

700–900

80–90

91

Thickness availability (mm)

>0.70

>0.55

>0.55

Density (g/cm3)

1.17

2.4–2.5

2.45

–6


Young’s modulus (GPa)

1.8–3.1

72

73.3

Refractive index

1.48–1.50

1.52

1.51

Light transmission (400–700 nm)

>88%

>90%

>91%

Compressive stress (MPa)

NA

400–600

650–825

Depth of layer(mm)

NA

10–30

40–80

Pencil hardness

3–4H (coated)

6–8H

9H

619

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

Ring-on-ring load at failure (N)

1,600

Glass strength

Gorilla Glass

1,200

800

Greater damage tolerance

Soda-lime

400

0 0.0

0.5

1.0

1.5 2.0 Scratch load (N)

2.5

3.0

Damage severity

. Fig. 8 Strength reduction of Gorilla Glass and chemically tempered soda-lime glass after abrasion (Courtesy of Corning Inc.)

140 120 Load to failure (kg)f

620

Gorilla Glass (DOL = 41 µm) Soda-lime glass (DOL = 12 µm)

100 80 60 40 20 0 0.3

0.5

0.7 0.9 Thickness (mm)

1.1

1.3

. Fig. 9 Ring-on-ring strength of various thicknesses of Gorilla Glass and soda-lime glass after abrasion (Courtesy of Corning Inc.)

Glass composition was developed to maximize the depth of layer (DOL) in the compressive strengthening effect – a key factor in making Gorilla Glass a contributor in the drive to thin form factor [25]. > Figure 9 illustrates the fact that Gorilla Glass with a deeper DOL results in much higher retained strength after abrasion in reduced thickness.

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

5

5.1.1

Summary and Future Directions

The modern AMLCD and OLED platforms have benefited from decades of collaboration between display manufacturers and glassmakers, resulting in highly optimized aluminosilicate glass substrates. These substrate requirements – including glass composition, surface quality, advanced physical attributes, and other factors – strongly depend upon the specifics of the application and the processes used to manufacture the panel. From its origins in small panels to the high-volume notebook computer, computer monitor, and large screen TV panels of today, the success of the AMLCD platform has been facilitated by the scalability of a-Si substrate manufacture. In high-performance portable devices as well as the emerging OLED platform, increased thermal durability attributes has driven substrate design. Increasingly, glass is also employed as a structural element of the display device, as evidenced by a cover glass that facilitates the incorporation of touch functionality and offers design flexibility. The aluminosilicate glass system has provided options for chemically tempered cover glass and touch panel components that provide enhanced glass strength and device reliability in thin form factors. It is unlikely that the trend of progressively larger size substrates will be sustained much beyond the 3 m dimension (the present Gen 10), due to the diminishing pull for size capability beyond a 7000 diagonal screen within the TV application and the diminishing benefits for the economies of scale offered by larger size substrates. In established applications, the focus of display substrate innovation will be in reducing system cost and in enhancing the immersiveness of the flat panel display experience. This is best accomplished, in the author’s opinion, by systematically and collaboratively enhancing existing substrate attributes that have already been under continuous improvement throughout the LCD era. For example, scalable OLED-TV technology will involve further enhancements to the thermal durability aspects of aluminosilicate substrates – a process that has been underway since the emergence of LTPS in the 1990s. The next step change in creating an immersive flat panel visual experience is achieving a compelling 3-D effect for LCD TV. This involves controlling left and right eye information via polarization modulation, by either use of light-shutter eyeglasses or by interposing an active retarder between the display and viewer (see > Sect. 9, 3-D Displays). The substrate or display cover glass can induce nonuniform optical retardation (discussed above in > Sect. 4.1.3), with the potential of creating anomalies in the 3-D effect, including chromatic distortions and crosstalk between left and right eye information. Likewise, the 3-D effect is enhanced by reducing the glass thickness in the display stack [26]. Therefore, optimizing the substrate for 3-D involves further improvements in attributes (thinness and retardation) that have already been present as central substrate themes in the evolution of the AMLCD platform. Looking ahead, new display applications are creating pull for new categories of glass attributes. For better or worse, an undeniable trend is that our environment will increasingly be saturated with timely, interactive, and compelling generated visual information displayed on an increasing diversity of electronic devices. e-Paper is seen as one of the key emerging platforms. As a material, glass brings a unique combination of transparency, dimensional stability, strength, and a surface that can be both chemically and physically compatible with semiconductor function. With continued innovation, the glass substrate will continue to serve the display industry as an enabling component capability within emerging platforms. Gaps, however, do exist – primarily in the convergence of glass capability, display process technology, and device architecture for new display applications.

621

622

5.1.1

Glass Substrates for AMLCD, OLED and Emerging Display Platforms

As we progress to an era of ubiquitous display, flexibility is a targeted dimension of highperformance display function that has been only partially fulfilled (see > Part 5.6, Flexible Displays). When properly packaged and handled, thin glass can be a flexible and strong display substrate; however display manufacturers have not yet made the necessary investments to employ thin flexible glass; nor have glass compositions been optimized for flexibility. Likewise, ubiquitous display will require much lower manufacturing costs than the current sheet-based process, and it is clear that roll-to-roll processing offers a path forward. As discussed above, glass as a R2R or flexible display component substrate for low-cost manufacture is feasible, but its development will require close collaboration between the device designer, the display manufacturing equipment industry, and the advanced glass substrate manufacturer. This collaboration will be a central theme as electronic display evolves to achieve ubiquity.

6

Further Reading

There has been no comprehensive review of display substrate technology since the early days of the AMLCD platform. Although dated, previously published collaborations of the author [4, 27] provide some details of glass properties and their impact upon LCD product and process performance. More recently, a broad discussion [16] of the dynamics behind the sustained value of highly engineered substrate glasses in an era of increased display panel commodization may have interest for the general reader. Very recently, Ellison and Cornejo (two principals in display substrate composition innovations at Corning Incorporated) have presented an insightful and entertaining retrospective on the LCD substrate era that delves deeply into glass composition design for LCD product and process [28].

References 1. Allen KJ (2005) Reel to real: prospects for flexible displays. Proc IEEE 93(8):1394–1399 2. Venugopal SM, Allee DR (2007) Integrated a-Si:H source drivers for 4 QVGA electrophoretic display on flexible stainless steel substrate. J Display Technol 3(1):57–63 3. Large Area Flexible Electronics Chapter, 2009. iNEMI Roadmap, International Electronics Manufacturing Initiative (www.inemi.org) 4. Garner S, Merz G, Matusick J, Tosch J, Joseph M, Boudreau B (2009) Flexible glass substrates for continuous processing. 2nd Annual Symposium on Flexible Electronics, Binghamton University, 19 August 2009 5. Dumbaugh WH, Bocko PL (1993) Substrates glasses for flat-panel displays. 1990 SID International Symposium, Digest of Technical Papers, Society of Information Display, Playa del Rey, vol 21, pp 70–72 6. Dumbaugh WH, Bocko PL, Fehlner FP (1992) Glasses for flat-panel displays. In: Cable M, Parker

7.

8.

9.

10. 11.

12.

JM (eds) High performance glasses. Chapman and Hall, New York, pp 86–101 Kay FE, Whitney RK, Zeman JE (1983) Alkali extraction as a determinant in the selection of a glass for displays. IEEE Trans Elect Devices 30(5):545–548 Araujo RJ, Binkowski NJ, Fehlner FP (1996) Alkali metal ion migration control. U.S. patent number 5,578,103, published on 26 November 1996 Lapp JC, Moffatt DM, Dumbaugh WH, Bocko PL, Anma M (1994) New substrate for advanced flat panel display applications. SID International Symposium Digest Tech Papers Anma M, Bocko PL (1996) Substrate for poly-Si TFT applications. IDW 96 Digest of Technical Papers, 373 Gulati ST, Helfinstine JD (1997) Fatigue resistance and design strength of advanced AMLCD glass substrates. Display Manufacturing Technology Conference, Digest of Technical Papers, pp 29–30 Webb JE, Hall DW (2009) Corning Incorporated Internal Document

Glass Substrates for AMLCD, OLED and Emerging Display Platforms 13. Bocko PL, Allaire RA (1995) Glass contribution to robustness of displays for automotive applications. SID Symposium on Vehicle Displays, Ypsilanti 14. Bocko PL, Fenn PM, Morse LR, Okamoto F (1991) Surface chemistry and microstructure of flat panel display substrates. SID 91 Digest, 675 15. See, for example, Corning EAGLE XG® Display Grade Glass Substrates, Product Information Sheet, PIE 301, March 2006 16. Bocko PL, Lee HS (2008) Highly engineered glass substrates for LCD television: why reducing value is incompatible with consumer expectations. Information Display 24(5):26–30 17. Lapp J, Moffatt D, Dumbaugh W, Bocko P, Anma M (1994) A new substrate for advanced flat-paneldisplay applications. Society for Information Display International Digest of Technical Papers (SID’94), vol 25, p 851 18. Bocko PL (2003) The challenges of higher-generation glass. Information Display 19(11):12–15 19. Lin C-P, Xiao Y-H, Tsui B-Y (2006) Process and characteristics of fully silicided source/drain (FSD) thin-film transistors. IEEE Trans Electron Devices 53(12):3086–3094 20. Leea W-K, Hana S-M, Choib J, Han M-K (2008) The characteristics of solid phase crystallized (SPC) polycrystalline silicon thin film transistors employing

21.

22.

23. 24.

25.

26.

27.

28.

5.1.1

amorphous silicon process. J Non-Cryst Solids 354:2509–2512 So BS, You YH, Kim HJ, Kim YH, Hwang JH, Shin DH, Ryu SR, Choi K, Kim YC (2005) Application of field-enhanced rapid thermal annealing to activation of doped polycrystalline Si thin films. Mater Res Soc Symp Proc 862:A2.4.1 Moffatt DM (1995) Properties of glass substrates for poly-Si AMLCD technology. Materials Research Society Symposium Proceedings, MRS 1995 Spring Meeting, San Francisco Bocko PL (2008) Jade glass signifies breakthrough in polysilicon technology. Display Devices Summer Bocko PL (2009) High-performance cover glass enhancing mechanical reliability of touch screens. FINETECH Conference – Touch Panel Session Price JJ et al (2009) A mechanics framework for ionexchanged cover glass with a deep compression layer. SID Symposium Digest of Technical Papers 40 (1):1049–1051 Hamilton L (2009) Better TV through glass: from panel through signal delivery. Conference Proceedings USFPD 2009 (DisplaySearch) Bocko PL, Whitney RK (1991) Engineered materials handbook. Volume 4: ceramics and glasses. ASM International, USA, p 1045 Ellison A, Cornejo IA (2010) Glass substrates for liquid crystal displays. Int J Appl Glass Sci 1(1):87–103

623

Part 5.2

Inorganic Semiconductor TFT Technology

5.2.1 Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs) A. J. Flewitt 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 628

2 a-Si:H Preparation by Plasma-Enhanced Chemical Vapor Deposition . . . . . . . . . . . 628 2.1 rf-PECVD Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 629 2.2 rf-PECVD Deposition Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 631 3 a-Si:H TFT Architecture and Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634 3.1 TFT Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634 3.2 TFT Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634 4 4.1 4.2 4.3

a-Si:H TFT Device Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 637 The a-Si:H Density of States, Electronic Transport and Doping . . . . . . . . . . . . . . . . . . . . 637 The Characteristics of a-Si:H TFTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639 Threshold Voltage Shift in a-Si:H TFTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643

5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 644

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.2.1, # Springer-Verlag Berlin Heidelberg 2012

628

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

Abstract: Hydrogenated amorphous silicon has enabled the active matrix liquid crystal display to dominate the flat panel display market, and indeed even to challenge the cathode ray tube as the leading display technology. This is the triumph of manufacturability over performance. Hydrogenated amorphous silicon thin film transistors have several fundamental performance limitations which are directly linked to the physics of the amorphous material. However, the amorphous structure coupled with the use of plasma-enhanced chemical vapor deposition (PECVD) allows devices to be manufactured with exceptional reproducibility and uniformity over very large area display backplanes. Consequently, the material limitations are tolerated and engineering solutions found to mitigate their effects. Acronyms: AMLCD, Active matrix liquid crystal display; a-Si:H, Hydrogenated amorphous silicon; a-SiN:H, Hydrogenated amorphous silicon nitride; dc, Direct current; FET, Field-effect transistor; MOSFET, Metal oxide semiconductor field-effect transistor; n+, Heavily doped n-type; NMOS, N-channel metal oxide semiconductor; PECVD, Plasma-enhanced chemical vapor deposition; RIE, Reactive ion etch; rf, Radio frequency; TFT, Thin film transistor

1

Introduction

The proliferation of active matrix liquid crystal displays (AMLCDs, see > Chap. 7.4.1) in markets as diverse as mobile telephones and home television has been enabled by hydrogenated amorphous silicon (a-Si:H), as it is very well suited to large area electronic applications. This is largely due to the fact that a-Si:H can be grown by a chemical vapor deposition process from a gaseous precursor over very large areas, and with a sufficiently wide processing window which ensures that reproducibility is excellent. Furthermore, the absence of any long-range order or grain boundaries in the amorphous material means that device uniformity over large areas is also very high. This is critical for display applications where each pixel in the display is controlled by a single thin film transistor (TFT). Any variation in the characteristics of these devices would result in nonuniformities in the resulting display image, to which the human eye is very sensitive. Another key factor behind the manufacturing favor shown to a-Si:H for display backplanes is the dominance of silicon technology for microelectronics more generally. Crystalline silicon is a well-known material, and a wealth of experience exists in how to process this material and control its electronic properties. The amorphous structure of a-Si:H and the presence of significant quantities of hydrogen does profoundly affect the physics of a-Si:H devices compared with the crystalline silicon equivalent, and indeed this restricts how widely a-Si:H can be employed for displays. However, in the case of AMLCDs, engineering solutions have been found to work around these material limitations. This section will look at all of these aspects of a-Si:H TFTs, starting with a discussion of how a-Si:H can be deposited over large areas. This leads on to a consideration of the architecture and fabrication of TFTs for displays and finally a review of the physics of a-Si:H TFTs, and how this affects device properties.

2

a-Si:H Preparation by Plasma-Enhanced Chemical Vapor Deposition

Plasma-enhanced chemical vapor deposition (PECVD) has consistently been demonstrated to produce the best quality a-Si:H for electronic applications – whether for TFTs or solar cells (the

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

5.2.1

other major market that is addressed by this material). Silane (SiH4) gas acts as the source of silicon. Silane is a highly reactive molecule, and indeed SiH4 is pyrophoric, reacting spontaneously and highly exothermically with oxygen. However, it does not readily react with itself and must be heated to temperatures
800 C before significant thermal decomposition takes place. Therefore, in order to produce a-Si:H material on a substrate, it is necessary to activate the silane gas. This is achieved by forming a low-pressure, glow-discharge plasma. In the earliest reports of a-Si:H growth, radio-frequency (rf) radiation was inductively coupled into a volume of silane gas at low pressure by wrapping around a quartz tube a coil of wire to which an rf power source was applied [1, 2]. However, very quickly it was shown that capacitive coupling of the rf radiation could also be successfully applied, and this has formed the basis of almost all industrial growth systems since [3]. Therefore, the following discussion will focus on this rf-PECVD technique. However, it should be noted that other methods of activating silane gas are also employed, even if rather less frequently. The most notable of these is catalytic chemical vapor deposition (sometimes also referred to as hot-wire chemical vapor deposition (HWCVD)), in which the silane gas is passed over an array of tungsten filaments that are heated to temperatures
1,600 C. This causes the silane to decompose before coating a substrate in an a-Si:H thin film [4, 5]. While this produces material with very good properties, such is the weight of experience behind rf-PECVD that this technique remains dominant.

2.1

rf-PECVD Reactor

The deposition of a-Si:H by rf-PECVD takes place in a reactor of the type shown in > Fig. 1. The main body of the reactor consists of a vacuum chamber, in which the substrate (most commonly a display-grade glass [6]) sits on a metal plate which is electrically earthed. The metal plate is then heated, typically to between 200 C and 400 C, before a regulated flow of silane gas is injected into the chamber. Depending of the precise deposition conditions that are desired, the silane may be diluted prior to injection with an inert gas, such as helium or argon, or with hydrogen whose effect is discussed in > Sect. 2.2. Disilane (Si2H6) may also be used as an alternative to silane, and occasionally the use of chlorosilanes and fluorosilanes are also reported, such as tetrafluorosilane (SiF4), in situations where some crystallinity is required in the final material [7]. Impurities can also be added to the system at this stage by mixing other gases with the silane. For example, phosphine (PH3) will result in the addition of phosphorous into the a-Si:H, which acts an n-type dopant (see > Sect. 4.1). P-type doping using boron can be achieved through the addition of diborane gas (B2H6) [8]. Alloys of amorphous silicon can also be produced, such as silicon nitride, which is an excellent insulator, through the addition of ammonia (NH3), sometimes diluted with nitrogen or hydrogen [9]. In order to achieve a uniform deposition, the gas is injected into the chamber through a large ‘‘showerhead,’’ which is a plate of similar size to the substrate plate with an array of small holes through which the gas passes. The showerhead and the substrate plate are parallel to each other with a small gap (5–40 mm) between the two. The pressure of the gas in the chamber is maintained between 0.1 and 1 torr by a vacuum pumping stack and a throttle valve. The gas is activated by applying an rf electric signal, most commonly at a frequency of 13.56 MHz, to the showerhead. The resulting high-frequency electric field that is set up between the showerhead and the earthed substrate plate leads to dissociation of the silane gas and the formation of a plasma. Deposition of material then takes place on all surfaces which are exposed to the activated gas precursors. The formation of thick, unwanted deposits of silicon

629

630

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

10 CF4 1 SiH4

8 2

PH3

8

N2O

3

NH3

2

N2 4

H2 He

11 8

9

12

1 rf generator

7 Exhaust

2 Coaxial cable

8 Valves

P

6 5

3 Matching network

9 Mass flow controllers

4 Gas shower head

10 Rotameter

5 Baffle valve

11 Heated sample stage (earthed)

6 Vacuum pumps

12 Pressure gauge

7

. Fig. 1 Schematic diagram of a capacitively coupled rf-PECVD reactor (reprinted with permission from [39])

on chamber surfaces over long periods of time is avoided by cleaning of the chamber through the use of a high-power plasma of tetrafluorocarbon (CF4) gas mixed with a small quantity of oxygen (
10%) which readily etches silicon material. Although a basic rf-PECVD reactor is relatively simple to construct, a production tool is rather more complex. In practice, the fabrication of a full TFT requires the deposition of a number of layers of material, including the a-Si:H semiconducting channel, doped a-Si:H contacts for the source and drain and a silicon nitride gate dielectric, as will be discussed in > Sect. 3. However, impurities, such as boron, once introduced into a deposition chamber will tend to persist, resulting in a small, but significant, impurity concentration in subsequent depositions. Therefore, separate chambers should be used for undoped a-Si:H, doped a-Si:H, and dielectrics. Furthermore, the best devices are produced when the interface between layers – particularly that between the a-Si:H semiconductor and the silicon nitride gate dielectric – is kept clean by avoiding contact with air between depositions. Therefore, the different deposition chambers should be linked by a vacuum load-lock chamber. When it is remembered that Generation 10 rf-PECVD systems allow for depositions onto substrates of 2.88  3.13 m size, the resulting deposition system is of considerable physical size.

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

5.2.1

Maintaining a uniform deposition over such a large area is also a significant engineering challenge. In practice, the substrate temperature is not constant over the whole area, but rather is tailored to compensate for the inevitable changes in gas flow that exist toward the edge of the substrate relative to the center. Furthermore, the shape of the gas injection holes in the showerhead is engineered to improve deposition uniformity, with a bell shape being favored, so that the plasma is formed within the bottom of the bell. The design of these holes is protected by significant intellectual property held by the major manufacturers of such systems, who also closely guard the know-how related to their exact role in the deposition process. Suffice to say that such systems are impressive pieces of engineering in their own right, and they enable the cost-effective production of AMLCDs that has resulted in such a large market for these displays.

2.2

rf-PECVD Deposition Processes

An rf-PECVD reactor of the type described in > Sect. 2.1 affords some degree of control over the properties of the resulting material through appropriate setting of the following deposition parameters: substrate temperature, gas pressure, gas flow rates, and rf power. In practice, an empirical approach is normally adopted in which a sensible range of values is defined for each of these deposition parameters, and a set of trial depositions are performed within this defined range. The resulting material properties are measured and the results used to determine an ‘‘optimum’’ set of growth conditions for a given application. In this regard, some understanding of the effect that changing each deposition parameter will have upon the resulting material structure is helpful, and a simple overview will be presented here. There have been a significant number of studies into the silane plasma that is produced in the rf-PECVD reactor with the aim of discerning the dominant precursors for deposition. In this regard, it is clear that under the typical conditions employed, the dominant silicon species in the plasma is the SiH3 radical, whose formation also results in a significant concentration of hydrogen radicals [10]. It is the reaction of these SiH3 radicals with the surface of the a-Si:H that results in material growth. In chemically bonding to the growing a-Si:H surface, an SiH3 radical will clearly use its dangling bond. Therefore, as the other three silicon bonds lead to hydrogen atoms, it must be the case that the surface of a-Si:H is predominantly hydrogen-terminated, as shown in > Fig. 2, and this has a profound influence upon how growth takes place [11, 12]. The key feature of deposition is that, as the a-Si:H surface is hydrogen terminated, there is not necessarily an available site for an arriving SiH3 radical to chemically bond to the surface where it lands. Instead, it is physisorbed and diffuses over the surface until either: (a) it is released back into the gas phase; (b) it abstracts a hydrogen atom from the surface to form an SiH4 molecule, which is released back into the gas phase, and a dangling bond on the surface is created; (c) it chemically bonds to the surface at a site where a dangling bond has previously been created. It is clear that the creation of surface dangling bonds is critical for growth to occur, and this may take place not only through the SiH3 abstraction process, but also through abstraction of hydrogen by hydrogen radicals which are also present in the plasma. Indeed, the dilution of silane with hydrogen gas will increase the number of hydrogen radicals in the plasma and leads to a reduction in the hydrogen content of the a-Si:H. Creation of dangling bonds on the surface is also possible through the effusion of hydrogen from the hydrogen-rich a-Si:H surface, where the energy barrier to atomic hydrogen migration is believed to be lower than in the bulk of the material due to the presence of more weak Si–Si bonds [11, 12].

631

632

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

Plasma

SiH3

H abstraction

Mobile SiH3

Sticking (growth) H-saturated surface H elimination

Surface layers buried to become bulk layers

Growth zone Bulk a-Si:H

. Fig. 2 Surface interactions between SiH3 radicals and the growth a-Si:H (reprinted with permission from [11])

All of these processes (hydrogen effusion and surface diffusion) are thermally activated, and therefore it is not surprising that the substrate temperature during growth will have a profound influence upon the properties of the resulting material. At the most basic level, increasing the substrate temperature will result in a reduced hydrogen content of the deposited a-Si:H. However, as will be discussed in > Sect. 4.1, the hydrogen content also has a profound influence upon the defect density of the material and the distribution of weak Si–Si bonds. Furthermore, the elevated substrate temperature during growth also provides energy for structural relaxation of the bulk silicon network and an associated reduction in the defect density of the material. While the substrate temperature profoundly affects surface processes during growth, it is unsurprising that the gas pressure should have a significant impact upon the plasma. Low pressure glow discharge plasmas of the type employed in rf-PECVD have an equal number of ions and electrons in the bulk of the plasma. However, close to the electrodes, the higher mobility of electrons compared to ions (due to the former’s lower mass) results in the plasma being depleted of electrons to produce a region of net positive charge. Unlike the bulk of the plasma, these regions do not emit visible light, and are called the plasma ‘‘sheaths.’’ The net positive space charge in the sheaths means that the bulk of the plasma sits at a positive dc potential relative to the earthed substrate plate, so that positively charged ions in the bulk plasma will be accelerated across the sheath, gaining significant kinetic energy in the process, which may be lost through collisions with other gas species in the sheaths if the mean free path is sufficiently low. > Figure 3a shows a schematic diagram of the bulk plasma, its sheaths and the resulting dc potential as a function of position, while > Fig. 3b is an equivalent electrical circuit representation of the plasma in which the bulk behaves as a simple resistance and the sheaths as a capacitance with parallel-loss conductance and a diode allowing one sense of dc bias to form due to the loss of electrons [13]. The consequence of this is that two distinct regimes are observed in the deposition of a-Si:H as a function of gas pressure: the so-called a- and g-regimes [14]. At high pressures – the

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

5.2.1

Plasma x

Cathode sheath Bulk plasma Anode sheath

. Fig. 3 (a) Schematic diagram of the plasma between two electrodes. The upper ‘‘driven’’ electrode is the showerhead while the lower ‘‘earthed’’ electrode is the substrate. The dc potential that results is also shown. (b) The equivalent circuit for the plasma [13]

g-regime – the effect of the plasma sheaths is limited as the mean free path is very low and so most of the kinetic energy gained by ions crossing the sheaths are lost to collisions. Instead, the dominant processes are gas phase reactions, and electrically the plasma is dominated by the resistive bulk. Higher order silyl species are formed in the plasma, of the form SixHy, where x  2; the plasma becomes ‘‘dusty,’’ and very high growth rates result. At low pressures – the a-regime – gas phase reactions are suppressed, ion bombardment of the substrate increases and the plasma is dominated by the capacitive plasma sheaths. Material for AMLCD backplane TFTs is almost exclusively deposited in the g-regime, as this produces material which is more stable, even though the electron mobility is reduced, as will be discussed in > Sect. 4. The gas pressure also affects the residence time of species in the deposition chamber in conjunction with the gas-flow rate, which is another of the deposition parameters that we are able to control. For a system in equilibrium, when the gas pressure, p, is stable, the rate of gas flow into the chamber, F, must equal the gas-flow out. Therefore, from the kinetic theory of gases, it can easily be shown that the average residence time, tr , is approximately given by tr ¼

pVLA ; RTF

ð1Þ

where V is the chamber volume, LA is the Avogadro number, R is the Molar Gas Constant, and T is the temperature. The residence time will affect the likelihood of a particular gas phase reaction occurring, such as the dissociation of SiH4 into SiH3 and H radicals. It will therefore affect the gas utilization. Ideally, the gas utilization should be high, as this will reduce the cost of running the system. Gas utilization is also affected by the last of the deposition parameters that can be controlled directly – the rf power. This determines the energy density of the plasma, and hence which gas species are activated and to what degree. As the silane molecule is very easily

633

634

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

dissociated; only very low plasma powers (as little as
10 mW cm2) are typically required to initiate a-Si:H deposition. However, higher rf powers will result in greater gas utilization and an increased deposition rate, although this will produce greater ion bombardment of the substrate as the plasma dc potential will also tend to rise. Higher rf powers may also be necessary for the deposition of other thin-film silicon materials, such as silicon nitride, where it is also necessary to dissociate other gases with a higher dissociation energy.

3

a-Si:H TFT Architecture and Fabrication

3.1

TFT Architecture

Although not implicit in its name, the TFT is a field-effect transistor, and therefore has three terminals: a source, drain, and gate. The geometry of the device must then be such that an insulating gate dielectric must sit between the gate and the channel semiconductor. Application of a voltage to the gate with respect to the channel should lead to an electric field penetrating into the channel semiconductor which must result in a modulation of the carrier concentration, and hence also the current that can flow between the source and drain contacts, which are applied directly to the semiconducting channel. Given that the semiconducting-channel material (in this case, intrinsic a-Si:H) is very thin (typically less than 100 nm), it is possible to conceive two generic device geometries. Perhaps the most obvious (and most similar to field-effect devices on bulk semiconductors) is where the source, drain, and gate are all on the same side of the channel, and this is called the ‘‘coplanar’’ structure. However, it is also possible to place the gate on the opposite side of the channel to the source and drain, and this is called the ‘‘staggered’’ structure. Furthermore, it is possible to place the gate at the top of the stack of thin-film materials or at the bottom, and these are called ‘‘top-gate’’ and ‘‘bottom-gate’’ structures, respectively, for obvious reasons. Therefore, there are four possible device architectures, as shown in > Fig. 4: top-gate coplanar, top-gate staggered, bottom-gate coplanar, and bottom-gate staggered [15]. Of these, the bottom-gate staggered structure is favored for a-Si:H TFTs. This is due to the fact that it is very important to minimize the density of defects at the interface between the semiconducting channel and the gate dielectric, as will be discussed in > Sect. 4.2. Using hydrogenated amorphous silicon nitride (a-SiN:H) as the gate dielectric where the a-Si:H is deposited on top of the a-SiN:H achieves this, which leads to a bottom-gate structure [16]. Meanwhile, the inverted structure allows the semiconductor and dielectric to be deposited consecutively with a consequent reduction in the density of defect states, which is not the case for the coplanar structure. Therefore, discussion of TFT fabrication in the following section will be limited to the bottom-gate staggered structure.

3.2

TFT Fabrication

A means of fabricating an array of a-Si:H TFTs must now be considered, given the requirement to use the bottom-gate staggered structure. For large area electronic applications, such as the AMLCD backplane, typical device dimensions are typically greater than 0.5 mm, and simple ultraviolet optical lithography (see > Chap. 5.5.1) can be used for patterning, and as few as three masks will deliver an operational device. However, this assumes that the same pattern is applied to both the gate dielectric and the channel, so that these can be patterned with a single

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

5.2.1

Electrode Semiconductor Dielectric

Coplanar

Staggered

Substrate Top-gate

Bottom-gate

. Fig. 4 The four possible TFT device architectures: top-gate coplanar, top-gate staggered, bottom-gate coplanar and bottom-gate staggered

mask. This is only ever really used in a research laboratory. In practice, it is almost always necessary to pattern the gate dielectric separately to open up specific vias to the gate and allow interconnect tracking elsewhere. Further, masks may also be required for etch-stop or passivation layers and interconnects, in order to realize a fully operational system. Therefore, in practice four or five masks are normally employed. The first decision that must be made is that of a substrate material. In this regard, there are two key restrictions: firstly, the need for the substrate to be able to withstand the temperature during deposition of the a-Si:H layer, which can be as high as 400 C (although significantly lower temperatures have been employed successfully by adjusting the full range of deposition conditions [17–20]), and secondly, the need to be compatible with optical photolithography, which means that a substrate must be resistant to swelling when immersed in liquids to allow accurate alignment of layers. These requirements make the use of plastic substrates very challenging. Furthermore, plastics tend to suffer from high surface roughness, which can adversely affect TFT performance, and a high concentration of impurities which can leach into the device. Therefore, it is normal to coat plastic substrates in planarised barrier layers, such as silicon nitride, to reduce these problems. Plastic substrates remain, for the time being, a research curiosity, and glass substrates are most commonly employed in practice. Even here, however, the range of glasses that can be used is limited. Firstly, the surface roughness should be low to avoid an adverse effect on TFT performance. Secondly, the thermal expansion coefficient of the glass should be similar to that of the a-Si:H to avoid the introduction of thermal stresses into the TFT materials, caused by the thermal cycling of the substrate during device fabrication. Intrinsic stress is known to make a-Si:H TFTs less stable to bias-induced threshold voltage shifts [21]. Finally, the substrate must be able to withstand the 400 C a-Si:H-deposition temperature, which means that the strain point of the glass should be greater than this. Glass manufacturers have responded to this need by developing a range of ‘‘display glasses.’’ Originally, these were barium borosilicate glasses, but these have been superseded by alkaline earth borosilicate glasses which also have lower densities and low heavy-metal-ion concentrations [6]. More detailed information on glass substrates is also included in > Chap. 5.1.1. The only other material selection to be made is that of the source, drain, and gate electrodes. As the source and drain are patterned using a different mask to the gate,

635

636

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

a different metal may be selected for the gate from the other two electrodes. In terms of the source and drain electrodes, it is vital that the electrode material is chemically stable and does not act to dope the a-Si:H. Furthermore, it must be capable of forming a good Ohmic contact with a-Si:H to avoid a high parasitic-contact resistance. To this end, a layer of highly phosphorous doped (n+) a-Si:H is usually included between the a-Si:H channel and the source/ drain electrodes to enhance electron injection. It should also be possible to deposit the electrode material easily over large areas (sputtering is often used for this purpose) and to chemically etch the material easily. Chromium and titanium, for example, meet these requirements. A typical fabrication process, therefore, would start with the gate metal being deposited. The first mask would be used to define the gate pattern. The a-SiN:H, a-Si:H, and n+ a-Si:H would then be deposited consecutively without allowing any contamination of interfaces, followed by the source/drain metal. Typically, the a-SiN:H will be over 150 nm thick to ensure that there are no pinhole defects, and that the dielectric can withstand the required gate voltages without allowing significant leakage current. Field penetration into a-Si:H is only a few tens of nm, and therefore the a-Si:H need only be between 50 and 100 nm thick, while the n+ a-Si:H-charge injection layer need only be a few tens of nm thick to achieve a low contact resistance. The second mask would then be used to isolate individual TFT islands by etching through the source/drain metal, n+ a-Si:H, a-Si:H, and a-Si:N thin films. This will also expose a connection to the gate electrode. While metal etching is frequently performed using wet chemistry, the silicon layers would normally be etched using a dry reactive ion etch (RIE). A third mask would then be used to define the source and drain electrodes by etching through the source/drain metal and the n+ a-Si:H. There is no selective dry RIE which will etch the n+ a-Si:H, but not the underlying a-Si:H. Therefore, some degree of ‘‘over-etching’’ is required which will result in at least 10 nm of a-Si:H being removed also. The accuracy with which this can be achieved will partly determine the thickness of a-Si:H that is initially deposited. This lack of etch selectivity between the n+ a-Si:H and the a-Si:H leads to problems not only with process yield and run-to-run variability, but also results in the creation of defects on the etched a-Si:H (called the backside of the a-Si:H). These defect states can act to pin the Fermi energy, leading to a parasitic off-state current, an increase in threshold voltage, and a reduction in field-effect mobility [22]. This is called the ‘‘back-channel effect’’ (see > Sect. 4.2). A popular solution to this is to introduce an ‘‘etch-stop’’ layer between the a-Si:H and the n+ a-Si:H. This requires a change in the fabrication process flow after the deposition of the a-Si:H layer. At this point, the deposition of the n+ a-Si:H and the source/drain metal must be postponed. Instead, a second layer of a-SiN:H is deposited which will act as the etch-stop, and the second mask is used to isolate the TFT islands by etching through the top a-SiN:H, a-Si:H, and bottom a-SiN:H (being the only layers present at this stage). An additional mask is now used to pattern the top a-SiN:H layer to leave islands of material, where the a-Si:H channel must be protected before returning the substrate to the rf-PECVD system for deposition of the n+ a-Si:H and the source/drain metal. The process then continues as before with the patterning of the source/ drain electrodes, but now the a-Si:H channel is passivated and not exposed to the RIE etch of the n+ a-Si:H, thereby overcoming any back-channel effect. However, this is at the cost of adding an extra mask into the process and also introduces the risk of contamination between the a-Si:H and n+ a-Si:H layers which could lead to a series parasitic resistance. The consequent reduction in throughput and yield means that the etch stop is rarely actually used in industry, with preference being to minimize the defect density on the back surface through good etch control and post-etch annealing.

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

5.2.1

4

a-Si:H TFT Device Physics

4.1

The a-Si:H Density of States, Electronic Transport and Doping

Before discussing the characteristics of a-Si:H TFTs, it is necessary to appreciate how the density of states of this material differs from its crystalline-silicon counterpart. Crystalline silicon is an indirect gap semiconductor with a band gap between the bottom of the conduction band and top of the valence band of 1.1 eV. This is down to the fact that the atoms in crystalline silicon have a long-range, tetrahedral order that is due to the atoms forming four covalent bonds with an sp3 electron hybridization (i.e., they are all four-fold coordinated). The Coulomb interaction between electrons in these covalent bonds and the charge in the atomic cores means that bonding electrons sit at a lower energy than anti-bonding electrons, and this is the basis of the continuum of valence-band states and conduction-band states, respectively. A-Si:H, in comparison, is considered to consist of a continuous random network of silicon atoms. Although these atoms have a generally tetrahedral bonding structure, there is a significant variation in bond lengths and bond angles, by
10% and 2%, respectively. As a result, the material possesses no long-range order, and rather than all bonds having the same energy, there is a spread of bond energies also. In some cases, local network strain will be so great as to prevent an atom forming four Si–Si bonds, resulting in the atom being undercoordinated and the presence of a dangling bond. The presence of these dangling bonds and weak Si–Si bonds has a profound influence upon the density of states of a-Si:H, its electronic properties, and, consequentially, the operation of TFTs. In an ideal crystalline lattice, all the electron states associated with the conduction and valence bands are extended in nature – they are spatially spread over the whole volume of the solid. The disorder in an amorphous semiconductor means that some electron states are not spatially extensive, but are localized with their wavefunction decaying exponentially from a particular point. Such localized states may only exist below some critical energy called the ‘‘mobility edge’’, above which states will generally be extended [23]. These localized states, which are associated with the most strained (and hence weakest) Si–Si bonds, therefore sit at the bottom of the conduction band and top of the valence band and form band tails which extend into the otherwise forbidden energy gap, as shown in > Fig. 5. The width of these bandtail states is known as the ‘‘Urbach energy’’ (or sometimes the ‘‘Urbach slope’’) and is typically
50 meV in a-Si:H [24]. The other effect of disorder is to introduce many scattering sites, with the consequence that momentum is no longer conserved in electronic transitions, and so there is no energy-wavenumber (E-k) dispersion relation which means that a-Si:H must be a direct gap semiconductor. Dangling bonds also produce localized states which lie in the energy gap. It might be expected that these would all exist at the energy associated with an unbonded sp3 electron state. However, in practice, the disorder in the amorphous material will mean that the local environment for any one dangling bond defect is unique, and so there will be a spread in defect energies. Furthermore, a simple dangling bond defect would be occupied by a single electron – the ‘‘uncharged’’ state – but in actual fact, the dangling bond may either lose this electron to become positively charged, or gain one further electron to become negatively charged. The charge state will also affect the energy of the defect. As a result, there is a ‘‘pool’’ of defect states with some spread of energies in the energy gap, as shown in > Fig. 5 [25]. States close to the valence band will generally be negatively charged, and those close to the conduction band will be positively charged.

637

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

Valence band extended states

Conduction band extended states

Localised states within mobility gap (Eg )

EV

Dangling bonds

EF

Band tail

Band tail

Electron density of states (m −3)

638

EC

Energy (eV)

. Fig. 5 Schematic diagram of the a-Si:H density of states (reprinted with permission from [40])

There is also a clear link between the weak Si–Si bond and the dangling bonds, as the former may break to create the latter, and vice versa. It is proper, therefore, to consider the material as sitting in some equilibrium between weak bonds and dangling bonds that is energetically favorable, but which may be affected by, e.g., moving the Fermi energy (as happens when a voltage is applied to the gate of a TFT), which will change the charge state of the defects, their energy distribution, and the likelihood of weak bond formation and annihilation. A similar effect occurs when electron-hole pairs are formed in a-Si:H by the absorption of photons, which affects solar cells [26, 27]. In unhydrogenated amorphous silicon with a low intrinsic stress (so that disorder is minimized), this equilibrium results in a density of dangling bond defects
1020 cm3. Such a high defect density would pin the Fermi energy and prevent transistor action. Hydrogen acts to reduce this defect density to as low as 1016 cm3 by bonding at dangling bond defect locations. Hydrogen, therefore, has a profound influence upon the density of states of a-Si:H as it will affect the whole equilibrium between dangling bonds and weak bonds, and will also lead to a reduction in the Urbach energy, as it will become more energetically favorable for very strained Si–Si bonds to break and be passivated by hydrogen. Therefore, it is not as straightforward as adding simply enough hydrogen to passivate dangling bonds, and, in fact, much more hydrogen is required to minimize the dangling bond defect density – typically
10 atomic % – due to its effect upon the weak-bond equilibrium also [28]. This is also consistent with the evidence from nuclear magnetic resonance that hydrogen tends to exist in a localized complex consisting of two hydrogen atoms in close (0.6–0.8 nm) proximity to each other, as the annihilation of a single weak Si–Si bond will require two dangling bonds to be passivated with hydrogen atoms [29].

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

5.2.1

The full picture of the density of states is therefore that of > Fig. 5, in which the conduction and valence bands each have extended states with a tail of localized states at the edge of the band gap. The energy separating the mobility edge of the conduction and valence bands is called the ‘‘mobility gap’’, and is found to be
1.9 eV. The gap itself then contains a distribution of dangling bond defect states with a density
1016 cm3. The density of states in a-Si:H has a profound impact upon conduction. Whereas in crystalline silicon, conduction electrons are in extended states, in a-Si:H conduction at room temperature is dominated by the band-tail states. As these are localized, conduction takes place by electrons hopping between these localized states [2, 23]. The effect of this is that the electron drift mobility, mD, is reduced from its free carrier value, m0, by the proportion of time the carriers spend in the trap state, ttrap, relative to the time they are free, tfree, according to [30]
 tfree : ð2Þ mD ¼ m0 ttrap þ tfree For high quality a-Si:H, this results in a drift mobility for electrons of
1 cm2 V1 s1 and for holes of
0.003 cm2 V1 s1. A-Si:H is therefore not suitable for p-channel devices, and a-Si:H TFT circuits are therefore based on NMOS (n-channel metal oxide semiconductor) technology, which is a major limitation. In > Sect. 3.2, it was highlighted that in order to be able to inject electrons into an n-channel TFT, it is necessary to include a highly doped layer of n-type hydrogenated amorphous silicon. Phosphorous is most commonly used as the impurity to achieve this. However, in an amorphous semiconductor, substitutional doping is not possible, as is the case in a crystalline semiconductor, as the lack of any long-range order will tend to allow an impurity atom to adopt a local bonding configuration that suits its own electronic structure. However, the work of Spear and LeComber clearly demonstrated experimentally that impurities could be used to introduce electron donor and acceptor states into a-Si:H and, thereby, modulate the Fermi energy [8]. The accepted mechanism by which this takes place was developed by Street [31], and is shown pictorially in > Fig. 6. Taking phosphorous as an example, for doping to occur, a P atom must take up a four-fold bonding coordination rather than the three-fold coordination that would be expected according to the 8-N rule of Mott [23]. However, the four-fold coordinated state is only slightly higher in energy than the three-fold state. Therefore, in the presence of a nearby weak Si–Si bond, it may represent a lower energy overall for the weak Si–Si bond to break to form two dangling bonds, one of which will then form the additional bond to the P atom required to change it to four-fold coordination, and the other dangling bond taking up the resulting ‘‘donated’’ electron from the P atom. By increasing the occupation of the defect state associated with the dangling bond, the Fermi level is raised to a higher level in the band gap. However, this process can only occur where there is a favorable weak Si–Si bond, and so only around one in every ten P atoms actually yields a donor electron, and very much higher impurity concentrations are required to achieve a desired doping level than might be expected.

4.2

The Characteristics of a-Si:H TFTs

> Figure 7 shows a typical gate transfer characteristic

for a good a-Si:H TFT. All a-Si:H TFTs are operated as n-channel enhancement (accumulation) mode devices due to the far superior

639

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

Defect level and fermi energy (Occupation = 50%)

Energy (eV )

Conduction band Donor level Energy (eV )

640

Valence band

Conduction band Donor level Electron transfer Fermi level rises Defect level (Occupation > 50%) New dangling bond states Valence band

. Fig. 6 A schematic diagram showing how doping in a-Si:H is mediated by the presence of defect states close to the site of the impurity in an equilibrium process (reprinted with permission from [40])

electron mobility compared to hole mobility, and the fact that the defect states naturally pin the Fermi energy toward the middle of the gap, meaning that the channel device has a very high resistivity when no bias is applied to the gate, which is the normal AMLCD-backplane device requirement. The characteristics of the TFT are very similar to those of a MOS field effect transistor (FET), but with some specific modifications. It is clear from > Fig. 7 that there exists some threshold voltage, VT, above which the device is operating in an accumulation mode. If this device were a simple MOSFET, then the drain-source current, IDS, would depend on both the gate-source and drain-source voltages, VGS and VDS, respectively, above this threshold voltage, but before saturation due to pinch-off of the channel, according to   W VDS 2 IDS ¼ Ct mFE ðVGS  VT ÞVDS  ; ð3Þ L 2 where Ci is the capacitance per unit area of the gate dielectric, mFE is the field-effect mobility of the carriers, W is the channel width, and L is the channel length. If the device is saturated (i.e., VDS > VGS  VT), then IDS is limited to IDS ¼ Ct mFE

W ðVGS  VT Þ2 : L 2

ð4Þ

Although these equations are frequently employed to extract the threshold voltage and field-effect mobility (in the case of > Eq. 4, from a graph of √IDS plotted as a function of VGS), the theoretical approach by which this model is derived does not take into account the fact that conduction is dominated by carriers in the band tails. Allowing for this results in > Eq. 4 being modified to [32]

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

5.2.1

10−3 10−4 10−5 10−6

VDS = 0·2 V VDS = 10 V

IDS [A]

10−7 10−8 10−9 10−10 10−11 10−12 10−13 −15

W/L = 230 L = 50 µm −10

−5

0 VGS [V]

5

10

15

. Fig. 7 A typical gate transfer characteristic for an a-Si:H TFT

IDS ¼ zCta1

mFE W ðVGS  VT Þa ð1 þ lVDS Þ: a L

ð5Þ

Three new terms have been introduced: a, z, and l. The first of these is dependent upon the slope of the density of tail states, VnC and the thermal voltage, vth, according to a¼2

VnC vth

and is usually numerically equal to 2. The relative permittivity, er , and density of electrons trapped in the tail states of the a-Si:H, NnC are combined into z, as  z¼

 1a aevth e0 eg NnC ½ 2 ; a1

ð6Þ

where e is the electronic charge and e0 is the permittivity of free space. Finally, l is the channel modulation parameter, which allows for the fact that the drain-source voltage does have some influence upon the current above pinch-off (an effect that is frequently observed in short channel MOSFETs in accumulation mode)  [33]. It should be noted that if a = 2, then 1  a2 becomes zero and z = 1, and therefore, if l is 0, then the original MOSFET equation (> Eq. 4) is restored. The second regime of operation that should be considered is that ‘‘subthreshold’’, where the device is turning on. In this region, the current increases exponentially with the voltage over and above that at which this regime begins, VTS, so that
 W VGS  VTS IDS ¼ I0S exp ; ð7Þ L Sf

641

642

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

where Sf is the subthreshold slope. The pre-exponential factor, I0S, is related to a similar set of a-Si:H material properties as z, namely I0S ¼ mFE Sf

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi aevth e0 eg NnC : 2ð a  1 Þ

ð8Þ

While > Equations 5–8 provide a mathematical description of the operation of an a-Si:H TFT, they do not, in themselves, allow insight into the physical operation of a device. To do this, it is simplest to start by considering a TFT where a positive bias has been applied to VDS and a negative bias has been applied to VGS, so that the device is off. In the steady-state, current flow in the device is dominated by charge transfer across the gate dielectric due to Poole-Frenkel tunneling [32] and will increase in magnitude if the gate voltage is reduced to more negative values further, as seen in the VDS = 0.2 V curve of > Fig. 7, where VGS < 5 V. For small-gate voltages, the electric field is insufficient for Poole–Frenkel tunneling to occur, and in this regime the small current that flows through the gate dielectric will be of an Ohmic nature due to the presence of a very small number of thermally generated carriers. Now consider the application of a positive VGS bias to the device. The capacitance of the gate and channel will cause a buildup of negative charge in the channel, which is supplied from the source and drain electrodes. As the Fermi level has been sitting in the partially filled dangling bond defect states toward the center of the gap, these states must first be filled with electrons before carriers can begin to occupy the conduction band tail states. Therefore, the distribution of these mid-gap defect states will have a profound influence upon the threshold voltage of the device, with a greater density of mid-gap states toward the upper half of the band gap requiring more charge density for occupation and hence a greater threshold voltage. The distribution of these defects will also influence the subthreshold slope, with a low defect density in the upper half of the band gap being required for a sharp turn-on. Assuming that there is sufficient charge for the defect states to be filled, then the band-tail states will begin to be occupied, and the device will carry a significant source-drain current. However, as band-tail hopping limits conduction in these tail states (see > Sect. 4.1), then the field-effect mobility will be lower than the drift mobility associated with the extended states of the conduction band. Typical values are between 0.5 and 1 cm2 V1 s1, although there are recent reports of higher values being achieved by using dielectrics other than silicon nitride or silicon oxide [34]. As the total density of band-tail states is so high in a-Si:H, it is not possible to move the Fermi level into the extended states of the conduction band – taking advantage of the higher mobility associate with these states – without the use of very high gate electric fields, which would first break down any dielectric. The time required for switching to take place is largely dependent upon the process of charge trapping in defect states and can be as short as 1 ms, but can take up to 10 s for a poor quality device [15]. An additional influence upon a-Si:H TFT transfer characteristics is the ‘‘back-channel effect,’’ which is observed when no etch stop is used in the fabrication of a bottom-gate staggered TFT (see > Sect. 3.2). In this case, defects are created at the (back) surface of the a-Si:H that is exposed to the etch. These defects can pin the Fermi level at this interface, which can have two effects. In the off-state, there may still be a layer of accumulated charge at the back surface which can result in a high off-state current. Meanwhile, in the on-state, the pinned Fermi level will reduce the degree of band bending at the channel-dielectric interface as the a-Si:H will only be
100 nm thick. This will degrade the subthreshold slope and reduce the on-state current of the device [22].

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

4.3

5.2.1

Threshold Voltage Shift in a-Si:H TFTs

A feature of a-Si:H is the equilibrium that exists between the weak Si–Si bonds and the dangling bonds, which was discussed in > Sect. 4.1. Furthermore, we know that the distribution of dangling bond defects as a function of energy is affected by the charge state of the defects, as described by the Defect Pool Model [25]. Therefore, it should not be surprising that a change in the charge state of defects caused by the accumulation of charge in the semiconducting channel of an a-Si:H TFT will affect the dangling-bond defect distribution and the weak-bond versus dangling-bond equilibrium. This is manifested by a change in the threshold voltage of an a-Si:H TFT as a function of time, while a bias is applied to VGS, with the threshold voltage increasing as function of bias time, and tending toward the gate-bias voltage. The rate of this voltage shift can be increased if the temperature of the device is increased. An example of this process acting upon the gate transfer characteristics of a device is shown in > Fig. 8 [35]. The voltage shift is also reversible, and the original device characteristics can be restored by heating the TFT to
200 C with no bias applied to the gate, as this provides sufficient energy to allow the original equilibrium of weak bond and dangling bonds to be restored [36]. The defect creation process is limited by the breaking of weak Si–Si bonds, which is then followed by a local rearrangement of hydrogen to stabilize the dangling bond defects that have been created [21]. Therefore, a-Si:H which has a low intrinsic stress is found to be more stable, and this is achieved by depositing a-Si:H in the g-regime. It is for this reason that g-regime material is most commonly used for AMLCD backplanes, even though the field-effect mobility

10.0 µ 5,000 s 10,000 s

8.0 µ

20,000 s 30,000 s

Ids (A)

6.0 µ 50,000 s 70,000 s

4.0 µ 1,000 s

3,000 s

0s

2.0 µ 80,000 s

0.0 0

5

10

15

20 25 Vg (V)

30

35

40

45

. Fig. 8 Threshold voltage shift as a function of time for a deuterated amorphous silicon. The device has been heated to 102 C to enhance the rate of threshold voltage shift (reprinted with permission from [35] ß 2005 American Institute of Physics)

643

644

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

of this material is not as high as for a-regime material (see > Sect. 3.2). The defect removal process, however, is limited by the breaking of a weak Si–H bond [37]. A second mechanism is also observed for threshold voltage shifts under gate biasing in a-Si:H TFTs due to charge trapping in the a-SiN:H gate dielectric [38]. In this case, there is no temperature dependence in the rate of threshold voltage shift. Which mechanism dominates will depend on the quality of the a-Si:H and a-SiN:H. However, the charge trapping effect tends to only exceed the a-Si:H defect creation mechanism at high gate voltages.

5

Summary

Hydrogenated amorphous silicon has enabled the rapid growth of the large-area electronics industry through the latter part of the twentieth and the first part of the twenty-first centuries, with backplanes for AMLCDs being one of the major applications. One reason for this is the scalability of rf-PECVD which is used to deposit this material, with deposition systems able to coat areas in excess of 10 m2 in a single run. However, rf-PECVD is a complex process with a number of controllable deposition parameters having interconnected effects upon the properties of the a-Si:H deposited. As a result, material optimization is a largely empirical process. However, it also allows for the deposition of other silicon-based materials, such as a-SiN:H, which is the preferred dielectric for TFTs. As a result, it is found that the bottom-gate staggered TFT structure produces the best devices due to the quality of the interface formed, and devices with this structure can be fabricated using as few as three masks. A second key advantage that a-Si:H possesses for large-area electronics is its amorphous structure with no grain boundaries that can lead to statistical variations between devices on a single substrate. As a consequence, exceptionally high levels of device uniformity can be achieved on a backplane. However, the disorder produced as a result of the amorphous network results in the presence of weak Si–Si bonds and dangling bonds, which lie in an equilibrium with each other, and lead to the presence of localized band-tail states and mid-gap defect states, respectively. As a consequence, the mobility of electrons in a-Si:H is about three orders of magnitude smaller than for crystalline silicon, and this limits the switching speed of devices significantly. Furthermore, changes in the threshold voltage of TFTs are observed as a function of time when a bias is applied to the gate of the device. This is due to changes in the weak-bond/ dangling-bond equilibrium which leads to the creation of more dangling bonds. This also limits the application of a-Si:H TFTs. In particular, these mobility and stability limitations makes it unlikely that a-Si:H TFTs will be able to drive backplanes for future generations of active matrix organic light-emitting diode displays. However, the cathode-ray tube has shown that a high quality, low cost display technology is extremely resilient to new challengers to the market, and so a future for a-Si:H in displays is probably assured well into the twenty-first century.

References 1. Chittick RC, Alexander JH, Sterling HF (1969) The preparation and properties of amorphous silicon. J Electrochem Soc 116:77–81

2. Le Comber P, Spear W (1970) Electronic transport in amorphous silicon films. Phys Rev Lett 25:509–11

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs) 3. Perrin J (1995) In: Bruno G et al (eds) Plasma deposition of amorphous silicon-based materials. Academic, London, pp 177–241 4. Heintze M, Zedlitz R, Wanka H, Schubert M (1996) Amorphous and microcrystalline silicon by hot wire chemical vapor deposition. J Appl Phys 79:2699–706 5. Matsumura H, Umemoto H, Masuda A (2004) CatCVD (hot-wire CVD): how different from PECVD in preparing amorphous silicon. J Non-Cryst Solids 338–40:19–26 6. Ellison A, Cornejo IA (2010) Glass substrates for liquid crystal displays. Int J Appl Glass Sci 1:87–103 7. Kakinuma H, Mohri M, Tsuruoka T (1995) Mechanism of low-temperature polycrystalline silicon growth from a SiF4/SiH4/H2 plasma. J Appl Phys 77:646–52 8. Spear W, LeComber P (1975) Substitutional doping of amorphous silicon. Solid State Comm 17:1193–6 9. Murley D, French I, Deane S, Gibson R (1996) The effect of hydrogen dilution on the aminosilane plasma regime used to deposit nitrogen-rich amorphous silicon nitride. J Non-Cryst Solids 198– 200:1058–62 10. Perrin J (1991) Plasma and surface reactions during a-Si:H films growth. J Non-Cryst Solids 137(138):639–44 11. Robertson J (2000) Growth processes of hydrogenated amorphous silicon. Mater Res Soc Symp Proc 609:A1.4.1–A.4.12 12. Robertson J (2000) Deposition mechanism of hydrogenated amorphous silicon. J Appl Phys 87:2608–17 13. Ko¨hler K, Coburn JW, Horne DE, Kay E, Keller JH (1985) Plasma potentials of 13.56-MHz rf argon glow discharges in a planar system. J Appl Phys 57:59–66 14. French I, Deane S, Murley D, Hewett J, Gale I, Powell M (1997) The effect of the amorphous silicon alphagamma transition on thin film transistor performance. Mater Res Soc Symp Proc 467:875–80 15. Powell M (1989) The physics of amorphous-silicon thin-film transistors. IEEE Trans Electron Devices 36:2753–63 16. Lucovsky G, Phillips JC (2000) Why SiNx:H is the preferred gate dielectric for amorphous Si thin film transistors (TFTs) and SiO2 is the preferred gate dielectric for polycrystalline Si TFTs. Mater Res Soc Symp Proc 558:135–40 17. Sazonov A, Striakhilev D, Lee C-H, Nathan A (2005) Low-temperature materials and thin film transistors for flexible electronics. Proc IEEE 93:1420–8 18. Yang C-S, Smith L, Arthur C, Parsons G (2000) Stability of low-temperature amorphous silicon thin film transistors formed on glass and transparent plastic substrates. J Vac Sci Tech B 18:683–9

5.2.1

19. Alpuim P, Chu V, Conde JP (2000) Low substrate temperature deposition of amorphous and microcrystalline silicon films on plastic substrates by hotwire chemical vapor deposition. J Non-Cryst Solids 266–269:110–4 20. Sarma KR, Chanley C, Dodd S, Roush J, Schmidt J, Srdanov G, Stevenson M, Wessel R, Innocenzo J, Yu G, O’Regan M, MacDonald WA, Eveson R, Long K, Gleskova H, Wagner S, Sturm JC (2003) Active matrix OLED using 150 C a-Si TFT backplane built on flexible plastic substrate. Proc SPIE 5080:180–91 21. Wehrspohn RB, Deane SC, French ID, Gale I, Hewett J, Powell MJ, Robertson J (2000) Relative importance of the Si-Si bond and Si-H bond for the stability of amorphous silicon thin film transistors. J Appl Phys 87:144–54 22. Ando M, Wakagi M, Minemura T (1998) Effects of back-channel etching on the performance of a-Si:H thin-film transistors. Jpn J Appl Phys 37:3904–9 23. Mott NF (1967) Electrons in disordered structures. Adv Phys 16:49–144 24. Urbach F (1953) The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids. Phys Rev 92:1324 25. Powell MJ, van Berkel C, Franklin AR, Deane SC, Milne WI (1992) Defect pool in amorphous-silicon thin-film transistors. Phys Rev B 45:4160–70 26. Staebler D, Wronski C (1977) Reversible conductivity changes in discharge-produced amorphous Si. Appl Phys Lett 31:292–4 27. Staebler D, Wronski C (1980) Optically induced conductivity changes in discharge-produced hydrogenated amorphous silicon. J Appl Phys 51:3262–8 28. Nickel N, Jackson W (1995) Hydrogen-mediated creation and annihilation of strain in amorphous silicon. Phys Rev B 51:4872–81 29. Su T, Taylor PC (2003) Magnetic resonance measurements in hydrogenated amorphous silicon. Sol Energy Mat Sol Cells 78:269–98 30. Street RA, Kakalios J, Hack M (1988) Electron drift mobility in doped amorphous silicon. Phys Rev B 38:5603 31. Street R (1982) Doping and the Fermi energy in amorphous silicon. Phys Rev Lett 49:1187–90 32. Nathan A, Servati P, Karim KS, Striakhilev D, Sazonov A (2003) Thin film transistor integration on glass and plastic substrates in amorphous silicon technology. IEE Proc Circ Dev Syst 150:329–38 33. Shichman H, Hodges DA (1968) Modeling and simulation of insulated-gate field-effect transistor switching circuits. IEEE J Solid-State Circ 3:285–9 34. Han L, Mandlik P, Cherenack KH, Wagner S (2009) Amorphous silicon thin-film transistors with fieldeffect mobilities of 2 cm2/V s for electrons and 0.1 cm2/V s for holes. Appl Phys Lett 94:162105

645

646

5.2.1

Hydrogenated Amorphous Silicon Thin Film Transistors (a Si:H TFTs)

35. Wehrspohn RB, Lin SF, Flewitt AJ, Milne WI, Powell MJ (2005) Absence of enhanced stability in fully deuterated amorphous silicon thin-film transistors. J Appl Phys 98:054505 36. Flewitt AJ, Lin S, Milne WI, Wehrspohn RB, Powell MJ (2006) Mechanisms for defect creation and removal in hydrogenated and deuterated amorphous silicon studied using thin film transistors. Mater Res Soc Symp Proc 910:449–60 37. Flewitt AJ, Lin S, Milne WI, Wehrspohn RB, Powell MJ (2006) Characterization of defect removal in hydrogenated and deuterated amorphous

silicon thin film transistors. J Non-Cryst Solids 352:1700–3 38. Powell MJ (1983) Charge trapping instabilities in amorphous silicon-silicon nitride thin-film transistors. Appl Phys Lett 43:597–9 39. Flewitt AJ (2006) In: Gaura E, Newman R (eds) Smart MEMS and sensor systems. Imperial College Press, London, pp 31–105 40. Flewitt AJ, Milne WI (2004) In: Kuo Y (ed) Amorphous silicon thin film transistors, vol 1, Thin film transistors: materials and processes. Kluwer Academic, Massachusetts, pp 15–78

Further Reading Street RA (1991) Hydrogenated amorphous silicon. Cambridge University Press, Cambridge

Kuo Y (2004) Amorphous silicon thin film transistors, vol 1, Thin film transistors: materials and processes. Kluwer Academic, Massachusetts

5.2.2 Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs) S. D. Brotherton 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648

2 2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.2

Poly-Si Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648 Excimer Laser Crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 Crystallization Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 TFT Crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 652 ELA Process Control Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655 Large Grain Poly-Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655

3 3.1 3.1.1 3.1.2 3.2

Poly-Si TFT Architecture and Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657 Self-Aligned Source and Drain Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 658 Drain Field Relief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 660 Fabrication Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 662

4 4.1 4.2 4.3 4.4

Performance Artifacts and Drain Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 662 Electrostatic Drain Field, F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 662 Hot Carrier Damage (HCD) and LDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664 Field-Enhanced Leakage Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 667 Other Bias-Stress Instabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 669

5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 670

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.2.2, # Springer-Verlag Berlin Heidelberg 2012

648

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

Abstract: Poly-Si TFTs are now in mass production for small diagonal displays, and, in this article, the fabrication and performance of these devices is discussed, with particular focus on the formation of poly-Si by excimer laser crystallization, the issues surrounding TFT architecture, and the performance artifacts associated with the high electrostatic field at the drain junction. List of Abbreviations: AMLCD, Active Matrix Liquid Crystal Display; AMOLED, Active Matrix Organic Light Emitting Diode; a-Si:H, Hydrogenated Amorphous Silicon; DOS, Density of States; ELA, Excimer Laser Annealing; GOLDD, Gate Overlapped Lightly Doped Drain; HCD, Hot Carrier Damage; LDD, Lightly Doped Drain; LPCVD, Low Pressure Chemical Vapor Deposition; MOSFET, Metal Oxide Semiconductor Field Effect Transistor; PECVD, Plasma Enhanced Chemical Vapor Deposition; SLG, Super-Lateral Growth; SOG, System-OnGlass; SOI, Silicon-on-Insulator

1

Introduction

The interest in poly-Si, as a thin film transistor (TFT) material, started soon after a-Si:H TFTs became recognized as the most promising technology for the large-scale production of active matrix liquid crystal displays (AMLCDs). The low carrier mobility of a-Si:H TFTs ( Sect. 3.2, the typical poly-Si process requires a higher mask count than the standard a-Si:H process, resulting in a larger processing cost for a given glass substrate size. (> Chap. 5.1.1 contains a full discussion of a-Si:H TFTs). In the following sections, key aspects of these processes are reviewed, and, in > Sect. 2, the preparation of poly-Si films by the industry standard technique of excimer laser annealing (ELA) is discussed. Device architecture and the fabrication process are presented in > Sect. 3, and aspects of device performance, including the influence of the drain field on bias-stress stability and field-enhanced leakage currents, are discussed in > Sect. 4.

2

Poly-Si Preparation

In contrast to the carrier mobility in a-Si:H, which has remained within the range 0.5–1.0 cm2/Vs over the last 20 years or so, the electron mobility in poly-Si has increased from Chap. 5.1.1. Finally, the overall complexity of the process needs to be borne in mind, as, the more complex the process, the more expensive the final product will be. The presently preferred poly-Si preparation technique uses an excimer laser, and this has superseded previously investigated techniques of direct deposition and solid phase crystallization. Further discussion of these procedures is beyond the scope of this article, but a review of them can be found in reference [3].

2.1

Excimer Laser Crystallization

2.1.1

Introduction

Excimer lasers are gas lasers operating in the ultraviolet wavelength range, from 193 to 351 nm depending upon the gas mixture chosen. For crystallization, the preferred gas mixture is XeCl, giving a wavelength of 308 nm; similar crystallization results have been obtained from KrF excimer lasers at 248 nm, but the 308 nm laser is preferred for industrialization as the longer wavelength is less damaging to the optical components in the beam path. These are pulsed lasers, with a typical pulse duration of
30 ns, a maximum repetition frequency of 600 Hz, and can deliver up to 0.9 J/pulse [4]. The raw pulse shape is semi-Gaussian, with dimensions of
11 cm, but, for crystallization, a line beam is preferred, and beam shaping optics are used to produce a highly elongated beam, whose dimensions can be up to 465 mm in the long axis and down to 350 mm for the short axis [4]. The steep edges in the short axis profile have led to the beam shape being referred to as a ‘‘top-hat’’ beam, and all the irradiations discussed in this article will be assumed to have this shape, unless specified otherwise. For crystallization, the plate is mechanically swept through the short axis of the beam at a rate, which delivers a multi-shot process, of typically 10–30 shots per point for commercial processing, so that the plate translation distance between shots in these systems is typically in the range 12–50 mm for short axis lengths of 350–500 mm. Both KrF and XeCl excimer lasers were used in the specific experimental work described below, with the former having the raw, semi-Gaussian beam shape (but had been homogenized in the uniform direction), and the latter being the more conventional top-hat line beam.

2.1.2

Crystallization Process

The optical absorption coefficients of a-Si at the wavelengths of 248 and 308 nm are 5.7 and 7.6 nm, respectively, leading to strong absorption in, and intense heating of, the silicon film, which, if the incident energy density is high enough, will heat the film to its melting point of

649

650

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

During irradiation

(i) E < Emelt – partial melting

Molten

– Medium grain

Solid

– Fine grain/ amorphous

Molten (ii) E ~ Emelt – near melt-through

Solid

(iii) E > Emelt – full melt-through

Crystallised film

Large grain Nucleation on solid regions Super lateral growth (SLG) (Im et al.)

Molten

Random nucleation in super cooled melt. Fine grain

. Fig. 1 Schematic illustration of a-Si melting regimes during excimer laser irradiation and resulting poly-Si grain structure

1,420 K. Simulations and measurements have shown that the threshold energy density for melting thin a-Si films is
100 mJ/cm2 [5], but, with the short pulse duration of 30 ns, the thermal diffusion length through a glass capping layer of SiO2 is
0.2 mm, so that glass plates capped with 0.5 mm or more of SiO2 are protected from unacceptable temperature excursions. The current understanding of the crystallization mechanism of a-Si has shown that the melt depth of the film is a crucial factor in determining the outcome of the process [6–9]. This is shown schematically in > Fig. 1 where three melting regimes are identified. Following partial film melting, the crystallized film has a vertically stratified appearance, with mid-sized grains (
100 nm) within the previously melted region, and either a fine grain or amorphous region beneath it. When the film is almost fully melted, small solid islands remain at the back of the film, seeding the growth of large (
300 nm) high quality grains, as the film cools. These columnar grains extend through the entire thickness of the film, which no longer shows vertical stratification. This process was identified by Im et al. [6, 7], and given the name super-lateral growth (SLG); as will be shown below, this is the crystallization regime, which yields high quality TFTs. Finally, in a fully melted film, the seeding centers responsible for the SLG growth are lost, and crystallization of the film relies upon random nucleation in a super-cooled melt, resulting in a fine grain film [6]. From the above scenario, it is apparent that the optimum energy range is that which results in the SLG regime; energy densities above and below this produce smaller grain poly-Si. However, as shown by the grain size results in > Fig. 2, the SLG regime occurred over a very limited energy density range of
45 mJ/cm2 from 195 to
240 mJ/cm2 [6]. These particular results were from single shot irradiations [6], and qualitatively identical results were obtained from multi-shot irradiations [9], apart from a growth in SLG grain size with increasing shot number.

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

400

Average grain radius (nm)

300

200

100

0 50

100

150

200

Laser energy density

250

300

350

(mJ/cm2)

. Fig. 2 Variation of average grain radius of excimer laser crystallized 100 nm a-Si films with laser energy density (films capped with 50 nm SiO2) (Reproduced with permission from [6]. Copyright 1993, American Institute of Physics)

The essential features of the above process are fully demonstrated in the correlation of TFT behavior with the energy density used to crystallize the film. This can be most easily seen with static irradiations of a 40 nm thick a-Si film, using a semi-Gaussian excimer laser beam, as shown in > Fig. 3a–c [10, 11]. It should be emphasized that this mode of irradiation is not the conventional TFT crystallization procedure (which involves swept, line-beam irradiations), but is used purely as an experimental tool. > Figure 3a shows a profile of electron mobility measurements made on a line of non-self-aligned TFTs, with a spatial pitch of 220 mm. The profile through the approximately Gaussian distribution of energy densities within the beam facilitates a precise mapping of electron mobility (and other device parameters) against incident energy density [11]. The maximum beam intensity had been set to
270 mJ/cm2, which is a value in the SLG regime, and a progressive increase in mobility from 0 to
200 cm2/Vs is seen as the crystallization energy increased from zero to its maximum. In > Fig. 3b, the maximum energy density was increased to 330 mJ/cm2, which was large enough to induce full melting of the film over the central portion of the beam. The ensuing fine grain material [6] resulted in a substantial reduction of electron mobility from
200 to
25 cm2/Vs, whilst the edge regions, irradiated at lower energy densities, retained the appearance seen in > Fig. 3a. The abrupt decrease in mobility gave an SLG window size of
45 mJ/cm2 [11], which is the same as the energy window from the TEM data of [6]. Finally, > Fig. 3c shows the result of reirradiating the material from > Fig. 3b using lower intensity conditions, which correspond to the SLG regime. This converted the previously fine grain material back into larger grain

651

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

Normalized beam intensity 1.0 300 250 Mobility, μ (cm2/Vs)

652

200

100 Epk = 273 mJ/cm2

50 0 0

a

Epk = 330 mJ/cm2

0.5 150

0.2 0.4 0.6 0.8 1.0 Position in beam (cm)

0 1.2

Epk = 273 mJ/cm2

0

0.2 0.4 0.6 0.8 1.0 Position in beam (cm)

b

1.2

Epk = 330 mJ/cm2

Epk = 273 mJ/cm2

Epk = 330 mJ/cm2

0

c

0.2 0.4 0.6 0.8 1.0 Position in beam (cm)

1.2

Epk = 330 + 273 mJ/cm2

. Fig. 3 Electron mobility as a function of position following stationary KrF excimer laser crystallization. Measured after irradiations at the following peak intensities: (a) 273 mJ/cm2, (b) 330 mJ/cm2, (c) 330 mJ/cm2 followed by 273 mJ/cm2 (Taken from [30]. ß MRS)

SLG material, with a consequent recovery in electron mobility. Hence, the material can be cycled in and out of the SLG regime, depending upon the final energy density used to crystallize the film.

2.1.3

TFT Crystallization

An overview of TFT results from swept beam processing is shown in > Fig. 4 [11, 12]. This figure shows the dependence of electron mobility, in n-channel TFTs, upon the irradiation energy density, with the a-Si precursor film thickness as an independent parameter. > Figure 4a

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

#F539/2

Mobility, μ (cm2/Vs)

29

40

80

145 nm IV

III

100 II

10

I 200

a

300 400 500 Energy density (mJ/cm2)

600

#F704 20

130 nm

75

Mobility, μ (cm2/Vs)

III

II

10

I 200

b

IV

100

300 400 500 Energy density (mJ/cm2)

600

. Fig. 4 Electron mobility as a function of excimer laser energy density for a-Si precursor films of different thickness (a) 248 nm (KrF) irradiation of LPCVD a-Si, (b) 308 nm (XeCl) irradiation of PECVD a-Si:H (Reproduced from [12]. With permissions by the Society for Information Display)

contains results from low pressure chemical vapor deposition (LPCVD) precursor a-Si, crystallized by a 248 nm KrF semi-Gaussian beam, and the data in > Fig. 4b are from the more commonly used plasma enhanced chemical vapour deposition (PECVD) precursor a-Si: H, crystallized by a 308 nm XeCl line beam. The two sets of curves demonstrate the same essential features of the crystallization process, which are independent of the laser wavelength, the type of precursor material, and the beam shape details. The two most obvious features in these curves are that the outcome of the crystallization process is a strong function of film thickness (and the energy densities required to achieve maximum carrier mobility have been shown to scale approximately linearly with film thickness [11]), and that the crystallization process is not a simple monotonic function of energy density. The results in > Fig. 4 only show electron mobility, but the other key device parameters of subthreshold slope and leakage

653

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

10-4

10-6 ID (A)

654

p-channel

n-channel

W = 50 L 6

μη = 230 cm2 Vs μρ = 104 cm2 Vs

VD = ±5 V

10-8

10-10

10-12 −18

−12

−6

0 VG (V)

6

12

18

. Fig. 5 Transfer characteristics of n- and p-channel excimer laser crystallized non-self-aligned TFTs (Taken from [24]. Reproduced by permission of ECS – The Electrochemical Society)

current show a corresponding dependence on crystallization energy density, and qualitatively identical results were also obtained from p-channel TFTs [11]. Whilst > Fig. 4 has focussed on electron mobility values, > Fig. 5 shows the transfer characteristics of high quality, non-selfaligned n- and p-channel TFTs obtained from this process, illustrating the attainment of low leakage currents and small subthreshold slopes as well as the high carrier mobility values. The crystallization regimes in > Fig. 4 can be broken down into four phases, which are most clearly seen in the thicker films, but can also be discerned by points of inflection in the thinner films. The four phases are an initial increase in mobility (I), a plateau region (II), a second rise in mobility (III), and, finally, a decrease in mobility (IV). The initial increase in mobility occurs above the melt threshold energy, when the surface of the film is converted into a polycrystalline form. As the thickness of the crystallized region increases, so the electron field effect mobility continues to increase, until the band bending is contained entirely within the crystallized region. When that occurs, regime II is seen, and further increases in crystallization depth have a minimal effect upon the carrier mobility. This plateau region is resolved most unambiguously in the thicker films, and corresponds to partial melting of the film. Ultimately, with increasing incident energy density, the melt depth increases until the film is almost fully melted, when the SLG regime is initiated, and the resulting change in grain size and quality is responsible for the abrupt increase in electron mobility in regime III. It will be seen how the energy density for this regime scales with film thickness [11], and to obtain maximum carrier mobility, the crystallization energy density must be matched to the film thickness. Finally, at the highest energy densities, the electron mobility starts to decrease in regime IV due to the incipient formation of fine grain material. However, the decrease in mobility is much smaller than seen with the stationary beam process of > Fig. 3b, because, in the swept beam process, subsequent lower intensity pulses, in the trailing edge of the beam, can reset the material back into the SLG regime.

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

2.1.4

5.2.2

ELA Process Control Issues

As shown by the results in > Fig. 4, to obtain high carrier mobility devices, the film needs to be crystallized within the SLG regime, and, ideally, at the energy density giving the maximum mobility. But, there is limited accuracy in the precise setting of the laser energy density, and, moreover, the pulse-to-pulse fluctuations in energy density [4] mean that samples will occasionally be exposed to higher intensity irradiations, and, where this causes full meltthrough, lead to a consequent degradation in device parameters. Comparison of > Figs. 3b and > 4 shows that the magnitude of this degradation is determined by the opportunity within the process to re-irradiate the fine grain material and to convert it back to large grain SLG material. The static irradiations in > Fig. 3b can be regarded as a zero pulse overlap process (giving gross mobility nonuniformity when full melt-through occurs), whereas the 100-shot, swept beam processes in > Fig. 4 can be regarded as a 99% pulse overlap process (giving greatly improved uniformity, even after full melt-through). Hence, the practical issue of plate processing is the trade-off between reduced pulse overlap (giving higher plate throughput) and acceptable uniformity within a realistic process window. Typical plate processing uses a 20-shot process, yielding an electron mobility of
120 cm2/Vs, within an energy window of
30–40 mJ/cm2. However, by broadening the trailing edge of the beam, in order to optimize the SLG recovery process, the size of the energy process window can be increased by a factor of
2 for a 10-shot process [13]. As stated above, the major cause of nonuniformity has been pulse-to-pulse instability, and improvements in pulse stability, to 3% over 3 sigma [4], has contributed to the improved uniformity of the crystallization process. Combined with a technique to randomize intensity variations along the beam length, the more stringent uniformity requirements for active matrix organic light emitting diode (AMOLED) displays may be met [4], although some groups still express reservations about the uniformity of poly-Si for this application [14].

2.2

Large Grain Poly-Si

As discussed above, the conventional ELA technique produces devices with limited electron mobility ( Chap. 5.2.1), and is much more similar to the architecture of single crystal silicon-oninsulator (SOI) MOSFETs. Further points of similarity with the SOI MOSFETs, and distinction from a-Si:H TFTs and emerging TFT processes in organic or oxide materials, are that both n-channel and p-channel poly-Si TFTs, with comparable performance, can be fabricated with a common process. (Discussion of organic and oxide TFTs can be found in > Part. 5.3, Emerging TFT Technologies). Secondly, the source and drain doping is usually accomplished using ion shower doping, and this process has a number of similarities to ion implantation for MOSFETs. Hence, the architecture and a key fabrication stage of poly-Si TFTs are closer to MOSFETs than to devices fabricated in other thin film materials. However, high-dose ion doping is potentially damaging to the crystallographic order of the poly-Si film, particularly for the heavier phosphorus ion compared with boron. The degree of damage will be determined by the ion dose, ion energy, and substrate temperature, and, for high enough doses into single crystal silicon, the damage can be sufficient to cause complete amorphization of the implanted layer [23]. This is also an issue in poly-Si, and is discussed further in > Sect. 3.1.1. The essential difference between the two architectures in > Fig. 6 relates to the formation of the source and drain regions, which determines the overlap of the gate electrode across these regions. For the NSA structure, with significant overlap, the source and drain regions can be ion doped prior to the crystallization of the film, and the dopants in these regions will then be

S

G

D

S

G

D

Al

Al SiO2

Al

Al

Al SiO2

Al

n+

a

p-Si

n+

n+

p-Si

SiO2

SiO2

Glass

Glass

n+

b

. Fig. 6 Cross-sectional diagrams of poly-Si TFT architectures (a) non-self-aligned, (b) self-aligned

657

658

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

activated during film crystallization. This process can be implemented with direct ion doping into bare a-Si, and results in a high level of dopant activation, with minimal residual ion damage after ELA. A doping process employing a phosphorus dose of 11015 cm2 at 10 keV typically gives a sheet resistance of
250 Ω/square. Following this stage, the gate dielectric is deposited and defined, followed by gate, drain and source metal deposition, and definition, where the overlap between the gate metal and the doped source and drain regions is determined by the alignment rules for the process, and can be up to
3 mm. This is a simple and robust fabrication procedure, and is well suited to the basic study of material parameters (as described in > Sect. 2), but the architecture suffers from parasitic gate-drain capacitance, which will degrade high frequency transistor performance. In addition, it is not well suited to the fabrication of shorter channel devices (L Figure 7 shows the variation of poly-Si sheet resistance as a function of phosphorus ion dose, and compares laser and furnace activation. With laser activation, the sheet resistance varied inversely with phosphorus dose, as would be expected, but, with furnace activation at 450 C, this was only seen at the lower doses. At the higher doses, the sheet resistance began to increase with dose, and, at the highest dose, it was above the apparatus measurement limit of 1106 Ω/square. This was confirmed to be due to film amorphization, since comparable doses could be activated in 80 nm thick films. Furnace activation, when successful, also resulted in a higher sheet resistance than that obtained with laser activation. Further issues in the SA doping process are revealed when the TFT transfer characteristics, processed with furnace and laser activation, are compared in > Fig. 8 [24]. One obvious feature in these curves, and in other publications [25, 26], is the higher minimum leakage current in the laser-activated n-channel TFTs, which is due to residual ion doping damage at the edge of the drain junction [24, 26]. Cross-sectional TEM of a more heavily phosphorus-implanted sample showed a 200 nm wide residual amorphous region, located in the doped region near the

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

Rs (Ω/square)

105

5.2.2

450 C activation Laser activation Slope = −1

104

103

102 1015 Total phosphorus dose (cm−2)

1016

. Fig. 7 Variation of sheet resistance of phosphorus implanted poly-Si as a function of dose and activation method (80 keV implant through 145 nm of SiO2)

10−4

W 50 = L 6

10−6

Laser

ID (A)

VD = ±5V

450°C

10−8

10−10

10−12 −18

−12

−6

0 VG (V)

6

12

18

. Fig. 8 Transfer characteristics of self-aligned n- and p-channel poly-Si TFTs, showing the effect of laser or furnace dopant activation (Taken from [24]. Reproduced by permission of ECS – The Electrochemical Society)

659

660

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

gate edge, and also extending beneath the gate (due to lateral end-of-range damage not exposed to the laser) [24]. Very similar results have been reported in arsenic doped n-channel TFTs, where the leakage current and TEM-imaged crystallographic damage in the exposed silicon near the gate edge have been correlated with diffraction of the laser beam by the gate edge [26]. A further feature in > Fig. 8 is the lower on-current in the furnace activated n-channel TFT, due to the higher sheet resistance in the source and drain regions, as seen in > Fig. 7. Finally, the field effect mobility extracted from laser crystallized n-channel TFTs was less than from identically crystallized NSA TFTs [24], and, as with the leakage current, this has also been seen in arsenic doped TFTs, and, again, associated with high resistance, parasitic damage regions at the gate edge [26]. The damage can have a significant impact upon the on-current of short channel TFTs [26–28], where a fixed series resistance has an increasingly large effect as channel resistance reduces. Various techniques have been reported for minimizing the general ion doping damage effects in SA n-channel TFTs, including a careful control of ion dose and energy [24], obliqueincidence laser irradiation to minimize the gate-edge diffraction [29], and the use of an off-set gate, in which the gate length is reduced by lateral etching after ion doping, so that the diffraction and masking effects at the gate edge no longer interfere with the full melting of the doped region [30]. As is apparent from the above discussion, the process for fabricating SA TFTs, with minimal performance artifacts, is considerably more complex than the NSA TFT process, and this is the reason for favoring the use of NSA TFTs for basic poly-Si materials studies, such as those presented in > Sect. 2. However, the potentially superior performance of SA TFTs, due to lower parasitic capacitances, and their better compatibility with sub-micron channel length TFTs makes them the preferred choice for many poly-Si applications.

3.1.2

Drain Field Relief

When poly-Si TFTs are exposed to a drain bias stress, there can be a critical fall-off in performance, as illustrated by the results in > Fig. 9: the exposure of the device, to a 13 V drain bias for 60s, led to an increase in off-state current and a reduction in on-state current, as well as a major change in the shape of the output characteristics. This phenomenon has been ascribed to hot-electron damage [31, 32], and the physics of this process is discussed in > Sect. 4.2, where it is shown that an alteration in device architecture is needed to suppress this instability. The architectural change is one in which the field at the drain/channel junction is reduced, and this is most easily accomplished by the use of lightly doped drain (LDD) regions, as shown schematically in > Fig. 10a and b. These figures illustrate two LDD variants, where the lightly doped region is either external to the gate, or is overlapped by the gate (GOLDD). In both cases, an extra masking and processing step is involved to form this region, and, as with the drain region itself, the LDD region is formed by phosphorus ion doping, but with a lower dose in the poly-Si (typically in the range 51012 to 51013 cm2, in contrast to the fully doped drain region where the dose is likely to be in the range 51014 to 11015 cm2). When the region is defined photolithographically, its size will be determined by the process design rules, and is likely to be of the order of 1–3 mm. The qualitative trade-off with dose is that lower doses will give more field relief, and hence, better stability, but a larger series resistance, and hence, lower on-current. This trade-off is minimized with GOLDD, where the gate modulates the conductivity of the GOLDD region,

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

5.2.2

10−3 F463 n-channel

10−5

VD = 0.5 V

Initial

10−7

0.3 After stress ID (mA)

ID (A)

W 50 = L 6

10−9

After VG = 4 V VD = 13 V, l min

0.2 Initial

0.1

0

5 VD (V)

10−11

−16

−12

−8

−4

0 VG (V)

4

8

12

10

16

. Fig. 9 Influence of drain bias stress of 13 V for 60s on NSA TFT transfer characteristic. The insert shows its effect upon the output characteristic (Taken from [38]. Reproduced with permission from APEX/JJAP)

S

G

Al

Al SiO2 n+

p-Si

D

S

G

D

Al

Al

Al

Al

SiO2

LDD n−

n+

n+

p-Si

n+

SiO2

SiO2 GOLDD

Glass

Glass

a

b

. Fig. 10 Cross-sectional diagrams of field relief structures in self-aligned poly-Si TFTs: (a) LDD, (b) gate overlapped LDD (GOLDD)

and it also gives better device stability (see > Sect. 4.2). However, as with the NSA architecture, GOLDD gives greater parasitic capacitance, and is less compatible with short channel length devices. To obtain sub-micron LDD regions, a non-lithographic definition procedure is required, such as the off-set gate formed by lateral etching [33] or by a sidewall spacer [34], and, if the spacer itself is conducting, a GOLDD region will be formed [35].

661

662

5.2.2 3.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

Fabrication Process

The fabrication of a simple NSA n-channel TFT (see > Fig. 6a) is a four-mask process involving the definition of the poly-Si island, the source and drain doping locations, contact window opening through the gate oxide, and definition of the metal contact areas. The SA n-channel TFT (see > Fig. 6b) is also a four-mask process, with island definition, gate metal definition, contact window opening, and source and drain metal definition. The mask count for these two processes compares well with the fabrication of an a-Si:H TFT, but the mask count can rise to nine for a full AMLCD process, containing complementary circuit TFTs, plus field relief for the n-channel TFTs. The extra stages being separate photolithographic definitions for the source and drain areas for the n- and p-channel TFTs, plus the LDD area for the n-channel TFTs. To this, contact windows for the ITO pixel electrodes and the ITO patterning itself needs to be added. For both architectures, two other stages are frequently necessary, namely, de-hydrogenation, and low dose B+ doping. The a-Si:H precursor film typically contains
10% hydrogen, and, if this film is exposed to a top-hat laser beam, there will be an explosive release of hydrogen causing severe blistering of the film. A low temperature thermal anneal at 400–450 C is used to reduce the hydrogen content to
1% or less, at which level the film can be exposed to a top-hat beam without risk of mechanical damage. The low dose boron ion doping is used to compensate for an intrinsic electron richness in the crystallized poly-Si film, which is due to a combination of positive charges in the oxides above and below the film, and/or the neutral level of the poly-Si band-gap states being above mid-gap. The net effect of these centers is to shift the threshold voltage of the TFTs in a negative direction. Boron doping is used to compensate this shift, and to achieve symmetrically positioned transfer characteristics of p- and n-channel TFTs either side of Vg =0 V, as seen with the characteristics in > Figs. 5 and > 8.

4

Performance Artifacts and Drain Field

Three major performance artifacts have been identified in poly-Si TFTs. Firstly, the off-state current increases exponentially with both gate and drain bias, and examples of the gate bias dependence can be seen in > Figs. 5 and > 8. Secondly, the output characteristics show poor current saturation, as seen by the insert in > Fig. 9, and, finally, n-channel TFTs are subject to drain bias-stress instability, as also illustrated in > Fig. 9. The details of this instability process are discussed further in > Sect. 4.2, and the leakage current effects in > Sect. 4.3, whilst the electrostatic drain field itself is discussed in > Sect. 4.1.

4.1

Electrostatic Drain Field, F

The 2-D electrostatic field in the space charge region of a semiconductor junction is given by Poisson’s equation, which relates the space charge density, Nsc, and field, F: @Fx @Fy qNsc þ ¼ @x @y es e0

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

5.2.2

Using a 2-D simulator, evaluation of this expression, for poly-Si TFTs, showed that the differential terms were opposite in sign, and both were much larger than the space charge term on the right side, such that their difference was only
10–20% of their absolute values [36]. Thus, the internal electrostatic field was not controlled by the poly-Si density of states, DOS, as it would be in a 1-D analysis. This is shown in > Fig. 11a, in which the maximum drain field was calculated as a function of gate bias, with the poly-Si DOS as a scaled independent parameter [36]. In this diagram, the different DOS values produced subthreshold slopes of 3.0, 1.0, and 0.3 V/decade in the simulated TFT transfer characteristics (for a gate oxide thickness, tox, of 0.15 mm). To put those slopes into context, a high quality poly-Si TFT would have a subthreshold slope of 0.4 V/decade for this oxide thickness. Therefore, for TFTs of practical interest, with a subthreshold slope of Fig. 11b, demonstrating that the more abrupt the junction is, the greater is the peak field. Because of the significance of the lateral dopant distribution, the drain field in NSA TFTs decreases with increasing ELA shots (due to lateral diffusion of the drain dopant) [37], and, equally, the drain field in SA TFTs is likely to be larger than in NSA TFTs (due to reduced lateral diffusion in the former). As the poly-Si quality itself had little effect upon the magnitude of the drain field, a change in architecture was required to produce a greater lateral dopant spread, and this is most readily accomplished through the use of lightly doped drain (LDD) regions, the fabrication of which was discussed in > Sect. 3.1.2. It is worth noting that similar high drain field effects occur in MOSFETs, and these also routinely incorporate LDD regions for field relief.

⫻105

6 6

⫻105

5 Fmax (V/cm)

Fmax (V/cm)

5 4 3

VD = 5 V

A 10 N(E) B N(E)

2

a

3 VD = 5 V

2

VG = −5 V

C 0.1 N(E)

1

1 0 −15

4

0 −10

−5 VG (V)

0

5

0

b

50 100 150 200 Lateral straggle of drain dopant (nm)

. Fig. 11 Simulations of maximum field at the drain/channel junction as a function of (a) gate bias and poly-Si DOS (56 nm lateral dopant spread), (b) lateral spread of drain dopant (N(E)S=1 V/dec) (Reproduced with permission from [36]. Copyright 1996, American Institute of Physics)

663

664

5.2.2 4.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

Hot Carrier Damage (HCD) and LDD

The high drain field, combined with the high electron mobility, leads to carrier heating and the generation of additional electron-hole pairs by impact ionization. These additional carriers contribute to the poor saturation of the output characteristics of poly-Si TFTs, and the socalled ‘‘kink’’ effect when the impact ionization-induced avalanche currents ultimately dominate the current flow [31, 39]. The impact ionization, underlying the ‘‘kink’’ effect, is also enhanced by parasitic bipolar transistor (PBT) action in the floating body of the TFT [39], and this effect increases with reducing channel length [40]. However, further discussion of PBT effects is beyond the scope of this review. The impact of hot carrier damage on n-channel TFT transfer characteristics has already been shown in > Fig. 9, and the changes in both on-state and off-state currents are attributed to the creation of localized electron trapping centers near the drain junction. Also shown by the inset is the change in output characteristics as a result of the HCD: at low drain bias, there is reduced drain current (compared with the unstressed sample) due to the high resistance of the channel near the drain, and, at large drain bias, there is a higher drain current due to the negatively charged trapping states increasing the drain field, and hence increasing the avalanche generation rate [41]. Hot carrier damage has been extensively studied in c-MOSFETs, and the damage centers were identified as acceptor states at the Si/SiO2 interface, as well as positively charged states in the gate oxide itself. From the dependence of these densities upon the gate bias, during drain bias stress, it was concluded that both hot-electron and hot-hole injection into the oxide had to take place, and that the generation of interface traps resulted from a two-stage process of hole capture at oxide traps near the interface and subsequent recombination with injected electrons [42]. It is likely that the recombination energy released by electron capture could break weak interface bonds. There is injection of both carrier types because, under the normal bias-stress conditions, Vd >Vg, giving field reversal in the oxide near the drain junction, which promotes hole injection. In spite of this field reversal, significant electron injection can also take place against the field if the hot electron is within a scattering distance of the Si/SiO2 interface [42]. In analogy with these HCD effects in MOSFETs, the poly-Si damage centers have been widely attributed to states at the poly-Si/SiO2 interface [31, 32, 38, 41, 43], or, in analogy with defect creation in a-SiH TFTs, attributed to bulk states in the poly-Si [44]. In both cases, good fits were obtained between simulation models and the experimental characteristics, but the latter conclusion was based partly upon the observation of damage at the back of the film, and subsequent simulation work showed that, even with top-gated devices, damage centers are produced at both the front and back interfaces [41, 45]. As mentioned in > Sect. 3.1.2, the problem of hot carrier damage can be reduced by the use of field relief regions, such as LDD and GOLDD, and, by and large, lower doping levels give better field relief. However, dose optimization for field relief using LDD structures involves a compromise with series resistance and reduced on-current [45]. In contrast, optimization of dopant concentration in GOLDD structures involves almost no compromise with series resistance in the on-state, although there will be some current reduction due to the increased channel length to accommodate the GOLDD region. Less obviously, the GOLDD structure also gave significantly better field relief, as shown by the comparison of the bias-stress results in > Fig. 12a–d. In the GOLDD device, there was almost no reduction in the drain current after a 30 V drain bias stress, compared with the 10–100 times reduction in the LDD device after

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

10−7

Id (μA)

Id (A)

40

10−11

Vgs = 3.6 V

20

Vds = 0.1 V 10−15

0

10 Vgs (V)

a

0

20

4

0

b

8

12

Vds (V)

120 0.2

Vds = 0.1 V

100

0.15 Ids (mA)

Id (μA)

80 60 40

c

0.1 0.05

20 0

Vg = 3.6 V

0

5

10 Vg (V)

15

20

0

d

5

10

15 20 Vds (V)

25

30

. Fig. 12 Experimental measurements before and after drain bias stress – arrows indicate direction of changes: (a) transfer characteristic, and (b) output characteristics of LDD TFTs after drain bias stress at 8, 10, and 12 V for 5,000s (LDD dose=11013 P/cm2); (c) linear transfer characteristics, and (d) output characteristics of GOLDD TFTs after drain bias stress at 30 V for 75s, 1,270s and 10,000s (GOLDD dose=51012 P/cm2) (Taken from [45]. ß 2006 IEEE)

10 V stress. There was also no degradation in the output characteristic at low Vd, and the characteristic second saturation regime in the GOLDD TFTs was reduced by the stress [45]. These differences have been explained by applying the MOSFET HCD model to poly-Si TFTs [45], in which the simulated carrier densities and fields in the TFT were used to model the hot-carrier injection currents into both the top and bottom oxide layers by using the ‘‘lucky electron’’ model [46]. The hot-electron injection current is given by [45]: Z Je ðxÞ ¼ A nðx; yÞPe ðyÞdy and the electron injection probability, Pe(y), is given by [46]: y Fb Pe ðyÞ / exp  exp  l Fx l where Fb is the zero field oxide barrier height, and l is the electron mean free path. The first term describes the probability of an electron, at depth y in the poly-Si, traveling to the interface without an inelastic collision, and the second term is the probability of the hot electron having enough energy to surmount the barrier Fb. A similar expression holds for hole injection, and

665

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

the barrier height for electrons (holes) is 3.1 eV (4.7 eV), and the electron (hole) mean free path is 6.5 nm (4.7 nm). From the calculated hot-carrier injection currents, and the ensuing two-stage carrier recombination within the oxide, the positive charge density (due to trapped holes in the oxide) and the negative charge in interface states was calculated iteratively with time, so that, as these charges built up, their affect upon the field and current distribution in the poly-Si was re-calculated, which then altered the trap generation rates [45]. > Figure 13a and b show the simulated Id–Vg and Id–Vd plots for the LDD devices, and > Fig. 13c shows the Id–Vd plot for the GOLDD device. In all cases, these results accurately reflect the experimental data in > Fig. 12. For the LDD structure, acceptor trap generation started at the channel/LDD interface, where the field was at a maximum, and, with increasing time and Vd-stress, spread into adjacent channel and LDD regions at both the top and bottom poly-Si/SiO2 interfaces. The positive trapped charge started in the same location, but finished up as a bi-modal distribution either side of the high field point. The negative charge in the interface states dominated, and constricted the electron channel current, near the drain, into

10−6

50

10−8

40 Id (μA)

Id (A)

10−10 10−12

Vgs = 3.6 V

30 20 10

Vds = 0.1 V 10−14

0

5

10 Vgs (V)

a

15

0

20

0

2

4

b

6 8 Vds (V)

10

12

1 Vg = 3.6 V

0.8 Id (μA)

666

0.6 0.4 0.2 0

c

0

5

10

15 20 Vds (V)

25

30

. Fig. 13 Simulated characteristics before and after drain bias stress – arrows show direction of changes from unstressed states: (a) transfer characteristic, and (b) output characteristics of LDD TFTs following stress at 8, 10, and 12 V for 5,000s (LDD dose=11013 P/cm2); (c) output characteristics of GOLDD TFT following 23 V stress for 323s and 10,000s (GOLDD dose=51012 P/cm2) (Taken from [45]. ß 2006 IEEE)

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

5.2.2

the center of the film, forming a resistive bottleneck, which was responsible for the loss of on-current. For the GOLDD structure, the interface state densities were lower than in the LDD structure, and a large imbalance in positive charges between the top and bottom interfaces meant that the largely uncompensated negative charges at the back interface dominated the device characteristics, giving negligible on-current loss [45]. As shown, the GOLDD architecture gives significantly better hot carrier stability than the simpler LDD region, but, as with the non-self-aligned architecture, it increases the parasitic capacitance between gate and drain, and the choice of LDD architecture will be dictated by the trade-off in speed and high voltage stability requirements. As discussed in > Sect. 3.1.2, a submicron GOLDD region would be needed for short channel devices, and this has been demonstrated with a conducting spacer technology [35]. P-channel TFTs, in common with p-channel MOSFETs, are far less susceptible to HCD [31, 38], and both MOSFET [47] and TFT [48] results have been successfully modeled with just electron injection. However, there does not seem to be a unified HCD model using the same injection conditions for both n- and p-channel TFTs.

4.3

Field-Enhanced Leakage Currents

High quality poly-Si TFTs, when biased in the off-state, show a leakage current characteristic, which is channel length independent [49], and which increases exponentially with drain bias and gate bias (see > Figs. 5 and > 8). The channel length independence indicates a current, which is limited by electron-hole pair generation processes at the drain, rather by resistive flow through the body of the device (which would scale inversely with channel length). However, normal thermal generation processes would produce currents, which have only a square root dependence on drain bias, and an even weaker dependence on gate bias. Hence, these anomalously large currents are described as field-enhanced currents, and > Fig. 14 schematically shows the range of electron emission processes, which can be expected in the space charge region of a reverse biased junction. The basic leakage current mechanism is controlled by

Conduction band edge

Thermal emission (field independent) Poole–Frenkel (field dependent) ET

Phonon assisted tunnelling (field dependent)

Tunnelling (temperature independent)

. Fig. 14 Schematic illustration of electron emission processes into the conduction band from a trap at depth ET

667

668

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

sequential electron and hole emission from deep traps near the center of the band-gap, and, for simplicity, > Fig. 14 shows only the electron emission processes to the conduction band. There will be equivalent hole emission processes to the valence band. At low fields and high temperatures, simple thermal emission (with no field dependence) will dominate, giving drain current activation energies of
EG/2. At high fields and low temperatures, tunneling will dominate giving near-zero activation energies. Between these two extremes, two further processes are shown, which are the field-induced lowering of the emission barrier for a Coulombic center (the Poole–Frenkel effect), and thermal emission to a virtual state followed by tunneling (phonon-assisted tunneling), both of which are field dependent. Experimental results at room temperature showing thermal activation energies decreasing from
EG/2 with increasing Vd [36] and Vg [50] rule out pure tunneling, and the best fits to the data have been with phonon-assisted tunneling [36, 49–52], although at the highest biases, band-to-band tunneling is also significant [51, 52]. The phonon-assisted tunneling can be represented as a field-dependent enhancement, en(F), of the low-field electron thermal emission rate, en0, by: en ðFÞ ¼ en0 gðFÞ The field enhancement factor, g(F), for a discrete Coulomb center at E T, is given by [53]: ( )(   ) Z ET DET q E 4E 1:5 ð2m qÞ0:5 DET 5=3  þ 1 dE exp gðFÞ ¼ exp kT DET kT kT 3 hF E where, DET is the emission barrier lowering due to the Poole–Frenkel effect, and this will be zero for a Dirac center. The simplified expression for a Dirac center can be approximated by [53]: ( !2 ) 2p0:5 q hF 1 q hF gðFÞ exp ðkT Þ1:5 2ð2m Þ0:5 3ðkT Þ3 2ð2m Þ0:5 The exponential term in the above expression is quadratic in F, and the depletion approximation relates the maximum field to the drain bias by Fmax / Vd0:5, so that this simple expression correctly predicts the observed exponential dependence of leakage current on drain bias. The field-free thermal emission rate, en0, is given by: en0 ¼ vsn NC exp 

ET kT

where sn is the electron capture cross section, v is the electron thermal velocity, and Nc is the effective density of conduction band states. Equivalent expressions for ep(F) and ep0 can also be formulated for the hole emission rates, and the leakage current density, J, is given by integration across the drain space charge region, of width d: Zd en ep J ¼ qNT dx en þ ep 0

Although the preceding expressions are for a discrete trap, and the poly-Si DOS is distributed across the band-gap, current generation processes via electron-hole pair emission

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

5.2.2

are localized on near-mid-gap centers in order to minimize the sum en +ep, and this process can be reasonably accurately represented by discrete near-mid-gap states. In numerical simulation of the leakage current process, the full trap distribution was used [51, 52], and the results remain consistent with the above qualitative considerations, although it has been shown that, at high fields, the most efficient generation centers can be up to 50 meVoff mid-gap [54] due to the Poole–Frenkel effect. An interesting aspect of those calculations was the observation that, for distributed traps, the current can be limited by emission from the Coulomb state of the center, located slightly deeper than EG/2, whereas, for a discrete trap at EG/2, the current would normally be limited by the slower emission from the Dirac state of the center. As with the hot carrier instability, the field-enhanced leakage currents are a direct consequence of the high drain fields in poly-Si TFTs, and field relief at the drain, using LDD or GOLDD regions, will also reduce these field-enhanced currents [38, 52]. Although the DOS values in high quality TFTs will not influence the drain field itself, the magnitude of the currents will scale with the trap density near mid-gap, and, hence, with the DOS values near mid-gap. Also, in common with the hot carrier instability, field-enhanced leakage currents are observed in MOSFETs [55], and are not specific to poly-Si TFTs.

4.4

Other Bias-Stress Instabilities

> Section 4.2 dealt with hot carrier induced instability, which is at a maximum when high drain bias stress is combined with a low gate bias setting approximately equal to the threshold voltage [37]. Other important bias-stress combinations are high gate bias without drain bias, and combined high gate and high drain bias. Gate bias stress alone can result in threshold voltage shifts, due to charged ion movement, in porous, poor quality gate oxides [56]. The absorption of water in these films has been identified by characteristic thermal desorption spectroscopy peaks at 100–200 C [57], and the presence of H+ and OH ions in the oxide has led to both negative and positive threshold voltage shifts after positive and negative bias-stressing, respectively [56]. However, current state-of-the-art gate oxide films, formed by PECVD deposition from either oxygen-diluted tetraethylorthosilicate (TEOS) [57], or helium-diluted silane and nitrous oxide [58, 59] have deliberately employed dilution conditions to reduce the oxide deposition rate, and to deposit dense, low porosity oxides, which are free from water-induced instabilities. The combination of high gate and high drain bias-stress results in a large current flow through the device, and this can lead to self-heating-induced instabilities [60–65], in which the modulus of the threshold voltage of both n-channel and p-channel TFTs increased with increasing bias stress. As might be expected, the self-heating effects, induced by different combinations of gate bias and drain bias, were best represented by the stress-power dissipation (=Id  Vd) within the device, and the bias-stress threshold voltage shifts could be uniquely correlated with this power dissipation [60]. The self-heating effects have been confirmed by both direct infrared detector measurements of the surface temperature of the device [60, 62], as well as by numerical simulations of Joule heating within the TFT [60, 61, 63, 65]. The measurements showed surface temperatures from
100 C up to
250 C, depending upon the specific device geometry and bias conditions [60, 62], and the magnitude of the bias-stress instability was correlated with the measured temperature of the device [60, 62]. The device geometry was identified as a key parameter in determining the internal temperature distribution. This was because the low thermal conductivity of the glass substrate (
1.3 W/m/K) made

669

670

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

it an inefficient route for the dissipation of heat from the TFT channel, with the device edges and its top surface being the other channels for heat dissipation. Hence, the size of the device, and particularly the width of the channel (for a given channel length), played an important role in determining its temperature, with wider devices displaying both higher temperatures and greater stress-induced threshold voltage shifts [60, 62, 65]. In agreement with the experimental measurements, the simulations also predicted channel temperatures of from
120 C to
300 C [60, 61, 63, 65], depending upon the biasing conditions and the device geometry. In addition, the simulations demonstrated that, even when the temperature rise was not sufficient to cause bias-stress instability, it was sufficient to induce a negative output conductance in high mobility TFTs, due to increased phonon scattering of the channel electrons [63]. The threshold voltage instability has been correlated with an increase in the trapping state density either near the middle of the poly-Si band-gap [60], or at the Si/SiO2 interface [65], and attributed to thermally induced Si-H bond breaking either in the poly-Si grain boundaries [60] or at the interface [65]. Other workers have suggested electron injection into the gate oxide [61, 62], in analogy with hot carrier defect generation [61]. However, some of the results have been obtained at drain bias values less than the saturation voltage [60], which would keep the drain field low, and militate against hot carrier injection in those cases. Whilst self-heating effects have been well established as a reliability issue in poly-Si TFTs on glass, they are an even bigger issue with TFTs on flexible, polymer substrates [64, 65], where the thermal conductivity is even lower at
0.2 W/m/K. Some common solutions have been suggested for both types of substrate, such as multi-fingered devices to avoid large W values, as well as overall device scaling to reduce both device area and drain biases, whilst retaining the required levels of channel current at lower power levels [64]. Alternatively, reduced self-heating effects have been demonstrated with thermally conducting flexible substrates, such as copper [61] or stainless steel [65] foils. The use of thin foils is discussed further in > Part. 5.6, which reviews the field of flexible substrate technologies for TFTs.

5

Summary

Poly-Si TFTs are now in mass production for small diagonal displays, where the cost advantages of circuit integration make them competitive with the dominant AMLCD technology based upon a-Si:H TFTs. The state-of-the-art technology uses an a-Si:H precursor film, which is converted into poly-Si by excimer laser crystallization, and current multi-shot commercial processing can deliver TFTs with an electron mobility up to
120 cm2/Vs. More innovative techniques, yielding large grain or single crystal areas, have demonstrated electron mobilities from 450 to 900 cm2/Vs. The commonest TFT architecture is a self-aligned, top-gated device (similar to SOI MOSFETs), which requires careful control of the ion doping and dopant activation process to minimize ion damage effects, particularly in n-channel TFTs. The electrical characteristics of poly-Si TFTs display a number of undesirable artifacts, such as low output impedance, drain bias-stress instability, and field-enhanced leakage currents. These are associated with a large electrostatic field at the drain junction, and are a fundamental aspect of the device architecture (rather than poor material quality), and require a modification of the architecture to incorporate a lightly doped region adjacent to the drain

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

5.2.2

to reduce its field. Other instabilities, particularly those associated with self-heating, are a more fundamental aspect of a TFT technology on a thermally insulating substrate like glass. Future work on poly-Si TFTs will include high speed, short channel devices for advanced system-on-glass applications, and improved uniformity for AMOLED applications.

References 1. Nishibe T, Nakamura T (2004) Realization of newconcept ‘‘input display’’ by p-Si SOG technology. Proceedings of AMLCD’04, Tokyo, pp 85–88 2. Mitani M, Endo T, Taniguchi Y, Katou T, Shimoto S, Ohno T, Tsuboi S, Okada T, Azuma K, Kawachi G, Matsumura M (2008) Ultrahigh performance polycrystalline silicon thin-film transistors on excimerlaser-processed pseudo-single-crystal films. Jpn J Appl Phys 47(12):8707–8713 3. Brotherton SD (1995) Polycrystalline silicon thin film transistors. Semicond Sci Technol 10:721–728 4. Paetzl R, Brune J, Herbst L, Simon F, Turk BA (2009) Advanced laser crystallisation for activematrix display manufacturing. Proceedings of the 5th International TFT Conference, ITC’09, France, 10.2 5. de Unamuno S, Fogarassy E (1989) A thermal description of the melting of c- and a-silicon under pulsed excimer lasers. Appl Surf Sci 36(1–4):1–11 6. Im JS, Kim HJ, Thompson MO (1993) Phase transformation mechanisms in excimer laser crystallisation of amorphous silicon films. Appl Phys Lett 63(14):1969–1971 7. Im JS, Kim HJ (1994) On the super lateral growth phenomenon observed in excimer laser-induced crystallisation of thin Si films. Appl Phys Lett 64(17):2303–2305 8. Im JS, Crowder MA, Sposili RS, Leonard JP, Kim HJ, Yoon JH, Gupta VV, Jin Song H, Cho HS (1998) Controlled super-lateral growth of Si films for microstructural manipulation and optimisation. Phys Stat Sol A 166(2):603–617 9. Kim HJ, Im JS (1994) Multiple pulse irradiations in excimer laser-induced crystallisation of amorphous Si films. Mat Res Soc Symp Proc 321:665–670 10. Brotherton SD, McCulloch DJ, Gowers JP, Trainor M, Edwards MJ, Ayres JR (1997) Film thickness effects in laser crystallised poly-Si. TFTs. Proceedings of the AMLCD’96, Tokyo, pp 21–24 11. Brotherton SD, McCulloch DJ, Gowers JP, Ayres JR, Trainor M (1997) Influence of melt depth in laser crystallised poly-Si thin film transistors. J Appl Phys 82(8):4086–4094 12. Brotherton SD, McCulloch DJ, Gowers JP, French ID, Gale I (2000) Issues in laser crystallisation of

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

poly-Si. Proceedings of the IDMC’2000, Seoul, pp 65–69 Brotherton SD, McCulloch DJ, Gowers JP (2004) Influence of excimer laser beam shape on poly-Si crystallisation. Jpn J Appl Phys 43(8): 5114–5121 Kim HD, Jeong JK, Chung H-J, Mo Y-G (2008) Technological challenges for large-size AMOLED display. SID’08 Digest, pp 291–294 Sposili RS, Im JS (1996) Sequential lateral solidification of thin silicon films on SiO2. Appl Phys Lett 69(19):2864–2866 Voutsas AT (2003) Assessment of the performance of laser-based lateral-crystallisation technology via analysis and modelling of polysilicon thin film transistor technology. IEEE Trans Ed-50(6):1494–1500 Park J-Y, Park H-H, Lee K-Y, Chung H-K (2004) Design of sequential lateral solidification crystallisation method for low temperature poly-Si thin film transistors. Jpn J Appl Phys 43(4a): 1280–1286 Oh C-H, Matsumura M (1998) Preparation of location-controlled crystal-silicon islands by means of excimer-laser annealing. Proceedings of the AMLCD’98, Tokyo, pp 13–16 Van der Wilt PC, van Dijk BD, Bertens GJ, Ishihara R, Beenakker CIM (2001) Formation of locationcontrolled crystalline islands using substrateembedded seeds in excimer-laser crystallization of silicon films. Appl Phys Lett 79(12):1819–1821 Rana V, Ishihara R, Hiroshima Y, Abe D, Inoue S, Shimoda T, Metselaar W, Beenakker K (2005) Dependence of single-crystalline Si TFT characteristics on the channel position inside a locationcontrolled grain. IEEE Trans ED-52(12):2622–2628 Brotherton SD, Crowder MA, Limanov AB, Turk B, Im JS (2001) Characterisation of poly-Si TFTs in directionally crystallized SLS Si. Proceedings of the Asia Display/IDW’01, pp 387–390 Crowder MA, Limanov AB, Im JS (2000) Sub-grain boundary spacing in directionally crystallized Si films obtained via sequential lateral solidification. Proc Mat Res Soc Symp 621E:Q9.6.1–Q9.6.7 Ryssel H, Ruge I (1986) Ion implantation, chapters 2 and 9. Wiley, New York

671

672

5.2.2

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

24. Brotherton SD, Ayres JR, Fisher CA, Glaister C, Gowers JP, McCulloch DJ, Trainor MJ (1998) The technology and application of laser crystallised polySi TFTs. Electrochemical Soc Proc 98–22:25–42 25. Peng D-Z, Chang T-C, Zan H-W, Huang T-Y, Chang C-Y, Liu P-T (2002) Reliability of laser-activated low-temperature polycrystalline silicon thin-film transistors. Appl Phys Lett 80(25):4780–4782 26. Park K-C, Nam W-J, Kang S-H, Han M-K (2004) Incomplete laser annealing of ion doping damage at source/drain junctions of poly-Si thinfilm transistors. Electrochem Solid-State Lett 7(6): G116–G118 27. Brotherton SD, Lee S-G, Glasse C, Ayres JR, Glaister C (2002) Short channel poly-Si TFTs. Proceedings of the IDW’02, Hiroshima, pp 283–286 28. Valletta A, Mariucci L, Fortunato G, Brotherton SD (2003) Surface scattering effects in polycrystalline silicon thin film transistors. Appl Phys Lett 82(18):3119–3121 29. Nam W-J, Park K-C, Jung S-H, Park S-J, Han M-K (2003) Observation and annealing of incomplete recrystallized junction defects due to the excimer laser beam diffraction at the gate edge in poly-Si TFT. Mat Res Soc Symp Proc 762:A17.8 30. Brotherton SD, McCulloch DJ, Gowers JP, Ayres JR, Fisher CA, Rohlfing RW (2000) Excimer laser crystallisation of poly-Si TFTs for AMLCDs. Mat Res Soc Symp Proc 621:Q7.1.1–Q7.1.12 31. Young ND, Gill A, Edwards MJ (1992) Hot carrier degradation in low temperature processed polycrystalline silicon thin film transistors. Semicond Sci Technol 7(9):1183–1188 32. Young ND (1996) The formation and annealing of hot-carrier-induced degradation in poly-Si TFTs, MOSFETs, and SOI devices, and similarities to statecreation in a-Si:H. IEEE Trans ED-43(3):450–456 33. Rohlfing FW, Ayres JR, Brotherton SD, Fisher CA, McCulloch DJ (2000) Fabrication and characterisation of poly-Si TFTs with self-aligned lightly-doped drain. Proceedings of the 20th SID-IDRC, Florida, pp 119–122 34. Yoshinouchi A, Morita T, Itoh M, Yoneda H, Yamane Y, Yamamoto Y, Tsucimoto S, Funada F, Awane K (1996) Process technologies for monolithic low-temperature poly-Si TFT-LCDs. Proceedings of the SID EuroDisplay’96, Birmingham, UK, pp 29–32 35. Glasse C, Brotherton SD, French ID, Green PW, Rowe C (2003) Short channel TFTs made with sidewall spacer technology. Proceedings of the AMLCD’03, pp 317–320 36. Brotherton SD, Ayres JR, Trainor MJ (1996) Control and analysis of leakage currents in poly-Si TFTs. J Appl Phys 79(2):895–904

37. Ayres JR, Brotherton SD (1996) Influence of drain field on poly-Si TFT behaviour. Proceedings of the SID EuroDisplay’96, Birmingham, UK, pp 33–36 38. Ayres JR, Brotherton SD, McCulloch DJ, Trainor M (1998) Analysis of drain field and hot carrier stability of poly-Si TFTs. Jpn J Appl Phys 37(4a):1801–1808 39. Valdinoci M, Colalongo L, Baccarani G, Fortunato G, Pecora A, Policicchio I (1997) Floating body effects in polysilicon thin-film transistors. IEEE Trans ED-44(12):2234–2241 40. Valletta A, Gaucci P, Mariucci L, Fortunato G, Brotherton SD (2004) Kink effect in short channel polycrystalline silicon thin film transistors. Appl Phys Lett 85(15):3113–3115 41. Mariucci L, Fortunato G, Carluccio R, Pecora A, Giovannini S, Massussi F, Colalongo L, Valdinoci M (1998) Determination of hot-carrier induced interface state density in polycrystalline silicon thin-film transistors. J Appl Phys 84(4):2341–2348 42. Hofmann KR, Werner C, Weber W, Dorda G (1985) Hot-electron and hole-emission effects in short nchannel MOSFETs. IEEE Trans ED-32(3):691–699 43. Young ND (1996) Formation and annealing of hotcarrier-induced degradation in poly-Si TFTs, MOSFETs and SOI devices, and similarities to statecreation in a-Si:H. IEEE Trans ED-43(3):450–456 44. Brown TM, Migliorato P (2000) Determination of the concentration of hot-carrier-induced bulk defects in laser-recrystallized polysilicon thin film transistors. Appl Phys Lett 76(8):1024–1026 45. Valletta A, Mariucci L, Fortunato G (2006) Hotcarrier-induced degradation of LDD polysilicon TFTs. IEEE Trans ED-53(1):43–50 46. Tam S, Ko P, Hu C (1984) Lucky-electron model of channel hot-electron injection in MOSFETs. Trans ED-31(9):1116–1125 47. Schwerin A, Hansch W, Weber W (1987) The relationship between oxide charge and device degradation: a comparative study of n- and p-channel MOSFETs. Trans IEEE ED-34(12):2493–2500 48. Gaucci P, Mariucci L, Valletta A, Cuscuna M, Maiolo L, Pecora A, Fortunato G, Templier F (2007) Hot carrier effects in p-channel polysilicon TFTs fabricated on flexible substrates. Proceedings of the 3rd International TFT Conference, ITC’07, Rome, pp 180–183 49. Ayres JR, Brotherton SD, Young ND (1992) Low temperature poly-Si for liquid crystal display addressing. Optoelectronics Devices Technol 7(2):301–320 50. Migliorato P, Reita C, Tallarida G, Quinn M, Fortunato G (1995) Anomalous off-current mechanisms in n-channel poly-si thin film transistors. Solid State Electron 38(12):2075–2079

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs) 51. Colalongo L, Valdinoci M, Baccarani G, Migliorato P, Tallarida G, Reita C (1997) Numerical analysis of poly-TFTs under off conditions. Solid State Electron 41(4):627–633 52. Bonfiglietti A, Cuscuna M, Valletta A, Mariucci L, Pecora A, Fortunato G, Brotherton SD, Ayres JR (2003) Analysis of electrical characteristics of GOLDD poly-Si TFTs with different LDD doping concentration. Trans IEEE ED-50(2):2425–2433 53. Vincent G, Chantre A, Bois D (1979) Electric field effect on the thermal emission of traps in semiconductor junctions. J Appl Phys 50(8):5484–5487 54. Lui OKB, Migliorato P (1997) A new generationrecombination model for device simulation including the poole-frenkel effect and phonon-assisted tunneling. Solid State Electron 41(4):575–583 55. Nathan V, Das NC (1993) Gate-induced drain leakage current in MOS devices. Trans IEEE ED40(10):1888–1890 56. Young ND, Gill A (1992) Water-related instability in TFTs formed using deposited gate oxides. Semicond Sci Technol 7:1103–1108 57. Hirashita N, Tokitoh S, Uchida H (1993) Thermal desorption and infrared studies of plama-enhanced chemical vapor deposited SiO films with tetraethylorthosilicate. Jpn J Appl Phys 32(4):1787–1793 58. Batey J, Tierney E (1986) Low-temperature deposition of high-quality silicon dioxide by plasmaenhanced chemical vapor deposition. J Appl Phys 60(9):3136–3145

5.2.2

59. Young ND, McCulloch DJ, Bunn RM, French ID, Gale IG (1998) Low temperature poly-Si on glass and polymer substrates. Proceedings of the Asia Display’98, Seoul, pp 83–93 60. Inoue S, Ohshima H, Shimoda T (2002) Analysis of degradation phenomenon caused by self-heating in low-temperature-processed polycrystalline silicon thin film transistors. Jpn J Appl Phys 41(11a): 6313–6319 61. Takechi K, Nakata M, Kanoh H, Otsuki S, Kaneko S (2006) Dependence of self-heating effects on operation conditions and device structures for polycrystalline silicon TFTs. IEEE Trans ED-53(2):251–257 62. Uraoka Y, Kitajima K, Yano H, Hatayama T, Fuyuki T, Hashimoto S, Morita Y (2004) Degradation of low temperature poly-Si TFTs by Joule heating. Proceedings of the AMLCD’04, pp 337–340 63. Valletta A, Moroni A, Mariucci L, Bonfiglietti A, Fortunato G (2006) Self-heating effects in polycrystalline silicon TFTs. Appl Phys Lett 89:093509-1– 093509-3 64. Miyasaka M, Hara H, Karaki N, Inoue S, Kawai H, Nebashi S (2008) Technical obstacles to thin film transistor circuits on plastic. Jpn J Appl Phys 47(6):4430–4435 65. Fortunato G, Cuscuna M, Gaucci P, Maiolo L, Mariucci L, Pecora A, Valletta A, Templier F (2009) Self-heating effects in p-channel polysilicon TFTs fabricated on different substrates. J Korean Phys Soc 54(1):455–462

Further Reading Kagan CR, Andry P (2003) Thin film transistors. CRC Press, New York

Kuo Y (2004) Thin film transistors: materials and processes – vol 2: polycrystalline silicon thin film transistors. Kluwer, Norwell

673

Part 5.3

Emerging TFT Technologies

5.3.1 Organic TFTs: VacuumDeposited Small-Molecule Semiconductors Hagen Klauk 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 2 Organic Crystal Structure, Thin-Film Morphology, and Carrier Transport . . . . . . . . 678 3 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681 4 Carrier Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 5 Device Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 6 Static and Dynamic Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.3.1, # Springer-Verlag Berlin Heidelberg 2012

678

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

Abstract: Thin-film transistors (TFTs) based on conjugated organic semiconductors can usually be manufactured at temperatures below about 100
C and thus on flexible polymeric substrates. Organic TFTs are therefore a potential alternative to hydrogenated amorphous silicon (a-Si:H), polysilicon, and metal oxide TFTs for flexible active-matrix displays. This chapter provides a brief overview of some important aspects of organic TFTs based on vacuumdeposited small-molecule semiconductors, such as pentacene. List of Abbreviations: CuPc, Copper Phthalocyanine; DNTT, Dinaphtho-[2,3-b:20 ,30 -f] thieno[3,2-b]thiophene; HOMO, Highest Occupied Molecular Orbital; LUMO, Lowest Unoccupied Molecular Orbital; MTR, Multiple Trapping and Release; NTCDI, Naphthalene Tetracarboxylic Diimide; PEN, Polyethylene Naphthalate; PTCDI, Perylene Tetracarboxylic Diimide; TFT, Thin-Film Transistor

1

Introduction

Organic TFTs are metal-insulator-semiconductor field-effect transistors in which the semiconductor is a thin layer of conjugated organic molecules. Conjugation refers to the presence of alternating single and double bonds between covalently bound carbon atoms that leads to the delocalization of one of the four valence electrons of each carbon atom and thus allows efficient charge transport within each conjugated molecule. The arrangement of the organic molecules within the semiconductor layer is governed by relatively weak van der Waals interactions. As a result, the electronic wave functions usually do not extend over the entire volume of the organic solid, but are localized to a finite number of molecules. The mobility of electrons traveling through the organic semiconductor is therefore determined by the ease with which electrons are transported from one molecule to the next under the influence of the applied electric field. In other words, charge transport through the organic semiconductor is limited by trapping in localized states. Depending on the choice of the organic semiconductor, its chemical purity, and the microstructure of the organic film, the carrier mobilities in organic semiconductor films are usually in the range of 0.01 to slightly above 1 cm2/Vs. Organic semiconductors can be categorized as follows: 1. Conjugated small molecules that are insoluble in common organic solvents and are thus deposited by thermal sublimation in vacuum or by organic vapor-phase deposition. 2. Soluble conjugated small molecules that can be deposited by spin-coating, dip-coating, or printing. 3. Conjugated polymers. This chapter focuses on TFTs based on vacuum-deposited small-molecule organic semiconductors.

2

Organic Crystal Structure, Thin-Film Morphology, and Carrier Transport

One of the most popular small-molecule organic semiconductors for organic TFTs is pentacene (see > Fig. 1a). Like many small-molecule semiconductors, pentacene has a strong tendency to form crystalline domains. > Figure 1b shows the crystal structure of the thin-film polymorph

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

5.3.1

b c a

a

a

b

q

b

. Fig. 1 (a) Molecular structure of pentacene. (b) Crystal structure of the thin-film polymorph of pentacene, as determined by Schiefer and coworkers using x-ray diffraction [1] (a = 0.596 nm, b = 0.760 nm, c = 1.56 nm, u  55
). The (001) plane is oriented parallel to the substrate surface

of pentacene, as determined by Schiefer and coworkers using grazing-incidence x-ray diffraction on vacuum-deposited polycrystalline pentacene films [1]. When pentacene is deposited onto a dielectric surface, the molecules assume an approximately upright orientation on the surface, so that the (001) lattice plane is oriented parallel to the substrate surface. > Figure 2 shows the shape of the highest occupied molecular orbitals (HOMO) of the pentacene molecules within the (001) lattice plane, as determined by Troisi and Orlandi using quantum-mechanical calculations [2]. As a result of the regular molecular arrangement and the relatively small intermolecular distances within the (001) plane, the delocalized frontier orbitals of neighboring molecules partially overlap, thereby facilitating efficient intermolecular charge-carrier transfer within the (001) plane. Because the (001) lattice plane is oriented parallel to the substrate surface (i.e., in the plane in which the electron current flows in field-effect transistors), the carrier field-effect mobility in pentacene TFTs benefits greatly from the small intermolecular distances in this lattice plane. The pentacene crystal structure is characterized by a pronounced anisotropy. In the direction perpendicular to the (001) lattice plane (i.e., perpendicular to the dielectric substrate surface), orbital overlap and carrier transport efficiency are significantly smaller than within the (001) plane. Quantum-mechanical calculations of the charge-carrier dynamics in defect-free pentacene crystals suggest that carriers propagating through the molecular lattice under the influence of an external electric field are delocalized over several molecules and that the carrier drift

679

680

5.3.1 Face-to-face

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

Edge-to-face

. Fig. 2 Shape of the highest occupied molecular orbitals (HOMO) of neighboring molecules within the (001) plane of the pentacene crystal, as determined by Troisi and Orlandi using quantummechanical calculations [2] (Reprinted with permission from [2]. Copyright 2005 by the American Chemical Society)

velocity is within a factor of two of the saturation velocity in single-crystalline silicon [3]. Temperature-dependent time-of-flight mobility measurements on highly purified naphthalene and pentacene crystals have shown evidence of charge transport in extended states with a charge-carrier mobility that is limited by phonon scattering, rather than thermal activation [4, 5]. Carrier mobilities in such crystals can exceed 30 cm2/Vs at room temperature [4] and 100 cm2/Vs at cryogenic temperatures [5]. For TFT fabrication, pentacene is usually deposited by thermal sublimation in vacuum [6] or by organic vapor-phase deposition [7]. The microstructure of the resulting films depends to some extent on the surface energy of the substrate, the surface roughness of the substrate, the substrate temperature during the deposition, the deposition rate, and the film thickness [8, 9], but in general, thin pentacene films deposited in vacuum or from the vapor phase are distinctly polycrystalline, as shown in > Fig. 3 [1]. Field-effect mobilities in 30 nm thick polycrystalline pentacene films deposited onto smooth dielectrics with low surface energy are typically between 1 and 3 cm2/Vs [10, 11], although mobilities as large as 6 cm2/Vs have also been reported [12–14]. The fact that the field-effect mobility in polycrystalline pentacene films can be within a factor of 5 of the room-temperature time-of-flight mobility in carefully prepared pentacene single crystals suggests that the influence of the grain boundaries on the charge transport through the polycrystalline films is somewhat smaller than might have been expected. Charge transport in polycrystalline organic semiconductor films is usually described by the multiple trapping and release (MTR) model. The MTR model was adapted for organic transistors by Horowitz and coworkers [15] and is based on the assumption that most of the charge carriers in the semiconductor channel are trapped in localized states and that carriers are temporarily promoted to an extended-state band in which charge transport occurs. The number of carriers available for transport then depends on the difference in energy between the trap level and the extended-state band, as well as on the temperature and on the gate voltage [15].

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

5.3.1

. Fig. 3 Atomic force microscopy (AFM) image and grazing-incidence x-ray diffraction (GIXD) pattern of a thin pentacene film deposited by vacuum sublimation (Reprinted with permission from [1]. Copyright 2007 by the American Chemical Society)

3

Stability

When a polycrystalline pentacene film is exposed to organic solvents or excessive heat, parts of the film will relax from the thin-film polymorph (with a d(00l) spacing of 15.6 A˚, as shown in ˚ ) [16, 17]. Since the > Fig. 1) to the more stable bulk polymorph (with a d(00l) spacing of 14.4 A bulk polymorph is characterized by different lattice parameters [2], the transition leads to mechanical stress in the film that produces microscopic (and even macroscopic) structural defects [16, 17]. As a result, the relaxation leads to a significant drop in the charge-carrier mobility [16, 17]. If the strength of the interaction between the pentacene molecules and the underlying surface is small (as is the case when pentacene is deposited onto a low-energy surface, such as a polymer or an alkylsilane self-assembled monolayer), the transition from the thin-film polymorph to the bulk polymorph can occur at temperatures as low as about 80
C [17]. This implies that pentacene TFTs may not have sufficient thermal stability for typical flat-panel display applications. However, by encapsulating the pentacene layer with an organic passivation film, the transition temperature can be increased to about 160
C (see > Fig. 4) [17–19], which should be sufficient for most flat-panel display applications. Like amorphous-silicon TFTs, organic TFTs are affected by bias stress-induced thresholdvoltage shifts (see > Fig. 5). Although the bias-stress effect in organic TFTs has been investigated for many years, a complete picture of the underlying mechanisms is still emerging [20]. A number of studies have shown that the bias stress-induced threshold voltage shift in organic TFTs is affected by a wide range of parameters, including the process temperature [21], the density of grain boundaries [22], the choice of the gate dielectric [23, 24], and the presence of oxidizing or polar molecules (oxygen, ozone, water) in the TFT structure or in the environment [24]. Although a substantial number of small-molecule semiconductors have been synthesized and investigated [25, 26], pentacene consistently provides the largest carrier mobilities, due to its favorable crystal structure. However, pentacene is easily oxidized when exposed to oxygen, water, ozone, and other air-borne species. When pentacene oxidizes, the hydrogen atoms at the 6 and 13 positions (i.e., at the central benzene ring of the molecule) are replaced with oxygen.

681

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

10−5

30°C, 80°C, 100°C, 160°C

-IDS (A)

10−7 210°C 190°C

10−9 10−11

220°C

10−13 20

10

0

−10 −20 VGS (V)

−30

−40

. Fig. 4 Transfer characteristics of a pentacene TFT on a flexible substrate with a polyimide gate dielectric and a parylene encapsulation layer. The transfer characteristics were measured after the substrate had been heated to temperatures between 80
C and 220
C in a nitrogen environment. All measurements were performed at a temperature of 30
C (Reprinted with permission from [18]. Copyright 2006 by the American Institute of Physics)

1.2

1.0 A

PS

1 0.9 0.8

B

IDS (t)/IDS0

I/If=0

682

0.6 C

0.4

OTS 0.8 BCB 0.7

VDS = VGS = −10 V

0.2

W/L = 1,000/100 µm 0

a

10−2

0.6 100 102 Time (s)

104

0

b

1,000

2,000

3,000

4,000

Stress time (s)

. Fig. 5 Bias stress-induced threshold voltage shift (manifesting itself as a time-dependent reduction in drain current during bias stress) in pentacene TFTs. (a) The bias stress-induced threshold voltage shift is affected by the process temperature (A: TFT annealed at 140
C; B, C: TFTs not annealed; bias-stress conditions: VGS = VDS = 40 V). (Reprinted with permission from [21]. Copyright 2005 by the American Institute of Physics.) (b) The bias stress-induced threshold voltage shift is affected by the choice of the gate dielectric. (Reprinted with permission from [23]. Copyright 2009 by Elsevier)

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

10

S S

Hole mobility (cm2/Vs)

Stored and tested in air

DNTT

1

0.1 Pentacene

0

a

b

30

60 90 120 150 Exposure to air (days)

180

. Fig. 6 Small-molecule organic semiconductors with improved air stability. (a) Dinaphtho-[2,3-b:20 ,30 -f] thieno[3,2-b]thiophene (DNTT) [30]. (b) Comparison of the air stability of pentacene and DNTT

Unlike the hydrogen that is replaced, oxygen forms double bonds with the carbon atoms, and this destroys the conjugation of the central benzene ring and substantially reduces the extent of the conjugated p-system of the molecule. The result of the oxidation is therefore a molecule with different orbital energies that no longer participates in the charge carrier transport of the transistor. As more and more pentacene molecules are oxidized while the transistor is exposed to air, the carrier mobility therefore decreases monotonically and irreversibly. The rate at which the mobility degrades depends on the gate dielectric, the pentacene film thickness, and other factors, but it can be as high as one order of magnitude within a few weeks. Small-molecule semiconductors with a larger ionization potential and thus better oxidation resistance than pentacene have been proposed [27–29], but usually at the expense of a less favorable crystal structure, so that the initial mobilities of these materials are inferior to that of pentacene. An example of a conjugated molecule that has a larger ionization potential and thus better air stability compared with pentacene, but adopts a crystal structure with excellent orbital overlap similar to pentacene, is dinaphtho-[2,3-b:20 ,30 -f]thieno[3,2-b]thiophene (DNTT; see > Fig. 6a) [30, 31]. DNTT provides initial mobilities that are very similar to pentacene, but shows much better air stability than pentacene (see > Fig. 6b).

4

Carrier Type

Most organic semiconductors are hydrocarbons with an ionization potential (HOMO energy) between 4.5 and 5.5 eV and an electron affinity (LUMO energy) between 2.5 and 3.5 eV. The source and drain contacts of organic TFTs are usually realized by contacting the intrinsic (i.e., not intentionally doped) organic semiconductor directly with an inorganic metal or with a conducting polymer. In the case of metal contacts, the use of noble metals, such as gold, is

683

684

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

preferred in order to avoid the formation of an insulating metal oxide layer at the metal/ semiconductor interface. Noble metals and conducting polymers have a work function in the range of 4.5–5.5 eV. Therefore, the energy barrier between the Fermi level of the source/drain contacts and the HOMO of the organic semiconductor is usually much smaller than the energy barrier between the Fermi level of the contacts and the LUMO of the semiconductor. As a result, most organic TFTs operate efficiently only as p-channel transistors, not as n-channel transistors. The fabrication of organic n-channel transistors requires that the energy barrier between the contacts and the LUMO of the semiconductor is as small as possible (and much larger than the barrier to the HOMO, to avoid injection of the wrong carrier type from the drain which would result in large leakage currents). For semiconductors with a small electron affinity, such as pentacene, this can, in principle, be achieved by employing contacts based on a low-work function metal. Indeed, pentacene n-channel TFTs with electron mobilities as large as 0.2 cm2/Vs have been realized by utilizing calcium for the source and drain contacts [32]. However, these transistors can be operated only in vacuum or in an inert ambient; due to the rapid oxidation of calcium they immediately and irreversibly degrade when exposed to air. A more promising approach to air-stable organic n-channel TFTs is the synthesis of organic semiconductors with larger electron affinity and larger ionization potential. This can be achieved by starting with copper phthalocyanine (CuPc), naphthalene tetracarboxylic diimide (NTCDI), or perylene tetracarboxylic diimide (PTCDI) as the conjugated core (which usually have electron affinities and ionization potentials that are slightly larger than those of pentacene) and employing electronegative substituents, such as cyano, fluorine or chlorine groups, to further increase the electron affinity and the ionization potential. The molecules shown in > Fig. 7 have electron affinities (LUMO energy) greater than about 3.8 eV, and so all of them can be used in combination with noble-metal source/drain contacts to realize n-channel TFTs with good air stability. The field-effect mobility in these n-channel TFTs depends critically on the morphology of the organic semiconductor layer and can be as small as 0.02 cm2/Vs in the case of F16CuPc [33] and greater than 1 cm2/Vs in the case of PTCDI-CH2C3F7 [34].

5

Device Architecture

Depending on the sequence in which the materials that make up the TFT (gate electrode, gate dielectric, semiconductor, source/drain contacts) are deposited onto the substrate, four different TFT architectures can be distinguished, as shown in > Fig. 8. Each of the four TFT architectures shown in > Fig. 8 has certain advantages and disadvantages. For example, the presence of an energy barrier at the interfaces between the organic semiconductor and the source and drain contacts is expected to impede the exchange of charge carriers between the contacts and the semiconductor. Experiments and simulations have shown that for the same energy barrier height, TFTs with a staggered structure (A, C) have the advantage of being less affected by this energy barrier than TFTs with a coplanar structure (B, D) [37–41]. This may be related to differences in the area available for charge exchange between the contacts and the carrier channel, or to differences in the morphology of the organic semiconductor in the vicinity of the contacts [37–41]. However, in case of the bottomgate coplanar structure (B), the effect of the energy barrier on the carrier exchange efficiency can be substantially reduced by modifying the surface of the source and drain contacts with

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

F

F

F F

F

F

F

F

F N F

N

N Cu

N N

F

O

N

O

O

N

O

F N

N

F

N F

F F

F

F

F F

F

a

b

F3C

F

F3C CF2

F2C O

CF2

F2C O

N

O

N

O

O

N

O

CN NC

O

N

O

CF2

CF2

c

F2C

CF3

F2C

d

CF3

. Fig. 7 Conjugated small-molecule semiconductors for air-stable organic n-channel TFTs. (a) Hexadecafluorocopperphthalocyanine (F16CuPc) [33]. (b) Bis(trifluoromethylbenzyl)naphthalene tetracarboxylic diimide (NTCDI-CH2C6H4CF3) [35]. (c) Bis(2,2,3,3,4,4,4heptafluorobutyl)-dicyano-perylene tetracarboxylic diimide (PTCDI-(CN2)-CH2C3F7) [36]. (d) Bis(2,2,3,3,4,4,4-heptafluorobutyl)-perylene tetracarboxylic diimide (PTCDI-CH2C3F7) [34]

a thin organic monolayer carrying an appropriate dipole moment [42, 43] or with a thin conducting metal oxide [44–47]. Also, in case of the staggered structures (A, C), the TFT performance is closely related to the thickness of the semiconductor layer; if the semiconductor layer is thicker than about 50 nm, the series resistance associated with the vertical current path from the contact/semiconductor interface to the carrier channel can become quite large [48, 49], due to the fact that carrier transport through the polycrystalline organic semiconductor film is usually quite inefficient in the direction perpendicular to the (001) lattice plane.

685

686

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

Source

Drain

Semiconductor Source

++++++++++++++++ Gate dielectric Gate electrode

Substrate

b

Gate electrode

Gate electrode

Gate dielectric ++++++++++++++++ Source

c

Gate dielectric Source

Drain ++++++

Drain Substrate

Drain

Gate electrode

Substrate

a

++++++ Gate dielectric

d

Substrate

. Fig. 8 Schematic crosssections of the four principle TFT architectures. The carrier channel is schematically shown in red. (a) Bottom-gate (inverted) staggered TFT. (b) Bottom-gate (inverted) coplanar TFT. (c) Top-gate staggered TFT. (d) Top-gate coplanar TFT

An important advantage of the bottom-gate coplanar structure (B) is that the gate dielectric layer and the source and drain contacts are prepared before the organic semiconductor is deposited. The reason why this is important is that many high-mobility organic semiconductors, especially vacuum-deposited small-molecule materials, adopt a thin-film microstructure that is very sensitive to external perturbations. For example, vacuum-deposited pentacene films undergo an irreversible phase transition, associated with a substantial drop in carrier mobility, when exposed to organic solvents, such as those employed for the solutionbased deposition of polymer gate dielectrics and in the photolithographic patterning of the source and drain contacts [16]. With the bottom-gate coplanar structure (B), methods involving solvents and/or thermal treatments can be safely employed to prepare the gate dielectric and the source and drain contacts without harming the semiconductor layer. TFTs based on vacuum-deposited small-molecule semiconductors are typically fabricated using one of the bottom-gate architectures (A or B), while TFTs based on solution-deposited semiconductors sometimes employ the top-gate staggered architecture (C). Regardless of the device architecture, it is usually necessary to pattern the semiconductor layer in order to eliminate leakage currents and cross talk between devices. Since vacuum-deposited smallmolecule organic semiconductors (such as pentacene) can usually not be patterned by photolithography, they are often patterned by deposition through an aperture mask. Depending on the required resolution, this can be a conformable photoresist mask fabricated directly on the surface of the substrate [50], a laser-patterned metal or polyimide shadow mask [51], or a highresolution Si or SiN membrane fabricated by optical or electron-beam lithography [52]. The gate dielectric of organic TFTs can be made using a variety of materials and material combinations, including metal oxides, polymers, polymer/nanoparticle composites, organic self-assembled monolayers or multilayers, polymer electrolytes, ion gels, organic/inorganic bilayer dielectrics, and many more. Depending on the materials choice, the dielectric can be prepared by physical vapor deposition, chemical vapor deposition, atomic layer deposition, anodization, plasma oxidation, spin-coating, self-assembly, inkjet printing, gravure printing, etc. A systematic comparison of more than 40 different gate dielectrics that have been

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

5.3.1

. Table 1 Materials and preparation methods for the various components of organic TFTs Material Gate electrodes, Metals (Al, Cr, Ti, etc.), source, and conducting oxides (ITO) drain contacts

Deposition

Patterning

Physical vapor deposition (PVD)

Photolithography + etching or lift-off [54] Digital lithography + etching [55] shadow mask [51]

Metal nanoparticle inks [56] Semiconductor

Gate dielectric

Conducting polymers [57] n-channel: p-channel: Pentacene [6], PTCDIs [34], DNTT [30], etc. NTCDIs [35], etc. Insulating metal oxides

Inkjet printing [58], flexographic printing [59], offset printing [59], gravure printing [60], etc. Vacuum sublimation [6], organic vapor-phase deposition [7]

Photoresist mask [50], shadow mask [51]

Organic vapor-jet printing [61] PVD [54], CVD [62], ALD [63], sol-gel [14]

Photolithography + etching or lift-off

Anodization [64], plasma oxidation [65] Polymers [66], siloxanes [13], Spin-coating Photolithography + etching [69] silsesquioxanes [67], polymer/nanoparticle Photopatterning [70] composites [68] Inkjet printing, gravure printing [59, 60, 71], etc. Self-assembled monolayers [72], self-assembled multilayers [73]

Self-assembly from solution [72], Self-assembly from the vapor phase [74]

Polymer electrolytes, ion gels Inkjet printing [75]

successfully employed for high-mobility organic TFTs is provided in a recent review by Ortiz, Facchetti, and Marks [53]. > Table 1 provides a brief summary of materials that are frequently employed in organic TFTs.

6 > Figure

Static and Dynamic Performance

9a shows a schematic cross section and a photograph of an organic p-channel TFT fabricated on a 125 mm thick flexible polyethylene naphthalate (PEN) substrate using the bottom-gate (inverted) staggered device structure. The gate electrode is a 30 nm thick layer of aluminum deposited directly onto the PEN surface by thermal evaporation in vacuum. The gate dielectric consists of a 3.6 nm thick AlOx layer (obtained by oxidizing the surface of the Al gate electrodes in an oxygen plasma) and a 1.7 nm thick self-assembled monolayer of tetradecylphosphonic acid (obtained by submersing the substrate in a 2-propanol solution and baking at 70
C; this is the highest temperature during the TFT fabrication process [72]). The gate dielectric has a capacitance per unit area (Cdiel) of 0.8 mF/cm2. The semiconductor is a 25 nm thick polycrystalline film of dinaphtho-[2,3-b:20 ,30 -f]thieno[3,2-b]thiophene (DNTT) deposited by thermal sublimation in vacuum [30]. The source and drain contacts are 30 nm

687

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

Organic semiconductor

Source (Au)

Drain (Au)

100 µm Gate (Al)

SAM

AlOx

−1.8 V

10−5 10−6

−2.1 V

10−7

10−5 Drain-source −6 voltage = −1.5 V 10 10−7

10−8

10−8

10−9

10−9

10−10

10−10

10−11

10−11

10−12

10−12

10−13

10−13

−4 −6 −8 −10 −12 −14 −3

b

14

c

12

−2.4 V −2.7 V Gate-source voltage = −3.0 V −2 −1 Drain-source voltage (V)

−14

10 0

10 8 6 4 2 0 −3

−2 −1 Gate-source voltage (V)

−3

1.5

Drain-source voltage = −1.5 V

Hole mobility (cm2/Vs)

Drain current (µA)

−2

Gate current (A)

0

0

1.2

−2 −1 Gate-source voltage (V)

Drain current (A)

Substrate

a

Transconductance (µS)

688

10−14 0

Drain-source voltage = −1.5 V

0.9 0.6 0.3 0.0

−2 −1 Gate-source voltage (V)

0

. Fig. 9 Electrical characteristics of a p-channel TFT on a flexible polyethylene naphthalate (PEN) substrate using dinaphtho-[2,3-b:20 ,30 -f]thieno[3,2-b]thiophene (DNTT; see > Fig. 6a) as the semiconductor. The TFT has a channel length of 10 mm and a channel width of 100 mm. (a) Schematic cross section and photograph. (b) Output and transfer characteristics. (c) Transconductance and hole mobility in the saturation regime as a function of gate-source voltage

thick gold, also deposited by thermal evaporation in vacuum. The Al gate electrodes, the organic semiconductor layer, and the Au source and drain contact are all patterned using polyimide shadow masks. The channel width (W) is 100 mm and the channel length (L) is 10 mm. The electrical characteristics of this TFT are shown in > Fig. 9b and c. The threshold

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

Gate current (A)

10−7 10−8 50 TFTs L = 30 µm 10−9 W = 100 µm

10−8 10−9

10−10

10−10

10−11

10−11

10−12

10−12

10−13 −3

−2 −1 Gate-source voltage (V)

10−13 0

Counts

10−6 Drain-source voltage = −1.5 V 10−7 Drain current (A)

10−6

5.3.1

24 22 Mean = 1.5 µA 20 s = 0.16 µA 18 16 14 12 10 8 6 4 2 0 0.5 0.8 1.1 1.4 1.7 2.0 2.3 Maximum drain current (µA)

. Fig. 10 Uniformity of the electrical characteristics of 50 TFTs on a glass substrate

voltage is 1.5 V, the subthreshold swing is 100 mV/decade, the on/off ratio is about 108, the transconductance reaches 12 mS (0.12 S/m when normalized to the channel width), and the maximum field-effect mobility is 1.2 cm2/Vs. Based on the transconductance (12 mS) and the gate capacitance (25 pF, ignoring the contribution from the Miller capacitance), the cutoff frequency of the TFT in > Fig. 9 is estimated to be about 70 kHz (fT  gm/2pCG). In terms of field-effect mobility and on/off ratio, the initial performance of TFTs based on the semiconductor DNTT is similar to that of TFTs based on pentacene [31]. A significant advantage of DNTT in comparison to pentacene is the better air stability (see > Fig. 6). > Figure 10 illustrates the uniformity of the electrical characteristics of 50 TFTs fabricated on a glass substrate [72]. The standard deviation of the drain current measured at VGS = 3 V and VDS = 1.5 V is about 10%, and the threshold voltage varies by no more than 30 mV. > Figure 11 shows the current-voltage characteristics of an organic n-channel TFT fabricated on a 125 mm thick flexible PEN substrate using vacuum-evaporated hexadecafluorocopperphthalocyanine (F16CuPc) as the semiconductor. The electron affinity of F16CuPc is sufficiently large (4.5 eV) that gold source and drain contacts provide acceptable performance. At the same time, the HOMO-LUMO gap is sufficiently large, so that the undesirable injection of holes from the drain contact at negative gate-source voltages is suppressed. The TFT in > Fig. 11 has a channel width (W) of 1,000 mm and a channel length (L) of 20 mm. The electron mobility is 0.02 cm2/Vs, the on/off ratio is 106, the subthreshold swing is 170 mV/decade, and the cutoff frequency calculated using Equation (20) is 400 Hz. Aside from the semiconductor, the functional materials, the device structure, and the manufacturing process are identical to those employed for the DNTT p-channel TFT in > Fig. 9.

7

Summary

Organic TFTs can usually be fabricated at temperatures below about 100
C and thus on polymer foils and even on paper [76]. This opens the possibility of manufacturing activematrix flat-panel displays on flexible, bendable, foldable, rollable, or stretchable substrates for a variety of emerging display applications [77–88].

689

10−6

Gate-source voltage = 3.0 V 2.7 V

1.5

Drain current (A)

Drain current (µA)

2.0

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

2.4 V 1.0

2.1 V 1.8 V

0.5

0.0

1.5 V

0

a

1 2 Drain-source voltage (V)

3

b

10−7

10−8

10−9

10−9

10−10

10−10

10−11

10−11

10−12 −1

c

10−12 0 1 Gate-source voltage (V)

2

0.03 Electron mobility (cm2/Vs)

Drain-source voltage = 1.5 V

0.9 0.6 0.3 0.0 −1

10−7

10−8

1.5 1.2

10−6

Drain-source voltage = 1.5 V

Gate current (A)

5.3.1

Transconductance (µS)

690

0 1 Gate-source voltage (V)

Drain-source voltage = 1.5 V 0.02

0.01

0.00 −1

2

d

0 1 Gate-source voltage (V)

. Fig. 11 Electrical characteristics of an organic n-channel TFT on a flexible PEN substrate using hexadecafluorocopperphthalocyanine (F16CuPc; see > Fig. 7a) as the semiconductor. The TFT has a channel length of 20 mm and a channel width of 1,000 mm. The current-voltage curves were recorded in ambient air. (a) Output and transfer characteristics. (c) Transconductance and electron mobility in the saturation regime as a function of gate-source voltage

The static and dynamic performance of state-of-the-art organic p-channel TFTs is already sufficient for small or medium-size displays in which the TFTs operate with critical frequencies in the range of a few kilohertz or a few tens of kilohertz (see > Fig. 12). Strategies for increasing the performance of organic TFTs include further improvements in the carrier mobility in the organic semiconductor (either through the synthesis of new materials, through improved purification, or by enhancing the molecular order in the semiconductor layer) and more aggressive scaling of the lateral transistor dimensions (channel length and contact overlap). Further improvements are also required in the operational and environmental stability of organic TFTs [20, 30]. For many years, the performance of organic n-channel TFTs lagged significantly behind that of organic p-channel TFTs, but thanks to the discovery that certain fluoroalkyl-substituted naphthalene and perylene tetracarboxylic diimide (NTCDI, PTCDI) derivatives form wellordered films with large electron mobilities in ambient-air conditions, the gap between n-channel

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

5.3.1

. Fig. 12 Flexible active-matrix organic light-emitting diode (AMOLED) display with pentacene TFTs developed by Sony (Reprinted with permission [86]. Copyright 2008 by the Society for Information Display)

and p-channel TFT performance has been closing [34–36]. This raises legitimate hope that at least some of the peripheral circuitry of flexible active-matrix displays can eventually be included on the display backplane in the form of low-power organic complementary circuits [89].

References 1. Schiefer S, Huth M, Dobrinevski A, Nickel B (2007) Determination of the crystal structure of substrateinduced pentacene polymorphs in fiber structured thin films. J Am Chem Soc 129:10316 2. Troisi A, Orlandi G (2005) Band structure of the four pentacene polymorphs and effect on the hole mobility at low temperature. J Phys Chem B 109:1849 3. Hultell M, Stafstro¨m S (2006) Polaron dynamics in highly ordered molecular crystals. Chem Phys Lett 428:446 4. Jurchescu OD, Baas J, Palstra TTM (2004) Effect of impurities on the mobility of single crystal pentacene. Appl Phys Lett 84:3061 5. Warta W, Karl N (1985) Hot holes in naphthalene: high, electric-field dependent mobilities. Phys Rev B 32:1172 6. Gundlach DJ, Lin YY, Jackson TN, Nelson SF, Schlom DG (1997) Pentacene organic thin film

7.

8.

9.

10.

transistors – molecular ordering and mobility. IEEE Electron Dev Lett 18:87 Lunt RR, Lassiter BE, Benziger JB, Forrest SR (2009) Organic vapor phase deposition for the growth of large area organic electronic devices. Appl Phys Lett 95:233305 Meyer zu Heringdorf FJ, Reuter MC, Tromp RM (2001) Growth dynamics of pentacene thin films. Nature 412:517 Kim C, Facchetti A, Marks TJ (2009) Probing the surface glass transition temperature of polymer films via organic semiconductor growth mode, microstructure, and thin-film transistor response. J Am Chem Soc 131:9122 Lin YY, Gundlach DJ, Nelson SF, Jackson TN (1997) Stacked pentacene layer organic thin film transistors with improved characteristics. IEEE Electron Dev Lett 18:606

691

692

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

11. Kelley TW, Boardman LD, Dunbar TD, Muyres DV, Pellerite MJ, Smith TP (2003) High-performance OTFTs using surface-modified alumina dielectrics. J Phys Chem B 107:5877 12. Lee S, Koo B, Shin J, Lee E, Park H, Kim H (2006) Effects of hydroxyl groups in polymeric dielectrics on organic transistor performance. Appl Phys Lett 88:162109 13. Choi CG, Bae BS (2007) Effects of hydroxyl groups in gate dielectrics on the hysteresis of organic thin film transistors. Electrochem Solid-State Lett 10:H347 14. Tan HS, Mathews N, Cahyadi T, Zhu FR, Mhaisalkar SG (2009) The effect of dielectric constant on device mobilities of high-performance, flexible organic field effect transistors. Appl Phys Lett 94:263303 15. Horowitz G, Hajlaoui R, Delannoy P (1995) Temperature dependence of the field-effect mobility of sexithiophene. Determination of the density of traps. J Phys III France 5:355 16. Gundlach DJ, Jackson TN, Schlom DG, Nelson SF (1999) Solvent-induced phase transition in thermally evaporated pentacene films. Appl Phys Lett 74:3302 17. Ji T, Jung S, Varadan VK (2008) On the correlation of postannealing induced phase transition in pentacene with carrier transport. Org Electron 9:895 18. Sekitani T, Someya T, Sakurai T (2006) Effects of annealing on pentacene field-effect transistors using polyimide gate dielectric layers. J Appl Phys 100:024513 19. Fukuda K, Sekitani T, Someya T (2009) Effects of annealing on electronic and structural characteristics of pentacene thin-film transistors on polyimide gate dielectrics. Appl Phys Lett 95:023302 20. Sirringhaus H (2009) Reliability of organic fieldeffect transistors. Adv Mater 21:3859 21. Sekitani T, Iba S, Kato Y, Noguchi Y, Someya T, Sakurai T (2005) Suppression of DC bias stressinduced degradation of organic field-effect transistors using postannealing effects. Appl Phys Lett 87:073505 22. Kalb WL, Mathis T, Haas S, Stassen AF, Batlogg B (2007) Organic small molecule field-effect transistors with CytopTM gate dielectric: eliminating gate bias stress effects. Appl Phys Lett 90:092104 23. Zhang XH, Tiwari SP, Kippelen B (2009) Pentacene organic field-effect transistors with polymeric dielectric interfaces: performance and stability. Org Electron 10:1133 24. Ryu KK, Nausieda I, He DD, Akinwande AI, Bulovic V, Sodini CG (2010) Bias-stress effect in pentacene organic thin-film transistors. IEEE Trans Electron Dev 57:1003 25. Facchetti A (2007) Semiconductors of organic transistors. Mater Today 10:28

26. Murphy AR, Frechet JMJ (2007) Organic semiconducting oligomers for use in thin film transistors. Chem Rev 107:1066 27. Meng H, Bao Z, Lovinger AJ, Wang BC, Mujsce AM (2001) High field effect mobility oligofluorene derivatives with high environmental stability. J Am Chem Soc 123:9214 28. Merlo JA, Newman CR, Gerlach CP, Kelley TW, Muyres DV, Fritz SE, Toney MF, Frisbie CD (2005) p-channel organic semiconductors based on hybrid acene-thiophene molecules for thin-film transistor applications. J Am Chem Soc 127:3997 29. Locklin J, Ling MM, Sung A, Roberts ME, Bao Z (2006) High-performance organic semiconductors based on fluorine-phenylene oligomers with high ionization potentials. Adv Mater 18:2989 30. Yamamoto T, Takimiya K (2007) Facile synthesis of highly p-extended heteroarenes, dinaphtho[2, 3-b:2’, 3’-f]chalcogenopheno[3, 2-b]chalcogenophenes, and their application to field-effect transistors. J Am Chem Soc 129:2224 31. Zschieschang U, Yamamoto T, Takimiya K, Kuwabara H, Ikeda M, Sekitani T, Someya T, Klauk H (2010) Flexible low voltage organic transistors and circuits based on a high mobility organic semiconductor with good air stability. Adv Mater 22:982 32. Ahles M, Schmechel R, von Seggern H (2004) n-type organic field-effect transistor based on interfacedoped pentacene. Appl Phys Lett 85:4499 33. Bao Z, Lovinger A, Brown J (1998) New air-stable nchannel organic thin film transistors. J Am Chem Soc 120:207 34. Schmidt R, Oh JH, Sun YS, Deppisch M, Krause AM, Radacki K, Braunschweig H, Ko¨nemann M, Erk P, Bao Z, Wu¨rthner F (2009) High-performance air stable n-channel organic thin film transistors based on halogenated perylene bisimide semiconductors. J Am Chem Soc 131:6215 35. Katz HE, Johnson J, Lovinger AJ, Li W (2000) Naphthalenetetracarboxylic diimide-based nchannel transistor semiconductors: structural variation and thiol-enhanced gold contacts. J Am Chem Soc 122:7787 36. Jones BA, Ahrens MJ, Yoon MH, Facchetti A, Marks TJ, Wasielewski MR (2004) High-mobility air-stable n-type semiconductors with processing versatility: Dicyanoperylene-3, 4:9, 10-bis(dicarboximides). Angew Chem Int Ed 43:6363 37. Necliudov PV, Shur MS, Gundlach DJ, Jackson TN (2000) Modeling of organic thin film transistors with different designs. J Appl Phys 88:6594 38. Hill IG (2005) Numerical simulations of contact resistance in organic thin-film transistors. Appl Phys Lett 87:163505 39. Gundlach DJ, Zhou L, Nichols JA, Jackson TN, Necliudov PV, Shur MS (2006) An experimental

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

40.

41.

42.

43.

44.

45.

46.

47.

48.

49.

50.

51.

52.

study of contact effects in organic thin film transistors. J Appl Phys 100:024509 Gupta D, Katiyar M, Gupta D (2009) An analysis of the difference in behavior of top and bottom contact organic thin film transistors using device simulation. Org Electron 10:775 Shim CH, Maruoka F, Hattori R (2010) Structural analysis on organic thin-film transistor with device simulation. IEEE Trans Electron Dev 57:195 Gundlach DJ, Jia L, Jackson TN (2001) Pentacene TFT with improved linear region characteristics using chemically modified source and drain electrodes. IEEE Electron Dev Lett 22:571 Cai QJ, Chan-Park MB, Zhou Q, Lu ZS, Li CM, Ong BS (2008) Self-assembled monolayers mediated charge injection for high performance bottomcontact poly(3, 3’’’-didodecylquaterthiophene) thin-film transistors. Org Electron 9:936 Chen FC, Lin YS, Chen TH, Kung LJ (2007) Efficient hole injection in highly transparent organic thin-film transistors. Electrochem Solid-State Lett 10:H186 Stadlober B, Haas U, Gold H, Haase A, Jakopic G, Leising G, Koch N, Rentenberger S, Zojer E (2007) Orders-of-magnitude reduction of the contact resistance in short-channel hot embossed organic thin film transistors by oxidative treatment of Au-electrodes. Adv Funct Mater 17:2678 Kumaki D, Umeda T, Tokito S (2008) Reducing the contact resistance of bottom-contact pentacene thin-film transistors by employing a MoOx carrier injection layer. Appl Phys Lett 92:013301 Kano M, Minari T, Tsukagoshi K (2009) Improvement of subthreshold current transport by contact interface modification in p-type organic field-effect transistors. Appl Phys Lett 94:143304 Pesavento PV, Puntambekar KP, Frisbie CD, McKeen JC, Ruden PP (2006) Film and contact resistance in pentacene thin-film transistors: dependence on film thickness, electrode geometry, and correlation with hole mobility. J Appl Phys 99:094504 Sohn CW, Rim TU, Choi GB, Jeong YH (2010) Analysis of contact effects in inverted-staggered organic thin-film transistors based on anisotropic conduction. IEEE Trans Electron Dev 57:986 Klauk H, Gundlach DJ, Nichols JA, Jackson TN (1999) Pentacene thin-film transistors for circuit and display applications. IEEE Trans Electron Dev 46:1258 Muyres DV, Baude PF, Theiss S, Haase M, Kelley TW, Fleming P (2004) Polymeric aperture masks for high performance organic integrated circuits. J Vac Sci Technol A 22:1892 Kim GM, van den Boogaart MAF, Brugger J (2003) Fabrication and application of a full wafer size micro/nanostencil for multiple length-scale surface patterning. Microelectron Eng 67–68:609

5.3.1

53. Ortiz RP, Facchetti A, Marks TJ (2010) High-k organic, inorganic, and hybrid dielectrics for low-voltage organic field-effect transistors. Chem Rev 110:205 54. Myny K, Steudel S, Smout S, Vicca P, Furthner F, van der Putten B, Tripathi AK, Gelinck GH, Genoe J, Dehaene W, Heremans P (2010) Organic RFID transponder chip with data rate compatible with electronic product coding. Org Electron 11:1176 55. Arias AC, Ready SE, Lujan R, Wong WS, Paul KE, Salleo A, Chabinyc ML, Apte R, Street RA, Wu Y, Liu P, Ong B (2004) All jet-printed polymer thin-film transistor active-matrix backplanes. Appl Phys Lett 85:3304 56. Li Y, Wu Y, Ong B (2005) Facile synthesis of silver nanoparticles useful for fabrication of highconductive elements for printed electronics. J Am Chem Soc 127:3266 57. Sirringhaus H, Kawase T, Friend RH, Shimoda T, Inbasekaran M, Wu W, Woo EP (2000) Highresolution inkjet printing of all-polymer transistor circuits. Science 290:2123 58. Sele CW, von Werne T, Friend RH, Sirringhaus H (2005) Lithography-free, self-aligned inkjet printing with sub-hundred-nanometer resolution. Adv Mater 17:997 59. Hu¨bler AC, Doetz F, Kempa H, Katz HE, Bartzsch M, Brandt N, Hennig I, Fuegmann U, Vaidyanathan S, Granstrom J, Liu S, Sydorenko A, Zillger T, Schmidt G, Preissler K, Reichmanis E, Eckerle P, Richter F, Fischer T, Hahn U (2007) Ring oscillator fabricated completely by means of mass-printing technologies. Org Electron 8:480 60. Voigt MM, Guite A, Chung DY, Khan RUA, Campbell AJ, Bradley DDC, Meng F, Steinke JHG, Tierney S, McCulloch I, Penxten H, Lutsen L, Douheret O, Manca J, Brokmann U, So¨nnichsen K, Hu¨lsenberg D, Bock W, Barron C, Blanckaert N, Springer S, Grupp J, Mosley A (2010) Polymer field-effect transistors fabricated by the sequential gravure printing of polythiophene, two insulator layers, and a metal ink gate. Adv Funct Mater 20:239 61. Shtein M, Peumans P, Benziger JB, Forrest SR (2004) Direct, mask- and solvent-free printing of molecular organic semiconductors. Adv Mater 16:1615 62. Knipp D, Street RA, Vo¨lkel A, Ho J (2003) Pentacene thin film transistors on inorganic dielectrics: morphology, structural properties, and electronic transport. J Appl Phys 93:347 63. Zhang XH, Tiwari SP, Kim SJ, Kippelen B (2009) Low-voltage pentacene organic field-effect transistors with high-k HfO2 gate dielectrics and high stability under bias stress. Appl Phys Lett 95:223302 64. Goettling S, Diehm B, Fruehauf N (2008) Activematrix OTFT display with anodized gate dielectric. J Display Technol 4:300

693

694

5.3.1

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

65. Kang H, Han KK, Park JE, Lee HH (2007) High mobility, low voltage polymer transistor. Org Electron 8:460 66. Roberts ME, Queralto N, Mannsfeld SCB, Reinecke BN, Knoll W, Bao Z (2009) Cross-linked polymer gate dielectric films for low-voltage organic transistors. Chem Mater 21:2292 67. Huang C, West JE, Katz HE (2007) Organic fieldeffect transistors and unipolar logic gates on charged electrets from spin-on organosilsesquioxane resisns. Adv Funct Mater 17:142 68. Schroeder R, Majewski LA, Grell M (2005) Highperformance organic transistors using solutionprocessed nanoparticle-filled high-k polymer gate insulators. Adv Mater 17:1535 69. Klauk H, Halik M, Zschieschang U, Eder F, Schmid G, Dehm C (2003) Pentacene organic transistors and ring oscillators on glass and on flexible polymeric substrates. Appl Phys Lett 82:4175 70. Jang J, Kim SH, Hwang J, Nam S, Yang C, Chung DS, Park CE (2009) Photopatternable ultrathin gate dielectrics for low-voltage-operating organic circuits. Appl Phys Lett 95:073302 71. Yan H, Chen Z, Zheng Y, Newman C, Quinn JR, Do¨tz F, Kastler M, Facchetti A (2009) A highmobility electron-transporting polymer for printed transistors. Nature 457:679 72. Klauk H, Zschieschang U, Pflaum J, Halik M (2007) Ultralow-power organic complementary circuits. Nature 445:745 73. Yoon MH, Facchetti A, Marks TJ (2005) s-p molecular dielectric multilayers for low-voltage organic thin-film transistors. Proc Natl Acad Sci 102:4678 74. DiBenedetto SA, Frattarelli D, Facchetti A, Ratner MA, Marks TJ (2009) Structure-performance correlations in vapor phase deposited self-assembled nanodielectrics for organic field-effect transistors. J Am Chem Soc 131:11080 75. Xia Y, Zhang W, Ha M, Cho JH, Renn MJ, Kim CH, Frisbie CD (2010) Printed sub-2 V gel-electrolytegated polymer transistors and circuits. Adv Funct Mater 20:587 76. Zschieschang U, Yamamoto T, Takimiya K, Kuwabara H, Ikeda M, Sekitani T, Someya T, Klauk H (2011) Adv Mater 23:654 77. Rogers JA, Bao Z, Baldwin K, Dodabalapur A, Crone B, Raju VR, Kuck V, Katz H, Amundson K, Ewing J, Drzaic P (2001) Paper-like electronic displays: largearea rubber-stamped plastic sheets of electronics and microencapsulated electrophoretic inks. Proc Natl Acad Sci 98:4835 78. Sheraw CD, Zhou L, Huang JR, Gundlach DJ, Jackson TN, Kane MG, Hill IG, Hammond MS, Campi J, Greening BK, Francl J, West J (2002) Organic thin-

79.

80.

81.

82.

83.

84.

85.

86.

87.

88.

89.

film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates. Appl Phys Lett 80:1088 Gelinck GH, Huitema HEA, van Veenendaal E, Cantatore E, Schrijnemakers L, van der Putten JBPH, Geuns TCT, Beenhakkers M, Giesbers JB, Huisman BH, Meijer EJ, Benito EM, Touwslager FJ, Marsman AW, van Rens BJE, de Leeuw DM (2004) Flexible active-matrix displays and shift registers based on solution-processed organic transistors. Nat Mater 3:106 Huitema HEA, Gelinck GH, van Lieshout PJG, van Veenendaal E, Touwslager FJ (2006) Flexible electronic-paper active-matrix displays. J Soc Inform Display 14:729 Mizukami M, Hirohata N, Iseki T, Ohtawara K, Tada T, Yagyu S, Abe T, Suzuki T, Fujisaki Y, Inoue Y, Tokito S, Kurita T (2006) Flexible AM OLED panel driven by bottom-contact OTFTs. IEEE Electron Dev Lett 27:249 Zhou L, Wanga A, Wu SC, Sun J, Park S, Jackson TN (2006) All-organic active matrix flexible display. Appl Phys Lett 88:083502 Kato Y, Sekitani T, Takamiya M, Doi M, Asaka K, Sakurai T, Someya T (2007) Sheet-type Braille displays by integrating organic field-effect transistor and polymeric actuators. IEEE Trans Electron Dev 54:202 Nomoto K, Yoneya N, Hirai N, Yagi I, Kawashima N, Noda M, Kasahara J (2007) Solution-processed organic TFT array for active-matrix LCDs on a plastic substrate. J Soc Inform Display 15:491 Fujisaki Y, Sato H, Takei T, Yamamoto T, Fujikake H, Tokito S, Kurita T (2008) A 5-in. flexible ferroelectric liquid-crystal display driven by organic thin-film transistors. J Soc Inform Display 16:1251 Yagi I, Hirai N, Miyamoto Y, Noda M, Imaoka A, Yoneya N, Nomoto K, Kasahara J, Yumoto A, Urabe T (2008) A flexible full-color AMOLED display driven by OTFTs. J Soc Inform Display 16:15 Sekitani T, Nakajima H, Maeda H, Fukushima T, Aida T, Hata K, Someya T (2009) Stretchable active-matrix organic light-emitting diode display using printable elastic conductors. Nat Mater 8:494 Suzuki M, Fukagawa H, Nakajima Y, Tsuzuki T, Takei T, Yamamoto T, Tokito S (2009) A 5.8-in. phosphorescent color AMOLED display fabricated by ink-jet printing on plastic substrate. J Soc Inform Display 17:1037 Xiong W, Guo Y, Zschieschang U, Klauk H, Murmann B (2010) A 3-V, 6-Bit C-2C digital-toanalog converter using complementary organic thin-film transistors on glass. IEEE J Solid-State Circ 45:1380

Organic TFTs: Vacuum-Deposited Small-Molecule Semiconductors

5.3.1

Further Reading Bao Z, Locklin J (eds) (2007) Organic field-effect transistors. CRC Press, New York

Klauk H (ed) (2006) Organic electronics. Materials, manufacturing and applications. Wiley-VCH, Weinheim

695

5.3.2 Organic TFTs: SolutionProcessable SmallMolecule Semiconductors David Redinger . Marcia Payne 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 698

2

P-Type Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 700

3

N-Type Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 701

4

Deposition Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 702

5

Device Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 703

6 6.1 6.2 6.3 6.4

Current State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704 Pentacene Precursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704 TES-ADT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705 DiF-TES-ADT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705 TIPS-Pentacene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705

7

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.3.2, # Springer-Verlag Berlin Heidelberg 2012

698

5.3.2

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

Abstract: Organic semiconductor (OSC) materials are seen as a potential replacement for amorphous silicon in future display applications. State-of-the-art small-molecule OSC materials, when incorporated into a transistor, obtain performance on par or superior to transistors using amorphous silicon. Higher performance coupled with the potential for lowcost fabrication make small-molecule OSC materials extremely promising and the focus of research organizations worldwide. This chapter provides an overview of p-type and n-type materials available, then addresses the impact of processing conditions on transistor performance, and finally concludes with an overview of state-of-the-art materials. List of Abbreviations: DFCO-4T, Phenacyl Quaterthiophene; DiF-TES-ADT, Bis(triethylsilylethynyldifluoro)anthradithiophene; DT-TTF, Dithiophene Tetrathiafulvalene; HMDS, Hexamethyldisilazane; P3HT, Poly(3-hexylthiophene); PCBM, Phenyl-C61-Butyric Acid Methyl Ester; PFBT, Pentafluorobenzenethiol; PVP, Poly(4-vinylphenol); SAM, Self-assembled Monolayer; TES-ADT, Bis(triethylsilylethynyl)anthradithiophene; TIPS Pentacene, Bis (triisopropylsilylethynyl)pentacene

1

Introduction

State-of-the-art soluble small-molecule organic semiconductors have recently demonstrated mobility which exceeds 1 cm2/Vs. Display backplane fabrication is a large cost-sensitive market well suited for organic TFTs since the performance requirements are within reach, only one type of semiconductor is needed, and the switching nature of backplanes (LCD, see > Chap. 7.4.1, or electrophoretic, see > Chap. 8.1.1) is less stressful than applications requiring DC bias. The soluble nature of these materials enables solution processing techniques such as ink-jet and gravure printing, spin coating, or slot-die coating to deposit the semiconductor. Solution processing has the potential to reduce fabrication costs by reducing material usage, enabling all-additive processing, increasing throughput by elimination of vacuum pump-down time, and is compatible with low-cost roll-to-roll fabrication techniques. Soluble semiconducting polymers have been studied extensively for use in field-effect transistors (See also > Chap. 5.3.3) because their solution rheology makes them particularly apt for large-scale printing with excellent device uniformity [1]. Many of the challenges associated with organic TFT fabrication are shared between polymeric and small-molecule semiconductors, and an overview of the field is available in literature reviews [2]. The most studied polymer in OFET applications, poly(3-hexylthiophene), exhibits hole mobility as high as 0.1 cm2/Vs [3], while mobility reported for films of alkyl-substituted oligothiophene (> Fig. 1, 1) are an order of magnitude higher [4]. In recent years, pentacene (> Fig. 1, 2) has emerged as the benchmark material for vapordeposited thin-film transistors (See also > Chap. 5.3.1) due to its commercial availability and hole mobility consistently reported over 1 cm2/Vs [5]. Polycrystalline thin films of rubrene (> Fig. 1, 3), a tetracene derivative, grown under highly controlled conditions to maximize crystalline domain size resulted in hole mobility as high as 4.6 cm2/Vs [6]. These smallmolecule reports employ vacuum sublimation, a process that allows precise control over layer thickness and ease of device construction through the sequential growth of layers, but at the same time is inefficient in terms of material usage. Vacuum evaporation does not fully exploit materials that tend toward organized self-assembly when allowed to adopt lowest

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

5.3.2

. Fig. 1 Structures of semiconductors (1–6, 9–13) and pentacene precursors (7, 8) discussed in this chapter

699

700

5.3.2

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

energy thermodynamic states. Even at slow evaporation rates, the growth of films is governed mainly by kinetic factors, resulting in smaller crystalline domains and higher defect density. Until recently, TFTs based on solution-processed organic semiconductors have been able to achieve a field-effect mobility of 1 cm2/Vs. The field has progressed rapidly over the last decade; the performance of soluble small-molecule semiconductors is now on par with vacuumsublimated pentacene results [7]. The following section contains an overview of the various classes of materials – both hole and electron carriers – that have been shown to exhibit notable mobility when deposited from solution in organic-based TFTs. A later section will examine in greater detail the most promising of these materials – those with performance characteristics most likely to lead to commercially viable device constructions.

2

P-Type Semiconductors

While pentacene has been deposited from solution at high temperature onto glass substrates [8], the process is incompatible with flexible plastic substrates. One potential solution to this problem was to develop a soluble precursor that could be converted back to pentacene through the application of heat or light energy. This approach has yielded pentacene transistors with mobility as high as 0.9 cm2/Vs and on/off current ratios in excess of 106 [9]. One of the drawbacks to a high-temperature conversion is the inability to use a range of lowcost flexible substrates (see > Part 5.6). However, despite the need for a thermal conversion step, pentacene precursors have been successfully used to fabricate circuits on flexible substrates using a bottom-gate architecture with photolithographically defined gold source and drain contacts, and PVP gate dielectric [10]. Integrated row drivers for electrophoretic displays have been achieved using a similar process [11]. A pentacene precursor was also used in the first demonstration of an all-ink-jet-printed TFT [12]. Alternatively, solubilizing groups can both increase solubility and enhance the intermolecular interactions of molecules, resulting in the formation of large crystalline domains where film deposition is governed by thermodynamic factors and molecules access lowest energy states. Anthony and coworkers have synthesized a number of pentacene and anthradithiophene molecules functionalized with trialkylsilylethynyl substituents along the backbone [13]. Small alterations in the size or shape of the alkyl appendages result in profound differences in the solid-state of the crystalline material, with ideally substituted materials showing significant pi-facial intermolecular interactions in two dimensions. Thin-film transistors using solution-processed TIPS-pentacene (> Fig. 1, 4) and TES-difluoroanthradithiophene (diF-TES-ADT, > Fig. 1, 5) have exhibited a hole mobility of 1.8 cm2/Vs [14] and 1.5 cm2/Vs [15], respectively, and on/off current ratios greater than 106. Yeates and coworkers have demonstrated a similar pentacene derivative with additional core substitution, showing solution-processed hole mobility as high as 2.5 cm2/Vs [16]. Improved film formation from solution reflects the ability to use substituents to influence the process of film formation, so as to form larger domains and fewer grain boundaries. Tetrathiafulvalene derivatives, with their excellent solubility and general synthetic ease, have found use in charge transfer salts as electron donors, but have only begun to be studied as thin-film transistor semiconductors. Dithiophene-tetrathiafulvalene (DT-TTF, > Fig. 1, 6) does not possess solubilizing groups to promote film order, but a carefully controlled zone

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

5.3.2

casting method resulted in well-oriented films with a hole mobility of 0.17 cm2/Vs [17]. Films of other tetrathiafulvalene derivatives prepared using a more scalable technique like spin casting exhibit mobility an order of magnitude lower and lower on/off current ratios [18].

3

N-Type Semiconductors

Compared to p-type small-molecule organic semiconductors, there are far fewer reported n-type materials. The performance of these n-type materials typically lags behind their p-type counterparts. Many organic materials can conduct both electrons and holes, but usually only hole conduction is observed under a standard semiconductor screening process with gold as injecting electrode [19]. While the work function of gold (5 eV) allows efficient hole injection, the injection barrier for electrons is too large for most organic semiconductors, whose LUMO levels lie at significantly higher energy (2–3 eV). The interface of the semiconductor with the dielectric, commonly silicon dioxide, is characterized by the presence of deep electron traps that further impede electron flow. Using a hydroxyl-free gate dielectric, such as a thermally cross-linked benzocyclobutene derivative, has been shown to limit trapping at the gate interface, enabling n-channel conduction in some polymeric semiconductors [20]. Electron conducting materials tend to be environmentally unstable, as the radical anions present in the channel (under bias) are excellent reductants for both oxygen and water. Increasing the electron affinity of a material by the attachment of electron-withdrawing groups is a common approach, with the dual purpose of reducing the electron injection barrier and improving environmental stability. Naphthalene- and perylenediimides are a robust and synthetically malleable class of materials that have attracted interest as n-type semiconductors. One of the earliest accounts was by Katz of a fluoroalkyl naphthalenediimide (> Fig. 1, 9) that could be processed in trifluorotoluene at 100 C [21]. Despite a large variation in film uniformity, several devices showed electron mobility >0.01 cm2/Vs and good environmental stability. Vacuum sublimed films were uniform and showed an order of magnitude higher mobility. Marks and coworkers have worked with cyanoperylene diimides, employing a variety of alkyl substituents to tailor the properties. The N-octyl derivative (> Fig. 1, 10) has been deposited using ink-jet printing into a bottom-contact transistor, with electron mobility of 0.02 cm2/Vs and on/off current ratio >104 when measured under inert conditions [22]. The material was also incorporated into a complementary ring oscillator with poly(3-hexylthiophene) (P3HT); however, the devices showed sensitivity to air. By employing a more precise drop-casting method to enhance self-organization of the material within films, devices showing electron mobility of 0.04 cm2/Vs and on/off current ratio >105 in addition to ambient stability for several months were demonstrated [23]. Oligothiophene derivatives transport electrons with moderate efficiency when functionalized with electron-withdrawing substituents. Phenacyl quaterthiophene (DFCO-4T, > Fig. 1, 11) can be drop-cast from xylene to form highly crystalline films. Incorporation into transistors resulted in electron mobility as high as 0.2 cm2/Vs when measured under inert atmosphere, but no air-stability was reported [24]. Terthiophene substituted with dicyanomethylene units (> Fig. 1, 12) is forced to adopt a quinoidal form that has greater oxidative stability. The measured electron mobility is the same as for the previous material, but the on/off current ratio is higher at 106, and the devices can be operated in ambient conditions [25]. Fullerene derivatives are some of the most heavily studied organic materials, known for their efficient electron conduction. For heterojunction photovoltaics, phenyl-C61-butyric acid

701

702

5.3.2

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

methyl ester (PCBM) (> Fig. 1, 13), a solubilized C60 fullerene derivative, has emerged as one of the most successful derivatives. Processing conditions appear to be crucial for this material, requiring precise interfacial control to minimize carrier traps that reduce both mobility and overall current. Nevertheless, transistor electron mobility reported for PCBM is competitive with other materials, at 0.2 cm2/Vs, and further optimization appears likely [26]. Chikamatsu and coworkers used long alkyl chains attached via an N-methylpyrrolidine to impose improved self-assembly in films of the material during a post spin-casting anneal and achieved similar electron mobility [27]. None of the C60-based derivatives possess air-stable electronic characteristics, and practical devices will require encapsulation.

4

Deposition Methods

With most small-molecule organic semiconductors the deposition method has a significant impact on the quality of the semiconducting layer. The highest mobility is generally achieved in systems showing the highest order and fewest defects. This can be difficult to achieve in a thin film, a familiar problem in the field of thin-film transistors. Unlike most inorganic systems, which use vacuum deposition techniques, soluble semiconductors can be deposited at atmospheric pressure, at room temperature, and are compatible with low-cost all-additive processes. Many deposition methods have been attempted, and there is a large overlap with techniques used for polymeric organic semiconductors (see also > Chap. 5.3.3). Deposition methods can generally be grouped into two categories – those that require post-deposition patterning, and all-additive processes which selectively deposit material only in the channel of the thin-film transistor. For cost reasons, all-additive printing techniques [28] may be preferred, but largearea techniques such as spray coating, spin coating, dip coating, and blade coating have all been attempted with varying degrees of success. A detailed discussion of various solution coating techniques can be found in the Further Reading section. The challenge is determining which deposition method will give the desired transistor performance while still meeting the goal of low-cost fabrication. Both the inherent properties of the semiconductor and the choice of deposition method will ultimately determine the performance of the system. The performance of the semiconductor will be affected by the degree of crystallinity in the film. A slow rate of crystallization is often used to improve film quality [29], which can be achieved by using high boiling point solvents that evaporate slowly such as anisole, trichlorobenzene, or n-butylbenzene. However, a slower drying process may lead to slower throughput and thus increased manufacturing cost. Nonetheless a high-quality film is critical to the performance of the resulting TFTs [30]. The problem is further compounded because some deposition methods require solvents with specific properties, such as ink-jet printing, which requires high-viscosity solvents. Thus finding a combination of semiconductor, deposition method, and solvent system that results in a reliable high-throughput process is a challenging problem in the field of organic electronics. Device isolation requires a patterned semiconductor layer which can be achieved either by an all-additive deposition method, or by performing lithography after semiconductor deposition. Traditional lithography is especially difficult with organic semiconductors because the thin films are easily disturbed by contact with solvent from either the photoresist deposition or a lift-off process [31]. While this is a concern even for vapor-deposited films of pentacene, it is an even larger concern for soluble semiconductors due to their solubility in a wide range of organic solvents. This has led to developments in photoresist materials that are compatible

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

5.3.2

with fluorinated solvent systems, which are generally non-solvents for hydrocarbon semiconductor molecules [32]. Similar challenges finding orthogonal solvent systems occur when depositing a dielectric material in top-gate architectures. Exposure to UV light can also damage the semiconductor layer, and thus most photolithography steps are done prior to semiconductor deposition. UV- or e-beam-induced damage has been exploited by several researchers to isolate TFTs by making non-channel material electrically insulating [33].

5

Device Architectures

Choice of semiconductor deposition method is only one critical aspect of transistor design. Transistor design and fabrication involves many deposition and patterning steps which can have an impact on the quality of the semiconducting film. The surface upon which the semiconductor is deposited can also impact the quality of the semiconductor film. Bottomgate top-contact devices are a common architecture used to evaluate semiconductor materials (> Fig. 2a). Since the dielectric is deposited prior to the semiconductor there is no chance of disturbing the semiconductor layer during deposition. A common test structure uses a heavily doped wafer as the gate, which is then thermally oxidized to grow a SiO2 dielectric layer. Source and drain contacts can be deposited before or after the semiconductor layer. Thermal evaporation is typically used for source and drain deposition to avoid damaging the semiconductor layer. While this is not a practical structure for creating circuits (due to the common gate), it is a convenient test method. Studies have shown that surface treatments such as SAMs can improve the measured mobility of transistors fabricated on the surface. In top-gate architectures, the semiconductor is typically deposited onto source and drain contacts which have already been patterned (> Fig. 2d). The presence of the contact material will create a discontinuity in the surface properties of the substrate and can affect how the semiconductor crystallizes. Contact treatments utilizing self-assembled monolayers have been used to improve not only contact resistance [34], but can act as nucleation sites for the semiconductor [35]. The semiconductor then grows laterally from both source and drain contacts into the transistor channel, forming a high-quality semiconductor layer. Contact treatments can affect the crystallinity of the semiconductor up to 10 mm from the edge of the contact. The benefits of contact treatments are dependent on the channel length of the transistor for several reasons and have been studied in great detail [36]. As with any transistor, contact resistance is in series with the channel resistance and therefore becomes more significant as the channel length is reduced. In systems where the contacts act as nucleation sites, shorter channel lengths (typically 20 mm or less) enable the regions of high crystallinity to contact without a region of less order in the center of the channel. Thus applications requiring high mobility will benefit from short channel lengths on the order of 5 mm. Short channel lengths can limit the choice of patterning and deposition techniques, since many printing methods cannot achieve the required resolution. This has led to the concept of a hybrid approach, where the critical lithography steps are done with traditional photolithography and the subsequent steps done with low-cost techniques such as ink-jet printing. There has been some debate as to whether bottom- or top-gate structures are a better choice for small-molecule organic systems. Bottom-gate bottom-contact structures (> Fig. 2b) are common with all ink-jet printed devices, since the semiconductor is printed last and therefore is not disturbed by subsequent processing steps. There is some evidence that topgate devices may have higher mobility than bottom-gate devices, for the same semiconductor

703

704

5.3.2

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

Source

Semiconductor

Drain Source

++++++++++++++++

Gate dielectric

Gate electrode

Gate electrode

Substrate

a

Gate electrode

Gate dielectric ++++++++++++++++

Gate dielectric Source

Drain ++++++

Drain Substrate

Drain

Substrate

b

Gate electrode

Source

c

++++++

Gate dielectric

d

Substrate

. Fig. 2 Schematic cross-sections of the four principle TFT architectures. The carrier channel is schematically shown in red (reprinted from > Chap. 5.3.1). (a) Bottom-gate top-contact (staggered) TFT. (b) Bottom-gate bottom-contact (coplanar) TFT. (c) Top-gate bottom-contact (staggered) TFT. (d) Top-gate top-contact (coplanar) TFT

film [37]. In this work, a dual-gate structure was used to compare mobility near the top surface of the semiconductor film with the bottom surface. Although this work was done with a polymeric semiconductor, most reports of high-mobility TFTs are based on small-molecule semiconductors and use a top-gate architecture.

6

Current State of the Art

6.1

Pentacene Precursor

Mu¨llen and coworkers synthesized a cyclohexadiene-bridged pentacene precursor (> Fig. 1, 7) that could be converted to pentacene by annealing at 180 C via the retro-Diels Alder reaction [38]. TFTs were prepared on a SiO2 substrate which had been treated with hexamethyldisilazane (HMDS). After converting the semiconductor film at 200 C for 5 s, field-effect mobility of 0.2 cm2/Vs was measured. Afzali and coworkers at IBM used a sulfinylacetamide-bridged precursor (> Fig. 1, 8) [39]. Films could be converted to pentacene by annealing at 130 C resulting in a measured mobility for TFTs of 0.13 cm2/Vs, but the highest mobility required annealing at 200 C. Researchers from IBM used a substitution to precursor 8 to generate films that could be photopatterned to 40 mm feature size, subsequent to solution deposition. Exposure to UV light and a mild acid allows the selective removal of the bridge, leaving unconverted precursor in the nonexposed areas to be removed using methanol. Devices constructed using this technique showed hole mobility as high as 0.25 cm2/Vs [40]. The pentacene precursor approach yielded some of the earliest noteworthy TFT results using solution processing of small molecules. The focus of current research has shifted predominately to the use of solubilized molecules that require no post-deposition conversion step, or to a solubilized small-molecule-polymerhybrid approach.

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

6.2

5.3.2

TES-ADT

Triethylsilylethynyl anthradithiophene (TES-ADT) has demonstrated mobility of approximately 0.2 cm2/Vs in a bottom-gate bottom-contact TFT device. When spin-cast, films are typically amorphous with very low mobility and must undergo a post-deposition crystallization step. The low melting point of TES-ADT (160 C) indicates a low reorganization energy, which has several implications. It has been shown that film order will increase at room temperature over several days, leading to a 100X improvement in measured mobility after 1 week [41]. Alternatively, film order can be increased using a solvent-vapor anneal, which is done by exposure to 1,2-dichloroethane vapor for 1–10 min [42]. Solvent annealing has been shown to improve mobility from 0.002 to 0.11 cm2/Vs. However, TES-ADT films can also be easily damaged by exposure to heat. Contrary to most other organic semiconductors, bottom-contact TES-ADT TFTs have shown higher performance than top-contact devices (in bottom-gate constructions). This effect has been attributed to thermal damage of the TESADT film during evaporation of the source and drain contacts [43]. The low melting point of TES-ADT may be a problem in display applications where temperatures above 150 C are often required.

6.3

DiF-TES-ADT

A significant improvement to TES-ADT was achieved by fluorine substitution on the anthradithiophene backbone, leading to increased thermal and photo stability [44]. DiF-TES-ADT (> Fig. 1, 5) will form high-quality films from spin casting and does not require post-deposition crystallization steps. Mobility was also significantly improved over TES-ADT. TFTs fabricated using short (10 mm) channel lengths and pentafluorothiophenol contact treatments have demonstrated mobility higher than 1.5 cm2/Vs, with typical results of 0.7 cm2/Vs. TFTs fabricated on flexible substrates have shown slightly lower performance in the range of 0.1–0.2 cm2/Vs, but this was sufficient to drive a seven-stage ring oscillator at 22 kHz using a 80 V supply voltage [45]. TFTs fabricated using single crystals of diF-TES-ADT have been demonstrated with mobility as high as 6 cm2/Vs [46].

6.4

TIPS-Pentacene

TIPS-pentacene (> Fig. 1, 4) was the first high-performance soluble small-molecule organic semiconductor, and with a field-effect mobility of approximately 2 cm2/Vs, it remains one of the most promising materials available. While simple spin coating will typically yield TFTs with mobility on the order of 10 2 cm2/Vs, a combination of surface treatments, contact treatments, and short channel lengths can result in TFTs with mobility greater than 1 cm2/Vs for both topand bottom-gate constructions. Results as high as 1.8 cm2/Vs were achieved by the Jackson group at Penn State University using a simple thermally oxidized Si wafer bottom-gate construction [47]. Gold source and drain contacts were patterned using photolithography and then treated with a solution of pentafluorobenzenethiol (PFBT) to form a monolayer on the gold contacts. Next, the substrate was exposed to hexamethyldisilazane (HMDS) to form a self-assembled monolayer (SAM) in the channel region. The TIPS-pentacene was drop-cast from toluene to complete the device. In addition to high mobility, these devices exhibited a high on/off ratio of 108 and a subthreshold slope of 300 mV/dec. This process was also used

705

706

5.3.2

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

to create functioning seven-stage ring oscillators using a patterned Ni gate electrode and sputtered SiO2 gate dielectric, although the mobility was slightly lower and subthreshold slope higher [48]. Mobility of 1.8 cm2/Vs has also been achieved in a top-gate construction using a 10 mm channel length [49]. Gold source and drain electrodes are patterned on a glass substrate using photolithography. After a UV ozone treatment, contacts are treated with a PFBT SAM. The substrate is then coated with a phenyl-terminated organosilane SAM, which bonds to any exposed OH groups on the surface and acts to suppress off-state leakage. A fluoropolymer deposited from an orthogonal solvent is used as the gate dielectric, followed by deposition of the Cr/Al gate layer. Top-gate dielectrics can act to encapsulate and isolate the semiconductor layer, which can be a significant advantage when integrating the TFT into a display.

7

Conclusion

Recent progress in soluble small-molecule organic semiconductors has resulted in multiple materials with mobility greater than that of amorphous silicon. Therefore these materials are strong candidates to replace amorphous silicon in low-cost, low-temperature, and flexible displays. Small-molecule OSCs have been utilized in several electrophoretic and OLED display prototypes, as well as integrated row drivers. Cost savings will occur with a system-wide approach to fabrication, which includes not only the semiconductor, but also the dielectric, TFT design, lithography techniques, and display integration and assembly. While these materials are compatible with low-cost roll-to-roll fabrication, several challenges remain to largescale production. High switching speeds require high-resolution lithography which has been difficult to achieve roll-to-roll and is an active area of research. Further advances in processing will enable new applications outside of displays, such as sensors, digital logic (utilizing both n-type and p-type semiconductors), RFID, and analog circuits.

References 1. Dimitrakopoulos CD, Malenfant PRL (2002) Organic thin-film transistors for large-area electronics. Adv Mater 14(2):99–117 2. (a) Katz HE (2004) Recent advances in semiconductor performance and printing processes for organic transistor-based electronics. Chem Mater 16(23):4748–4756. (b) Sirringhaus H (2009) Materials and applications for solution-processed organic field-effect transistors. Proc IEEE 97(9):1570–1579. (c) Anthony JE (2006) Functionalized acenes and heteroacenes for organic electronics. Chem Rev 106(12):5028–5048 3. McCulloch I, Heeney M, Bailey C, Genevicius K, MacDonald I, Shkunov M, Sparrowe D, Tierney S, Wagner R, Zhang W, Chabinyc ML, Kline J, McGehee MD, Toney MF (2006) Liquid-crystalline semiconducting polymers with high charge-carrier mobility. Nat Mater 5(4):328–333 4. Halik M, Klauk H, Zschieschang U, Schmid G, Ponomarenko S, Kirchmeyer S, Weber W (2003)

5.

6.

7.

8.

Relationship between molecular structure and electrical performance of oligothiophene organic thinfilm transistors. Adv Mater 15(11):917–922 Kelley TW, Baude PF, Gerlach C, Ender DE, Muyres D, Haase MA, Vogel DE, Theiss SD (2004) Recent progress in organic electronics: materials, devices, and processes. Chem Mater 16(23):4413–4422 Di C-a, Yu G, Liu Y, Guo Y, Sun X, Zheng J, Wen Y, Wu W, Zhu D (2009) Selective crystallization of organic semiconductors for high performance organic field-effect transistors. Chem Mater 21(20):4873–4879 Baude PF, Ender DA, Haase MA, Kelley TW, Muyres DV, Theiss SD (2003) Pentacene-based radiofrequency identification circuitry. Appl Phys Lett 82(22):3964–3966 Natsume Y, Minakata T, Aoyagi T (2009) Pentacene thin film transistors fabricated by solution process with directional crystal growth. Org Electron 10(1):107–114

Organic TFTs: Solution-Processable Small-Molecule Semiconductors 9. See Refs. 38–40 (Herwig et al., Afzali et al., Weidkamp et al.) 10. Cantatore E, Geuns TCT, Gelinck GH, van Veenendall E, Gruijthuijsen AFA, Schrijnemakers L, Drews S, de Leeuw DM (2007) A 13.56-MHz RFID system based on organic transponders. IEEE J SolidState Circuits 42(1):84–92 11. Van Lieshout, P, van Veenendall, E, Schrijnemakers, L, Gelinck, G, Touwslager, F, Huitema, F (2005) A flexible 240320-pixel display with integrated row drivers manufactured in organic electronics. ISSCC Digest Technical Papers 578 12. Molesa SM, de la Fuenta Vornbrock A, Chang PC, Subramanian V (2005) Low-voltage inkjetted organic transistors for printed RFID and display applications. IEDM Technical Digest 5.4.1–5.4.4 13. (a) Anthony JE, Brooks JS, Eaton DL, Parkin SR (2001) Functionalized pentacene: improved electronic properties from control of solid-state order. J Am Chem Soc 123(38):9482–9483. (b) Payne MM, Odom SA, Parkin SR, Anthony JE (2004) Stable, crystalline acenedithiophenes with up to seven linearly fused rings. Org Lett 6(19):3325–3328 14. Park SK, Jackson TN, Anthony JE, Mourey DA (2007) High mobility solution processed 6,13-bis (triisopropyl-silylethynyl)pentacene organic thin film transistors. Appl Phys Lett 91(6):063514 15. Park SK, Mourey DA, Subramanian S, Anthony JE, Jackson TN (2008) High-mobility spin-cast organic thin film transistors. Appl Phys Lett 93(4):043301 16. Llorente GR, Dufourg-Madec M-B, Crouch DJ, Pritchard RG, Ogier S, Yeates SG (2009) High performance, acene-based organic thin film transistors. Chem Commun 29:3059–3061 17. Mas-Torrent M, Masirek S, Hadley P, Criviller N, Oxtoby NS, Reuter P, Veciana J, Rovira C, Tracz A (2008) Organic field-effect transistors (OFETs) of highly oriented films of dithiophenetetrathiafulvalene prepared by zone casting. Org Electron 9(1):143–148 18. (a) Gao X, Wu W, Liu Y, Qiu W, Sun X, Yu G, Zhu D (2006) A facile synthesis of linear benzene-fused bis(tetrathiafulvalene) compounds and their application for organic field-effect transistors. Chem Commun 26:2750–2752. (b) Doi I, Miyazaki E, Takimiya K, Kunugi Y (2007) Chem Mater 19(22):5230–5237 19. Dodabalapur A (2005) Negatively successful. Nature 434(7030):151–152 20. Chua L-L, Zaumseil J, Chang J-F, Ou EC-W, Ho PKH, Sirringhaus H, Friend RH (2005) General observation of n-type field-effect behaviour in organic semiconductors. Nature 434(7030):194–199 21. Katz HE, Lovinger AJ, Johnson J, Kloc C, Siegrist T, Li W, Lin Y-Y, Dodabalapur A (2000) A soluble and

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

5.3.2

air-stable organic semiconductor with high electron mobility. Nature 404(6777):478–480 Yoo B, Jones BA, Basu D, Fine D, Jung T, Mohapatra S, Facchetti A, Dimmler K, Wasielewski MR, Marks TJ, Dodabalapur A (2007) High-performance solution-deposited n-channel organic transistors and their complementary circuits. Adv Mater 19(22):4028–4032 Yan H, Lu S, Zheng Y, Inagaki P, Facchetti A, Marks TJ (2007) High mobility solution-processed nchannel organic thin film transistors. Proc SPIE 6658:66580S-1–66580S-8 Letizia JA, Facchetti A, Stern CL, Ratner MA, Marks TJ (2005) High electron mobility in solution-cast and vapor-deposited phenacyl-quaterthiophene-based field effect transistors: toward n-type polythiophenes. J Am Chem Soc 127(39):13476–13477 Ortiz RP, Facchetti A, Marks TJ, Casado J, Zgierski MZ, Kozaki M, Herna´ndez V, Navarrete JTL (2009) Ambipolar organic field-effect transistors from cross-conjugated aromatic quaterthiophenes; comparisons with quinoidal parent materials. Adv Funct Mater 19(3):386–394 Singh TB, Marjanovic N, Stadler P, Auinger M, Matt GJ, Gu¨nes S, Sariciftci NS (2005) Fabrication and characterization of solution-processed methanofullerenebased organic field-effect transistors. J Appl Phys 97(8):083714 Chikamatsu M, Itakura A, Yoshida Y, Azumi R, Kikuchi K, Yase K (2006) Correlation of molecular structure, packing motif and thin-film transistor characteristics of solution-processed n-type organic semiconductors based on dodecyl-substituted C60 derivatives. J Photochem Photobiol, A 182(3):245–249 (a) See Reference 12 (Molesa et al.). (b) Sirringhaus H, Kawase T, Friend RH, Shimoda T, Inbasekaran M, Wu W, Woo EP (2000) High-resolution inkjet printing of all-polymer transistor circuits. Science 290:2123–2126 Anthony JE, Eaton DL, Parkin SR (2002) A road map to stable, soluble, easily crystallized pentacene derivatives. Org Lett 4(1):15–18 Sele CW, Kjellander BKC, Niesen B, Thornton MJ, Bas J, van der Putten PH, Myny K, Wondergem HJ, Moser A, Resel R, van Breeman AJJM, van Aerle N, Heremans P, Anthony JE, Gelinck GH (2009) Controlled deposition of highly ordered soluble acene thin films: effect of morphology and crystal orientation on transistor performance. Adv Mater 21(48):4926–4931 Huang J, Xia R, Kim Y, Wang X, Dane J, Hofmann O, Mosley A, de Mello AJ, de Mello JC, Bradley DDC (2007) Patterning of organic devices by interlayer lithography. J Mater Chem 17(11):1043–1049 Lee J-K, Chatzichristidi M, Zakhidov AA, Taylor PG, DeFranco JA, Hwang HS, Fong HH, Holmes AB,

707

708

5.3.2 33.

34.

35.

36.

37.

38.

39.

40.

Organic TFTs: Solution-Processable Small-Molecule Semiconductors

Malliarus GG, Ober CK (2008) Acid-sensitive semiperfluoroalkyl resorcinarene: an imaging material for organic electronics. J Am Chem Soc 130(35):11564–11565 (a) Dickey KC, Subramanian S, Anthony JE, Han LH, Chen S, Loo Y-L (2007) Large-area patterning of a solution-processable organic semiconductor to reduce parasitic leakage and off currents in thinfilm transistors. Appl Phys Lett 90(24):244103. (b) Mattis BA, Pei Y, Subramanian V (2005) Nanoscale device isolation of organic transistors via electron-beam lithography. Appl Phys Lett 86(3):033113 Gundlach DJ, Jia L, Jackson TN (2001) Pentacene TFT with improved linear region characteristics using chemically modified source and drain electrodes. IEEE Electron Device Lett 22(12):571–573 Gundlach DJ, Royer JE, Park SK, Subramanian S, Jurchescu OD, Hamadani BH, Moad AJ, Kline RJ, Teague LC, Kirillov O, Richter CA, Kushmerick JG, Richter LJ, Parkin SR, Jackson TN, Anthony JE (2008) Contact-induced crystallinity for highperformance soluble acene-based transistors and circuits. Nat Mater 7(3):216–221 Gundlach DJ, Zhou L, Nichols JA, Jackson TN, Nucliudov PV, Shur MS (2006) An experimental study of contact effects in organic thin film transistors. J Appl Phys 100(2):024509 Hamilton R, Smith J, Ogier S, Heeney M, Anthony J, McCulloch I, Veres J, Bradley D, Anthopoulos T (2009) High-performance polymer small molecule blend organic transistors. Adv Mater 21(10):1166–1171 Herwig PT, Mu¨llen K (1999) Solid-state conversion into pentacene and application in a field-effect transistor. Adv Mater 11(6):480–483 Afzali A, Dimitrakopoulos CD, Breen TL (2002) High-performance, solution-processed organic thin film transistors from a novel pentacene precursor. J Am Chem Soc 124(30):8812–8813 Weidkamp KP, Afzali A, Tromp RM, Hamers RJ (2004) A photopatternable pentacene precursor for

41.

42.

43.

44.

45.

46.

47. 48.

49.

use in organic thin-film transistors. J Am Chem Soc 126(40):12740–12741 Lee W, Lim J, Kim D, Cho J, Jang Y, Kim Y, Han J, Cho K (2008) Room-temperature self-organizing characteristics of soluble acene field-effect transistors. Adv Funct Mater 18(4):560–565 Dickey K, Anthony J, Loo Y-L (2006) Improving organic thin-film transistor performance through solvent-vapor annealing of solution-processable triethylsilylethynyl anthradithiophene. Adv Mater 18(13):1721–1726 Dickey K, Smith T, Stevenson K, Subramanian S, Anthony J, Loo Y-L (2007) Establishing efficient electrical contact to the weak crystals of triethylsilylethynyl anthradithiophene. Chem Mater 19(22):5210–5215 Subramanian S, Park S, Parkin S, Podzorov V, Jackson T, Anthony J (2008) Chromophore fluorination enhances crystallization and stability of soluble anthradithiophene semiconductors. J Am Chem Soc 130(9):2706–2707 Park SK, Mourey DA, Subramanian S, Anthony JE, Jackson TN (2008) Polymeric substrate spin-cast diF-TESADT OTFT circuits. IEEE Electron Device Lett 29(9):1004–1006 Jurchescu OD, Subramanian S, Kline RJ, Hudson SD, Anthony JE, Jackson TN, Gundlach DJ (2008) Organic single-crystal transistors of soluble anthradithiophene. Chem Mater 20(21):6733–6737 See Reference 14 (Park et al. 2007 APL) Park SK, Anthony JE, Jackson TN (2007) Solutionprocessed TIPS-pentacene organic thin-filmtransistor circuits. IEEE Electron Device Lett 28(10):877–879 Halls J, Newsome C, Kugler T, Whiting G, Murphy C, Burroughes J (2008) OTFT development for OLED backplanes: optimisation of high mobility 10 mm channel OTFTs. Presented at Intertech Thin-Film Transistors Conference, La Jolla, CA

Further Reading Murphy AR, Fre´chet JMJ (2007) Organic semiconducting oligomers for use in thin film transistors. Chem Rev 107(4):1066–1096 Zaumseil J, Sirringhaus H (2007) Electron and ambipolar transport in organic field-effect transistors. Chem Rev 107(4):1296–1323 Menard E, Meitl MA, Sun Y, Park J-U, Shir DJ-L, Nam YS, Jeon S, Rogers JA (2007) Micro- and nanopatterning techniques for organic electronic and optoelectronic systems. Chem Rev 107(4):1117–1160

Mas-Torrent M, Rovia C (2007) Novel small molecules for organic field-effect transistors: towards processability and high performance. Chem Soc Rev 37(4):827–838 Arias AC, MacKenzie JD, McCulloch I, Rivnay J, Salleo A (2010) Materials and applications for large area electronics: solution-based approaches. Chem Rev 110(1):3–24 Klauk H (ed) (2006) Organic electronics: materials, manufacturing and applications. Wiley-VCH, Weinheim

5.3.3 Organic TFTs: Polymers Feng Liu . Sunzida Ferdous . Alejandro L. Briseno 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710 2 Device Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711 3 p-Type Polymer Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713 4 n-Type and Ambipolar Polymer Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715 5 Complementary Circuits and Inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 717 6 Dielectrics and Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 718 7 Device Patterning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 721 8 Conclusion and Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.3.3, # Springer-Verlag Berlin Heidelberg 2012

710

5.3.3

Organic TFTs: Polymers

Abstract: Polymer semiconductor field-effect transistors are expected to be next-generation devices for use as drivers in displays and a variety of other technological applications. They are of particular interest because of their low-cost fabrication, solution processability, mechanical flexibility, and large-area fabrication capabilities. An overview on the development of polymerbased organic field-effect transistors (OFETs) is discussed with emphasis on device configurations, choice of materials, and device engineering. List of Abbreviations: mCP, Microcontact Printing; BBL, Poly(benzobisimidazobenophenanthroline); BC/TG, Bottom Contact/Top Gate; BC/BG, Bottom Contact/Bottom Gate; BTS, Benzyltrichlorosilane; CMOS, Complementary Metal-Oxide-Semiconductor; DMSO, Dimethyl Sulfoxide; F8T2, Poly(9,90 -dioctylfluorene-co-bithophene); FTS, (Tridecafluoro1,1,2,2-tetrahydrooctyl) Trichlorosilane; HMDS, Hexamethyldisilazane; HOMO, Highest Occupied Molecular Orbital; ITO, Indium Tin Oxide; LUMO, Lowest Unoccupied Molecular Orbital; MIMIC, Micro-molding in Capillaries; MTP, Metal Transfer Printing; nTP, Nano-transfer Printing; OFET, Organic Field-Effect Transistor; OTS, Octadecyltrichlorosilane; P3HT, Poly(3-hexylthiophene); PBTTT, Poly(2,5-bis(3-alkylthiophene-2-yl)-thieno[3,2-b] thiophene); PCBM, [6,6]-Phenyl C61-Butyric Acid Methyl Ester; PDMS, Polydimethylsiloxane; PEDOT/PSS, Poly(3,4-ethylenedioxythiophene) Doped with Polystyrene Sulfonic Acid; PMMA, Poly(methylmethacrylate); PQT-12, Poly(3,3000 -didodecylquaterthiophene); PSPMMA-PS, Poly(Styrene-Block-Methylmethacrylate-Block-Styrene); PVP, Polyvinylphenol; SAM, Self-assembled Monolayer; SEM, Scanning Electron Micrograph; TC/BG, Top Contact/ Bottom Gate; VTS, 7-Octenyltrichlorosilane

1

Introduction

Conjugated polymer semiconductors are of high interest due to their broad-range applications in light emitting diodes, thin-film transistors, and photovoltaics [1]. For example, in polythiophene, the backbone consists of alternating single and double carbon–carbon bonds. These carbon atoms are sp2 hybridized where an overlap of p-orbitals create a pathway for charge transport. The electrons in the p-orbitals are ‘‘delocalized’’ and determine the intrinsic properties of a polymer such as light absorption, energy levels of frontier orbitals, and charge transport properties [1]. The chemical and physical properties of a conjugated polymer can be well controlled and finely tuned by synthetic chemistry, which yields a broad library of materials that can meet specific requirements for different applications. To achieve nextgeneration all-plastic electronic devices, one must have access to high-performance, air-stable semiconductor materials. Polymers semiconductors are excellent candidates as they possess good solubility, roll-to-roll processability, mechanical flexibility, and in certain materials, environmental stability. Organic transistors, the very basic elements of circuits, have been the focus of intensive investigation over the past two decades [2]. Both p- and n-type polymers are well developed and complementary logic devices have been recently demonstrated [2, 3]. In recent years, device engineering has improved the performance of OFETs to a point where their carrier mobilities have surpassed that of amorphous silicon (see > Chap. 5.2.1 and [3]). With the advent of modern technology such as ink-jet printers, polymer semiconductors are now processed over large areas in patterned arrays and employed in printed electronic applications by several start-up companies.

Organic TFTs: Polymers

2

5.3.3

Device Configuration

The operation of an OFET includes three components (> Fig. 1a): a thin semiconductor active layer, a dielectric layer, and three electrodes [4]. Two of these electrodes, source and drain, are in ohmic contact with the active layer and form the conducting channel. The third electrode, gate, modulates the charge carrier density in the conduction channel, and the dielectric layer separates the active layer from the gate. The source electrode is grounded and serves as the charge injecting electrode, and the drain serves as the charge extracting electrode. Voltage is applied to the gate (Vg) and to the drain (Vds). An OFET essentially works as a parallel plate capacitor. For a p-channel device, applying a negative Vg induces positive charges at the dielectric–semiconductor interface and vice versa for an n-channel device. The number of induced charges depends on the applied Vg, dielectric constant of the insulator (k), and its thickness.

W L Source

Drain Semiconductor

Vd

Insulator Gate

a Vg Vd VTh Vd >Vd,sat

Id

V(x)=Vg–VTh

d

Pinch-off point

Vg > VTh

Vds

. Fig. 1 (a) Schematic of a field-effect transistor, (b–d) different operation regimes of an OFET with their corresponding I–V curves: (b) linear region, (c) onset of saturation, (d) saturated region (Reprinted with permission from [4] © 2007 Chemical Reviews)

711

5.3.3

Organic TFTs: Polymers

> Figures 1b–d show different working regimes of an OFET and their corresponding I–V characteristics. When a relatively low Vds is applied compared to Vg, the current flow follows Ohm’s law and is proportional to both Vds and Vg. This is known as the linear regime. As Vds is increased, a point is reached when Vds equals the effective gate voltage (Vg Vth). This point is called the ‘‘pinch-off ’’ point (> Fig. 1c), where a depletion region is formed near the drain electrode. The potential drop between the source and this point becomes constant and the current begins to saturate [4]. Any further increase in Vds only increases the depletion width, but does not increase the drain current significantly. This is the saturation regime (> Fig. 1d). > Figure 2a illustrates the output characteristics of a typical transistor and > Figs. 2b, c show the transfer characteristics for both linear and saturation regimes, respectively. The mobility in these two regimes can be calculated from the following equations:

IDlin ¼

W mCi ðVG  VT ÞVD L

ð1Þ

W mCi ðVG  VT Þ2 2L

ð2Þ

IDsat ¼

where W, L, Ci, and m are the channel width, channel length, insulator capacitance, and charge carrier mobility, respectively. These equations are based on the assumptions of the gradual channel approximation (electric field normal to the channel created by the Vg is much higher than the electric field parallel to the channel created by Vds) and constant mobility [4]. Gradual channel approximation is only valid when the channel length is much larger than the insulator thickness (L > 10  dinsulator) [4, 5]. When L is close to or smaller than insulator thickness, electric field due to source-drain voltage could dominate the charge distribution in the channel causing the breakdown of the first assumption. OFETs can have different device structures with respect to the substrate. > Figure 3 shows a bottom contact/top gate (BC/TG), bottom contact/bottom gate (BC/BG), and a top contact/ bottom gate (TC/BG). For fabrication reasons, bottom-gate structure is more suitable since organic semiconductors are very fragile compared to their inorganic counterparts. The devices

a

Source-drain voltage (V)

b

Gate voltage (V)

log (Drain current (A))

Drain current (A)

reshold

Von

Subth

log (Drain current (A))

Lin

ear

Saturation

c

VTh

Drain current1/2

Saturation regime

Linear regime

Drain current (A)

712

Gate voltage (V)

. Fig. 2 (a) Typical output characteristics with linear and saturation regimes, (b) transfer characteristics in linear (Vd Vg Vth) regime. Threshold voltage (Vth) is obtained from the square-root plot (Reprinted with permission from [4] © 2007 Chemical Reviews)

5.3.3

Organic TFTs: Polymers

BC/TG

BC/BG

TC/BG Ss

GG

s S s S

a

D

D

D G G

G G

b

c

. Fig. 3 OFET configurations. (a) bottom contact/top gate, (b) bottom contact/bottom gate, and (c) top contact/bottom gate. S, D, and G stand for source, drain, and gate, respectively (Reprinted with permission from [4] © 2007 Chemical Reviews)

in > Fig. 3a and c are in staggered configuration, whereas the device in > Fig. 1b is in the coplanar configuration. The conducting channel in the staggered devices is separated from the source and drain electrodes by the semiconducting layer, whereas, in the coplanar device, the edges of the source and drain are in direct contact with the channel. This leads to direct charge injection into the conducting channel in the BC/BG structure. Several parameters such as the semiconductor/dielectric interface and the interface between the semiconductor and source-drain electrodes (contact resistance) will affect the performance of an OFET [6–8].

3

p-Type Polymer Semiconductors

For polymer OFETs, p-type semiconductor polymers are well developed, and a variety of these polymers are summarized in > Fig. 4. Poly(thiophene) (1) was the first polymer to be used in field-effect transistors [9]. Poly(thiophene) has a long chain of thiophene units, and the alternating single- and double-bond structure effectively extends the conjugation of this material and provides the conduction channel for charge to transport. A great deal of work has been done to chemically modify poly(thiophene) to increase solubility and improve the crystallinity. Regiorandom poly(3-hexylthiophene) (P3HT) (2) is regarded as the first solution-processable OFET with mobilities of 104–105 cm2/Vs [10]. Increasing the regioregularity of P3HT decreases its band gap and increases its crystallinity. The mobility of regioregular P3HT reached 0.05–0.1 cm2/Vs with on/off ratios of 106 [11]. Molecular weight and process conditions also have a profound effect on device performance [11–17]. A major drawback for P3HT is the poor stability. P3HT thin-film devices are very sensitive to oxygen and moisture [3, 16]. If the device is fabricated and tested in air, the oxidative doping of P3HT thin films will increase the off-current, and thus decrease the on/off ratio. This effect can be minimized by increasing the ionization potential or the highest occupied molecular orbital (HOMO). For poly(thiophene)s, the HOMO could be tuned by chemically modifying the polymer structure or by introducing rotational degrees of freedom or geometric twists [3, 18–20]. Poly(3,3000 -dialkylquaterthiophene) (PQT-12) (3) is a well-studied air-stable polymer semiconductor based on this design [21]. The rotational freedom of unsubstituted thienylene moieties in PQT-12 reduces the conjugation length and its ionization potential is 0.1 eV higher than that of regioregular P3HT. Bottom-gate, top-contact devices fabricated in air show mobilities up to 0.14 cm2/Vs with on/off ratios of 107 [21]. Fused ring thiophene and thioacene building blocks are also proven to increase the stability of thiophene copolymers [22–28]. Poly(2,5-bis(3-alkylthiophene-2-yl)-thieno[3,2-b]

713

714

5.3.3

Organic TFTs: Polymers

. Fig. 4 Polymer semiconductors that exhibit p-type characteristics

thiophene) (PBTTT) (4) is a good example. The ionization potential for PBTTT is 0.3 eV higher than that of regioregular P3HT [23]. Mobilities as large as 0.6 cm2/Vs and on/off ratios larger than 106 can be obtained on annealed devices under nitrogen atmosphere (> Fig. 5a). In low humidity (4%), the devices show good stability with almost no change in performance. The alkyl-substituted thienothiophene polymer (5) is another well-studied material. The incorporation of the rigid thienothiophene moiety into the polymer increases the ionization potential and results in optimal packing of polymer chains [28]. The best device performance exhibited a mobility of 0.33 cm2/Vs with on/off ratios exceeding 105. The device with octyltrichlorosilane-modified gate dielectrics showed no significant degradation in output characteristics over nine months when stored at 30% relative humidity [28]. Introducing electron withdrawing groups to the polymer backbone is another effective method to increase the air stability of thiophene-based polymers [2, 3, 29–31]. A typical example is the introduction of a didodecylbithiazole unit to the backbone of poly(didodecylquaterthiophene) (6). OFET devices showed much better improved stability in air with mobilities of 0.33 cm2/Vs [31]. Additional examples of high-mobility, high-stability semiconductor polymers [32–44] include poly(4,8-didodecylbenzo[1,2-b:4,5-b0 ]dithiophene) (7) developed by Ong and coworkers. This particular material yielded mobilities of 0.25 cm2/Vs with on/off ratios of 106 [33, 34]. A naphthodithiophene polymer (8) increased the ionization potential and enhanced the interchain packing to yield a mobility of 0.54 cm2/Vs with on/off ratios of 107 [36]. A hole mobility of 0.11 cm2/Vs was measured for a benzothiadiazolecyclopentadithiophene copolymer (9). The fused cyclopentadithiophene ring reduced the reorganization energy and enhanced the mobility [38]. McCullough and coworkers synthesized n-alkyldithieno[3,2-b:20 ,30 -d]pyrrole (10) to yield as-cast OFET devices with a mobility of 0.21 cm2/Vs. However, thermal annealing largely reduced the carrier mobility [39, 40].

5.3.3

Organic TFTs: Polymers

1 × 10–2

Vg = –60V

8 × 10–3

Isd (A)

Vg = –45V 6 × 10–3 4 × 10–3

Vg = –30V

2 × 10–3

Vg = –15V Vg = 0V

0 × 100

0

–10

–20

–30 Vd (V)

a

–40

–50

–60

10–4

15 4 60V

2

–2 –6 0

50V

ISD (A)

ISD (µA)

10–6

0

10

6

10–8

PS PTBS D2200 PMMA CYTOP

40V

5

10–10

30V 20V 0 0

b

20 40 VSD (V)

10–12

60

c

–20

0

20

40

60

VSG (V)

. Fig. 5 (a) Field-effect transistor output characteristics of polymer (4) under N2 atmosphere (W = 10,000 mm and L=20 mm). (b) Output characteristics of polymer (18) with a PMMA-based dielectric insulator. The inset shows the linear I–V characteristics at low voltages. (c) Overlay of transfer characteristics of (18) with various polymer dielectric layers (Reprinted with permission from [23] © 2006 Nature Materials, and [53] © 2009 Nature)

Marks and coworkers incorporated dithienosilole (11) and dibenzosilole (12) units into thiophene polymers to obtain air-stable transistor devices with mobilities of 0.06 and 0.08 cm2/Vs, respectively [41, 42]. Jenekhe and Watson incorporated phthalimide units into thiophene polymers (13) and an air-stable mobility of 0.28 cm2/Vs was measured [43].

4

n-Type and Ambipolar Polymer Semiconductors

Compared to p-type polymers, the development of n-type and ambipolar polymer semiconductors in OFET applications have remained far behind. > Figure 6 shows a series of n-type polymer semiconductors recently employed in OFET and ambipolar applications

715

716

5.3.3

Organic TFTs: Polymers

. Fig. 6 Polymer semiconductors that exhibit n-type characteristics

[45–58]. The design of air-stable n-type polymers requires low-lying LUMO energies which enhance device stability by energetically stabilizing the induced electrons during charge transport [45]. Moreover, the semiconductor–dielectric interface is of particular importance since traps at the interface could largely reduce the current through the conducting channel [46]. Poly(benzobisimidazobenophenanthroline) (BBL) (14) is the first few well-studied n-type polymer with high electron mobility up to 0.1 cm2/Vs [47]. The rich nitrogen and oxygen heteroatoms in BBL gave rise to good electron accepting properties and a low electron affinity of 4.0–4.4 eV [47, 48]. Marks and coworkers developed n-alkyl-2,20 -bithiophene-3,30 dicaboximide (15) that showed n-channel electron mobilities up to 0.01 cm2/Vs [49]. It was recently discovered that perylene bisimide building blocks work well in n-type materials. For example, Marder and coworkers synthesized a low band gap dithienothiophene (16) copolymer with an electron mobility of 1.3102 cm2/Vs under inert gas [51]. Facchetti and coworkers reported perylenedicarboximide and naphthalenedicarboximide-based polymers (17) with a mobility of 0.002 cm2/Vs under vacuum, while the naphthalenedicarboximide polymer (18) showed a mobility of 0.06 cm2/Vs [52]. Their subsequent work showed that the electron mobility of naphthalenedicarboximide polymer could reach up to 0.85 cm2/Vs under ambient conditions in combination with Au contacts and various polymer dielectrics. The device performance of this polymer is shown in > Fig. 5b, c [53].

Organic TFTs: Polymers

5.3.3

For ambipolar polymer OFETs, the device structure and material design are still not as well developed as those of small-molecule devices (see > Chaps. 5.3.1 and > 5.3.2 and [3, 4, 54]). It is difficult to efficiently inject both electrons and holes from one electrode such as gold due to the injection barriers for at least one type of carrier. Sirringhaus and coworkers demonstrated that the electron and hole contact resistance in ambipolar OFETs could be controlled by using thiol-based self-assembled monolayers (SAMs). By employing specific SAMs, the injection of both electrons and holes can be achieved [55]. Marks and coworkers finely tuned the HOMO-LUMO energy levels of a family of ladder-type polymers which resulted in ambipolar transport (19) [45]. Several additional polymers such as donor-acceptor polymers (20), naphthalenedicarboximide copolymer (21), and polyselenophene (22) also showed interesting ambipolar transport properties [56–58].

5

Complementary Circuits and Inverters

In order for p- and n-channel polymer OFETs and complementary inverters to be useful in real-world electronic applications, they must exhibit long-term air stability and endure the effects of atmospheric contaminants such as oxygen and moisture [3, 16]. Although several reports have demonstrated complex all-polymer circuits, the devices from these reports were exclusively fabricated from p-type polymer semiconductors in unipolar operation [59–61]. The advantage of having a truly complementary system is low static power dissipation, better noise margins, a more robust operation, and the ease of using them to design highly sophisticated circuits [4]. A conducting substrate (i.e., ITO, gold, etc.) serves as the gate electrode and functions as the input node (Vin). The supply voltage (Vdd) is provided by the source of the load transistor, and the source of the driver is grounded. Output voltage (Vout) is given by the drain electrodes from both load and driver. A circuit diagram describing an inverter is shown in > Fig. 7a. Two approaches are usually taken to make an organic inverter: using a p- and n-type material or using ambipolar materials on a common substrate. Ambipolar polymer (21) mentioned above has been used (> Fig. 7b) as a complementary inverter by electrically connecting it to both load and driver mode on a common substrate with a voltage gain of 30 [58]. Their results illustrate the importance of utilizing high-mobility ambipolar transistors for use in complementary circuits. Ambipolar output curves and static transfer characteristics of this donor-acceptor copolymer are shown in > Fig. 7c and d, respectively. Depending on the polarity of the supply voltage, it is possible to observe well-defined voltage-transfer characteristics in the first and third quadrant of the output versus input diagram (> Fig. 7d). Complementary inverters were also previously demonstrated using small-molecule organic semiconductors [62]. The ideal transfer characteristics should show symmetrical gate threshold switching (Vm) at nearly half of the supply voltage (Vm = Vdd/2). The symmetry is a result of equally matched mobilities and threshold voltages of the p- and n-channel transistors of the inverter. This point stresses the importance of having polymers that exhibit similar mobilities and electrical characteristics. The switching thresholds of the inverters can also be graphically estimated at the intersection of the transition region from the transfer curves as shown in > Fig. 7d. Other examples of polymeric complementary inverters include solution-processable P3HT and n-type naphthalenedicarboximide polymer (18) with gains as high as 65 [53]. Ambipolar transistors, based on [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) and poly[2-methoxy5-(30 ,70 -dimethyloctyloxy)]-p-phenylene vinylene blends, have also been reported to show CMOS inverter behavior with high gain [63].

717

5.3.3

Organic TFTs: Polymers

Vsupply –2.5

Vout

Vin driver

H9C4

Ids (µA)

a

C2H5 O H25C12O

80 V

–2.0

20

–1.5

15 –80 V

–1.0

S

S

n

b

5

–0.0 –80 –60 –40 –20

O C2H5

10

40 V 20 V 0 V

–40 V –20 V 0 V

OC12H25

N

60 V

–60 V

–0.5

N O

25

Ids (µA)

load

c

0

0 20 40 60 80

Vds (V)

O C4H9

PNIBT 100 Vdc

60 40

Vdd (V) ±100 ±90 ±80 ±70 ±60

Gain

80 Vin

30

20 Vout 10

20 Vout (v)

718

0

0 0

50 100 Gain

–20

30

–40

20

–60

10

–80 –100 –100 –80 –60 –40 –20

d

0

0 –100 –50

0

20

60

40

80 100

Vin (V)

. Fig. 7 Chemical structure of copolymer along with its device characteristics. (a) Circuit diagram of an inverter, (b) chemical structure of PNIBT ambipolar semiconductor, (c) ambipolar output characteristics showing good gate modulation, and (d) static inverter transfer characteristics and the corresponding voltage gain (Reprinted with permission from [58] © 2010 Advanced Materials)

6

Dielectrics and Interfaces

Polymer transistors intrinsically suffer from low mobility and stability issues in ambient conditions. One of the main challenges is to achieve high mobility with a low operating voltage. Molecular ordering through self-assembly is a key factor for high mobility in polymer OFETs. Apart from the molecular design of semiconductor materials, the interface between the dielectric and active layers plays an important role in device performance. An ideal dielectric film needs to have low trap density at the semiconductor/dielectric interface, low surface roughness, low impurity concentration, and compatibility with organic semiconductors. Both inorganic and organic materials have been used as dielectric layers. For instance, SiO2

Organic TFTs: Polymers

5.3.3

and SiNx are commonly used inorganic insulators with thicknesses ranging from 100–300 nm, and polyvinylphenol (PVP) and poly(methylmethacrylate) (PMMA) are widely used solutionprocessable organic dielectrics among many others. Efforts have been applied to achieve a high drain current with a low operating voltage by increasing the insulator capacitance (Ci). The use of high-k dielectric, polymer-TiO2 composites, and ion-gel electrolytes are just a few examples [64–69]. OFETs with high dielectric capacitance allows higher charge injection into the semiconductor layer at a given gate voltage, and therefore, the device can turn on at lower voltage. Frisbie and coworkers demonstrated low operating voltage (> Fig. 8b, d) printed OFETs with a triblock copolymer ion-gel as a gate dielectric for P3HT, PQT-12, and poly-9,90 dioctyl-fluorene-co-bithiophene (F8T2) semiconductors [69]. The ion-gel polymer dielectric layer (poly(styrene-blockmethylmethacrylate- block-styrene) (PS-PMMA-PS) dissolved into an ionic liquid, 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI])) has a much higher specific capacitance (1 to 20 mF cm2 depending on the frequency) compared to other solution-processable polymer dielectrics (0.02 mF cm2). This high capacitance is attributed to formation of electrical double layers at the electrolyte/electrode interface. One of the benefits of ion-gel dielectric is that due to their high polarizability, the gate electrode registration does not have to be extremely precise and can be physically offset instead of placing directly on top (for top gate) or bottom (for bottom gate) of the source-drain channel (> Fig. 8c). OFETs with both aligned and offset gate electrodes show very similar current-voltage characteristics (> Fig. 8b, d) showing the importance of ion-gel as a promising dielectric material for printed electronics. Many research groups have also investigated the effects of chemical modification of the dielectric layer prior to the deposition of the semiconductor layers [6, 7, 54]. Salleo reported that a self-assembled monolayer of octadecyltrichlorosilane (OTS) on the oxide layer yielded improved mobilities of 0.015 cm2/Vs for F8T2 among the other silanes studied (7-octenyltrichlorosilane (VTS), (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTS), and benzyltrichlorosilane (BTS)) and hexamethyldisilazane (HMDS) [7]. They have found that the carrier mobility in polymer OFETs do not solely depend on the dielectric contamination (i.e., silanol groups) by showing the non-monotonic dependence of mobility on dielectric surface energy (> Table 1). FTS-treated surfaces show the lowest surface energy, while BTS show the highest; however, highest mobility was observed on OTS-treated surfaces, and not on FTS treated substrates. Mobility is related to specific SAM structure in addition to the surface energy. Longer alkane chains in OTS or benzene rings in BTS can interact with similar features in F8T2, enhancing the performance of the OFET. Low-voltage operating polymer transistors have also been demonstrated using only SAM as the gate dielectric [70] as well as spin-coated polymer dielectrics [71] for P3HT. Solution-processable polymer dielectrics are of particular interests because of their low fabrication cost compared to thermally grown inorganic oxides and their compatibility towards all polymer printed electronics. For top-gate architecture, dielectric and its processing solvent limits the device performance, while for a bottom-gate architecture, dielectric/semiconductor interfaces play important roles. The interface between source-drain and semiconductor is also important for better charge injection into the active layer. These interfaces require low contact resistance which occurs from parasitic resistance and the energy barrier between electrode and HOMO level of a p-type semiconductor (LUMO for n-type). Au and Pt are commonly used electrodes due to their environmental stability. Treatment of Au contacts with a SAM of alkane thiol and aromatic thiols have been investigated for small-molecule OFETs. Aromatic thiol treatment showed

719

5.3.3

Organic TFTs: Polymers PEDOT:PSS lon gel Au

Au

P3HT Polyimide substrate

10–2 10–3 800 µm

Reverse

–ID (A)

10–4 10–5 10–6 10–7

Forward

10–8 10–9 –3

a

–2

Gate

Source

Drain P3HT Polyimide substate

0

–1

1

2

VG (V)

b 10–2 10–3

lon gel

10–4 Reverse

lon gel – ID (A)

720

P3HT

10–5 10–6 10–7

Forward

G 10–8 S

D

10–9

20 µm

–3 –2 –1

c

d

0

1

2

3

VG (V)

. Fig. 8 (a) Optical image of an aerosol printed ion-gel top-gated OFET array on a plastic substrate with channel lengths of 20 mm and widths of 1,400 mm, (b) schematic cross-section of a top-gated iongel OFET with aligned gate electrode and transfer curve for P3HT based transistor showing very low operating voltage with Vd = 1 V, and (c) a schematic cross-section of a misaligned gate electrode relative to the channel area by 60 mm and optical image of the corresponding device and, (d) transfer characteristics of the displaced gate OFET with 20 mm channel length and 1,400 mm channel width (Reprinted with permission from [69] © 2008 Nature Materials)

Organic TFTs: Polymers

5.3.3

. Table 1 Contact angle measurements of water, xylene, and F8T2 solution on SiO2 substrates with different treatments as well as mobility and On/Off values of OFETs on substrates with different treatments (Reprinted with permission from [7] © 2002 Applied Physics Letters) Contact angle

Water

Xylene

Spun-cast films 0.5 wt% F8T2 Mobility in xylene (cm2/Vs)

Drop-cast films Mobility Ion/Ioff (cm2/Vs)

-4

Ion/Ioff -4

No treatment Fig. 4 [5]. Here, we take the value of n as a measure of the spatial spread of the metal cation s orbital. Thus, candidates for transparent amorphous semiconductors having large electron mobilities comparable to those of the corresponding crystals are transparent oxides constituting of PTM cations with an electronic configuration (n 1)d10ns0, where n  5. Note that transition metal cations with an open-shell structure are ruled out as candidates because they are not transparent owing to absorptions arising from d–d transitions. In the case of crystalline semiconducting oxides, this requirement is relaxed to be n  4 as exemplified by ZnO; Zn2+ has the 3d104s0 configuration, because crystalline materials have much more regular and compact structures than amorphous oxides have. > Figure 5 illustrates the orbitals of silicon and a PTM oxide for crystalline and amorphous states. The drastic reduction of the electron mobility in the amorphous state from crystalline silicon may be understood intuitively from the figure, whereas medium mobility in crystalline PTM oxides is maintained even in the amorphous state. In a sense, the situation of the CBM in PTM oxides is similar to that in amorphous metal alloys in that metal orbitals dominantly

733

5.3.4

Oxide TFTs

Covalent semicon.

Ionic oxide semicon. M:(n-1)d10ns0 (n≥5) Crystal

Oxygen 2p-orbital

sp3-orbital

Metal ns-orbital

Amorphous

. Fig. 5 Orbital drawing of the electron pathway (conduction band bottom) in covalent compound semiconductors and ionic oxide semiconductors

O 2p σ amorphous ≈ crystalline Amorphous

5s

Ni60 Nb40

As-deposited Annealing O 2p

ρ (μΩ cm)

734

160 Liquid

145 130 Crystalline 0

5s

300

600 900 T(°C)

1,200

. Fig. 6 Similarity between ionic oxides and metals: comparison of resistivity among liquid, amorphous, and crystalline metals (right); expected change in wave functions at the conduction band minimum (CBM) of ionic amorphous oxide semiconductors with annealing (left). The electron mobility is expected to be enhanced by annealing as in the case of amorphous metals

constitute the electron pathways. The conductivity of amorphous/liquid metal alloys remains at a slightly lower level compared with that in the corresponding crystalline phases, as illustrated in > Fig 6 [6]. The structure of amorphous metal is modeled by dense random packing of metal spheres and the occupation of a metalloid at the interstitial position. We may consider that in amorphous PTM oxides the vacant ns orbitals of the metal cations work as metal atoms in amorphous metal.

Oxide TFTs

5.3.4

Electron dopability in semiconductors is determined by the thermodynamic stability of doped electrons, expressed by the magnitude of electron affinity, and the easiness of formation of the counter defect, i.e., electron-capturing defect, in the hosts. If a higher valence state of candidate PTM cations is not so stable, the generated electron tends to reduce the cation, i.e., Sn4+ with the isoelectronic configuration as In3+ is inferior to In3+. As for stability of the doped electron, the electron-neutrality level of the candidate metal oxides is close to or above the CBM [8] owing to a large energy dispersion of the CBM reflecting a large overlap in ns orbitals between the neighboring metal cations. As a consequence, the above hypothesis predicts that transparent amorphous oxides are capable of being electrondoped and have a large electron mobility comparable to the that of the corresponding crystalline phases.

3

Electron Transport in AOSs

Ionic AOSs have several common properties which are not seen in conventional amorphous semiconductors. First is their large electron mobility, such as 10–40 cm2 (V s) 1, which is higher by 1–2 orders of magnitude than that in a-Si:H or higher by several orders of magnitude than that in glassy semiconductors or amorphous chalcogenides. Second is that a degenerate state can be realized by doping. This is totally different from the other amorphous semiconductors. For instance, crystalline silicon is easily changed to the degenerate state by carrier doping (approximately several hundred parts per million), but no such a state has been attained in a-Si:H [9] to date. That is, conduction takes place by hopping or percolation between tail states in conventional amorphous semiconductors. This is the reason why drift mobility in the amorphous state is so small compared with that in the crystalline state. On the other hand, in amorphous PTM oxides, the Fermi level can exceed the mobility gap easily by carrier doping, leading to band conduction. It is considered that this striking difference originates from the nature of the chemical bonding between the materials, i.e., strong ionic bonding with a spherical potential is very favorable for the formation of a shallow tail state having a small density of states. The last unique feature is that electron carriers are primarily injected by chemical doping. Substitutional doping, which is effective in crystalline semiconductors and a-Si:H, is very inefficient. Each ion is primarily connected by ionic bonding and has large flexibility in a coordination structure compared with the tetrahedral unit in a-Si:H. Thus, doping (or substitution) of aliovalent ions does not yield charge carriers if neutrality of the total formal charge of the constituent ions is maintained. Therefore, an effective method of doping is to alter the stoichiometry of the oxygen ion by controlling the oxygen gas pressure during the deposition processes or ion implantation of cations with a low electron affinity. Both processes modify the charge valence of cations and anions, leading to the injection of carrier electrons into the CBM [4]. This situation is similar to that in organic semiconductors such as polyacetylene for which carrier doping is carried out by chemical doping using, for example, PF6 (hole doping) and I3 (electron doping) [10]. > Table 1 summarizes the bonding nature and carrier transport properties of various classes of amorphous semiconductors. Among them amorphous a-IGZO has been extensively studied as the semiconducting channel layer of transparent TFTs since the first report [2] in late 2004. > Figure 7 summarizes the electrical properties (Hall mobility mHall and carrier concentration Ne) for the films in the In2O3–Ga2O3–ZnO system [11]. All the films were deposited under the same

735

5.3.4

Oxide TFTs

. Table 1 Comparison of various amorphous semiconductors

Material System

Chemical bond

Mechanism

Hall voltage sign

Mobility cm2 (V s)

Tetrahedral

Covalent

Hopping

Abnormal

1

Abnormal

3

Chalcogenide

Covalent

Hopping

Oxide (glass semiconductors)

Covalent + Ionic

Hopping

(Ionic amorphous oxide semiconductors)

Ionic

Band conduction

2

μHall (cm V

Ga2O3 1.00

10

Tl2Se-As2Se3

10–4

V2O5-P2O5

1060

In-Ga-Zn-O

Ga2O3

–1- –1

s )

10

0

0.00

100

1.00

ol%

) ol%

(m 0.25

0.75

X in (In2O3)x-(ZnO)1-x (mol%)

3) 1-x

aO 2 x -(G

O) Zn

0.00

1.00 In O 2 3

0.75

0.25

)

0.50

)

0.25

33 (100) 39 (100) 34 (500)

ol%

17 (60) 36 (200)

1.00 ZnO 0.00

n(

26 (100)

Xi

(m 2O 3) 1-x

a 3) x (G

nO (Z in

25 (200)

0.50

Amorphous

ol%

18 (10)

(m

14 (60)

0.50

(m

15 (70)

0.75

) 1-x O3 (In 2

9 (5) 9 (0.6)

) 1-x O3 (In 2

0.50

0.25

) xO3 Ga 2

0.9 (5)

) xO3 Ga 2

5 (0.006)

n(

0.75

n(

Xi

0.75

0.50

0.25

Example Si:H

)

0.00

Normal

1

Xi

Measured Not measured

X

736

Crystalline

Crystalline

1.00

ZnO 0.00

0.25

0.50

0.75

X in (In2O3)x-(ZnO)1-x (mol%)

0.00 1.00 In O 2 3

. Fig. 7 The amorphous formation region (right) and Hall mobility and carrier electron concentrations (left) in the In2O3–Ga2O3–ZnO (IGZO) system. The thin film was deposited on a glass substrate by a pulsed laser deposition (PO2 = 1 Pa). Numbers in parentheses denote the carrier electron concentration(1018 cm 3)

conditions: i.e., on a SiO2 glass substrate at room temperature and an oxygen partial pressure of 1.0 Pa. Although pure In2O3 and ZnO films exhibit large Hall mobilities of approximately 34 cm2·V 1·s 1 and approximately 19 cm2·V 1·s 1, respectively, they are crystalline even if the films are deposited at room temperature Moreover, it is not easy to control the carrier concentration down to less than 1017 cm 3 in these films without compensation doping. Pure Ga2O3 forms amorphous films but carrier doping, i.e., formation of a shallow oxygen vacancy, is very hard irrespective of the deposition conditions examined. Thus, these endmember materials in this ternary system are not appropriate because of local nonuniformity due to grain boundaries, no stable amorphous phase, and/or the difficulty in carrier generation. As known in glass science, incorporation of aliovalent and different-size cations is effective to enhance amorphization, and it is very favorable to introduce network forming cations. Indeed, stable amorphous phases are formed in the binary systems of In2O3–Ga2O3 (aIGO) and ZnO–Ga2O3, and in the ternary system of a-IGZO. Both the Hall mobility and the

Oxide TFTs

5.3.4

EF Eth EF Ec

Electron EF > Eth

EF < Eth

. Fig. 8 Near the conduction band bottom. Arrows denote conduction paths of electrons for the cases of EF>Eth and EF Fig. 8. Carrier mobility strongly depends on carrier concentration, and large mobilities are obtained at carrier concentrations larger than a threshold value (e.g., approximately 1018 cm 3 for a-IGZO) [13]. However, introduction of high-density carriers (e.g., more than 1020 cm 3) became very difficult in the films with higher gallium content. This indicates that high mobility is not easily obtained in a-IGZO films with high gallium contents if one tries to dope carriers by impurity doping or by introducing oxygen vacancies. This feature is unfavorable for TCO applications, but it would not be a disadvantage for semiconductor device applications because the difficulty in the carrier doping by oxygen vacancies suggests better controllability and stability of carrier concentration, especially at low concentrations. Even if high-density doping is difficult, by choosing the deposition condition, one can still induce high-density carriers by an external electric field if TFT structures are employed, which may make it possible to utilize the potential high mobilities that may be available at high carrier concentrations. > Figure 9 compares the tail state density in a-IGZO with that in a-Si:H [12]. The tail state density around the CBM controlling N-channel mobility in a-IGZO is lower by 3 orders of magnitude than that in a-Si:H. Such a low tail state density makes it possible to upshift the Fermi level to the CBM, which induces the band conduction, giving a high mobility comparable to that in the crystal. So far, no amorphous semiconductors in which band conduction occurs have been found, except for TAOS. Hall mobilities greater than 30 cm2·V 1·s 1 are obtained in In2O3–ZnO (a-IZO) systems [14], which are used as amorphous TCOs. However, controllability of carrier concentration and stability of low carrier state are not satisfactory in a-IZO films.

737

5.3.4

Oxide TFTs 1023 1022

a-IGZO

a-Si:H

021

1019

DOS (cm–2 eV–1)

738

020

Dit = 0.9 × 1011 cm–2/eV

1018

019 EP

018

(Depletion)

1017

017 1016

(Enhancement) ECBM EF @Vg = 0

1015 0.0

0.5

1.0

1.5

016 015 2.0

2.5

Energy (eV)

3.0

EF 014 –0.5

0

ECBM 0.5

Energy E (eV)

. Fig. 9 Tail state density in amorphous IGZO (a-IGZO) estimated from TFT characteristics. The estimation is valid for an energy range below approximately 1 eV from the CBM. For comparison, data on a-Si:H are shown. DOS density of states

4

Fabrication of TAOS TFTs

TFTs are fundamental building blocks for state-of-the-art microelectronics such as FPDs and system-on-glass. Furthermore, fabricating low-temperature TFTs will allow flexible large-area electronic devices to be developed. These devices are flexible, lightweight, shock resistant, and potentially affordable, which are inevitable for large, economic, high-resolution displays, wearable computers, and paper displays. Further, when combined with ‘‘transparent circuit technology,’’ TFTs can integrate display functions even on windshields of automobiles. Hydrogenated amorphous silicon (a-Si:H) and organic semiconductors are most extensively investigated for flexible electronics and their use to fabricate flexible solar cells and TFTs has been demonstrated. However, device performance is limited by the low mobilities of the channel materials, e.g., field-effect mobilities, meff, are less than 1 cm2 V 1·s 1 for a-Si:H and approximately 0.5 cm2·V 1·s 1 for pentacene TFTs. In addition, silicon-based devices are of less interest for transparent circuits since they are not transparent owing to the narrow band gap. In contrast, degenerate band conduction and large mobility (more than 10 cm2·V 1·s 1) are possible in AOSs that are composed of PTM cations as described in the previous section. In this section, fabrication of a-IGZO TFTs is described. A-IGZO films were prepared by a pulsed laser deposition with a KrF excimer laser or conventional sputtering techniques using a polycrystalline target of InGaZnO4 or an appropriate compound in this ternary system at room temperature in an oxygen atmosphere. Since the resulting thin film is amorphous, thin film fabrication processes are not critical compared with process parameters such as partial oxygen pressure during deposition and postannealing treatments. These characteristics provide a large allowance in the device fabrication process. TFTs were fabricated on glass

Oxide TFTs

Drain a-IGZO SiO2 n+-Si

Drain current, IDS (mA)

0.2 VGS (V) 0.15

5.0 4.5 4.0

0.1

3.5

0.05

10–3 VDS = 2V

Drain current, IDS (mA)

Gate

10–3

Source

10–5

10–5

10–7

10–7

10–9

10–9

10–11

10–11

Leakeage current, IG (A)

Gate width/length = 300/50 μm, Thickness : SiO2 = 100 nm, a-IGZO = 30 nm

5.3.4

3.0 2.5 2.0

0 0

–2

0 2 4 6 Gate voltage, VGS (V)

6 8 2 4 Drain voltage, VDS (V)

. Fig. 10 Performance of an a-IGZO TFT fabricated on a glass substrate

substrates. > Figure 10 shows a typical drain–source current (IDS)–voltage (VDS) characteristic measured at various gate biases (VGS) at room temperature in air. The saturation regime fieldeffect mobility (msat) was estimated to be approximately 13 cm2·V 1·s 1, which is almost the same as that obtained from the linear region (mlin  9 cm2·V 1·s 1 at VDS = 2 V). The transfer characteristic (IDS versus VGS) showed a low off current of less than 10 10 A. IDS was modulated by approximately 6 orders of magnitude by the applied VGS of approximately 6 V. A key technical issue for practical application is the instability under the device operation condition. In driving FFDs, one mostly applies negative bias under light. Thus, the instability of TFTs under bias in light illumination is most important. > Figure 11 shows changes in the transfer curves of an amorphous Hf–In–Zn TFT subjected to negative-bias stress ( 20V) and with no light and both negative-bias stress and white light (1,000 cd m 2) [15]. A large negative shift of the threshold voltage and increase of the off current are conspicuous for the TFTs subjected to both negative-bias stress and light illumination. Such an observation was commonly reported for TAOS TFTs including a-IGZO and polycrystalline ZnO. Recent research clarified these changes can be largely reduced by applying passivation of the backchannel such as through the use of dense Al2O3 [16, 17], implying that photoinduced desorption of atmospheric components such as O2 and H2O plays an essential role in these phenomena. The typical fabrication process of TAOS TFTs is illustrated in > Fig. 12. This structure, bottom gate and top contact type, is similar to that of a-Si:H TFTs. The thickness of the active layer is 30–50 nm. Exposure of the backchannel of the TAOS layer to hydrogen-bearing plasma must be strictly avoided because hydrogen impregnated in the TAOS serves as a donor by releasing an electron. For example, direct deposition of SiON by plasma-enhanced chemical vapor deposition, which is a conventional process in a-Si:H, makes the active layer conducting. This originates from the instability of H0 in the TAOS (this is also true for most TCOs such as ZnO) relative to H0 + (OH) and e-. The top contact is favorable for obtaining better Ohmic contact than the bottom contact because (1) oxidation of the metal contact by plasma does not occur during the deposition process of the TAOS, and (2) deposition of contact metals results in the carrier generation in the TAOS layer beneath the contact.

739

5.3.4

Oxide TFTs

Keithley 4200-SCS

Vg = –20 V SiOx (etch stopper)

InZnO

HflnZnO SiO2 InSnO Glass substrate

White light (1000cd/m2)

Bias only 10–4

10–6

10–6

10–8

0h 2h 4h 6h 8h 10 h 12 h

10–10 10–12

a

Bias + light

10–4

10–14 –30 –20 –10

0

10

20

Id (A)

Id (A)

740

10–8

0h 2h 4h 6h 8h 10 h 12 h

10–10 10–12

30

b

Vg (V)

10–14 –30 –20 –10

0

10

20

30

Vg (V)

. Fig. 11 Changes in transfer curves of an amorphous HfInZnO TFT with voltage bias and light illumination. The measurement scheme is at the top

5

Unique Features of TAOS TFTs

TAOS TFTs have three unique characteristics compared with other TFTs. First is high field mobility greater than 10 cm2 (V s) 1. Second is easy fabrication at low temperature using conventional DC sputtering. The last is a large process allowance. The TFTs fabricated at unoptimized conditions exhibit poor performance, but the TFT performance can be much improved to that of TFTs prepared under the optimized condition just by annealing at an appropriate temperature far below the crystallization temperature of the TAOS. > Figure 13 shows an example of a-IZGO TFTs showing the effectiveness of postannealing to improve the TFT performance. The annealing temperature is 250–300 C, which is much lower than the crystallization temperature (above 500 C). No distinct structural change around each metal

Oxide TFTs

5.3.4

I. Fabrication of gate electrode (1st Photo Engraving Proces (PEP)) Mo/Ta, A1:Nd (sputter + etching) Mo/Ta, AI:Nd etc.

II. Formation of gate insulator and a-IGZO channel layer SiO2 (sputter or plasma CVD), a-IGZO (sputtering) a-IGZO channel layer

SiO2, SiNx, etc.

III. Patterning of a-IGZO channel layer (2nd PEP)

IV. Formation of source/drain contacts (3rd PEP) Cr/Mo (sputter + etching) Source : Cr/Mo etc. Drain : Cr/Mo etc.

V. Formation of passivation layer (4th PEP) SiO2/SiNx (sputter or plasma CVD + dry etching) SiNX

Passivation : SiO2

VI. Formation of pixel electrode (5th PEP) ITO (sputter + etching) Pixel electrode

. Fig. 12 Typical transparent amorphous oxide semiconductor TFT fabrication process. CVD chemical vapor deposition, ITO indium tin oxide

cation was noted before and after annealing. Pronounced annealing effects are observed commonly for TAOS TFTs [18, 19]. It is worth noting that the TFT performance is practically determined by this postannealing treatment. This is similar to the case of amorphous metal and provides a large process window for deposition. Humidity in the annealing atmosphere enhances the improvement of performance of the TFT devices [20]. > Figure 14 shows the performance histograms of a-IGZO TFTs which were fabricated on a glass substrate by a conventional sputtering and subsequently annealed [21]. About 100 TFTs were fabricated from a 1 cm  1 cm area of a-IGZO thin film. The TFT exhibits excellent uniformity and high average performance. The value of msat is around 0.5 cm2 (V s) 1 and

741

5.3.4

Oxide TFTs

Low quality

Drain current, IDS (A)

High quality as-deposited 10

as-deposited

300°C annealed

s >1 V/decade μsat < 1 cm2(Vs)–1

–4

10–6 s ∼0.2 V/decade μsat ∼8 cm2(Vs)–1

10–8

s ∼0.2 V/decade μsat ∼7 cm2 (Vs)–1

10–10 VDS = 4V

10–12

–5

0

5

10

15

VDS = 4V

VDS = 4V

5 10 0 –5 Gate voltage, Gvs (V)

–5

0

5

10

15

10 9 8

Frequency

742

Unannealed Dry annealed Wet annealed

Unannealed Dry annealed Wet annealed

Vth

Unannealed Dry annealed Wet annealed

μFE

S

7 6 5 4 3 2 1 0 –3 –2 –1 0

a

1

2

Vth (V)

3

4

5

6

2

b

4

6

8

10

μsat (cm2/(Vs))

12

14

100 200 300 400 500 600 700

c

S (mV/decade)

. Fig. 13 Effects of postannealing on the performance of a-IGZO TFTs. Top: Transfer curves of TFTs fabricated under optimized and unoptimized conditions. Annealing was performed in an ambient atmosphere. Bottom: Effect of various atmospheres during annealing

s is 0.11 cm2 (V s) 1 (0.76% of the average value), demonstrating the excellent uniformity of the a-IGZO TFTs. This strongly suggests that the a-IGZO TFTs essentially have good shortrange uniformity and are advantageous in integrated circuits and large areas. Short-channel TFTs are needed for integrated circuits. It was reported by Samsung in 2008 that a-IGZO TFTs keep almost the same mobilities, on/off ratios, threshold voltages, and subthreshold slopes when the channel length is reduced down to 50 nm [22]. These results indicate that the a-IGZO TFT is a candidate for a selection transistor in 3D cross-point stacking memory. > Table 2 shows a comparison of TAOS TFTs with a-Si:H and polycrystalline silicon TFTs as summarized by Jeong et al. [23] in 2008. It is obvious that a-IGZO TFTs meet the requirements of high mobility, homogeneity, and low cost. The major technical issue was instability underbiasing. TAOS TFTs are sensitive to the light corresponding to the subband gap and two performance changes are induced, positive shift of the threshold voltage and increase in off current (mobility and subthreshold slope remain unchanged). For example, such a change is induced by illumination with light of wavelength shorter than 460 nm [24]. The magnitude of the shift of the threshold voltage depends on the intensity and wavelength of the light. The extent is rather smaller than that in a-Si:H but is larger than that in polycrystalline silicon TFTs.

Oxide TFTs

80 2

80

–1 –1

60

Vthave. = 2.25 (V)

–1 –1

σ : 0.11 (cm V s )

Frequency

Frequency

μsat ave. = 14.55 (cm V s ) 2

40 20

a

0 14.2 14.4 14.6 14.8 15.0 15.2 Saturation mobility, μsat (cm2V–1s–1)

20 2.0 2.2 2.4 2.6 2.8 Threshold voltage, Vth (V)

80 Frequency

Frequency

c

σ : 0.13 (V)

40

b

Von ave. = –0.37 (V) σ : 0.13 (V)

40 20 0 –1

60

0 1.8

80 60

5.3.4

–0.8 –0.6 –0.4 –0.2 Turn-on voltage, Von (V)

0

d

60

s ave. = 0.197 (V decade–1) decade–1)

σ : 0.006 (v

40 20 0 0.14 0.16 0.18 0.20 0.22 0.24 Sub-threshold swing (V decade–1)

. Fig. 14 Histogram of device performance in 97 a-IGZO TFTs fabricated on a 1-cm2 area

6

Progress in TAOS TFTs as the FPD Backplane

TAOS TFTs have several unique features as follows: (1) high electron mobility ranging from 10 to 30 cm2 (V s) 1 depending on the material systems, (2) capable of being produced by conventional sputtering at low temperatures, (3) controllable performance by postannealing, (4) optically transparent, (5) only n-channel operation, no inversion occurs due to high tail and defect state density above the VBM, (6) a large material selection range for the gate insulator owing to feature 4, and (7) no short channel effect down to 50 nm. Utilizing these advantages, extensive studies are continuing on the application of TAOS TFTs s to the backplane for FPDs as well as novel display structures.

6.1

Novel Display Structure

An innovative electronic paper display structure called ‘‘front drive’’ type was recently proposed by Ito et al. [25] of Toppan Printing. Alignment of the TFTarray to the color filter array is a troublesome process in display assembly because a-Si:H is nontransparent and there is variation of the substrate dimension with aging. Their idea to avoid this difficulty was to directly deposit the TAOS TFT arrays on the color filter arrays utilizing the low-temperature process and optical transparency simultaneously. This is the first demonstration of a device structure benefiting from optical transparency of a TAOS. > Figure 15 shows the front-drive structure applied to an electronic paper based on electronic ink imaging film. Taking advantage

743

744

5.3.4

Oxide TFTs

. Table 2 Comparison of thin-film-transistor (TFT) technology including transparent amorphous oxide semiconductors (TAOS) and silicon (Taken from JK Jeong, Samsung SD/Inha University, MC Sung, LG Electr. 2nd oxide TFT Workshop (2007), IMID 2009)

Generation

a-Si:H TFT

Poly-Si TFT

Amorphous oxide TFT

8–10G

4G(LTPS)/8G(HTPS)?

1G/8G(?)

Channel

a-Si:H

ELA LTPS/SPC HTPS

a-InGaZnO4

TFT mask steps

4–5 masks

5–9(11) masks

4–5 masks

30 V

Figure 17 shows a photograph of sputtering targets (crystalline In2O3–Ga2O3–ZnO) for a G8-sized glass substrate. We anticipate the mass production of various FPD panels driven by this backplane in late 2010 or 2011. The next technical challenge in TAOS TFTs will be the development of nonvacuum processes [26]. Although research on this subject is rapidly increasing, examination of device reliability has not been performed yet. The ultimate goal of this research is to fabricate highperformance TFTs on plastics or polymer-coated papers leading to flexible or paper displays. Invention of innovative material processing beyond the traditional sol–gel or solution process would be needed. Another technical challenge in oxide TFTs is to realize a CMOS. TOSs were born from TCOs. The requirements for TOSs differ from those for the TCOs. Control of carrier concentration and carrier type is essentially important for the former. The current status of TOSs is far from this ideal situation in particular for carrier polarity control. Although many papers have reported on p-type TOSs, including p-ZnO, no p-channel TFT with a field-effect mobility of more than 0.1 cm2 (V s) 1 has been realized to 2007. It is considered that instability and/or

Oxide TFTs

5.3.4

. Fig. 17 Photograph of a ceramic target for sputtering a-IGZO thin films on an eighth-generation glass substrate

high gap state density are the primary origin. For example, Cu2O is a well know p-type semiconductor and has attracted interest as the active layer since the first TFT patent proposed by Heil in 1935. Matsuzaki et al. fabricated epitaxial thin films and obtained Hall mobility of approximately 100 cm2 (V s) 1 at a hole concentration of approximately 1013 cm 3, which is comparable to that in single-crystalline Cu2O. However, Cu2O-based TFTs did not operate well enough and the estimated field-effect mobility remained approximately 0.1 [27]. This striking difference comes from large tail state densities. Such a situation appears to be similar to that for other p-type oxide semiconductors. In 2008, Ogo et al. [28] reported a p-channel TFT with a mobility of 1.4cm2 (V s) 1 employing SnO (not SnO2) as the active layer. This is the first demonstration of a p-channel oxide TFT with a mobility greater than 1cm2 (V s) 1, which was a long-standing target in this area. The next goal is fabrication of a CMOS by combining p-channel and n-channel oxide TFTs. Although a monopolar channel is sufficient for TFTs for the backplane of displays, CMOS is applicable for logic circuits. Exploiting bipolar semiconductive oxides with low tail state densities which can be fabricated at low temperatures is the most essential work for this goal. Oxide semiconductors are easy to fabricate by conventional sputtering and are robust to oxygen and radiation, in general. If an oxide-based CMOS structure can be fabricated on various types of substrates, including plastics, flexible electronic circuits would be a promising possibility. Of course, the formation of a heterojunction between a TOS and an organic semiconductor is a practical and promising way to active device application [29] such as a photosensor, a CMOS, and a solar cell.

747

748

5.3.4

Oxide TFTs

References 1. Hosono H (2007) Recent progress in transparent oxide semiconductors: Materials and device application. Thin Solid Films 515:6000–6014 2. Nomura K, Ohta H, Takagi A, Kamiya, Hirano M, and Hosono H (2004) Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 432:488–492 3. Hosono H, Yasukawa M, Kawazoe H (1996) Novel oxide amorphous semiconductors: transparent conducting amorphous oxides. J Non-Cryst Solids 203:334–344 4. Hosono H (2006) Ionic amorphous oxide semiconductors: Material design, carrier transport, and device application. J Non-Cryst Solids 352:851–858 5. Orita M, Ohta H, Hirano M, Narushima S, Hosono H (2001) Amorphous transparent conductive oxide, InGaO3(ZnO)m (m Chap. 5.4.2 and as semiconductor in thin film transistors. The latter will be discussed in this chapter. On-panel display electronics are still predominantly realized in amorphous Silicon. Thin film transistors (TFT) produced by this technology exhibit a rather low charge carrier mobility in the order of 1 cm2/Vs. This prevents the desired complete integration of driver electronics on the display. It has been shown by Fuhrer et al. that individual carbon nanotubes can have charge carrier mobilities up to 79,000 cm2/Vs, exceeding the values of all known semiconductors [6]. In another publication on/off current ratios reaching up to 107 were reported [7], making

Carbon Nanotube TFTs

5.3.5

CNT-TFTs also well suited as switches in active matrix (AM) displays. Another big advantage is the possibility of solution processing, enabling lower cost production and application to flexible plastic substrates as it is also propagated for organic semiconductors. The latter suffer however from similar and mostly even lower charge carrier mobilities than amorphous silicon and show other drawbacks like degradation under ambient conditions and the necessity of high driving fields for a sufficient gating effect. Carbon nanotubes in contrast have a chemically very stable structure, need only low driving fields and are on top of that mechanically flexible and compatible to plastic substrates. All of these intrinsic properties make carbon nanotubes a very promising candidate as semiconductor especially for flexible displays. The challenges lie in utilizing the intrinsic properties of such nano-molecules in micron sized devices. Despite the fast progress in recent years in developing high-performance CNT-TFTs, there are still some issues to overcome before a reliable integration into a display process is feasible. In the following sections, the current status of CNT-TFT research related to possible display applications will be presented, starting with the necessary basic knowledge about structure and electronic properties of carbon nanotubes, followed by the discussion of the key techniques of material preparation, separation of metallic from semiconducting nanotubes, deposition, and the realization of actual TFT devices including transport through nanotube networks and contact formation with metals. The chapter will close with a summary and prospects for future research.

2

Structure and Electronic Properties of Carbon Nanotubes

To better understand the challenges that one has to face when trying to utilize carbon nanotubes from an engineering point of view, it is necessary to have a basic understanding of the structure and electronic properties of single walled CNTs, the basic building block of any CNT device. The structure of a SWNT is best described by looking at a mono layer of graphite also called graphene (see > Fig. 1a). In graphene, the sp2 hybridized carbon atoms form a wellordered hexagonal honeycomb lattice. The sp2 hybrid orbitals form strong and planar s-bonds under an angle of 120
leading to the hexagonal structure. The distance between two C atoms is

Ch q

a2 a1

a

b

. Fig. 1 (a) Graphene lattice with basis vectors ~ a1 and ~ a2 , chiral vector ~ Ch and chiral angle u for a (6,3) SWNT. (b) Single walled carbon nanotubes, left: zigzag, center: armchair, right: chiral (Copyright [8])

753

5.3.5

Carbon Nanotube TFTs

ac–c = 1.42 A˚. The pz orbitals standing perpendicular to this layer form a delocalized p-electron system causing, for example, the good electrical conductivity of graphite along the layer direction. A carbon nanotube can be thought of as a graphene sheet rolled up in a way that two crystallographically identical C atoms overlap. The vector connecting these two atoms in a1  m~ a2, where ~ a1 and ~ a2 are the basis vectors the graphene lattice is called chiral vector ~ Ch ¼ n~ of the graphene lattice and n and m are integers. ~ Ch forms the circumference of the nanotube and the specification of the 2-tuple (n, m) is adequate to fully describe the structure of each CNT. In the example in > Fig. 1a, the graphene lattice would be cut along the dashed lines and a1 is called rolled up in a way that the cutting edges would touch. The angle y between ~ Ch and ~ chiral angle and can take values between 0
and 30
. Three distinctive types can be characterized. All tubes with (n, 0) and y = 0
are called ‘‘zigzag’’; tubes with (n, n) and y = 30
are called ‘‘armchair.’’ Both names coming from the structure of the C–C bonds along the chiral vector (see ends of the tubes in > Fig. 1b). All tubes with 0
< y < 30
form the class of ‘‘chiral’’ tubes. Images of all three types are shown in > Fig. 1b. An infinite sheet of graphene is a 2D zero band gap semiconductor with a continuous distribution of density of states (DOS) above and below the Fermi energy Ef. Using the zonefolding method and periodic boundary conditions in the circumferential direction, the DOS for the quasi-1D nanotubes can be derived leading to characteristic van Hove singularities as in any 1D electronic system (see > Fig. 2) [9]. The plot of DOS over carrier energy shows the number of states at each energy level that can be occupied by a charge carrier. For a semiconducting SWNT, there is a gap at the Fermi level (E = 0 eV) where no states are allowed (see > Fig. 2a). This region marks the band gap that is also symbolized in common band diagrams, while the borders where states are allowed again stand for the beginning of the conduction and valence band. Metallic SWNTs on the other hand have a limited number of states at the Fermi level (see > Fig. 2b). Whether a SWNT is metallic or semiconducting is determined by its chiral angle and diameter dt. The condition for a nanotube to be metallic is 2n + m = 3q where q is an integer [9] and can be found by plotting the allowed wave vectors in the nanotube onto the Brillouin zone of graphene (Chap. 3.2 of [10]). From this condition, it can easily be seen that for a continuous diameter distribution 1/3 of the SWNTs show metallic behavior. All other nanotubes are semiconducting with an increasing band gap for decreasing diameters (Eg  0.7 eV/dt [nm]) [5]. The zone-folding method only takes the periodic

1

a

1 DOS [states/1C-atom/eV]

DOS [states/1C-atom/eV]

754

0.8 0.6 0.4 0.2 0 −3

−2

−1

0 E [eV]

1

2

3

b

0.8 0.6 0.4 0.2 0 −3

−2

−1

0 E [eV]

1

2

. Fig. 2 Density of states for (a) semiconducting (10,0) and (b) metallic (9,0) carbon nanotube (Data courtesy of S. Maruyama [12])

3

Carbon Nanotube TFTs

5.3.5

boundary conditions into account, leading to identical properties for a nanotube and the flat nanoribbon that one would get if the nanotube was sliced open along its length. For smaller diameters the strain on the C–C bonds can no longer be neglected. Small diameter ( Sect. 5. Some of them use the same processes as are presented in the following section about nanotube synthesis.

3.1

Synthesis of Carbon Nanotubes

There are three primary methods to produce carbon nanotubes: arc-discharge, laser-ablation, and chemical vapor deposition (CVD). In all three cases, transition metal catalysts like Fe, Co, Mo, Ni, or Y are necessary to get mostly SWNTs. The relatively simple and cheap setup of the arc-discharge method enabled lots of groups worldwide to produce their own CNTs leading to a huge increase in nanotube research. Between two carbon electrodes, a DC plasma is generated by an arc-discharge in a certain gas atmosphere. The anode, containing small amounts of the metal catalyst, is being consumed while the nanotubes condensate at the cathode [13]. In the laser-ablation method, the carbon target including the metal catalyst is evaporated by a high power laser in a furnace under 1,200
C in an inert gas flow. The nanotubes are collected at a cooled surface outside the furnace [14]. With both methods the nanotubes are collected as powder with purities of 70–90%, the rest being amorphous carbon or other unwanted carbon molecules and catalyst clusters. A controlled synthesis on a substrate surface or a directed deposition is not possible. Due to strong van der Waals forces the nanotubes form ropes or bundles consisting of several tens to hundreds of single tubes. The formation of these bundles is a major issue for subsequent processing since in most applications and especially as semiconductor in TFTs, well-separated individual SWNTs are wanted. Mass production with both techniques seems unlikely because of high power consumption and scaling issues. In the CVD approach, some kind of gaseous carbon feedstock is decomposed under temperatures between 700
C and 1,200
C. Typical carbon sources are hydrocarbons like methane or carbon monoxide (CO). The metal catalysts in the form of nanoparticles can be predeposited on the substrate or they might be formed during the CVD for example by the decomposition of an organometallic species like in the HiPCO process [15, 16]. HiPCO stands for high-pressure catalytic decomposition of carbon monoxide and is a promising candidate when it comes to high volume mass production. With various choices in carbon source, catalyst, temperature, gas atmosphere, and reactor design there are many flavors of CVD synthesis. So far the most promising besides HiPCO are methane CVD, CO CVD, alcohol CVD, and plasma-enhanced CVD (PECVD) [5].

755

756

5.3.5

Carbon Nanotube TFTs

Advantages of the CVD processes are direct deposition or growth of well-separated SWNTs on the substrate as will be discussed in > Sect. 5.1 where deposition techniques are presented. By choosing the right parameters and substrates alignment and position selective growth can be achieved. The drawback however is the limited choice of substrate materials due to the high reactor temperatures. Successful growth at lower temperatures was reported by several groups. The lower temperatures are however also a cause for higher impurity or defect rates. Using the advantages of direct growth on display grade glass or even plastic substrates stays challenging. Transfer techniques might be a way out of this dilemma. A highly appreciated milestone in CNT synthesis would be the growth of a single (n, m) species or at least of a certain electronic type. By varying synthesis parameters, catalyst material, and carbon feedstock, researchers were able to get better control over the SWNT diameter [17]. Since with smaller diameters ( Fig. 3a. The mixed suspension flows through the microfluidic channel, passing a displaced electrode array. The m-SWNTs are attracted by the electrical field and are drawn into a parallel stream of a buffer solution. The parallel streams are separated afterwards leading an all metallic suspension and a mixture of s-SWNTs and residual m-SWNTs. The most promising approach judging by its commercialization in 2007 uses ultracentrifugation in a density gradient medium. The technique allows to sort the species injected into the density gradient medium by its buoyant density during ultracentrifugation. It was first used to sort SWNTs by their diameter [44]. Using mixtures of two surfactants, Hersam et al. managed to separate SWNTs by their electronic type with purities up to 99% [55]. The selectivity comes from the differences in polarizability of m-SWNT and s-SWNT and the two surfactants that competitively adsorb to the SWNT surface, leading to different buoyant densities for different species. In order to receive high purity samples, several iterations and starting material with a limited range of (n, m) as achieved by the CoMoCAT method [56] are

Carbon Nanotube TFTs

5.3.5

Vm − DEP Vm − total

Buffer Solution

Vfluid

Vfluid

Sample P

Vs − total

Sample M

Sample S

a

b . Fig. 3 Sorting of SWNTs by electronic type. (a) By dielectrophoresis in a microfluidic channel; Metallic SWNTs (red rods) are subjected to a significantly larger dielectrophoretic force, perpendicular to the direction of the flow, than semiconducting SWNTs (blue rods). Reprinted with permission from [54]. (Copyright 2008 American Chemical Society), (b) By ultracentrifugation in a nonlinear density gradient medium; Image of a centrifuge tube containing HiPCO SWNTs sorted by one 18-h run at 268,000 g. The distinct colored bands are layers enriched in different SWNT species. The near-infrared absorbance spectra of the marked colored layers, show the main (n, m) component. Spectra are normalized and offset for clarity. The unsorted HiPCO spectrum is scaled down by a factor of 10 (Reprinted by permission from Macmillan Publishers Ltd: Nature Nanotechnology [57], Copyright 2010)

necessary. Ghosh et al. reported a drastically improved efficiency by using tailored nonlinear density gradients. With this improvement, they were able to sort highly polydisperse HiPCO material into individual (n, m) fractions in a single step (see > Fig. 3b) [57].

759

760

5.3.5 5

Carbon Nanotube TFTs

Deposition Techniques

By contacting individual SWNTs with metal electrodes, the outstanding performance of CNT transistors was experimentally demonstrated quite early. One of the main challenges of successfully introducing CNT-TFTs into production processes is finding a reliable deposition technique. While single nanotube devices are indispensable for fundamental research, they are not well suited for applications like switching or driving TFTs in active matrices or on-panel electronics in general for the following reasons: (1) the lack of identical CNTs in terms of electronic type, length, and diameter; (2) the limited current output; and (3) the challenging deposition of individual nanotubes in each TFT channel [58]. Therefore, random networks or aligned arrays are propagated for CNT-TFTs. This allows for an averaging of the electrical properties and high current throughput enabled by many parallel current paths. Several groups demonstrated well-working TFTs with randomly oriented carbon nanotube networks (CNN) even with multidisperse CNT feedstock. Ideally, the TFT channel should however consist of more or less identical, purely semiconducting SWNTs that directly connect the drain and source contacts. Several reviews concentrate on the oriented assembly of carbon nanotubes in general [5, 59, 60]. In this section, an overview of promising deposition techniques for CNT-TFTs in display applications will be given, categorized in direct growth methods and deposition from solution. Also methods for achieving an alignment after the deposition were presented in recent years. Kocabas et al. used linearly polarized laser pulses to ablate nanotubes in the polarization direction. By rotation of the polarization all unwanted nanotubes can be eliminated. This method seems however unfeasible for TFT applications with the aim of high density aligned arrays [61].

5.1

Direct Growth

The CVD methods discussed in the section about SWNT synthesis (> Sect. 3.1) can also be used to directly grow SWNTs on the substrate. This eliminates the drawbacks of solution processing like damaging or shortening of CNTs by strong acid treatment and intensive sonication, contamination with solvents or dispersants, and formation of bundles. The CVD methods also often produce longer average tube lengths and nanotubes with less molecular imperfections [58] The direct growth also allows good control over alignment, length, and density of the nanotubes. > Figure 4 shows several types of CNT layers produced by different direct growth processes. Several methods for alignment control were experimentally demonstrated in recent years. Including alignment by electric [62] or magnetic [63] fields as well as directed gas flow in the CVD system [64]. Recently several groups demonstrated almost perfect alignment using patterned catalysts on single-crystalline quartz substrates [65] (see also > Fig. 4d from the work of Cao et al. [58]). They have also presented theoretic calculations showing that the alignment is caused by angledependent van der Waals interactions with the substrate [66]. Ding et al. have managed to combine this method of alignment with highly enriched growth of semiconducting SWNTs. They claim purities of more than 95% by introducing methanol in the growth process [67]. While other selective growth methods are limited to small nanotube diameters, Ding et al. report diameter distributions from 1.6 to 1.8 nm, stating that the selective growth is not only caused by the methanol, but also the interaction with the quartz lattice. This remarkable

Carbon Nanotube TFTs

5.3.5

. Fig. 4 SEM images of SWNT thin films grown by CVD: (a) high D SWNT film grown with ethanol as the feed gas, and Fe/Co/Mo catalysts on silica supports; (b) moderate D SWNT film grown with methane as feed gas and Fe nanoparticle catalyst; (c) partially aligned SWNT film grown on singlecrystalline ST-cut quartz substrate; (d) perfectly aligned SWNT arrays grown with Fe catalyst patterned into 10 mm wide strips (bright horizontal lines at top/bottom edges of the image) on a similar quartz substrate (Reprinted with permission from [58])

improvement might lead to the elimination of one of the biggest disadvantages of direct growth methods. While having good control over the topological conditions, the total elimination of m-SWNTs was so far only possible by post-deposition methods. Another major flaw of direct growth methods is the limitation in substrate materials. Deposition of aligned CNT arrays on display grade glass or plastic substrates is so far unfeasible due to process temperatures around 900
C and the need for a specially cut single-crystalline surface. Several groups have however presented transfer methods to print patterned randomly oriented CNNs and aligned CNT arrays on plastic substrates [68–71]. Im et al. presented a further printing technique where CNTs are grown as vertical forests on a silicon wafer and are then transferred by pressing this wafer on the target substrate and sliding it along the desired CNT alignment direction [72]. Although these printing techniques allow the use of high-quality and well-aligned CNT arrays on the desired substrates, the precondition of using defect free silicon or quartz wafers for the growth as well as necessary alignment techniques might be significant hurdles for scaling and cost-effective high volume production. A special case of direct growth but deposition at room temperature presented by the Kauppinen group allows for fabrication of CNNs on glass and plastic substrates [73]. With this technique the SWNTs grown with a floating catalyst method are transferred from the

761

762

5.3.5

Carbon Nanotube TFTs

reactor to the substrate as aerosol. The density of the deposition can be controlled by electrical fields. An aligned deposition or enrichment in s-SWNTs was not presented so far. Scaling to large area applications also seems difficult.

5.2

Solution Deposition

Besides the already mentioned disadvantages of solution deposition, there are several benefits compared to deposition during synthesis. Because of vacuum-free deposition the costs for investment in equipment and the fabrication can be quite low even for large area tools. The room-temperature processing also allows for a great variety of substrate materials. While in direct growth methods the synthesis and the deposition are directly linked, the deposition from solution enables one to use diverse materials that are processed with state-of-the-art dispersing, cleaning, and sorting methods. One of the simplest deposition techniques is immersing surface treated substrates into surfactant aided CNT suspensions [74]. For this and other deposition methods, self-assembled monolayers of amine-terminated silanes are often used as adhesion promoter. For the deposition of transparent conducting films (TCF) from CNTs, spray coating is an often used method (see > Chap. 5.4.2) [75, 76]. Larger droplets can however lead to an increased density at the edge of the dried droplet and formation of bundles. The ultrasonic spraying presented by Tenent et al. might give better results [77]. Ink-jet printing was also demonstrated using an aerosol jet [78]. This method also allows for patterned deposition. A control of alignment is impossible with all three deposition methods, though. Self-assembly techniques that allow control over the alignment as well as patterned deposition are possible by selective and patterned chemical [79, 80] or biological [81–84] treatment of the substrate surface prior to deposition of the nanotubes. Xiong et al. used surface treated flexible polymer substrates with patterned photoresist to achieve aligned and patterned deposition. The substrate is immersed in the nanotube solution and by controlled pulling the nanotubes align parallel to the pulling direction due to friction at the substrate surface [85]. Similar to liquid crystal molecules, carbon nanotubes tend to align parallel to each other in some liquid phases. Several methods use this alignment and transfer the aligned nanotubes by contacting the liquid phase with the substrate surface. Examples are bubble blown films [86] where soap bubbles are used as transfer medium (see > Fig. 5a) or Langmuir-Blodgett films [87–89] (see > Fig. 5b). For Langmuir-Blodgett film deposition the nanotubes need however to be functionalized or encapsulated. Similar to these methods parallel alignment can be achieved by evaporation of the surfactant solution [90–92]. In the meniscus formed at the interface between substrate and suspension, the nanotubes align parallel to the liquid front line leading to high density arrays. > Figure 5c shows an image of aligned nanotubes produced by this technique. CNTs dispersed in lyotropic nematic suspensions can be aligned by shearing between two substrates and subsequent drying of the liquid crystal [93]. Alignment by directed gas flow over a suspension coated surface was also demonstrated [94, 95]. In thin film technology, spin coating is often used for the deposition of organic materials. It is also used for CNT deposition from surfactant solutions [96, 97]. Since the micelle surrounding the individual nanotubes prevents direct contact of the CNT with the substrate, only few nanotubes would be deposited by standard spin coating. Meitl et al. introduced a second stream of an organic solvent that is mixed with the suspension stream shortly above the spinning substrate and removes the micelle. The interaction with the substrate leads to

Carbon Nanotube TFTs

a

5.3.5

b

d

c . Fig. 5 Aligned arrays of carbon nanotubes deposited from solution by (a) bubble blown films (inset shows nanotubes on a wafer), (b) Langmuir-Blodgett films, (c) evaporation and (d) dielectrophoresis. Reprinted with permission from (a) Macmillan Publishers Ltd: Nature Nanotechnology [86], Copyright 2007, (b) [89] Copyright 2007 American Chemical Society, (c) [92] Copyright 2007 American Chemical Society, and (d) [104] Copyright 2009, American Institute of Physics

a certain degree of alignment in radial direction [96]. Bao et al. additionally introduced sorting of electronic type into the spin coating process by using different kinds of surface functionalizations [98, 99]. As already mentioned in the section about nondestructive sorting, dielectrophoresis can also be used as deposition technique. The nanotubes, polarized by the electric field, do not only feel an attractive force toward higher field strengths causing the deposition, the polarization also leads to an alignment of the nanotube axis parallel to the streamlines of the field regardless of the field direction [45, 100] (see > Fig. 5d). Like for the attractive force, the rotating force is much higher for m-SWNTs than for s-SWNTs. The method was used to deposit individual nanotubes between thin electrode fingers [101–103], as well as large nanotube arrays [104] as can be seen in > Fig. 5d.

763

764

5.3.5 6

Carbon Nanotube TFTs

Implementation in Thin Film Transistors

After having discussed basic facts about carbon nanotubes, their synthesis, purification, dispersing, sorting, and deposition techniques, this section will cover the actual realization of carbon nanotube transistors. These are usually realized as thin film transistors in a bottomgate architecture. As with other semiconducting materials highly doped silicon wafers with thermally grown silicon oxide as gate dielectric are easy to use for first realizations of devices. But also patterned metal gate fingers with inorganic or organic dielectrics are often used especially on glass and flexible substrates. Even high density CNNs were presented as gate. The semiconducting CNT channel consisting of a randomly oriented network or an aligned array is usually contacted by metallic top contacts. A schematic diagram of such a bottom-gate top-contact network TFT is shown in > Fig. 6a. Bottom-contact architecture is also possible but a top-contact metal evaporated onto the nanotubes gives a larger coverage and therefore a better contact. Instead of metal contacts high density CNNs can also be used as drain and source contacts as will be discussed in the next section. Top-gate devices were also presented since this is the only way of producing devices with highly aligned nanotube arrays grown on the surface of, for example, quartz wafers without the use of a transfer process [105]. As discussed before, single tube devices are albeit their unmatched charge carrier mobilities with values up to several 10.000 cm2/Vs [6, 106] not well suited for display applications due to limited current output and largely increased demands for homogeneous synthesis and large area assembly. The focus is therefore laid on thin film transistors made of randomly oriented networks or aligned arrays of carbon nanotubes. Starting with the contact to metal electrodes and continuing with a description of electronic transport in CNNs, other issues like doping and passivation methods will be discussed. At the end, some state-of-the-art devices will be presented.

6.1

Contacting Carbon Nanotubes with Metal Electrodes

Carbon nanotube transistors usually show p-type behavior. This is for one thing caused by p-doping of s-SWNTs by oxygen adsorption as will be discussed later, for another thing Schottky barriers (SB) at the metal-nanotube interfaced play an important role. High work Lc

SWNTs Drain

a

Substrate

S

D

Wc

Source

Oxidized gate metal

b

. Fig. 6 Carbon nanotube network thin film transistors. (a) Schematic of a bottom-gate top-contact TFT using oxidized aluminum as gate and dielectric (Reprinted from [117], Copyright 2007, with permission from Elsevier). (b) Percolating stick model

Carbon Nanotube TFTs

5.3.5

function metals like Pd, Au, and Ti have been widely used as source and drain contacts in CNT transistors. As with other semiconductors connected to a free electron metal, a Schottky barrier forms at the interface. The size of the barrier is mainly defined by the work function fm of the metal [107]. The width of the SB can be influenced by the gate bias. When Ef of the metal falls in the center of the nanotube bandgap, the barriers for electrons and holes are identical leading to ambipolar behavior. For negative gate bias hole conduction is increased, for positive bias electrons have a higher probability for crossing. Due to the large barriers, the on-current is however significantly reduced and large gate voltages are necessary. Metals with higher fm that brings Ef closer to the valence band of the s-SWNT give p-type devices while metals with lower fm and therefore Ef close to the conduction band make n-type devices. The better the matching is, the higher the on-current can become [108]. As Eg is diameter dependent, fm must be tailored to the used material and deviations in mixtures have to be accepted [7, 108]. Ohmic contacts in p-type transistors have been reported for Pd [109] and Rh [7] as well as Y and Sc in n-type transistors [110]. In such cases ballistic transport for short channel ( Fig. 6b) [112]. With aspect ratios in the order of L/dt = 1,000 and above. By using percolation theory a basic understanding of the conduction through a CNN with the classical mixture of 1/3 m-SWNTs can be gained [113–115]. In a typical TFT setup like demonstrated in > Fig. 6b with channel length LC and width WC, the nanotube network is contacted by the source and drain electrodes. In the case of LC > > L, the behavior of the CNN can be tailored by adjusting its density D [tubes/area]. In the following, four distinct regimes are characterized: (1) For very low values of D no conductive paths between source and drain exist, leading to an open device. (2) When D is just slightly above the percolation threshold of the s-SWNTs ps but due to the 1:2 ratio of metallic and semiconducting nanotubes below the percolation threshold of the m-SWNTs pm, individual current paths between source and drain exist that can be controlled by the gate bias. Since all current paths are effectively semiconducting, a low off-current Ioff can be achieved. Due to the small number of connections between source and drain the on-current Ion is limited, leading to a low on/off ratio (Ion/Ioff ). (3) With higher D, still below pm, Ion is increased, leading to larger on/off ratios. (4) For increasing densities with D > pm more and more purely metallic paths connect the source and drain contacts, leading to increasing Ioff and smaller Ion/Ioff up to a point where no gating effect is perceivable any more [116]. The charge carrier mobility in network and array devices is mostly extracted from transfer characteristics in the linear or saturation region by using the known equations from MOS fieldeffect transistors (FET) [97, 105]. In both cases, the parallel plate capacitance for the area WC  LC is usually used. Since the channel consists of individual current paths instead of a bulk semiconductor that fills the complete are, the calculated values do not reflect the intrinsic mobility in the SWNTs but give an effective device charge carrier mobility meff. It is however

765

5.3.5

Carbon Nanotube TFTs

a good indicator for comparing the TFT performance with other technologies that use the same device area. Obviously, since D can be quite different for devices with identical WC and LC, meff is also dependent on D. In regime (2) only a few current paths exist leading to a low meff. For increasing D in regimes (3) and (4) meff rises since more and more of the channel region is covered with current paths. The characteristic of Ion/Ioff vs. meff for network TFTs with varying D is demonstrated in > Fig. 7a. The trend in region (3) to higher values for the on/off ratio and meff for increasing D is indicated by the straight lines for different LC. The data scatter in the lower left area of the plot (low meff and Ion/Ioff ) caused by devices with LC = 5 mm shows that shorter channels are bridged more easily by metallic pathways. It was demonstrated by simulations that the shorting of the channel can be significantly decreased by going to smaller

106 105

(3)

Ion/Ioff

104 103

L = 5 um L = 20 um L = 50 um

(2)

102 (4)

101 100 0.01

a

0.1 1 dev ice charge carrier mobility [cm2/Vs]

10

106 10 s 20 s 25 s 30 s 35 s

105 104 Ion/Ioff

766

103 102 101 100 0.01

b

0.1

1

10

device charge carrier mobility [cm2/Vs]

. Fig. 7 Ion/Ioff over meff for CNT-TFTs with LC = 5,20,50 mm, WC = 50,100,200 mm. (a) Varying D with 1/3 m-SWNT content. Numbers indicate the regimes as characterized in the text. The straight lines show the described trend in region (3) for different LC. (b) 98% s-SWNT, D adjusted by deposition time. CNT deposition was done by spin coating in both cases

Carbon Nanotube TFTs

5.3.5

L/L C ratios [118]. This introduces however other significant drawbacks as will be discussed in the next paragraph. In a less significant way a wider channel also increases the probability of shorting. The Rogers group demonstrated that slicing of the CNN into narrow ribbons from source to drain drastically minimizes shorting effects of m-SWNTs. This leads to several sub-channels with small WC/LC ratio and only a slight reduction in transconductance [119, 120]. The intertube contact resistance was neglected so far in the described model. It was however experimentally found that the junction resistance between crossed nanotubes is much larger than the intratube resistance of the SWNTs themselves [121, 122]. At junctions of two m-SWNTs or two s-SWNTs, charge carriers need to tunnel a thin barrier. Judging by the size of the contact area these junctions make relatively good tunnel contacts. In the case of a junction between metallic and semiconducting SWNTs, there exists an additional Schottky barrier increasing the contact resistance by two orders of magnitude [121]. This Schottky barrier does not only need to be overcome for carriers going from one tube to the other. The originating space charge region at the intersection also limits carrier transport along the s-SWNT. Switching in CNT-TFTs is therefore dominated by gate-induced modulation of SBs between metal contact and CNN as well as SBs in the network itself while in classical MOSFETs switching is caused by modulation of the carrier density in the channel. Other than in classical SB devices, the subthreshold swing S can however come close to the theoretical limit of 60 mV/decade [123]. The increased conduction resistance for carrier transfer between tubes limits meff for network TFTs by about 2 orders of magnitude compared to single tube devices [124]. This however still leads to mobilities that are 1–2 orders of magnitude larger than achievable with amorphous Silicon. The limitations caused by the intertube resistance can be reduced by achieving more parallel alignment [125] or by enrichment in s-SWNTs [112]. The ultimate goal would be to directly bridge the channel by arrays of parallel and purely semiconducting nanotube arrays without any crossings. When using highly enriched semiconducting SWNTs the performance in terms of meff can be significantly improved compared to devices with 1/3 m-SWNTs. This allows for high Ion/Ioff combined with fairly large values for meff as can be seen in > Fig. 7b for random networks of 98% s-SWNTs. For longer deposition times and therefore higher network densities the on/off ratio is however still significantly reduced. Besides the residual metallic content, this is caused by screening of the gate field by the nanotubes themselves. SWNTs lying not directly at the interface to the dielectric cannot be gated effectively leading to an increased off-current. When adjusting the deposition time to get a network density where gate screening is not observable the homogeneity of device performance is however greatly increased compared to networks with higher metallic content as will be discussed in > Sect. 6.5. Typical transfer and output characteristics of such devices are shown in > Fig. 8.

6.3

Realizing CMOS Circuits

Carbon nanotube TFTs usually exhibit p-type behavior. This is caused by SBs as was discussed before as well as adsorbed oxygen [126]. Although logic circuits can be realized by only using PMOS technology, static current flow leads to increased power consumption. In order to achieve high-quality and low power circuits, complementary logic using both p-type and n-type transistors is necessary. Besides the already presented influence of the metal contacts, several doping methods were published in recent years. The Fermi level in s-SWNTs can be tuned by electrostatic fields [127] or by charge transfer from adsorbed amine-rich polymers

767

5.3.5

Carbon Nanotube TFTs

1e–05 1e–06

ID/A

1e–07 1e–08 1e–09 1e–10 1e–11 1e–12 −10

−5

0 VGS/V

a

5

10

2e–06 0 −2e–06 ID/A

768

−10V −9V −8V −7V −6V −5V −4V −3V −2V 0V 1V

−4e–06 −6e–06 −8e–06 −1e–05 −10

b

−8

−6

−4 VDS/V

−2

0

2

. Fig. 8 TFT characteristics for devices with WC = LC = 50 mm. (a) Id VS. Vgs at Vds – 1 V for 10 devices. (b) ID vs. VDs of one device from (a)

like Poly(ethylenimine) (PEI) and Poly(aniline) (PANI) [128–130], potassium [126] or hydrazine [130]. The latter two are however not stable in air. The doping techniques were also applied to networks and aligned arrays to achieve n-type devices, p-n diodes, and CMOS logic circuits [105, 131, 132]. Avouris et al. demonstrated a five-stage ring oscillator on a single SWNT by using source and drain metal contacts that give ambipolar behavior and adjusting the threshold voltage by using gate metals with different work functions [133].

6.4

Further Effects Influencing Nanotube TFT Performance

Single walled nanotubes have a strong tendency to form bundles caused by van der Waals forces and their high aspect ratio. This is pronounced in post-synthesis processing due to higher

Carbon Nanotube TFTs

5.3.5

interaction potential, but also occurs in direct growth methods. Especially in mixtures of metallic and semiconducting nanotubes, bundles can deteriorate the device performance due to several effects [134]. (1) Even for high enrichment in s-SWNTs a single m-SWNT in a bundle can drastically weaken the switching effect. (2) Bundling can also lower the efficiency of the channel by screening of the gate field. (3) The band structure of the individual tubes are slightly altered due to deformations caused by the high van der Waals forces. As with other semiconductors the TFT channel needs to be passivated in order to shield it from contaminations that can lead to parasitic effects like doping by the environment, shift of the threshold voltage or hysteresis in the transfer characteristics [135, 136]. Successful passivation was demonstrated for organic and inorganic capping layers, including Poly(methyl methacrylate) (PMMA) [137, 138], SiNx [139] and Al2O3 [140, 141].

6.5

Performance of Carbon Nanotube Network and Array TFTs

This section shall give a brief overview of the performance of CNT network and array TFTs and realized circuits produced with some of the discussed methods. Only publications with relevant overall performance in terms of on/off ratios and meff are considered. For switching TFTs, the on/off ratio should reach 105 while meff should at least reach the amorphous silicon margin of 1 cm2/Vs. For logic circuits, the on/off ratio can be lower (102–103) while meff should be at least 1 order of magnitude increased in order to achieve high switching speeds. While for single nanotube devices on/off ratios up to 107 have been reported [7], in network devices the highest reported values lie above 106 [142]. Smaller diameter nanotubes might lead to higher ratios due to an increased band gap, but also to increased contact resistance. Despite the Schottky barrier switching behavior, subthreshold swings of single tube devices were reported to come close to the theoretical limit of 60 mV/decade [123]. For network devices values as low as 140 mV/ decade were reported [119]. Using the spin coating technique, Schindler et al. produced randomly oriented network TFTs on glass and flexible plastic substrates using anodically oxidized Al as gate and dielectric. The reported values for on/off ratio and meff go up to more than 105 and several cm2/Vs respectively, including however large statistical spread caused by the inhomogeneous network density [76, 97]. By using a modified dispense unit and semiconductor enriched SWNTs, they were able to achieve high yield with Ion/Ioff > 105 and meff  5 cm2/Vs [143] for devices with WC/LC up to 2.5. Slicing of the channel into several subchannels with high aspect ratio as discussed before might further improve the on/off ratio especially for larger WC/LC. Stokes et al. produced similar devices with aligned arrays deposited by dielectrophoresis. They achieved on/off ratios above 104 and a meff as high as 123 cm2/Vs [104]. Due to the metallic content these results are however only possible by applying repeated electrical breakdown which is a serious limitation for electrical circuit fabrication. Nougaret et al. used dielectrophoresis to deposit 99% pure SWNTs [144]. As mentioned before, the alignment of s-SWNTs parallel to the electric field is less pronounced and the produced networks are therefore randomly oriented. They report operation frequencies in the GHz range. On/off ratios and mobilities were however not reported. Using the immersion technique Wang et al. reported Ion/Ioff = 104 and meff = 52 cm2/Vs [74]. Although some of the mentioned devices were realized on silicon wafers, the fabrication on glass or plastic substrates should in principle be possible. The Rogers group has specialized on growing nanotubes on wafers and transferring them onto other substrate materials afterwards. With this technique they have realized randomly

769

770

5.3.5

Carbon Nanotube TFTs

oriented, sliced network TFTs on flexible plastic substrates with reported values of Ion/Ioff = 105, meff = 80 cm2/Vs and S = 140 mV/decade. They used such devices to build a 4-bit row decoder on plastic, consisting of 88 transistors and working with frequencies in the kHz range [119]. By using dense metallic nanotube networks for gate, source, and drain, and an organic dielectric, they also realized highly transparent, flexible transistors [145]. For aligned array TFTs processed in the same manner as described above they report Ion/Ioff = 105 and meff = 400 cm2/Vs after modification by electrical breakdown. Without this treatment, the mobility lies at 1,000 cm2/Vs, but the on/off ratios are below 10 [58, 105]. With the floating-catalyst CVD developed by the Kauppinen group researchers from the Nagoya University, Japan and the Aalto University, Finland fabricated randomly oriented network TFTs with well adjusted network density on flexible and transparent substrates. They used however a filter membrane as transfer medium. Despite 1/3 metallic content they report typical mobilities of 20 cm2/Vs combined with on/off ratios of 106 [142]. Yu et al. reported network TFTs with directly grown nanotubes enriched in s-SWNT by 95% to have Ion/Ioff = 105 and meff = 8 cm2/Vs with high yield and no modifications like electrical breakdown. These results were however achieved on silicon wafers without transfer after deposition. TFTs with highly aligned arrays of also 95% s-SWNTs exhibit rather low on/off ratios in the region of 3. Because of the direct connection between source and drain even low amounts of m-SWNTs drastically lower the achievable ratio without further treatment [146]. Such devices with high density arrays and sub-micron channel lengths prepared by aligned growth or dielectrophoresis have cut-off frequencies up to 10 GHz [146, 147]. By minimizing the metallic content, these TFTs might be used for radio frequency applications. Nanotube transistors were already used as RF amplifiers in an entire AM radio system [148].

7

Conclusions

The overview given in this chapter shows the great diversity of carbon nanotube research. Even by confining the scope to transistors and limiting it further down to technologies that are applicable to flat panel displays. For each process step, starting from CNT synthesis, purification and dispersing in liquids down to sorting techniques, deposition and device fabrication a variety of quite different approaches were presented in recent years. The same holds for the proposed applications. The combination of ultrahigh charge carrier mobilities, chemical stability, mechanical flexibility, and possible low-cost solution processing opens up new possibilities from highperformance devices in silicon based circuits down to comparably simple, high volume, and lowcost flexible products like RFID tags or other flexible electronics. The demands of display applications lay somewhere in between. The achievable on/off ratios allow the realization of active matrices for LCDs or OLEDs. While high charge carrier mobilities and the feasibility of creating CMOS circuits are promising for on-panel driver electronics, the added ability of vacuum-free solution processing and good compatibility of material and processes to flexible plastic substrates could revolutionize display fabrication and open up new possibilities especially for flexible displays. Although several groups have demonstrated well-working devices and even complex circuits based on CNT networks or aligned arrays, many optimizations are necessary until one can truly think about integration into a real product. This basically holds for all involved steps. Improvements in synthesis and postprocessing of the nanotubes, especially the separation of s-SWNTs and m-SWNTs or the selective growth are mandatory for achieving high yield and performance. High volume and cost-effective production of high-quality SWNTs is another issue.

Carbon Nanotube TFTs

5.3.5

The main challenge when it comes to device fabrication is the controlled deposition of preferably well-aligned SWNT arrays. The proposed methods can be categorized in direct growth and deposition from solution. The former allows for good control over alignment, density and length of the nanotubes. These advantages are however combined with the necessity to use transfer techniques since a direct growth with these qualities on glass or plastic is not possible. Scaling, cost-effective production and synthesis of purely s-SWNTs seem difficult so far. The advantages of deposition from solution are cost-effectiveness due to vacuum-free methods, applicability to large area substrates, room-temperature processing, and the possibility to use very high purity s-SWNT dispersions. The control over alignment and homogeneity is however less pronounced leading to higher variations in device performance. A clear favorite can so far not be announced. It seems likely that both ways will find their way to production with different methods for different requirements like quality or performance vs. cost or the choice of substrate materials. Although perfectly aligned nanotube arrays will ultimately give better performance, randomly oriented networks so far give good on/off ratios without the need for further treatment. If progress in CNTresearch and especially in the field of thin film transistors will be as rapid as in the last decade, it is very likely, that carbon nanotubes will become a basic building block of future display technologies. If high performance, homogeneity and yield can be achieved with solution deposition techniques CNT-TFTs will be able to compete with current technologies. Especially in the field of flexible displays where so far organic semiconductors are preferred, CNTs show the potential for great performance improvement possibly allowing for fully integrated driver electronics. Being able to produce flexible displays without the need to attach large drivers that seriously limit flexibility and freedom of design will enable the full potential of this new type of display.

References 1. Iijima S (1991) Helical microtubules of graphitic carbon. Nature 354:56–58 2. Bethune DS, Kiang C, de Vries MS, Groman G, Savoy R, Vazquez J, Beyers R (1993) Cobalt-catalysed growth of carbon nanotubes with single-atomiclayer walls. Nature 363:605–607 3. Iijima S, Ichihashi T (1993) Single-shell carbon nanotubes of 1-nm diameter. Nature 363:603–605 4. Avouris P, Chen Z, Perebeinos V (2007) Carbonbased electronics. Nat Nanotechnol 2:605–615 5. Jorio A, Dresselhaus G, Dresselhaus MS (2008) Carbon nanotubes – advanced topics in the synthesis, structure, properties and applications, vol 111, Topics in applied physics. Springer, Berlin 6. Durkop T, Getty SA, Cobas E, Fuhrer MS (2004) Extraordinary mobility in semiconducting carbon nanotubes. Nano Lett 4:35–39 7. Kim W, Javey A, Tu R, Cao J, Wang Q, Dai H (2005) Electrical contacts to carbon nanotubes down to 1 nm in diameter. Appl Phys Lett 87:173101 8. Saito Y (2010) Yahachi saito laboratory, Nagoya University. http://www.surf.nuqe.nagoyau.ac.jp. Accessed 12 May, 2010

9. Saito R, Fujita M, Dresselhaus G, Dresselhaus MS (1992) Electronic structure of chiral graphene tubules. Appl Phys Lett 60:2204–2206 10. Reich DS, Thomsen PC, Maultzsch DPJ (2003) Carbon nanotubes – basic concepts and physical properties. Wiley-VCH, Weinheim 11. Kane CL, Mele EJ (1997) Size, shape, and low energy electronic structure of carbon nanotubes. Phys Rev Lett 78:1932 12. Maruyama S (2010) Shigeo maruyama’s carbon nanotube and fullerene site. http://www.photon.t. u-tokyo.ac.jp/~maruyama/nanotube.html. Accessed 12 May, 2010 13. Popov VN (2004) Carbon nanotubes: properties and application. Mat Sci Eng R, Reports 43:61–102 14. Thess A et al (1996) Crystalline ropes of metallic carbon nanotubes. Science 273:483–487 15. Bronikowski MJ, Willis PA, Colbert DT, Smith K, Smalley RE (2001) Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPco process: A parametric study. J Vacuum Sci Technol A 19:1800–1805

771

772

5.3.5

Carbon Nanotube TFTs

16. Nikolaev P, Bronikowski MJ, Bradley RK, Rohmund F, Colbert DT, Smith KA, Smalley RE (1999) Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide. Chem Phys Lett 313:91–97 17. Hersam MC (2008) Progress towards monodisperse single-walled carbon nanotubes. Nat Nanotechnol 3:387–394 18. Lolli G, Zhang L, Balzano L, Sakulchaicharoen N, Tan Y, Resasco DE (2006) Tailoring (n, m) structure of Single-Walled carbon nanotubes by modifying reaction conditions and the nature of the support of CoMo catalysts. J Phys Chem B 110:2108–2115 19. Li Yet al (2004) Preferential growth of semiconducting Single-Walled carbon nanotubes by a plasma enhanced CVD method. Nano Lett 4:317–321 20. Li Y, Peng S, Mann D, Cao J, Tu R, Cho KJ, Dai H (2005) On the origin of preferential growth of semiconducting single-walled carbon nanotubes. J Phys Chem B 109:6968–6971 21. Qu L, Du F, Dai L (2008) Preferential syntheses of semiconducting vertically aligned single-walled carbon nanotubes for direct use in FETs. Nano Lett 8:2682–2687 22. Wang Y, Liu Y, Li X, Cao L, Wei D, Zhang H, Shi D, Yu G, Kajiura H, Li Y (2007) Direct enrichment of metallic single-walled carbon nanotubes induced by the different molecular composition of monohydroxy alcohol homologues. Small 3:1486–1490 23. Harutyunyan AR, Chen G, Paronyan TM, Pigos EM, Kuznetsov OA, Hewa-parakrama K, Kim SM, Zakharov D, Stach EA, Sumanasekera GU (2009) Preferential growth of single-walled carbon nanotubes with metallic conductivity. Science 326:116–120 24. Wang Y, Kim MJ, Shan H, Kittrell C, Fan H, Ericson LM, Hwang W, Arepalli S, Hauge RH, Smalley RE (2005) Continued growth of single-walled carbon nanotubes. Nano Lett 5:997–1002 25. Jasti R, Bhattacharjee J, Neaton JB, Bertozzi CR (2008) Synthesis, characterization, and theory of [9]-, [12]-, and [18] Cycloparaphenylene: carbon nanohoop structures. J Am Chem Soc 130:17646– 17647 26. Fort EH, Donovan PM, Scott LT (2009) Diels-Alder reactivity of polycyclic aromatic hydrocarbon bay regions: implications for metal-free growth of single-chirality carbon nanotubes. J Am Chem Soc 131:16006–16007 27. Park T, Banerjee S, Hemraj-Benny T, Wong SS (2006) Purification strategies and purity visualization techniques for single-walled carbon nanotubes. J Mater Chem 16:141 28. Hou P, Liu C, Cheng H (2008) Purification of carbon nanotubes. Carbon 46:2003–2025 29. Dillon AC, Gennett T, Jones KM, Alleman JL, Parilla PA, Heben MJ (1999) A simple and complete

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

purification of Single-Walled carbon nanotube materials. Adv Mater 11:1354–1358 Banerjee S, Hemraj-Benny T, Wong SS (2005) Covalent surface chemistry of single-walled carbon nanotubes. Adv Mater 17:17–29 Tasis D, Tagmatarchis N, Bianco A, Prato M (2006) Chemistry of carbon nanotubes. Chem Rev 106:1105–1136 Bahr JL, Yang J, Kosynkin DV, Bronikowski MJ, Smalley RE, Tour JM (2001) Functionalization of carbon nanotubes by electrochemical reduction of aryl diazonium salts: a bucky paper electrode. J Am Chem Soc 123:6536–6542 Pe´nicaud A, Poulin P, Derre´ A, Anglaret E, Petit P (2005) Spontaneous dissolution of a single-wall carbon nanotube salt. J Am Chem Soc 127:8–9 Ramesh S, Ericson LM, Davis VA, Saini RK, Kittrell C, Pasquali M, Billups WE, Adams WW, Hauge RH, Smalley RE (2004) Dissolution of pristine single walled carbon nanotubes in superacids by direct protonation. J Phys Chem B 108:8794–8798 Davis VA et al (2009) True solutions of single-walled carbon nanotubes for assembly into macroscopic materials. Nat Nanotechnol 4:830–834 Ausman KD, Piner R, Lourie O, Ruoff RS, Korobov M (2000) Organic solvent dispersions of single-walled carbon nanotubes: toward solutions of pristine nanotubes. J Phys Chem B 104:8911–8915 Bergin SD et al (2008) Towards solutions of singlewalled carbon nanotubes in common solvents. Adv Mater 20:1876–1881 Islam MF, Rojas E, Bergey DM, Johnson AT, Yodh AG (2003) High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. Nano Lett 3:269–273 Xu Z, Yang X, Yang Z (2010) A molecular simulation probing of structure and interaction for supramolecular sodium dodecyl sulfate/single-wall carbon nanotube assemblies. Nano Lett 10:985–991 O’Connell MJ et al (2002) Band gap fluorescence from individual single-walled carbon nanotubes. Science 297:593–596 Geng HZ, Kim KK, So KP, Lee YS, Chang Y, Lee YH (2007) Effect of acid treatment on carbon nanotubebased flexible transparent conducting films. J Am Chem Soc 129:7758–7759 Huang X, Mclean RS, Zheng M (2005) Highresolution length sorting and purification of DNA-wrapped carbon nanotubes by size-exclusion chromatography. Anal Chem 77:6225–6228 Arnold K, Hennrich F, Krupke R, Lebedkin S, Kappes MM (2006) Length separation studies of single walled carbon nanotube dispersions. Physica Status Solidi (B) 243:3073–3076

Carbon Nanotube TFTs 44. Arnold MS, Stupp SI, Hersam MC (2005) Enrichment of single-walled carbon nanotubes by diameter in density gradients. Nano Lett 5:713–718 45. Krupke R, Hennrich F, v Loehneysen H, Kappes MM (2003) Separation of metallic from semiconducting single-walled carbon nanotubes. Science 301: 344–347 46. Collins PG, Arnold MS, Avouris P (2001) Engineering carbon nanotubes and nanotube circuits using electrical breakdown. Science 292:706–709 47. Seidel R, Graham AP, Unger E, Duesberg GS, Liebau M, Steinhoegl W, Kreupl F, Hoenlein W, Pompe W (2004) High-current nanotube transistors. Nano Lett 4:831–834 48. Balasubramanian K, Sordan R, Burghard M, Kern K (2004) A selective electrochemical approach to carbon nanotube field-effect transistors. Nano Lett 4:827–830 49. Hassanien A, Tokumoto M, Umek P, Vrbanicˇ D, Mozeticˇ M, Mihailovic´ D, Venturini P (2005) Selective etching of metallic single-wall carbon nanotubes with hydrogen plasma. Nanotechnology 16:278–281 50. Zhang G, Qi P, Wang X, Lu Y, Li X, Tu R, Bangsaruntip S, Mann D, Zhang L, Dai H (2006) Selective etching of metallic carbon nanotubes by gas-phase reaction. Science 314:974–977 51. Huang H, Maruyama R, Noda K, Kajiura H, Kadono K (2006) Preferential destruction of metallic singlewalled carbon nanotubes by laser irradiation. J Phys Chem B 110:7316–7320 52. Dimaki M, Boggild P (2004) Dielectrophoresis of carbon nanotubes using microelectrodes: a numerical study. Nanotechnology 15:1095–1102 53. Krupke R, Hennrich F, Kappes MM, v Lohneysen H (2004) Surface conductance induced dielectrophoresis of semiconducting single-walled carbon nanotubes. Nano Lett 4:1395–1399 54. Shin DH, Kim J, Shim HC, Song J, Yoon J, Kim J, Jeong S, Kang J, Baik S, Han C (2008) Continuous extraction of highly pure metallic single-walled carbon nanotubes in a microfluidic channel. Nano Lett 8:4380–4385 55. Arnold MS, Green AA, Hulvat JF, Stupp SI, Hersam MC (2006) Sorting carbon nanotubes by electronic structure using density differentiation. Nat Nanotechnol 1:60–65 56. Bachilo SM, Balzano L, Herrera JE, Pompeo F, Resasco DE, Weisman RB (2003) Narrow (n, m)distribution of single-walled carbon nanotubes grown using a solid supported catalyst. J Am Chem Soc 125:11186–11187 57. Ghosh S, Bachilo SM, Weisman RB (2010) Advanced sorting of single-walled carbon nanotubes by nonlinear density-gradient ultracentrifugation. Nat Nanotechnol 5:443–450

5.3.5

58. Cao Q, Rogers J (2008) Random networks and aligned arrays of single-walled carbon nanotubes for electronic device applications. Nano Res 1:259–272 59. Yan Y, Chan-Park MB, Zhang Q (2007) Advances in carbon-nanotube assembly. Small 3:24–42 60. Huang L, Jia Z, O’Brien S (2007) Orientated assembly of single-walled carbon nanotubes and applications. J Mater Chem 17:3863–3874 61. Kocabas C, Meitl MA, Gaur A, Shim M, Rogers JA (2004) Aligned arrays of single-walled carbon nanotubes generated from random networks by orientationally selective laser ablation. Nano Lett 4:2421–2426 62. Nojeh A, Ural A, Pease RF, Dai H (2004) Electricfield-directed growth of carbon nanotubes in two dimensions. In: The 48th international conference on electron, ion, and photon beam technology and nanofabrication, San Diego, California (USA), Nov., vol 22, pp 3421–3425, AVS 63. Lee K, Cho J, Sigmund W (2003) Control of growth orientation for carbon nanotubes. Appl Phys Lett 82:448 64. Huang L, Cui X, White B, O’Brien SP (2004) Long and oriented single-walled carbon nanotubes grown by ethanol chemical vapor deposition. J Phys Chem B 108:16451–16456 65. McNicholas TP, Ding L, Yuan D, Liu J (2009) Density enhancement of aligned single-walled carbon nanotube thin films on quartz substrates by sulfur-assisted synthesis. Nano Lett 9:3646–3650 66. Xiao J et al (2009) Alignment controlled growth of single-walled carbon nanotubes on quartz substrates. Nano Lett 9:4311–4319 67. Ding L, Tselev A, Wang J, Yuan D, Chu H, McNicholas TP, Li Y, Liu J (2009) Selective growth of well-aligned semiconducting single-walled carbon nanotubes. Nano Lett 9:800–805 68. Hur S, Park OO, Rogers JA (2005) Extreme bendability of single-walled carbon nanotube networks transferred from high-temperature growth substrates to plastic and their use in thin-film transistors. Appl Phys Lett 86:243502 69. Zhou Y, Liangbing H, Gru¨ner G (2006) A method of printing carbon nanotube thin films. Appl Phys Lett 88:123109–1–3 70. Kocabas C, Kang SJ, Ozel T, Shim M, Rogers JA (2007) Improved synthesis of aligned arrays of single-walled carbon nanotubes and their implementation in thin film type transistors. J Phys Chem C 111:17879–17886 71. Tseng SH, Tai NH (2009) Fabrication of a transparent and flexible thin film transistor based on single-walled carbon nanotubes using the direct transfer method. Appl Phys Lett 95:204104

773

774

5.3.5

Carbon Nanotube TFTs

72. Im J et al (2009) Direct printing of aligned carbon nanotube patterns for high-performance thin film devices. Appl Phys Lett 94:053109 73. Zavodchikova MY, Nasibulin AG, Kulmala T, Grigoras K, Anisimov AS, Franssila S, Ermolov V, Kauppinen EI (2008) Novel carbon nanotube network deposition technique for electronic device fabrication. Phys Status Solidi (B) 245:2272–2275 74. Wang C, Zhang J, Ryu K, Badmaev A, Arco LGD, Zhou C (2009) Wafer-scale fabrication of separated carbon nanotube thin-film transistors for display applications. Nano Lett 9:4285–4291 75. Ka¨mpgen M, Duesberg GS, Roth S (2005) Transparent carbon nanotube coatings. Appl Surf Sci 252:425–429 76. Schindler A, Spiessberger S, Fruehauf N, Novak JP, Yaniv Z (2007) Solution-deposited carbon nanotube networks for flexible active matrix displays. In: Proceedings of Asia display, Shanghai, China, vol 1, pp 882–887 77. Tenent RC, Barnes TM, Bergeson JD, Ferguson AJ, To B, Gedvilas LM, Heben MJ, Blackburn JL (2009) Ultrasmooth, large-area, high-uniformity, conductive transparent single-walled-carbonnanotube films for photovoltaics produced by ultrasonic spraying. Adv Mater 21:3210–3216 78. Vaillancourt J et al (2008) All ink-jet-printed carbon nanotube thin-film transistor on a polyimide substrate with an ultrahigh operating frequency of over 5 GHz. Appl Phys Lett 93:243301–3 79. Liu J, Casavant MJ, Cox M, Walters DA, Boul P, Lu W, Rimberg AJ, Smith KA, Colbert DT, Smalley RE (1999) Controlled deposition of individual singlewalled carbon nanotubes on chemically functionalized templates. Chem Phys Lett 303:125–129 80. Auvray S, Derycke V, Goffman M, Filoramo A, Jost O, Bourgoin J (2005) Chemical optimization of selfassembled carbon nanotube transistors. Nano Lett 5:451–455 81. Xin H, Woolley AT (2003) DNA-templated nanotube localization. J Am Chem Soc 125:8710–8711 82. Keren K, Berman RS, Buchstab E, Sivan U, Braun E (2003) DNA-templated carbon nanotube fieldeffect transistor. Science 302:1380–1382 83. Mclean RS, Huang X, Khripin C, Jagota A, Zheng M (2006) Controlled two-dimensional pattern of spontaneously aligned carbon nanotubes. Nano Lett 6:55–60 84. Maune HT, Han S-P, Barish RD, Bockrath M, Goddard WA, Rothemund PWK, Winfree E (2010) Self-assembly of carbon nanotubes into two-dimensional geometries using DNA origami templates. Nat Nanotechnol 5:61–66 85. Xiong X, Chen C, Ryan P, Busnaina AA, Jung YJ, Dokmeci MR (2009) Directed assembly of high

density single-walled carbon nanotube patterns on flexible polymer substrates. Nanotechnology 20:295302 86. Yu G, Cao A, Lieber CM (2007) Large-area blown bubble films of aligned nanowires and carbon nanotubes. Nat Nanotechnol 2:372–377 87. Kim Y, Minami N, Zhu W, Kazaoui S, Azumi R, Matsumoto M (2003) Homogeneous and structurally controlled thin films of single-wall carbon nanotubes by the Langmuir-Blodgett technique. Synthetic Met 135–136:747–748 88. Jin S, Whang D, McAlpine MC, Friedman RS, Wu Y, Lieber CM (2004) Scalable interconnection and integration of nanowire devices without registration. Nano Lett 4:915–919 89. Li X, Zhang L, Wang X, Shimoyama I, Sun X, Seo W, Dai H (2007) Langmuir-Blodgett assembly of densely aligned single-walled carbon nanotubes from bulk materials. J Am Chem Soc 129:4890–4891 90. Shimoda H, Oh S, Geng H, Walker R, Zhang X, McNeil L, Zhou O (2002) Self-assembly of carbon nanotubes. Adv Mater 14:899–901 91. Huang L, Cui X, Dukovic G, Brien SPO (2004) Selforganizing high-density single-walled carbon nanotube arrays from surfactant suspensions. Nanotechnology 15:1450–1454 92. Engel M, Small JP, Steiner M, Freitag M, Green AA, Hersam MC, Avouris P (2008) Thin film nanotube transistors based on self-assembled, aligned, semiconducting carbon nanotube arrays. ACS Nano 2:2445–2452 93. Zamora-Ledezma C, Blanc C, Maugey M, Zakri C, Poulin P, Anglaret E (2008) Anisotropic thin films of single-wall carbon nanotubes from aligned lyotropic nematic suspensions. Nano Lett 8:4103–4107 94. Lay MD, Novak JP, Snow ES (2004) Simple route to large-scale ordered arrays of liquid-deposited carbon nanotubes. Nano Lett 4:603–606 95. Xin H, Woolley AT (2004) Directional orientation of carbon nanotubes on surfaces using a gas flow cell. Nano Lett 4:1481–1484 96. Meitl MA, Zhou Y, Gaur A, Jeon S, Usrey ML, Strano MS, Rogers JA (2004) Solution casting and transfer printing single-walled carbon nanotube films. Nano Lett 4:1643–1647 97. Schindler A, Spiessberger S, Hergert S, Fruehauf N, Novak JP, Yaniv Z (2008) Suspension-Deposited carbon nanotube networks for flexible active matrix displays. J Soc Info Disp 16:651–658 98. LeMieux MC, Roberts M, Barman S, Jin YW, Kim JM, Bao Z (2008) Self-sorted, aligned nanotube networks for thin-film transistors. Science 321:101–104 99. Roberts ME, LeMieux MC, Sokolov AN, Bao Z (2009) Self-sorted nanotube networks on polymer

Carbon Nanotube TFTs

100.

101.

102.

103.

104.

105.

106.

107.

108.

109.

110.

111.

112.

dielectrics for low-voltage thin-film transistors. Nano Lett 9:2526–2531 Peng N, Zhang Q, Sun Y (2008) Motion of carbon nanotubes in suspension under AC electric field. Int J Nanomanufacturing 2:50–58 Vijayaraghavan A, Blatt S, Weissenberger D, Oron-Carl M, Hennrich F, Gerthsen D, Hahn H, Krupke R (2007) Ultra-large-scale directed assembly of single-walled carbon nanotube devices. Nano Lett 7:1556–1560 Sorgenfrei S, Meric I, Banerjee S, Akey A, Rosenblatt S, Herman IP, Shepard KL (2009) Controlled dielectrophoretic assembly of carbon nanotubes using real-time electrical detection. Appl Phys Lett 94:053105 Stokes P, Khondaker SI (2010) High quality solution processed carbon nanotube transistors assembled by dielectrophoresis. Appl Phys Lett 96:083110 Stokes P, Silbar E, Zayas YM, Khondaker SI (2009) Solution processed large area field effect transistors from dielectrophoreticly aligned arrays of carbon nanotubes. Appl Phys Lett 94:113104 Kang SJ, Kocabas C, Ozel T, Shim M, Pimparkar N, Alam MA, Rotkin SV, Rogers JA (2007) Highperformance electronics using dense, perfectly aligned arrays of single-walled carbon nanotubes. Nat Nanotechnol 2:230–236 Zhou X, Park J, Huang S, Liu J, McEuen PL (2005) Band structure, phonon scattering, and the performance limit of single-walled carbon nanotube transistors. Phys Rev Lett 95:146805 Heinze S, Tersoff J, Martel R, Derycke V, Appenzeller J, Avouris P (2002) Carbon nanotubes as schottky barrier transistors. Phys Rev Lett 89:106801 Chen Z, Appenzeller J, Knoch J, Lin Y, Avouris P (2005) The role of metal-nanotube contact in the performance of carbon nanotube field-effect transistors. Nano Lett 5:1497–1502 Javey A, Guo J, Wang Q, Lundstrom M, Dai H (2003) Ballistic carbon nanotube field-effect transistors. Nature 424:654–657 Ding L et al (2009) Y-contacted high-performance n-type single-walled carbon nanotube field-effect transistors: scaling and comparison with Sccontacted devices. Nano Lett 9:4209–4214 Lim SC, Jang JH, Bae DJ, Han GH, Lee S, Yeo I, Lee YH (2009) Contact resistance between metal and carbon nanotube interconnects: effect of work function and wettability. Appl Phys Lett 95:264103 Topinka MA, Rowell MW, Goldhaber-Gordon D, McGehee MD, Hecht DS, Gru¨ner G (2009) Charge transport in interpenetrating networks of semiconducting and metallic carbon nanotubes. Nano Lett 9:1866–1871

5.3.5

113. Hu L, Hecht DS, Gru¨ner G (2004) Percolation in transparent and conducting carbon nanotube networks. Nano Lett 4:2513–2517 114. Kumar S, Pimparkar N, Murthy JY, Alam MA (2006) Theory of transfer characteristics of nanotube network transistors. Appl Phys Lett 88:123505 115. Ska´kalova´ V, Kaiser AB, Woo Y, Roth S (2006) Electronic transport in carbon nanotubes: from individual nanotubes to thin and thick networks. Phys Rev B 74:085403 116. Li J, Zhang Z, Zhang S (2007) Percolation in random networks of heterogeneous nanotubes. Appl Phys Lett 91:253127–3 117. Schindler A, Brill J, Fruehauf N, Novak JP, Yaniv Z (2007) Solution-deposited carbon nanotube layers for flexible display applications. Physica E 37:119–123 118. Ishida M, Nihey F (2008) Estimating the yield and characteristics of random network carbon nanotube transistors. Appl Phys Lett 92:163507 119. Cao Q, Kim H-S, Pimparkar N, Kulkarni JP, Wang C, Shim M, Roy K, Alam MA, Rogers JA (2008) Medium-scale carbon nanotube thin-film integrated circuits on flexible plastic substrates. Nature 454:495–500 120. Pimparkar N, Cao Q, Rogers J, Alam M (2009) Theory and practice of ‘‘Striping’’ for improved ON/OFF ratio in carbon nanonet thin film transistors. Nano Res 2:167–175 121. Fuhrer MS et al (2000) Crossed nanotube junctions. Science 288:494–497 122. Stadermann M et al (2004) Nanoscale study of conduction through carbon nanotube networks. Phys Rev B 69:201402–1–3 123. Javey A, Kim H, Brink M, Wang Q, Ural A, Guo J, McIntyre P, McEuen P, Lundstrom M, Dai H (2002) High-[kappa] dielectrics for advanced carbonnanotube transistors and logic gates. Nat Mater 1:241–246 124. Xu H, Zhang S, Anlage SM, Hu L, Gru¨ner G (2008) Frequency- and electric-field-dependent conductivity of single-walled carbon nanotube networks of varying density. Phys Rev B 77:075418 125. Kocabas C, Pimparkar N, Yesilyurt O, Kang SJ, Alam MA, Rogers JA (2007) Experimental and theoretical studies of transport through large scale, partially aligned arrays of Single-Walled carbon nanotubes in thin film type transistors. Nano Lett 7:1195–1202 126. Derycke V, Martel R, Appenzeller J, Avouris P (2002) Controlling doping and carrier injection in carbon nanotube transistors. Appl Phys Lett 80:2773 127. Lin YM, Appenzeller J, Knoch J, Avouris P (2005) High-performance carbon nanotube field-effect

775

776

5.3.5 128.

129.

130.

131.

132.

133.

134.

135.

136.

137.

138.

Carbon Nanotube TFTs

transistor with tunable polarities. IEEE Trans Nanotechnol 4:481–489 Shim M, Javey A, Kam NWS, Dai H (2001) Polymer functionalization for air-stable n-type carbon nanotube field-effect transistors. J Am Chem Soc 123:11512–11513 Siddons GP, Merchin D, Back JH, Jeong JK, Shim M (2004) Highly efficient gating and doping of carbon nanotubes with polymer electrolytes. Nano Lett 4:927–931 Klinke C, Chen J, Afzali A, Avouris P (2005) Charge transfer induced polarity switching in carbon nanotube transistors. Nano Lett 5:555–558 Zhou Y, Gaur A, Hur S, Kocabas C, Meitl MA, Shim M, Rogers JA (2004) p-Channel, n-Channel thin film transistors and p-n diodes based on single wall carbon nanotube networks. Nano Lett 4:2031–2035 Ryu K, Badmaev A, Wang C, Lin A, Patil N, Gomez L, Kumar A, Mitra S, Wong HP, Zhou C (2009) CMOS-analogous wafer-scale nanotube-onInsulator approach for submicrometer devices and integrated circuits using aligned nanotubes. Nano Lett 9:189–197 Chen Z, Appenzeller J, Lin Y, Sippel-Oakley J, Rinzler AG, Tang J, Wind SJ, Solomon PM, Avouris P (2006) An integrated logic circuit assembled on a single carbon nanotube. Science 311:1735 Kim T, Kim G, Choi WI, Kwon Y, Zuo J (2010) Electrical transport in small bundles of single-walled carbon nanotubes: intertube interaction and effects of tube deformation. Appl Phys Lett 96:173107 Collins PG, Bradley K, Ishigami M, Zettl A (2000) Extreme oxygen sensitivity of electronic properties of carbon nanotubes. Science 287:1801–1804 Kar S, Vijayaraghavan A, Soldano C, Talapatra S, Vajtai R, Nalamasu O, Ajayan PM (2006) Quantitative analysis of hysteresis in carbon nanotube fieldeffect devices. Appl Phys Lett 89:132118–3 Kim W, Javey A, Vermesh O, Wang Q, Yiming L, Dai H (2003) Hysteresis caused by water molecules in carbon nanotube field-effect transistors. Nano Lett 3:193–198 Shimauchi H, Ohno Y, Kishimoto S, Mizutani T (2006) Suppression of hysteresis in carbon nanotube field-effect transistors: effect of contamination induced by device fabrication process. Jpn J Appl Phys 45:5501–5503

139. Mizutani T, Iwatsuki S, Ohno Y, Kishimoto S (2005) Effects of fabrication process on currentvoltage characteristics of carbon nanotube field effect transistors. Jpn J Appl Phys 44:1599–1602 140. Kim SK, Xuan Y, Ye PD, Mohammadi S, Back JH, Shim M (2007) Atomic layer deposited Al2O3 for gate dielectric and passivation layer of single-walled carbon nanotube transistors. Appl Phys Lett 90:163108 141. Zavodchikova MY, Kulmala T, Nasibulin AG, Ermolov V, Franssila S, Grigoras K, Kauppinen EI (2009) Carbon nanotube thin film transistors based on aerosol methods. Nanotechnology 20:085201 142. Sun D-M, Timmermans MY, Tian Y, Nasibulin AG, Kauppinen EI, Kishimoto S, Mizutani T, Ohno Y (2011) Flexible high-performance carbon nanotube integrated circuits. Nat Nano 6:156–161 143. Schindler A, Knorr S, Novak JP, Yaniv Z, Fruehauf N (2010) High yield solution-based deposition of carbon nanotube thin film transistors for flexible display applications. In: 10th International meeting on information display, Seoul, Korea 144. Nougaret L, Happy H, Dambrine G, Derycke V, Bourgoin JP, Green AA, Hersam MC (2009) 80 GHz field-effect transistors produced using high purity semiconducting single-walled carbon nanotubes. Appl Phys Lett 94:243505 145. Cao Q, Hur S, Zhu Z, Sun Y, Wang C, Meitl MA, Shim M, Rogers JA (2006) Highly bendable, transparent thin-film transistors that use carbon-nanotube-based conductors and semiconductors with elastomeric dielectrics. Adv Mater 18:304–309 146. Kocabas C et al (2009) High-frequency performance of submicrometer transistors that use aligned arrays of single-walled carbon nanotubes. Nano Lett 9:1937–1943 147. Rutherglen C, Jain D, Burke P (2009) Nanotube electronics for radiofrequency applications. Nat Nanotechnol 4:811–819 148. Kocabas C, Kim H-S, Banks T, Rogers JA, Pesetski AA, Baumgardner JE, Krishnaswamy SV, Zhang H (2008) Radio frequency analog electronics based on carbon nanotube transistors. Proc Natl Acad Sci 105:1405–1409

Part 5.4

Transparent Conductors: ITO and ITO Replacements

5.4.1 Indium Tin Oxide (ITO): Sputter Deposition Processes Paul Lippens . Uwe Muehlfeld 1 Introduction: ITO as a Transparent Conductive Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 1.1 Generalities About TCOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 1.2 Conductivity Mechanism in ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781 2 Deposition Technologies for ITO Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782 2.1 Sputtering with Ceramic Targets Versus Reactive Sputtering with Metal Alloy Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782 2.2 Nodule Formation when Depositing ITO Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783 2.3 Consequences of Nodules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784 2.4 Choice of Electrical Power with Which the Plasma is Sustained . . . . . . . . . . . . . . . . . . . . 784 2.5 Emerging Trend in Sputter Power Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784 3 3.1 3.2 3.3

State-of-the-Art Sputter Deposition of ITO in the Display Industries . . . . . . . . . . . . 785 ITO in LCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785 ITO in PDP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786 ITO in TP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 787

4 New Technology for Sputter Deposition of ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 787 4.1 Rotary Magnetrons and Rotary Ceramic ITO Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 787 4.2 Advantages of Rotary Magnetrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788 5 Costing of Depositing ITO in the Display Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790 5.1 CF-ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790 5.2 ITO in TP (PET-FILM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 792 6

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.4.1, # Springer-Verlag Berlin Heidelberg 2012

780

5.4.1

Indium Tin Oxide (ITO): Sputter Deposition Processes

Abstract: ITO (or tin-doped indium-oxide) coatings have excellent electrical conductivity and optical transparency and are therefore used as transparent electrodes in most display products. Such coatings can be deposited either in a purely reactive sputter process from metallic In-Sn alloy targets or from ceramic ITO targets using a quasi-reactive process. The latter technology implemented as DC magnetron sputtering prevails today in the industry. The formation of nodules on the target surface during the sputtering process over time is associated with the fact that indium-oxide exists in three allotropic phases. Such nodules are a source of process instability and of coating pinholes. The usage of rotary cathodes is a way to eliminate or at least strongly reduce this problem. Such cathodes also give rise to considerably reduced coating costs, as will be illustrated with applications taken from the LCD and touch panel industries. List of Abbreviations: AM-LCD, Active Matrix Liquid Crystal Display; CF, Color Filter; CSTN, Color Super-Twisted Nematic; DC, Direct Current; DDR, Dynamic Deposition Rate (Normally in nm.m/min); EMI, Electromagnetic Interference; ITO, Indium-Tin-Oxide (or Tin-Doped Indium-Oxide); LCD, Liquid Crystal Display; MF-AC, Middle Frequency Alternating Current (Typic. 10–200 kHz); PDP, Plasma Display Panel; PE-CVD, PlasmaEnhanced Chemical Vapor Deposition; PEM, Plasma Emission Monitor; PET, Poly-EthyleneTerephthalate; PM-LCD, Passive Matrix Liquid Crystal Display; RF, Radio Frequency (Typic. 13.56 MHz); STN, Super-twisted Nematic; TCO, Transparent Conductive Oxide; TEC, Transparent Electronic Conductor; TFT, Thin Film Transistor; TN, Twisted Nematic; TP, Touch Panel; TU, Target Utilization; UV, Ultra Violet

1

Introduction: ITO as a Transparent Conductive Oxide

1.1

Generalities About TCOs

Indium-Tin-Oxide or Sn-doped In2O3 (Sn:In2O3), commonly indicated as ITO, is a transparent conductive oxide (TCO), i.e., in a thin film form, it is conductive and visible light can easily pass through it. TCO’s are only one category of the broader family of Transparent Electronic Conductors (TEC’s). Thin metal films ( Sect. 2.2.) etc. will drive the process automatically into point B where the sputter rate is much lower and where the target operates in poisoned mode. Reducing the reactive gas flow to bring back the original sputter rate is not effective, as the process will follow the line B-D until it jumps back to point E, where the target operates in metallic mode (but where the coating shows too much absorption). There exists control equipment such as a plasma emission monitor (PEM), [5] in essence a negative feedback loop control equipment, with which a working point in the transition zone can be fixed. Ultra-fast gas flow controllers such as piezo-valves can be used in such control loop. However, such equipment is cumbersome to operate for large magnetrons (over 1 m in length), which have become, due to the dimension of the AM-LCD substrates, state of the art in the industry. In other words, sputtering with metallic targets is not an option any longer for current AM-LCD technology. Remark also that In-Sn metallic targets have a low melting point (typically around 150 C) which limits the power loads that can be used on such targets. Low power loads of course result in low deposition rates. The hysteresis effect is much more limited, if not absent, with ceramic ITO targets. Some oxygen needs to be added to the process though, in order to supplement oxygen lost to the pumping system. But, in general, the process with ceramic ITO targets is much more controllable and stable compared to the reactive process with metallic alloy targets.

Sputtering in ‘metallic’ mode Sputter rate

E

A ‘transition’ zone

D

B

C

Sputtering in ‘poisoned’ mode Reactive gas flow or partial pressure

. Fig. 1 Reactive sputtering of metallic targets in the deposition rate/reactive gas flow plane

Indium Tin Oxide (ITO): Sputter Deposition Processes

5.4.1

50 µm

. Fig. 2 Optical micrograph of a ceramic ITO (90:10) sputtering target. The bright spots are the In4Sn3O12 compound phase

In AM-LCD’s, because the lowest coating resistivity is aimed at, the classical 90:10 ITO composition is chosen (sputtered from In2O3:SnO2, 90:10 by wt% ceramic targets). Those targets have a fine microstructure consisting of two phases: a continuous In2O3 phase (in which some Sn is in solid solution) and the In4Sn3O12 compound phase (> Fig. 2). Those targets have a high density (7.1 g/cc) and a high purity (4 N).

2.2

Nodule Formation when Depositing ITO Coatings

Indium-oxide exists in three allotropic phases: In2O3, InO, and In2O. The In2O3 (indiumsesqui-oxide) is the thermodynamically most stable phase, which is present in the sputter targets. Sputtered In can redeposit as In2O, indium-sub-oxide, on the surface of the target under nonequilibrium vacuum conditions in a sputter coater, when certain conditions are fulfilled. On planar targets and depending on the design of the magnet array, up to 50% of the target surface might not be eroding during sputtering and will be exposed to significant redeposition. The sputter rate of In2O is lower than that of In2O3 and hence, redeposited In crystallized in the In2O phase will tend to grow into cubic crystals, called nodules, which remain on the target surface, [6]. The formation of the In2O nodules can be described as an equilibrium reaction: In2 O3 $ In2 O þ O2 A few parameters influence the formation of these nodules, i.e., drive the reaction above to the right side of the equation. One is the presence of residual SnO2 in the target (which gives rise to micro-arcing) [7]. Also high target temperature will promote the crystallization of redeposited In in the In2O phase. Omata et al. [8] have shown that low target density promotes nodule formation. Targets with lower density will normally operate at higher target temperature because of lower thermal conductivity (less efficient heat transfer). Remark that local porosity might also entail higher local temperatures (hot spots) on the target. Finally, there are publications showing that impurities in the target can positively influence the formation of nodules [9, 10]. This might be due to catalytic effects or to epitaxial growth mechanisms.

783

784

5.4.1 2.3

Indium Tin Oxide (ITO): Sputter Deposition Processes

Consequences of Nodules

Nodules on planar ITO targets are a pain in the ITO deposition process, especially in the LCD industry where low pinhole densities are required. Such pinholes arise, a.o., due to arcing of targets. Arcing is a local voltage breakdown in which a small (micro-arcing) or a large part (macro-arcing) of the sputter current is dumped. In2O particles, because of the difference in electrical conductivity compared to the surrounding matrix and because of the fact that they form geometrical surface asperities, tend to charge up in the sputter process and cause arcing. With both nodule growth and increase of nodule density, arcing frequency becomes higher, which finally necessitates interruption of the sputter process, venting of the system, and mechanical cleaning of the target surface. The frequency of such cleaning cycles depends on the planar target quality and on the exact sputter process used. Typically, the sputter process is interrupted after 100–150 h. The venting, cleaning itself, and the pumping to base pressure reduce the up time of the equipment considerably. Moreover, the cleaning itself is a source for particles in the equipment. And finally, precious target material is lost by the mechanical removal operation of nodules. Remark that metallic In:Sn alloy targets also suffer from strong nodule formation. In most cases, it is even more pronounced than with ITO planar targets. Moreover, mechanical removal of nodules from In:Sn targets is difficult as the metallic alloy itself is rather soft. The cleaning easily leads to damage to the target itself. Nodules have been one of the main reasons why reactive sputtering of metallic alloy targets has disappeared in the LCD industry.

2.4

Choice of Electrical Power with Which the Plasma is Sustained

As already indicated, almost the entire industry is sputtering ITO with Direct Current (DC) power supplies and using a magnetron. Radio Frequency (RF) magnetron sputtering in principle gives rise to ITO thin films with better electrooptical properties; however, the deposition rate of RF is considerably lower than for DC. Also, the cathode size – which is in practice always over 1 m in length – (and deposition speed requirements) in the display industry requires high power loads on the magnetrons. For such RF power loads, impedance matching becomes extremely cumbersome and, even more importantly, it becomes difficult to shield the rest of the coater aggregates and the environment outside the coater from the RF power. RF sputtering of ITO is therefore limited to the development laboratory.

2.5

Emerging Trend in Sputter Power Supplies

Recently, the superposition of RF electrical power on top of DC power has been used to sputter ITO coatings. This ‘‘RF on DC sputter technology’’ leads to excellent electrooptical ITO coating properties [11], better than what can be achieved with DC magnetron technology. However, even if the RF power is only a fraction of total DC + RF power (e.g., 20%), then still impedance matching and RF-shielding (EMI) becomes a real problem for large coaters (with sputter sources over 2 m in length). Remark that the RF fraction might also be as high as 50% of total power. In practice, it is therefore quasi impossible to use DC + RF for cathodes longer than approximately 1 m. This ‘‘RF on DC sputter technology’’ has therefore only found limited application in the industry. On top of that, RF technology is not compatible with rotary magnetrons (see further).

Indium Tin Oxide (ITO): Sputter Deposition Processes

5.4.1

3

State-of-the-Art Sputter Deposition of ITO in the Display Industries

3.1

ITO in LCD

Two distinct ITO coatings are being deposited in active matrix liquid crystal displays. A rather thick coating, typically between 125 and 150 nm, is used as the common electrode in the color filter (CF) plane (see > Fig. 3). The CF is made on the front glass of the display. This electrode has to draw current from a longer distance and hence needs a very low sheet resistance (by preference less than 15 ohms/square). It needs to be as transparent as possible (TA, i.e., the average transmission in the visible between 400 and 700 nm needs to be >90%). Those technical specifications require deposition of the ITO thin film at min. 200 C. Typically, 220–240 C is used. The CF-ITO coating is deposited either directly on the organic planarizing layer, in which case a bake-out of this layer is done prior to ITO deposition, or it is deposited on a thin (20 nm) SiO2 nucleation layer. Such nucleation layer is also sputter deposited. The polyimide alignment layer is then deposited directly onto the ITO. Dynamic deposition on vertical sputter coating lines equipped with radiative substrate heaters and several successive in-line magnetrons is performed. This setup eliminates particles falling down from the targets onto the substrates due to gravimetrical effects. The deposition technology is normally DC with planar magnetrons. An example of such a coating line is shown below (see > Fig. 4). The second ITO coating in an AM-LCD is used as pixel electrode (see > Fig. 5) in the thin film transistor (TFT)-plane. It forms the contact to the transistor active area and is one of the contacts of the storage capacitor. This coating is much thinner than in the CF-plane (typical thicknesses of 20–50 nm are used). The ITO in the TFT-plane is deposited by magnetron sputtering with large planar magnetrons in a horizontal cluster tool on which also PE-CVD deposition processes are integrated. The latter processes are for deposition of SiNx gate insulator and for the amorphous-Si active areas. Typical cycle times for the ITO-process are around 1 min (including substrate exchange). Again, the substrate is heated to approximately 200 C. Remark that in the older passive matrix LCD’s (PM-LCD), there are also two ITO coatings: One is on the front glass (again CF-plane) for the signal electrodes, which are kept at supply voltage; the other is on the back glass for the scanning electrodes, which are addressed sequentially (multiplexing principle). PM-LCDs (also commonly referred to as ‘‘TN’’ or ‘‘STN’’ or ‘‘CSTN’’ with C for color) are normally made on normal soda-lime glass (which is ITO layer

SiO2 nucleation layer (optional) CF layer

Planarizing layer (organic)

BM layer

Substrate

. Fig. 3 Principle scheme of a color filter substrate for LCD showing the ITO layer

785

786

5.4.1

Indium Tin Oxide (ITO): Sputter Deposition Processes

. Fig. 4 AKT® – NEW ARISTO 2200 coater for CF-ITO: Vertical in-line concept, substrate size 2,200  2,500 mm, friction-free magnetic carrier transport (Picture courtesy of Applied Materials) Source

Passivation

Storage capacitor

Drain

Semiconductor layers

PixeI TO

Gate Electrode

Gate Insulator

Glass substrate

. Fig. 5 Principle picture of the TFT-plane for AM-LCD’s showing the position of the ITO coating

much cheaper than the special glass used in AM-LCD’s). Therefore, a sodium barrier is needed between ITO and the glass. A thin SiO2 coating (15–20 nm) is typically used as such barrier. It is RF-sputtered with planar magnetrons using quartz targets or sputtered from doped Si planar targets in a reactive process using MF-AC. Remark that PM-LCD’s are mainly produced on earlier generation deposition lines (up to Gen5), such LCD-lines have a substrate size of max. 1.5 m and hence, RF technology can still be used for SiO2 deposition.

3.2

ITO in PDP

Plasma Display Panels (PDP) also consist of two glass plates coated with electrodes, phosphors, and dielectrics and joined together separated by a gas gap (typically 100 mm): Both plates define

Indium Tin Oxide (ITO): Sputter Deposition Processes

5.4.1

three cavities per pixel in which a gas plasma can be generated. [12] is a nice overview article on PDP technology. PDPs are also discussed in > Chap. 6.3.1 of this book. The front glass plate of such devices is coated with ITO which is patterned (gap between ITO electrodes is typically 70 mm) to form the display electrodes (also called coplanar electrodes). Such electrodes are typically Part 5.7 of this handbook, the technology of touch panels is explained in detail. Herewith, some aspects related to the ITO coatings used in such devices are explained. Resistive touch screens make use of both an ITO-coated glass and an ITO-coated polymer substrate (e.g., PET-film). In some capacitive touch panels also, ITO-coated glass is used. The ITO coating on glass used in resistive touch panels is mostly some 10–20-nm thick with a transmittance at 550 nm of 89–93%. Sheet resistance varies between 200 and 650 ohms/ square. The coating is deposited on glass at room temperature. Resistivity of the coating varies between 500 and 750 mohm.cm: Various sputter target compositions going from In2O3:SnO2, 97:3 to 80:20, wt%, are being used. This is the only display application of ITO where the classical 90:10 composition does not dominate. The etching rate of the ITO coating in HCl: H2O (with some HNO3 addition) at 45 C is higher than 0.1 nm/s. As cheap soda-lime glass (thickness 0.21–2.8 mm) is used for these products, a 20-nm SiO2 Na-barrier underneath the ITO coating is required. Such barrier is sputter deposited from either quartz (RF technology) or doped Si targets (MF-AC reactive process). Most resistive touch panels make also use of an ITO-coated PET substrate. This polymer film is coated in a roll-to-roll vacuum coater (also called web coater) using magnetron sputtering technology [14]. The film is transported over a cooled drum (typically at room or lower temperature) during deposition. The ITO coating has a typical sheet resistance of 250 ohms/square with a total light transmittance of 85–88%. Resistivity is typically in the 400–600 mohm.cm range (see for some examples of coating conditions [15]), which means that minimum coating thicknesses between 15 and 25 nm are sufficient.

4

New Technology for Sputter Deposition of ITO

4.1

Rotary Magnetrons and Rotary Ceramic ITO Targets

Rotary magnetrons [16] use cylindrical tube-shaped targets which are rotated around their axis during sputtering: > Figure 6 shows a principle scheme of such a sputtering source. The magnet array is steady and is located inside the target. The cylindrical target itself is fixed in

787

788

5.4.1

Indium Tin Oxide (ITO): Sputter Deposition Processes

N

S

N

. Fig. 6 Principle scheme of a rotary sputtering source

the sputter magnetron source using a pair of end-blocks. Cooling water and power is entered via one of the two end-blocks. The cooling water flows inside the target or the target backing tube and makes direct contact to the inner surface of the target or the target backing tube, respectively. As with planar magnetrons, rotary magnetrons can be mounted both horizontally and vertically in the coater. Horizontal magnetrons also exist in cantilever versions, for instance used in web coaters.

4.2

Advantages of Rotary Magnetrons

There are a number of advantages of rotary magnetrons compared to planar counterparts. First of all, almost the entire target surface erodes, which leads to much higher target utilization. Target utilization (TU) is defined as follows (Remark: The target wt does not include the backing tube and the bonding material(s) or used tie coats): TUð%Þ ¼ Sputter eroded wt of target=Initial target wt  100 Or: TUð%Þ ¼ ðInitial target wt  Target wt at end of lifeÞ=Initial target wt  100ð%Þ For planar targets, TU is limited due to the development of a race-track. It is typically between 20% and 45%, depending on the design of the magnet array, with the higher values for ITO sputtering in the LCD industry (where often tapered target edges and cutoff target corners are used). Remark however that these numbers do not take the In into account that is lost in cleaning away nodules. In reality with planar targets, TU is not higher than 40% and in many cases closer to 35%. An absolute minimum TU for rotary targets is 70%, but values as high as 90% for ITO have been reported. Again here, the design of the magnet array will determine the erosion pattern at both target ends where the race-track closes. It is here that, for non-optimized magnet arrays, some deeper erosion can happen. The extent of this deeper erosion will determine

Indium Tin Oxide (ITO): Sputter Deposition Processes

5.4.1

the actual TU. The very last few millimeters at each end of a rotary target do not erode of course, so TU’s above 90% are unlikely. In any case, the TU for rotary magnetrons is at least more than double, even closer to threefold that of planar magnetrons. This is an important aspect in the lower coating cost with this technology, as will be discussed in the next section. On rotary targets, more than 90% of the target surface erodes, but also their sputtered surfaces are subject to momentarily redeposition in between passages in front of the plasma. However, coating redeposition is much more limited with rotary targets and on top of that, they normally run at lower target temperatures (because of better cooling). The result is that rotary ITO targets sputter quasi nodule free. This is an enormous advantage of such targets: It leads to coatings with lower pinhole density and the productivity with such targets increases considerably as frequent mechanical cleaning of target surfaces is avoided. Therefore, lately, this technology has gained considerable interest in the LCD industry and it is most likely that it will take over planar deposition technology over the next years. Besides the higher TU, rotary ITO targets offer a number of other advantages: ● Higher total material availability per target, i.e., less frequent target changes needed. ● As indicated already, absence of nodule formation with ITO, which is a major element in the higher productivity with such targets. ● Rotary targets in general operate at a lower target temperature due to better target cooling and can hence be placed closer to the substrate, this improves collection efficiency. ● Rotary targets allow higher power loads than planar counterparts (power is essentially ‘‘smeared out’’ over a larger surface area). ● Rotary targets allow for quicker target change than planar targets. There are also a few disadvantages of rotary technology too: ● The rotary sputtering sources are more complex than planar magnetrons (they have moving parts). ● Rotary targets have in general a higher kg cost than planar equivalents. As will be shown below, the advantages of rotary targets outweigh by far the disadvantages. It is therefore expected that rotary technology for ITO sputtering will get general acceptance in the display industry in the next few years. The first demonstration projects have already taken place. The same trend is in fact happening for metal sputtering (Mo and other metals) in the TFT-plane. Rotary ITO targets are manufactured in pretty much the same way as their planar counterparts. Targets consist of a backing tube on which several cylindrical ITO target segments are bonded. These cylindrical segments are manufactured with a powder metallurgical technology (sintering) equivalent to planar tiles. Segments with a wall thickness of 6–10 mm are state of the art already [17]. > Figure 7 shows a picture of a rotary ceramic ITO target for the LCD industry. RF technology is today not compatible with rotary magnetrons. The main problem is that electrical power is transferred on rotary targets via a carbon brush contact. The impedance of that contact tends to vary over time, and this makes impedance matching difficult. Even if the matching and shielding problems with ‘‘RF on DC sputtering technology’’ could eventually be solved, then still it is unlikely that it could be combined with rotary magnetrons. This is one more reason to believe that RF on DC sputter technology will not prevail in the mainstream AM-LCD industry.

789

790

5.4.1

Indium Tin Oxide (ITO): Sputter Deposition Processes

5

Costing of Depositing ITO in the Display Industry

5.1

CF-ITO

Taking into account the differences between planar and rotary ITO target technology discussed above, a cost comparison between both technologies for the production of CF-ITO on a Gen8.5 line (substrate size 2.2  2.6 m) can be made. > Table 1 below shows the parameterization which has been used. For rotary technology, two distinct cases have been taken: one which can

. Fig. 7 Multi-segment Gen5 ITO sintered rotary target for LCD applications

. Table 1 Boundary conditions of cost calculations for CF-ITO in Gen8.5. DDR indicates the Dynamic Deposition Rate (in nm.m/min): It is the coating thickness in nm if the substrate speed is 1 m/min ITO for CF plane in TFT-LCD, ex. 150 nm, produced Case 1: on Gen8.5 line planar Target dimension (m)

8  0.23  2.7

8  0.147 (OD) 8  0.155 (OD)  2.7 (L)  2.7 (L)

Target thickness (mm)

9

6

10

Relative target cost (in €/kg)

1

2.5

1.7

a

Target utilization (%)

34

82.5

87.5

Substrate collection efficiency (%)

50

60

65

Max. target power (kW/m)

7

10

12

DDR (nm.m/min)

58.33

100

130

Substrate yield (%)

96

98

98

Time between coater cleaning intervals (h)

80

134.4

121.3

Duration maintenance interval @ 4 operators, including venting and evacuation (h)

12

10

10

Mounting new target set @ 4 operators (h)

6

4

4

ITO wt available per target (kg)

39.74

51.02

87.45

33,461

65,896

Yielded produced Gen8.5 substrate units with set of 8,767 8 targets (#) a

Case 2: rotary, Case 3: rotary, cons. opt.

Takes into account lost indium due to cleaning away nodules

Indium Tin Oxide (ITO): Sputter Deposition Processes

5.4.1

be considered as conservative (case 2) and one which can be considered as an optimized case (case 3). In all cases, a target density of 7.11 g/cc has been taken. The following assumptions have also been taken into account: ● ● ● ● ● ●

Hourly labor rate of 24 €/h (fully loaded). 350 working days in a 24/7 (hours/days) regime. Electricity rate of 0.075 €/kWh. Yearly general coater maintenance has been included in the machine cost. Coater has been depreciated over 5 years. Energy for vacuum pumps, substrate heaters, substrate transport, and other coater aggregates has not been taken into account (is equal for all considered cases). ● Costs for sputter gases, cooling water, and compressed air (all small cost elements) have also been left out.

The calculations (> Fig. 8) show that rotary technology yields substantial cost savings, typically between 22% and 52% for deposition of CF-ITO. Remark that materials’ cost is a very large fraction of total costs per unit CF-substrate with ITO deposition: 79% in case of planar technology and between 86% and 82% in case of rotary technology (depending on the exact conditions under which such rotary targets are used, i.e., case 2 vs. case 3). > Figure 9 shows the number of (yielded) Gen8.5 substrates produced for 1,000 € spent in sputtering targets under the three cases calculated above. It indicates that the productivity with rotary targets is, even in the conservative case, much higher than with planar targets: The manufacturer can generate more value for money spent. Similar cost comparisons can be made for ITO in the TFT-plane. They lead to equivalent cost advantages for rotary ITO technology as has been demonstrated here.

25 –22.9 %

Materials cost Labour cost Energy cost Machine cost

20

Cost (€/unit)

–52.2%

15

10

5

0 Case 1: planar

Case 2: rotary, conservative

Case 3: rotary, optimized

. Fig. 8 Coating cost calculation per unit of Gen8.5 substrate for the CF-ITO when using different sputtering source technologies

791

5.4.1

Indium Tin Oxide (ITO): Sputter Deposition Processes

+18.9 %

+100.9 %

Case 2: rotary, conservative

Case 3: rotary, optimized

120 Yielded production for 1000 € spent in sputtering targets

792

100 80 60 40 20 0 Case 1: planar

. Fig. 9 Gen8.5 substrate production for 1,000 € spent in sputtering targets

. Table 2 Boundary conditions for cost calculations of ITO on PET for TP ITO for TP, ex. 25 nm, produced on roll-to- Case 1: roll coater with one coating source planar

Case 2: rotary, cons.

Case 3: rotary, opt.

Target dimension (m)

1.6  0.2

Target utilization (%)

20

0.147 (OD)  1.6 (L) 0.159 (OD)  1.6 (L) 80

87,5

Max. target power (kW/m)

7

10

10

DDR (nm.m/min)

58.3

100

108.3

Time between coater cleaning intervals (h)

48

80.6

87.4

Duration target change (excluding pumping 1.5 @ 2 operators) (h)

1

1

Extra downtime for cleaning target at shield 1 change (h)

0

0

ITO wt available per target (kg)

20.48

30.23

51.82

Yielded production with one target (m2)

10,640

82,320

167,384

5.2

ITO in TP (PET-FILM)

A comparative calculation to deposit 25-nm ITO with a single deposition source in a roll-toroll coater with max. substrate length of 1,000 m (yielded substrate width of 1.4 m) has also been performed. The boundary conditions are similar to those indicated in > Table 1 unless otherwise stated in > Table 2.

Indium Tin Oxide (ITO): Sputter Deposition Processes

5.4.1

3 –43.0 %

–54.4 %

Cost (€/m2)

2 Materials cost Energy cost

Labour cost Machine cost

1

0 Case 1: Planar

Case 2: Rotary, cons.

Case 3: Rotary, optim.

. Fig. 10 Cost comparison for the ITO on PET application (TP)

Some other boundary conditions which are identical for the three cases: ● Downtime for substrate change (including venting and evacuation): 4.25 h, equal for all cases. ● Downtime for shield change (at substrate change only, excluding venting and evacuation): 1.25 h, equal for all cases. ● Coater maintenance cost has been included in the machine cost. Coater is depreciated over 5 years. All other boundary conditions can be taken from the CF-application given above. 2 > Figure 10 shows the cost calculations per m of yielded PETsubstrate for each of the three cases. Remark that rotary technology again leads to considerably lower coating costs. The contribution of materials’ cost in the total coating cost is considerably lower (46% for case 1 and 38% and 27.5% for cases 2 and 3 respectively) than in the CF case. The primary reason (valid for all cases) is the considerably lower coating thickness (25 nm vs. 150 nm) which has been considered here. It is also due to the fact that web coaters generally are more expensive than glass coating lines (for the same source length); in addition the calculations performed here are for a substrate width of 1.4 m only, much smaller than the 2.2 m for Gen8.5 above. Evaluating above cost comparison, it should not surprise us that the yielded coated substrate production per 1,000 € spent in sputtering targets is much higher in case of rotary technology: +110% compared to planar technology in the conservative case 2 and + 265% in the optimistic case 3.

6

Conclusions

DC magnetron sputtered ITO, using ceramic targets, is still the transparent conducting oxide of choice for most applications in display technology (LCD, PDP and TP). Rotary magnetron

793

794

5.4.1

Indium Tin Oxide (ITO): Sputter Deposition Processes

technology for ITO deposition can bring advantages both in process stability and product yield – due to the reduction of nodules – and in coating costs. As such technology entered other industries such as architectural glass coating, web coating, and photovoltaics quite some time ago successfully, it can be assumed that it will also happen in the display industry. The first coating lines with such magnetrons have already been made or retrofitted.

Acknowledgments Paul Lippens is the sole author of part 5.4.1.5.2 ‘‘ITO in TP (PET-FILM).’’

References 1. Haacke G (1976) New figure of merit for transparent conductors. J Appl Phys 47:4086–4088 2. Minami T (2000) New n-type transparent conducting oxides. MRS Bulletin/Aug 2000, pp 38–44 3. Bright C (2008) Alternative transparent conductive oxides (TCO) to ITO. In: Proceedings of the 51st annual technical conference of the society of vacuum coaters (SVC), Chicago, April 19–24, 2008, pp 840–850 4. Schiller S et al (1994) The optical plasma emission – a useful tool to monitor and to control the reactive magnetron sputtering. In: Presentation at the conference on in-situ monitoring and diagnostics of plasma processes, November 18, 1994, Ghent, Belgium 5. Stru¨mpfel J, May C (2000) Low ohm large area ITO coating by reactive magnetron sputtering in DC and MF-mode. Vacuum 59(2–3):500–505 6. Lippens P et al (1998) Chemical instability of the target surface during DC-magnetron sputtering of ITO coatings. Thin Solid Films 317:405–408 7. Nakashima K, Kumahara Y (2002) Effect of tin oxide dispersion on nodule formation in ITO sputtering. Vacuum 66:221–226 8. Omata T et al (2006) Characterization of indium-tin oxide sputtering targets showing various densities of nodule formation. Thin Solid Films 503:22–28 9. Reger N et al (1998) Sputtering effects during 3Dimaging of indium-tin-oxide sputtering targets. In: Gillen et al (eds) Secondary ion mass spectrometry, SIMS XI, Wiley, pp 855–858

10. Schlott M et al (1996) P-31: nodule formation on indium-oxide tin-oxide sputtering targets. SID96 DIGEST, Santa Ana 11. Stowell M et al (2007) RF-superimposed DC and pulsed DC sputtering for deposition of transparent conductive oxides. Thin Solid Films 515:7654–7657 12. Boeuf JP (2003) Plasma display panels: physics, recent developments and key issues. J Phys D Appl Phys 36:R53–R79 13. Nagao N et al (2002) Development of Ag fence electrode structure PDP without ITO. Proc Int Disp Workshops 9:765–768 14. Kukla R et al (2004) New modular roll-to-roll PVD web coater for clean room production. In: Proceedings of the 47th annual technical conference of the society of vacuum coaters (SVC), April 24–29, 2004, Dallas, pp 174–178 15. Lippens P, Verheyen P (1994) Electrochromic halfcells on organic substrate produced by subsequent deposition of ITO and WO3 in a roll coater using optical plasma emission monitoring. In: Proceedings of the 37th annual technical conference of the society of vacuum coaters (SVC), 1994, Boston, pp 254–259 16. US Patent 4,356,073, 26 Oct. 1982 of Shatterproof Glass Corp 17. Lippens P et al (2008) Thin film ITO coatings obtained using rotary sputtering targets. In: De Gryse R et al (eds) Proceedings of the international conference on thin films 14 (ICTF14 & RSD2008), Ghent, Belgium, pp 52–55

5.4.2 ITO Replacements: Carbon Nanotubes Axel Schindler 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 796

2

Carbon Nanotube Networks as Transparent Electronic Conductor . . . . . . . . . . . . . . . 797

3

Deposition Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797

4 4.1 4.2 4.3

Properties of Thin Carbon Nanotube Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 799 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 799 Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 800 Electrical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801

5

Demonstrated Display Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803

6

Conclusions and Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.4.2, # Springer-Verlag Berlin Heidelberg 2012

796

5.4.2

ITO Replacements: Carbon Nanotubes

Abstract: Randomly oriented networks of carbon nanotubes are a promising candidate for ITO replacement. High flexibility and mechanical robustness, as well as vacuum-free deposition from suspensions and almost unlimited material resources enable new applications and lower production costs. This chapter shall give an overview of the main aspects of this new material starting with a general description and deposition techniques. In the following, the mechanical, optical, and electrical properties are discussed. The chapter ends with presented display applications, conclusions, and future prospects. List of Abbreviations: AM, Active Matrix; CNN, Carbon Nanotube Network; CNT, Carbon Nanotube; DOS, Density of States; Ef, Fermi Energy Level; ITO, Indium Tin Oxide; LC, Liquid Crystal; LCD, Liquid Crystal Display; m-SWNT, Metallic Single-Walled Nanotube; PDLC, Polymer-Dispersed Liquid Crystal; R□, Sheet Resistance [O/□]; RIE, Reactive Ion Etching; SWNT, Single-Walled Nanotube; s-SWNT, Semiconducting Single-Walled Nanotube; T, Optical Transmittance [%]; TEC, Transparent Electronic Conductor; TN, Twisted Nematic

1

Introduction

For display applications where an optically transparent, but electrically conductive material is needed, indium tin oxide (ITO) is the predominantly used material. Its electro-optical performance, i.e., the achievable conductivity for a given transmission, is still unmatched, and the processes for deposition and patterning of the ITO are well established. There are however drawbacks besides the limited indium resources that make the search for alternatives worthwhile. Especially when it comes to applications where increased mechanical stability is required, the mostly crystalline ITO is not the optimal candidate. Besides touch-sensitive display panels, where a higher resistivity of the transparent electrode is acceptable, mechanical stability combined with high conductivity and transparency is mandatory for future flexible displays. For proposed applications like rollable or wearable displays, all materials including the transparent electronic conductor (TEC) need to withstand highly increased mechanical stress compared to current flat panel displays on glass, caused by bending and strain. The remarkable combination of electrical, mechanical, and optical properties of thin films created from carbon nanotubes (CNT) makes them well suited for such applications. In contrast to ITO, they are also very compatible to flexible plastic substrates. The added possibility to deposit these films vacuum-free under room temperature might also lead to cost reduction in the fabrication process. Despite the large improvements in conductivity, the electro-optical performance of CNT-TECs is still inferior compared to ITO. Nevertheless, several prototype displays, from simple directly addressed liquid crystal displays (LCD) to full-color activematrix (AM) LCDs, were presented in the last years demonstrating the applicability of CNTTECs to display applications. Some basic information about carbon nanotubes, including their electronic structure, their synthesis, and purification techniques as well as production of CNT dispersions and processes to separate metallic from semiconducting nanotubes, can be found in > Chap. 5.3.5. In the following sections, the details regarding their application as transparent electronic coatings will be discussed. Starting with some basic facts about thin films of CNTs and their properties and continuing with deposition techniques, doping effects and published display applications and ending with future prospects.

ITO Replacements: Carbon Nanotubes

2

5.4.2

Carbon Nanotube Networks as Transparent Electronic Conductor

As discussed in > Chap. 5.3.5, single-walled carbon nanotubes (SWNT) are hollow cylinders of carbon atoms in a hexagonal lattice with diameters from roughly 0.5 to 2 nm and lengths from 100 nm to several centimeters [1]. Depending on their exact structure, SWNTs can be metallically conducting or semiconducting. With all common synthesis procedures, a mixture of diverse geometries in a certain diameter range is produced. About one third of the produced tubes are metallic while two thirds are semiconducting. The nanotubes can be directly grown on a substrate using chemical vapor deposition (CVD) processes. With this technique, the density and alignment of the CNTs can be closely controlled. The necessary process temperatures of 500–1,200
C are too high though even for display grade glass. There are methods to transfer directly grown CNNs to other substrates, but for the application as TECs, where large areas need to be covered by cost effective processes, solution processing seems to be the method of choice, especially since in the produced films, an alignment of the tubes is not necessary or even a disadvantage as long as isotropic conductivity is desired. Starting with SWNTs in powder form, the nanotubes are dispersed in aqueous surfactant solutions leading to long-time stable suspensions. These suspensions can then be deposited using potentially cheap techniques (see next section) compared to the vacuum processes used for ITO deposition. Thin layers of randomly oriented carbon nanotubes simplified can be thought of as a 2D network of interconnected conductive sticks. It is easy to understand that the conductivity of such a layer is not only determined by the intrinsic conductivity of the individual tubes, but also by the intertube resistance. When using CNT material with the classic ratio of one third metallic SWNTs (m-SWNT) and two thirds semiconducting SWNTs (s-SWNT), there are three possible regimes, depending on the network density or thickness of the layer that can be described by percolation theory [2, 3]. (1) If the density is too low and only some nanotubes touch each other there is no continued conduction. (2) If the density is above the percolation threshold of the s-SWNT but due to the 1:2 ratio between metallic and semiconducting nanotubes below the percolation threshold of the m-SWNT, the CNT layer shows nonlinear I-V characteristics, acting like a semiconductor that can be used for CNT-network (CNN) transistors [4]. (3) For network densities above the percolation threshold of m-SWNTs, the CNT layers act like a conductor with linear I-V characteristics [5]. For these kind of networks, the above-introduced model of randomly oriented straight sticks is somewhat misleading. Due to the high aspect ratio of the single tubes, their flexibility, and strong van der Waals interactions, the nanotubes form seamlessly interwoven networks with no noticeable beginnings and ends of single tubes (see > Fig. 1). The tendency to form thicker or thinner bundles depends on the deposition technique and results in different roughness values of the deposited layers as will be discussed later.

3

Deposition Techniques

Being able to disperse CNTs in aqueous solutions enables the use of cost-effective deposition techniques at low temperatures. The drawback of having to use a dispersant introduces a contamination with an insulating or even electrically charged material though. In order to

797

798

5.4.2 0 nm 0

ITO Replacements: Carbon Nanotubes

500

1000

1500 46.8 nm

500

1000

1500

−28.2 nm

. Fig. 1 Atomic force microscopy image of a 50 V/□; SWNT network on glass (Reprinted with permission from [6]. Copyright 2006, American Institute of Physics)

receive optimal conductivity, care has to be taken to completely remove those unwanted species after deposition [7]. Nowadays, surfactant-free dispersions are also available on the market. Kauppinen et al. presented a dry and room-temperature method where the nanotubes are deposited directly downstream of the synthesis reactor. They use electrical charging to control the deposition [8]. The method was demonstrated for high low-density networks. Scaling for large area applications has yet to be presented, though. A spin-off company called Canatu was however founded that among other nanotube related business shall also offer transparent conductive coatings fabricated with this technique. Dip coating as presented by Ng et al. is a very simple setup [9]. Substrates are treated with an adhesion promoter, dipped into a CNT surfactant suspension and dried in nitrogen atmosphere. This process can be cycled to achieve the necessary layer thickness. With 10 cycles, a sheet resistance of 130 O/□ at 70% transmission was achieved after removal of surfactant and adhesion promoter. While this is a very simple setup, homogeneity, scalability, and contamination of the substrates’ back surface might be issues. Xiong et al. also demonstrated patterned and aligned deposition using a controlled pulling process [10]. Lima et al. used electrophoretic deposition for the creation of CNT TECs in thin films [11]. The substrates are first coated with a thin metal layer that serves as electrode. Immersed into the surfactant solution, an electric field between substrate and another metal electrode charges the nanotubes and transports them to the substrate surface. After the deposition the metal layer is etched away. While the extra metal layer makes this process less favorable, it has the advantage that the nanotube layer can be patterned by deposition. An often used process to create films of CNTs is by vacuum filtration [12]. For this method, the CNT suspension is sucked through a filter membrane by vacuum. The nanotubes form a dense layer on top of the filter. The thickness can be controlled by the volume and concentration of the suspension. After the filter process and sufficient rinsing, the nanotube

ITO Replacements: Carbon Nanotubes

5.4.2

layer can be transferred by two techniques. In soft lithography, an elastomeric stamp is brought in contact with the nanotube network which is then transferred from the membrane to the desired substrate. In each transfer, the surface energy of the target material needs to be higher than the surface energy of the source material in order to achieve a sufficient transfer. By molding of the elastomer, a patterned deposition can be achieved [13, 14]. In the second method, the nanotube layer is brought in contact with the substrate followed by dissolving of the filter in an appropriate solvent [15]. While scaling of these methods seems difficult, vacuum filtration is a simple method for material testing, and the best conductivities were reported using this technique [12]. A promising method when it comes to large area applications is spray coating. CNT suspensions can be easily spray coated on large substrates using either a simple air brush [16, 17] or highly automated systems. The substrates are usually moderately heated to accelerate water evaporation of the striking droplets. Large droplets can increase the layer roughness presumably caused by the coffee stain effect. Layers with very low roughness and high uniformity were reported by Tenent et al. using ultrasonic spraying [18]. Combined with the scalability and compatibility with many substrate materials, including plastic substrates, spray coating seems predestined for display applications. An ink-jet method was also presented for low-density CNT deposition [19]. It uses an aerosol jet and enables patterned deposition. While scalability might not be an issue, it is questionable if a satisfying throughput in a production process can be achieved.

4

Properties of Thin Carbon Nanotube Films

In order to replace the well-established ITO, the alternatives need to exhibit a certain degree of predominance in at least some properties. In the following section, the important mechanical, optical, and electrical properties of carbon nanotube layers related to display applications are discussed and comparisons to ITO are drawn.

4.1

Mechanical Properties

For reliable processing of CNT thin films, a good adhesion to the substrate is mandatory. Carbon nanotubes show good adhesion to plastic substrates [17, 20]. For the deposition on glass and other inorganic substrates, a self-assembled monolayer of an amine-terminated silane is commonly used as adhesion promoter [13, 17, 21]. Alternatively, organic layers might be used as adhesion promoter. While the achievable adhesion is enough for further processing, mechanical scratching leads to punctual removal. In areas where a direct electrical contact is not necessary, polymeric capping layers can increase the mechanical stability [22]. Because of the network nature, the polymer soaks into the nanotube layer, increasing also the substrate adhesion. The high flexibility and mechanical robustness of CNT layers was demonstrated by several groups. Compared to oxide TECs like ITO, CNNs show by far less deterioration of the conductivity by mechanical stress like bending, folding, abrasion (at least in combination with a binder), or stretching [9, 23–25]. While the crystalline phase of the ITO cracks easily leading to total failure, the CNNs stay intact even when the position of some nanotubes is moved. Hu et al. demonstrated electrical conductivity for strains up to 700% [25]. > Table 1

799

800

5.4.2

ITO Replacements: Carbon Nanotubes

. Table 1 Mechanical stress tests of CNT/Acrylic Binder and ITO on 125 mm PET film (Data taken from [23]) Test

ITO

CNT

Uniaxial strain

First cracks at 2.5% strain catastrophic failure 90% [33]. In recent years, a better understanding of the conduction in CNNs was established and routes for improvement were introduced. Besides obvious factors like having high-quality and purified raw material, several factors can increase the conductivity. Since conduction is limited by intertube resistance, long nanotubes are preferred [34]. Also a smaller bundle diameter or preferably individual SWNTs give lower junction resistance [33, 34]. The complete removal of residual surfactant by acid treatment can also lead to a better conductivity [7]. Several groups have however demonstrated that the main effect of treatment with acids like HNO3 or SOCl2 is not the removal of unwanted species, but electrical p-type doping of the nanotubes [35–37]. These redox dopants were found to increase the delocalized carrier density. But more importantly, the barrier for intertube conduction is lowered by doping [29, 33, 38]. Besides intentional doping, CNNs processed under ambient air are p-doped to a certain degree by oxygen [39, 40]. All dopants can be repeatedly desorbed by thermal treatment [38], alternatively hydrazine (NH2NH2) doping reverses the effect [41]. Stabilization of the dopants was demonstrated by using a capping layer of PEDOT/PSS [42]. While for several years it was propagated that purely metallic CNNs would give an optimal conductivity, the recently availability of highly enriched material led to analyses with unexpected results. Blackburn et al. discovered that redox-doped semiconducting CNNs have a higher conductivity than metal-enriched films [41]. The cause can be explained by looking at the density of states (DOS) over energy plot in > Fig. 3. This plot shows the number of states at each energy level that can be occupied by a charge carrier for two m-SWNTs and two s-SWNTs. The band gap of the semiconducting nanotubes is defined by the region where no states are allowed, while metallic nanotubes have states at each energy level. For a more detailed discussion of the density of states of carbon nanotubes and the characteristic peaks called van Hove singularities, please see > Sect. 2 of Chap. 5.3.5. In the intrinsic case, i.e., without doping the Fermi level Ef lies at the energy level in the symmetric center of the plot. Ef is the energy level where the probability of a state to be occupied by an electron is 0.5. Above (for higher energies), the probability decreases, while it increases below. This means that in the intrinsic case m-SWNTs are conducting, while s-SWNTs are not due to a lack of free electrons in the conduction band or holes in the valence band. This is because there are no allowed states or the probability for the existing states to be occupied is too low. By p-type doping, Ef shifts toward the valence band. With sufficient doping, the fermi level falls into the van Hove singularities outside of the energy gap, leading to a higher density of free charge carriers in s-SWNTs than in m-SWNTs. Additionally, the intertube barrier is lowered more efficiently for junctions of s-SWNTs [41]. A possible drawback of the p-type doping is the simultaneous increase in the work function [42]. It also reduces the stability in regard to the level of doping. Another important factor for display applications is the nanotube/metal contact resistance. High work function metals like Pd, Au, Mo, Ag, and Ti are commonly used to contact carbon nanotubes. Recently reported values for the contact resistance to CNNs lie in the order of several 10 mO/cm2 [43, 44]. For Ag, e.g., the reported specific contact resistance to ITO is

ITO Replacements: Carbon Nanotubes

5.4.2

. Fig. 3 Calculated density of states for (17,0) and (10,5) s-SWNTs and (10,10) and (8,8) m-SWNTs, representative of the 1–1.4 nm diameter distribution produced by laser ablation, plotted on an absolute energy scale, versus the normal hydrogen electrode (NHE) and versus vacuum. Horizontal lines show the approximate Fermi level following intentional (hydrazine and thionyl chloride) chemical treatments and unintentional oxygen adsorption (Reprinted with permission from [41], DOS for m-SWNTs were slightly corrected. Copyright 2008 American Chemical Society)

8 mO/cm2 compared to 20 mO/cm2 for undoped CNNs. Lim et al. reported that besides the work function, the wettability of the metal on top of the nanotubes also plays an important factor [45].

5

Demonstrated Display Applications

For the successful use of CNT TECs in display applications, not only a homogeneous deposition technique is required but also a reliable process for patterning in micron-sized structures. Since most deposition techniques are not capable of producing patterned layers, subtractive patterning processes are necessary. As for other thin films, the patterning by photolithography and subsequent chemical or physical etching can be applied to nanotube layers as well. Reactive ion etching (RIE) with an O2 plasma can be used for the effective removal of CNTs [22, 46, 47]. An example for a vacuum-free removal of at least low-density CNT networks is the usage of a CO2 snow jet [4, 48]. Liftoff techniques were also used. They are however not favorable for production. Although CNT networks were propagated as ITO replacement for display applications in 2003 [49], it took several years before the first prototypes were presented. The first intermediate step was the presentation of a single pixel polymer-dispersed liquid crystal (PDLC) cell on plastic by Chan Yu King et al. [50]. A PDLC display showing real content via directly addressed segments was presented in 2007 [51]. The realization of PDLC displays is straightforward since no alignment layers are necessary. They are however not relevant in commercial products due

803

804

5.4.2

ITO Replacements: Carbon Nanotubes

. Fig. 4 Twisted nematic LCD prototypes with carbon nanotube pixel electrodes. (a) Full color 4 in. qVGA AMLCD (b) Flexible directly addressed display on plastic [43]

to low contrast ratios and high addressing voltages. These displays only consist of sandwiched glass or plastic substrates with patterned electrodes and the liquid crystal in polymer matrix in between. For the commercially dominant twisted nematic (TN) LCDs, an alignment layer for the orientation of the liquid crystal molecules is necessary. By using a polymer capping layer on the nanotube network that is rubbed after curing, Schindler et al. from the University of Stuttgart demonstrated reliable LC orientation even on relatively rough CNT networks. By increasing the polymer thickness, a complete planarization is possible though. In 2008, the group demonstrated the first full-color AMLCD where ITO was completely replaced by CNNs (see > Fig. 4a) [22, 52]. The fact that the CNNs were integrated as pixel electrodes into a standard backplane and as common electrode in the color filter process clearly demonstrates the applicability of this new material into state of the art display processes. In 2009 directly addressed flexible TN-LCDs on plastic substrates using transparent CNT electrodes were presented (see > Fig. 4b) [43]. Samsung in collaboration with NanoIntegris introduced CNT TECs into electrophoretic displays and demonstrated a 14.3 in. full-color e-paper at the 8th International Meeting on Information Display (IMID) in 2008. Besides several LCD applications, the use of CNT electrodes in organic light-emitting diodes was also investigated by several groups. The nanotube networks were used as holeinjecting anodes [20, 22, 26, 53, 54], electron-injecting cathodes [55], or for both electrodes [56]. Brightness levels as high as 4,500 cd/m2 and current efficiencies up to 2.5 cd/A were reported [26]. The hole-conducting nature of the nanotubes is not only propagated for hole injection in OLEDs, but also as efficient hole collector in organic photovoltaics [18].

6

Conclusions and Future Prospects

Much progress in the realization and optimization of transparent conducting films consisting of randomly oriented carbon nanotube networks was made in the last years. Low-cost deposition techniques from suspension, capable of homogeneous large area deposition, were demonstrated, and detailed studies of the network led to a deeper understanding of the conduction and metal/ CNT contact. This also led to improvements in the conductivity by doping. The newfound

ITO Replacements: Carbon Nanotubes

5.4.2

sorting techniques by diameter, length, and electronic type give further possibilities to tailor the material for a certain application. While, interestingly, s-SWNTs are proposed to give best conductivity after doping, m-SWNT networks can have a lower work function. The applicability of CNT networks as ITO replacement in liquid crystal displays was demonstrated by several prototypes, and cooperations with major display panel manufacturers manifest the interest of the display industry. OLEDs were also realized with CNT TECs. The publications are limited to single devices however. Real displays were not demonstrated so far. Even though much progress in increasing the conductivity of CNT networks was made in recent years, the values of ITO were not achieved so far. This makes CNTs less qualified for high-current applications like OLED lighting. In voltage-driven applications like LCDs and especially in active-matrix displays with only low aspect ratio pixels and common electrodes even with increased resistance, well-working displays can be realized. The great advantage of CNT TECs over ITO lies in their flexibility and mechanical as well as chemical robustness combined with low temperature vacuum-free coating methods that are compatible with plastic substrates. Applications like flexible displays, touch-sensitive devices, and flexible electronics in general will therefore be the most likely fields where transparent carbon nanotube coatings have the chance to enter commercialization. At the current state of research, spray coating with subsequent patterning by reactive ion etching seems to be the best method in terms of reliability, controllability, substrate size, and achievable feature resolution. The real breakthrough in deposition costs will be achieved if this can also be realized in combination with the big advantage of wet deposition – nonsubtractive patterning. With methods like gravure coating high-volume production with low investment and deposition costs would be possible. This would firstly allow for simple low information content but highvolume products like price tags or even decorative displays in clothing. With evolving manufacturing techniques also high-resolution displays seem possible. Besides flexible applications, CNT TECs will have to bring significant cost reduction to capture some of ITO’s market share. Even though wet deposition techniques might lead to savings in investment and deposition costs, it can so far not be estimated if the entire coating costs per area will be lower than for ITO or even competitive. In today’s display fabrication, more than two thirds of the costs for an ITO coating are caused by material costs. Mainly because of the high indium price (compare > Chap. 5.4.1). Therefore, machine costs only play a secondary role. The current bottleneck in commercial carbon nanotube production is the growth itself. The used methods do not allow for high-volume production of high-quality material in terms of purity, defects, and diameter distribution. The direct production costs today lie in the order of a few hundred dollars per gram. It seems however reasonable to predict that when the demand is growing fast enough to allow for significant scale-up, the costs can quickly be reduced by about an order of magnitude. Post-synthesis processing and the methods of separation by electronic type in particular have evolved rapidly in recent years (compare > Sect. 4 of Chap. 5.3.5). Further improvements will keep the additional costs low so that the overall price will be dominated by the material. It is worth noting though that as long as the starting material will have the classic ratio of one third metallic to two thirds semiconducting nanotubes, the cost will at least be increased by 50% for s-SWNTs and by 200% for m-SWNTs. Summarizing it can be said that carbon nanotube TECs have a very high potential, especially when it comes to flexible displays. In this widely proposed new market, they have a clear edge over the commonly used ITO. The technology is however still very young, and

805

806

5.4.2

ITO Replacements: Carbon Nanotubes

improvements in several areas are still necessary. Especially in growth methods and deposition techniques that use the full potential of the wet processing capabilities. The demonstrated prototype displays have successfully demonstrated the applicability of this new material to display applications.

References 1. Popov VN (2004) Carbon nanotubes: properties and application. Mater Sci Eng: R: Reports 43:61–102 2. Hu L, Hecht DS, Gru¨ner G (2004) Percolation in transparent and conducting carbon nanotube networks. Nano Lett 4:2513–2517 3. Li J, Zhang Z, Zhang S (2007) Percolation in random networks of heterogeneous nanotubes. Appl Phys Lett 91:253127–3 4. Snow ES, Novak JP, Campbell PM, Park D (2003) Random networks of carbon nanotubes as an electronic material. Appl Phys Lett 82:2145–2147 5. Ska´kalova´ V, Kaiser AB, Woo Y, Roth S (2006) Electronic transport in carbon nanotubes: from individual nanotubes to thin and thick networks. Phys Rev B 74:085403 6. van de Lagemaat J, Barnes TM, Rumbles G, Shaheen SE, Coutts TJ, Weeks C, Levitsky I, Peltola J, Glatkowski P (2006) Organic solar cells with carbon nanotubes replacing ITO as the transparent electrode. Appl Phys Lett 88:233503 7. Geng HZ, Kim KK, So KP, Lee YS, Chang Y, Lee YH (2007) Effect of acid treatment on carbon nanotubebased flexible transparent conducting films. J Am Chem Soc 129:7758–7759 8. Zavodchikova MY, Nasibulin AG, Kulmala T, Grigoras K, Anisimov AS, Franssila S, Ermolov V, Kauppinen EI (2008) Novel carbon nanotube network deposition technique for electronic device fabrication. Phys Status Solidi (B) 245:2272–2275 9. Ng MHA, Hartadi LT, Tan H, Poa CHP (2008) Efficient coating of transparent and conductive carbon nanotube thin films on plastic substrates. Nanotechnology 19:205703 10. Xiong X, Chen C, Ryan P, Busnaina AA, Jung YJ, Dokmeci MR (2009) Directed assembly of high density single-walled carbon nanotube patterns on flexible polymer substrates. Nanotechnology 20:295302 11. Lima MD, de Andrade MJ, Bergmann MJ, Roth S (2008) Thin, conductive, carbon nanotube networks over transparent substrates by electrophoretic deposition. J Mater Chem 18:776–779 12. Wu Z et al (2004) Transparent, conductive carbon nanotube films. Science 305:1273–1276 13. Meitl MA, Zhou Y, Gaur A, Jeon S, Usrey ML, Strano MS, Rogers JA (2004) Solution casting and transfer

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

printing single-walled carbon nanotube films. Nano Lett 4:1643–1647 Zhou Y, Liangbing H, Gru¨ner G (2006) A method of printing carbon nanotube thin films. Appl Phys Lett 88:123109–1–3 Chhowalla M (2007) Transparent and conducting SWNT thin films for flexible electronics. J Soc Info Display 15:1085–1088 Ka¨mpgen M, Duesberg GS, Roth S (2005) Transparent carbon nanotube coatings. Appl Surf Sci 252: 425–429 Schindler A, Spiessberger S, Fruehauf N, Novak JP, Yaniv Z (2007) Solution-deposited carbon nanotube networks for flexible active matrix displays. In: Proceedings of Asia Display, Shanghai, China, vol 1, pp 882–887 Tenent RC, Barnes TM, Bergeson JD, Ferguson AJ, To B, Gedvilas LM, Heben MJ, Blackburn JL (2009) Ultrasmooth, large-area, high-uniformity, conductive transparent single-walled-carbonnanotube films for photovoltaics produced by ultrasonic spraying. Adv Mater 21:3210–3216 Vaillancourt J et al (2008) All ink-jet-printed carbon nanotube thin-film transistor on a polyimide substrate with an ultrahigh operating frequency of over 5 GHz. Appl Phys Lett 93:243301–243303 Li J, Hu L, Wang L, Zhou Y, Gru¨ner G, Marks TJ (2006) Organic light-emitting diodes having carbon nanotube anodes. Nano Lett 6:2472–2477 Opatkiewicz JP, LeMieux MC, Bao Z (2010) Influence of electrostatic interactions on spin-assembled single-walled carbon nanotube networks on aminefunctionalized surfaces. ACS Nano 4:1167–1177 Schindler A, Spiessberger S, Hergert S, Fruehauf N, Novak JP, Yaniv Z (2008) Suspension-deposited carbon nanotube networks for flexible active matrix displays. J Soc Info Display 16:651–658 Arthur D, Glatkowski P, Wallis P, Trottier M (2004) Flexible transparent circuits from carbon nanotubes. In: SID Digest, Seattle, Washington, vol 35, pp 582–585, Society for Information Display Park Y, Hu L, Gru¨ner G, Irvin G, Drzaic P (2008) Integration of carbon nanotube transparent electrodes into display applications. SID Digest 37:537–540

ITO Replacements: Carbon Nanotubes 25. Hu L, Yuan W, Brochu P, Gru¨ner G, Pei Q (2009) Highly stretchable, conductive, and transparent nanotube thin films. Appl Phys Lett 94:161108 26. Williams CD, Robles RO, Zhang M, Li S, Baughman RH, Zakhidov AA (2008) Multiwalled carbon nanotube sheets as transparent electrodes in high brightness organic light-emitting diodes. Appl Phys Lett 93:183506 27. de Andrade MJ, Lima MD, Ska´kalova´ V, Bergmann CP, Roth S (2007) Electrical properties of transparent carbon nanotube networks prepared through different techniques. Phys Status Solidi RRL 1:178–180 28. Trottier CM, Glatkowski P, Wallis P, Luo J (2005) Properties and characterization of carbon-nanotube-based transparent conductive coating. J Soc Info Display 13:759–763 29. Barnes TM, van de Lagemaat J, Levi D, Rumbles G, Coutts TJ, Weeks CL, Britz DA, Levitsky I, Peltola J, Glatkowski P (2007) Optical characterization of highly conductive single-wall carbon-nanotube transparent electrodes. Phys Rev B 75:235410 30. Green AA, Hersam MC (2008) Colored semitransparent conductive coatings consisting of monodisperse metallic single-walled carbon nanotubes. Nano Lett 8:1417–1422 31. Yan X, Mont FW, Poxson DJ, Schubert MF, Kim JK, Cho J, Schubert EF (2009) Refractive-indexmatched indium–tin-oxide electrodes for liquid crystal displays. Japan J Appl Phys 48:120203 32. Fuhrer MS et al (2000) Crossed nanotube junctions. Science 288:494–497 33. Nirmalraj PN, Lyons PE, De S, Coleman JN, Boland JJ (2009) Electrical connectivity in single-walled carbon nanotube networks. Nano Lett 9:3890–3895 34. Hecht D, Hu L, Gru¨ner G (2006) Conductivity scaling with bundle length and diameter in single walled carbon nanotube networks. Appl Phys Lett 89:133112 35. Parekh BB, Fanchini G, Eda G, Chhowalla M (2007) Improved conductivity of transparent single-wall carbon nanotube thin films via stable postdeposition functionalization. Appl Phys Lett 90:121913–3 36. Dettlaff-Weglikowska U et al (2005) Effect of SOCl2 treatment on electrical and mechanical properties of single-wall carbon nanotube networks. J Am Chem Soc 127:5125–5131 37. Ska´kalova´ V, Kaiser AB, Dettlaff-Weglikowska U, Hrncarikova K, Roth S (2005) Effect of chemical treatment on electrical conductivity, infrared absorption, and raman spectra of single-walled carbon nanotubes. J Phys Chem B 109:7174–7181 38. Barnes TM, Blackburn JL, van de Lagemaat J, Coutts TJ, Heben MJ (2008) Reversibility, dopant desorption, and tunneling in the temperature-dependent

39.

40.

41.

42.

43.

44.

45.

46.

47.

48.

49.

50.

51.

5.4.2

conductivity of type-separated, conductive carbon nanotube networks. ACS Nano 2:1968–1976 Collins PG, Bradley K, Ishigami M, Zettl A (2000) Extreme oxygen sensitivity of electronic properties of carbon nanotubes. Science 287:1801–1804 Mowbray DJ, Morgan C, Thygesen KS (2009) Influence of o2 and n2 on the conductivity of carbon nanotube networks. Phys Rev B 79:195431 Blackburn JL, Barnes TM, Beard MC, Kim Y, Tenent RC, McDonald TJ, To B, Coutts TJ, Heben MJ (2008) Transparent conductive Single-Walled carbon nanotube networks with precisely tunable ratios of semiconducting and metallic nanotubes. ACS Nano 2:1266–1274 Jackson R, Domercq B, Jain R, Kippelen B, Graham S (2008) Stability of doped transparent carbon nanotube electrodes. Adv Funct Mater 18:2548–2554 Schindler A, Schau P, Fruehauf N (2009) Activematrix and flexible liquid-crystal displays with carbon-nanotube pixel electrodes. J Soc Info Display 17:853–860 Jackson R, Graham S (2009) Specific contact resistance at metal/carbon nanotube interfaces. Appl Phys Lett 94: 012109-3 Lim SC, Jang JH, Bae DJ, Han GH, Lee S, Yeo I, Lee YH (2009) Contact resistance between metal and carbon nanotube interconnects: Effect of work function and wettability. Appl Phys Lett 95:264103 Behnam A, Choi Y, Noriega L, Wu Z, Kravchenko I, Rinzler AG, Ural A (2007) Nanolithographic patterning of transparent, conductive single-walled carbon nanotube films by inductively coupled plasma reactive ion etching. J Vaccum Sci Technol B 25:348–354 Cao Q, Kim HS, Pimparkar N, Kulkarni JP, Wang C, Shim M, Roy K, Alam MA, Rogers JA (2008) Mediumscale carbon nanotube thin-film integrated circuits on flexible plastic substrates. Nature 454:495–500 Sherman R, Hirt D, Vane R (1994) Surface cleaning with the carbon dioxide snow jet. J Vacccum Sci Technol A 12:1876–1881 Glatkowski P (2003) Carbon nanotube based transparent conductive coatings. In: International SAMPE symposium and exhibition, Long Beach, California, US, pp 2146–2152 Chan-Yu-King R, Roussel F (2007) Transparent carbon nanotube-based driving electrodes for liquid crystal dispersion display devices. Appl Phys A 86: 159–163 Schindler A, Spiessberger S, Fruehauf N, Novak JP, Yaniv Z (2007) Solution-deposited carbon nanotube networks for flexible active matrix displays. In: Asia Display, Shanghai, China, Mar vol 1, pp 882–887

807

808

5.4.2

ITO Replacements: Carbon Nanotubes

52. Schindler A, Pross A, Baur H, Fruehauf N (2008) AMLCD with carbon-nanotube pixel electrodes. In: SID Digest, Los Angeles, pp 947–950 53. Aguirre CM, Auvray S, Pigeon S, Izquierdo R, Desjardins P, Martel R (2006) Carbon nanotube sheets as electrodes in organic light-emitting diodes. Appl Phys Lett 88: 183104/1–3 54. Zhang D, Ryu K, Liu X, Polikarpov E, Ly J, Tompson ME, Zhou C (2006) Transparent, conductive, and flexible carbon nanotube films and their

application in organic light-emitting diodes. Nano Lett 6:1880–1886 55. Yu Z, Hu L, Liu Z, Sun M, Wang M, Gru¨ner G, Pei Q (2009) Fully bendable polymer light emitting devices with carbon nanotubes as cathode and anode. Appl Phys Lett 95:203304 56. Liu D, Fina M, Guo J, Chen X, Liu G, Johnson SG, Mao SS (2009) Organic light-emitting diodes with carbon nanotube cathode-organic interface layer. Appl Phys Lett 94:013110

5.4.3 ITO Replacements: Polymers Wilfried Lo¨venich . Andreas Elschner 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810 1.1 Conductive Polymers Versus Insulating Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810 1.2 Most Common Conductive Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811 2 PEDOT: A Stable and Processable Conductive Polymer . . . . . . . . . . . . . . . . . . . . . . . . . . 812 2.1 PEDOT Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 812 2.2 Charge Transport in PEDOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 812 3 3.1 3.2 3.3 3.4

PEDOT as Transparent Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 Transparency and Conductivity of PEDOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 Index of Refraction of PEDOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815 Combination of PEDOT with Metallic Support Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 Examples of PEDOT Replacing ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816

4

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.4.3, # Springer-Verlag Berlin Heidelberg 2012

810

5.4.3

ITO Replacements: Polymers

Abstract: Indium tin oxide (ITO) is widely used as electrode in the display industry. This chapter focuses on conductive polymers as ITO replacement. It describes the properties of intrinsically conductive polymers in general and those of the poly(3,4-ethylene dioxythiophene) (PEDOT), the most advanced intrinsically conductive polymer (ICP) in terms of conductivity and transparency, in particular. The synthesis of PEDOT is described and its conductivity, transmission, and refractive index are compared to that of ITO. Finally, first examples of PEDOT as ITO replacement are given. List of Abbreviations: AZO, Aluminum Doped ZnO; EDOT, Ethylenedioxythiophene; ICP, Intrinsically Conductive Polymer; ITO, Indium Tin Oxide; OLED, Organic Light Emitting Diode; OPV, Organic Photovoltaic; PEDOT, Poly-(3,4-ethylenedioxythiophene); PET, Polyethylene Terephthalate; PLED, Polymer Light Emitting Diode; PSS, Polystyrenesulfonic Acid; SM-OLED, Small Molecule Organic Light Emitting Diode; TCO, Transparent Conductive Oxide

1

Introduction

Since the discovery of intrinsically conductive polymers (ICPs) in 1977, the knowledge in this field has grown dramatically. There has been a steady increase in both scientific publications as well as patents. Since only some limited aspects of ICPs are covered by this book, the reader is referred to the further reading list and some of the many reviews [1–3]. It is important to note however that during the last 20 years ICPs have developed from laboratory materials to mature industrial products.

1.1

Conductive Polymers Versus Insulating Polymers

The specific properties of ICP arise since these materials are conjugated polymers with alternating single and double bonds [1]. A comparison of the electronic bonds in such conjugated polymers with those in saturated polymers shows the origin of these specific properties. In saturated polymers such as polyethylene, all bonds between atoms are single bonds and therefore all valence electrons are used in covalent s-bonds. In terms of electronic states in such a saturated polymer, a very large gap between the valence band and the conduction band is observed. The energy required for an electron to move from the valence band to the conduction band is large and the material shows typical insulating properties. In conjugated polymers a p system is formed along the polymer backbone [4]. The carbon atoms typically involved in the formation of the polymer backbone form three sigma bonds with neighboring atoms and the remaining p orbitals – typically described as pz orbitals – engage in the p-system. In some conjugated polymers such as polyaniline also nitrogen atoms are involved in the conjugation path [2]. The energy gap between the valence band and the conduction band is much smaller than in saturated polymers. In their neutral state, conjugated polymers behave like semiconductors [5]. If additional charges are introduced into the polymers, additional electronic states are generated between the valence band and the conduction band and electrons can move freely. The material becomes conductive. The addition of charges is often referred to as doping. If the doping levels are high, some conductive polymers behave like metals. They show conductivity even at very low temperatures and the conductivity decreases with increasing temperature.

ITO Replacements: Polymers

5.4.3

There are several ways to introduce charges into those polymers and to render them conductive. In the case of chemical doping, the polymer chain is either oxidized (p-doped) or reduced (n-doped) by an oxidation or reducing agent [4]. The charges can also be introduced electrochemically or by photo-doping. In summary, the origin of conductivity in ICPs is related to the mobility of free charges, that is, electrons or holes (electron vacancies). It is not related to the migration of ions as in electrolytes.

1.2

Most Common Conductive Polymers

The most prominent examples of ICPs are polyacetylene, polyaniline, polypyrrole, polythiophene, and their derivatives. > Figure 1 shows the chemical structures of these polymers as well as that of poly-(3,4-ethylenedioxythiophene) (PEDOT), a polythiophene derivative. The latter will be discussed in more detail. For simplicity the neutral forms are shown. Polyacetylene is the simplest conjugated polymer. Polyacetylene can be oxidized, for example, with iodine (I2). The polymer becomes positively charged and iodide is formed as counter ion. Doped polyacetylene reaches conductivities of 104–105 S/cm [6]. As comparison, the widely used transparent conductive oxide (TCO), indium tin oxide (ITO) shows conductivities of 2,000–10,000 S/cm, depending on deposition temperature. However, because of its low environmental stability polyacetylene has not been used in industrial applications.

. Fig. 1 Chemical structures of polyacetylene, polyaniline, polypyrrole, polythiophene, and poly[3,4ethylenedioxythiophene]

811

812

5.4.3

ITO Replacements: Polymers

Polyaniline and polypyrrole are more environmentally stable [3]. Polyaniline is used, for example, as antistatic coating on plastic films. Polypyrrole is used as counterelectrode in solidstate capacitors. However, both materials show lower conductivity and less transparency for visible light compared to PEDOT [7]. The latter is available with a conductivity of up to 1,000 S/cm and high transparency. Therefore, this chapter will focus on PEDOT and its use as ITO replacement.

2

PEDOT: A Stable and Processable Conductive Polymer

PEDOTwas discovered in the late 1980s in the central R&D department of Bayer AG [8]. Today, it is used in a wide range of applications including antistatic coatings, as counter-electrodes in capacitors, as transparent electrode and as buffer layer on organic light emitting diodes (OLEDs) and organic photovoltaic (OPV) products [9]. The conductivity of commercially available PEDOT dispersions has continuously risen over the last 10 years and has reached 1,000 S/cm in 2009 [10]. As a consequence, PEDOT can now compete with transparent conductive oxides (TCOs) and first applications as conductive electrode emerge.

2.1

PEDOT Synthesis

PEDOT can be prepared via chemical polymerization of ethylenedioxythiophene (EDOT), electrochemical polymerization of EDOT, or transition metal-mediated coupling of dihalo EDOT derivatives [3]. The chemical polymerization of EDOT using Fe(III) salts such as Fe(III) chloride or Fe (III) tosylate results in a black and insoluble compound. If this polymerization is performed on a substrate, transparent highly conductive films can be formed. This so-called in situ PEDOT is, for example, used as counter-electrode in solid-state capacitors [11]. A more versatile form of PEDOT is a polyelectrolyte complex with polystyrenesulfonic acid (PSS). Such a PEDOT:PSS complex can be formed when EDOT is chemically polymerized in the presence of PSS [12]. The chemical equation for this polymerization of EDOT using Na2S2O8 and Fe2(SO4)3 as oxidation agents is shown in > Fig. 2. The equation shows that EDOT monomers are joined to form the polymer PEDOT chains. Furthermore, it becomes apparent that the PEDOT chain is positively charged. These positive charges can move along the polymer chain and in fact between polymer chains and hence a macroscopic conductivity is obtained. The counterion for the cationic PEDOT is PSS, which is not polymerized during the reaction but is present as polymer from the beginning. The polyelectrolyte complex is obtained in the form of swollen gel particles [9]. The composition is such that PSS is present in excess. The sulfonic acid groups of PSS balance the charges of the cationic PEDOT. Furthermore, the PSS loops and tails at the outside of a gel particle result in an electrostatic repulsion between particles which stabilizes the particles against further coagulation [13]. Typical commercial PEDOT:PSS dispersions show a PEDOT:PSS mass ratio of 1:2.5 [10].

2.2

Charge Transport in PEDOT

PEDOT:PSS is an intrinsically conducting polymer with metal-like properties. The charge transported occurs via a hopping mechanism in a heterogeneously disordered system [14].

ITO Replacements: Polymers

5.4.3

. Fig. 2 Polymerization of EDOT in the presence of PSS using Na2S2O8 and Fe2(SO4)3 as oxidation agents

The charge transport of PEDOT:PSS films strongly depends on the film morphology. This can be observed when water-miscible high-boiling solvents like ethylene glycol or dimethylsulfoxide (DMSO) are added to a PEDOT:PSS solution [15]. When PEDOT:PSS is deposited from aqueous dispersion it does not reach an equilibrium state in the solid film. Instead it is ‘‘frozen’’ in a nonequilibrium state [16]. The presence of the water-soluble, high-boiling solvents allows the blend to rearrange and highly conductive paths are formed [17, 18]. PEDOT films are highly conductive and also highly transparent. > Figure 3 depicts the relative transmission, absorption, and reflection spectra of a high-conductive PEDOT:PSS sample with a conductivity of 1,000 S/cm [10]. The broad absorption band in the visible and in the IR region can be interpreted as the contribution of free charge carriers to absorption or alternatively to excitations of mid-gap states. Due to a slightly higher absorption in the red PEDOT:PSS films have a light blue appearance.

3

PEDOT as Transparent Electrode

3.1

Transparency and Conductivity of PEDOT

The sheet resistance and the transparency of a transparent conductor can be tuned by adjusting the layer thickness of those materials. In > Fig. 4, the internal luminous transmission Y of ITO, PEDOT:PSS, and in situ PEDOT is shown as a function of the sheet resistance [19]. The luminous transmission Y is defined as the transmission of a sample weight and integrated with the sensitivity curve of the human eye and it is shown relative to the luminous transmission of the substrate. It is a measure of the overall transmission taking into account the different colors

813

5.4.3

ITO Replacements: Polymers

90 80

Transmission Absorption Reflection

70 T, A, R (%)

60 50 40 30 20 10 0 500

1,000 1,500 Wavelength (nm)

2,000

2,500

. Fig. 3 Relative transmission, absorption, and reflection spectra of a 190-nm-thick PEDOT:PSS film with a weight ration PEDOT:PSS of 1:2.5 and a conductivity of 1,000 S/cm deposited on quartz substrate

d/nm

1.0 100 200

0.8 Y/Ysubstrate

814

50

500

0.6 0.4

1000

In-situ-PEDOT σ = 1,200 S/cm ITO σ = 2,000 S/cm

0.2 0.0 10−1

CleviosTM PH1000 σ = 1,000 S/cm

100

102 101 Rsq [Ωsq]

103

104

. Fig. 4 Internal luminous transmission as a function of sheet resistance Rsq for high-conductive PEDOT:PSS (1:2.5 w/w), in situ PEDOT, and ITO. The stars denote the PEDOT:PSS layer thickness (d/nm)

of the materials. The luminous transmission for a specific material can be varied by varying the layer thickness. For ITO on PET films, a conductivity of 2,000 S/cm is assumed. Although > Fig. 4 does not take into account thin film interferences, it allows to estimate the level of transmission for a given sheet resistance. For all materials, thin films show a high sheet

ITO Replacements: Polymers

5.4.3

resistance and a high transmission (right hand side), whereas thick films give rise to a low sheet resistance and a low transmission (left hand side). While PEDOT:PSS and in situ PEDOT show a somewhat higher sheet resistance at comparable transmissions, organic conductive polymers offer important benefits compared to ITO. They are processable from solution, they are printable, and they are also highly flexible. To compare various materials independent of their layer thickness d, R. G. Gordon proposed a classification of coatings by a figure of merit, defined as the ratio of its conductivity and its visible absorption coefficient  1 ; s=a ¼  Rsq  ln ðT þ R Þ where s is the conductivity, a is the absorption coefficient, Rsq is the sheet resistance, T is the total visible transmission and R is the total visible reflectance [20]. For high-quality ITO and for AZO (Aluminum doped ZnO), values of 4–5 O1 were determined. From > Fig. 4, the values for PEDOT:PSS and in situ PEDOT can be extracted as 0.1 and 0.03 O1, respectively. The values shown for ITO in > Fig. 4 are typical values for ITO on PET and therefore significantly lower than those of high-quality ITO. A value of 0.5 O1 can be extracted. The comparison of PEDOT with ITO on PET seems appropriate, since organic conductors such as PEDOT are of particular interest in combination with flexible substrates.

3.2

Index of Refraction of PEDOT

In order to calculate light out-coupling efficiency in optoelectronic devices containing thin layers of PEDOT:PSS, it is necessary to determine the spectral dependence of the index of refraction n and the absorption constant k. The n and k value of a PEDOT:PSS dispersion depend on the ratio of the two components PEDOT and PSS, as shown in > Fig. 5 for the ratios

1.7

0.12 0.10

1.5 0.08 0.06

1.4

0.04

Absorption constant k

1.6 Refractive index n

0.14

PEDOT: PSS (1:6) PEDOT: PSS (1:2.5)

1.3 0.02 1.2 300

0.00 400

500 600 700 Wavelength (nm)

800

900

. Fig. 5 Spectral dependence of refractive index n and absorption constant k for the PEDOT:PSS types 1:2.5 and 1:6

815

816

5.4.3

ITO Replacements: Polymers

1:2.5 and 1:6. A higher PEDOT content results in a higher absorption constant and a lower refractive index over the whole visible spectrum. Both materials have a refractive index below 1.6 over the visible range of light. In contrast, ITO has a refractive index between 1.8 and 2.2 in this region. It has been shown that in the case of OLEDs, a low refractive index can lead to improved device performance due to improved light out-coupling [21].

3.3

Combination of PEDOT with Metallic Support Lines

Electroluminescent displays and lamps, solar cells, electrochromic windows, and touch-panel screens require high-performing, low-light-absorbing conductors. The voltage drop across the conductive electrode needs to be as low as possible. Even the most efficient TCO layers do not fulfill these demands completely and therefore need to be combined with metallic support lines, so-called bus-bars. These non-transparent, metallic lines contact the TCO-layer and distribute the current laterally on large substrate areas to decrease the overall resistance. Unavoidably, such lines cover some of the active area. In order to minimize this effect the bus-bar grid needs to be optimized when combined with transparent conductive polymers. In the case of PEDOT the distance between the individual metal lines needs to be smaller compared to TCOs in order to compensate for the lower conductivity. Carter et al. combined thin Au stripes with PEDOT:PSS which allowed the construction of ITO-free PLEDs [22]. Neyts et al. calculated the voltage drop across the transparent anode and compared their results with the actual luminance drop observed in OLEDs [23, 24].

3.4

Examples of PEDOT Replacing ITO

The use of ICPs and in particular PEDOT:PSS as transparent anode in display applications has been shown by many groups. Already in 1997, Scott et al. demonstrated that PEDOT:PSS and polyaniline can be used as transparent anode in polymeric light emitting diodes (PLED) [25]. They found that the oxidative degradation rate of the conjugated emitting polymer resulting from the transfer of oxygen atoms stemming from ITO was reduced by several orders of magnitude when conducting polymers were used. In 2002, Kim et al. showed that PEDOT:PSS in combination with high-boiling solvents forms particularly efficient anodes for OLEDs with a low voltage drop across the anode [26]. The high performance of PEDOT:PSS as conductive anode in a PLED was also shown by Ouyang et al.[27]. High-conductive PEDOT:PSS has also been tested in small molecule OLEDs (SM-OLED). SM-OLEDs have attracted a lot of attention recently due to their high efficiency [28]. A specific type of SM-OLEDs comprise a doped hole transport layer and a doped electron transport layer, so-called pin-OLEDs. A PEDOT:PSS-based film, having a conductivity of 500 S/cm, was compared directly to ITO anodes using the same high-efficient pin-OLED layer stack [21]. As mentioned above, OLEDs using PEDOT:PSS anodes showed slightly higher device efficiencies owing to an improved light out-coupling efficiency. The lifetime of these devices was found to be independent of the anode material [29]. The same group prepared comparable devices using polyaniline with a conductivity of 200 S/cm as anode. Due to the stronger absorption of polyaniline, in particular in the red region of the spectrum, a reduced power efficiency compared to ITO was observed [30]. In 2008, the use of PEDOT:PSS as ITO replacement was shown in a 10  10 cm white OLED tile [31].

ITO Replacements: Polymers

4

5.4.3

Conclusion

TCOs are highly important in the display industry. They are highly transparent, exhibit low sheet resistance, and are durable. However, there are also disadvantages: TCOs are typically produced in a cost-intensive sputtering process. They are brittle and the structuring requires a separate etching process step. Conductive polymers can overcome some of these disadvantages and have the potential to substitute ITO in many display applications. They are flexible, printable, and also offer a cost advantage against TCOs. Although TCOs are more conductive, the combination of conductive polymers with metallic support lines can overcome this limitation. First examples of the use of PEDOT:PSS as ITO replacement in OLEDs have been shown. Further uses in the display industry can be anticipated.

References 1. Scholz F (2008) Conducting polymers. A new era in electrochemistry. Springer, Berlin 2. Skotheim TA, Reynolds JR (2007) Handbook of conducting polymers. CRC, New York 3. Groenendaal L, Jonas F, Freitag D, Pielartzik H, Reynolds JR (2000) Poly(3, 4-ethylenedioxythiophene) and its derivatives: past, present, and future. Adv Mater 12(7):481–494 4. Heeger AJ (2001) Semiconducting and metallic polymers: the fourth generation of polymeric materials. J Phys Chem B 105(36):8475–8491 5. Kaiser AB (2001) Systematic conductivity behaviour in conjugated polymers: effects of heterogeneous disorder. Adv Mater 13(12–13):927–941 6. Park YW, Choi ES, Suh DS (1998) Metallic temperature dependence of resistivity in perchlorate doped polyacetylene. Synth Met 96(1):81–86 7. Jonas F, Schrader L (1991) Synth Met 41–43: 831–836 8. Jonas F, Heywang G, Schmidtberg W (1988) Deutsches Patentamt. DE 3813589 (Bayer AG) 9. Kirchmeyer S, Reuter K (2005) Scientific importance, properties and growing applications of poly(3,4-ethylenedioxythiophene). J Mater Chem 15(21):2077–2088 10. Clevios™ PH 1000 supplied by Heraeus Clevios GmbH, Chempark Leverkusen, 51368 Leverkusen, Germany. www.clevios.com 11. Winther-Jensen B, Breiby DW, West K (2005) Base inhibited oxidative polymerization of 3, 4-ethylenedioxythiophene with iron(III) tosylate. Synth Met 152(1–3):1–4 12. Jonas F, Krafft W (1990) European Patent. EP 440957 (Bayer AG) 13. Petrak K (1992) Polyelectrolyte complexes. In: Hara M (ed) Polyelectrolytes. Marcel Dekker, New York, pp 265–297

14. Aleshin AN, Williams SR, Heeger AJ (1998) Transport properties of poly(3, 4-ethylenedioxythiophene)/ poly(styrenesulfonate). Synth Met 94(2):173–177 15. Jo¨nsson SKM, Birgerson J, Crispin X, Greczynski G, Osikowicz W, Denier van der Gon AW, Salaneck WR, Fahlman M (2003) The effect of solvents on the morphology and sheet resistance in poly (3, 4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT-PSS) films. Synth Met 139(1):1–10 16. Nardes AM, Jansen RAJ, Kemerink M (2008) A morphological model for the solvent-enhanced conductivity of PEDOT: PSS thin films. Adv Funct Mater 18(6):865–871 17. Ghosh S, Ingana¨s O (2001) Nano-structured conducting polymer network based on PEDOTPSS. Synth Met 121(1–3):1321–1322 18. Kim JY, Jung JH, Lee DE, Joo J (2002) Enhancement of electrical conductivity of poly(3, 4-ethylenedioxythiophene)/poly(4-styrenesulfonate) by a change of solvents. Synth Met 126(2–3):311–316 19. Elschner A, Jonas F, Kirchmeyer S, Lo¨venich W (2008) PEDOT-based Layers for TCO-Substitution and Hole-Injection. SID 08 Digest 29.2:407–410 20. Gordon G (2000) Criteria for choosing transparent conductors. In: Ginley DS, Bright C (eds) MRS bulletin. Materials research society, Warrendale, pp 52–57 21. Fehse K, Walzer K, Leo K, Lo¨venich W, Elschner A (2007) Highly conductive polymer anodes as replacement of inorganic materials for high efficiency organic light-emitting diodes. Adv Mater 19(3):441–444 22. Carter SA, Angelopoulos M, Karg S, Brock PJ, Scott JC (1997) Polymeric anodes for improved polymer light-emitting diode performance. Appl Phys Lett 70(16):2067–2069 23. Neyts K, Marescaux M, Nieto AU, Elschner A, Lo¨venich W, Fehse K, Huang Q, Walzer K, Leo K

817

818

5.4.3 24.

25.

26.

27.

ITO Replacements: Polymers

(2006) Inhomogeneous luminance in organic light emitting diodes related to electrode resistivity. J Appl Phys 100(11):114513/1–114513/4 Neyts K, Real A, Marescaux M, Mladenovski M, Beeckman J (2008) Conductor grid optimization for luminance loss reduction in organic light emitting diodes. J Appl Phys 103(9):093113/1– 093113/5 Scott JC, Carter SA, Karg S, Angelopoulos M (1997) Polymeric anodes for organic light-emitting diodes. Synth Met 85(1–3):1197–1200 Kim WH, Ma¨kinen AJ, Nikolov N, Shashidhar R, Kim H, Kafafi ZH (2002) Molecular organic lightemitting diodes using highly conductive polymers as anodes. Appl Phys Lett 80(20):3844–3846 Ouyang J, Chu CW, Chen FC, Xu Q, Yang Q (2005) High-conductivity poly(3, 4-ethylenedioxythiophene):

28. 29.

30.

31.

poly(styrenesulfonate) films and its application in polymer optoelectronic devices. Adv Funct Mater 15(2):203–208 http://en.wikipedia.org/wiki/Organic_light-emitting_ diode. Accessed on 16 Feb 2011 Fehse K, Meerheim R, Walzer K, Leo K, Lo¨venich W, Elschner A (2008) Lifetime of organic light-emitting diodes on polymer anodes. Appl Phys Lett 93(8):083303/1–083303/3 Fehse K, Schwartz G, Walzer K, Leo K (2007) Combination of a polyaniline anode doped charge transport layers for high-efficiency organic light emitting diodes. J Appl Phys 101(12)):124509-1– 124509-4 Visser P (2008) http://www.hitech-projects.com/ euprojects/olla/news/press_release_june_2008/OLLA_ pressrelease6_v4.pdf. Accessed on 16 Feb 2011

Further Reading Elschner A, Kirchmeyer S, Lo¨venich W, Merker U, Reuter K (2010) PEDOT – principles and applications of an intrinsically conductive polymer. CRC, Boca Raton

Nalwa HS (1997) Handbook of organic conductive molecules and polymers. Wiley, Chichester

5.4.4 ITO Replacements: Insulator-Metal-Insulator Layers Bernd Szyszka 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820

2

Material Properties of Insulator–Metal–Insulator Layers . . . . . . . . . . . . . . . . . . . . . . . . 821

3 3.1 3.2 3.3

Production Techniques for IMI-based TCO Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822 Coatings on Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822 Coatings on Polymer Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823 Patterning of IMI Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824

4 4.1 4.1.1 4.1.2 4.2 4.3

Applications of IMI Films in Displays Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825 LCD Display Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825 STN-LCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825 Color Filters for Passive and Active Matrix LCDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825 Low-E Filter in PDP Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829 IMI Layers for OLED Displays and OLED Backlighting . . . . . . . . . . . . . . . . . . . . . . . . . . 830

5

Challenges, Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 830

6

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 830

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.4.4, # Springer-Verlag Berlin Heidelberg 2012

820

5.4.4

ITO Replacements: Insulator-Metal-Insulator Layers

Abstract: Insulator–metal–insulator (IMI) films open up novel pathways for highly conductive transparent electrode applications with sheet resistivity of less than 10 O/□. IMI films utilize thin metallic layers such as silver with thickness in the order of 10–15 nm to provide electrical conductivity. This metal is embedded in dielectric layers for antireflection and corrosion protection. Such layers can be deposited by magnetron sputtering either on rigid substrates or on polymeric web without substrate heating at high deposition rate. Also, concepts for fine pattering of IMI films are available. This section gives a brief overview of the technology. List of Abbreviations: AM-LCD, Active Matrix Liquid Cystal Display; IMI, Insulator–Metal– Insulator Films; Class A, PDP Displays Aimed at the Public Use, e.g., Information Displays in Airports; Class B, PDP Displays Aimed at Personal or Home Use, e.g., Flat-Screen TVs; EMI, Electromagnetic Interference; LCD, Liquid Crystal Display; PDP, Plasma Display Panel; OLED, Organic Light-Emitting Diode

1

Introduction

Insulator–metal–insulator (IMI) films utilize the metallic conductivity of thin metal layers for electrical conduction while ceramic layers surrounding the metal provide antireflection. This concept of induced transmission in absorbing films dates back to the 1950s [1]. The most prominent metal for such applications is silver, since it offers both low absorption losses and high electrical conductivity at film thickness in the order of 10–15 nm [2]. Therefore, even single Ag layer IMI films with total thickness of 100 nm exhibit sheet resistance in the order of RSh  4 O/□. For high-quality ITO (resistivity r = 150 mOcm) on the other hand, a film thickness of at least 380 nm is necessary to achieve similar sheet resistance. The purpose of this section is to give an overview on the state of the art for IMI layers for display applications. Also, the author would like to mention some other important review papers in this field, in particular [3] which gives a comprehensive overview on transparent conductive materials for solar applications and [4] which addresses the field of flexible display manufacturing. In general, the main applications of IMI films are energy-efficient coatings for architectural glazing [3, 5]. Typical IMI films reveal sheet resistivity and thermal emissivity in the order of RSh  4 O/□ and e  4%, respectively, for single Ag-layer IMI coatings [6] and thus, almost ideal reflection of the thermal infrared black body radiation. Electromagnetic interference (EMI) filters for plasma display panels (PDPs) utilize the same approach to minimize the unwanted near infrared emission from the display. Complex layer stacks containing up to 5 Ag layers are necessary for this application where high infrared reflectivity in the thermal infrared and also steep increase of reflectivity in the near infrared is mandatory. The demand on shielding differs for the application of the PDP. For commercial use (Class ‘‘A’’ displays, aimed at the public use, e.g., information displays in airports), a sheet resistance of RSh < 3 O/□ can be sufficient. For home use (Class ‘‘B’’ displays, aimed at personal or home use, e.g., flat-screen TVs), RSh  1 O is necessary, depending on the size of the PDP [7]. The second application segment is the use of IMI layers to provide transparent conductivity with low sheet resistance for electrical applications. Such layers are most relevant for currentdriven devices [8] such as organic light-emitting diode (OLED) displays and also for passive matrix liquid crystal display (LCD). In addition to this, the substitution of transparent

5.4.4

ITO Replacements: Insulator-Metal-Insulator Layers

conductive oxides (TCOs) by IMI layers reveals certain advantages in the field of flexible displays [4]. Conventional TCOs are brittle materials while IMI stacks reveal certain flexibility due to the incorporation of the ductile metal layers. Bending experiments reveal a superior stability of electrical properties for IMI films compared to ITO. Experiments show for PET (100 mm)/ITO (100 nm) a strong increase of sheet resistance after bending to 6 mm radius in contrast to IMI layers (PET (100 mm)/ITO (35 nm)/Ag (8–12 nm)/ITO (35 nm)), where even more than 1,000 bending cycles can be performed without substantial degradation of sheet resistance [9].

2

Material Properties of Insulator–Metal–Insulator Layers

From a fundamental point of view, the deposition of very thin metal layers on insulating substrates is quite difficult since metallic bonding occurs in the metal and ionic and covalent forces bond the dielectric layers. Therefore, only van der Waals forces are available to stabilize the interface between metal and dielectric layers [10]. As a consequence, island growth of thin metal layers is generally favored [11] and special techniques such as introduction of ZnO seed layers promoting the heteroepitaxial growth of silver layers [12] are necessary. Another approach to optimize the interface properties is the introduction of thin substoichiometric oxides or even bare transition metal layers between silver and oxide layers. Especially NiCr and NiCrOx layers were found to be useful for this purpose [13] since the different oxidization states of the metals are helpful to establish a smooth transition from metallic bonding in the silver to ionic or covalent bonding in the oxide layers. As a consequence, the properties of Ag films applied in IMI layers with film thicknesses ranging from 8 to 15 nm are far from ideal since the layer thickness is close to percolation threshold and since scattering occurs at the grain boundaries and at the interface of the film [14]. > Figure 1 gives an example on the Ag resistivity which can be achieved for optimized Ag growth conditions using in-line sputtering of Ag on ZnO:Al coated glass [15].

Ag resistivity [μΩcm]

5.0 4.5 4.0 3.5 3.0 2.5 5

10

15

20 25 30 t-Ag (XRR) [nm]

35

40

. Fig. 1 Dependence of Ag resistivity on Ag film thickness for the layer stack glass (Schott AF45)/ZnO: Al (7 nm)/Ag. The Ag thickness has been measured by X-ray reflectometry (XRR) (Reprinted from [15])

821

822

5.4.4

ITO Replacements: Insulator-Metal-Insulator Layers

For a typical film thickness of 12 nm, the resistivity is in the order of 3.5–3.8 mOcm. The Ag bulk resistivity, on the other hand, is only 1.5 mOcm at room temperature [16]. The history of the development for large IMI films is shown in > Fig. 2 motivated by the demands on low emissivity for architectural glazing. The development of IMI coatings with sheet resistance in the order of 4 O/□ was possible due to optimization of growth conditions and due to high-index dielectrics such as TiO2 allowing for antireflection of thicker Ag films. Double Ag layer IMI films have been developed for Sun-Control applications and were also discussed for PDP Low-E filters (see below). However, for this demand, the use of 3 up to 5 Ag layers has shown to be necessary.

3

Production Techniques for IMI-based TCO Films

The industrial manufacturing of large area IMI layers for display applications is feasible by means of in-line sputter processes on rigid substrates [18] or by magnetron sputtering using roll-to-roll web coating [19, 20]. In both cases, large area magnetron sputtering is the key technology for the multilayer thin film deposition process. Planar magnetron sputtering suffering from inhomogeneous target erosion and unwanted particle contamination has been used (see in > Fig. 3). Nowadays, rotatable magnetrons are used almost exclusively (see > Fig. 4 and also > Chap. 5.4.1 in this book).

3.1

Coatings on Glass

For IMI coatings on glass for display applications, the process transfer from large area glass coating [21] combined with machinery know-how from vertical in-line TCO coating [22] has been straightforward. Vertical coating equipment (shown in > Fig. 5) or horizontal sputter-up coaters (not shown) are used to achieve low defect density.

K-Glass Rsh = 16 W/ e = 17% Ug = 1.6 W/(m2K)

Standard Low-E Rsh = 8 W/ e = 8% Ug = 1.3 W/(m2K) SnO2

SnO2–x:F

High-Performance Low-E I Rsh = 4 W/ e=4% Ug = 1.1 W/(m2K)

High-Performance Low-E II Rsh = 4 W/ e = 4% Ug = 1.1 W/(m2K)

Bi2O2:Mn

TiO2

Si3N4 Interface

SiCOx Float glass Blocker (NiCrOx, TiOx, NbOx, AIN, ...) Ag Dielectrics (SnO2, Si3N4, ZnO, Bi2O3:Mn, Tio2, ...) Interface layers (e.g. ZnO, ZnO:AI)

Float glass High-Performance Low-E III Rsh < 2.5 W/ e = 2% Ug = 1.0 W/(m2K)

Float glass

Float glass High-Performance Low-E Sun-Control Coating IV Rsh < 2 W/ e < 2%, Ug = 1.0 W/(m2K)

Float glass

Float glass PDP EMI Filter Rsh < 1.2 W/

Float glass

. Fig. 2 Overview on the development of large area insulator–metal–insulator (IMI) films for architectural glass and plasma display panel (PDP) Low-E filter. Eps is the thermal emissivity of the coating. Ug is the insulation value for a typical double pane coating according to EN 673 (Reprinted from [17])

ITO Replacements: Insulator-Metal-Insulator Layers

5.4.4

‘Racetrack’

Magnetic field

Electric field

Cycloidic paths of the electrons

Magnetic field Electric field

a

b

. Fig. 3 (a) Schematic on the operation principle of a planar magnetron cathode, (b) Picture of a planar magnetron cathode of 3.75 m length (Reprinted from [5])

. Fig. 4 (a) Schematic of a rotatable magnetron cathode, (b) Picture of dual rotatable magnetron cathodes (‘C-MAG’, Airco) (Reprinted from [5])

3.2

Coatings on Polymer Substrates

For IMI coatings on polymer web, a similar combination of technologies has been achieved. Starting with conventional web coating for packaging [19], the transition toward fine optical layers for antireflective (AR) purposes has been established in the late 1990s [23]. Based on this, the introduction of IMI concepts for Low-E PDP coating is straightforward [7, 20, 24]. Pioneering work has been made by Southwall Technologies Inc. for the development of IMI Low-E coatings on foils for architectural and automotive applications [25, 26] (> Fig. 6).

823

824

5.4.4

ITO Replacements: Insulator-Metal-Insulator Layers

CRS cleanroom & cooldown tunnel grayroom

cleanroom

traverse type buffer chamber

load/unload robot system

carrier management

substrate heating

sputter area

. Fig. 5 Typical setup for a clean room installation of a vertical glass in-line coater

. Fig. 6 Setup for IMI layer polymer web coating (Reprinted from [20])

3.3

Patterning of IMI Layers

The substitution of ITO films by IMI layers requires not only the correct properties of the homogenous film but also proper techniques for fine patterning. Standard etching techniques developed for wet chemical ITO patterning [27] have to be modified for IMI layer patterning [28]. Also, techniques such as dry etching using IR lasers (i.e., laser ablation) can be used for fine patterning [29].

ITO Replacements: Insulator-Metal-Insulator Layers

4

5.4.4

Applications of IMI Films in Displays Devices

From the viewpoint of market penetration, the most relevant application of IMI films for flat panel display applications is the segment of low-emissive EMI filters for plasma display panels [30]. For this market, IMI films are used almost exclusively for Class ‘‘A’’ displays (commercial use) while competition against filters made of fine-patterned metal mesh is seen for Class ‘‘B’’ (personal use) displays [31]. Besides this, further applications are IMI layers on color filters for all types of LCD displays [32] and patterned IMI layers for passive matrix display [33]. Future applications will address OLED back light units where considerable cost improvements can be expected when Ag based IMI layers are introduced [34]. Another important field for IMI filters with slightly modified design is the segment of reflection enhancement filters, to be applied in the backlight units of LCD [35]. Here, thicker Ag films coated by dielectric layers for reflection enhancement are used. > Table 1 gives an overview on the scientific literature related to the application of IMI films in display devices. The two main substrates, rigid glass and flexible polymer, are treated separately. The level of development addresses the scale of the investigation, whether it is on pure material development, process development for production, or device development.

4.1

LCD Display Applications

4.1.1

STN-LCD

The development of passive matrix LCD display on polymer substrates for Personal Digital Assistant (PDA) applications was one of the drivers for IMI films as transparent electrodes. IMI films reveal sheet resistivity in the order of RSh  8 O/□ at 83% maximum transmittance on polymer web compared to RSh  60 O/□ for conventional ITO layers [44] with similar transmittance. STN-LCD displays require low-resistance electrodes with sheet resistance in the order of a few Ohms per square for high resolution and fast switching. Film thickness in the order of 500 nm is required for standard ITO with resistivity of r = 1.5  10 4 Ocm. Therefore, intense work has been performed to substitute ITO films by IMI stacks containing one [32] or two Ag layers. Double IMI films based on ITO and Ag reveal proper electrical properties [42]. Fine pattering however has been improved by amorphous TCOs such as IZO films [28]. Even these layers suffer from insufficient chemical stability during glass cleaning. This issue has been improved by introduction of an ITO capping layer [33]. > Figure 7 shows the ability for fine pattering of IMI electrodes by wet chemical etching. The layer stack allows for acceptable stability and fine patterning for STN-LCD manufacturing, as shown in > Fig. 8.

4.1.2

Color Filters for Passive and Active Matrix LCDs

In LCDs, the color filter serves as the counter electrode. The filter consists of organic dyes and a homogenous TCO coating, in general ITO, is deposited on top of the dye layer (> Fig. 9). The process temperature has to be below 200 C during ITO deposition [45] and therefore, there is considerable interest to introduce IMI-based layers deposited at room temperature [32] to substitute the ITO deposition process on heated substrates.

825

Material

Material

Material

Material

Material

G / (IZO / Ag:Pd)2 / IZO / ITO

G / ITO / Ag:X / ITO

G / ITO / Ag:X / ITO

G / (TiO2 / Ag)2–3 / TiO2

2–3  IMI layers using: ITO / Ag / ITO TiO2 / Ag / Ti / TiO2 TiO2 / ITO / Ag / ITO / TiO2

G / (IZO / Ag)2–3 / IZO

Glass

Glass

Glass

Glass

Glass

Glass

Lab scale, not specified

Lab scale, not specified

Lab scale, not specified

Lab scale, not specified

RFMS

Static deposition, 2 inch targets

RFMS, RRFMS Lab scale, not specified

TiO2 by RRFMS, ITO by RFMS

ITO by RFMS, Not specified Ag:X by DCMS

DCMS

DCMS

DCMS

Coater

Comparison of Ag, Ag:Au, Ag: [36] Pd, and Ag:(Pd, Cu) Improved smoothness and [8] temperature stability for Ag:Ti

Low O TCO electrode Low O TCO electrode, for OLED display

PDP Low-E filter

PDP Low-E filter

PDP Low-E filter

Improved alkaline durability due to introduction of ITO caping layer

Low O TCO electrode, color filter STN-LCD

[38]

Measurement of EMI shielding [39] efficiency, comparison of Ag and Ag:(Pd, Cu) (APC) films

Comparison of blocker materials and introduction of ITO base layer and protective layer

Comparison of metallic Ti and [37] ceramic TiOx and ITO blocker films for PDP low-E filter application

[33]

[28]

Comparison of IMI layers using ITO, IZO, and ZnO, fine patterning by wet chemical etching

Low O TCO electrode, color filter STN-LCD

Refs.

Remarks

Application

5.4.4

Material

Material

/ IZO

G / (IZO / Ag:Pd)

1–2

Level of development Process

Glass

Substrate Stack

. Table 1 Literature survey on insulator–metal–insulator (IMI) films for display applications (DCMS DC magnetron sputtering, MFMS medium frequency magnetron sputtering, RFMS RF magnetron sputtering, RRFMS reactive RF magnetron sputtering, ITO In2O3:Sn, IZO In2O3:ZnO amorphous oxide)

826 ITO Replacements: Insulator-Metal-Insulator Layers

DCMS

Sputtering, no further spec.

G / ITO / Ag:(Pd, Cu) / ITO Device G / AZO / Ag:(Pd, Cu) / AZO

Material

Process

Process

G / (ITO / Ag)4 / Ag / Me

G / (ITO / Ag)3 / ITO

G / (TiO2 / Ag*)3–4 / TiO2

G / ITO / Ag / ITO‘ G / NbOx / Ag / NbOx

Polymer PET

Polymer PET

Polymer PET

Polymer

Process

Roll-to-roll web coating, 1,4 m web width

Roll-to-roll web coating

Not specified

Not specified; processing by lithography, wet and dry etching

In-line coater, dynamic deposition, target dimension of 488  87.5 mm2

In-line coater, dynamic deposition, target dimension of 488  87.5 mm2

PDP Low-E filter

PDP Low-E filter

PDP Low-E filter

[43]

Realization of IMI layers on web, modeling of optical properties

[20]

Realization of 3 Ag layer Class [7] A (RSh = 1.8 O) and 4 Ag layer Class B (RSh = 1.1 O) films on web

Optimization of EMI shielding [24] and display color for given PDP

Investigation on corrosion improvements due to transition metal top coat

Succesfuls development of OLED device on glass / ITO / Ag:(Pd, Cu) / ITO

Low O TCO electrode, for OLED lighting

[34]

[42] Optical modeling, post depostion treatment at 300 C

Low O TCO electrode

[41]

Comparison with ITO layers using figure of merit by Haacke [40]

Low O TCO electrode

DCMS, MFMS Roll-to-roll web Low O TCO electrodes, coating, 600 mm web PDP Low-E filter width

DCMS, TiO2 tube targets

DCMS

DCMS

Glass

Process

G / ITO / Ag / ITO

DCMS

Glass

Process

G / ITO / Ag:Cu / ITO

Glass

ITO Replacements: Insulator-Metal-Insulator Layers

5.4.4 827

828

5.4.4

ITO Replacements: Insulator-Metal-Insulator Layers

Space

Line

25 μ m

. Fig. 7 Fine patterning of ITO-capped IMI electrodes (glass/IZO/Ag:Pd/IZO/Ag:Pd/IZO/ITO) by wet chemical etching (solution of HBr and FeCl3) (Reprinted from [33])

Patterning properties

Moisture durability

Alkali durability

(1) ITO/Ag (2) IZO/AgPd (3) ITO/IZO/AgPd (this work)

Practical use level

(4) Thick ITO (760 nm) 1 Bad

2

3

4

5 Good

. Fig. 8 Comparison of patterning properties and durability for thick ITO layers and for different IMI layer sequences (Reprinted from [33])

Color filter layer

ITO

Black matrix Over coat layer

R

G

B

R

G

B Clear substrate

. Fig. 9 Fundamental structure of liquid crystal display (LCD) color filter (Reprinted from [45])

ITO Replacements: Insulator-Metal-Insulator Layers

4.2

5.4.4

Low-E Filter in PDP Displays

Plasma display panels (PDPs) utilize the optical emission of plasma discharges for the activation of phosphors for visible light emission [30]. The plasma discharge emits also unwanted near infrared radiation which interferes with wireless communication systems and which is harmful for the human body. Furthermore, the color gamut of the bare display is not ideal and both effects, the EMI shield and the color correction, can be handled using PDP Low-E filters. A typical setup of such a PDP filter is shown in > Fig. 10. The filter consists of a tempered glass pane which is either laminated or directly sputter coated with AR and Low-E films. The example shown here utilizes lamination of AR and IMI coated web. The unwanted IR radiation is shielded by means of the low-emissivity IMI film with sheet resistivity in the order of RSh = 1.1 O/□ for class B PDPs (home use) [7]. Damping of IR radiation in the order of 45 dB is achieved using such coatings [39]. > Figure 11 gives an example for the correlation of optical model and achieved performance for a 4 Ag layer sputtered film on glass. For this coating, the total Ag thickness is 55 nm. Further color corrections can be achieved using dye coatings [46].

Dye-containing adhesive

Polymeric film

Black ceramics

Anti-reflection (+ wear-proof) coating

Semi-tempered glass

Bus bar Sputtered multilayer coatings

Transparent adhesive

. Fig. 10 Example for a plasma display panel Low-E filter realized by lamination of sputtered multilayer IMI film on polymer web and by additional lamination of antireflective (AR) coated polymer web (Reprinted from [46])

829

5.4.4

ITO Replacements: Insulator-Metal-Insulator Layers

100 Reflectance, transmittance [%]

830

90 80

Experiment Model

70 60

Si3N4

50

Ag Si3N4

40

Ag Si3N4

30

Ag Si3N4

20 10

Ag Si3N4

0 200

400

600 800 Wavelength [nm]

1,000

Glass

. Fig. 11 Examples of coating design and experimental realization for a four layer PDP Low-E filter on glass with RSh < 1.1 V (Reprinted from [17])

4.3

IMI Layers for OLED Displays and OLED Backlighting

OLEDs are current-driven devices and therefore, the demand on low ohmic electrodes is even more severe than for LCDs. In addition to this, the surface smoothness is important to prevent short cuts and electro migration. Several devices are discussed on lab scale [34]. Ag:Ti alloys were found to be useful in terms of minimizing the surface roughness of the layer [8].

5

Challenges, Perspectives

IMI layers reveal attractive properties in terms of achieving low sheet resistivity at reasonable transmittance at low cost. The technology can also be well adopted to flexible displays, OLEDs [47] and roll to roll processing. Open questions, however, are the increased vulnerability to corrosion attack of the metal layer as well as the nonavailability of industrial standard process for thin film patterning. Therefore, so far IMI layers have not found their way yet to the mass market of AM-LCD’s. Remark that the limited optical transmittance of IMI layers compared to ITO also plays a role here.

6

Summary

The substitution of homogenous TCOs films by IMI layers opens up attractive pathways to low-cost and high-performance design concepts. Issues such as patterning and corrosion stability have been solved to a certain extent. However, the situation is nonideal due to the complexity of these layer stacks and further development work needs to be done.

ITO Replacements: Insulator-Metal-Insulator Layers

5.4.4

References 1. Berning PH, Turner AF (1957) Induced transmission in absorbing films applied to band pass filter design. J Opt Soc Am 47:230–239 2. Gla¨ser HJ (1980) Improved insulating glass with low emissivity coatings based on gold, silver or copper films ebbed in interference layers. Glass Technol 21:254–261 3. Granqvist CG (2007) Transparent conductors as solar energy materials: a panoramic review. Sol Energy Mater Sol Cells 91:1529–1598 4. Choi MC, Kim Y, Ha CS (2008) Polymers for flexible displays: from material selection to device applications. Prog Polym Sci 33:581–630 5. Gla¨ser HJ, Szyszka B (2007) History of glass coating for architectural glazing. In: SVC annual technical conference proceedings, vol 50, pp 216–229 6. Szczyrbowski J, Bra¨uer G, Ruske M, Schilling H, Zmelty A (1999) New low emissivity coating based on TwinMag sputtered TiO2 and Si3N4 layers. Thin Solid Films 351:254–259 7. Persoone P, Dierickx S, Luys S, De Bosscher W (2005) Sputter deposited TiO2 coatings for plasma display filters. In: SVC annual technical conference proceedings, vol 48, pp 180–183 8. Chen SW, Koo CH (2007) ITO-Ag alloy-ITO film with stable and high conductivity depending on the control of atomically flat interface. Mater Lett 61:4097–4099 9. Lewis J, Grego S, Chalamala B, Vick E, Temple D (2004) Highly flexible transparent electrodes for organic light-emitting diode-based displays. Appl Phys Lett 85:3450–3452 10. Stoneham AM, Ramos MMD, Sutton AP (1993) How do they stick together? The statistics and dynamics of interfaces. Philos Mag A 67: 797–811 11. Fuks D, Dorfman S, Zhukovskii YF, Kotomin EA, Stoneham AM (2002) Theory of the growth mode for a thin metallic film on an insulating substrate. Surf Sci 499:24–40 12. Arbab M (2001) The base layer effect on the d.c. conductivity and structure of direct current magnetron sputtered thin films of silver. Thin Solid Films 381:15–21 13. Martin-Palma RJ, Martinez-Duart JM (1999) Ni-Cr passivation of very thin Ag films for low-emissivity multilayer coatings. J Vacuum Sci Technol A 17:3449–3451 14. Liu H-D, Zhao Y-P, Ramanath G, Murarka SP, Wang G-C (2001) Thickness dependent electrical resistivity of ultrathin (< 40 nm) Cu films. Thin Solid Films 384:151–156

15. Ulrich S, Pflug A, Schiffmann KI, Szyszka B (2008) Optical modeling and XRR/AFM characterization of highly conductive thin Ag layers. Phys Stat Sol (c) 5/5:1235–1239 16. Kohlrausch F (1968) In: Lautz G, Taubert R (eds) Praktische Physis, Band 3, Tafeln, 22nd edn. Teubner Verlag, Stuttgart 17. Szyszka B (2009) Transparent leitfa¨hige Schichten – Historie, Gegenwart und Zukunft. In: Proceedings OTTI TCO-3 conference, 30 November to 1 December 2009, Neu-Ulm, Germany 18. Perata M, Nadel S (2001) A new generation of in-line high volume production systems for low-defect coating applications. In: Proceedings glass processing days, pp 572–577 19. Ludwig R, Kukla R, Josephson E (2005) Vacuum web coating - state of the art and potential for electronics. In: Proceedings of the IEEE, vol 93, pp 1483–1490 20. Fahland M, Vogt T, Schoenberger W, Schiller N (2008) Optical properties of metal based transparent electrodes on polymer films. Thin Solid Films 516 / 17:5777–5780 21. Bra¨uer G (1999) Large area glass coating. Surf Coat Technol 112:358–365 22. Latz R, Ocker B, Daube C, Noll-Daube S (1993) Large-scale sputtering of ITO and SiO2 for highquality display. In: SID Digest, pp 975–978 23. Ishikawa H, Honjo Y, Watanabe K (1999) Threelayer broad-band antireflective coating on web. Thin Solid Films 351:212–215 24. Okamura T, Fukuda I, Koike K, Saigou H, Yoshikai M, Matsuzaki Y (2001) Optical filters for plasma display panels using organic dyes and sputtered multilayer coatings. J Vac Sci Technol A 19:1090–1094 25. Sahm H, Charton C, Thielsch R (2004) Oxidation behaviour of thin silver films deposited on plastic web characterized by spectroscopic ellipsometry (SE). Thin Solid Films 455-456:819–823 26. Stoessel C, Wahl A, Boman L, Thielsch R (2008) Improved corrosion resistance sputter type FabryPerot filters for EMI shielding and NIR blocking of Plasma Display Panels (PDP). In: Poster presentation ICCG, Eindhoven 27. Jacobs JWM, van Rooijen HF, van der Meerakker JEAM, Vink TJ (1997) Microstructural and electrochemical aspects of wet etching ITO films. In: Electrochemical Society Proceedings, vol 96-23, Pennington, pp 158–173 28. Aoshima Y, Miyazaki M, Sato K, Akao H, Takaki S, Adachi K (2000) Development of silver-based multilayer coating electrodes with low resistance for use in flat panel displays. Jpn J Appl Phys 39:4884–4889

831

832

5.4.4

ITO Replacements: Insulator-Metal-Insulator Layers

29. Kim J, Noh J, Ihm D (2002) Transparent multi-layer conductive electrode film prepared by DC sputter deposition and its flat panel display application. Mat Cryst Liq Cryst 377:41–44 30. Weber LF (2006) History of the plasma display panel. IEEE Trans Plasma Sci 34:268–278 31. Kim YJ, Choi U, Kim YS (2008) Screen filter design considerations for plasma display panels (PDP) to achieve a high brightness with a minimal loss of EMI shielding effectiveness. J Electromagnet Waves Appl 22:775–786 32. Choi KH, Kim JY, Lee YS, Kim HJ (1999) ITO/Ag/ ITO multilayer films for the application of a very low resistance transparent electrode. Thin Solid Films 341:152–155 33. Aoshima Y, Miyazaki M, Sato K, Akao Y, Takaki S, Adachi H (2001) Improvement of alkali durability of silver-based multilayer coatings for use in flat panel displays. Jpn J Appl Phys 40:4166–4170 34. Koike K, Fukuda S (2008) Multilayer transparent electrode consisting of silver alloy layer and metal oxide layers for organic luminescent electronic display device. J Vac Sci Technol A 26:444–454 35. Koike K, Shimada K, Fukuda S (2010) Multilayer high reflectance coating on polyethylene terephthalate film consisting of Ag/SiO2/TiO2 layers that are not quarter-wave thickness. J Vac Sci Technol A 28:99–107 36. Roh HS, Kim GH, Lee WJ (2008) Effects of added metallic elements in Ag-alloys on properties of indium-tin-oxide/Ag-alloy/indium-tin-oxide transparent conductive multilayer system. Jpn J Appl Phys 47:6337–6342 37. Lee JH, Hwangbo CK (2002) Characterization of the optical and structural properties for low emissivity filters with Ti, TiOx and ITO barrier layers. J Korean Phys Soc 46:S154–S158 38. Lee JH, Lee SH, Hwangbo CK, Lee KS (2004) Optical and structural properties of TiO2/Ti/Ag/TiO2 and

39.

40. 41.

42.

43.

44.

45. 46.

47.

TiO2/ITO/Ag/ITO/TiO2 metal-dielectric multilayers by RF magnetron sputtering for display application. J Korean Phys Soc 44:750–756 Kim WM, Ku DY, Lee IK, Seo YW, Cheong BK, Lee TS, Kim IH, Lee KS (2005) The electromagnetic interference shielding effect of indium–zinc oxide/ silver alloy multilayered thin films. Thin Solid Films 473:315–320 Haacke G (1976) New figure of merit for transparent conductors. J Appl Phys 47:4086–4089 Bender M, Seelig W, Daube C, Frankenberger H, Ocker B, Stollenwerk J (1998) Dependence of film composition and thickness on optical and electrical properties of ITO – metal – ITO multilayers. Thin Solid Films 326:67–71 Klo¨ppel A, Kriegseis W, Meyer BK, Scharmann A, Daube C, Stollenwerk J, Trube J (2000) Dependence of the electrical and optical behaviour of ITO-silverITO multilayers on the silver properties. Thin Solid Films 365:139–146 Koike K, Yamazaki F, Okamura T, Fukuda S (2007) Improvement of corrosion resistance of transparent conductive multilayer coating consisting of silver layers and transparent metal oxide layers. J Vac Sci Technol A 25:527–531 You J, Park J, Lee J (1998) New approaches for largesize plastic LCD applications. In: Proceedings Asia display, vol 18, pp 179–182 Sabnis RW (1999) Color filter technology for liquid crystal displays. Displays 20:119–129 Okamura T, Fukuda S, KoikeK, Saigou H, Kitagawa T, Yoshikai M, Koyama M, Misawa T, Matsuzaki Y (2000) PDP optical filter with sputtered multilayer coatings and organic dyes. In: Proceedings IDW, pp 783–786 Li Y, Wang L, Chang C, Duan L, Qiu Y (2004) Flexible organic light-emitting diodes with ITO/Ag/ ITO multi-layers as anodes. Chin Sci Bull 49/ 13:1328–1331

Part 5.5

Patterning Processes

5.5.1 Photolithography for Thin-Film-Transistor Liquid Crystal Displays Wen-yi Lin . W. B. Wu . K. C. Cheng . Hsin Hung Li 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 836

2 2.1 2.2 2.3 2.3.1 2.3.2 2.3.3

Photoresist Material Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837 Positive and Negative Photoresists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837 Compositions of Positive Photoresists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837 Important Properties of Photoresists for TFT Array Processes . . . . . . . . . . . . . . . . . . . . 839 Relationship between Photosensitivity and Exposure Dose . . . . . . . . . . . . . . . . . . . . . . . 841 Photoresist Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843 Resistance to Plasma Etching and Dark Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843

3 3.1 3.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.4 3.5

Photolithographic Process and Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844 Spin and Slit Coating of Photoresists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 845 Vacuum Drying and Prebaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 846 UV-Light Proximity and Projection Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 847 Proximity Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 848 Projection Exposure Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849 Resolution and Depth of Focus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 850 Half Tone and Gray Tone Masks and Maskless Technologies . . . . . . . . . . . . . . . . . . . . . 852 Photoresist Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853 Postbaking of Photoresists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 854

4 4.1 4.2 4.3

Critical-To-Quality Factors of TFT LCD Photolithography . . . . . . . . . . . . . . . . . . . . . . 854 Effects of CD and Differential CD on Display Performance . . . . . . . . . . . . . . . . . . . . . . 854 Effects of Overlay and Differential Overlay on Display Performance . . . . . . . . . . . . . 856 Effects of Total Pitch of Patterned LCD Glass on Display Performance . . . . . . . . . . 858

5

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 858

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.5.1, # Springer-Verlag Berlin Heidelberg 2012

836

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

Abstract: The fundamentals of resist materials and chemistry, equipment, and process engineering for thin-film transistor (TFT) liquid crystal display (LCD) photolithography are reviewed with a primary focus on manufacturing technology. The effects of the materials and concentrations of (mainly TFT array) photoresist constituents on the photosensitivity, dissolution behaviors in the developer, contrast, resolution, dark erosion, and stability against plasma etching are presented. Along with the equipment designs and functions, the process engineering to achieve the optimum resolution, better depth of field, higher throughput, and lower cost is explained and discussed. The efforts to address the great difficulty associated with capability/capacity improvement and increasing dimensions and weight of glass substrates and equipment are also reviewed, in response to the rising volume demands of larger displays. Mura issues related to the critical dimension (CD), differential CD, overlay, differential overlay, and total pitch are elucidated and the essential control factors and approaches are concisely introduced. Guided by the continuing, market-driven industry trends, we report the directions and future challenges for photolithography for TFT LCD applications. List of Abbreviations: CD, Critical Dimension; DNQ, Diazonaphthoquinone; DOF, Depth of Field; Eop, For Practical Manufacturing Processes, the Recommended Operation Dose which Enables the Complete Dissolution of a UV-Irradiated Positive Photoresist During Development; Eth, The Threshold Dose Above Which an Exposed Positive Resist Is Fully Soluble in the Developer; LCD, Liquid Crystal Display; NA, Numerical Aperture; PAC, Photoactive Compound; TFT, Thin-Film Transistor

1

Introduction

After decades of worldwide intense pursuit of flat panel display commercialization, thin-filmtransistor (TFT) liquid crystal displays (LCDs) gradually emerged to become one of the leading flat panel display technologies. The recent LCD market trends toward (1) brighter displays with less energy consumption, (2) superior image resolution, (3) elevated frame rate for motion blur reduction, and (4) higher-definition 3D images and motion picture displays continuously challenge and drive research and development efforts to further miniaturize microdevices on display panels and to improve the pixel aperture ratio (a quality factor that is the ratio of the visible-light-transparent area of a display to the total panel area of the pixels). This, in turn, requires higher device pattern resolution, better overlay and registration accuracy, and total pitch control. All of these critical structural features are mainly defined by photolithography (an optical chemical process of transferring patterns from a mask onto a substrate) in the TFT LCD production process. As a result, photolithography is one of the significant success factors of the advancement of TFT LCD technologies and their market growth. LCD lithography clearly distinguishes itself in terms of dimension, weight, and technical challenges from its IC semiconductor counterparts. (For example, generation 8.5 photolithography equipment occupies approximately 5,800 m2 and weighs approximately 410 t.) Owing to the immense market potential (worldwide LCD panel revenue was approximately US$69 billion in 2009) [1], the drastically different set of technical challenges for the TFT LCD industry attracts tremendous research and development interest. To produce crystal-clear, mura-free (‘‘uniform’’) picture quality, extensive TFT LCD efforts focused on photolithography engineering, including materials, equipment, and processing technologies, to achieve microscopic precision and submicron uniformity of pixel patterning throughout the entire substrates of 8.69 m2, in contrast with IC lithography achieving 28-nm critical dimension (CD)

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

on a 46-cm wafer. As TV display panel size has expanded to the commercially available 65 and 71 in., TFT LCD substrate size has increased more than 75 times from generation 1 (300
400 mm) to generation 10 (2,880
3,130 mm) since the 1990s, as shown in > Fig. 1 displaying the increasing dimension of glass substrates as a function of generation evolution (related information is also available in > Chap. 5.1.1). The demand for throughput increase has been another constant driving force in the LCD industry for greater production output and a lower contribution of capital cost to the display panel price. However, as the glass substrate size increases, raising throughput without comprising yield, quality, and performance becomes even more exigent. Approaches to address this include raising the photoresist coating speed without loss of thickness uniformity and increasing the scanning velocity of photoexposure systems while maintaining great control of the CD and the overlay, to name a few. These solutions require, for example, weight reduction of the optical scanning unit, powerful servomotors with accurate laser optic control, and photoresist materials engineering to increase exposure speed, etc. In this section, the principles and manufacturing technology of photolithography, including materials, processing, and equipment for TFT LCD, are reviewed and future research and development directions are briefly discussed.

2

Photoresist Material Engineering

Photoresists comprise of photoactive compounds (PACs), polymer resins, solvents, and additives (surfactants and/or adhesion promoters). They play a very important role in photolithography processing as they serve to define device circuits and color pixel patterns. Recent efforts in materials engineering have been devoted to developing higher optical sensitivity, high resolution, compatibility with lasers (as a light source), and lower-cost solutions.

2.1

Positive and Negative Photoresists

There are two types of photoresists: positive and negative. The UV-irradiated positive resist dissolves easily in a developer. The unexposed resist protects the underlying films during the following etching step, as schematically shown in > Fig. 2a. Because transistors and electronic circuits require higher-resolution patterning, positive resists are often utilized in TFT array processes. The constituents of negative resists are cross-linked and polymerized under exposure to UV radiation. Without cross-linking, the unexposed resist is easily dissolved in a developer and subsequently rinsed off in water, as shown in > Fig. 2b. The cross-linked polymer is further heated and hardened to form a stable, patterned structure on a glass substrate. Because they are more stable and reliable than positive resists owing to the aforementioned cross-linking and polymerization, negative resists are commonly utilized in color filter process to form black matrix, red, green, and blue patterns as well as photospacers.

2.2

Compositions of Positive Photoresists

As this section primarily focuses on TFT array process, the materials engineering of positive photoresists is further described and discussed. Typical compositions of commercially available positive photoresists are displayed in > Fig. 3. Acting as a carrier for the resist coating, the

837

5.5.1 Length and width of glass substrates (mm)

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

2,880  3,130

G10

2,200  2,500 2,160  2,460

G8.5

1,950  2,250 1,870  2,200

G8 G7.5 G7

1,500  1,850

G6

1,300  1,500

G5.5

1,100  1,300

G5

680  880 600  720 550  650 370  470 300  400

G4

22m

11m G3.5 G3 G2 G1

a

Glass substrate size by generation 10.0 G10

9.0 8.0 7.0 Glass area (m2)

838

6.0 G8.5

G8

5.0 G7

4.0

G7.5

3.0 G6 2.0 G5 G4

1.0 G1

G3.5 5 G2 G3

0.0

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

b . Fig. 1 (Continued)

LCD monitor unveiled

LCD TV unveiled

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

solvent usually takes up approximately 70–85% by weight and is subsequently driven off in the following process steps. The remaining resist is active compounds for an optically induced chemical reaction to serve a patterning function. (Typically, the solid content of a resist for TFT array process application is approximately 15–20%, whereas that for IC processing is approximately 25–30%.) In positive photoresists, the active ingredients are mainly novolac resins and PACs. The latter are the product of the esterification reaction between diazonaphthoquinone (DNQ) and polyhydroxybenzophenone. > Figure 4 shows an example of the esterification reaction, in which an SO2R group of DNQ reacts with an OH group of a polyhydroxybenzophenone compound (2,20 ,4,40 -tetrahydroxybenzophenone) to form a PAC. Because there are several OH groups in a polyhydroxybenzophenone molecule, multiple DNQ molecules can link with it. The esterification ratio (defined as the number of DNQ molecules linking with the polyhydroxybenzophenone divided by the total number of OH groups of the polyhydroxybenzophenone) is an important property affecting photosensitivity, dissolution, and photoresist contrast (discussed later). In the absence of UV radiation exposure, the products of the azocoupling reactions between the DNQ in PAC and novolac resins are insoluble in a developer. (An example of azocoupling is shown in > Fig. 5.) With novolac resins, the unexposed photoresists have the required structural integrity to withstand the subsequent TFT array processes before they are stripped. On the other hand, UV radiation causes carboxylic groups (COOH) to form in the DNQ and as a result, the exposed acidic resist is soluble in basic developer solution. An optimum ratio of DNQ, thus, has to be carefully engineered to inhibit the dissolution of the unexposed resist and to promote easy dissolution of the exposed resist. Solvent is added to fully dissolve polymer resins and the PAC to enable resist coating on substrates. By adjusting the solvent concentration, one can control the viscosity of the resist solution for optimum coating. Other formulation considerations of the solvents include low toxicity, suitable evaporation behaviors, and low cost. Propylene glycol monomethyl ether acetate, for example, is one of the most commonly used solvents to produce resists for TFT array processes. Moreover, a small fraction of additives, such as adhesion promoters (e.g., Si– O–H-containing compounds) and various types of surfactants, are frequently utilized to improve resist adhesion and coating thickness uniformity, respectively.

2.3

Important Properties of Photoresists for TFT Array Processes

To simplify production management, utilizing a single kind of resist is often preferred for the TFT array patterning process in a production line. That is, the resist has to be meticulously designed and formulated to meet all the process requirements, including high optical . Fig. 1 (a) Thin-film-transistor (TFT) liquid crystal display (LCD) generation evolves and the length and width of glass substrates increase with the needs for new applications. (The glass substrate size is determined by the most efficient utilization of a glass substrate that can accommodate a certain number of products with the highest commercial potentials, by the time the generation is ready for mass production. For example, the utilization rate of eight 42-in. panels of a generation 7.5 substrate is 89%, whereas eight 43-in. TV panels do not fit on the substrate.) (b) Exponential growth of glass area in the last 2 decades. The swift dimension increase from generation 5 to generation 8.5 and beyond is fueled by the demands for larger TV

839

BM

Glass substrate

Negative (color) resist

Glass substrate

Coat Color resist

Glass substrate

Glass substrate

Expose

Develop

Develop

Mask

Light source

Glass substrate

PR

Color resist

Glass substrate

PR

Expose

Glass substrate

Glass substrate

. Fig. 2 Illustration of how coatings of (a) positive and (b) negative resists respond to UV exposure and photodevelopment. PR photoresist, BM black matrix

b

a

Coat

Mask

Light source

5.5.1

Thin film

Positive resist

840 Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

Positive photoresist compositions ~1%

2–5% 12–24%

70–85% Solvent

Resin

PAC

Additives

. Fig. 3 Compositions of typical positive photoresists by weight commercially available in the TFT LCD and IC industries. PAC photoactive compound

N2

O

O

O O + S O

R

O OH

OH

O

OH

HO

S

O

HO

OH

HO

N2 O DNQ

2,2′, 4,4′-tetrahydroxybenzophenone

PAC

. Fig. 4 The esterification reaction between diazonaphthoquinone (DNQ) and 2,20 ,4,40 tetrahydroxybenzophenone to form a photoactive compound

sensitivity, high resolution, low dark erosion, good adhesion to underlying materials (metal, semiconducting, and dielectric coatings), a wide process window for CD control, stability during plasma and wet chemical etching, etc. In this section, the sensitivity, resist resolution of patterns, and dark erosion of positive photoresists are further discussed.

2.3.1

Relationship between Photosensitivity and Exposure Dose

The photosensitivity of a positive resist is defined as the exposure dose needed to induce a sufficient chemical reaction of the constituents of the resist that will completely dissolve in a developer. It is expressed as dose=energy/area (usually in units of millijoules per square centimeter). > Figure 6 shows a typical resist thickness–dose relationship after exposure and development: the remaining film thickness decreases with increasing exposure dose after the onset point. The contrast of a positive resist, gp, the slope of the linear portion of the sensitivity curve in > Fig. 6, represents the ratio of the exposed resist thickness to that of its unexposed counterpart, after development. It is formulated as [2]

841

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays H2 C

HO O

N2

N2

H2 C

HO

O H2 C

HO

CH3

H2 C O

O S O O

O

S O

O

O O S O

N2

n CH3

O

N2 O

O

O H3C

O

N N

H2 C

+

O S O

N2

CH3 CH3 OH

OH

O

H3C

H3C

N

O

S O

O S O O

O

O

O S

N

O S

O O

HO C H2

OH

C H2

C H2

OH

OH

N

C H2

OH

N H3C

C H2

H3C

OH

C H2

H3C

OH

C H2

OH

C H2

. Fig. 5 Azocoupling between the non-UV-irradiated DNQ in photoactive compound and novolac resins

Normalized thickness

842

1 Slope = γ p

0

log10E0

log10Eth

Exposure dose, log10E

. Fig. 6 The remaining photoresist thickness after development as a function of exposure dose

gp ¼

1 log Eth  log E0

ð1Þ

where Eth is the threshold dose above which a resist is fully soluble in the developer and E0 is the exposure energy at the onset, as displayed in > Fig. 6. Eth is an indication of photosensitivity, and the recommended operation dose, Eop (approximately 30 mJ/cm2), is generally 1.1–1.3

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

times the threshold dose for LCD photolithography [3]. In semiconductor processing, Eop is usually larger than 1.5Eth or approximately 60 mJ/cm2, owing to a stronger interest for higher resolution [3]. The LCD Eop is lower owing to a greater need for a short exposure time. To accomplish this, highly sensitive LCD resists are engineered with lower molecular mass novolac resins (for example, 3,000–4,000 g/mol vs. more than 5,000 g/mol for IC resists), a smaller esterification ratio, and a smaller concentration of PAC.

2.3.2

Photoresist Resolution

The resolution of photolithography is defined as the smallest feature that can be resolved in a densely patterned area. It is strongly dependent on a number of factors, such as equipment hardware capability, process characteristics, and resist materials. Resolution is also related to the chain scission of positive resists and the cross-link network of negative counterparts during UV exposure. Because resolution is strongly correlated to the contrast of a resist, it can be improved by increasing the esterification ratio and the concentration of PAC, among other factors. (However, this is at the expense of photosensitivity reduction.) A positive photoresist tends to have a lower molecular mass and is more inclined to have a higher contrast. Besides, the chemical reactions of a resist in the developer also affect resolution. The dissolution mechanism of positive resists in the developer is inherently different from that of negative resists: In contact with the developer, the volume of positive resists does not increase as much as that of negative resists, which are penetrated by developer solution during development and, in turn, swell (volume increase). On the basis of the considerations above, positive resists are capable of rendering higher resolution and are usually used in all TFT array process steps.

2.3.3

Resistance to Plasma Etching and Dark Erosion

Though a positive resist is intended to protect the underlying films from plasma etching in the subsequent dry etching process, a tiny portion of the resist is still eroded away. In poorly controlled cases (for example, insufficient baking and/or vacuum drying), a partially dried positive resist may be fractionally eroded and the by-products are deposited around the edge of the resist, forming a hard coating and prevent the underlying films from being further etched away. In some cases, the phenomenon may create a current leakage path and, in turn, degrade the display panel performance. Therefore, it is imperative to carefully control the drying and evaporation of solvent vapor. The unexposed positive resist is formulated to be stable in the developer. However, it is unavoidable that a trace amount of the resist will dissolve in the developing process; this is termed ‘‘dark erosion.’’ In certain cases, the resist profile may even be unfavorably altered and the CD of patterned features may deteriorate owing to severe dark erosion. To reduce the effect, resistance to dark erosion is improved with increasing molecular mass and/or the orthosubstitution ratio of novolac resins. Again, this is at the expense of lower photosensitivity. To meet the requirements of photosensitivity, developer solubility, thermal stability, and resist contrast, a mixture of novolac resins of different molecular masses is formulated to produce optimal resists. > Table 1 summarizes the characteristic influence of positive photoresist compositions on photolithography behaviors [3, 4]. The trade-offs were taken into

843

844

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

. Table 1 Effects of positive photoresist compositions on photolithographic behaviors [3, 4]

Photo sensitivity

Dissolution inhibition of the unexposed resist

Dissolution promotion of the Thermal exposed resist stability Contrast

Molecular weight of novolac Ortho-substitution of novolac Esterification ratio in PACa PAC concentrationa Directly proportional; Inversely proportional; Independent PAC photoactive compound a Based on the commercially available products, under a sufficient dose of UV exposure for thin film transistor array processing

consideration in designing numerous commercial resists such as SR and TFP series resists by AZ Electronic Materials and Tokyo Ohka Kogyo, respectively, for higher contrast and photosensitivity. As solvent evaporation is sensitive to temperature, mura is sometimes observed at lift pins and vacuum holes which are at a different temperature from the substrate stage. To alleviate the thermal dependence and mura formation, multiple solvents of different boiling temperatures are used. Despite extensive efforts having been dedicated to the development of mura-less resists, the LCD industry is still in need of a better solution for this, along with higherthroughput resist products.

3 > Figure

Photolithographic Process and Equipment

7 shows a typical photolithographic process flow. It begins with a glass cleaning process, followed by dehydration baking on a hot plate to drive off any residual moisture on the glass substrate. To improve resist adhesion to the glass, priming with hexamethyldisilazane is sometimes applied. Subsequently, the photoresist is coated on the glass, using a spin or slit coater. The resist-coated glass then enters a vacuum drying chamber, where most of the solvent evaporates. It is further dried on a prebaking plate before the substrate is transferred to an exposure system in which the photoresist is irradiated with masked UV light to induce an optical-chemical reaction among the resist’s constituents. The substrate is rolled to a developer unit for development. The exposed positive resist material is dissolved in the developer, rinsed off with water, and air-dried. The remaining resist is postbaked again to remove any residual moisture and solvent and to improve resist adhesion to the glass. The resist-coated substrate is then ready either for subsequent etching or for metrology, such as thickness and CD measurements, macroscopic observation, and auto-optical inspection.

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

Substrate cleaning, dehydration baking and HMDS priming

Coating

5.5.1

Vacuum drying and pre-baking

Exposure

Inspection

Post-baking

Developing

. Fig. 7 The TFT LCD photolithographic process. HMDS hexamethyldisilazane

3.1

Spin and Slit Coating of Photoresists

As shown in > Fig. 8, two techniques of resist dispensing are used in TFT LCD production, namely, spin and slit coating. In spin coating (used in LCD generations 1–5), the resist is either dropped at the substrate center or dispensed through a slit nozzle to cover the entire glass. The glass stage then accelerates and eventually spins at a very high rotational velocity (generally approximately 1,000–1,300 rpm). Consequently, the resist is driven off by centrifugal force and is spread ‘‘uniformly’’ on the glass. Nearly 80–95% of the dispensed resist spreads off the glass and the excess is either drained/wasted or recycled, raising the production cost. The remaining resist on the glass forms a thin coating with a thickness in the range 1–3 mm (dictated mostly by the stage spin speed and the solvent viscosity) for the TFT array process. As substrate size increases with the size of TFT LCD products, especially driven by TV demands, numerous issues with spin coating are encountered. The tangential speed at the tip of a generation 5 glass can be as high as approximately 370 km/h. Further enlargement of glass substrates (generation 6 and above) poses greater challenges of holding the glass on a fast spinning stage, leading to safety concerns. Additionally, the thickness uniformity of the resist coatings is not easily controlled, owing to the heavier spin stages, the higher momentum during rotation, etc. As a result, spinning coating was replaced by slit coating, which has become the industry’s standard in newer generation 5 coaters and beyond. In slit coating, while moving above a glass substrate, a nozzle with a slit through which the resist is dispensed is integrated with a premetered pump to operate at a constant flow rate to achieve uniform resist thickness. To slit-coat resists for LCD production, it is crucial to manage and control the following important hardware and process parameters: the interior construction (internal channel and shim-nozzle housing structure) of the nozzle, the sagging of the nozzle owing to its own weight (generation 8.5, 2,700 mm, approximately 110–160 kg), the leveling of the nozzle, the pumping rate (generally, 2–4 ml/s), the gap between the nozzles and the glass (typically, of the order of 100 mm), instantaneous start and end control, air bubble formation prevention, and motion control of the nozzle scanning process. Among the factors, nozzle gap, nozzle leveling, and resist pumping rate are more process-engineer-accessible and

845

846

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

Spin coating

Nozzle

PR

Film

Glass

a

Slit coating

Nozzle

Nozzle

Film

b

Glass

PR

Glass

. Fig. 8 (a) Spin coating and (b) slit coating of photoresists, used in the TFT LCD industry. PR photoresist

are sometimes adjusted to maintain the desired thickness uniformity. Despite the aforementioned tremendous technical challenges, slit coating has significant advantages over spin coating: high resist utilization rate (more than approximately 90–95%), more throughput, and safety. With the increasing demands of larger substrates and shorter takt time, longer, heavier slit nozzles are used and moved at a higher speed. This may easily result in serious vibration and, in turn, mura formation if the nozzle motion is inappropriately controlled. Therefore, the nozzle system has to be meticulously designed and engineered to reduce vibration. Some common solutions to this include (1) reducing the nozzle weight through design, construction, and material replacement, (2) reinforcing the nozzle support, (3) lowering the center of gravity of the entire coating structure, etc.

3.2

Vacuum Drying and Prebaking

As described earlier, the resist contains mostly solvents which are to be driven off before UV exposure. Vacuum drying (and the subsequent prebaking) serves this function. Vigilant control of the vacuum drying evaporation rate is essential for the process to succeed. If the initial vacuum drying rate is too fast, the resist quickly forms a prematured, dried shell on the coating’s surface and this, in turn, prevents the remaining solvents under the shell from evaporating. In the subsequent vacuum drying step and/or baking process, for the incompletely dried resist the solvent evaporation process continues and this sometimes leads to volcanic eruption of the bottom layer, creating holes in the resist coating.

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

1.0  105 Pa

Pressures

6.6  104 Pa

2.6  101 Pa 15 s

60 s Process time

80 s

. Fig. 9 A typical two-step vacuum drying process as a function of time

On the other hand, excessively slow vacuum drying also results in a partially dried resist when the vacuum drying process is over. In the subsequent baking for further drying, mura is sometimes observed, owing to differential evaporation of the residual solvent as a result of a temperature difference between the stage and its lift pins/holes. A common solution to the aforementioned problem is to adopt a two-step vacuum drying process, as displayed in > Fig. 9. First, a slow vacuum is applied by partially opening the pump valve. This enables gradual surface evaporation without the formation of an exceptionally dried surface layer on the resist. After approximately 10–20 s, the vacuum drying speeds up to complete the drying process for takt time reduction. Once the final vacuum pressure is reached (in the range 20–100 Pa), depending on the solvent compositions, nitrogen is introduced into the chamber to remove the vacuum. After vacuum drying, besides other purposes, prebaking is applied to remove any residual solvent and to provide thermal energy to improve resist adhesion to the glass. Temperature control (ranging from 100–130 C) is crucial to prebaking. A higher temperature may roughen the resist surface, owing to nonuniform solvent evaporation.

3.3

UV-Light Proximity and Projection Exposure

The dimension and weight of a UV exposure system increase tremendously (for example, generation 8.5, 10.7
10.0
5.6 m, approximately 115 t) with glass substrates, leading to enormous challenges for the stage motion engineering, whose primary goals are to serve lithography functions, to shorten the takt time, and to minimize vibration. Therefore, it is vital to reduce the stage weight (as the dimension cannot be reduced). Modern stage designs take advantage of aerospace technologies by replacing heavy ceramics with lightweight, carbon-fiber-reinforced composites and by utilizing a welded framework instead of a one-piece, molded structure. Furthermore, stage vibration issues are alleviated using mass dampers and active mount units. The computer-aided control system can detect the speed and vibration of the moving stage and instruct the linear motor systems of the active mount units to apply a series of counteracting mechanisms to reduce vibration.

847

848

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

The UV light optical system undoubtedly contributes the most to the control of photolithographic characteristics such as pattern resolution and overlay accuracy. Currently, two of the main optical exposure systems are used in the TFT LCD industry, namely, projection and proximity. Because proximity systems provide sufficient resolution (of the order of 10 mm) and overlay control capability with better cost of ownership, they are the standard for the color filter process. On the other hand, owing to the needs of higher resolution (of the orders of 1 and 10 mm) and overlay accuracy, projection scanners are the norm for the TFT array.

3.3.1

Proximity Exposure

Compared with a contact exposure system, a proximity system has an edge of noncontact with glass and, hence, less scratch risk and fewer cleaning iterations. Collimated UV light is configured and uniformly radiates the mask-covered resist to induce photochemical reactions, as schematically shown in > Fig. 10. The gap between the glass and the mask is one of the key process parameters determining a proximity system’s pattern resolution (R): R ¼ 1:5ðlgÞ0:5

ð2Þ

where g is the mask–glass gap and l is the wavelength of the UV light [5]. Proximity (and projection) systems utilize high-pressure (20-atm) mercury lamps, emitting UV light whose peaks, g-, h-, and i-lines, are at 436, 406, and 365 nm, respectively. As the mask size increases, its weight increases (generation 8.5 mask, 1,220 mm
1,400 mm
13 mm, 48 kg) and the center of gravity is farther away from the mask’s edge support, resulting in even more severe mask sagging. One of the solutions to this problem is to apply a vacuum on the mask (by placing another sheet of UV-transparent bending glass over the mask and pumping down the small

Illumination through fly eye lens Area shutter

Board shutter

Gap sensor at standby position

Bending glass Mask Vacuum Substrate Substrate stage

. Fig. 10 An optical system of proximity UV exposure, commonly used for radiating color filter resists. Above the mask, a sheet of bending glass is placed and a vacuum is applied to offset the mask bending owing to its own weight

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

chamber in between) to counteract gravity. Despite all efforts, however, commercially available generation 8.5 masks still suffer sagging of approximately 50 mm. Stage flatness also directly affects the mask–glass gap and it is generally controlled within approximately 30 mm. If these two phenomena are into consideration, a safe glass–mask gap to avoid scratch is, hence, kept at no less than 100 mm. Consequently, the resolution of a commercially available proximity exposure equipment is limited to approximately 10 mm, according to Eq. 2.

3.3.2

Projection Exposure Systems

Projection systems are categorized into scanners and steppers. (In a scanner, a substrate is exposed to UV light while the substrate stage moves. In contrast, in a stepper, the substrate stage motion and UV exposure are conducted sequentially.) Owing to the growing demands of larger glass substrates and higher throughput, scanners have become the mainstream technology, replacing steppers, since generation 4.5. > Figure 11 displays a simplified illustration of two projection scanners for TFT LCD photolithography. The lens mirror system (> Fig. 11a) is constructed with a hardware preset optical system, whereas the lens module system (> Fig. 11b) employs a series of independently software-programmable, electromechanically driven lens units to enable in-process optimization. Masks (generation 8.5 mask, 1,220
1,400
13 mm, 48 kg) in projection scanners, as shown in > Fig. 11, are approximately one meter away from glass substrates and, hence, they are nearly free from being scratched. Despite its extremely heavy weight, the highly sophisticated optical system is integrated with the advanced

The lens mirror system

The lens modules system

Light source

Light source

Mask

Mask

Projection lens modules system Projection lens mirror system

Glass substrate

a

Stage

Glass substrate

b

Stage

. Fig. 11 UV exposure systems using (a) the lens mirror (Canon) and (b) the lens modules (Nikon) projection techniques, capable of micron-level resolution and submicron overlay accuracy

849

850

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

stage driving schemes, the self-calibrating stage sensors to guide the stage motion, the vibration-controlled floating foundation, and the registration alignment mechanisms. All of these contribute to achieving the desired micron-level resolution and submicron overlay accuracy over the entire 9.0-m2 substrate area (generation 10).

3.3.3

Resolution and Depth of Focus

According to the Rayleigh equation, the pattern resolution of a projection system is formulated as R ¼ K1

l NA

ð3Þ

where K1 is a process and material constant (influenced by dose, resist composition, etc.), l is the wavelength of the light source, and NA is the numerical aperture of the exposure system: NA ¼ n sin y

ð4Þ

where n is the index of refraction in the image medium (usually air and n=1) and y is half of the angle of the light converging at the best focal plane, as shown in > Fig. 12 [6]. According to the equations, the resolution of an optical projection is improved by increasing NA and/or using a light source of shorter wavelengths. For example, an i-line filter blocks g and h lines and improves resolution (however, at the expense of throughput). Numerous optical mask techniques were developed to further enhance patterning characteristics, including phase shift mask and optical proximity correction [7, 8]. The former utilizes a phase-shift layer on the adjacent apertures of a mask to reverse the sign of the electrical field, and, therefore, the adjacent features can be resolved completely. As a result, the resolution of TFT array lithography can be improved to 2.5 mm from 3.0 mm (vs. binary masks). Optical proximity correction applies a precompensated mask feature to correct the distortion caused by light diffraction, usually occurring at the corners of a pattern to prevent rounding of the designed features. Another equally essential factor for the performance/quality of photolithography is the depth of field (DOF), whose physical meaning is that it represents the tolerance of optical focus

(Mask) object plane

Lens

Best focusing plane Resolution

Lens axis

θ

Depth of focus

. Fig. 12 Optics illustrating the basics of light and lens interaction and the critical control parameters and their relations to depth of field, resolution, and numerical aperture [6]

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

variation resulting from issues with stage flatness, stage cleanliness (scum and particles), etc. DOF, calculated from the following equation, DOF ¼ K2

l ðNAÞ2

ð5Þ

where K2 is a process and material constant (affected by dose, resist composition, etc.), increases with longer wavelength and smaller NA. Equations 3 and 5 indicate a shorter wavelength improves resolution but compromises DOF. It follows that since DOF is inversely proportional to the square of NA and resolution is inversely proportion to NA, the photolithography industry utilizes a shorter wavelength and a smaller NA to optimize both resolution and DOF. Other processing parameters, such as exposure dose, also affect DOF and resolution. In > Fig. 13, generalized Bossung curves shows resist line width and DOF as a function of exposure dose. As can be seen in the hypothetical focus-exposure plot, a greater dose leads to a smaller line width at a fixed focal point. Beyond 36.4 mJ/cm2 dose, a slight variation of focal position leads to a different line width, implying a small process window. At the sweet spot (within the range 22.4–28.0 mJ/cm2 dose), flat Bossung curves are found; that is, a nearly

6 Mask L/S 3.5 μm

5.5

14.0 mJ/cm2 5

16.8 mJ/cm2 19.6 mJ/cm2

4.5

22.4 mJ/cm2

CD (μm)

4

25.2 mJ/cm2 28.0 mJ/cm2

3.5

30.8 mJ/cm2 33.6 mJ/cm2

3

36.4 mJ/cm2 2.5 39.2 mJ/cm2 42.0 mJ/cm2

2 1.5 1 −50

−40

−30

−20 −10 0 10 20 Focus Z position (μm)

30

40

50

. Fig. 13 Generalized Bossung relations for resist line width, focal positions, and exposure dose of an exposure system. A useful quality measure to determine how invariant a resist line width is at different focal points under various doses. CD critical dimension

851

852

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

constant line width can be achieved at different focal positions (between 25 and 25 mm). In other words, with these doses, a larger process window can be found. This is one of the most important, basic tools to qualify a scanner before it can be released for further process tuning for production.

3.3.4

Half Tone and Gray Tone Masks and Maskless Technologies

A binary mask is produced, using laser writing, to pattern chromium-coated quartz substrates. Since chromium is UV-light-opaque, the mask offers, nearly, either 100 or 0% radiation under exposure (> Fig. 14). As a result, it is used to circuitize resist patterns of equal thickness in an exposure scan. Compared with other masks, binary masks are relatively simple to produce and, hence, less expensive. A half tone mask utilizes multilayers of films with differing UV transparency to enable multiple doses in a scan, as demonstrated in > Fig. 14. Consequently, different remaining resist thickness after development can be accomplished. With the resist of varying thickness and complimentary etching steps, the LCD industry’s five iterations of photolithography for a TFT array can be reduced to three to four [9]. Similar to the half tone technology, gray tone masks can also serve to decrease TFT array lithography iterations by varying doses as well. They make use of a pattern design of fine slits to diffract UV light, leading to multiple doses at different areas. The color filter process sometimes adopts these mask technologies to establish negative photoresist spacers (used to keep the TFT and color filter substrates at a constant gap) of various heights in one lithography iteration, instead of two. > Figure 14 shows the postbaked taper profile of photoresist spacers, using different mask technologies. The taper of the negative resist structures is related to the

Half tone mask

Binary mask

Light

Light

Semi-transparent film

Gray tone mask Slit patterns

Quartz

Light

Photo mask Cr Reduced light intensity

Reduced light intensity

PR

PR Substrate

a

PR Substrate

b

Substrate

c

. Fig. 14 (a) Half tone, (b) binary, and (c) gray tone mask technology and their corresponding, actual negative photoresist profiles

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

combined effects of dose (which affects cross-linking) and subsequent baking. (The cross-link density influences the tendency for thermal reflow during postbaking.) Gray tone masks have the advantage of a lower cost than half tone masks. However, the design and utilization of gray tone masks are more complicated than those of half tone masks. The former’s resist patterns are more susceptible to the influence of process deviations. With increasing LCD generation, the size and the cost of masks and exposure systems greatly increase. This has led to significant efforts to research and develop the next-generation maskless technologies. Direct writing using laser and electronic beams is on the verge of entering the commercial TFT LCD photolithography equipment market. In addition to the absence of mask cost, they have the advantages of offering much greater flexibility for evaluating and establishing novel pixel design concepts to advance TFT LCD displays. However, in general, there is still room for throughput improvement and cost reduction of the capital investment. Other maskless solutions include printing microdrops of ink materials on glass substrates, using ink-jet technology. To date, as it is limited to a lower resolution capability, ink-jet printing is applied mainly for color filter processing to print photoresist spacers and color resists. The major advantages of ink-jet printing include its volume saving of inks (further improvement is needed for predispensing and ink-jet head cleaning), and the absence of a mask, exposure system, developer, and stripper. Again, it also provides greater flexibility to engineer pattern designs.

3.4

Photoresist Development

> Figure 15 shows the basic steps in a developing process, including developer dispensing, draining, water rinsing, and air drying. After UV exposure, a nozzle is used to dispense a layer of basic developer on the resist-coated glass. The developer stays on the surface of the

Developer nozzle Air knife Developer 5°–15°

PR Glass substrate

PR Glass substrate

Spray rinsing

Air drying PR

PR

Glass substrate

Glass substrate

. Fig. 15 The basic steps of a developing process include developer dispensing, draining, water rinsing, and air drying

853

854

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

glass for a few seconds to react with the photoresist; the substrates are then titled to 5–15 to drain the remaining developer and the reaction products. Water rinsing is applied to dilute any developer remaining on the glass and, in turn, to bring any further resist development to a halt. The glass is then further cleaned using a large quantity of deionized water to wash off any residual developer and the by-products. The glass is subsequently dried using an air knife. To achieve the desired CD and its uniformity and to avoid mura and water marks, the following process parameters must be optimized and well controlled: (1) the duration for which the developer stays on the glass (puddle time), (2) the substrate transportation speed under the developer nozzle and the water rinse nozzle, (3) the developer drain speed, (4) the water rinse flow rate, (5) the developer concentration, (6) the developer temperature, and (7) the air drying flow rate, uniformity, and angle.

3.5

Postbaking of Photoresists

To eliminate traces of solvent and moisture and to harden (strengthen) the resist to withstand the etching of plasma and/or wet etchants in the subsequent steps, the resist-patterned glass is baked again. The temperature is generally maintained in the range of approximately 130–150 C. Overheating may turn the resist into a hard scum which cannot be completely removed in the following stripping process. The thermal treatment also has an influence on the resist taper profile and, therefore, needs to be carefully controlled to maintain the desired resist structure.

4

Critical-To-Quality Factors of TFT LCD Photolithography

Color saturation, color contrast, response time, resolution, and many other important characteristics of TFT LCD displays are strongly dependent on photolithography. With the primary emphasis on TFT arrays, their critical process parameters are reviewed, including CD, differential CD, overlay, differential overlay, and total pitch.

4.1

Effects of CD and Differential CD on Display Performance

‘‘Critical dimension’’ (CD) means, physically, the width and length of a patterned structure on a pixel and circuit layout. The combined effects of resist materials, masking, coating, exposing, and developing affect the CD: the essential process parameters include mask layout, binary, half tone or gray tone designs, resist thickness, exposure dose, developing time, developer temperature, developer concentration, etc. (A thicker resist, smaller dose, and shorter developing time lead to a larger CD.) Minute CD variation may cause severe display problems; for example, a longer transistor channel results in insufficient pixel charging and, in turn, leads to display anomalies. Sometimes, even though the CD variation of a single feature is within the design specifications, poor quality of the display may still be observed. An example (> Fig. 16) is that a consistent, uniform CD in one region slightly differs from the CDs in another region and consequently mura on an array glass (> Fig. 16a) and a fully assembled display panel (> Fig. 16b) can be perceived. (The mura is attributed to differential doses as a result of slight

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

a

6.5 6.3 0.4 μm CD (μm)

6.1 5.9 5.7 5.5 5.3

b

Distance (arbitrary unit)

. Fig. 16 (a) Three bands of mura caused by a submicron critical dimension (CD) change are found in the as-processed TFT glass. (b) An assembled LCD panel shows band mura resulting from CD deviation of approximately 0.4 mm (The glasses in (a) and (b) are not identical but their CD changes are of the same level)

855

856

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

differences in the depth of focus among lens modules.) Therefore, it is essential to control not only the CD but also its uniformity throughout the entire display. One common approach to detect such an issue early (of course, it is only at micron and submicron levels) is to macroscopically inspect the as-processed TFT array glass, using the naked eye before it is assembled into a fully functional display panel. Although human eyes have a very limited spatial resolution of the order of 0.1 mm, they are extremely sensitive to minute (submicron), gradual (and yet in an orderly fashion) changes of local nonuniformity. To assist the identification of the slight variation of the CD or the differential CD, various visible light sources (for example, sodium lamps) and collimated and diffused light are sometimes utilized. However, as the glass size expands to as much as 3.13 m (generation 10), it becomes impractical to inspect a mura that is probably meters away from the naked eye. Digital macroinspection, using an array of computer-interfaced cameras and advanced image processing techniques, is now deployed to identify potential issues.

4.2

Effects of Overlay and Differential Overlay on Display Performance

Overlay is a measurement of the distance of a subsequent pattern feature from the previous one. A slight change of overlay may cause poor display quality as well. For example, an overlay shift contributes to the deviation of parasitic capacitance coupled between, say, data lines and their nearby pixels, interfering with the pixels’ electrical charging ratio. As a result, the affected pixels are at a different pixel voltage from their neighboring devices. In turn, this alters the liquid crystal orientation/alignment of the affected pixels, compared with those in the vicinity, resulting in display mura. A smaller differential overlay shift but in a sufficiently sizeable area may still cause display issues. Although the human eye is generally not sensitive enough to identify a small gray scale difference (for example, one to two levels out of a total of 256 levels) of a display panel, it may be effective enough to recognize the minute difference stated above if the spots or panels are large enough or compared side-by-side. Hence, a diminutive differential overlay change (of the order of submicrons) may still cause visible display quality issues (based on a mechanism similar to that described in the previous paragraph). Therefore, it is also vital to control the differential overlay. > Figure 17 shows bands of mura resulting from submicron differential overlay deviation of a TFT array glass and a visible-to-naked-eye, vertical mura of an assembled LCD panel caused by an overlay shift of 0.5–0.8 mm in an orderly arrangement. Pattern registration techniques of an optical exposure system regulate overlay performance. In TFT array processing, overlay accuracy can be improved by enhancing the recognition capability of alignment marks, using an optimized light source and/or mark design. Stage motion can also be controlled to compensate for overlay shift. In lens module projection systems capable of individual lens unit adjustment, nonlinear compensation of the lens array can be applied after every 50 mm of travel of the optical systems to reduce overlay deviation. (Linear compensation which enables one adjustment per scan can also be utilized.) As an exposure system is infrequently used to process all of the layers to complete a functional circuit, not only the inherent photolithographic characteristics of exposure systems but also their equipment-to-equipment repeatability are of the essence for overlay control. Since the edge-to-edge alignment is influenced by the combined effects of the CD and overlay variations (in addition to total pitch, which is described later), a statistical approach of

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

a

0.8

3–1 OL

0.6 0.5 μm

Overlay (μm)

0.4

5–3 OL

0.2 0 −0.2 −0.4 −0.6

0.8 μm

−0.8

b

Distance (arbitrary unit)

. Fig. 17 (a) Four bands of mura observed in the as-processed TFT glass with submicron differential overlay shifts. (b) A finished LCD panel exhibiting a vertical band mura as a result of rapid overlay change locally (The panels in the two cases are not the same but they have a similar scale of differential overlay change) [10]

calculating the root mean square is used to determine the total influences of every contributing factor, instead of simple addition–subtraction arithmetic. The quadratic mean takes both probability and variation into consideration and, hence, is of great value to determine overlay tolerance for optimal pixel design. Alternatively, overlay-related issues are sometimes

857

858

5.5.1

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

addressed by circuit design solutions. For example, the coupled gate–source parasitic capacitance effects resulting from overlay deviation are compensated at times, utilizing an additional circuit. However, the additional compensation circuit lowers the aperture ratio of a TFT LCD panel and, in turn, decreases the brightness.

4.3

Effects of Total Pitch of Patterned LCD Glass on Display Performance

To produce a quality display, all of the pixel patterns of the separately processed TFTand color filter panels need to be ‘‘well’’ matched with each other, i.e., pixel to pixel, within the predesigned mismatch tolerance, when assembled. Therefore, it is highly crucial to effectively control both the microscopic patterns and the macroscopic glass shape and dimension. To accomplish this requires an understating of materials and processes. As glass substrates are produced through rapid quenching from the molten state (above 1,400 C) to room temperature in minutes (for example, a fusion flow production process), the chemical structure is in a metastable state. Hence, at elevated temperatures, glass substrates tend to irreversibly shrink (i.e., densify as a function of temperature and soaking time) during subsequent thermal exposure, necessary for TFT-LCD processing. For example, the irreversible thermal shrinkage of a generation 8.5 glass is of the order of several microns diagonally, after LCD processing. In addition, reversible thermal expansion/shrinkage of glass is also observed at room temperature (of the order of several microns per degree Celsius change, in diagonals, of a generation 8.5 glass). A frequently used gauge to assist the control of the aforementioned reversible and irreversible shrinkage and expansion is termed ‘‘total pitch.’’ It is a measurement of both the microscopic and the macroscopic dimension match of color filter and TFT array glass substrates. As the processed TFT and color filter glass substrates have their own distinctive thermal history, the design of pixel patterns of TFTs and color filters has to take the postprocessed total pitch of both glass substrates into consideration. Pre-enlargement, preshrinkage, and/or distortion correction of TFT and color filter patterns may be necessary to match the TFT and color filter pixels to be assembled. Inferior total pitch control frequently leads to TFT–color filter glass dimension disparity and, hence, partial light leakage through supposedly a ‘‘dark’’ matrix (or lines) is observed with the naked eye. The slight perception of gray level change again results in mura. Owing to the high sensitivity of glass dimension change to temperature, UV exposure systems are equipped with thermal chambers and temperature-controlled substrate stages (capable of 0.1 C control over the entire substrate area, for example, 5.5 m2 of a generation 8.5 glass) to reduce temperature-deviation-induced size variation and, in turn, help better define patterns in a controlled fashion. Besides, mask and substrate stages are furnished with a high-precision laser interferometer to guide their motion during pattern exposure and, in turn, to obtain the predefined shape and dimension. (Related information is also available in > Chap. 5.1.1.)

5

Conclusion

The chemistry and materials engineering of photoresists are related to resolution, photosensitivity, recommended exposure dose, dark erosion, and stability during plasma etching process. In general, the increase in molecular mass of novolac resins, the esterification ratio in the PAC, and PAC

Photolithography for Thin-Film-Transistor Liquid Crystal Displays

5.5.1

concentration tend to inhibit dissolution of the unexposed positive resists and decrease photosensitivity. The critical-to-process and critical-to-quality process and equipment parameters to control essential lithographic properties were presented, such as DOF, resolution, and resist thickness and its uniformity. The combined effects of materials, process, and equipment on CD, differential CD, overlay, differential overlay, and total pitch were discussed. A slight shift of the aforementioned five photolithographic behaviors may lead to undesired, visible mura on either the as-processed glass or the assembled display panels. Therefore, it is imperative to understand the basics and to control the mechanisms to address mura issues.

References 1. DisplaySearch PanelTrack (2010) http://www. displaysearch.com 2. Wake RW, Flanigan MC (1985) A review of contrast in positive resists. Proceedings of SPIE 539:291 3. Chen Y (2009) unpublished work 4. Dammel R (1993) Basic chemistry of Novolaks. Diazonaphthoquinone-base Resists Tutorial Text SPIE, Vol. TT 11, p 39 5. Thompson LF, Bowden MJ (1994) The lithography process: the physics. Introd Microlithograp: 19–38 6. King MC (1981) Principles of optical lithography, vol 1, VLSI Electronics. Academic, New York

7. Lin BJ (1993) Phase-shift masks gain an edge. IEEE Circuits Devices Mag: 28–35 8. Stanly W, Richard NT (2000) Optical aligners and photomask. Silicon processing for the VLSI era 1:628 9. Kuo Y (2003) Large TFT LCD Manufacturing Technology. Thin film Transistor Technologies VI, Proceedings of The International Symposium, pp 1–6 10. Lin WY, Wu WB, Chang JT, Tsai MT, Lin WT (2008) Recent advancements and future challenges of photolithography of TFT-LCD for display applications. Keynote Speech, SPIE Lithography Asia –Taiwan, November 2008

859

5.5.2 Wet Etching Hua-Chi Cheng 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 862

2 2.1 2.2 2.3

Principle of Wet Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 863 Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 863 Overetching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 863 Etch Bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 864

3 3.1 3.2 3.3

Method of Wet Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865 Etchants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 867 Wet Etching Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 867

4

New Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 869

5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 869

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.5.2, # Springer-Verlag Berlin Heidelberg 2012

862

5.5.2

Wet Etching

Abstract: Wet etching is a popular process in the mass production for electronic devices, especially in the display and integrated circuit industry. The major techniques of wet etching are described in this chapter. It begins with a technical overview and discussion of the applications of wet etching in flat panel display fabrication. The chapter explains the principles of wet etching in terms of etching mechanisms, materials processing, and other critical factors. In the applications, some special requests are involved in the etching process. Moreover, the process requirements such as tools, etchants, special considerations, and control factors are also explained. The chapter concludes with a discussion on the future development of wet etching. List of Abbreviations: BOE, Buffered Oxide Etch; CD, Critical Dimension; IGZO, Indium Gallium Zinc Oxide; ITO, Indium Tin Oxide; IZO, Indium Zinc Oxide; LCD, Liquid Crystal Display; MoW, Molybdenum Tungsten; TFT, Thin-Film Transistor

1

Introduction

Etching including both the wet and the dry process is defined as the removal of unwanted materials. To compare the material properties before and after the removal process, the characteristics of residual materials cannot be changed. Wet etching, also known as chemical etching, is a process using chemicals (acids or bases) to react with unwanted materials, such as metals, semiconductors, and dielectric materials. It can be applied to remove the unwanted material from the top of multilayers when the multimaterials have etching selectivity. The metal etching process is widely implemented in many traditional industry fields owing to its low costs, high throughput, and simplicity. Although wet etching is a robust process, it has a major drawback for fine-line patterning with critical dimension (CD) control owing to the isotropic (omnidirectional) nature of chemical reaction processes. In some special requirements, such as a fine line or a precise pattern in the semiconductor field, the defined size is miniature, e.g., several micrometers or nanometers. For state-of-the-art semiconductor integrated circuit device processing, such as dynamic random access memory or microprocessor units with feature size down to the nanoscale, the applicability of wet etching would become limited. Dry etching, on the other hand, typically employs plasma processes to generate reactive species with the help of ion bombardment or uses laser ablation for anisotropic (unidirectional) material removal. As such, dry etching is more conducive to precise CD control than wet etching. However, the damage to the underlayers or photoresist induced by high-energy ionic plasma species or laser irradiation is a formidable challenge for dry etching. Some materials used in the display technology are sensitive to high-energy exposure, such as indium tin oxide (ITO), indium zinc oxide (IZO), and indium gallium zinc oxide (IGZO), and would cause process yield concerns. The sensitive materials exposed to the plasma etcher or high-energy laser beam would cause a phase translation or the creation of dangling bonds in the microstructure. Another material, silicon dioxide (SiO2), can be patterned by dry etching through an ion bombardment process; however, SiO2 may cause poor etched patterns, and the photoresist can be severely damaged if used as the masking layer, which can leave behind organic residues which interfere with the subsequent semiconductor processes. Therefore, wet etching is a more suitable for patterning of these sensitive materials [1–3]. Dry etching also poses a challenge for large-area applications since the uniformity is difficult to control with plasma reaction/ion bombardment or laser ablation.

Wet Etching

5.5.2

In flat panel display technology, etching uniformity and material properties contribute equally to the development. Nonuniformity becomes a problem when attempting to process large panels with dry or wet etching. In contemporary production, the display manufacturing size has been scaled up to generation 10 (size 2,880 mm
3,130 mm), making panel uniformity of wet etching challenging; this nonuniformity results from the vertical immersion of the panel into the etchant solution, and this problem has to be solved. Therefore, for some specific materials such as IGZO, the uniformity of wet etching becomes the top priority.

2

Principle of Wet Etching

The mechanisms of wet etching rely completely on chemical reactions. For most etching processes, the selectivity, overetching, and etch bias are the most important considerations. We will elucidate these topics in this section.

2.1

Selectivity

Etching selectivity means the unwanted material is removed at a higher rate than the remaining materials. The wet etching process has a high etching selectivity owing to the vastly different chemical reactivity between the unwanted and the remaining materials with the selected wet etchant. Most wet etching processes for specific materials have a limiting step which results from the formation of chemical by-products in the etching solution. The etching rate of the specific materials will decrease in this step. Etching selectivity (S) between the first material and the second material can be defined as follows: r1 ð1Þ S¼ ; r2 where r1 is the etching rate for the first material, and r2 is the etching rate for the second material. If an etchant with a higher etching rate for the first material than for the second material, the value of S will be more than 1. That means the etchant has a higher selectivity for the first material over the second one, e.g., the first and second materials are the etched layer and the photoresist, respectively. Therefore, the photoresist can be used as a mask to define the area for patterning. For etching of different multilayers, there must be differentiation of the etching selectivity of etchants among these layers to ensure an etched cross-section profile is obtained. > Table 1 shows suitable etchants for different materials. It will help us to select a suitable etchant when there are multiple layers of different materials on the etching surface.

2.2

Overetching

Overetching is typically employed during the etching process to ensure the unwanted materials are fully removed. Overetching may increase the etch time by 10–20% compared with that for regular etching. For multilayer etching, the overetching can be controlled in the same way. For example, multilayers of molybdenum tungsten (MoW) and SiO2 are usually wet-etched by a buffered oxide etch (BOE) solution. The etching rate of SiO2 is higher than that of MoW in the BOE solution. Since the SiO2 material has a higher etching selectivity than MoW in the BOE solution, the overetching technique can be used in the multilayer patterning.

863

864

5.5.2

Wet Etching

. Table 1 Suitable etchants for different materials [21] Material

Etchant

SiO2

HF (46% in H2O)

SiO2 (on MoW) Si3N4

(NH4F–HF)–H2O (1:20)

NH4F–HF (6:1) HF (49%) H3PO4–H2O (at 130–150oC) Polycrystalline Si Crystalline Si

HNO3–H2O–HF (in CH3COOH) (50:20:1) HNO3–H2O–HF (in CH3COOH) (50:20:1) KOH–H2O–C3H7OH (23 wt% KOH, 13 wt% C3H7OH)

MoW

Na2S2O3(sat aq)–K2S2O5 (50 ml Na2S2O3, 1 g K2S2O5)

Al

H3PO4–H2O–HNO3–CH3COOH (16:2:1:1)

Ti

NH4OH–H2O2–H2O (1:1:5)

TiN

NH4OH–H2O2–H2O (1:1:5)

TiSi2

NH4F–HF (6:1)

Photoresist

H2SO4–H2O2 (125oC) C2H4NH2OH C3H6NH2OH C6H4(OH)2

2.3

Etch Bias

The dimension of undercutting is called bias – the undercut will appear during overetching. Etchants with large bias are called isotropic etchants, since the target etched layer is equally removed in all directions during the etching process. Anisotropic etching is greatly preferred for defining well-controlled patterns with a vertical profile. Isotropic etching is usually found in the wet etching process. > Figure 1 shows the phenomenon of isotropic etching, where the etched region is larger than the defined pattern, the so-called undercutting. The etch bias is defined as the deviation (D) between the expected and the real pattern. For the ideal isotropic case, the etched thickness (t) equals the etch bias. A large etch bias might affect the circuit design or device behavior. Generally, the ‘‘undercut’’ phenomenon cannot be controlled by reducing the concentration of the etchant solution. In a practical etching process, it is difficult to have either ideal isotropic or ideal anisotropic etching, and somewhere between the two is likely. Therefore, the anisotropy (Af ) is used to define the degree of directional etching, as shown in the following relationship: rlateral Af ¼ 1  ; ð2Þ rvertical where rlateral and rvertical are the etching rate in the lateral and vertical directions, respectively. 2 can be rewritten in terms of etch bias (D) and etching thickness (t) as > Eq. 3:

> Equation

Af ¼ 1 

D : t

ð3Þ

Wet Etching

5.5.2

D

t

D=t Isotropic etching D Table 2. The wet etching process is used to remove some layers from a panel immersed in a tank with an etchant for a period of time. This process is usually applied to pattern semiconductor devices. Semiconductor devices contain thin layers of semiconductive materials, dielectric materials, and conductors. The whole device process comprises a series of thin film deposition, photolithography, etching, and photoresist stripping steps. The flow chart of a semiconductor device process is shown in > Fig. 2. For wet etching, the process equipment, etchants, and controlling factors that affect the patterned device structure are discussed extensively in the following.

3.1

Equipment

Wet etching can be categorized as either immersion etching or spray etching techniques. Immersion etching is the simplest technique, consisting of a quartz vessel with a temperature regulator filled with a chemical solution (etchant). The sample to be etched is sunk into the vessel for a period of time, then transferred to a deionized water rinse tank (> Fig. 3a) [4]. During immersion etching, the unwanted region is removed through dissolution and mechanical agitation, with behavior similar to that in isotropic etching, thus causing the undercutting. A spray etcher is another etching tool; it sprays a chemical solution from stationary nozzles onto a rotating substrate in a tank. The etching is mainly dominated by the reactivity and dissolution ability between the etchant and the sample, with behavior similar to that in anisotropic etching, thus causing less undercutting. When the first spraying had been done, the process is continued by spraying another etchant for etching of the next material layer or spraying deionized water for rinsing. The substrate can be dried in the same tank with nitrogen gas (> Fig. 3b) [4].

865

866

5.5.2

Wet Etching

. Table 2 Process comparisons between wet and dry etching Wet etching

Dry etching

Etching rate

High

Low

Uniformity

Poora

Gooda

Repeatability

Poor

Good

CD loss

Large

Small

Selectivity to under layer

Good

Poor

Profile control

Very poor

Good

Multilayer etch

Difficult

Possible

Advantage

High selectivity

Anisotropic

Free of damage

Fine-pattern definition

High throughput

Fewer waste problems Better process control

Disadvantage

Isotropic

Damage issue

Fine-pattern limitation

Selectivity issue

Incomplete etching

Low throughput

Bubble formation Scum remainder Adhesion-problem CD, critical dimension a For small plane and fine-pitch patterning.

Substrate

Fully grown film on a substrate

Grown film

Photoresist layer coating

Photoresist

UV lithography

UV exposed photoresist

Etching and PR removal

Patterned structure on the substrate

. Fig. 2 Flow chart of a semiconductor device fabrication process. PR photoresist

The advantages of spray etching are the ability to supply fresh etchants during the entire etching process, meaning efficient use of the etchant, and precise timing control. However, the main disadvantage of the spray etching is system cost. It needs an etch-resistant material to prevent damage to the etching equipment [5]. Both wet etching techniques require timing control and regular agitation by integrating stirring, heating, or ultrasonic vibration to enhance etching uniformity [6].

Wet Etching

Vibration

5.5.2

Rotation Glass Etchant

Glass

Bubble

a

b

DI water

. Fig. 3 (a) Immersion etching; (b) spray etching. DI deionized

3.2

Etchants

To obtain better results in the selectivity of unwanted materials in the etching process; researchers have developed many types of etchants. These etchants are selected depending on the ability to remove the upper layer without attacking the under layer for multilayer etching. The best potential etchant must have certain properties, for example, easy processing, high etching rate, minimum undercut, economic regeneration, good dissolution capacity, and safe maintenance [7–12]. Conventional etchants generally consist of a strong acid, such as a halogen acid [13–15]. For example, amorphous silicon may be wet-etched using a mixture of nitric acid (HNO3) and hydrofluoric acid (HF). The nitric acid oxidizes the silicon surface to form a layer of silicon dioxide, which can be dissolved by hydrogen fluoride (HF). The reaction formula is as follows: Si + HNO3 + 6HF ! H2SiF6 + HNO2 + H2 + H2O. Silicon dioxide is usually deposited as the gate insulator of an electronic device, which can be etched by water-diluted HF with buffering agents such as ammonium fluoride (NH4F). Another application of wet etching is to pattern the conductive oxide electrode of an electronic device, such as ITO and IZO. The ITO films are usually etched using a strong acid such as a halogen acid which would corrode the other structure inside the device. Any halide residue during etching can degrade the electrical performance of the device [16, 17]. Therefore, weak acid etchants are generally preferred over strong acid etchants for conductive oxide electrode patterning. For example, oxalic acid is the most popular candidate for conductive oxide etching. Metals, such as aluminum and aluminum alloy layers, may be etched by a solution mixture of phosphoric acid, acetic acid, nitric acid, and water. > Table 3 shows the typical materials employed in thin-film-transistor (TFT) liquid crystal display (LCD) backplane fabrication and the appropriate wet etchants for each material.

3.3

Wet Etching Process

The principal mechanism of the wet etching process is the attack of the exposed unwanted material by corrosive chemicals, where the corrosive chemicals are transferred to the interior

867

868

5.5.2

Wet Etching

. Table 3 Wet etchants for liquid crystal display applications [15] Composition

Etchants

SiO2

NH4F–HF (7:1) BHF, 35 C

SiO2

NH4F–CH3COOH–C2H6O2–H2O (14:32:4:50)

SiO2 (on MoW)

(NH4F–HF)–H2O (1:20)

Si3N4

H3PO4, 160–180 C

Al

H3PO4–HNO3–H2O (80:4:16)

Mo

H3PO4–HNO3–H2O (80:4:16)

W

H2O2–H2O (1:1)

Cr

HNO3–H2O (1:1)

Cu

HNO3–H2O (1:1)

Ni

HNO3–CH3COOH–H2SO4 (5:5:2)

Ti

HF–H2O2

Au

KI–I2–H2O

MoW

Na2S2O3(sat aq)–K2S2O5 (50 ml Na2S2O3, 1 g K2S2O5)

ITO

H2C2O4–H2O (3:10)

ITO, indium tin oxide

microstructure of the unwanted material through a phase boundary. In particular, the corrosive chemical transfer involves the dissolution of chemicals into ionic species in the solution. A typical etching process consists of three major steps. Firstly, the reactive ions are transported to the etching surface from bulk solution (etchants) by diffusion. Secondly, the ions react with the etching structure. Finally, the reaction products dissolved and diffuse out from the structure into bulk solution [18]. The most important aspect of the etching is to control how much the materials are etched away. The following parameters are used to control the etching process: 1. Concentration: How concentrated the acid or alkali is, the ratio of the components in the etchants can vary depending on the type of target materials and the etching rate wanted. 2. Time: How long the etchant is used to etch. This really depends on the etching rate of the etchant. A typical etching time is between 30 and 60 s. 3. Temperature: At what temperature the etching of the target materials takes place. Typically, the higher the temperature, the faster the etching rate, which follows a well-known Arrhenius relationship. 4. Agitation: The delivery method of the etchants. The methods include thermal convection, spray, mechanical, bubble, and ultrasonic methods. Each method has its advantages and disadvantages. The spray method is considered the best because it produces the least amount of undercutting, cutting underneath the photoresist, and the highest etching rate. The etching rate and the etched profile depend on some key points: the types of substrate, the compositions of the etchant, the choice of masking layer (adhesion and etch resistance), the etching temperature, and agitation during processing. The etching rate is mainly controlled by the etchant’s reactivity and temperature during processing. The mass transportation of the reaction products is crucial to the final profile of the etched patterns [19]. Other practical considerations

Wet Etching

5.5.2

include the use of a certain overetching time beyond the end point of the etching (typically 5–10% of the total etching time). However, undercutting, especially during the overetching period, often appears beneath the edges of photoresist patterns owing isotropic etching. To optimize the etching time, the etching process is typically performed at 30oC or a higher temperature. The etching rate can also be increased by raising the concentration of the etchants or by using mechanical agitation by N2 bubbles or ultrasonic agitation during processing. Mechanical agitation is often used to improve the uniformity and the reproducibility since it can assist the etched surface to fully react with fresh etchant species and in the effective removal of reaction products from the etched surface.

4

New Approach

Conventional and emerging wet etching techniques for thin films include immersion, spray, electrolytic, and gas-phase etching, mechanical–chemical polishing, and certain fusion techniques [20]. The choice of the etching technique depends on the etch profile requirements. The most common use of etching for flat panel display fabrication is the liquid chemical immersion and spraying method. Generally, the specimen is immersed in the etchant solution by immersion etching; mechanical agitation is required to ensure etch uniformity and etching rate consistency. Immersion etching is suitable for a specimen of less than generation 3.5. Spray etching is a new approach and a good candidate to replace immersion etching because it can offer reproducible uniformity, etching time, and computer automation; and is suitable for TFT LCD manufacturing with substrate size more than generation 6.

5

Summary

For in-line application, low cost and high throughput are the most important considerations. Dry etching methods can produce more precise patterns than wet etching, and they are particularly useful for materials and semiconductors which are chemically resistant and could not be wet etched. This process is recommended if the design requires deep vertical sidewalls. However, dry etching is a high-cost method compared with wet etching. Therefore, wet etching is an attractive solution for the mass production of flat-panel displays with a generation 5 substrate size and beyond.

References 1. Lee CY, Chang C, Shih WP, Dai CL (2010) Wet etching rates of InGaZnO for the fabrication of transparent thin-film transistors on plastic substrates. Thin Solid Film 518:3992 2. Cheong WS, Yoon YS, Shin JH, Hwang CS, Chu HY (2009) Process development of ITO source/drain electrode for the top-gate indium–gallium–zinc oxide transparent thin-film transistor. Thin solid film 517:4094 3. Lee HN, Kyung J, Sung MC, Kim DY, Kang SK, Kim SJ, Kim CN, Kim HG, Kim ST (2008) Oxide TFT

with multilayer gate insulator for backplane of AMOLED device. J Soc Inf Display 16:265 4. Wei HF, Hsiue GH, Liu CY, Chen KF (2008) Jpn J Appl Phys 47:9001 5. Franssila S (2004) Enhancement of dimension uniformity of wet-etched thick insulator holes in triode carbon nanotube field emisson display devices. Introduction to microfabrication. Wiley, New York, p 120 6. Taylor D (1998) Wet-etch process improvements through SPC. Solid State Tech 119

869

870

5.5.2

Wet Etching

7. Dini JW (1984) Fundamental of chemical milling. American Machinist 768:113 8. El-Hofy HAG (2005) Advanced machining processes. McGraw-Hill, Blacklick, p 115 9. Collie MJ (1982) Etching compositions and process. Noyes data, Park Ridge, NJ, p 80 10. Sangwal K (1985) Etching of crystals theory, experiment, and application. Defects in solids. North Holand, Amsterdam, p 20 11. Quirk M (2001) Semiconductor manufacturing technology. Prentice Hall, Upper Saddle River, NJ, p 421 12. Singer P (1997) The many challenges of oxide etching. Semicond Int 110 13. Lii Y, Chang C, Sze S (1996) Etching. ULSI technology, McGraw-Hill, New York, 342 14. Singer P (1993) Meeting oxide, poly and metal etch requirements. Semicond Int Cahners 51

15. Zant PV (2000) Microchip fabrication. McGrawHill, New York, 259 16. Scholten M, van den Meerakker JEAM (1993) J Electrochem Soc 140:471 17. Huang CJ, Su YK, Wu SL (2004) On the mechanism of ITO etching: the specificity of halogen acids. Mater Chem Phys 84:146 18. Ko¨hler M (1999) The effect of solvent on the etching of ITO electrode. Etching in microsystem technology. Wiley-VCH, Weinheim, p 9. 19. Senturia SD (2001) Microsystem design. Kluwer, New York 20. Kern W, Deckert C (1978) Thin film processes. Chap V-1, Academic, New York 21. Plummer JD, Deal MD, Griffin PB (2000) Silicon VLSI technology: fundamentals, practice and modeling. Prentice Hall, Upper Saddle River, NJ

Further Reading Rai-Choudhury P (1997) Handbook of microlithography, micromachining, and microfabrication, vol 1 and 2. SPIE Press and IEE Press, Bellingham, WA Nguyen NT, Wereley S (2002) Fundamentals and applications of microfulidics. Artech House, Boston Madou M (1997) Fundamentals of microfabrication. CRC, London

Jaeger RC (1993) Introduction to microelectronic fabrication. Addison-Wesley, Reading Walker P, Tarn WH (1991) CRC handbook of metal etchants. CRC press, Boca Raton, pp 287–291 Kohler M (1999) Etching in Microsystem Technology. Wiley, New York, p 329

5.5.3 Dry Etching Eugen Stamate . Geun Young Yeom 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872

2

Plasma Surface Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872

3

Dry Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873

4

Etching Parameters, Requirements, and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874

5 5.1 5.2 5.3 5.4 5.5

Plasma Sources for Dry Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874 Capacitive Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875 ICP (Inductively Coupled Plasma) Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 876 Helicon Wave Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 877 ECR (Electron Cyclotron Resonance) Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 877 SWP (Surface Wave Plasma) Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 878

6 6.1 6.2 6.3 6.4 6.5 6.6

Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 879 Dry Etching of Indium Tin Oxide (ITO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 879 Dry Etching of Gate Metal Such as Al-Nd, Mo-Al Films, etc. for TFT-LCD . . . . . . . 879 Dry Etching of Copper Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 879 Patterning of InGaZnO TFT by Dry Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880 Dry Etching of Silicon Nitride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880 Dry Etching of a-Si:H/Polysilicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880

7

Future Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.5.3, # Springer-Verlag Berlin Heidelberg 2012

872

5.5.3

Dry Etching

Abstract: Production of large area flat panel displays (FPDs) involves several pattern transfer and device fabrication steps that can be performed with dry etching technologies. Even though the dry etching using capacitively coupled plasma is generally used to maintain high etch uniformity, due to the need for the higher processing rates in FPDs, high density plasma processing tools that can handle larger area substrate uniformly are more intensively studied especially for the dry etching of polysilicon thin films. In the case of FPD processing, the current substrate size ranges from 730 mm  920 mm (fourth generation) to 2,200 mm  2,500 mm (eighth generation), and the substrate size is expected to increase further within a few years. This chapter aims to present relevant details on dry etching including the phenomenology, materials to be etched with the different recipes, plasma sources fulfilling the dry etching requirements, and advantages of dry etching over wet processing. Current status and future trends are also presented. List of Abbreviations: ARDE, Aspect-Ratio Dependence-Etching; CCP, Capacitively Coupled Plasmas; DBD, Dielectric Barrier Discharge; ECR, Electron Cyclotron Resonance; HWP, Helicon Wave Plasmas; ICP, Inductively Coupled Plasmas; PECVD, Plasma Enhanced Chemical Vapor Deposition; RHP, Right Hand Polarized; RIE, Reactive Ion Etching; SWP, Surface Wave Plasmas

1

Introduction

Liquid crystal displays and rigid or flexible organic light-emitting diode displays using active-matrix require thin film transistors (TFT) with high mobility, and low leakage current can be produced by patterning technologies based on wet or dry etching. Commercial applications add the need for high resolution, high throughput, and large area. Due to its better directionality, more reliability, and less environmental impact, the processing using dry etching is preferable against wet etching. Dry etching is also used for patterning the source/drain and gate a-Si electrodes, the transparent ITO films and the metal electrode. Dry etching is performed in a plasma source under high controllability of process parameters. The interdependence between etching parameters and plasma properties needs to get an overview on both subjects.

2

Plasma Surface Interaction

Plasma is defined as an electrically neutral ionized gas. Plasmas used in thin film processing applications have a degree of ionization that can range between 0.1% and 10% and can be more complex including not only electrons, positive ions, and radiation, but also negative ions, negatively charged dust, and complex radicals. When getting in contact with a surface (biased, grounded, floated walls, or inserted objects), plasma forms a space charge layer, named sheath, which contributes to the adjustment of the charge fluxes to balance the loss and generation of charged particles. This charge distribution results in a potential profile within the sheath and a plasma potential in the plasma core. A surface biased negatively with respect to the plasma potential repels the electrons and accelerates the positive ions to an energy given by the potential difference. It creates a flux of directive ions that can induce surface modifications dependent on the properties of the ion-surface system. In a very synthetic way one can describe this interdependence as a plasma-assisted, ion-beam induced, surface modification process. Depending on the ion species, their energy and the surface properties the ion

Dry Etching

5.5.3

impact can induce etching or deposition, sputtering or implantation. In addition to the ions, dissociated radicals which are very reactive with the thin film material are formed in the plasma. Due to these dissociated radicals, spontaneous isotropic etching similar to wet etching is expected in addition to the ion-enhanced directional etching by incident ions. Since research on plasma etching was mainly driven by the need to replace wet etching in microelectronic industry it was named dry etching because it takes place in gas phase. Main plasma parameters are plasma potential, plasma density, electron temperature, and the electron energy distribution function that can be controlled within some limits by changing the type of plasma source (electron heating mechanism), the gas pressure and its composition (mixing ratio and flow rate).

3

Dry Etching

Pattern transfer involves material removal from a surface. Let us consider a pattern of width d delineated by a mask layer of thickness hPR to be transferred to a film of thickness hF deposited on a substrate. Since wet processing is isotropic (the etching rated is independent of direction), this results in the undercut profile as illustrated in > Fig. 1a which is unacceptable since a ratio h/d >> 1 is desired. In contrast, dry etching by positive ions from plasma and accelerated in the sheath provides the needed directionality and sidewall control as to ensure a directional etch (> Fig. 1b) or a vertical pattern transfer (> Fig. 1c). (In some cases, such as the gate metal etching of TFT for flat panel display [FPD], to obtain uniform deposition coverage of the materials above the etched gate metal, sloped directional etching in > Fig. 1b is preferred.) To ensure noncollision of the incident ions within the sheath and the etched trench, a mean free path of ions larger than these dimensions is necessary. This need can be fulfilled by operating the plasma source at pressures below atmospheric pressure. The dry (plasma) etching mechanism can be understood in correlation with the following steps [1]: (1) generation and transport of reactive species (atoms, molecules, ions) within the plasmasheath-surface system; (2) physisorption or chemisorptions of reactive species on the surface; (3) dissociation of reactants, formation of chemical bonds to the surface, diffusion and formation of desorbing species; (4) desorption and transport of product species from surface to plasma; (5) possible redeposition of etching products. In this context, there are four processes that can result in material removal: sputtering, chemical etching, ion-enhanced energy-driven etching, and ion inhibitor etching [2]. Sputtering is an anisotropic process that produces material removal by energetic ions. For a given ion energy, the sputtering yield is mainly dependent on the surface binding energy

d

d

hPR

d Mask

h

Bulk material Substrate

a

b

c

. Fig. 1 (a) Isotropic wet etching, (b) dry etching (directional etching), (c) dry etching (vertical etching)

873

874

5.5.3

Dry Etching

which makes the process unselective. The sputtering yield dependence on the incidence angle results in formation of facets. The main advantage of ion sputtering is the ability to remove nonvolatile byproducts such as copper and other metals. Chemical etching involves noncharged reactive species from plasma that react with the surface forming volatile products. The process is isotropic but can be highly selective. During the ion-enhanced energy-driven etching, the simultaneous action of reactive species and energetic ions result in an etching yield higher than that resulted by individual exposure. Despite this advantage, the process can show less anisotropic etch profile as shown in > Fig. 1b depending on the spontaneous chemical etch rate at the sidewall. The ion-enhanced inhibitor etching combines the reactive species and the energetic ions with inhibitor precursor molecules that form a passivation layer on the sidewalls. The result is more anisotropic etching at the trench bottom assisted by energetic ions while the lateral sidewall is protected from etchants [3].

4

Etching Parameters, Requirements, and Challenges

1. Etching rate. Typically measured in nanometer per minute is an essential throughput factor, decisive for moving an etching receipt from laboratory to production line. It needs to be uniform (below 10%) over the whole substrate area. The vertical and horizontal etching rates can be different. 2. Directionality (anisotropy) and profile control. The etched feature edges must be sloped (as shown in > Fig. 1b) or close to vertical (as shown in > Fig. 1c) depending on the TFT material to be etched. There are several profile distortions including notching, bowing, trenching, or faceting. Each of them can be addressed by proper profile control procedures. 3. Selectivity. Pattern transfer involves masking and use of photoresist or different materials. In order to etch trenches which have a high aspect (h >> d), the etching rate of bulk material should be higher than that for photoresist. Then, the selectivity is defined as the ratio of the etch rate of one material versus that of other. 4. Passivation of sidewalls. The radicals during ion-enhanced energy-driven etching are beneficial in obtaining a sloped profile but it has a drawback in obtaining a vertical profile due to the isotropic etching-induced undercutting that needs to be suppressed. The vertical profile can be realized by forming a sidewall protecting film by controlling the plasma discharge chemistry (addition of gases, decomposition and redeposition of photoresist, lowering the substrate temperature). 5. Aspect-ratio dependence-etching (ARDE) or RIE lag effect. It relates to etching rates dependent on the aspect-ratio of the etched patterns and is mainly caused by reduction of ion flux at the substrate, deposition of products from masking layer or redeposition of nonvolatile products. (No significant ARDE effect is observed during the FPD processing due to the small aspect-ratio of FPD materials to be etched.) 6. Plasma damage. It can be caused by high-energy ion bombardment, contamination by metal components, and charging-up induced by latent antenna effect or charge accumulation.

5

Plasma Sources for Dry Etching

In principle, a plasma can be generated between two biased electrodes as long as the electrons can accumulate enough energy between two collisions as to ionize the gas (Ohmic heating).

Dry Etching

5.5.3

Electron heating can also take place by their interaction with a moving sheath (stochastic heating), for instance, the sheath in front of an electrode biased with RF power or electron interaction with waves. Fine patterning with high aspect-ratio requires relatively low pressures where RF electromagnetic fields ranging from tens of hertz up to microwaves can be used to produce plasmas of densities ranging from 1013 up to 1018 m 3. At low pressures, the electrical power used to generate the plasma is mainly coupled to electrons. Depending on the type of the energy transfer to the electrons, the main plasma sources used for dry etching can be divided into the capacitively coupled plasmas (CCP), inductively coupled plasmas (ICP), electron cyclotron resonance (ECR), surface wave plasmas (SWP), and helicon wave plasmas (HWP). Currently, in the FPD processing, CCP is generally used due to the easier control of the etch uniformity.

5.1

Capacitive Discharges

A CCP for FPD processing includes two electrodes immersed in a relatively high pressure chamber (few hundred milliTorr up to few Torr) at a distance of a few centimeters and biased with rf power in megahertz range (see > Fig. 2a). The electrodes may have the same area (symmetric discharge) or be different (asymmetric). The sheath forming to both electrodes will oscillate during each period of the electromagnetic field accelerating some of the energetic electrons by stochasting heating while Ohmic heating takes place in bulk plasma. The main characteristics of CCP discharges are moderate densities (around 1016 m 3), electron temperatures around 3–5 eV, and plasma potential above 10 V. Due to the oscillating sheath at the electrodes the ion energy distribution function is bi-modal with two peaks connected with a plateau. The distance between peaks is mass dependent and can be controlled by changing the excitation frequency or the phase between two different power sources of the electrodes. The CCP was intensively used for dry etching starting with 1980s, and both plasma etching mode (the substrate is located at the ground electrode, where, chemical reaction controls the etching) and RIE (Reactive Ion Etching) mode (the substrate is located at the power electrode, where, chemical + physical reaction controls the etching) are used for the pattern generation of sloped etch profile (> Fig. 1b) and vertical profile (> Fig. 1c), respectively. These sources suffer from the lack of independent control of ion flux and ion energy. This is because the increase of plasma density by increasing rf voltage also raises the sheath potential which increases simultaneously the ion energy impacting the substrate. Later developments of CCP for semiconductor device processing include the addition of a magnetic field parallel to the electrodes to enhance the plasma density or bias the electrodes with different frequencies (for instance, 2 and 40 MHz) as to control the ion energy distribution function. CCP discharges are commonly used for production of TFT for flat panel displays with area larger than 1 m2. However, an application to even larger substrates is limited by plasma nonuniformities induced by electromagnetic wave propagation effect between the electrodes referred as the standing wave effect [4] that is enhanced at high frequencies when the electrode size is larger than about a few tenths of the excitation frequency in vacuum. The standing wave effect can be suppressed using a Gaussian-shaped top electrode as shown in > Fig. 2a [5]. Asymmetric discharges are also affected by a redistribution of the RF current along the plasma denoted as the telegraph effect that can be reduced by using symmetric electrodes [6].

875

876

5.5.3

Dry Etching RF1 RF coil Dilectric

RF E1 sh1

sh2

Helicon plasma sources

E2

RF2

a

Gaussian-shaped top electrode

b

RF

Double comb-type RF antenna desining

Seven helicon sources configuration

c

915 MHz 2.45 GHz

ECR plasma cells Wave guide Surface wave

RF

d

Matrix ECR configuration

RF

e

Slot antenna design

. Fig. 2 (a) CCP discharge with electrodes with separate sheaths (sh1 and sh2) at each of the electrodes (E1 and E2). A Gaussian-shaped top electrode for suppressing the standing wave effect is also illustrated; (b) ICP configuration with separate biases (RF1 and RF2) on each electrode a dielectric plate on top and a double comb-type antenna; (c) Permanent-magnet helicon sources at the top of a large area plasma device; (d) Matrix-ECR configuration based on individual ECR plasma cells; (e) Surface wave plasma source with wave guide for microwave propagation and slot antenna at the top of the dielectric plate

5.2

ICP (Inductively Coupled Plasma) Discharges

An alternative way for coupling the RF power to the discharge is to use an external antenna that is injecting energy into plasma through a dielectric plate transparent to the RF field (see > Fig. 2b). This configuration separates the electrodes from a direct contact with plasma as in CCP and provides an inductive coupling that avoids large voltages across the sheath with a direct consequence on the ion energy which drops from more than 100 V for CCP to less than 30 V in ICP. Moreover, the ion energy can be controlled by applying an independent RF bias on the substrate. The electron heating takes place in the close proximity of the dielectric window (the skin depth layer) by both ohmic and stochastic mechanisms. Plasma densities in ICP are one order of magnitude higher than in CPP for similar RF power and pressures. The coil antenna can be planar and placed on the top of a flat dielectric plate. Additional elements may

Dry Etching

5.5.3

be a shield for reducing the parasite capacitive coupling dominant at low powers and permanent magnets to reduce the loss at the walls. Due to their ability to etch on substrates larger than 1 m2 with high etching rates and low damage, ICP sources are being investigated to replace the CCP for the next generation plasma source. In order to avoid process nonuniformities caused by standing wave effects novel types of antenna have been developed, including U-type [7] or internal linear double-comb configurations [8]. Moreover, high-density, low-damage and meters-scale ICP plasma sources with controllable plasma uniformity profile were developed using low-inductance antenna modules with individual control for the injected power [9]. These configurations are using internal antennas covered with a thin dielectric so that there is no need of an expensive and damage-sensitive dielectric plate on top of a large vacuum chamber. Additional improvement includes RF antenna embedded with a magnetic core for increasing the power transfer efficiency [10].

5.3

Helicon Wave Discharges

The helicon wave plasma sources are based on the excitation of whistler waves bounded within a cylinder [11, 12]. The electromagnetic field is excited with an antenna designed to launch different modes in the presence of a magnetic field of hundred Gauss (see > Fig. 2c). The magnetic field confines the electrons, extends the skin depth layer, and helps to control the plasma uniformity. Most common designs for antennas are the Nagoya Type III, half helical and double saddle coil [1]. The electrons are heated by electron cyclotron waves in the Trivepiece-Gould mode which is dominant in the bulk plasma. As well, nonlinear or parametric coupling to hybrid waves or ion acoustic waves takes place near the antenna. The ionization factor is higher than in ICP and results in plasma densities one order of magnitude higher for similar power. Developments for dry etching of large area substrates include distributed sources with multiple discharges with and without magnetic field [13]. The high cost of large magnets and their power supply was reduced by using permanent magnets [14].

5.4

ECR (Electron Cyclotron Resonance) Discharges

When a plane polarized microwave field is applied to a low pressure gas in the presence of a stationary magnetic filed its right hand polarized (RHP) component can interact resonantly with electrons. Thus, for a typical frequency of 2.54 GHz the resonance will take place for a magnetic field of about 875 Gauss. In this situation the electrons will gyrate in phase with the RHP wave, seeing an accelerating electric filed over several orbits [1, 2]. The energy accumulated by electrons will be then distributed by collisions to produce and sustain the plasma at a high density (1017 m 3). The presence of a large magnetic field in plasma volume is not beneficial for applications dedicated to large area substrates both due to large costs and uniformity problems. This drawback was solved by multipole and distributed configurations (DECR) where permanent magnets are used to localize the ECR zone near the walls [15]. A new development is the matrix-ECR configuration that uses individual ECR cells with microwave injection through a water-cooled magnet (see > Fig. 2d). The cells can be distributed at the top of a large size vacuum chamber in a scalable configuration. The main advantage besides magnetic field–free plasma volume is the possibility to control the plasma uniformity profile by individual tuning of each ECR cell [16].

877

878

5.5.3 5.5

Dry Etching

SWP (Surface Wave Plasma) Discharges

Electromagnetic waves penetrate into plasma only within the skin depth layer. However, surface waves can propagate along the plasma-dielectric boundary heating the electrons and sustaining the discharge. Initial designs included surface wave propagation in cylindrical tubes made of dielectric materials with waves excited by surfatrons or surfaguides [17]. The propagation is limited to a distance given by surface wave resonance density where the discharge ends steeply. The concept of planar surface wave was introduced later on, where the plasma is produced below a large dielectric plate [18]. Reflection at the plate’s end results in a standing wave pattern that considerably improves the overall plasma uniformity despite an accentuated pattern of maxima and minima induced by the standing waves in vicinity of the dielectric (see > Fig. 2e). The wave excitation for planar SWP uses slot antennas with shape and dimensions carefully designed to ensure a very good coupling of the plasma with the microwave source. The heating mechanism for electrons is not well understood but it tends to be associated with transit time heating in a local resonance area at the cutoff density localized at the interface between plasma and the dielectric plate [19]. SWP can produce plasma densities larger than 1017 m 3 with a Te lower than in ICP (1–3 eV). Meter scale SWP for large area substrates have been reported recently where the large dielectric plate at the top of the chamber was replaced with parallel microwave injection using two or three lines with waveguides filled with dielectric [20] (> Table 1).

. Table 1 Summary of plasma parameters and operation characteristics for different discharges

Discharge

No (m 3) 15

16

Te (eV)

Vpl (V)

Advantages

Disadvantages

Ref. [4–6]

CCP

10 –10

3–5

20–30 Easy design, moderate cost

Low plasma density, standing wave and telegraph effects

ICP

1016–1017 2–4

10–20 High density, moderate cost

Complex antenna [7–10] design for large area

Helicon

1017–1019 1–3

10–30 High density, independent tuning for each helicon source

Requires magnetic [11–14] field, operation with mode jumps, high cost for individual sources construction

ECR

1016–1017 1–3

5–15

High density, independent tuning for each ECR cell, operation at very low pressures

High cost for [16] microwave power distribution and cells construction

SWP

1017–1018 1–3

5–10

High density, low electron temperature

Complex design for [19–20] slot antenna, large dielectric needed at the top or inside the wave guide

Dry Etching

6

Examples

6.1

Dry Etching of Indium Tin Oxide (ITO)

5.5.3

ITO thin films with high optical transmittance and very good electrical conductivity are used for display fabrication. These properties are sensitive to the deposition method which makes the wet etching difficult due to variation in the etch rate, formation of residues, and excess sidewall etching. In contrast, dry etching can provide fine patterning but needs optimized selectivity over SiO2 and Si3N4 under layers. Park et al. have studied dry etching characteristics of ITO films including selectivity using Ar/CH4 and Ar/H2 ICP discharges [21] and reported etching rates of about 120 nm/min for ITO with a selectivity of 9 over Si3N4 and 4 over SiO2 in Ar/CH4. Similar values for selectivity corresponded to an etching rate of about 40 nm/min in Ar/H2 plasma. They concluded that the etch selectivity of ITO over Si3N4 was higher than that over SiO2 due to differences in polymer formation. Currently, ITO is etched by wet etching even though the dry etching is preferred.

6.2

Dry Etching of Gate Metal Such as Al-Nd, Mo-Al Films, etc. for TFT-LCD

Aluminum is one of the materials used for gate electrodes in TFT-LCD technology. Addition of Ti and Nd prevent the formation of hillocks, maintain low-resistivity, and enhance corrosion resistance. Despite these good properties Al-Nd is difficult to be wet etched due to low etch rates, reduction of line width and residues formation. Dry etching is also challenged by the need of a good selectivity over photoresist. Han et al. have studied the etching characteristics of Al-Nd in a magnetized ICP using a gas combination of Cl2/BCl3 and HBr/BCl3 [22]. They reported etching rates of Al-Nd three times less than for pure Al with a selectivity of 0.9 in Cl2/BCl3. When using HBr/BCl3 gas mixture (1:1), the etching rate was about 140 nm/min with a selectivity over photoresist of 1.1. It was also found that Nd was preferentially removed by HBr while Al by BCl3. The etching rate decreased with pressure but increased with plasma density. Currently, the gate metals are etched partially by dry etching or wet etching. For the uniform deposition of gate dielectric material above the gate metal, the gate metal etch profile should show well slope etch profile.

6.3

Dry Etching of Copper Films

Due to its high conductivity and low cost, copper is considered one of the best materials for metal electrode of high resolution, low power consumption, and large area TFT-LCD. Since wet etching of Cu shows severe corrosion of interlayer and grain boundary of line sidewall, a possible replacement with dry etching is highly desired. Dry etching of Cu was also intensively studied for microelectronic industry but LCD needs additional requirements related to process scalability to meter size plasma sources. Jang et al. have studied dry etching of Cu in a scalable ICP discharge using Ar/Cl2 plasma with simultaneous exposure to ultraviolet radiation or substrate bias [23]. For a discharge power higher than 600 W and substrate bias of 75 W, the authors reported an etching rate of 300 nm/min without UV

879

880

5.5.3

Dry Etching

exposure. The sidewall profiles showed no residue formation on the glass surface. However, due to the difficulty of dry etching of copper, if copper is used for gate metal, it is currently etched by wet etching.

6.4

Patterning of InGaZnO TFT by Dry Etching

Transparent ZnO-based TFTs, such as amorphous InGaZnO (a-IGZO) which can be fabricated on plastic substrates at low temperature, are very attractive for fabrication of flexible displays. The solubility of such films during wet etching requires a low-damage dry patterning technology. Kim et al. have used photolithography and dry etching to form an etch stopper and the source/drain electrode [24]. They found that the etch stopper was necessary to prevent plasma damage while keeping the high field-effect mobility of about 36 cm2/V.

6.5

Dry Etching of Silicon Nitride

Silicon nitride is deposited by plasma enhanced chemical vapor deposition (PECVD) and is used as gate dielectric material and as the TFT passivation material. The deposited silicon nitride is not stoichiometric Si3N4 but hydrogen containing silicon nitride (SixNyHz). The etching of the silicon nitride for eighth generation plasma system is carried out by halogen containing gases such as SF6/O2 with a RIE mode [25]. Oxygen is added to increase the etch rate by removing sulfur from SF6 [26]. With these gases, the etch rate of about 500 nm/min and a sloped etching of 6070 are obtained by controlling the etch selectivity with photoresist.

6.6

Dry Etching of a-Si:H/Polysilicon

Hydrogenated amorphous silicon (a-Si:H) is the most widely used gate semiconductor material for TFT in current thin film transistors for FPD. In addition, for large size AMOLED TV, polysilicon is investigated to replace a-Si:H. Both of these materials are compatible with large area glass substrate processes, which is necessary to fabricate FPD at reasonable cost. Polysilicon TFT technology is chosen for its higher mobility and greater stability compared with amorphous TFTs as well as for its ability to provide p-channel devices. The higher mobility is needed for the single TFT per pixel design because the pixels only emit light for a small fraction of the frame time and therefore large currents are required. The etching of a-Si:H and polysilicon for eighth generation plasma system is carried out by halogen containing gases such as Cl2-rich Cl2/SF6/O2 with a RIE mode operated at dual frequencies (23 and 13.56 MHz; 23 MHz is added to improve the etch uniformity) [27]. With this gas mixture, about 200 nm/min and a sloped etching of about 45 are obtained. A sloped etch profile is also required to ensure a conformal step coverage of the following depositing material.

7

Future Trends

The FPD glass size is increasing larger and larger and the size of the glass substrate for TFT-LCD is currently as large as 2,200  2,500 mm (eighth generation) (or 2,850  3,050 mm

Dry Etching

5.5.3

(tenth generation) for a LCD production company in Japan). The companies are trying to increase the glass substrate further to 3,000  3,320 mm (11th generation) within a few years to decrease the production cost. For the substrate size larger than eighth generation, the standing wave effect is dominant and, to obtain a uniform plasma over the large area substrate, different plasma source techniques or methods to remove the standing wave (for example, use of traveling wave) are required for the development of the super-large size plasma etching source. In addition, to increase the pixel density, the wet etching of gate metal, ITO, etc. used in current TFT processing will be replaced to dry etching for the fine line definition. Also, for the etching of polysilicon, etc., which are required for the processing of TFT for large size AMOLED TV, high density plasma sources will be introduced to improve the etch throughput required for those device processing. Therefore, large area (>sixth generation) high density plasma sources are needed to be developed. For the plasma etching gases, the green etch gas is being investigated because SF6 gas which is used to etch silicon and silicon nitride of TFT [28] tends to show very high global warming effect. In addition, for the next generation displays, flexible displays are investigated and, for the etching of those device materials, completely different plasma etching techniques such as atmospheric pressure plasma etching techniques (for example, dielectric barrier discharge (DBD), plasma torch, etc.) that can operate at atmospheric pressure condition may need to be developed.

References 1. Chen FF, Chang JP (2003) Lecture notes on principles of plasma processing. Kluwer Academic/ Plenum, New York 2. Liberman MA, Lichtenberg AJ (2005) Principles of plasma discharges and materials processing. Wiley, Hoboken 3. Abe H, Yoneda M, Fujiwara N (2008) Developments of plasma etching technology for fabricating semiconductor devices. Jpn J Appl Phys 47:1435–1455 4. Schmitt J, Elyaakoubi M, Sansonnens L (2002) Glow discharge processing in the liquid crystal display industry. Plasma Sources Sci Technol 11:A206–A210 5. Schmidt H, Sansonnens L, Howling AA, Hollenstein Ch, Elyaakoubi M, Schmitt JPM (2004) Improving plasma uniformity using lens-shaped electrodes in a large area very high frequency reactor. J Appl Phys 95:4559–4564 6. Howling AA, Derendinger L, Sansonnens L, Schmidt H, Hollenstein Ch, Sakanaka E, Schmitt JPM (2005) Probe measurements of plasma potential nonuniformity due to edge asymmetry in large-area radio-frequency reactors: the telegraph effect. J Appl Phys 97:1940136 7. Jung SJ, Kim KN, Yeom GY (2005) Etching characteristics of multiple U-type internal linear inductively coupled plasma for flat panel display. Surf Coat Technol 200:780–783 8. Kim KN, Lim JH, Park JK, Yeom GY (2008) Scalable internal linear double comb-type inductively

9.

10.

11. 12. 13.

14.

15.

16.

coupled plasma source for large area flat panel display processing. Surf Coat Technol 202:5242–5245 Setsuhara Y, Takenaka K, Cho K, Han JG (2009) Large-area and low-damage processes for hybrid flexible device fabrications with reactive highdensity plasmas driven by multiple low-inductance antenna modules. J Phys Conf Ser 165:012042 Colpo P, Meziani T, Rossi F (2005) Inductively coupled plasmas: optimizing the inductive-coupling efficiency for large-area source design. J Vac Sci Technol A23:270 Boswell RW, Chen FF (1997) Helicons – the early years. IEEE Trans Plasma Sci 25:1229–1244 Chen FF, Boswell RW (1997) Helicons – the past decade. IEEE Trans Plasma Sci 25:1245–1257 Chen FF, Torreblanca H (2007) Large-area helicon plasma source with permanent magnets. Plasma Phys Control Fusion 49:A81–A93 Chen FF, Torreblanca H (2009) Permanent-magnet helicon sources and arrays: a new type of RF plasma. Phys Plasmas 16:057102 Pichot M, Durandet A, Pelletier J, Arnal Y, Vallier L (1988) Microwave multipolar plasmas excited by distributed electron cyclotron resonance: concept and performance. Rev Sci Instrum 59:1072–1075 Latrasse L, Lacoste A, Sirou J, Pelletier J (2007) High density distributed microwave plasma sources in a matrix configuration: concept design and performance. Plasma Sources Sci Technol 16:7–12

881

882

5.5.3

Dry Etching

17. Moisan M, Shivarova A, Trivelpiece AW (1982) Experimental investigations of the propagation of surface-wave along a plasma- column. Plasma Phys Control Fusion 24:1331–1400 18. Komaki K, Kobayashi S (1989) Generation of a microwave plasma using traveling waves. J Microw Power Electromagn Energy 24:140 19. Ganachev IP, Sugai H (2002) Production and control of planar microwave plasmas for materials processing. Plasma Sources Sci Technol 11: A178–A190 20. Ishijima T, Nojiri Y, Toyoda H, Sugai H (2010) Novel antenna coupler design for production of meterscale high-density planar surface wave plasma. Jpn J Appl Phys 49:086002 21. Park JY, Kim HS, Lee DH, Kwon KH, Yeom GY (2000) A study on the etch characteristics of ITO thin film using inductively coupled plasmas. Surf Coat Technol 131:247–251 22. Han HR, Lee YJ, Yeom GY, Ohand KH, Hong MP (2000) Dry etch characteristics of Al-Nd films for TFT-LCD. Surf Coat Technol 133, 606–611

23. Jang KH, Lee WJ, Kim HR, Yeom GY (2004) Etching of cooper films for thin film transistor liquid crystal display using inductively coupled chlorine-based plasmas. Jpn J Appl Phys 43:8300–8303 24. Kim M, Jeong JH, Lee HJ, Ahn TK, Shin HS, Park J-S, Jeong JK, Mo Y-G, Kim HD (2007) High mobility bottom gate InGaZnO thin film transistors with SiOx etch stopper. Appl Phys Lett 90:212114 25. Williams KR, Muller RS (1996) Etch rates for micromachining processing. J Microelectromechanical Systems 5:256–269 26. Urisu T, Kyuragi H (1987) Synchrotron radiationexcited chemical-vapor deposition and etching. J Vac Sci Technol B5:1436–1440 27. Chang KM, Yeh TH, Deng IC, Lin HC (1996) Highly selective etching for polysilicon and etch-induced damage to gate oxide with halogen-bearing electroncyclotron-resonance plasma. J Appl Phs 80:3048–3055 28. Draghici M, Stamate E (2010) Properties and etching rates of negative ions in inductively copuled plasmas and dc discharges produced in Ar/SF6. J Appl Phys 107:123304

Further Reading Kyung S-J, Park J-B, Lee Y-H, Lee J-H, Yeom GY (2007) High-speed etching of amorphous silicon using pin-to-plate dielectric barrier discharge. Surf Coat Technol 202:1204–1207 Laverty SJ, Maguire PD (2000) Low resistance transparent electrodes for large area flat panel display devices. J Vac Sci Technol B19:1–6

Park J-S, Jeong JK, Moo Y-G, Kim HD, Kim S-I (2007) Improvements in the device characteristics of amorphous indium gallium zinc oxide thin-film transistors by Ar plasma treatment. Appl Phys Lett 90:262106

Section 5

TFTs and Materials for Displays and Touchscreens

Part 5.6

Flexible Displays

5.6.1 Flexible Displays: Attributes, Technologies Compatible with Flexible Substrates and Applications Kalluri R. Sarma 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 886

2

Physical Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 887

3

Performance Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 888

4

Cost Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 888

5 5.1 5.2 5.3

Display Media Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 889 LCD Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 889 Electrophoretic Display (EPD) Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 890 OLED Display Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 890

6 6.1 6.2 6.3 6.4 6.5

Flexible Display Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 891 Flat Display Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 891 Conformal Display Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 891 Bendable Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 892 Foldable Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893 Rollable Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893

7

Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 894

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.6.1, # Springer-Verlag Berlin Heidelberg 2012

886

5.6.1

Flexible Displays

Abstract: Flexible displays are an exciting development because of their physical and performance attributes and their capability to enable new products requiring displays with unique form factors that the current rigid glass substrate based displays cannot support. Flexible displays can be very thin, light weight, have unique form factors and be highly rugged and not prone to breakage on impact unlike rigid and flat glass substrate based displays. The flexible form factors such as having an arbitrary shape, ability to be curved, conformal, bendable, and rollable can enable a variety of new applications and products. In this chapter, we will discuss the various attributes of the flexible displays, their potential applications, and the display media appropriate for flexible displays. List of Abbreviations: AM EPD, Active Matrix Electrophoretic Display; AM LCD, Active Matrix Liquid Crystal Display; AM OLED, Active Matrix Organic Light Emitting Display; EPD, Electrophoretic Display; LCD, Liquid Crystal Display; MEMS, Micro-electromechanical Systems; OLED, Organic Light Emitting Diode; OTFT, Organic Thin Film Transistor; RTR, Roll-to-Roll

1

Introduction

During the past three decades, development of flat panel displays, more particularly active matrix flat panel displays based on rigid and flat glass substrates has had a profound impact on the society by enabling creation of low-power displays for mobile applications (communications) such as mobile phones, note book PCs, and by replacing the CRT displays, that are bulky, heavy, and power hungry, for the desktop PC and home TV applications. Glass substrate based flat panel displays such as AM LCDs have enabled so many new applications for which the venerable CRT displays were just not suitable. Flexible displays are viewed to be next big frontier for the display technology development. There has been much interest in flexible displays and electronics fabricated using thin flexible plastic substrates for a long time, because of their perceived potential advantages compared to conventional displays built on rigid glass substrates. The potential advantages for flexible displays include, being very thin, light weight, highly rugged with greatly minimized propensity for breakage, and amenable to low-cost roll-to-roll manufacturing, in comparison to devices built on the conventional rigid glass substrates. In addition, flexible electronics and displays can enable a variety of new applications due to their ability to have unique form factors and to be curved and conformable. Further flexible displays can be rollable or foldable when not in use during storage or transportation. While there has been interest and activities in the development of rugged plastic substrate based LCDs for more than 20 years, during the past decade the interest in development of flexible displays intensified because of the significant advances in the enabling technologies for flexible displays and the R&D investments in flexible display developments have grown substantially. At the time of this writing, some of the flexible display technologies, such as the ones based on electrophoretic display (EPD) media (see > Chaps. 8.1.1 and > 8.1.2), are very close to being commercialized with initial product introductions. While the ultimate vision of a flexible display is one where it is foldable and rollable numerous times during use and during transportation and storage, the initial applications are focused on taking advantage of their thinness, form factor, weight savings, low power, and ruggedness attributes. Recent market studies [11] project that flexible display market will have annual revenue of over US $3B per year by the year 2015, and over US $6B per year by 2017.

Flexible Displays

5.6.1

In this chapter, we will first discuss the physical, performance, and cost attributes of the flexible displays being developed. We will then discuss the display media technologies compatible with flexible displays. Finally, we will discuss the various potential applications for flexible displays. The detailed technical aspects of flexible substrates and flexible display fabrication are discussed in > Chap. 5.6.2 of this handbook.

2

Physical Attributes

The physical attributes include thickness, weight, form factor, ruggedness, and flexibility with the ability to be flat, conformal, bent, folded, and rolled into a tube for storage and transportation when not in use. Compared to the rigid glass substrate based displays, flexible displays are inherently and substantially thinner and lighter in weight. Superior size, weight and power (SWaP) attributes are highly desirable for mobile electronics applications and for a variety of other commercial and aerospace and defense applications. The flat panel display industry has been making steady progress over the past two decades in reducing the thickness of the display glass substrates from 1 to 0.5 mm for mobile display applications, in particular. A flexible display based on a polymer (plastic) substrate can allow a paradigm change in the physical attributes of the display in relation to the rigid glass substrate based displays. First, flexible displays can be ultrathin (for example, as thin as 50 mm or 0.05 mm) compared to the thin glass substrate based mobile displays with thickness in the range of 500 mm or 0.5 mm. Also, with the density of typical plastic substrate being typically about a factor of 2 lower than display glass, a flexible plastic substrate based display can be ultimately 20 times lighter than the thickness of a glass substrate based mobile displays. It is well known that thinner glass substrates and displays made using these substrates are extremely fragile and can shatter and break on impact when dropped on the floor accidentally. Currently, for critical applications, the glass substrate based displays are ruggedized typically by using a thick impact resistant cover glass, which is either laminated to the display surface using an adhesive, or mechanically fixed in front of the display glass, to protect the actual display against impact stresses. This will increase the display thickness and weight even more. In comparison, displays fabricated on thin flexible substrates materials such as a plastic or stainless steel foils are highly rugged against impact and do not shatter or break. Capability for having a unique form factor and the ability to be conformal, bent, folded or rolled is an important attribute of flexible displays. Unlike rigid glass substrates, flexible displays fabricated on thin plastic or metal foil substrates can be cut to any size and shape readily and costeffectively. Their ability to be conformal to a surface such as the shape of a wrist surface or cuff surface, for example, allow numerous applications in the consumer, industrial and aerospace and defense applications as discussed in detail in > Sect. 6. Similarly, flexible displays can be bendable and rollable, and these attributes enable a variety of new and novel products that cannot be imagined using the current rigid glass based displays. For example, rollable flexible displays with a rolling radius of Part 6.6) with its highly demanding TFT backplane requirements and the barrier layer (for oxygen and water vapor) requirements, there are many flexible display applications that do not require a TFT backplane or have stringent barrier layer requirements. Examples of these displays include flexible displays for low-power electronic shelf labels and large area signage using bistable reflective display media such as EPD and Cholesteric LCD, which can be addressed by passive

Flexible Displays

5.6.1

matrix. RTR technologies for manufacturing these types of flexible displays are being commercialized with initial products expected to be introduced soon.

5

Display Media Technologies

For active matrix flexible displays the popular display media being considered includes liquid crystal display (LCD), electrophoretic display (EPD), and organic light emitting diode (OLED) display. These display media also happen to be most popular ones in use or under active development using flat rigid glass substrates. Science and technology of these display media, as used with conventional display glass substrates, is discussed in detail in > Sects. 6, Emissive Displays, > 7, Liquid Crystal Displays and > 8, Paper-Like and Low Power Displays of this book. In the following we will discuss the applicability of these display media for use with flexible substrate and display application. > Table 1 shows the relative comparison of the flexible display attributes using these display media. Some of the emerging display media such as electrowetting (> Chaps. 8.1.3 and > 8.1.4) [3] and MEMS (> Chaps. 8.2.1 and > 8.2.2) [4] have a potential for being used in flexible display applications because of their unique advantages. However, we will not discuss them further in this chapter.

5.1

LCD Media

One of the significant issues with the use of LCD media for flexible displays is the LC (liquid crystal) cell gap control. LC cell gap value has a significant effect on the display optical performance (luminance, contrast ratio, chromaticity stability, viewing angle, etc.), and . Table 1 Comparison of display media technologies LCD Attribute

Transmissive/ Reflective transflective (cholesteric)

EPD

OLED

Flexibility

OK

OK

Better

Best

Ruggedness

OK

OK

Better

Best

Image quality

Better

OK

OK

Best

Power OK consumption

Best

Best

OK

Response time

OK

Poor

Poor

Best

Issues

1. Flexible backlight needed 2. Sensitive to cell gap changes

1. Front lighting 1. Front lighting needed for dark needed for dark ambient viewing ambient viewing 2. Sensitive to cell gap changes

1. Flexible gas permeation barrier film needed

889

890

5.6.1

Flexible Displays

maintaining this cell gap at its optimum value as the display is bent or flexed is difficult. Nevertheless, LCD can be a good display media if the objective of the flexible (plastic or metal foil based) display is thin, light weight, and rugged, and close to being flat without any significant level of bending. Secondly, the typical transmissive LCD mode displays that are widely in use today for a broad range of applications require a backlight that needs to be flexible [14] and a color filter array that need to be fabricated on a flexible substrate. Low-power reflective LCD mode displays do not require a backlight. However, they do need to be front lit, for dark ambient night time viewing, thereby requiring a front lighting scheme that is compatible with the flexible form factor of the display surface involved. Cholesteric mode LCDs [2, 8–10, 13] are being actively developed for the flexible electronic paper applications, in addition to a variety of other applications including shelf label displays, e-skins, and outdoor advertisement displays. They are bistable and do not require any power in high ambient light conditions, except when updating the image on the display, and thus have the ultimate low-power potential. However, they have slow switching speeds (100 ms), and thus are not suitable for video applications. In addition it is not very easy to achieve full color displays with good color saturation using this technology.

5.2

Electrophoretic Display (EPD) Media

EPD [1, 7, 12] is a reflective bistable (low-power) display that does not have the cell gap control issues as in LCDs. It is based on microencapsulated oppositely charged colored particles that move in an electric field. The EPD media is typically fabricated in a film form (electronic-ink film) and is attached to the TFT backplane by hot roll lamination. The film consists of microcapsules in a polymer binder coated onto a substrate with an outer layer of polyester and indium tin oxide that serves as the counter electrode for the pixel electrode in the active matrix display. The pixel is switched by moving the sub-micron sized black and white particles in the microcapsules, with opposite charge. Depending on whether white or black sub-micron sized particles are closer to the viewer, light is scattered back (white state) or absorbed (black state). As the EPD media is bistable, grayscale is achieved by pulse width modulation. Recently significant advances are made in the EPD technology in reducing the drive voltages, and improving the response time. As this is a reflective mode display, it has excellent sunlight viewability characteristics. Because the requirements of a barrier layer for protection of the EPD media, and the requirements of the active matrix TFT backplane for driving the EPD are not stringent, and the simplicity of the monochrome reflective, bistable EPD technology, currently several companies are actively commercializing flexible displays using this display media, for applications such as e-books and a variety of other applications. However, response time in the present practical devices is still in the 100 ms range and does not support video applications, and additional development is needed for realizing full color EPDs. Other variations of this EPD media with a potential for use in flexible displays are being actively developed by SiPix [5] and Bridgestone [6] (see > Chap. 8.1.1 for further discussion).

5.3

OLED Display Media

Active matrix organic light emitting diode (AM OLED) display technology offers significant advantages over the current well-entrenched Active Matrix Liquid Crystal Display (AM LCD)

Flexible Displays

5.6.1

with respect to superior image quality with wide viewing angle and fast response time, and being lighter and thinner, lower cost (does not need backlight or color filters), and lower power. Because of this, many companies are actively developing AM OLED displays built using rigid flat glass substrates. Commercialization of these glass substrate based AM OLED displays has also started for small display sizes for smart phone applications. OLED display media is believed to be ideal choice for use in a flexible display as it represents the ultimate flexible display with a rugged solid state structure along with the other attributes including full color, superior image quality, full motion video, and low power. However, the OLED display media has very stringent requirements both with respect to the barrier layer specifications for moisture and oxygen permeation, and the active matrix TFT backplane performance. However significant progress has been made in the thin film barrier layer technology, and active matrix backplane and TFT technologies as discussed in > Chap. 5.6.2. Recently significant progress has been made, and impressive flexible AM OLED displays have been demonstrated as discussed in > Sect. 2.1 of Chap. 5.6.2.

6

Flexible Display Applications

A very broad range of flexible display applications are being envisioned and pursued. On one end of the application spectrum, flexible displays encompass low resolution, direct addressed or passive matrix addressed displays for small electronic shelf labels and large electronic advertisement displays, while on the other end of the spectrum they encompass active matrix addressed, high performance, high resolution, mobile information displays. Currently there is a high level of interest in flexible active matrix displays as they enable broad range of new application. Flexible displays offer significant freedom and opportunity to product designers in the design of products utilizing flexible displays due to its unique attributes described in > Sects. 2, > 3 and > 4. In the following we will discuss the various categories of applications envisioned for the flexible displays.

6.1

Flat Display Surface

As flexible displays fabricated on metal foil or polymer foil substrates that are impact resistant unlike glass, they are being developed for a straight forward replacement for displays fabricated on conventional glass substrates. In this application, flexible displays enable significant weight and thickness savings in addition to being inherently rugged.

6.2

Conformal Display Surface

Flexible displays can enable conformal display applications where the display surface conforms to a specific surface curvature of interest. > Figure 1 shows an example of a conformal display on a wrist worn device. Other examples of conformal display applications include displays for cuff worn devices, and displays on various devices with curved surfaces. In these applications the display is designed to bent only once during its operation.

891

892

5.6.1

Flexible Displays

. Fig. 1 Example of a conformal display application [16]

Fujitsu

a

Samsung SDI

b

. Fig. 2 Examples of bendable displays: (a) bendable AM OLED Display, (b) bendable cholesteric LCD display

6.3

Bendable Displays

Some applications may require the flexible display to be bent over a desired curvature of interest multiple times, during use, to suit the application requirements. > Figure 2 shows examples of bendable displays.

Flexible Displays

5.6.1

. Fig. 3 Example of a foldable electrophoretic display by Polymer Vision [8]

6.4

Foldable Displays

Foldable displays are another category of application. These applications decouple the device size and the display size and allow the display size to be much larger than the device size. ® > Figure 3 shows an example of a foldable (and rollable) display, Readius , using an organic TFT backplane and EPD display media, demonstrated by Polymer Vision [8]. This type of device involves bending and unbending and rolling and unrolling the flexible display multiple times during the life of the device usage.

6.5

Rollable Displays

Fully rollable high performance flexible AM OLED display is the ultimate vision of the flexible display development efforts. > Figure 4a shows a concept device by UDC for a rollable, full color flexible AM OLED display. > Figure 4b and c shows photographs of a rollable, full color, flexible AM OLED display demonstrated recently by Sony [15]. This display utilizes an organic TFT backplane and OLED display media. Rollable displays also allow decoupling of the device size and the display size. The display is rolled-in when not in use, for transportation and storage.

893

894

5.6.1

Flexible Displays

USDC

a

Sony

Sony

b

c

. Fig. 4 Examples of rollable displays: (a) rollable concept display device, (b and c) flexible and rollable OLED display fabricated using an organic TFT backplane in the rolled-out and rolled-in conditions [15]

7

Summary and Conclusion

Display has been a significant enabler and a central part of the revolutionary advances in devices, systems, and applications encompassing computers, communications, and displays, in recent times. During the past two decades, flat panel displays, more particularly the active matrix TFT flat panel displays, have enabled new applications such as note book PC that would not have been possible without them. The value proposition was compelling for replacing the dominant (and bulky) CRT display with a TFT-LCD with its attributes of being significantly thinner, lighter, and having superior image quality and consuming less power in essentially all of its applications. Having experienced the phenomenal successes in the active matrix flat panel display developments and its widespread adaptation in broad range of applications, the display industry is now fascinated at the prospects of repeating that success with the development of flexible displays. Flexible displays represent a new paradigm in display technology development. First, compared to the rigid flat glass based displays, flexible displays are significantly lighter, thinner, more rugged, and can be rollable or foldable for stowage. Secondly, they can have unique form factors with respect to size and shape that allows a variety of new applications. In addition, flexible displays have a potential for roll-to-roll (RTR) manufacturing for significant cost reduction. During the past few years significant progress has been made in the development of flexible displays utilizing EPD and OLED media. Flexible EPDs are nearing commercialization stage for applications such as e-book readers. Impressive demonstrations have been shown for the flexible AM OLED displays as well. Flexible AM OLEDs (built on thin plastic foil substrates) with a flat form factor are expected to be their first application, to start benefiting early from its attributes of being thinner, weighing less, and being highly rugged. Significant innovation is expected in the development of products utilizing flexible displays and their unique physical and performance attributes, as this technology matures. Also, emerging display media has a potential to satisfy unique flexible display requirements.

References 1. Burns S (2010) QUE: an e-Reader built using flexible display technology. SID ‘10 Digest 477 2. Chen KT, Liao YC, Yang JC, Shiu JW, Tsai YS, Wu KW, Chen CJ, Hsu CC, Wu CC, Chen WC,

Chin CL (2009) High performance full color cholesteric liquid crystal display with dual stacking structure. SID ‘09 Digest 40(1):300–302

Flexible Displays 3. Feenstra J, Schram I, Evans M, Vermeulen P, Cometti C, Weert MV, Ferket M, Massard R, Mans J, Sakai T (2010) Large size full-color e-Reader displays based on electrowetting. SID ‘10 Digest 41(1):480–483 4. http://www.mirasoldisplays.com/. Accessed 15 Mar 2011 5. http://www.sipix.com/technology/index.html. Accessed 15 Mar 2011 6. http://www2.bridgestone-dp.jp/global/adv-materials/QR-LPD/. Accessed 15 Mar 2011 7. Huitema E, Touwslager F, Veenendaal EV, Aerle NV, Lieshout PV (2009) Rollable displays: from concept to manufacturing. SID ‘09 Digest 104 8. Kato T, Kurosaki Y, Kiyota Y, Tomita J, Yoshihara T (2010) Application and effects of orientation control technology in electronic paper using cholesteric liquid crystals. SID ‘10 Digest 41(1):568–571 9. Kent Displays Inc., 343 Portage Blvd, Kent, Ohio 44240, USA 10. Khan A, Shiyanovskaya I, Schneider T, Doane JW (2006) Recent progress in flexible and drapable reflective cholesteric displays. SID ‘06 Digest 37(1): 1728–1731

5.6.1

11. Markowitz P (2010) Outlook for flexible, printed electronics. In: Flex tech alliance, flexible electronics and displays conference and exhibition, February 2010, Phoenix, Arizona 12. McCreary M (2007) Advances in microencapsulated electrophoretic displays. In: Proceedings of USDC flexible displays & microelectronics conference & exhibition, Feb 2007, Phoenix, Arizona 13. Montbach E, Pishnyak O, Lightfoot M, Miller N, Khan A, Doane JW (2009) Flexible electronic skin display. SID ‘09 Digest 40(1):16–19 14. Montgomery DJ, Walton H, Ishida T, Tsuda Y (2009) A variable curve backlight. SID ‘09 Digest 10 15. Noda M, Kobayashi N, Katsuhara M, Yumoto A, Ushikura SI, Yasuda RI, Hirai N, Yukawa G, Yagi I, Nomoto K, Urabe T (2010) A rollable AM-OLED display driven by OTFTs. SID ‘10 Digest 47(3): 710–713 16. http://www.oled-info.com/udc-wrist-mounted-flexibleph-oled-display-prototype-photo. Accessed 15 Mar 2011

895

5.6.2 Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies Kalluri R. Sarma 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 899

2 2.1 2.2 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.4 2.5

Substrate Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 899 Thin/Flexible Metal Foils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 900 Flexible Polymer/Plastic Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 901 PEN Plastic Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903 Optical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903 Surface Smoothness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903 Resistance to Solvents and Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904 Dimensional Stability and Reproducibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 905 Barrier Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 906 In situ Fabricated Flexible Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 906 Barrier/Encapsulation Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 907

3 3.1 3.2 3.3 3.4

TFT Technology Options for Flexible Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 908 LTPS TFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 908 a-Si TFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 909 OTFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 910 OSC-TFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 911

4 4.1 4.1.1 4.1.2 4.1.3 4.1.4 4.2 4.3 4.4

TFT Processing Strategies for Flexible Backplanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 911 Direct Processing (on Flexible Plastic and Stainless Steel Substrates) . . . . . . . . . . . . . 912 a-Si TFT on PEN Plastic Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 912 ULTPS on Plastic Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 915 O-TFT on Plastic Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 916 Direct Fabrication using Stainless Steel Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 917 Device Layer Transfer (DLT) Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 918 Temporary Substrate Bonding and De-bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 919 In situ Plastic Coating on a Temporary Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 921

5

TFT Backplane Fabrication by Direct Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 924

6

Roll-to-Roll (RTR) Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 926

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.6.2, # Springer-Verlag Berlin Heidelberg 2012

898

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

7 7.1

Other Technical Challenges for Flexible Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 928 Self-heating Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 928

8

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 929

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

Abstract: TFT Technologies developed for fabricating active matrix (AM) backplanes on rigid glass substrates for conventional flat panel displays cannot readily be used for fabricating active matrix backplanes on flexible substrates and displays. In addition to mechanical handling issues, flexible substrates impose many additional constraints such as process temperature limitation and thermal stress issues due to CTE mismatch with the TFT thin films for fabricating backplanes for flexible displays. In this chapter, we discuss the flexible substrate options and TFT processing strategies for fabricating flexible backplanes and flexible displays using a various display media. Current status on TFT fabrication by printing and roll-to-roll fabrication for flexible displays is also discussed. List of Abbreviations: AM EPD, Active Matrix Electrophoretic Display; AM LCD, Active Matrix Liquid Crystal Display; AM OLED, Active Matrix Organic Light Emitting Display; aSi:H, Hydrogenated Amorphous Silicon; CNT, Carbon Nanotubes; ELA, Excimer Laser Annealing; EPD, Electrophoretic Display; LCD, Liquid Crystal Display; LTPS, LowTemperature Polysilicon; MEMS, Micro-Electromechanical Systems; MOSFET, Metal Oxide Semiconductor Field Effect Transistor; OLED, Organic Light Emitting Diode; OSC-TFT, Oxide Semiconductor Thin Film Transistor; OTFT, Organic Thin Film Transistor; PEN, Polyethylene Naphthalate; PI, Polyimide; RTR, Roll-to-Roll; TCE, Thermal Coefficient of Expansion; TFT, Thin Film Transistor; ULTPS, Ultralow-Temperature Polysilicon

1

Introduction

Flexible thin film transistor (TFT) backplane is a crucial enabler for fabricating flexible displays. Once the flexible TFT backplane is fabricated, it is integrated with the display media, such as LCD, electrophoretic display (EPD), or organic light emitting diode (OLED), and appropriate drive electronics to complete the flexible display fabrication. There are several TFT technology options that include a-Si TFT, low-temperature polysilicon TFT (LTPS TFT), organic TFT (O-TFT), and oxide semiconductor TFT (OSC-TFT). The selection of the appropriate TFT option depends primarily on the display media selected and the display specifications such as size, resolution, and refresh rate. While a full-color, high-resolution, flexible OLED display with high-speed video is the holy grail of the flexible display development efforts, there are many applications, for example, an e-reader, where a flexible, lowpower, monochrome, bistable display using electrophoretic display media may be better suited. In this chapter, we discuss flexible substrate options, barrier layers, TFT technology options, TFT processing strategies, and the remaining technical issues for realizing various types of flexible displays of interest.

2

Substrate Options

Thin metal foils such as stainless steel, and thin polymer materials are the leading candidate substrates for fabricating flexible backplanes and displays [1]. In the following, we discuss the relative advantages and issues associated with these two options.

899

900

5.6.2 2.1

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

Thin/Flexible Metal Foils

Metal foil substrates offer the advantages of higher process temperature capability (for TFT fabrication), dimensional stability (no shrinkage of the substrate during high-temperature processing associated with the TFT fabrication), and being impervious to oxygen and moisture (inherent barrier for the ambient oxygen and moisture). The high thermal conductivity of a metal foil substrate is also an advantage for heat extraction and thermal management which is discussed in more detail in > Sect. 7. The disadvantages and limitations of the metal foil substrate include the following: 1. Being opaque, it cannot be used for transmissive displays or bottom emission OLED displays. 2. Poor surface smoothness characteristics. 3. Capacitive coupling effects. 4. Compatibility issues with the TFT process chemicals. Not being transparent limits the use of metal foil substrates to reflective displays and top emission OLED displays. Stainless steel, such as STS 304 and STS 430 are popular candidate metal foil substrates for use in flexible displays. The surface of these starting stainless steel substrates is very rough with large (>0.1 mm) surface protrusions which is not acceptable for flexible display applications because they result in TFT defects in the backplane and also defects in the display media (pixels) integrated on these surfaces. The thickness of thin films employed in the TFT structure are typically in the range of
100 nm, and the thickness of the thin films employed in OLED media (pixels) can be as low as
10 nm. Substrate surface protrusions can cause shorts across the TFT electrodes and the display pixels (e.g., OLED device), or create leakage paths in the TFT and the pixel structures. The starting stainless steel substrates are typically polished to remove the surface protrusions and improve the surface smoothness. In addition, typically the polished stainless steel substrates are coated with a planarizing/buffer layer [2], to improve the surface smoothness and make them suitable for fabricating the flexible backplanes and displays, without defects and with high yield. Compatibility with the TFT process chemicals can be addressed by using an appropriate protective film at the backside of the stainless steel substrate. Metal foil substrate, by itself, is a good barrier (for oxygen and moisture) and thus it does not require an additional barrier layer. However, the display fabricated using the metal foil substrate would still require a good barrier (encapsulation) layer to be applied on top of the fabricated TFT and the display media such as OLED. Another consideration in the use of metal foil substrate is the parasitic coupling capacitance due to coupling of the backplane electronics to the conductive substrate. The planarizing/buffer layer used for improving surface smoothness of the substrates can also serve to isolate it electrically from the TFT circuit, and reduce the parasitic capacitance between the stainless steel substrate and the TFT and pixel circuits. Stainless steel is being actively investigated as a substrate for the flexible backplanes using LTPS TFT (e.g., [2]) as well as a-Si TFT for reflective (e.g., [3–5]) and top emission mode OLED (e.g., [2, 6, 7]) display applications. Paek et al. [8] report on an interesting method of fabricating a-Si TFT backplanes on thick rigid STS430 stainless steel substrates, and subsequently thinning the backside of the stainless steel substrate by etching down to a thickness of 0.1 mm. These backplanes are then used to fabricate and demonstrate flexible 4.300 QVGA AM OLED, 11.500 UXGA AM OLED, and 1900 AM EPD displays. > Figure 1 shows the 4.300 AM OLED and 1900 AM EPD demonstrated. This process is

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

. Fig. 1 Photographs of flexible low-temperature a-Si TFT (a) 4.300 AM OLED and (b) 1900 AM EPD displays fabricated using backside thinning of stainless steel substrates [3]

used on a Gen 2 line (370  470 mm) line to demonstrate fabrication of flexible OLED and EPD displays on a conventional TFT manufacturing line. However, the concerns on the approach include thinning process yield, cost, and scalability.

2.2

Flexible Polymer/Plastic Substrates

A transparent plastic substrate has the advantage of being compatible with transmissive as well as reflective displays. Thus it is compatible with both top and bottom emitting OLED device architectures, thereby making it suitable for a broader range of display applications. The technical challenges in the development of plastic substrates for active matrix (AM) display application are, however, extremely demanding. The plastic substrates, while being flexible, need to offer glass-like properties, and must therefore have high transmission, low haze, smoothness of surface, excellent dimensional and thermal stability, and low coefficient of thermal expansion (CTE) mismatch with the TFT thin films, and must be excellent barriers for oxygen and moisture transport. > Table 1 shows the properties of the some of the common candidate plastic substrate materials for flexible backplane and display fabrication. These candidate substrates include polyethylene terephthalate (PET – e.g., Melenix® from DuPont Teijin Films), polyethylene naphthalate (PEN, e.g., Teonex®, Q65, from DuPont Teijin Films), poly carbonate (PC, e.g., GE’s Lexan®), polyethersulfone (PES, e.g., Sumilite® from Sumitomo Bakelite), and polyimide (PI, e.g., Kapton® from DuPont). While Kapton has high Tg, it absorbs in the visible (yellow color), and thus is not suitable for transmissive displays or bottom emission OLED displays. Higher process temperature (>350 C) capable clear plastic substrates are also being developed and investigated [9] for use as a drop-in replacement for glass with conventional (high-temperature) a-Si TFT fabrication process. However, as these high-temperature, clear plastic substrates are not commercially available at this time, we will not discuss them further. Some of the important limitations of the available plastic substrates include: limited process temperature capability, lack of dimensional stability (during TFT processing involving high temperatures), and significant differences in the linear thermal coefficient of expansion (TCE) between the plastic substrate and the TFT thin films. Plastic substrates are believed to

901

902

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

. Table 1 Available candidate plastic substrates PET (Melinex®) ST506

PEN (Teonex®), Q65FA

PC

PES (Sumilite)

PI (Kapton)

Tg ( C)

78

120

150

223

410

Upper process temperature ( C)

150

220

CTE (55 C to 85 C) (ppm/ C)

20–25

18–20

60–70

54

30–60

Transmission (%) (400–700 nm)

89

87

90

90

Yellow

Moisture absorption (%)

0.14

0.14

0.4

1.4

1.8

1.7

Young’s modulus (Gpa)

4

5

Tensile strength (Mpa)

225

275

Density (g/cm3)

1.4

1.36

Refractive index

1.66

1.5–1.75

Birefringence (nm)

46

1.2

2.2

2.5

83

231

1.37

1.43

1.58

1.66

14

13

. Table 2 Comparison of PEN and stainless steel substrates to glass substrates Glass

PEN

Stainless steel

Weight (g/m ) (for 100 mm thick film)

220

120

800

Transmission in the visible range

92%

90%

0% >1,000

2



Maximum process temperature ( C)

Figure 2 [16] shows the protrusions in nonplanarized and planarized Q65FA over a 5 cm by 5 cm area. Protrusions greater than 40 nm high are completely removed after planarization. There is a significant decrease in protrusions smaller than 40 nm due to planarization as well. The planarization layer also promotes the adhesion of the subsequent barrier layers deposited on the substrate.

2.3.3

Resistance to Solvents and Moisture

The Q65 Teonex® brand has excellent solvent resistance to most acids and organic solvents and will typically withstand the solvents used in AM OLED display fabrication. Indeed no specific issues of significance are observed using the Teonex® Q65 substrate during the fabrication of the a-Si TFT backplanes and AM OLED test displays [10]. While the PEN substrate does not react with moisture, it does absorb moisture, which results in a dimensional change. > Figure 3 shows the moisture absorption in the PEN substrate as a function of relative humidity (RH) and time [15]. At 40% RH, the equilibrium moisture concentration in the film is expected to be about 957 ppm which is very high as every 100 ppm of moisture absorbed; the film is estimated to expand by approximately 45 ppm. This is a very significant dimensional change and can deleteriously affect the TFT backplane process if it is not managed. Moisture absorption is reversible by heating the substrate in vacuum or in an inert atmosphere. Uncontrolled moisture absorption/desorption during the TFT backplane fabrication can potentially have far more impact on the substrate dimensional stability than the dimensional instability due to the inherent PEN substrate shrinkage. It is important to understand the moisture absorption/desorption characteristics of the PEN substrate to control its dimensions during the active matrix backplane fabrication.

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

1,600 RH 20% RH 40% RH 60%

1,400

1,440 ppm

Moisture (ppm)

1,200 1,000 957 ppm 800 600 486 ppm

400 200 0 0

2

4

6

8 Time (h)

10

12

14

16

. Fig. 3 Moisture absorption in PEN plastic substrates [7]

2.3.4

Dimensional Stability and Reproducibility

Dimensional stability and reproducibility during TFT array processing (involving temperature cycles between room temperature and the TFT process temperatures) is extremely critical to ensure that the features in each layer of the TFT device structure align properly with the features in the previous layers. Glass substrate does not have this issue as it has excellent dimensional stability during TFT array processing. In addition to dimensional stability, reproducibility is also important for plastic substrates. While dimensional changes (due to moisture absorption, etc.) need to be very small (negligible), at minimum, they need to be predictable and controllable, so that they can be managed during fabrication of each layer of the TFT structure. Two physical aspects come into play for polymer films during the display fabrication [16]: (1) shrinkage of the film and (2) natural expansion of the film. To understand film shrinkage, it is important to recognize that PEN films (Teonex®) are produced using a sequential biaxial stretching technology. This process involves stretching film in machine and transverse directions (MD and TD) and heat setting at elevated temperature. As a consequence, a complex semicrystalline microstructure develops in the material, which exhibits remarkable strength, stiffness, and thermal stability. The film comprises a mosaic of crystallites or aggregated crystallites accounting for nearly 50 wt% of its material which align along the directions of stretch. The noncrystalline region also possesses some preferred molecular orientation, which is a consequence of its connectivity to the crystalline phase. Importantly, the molecular chains residing in the noncrystalline region are on average slightly extended and therefore do not exist in their equilibrium Gaussian distribution. Shrinkage is associated with the relaxation of this residual strain back to equilibrium within the partially oriented parts of the film structure. To counterbalance this effect, PEN films are further exposed to a thermal relaxation process, in which film is transported relatively unconstrained through an additional heating zone. The second factor that impacts dimensional reproducibility as the temperature is cycled is the natural expansion of the film as quantified by the TCE.

905

906

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

. Table 3 CTE of PEN (Q65) as a function of temperature and orientation CTE (ppm/ C) 50 C to 0 C

0–50 C

50–100 C

100–150 C

Machine direction

13

16

18

25

Transverse direction

8

11

18

29

Shrinkage at a given temperature is measured by placing the sample in a heated oven for a given period of time. The %shrinkage is calculated as the %change in dimension of the film in a given direction due to heating. Heat-stabilized films exhibit shrinkage of the order of Table 3. Excessive strains/stresses result in film cracking, delamination, and substrate curling/buckling problems.

2.3.5

Barrier Properties

The inherent barrier properties of PEN films are typically of the order of ca. 1 g/m2/day for water vapor transmission rate and an equivalent ca. of 3 mL/m2/day for oxygen transmission rates. This is a long way from the levels required for the protection of OLED displays, which require water vapor transmission rates of Fig. 4. The figure shows the oxygen and moisture sensitivity range for the LCD, EPD, and OLED display media, and TFTs. For example, for the protection of an OLED display, the plastic substrate (barrier layer) must have a permeability of less than 106 g/m2/day for moisture and 105 mL/m2/day for oxygen. In comparison, LCD displays have a requirement of less than 102 g/m2/day for oxygen and moisture, which is significantly less stringent compared to OLEDs. The base plastic substrates typically have about 10 g/m2/day transmission rates for oxygen and moisture implying the need for incorporating a separate barrier layer. In principle, a thin layer of an inorganic film such as SiO2, SiNx, Al2O3, etc., deposited on the flexible plastic substrate can serve as a barrier layer with the required impermeability to oxygen and moisture. However, in practice, multilayer barrier film structures are believed to be required to counter the effects of the pinholes/cracks in single-layer-deposited barrier layers. Several organizations are currently developing optically transparent multilayer barrier coatings for flexible OLED displays [17]. Vitex Systems [18] uses such kind of an approach for their barrier film called Barix™ which employs alternating layers of a UV curable acrylate polymer and a 500 A˚ thick ceramic Al2O3 deposited in vacuum, as shown in > Fig. 5. The inorganic films serve as barrier films for oxygen and moisture, organic layers serve the planarization/ smoothing function, and multilayers (diads) provide redundancy against pinhole defects in the barrier films. The Barix™ layer is found to be an effective barrier layer, by minimizing the detrimental effects of pinholes and diffusion at grain boundaries. The Barix™ films typically about 3 mm thick, were found to have water permeability in the range of 106 g/m2/day.

102

10−2

10

10−4

10−6

WVTR g/m2/day Inorganic coating

Bare polymer

Organic-inorganic multi-layer TFTs

OLEDs

LCDs, EPDs OTR mL/m2/day 103

10

10−1

. Fig. 4 Oxygen and moisture barrier levels required for various displays

10−3

10−5

907

908

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

H2O

H2O

Barrier layers

Polymer

Polymer 2,199.202 nm

Polymer

Polymer

Substrate

1 µm

. Fig. 5 Vitex barrier comprising a multilayer stack of organic and inorganic films [15]

Note that whether using a plastic substrate or a stainless steel substrate, the top side of the TFT backplane, and the display media (e.g., OLED), must be protected with either an impermeable thin film encapsulation (barrier) layer directly, or by another substrate coated with an encapsulation (barrier) layer.

3

TFT Technology Options for Flexible Displays

Flexible substrate compatible TFT backplane technology is a critical item for the development of flexible active matrix displays. Both the well-established TFT technologies, namely, a-Si TFTand LTPS TFT, are being considered for flexible display applications. In addition, emerging TFT technologies, such as O-TFT (organic TFT) and Oxide Semiconductor (such as InGaZnO) (OSC-TFT) technologies, are also being developed for flexible backplane applications. Generally, the TFT processes developed and optimized for use with the flat and rigid glass substrates (with a
600 C process temperature capability) cannot readily be applied for use with the flexible plastic substrates due to reasons such as lower-process temperature constraints, thermal stress issues resulting from the CTE mismatch, and dimensional stability issues. Consideration of the characteristics of the available TFT technologies (see > Chap. 5.1.1, > Part 5.2, Inorganic Semiconductor TFT Technology, > Part 5.3, Emerging TFT Technologies) can illustrate the issues in adapting them for the fabrication of flexible backplanes. > Table 4 shows a comparison of the candidate TFT technologies including device layer transfer (DLT), LTPS, ULTPS, conventional a-Si TFT, low-temperature a-Si TFT, O-TFT, and OSC-TFT. In the following, we discuss the advantages and issues with each of these options (except DLT, which is discussed in > Sect. 4.2) for fabricating flexible backplanes.

3.1

LTPS TFT

Conventional LTPS process used in the current AM LCD and AM OLED displays uses a typical process temperature in the range of
450 C using a polysilicon film produced by excimer laser

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

. Table 4 TFT technology options for flexible displays Device layer transfer (DLT) 

Polysilicon

a-Si

LTPS

LowConven. temperature Organic a-Si a-Si TFT

ULTPS





OSC-TFT


450 C Process temperature ( C)








450 C Fig. 9a) and the 160  3  160 pixel (> Fig. 9b) monochrome polymer OLED displays fabricated. To protect the OLED media, the display is laminated to a rigid glass substrate on the anode side of the OLED. As seen in > Fig. 9, while these displays have some pixel and line defects, they do validate the 150 C a-Si TFT process and the backplane design for a flexible AM OLED. The fabricated

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

. Fig. 9 (a) Photographs of images being displayed on a 64  64 pixel AM OLED and a (b) 160(3)  160 pixel AM OLED fabricated using a flexible PEN plastic backplane built with low-temperature a-Si TFTs [24, 25]

displays were capable of displaying grayscale images and full motion video. The control displays fabricated using glass substrates were found to perform similarly except for having fewer pixel and line defects. The surface quality of the PEN plastic substrate was found to have a significant impact on the quality of the displays fabricated with respect to pixel and line defects observed. Displays fabricated on PEN substrates with improved surface quality exhibited significantly fewer display defects. To fully demonstrate the flexible display concept, Sarma et al. integrated Barix thin film encapsulation [12, 18], with the flexible backplanes and a red phosphorescent OLED display media. The Barix (barrier film) is of the order of only a few microns. Thus the thickness of the flexible display fabricated is
130 microns. > Figure 10a, b shows the photographs of a flexible AM OLED test display fabricated along with its flexural capabilities [12]. > Figure 10c shows a schematic cross section of the flexible AM OLED display fabricated. While the concept of flexible AM OLED displays using direct fabrication on flexible PEN plastic displays has been demonstrated for a display size of up to
5 cm  5 cm with a resolution of 80 cgpi, significant improvements in flexible substrate with respect to reduction of shrinkage and CTE mismatch with the TFT thin films is necessary for extending this approach for larger-size and higher-resolution flexible displays. In addition, development of methods for mechanical handling of the thin flexible backplanes during the TFT processing is essential for realizing large area backplanes and flexible displays.

4.1.2

ULTPS on Plastic Substrates

Polysilicon TFTs have the advantage of providing high mobility and CMOS option for integrating the row and column drivers in the flexible display. ULTPS approaches where the process temperature is kept under Sects. 4.2, > 4.3, and > 4.4.

915

916

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

a

b Thin Film Encapsulation

Cathode

SiNx Overcoat

ITO Anode PLED Films

Plastic Substrate

c

Light Emission

. Fig. 10 (a) Photograph of a checker board image on a 64  64 pixel AM OLED fabricated using a flexible PEN plastic backplane built with low-temperature a-Si TFTs and thin film encapsulation, (b) flexural capability of the display, (c) schematic of the display cross section [25]

4.1.3

O-TFT on Plastic Substrate

As O-TFTs can be fabricated at low process temperatures (typically Chaps. 5.3.1, > 5.3.2, and > 5.3.3. Recently, O-TFT backplanes have been successfully fabricated directly on low-temperature flexible plastic substrates to demonstrate flexible AM LCD, AM EPD, and AM OLED displays (e.g., [25, 27–29, 35]). Suzuki et al. [35] demonstrated a 5.8-in. diagonal flexible phosphorescent color AM OLED using O-TFT backplanes fabricated on flexible PEN plastic substrate. Pentacene used as the organic semiconductor was deposited by thermal evaporation.

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

. Fig. 11 Printed O-TFT driven (a) 4.800 VGA AM FPD, (b) 2.500 QQVGA AM OLED, and (c) 4.100 , 80 mm thick, rollable OLED (121 ppi) in a rolled-up condition with a radius of 4 mm [16, 18]

The O-TFT exhibited a current on/off ratio of 106, and a mobility of 0.1 cm2/Vs. The display had a resolution of 213 (RGB)  120 pixels with a pixel pitch of 42 ppi. Sony [25, 28] demonstrated very impressive flexible and rollable AM OLED and AM EPD displays driven by OTFTs as shown in > Fig. 11. The 4.100 wide AM OLED display has a resolution of 432  RGXB  240 pixels with a pitch of 121 ppi. The thickness and bending radius of the rollable displays were 80 mm and Figure 12 shows [37] the flexible stainless steel backplane and the AM OLED display fabricated using the direct fabrication of LTPS TFT on stainless steel foil substrates. Chuang et al. [36] reported fabrication of LTPS TFT backplanes on 100 mm thick, type 304 stainless steel substrates. The substrates were first polished to a surface roughness of about 1 nm

917

918

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

. Fig. 12 Flexible stainless steel backplane (a) and the 3.300 , 640  480 pixel AM OLED display (b) fabricated using direct fabrication of LTPS TFT backplane on a stainless steel foil substrates [30]

and then a passivation layer of PE CVD SiO2 is deposited to isolate the conductive substrate from LTPS TFT backplane fabricated using excimer laser recrystallized LTPS. Process temperatures up to 700 C were utilized for dopant thermal activation. Arihara et al. [38] demonstrated fabrication of In-Ga-Zn-Oxide TFT backplanes on stainless-used-steel (SUS) substrates using process temperatures up to 300 C. These backplanes are then integrated with white OLED display media and a flexible color filter array fabricated on PEN substrates. The 4.7-in. diagonal full-color OLED display had a QVGA (320  RGB  240) resolution and a panel thickness of 0.4 mm.

4.2

Device Layer Transfer (DLT) Process

The DLT process involves standard (high-temperature) TFT fabrication on a conventional display glass substrate, followed by transfer of the TFT circuit (backplane) on to a flexible plastic substrate by adhesive bonding at a lower temperature (e.g., less than 150 C). This approach is pursued by multiple companies [39–43] for flexible display and flexible electronics application. Seiko Epson refers to this process as SUFTLA (surface-free technology by laser annealing) and has made significant advances to this approach in recent years. More specifically, the SUFTLA technology involves transferring high-performance, LTPS TFT backplane (circuits) fabricated on a conventional display glass substrate with an exfoliation layer (sacrificial a-Si layer), to a flexible plastic sheet as shown in > Fig. 13. The SUFTLA process consists of two transfer steps. First, a sacrificial amorphous silicon (a-Si) layer is formed on an original glass substrate (> Fig. 13a), followed by conventional CMOS LTPS TFT backplane fabrication. This substrate is then attached to a temporary substrate with a watersoluble adhesive as shown in > Fig. 13b on the device’s top side. Next, Xe Cl excimer laser light is irradiated onto the amorphous silicon layer from the back of the original glass substrate to trigger release of the TFT backplane circuitry from the glass substrate as shown in > Fig. 13c. The amorphous silicon layer absorbs the laser light to weaken the adhesion between TFT devices and the original glass substrate. Thus, polysilicon TFT devices are transferred onto the temporary substrate. The second transfer step starts with laminating the back side of the TFT devices onto the final plastic substrate, using a permanent adhesive that is not water soluble as shown in > Fig. 13d. The substrate is then submerged in water to separate from the temporary

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

Nch TFT

5.6.2

Pch TFT

Original substrate

a Sacrificial a-Si layer Water soluble temporary adhesive Temporary substrate Plastic substrate

d

b

c

XeCl excimer laser

Non water soluble permanent adhesive

e

. Fig. 13 Device layer transfer (DLT) process by SUFTLA approach [34]

substrate as the temporary adhesive dissolves, thereby transferring the high-performance LTPS CMOS backplane on the flexible plastic substrate as shown in > Fig. 13e. These highperformance backplanes are then used to fabricate and demonstrate a variety of flexible displays, including AM LCD, AM EPD, and AM OLED, and other flexible electronic devices such as fingerprint sensors as shown in > Fig. 14, with the Y-axis showing the number of TFTs on plastic, and the X-axis showing the year the device was demonstrated . The paperback-sized displays up to 131 mm  98 mm, with the scan and data drivers integrated with over seven million TFTs have been successfully demonstrated on plastic substrates. SUFTLA has the potential to fabricate very-high-quality flexible displays, using high mobility and stable LTPS TFT technology. Practical considerations for this approach include cost and yield. The extra cost associated with the a-Si sacrificial layer deposition and the two transfer steps in the fabrication of the flexible TFT backplane need to be minimized. However, the main issue that remains to be resolved for this approach, particularly for large-size flexible displays, includes defect control and yield. The transfer yield can have a major impact on the cost of the SUFTLA process. Defects such as air bubbles, dust and particles in the water-soluble adhesive that prevent adhesion to the temporary substrate, can create defects that impact the yield. While smallsize displays will have less of an issue with yield, large-size displays can have significant yield issue to resolve, as the yield decreases exponentially with the display size.

4.3

Temporary Substrate Bonding and De-bonding

The temporary substrate bonding and de-bonding approach [3, 44–48] involves laminating the flexible substrate to a rigid temporary substrate such as a glass or a ceramic substrate (e.g., by

919

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

Paperback-sized display 107 Monochromatic AM-OLED

106

Number of transistors

920

105

Fingerprint sensor

SRAM

AM-LCD AM-EPD

Color AM-OLED 104

Microprocessor

103

Thin film display

102 10 Ring oscillator 1 1998

1999

2000

2001

2002 2003 Year

2004

2005

2006

2007

. Fig. 14 SUFTLA technology progression [34]

using a temporary adhesive), fabricating the TFT backplane and de-bonding/separating the flexible substrate with the TFT backplane from the temporary substrate, as illustrated in > Fig. 15. Bonding to a rigid temporary substrate greatly improves the ease of handling the flexible substrate and facilitates using conventional TFT processing equipment to fabricate the backplane. This approach is widely used by various organizations. The issues in this approach include: (1) temperature constraints imposed by the temporary adhesive, (2) potential for chemical contamination by the temporary adhesive during the TFT processing, (3) yield of the bonding and de-bonding (of the flexible substrate/backplane from the rigid carrier substrate) operations with complete removal of the temporary adhesive, (4) cost of the bonding and de-bonding operations, and (5) cost of the temporary substrate if it is not reuseable, or has limited reuseability. Flexible backplanes and displays using flexible plastic as well as metal foil substrates and display media such as EPD and OLED have been fabricated. Paek et al. [3] demonstrated a 10.100 SVGA flexible monochrome AM EPD, with a thickness of 0.3 mm, using this approach with a metal foil substrate as shown in > Fig. 16. Hwang et al. [44] demonstrated a flexible AM FPD display using this approach with a 120 C a-Si TFT backplane on a flexible PEN plastic substrate as shown in > Fig. 17. This is a 14.3-in. (A4 size) display with a 1,280  900 pixel resolution with a 15 V drive. FDC has demonstrated [45–47] flexible AM EPDs and AM OLEDs using PEN plastic substrates as well as flexible metal foil substrates and a-Si TFT backplanes processed at 180 C. Ma et al. [48] demonstrated fabrication of a-Si TFT backplanes on flexible stainless steel substrates at 200 C using this approach. These backplanes have been integrated with phosphorescent OLED media to demonstrate full-color 4-in. diagonal QVGA displays with a thickness of 0.3 mm as shown in > Fig. 18, for a rugged wrist display application.

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

Adhesive Plastic substrate Carrier substrate Temporary substrate bonding

TFT array Plastic substrate Carrier substrate TFT fabrication: Figure

In situ Plastic Coating on a Temporary Substrate

19 illustrates the in situ plastic coating backplane process strategy [49–52]. This strategy involves coating a low-TCE polyimide (PI) film on a glass substrate with a sacrificial layer. This coated PI film serves as a flexible substrate. The backplane circuit is

921

922

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

. Fig. 17 Photograph of an AM EPD display fabricated using low-temperature a-Si TFTs backplane on a flexible PEN plastic backplane [38]

. Fig. 18 Photographs of a 4-in. flexible AM OLED panel fabricated using low-temperature a-Si TFT backplane on a stainless steel substrate, under inward and outward bending [42]

then processed on the PI surface using conventional TFT processes and equipment. The fabricated backplane on the flexible PI film (substrate) is then released (separated) from the temporary rigid substrate, by a proprietary trigger release mechanism involving a thermal, optical, or mechanical process. Philips [49–51] has developed this approach initially for flexible a-Si TFT backplanes for e-paper type displays, and has named it EPLAR (Electronics on Plastic by Laser Release) process. This process involves two extra process steps compared to a conventional a-Si TFT process on a rigid glass substrate. The first is an additive process of spin-coating a 10-mm-thick polyimide layer (which subsequently becomes the self-supporting flexible substrate/backplane). The temperature capability of this polyimide layer exceeds the requirements of the conventional a-Si TFT process, thus it can be processed in conventional a-Si

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

Sacrificial layer Spin-on plastic Carrier substrate Spin-coated Pl on carrier

TFT array Spin-on plastic Carrier substrate TFT fabrication: ~300°C

TFT array Plastic substrate

Carrier substrate Laser release: interfacial melting

. Fig. 19 In situ coating of plastic

TFT backplane fabrication facilities using standard processes. Electrophoretic display media is then laminated to the TFT backplane, and the resulting display on the polyimide foil is then separated from the rigid carrier glass substrate, by a laser release process, which relies on the appropriate glass surface treatments prior to the polyimide spin coating, and use of the appropriate type of polyimide. Flexible Electrophoretic displays have been demonstrated using this process. This process can be adapted for the fabrication of LTPS-TFT or OSC-TFT backplanes, and other display media such as an OLED. > Figure 20 shows a photo of a 9.700 flexible E-Paper display [51] using a-Si TFT backplane on a thin PI substrate and EPD display media. Recently, ITRI [53, 54] demonstrated this process using a separate de-bonding layer (DBL), unlike the EPLaR process. In the ITRI process, the PI material is custom synthesized. The PI solution is coated after depositing a DBL on a glass temporary substrate. The area covered by PI is intentionally made larger than of DBL’s. The glass substrate was then subjected to the TFT backplane fabrication process on Gen 2 glass line. Top gate a-Si:H and mc-Si TFTs were fabricated by a six-mask process at 200 C. Since PI’s edges extend over the underlying DBL and are in direct contact with the glass, it adheres securely to the glass carrier during the entire TFT process. As a result, alignment of TFT layers on the PI substrate can be maintained throughout the process. In other words, the thermally induced misalignment issue can be largely avoided here. Due to the DBL’s weak adhesion with PI film, the PI layer with TFT device can be easily separated from glass by simply cutting the circumference of the PI layer where the cutting line is within the edges of the DBL. > Figure 21 shows examples of a flexible AM OLED (a) and flexible AM EPD

923

924

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

a-Si

Gate metal

n+ a-Si

SD metal

SiN passivation

ITO pixel

SiN gate

Passivation

Polyimide film Glass (1.1 mm)

a

EPLaRTM flexible display The first flexible display made in a working TFT factory Transparent conductor

Charged particles

+ +

– –

– –

+ +

Patterned conductor

Dielectric fluid

Philips, Thales LCD and SiPix

b . Fig. 20 Cross section of an EPLAR a-Si TFT array while it is still anchored to a glass substrate (a), and photograph of a laser released EPLAR display (b) [11]

(b) demonstrated [53] by this process. Jang et al. [55] developed this process using an ultrathin buffer layer coating prior to the PI spin coating. They utilized this structure to fabricate amorphous IGZO backplanes at 200 C for driving AM OLED displays. Samsung [56–58] developed this process using a plastic film coating with attractive manufacturable properties such as very low CTE (
3 ppm/K) and high temperature processing capability (up to 350 C), and a room temperature delamination process that makes no electrical and mechanical damage to TFTs. This approach is used to fabricate flexible backplanes using OSC-TFTs, and LTPS-TFTs and AM OLED displays using these backplanes. The top emission mode was used for organic light emitting diode (OLED) structure, and thin film encapsulation was applied for flexible encapsulation. > Figure 22 shows [57] a 2.8-in., QVGA, full-color top emission AM OLED display demonstrated using this approach.

5

TFT Backplane Fabrication by Direct Printing

Direct pattern printing is a very attractive approach for fabricating each layer of the TFT structure for the flexible backplane and display fabrication. Compared to the conventional thin film deposition and photolithographic processes, direct pattern printing process can be more

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

. Fig. 21 Photographs of flexible displays fabricated by the in situ plastic coating method using a separate de-bonding layer: (a) 600 SVGA AM EPD, (b) 4.100 a-Si TFT AM OLED, and (c) 4.100 mc-Si TFT AM OLED [43]

compatible with use of flexible plastic substrates, and low-cost roll-to-roll processing. Printing is also expected to have a low environmental impact because of small number of process steps, small amount of materials used, and high throughput. Direct pattern printing method requires both printing materials (semiconductor ink for the transistor active layer, conductor inks for the bus lines and pixel electrode, and dielectric ink for the dielectric and passivation layers) and printing methods. All inks must meet the full set of requirements to serve their respective functions for the desired TFT device operation. Silver ink is an example candidate for the bus lines. Candidate inks for the transparent pixel electrode include ITO nanoparticles, CNTs, and metal nanowires. Candidate printing methods include ink-jet printing, offset printing, microcontact printing, imprinting, gravure printing, flexo-printing, and screen printing. Each printing method has its own advantages and limitations, and different printing methods are better suited for each layer of the TFT structure. Ink-jet printing of OTFTs is currently one of the hot topics in large area, printable, and flexible displays and electronics due to its low-temperature processing being compatible with low thermal budget of the available plastic substrates [59, 60]. Since, at this stage of development, it is generally very difficult to optimize printing conditions of materials for all different TFT functional layers, especially for organic dielectric and semiconductor layers, the ink-jet printing process has been used in combination with other solution- or vacuum-based fabrication methods to demonstrate solution-processable OTFTs. Recently, Suzuki et al. [59] demonstrated a 200 ppi all-printed organic TFT backplane and a flexible EPD display. The bottom gate OTFT structure used surface energy controlled silver nanoparticle ink-jet

925

926

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

. Fig. 22 Photograph of a flexible 2.800 QVGA AM OLED fabricated by the in situ plastic coating method involving a plastic film with a CTE of 3 ppm/K and process temperature capability of 350 C, and LTPS TFTs [47]

deposition for the gate and source-drain layers, and ink-jet printing of organic semiconductor. A spin-coated novel polyimide was used as a gate dielectric. The insulator and pixel electrodes were fabricated by screen printing. All these layers were printed under ambient conditions with a maximum process temperature of 180 C to fabricate OTFTs with a channel length of 5 mm and a mobility of 0.1 cm2/Vs. This backplane is used to demonstrate a 3.2-in. diagonal, 540  360 pixel electrophoretic display. While good progress is made on printable OTFTs and backplanes, several technical issues still remain to be resolved [60]. Although printed OTFTs with a reasonable performance have been demonstrated, there are still several remaining technical challenges in materials and device structures for developing high-performance all-ink-jet-printed organic thin film transistors. The issues that remain to be resolved include: formation of narrow, high-aspect-ratio metal lines with low sheet resistance; optimum processing and curing conditions for a printed, defect-free, high-quality organic gate dielectric layer; surface energy and wetting issues for the printed organic semiconducting layer; and contact resistance between source/drain electrodes and the organic semiconducting layer, especially for the bottom-contact organic thin film transistor structure.

6

Roll-to-Roll (RTR) Processing

Currently, the popular approaches for fabricating flexible backplanes and displays are based on a plate-to-plate type approach involving TFT fabrication on a flexible substrate attached (laminated) to a rigid carrier substrate as discussed in > Sects. 4.3 and > 4.4. These are batch-type processes and use conventional vacuum deposition and lithographic patterning technologies. On the other hand, RTR process is a well-known technology that is commonly used in cost-effective manufacturing of some thin film devices on flexible substrates in

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

a continuous fashion. RTR processing offers significant advantages compared to the conventional batch process, as it increases throughput by allowing greater levels of automation and by eliminating the overhead time involved in loading and unloading panels into lithographic tools and chemical processing stations. However, there are many challenges in fabricating flexible TFT devices and backplanes, requiring multiple layers with small design rules and precise alignment between various layers, using an RTR process. The vacuum and the photolithographic processes which constitute the bulk of the current TFT fabrication are not compatible with true RTR processing, because the roll needs to stay stationary during the photolith exposure time. The current efforts in the application of RTR processes for flexible backplanes and displays are directed toward realizing the benefit of integrating RTR process steps, where feasible, into predominantly plate-to-plate processes. As an example, recently, NEC [61] reported on the development of a rollable flexible silicon TFT backplane utilizing a RTR continuous lamination process. The roll-to-roll TFT-backplane technology involves a glass-etching TFT transfer process and a roll-to-roll continuous lamination process. The transfer process includes highrate, uniform glass-etching to transfer TFT arrays fabricated on a glass substrate to a flexible plastic film. In the roll-to-roll process, thinned TFT-glass sheets (0.1 mm) and a base-film roll are continuously laminated using a permanent adhesive. Choosing both an appropriate elastic modulus for the adhesive and appropriate tension strength to be used in the process is key to suppressing deformation of the TFT-backplane rolls caused by thermal stress. TFT backplanes that can be wound, without any major physical damage such as cracking, on a roll whose core diameter is approximately 300 mm have been demonstrated. In this case, while the actual TFT fabrication is conducted in a plate-to-plate process, RTR process is utilized for transferring/ laminating the backplanes to a flexible plastic roll at a high rate and low cost. HP Lab is developing their SAIL (self-aligned imprint lithography) technology [62] that utilizes an imprinting process for the manufacture of TFT backplanes on plastic films. The SAIL process eliminates the need for many photolithographic/resist etch steps which are expensive and have a low throughput. While the SAIL process still uses vacuum deposition and dry etching for the TFT layers, its cost advantage comes from completing all the layer deposition steps prior to any of the patterning steps, and using a monolithic 3D masking structure. The multiple patterns required to create the backplane are encoded in the different heights of a 3D masking structure that is molded on top of the thin film stack once before any of the etching steps. By alternately etching the masking structure and the thin film stack, the multiple patterns required for the backplane are transferred to the device layers. Because the mask distorts with the substrate perfect alignment is maintained regardless of process induced distortion. These backplanes have been used to demonstrate AM EPDs. While the SAIL process is not a true and complete RTR process, it still benefits from the RTR imprint patterning process. Active-matrix TFT devices and backplanes fabricated completely by printing procedures, without use of any vacuum deposition steps and photolithographic patterning procedures, have the potential for full roll-to-roll fabrication and the associated ultimate low-cost benefits. At present, printable inks for the semiconductor and gate insulator materials are not available particularly for inorganic (a-Si, LTPS and OSC) TFTs. At this time, OTFT technology appears to be closest to having the printable semiconductor and gate insulator materials and the potential for developing a more complete RTR process for backplane fabrication. Development of fine pattern printing technologies is also essential for realizing RTR technologies capable of fabricating high-resolution flexible displays.

927

928

5.6.2 7

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

Other Technical Challenges for Flexible Displays

Throughout this chapter, up to now, we have discussed various barriers and technical challenges in the fabrication of flexible active matrix TFT backplanes and displays due to the characteristics of the available thin flexible metal and plastic foils. When we consider the actual operational aspects of the flexible electronics and displays based on plastic substrates, there are two other challenges that need to be addressed and resolved, namely, self-heating effects and mechanical durability [63, 64]. It is important to consider the mechanical durability during the flexible display operation due to the very thin and fragile nature of flexible displays when they are bent with a very small radius or when folded. The characteristics of the flexible substrate selected (such as the TCE, young’s modulus, thickness of the film, and its viscosity) can have an impact on the thermo-mechanical stresses generated during operation of the display [63], and thus its durability. Mechanical durability issues during use need to be addressed by proper packaging/support for the flexible display during storage and during use that ensures that the backplane/display does not experience stains beyond the elastic limit.

7.1

Self-heating Effects

Self-heating effects in TFTs on glass substrates are well known [64]. When the TFT is in the onstate, the source–drain current results in Joule heating, which raises the temperature of the TFT and this effect is known as self-heating. This effect can be a significant barrier to flexible electronics and displays built on plastic substrates that have a very low thermal conductivity in comparison to the thermal conductivity of glass substrates. > Table 5 shows the thermal conductivity of plastic substrates in relation to the typical TFT thin film materials and glass. The low thermal conductivity of plastic film prevents heat from dissipating from the semiconductor channel layer of the TFT, leading to the device temperature rise. Thus, for TFTs with identical performance, flexible plastic backplanes and displays exhibit greater susceptibility to self-heating than the backplanes and displays on glass substrates. Excessive temperature rise can lead to deformation of the plastic material or the delamination of the TFT devices from the substrate in addition to affecting the TFT device performance and consequently the display performance. . Table 5 Thermal conductivity of various TFT and substrate materials Solid

Thermal conductivity (WM1 K1)

Aluminum

2.39

Stainless steel

0.162

Polysilicon

1.55

Silicon

1.48

Amorphous silicon

0.018

SiO2 glass

0.014

Polyimide

0.0052

Plastic films

~0.002

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

5.6.2

The display media used can also have an impact on the severity of the self-heating problem. For, example, self-heating is expected to be a bigger issue for flexible AM OLED displays that involve continuous current flow and heat generation through the OLED pixel compared to an AM EPD display that does not generate any light (or heat); EPD merely modulates the reflected ambient light. Self-heating effects can be minimized by: (1) optimizing the shape of the TFT for effective heat dissipation, (2) improving the TFT electrical performance characteristics such as by improving the mobility and reducing the threshold voltage, (3) scaling the TFT dimensions such as channel length, and dielectric thickness, and (4) utilizing energy-efficient drive circuits. With respect to optimizing the shape of the TFT, heat dissipation can be improved by utilizing several TFTs with a smaller channel width, W, connected in parallel as opposed to using a single TFT device with a large channel width, while maintaining the desired source-drain current. Self-heating effects can be greatly minimized by metal foil substrates with high thermal conductivity.

8

Summary and Conclusions

During recent years, significant progress has been made on the development of flexible substrates and the compatible TFT processing methods for fabricating flexible backplanes and displays. While direct fabrication of TFT backplanes on available flexible substrates has been demonstrated for small-size displays, currently, this approach is not believed to be practical for large-size displays, particularly when using inorganic TFTs. At this time, the barriers for the direct fabrication approach for large-size displays include issues of mechanical handling of thin, flexible, and self-supporting substrates through the current plate-to-plate, batch-type TFT process equipment, and dimensional stability issues due to shrinkage and CTE mismatch. While the direct processing strategy may be more amenable for low-cost RTR processing, significant advances are required in technologies for direct TFT processing as well as RTR processing, for realizing a viable overall approach. At this time, OTFT technology is more compatible with direct processing and RTR fabrication approach. While the device layer transfer (DLT) strategy can be practical for fabricating high-quality flexible displays, currently, yield and cost are the barriers to be overcome for use of this technology, particularly for large area flexible displays. Both bond–de-bond and in situ plastic coating methods show promise for providing a viable path for fabricating flexible displays with various display media and a broad range of sizes. The current developments in flexible stainless steel foils and polymer substrates, temporary adhesives, as well as plastic coating methods are helping in the advance of these two TFT process strategies. At present flexible displays using both OLED and EPD are being developed very actively, even though there are some efforts in the flexible LCD development. Very impressive flexible electrophoretic displays have been demonstrated using a-Si TFT, LTPS TFT, and O-TFT backplanes, using plastic as well as stainless steel substrates. Flexible AM EPDs are expected to be commercialized soon for the e-book and other very-low-power display applications. Because a flexible OLED is considered an ultimate display, further development of various technology elements to enable manufacturing of these displays is expected to continue. Development of a cost-effective multilayer barrier film and its integration with the backplane and OLED display fabrication processes is an important enabling element to realize flexible AM OLED displays. Important progress continues to be made in the lower-temperature a-Si

929

930

5.6.2

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

TFT, LTPS TFT, OTFT, and OSC-TFT technologies for application to the flexible AM OLED displays. Also, important advances continue to be made in the OLED media itself with respect to improved luminous efficiency, lower drive voltages, and longer lifetime. These advances can relax the requirements of the active matrix TFT devices with respect to drive currents and TFT gate bias stress stability requirements to accelerate the flexible AM OLED development. Development of science and technology required for manufacturing of flexible displays, particularly flexible AM OLED display, is a tough technical challenge. While significant progress has been made in this endeavor, many technical issues still remain to be resolved as discussed in this chapter. However, the potential for successful development of flexible AM OLEDs is high because of the significant value proposition of the flexible display products and systems, high probability of the current approaches being pursued to resolving the current technical issues, and high levels of the industry investment in this technology.

References 1. Erlat AG, Yan M, Duggal AR (2009) Substrates and thin film barrier technology for flexible electronics. In: Wong WS, Sallelo A (eds) Flexible electronics. Springer, New York 2. Jin DU, Jeong JK, Kim TW, Lee JS, Ahn TK, Mo YK, Chung HK (2006) Flexible AM OLED display on stainless steel foil. J Soc Inf Disp 14(12):1083–1090 3. Paek SH, Kim KL, Seo HS, Jeong YS, Yi SY, Lee SY, Choi NB, Kim SH, Kim CD, Chung IJ (2006) 10.1 inch SVGA ultra thin and flexible active matrix electrophoretic display. SID ’06 Dig 37(1):1834–1837 4. Raupp GB, Colaneri N, O’Rourke SM, Kaminski J, Allee DR, Venugopal SM, Bawolek EJ, Loy DE, Moye C, Angeno SK, O’Brien BP, Bottesch D, Rednour S, Blanchard R, Marrs M, Dailey J, Long K (2006) Flexible display technology in a pilot line manufacturing environment. In: Army science conference, Orlando, 27–30 Nov 2006 5. Raupp G (2007) Active matrix TFT technology development and pilot line manufacturing for reflective and emissive flexible displays. In: Proceedings of the USDC flexible displays & microelectronics conference & exhibit, Phoenix, Az, 5–8 Feb 2007 6. Jin DU, Jeong JK, Shin HS, Kim MK, Ahn TK, Kwon SY, Kwack JHo, Kim TW, Mo YG, Chung HK (2006) 5.6-inch Flexible full color top emission AM OLED display on stainless steel foil. SID ’06 Dig 37(1): 1855–1857 7. Chwang A, Hewitt R, Urbanik K, Silvernail J, Rajan K, Hack M, Brown J, Lu JP, Shih C, Ho J, Street R, Ramos T, Moro L, Rutherford N, Tognoni K, Anderson B, Huffman D (2006) Full color 100 dpi AMOLED displays on flexible stainless steel substrates. SID ’06 Dig 37:1858–1861 8. Paek SH, Park YI, Park CH, Lim YS, Shin SI, Kim CD, Hwang YK (2010) Flexible display technology

9.

10.

11.

12. 13.

14. 15.

16.

for a mass production using the improved etching technology. SID ’10 Dig 41(1):1047–1049 Long K, Kattami AZ, Chen IC, Gleskova H, Wagner S, Sturm J (2006) Stability of amorphous silicon TFTs deposited on clear plastic substrates at 250 C to 280 C. IEEE Electron Device Lett 27(2):111–113 Sarma KR, Chanley C, Dodd S, Roush J, Schmidt J, Srdanov G, Stevenson M, Yu G, Wessel R, Innocenzo J, O’Regan M, MacDonald WA, Eveson R, Long K, Gleskova H, Wagner S, Sturm JC (2003) Active matrix OLED using 150 C a-Si TFT backplane. In: Proceedings of the SPIE 2003, Orlando. Cockpit displays X, vol 5080, pp 180–191 Sarma KR, Schmidt J, Roush J, Dodd S, Chanley C (2004) Active matrix OLED displays using a-Si TFT backplanes fabricated on flexible plastic substrates. In: Proceedings of the SPIE defence displays conference, Orlando, FL, April 2004 Sarma KR et al (2007) Flexible OLED displays. In: Proceedings of the AM FPD ’07, Hugo MacDonald WA, Mace JM, Polack NP (2002) 45th Annual technical conference proceedings of the Society of Vacuum Coaters, p 482 MacDonald WA (2004) Engineered films for display technologies. J Mater Chem 14:4–10 MacDonald WA, Eveson R, MacKerron D, Adam R, Rollins K, Rustin R, Looney MK, Hashimoto K (2006) The impact of environment on dimensional reproducibility of polyester film during flexible electronics processing. SID ’06 Dig 37(1): 414–417 Eveson R, MacDonald WA, MacKerron D, Hadgson A, Adam R, Rakos K, Rollins K, Rustin R, Looney SJ, Asai M, Hashimoto K (2008) Optimizing polyester films for flexible electronics applications. SID ’08 Dig 39(1):1431–1434

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies 17. Graff G, Burrows PE, Williford RE, Praino RF (2005) Barrier layer technology for flexible displays. In: Crawford GP (ed) Flexible flat panel displays. Wiley, Chichester 18. Moro M, Chu X, Hirayama H, Krajewski T, Visser RJ (2006) A mass manufacturing process for Barix encapsulation of OLED displays. In: IMID/IDMC, Daegu Exhibition and Convention Center (EXCO), Daegu, 22–25 Aug 2006 19. Kwon JY, Jung JS, Park KB, Kim JM, Lim H, Lee SY, Kim JM, Noguchi T, Hur JH, Jang J (2006) 2.2 inch qqVGA AMOLED driven by Ultra Low Temperature Poly Silicon (ULTPS) TFT direct fabricated below 200 C. SID’06 Dig 37(2):1358–1361 20. Gosain DP, Usui TNS (2000) High mobility thin film transistors fabricated on plastic substrates as process temperature of 110 C. Jpn J Appl Phys 39L:179 21. Gleskova H, Wagner S (2001) DC-gate bias stressing of a-Si:H TFTs fabricated at 150 C on ployimide foil. IEEE Trans Electron Device 48(8):1667 22. Gleskova H, Wagner S (1999) Amorphous silicon thin-film transistors on compliant polyimide foil substrates. IEEE Electron Device Lett 20(9):473 23. He S, Nishiki H, Hartzell J, Nakata Y (2000) Low temperature PECVD a-Si TFT for plastic substrate. SID 2000 Dig 31(1):278–281 24. Wagner S, Han L, Hekmatshoar B, Song K, Mandlik P, Cherenack KH, Sturm J (2010) Amorphous silicon TFT technology for rollable OLED displays. SID ’10 Dig 41(1):917–920 25. Nomoto K (2010) Development of flexible displays driven by organic TFTs. SID ’10 Digest, p 1165 26. Gundlach DJ, Kuo DJ, Nelson CC, Jackson TN (1999) Organic thin film transistors with field effect mobility >2 cm2/V-sec. IEEE Electron Device Lett: p 164 27. Burns SE et al (2006) A flexible plastic SVGA e-paper display. SID’06 Dig 37(1):74–76 28. Noda M, Kobayashi N, Katsuhara M, Yumoto A, Ushikura SI, Yasuda RI, Hirai N, Yukawa G, Yagi I, Nomoto K, Urabe T (2010) A rollable AM-OLED display driven by OTFTs. SID ’10 Dig 41(1):710–713 29. Burns S (2010) QUE: an e-Reader built using flexible display technology. SID ’10 Dig:477–479 30. Hirao T, Furuta M, Furuta H, Matsuda T, Hiramatsu T, Hokari H, Yoshida M (2006) High mobility top-gate zinc oxide thin film transistors (ZnO-TFTs) for active matrix liquid crystal displays. SID ’06 Digest, pp 18–21 31. Carcia PF, McLean RS, Reilly MH (2005) Oxide engineering of ZnO thin-film transistors for flexible electronics. J Soc Inf Disp 13:547 32. Nomura K, Ohta H, Takagi A, Kamiya T, Hirano M, Hosono H (2004) Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 432:488

5.6.2

33. Kamiya T, Nomura K, Hosono H (2009) J Disp Technol 5(7):273 34. Mo YG, Kim M, Kang CK, Jeong JH, Park YS, Choi CG, Kim HD, Kim SS (2010) Amorphous oxide TFT backplane for large size AMOLED TVs. SID ’10 Dig 41(1):1037–1040 35. Suzuki M, Fukagawa H, Nakajima Y, Suzuki T, Yamamoto T, Tokito S (2008) 5.8-inch Phosphorescent color AM-OLED display driven by OTFTs on plastic substgrate. In: IDW’08, Toki Messe Niigata Convention Center, Niigata, 3–5 Dec 2008, p 1515 36. Chuang TK, Troccoli M, Hatalis M, Voutsas A (2007) Polysilicon TFT technology on flexible metal foil for AM PLED displays. J Soc Inf Disp 15(7):455 37. Kattamis AZ, Giebink N, Cheng CC, Wagner S, Forrest SR, Hong Y, Cannella V (2007) Active-matrix organic light-emitting displays employing two thinfilm-transistor a-Si:H pixels on flexible stainlesssteel foi. J Soc Inf Disp 15(7):433 38. Arihara K, Kano M, Motai K, Naitou Y, Kadowaki M, Nakajima H, Tsuboi T, Kato C, Kishimoto Y, Maeda H (2009) Fabrication of flexible 4.7-inch QVGA AM-OLED panel driven by In-Ga-Zn-Oxide TFTs with flexible color filter. In: IDW ’09, World Convention Center Summit, Miyazaki, 9–11 Dec 2009, p 1613 39. Utsunomiya S et al (2003) Flexible color AM OLED display fabricated using surface free technology by laser ablation/annealing (SUFTLA®) and Ink-jet Printing Technology. SID ’03 Dig 34(1):864–867 40. Inoue S, Utsunomiya S, Saeki T, Shimoda T (2002) Surface-free technology by laser annealing (SUFTLA) and its application to poly-Si TFT-LCDs on Plastic film with integrated drivers. IEEE Trans Electron Device 49(8):1353 41. Miyasaka M (2007) Suftla flexible microelectronics on their way to business. SID ’07 Dig 38(1): 1673–1676 42. Miyasaka M, Nebashi S, Shimoda T (2006) Suftla flexible active-matrix electrophoretic displays. In: IMID/IDMC ’06 Digest, Daegu Exhibition and Convention Center (EXCO), Daegu, 22–25 Aug 2006, p 466 43. Asano A, Kinoshita T, Otani N (2003) A plastic 3.8in low-temperature polycrystalline silicon TFT color LCD panel. SID ’03 Digest, pp 988–991 44. Hwang TH, Lee W, Hong WS, Kim SJ, Kim SI, Roh NS, Nikulin I, Choi JY, Jeon HI, Hong SJ, Lee JK, Han MJ, Baek SJ, Kim M, Lee SUk, Shin SS (2007) 14.3 inch active matrix-based plastic electrophoretic display using low temperature processes. SID ’07 Dig 38(1):1684–1685 45. Raupp GB, O’Rourke SM, Moyer C, O’Brien BP, Ageno SK, Loy DE, Bawolek EJ, Allee DR, Venugopal SM, Kaminski JP, Bottesch D, Dailey J, Long K, Marrs M, Munizza MR, Haverinen H,

931

932

5.6.2 46.

47.

48.

49.

50.

51. 52.

53.

54.

Flexible Displays: TFT Technology: Substrate Options and TFT Processing Strategies

Colaneri N (2007) Low-temperature amorphoussilicon backplane technology development for flexible displays in a manufacturing pilot-line environment. J Soc Inf Disp 15(7):445 O’Rourke SM, Venugopal SM, Raupp GB, Allee DR, Ageno S, Bawolek EJ, Loy DE, Kaminski JP, Moyer C, O’Brien B, Long K, Marrs M, Bottesch D, Dailey J, Trujillo J, Cordova R, Richards M, Toy D, Colaneri N (2008) Active matrix electrophoretic displays on temporary bonded stainless steel substrates with 180 C a-Si:H TFTs. SID ’08 Dig 39(1):422–424 Loy D, Lee YK, Bell C, Richards M, Bawolek E, Ageno S, Moyer C, Marrs M, Venugopal SM, Kaminski J, Colaneri N, O’Rourke SM (2009) Active matrix PHOLED displays on temporary bonded polyethylene naphthalate substrates with 180 C a-Si:H TFTs. SID ’09 Dig 40(1):988–991 Ma R, Rajan K, Silvernail J, Urbanik K, Paynter J, Mandlik P, Hack M, Brown JJ, Yoo JS, Jung SH, Kim YC, Yoon SY, Kim CD, Kang IB, Hwang YK, Chung IJ, Tognoni K, Anderson R, Huffman D (2010) Wearable 4-in. QVGA full-color-video flexible AMOLEDs for rugged applications. J Soc Inf Disp 18(1):50 Battersby S, Fench I (2006) Plastic displays made by standard a-Si TFT technology. In: IMID/IDMC ’06, Daegu Exhibition and Convention Center (EXCO), Daegu, 22–25 Aug 2006, p 1546 French I, George D, Kretz T, Templier F, Lifka F (2007) Flexible displays and electronics made in AM-LCD facilities by the EPLaRTM process. SID ’07 Digest, p 1680 French I (2009) Flexible e-books. SID ’09 Digest, p 100 Pecora A, Maiolo L, Cuscuna M, Simeone D, Minotti A, Mariucci L, Fortunato G (2008) Lowtemperature polysilicon thin film transistors on polyimide substrates for electronics on plastic. Solid State Electron 52:348–352 Lee CC, Chang YY, Cheng HC, Ho JC, Chen J (2010) A novel approach to make flexible active matrix displays. SID ’10 Dig 41(1):810–813 Cheng HC, Huang YS, Liu CJ, Li CW, Ho KY, Cheng CH, Peng SY, Chen YP, Lin HC, Kung BC, Lee PF, Huang JJ, Jiang LY, Lee CC (2009) Plastic substrate and backplane for flexible AM OLED by

55. 56.

57.

58.

59.

60.

61.

62.

63.

64.

sheet to sheet process. IDW ’09, World Convention Center Summit, Miyazaki, 9–11 Dec 2009, p 1601 Jang J, Choi MH, Cheon JY (2010) TFT technologies for flexible displays. SID ’10 Dig 41(1):1143–1146 Jin DU, Lee JS, Kim TW, An SG, Straykhilev D, Pyo YS, Kim HS, Lee DB, Mo YG, Kim HD, Chung HK (2009) World-largest (6.500 ) flexible full color top emission AMOLED display on plastic film and its bending properties. SID ’09 Digest, pp 983–985 An S, Lee J, Kim Y, Kim T, Jin D, Min H, Chung HK, Kim SS (2010) 2.8-inch WQVGA flexible AMOLED using high performance low temperature polysilicon TFT on plastic substrates. SID ’10 Dig Jin DU, Kim TW, Koo HW, Stryakhilev D, Kim HS, Seo SJ, Kim MJ, Min HK, Chung HK, Kim SS (2010) Highly robust flexible AM OLED display on plastic substrate with new structure. SID ’10 Dig 41(1):703–705 Sujuki S, Yutani K, Nakashima M, Onodera A, Mizukami S, Kato M, Tano T, Tomono H, Yanagisawa M, Kameyama K (2009) A 200 ppi allprinted organic TFT backplane for flexible electrophoretic displays. In: IDW ’09, World Convention Center Summit, Miyazaki, 9–11 Dec 2009, p 1581 Hong Y, Chung S (2010) Technical issues towards all inkjet-printed organic thin-film transistors. SID ’10 Dig 41(1):1147–1150 Takechi K, Yamaguchi S, Tanabe H, Kaneko S (2010) Development of rollable silicon thin-film-transistor backplanes utilizing a roll-to-roll continuous lamination process. J Soc Inf Disp 18(6):391 Taussig C, Cobene R, Elder R, Jackson W, Jam M, Jeans A, Luo H, Maltabes J, Mei P, Smith M, Perlov C, Zhao L (2010) Roll-to-roll manufacturing of backplanes for paper-like displays. SID ’10 Dig 41(1):1151–1154 Miyasaka M, Hara H, Karaki N, Inoue S, Kawai K, Nebashi S (2008) Technical obstacles to thin film transistor circuits on plastic. Jpn JAP 47(6):4430 Fortunato G, Cuscuna M, Gaucci P, Maiolo L, Mariucci L, Pecora A, Valletta A, Templier F (2009) Self-heating effects in p-channel polysilicon TFTs fabricated on different substrates. J Korean Phys Soc 54(1):455

Suggestions for Further Reading on Poly-Si TFTs Wong WS, Salleo A (eds) (2009) Flexible electronics: materials and applications. Springer, New York. ISBN:978-0-387-74362-2

Part 5.7

Touchscreen Technologies

5.7.1 Introduction to Touchscreen Technologies Robert Phares . Mark Fihn 1

Introduction to Touchscreens and Touchscreen Systems . . . . . . . . . . . . . . . . . . . . . . . . . 937

2 2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 2.1.6 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.4 2.4.1 2.4.2 2.4.3

Analog and Digital Resistive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 938 Four-Wire Resistive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 938 Principle of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 938 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 940 Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 940 4W Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 947 4W Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 947 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 947 Eight-Wire Resistive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 948 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 948 Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 948 8W Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949 8W Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949 Five-Wire Resistive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949 Principle of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 950 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 950 Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953 5W Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955 5W Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956 Resistive Multi-touch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956 Digital Multi-touch Resistive (DMR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956 Analog Multi-touch Resistive (AMR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 957 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 957

3 3.1 3.1.1 3.1.2 3.1.3 3.1.4

Capacitive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 958 Surface Capacitive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 958 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 958 Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 959 Surface Capacitive Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 960 Surface Capacitive Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 960

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.7.1, # Springer-Verlag Berlin Heidelberg 2012

936

5.7.1

Introduction to Touchscreen Technologies

3.1.5 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Projected Capacitive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PCAP Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PCAP Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

960 960 960 961 963 963 963

4 4.1 4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.3 4.3.1

Optical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 963 Scanning Infrared . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 964 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 964 Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 964 Scanning IR Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965 Scanning IR Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965 Camera-Based Optical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966 Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966 Camera-Based Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966 Camera Optical Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 967 Surface Computing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 967 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 968

5 5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.2 5.3 5.3.1 5.3.2 5.3.3

Acoustic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 969 Surface Acoustic Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 969 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 969 Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 970 Advantages of SAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 970 Disadvantages of SAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 971 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 971 Guided Acoustic Wave (GAW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 971 Dispersive Signal Technology (DST) and Acoustic Pulse Recognition (APR) . . . . 971 APR and DST Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972 APR and DST Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972

6 6.1

In-Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 973

7

General Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 973

Introduction to Touchscreen Technologies

5.7.1

Abstract: This chapter will review the major commercially available touchscreen technologies, including the design considerations, materials, and construction methods. Strengths and weaknesses of the technologies are listed in an easy-to-read format. Brief reviews of new and emerging technologies are also included. List of Abbreviations: 3W, Three Wire; 4W, Four Wire; 5W, Five Wire; 8W, Eight Wire; ACF, Anisotropic Conductive Adhesives; AMR, Analog Multi-touch Resistive; APR, Acoustic Pulse Recognition; DI, Diffused Illumination; DMR, Digital Multi-touch Resistive; DSI, Diffused Surface Illumination; FFC, Flat Flexible Cable; FTIR, Frustrated Total Internal Reflection; GAW, Guided Acoustic Wave; GUI, Graphical User Interface; ITO, Indium Tin Oxide; PCAP, Projected Capacitive; PSA, Pressure-Sensitive Adhesive; SAW, Surface Acoustic Wave; SCAP, Surface Capacitive; VLT, Visible Light Transmission; ZIF, Zero Insertion Force

1

Introduction to Touchscreens and Touchscreen Systems

A touchscreen or touch system is a combination of hardware and software that provides a user input device for a computer that replaces or supplements a more traditional input device such as a keyboard, mouse, or trackball. Touchscreens have become more useful and easier to implement into a computer system with the widespread adoption of graphical user interface (GUI) systems. A touchscreen can easily be configured to emulate the characteristics of a mouse, and thus requires no special software at either the operating system or the application software level to operate relatively seamlessly as an alternate input device in such a system. The only software requirement is a mouse ‘‘driver.’’ The main components of a touchscreen system are: 1. Touch sensor: The sensor is a physical sensor or transducer with a touch responsive surface or plane. Ideally the sensor is transparent, and provides a relatively rigid surface to touch. The sensor may be electrical, mechanical, optical, or acoustic in its operating principle. 2. Controller: An electronic device, usually microprocessor controlled, which provides an excitation or ‘‘drive’’ signal for the touch sensor, receives signals from the sensor in response to a touch, converts the received signal into digital coordinates of the location of the touch, and transmits the touch coordinates to the host computer. 3. Software driver: While it is possible for a computer system to process touch events directly at the application software level, the most powerful and appropriate way for touch events to be applied to the operating system and to applications programs is by processing touch events through a driver. The driver receives all communications from the touchscreen controller, manages interrupts from the interface device – USB port, serial port, etc., – buffers streams of touches, processes mouse up/down clicks and most importantly, converts touch events into mouse compatible cursor control commands that the OS and applications programs can interpret and act on. This chapter will review presently available touchscreen technologies, focusing on design considerations, materials, and construction processes. Comments on system integration issues are included where they are considered significant. Emerging and developing technologies are also reviewed. As another chapter in this section reviews touchscreen controller designs and capabilities for several different touchscreen technologies (see > Chap. 5.7.4), there is little emphasis in this chapter on controller designs.

937

938

5.7.1 2

Introduction to Touchscreen Technologies

Analog and Digital Resistive

Resistive is one of the oldest types of touchscreens, having been produced in some form since the mid-1970s. These touchscreens are some of the largest selling products in the touchscreen business, surpassed only recently by projected capacitive (PCAP). Four-wire resistive touchscreens are the simplest type of analog resistive touchscreens, and they are considered first. Many of the concepts applicable to other types of resistive touchscreens are introduced in this section.

2.1

Four-Wire Resistive

The simplest analog resistive touchscreen is known as the ‘‘four-wire’’ (4W) resistive. The concept of this touchscreen is that of an orthogonal two-axis voltage divider.

2.1.1

Principle of Operation

In the 4W product, two conductive planes are developed on transparent conductive substrates, each dedicated to one of the orthogonal coordinate axes. In operation, an electric field is sequentially impressed on the two conductive planes, with the fields being orthogonal to each other. From each plane, a voltage proportional to the position of touch on that plane is passed to the controller for measurement. The details of operation are explained referencing > Fig. 1. In > Fig. 1, it is assumed that the X or horizontal axis is the lower plane, and the Yor vertical axis is the upper plane. While the planes are shown widely separated, the actual construction of a practical touchscreen incorporates separations of 0.2 mm or less. In operation as described here, the controller applies a potential, usually no greater than 5VDC, across highly conductive ‘‘bias’’ or ‘‘drive’’ electrodes spanning each end of the X-axis. Each conductive plane is typically a thin transparent plastic or glass substrate with an applied transparent conductive coating. The sheet resistance of the coating is usually
150–800 Ω per square (Ω⁄□) in commercially available coated glass and film, and the sheet resistance of the electrodes is typically Chap. 5.4.1). ITO can be coated on plastic (polyethylene terephthalate, or PET, being the most common for touchscreens) and many kinds of glass. Chemically strengthened (CS) glass may also be ITO coated. Heat tempered glass cannot be ITO coated, as the temper will be substantially removed by the heat of the ITO coating process. Touchscreens can be assembled and subsequently laminated to another substrate, and this is one choice for construction – a 4W touchscreen constructed of two flexible film layers (usually called ‘‘film– film’’ construction) laminated to a glass or rigid plastic backing plate. Film–film construction is common for very small 4W touchscreens, but most large (10 cm diagonal and above) 4W resistive touchscreens are produced on glass lower substrates with PET upper substrates. Chemically strengthened (CS) glass is a popular lower substrate choice for smaller touchscreens and for a laminated back-plate of a film–film construction, as the CS process is suitable for soda-lime float glass (or any high sodium oxide glass) up to 3 mm in thickness [1].

Introduction to Touchscreen Technologies

5.7.1

For the same thickness, a CS glass substrate may be as much as five times stronger than an annealed soda-lime glass substrate. The method by which the CS substrate is cut to the final size will have a significant effect on the strength of the substrate. When cut by the traditional scribeand-break method a CS glass substrate will lose essentially all its additional strength for a distance of 20–25 mm from the scribe line, due to micro-cracks which propagate laterally from the scribe. Thus, a small touchscreen made on a substrate cut from a larger sheet of CS glass may have little or no increased strength over an equivalent part built on an annealed soda-lime blank. However, when scribed by the laser technique developed by Schott Advanced Processing and Schott AG, which creates a tension-induced fissure line by rapid heating of the glass followed by rapid cooling of the glass, a vertical, chip-free edge is produced when the glass is snapped [2]. With no lateral cracks, the edge strength of the glass is preserved. Film–film construction dictates that the electrodes for each substrate and any connecting circuits must be made by a process that does not damage the plastic substrate or its ITO coating. Accordingly, electrodes are limited to screen-printed conductive inks (almost exclusively silver) which are UV, visible light, or low-temperature thermally cured. Lower substrates of glass may also use these same silver inks, but the choice of high-temperature fired (usually) silver electrodes is also available. Development work in nanoparticle silver and gold inks suggests that further improvement in touchscreen electrodes is possible, but there appears to be little commercial use of these materials in touchscreens today. The stability and durability of printed inks was a concern in early production 4W touchscreens, and the performance of fired silver was clearly superior. Better silver inks and the widespread use of transparent insulating inks over the silver (commonly referred to as ‘‘dielectrics’’) have improved the performance of non-fired inks to the point that there appears to be little advantage to the fired electrode process for 4W touchscreens. A further issue with a fired silver process is the difficulty of implementation in a post-ITO deposition process. Without atmospheric control in the firing process, ITO uniformity and resistivity can be severely affected, and the presence of the silver locally affects the ITO also. These considerations usually consign the fired silver process to the ITO coater, eliminating the significant cost advantage of mass-produced ITO-coated glass and a printed electrode design. The uniformity of the conductive coating on the glass and plastic substrates is important to the linearity of the final touchscreen. Tolerance on conductive substrate absolute resistivity is rarely better than +/10%, but the quality of sputtered ITO-coated substrates, whether glass or plastic, has improved significantly in the
25 years that such products have been available as a standard product, and localized variations in resistivity, which is the type of defect that actually has much impact on the linearity of a touchscreen, is now relatively rare. Regardless of the base material used, the visible light transmission (VLT) of basic commercially available ITO-coated substrates is seldom greater than 85–91%. This results in an overall touchscreen VLTof
72–83%. Most of the loss of VLT in ITO-coated substrates is reflective. Thus, there is keen interest in the industry in conductive substrates that are more transmissive and less reflective. Multilayer coating constructions with modest improvements in VLT over ‘‘standard’’ single layer coatings are available at premium prices. Work continues on promising alternate materials such as carbon nanotube (CNT) and PEDOT-PSS coatings (PEDOT-PSS is a printable transparent conductive polymer available under the trade names Clevios (from Haraeus) and Orgacon (from AGFA)) though neither has achieved widespread engineering nor commercial success. CNT films in particular are expected to have lower reflectance than ITO films [3]. A frequent customer-designated feature of a resistive touchscreen is the surface finish of the upper plane substrate. It is this acrylic surface which can be specified to have a polished or a matte finish at the customer’s request. The matte finish material will typically have

941

942

5.7.1

Introduction to Touchscreen Technologies

a ‘‘60 gloss’’ rating of
90 while the polished finish is a specular reflecting surface with a gloss rating of 130 or more. A perfectly reflecting surface is defined as having a gloss of 200 when measured by the typical gloss meter. The use of a 60 gloss meter reading to specify the degree of a matte finish is a popular but unfortunate choice for this type of material. In the early days of the touchscreen industry, there were few quick and easy techniques for measuring gloss and related parameters, and the 60 gloss meter was adopted from the automotive industry, where it is used for measuring paint finishes. The presence of the ITO layer on the opposite side of the substrate makes the matte finishes measure higher gloss (i.e., more reflective) than it actually is. A more useful measurement of the finish texture is haze [4] but this parameter is still not universally given either in material or in finished product specifications. A related parameter, image resolution, has all but disappeared from published touchscreen specifications, but is still a very useful way to relate haze and gloss of touchscreen finishes to transmitted image quality. The most common way to measure image resolution is with the 1951 United States Air Force Resolution Test Chart, (> Fig. 2), also known as the ‘‘Air Force Bar Chart,’’ specified by MILSTD-150A [5] MIL-STD-150A was canceled in 2006 but slides are available from Edmund Optics [6] in the original format, and from Applied Images [7] in an updated format. The outer surface of the upper substrate is the surface that the user touches and it is subject to considerable wear. The upper substrate of most 4W resistive touchscreens is made of 125–175 mm PET, with a
13 mm layer of acrylic hard-coated material bonded to the outer (non-ITO coated) surface. The hard coat layer has a pencil hardness rating of 2–4 H, depending on the measurement standard used, which is considerably harder than the raw PET, but still wears out with use and is a relatively easy surface to damage with a hard, small radius stylus. One improvement which addresses the wear issue is a laminated layer of very thin (0.1 mm) glass applied to the outer surface of the upper substrate [8].

−2

−1 1

2

2

3

4 5 6 . Fig. 2 USAF Resolution test chart image

0

1

2 3 4 5 6

2 2 3 4 5 6

3

1 2 3 4 5 6

2 1

3 1 2 3 4 5 6

0 1

4 5 6

−2 1

Introduction to Touchscreen Technologies

5.7.1

Patterning, or isolation of the printed connecting circuits from adjacent electrodes on both touchscreen substrates is usually necessary for proper operation. Isolation is usually accomplished by a subtractive process etching or laser ablation of ITO around the circuit traces and the non-driven edges of the active area. An alternative that requires no etching or ablation is a totally additive process. Dielectric layers are printed over ITO, usually in the entire non-active area of the substrate. The connecting circuit and electrode are then printed, with the connecting circuit on the dielectric and the electrode on the exposed ITO surface at the edge of the active area. A second layer of dielectric may be further printed over the connecting circuit and drive electrode. It is also possible to achieve isolation by selective deposition of ITO during the coating process, but this process is economically viable only for high volume products. Separator dot size and dot array pitch in resistive touchscreens varies by manufacturer. The size – diameter and height – and the array design of the separator dots is generally a set parameter of the manufacturer. Dots are usually applied by a screen print process. The diameter of dots used in available touchscreens has been observed to be
0.07–0.2 mm. Dot maximum height, near the center of the dot, is usually
0.1–0.2 times the diameter of the dot. Most dots have a hemispherical cross section. This shape is largely a consequence of the surface tension and the viscosity of the deposited ink drop and sets the dot height. Dot height can be altered somewhat by changing the viscosity of the dot ink or adding fillers to the ink, but profiles outside the range given are generally not achievable. Larger dot diameters can be successfully printed with fine pitch polyester or stainless steel screens, but the smaller dots,
0.4 mm or less, must be printed with foil type screens which have open holes. These screens limit the flexibility of printing formulations, and the diameter to height ratios cannot be varied significantly. Occasionally dots are printed with the same material and at the same time as one of the dielectric layers for one of the substrates. A well-designed separator dot array must account for numerous requirements, including: ● Electrical – Dots must be fabricated from nonconductive material. Typical dots are made of UVcurable epoxies. Touchscreen insulation resistance should be at least 1MΩ, and values of 10 MΩ or more are frequently seen. – Capacitance. The touchscreen is a distributed resistor(R)-capacitor(C) network which can be modeled as a single section R-C low pass network when considering touchscreen capacitance: The resistance is the value of the resistivity of the pick-off substrate, while the capacitance is the capacitance between the two touchscreen substrates. The capacitance determines the settling time of a touch voltage at the input to the controller measurement circuit, and a long settling time may contribute to measurement errors. Most commercially available controllers have sufficiently fast A/D converters and long enough delay times to avoid settling time errors, and some controllers have a software adjustable settling time parameter. The problem is most likely to occur in embedded systems where a microprocessor with a ‘‘built-in’’ 4W controller function is used with a large touchscreen. The settling time is readily observed with an oscilloscope by probing the touchscreen connection to the controller input while the touchscreen is touched. – Wider dot pitch means fewer total dots in the active area of the touchscreen, and thus fewer dots to traverse when ‘‘dragging,’’ which is a frequent user operation in computer systems with graphical user interfaces (GUIs). Passing over a dot with a finger, and especially with a relatively small radius stylus, frequently results in loss of touch for a sufficiently long period that the controller reports this ‘‘untouch’’ event to the system.

943

944

5.7.1

Introduction to Touchscreen Technologies

● Mechanical – A narrow dot pitch increases the activation force and improves system rejection of unintended touches. Conversely, a wide dot pitch reduces the activation force and degrades unintended touch rejection. – Manipulation of the dot pattern can be one solution to a known problem of resistive touchscreens, that of the upper substrate sticking to the lower substrate as a result of a touch. This problem primarily occurs from cohesive forces between the materials involved but may also result from environmental conditions or simply a missing dot. Solutions may include surface treatment – usually texturing the substrate under the conductive layer for one or both of the substrates. However, this solution is not ideal, as surface texture on any of the inner or outer touchscreen surfaces degrades image resolution. Elo Touchsystems uses a non-square dot pattern to prevent the problem in their resistive touchscreens [9] but this approach is not widely used. – Dots may be attached to either conductive surface of the touchscreen. However, it has been repeatedly demonstrated that longevity of the dots is improved by attaching them to the lower (rigid) substrate. – One of the well-known failure mechanisms of resistive touchscreens is the failure of the flexible substrate in locations where frequent touches occur. In most resistive touchscreens, the flexible upper substrate rests on the crown of the dots over most of the touchscreen. When the touchscreen is touched, the repeated flexure of the substrate around the crown of the dot breaks up the ITO coating (ITO being the most common conductive coating) around the point of contact. The result is a loss of conductivity in this area, further resulting in inaccurate touch reports, progressing to complete loss of touch response in the area. Accordingly, the widest dot pitch consistent with other requirements of the system should be used to delay the onset of this type of failure. The technique of closely spaced dots to achieve ‘‘palm rejection’’ in resistive touchscreens (thereby making the touchscreen responsive to only small tipped styli or fingernails) has not been successful, generally resulting in very rapid ITO failures in frequently touched locations. ● Cosmetic, Optical, and Ergonomic – Large dots are readily visible when the underlying display is dark, and are sometimes reported as a customer complaint. Many users can see dots when the display is active, and can be distracting from the user experience. – The additional profile on the top substrate resulting from the dots can easily be felt by most users, especially in dragging operations. Customers may have some input to the dot array design, particularly as to where the closest dots to the edge of the active area located. As > Fig. 3 suggests, the dots are frequently an order of magnitude shorter than the height of the adhesive, and therefore are relatively insignificant in preventing accidental activation of the touchscreen near the edge of the active area. However, these dots may still interfere with easy activation of the touchscreen near the edge. Selective elimination of these dots, or moving the entire dot array further away from the edges of the active area may improve the edge activation characteristics. If the adhesive is closer in height to that of the dots, the location of the dots near the edge may be significant in setting the force to activate the touchscreen near the edge, but adhesive thickness close to the height of the dots is difficult to achieve.

Introduction to Touchscreen Technologies Upper Substrate (PET) with ITO Coating and Silver Electrodes

Active Area ITO

Adhesive Silver Electrodes ZIF Terminated Cable ITO

5.7.1

ACF

Adhesive

Separator Dots

ITO-Coated Glass, Printed/Fired Silver Electrodes, and Dots

. Fig. 3 Cross section of typical 4W touchscreen

Failure of 4W touchscreens due to stylus activation is also a significant problem. When used with a stylus, the damage caused by activation near the separator dots is aggravated by a small radius stylus tip. The small radius of the stylus tip decreases the radius of the deflection of the flexible upper substrate, especially when touches are made close to the dots, and results in more rapid cracking of the brittle ITO coating. Eliminating or significantly reducing the rate of this failure mechanism is another desired feature of alternative conductive coatings such as CNT and PEDOT. Early 4W touchscreens were constructed with integral interconnect cables, or ‘‘tails,’’ that were an extension of the substrate material, overprinted with an extension of the printed electrode material. These touchscreens were film–film construction, laminated to glass or rigid plastic back plates or used with displays with rigid front surfaces, such as cylindrical CRTs, plasma, or EL types. Anisotropic conductive adhesives (ACFs) were not available at this time or were still in development, so there was no reliable non-solder method to attach separately constructed tails. Touchscreens of this construction are undesirable in today’s market for several reasons: ● Integral tails will be damaged by small radius ($50USD/m2 in the current market, making any scrap expensive. ● Crimped pin and receptacle construction, usually on 0.100 centers for older touchscreen systems, is now too large for the small form factors of many electronic products. Today’s preferred construction is ‘‘zero insertion force’’ (ZIF) tails and connectors. ZIF construction, while possible, is more difficult and less reliable when implemented on an integral tail, as it must be used with a upper and lower contact connector (the conductive surface for each plane is on opposite sides of the substrate at the tail) and is printed silver rather than the preferred gold at the mating contact location. While the cost of a separately attached tail, ACF and processing cost must also be considered, the improved form factor and product suitability usually trumps the simplicity of the integral tail in 4W touchscreens today. The preferred construction for separate touchscreen tails is now

945

946

5.7.1

Introduction to Touchscreen Technologies

copper circuits on polyimide substrates. This construction is a considerable improvement over the previous generation of polyester-clad copper circuits, as the polyester-based construction, while flexible and suitable for small radius bends, is relatively stiff in the plane of the tail, and can easily transmit significant shear forces to the mating touchscreen contact pads. The polyimide tails are essentially the same as other flat flexible cable (FFC) products for electronics, and differ only in the way that they are typically terminated. Polyester tails are intended for solder and crimped pin and receptacle terminations, while the polyimide tails may be ACF, solder, or ZIF terminated. At the touchscreen termination, copper circuits may be exposed on one or both sides for ACF bonding to the touchscreen circuits. In a one-sided termination, all the circuits that are bonded to the tail terminate on the lower substrate, which is usually glass, as previously explained. Electrical ‘‘crossovers’’ or ‘‘interconnects’’ are required on the touchscreen to transfer the circuit termination on the upper substrate to the lower substrate. This is usually accomplished with silver epoxy or silver inks deposited between overlapping silver pads on the two substrates. Subsequently the tail is ACF bonded to the lower glass substrate. An alternate process that does not use crossovers can be done by exposing two copper circuits on each side of the tail, and bonding all the circuits with ACF in one bonding cycle to the two substrates, which have previously been laminated together in all areas except the tail location. The controller end of an FFC type tail is preferably a ZIF compatible termination, with a contact pitch of 1 mm or smaller. The exposed copper pads for each touchscreen circuit are over-coated with gold. A stiffener of a rigid or semirigid plastic is usually adhered to the backside of the area where the exposed contacts are laminated, especially when the contacts are all on one side of the FFC. Connection to the mating ZIF socket is achieved by inserting the tail into the slot in the ZIF socket, and then activating a compression mechanism – a slide or a lever – to force the socket contacts against the FFC contacts. The result is a gold–gold mating connection that is very low impedance and sufficiently reliable for the few mating cycles that a production environment touchscreen system should experience. The last material necessary for a minimally constructed 4W touchscreen is an adhesive to bond the lower and upper substrates together. The adhesive is applied in a ‘‘picture frame’’ shape between the outer perimeter of the touchscreen and the outer perimeter of the active area. The adhesive not only serves to bond the two substrates together but also provides some degree of electrical insulation between the two substrates and a barrier to the ingress of contamination into the active area. The adhesive may also provide some mechanical strain relief for the interconnect cable. Some touchscreen vendors use the adhesive as the only electrical insulation between the two substrates, but the preferred method is to apply a dielectric layer over the circuits of each substrate. The adhesive construction of choice for most touchscreens is pressure-sensitive acrylic resin on both sides of a plastic film carrier, although screen-printed adhesive is sometimes used on smaller units. In either case, acrylic resins are preferred for high shear and peel strength, and in some touchscreens are used because of good transparency. In simple constructions where no dielectric layers are applied, the inside edge of the adhesive may also define the edge of the active area of the touchscreen. While acrylic adhesives are preferred, they must be compatible with the ITO coatings used as the conductive planes. Many acrylic adhesives have significant amounts of free acrylic acid in the resin, and this acid will damage the ITO coatings, particularly on plastics [10] over time, so acid-free formulations must be used. The basic materials discussed above are all commercially available from numerous suppliers. Conductively coated glass and plastic substrates have been refined from offshoots of the display industry into products very specific for touchscreens, and the 4W touchscreen industry

Introduction to Touchscreen Technologies

5.7.1

has benefited significantly from this effort. These materials and the accompanying inks and adhesives necessary to produce a finished product are widely available in China, South Korea, and Taiwan, where both touchscreens and electronic displays are primarily manufactured today. The basic assembly process of a typical subtractive process 4W touchscreen is summarized as follows: ● ITO-coated glass or plastic substrate is patterned by etching, laser ablation, or selective coating of the ITO. ● Electrodes and connecting circuits are screen printed on both substrates. ● Separator dots are printed on one substrate. ● Dielectric overcoat is printed on one or both substrates. ● Interconnect cable is bonded to the touchscreen circuits by solder or ACF to the lower substrate. ● Crossover silver ink or conductive adhesive is applied to crossover area. ● Adhesive is laminated to one (usually the lower) substrate. ● Upper substrate is laminated to the lower substrate. The low cost and short field life of 4W touchscreens militates against significant enhancements to the touchscreen that could improve its usability and durability. While many options are possible, especially in the area of optics, they are seldom employed in 4W touchscreenequipped systems.

2.1.4 ● ● ● ● ● ● ● ● ● ●

4W Advantages

Versatile-arbitrary stylus operation Low cost Simple design Small borders, versatile (separate) tail constructions Easy environmental seal Relatively insensitive to EMI Low cost controllers ‘‘Free’’ embedded controllers available in some microprocessors Low power consumption Many suppliers – more than 60 known in Asia

2.1.5

4W Disadvantages

● Poor durability, especially with stylus operation ● Poor optics – low VLT, high reflectivity – make it unsuitable for outdoor use ● Single-touch operation only – no multi-touch

2.1.6

Summary

Despite having poor durability and optical performance, four-wire touchscreens persist in the touchscreen market because of very low cost and ease of implementation, particularly in

947

948

5.7.1

Introduction to Touchscreen Technologies

embedded system products where the microprocessor has an on-board 4W touchscreen controller. In benign environments, the performance of 4W is ‘‘good enough,’’ making it still the choice for many applications.

2.2

Eight-Wire Resistive

The eight-wire (8W) resistive touchscreen is an enhanced 4W touchscreen. The enhancement is essentially what is known as ‘‘remote sensing’’ in electronic power supplies – an extra set of electrodes that detect the voltage at the load and adjust the source voltage to compensate for the voltage drop caused by the resistance of the circuit between the source and the load.

2.2.1

Design

> Figure 4 illustrates the typical design configuration for an 8W touchscreen, showing one substrate only. In the application to touchscreens, the remote sense leads are connected to the touchscreen electrodes at the same point as the drive circuits from the controller. At the controller end, the remote sense leads are connected to a measurement circuit input and provide the reference to correct the controller drive voltage.

2.2.2

Materials and Construction

There are no significant differences in materials or construction between 4W and 8W touchscreens. Refer to the 4W > Sect. 2.1.3 of this chapter for details of materials and construction for both types.

Isolation lines

Cable attach

Remote sense circuits

Cable attach area for other substrate

. Fig. 4 One substrate of eight-wire (8W) touchscreen

Drive circuits

C/L

Introduction to Touchscreen Technologies

2.2.3 ● ● ● ●

5.7.1

8W Advantages

Versatile-arbitrary stylus operation Easy environmental seal Low power consumption Multiple suppliers, but less frequently offered than 4W

2.2.4

8W Disadvantages

● Poor durability, especially with stylus operation ● Poor optics – low VLT, high reflectivity – make it unsuitable for outdoor use ● Single-touch operation only – no multi-touch

2.2.5

Summary

The remote sensing feature compensated for resistance changes in the tail circuits, and in the early days of resistive touchscreens was often included in larger models. Integral tail configurations were common, and tails were often quite long. Damage from inappropriate bending or flexing during installation did occur, and remote sensing compensated for it. Remote sensing also was sold as an automatic calibration feature, and for this purpose worked very well. By keeping the voltages between the electrodes on each substrate fixed, the coordinate output for a touch in any given location was known. Calibration constants based on the touchscreen output coordinate range were included in the application software or the system calibration file, and no manual calibration by the user was required. The feature was useful in embedded systems where code space was very limited, as no user calibration program was included. It also streamlined manufacturing system checkout, as in theory no calibration needed to be performed there either. The benefits of the 8W remote sensing feature were greatly oversold, and claims were made that the feature could compensate for electrode deterioration on the touchscreen itself, in addition to compensating for resistance changes in the tail circuits. These claims cannot be supported on any engineering basis, but persist today. The issue is moot, however, as 8W touchscreens are sold today almost exclusively as replacement product for legacy designs.

2.3

Five-Wire Resistive

Many different five-wire (5W) touchscreen designs exist. All commercially successful designs today are ones which produce orthogonal, linear electric fields on the touch surface when driven by an appropriate controller. No nonlinear transformation from the received analog voltages from the touchscreen is performed. Consequently, the generated touch coordinates can be directly mapped into a Cartesian coordinate space, as is true with the 4W touchscreens discussed earlier. However, there are other designs which do not depend on orthogonal linear fields. These designs require conversion of the received non-Cartesian touch voltages into a display coordinate space which is Cartesian, by a lookup table or mapping algorithm of some sort. These designs require a significant increase in the computational capability of the

949

950

5.7.1

Introduction to Touchscreen Technologies

controller, but also have a significant decrease in the design complexity and manufacturing precision of the touch substrate and electrode array [11] Despite the improvements in today’s microprocessor technology, no manufacturer has introduced one of these simplified touchscreen systems. This section will thus concentrate on those designs which use the orthogonal field concept. Five-wire (5W) touchscreens share many of the characteristics of 4W touchscreens, including the basic materials and construction techniques. The significant difference between the two types is that the orthogonal electric fields that provide the touch location information are developed on a single substrate – the lower substrate. The 5W orthogonal fields are sequentially developed on the lower substrate by manipulating the controller drive signals, and the upper substrate is used exclusively as a pick-off layer.

2.3.1

Principle of Operation

Sequential orthogonal fields are developed on the 5W lower substrate by employing resistive electrodes arrayed along the edges of the active area of the touchscreen. The ends of each electrode array are electrically connected to the arrays on the adjacent edges. When a touch occurs, the controller provides drive signals that produce alternating orthogonal electric fields on the lower substrate conductive plane. The conductive surface of the upper substrate is in contact with the lower substrate during both measurement cycles, picking off a voltage proportional to position for both axes. The upper substrate is continuously connected to the single measurement input of the controller.

2.3.2

Design

Five-wire touchscreens use a resistive electrode array in conjunction with specific controller drive signals to achieve the conditions for linear touchscreen performance. The controller drive signals are shown in > Table 1. In practical touchscreens, the resistive electrode array consists of series and/or parallelconnected ‘‘strings’’ of resistors formed by small parallel conductive electrodes with the ITO transparent conductive coating of the substrate between the electrodes. Selective removal of the ITO coating around these resistors adjusts the resistor values and connections to the active area. In addition, some designs employ contact electrodes that extend from the array toward . Table 1 Five-wire touchscreen controller drive signals Touchscreen electrode node Controller drive state

Upper left

Upper right

Lower left

Lower right

Idle (waiting for touch)

H

H

H

H

Horizontal (X ) drive

L

H

L

H

Vertical (Y ) drive

H

H

L

L

H Controller high drive potential L Controller low drive potential

Introduction to Touchscreen Technologies

5.7.1

the active area of the touchscreen. The shape and position of these electrodes effectively create another array resistor, and are always figured into the design of the array. The equivalent circuit of a typical array is shown as > Fig. 5. Arrays are always symmetric around the centerline of the active area in both axes. Referring to > Table 1 and > Fig. 5, the nature of the array design problem can be understood. Consider a Y-axis drive sequence, with a ‘‘high’’ potential, designated ‘‘H’’ applied at the top left and top right corners of the array, and a ‘‘low’’ potential ‘‘L’’ applied at the bottom left and bottom right corners. In this example, it is assumed that current flows across the active area only through the branching resistors RB. Conventional current will flow from both top corners through the series resistors RS toward the center of the array, and then through the branching resistors RB. This current then flows across the active area of the touchscreen, and back into the resistor string at the bottom of the active area. Additionally, current will flow through the lateral resistor strings along the left and right edges of the active area. If the resistor array is properly designed, little or no current will flow into the active area from these lateral strings through another set of branching resistors. This requires that the voltage drop in the lateral strings approximately matches the voltage drop across the active area of the substrate. The branching resistors RB may represent actual resistors formed similarly to the series resistors RS, but more typically model electrodes of different sizes that have different effective resistances. The need for different branching resistances can be understood by considering the effect of current flow through the series and branching resistors. There is a potential drop at each series resistor, and neglecting the branch current for a moment, it is apparent that the potential at the junctions of each series resistor is progressively reduced. Without some compensation for this potential drop, equipotential lines near the upper resistor string will not be straight; rather, they will be bowed. The center of the bow will be closer to the upper string than the ends of the bow, as a given potential on the active area near the center of each string will be much closer to the string than the same potential near the corner of the array. The compensation is achieved by increasing the resistance of the branching resistors closer to the corners of the array. In practice, this increase is often accomplished by making the effective contact area of the branching electrode in question smaller. Localized variations in the electric field frequently occur near the non-driven arrays in each axis. Much of this variation is due to the discrete nature of the branching resistors, which cannot perfectly source or sink a uniform current source across a broad area (the entire length of a resistor string) with the branching resistor terminations located very close to the active area of the touchscreen. One strategy for improving the edge performance is simply to move the entire array back from the edge of the active area. When the physical space in the design is available, this setback of the array is the easiest solution. Another strategy that was sometimes employed to correct both the local variations and the more general equipotential bow, when space was available, was to create a reverse bow in the physical layout of the resistor strings or the branching resistors. The physical layout of the branching resistor or electrode terminations was the mirror image of the predicted bow of the equipotential lines [12]. However, constraints on the overall size of the touchscreen compared to the active area size usually do not permit large setback distances or the reverse bow concept. Accordingly, much effort is devoted to optimizing the edge performance of the touchscreen by modifications to the basic design with additional electrodes, and additional deletion of the ITO. Many intricate designs with several rows of near continuous electrodes beyond the basic series and branching resistor strings are often seen. Another aspect of the electrode design is the eventual power consumption of the controller. In a 4W touchscreen system, the current across each driven sheet is dependent only

951

RB

RB

RB

RB

RS1

RB

RS1

RB RB

RB RB

RB RS1

RB RS1

Current flow

RS1 RB RB

RS1

Active area outline

RB RB

RS1

RB

RS1

RB

RS1

RB

RS1

Equipotential lines for optimized RB

Equipotential lines for equal RB

. Fig. 5 Five-wire (5W) touchscreen electrode array equivalent circuit

L

RS2

RS2

RS2

RS2

RS2

BR

RB

RB RB

BR

RB

RS1

BR

RS2

RS1

BR

RS1

RB

RB RB

RS1

RS1

RB

RB

RB

RB

RB

L

RS2

RS2

RS2

RS2

RS2

RS2

H

5.7.1

BR

RS1

BR

H

952 Introduction to Touchscreen Technologies

Introduction to Touchscreen Technologies

5.7.1

on the resistivity of the conductive coating and the aspect ratio of the substrate. The resistivity of the substrate can be selected to limit the current if necessary. In the 5W touchscreen, significant current flows through the lateral resistors during each drive cycle of the controller, and the electrode design must be managed with the eventual controller current in mind. This is not an issue when touchscreens and controllers are provided by the same manufacturer, but some third-party controllers are limited in current source/sink capability and this parameter is frequently not specified in third-party controller documentation. Five-wire touchscreen electrode design has been greatly simplified in recent years by the availability of low cost finite element analysis (FEA) computer software. A basic design can be adapted to different sizes and modified to correct predicted deficiencies by using a DC circuit model available in almost any FEA program. Electrode designs can be drawn in 2D CAD packages and then imported into the FEA program. Solving the equations can be performed in a matter of minutes, and the process of modifying and then re-solving the equations is dependent only on the designer’s understanding of the underlying electrical principles of the design. The FEA program may also calculate the total power consumption of the touchscreen for each drive state. Upper substrate designs for a 5W touchscreen are very simple. All that is required is provision for a crossover contact between the upper and lower substrates, assuming that the tail is connected to the lower substrate. If the crossover does not require patterning or processing of the upper substrate itself (a typical configuration) then the upper substrate may be nothing more than a piece of conductively coated plastic film. Many variations are possible. One such design reduces the magnitude and variation of the resistance in the upper substrate circuit by applying a continuous conductive ring around the perimeter. As the ring is confined to the non-active area of the substrate, the only disadvantages to this concept are the cost of the printed material (silver, and in some designs, an overcoat of dielectric material) and the process step. Compared to a localized contact directly over the crossover area, which may be no more than 2–3 cm long, the conductive ring addition may reduce the total circuit resistance and the resistance variation of the upper substrate by 50% or more. Resistive touchscreens are not severely affected by EMI, and very few 4W or 5W types are designed with an EMI shield except for aerospace and military applications. The configuration of the upper substrate, being separated from the usual display behind the touchscreen by the lower substrate, also serves to provide some additional degree of EMI immunity for the 5W touchscreen. When this EMI suppression is insufficient, the first improvement is usually to apply ferrite beads to each individual cable circuit, then to shield the entire cable, and then to add an EMI shield to the backside of the lower substrate. A backside EMI shield is a significant change to the standard resistive design, as these shields may have a resistivity of 10 Ω⁄□ or less, and a VLTof 80% or less. Further, the shield coating must be applied directly to the backside of the lower substrate or applied to an additional substrate that is subsequently laminated to the lower substrate to avoid additional losses in VLT and additional reflections.

2.3.3

Materials and Construction

Materials and construction processes used to manufacture 5W touchscreens are identical to those used for their 4W counterparts. The only exception is the relaxed requirement that the upper substrate be uniform to the extent required for an acceptable 4W touchscreen. The reason for this is that the circuit resistance of the upper (pick-off) substrate is
1,000 times less than the input impedance of the measurement circuit of the controller and nonuniform substrate

953

954

5.7.1

Introduction to Touchscreen Technologies

resistance therefore has negligible effect on the accuracy of the voltage at the controller input. In practice, the same grade of material that is normally used for 4W touchscreen substrates is also used for the 5W touchscreen pick-off layer. The upper substrate will degrade with use just as it does in a 4W touchscreen, but the functionality of the 5W touchscreen is not affected until the ITO coating is substantially destroyed in a local area. The field life of a 5W touchscreen is generally expected to be more than ten times that of a 4W touchscreen. One aspect of the more complicated electrode array design of 5W touchscreens is manufacturing strategy. Five-wire lower substrates, when complete, can be tested without the upper substrate being present. A conductive stylus is substituted for the upper (pick-off) substrate, and the lower substrate can then be tested for linearity. In high volume manufacturing, it is desirable to produce all substrates with perfect linearity. This is seldom achievable for many reasons, including: ● Variations in the general resistivity of the conductive coating. The typical magnetron sputtering process is rarely held to better than +/10% tolerance at acceptable commercial production rates. ● Local variations in the resistivity of the coating resulting from such things as glass flaws and spatter from the sputtering process. Magnetron sputtering is a very complex process based on a simple idea. ● Patterning of ITO during sputtering. Some manufacturers use coated glass with formed resistors and deleted ITO from the glass vendor. The masking material and the deposited (usually silver) electrode material introduce variation in the ITO deposition process. ● Printed silver electrode designs (non-fired) may have variations in the resistance of the silver–ITO glass interface. ● Imperfect in-process ITO patterning, which may be done by acid etching, mechanical ablation, or laser ablation. Accordingly, touchscreen manufacturers may build in the capability to adjust the linearity of the design by localized removal of ITO and electrode material as an in-process step. This process can be anything from a simple removal of a small amount of ITO from the resistors at each corner to extensive ITO and electrode removal at multiple locations on the array. With an electrode design that can be modified as an in-process step, manufacturing yields can be significantly improved. One significant variation on the 5W touchscreen concept eliminates most of the difficulties caused by the voltage drop along the resistive electrode array, and also eliminates the problem of the large current draw in the two non-driven edges during each drive state of the controller. This variation is known as the ‘‘diode’’ or ‘‘three-wire’’ (3W) touchscreen. In the diode touchscreen, small-signal diodes are used to control the flow of current on a lower substrate identical to that of a ‘‘conventional’’ 5W touchscreen. With appropriate spacing and setback of the diode substrate terminations from the edges of the active area of the touchscreen, a set of equal potentials at each of these terminations on one edge and another set of equal potentials on the opposite edged and will produce straight equipotential lines on the active area of the substrate. This is exactly the condition necessary for a linear touchscreen. Referring to > Fig. 6, it can be appreciated that there is negligible variation in potential at the two common nodes for the diodes, and given a uniform conductive substrate, that the current through each diode that is biased on will be approximately equal. Thus, the drive conditions to produce straight equipotential lines on the substrate appear to be achievable with DC drive, and this is in fact true.

Introduction to Touchscreen Technologies

5.7.1

Small signal diodes, 1N4148 or similar Active area

Y Drive

X Drive

. Fig. 6 Three-wire (3W) touchscreen lower substrate and drive circuit

Additionally, it can be seen by referring to > Table 1 that the necessary drive conditions for the diode touchscreen are the same as those required for a conventional 5W touchscreen. Diode touchscreens can be driven from 5W touchscreen controllers using the two alternating potential circuits only, and the upper substrate circuit is identical to that of any 5W touchscreen. Thus, the complete touchscreen requires only three circuits to function – a three-wire touchscreen. The diode drive circuit produces very little field distortion on the non-driven edges, so the edge linearity of the touchscreen is very good. There is no power loss in the nondriven edges, so power requirements are similar to that of 4W, and determined by the resistivity of the lower substrate and aspect ratio of the touchscreen. Diode touchscreens are produced today by only one significant manufacturer – Fujitsu. An outstanding weakness of all resistive touchscreens is poor sunlight readability. A standard design with no optical enhancements has four reflective surfaces, and two layers of conductively coated substrates with varying degrees of light attenuation. The relatively long field life of a 5W resistive touchscreen makes the consideration of optical enhancements such as higher light transmission coatings, polarizers, antireflective surfaces, and other optically matched interfaces a reasonable proposition for some applications. Most of these enhancements require additional or different materials and processing steps, but none of them are basically incompatible with the usual assembly techniques of resistive touchscreens. Methods for implementing improved sunlight readability are explained in the additional reading listing.

2.3.4 ● ● ● ●

5W Advantages

Versatile – arbitrary stylus operation Simple design – with the aid of FEA software Small borders, versatile tail constructions Easy environmental seal

955

956

5.7.1

Introduction to Touchscreen Technologies

● Much better field life than 4W ● Better EMI rejection than 4W ● Many suppliers, decreasing price differential between 4W and 5W

2.3.5

5W Disadvantages

● Poor optics – low VLT, high reflectivity – make 5W unsuitable for outdoor use without enhancements ● Higher power consumption than 4W touchscreen systems ● Few embedded controller solutions ● Single-touch operation only – no multi-touch

2.3.6

Summary

Five-wire touchscreen systems persist in the marketplace. They are cost effective and reliable in some markets, notably point of sale (POS), industrial controls, and low cost portable/mobile devices. Improvements in upper substrate materials would provide a significant extension of field life and likely continue the popularity of this technology. Five-wire touchscreens have substantially become an international commodity product, and interoperability between touchscreens and controllers from different manufacturers is widely assumed, at the system designer’s peril.

2.4

Resistive Multi-touch

Interest in computer systems applications capable of supporting multiple simultaneous touches is long standing. However, implementation of such capability was limited by the lack of computer operating systems natively supporting more than one GUI screen pointer, and by the lack of touchscreen hardware capable of resolving and reporting multiple touches. Fourand five-wire resistive touchscreens are not capable of resolving multiple touches; rather, these technologies effectively average multiple touches. Operating system support for at least two touches is now available from several sources, including Apple, Microsoft, and several Linux distributions. Hardware that can support multiple touches is now being developed in two resistive touchscreen variants – digital and analog.

2.4.1

Digital Multi-touch Resistive (DMR)

The operating principle for this technology is transparent conductive crossbar switches. The switches are comprised of narrow stripes of transparent conductors, closely spaced and individually connected to the controller, with conductors arrayed in the horizontal axis on one substrate and in the vertical axis on a second substrate. A matrix of switches is created, with resolution that is limited only by the pitch and width of the individual stripes in each axis.

Introduction to Touchscreen Technologies

5.7.1

The construction of DMR touchscreens closely resembles the 4W and 5W touchscreens discussed earlier, with closely spaced substrates separated by a grid of separator dots. The rows and columns of this matrix are scanned for activity by the controller to determine the location of a touch, or touches. Touchscreens based on the switch matrix principle have been available for many years, but generally had limited resolution and only supported a single touch. The controller is the difference between the older digital designs which supported single touch and today’s DMR. The original digital resistive concept used a diode matrix or a simple voltage divider scheme to decode the touch location. For the multiple touch support, the DMR controller can account for two or more touches along the same conductive stripe in one axis. Regardless of the support for single or multiple touches, the outstanding disadvantages of DMR are the extensive patterning of the individual substrates that is required, and the accompanying tail required to bring out the termination for each stripe to the controller. Another issue is the potential deterioration of the upper flexible resistive substrate, which is again coated with the relatively fragile ITO, noted elsewhere. Despite the obvious problems of DMR touchscreens, they represent a potentially less expensive alternative to projected capacitive touchscreens for multi-touch support. DMR touchscreens have not yet achieved wide commercial acceptance.

2.4.2

Analog Multi-touch Resistive (AMR)

AMR is another approach to multiple touch resistive touchscreens. The operating principle is that of multiple narrow 4W resistive touchscreens. Stripes of transparent conductive coatings are arrayed on one substrate. The stripes are typically
1 cm in width. Stripes of similar width are orthogonally arrayed on a second substrate. These substrates are assembled similarly to other resistive touchscreens with close separations and separator dots to insulate the two substrates. At the intersection of each stripe on the two substrates a small 4W resistive touchscreen is created. Sequential driving of the stripes of one substrate while looking for activity on one stripe at a time on the second substrate can resolve multiple touches that are separated by at least the gap between adjacent stripes. AMR suffers from the same problems as 4W resistive touchscreens, in that the upper substrate will deteriorate over time. The deterioration is accelerated by sliding touch contacts, especially when the touchscreen is activated by a small radius stylus, just as in 4W. Since this is one of the intended modes of operation for such a touchscreen, it would seem obvious that the AMR touchscreen needs a more durable upper substrate than the traditional ITO-coated PET. AMR touchscreens have not yet achieved wide commercial acceptance, but do represent a lower cost alternative to projected capacitive touchscreens for multi-touch operation.

2.4.3

Summary

Multi-touch resistive touchscreens are being developed. AMR and DMR have achieved limited commercial success, but still represent a lower cost alternative to projected capacitive touchscreens. The production of these touchscreens is also within the capabilities of a large, well-established manufacturing base in Asia currently manufacturing more conventional 4W and 5W resistive touchscreens.

957

958

5.7.1 3

Introduction to Touchscreen Technologies

Capacitive

The term ‘‘capacitive’’ describes two very different touchscreen technologies. Surface capacitive (SCAP) is an analog device that is essentially a 5W resistive touchscreen without an upper substrate, driven with an AC signal of 10 KHz or higher frequency. The user’s finger or conductive stylus provides the return circuit for the drive signal. Projected capacitive (PCAP) is a matrix of discrete electrodes that resembles a digital resistive touchscreen in construction, but which depends on the perturbation of the capacitance of each electrode, or mutual capacitance among crossed electrodes for position detection.

3.1

Surface Capacitive

Surface capacitive touchscreens enjoyed early success as a more durable alternative to resistive touchscreens. With no plastic upper substrate to damage or wear, SCAP was perceived as the answer for demanding touch applications. The development of SCAP touchscreens coincided with the proliferation of state-run lottery games in the USA and SCAP touchscreens became the input device of choice for many of these systems. With that success, SCAP became a mainstream touch technology.

3.1.1

Design

SCAP touchscreens are almost exclusively built on glass substrates. Low-temperature sputtered coatings and any other mechanically applied coatings to plastic substrates are not durable enough for the field life demanded of touchscreen systems. With glass substrates, the same considerations apply as with resistive touchscreens. As previously mentioned, SCAP touchscreens are essentially 5W resistive touchscreens without an upper substrate. One important addition is a transparent dielectric coating on the front surface of the substrate, which protects the substrate conductive coating and is frequently used to provide an antiglare finish. Another aspect of the design is that a substrate back surface conductive layer is frequently applied as an EMI shield. With this EMI shield, the optical improvement often cited as an advantage of the SCAP designs versus resistive is diminished. With a dielectric front surface, a DC drive signal will not couple to the user’s finger or a conductive stylus. Consequently, the SCAP controller drive is an AC signal, with a frequency of 10 KHz or higher. Adjustment of the drive frequency may be done for improved EMI rejection, and some advanced controllers have automatic frequency selection [13]. The lack of an upper substrate circuit in SCAP touchscreens demands a different touch event processing method for the controller. The usual method is to compute ratios of the currents through adjacent corner nodes of the electrode arrays to the total current through the nodes. The ratios are proportional to the distance of the touch to the appropriate edge of the electrode array [14]. The success of this method requires a linearized touch surface for the substrate, just as in resistive touchscreens. In a good design and under best conditions, SCAP touchscreens have extremely good sensitivity, requiring only slight contact with the dielectric coating to produce a touch response. This characteristic makes SCAP a frequent choice for amusement games which require fast response and sliding touches. However, SCAP touchscreens sometimes suffer

Introduction to Touchscreen Technologies

5.7.1

from an apparent lack of sensitivity when used by people with dry fingers or very small fingers (small children!); these users may not conduct sufficient signal to give consistent touch readings. Related to this problem is the lack of good response to gloved hands. The only gloves which give good SCAP activation are thin plastic or rubber gloves. Another problem results from poor ground return paths, which also causes poor touch sensitivity. This often makes SCAP a poor choice for portable systems Changes in the touch current magnitude resulting from changes in available ground paths should not result in changes in apparent touch location, but this problem is observed in some capacitive touch systems also. This ‘‘drift’’ in touch location, sometimes requiring frequent recalibration of touchscreens in the field has historically been one of the most troublesome features of SCAP touchscreens.

3.1.2

Materials and Construction

The substrate conductive coating for an SCAP touchscreen may not be ITO as is typically used in resistive touchscreens; rather, harder coatings such as tin-antimony oxide and pyrolitic tin oxide (TO) may be used, with some sacrifice in VLT. Even with the harder coatings, dielectric overcoats of sol gel or sputtered SiO2 are still applied. Backside EMI filter coatings may be sputtered onto a pyrolitic TO substrate without altering the TO resistivity. In addition to backside EMI coatings, one technique used to reduce the occurrence of drift is to also shield the entire electrode area of the touchscreen and extend this shield to the controller end of the cable. The technique usually involves shields constructed from metal foil with conductive adhesive. Copper foil is preferred as it can be soldered at critical junctions where the performance of the conductive adhesive may be unsatisfactory. In a typical construction, the foil is connected to the backside EMI shield, wrapped around the edge of the glass, and extended over a dielectric layer applied on top of the electrode array. Star grounding is essential for the proper performance of this shield, and further requires that the wraparound shield not come in contact with any system ground other than the controller. Aside from the above considerations, there is little difference in the basic construction of the SCAP substrate from that of a 5W resistive lower substrate. > Figure 7 shows a cross section of a typical SCAP touchscreen.

EMI guard ring Electrode array

SiO2 or Solgel dielectric layer

EMI Shield/Guard ring connection

Glass

EMI shield layer (ITO) Ground Lead to Controller

. Fig. 7 Typical SCAP touchscreen construction

ITO Coating

959

960

5.7.1 3.1.3

Introduction to Touchscreen Technologies

Surface Capacitive Advantages

● Greatly improved durability over resistive touchscreens ● High VLT compared to most resistive touchscreens ● Very good touch sensitivity and dragging characteristics, without skips from separator dots of resistive touchscreens ● Easy environmental seal, although conductive liquids may pose difficulties

3.1.4

Surface Capacitive Disadvantages

● Limited non-finger input – gloves other than thin plastic or rubber do not work, and a dedicated conductive stylus is almost required for any realistic stylus operation. ● EMI and touch sensitivity problems in portable applications. ● Front surface optical treatments, such as polarizers, cannot be applied over the dielectric layer, further limiting portable applications. ● Single-touch only. ● No embedded controllers available in system microprocessors. ● Higher overall cost than typical resistive solutions.

3.1.5

Summary

SCAP has some several advantages over its resistive cousins, and has a few markets, notably amusement and wagering, where it is widely used and performs well. However, its relatively high cost has spurred the gaming markets to find cheaper solutions and this customer base is eroding. The significant disadvantages of SCAP – high cost, lack of input flexibility and EMI issues in some applications, and the lack of multi-touch capability – would appear to limit its future.

3.2

Projected Capacitive

The PCAP touchscreen concept depends on measurement of small changes to a very small nominal capacitance between conductive electrodes in two orthogonal arrays (mutual capacitance) or the capacitance of each individual electrode to ground or a common electrode (selfcapacitance). Although the self-capacitance design is used in some products, designers are more interested in the mutual capacitance designs, for one particular reason – the ability to track two or more touches simultaneously. This chapter will focus on the mutual capacitance method only, though the operating principles and the construction of both types are quite similar. The controller is a very significant part of the PCAP touchscreen system, and the theory and design of a typical PCAP controller is covered in > Chap. 5.7.4.

3.2.1

Design

Substrates for PCAP touchscreens may be glass or plastic, or a combination thereof. In the mobile telephony market, which has been the most popular application for PCAP touchscreens in recent years, the most popular substrate material is CS glass.

Introduction to Touchscreen Technologies

5.7.1

PCAP arrays are usually constructed both as narrow stripes of transparent conductors such as ITO, or of small gauge wire, and are separated by a dielectric of
1 mm or less. Adjacent stripes in each of the arrays are separated by a distance of 5–10 mm, allowing some coupling between a user’s finger and at least two electrodes at the same time. While the total of the parasitic and nominal capacitance between any two electrode intersections may be as much as 100 pF, the change in capacitance caused by the proximity of a user’s finger near an intersection is
1 pF or less, and the effect on the adjacent electrode may be less than 0.1 pF [15]. The high resistance of narrow electrode stripes in combination with the extremely low capacitance has limited the size of PCAP touchscreens. A further problem is the number of ports available for electrode I/O lines, which necessarily expands as the size of the touchscreen increases. One design concept to overcome both of these problems is to use multiple controllers slaved together. Each processor controls a portion of the touchscreen. The multiple touch capability of PCAP comes from the discrete method of analyzing the capacitance changes at or near the intersections of the electrode. Regardless of the specific method used to sample the capacitance of the touchscreen, each change in capacitance which exceeds the system threshold is mapped and will be reported as a touch if there is a valid pair of sensed touch locations. The design of most systems suggests that the minimum resolvable distance between multiple touches is approximately the spacing between adjacent electrodes in each plane. In addition to multiple touch capability, the sensing method for PCAP also provides the capability for adaptive background mapping of the nominal capacitance for every electrode array intersection. By continuously scanning the arrays for the static capacitance when no touch activity is occurring, the controller can effectively ‘‘learn’’ the background capacitance, improving the uniformity of the touchscreen sensitivity, and improving the EMI rejection of the system. Some PCAP systems have sufficient sensitivity that they can sense touch through very thick front protective lenses or at some distance from the touchscreen system through air. Most systems, however, are adjusted for best performance with finger contact to the front lens surface. Under this condition, most PCAP systems will not detect touch when activated by a gloved finger unless the glove material is thin rubber or plastic. Although the most prevalent design model for PCAP systems in the mobile/portable market is a tightly integrated touchscreen and controller, with the controller chip usually embedded in the tail of the touchscreen, many of the controllers are not designed and manufactured by the manufacturers of the touchscreen itself. There is a growing base of third-party controller manufacturers for PCAP touchscreens. Interoperability is a potentially significant issue for users of controllers from these relatively inexperienced companies.

3.2.2

Materials and Construction

As two electrode arrays are required for PCAP operation, the formation of the arrays is of considerable importance. The possible methods for transparent conductors are selective removal and selective deposition of material. If one of the electrode array substrates is made of plastic, selective deposition is an efficient way to create the array in mass production. However, one technique for minimizing the optical effects of adjacent bands of coated and uncoated material on each substrate is to effectively eliminate the uncoated area between adjacent electrodes by ‘‘floating’’ the area rather than eliminating it. This is accomplished with

961

962

5.7.1

Introduction to Touchscreen Technologies

a very narrow – 0.1 mm or less – deletion line of the transparent conductor between electrodes and the adjacent floating areas. This feature size may be difficult to obtain in a selective deposition process, and the preferred method is selective deletion by etch or ablation. The dielectric constant between the two electrode arrays must be stable for consistent performance. This dictates that arrays on separate substrates be laminated together. The performance of pressure-sensitive adhesive (PSA) laminations between glass and plastic substrates has not been a reliable lamination method for PCAP, and the cost of dry film or wet – silicone or epoxy – lamination is a concern. This design consideration thus favors the use of substrates with electrode arrays formed directly on both sides of a single substrate. Each electrode in an array must be connected to an I/O line from the system microprocessor. The connecting traces between the processor and the electrodes are long and subject to cross talk from adjacent traces. Two construction techniques result from these problems – ground traces between adjacent connection traces, and placement of processor chips in the tail of the touchscreen. A further benefit of this construction is that the need for a backside EMI shield is frequently eliminated if the controller also has adaptive background measurement capability. The low resistance of the fine embedded wire construction of several PCAP manufacturers, notably Zytronic, permits very large touchscreens to be assembled. The
0.01 mm diameter of the wire makes it nearly invisible at typical user distances, and it does not interfere with the projected image from the display. The construction of the wire type touchscreen is tedious, however, and the relative cost of this construction is high. With the above considerations in mind, the possible touchscreen substrate stack-ups are: ● ● ● ● ● ●

ITO on two substrates of PET ITO on one substrate of PET and one substrate of glass ITO on two substrates of glass ITO on both sides of a single glass substrate ITO plus dielectric plus ITO – two layers on the same side of a glass substrate Wire between two glass, one glass and one PET or two PET substrates, with a thin flexible insulator between the wires

Representative construction of a double-sided single glass construction is shown as 8. Discrete substrates with one array are always laminated together, and frequently a cover glass or PET layer is also laminated on top of the touchscreen stack for protection and durability. Some applications may additionally require a backside EMI shield, as previously mentioned.

> Fig.

Cover glass (Touch surface) X Electrode cable attachment point

Optical index matching lamination

Double-sided ITO Coated Glass X Electrodes (ITO) Y Electrodes (ITO+SiO2)

. Fig. 8 Typical construction of single glass PCAP touchscreen

Optical fill regions

SiO2 and ITO etch

Introduction to Touchscreen Technologies

3.2.3 ● ● ● ● ● ● ●

PCAP Advantages

Excellent sensitivity with bare finger activation Multi-touch capability Excellent durability with glass front designs Unaffected by most contamination Supplier base for touchscreens and controllers is increasing rapidly, reducing cost Excellent VLT Relatively simple integration

3.2.4 ● ● ● ● ●

5.7.1

PCAP Disadvantages

Limited stylus capability Controllers are expensive and complex Some EMI susceptibility Size may be limited depending on the controller chip used Potential controller/touchscreen interoperability issues

3.2.5

Summary

Improved capability of modern microprocessors has been substantially responsible for making the revival of this old and largely discarded technology possible. Apple Computer clearly had a major part in making this revival successful. The mystery is in why 3M, a major player in the touchscreen industry, failed to recognize the potential in two predecessor products based on the same principles as the current generation of PCAP products, until another company launched their own wildly successful version. The electronics industry has enthusiastically embraced this ‘‘new’’ technology, and the number of products based on PCAP is growing rapidly, with traditional integrated touch system providers playing a lesser role in this growth. Systems specifiers should consider sourcing carefully, as there are many inexperienced vendors in the market today. PCAP is well established in the touchscreen market now, with well over 100 million units as the installed base in mobile telephones alone. The basic characteristics of this technology make it suitable for many applications now served by other technologies, and it should be expected that some current technologies will disappear for all but legacy applications.

4

Optical

Optical touchscreens include perhaps the oldest touch technology in the industry – scanning infrared (SI). This technology was developed as an input device for a very early computer learning concept, ‘‘PLATO’’ developed at the University of Illinois, beginning in the 1960s. Donald Bitzer and his team at Illinois also invented the first bit mapped plasma display as part of the same project [16]. Also included in this section are camera-based systems that triangulate a touch position by various means.

963

964

5.7.1 4.1

Introduction to Touchscreen Technologies

Scanning Infrared

Scanning infrared (IR) touchscreens are based on the concept of determining position by the interruption of one or more of a series of light beams and thus interrupting the excitation of a phototransistor that is illuminated by the beam.

4.1.1

Design

Classic scanning IR systems use an orthogonal grid of pairs of IR LEDs and phototransistors (receptors) to define a touch-sensitive space. The LEDs are on one side of one axis of the touchscreen, and the corresponding receptors are on the other side. The beam of the LED and the reception angle of the receptor are designed so that one beam illuminates substantially one receptor only. The orthogonal axis is similarly arranged with LEDs and receptors. The spacing of adjacent beam axes is 3–6 mm, and is limited only by the physical size of the components and the collimation and beam width of the LEDs, which is also a function of the dimensions of the touchscreen. Touchscreens of 1.5 m diagonal size are within the capability of this technology. In operation, each LED in one axis is turned on in sequence, and the corresponding receptor is illuminated by the LED beam. A blocked beam changes the state of the receptor and signals the system that a touch event may have occurred. When the scanning, or operating every pair of one axis of the touchscreen is complete, the other axis is similarly scanned. If a valid beam-blocking event in the second axis is detected, a valid coordinate pair is generated and processed by the system. The scanning operation is continuous. Various algorithms are used to condition the response of the receptors to the beam to detect a blocking event, set the sensitivity of the system, and respond to such conditions as bright sunlight and contamination on the touchscreen which may semi-permanently block a beam. Depending on the size of the touchscreen, 20–50 complete scans may be performed per second. The resolution of an IR system would seem to be set by the pitch of the beam component pairs. However, since it regularly happens that a touch event simultaneously blocks two adjacent beams, the system can resolve half the distance between adjacent beams. Newer IR systems extend this concept and process the illumination of five or six receptors from each beam event. By analyzing the actual signal levels at the receptors rather than the binary ON–OFF condition of a given beam, the resolution of the system is greatly increased. One new variation on the scanning IR concept has emerged in recent years – a single emitter and single receptor touchscreen with waveguides to create the multiple beam paths required. This concept has a substantially reduced component count versus traditional scanning IR systems and has potential to substantially lower IR mass production costs.

4.1.2

Materials and Construction

Two styles of scanning IR construction are used. One incorporates all of the system electronics in addition to the beam components on a printed circuit board (PCB) surrounding the active area of the touchscreen, and the other transfers most of the electronics off the beam PCBs. The choice of these constructions depends almost entirely on the available space in the display perimeter. Also, ‘‘beam component only’’ PCBs can more easily be modified to change the length of the beam component array, making something close to a standard product a simpler design issue.

Introduction to Touchscreen Technologies

5.7.1

Beam PCBs are designed so that the beams are as close to the display lens as possible. In this condition the user’s perception of system operation is that the lens is physically contacted to cause a touch event, although the beam may actually be broken before the stylus or finger contacts the lens. A display lens in an IR touch system is required only for protection of the display panel. This lens is not required for CRT displays, but essentially all LCD and most plasma panels need a lens to protect various elements of the display. A lens can also be incorporated in the assembly to provide a complete environmental seal for the finished product. The other components of the assembly that make the seal are the IR-transparent lens which is used to mechanically cover the apertures of the LEDs and receptors, and to block light other than that below the approximate wavelength of the IR beam,
850 nm. The lenses are joined at the edge of the touchscreen and often attached to a display bezel separate from the rest of the product enclosure.

4.1.3

Scanning IR Advantages

● Infrared (IR) lights and signal receiver devices are arranged inside the frame, and transparency is reduced only by the display lens. ● Touch components are hidden inside of the frame, making the technology very durable. With durable display/glass, IR can be used in rugged outdoor public displays. ● Capable of drag/motion detection. ● Calibration is not necessary after initial video alignment. ● Scalable to very large sizes. ● Stylus independent. ● Does not require a glass lens – rigid plastics can be used.

4.1.4 ● ● ● ● ● ●

Scanning IR Disadvantages

High engineering design cost. IR component count is high, thus cost increases greatly with size. Direct sun light affects infrared lights. Resolution is not adequate to draw detailed pictures. Any touch is detected (bugs, dirt, etc.), so these can be detected unintentionally. Surface obstruction and hover can easily cause a false touch.

4.1.5

Summary

Scanning IR touchscreens are well suited for unattended public access kiosks and similar applications, and are durable enough for high transaction rate products such as point of sale terminals. The absolute stability of calibration, stylus independence, excellent environmental seal, and capability of highly vandal-resistant installations make IR an excellent choice for high volume products where these characteristics are important. High NRE costs are a deterrent to new designs as is the physical space that IR takes up in some product configurations. Whether improved designs can keep scanning IR viable against camera-based optical systems is unknown.

965

966

5.7.1 4.2

Introduction to Touchscreen Technologies

Camera-Based Optical

Optical touchscreens are a relatively modern development in touchscreen technology. The most common design for this system consists of two or more image sensors (cameras) placed around the edges (usually in the upper left and right corners) of the screen. Infrared LEDs illuminate the touchscreen without firing directly into the cameras. They are usually placed in the top center edge of the touchscreen. Light from the IR source illuminates the user’s finger or stylus, and the reflection from the stylus is detected by the cameras. The cameras have excellent angular resolution and can accurately report the angle of the light. An appropriate algorithm in the system electronics computes the actual touch location. Camera systems are growing in popularity, due to its scalability, versatility, and affordability, especially for larger units.

4.2.1

Design

Most of the design of the camera touchscreen system is mechanical – proper mounting of the cameras and IR light source is required.

4.2.2

Materials and Construction

Some of the same considerations important to the scanning IR systems apply to the optical systems as well. An IR transparent cover or a full bezel is required to preserve the cosmetic appearance of the display (many optical touch systems are retrofits to existing displays). Cameras must be properly aimed for coverage of the active area in azimuth and elevation. A protective lens is necessary to prevent display damage, and for large display sizes, this lens will be heavy and difficult to mount.

4.2.3 ● ● ● ● ● ●

Excellent durability, as the cameras are always recessed inside the bezel. Drag/motion touch capable. Fast touch processing, with good resolution – drawing on the screen is reasonable. Multi-touch capable. Scalability to large sizes. Object sizes can be recognized.

4.2.4 ● ● ● ●

Camera-Based Advantages

Camera Optical Disadvantages

Initial calibration may be difficult when installing sensor devices or may require adjustments. Direct sun light affects infrared lights. Display contamination may result in false touches. Shadowing can be a problem when it comes to multi-touch. Extra cameras can reduce shadowing issues. ● Requires a profile height above the screen surface. ● A protective lens is required.

Introduction to Touchscreen Technologies

4.2.5

5.7.1

Summary

The obvious advantages of this technology versus scanning infrared are obvious. Better touch processing speed, size scale-up capability with lower cost, and fewer design complexities give the edge to this technology in applications where an optical touchscreen is generally the right choice. This technology has the potential to displace scanning IR.

4.3

Surface Computing

The term ‘‘surface computing’’ (sometimes called ‘‘tabletop computing’’) describes a specialized computer GUI in which the keyboard and mouse are completely replaced by a touch-sensitive display, and users interact with common and intuitive objects rather than traditional GUI elements such as windows, icons, and drop-down menus. The goal of surface computing is to integrate the physical world and the virtual (digital) world more closely so that digital information becomes immediately and easily available when users interact with a physical object or an environment. One of the examples commonly used to illustrate this concept is the idea of a horizontal touch-display in a retail mobile-phone store, where the user/ prospect places two physical phones on the display’s surface. The software driving the display identifies the phones and immediately displays a comparison of the two phones’ features, specifications, and pricing. The user can then interact with the information using his or her hands to explore details or modify the way the phones are compared. Another common example is the idea of placing a digital camera on the display surface and having the photos in the camera automatically copied to the display, where the user can interact with them using multi-touch finger-gestures such as flicks, pinches, rotations, etc. The photos can be transferred to a mobile phone simply by placing the phone on the display surface and dragging the photos over to it. Surface computing can work with any type of display, including flat-panel, rear-projection, and front-projection. On the touch side, the choices are more limited. While some early implementations such as DiamondTable used capacitive sensing, essentially all current implementations of surface computing use infrared (IR) camera-based sensing; this requires one or more cameras to be positioned so that an image of the entire screen can be captured. There are currently three methods of supplying the IR light that is received by the vision-based camera in surface computing. These methods are Diffused Illumination (DI), Frustrated Total Internal Reflection (FTIR), and Diffused Surface Illumination (DSI): ● DI: Diffused Illumination can be used with either front- or rear-illumination systems. Rear DI utilizes infrared light projected on the screen from below the touch surface. A diffuser is placed on the top or the bottom of the touch surface. When an object touches the surface, it reflects more light than the diffuser (or objects in the background), and the extra light is sensed by a camera. Depending on the diffuser, this method can also detect hover above the screen and can identify objects placed on the surface. In the case of front DI, infrared light is projected on the screen from above the touch surface, such that a shadow is created when an object touches the diffused surface and can then be similarly recognized by a camera. ● FTIR: The concept of Frustrated Total Internal Reflection is a physical condition related to differences in the refractive indexes of adjacent materials. When light passes from one material to another with a higher refractive index at an angle of incidence greater than the

967

968

5.7.1

Introduction to Touchscreen Technologies

Total internal reflection Acrylic pane LED

Scattered light Baffle

Diffuser Projector Video camera

. Fig. 9 Concept of Frustrated Total Internal Reflection (FTIR). This figure illustrates how FTIR can be used to sense touch. A rear-projection screen (diffuser) is attached with a small air gap to the underside of a sheet of acrylic. Infrared LEDs inject light into the polished edge of the acrylic; TIR causes the light to remain trapped within the sheet. A baffle blocks light with a higher angle of incidence near the edge of the acrylic. When a finger touches the surface of the acrylic, it ‘‘frustrates’’ TIR and causes light to scatter out through the acrylic toward a vision-based camera equipped with an IR band-pass filter. (Media Research Laboratory, New York University)

specific angle (described by Snell’s Law), then no refraction occurs in the material, and light is reflected. This method traps infrared light in an acrylic overlay, which is frustrated (scattered) at the point of a touch; the scattered light is then recognized by camera-based imaging. See > Fig. 9. ● DSI: Diffused Surface Illumination uses a special acrylic to distribute the IR evenly across the surface. This method relies on small particles inside the acrylic which function like tiny mirrors. When IR light is injected into the edges of the acrylic (as in FTIR), the particles redirect the light to the surface and spread it evenly. When a user touches the surface, the light is scattered and seen by the vision-based camera as a blob of IR light.

4.3.1

Summary

Surface computing technology holds the promise of changing the way people interact with computers, going well beyond applying touch in the replacement of traditional user interfaces in appliances. There is an expanding body of work on multi-touch, object recognition, direct manipulation, 2D & 3D gestures, and related fields that continue to enable innovation in the area of surface computing. Most of what has been developed in the surface computing area to date has been a demonstration product or dedicated products for trade shows or multimedia presentations. What, if any, commercial niche this concept will settle into is still unknown, but it can be increasingly expected in public exhibitions, museums, and other cultural venues. It is too much fun for the public not to be used in this way!

Introduction to Touchscreen Technologies

5

5.7.1

Acoustic

Acoustic touchscreens include surface acoustic wave (SAW), guided acoustic wave (GAW), and two bending wave technologies – acoustic pulse recognition (APR) and dispersive signal technology. These technologies all depend on some aspect of propagating mechanical waves on the surface or through the bulk of a high Q transparent substrate (i.e., glass) and capturing and analyzing those waves to determine a touch position.

5.1

Surface Acoustic Wave

SAW touchscreens are based on the propagation of shear waves in the surface of a piece of glass. A repetitive short burst of a high frequency (
5.53 MHz is a popular choice) high amplitude sinusoidal wave is applied to a transmitting piezoelectric transducer, which is in turn coupled to one edge of a glass substrate through a mechanical transformer. The shear waves produced in the surface of the glass from this process propagate along one edge of the glass and are redirected perpendicularly across the glass by an array of partial reflectors along this glass edge. On the opposite side of the glass, a mirror-image array of partial reflectors further redirects the waves to a receiving piezoelectric transducer coupled identically to the glass as the transmitting transducer. The transmitting and receiving transducers are also identical. A user’s finger or soft stylus intercepts a portion of the wave train which is propagating across the active area of the touchscreen glass, and the wave is temporally attenuated in relation to the sum of all the reflected wave paths, which are progressively delayed by the longer length of the reflection path as the burst wave travels along the reflector array. The wave train is partially reflected as it encounters each individual reflector. The touchscreen controller receives the wave train, which is now stretched out in time, compared to the length of the original wave burst, by the multiple partial reflections of the burst. A touch position in one axis is derived from relating the attenuated area of the reflected wave train compared to the original wave train in an untouched condition. The second coordinate of the touch position is similarly derived from an orthogonal set of transducers and reflectors that are operated similarly to the foregoing description [17].

5.1.1

Design

The basic design of SAW touchscreens is relatively simple. The standard glass is ordinary sodalime float glass, although higher Q glasses such as borosilicate may be used for larger touchscreens where signal attenuation is also greater. Most SAW touchscreens are designed for 2–3 mm glass substrates. Thicker glass improves signal strength, especially for larger touchscreens. Reflector arrays designed for a large touchscreen can be truncated at the transducer end to use with smaller touchscreens. The arrays are merely shortened to the appropriate length and applied to the edges of the glass substrate without further modification. Transducers and the associated coupling transformer (in early designs), a wedge-shaped piece of plastic, are all of a standard design and have a predetermined nominal placement. All reflector arrays are designed so that the spacing between adjacent individual stripes in the array is an integral multiple of one wavelength of the propagating wave on the glass. This condition largely

969

970

5.7.1

Introduction to Touchscreen Technologies

prevents destructive interference of the wave train as it travels down the reflector array and undergoes partial reflection by the array. SAW controllers are essentially universal, and for a given manufacturer, should be compatible with all sizes of touchscreens. The choice of
5.53 MHz as the operating frequency for most SAW touchscreens was an arbitrary, early choice of Zenith Electronics and patent licensee Elographics (now Elo TouchSystems) that was convenient to use, as it is a microprocessor clock frequency that is required for generation of proper serial communications baud rates with the 8251 family of microprocessors. It appears that all SAW manufacturers now use this operating frequency. There is no particular reason otherwise for the use of this frequency, as SAW touchscreens work satisfactorily over at least the range of 4–10 MHz. SAW systems are adaptive. The controller constantly ‘‘relearns’’ the baseline waveform of the received wave train, allowing it to map out most forms of contamination, adjust for changing environmental conditions, and change its noise threshold if necessary.

5.1.2

Materials and Construction

As mentioned earlier, soda-lime float glass is the standard substrate for SAW touchscreens. A common thickness is
3 mm. Surface finish of the raw glass must be determined at the time of manufacture, as there is no coating or additional substrate is applied to a SAW touchscreen. Polished and antiglare finishes are acceptable. Reflectors are made of a low-temperature glass frit. The frit paste is applied by screen printing and then fired. Transducers and the associated cable are placed on the glass, temporarily held in place during adjustment, and then glued to the glass. Adjustments to the reflector array can be made to alter the received wave train characteristic after the transducers are fixed in place. A newer transducer design uses piezo transducers glued flush to the backside of the glass, directly underneath the nominal location of a top-mounted transducer. The burst transmit signal is launched from the transducer as a bulk wave. At the top surface of the glass, the wave encounters an array of mode conversion reflectors and is converted to surface waves. The mode conversion reflectors can be printed in the same step as the wave directing partial reflectors, and thus can be perfectly aligned. As a result, the alignment of the piezo transducer is then noncritical and requires little or no adjustment. As there are no conductive coatings on the glass substrate, and no processes that affect the glass during manufacture, SAW production is usually a high yield process.

5.1.3 ● ● ● ● ● ● ● ●

Advantages of SAW

Finger and soft stylus activation. High VLT – no conductive coatings on the substrate. CS and heat strengthened glass may be used for additional durability. Excellent sensitivity. Good drag performance. Good resolution. Excellent EMI rejection without touchscreen enhancements. Multi-touch capability has been implemented in new designs.

Introduction to Touchscreen Technologies

5.1.4 ● ● ● ●

5.7.1

Disadvantages of SAW

Challenging to environmentally seal. Contamination sometimes causes problems, and touchscreen is not waterproof. No post manufacturing optical or environmental front surface enhancements are possible. Does not scale-up in size well–
0.7 m diagonal is near maximum comfortable size.

5.1.5

Summary

SAW is an excellent technology for many applications. With excellent clarity and VLT, and no conductive coatings to add reflections, it performs well in uncontrolled lighting conditions. With the exception of large quantities of liquids, SAW can adapt to and reject most forms of contamination on the touchscreen glass, and unattended operation is excellent. With the appropriate diagnostic software, some SAW systems can be remotely monitored and troubleshot. The addition of multi-touch capability by Elo TouchSystems significantly enhances the competitiveness of this product. Environmental sealing solutions exist for this product but can be a challenge to implement if not designed in at the beginning of a project.

5.2

Guided Acoustic Wave (GAW)

GAW is a variant of SAW that uses lamb waves rather than shear waves. Lamb waves propagate through an inner layer of the glass, making one important aspect of the technology better than SAW – GAW is virtually immune to influence by surface contamination, and by extension, is much easier to environmentally seal. GAW was discontinued by its only manufacturer, Carroll Touch, after years of limited commercial success.

5.3

Dispersive Signal Technology (DST) and Acoustic Pulse Recognition (APR)

APR and DST are very similar technologies. Bulk acoustic waves which are generated in a glass substrate are sensed by several piezoelectric transducers, mounted near the edges on the backside of the glass. The received acoustic signature is matched to a lookup table entry stored in system memory or in an outboard file associated with an operating system driver. Simple designs, inexpensive materials (glass), and easy scale-up in size make these systems attractive for large format touchscreen systems. The outstanding manufacturing difficulty is the present need to map each touch location into the lookup table by tapping the touchscreen in every major touch location, although some interpolation is apparently possible. The outstanding operational difficulty is the inability of these systems to detect a touch and hold event. With no tapping or movement after the initial acoustic signature is detected, there is no further acoustic signal for the controller to look up. ‘‘Point and shoot’’ mode and ‘‘click and drag’’ mode compatibility is also an issue. Integration can also be a problem if the glass is coupled to a structure that is resonant near the frequencies of the acoustic signature of the glass.

971

972

5.7.1 5.3.1 ● ● ● ● ●

Introduction to Touchscreen Technologies

APR and DST Advantages

Very simple sensor (plain glass + four piezoelectric transducers) Works with finger, stylus, or any other touch object Very durable and transparent touch sensor Resistant to surface contamination; works with scratches Scales in size very easily, except for lengthy calibration procedure (generation of lookup table)

5.3.2

APR and DST Disadvantages

● No ‘‘touch and hold’’; no multi-touch. ● Control of mounting method in bezel is critical.

5.3.3

Summary

With low cost to manufacture (except for lookup table generation), relatively easy scale-up in size, excellent optical and ergonomic appeal and the durability of all glass construction, these products are superior technologies for POS, interactive multimedia, and interactive digital signage.

6

In-Cell

The term ‘‘in-cell touch’’ generally refers to the implementation of a touch sensor inside the cell of an LCD, (although in-cell implementations have also been proposed for PDP, electrophoretic, and OLED technologies). LCD in-cell touch currently exists in three forms, only one of which is physically inside the LCD cell. The three forms are as follows: ● In-cell: The touch sensor is physically inside the LCD cell. The touch sensor can take the form of light-sensing elements, microswitches, or capacitive electrodes. ● On-cell: The touch sensor is an X–Y array of capacitive electrodes deposited on the top or bottom surface of the color-filter substrate. Strictly speaking, when the electrodes are on the bottom surface of the substrate they are physically inside the cell – but this is still usually called ‘‘on-cell’’ because of the type of electrodes. (This is a good illustration of the fact that the terminology for in-cell touch is still evolving). ● Out-cell: This new term, coined in 2009 by AU Optronics Corp., describes the configuration in which a standard touchscreen (usually only resistive or projected capacitive) is laminated directly on top of the LCD during module manufacturing. Unlike the other two, this configuration typically requires an additional piece of glass – even though it is technically possible to use a film–film resistive touchscreen in this case. Because these terms and the technology that they describe are quite new, there is still quite a bit of variation in their use in technical and marketing documents. Caution is advised while reading any relevant material; ‘‘on-cell’’ may often be used to describe something that is actually ‘‘out-cell,’’ and vice versa.

Introduction to Touchscreen Technologies

5.7.1

Types: There are currently three different touch technologies being used in in-cell and on-cell touch: ● Light sensing (in-cell): This technology, also called ‘‘optical,’’ uses the addition of a phototransistor into some or all of the LCD’s pixels. The screen can be touched with a finger, stylus, light pen, or laser pointer. The touch-sensing array can also be used as a scanner. A cover glass can be used to protect the LCD’s surface. ● Voltage sensing (in-cell): This technology, also called ‘‘switch sensing,’’ uses the addition of microswitches for X and Y coordinates into each pixel or group of pixels. The screen can be touched with a finger or a stylus, within the damage limits of the LCD’s surface. A cover glass cannot be used to protect the LCD’s surface. ● Charge sensing (in-cell/on-cell): This technology, also called ‘‘pressed capacitive,’’ uses variable-capacitor electrodes in each pixel or group of pixels. The screen can be touched with a finger or stylus, within the damage limits of the LCD’s surface. A cover glass cannot be used to protect the LCD’s surface. Charge sensing is also used as an on-cell solution, where it is called ‘‘capacitive sensing.’’ It is basically the same as today’s projected capacitive but uses an X–Y array of capacitive-sensing electrodes on the top surface of the color-filter substrate. The screen can be touched only with a finger. A cover glass can be used to protect the LCD’s surface. In-cell solutions are appealing in that they change the manufacturing supply chain, enabling minimal material and production costs, with opportunities to significantly enhance touch performance. But they require fundamental LCD design changes. Modifying the backplane or frontplane of a single LCD to add in-cell touch costs more than $1 million due to masking, so it is an expensive initial transition. If touch is not required in every LCD, it is not certain that LCD manufacturers will be willing to make in-cell touch and non-touch versions of multiple displays. Most LCD manufacturers have been working diligently on one or more forms of in-cell and on-cell touch solutions. Sharp, AUO, and Samsung seem to be the industry leaders, although TMD is credited for developing some of the most advanced in-cell solutions.

6.1

Summary

Although in-cell touch has been eagerly anticipated for more than 7 years, it still has some distance to go to reach full commercialization. Light-sensing in-cell is probably the furthest away because it has the most unresolved problems. Voltage-sensing in-cell has promise, but there are no announced LCDs or end-user products that incorporate it. Charge-sensing in-cell and on-cell are the closest to commercialization, with a few announced LCDs that will probably ship in mobile phones during 2011. The focus of most of the LCD manufacturers working on in-cell touch is now on mobile displays because sizes larger than 26 cm have proven to be quite difficult.

7

General Summary and Conclusion

Touchscreens have been commercial available in some form for over thirty years, yet are still regarded as new technology in many engineering and business communities. It is true that innovation is continuing in touchscreen technology, but many available touchscreen systems are based on mature and stable hardware, being updated for new system interfaces and computer operating systems as conditions in the computer industry change. Today there are four broad classes of touchscreens available–resistive, capacitive, electro-mechanical and

973

974

5.7.1

Introduction to Touchscreen Technologies

optical, with several different specific types within each class–and all present different design, implementation and user considerations for the researcher and system designer. This article is intended to present the essential characteristics of each touchscreen type and alert the researcher and system designer to many of the important details affecting design and use choices. Materials and manufacturing processes are an important part of touchscreen choice, and definitely have an effect on performance and environmental survivability. Improvements in touchscreen materials, particularly transparent conductors, are beginning to have some impact on touchscreen design and construction, and may change the present consideration of particular touchscreen technologies for certain environmental situations. Designers should consider the information presented here in making system choices. Finally, information is presented on the emerging ‘‘in-cell’’ touch technologies for designers to watch and consider. None of these technologies have reached full commercialization, and the full feature and benefit set for these touchscreens has not been defined. Given the potential for greatly reduced manufacturing cost and improved display optics for in-cell touchscreens, it is expected that one or more of the in-cell technologies could have significant impact on the future direction of touchscreen system choices if the technology lives up to its potential.

References 1. Berliner Glas chemical strengthening. http://www. berlinerglas.de/download/chemical_strengthening. pdf 2. Hermanns C, Middleton J (2005) Laser separation of flat glass in electronic-, optic-, display- and bioindustry. Proc SPIE 5713:387–396 3. Carbon for electronics. http://www.unidym.com/ files/unidym_03_final10.pdf 4. ASTM D1003-95, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics 5. MIL-STD-150A. http://assist.daps.dla.mil 6. Edmund Optics, Inc., 101 East Gloucester Pike, Barrington, NJ 08007 7. Applied Image, Inc., 1653 East Main St., Rochester, NY 14609 8. US Patent 7,819,998

Further Reading Stetson JW (2006) Analog resistive touch panels and sunlight readability. Inform Display 22(12):26–30

9. US Patent 5,220,136 10. Sierros KA, Morris NJ, Ramji K, Cairns DR (2009) Stress-corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices. Thin Solid Films 517(8):2590–2595 11. US Patent 7,265,686 12. US Patent 5,736,688 13. eGalax_eMPIA Technology Inc., Surface Capacitive Touch Control Board. http://home.eeti.com.tw/ web20/eg/ESCAP7000.html 14. US Patent 4,353,552 15. O’Connor T (2010) Microchip mTouch™ projected capacitive touch screen sensing theory of operation 16. US Patent 4,100,535 17. US Patent 4,642,423

5.7.2 Transparent Conducting Coatings on Polymer Substrates for Touchscreens and Displays Charles A. Bishop 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 976

2

Polyethylene Terephthalate (PET) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 977

3 3.1 3.2 3.3 3.4

Transparent Conducting Coatings on PET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 979 Indium Tin Oxide (ITO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 981 Aluminium-Doped Zinc Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 983 PEDOT:PSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 983 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 984

4

Additional Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985

5

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 987

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.7.2, # Springer-Verlag Berlin Heidelberg 2012

976

5.7.2

Transparent Conducting Coatings on Polymer Substrates

Abstract: There has been an interest in using polymer films as an alternative to glass substrates. Polymer substrates have the potential to reduce manufacturing costs by using roll-to-roll processing. Polymers also have improved flexibility and can reduce the weight of the final product. Many touch screens are constructions where two transparent conducting coatings are placed facing each other but separated by an array of small spacer dots and as one of the transparent coated films is pressed and distorted it contacts the other conducting film making electrical contact and completing the circuit. Thus, having a robust transparent conducting coating on a flexible polymer substrate has a ready-made market so long as it can meet the required specification. The polymers have to be coated at a lower temperature than glass making the transparent conducting coatings less conducting or transparent than can be achieved on glass and so there is a trade-off of properties. As the coating performance improves and the yield and costs reduce, the move to polymer substrates is expected to increase. This chapter highlights the basics of vacuum deposition of transparent conducting coatings onto polymer substrates. List of Abbreviations: AZO, Aluminum-Doped Zinc Oxide; CIGS, Copper Indium Gallium Diselenide; CIS, Copper Indium Diselenide; CNT, Carbon Nanotubes; ITO, Indium Tin Oxide; MWCNT or MWNT, Multiwall Carbon Nanotubes; PEDOT:PSS, PolyethyleneDioxythiophene: Polystyrene Sulfonic Acid; PEN, Polyester Naphthalate; PET, Polyester Terephthalate; ppm, Parts per Million; RI, Refractive Index; SWCNT or SWNT, Single Wall Carbon Nanotubes; TCO, Transparent Conducting Oxide

1

Introduction

Transparent conducting coatings can be deposited onto many different polymer substrates; however, the reality is that the cost performance benefits of polyethylene terephthalate (PET) mean that this is the preferred substrate in most cases. In a few applications, the higher temperature performance of polyethylene naphthalate (PEN) can justify the higher cost of this polymer. The advantage for using polymer films is in the ability to manufacture devices using roll-toroll processing and in the flexibility of the resultant devices. Although these advantages are attractive, there are limitations to the polymer films. The polymer films have a thermal limitation which results in the transparent conducting coatings having a lower performance compared to those produced on glass at higher substrate temperatures. The polymer films are not as thermally stable as the glass substrates [1, 2]. Added to this it is more difficult to clean the polymer substrates and so yields tend to be lower than for the glass substrates they are to replace. Hence, although there has, for many years, been an expectation that polymer-based devices would replace the glass based devices, this has been a slower conversion than predicted. However, the size of the market as well as the possibility of also supplying the ultra barrier market for the encapsulation of organic light-emitting devices and also solar cells has given the polymer suppliers a suitable incentive to develop improved substrates. Now there are a number of different PET substrates available beyond the basic film. This includes heat-stabilized as well as heat-stabilized and planarized versions of both PET and PEN film. The polymer is quite a soft substrate and so the transparent conducting coating can be damaged more easily than if deposited onto a harder and more abrasion-resistant surface. Some substrates are pre-coated with a hard coating before the transparent conducting coating is applied. This can be an

Transparent Conducting Coatings on Polymer Substrates

5.7.2

organic coating and often the same organic hard coating is applied to the back surface too. In addition, some suppliers prefer to deposit a thin silica layer by vacuum deposition prior to the vacuum deposition of the transparent conductor [3]. The organic coating is able to reduce the surface roughness but is not a good barrier material whereas the silica, or similar inorganic coating, will deposit as a conformal coating and so will not smooth the surface but is a good barrier material, particularly as it is deposited onto a smooth surface [4–6]. The combination of planarizing layer and barrier coating can give an improvement both to the coating conductivity as well to the adhesion of the coating, thus providing increased flexibility. There are additional benefits too, one is that either the organic or the inorganic layer provides a better optical match to the substrate than the transparent conducting oxide and so reduces the reflection from the back surface of the conducting coating increasing the transmittance of the coated material. The other benefit is that the crystallinity of the depositing transparent conducting coating can be affected to advantage. Typically, the polymer substrate thickness is in the range 50–175 mm. This variety of substrate thickness, with or without heat stabilization or planarizing layers or hard coatings, leads to a wide choice of possible substrates (see > Part 5.6).

2

Polyethylene Terephthalate (PET)

Polyester is the more common name for the family of polymers that includes PET or PEN. The PET monomer is produced by combining two raw materials in a process called direct esterification. The raw materials are usually monoethylene glycol, an alcohol, and an organic acid which is either dimethyl terephthalate or pure terephthalic acid. The monomer along with a catalyst is heated under vacuum and undergoes polycondensation to produce the polymer. The polymer comprises a number of repeat units of monomer, typically of the order of 100, to produce long-chain molecules. The viscosity of the polymer is determined by the chain length and so once a given viscosity is achieved the process is stopped. The resultant polymer is extruded and chopped into chips ready for use in the polymer film extruder. The film-making process starts with the chip being melted in an extruder and a thin wide ribbon of polymer is extruded through a die [7, 8]. The molten ribbon falls onto a rotating cooled casting drum where the polymer ribbon is cooled and the polymer freezes with a completely amorphous structure. At this point, the polymer has little of the mechanical performance that PET is known for. To improve the properties, the film is usually biaxially oriented. This can be done using either a stenter or bubble process as shown schematically in > Fig. 1. In the stenter process, this is usually done using sequential orientation. The ribbon of polymer is first stretched in the machine direction. This is achieved by having two sets of rolls where the second set of rolls is rotating at a faster speed than the first set of rolls. In this way, the polymer is stretched forward. To make this process easier, the polymer is heated as it passes through the first rolls and between the two sets of rolls. At the end of the second set of the rolls, the polymer is cooled to fix the stretch. The polymer web is then gripped at each edge and the web enters the stenter and is heated and then the clips holding the web are moved sideways to provide the transverse direction orientation. Following this second stretching process, the film is held at the elevated temperature for a short time to allow some stabilization and then it exits the stenter. Where the clips have held the web the web has not been stretched and so the edges are removed by slitting of the edges of the web. The result of this

977

978

5.7.2 Casting drum

Transparent Conducting Coatings on Polymer Substrates

Forwards Stenter oven draw

Edge trim Rewind to roll re-cycle

Nip rolls

Bubble Solidification region Frost line

extruder

Gas inflation Feed hopper Sideways draw Heat set / annealing

Bubble

Edge slit

Cooling air Die

extruder

Rewind rolls

. Fig. 1 Schematics of the biaxial film manufacturing process showing the stenter process on the left and the bubble process on the right

process is that the web is many times wider and longer than the original extrusion but also considerably thinner. The biaxial stretching process changes the structure of the polymer. The original amorphous structure can be thought of as being like a bowl of spaghetti with the polymer chains intermingled. When the web is stretched, these polymer chains will start to be oriented along the stretching direction. As sections of the intermingled chains become closer and closer to parallel, they can form crystallites. These crystals have a much higher tensile strength than the amorphous polymer regions as well as having a higher refractive index and a greater resistance to gas diffusion. At the end of the biaxial orientation process, the polymer contains of the order of 60% crystalline material. The crystals may be stronger but they are also less flexible and hence the precise percentage of crystallization is aimed at providing a balance of different properties. Hence, different manufacturers may supply films with slightly different mechanical and optical properties reflecting their different proprietary processing and additives. The quality of the polymer web is important to the vacuum deposition process. The vacuum deposition process will heat the web up and if the temperature goes too high the web can shrink due to residual stress within the film. The residual stress within the web is not necessarily uniform across the web. The PET web may be several meters wide [mill rolls] when produced on a film line but then the web is slit down to narrower widths to suit the vacuum deposition systems. Rolls of polymer slit from the edge of the mill rolls will have the residual stress at a different orientation to that found in rolls slit from the center of the mill rolls. The heat-stabilized web is an attempt to minimize this problem. The standard web is reheated on a winding system in an oven under minimum tension. This allows the polymer chains time to move to a position that minimizes the residual stress. The temperature the stabilization is done at determines the safe operating temperature for future processing. If the stabilization temperature is exceeded, the web can be expected to begin to shrink. As can be expected, this stabilization process is slow and takes energy and so adds to the substrate costs. This can increase the cost of the substrate by a factor of 2
– 2.5
the standard cost of PET film. Going to the higher-temperature PEN version and having that heat stabilized too can increase the cost to a factor of 10
the standard PET.

Transparent Conducting Coatings on Polymer Substrates

5.7.2

The extruder die not only needs to be clean and the slot parallel but also the temperature needs to be uniform across the whole die. If there are any variations to the die the web will have thickness variations. Any thickness variation can be seen as a potential winding problem. The thickest part of the web will take the applied tension first and so this can lead to uneven winding or as the web is heated during the deposition there is the possibility of the tension exceeding the polymer yield point and causing a permanent deformation of the polymer along the high thickness line along the length of the web. The cleanliness of the web is also something that can be used to discriminate between polymer web suppliers. All polymer webs have particulate contamination on the surfaces. This may come from the manufacturing line where unpolymerized monomer may evaporate, condense, and fall back onto the web as a fine white powder. Slitting dust may also be on the web surface as well as any atmospheric particulates. As the polymer web approaches, touches, and leaves various rollers in the winding system, the web generates a triboelectric charge which can attract fine debris to the surfaces. It is not cost effective to stop this contamination entirely during manufacture but it is possible to limit the contamination by the strategic use of clean air hoods and air filtering. It is also possible to remove most of the debris down to a particle size of approximately 300 nm using cleaning techniques such as the tacky roll process. Debris on the surface is a source of coating defects such as pinholes or cracking which can affect the electrical performance of the coating. Another source of contamination is the exuding of oligomer from the bulk polymer onto the web surface. No polymerization process is perfect and there will always be some residual monomer or short-chain fragments. The shorter the chain length the more mobile these fragments will be and the faster they will appear at the surface. This oligomer will reduce the surface energy and as the oligomer will not be well bonded to the bulk polymer, the adhesion will be poor. Vacuum depositing coatings onto this contaminated surface will result in poor coating adhesion which will also be seen as the transparent conducting coatings cracking at lower strains. When tested the higher adhesion shows the coatings can be flexed around smaller test mandrels without cracking. Surface roughness can also be a source of defects and cracking and hence another option that is now offered is the planarization of the web surface. This not only provides a smoother surface but also can be a barrier to oligomer exuding onto the surface.

3

Transparent Conducting Coatings on PET

There are several options to producing a transparent conducting coating [9–19]. Some of these materials such as cadmium tin oxide or antimony tin oxide are regarded as toxic and are not preferred for large-scale manufacturing. Others such as indium zinc oxide, gallium zinc oxide, indium gallium zinc oxide, niobium-doped titanium oxide, tin oxide, or fluorine-doped tin oxide have yet to demonstrate a performance to match that of indium tin oxide [ITO]. The established preferred coating that has the best overall performance is indium tin oxide and the most widely used composition has the indium oxide to tin oxide ratio of 90:10. The indium oxide has a good performance without any doping but is relatively soft and so the doping with tin oxide makes the coating more robust without the loss of the electrical performance. An example of the performance of three of the transparent conducting oxides deposited at room temperature onto polyester film is given in > Fig. 2. This dominance of the transparent conducting coating market is currently being reevaluated because of a potential shortage in the supply of indium. Indium has been mined as a by-product

979

5.7.2

Transparent Conducting Coatings on Polymer Substrates

100

REFLECTANCE/TRANSMITTANCE %

980

80

60

40

20

0 0.3

1.0

10 WAVELENGTH (microns)

30

Indium Oxide = Indium Tin Oxide = Cadmium Tin Oxide =

. Fig. 2 Reflectance and transmittance performance of three oxide coatings as deposited onto PET at room temperature [20]

of zinc and for many years the supply outstripped demand producing a stockpile. In more recent times, the low cost of zinc meant that mines were closed and after one mine was closed because of an explosion the situation changed and the consumption became larger than the supply. This caused a large rise in price which started the revaluation process. The consumption of indium is set to rise considerably as there is a rapid expansion of the production of copper indium gallium diselenide (CIGS) solar cells. These CIS or CIGS solar cells also may use ITO as the transparent top contact. So at the same time the display market is growing rapidly, there is an equally large market also requiring copious quantities of indium. Either of these markets would be larger than the current mined quantity of indium and so there is predicted to be a shortage of indium and hence a large price rise expected. The amount of indium per device is small and so even a large price increase could be absorbed in the total device cost. Over the last 20 years, the amount of indium mined has increased by a factor of approximately 7
and the efficiency of processing has improved. The improved recovery from low ore content and higher price has allowed indium to become a by-product of ores other than from zinc mining. Therefore, theoretically, there should be enough indium to satisfy needs for some time to come. What is of more concern is the security of supply. Rising economies such as China have put up barriers to the free trade of indium to ensure China has sufficient supplies for its own needs. This type of restriction has created a fear of having an erratic supply situation such that alternatives to ITO are being evaluated. Of the possible

Transparent Conducting Coatings on Polymer Substrates

5.7.2

alternatives, aluminum-doped zinc oxide (AZO) is the front runner. Further in the future are some other options such as printed transparent conducting coatings such as PEDOT:PSS and then behind that the use of carbon nanotubes.

3.1

Indium Tin Oxide (ITO)

Indium tin oxide is deposited with a sub-stoichiometry of oxygen of a few parts per million (ppm) making it a semiconducting material with a high visible light transmittance [21].This combination of transparency and electrical conductivity also gives the material the other name of transparent conducting oxide (TCO). ITO can be deposited by magnetron sputtering from either an alloy oxide target or an alloy metallic target by reactive deposition. The simplest deposition process is to use an alloy oxide target where it is possible to sputter a conducting coating usually in an argon and oxygen atmosphere. The oxygen only needs to be added at a low level, just enough to compensate for any oxygen lost during the sputtering process. As the target is already an oxide, the time for the target to reach stability and the recovery time following arcing are both faster than for metallic targets. The costs are generally higher than for sputtering from a metal target. The oxide targets tend to grow nodules which limits the production run time before the target needs to be resurfaced. It has been calculated that the cost per unit area of ITO from ceramic targets is approximately double that of depositing the same coatings from metal targets. Sputtering from a metal target is more difficult as a higher proportion of oxygen is required to react with the metal atoms to produce the correct stoichiometry of deposited coating. The oxygen introduction has to be balanced as too much oxygen will poison the target as well as depositing a coating with a less than optimum conductivity. To get a good coating uniformity across the width of the web, the vacuum deposition system needs to have inherent symmetry about the web centerline. If this is not the case it will be even more difficult to obtain the correct balance of metal and oxygen arrival at the substrate across the whole web width to produce uniform and optimal coating conductivity. As the metal target surface can become oxidized and the sputtering rate from the oxide target will be different to the sputtering rate from the metal target surface, the time for the target to become stable at the start of the process and following any arcing will take longer than for oxide targets. This makes the control of arcs a critical part of the manufacturing process. Sputtering can be from a single magnetron cathode. In this case, the racetrack will be kept clean by the sputtering of atoms but at the edges of the racetrack there will be an accumulation of backscattered material that may be poorly conducting. This material can, over time, become charged up and can initiate an arc to the plasma. This arc will consume all the current and will locally heat up the target at the root of the arc possibly causing melting of the target. If the arc is quenched by cutting off the power to the cathode and then reintroducing the power, the arc may immediately restart in the same place. This is because the higher secondary electron emission from the hot spot attracts the ions from the plasma to the same spot and encourages the higher conduction path to reestablish. Hence, modern power supplies not only cut the power off but initially reverse the polarity of the supply to actively quench the arc and so minimize the target heating. Other power supply options are to use a switched mode power supply that allows the polarity of the supply to change for a small proportion of the time. This proactive switching of polarity means that any surface charging that might cause an arc is actively neutralized on a regular basis.

981

982

5.7.2

Transparent Conducting Coatings on Polymer Substrates

This same problem can also be overcome by changing the geometry of the target from a flat plate planar target to a cylindrical target. The cylindrical target rotates at a low speed over the magnets and in this way the whole circumference of the target is sputtered. This means that any backscattered material is immediately removed and so there is no accumulation to become charged up and become a source of arcing. Another production problem is known as the disappearing anode. In production systems where there is a large amount of material deposited, it is a common problem that over time the sputtered material that does not reach the substrate can coat the surfaces that act as the anode to the sputtering process. The accumulation of this scattered material may be quite porous and with the availability of oxygen can become insulating. If the anode becomes fully coated with an insulator the anode will cease to function and the plasma can become unstable or even be extinguished. One method of combating this is to use a pair of magnetrons where the two targets are connected to a medium frequency power supply. In this way, over one half cycle one target is the cathode and the other is the anode and for the other half cycle, the roles are reversed. As one target is always being sputtered, it means that both targets are being kept clean and so the anode never becomes coated and nonconducting. The ultimate production process is to combine two of the above solutions and that is to have a pair of rotating cylindrical magnetrons connected to a medium frequency power supply. In this way, the rotation prevents any powdery accumulation of backscattered material as well as the alternating polarity keeping both targets clean. This minimizes the possibility of producing a source of arcing and so maximizes the production uptime. The sputtered ITO is generally reported as being either amorphous or crystalline with the crystalline form being the more conducting form. The conductivity is highly dependent upon the crystal size because the grain boundaries are disordered limiting the electron mobility. The grain boundaries will also accumulate oxygen which increases the resistivity too. Hence, the smaller the crystal size the more grain boundaries there are and the higher the coating resistance. Hence, the coatings on glass substrates, that can tolerate a high substrate temperature, have a larger crystal size which is a source of the higher coating conductivity. This is why annealing the ITO coatings on glass has been one route to increasing the coating conductivity. However, because the polymer substrates are limited in their stability to heat, it is generally of little benefit to anneal the ITO coatings on polymer substrates as the annealing temperature has to be kept so low that grain growth does not easily occur. ITO coatings have been known to change resistivity with time with some coatings being worse than others. The conductivity varies with the oxygen content and the oxygen content is controlled by the excess partial pressure during the sputtering process. If the conductivity of the coating is measured on-line during the vacuum deposition process it can be seen that the resistivity will pass through a minimum as the oxygen flow is increased. If the resistance is held at the minimum but drifts away from the minimum it is impossible to know if the drift is due to an excess of oxygen or due to oxygen deficiency. The control is easier if the resistivity is held just to the side of the minimum as then the drift will cause the resistivity up or down a slope and hence the oxygen flow can be adjusted accordingly. There is a preference for the resistivity to be set just to the metallic side of the minimum as this will give the final product more stability. The final product has been known to continue to oxidize over a period of months or years and hence the resistivity to change. If the control point were to be on the oxygen-rich side of the minimum and further oxidation to occur, the resistivity would continue to increase with time. This has been known to be enough for touch screens to become inoperable. If the control is set to the metal-rich side of the resistivity minimum, then any further oxidation firstly has to take

Transparent Conducting Coatings on Polymer Substrates

5.7.2

the coating through the resistivity minimum before further oxidation takes the resistivity higher and causes problems during use. Typically, magnetron sputtering sources have a stable deposition rate and so once the control system is set the coating will be of a constant thickness. If it is desired to produce coatings of a different resistivity the simple rule of thumb is that double the thickness will halve the resistivity or halve the thickness and double the resistivity. Typically, to produce a coating of approximately 20 Ω/sq, the coating thickness will be around 250–300 nm. Hence, if we take the latter thickness to go from 20 to 40 Ω/sq, the thickness would be reduced from 300 nm down to 150 nm. The simplest way of halving the thickness would be to double the winding speed with the rest of the conditions staying constant. The quality of the ITO coating is in part dependent on the quality and stability of the control of the process. Generally, the voltage, current, and power are monitored along with the total gas pressure and partial pressures and gas flows to each of the gases. Using plasma emission monitoring it is possible to use the emission lines specific to the sputtering metal and the oxygen gas to control the process. The intensity of the metal is dependent on the sputtering rate and if the oxygen emission intensity is kept at a constant ratio to the metal line intensity, the depositing composition can be held constant. The one other item to monitor is any arcing that occurs and the position along the length of the coated web. This is important as arcing can result in a loss on conductivity and loss of yield on subsequent products. As the coating is transparent it is impossible to find these areas of coating that are out of specification without either measuring the whole length of coated film or to map the coating during deposition. Then using the data files, each area can be found on any downstream production process and subsequent processing can be avoided. There is a complementary chapter on the magnetron sputtering of ITO coatings onto glass given in > Chap. 5.4.1.

3.2

Aluminium-Doped Zinc Oxide

As this is a relatively new material, there has not been the same amount of development carried out [22, 23] and so the choice of composition and targets is limited. Using an oxide target with an alumina content of 2% by weight is one option and this ceramic target has the advantage that it can be sputtered in 100% argon. The cost of the materials is lower than for ITO and can be approximately one fifth the cost but as the coating thickness needs to be thicker to get to equivalent sheet resistance the cost only drops to about half that of the ITO. Indications are that the AZO is more susceptible to arcing than the ITO, but this attributed to the target quality that is also expected to improve as the demand for targets and higher quality increases.

3.3

PEDOT:PSS

There is a group of polymers known as intrinsically conducting polymers that have attracted interest as they can be printed and hence the high cost vacuum deposition process could potentially be eliminated. These suitably doped polymers can become conducting to some degree although this can often be at the expense of transparency. This group includes polymers based on polyaniline, polypyrrole, polyacetylene, polyphenylene, and polythiophene. Of these the last one listed is the one that is probably the most widely used of those available. It is made

983

984

5.7.2

Transparent Conducting Coatings on Polymer Substrates

by the aqueous oxidation polymerization of the monomer 3,4-ethylene-dioxythiophene (EDOT) with polymer polystyrene sulfonic acid (PSS or PSSA). Once polymerized this acronym then becomes PEDOT:PSS. The coating produced by the PEDOT:PSS material is not unlike the vacuum-deposited counterparts in that the PEDOT and PSS polymer chains intermingle to make crystals. These crystals are of a higher conductivity than the gaps between the crystals. Hence, the conductivity can be improved if the grain boundaries are minimized either by growing larger crystals or by getting some intermingling between crystals to bridge the grain boundaries. These coatings have been aimed at replacing the higher resistivity inorganic TCO coatings for flexible electronic applications [24–28]. To improve the conductivity, a very thin metallic mesh has been used between the polymer TCO layers. The metallic mesh is thin enough that it is not noticed easily by the viewer but again is able to contribute to the conductivity of the structure [29]. This enables a resistivity of Chap. 5.4.3.

3.4

Carbon Nanotubes

Although the carbon nanotubes start farther back than the rest of the materials, it is expected that they will eventually become more widely used as it has the potential to outperform any of the other materials. Carbon nanotubes (CNT) are one of the families of fullerene materials [30–33]. Graphene is the single thickness sheet form, the ‘‘buckyball’’ is the spherical form made from 60 carbon atoms and then there are the nanotubes. The nanotubes can be in two basic forms of either single wall tubes (SWCNT or SWNT) or multiwall tubes (MWCNT or MWNT). Each tube is only a single carbon atom thick, but in the case of the multiwall tubes there are a series of concentric tubes each of a single carbon atom thickness. The SWNT can grow with different tube diameters and hence different atom positions and the diameter and bonding angle will affect the tube conductivity and determine if the tube has metallic conductivity or is a semiconductor. The conductivity of the MWNT depends only on the outermost shell. The carbon bonds in the graphite from and the SWNT have a tensile performance something like 10–50
that of stainless steel with a thermal conductivity 5
that of copper. The SWNT have a diameter of only 0.7–2 nm but can have an aspect ratio of at least 1,000:1 which can be used to advantage as an additive to provide polymers with electrical conductivity. Polymers can have fillers added to make them conducting and this usually takes the form of spherical powders dispersed into the polymer. The powders can be metals or carbon or even semiconducting materials such as fluorine-doped tin oxide. The conductivity relies on the powders being packed densely enough that the powders either touch or the polymer between the fillers is thin enough that there is conductivity between fillers. This again can be regarded as the conductivity being limited by the number of grain boundaries. The conductivity can be improved by minimizing these grain boundaries and this can be done by changing the shape of the fillers from spherical to flake where there is a high aspect ratio with two dimensions being large and one dimension being small. The SWNT takes this one step further by being small in two dimensions and large in only one dimension. In addition, the overall size is smaller as the

Transparent Conducting Coatings on Polymer Substrates

5.7.2

flakes tend to be of the order of 200 nm thick and then several microns across the other two dimensions. Thus, the SWNT at 0.7–2 nm diameter with an aspect ratio of 1,000:1 makes the filler size considerably smaller. Along with the higher conductivity of the filler, this means that conducting films can be made using only 0.04% loading of SWNT filler compared to a loading >>5% of spherical filler. This reduced size and quantity makes the conducting polymer more transparent than can be achieved by conventional filled conducting polymers. The SWNT-filled polymer coatings can already match ITO coatings at 90% visible light transmittance at 200 Ω/sq. Where the resistivity required is above 200 Ω/sq, the SWNT can be better than ITO. Depositing ITO with a resistivity greater than this becomes increasingly harder as the coating thickness decreases and can approach being discontinuous. For the SWNT, the higher resistivity coatings are much easier to achieve. The SWNT coatings are also considerably more flexible and have even been demonstrated to survive creasing which ITO cannot survive as the ceramic ITO cracks at a radius well before this severe treatment. The SWNT coatings also compare well to the PEDOT:PSS coatings where they show a transparency 10% higher for the same conductivity down in the 200–300 Ω/sq range. This does not imply that the SWNT-filled polymer coatings are going to replace either the conducting polymers or ITO imminently. SWNT have to be manufactured in much larger quantities and then improved sorting and filtering needs to be available. Currently, the nanotubes are manufactured and they can be of various diameters and a mixture of single and multiwall tubes. To get the best out of the material, the SWNT needs to be separated from the MWNT and then the SWNT material needs to be sorted into the different diameter tubes. This amounts to plenty of development work to be done before these materials become commonplace and widely available as rolls of transparent conducting polymer film. The SWNT has a performance approximately 10
better than the MWNT but the MWNT is much more easily produced and so currently more of the MWNT is sold and used in development trials; however, in the longer term, as the manufacture, sorting, and sizing improves, the cost of the SWNT will reduce and the quantities available increase and it is expected that the SWNT will become the material of choice. This topic is covered in more detail in > Chap. 5.4.2.

4

Additional Coatings

When using TCO-coated polymer film for applications such as displays or touch screens, there are additional coatings that ought to be considered. The performance of optical coatings depends on the transmittance, reflectance, and absorptance of all the layers involved. At each transition between materials, there may be a reflection the magnitude depending on the size of mismatch of refractive index between the two materials [34]. So for our ITO-coated PET film, we have the following refractive indices (RI): ITO RI ¼ 2:0 Air RI ¼ 1:0 PET RI ¼ 1:5 Where the ITO-coated PET film is used for touch screens, the top surface is usually the PET and this is a fairly soft polymer which can easily be scratched and damaged by fingernails poking the surface. Hence, the PET is commonly coated with a hard coating which if chosen carefully can be chosen to reduce the front surface reflection where there is the refractive index mismatch between the air and PET [35]. This can include the material choice but can also

985

986

5.7.2

Transparent Conducting Coatings on Polymer Substrates

include a texturing of the surface where the controlled surface roughness means there is, on average, proportions of the coating and air and so the effective surface refractive index is reduced [36]. This is shown schematically in > Fig. 3 The ITO to PET interface has a similar mismatch and there has been work done to grade the ITO refractive index as it is initially deposited so that it too has a better match to the PET and hence the reflection is reduced [37]. Further down, there may be a PET to air and the air to polymer or glass interfaces. In some cases, this air gap has been replaced with an index matched polymer and so further reduce the reflections. There is another alternative and that is to deposit an alternating high and low refractive index optical stack that is optimized to work as an antireflection coating [38]. This can work very well for the chosen wavelength but at other wavelengths may not be as good. This can be improved but this usually requires additional coating layers and this will increase the costs. A number of these antireflecting coatings whilst working well initially can be degraded over time as the surface becomes contaminated with finger grease. This has led to work on antireflection coatings that also include other functional performance such as conductivity and a low top surface energy [39] that has an antistick or antigraffiti performance. Other multilayer options include using the ITO coating as an antireflection coating to a very thin metal coating such as silver or gold. The very thin metal is around 7 nm thick and already transparent but with the ITO on each side acting as an antireflection coating, the visible transmittance can be increased. The advantage of using a thin metal is that the conductivity can be increased without needing the substrate to be heated but this is at the expense of some visible light transmittance.

Light source Diffuser film Light collimated film Rear polarizing film Rear glass with TFT Liquid crystal layer Front glass & color filter Front polarizing film View angle film elements Index matched adhesive layer Glass with graded index ITO Separator dots in air layer ITO sputtered PET Hard coat or textured hard coat or AR or vAR + hydrophobic layer

= interfaces where reflections can be reduced by either grading the refractive index or choosing a matched index material or texturing the surface

Active matrix touch panel liquid crystal display

. Fig. 3 A schematic of a touch panel display showing where it is possible to reduce some of the unwanted reflections

Transparent Conducting Coatings on Polymer Substrates

5

5.7.2

Conclusion

Vacuum-deposited coatings can offer a complete range of functionality, transparent conducting, antireflection, hydrophobic, antistatic, as well as electromagnetic shielding. However, there is the competition from conducting polymers and carbon nanotube-filled coatings. This holds the tantalizing prospect that some time in the future it will be possible to print of all the layers with no vacuum-deposited coatings required. Until then, TCO-coated flexible polymer substrates will continue to grow as it continues to compete with coated glass on price, flexibility, and lightweight. In addition, journals such as Thin Sold Films & J. Vacuum Science & Technology and proceedings of conferences such as the Annual technical conferences for the Society of Vacuum Coaters (SVC) and Association of Industrial Metallizers, Coaters and Laminators (AIMCAL) have a good supply of useful papers.

References 1. Paine DC et al (2005) Chapter 5, Transparent conducting oxide materials and technology. In: Crawford GP (ed) Flexible flat panel displays. Wiley, Chichester, pp 79–97. ISBN 0-470-87048-6 2. Lewis BG, Paine DC (2000) Applications and processing of transparent conducting oxides. MRS Bull 25:22–27 3. Murakami H et al (2007) Study of influence of under layer on indium tin oxide crystallization. In: 50th Annual technical conference proceedings, Louisville, KY. Society of Vacuum Coaters, Albuquerque, NM, pp 754–756 4. MacDonald WA (2004) Engineered films for display technologies. J Mater Chem 14:4–10 5. MacDonald WA et al (2002) New developments in polyester films for display applications. In: 45th Annual technical conference proceedings. Society of Vacuum Coaters, Albuquerque, NM, pp 482–486 6. Adam R et al (2004) Optimising polyester films for flexible electronic applications. In: Proceedings of the AIMCAL fall technical conference 2004, Charleston, South Carolina 7. Breil J (2009) Chapter 11. Oriented film technology. In: Wagner JR (ed) Multilayer flexible packaging – Technology and applications for the food, personal care and over-the-counter pharmaceutical industries. Elsevier, Amsterdam, pp 119–136. ISBN 13: 978-0-8155-2021-4 8. Kanai T, Campbell GA (eds) (1999) Film processing. Hanser Gardner, Cincinnati. ISBN 1 56990 252 6, Carl Hanser Verlag, ISBN 1 446 17882 9. Haake G (1977) Transparent conducting coatings. Ann Rev Matls Sci 7:73–93 10. Bright C (1983) How to specify and select transparent electrically conducting coatings. Photonics Spectra, (June & July)

11. Howson RP, Ridge MI (1981) Deposition of transparent heat-reflecting coatings of metal oxides using reactive planar magnetron sputtering of a metal &/or alloy. Thin Solid Films 77:119–125 12. Ridge MI et al (1983) Control of the optical properties of transparent conducting films prepared by reactive magnetron sputtering. SPIE. Thin Film Technol 401:301–306 13. Chopra KL et al (1983) Transparent conductors – A status review. Thin Solid Films 102:1–46 14. Vossen JL (1977) Transparent conducting films. Phys Thin Films 9:1–71 15. Dawar AL, Joshi JC (1984) Review: semiconducting transparent thin films: their properties & applications. J Matls Sci 19:1–23 16. Haake G (1986) Reactive sputtering from a Cd-Sn target. Thin Solid Films 137:101–109 17. Pisarkiewicz T et al (1987) Preparation, electrical properties & optical characterisation of Cd2SnO4 and CdIn2O4 thin films as transparent & conductive coatings. Thin Solid Films 153:479–486 18. Hartnagel HL et al (1995) Semiconducting transparent thin films. Institute of Physics, Bristol. ISBN 0 7503 0322 0 19. Louch S et al (2009) Transparent conducting oxides on polymerwebs. In: 52nd Annual technical conference proceedings. Society of Vacuum Coaters, Albuquerque, NM, pp 746–750 20. Howson RP et al (1984) Optimised transparent and heat reflecting oxide and nitride films. Sol Energy Mater 11:223–229 21. Cormia RL et al (1998) Roll to roll coating of indium tin oxide, a status report. In: Proceedings of the 41st annual technical conference. Society of Vacuum Coaters, Albuquerque, NM, pp 452–457

987

988

5.7.2

Transparent Conducting Coatings on Polymer Substrates

22. Bright C (2008) Alternative transparent conductive oxides [TCO] to ITO. In: 51st Annual technical conference proceedings. Society of Vacuum Coaters, Albuquerque, NM, pp 840–850 23. Jin Z, Granqvist CG (1987) Transparent & infraredreflecting ZnO:Al films reactively sputtered onto polyester foil. Appl Opt 26(16):3191–3192 24. Ponce de Leon C (2008) Conducting polymer coatings in electrochemical technology Part 2 – Application areas. Trans Inst Met Finishing 86(1):34–40 25. Wobser M (2009) New PEDOT formulations enable highly-conductive PET. In: 23rd International vacuum web coating conference & AIMCAL fall technical conference, 2009, Myrtle Beach, South Carolina 26. Ouyang J, Yang Y (2006) Conducting polymer as transparent electric glue. Adv Mater 18:2141–2144 27. Wang Y (2009) Research progress on a novel conductive polymer – poly[3,4- ethylenedioxythiophene] [PEDOT]. J Phys: Conf Ser 152, 012023 pp 1–10 (http://iopscience.iop.org/1742-6596/ 152/1/012023) 28. Lubianez RP et al (2008) Advances in PEDOT:PSS conductive polymer dispersions. In: 22nd international vacuum web coating conference & AIMCAL fall technical conference, 2008, Myrtle Beach, South Carolina 29. Zou J et al (2010) Metal grid/conducting polymer hybrid transparent electrode for inverted polymer solar cells. Appl Phys Lett 96(20)

30. Gu¨erri TB (2008) Transparent conducting coatings based on carbon nanotubes. Thesis 2008, Stuttgart University, Stuttgart 31. Hecht DS et al (2009) Carbon-nanotube film on plastic as transparent electrode for resistive touch screens. J SID 17(11):941–946 32. Gruner G (2006) Carbon nanotube films for transparent and plastic electronics. J Mater Chem 16: 3533–3539 33. Peltola J et al (2007) Carbon-nanotube transparent electrodes for flexible Displays. J Soc Info Display 23:20–23 34. Stetson J (2006) Analog resistive touch panels and sunlight readability. J Soc Info Display 22:26–30 35. Inoue M (2007) Roll-to-roll ITO film for touch panel application: durable ITO sputter deposition and hard coat wet coating. In: 50th Annual technical conference proceedings. Society of Vacuum Coaters, Albuquerque, NM, pp 674–676 36. Van Ostrand DK et al (2010) Microstructures to reduce the appearance of fingerprints on surfaces. Patent application No: 20100033818 2010 37. Devisser B (2003) Designing touch LCDs for portable devices. Inf Display 19(7):18–21 38. Aufderheide BE (2001) Triple layer anti-reflecting coating for touch screens. WO/2001/057579 39. Blanchard RD (2006) Resistive touch panel using removable, tensioned top layer. US Patent 7,071,927 07/04/2006

Further Reading Bishop CA (2007) Vacuum deposition onto webs, films and foils. Elsevier/William Andrew Publishing, Amsterdam. ISBN 978 08155 1535 7 Bishop CA (2010) Roll-to-roll vacuum deposition of barrier coatings. Scrivener Publishing/Wiley, Hoboken NJ. ISBN 978 0470 60956 9 Freund LB, Suresh S (2003) Thin film materials: stress, defect formation and surface evolution. Cambridge University Press, Cambridge. ISBN 0 521 82281 5 Glocker DA, Ismat Shah S (eds) (2002) Handbook of thin film processes Vol. 1 & 2. Taylor & Francis

Group/Institute of Physics Publishing, Bristol. ISBN 0 7503 0833 8 Hartnagel HL et al (1995) Semiconducting transparent thin films. Institute of Physics Publishing, Bristol. ISBN 0 7503 322 0 Vossen JL, Kern W (1978) Thin film processes. Academic Press, New York. ISBN 0-12-728250 5 Wasa K et al (2004) Thin film materials technology, sputtering of compound materials. Elsevier/ William Andrew Publishing, Amsterdam. ISBN 0 8155 1483 2

5.7.3 Anisotropic Conductive Adhesives Peter J. Opdahl 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 990

2

Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 990

3 3.1 3.2 3.3

Manufacturing Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 991 Lamination or Dispense Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 992 Mounting Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993 Bonding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993

4

Reliability Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994

5

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.7.3, # Springer-Verlag Berlin Heidelberg 2012

990

5.7.3

Anisotropic Conductive Adhesives

Abstract: Anisotropic conductive adhesives (ACAs) are a set of materials typically combining either epoxy or acryl adhesives and conductive particles to allow electrical connection across what would otherwise be a standard mechanical adhesive assembly. They differ from isotropic conductive adhesives such as silver epoxy in that the conductive particles are loaded and distributed in such a way that they do not conduct within the bulk of the adhesive, but do conduct in the Z-axis when they are trapped between electrodes on the top and bottom substrates. This allows them to offer some unique advantages compared with isotropic adhesives or various solder technologies. In the case of touch panels, these advantages are primarily related to its low temperature and high interconnect density capabilities, although cost and speed of assembly may also be considerations. ACAs are widely used in the display and electronics assembly industries. In flat panel displays, they are used to make the connection between the drive circuitry and the display itself. They are also used extensively in other applications that require high-density and/or lowtemperature assembly at high volume. This includes touch panels, camera modules for mobile phones, touchpads for notebook computers, and RFID assemblies for smartcards. ACAs have also had limited success in semiconductor packaging, but the reliability requirements for these applications are not always possible to achieve with ACA technology. List of Abbreviations: ACA, Anisotropic Conductive Adhesives; ACF, Anisotropic Conductive Film; ACP, Anisotropic Conductive Paste; COB, Chip-On-Board; COF, Chip-On-Flex; COG, Chip-On-Glass; DSC, Differential Scanning Calorimetry; FOB, Flex-On-Board; FOF, Flex-On-Flex; FOG, Flex-On-Glass; FPC, Flexible Printed Circuit; FTIR, Fourier Transform Infrared Spectroscopy; PCB, Printed Circuit Board

1

Introduction

Anisotropic conductive adhesive (ACA) is the generic term used to refer to anisotropic conductive film (ACF) and anisotropic conductive paste (ACP). The technology is based on the premise that a balanced loading and distribution of conductive particles within an adhesive matrix will allow those particles to become trapped between the upper and lower sides of an assembly, thereby conducting electricity through the vertical axis while not creating shorts in the horizontal axes. ACA is widely used in assemblies that do not lend themselves to solder or connector interconnections, such as high density connections to glass or to low-temperature substrates such as polyester or other polymers (> Fig. 1) [1]. ACAs were first developed in the mid-1970s, beginning as very simple hotmelt adhesive systems for driving LCD calculator displays using cheap connectors made with conductive inks screen-printed on polyester films. These systems were improved and ACF as we now know it was first released to the market in 1984.

2

Materials

ACF is currently much more common than ACP, although this is changing as materials and process technologies mature. The primary advantage of ACF is that the distribution of particles within an ACF can be fixed at the manufacturer during the curing stage after the material has been cast on the release film. Until the ACF undergoes the thermal compression required in the final assembly process, these particles are locked in the solid adhesive matrix, preventing their

Anisotropic Conductive Adhesives

5.7.3

FPC

Top and Bottom conductors aligned and bonded.

PCB

. Fig. 1 Cross-sectional view of an ACA assembly. White dots are conductive particles

movement. Conversely, ACPs can settle after manufacturing and must be thoroughly mixed before use. ACPs may also create uneven distribution during the dispense process as particles can agglomerate as they pass through the needle on their way to the substrate. ACP is not often printed due to the cost of wasted material. ACF is supplied on reels, typically in 50, 100, and 200m lengths and in widths as narrow as 1.0 mm. ACP is supplied in a variety of industry-standard syringe sizes. Both must be refrigerated during storage, typically at 0
C, 5
C. The pot life for ACPs varies significantly, but is usually less than 24 h. ACFs can be used for up to 2 weeks after being removed from their original packaging. ACAs are generally defined by their adhesive type, particle type, and particle loading. An example base specification would be an epoxy binder with 9 mm diameter nickel–gold plated polymer spheres at a loading of 750,000 particles/mm3. The adhesive system and particle type are determined by the type of assembly being made, while the loading required is a function of the interconnect density. > Table 1 shows some of the key characteristics and applications for which different ACAs are used. Nickel and polymer spheres used in ACAs may be Au or NiAu plated. Nickel dust and solder particles are never plated. The solder alloys used are lead-free and may be of varying composition. Nickel and solder particles may be used on OSP (organic solderability preservative) treated substrates, but polymer particles require a conductive surface such as Au, Ag, or Sn. The epoxy adhesive used is most typically a solventless single-part biphenol with imidazole accelerators. Curing temperatures are between 150
C and 220
C and the cure time ranges from 5 to 15 s. Acryl-based ACAs cure at 130–180
C in a time of 3–10 s.

3

Manufacturing Process

The ACA assembly process is generally the same for all types of assemblies, differing only in whether a paste is dispensed or a film is laminated (> Fig. 2). In the case of ACF film, the material is laminated to a substrate and its release liner removed to expose the top surface of the ACF. The top half of the assembly is then aligned to the substrate and mounted to it in a process analogous to SMT mounting. Lastly, the assembly is bonded together using heat to cure the adhesive. It is possible to combine the mounting and bonding steps into one step, but due to throughput restrictions they are normally separated for high volume applications. The highest throughput general use lines currently available utilize multiple ACF lamination, mounting, and bonding heads to achieve a total cycle time of under 2 s per bond assembly. > Table 2 shows typical cycle times per head for semiautomatic or automatic machines. Specialty lines for RFID assembly using ACP offer up to 10,000 UPH of throughput.

991

992

5.7.3

Anisotropic Conductive Adhesives

. Table 1 Common adhesive/particle configurations and their use Epoxy-based adhesives

Acryl-based adhesives

Processing

Assembly temp. (
C)

160–220

130–180

Reworkability

Low

High

Reliability

Environmental stability

Very high

High

Particle type and application use

Nickel dust (1–30 mm dia.)

RFID

RFID

Nickel spheres (6–10 mm dia.)

FOB, FOF, FOG

FOB, FOF, FOG

Solder spheres (9–10 mm dia.)

FOB, FOF

FOB, FOF

Polymer spheres (2–4 mm dia.)

COF, COG

N/A

Polymer spheres (5–10 mm dia.) FOB, FOF, FOG, COB

FOB, FOF, FOG

Polymer spheres (10+ mm dia.)

FOB, FOF

FOB, FOF

COB Chip on board, COF Chip on flex, COG Chip on glass, FOB Flex on board, FOF Flex on flex, FOG Flex on glass

Low Temp Low Pressure

Substrate

Remove release liner

ACF lamination

Higher Temp Higher Pressure Hotbar

Completion

Final bonding

Device alignment and mounting

. Fig. 2 ACA assembly process

3.1

Lamination or Dispense Process

Most ACFs are laminated at a pressure of 0.1–0.5 MPa, using 60–90
C for 0.2–1.0 s. It is critical that no air be trapped under the ACF during this process as this can lead to higher levels of voiding and lower peel strength after assembly. Automated systems will often use image recognition systems to inspect the laminated ACF for position, wrinkles, or other defects. Typical ACF lamination accuracy is 0.2 mm in both the X and Y axes.

Anisotropic Conductive Adhesives

5.7.3

. Table 2 Typical cycle times per equipment head Lamination or dispensing

Mounting

Bonding

Head-down time

0.2–2.0

.01–2.0

5.0–15.0

Total machine cycle time

4.0–10.0

1.0–10.0

8.0–25.0

Note: Times are in seconds. Total machine cycle time includes all equipment and operator operations except load and unload

Dispensing of ACPs is similar to dispensing of other adhesives. The rheological and thixotropic characteristics of the material used will largely determine the dispense technology and speed. It is notable that using heat in the dispenser to help control viscosity is not possible because it will cause the ACP to start curing.

3.2

Mounting Process

Mounting can be done at room temperature or with heat up to 100
C. Heat is used to decrease the amount of time the head must remain down and pressing the top side of the assembly into the ACF, but the use of heat must be managed so that it does not begin to cure the adhesive and affect the quality of the assembly. The amount of pressure used in the mounting process varies widely from 0.05 to 0.5 MPa. In general, pressure limitations are a function of the assembly equipment, not the material set. Mounting accuracy is dependent on assembly requirements, but for pitches down to 0.2 mm, which are typical for touch panel assembly, 15–20 mm is common. Equipment capable of submicron mounting accuracy is also available, but most volume manufacturing equipment will be in the 3–20 mm range.

3.3

Bonding Process

The final step is a thermal compression process where a heated press is brought into contact with the ACF assembly, causing the two sides to be pushed together, trapping the conductive particles between the opposing contacts and curing the adhesive. The heat may be supplied using several different methods. The most common method is called ‘‘constant heat,’’ where a heater is embedded into the press and heats a block of metal to a fixed temperature. This block then transfers heat through a tool made to the size of the ACF bond, curing the ACF assembly. The heater will engage and disengage only to keep the block at a predefined temperature and the block itself is sized to provide an appropriate thermal mass for the assembly. ‘‘Pulsed heat’’ systems employ an active controller that monitors and alternately heats or cools the bonding tool to actively raise and lower the temperature. This allows a profile to be created with multiple temperature points on it, offering greater flexibility, but with the penalty of higher equipment cost and somewhat lower equipment stability. With pulsed heat, the head is made of ceramic or metal and is designed to have a low thermal mass to assist with rapid heating and cooling.

993

994

5.7.3

Anisotropic Conductive Adhesives

X-axis contacts Y-axis contacts

Ground pad Stepped bond head sized for this assembly

. Fig. 3 Cross section of capacitive touch panel bonding area

Bond heads may be flat or stepped to accommodate different heights within the assembly. A capacitive touch panel that brings the X and Y contacts to the same side may have two or three different surfaces to be bonded. Using an appropriately designed head and nest, it is possible to do this in one pass, although two-pass assembly is also done. In the example above, we see an assembly with two planes upon which bonds must be made (> Fig. 3). There may also be a ground pad on the rear side of the assembly that will also use ACA. In this case, the reverse side must be bonded in a separate process and if possible should not be located such that it is under the bonding area on the top side of the assembly. This is to prevent the two bond processes from adversely affecting each other. The bonding process has the most impact on long-term reliability and will require testing to determine its robustness. The bonding profile contains heat, pressure, and time variables. Pressure will determine the gap remaining between the two sides of the assembly after bonding is complete. In general, this gap should be 30–50% the original diameter of the particles in the selected ACA. The gap can only be precisely measured by using cross sections, but in-process monitoring can be done by observing the diameter of the crushed particles and deriving a presumed gap from those values. Heat and time are adjusted together to inject the required amount of thermal energy into the bond. In general, a higher peak temperature will allow a shorter bonding time, but the adhesive must not cure so fast that it does not have time to adequately wet the surfaces being bonded or force out any trapped gasses. The temperature must also not ramp too slowly, as this may also prevent adequate wetting. A typical process will achieve 70–80% of the peak temperature within the first 2 s of the head contacting the assembly.

4

Reliability Testing

Reliability testing for ACA bonds can vary widely (> Table 3). Many but not all materials can comfortably survive multiple solder reflow passes at 250–260
C, and a well-implemented ACA interconnect is often more reliable than the SMT components surrounding it, particularly for drop tests. Electrical reliability is tested by looking at how the contact resistance of the joint increases over time. ACA contact resistance will vary widely depending on ACA selected and the type of surface to which it is making the connection (> Fig. 4). Gold-on-gold connections can have contact resistances of down to 5 mO. Typical contacts to ITO will yield a resistance of 10–15 mO.

Anisotropic Conductive Adhesives

5.7.3

. Table 3 Reliability testing conditions Typical test conditions High heat/high humidity

Epoxy-based adhesives

Acryl-based adhesives

85
C/85% RH

85
C/85% RH

1,000 hours

1,000 hours




Thermal shock

55 C, 30 min , room temperature, 5 min , 125
C, 30 min

40
C, 30 min , room temperature, 5 min , 100
C, 30 min

1,000 cycles

1,000 cycles

85°C/85% RH Resistance in mΩ

50 40

MAX AVG

30

MIN

20 10 0

0

500

1,000

1,500

Hours

. Fig. 4 Typical high temperature/high humidity resistance chart

In a good ACA assembly, contact resistance will often decrease for the first 100 h or so of high-temperature testing as the adhesive continues to cure. It will then stabilize and very little change will be seen afterward [2]. Peel strength is one area that is almost always tested. While a direct causal link between peel strength and reliability is often not possible to demonstrate, because it is easy to measure and easy to understand, users often spend a disproportionate amount of time trying to optimize it. The key metrics with peel strength should be the initial adhesion strength and long-term drop test reliability. Initial peel strength must be high enough to withstand any subsequent assembly processes prior to final assembly, and long-term adhesion must remain high enough to withstand normal use. Typical ACF assemblies using ACF for flex-on-flex (FOF) or flex-onboard (FOB) have peel strengths between 12–25N/linear centimeter [3]. At this level, the copper conductors on a PCB or FPC will typically delaminate prior to the ACF bond failing. Adhesion to other substrates such as flex-on-glass (FOG) is often slightly lower due to the flatter topography of the materials being joined [4]. FTIR (Fourier transform infrared spectroscopy) or DSC (differential scanning calorimetry) analysis is used to determine the final level of cure that was achieved within the adhesive. Levels over 85% will pass most consumer-grade reliability requirements. This is typically a critical milestone in the development of an ACA process because it not only defines reliability to a great extent, but also sets the head-down time of the bond cycle. Because the head-down time is almost always the longest part of the assembly process, this determines the throughput for that portion of the line.

995

996

5.7.3 5

Anisotropic Conductive Adhesives

Conclusion

ACAs provide an environmentally friendly and cost-competitive solution to high-density interconnects. They allow assemblies at temperatures as low as 130
C and offer reliability that meets or exceeds that typically required for consumer electronics. The technology is already widely used in both the flat panel display and electronics assembly industries, making it the leading choice for use in interconnecting touch panels and other assemblies requiring the density, reliability, economics, and industry-wide acceptance that ACAs offer.

References 1.

2.

3.

Savolainen P, Saarinen I, Rusanen O (2004) Highdensity interconnections in mobile phones using ACF. In: Polytronics 2004 IEEE international conference on polymers & adhesives, Portland, September 2004, AP22 Islam RA, Chan YC (2004) Effect of drop impact energy on contact resistance of anisotropic conductive film adhesive film joints. J Mater Res 19(6): 1662–1668 Chen X, Zhang J, Jao C, Liu Y (2005) Bonding parameters of anisotropic conductive adhesive

4.

film and peeling strength. Key Eng Mater 297–300: 918–923 Kim H-J, Chung C-K, Yim M-J, Hong S-M, Jang S-Y, Moon Y-J, Paik K-W (2006) Study on bubble formation in rigid-flexible substrates bonding using anisotropic conductive films (ACFs) and their effects on the ACF joint reliability. In: Proceedings of the 56th electronic components and technology conference, 2006, San Diego

Further Reading Anisotropic Conductive Film. http://www.sonycid.jp/en/ products/dd1/index.html Anisolm. http://www.hitachi-chem.co.jp/japanese/products/do/001.html

A list of high-quality papers on ACF that are available online is maintained at http://autoacf.com/ ACF_Online_Resources.aspx

5.7.4 Touchscreen Computer Interfaces: Electronics Lance Lamont . Carol Crawford 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 998

2

Trends in Touch Screen Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 998

3 3.1

Electronics as Part of a Touch System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999 Role of Touch Screen Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999

4 4.1 4.2 4.2.1 4.2.2 4.3 4.3.1 4.3.2

Driving the Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999 Driving a 4-Wire Analog Resistive Touch Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999 Driving a Projected Capacitive Touch Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 Capacitance of Touch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003 Capacitance Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003 Capacitive Sensing Module (CSM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003 Charge Time Measurement Unit (CTMU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003

5 5.1 5.2

Interpreting the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 Filtering the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005 Calibrating the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005

6

Gestures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1006

7

Options for Touch Screen Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1006

8 8.1 8.2

Touch Electronics Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1006 Information Within a Data Packet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1007 Some Typical Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1007

9

Summary/Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1008

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_5.7.4, # Springer-Verlag Berlin Heidelberg 2012

998

5.7.4

Touchscreen Computer Interfaces: Electronics

Abstract: This chapter will review the history and role of touch screen electronics as the interface to touch technology. Touch screens are user-friendly input devices that are quickly becoming a human interface standard. Common examples of touch screen applications include cell phones, GPS units, public kiosks, and industrial or medical input devices. Touch adds to the value proposition of a wide variety of products by enhancing functionality and ease of use. Touch screen systems are typically made up of three key elements: the touch screen sensor, the electronics, and the display. This chapter will explore how touch electronics profoundly impacts design, functionality, and customer experience in touch systems. List of Abbreviations: CSM, Capacitive Sensing Method; CTMU, Charge Time Measure Units; EEPROM, Electrically Erasable Programmable Read-Only Memory; GUI, Graphical User Interface; ITO, Indium Tin Oxide

1

Introduction

Touch technology as a human–machine interface has been around since the 1970s. Early examples of touch interface designs were matrix, analog resistive, and infrared touch solutions. These were primarily used for industrial, point of sale, and medical devices. Developments in materials, manufacturing processes, and electronics contributed to the introduction of new touch technologies into broader markets like casino gaming machines, self-serve terminals/ kiosks, and handheld devices. Most of the touch solutions for the past 30 years have been focused on single touch, nonmoving applications. Navigating a static button menu or changing a setting in an industrial application requires one activation point and little fanfare. The introduction of the iPhone in 2006 with its multi-touch-enabled gestures inside a dynamic graphic user interface (GUI) is commonly referenced as a pivotal evolutionary step in the touch industry that raised performance expectations. Now multi-touch capability, advanced gestures, and an intuitive GUI are rapidly becoming an interface standard.

2

Trends in Touch Screen Electronics

Touch screen electronics have closely matched the general advancement and pressures of consumer electronics. With the release of ‘‘must-have’’ gadgets over the last 10 years, touch sensing has become a very popular element to differentiate a product. Consumer items like mobile phones, MP3 players, and other high-volume applications have caused significant price and performance pressure for touch electronics. As processing power of the main processor for the system (host) has increased, transferring processing from the touch electronics to the host has occurred. This allows simpler and cheaper touch electronics and a more compute-intensive driver on the host side. This trend will continue for many high-powered devices; however, discrete touch electronics are often considered for reliable performance in systems that benefit from off-loading processing time and power from the host. As processing power has increased, additional touch technologies have also become attainable. Advanced video processing and high-speed electronics have allowed for the implementation of many of the multi-touch systems available today.

Touchscreen Computer Interfaces: Electronics

5.7.4

As the consumer market continues to push for new and exciting electronics, touch sensing technologies will continue to be created and improved. One of the many challenges the touch industry faces is the lack of standards for some of the new developments. The advanced pace of recent years has led most touch providers to create custom designs compared to off-the-shelf solutions that were common only 10 years ago.

3

Electronics as Part of a Touch System

A typical touch system involves three primary components: a sensor, driving electronics, and software to interpret the touch, typically in the form of a driver or operating system. Depending upon the touch system, each of the main elements can have a different level of cost or complexity and often come with trade-offs at the software driver or electronics level.

3.1

Role of Touch Screen Electronics

Touch screen electronics play the central role in all touch systems. The sensor is connected to the electronics, and the electronics present touch data to the software on the host via a communications protocol, as shown here in > Fig. 1. The touch electronics have several possible functions: driving the sensor, interpreting the data (as shown in Touch Decoding in > Fig. 1) from the sensor, filtering the data, and calibrating the data. The primary function of the touch electronics is to drive the sensor. This generates data from the sensor that can be processed into touch information. Some touch controllers are only passing this raw touch data to the host for processing. More advanced touch controllers are able to perform all of the other steps such as interpreting, filtering, and calibrating touch data. This off-loads work from the host which is sometimes preferred, depending on system requirements.

4

Driving the Sensor

The primary purpose of touch sensor electronics is to drive the sensor in order to generate data that can be interpreted as touch information. Each touch sensing technology requires a different method to drive the sensor and there can be further variation within each technology. We’ll explore the basic functionality of a 4 wire analog resistive sensor and a typical projected capacitive sensor to illustrate the concepts of these two common touch technologies.

4.1

Driving a 4-Wire Analog Resistive Touch Sensor

A 4-wire analog resistive touch sensor consists of a Stable and Flex layer, mechanically and electrically separated by spacer dots. The layers are assembled perpendicular to each other. The touch position is determined by first applying a voltage gradient across the flex layer and using the stable layer to measure the flex layer’s touch position voltage. The second step is applying a voltage gradient across the stable layer and using the flex layer to measure the stable layer’s touch position voltage (> Fig. 2).

999

Low power wake-up

A/D Converter Touch decoding

Calibration corrected data option

Configuration registers

Coordinate data filtering

I2C

TTI

SPI

Communication control

. Fig. 1 Microchip’s AR1000 Universal Resistive Touch Screen Controller Block Diagram (© 2009 Microchip Technology Inc.)

MUX

Internal clock

Host

5.7.4

Any manufacturer 4, 5, or 8-wire Sensor

Touch screen drivers

1000 Touchscreen Computer Interfaces: Electronics

Touchscreen Computer Interfaces: Electronics

Drive and measure X-axis

5.7.4

Drive and measure Y-axis 5 V DC X 5 V DC

ITO Spa

cer ITO

ITO Spa

cer ITO

Y

. Fig. 2 Driving a 4-wire analog resistive touch sensor (© 2009 Microchip Technology Inc.)

The measured voltage at any position across a driven axis is predictable. A touch moving in the direction of the driven axis will yield a linearly changing voltage. A touch moving perpendicular to the driven axis will yield a relatively unchanging voltage. Analog resistive sensors are typically the most easily integrated sensors, and can generate very good single finger touch data. This data could be interpreted as gestures as well as standard cursor movement [1].

4.2

Driving a Projected Capacitive Touch Sensor

Projected capacitive touch typically has a smooth glass front and supports the multi-touch and advanced multi-finger gesture features that are so popular in many consumer applications today. The basic construction of a projected capacitive sensor typically uses two dielectrically separated layers of glass patterned with ITO that are configured in rows and columns positioned perpendicularly to create a matrix. Projected capacitive functionality is achieved by scanning along each axis, identifying high capacitance lines, then determining a position by the intersection of those high capacitance lines (> Fig. 4). This scanning system and the sensor pattern allow potential decoding of multiple simultaneous touch points known as ‘‘multi-touch’’ capable [2]. The side view (> Fig. 3) shows a possible combination of the layers that make up a projected capacitive sensor. This sensor pattern (> Fig. 4) shows a typical row and column configuration. There are a variety of sensor materials and patterns that operate on the same basic principle involving a change in capacitance. We will review possible capacitive sensing methods below.

4.2.1

Capacitance

Capacitance is the ability of a material to store electrical charge. A simple capacitor model is two conductive plates separated by an insulator Capacitance (farads) = k
e0
(A/d) where e0 = permittivity of free space = 8.854e – 12 F/m

1001

1002

5.7.4

Touchscreen Computer Interfaces: Electronics

Overlay (optional) Front panel Top Y-axis electrodes Adhesive Bottom X-axis electrodes Back panel Shield

. Fig. 3 Typical projected capacitive sensor layer construction (© 2009 Microchip Technology Inc.)

Y09 Touch #1 Y08 Y07 Y06 Y05 Touch #2

Y04 Y03 Y02 Y01

X12 X11 X10 X09 X08 X07 X06 X05 X04 X03 X02 X01

. Fig. 4 Typical projected capacitive two-layer construction (© 2009 Microchip Technology Inc.)

The value of capacitance is dependent on ● Surface area of the plates ● Distance between the plates ● Materials constant for the insulator between plates

Touchscreen Computer Interfaces: Electronics

4.2.2

5.7.4

Capacitance of Touch

The capacitance of touch is dependent on sensor design, sensor integration, touch controller design, and the touch itself. Some examples of sensor properties that affect its capacitance are: ● ● ● ●

Front panel thickness Electrode geometry and pitch X,Y layer-to-layer spacing Rear shielding

The capacitance of the sensor and of the touch can vary significantly, based on the many variables.

4.3

Capacitance Measurement Methods

There are a number of methods to measure capacitance. We will take a closer look at the example methods of Relaxation Oscillator using Capacitive Sensing Method (CSM) and the Charge Time Measure Units (CTMU).

4.3.1

Capacitive Sensing Module (CSM)

The CSM is a proprietary Microchip hardware module available in a variety of different PIC microcontrollers. The CSM enables the measurement of capacitance based on the relaxation oscillator methodology. The CSM produces an oscillating voltage signal for measurement, at a frequency dependent on the capacitance of an object connected to the module. The basic concept is as follows: ● CSM oscillates at some frequency, dependent on the capacitance of a connected sensing electrode. ● CSM frequency changes when a touch is introduced near the sensing electrode because the touch changes the total capacitance presented by the electrode. ● CSM frequency change is used as an indication of a touch condition. A simplified block diagram of the CSM to sensor interface is shown in > Fig. 5.

4.3.2

Charge Time Measurement Unit (CTMU)

The Charge Time Measurement Unit (CTMU) is a proprietary silicon-based Microchip module with the capability to perform capacitance measurements along with other advanced measurement capabilities. The following relationship can be used to determine the capacitance. i¼C or

dv DV ¼C dt Dt

Current = Capacitance ∗ (Change in Voltage/Change in Time)

1003

1004

5.7.4

Touchscreen Computer Interfaces: Electronics

Microchip PIC® Microcontroller Channel control Sensor Capacitive sensing module (CSM)

Sensor electrode

Measure oscillation frequency

Sensor electrode

CTouch

CElectrode

. Fig. 5 Simplified block diagram of the CSM to sensor interface (© 2009 Microchip Technology Inc.)

Measure Voltage After Charging for Time  Method 1. 2. 3. 4.

Connect a charging voltage or current to the capacitance load. Start a timer. Wait a fixed delay time. Measure the voltage that the capacitive load has been charged to. Measure Time to Charge to Measured Voltage  Method

1. 2. 3. 4.

Connect a charging voltage or current to the capacitance load. Start a timer. Measure the voltage that the capacitive load has charged to. If measured voltage has not exceeded a defined value, then repeat at (3).

Stop timer, which represents how long it took for the capacitive load to charge to the desired voltage. A block diagram of the CTMU module is shown in > Fig. 6.

5

Interpreting the Data

Interpreting the data is the initial process of determining if there is touch activation on the sensor. The sophistication and complexity of this step is a key differentiator between competing touch electronic solutions. Creating and determining the right thresholds are protected secrets that must determine a real intentional touch versus outside influencers such as the effects of water, temperature, conducted or radiated noise, and the inadvertent bump of the user. This functionality may be present in the touch electronics themselves or made to be a part of the application software. There is considerable variation in the quality of these features depending

Touchscreen Computer Interfaces: Electronics

5.7.4

+VDrive Current Source

Charge Mux Electrode (capacitive load)

CTouch

CParasitic

Measure Voltage

ADC

Discharge

Channel Select

CPU

. Fig. 6 Block diagram of the CTMU module (© 2009 Microchip Technology Inc.)

on the supplier and their implementation. Some of this variation can be covered up with additional filters and also software applications that have high error tolerances such as in menu applications that only require activating large buttons.

5.1

Filtering the Data

Once potential data has been generated, there is always the possibility for invalid data to be generated. Adding a filtering layer is a typical solution to address this issue. This functionality may include averaging filters, ‘‘zinger’’ filters or other filters that remove significant outliers from the data stream. Advanced or proprietary filters like timers to generate a touch-up state are often application specific and result from collaborative work with the IC supplier to support the design. The presence and sophistication of these elements can indicate a supplier’s long history in supporting touch designs. Filters are sometimes built into the touch electronics as configurable registers. This is common for advanced touch electronics. More often, they are provided as sample code for the user to implement by the host in a post processing step of the touch data.

5.2

Calibrating the Data

Some touch technologies have inherent position information. Others present a range of data that can change from sensor to sensor or even with environmental changes. Calibration is the process of correcting the touch data values to be presented within the expected range of values. The most basic example is calibrating the touch screen coordinates to the orientation of the display. The touch screen by itself has no association with what is displayed beneath it, nor the orientation of up, down, left, and right. Calibration is important for off-the-shelf touch solutions that serve multiple markets and multiple needs. It is frequently delivered as software by the provider of the touch electronics. Embedded devices often control this element and build in any calibration the touch system requires into the data processing by the host.

1005

1006

5.7.4 6

Touchscreen Computer Interfaces: Electronics

Gestures

Gestures have become increasingly popular in modern touch applications. Almost any touch technology that supports detection of motion can be utilized to generate gesture information. Single touch technologies will be limited to single touch gestures such as swipes and scrolling motions, whereas multi-touch technologies would be able to generate data that can be interpreted as many different gestures.

7

Options for Touch Screen Electronics

When selecting touch screen electronics, there are quite a few options, each with its own tradeoff between price, performance, and development time. The general categories are standalone controllers, multipurpose devices that include touch support, and host CPUs with touch. Some touch sensor manufacturers have developed controllers that work with their sensor construction and technology, to be sold bundled with their sensors. Other solutions that focus on developing universal touch controllers are designed to work with any sensor, regardless of manufacturer or construction, to allow for greater supply chain flexibility with changing sensors and displays. Full driver support for all major operating systems with a reach feature set like multi-monitor support, timed click functionality, and other proprietary functions are typically free with these scenarios. Another option for analog resistive touch controllers comes from the suppliers of analogto-digital convertors (A/Ds). Resistive touch screen decoding starts with an A/D measurement. Therefore, many A/D suppliers offer their parts as touch screen controllers with sample code that the host can use to process the data from this external A/D. This is a simple way to provide touch functionality, but requires the designer to do further development to create a complete touch solution. As an option for further integration of electronics, many microcontroller companies offer software stacks or libraries that integrate touch screen driving into a microcontroller or even a general-purpose CPU. This allows for flexible designs that can be ported to many generic devices. Also, core processors for embedded designs may offer basic 4-wire resistive touch decoding by utilizing the built-in A/D capability of the host. If the host can handle the load and the lines are free, this can be an attractive option for certain designs. When determining which sensor electronics are appropriate for an application, it’s important to evaluate several variables past cost alone such as performance, development time, ESD protection, RFI/EMF protection, additional required components, supply chain flexibility, and level of support.

8

Touch Electronics Communications

All touch electronics present their data to the software or other electronics via a communications protocol. Depending upon the controller, the base communications standard could be I2C, SPI, UART, PS/2, USB, or any other. Controllers intended for embedded environments typically communicate over I2C, SPI, or UART, whereas controllers intended to integrate into full computer systems typically utilize UART or USB. PS/2 has decreased in popularity significantly, and has some additional challenges due to PS/2 specifications.

Touchscreen Computer Interfaces: Electronics

8.1

5.7.4

Information Within a Data Packet

A touch controller generates data packets that can contain many different pieces of information. If it is a simple ‘‘controller,’’ it may just be a stream of A/D measurements. More intelligent controllers will typically output at least: ● Touch status (touch down/up) ● X touch coordinate ● Y touch coordinate There are numerous other data elements that could be included in a data packet, including: ● ● ● ● ● ● ● ●

Gesture (if detected/identified) ‘‘Confidence’’ Additional buttons (right click) Touch ID (if multi-touch) Z axis (‘‘pressure’’) Touch width Touch height Tilt An example of how the Microchip AR1000 data packet looks like this: Byte # 1 2 3 4 5

Bit 7 1 0 0 0 0

Bit 6 R X6 0 Y6 0

Bit 5 R X5 0 Y5 0

Bit 4 R X4 X11 Y4 Y11

Bit 3 R X3 X10 Y3 Y10

Bit 2 R X2 X9 Y2 Y9

Bit 1 R X1 X8 Y1 Y8

Bit 0 P X0 X7 Y0 Y7

The Microchip AR1000 data packet. © 2009 Microchip Technology Inc.

Where P: 0 Pen-Up, 1 Pen-Down R: Reserved X11-X0: X-axis coordinate Y11-Y0: Y-axis coordinate Note that the AR1000 data packet contains the typical data values of touch (pen) state and X and Y coordinates. The minimal amount of information allows for rapid data packet transmission which enables a fast response rate. Additionally, bit 7 is denoted as a ‘‘Synch Bit,’’ which allows software to easily identify the start of a packet.

8.2

Some Typical Commands

At a minimum, most touch controllers will support basic commands such as enable and disable. Beyond those, many controllers will have highly developed command sets, allowing configuration of serial numbers, recording of calibration, and access to on-board EEPROM (electrically erasable programmable read-only memory) for custom usage.

1007

1008

5.7.4

Touchscreen Computer Interfaces: Electronics

For example, the AR1000 controller supports the following commands: ● ● ● ● ● ● ● ● ● ●

Enable touch Disable touch Calibrate mode Register read Register write Register start address request Registers write to EEPROM EEPROM read EEPROM write EEPROM write to registers

This simple set of commands allows for advanced functionality, particularly accessing and modifying configuration registers within the chip.

9

Summary/Conclusion

Touch screens are user-friendly input devices that are quickly becoming a human interface standard. This chapter has reviewed the history and current trends of the touch industry from the touch screen controller perspective. This overview of the role of the touch electronics has focused on its importance as the primary component of a touch system.

Acknowledgment This chapter is printed with the permission of Microchip Technology Incorporated. No further reprints or reproductions may be made of said materials without Microchip Technology Inc.’s prior written consent.

References 1. 2.

Microchip Technology (2009) AR1000 Data Sheet DS41393A O’Connor T (2010) Microchip mTouch™ Projected Capacitive Touch Screen Sensing Theory of

Operation. A Technical Publication available from the Microchip mTouch Design Center at http:// www.microchip.com\mtouch

Further Reading Bobrow LS (1981) Elementary linear circuits analysis. Holt Rinehart & Winston, Amherst Buxton B (2007) Sketching user experiences: getting the design right and the right design (interactive technologies). Morgan Kaufman, San Francisco

Journal of the Society for Information Display. http:// www.sid.org/

Section 6

Emissive Displays

Part 6.1

Inorganic Phosphors

6.1.1 Luminescence of Phosphors Robert Withnall . Jack Silver 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014 2 Phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014 3 Stokes and Anti-Stokes Luminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015 4 Spontaneous Emission of a Two-Level System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015 5 Electric Dipole Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1016 6 Oscillator Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1016 7 Emission Lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1016 8 Conclusion/Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1018

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.1.1, # Springer-Verlag Berlin Heidelberg 2012

1014

6.1.1

Luminescence of Phosphors

Abstract: This chapter introduces the subject of luminescence in inorganic materials, and the physical mechanisms that are responsible for the light-emitting behavior of fluorescent and phosphorescent phosphors, including Stokes (and anti-Stokes) emission, spontaneous emission, electric dipole transitions, oscillator strength, and emission lifetimes. List of Abbreviations: CL, Cathodoluminescence; CRT, Cathode Ray Tube; EL, Electroluminescence; LED, Light-Emitting Diode; OLED, Organic Light-Emitting Diode; PL, Photoluminescence

1

Introduction

Luminescence can be defined as the conversion of energy into visible (or near-visible) light by non-incandescent sources. It can be induced by electron impact, electromagnetic radiation, chemical, electrical, or physiological excitation, as well as friction and thermal stimulation, and it is distinct from incandescence which is broadband light emission from thermally excited blackbodies. The emitted light arises from electronic transitions between discrete stationary states of luminescent centers or, in the case of extended electronic state solids having delocalized electrons, between electronic bands. In this section, we discuss the physics of luminescence in phosphors, and how this relates to display applications. The specific case of rare-earth phosphors is discussed in detail in > Chap. 6.1.2.

2

Phosphors

A phosphor can be defined as a solid material that manifests luminescence following the absorption of energy from an external source. Light emission from a material during the time it is exposed to exciting radiation (or other energy source) is referred to as fluorescence, in contrast to where the emission (afterglow) is detectable after the end of the excitation it is called phosphorescence (which could be up to several hours). It is important to highlight that these are definitions as used for inorganic materials, whereas the terminology has specific meaning related to organic molecules in organic light-emitting diode (OLED) technology. For organic compounds, fluorescence is used to describe light emission from a singlet state, and phosphorescence refers to emission from a triplet state (> Chap. 6.6.1). In display technology, the constraints on energy provision for phosphors are as follows: Photons – Photoluminescence An electric field – Electroluminescence Cathode rays – Cathodoluminescence Other kinds of luminescence include chemiluminescence, thermoluminescence, and triboluminescence. Electroluminescence is the phenomenon of light emission as a result of the application of an external electric field to a special kind of phosphor (i.e., an electroluminescent (EL) phosphor). EL phosphors are specially designed to respond to such a field. The term EL is also used to describe the light emission produced by low field, injection, or recombination luminescence, which is covered in > Chap. 6.4.1 on light-emitting diodes. Cathodoluminescence is the term used to describe emission from a solid phosphor that is excited by electron beams. In cathode ray tubes (CRTs), the electrons can be focused into

Luminescence of Phosphors

6.1.1

. Table 1 Example ZnS phosphors Co-activator ion Activator ion

(Donor)

(Acceptor)

Quencher ion (Luminescence killer)

Ag

Cl

Al

Fe

Cu

Br

Sc

Co

Au

I

Ga

Ni

Mn

In

a beam that can be raster scanned across the screen. The application of cathodoluminescence to CRTs is discussed in > Chap. 6.2.1. Phosphors essentially consist of very pure inorganic materials doped with suitable ions called activators. The activator is usually present in concentration levels varying from a few parts per million to 1–5% of the host lattice. Often, additional ions act as charge compensators or donors in the lattice. These are termed co-activators. An example of the combination of activator, co-activator and quenching ions that determine the properties of a ZnS host lattice phosphor is given in > Table 1.

3

Stokes and Anti-Stokes Luminescence

To understand all the processes that are usually included under the general heading of luminescence, it is first necessary to appreciate how photons interact with solid-state lattices. Thus, in Stokes luminescence, higher energy (could be a photon or energetic electron) is absorbed in some form and then there is an emission at lower energy of a photon. The emission is due to an electronic transition from an upper excited state to the ground state or lower energy excited state. The excitation may be due to the absorption of energy from an external source (examples of such sources include high-energy particles, other electrons, photons, or even external electric fields) or else it could be due to energy transfer within the phosphor sample. This process takes place in the solid often on a dopant atom (cation) and this atom is referred to as the activator, and its location as an active center. In such a case, the excess energy that was absorbed by the solid is converted to lattice vibrational energy (heat). If instead of Stokes luminescence, two or more photons excite the same activator center then the emission can occur at higher energy than that of the exciting photons; this is referred to as upconversion which is an anti-Stokes process. The kinetics of the latter process exhibit a nonlinear dependence of the emission intensity on the exciting power density.

4

Spontaneous Emission of a Two-Level System

We consider two levels coupled by an electric dipole-allowed transition. In addition to stimulated emission, which occurs at a rate that is proportional to the density of radiation at the transition frequency, Einstein found that a spontaneous emission process must occur at the transition frequency in order to obtain thermal equilibrium in the radiation field [1]. The spontaneous radiative decay rate coefficient, A21, is the inverse of the natural radiative

1015

1016

6.1.1

Luminescence of Phosphors

lifetime (i.e., the 1/e emission lifetime, t), and it can be deduced that the emission lifetime can be determined from the absorption coefficient, B12, according to > Eq. 1 [2]. A21 ¼

1 8phn321 g1 B12 ¼ c 3 g2 t

ð1Þ

where h is Planck’s constant, c is the velocity of light, n21 is the transition frequency, and g1 and g2 are the degeneracies of the lower and upper levels, respectively. Thus, it can be seen from (> Eq. 1) that the spontaneous emission rate increases as the transition frequency, n21, increases. It should be noted that spontaneous emission is a random process and obeys first-order kinetics.

5

Electric Dipole Transitions

Strong absorption and emission are due to electronic transitions that are electric dipole in type. For the case of absorption, the oscillating electric field of the incident light beam induces an oscillating dipole in the molecule and, when this corresponds to a natural frequency of the molecule, resonance occurs and the molecule gains energy from the light wave. For the case of spontaneous emission, it can be viewed that the emission is stimulated by zero-point fluctuations in the vacuum electromagnetic field, which induce an oscillating dipole in the molecule causing it to lose energy to the surrounding field. For both the aforementioned absorption and emission cases, the processes are electric dipole allowed. Such electric dipole-allowed transitions occur for s-p, p-d, d-f transitions, but s-d, p-p, d-d, f-f transitions are electric dipole forbidden according to the so-called parity or orbital selection rule.

6

Oscillator Strength

The oscillator strength, f21, is useful for specifying the intensity of a transition and it is directly proportional to the spontaneous radiative decay rate coefficient, A21, as can be seen from > Eq. 2 [2]. f 21 ¼

mc 3 g2 A21 8p2 n221 e 2 g1

ð2Þ

where m is the mass of an electron, e is the electronic charge, c is the velocity of light, and g2 and g1 are the degeneracies of the upper and lower levels, respectively. The oscillator strength links the quantum theory of emission and absorption of radiation to the classical theory and it is observed that f is close to unity for electric dipole-allowed transitions and f is very much less than unity for forbidden transitions.

7

Emission Lifetimes

For the aforementioned two-level system, the emission rate is given by > Eq. 3. Iem ¼ 

dN2 ¼ A21 N2 dt

6.1.1

Luminescence of Phosphors

1

Intensity

0.8 0.6 0.4 0.368 0.2 0 0

t

20

40

60

80

100

Time/ns

. Fig. 1 A plot of spontaneous emission intensity versus time showing the 1/e emission lifetime, t

where N2 is the number of emitters populating level 2 at time t, and A21 is the spontaneous emission rate coefficient. Integration gives N ðtÞ ¼ Nð0Þ expð  A21 tÞ and since the emission intensity is proportional to the number of emitters at any given time, IðtÞ ¼ Ið0Þ expð  A21 tÞ Substituting the emission lifetime t = 1/A21 gives IðtÞ ¼ Ið0Þ expð  t=tÞ Thus after a time t equal to t, the emission intensity has decreased to 1/e ( = 0.368) of its value at t = 0 (see > Fig. 1). Consequently, t is referred to as the 1/e emission lifetime. Luminescence is a general term for an emission process that does not distinguish the magnitude of the emission lifetime, but fluorescence and phosphorescence are more specific terms. Fluorescence is a spin-allowed (DS = 0) emission process that occurs on a relatively fast timescale (1/e emission lifetime < ca. 1 ms) due to the relatively high oscillator strength of the electronic transition that gives rise to the emission. Conversely, phosphorescence is a spin-forbidden (DS 6¼ 0) emission process that occurs on a relatively slow timescale (1/e emission lifetime > ca. 1 ms) due to the relatively low oscillator strength of the electronic transition that gives rise to the emission. Typical 1/e emission lifetimes of activator ions in seconds are as follows: Mn (3d!3d)  102 Cu+ (4s!3d)  103–104 Ag+ (5s!4d)  106–105 Eu3+ (4f!4f)  104–102 Tb3+ (4f!4f)  104–102 Ce3+ (5d!4f)  3  108 Eu2+ (5d!4f)  8  107 2+

1017

1018

6.1.1 8

Luminescence of Phosphors

Conclusion/Summary

The principles of luminescence in inorganic materials have been discussed, outlining the primary mechanisms that affect the energy conversion and light-emitting properties of phosphors for use in displays and other visible light-emitting applications. A more detailed discussion of the physics of rare-earth phosphors is given in > Chap. 6.1.2, with an outline of the chemistry and synthesis of phosphor materials provided in > Chap. 6.1.3.

References 1.

Einstein A (1917) Zur Strahlung. Physik Z 18:121

Quantentheorie

der

2.

Steinfeld JI (1986) An introduction to modern molecular spectroscopy, 2nd edn. MIT Press, Cambridge, MA

Further Reading Blasse G, Grabmeier BC (1994) Luminescent materials. Springer, Berlin Kitai AH (ed) (1993) Solid state luminescence. Chapman & Hall, London Kitai AH (ed) (2008) Luminescent materials. Wiley, Chichester

Nakazawa E (2007) Fundamentals of luminescence. In: Phosphor handbook, 2nd edn, Chap. 2. CRC Press, Boca Raton Ronda C (ed) (2008) Luminescence. Wiley VCH, Weinheim

6.1.2 Physics of Light Emission from Rare-Earth Doped Phosphors Robert Withnall . Jack Silver 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1020 1.1 Activator Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1020 1.2 Phosphors Containing Rare Earth Element Activator Ions . . . . . . . . . . . . . . . . . . . . . . . . . 1020 2

f-f Electronic Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1020

3

d-f Electronic Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024

4

Charge-Transfer Electronic Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1027

5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1027

6

Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1027

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.1.2, # Springer-Verlag Berlin Heidelberg 2012

1020

6.1.2

Physics of Light Emission from Rare-Earth Doped Phosphors

Abstract: A detailed discussion of characteristics of rare earth element doped phosphors is presented via a discussion of the f-f, d-f, and charge-transfer electronic transitions associated with rare earth ions. This chapter explains how the Dieke diagram is used to characterize these transitions, and uses the phosphors Y2O3:Eu3+ and YAG:Ce3+ as specific examples of important phosphors. List of Abbreviations: PL, Photoluminescence; REE, Rare Earth Element; YAG, Yttrium Aluminum Garnet

1

Introduction

As discussed in > Chap. 6.1.1, phosphors can be excited in different ways in order to achieve their excited states, although one should always be mindful that optical excitation (PL) must be carried out at a wavelength that coincides with the wavelength region of the phosphor absorption. However, the decay mechanisms are the same, irrespective of the pathways by which the excited states are originally populated. The light emission occurs via radiative processes, which are in competition with non-radiative processes, and it is important for the former to dominate the latter in order for phosphors to emit light efficiently. Indeed, the loss of luminescence or ‘‘quenching’’ of the luminescence can be caused by different mechanisms (e.g., thermal quenching, concentration quenching, impurity quenching – the presence of luminescent ‘‘killers,’’ or quenching ions). Also, for inorganic solids, de-excitation via the population of lattice phonons can result in significant non-radiative relaxation back to the ground state. It is consequently a fundamental consideration when designing a phosphor that these nonradiative processes are minimized.

1.1

Activator Ions

An emitting ion that is doped into a lattice is referred to as an ‘‘activator’’ ion and phosphors that contain activator ions are the most common type. In addition to activator ions, additional ions may be co-doped into the host lattice as ‘‘sensitisers’’ (or ‘‘co-activators’’), which harvest the energy from the exciting source and transfer it to the activator ions. In the following discussion, we consider the light emission from phosphors containing rare earth element activator centers.

1.2

Phosphors Containing Rare Earth Element Activator Ions

There are three types of electronic transition exhibited by rare earth element (REE) ions, namely f-f, d-f, and charge-transfer electronic transitions, and we now consider each of these in turn.

2

f-f Electronic Transitions

An ion located within a crystalline host lattice is subject to the crystal field, which is the local electric field due to surrounding atoms. With some dopant/host combinations, the effect of the local field is to split the degenerate states of the ion via crystal field splitting, which can lead to broadening of emission spectra. However, many phosphors contain trivalent REE activator

Physics of Light Emission from Rare-Earth Doped Phosphors

6.1.2

ions which exhibit characteristic line emission spectra due to f-f electronic transitions. The ‘‘atom-like’’ sharpness of the emission lines results from the fact that the transitions take place between f orbitals which are not very sensitive to the environment of the REE ion, since they are screened by 5s and 5p electrons. The energy levels of the states arising from the 4f electron configurations of trivalent REE ions were originally reported by Dieke [1] and refined by Carnall et al. [2], and these are shown in > Fig. 1. The nomenclature of term and state is commonly used when referring to the Dieke

50

2

D5/2

2

G 7/2 9/2

1

S0

1/2 2

I 13/2 3/2 3/2 17/2 11/2

2

F 7/2

40

9/2

7/2 5/2

5/2 6

9/2

I

2

2 7/2 4

6

P3/2

3/2 9/2 17/2

0

5/2

5/2 7/2

5

3/2

1

30 cm−1 ⫻ 103

11/2 3/2

7/2

P3/2

1/2

2

D 5/2

3

P2

↑2P

6 3

1 2 3

1/2

0

20

7/2 1

D2

3

2

H11/2 F9/2

K6 F

7/2

G4

4F 7/2 3/2 4G 5/2

2

D

9/2 11/2 2

G5

4

5

F5

F 1/2 3/2 5/2

F11/2

15/2

2 6 7/2

13/2

5

11/2

13/2

6

11/2

5

9/2 7/2

5

I4

5/2

Ce

3H 4

Pr

4I

9/2

Nd

5I

4

Pm

6H

5/2

Sm

3/2

F9/2 I 9/2

7F

4

3

H4

3

H5

5/2

11/2 6

6 5 4 3 2 1

F2 3

4

5/2

0 1 2 3

3

5

6H 7/2

0 2F

H11/2

4

7/2 5/2

7

G4

↑S 6

8

1

7/2

4

9/2 3

2

1

6

F4

F3/2 5/2

3

D2

G 9/2

4

15/2 9/2

0

3

1

2

5

5

5 4 2 3 2 1

3/2

1

10

D

P3/2

19/2

5

5

4

2

11/2 3/2 13/2

1 0 6

5/2 7/2

G2

11/2

5 10 3 ↑5D

2

P2

H9/2

3 4

11/2 7/2

3

13/2

11/2

3

F4

7 13/2

5

0

8S

7F

6H 15/2

5I

Eu

Gd

Tb

Dy

Ho

6

8

4I

15/2

3H 6

Er

Tm

2F

7/2

Yb

. Fig. 1 Dieke diagram showing the energy levels of the trivalent rare earth element (REE) ions arising from their 4fn electron configurations (Adapted from [1])

1021

1022

6.1.2

Physics of Light Emission from Rare-Earth Doped Phosphors

diagram: a term specifies the orbital angular momentum (L) and the spin multiplicity (2S+1) as a superscript in the form of the symbol 2S+1L, whereas a state also specifies the value of the total angular momentum (J) as a subscript in the form of the symbol 2S+1LJ. The values of S and L are obtained by summing the spins and orbital angular momenta of the individual electrons respectively, that is, S = Ss and L = Sl, and J can have values of L+S, L+S
1, L+S
2, . . .., L
S. If L = 0, the term symbol is 2S+1S; if L = 1, the term symbol is 2S+1P; if L = 2, the term symbol is 2S+1D; if L = 3, the term symbol is 2S+1F; if L = 4, the term symbol is 2S+1G; if L = 5, the term symbol is 2S+1H; and if L = 6, the term symbol is 2S+1I. In order to identify the lowest energy levels in the ground state manifolds of the trivalent REE ions, we need to observe Hund’s rules, as follows (in order of importance). Hund rule 1: the spin multiplicity (2S+1) should be a maximum. Hund rule 2: the orbital angular momentum (L) should be a maximum. Hund rule 3: the total orbital angular momentum (J) should be a minimum if the f sub-shell is less than half full, but it should be a maximum if the f sub-shell is more than half full. As an example, let us consider the Eu3+ ion which has a valence electron configuration of 4f . The electrons populate the f orbitals so that they are all unpaired (as shown in > Fig. 2), in order to satisfy Hund’s rule 1 (maximum spin multiplicity), and the value of S = Ss = 6  ½ = 3. In addition, L is maximized in order to satisfy Hund’s rule 2, and the value of L = Sl = +3+2+1 +0
1
2 = 3. Thus, the symbol of the lowest energy term of Eu3+ is 7F (i.e., 2S+1L where S = 3 and L = 3). Possible J values when S = 3 and L = 3 are J = 6, 5, 4, 3, 2, 1, 0 (i.e., L+S, L+S
1, . . ..L
S), and as the f sub-shell of Eu3+ is less than half full, the total angular momentum (J) should be a minimum (i.e., J = 0) in order to give the lowest energy state (by Hund’s rule 3). Thus, the order of energies of the states in the 7F term of Eu3+ is 7F0, 7F1, 7F2, 7F3, 7F4, 7F5, 7F6 (as can be seen in the Dieke diagram shown in > Fig. 1). As another example, let us consider the Er3+ ion which has a valence electron configuration of 4f11. The electrons populate the f orbitals so that three are unpaired (as shown in > Fig. 3), in order to satisfy Hund’s rule 1 (maximum spin multiplicity), and the value of S = Ss = 3  ½ = 3/2. In addition, L is maximized in order to satisfy Hund’s rule 2, and the value of L = Sl = 2  (+3+2 +1+0)
1
2
3 = 6. Thus, the symbol of the lowest energy term of Er3+ is 4I (i.e., 2S+1L where S = 3/2 and L = 6). 6

-3 -2 -1

0

+1 +2 +3

. Fig. 2 Diagram showing the 4f6 valence electron configuration of Eu3+

-3 -2 -1

0

+1 +2 +3

. Fig. 3 Diagram showing the 4f11 valence electron configuration of Er3+

Physics of Light Emission from Rare-Earth Doped Phosphors

6.1.2

Possible J values when S = 3/2 and L = 6 are J = 15/2,13/2,11/2,9/2 (i.e., L+S, L+S
1, . . .. L
S), and as the f sub-shell of Er3+ is more than half full, the total angular momentum (J) should be a maximum (i.e., J = 15/2) in order to give the lowest energy state (by Hund’s rule 3). Thus, the order of energies of the states in the 4I term of Er3+ is 4I15/2, 4I13/2, 4I11/2, 4I9/2 (as can be seen in the Dieke diagram shown in > Fig. 1). As can be seen from the Dieke diagram in > Fig. 1, Ce3+ only has two low-energy states arising from its 4f1 valence electron configuration. As S = ½ and L = 3 (by Hund’s rule 2), J = 5/2 or 7/2 (i.e., L
S and L+S, respectively), and the states are 2F5/2 and 2F7/2 with the former being lower in energy than the latter (by Hund’s rule 3). Likewise, Yb3+ also has only two low-energy states arising from its 4f13 valence electron configuration and these are again the 2F5/2 and 2F7/2 states, but now the latter is lower in energy than the former (by Hund’s rule 3) because the f sub-shell is more than half full. Note that, apart from the first and last trivalent REE ions in the Dieke diagram, namely Ce3+ and Yb3+, the REE ions have numerous energy levels in the infrared and visible energy ranges (< 25000 cm
1), with the exception of Gd3+. The reason for the lack of low-lying energy levels in the case of Gd3+ is that this ion has a 4f7 valence electron configuration which gives rise to a particularly stable 8S7/2 ground state on account of the half full f sub-shell. Thus, the lowest energy emissions due to f-f electronic transitions of Gd3+ occur in the near ultraviolet (see > Fig. 1). The ground states of the other trivalent REE ions can be derived in a similar way to that used for the examples of Eu3+, Er3+, Ce3+, and Yb3+ above. The crystal field splitting is small, on the order of a few hundred cm
1, which is due to the lack of sensitivity of the f orbitals to the environment of the REE ions, the width of the levels in the Dieke diagram indicating the magnitude of the crystal field splitting. The energy levels shown in the Dieke diagram are those of the trivalent REE ions in the gas phase, but these levels are slightly different in phosphors when the ions are incorporated in host lattices. Indeed, the energy levels differ slightly according to the type of lattice due to the affects of covalency and crystal field, but these affects are small because the electronic transitions only involve f orbitals which are not very sensitive to the environment of the REE ion. This is in stark contrast to the d-f electronic transitions which are much more sensitive to the environment of the REE ion. As mentioned in > Chap. 6.1.1, strong emissions can result from electric dipole-allowed electronic transitions, which have high oscillator strengths, but this is not the case for the electric dipole-forbidden f-f electronic transitions. However, when a REE activator ion is located on a non-centrosymmetric lattice site (i.e., lacking inversion symmetry), the parity selection rule is relaxed, and emissions can be observed from so-called forced electric dipole f-f electronic transitions. In contrast, when the REE ion is located on a centrosymmetric lattice site, the f-f electronic transitions are electric dipole forbidden. However, in this case, weak emission may still be observed either from vibronically allowed electric dipole f-f electronic transitions (electric dipole transitions coupled to vibrations) or else from magnetic dipole allowed f-f electronic transitions. In order to illustrate the aforementioned selection rules for f-f electronic transitions, let us consider Eu3+ activator ions in the Y2O3 host lattice. This Y2O3:Eu3+ phosphor is an important red-emitting photo- and cathodo-luminescent material for both lighting and display applications, and it serves as a good example here because the Eu3+ ions occupy two different sites in the Y2O3 lattice (see > Fig. 4). Both sites are surrounded by six O2
anions at the corners of a cube, the other two corners being vacant. One site has the two vacancies on a face diagonal of

1023

1024

6.1.2

Physics of Light Emission from Rare-Earth Doped Phosphors

Y3+ O2− Vacancy C2

S6

. Fig. 4 Diagram showing the two different cation sites in the Y2O3 lattice

the cube and consequently this site is non-centrosymmetric and has C2 symmetry. The other site has the two vacancies on a body diagonal of the cube resulting in a centrosymmetric site of S6 symmetry. The ratio of the number of C2 sites to the number of S6 sites is 3:1 [3]. The key point we wish to make is that the emission from Eu3+ activator ions on C2 sites dominates the emission spectrum of the Y2O3:Eu3+ phosphor with only very weak features being observed from Eu3+ ions on S6 sites. This is because strong emissions occur due to f-f electronic transitions of the forced electric dipole type when the Eu3+ activators are located on the non-centrosymmetric C2 sites. The selection rules for electric dipole transitions are as follows: DJ = 0, 2 although DJ = 4, 6 transitions can give rise to weaker emissions. Weaker emissions due to magnetic dipole f-f electronic transitions are observed from Eu3+ ions on both C2 and S6 sites. The selection rules for magnetic dipole transitions are as follows: DJ = 0, 1 (but J = 0 to J = 0 is forbidden since it involves no change in angular momentum). All possible 5D0!7FJ electronic transitions for the Eu3+ ion are shown in > Fig. 5 [4]. These include the magnetic dipole-allowed 5D0!7F1 (for Eu3+ ions on C2 and S6 sites of the Y2O3 lattice) and the electric dipole-allowed 5D0!7F2 (for Eu3+ ions on C2 sites only) electronic transitions. The emission lines arising from the 5D0!7F0, 5D0!7F1, and 5D0!7F2 transitions are shown in the emission spectrum shown in > Fig. 6. Please note that a weak emission is observed from the 5D0!7F0 transition, even though it is forbidden by the selection rules.

3

d-f Electronic Transitions

Interconfigurational 5d!4f transitions are orbitally allowed, i.e., they are allowed by the Laporte selection rule which states that transitions from gerade orbitals to ungerade and those from ungerade orbitals to gerade are allowed, whereas those from gerade to gerade orbitals or from ungerade to ungerade are forbidden. Such transitions are found for rare earth ions that can be readily oxidized, such as the divalent Sm2+, Eu2+, and Yb2+, and the trivalent Ce3+, Pr3+, and Tb3+ ions. These emission bands are broad and their wavelength locations are

Physics of Light Emission from Rare-Earth Doped Phosphors

5

6.1.2

17233 cm−1

D0

7

F6

7F 7F 7

5 4

F3 7F 2 7 F1 7 F0 Eu3+

. Fig. 5 Schematic energy level diagram for the Eu3+ ion showing the possible 5D0!7FJ electronic transitions

III

50,000

Intensity

40,000 30,000 20,000 10,000

II

I

0 570

580

590

600 610 Wavelength/nm

620

630

. Fig. 6 Emission spectrum (over the range of 570–630 nm) of Y2O3:Eu3+ excited at a wavelength of 514.5 nm. The emission features I, II, and III are due to the 5D0!7F0, 5D0!7F1, and 5D0!7F2 electronic transitions of the Eu3+ ion, respectively

very sensitive to the environment of the activator ions in their host lattices, unlike emission lines due to 4f-4f transitions. This is because the 5d orbitals are much more sensitive to the surroundings of the rare earth ions than 4f orbitals. This results in a shift of the centroid energy of the 5d orbitals due to the so-called nephelauxetic effect, which is a measure of the extent

1025

6.1.2

Physics of Light Emission from Rare-Earth Doped Phosphors

of d electron delocalization from the metal ion on to the ligands, and a reasonably large splitting of the energies of the 5d orbitals due to the crystal field. A well-known example of a broad emission band, which peaks in the visible at ca. 535 nm and results from a 5d-4f electronic transition, is exhibited by the Ce3+ ion in YAG:Ce3+ phosphor (see > Fig. 7). As we have seen above, Ce3+ has a valence electron configuration of 4f1, which gives rise to the 2F5/2 and 2F7/2 low-lying energy states. Consequently, the visible emission is a composite of two bands due to an electronic transition from the lowest lying 5d energy level down to the 2F5/2 and 2F7/2 multiplet states. Since the emission is due to an orbitally allowed 5d-4f electronic transition, it has a short radiative lifetime of ca. 20 ns, whereas forbidden 4f-4f transitions have longer radiative lifetimes which are typically in the microsecond range [5]. The Ce3+ activator ions substitute some of the Y3+ ions which are dodecahedrally coordinated by eight O2
anions in the body centered cubic (bcc) host lattice of YAG (yttrium aluminum garnet, Y3Al5O12). The Ce3+ ions consequently occupy sites of D2 point group symmetry and the resulting crystal field causes the energies of the 5d orbitals to be split. Five absorption bands at 457.5, 339.7, 261, 225.4, and 204.6 nm were observed in the absorption spectrum of YAG:Ce by Tomiki et al. which were assigned to transitions from the 2F5/2 ground state to each of the 5d levels [6]. These, and other previous literature assignments of bands due to electronic transitions from the ground state to 5d electronic levels in YAG:Ce, were critically assessed by Tanner et al. who put forward a revised assignment in the light of their energy level and transition-intensity calculations [7]. When Y3+ ions (ionic radius of 101.9 pm [7]) are substituted by larger Gd3+ ions (ionic radius of 105.3 pm [7]), there is a red shift of the Ce3+ emission due to the increased crystal field that results from the decrease in Ce-O distance that leads to a larger splitting of the 5d energy levels (see > Fig. 7). Conversely, substitution of Al3+ ions (ionic radius of 51 pm [8]) for the larger Ga3+ ions (ionic radius of 62 pm [8]) on the octahedral sites gives a blue shift due to a weaker crystal field for the Ce3+ ions.

a

250 Irradiance /(mW m−2 nm−1)

1026

b 200 c 150

100

50

0 500

550

600 650 700 Wavelength/nm

750

800

. Fig. 7 Emission spectrum (over the range of 500–800 nm) of (a) YAG:Ce, (b) (Y0.80Gd0.20)AG:Ce, and (c) (Y0.60Gd0.40)AG:Ce. All three spectra were obtained using an exciting wavelength of 470 nm

Physics of Light Emission from Rare-Earth Doped Phosphors

4

6.1.2

Charge-Transfer Electronic Transitions

Interconfigurational charge-transfer transitions are also optically allowed and give rise to broad absorption bands. For example, the charge-transfer absorption of the red-emitting Y2O3:Eu phosphor results from the transfer of an electron from one of the O2
ligands to Eu3+ and gives rise to a broad absorption band at ca. 260 nm. Such transitions are exhibited by rare earth ions that are readily reduced, e.g., tetravalent Ce4+, Pr4+, and Tb4+, and the trivalent Sm3+, Eu3+, and Yb3+ ions [9].

5

Summary

Through detailed discussion of the electronic transitions responsible for luminescent emission, the characteristics of rare-earth doped phosphors have been presented, with specific reference to important phosphor systems. The range of phosphors used in display and lighting applications is vast, and the reader is consequently referred to the further reading section for more information related to their own area of interest. The specific characteristics of a particular phosphor are often determined to a large extent by the method of synthesis used, and the manner in which the phosphor is integrated into an application, hence this is the subject of the next > Chap. 6.1.3.

6

Future Directions

Research into new and improved phosphors is critical for advances in displays, lighting, and security applications. Some of the areas of interest under investigation include: nanoparticulate and submicron phosphors for high-resolution applications; improved blue saturated emission; anti-Stokes phosphors; phosphors with fast response, controlled decay, and broad-band white emission characteristics.

References 1. 2.

3.

4.

Dieke GH (1968) Spectra and energy levels of rare earth ions in crystals. Wiley, New York Carnall WT, Goodman GL, Rajnak K, Rana RS (1989) A systematic analysis of the spectra of the lanthanides doped into single crystal LaF3. J Chem Phys 90(7):3443–3457 Silver J, Withnall R (2004) Probes of structural and electronic environments of phosphor activators: Mo¨ssbauer and Raman spectroscopy. Chem Rev 104(6):2833–2855 Silver J, Martinez-Rubio MI, Ireland TG, Fern GR, Withnall R (2001) Yttrium oxide upconverting phosphors. Part 3: upconversion luminescent emission from europium-doped yttrium oxide under

5.

6.

7.

632.8 nm light excitation. J Phys Chem B 105: 9107–9112 van der Kolk E, Dorenbos P, de Haas JTM, van Eijk CWE (2005) Thermally stimulated electron delocalization and luminescence quenching of Ce impurities in GdAlO3. Phys Rev B 71:45121–45125 Tomiki T, Kohatsu T, Shimabukuro H, Ganaha Y (1992) Ce3+ centres in Y3Al5O12 (YAG) single crystals. 2. J Phys Soc Jpn 61:2382–2387 Tanner PA, Fu L, Ning L, Cheng B-M, Brik MG (2007) Soft synthesis and vacuum ultraviolet spectra of YAG:Ce3+ Nanocrystals: reassignment of Ce3+ energy levels. J Phys Condens Matter 19:216213 (1–14)

1027

1028

6.1.2 8.

Physics of Light Emission from Rare-Earth Doped Phosphors

Huh Y-D, Cho Y-S, Do YR (2002) The optical properties of (Y1
xGdx)3
z(Al1
yGay)5O12:Cez phosphors for white LEDs. Bull Korean Chem Soc 23:1435–1438

9.

Blasse G, Grabmeier BC (1994) Luminescent materials. Springer, Berlin

Further Reading Kitai AH (ed) (1993) Solid state luminescence. Chapman & Hall, London Kitai AH (ed) (2008) Luminescent materials. Wiley, Chichester

Ronda C (ed) (2008) Luminescence. Wiley VCH, Weinheim Yen WM, Shionoya S, Yamamoto H (2007) Phosphor handbook, 2nd edn. CRC, Boca Raton

6.1.3 Chemistry and Synthesis of Inorganic Light Emitting Phosphors Jack Silver . Robert Withnall 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1030 2 Traditional Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031 3 Newer Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031 4 Solution-Based Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032 5 Other Solution-Based Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033 6 Vapor Phase Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1034 7 Summary/Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1036 8 Future Phosphor Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1036

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.1.3, # Springer-Verlag Berlin Heidelberg 2012

1030

6.1.3

Chemistry and Synthesis of Inorganic Light Emitting Phosphors

Abstract: The synthesis of phosphors is described in terms of the broad range of techniques that have been utilized in different device/industrial applications. A review of the primary methods of solid state, solution process, and vapor phase techniques is provided, with examples from specific display systems. List of Abbreviations: ALD, Atomic Layer Deposition; ALE, Atomic Layer Epitaxy; CL, Cathodoluminescence; CLCB, Charged Liquid Cluster Beam; CVD, Chemical Vapor Deposition; EL, Electroluminescence; FED, Field Emission Display; MBE, Molecular Beam Epitaxy; MOCVD, Metallorganic Chemical Vapor Deposition; PLD, Pulsed Laser Deposition; TFEL, Thin Film Electroluminescence

1

Introduction

The synthesis and characterization of inorganic phosphors has been subject to intensive research since the beginning of the twentieth century. Because of its industrial importance the majority of this work has been carried out in industry and is not available in the public domain. One of the best ways therefore of accessing such knowledge of phosphor manufacturing methods for industrial phosphors is to consult specialist books written by people who have spent much of their lives working in the phosphor industry rather than research journals; here we refer the reader to the well-known texts by Ropp [1–3]. In industry, almost all phosphors were synthesized by solid state reactions between very pure inorganic compounds at high temperature. Clearly, in the space available, it is impossible to discuss every aspect of phosphor synthesis from its inception to present day; so here priority is given to discoveries and developments that have appeared in the last 20 years, and significant reviews are given as appropriate. Most industrial syntheses for almost all phosphors still use solid state reactions between very pure inorganic compounds at high temperature. Such methods of phosphor synthesis can be divided into two distinctly different types of reactions: (a) The first of these involves introducing the activator ions into an existing host material. An example of this is in zinc sulfide phosphors, where after particle growth of the host crystal, diffusion of the activators into the ZnS lattice occurs. (b) The second involves the synthesis of the host material and the simultaneous incorporation of the activator during firing. The main drawbacks of these predominantly solid state methods are: 1. That the distribution of the activators in the host lattice may not be even, as the precursors are not mixed on the atomic scale 2. That particle growth cannot be easily controlled, and involves milling and sieving In recent years, with the needs for phosphors for high-definition display screens and for color-converting LEDs, attention has turned to addressing these drawbacks by studying methods of homogeneous precipitation and utilizing particle growth methods developed for other industrial uses. The structures of most of the host lattices used to prepare phosphors have been described by Wells [4] and others [5]. The host lattice will determine the coordination environment of

Chemistry and Synthesis of Inorganic Light Emitting Phosphors

6.1.3

the dopant atom, which usually influences its emission behavior. For example, zinc orthosilicate Zn2SiO4 contains Zn(II) ions in distorted tetrahedral O-donor sites; substitution of these by Mn(II) ions facilitates green emission that has been attributed to the spin-forbidden 4 T1(4G)!6A1(6S) d-d transition of the Mn2+ ions [6, 7]. The influence of different site geometries for the dopant is apparent in the emission spectrum of SrY2O4:Eu3+, which consists of two kinds of Eu3+ emission bands that are assigned to Eu3+ ions in the Sr site and the Y site [8]. The general properties of luminescent materials have been discussed in a number of texts [1–3, 9–12]. Full synthetic details of most phosphors can be found elsewhere [1–3, 12] but an outline of some traditional and newer methods are given below by way of example.

2

Traditional Method

Solid state synthesis Usually, such methods first involved blending the high-purity precursor materials of the host crystal lattice, the activators (and/or co-activators) and the fluxes together. Next, the mix was fired in an unreactive crucible/container. The dopant, usually in the form of a simple salt, is added either during or after the firing of the host lattice. The product obtained by firing was usually sintered and needed to go through a treatment that involved crushing and milling. Product particles were then often sorted to remove the coarser/larger particles. The product then underwent surface treatments if required. Some of these processes were carried out several times. The host lattices are frequently prepared from metal sulfide lattices such as ZnS or Zn1~xCdxS2: alkaline earth carbonates or phosphates such as MHPO4 (M = Ca, Sr, or Ba), MNH4PO4.H2O (M = Cd or Mn) and yttrium or rare earth element oxides [1–3, 12]. The general (industrial) method of preparation usually involves precipitation of the phosphor itself, or an intermediate precursor, from solutions of pure cationic and anionic precursors. All starting materials should be as pure as possible, and impurities must not be present at levels greater than 1.0 ppm. Impurities detrimental to phosphor quality are usually referred to as ‘‘killers’’ [2], and may include M2+ (M = Fe, Co, Ni, Cu, Pd, and Pt), M3+ (M = Ti, V, Cr, Fe, Ru, Zr, Nb, Mo, Rh, Hf, Ta, and W), and M4+ (M = Os, Ir, and Re). These cations can usually be removed by precipitation or sequestration by selective chelation in solutions prior to processing [1–3, 12]. These impurities may depress the performance of a phosphor or cause the energy to be released at an undesirable wavelength or as phonons. Oxide host lattices are generally prepared from suitable metal and nonmetal oxides [13]. Nitride phosphors may be prepared by reaction of metal powders with Si3N4 (e.g., phosphors based on MgX Zn1xSiN2 doped with Eu and Tb, which were evaluated as part of a program to develop phosphors for flat panel display applications) [14, 15]. Sulfides may be obtained by solid state metatheses using, for example, sodium sulfide [16] (e.g., the Eu-doped strontium thiogallate phosphors Sr1xEuxGa2S4) or by thermolysis of zinc and related sulfides.

3

Newer Methods

These can be divided into those that are solution based and those that involve vapor phase or spray techniques.

1031

1032

6.1.3 4

Chemistry and Synthesis of Inorganic Light Emitting Phosphors

Solution-Based Methods

Homogeneous precipitation of phosphors from solution provides the possibility of mixing in the dopant atoms into the host lattice at the atomic level rather than depending on hightemperature diffusion. This permits more effective control over stoichiometry in the final phosphor. It also allows control over phosphor morphology which has, for instance, facilitated the generation of spherical particles which will self-assemble (packing well) in small pixel areas for high-definition display screens [17–28]. In addition, the size of the final phosphor particle can be tailored by manipulation of solution conditions influencing the precipitation process [29–31]. One very versatile method involves the addition of urea to metal salts in aqueous solution under conditions of pH which facilitate the decomposition of the urea and precipitation of metal hydroxycarbonates which are good precursors for metal oxide phosphors [17–28]. This has led to the controlled preparation of spherical particles of Y2O3:Eu particles of 100, 200, 300, 400, and 500 nm dimensions having a size distribution that can be controlled to within 5% [20–25]. Gd2O3:Eu has been prepared similarly, giving particles of sizes between 70 and 250 nm [32]. Precipitation methods were also developed for the synthesis of metal sulfides and selenides [33–37]. Of the two general methods, the first involves treatment of aqueous metal salts, without added dopant, with hydrazine hydrate and either sulfur or selenium. This method has been used to synthesize metal chalcogenide phosphors such as ZnS:Ag, ZnSe, Zn(S, Se), CdS, ZnS:Cu, ZnS:Cu, Al, Au [35], (Zn, Cd)S:Cu, and Zn (S, Se):Cu [33–35]; this method was also extended to the synthesis of ternary metallo-sulfide phosphors such as Sr0.97Ga2S4:Eu0.3, Sr0.985Ga2S4:Ce0.015, Zn0.98Ga2S4:Mn0.02, Ca0.8Ga2S4:Eu0.2, and Ba2Zn0.985S3:Mn0.015 [36]. The second method makes use of the decomposition of thiourea dioxide in aqueous solution at elevated temperatures in the presence of metal acetates, and has been used to prepare ZnS phosphors [37]. The advantage of these new methods is that the elaborate techniques necessary for the removal of toxic gases or vapors (H2S and CS2) common in solid state reactions are no longer required [33]. Precipitation can also be affected in nonaqueous solvents. Nanocrystals of ZnS:Tb could be prepared by a coprecipitation from the addition of aqueous sodium sulfide to Tb(NO3)3 and zinc acetate dissolved in methanol [38]. ZnS:TbF3 and ZnS:Eu have been prepared in a similar way. The photoluminescent intensities of the nanocrystals of ZnS:Tb and ZnS:Eu prepared in this way were apparently 2.5 and 2.8 times stronger than those of bulk (conventionally prepared) phosphors, and these nanocrystals have been proposed for FED, EL, PDP, and CRT applications [38]. Fine spherical Y2O2S:Eu3+ has been prepared using precursors of yttrium isopropoxide and europium chloride dissolved in absolute isopropyl alcohol, hydrolysis and precipitation being controlled by water additions. Sulfur was introduced by grinding and firing the calcined product with sodium thiosulfate pentahydrate [39]. Nanoparticles of ZnO-based phosphors were prepared from aqueous propanol solutions of Zn(ClO4)2 containing sodium hexametaphosphate as capping agent [40]. Similarly ZnS-based phosphors were prepared in basic aqueous methanolic solutions using trioctylphosphine oxide (TOPO) as capping agent and sulfide ions [40]. Sol-gel methods are used to prepare phosphors from solution by formation of a gel which is produced from precursors such as metal alkoxides that undergo hydrolysis and polycondensation reactions. These methods have been used to prepare Zn2SiO4:Mn2+ phosphors co-doped with Al3+ + Li+ and Ga3+ + Li+ ion using zinc acetate, ethanol, silicic acid, and nitrates of Ga3+

Chemistry and Synthesis of Inorganic Light Emitting Phosphors

6.1.3

and Al3. Li+ was added as the acetate or hydroxide, whereas Mn metal powder was dispersed in the host lattice by dry mixing into the calcined xerogel powders. The phosphor precursors were reacted hydrothermally at 700 C and 100 MPa [41]. Silica glass ceramics containing nanoparticles of TiO2 doped with Er3+ and Yb3+ for upconversion emission (anti-Stokes emission) were also prepared by sol-gel methods [42]. Addition of titanium is believed to improve the chemical durability of the material and to increase the glass transition temperature [43]. Other phosphors produced by sol-gel techniques include Y2SiO5:Tb3+ [44], Li-doped Gd2O3:Eu3+, Y2O3:M3+, Y2SiO5:M3+, and Y3Al5O12:M3+ (where M3+ = Eu3+ or Tb3+) [45]. The green-emitting CL-phosphors Y3Ga5O12:Tb, Y3AlGa2O12:Tb and Gd3Ga5O12:Tb, that are all isostructural with yttrium aluminum garnet (Y3Al5O12), were obtained as fine-grained crystalline powders following hydrothermal treatment of amorphous gels obtained by aqueous precipitation of Y, Gd, Al, Ga, and Tb salts [46]. Sol-gel methods continue to be developed; for example, high brightness Y2SiO5:Ce phosphor powders with spherical shape and fine size were synthesized by a melting salt-assisted sol-gel method (MS&Sol-Gel). Commercial tetraethylorthosilicate (TEOS) was used as the silica source and rare earth oxides were used as rare earth source [47]. The optical properties of Y2O3:Eu3+ micro- and nanophosphors synthesized by sol-gel process have been reported. Citric acid and tartaric acid were employed as chelating agents. Different factors effecting structures and properties of the phosphors, such as concentration of Eu3+, sintered temperature, and the ratio of metal ions to tartaric acid, were examined [48]. Microemulsion methods have been used to prepare Y2O3:Eu nanoparticles for potential use in FEDs [49]. The particles were prepared by the aqueous reaction of yttrium nitrate, europium nitrate, and ammonium hydroxide, by bulk precipitation in the reverse microemulsion composed of NP-5/NP-9, cyclohexane, and water. In comparison to material prepared by bulk precipitation methods, the particles prepared by this technique apparently manifested a narrower size distribution, spherical shape, smaller size (20–30 nm), higher crystallinity, and stronger photoluminescence [49]. Again, this method continues to be developed, for example, BaMgAl10O17:Eu2+ (BAM) was prepared in the microemulsion system and its phase behavior was studied [50].

5

Other Solution-Based Techniques

Combinatorial chemical methods have been used to synthesize the red phosphors Y(As,Nb,P,V) O4:Eu3+ [51]. The oxide precursors were dissolved in weak acids and the required amount of each solution was injected into a ceramic container. The solutions were evaporated, the residue fired, and the PL performance of the calcined powders monitored. The best compounds were 3þ 3þ Y0.9PO4:Eu3þ 0:1 , Y0.9(P0.9V0.1)O4:Eu0:1 , and Y0.9(P0.9Nb0.1)O4:Eu0:1 , the maximum luminescence 3þ being observed with Y0.9(P0.92V0.03Nb0.05)O4:Eu0:1 [51]. Combinational chemical screening was also used to identify Eu and Tb-activated phosphors in the system MO–Gd2O3–Al2O3 (M = Ca and Sr) [52]. The technique is claimed as a powerful means of increasing the rate of synthesizing/screening new phosphors, but only ‘‘candidate’’ phosphors based on Gdxy1EuxCa(or Sr)yAl2Od for red emission, Ca0.3EuxAl0.7Od for the blue-emitting phosphor, and Sr0.25EuxAl0.75Od, Ca0.25Gd0.25xTbxAl0.5Od, Ca0.65TbxAl0.35Od, Gd0.4xAl0.6TbxOd, Gd0.7xAl0.3TbxOd, and Sr0.05TbxAl0.95Od for the green-emitting ones were found [52].

1033

1034

6.1.3

Chemistry and Synthesis of Inorganic Light Emitting Phosphors

Solution coating methods have been used to prepare thin films of ZnGa2O4:Mn2+ and Ga2O3:Mn deposited on BaTiO3 ceramic sheets by dip coating from the pentanedionato complexes [Zn(C5H7O2)2], [Ga(C5H7O2)3], and MnCl2 dissolved in methanol or ethanol at 25 C. The coated ceramic sheets were then heated at 500–950 C for 30 s [53]. EL devices made using such solution coating techniques eliminate the need for vacuum processes [53].

6

Vapor Phase Methods

Chemical vapor deposition (CVD or MOCVD) can be used to prepare a film of the desired material by evaporation of volatile precursor molecules, which then decompose to give a film deposited on the substrate. The film (as it grows) is influenced by the surface ordering of the substrate layer; hence, the deposition is ‘‘epitaxial.’’ The needed decomposition of the precursor molecules takes place either on the surface of the substrate or in the gas phase near to it. This method was used to make thin and thick films of ZnS:Mn phosphors for thin-film electroluminesence (TFEL) [54–70]. This general method has been used to prepare CaGa2S4: Ce [71] and SrGa2S4:Ce [72]. Thin films of CaGaaSb:Ce films were also prepared [73]; SrS:Ce TFEL devices were made by MOCVD methods from Sr(thd)2, Ce(thd)4, and H2S as precursors [74]. These methods are being more widely used at the present time, for example, ultrafine Eu3+doped yttrium aluminum garnet (YAG:Eu3+) phosphor powders, with uniform diameters of about 1 mm, have been prepared by metallorganic chemical vapor deposition (MOCVD) [75]. Molecular beam epitaxy (MBE) In this method, the elemental components of the desired film, in the form of ‘‘molecular beams’’ are deposited epitaxially onto a heated crystalline substrate. Such molecular beams originate typically from thermally evaporated elemental sources (for example, evaporation of elemental As produces molecules of As2, As3, and As4). A refinement of this is atomic layer epitaxy (ALE) (also known as atomic layer deposition, ALD). Here, the substrate is exposed alternately to two (or more) precursors, facilitating film growth with remarkable control, one layer of atoms at a time. Compared to CVD, these methods do not involve chemical decomposition of the vapor phase precursor. Thin films of SrGa2S4:M, where M = Ce or Eu were grown using an MBE system from Sr metal, GaS3, and CeCl3 or EuCl3 [76]. Small grain size and high CL efficiency suitable for highresolution emissive displays such as FEDs were observed. ALD was used to prepare high luminosity blue-emitting films of CaS:Pb using [Ca(thd)2], Pb(C2H5)4, and H2S as precursors [77]; the emission intensity was strongly dependent on the Pb concentration, the growth parameters (e.g., temperature), and the precursors [78]. ALE has been used to make ZnS:Mn for use in EL displays [79–81]. EL devices using ZnS:Tb(O,Cl) EL were prepared from ZnS, Tb(thd)3, and H2S [82]. Films of SrS:Ce for use as TFEL devices were prepared from [Sr(thd)2], [Ce(thd)4], and H2S [83]. In RF magnetron sputtering, a ‘‘target’’ is bombarded with fast-moving ions that are produced by an electrical discharge in an inert gas (typically, argon) and directed onto the surface by a strong magnetic field. Momentum transfer results in atoms or molecules from the target being vaporized, and these then deposit in the substrate. This method’s advantage is that, as it is a ‘‘cold’’ technique, it can be used to prepare films of a wide variety of conducting or insulating materials on many types of substrate. In addition, it is both cheaper and faster than MBE, though it lacks the remarkable control that MBE offers.

Chemistry and Synthesis of Inorganic Light Emitting Phosphors

6.1.3

RF magnetron sputtering has been used both to introduce dopants into host lattices, and to coat surfaces with preformed phosphors. The former method has been used in doping a variety of gallium oxide and oxyanion lattices with Eu, Tb, Pr, or Dy [84], and in making some forms of Mg0.5Zn0.5SiN2:Eu [14] and MgxZn1xSiN2 [15]. Surface coating was useful for sputtering manganese-doped ZnS onto Si substrates [85], for depositing thin films of lanthanide-doped alkaline earth thiogallates Ca1xSrxGa2S4:Ln phosphors onto ceramic substrates [86], and for depositing Y2O3:Mn thin films onto BaTiO3 ceramic sheets [87]. RF-magnetron sputtered deposited Zn2GeO4:Mn films were investigated as a potential green-emitting phosphor for TFEL devices [88, 89]. Y4GeO4:Mn, Y2GeO5:Mn, and Y2GeO7: Mn phosphor thin films were deposited onto thick ceramic sheets by RF sputtering using (Y2O3–GeO2):Mn phosphor targets [90]. In TFEL devices high-luminance yellow emissions were obtained, and it was concluded that the new oxides were promising as host materials for EL phosphors [90]. Electron beam evaporation (e-beam) was used to deposit films of SrS:HoF3 from SrS pellets and HoF3 powder [91]. These were shown to be useful as white EL devices. The properties and the optimization of Y2O3:Eu cathodoluminescent thin film phosphor fabricated by electronbeam evaporation have been reported recently [92]. Pulsed laser deposition (PLD) has been used to produce Eu3+-activated Y2O3 phosphor films in situ on silicon and diamond-coated silicon substrates [93]. This method has also been used to prepare thin films of the silicate phosphors CaSiO3:Mn,Pb, (red emitting), ZnSiO4:Mn (green emitting), and Y2SiO5:Ce (blue emitting); a good correlation was found between photoluminescence intensity, and film crystallinity and surface morphology [94]. Ion implantation has been used to produce ZnGa2O4 thin films on flexible organic (polymide) substrates [95]. Preliminary cathodoluminescence (CL) results indicate brightness levels comparable to similar films on glass. Implanting Mn ions into sputter-deposited ZnGa2O4 films, followed by a ‘‘low’’ temperature (450 C) anneal allowed green CL to be achieved [96]. Aerosol spray pyrolysis/flame spray pyrolysis uses a solution of the desired metal ion precursors that sprays out of a nozzle using a carrier gas to produce small droplets. These droplets are passed through a high temperature furnace which transforms their contents into spherical metal oxide particles. Phosphors prepared in this way include Y2O3:Eu3+ [97–99], BaMgAl10O17:Eu2+ [100], Zn(Ga1xAlx)2O4:Mn [101], ZnGa2O4:Mn [102], and Gd2O3:Eu [32]. This method has also been used to prepare thin films of ZnO:Zn [103], Y2O3:Eu [103], and the blue emitter YNbO4:Bi [104]. Gd2O3:Eu phosphor particles of spherical and filled morphology have also been prepared by spray pyrolysis [32]. The method is also useful for developing particles with controlled morphology, and films from colloidal suspensions [99, 104–107]. Readers are referred to a recent review covering the generation of phosphor particles for photoluminescence applications by spray pyrolysis [108]. Charged liquid cluster beam (CLCB) methods have been used to deposit Mn-doped Zn2SiO4 for plasma display panels [109]. This methodology is said to be inherently suited to the fabrication of uniform, conformal coatings, of controlled chemical compositions and stoichiometries. It utilizes nanometer-scale charged drops of liquid precursors for thin film deposition and makes use of the atomic scale mixing present in these liquids to achieve a film of uniform stoichiometry. The morphology and photoluminescence intensity of films of (Zn1xMnx)2SiO4 prepared by the CLCB technique were found to be dependent on the deposition conditions [109].

1035

1036

6.1.3 7

Chemistry and Synthesis of Inorganic Light Emitting Phosphors

Summary/Conclusion

The synthesis of phosphors for the wide range of applications in which they are employed has historically relied on high-temperature solid state reactions. However, demand for highresolution, wide color gamut, and low power consumption displays has stimulated interest in a broader range of synthesis techniques to address the associated technical challenges. This chapter has reviewed the relevant methods in use across the phosphor and displays industry.

8

Future Phosphor Synthesis

It is apparent from the industrial/commercial importance of light-emitting materials that new methods of phosphor synthesis will continue to be a topic of immense interest, and existing methods of phosphor synthesis will continue to be developed. Furthermore, the immense amount of work that has been undertaken in recent years in synthesizing nanoparticle phosphors is set to continue.

References 1. Ropp RC (1993) The chemistry of artificial lighting devices. Elsevier, Amsterdam, pp 1–664 2. Ropp RC (1991) Luminescence and the solid state. Elsevier, Amsterdam, pp 1–453 3. Ropp RC (2004) Luminescence and the solid state, 2nd edn. Elsevier, Amsterdam, pp 1–711 4. Wells AF (1975) Structural inorganic chemisty, 4th edn. Clarendon, Oxford, pp 1–1095 5. Leskela M, Ninisto L (1986) Inorganic Complex Compounds 1. In: Gschneidner KA Jr, Eyring L (eds) Handbook on the physics and chemistry of rare earths, Chap. 56. Elsevier, Amsterdam, pp 203–334 6. Perkins HHK, Sienko MJ (1967) ESR study of manganese-doped a-zinc silicate crystals. J Chem Phys 46:2398–2401 7. Palumbo DT, Jr Brown JJ (1970) J Electrochem Soc 117:1184–1189 8. Park S-J, Park C-H, Yu B-Y, Bae H-S, Kim C-H, Pyum C-H (1999) Structure and luminescence of SrY2O4:Eu. J Electrochem Soc 146:3903–3906 9. Garlick GFJ (1949) Luminescent materials. Clarendon, Oxford, pp 1–254 10. Shinoya S, Yen WM (eds) (1998) Phosphor handbook. CRC, London, pp 1–921 11. Yen WM, Shinoya S, Yamamoto H (eds) (2007) Phosphor handbook, 2nd edn. CRC, London, pp 1–1051 12. Yen WM, Weber MJ (2004) Inorganic phosphors, compositions, preparation and optical properties. CRC, London, pp 1–475

13. Sohn K-S, Choi Y-G, Park HD (2000) Energy transfer between Tb3+ ions in YAlO3 host. J Electrochem Soc 147:3552–3558 14. Lee SS, Chang CK, Lim S (1996) SID’96 Dig 27: 471–473 15. Lee SS, Lim S, Oh MH, Chang H (1997) SID’97 Dig 28:576–579 16. Sastry ISR, Bacalski CF, McKittrich J (1999) Preparation of green-emitting Sr1-xEuxGa2S4 phosphors by a solid-state rapid metathesis reaction. J Electrochem Soc 146:4316–4319 17. Nishisu Y, Kobayashi M (1984) US Patent 5413736 1995, FR patent 2703065, 30 Sept 1984 18. Her YS, Matijevic E, Wilcox WR (1992) J Mater Res 7:2269–2272 19. Jiang YD, Wang ZL, Zhang F, Paris HG, Summers CJ (1998) Synthesis and characterization of Y2O3: Eu powder phosphor via a sol-gel technique. J Mater Res 13:2950–2955 20. Vecht A, Jing X, Gibbons C, Ireland T, Davies D, Marsh P, Newport A (1998) SID 98 Dig 29: 1043–1047 21. Vecht A, Gibbons C, Davies D, Jing X, Marsh P, Ireland T, Silver J, Newport A, Barber D (1999) Engineering phosphors for field emission displays. J Vac Sci Technol B 17:750–757 22. Jing X, Ireland T, Gibbons C, Barber DJ, Silver J, Vecht A, Fern G, Trowga P, Morton DC (1999) Control of Y2O3:Eu spherical particle phosphor size, assembly properties, and performance for FED and HDTV. J Electrochem Soc 146:4654–4658

Chemistry and Synthesis of Inorganic Light Emitting Phosphors 23. Martinez-Rubio MI, Ireland TG, Silver J, Fern G, Gibbons C, Vecht A (2000) Effect of EDTA on controlling nucleation and morphology in the synthesis of ultrafine Y2O3: Eu phosphors. Electrochem Solid State Lett 3:446–449 24. Vecht A, Martinez-Rubio MI, Ireland TG, Silver J, Fern G, Gibbons C (2000) SID 00 Digest 31:15–17 25. Silver J, Martinez-Rubio MI, Ireland TG, Fern G, Withnall R (2001) Yttrium oxide upconverting phosphors. 3. Upconversion luminescent emission from europium-doped yttrium oxide under 632.8 nm light excitation. J Phys Chem B 105:9107–9112 26. Martinez-Rubio MI, Ireland TG, Fern G, Snowden MJ, Silver J (2001) A new application for microgels: novel method for the synthesis of spherical particles of the Y2O3:Eu phosphor using a copolymer microgel of NIPAM and acrylic acid. Langmuir 17: 7145–7149 27. Martinez-Rubio MI, Ireland TG, Fern G, Silver J, Snowden MJ (2002) J Electrochem Soc 149: H53–H58 28. Ma D, Liu X, Pei Y, Cao L (1997) SID’97 Dig 28: 423–426 29. Williams R, Yocum PN, Stofko FS (1985) Preparation and properties of spherical zinc sulfide particles. J Colloid Interface Sci 106(2):388–398 30. Cooper JA, Paris HG, Stock SR, Yeng S, Summers CJ, Hill DN (1998) J SID 6:163–166 31. Ireland TG, Silver J, Gibbons C, Vecht A (1999) Easy self-assembly of yttrium oxide phosphor europium from solution using a phase sacrificial micellar. Electrochem Solid State Letts 2:52–54 32. Kang YC, Roh HS, Seung BP (2000) J Electrochem Soc 147:1601–1603 33. Vecht A, Davies DA, Smith D (1998) A novel method for the preparation of sulfides and selenides and its application for phosphor synthesis. Mater Res Innovat 2:176–180 34. Davies A, Vecht A, Rose JA, Marsh PJ, Gibbons CS, Silver J, Morton D, Blomquist S, Ravihandren R (1999) SID 99 Dig 30:1018–1021 35. Davies DA, Vecht A, Silver J, Marsh PJ, Rose JA (2000) J Electrochem Soc 147:765–771 36. Marsh PJ, Davies DA, Silver J, Smith DW, Withnall R, Vecht A (2001) J Electrochem Soc 148:D89–D93 37. Davies DA, Silver J, Vecht A, Marsh PJ, Rose JA (2001) J Electrochem Soc 148:H143–H148 38. Ihara M, Igarashi T, Kusanoki T, Ohno K (1999) SID’99 Dig 30:1026–1029 39. Kottaisamy M, Horikawa K, Kominami H, Aoki T, Azuma N, Nakamura T, Nakanishi Y, Hatanaka Y (2000) J Electrochem Soc 147:1612–1616 40. Wakefield G, Dobson PJ (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 299–302

6.1.3

41. Li QH, Komarnei S, White WB, Roy R (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 21–23 42. Newport A, Fern GR, Ireland T, Withnall R, Silver J, Vecht A (2000) Up-conversion emission phosphors based on doped silica glass ceramics prepared by solgel methods: control of silica glass ceramics containing anatase and rutile crystallites. J Mater Chem 11(5):1447–1451 43. Bausa LE, Garcia Sole J, Duran A, Fernandez Navarro JM (1991) J Non-Cryst Solids 127:267–272 44. Popovich NV, Soschin NP, Galactionou SS, Popova MN, Pogrebisskaya OP (2001) Extended Abstracts of the first international conference on science and technology of emissive displays and lighting, San Diego, 12–14 Nov 2001, pp 231–234 45. Ravichandran D, Roy R, White WB (1997) J SID 5:107–110 46. Phillips MLF, Walko RJ, Shea LE (1996) SID’96 Dig 27:121–124 47. Jiau H, Wei L, Zhang N, Zhong M, Jing X (2007) J Eur Ceram Soc 27:185–189 48. Hao BV, Huy PT, Khiem TN, Ngan NTT, Duong PH (2009) J Phys Conf Ser 187(012074):1–6 49. Lee MH, Oh SG, Yi SC, Seo DS, Hong JP, Kim CO, Yoo YK, Yoo JS (2000) J Electrochem Soc 147: 3139–3142 50. Zhang X, Zhuang W, Cui X, He H, Huang X (2006) J Rare Earths 24:736–739 51. Sohn K-S, Zeon IW, Park HD Extended abstracts of the first international conference on science and technology of emissive displays and lighting, San Diego, 12–14 Nov 2001, pp 139–142 52. Kim CH, Park SM, Jeong YS, Park JK, Park HD, Park JT (2001) Extended Abstracts of the first international conference on science and technology of emissive displays and lighting, San Diego, 12–14 Nov 2001, pp 43–48 53. Minam T, Sakagami Y, Miyata T (1997) Extended abstracts of the third internationsl conference on the science and technology of display phosphors. Huntington Beach, 3–5 Nov 1997, pp 37–40 54. Manasevit H, Simpson W (1971) J Electrochem Soc 118:644–647 55. Wright PJ, Cockayne B (1982) J Cryst Growth 59: 148–154 56. Wright PJ, Cockayne B, Cattell AF, Dean PJ, Pitt AD, Blackmore GM (1982) J Cryst Growth 59:155–160 57. Cattell AF, Cockayne B, Dexter K, Kirton J, Wright P (1983) J IEEE Trans Electron Dev 30:471–475 58. Cockayne B, Wright PJ, Armstrong AJ, Jones AC, Orrell ED (1988) J Cryst Growth 91:57–62 59. Takata S, Minami T, Miyata T, Nanto H (1988) J Cryst Growth 86:257–262

1037

1038

6.1.3

Chemistry and Synthesis of Inorganic Light Emitting Phosphors

60. Fujita S, Isemura M, Sakamoto T, Yoshimura N (1988) J Cryst Growth 86:263–267 61. Cockayne B, Wright PJ, Skolnick MS, Pitt AD, Williams JD, Ng TL (1985) J Cryst Growth 72: 17–22 62. Yoshiwaya A, Yamaga S, Tanaka K (1984) Jpn J Appl Phys 23:L388–390 63. Hirabayashi K, Kogure D (1985) Jpn J Appl Phys 24:1484–1487 64. Hirabayashi K, Kozawaguchi H (1986) Jpn J Appl Phys 25:L379–L381 65. Hirabayashi K, Kozawaguchi H (1986) Jpn J Appl Phys 25:711–713 66. Hirabayashi K, Kozawaguchi H, Tsujiyama B (1987) Jpn J Appl Phys 26:1472–1476 67. Hirabayashi K, Kozawaguchi H, Tsujiyama B (1987) Kenkyu Jitsuyoko Hokoku 36:829–835 68. Vecht A (1980) Methods of producing thin films. GB Patent 2049636, 31 Dec 1980 69. Katahasi Y, Yuki R, Sugiura M, Motojima S, Sugiyama K (1980) J Cryst Growth 50:491–497 70. Hiskes R, DiCarolis SA, Mueller-Mach R, Mazzi V, Nauka K, Mueller J (1997) SID 5:93–97 71. Moss TS, Smith DC, Samuels JA, Dye RC, DelaRosa MJ, Schaus CF (1997) J SID 5:103–106 72. Chakhovstoi AG, Hunt CE, Yang T, Wagner BK, Summers CJ, Malinowski ME, Felter TE (1998) J SID 6:185–189 73. Kato A, Katayama M, Mizutani A, Ito N, Hattori T (1998) J SID 6:5–8 74. Moss TS, Dye RC, Tuenge RT (1997) Extended Abstracts of the Third International Conference on the Science and Technology of Display Phosphors. Huntington Beach, 3–5 Nov 1997, pp 17–12 75. Li Y, Zhang J, Xiao Q, Zeng R (2008) Mater Lett 62:3787–3789 76. Tanaka K, Okamota S, Izumi Y, Kominami H, Nakanishi Y, Du X, Yoshikawa A (2001) Extended abstracts of the first international conference on science and technology of emissive displays and lighting, San Diego, 12–14 Nov 2001, pp 235–238 77. Kim YS, Park S-H, KO, Cho K-I, Yun SJ (2001) SID’01 Dig 32:763–765 78. Yun SJ, Kim YS, Kang JS, Park K, Cho K, Ma DS (1999) SID’99 Dig 30:1142–1145 79. Vlasenko NA, Kononets Ya F, Beletskii AI, Denisova ZL, Kopytko Yu F, Soinen EL, Tornqvist RO, Vasama KM (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 53–56 80. Soininen EL, Harkonen G, Vasama K (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 105–108

81. Soenen B, Visschere P, Ihanus J, Ritala M, Leskela M (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 49–52 82. Yun SJ, Nan S-D, Kang J-S, Nam K-S, Pank H-M (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 167–170 83. Vlasenko NA, Beletskii AI, Denisova ZL, Kononets YaF (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 77–80 84. Kitai AH, Xiao T, Liu G, Li JH (1997) SID’97 Dig 28:419–422 85. Cranton WM, Thomas CB, Stevens R (1997) SID’97 Dig 28:966–969 86. Liu G, Lobban K, Bailey P (1998) SID’98 Dig 29: 648–651 87. Minami T, Shirai T, Kobayashi Y, Miyata T, Yamazaki M (2001) Extended abstracts of the first international conference on science and technology of emissive displays and lighting, San Diego, 12–14 Nov 2001, pp 107–110 88. Lewis JS, Holloway PH (2000) J Electrochem Soc 147:3148–3150 89. Bender JP, Wager JF, Kissick J, Clark BL, Keszler DA (2001) Extended abstracts of the first international conference on science and technology of emissive displays and lighting, San Diego, 12–14 Nov 2001, pp 103–106 90. Minami T, Kobayashi Y, Miyata T, Yamazaki M (2001)Extended abstracts of the first international conference on science and technology of emissive displays and lighting, San Diego, 12–14 Nov 2001, pp 169–172 91. Zhao LJ, Li CH, Zheng CW, Zhong GZ, Fan XW, Liu JH (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 199–202 92. Cho KG, Kumar D, Jones SL, Lee PG, Holloway PH, Singh RK (1998) J Electrochem Soc 145: 3456–3462 93. Sychov M, Nakanishi Y, Kominami H, Hatanaka Y, Hara K (2008) Jpn J Appl Phys 47:7206–7210 94. Sun XW, Kwok HS (1998) SID’98 Dig 29:608–611 95. Chang H, Lee SK, Park HD, Han CH (2000) SID’00 Dig 31:662–664 96. Kalkhoran NM, Vernon SM, Trivedi DA, Halverson WD, Pathange B, Davidson M, Holloway P (1997) SID’97 Dig 28:623–626 97. Xu C, Watkins BA, Jing X, Trowga P, Gibbons CS, Vecht A (1997) Appl Phys Lett 71:1643–1645

Chemistry and Synthesis of Inorganic Light Emitting Phosphors 98. Sievers RE, Milewski PD, Xu CY, Watkins BA (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 303–306 99. Cho SH, Koon SH, Yoo JS, Oh CW, Lee JD, Hong KJ, Koon SJ (2000) J Electrochem Soc 147: 3143–3147 100. Yoo J-S, Jeon BS, Hong GY, Yoo YK (2001) SID’01 Dig 32:750–753 101. Hong GY, Yoo YK, Yoo JS (2001) SID’01 Dig 32: 754–757 102. Cho SH, Yoo JS, Lee JD, Choi JS, Park SB (1997) Extended abstracts of the third international conference on the science and technology of display phosphors, Huntington Beach, 3–5 Nov 1997, pp 307–310

6.1.3

103. Gibbons CS, Vecht A, Smith DW (1997) J SID 5: 151–155 104. Gibbons CS, Vecht A, Smith DW, Jing X (1998) J SID 6:191–193 105. Roh HS, Kang YC, Park SB (2001) Extended abstracts of the first international conference on science and technology of emissive displays and lighting, San Diego, 12–14 Nov 2001, pp 29–31 106. Kang YC, Park SB (2000) J Electrochem Soc 147: 799–802 107. Golego N, Studenikin SA, Cocivera M (2000) J Electrochem Soc 147:1993–1996 108. Jung S, Kang YC, Kim JH (2007) J Mater Sci 42: 9783–9794 109. Kim K, Cich M, Choi H, Hwang ST (1998) SID’98 Dig 29:605–607

1039

Part 6.2

Cathodoluminescent Displays

6.2.1 Cathode Ray Tubes (CRTs) Gerhard Gassler 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044

2

Basic Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044

3

Einzel-Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1046

4

Bipotential-Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1046

5

Magnetic-Focus-Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1047

6

Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1047

7

Electrostatic Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1047

8

Magnetic Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1048

9

Magnification and Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1049

10

Dynamic Focus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050

11

Phosphor Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050

12

Color CRT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051

13 Special Application CRTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1052 13.1 CRTs for Head Up Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1052 13.2 Helmet Mounted Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053 14

Today and Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1054

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.2.1, # Springer-Verlag Berlin Heidelberg 2012

1044

6.2.1

Cathode Ray Tubes (CRTs)

Abstract: The cathode ray tube as the key component of a classical TV set, its basic principles and components are described and illustrated. From the emission of electrons through an oxide cathode, the formation of a concentrated electron beam by different focusing techniques, a moving image is generated by a rapidly deflected flow of electrons, which are converted to visible light on a phosphor screen. The basic concepts of cathodes, various electron guns, deflection methods, color generation and limitations of possible resolution and failure effects are discussed. List of Abbreviations: BFR, Beam-Forming Region; CRT, Cathode Ray Tube; HMD, Helmet Mounted Display; HUD, Head Up Display

1

Introduction

From the 1920s through the end of the twentieth century, it was the consumer TV, with a cathode ray tube (CRT) as its key component, which revolutionized communication and entertainment, and allowed people to view global events in real time. After hundreds of millions of CRTs manufactured, with LCD and other flat panel display technologies now well established, the position of the CRT in the mass consumer markets is falling. But it is an undeniable fact that this fascinating technology dominated the display industry for more than 70 years.

2

Basic Principles

Despite thousands and thousands of patents for new improved designs, the basic principle of a CRT did not change since the invention of Ferdinand Braun in 1897: Electrostatic or electromagnetic deflected electrons write like a pencil on a phosphor screen a fast moving trace, which is seen by the human eyes as an integral picture. > Figure 1 shows the basic components of a monitor CRT.

Glass bulb

Electron beam Electron gun Deflection coil

. Fig. 1 Basic components of a monitor CRT

Phosphor screen

Cathode Ray Tubes (CRTs)

6.2.1

Protected in an evacuated glass bulb a heated cathode generates free electrons, which are controlled and focused by a set of electrodes to a phosphor screen in front. Two pairs of orthogonal deflection plates are used in oscilloscopes and some special purpose tubes, whereas a more complex arrangement of two pairs of deflection coils enforce the deflection of the fast electron beam in a common picture or TV tube. Already the glass envelope for the whole setup is not just a simple compartment [1]. Inside, it has to keep a vacuum in the range of 10
9 mbar, not allowing any micro-leakage at any of its electrical feed-throughs for decades. Mechanically strong enough, it has to withstand not only the outside pressure, but also rough handling by careless users [2]. Additionally it must be opaque for x-ray radiation created by the electron beam to protect the viewer in front. Mass products like a CRT are always driven by utmost design-to-cost considerations. Most likely, the cheapest way to create a long lasting electron source was found with the Alkaline earth oxide cathode. To extract electrons from a metallic surface at manageable temperatures of about 600–1,000 C the work-function may not exceed 1.2–1.8 eV, which is the case for Barium Oxide [3]. The principle design of such a cathode is shown in > Fig. 2. As BaO is not stable in air, it is sprayed during production in the form of BaCO3 on top of the cathode sleeve. During the evacuation of the CRT on the pump, the BaCO3 layer gets converted into BaO and CO2 by heating. BaCO3 ! BaO þ CO2

ð1Þ

A further reduction of the work-function is achieved within the so-called activation procedure of the cathode, when a monolayer on the top of the BaO is reduced to metallic Barium. A different type of cathode is needed for some special application CRTs, where high brightness is required and therefore higher beam current is mandatory. These so-called dispenser-cathodes are able to deliver a current of up to 2 mA instead of about 0.3 mA of a BaO cathode. Imbedded in a porous tungsten pellet Barium atoms migrate via diffusion to the emitting surface and keep the work-function low [3]. After having created a cloud of electrons in front of the cathode, these electrons need to be concentrated, accelerated, and focused into a proper beam which ends sharply on the phosphor screen. Electrostatic forces induced by electrodes at different potentials or magnetic deflections generated by current-carrying coils help to form such a precise beam. The basic characteristic of the electron beam is already defined in the so-called Beam-Forming-Region (BFR) of the electron gun, right after the cathode. See > Fig. 3. A first positive voltage UACC1 on an electrode with a tiny hole in the center accelerates the emerging electrons from the cathode along the axis of the structure. The amount of electrons entering the electron gun structure, and therefore finally the brightness of the displayed spot on the phosphor screen, can be easily controlled through a negative voltage at the Wehnelt cap between. In optical terms, it is the size of the focused ‘‘Crossover’’ which will be magnified by

Heater

BaO coating Cathode sleeve

. Fig. 2 Design of a barium oxide cathode

1045

1046

6.2.1

Cathode Ray Tubes (CRTs)

the following electron-optical main-lens toward the screen. The final shape of the electron beam and thereby its appearance to the viewer is determined by the details of the electrodes, their diameters, apertures, and accelerating voltages. The electron-optical main-lens can follow one of the three basic different designs for an electron gun [4, 5]: ● Einzel-Lens ● Bipotential-Lens ● Magnetic-Focus-Lens

3

Einzel-Lens

Low to medium performance at reasonable costs can be achieved by the Einzel-Lens design with a focus voltage between 0 and 600 V (> Fig. 4). The focusing properties are achieved by a split anode and an inserted focus electrode. Limitations are given by the narrowed possibility to prefocus the electron beam before entering the main-lens and the relatively high numerical aberration failures of this electron-optical concept.

4

Bipotential-Lens

Almost all CRTs used in high resolution monitors and TV sets are using the concept of a Bipotential-Lens (> Fig. 5). After a smooth prefocusing of the beam when entering the

U Cathode 0 V

U Wehnelt –60 V U ACC1 +1,000 V

. Fig. 3 Beam-forming-region of an electron gun

Anode 15 kV

Screen Focus 0–600 V

. Fig. 4 Einzel-lens electron gun

Cathode Ray Tubes (CRTs)

6.2.1

focus lens, the electrons can occupy a bigger diameter of the main-lens, without causing major aberration failures. The focus voltage of a Bipotential-Lens is typically in the range of 25–30% of the anode voltage.

5

Magnetic-Focus-Lens

The advantage of a low aberration big main-lens diameter is maximized in the MagneticFocus-Lens design (> Fig. 6). Nevertheless, the costly magnetic focus coil limits its application to reprography systems for the film industry and other special applications like projectors in professional flight simulators [6].

6

Deflection

As already mentioned, there are two different ways to create an image with a fast moving electron beam: Electrostatic deflection using the electric field between plates at different potentials [7] and electromagnetic deflection induced by the Lorentz Force on charged particles in a magnetic field [8].

7

Electrostatic Deflection

The kinetic energy of an electron e with the mass m accelerated through the potential U0 is described as: 1=2mv 2

Focus

¼ eU0

ð2Þ

Anode 25 kV

Screen 7 kV

. Fig. 5 Bipotential-lens electron gun

Anode

Magnetic focus coil Screen

30 kV

. Fig. 6 Magnetic-focus-lens

1047

1048

6.2.1

Cathode Ray Tubes (CRTs)

and leads to a velocity of the electron v0:

rffiffiffiffiffiffiffiffiffiffi 2eU0 v0 ¼ m

ð3Þ

A potential Up at the deflection plates with distance a produces a deflecting electrical Field E Up ¼
Ea

ð4Þ

which results in a deflection d of the electron on the screen of: L  b Up d¼  2a U0 where

ð5Þ

L – Distance between center of deflection plates and screen a – Gap between deflection plates b – Length of deflection plates d – Deflection on the screen U0 – Accelerating potential UP – Potential at deflection plates Electrostatic deflection is mainly used in CRTs for instruments like oscilloscopes and some very unique CRTs for military applications like Sonar systems or thermal imaging (> Fig. 7).

8

Magnetic Deflection

The common way to deflect an electron beam and generate an image on a monitor or TV CRT is using the Lorentz force FL provoked by moving electrons e with velocity v in a homogenous magnetic field B (> Fig. 8).   ~ FL ¼ e ~ vx~ B ð6Þ The deflection angle in such a magnetic field of length L can be determined as e0  L  B sin F ¼ mv

ð7Þ

With Eq. (2) this can be converted to

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e0  L  B pffiffiffiffiffiffi sin F ¼ 2m  U0

b

Electron gun

Electron beam a

U0

Deflection plates at Up

d Screen

L

. Fig. 7 Electrostatic deflection of an electron beam

ð8Þ

6.2.1

Cathode Ray Tubes (CRTs)

Electron gun

F

U0

Screen

L Electron beam

. Fig. 8 Deflection of an electron beam in a perpendicular magnetic field

where e0 – Charge of an electron L – Length of the magnetic field B – Strength of the magnetic field m – Mass of an electron U0 – Accelerating voltage A standard TV set is designed to write 525 horizontal lines in the vertical direction; after completion of a horizontal line another set of perpendicular coils shifts the starting point a little bit downward and the next line follows. The spacing between adjacent lines is close enough to simulate an undisturbed image for the human eye. The composition of a complete image is repeated at least 50 times/s, today standard became already 100 Hz. During the development of a deflection yoke the designer has to minimize the effects of different sources of failures. These failures are proportional not only to the deflection angle, but also to the divergence angle of the electron beam itself. The main contributors are: Coma failure  a2 F where a is divergence angle of the electron beam Distortion  a F2 Astigmatism  F3 F – magnetic deflection angle

9

Magnification and Resolution

The magnification V of an electron gun in a CRT can be derived analog to an optical system by Abbe’s law [5, 8] (> Fig. 9): rffiffiffiffiffiffi rffiffiffiffiffiffi b UK sin aK UK V¼  ¼  a US sin aS US where a – Distance to object b – Distance to screen UK – Potential at crossover US – Potential at screen aK – Divergence angle cathode side aS – Divergence angle screen side

1049

1050

6.2.1

Cathode Ray Tubes (CRTs)

Anode Focus

2aS

2aK Prefocusing lens a

Screen Main lens b

. Fig. 9 Magnification of an electron gun derived from Abbe’s law

The physical limits of resolution are governed by three factors: 1. Temperature of the cathode The electrons from the hot cathode surface are emitted into all forward directions and create, therefore, a Crossover point with a diameter of approximately 20–50 mm, which creates a magnified Gaussian profile on the screen. 2. Spherical aberration failures of the electron-optical lenses Also the lenses of an electron gun are limited in quality by spherical aberration failures with an increase at the power of three for the divergence angle. Only close to the center line these failures can be neglected. Larger electron beam diameters, giving higher brightness on the screen, lead to visible distortion and reduction of resolution. 3. Repulsion of electrons to each other in a narrow beam The smaller the electron beam diameter, the more electrons repulse from each other. Only at CRTs with very high resolution this effect plays a significant role. During the design of a high resolution electron gun all these effects need to be calculated in electron-optics simulation. Starting from the choice of the cathode type, every aperture, distance, and lens diameter needs to be optimized to finally yield in a brilliant image. To achieve a satisfying result, most of the emphasis needs to be spent in the Beam-Forming-Region BFR, between cathode surface and entrance to the Prefocusing-Lens of a Bipotential gun. Mechanical precision in the order of mm have to be controlled in a mass-manufacturing process.

10

Dynamic Focus

It is obvious that the distance between the main lens and a point D on the screen is different than between main lens and the center. Therefore, a dynamic focus adjustment adding a parabolic term to the static focus voltage is necessary, when the electron beam passes over the screen (> Fig. 10).

11

Phosphor Screen

Even the most perfect electron beam is not visible until it does not hit the light emitting phosphor grains at the inner surface of the glass bulb. Phosphor screens are coatings of a crystalline powder with particle sizes around 5–9 mm, which are selected for their color

Cathode Ray Tubes (CRTs)

6.2.1

emission and persistency. Specific dopants to the basic crystal structure can change the color coordinates of the emission spectrum and will optimize the characteristics for the specific application [3] (> Table 1).

12

Color CRT

Designing a color CRT is nothing else than combining three identical electron guns for the primary colors red, green, and blue almost in a row. The correlation between a specific gun and an assigned color is achieved by a shadow mask placed very close to a thousand-fold pattern of individual, tiny phosphor dots forming the screen. The right and left electron guns are slightly tilted to the center one, so that all three beams match at the same hole of the shadow mask. After the shadow mask each one hits its specific phosphor color dot [9–11] (> Fig. 11). Instead of round phosphor dots, easier to manufacture phosphor stripes and elongated mask holes are widely used. A lot of efforts were invested into the improvement of contrast and color purity; thousands of patents helped to deliver an almost perfect image. The resolution of a shadow mask color CRT is limited by the distance between neighboring RGB-triplets, the so-called pitch. When a standard TV-CRT is built with a pitch of 0.8 mm, a high resolution professional color CRT can go down to 0.15 mm. The analog principles of a color CRT, together with the Lambertian kind of light emission, yield an almost unbeatable range of displayable colors, the color gamut.

Screen coordinates Anode Focus

Curved screen Centre

Deflected beam

Main lens

D

Dynamics focus voltage

. Fig. 10 Different path lengths for an electron beam and parabolic correction

. Table 1 Designation, color, and chemical composition of various phosphors Color coordinates Phosphor

Color

x

y

Chemical composition

Persistency

P1

Yellow/green

0.218

0.712

Zn2SiO4:Mn

Medium

P11

Blue

0.139

0.148

ZnS:Ag

Medium–short

P31

Green

0.226

0.528

ZnS:Cu

Medium–short

P43

Green

0.333

0.556

Gd2O2S:Tb

Medium

P45

White

0.253

0.312

Y2O2S:Tb

Medium

P56

Red

0.640

0.335

Y2O3:Eu

Medium

1051

1052

6.2.1

Cathode Ray Tubes (CRTs)

Blue Red

Green Electron guns

Shadow mask

Phosphor dots on screen

. Fig. 11 Color CRT with shadow mask and phosphor matrix

It is obvious that a shadow mask CRT by design is sensitive to any disturbing outside magnetic field. Therefore, especially a color CRT needs protection in the form of a magnetic shielding cabinet, which protects the beam from mislanding on the phosphor matrix. Any mislanding leads immediately to impurity in color, which can be easily noticed onboard older aircrafts using color CRTs for the flight entertainment. The changing electromagnetic field on earth causes sometimes dramatic changes in color on the screen. In the attempt to reduce the physical depth of a color CRT it was possible to increase the deflection angle from typically 90 up to 120 for a 3200 size HDTV version. Special applications requested even crooked or folded designs.

13

Special Application CRTs

Technical high end CRTs are still used in modern avionic applications. For sure direct view displays in modern aircrafts are all equipped with different flat panel solutions. The niche segment of Head Up and Helmet Mounted Displays is still governed by special CRTs [12, 13].

13.1

CRTs for Head Up Displays

The concept of Head Up Displays (HUDs) was developed about 40 years ago for military jet fighters in order to support the pilot with all necessary information in his direct field of view in forward direction outside of the cockpit. Therefore, a semi transparent glass plate is mounted under an angle of 45 in front of him. From below an extremely bright CRT projects the information toward the plate and the projected image can be seen by the pilot at infinity. The challenge for the CRT designer is to deliver a brightness up to 100,000 cd/m2 at high resolution within an extremely rough environment. From the cathode heater, all electrodes and connections in the design have to be ruggedized to withstand high shock and vibration levels [14, 15],

Cathode Ray Tubes (CRTs)

6.2.1

. Fig. 12 Miniature electron gun for a helmet mounted display used for military aircrafts

extreme temperature cycling, and other rough environmental conditions. Typically HUD CRTs can be driven in two different modes: ● The raster mode for a kind of TV image coming from a forward-looking infrared detector, which is mounted below or in front of the aircraft ● The stroke mode, to display very bright flight-symbology, numbers, and characters, clearly visible even in direct sunlight. In stroke mode, symbols are written like with a pencil directly onto the screen

13.2

Helmet Mounted Displays

Since the beginning of the 1990s, Helmet Mounted Displays (HMDs) are becoming integrated into modern aircrafts. Still only very special CRTs can fulfill all the requirements of the military concerning brightness, resolution, environment, and other specified parameter. The key element of an HMD is a miniaturized CRT with about 20 mm diameter and 75 mm length, which is integrated into the pilot’s helmet. The image, similar to that for a HUD, is projected to the inner side of the helmet’s visor. Wherever the pilot is moving his head, he takes all information with him and can concentrate in parallel to the outside world. A magnetic tracking sensor in the canopy of the aircraft detects simultaneously the movements of the pilot’s head and gives the information to the weapon control system. Finally, with his head movements the pilot can control the targeting of his missiles (> Fig. 12).

1053

1054

6.2.1 14

Cathode Ray Tubes (CRTs)

Today and Future

Actually, in the year 2010, flat panel displays have taken over all show rooms in the western world for commercial TV sets. Only in areas where the cost aspect is of major importance, CRT-based TV sets are still accepted by the market. Apart from commercial applications, only few military designs, like Helmet Mounted Displays, still use miniature CRTs in their actual design. It is likely that even these specialist CRT solutions will be replaced by modern alternatives within a few years, but it was the unique technology of CRTs which made the mass-manufacturing of displays possible in the twentieth century.

References 1. Scholze H (1977) Glas, Natur, Struktur und Eigenschaften. Springer, Berlin/Heidelberg 2. Shand EB (1958) Glass engineering. McGraw-Hill, New York 3. Bretting J (1991) Technische Ro¨hren. Hu¨thig, Heidelberg 4. Harting E, Read FH (1976) Electrostatic lenses. Elsevier, Amsterdam 5. Sturrock PA (1955) Static and dynamic electron optics. Cambridge University Press, Cambridge 6. Hawkes PW (1986) Advances in electronics and electron physics. Academic, New York 7. Klein PE (1979) Das Oszilloskop. Franzis, Munich 8. Eichmeier J, Thumm M (2008) Vacuum electronics. Springer, Berlin/Hedelberg

9. Schro¨ter F, Theile R, Wendt G (1956) Fernsehtechnik. Springer, Berlin/Hedelberg 10. Lang H (1978) Farbmetrik und Farbfernsehen. Oldenbourg, Munich 11. Grum F, Bartleson CJ (1980) Optical radiation measurements. Academic, New York 12. Sherr S (1970) Fundamentals of display system design. Wiley-Interscience, New York 13. Richards CJ (1973) Electronic display and data systems. McGraw-Hill, Maidenhead 14. Thomson WT (1965) Vibration theory and applications. Prentice-Hall, Englewood Cliffs 15. Grandall M (1963) Random vibration in mechanical systems. Academic, New York

6.2.2 Vacuum Fluorescent Displays (VFDs) Andrew Stubbings 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1056

2

VFD Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1056

3

Illumination and Lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057

4 Multiplex Drive Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057 4.1 Graphic VFD Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057 4.2 Inter-digit Blanking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1059 5 5.1 5.2 5.3

Filament Drive Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1060 AC Filament Drive with a Pulse Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1061 DC Filament Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1061 Grid/Anode Cutoff Voltage and Cathode Filament Bias (Ek) . . . . . . . . . . . . . . . . . . . . . . 1062

6

Construction: Standard Frame VFD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1063

7

Construction: CIG VFD Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1064

8

Construction: Active Matrix VFD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065

9

Phosphor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065

10

Operating Parameter Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1066

11

Example VFD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1066

12

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1067

13

Further Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1068

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.2.2, # Springer-Verlag Berlin Heidelberg 2012

1056

6.2.2

Vacuum Fluorescent Displays (VFDs)

Abstract: This section considers Vacuum Fluorescent Display (VFD) technology which produces a high brightness illumination using the principal of a triode vacuum tube in conjunction with a phosphor-coated anode. List of Abbreviations: CIG, Chip in Glass; eb, Anode Voltage; ec, Grid Voltage; ebc, Common Grid/Anode Voltage; Ebco, Anode Cutoff Voltage; Ecco, Grid Cutoff Voltage; Ef, Cathode Filament Heating Voltage; Ek, Filament Bias Voltage; ITO, Indium Tin Oxide; POC, Phosphor on Chip – Active Matrix; VFD, Vacuum Fluorescent Display

1

Introduction

VFD technology was invented by Dr. Tadashi Nakamura circa 1966, with early commercialization in calculators and later expansion into household consumer audio–video and cooking appliances, automotive displays, and extensive industrial applications. The robust construction, high illumination, and wide environmental capability of VFDs is offset by their higher power consumption compared to other technologies. VFDs maintain a niche position in the displays market with significant application cost reduction and lifetime improvement achieved through internal integration of the display driver ICs.

2

VFD Operation

The VFD is a type of triode vacuum tube with three electrodes which are: ● Cathode filament(s) ● Mesh control grids ● Phosphor-coated carbon anodes (> Fig. 1) The directly heated cathode filaments are coated in barium oxide to facilitate thermionic emission at 650 C when a heating voltage Ef is applied and a vacuum of 10 7 Torr is present within the tube. A Nesa coating is applied to the inside front face of the tube to repel electrons from any external static charge. The electrons emitted from the cathode filaments are attracted and diffused by mesh grids when a positive voltage is applied with 50% of electrons flowing through the grid mesh toward the anodes (opposite charges attract). When a mesh grid is supplied with a negative voltage, it repels the negative electrons and prevents them from reaching the anode (similar charges repel). The carbon anode bases are coated with phosphor typically zinc–zinc oxide with copper activator which forms a segment or dot, which collectively form individual characters. When an anode is supplied with a positive voltage, it will attract the electrons which have been accelerated through the grid. The segment emits light when these electrons impact the phosphor coating. When anodes are supplied with a negative voltage, they repel electrons from their phosphor coating and therefore remain unilluminated. The electrodes connect to external lead pins directly or via silver aluminum tracks on the substrate to IC drivers either internally (CIG) or externally.

Vacuum Fluorescent Displays (VFDs)

Grid

Filament

6.2.2

Phosphor

Lead Pin

. Fig. 1 Envelope end view

3

Illumination and Lifetime

VFD illumination varies proportional to the applied anode/grid voltage and the applied duty cycle. It ranges from 500 cd/m for multiplexed displays where the grid pulse width may be only 30 ms to 4,000 cd/m for multiplex drive displays with l ms grid pulse width. Sunlight view ability at 35,000 cd/m is possible with static drive and high voltage [1]. Luminance is proportional to K  ebc2.5  Du where K is a constant for the display, ebc is the anode/grid voltage, and Du is Duty. The duty cycle is adjusted to control the brightness of a VFD rather than the anode/grid voltage to maintain even illumination. Under normal operating conditions, a half-life of 30,000 + hours can be expected with the useable life extending to 200,000 hours due to the nonlinear degradation for displays with ebc operating in the range 12–35 VDC. Degradation of the VFD due to phosphor surface layer damage due to electron impact has decreased with the implementation of CIG technology to reduce the grid/anode voltage. Evaporation and contamination of the filament coating also contribute to VFD degradation. This is particularly evident with phosphors having a sulfide component with research developing alternatives [2].

4

Multiplex Drive Techniques

VFDs can operate with an anode voltage of 12 VDC when static drive is used. This does, however, involve each electrode connecting to a driver output resulting in significant pin count. To minimize the number of pin connections and driver chips, the majority of VFDs use a multiplexing drive method. Corresponding anode segments are connected in common under each separate grid, with each in turn being connected to a data line. Each character has its own separate grid which not only diffuses the electrons from the filaments, but also controls the selection of the character position in a ‘‘time share’’ multiplexing cycle. The duty cycle ‘‘on time’’ of each character will determine the appropriate operating voltage required to provide sufficient luminance. The refresh rate range is between 75 and 200 Hz (> Figs. 2, 3 and 4).

4.1

Graphic VFD Multiplexing

Most graphic dot matrix CIG VFDs require a particular scan procedure known as ‘‘overlapping grid scan.’’ This occurs when the space between anodes under different grids is less than 1 mm and it is necessary to turn on two adjacent grids at the same time in order to produce an even

1057

1058

6.2.2 e g f

Vacuum Fluorescent Displays (VFDs)

a

Gn

. Fig. 2 Internal connection

. Fig. 3 Typical circuit

. Fig. 4 Seven-segment character multiplex timing

b c d

G2

G1

Vacuum Fluorescent Displays (VFDs)

6.2.2

electron flow and consequently an even illumination of the anodes at the center of the adjacent grids (> Fig. 5). Each grid has two columns of anodes and there are four sets of anodes ABCD. When Grid 1 and 2 are ON, anodes B + C are active and A + D are turned off. Then, when Grid 2 and 3 are ON, anodes D + A are active and B + C are turned off. Six-way and eight-way schemes are also adopted to reduce operating voltage through extended duty cycle. Further reduction in voltage is achieved by a dual scan method with two anode buses driven by separate ICs left and right sides [3].

4.2

Inter-digit Blanking

One problem when using multiplexing is ghost illumination caused by decaying grid signals caused by stray capacitance between VFD electrodes and display drivers. If the grid timing overlaps the following grid and anode signal pulses, ghost illumination appears at unaddressed anode segments (> Fig. 6). To overcome this problem, an inter-digit blanking time should be added between grid pulse timings. Generally the inter-digit blanking time should be approximately 10–50 ms, but this can vary depending on the delay time. Delay time occurs when high value pull down resistor-type

. Fig. 5 Four-way anode/grid arrangement

Blanking

makes ghost illumination

Gn

Gn+1 Anode data for Gn+1

a

Without blanking

. Fig. 6 Inter-digit blanking to remove ghost illumination

b

With blanking

1059

6.2.2

Vacuum Fluorescent Displays (VFDs)

drivers are used or when the drive circuit is situated away from the VFD. We recommend that an appropriate inter-digit blanking time is utilized on the grid signal only, rather than on both grid and anode signals.

5

Filament Drive Techniques

Luminance varies with the filament voltage (Ef). Since the lifetime of a VFD is dictated by the extent of evaporation of oxide materials coated onto the tungsten filament wires, it is critical that the filament voltage is supplied within the specified ratings (> Fig. 7). Current drain from the anodes and grids to the filaments can cause ghost illumination so a bias voltage is applied to the filaments to raise them above ground. This is described later (> Fig. 8). Generally, the transformer has been the most popular device utilized to supply the filament voltage (Ef) with a sine wave which also has a center-tap for cathode bias. The center-tap technique is used to prevent luminance slant, i.e., difference in brightness from one side of the display to the other. 150 ec, eb, Duty = Constant Relative luminance (%)

1060

Typical Ef 100

50

0

50

100 Relative Ef (%)

150

. Fig. 7 Luminance vs filament voltage Anode, grid

ec, eb ec(eb)+ ec, eb ec, eb

VFD

F1

Ef

F2

√2.Ef

GND off level

F1

. Fig. 8 AC filament drive with a transformer

Filament

F2

√2 . Ef 2

6.2.2

Vacuum Fluorescent Displays (VFDs)

5.1

AC Filament Drive with a Pulse Circuit

In the case of a DC or battery power supply, a pulse wave form for the filaments can be generated from a DC to AC converter (> Fig. 9). The concept of pulse voltage supply to the filament is the same as AC filament drive. Please note that the pulse voltage should be calculated as an RMS (root mean square) value from the wave form as shown in formula (1). However, a 1/2 duty factor should be set, and the peak to peak pulse wave form should be 1.5 times or less than the RMS value. A frequency range of 10–200 kHz is used.

5.2

DC Filament Drive

If a DC filament drive is adopted (> Fig. 10), a potential difference between the anode and grid voltage will be apparent as a luminance slant across the display. This shows brighter luminance at one side of the display due to the DC voltage drop. In order to avoid this problem, the display construction involves a height bias between filament supports. The polarity (+, ) of the filament is then specified. However, this is generally only possible for VFDs up to 100 mm in length.

Anode, Grid t1·V12 + t2·V22 t1 + t2

Et =

....... (1) VFD

ON

V1

ON

F1

Vdisp= F2 ec(eb)+Ek

c.t.

ON

V2

Ef

+ t1

t2

DC IN

Ek

DC - AC inverter

. Fig. 9 DC-AC converter filament drive

Anode, grid ec, eb VFD

ec(eb)-Ef ec, eb

F1

ec, eb

F2 Ef +

GND

– F1

. Fig. 10 DC voltage applied to filament

F2

1061

1062

6.2.2 5.3

Vacuum Fluorescent Displays (VFDs)

Grid/Anode Cutoff Voltage and Cathode Filament Bias (Ek)

Luminance (L) varies with the anode voltage (eb) when the grid voltage (ec) is a constant. Luminance also varies with the grid voltage when the anode voltage is a constant (> Fig. 11). To completely turn off the luminescence at the unaddressed display segments, a negative voltage is applied to the unaddressed anodes and grids with respect to the filament. These negative voltages are called anode cutoff voltage (Ebco) and grid cutoff voltage (Ecco) respectively. The cutoff voltage varies depending on each type of display due to various differences in filament voltage and wave form. The filament bias voltage (Ek) is a voltage applied to the filament center-tap in order to cut off background illumination when the anodes and grids are not addressed. The ‘‘off ’’ anode and grid voltages remain negative with respect to the filament. The total supply voltage Vdisp is ec, eb + Ek. (In the case of CIG displays, the Ek is included in VDD2.) In typical driving circuits, a Zener diode supplies the cathode bias (Ek) and is set higher than that specified for Luminance (L)

ec=constant Ebco 0

Anode voltage(eb)

. Fig. 11 Luminance versus anode voltage

Anode, grid +V VFD ec,eb

Vdisp=ec(eb)+Ek F1

Ef

F2

Vdisp

c.t. +

C.T.

Ek

Filament center Ek

GND

. Fig. 12 Filament bias voltage

Vacuum Fluorescent Displays (VFDs)

6.2.2

the grid cutoff voltage (Ecco). Usually, the Ek is set at the same value as MIN voltage of Ecco shown in the specification or a slightly large value when utilizing a filament center-tap (FCT). If a center-tap is not available as in a bridge circuit, the idle state and peak voltage swing of the bridge can produce a virtual center-tap (> Fig. 12). The bias voltage for fine-pitch graphic VFD where the dot pitch is less than 0.35 mm does require special consideration due to adjacent negative fields of unaddressed anodes reducing the illumination of addressed anodes. In this case it is necessary to apply a bias of only 0.6 VDC. Levels higher than this will cause uneven illumination.

6

Construction: Standard Frame VFD

VFDs have evolved several methods of construction (> Fig. 13, > Table 1). The basic model is that of the frame-type construction. Other techniques with CIG (chip in glass driver), active matrix, rib grid, and line anode VFDs still maintain the primary mode of operation. The tungsten filaments have a diameter of 14.7 mm and are coated with barium oxide which increases the diameter to 34 mm. The typical gap between the filaments and grids is 1.0 mm. The grids are made from chemically etched stainless steel alloy 304 with a thickness of 50 mm. The typical gap between the grids and phosphor is 0.5 mm with the phosphor screen printed to a thickness of 30–35 mm. The carbon anode base can be 5% larger than the phosphor pattern and is also 30–35 mm thick. The grid mesh, filament supports, and lead pins are assembled on a single metal lead frame for assembly purposes. The ends of the grid mesh are extended to the outside of the envelope,

5

11

14

9

7

10

13

12

1

4

3

2

6

15

. Fig. 13 Main component parts . Table 1 Component cross reference 1 Glass substrate (anode plate)

9

2 Conductive layer

10 Getter

Filament (cathode)

3 Anode (base)

11 Face glass (cover glass)

4 Insulation layer

12 Spacer glass

5 Phosphor (display pattern)

13 Evacuation tube

6 Conductive paste

14 NESA (or ITO) coating

7 Grid mesh

15 Lead pin

1063

1064

6.2.2

Vacuum Fluorescent Displays (VFDs)

and are formed as lead pins. A new technique of mounting the filament supports beneath the face glass enables a larger phosphor area [4]. The anode leads are extended into the envelope to connect with conductive paste on pads which are formed as part of the tracking on the glass substrate. Both ends of the filament are welded to the lever springs of the filament support and the fixed anchor with a tension of 20–27 g. The face glass and glass substrate (anode plate) have a thickness of 1.8–5 mm depending on the physical size of the display due to the strength required to support the vacuum. The frame is assembled with the side walls, face glass, and substrate using frit ceramic and fired for 30–45 min at a temperature of 450 C. The exhaust pipe previously used for evacuation is now replaced by a hole in the glass substrate covered with a frit sealed metal plate of 8 mm diameter. At the end of the evacuation process the getter ring coated in barium is inductively heated. This deposits a film on the front glass face which will absorb residual-free molecules. The lead pins are tinned and formed into a suitable shape for PC board assembly. FRAME-Type VFDs require press-formed metal dies for construction. They offer good production yield and high reliability against various environmental conditions. A hybrid of this construction mounts the grids directly on the glass substrate which allows complex grid patterns.

7

Construction: CIG VFD Displays

The driver chip is located on the glass plate under the frame and connected by wire bonding to the electrodes which are located on the glass plate (> Fig. 14). The data and power supplies

. Fig. 14 Integrated driver located under filament supports

Vacuum Fluorescent Displays (VFDs)

6.2.2

Output (to internal grid or anode) 96bit driver Level shifter

VDD2 VDD1

LAT

Latch

BK

SI CLK

96bit shift register

SO

. Fig. 15 VFD driver functional blocks

to the drivers are connected to the lead pins through the conductive tracks on the plate. The driver outputs connect via the conductive tracks located on the same plate to their respective segments (phosphor anodes) and/or grids. The CIG VFD incorporates one to four 96-bit, 128-bit, or 144-bit drivers, depending on the display pattern demanded by the application (> Fig. 15). Like the driver ICs in ordinary VFDs, the drivers in CIG VFD contain level shifters, latches, and shift registers made of C-MOS FET circuits. The devices are less than 10 mm in length with very fine bonding pitches along one side which requires fast fine-pitch bonding machines.

8

Construction: Active Matrix VFD

Active Matrix VFDs are a special type of chip-in-glass (CIG) VFD. They contain small silicon chips about 5  5 mm in size which include the phosphor matrix, driver, and memory functions. In order to construct a wide display area, these small square silicon chips are precisely ‘‘tiled’’ on the glass plate with each having a 16  16 dot matrix phosphor pattern precisely formed on the top surface. Only two chips can be put vertically (Y direction) due to the space required for wire bonding (> Fig. 16). As with conventional VFDs, the active matrix VFDs have cathodes (filaments) to emit electrons, a single mesh grid to diffuse electrons, with the tiled anodes coated with phosphor to which attract electrons and emit light (> Fig. 17). As with other CIG VFDs, a reduced number of lead pins contribute to easy assembly compared to the conventional graphic VFDs (> Fig. 18). The active matrix VFD operation is static drive making it ideal for low noise and long life applications. The anode/grid voltage is typically 15 VDC with a filament bias of 0.6 VDC.

9

Phosphor

The requirement for Restriction of Hazardous Substances (RoHS) compliance has seen the removal of cadmium-based color phosphors used in VFD [5, 6]. Compliant phosphors exist for red, green, and blue as shown in > Table 2.

1065

1066

6.2.2

Vacuum Fluorescent Displays (VFDs)

. Fig. 16 Array of silicon die forming graphic dot matrix

10

Operating Parameter Range

High voltage driver ICs and photo lithographic printing processes enable VFD to support a wide range of applications. Typical operating parameters are shown in > Table 3.

11

Example VFD

. Fig. 17 Custom VFD with flexible phosphor pattern

Vacuum Fluorescent Displays (VFDs)

6.2.2

. Fig. 18 Multicolor electric oven display (Image courtesy Rational AG)

. Table 2 Phosphor colors - RoHS

Color

Phosphor Color Name

Chromatic Coordinate

Brightness

Life

X

(cd/m2)a

(hrs)b

Y

Composition

Blue

Light Blue

0.182

0.171

70

10000

ZnGa2O4 + ZnO

Blue

Bluish Green

0.202

0.368

500

10000

ZnO + blue pigment

Green

Vivid Green

0.090

0.707

300

3000

Green

Green

0.243

0.418

1500

30000

Red

Vivid Red Orange 0.673

0.326

160

1500

ZnGa2O4 : Mn ZnO CaTiO3

a

ebc = 50 V, Du = 1/30 at actual measurement value Ef = 3.15 V, ebc = 50 V, Du = 1/30 at half-life of brightness

b

12

Conclusion

VFD technology provides high brightness, reliable operation in harsh environmental applications. The ability to create easy to read 2-mm characters with either a fixed icon or miniature dot matrix has enabled complex consumer applications to attain small form factors for VCR, DVD, and audio players at low cost. The screen printing processes used in phosphor and insulator assembly have low setup costs allowing flexibility of design from one product iteration to another which allows designers to regularly add variation to meet application improvement.

1067

1068

6.2.2

Vacuum Fluorescent Displays (VFDs)

. Table 3 Operating parameter range Parameter

Range

Filament voltage

AC drive 1.5–15 Vpp, DC drive 1.5–5 VDC

Filament frequency

100–200 Hz

Filament current

25 mA per filament; 2–15 filaments per display

Anode/grid voltage

12–35 VDC long life, 120 VDC maximum

Anode + grid voltage

10–50 mA depending on height and duty cycle

Grid cutoff voltage

0.6 VDC for static displays, 1–12 VDC for multiplex

Luminosity

Standard 640 cd/m2 – high brightness 4,000 cd/m2

Package dimension

Length/width 350  75 mm standard, 500  90 mm maximum

Package dead space

Horizontal 14 mm each end, vertical 7 mm top, bottom

Minimum phosphor gap

0.1 mm common anodes, 0.3 mm different anodes

Minimum grid gap

2 mm standard multiplex, 0.1 mm overlapping multiplex

Viewing angle

85 (center, center)

The advent of 4,000 cd/m2 displays provides a solution for external use in high illumination and environmentally challenging applications like outdoor vending and electric vehicle charging points. VFD has yet to meet the requirement for low power and full color graphic displays provided by TFT displays. Successful development of nanotube emitters and coated microphosphors could provide the necessary components to meet this challenge.

13

Further Research

Future research will focus on lower power cathodes using microfilaments or nanotube emitters [7], the development of blue, green, and red phosphors with comparable performance to ZnO green for multicolor displays, and vacuum sealing with lead-free frit materials.

Acknowledgments The author wishes to thank Tadami Maeda of Noritake Itron for helpful discussion. Images supplied courtesy of K. Kinoshita, Director, Noritake Itron Corporation.

References 1.

2.

Iwase H et al (1995) VFD for head up display with luminance of 35000 cd/m. Society of Automotive Engineers, Warren Dale, PA, USA Kubota S, Yamane H et al (2005) Substituting sulphide phosphors in VFD. JP Patent 2,005,008,674

3. 4. 5.

Maeda T et al (2007) Increasing phosphor area in VFD. US Patent 7,262,549 Mizohata T, Hiraga M (2006) Multiplex anode matrix VFD. US Patent 7,071,903 Hamada T, Kitagawa K, Toki H (2004) Cadmium free phosphor for VFD. US Patent 6,690,119

Vacuum Fluorescent Displays (VFDs) 6.

Oshima H (2003) Zn2GeO4 cadmium free phosphor for VFD. JP Patent 2,003,336,060

7.

6.2.2

Uemura S, Yotani J et al (2003) VFD with field emission source. US Patent 6,624,566

Further Reading Application notes. www.noritake-itron.com Blankenbach K, Gassler G, Koops HWP (2008) Vacuum displays. In: Eichmeier JA, Thumm MK (eds) Vacuum electronics: components and devices. Springer, Berlin

Kamikubo H, (1992) ZnO: Zn phosphor with tungsten oxide powder. US Patent 5,128,063 Ogawa Y, Ishikawa K et al (2003) Double faced VFD driving method. US Patent 6,611,094

1069

6.2.3 Field Emission Displays (FEDs) Yongchang Fan . Mervyn Rose 1 1.1 1.2 1.2.1 1.2.2 1.3

Electron Field Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1072 The Principles of Field Emission Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1072 Theory of Electron Field Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1074 Surface Potential Barrier and Electron Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1074 Field Emission Equation on Metal Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1076 The Accuracy and Limitations of Fowler–Nordheim Equation . . . . . . . . . . . . . . . . . . 1081

2 2.1 2.2 2.3

Spindt-Type Field Emitter Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1082 Spindt-Type Sharp Tip emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1082 Fabrication of Metal Field Emitter Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1083 Fabrication of Silicon Field Emitter Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1085

3 3.1 3.2 3.3

The Performance of Spindt-Type Emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1087 The Emission Characteristics of the Spindt Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1087 Emitter Geometric Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1088 The Emitter Material Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1090

4 4.1 4.2

Emission Uniformity and Stability Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1092 Resistance Current Limiting Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1092 The Structures of Ballast Resistance Layer in FEA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094

5 5.1 5.2

Focusing Electrode Incorporated FEA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095 The Necessity of Focusing Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095 Integration of Focusing Electrodes in FEA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1096

6 6.1 6.2

Fabrication of Faceplate in FEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1099 The Phosphor Issue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1099 Fabrication of Anode Faceplate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1100

7 7.1 7.2 7.3

Maintaining Vacuum and Packaging Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1101 FED Vacuum Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1101 The Spacer Support Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1102 Maintaining Vacuum in FED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1103

8

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1103

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.2.3, # Springer-Verlag Berlin Heidelberg 2012

1072

6.2.3

Field Emission Displays (FEDs)

Abstract: Field emission displays have long been seen as ideal image visualization devices. The combination of their emissive nature coupled with high speed and other benefits make them a candidate to compete with liquid crystal displays in a major market. The technology has been difficult to realize but the combination of modern micro-fabrication techniques coupled with field enhancement from sharp structures as a source of electrons has enabled design solutions that have delivered displays with remarkable performance. However, manufacturing such devices has proven difficult. Modern attempts include nanotechnology and materials solutions that have delivered new hope. There is a common history to many aspects of field emission and common challenges in realizing a vacuum microelectronics–based technology. This chapter examines this background, from the fundamental quantum mechanical process of electron emission from surfaces, through geometric enhancement by sharp points to the ground breaking device work of Spindt to give an insight into the field emission process. It also addresses cathode issues such as emission uniformity, through to the phosphor issues on the anode plate and the problems of spacing and vacuum sealing. List of Abbreviations: BOE, Buffered Oxide Etch; CRT, Cathode Ray Tube; FEA, Field Emission Array; FED, Field Emission Display; LCD, Liquid Crystal Display

1

Electron Field Emission

1.1

The Principles of Field Emission Displays

Field Emission Displays (FED) have long been thought of by many as the ideal visual display. Expectations were high for a new competitive technology that would deliver low cost high speed devices at high resolution. Dramatic advances have been made, but it has still not found its place in mass markets as manufacturing problems have been hard to resolve. Hopes of nanotechnology solutions keep the hope alive. FED is an emissive device and is the combination product of display and vacuum microelectronics. In essence, the field emission display can be thought of as a flat, thin, and low power cathode ray tube (CRT). The difference is that instead of just one or three ‘‘guns’’ spraying electrons against the phosphor coated screen, there are millions of micro-tip electron sources in the flat cold cathode. The basic concept of vacuum microelectronics was originated in the early 1960s of the twentieth century. The main theme in the vacuum microelectronics is to reduce the feature size of the devices to the micron scale so that it could match with semiconductor devices but retain the unique advantages of vacuum devices. Undoubtedly, this kind of vacuum microelectronic devices can only be based on micro field emission cold cathodes, so the advance of vacuum microelectronics technology is intimately correlated and co-developed with field emission studies. For a period from the 1950s to the 1970s in the last century, because of the rapid development of the semiconductor technology, the development of vacuum triode microelectronic devices lagged behind. However, striking achievement had been made in the development of micro field emission cathodes and field emission arrays. Since the initial work on fabrication of molybdenum metal tip field emission arrays by microelectronic manufacture techniques in 1968, great advances in cold emission cathode development have been achieved with increasing emission current density, improved emission stability, prolonging device life time, expanding cathode size, and reduced extraction voltage. The new applications based on

Field Emission Displays (FEDs)

6.2.3

cold emission cathodes are gradually expanding and the field emission display is one of the most representative examples. So, a field emission display is a flat panel display and it utilizes substantially the same physical principle as that used in CRT. FED operates in vacuum conditions and the light emission is produced by the electron bombardment of phosphors on a faceplate. The phosphor coated faceplate anode possesses a similar structure to that of the CRT. In the CRT, the electron beams are produced from three thermionic electron guns; after being modulated the three electron beams, which are responsible for red, green, and blue light excitations, are scanned dot by dot horizontally and line by line vertically on the faceplate forming the viewed image (> Chap. 6.2.1). Unlike CRTs, FEDs rely on electric field or voltage-induced, rather than temperatureinduced emission of electrons to excite the phosphors. The distinct feature in FED is that each image sub-pixel, i.e., the red, green, and blue phosphor dot on the faceplate, has a corresponding field emission array which is composed of thousands of micro-emitters. The emission current from each of these field emission arrays is controlled by the voltage supplied to the column cathode electrode and row gate electrode address lines, so is matrix addressed. In the field emission display, a strong electric field is formed between field emitters and gate electrode so that electrons are emitted from the field emission array to impinge on the phosphors on the faceplate, thereby emitting light and forming the visible image. In most situations, the field emission display (FED) has a basic triode structure comprising a cathode, gate and anode electrodes. The cathode plate is usually fabricated on a glass substrate on which the field emission arrays are placed with the incorporated gate electrodes. To produce light emission efficiently from the phosphor materials, the emitted electrons must attain high kinetic energy and this can be achieved by applying a high anode voltage to the phosphor coated faceplate. The field emission display is lightweight and has a thin profile, like the liquid crystal display (LCD), and the advantages of high brightness and self-luminescence, like the CRT. FEDs are generally energy efficient since they are electrostatic devices that require no heat or energy when they are off. When they operate, nearly all of the emitted electron energy is dissipated on phosphor bombardment and the creation of visible light. > Figure 1 shows a schematic diagram of the field emission display. On the anode faceplate, the red, green, and blue phosphor image sub-pixels are coated and these imaging elements are separated by the black base matrix. On the cold cathode plate, the field emitter arrays are fabricated in the intersection between column cathode electrodes and row gate electrodes. The anode faceplate and cathode plate are separated and supported by ceramic or glass spacers. The edges of the anode and cathode plates are sealed with low melting temperature glass, and suitable getter materials are put within the FED device to maintain the high vacuum conditions. The research and development of field emission displays mainly depend on four key technologies, i.e., emitter fabrication technology, vacuum packaging technology, low voltage phosphor technology, and driver integrated circuit technology. Among these four technologies, the technology for fabrication of large area field emission cathode with high reliability, uniformity, and reproducibility is recognized as the most important since the main characteristics of the cold cathodes such as the adopted emitting material and its structure determine the overall performance of FED devices. In this chapter, our attention is mainly focused on various cold emission cathodes, i.e., their working principles, main characteristics, and the corresponding fabrication techniques.

1073

Field Emission Displays (FEDs)

Black matrix

Vacuum seal

Phosphor face plate

Red

a

Blue

Green

Red

Blue

Green

Spacer

6.2.3 Spacer

1074

Field-emitter cathode array

Anode Gate electrode

Insulator layer

b

Emitters

Cathode electrode

. Fig. 1 The schematic diagram of the structure of field emission display (FED), (a) a cross-section view, (b) a perspective view

1.2

Theory of Electron Field Emission

1.2.1

Surface Potential Barrier and Electron Emission

Field emission is a quantum-mechanical phenomenon in which electrons tunnel through a potential barrier at the surface of a solid as a result of the application of a large electric field. Fowler and Nordheim calculated the relationship between this field and the emission current, based on a model triangular barrier [1, 2]. In field emission, external electric fields on the order of 107 V/cm are required for appreciable electron current. Solid materials can also emit electrons through thermionic emission and photoemission in which electrons acquire sufficient energy via heating or energy exchange with photons, respectively, to overcome the potential barrier. According to the solid physics principle, the electrons in a conductor or semiconductor comply with the Fermi–Dirac statistics, and its distribution can be described by the probability wave function. On the metal surface, the atomic periodic structure is destroyed, surface density states are formed, and the distribution of the electrons cannot be described by plane wave functions. At a certain distance away from the surface, the amplitude of the wave function is zero. When studying the electron emission phenomena, it is usual to apply the concept of a surface potential barrier to deal with the problems. When an electron escapes from the metal surface, it is subjected to an inward force and this force can be expressed by a potential. When the electron is a little away from the surface, the conductor surface can be treated as an ideal

Field Emission Displays (FEDs)

6.2.3

surface and the force subjected by the escaping electron can be described by the electrostatic image force. When the distance between the electron and surface is of the order of the distance among atoms, the acting force on the electrons comes from the action of the outermost ion array and it is very difficult to express these actions by an accurate formula. With the introduction of the concept of the surface potential barrier, the electrons both out and within the metal surface can be expressed by plane wave functions. These approximation and hypothesis have greatly simplified the field emission problem and make it possible to be dealt with theoretically. The surface potential barrier and the energy distribution of electrons near the metal surface are shown in > Fig. 2. For the surface potential barriers shown on the right side, the x coordinate is the distance from the conductor surface and the y coordinate is the potential height. Curve a represents the potential barrier without an external electric field. At a distance of 1 nm away from the surface, the potential becomes constant, meaning the electrons at this location are not subject to the electric force. On the left of the figure, the curve represents the electron energy distribution in terms of electron numbers in the metal conduction band, and EF is the Fermi energy. Within a metal or a semiconductor material, as the electron energy distribution complies with the Fermi statistics, the electron number will reduce abruptly above Fermi level. Only electrons with energy larger than the potential barrier can emit into vacuum. The difference between the Fermi energy and surface potential barrier, i.e., the height of the barrier, is defined as the escape work function, denoted by f which is defined as the energy required to remove an electron from the Fermi level of the metal to a rest position just outside the material (the vacuum level). The work function for normal metal materials is a few electric volts. In general, the active metals have a small work function, for example, the metal cesium (Cs) has a low function around 1.5 eV. At room temperature, the probability of electron distribution with an energy higher than Fermi energy is very low, so even for Cs, no electron emission can take place and be measured at room temperature. The Fermi distribution is strongly dependent on temperature. At high temperatures, a large number of electrons are energized and distributed well above the Fermi level, and these electrons

E

a j EF b

c N

0.5

1.0

x (nm)

. Fig. 2 The surface potential barriers and the energy distribution of electrons near Fermi level

1075

1076

6.2.3

Field Emission Displays (FEDs)

can surmount the barrier height, so a lot of metals can emit substantial amount of electrons when heated and this is the working principle of the thermionic emission cathode. Electron thermionic emission is an equilibrium process and emission is very stable. Photon irradiation and external electron bombardment can also lead to the electrons in the metal or semiconductor gaining enough energy to surmount the potential barrier to cause electron emission. These phenomena are called respectively photoemission and secondary electron emission. Reducing the surface potential barrier with the presence of an external electric field can also lead to electron emission and this phenomenon is known as electron field emission. When no field is present, the width of the potential barrier is infinite, and the electrons with energy lower than the potential barrier cannot escape. The external electric field can reduce the height and narrow the width of the potential barrier, and this makes it possible for the electrons with energy lower than the potential barrier emit into vacuum through the tunneling effect. Therefore, the field emission phenomena cannot be explained by the classic theory. To obtain practicable and usable electron emission densities, the electrical field is usually required to be higher than 108 V/m, and in this situation, the tunneling effect dominates the emission process, so the field emission phenomena can only be formulated by the quantum mechanics. The curves b and c in > Fig. 2 represent the distribution of the surface potential barrier in the presence of external electric fields with different strengths. When the external field increases, the potential barrier changes from infinite to finite, and the barrier height reduces. In comparison with the electron energy distribution on the left side of the > Fig. 2, for the potential barrier shown in b, a substantial amount of field emission electrons can be obtained. For the case shown in curve c, the potential barrier reduces so much that both field emission and thermal emission become very strong. In reality, this case rarely happens as the required field is difficult to achieve, and in most situations, surface breakdown will occur before the required high field can be sustained. > Figure 3 shows the electron tunneling phenomena and the energy distribution of the field emission electrons [3] when the potential barrier becomes narrower with the existence of the external electric field, where E1(x) and E2(x) represent the strength of electric field. According to the theory of free electron distribution within a metal and the Fowler–Nordheim formula (which will be derived in next section) from a metal surface, the full width at half maximum (FWHM) of the energy distribution for the field emission electrons can be obtained and is approximately 4.5 eV. The status of the electron energy distribution within the emitting material and the surface electric field can be analyzed based on the energy distribution of the emitted electrons.

1.2.2

Field Emission Equation on Metal Surface

The field emission equation for a metal surface, which relates the emission current, work function, and electric field strength to determine the field emission, was derived by Fowler and Nordheim [1, 2] based on the tunneling effect in quantum mechanics. To derive this equation, the following assumptions or hypotheses were made: 1. 2. 3. 4.

Metal is an ideal surface, atomic scale fluctuations neglected. Electrons within the metal comply with the Fermi distribution. Surface work function is uniformly distributed. The surface potential barrier is produced by the image force.

Field Emission Displays (FEDs)

6.2.3

In the following part, the emission equation is derived in reference to > Fig. 4. Also, it is supposed that the electron energy is null in vacuum. The curve a in > Fig. 4 represents the surface barrier with no external field, curve b designates the surface barrier with the presence of external field, and curve c signifies the image force barrier near the metal surface. In a certain range near the metal surface, the real surface potential barrier can be replaced by the image force barrier.

E φ

Metal

I(E)

EF

E1(x) > E2(X)

. Fig. 3 The energy distribution of the field emission electrons (Reprinted from Bonard et al. [3] with permission)

E x1

x2 a

–eExx

φ

b

E EF

c

−

1 16peox

. Fig. 4 The surface barrier with the presence of external electric field

x

1077

1078

6.2.3

Field Emission Displays (FEDs)

According to quantum theory, the following equation is valid: v¼

hk m

ð1Þ

where m is the mass of electron, h is Plank’s constant, k is the wave vector in k space, and v denotes the velocity of electron. The number of quantum states in the unit volume of dkxdkydkz in the k space is 2 dkxdkydkz:
m3 2dkx dky dkz ¼ 2 dvx dvy dvz ð2Þ h Multiplying > (2) by the Fermi distribution function, the electron number within the unit volume of dvxdvydvz is:
m3 1 dvx dvy dvz dn ¼ 2 ð3Þ 1mv 2 E F 2 h e KT þ 1 where k is the Boltzmann constant and T is the absolute temperature. The number of electrons in the velocity range from vx to vx + dvx in the x direction is: Z þ1 Z þ1
m3 1 dvx dnx ¼ 2 dvy dvz 1mðv 2 þv 2 þv 2 ÞE F x y z 2 h 1 1 KT e þ1 ð4Þ i 4pkTm2 h 1 2 ¼ ln 1 þ e EF 2mvx =KT dvx h3 In the above calculation process, the following integration formula has been used: Z Z 1 Z þ1 Z þ1 1 1 p 1 1 dxdy ¼2p rdr ¼ dx cðx 2 þy 2 Þ c 0 1 þ ae x 1 þ ae cr2 1 1 1 þ ae 0   Z p 1 1 p 1 dy ¼ ln 1 þ ¼ c a yð1 þ yÞ c a The emission current density can be expressed as: Z 1 J ¼e vx Ddnx

ð5Þ

ð6Þ

0

where D is the tunneling transmission coefficient and is obtained by solving the Schrodinger equation in considering the shape of the surface barrier, the electron wave function, and the derivative continuous conditions [4]. At normal temperature the electrons in a metal are mainly distributed near and below the Fermi energy level. The main contribution to the field emission comes from the electrons near the Fermi level and the electrons with energy much less than the Fermi energy can be neglected. From > Fig. 4 it can be seen that as long as the metal work function is not too high or in other words the Fermi energy is not too low, it is acceptable to use the image force barrier to represent the surface potential barrier. With the presence of the external field, the surface potential barrier can be expressed as: U ðxÞ ¼ 

1  eEx x 16peo x

ð7Þ

where, 1/16peox is the image force potential and Ex is the strength of electric field in x direction.

Field Emission Displays (FEDs)

6.2.3

As shown in > Fig. 4, for the electron with energy E, the tunneling transmission coefficient D is dependent on the barrier area above the energy E. The expression of the tunneling transmission coefficient D is: ( rffiffiffiffiffiffiffiffiffiffiffi Z sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi )  8p2 m x2  1 2Q  eEx x dx ¼ exp 2 D¼e E  ð8Þ  2 h 16peo x x1 The integration in can be expressed as:

> (8)

is ellipse integration, solved by Nordheim, and the solution Q ¼ Qo yðgÞ

ð9Þ

rffiffiffiffiffiffiffiffiffiffiffi Z pffiffiffiffiffiffiffi 3 8p2 m x2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4p 2mjE j2 Qo ¼ jE þ eEx xÞjdx ¼ h2 x1 3heEx

ð10Þ

where

is the Q when not considering the image force potential. yðgÞ is known as the Nordheim function, and is the correction coefficient when the image force potential is taken into consideration. pffiffiffiffiffiffiffi e eEx 2 ð11Þ yðgÞ ¼ 0:95  g ; g ¼ pffiffiffiffiffiffiffiffiffi 4pe0 jE j g represents the ratio of the difference between the potential barrier top, with the presence of external field, and vacuum level energy to the electron kinetic energy in the x direction. Substituting expressions > (4) and > (8) into > (6), we can obtain: " pffiffiffiffiffiffiffi 3 # "  # Z EF  12 mvx2 4pkeTm2 1 8p 2mjE j2 ln 1 þ exp ð12Þ vx dvx J¼ exp  h3 3heEx KT 0 Let T = 0, taking integration for the above expression, we obtain: " # pffiffiffiffiffiffiffi 3 e 3 Ex2 8p 2mjEF j2 J0 ¼ yðgÞ ð13Þ exp  8pjEF jtg2 3heEx pffiffiffiffiffi eE ffix where tg2  1:1, yðgÞ ¼ 0:95  g2 , g ¼ peffiffiffiffiffiffi 4pe0 jEF j It is a convention to express the electron current density by the unit of A/cm2, the Fermi energy in eV and electric field by V/cm. Under these conditions, the emission electron current density expression > (13) can be rewritten as: " pffiffiffiffiffi# 3  2 6:83  107 ’2 6 Ex 4 Ex J0 ¼ 1:54  10 exp  y 3:79  10 ð14Þ ftg2 Ex f pffiffiffiffi where f ¼ jEF j is the work function, g ¼ 3:79  104 fEx . Expression > (14) is the so-called Fowler–Nordheim equation and it can be simply written as: " # 3 AEx2 Bf2 yðgÞ ð15Þ J0 ¼ 2 exp  ftg Ex Both A and B are constants, with A = 1.54  106 and B = 6.83  107.

1079

1080

6.2.3

Field Emission Displays (FEDs)

. Table 1 The emission current densities from a flat metal cathode (work function 4.5 eV) E (V/cm)  107 2

J(A/cm )

1.0 4.4  10

2.0 18

5 10

2.2 4

0.01

2.4 0.12

2.6 1.06

2.8 6.76

3.0

4.0

5.0

34.0

1  10

4

3.6  105


pffiffiffiffiffi  eE ffix Substituting tg2  1:1,yðgÞ ¼ 0:95  g2 , into expression > (14), for g g ¼ peffiffiffiffiffiffi and 4pe0 jEF j values for the fundamental constants, we can obtain: ! ! 2 10:4 6:44  107 f3=2 6 Ex exp 1=2 exp ð16Þ J ¼ 1:42  10 f Ex f where, the emission current density J is in units of A/cm2, Ex is in V/cm, and f in eV. The Fowler–Nordheim equation expressed by > (16) gives the explicit relationship of the emission current density J with the external field strength Ex and the material’s work functionf. For the normal refractory metals, the work function is in the range from 4.1 to 4.6 eV. > Table 1 lists the emission current density calculated from a flat metal cathode with work function of 4.5 eV. For the sharp metal tip emitters which are frequently used in FED devices, the emission current density at the tip can arrive at 104–105 A/cm2, so the local field strength at the emission site should be in the range from 4  107 to 5  107 V/cm. Such a high electric field can only be achieved by a sharp metal tip with a radius of curvature in the range of less than 15 nm. The Fowler–Nordheim equation was derived under the T = 0 condition. In fact, as long as the metal surface work function is not very low or the external field is not too high ( (15): I J ¼ ; E ¼ bV a

ð17Þ

where I is the emission current in units of A, a is the emission area in units of cm2, V is the supplied voltage V, and b is the field enhancement or field conversion factor in units of cm1. The field enhancement factor b is dependent on the geometrical parameters of the emitters, and the distance between emitter and extraction gate electrode. Substituting > (17) into > (15), we get the concise emission current expression:   b ð18Þ I ¼ aV 2 exp  V  2 3 1:44107 B where a ¼ aAb , b ¼ 0:95Bf2 b. Taking logarithms on both sides of > (18) 1 1:1f exp f2 leads to:     I 1 ð19Þ ¼ ln a  b ln V2 V ln(I/V2) has a linear relationship with 1/V. The drawing of the experimental data sets ln(I/V2) versus 1/V is usually known as a Fowler–Nordheim plot. Experimentally, the

Field Emission Displays (FEDs)

6.2.3

Fowler–Nordheim plot is used as a criterion to judge whether or not the electrons measured are produced through the field emission mechanism. Ideally all the measured data should be on a straight line. In principle, both the surface work function f and field enhancement factor b can be calculated from the intersection lna and angle b of the F–N plot. However, in many cases, the results obtained from Fowler–Nordheim analysis based on the experiment data are often unreasonable and this issue will be discussed in the next subsection. The emission equation derived above is valid for the ideal metal surface. For the semiconductor surface, the energy distribution of the electrons within a semiconductor material is different from that within a metal, so the field emission equation for semiconductor material is different. The emission equation for an n type semiconductor at room temperature is:  w3=2 yðgÞ ð20Þ J ¼ 4:25  1013 n exp 6:78  107 Ex where g ¼ 3:79  104 ½ðe  1Þ=ðe þ 1Þ, e is the permittivity, and w is the electron affinity, which is defined as the energy difference between the conduction band bottom and the vacuum energy level. N is the electron density in conduction band.

1.3

The Accuracy and Limitations of Fowler–Nordheim Equation

The Fowler–Nordheim equation [1, 2] was obtained through analyzing the electron quantum tunneling effect based on the image force surface potential barrier. As a means to judge whether or not the electron emission is caused by the field emission mechanism, the Fowler–Nordheim equation has been widely and successfully used in vacuum microelectronics. However, when trying to apply it to perform accurate and quantitative analysis, the results are sometimes not reasonable and even absurd. This phenomenon can be attributed to many factors. There is a prerequisite to attribute the cause of the surface barrier to the image force. For a metal with a work function of 5 eV, its Fermi energy lies at 5 eV below the transverse axis in > Fig. 4. It can be calculated that at a distance of 0.07 nm away from the surface, the image force potential is –5 eV; unfortunately, the distance of 0.07 nm is less than the distance between two atoms, so obviously in this situation the hypothesis of the image force potential is not valid. For Cs, the Fermi energy lies at 1.5 eV below the transverse axis and this value is the same as the image force potential at a distance of 0.24 nm away from the surface. This distance is close to the distance between atoms, so in this case the assumption of image force potential is valid. The Fowler–Nordheim equation was derived on the basis that the metal has an ideal surface. However, there is a big difference between this assumption and the real situation. In thermionic emission, photoemission, and secondary emission, the emission current is only dependent on the height of the potential barrier and it has nothing to do with the shape of the surface potential. Work function is a macroscopic measurable quantity which has already included the influence of the surface changes to the electron emission. In field emission, the current is strongly dependent on the shape of the surface potential barrier, meanwhile, the shape of the surface potential barrier is closely correlated with the surface electric field, so it is not always possible to get the work function through the Fowler–Nordheim relation or to expect that the obtained work function is comparable with that obtained by other experimental methods. So it is clear that the Fowler–Nordheim equation cannot always be used as an accurate and quantitative field emission formula. In the research work on field emission devices, the evaluation of field emission performance should mainly be based on the experimental emission

1081

1082

6.2.3

Field Emission Displays (FEDs)

current data. Furthermore, the field enhancement factor b, obtained based on Fowler– Nordheim equation, is usually not true and it can only be used as a relative reference and comparison. As we will see later, field emission can be obtained from ‘‘planar’’ devices with no geometrical field confining enhancement from sharp points and the emission current must be described in a different way. However, even in these cases the Fowler–Nordheim relation can be a useful measure.

2

Spindt-Type Field Emitter Array

2.1

Spindt-Type Sharp Tip emitters

According to F–N equation, there are two approaches to achieve a large field emission current, i.e., reducing the work function of the emitters and increasing the surface electric field. For normal materials, low work function means active chemical properties and easy oxidation. For example, the metal Cs that is used in photoemission cathodes and n-type semiconductor oxides, with work functions of 1.5 and 0.5 eV respectively, are so sensitive to the environment that they can only be processed under vacuum conditions and this limits their application in field emission devices. For normal metal emitters, to achieve appreciable electron field emission current, the surface electric field needs to be as high as 109 V/cm, and such a high field strength can be attained by using the sharp tip effect. Since the discovery of the field emission phenomena, refractory metals have mainly been used as the cold cathode emission materials. In the earliest experiments, sharp metal tips with a radius of curvature less than 10 nm were fabricated by chemically etching tungsten filaments. Today, this kind of tungsten sharp tip is still being used as the electron emission gun in field emission electron microscopes. To apply the field emission cathode in vacuum microelectronics, two key challenging issues should been addressed and solved, i.e., > (1) instead of single metal tip, a high density of tip arrays should be constructed to achieve uniform and high emission current, > (2) extracting electrode or gate electrode, which is close enough to the metal tips, should be constructed so that a low extraction voltage is required to initiate the field emission. In 1968, Charles A. Spindt and his co-workers at SRI International fabricated gate electrode controlled molybdenum (Mo) metal tip field emission arrays (FEAs) by microlithography and thin film deposition technology [5, 6]. This achievement has set down the foundation for the modern vacuum microelectronics and particularly for the first generation of novel flat panel displays, i.e., the field emission displays (FEDs) [5, 6]. The field emission cold cathode developed by Spindt is usually called the Spindt-type field emission array in which the field emitters are periodically arranged on the surface of a cathode plate and the individual field emitters are micro-sized sharp molybdenum metal cones located inside a cylindrical void formed in an insulator layer as shown in > Fig. 5. The basic structure of the Spindt emitters (see > Fig. 5a) includes substrate, cathode electrode, ballast resistance layer, emission metal tip, gate electrode with circular aperture, and the dielectric layer which is sandwiched between the cathode and gate electrodes. The substrate can be glass or silicon wafer, with the cathode and gate electrodes usually made of metal films, and the dielectric layer materials are usually thick SiO2 or SiN films. The Spindt emitters can be fabricated with a very small radius of curvature at the tips, with the diameter of the gate aperture only about 1 mm, so a very high electric field can be formed at the tip surface at a relatively low voltage. When the gate voltage is a few tens of electric volts, the electric field achieved at the tip can be as high as 109 V/m.

Field Emission Displays (FEDs)

6.2.3

Gate electrode Insulator layer Metal tip Resistive layer Cathode Substrate

a

Light

Front glass panel Phosphor

Anode

Electrons

Cone (‘Splindt tip’) Gate insulator Insulator Cathode Rear glass substrate

b

Narrow gate

. Fig. 5 (a) Schematic diagram of Spindt Tip structure, (b) triode structured FED device

Such a high field can substantially reduce the height and narrow the width of the surface barrier and the free electron in the metal can emit into the vacuum through the tunneling effect at relatively low gate voltages (less than 100 V). > Figure 5b shows a schematic diagram of the FED devices based on the Spindt-type emitter.

2.2

Fabrication of Metal Field Emitter Arrays

Depending on the adopted emission materials, i.e., metal or silicon, there are two corresponding processes to fabricate the Spindt-type field emission arrays. For molybdenum metal tip emission array, the gate electrode and the well beneath the gate is first constructed and then the metal tip is formed in the well by thin film deposition process [5]. > Figure 6 shows a typical process flow for fabrication of the Spindt-type metal tip emitter arrays [7]. The process begins with deposition and patterning of the cathode address lines and an amorphous silicon resistance layer on the glass substrate, and then the SiO2 insulator and top gate metal layers are sequentially deposited on top as shown in > Fig. 6a. The thickness for each material layer is correlated with the structure of the metal tip emitters. To fabricate the metal tips with a height of around 1 mm, the typical thickness for each layer from bottom to top is respectively 100 nm, 200 nm, 1 mm, and 100 nm. The top gate metallic layer is patterned using photolithography and etching techniques to form the circular gate apertures as illustrated in > Fig. 6b. Etching the insulator through the gate aperture down to the resistance layer creates a cavity in the insulator layer and this cavity undercuts the gate electrode and uncovers the resistive layer as illustrated in > Fig. 6c. Next, a sacrificial layer, typically aluminum, is deposited using electron-beam evaporation in vacuum at grazing

1083

1084

6.2.3

Field Emission Displays (FEDs)

Gate metal SiO2 Cathode metal a-Si glass substrate

a

b

Sacrificial layer

c

d

Tip metal deposition

emitter cone

e

f

. Fig. 6 A typical process for the fabrication of Spindt-type metal tip emitter array. (a) Deposition of multilayer structured materials on glass substrate. (b) Patterning gate apertures by photolithography. (c) Formation of emitter well by etching. (d) Deposition of sacrificial liftoff layer, the rotation of substrate is indicated by arrow. (e) Tip deposition by electron beam evaporation. (f) Removal of sacrificial and closure layers (Reprinted from Talin et al. [7] with permission)

incidence with respect to the gate electrode plane with the substrate rotating, and this leads to a partial closure of the gate apertures as shown in > Fig. 6d. Molybdenum is then deposited, again by e-beam evaporation, but this time at normal incidence with respect to the gate electrode plane. During this processing step, the size of the gate hole continues to decrease because of the deposition of the material on the gate aperture periphery. A cone with a sharp point grows inside the well as the aperture above it closes, resulting in the structure shown in > Fig. 6e. The sacrificial closure layer is finally removed in a suitable selective etchant and the molybdenum metal tips are exposed within the cavity under the gate holes (see > Fig. 6f ). In the fabrication process described above, the difficult steps are patterning the gate electrodes with a high density of submicron sized holes and the deposition of metal tips through the gate apertures. For large area field emission array fabrication, accurate photolithography equipment and techniques are required to guarantee uniformity and a consistent

Field Emission Displays (FEDs)

a

2 μm

b

6.2.3

1 μm

. Fig. 7 The SEM images of the fabricated Mo metal tip emitters, (a) a 3  3 array of emitters, (b) crosssection of a single emitter cone (Reprinted from Talin et al. [7] with permission)

gate size. When the metal tips are deposited, the evaporation beam must be exactly perpendicular to the substrate on the whole surface area. Investigation shows that when the angle between the beam direction and the substrate normal changes 0.8 for a typical evaporator, the resultant change in the tip radius of curvature is about 2 nm over 6 cm distance along the substrate. This tip radius variation resulted in a 75% decrease in emission current for tips at the edge of the substrate compared to those at the center. Larger volume deposition equipment with a collimated evaporation beam is needed when the size of the emission array is increased, otherwise, sharp and uniform tip radius of curvature cannot be retained which will result in non-uniform field emission. Fortunately, today, by using state-of-the-art photolithography and evaporation facilities and techniques, the individual emitters can be packed close together. The available minimum gate aperture diameter is about 200 nm and the density of one hole in one square micron can be achieved on the large emission array with dimensions up to 19 in. The average macroscopic current density that can be obtained from a Spindt array can be as high as 2  103 A/cm2, which is sufficient for FED applications. This complexity, however, exemplifies the manufacturing challenges faced by FED fabricators. > Figure 7 shows scanning electron micrographs taken from a real molybdenum metal tip emission array [7]. The gate aperture, metal tips, insulator layer, bottom cathode electrodes, and their relative positions are clearly observed from these images.

2.3

Fabrication of Silicon Field Emitter Array

To fabricate silicon tip emission arrays, the silicon tips are first formed by a plasma or chemical etching process and then the gate electrodes are deposited. > Figure 8 shows a typical fabrication process flow for the silicon field emitter array [8]. The process starts with forming n-type conductive cathode electrode strips on the semiinsulating silicon substrate by ion implantation or thermal diffusion techniques, then forming a silicon oxide layer with a thickness around 0.5 mm by thermal oxidation processes. The silicon oxide layer is patterned by photolithography to form circular caps which serve as masks for the

1085

1086

6.2.3

Field Emission Displays (FEDs)

Masking oxide SiO2

n-type substrate

d

a

Mo gate Si post

b

e Mo gate

Si tip

SiO2

c

f

. Fig. 8 The schematic diagram for the fabrication of Spindt-type silicon tip emitter array (Reprinted from Lee et al. [8] with permission)

subsequent etching of silicon to form the tips. The specified diameter of the circular caps is correlated with the height of the silicon tips and the size of the gate hole which are to be fabricated subsequently. In the example shown in > Fig. 8a, the cap size is 1 mm in diameter. The silicon substrate is then etched to the desired degree to form silicon posts under the caps either by reactive ion etching in gaseous mixtures containing SF6 and O2 or chemical etching in solutions containing HNO3, HF, CH3COOH, and H2O (> Fig. 8b). After the silicon posts are formed, the wafers are oxidized to sharpen the tips with the SiO2 masks still in place as shown in > Fig. 8c. If ungated silicon tip arrays are desired, the thermal oxide formed during the oxidation sharpening is etched away in the buffered oxide etch (BOE). During this process the oxide caps are removed as well, resulting in a structure whose SEM image is shown in > Fig. 9a. If gated FEAs are to be fabricated, the BOE step is not performed, the caps remain in place, and a layer of SiO2 is deposited by e-beam evaporation in vacuum. This layer, together with the underlying thermal oxide grown during oxidation sharpening, forms the gate-to-cathode insulator. The next step is to deposit the gate metal layer and patterning the gate electrode addressing lines. During the line-of-sight SiO2 deposition, the oxide masks shadow the silicon posts, so the insulator deposits only between the posts (> Fig. 8d). The resultant profile of the evaporated SiO2 allows for the formation of the gate electrode apertures by directional e-beam evaporation (> Fig. 8e). After the gate metal deposition and patterning, the thermal oxide formed during the oxidation sharpening together with the oxide caps are all etched away in the buffered oxide etch (BOE) and a hemispherical cavity in the insulator is created (> Fig. 8f ). Device fabrication is completed with the addition of a metal electrode to provide electrical contact with the rear side of the wafer.

Field Emission Displays (FEDs)

6.2.3

. Fig. 9 The SEM images of Si tip emitter arrays, (a) Si tip array, (b) Spindt-type Si tip field emission array with gate electrodes (Reprinted from Chen [9] with permission)

> Figure 9 shows the SEM images of the fabricated Si tip array and an Si tip emitter array with

gate electrodes [9] which were fabricated basically following the steps sketched in > Fig. 6. In comparison with metal tip emitter arrays, the fabrication process for silicon tip emitter arrays is relatively easy to perform. There are many variations of the process flow for silicon tip emitter array fabrication. For example, instead of using silicon wafer, amorphous silicon and polycrystalline silicon layers deposited on metal coated glass substrate have been processed to fabricate amorphous and polycrystalline silicon tip emitter arrays.

3

The Performance of Spindt-Type Emitters

3.1

The Emission Characteristics of the Spindt Tips

In comparison with thermionic electron emission, electron field emission in practice is relatively poor in both uniformity and stability. For a given material for a thermionic emission cathode, as long as the temperature is high enough, the emission current density has nothing to do with the emitter itself. The thermionic emission current is limited by space charge and its magnitude is controlled by the anode or gate voltage, while the electric filed on the cathode surface is zero or negative. In electron field emission, to produce efficient electron emission the electric field on the emitter’s surface is very strong, and it is not possible to achieve complete space charge limiting. The emission current is not only controlled by the anode or gate voltage but is also dependent on the geometrical parameters of the emitter itself. Experimental analysis shows that when a Spindt-type field emission array is in operation, in average, less than 10% of the sharp tips on the array really emit electrons and the majority of the tips do not emit. Meanwhile constant changes occur on the sharp emitting tips during the operation. In reality, the field emission current is always a statistical average value, no matter if it is a single emitter or an emitter array. Obviously, the emission current fluctuation will become small when the number of tips in operation is increased substantially. For traditional tungsten emitters such as those used in the electron microscope, stable field emission can be obtained

1087

6.2.3

Field Emission Displays (FEDs)

Current–voltage plot

−12

8

−13

6

Fowler–Nordheim fit

LN (IA / VGT2)

−14

4

−15 −16 −17

2

−18 0

Fowler–Nordheim plot Measured data

Anode current Gate current

Current (mA)

1088

0

10 20 30 40 50 60 70 Gate-tip voltage (V)

−19 16

A = 2.74 × 10−3 A/V2 B = 460 V

18 20 22 24 26 1,000 / gate-tip voltage (V)

28

. Fig. 10 (a) Current-voltage characteristics for molybdenum FEA fabricated using the interferometric lithography; (b) the data plotted in the Fowler–Nordheim coordinates (Reprinted from [10] with permission)

from a single filament. The main reason is that the surface field is very high and the emission area is relatively much larger which effectively reduces the emission current fluctuations. > Figure 10 shows the emission characteristics for a typical Spindt-type metal tip array with a total of 70,000 tips. > Figure 10a shows the representative I-V curve and it is clear that there is a threshold for the gate voltage and the emission current will increase rapidly when the gate voltage is higher than the threshold value. In the case shown in > Fig. 10a, the gate voltage threshold is around 40 V. > Figure 10b shows the data plotted in the Fowler–Nordheim coordinates. All of the experiment data are located on a straight line indicating the measured current was produced by field emission. The main factors which affect the emission current such as the electric field, work function, and the emitter’s geometrical parameters (through the field enhancement factor b) are reflected in the Fowler–Nordheim equation. However, other factors, such as vacuum condition, field-induced local heating, surface contamination, etc., are not included and when these factors become significant, the deviation from the Fowler–Nordheim relationship will occur and this is frequently manifested in the F–N plot with the experiment data points deviating away from the straight line, especially in the low or high extraction voltage range.

3.2

Emitter Geometric Effect

The significant influence of geometrical parameters of field emitters on the field emission performance is manifested by the strong relationship of the current density J with the field

Field Emission Displays (FEDs)

Tip radius of curvature r

6.2.3

Gate opening diameter d

Gate metal x

Insulator

–x

h

Cathode metal substrate

. Fig. 11 The geometrical structure of the Spindt-type emitter. The emitter height is denoted by h, the gate aperture diameter by d, and the tip radius of curvature by r. x is the vertical distance from the tip end to the gate metal plane

enhancement factor b in the F–N equation. To appreciate this point clearly, the explicit form of F–N expression > (16) is rewritten as: ! ! 10:4 V2 6:44  107  f3=2 2 6 exp  : ð21Þ J ðV ; f; bÞ ¼ 1:42  10  b  exp 1=2  f V b f The values of the field enhancement factor b depend on geometrical parameters of the device structure, such as the gate opening diameter d, tip radius of curvature r, emitter height h, and the vertical distance x of the tip end with respect to the gate metal plane. These geometrical parameters are illustrated in > Fig. 11. > Figure 12 shows a plot of emission current J, calculated using expression > (21), as a function of gate voltage V for the fixed value of work function f = 4 eVand field enhancement factor b varying from 3  105 to 6  105 cm1. The plots show that increasing b causes significant increase in the emission current density for a given gate voltage. For example, increasing b by 25% from 4  105 to 5 105 cm1 at the gate voltage of 100 V results in an order of magnitude increase of the emission current density, namely, from about 2  105 to 4  106 A/cm2. To analyze the effect of device geometry on device performance, electric field strengths at the emitter tip can be calculated as function of these parameters, using finite-element field calculation methods. It has been shown that the tip radius of curvature and the gate aperture diameter influence b most significantly, i.e., b increases rapidly as r and d decrease. In practice, there is a limitation for the minimum d which is dependent on the available resolution of the photolithography techniques. Particularly, the small tip radius of curvature r is more difficult to control in fabrication process. The multi-photolithography and multi-evaporation processes can be adopted to retain a very small tip radius of curvature for the emitter array. However, these fabrication processes are extremely complicated and time-consuming and, it is only used for special cases and is rarely used in the normal fabrication process.

1089

6.2.3

Field Emission Displays (FEDs)

1 × 107 Emission current density (A/cm2)

1090

9 × 106 8×

Φ = 4 eV

106 b = 6 × 105 cm−1

7 × 106 6 × 106

b = 5 × 105 cm−1

5 × 106 b = 4 × 105 cm−1

4 × 106 3 × 106 2×

b = 3 × 105 cm−1

106

1 × 106 0 0

20

40

60 80 100 Gate voltage (V)

120

140

160

. Fig. 12 Plots of emission current density as a function of gate voltage for the fixed value of f = 4 eV and varying values of b (Reprinted from Temple [10] with permission)

To improve the emission performance, the diameter of the gate holes should be as small as possible. Furthermore, the relative position of the tip to the gate hole should be optimized and it is beneficial to protrude the tip above the gate aperture plane in the range 100–200 nm. When both the gate voltage and distance between cathode and gate electrodes are specified, the surface field is determined by the emitter’s geometrical shape. Any subtle difference will affect the magnitude and distribution of the electric field. No matter what kind of fabrication process is adopted, the field emission performance for a single tip is difficult to control and the reason is that the field emission occurs at the location where atomic scale protrusion occurs. The highest electric field is usually located at the tip end, but this does not guarantee the field emission definitely occurs at the tip end, there are also atomic scale protrusions beyond the tip end and effective emission can occur there. These atomic scale protrusions are uncontrollable in the fabrication process, and can form and disappear during operation because of the incidental electric breakdown, field emission thermal effects, iron bombardment, etc.

3.3

The Emitter Material Effect

From the F–N equation, it can be seen that the influence of the material work function on the emission current is significant. Choosing an emitter material with a low work function is an effective approach to increasing the emission current density and reducing the gate extraction voltage. > Figure 13 shows a plot of emission current density J versus gate voltage V for a fixed field enhancement factor b (b = 4  105 cm1) with work function f varying from 2 to 5 eV. A stronger effect is observed for decreasing the work function value. When the work function value is reduced, the whole J–V plot shifts to the left and that means the required extraction voltage for obtaining the same emission current is reduced significantly. For a fixed electric

6.2.3

Field Emission Displays (FEDs)

field, for example, for a gate voltage of 100 V, when the work function decreases from 4 eV to 3 eV, the emission current density will increase from 2  105 to over 1  108 A/cm2, i.e., a three orders of magnitude increase in the emission current density. > Table 2 gives some quantitative results to show the relationship of the emission current density and work function when the electric field strength is kept at 4  107 V/cm. The current density will increase nearly 3.2 times when the work function reduces in step of 0.2 eV each time. Because of the different measurement method and surface conditions, there are certain differences for the work functions reported in the literature. Although the differences are usually no more than 0.2 eV, its influence on the emission current calculation is substantial.

Emission current density (A/cm2)

1 × 107 β = 4×105 cm−1

9 × 106 8 × 106 7 × 106 6 × 106 5 × 106

φ = 2eV

4 × 106

φ = 3eV

3 × 106

φ = 4eV

2 × 106 1 × 106

φ = 5eV

0 0

20

40

60 80 100 Gate voltage (V)

120

140

160

. Fig. 13 Plots of emission current density as a function of gate voltage for the fixed value of b = 4  105 cm1 and varying values of f (Reprinted from Temple [10] with permission)

. Table 2 The relationship of emission current density with the work function Work function (eV) 2

Current density (A/cm )

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

0.0052

0.017

0.057

0.168

0.06

1.93

6.08

19.0

58.3

. Table 3 Work functions and evaporation temperatures for commonly used field emission materials Materials

Mo

Ta

W

Si

Evaporation temperatures

2,090

2,507

2,667

1,204

Work functions

4.4

4.2

4.5

4.15

LaB6

HfN

TiN

ZrN

C(SP3)

2.7

3.7

3.1

2.8

NEA

2,200

1091

1092

6.2.3

Field Emission Displays (FEDs)

Metal surfaces easily absorb gas molecules which can lead to a 20–30% increase in work function. When choosing the field emission materials, in addition to considering the work function factor, the physical and chemical stability, the feasibility and degree of processing, cost are also important factors to be considered. In > Table 3, the work function and evaporation temperatures for several commonly used emit materials are listed for reference. A FED can be subjected to temperatures as high as 450  C during the sealing process. Because the sealing process is usually performed in a protective environment, there is no special need for the materials to resist the high temperature oxidation. However, the selected materials should be resistive to oxidization at normal temperatures. Meanwhile, the materials should be resistant to evaporation to prevent material evaporation in the sealing process. The evaporation of emitter materials will reduce the voltage bearing ability between the electrodes. The refractory metals such as Mo can satisfy nearly all of the above mentioned requirements. The main weakness of Mo is its high work function. To deposit low work function thin film materials on the Spindt tip emitters is a very effective means to reduce the work function. According to the literature, the extraction gate voltage can be substantially reduced by coating the tips with Cs metal. Unfortunately, this coating process can only be performed under high vacuum conditions and it is difficult to use in practical device fabrication. Alkaline metals usually also have a low work function, but these kinds of metals have a very low evaporation temperature and could not withstand the sealing process. The metal oxides formed from the IV metal elements such as titanium oxide, zirconium oxide, chromium nitride, etc., possess excellent electric conductivity, high evaporation temperature, and also resistance to the oxidation, and make suitable emitter materials. These oxide materials can be deposited on top of Mo or Si tips to reduce the emitter’s work function.

4

Emission Uniformity and Stability Issues

4.1

Resistance Current Limiting Principle

In comparison to thermionic emission, the main disadvantage in the field emission cold cathode is the fluctuations in emission current. For a thermionic emission cathode, as long as the temperature is high enough, the emission current is totally controlled by anode and gate voltages and has nothing to do with the emitter itself, so thermionic emission is highly stable. Field emission current is determined by the surface electric field and the cathode surface status. As was discussed in > Sect. 3, the emission current is a strong function of the field enhancement factor b which in turn is strongly dependent on the geometrical parameters of the emitters, primarily the tip radius and the diameter of the gate aperture. Even a small variation in the tip radius between individual tips in the field emission array may cause orders of magnitude variations in the emission current. During the device operation, the incidental changes of the emitter surface status are always inevitable because of the strong field modification, ion bombardment, localized heating, and gas molecule adsorption, and all of these detrimental factors can cause the emission current to fluctuate. A field emission cold cathode array could not work normally and efficiently unless certain feedback control functions are incorporated to suppress or eliminate the current fluctuations. This kind of feedback control cannot be accomplished by relying on external circuitry but should depend on the internal auto-feedback mechanism. One of the simple approaches is to

Field Emission Displays (FEDs)

Resistive layer

6.2.3 Gate

Cathodic contact Baseplate

. Fig. 14 Schematic diagram illustrating the placement of the resistive layer in the field emission array (Reprinted from Temple [10] with permission)

add a resistance in series with the field emitters as illustrated in > Fig. 14 [3, 11]. This added resistance layer is usually called a ballast resistance and it has two basic functions: ● To limit the emission current. When the emission current from certain emitter is too large, the voltage drop across the resistance will increase so the effective tip-gate voltage will decrease and this will lead to a drop in the emission current. The voltage drop across the resistive layer acts as a negative feedback to limit the emitted current. ● If a tip is short circuited with the gate, the ballast resistance layer beneath the tip bears the voltage drop, and in this situation all other tip emitters can still work normally. Since the number of tips in an array is huge, there is no discernible influence when a few tips lose functions. However, if there is no series resistance layer, the whole emitter array will disable if a tip short circuits with the gate. The effect of the ballast resistance layer in the field emission array (FEA) is demonstrated in 15 where emission currents versus gate voltage are plotted for three arbitrary tips in an arbitrary FEA. When a field emission array without ballast resistance is in operation, there are three possible working scenarios:

> Fig.

● The majority of the tip emitters work normally, but the emission ability for each emitter is different. This is true because no emitters can have the exactly the same tip radius, so the arbitrarily selected I-V curves do not track each other, as shown in > Fig. 15. ● The minority of tips emit abnormally because of their large field enhancement. At a given gate voltage Vg emission currents from the three tips might therefore be significantly different as signified by the hollow dots, i.e., the intercept points of the I-V curves with the vertical line at Vg. ● The field emitter array fails to work because certain emitters are in short circuit with the gate electrode. However, the working status of the emitters on the FEA will change favorably when the ballast resistance layer is introduced in series with the emitters. With the ballast, the resistance bears part of the gate voltage drop as depicted by Vresis in > Fig. 15. The larger the

1093

1094

6.2.3

Field Emission Displays (FEDs)

I

Abnormal emission

Short circuit

Normal emission

Vtip

Vresis

Vg

. Fig. 15 Illustration of the effect of series resistance layer on the emission current uniformity across an FEA (Reprinted from Temple [10] with permission)

emission current, the larger the voltage drop across the resistance, the less the effective voltage supplied to the tip (Vtip). Under this situation the emission currents from the three tips are determined by intersection of the current– voltage curves and the load line shown in > Fig. 15, and, as can be seen, the difference in the values of the currents can be considerably smaller. With the presence of a series ballast resistance layer, the whole emitter array will still work normally even if a few emitters short circuit with the gate electrode because the resistance layer bears the voltage drop and the tip-gate voltage for other emitters will be kept unchanged. It should be pointed out that for a normal field emitter array, even if a ballast resistance is present, emission differences still exist among the emitters. However, the number of tip emitters corresponding to each image pixel is huge and the larger number of tips provides a constant and uniform statistical average by smoothing out emission fluctuations. This ensures uniform emission from each field emission array under the same working conditions.

4.2

The Structures of Ballast Resistance Layer in FEA

In most situations, films of hydrogenated amorphous silicon (a-Si:H) are used as the resistance layer material because it can be deposited in large areas and its resistivity can be controlled by the deposition conditions. There are several possible structures for the ballast resistance layer in FEA as demonstrated in > Fig. 16. The simplest approach is to use a blanket resistive layer deposited directly on top of the base electrode as shown in > Fig. 16a. In this structure the thickness of the resistance layer cannot be made too thick and is usually around 500 nm so the breakdown resistance of the layer is limited. In the structure shown in > Fig. 16b, both the breakdown bearing ability and current limiting ability of the integrated resistance can be greatly improved. The shortcoming of this structure is that the emitters located in the central

Field Emission Displays (FEDs)

6.2.3

a

b

c

d . Fig. 16 Schematic diagrams of the structures of the ballast resistance in field emission array (Left column). The right column is the corresponding equivalence circuit diagrams

region have a larger resistance so the gate-tip voltage is relatively lower and emission is not uniform over the whole array area. This weakness can be overcome by the structure shown in > Fig. 16c. In the structure shown in > Fig. 16b, the resistance value can be controlled and adjusted in a wide range with all the emitters having the same gate voltage. The disadvantage of this structure is that the array will fail to work if any one of the emitters short with the gate electrode. One of the approaches to solve this problem is to fabricate many such small emitting units in an array corresponding to single image pixel so that even loss of a few units will not affect the pixel light intensity. The structure shown in > Fig. 16d is an ideal configuration. In this structure, the same gate voltage can be supplied to each emitter; meanwhile, each emitter possesses a same vertical series resistance and a same lateral resistance, the resistance value of which can be made very large. The ballast resistance constructed in this way has excellent current limiting and breakdown bearing abilities to facilitate uniform and stable electron field emission from the field emission array cold cathode.

5

Focusing Electrode Incorporated FEA

5.1

The Necessity of Focusing Electrodes

In a normal triode structured FED, the lateral velocity of the emitted electrons from a field emission array is very large and it is nearly the same as the vertical velocity toward anode electrode. Simulations of trajectories of electrons emitted from a single gated field emitter indicate that the half angle of the spread of the electron beam is in the range of 20–30 depending on the operating gate voltage. The large beam divergence results in large beam spot on the faceplate and this reduces the image resolution and degrades the color purity.

1095

1096

6.2.3

Field Emission Displays (FEDs)

To achieve the required resolution, the anode-cathode gap distance should be reduced to a few hundred microns. In such a small gap, the anode voltage cannot be too high and is usually less than 2,000 V to prevent electric breakdown. Under these conditions, only low voltage phosphor coated faceplate can be used. However, the performance of the low voltage phosphor is far less than that of the ordinarily used high voltage phosphors. So the performance of low voltage operated FED is usually not ideal. When the anode voltage is above 5,000 V, the conventional high voltage phosphors can be used and these phosphors can give high luminance and an ideal color gamut. To avoid fieldinduced damage, the separation distance between anode and cathode should be increased to above 2 mm. In such a large distance, the image resolution will deteriorate more seriously and the beam divergence issue must be dealt with. Introducing an electron beam focusing function between the cathode and anode electrodes is a feasible remedy to allow FED to be operated under high voltage conditions [12]. This basic idea has been realized by incorporation of additional beam focusing electrodes in the gated field emission arrays. Two kinds of focusing electrode arrays have been developed as shown in > Fig. 17. > Figure 17a shows a vertical co-axis focusing structure where the lens electrode is placed above the gate electrode and is separated from the gate by the second insulator layer. > Figure 17b shows a co-planar focusing structure where the lens electrode is fabricated on the same plane with the gate electrode. In this section, only the more popular vertical co-axis focusing structure will be discussed.

5.2

Integration of Focusing Electrodes in FEA

The fabrication processes for integration of focusing electrodes in a field emission array are different depending on whether a metal or silicon tips are used. > Figure 18 shows a schematic diagram of the process flow used for fabrication of silicon FEAs with the lens electrode placed above the gate electrode [5]. The first part of the process is similar to the process shown in > Fig. 8. A thin SiO2 layer is thermally grown on an n-type Si substrate, and caps are formed by conventional lithography and wet etching techniques (> Fig. 18a). These caps serve as etch masks to create silicon tips via RIE, and the tips are then sharpened by silicon oxidation (> Fig. 18b). Layers of SiO2 and metal Nb are deposited sequentially by evaporation to form the insulator and the lower gate electrode, respectively (> Fig. 18c). This is followed by patterning of the photoresist layer to form a lift-off structure for access to the lower gate (> Fig. 18d).

Focusing electrode Focusing electrode Gate electrode

a

Gate electrode

b

. Fig. 17 Focusing electrode incorporated field emitters. (a) Vertical co-axis focusing structure, (b) co-planar focusing structure (Adapted from Kesling and Hunt [13])

Field Emission Displays (FEDs)

6.2.3

SiO2 and niobium layers are then again deposited to form the second insulating layer and the upper focusing electrode, respectively (> Fig. 18e). Finally, the photoresist pattern and the SiO2 caps are lifted-off by ultrasonic agitation in a solvent and buffered hydrofluoric acid, respectively (> Fig. 18f ). > Figure 19 shows SEM images of a completed FEA, where the diameters of the lower and upper gate openings are 2 and 3 mm, respectively.

PR

Photo resist (PR) SiO2 disk formation

Resist pad formation

Si substrate

a

d

Nb SiOx and upper gate deposition Si etching and thermal oxidation

b

e Upper gate Nb

SiOx

SiOx and lower gate deposition

Emitter

Lift off Lower gate

SiOx

c

f

. Fig. 18 Process flow for the integration of a focusing electrode into a gated Si FEA (Reprinted from Itoh et al. [14] with permission)

Upper Gate Upper Gate SiOx Lower Gate Lower Gate

Si SiO2

Tip

a

1 μm

SiOx

b

Si Substrate

. Fig. 19 SEM micrographs of gated FEAs with an integrated beam focusing electrode: (a) perspective view; (b) cross-section view (Reprinted from Kesling and Hunt [13] with permission)

1097

1098

6.2.3

Field Emission Displays (FEDs)

When metal tip emitters are used, the incorporation of a focusing electrode in the array is straightforward, i.e., sequentially deposit bottom cathode electrode, resistance layer, insulator, gate, second insulator layer, and the top most electrode layer. Aperture photolithography and etching follows, the etching stopping at the ballast resistance layer. Finally the metal tips are formed in the cavity. > Figure 20 shows the fabrication process. The above method is straightforward, but the size of the extraction gate hole is difficult to control. The main reason is that the gate electrode can be reached only when etching arrives at a certain depth and the diameter of the gate aperture is influenced by the local etching environment. In addition to the nonuniform etching problem, the size ratio of the focusing electrode aperture to gate electrode aperture also cannot be controlled in this process. One effective approach to overcome these problems is to pattern the gate electrode first, then use the patterned gate aperture as a mask to pattern and etching the top focusing electrode. The main fabrication processes are sketched in > Fig. 21. As in the routine Spindt tip fabrication process, the gate electrode is first patterned as shown in > Fig. 21a. A second insulator layer and top focusing electrode are sequentially deposited on top of the patterned gate electrode and then backside UV light exposure is performed to define the focusing electrode aperture by using the gate hole as the photomask (see > Fig. 21b). After exposure, the top focusing electrode is patterned and the size of the aperture can be controlled by changing the exposure and etching times. Finally the cavity with both gate and focusing electrodes can be formed by further etching from top to bottom

Focusing electrode layer

a

c

Cathode

b

Gate

Insulator

Resistance

d

. Fig. 20 Schematic diagram of the fabrication process for incorporation of focusing electrode in the metal tip emission array. (a) Sequentially deposit cathode, resistance, insulator SiO2, gate, insulator, and top focusing electrode layers from bottom to top, (b) pattern focusing electrode aperture first and then etching from top to bottom and stop at the resistance layer, (c) deposition of metal tip in the cavity, (d) the final emitter structure with both gate and focusing electrodes

Field Emission Displays (FEDs)

6.2.3

Focusing electrode Gate electrode

c

a Photoresist

b

Cathode

d

. Fig. 21 The schematic diagram of the fabrication process for incorporation of focusing electrode in the metal tip emission array. (a) Pattern and define gate aperture, (b) back side UV exposure to define the focusing electrode aperture, (c) pattern the top focusing electrode, (d) etching to form the cavity with both gate and focusing electrode incorporated

resistance layer as illustrated in > Fig. 21d. The metal tip can be deposited within the cavity by the same process as depicted in > Fig. 20c and d. To use the backside UV light exposure approach, the bottom cathode layer, resistance layer, two insulator layers, and the top focusing electrode are all required to be transparent to UV light. In practice, the transparent and conductive ITO is selected as the cathode and focusing electrode material, SiO2 is used as the insulator and the specially prepared composite material, which consists of SiO2 and nano-silver (Ag) particles, is used as the resistance layer material. The beneficial functions of the focusing electrode have been confirmed by the improved image quality when a suitable voltage is supplied to the focusing electrodes. Now, nearly all of stateof-the-art FEDs operating at a high voltage adopts an electron beam focusing approach.

6

Fabrication of Faceplate in FEDs

6.1

The Phosphor Issue

An important parameter in the construction of the FED is the voltage applied to the anode. There are two main FED system architectures: high voltage (>5 kV) and low voltage ( Figure 22 shows the typical structure for the commonly used phosphor faceplate. For high voltage FED, the faceplate is made from bare glass substrate without the ITO coating. The glass substrate then is coated with the black base matrix layer and red, green, and blue phosphors on the specified location, then the surface flattened by coating with a layer of organic lacquer material. Finally a thin layer of aluminum is evaporated on top. There are three

Field Emission Displays (FEDs)

RED

(a)

6.2.3

GREEN

BLUE

615 μm

615 μm

b

205 μm

a c . Fig. 22 The basic structure of phosphor coated faceplate used in FED devices. (a) The overall view shows the phosphor image pixels on black matrix base layer, (b) a single image pixel composed of red, green, and blue sub-pixels, (c) cross-section view of the single image pixel

functions for this aluminum layer; it acts as the anode electrode because both the phosphor materials and glass are insulators; acts as the protection layer for the phosphors, as electron bombardment damage and field induced fall off can be avoided; acts as light reflective layer to increase device brightness. The base black layer and phosphor matrix can be coated by a variety of mature techniques such as screen printing, ink jet printing, and electrophoresis.

7

Maintaining Vacuum and Packaging Issues

7.1

FED Vacuum Packaging

To form a flat panel display device, the field emission cathode plate must be combined with the phosphor coated anode faceplate forming a diode or triode structured field emission display. This final fabrication process is known as vacuum packaging. Usually, a standard panel packaging process includes a panel sealing process and a gas exhausting process as shown in > Fig. 23. In the sealing process, shown in > Fig. 23a, a low melting temperature glass frit is placed on the edge on the cathode plate to form the frit frame. When heated to 300–400 C, the frit glass melts, forming a sealed interior region between the cathode and anode faceplate. In the sealing process a carrier gas flow, such as a mixture of Ar and H2, may be used to prevent any contamination or oxidation occurring to the emitters or phosphors. The commonly used frit glass is mainly composed of PbO, SiO2, ZnO, and B2O3, and has a good sealing ability. The sealed devices are then evacuated to achieve the required high vacuum working environment. During the evacuation process, the whole device is baked at about 350 C to release any absorbed gas molecules on the emitter and phosphor surface. In the newly developed vacuum packaging process, the frit sealing and evacuation are performed in a large temperature controlled vacuum chamber. After achieving high vacuum

1101

1102

6.2.3

Field Emission Displays (FEDs)

Metal duct

Frit frame

Anode plate

Spacers

Getter box

Cathode plate

Frit glass

Getter

Evacuation tube Cathode plate

b

Ceramic spacer Anode faceplate

a . Fig. 23 Schematic diagrams of the vacuum packaging process, (a) anode and cathode plate frit sealing, (b) evacuation of a sealed device (Reprinted from [16] with permission)

for a period of time, frit glass is melted by increasing the chamber temperature, then the wellaligned anode and cathode plates are pressed mechanically against each other and are stopped at a gap distance defined by spacers. The melted frit frame is squeezed between the anode and cathode plate at the edge to seal the device tightly resulting in the sealed interior under vacuum. Therefore, the process step associated with evacuation through a tube is not required.

7.2

The Spacer Support Technique

The mechanical structure of the FEDs consists of a hermetically sealed glass envelope that is evacuated to form the vacuum space required to accelerate the electron beams. Depending on the size of the display and thickness of the glass walls, spacers are generally required in order to support the glass plates against the atmospheric pressure. The spacers must also be capable of standing high voltage gradients and be optically invisible to the viewer under normal operating conditions. The spacer material must also possess the compressive strength to withstand the difference between the vacuum and external air pressure. In addition, the spacers must be extremely thin so that it does not disturb the electron trajectory. Furthermore, the electrical characteristics of the spacer have also to be optimized to prevent charge build-up, which in turn impairs the image quality by causing the electron beams to bend. There are various support structures and fabrication approaches in FED. The main structures are column supports, ball, or wall supports. The wall support structure has been widely adopted in FED construction. These are made of glass or ceramic materials with thin slice structure with a thickness 50–200 mm. By using these, a large gap distance between the anode and cathode can be realized (> 2 mm). The fabrication technique for the supporting wall is relatively simple and can be processed by routine mechanical cutting and polishing means. To prevent surface electron accumulation, suitable films can be coated on the supporting wall surface to reduce secondary electron emission. Since the supporting wall has a high aspect ratio, configuring, locating, and fixing the spacers in the sealing process is a very delicate process.

Field Emission Displays (FEDs)

7.3

6.2.3

Maintaining Vacuum in FED

The degradation of emission performance of a field emission array is a common problem in FED research. The performance deteriorates in such a way that the emission current reduces rapidly at the earliest working stage, and after a period, the current gradually approaches stability. In general, the higher the vacuum within the FED devices, the slower the emission current decreases, and the larger the stabilized emission current for the FED devices. The degrading of the emission performance of an array is often considered to be a consequence of the interaction between the emitters with the residual gas left in the FED devices and can be attributed to gas adsorption on the emitter surface, surface oxidation, ion sputtering, and ion implantation. Experimental investigation has confirmed that gas adsorption and desorption play a predominate role in affecting the emission performance. So achieving and maintaining a high vacuum condition is crucial to keeping the FED in normal working order for a long period of time. FED requires a form of getter technology to maintain the high vacuum inside the glass envelope after evacuation and sealing. After sealing, the getter is the only means to maintain high vacuum conditions. FED has a large ratio of surface area to the inside space volume. This requires a getter with a large absorption rate and capacity. Special techniques for FED getter technology have been developed.

8

Summary

There has long been an interest in field emission as an electron source. Field emission is a quantum mechanical process where electrons escape from a surface to a vacuum by tunneling. The relationship between the emission current and applied voltage was first described by Fowler and Nordheim. Field emission requires large electric fields to obtain high currents. Sharp structures allow these high electric fields. Microfabrication techniques can be used to produce sharp pointed structures and these can be made into arrays. Spindt first demonstrated such arrays with metallic points on silicon. This work has now developed worldwide to an advanced state. The technology has been difficult to realize but the combination of modern micro-fabrication techniques coupled with field enhancement from sharp structures as a source of electrons has enabled design solutions that have delivered displays with remarkable performance. However, manufacturing such devices has proven difficult. The vacuum electronics technology has seen some challenges including sealing and spacing, phosphor, as well as cathode uniformity. This chapter has examined this background and described some of the fundamental features common to many FED systems.

References 1. Fowler RH, Nordheim LW (1928) Electron emission in intense electric fields. Proc Roy Soc A119:173 2. Nordheim L (1928) Proc Roy Soc A121:626 3. Bonard J-M, Kind H, Stoeckli T, Nilsson LO (2001) Field emission from carbon nanotubes: the first five years. Solid State Electron 45:893–914 4. Schiff L (1968) Quantum mechanics, 3rd edn. McGraw Hill, NY

5. Spindt CA (1968) A thin film field emission cathode. J Appl Phys 39:3504 6. Spindt CA, Brodie I, Humphrey L, Westerberg ER (1968) Physical properties of thin film field emission cathodes with molybdenum cones. J Appl Phys 47:5248 7. Talin A, Dean KA, Jaskie JE (2001) Field emission display: a critical review. Solid State Electron 45:963

1103

1104

6.2.3

Field Emission Displays (FEDs)

8. Lee J, Shim BC, Park BG (2001) Silicide application on gated single crystal, polysilicon and amorphous silicon FEAs. IEEE Tans Electron Dev 48(1): 149–153 9. Chen L (2007) Experimental study of ultra-sharp silicon nano-tips. Solid State Commun 143:553–557 10. Temple D (1999) Recent progress in field emitter array development for high performance applications. Mater Sci Eng R24:185–239 11. Levine JD, Meyer R, Baptist R, Felter TE, Talin AA (1995) Field emission from microtip test arrays using resistor stabilization. J Vac Sci Technol B13:474

12. Li DJ, Zhang JC (2001) Focusing field emission arrays constructed by self – aligned photolithography. J Vac Sci Technol B19:1820 13. Kesling WD, Hunt CE (1995) Beam focusing for field – emission flat – panel display. IEEE Trans Electron Dev 42(2):340–347 14. Itoh J, Tohama Y, Morikawa K, Kanemaru S, Shimizu K (1995) Development and applications of field emitter arrays in Japan. J Vac Sci Technol B13:1968 15. Pertersen RO (1997) FED phosphors: low or high voltage. Inform Display 3:22 16. http://www.fe-tech.co.jp/en/structure/structure.html

6.2.4 New Field Emission Technologies Mervyn Rose . Yongchang Fan 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1106

2 2.1 2.2

Nano-Spindt Field Emission Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1106 The Basic Structure of Nano-Spindt FED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1107 Performance of the Nano-Spindt FED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1108

3 3.1 3.1.1 3.1.2 3.2 3.2.1 3.2.2 3.2.3 3.3 3.3.1 3.3.2 3.4

New Materials and Novel Field Emission Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1109 Carbon Nanotube Cathode–Based FEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1110 The Emission Mechanism of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1111 Fabrication of CNT-Type FEAs and Its Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 1113 Surface-Conduction Electron-Emitter Display (SED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1120 Surface Conduction Electron Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1122 Fabrication of Surface Conduction Emitter Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1122 The SED Developed by Canon and Toshiba . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1123 Thin Film Silicon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1126 Ballistic Electron Surface-Emitting Display (BSD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1126 Laser Processed Thin Film Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1130 Metal–Insulator–Metal (MIM) Structured Cold Cathode FEDs . . . . . . . . . . . . . . . . . 1132

4

Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134

5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.2.4, # Springer-Verlag Berlin Heidelberg 2012

1106

6.2.4

New Field Emission Technologies

Abstract: Field emission has had a long history of development, but has not emerged as an economic technology. The Spindt tip device has resolved many operational problems and much of our understanding of field emission process, but manufacturing difficulties have precluded mass market devices. Attention is turning to nanotechnology solutions. NanoSpindt shows much promise and can potentially resolve the manufacture difficulty. The emergence of carbon electronics and especially carbon nanotubes has now provided a new momentum. Planar devices, no longer relying on geometric field enhancement, are an attractive technology with nanocomposites providing high internal electric fields, and simple printing and laser techniques offering a route to manufacturability. New theories for this are now emerging. The field emission display is still a great hope for an emissive, low-power, lowcost display that can challenge the dominance of LCD, and is tantalizingly close. However, at this point, there are no plans to manufacture these devices. List of Abbreviations: CNT, Carbon Nanotube; FED, Field Emission Display; MIM, Metal– Insulator–Metal; PBS, Porous Polycrystalline Silicon–Based Ballistic Electron Surface Display; SED, Surface Conduction Electron-Emitter Display

1

Introduction

The previous chapter (> Chap. 6.2.3) explained the background to field emission, the necessary theory and the conventional Spindt devices, problems, and solutions. It also highlighted the many challenges faced by engineers working in this technology. These devices did not really make it to mass market applications despite the many advantages the technology holds. The complexity of manufacture that was highlighted was largely to blame along with effects at the atomistic level. However, the display community has not given up on the dream of a lowprofile, low-power, and low-cost emissive display. Many new technologies are finding radical solutions to the problems, and a new generation of devices based on nanoscale materials is emerging. Moving to planar structures can resolve many of the problems generated by reliance on geometric confinement of the field, and new theories for these materials are emerging. The manufacture issue is being addressed by new processing methods such as printing and laser processes. The Spindt process itself has been refined with progress in nanotechnology.

2

Nano-Spindt Field Emission Displays

Since the first concentrated effort to develop FED systems by the Silicon Video Corporation, later Candescent Technologies, in 1991, FED research and development has really undergone a difficult journey. More than 20 companies around the world joined the battle to try to conquer the FED problems, put it into the market and make a profitable business. To date, all have failed, and among these companies there are big names such as Motorola, Sony, Canon, Advance Nanotech, Toshiba, Futaba, and Samsung. Even though no real FED products have appeared in the market, thanks to the great efforts made by these companies, many different types of FED prototypes, some based on the Spindt-type emitter array, have been fabricated and demonstrated. The nano-Spindt field emission display developed by Sony and Toshiba is one of the latest prototypes which represent the highest FED technology level. In this section,

New Field Emission Technologies

6.2.4

the main characteristics of nano-Spindt FED are outlined and this gives a glimpse of the current status of FED research and development.

2.1

The Basic Structure of Nano-Spindt FED

> Figure

1 shows the basic structure of nano-Spindt FED. The cathode has a simple wiring pattern consisting of column cathode and row gate lines, enabling easy manufacture to the required precision level using any TFT production system. The panel vacuum is created using the established know-how. Furthermore, the nano-Spindt FED is simple to build, with far fewer components than other flat panel displays. > Figure 2 shows the single image pixel and

Ceramic spacer Uniquely designed to be electrically invisible and positioned in the vacuum between the cathode plate and the anode faceplate to make the panel withstand external air pressure. Insulation layer Prevents column and row wirings from merging.

Focus grid

Resistor layer Cathode plate Column wiring Row wiring

Phosphor triad Anode faceplate Made of robust glass with/without a low-reflective finish.

Simple wiring matrix The emitter array is formed at the intersection of each column line and row line.

Overcoat layer Aluminum metal back layer Together with the overcoatllayer, reduces deterioration in color purity and contributes to sharper focus than CRTs.

. Fig. 1 The perspective view of the structure of a nano-Spindt FED

1107

1108

6.2.4

New Field Emission Technologies

tip emitter structure. With more than 10,000 Spindt-type emitters used to illuminate each pixel, the nano-Spindt FED delivers clear, distortion-less images that are precisely focused across the entire screen. The high emitter density also contributes to uniform brightness and color without a hint of deterioration in purity. An ultrasmall emitter chip and gate hole, both on the order of 120 nm in diameter, make it possible to drive the cathode at a low voltage. Fabrication of electron sources as minuscule as the nano-Spindt emitter is generally considered to be difficult. Taking advantage of unique technologies developed by Field Emission Technologies Inc., however, the emitter tips used in nano-Spindt FED are precisely created to be 120 nm. These tiny emitters can also be manufactured using an existing TFT production system. > Figure 3 shows the fabrication process for metal tip emitters on the field emission array.

2.2

Performance of the Nano-Spindt FED

The nano-Spindt FED reproduces images using an array of nanocone emitters to excite phosphors. Taking advantage of highly efficient self-emissive phosphors combined with a line-sequential drive system, this display claims to deliver the visual performance required of professional monitor, including a wide viewing angle, life-like color, outstanding contrast

. Fig. 2 Close-up of the color pixels, (a) nano-Spindt emitter (b) ultrahigh density of emitter array, (c) the red, green, and blue image pixels

Small cavities are created in the gate electrode layer and insulation layer formed on a glass substrate

Molybdenum is deposited until the cavities disappear

Removing the molybdenum deposition completes the process Nanoscale gate hole (φ120 nm)

Gate electrode Insulation layer Glass Substrate

Resistor layer Cathode electrode A cone-shaped emitter develops in each cavity

. Fig. 3 Fabrication of molybdenum metal tip emitters in nano-Spindt FED

New Field Emission Technologies

6.2.4

with true black reproduction, and no-blur display of quickly moving images. Furthermore, the nano-Spindt FED provides distinctively sharper and clearer images all over the active area. The main characteristics of the nano-Spindt FED are listed in > Fig. 4b, the corresponding 19.2 prototype nano-FED, which was demonstrated in 2007, is shown in > Fig. 4a. One of the greatest advantages of the nano-Spindt FED is that it can display high-speed motion with absolutely no blur. The high emission efficiency of the nano-Spindt FED allows the use of a line-sequential impulse drive wherein a single signal line hit by electron beams emits light momentarily. Light is emitted during only part of one frame time. With this drive system coupled with newly developed phosphors featuring a short decay time, the nano-Spindt FED can be driven at 240 Hz and never displays a blurred image and is unsurpassed in contrast. The self-emissive design also assures a very wide field of view, with equal brightness and color uniformity at all viewing angles. Because the nano-Spindt FED produces light only from the ‘‘on’’ pixels, power consumption is dramatically lower than in LCDs where the backlight must always be on. The ultra-high-density emitter array, driven efficiently by a low voltage, with a low current load applied to individual emitters, is far less vulnerable to deterioration. Due to its fast response time and short phosphor decay time, the nano-Spindt FED can easily handle today’s advanced image formats. As of July 2007, it is the world’s first flat panel display equipped with an input interface for high-definition sources recorded at 240 Hz, a frame rate four times higher than that of current broadcast programs. All of this superb performances seemed to bestow nano-FED a splendid future with the nano-Spindt FED ready for use in the imaging systems of tomorrow. Unfortunately, after considerable time and effort, Sony’s FED efforts started winding down in 2009 as LCD became the dominant technology. It is clear that the key issue with nano-Spindt FED is not front screen performance, but it is the scaling to mass production with a competitive manufacturing price. In January 2010, AU Optronics announced that it acquired essential FED assets from Sony and intends to continue development of the technology. This is a significant step for the technology.

3

New Materials and Novel Field Emission Displays

The first generation of field emission displays are primarily based on the Spindt-type field emission arrays. Even through excellent FED prototypes with superior performance have been

Panel size

19.2”, 391.68(H) ⫻ 293.76(V) mm

Resolution

1280 ⫻ 960 dots (aspect ratio 4 : 3, SXGA), 0.306 mm pitch

Brightness

400 cd/m2

Contrast

More than 20,000 : 1

Display capability

10 bits

Phosphor

SMPTE/EBU

Cathode

Nano-Spindt ultra-high density emitter array

Dimensions

500(W) ⫻ 350(H) ⫻ 55 (D) mm excluding projected parts and stand

. Fig. 4 (a) 19.200 prototype nano-Spindt FED and (b) its main characteristics

1109

1110

6.2.4

New Field Emission Technologies

demonstrated in recent years, such as the latest nano-Spindt FEDs, no real commercial FED devices have appeared on the market so far. As we have seen, the fabrication of Spindt-type FEDs involves complicated, timeconsuming and multistep processes such as thin film deposition, photolithography, chemical or reactive ion etching, vacuum packaging etc., particularly challenging for larger area fabrication. All of these factors lead to a very high price for the first generation of Spindt-type field emission displays. In future FED research and development, there is not too much room to maneuver for the anode faceplate, supporting spacers and device sealing. To make FED more competitive, new materials and novel cold cathode structures have to be explored. Hopefully by adopting thin film planar structured field emission arrays, simple and cheap manufacturing techniques can be used to fabricate large area FED devices with a low manufacturing price. In fact, since the Spindt-type arrays were developed in the early 1990s, the research work on new materials and novel structures has never really stopped. Diamond and diamond-like materials were extensively studied in the early period of 2000s because of their relatively low work functions and excellent physical and chemical properties. It was realized however that these materials proved difficult since the material fabrication process involves high temperature and also the emission is nonuniform. In the middle of 1990s, the excellent field emission property of carbon nanotubes (CNTs) was discovered and this stimulated a new wave of research and development work on FED. The development of novel FED devices based on CNT field emission has becomes the main trend in FED research for many companies who have joined the race and demonstrated prototypes. In addition to the CNT FEDs, other types are appearing, and among these, the most influential are the surface conduction electron-emitter displays (SED) developed by Canon and Toshiba and the porous polycrystalline silicon–based ballistic electron surface display (PBS) developed by Komoda and his coworkers at Matsushita. In this section, we introduce these novel FED devices with discussion mainly focused on the working principle of the corresponding field emission cathodes, its main characteristics, and the relevant fabrication processes.

3.1

Carbon Nanotube Cathode–Based FEDs

Since the discovery of carbon nanotubes (CNTs) in the1990s, the material has been applied to field emission research and applications [1, 2, 4–6, 18, 23]. They have several properties which make them extraordinary materials for field emission. Firstly, single-wall metallic-type nanotubes and multiwall nanotubes exhibit high electrical conductivity at room temperature. Secondly, nanotubes are high aspect ratio whisker-like structures for optimum geometrical field enhancement. Thirdly, nanotubes can be very stable emitters, even at high temperatures. These unique characteristics make them remarkable field emitters. Indeed, among all of the materials studied so far, the CNT material has the lowest field emission threshold and the largest emission current density at a low electric field. There are two types of CNT, single wall and multiwall. According to the tip status, CNT can also be classified as open or closed tip type. The materials are produced mainly by chemical vapor deposition (CVD) with the presence of a metal catalyst on the substrate surface, arc discharge of a metal catalyst contained carbon electrode or the laser ablation of a solid carbon target. In this section, we do not discuss the details of CNT synthesis, instead our emphasis is mainly focused on the field emission and display-related subjects.

New Field Emission Technologies

6.2.4

10−7 Closed

Current density [mA cm−2]

Open

Current [A]

10−8 10−9 10−10 10−11 10−12

a

0

50

100

150

200

250

b

SWNT

6 4 2 0 0

300

Voltage [V]

Closed MWNT 8

0.5

1

1.5

2

2.5

3

3.5

4

Voltage [V/μm]

. Fig. 5 (a) I–V characteristics for a single closed and opened MWNT (the current is given in logarithmic scale), (b) I–V characteristics for closed MWNT and SWNT films [23]

3.1.1

The Emission Mechanism of CNTs

The initial application of CNT in field emission began with the consideration of its superb field enhancement capabilities as a result of its nanometer scale dimensions and the extremely high length to diameter ratio. Usually, the diameter of multiwall CNTs is in the range from a few tens to a few hundreds of nanometers. The diameter for single wall nanotubes is even less than a few nanometers but their length can be a few tens of micrometers. The emission threshold for CNT materials is usually in the range from 1 to 10 V/mm, much lower than that for Spindt metal tips. In fact, the radius of curvature of the metal tip emitters is comparable with that of the CNTs. As to the work function, there is also no big difference between the refractory metal and the graphite materials. Further investigation and studies have indicated that the field emission mechanism of CNTs is very complicated and their low emission threshold cannot be explained only by considering the field enhancement effect. > Figure 5 shows the emission I–V curves measured from a single CNT and CNT films. There is a big difference for the emission threshold for the CNT with an open or closed tip as shown in > Fig. 5a with the emission threshold of the close-ended CNT only a half of that for the open-ended CNT. This observed phenomenon cannot be accounted for by the field enhancement effect alone. > Figure 5b shows the I–V characteristics for CNT films which are composed of respectively closed MWNTs and SWNTs. Obviously the closed MWNTs have a better field emission performance than SWNTs. In general, to obtain a low operating voltage as well as a long emission lifetime, the nanotubes should be multiwalled and have closed and well-ordered tips. There are a number of explanations of the emission mechanisms for CNTmaterial and they are summarized below: 1. Based on the fact that the emission I–V relationship of CNTs complies with the F–N equation, some suggest that the emission mechanism of CNT is the same as that of metals. However, this assumption cannot explain the lower threshold. 2. The emission comes from the single carbon chain, so the field enhancement factor is extremely large which results in low emission threshold. This mechanism can only explain the emissions from the CNTs with open tips however and cannot be verified by experiment.

1111

1112

6.2.4

New Field Emission Technologies

3. Graphite sheet forms sharp bend at the open end, carbon atoms form sp3 rather than sp2 bonding, and this can substantially reduce the surface barrier, so it will result in a low emission threshold. This explanation is also only suitable to the openended case. 4. As shown in > Fig. 6, there are localized energy bands with a narrow energy distribution at the tip of the CNTs, with electrons emitted from these bands. If this explanation is correct, then electron transitions will occur among these energy levels so fluorescent emission will accompany these processes. The speculated emissions have been detected experimentally. The measurement performed on MWNT and SWNT by tunneling microscopy indicates that the localized energy bands do indeed exist. The band distribution full width at half maximum (FWHM) and the relevant band separations are consistent with the fluorescent measurement results. The right side in > Fig. 6 is the field-electron energy distribution (FEED). Experiment results show that FEED is very narrow, around 0.2 eV. The FEED consists of many subbranches and this is consistent with theoretical analysis. Since the distribution of the localized energy band is closely related to the specific structure of the particular CNT (diameter, defect, multi or single wall, open or closed etc.), a big difference exists in the energy band distribution among the different CNTs, with a difference in their emission. 5. As shown in > Fig. 7, a layer of gas molecules adsorbed on the CNT surface, the electrons are emitted through vibration tunneling effect. The basic fact to support this kind of assumption is that when the temperature is raised to 600 C, emission will reduce and FEED becomes wide, attributed to gas molecule desorption at high temperature and vibration emission effects disappear. So the field emission mechanism of CNTs is very complicated, and correlated with factors such the type, structure, defect, and absorbance state, etc., with no one explanation satisfactorily accounting for all the observed emission characteristics. The above explanations then are not necessarily correct. New observed phenomena and definitive explanations will appear in the course of further investigation.

E

CNT body EF

I(E) E 11 E 22 E 33

. Fig. 6 The localized and quantized energy bands at the tip location of CNTs and the electron tunneling with the presence of electric field [1]

New Field Emission Technologies

6.2.4 E

CNT body EF

Absorbate state I (E )

. Fig. 7 The vibration tunneling of electrons from the CNTs adsorbed with gas molecules on surface [1]

3.1.2

Fabrication of CNT-Type FEAs and Its Characteristics

For field emission applications, there are two approaches to fabricating a CNT field emission array cathode. For large area devices, thick film screen printing methods can be used, while for small size and high-resolution displays the CVD growth process is required to produce the well-aligned array of CNTs. Direct Growth Techniques

In order to utilize the unique properties of carbon nanotubes in FED devices, it is necessary to develop a technology which enables high yield, uniformity, and preferential growth of aligned nanotubes. Such a technology has been established by using plasma-enhanced chemical vapor deposition (PECVD) of acetylene and ammonia gases in the presence of a nickel catalyst with precisely controlled deposition parameters [2]. By patterning the nickel catalyst, uniform arrays of nanotubes and even single free-standing aligned nanotubes at precise locations can be deposited as shown in > Fig. 8. The patterned array of individual vertically aligned nanotubes shown in > Fig. 8b has the most desirable field emission characteristics, that is, the highest apparent field enhancement factor and emission site density. The CNT arrays can be used directly in diode-structured FED devices or in CNT diode-structured lamps. To achieve truly low-voltage addressable operation for FED, triode-structured arrays similar to Spindt arrays have been fabricated by many research groups around the world. > Figure 9 show a typical process developed by the research group at Cambridge University headed by Professor Bill Milne. In this process, a single-mask, self-aligned technique was used to pattern the gate, insulator, and catalyst for nanotube growth. Vertically aligned carbon nanotubes were then grown inside the gated structure by PECVD. The self-aligned fabrication process ensured that the nanotubes are always centered with respect to the gate apertures (2 mm diameter) over the entire device. > Figure 10 shows an SEM image of CNT emission array with integrated gate electrode. The fabricated field emission array exhibited an initial turn-on voltage of 9 V. After the first measurements, the turn-on voltage shifted to 15 V, and a peak current density of 0.6 mA/cm2

1113

1114

6.2.4

New Field Emission Technologies

2 μm

a

High Mag.

b

10 μm

. Fig. 8 CTN emitter array prepared by CVD direct growth technique. (a) CNT emitter array composed of bunch of CNTs in each location, (b) CNT emitter array composed of singly and vertically grown CNT at each location on the array [34, 35]

Resist Gate electrode

Catalyst and barrier layer (if needed)

Gate

Insulator

a Emitter electrode

b Emitter electrode Nanotube/nanowire Gate

Gate isotropic underetch

c Emitter electrode

d Emitter electrode

. Fig. 9 The self-aligned process for fabricating nanotube emission array with integrated gate electrode. (a) A photoresist hole is first patterned on top of a gate electrode/insulator/emitter electrode sandwich. (b) The gate and insulator material are then isotropically etched. (c) A thin film of catalyst, and diffusion barrier (if required), are deposited on the structure. (d) A lift-off is then performed to remove the unwanted catalyst on top of the gate followed by the nanotube growth inside the gate cavity [33]

at 40 V was achieved, using a duty cycle of 0.5%. The size of the Ni dot determines the diameter and number of CNTs per dot. By careful tuning the growth conditions, a single nanotube can grow in each cavity. The CVD growth of high-quality CNT is a high-temperature process which prevents using low melting point glass as the substrate. A CNT field emission array prepared on silicon wafer has a limited area and it is only suitable for small size and high-resolution FED device

New Field Emission Technologies

6.2.4

3 μm

. Fig. 10 SEM images of CNT emission arrays with integrated gate electrode [10]

fabrication. One particular difficulty with CVD growth of CNTs is nonuniformity and low reproducibility. The complexity of growth parameters such as temperature, flow rate, catalyst metal, gas species, pressure, and size of catalyst particles can cause a very wide diversity in tube structure, diameter, density, length, and crystal quality. The difficulties in fabricating uniformly distributed CNTemitters on large areas by CVD direct growth techniques seem insurmountable. Screen Printing Techniques

Today, screen printing is the most widely used technique to fabricate large area CNT field emission arrays [12, 15–17, 31]. In most CNT FED applications, nanotubes are first mass produced by arc discharge, the most cost-effective production technique. The arc discharge nanotubes are purified and mixed with an epoxy/binder and screen printed or applied at the required location. Alternatively, electrophoresis can be used to adhere the nanotubes dispersed in solution to specific electrodes. The earliest CNT FED devices were fabricated in a diode structure taking advantage of the low emission field threshold. A 4.5 in., fully sealed, field emission display was demonstrated by Samsung in 1999. The schematic structure of the device is shown in > Fig. 11. This FED consists of two glass plates: SWNTs stripes on the patterned cathode glass and phosphor-coated indium tin oxide (ITO) stripes on the anode glass. The spacing between the plates is kept by 200 mm spacers, and the pixels are formed at the intersection of cathode and anode stripes. The column-structured CNT emission cold cathodes were fabricated by using a nanotube and organic binder paste. The purified SWNTs synthesized by conventional arc discharge are dispersed in isopropyl alcohol, and then mixed with an organic mixture of nitrocellulose. The paste of well-dispersed CNTs is then squeezed onto the metal patterned soda lime glass through the metal mesh of 20 mm in size and subsequently heat treated to remove the organic binder. > Figure 12a shows a SEM image cross section of a CNT cathode. The CNT bundles are firmly adhered to the metal electrode and aligned mostly perpendicular to the substrate. The density of CNT bundles is about 100 times larger than the typical density of micro-tips in Spindt-type FEDs. > Figure 12b shows the I–V characteristics of the CNT cathode in which SWNTs/metal form stripes of 300 mm wide with a pitch size of 625 mm. The turn-on field was less than 1 V/mm and the total current 1.5 mA at 3 V/mm (current density, J=590 mA/cm2). A graphite powder cathode was also prepared by the same method for comparison as shown in > Fig. 12b.

1115

1116

6.2.4

New Field Emission Technologies

ITO glass plate (t = 1.1 mm) Patterned phosphor 2.4 mm

200 μm

Spacer Carbon nanotube/ patterned metal Glass plate

. Fig. 11 Schematic structure of the fully sealed 128 lines matrix-addressable carbon nanotube flat panel display. The pixels are formed at the cross section of cathode and anode [15]

The fully sealed, 4.5 in. diagonal FED has been demonstrated with high brightness of 1,800 cd/cm2, as shown in > Fig. 13. The cathode was prepared by screen printing. After firing the composite, pretreatments such as rubbing and electric field conditioning was carried out to align the embedded CNTs in the vertical direction. The fully sealed diode-type display (200 mm glass spacer) turned on at less than 1 V/mm and emitted 1.5 mA at 3 V/mm. This passive matrixaddressed CNT-FED proved that a cathode array produced from screen printing is scalable, enabling large panels with low production cost [5]. For the diode-structured CNT FED, the gap distance between anode and cathode plates is usually in the range from 100 to 300 mm. In order to obtain the required luminance, the anode voltage needs to be above 500 V, so high voltage driving circuitry is required leading to a high manufacture price. Diode-structured FED can only use low-voltage phosphors and there are still problems to be resolved with these, such as degradation and contamination when bombarded by the high density of emission electrons. To realize low voltage addressing and adopt the highly efficient conventional high voltage phosphors, many kinds of gate electrode incorporated CNT field emission cathode arrays have been constructed and attempted. One of the earliest triode-structured CNT FEDs developed by Noritake Co., Ltd. in Japan is shown in > Fig. 14. A metal mesh was inserted between the anode and cathode to serve as the gate electrode. The strips of metal gate are parallel with the phosphor strips on the anode and perpendicular to the cathode strips on which the CNT paste is screen printed. The distance between the anode and cathode is 2–4 mm with an anode voltage of 6 kV. The gap between the cathode and gate is about 0.3 mm, and the corresponding gate addressing voltage is a few hundreds volts. To guarantee insulation and prevent cross talk, insulator walls lie in the gaps between electrode strips on both cathode and anode. Based on this basic structure, Noritake Electronics developed better CNT FED with a luminance above 10,000 cd/m2. Among the variety of CNT field emission array cathodes developed so far, the normal gate electrode incorporated field emission array seems to be one of the most promising cathode structures for an application of CNTs to FEDs in terms of operation voltages as well as FED device performance. The structure of the gate electrode incorporated CNT field emitters is similar to the Spindt-type field emitters, but it has a much larger gate aperture due to the

New Field Emission Technologies

a

SAIT/AEIab 5.0kV 5.8mm ⫻60.0k SE(M)

0.0020

6.2.4

500 nm

−16 −18 Ln (Ln/V)

−20

0.0015

−22

Carbon nanotube

−24 −26 −28

Current (A)

−30 −32 0.004 0.006 0.008 0.010 0.012 0.014 1/V

0.0010

Graphite 0.0005

0.0000 0

b

1

2

3 Electric field (V/μm)

4

5

. Fig. 12 CNT field emission cathode prepared by screen printing method and its emission characteristics. (a) Cross-sectional SEM image of the cathode, (b) I–V curves measured from the CNT cathode [16]

resolution limitation of the screen printing technique and the characteristics of the CNT paste materials. On the field-emitter array (FEA) templates fabricated by the same processes as those of the Spindt-type FEAs, CNT emitter dots were formed inside gate holes by screen printing of a photosensitive paste containing CNTs and subsequent back side exposure of UV light. A dot of CNT emitters with a diameter of 20 mm was defined for each gate hole with a diameter of 30 mm. The basic structure for the gated CNT emitter array is shown in > Fig. 15. For this type of emitter array, a gate driving voltage of about 60 V is required. To further improve the performance of the CNT emitter array, Ni wall structure (NWS) is electroplated to form a thick gate to suppress diode emission induced by strong electric

1117

6.2.4

New Field Emission Technologies

2000 Brightness (cd/m2)

1118

1600 1200 800 400 0

a

0

b

1

2 3 4 Electric field (V/μm)

5

6

. Fig. 13 (a) Emitting image of fully sealed SWNT-FED at color mode with red, green, and blue phosphor columns, (b) light intensity with electric field [16]

ITO glass plate

Patterned phosphor Metal mesh Carbon nanotube/ patterned metal

Spacer

Glass plate (t=1.1vmm)

. Fig. 14 Triode-structured CNT-FED [17]

CNT emitter Ni gate electrode SiO2 insulator a-Si layer ITO cathode electrode

. Fig. 15 Schematic diagram of a normal gated CNT field emitter array [12]

New Field Emission Technologies

6.2.4

strengths due to an anode potential and to better focus electron beams to the corresponding color pixels (see > Fig. 16). A full color, vacuum-sealed 5 in. diagonal CNT-FED was constructed based on this improved CNT emitter array with a spacing of 1.1 mm between the lower and upper electrodes. The image shown in > Fig. 17 was observed for a gate voltage of 60 V and anode voltage of 2 kV, anode current of 0.64 mA, with a duty ratio of 1/120 and a frequency of 100 Hz. Other cathode structures have also been attempted aimed at reducing the driving voltage and improving stability and uniformity [30, 37, 38]. > Figure 18a shows a structure with the gate electrodes located beneath the cathode electrodes, both electrodes perpendicular to each other and separated by an insulator layer with a thickness of around 5 mm. This kind of structure can significantly reduce gate-cathode distance preventing the chance of short-circuit during fabrication and operation. > Figure 18b shows a planar structure where both cathode

Thick Ni wall (30 μm high)

Ni gate electrode SiO2 insulator a-Si layer ITO cathode electrode

. Fig. 16 Improved structure of the gated CNT emitter array [27, 31]

. Fig. 17 Full color moving image of a fully vacuum-sealed 5 in. diagonal CNT-FED [27]

1119

1120

6.2.4

New Field Emission Technologies

and gate are fabricated on the same surface plane. This structure is relatively easy to fabricate but the electron beam divergence is relatively large. > Figure 19 shows a 35-in. high-definition CNT-FED demonstrated by Samsung SDI [18]. The information on the structure of the CTN emitter array is not available. The image quality has been greatly improved but is still far from that shown by the state-of-the-art LCD. Looking back in the history of the FED, no one can predict the future for CNT FED but like the Spindt technology there is still a long way to go before CNT FEDs can reach the market.

3.2

Surface-Conduction Electron-Emitter Display (SED)

The surface-conduction electron-emitter display (SED) is a flat-panel, high-resolution display which has been developed by Canon and Toshiba since the 1990s [3, 7, 13, 24, 26]. In SEDs, electron emitters are distributed in a number equal to the number of pixels on the display as

Anode

Anode CNT

Cathode

CNT

e− Cathode

Insulator Gate a

Gate

Gate

b

. Fig. 18 CNT-FEDs with a different emission array structure (a) the gate electrode is located beneath the cathode electrode and both are separated by an insulator layer, (b) the gate and cathode electrodes are fabricated on the same surface plane

. Fig. 19 Full color image shown on a 35 in. high-definition full color CNT-FED developed by Samsung [18]

New Field Emission Technologies

Electron emittor

Phosphor

6.2.4 Phosphor

Electron gun

Spacer

Deflecting yoke

a

SED

CRT

Light Color filter Black Matrix

Phosphor

Al

Anode plate R

B Frit seal

Spacer

Vacuum

Electrons

Spacer

Frit seal

Getter

G

Cathode plate

b . Fig. 20 (a) Comparison between cathode ray tube (CRT) and surface-conduction electron-emitter display (SED), (b) schematic diagram of cross-section view of a single image pixel in SED

shown in > Fig. 20. The space between the two electrodes in each electron emitter is an extremely small gap of just several nanometers. When a voltage is applied between the electrodes, electrons are emitted from one side, and some of these electrons are accelerated by an additional voltage applied between the glass substrates, resulting in luminescence when the accelerated electrons strike the phosphor. The same image forming principle that the emitted electrons excite a phosphor coating on the display panel can classify SED as a kind of field emission displays (FED), so the SED devices possess all the merits of the FED. In comparison with conventional FED devices, the big difference is that in SED, the field emitters in the array has a thin film planar structure, and the electrons, that are accelerated by the anode voltage to bombard the phosphor pixel on the faceplate, come from part of the surface conduction electrons which tunnel though the planar-structured thin film emitters fabricated on the cathode surface [29].

1121

1122

6.2.4 3.2.1

New Field Emission Technologies

Surface Conduction Electron Emission

The surface conduction electron emission phenomenon was discovered by scholars from the former Soviet Union in the early 1960s and it is usually classified as thin film field emission. This kind of cold cathode has a planar structure, that is, both the cathode and extraction (gate) electrode lie on the same surface plane. The fabrication process and emission mechanism for the thin film emitter are described below. First, a metal film is deposited and patterned to form parallel cathode and extraction electrodes on the substrate surface with a gap of around 10 mm. A layer of SnO2 film or Sn metal film is evaporated to cover the gap and electrodes followed by oxidation to form a conductive SnO2 film. Because the deposited film is very thin, it is composed of continuous islands and the electric conduction paths exist among these islands. Under vacuum condition, a voltage is supplied between two electrodes. When the current is increased to a certain value, some conduction paths will be burnt out. After this, the current passing through the two electrodes is achieved by the field emission through the tiny gaps between the isolated islands. A simple emission model for surface emission is illustrated in > Fig. 21. In the presence of an electric field, which is distributed in the gap between SnO2 islands, field emission can take place and the electrons travel from one island to the next. Also, as shown in > Fig. 21, when an anode is placed above the cathode, part of the field emission electrons traveling between islands will be attracted to the anode under the action of the strong field. The ratio of the anode current to the surface conduction current is defined as the emission ratio. The emission ratio for the cathode based on the SnO2 thin film is above 5%. However, the emission current is very unstable and fluctuations up to 10% can be measured. There are many factors which lead to the large fluctuation of emission current and investigations shows the intrinsic instability of SnO2 itself to be the predominant factor.

3.2.2

Fabrication of Surface Conduction Emitter Array

Canon began exploring the surface conduction electron emission technology from the early 1990s. By adopting new thin film material consisting of PdO nanoparticles and thin film printing technology, surface conduction emitter arrays with stable emission current were developed. At Euro Display’96, Canon presented the principle of Surface Conduction Emitter (SCE) for FEDs [24]. This concept was revolutionary, since the paradigm for cold emission in FEDs at that time was the micro-tip array.

e−

e−

e−

Anode Electric field

Cathode

. Fig. 21 Principle of surface conduction electron emissions

Gate

New Field Emission Technologies

6.2.4

The fabrication process of the cold cathode with an addressable planar emitter array is relatively easier than that of the Spindt array. First, a simple matrix of silver wires is deposited by a printing method using silver paste and insulating films at the crossovers. Electrodes are added into this array, typically using platinum, leaving a gap of about 60 mm between the electrodes, which are addressed through the row and column silver wires. Next, square pads of palladium oxide (PdO) only 20 nm thick are deposited into the gaps between the electrodes, connecting to them to supply power. A small slit is cut into the pad in the middle by repeatedly pulsing high currents though them, the resulting erosion causing a gap to form. The gap in the pad forms the emitter. The width of the gap has to be tightly controlled in order to work properly, and this proved difficult to control in practice [7]. Modern SEDs add another step that greatly eases production. The palladium oxide pads are deposited with a large gap between them, as much as 50 mm, which allows them to be added directly using technology adapted from inkjet printing. The entire screen is then placed in an organic gas and electrical pulses sent through the pads. Carbon in the gas is pulled onto the edges of the slit in the PdO squares, forming thin films that extend vertically off the tops of the gaps and grow toward each other at a slight angle. This process is self-limiting; if the gap gets too small the pulses erode the carbon, so the gap width can be controlled to produce a fairly constant 5 nm slit between them. > Figure 22 shows the carbon deposition and the structure of the nanogap in PdO layer with graphite.

3.2.3

The SED Developed by Canon and Toshiba

In 1999, Canon and Toshiba started cooperation on development of the surface conduction electron emitter display and a number of prototype SEDs have been demonstrated in several big electronic and display exhibition shows and notably the 36 in., wide aspect ratio SED at the CEATEC Japan 2004 electronics show and 50 in. SED at CEATEC 2005. The SED structure is unique in that the electron beam current supplied to the anode for each pixel is generated in a two-step process creating a gap structure. The current across the gap follows the Fowler-Nordheim law and is thus highly nonlinear, allowing for matrix addressability as will be discussed later.

Methanol vapor

Nano gap PdO Pt (scan) Pdo

graphite

Pt (signal)

Pt Substrate

. Fig. 22 Surface conduction emitter with graphite on the edges of PdO gap. (a) Graphite is deposited in the gap by a CVD process during electrical activation of the emitters, (b) The structure of the nano gap in PdO layer with graphite

1123

1124

6.2.4

New Field Emission Technologies

The electrons that tunnel across the gap and strike the counter electrode are either absorbed into the counter electrode (thereby creating only heat and no light) or they are scattered, captured by the electric field created by the anode potential and accelerated to a particular phosphor dot, thus creating a spot of red, green, or blue light. This combined electron emission plus the scattering process is illustrated in > Fig. 23 where Va is the anode potential and Vf is the driving potential across the gap. Multiple scattering events may take place before the electron is captured by the anode field. The efficiency of the number of electrons captured by the anode field is quite low, on the order of 3% of the surface conduction electrons, but the power efficiency is reasonable since Vf is low, on order of 20 V [3]. The SED is driven line-by-line as shown in > Fig. 24. The scanning circuit generates the scan signal (Vscan) and the signal modulation circuit generates a pulse width modulation signal (Vsig) that is synchronized with the scan signal. The quantum tunneling effect that emits electrons across the slits is highly nonlinear, and the process tends to be fully on or off for any given voltage. This allows the selection of particular emitters by powering a single horizontal row on the screen and then powering all of the required vertical columns at the same time. The selected emitters are rapidly turned on and off line by line using pulse width modulation, so that the total brightness of a spot in any given time can be controlled. Because of the highly nonlinear emission characteristics of the surface conduction emitter, it is possible to drive each pixel selectively using a simple matrix x–y configuration without active elements and still achieve a luminance contrast ratio of 100,000:1 with a signal voltage of 18.9 V and a scanning voltage of 9.5 V. The SED switching voltage is much lower than that used in other type of FED devices but the device must be designed for much higher steady-state current loads, as much as a factor of 30 times higher as a result of the inefficiency of the SCE electron scattering mechanism. The larger currents also forces the interconnect lines to have lower resistance compared to FED as even a small voltage drop along the line can result in edgeto-edge nonuniformity. > Figure 25 shows the prototype 36 in. SED developed by Canon and Toshiba in 2005 [3]. When SED Inc. exhibited 10 of their surface-conduction-emitter display (SED) panels at the

Luminescence Color filter

Black matrix Glass substrate

Electrode

Electron beams

Phosphor Metal back film

Enlarged image of electron emitter

Electron emitter

Va Glass substrate

Several nm Nanogap

Vf

. Fig. 23 Structure of SED and its operation principle. Each sub-pixel has a unique pair of electrodes that supplies an electron current [3]

New Field Emission Technologies

6.2.4

Anode plate Scan signal

0

0

Vsig

Scanning circuit

Vscan

Va

0 Vscan 0 0

0 Modulation signal

Cathode plate SCE

0 Vsig

0 Vsig

Signal modulation circuit

. Fig. 24 Diagram of SED matrix-addressed driving method [3]

. Fig. 25 The 36 in. SED developed by Canon and Toshiba in 2005 [3]

big electronic and display show CEATEC Tokyo (Japan) in October 2005, the world witnessed the state of the art in displays, a picture performance with high contrast, lack of motion artifacts, and wide color gamut. Since SED is a flat panel display with a panel thickness of 7 mm, many people even think it is highly possible for the SEDs to replace the dominant FPDs for large areas, notably LCDs and PDPs. Unfortunately, after considerable time and effort in the early and mid-2000s, SED efforts started winding down in 2009 as LCD became the dominant technology. In August 2010, Canon announced they were shutting down their joint effort to develop SEDs commercially, signaling the end of development efforts.

1125

1126

6.2.4

New Field Emission Technologies

3.3

Thin Film Silicon Materials

3.3.1

Ballistic Electron Surface-Emitting Display (BSD)

In 1995, a new electron emission phenomenon from nanocrystalline silicon (NS) diodes with a structure of Au/NS/Si was reported by Koshida et al. in 1998. Komoda and coworkers at the Advanced Technology Research Laboratory of Matsushita Electric Company developed a cold cathode for flat panel display using porous polysilicon (PPS) as cold cathode materials and demonstrated that the mechanism of electron emission is quasi-ballistic, quite different from that of conventional FED types [21, 25, 28]. The electron emission from these kinds of silicon cathodes possesses high efficiency and small fluctuations and flat panel displays based on this kind of cathode are called ballistic electron surface-emitting displays (BSD) [32, 36]. In this section, we will simply introduce the fabrication process for nanocrystalline porous silicon emitters, overview its main characteristics, and discuss the mechanism of ballistic electron emission. Finally, a prototype of 7.6-in. diagonal full-color BSD fabricated on a glass substrate is described. The Fabrication of the BSD Cathode

Initially, the cold cathode was fabricated on silicon wafers and subsequently developed further for silicon films deposited on glass. The fabrication process for BSD on glass is demonstrated in > Fig. 26. First, a 300 nm tungsten metal layer is deposited onto the surface of the glass, forming the bottom cathode electrodes, by a sputtering technique (> Fig. 26a). Then a polysilicon film is deposited by plasma-enhanced chemical vapor deposition (PECVD) below 400 C with a thickness of 1.5 mm (> Fig. 26b). After forming polysilicon onto the metal electrodes, samples are anodized in a solution of HF (50%):ethanol=1:1 at a current density of 30 mA/cm2 for 12 s under the illumination of a 500 W tungsten lamp at a distance of 20 cm. The thickness of the nanocrystalline porous silicon (NPS) layer is about the same as that of the polysilicon layer. After anodization, substrates go through an electrochemical oxidation (ECO) technique where they go into an aqueous solution containing 1 M sulfuric acid (H2SO4) with a current source of 20 mA/cm2 applied for 20 s to the NPS layer with respect to the solution.

Porous-Si

Tungsten film Glass substrate

a

c Poly-Si

b

Surface electrode

d

. Fig. 26 The process flow of the BSD cold cathode on glass substrate [32]

New Field Emission Technologies

6.2.4

Finally, a semitransparent 15 nm gold film is deposited as a surface gate electrode onto the NPS layers by a sputtering. The Main Characteristics of the BSD Cathode > Figure

27 shows the schematic diagram of the structure for a BSD cold cathode and its operation. With a positive dc bias voltage Vps applied to the top Au electrode, the electrons can emit into the vacuum from the surface and the emitted current Ie can be measured by a collection electrode, which is biased with a positive voltage supply of Vc. The emission mechanism is based on a specific field-induced electron drift in the porous silicon layer associated with generation of hot electrons and the subsequent ejection via tunneling through the top surface electrode. In contrast to the case of conventional field emitter arrays, the electron acceleration process in the porous silicon layer is a key factor in efficient emission. The measured free drift length of the carriers under strong electric field (about 105 V/cm) within the PS layer reaches 1 mm. This value is much larger than the size of the average silicon nanocrystallites (around 10 nm) and it indicates that the conduction electrons are accelerated in the NPS layer ballistically and reach the surface electrode without suffering significant scattering losses. This kind of porous silicon–based ballistic cold cathode emitter has a number of advantages over conventional field emitters and other cold cathodes as it possesses surface emitting capabilities with no tip structure required for electron emission. It also allows low-voltage operation. The threshold voltage for electron emission is only 6 V and a high electron emission current density (Je) of 2.43 mA/cm2 can be obtained at surface electrode bias voltage (Vps) of 18 V. Electron emission efficiency , which is defined as the ratio of emission current Je to diode current Jps, can reach to about 2.6%. The emission current is insensitive to the vacuum pressure. For example, the emission current is about 1 mA/cm2 at Vps =16 V and remains almost constant when ambient pressure changes from 103 to 10 Pa. This means the cathode can work normally at a low vacuum

nc-PPS

Collector Top

Poly-Si e−

e−

Glass

e−

Bottom Ips

Ie Vps

Vc

. Fig. 27 Schematic of the structure and operation for the polyporous silicon–based cold cathode [32]

1127

1128

6.2.4

New Field Emission Technologies

pressure of 10 Pa. The emission has a small dispersion angle because the cathode emits energetic hot electrons. In addition to these advantages, the large area BDS cathode can be easily made and the fabrication technique is compatible with the well-developed silicon planar device processing technology. The Mechanism of Ballistic Electron Emission

The electron emission mechanism can be explained in referring to > Fig. 28. Initially, the PECVD deposited poly-Si layer on the bottom electrode has a columnar structure. In the subsequent anodization process, the ethanoic HF solution promotes local dissolution preferentially at the poly-Si grain boundaries and forming porous silicon at the boundary region between the columnar crystals. The rapid thermal oxidization process forms a thin layer of SiO2 film which is signified in > Fig. 28 as the circular shells surrounding the nano-sized Si particles. When a positive bias is applied to the surface electrode on top of the NPS layer with respect to the bottom electrode, the electrons are injected from the bottom electrode into the porous silicon layer toward the top electrode. The probability of electrons colliding with the nanosized silicon particles is very small, which leads to a large free drift path nearly equal to the thickness of the entire porous silicon layer. So the electrons accelerated by the internal field (as high as 107 V/cm) can travel through the porous silicon layer nearly free. When they arrive at the top electrode, the ballistic electrons gain enough energy to tunnel through the top electrode and emit into vacuum [32]. Fabricated BSD Model

A full 7.6 in. diagonal BSD model based on this porous silicon has been demonstrated with a low-temperature process on TFT or PDP glass substrates. > Figure 29 shows the processing steps for a BSD array fabricated on glass substrate 200200 mm with an active pixel area of 154116 mm2. > Figure 30 shows a schematic cross-sectional structure of the vacuum-encapsulated BSD. Supporting glass plate of 2.8-mm thick is attached to the backside of the BSD panel to increase the strength against vacuum pressure. The fabricated cathode array and face (anode) plate with P-22 phosphor on ITO electrode are jointed with frame glass, with a 5 mm gap. The control voltage Vps is applied between the surface and the bottom electrode and an anode voltage of e− e−

e− e− e− e−

e− e−

SiO2 Si

. Fig. 28 The working principle of the ballistic electron surface emitting cold cathode [32]

New Field Emission Technologies

6.2.4

Bottom electrode

a NPS layer

b Surface electrode

c

Bus electrode

d . Fig. 29 Processing steps for fabrication of ballistic surface-emitting arrays. (a) Bottom electrode deposition and patterning, (b) poly-Si layer deposition and formation of nanocrystallized poly-Si layer, (c) surface electrode deposition and patterning, (d) top bus electrode deposition and patterning [36]

Surface electrode

ITO

Phosphor

Poly-Si

Face plate e−

e− Frame glass

NPS

TFT glass substrate Supporting glass plate Bottom electrode

a

b

. Fig. 30 The fabricated 7.6 in. diagonal prototype BSD model. (a) Schematic cross section of the vacuum-encapsulated BSD model. (b) Perspective view of the BSD [32]

1129

1130

6.2.4

New Field Emission Technologies

5 kV applied between the ITO of the faceplate and the surface electrode of the BSD. Excellent electron emission characteristics, such as emission current density, efficiency, and stability are a feature of this technology. Consequently, BSD provides a promising route to realize large-size FEDs for the near future.

3.3.2

Laser Processed Thin Film Silicon

Thin film amorphous silicon is an attractive proposition for large area devices. Laser processing techniques are widely used in the low-temperature polysilicon process, so a display infrastructure exists for such a technology. One such device was developed by Quantum Filament Technologies (QFT) Ltd. This was an attempt to use the attractive features of all the novel technologies described in this section into a manufacturable and stable device. Excimer laser processing created a micro- or nano-tip external morphology in thin films, with nanoscale inclusions creating filamentary pathways and effective field emission [8, 9]. Novel self-forming gate technology aimed at low-voltage operation, and a lithography-free approach to realizing low-cost, low-power devices with no manufacturing difficulty together with the familiar materials and tools of the AMLCD process was an attractive proposition. QFT used a combination of these tools and techniques to develop a new field emission backplane based on amorphous silicon and laser processing which gives highly uniform electron emission with low threshold voltage and no need for conditioning. The backplane is manufactured from amorphous silicon and its unique field emission characteristics are created by a simple engineering process which is free of any lithographic steps. Filamentary conduction in thin films and amorphous silicon devices has been studied for some time. These filaments are found in inhomogeneous nanoscale and granular structures. Stable filamentary conducting channels result in interesting effects, such as bistability and electron emission at low electric fields. The filamentary channels are determined by the distribution of nanoparticles and act as emitters. QFT create nanocrystals and granular structures through laser processing amorphous silicon with particular underlying metals which have a catalytic effect on the formation of the matrix with the conditions for stable filaments with the internal structure required for field emission without conditioning and at low thresholds (> Fig. 31). In order to operate the device at low current, it is arranged as a three-terminal device with self-aligned gates, through an etching process, drawing the electrons from the a-Si:H at a very low potential difference and so can be switched at low energy. The bias opens up channels in the internal structure between the nanoscale inclusions and creates a high internal electric field. The electrons are then attracted to the anode at a high potential difference from the cathode, in a process that does not require high currents. The attractive feature is of this technology is simplicity and that it is based on amorphous silicon, which is a mature display technology, and on excimer laser processing, the tool for the low-temperature polysilicon process. Deposition of a patterned cathode metal is followed by hydrogenated amorphous silicon. This is then directly processed with an excimer laser with a distinct beam profile at modest average energies. The result is a film that has a very rough surface, containing the necessary internal structure for filamentary conduction. Thin film silicon nitride is then deposited conformally over the surface structures (as a TFT gate dielectric process) and thin film metal deposited on top of this. The structures are then simply etched by RIE to expose the microtip. This process has the

New Field Emission Technologies

6.2.4

1.00E−05 Cycle 1 Cycle 2 Cycle 3 Cycle 4

Emission current (A)

1.00E−06 1.00E−07 1.00E−08 1.00E−09 1.00E−10 1.00E−11 1.00E−12

0.3u

a

0

b

10 20 30 40 50 Macroscopic electric field (V/μm)

60

. Fig. 31 (a) Surface of a QFT backplate (b) Plot of emission current for applied field [9]

0.3u

0.3u

0.3u

. Fig. 32 Formation of self aligned gates on encapsulated nanostructures features by laser processing techniques, (a) encapsulation by SiN and metal, (b) RIE etching initiation, (c) exposed gated microtips [8]

effect of etching from the top of the structure through the metal to the insulator, leaving a freestanding filamentary micro-tip surrounded by a metal gate isolated by the insulator. Each ‘‘tip’’ is a fraction of a micron, so each pixel has tens of thousands of emitter sites. If you consider the steps just described, it is rather like the thin film processing a TFT, but with no lithographic steps. This then forms the cathode plate. The device is completed by forming a vacuum space with a phosphor-coated anode plate (> Fig. 32). Richard Forbes [11, 14] has developed a theory of planar nanostructured heterogeneous emitters that exhibit field induced electron emission and show filamentary conduction. These low macroscopic field (LMF) electron materials emit into vacuum at 1–10 V/mm and are typically a dielectric matrix with nanoscale inclusions of higher electrical conductivity. Conducting channels or filaments open up as a pathway to emission from the surface. A feature of this mechanism is that the internal structure creates a high internal field enabling thermalized electrons to escape into the vacuum by tunneling (> Fig. 33).

1131

6.2.4 3.4

New Field Emission Technologies

Metal–Insulator–Metal (MIM) Structured Cold Cathode FEDs

The metal–insulator–metal (MIM) structured cold emission cathode was studied as early as 1930s but its application in FEDs was not seen till the 1990s [19, 20, 22]. In 1997, Hitachi developed a MIM cold emission cathode by an anodization process and applied it to the FED fabrication described below. A patterned aluminum film on a glass substrate formed the bottom column row cathode electrodes forming Al2O3 thin film on anodization. A sputtered trilayer-structured Ir–Pt–Au film with a total thickness of 6 nm formed the top row surface electrodes. The anode plate is a phosphor-coated glass with transparent ITO electrodes. > Figure 34 shows the basic structure. When the electric field within the Al2O3 thin film reaches 1 V/nm, the emission current collected by the anode can achieve 5 mA/cm2 and this current density can satisfy the requirement for display applications. The emission ratio, defined as the ratio of emission current to the diode current, is around 0.3%. > Figure 35 shows a schematic representation of the MIM cathode’s band structure for both unbiased and biased conditions.

Vacuum

One way to design a good LMF emitter Protect against poor vacuum

Dielectric film

Get sufficient field enhancement Enhance path conductivity Minimise junction resistance

Substrate

1132

+ Ensure local current limiting (‘ballasting’)

+ Optimise number of emission sites . Fig. 33 Filamentary model of electron emission [11]

Ir-Pt-Au Al2O3

V

Al Glass substrate

. Fig. 34 A schematic diagram of the metal–insulator–metal (MIM) structured cold emission cathode

6.2.4

New Field Emission Technologies

Metal

Insulator

Metal

Vacuum

Metal

Insulator

Ecc Ef2

Ef1 Evv

EVAC Ef1

Vacuum e−

Ec Ev

a

Metal

EVAC Electron energy

Electron energy

EVAC

Ef2

b

. Fig. 35 Generalized energy diagram for MIM structures: (a) no bias applied; (b) with the effect of applied field

Ef1 is the Fermi energy of the bottom electrode metal, and Ef2 the Fermi energy of the top electrode metal. Ec is the conduction band’s minimum of the insulator and Ev is the valence band’s maximum. w = Evac Ec is the affinity of the insulator layer. When a voltage is applied between two conduction metal layers, the electric field within the insulator layer lowers the potential barrier (see > Fig. 35b) for electron tunneling, thereby increasing the tunneling rate. The electrons in the bottom electrode overcome the potential barrier at the interface of the bottom electrode and the insulator and get into the conduction band of the insulator through tunneling effect. During this process, the electrons lose energy by scattering and trapping, but gain kinetic energy by the electric field acceleration. After arriving at the top electrode, electrons with a high kinetic energy can emit into vacuum. Electrons will also be subject to strong scattering within the top electrode and the majority will lose their kinetic energy and contribute to the diode current. Only a small fraction of the electrons can escape from the top electrode so the emission ratio is very low. The transmission behavior of the electrons in the insulator layer can be described by the free drift distance, or distance between scattering. The product of this distance, electric field, and the electron charge is the electron’s average kinetic energy. Obviously the electron emission ratio can be increased by increasing the voltage and the free travel distance. The maximum voltage which can be applied to the top and bottom electrodes is limited by the breakdown field of the insulating layer, so the only feasible approach is to increase the free drift distance. Since Al2O3 is amorphous in structure, the electron’s free drift distance is only around 1 nm, when the electric field approaches 0.5 V/nm, the average energy that the electrons can gain is about 0.5 eV. Further increase in the electric field easily leads to breakdown. The affinity of Al2O3 is 0.8 eV, which is higher than the electron’s average energy, so the low emission ratio is expected. Another reason for the low emission rate is scattering in the top electrode. This 6 nm thick electrode is also amorphous so the electron average free drift distance is far less than 10 nm which is the free drift distance of electron in crystalline gold material. Adopting complex multilayer-structured metal films for the top electrode can increase the device’s stability and emission ratio. With the presence of a strong electric field, metal atoms in the top electrode can get into the insulator layer and this can reduce the voltage-bearing ability and leads to breakdown. Experimental results and theoretical analysis show that metal iridium atoms will not get into the insulator under strong field conditions. Meanwhile the iridium can

1133

1134

6.2.4

New Field Emission Technologies

act as an isolation layer to prevent other kinds of atoms from getting into the insulator. The main purpose in choosing Pt and Au is to lower the effective work functions. There is a 0.5 eV difference in work function between Pt and Au, and when these two metal films are combined and used together, the vacuum energy level can be reduced by the same value and the emission ratio increased substantially. If a material with an even lower work function is used as the top layer for the surface electrode, the vacuum energy level can be further reduced. When considering the high-temperature stability in the sealing process, Au is the best choice. The ideal insulator materials in MIM devices should have a relatively wide band gap, low affinity, high voltage bearing ability and long free drift distance. No materials are available to satisfy all of these requirements, so compromise is needed in choosing a suitable insulator. For example, when the affinity is low, the energy required by the electron to emit is also low and a high emission ratio can be obtained. However, in this situation, electrons will encounter a larger potential barrier at the insulator which leads to a low tunneling probability; therefore a large electric field is needed to achieve the required emission current density. Unfortunately, a strong field can easily cause insulator breakdown. Up to now, only a few wide band dielectric materials such as Al2O2, SiO2, and MgO et al. are used as the insulator materials in MIM-structured emission cathodes. There are, however, many advantages with the MIM-structured FED, such as excellent emission uniformity, strong contamination resistance, and insensitivity to vacuum conditions. The electron beam divergence is very small. For example, when the distance between cathode and anode is 3 mm at an anode voltage of 5,000 V, the lateral divergence is less than 25 mm. This characteristic is crucially important to high-resolution FED applications. The main weakness for MIM cathode is the low emission rate. At the specified emission current density, the diode current is usually too large. For large area display, the excessive conduction current will lead to significant increase in price in driving circuitry. So the MIM cathode–based FED devices developed by Hitachi can only be used in small screen applications. The thickness of the insulator layer is very small and the capacitance between two electrodes very large, so the charging and discharging time will limit the screen size and the total number of image pixels. Simple calculation indicates that when the screen size is 5 in. with 640480 image pixels, the row capacitance will be 0.1 mF. Even for displays without gray scale, the allowable electrode resistance is 10Ω, a value which can be realized, but further increase in screen size or increase in the number of pixels will require even smaller resistances which are inaccessible in a practical fabrication process.

4

Concluding Remarks

The history of FEDs research and development is one of the most interesting in display development, both exciting and distressful. In recent years Pix Tech, Motorola, Candescent, Canon/Toshiba, Sony, Futaba, and Samsung spent substantially more than a billion dollars on the development of medium and large area FEDs. In spite of these efforts, Motorola stopped its industrialization of FEDs, while Pix Tech and Candescent went bankrupt. Sony closed its spinoff company dedicated to nano-Spindt FEDs and Canon withdrew from further activity on SEDs. The small innovative companies that looked for low cost and novel solutions did not survive the economic downturn. Even with these continuous failures in commercialization of FED, it is still too early to judge whether the FED is dead. What we can say at this stage is that the FED technology is still not mature in the sense of both performance and competitiveness in

New Field Emission Technologies

6.2.4

manufacturing price. In fact there is still a lot of work going on at the moment on field emission research. The innovative solutions to the complex problems show that FED can grow and mature. A perfect emissive technology is always just around the corner.

5

Summary

Field emission displays have been difficult to realize and have not yet reached mass markets. A number of limitations have been uncovered and addressed, leading to high-quality demonstrator displays. However, manufacturing these devices has proved to be technically difficult and not economically viable. Modern approaches moved toward planar emitters and include nanotechnology and materials solutions and these have delivered new hope. There is a common history to many aspects of field emission and common challenges in realizing a vacuum microelectronics–based technology that was examined in the previous chapter. This chapter has looked at the new nanotechnology approaches. The advent of carbon nanotube technology saw a dramatic rise in activity in FED, but again, the usual issues limited its success. Other nanotechnology approaches such as surface emitting devices, ballistic emission, printing, and laser processing nanocomposites have been examined here. They all show promise, but to date, no manufacturer has found a way to mass markets. It may be a matter of time, as great strides have been made and there is still a need for a low-cost emissive technology.

References 1. Bonard JM, Salvetat JP, Sto¨ckli T et al (1999) Field emission from carbon nanotubes: perspectives for applications and clues to the emission mechanism. Appl Phys A 69:245–254 2. Bonard JM, Kind H, Stoeckli T, Nilsson LO (2001) Field emission from carbon nanotubes: the first five years. Solid State Electron 45:893–914 3. Milne WI, Teo KBK, Amaratunga GAJ et al (2004) Carbon nanotubes as field emission sources. J Mater Chem 14:933–943 4. Cheng Y, Zhou O (2003) Electron field emission from carbon nanotubes. C R Phys 4:1021–1033 5. Choi WB, Chung DS, Kang JH et al (1999) Fully sealed high – brightness carbon nanotube field emission display. Appl Phys Lett 75:3129–3131 6. Choi YS, Kanga JH, Kima HY, Leea BG, Leea CG (2004) A simple structure and fabrication of carbonnanotube field emission display. Appl Surf Sci 221:370–374 7. Lim SC, Lee K, Lee IH, Lee YH (2007) Field emission and application of carbon nanotubes. NANO Brief Rep Rev 2(2):69–89 8. Teo KBK, Lee SB, Chhowalla M, Semet V, Binh VT, Groening O, Castignolles M, Loiseau A, Pirio G, Legagneux P, Pribat D, Hasko DG, Ahmed H, Amaratunga GAJ, Milne WI (2003) Plasma enhanced chemical vapour deposition carbon

9.

10.

11.

12.

13.

nanotubes/nanofibres – how uniform do they grow? Nanotechnology 14:204–211 Teo KBK, Chhowalla M, Amaratunga GAJ, Milne WI, Pirio G, Legagneux P, Wyczisk F, Olivier J, Pribat D (2002) Characterization of plasma-enhanced chemical vapor deposition carbon nanotubes by auger electron spectroscopy. J Vac Sci Technol B20(1):116–121 Gangloff L, Minoux E, Teo KBK, Vincent P, Semet VT, Binh VT, Yang MH, Bu IYY, Lacerda RG, Pirio G, Schnell JP, Pribat D (2004) Self-aligned, gated arrays of individual nanotube and nanowire emitters. Nano Lett 4(9):1579 Teo KBK, Chhowalla M, Amaratunga GAJ, Milne WI, Legagneux P, Pirio G, Gangloff L, Pribat D, Semet V, Binh VT, Bruenger WH, Eichholz J, Hanssen H, Friedrich D, Lee SB, Hasko DG, Ahmed H (2003) Fabrication and electrical characteristics of carbon nanotube-based microcathodes for use in a parallel electron-beam lithography system. J Vac Sci Technol B 21(2):693–697 Kim JM, Choi WB, Lee NS, Jung JE (2000) Field emission from carbon nanotubes for displays. Diamond Relat Mater 9:1184–1189 Lee NS, Chung DS, Kang JH, Kim HY, Park SH, Jin YW, Choi YS, Han IT, Park NS, Yun MJ, Jun JE, Lee CJ, You JH, Jo SH, Lee CG, Kim JM (2000)

1135

1136

6.2.4 14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

New Field Emission Technologies

Carbon nanotube-based field-emission displays for large-area and full-color application. Jpn J Appl Phys 39:7154–7158 Wang QL, Lei W, Zhang XB, Wang BP, Liu M, Zhou XD, Di YS, Ma XY (2005) A novel gate structure in large diagonal size printable CNT-FED. Appl Surf Sci 239:458–463 Ito F, Tomihari Y, Okada Y, Konuma K, Okamoto A (2001) Carbon-nanotube-based triode-field-emission displays using gated emitter structure. IEEE Electron Device Lett 22(9):426–428 Jung JE, Jin YW, Choi JH, Park YJ, Ko TY, Chung DS, Kim JW, Jang JE, Cha SN, Yi WK, Cho SH, Yoon MJ, Lee CG, You JH, Lee NS, Yoo JB, Kim JM (2002) Fabrication of triode-type field emission displays with high-density carbon-nanotube emitter arrays. Phys B 323:71–77 Jung JE, Choi JH, Park YJ, Lee HW, Jin YW, Chung DS, Park SH, Jang JE, Hwang SY, Ko TY, Choi YS, Cho SH, Lee CG, You JH, Lee NS, Yoo JB, Kime JM (2003) Development of triode-type carbon nanotube field-emitter arrays with suppression of diode emission by forming electroplated Ni wall structure. J Vac Sci Technol B 21(1):375–381 Fentimore AM, Cheng LT, Roach DH (2008) A stable under-gate triode CNT field emitter fabricated via screen printing. Diamond Relat Mater 17: 2005–2009 Yung YJ, Son GH, Park JH, Kim YW, Berdinsky AS, Yoo JB, Park CY (2005) Fabrication and properties of under-gated triode with CNT emitter for flat lamp. Diamond Relat Mater 14:2109–2112 Yung YJ, Park JH, Jeon SY, Alegaonkar PS, Berdinsky AS, Yoo JB, Park CY (2006) Simple fabrication process of a screen-printed triode-CNT field emitter array. Diamond Relat Mater 15:1855–1858 Nomura I, Sakai K, Yamaguchi E, Yamanobe M, Ikeda S (1996) A new emissive display based on surface-conduction electron emitters. In: Proceedings of IDW’96, Kobe, pp 523–526 Yamaguchi E, Sakai K, Nomura I, Ono T, Yamanobe M, Abe N, Hara T, Hatanaka K, Osada Y, Yamamoto H, Nakagiri T (1997) A 10-in. surface-conduction electron-emitter display. J Soc Inf Disp 5(4):345–348 Bezryadin A, Dekker C (1997) Nanofabrication of electrodes with sub-5 nm spacing for transport experiments on single molecules and metal clusters. J Vac Sci Technol B 15(4):793–799 Lee HI, Park SS, Park DI, Ham SH, Lee JH, Lee JH (1998) Nanometer-scaled gap control for low voltage and high current operation of field emission array, J Vac Sci Technol B 16(2):762–764

25. Oguchi T, Yamaguchi E, Sasaki K, Suzuki K, Uzawa S, Hatanaka K (2005) A 36-inch surface-conduction electron-emitter display (SED). SID Dig 32: 1929–1931 26. Lo HY, Li Y, Chao HY, Tsai CH, Pan FM (2007) Field-emission properties of novel palladium nanogaps for surface conduction electron emitters. Nanotechnology 18:475708 27. Komoda T, Honda Y, Hatai T et al (2000) Matrix flat–panel application of ballistic electron surface– emission display. Soc Inf Display Int Symp Dig Tech 31:428–431 28. Nishiguchi K, Zhao X, Oda S (2002) Nanocrystalline silicon electron emitter with a high efficiency enhanced by a planarization technique. J Appl Phys 92(5):2748–2757 29. Nakajima Y, Toyama H, Kojima A et al (2003) A solid-state light-emitting device based on ballistic electron excitation using an inorganic material as a fluorescent film. Phys Stat Sol (a) 197(2):316–320 30. Koshida N, Kojima A, Nakajima Y et al (2003) Application of nanocrystalline silicon and ballistic electron emitter to flat panel display devices. Electrochem Soc Interface 12(2):52–55 31. Komoda T, Koshida N (2009) Nanocrystalline silicon ballistic electron emitter. In: Koshida N (ed) Device applications of silicon nanocrystals and nanostructures – nanostructure science and technology. Springer, New York, pp 251–291 32. Tang Y, Silva S, Rose MJ, Boscovich B, Shannon J (2002) Field emission from laser crystallised amorphous silicon. Appl Phys Lett 80:4154–4156 33. Forrest RD, Cox DC, Tang YF, Shannon JM, Silva SRP (2003) Fabrication of a self aligned microtip field emission array. J Vac Sci Technol B 21:1560–1566 34. Forbes RG (2001) Field induced electron emission from electrically nanostructured heterogeneous (ENH) materials. Ultramicroscopy 89:7–15 35. Forbes RG (2001) Low macroscopic filed emission from carbon films and other electrically nanostructured heterogeneous materials. Solid State Electron 45:779–808 36. Sharpe RG, Palmer RE (1996) Evidence for field emission in electronformed metal-insulator-metal devices. Thin Solid Films 288:164–166 37. Kusunoki T, Suzuki M (2000) Increasing emission current from MIM cathodes by using an Ir-Pt-Au multilayer top electrode. IEEE Trans Electron Devices 47(8):1667–1672 38. Lapicki A, Barstis TLO, Engstrom T (2003) Coldcathode electron emission from nanostructured metal-insulator-metal devices. 41st Aerospace sciences meeting and exhibit. Reno

Part 6.3

Plasma Display Panels

6.3.1 Plasma Display Panels David N. Liu 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1140

2

Basic Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1140

3

Characteristics of Gas Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1142

4

Front and Rear Plate Fabrication Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1144

5

Front Plate Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1144

6

Rear Plate Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145

7

Assembly and Aging Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1147

8

System Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1147

9

Cell Operation Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1148

10 Driving Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1148 11 Conclusions and Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1149

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.3.1, # Springer-Verlag Berlin Heidelberg 2012

1140

6.3.1

Plasma Display Panels

Abstract: An overview of plasma display panel fundamentals is presented. This article begins by describing the physics of a gas discharge. Then the technologies used to form the front plate and the rear plate of a plasma display panel are described. After the assembly of a vacuumsealed plasma display has been introduced, the display cell operation mechanism and the driving techniques for a plasma display panel are discussed. Finally, directions for future research are mentioned. List of Abbreviations: AC, Alternating Current; ADS, Address Display Separation; DC, Direct Current; EMI, Electromagnetic Interference; FED, Field Emission Display; IRI, Infrared Interference; ITO, Indium Tin Oxide; LCD, Liquid Crystal Display; OLED, Organic Light Emitting Display; PDP, Plasma Display Panel

1

Introduction

Flat panel displays have become ubiquitous through their use in computer, communication, and consumer devices. There are many kinds of flat panel displays, including plasma display panels (PDPs), liquid crystal displays (LCDs), organic light emitting displays (OLEDs), and field emission displays (FEDs). An LCD is a nonemissive display which additionally needs a backlight, color filter, polarizer, and other optical components. Although its viewing angle and operating temperature are limited, its mature manufacturing technology, low driving voltage, and long life have allowed the LCD to become the most commonly used flat panel display. An OLED is a kind of solid-state display that has no need for a vacuum, gas, or liquid inside the panel. It has high optical efficiency and high picture quality although it has a relatively high manufacturing cost and it is difficult to scale up. An FED generally has a cathode ray tube (CRT)-like quality with flat characteristics. It has high optical efficiency, although its major drawbacks are poor emission uniformity and the short life of color application. The PDP [1], [2] is a gas-discharge display with high picture quality, relatively simple manufacture, and easy scale-up, which has enabled it to become one of the major choices for large display applications. Its major drawbacks are the high voltage driving and the large pixel size. The benchmarks for these various display types are shown in > Table 1. The operation mechanism used in a PDP is similar to that of a fluorescent lamp; however, the gases most commonly used in PDPs are neon and xenon rather than argon and mercury as in fluorescent lamps. The peak UV wavelengths in PDPs are 147 and 173 nm. These wavelengths are generally called vacuum UV, can only propagate in a vacuum, and are strongly absorbed by normal air. That is one of the reasons that a PDP needs to evacuate out normal gas before filling in the display gas. This process ensures vacuum UV can effectively reach phosphor and excite it. Although a PDP cannot have very small pixel size and its operating voltage is high, it has a wider viewing angle, faster response time, and wider temperature range than an LCD. Therefore, a PDP is a good candidate for large panel displays because it is effective for static pictures and motion pictures, from cold temperature to hot temperature, and for personal and public use. Moreover, a PDP can be fabricated at low cost and with a simple manufacturing process.

2

Basic Structure

A direct current (DC) PDP and an alternating current (AC) PDP are the two major types. A DC PDP has the advantage of simplicity, whereas an AC PDP has the advantage of a longer

Plasma Display Panels

6.3.1

. Table 1 Benchmarks of typical flat panel displays

PDP

Advantage

Disadvantage

Application size

Easy to scale-up

High driving voltage

40–100 in.

High picture quality

Difficult for small pixel size

Relatively simple process LCD

Mature manufacturing technology

Complex components (backlight, color filter, 100 in. or less polarizer)

Low driving voltage

Limited viewing angle and operating temperature

Long life

Relatively long response time OLED High optical efficiency High picture quality

Relatively high manufacturing cost

40 in. or less

Not easy to scale up

Relatively simple process Low driving voltage FED

High picture quality

Poor emission uniformity

High optical efficiency

Short lifetime for color FED

100 in. or less

PDP plasma display panel, LCD liquid crystal display, OLED organic light emitting display, FED field emission display

operating lifetime. In a DC PDP, two electrodes are directly exposed to the gas, so it can be operated by DC mode. The major drawback of this mode of operation is the relatively short operating lifetime since the plasma directly bombards the electrodes and the phosphor. In contrast, an AC PDP uses an AC, so a dielectric layer is needed. Additionally, a protection layer can be desirably deposited to provide protection from the plasma bombardment and therefore the operating lifetime is increased. The AC PDP is the most popular type of PDP. > Figure 1 presents the structure of one elementary cell of an AC PDP for surface discharge which is commonly adopted. The rib height is typically 100 mm and the gas pressure inside the cell is 500 Torr. Plasma is generated by an AC and is successfully maintained on the surface of the upper plate while UV radiation is able to excite the phosphor, such that it does not damage the phosphor that is in the lower plate. Accordingly, the display life is extended. The barrier rib fabrication process is one of the most important processes in PDP formation. The barrier rib serves not only to maintain the space between the front plate and the rear plate [3] but also to prevent cross talk among cells. Moreover, the side walls of the barrier rib provide an additional surface on which phosphor is deposited, increasing the area of phosphor. This increased area of phosphor increases the brightness. Red, green, and blue phosphors are used to produce red, green, and blue colors [4]. The phosphor brightness will be slightly reduced when a PDP is operating because plasma ions sputter phosphor and result in phosphor degradation. Outgas from the barrier rib and the other layers of the device may contaminate the phosphor and result in further phosphor degradation. A protection layer is used to offer high resistance against ion bombardment to increase the operating lifetime. In addition, this protection layer has to provide higher secondary electron emission so the operating voltage can be reduced [5]. Furthermore, the high transparency of this layer is also important to ensure high light output. The dielectric layer is required for

1141

1142

6.3.1

Plasma Display Panels

Sustain auxiliary electrode

Sustain electrode Light

Substrate Front plate

Dielectric

Plasma

Light UV

Protection layer UV

Barrier rib

Rear plate Dielectric substrate Red phosphor

Address electrode

Blue phosphor

Green phosphor

. Fig. 1 Structures of an elementary cell of a surface-discharge alternating current plasma display panel (AC PDP)

AC operation and provides capacitance, whereas the electrodes provide the energy to discharge the gas. The dielectric layer in the front plate must have a smooth surface and be bubble-free so that the film surface during the protection layer fabrication process is smooth. Additionally, this layer must have high transparency and be an effective insulator so that the panel can output more light with a lower leakage current [6]. Unlike the dielectric in the front plate, this dielectric layer in the rear plate is not required to be transparent. The electrode in the front plate acts as a sustain electrode in the sustain period of driving operation [7] and its geometry is optimized to have the maximum open ratio and light out. It should be transparent so as not to block the emitted light. Because the conductivity of the typically transparent electrode is not as high as that of a typical metal, an auxiliary electrode or bus electrode with a relatively small line width is affixed to the transparent electrode so the combined electrode conductivity is increased while keeping the open ratio high. The electrode in the rear plate acts as an address electrode in the address period of driving operation. Accordingly, this electrode is called the address electrode.

3

Characteristics of Gas Discharge

When a voltage is applied to a gas that is insufficient to ionize individual atoms, the atoms are excited to higher levels. This reaction is called excitation. An excited atom can decay to a metastable state by the emission of radiation. This metastable atom is an excited atom with a longer life than that of a typically excited atom. This reaction is called metastable atom generation. If a voltage greater than a certain voltage threshold is applied to the gas, then the atom is ionized. This reaction is called ionization. A gaseous mixture is used to provide more ionization at the applied voltage [8] because a metastable atom is likely to be ionized into

Plasma Display Panels

6.3.1

another species of gas atom. This gas mixture is called a Penning mixture and the mechanism is called a Penning reaction. The Penning mechanism uses energetic particles to accelerate discharge and to reduce the firing voltage. Moreover, the firing voltage is related to the product of the gas pressure and the distance between the cathode and the anode. Many gas discharges have similar characteristics. > Figure 2 shows a typical current–voltage plot for a gas discharge [9]. In the low-voltage regime, the current is small and increases slightly with voltage. As the applied voltage reaches a particular value, gas discharge is initiated. This voltage is called the firing voltage, and is usually more than 100 V. As the applied voltage continues to increase, the PDP voltage remains constant until glowing occurs, but the current increases markedly with the applied voltage. This regime is called Townsend discharge. If the current is not limited, the discharge will naturally evolve to a glowing regime (subnormal/ normal/abnormal glowing regime) which can be sustained for a lower voltage than the firing voltage. This voltage is called the sustaining voltage. In other words, the glowing stops when the voltage is reduced to less than the sustaining voltage. In the glowing regime, the main source of electrons is no longer the direct ionization but is the secondary emission due to ion bombardment of the cathode. When glowing has occurred in the glowing regimes, the voltage of the PDP drops in the subnormal glow stage, stabilizes in the normal glow stage, and increases in the abnormal stage. This normal glowing regime is the regime in which a PDP operated. If the current is not limited and is continuously increased, an arc occurs.

I (current) Arc

Abnormal glow

Normal glow Subnormal glow

Townsend discharge

Small current regime

Sustain voltage

Firing voltage V (voltage)

. Fig. 2 I–V characteristics of gas discharge

1143

1144

6.3.1 4

Plasma Display Panels

Front and Rear Plate Fabrication Processes

The PDP consists of a front plate and a rear plate, and fabrication includes assembly and aging processes. The front plate and the rear plate can be processed separately at the beginning. After these plates have been fabricated, assembly and aging processes are initiated. Screen printing [10] and photolithography are the two major approaches in both the front plate and the rear plate of the PDP fabrication process. A screen mask, paste, and a printing machine (including squeezing) are three major components for screen printing [11]. The ideal paste can easily pass through a screen mask when a shear force is applied to the paste. However, the paste of the pattern becomes solid and remains still when no shear force is applied and so does not diffuse and the original size of the pattern is maintained. After the paste has been deposited, drying and firing processes are required. Drying removes the solvent, with the process temperature typically under 150 C. The critical aspect of the drying process is the uniformity of drying from the outer surface to the inner core and from the edge to the center of the paste layer. The firing process is used to remove the binder from the paste and to melt the particles. The process temperature typically exceeds 300 C in the debinding process and 500 C in the firing process. The critical parts of the firing process are complete removal of the binder and reduction of permanent deformation of the substrate in the cooling stage. To reduce permanent deformation of the substrate that occurs during the cooling stage, step cooling in the cooling stage is used at the beginning of the cooling stage to reduce the stress. In addition to screen printing, photolithography is used commonly in display and semiconductor processes. It uses a photoresist to produce a pattern. This pattern of the photoresist is used to etch the desired material, which is deposited before photoresist formation. After the material has been etched, the photoresist is stripped off and a desired pattern of a material layer is formed. Soda lime glass is a commonly used PDP substrate [12]. The thickness of such a substrate is typically 2.8 mm. The major issue concerning the PDP substrate is its thermal expansion, which is associated with the high temperature of the pastes (dielectric layer, barrier rib layer, phosphor layer) firing process used in front and rear plate formation [13]. The thermal expansion may cause an error in the image position [14]. Floating of soda lime glass is commonly adopted; this is a traditional process in glass formation and is cost-effective. The substrate in the rear plate typically requires an additional exhaust hole for pumping out normal air from the panel and pumping discharge gas into the panel.

5

Front Plate Techniques

The front plate comprises substrate, electrode, dielectric, and protection layers. The major functions of this plate are to provide gas discharge and display an image. > Figure 3 presents a typical fabrication process for a front plate. At the beginning of the process, the sustain electrode and the sustain auxiliary electrode are deposited and patterned on the front glass, followed by deposition of the dielectric. The last step of the front plate fabrication is the formation of the protection layer. The electrode in the front panel is called a discharge or a sustain/sustain auxiliary electrode, which provides the energy to discharge the gas and to sustain the discharge. Indium tin oxide (ITO) and SnO are the materials that are commonly used in the transparent electrode. Since the conductivity of ITO is not as high as that of a typical metal, copper is commonly used as the

Plasma Display Panels

6.3.1

Front glass Front glass in

Sustain electrode Sustain electrode formation Sustain auxiliary electrode Sustain auxiliary electrode formation Dielectric layer Dielectric layer formation Protection layer Protection layer formation

. Fig. 3 Fabrication of the front plate

conductive metal to increase ITO conductivity. Since copper does not easily adhere, a chromium layer is used to promote the adhesion before and after the copper layer has been formed. This metal electrode (Cr/Cu/Cr) on the ITO electrode is called the auxiliary electrode or the bus electrode [15]. The electrode is usually formed by a photolithography approach. Silica paste is commonly used in the dielectric layer. Screen printing is the typical process used to fabricate this layer. As the protection layer material, various materials such as CeO2, La2O3, and MgO are used [16]. Among these materials, MgO is the most commonly used because it not only is a high-temperature refractory material with high secondary electron emission and high transparency but it can also can endure ion bombardment. This protection layer is typically formed by an evaporation process.

6

Rear Plate Techniques

The rear plate comprises substrate, electrode, dielectric, barrier rib, and phosphor layers. The major functions of the rear plate are to provide a gas discharge and generate light. > Figure 4 presents a typical fabrication process for a rear plate. At the beginning of the process, the address electrode is deposited and patterned on the rear glass, followed by deposition of the dielectric. Finally, the barrier rib and the phosphor are formed and patterned. Silver paste is commonly used in the electrode, which is typically fabricated by screen printing or a photosensitive-paste approach. The photosensitive-paste approach can achieve a line width as small as 20 mm, whereas screen printing can only achieve 50 mm. However, screen printing remains a common approach for forming an address electrode because of its lower material cost and simpler process steps [17]. In practical use, the dielectric layer material is silica paste and the dielectric layer is typically formed by screen printing.

1145

1146

6.3.1

Plasma Display Panels

Rear glass Rear glass in

Address electrode Address electrode formation Dielectric layer Dielectric layer formation Barrier rib Barrier rib formation Green phosphor Red phosphor Blue phosphor Phosphor layer formation

. Fig. 4 Fabrication of the rear plate

To reduce the difference between the gas pressure inside and outside the panel, the gas pressure of the panel does not exceed 1 atm (760 Torr) and is typically set to around 500 Torr. At this gas pressure, the rib height is about 150 mm for a typical gas mixture of neon and xenon, so a low firing voltage can be achieved. In addition, the barrier rib should be as thin as possible to make the aperture ratio larger or the resolution higher of a display cell [18]. The barrier rib material is typically silica paste and the barrier rib is typically formed by screen printing, press molding, or sandblasting. Screen printing is very cost-effective but the process time is relatively long owing to the need for multiple screen printings. Additionally, each layer interface between each printing is also critical in screen printing [19]. Press molding has a shorter process time and high position accuracy but suffers from high production cost. Sandblasting also has a short process time and high position accuracy. Although the production cost is higher than for screen printing, the comprehensive performances of sandblasting in terms of position accuracy, process time, and rib-shape control are better than those of screen printing and press molding [20]. Unlike in a CRT, in which phosphor is excited by electrons, in a PDP, the phosphors are UV-excited [21] and are usually deposited by screen printing. The compositions of PDP red, green, and blue phosphors are Y0.65Gd0.35BO3:Eu, BaAl12O19:Mn, and BaMgAl14O23:Eu, respectively [22]. The decay times of these red, green, and blue phosphors are 9 ms, 17 ms, and less than 1 ms, respectively. Since the decay time of green phosphor, BaAl12O19:Mn, is a little longer than the decay times of red and blue phosphors, a new green phosphor of Zn2SiO4:Mn with a decay time of less than 14 ms has been developed. The degradation of the blue phosphor, BaMgAl14O23:Eu, is caused by oxidation of europium during the panel fabrication process [23, 24, 25]. In addition to the decay time of the green phosphors and the degradation of the blue phosphor, the color purity of the red phosphor, Y0.65Gd0.35BO3:Eu, is insufficient [26, 27]. These phosphors need to be further improved.

Plasma Display Panels

7

6.3.1

Assembly and Aging Techniques

Assembly involves sealing layer formation, panel alignment, sealing, and gas filling. In the assembly process, the front plate and the rear plate are bound into a display panel with precise alignment, vacuum-tight sealing, and a cleaned cell [28]. Following assembly, an aging process is required to expose defects and stabilize the quality of the display [29]. At the beginning of the assembly process, a sealing layer is deposited onto the surrounding area of the rear plate. Glass frits or glass powder [30] is the typical sealing layer material used in a PDP. It is used to perform vacuum-tight sealing and does not outgas after the sealing process. Additionally, this material must also have a low melting point, and its thermal expansion must be compatible with that of a glass substrate. After the sealing layer has been deposited, the front and rear plates are aligned and then sealed. The major challenge in the alignment is a shift during high-temperature sealing. The front plate and the rear plate are typically clamped and fixed using clippers so that they do not shift during sealing. The sealing process melts the material of the sealing layer so it binds the front plate to the rear plate permanently [31]. The gas given off during the sealing process can contaminate the surface and protection layers of the display cell. Because of the thermal characteristics of the glass frits and glass powder, the sealing is typically performed at a high temperature of typically 450 C. After sealing is completed, evacuating the panel is the next step. Since impurities of H2, O2, N2, CO2, and CO can increase the operating voltage and decrease the brightness of the panel, a chemical getter is usually used to assist the vacuum pump and adsorb the impurities such as H2, O2, N2, CO2, and CO [32, 33]. After a vacuum of 10 7 Torr has been reached, the display cell is filled with the purge gas, which is then evacuated to clean the display cell. After the purging has been completed, the display cell is filled with display gas and the exhaust tube is tipped off. Neon and xenon are the gases that are typically used as display gases. The amount of each gas as a percentage of the total gas mixture pressure is very important since it determines the UV intensity, the required discharge voltage, and the discharge efficiency [34, 35]. Tipping off is the process of cutting off and isolating the inside and outside of the panel using a tip end. A preliminary tip-off process is typically needed to evacuate the desorbed contaminants so that only very little given-off gas is left inside the panel. Finally, aging is performed. Defects or contamination on the MgO surface, dielectric, and electrode are revealed during the aging process [36]. A defect of an electrode can be open, short, or anything in between. Additionally, aging can stabilize the operating voltage and reduce the operating voltage since it can polish or smoothen the MgO surface, move surface contamination from MgO at the discharge site, adsorb some gases onto the MgO surface, and emit some gases from the MgO surface [37, 38, 39].

8

System Techniques

The basic circuits for an AC PDP consist of a power supply circuit, a signal processing circuit, and a scan/data driving circuit. A PDP is driven by a scan/data driving circuit, the signal processing circuit provides a video signal to the panel, and the power supply circuit provides power to the whole system. In addition to the basic circuits and the driving function, an energy recovery circuit is also usually used in an AC PDP to collect and reuse the energy so PDP power can be saved. [40], [41] In this circuit, a capacitor is used to store the energy and an inductor is used for conduction and to protect the circuit during AC operation. When this circuit is used, energy can be effectively collected and reused.

1147

1148

6.3.1

Plasma Display Panels

There are several issues regarding PDPs which need to be solved, such as false contours, image sticking, electromagnetic interference (EMI), and infrared interference (IRI). A false contour possibly arises between adjacent pixels when a moving picture is displayed [42, 43, 44]. Image sticking can occur when a static image is displayed for up to tens of minutes [45, 46]. These false contour and image sticking effects influence the image quality in a PDP. Many approaches have been proposed to reduce the false contours [47], [48] and the image sticking [49]. An electromagnetic wave is associated with the PDP electronic circuits and the PDP and an infrared wave is typically generated from the red phosphor of the PDP. The EMI and IRI effects possibly interfere with remote controls for other appliances when a PDP is used. To reduce these EMI and IRI effects, additional filter glass is usually required to provide EMI shielding and IRI blocking [50].

9

Cell Operation Mechanism

To drive a PDP cell, it is important to understand the cell operation mechanism. For a PDP display cell, when the first applied voltage of each frame is less than the firing voltage, the cell cannot discharge even when the applied voltage exceeds the sustaining voltage. When a voltage above the firing voltage is applied, the gas in the cell is discharged into positive ions and negative ions. The dielectric layer covering the electrodes is then charged with the electrons and ions of the plasma. This stored charge leads to a voltage with opposite polarity in comparison with the applied voltage, so the discharge will rapidly stop (typically after 20 ns). This capacitive voltage is crucial for the following sustain period of the cell operation. Indeed in the next half cycle of the applied AC signal, the capacitive voltage will be added to the applied voltage so that firing conditions can be reached with a lower applied voltage: the sustaining voltage. These positive ions and negative ions recombine and generate UV radiation. Once the gas has been discharged, the applied voltage need not exceed the firing voltage but need only exceed the sustaining voltage. However, the electrical polarity must alternate so that the discharge can continue. To proceed to the end stage of each frame, the cell must erase the discharge using a low voltage with polarity opposite that of the existing electricity. The discharge is then gradually reduced. Eventually, the discharge in the cell is completely erased [51].

10

Driving Techniques

Although many modulation approaches such as address display separation (ADS) and address with display can be used in a PDP [52]. ADS is the most used approach owing to its relatively simple waveform and circuit. In the ADS approach, each field has many subfields and the desired gray levels govern the numbers of subfields. The driving waveform of each subfield comprises an address period and a sustain period, which occur sequentially, as presented in > Fig. 5 [53]. The lengths of the address periods are the same, whereas the lengths of the sustain periods differ among the subfields. A typical address period comprises erasing, priming, erasing, and writing subperiods [54]. The first erasing is to clean all data in the display. Then, priming with a higher voltage is used to generate a discharge. When priming is activated, energetic particles are generated and help to accelerate discharge and reduce the firing voltage. The priming

Plasma Display Panels

6.3.1

Sub-field (SF) Sustain period

Address period

Erasing

Erasing

Writing

Priming Address electrode (Rear plate electrode)

Address pulse

Sustain Scan pulse electrode 1 (Front plate electrode 1) Sustain

Scan Line 1, 2, 3,…. Sustain pulse

electrode 2 (Front plate electrode 2)

. Fig. 5 Driving waveform of an AC PDP

function is typically designed at the beginning of the driving waveform so that it can cause gas discharge and generate plasma before the writing period. Although the discharge stops after the priming period, residual ions remain present in the panel. These residual ions reduce the required duration in the writing period. This mechanism offers a great advantage in high-speed addressing operations of PDPs. After priming has been done, erasing is performed again. These three actions are designed to have clean data with residual ions of the panel and provide a condition of shorter required writing time for the following writing subperiod. The length of the writing subperiod is determined by the time required for each scan line and the number of scan lines in the display panel. For a writing time of 2 ms for each scan line and for the video graphics array format of the display panel with a scan line number of 480, the writing subperiod is around 1 ms for each subfield. Since the gray levels are determined by the length of the sustain period, various sustain periods in each subfield generate various gray levels. The sustain periods in the subfields are typically arranged as 1, 2, 4, 8, 16, 32, 64, and 128 units of time so gray levels from 0 to 255 (256 gray levels) can be achieved.

11

Conclusions and Directions for Future Research

An AC PDP with surface discharge is commonly adopted in PDP formation owing to its longer device life. In the device, plasma is generated by an AC and is successfully maintained on the surface of the upper plate while UV radiation remains is to excite phosphor. This cell structure of a display cell provides better protection of phosphor from damage, so the life of display panel is extended. ADS is the most used approach for driving owing to its relatively simple waveform

1149

1150

6.3.1

Plasma Display Panels

and circuit. In the ADS approach, the driving waveform comprises a sequential address period and sustain period, whereas the display cell is selected during the address period and is displayed in the sustain period. Although the brightness and lifetime have been improved significantly over the last few decades, reduction of the driving voltage and the effects of false contours and image sticking still need to be researched further. Additionally, reduction of the decay time of the green phosphors and the degradation of the blue phosphor as well as improvement of the color purity of the red phosphor are also future goals for the phosphors.

References 1. Oversluizen G, Dekker T (2006) ‘‘High Efficacy PDP Design,’’ SID 06 DIGEST, 1110 2. Sano Y, Nakamura T, Numomura K, Konishi T, Usui M, Tanaka A, Yoshida T, Yamada H, Oida O, Fujimura R (1998) High-contrast 50-in color ac plasma display with 1365  768 pixels. SID 98 DIGEST, 275 3. Whang KW, Bae HS, Lee KH, Kim TJ (2005) The effect of cell geometry and plasma loss on the luminous efficiency in ac plasma display panel. SOD 05 DIGEST, 1130 4. Yacobi BG, Holt DB (1994) Luminescence phenomena, Cathodoluminescence microscopy of inorganic solids. Plenum, New York/London, p 21 5. Andoh S, Murase K, Umeda S (1976) Discharge-time lag in a plasma display-selection of protection layer (g Surface). IEEE Trans Electron Devices 23(3):319 6. Yokoe M, Ohno S, Shenda S, Nakayama K (2001) Firing of dielectric layer in vacuum for high transmittance, Asia Display’01, p 805 7. Choi KC, Cho KH, Lee SM, Jang C, Mun JH, Kim SH (2007) High efficient discharge mode in an AC PDP with an auxiliary electrode. SID 07 DIGEST, 1530 8. Kruithof AA, Penning FM (1937) Determination of the townsend ionization coefficient a for mixtures of neon and argon. Physica 4:430 9. Weber LF (1985) Plasma displays. In: Tannas LE Jr (ed) Flat-panel displays and CRTs. Van Nortrand Reinhold, New York, p 332 10. Reimer DE (1988) Analytical engineering model of the screen printing process: Part I. Solid State Technology/August, p 107 11. Sakamoto S, Ogawa Y (1995) Screen printing for fabrication of PDPs. IDW’95, p 41 12. Dumbaugh WH (1992) Status and future directions of flat-panel-display substrates. SID 92 DIGEST, 805 13. Maeda K, Nishizawa M, Nakashima T, Nakao Y (1997) Thermal compaction of PDP glass substrates. SID 97 DIGEST, 544 14. Ford PW, Veyhl EW (1993) Image position errors due to plate bending. SID 93 DIGEST, 983

15. Tachibana K (2006) Design and performance of AC-PDP cells with auxiliary electrode structures. SID 06 DIGEST, 1205 16. Urade T, Iemori T, Osawa M, Nakayama N, Morita I (1976) A protecting layer for the dielectric in AC plasma panels. IEEE Trans Electron Devices 23(3):313 17. Hayakawa H, Sakuda K, Kuwada R, Mutoh T, Sato A, Suess TR, Taylor BE, Smith JD (1997) Photoimageable thick film black conductor system for FPD, IDW’97, 547 18. Ryu SM, Han M, Yang DY, Lee SS, Kim DJ, Park LS (2007) Ultra-slim barrier ribs for plasma display panel by X-ray lithography process, SID 07 DIGEST, 1205 19. Fischer-Cripps AC, Collins RE, Turner GM, Bezzel E (1995) Stress and fracture probability in evacuated glazing. Building Environ 30(1):41 20. Fujii H, Tanabe H, Ishiga H, Harayama M, Oka M (1992) A sandblasting process for fabrication of color PDP phosphor screens, SID 92 DIGEST, p 728 21. Vecht A (1994) Advances in phosphor materials for display application, IDRC’94, p 86 22. Shionoya S (1995) Luminescence mechanism of phosphors for displays, IDW’95, p 63 23. Kajiyama H, Tanno H, Shinoda T, Fukasawa T, Ramasamy R, Shanmugavelayutham G, Yasuda T (2007) Lifetime improvement of Eu-doped BAM by plasma treatment, SID 07 DIGEST, p 1321 24. Ushirozawa M (2000) Luminance degradation of blue phosphor BaMgAl10O17:Eu for PDP, SID 00 DIGEST, p 224 25. Zhang S (2005) Recent development of blue phosphors for PDP application, SID 05 DIGEST, p 1142 26. Tanner H, Vecht A, Smith DW, Gibbons CS, Charlesworth D (1995) High-resolution phosphors: characterization and assessment, SID 95 DIGEST, p 623 27. Greer JA, Vanhook HJ, Nguyen HQ, Tabat MD, Gammie G (1994) Thin-film phosphors prepared by pulsed-laser deposition, SID 94 DIGEST, p 827

Plasma Display Panels 28. Reisman A (1978) Single-cycle gas panel assembly. IBM Res Dev 22(6):596 29. Moine B, Bizarri G (2007) Aging processes of the blue phosphor in plasma display panels, SID 07 DIGEST, 1317 (2007). 30. Roth A (1966) ‘‘Permanent seals’’, vacuum sealing techniques. Pergamon, Oxford, p 23 31. Alpha JW (1976) Glass sealing technology for displays. Opt. Laser Technol., December, 259 32. Zeng SQ, Hunt A, Greif R (1995) Mean free path and apparent thermal conductivity of a gas in a porous medium. Trans. ASME 117(August):758 33. Clugston DA, Collins RE (1994) Pump down of evacuated glazing. J Vac Sci Technol A 12(1):241 34. Jang SK, Tae HS, Jung EY, Suh KJ, Ahn JC, Heo EG, Lee BH (2007) Influence of He contents on reset and address discharge characteristics under variable panel temperature in AC PDPs, SID 07 DIGEST, p 1629 35. Oversluizen G, de Zwart S, Dekker T, Gillies MF (2002) The route towards a high efficacy PDP; Influence of driving condition, Xe partial pressure, and cell design, SID 02 DIGEST, p 848 36. Park MS, Park DH, Kim BH, Ryu BG, Kim ST, Seo GW, Kim DY, Park ST, Kim JB (2006) Effect of aging discharge on the MgO protective layer of AC-plasma display panel, SID 06 DIGEST, p 1399 37. Pleshko P (1981) AC plasma display aging model and lifetime calculations. IEEE Trans Electron Devices ED-28(6):654 38. Byrum BW Jr (1975) Surface aging mechanisms of AC plasma display panels. IEEE Trans Electron Devices ED-22(9):685 39. Aboelfotoh MO (1981) Aging characteristics of AC plasma display panels. IEEE Trans Electron Devices ED-28(6):645 40. Mas C, Troussel G, Benoit E (2000) A new IC for generating AC power supply, SID 00 DIGEST, p 216 41. Choi JP, Kim TH, Kim HY, Myoung DJ, Lim K, Park MH (2001) Development of new energy recovery driving method for column PDP addressing, SID 01 DIGEST, p 1232 42. Mikoshiba S (1995) Picture quality issues for color plasma displays, IDW’95, p 57

6.3.1

43. Koura T, Yamamoto T, Ishii K, Takano Y, Kokubun H, Kurita T, Kobayashi K, Murakami H, Yamaguchi K (1998) Evaluation of moving-picture quality on 42-in PDP, SID 98 DIGEST, p 620 44. Yamaguchi T, Matsuda T, Kohgami A, Mikoshiba S (1996) Degradation of moving quality in PDPs: dynamic false contours. J. SID 4/4:263 45. Tae HS, Park CS, Kwon YK, Heo EG, Lee BH (2007) Solution to boundary image sticking in AC plasma display panel, SID 07 DIGEST, p 1617 46. Park CS, Tae HS, Kwon YK, Seo SB, Heo EG, Lee BH, Lee KS (2006) Experimental study on halo-type boundary image sticking in 42-in AC plasma display panel, SID 06 DIGEST, p 1213 47. Zhu YW, Toda K, Yamaguchi T, Shiga T, Mikoshiba S (1997) A method-dependent equalizing-pulse technique for reducing gray-scale disturbances on PDPs. SID 97 DIGEST, p 221 48. Ryeom J, Kim SW, Roh YB, Park CB (1998) An image data rearranged sub-field method for reducing dynamic false contours in PDPs. IDW’98, p 547 49. Lee HJ, Kim DH, Kim YR, Hahm MS, Lee DK, Choi JY, Park CH, Rhyu JW, Kim JY, Park CH (2004) Analysis of temporal image sticking in ACPDP and the methods to reduce it. SID 04 DIGEST, p 214 50. Zagdoun G, Heitz T, Talpaert X (2004) New type of optical filter for PDP TV with improved durability. SID 04 DIGEST, p 918 51. Hirakawa H, Katayama T, Kuroki S, Nakahara H, Nanto T, Yoshikawa K, Otsuka A, Wakitani M (1998) Cell structure and driving method of a 25-in. (64-cm) diagonal high-resolution color AC plasma display. SID 98 DIGEST, p 279 52. Uchidoi M, Saegusa N, Sato Y, Okano T (1996) Panel design and driving method for 40-in. diagonal ac plasma displays. IDW’96, p 291 53. Kojima T, Toyonaga R, Sakai T, Tajima T, Sega S, Kuriyama T, Koike J, Murakami H (1979) Sixteeninch gas-discharge display panel with 2-lines-at-atime-driving. Proc SID 20:153 54. Mikoshiba S (1999) Advancements in plasma panels. Information Display 2/99:28

Further Reading Jensen KL (2000) Electron-emissive materials, vacuum microelectronics and flat-panel displays. Material Research Society, PA Castellano JA (1992) Handbook of display technology. Academic, London Matsumoto S (1990) Electronic display devices. Wiley, Chichester

Refioglu HI (1983) Electronic displays. IEEE Press, New York Geng H (2005) Semiconductor manufacturing handbook. McGraw-Hill, New York MacDonald LW, Lowe AC (1997) Display system: design and applications. Wiley-SID, New York

1151

Part 6.4

Light Emitting Diode (LED) Displays

6.4.1 Light Emitting Diodes: Fundamentals M. R. Krames 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156

2 2.1 2.2 2.3

LED Principles and Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 Semiconductor Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 p-n Junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1159 LED Fabrication and Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1159

3 3.1 3.2 3.3 3.4

LED Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1161 Internal Quantum Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1161 Light Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1162 Monochromatic LED Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1162 Down-Conversion for White-Emitting LEDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1164

4

Summary and Directions for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1166

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.4.1, # Springer-Verlag Berlin Heidelberg 2012

1156

6.4.1

Light Emitting Diodes: Fundamentals

Abstract: Light-emitting diode (LED) fundamentals are reviewed with an emphasis on the two dominant material systems for solid-state lighting applications, aluminum-galliumindium-phosphide (Al,Ga,In)P and aluminum-gallium-indium-nitride (Al,Ga,In)N. Emphasis is placed on high-power (e.g., 1-Watt class or higher) LED chip and packaging technology, which currently drives the adoption of LEDs for general lighting and large-area liquid-crystal display (LCD) applications. State-of-the-art monochromatic performance is reviewed, as well as that of white-emitting devices employing down-conversion materials (e.g., phosphors) pumped by III-nitride LED chips, the most common method of providing solid-state white light for both general illumination and LCD monitors and televisions. List of Abbreviations: AlGaInN, Aluminum-Gallium-Indium-Nitride; AlGaInP, AluminumGallium-Indium-Phosphide; AlN, Aluminum Nitride; AlP, Aluminum Phosphide; CRI, ColorRendering Index; FL, Fluorescent Lamp; GaAs, Gallium-Arsenide; GaAsP, Gallium-ArsenidePhosphide; GaN, Gallium-Nitride; GaP, Gallium Phosphide; HID, High-Intensity-Discharge Lamp; HVPE, Hydride Vapor Phase Epitaxy; InGaN, Indium-Gallium-Nitride; InN, IndiumNitride; InP, Indium-Phosphide; LED, Light-Emitting Diode; LCD, Liquid-Crystal Display; MOCVD, Metal-Organic Chemical Vapor Deposition; MBE, Molecular Beam Epitaxy; PC, Phosphor Converted; QD, Quantum Dot; SiC, Silicon Carbide; UV, Ultra Violet; YAG, Yttrium Aluminum Oxide Garnet Phosphor; W, Tungsten Lamp; W-H, Tungsten-Halogen Lamp

1

Introduction

While experimental observations of visible-spectrum electro-luminescence occurred sporadically in the early twentieth century, it was not until the demonstration of red emission from compound semiconductors that the modern era of light-emitting diodes (LEDs) began [1]. Shortly thereafter, in the late 1960s, red-emitting LEDs based on gallium-arsenide-phosphide (GaAsP) became commercially viable and drove display innovation in emerging applications such as electronic calculators and digital watches. The subsequent development of new compound-semiconductor material systems led to an increased range of emission wavelengths, and also increased efficiencies. By the early 1990s, the aluminum-gallium-indium-phosphide (Al,Ga,In)P system, lattice-matched to widely available GaAs substrates, provided red-, orange-, and amber-emitting LEDs that outperformed filtered incandescent tungsten lamps [2]. Meanwhile, advancements in Japan [3, 4] had led to efficient light emission from indium-gallium-nitride (InGaN) layers grown on gallium-nitride (GaN) deposited on sapphire substrates [5]. Subsequent developments resulted in violet-emitting laser diodes (now the basis for BluRay™ technology) [6] and blue-, green-, and even yellowemitting LEDs [7]. Combined with broadband-emitting phosphors, the violet- and blue-emitting LEDs were exploited to provide white-emitting LEDs. The most common approach has been to combine a blue-emitting LED with Y3Al5O12:Ce3+, a broad-emitting yellow phosphor (see > Chap. 6.1.2). The combined ‘‘YAG’’ phosphor emission and residual blue light intercepts the blackbody curve in chromaticity space (see > Chap. 2.4.1) within the 4,000–8,000 K regime (depending on blue emission wavelength and detailed phosphor composition and doping level) and provides a relatively simple, high efficacy, white light source [8]. Indeed, white-emitting LEDs based on such technology reach efficacies that exceed all conventional light sources of similar light quality [9]. In addition, more sophisticated phosphor combinations can result in efficient LEDs in the ‘‘warm’’ color temperature regimes

Light Emitting Diodes: Fundamentals

6.4.1

1,000 map

Luminous efficacy (lm/W)

oad

InGaN-based PC White Shaped / Textured AlGaInP

100

ER . DO U.S

HID FL

AlGaInP / GaP

W-H W

AlGaInP / GaAs

10

AlGaAs / AlGaAs InGaN AlGaAs / GaAs GaAsP:N

1

GaP:Zn,O

InGaN

GaAsP

0.1 1960

1970

1980

1990

2000

2010

2020

. Fig. 1 Historical performance progress in terms of luminous efficacy for red-, blue-, green-, and white-emitting commercially available LEDs. Labels refer to the prevailing materials technologies. Performance prognosis for ‘‘cool white’’ LEDs is given by the U.S. Department of Energy (dashed line). Also shown are typical performance levels for conventional white light sources: incandescent tungsten (W), tungsten-halogen (W-H), fluorescent (FL), and high-intensity discharge (HID)

(2,700–3,500 K) common to general lighting applications, as well as devices suited for use as backlights for liquid-crystal-display (LCD) panels. The historical progress of LED performance is illustrated in > Fig. 1 [10], along with pro forma projections from the U.S. Department of Energy [11].

2

LED Principles and Fabrication

2.1

Semiconductor Optical Properties

Fundamental to the optical properties of a semiconductor is the ‘‘forbidden gap’’ of energy levels, or bandgap, which is induced by the periodic spacing of atomic unit cells of the crystalline structure native to it. The edges of the bandgap consist of the normally occupied valence-band electron levels, and the normally non-occupied conduction-band levels. Provided sufficient energy, electrons from the valence band may be excited to the conduction band, leaving an unoccupied state, or hole, in the valence band. Recombination of electrons and holes results in a release of energy approximately equal to the bandgap energy and may be in the form of light (radiative) or heat (non-radiative). The most common semiconductor, silicon, has a crystal structure which results in the lowest energy level for electron states being displaced in momentum with respect to hole states. This indirect bandgap situation requires energy from crystal lattice vibrations (i.e., phonons) to provide momentum conservation for radiative recombination of an electron and hole pair.

1157

6.4.1

Light Emitting Diodes: Fundamentals

The resulting recombination rates are extremely low and cannot effectively compete with nonradiative processes, and thus the light generation efficiency of indirect bandgap semiconductors like silicon is poor. Direct bandgap semiconductors, however, have energy dispersion environments wherein electrons in the lowest-lying conduction-band states are momentum matched to holes in the valence band. The radiative recombination rates are very high and internal quantum efficiencies approaching 100% are reached in such materials, e.g., galliumarsenide. The difference between light emission mechanisms between direct and indirect bandgap semiconductors is shown in > Fig. 2. The III-V material systems most interesting for visible-spectrum LEDs are the wurtzite (Al,Ga,In)N and zinc-blende (Al,Ga,In)P alloys. Bandgap energies vs. lattice constant for these two systems are plotted in > Fig. 3. As illustrated in the figure, the (Al,Ga,In)P system may be lattice-matched to GaAs, a widely available substrate, at mole fractions of (Al, Ga)P = 52% and InP = 48%. By tuning the Al/Ga ratio, deep red (650 nm) to amber (
580 nm) emission is achieved with reasonable radiative efficiency. Higher Al/Ga ratios eventually reach a direct/ indirect bandgap crossover which limits the efficiency (and commercial availability) of shorter wavelength LEDs based on the (Al,Ga,In)P material system. For the (Al,Ga,In)N system, InGaN is the workhorse active layer material and is typically grown lattice-matched to a GaN template or buffer layer deposited on substrates such as sapphire, SiC, or GaN itself. The (Al,Ga,In)N system is fully direct bandgap, and in principle could provide active layer material for emission across the entire visible spectrum as well as into the deep UV (AlN) and infrared (InN). In practice, InGaN active layers lattice-matched to GaN provide efficient emission from the ultraviolet (
380 nm) to green (
550 nm). The lack of efficient emitters from either material system in the 550–580 nm regime is sometimes referred to as the ‘‘green gap.’’ It is ironic that this particular wavelength region spans the most sensitive responsiveness of the human eye (555 nm ↔ 683 lm/W) (see > Chap. 2.4.1).

Phono

n abso

Energy

1158

rption sion

n emis

Phono

Momentum

Direct bandgap

Photon emission

Photon emission

Indirect bandgap

. Fig. 2 Illustration of light emission mechanisms in direct vs. indirect bandgap semiconductors. For indirect bandgap materials, phonon interactions are required for conservation of momentum, resulting in slow radiative recombination rates and thus inefficient light generation

6.4.1

Light Emitting Diodes: Fundamentals

AIN

6.0

Bandgap energy (eV)

5.0 4.0

(AIxGa1−x)0.52In0.48P / GaAs GaN

3.0 AIP GaP

2.0 inxGa1−xN / GaN

1.0 0.0 3.0

3.1

3.2

GaAs

InN

3.3

3.4

3.5 5.4 5.5 Lattice constant (A)

5.6

5.7

InP

5.8

5.9

. Fig. 3 Bandgap energy vs. (short) crystal lattice constant for the cubic (Al,Ga,In)P and wurtzite (Al,Ga,In)N material systems showing the primary active layer alloys for red-to-amber emission, (Alx Ga1x)0.52In0.48P deposited on GaAs, and ultraviolet-to-green emission, InxGa1xN deposited on GaN

2.2

p-n Junction

The heart of a diode is the junction between positively charged (p-type) and negatively charged (n-type) regions within a semiconductor. With no applied bias voltage, negatively charged carriers (electrons) and positively charged carriers (holes) remain in the n-type, and p-type regions, respectively, and no electrical current flows through the region depleted of carriers (depletion region). Upon applied forward voltage, Vf, the junction field is reduced, and electrons and holes are injected into the depletion region and may recombine to emit a photon of an energy approximately that of the bandgap energy, as illustrated in > Fig. 4. In practical devices, the alloy composition, and thus bandgap, of the light-emitting layers (active layers) within the depletion region are tuned to provide the desired emission energy or wavelength, i.e., color.

2.3

LED Fabrication and Packaging

LED fabrication and packaging has a strong analogy to that of integrated circuits. Significant differences are the primary materials platforms (i.e., not silicon) and packaging technology which must provide means for optical transmission and conditioning. A typical flow is shown in > Fig. 5. First, a substrate material is provided and machined to provide wafer substrates for subsequent deposition of device layers. For the (Al,Ga,In)N system, the most common substrates are sapphire, SiC, and GaN. For the (Al,Ga,In)P system, the typical substrate is GaAs. Device layer deposition is performed epitaxially, typically using metal-organic chemical vapor deposition (MOCVD) apparatus. MOCVD is preferred in the high-volumemanufacturing LED industry due to large capacity and fast throughput, which are not readily

1159

1160

6.4.1 Negatively charged carriers (electrons)

Light Emitting Diodes: Fundamentals

Conduction bandedge

– – – – – –

p-type

EFn

EFp

n-type Valence bandedge

+ + + + +

+

Bandgap, Eg energy Postively charged carriers (holes)

+Vf

. Fig. 4 Illustration of a p-n junction under forward bias, Vf. Electrons in the conduction band and holes in the valence band are injected into the depletion region in concentrations consistent with their respective quasi-Fermi energy levels, EFn and EFp. There, they may recombine to release energy approximately that of the bandgap energy, Eg. In high-quality direct bandgap semiconductors, with high probability this energy release is in the form of light

Bulk substrate crystal growth & machining

Epitaxial deposition of LED layer structures

Water fabrication

Die fabrication

. Fig. 5 LED fabrication process flow from growth substrate boule (sapphire shown) to packaged LED (courtesy of Philips Lumileds Lighting Co.). The 1-W class LED packages shown are Philips ® Lumileds LUXEON Rebel (left), Nichia NS6W183T (center), and Cree Xlamp® XP-E (right)

Light Emitting Diodes: Fundamentals

6.4.1

achieved in molecular beam epitaxy (MBE) systems. Also, MOCVD provides tight flow control necessary for thin layers such as quantum wells, which is not easily achievable with other fast-growth techniques such as hydride vapor phase epitaxy (HVPE). After epitaxial layer deposition, the wafers are patterned, etched, and metalized to a specified chip size and electrodes are provided for electrical contact to the p-n junction (wafer fabrication). Then, the wafer is diced to provide individual LED chips (die fabrication). Means for light extraction are usually included in the wafer fabrication and/or die fabrication steps. Finally, the LED chips are mounted onto a lead-frame or chip-carrier, phosphors (if required) are deposited, the devices are lensed, and the lead-frame or carrier is singulated to provide individual packaged LEDs. A variety of package types are employed, the characteristics of which are generally selected to serve specific applications. Some of the most popular 1-Watt-class package types (the most popular today for general lighting and large-area LCD applications) are shown in > Fig. 5.

3

LED Performance

The basic parameters governing primary LED performance are the internal quantum efficiency and light extraction efficiency, the product of which gives the external quantum efficiency (ratio of photons emitted vs. electrons injected through the contacts),
ext ¼
int Cext ; where
int is the internal quantum efficiency and Cext is the light extraction efficiency. Important for overall power conversion efficiency is also the ratio between the forward voltage of the LED, Vf, and the centroid photon voltage, Eph, and one has
wpe ¼
ext Eph =Vf ; as the so-called wall-plug efficiency (ratio of optical Watts out vs. electrical Watts in). The centroid photon voltage is typically approximated as 1,240/lp, where lp is the peak wavelength of the primary LED emission spectrum.

3.1

Internal Quantum Efficiency

The internal light-generating efficiency of an LED is a product of the efficiency of injection of electrons and holes into the active layers, and the probability of radiative recombination of electron–hole pairs in the active layers, which is in competition with non-radiative processes. Thus, the internal quantum efficiency may be written
int ¼
inj
rad ; where
inj is the fraction of current injected into the active layers (vs. total diode current), and
rad reflects the probability of radiative recombination of electron–hole pairs. Often,
rad is described by radiative and non-radiative recombination rates. Under high-injection conditions where the concentration (cm3) of injected electrons, n, and holes, p, are similar (i.e., n
p), one may write

rad ¼ Bn2 = An þ Bn2 þ Cn3

1161

1162

6.4.1

Light Emitting Diodes: Fundamentals

where A (s1) represents Shockley–Reed–Hall non-radiative recombination, B (cm3s1) is the bimolecular radiative rate coefficient, and C (cm6s1) represents non-radiative recombination mechanisms involving three carriers, i.e., Auger recombination. Thus, the radiative efficiency, and therefore the internal quantum efficiency, is dependent on carrier density (and current density). The diode current density is the sum of recombination current and non-injected (i.e., leakage) current, and may be written Jtot ¼ qRLz þ Jleak ; where R = An + Bn2 + Cn3 is the total recombination rate, Lz is the total active layer(s) thickness, Jleak represents the leakage current density, and q is the elemental electron charge. Typical operating current densities for high-brightness LEDs are in the 20–50 Acm2 range. Estimated values for A, B, and C tend to vary significantly within the literature. However, estimates of internal quantum efficiency can be obtained from external quantum efficiencies if light extraction efficiency is determined independently [12]. Reasonable estimates for internal quantum efficiencies for state-of-the-art InGaN devices are from
80–90% in the violet/blue regime to
40% in the green regime and, for AlGaInP, from
90% in the red to
20% in the amber.

3.2

Light Extraction

Light extraction efficiency for LEDs is defined as the ratio of photons generated inside the LED chip to those which escape outside the chip. The governing issue here is electromagnetic boundary conditions between a high-refractive-index media (e.g., nGaN
2.4, nGaP
3.3) and a low-refractive-index ambient (i.e., nsilicone
nepoxy
1.5, nair = 1.0), which results in total-internal-reflection of photons impinging that interface at angles beyond the critical angle given by Snell’s Law, yc = sin1 (na/nLED), where nLED is the refractive index of the LED material and na is the refractive index of the ambient. A quick calculation of escape probability through the escape cone defined by the critical angle reveals that light extraction efficiency from highrefractive-index materials can be very low unless special means are provided to redirect photons to escape. The most common, and successful, methods to achieve this are through novel ‘‘chip shaping’’ or by randomized texturing of surfaces in low-loss materials. Very good results are obtainable, with extraction efficiencies as high as 60% estimated for shaped chips in the (Al,Ga,In)P system, to 80% for thin-film chips in the (Al,Ga,In)N system. Some selected LED chip designs achieving high light extraction efficiency are shown in > Fig. 6 [9, 13–15].

3.3

Monochromatic LED Performance

External quantum efficiencies of best-reported Watt-class LEDs based on the (Al,Ga,In)P and (Al,Ga,In)N materials systems are shown in > Fig. 7 [9, 14, 16–18]. These data are for devices operating at current densities reasonable for real-world applications (35 A/cm2). The InGaN points include devices based on polar, nonpolar, and semipolar orientations of wurtzite GaN. The AlGaInP points are from shaped-chip LEDs. The ‘‘green gap’’ shows clearly against the background of the 1931 CIE eye response curve, V(l) (see > Chap. 2.4.1). For InGaN, as the InN mole fraction is increased, the miscibility gap between GaN and InN becomes an issue, and deposition conditions required for obtaining high InN-containing layers result in lower quality films. In addition, increased compressive strain results in piezoelectric field build-up in the

c

MQW active region

p-GaN

Ceramic submount

n-GaN

Roughened n-GaN

Carrier

p-GaN Mirror and p-contact

n-GaN

Metal and contacts

Solder

MQW

d

p-Gap

b

Epi-layer

n-Gap AIGaInP

1mm

Patterned sapphire substrate

ITO p-electrode

. Fig. 6 LED chip designs for high light extraction efficiency. Shown for InGaN-based LEDs are (a) vertical thin-film, (b) patterned growth substrate, and (c) thin-film flip-chip. Each of these designs employ textured surfaces to frustrate total-internal-reflection. Shown in (d) is a shaped-chip design for a AlGaInP-based emitter.

a

180 μm

7 μm

n-contact

Light Emitting Diodes: Fundamentals

6.4.1 1163

6.4.1

Light Emitting Diodes: Fundamentals

80% InGaN

External quantum efficiency

1164

60%

AlGalnP

Ph

V(l)

os

ph

or

-c

on ve r te

d*

40%

20% ∗PC Amber LUXEON® Rebel 0% 400

500

600 Peak wavelength (nm)

700

800

. Fig. 7 State-of-the-art external quantum efficiency vs. peak wavelength of Watt-class LEDs for monochromatic emission. Data points are limited to documented performance at reasonable current densities required for real-world applications. In addition to the InGaN and AlGaInP primary emitter performance, phosphor-converted (PC) amber performance is shown (available product with the amber down-converter is shown in the inset, courtesy of Philips Lumileds Lighting Co.)

active region of the device which complicates carrier injection efficiency and radiative recombination rates. The overall effect is reduced quantum efficiency from the UV/blue regions to the green, and no commercial availability of emitters beyond 550 nm. For AlGaInP, increasing the Al/Ga ratio of the GaAs-lattice-matched alloys to achieve shorter wavelength devices invokes competition between indirect and direct bandgap valley minima. Thus, radiative efficiency (and also high-temperature performance) of AlGaInP LEDs decreases dramatically from the red to the yellow wavelength regimes.

3.4

Down-Conversion for White-Emitting LEDS

While it is certainly possible to mix multiple LED primary emission spectra to obtain various white light spectra, the ‘‘green gap’’ problem and complications with multiple drive schemes and color control feedback today make down-conversion the preferred method for producing white-emitting LEDs. In this method, a primary ‘‘pump’’ LED emitting the UV, violet, or blue is employed to excite one or more down-conversion materials, typically phosphors (see > Chaps. 6.1.1, > 6.1.2). The pump emission may be completely consumed or, more commonly, combined with the phosphor emission to provide the final desired white light spectrum. The most common approach is to use a blue-emitting pump LED combined with YAG phosphor. The combination of the primary blue light and broad yellow YAG emission results in

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

0.1

InGaN (blue)

0.2

0.3 0.4 0.5 x-chromaticity

Blackbody loci

0.6

Y3AI5O12:Ce

0.7

M2Si5N8:Eu

CIE 1931 x-y

0.8

p-type gallium nitride

450

Gold wire

Quantum wells

n-type gallium nitride

Photon

350

Metal contact

Submount

550 650 Wavlength (nm)

Solder

Metal contact

Gold wire

Photon

750

3000K

. Fig. 8 Left: CIE 1931 chromaticity chart showing color points of emission from a blue LED, and YAG and red M2Si5N8:Eu phosphors. Any color point within this region (triangle) is achievable by weighting the three emission levels. For white, emission weighting is tuned to the blackbody loci at the desired color temperature. Upper Right: Emission spectra of such an LED for 3,000 K. Lower Right: Artist’s rendition of a phosphor-converted LED employing green and red phosphors (courtesy of Philips Lumileds Lighting Co.)

y-chromaticity

0.9

Light Emitting Diodes: Fundamentals

6.4.1 1165

1166

6.4.1

Light Emitting Diodes: Fundamentals

a simple method for achieving white emission with relatively high efficacy, simply by adjusting the phosphor-loading ratio so that the emission color point falls on the blackbody curve in chromaticity space (> Fig. 8). Drawbacks to the YAG-only approach are a limited range of color temperatures and color-rendering indices (CRIs) typically less than 80, which is below the desired levels for general lighting applications. Also, for LCD applications, the lack of red content in the blue-plus-YAG spectrum limits the achievable display color gamut to well below (
67% of) the NTSC standard. The color quality can be improved by adding a red component to the blue-plus-YAG emission. The most widely used red phosphors belong to the M2Si5N8:Eu2+ nitrido-silicate systems [19]. By adding the red component, a third point in chromaticity space opens up, and a wide gamut of color points are achievable, including the warmer color temperatures associated with tungsten (2,900K) and tungsten-halogen (3,200K) incandescence. In addition to offering warmer whites, the addition of the red phosphor emission gives a significant lift to color rendering, with CRIs reaching above 80 and even up to 90. Also, the increased red content leads to higher color gamuts in LCD applications. In addition to enabling single-chip white-emitting LEDs, down-conversion has been exploited to provide improved monochromatic LEDs. Shown in > Fig. 7 is the external quantum efficiency for an amber-emitting LED based on down-conversion. This phosphor-converted (PC) amber LED dramatically outperforms (Al,Ga,In)P, and provides high-temperature stability as well. Additional down-conversion-based monochromatic LEDs in the cyan to amber, and perhaps to red, might be expected in future, as solutions for the ‘‘green gap’’ problem, until performance of primary emitters improves in this wavelength range. New materials are being developed that may challenge phosphors as the preferred downconversion media for LEDs. Semiconductor nanoparticles, or ‘‘quantum dots’’ (QDs), based on II-VI or III-V compound-semiconductor systems are now achieving quantum yields comparable to conventional phosphors [20]. QDs have additional advantages, such as narrow emission widths (30–50 nm), tuneable spectrum by composition or size distribution engineering, and eliminated backscatter of primary pump light which can be a loss mechanism for phosphor-based LEDs. It remains to be seen how QDs will fare against incumbent phosphors. Challenges are reliability and high-temperature operation. Nevertheless, QD-based solid-state lighting products are beginning to be seen on the market [21].

4

Summary and Directions for Future Research

LED performance has improved by orders of magnitude over the last several decades, and with the incorporation of down-conversion technology, all colors are available at efficacies surpassing that of conventional lighting technologies. This has enabled LED light sources with total cost-of-ownership payback of only a few years (vs. conventional lamps) for general lighting applications. Also, it has resulted in a revolution in the LCD backlighting market, providing more efficient displays with higher color gamuts and slimmer form factors. Still, improved performance requirements in the ‘‘green gap’’ regime for primary emitters, and in new downconversion media, provide opportunities for relevant research and development. As performance nears theoretical limits (the best InGaN LEDs have peak external quantum efficiencies exceeding 80% [9]), emphasis will be placed on reducing costs and improving light quality. In addition, incorporation of smart controls into LED-based lighting for energy management, ambience creation, and advanced displays will become more prevalent.

Light Emitting Diodes: Fundamentals

6.4.1

References 1. Holonyak N Jr, Bevacqua SF (1962) Coherent (visible) light emission from Ga(AsP) junctions. Appl Phys Lett 1:82–83 2. Kish FA, Steranka FM, DeFevere DC, Vanderwater DA, Park KG, Kuo CP, Osentowski TD, Peanasky MJ, Yu JG, Fletcher RM, Steigerwald DA, Craford MG, Robbins VM (1994) Very high-efficiency semiconductor waferbonded transparent-substrate (AlGa)InP/GaP lightemitting diodes. App Phys Lett 64:2839–2841 3. Amano H, Sawaki N, Akasaki I, Toyoda Y (1986) Metalorganic vapour phase epitaxial growth of a high quality GaN film using an AlN buffer layer. Appl Phys Lett 48:353–355 4. Amano H, Kito M, Hiramatsu K, Akasaki I (1989) P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI). Jpn J Appl Phys 28:L2112–L2114 5. Nakamura S, Senoh M, Mukai T (1993) High-power InGaN/GaN double-heterostructure violet light emitting diodes. Appl Phys Lett 62:2390–2392 6. Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matshushita T, Kiyoku H, Sugimoto Y (1996) InGaN-based multi-quantum-well-structure laser diodes. Jpn J Appl Phys 35:L74–L76 7. Nakamura S, Senoh M, Iwasa N, Nagahama S (1995) High-brightness InGaN blue, green and yellow lightemitting diodes with quantum well structures. Jpn J Appl Phys 34:L797–L799 8. Bando K, Sakano K, Noguchi Y, Shimizu Y (1998) Development of high-bright and pure-white LED lamps. J Light Visual Environ 22:2–5 9. Narukawa Y, Ichikawa M, Sanga D, Sano M, Mukai T (2010) White light emitting diodes with super-high luminous efficacy. J Phys D Appl Phys 43:354002 10. Dupuis RD, Krames MR (2008) History, development, and applications of high-brightness visible light-emitting diodes. IEEE J Lightwave Technol 26:1154–1171 11. Solid-State Lighting Research and Development: Multi-Year Program Plan (2010) Lighting Research and Development, Building Technologies Program, U.S. Department of Energy http://apps1.eere.gov/

12.

13.

14.

15.

16.

17.

18.

19.

20. 21.

buildings/publications/pdfs/ssl/ssl_mypp2011_web. pdf Krames MR, Shchekin OB, Mueller-Mach R, Mueller GO, Zhou L, Harbers G, Craford MG (2007) Status and future of high-power lightemitting diodes for solid-state lighting. IEEE J Display Technol 3:160–175 Hahn B, Weimar A, Peter M, Baur J (2008) Highpower InGaN LEDs: present status and future prospects. Proc SPIE 6910:691004 Shchekin OB, Epler JE, Trottier TA, Margalith T, Steigerwald DA, Holcomb MO, Martin PS, Krames MR (2006) High performance thin-film flip-chip InGaN-GaN light-emitting diodes. Appl Phys Lett 89:2365–2367 Krames MR, Holcomb MO, Hofler GE, CarterComan C, Chen EI, Tan IH, Grillot P, Gardner NF, Chui HC, Huang JW, Stockman SA, Kish FA, Craford MG (1999) High-power truncatedinverted-pyramid (AlGa)InP/GaP light-emitting diodes exhibiting >50% external quantum efficiency. Appl Phys Lett 75(16):071109 Michiue A, Miyoshi T, Yanamoto T, Kozaki T, Nagahama SI, Narukawa Y, Sano M, Yamada T, Mukai T (2009) Recent development of nitride LEDs and LDs. Proc SPIE 7216:561 Vampola KJ, Fellows NN, Masui H, Brinkley SE, Furukawa M, Chung RB, Sato H, Sonoda J, Hirasawa H, Iza M, DenBaars SP, Nakamura S (2009). Highly efficient broad‐area blue and white light‐emitting diodes on bulk GaN substrates. Physica Status Solidi A 206(2): 200–202 Krames MR (2009) Status and prognosis for solid‐ state lighting technology. CLEO Conference, Baltimore (presentation CM001) Mueller-Mach R, Mueller G, Krames MR, Hoppe HA, Stadler F, Schnick W, Juestel T, Schmidt P (2005) Highly efficient all-nitride phosphor-converted white light emitting diode. Phys Status Solidi A 202:1727–1732 Reiss P, Protiere M, Li L (2009) Small 5(2):154–168 Quantum dots: a quantum leap for lighting (2010) The Economist, London

Further Reading Nakamura S, Pearton SJ, Fasol G (2000) The blue laser diode: the complete story. Springer, Berlin/Heidelberg/New York Pankove J (1971) Optical processes in semiconductors. Englewood Cliffs, New Jersey

Schubert EF (2006) Light-emitting diodes, 2nd edn. Cambridge University Press, New York Stringfellow G, Craford MG (1997) High brightness light-emitting diodes. Semiconductors Semimetals 48:1–454

1167

6.4.2 LED Display Applications and Design Considerations Robbie Thielemans 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1170

2 2.1 2.2

Visual Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1171 Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1171 Intensity and Luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1171

3 3.1 3.2 3.2.1 3.2.2 3.3 3.4 3.5

LED Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1172 Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1172 Binning Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1173 Intensity Binning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1173 Color Binning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1173 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174 Spatial Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1175 Power Dissipation and Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1175

4 4.1 4.1.1 4.1.2 4.2 4.2.1 4.2.2 4.2.3 4.3

Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176 Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176 Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176 Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176 Constant Current Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176 Power Supply Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1177 Processing Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1178 Uniformity Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1178

5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1180

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.4.2, # Springer-Verlag Berlin Heidelberg 2012

1170

6.4.2

LED Display Applications and Design Considerations

Abstract: This chapter presents a brief overview of the issues involved in the production and implementation of full color modular LED displays for large-area display and signage applications. The technical issues related to achieving large-area uniformity and visual quality are discussed in terms of the practical device selection, driving and system implementation factors that should be considered. List of Abbreviations: CIE, Commission Internationale de l’Eclairage; LED, Light Emitting Diode

1

Introduction

This chapter will focus primarily on full color modular LED displays for large-area display and signage applications, where picture quality and uniformity are important criteria along with the overall size of the display. Display systems which utilize a RGB module approach for full color are considered in detail, with the discussion also relevant to the use single color dot matrix displays or alphanumerical displays, which are effectively a subset of the outline presented. Although full color LED displays have been in common use for a number of years, since the development of blue LEDs in the early 1990s following the pioneering work of Shuji Nakamura [1], display performance improvements have not yet reached their limit. This chapter will describe in detail what parameters to take into account when selecting LEDs, driving them and getting the desired performance using state-of-the-art digital signal techniques and calibration routines. It will also give an indication of the market considerations, getting the balance right for optimum cost effectiveness and product life cycle expectancies (> Fig. 1).

. Fig. 1 Example of a large-area modular LED display at the Paris Motor Show 2010 (Courtesy of Creative Technology Germany, www.ctgermany.com)

LED Display Applications and Design Considerations

2

Visual Parameters

2.1

Color

6.4.2

One of the most significant challenges for a large-area display system built up from discrete elements or modules is to achieve a perceived uniformity of color, particularly for static images and large areas of single color. Human color perception varies significantly across individuals, hence it is critical that an objective and quantifiable method of measurement is employed to facilitate the integration of modules into a final display unit with the required color and uniformity properties. As described in detail in > Chap. 2.2.2, the Commission Internationale de l’Eclairage (CIE) established the XYZ tristimulus system for the measurement and quantification of color, which is based on the assumption that every color is a combination of three primary colors: red, green, and blue. The XYZ tristimulus values are obtained by integrating the spectral power distribution of radiation S(l) and the three eye response curves x(l), y(l), and z(l) over the visible wavelengths 380–780 nm. This system is designed in such a way that one of the three tristimulus values – the Y value – has a spectral sensitivity that corresponds to the lightness sensitivity of the human vision, thus representing the luminance. It is convenient, for both conceptual understanding and computation, to have a representation of ‘‘pure’’ color in the absence of luminance. The CIE standardized a procedure for normalizing XYZ tristimulus values to obtain two chromaticity values x and y: x ¼ XþYX þZ

y ¼ XþYY þZ

Thus, using this projective transformation, colors can be plotted in an (x,y) chromaticity diagram as in > Fig. 2 (see also > Sect. 5 of Chap. 2.2.2 ‘‘Chromaticity Diagrams’’). Visual sensitivity to small color differences is the essential factor determining the precision of color matching. Data on color-difference thresholds provides indirect evidence on how this precision will vary with the color of the matching field and with other factors that affect sensitivity (see > Chaps. 2.2.6 and > 2.2.7 and [2]). This is an important consideration for modular LED applications, where methods of dealing with subtle color variations, such as ‘‘color binning’’ as discussed below are critical for uniformity across large display systems.

2.2

Intensity and Luminosity

As defined in > Chap. 2.4.1 (Part 4.2), intensity is the rate of flow of radiant energy per unit solid angle – that is, in a particular, specific direction. The CIE has defined luminance, denoted by Y, as intensity per unit area, weighted by a spectral sensitivity function that is characteristic of human vision. The magnitude of luminance is proportional to physical power; in that sense, it is like intensity. But the spectral composition of luminance is related to the brightness sensitivity of human vision. The luminance generated by a physical device is not always proportional to the applied signal, as seen for example with cathode ray tube (CRT) and liquid crystal displays (LCDs). LEDs, however, if driven with constant current in pulse width modulation, behave more linearly [3]. Luminance nonuniformity is the gradual change of luminance from one display area to the next. It can be broken down into large-area and small-area nonuniformity. Large-area nonuniformity is usually a luminance change over the entire display (i.e., edge-to-edge, edge-to-middle). Because these changes are very gradual, a 50% change from edge to edge

1171

6.4.2

LED Display Applications and Design Considerations

CIE 1932 Chromaticity Diagram

0.9 520

0.8

530 S

(Dominant Hue) 540

510

0.7

550

F (LED coordinates)

505

560

0.6

570

500

CIE y

1172

0.5

580 590

495

0.4 0.3

490

0.2

600 610 620 640 780

E (0.33,0.33)

485 480

0.1

475 470 460 360

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

CIE x

. Fig. 2 CIE chromaticity diagram illustrating the dominant hue and purity of a particular LED color coordinate, as determined by plotting a straight line from the equal energy white point (E) through the LED coordinates (F) to the intersection (S) at the spectral boundary of the diagram

sometimes cannot be noticed [4]. At low display luminance however, human vision is adapted in such way that even these gradual changes may be seen [2]. Small-area nonuniformity is often referred to as pixel-to-pixel changes.

3

LED Parameters

3.1

Color

The spectral power distribution of the optical radiation emitted by LEDs differs in many ways from other radiation sources. It is neither monochromatic like a laser, nor broadband like a tungsten lamp, but rather lies somewhere between these two extremes. Peak Wavelength lp is the wavelength at the maximal intensity of the spectrum. It is easy to define and is therefore generally given in LED data sheets. However, the peak wavelength has little significance for practical purposes since two LEDs may well have the same peak wavelength but different chromaticities – i.e., the CIE coordinates are different, and hence the color perceived by an observer also differs. Dominant wavelength is determined from the CIE coordinates of the measured spectrum. If plotted on a CIE chromaticity diagram, the intersection at the boundary of a line drawn from the equal energy (white) point E=(0.33,0.33) through the coordinates (F) of the LED gives the dominant wavelength. It is an indication of the color sensation produced in the human eye by the LED.

LED Display Applications and Design Considerations

6.4.2

Purity is defined as the ratio of the distance from the equal energy point E to the color coordinate F and the distance from the equal energy point E to the intersection S in the color diagram boundary (see > Fig. 2), i.e., the closer the coordinates are to the boundary, the higher the color purity, or saturation. As a conclusion one could say that for the evaluation of LEDs from a strictly color point of view, one needs at least the (x,y) coordinates or the dominant wavelength together with the purity. It can be easily seen that a certain LED is much less saturated/pure when the location of the (x,y) coordinates is close to the equal energy point, compared to one lying on the outer section of the CIE diagram, although the dominant wavelength can be exactly the same.

3.2

Binning Levels

Although the LED manufacturing process is well known and for standard types very mature, typical manufacturing process tolerances and yield parameters are influenced by the cost/ performance balance, and will have an affect on the reproducibility/variability of resultant individual LED device parameters [5]. The specific parameters which fall under these described variations include: intensity variation, color coordinate variation, efficiency variation, temperature dependencies, and packaging tolerances, which all have a direct or indirect impact on display appearance and performance.

3.2.1

Intensity Binning

Intensity variation is the most straightforward parameter to monitor, since LEDs are typically specified at a certain constant/DC drive current; hence, light output is measured at this specific driving condition. The resultant variation in device performance due to process variations is then evident as a Gaussian distribution of intensities across a particular batch of LEDs, as shown in > Fig. 3. The overall spread (maximum versus minimum) is typically of the order of magnitude of 3–5. To provide a mechanism of classifying individual LEDs according to this distribution, multiple manufacturers split the LED offering into light output ‘‘binnings’’ [6] (indicated with vertical lines in > Fig. 3). To achieve a uniform display, it is thus necessary to use a particular range or ‘‘bin’’ of LEDs in a single system.

3.2.2

Color Binning

Color binning is less straightforward as the measurements need to happen in x,y color space and not just for the dominant wavelength in order to have a decent color parameter. Typical LED layer thickness variances are the root cause of the color distribution pattern, an example of which is plotted for three primary color LEDs in the CIE diagram of > Fig. 4. Again, as with intensity binning, manufacturers can also define different color grade bins – although some manufacturers still continue to provide their information using only the dominant wavelength. This can lead to nonuniformities in display color. For example, > Fig. 5 shows an example of an LED backlight for an automotive instrument cluster application where multiple LEDs from the same color bin have been used, but there are still color nonuniformities which are observable.

1173

LED Display Applications and Design Considerations

No. of samples

6.4.2

Light Output

. Fig. 3 Light output binnings

EBU matrix

y (CIE)

1174

x (CIE)

. Fig. 4 Example of color binning distribution in a CIE chart

3.3

Efficiency

Usually LEDs are specified at a certain constant current drive level – usually 10 or 20 mA. However, when driving LEDs in a full color video application, these specifications do not satisfy the full qualification for usage, due to the potential for nonuniformities in output across a display. It has been shown that the efficiencies of LEDs can differ significantly under different drive conditions, both in light output and in color coordinate variation.

LED Display Applications and Design Considerations

6.4.2

. Fig. 5 Example of RGB LEDs (uncalibrated)

3.4

Spatial Radiation

There are many different packages and types of LEDs generating different spatial radiation patterns. Precise knowledge of the angle-dependent distribution of radiation is necessary for some applications. For example, a full color (RGB) LED may appear white when observed at a normal angle if all three colors are illuminated simultaneously. However, if the LEDs have a different spatial distribution of radiation for the individual colors a color change occurs when the display is observed off axis. This can be easily deducted from the fact that due to (even small) manufacturing construction differences in the light field of the actual die of the LED, variations will occur.

3.5

Power Dissipation and Heat

LEDs are very susceptible to the effects of heat on performance. Typically, a 10% reduction in luminous intensity results from an increase in temperature from 25 C to 60 C [7]. LEDs have a generic behavior at elevated temperatures: At higher temperatures they are less efficient. The luminous intensity at 60 C is reduced by 10%, e.g., of its value at 25 C Therefore, one not only needs to know the specific temperature derating curves of the LEDs, but one must be able to cope with this issue (either via electronic compensation/LED spec negotiation for efficiency and temperature derating.) LED driver electronics are also crucial at this point. Having a very tight LED specification does not guarantee display performance if the tolerances on, for instance, the drivers are not as well specified. These constant current drivers need to supply the correct current because current has an influence on both light output and color coordinates. Temperature and design stability are de facto crucial. This means that in-between different outputs in the same devices the tolerance needs to be controlled radically in combination with the tolerance between different drivers. In the industry, these are called inter- and intrachip tolerances.

1175

1176

6.4.2

LED Display Applications and Design Considerations

All these examples show that extra care needs to be taken when designing/building/ specifying components for a full color LED display device. These will lead to some implementation guidelines in the next section.

4

Implementation

4.1

LEDs

For all the reasons as described above, it is of utmost importance to agree with a supplier certain specifications which have an impact on display quality balanced with display cost. One thing is in both parties eventual benefit: to choose only one specification which suits both parties.

4.1.1

Color

Define a color ‘‘box’’ with the supplier on both levels: using industry standard color bins and color efficiency bins. This box must of course be as small as possible, keeping in mind that these boxes are optimized toward the manufacturer’s highest yield. Make also sure that LEDs that reside in those boxes can be compensated using the headroom design in the electronics. This will be a trade-off between cost of electronics processing and LED cost.

4.1.2

Intensity

Here the same rule applies, but with more emphasis toward the intensity efficiency level as this will determine the minimal light output specification in combination with the maximum power dissipation level. Tolerances can be made bigger, as intensity is more straightforward to compensate for, although a maximum limit needs also to be set because of temperature issues.

4.2

Electronics

Multiple implementation schemes are possible. Again, some guidelines are given.

4.2.1

Constant Current Drivers

As LEDs are immediately driven using these components, one could easily see that the choice and design of these specific components are very critical. This is certainly true in a video environment. Although one can debate availability of such a component specifically for the high performance video market, all of them have advantages and severe drawbacks. Most important parameters are summarized: 1. Interpin output current variation Most constant current drivers have more than one output. Industry standards are 8, 16, or 24 outputs. These outputs are immediately coupled to the LEDs. If one wants to display a uniform image, it is of utmost importance that the outputs within one chip driving

LED Display Applications and Design Considerations

6.4.2

a multitude of LEDs all have the same current. If there is a significant variation in-between outputs of the pins, this of course would add to certain LEDs driven at another current. Again, this would mean that those LEDs have different resultant intensities and color, even if all the LEDs have the same specification. It is therefore very obvious that not only LED variation is a source of nonuniformity, but also the constant current drivers. Hence, variations in the range of 1–3% are preferred, as 0% variations are obviously cost prohibitive. 2. Interchip output current variation The same reasoning as above is valid for the current variation in-between multiple constant current driver chips, as a multitude of these are used to drive a screen. Hence, trimming toward less than 2.5% variation is also preferred. If both the variations of a and b are higher than indicated, this would even demand for a higher degree of compensation. Now, not only the LEDs need compensation, but also the constant current driver variation. A higher degree of compensation then means even generating more ‘‘gray levels’’ as we will see later on, which in return would mean faster constant, more expensive, current drivers. This immediately links to the next topic. 3. Speed/settling time Speed also is part of the accuracy definition of the constant current output. Having an extremely slow constant current driver, would most probably mean that the chip manufacturer can have a design which is compliant to a and b, but slow (in the order of magnitude of milliseconds). For comparison sake, an NTSC video source updates every 16.6 ms (60 Hz). This would mean that in this time frame, the constant current driver can only switch reliably 16 times, so it can only generate in that time frame 16 gray levels, which is in video standards completely unacceptable. Hence, rise and fall times in the order of ns are required, while keeping the a and b specification. As a consequence, the chip design requires specific optimized drive circuitry and layout for the fast response. 4. Power dissipation Usually an afterthought, but certainly critical, is having a constant current driver which operates on its outputs with as low a voltage drop as possible, as this would only increase the overall display power dissipation, and heat generation. Heat generation is not only negative for design reasons, but also regarding LED light output variations. Specific ‘‘hot spots’’ could become the reason of nonuniformity. This issue now also links in into the power supply design itself.

4.2.2

Power Supply Design

The obvious choice for a power supply would be to pick an industry standard. But, there are some compelling reasons why this cannot be done without caution: 1. R,G, and B LEDs have different threshold voltages, so in order not to waste any energy, and have excessive heat generation, it would be best to optimize the SMPS output voltages to the adequate LED drive voltages. Typically, the threshold voltages for green and blue LEDs are around 3.5 V, while red is typically around 1.9 V. Hence a power supply which outputs 2 V, i.e., 4.2 V (Green-Blue) and 2.6 V (Red), has advantages with regard to total system power dissipation. (0.7 V is added for constant current driver operation.)

1177

1178

6.4.2

LED Display Applications and Design Considerations

2. Reaction time. As display content can change immediately, the power supply must be able to go from 0% to 100% power load at once. (typically, 16–20 ms response time is needed as video sources run at either 60 or 50 Hz, and the images can change instantaneously). Most power supplies need a (high) minimum or even constant load to guarantee reliability and operation. Of course, adding power factor correction (PFC) immediately will allow a better cost of ownership for the complete display, as real PFC correctors for huge installations are cost prohibitive.

4.2.3

Processing Electronics

Gamma

The human eye is not linearly sensitive to light intensity but when LEDs are driven in pulse width using constant current drivers, they react linearly, hence, the need for a real-time Gamma correction (see > Chap. 11.2.2) (> Fig. 6).

4.3

Uniformity Correction

In order to correct for nonuniformities, one needs high standard measurement means, such as for instance a high resolution camera or spectrometer, for measuring each individual LED for its x,y and Y coordinates, with such an accuracy that the measurement procedure error is below the visible threshold. (The procedure here is that all the measurements must be done under the same conditions throughout time – temperature, drive current, and so on.) These measurements can then be subsequently stored in a nonvolatile memory preferably close to the LEDs, so that the subsequent hardware can make use of these measurements and can adapt them through time and usage circumstances. Again, continuous adaptation of these parameters means that at least a performing controller can calculate in real time how to adapt the parameters in function of temperature, lifetime usage, requested light output, and so on.

. Fig. 6 Simulated pictures of gamma and non-gamma corrected displays. Note the ‘‘lack’’ of color depth if no gamma correction is applied

LED Display Applications and Design Considerations

6.4.2

The next diagram shows in detail how the electronics can be built, keeping in mind that no off-the-shelf components can be found for such a high standard performance requirement. Therefore, the preference goes to implementing the calculations in programmable logic, which in its turn has an advantage to the customer. Whenever display standards change or better processing algorithms are developed, these can easily be upgraded in the field, so that the display will continue to perform to the latest innovation and image display capabilities. > Figure 7 shows an implementation proposal on the processing path. Without showing the detail of complexity on the individual brightness control of every LED and the fast processor interface for updating the parameters in real time (see above discussion regarding temperature, lifetime, and color temperature real-time updating), one can deduct three paths. The top path is the main red processing part, the middle one is the main green path, and the bottom one is the blue core processing. As one can see, each path has its scaler incorporated. Usually LED displays are built modularly, which in fact means that such a self-contained module has only a fraction of LEDs to account for. Thus, the complexity goes down per system, but performance goes up. Since such a module only needs to display a fraction of the total picture, the processing time allocated for one pixel is magnitudes higher than in the case of a full display. This also means that for instance scaling algorithms can be implemented on a module level, which are not commonly used in the industry because the processing power needed on a full display level would require cost prohibitive electronics. Now, due to the modular system, since a module only has a ‘‘few’’ pixels to take care of, the processing time per pixel is much higher, so the time spent on optimizing and using extreme performing algorithms is possible.

Scaling

Gamma Calculation

Constant current drivers

R on G

Drive

R on B

Scaling

Gamma Calculation

Constant current drivers

G on R

Drive

G on B

Scaling

Gamma Calculation

Constant current drivers

B on G

Drive

B on R

. Fig. 7 Core processing paths for large area modular displays, showing the separate paths for red (top), green (middle) and blue (bottom)

1179

1180

6.4.2

LED Display Applications and Design Considerations

Next to the scaling part, one can apply the gain of the current overall color, which in its return is fed into the high-resolution gamma calculation algorithm, to ensure proper light output response. Now, having the right color, one can start mixing. As one can see in the above diagram, the colored dots are in fact busses connected to each other. These bus connections are the transportation means if information needed for other colors whenever a certain color point needs to be corrected on an LED level. An example will clarify the issue. Suppose one want to show a uniform green color. It is the obvious that the middle channel gets the most attention, as this channel will generate ‘‘most’’ of the green. On the other hand, one also now knows that even between same LEDs color differences exist. Therefore, using the green dot channel, multiplied by the requested amplitude, red and blue color can be added to the particular LEDs so that color and brightness match each other. Obviously, the amount of color addition must be in the range of the color correction needed, but is usually a 100–1,000 times less than the main color. The consequence is in this case that the bit depth calculation and LED light output regulations, must be even 10–100 times more, to also accommodate a color correction scale. Thus, calculating back the numbers, this would yield a processing path in the range of 12–16 bits, with intermediate calculations even at a higher level in order not to accumulate rounding errors at the end stage.

5

Summary

This chapter provides an overview of some important topics regarding LED processing for large-area modular display implementation. One needs to take into account a wide range of visual and electronics parameters, which are not that obvious in the standard display industry. On top of that, the economics, manufacturability, and requirements here are much more involved and are indeed important pieces of the puzzle. The intention of this chapter is to make designers, consultants, and customers aware of most important pieces of the puzzle, and hopefully allow the right questions to be considered.

References 1. 2.

3. 4.

Nakamura S (1994) Nichia 1cd blue LED paves way for full colour display. Nikkei Electron Asia 6:65–69 Wyszecki G, Stiles WS (1967) Color science. Concept, methods, quantitative data and formulae. Wiley, Oxford http://www.ledsmagazine.com/features/4/8/1 Chen K-Y, Chen S-M, Hao Z-D (1998) Optical illumination system having improved efficiency and

5. 6. 7.

uniformity and projection instrument comprising such a system. US Patent 5,755,503, 26 May 1998 Reference to multiple LED datasheets. www.nichia. com www.ledlight.osram-os.com/tools/fine-white-binning Schwedler W, Nguyen F (2010) Invited paper: LED backlighting for LCD TVs. SID Symposium Digest of Technical Papers 41(1):1091–1096

Further Reading Berns RS (2000) Billmeyer and Saltzman’s principles of color technology, 3rd edn. Wiley Ohta N, Robertson AR (2005) Colorimetry: fundamentals and applications, Wiley

Schubert EF (2003) Light-emitting diodes. Cambridge University Press, Cambridge. ISBN 0819439568

Part 6.5

Inorganic Electroluminescent Displays

6.5.1 Thin Film Electroluminescence (TFEL) Adrian H. Kitai . Feng Chen 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184 2 Background of EL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184 3 Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1186 4 Electroluminescent Phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1189 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1191

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.5.1, # Springer-Verlag Berlin Heidelberg 2012

1184

6.5.1

Thin Film Electroluminescence (TFEL)

Abstract: This chapter focuses on TFEL materials and devices. There are distinct operating principles underlying these devices, which will be examined in detail. Established and emerging EL phosphor materials are described and their relative merits and specific properties discussed. The chapter concludes with some indication of likely future developments in the context of competing flat panel display technologies. List of Abbreviations: TFEL, Thin Film Electroluminescence; TDEL, Thick Dielectric Electroluminescence

1

Introduction

The solid state conversion of electrical energy to visible light has evolved into a diverse field. The requirements for various applications are diverse also. For example, high-intensity LightEmitting Diode (LED) sources are well entrenched in LCD backlighting applications, and are entering the lighting market. Powder Electroluminescence (Powder EL) continues to serve in nightlights and keyboard illumination. Organic Light-Emitting Diode (OLED) devices are entering the display market in portable displays. Thin Film Electroluminescence (TFEL) is being developed for color television applications.

2

Background of EL

This chapter focuses on TFEL materials and devices. There are distinct operating principles underlying these devices, which will be examined in detail. However, it is useful to start by examining features that distinguish both powder EL and TFEL materials and devices from diode-type devices. Powder EL is covered in > Chap. 6.5.2. The materials used in powder EL and TFEL devices are generally inorganic polycrystalline solids rather than the single crystal materials used in LEDs. This permits large area devices to be manufactured at a reasonable cost. Highly uniform light emission over large areas is available. This is difficult to achieve with LEDs that are small area, albeit intense, sources of light. The polycrystalline nature of these EL powder and thin film materials is the key reason for the uniformity of the light emission. The light emitted from each grain of material is normally not seen as a distinct source by the observer, as the light per unit area emitted is the result of numerous grains or crystals. For this reason, a less perfect material than the single crystal LED materials is able to create highly reproducible lighting without requiring the binning associated with LED devices (> Chap. 6.4.2). Another key distinction between LEDs and powder EL and TFEL materials lies in the light emission process. In LEDs, band-to-band recombination generally occurs. This means that traps and defects that lead to unwanted, non-emissive recombination events between electrons and holes must be minimized by using high purity, low-defect density materials. This is fundamentally due to the delocalization of electrons and holes in energy bands. Since electron and hole conduction is a necessary condition for efficient diode-type devices, conduction and valence band transport is necessary. This carrier delocalization is therefore a mixed blessing, as it allows for high current density carrier transport and the associated availability of electrons and holes to supply recombination. However, it also allows carriers relatively free access to any traps within a diffusion length of the carrier.

Thin Film Electroluminescence (TFEL)

6.5.1

In powder EL and TFEL materials, luminescence is generally derived from nonmobile charges. They are trapped in donor-/acceptor-type traps (powder EL) or in recombination centers in the case of thin film EL. These are, not surprisingly, insulating materials. Charge transport now occurs by a mechanism other than band transport alone. Upon application of a high electric field, high field breakdown (avalanche breakdown) occurs and charges flow, even though the EL materials are insulators. In this chapter, thin film EL will be reviewed, and both the science and technology development to date will be discussed. In 1967, Russ and Kennedy [1] demonstrated a double-insulating-layer EL structure. > Figure 1 shows the structure of a double-insulating-layer TFEL device. It consists of a substrate, usually glass, on which a series of thin film layers is grown. There are two electrodes, two dielectric layers, and a phosphor layer.

. Fig. 1 (a) Thin film double-insulating-layer TFEL device structure showing sequence of layers deposited as thin films on a glass substrate. Typical materials used are: (i) Glass substrate: Corning 1737 glass; (ii) Transparent electrode: Indium tin oxide(ITO), 150 nm; (iii) Dielectric layers: Aluminum titanate: 200 nm each; (iv) Phosphor layer: Manganese-doped zinc sulfide (ZnS:Mn), 500 nm; (v) Rear electrode: Aluminum,100 nm. (b) TFEL display with rows and columns. The sheet resistance of the transparent (ITO) electrodes is important for matrix addressing. ITO can be grown with under 10 ohms per square and over 80% transmission of light

1185

1186

6.5.1

Thin Film Electroluminescence (TFEL)

More recently, a number of variants on this structure have been developed in which thick films or even sheets are used in place of the thin film insulating layers. In all cases, we will classify these structures as TFEL devices because the phosphor layer, which emits the light, is always a thin film. Note that sometimes only one dielectric layer is shown. There is no requirement for two insulating layers, although historically TFEL devices were made this way. Two thin (e.g., 10 nm) interface layers at the two sides of the phosphor layer may be formed as part of the EL structures, even if there is only one thicker insulating layer. More recently, a number of other important TFEL structures have been developed. These structures offer specific advantages for certain applications. They all rely on the same mechanism that will be discussed shortly. Recently developed by iFire Inc, Thick Dielectric EL (TDEL) devices [2] offer high EL brightness and efficiency. Here, the thin film dielectric layers are replaced with a single thick film dielectric made from high-dielectric-constant (relative dielectric constant over 1,000) ceramics such as barium titanates or lead zirconium titanates. The layer may be deposited by screen printing. A subsequent planarization layer provides a sufficiently smooth dielectric surface to permit the deposition of good-quality phosphor thin films. The use of freestanding ceramic sheets for EL substrate has been employed to develop a number of new EL phosphors [3]. Typically, a barium titanate ceramic is prepared by sintering barium titanate powder to form a dense ceramic and polishing at least one side of the resulting ceramic sheet to prepare for the phosphor layer. The sheet is typically about 200 mm thick. Another interesting EL structure has been demonstrated [4], in which the light from the EL phosphor passes through a glass substrate, and the thick film dielectric layer is deposited on the back side of the phosphor layer. In principle, this structure can exhibit high brightness and efficiency similar to the TDEL structure; however, it has not been developed very extensively to date. The device structure in Sphere-supported thin film EL [5] is effectively composed of numerous spherical, small-area TFEL devices embedded within a polymer (polypropylene) film. Each spherical TFEL device consists of a barium titanate sphere coated with a series of thin films, including the EL phosphor layer on the upper surface and an electrode on the lower surface.

3

Theory of Operation

The most important electronic processes occur within the phosphor layer as well as at the phosphor interfaces. The phosphor layer must satisfy a large number of criteria to enable efficient light emission: ● ● ● ●

It must be transparent to the wavelength of light being emitted. It must contain impurities having localized quantum states. It must be an electrical insulator. It must exhibit an avalanche-type breakdown process once a critical electric field is reached. The critical field is on the order of 108 V/m. A typical thickness of 1 mm means that this critical field is reached when about 100 V falls across it. ● The electrons that generate light must be able to fall into a localized ground state to cause light emission even in the presence of a high electric field in the phosphor layer.

The process by which light is emitted is shown in > Fig. 2. The light emission process begins upon application of a voltage across the electrodes of the TFEL device, causing the phosphor layer to sustain a high electric field. This electric field allows

Thin Film Electroluminescence (TFEL)

6.5.1

hn Phosphor layer in avalanching electric field

Dielectric layer Energy

Position

. Fig. 2 Energy band diagram of TFEL device showing electron trapping at phosphor/dielectric interfaces solid circles are occupied traps, hollow circles are empty traps, tunneling, impact excitation of activator, and re-trapping (Reprinted from [6], copyright (1995), with permission from World Scientific Publishing Co Pte. Ltd.)

electrons trapped in interface states at the interface layer, on the left-hand side of > Fig. 2, to tunnel into the conduction band of the phosphor layer. Once traveling in the conduction band, these electrons become ‘‘hot,’’ possessing a few electron volts of kinetic energy, and may impact-excite impurity centers causing the electrons in the ground state of the impurity center to become excited. When these excited electrons return to their ground state, light is emitted. The conduction band electrons may also excite other valence band electrons into the conduction band by an avalanche process. These further electrons may also excite impurity centers. Eventually, electrons in the conduction band reach the opposite interface layer on the right and get trapped at interface states there. No further excitation occurs until the electric field is reversed in polarity, allowing these trapped electrons to return to the left interface layer. Light emission occurs again during this process. A certain proportion of the conduction band electrons will return to the valence band, allowing avalanche processes to repeat every time electrons cross the phosphor layer. Light emission is pulsed, and an AC voltage is necessary for sustained operation of this light emission process. This voltage generally consists of a series of voltage pulses of peak voltage Vp, with alternating polarity. However, any AC voltage such as a sine wave will cause light emission, provided that the critical electric field is reached in the phosphor layer. If a series of voltage pulses of the same voltage Vp is applied of only one polarity, then light emission will not occur. The light emission from a typical device is shown in > Fig. 3 [7]. Here, the phosphor material is ZnS:Mn. ZnS is a well-known semiconductor having a valence band as well as

1187

6.5.1

Thin Film Electroluminescence (TFEL)

103

ZnS:Mn (0.65 μm) 1 kHz Sinusoidal wave drive L L30

103

102

102

101 η

101

100

Threshold voltage 100 100

Luminous efficiency η (lm/W)

104

Luminance L (cd/m2)

1188

150

200

250

Vth 10–1 300

Voltage V (V)

. Fig. 3 Brightness-voltage characteristic of ZnS:Mn EL phosphor measured in a thin film device. (Reprinted from [7], copyright (1995), with permission from World Scientific Publishing Co Pte. Ltd.)

a conduction band, with an energy gap of about 3.6 eV. Manganese is well known to possess a localized electronic ground state and an excited state. Note the sudden onset of luminance at a specific threshold voltage V. This is inherent in an avalanche process and is useful for display applications of TFEL. In a passive matrix flat panel TFEL display, partial voltages are present across ‘‘off ’’ pixels. If they lie below the threshold voltage, these partial voltages do not cause light emission, leading to high-contrast flat panel displays. Details of multiplexing drive methods of a TFEL display are well described in the literature and will not be discussed further in this chapter (see [8]). It is now appropriate to describe the purpose of the dielectric insulator layer(s) in the EL device. For drive voltages below the threshold voltage, the phosphor layer is an insulator. However, for higher drive voltages, the phosphor layer is an avalanching state, and therefore the voltage drop across this layer is clamped. Without the insulating layer, high currents could flow and damage the device. The dielectric layer limits the maximum current flow to the phosphor layer according to the general relationship I ¼ CdV=dt

ð1Þ

where C is the effective dielectric layer capacitance per unit area and V is applied voltage (assumed to be larger than the threshold voltage). An equivalent circuit of a TFEL device is shown in > Fig. 4. A capacitor C is connected in series with two back-to-back zener diodes. Ci represents the effective dielectric capacitance per unit area of the TFEL device, and the back-to-back zener diodes represent the phosphor layer.

Thin Film Electroluminescence (TFEL)

6.5.1

Ci Cp

}

Zener diodes: breakdown voltage Vt

. Fig. 4 Equivalent circuit of TFEL device showing insulator capacitance Ci (note that this represents the series capacitance of both dielectric layers if two layers are present), Cp the phosphor layer capacitance, and phosphor breakdown voltage Vt represented as the zener diode turn-on voltage in reverse bias

The zener diode voltage represents the phosphor threshold voltage Vt (phosphor) and is determined by the critical electric field across the phosphor layer for avalanching to occur as well as the phosphor layer thickness. It is experimentally determined that the brightness of an EL device is substantially proportional to the amount of charge per unit area that flows across the phosphor layer during each voltage pulse, as well as to the frequency of these pulses. The charge flowing through the phosphor layer flows only for applied voltages that exceed Vt and is determined from Q ¼ Ci ðV  Vt Þ

ð2Þ

where Vt is the threshold voltage and V is the applied voltage. Clearly, a high capacitance dielectric is desirable to maximize the transferred charge through the phosphor of the EL device and hence its brightness. We know that Ci ¼ e0 ed =d

ð3Þ

where d is the dielectric thickness and ed is the relative dielectric constant of the dielectric material. For this reason, high-dielectric-constant dielectric materials play an important role in high-performance EL devices. In addition, the thickness of the dielectric layer needs to be minimized, but this thickness may be much higher for materials with high dielectric constants. For example, a 20 mm barium titanate-based layer having relative dielectric constant of 2,000 will actually provide a value of Ci that is almost four times higher than a 0.4 mm alumina layer having a relative dielectric constant of 11. Now, Q = Ci(V  Vt) will be higher, and the maximum EL brightness will increase.

4

Electroluminescent Phosphors

The first high-performance phosphor material and still the material that has received the most study is ZnS:Mn. ZnS, a II–VI semiconductor with an energy gap of 3.6 eV. [9]. There are two structural variants, namely, a cubic phase and a hexagonal phase. However, these two phases differ only in terms of second-nearest neighbors, and both phases are excellent EL phosphors. In either phase, each Zn2+ ion is coordinated by 4 S2 ions in a tetrahedral configuration.

1189

1190

6.5.1

Thin Film Electroluminescence (TFEL)

ZnS is a relatively stable sulfide in that it may be stored in ambient atmospheric conditions without degradation. However, it does require moisture protection when it is subjected to high electric fields in an EL device. This may be due to the field-assisted dissociation of water to form OH-ions that hydrate the ZnS material. The EL phosphor ZnS:Mn is more correctly described as MnxZn(lx)S with x in the range of 0.005–0.02. The dopant Mn provides the observed yellow luminescence, and Mn as a 2+ ion substitutes effectively for Zn2+ ions. This is achieved since both Zn and Mn are 2+ ions, have similar ionic radii (radius of Zn2+ = 0.74 nm, radius of Mn2+ = 0.8 nm), and are able to bond in a tetrahedral configuration. Mn, being a transition metal, has an unfilled inner shell or 3D shell, which gives rise to a ground state as well as an excited state. It is important to note that the electronic transition from excited state to ground state generates a photon, but it does not affect the charge on the Mn2+ ion, which remains at 2+ throughout this transition. The luminescent process is therefore not a charge transfer process, whereas the luminescence in other well-known ZnS phosphors, such as CRT phosphors ZnS: Cu and ZnS:Ag, is a charge transfer processes. Here, an electron falls from a higher energy state elsewhere in the phosphor and is accepted by the Cu2+ or Ag2+ dopant. This results in a Cu1+ or Ag1+ state, and the luminescence results from the transfer of the electron to the metal ion, which is a trap-assisted electron–hole recombination process. However, the Mn2+ ion emits light without charge transfer. This distinction is very important, as only luminescent centers that function without charge transfer generally exhibit good EL properties, including high efficiency and long life. When a high electric field is applied to the EL phosphor, the avalanche process takes place, and the hot electrons generated impact-excite the Mn centers. This impact-excitation process is characterized by an impact-excitation cross-section, which describes the success rate of hot electrons exciting the Mn2+ ion. Mn has a large impact-excitation cross-section in ZnS, and the applied electric field does not modify the efficient recombination process that takes place within the Mn2+ ion. Trap-assisted electron–hole luminescence processes are not useful for high field EL since the electric fields established to cause avalanching, as well as the acceleration of electrons to become ‘‘hot,’’ sweep free carriers away and prevent their recombination. Therefore, electrons that normally recombine and generate photons in CRT phosphors, such as ZnS:Cu and ZnS:Ag, are driven back and forth between the phosphor/dielectric interfaces under EL drive conditions, and eventually may find their way back to the valence band via non-radiative trapping. For this reason, ZnS:Cu and ZnS:Ag are not used as thin film EL phosphors. Mn2+ ions may also generate green luminescence in other EL phosphor hosts. An effective oxide phosphor Zn2xMnxSiO4 [3] relies on the same inner-shell radiative transition as in ZnS:Mn. The color of the luminescence is green instead of yellow, due to a change in the crystal field surrounding the Mn2+ ion. Other efficient EL luminescent centers exist. Both sulfide and oxide compounds exist. Nitride and fluoride host materials have also been identified, but EL performance has not been as successful to date. Eu2+ exhibits excellent blue luminescence in BaAl2S4 phosphors. Eu2+ has a 4f-4d transition. Ce3+ exhibits bright green-blue luminescence in SrS phosphors, and Ce3+ has a 4f-4d transition. A large number of host materials are available from modifications of other hosts. For example, MgxZn1xS:Mn phosphors exhibit a blue-shift compared to ZnS:Mn material, which gives this phosphor a peak wavelength below 580 nm [10]. SrGa2S4:Eu is a green EL phosphor [11] not the same as, but related to, BaAl2S4:Eu. Zn2Si1xGexO4:Mn represents a family of

Thin Film Electroluminescence (TFEL)

6.5.1

. Table 1 Important sulfide and oxide EL phosphors showing their composition color, CIE coordinates, and reported luminous efficiency. Note that in many cases, a range of efficiency values is given, since various values are known depending on the device structure and preparation conditions Phosphor

Color

CIE coordinates

Efficiency, lm/W

Reference

1

ZnS:Mn

Yellow

0.5, 0.5

3–10

[13]

2

ZnS:Tb

Green

0.32, 0.6

0.5–2

[13]

3

SrS:Ce

Blue-green

0.19, 0.38

0.5–1.5

[13]

4

SrS:Ce, Eu

White

0.41, 0.39

0.4

[13]

5

BaAl2S4:Eu

Blue

0.135, 0.1

0–1.5

[15]

6

SrGa2S4:Eu

Green

0.226, 0.701

1–2

[11]

7

Zn2SiO4:Mn

Green

0.2, 0.7

0.5–2

[16]

8

Zn2SixGe1-xO4:Mn

Green

0.2, 0.7

1–3

[16] [16]

9

ZnGa2O4:Mn

Green

0.08, 0.68

1–2

10

Ga2O3:Eu

Red

0.64, 0.36

0.5–1

[14]

11

Y2O3:Mn

Yellow

0.51, 0.44

10

[17]

12

YxGayO3:Mn

Yellow

0.54, 0.46

10

[17]

13

YxGe,O3:Mn

Yellow

0.43, 0.44

10

[17]

green oxide phosphors with higher brightness and efficiency and relatively low processing temperature requirements compared to other oxide phosphors, such as Zn2SiO4:Mn [12]. ZnS:Tb is also a bright green phosphor [13] that is unusual in that Tb is not readily soluble in the ZnS host due to the incompatible size and charge of the Tb3+ ion. A charge compensation co-activator, such as F or O, can play a role in improving the performance of these phosphors. Eu3+ exhibits bright orange-red luminescence in Ga2O3 host material. Eu3+ has a 4f-4f transition. This phosphor is unusual in that it works in an amorphous phase, demonstrating that crystallization is not a requirement for all EL phosphors. Eu3+ is not size-compatible with the Ga cations in the host material [14]. A list of some important EL phosphors together with their properties is shown in > Table 1.

5

Conclusions

From early monochrome EL displays in the 1980s to full-color EL displays of 2006, the field of thin film EL has been active. In particular, the achievement of a wide range of new EL phosphor compounds has been impressive, particularly in that new EL phosphor materials cannot be modeled and then fabricated due to our incomplete understanding of hot electron processes, as well our lack of models of electric field effects on luminescent centers. Phosphor compounds for thin film EL devices are almost always distinct from successful powder phosphors used in fluorescent lamps and CRTs. The innovative range of device structures is also particularly noteworthy. Ceramic, glasses, thick films and thin films, as well as a wide range of fabrication processes and methods have been developed for thin film EL materials.

1191

1192

6.5.1

Thin Film Electroluminescence (TFEL)

The commercial availability of LCD and Plasma display panels has currently reduced the large-scale development of thin film EL devices. However, the desire for a truly solid state flat panel is still there. Organic Light-Emitting Diode (OLED) displays are another development that has taken attention away from thin film electroluminescence. However, OLED displays have not yet reached performance levels that allow them to compete in most flat panel display markets. Thin film electroluminescence will continue to be a fascinating technology, and further developments in fundamental physics, materials, fabrication techniques, and architectures could play a role in their continued commercial viability.

Acknowledgments This chapter is a condensed version of Kitai (2008).

References 1. Russ MJ, Kenney DI (1967) The effects of double insulating layers on the electroluminescence of evaporated ZnS:Mn films. J Electrochem Soc 114:1066 2. Wu X, Carkner D, Hamada H, Yoshida I, Kutsukake M, Dantani K (2004) Large-screen flat panel displays based on thick-dielectric electroluminescent (TDEL) technology. SID Dig 35:1146 3. Minami T, Miyata T, Takata S, Fukuda I (1991) High-luminance green Zn2SiO4:Mn thin-film electroluminescent devices using an insulating BaTiO3 ceramic sheet. Jpn J Appl Phys 30:L117–L119 4. Heikenfeld J, Jones RA, Steckl AJ (2003) Black dielectric electroluminescent 160  80 pixel display. SID Dig 34:1110 5. Xiang Y, Kitai AH, Cox B (2005) Sphere-supported thin-film electroluminescence: a new platform technology for displays and lighting. J Soc Info Display 13:493 6. Ono YA (1995) Electroluminescent displays. World Scientific, Singapore, p 9 7. Ono YA (1995) Electroluminescent displays. World Scientific, Singapore, p 10

Further Reading Kitai AH (2008) Thin film electroluminescence. In: Kitai A (ed) Luminescent materials and applications. John Wiley, Chichester

8. Ono YA (1995) Electroluminescent displays. World Scientific, Singapore, p 98 9. Ono YA (1995) Electroluminescent displays. World Scientific, Singapore, p 44 10. Mikami A (1997) Proc SID Int Symp Dig 28:851 11. Yano Y, Oike T, Nagano K (2002) Proc. EL 2002, Gent, Belgium, p 225 12. Xiao T, Kitai AH, Liu G, Nakua A (1997) SID Dig 28:415 13. Ono YA (1995) Electroluminescent displays. World Scientific, Singapore, p 84 14. Stodilka D, Kitai AH, Huang Z, Cook K (2000) SID Int Symp Dig 31(May):11 15. Xin Y, Hunt T, Acchione J (2004) Multi-source deposition of BaAl2S4 blue phosphors. SID Dig 35:1138 16. Minami T (1998) Extended abstracts of the 4th international conference science and technology of display phosphors, Oregon, p 195 17. Minami T (2002) Proc. EL 2002, Gent, Belgium, p 219

6.5.2 AC Powder Electroluminescence (ACPEL) and Devices Feng Chen . Adrian H. Kitai 1

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1194

2

Structure and Materials of AC Powder EL Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1195

3

The Mechanism of Light Emission for AC ZnS-Powder-EL Device . . . . . . . . . . . . . . 1196

4

EL Characteristics of AC Powder EL Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1199

5

ZnS Powder EL Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1201

6 6.1 6.2 6.3 6.4

Limitations of AC Powder EL Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1202 Lifetime and Luminance Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1202 Luminance and Relative High Operating Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1203 Moisture and Operating Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1203 Applications of ACPEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1204

This chapter is a condensed version of Chen F, Xiang Y (2008) AC powder electroluminescence. In: Kitai A (ed) Luminescent materials and applications. Wiley, Chichester, UK. Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.5.2, # Springer-Verlag Berlin Heidelberg 2012

1194

6.5.2

AC Powder Electroluminescence (ACPEL) and Devices

Abstract: Inorganic powder phosphor electroluminescence is divided into AC powder EL (ACPEL) and DC powder EL (DCPEL) in terms of the form of the driving electric field. Since DCPEL is not an active field today, and no commercial devices are manufactured, only ACPEL is discussed in this chapter. List of Abbreviations: ACPEL, AC Powder Electroluminescence; ACTFEL, AC Thin Film Electroluminescence; CVD, Chemical Vapor Deposition; DCPEL, DC Powder Electroluminescence; EXAFS, Extended X-Ray-Absorption Fine-Structure; TFT, Thin Film Transistor; XANES, X-Ray-Absorption Near-Edge Spectroscopy

1

Background

Historically, the phenomenon of electroluminescence was first observed in SiC by H. J. Round in 1907 [1]. He reported visible light emitted from the negative contact when an electrical current was passed through a rectifying contact on SiC. In the 1920s and 1930s, Lossev [2–4] of the Nizhny Novgorod Radio Laboratory in the Soviet Union reported light emission after performing a series of experiments of rectifying contacts on SiC in more detail. This type of electroluminescence was known as ‘‘Lossev effect’’ and is considered nowadays as junction electroluminescence in a semiconductor [5]. The underlying light-emitting mechanism is the injection of minority carriers across a forward-biased p–n junction, followed by radiative recombination of electrons and holes. In 1936, Destriau [6] discovered light emission in zinc sulfide powder activated with an excess of copper. This was the first recorded report of electroluminescence in powder materials under applied voltage and was known as the ‘‘Destriau effect.’’ However, this effect received very little attention in the following decade. Even in 1950, Leverenz [7] questioned the mechanism of the light emission reported by Destriau. He suggested that the powder phosphor may in fact be excited by the UV light emitted by the electrical breakdown of the gases in the fairly porous powder phosphors. However, a great interest in ACPEL was triggered by the development of high performance polymers and ceramics during the Second World War till the late 1940s. Transparent conducting electrodes were achieved for the first time. From the 1950s to the mid-1960s, hence, large academic and industrial research efforts were devoted to ACPEL displays. Most of the work in that period was reviewed by Ivey [8] and Henisch [9]. Generally, ACPEL has a simple device structure and features ease of fabrication with a lowcost manufacturing method. At a low luminance of 3.4 cd/m2, its lifetime (operating time for the luminance to drop to half of its original luminance) is near infinite. ACPEL lights have been known to operate continuously for over 10 years at 100 V rms at 60 Hz. However, ACPEL has several significant challenges for display applications: ● Low discrimination ratio, which is defined as the ratio of luminance at V to the luminance at V/2 ● Low contrast ratio ● Short lifetime at moderate to high luminance The low discrimination ratio is due to the fundamental physics of the light-emitting mechanism of the AC powder phosphor. This feature indicates that ACPEL is not suitable for high-resolution multiplexed displays. One way to overcome this issue is to incorporate a Thin Film Transistor (TFT) drive circuit into the ACTFEL display as suggested by Fisher in 1971 [10] and explored by Brody [11]. Unfortunately, experimental devices had a considerable

AC Powder Electroluminescence (ACPEL) and Devices

6.5.2

number of blemishes, resulting from the poor quality of the TFTs. Meanwhile, TFTs were at their early stage and amorphous silicon technology did not exist at that time. Consequently, this technology was soon abandoned. The low contrast ratio in moderate to high ambient illumination is due to the high reflectivity of the powder phosphor itself. Filters can be used to increase the contrast ratio at the cost of decreased luminance. Therefore, higher voltages and frequencies were required to drive the display, which greatly decreased the lifetime of the ACPEL display. The short lifetime at moderate to high luminance is due to the exponential decay feature of the AC powder EL phosphors. Fisher [10] explained that the decay in brightness is related to the blunting of microscopic tips of Cu2S precipitates inside the ZnS phosphor particles. The blunting occurs by the diffusion of the copper ions in ZnS lattice under the influence of the high AC field. The degradation is deteriorated by moisture, high operating temperature and frequency. At about 170 cd/m2, the typical lifetime of ACPEL lamps is 1,000 h. In spite of the considerable technical effort from the 1950s to the mid 1960s, the inability to improve the lifetime of ACPEL displays led to its disfavor in the late 1960s. At the same time, other technologies such as gas discharge, light-emitting diodes, vacuum fluorescence, liquid crystals, and thin film electroluminescence attracted more and more research interest. These newly emerging technologies greatly diminished interest in ACPEL. By 1974, when Sharp developed a double-insulator AC thin film electroluminescent display with high performance and reliability, almost all of the research and development teams at the various companies throughout the USA who had been working on EL during the 1950s and 1960s had been disbanded [12]. Since then, no fundamental work has been published which has dramatically improved the performance of ACPEL lamp or displays, although important progress has been made in packaging. A CVD method is used to encapsulate ZnS:Cu,Cl phosphor particles with a Ti-Si-O film in order to improve the lifetime by preventing moisture penetration [13]. By maintaining constant current or constant power, lifetime of an ACPEL device producing 200 cd/m2 has been extended to more than 3,000 h [14].

2

Structure and Materials of AC Powder EL Devices

The first AC powder EL devices were developed by Sylvania in the first era of EL devices. A typical structure of an AC powder ZnS EL device is shown in > Fig. 1. To date, the

Al rear electrode

Dielectric ZnS:Cu,Cl phosphor (50–100 μm) Transparent electrode (ITO) glass or plastic substrate

. Fig. 1 Typical structure of AC powder phosphor EL device

1195

1196

6.5.2

AC Powder Electroluminescence (ACPEL) and Devices

. Table 1 Some of the powder phosphors known to exhibit EL under AC Phosphor

Color

References

ZnS:Cu,Cl(Br, I)

Blue

[15, 16]

ZnS:Cu,Cl(Br, I)

Green

[15, 16]

ZnS:Mn, Cl

Yellow

[17]

ZnS:Mn, Cu, Cl

Yellow

[18]

ZnSe:Cu, Cl

Yellow

[18]

ZnSSe:Cu, Cl

Yellow

[18]

ZnCdS:Mn, Cl (Cu)

Yellow

[15]

ZnCdS:Ag,Cl (Au)

Blue

[15]

ZnS:Cu,Al

Blue

[19]

well-known excitation of powder phosphors by alternating electric field is limited to a comparatively small group of phosphors, mainly of the ZnS type. As shown in > Fig. 1, the EL active phosphor layer consists of suitably doped ZnS powders with particle size of 5–20 mm suspended in a dielectric which acts as a binder as well. This phosphor layer is 50–100 mm thick and is sandwiched between two electrodes, one of which is transparent, and is supported by a substrate, consisting of either glass or flexible plastic. The EL color of the film depends on the activator of the ZnS phosphors. A common ZnS phosphor used is the green-emitting ZnS: Cu, Cl(or Al); in this material, the Cu activator acts as an acceptor and is responsible for the color of the emission, while Cl (or Al) works as a donor. The amount of the Cu added in the preparation process of these phosphors is 10
3 to 10
4 g/g of ZnS and is one order of magnitude larger than that added to ZnS phosphors used in CRTs. > Table 1 shows some of the binary and ternary systems that have been investigated over the years used for AC powder EL. The embedding dielectric in the device is an organic material with a large dielectric constant, such as cyanoethylcellulose or low melting glass [20]. In order to increase the stability and protect the EL device against catastrophic dielectric breakdown, an insulating layer, consisting of BaTiO3 powders dispersed in another dielectric material, is often inserted between the EL active layer and the Al rear electrode.

3

The Mechanism of Light Emission for AC ZnS-Powder-EL Device

Generally, the explanation for ACPEL is still open to speculation. However, until now, the most popular and reasonable theory has been the bipolar field-emission model, proposed by Fisher. A careful study of the interior of ZnS:Cu,Cl particles using an optical microscope was performed. It was observed that the shape of the lighting-emitting region with a single EL particle takes the form of double lines with shapes similar to twinkling tails of a comet, as illustrated in > Fig. 2. On further observing the ZnS phosphor particles under the microscope, Fisher found that there were many dark segregations and precipitates inside the phosphor particles as shown in > Fig. 3.

AC Powder Electroluminescence (ACPEL) and Devices

≅

E

Eth

6.5.2

E > Eth

. Fig. 2 Typical microscopic view of EL from ZnS:Cu,Cl particles. Double lines at threshold voltage and above the threshold voltage are illustrated (From [21], reproduced by permission of The Electrochemical Society)

ZnS Particle

Segregation

Field Direction

. Fig. 3 Phosphor particles containing dark segregations and emitting spots (From [21], reproduced by permission of The Electrochemical Society)

According to these observations, Fisher proposed a model, namely the bipolar fieldemission model, for the EL mechanism. First, he believed that the dark segregations and precipitates inside the phosphor particles are Cu2
xS. ZnS EL powders are typically prepared by firing at high temperature (1,100
1,200 C) where the hexagonal wurtzite phase predominates. When the powders are cooled, there is a phase transition to the cubic zinc-blende structure. Consequently, copper preferentially precipitates on defects formed in the hexagonalto-cubic transformation as the result of the reduction of their solubility in ZnS, in form of

1197

1198

6.5.2

AC Powder Electroluminescence (ACPEL) and Devices

conduction band electron hole

Cl EL valence band

p-CuχS

Cu ZnS

. Fig. 4 EL emission mechanism and schematic energy-band diagram of AC powder EL devices (From [22], reproduced by permission of The Electrochemical Society)

Cu2
xS needles embedded in the crystal matrix. Cu2
xS is known to be a p-type semiconductor with high conductivity. Between these Cu2
xS precipitates and ZnS powder, heterojunctions are formed, as shown in > Fig. 4 [22]. When an electric field is applied to the phosphor particles, relatively high electric fields will be concentrated on the tips of Cu2
xS conducting needles (the effective tip radius is on the order of 100 nm) compared with that of other regions. Therefore, an applied field of 106–107 V/m can induce a local field of 108 V/m or more. This electric field is strong enough to induce tunneling of holes from one end of the needle and electrons from the other to the ZnS: Cu,Cl lattice. The electrons are captured in nearby shallow traps in Cl donor sites, while the holes are trapped by the Cu recombination centers (acceptor sites). Eventually, a polarization field that opposes the applied field is formed at the Cu2
xS tips. When the field is reversed, there is temporarily a very large field near the tips, which enhances field emission in the reverse direction until a polarization field of the opposite sign develops. Meanwhile, injected electrons can combine with trapped holes (from the previous half cycle) at the recombination centers to produce EL each time the field is reversed. > Figure 5 [22] shows the illustration of the basic principle of the bipolar field-emission model. As a conclusion to the bipolar field-emission model, EL emission from ZnS-powder-EL device is caused by the radiative recombination of electron-hole pairs through donor–acceptor pairs. Experimental evidence has been provided by Ono et al. [23] for the validity of Fisher’s model by carefully examining ZnS:Cu phosphor particles showing EL using a transmission electron microscope (TEM). Black speckles in the shape of narrow needles with diameters of

AC Powder Electroluminescence (ACPEL) and Devices

6.5.2

. Fig. 5 Illustration of the basic principle of field-emission model. Above, at field application, electrons and holes are ejected from the opposite ends of the conducting inclusion, where the field is intensified, into ZnS lattice. Holes are trapped after short path. Electrons can travel farther. Below, at field reversal, trapped electrons flow back to recombine with trapped holes (light emission). Other electrons are field-emitted into the trapped holes. New holes are field-emitted at the other end of the conducting line (From [22], reproduced by permission of The Electrochemical Society)

20–40 nm along the boundaries of micro-twin crystals inside a ZnS particle were observed under TEM. The speckles were later identified as Cu2S based on X-ray-absorption near-edge spectroscopy (XANES) by measuring the wavelength of characteristic X-rays emitted from the precipitates. Cu2S is well known to be a p-type semiconductor with high metallic conductivity. Therefore, these observations support the prediction of Fisher.

4

EL Characteristics of AC Powder EL Devices

EL is observed when an AC voltage of about 100–200 V corresponding to an electric field of order of 104 V/cm is applied across the electrodes of the device. Luminance-voltage characteristics of a typical EL device are shown in > Fig. 6 [24]. The observed dependence of the luminance (L) on the applied voltage (V) is expressed by > Eq. 1
1=2 ! V0 ð1Þ L ¼ L0 exp
V

1199

6.5.2

AC Powder Electroluminescence (ACPEL) and Devices

frequency 400 Hz 100 L = L0 exp [–(V0 / V)1/2]

h ∝ L1/2 V–2

10

1

Efficiency h [Im / W]

10

Luminance L [cd/m2]

1200

1

100

200

300

Voltage Vrms [V]

. Fig. 6 Typical luminance-voltage and efficiency-voltage characteristics of powder AC EL devices

The parameters L0 and V0 depend on the particle size of the phosphor, the concentration of the powder in the dielectric, the dielectric constant of the embedding medium, and the device thickness. It has been established empirically that one of the key parameters affecting EL characteristics is the particle size, and a critical trade-off between the EL efficiency and the operational lifetimes, defined by the time when the luminance becomes one half of the initial value, exists as follows. The efficiency increases in proportion to d
1/2, where d is the particle size; this leads to the nonlinearity of the luminance-voltage dependence. The operational lifetime, however, decreases in proportion to d. In addition, the luminance increases with frequency in the frequency region of 100 Hz to 10 kHz. Luminance of 100 cd/m2 has been achieved for devices driven at a frequency of 400 Hz and a voltage of 200 V [24]. A typical voltage dependence of the EL efficiency, , is also shown in > Fig. 6. Typical values of the efficiency are 1–10 lm/W. The efficiency increases initially with increased applied voltage up to a saturation value, but then decreases gradually with further increases in voltage. The EL efficiency dependence on the voltage V is expressed by  ¼ L1=2 V
2. The maximum efficiency is obtained at a voltage well below the highest luminance level.

AC Powder Electroluminescence (ACPEL) and Devices

5

6.5.2

ZnS Powder EL Materials

Most powder EL research has been centered around the II–VI compounds and by far the most important EL lattice is zinc sulfide (ZnS). Zinc sulfide is a semiconductor material and exists in two main structural modifications, the low temperature form which is cubic zinc blende with band-gap energy of 3.7 eV, and the high temperature form which is hexagonal wurtzite with band-gap energy of 3.8 eV. Due to its excellent electrical properties such as the large band-gap energy, direct recombination and low leakage current, ZnS is ideal being used as phosphor material by doping with transition metals or rare-earth metals [25, 26]. In addition, owing to the advantage of the simple manufacturing process, the convenience of being able to print large areas and the high power efficiency, ZnS phosphor powders are suitable for back lighting of liquid crystal panels or for flat panel displays [27]. Therefore, ZnS type phosphors such as green-emitting ZnS:Cu, Al are very important from a practical point of view. Luminescence centers in these phosphors are formed from deep donors or deep acceptors, or by their association at the nearest-neighbor sites. The emission spectra of ACPEL devices are shown in > Fig. 7 [15]. Emission colors depend on the different luminescent centers incorporated in the phosphors. When the ZnS lattice is activated with Cu (activators) and Cl, I, and Al (co-activator), donor (co-activator)-acceptor (activators) pairs are formed. As mentioned above, the EL is due to the radiative recombination of electron-hole pairs at donor–acceptor (D–A) pair sites. The combination of Cu and Al (ZnS: Cu,Al) produces green (550 nm) emission color. The combination of Cu and Cl (ZnS:Cu, Cl) gives blue (460 nm) and green emission bands, their relative intensities depending on the relative amount of Cu to Cl. ZnS:Cu,I shows blue emission. It should be noticed that ZnS:Cu in which no co-activators are incorporated shows a red emission. By further incorporating Mn2+ ions into ZnS:Cu,Cl phosphors, the resultant ZnS:Cu,Mn,Cl shows a yellow emission(580 nm) due to Mn2+.

Intensity (arb, units)

violet

blue

green

ZnS:Cu, Cl

orange -red

ZrS: Cu, Al

red

ZnS: Cu,Mn,Cl

ZnS:Cu

Zns:Cu,l

400

500

600 Wavelength (nm)

. Fig. 7 The emission spectra of AC powder EL devices

700

1201

6.5.2 6

AC Powder Electroluminescence (ACPEL) and Devices

Limitations of AC Powder EL Devices

In the 1950s and 1960s, research effort was devoted to AC powder phosphor EL with the aim to fabricate efficient illumination panels. However, low luminance and significant luminance degradation in the course of operational times shorter than 500 h have been serious problems for devices by using powder phosphors. Fortunately, the research efforts were trigged again owing to the advance of AC thin film EL (ACTFEL) in 1974 with high luminance and lifetimes as long as 10,000 h [28].

6.1

Lifetime and Luminance Degradation

Lifetime and degradation is a key issue for the application of AC powder phosphor EL. Lifetime is defined as the operating time over which the luminance decreases to one half of the initial value. > Figure 8 shows a typical example of EL light output versus time [29]. The degradation rate depends on the driving conditions (such as frequency and luminance levels) and on the environmental conditions, especially on temperature and humidity. The luminance decay with time is usually expressed by L=L0 ¼ ð1 þ atÞ
1, where a is a constant roughly proportional to the driving frequency [14]. Furthermore, it is very difficult to have both long lifetime and high luminance simultaneously for powder EL devices because they are trade-off characteristics. According to Fisher’s bipolar field-emission model, the degradation is related to the diffusion of copper in the ZnS lattice with the help of electric field. It is known that metals of group I (Ag and Cu) are fast-diffusion impurities in II–VI compounds [30]. The tips of the copper sulfide-decorated imperfection lines, which induce a highly localized electric field in

100% Operation: 24°C, 51% RH V0-p=230 V, f=250 Hz square waveform (5/25/5)

90% Luminance (%)

1202

80%

70% L/L0=(1+αt)−1 α=1⫻10−3 h−1

60%

50% 0

100

200

300 400 Hour (hr)

500

600

700

. Fig. 8 Luminance maintenance curve of AC powder EL Lamp. Operation:V0
p = 230, f = 250 Hz, square waveform with pulse width of 25 s, rise time of 5 s, and fall time of 5 s (From [29], reproduced by permission of The Electrochemical Society)

AC Powder Electroluminescence (ACPEL) and Devices

6.5.2

the phosphor region, can be blunted by the diffusion of copper ions or by the attraction of copper ions from the adjacent host crystal under the influence of the high AC field. As operating time increases, copper sulfide lines become shorter and approach a final state which does not initiate sufficient electric field to excite luminescence. It has been experimentally confirmed that sulfur vacancies are one factor which accelerates EL deterioration, although the sulfur vacancy density is not increased by deterioration [31], suggesting that the sulfur vacancies assist copper diffusion. However, this model was recently challenged by Warkentin et al. using the extended X-rayabsorption fine-structure (EXAFS) technique [32]. They argued that XANES used by Onio et al. in 1990 is not a good probe of structure because the X-ray absorption edges of Cu in CuS and Cu2S are essentially the same energy, and therefore EXAFS is needed to identify these two structures. According to their EXAFS results, the identification of CuxS precipitates as CuS instead of Cu2S was made. It was suggested that the CuS precipitates, which themselves do not electroluminesce, are unchanged during the degradation of the device. Meanwhile, isolated Cu ions, which are present as a solute in the ZnS lattice and formed along with CuS clusters during the phase transition upon cooling, trap electrons and work as highly effective recombination centers. Therefore, the degradation of the powder EL devices is explained as the diffusion of isolated EL-active Cu ions to CuS clusters or the ZnS particle surface, resulting in the concentration of isolated Cu decreasing as EL degradation progresses. This model is quite possible because the degraded powder devices can be recovered under a heat treatment of relatively low temperature because of the low decomposition temperature for CuS (220 C or even lower for small clusters in the ZnS crystal due to the chemical pressure [33]). The recovered brightness depends on the temperature and time of heat treatment [34]. In summary, the cause of ACPEL degradation is far from fully understood and extensive research work is still needed. Regardless of which model is more likely to approach the real case, however, it can be concluded that the diffusion of Cu in and around CuxS is substantially responsible for the loss of EL brightness. It has been experimentally proved that EL lamps operated at elevated temperature degrade much faster than those operated at low temperature, which is the most suggestive evidence that powder EL degradation is a diffusion-related phenomenon [29].

6.2

Luminance and Relative High Operating Voltage

A typical value of luminance for AC ZnS-powder EL devices is from 3 to 10 cd/m2 and the operating voltage is usually above 100 V. As mentioned above, luminance and lifetime are trade-off characteristics for power EL devices. Recently, the luminance has been improved steadily up to 100 cd/m2 driven at a frequency of 400 Hz and a voltage of 200 V as shown in > Fig. 8.

6.3

Moisture and Operating Environment

Another serious challenge for powder EL is that it is very sensitive to the moisture and operating environment. The reaction shown below occurs between ZnS and water to produce SiO2. ZnS þ 2H2 O ! SO2 þ Zn þ 2H2

1203

1204

6.5.2

AC Powder Electroluminescence (ACPEL) and Devices

Through this reaction, sulfur escapes from the ZnS phosphor, generating sulfur vacancies and zinc vacancies in the phosphor. It has been reported that the luminance of the ZnS phosphor deteriorates when the number of sulfur vacancies increases [31]. For this reason, two methods preventing degradation have been developed. One method is using anti-humidity film, such as fluorocarbon film [35], to package the entire device. Another method is coating the phosphor particles themselves by transparent thin films with anti-humidity properties [36–39].

6.4

Applications of ACPEL

As summarized by S.V. Peteryl and P.R. Fuller [40], EL lamps and information displays historically suffer from serious shortcomings-low luminance, short useful operating life, poor visibility in normal room light, and no visibility under high ambient light. In addition, the high-voltage control circuit presented great obstacles, and operation at temperatures much above room temperature caused rapid deterioration of intensity of emitted light. In spite of this, ACPEL features include low power consumption, uniform light emission over a large area, and an easy screen-print fabrication method. Due to the low brightness and short lifetime, however, the applications of ACPEL devices are mainly limited to backlighting and lamp applications that require low brightness and low illumination environment, such as nightlights and backlighting for LCDs and keypads in portable electronics and home electronics. In addition, ACPEL with plastic cell structure provides great versatility in the product design in terms of weight, compactness, and robustness. Flexible ACPEL lamp can be folded, creased, and pierced while maintaining complete functionality – allowing design concepts previously unachievable. ACPEL continues to be the most commercially successful high-field EL device.

References 1. Krasnov AN (2003) Electroluminescent displays: history and lessons learned. Displays 24(2):73–79 2. Lossev OV (1923) Telegrafia Telefonia 18:61 3. Lossev OV (1928) Philos Mag Ser 6(7):1024 4. Lossev OV (1933) Phys Z 34:397 5. Grimmeiss HG, Allen JW (2006) Light emitting diodes – How it started. J Non-Cryst Solids 352:871 6. Destriau G (1936) J Chim Phys 33:587 7. Leverenz HW (1950) An introduction to luminescence of solids. Wiley, New York, p. 61 8. Ivey HF (1966) Advances in electronics and electron physics. Suppl. 1: electroluminescence and related effects. Academic, New York 9. Henisch HK (1962) Electroluminescence. Pergamon, Oxford 10. Fischer AG (1971) Electroluminescent II–VI heterojunctions. J Electrochem Soc 118:139c 11. Brody TP, Yu KK (1975) A 6  6-in 20-lpi electroluminescent display panel. IEEE Trans Electron Devices 22:739 12. Tannas LE Jr (1985) Flat-panel displays and CRTs. Van Nostrand Reinhold, New York, p. 244

13. Sawada M, Oobayashi S, Yamaguchi K, Takemura H, Nakamura M, Momose K, Saka H (2002) Characteristics of light emission lifetime of electroluminescent phosphor encapsulated by titanium–silicon–oxide film. Jpn J Appl Phys 41(6A):3885–3889 14. Shionoya S, Yen WM (1998) Phosphor handbook. CRC, Boca Raton, p. 606 15. Destriau G (1937) J Chim Phys 34:327 16. Jaffe PM (1961) On the theory of electroluminescence deterioration. J Electrochem Soc 108:711 17. Gobrecht H, Gumlich HE (1956) Sur le renforcement et l’extinction par les champs e´lectriques alternatifs de la luminescence des sulfures de zinc active´s au manganese. J Phys Radium 17:754 18. Thornton WA (1958) Bull Am Phys Soc 3:233 19. Gobrecht H, Gumlich HE, Nelkowaki H, Langar DZ (1957) Kathodo-Elektro-Lumineszenzerscheinungen bei Zinksulfid-Phosphoren. Z Phys 149:504 20. Ono Y (1995) Electroluminescent displays, Series on information display. World Scientific, Singapore, p. 11

AC Powder Electroluminescence (ACPEL) and Devices 21. Fisher AG (1962) Electroluminescent lines in ZnS powder particles: I. Embedding media and basic observations. J Electrochem Soc 109(11):1043–1049 22. Fisher AG (1963) Electroluminescent lines in ZnS powder particles: II. Models and comparison with experience. J Electrochem Soc 110:733–748 23. Ono Y, Shiraga N, Kadokura H, Yamada K (1990) Electron Inform Commun Eng Tech Rep 89:378 24. Shionoya S, Yen WM (1998) Phosphor handbook. CRC, Boca Raton, p. 603 25. McClure DS (1963) Optical spectra of exchange coupled Mn++ ion pairs in ZnS:MnS. J Chem Phys 39:2850 26. Shrader RE, Larach S, Yocom PN (1971) Cathodoluminescence efficiency of Tm+3 in zinc sulfide. J Appl Phys 42:4529 27. Nien YT, Chen IG, Hwang CS, Chu SY (2006) Microstructure and electroluminescence of ZnS:Cu, Cl phosphor powders prepared by firing with CuS nanocrystallites. J Electroceram 17:299–303 28. Shionoya S, Yen WM (1998) Phosphor handbook. CRC, Boca Raton, p. 601 29. Chen F, Kitai AH, Xiang Y (2009) Temperaturedependent degradation of AC powder EL. J Electrochem Soc 156(7):H585–H587 ¨ ztu¨rk K, 30. Bacaksiz E, Dzhafarov TD, Novruzov VD, O Tomakin M, Ku¨cu¨ko¨merog˘lu T, Altumbas¸ M,

Further Reading Chen F, Xiang Y (2008) AC powder electroluminescence. In: Kitai A (ed) Luminescent materials and applications. Wiley, Chichester

31.

32.

33.

34.

35. 36. 37.

38. 39. 40.

6.5.2

Yanmaz E, Abay B (2004) Copper diffusion in ZnS thin films. Phys Status Solidi A 201:2948 Hirabayashi K, Ozawaguchi H, Tsujiyama B (1983) Study on A-C Powder EL Phosphor Deterioration Factors. J Electrochem Soc 130(11):2259 Warkentin M, Bridges F, Carter SA, Anderson M (2007) Electroluminescence materials ZnS:Cu, Cl and ZnS:Cu, Mn, Cl studied by EXAFS spectroscopy. Phys Rev B 75:075301 Hanes A, Kenl M, Mieroiu E, Sandulescu D, Zaharia M (1972) Manualul lnginerului himist, I. Editura Tehnica, Bucharest Stanley J, Jiang Y, Bridges F, Carter SA, Ruhlen L (2010) Degradation and rejuvenation studies of AC electroluminescent ZnS:Cu,Cl phosphors. J Phys Condens Matter 22:055301 Hirabayashi K, Kozawaguchi H, Tsujiyama B (1985) Kenkyu Jitsuyoka, Hokoku 34, 525 (In Japanese) Sigai AG (1991) US Patent 5,051,277 Yan S, Maeda H, Hayashi JI, Kusakabe K, Morooka S, Okubo T (1993) Low-temperature plasma coating of electroluminescence particles with silicon nitride film. J Mater Sci 28:1829 Budd KD (1995) US Patent, 5,418,062 Budd KD (1999) US Patent 5,958,591 Tannas LE Jr (1985) Flat-panel displays and CRTs. Van Nostrand Reinhold, New York, p. 241

1205

Part 6.6

Organic Electroluminescent Displays

6.6.1 Organic Light Emitting Diodes (OLEDS) Ruiqing Ma 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1210

2 2.1 2.2

OLED Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1211 Small Molecule OLEDs (SMOLEDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1211 Polymer OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213

3 3.1 3.2 3.2.1 3.2.2 3.2.3 3.3 3.3.1 3.3.2 3.4 3.4.1 3.4.2

OLED Design and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213 OLED Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213 Charge Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215 Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215 Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215 Carrier Injection Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1216 Charge Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1216 Hole Transport Layer (HTL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217 Electron Transport Layer (ETL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217 Light Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217 Host Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1218 Dopants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1219

4 4.1 4.2 4.3

OLED Fabrication and Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1219 OLED Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1219 Degradation and Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1220 Light Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1220

5

Future Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1221

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.6.1, # Springer-Verlag Berlin Heidelberg 2012

1210

6.6.1

Organic Light Emitting Diodes (OLEDS)

Abstract: Organic light emitting diode (OLED) technology has experienced substantial progress in the last couple of decades and is emerging as a strong flat panel display technology with new products ranging from mobile displays to small-size TVs introduced into the marketplace every year. After a brief review of the history and an introduction of basic device structures, this chapter will explain how OLEDs work by diving into the three key electroluminescent process steps: charge injection, charge transport, and charge recombination and light emission. This is followed by discussions on device fabrication and operation which cover topics such as degradation and light extraction. This chapter will end with a brief discussion on the future directions in the OLED research and development. List of Abbreviations: ETL, Electron Transport Layer; HOMO, Highest Occupied Molecular Orbital; HTL, Hole Transport Layer; IQE, Internal Quantum Efficiency; LUMO, Lowest Unoccupied Molecular Orbital; OLED, Organic Light Emitting Diode; OVPD, Organic Vapor Phase Deposition; PHOLED, Phosphorescent OLED; SMOLED, Small Molecule OLED; VTE, Vacuum Thermal Evaporation

1

Introduction

Organic light emitting diodes (OLEDs) are based on electroluminescence in organic materials, whose discovery can be traced back to the 1950s when light emission was observed by applying high voltage to thin films of acridine orange and quinacrine [1]. In 1963, Pope et al. observed bright blue electroluminescence in single crystal anthracene by applying a high DC voltage (
400 V) across the crystal [2]. Later, Vincett et al. used evaporated thin film of anthracene and was able to reduce the drive voltage to below 30 V. However, the quantum efficiency of this device is only about 0.05% [3]. The work that really opened the door of organic electroluminescence for practical applications was done by Tang et al. in the 1980s when they demonstrated a heterojunction OLED and developed a doping method by introducing a small amount of fluorescent materials into a host matrix [4, 5]. The electroluminescence in conjugated polymer poly(p-phenylene-vinylene) (PPV) was reported in 1990 [6], and since then significant progress has been made in the polymer OLEDs. This work was then followed by a breakthrough in 1998 when Forrest and Thompson discovered phosphorescent light emitting materials which dramatically improved the quantum efficiency of the OLED devices [7]. Today, phosphorescent OLEDs are a cornerstone technology for low-power flat panel displays and energy-efficient light sources based on OLEDs. As a flat panel display technology, OLED stands out with its ultrathin profile, vivid color, ultrafast switching speed, wide viewing angles, extremely high contrast ratio, and low power consumption. In addition, because of its simple, elegant structure, OLED can be easily made into transparent and flexible displays, which enables novel device architectures and applications. In this chapter, we will first introduce simple small molecule and polymer OLEDs. Then, we will discuss detailed OLED design and material considerations based on three key electroluminescent process steps: charge injection, charge transport, and charge recombination and light emission. Finally, we will discuss topics related to OLED fabrication and operation. This chapter puts more emphasis on small molecule OLEDs which are the technology used today in mass production. For readers interested in learning more about polymer OLEDs, some excellent references have been provided [8–10].

Organic Light Emitting Diodes (OLEDS)

2

OLED Basics

2.1

Small Molecule OLEDs (SMOLEDs)

6.6.1

In a typical bilayer OLED device, two different organic materials are sandwiched between an anode and a cathode, as shown in > Fig. 1. The material next to anode has good conductivity for positive charges (holes) and is used as the hole transport layer (HTL), while the material next to cathode transports electrons better and is used as the electron transport layer (ETL). The holes and electrons injected from anode and cathode, respectively, migrate through the HTL and ETL, and recombine at the HTL–ETL interface to form excited states called excitons. When excitons radiatively relax to the lower energy ground state, light is generated. > Figure 1 also shows the typical materials used for constructing a bilayer OLED device: aromatic diamine as HTL and tris (8-hydroxy-quinolinato) aluminum (Alq3) as ETL. Both molecules contain six benzene rings but with different configurations, and their properties will be further discussed in the following sessions. A typical energy diagram of the bilayer OLED structure under forward bias condition is shown in > Fig. 2. The electronic structure of organic molecules can be described by HOMO

Diamine

Cathode: Mg: Ag

Alq3

ETL: Alq3 N

HTL: Diamine

N

O

O N Al

N

Anode: ITO

O

N

Glass substrate

. Fig. 1 The structure and materials used in a bilayer OLED device (Reproduced from [4] with permission by American Institute of Physics)

ΔEC

HTL

Anode

- Cathode

-

ETL

+

ΔEV

+

. Fig. 2 Energy level diagram of a bilayer small molecule OLED

1211

6.6.1

Organic Light Emitting Diodes (OLEDS)

1.E+02

1.E+05

1.E+01

1.E+04

1.E+00

1.E+03

1.E−01

1.E+02

1.E−02

1.E+01

1.E−03

1.E+00

1.E−04

1.E−01

1.E−05 0

2

4

6

8

1.E−02 10

Voltage [V]

. Fig. 3 J–V–L characteristics of an OLED with the structure of ITO/NPD (40 nm)/Alq3 (40 nm)/Mg:Ag

Luminance [cd/m2]

(highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital). HOMO and LUMO may be considered as analagous to valence and conduction bands, respectively, in conventional semiconductor physics. From the energy diagram, we can see this bilayer OLED is basically a heterojunction diode. With forward bias, holes are injected from the indium tin oxide (ITO) anode to diamine, overcoming a small energy barrier. The hole mobility of organic materials is reasonably high and there is little voltage drop at the HTL layer. Once the holes reach the HTL/ETL interface, they encounter another energy barrier ΔEV, which is the HOMO energy level difference between the two organic materials. For electrons, they are injected from the cathode into Alq3 through a Schottky barrier, and once they migrate to the HTL/ETL interface, they experience ΔEC, which is the LUMO energy level difference between the two materials. Because of the existence of the barriers ΔEC for electrons and ΔEV for holes, carriers (electrons and holes) will accumulate at the HTL/ETL interface and have a better chance to recombine and thereby emit photons. In a different design, ΔEV is small so the holes can overcome the barrier and move to the ETL region while ΔEC is large enough so electrons will not be able to cross the interface. This way the recombination sites will be increased significantly because the depth of ETL material is utilized. > Figure 3 shows the current density–voltage–luminance (J–V–L) characteristics of a device with the following structure: ITO/NPD(40 nm)/Alq3(40 nm)/Mg:Ag. Bright green emission can be observed from this simple device. With a small forward bias, there is a very small current and little exciton formation. As the voltage is raised, the current quickly increases typically obeying a power law. When the current density is larger than 0.01 mA/cm2, the luminance output is directly proportional to the current density, and efficiencies of a few lumens per Watt (lm/W) are easily attainable.

Current density J[mA/cm2]

1212

Organic Light Emitting Diodes (OLEDS)

2.2

6.6.1

Polymer OLEDs

Since the early demonstrations of electroluminescence in conjugated polymer PPV in 1990, there has been intensive research and development effort in the field of polymer OLEDs. The structure of a simple one layer polymer OLED is shown in > Fig. 4, where conjugated polymer PPV is sandwiched between ITO anode and a low work function metal calcium cathode. The polymer layer can be deposited by printing or spin-coating, while the low function metal is deposited using vacuum thermal evaporation process. During the OLED operation, positive charges are injected from ITO anode and electrons are injected from cathode into the PPV layer. The electrons and holes move along and ‘‘hop’’ between the polymer chains, and when they meet, excitons are formed. These excited states can radiatively relax to the ground state, giving out light. The color of the light is determined by the energy gap of the polymer, and can be modified by changing the chemical structure of the polymer. The conjugated polymers typically transport holes much better than electrons. As a result, some of the holes will reach the cathode without recombination because of limited availability of electrons, resulting in a low-efficiency device. To solve this problem, an electron transport layer can be inserted between the PPV and cathode [11], similar to the bilayer structure shown in > Fig. 1. This ETL layer will improve electron injection from the cathode, block the holes from reaching the cathode without recombination, increase the charge density at the interface, and promote recombination. Notice that the light emitting material is a hole transport material (PPV) in this case while it is an electron transport material (Alq3) in the SMOLED shown in > Fig. 1.

3

OLED Design and Materials

3.1

OLED Structure

A more sophisticated SMOLED structure is demonstrated in > Fig. 5, where HIL, EIL, HBL, EBL, and EML stand for hole injection layer, electron injection layer, hole blocking layer, electron blocking layer, and emissive layer, respectively. The EML layer of a typical highefficiency OLED device consists of a phosphorescent dopant in an organic host matrix. When a transparent anode such as ITO and an opaque cathode are used, the light will be

Conjugated polymer PPV

n

Cathode (Calcium) ITO anode

Glass substrate

. Fig. 4 The structure of a single-layer polymer OLED

1213

1214

6.6.1

Organic Light Emitting Diodes (OLEDS)

Cathode EIL ETL HBL EML (Phosphorescent OLED) EBL HTL HIL Anode Glass substrate

. Fig. 5 A more sophisticated high-efficiency OLED device structure

2.6 eV

2.7 eV 3.2 eV

3.2 eV

3.7 eV MgAg

HTL

ITO

α-NPD 40 nm

4.7 eV

ETL

EML Ir(ppy)3 in CBP 20 nm

HBL

Alq3 20 nm

BCP 6 nm

5.7 eV 6.3 eV

6.0 eV 6.7 eV

. Fig. 6 Energy level diagram of a phosphorescent OLED device with ITO anode, Mg:Ag cathode and HTL, EML, HBL, and ETL organic layers

emitted through the substrate that the OLED is built upon and this configuration is called a bottom emitting OLED. With an opaque anode and a transparent cathode, the light will be emitted through the cathode and the device is called a top emitting OLED. When both electrodes can let light go through, the device is a transparent OLED. The energy level diagram of a typical phosphorescent OLED device with ITO anode, Mg:Ag cathode and HTL, EML, HBL, and ETL organic layers is shown in > Fig. 6. The electroluminescence process of OLED can be broken down into the following steps: (1) charge injection to organics by anode and cathode; (2) charge transport within the organics – injected charges

Organic Light Emitting Diodes (OLEDS)

6.6.1

referred as polarons migrate through the transport layers through a ‘‘hopping’’ mechanism; and (3) carrier recombination and light emission. The key considerations in designing a high-efficiency OLED device include (a) equal number of electrons and holes – multiple organic films are preferred; (b) high transport rates at low operating voltage; (c) efficient charge recombination and efficient emission from excited states.

3.2

Charge Injection

The work function of metal fm, defined as the energy difference between its Fermi level and the vacuum level, is different from the electron affinity of the organic material, which is the energy difference between its LUMO level and the vacuum level. This energy difference fb is the barrier for the carrier injection at the metal–organic interface. Two proposed carrier injection mechanisms at the metal–organic interface are (1) tunneling carrier injection with the help of a local high electric field and (2) thermally assisted thermionic injection via impurity and defects at the interface [12]. Adjusting the Fermi energy of the metal to be closer to the LUMO level of organic material will improve the injection for both cases.

3.2.1

Cathode

As explained above, the key to a good carrier injection is to reduce the barrier at the metal– organic interface. This requires low work function metals or metal alloys such as Ca (f = 2.9 eV), Yb (f = 2.4 eV), and Mg (f = 3.7 eV). However, low work function also indicates high chemical reactivity. The ease of oxidation of low function metals makes them very sensitive to the presence of oxygen and water during the OLED fabrication and operation. Typically, the reacting site becomes open circuit and a dark spot can be observed in a light emitting background when the device is turned on. This is the main driving force for the hermetic sealing of the OLED devices. Two of the popular cathode choices are LiF/Al and Mg:Ag. Studies show that a LiF/Al cathode, when brought into contact with organic materials, may generate anionic species that facilitate electron injection [13]. In the case of a Mg:Ag cathode, 10% of Ag helps to stabilize Mg and improve the adhesion of Mg to the organic materials while maintaining a low work function.

3.2.2

Anode

The most used anode material is ITO for several very important reasons: (1) as a highly degenerate n-type semiconductor, ITO has high conductivity for display applications; (2) ITO is transparent so light can be emitted through; (3) ITO has reasonable work functions (
4.7 eV) to match with the HTL; and (4) it has been used as electrodes for liquid crystal displays (LCDs) for many years so the technology and infrastructure are readily available. The work function of ITO, and thus the performance of the OLED, can be modified by surface treatment methods such as types of solvent applied, the application of plasma, and the

1215

1216

6.6.1

Organic Light Emitting Diodes (OLEDS)

application of a thin interfacial layer. Besides ITO, some examples of the materials investigated for anode application include aluminum, gold, silver, and other conductive oxide materials.

3.2.3

Carrier Injection Layers

As shown in the energy level diagram in > Fig. 6, there exists a large barrier for hole injection at the ITO–HTL interface. To address this issue, an interfacial HIL material can be inserted between ITO and HTL to facilitate the injection. For example, by introducing a thin layer of copper phthalocyanine (CuPc) between ITO and HTL layers, a highly stable OLED was achieved [14]. This is due to the better matched HOMO levels between CuPC (
5.1 eV) and ITO (
4.7 eV). In comparison, the HOMO level of the HTL is much larger (
5.7 eV). Besides CuPc, some examples of the HIL materials include PEDOT/PSS (poly-3,4ethylenedioxythiophene/polystyrene sulfonic acid), fluorohydrocarbon (CFx), and thin inorganic oxide films. Similar to HIL, a thin layer of interfacial EIL material can be introduced between cathode and ETL materials. The LiF layer in the LiF/Al cathode is a good example. Other materials that have been studied as EIL materials include CsF, M2O, BCP doped with Li/Cs, and organic polymer surfactants.

3.3

Charge Transport

The key ingredient for organic semiconductors is carbon–carbon double bond. In a carbon– carbon single bond structure, each carbon atom is bonded to four other atoms and the electronic orbitals are fully saturated. While in a double bond structure, each carbon atom is covalently bonded to three atoms, leaving one unpaired electron per carbon atom in the p orbital, as shown in > Fig. 7. The p orbitals of both carbon atoms overlap and electrons can freely move to the next orbital. As rings or double bond containing systems are linked together, the electrons are free to further delocalize, giving the material semiconducting or metallic properties. As these are the lowest energy orbitals, they are readily oxidized (hole) or reduced (electron) and become a path for charge to move. Although this intramolecular effect is important, the charge transport in OLEDs is dominated by intermolecular process. The key requirements of charge transport materials include high carrier mobility, good energy level alignment for charge injection and migration between layers, high electrochemical stability, high glass transition temperature (>100 C) to ensure thermal stability, and long thermal evaporation stability.

H3C H3C H3C H3C

C

C

CH3 CH3 CH3 CH3

. Fig. 7 The electron orbitals in carbon–carbon double bond and benzene

Organic Light Emitting Diodes (OLEDS)

3.3.1

6.6.1

Hole Transport Layer (HTL)

The functions of the HTL layer are to deliver the holes to the EML efficiently, and at the same time, to prevent electrons from entering the HTL and reaching the anode. Conjugated polymers are typically good conductor for holes so the need for a HTL layer is much less in a polymer OLED device. For a SMOLED, the introduction of a HTL layer is one of the important milestones in its development. Two commonly used HTL materials are N,N 0 -(3-methylphenyl)1,10 -biphenyl4,40 diamine (TPD: Tg = 60 C), and 4,40 -bis[N-(1-naphthyl–1)-N-phenyl-amino]-biphenyl (a-NPD: Tg = 95 C). Some other materials have been investigated as HTL include star-burst 4,40 ,400 -tri(N-carbazolyl)triphenylamine (TCTA: Tg = 151 C), Spiro-NPB (Tg = 147 C), indolocarbazole (Tg
164 C), triphenylmethanes, and phenylazomethines.

3.3.2

Electron Transport Layer (ETL)

The functions of the ETL materials are to deliver electrons to the emissive layer, and at the same time, block holes from reaching cathodes without recombination. An ideal ETL material should possess: (1) a LUMO level closely matched with cathode to facilitate electron injection, (2) a high electron mobility for electron transport, (3) the capability of forming a stable, amorphous thin film, and (4) an optimized band gap so not to absorb emitted light. Since its first appearance as emissive and electron transport material in the 1980s, Alq3 has been one of the most studied and commonly used ETL materials. Its electron mobility is about two orders of magnitude higher than the hole mobility [15]. Some other examples of the ETL materials studied include 1,3,5-tris(N-phenylbenzimidizol–2yl)benzene (TPBi), bis(10-hydro-xybenzo-quinolinato) beryllium (Be(bq)2), oxadiazole, triazole, and silole.

3.4

Light Emission

When an electron and a hole reach the recombination zone, they may form an exciton, which is a chargeless electron–hole pair that can transport energy. Unlike typical inorganic semiconductors that have delocalized excitons over many lattice sites, typical organic materials have excitons confined to local sites or molecules. These highly localized excitons can migrate by hopping to the neighboring molecules. Their energy can be released through radiative or nonradiative decay processes, or transferred to the neighboring molecules. Excitons generated by recombination of opposite charges are generally considered to form as a statistical mixture depending on their spin states [16]. There are one singlet state with zero spin angular momentum (S = 0) and three possible triplet states with total spin S = 1. The percentages of singlet and triplet states are 25% and 75%, respectively, determined by the quantum mechanical rule. The singlet excited states can readily relax to singlet ground states through a fast, efficient fluorescent process. Although considered as a spin-forbidden process, the triplet–singlet transition with emission of photons, called phosphorescence, can be realized with mechanism to change spin angular momentum. In most SMOLEDs, the EML layer is composed of one charge transport material as a host and at least one light emitting material as a dopant. During OLED operation, the singlet and triplet excited states are typically first formed in the host materials, and then can be transferred

1217

1218

6.6.1

Organic Light Emitting Diodes (OLEDS)

to the guest molecules through dipole–dipole interaction (Forster) or exchange-induced (Dexter) energy transfer processes, as shown in > Fig. 8. In a fluorescent system, light emission occurs when singlet excited states relaxes to the ground states, which limits the internal quantum efficiency to approximately 25%. In the phosphorescent system, heavy metals are used to introduce relativistic angular momentum to the electrons, converting singlet excited states to the triplet states through intersystem crossing [7]. Through spin-orbital coupling, triplets radiatively relax to the singlet ground states, thus enabling up to 100% internal quantum efficiency (IQE). In this phosphorescence process, the lifetime of the excited states is in the order of microseconds. Because of the very high IQE, phosphorescent OLED (PHOLED) technology enables displays to have lower power consumption than a backlit AMLCD, significantly extending battery life for mobile devices. The lower drive current requirement of PHOLEDs makes it easier to use amorphous silicon (a-Si) as the backplane TFT technology. The higher efficiencies also lead to reduction in display temperature rise, and thus extending the display lifetime.

3.4.1

Host Materials

One consideration in designing a host material is to facilitate the energy transfer to the guest. Studies on guest–host phosphorescent systems show that the band gap of the host material should be larger and enclose that of the guest [17]. Other considerations include high charge mobility, compatibility with the guest molecule in forming homogeneous thin films, and good thermal and electrochemical stability. It is very common to choose host materials from the charge transport materials. Several good electron transport hosts include Alq3 (LUMO: 3.0 eV; HOMO: 5.7 eV; T1 = 2.0 eV), TPBI (LUMO: 2.7 eV; HOMO: 6.2 eV), and aluminum bis(2-methyl–8-quinolinato)-4phenylphenolate (BAlq: LUMO: 3.0 eV; HOMO: 5.9 eV; T1 = 2.2 eV), while two examples of hole transport hosts are 4,40 -bis(9-carbazolyl)-biphenyl (CBP, LUMO: 3.0 eV;

S1 S1

Transfer

ISC

T1

T1 F

P So

So Host

Guest

. Fig. 8 Light generation in a guest–host system. S0, S1, and T1 are singlet ground state, first excited singlet state, and first excited triplet state, respectively. ISC, F, and P stand for intersystem crossing, fluorescence, and phosphorescence, respectively

Organic Light Emitting Diodes (OLEDS)

6.6.1

HOMO: 6.3 eV; T1 = 2.7 eV) and N,N’-dicarbazolyl-3,5-benzene (mCP, LUMO: 2.4 eV; HOMO: 5.9 eV;T1 = 3.0 eV). For fluorescent systems, a band gap of 2.7 eV makes Alq3 suitable as host material for red and green emitters. For fluorescent blue host, a larger band gap is needed. One commonly used material is ADN with a band gap of 3.2 eV. However, for phosphorescent systems where the transition is from triplet excited states, we should choose materials with appropriate triplet energies. For example, Balq and CBP, with triplet energies of about 2.2 eV and 2.7 eV, can be used as host materials for phosphorescent red and green emitters, respectively. For a material to be used as a deep blue phosphorescent host, its triplet energy needs to be larger than 3 eV. Arysilane compounds have been investigated for this purpose.

3.4.2

Dopants

Typical fluorescent dopants for red, green, and blue colors are red arylidene laser dyes such as 4-(Dicyanomethylene)-2-methyl–6-[p-(dimethylamino)styryl]-4H-pyran (DCM), Coumarin laser dyes such as C-545T, and distyrylarylene series such as BCzVB, respectively. Examples of early phosphorescent dopants for red, green, and blue colors are phenylisoquinoline iridium complexes, phenyl pyridine iridium complexes such as Ir(ppy)3, and Firpic, respectively.

4

OLED Fabrication and Operation

4.1

OLED Fabrication

To fabricate a bottom emitting OLED device, a transparent conductive oxide such as ITO is first patterned using standard photolithographic techniques to form the anode with predetermined dimensions. The ITO is then cleaned to give a smooth, particle-free surface, and treated by plasma or other surface treatment to improve charge injection. Then, the substrate is transferred into a vacuum deposition chamber. Organic layers are deposited sequentially by thermal evaporation onto the substrate. During this process, organic materials are sublimed from metallic or ceramic crucibles at a base pressure of
107 Torr. The deposition rate on the substrate is typically in the range of 0.1–0.5 nm/s, monitored by a quartz oscillator. In the case of guest–host doping layer, the host and dopant are deposited simultaneously and the doping concentration is controlled by adjusting the relative deposition rate of the dopant with respect to that of the host material. The footprint of the organic layer is defined by a shadow mask and the thickness is controlled by deposition time at a given rate. After all the organic layers have been deposited, a cathode, typically a low work function metal, is then evaporated onto the organic layers through a second shadow mask. The organic and metal depositions are preferred to be done in separate chambers to eliminate cross contamination. Although many techniques have been investigated for organic deposition, thermal evaporation and solution processing (coating, printing) are the two dominant processes. Vacuum thermal evaporation (VTE) offers pristine amorphous thin films and thus excellent device performance. However, patterning materials requires a shadow mask to be used, which results

1219

1220

6.6.1

Organic Light Emitting Diodes (OLEDS)

in lower material utilization and is difficult to implement for large size substrate. Since no shadow mask is needed, solution processing can have high scalability and high material utilization, and has the potential of developing into a high-throughput, low-cost manufacture process. While polymer materials are excellent candidates for solution process, small molecule OLED materials have also been developed for this purpose [18]. One exciting new technique for organic deposition is organic vapor phase deposition (OVPD). In this process, organic materials are vaporized and an inert carrier gas such as nitrogen is used to dilute and carry the materials to the cooled substrate where organic materials condense and form thin films. This technique has the potential to be a highthroughput manufacturing process.

4.2

Degradation and Encapsulation

The organic semiconductor materials used in OLEDs are prone to degradation due to their intrinsic properties. These include crystallization or other morphological changes over time, intrinsic photo/electrochemistry, migration, and reaction of impurities within the organic and electrodes. In addition, because of the high reactivity of low work function metals, the cathodes in OLEDs are easily oxidized and delaminate from organics when exposed to atmospheric oxygen and water [19]. During this process, water and oxygen first diffuse to the cathode– organic interface through pinholes and other defects in the cathode. Then, cathode and organic react with water and oxygen to form insulating layers (e.g., oxide) between the cathode and organic, which results local open circuits, seen by an observer as dark spots in the bright emissive background at on state. To protect the devices from external water and oxygen, OLEDs need to be hermetically sealed. This can be achieved by placing a metal can or a piece of cover glass on top of the OLED device followed by sealing the edges with epoxy or other sealant materials with good barrier properties. This work should be done in the controlled dry inert environment such as a nitrogen or argon glove box. Inside the chamber, desiccant materials in the forms of powder, getter, or gel are used to further protect the OLED devices. Another way to encapsulate OLEDs is to apply thin films of dense materials on top of the OLEDs. For example, silicon oxide and silicon nitride films have been used to encapsulate OLEDs. The challenge for this approach is that the film cannot be grown thick enough because it would crack. A very thin film is not effective in covering particles and topographical features. These defects and also pinholes in the very thin film leave paths for moisture and oxygen to penetrate. In a multilayer approach, the barrier consists of alternating layers of polymer films and inorganic oxide [20]. Oxide layers act as permeation barriers to the diffusion of water and oxygen, while the polymer layers prevent propagation of defects through the multilayer structure and planarize any particles or rough surfaces.

4.3

Light Extraction

The typical configuration of a bottom emission OLED device is shown in > Fig. 9. Light is generated in the organic layer. Since the refractive indices of the organic materials and ITO are larger than that of typical substrate materials, when light reaches the ITO–substrate interface,

Organic Light Emitting Diodes (OLEDS)

6.6.1

Organic and ITO (n > 1.7) Substrate (n~1.5) Air (n = 1.0)

. Fig. 9 Typical bottom emission OLED device structure and optical indices

a portion of the light will be reflected back due to the total internal reflection. In most cases, this portion of light will be trapped inside the organic/ITO, reabsorbed, or emitted at the edges (organic waveguide mode). For the light that is able to escape the organic/ITO layers, portion of it will be total internal reflected at the substrate–air interface, and trapped in the substrate (substrate waveguide mode). For example, at a glass–air interface, any light with an incident angle larger than the critical angle yc = arcSin (1/n) will be reflected. Here, n is the refractive index of the glass. When n = 1.45, yc is 43.6 . As a result of the waveguide effect, only about 20% of the light generated by an OLED can escape the device. Some proposed methods to improve light extraction include shaped devices [21], textured interfaces and surfaces [22], low index substrates [23], micro-lens array [24], and 2D photonic crystals.

5

Future Development

In recent years, OLED and LCD strongly influence each other in their research and development effort. While LCD continuously improves its view angle with multidomain structures, switching speed with new driving schemes, and color performance with LED backlights, OLED development has been focused on improving organic material performance in efficiency and lifetime, low-power-consumption device architectures, 3D and flexible displays, high-yield low-cost manufacturing process, and scaling up of this process for TV applications. This trend should continue at least in the near future. To realize OLED’s full potential in low power consumption, better materials such as phosphorescent light emitting materials and better device architectures need to be developed. On the device side, researchers continue to take advantage of OLEDs’ strength and work on novel display formats: (1) transparent displays – these have been demonstrated by Samsung at SID2010 and may have some very interesting applications [25]; (2) 3D displays utilizing the ultrafast switching of OLEDs; and (3) flexible displays – OLED is the strongest candidate for high information content full-color video flexible display applications [26]. With the success in small- to medium-size displays, the next big challenge for OLEDs will be scaling up to larger size TV applications, which really highlight OLED’s strength in deep black, fast switching speed, ultrahigh contrast at all viewing angles, vivid color, and low power consumption. Specific challenges include the scaling up of backplane, OLED patterning, and encapsulation technologies.

1221

1222

6.6.1

Organic Light Emitting Diodes (OLEDS)

References 1. Bernanose A, Comte M, Vouaux P (1953) A new method of emission of light by certain organic compounds. J Chem Phys 50:64–68 2. Pope M, Kallmann HP, Magnante P (1963) Electroluminescence in organic crystals. J Chem Phys 38:2042–2043 3. Vincett PS, Barlow WA, Hann RA, Roberts GG (1982) Electrical conduction and low voltage blue electroluminescence in vacuum-deposited organic films. Thin Solid Films 94:171–183 4. Tang CW, VanSlyke SA (1987) Organic electroluminescent diodes. Appl Phys Lett 51(12):913 5. Tang CW, VanSlyke SA, Chen CH (1989) Electroluminescence of doped organic thin films. J Appl Phys 65(9):3610–3616 6. Burroughes JH, Bradley DDC, Brown AR, Marks RN, Mackay K, Friend RH, Burn PL, Holmes AB (1990) Light-emitting diodes based on conjugated polymers. Nature 347:539–541 7. Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, Forrest SR (1998) High efficiency phosphorescent emission from organic electroluminescent devices. Nature 395:151 8. Greenham NC, Friend RH (1995) Semiconductor device physics of conjugated polymers. In: Ehrenreich H, Spaepen F (eds) Solid state physics. Academic, San Diego, pp 1–149 9. Friend RH (2001) Conjugated polymer: new materials for optoelectronic devices. Pure Appl Chem 73:425–430 10. Akcelrud L (2003) Electroluminescent polymers. Prog Polym Sci 28:875–962 11. Brown AR, Bradley DDC, Burroughes JH, Friend RH, Greenham NC, Burn PL, Holmes AB, Kraft A (1992) Poly(p-phenylenevinylene) light-emitting diodes: Enhanced electroluminescent efficiency through charge carrier confinement. Appl Phys Lett 61:2793–2795 12. Shinar J (ed) (2004) Organic light emitting devices. Springer, New York 13. Mason MG, Tang CW, Hung LS, Raychaudhuri P, Madathil J, Giesen DJ, Yan L, Le QT, Gao Y, Lee ST, Liao LS, Cheng LF, Salaneck WR, dos Santos DA, Bredas JL (2001) Interfacial chemistry of Alq3 and LiF with reactive metals. J Appl Phys 89:2756–2765 14. Vanslyke SA, Chen CH, Tang CW (1996) Organic electroluminescent devices with improved stability. Appl Phys Lett 69:2160–2162

15. Kepler RG, Beeson PM, Jacobs SJ, Anderson RA, Sinclair MB, Valencia VS, Cahill PA (1995) Electron and hole mobility in tris(8-hydroxyquinolinolato-N1, O8) aluminum. Appl Phys Lett 66:3618–3620 16. Turro NJ (1978) Modern molecular photochemistry. University Press, Menlo Park 17. Thomas T, Okada S, Chen J, Furugori M (2003) Improved host material design for phosphorescent guest-hot systems. Thin Solid Films 436:264–268 18. Xia S, Cheon K-O, Brooks JJ, Rothman M, Ngo T, Hett P, Kwong RC, Inbasekaran M, Brown JJ, Sonoyama T, Ito M, Seki S, Miyashita S (2009) Printable phosphorescent organic light-emitting devices. J SID 17(2):167 19. Liew Y-F, Aziz H, Hu N-X, Chan HS-O XuG, Popovic Z (2000) Investigation of the sites of dark spots in organic light emitting devices. Appl Phys Lett 77:2650 20. Hack M, Chwang A, Tung Y-J, Hewitt R, Brown J, Lu JP, Shih C, Ho J, Street RA, Moro L, Chu X, Krajewski T, Rutherford N, Visser R (2005) Status and opportunities for high efficiency OLED displays on flexible substrates. Mater Res Soc Symp Proc 870E:H3.1.1 21. Gu G, Garbuzov DZ, Burrows PE, Venkatesh S, Forrest SR, Thompson ME (1997) High-externalquantum-efficiency organic light-emitting devices. Opt Lett 22:396–398 22. Yamasaki T, Sumioka K, Tsutsui T (2000) Organic light-emitting device with an ordered monolayer of silica microspheres as a scattering medium. Appl Phys Lett 76:1243–1245 23. Tsutsui T, Yahiro M, Yokogawa H, Kawano K, Yokoyama M (2001) Doubling coupling-out efficiency in organic light-emitting devices using a thin silica aerogel layer. Adv Mater 13:1149–1152 24. Mo¨ller S, Forrest SR (2002) Improved light outcoupling in organic light emitting diodes employing ordered microlens arrays. J Appl Phys 91:3324–3327 25. Song YW, Hwang KH, Yoon SG, Ha JH, Kim KN, Lee JH, Kim SC (2010) LTPS-based transparent AM OLED. SID 10 Digest, p 144 26. Ma R, Hack M, Brown JJ (2010) Flexible AMOLEDs for low-power, rugged applications. Inf Display 26(2):8

6.6.2 Active Matrix for OLED Displays Ruiqing Ma 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1224

2 2.1 2.2 2.3 2.4 2.5 2.6

Backplane Technologies and Driving Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1225 TFT Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1225 Laser-Based Low Temperature Poly-Silicon (LTPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1226 Amorphous Silicon (a-Si) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1228 Solid Phase Crystallization (SPC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1230 Microcrystalline Silicon (mc-Si) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1231 Oxide TFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1231

3 3.1 3.2 3.3 3.4

Other Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232 Image Sticking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232 p-Channel Versus n-Channel TFTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1233 OLED Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1233 Flexible AMOLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1234

4

Summary and Future Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1236

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_6.6.2, # Springer-Verlag Berlin Heidelberg 2012

1224

6.6.2

Active Matrix for OLED Displays

Abstract: Significant progress has been made in the past decade in AMOLED displays. Various prototypes such as 40-in. TVs have been demonstrated and mass production of small size AMOLEDs started in 2007 with continuous ramping up of volume. As current driven devices, OLEDs pose tremendous challenge in finding the right active matrix backplanes. As a result, various technologies have been and continue to be investigated for driving AMOLEDs. In this chapter, active matrix for OLEDs will be discussed based on the type of backplane technologies. This is followed by several other considerations in designing the active matrix such as the type of transistors and the image sticking problem. At the end of this chapter a brief summary and outlook for future research and development will be provided. List of Abbreviations: 2T1C, 2 Transistors and 1 Capacitor; AMLCD, Active Matrix Liquid Crystal Display; AMOLED, Active Matrix Organic Light Emitting Diode; a-Si, Amorphous Silicon; ELA, Excimer Laser Annealing; LTPS, Low Temperature Poly-Silicon; MIC, Metal Induced Crystallization; PECVD, Plasma Enhanced Chemical Vapor Deposition; RTA, Rapid Thermal Annealing; SLS, Sequential Lateral Solidification; SPC, Solid Phase Crystallization; TFT, Thin Film Transistor; mc-Si, Microcrystalline Silicon

1

Introduction

This past decade has been an exciting time for active matrix organic light emitting diodes (AMOLEDs) with frequent introduction of new prototypes and new commercial products. The first full color 2.4-in. AMOLED was demonstrated in 1999 by Kodak and Sanyo Electronics. This is followed by larger size AMOLED prototype introductions by Toshiba and Matsushita (17 WXGA, p-Si), Kodak and Sanyo (15 WXGA, p-Si) in 2002; and Sony (24.2 XGA, p-Si), Samsung SDI (15.5 WXGA, p-Si), and Chi Mei Optoelectronics and IBM Japan (20 WXGA, a-Si) in 2003. Samsung stunned the display world in 2005 with a 40-in. AMOLED TV driven by amorphous silicon backplane. In recent years with improved backplane and OLED deposition technologies, large size AMOLEDs have been demonstrated and these include a 27.3-in. HD display using microcrystalline silicon backplane by Sony in 2007 and a 40-in. FHD TV with LTPS backplane by Samsung in 2008. Some of the recent AMOLED prototypes categorized based on their active matrix backplane technologies are shown in > Table 1. In the commercial front, the first digital camera with an AMOLED display was introduced in 2003 (Kodak LS633). Samsung launched mass production of small size AMOLEDs in 2007 and today they are being used in many mobile devices with the latest example of the Samsung Galaxy smart phone. In 2007, Sony released its 11-in. qFHD TV XEL-1, signaling the start of the AMOLED TV era. This is followed by the launch of a 15-in. AMOLED TV (15EL9500) by LG Display in 2009. As shown by various prototypes in > Table 1, backplane technology plays a critical role in the development of AMOLEDs. The current driven property of AMOLEDs demands stable transistors with large current capacity. Different backplane technologies are being investigated to meet that requirement. Low Temperature Poly-Silicon (LTPS) based on laser process becomes the first commercial backplane solution because of its high mobility and stability. However, laser process has its own challenges in uniformity and scalability. Amorphous silicon (a-Si) has also been investigated intensively as the AMOLED backplane technology. The advantages of a-Si backplanes for AMOLED production are lower cost, excellent uniformity, and the ability to leverage off the large manufacturing base that already serves the AMLCD

Active Matrix for OLED Displays

6.6.2

. Table 1 Specifications of recently demonstrated AMOLED prototypes Oxide TFT

mc-Si

LTPS (SGS)

SPC

a-Si

Size (in.)

14 WXGA

15 XGA

15 XGA

19 qFHD

27.3 FHD

Number of pixels

1,280768

1,024768

1,024 768

960540

1,9201,080

Pixel size (mm)

80240

99297

96297

435435

Resolution (dpi)

106

85.5

85.5

58.4

Brightness (cd/m2) 200 (peak 600)

200 (peak 600)

Color saturation

>110% (NTSC)

>100% (NTSC)

Pixel circuit

6T1C

4T1C

Aperture ratio

52%

40%

Contrast ratio

>1,000,000:1

>1,000,000:1

OLED structure

Top emission

Bottom emission

Top emission

Company

Samsung

LG Display

LG Display Samsung

Sony

Year

2008

2008

2008

2007

5T2C 52%

R,G 35%, B64% >1,000,000:1 Bottom emission 2010

industry. These properties are really important for scaling up the AMOLED displays. However, there are serious issues with a-Si in its low stability and mobility. In recent years, intensive research and development effort has been focused on alternative backplane technologies. These include advanced laser process such as Sequential Lateral Solidification (SLS), advanced Solid Phase Crystallization (SPC), microcrystalline silicon and oxide TFTs. Since the active matrix is tied to the backplane technology being used, the discussion in this chapter is arranged based on different types of backplanes. With each backplane technology, its strength and weakness and the pixel circuits to address the weakness will be discussed. This is followed by other considerations in designing active matrix for AMOLEDs such as types of TFTs, image sticking and designs for transparent and flexible AMOLED displays. In the end a brief summary of different backplane technologies and outlook for large size AMOLED TVs will be provided. Since AMOLEDs are still under rapid development, readers are advised to conduct further reading to get up-to-date progress of this dynamic field.

2

Backplane Technologies and Driving Circuits

2.1

TFT Requirements

Before the emergence of AMOLEDs, the development of thin film transistors (TFTs) was driven by active matrix liquid crystal displays (AMLCDs). Since LCDs are voltage driven devices, the pixel driving circuit is relatively simple, consisting of a switching TFT and a storage capacitor, as shown in > Fig. 1a. When the pixel is selected for refreshing, Vselect is applied to the gate of the transistor to turn on the switch, allowing the current to flow from the data line through the transistor to charge the capacitor Cstorage to the signal voltage Vdata, which is maintained during

1225

1226

6.6.2

Active Matrix for OLED Displays

Vdata

V

Vdata Vselect

Vselect

Cstorage Switching TFT Cstorage

Switching TFT

LC

Driving TFT

OLED

a

b

. Fig. 1 Electrical schematic of basic pixel level driving circuits of (a) AMLCDs, and (b) AMOLEDs

the deselect period to drive the liquid crystal cell. In this drive scheme, the current flow within the TFT is low and the stress on TFT is small. As a result, amorphous silicon technology has become the dominant backplane choice for AMLCDs despite its low mobility ( Fig. 1b. The switching TFT and the capacitor store the data voltage, and the driving TFT converts the voltage signal to current to drive the OLED. When a pixel is selected, the switching TFT is turned on by the gate voltage Vselect, allowing data voltage Vdata to be charged onto the storage capacitor Cstorage. This voltage is also the gate voltage of the driving TFT. Depending on the amplitude of Vdata at the gate, current at different levels will flow from power line to the ground through the OLED device. The switching TFT is only on when the pixel is selected and the storage capacitor is being charged. The key requirement for this transistor is low leakage, which will prevent the storage capacitor from discharging. For the driving TFT, depending on the signals, it could be on during the whole frame time with current flowing constantly to keep the OLED on. This requires a very stable TFT with the ability to conduct large current levels. A TFT operated at saturation regime is preferred as it can keep constant current better even when the OLED resistance changes over time.

2.2

Laser-Based Low Temperature Poly-Silicon (LTPS)

The work on low temperature poly-silicon TFTs started in the 1980s, and the TFT performance improved quickly and became viable for AMLCDs in the 1990s. However, the benefit of LTPS is not significant enough to compensate for its cost for AMLCD industry except in certain small AMLCD arrays where the use of LTPS is justified because it enables the integration of the drive electronics. The rapidly emerging field of OLEDs with their more demanding circuit requirements has resulted in LTPS being an area of active development. In fact, the first mass produced AMOLED panels use the LTPS backplane technology. The key to the LTPS TFT technology is the formation of polycrystalline silicon on glass substrates which demands a process temperature typically not exceeding 650 C. The typical

Active Matrix for OLED Displays

6.6.2

LTPS process involves the deposition of a silicon precursor film (typically a-Si) and the subsequent crystallization of this film into poly-silicon with the assistance of high temperature and sometimes catalysts. The process choice of depositing amorphous silicon precursor film is plasma enhanced chemical vapor deposition (PECVD) as it is a mature technology for AMLCDs. For crystallization, there are two main processes: solid phase crystallization (SPC), a non-laser process, and excimer laser annealing (ELA). A detailed discussion on ELA technology including TFT fabrication process can be found in > Chap. 5.2.2. What stands out for this laser technology is that the crystallization starts from liquid phase with no preexisting local energy barriers. As a result, the quality of the silicon in each grain is excellent and the TFTs have high mobility (100 cm2/V-s) and stable threshold voltage. This is the reason why ELA-based LTPS becomes the first commercialized backplane technology for AMOLEDs. However, ELA also has its own challenges. Fundamentally, the process window for near-complete melting regime is small, especially with less than perfect excimer laser beam uniformity and shot to shot reproducibility. Secondly, there is no good control of the size and location of the grains with respect to the TFT channel. As a result, there is a large variation of threshold voltage and carrier mobility of the ELA transistors within a single panel. In one AMOLED uniformity study, it is found that the local current variation of 50 TFTs at low gray level is above 30%, and the non-uniformity of an AMOLED panel with 2T1C driving circuits is over 30% [1]. To overcome the non-uniformity of the TFT characteristics, complex compensation circuits have been developed over the years and applied to the commercial AMOLED displays. Depending on the type of signals at the data line, the compensation methods can be described as voltage programming or current programming schemes. In the voltage programming scheme, an extra step is added to detect and store the threshold voltage of the driving TFT. One example of voltage programming compensation circuit design is shown in > Fig. 2, where a CMOS process with 5 transistors and 2 capacitors (5T2C) is used [2]. In this design, M3 is the switching TFT and M1 is the driving TFT. During the Vth detection period, scan(n 1) signal turns on M2 and M5, where M5 resets Cst and M2 causes M1 to operate in diode mode. As a result the gate node of M1 settles at its own threshold voltage Vth and this information is stored at CVth . M4 is an n-channel transistor that prevents the current from flowing through the OLED during Vth detection period. In the next period, M2 and M5 are turned off and scan(n) turns on switching TFT M3, allowing data signal to be stored at Cst. After these two periods, both the data signal and driving TFT Vth signal are stored at the capacitors and can be used in the emitting period that follows. As a result, the Vth variation is fully compensated. While voltage programming compensates threshold voltage variation, current programming can compensate both threshold voltage and mobility differences of the TFTs. In the current programming compensation scheme, the voltage signal needs to be converted to current. Since this conversion circuit is very complicated, it makes sense to have this done at the display periphery for many pixels and then the current signals are sent to the pixels through data lines. Generally speaking, both voltage and current programming schemes can offer good image quality. The disadvantage of voltage programming is the complex circuit at the pixel level. For current programming scheme, the de-multiplexing is very difficult and the operation speed can be slow for low current levels [3]. In recent years advanced laser techniques have been developed to achieve precise control of the laser process. One of such methods is sequential lateral solidification (SLS), where crystal grains can grow laterally along one direction to arbitrary length. A basic process includes the complete melting of amorphous silicon (a-Si) film in the predesignated regions to achieve

1227

1228

6.6.2

Active Matrix for OLED Displays

data(m)

ELVDD

scan(n)

M5

Cst

M3

CVth M1

M2 scan(n–1) M4 OLED

. Fig. 2 Configuration of the compensation circuit with 5 TFTs and 2 capacitors. Reproduced from [2] with permission by SID

controlled super-lateral growth, micro-translation of the substrate and re-irradiation to extend the grains [4]. After each sequential shot, the crystals grow from the poly-Si grains generated in the previous shot. As a result, the grains can grow very long with good quality. Using this technology, a 14-in. WXGA AMOLED has been demonstrated with a 6T1C pixel circuit. The peak luminance reached 600 cd/m2 and color saturation was more than 110% of NTSC triangle.

2.3

Amorphous Silicon (a-Si)

Amorphous silicon was first introduced as the active material in 1979 and significant progress was made in the 1980s. Later in the 1990s in the battle of the active material for AMLCD backplanes, amorphous silicon won over low temperature poly-silicon because of its simple, low cost process, excellent uniformity and ease of scaling up. Normally it requires 8–11 mask steps to make LTPS TFTs in addition to the expensive ELA laser process, while only 4 or 5 mask steps are needed for a-Si TFTs with excellent uniformity. However, there are two major challenges in using amorphous silicon to drive OLEDs: (1) the carrier mobility of a-Si is low, limiting the driving current for high brightness OLEDs; and (2) the threshold voltage of a-Si TFTs changes under the stress of constant gate bias. To address the low mobility, large size driving TFTs and high efficiency phosphorescent OLEDs need to be used. The threshold voltage shift of hydrogenated amorphous silicon TFTs has been studied extensively since their introduction. It is understood that two mechanisms contribute to this instability: the trapping of charges in the gate insulator SiNx:H and the bias induced breakage of weak Si–Si bonds creating negatively charged dangling-bond states in the active material a-Si:H [5].

Active Matrix for OLED Displays

6.6.2

A detailed discussion on this topic can be found in > Chap. 5.2.1. In a saturated TFT, the current is proportional to (VGS Vth)2, where VGS and Vth are the gate-source voltage and threshold voltage, respectively. A small change in threshold voltage can cause big variation in the current flowing through OLEDs. Some of the ongoing efforts to address the stability challenge include: (1) improvement in the transistor design and fabrication process – for example, the PEVCD deposition condition can influence the stability; (2) application of high efficiency OLEDs such as phosphorescent OLEDs to reduce the high current requirement – this approach has many additional benefits such as long lifetime, low temperature increase, low voltage drop on the bus lines and low power consumption; (3) pulsed driving or negative bias – negative bias can release trapped charges and thus reduce the threshold voltage shift; and (4) application of compensation circuits – this requires complex circuits at the pixel level (>4 transistors). > Figure 3 shows an a-Si TFT pixel circuit design with both threshold voltage compensation and negative bias annealing [6]. In this scheme, T3 is the driving TFT with a threshold voltage of Vth3. Each frame is divided into three phases. In the first phase, VSCAN turns on T2 and T4, allowing Vth3 and

VDD

VSCAN T1

VDATA

VEMS

T2

C

A T3 CST T6 T4

T5 B VSS

CLK

1 frame : 60 Hz 15 V VSCAN VEMS VCLK

−15 V (1)

(2)

(3)

Emission

FTA

15 V −15 V 10 V −15 V

. Fig. 3 The a-Si TFT pixel circuit with both threshold voltage compensation and negative bias annealing. Reproduced from [6] with permission by SID

1229

1230

6.6.2

Active Matrix for OLED Displays

VDATA to be applied to nodes A and B, respectively. This voltage difference (Vth3 VDATA) is stored in the capacitor CST and becomes the VGS of the driving TFT T3 during the second phase (emission) when VEMS is on. In the third phase, a negative bias is applied to the gate of T3 to suppress the threshold voltage shift. The current stability measured after 60 h electrical stress at 60 C is 96%. In comparison, the stabilities measured at the same condition are 86%, 82%, and 57% for the cases of Vth compensation only, negative bias only, and conventional 2T1C pixel, respectively.

2.4

Solid Phase Crystallization (SPC)

As the potential backplane technologies for AMOLEDs, both amorphous silicon and laserbased LTPS have obvious advantages and shortcomings. Interestingly, their key properties such as mobility, uniformity, and TFT stability are opposite to each other. Researchers have been searching for alternatives to combine a-Si’s good uniformity, scalability, and low cost with LTPS TFT’s high mobility and stable threshold voltage. In fact, the very high mobility of laser-based LTPS is not absolutely required to drive AMOLEDs, especially when high efficiency phosphorescent OLED is used. Several alternative backplane technologies being developed are advanced solid phase crystallization, microcrystalline silicon (mc-Si), and oxide semiconductors. SPC can be realized by annealing a-Si precursor film at elevated temperature for long periods of time (tens of hours). The nucleation process depends strongly on the microstructure of the precursor film, which is determined by the PECVD deposition process conditions such as deposition temperature and rate. The major drawback of this technique is the long process time. To address this, two approaches have been studied intensively: one uses higher temperature through rapid thermal annealing (RTA) process, and the other takes the route of lower temperature by using metal induced crystallization (MIC). RTA is a well known semiconductor process which utilizes tungsten halogen or xenon arc lamps to heat up the Si film for a short period of time. Since the exposure to higher temperature is short, for example, 5 min at 750 C [7], there is minimum detrimental impact to the glass substrate. In MIC, metal Nickel presented in the silicon film reacts with silicon to form Nickel silicide, which has a lattice structure very similar to that of crystalline silicon. The presence of Nickel silicide facilitates the crystalline silicon growth at relatively low temperatures. Other metals such as Au, Al, Sb, In, Pd, and Ti have also been investigated for this purpose. The biggest challenge for MIC technology is that the metal catalyst residues may contaminate the silicon material and cause large leakage currents. In recent years, a new technique has been developed to use electric or magnetic fields during the RTA or MIC process to further improve the growth rate and reduce process temperature. Comparing to laser process, SPC-based TFTs have better uniformity. They also have the potential to be low cost. However, low annealing temperature results in long process time and makes it impossible to overcome some local energy barriers. As a result, micro-defects exist in each grain leading to lower mobility comparing to laser process. Other challenges include large hysteresis and leakage current, and less bias stability for the transistors. Large hysteresis can cause short term ‘‘image sticking,’’ but this can be significantly reduced by inserting black image data during the half frame to avoid the continuous DC bias [7]. LG Display reported a 15-in. AMOLED using advanced SPC process [8]. The driving TFT employs SiNx/SiO2 double gate insulator structure with W/L of 6 mm/18 mm. The threshold

Active Matrix for OLED Displays

6.6.2

voltage, sub-threshold swing, and mobility are 2 V, 0.6 V/decade, and 30 cm2/V-s, respectively. Even without Vth compensation, the global luminance non-uniformity of the 15-in. panel is lower than 10% at full gray level.

2.5

Microcrystalline Silicon (mc-Si)

Microcrystalline silicon, also referred as nano-crystalline silicon, has small grains of crystalline silicon within the amorphous phase. Because of the presence of crystalline silicon grains and less hydrogen concentration, mc-Si has higher mobility and better stability than amorphous silicon. Microcrystalline silicon can be directly deposited by PECVD [9] or by diode laser thermal annealing of amorphous silicon. The advantages of laser diode annealing include stable light output, ease of control, and good scalability for bigger mother glass. It has been reported that transistors made by this process have a mobility of 3.1 cm2/V-s, a threshold voltage of 2.3 V, and a sub-threshold slope of 0.93 V/decade. The threshold voltage shift is about 2 orders of magnitude smaller than that of a-Si TFTs, and two times higher than that of LTPS TFTs [10]. A 27.3-in. display has been demonstrated by using this technology.

2.6

Oxide TFT

In recent years, significant interest has been generated in applying amorphous oxide such as InGaZnO (IGZO) to drive AMOLEDs. In contrary to silicon, InGaZnO is insensitive to the distorted metal-oxygen-metal bonds because of its unique conduction band structure, so good mobility (>10 cm2/V-s) can be achieved in the amorphous phase. For example, a field-effect mobility of 17 cm2/V-s, and an Ion/Ioff ratio of 109 was reported on an IGZO TFT with W/L = 25/10 mm [11]. Besides the high mobility, IGZO material is transparent and can be processed at room temperature. In addition, the oxide semiconductor materials can be deposited by sputtering process which can be scaled up to large sizes. An excellent discussion on oxide TFTs can be found in > Chap. 5.3.4. One challenge for oxide TFTs is that their threshold voltage changes under gate voltage stress. Research and development work has focused on optimizing the oxide semiconductor composition and developing a compatible gate dielectric material. A recent study has found that this threshold voltage shift is related to field induced molecular oxygen adsorption and desorption, and/or hydrogen or water on the back channel surface [12]. A channel protection layer with good barrier property (e.g., SiO2, Al2O3) can improve the stability significantly. A full color AMOLED driven by oxide TFTs was first demonstrated in 2007, and since then significant process has been made to further advance the technology. The driving force behind this rapid progress is the need for advanced backplanes for AMOLEDs and large size AMLCDs. In 2010, SONY demonstrated an 11.7-in. qHD AMOLED prototype using 2 transistor 1 capacitor circuit [13]. The mobility, sub-threshold slope, and threshold voltage of the TFTs are 11.5 cm2/ V-s, 0.27 V/decade, and 0.3 V, respectively. The brightness non-uniformity of this panel is only 10% in wide distribution. In the same year, Samsung Mobile Display demonstrated a qFHD 19in. AMOLED driven by amorphous IGZO backplane. The mobility, sub-threshold slope, and Ion/Ioff ratio of the TFTs are 21 cm2/V-s, 0.29 V/decade, and >108, respectively. The bias temperature stability of the TFTs was found to be similar to or better than ELA LTPS technology. The variation of threshold voltage was addressed by using a 5T2C pixel circuit [12].

1231

1232

6.6.2

Active Matrix for OLED Displays

3

Other Considerations

3.1

Image Sticking

Image sticking, or image retention, refers to the phenomenon where a faint outline, or a ‘‘ghost,’’ of a previously displayed image (tends to be static) is still visible even after it has been changed. In AMOLEDs, this can be caused by the change of TFT characteristics such as threshold voltage shift in a-Si-based TFTs and/or the degradation of OLED efficiency over time. Generally speaking, localized 3% brightness variation with surrounding areas can be distinguished [14]. The solutions for threshold voltage shift have been discussed in previous sessions. With OLED degradation, while OLED materials and device performance are improving on a daily basis, researchers also come up with novel ways to address this challenge. One of such solutions is a pixel circuit with optical feedback function so the compensation is done locally and in real time, as shown in > Fig. 4 [15]. In the preparation phase, T1 and T2 are turned on to charge C2 with data voltage and C1 with a high enough potential to turn on TD (Power is set low in this phase so there is no current flow through the OLED). Then T1 and T2 are off and Power is set to high, which allows the OLED to emit light. The Photo TFT, upon receiving the light, generates current according to the light intensity and starts charging C2. Once the voltage on C2 is charged over the threshold voltage of Ts, Ts will be turned on and become a short circuit for C1, causing C1 to discharge. The way this circuit works is that the brighter the OLED, the sooner C1 is discharged and thus the less time the OLED is emitting light. The non-uniformity of the OLED is solved in the time domain. A big advantage of this design is that it addresses the brightness directly, no matter what are the causes of the nonuniformity. However, in order for this scheme to work well, the photo TFT needs to be reliable, uniform over large area and have high sensitivity to OLED light, but not ambient light.

Power Common

T2 A1 Photo TFT TD T1

TS

C2 V2

V1 C1

A1 Column

OLED

. Fig. 4 An a-Si:H optical feedback pixel circuit. Reproduced from [15] with permission by SID

Active Matrix for OLED Displays

3.2

6.6.2

p-Channel Versus n-Channel TFTs

In a typical AMOLED fabrication process, anodes are deposited before cathodes, so the anodes are connected to the TFTs and current will flow through the driving TFT to the OLED device. For a p-channel TFT, the pixel circuit in > Fig. 1b works very well because the drain of the TFT will be connected to the OLED anode and the gate-source voltage (VGS) is independent of OLED changes. However, for an n-channel TFT such as an a-Si-based TFT, the same pixel circuit implies the source of the TFT is connected to the OLED. In this case, the gate voltage of the driving TFT, Vdata, is the sum of VGS of the driving TFTand the voltage across OLED VOLED . Any change of VOLED will lead to the change of VGS, resulting in non-uniform luminance, or image sticking. The solution to this problem is to make connections between the cathode of the OLED and the n-channel TFT such that VGS is independent of OLED changes, as shown in > Fig. 5. One way to realize this is to develop a high efficiency inverted OLED structure where the cathode is deposited first. LG Display reported a novel architecture where two substrates, one with standard OLEDs (cathodes last) and the other with TFTs, are connected together in a fashion similar to flip chip bonding such that the cathodes are connected to the TFTs [16]. A 15-in. XGA AMOLED has been demonstrated using this technology.

3.3

OLED Structures

Both top emission and bottom emission OLEDs are being used in AMOLEDs, as shown by various prototypes in > Table 1. Generally speaking, the design and fabrication process of bottom emission OLED backplanes are simpler. However, because more complicated pixel circuits are needed, the aperture ratio of the pixels is compromised for bottom emission AMOLEDs. Small aperture demands higher driving current density which causes more stress to the transistors and OLEDs. This issue can be resolved by using top emission OLED structure where a reflective anode is deposited on top of the pixel circuit (a planarization layer is typically inserted in between). As light goes to the opposite side of the TFTs, the pixel circuit has no

VDD

Vdata

Vselect OLED

Switching TFT

G

Driving TFT S

Cstorage

. Fig. 5 Electrical schematic of an AMOLED pixel with the gate-source voltage of an n-type TFT independent of OLED voltage

1233

1234

6.6.2

Active Matrix for OLED Displays

impact on the aperture ratio. This is why top emission OLEDs are popular in small to medium AMOLED displays for mobile applications where high pixel density is critical. However, the top emission structure is more complicated to fabricate. In addition, since the top electrodes (typically cathodes) are not transparent enough, they form a cavity structure where certain wavelength of light can be enhanced if it matches the resonant wavelength of the cavity. While this can produce more saturated light, the light out of the cavity does have angular dependence in its wavelength which can be problematic for applications requiring good viewing angle performance. For large size TV applications where the pixel density is less, bottom emission OLEDs start to show their strength in good uniformity, little angular dependence of color, and simpler process. Another interesting display format OLEDs can offer is a transparent display. In principle, this can be done by using a transparent cathode for a bottom emission AMOLED. With transparent oxide TFTs, transparent AMOLEDs can be fabricated without losing aperture ratio due to the TFTs [17]. In 2010, Samsung demonstrated a new architecture of transparent AMOLEDs using LTPS backplanes. In this design, the pixels are divided into three regions: (1) opaque regions where metal bus lines and electrodes are covered by black matrix, (2) top emission RGB pixels with pixel circuits underneath, and (3) passive transparent regions which can be seen through. The transparency of the display is determined by the area ratio of this seethrough region and its transparency. A 14.1-in. qFHD AMOLED has been demonstrated using this technology with a brightness of 200 cd/m2 and a transparency up to 38% [18].

3.4

Flexible AMOLEDs

One feature that significantly differentiates OLEDs from LCDs is that OLEDs can be easily made into flexible displays because of their simple, elegant, thin film structure. However, making a high quality backplane directly on flexible substrates is a huge challenge. Three major material groups have been considered as substrates for flexible AMOLED displays: thin glass, metal foils, and plastic films. Although thin glass has good thermal and barrier property, it is less flexible and difficult to handle. And most problematic of all, it is brittle so a rugged display cannot be realized. Thin plastic films have excellent flexibility; however, their moisture barrier property is poor and their thermal properties can limit the maximum process temperature of the backplanes. Flexible metal foils offer a number of desirable advantages including reasonable flexibility, enhanced thermal and mechanical durability, and excellent permeation barrier property, but their surfaces tend to be rough and the materials are opaque and not as flexible as plastic films. There are many challenges in fabricating backplanes for flexible AMOLED displays. The most critical one is the sagging of flexible substrates during TFT fabrication. Other challenges include matching the thermal properties of the flexible substrates with the rest of materials, reducing flexible substrate surface roughness, and developing a flexible transparent thin film encapsulation. One additional challenge for the TFTs on flexible substrates is the strain in the TFTs as a result of bending or stretching a flexible display. UDC and Kyung Hee University built a-Si transistors on planarized 25 mm metal substrates and then measured the TFT characteristics during bending inward or outward around a cylinder of 5 mm diameter [19]. > Figure 6 shows the relative field-effect mobility m/m0, threshold voltage Vth/Vth0, and sub-threshold slope S/S0 as a function of strain (e), where m is the mobility under an imposed strain and m0 is the

6.6.2

Active Matrix for OLED Displays

2.0

3.0 m /m0 = 1 + 0.24ε 2.5

m/m0

2.0

1.0

1.5

1.0 0.5

Vth/Vth0 and S/S0

1.5

m/m0 0.5

Vth/Vth0 S/S0 0.0 −0.8 −0.6 −0.4 −0.2

0.0 0.2 Strain (%)

0.4

0.6

0.0 0.8

. Fig. 6 The relative field-effect mobility m/m0, threshold voltage Vth/Vth0, and sub-threshold slope S/S0 of an a-Si:H TFT on a 25 mm metal foil as a function of strain («). Reproduced from [19] with permission by SID

. Table 2 A comparison of AMOLED backplane technologies based on recently demonstrated prototypes

Type

ELA LTPS

SPC

Amorphous Si

micro-Silicon

Oxide (IGZO)

CMOS

PMOS

NMOS

NMOS

NMOS 10–30

Mobility (cm /V-s)

50–100

30

5T, 12C

>5T, 12C

45T

N/A

2T1C, 5T2C

Stability

Good

OK

Issue

OK

OK OK

2

Uniformity

Issue

OK

Good

OK

Manufacturability

Maturing

Developing

Good

Developing

Developing

Scaling up

Difficult

OK

Good

OK

OK

mobility at flat state. The field-effect mobility increases when the substrate bends outward and decreases as substrate bends inward. The crack was generated in the samples when the strain was higher than 2.4%. The TFT performance was found to be unchanged after the TFT was bent 10,000 times, which indicates that the flexibility of the TFT is good. Almost all the backplane technologies have been investigated for flexible AMOLED applications, and many prototypes have been demonstrated. In 2008, UDC, LGD, and L3 demonstrated a rugged wearable wrist unit built upon a flexible 4-in. QVGA full color AMOLED, which utilized an a-Si backplane built on metal foils [20]. In 2010, Sony demonstrated a rollable 4.1-in. 121ppi AMOLED display with a thickness of 80 mm, using organic

1235

1236

6.6.2

Active Matrix for OLED Displays

semiconductor perixanthenoxanthene and 2T1C pixel circuit [21]. The display showed no degradation under 4-mm radius rollup and release cycle over 1,000 times. Also in 2010, Samsung Mobile Display demonstrated a 4.5-in. WVGA flexible AMOLED using LTPS backplane built on polyimide substrates. The display is 240 mm thick and has brightness and contrast ratio of 250 cd/m2 and 100,000:1, respectively [22].

4

Summary and Future Development

A comparison of different backplane technologies for AMOLEDs is summarized in > Table 2. Laser-based LTPS has been used in mass production of AMOLEDs since 2007 and is the dominant backplane technology today. Through the development of compensation circuits, the threshold voltage variation has been resolved for small to medium size AMOLEDs. However, as both the display and the mother glass grow bigger, laser-based LTPS faces significant challenges in its cost, uniformity, performance, and availability of laser equipments. Amorphous silicon has excellent uniformity, scalability, and low cost potential, and has been demonstrated to be a viable backplane for AMOLEDs. However, its stability is still not satisfactory for AMOLED displays. To find the best backplane for large size AMOLEDs, new technologies are being investigated actively. These include advanced laser process such as SLS, advanced SPC, microcrystalline silicon, oxide TFTs, single crystalline silicon based on siliconon-glass technology [23] and organic semiconductors [21]. Advanced SPC, microcrystalline silicon, and oxide TFT technologies offer better mobility and stability than a-Si, better uniformity and scalability than laser-based LTPS. They have the potential of becoming the next backplane technology for AMOLEDs, especially for large size displays. Organic TFT is an emerging technology and holds significant promise as a future backplane solution, especially for flexible displays when combined with plastic substrates. However, many technical issues such as stability and uniformity need to be solved before it could become commercially viable. AMOLED is still a very dynamic field and will continue to be so for the foreseeable future. We should see continuous research and development effort in searching the best active matrix backplane until we can make large size AMOLEDs with excellent image quality at low cost.

References 1. Hong S-K, Kim B-K, Ha Y-M (2007) LTPS technology for improving the uniformity of AMOLEDs. SID ‘07 Dig, 1366–1369 2. Komiya N, Oh CY, Eom KM, Jeong JT, Chung HK, Choi SM, Kwom OK (2003) Comparison of Vth compensation ability among voltage programming circuits for AMOLED panels. IDW ‘03 Dig 10:275–278 3. Stewart R (2010) Active matrix OLED pixel design. SID ‘10 Dig, 790–793 4. Choi JB, Chang YJ, Park C-H, Kim YI, Eom J, Na HD, Chung I-D, Jin SH, Song Y-R, Choi B, Kim HS, Park K, Kim C-W, Souk JH, Kim Y, Jung B (2008) Sequential lateral solidification (SLS) process for large area AMOLED. SID ‘08 Dig 39:97–100

5. Hekmatshoar B, Cherenack KH, Wagner S, Sturn JC (2008) Amorphous silicon thin-film transistors with DC saturation current half-life of more than 100 years. IEDM, 89–92 6. Lee J-H, Park H-S, Choi S-H, Lee W-K, Han M-K, Goh J-c, Choi J, Chung K (2007) Highly stable a-Si:H TFT pixel for large area AMOLED by employing both Vth storing and the negative bias annealing. SID Symp Dig, 165–168 7. Choi H-S, Choi J-S, Hong S-K, Kim B-K, Ha Y-M (2007) LTPS technology for improving the performance of AMOLEDs. IMID 07 Dig, 1781–1784 8. Jung SH, Lee HK, Kim CY, Yoon SY, Kim CD, Kang IB (2008) 15-inch AMOLED display with SPC TFTs

Active Matrix for OLED Displays

9.

10.

11.

12.

13.

14.

15.

16.

and a symmetric driving method. SID ‘08 Dig 39(1):101–104 Girotra KS, Souk JH, Chung K, Kim S, Kim S, Kim B-J, Yang S-H, Choi B, Goh J, Song Y-R, Choi Y-M (2006) A 14.1 inch AMOLED display using highly stable PECVD based microcrystalline silicon TFT backplane. SID ‘06 Dig 37(1):1972–1975 Urabe T, Sasaoka T, Tatsuki K, Takaki J (2007) Technological evolution for large screen size active matrix OLED display. SID ‘07 Dig 38(1):161–164 Jeong JK, Chung H-J, Mo Y-G, Kim HD (2008) A new era of oxide thin-film transistors for large-sized AMOLED displays. Inf Disp 24(9):20 Mo YG, Kim M, Kang CK, Jeong JH, Park YS, Choi CG, Kim HD, Kim SS (2010) Amorphous oxide TFT backplane for large size AMOLED TVs. SID ‘10 Dig 41(1):1037–1040 Arai T, Morosawa N, Tokunaga K, Terai Y, Fukumoto E, Fujimori T, Nakayama T, Yamaguchi T, Sasaoka T (2010) Highly reliable oxide-semiconductor TFT for AM-OLED display. SID ‘10 Dig 41(1):1033–1036 Matsueda Y, Shin D-Y, Chung H-K (2008) AMOLED technologies for uniform image and sufficient lifetime of image sticking. SID ‘08 Dig 39(1):9–12 Fish D, Young N, Deane S, Steer A, George D, Giraldo A, Lifka H, Gielkens O, Oepts W (2005) Optical feedback for AMOLED display compensation using LTPS and a-Si:H technologies. SID ‘05 Dig 36(1):1340–1343 Han C-W, Kim O-H, Bae S-J, Lee M-K, Nam W-J, Tak Y-H, Kang IB, Chung IJ (2008) 15-inch XGA

17.

18.

19.

20.

21. 22. 23.

6.6.2

dual-plate OLED display (DOD) based on amorphous silicon (a-Si) TFT backplane. SID ‘08 Dig 39(1):5–8 Park S-HK, Ryu M, Hwang C-S, Yang S, Byun C, Lee J-I, Shin J, Yoon SM, Chu HY, Cho KI, Lee K, Oh MS, Im S (2008) Transparent ZnO thin film transistor for the application of high aperture ratio bottom emission AM-OLED display. SID ‘08 Dig 39(1):629–632 Song YW, Hwang KH, Yoon SG, Ha JH, Kim KN, Lee JH, Kim SC (2010) LTPS-based transparent AM OLED. SID ‘10 Dig, 1340–1343 Ma R, Rajan K, Hack M, Brown JJ, Cheon JH, Kim SH, Kang MH, Lee WH, Jang J (2008) Highly flexible low power consumption AMOLED displays on ultra thin stainless steel substrate. SID Symp Dig Tech Pap 39(1):425–428 Ma R, Rajan K, Silvernail J, Urbanik K, Paynter J, Mandlik P, Hack M, Brown JJ, Yoo JS, Kim Y-C, Kim I-H, Byun S-C, Jung S-H, Kim J-M, Yoon S-Y, Kim C-D, Kang I-B, Tognoni K, Anderson R, Huffman D (2009) Wearable 4-inch QVGA full color video flexible AMOLEDs for rugged applications. SID Symp Dig Tech Pap 40(1):96–99 Nomoto K (2010) Development of flexible displays driven by organic TFTs. SID ‘10 Dig, 1155–1158 FPD International 2010, Makuhari Messe, Japan Choi JB, Chang Y-J, Shim S-H, Chung I-D, Park KW, Park KC, Moon KC, Min H-K, Kim C-W, Gadkaree KP, Couillard JG, Cites JS, Ahn SE (2007) AMOLED based on silicon-on-glass (SiOG) technology. SID ‘07 Dig 38:1378

1237

Section 7

Liquid Crystal Displays

Part 7.1

Liquid Crystal Fundamentals and Materials

7.1.1 Materials and Phase Structures of Calamitic and Discotic Liquid Crystals J. W. Goodby 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1244

2

Melting Processes of Calamitic Thermotropic Liquid Crystals . . . . . . . . . . . . . . . . . . . 1245

3

The Structure of the Nematic Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1246

4 Structures of Smectic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1250 4.1 Structure of the Smectic A Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1251 4.2 Structure of the Smectic C Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1254 5

Molecular Templates for Calamitic and Discotic Nematic Liquid Crystals . . . . . 1255

6 6.1 6.2 6.3 6.4

Molecular Templates for Smectic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1262 Effects of Terminal Aliphatic Chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1264 Effect of Polar Groups at the Core Termini . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1264 Effect of Functional Groups that Terminate the Core Structure . . . . . . . . . . . . . . . . . . . 1266 Effect of Core Ring Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1267

7 7.1 7.2 7.3 7.4 7.5

Chiral Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1271 Aspects of Symmetry and Asymmetry in Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . 1271 Molecular Asymmetry in Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1271 Helicity in Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1275 Space Asymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1280 Frustrated Chiral Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1283

8

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1285

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_7.1.1, # Springer-Verlag Berlin Heidelberg 2012

1244

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

Abstract: In this article, an introduction to the field of liquid crystals relevant to displays and devices is given. It covers the structures of nematic, and smectic, calamitic liquid crystals that can be used as the switching elements in displays, and nematic discotic and columnar nematic liquid crystals that can be used in optical films. Examples of the types of materials that can be employed, in each case, are given, and also the effects that can be achieved when the materials are chiral. Lists of Abbreviations: 5CB, 40 -Pentyl-4-Cyanobiphenyl; 8SI, 4-(2-Methylbutyl)phenyl 4-noctylbiphenyl-40 -Carboxylate; AF, Antiferroelectric; BDH, British Drug Houses; CE8, 4-(2methylbutyl)phenyl 4-n-Octylbiphenyl-40 -Carboxylate; Col, Columnar; F, Ferroelectric; IPSLCD, In-Plane Switching Liquid Crystal Display; LCD, Liquid Crystal Display; N, Nematic; Ps, Spontaneous Polarization; Sm, Smectic; SSF-LCD, Surface-Stabilized Ferroelectric Liquid Crystal Display; STN-LCD, Super-Twisted Nematic Liquid Crystal Display; TN-LCD, Twisted Nematic Liquid Crystal Display; VAN-LCD, Vertically Aligned Liquid Crystal Display

1

Introduction

In a discussion concerning the structures of liquid crystal phases formed by rod-like molecules, a point is often reached where the arguments center on which structural features actually define the liquid-crystalline state. For example, when a conventional solid melts to a liquid, the strongly organized molecular array of the solid collapses to yield a disordered liquid where the molecules translate, tumble, and rotate freely. Thus, at the melting point, the molecules undergo large and rapid simultaneous changes in rotational, positional, and orientational order. However, when the melting process is mediated by liquid-crystalline behavior, there is usually a stepwise breakdown in this order. The incremental steps of this decay occur with changing temperature, thus producing a variety of thermodynamically stable intermediary states between the solid and the liquid. This collection of structurally unique phases essentially constitutes the thermotropic liquid-crystalline state [1–4]. Liquid crystals, therefore, are classically defined as those orientationally ordered phases that occur between the breakdown of positional/translational order on melting a solid, and the breakdown of orientational order on melting to a liquid [5]. However, for the purposes of discussing mesophase structures, all of the ordered states that occur between the increase in molecular rotational freedom on heating of the solid and the breakdown of orientational order on melting to the liquid will be described in the following sections. Thus, in these terms, the melting process can be characterized in the following way: first, an initial breakdown in order with the molecules oscillating or rotating rapidly about one or more axes; second, a collapse of the long-range positional ordering of the molecules to give a state where the molecules have short-range positional order (15–700 A˚), but yet they still have long-range orientational order; third, a disruption in the short-range and long-range order to produce a completely disordered liquid. Consequently, for a scenario involving materials where the constituent molecules have rod-like shapes, the first step in the breakdown of order is for the relatively static lath-like molecules of the solid to oscillate, or rotate rapidly, about a given axis (usually the long axes of the molecules) to give a layered ‘‘smectic-like’’ crystal phase. Secondly, the long-range positional order is then lost to produce a lamellar ‘‘smectic liquid crystal’’ mesophase. Thirdly, the local packing order is then destroyed, but the orientational order still remains with the

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

molecules reorganizing so that their long axes lie roughly in the same direction (known as the director of the phase) to give the nematic phase. Finally, this order breaks down to give the amorphous liquid. This description [6] of the melting process for rod-like molecules is shown schematically in > Fig. 1. As with other states of matter, the mesophases are indefinitely stable at defined temperatures and pressures. Mesophases observed above the melting point during the heating process are stable and termed ‘‘enantiotropic’’ [7], whereas those phases that occur below the melting point on supercooling of the crystal are metastable and are called monotropic. Transitions between the various liquid crystal mesophases invariably occur at defined temperatures and with little hysteresis observed between heating and cooling cycles.

2

Melting Processes of Calamitic Thermotropic Liquid Crystals

The liquid crystal phases of materials that possess molecules with rod-like molecular structures can be divided into two: the nematic phase where the molecules are simply orientationally ordered and the smectic phase where the molecules are arranged in soft layers. The nematic phase can potentially have four variants: the calamitic nematic, the discotic nematic, the biaxial nematic, and the cubatic nematic phases. Only the uniaxial nematic phase can be truly considered as calamitic as the constituent molecules are rod-like, for the other nematic phases the molecules do not have lath-like shapes. In all cases, it is possible to have reentrant nematic

Translational order is lost

Molecules are able to rotate

Molecules in fixed positions unable to rotate

T1

T3 Soft diffuse layers

T2

The crystal state long range order

The smectic mesophase layer ordering

The soft smectic crystal state long range translational order rotational disorder

T4 n^

Molecules free to rotate and tumble

Director Layer order lost

Melting process for a calamitic liquid crystal

T5

The isotropic liquid a disordered structure

The nematic mesophase orientationally ordered

. Fig. 1 The melting process of a calamitic (rod-like) liquid-crystalline material

1245

1246

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

phases where a nematic phase can be formed from a liquid, which is then cooled into a smectic and returns to a nematic on further cooling. However, transitions between the various nematic phases are not usually observed even though there have been a number of major searches made for the uniaxial to biaxial nematic phase transition. Conversely, the loss of order in the melting of the layered smectic state of calamitic systems can be broken down further into smaller increments via the changes in the packing order of the molecules. These incremental changes correspond to the formation of a number of structurally distinct layered modifications classified by miscibility/mixture studies and given the code letters A, B (hexatic), B(crystal), C, Calt, E, F, G, H, I, J, and K (note that the D phase has a cubic structure [6] even though the constituent molecules are rod-like in nature, and therefore because it does not have a truly layered structure it is omitted from the list of smectic phases). It is interesting to note at this point that the original classification of the smectic modifications was not made through structural investigations of the phases, but rather via miscibility studies using the phase rule and binary phase diagrams [8]. Phases that were found to be miscible over the entire concentration range of the phase diagram for binary systems were said to belong to the same miscibility group, irrespective of whether or not they were actually known to have the same structures. Consequently, the classification of unidentified phases of a novel material becomes a test of miscibility rather than immiscibility [8]. Phase sequencing with respect to temperature, and priority sequencing in pure materials and in phase diagrams has led to a thermodynamic ordering of the phases in the smectic state [9]. For example, current knowledge gives this sequence as: Isotropic Liquid; N; SmA; D; SmC; SmCalt ; ½SmBðhexÞ; SmI; BðcrystÞ; SmF; J; G; E; K; H; crystal ! increasing order !

where N is the nematic phase, D is a cubic phase, SmA, SmB(hex), SmC, SmCalt, SmI, and SmF are smectic liquid crystals, and B(cryst), J, G, E, K, and H are ‘‘smectic-like’’ soft crystal phases. Over the last 30 years, however, X-ray diffraction techniques have been used to investigate the structures of the smectic modifications [3, 4, 10–15]. This has led to combining miscibility classification with structural observations. Thus, certain structural features, such as the extent of the positional ordering, tilt orientational ordering, packing structure, and bond orientational ordering can be associated with each individual miscibility group. The structural properties of each miscibility class are depicted in > Fig. 2. No material has yet been found that exhibits all of these phases, but many compounds exhibit complex polymorphism as in, for example, 4-(2-methylbutyl)phenyl 4-n-octylbiphenyl40 -carboxylate (8SI, CE8, E. Merck (BDH)), which has an N, SmA, SmC, SmI, J, and G phase sequence [16].

3

The Structure of the Nematic Phase

The nematic phase is essentially a one-dimensionally ordered elastic fluid in which the molecules are orientationally ordered, but where there is no long-range positional ordering of the molecules. In this phase, the rod-like molecules tend to align parallel to each other with their long axes pointing roughly in the same direction [17]. The average direction along which the molecules point is called the director, a unit vector, of the phase and is usually given the symbol n. The rod-like molecules in the nematic phase are free to rotate about their long axes

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

n^

Structures of calamitic nematic and smectic liquid crystal phases (Plan and side views)

Nematic phase

Orthogonal phases

Tilted phases

Smectic Calt (anticlinic)

Smectic A

Smectic C (synclinic)

Hexatic B

Smectic I

Smectic F

Crystal B

Crystal J

Crystal G

Crystal E

Crystal K

Crystal H

Short range order

Long range order

. Fig. 2 The plan and elevation structures of the smectic phases

. Fig. 3 The structure of the nematic phase

and to some degree about their short axes, concomitantly, the relaxation times for end-overend rotations are much longer (105–106 times per second) than those around their long axes (1011–1012 times per second) [18–22]. The structure of the nematic phase is depicted in > Fig. 3.

1247

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

In the bulk nematic phase, there are as many molecules pointing in one direction relative to the director as there are pointing in the opposite direction (a rotation of 180 ), that is, the molecules have a disordered head-to-tail arrangement in the phase, and therefore the phase is not ferroelectric. Thus, the phase has rotational symmetry relative to the director. The degree to which the molecules are aligned along the director is termed the order parameter of the phase, which is defined by the equation: S¼

1 < 3cos2 y  1 > 2

where y is the angle made between the long axis of each individual rod-like molecule and the director. The brackets in the equation imply that this is an average taken over a very large number of molecules. An order parameter of zero implies that the phase has no order at all (it is an isotropic liquid), whereas a value of one indicates that the phase is perfectly ordered, that is, all the long axes of the molecules are parallel to one another and to the director. For a typical nematic phase, the order parameter has a value in the region between 0.4 and 0.7 indicating that the molecules are considerably disordered. The order parameter has the same symmetry properties as the nematic phase, in that the order parameter is unchanged by rotating any molecule through an angle of 180 . In a typical liquid crystal, S decreases as the temperature is raised. An example of the temperature dependency of the order parameter as a function of the reduced temperature from the liquid to the liquid crystal transition is shown in > Fig. 4. The order parameter decreases as the transition to the liquid is approached because of the increase in the thermal motion of the molecules. As the phase transition to the isotropic liquid is first order, there is a precipitous drop in the order parameter at the transition. The nematic phase is birefringent due to the anisotropic nature of its molecular properties and molecular ordering. The extraordinary ray travels at a slower velocity than the ordinary

1.0

0.8

0.6 S

1248

0.4

0.2

Liquid

Nematic liquid crystal

0.0 0.92

0.94

0.96

0.98 T/Tc

. Fig. 4 Temperature dependence of the order parameter, S

1.0

1.2

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

ray, thereby indicating that the phase has a positive birefringence. Moreover, in most nematic phases, the molecules are rotationally and orientationally disordered with respect to their short axes, and consequently, the phase is optically uniaxial. However, in some cases, particularly for molecules with broad molecular shapes or bent core structures, the degree of rotational freedom of the molecules about their long axes is restricted. This could lead to a preferred macroscopic ordering of the lath or bent-shaped molecules to give a nematic phase that is biaxial [23–27], see > Fig. 5a and > b, respectively. The nematic discotic phase is essentially a nematic phase made up of molecules that have disc-like shapes. The organization is similar to taking a stack of coins and knocking them over so that the planes of the coins are roughly parallel to one another, but where the coins have no positional order, as shown in > Fig. 6 [28, 29]. In the case of the discotic nematic phase, the coins are replaced with molecules that are designed to possess disc-like inflexible core units. The director of this uniaxial phase is essentially normal to the planes of the discs. However, typically the major transition moment of each disc-like molecule lies in the plane of the disc, and therefore, overall the birefringence is negative, unlike conventional calamitic nematics that are positive. Like calamitics, the molecules will be undergoing some dynamic motion, but probably to a lesser extent with rotational motions of the discs being easier than flipping. The discs can also slide over one another easier than move in a direction perpendicular to the planes. Typically, the molecules in discotic-nematic systems have symmetrical structures and

Director n^ Restricted rotation Board-like molecules

a

Local ordering of the molecules in the biaxial nematic phase

Different properties in three directions

b

. Fig. 5 Proposed structure of the biaxial nematic phase for board-like molecules (a), and bent-core molecules (b)

1249

1250

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

Molecules hexagonally close packed

Director

Disc-like molecules

Molecules

Disordering of the positions of the molecules along the column axis

Molecules ordered along the column axis Columns hexagonally close packed

Columns hexagonally close packed The nematic discotic phase

Disordered hexagonal columnar phase Dhd

Ordered hexagonal columnar phase Dho

. Fig. 6 Examples of ordered and disordered mesophases based on materials with disc-like molecular structures

therefore, there is no dipole contribution to the permittivities. Consequently, such mesophases are of little use as electrically addressable materials in displays. However, because of their negative birefringences, they are of considerable use in negative optical retardation films, which are used to provide wide viewing angles for LCDs [30]. There are also numerous other liquid crystal phases that possess disc-like molecules. They usually have columnar structures, rather like stacks of coins. The molecules in the stacks can be either ordered or disordered, and the columns can be hexagonally or orthorhombically closely packed together. They are, thus, similar to smectic phases in their relationships to the nematic phase.

4

Structures of Smectic Liquid Crystals

The lamellar smectic state is readily divided into four subgroups by considering firstly, the extent of the in-plane positional ordering of the constituent molecules, and secondly, the tilt orientational ordering of the long axes of the molecules relative to the layer planes, see > Fig. 2 [3, 4, 6]. Two groups can be defined where the molecules have their long axes on average normal to the layers. These two groups are distinguished from each other by the extent of the positional ordering of the constituent molecules. For example, the smectic A and hexatic B phases are smectic liquid crystals in which the molecules have only short-range positional order [6, 31], whereas the crystal B and crystal E phases are ‘‘smectic-like’’ soft crystal modifications [6, 32] where the molecules have long-range orientational ordering in three dimensions. Two other classes can be distinguished where the molecules are tilted with respect to the layer planes. In the smectic C, smectic I, and smectic F phases, the molecules have short-range orientational ordering [33], whereas in the crystal G, crystal H, crystal J, and crystal K phases the molecules have long-range three-dimensional ordering [12, 33]. Thus, smectics A, C, Calt, hexatic B, I, and F are essentially smectic liquid crystals, whereas Bcryst, E, G, H, J, and K are crystal phases. These latter phases, however, have somewhat different properties than normal crystals, for example, their constituent molecules are reorienting rapidly about their long axes (1011 times s1) [21, 34]. In the following sections, the structures of the smectic A and smectic C phases are described because they have been utilized in display devices. The smectic A phase has been used in light

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

scattering devices which can be electrically and/or thermally addressed. Homeotropic alignment gives an optically clear device, whereas homogeneous alignment gives a light scattering state, where the homogeneous alignment can be produced thermally (to give a data storage device) or electrically (to give a display device). Smectic C phases, on the other hand, when they are composed of chiral material can exhibit ferroelectric properties, which in turn can be utilized in a fast switching bistable mode of operation. A subgroup of the smectic C phase, (smectic Calt), when composed of chiral material can exhibit antiferroeletric properties which can be used in tristate switching devices. Chiral nematic and smectic systems will therefore be discussed as a separate section because of their unique properties.

4.1

Structure of the Smectic A Phase

In the smectic A phase, the molecules are arranged in layers so that their long axes are on average perpendicular to the layer planes, see > Fig. 7. The molecules are undergoing rapid reorientational motion about their long axes on a timescale of 1011 times per second. They are also undergoing relaxations about their long axes on a timescale of 106 times per second. The molecules are arranged so that there is no translational periodicity in the planes of the layers or between the layers. Therefore, there is only short-range ordering extending over a few molecular centers at most (15–25 A˚), with the ordering falling off in an algebraic fashion [35, 36]. Perpendicular to the layers, the molecules are essentially arranged in a one-dimensional density wave, therefore, the layers themselves must be considered as being diffuse, as shown in > Fig. 7. As a consequence, the concept of a lamellar mesophase is somewhat misleading because the layers are so diffuse that on a macroscopic scale they are almost nonexistent. In actual fact, the molecules are arranged within the lamellae in such a way that they are often randomly tilted at slight angles with respect to the layer normal. This makes the layer spacing on average slightly shorter than the molecular length. Typically, the molecules have

. Fig. 7 Structure of the orthogonal smectic A phase. The molecules are shown as ellipsoids

1251

1252

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

time-dependent tilts anywhere up to about 14 –15 from the layer normal [37–39]. However, as the tilting is random across the bulk of the phase, the mesophase is optically uniaxial with the optic axis perpendicular to the lamellae, and hence the phase has overall D1 symmetry. The smectic A phase can also have other variations in which the molecules are not arranged in singular molecular layers but are organized in semi-bilayer and bilayer structures. Semibilayer ordering is typically caused either by interdigitation or partial pairing of the molecules [40, 41]. Smectic A phases that have these structures invariably occur for materials where the molecules carry terminal polar groups, such as cyano moieties. In a typical example, the molecules overlap so that the polar terminal groups interact with the ends of the central cores of adjacent molecules, see > Fig. 8. In doing so, the molecules overlap to give a bilayer structure that has a bilayer spacing which is approximately 1.4 times the molecular length of a single molecule. This phase is often given the symbol SmAd, where d stands for a dimeric system. This particular mesophase is used in data storage applications and light scattering displays. A closely related subclass of the smectic A phase also exists where the molecules form a bilayer structure, where the bilayer spacing this time is equal to approximately twice the molecular length [42]. In this case, the polar terminal groups of the molecules overlap with each other to form dimers where the length of the paired system is equal to approximately twice the individual molecular length. This phase is called the smectic A2 phase. This phase could also be considered as having a monolayer structuring where the molecules in each individual layer point in the same direction, but the directions alternate from layer to layer giving an antiferroelectric ordering. > Figure 8 depicts this layer ordering using ovoid-shaped molecules to demonstrate the directional ordering of the molecules. As with the other smectic A phases, the molecules in the smectic A2 phase are in dynamic motion, and consequently the pairing of the molecules should be considered to be in constant flux. Alternatively, it is also possible to have polar molecular systems where the molecules do not overlap with each other to form a bilayer structure, but instead the molecules form lamellae where they are arranged in a disordered head-to-tail way so that a monolayer structure results, see > Fig. 8. This phase has been given the symbol SmA1, and transitions can be found from monolayer SmA1 to bilayer SmA2 and SmAd phases. It is also possible to have other variants of the smectic A phase, for instance, it is feasible to have a structure composed of SmA2 layers where the layers have a periodic in-plane correlation extending over approximately 150 A˚. This correlation, which extends over a large number of molecules, is produced by a half layer shift in the lamellae structuring. By periodic shifting of the bilayers into adjacent layers above and below, an undulating structure is formed [43]. At the point where there is a shift in the layer ordering, a region of the SmA1 phase is produced within the SmA2 phase. Alternatively, the structure of this phase can be viewed as being composed of layers where the molecules point in the same direction, and where this direction flips or inverts on a scale of approximately 150 A˚. This structure has been called the ‘‘ribbon’’ or antiphase ˜ . When this phase is formed, it is thought to be phase, and has been given the symbol SmA due to incommensurabilities between the lengths of the monomeric and the dimeric species [44, 45]. In addition, a case has been reported for the coexistence of two colinear incommensurate density waves of types SmA2 and SmAd [46]. The subphases of smectic A can therefore be described as follows: 1. SmA1 is a conventional smectic A phase where the molecules have random head-to-tail orientations.

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

Molecular length d Smectic A liquid crystals Bilayer structure lamellar spacing ~1.4d Smectic Ad Phase

Monolayer spacing Molecules Bilayer spacing

Smectic A1 phase

Smectic A2 phase

Half layer shift in the bilayer structure

In-plane density wave

~ Smectic A antiphase

. Fig. 8 Bilayer and monolayer structures of the smectic A phase

2. SmA2 is a bilayer phase with antiferroelectric ordering of the constituent molecules. 3. SmAd is a semi-bilayer phase with partial molecular overlapping due to associations. ˜ is a phase with a modulated antiferroelectric ordering of the molecules within the 4. SmA layers giving a ribbon-like structure. Thus, it can be seen that the smectic A phase is rather more complicated than the simple picture often presented of molecules arranged in orthogonal layers. It is very important to remember that the layer structure is only weak and that dimeric interactions can play an important part in the structuring of the phase, and hence its physical properties.

1253

1254

7.1.1 4.2

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

Structure of the Smectic C Phase

In the smectic C phase, the constituent molecules are arranged in diffuse layers where the molecules are tilted at a temperature-dependent angle y with respect to the layer planes [47]. When the smectic C phase is formed from the smectic A phase upon cooling, the temperature dependence of the tilt angle approximately takes the form: ðyÞT ¼ ðyÞ0 ðTc  TÞa where (y)T is the tilt angle at temperature T C, (y)0 is a constant, Tc is the smectic A to smectic C transition temperature, T is the temperature, and a is an exponent that is usually set equal to 0.5 (typically the experimental value of the exponent is found to be less than 0.5) [47–49]. The molecules within the layers are locally and hexagonally close-packed with respect to the director of the phase; however, this ordering is only short range, extending over distances of approximately 15 A˚. Over large distances, therefore, the molecules are randomly packed, and in any one domain the molecules are tilted roughly in the same direction in and between the layers, and hence the phase is described as being synclinic (see > Fig. 9). Thus, the tilt orientational ordering between successive layers is preserved over long distances. Consequently, the smectic C phase has C2h symmetry and is weakly optically biaxial. A subphase of the smectic C phase also exists, which is called the alternating or anticlinic smectic C (SmCalt) phase [50–52]. This phase was originally discovered by Levelut et al. [50] and given the code letter smectic O. However, the chiral version of this phase was labeled as being an antiferroelectric smectic C phase by Fukuda et al. [53], and it is this descriptor that is in current, general use. The in-plane ordering of the molecules is thought to be identical to that of the smectic C phase. The major difference between the alternating C and normal smectic C phases resides in the relationship between the tilt directions in successive layers. In the alternating tilt phase,

θ

. Fig. 9 Structure of the tilted smectic C phase. The molecules are shown as ellipsoids, the layers are diffuse, there is no long-range positional order but there is long-range tilt orientation

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

the tilt direction is rotated by 180 on passing from one layer to the next [54]. Thus, the tilt direction appears to flip from one layer to the next, thereby producing a zigzag layered structuring, and the phase should be considered as a one-dimensional crystal. Consequently, the director of the phase is effectively normal to the layer planes, as shown in > Fig. 10. However, there appears to be no long-range positional correlations of the molecules between layers, even though the orientational ordering appears to be long range. So far, the alternating C phase has always been found to occur below the smectic C phase on cooling for compounds that exhibit both phases. In addition to the alternating smectic C subphase, other subphases of the smectic C phase can be found for systems where the molecules carry terminal polar groups (e.g., cyano) [6]. These subphases are identical to those of the smectic A phase, except for the fact that the ~ phases molecules are tilted with respect to the layer planes. Thus, the smectic C1, C2, Cd, and C ˜ are the direct analogs of the A1, A2, Ad, and A phases, respectively [6].

5

Molecular Templates for Calamitic and Discotic Nematic Liquid Crystals

The type of liquid crystal phase formed by a mesomorphic material is essentially dependent on the molecular properties of the substance. A primary factor in the formation of liquid crystal phases is the gross molecular shape. Three separate species can be identified where the molecules have the following rotational volumes: spheroid, ellipsoid, and discoid. Spheroid mesomorphic materials generally give rise to plastic crystals. Ellipsoid or rod-like molecules give rise to calamitic liquid crystals, which include nematic (N nematos, Greek for threadlike), smectic liquid crystals, and anisotropic plastic crystals (S smectos, Greek for soap-like). Discoid materials produce nematic-discotic and columnar liquid crystals, see > Fig. 11. Molecules with combinations of these shapes can also be mesomorphic. For instance, molecules that possess disc-like and rod-like shapes can exhibit both calamitic and discotic phases, and such materials are often called phasmidic; similarly molecular structures that combine features of both discs and spheres can have bowl-like shapes and can produce bowlic or pyramidal

. Fig. 10 Structure of the alternating tilt smectic Calt phase. The molecules tilt in opposite directions on passing from one layer to the next

1255

1256

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

Molecular template

Mesogenic system

Spherical Dendritic

Calamitic Cylindrical Phasmidic

Discoid

Columnar

Bowlic Columnar

. Fig. 11 Molecular templates and the associated liquid crystal mesophases that they form

mesophases. A number of molecular templates for the formation of certain liquid-crystalline modifications are shown in > Fig. 11. All of these templates have the potential to be chiral. Asymmetry in structure can be seated either in the core of the structure, as shown by the structures on the left-hand side of the figure, or in the peripheral aliphatic chains shown in the right-hand side. Up until the last decade, the basic design of the molecular structures of calamitic thermotropic liquid crystals changed very little. For materials with rod-like molecular shapes, the prototypical molecular design involves the incorporation of a central aromatic, heterocyclic, or alicyclic core unit, to which are attached terminal aliphatic chains [55–58], thereby engendering structures with rigid or semirigid sections surrounded or segregated by flexible fatty chains, see structures 1 and 2 in > Fig. 12. When molecules with this type of architecture self-organize, they generally do so with their rigid, aromatic parts tending to pack together, and their flexible/dynamic aliphatic chains orienting together. Consequently, the main target of material design has been, by default, the variation in the structure of the central core region of the molecules in the belief that the core is more important in influencing mesophase incidence, mesophase temperature range, clearing point, melting point, mesophase sequence, dielectric and optical anisotropy, elastic coefficients, and the reorientational viscosity associated with the mesophase [59–62].

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1 O O−R 1

N O R1(O) O−R2

R(O)

CN

F R1(O)

2

3

F (O)R2

4

. Fig. 12 Typical material design for rod-like liquid crystals [63, 76, 83, 84]. Materials 3 and 4 show the design of liquid crystals found in display devices

The bedrock of the LCD industry has been built on the twisted nematic display device (TN-LCD), which was developed in the early 1970s. Nematic liquid crystals designed for applications in TN (Twisted Nematic) and STN (Super-Twisted Nematic) Liquid Crystal Display (LCD) technologies typically have large polar groups situated at one terminus of the molecular structure to engender large positive dielectric anisotropies where the dielectric permittivity parallel to the organization of the long axes of the molecules is larger than the perpendicular permittivity [63–67]. The structural design for nematogens with such anisotropies is exemplified by the cyanobiphenyls shown in structure 3 in > Fig. 12. Similarly, modern LCD-TVs utilizing in-plane switching modes (i.e., IPS-LCDs) [68–70] with wide viewing angles also employ nematogens with positive dielectric anisotropies. Conversely, competitive LCD-TV technologies based on vertically aligned nematic modes (VAN-LCDs) utilize materials with negative dielectric anisotropies [71–75]. Such materials are designed to have polar groups situated in a lateral position relative to the long axes of the molecules as exemplified by the difluoroterphenyls, 4, shown in > Fig. 12 [76]. In addition to the design and development of materials for applications in VAN-LCDs, materials such as 4, with negative dielectric anisotropies, are of practical use in smectic devices, for example, those based on the surface-stabilized ferroelectric liquid crystal geometries (SSF-LCD) [77] or tVmin ferroelectric displays based on dielectric biaxiality [78–80]. Ferroelectric devices offer fast response times and bistable operation, and are of practical use in the eyepieces of digital cameras [81, 82]. If we consider the common ‘‘materials design’’ for positive dielectric anisotropy materials which comprise a central core unit with one attached peripheral group, usually an aliphatic chain, as depicted in structure 3 in > Fig. 12 [76, 83], then it is easy to understand how the differences in interactions between the aliphatic flexible chains and the aromatic rigid cores can lead to so-called microphase segregation where the like parts of the molecules pack together and repel the dissimilar parts. > Figure 13 shows how the like parts of the molecules can interact, but at the same time repel the dissimilar parts; however, the simple fact that the two are joined together means that the dissimilar parts cannot escape from one another. This means that large-scale segregation cannot occur, and that any segregation must take place in the

1257

1258

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

. Fig. 13 Microphase segregation of the two dissimilar sections of the molecular structure, in this case aromatic and aliphatic parts, occurs but long-range orientational order can still be retained

microscale regime. As a consequence, nucleation to form extended organized quasicrystalline order cannot occur; however, short-range order is possible. The extent of the microphase segregation and changes between the proportion of the interactions, such that one of the interactions between the like parts becomes more dominant, can affect the type of mesophases observed. For example, if the flexible, aliphatic chains of a dichotomous microphase segregating system are lengthened, their intermolecular interactions will come to dominate over the interactions of, say, the non-flexible, aromatic sections of the molecules. In this case, a layered structure will come to dominate. Thus, as the chain length is increased, nematic phases will give way to lamellar smectic phases. This effect is demonstrated in > Fig. 14 for the homologous series of the 40 -alkyl-4-cyanobiphenyls. For the homologues with short chain lengths (C1–C7) nematic phases dominate, at which point smectic A phases are introduced and come to dominate totally by the C10 homologue. For materials found in TN, STN, and IPS liquid crystal displays (LCDs), the dielectric permittivity parallel to the organization of the long axes of the molecules is larger than the perpendicular mode, which means that the molecular dipole is required to be located along the long axis of the molecule as shown in > Fig. 15 for 40 -pentyloxy-4-cyanobiphenyl (5OBC). In addition to microphase segregation, the interactions of the molecular dipoles, often in an antiparallel arrangement, can lead to quadrupolar coupling which will stabilize the mesophase with respect to temperature. Quadrupolar couplings of this nature are epitomized by the antiparallel arrangement shown in the lower part of > Fig. 15. Designing materials with appropriate device properties, or fine-tuning of properties for applications, has mostly been achieved through selection of the structure of the rigid or aromatic part of a mesogen and the lengths of the aliphatic chains. Properties such as transition temperatures, viscosity, dielectric permittivity, birefringence, and response time can be manipulated via subtle changes to chemical structure. For example, > Fig. 16 shows how the structure of 40 -pentyl-4-cycanobiphenyl (5CB) can be modified by changing the phenyl rings for either trans-cyclohexyl moieties or 2,2,2-bicyclooctane cages. Over a 150 C temperature

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

120 110

CnH2n+1

CN

100 90 Isotropic liquid

80 70 Temp (⬚C)

SmA

50 40

N

30

N

Crystal

20 Enantiotropic Nematic and Smectic A Phases occuring above the melt point

Monotropic Nematic occuring below the melt point

10 0 0

1

2

3

4

5

6

7

8

9

10 11 12 13

Number of carbon atoms (n)

. Fig. 14 The effect of aliphatic chain length on the melting points and phase transitions ( C) for the 40 -alkyl-4-cyanobiphenyls

Dipole

O

C

N

Polar and Polarizable molecular system

Dipole

δ

O

δ

N

C

Nδ O

C

Dipole Quadrupolrar coupling

. Fig. 15 Dipole location in 40 -pentyloxy-4-cyanobiphenyl

δ

1259

1260

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

C5H11 –60

CN +20

Nematic to liquid 35 ⬚C

C5H11

C5H11

CN Nematic to liquid −25 ⬚C

Nematic to liquid 55 ⬚C

+110 +75

CN

+30 C5H11

CN +45 Nematic to liquid 85 ⬚C

C5H11

C5H11

CN +44

Nematic to liquid 50 ⬚C

CN Nematic to liquid 100 ⬚C

+29 C5H11

CN Nematic to liquid 129 ⬚C

. Fig. 16 Affect of the structure of the mesogenic core on nematic to isotropic liquid transition temperatures ( C)

variation can be achieved by simple replacements. It is also interesting that incorporating alicyclic core systems has an advantageous affect on the clearing points in comparison to the incorporation of aromatic rings. Nematic to isotropic liquid transition temperatures are also sensitive to the inclusion of heteroatoms into rings systems. For example, > Fig. 17 shows the effect of incorporating nitrogen into biphenyl systems where the phenyl pyrimidine analogue has a higher clearing point by 17 C, conversely when heteroatoms, such as oxygen or sulfur, are incorporated into a cyclohexyl ring, the clearing points are invariably lower. The larger and more polarizable sulfur atom gives much lower transitions in comparison to the smaller and more polar oxygen. The design of nematic discotic materials requires steric effects in order to stabilize the phase over the formation of columnar phases. For aromatic disc-like materials, the face-to-face interactions of the discs are quite strong, and hence these drive the formation of molecular stacks. However, by increasing the steric repulsion between the discs, thereby forcing them apart, the discs become free to slide over one another to give a nematic phase. For example, consider the disc-like triphenylene material shown in > Fig. 18. When R = C10H21, the material melts at 142 C to give a columnar phase, which forms a nematic phase at 191 C, clearing to the

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

C5H11

C5H11

CN

Nematic to liquid 55 ⬚C

Nematic to liquid 35 ⬚C N

C5H11

C5H11

CN N

O CN O

Nematic to liquid 52 ⬚C

Nematic to liquid 52 ⬚C

S

C5H11

CN

C5H11

CN

O CN S

S Nematic to liquid 28 ⬚C

Nematic to liquid 19 ⬚C

. Fig. 17 Affect of heteroatoms in the mesogenic core on nematic to isotropic liquid transition temperatures ( C)

OR OR O RO

O

O O

O O

O O O

O

O O

RO

OR

OR

. Fig. 18 Structure of a nematic and columnar family of disc-like materials

liquid at 212 C. The simple inclusion of a methyl substituent ortho to the C10 chain suppresses the formation of columnar phases and the melting point by 40 C. The clearing point also falls to 192 C, and so the material exhibits an enantiotropic nematic discotic over a temperature range of 90 C. Such materials are used in negative retardation films to give wide viewing angle properties to LCDs. Increasing the size of an ortho substituent has been found to have the effect of increasing the clearing point, but at the expense of an increasing melting point [28, 29]. However, no columnar phases are observed and the materials remain solely nematic. Conversely, if the substituents are located meta to the C10 chain, as the size of the substituent is increased the

1261

1262

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

. Table 1 Affect of bulky substituents on the melting points and nematic to isotropic liquid transition temperatures ( C) for triphenylene materials with disc-like molecular structures OR OR RO RO OR OR

X R=

O OC10H21

Outer Substituent

X R=

O OC10H21

Inner Substituent

X

K

Colrd

ND

Iso liq

H

∙

142

∙

191

∙

212

∙

CH3

∙

102

∙

–

∙

192

∙

C2H5

∙

129

∙

–

∙

206

∙

CH(CH3)2

∙

161

∙

–

∙

202

∙

C(CH3)3

∙

194

∙

–

∙

225

∙

H

∙

142

∙

191

∙

212

∙

CH3

∙

107

∙

–

∙

162

∙

C2H5

∙

117

∙

–

∙

131

∙

CH(CH3)2

∙

93

∙

–

∙

70

∙

melting point is driven down along with the clearing point to temperatures around 100 C, as shown in > Table 1.

6

Molecular Templates for Smectic Liquid Crystals

At a crude level, the type of smectic mesophase formed by a smectogen depends on molecular shape, with the following subunits being important: (1) a rigid core, (2) polar functional groups, (3) laterally appended groups, and (4) peripheral substituents and chains. From property–structure correlations developed over many years of studying the relationships between molecular structure and mesophase formation, some idealized molecular shapes that support smectic phase generation have been identified. The immediate impression is that smectogens do not necessarily have molecular shapes that are confined to being rod-like or lath-like or calamitic! The molecules can have bent shapes, be bowlic or pyramidal, X- or cross-shaped, or even spherical. Essentially, examination of the varieties of molecular templates that support smectic phase formation indicates that molecules will find a number of ways, through molecular deformation or by sharing space, to form lamellar structures. For example, for compounds with less-flexible structures, smectic mesophases can be formed if the molecules are able to share space, thereby reducing the void volume in the structure of the mesophase.

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

1

R (A)

CORE

Z

(B) R2

CORE

(X)

1

R (A)

(Y) (i)

CORE

Z

CORE

(X)

W

(Y)

Z is a linking group 1R and 2R are aliphatic chains X and Y are lateral substituents A and B are usually polar linking groups W is a terminal group

(ii)

Examples: Z = CH2CH2, (CH2)4, CH=CH, C≡C, COO, COS, CH=CHCOO, N=N, CH=N, CH2O, COCH2, CONH, or nothing etc X and Y = H, CH3, C2H5, CnH2n+1, F, Cl, Br, I, CF3, CCl3, CN, COOH, NO2, NH2, N(CH3)2, OH, or multiples thereof A and B = CH2, O, S, Se, NH, NCH3 etc W = H, CH3, OCH3, CF3, C2F5, CN, F, Cl, Br, I, NO2, N(CH3)2, COOH, OH etc Structures of Some Common Core Ring Components O

N

O

O

N

H N

S

N N

N

H N N

N N

N N

S N N

etc N

. Fig. 19 Molecular motifs for liquid crystal design

Thus, given that there is a very wide variety of template structures that can support smectic phase formation, it is probably easier to focus on the more significant types of smectic material. These materials tend to be drawn from the more traditional/conventional lath-like structural templates for smectogens, where the template incorporates a simple linear core that has one or two peripheral groups associated with it. Further details concerning the molecular architecture/functional groups of this simple template are shown in > Fig. 19. In this template, the core structure is further divided to show the linking groups, terminal substituents, and polar/apolar groups position at the termini of the core, and lateral groups attached to the core. Architecture (i) tends to be the template that will give the largest diversity of mesophase type in comparison to architecture (ii). For example, system (i) with two peripheral aliphatic chains will be more likely to support the formation of tilted mesophases in comparison to structure (ii). Thus, it is common when the two peripheral aliphatic chains are extended in length, there will be a crossover from orthogonal phase (SmA) to tilted mesophase formation (SmC). Only under specific architectural conditions will a system with a single aliphatic chain exhibit tilted mesophases.

1263

1264

7.1.1 6.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

Effects of Terminal Aliphatic Chains

For systems that have two terminal aliphatic chains that can be independently varied in length, tilted phases tend to occur when the two lengths are similar to one another. At short chain lengths, usually orthogonal phases such as smectic A, smectic B, crystal B, and crystal E occur. As the chain lengths of both peripheral aliphatic groups are increased, tilted smectic C phases are usually observed first, followed by smectics I and F, underlying these phases there is the possibility of forming more ordered modifications such as J, G, H, or K. At very long chain lengths, the tilted phases sometimes, but not always, disappear to give systems that just exhibit smectic A phases. For systems where there is only one terminal aliphatic chain, typically no tilted smectic phases are observed, and orthogonal phases predominate. The smectic A phase is the commonest modification observed, followed by the smectic B, crystal B, and E phases. > Figure 20 shows a comparison of three materials that have only one terminal aliphatic chain, but where the terminal ring of the core has the possibility of the incorporation of a heteroatom (nitrogen). It can be seen from this figure that the nematic phase still tends to dominate over smectic phases, and that the smectic phases observed are all smectic A in type.

6.2

Effect of Polar Groups at the Core Termini

The above discussion applies to most systems irrespective of core types, linking groups, and lateral groups, but not polar groups positioned at the termini of the core structures. The groups A and B in structure (i), shown in > Fig. 19, can be very important in determining whether or not tilted mesophases are formed. It was found over a large number of studies that when functional groups A and B are polar (but not hydrogen bonding), a higher incidence of tilted

O

C10H21O O

Cr 110 (• SmA 106) • N 127 • Iso Liq O

C10H21O O

N

Cr 115 • SmA 126 • Iso Liq O

C10H21O

N O

Cr 121 (• N 114) • Iso Liq

. Fig. 20 Liquid crystal properties of materials that have only one terminal chain

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

smectic phases is observed. > Figure 21 (left) demonstrates this effect for a set of substituted phenyl biphenyl carboxylates. In this family, the oxygen atoms at the termini of the core are systematically removed. With both oxygen atoms present the material exhibits a tilted smectic C phase, and as the oxygen atoms are removed the temperature range and the upper transition temperature of the smectic C phase fall, and when there are no ether links present the tilted smectic C phase disappears. This result is typical of many material systems resulting in the experimental observations being made forming the basis for McMillan’s theoretical model of the smectic C phase [85]. In this model, McMillan termed the dipoles associated with the groups at the termini of the core as ‘‘outboard terminal dipoles.’’ The coupling of the terminal outboard dipoles at the smectic A to smectic C phase transition was assumed to generate a torque which resulted in the molecules being tilted over in their layers. However, as with most studies concerning the relationship between molecular structure and the types of smectic liquid crystal phase observed, this is not the whole story. If methyl branching points are incorporated into the terminal aliphatic chains without the inclusion of ‘‘outboard’’ dipolar groups, then tilted smectic phases can be returned, as shown in > Fig. 21 (right). This demonstrates the pronounced effects that can be produced from small changes in molecular structure and effect of the packing of the molecules together. Other theoreticians, most notably Wulf [86], attempted to relate the tilting of the molecules to the way in which the molecules pack laterally together side by side. Examination of the

O

C8H17O O

OC8H17

Cr 110 • SmB 116 • SmC 165 • SmA 200 • Iso Liq

O

C8H17 C8H17

O O

C8H17O O

C8H17

Cr 102 (• G 67 • SmB 98) • SmA 153 • Iso Liq

O

Cr 110 • SmB 110.5 • SmC 132.5 • SmA 184 • Iso Liq *

O

O

C8H17 O

OC8H17

Cr 98 • SmB 112 • SmC 125 • SmA 171 • N 173 • Iso Liq

Cr 83.4 (•SmC* 74.3 • SmA* 81) • N* 114 • Iso Liq

O O

O

C8H17 O

C8H17

Cr 86 (•SmC 78.1) • SmA 96.1 • N 118.5 • Iso Liq

Cr 102 (• G 67 • SmB 98) • SmA 153 • Iso Liq

. Fig. 21 Effect of branching in the terminal chain on the formation of smectic C phases

*

1265

1266

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

molecular shapes of compounds that form tilted phases showed most had zigzag shapes, and it was the way in which the bends in the structures of adjacent molecules nested together that produced the overall tilted structure. Wulf ’s model also showed that as the aliphatic chains are lengthened, the molecular structure becomes increasingly zigzag in shape; however, as the aliphatic chain lengths are increased further, the zigzag shape becomes less prominent. Thus, Wulf ’s model of the tilted smectic C phase correctly reflects experimental observations, that is, as the terminal chain lengths are increased, the molecular shape becomes zigzag thereby injecting and stabilizing the smectic C phase; at longer chain lengths, the zigzag shape is less prominent and so the smectic C phase becomes less stable and disappears.

6.3

Effect of Functional Groups that Terminate the Core Structure

This section applies specifically to materials with template structures such as the one shown in > Fig. 19(ii). Here, the template has only one terminal chain attached to one end of the core, and at the other end of the core is a functional group that can be either polar or apolar. Generally, terminal groups that are polar and conjugated to the aromatic core tend to form nematic phases, whereas nonpolar unconjugated groups lean toward forming smectic A and B phases. For example, > Fig. 22 shows a property/structure correlation between mesophase type formed and the substituent in the 4-substituted-(X)-phenyl 4-n-octyloxybiphenyl-4carboxylates [87]. The substituent was varied from being polar to apolar, and its length was altered from hydro to fluoro to propyl. Tilted phases only occur when the substituent exceeds a certain length (equivalent to propyl in this case). Nematic phases are exhibited for the polar cyano and nitro substituents. As noted earlier in > Sect. 4.1 for polar terminal groups, such as the cyano group, the structure of the resulting smectic A phase differs from that of the less-polar systems. In the lesspolar systems, the layer spacing in the smectic A phase is approximately equal to the molecular

. Fig. 22 Effect of terminal end group on mesophase formation

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

length, whereas for polar systems, and cyano systems in particular, the layer spacing is larger. Generally, for polar groups, the layer spacing is approximately 1.4 times the molecular length, thereby generating a bilayer structure. This increase in layer spacing is due to the formation of molecular pairs caused by a quadrupolar coupling between the longitudinal dipoles, as described earlier.

6.4

Effect of Core Ring Structures

It is generally understood that smectic liquid crystals are more likely to be formed in systems where the central core region has aromatic or heteroaromatic ring structures. Alicyclic ring systems conversely tend to disfavor smectic phase formation, and in particular, they tend to depress tilted phases over orthogonal phases. For example, early work in the search for nematic materials for display devices resulted in the development of the 4-substituted-phenyl benzoate unit as a suitable core for imparting mesogenic properties to a material and for use in modifications to physical properties (most notably, elastic constants). Extensions to the work on the 4-substituted-phenyl benzoate system included the replacement of either the phenyl or benzoate unit with the trans 1,4-disubstituted cyclohexyl moiety or the corresponding bicyclo [2.2.2]octane unit. Investigations of transition temperatures in pure materials and in binary phase diagrams show that the alicyclic systems are prone to raising clearing temperatures while at the same time suppressing smectic phase formation. > Figure 23 shows a comparison of various esters where one of the rings of the phenyl benzoate unit has been replaced by an alicyclic ring. Although smectic phases are rarely seen in such systems, it still can be seen how the clearing temperature is enhanced by the inclusion of a bulky alicyclic system such as the bicyclooctanyl moiety. Smectic phases are not seen in the phenyl benzoates because their melting points and clearing temperatures are so low. However, the cyclohexyl system does show weak smectic behavior, but the bicyclohexanyl systems, even though they have wide mesomorphic ranges, are not found to exhibit smectic phases.

O O

C5H11

Cr 34.8 (• N 25.9) • Iso Liq

O

O

Cr 42.8 • N 51.7 • Iso Liq

C5H11

C5H11 C5H11

O

O

O

OC5H11

C5H11

Cr 31 • N 65.5 • Iso Liq

Cr 36 (• SmA 29) • N 48 • Iso Liq

C5H11 O

O

O

C5H11

C5H11

C5H11 O

O

OC5H11

Cr 50.5 • N 93.5 • Iso Liq

Cr 28 • N 70 • Iso Liq O

C5H11 O

C5H11

Cr 28 (• N 22) • Iso Liq

. Fig. 23 Effect alicyclic and aromatic ring combinations on mesophase formation

OC5H11

1267

1268

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

The above study reflects a comparison for systems that would be expected to form monolayer structures in the smectic state. A similar comparison, however, can be made for systems that might exhibit bilayer ordering. For example, the cyanobiphenyl core and related materials have been explored in depth because of their use in device applications. Again, one of the rings of the biphenyl unit can be systematically replaced by cyclohexyl and bicyclooctyl moieties. > Figure 24 shows some comparative results for the 4-n-alkyl-40 -cyanobiphenyls, the trans-1-n-alkyl-4-(4-cyanophenyl)cyclohexanes, and the 1-n-alkyl-4-(4-cyanophenyl)bicyclo[2.2.2]octanes. This comparison is more conclusive than the preceding one, and demonstrates clearly, at least for the formation of bilayer smectic phases, that the inclusion of alicyclic systems suppresses smectic phase formation and improves the likelihood of nematic phases being formed. Similar studies using other ring systems in the central core show that, by and large, the use of nonpolar or nonpolarizable or highly polarizable cores results in the formation of smectic B phases or crystal B phases. Polar cores on the other hand enhance the formation of more disordered smectic phases such as the smectic A and smectic C modifications. > Figure 25 shows comparisons for a variety of ‘‘biphenyl clones’’ (where possible for the hexyl- and hexyloxy-substituted derivatives). In this study, one of the phenyl units has been replaced with an alicyclic or a heterocyclic ring. Resulting cores that are relatively nonpolar tend to exhibit orthogonal ordered phases such as smectic B and crystal B and E phases. For symmetrical polar units (e.g., pyrimidine and tetrazine), smectic A phases tend to predominate, and where one side of the core is more polar (e.g., pyradizine) smectic C phases are found. In addition to affecting mesophase type, the polarity of the core can also raise transition temperatures. In the case where a heterocyclic ring is coupled to an alicyclic ring with strong mesogenic tendencies (e.g., bicyclohexane linked to tetrazine), high clearing temperatures can be achieved. A wide variety of similar studies on three-ring, fused-ring, and two-ring systems show that by careful control of the degree of polarity in the core, the direction of dipoles, the symmetry (or lack) of the charge distribution, the polarizability of the p-system, and the overall steric shape (bent or linear), the synthesis of smectic materials possessing phases of predicted type, structure, and accompanying transition temperatures can be achieved (> Fig. 25). Host materials for so-called ferroelectric devices are mixtures of compounds that are achiral, and when blended together they exhibit a nematic, smectic A, and smectic C phase sequences. This sequence of phases is required for the purposes of ease of alignment with the smectic C phase being available over a temperature range that includes room temperature. The materials must be able to dissolve a suitable chiral dopant or combination of dopants that will induce the necessary symmetry breaking operation in order to generate helielectric and surface-stabilized ferroelectric properties. The host materials must also have low viscosities

CnH2n+1 n 7 8 9

Cr • • •

28.5 21 40.5

CnH2n+1

CN SmA • 32.5 • 44.5

N • • •

42 40 47.5

Iso Liq • • •

n 7 8 9

Cr • • •

30 33 35

CN SmX • 17 -

N • • •

59 54 57

Iso Liq • • •

CnH2n+1 n 7 8 9

Cr • • •

CN

61 52 56

N • • •

95 90 90

Iso Liq • • •

. Fig. 24 Effect on transition temperatures ( C) of combinations of alicyclic and aromatic rings in the central core sections of mesogens

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1 N

C6H13O

C6H13

C6H13

C6H13

C6H13S

C6H13

N Cr 9 • E 68 • B 84 • Iso Liq

C6H13O

N

Cr 25 • SmX 52 .5 • Iso Liq

C7H15

C6H13

Cr 50 • SmA 57.5 • Iso Liq

OC6H13

C6H13

OC6H13

N Cr 38 • B 52 • Iso Liq

N N

Cr 22 • SmX 66 • N 69 • Iso Liq

Cr 80 (• SmC 78) • Iso Liq

N C3H7O

C4H9

C6H13O

C6H13

C6H13O

OC6H13 N N

N Cr 69 (• N 47) • Iso Liq

Cr 87 • SmC 106 • Iso Liq

Cr 45 • SmA 75 • Iso Liq O

C4H9O

N C6H13

C6H13O

N N C6H13

O

Cr 44 • SmB 59 • SmA 89 • Iso Liq O

Cr 65 (• N 58.5) • Iso Liq N N

N C6H13

O Cr 34 • SmA 45 • N 53 • Iso Liq

C6H13

C6H13O N Cr 45 • SmA 71 • Iso Liq

OC5H11 N N

N

Cr 44 • B 50 • Iso Liq

C6H13O

C6H13

C5H11

C3H7O N N Cr 130 • SmX 189 • Iso Liq

. Fig. 25 Effect of ring systems on the incidence of smectic phases

in order to produce mixtures that have fast responses to weak electrical fields. They must also be chemically and photochemically stable and have a tilt angle of about 22 –28 in the smectic C phase for optimum device performance. The properties required of ferroelectric hosts therefore means that materials such as esters, Schiff ’s bases, and azo compounds are unsuitable because of their slow response times and poor stability. The best materials discovered so far rely on the removal of any functional or linking groups which might increase the viscosity or lower the stability. > Figure 26 shows a variety of families of host materials [76, 88–92]. These materials have a number of attributes in common. Firstly, they are devoid of linking groups, thus the aromatic or heterocyclic rings in the core are directly linked. Secondly, only alkyl or alkoxy terminal chains are used in order to maintain as low a viscosity as possible. Thirdly, some materials carry lateral fluoro-substituents in order to increase the dielectric anisotropy and dielectric biaxiality. Fourthly, the selective positioning of the fluoro-substituents or the inclusion of heterocyclic rings can be used to stabilize the formation of tilted phases. In addition, modifications to the terminal end chains by the introduction of alkenic moieties have been used extensively to moderate phase transition temperatures and physical properties [93]. The fact that all of the best host materials have no linking functional groups and that most of the core rings are directly linked means that in order to generate materials with appropriate substituents and substitution patterns or appropriate ring types, the coupling of rings together to yield suitable core structures is very important [76, 94, 95]. For example, > Fig. 27 shows examples of alkyl- and alkoxy-substituted difluoroterphenyls, all of which exhibit nematic phases that are useful for the purposes of alignment. Four of the materials

1269

1270

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

F

F N

N R⬘(O)

R⬘(O)

(O)R⬙

(O)R⬙

F

F N

N R⬘(O)

R⬘(O)

(O)R⬙

(O)R⬙ N

N

F

F

F

R⬘(O)

R⬘(O)

(O)R⬙

F

F (O)R⬙

F

F

F

O R⬘(O)

(O)R⬙

R⬘

(O)R⬙ O

. Fig. 26 Potential host materials for applications in surface-stabilized ferroelectric displays

F

F

C5H11

F C8H17O

OC8H17

Cryst 89 SmC 155.5 SmA 165 N 166 Iso Liq F C7H15

F C7H15

Cryst 89.5 SmC 148 SmA 151.5 N 154 Iso Liq

F

F C5H11

C5H11

OC8H17

Cryst 48.5 SmC 95 N 141.5 Iso Liq

Cryst 56 SmC 105.5 SmA 131 N 136 Iso Liq F

F

F

C7H15

C9H19

Cryst 49 SmC 77 SmA 93 N 108.5 Iso Liq

. Fig. 27 Difluoroterphenyls used as hosts in smectic C* displays

also exhibit smectic A phases, which allow for the setting up of the layer organization that provides for two degenerate tilts of the smectic C phase formed upon cooling. When doped with a chiral material, the smectic C phase becomes helielectric, and locally (in the planes of the layers) ferroelectric. Mixture of these types of achiral materials can be used to form host mixtures with suitable phase sequences and transition temperatures for device applications.

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7

7.1.1

Chiral Liquid Crystals

An object is said to be chiral when it has a nonsuperimposable mirror image, that is, the object is said to be ‘‘handed.’’ Many natural systems are composed of mixtures of materials that have left-handed and right-handed antipodes, called enantiomers, that are present in unequal proportions. Such systems are said to be chiral [96, 97]. In chemical systems, the topic of chirality has reached a pinnacle of importance in recent years, particularly with the development of new asymmetric reactions that allow chemists to design and create chiral (handed) materials almost at will. In liquid crystal systems, the availability of chiral substrates of high optical purity has led to the discovery of novel phase structures and related phase behaviors. However, it is also important to recognize that many of the physical properties of chiral liquid-crystalline materials are dependent on the ‘‘degree of chirality’’ exhibited by the material, which in turn is directly related to the optical purity (enantiomeric excess). The optical purity is defined as the excess of one optical isomer (enantiomer) over the relative amount of the other in a chemically pure material. Consequently, a racemic mixture has an enantiomeric excess of 0, whereas the value for a single optical isomer is 1 (or 100%) [96]. Commonly, most optically active substrates are obtained from natural sources and are therefore not optically pure. For example, in the case of (S)-2-methylbutan1-ol obtained from fusel oil, the enantiomeric excess is 0.8 (80% optical purity), that is, 10% of the chemically pure material is made up of the (R)-isomer, and 90% is made up of (S)-isomer. Thus, when we compare the physical properties of chiral liquid crystals, the results obtained for each individual material must be normalized to take into account the optical purity. Linear relationships between enantiomeric excess and the physical properties dependent on the chirality of a material are expected but have not yet been experimentally verified.

7.1

Aspects of Symmetry and Asymmetry in Liquid Crystals

In principle, we can define chirality in liquid crystals in a number of relatively simple ways: first, it can be described by the point asymmetry of molecular structure; second through the space asymmetry of the structure of the mesophase; and third through the form chirality of the helical macrostructures found for many phases. Point asymmetry is itself described by the spatial configuration rules of Cahn, Ingold, and Prelog [98, 99]; space asymmetry by group theory; and form chirality by helicity or ‘‘handedness.’’ Two other important factors that are also coupled to the asymmetry in such systems are the molecular biaxiality and the conformational structure(s) of the molecules. In the molecular design of chiral liquid crystals, all of these factors have to be taken into account. Thus, in the following sections, the three levels of symmetry are discussed.

7.2

Molecular Asymmetry in Liquid Crystals

The concepts of molecular symmetry and asymmetry (point symmetry) are used to describe the spatial configuration of a single molecular structure, inasmuch as they describe the geometric, conformational, and configurational properties, that is, the stereochemistry, of a material. An asymmetric atom can be described as an atom that has a number of different

1271

1272

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

atoms or functional groups bonded to it for which their arrangement in 3-D space has no superimposable mirror image. Using carbon as an exemplary asymmetric system, > Fig. 28 shows the spatial configuration about two asymmetric carbon atoms that are related to one another as mirror images. The two atoms are labeled by the absolute configuration system of Cahn, Ingold, and Prelog as being either R (rectus, right) or S (sinister, left). The configuration designation of an asymmetric carbon atom is obtained by first determining the relationship of the group priorities about the asymmetric atom based on atomic number. The group with the lowest priority is positioned, see > Fig. 28, according to the conversion rule, to the rear of the tetrahedral asymmetric carbon atom. The other three groups are viewed from the opposite side of the asymmetric center to the group of lowest priority. If the remaining groups are arranged in descending order of priority relative to a clockwise direction about the asymmetric center, the spatial configuration is designated R. For the reverse situation, when the priority order descends in a counterclockwise direction, the spatial configuration is given the absolute configuration label S. Included in configurational isomers are two distinct classes of stereoisomers: enantiomers and diastereoisomers. Enantiomers are two molecules that have asymmetric (chiral) centers, which are related to each other as objects to nonsuperimposable mirror images (antipodes), as shown in the upper part of > Fig. 29 for rod-like molecules. Diastereoisomers contain more than one asymmetric atom and pairs of diastereoisomers do not share superimposable mirror images, as shown in the lower part of the figure. For example, a molecule that has two asymmetric centers with (R) and (S) configurations will be the diastereoisomer of the one with an (RR) configuration. Similarly, the (SS) configuration is the diastereoisomer of the (RS) molecule. However, the (RS) compound is the enantiomer of the (SR) variation, and the (RR) compound is the enantiomer of the (SS) form. Enantiomers are expected to have the same

A

A Least priority at the Back

C

Chiral atom

View

D

D B

B

C Priority based on atomic number

Mirror plane A (1)

A (1)

Chiral atom B (2)

(2) C D (4)

C (3)

(2) B D (4)

(R)

(S)

Clockwise order - Rectus

Counter-clockwise order-Sinister

The groups A, B, C, & D are arranged in ascending atomic number (1 to 4) with the group of the lowest value pointing back into the plane of the page

. Fig. 28 Cahn, Ingold, and Prelog rules for the classification of asymmetric centers in molecules

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

(S)

CH3

H

O

* H CH3 (R)

*

O CN

H3C (R)

(S)

*

H O

CN (S) Enantiomers

CH3

H *

H

(S)

O

*

H CH3

* H3C

* H CH (R) 3

O O

Enantiomers

Diastereoisomers * H CH (R) 3

. Fig. 29 Relationship between enantiomers and diastereoisomers for calamitic systems

physical properties as one another, but diastereoisomers can have properties that are different from one another. Molecules that possess more than two asymmetric centers, and which are more likely to have diastereoisomers, can be found in rod-like, discoid and spherical systems, for example, the first thermotropic liquid crystal that was reported was a derivative of cholesterol which itself possesses several asymmetric centers, see the upper part of > Fig. 30. However, where the mesogen has multiple peripheral chains, as for the discoid and spherical templates, there is a greater possibility of introducing asymmetric centers into the system, as shown in the lower part of > Fig. 30. So far we have only discussed chirality in molecules that contain an asymmetric atom; however, some molecules are optically active even when they do not possess an asymmetric atom, for example, substituted allenes and spirocyclobutanes. For molecules with a dissymmetric structural grouping, the (R) or (S) configuration is found by assigning priority 1 to the higher priority group in front, 2 to the lower priority group in front, 3 to the higher priority group at the back, etc., and then examining the path 1 * 2 * 3 * 4. For example, the absolute spatial configurational assignment for 1,3 dimethylallene enantiomers is shown in > Fig. 31. This type of dissymmetric, molecular-structure, concept was used by Solladie and Zimmerman [100] as a template in the creation of a range of chiral nematic liquid crystals based on the chiral cyclohexylidene ethanone unit, as shown in > Fig. 32. Dissymmetry can also occur for materials that possess rotational symmetry elements. Typically, dissymmetric materials that have twofold or threefold rotational axes of symmetry can also exhibit optical activity. For example, Geivandov et al. have synthesized a remarkable series of compounds based on twistane (tricyclo(4,4,0,0)decane) [101]. This chiral moiety has unique symmetry (D2) and is chiral, being composed of five fused boat forms of cyclohexane rings, which are all twisted in the same sense. This chiral moiety exists in two enantiomeric forms (a) and (b), as shown in > Fig. 33. The chirality of twistane is therefore due to the asymmetry (or space asymmetry) of the molecule as a whole and does not depend on the presence of asymmetric atoms.

1273

1274

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

H

OCO

a

H

O

* * O

O Me Me

Me

O

O

O

Me O

Me

Me

*

O

O

O

O

*

O

O Me

Me O

O Me

O

O

Me

Me Me O

O

* *

b

Cryst 120 ND* 138 Iso Liq Recryst 45

. Fig. 30 Chiral calamitic (a) and chiral discotic (b) materials where the molecules possess multiple chiral centers

The series of compounds derived from twistane consists of 8-alkyltwistanol esters. Some of these compounds have been found to exhibit chiral nematic phases, as shown in > Table 2. Dissymmetric molecules commonly have a simple axis of symmetry, and in asymmetric molecules this axis is absent; however, both species are usually optically active. In liquid crystal systems, both types of material are capable of exhibiting chiral properties. > Table 3

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

(1)

CH3

CH3

CH3 (3)

H C H3C

(1)

C

(4) H3C

C

H

H

H (2)

(3)

(4) H

CH3 H (2)

(R)

(S)

. Fig. 31 Molecular dissymmetry can be used as a pathway to in the induction of chirality into liquid crystal phases

H OCH3 O

H

Cryst 65 N* 124 (°C) Iso Liq

. Fig. 32 A chiral nematic liquid crystal derived from 1-biphenylyl-2-cyclohexylidene ethanone which has a dissymmetric structure

summarizes the relationships between optical activity, molecular structure, and rotational symmetry operations.

7.3

Helicity in Liquid Crystals

In this part, the way in which molecular chirality affects the formation of supramolecular selforganizing structures will be discussed. In particular, the formation of helical macrostructures will be examined in terms of calamitic (nematic and smectic) systems. When the nematic phase is composed of optically active materials (either a single component or a multicomponent mixtures made up of chiral compounds or chiral compounds

1275

1276

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

. Fig. 33 Enantiomeric forms of tricyclo (4,4,0,0) decane (twistane)

. Table 2 Transition temperatures ( C) of compounds derived from 8-alkyltwistanols [101] O X R

C5H11

O

R

X

Cryst – N*

N* – Iso Liq

C3H7

Phenyl

94.2

119.7

C5H11

Phenyl

71.8

126.4

C5H11

Cyclohexyl

65.0

124.7

. Table 3 Relationships between optical activity, molecular structure, and rotational symmetry operations (After Eliel [96]) Term

Alternating axis

Simple axis

Optical activity

Symmetric

Present

May or may not be present

Inactive

Dissymmetric

Absent

May or may not be present

Usually active

Asymmetric

Absent

Absent

Usually active

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

mixed with achiral materials), the phase itself becomes chiral and has reduced environmental space symmetry, that is, form optical activity [102]. The structure of the chiral nematic N∗ (or cholesteric) modification is one where the local molecular ordering is identical to that of the nematic phase, but in the direction normal to the director, the molecules pack to form a helical macrostructure, see > Fig. 34, that is, the vector that describes the average direction of the long molecular axes (director) undergoes a helical distortion. As in the nematic phase, the molecules have no long-range positional order, and no layering exists. The pitch of the helix can vary from about 0.1  106 m to almost infinity, and is dependent on optical

Tilt angle θ

Molecules Diffuse layered structure

P Spiralling orientational ordering of the molecules

P

Spiralling polarization caused by spiralling tilt direction

P

P

a

b

. Fig. 34 The helical macrostructures formed by the chiral nematic (a) and smectic C* phases (b)

1277

1278

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

purity and the ‘‘degree of molecular chirality’’ of the material. The optic axis of the phase is parallel to the helical axis, and so the phase has negative birefringence and is optically uniaxial. The helical macrostructures that are formed can be either right-handed or left-handed, and the handedness is dependent on the stereochemical structure of the constituent molecules of the phase. Enantiomers, however, will produce helical structures of opposite twist. A systematic study of the property–structure correlations of helical twist sense, pitch length, and other physical characteristics has been made, and a set of empirical rules [103] derived for materials that possess only one asymmetric center in the terminal aliphatic chain, indicating that the spatial configuration of the chiral center of a molecule is related to the twist direction of the helical structure in the following ways: Sol Sed Rod Rel where R or S is the absolute spatial configuration of the chiral center, o or e refers to either an odd or even position of the chiral center relative to the rigid molecular core (i.e., the parity of number of atoms that the chiral center is removed from the core, see > Fig. 35), and d or l refers to either dextro (right) or levo (left) optical twist senses of the helix. > Figure 35 shows the asymmetric center flips backward and forward as it is moved sequentially down a terminal aliphatic chain. As it oscillates backward and forward so too does the steric bulk associated with the functional groups attached to the asymmetric center. This flipping is associated with the conformational shape and the rotational dynamics of the system. As the chiral center is moved further away from the rigid core, its rotational freedom increases and its effect on chiral properties, such as the helical pitch, diminishes. The odd–even effects on the helical twist sense and the increasing rotational freedom of the chiral center are shown together in > Table 4 for a series of chiral (S)-4-alkyl-40 -cyanobiphenyls. The empirical rules appear work well for simple molecules that exhibit chiral nematic phases, but are sometimes less appropriate for more complex systems. When chiral materials are introduced into the lamellar smectic C phase, it too becomes optically active in a similar way to the N∗ phase. The helical macrostructure is generated by a precession of the tilt about an axis normal to the layers, as shown in > Fig. 34b. The tilt direction of the molecules in a layer above or below an object layer is rotated through an azimuthal angle relative to the object layer. This rotation always occurs in the same direction for a particular material, thereby forming a helix. The helix can be either right-handed or left-handed depending on the absolute spatial configuration of the constituent molecules, as for the chiral nematic phase. The pitch of the helix for most C∗ phases is commonly greater than 1 mm in length, indicating that a full twist of the helix is made up of many thousands of layers. When the asymmetric shapes of the molecular structures of materials that form chiral smectic C∗ phase are carefully examined, it is found that they are twisted, almost like miniature crankshafts [104]. The molecules will be packed in a random head-to-tail arrangement within the layer, and there will be substantial interlayer mixing of the molecules at the interfaces between the layers [105]. Thus, the molecular twist will be passed from one layer to the next. The helical structure should be envisaged as a result of a three-dimensional molecular twist being superimposed upon the periodic two-dimensional interfaces of the layers, that is, the layer interfaces become molecular recognition surfaces. The helix

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

O O O

n H 3C

2-methylbutyl- n = 1

Sed

3-methylpentyl- n = 2

Sol

4-methylhexyl- n = 3

Sed

*

. Fig. 35 The alternation of the chiral center as it is sequentially moved away from the aromatic core

. Table 4 Determination of optical twist rules [103] CH3 NC

(O)x

(CH2)n

* CCH2CH3 H

x

n

Parity

Pitch length (mm)

Twist sense

0

1

e

0.15

d

0

2

o

0.30

l

0

3

e

0.40

d

1

1

o

1.50

l

1

3

o

1.00

l

1279

1280

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

direction will be determined by the molecular twist which itself is related to the stereochemistry about the chiral centers, the location of the chiral centers within the molecular structure, and to the empirical property–structure correlations described earlier for chiral nematic phases.

7.4

Space Asymmetry

In certain liquid crystal phases, the space symmetry also has to be taken into account, in particular this applies to the layered ferroelectric smectic C∗ phase, and its related anti- and ferri-electric states, and to electroclinism in the orthogonal A∗ phases. It is difficult to define symmetry operations in fluid phases where the molecules are undergoing rapid changes in conformational structure, and therefore, we can only consider the simple situation of the local or environmental symmetry in the mesophase, say for a small molecular ensemble, and then apply this to the bulk phase. For example, consider the structure of the smectic C phase, when it is composed of achiral molecules or is a racemic modification; the environmental or local space symmetry consists of a center of inversion, a mirror plane normal to the layers, and a C2 axis parallel to the layers and normal to the tilt direction. Thus, the symmetry can be described as C2h [106]. However, when the molecules of the phase are optically active, the local symmetry in the layers is reduced to a polar C2 axis. As the molecules are themselves polar, there is an inequivalence with respect to the dipoles along the C2 axis. This in-equivalence occurs even though the molecules are undergoing rapid reorientational motion about their long axes. The time-dependent alignment of the dipoles along the C2 axis causes a spontaneous polarization to develop along this direction, parallel to the layer planes and perpendicular to the tilt direction of the molecular long axes, as shown in > Fig. 36. As a consequence, each individual layer can be considered as having a spontaneous polarization (Ps), and because the phase has C2 symmetry, the polarization must be directional (Ps(+) or (Ps()).

Centre of inversion

Mirror plane

Two-fold axis of rotation

Two-fold axis of rotation

. Fig. 36 Symmetry argument for ferroelectric properties in chiral smectic C* phases. The direction of the polarization in the right-hand figure is defined as Ps(þ)

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1 Helical axis

O O O O

Molecular chirality and point symmetry

H

CH3

Layers Molecules

Space symmetry

C2 axis

Molecular layers

P Form chirality Helical Macrostructure Spiralling Polarization

. Fig. 37 The interrelation of the three symmetry elements for the chiral smectic C* phase

> Figure 37 shows the interrelation of the three symmetry elements for the chiral smectic C∗ phase. The point asymmetry is responsible for the specific optical rotation and the optical purity determined in solution by polarimetry; the space asymmetry is responsible for the local spontaneous polarization and hence the ‘‘layer’’ ferroelectricity; and the form asymmetry is responsible for the circular dichroism, optical rotary dispersion, and the helielectric properties of the bulk phase. The magnitude of the spontaneous polarization and the circular dichroism is not only related to optical purity but also to the average tilt angles of the molecules relative to the layer planes of the mesophase. The tilt angle is temperature-dependent, taking the form for the spontaneous polarization of:

Ps ¼ PoðTc  TÞb where Ps is the spontaneous polarization, Po is a constant for a particular material, Tc is the Curie temperature (the transition from the smectic A to smectic C phase), T is the temperature, and b is a coefficient derived from Landau theory to be 0.5, but which in practice is found to be between 0.25 and 0.35. The temperature dependence of the tilt angle also takes a similar form, and thus the relationship between the tilt and the polarization takes the form: Ps ¼ Psiny where y is the tilt angle. The circular dichroism/optical rotary dispersion is related to the pitch length of the helical structure of the bulk phase. As the tilt angle increases as the temperature

1281

1282

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

is lowered, the helical pitch length will shorten. When the pitch length is comparable to the wavelength of visible light, one hand of the circularly polarized light will be reflected. At higher temperatures, this will appear red and move through the spectrum to blue as the temperature is lowered, which is the opposite of color sequence observed for the chiral nematic phase. The helielectric effect of the bulk phase has been used in display applications and beamsteering devices through helix unwinding. The voltage required for helix unwinding is coupled to the dielectric and spontaneous polarization, and thus the effect is sub-millisecond in response, which is similar to that of the ferroelectric surface-stabilized response time. These effects, thus, make chiral smectic C∗ phases of interest in displays and projectors that operate in the 100–300 Hz regime, which is beyond the capabilities of conventional nematics. With respect to these structures and effects found for the chiral smectic C∗ phase, it should be noted that the phase is not truly ferroelectric in the conventional sense. It is an improper or extrinsic ferroelectric only when the helical structure of the phase does not exist, that is, when the helix is unwound and the phase is aligned such that layers are bulk oriented. Thus, a chiral smectic C∗ phase should not be represented as being ferroelectric as ferroelectricity implies a certain organizational criteria which does not necessarily exist in the bulk phase. The subscripted terms, such as P for polar, F for ferroelectric, and AF for antiferroelectric, often used in labeling are consequently misleading. For chiral anticlinic phases, similar issues concerning structure and properties apply to those of the chiral smectic C∗ phase. Here, we also enter into a problem of phase characterization due to phase labeling. The equivalent of the antiferroelectric smectic C∗ phase, as noted earlier, was discovered before antiferroelectricity was found in liquid crystals. The achiral phase was labeled as smectic O and its chiral analogue as smectic O∗ because their structures were different to those of the smectic C/C∗ phases, and also because there could be a direct transition between the anticlinic and synclinic phases. These two points demonstrate convincingly that these phases are indeed different. The chiral anticlinic phase thus exhibits point and form asymmetry. The helicity in this case consists effectively of two intertwined helices that are out of phase by 180 and shifted by one layer relative to one another as shown in > Fig. 38. This structuring is probably due to the presentation/orientation of one layer to the adjacent layer at the interlayer interfaces. As with the synclinic phases, the helical structure is nonpolar in the bulk. The difference between chiral anticlinic and synclinic phases lies in the form asymmetry where there are opposing directions of the spontaneous polarization in adjacent layers. Consequently, in the bulk unwound and aligned phase, the spontaneous is averaged to zero. Similarly in the helical state it is also averaged to zero. On the application of an applied electric field to an anticlinic phase three stable states are observed, which is in contrast to the synclinic phase that possesses two. Lastly, in this section, chirality is briefly examined in the orthogonal smectic phases which have their rod-like constituent molecules arranged perpendicular to the layer planes. The smectic A phase, for instance, possesses D1h symmetry, but when the molecules are chiral the symmetry is reduced to D1. However, the phase does not exhibit form chirality as the layers do not support a helical structure. However, under the application of an applied electric field, an induced tilt of the molecules relative to the layer planes is observed. The size of the tilt angle in this case is linear with respect to the applied electric field. This effect is called electroclinic switching, and for some materials it is extremely quick, usually sub-millisecond and even submicrosecond in some cases.

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

Interpenetration Direction Tilt Angle θ

Tilt Direction

Propagation of Twist

Central Core

Terminal Aliphatic Chain

Tilt Direction Terminal Aliphatic Chain

Tilt Angle θ Interpenetration Direction

. Fig. 38 The local helical structure in the anitclinic smectic phase where the molecules are shown as having two terminal chains and a rigid ellipsoidal core

7.5

Frustrated Chiral Structures

Consider first a uniaxial phase that is composed of chiral rod-like molecules. In the simplest situation, a helix can form in a direction perpendicular to the long axis of an object molecule. This example is analogous to the structure of the chiral nematic phase. In the direction parallel to the long axis of the molecule, no twist can be effected. Now consider a similar situation, but this time the twist in the orientational order can occur in more than one direction in the plane perpendicular to the long axis of an object molecule. This structure is called a double-twist cylinder, and is shown in > Fig. 39a. In the simplest form of the double-twist cylinder, two helices are formed with their axes perpendicular to one another in the plane at right angles to the direction of the long axis of the molecule [107, 108]. Expanding this structure in two dimensions, the two helices can intersect to form a 2-D lattice, see > Fig. 39b. However, the helices cannot fill space uniformly and completely, and hence defects are formed. As helices are periodic structures, the locations of the defects created by their inability to fill space uniformly are also periodic. Thus, a 2-D lattice of defects is created. This inability to pack molecules uniformly can be extended to three dimensions, see > Fig. 39c, to give various cubic arrays of defects [109, 110]. The different lattices of defects provide the structural network required for the formation of a range of

1283

1284

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals Frustration of twist

Defect lines

a

b

c

5,000 Å

. Fig. 39 (a) A double-twist cylinder, (b) twist in two dimensions, (c) twist in three dimensions showing disclination lines

a

b

. Fig. 40 (a) Blue phase I and (b) Blue phase II

novel liquid crystal phases that are called Blue Phases. In principle, these phases are frustrated structures where the molecules would like to fill space with double-twist structures but are prevented from doing so, and the result is the formation of defects. Thus, the formation of defects stabilizes the structure of the phase. Two possible cubic structures of defects for the Blue Phase are shown in > Fig. 40. These frustrated phases were called Blue Phases because when they were first observed microscopically by Coates and Gray [111], they appeared blue. Their strong blue color is due

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

7.1.1

to the selective reflection of light. Other materials were later discovered which exhibited Blue Phases where the selective reflection was in the red or green region of the spectrum. Experimentally, it was found, however, that the helical pitch length must be of a similar length to the wavelength of visible light for a material to exhibit a Blue Phase so the lattice period for the defects is of the order of 5,000 A˚. Blue Phases have found some recent interest in fast switching devices based on the optical Kerr effect [112].

8

Summary

This chapter is designed to give the reader a relatively simple introduction to liquid crystal phases and materials that are relevant to electrooptic displays and devices. The article is by no means an exhaustive review of the literature; however, additional information can be gained from the Handbooks of Liquid Crystals (see Further Reading). Encompassing lists of materials that have applications in displays and devices are not available other than through a vast number of journal articles, or through the Liquid Crystal Database, LiqCryst 5.0, liqcryst. chemie.uni-hamburg.de/.

References 1. Gray GW, Winsor PA (1974) Liquid crystals and plastic crystals, vol 1 & 2. Ellis Horwood, Chichester 2. Kelker H, Hatz R (1980) Handbook of liquid crystals. Verlag Chemie, Weinheim 3. Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (eds) (1998) Handbook of liquid crystals, vol 2A: low molecular weight liquid crystals I. Wiley, Weinheim 4. Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (eds) (1998) Handbook of liquid crystals, vol 2B: low molecular weight liquid crystals II. Wiley, Weinheim 5. Leadbetter AJ (1987) In: Gray GW (ed) Thermotropic liquid crystals, critical reports on applied chemistry vol 22. Wiley, Chichester, pp 1–27 6. Gray GW, Goodby JW (1984) Smectic liquid crystals, textures and structures. Leonard Hill, Philadelphia 7. Etherington G, Leadbetter AJ, Wang XJ, Gray GW, Tajbakhsh A (1986) Liq Cryst 1:209 8. Sackmann H, Demus D (1966) Mol Cryst Liq Cryst 2:81 9. Sackmann H (1980) In: Helfrich W, Heppke G (eds) Liquid crystals of one- and two-dimensional order. Springer, New York, p 19 10. Leadbetter AJ, Mazid MA, Kelly BA, Goodby JW, Gray GW (1979) Phys Rev Lett 43:630 11. Leadbetter AJ (1979) In: Luckhurst GR, Gray GW (eds) The molecular physics of liquid crystals. Academic Press, New York, p 285 12. Pershan PS, Aeppli G, Litster JD, Birgeneau RJ (1981) Mol Cryst Liq Cryst 67:205

13. Benattar JJ, Moussa F, Lambert M (1984) J Phys (Paris) Lett 45:1053 14. Benattar JJ, Doucet J, Lambert M, Levelut A-M (1979) Phys Rev 20A:2505 15. Hardouin F, Tinh NH, Achard MF, Levelut A-M (1982) J Phys (Paris) Lett 43:327 16. Budai J, Pindak R, Davey SC, Goodby JW (1980) J Phys (Paris) Lett 41:1371 17. de Gennes PG (1974) The physics of liquid crystals. Oxford University Press, Oxford 18. Kresse H (1983) Adv Liq Cryst 6:109 19. Bata L, Buka A (1981) Mol Cryst Liq Cryst 63:307 20. Chandarsekhar S, Madhusudana NV (1985) Proc Indian Acad Sci (Chem Sci) 94:139 21. Richardson RM, Leadbetter AJ, Frost JC (1982) Mol Phys 45:1163 22. Leadbetter AJ, Richardson RM (1979) Incoherent quasielastic neutron scattering. In: Luckhurst GR, Gray GW (eds) The molecular physics of liquid crystals. Academic, New York, p 451 23. Chandrasekhar S, Raja VN, Sadishiva BK (1990) Mol Cryst Liq Cryst 7:65 24. Praefcke K, Kohne B, Singer D, Demus D, Pelzl G, Diele S (1990) Liq Cryst 7:589 25. Dingemans TJ, Samulski ET (2000) Liq Cryst 131:27 26. Kumar S, Acharya BR, Primak A, Kumar S (2004) Phys Rev Lett 92:145506 27. Xiang Y, Goodby JW, Go¨rtz V, Gleeson HF (2009) Appl Phys Lett 94. ISBN 193507-1-3 28. Beattie DR, Hindmarsh P, Goodby JW, Haslam SD, Richardson RM (1992) J Mater Chem 2:1261

1285

1286

7.1.1

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

29. Hindmarsh P, Hird M, Styring P, Goodby JW (1993) J Mater Chem 3:1117 30. Ohnishi H, Baba Y (1992) SHARP Tech J 54:47 31. Pindak R, Moncton DE, Davey SC, Goodby JW (1981) Phys Rev Lett 46:1135 32. Leadbetter AJ, Mazid MA, Kelly BA, Goodby JW, Gray GW (1979) Phys Rev Lett 43:630 33. Benattar JJ, Moussa F, Lambert M (1983) J Chim Phys 80:99 34. Leadbetter AJ, Frost JC, Gaughan JP, Mazid MA (1979) J Phys Paris 40:C3–185 35. Als-Nielsen J, Litster JD, Birgeneau RJ, Kaplan M, Safinya CR, Lindegaard-Andersen A, Mathiesen B (1980) Phys Rev B22:312 36. Als-Nielsen J (1981) In: Bocarra N (ed) Symmetries and broken symmetries. IDSET, Paris, p 107 37. Lo¨sche A, Grande S, Eider K (1973) First specialised Colloque Ampe`re. Krakow, Poland, p 103 38. Lo¨sche A, Grande S (1974) 18th ampe`re congress. Nottingham, England, p 201 39. De Vries A, Ekachai A, Spielberg N (1979) Mol Cryst Liq Cryst 49:143 40. Leadbetter AJ, Durrant JLA, Rugman M (1977) Mol Cryst Liq Cryst Lett 34:231 41. Leadbetter AJ, Frost JC, Gaughan JP, Gray GW, Mosley A (1979) J Phys (Paris) 40:375 42. Hardouin F, Levelut A-M, Benattar JJ, Sigaud G (1980) Solid State Commun 33:337 43. Hardouin F, Sigaud G, Tinh NH, Achard MF (1981) J Phys (Paris) Lett 42:63 44. Prost J (1984) Adv Phys 33:1 45. Prost J, Barois P (1983) J Chim Phys 80:65 46. Ratna BR, Shashidhar R, Raja VN (1985) Phys Rev Lett 55:1476 47. Goodby JW, Blinc R, Clark NA, Lagerwall ST, Osipov MA, Pikin SA, Sakurai T, Yoshino K, Zeks B (1991) Ferroelectric liquid crystals – principles, properties and applications. Gordon and Breach, Philadelphia 48. Dumrongrattana S, Huang CC (1986) Phys Rev Lett 56:464 49. Dumrongrattana S, Nounesisand G, Huang CC (1986) Phys Rev 33A:2187 50. Levelut A-M, Germain C, Keller P, Lie´bert L, Billard J (1983) J Phys (Paris) 44:623 51. Galerne Y, Lie´bert L (1990) Phys Rev Lett 64:906 52. Nishiyama I, Goodby JW (1015) J Mater Chem 1992:2 53. Hiji N, Chandani ADL, Nishiyama S, Ouchi Y, Takezoe H, Fukuda A (1988) Ferroelectrics 85:99 54. Galerne Y, Lie´bert L (1990) Phys Rev Lett 64:906 55. Toyne KJ (1987) In: Gray GW (ed) Thermotropic liquid crystals, critical reports on applied chemistry vol 22. Wiley, Chichester, pp 28–63

56. Demus D, Demus H, Zaschke H (1974) Flu¨ssige Kristalle in Tabellen. VEB Deutscher Verlag fu¨r Grundstoffindustrie, Leipzig 57. Demus D, Zaschke H (1984) Flu¨ssige Kristalle in Tabellen, vol II. VEB Deutscher Verlag fu¨r Grundstoffindustrie, Leipzig 58. Gray GW (1976) Adv Liq Cryst (1978)2:39; Gray GW Advances in liquid crystal materials applications, BDH special publication. BDH Chemicals Ltd, Poole 59. Minas H, Murawski H-R, Stegemeyer H, Sucrow W (1982) J Chem Soc Chem Commun 1982:308 60. Sucrow W, Minas H, Stegemeyer H, Geschwinder H, Murawski H-R, Kru¨ger C (1985) Chem Ber 118:3322 61. Pohl L, Eidenschink R, Krause J, Erdman D (1977) Phys Lett 60A:421 62. Goodby JW (1998) Phase structures of calamitic liquid crystals. In: Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (eds) The handbook of liquid crystals low molecular weight liquid crystals I, vol 2A. Wiley, Weinheim, pp 413–440 63. Gray GW, Harrison KJ, Nash JA (1973) Electron Lett 9:130 64. Eidenschink R, Erdman D, Krause J, Pohl L (1977) Angew Chem Int Ed Engl 17:133 65. Osman MA, Huynh-Ba T (1984) Helv Chim Acta 67:959 66. Boller A, Cereghetti M, Schadt M, Scherrer M (1977) Mol Cryst Liq Cryst 42:215 67. Carr N, Gray GW, McDonnell DG (1983) Mol Cryst Liq Cryst 97:13 68. Lueder E (2003) Liquid crystal displays. Wiley, Chichester 69. Yang D, Wu S (2006) Fundamentals of liquid crystal devices. Wiley, Chichester 70. Pauluth D, Tarumi K (2004) J Mater Chem 14:1219 71. Yang D, Wu S (2006) Fundamentals of liquid crystal devices. Wiley, Chichester 72. Pauluth D, Tarumi K (2004) J Mater Chem 14:1219 73. Kirsch P, Tarumi K (1998) Angew Chem Int Ed 37:484 74. Klasen-Memmer M, Bremer M, Rillich M (2003) US Patent 6,896,939 B2 75. Klasen M, Weller C, Tarumi K, Bremer M (2004) US patent 6,764,722, B2 76. Gray GW, Hird M, Lacey D, Toyne KJ (1989) J Chem Soc Perk Trans 2 2041 77. Clark NA, Lagerwall ST (1980) Appl Phys Lett 36:899 78. Jones JC, Towler MJ, Hughes JR (1993) Displays 14:86 79. Perennes FA, Crossland WA (1997) Opt Eng 36:2294 80. Goodby JW, Toyne KJ, Hird M, Styring P, Lewis RA, Beer A, Dong CC, Glendenning ME, Jones JC, Lymer KP, Slaney AJ, Minter V, Chan LKM

Materials and Phase Structures of Calamitic and Discotic Liquid Crystals

81.

82.

83. 84. 85. 86. 87. 88.

89. 90. 91.

92. 93. 94. 95.

(2000) Liquid crystal materials, devices and flat panel displays. In: Shashidhar R, Gnade B (eds) Proceedings of the SPIE, vol 3955, p 2 Clark NA, Crandall C, Handschy MA, Meadows MR, Malzbender RM, Park C, Xue JZ (1003) Ferroelectrics 2000:246 O’Callaghan MJ, Ferguson R, Vohra R, Thurmes W, Harant AW, Pecinovsky CS, Zhang YQ, Yang S, O’Neill M, Handschy MA (2009) J SID 17:369 Gray GW, Harrison KJ (1971) Mol Cryst Liq Cryst 13:37 Goodby JW, Gray GW (1976) J Phys (Paris) C3 37:17 McMillan WL (1921) Phys Rev A 1973:8 Wulf A (1975) Phys Rev A 11:365 Goodby JW, Gray GW (1976) Mol Cryst Liq Cryst 37:157 Hird M (1990) The synthesis and properties of liquid crystals for twisted nematic and ferroelectric displays. Ph.D. Thesis, University of Hull Gray GW, Hird M, Toyne KJ (1991) Mol Cryst Liq Cryst 204:43 Zaschke H (1975) J Prakt Chem 317:617 Dong C (1994) Fluorinated to lane and dioxane liquid crystals for ferroelectric display applications. Ph.D. Thesis, University of Hull Finkenzeller U, Pausch AE, Poetsch E, Svermann J (1993) Kontakte 2:3 Kelly SM (1996) Liq Cryst 20:493 Miyaura N, Suzuki A (1981) J Organomet Chem 213:C53 Miyaura N, Yamada K, Suginome H, Suzuki A (1985) J Am Chem Soc 107:972

7.1.1

96. Eliel EL (1962) Stereochemistry of carbon compounds. McGraw-Hill, New York 97. Orchin M, Kaplan F, Macomber RS, Wilson RM, Zimmer H (1980) The vocabulary of organic chemistry. Wiley, New York 98. Cahn RS, Ingold CK, Prelog V (1966) Angew Chem Int Ed 5:385 99. Cahn RS, Ingold CK (1951) J Chem Soc 612 100. Solladie G, Zimmerman GR (1985) J Org Chem 50:4062 101. Geivandov RC, Goncharov IV, Titov VV (1989) Mol Cryst Lid Cryst 166:101 102. Goodby JW (1991) J Mater Chem 1:307 103. Gray GW, McDonnell DG (1977) Mol Cryst Liq Cryst Lett 34:211 104. Goodby JW (1998) In: Cladis PE, Palffy-Muhoray P (eds) Dynamics and defects in liquid crystals: a Festschrift in honour of Alfred Saupe. Gordon and Breach, Amsterdam, pp 273–291 105. Yoshizawa A, Yokoyama NA, Kikuzaki H, Hirai T (1993) Liq Cryst 14:513 106. Meyer RB (1976) Mol Cryst Liq Cryst 40:74 107. Meiboom S, Sammon M (1980) Phys Rev Lett 44:882 108. Crooker PP (1989) Liq Cryst 5:751 109. Berreman DW (1984) In: Griffin AC, Johnson JF (eds) Liquid crystals and ordered fluids, vol 4. Plenum Press, New York, pp 925–943 110. Meiboom S, Sethna JP, Anderson PW, Brinkman WF (1981) Phys Rev Lett 46:1216 111. Coates D, Gray GW (1973) Phys Lett 45A:115 112. Yan J, Cheng H-C, Gauza S, Li Y, Jiao MZ, Rao LH, Wu ST (2010) Appl Phys Lett 96:071105

Further Reading Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (1998) Handbook of liquid crystals, vol 1: fundamentals. Wiley, Weinheim, p 914. ISBN 3-52729270-5 Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (1998) Handbook of liquid crystals, vol 2A: low molecular weight liquid crystals I. Wiley, Weinheim, p 490. ISBN 3-527-29271-3

Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (eds) (1999) Physical properties of liquid crystals. Wiley, Weinheim, p 503. ISBN 3-527-29747-2 Goodby JW, Blinc R, Clark NA, Lagerwall ST, Osipov MA, Pikin SA, Sakurai T, Yoshino K, Zeks B (1991) Ferroelectric liquid crystals – principles, properties and applications. Gordon and Breach, Philadelphia, p 474. ISBN 2-88124-282-0

1287

7.1.2 Introduction to Defect Textures in Liquid Crystals J. W. Goodby 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1290

2

Edge and Screw Dislocations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1293

3

Natural and Paramorphotic Defect Textures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1294

4

Schlieren Defect Textures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296

5 5.1 5.2 5.3

The Focal-Conic Fan Texture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1301 Focal-Conic Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1302 Bookshelf Alignment and Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1306 Chiral and Helical Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1308

6

Mechanisms of Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1311

7

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1312

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_7.1.2, # Springer-Verlag Berlin Heidelberg 2012

1290

7.1.2

Introduction to Defect Textures in Liquid Crystals

Abstract: Defects in liquid crystal systems are important to the identification of mesophase types, and to the construction and operation of displays. They may need to be eradicated or they may need to be present and of a controlled size. Defects of various types naturally grow in mesophases, and when they do they are observed in the polarizing microscope as defect textures. Defects can also be induced via device construction, for example, through the alignment coating. This chapter describes some of the defects formed naturally, some that are formed paramorphotically from another mesophase on cooling, and some induced by surface coatings.

1

Introduction

The structure of a perfect crystal is usually thought of as a rigid network of atoms, ions, or molecules, where each has a fixed position in the array or lattice (this excludes small displacements due to thermal vibrations). Within a perfect crystal the pattern extends indefinitely in three dimensions. However, this description does not apply to most ‘‘real’’ crystals. Generally most crystals, particularly naturally occurring ones, possess defects which can exist as either localized faults and minor misorientations in the structure, or in some instances extensive structural discontinuities where the whole of the crystal structure is affected. The introduction of defects into the structure of a crystal increases its internal energy compared to that of a perfect crystal. Conversely, the free energy of the system does not necessarily increase but may actually decrease. The Gibbs free energy can be expressed as follows: G ¼ E  TS

ð1Þ

where E is the internal energy, S is the entropy, and T is the temperature. Although the internal energy of the system may increase by the inclusion of defects, the value of the free energy may decrease because of the effect of the temperature and the entropy, TS. As the number of defects increases, the entropy of the crystal rises because of an increase in the number of possible arrangements. Thus the value of TDS will increase, thereby decreasing the value of the free energy G in spite of an increase in the internal energy E. The number of defects at equilibrium at a certain temperature can be determined from the following relationship: nd ¼ Ne Ed =kT

ð2Þ

where N is the total number of atomic sites per cubic centimeter or per mole, Ed is the energy of activation necessary to form a defect, T is the absolute temperature, and k is the Boltzmann constant. Liquid crystal mesophases [1], because they possess some degree of positional ordering, are like solids and are known to exhibit a wide variety of defects and discontinuities in their macroscopic structures. In addition, as liquid crystals can also exhibit fluid-like features, they can show strong changes in orientational order, which result in the formation of disclinations which are not usually found in solids. Consequently in liquid crystal systems, defects may be assigned as either dislocations, for example, grain boundaries, which break translational symmetries and are typically found in the more ordered (crystal-like) mesophases, or disclinations (originally called disinclinations) [2], for example, line singularities, which break rotational symmetries and are associated with more fluid mesophases. Defects in mesophases are very important for display and device construction and performance. In many cases it is necessary to minimize the number of defects because they impair device image quality and responses to external fields. Conversely there are some devices that depend on

Introduction to Defect Textures in Liquid Crystals

7.1.2

the formation of defects for their performance. For example, displays such as those based on the twisted nematic device need to be defect free, whereas light-scattering devices based smectic A phases are required to possess defects of controlled size. For these reasons, the descriptor ‘‘defects’’ has come to represent both dislocations and disclinations in liquid crystals. A collection of defects that may be linked together in a bulk mesophase, where the liquid crystal has not been subjected to orientation using a surface coating, have been come to be known as defect textures, or just simply liquid crystal textures. Microscope observation of liquid crystal textures can achieve two things: (1) it provides a simple and easy way to identify mesophases, and (2) it gives a qualitative way to define the types and density of defects present which can aid in deciding which alignment procedure would be required in device fabrication for a particular material. Thus, although the study of defects in liquid crystals is not a necessary prerequisite for device design, understanding them is necessary for construction, fabrication, and operation. Generally defects fall into two main categories: those that are in thermal equilibrium with the system, and as such are always present, and those that arise kinetically, their existence being particularly dependent on the nucleation processes from which the medium was formed. Examples of defects of the first category, which are found in liquid crystal systems, are those of the Frenkel [3] type (interstitial particles and vacancies). The size of this type of defect is comparable to a molecular length (several nanometers), which thereby renders them essentially invisible when viewed in the polarizing microscope. Frenkel defects do not, therefore, appreciably affect the microscopic defect textures [4, 5] typically observed for liquid crystal phases. The introduction of this type of singularity into a medium increases its internal energy, but this is often offset by a concurrent increase in entropy, which contributes to stabilizing the system. Consequently, it may be energy profitable for a system to possess a large number of defects/ discontinuities, thereby making it extremely difficult to exclude this type of imperfection from a mesophase. Other defects, as mentioned previously, are not necessarily endemic in a system, and their existence and presence may depend on external effects such as boundary, surface, and field effects. This type of singularity, an example being the schlieren [6–9] defect found in a nematic or a smectic C phase, is much more important in terms of the study of liquid crystal textures, and without its presence many mesophases would be impossible to identify and characterize by polarized light microscopy. Defects from both the categories described above can be classified in a variety of ways; one particularly useful method involves characterizing their dimensionality [10]. Point defects are classified as zero-dimensional, translational defects such as edge or screw dislocations as onedimensional, and the various types of wall defects as two-dimensional. It is also important to distinguish between defects that have a core singularity and those that have continuous cores [11]. > Table 1 gives a list of defects that may be classified in this way, some of which will be described in more detail later. The most widely used method used for the classification of linear defects was formulated by Volterra [12, 13]. In this technique, a perfect medium is first cut along an arbitrary surface (S), which is bounded by a line (L). The lips (S1) and (S2) of the surface are then displaced relative to each other by a translation, a rotation, or by a combination of both a rotation and translation. After relaxation of internal stresses the void produced is filled with perfect medium or excess material is removed, and a line discontinuity is formed. This process is shown pictorially in > Fig. 1. The dislocation line produced possesses three important features [12]. Firstly, the defect line cannot end in the interior of the medium, but instead must close in on itself, or end at another defect.

1291

1292

7.1.2

Introduction to Defect Textures in Liquid Crystals

. Table 1 The classification of defects by dimensionality Point defects (Zero-dimensional) With core singularity

Point singularities

Linear defects (One-dimensional)

Walls (two-dimensional)

Translational dislocations (edge and screw) Rotational dislocations (disclinations)

No core singularity

Surface inversion lines Coreless defect lines

Inversion walls (various)

S1

(L) S2

. Fig. 1 Classification method for linear defects (After Volterra [13])

Secondly, the displacement or strength of the defect has the same value along its whole length. Thirdly, all displacements can be analyzed in terms of either translational or rotational components. Thus, the vector of the displacement d(r) may be expressed by the following equation: dðrÞ ¼ b þ Oðvr Þ

ð3Þ

where b is known as the Burgers vector, which is a measure of the translational part of the displacement, r is the distance from the rotation axis v, and about this axis the material is rotated by an angle Ω. In pure translational defects (dislocations) b 6¼ 0 and Ω = 0, whereas in pure rotational defects (disclinations) b = 0 and Ω 6¼ 0. Usually b and Ω are symmetry elements of the structure in which the defect occurs. The magnitude of the size of the defect, per unit volume, can be estimated once the energy of the defect is taken into account. The energy can be expressed by the equation E  K ½dðrÞ2

ð4Þ

where K is a proportionality constant for the elastic deformation of the medium. In the case of liquid crystals, K is related to either one or more of the elastic constants of the mesophase

Introduction to Defect Textures in Liquid Crystals

7.1.2

derived from continuum theory, for example, for the nematic phase K is related to the splay, k11, twist, k22, and bend, k33, elastic constants. The values found for K associated with typical crystals are many orders of magnitude greater than the related values for liquid crystals. Thus, assuming that the defects associated with both crystalline and liquid-crystalline media have similar energies, it is clear that the vector displacements found in liquid-crystalline mesophases must be much greater than those found with imperfections associated with crystalline solids. This result illustrates the reason why defects in liquid crystal systems are easily seen in the polarizing light microscope under relatively low magnification (usually 100).

2

Edge and Screw Dislocations

Two main types of dislocation exist in liquid-crystalline phases, that is, edge and screw dislocations [14]. A combination of the two types leads to a mixed dislocation. In the solid state an edge dislocation is formed if an extra half-plane of atoms or molecules is inserted into a crystal. If the extra half-plane is inserted below the slip plane of the crystal, a negative edge dislocation results that is represented by the symbol T. A positive edge dislocation is given the symbol ⊥ with the extra half-plane of atoms being inserted above the slip plane. Liquid crystals exhibit edge dislocations in a variety of mesophases. The soft-crystal modifications can exhibit edge dislocations in a way that is analogous to that of a normal crystal where an additional half plane of molecules is inserted into the crystal structure. For example, smectic mesophases can exhibit this type of dislocation by virtue of their layer ordering. Thus, in > Fig. 2, it could be imagined that the extra half plane is in fact an extra layer of molecules. > Figure 3 shows the structure about the related screw dislocation. This particular defect again occurs in a variety of calamitic phases; in some it appears as a defect that can be seen in the microscope and is therefore an aid to phase identification. In other phases, screw dislocations stabilize the mesophase on a microscopic scale, for example, the twist grain boundary phases.

. Fig. 2 Structure about a positive edge dislocation (⊥) in a layered smectic phase

1293

1294

7.1.2

Introduction to Defect Textures in Liquid Crystals

Direction of movement of the screw dislocation

Screw dislocation line

Burgers circuit b Layers

Burgers vector

Molecules

. Fig. 3 Structure about a screw dislocation in a layered smectic phase

3

Natural and Paramorphotic Defect Textures

The natural texture of a liquid-crystalline phase is the texture that would be inherently formed on cooling the isotropic liquid into the mesomorphic state. Subsequent cooling may produce transitions to other phases, which generally will exhibit paramorphotic textures based on the defects of the preceding natural texture [15]. Similarly, when a crystal is heated into a liquid crystal phase, the mesophase will possess some characteristics of the solid, and if the mesophase has extensive ordering itself, then the defects formed will more likely resemble those of the crystal than the liquid crystal, that is, complicated textures are formed. This makes it important to perform phase identification from defect textures via cooling of the liquid rather than by heating of the solid. Certain defects are found to be common to many mesophases, whereas others are characteristic to only one mesophase. Combinations of defects may also be seen in many instances, with the nature of the texture observed in the polarizing microscope being dependent on the types of singularities present in the mesophase structure. In some cases defects play an even more important role in that they stabilize the formation of certain mesophases, for example, Blue Phases [16, 17] and Twist Grain Boundary (TGB) phases [18–21]. Without the stabilizing effect caused by the inclusion of defects, these unusual mesophase structures probably would not exist. In both the aforementioned types of mesophase, and unlike in any other type of liquid-crystalline phase, the defects are found to form an ordered array with each defect taking up a point in the defect lattice, which leads to the generation of the unusual natural textures seen by microscopy. The characterization of defects is particularly important in the classification of mesophase type, where combinations of natural and paramorphotic textures can be used to give definitive phase identification and classification. Typically defects can be characterized via two forms of alignment or orientations of the molecules; these orientations are called homogeneous and

Introduction to Defect Textures in Liquid Crystals

7.1.2

Optic axis

Molecules

a

Optic axis

b . Fig. 4 Homeotropic (a) and rubbed homogeneous (b) sample preparations for a nematic phase

homeotropic sample preparations. Studying the defects formed in combinations of these two sample orientations is again a powerful way in which to achieve phase classification. For phase identification, using polarized light microscopy, the liquid crystal material is usually sandwiched between a glass slide and cover-slip. For calamitic (rod-like) liquid crystals, a homeotropic sample preparation is obtained when the long axes of the molecules are found to be on average perpendicular to the surface of the glass plate, whereas homogeneous alignment is obtained where the molecules have their long axes lying parallel or at an angle to the surface of the glass, as shown in > Fig. 4 for a nematic phase. When the long axes of the molecules are on average parallel to the glass substrates, this orientation is said to be ‘‘planar’’ alignment. Thus, when dealing with phase characterization and classification two distinct experimental observations can be made based on phase sequence (paramorphosis) and on sample orientation. The combination of these observations produces a powerful method of phase classification. The following tables give lists of textures that are found naturally and paramorphotically for each calamitic mesophase for both homeotropic and homogeneous sample preparations. > Table 2 illustrates the natural and paramorphotic textures that are commonly observed when device-relevant phases are formed naturally from the amorphous liquid or the nematic or in the case of smectics paramorphotically. In addition to simple optical observations, mechanical shearing of the sample, under microscopic observation, can give additional information to the physical nature of the mesophase. Thus, the results of simple shearing of the sample are also included for further information. Furthermore, the paramorphotic textures described in > Table 2 obviously depend on the thermal and phase history of the material, that is, which mesophases are present and in what

1295

1296

7.1.2

Introduction to Defect Textures in Liquid Crystals

. Table 2 The natural and paramorphotic defect textures exhibited by device-relevant calamitic liquid crystals (as seen between crossed polarizers) Homogeneous/ planar alignment Mesophase (unrubbed)

Homeotropic/orthogonal alignment

Mechanical shearing

Other

Natural textures Nematic

Schlieren homogeneous

Extinct – black

Shears easily Brownian flashes

Smectic A

Focal-conic homogeneous polygonal defects

Extinct – black

Shears to homeotropic

Smectic C

Focal-conic – broken

Schlieren 4 brushes

Shears to schlieren

Schlieren from smectic A homeotropic (4 brushes). Sanded schlieren from isotropic cubic D phases

Shears to schlieren

Schlieren from smectic A Smectic Calt Broken focal-conic (anticlinic) from the smectic A or homeotropic or schlieren smectic C focal-conic textures C (2 + 4 brushes)

Shears to schlieren

Brownian motion

Paramorphotic textures Smectic C (synclinic)

Broken focal-conic from the smectic A focal-conic textures

order were they formed on cooling the liquid. It should also be noted that focal-conic defects tend to give rise to other focal-conic or mosaic textures, whereas homeotropic or optically extinct or schlieren textures usually give rise to similar textures depending on the phase type, or else they form mosaics when the mesophase formed has long-range periodic order. It can also be seen from > Table 2 that a large number of textures are formed by the various liquid-crystalline modifications (which have been limited to those used in devices). The types of defects that are associated with these textures are influenced by the varying degrees of order that the different mesophases possess. This ordering can range from orientational order, for example, that which is associated with the nematic phase, to the orientational and threedimensional positional ordering that is inherent in the soft-crystal modifications. Textural studies are an extremely important tool used in mesophase identification, and so an understanding of the structures of defects present in a mesophase is itself an important topic in the fundamental studies of liquid crystals.

4

Schlieren Defect Textures

Schlieren textures were once considered to be exclusively associated with the nematic phase until it was found that they were also exhibited by tilted smectic phases. The schlieren texture [22] observed in the polarizing microscope is produced either by a continuous, but sharp, change of molecular orientation within the sample, or by a change in molecular orientation about a point or line singularity, as shown in > Fig. 5 for a nematic phase.

Introduction to Defect Textures in Liquid Crystals

7.1.2

Molecules Director field

. Fig. 5 The director field about a singularity in a nematic phase

Between crossed polars characteristic dark brushes (lines of extinction) appear when the molecules are aligned with one or other of the two polarizers of the microscope. When the polarizer and the analyzer of the microscope are rotated simultaneously, the black brushes are seen to rotate across the area being viewed, while the origin or point source of the brushes remains in a fixed position. At the center of the schlieren a point or a line singularity is found; in the case of the line singularity it runs perpendicular to the viewing direction. Different types of point singularities are known to occur some possessing four brushes, whereas others have only two. In addition, the brushes associated with some singularities rotate in the same direction as that of the polarizer and analyzer (when they are rotated together), whereas others are found to rotate in the opposite direction to that of the polarizers. An experiment in which the polarizer and analyzer are rotated while the defect is viewed in the microscope therefore gives an effective method of characterizing schlieren defects. If the brushes rotate in the same direction as the polarizers the singularity is classified as positive (+), whereas the singularity is classified as negative () if the direction of rotation of the brushes is opposite to that of the polarizers. The defect is also given an s number, where s is the number of brushes observed divided by four. Singularities with values of +1/2, 1/2, +1, and 1 are observed for the nematic phase, whereas the schlieren texture of the smectic C phase only possesses singularities of the +1 or 1 type, that is, four-brushed singularities. Thus, the simple test of seeing if a sample has two- or four-brush defects can define whether or not the phase is nematic or smectic C. Additionally, the anticlinic smectic Calt phase can exhibit two- and fourbrush defects, thus distinguishing it from the synclinic smectic C phase. > Figure 6 schematically illustrates two- and four-brush defects and shows how the director field in the nematic phase can give rise to two- and four-brush defects. The values for s are given for each of the defects which are denoted as either positive or negative. For the smectic C phase the orientation of the tilt (the c-director) in the smectic C schlieren texture determines the topology near to the center of the point singularities, rather than the director as in the nematic phase. Thus, in a sense it is the c-director, that is, the axis along which the tilt occurs, that determines the nature of the defects. The topology about point defects in the smectic C phase are shown in > Fig. 7. It is interesting to note that these defects have similar geometries to those found for the s = 1 defects in the nematic phase. In the center of > Fig. 8 is a photomicrograph of the schlieren texture, where two defects are joined by a defect wall, of the nematic phase. Both s = 1/2 and s = 1 defects are shown, and

1297

1298

7.1.2

Introduction to Defect Textures in Liquid Crystals

4-Brush singularities in the nematic phase Schlieren brush

Polarizer

Singularity

Director field

S = +1

S = –1

2-Brush singularities in the nematic phase Schlieren brush

Crossed polars

S = +1/2

S = –1/2

. Fig. 6 Orientations of the molecules around a point singularity in the nematic phase

examples of the structures of these defects are illustrated in the upper (s = 1) and lower (s = 1/2) parts of the figure. In upper part of > Fig. 9, the schlieren texture for the smectic C phase is shown. The lower part of the figure shows an example of a possible structure around one of the defects. On close examination of the schlieren texture of a phase, it is evident that neighboring singularities that are connected by brushes are of the opposite sign and that in a specimen the sum of all the s numbers should equate to zero. Singularities of opposite signs can be seen to attract each other and eventually merge. If the two singularities are of equivalent but of opposite signs, the net result is that the singularities will annihilate one another. However, if they are of different strengths a new singularity will be formed with a value of s that is equivalent to the addition of the strengths of the two original singularities. Furthermore, a dislocation or disclination line must either close on itself, end at a surface, or at another defect. Lines that pass either horizontally or perpendicularly through a sample and end either at one or both surfaces are often referred to as threads, points with a strength

Introduction to Defect Textures in Liquid Crystals

7.1.2

Polarizer Schlieren brush Tilt direction

S = +1

S = +1

S = –1

S = –1

. Fig. 7 Topologies of the tilt of the molecules found in the schlieren texture of the synclinic smectic C phase

of 1/2 occurring at each end of the thread. The threads can also be seen to form closed loops when the line disclination closes in on itself. A combination of two disclinations may give rise to what are known as bulk inversion walls [22], where the director is turned to some degree on passing from one side of the defect to the other. An inversion wall appears in the texture of a nematic phase when two schlieren that run in parallel directions to one another and are joined at two points. Where a bulk inversion wall is anchored at both surfaces, a surface inversion wall [22, 23] is formed containing both defects with a strength of 1 and 1/2. In addition to the above, tilt inversion walls [24] may also occur where the director has a slightly tilted orientation with respect to the supporting surface. These usually appear as regions that exhibit a maximum degree of birefringence with respect to the rest of the sample, for example, two s + 1/2 disclinations may be connected by surface inversion lines, in which the molecules are tilted in opposite directions on each side of the band. Similarly, tilt inversion walls are also possible in smectic phases. Line singularities can also occur fixed by screw dislocations. They are prevalent in certain smectic phases where the molecules are tilted with respect to the layer planes, for example,

1299

1300

7.1.2

Introduction to Defect Textures in Liquid Crystals

4 Brush singularity s = +1

2 Brush singularity s = +1/2 Singularity

Director field

Polarizer

Polarizer Director field

Singularity

Polarizer Polarizer

. Fig. 8 Topologies of the defects found in the schlieren texture of the nematic phase. Both s = 1/2 and s = 1 defects are shown in the photomicrograph. The nematic director field about both singularities is shown schematically

Polarizer Smectic C phase

Tilt direction s = +1

Singularity

Polarizer

“Tilt” director field

. Fig. 9 Topologies of the defects found in the schlieren texture of the smectic C phase. Only s = 1 defects are shown in the photomicrograph. The tilt director field about one example of such singularities is shown schematically

Introduction to Defect Textures in Liquid Crystals

7.1.2

Screw dislocation

Arrows show the tilt direction of the molecules

One molecular layer

. Fig. 10 Structure about a screw dislocation in an anticlinic smectic Calt phase [25]

anticlinic smectic C phase (SmCalt), which has a structure with the alternating tilt of the molecules from one layer to the next. This phase can exhibit line singularities between screw dislocations to give defects known as dispirations [25]. In > Fig. 10, a screw dislocation for the anticlinic phase is depicted where the tilt (c) director, which is marked with dark arrows, spirals around the screw dislocation that is oriented perpendicular to the layers. The arrows marking the direction of the tilt can be seen to reverse direction on passing from one layer to the next. As the molecules pass around the screw dislocation the tilt direction changes, thus, the screw dislocation allows for the molecules in adjacent layers to have opposing tilts. This produces an s = 1/2 dispiration. If we now look down upon one circuit around positive and negative screw axes, we can see that under crossed polars these defects will appear as two-brush schlieren defects, that is, the topologies of the defects are similar to those found for the s = 1/2 defects in nematic phases. This type of defect in a smectic phase, therefore, becomes a prime way of differentiating between synclinic smectic C and anticlinic smectic C phases, and between chiral ferroelectric smectic C∗ and antiferroelectric phases. Similarly, it is possible to have a full rotation of the tilt about the screw dislocation rather than a 180 rotation. This will effectively give a defect of strength s = 3/2. Under crossed polars this defect will appear as a schlieren but this time with six brushes rather than two or four. Thus, this defect is not observed in either nematic or smectic C phases, and is diagnostic for anticlinic phases.

5

The Focal-Conic Fan Texture

Layered mesophases are also of importance in displays, particularly for light-scattering devices that use smectic A phases, and light modulating devices that use electroclinic, ferroelectric, or antiferroelectric phases. The simplest arrangement of the molecular layers is one where they are arranged perpendicular to the surfaces of the device, similar to how books are aligned on

1301

1302

7.1.2

Introduction to Defect Textures in Liquid Crystals

a

b

. Fig. 11 (a) Bookshelf alignment of the layers, and (b) the curved onion arrangement of the layers

a shelf. For ferroelectric liquid crystals this is usually referred to as bookshelf geometry, as shown in > Fig. 11a, and is the most desirable organization in the construction of displays. Conversely, the natural tendency of the layers to organize in the smectic state is curved in an onion skin arrangement, see > Fig. 11b. This arrangement of the layers is not particularly useful for ferroelectrics; however, for light-scattering devices the scattering from the defects formed is essentially isotropic. The incorporation of dyes into such structures gives the most uniform and intense adsorption. Scattering from the IR is also a possibility with such systems for environmentally controlled windows, etc. For devices, in one case there is a need to eradicate defects and in another there is a need to introduce defects of controlled size. Thus the study of defects in the smectic state is important in order to understand how they might form and to give insights into how they may be prevented or not. In the following sections, the natural tendency of the layers to form curved structures will be discussed in the form of focal-conic domains. In > Sect. 5.2, defects associated with bookshelf alignment will be discussed in relation to the formation of chevron defects where the layers become bent. It also should be noted that although only these predominant defects are discussed, there are many other important defects exhibited by lamellar phases, which include parabolic, polygonal defects, and boat defects. These can be found in the extra reading material given at the end of this book section.

5.1

Focal-Conic Defects

The focal-conic, or fan-like, texture [4, 5, 26] is the natural texture exhibited by the smectic A and C phases when formed from the liquid or the nematic phase, in addition, the synclinic and anticlinic smectic C phases also exhibit paramorphotic broken focal-conic textures. The focal-conic texture is often formed when a layered structure (smectic), which can sustain a bend deformation, is supported between two surfaces or other nucleation points. Focal-conic defects are usually prevalent when the molecules form strong attachments to the surfaces of the sample preparation or to the nucleation points. For example, around a nucleation point the molecules will adopt a radiating fan-like arrangement, within which both the layer thickness and the parallelism of the layers are preserved; thus the layers must curve/bend about the center of the nucleation point. However, because of the ease of nucleation and the liquid nature of the mesophase, neighboring centers of nucleation are found to grow and coalesce thereby allowing the layers to form a series of ‘‘doughnut-shaped’’ parallel tubes which in geometrical terms are known as Dupin cyclides. This process occurs in preference to the formation of isolated spherically curved surfaces, and proceeds via the formation of baˆtonnets [26].

Introduction to Defect Textures in Liquid Crystals

Molecular layers

7.1.2

Hyperbola

Ellipse

Ellipse Hyperbola

. Fig. 12 The cross-sectional (top) and plan (bottom) views of a focal-conic

When two nucleating centers are growing and they meet, as the outer layers of molecules are formed the junction where they meet will become a hyperbolic line of optical discontinuity. The molecules at the junction of the growing layers will be disorganized and between crossed polars this section will appear dark. As each layer is added the dark sections will make up the hyperbola. In the direction perpendicular to the plane that contains the hyperbola, another line of optical discontinuity forms as molecules are either parallel or perpendicular to the polars. This line is the edge of an elliptical discontinuity, as shown in > Fig. 12. The ellipse and hyperbola are related to one another in a geometric confocal construction. Focal-conics quite easily form when the distance between the glass of the cell is greater than 3–4 mm. For ferroelectric devices this means that defect-free displays are required to be thin. Another problem of having focal-conic defects in ferroelectric displays is that the polarization vector is effectively isotropic on a bulk scale, and so the device is no longer bistable. This can be suppressed to a degree by applying a large electric field that can reorient the layers into becoming bookshelf. However, as the curved layer organization is the low energy arrangement, it is to be expected that relaxation might occur. When viewed in the polarizing microscope, the confocal relationship of the focalconic domains can be seen as a dark cross. These lines of optical discontinuity can be seen in the right-hand part of > Fig. 13, which also illustrates the focal-conic defect for the smectic A phase in the left-hand part of the figure. In comparison to the smectic A phase, differences appear for the paramorphotic focalconic texture in the case of the smectic C phase due to the tilt of the molecules within the layers.

1303

1304

7.1.2

Introduction to Defect Textures in Liquid Crystals

Focal-conic defect

Molecules

Hyperbola

Ellipse Layers

. Fig. 13 A photomicrograph of the focal-conic defect texture of the smectic A phase. The lower part of the figure illustrates a section through a focal-conic domain

In the smectic C phase the ellipse and hyperbola again have a confocal relationship, but when viewed optically the focal-conic domains appear to be patchy or broken. The dark patches that are seen are thought to be produced by different orientations of the molecules in relation to the vibration axes of two polarizers. Thus, the patches seen in the microscope are related to different tilt domains, the tilt is produced at the phase transition to the smectic C phase with the molecules tilting in different directions to give a polydomain system. As the molecules tilt in different directions inversion walls and grain boundaries are formed. The broken focal-conic texture of the smectic C phase observed in the polarizing microscope is shown in > Fig. 14. If the smectic phase under study is composed of optically active material, as in the case of a helielectric smectic C∗ phase, a macroscopic helical ordering is produced within the focalconic domain. Under crossed polars, this structuring can produce dark lines in the texture which effectively run parallel to the layers. These lines are known as pitch bands or dechiralization lines depending on whether they are produced in the bulk (pitch bands) or as surface defects (dechiralization lines). The spacing between the parallel lines can be used, effectively, as a measure of the pitch length of the helical macrostructure of the mesophase. As the pitch axis of a material in the smectic C∗ phase runs perpendicular to the layer planes, pitch bands that appear in the texture can also be an effective indicator of the directions of the smectic layers at any particular point in the sample. Furthermore, the pitch bands effectively describe the confocal relationship of the focalconic structure as discussed earlier, with each band representing possibly hundreds of layers. ∗ > Figure 15 shows a chiral smectic C phase nucleating in the form of ba ˆ tonnets where a single focal-conic domain can be clearly seen. The horizontal line describes the ellipse (right-hand arrow) and the hyperbola can be seen where the curved layers meet in the vertical direction (upper arrow). The pitch bands, and by inference the molecular layers, are seen to curve around the defect as described earlier. Although the smectic phases produce the best examples of the focal-conic texture, the chiral nematic phase by virtue of its helicity (pitch) and also some crystal phases because of paramorphosis may exhibit this type of texture [27, 28]. In order to understand the

Introduction to Defect Textures in Liquid Crystals

7.1.2

. Fig. 14 The focal-conic texture of the smectic C phase (100)

. Fig. 15 The structure about a focal-conic defect of a chiral smectic C∗ phase. The pitch lines are parallel to the molecular layers (100). The right-hand arrow points toward the ellipse, and the upper arrow to the hyperbola

focal-conic texture of the chiral nematic phase, the mesophase must be considered as a pseudolamellar medium, in which each pitch length is considered to be equivalent to one layer in a smectic phase. In this model, the helices associated with the chiral nematic phase form around an ellipse and a hyperbola in a way similar to how the layers form focal-conic defects in smectics. Helical structures are found for layered smectics C, Calt, I, and F phases. In focal-conics the helical structure winds its way around the domains in 3D space. However, it is also possible to have helical structures that are confined to 2D in the planes of the device. Usually in this arrangement, the pitch of the helix is much shorter than the thickness of the device. In most cases these geometries are in helielectric and flexoelectric switching devices. They, however, require some form of bookshelf arrangement, and so this is discussed in the next section.

1305

1306

7.1.2 5.2

Introduction to Defect Textures in Liquid Crystals

Bookshelf Alignment and Defects

For electroclinic, ferroelectric, and antiferroelectric devices it is important to achieve high quality alignment through creating bookshelf arrangements of the molecular layers. Good alignment gives high quality contrast and large switching angles for ferroelectrics and antiferroelectrics. Alignment is usually achieved by having a liquid crystal that exhibits chiral nematic, smectic A∗, and SmC∗ (for ferroelectric and antiferroelectric) phases. Alignment is achieved for a long pitch chiral nematic phase, and cooling down through the other phases (as needed). The alignment coating can be either a rubbed thermosetting or thermoplastic polymer for a chiral nematic phase. In the absence of a chiral nematic phase, alignment in a smectic phase can be achieved using an epitaxial growth surface or over a rubbed thermosetting polymer (described later). Most devices that utilize bookshelf alignment are surfacestabilized ferroelectric displays, and so in the following section the most common defect that is encountered is described. Almost all ferroelectric liquid crystal cells exhibit ‘‘zigzag defects,’’ see > Fig. 16 [27]. The texture of this defect shows sharp jagged edges and rounded or ‘‘hairpin’’ defects between neighboring domains. Zigzag defects are often organized in the texture in a characteristic way. When the sample is observed in the direction parallel to smectic layer normal, the hairpin and jagged patterns appear to alternate. Defect loops are formed like ‘‘teardrops’’ and they generally face in the same direction, unless they are separated by zigzag defects. When a dc field of opposite bias is applied to an already poled cell, thereby reversing the spontaneous polarization, the dark and light regions of the cell reverse in appearance, that is, the dark regions become light and the light region becomes dark. Thus, the contrast in the device is severely reduced by these defects, hence understanding their structure and eliminating them from cells is important to the development of ferroelectric liquid crystal display (FLCD) technology.

. Fig. 16 Zigzag defect texture in a ferroelectric liquid crystal (100)

Introduction to Defect Textures in Liquid Crystals

7.1.2

A bent structure for the layers, as shown in > Fig. 17, was thus proposed to explain the microscopical studies of the defect textures and the optical and electrical switching properties. Consequently, the layers in most ferroelectric cells were found not to be perpendicular to the surface but at an angle. Across the cell thickness, the layer bend is localized to give ‘‘V’’-shaped layers called chevrons. Since the layer structure in the surface-stabilized ferroelectric liquid crystal has a chevron structure, defects are expected at locations where the chevrons either point toward one another or away from each other, that is, ‘‘«»’’ or ‘‘»«’’, see > Fig. 17 (lower). The zigzag type of defect was shown to be related to the «» type of boundary, and the hairpin defect was shown to correspond to »« type of boundary. Although the detailed processes by which the zigzag defects are formed has not been clearly demonstrated, it is thought that their formation is related to the rub direction, or due to the layers being formed in the smectic A∗ phase and at the subsequent transition to the smectic C∗ phase there is a reduction in layer thickness which causes internal stresses which are localized at the chevron defects. As noted, nearly all surface-stabilized ferroelectric liquid crystal cells have chevron structures and consequently show zigzag defects. Therefore, the development of methods to control the layer structure are important. The following points have to be taken into account when trying to reduce the number of zigzag defects: (1) the direction of, and (2) the magnitude of the pretilt angle of the alignment layers. Pre-tilt angle means the angle between the director and the surface of the alignment layer. For example, in the case of alignment layers produced by a rubbing procedure, the direction of the pre-tilt is determined by the upward tilt of molecules in the direction of the rubbing processes. The magnitude of the pre-tilt angle is determined by the type of polymer material used. When the rubbing directions are mutually parallel for the surfaces of the two cell substrates, two kinds of chevron structures of opposite direction are formed as shown in > Fig. 17. These two chevron structures, known as the Cl and C2 states, are distinguished by the relationship

Bookshelf alignment of layers

Rub direction

Molecules Layers

Rub

C1 Chevrons Surface tilt

Glass C2 Chevrons

Hairpin defect

Lightning defect

. Fig. 17 Formation of chevron defects due to rubbing and cooling effects (top), C1 and C2 defects formed by opposing bends of the chevrons (lower)

1307

1308

7.1.2

Introduction to Defect Textures in Liquid Crystals

between the direction of the chevron structure and the pre-tilt (see > Fig. 17) [28]. When the rubbing directions are parallel and the magnitude of the pre-tilt angle is large, one of the chevron structures may become preferentially stable, and as a consequence a zigzag defect-free cell is produced. For example [29], when a cell with high pre-tilt angle (about 15 ) was cooled from the smectic A∗ to the smectic C∗ phase, the C1 state preferentially appears immediately below the smectic A∗ phase to smectic C∗ phase transition, and then the C2 state appears at a lower temperature. The area of the C2 state increases with decreasing temperatures and can, under certain circumstances, take over much of the area dominated by the Cl state. As described above, the only way to control the pre-tilt angle is to be able to select a suitable polymer material for the alignment layer, such as polyimides. For example, the values of pre-tilt angles of alignment layers made with polyimides containing the hexafluoropropane group is reported to be two- to threefold greater than those of typical unfluorinated polyimides [30]. In cells made with alignment layers of this type of high tilt polymer, no zigzag defects are observed. This means, because of the large value of pre-tilt angle, one direction of the chevron structure dominates and the boundaries between the two types of chevron cannot be observed. Although the relationship between the molecular structure of the materials used for the alignment layer and the value of the pre-tilt angle is not clear, a strong interaction between polar groups in the polymer and the molecules of the liquid crystal is expected to be a contributing factor toward producing large values of the pre-tilt angle.

5.3

Chiral and Helical Systems

Systems that are chiral often possess helical structures, and under certain circumstances its presence can increase the number and types of defects that may occur. For example, the chiral nematic phase, which is the optically active variant of the nematic phase, may show textures and defects typical of the nematic phase, and other textures that are not exhibited by the nematic phase at all. Textures not shown by the nematic phase include the focal-conic fan and polygonal textures, as well as various edge dislocations. The chiral nematic phase is able to form these defect textures, which are usually associated with layered phases, by virtue of its quasilamellar structure produced by its helical structure. In this ensemble, the chiral nematic phase is viewed as being composed of sheets, with each sheet having a slightly different director orientation. Thus, the fan texture occurs where the sheets of chiral nematic phase are substituted for the layer ordering associated with a smectic phase. The helical ordering of the chiral nematic phase, therefore, forms around an ellipse and a hyperbola, giving rise to a texture that is somewhat reminiscent of that obtained for a smectic A phase. Similarly, the chiral nematic phase also can display edge dislocations due to its helical pitch. When an extra pitch or half-pitch is inserted into the system an edge dislocation is formed, this type of defect being visible in the Grandjean plane (oily-streak) texture of the chiral nematic phase, see > Fig. 18. Edge dislocations formed in a chiral nematic medium are, however, unstable and split into pairs of t- and l-lines which are called w-lines. > Figure 19 shows two w-lines that correspond to edge dislocations where the Burgers vector has a value of one full pitch, for both s = +1/2 and s = 1/2 disclinations [14]. Combinations of defects may give rise to a number of characteristic complex defects, including those termed as pinch, zigzag, and quadrilateral. In a chiral nematic texture, where the phase has a long pitch, these defects may also be visible in the polarizing microscope.

Introduction to Defect Textures in Liquid Crystals

7.1.2

. Fig. 18 The edge dislocations in the texture of the chiral nematic phase (100)

t+

t–

d ~ p/2, b = p λ–

λ–+

d ~ p/2, b = p

. Fig. 19 A pair of x-lines in the chiral nematic phase that correspond to edge dislocations where the Burgers vector has a value of one full pitch, for both s = +1/2 and s = 1/2 disclinations

1309

1310

7.1.2

Introduction to Defect Textures in Liquid Crystals

If we now turn to the effects that chirality has on the defects formed in helical smectic phases, we find a similar situation as that for chiral nematic phases; the defects found for achiral systems are still present but they are augmented by those associated with a helical structure. For example, as discussed previously, in the focal-conic texture of the chiral smectic C∗ phase the helical ordering can sometimes produce dark lines that run across the preparation in a direction that is parallel to the smectic layers. The lines can be attributed to one of two effects. Firstly, because of the helical structure of the mesophase, pitch bands are formed. These are related to the gradually spiraling tilt and orientation of the molecules. Thus, at certain points the molecules become aligned with one of the polarizers and therefore the texture appears dark. The lines, as expected, appear periodic in nature and measurement of the distance between them gives an effective measure of the helical pitch length of the mesophase. Secondly, a surface phenomenon can also produce the dark lines seen in the microscope; these lines are called dechiralization lines [31]. A topological model of this defect is one where the lines form pairs of +/ twist disclinations situated near the upper and lower boundaries (surfaces) of the preparation, parallel to the sample plane and smectic layers. Since the pair of twist disclinations, in a perfectly oriented sample, are situated in the same layer they lie on top of one another as illustrated in > Fig. 20. Thus the lines are effectively separated in the plane of the specimen by one pitch length. However, the upper and lower twist disclinations can be out of phase by half a pitch length under certain circumstances, and hence the lines in this situation are separated by half of a pitch length. As with pitch bands, dechiralization lines can be utilized as an effective measure of the helical pitch of a chiral smectic C∗ phase, but the top and bottom surfaces of the specimen have to be brought into sharp focus in order to ascertain which relationship the defects have (half-pitch or full-pitch spacing). If, however, the sample is not well-aligned, measurement of the pitch becomes unreliable as extra lines may be seen, that is, the defects no longer always occur in the same layer. Furthermore, it is possible to remove dechiralization lines by the application of a voltage across the sample, whereby the defects present close in on one another to form a loop leading to annihilation. Essentially in this situation the helical ordering of the mesophase is destroyed. Conversely surface pinning can maintain the presence of dechiralization lines, and if

Defect line

(+2π)

(+2π)

(–2π)

(–2π)

Bulk orientation Surface orientation of the molecules of the molecules

. Fig. 20 The structure of dechiralization lines in a chiral smectic C∗ phase

Introduction to Defect Textures in Liquid Crystals

7.1.2

the pinning is irregular, the lines no longer appear periodic. This type of defect texture is usually obtained when the pitch of the phase is comparable to the thickness of the sample.

6

Mechanisms of Alignment

Homeotropic alignment is usually obtained by two or three different methods. Typically a sample preparation using extremely clean glass, that is, washed in concentrated acid, water, and organic solvents, is an effective way of producing homeotropic alignment, particularly when coupled to a thin sample thickness. Mechanical shearing of the sample or the simple application of pressure to the cover-slip during the transition from the liquid to the liquidcrystalline state can induce homeotropy. It is more usual, however, to use chemical surface preparations in order to give homeotropic alignment over large areas. This is usually done by coating the surface with lecithin or dimethyloctyl (or octadecyl) chlorosilane, see > Fig. 21. In the first case the surface preparation is not permanent, but in the second case this treatment is permanent because of a reaction between the glass and the surfactant. Rubbed homogeneous alignment is typically achieved by using surface coatings that are either buffed or unbuffed [32]. In the laboratory poly(vinyl alcohol) is used as a non-permanent aligning agent, and when unrubbed it produces a homogeneous texture where the director can wander in the two-dimensional plane of the specimen, or when rubbed the degeneracy in the plane of the preparation is broken and the director no longer wanders about in the plane of the cell. Polymers such as nylon and polybutylene terephthalate have found extensive use in the laboratory framework; however, in manufacturing, polyimides are still the surface coating agents of choice. Other surface coating have also been used, such as silicon oxide, SiO, evaporation, microgrooved surfaces, etc., but these preparations are limited by the availability of evaporators and are therefore not commonly used either in the laboratory or in manufacturing processes. The use of thermoplastic polymers, such as nylon [33], for buffed alignment coatings results in the preparation of surfaces that effectively allow for epitaxial growth of the liquid crystal as it is formed on cooling from the liquid state [34]. When thermosetting polymers, such as polyimides, are used as alignment coatings they are usually buffed to break the degeneracy in the plane of the sample. Buffing usually results in the surface becoming deformed, resulting in the formation of microgrooves. The grooved surface essentially aids the alignment of the liquid crystal. Thermoplastic coatings are usually better for the alignment of smectic liquid crystals that form directly from the isotropic liquid, whereas thermosetting coatings usually require the presence

Liquid crystal

Surfactant e.g.

CH3

C18H37 Si Cl Lecithin - Non-permanent Chloro-dimethyl silanes - Permanent

CH3

. Fig. 21 The method of preparing homeotropic specimens using surface aligning treatments

1311

1312

7.1.2

Introduction to Defect Textures in Liquid Crystals

Thermosetting e.g. Polyimide - Cross-linked O O N

O

N

O

N

N n

O O

O O

N

O O

N

O O

N

N n

O

O

O

O

Surface microgrooves Rubbing

Thermoplastics e.g. Polyamide - Nylon - Hydrogen-bonded H H C

C O

N

C

H

N

C

O

O

O

H

H

H

N

C

N

C O

O

Rubbing

N n

N n

Surface chain orientation

. Fig. 22 A comparison of the use of thermosetting and thermoplastic polymers in creating oriented surfaces for homogeneous alignment

of a nematic phase between the smectic and the liquid states in order to subsequently align smectic phases formed on cooling. > Figure 22 illustrates the comparison between the two techniques for creating oriented homogeneous alignment.

7

Summary

Understanding defects is a necessity in the success of LCDs, thus this chapter was designed to give the reader a relatively simple introduction into defects found in liquid crystal phases that

Introduction to Defect Textures in Liquid Crystals

7.1.2

are relevant to electrooptic displays and devices. The chapter is by no means an exhaustive review of the literature; however, additional information can be gained from the listed books in the Further Reading list. Defect textures can be used to identify liquid crystal phases, and the study of their structures is important for their eradication in devices, or for their introduction with controlled size and distribution. This chapter touches upon all of these three aspects.

References 1. Collings P (1990) Liquid crystals, nature’s delicate state of matter. Princeton University Press, Princeton 2. Frank FC (1958) Discuss Faraday Soc 25:19; Brinkman WF, Cladis PE (1982) Phys Today 1 3. Kittel C (1971) Introduction to solid state physics, 5th edn. Wiley, Frankfurt/Main 4. Friedel G (1931) Z Kristallogr 79:26 5. Friedel G, Friedel E (1931) J Phys Radium VIII 2:133 6. Nehring J, Saupe A (1972) J Chem Soc Faraday Trans II 1:68 7. Sackmann H, Demus D (1969) Fortschr Chem Forsch 12:349 8. Arora SL, Fergason JL, Saupe A (1970) Mol Cryst Liq Cryst 10:243 9. Saupe A (1969) Mol Cryst Liq Cryst 7:59 10. Toulouse G, Kle´men M (1976) J Phys Lett 37:L149 11. Schopohl N, Sluckin TJ (1987) Phys Rev Lett 59:2582 12. Friedel J (1964) Dislocations. Pergamon, Oxford 13. Volterra V (1907) Ann Ecole Normale Super Paris (3) 24:400 14. Demus D, Richter LR (1978) Textures of liquid crystals. Chemie, Weiheim; Lagerwall ST, Meyer RB, Stebler B (1978) Ann Phys 3:249 15. Gray GW, Goodby JW (1984) Smectic liquid crystals, textures and structures. Leonard Hill, Glasgow 16. Coates D, Gray GW (1973) Phys Lett 45A:115 17. Crooker PP (1989) Liq Cryst 5:751 18. Goodby JW, Waugh MA, Stein SM, Chin E, Pindak R, Patel JS (1989) Nature 337:449 19. Goodby JW, Waugh MA, Stein SM, Chin E, Pindak R, Patel JS (1989) J Am Chem Soc 111:8119

20. Renn SR, Lubensky TC (1988) Phys Rev A 38:2132 21. Renn SR, Lubensky TC (1991) Mol Cryst Liq Cryst 209:349 22. Nehring J, Saupe A (1972) J Chem Soc Faraday Trans II 68:1; Demus D, Richter LR (1978) Textures of liquid crystals. Chemie, Weiheim 23. Klemen M, Williams C (1973) Philos Mag 28:725 24. Williams CE, Kle´men M (1974) J Phys Lett 35:L33 25. Takanishi Y, Takezoe H, Fukuda A, Komura H, Watanabe J (1992) J Mater Chem 2:71 26. Hartshorne NH, Stuart A (1970) Crystals and the polarizing microscope, 4th edn. Edward Arnold, London 27. Hanschy M, Clark NA, Lagerwall ST (1983) Phys Rev Lett 51:471 28. Fukuda A, Ouchi Y, Arai H, Ishikawa K, Takezoe H (1055) Liq Cryst 1989:5 29. Koden M, Shinomiya T, Itoh N, Kuratate T, Taniguchi T, Awane K, Wada T (1991) Jpn J Appl Phys 30:L1823 30. Negi YS, Suzuki Y, Hagiwara T, Kawamura I, Yamamoto N, Mori K, Yamada Y, Yamimoto K, Imai Y (1993) Liq Cryst 13:153 31. Glogarova M, Lejck L, Pavel J, Janovec U, Fousek F (1983) Mol Cryst Liq Cryst 91:309 32. Congnard J (1982) Mol Cryst Liq Cryst Suppl Ser (1):1 33. Patel JS, Goodby JW (1986) J Appl Phys 59:2355 34. Geary JM, Goodby JW, Kmetz AR, Patel JS (1987) J Appl Phys 62:4100

Further Reading Demus D, Richter L (1978) Textures of liquid crystals. Chemie, Weinheim, 228 p. ISBN 3-527-25796-9 Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (eds) (1998) Handbook of liquid crystals: fundamentals, vol 1. Wiley-VCH, Weinheim, 914 p. ISBN 3-527-29270-5 Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (eds) (1998) Handbook of liquid crystals: low molecular weight liquid crystals I, vol 2A. Wiley-VCH, Weinheim, 490 p. ISBN 3-527-29271-3

Demus D, Goodby JW, Gray GW, Spiess H-W, Vill V (eds) (1999) Physical properties of liquid crystals. Wiley-VCH, Weinheim, 503 p. ISBN 3527-29747-2 Dierking I (2003) Textures of liquid crystals. Wiley-VCH, Weinheim, 218 p. ISBN 3-527-30725-7 Goodby JW, Blinc R, Clark NA, Lagerwall ST, Osipov MA, Pikin SA, Sakurai T, Yoshino K, Zeks B (1991) Ferroelectric liquid crystals: principles, properties

1313

1314

7.1.2

Introduction to Defect Textures in Liquid Crystals

and applications. Gordon and Breach, Philadelphia and Reading, 474 p. ISBN 2-88124-282-0 Gray GW, Goodby JW (1984) Smectic liquid crystals: textures and structures. Leonard Hill, Glasgow and London, 220 p. ISBN 0-249-44168-3

Takatoh K, Hasegawa M, Koden M, Itoh N, Hasegawa R, Sakamoto M (2005) Alignment technologies and applications of liquid crystal devices. Taylor & Francis, Abingdon, 263 p. ISBN 0-74840902-5

7.1.3 Liquid Crystal Materials for Devices Melanie Klasen-Memmer . Harald Hirschmann 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1316

2

Basic Requirements and Physical Properties of LCs for Display Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1316

3 Nematic LC Materials for Passive-Matrix Addressed Displays . . . . . . . . . . . . . . . . . . 1322 3.1 Twisted Nematic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1322 3.2 Super-Twisted Nematic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1323 4 4.1 4.2 4.3 4.4 4.5

Nematic LC Materials for Active-Matrix Addressed Displays . . . . . . . . . . . . . . . . . . . 1330 Twisted Nematic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1330 In-plane-Switching and Fringe-Field-Switching Technologies . . . . . . . . . . . . . . . . . . . . . . 1331 Optically Compensated Bend Mode Displays and Projection Displays . . . . . . . . . . . . 1331 Vertical Alignment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1332 PS-VA Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1336

5 Non-Nematic LC Materials for Other Display Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1336 5.1 Blue Mode Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1336 5.2 Ferroelectric LC Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1338 6

Toxicological Investigations on Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1340

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_7.1.3, # Springer-Verlag Berlin Heidelberg 2012

1316

7.1.3

Liquid Crystal Materials for Devices

Abstract: Starting with the introduction of TN displays for wrist watches and calculators in the 1970s, the LCD technology is now the dominating technology for any kind of display application. This review summarizes the individual material requirements for passive-matrixdriven TN- and STN-technology, for TFT-TN-, IPS-, FFS-, OCB- and projection technology, for VA- and PS-VA-technology and for possible future technologies using Blue Phases or Ferroelectric LCs. Key LC materials and their structure–property relations, as well as LC-mixture properties are highlighted. List of Abbreviations: LC, Liquid Crystal; LCD, Liquid Crystal Display; TFT, Thin Film Transistor; TN, Twisted Nematic; STN, Super-Twisted Nematic; IPS, In-Plane Switching; FFS, Fringe Field Switching; OCB, Optically Compensated Bend; VA, Vertically Aligned; PS-VA, Polymer Stabilized VA; PSA, Polymer Sustained VA; RM, Reactive Mesogen; BP, Blue Phase; FLC, Ferroelectric LC; TNI, Clearing Point; De, Dielectric Anisotropy; Dn, Optical Anisotropy; g1, Rotational Viscosity; Kii (i = 1,2,3), Elastic Constants; extr., Extrapolated; d, Cellgap; VHR, Voltage Holding Ratio; C, Crystalline; N, Nematic; Sm, Smectic; I, Isotropic

1

Introduction

In the past two decades, remarkable development has been made in the application of LCDs. The first displays on market using TN technology for low information content and STN technology (> Chap. 7.3.1) for medium to high information content (especially for notebook displays in the late 1980s/early 1990s) were passive-matrix displays. Further developments resulted in the introduction of TFT-LCDs [1]. This technology enabled switching of a large number of segments. Based on the unique combination of properties of LCs and TFTs, notebooks were introduced into market in the beginning of the 1990s. This application was the driving force of new LCD developments for the rest of the decade, i.e., for LC allowing lower operating voltage for longer battery lifetime. Long time, the viewing angle dependency was thought to be an intrinsic problem of LCDs due to the birefringence of the LC molecules. However, the viewing angle dependency was significantly improved by the introduction of optical compensation films for TN, but even more through new display technologies such as IPS (> Chap. 7.3.3) and VA modes ([2, 3] and > Chap. 7.3.4) around 1995–1997. These new technologies enabled LCDs with larger display sizes. First products were computer monitors. Next big challenge was the application of LCDs to TVs which mainly required much faster switching times lower than one frame, higher contrast and brightness and larger viewing angles. Due to the continuous improvement of various display components, the first largesized LCD-TV was introduced into market in 2001. All these developments were accompanied in a decisive manner by the continuous development of new LC materials and mixtures which will be described in the following.

2

Basic Requirements and Physical Properties of LCs for Display Applications

LCs for commercial application must have a broad nematic phase range in order to guarantee the so-called operating temperature range of the displays. This phase range is about 40 to 100 C for automotive applications, 20 to 70 C for mobile applications, and 0 to 50 C for indoor applications such as PC monitor or TV. The clearing point of a liquid crystal is the

Liquid Crystal Materials for Devices

7.1.3

temperature at which the liquid crystal phase vanishes. It has to be at least 10 C higher than the operating temperature of the device. To be able to respond to an applied electric field, LCs must exhibit a dielectric anisotropy (De), defined as the difference of the dielectric constants parallel and perpendicular to the director of the nematic phase [4]. Depending on the molecular structure, the dielectric anisotropy can be positive (molecular dipole parallel to the long axis of the molecule) or negative (molecular dipole perpendicular to the long axis of the molecule). Elastic constants Kii are the proportional constants between the force (the electric field in case of LCDs) and the deformation of director fields. There exist three Kii (i = 1, 2, 3) dependent on the deformations of the director (splay, twist, bend) [4]. The operating voltage is proportional to the square root of the ratio between elastic constants and dielectric anisotropy. The reorientation of the LCs during switching depends on the configuration of the display and the switching mode. The basis of the visible electro-optical effect is the birefringence (Dn), which is defined as the difference between the extraordinary refractive index (light polarized parallel to the director) and the ordinary refractive index (light polarized perpendicular to the director). Upon switching the reorientation of the LC molecules leads to an effective change of the optical path length which is defined as d∗Dn (d = cellgap). This results in a change of the transmission of the display between 0 and 100%. The switching time of an LCD is proportional to the rotational viscosity (g1) of the liquid crystal and proportional to the square of the cellgap in the display [5]. In addition, the importance of the flow effect was demonstrated [6, 7]. Low viscosity and small cellgap is the key material parameter for the TV application. In active-matrix displays, each pixel is driven by a TFT charging the pixels by signal pulses. The voltage has to be sustained till the next refresh signal pulse arrives. This time span is called one frame time. The voltage drop during this time is characterized by the voltage holding ratio (VHR), which is defined as the ratio of the voltages at a pixel at the end and the beginning of the frame time [8]. A high VHR is prerequisite for a flicker-free picture. In passive-matrix-driven TN and STN displays, VHR should not be as high as in active-matrix displays in order to avoid image-sticking effects. Consequently, different polar LC materials are required for passiverespectively, active-matrix display applications. The main material research target is synthesis and identification of compounds with lower viscosity while maintaining or even improving the other properties. Computer simulations for the calculation of De and Dn based on molecular properties, such as dipole moments and polarizabilities are often in good agreement with measured values of LCs [9, 10]. Computer simulations of bulk properties like elastic constants, viscosities, and melting points on the other hand are still in an early stage [11]. Single liquid crystal compounds cannot fulfill the complex requirements of displays. The wide operating temperature range in combination with other physical properties requires mixtures of typically 10–20 compounds. Nevertheless, melting points of LC mixtures are not thermodynamically defined phase transitions but metastable states with a very long lifetime. All structural elements (side chains, rings, linking groups, terminal group) of a liquid crystal molecule contribute to the physical properties. For example, benzene rings with polar substituents contribute to the dielectric anisotropy significantly. In addition, the aromatic ring contributes to higher values of the optical anisotropy Dn compared to a cyclohexane ring. As a consequence, limitations for highly polar liquid crystals with low Dn-values were empirically found. Other contradictory requirements are the coexistence of high clearing point and low viscosity and the coexistence of high polarity and low viscosity. Thus, it is very difficult to achieve LCs with a high clearing point (broad operating temperature range) and low viscosity (fast switching) or LCs with high polarity (low operating

1317

1318

7.1.3

Liquid Crystal Materials for Devices

voltage) and low viscosity. However, these are the decisive LC properties for different LCD products. In > Table 1, characteristic property combinations for the most important LC parameters Dn, De, and g1 for different core structures are shown considering those with fluorine molecules as polar groups. Modifications of the polar groups will have significant impact especially on De. Additional properties like solubility, etc., play a decisive role for practical use. Moreover, liquid crystals must be chemically, photochemically, and electrochemically stable. Empirically, VHR values decrease with increasing dielectric anisotropy of the LCs. The combination of high polarity with high VHR is therefore another usually contradictory material requirement.

. Table 1 Characteristic Dn, D«, and g1 ranges for different core structures with polar fluorine substituents Dn

D«

g1 [mPas]

0.03–0.05

0–7

20–100

0.06–0.08

4–10

20–100

0.13–0.15

9–14

20–100

0.06–0.10

6–11

120–420

R

R

R

R

2 to ( 6)

Z

0.12–0.18 R

Z

8–25

80–260

2 to ( 9)

Z

0.23–0.30 R

100–320

2 to ( 6)

0.15–0.26 R

8–15

15–20

Z

R is an alkyl- or alkenyl-group. Z can be a COO-group, a CH2CH2-group or a single bond

250–500

C3H7

Structure

C3H7

O

O

O

O

O

F

F

CN

CN

CN

C3H7

C3H7

CN

C5H11

O

. Table 2 Physical properties of LCs for passive-matrix TN applications

CN

5

4

3

[40]

[39]

[38]

[37]

[36]

1

2

Ref.

No.

C 100 N 201 I

C 70 N (19) I

C 100 N (46) I

C 45 N 46 I

C 23 N 35 I

Phases

166.7

19.5

21.7

8.4

1.0

37.5

50.2

35.3

21.1

21.6

TNI. [ C] D«

0.165

0.182

0.194

0.136

0.237

Dn

750

270

126

116

112

1.95

1.78

1.83

1.92

1.66

g1 [mPas] K33/K11

Liquid Crystal Materials for Devices

7.1.3 1319

Structure

H3C

O

O C5H11

C3H7

OC2H5

C3H7

O

OC2H5

C3H7

11

10

9

8

[45]

[44]

[43]

[42]

[41]

6

7

Ref.

No.

C 35 N (19) I

C 34 N (31) I

C 94 I

C 42 N (38) I

C3I

C -3 SmB 68 I

Phases

32.3

31.0

60.5

25.6

78.8

16.7

2.7

0.7

1.2

0.1

0.4

1.1

TNI. [ C] D«

0.145

0.083

0.192

0.098

0.081

0.050

Dn

58

58

51

14

23

g1 [mPas] K33/K11

7.1.3

C3H7

C2H5

C3H7

O

C2H5

C3H7

. Table 2 (Continued)

1320 Liquid Crystal Materials for Devices

C3H7

O

O

CH3

C3H7

C3H7

OC2H5

OC2H5

TNI, Dn, De, and g1 are extrapolated from Merck mixture ZLI-4792

C3H7

C3H7

C3H7

C3H7

[50]

[49]

15

16

[47, 48]

14

C 110 N 253 I

C 88 N 95 I

287.6

112.2

C 158 SmB 212 332.1 SmA 223 N 327 I

C 110 SmB 212 N 333.5 325 I

13

[46]

C 63 SmB 110 N 195.9 179 I

12

0.5

2.3

0

0

0

0.252

0.300

0.137

0.072

0.082

356

54

491

909

146

1.38

1.38

Liquid Crystal Materials for Devices

7.1.3 1321

1322

7.1.3

Liquid Crystal Materials for Devices

Stability of LC molecules and mixtures upon storage and especially towards light-load and heat-load during production process and display lifetime is essential. As most LC singles are not nematic at room temperature, their individual parameters such as dielectric anisotropy De (measured at a frequency of 1 kHz), optical birefringence Dn (measured at a wavelength of 589.3 nm), rotational viscosity g1 and elastic constants Kii are extrapolated from suitable nematic LC host mixtures taking into account the variation of the clearing point of the mixture under investigation. All measurements were carried out at T = 20 C.

3

Nematic LC Materials for Passive-Matrix Addressed Displays

3.1

Twisted Nematic Displays

Passive-matrix-driven Twisted Nematic (TN) displays for low multiplex ratio are typically operated in the so-called 1st minimum (optical retardation d∗Dn = 0.5 mm), respectively, 2nd minimum (optical retardation d∗Dn = 1.12 mm) with respect to the Gooch–Tarry curve [12, 13]. For typical cellgaps of around 6 mm, LC mixtures are required exhibiting Dn-values in the range of 0.083 for 1st minimum application, respectively, 0.187 for 2nd minimum application. > Table 2 lists structures and physical properties of LCs for passive-matrix TN applications. TN applications became commercially feasible for wrist watches and calculators in the 1970s by development of polar singles like > 1 and > 2 bearing a cyano end-group. The De can be increased by almost a factor of 2 by the introduction of an ester-group like, e.g., > 3. Additional increase of De can be achieved by partial fluorination of the benzene rings (> 4), which will in addition result in a decrease of the melting point and the clearing point and an increase of g1. The introduction of an additional cyclohexane ring like in > 5 results in a simultaneous increase of the clearing point and the g1, but also in an increase of the melting point which limits its maximum concentration in a mixture. In addition to polar singles, dielectrically neutral singles with De  0 are essential for adjustment of the clearing point TNI, Dn, and g1 of the final mixture. Neutral two-ring molecules like > 6–9 are characterized by relatively low clearing points and low g1-values. The Dn can be enhanced by substitution of cyclohexane rings by phenyl-rings, but typically this substitution will lead to higher melting points. Introduction of oxygene into the side chain will simultaneously increase clearing point and g1. Other popular neutral two-ring molecules are the widely used cyclohexane esters > 10, which are also referred to as Demus esters, and the phenyl benzoate esters > 11, referred to as the Merck esters. Increase of the molecular length by introduction of additional rings has a strong impact on both clearing point and g1 (> 12–14). In addition, melting points increase and depending on the structure broad smectic phases appear. As shown, the Dn of a molecule can be modified to a certain amount by selecting different rings structures (phenyl, cyclohexene). However, a stronger impact on Dn can be realized by introduction of a triple-bond linking group resulting in a so-called tolane structure. From comparison of > 9 vs. > 15, it is obvious that this structural modification almost doubles the Dn. Three-ring tolane structures are often characterized by high melting points which limits their use in LC mixtures (> 16). Suitable chiral dopants in typical concentrations between 0.1 and 0.5% by weight are added to the mixtures in order to prevent any reversed twist deformations in the display.

Liquid Crystal Materials for Devices

3.2

7.1.3

Super-Twisted Nematic Displays

Passive-matrix-driven Super-Twisted Nematic (STN) displays for medium to high multiplex ratio are operated at a typical twist angle of 240 at an optical retardation value d∗Dn = 0.85. For cellgaps of 4–7 mm, LC mixtures are required exhibiting Dn-values in the range of 0.12–0.21. STN displays are typically addressed by the Alt–Pleshko addressing scheme [14]. According to the Alt–Pleshko law of multiplexing, a defined number of multiplexed rows requires a defined steepness of the transmission vs. voltage curves in order to achieve optimum contrast. Especially for high information content displays, the steepness of the electro-optical curve has to be in a regime which cannot be achieved by a TN display. For a given parameter set of twist angle, pretilt angle, and dielectric parameter De/e⊥, the steepness of the transmission vs. voltage curve can be increased by an increase of the ratio K33/K11. If however the selected ratio K33/K11 is higher than required for the number of multiplexed rows, hysteresis of the voltage vs. transmission curve may occur which limits the multiplexability. Due to the steep transmission vs. voltage curve compared to TN displays, STN displays are switched between two fairly adjacent voltage levels, which results in slow switching, especially for high information content displays. Consequently, LC materials characterized by low g1 are of particular importance. The ratio K33/K11 of polar- and neutral LC singles can be increased by introduction of a vinyl-group as a linking group or as end-group in the molecule. > Table 3 shows structures and physical properties of typical dielectrically neutral two-ring materials where the position and the number of the vinyl-groups in the molecule were modified (> 6, > 17–19) [15–17]. It is obvious that the introduction of each vinyl-group results in an increase of the ratio K33/ K11, but in addition also results in an increase of the clearing point and the exhibition of a nematic phase. For the realization of low g1, terminal vinyl-groups are more effective (> 17, > 18). This type of material has gained huge importance not only for STN application, but also for any other kind of LCD applications as its use results in a decrease of the g1 of a LC mixture and consequently shorter switching times in the LCD panel. As these neutral two-ring molecules are characterized by clearing points TNI which are too low for practical application, corresponding neutral three-ring materials containing at least one vinyl-group are additionally used. Such materials like > 20–22 are also displaying very high ratios of K33/K11, broad nematic phase ranges and low g1-values. Dn of STN mixtures can be relatively high, especially when the cellgap is decreased in order to realize fast switching. Therefore, also for these applications tolane materials like > 15 and > 16 are of special importance. It could be shown that melting points of tolane structures can be reduced and broad nematic phases can be realized by selected fluorination (> 23). In addition, these materials are characterized by very high Dn-values and low g1. As a consequence, fast switching LC mixtures exhibiting very high Dn-values can be easily realized. STN displays for automotive application, desktop telephones, and household application require a moderate dielectric anisotropy De  8–15. However, with the increasing boom of mobile telecommunication in the 1990s, LC mixtures had to work at low battery voltage even at deep temperatures. These mixtures are characterized by a dielectric anisotropy De in the range from 30–50, which can be realized by use of polar materials described before (> 2, > 4) and by corresponding polar materials where alkyl end-groups have been replaced by alkenyl endgroups for enhancement of K33/K11 (> 24). The passive-matrix addressing scheme results in

1323

Structure

C3H7

C2H5

CH3

C 52 N 63 I

C 66 N 162 I

20

C 30 N 47 I

19

18

C -23 SmB 35 N 49 I

C -3 SmB 68 I

6

17

Phases

No.

172.4

64.7

35.1

34.7

16.7

TNI [ C]

0

0.5

0.6

0.7

1.1

D«

0.101

0.078

0.068

0.052

0.050

Dn

118

33

26

18

23

g1 [mPas]

1.72

1.79

1.64

1.57

1.45

K33/K11

7.1.3

C3H7

C3H7

. Table 3 Physical properties of LCs for STN applications

1324 Liquid Crystal Materials for Devices

C3H7

F

F

F

CN

CN

TNI, Dn, De, and g1 are extrapolated from Merck mixture ZLI-4792

C3H7

F C2H5

OMe

CH3

25

24

23

22

21

C 57 I

C 68 N 74 I

C 70 N 175 I

C 67 SmB 93 N 208 I

C 61 SmB 70 N 178 I

67.2

31.5

213.1

218.6

181.1

32.6

22.5

4.2

0.2

0.1

0.110

0.170

0.345

0.100

0.097

99

180

67

224

138

1.98

2.10

1.32

1.90

1.81

Liquid Crystal Materials for Devices

7.1.3 1325

C3H7

C3H7

O

F

O

O

F

F

F

F

F

F

F

F

F

F

F

F

F

29

28

C 44 N 105 I

C 56 N 117 I

C 42 N (33) I

C 66 N 94 I

26

27

Phases

No.

91.5

115

54.2

74.3

TNI [ C]

10.5

11.1

12.6

9.7

D«

0.067

0.067

0.142

0.075

Dn

145

175

153

160

g1 [mPas]

7.1.3

C3H7

C3H7

Structure

. Table 4 Physical properties of LCs with D« > 0 for TFT applications

1326 Liquid Crystal Materials for Devices

C3H7

C3H7

C3H7

C3H7

O

F

F

F

F

O

F

O

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

33

32

31

30

C 62 N (28) I

C 48 I

C 64 I

C 35 N 66 I

36.8

20.6

23.3

58.9

19.6

25.2

15.2

14.0

0.219

0.158

0.135

0.065

216

96

173

158

Liquid Crystal Materials for Devices

7.1.3 1327

C3H7

C3H7

C3H7

F

O

O

O

O

F

F

F

F

O

F

F

F

F

F

F

F

F

C3H7

F

37

36

C 64 N 77 I

C 85 N 131 I

C 74 N (51) I

C 72 N 134 I

34

35

Phases

No.

78.1

115.6

63.2

163.5

TNI [ C]

10.6

27.5

17.0

1.3

D«

0.218

0.095

0.068

0.252

Dn

185

336

201

80

g1 [mPas]

7.1.3

C2H5

Structure

. Table 4 (Continued)

1328 Liquid Crystal Materials for Devices

F

F

F

F

Cl

TNI, Dn, De, and g1 are extrapolated from Merck mixture ZLI-4792

C4H9

C3H7

C3H7

F

F

F

F

F

F

F

40

39

38

C 112 SmA 166 N 200 I

C 111 SmE 135 SmA 214 N 250 I

C 94 N 117 I

160.8

217.3

109.8

20.1

18.1

11.9

0.273

0.294

0.256

257

404

257

Liquid Crystal Materials for Devices

7.1.3 1329

1330

7.1.3

Liquid Crystal Materials for Devices

high-frequency pulses at the individual pixel, where the absolute value depends on the multiplex ratio. As the dielectric anisotropy De is strongly temperature- and frequencydependent, this may cause insufficient switching resulting in poor contrast which limits the use of a display, especially at low temperatures. These effects can be minimized by using polar LC molecules consisting of a cyano end-group and two fluorine atoms at the phenyl-ring next to the cyano-group (> 25). As the relaxation frequency of a polar LC single decreases with increasing molecular length, polar two-ring molecules play a more dominant role than polar three-ring molecules, especially for higher multiplexed STN application. A STN display will only work if the ratio of cellgap and required pitch is properly adjusted. Typically, this ratio is about 0.53 for a STN display with 240 twist angle. The helical twisting power of a LC mixture is depending on its physical properties. Consequently, each modification of a LC mixture requires the determination of the helical twisting power and the chiral dopant concentration in order to adjust the correct pitch value.

4

Nematic LC Materials for Active-Matrix Addressed Displays

4.1

Twisted Nematic Displays

Active-matrix-driven high resolution Twisted Nematic (TFT-TN) displays are typically operated in the 1st minimum condition with respect to the Gooch–Tarry curve at an optical retardation d∗Dn in the range from 0.4 to 0.5. While cellgaps in the 1990s were around 6 mm, these could meanwhile be reduced to 3 mm, which should result in a theoretical response time improvement by a factor of four assuming that all other parameters are fixed. This cellgap reduction resulted in a required increase of Dn-values of LC mixtures from about 0.08 in the past to about 0.15 in state of the art mixtures. In contrast to passive-matrix-driven displays, active-matrix driving technology requires a high VHR-value over a wide temperature range for the realization of a flicker-free picture. From the material point of view, this requires liquid crystals with high resistivity. It could be shown that the resistivity of polar LC molecules can be increased by utilization of liquid crystals with fluorine atoms instead of cyano-atoms as endgroups. Polar two-ring materials with fluorine end-groups generally show very low clearing points and are not suitable for practical application in LC mixtures. > Table 4 lists chemical structures and physical properties of LCs with positive dielectric anisotropy for TFT applications. Typical polar three-ring materials for TFT-TN applications are 1,2,3-trifluorobenzene derivatives like > 26 and > 27. Compared to corresponding polar materials with cyano endgroups suitable for passive-matrix application, these materials generally are characterized by lower De-values. De can be increased by introduction of an ester- or a CF2O-linking group (> 28, 29). Both materials show almost the same De, but the CF2O-linking group results in lower g1 and better reliability, respectively, higher VHR. De can be further modified by different rings within the molecular structure like, e.g., the pyrane-ring in > 30. For mobile applications, such as notebooks, netbooks, and mobile telephones, low power consumption is required. This is realized by low switching voltages in the range of 1–1.5 V which can be achieved by LC mixtures characterized by De -values of 12–20. From the LC material side, polar three-ring molecules are required which consist of at least two phenyl-rings where the hydrogen atoms are partially or fully substituted by fluorine atoms. The individual materials are characterized by De-values of 15–25 and Dn-values between 0.13 and 0.22

Liquid Crystal Materials for Devices

7.1.3

(> 31–33). A further increase of De is possible by even longer molecules with fluorinated phenyl-rings, but typically these will exhibit higher melting points and higher g1-values which will limit their usability in LC mixtures. As cellgap has decreased in order to reduce switching times of the LCDs, additional highly birefringent materials are necessary in order to achieve the requested Dn-value of the final mixture. Tolane structures are not suitable as they will have a negative impact on VHR value. Materials with high Dn with good reliability are accessible by structures consisting of aromatic rings which may be partially fluorinated (> 34). Most of these materials are characterized by broad nematic phase ranges and Dn-values in the range from 0.20 to 0.26. In addition to the described polar materials and high Dn molecules, LC mixtures do also contain two-ring and three-ring neutral molecules with very low g1, respectively, high TNI for adjustment of the LC mixture parameters (> 17, 18, 20). Suitable chiral dopants for specified pitch of the LC mixture are added. Lowering the pitch value results in faster switching, but also in increased switching voltage, and has an impact on the contrast ratio.

4.2

In-plane-Switching and Fringe-Field-Switching Technologies

In-Plane-Switching (IPS) and Fringe-Field-Switching (FFS) displays are typically operated at an optical retardation d∗Dn slightly above 0.3, requiring Dn of LC mixtures in the range of 0.08–0.10 for standard cellgaps. In contrast to TN displays, the aperture ratio in IPS depends on the distance of the interdigital electrodes. As the threshold voltage is linearly dependent on the distance between the electrodes, displays with high aperture ratio, respectively, high brightness will require mixtures with high dielectric De values which however results in high g1, respectively, slow switching. Consequently, this mode requires LCs with an improved relation of De to g1. De-values of LC mixtures are typically in the range of 8 for monitor and TV application and at higher values for mobile applications. Up to now, only IPS displays using dielectrically positive LCs have been commercialized. LC singles with De-values of more than ten combined with Dn-values of 0.08–0.09 cannot be realized by the only use of fluorinated phenyl-ring building blocks (> 26), but require the introduction of suitable polar linking groups (> 28, 29). A different effective approach is the replacement of a cyclohexane ring by a dioxane ring (> 35). This modification results in an increase of De by a factor of two compared to > 26, while clearing points and Dn are not changed and g1 is only slightly increased. Another approach is the combination of a dioxane ring with fluorinated benzene rings and the CF2O linking group (> 36). This four-ring molecule combines a De of almost 30 with a high TNI and acceptable g1, while Dn is still below 0.1. Depending on the target specification, IPS mixtures will contain other polar and neutral liquid crystals which are also suitable for active-matrix-driven TFT-TN displays.

4.3

Optically Compensated Bend Mode Displays and Projection Displays

Optically Compensated Bend (OCB) mode displays require mixtures combining high Dn typically around 0.15 and high positive De in the range of about 15. Projection displays for fast switching displays are characterized by cellgap typically < 2 mm. Due to strong backlight

1331

1332

7.1.3

Liquid Crystal Materials for Devices

power operating temperatures are around 50 higher compared to direct view LCD TVs. As a consequence, LC mixtures combining high Dn-values, high clearing points, and high thermal and light stability are required. For OCB and projection displays, high Dn-values combined with high stability can be realized by partially fluorinated terphenyl structures (> 37, 38). A further increase of De is achieved by highly fluorinated terphenyls and quaterphenyls (> 33, 39, 40). In addition, quaterphenyl structures typically exhibit high TNI, but suffer from high melting points and broad smectic phases which limits their solubility in a LC mixture.

4.4

Vertical Alignment Technologies

For the different vertical alignment (VA) LCD technologies like MVA (Multi-domain VA), PVA (Patterned VA), or ASV (Advanced Super View) (> Chap. 7.3.4), LC materials with negative dielectric anisotropy De as shown in > Table 5 are needed to achieve a director orientation perpendicular to the electric field. The common feature of the materials with De < 0 is that lateral polar substituents induce a dipole moment perpendicular to the long axis of the molecules. The standard TFT driver requires an operating voltage of 6 V or lower. As a consequence, the dielectric anisotropy of the LC mixtures has to be around at least 3, taking the corresponding elastic constants into consideration. The required optical path d∗Dn is a little higher than 0.3 mm. For currently used cellgaps, this results in Dn-values of the mixtures between 0.08 and 0.11. VA modes are intrinsically fast switching modes and therefore mainly used for TV applications. However, to meet the requirements of switching times within one frame at higher frame rates and for 3D applications, still materials with improved switching times are necessary. Therefore, the main research target is the synthesis and identification of compounds with lower viscosity maintaining or even improving the other properties. Most commercial dielectrically negative materials are based on a 1,2-difluorobenzene building block [18–20]. Difluorinated alkoxy-compounds like > 41–44 show extrapolated De-values between 6 and 7, whereas bis-alkyl-substituted compounds like > 45 and > 46 or > 49 and > 50 only have De-values of around 2. For fast switching mixtures materials with low g1 are needed. The comparison of > 41 and > 42 shows that the introduction of an Ethylbridge has a significant negative impact on the g1-values. Especially, three-ring materials, which have to be used in the mixtures to achieve the requested clearing point of 75 C or higher exhibit large g1-values. A significant improvement, but with higher Dn, is the cyclohexylbiphenylderivative > 44 in comparison to the bicyclohexyl-derivative > 43. In > 44 De and clearing point are comparable to > 43, but the extrapolated g1 is reduced by almost 50%. Also the exchange of a fluorine atom in > 43 by a chlorine atom, which leads to compounds > 47 and > 48, has a large impact on the physical properties like reduced |De| and dramatically increased g1 so that these materials are not suitable for fast switching LC mixtures. A further reduction of the viscosity in combination with an increased optical anisotropy Dn is possible by using compounds with terphenyl structures like > 49 and > 50. These combine a moderate negative dielectric anisotropy with high extrapolated clearing points and relatively low rotational viscosities g1. However, the use of materials with larger dielectrical anisotropy |De| and acceptable g1 is desired, and many attempts have been made in the past to improve the properties of LCs based on the 2,3-difluorophenyl moiety, but due to the free rotation of the rings, this is not so straightforward as can be seen with the extrapolated data of > 51–53. By introduction of one or

C3H7

C3H7

C3H7

C3H7

C3H7

Structure

F

F

F

F

F

F

F

F

F

F

CH3

OC2H5

OC2H5

OC2H5

OC2H5

. Table 5 Physical properties of LCs with D« < 0 for VA applications

45

44

43

C 67 N 145 l

C 80 N 173 l

C 79 SmB (78) N 185 l

C 35 I

C 60 l

41

42

Phases

No.

137.1

182.0

178.4

17.9

14.5

TNI [ C]

2.8

5.9

6.0

5.9

7.0

D«

0.095

0.156

0.092

0.093

0.106

Dn

217

233

413

96

78

g1 [mPas]

Liquid Crystal Materials for Devices

7.1.3 1333

C2H5

C2H5

C3H7

F

F

F

Cl

F

F

F

F

Cl

F

C3H7

C3H7

OC2H5

OC2H5

CH3

50

49

48

C 73 N 115 l

C 120 N 136 l

C 91 N 153 I

C 106 SmB 107 N 169 I

C 94 N 128 I

46

47

Phases

No.

138.9

149.0

145.9

162.3

145.7

TNI [ C]

2.5

1.9

4.9

5.3

2.2

D«

Dn

0.233

0.239

0.081

0.088

0.157

g1 [mPas]

90

102

859

851

158

7.1.3

C3H7

C3H7

Structure

. Table 5 (Continued)

1334 Liquid Crystal Materials for Devices

C3H7

F

F

F F

F

F

F F

F

F

F

F

F

F

F

F

F

OC2H5

OC2H5

OC2H5

55

54

53

52

51

C 85 I

C 100 I

C 96 N 130 I

C 89 N 129 I

C 70 N 151 I

50.1

18.1

128.5

116.7

147.6

TNI, Dn, and g1 are extrapolated from Merck mixture ZLI-4792. De-values are extrapolated from Merck mixture ZLI-2857

C3H7

C3H7

C3H7

C3H7

8.6

6.7

9.2

6.4

6.3

0.085

0.086

0.152

0.149

0.153

142

136

587

505

390

Liquid Crystal Materials for Devices

7.1.3 1335

1336

7.1.3

Liquid Crystal Materials for Devices

even two more lateral fluorine atoms, the |De| can only be increased from 6 to 9, but in comparison to > 44 the result is a smaller TNI and a significantly larger g1. Another idea is to increase the polarity by avoiding the free rotation of polar molecule parts and fixing the most polar conformation. This was finally successful with fluorinated Indane materials like > 54 and > 55 [21, 22]. These materials do not exhibit any mesophase in the pure state, but the extrapolated TNI is more than 30 K higher than of the reference compound > 41, the |De| is increased and the g1 is only slightly increased. By utilizing polar materials with best balance of the relevant properties TNI, De and g1 and neutral materials with excellent g1, in the last years, it was possible to develop new mixture concepts with significantly improved switching times mainly for TV and monitor applications, but also for small-sized displays for mobile phones and car navigation systems.

4.5

PS-VA Technology

An advanced technology that could overcome the intrinsic problems of the conventional VAmodes like relatively low transmittance and strong grey-scale dependency of the switching-on times at high operating voltages is the PSA (Polymer Sustained VA) or PS-VA (Polymer Stabilized VA) technology [23–25, > Chap. 7.3.4]. In addition to the materials presented in > Sect. 4.4, for PS-VA applications UV-curable monomers, so-called ‘‘reactive mesogens’’ (RMs) like > 56 and > 57 (> Table 6) are added to the VA-LC mixture in appropriate concentrations of 56 to the LC mixture. It is necessary to maintain the blue-phase

O

O

. Table 6 Examples of RMs for PS-VA applications

O

O

O

O O

O CH3

O

O

O

O O O

57

56

Liquid Crystal Materials for Devices

7.1.3 1337

1338

7.1.3

Liquid Crystal Materials for Devices

structure during polymerization of the RM to meet the requirements for a practicable stabilization process at a defined temperature. The typical BPs with reflection wavelength in the visible light spectrum with bluish or greenish appearance are not useful for display applications because of the non-accessible black state in the off-state. A good black state of a display due to the optical isotropic behavior of the BP can be realized by tuning the amount of chiral dopant in the LC mixture and shifting the typical reflection wavelengths of a BP into the UV. These so-called ‘‘invisible’’ BPs are feasible for display applications. Due to the optical isotropy, it is furthermore necessary to realize a birefringence to achieve the required retardation in the on-state. This can be done by using an electric field parallel to the glass substrates of the display together with a highly polar LC mixture with De in a nematic host larger than 100, but still challenges for these displays are the high operating voltages. All these requirements can be influenced by development of mixtures with an appropriate combination of RMs, chiral dopant, and LC components.

5.2

Ferroelectric LC Displays

Since the discovery of ferroelectricity in chiral smectic LCs in 1975 [30], there has been intensive experimental investigation of the physical properties of ferroelectric LCs, but the technology has not yet proved commercially viable. Interest in this subject has increased considerably since the invention of surface-stabilized ferroelectric displays as they offer many advantages over conventional nematic display, mainly shorter switching times and excellent viewing angles [31]. The most striking features of such displays are the possibility of bistability of two optically different states and the fast, field-induced transition between these states. The performance and appearance of a FLC-display will depend to a large extent on the properties of the LC itself. The most important material parameters are spontaneous polarization Ps, tilt angle, helical pitch in the smectic and cholesteric phase, rotational viscosity g1, response time, optical anisotropy Dn, and dielectrical anisotropy De. In order to cover the working and storage temperature range of a display, a practically applicable FLC should have broad SmC∗-phase at least from 30 to 70 C. Both a SmA∗-phase and a cholesteric phase above the SmC∗-phase seem to be necessary to achieve a good orientation. Furthermore, the helical pitch in the cholesteric phase as well in the SmC∗-phase has to be compensated to a sufficient large value. This is a challenging task as for many chiral dopants the helical twisting powers in the cholesteric phase and in the SmC∗-phase are different. In order to achieve good transmission behavior of the display, the FLC should show small temperature dependence of the tilt angle with values close to 22.5 in the working temperature range. The optical anisotropy for a cell thickness of 2 mm should be 0.14. However, it has to be considered that such low cellgaps with sufficient uniformity can only be produced for smallsized displays. Switching times of about 30 ms at room temperature can be achieved if the FLC has low g1 and a sufficiently large spontaneous polarization Ps. All the requirements can only be fulfilled with many component mixtures. On the one hand, one could use only SmC∗-compounds and on the other hand one could dope a nonchiral SmC-mixture with suitable chiral dopants in order to induce a SmC∗-phase. Disadvantage of the first solution are higher viscosities of chiral compounds because of the branched chain compared to non-branched systems. Additionally, chiral compounds are more expensive. In the second concept, a non-chiral SmC-mixture with low viscosity and a chiral dopant

Liquid Crystal Materials for Devices

7.1.3

. Table 7 Examples of LCs for FLC applications Structure

No. Phases O

58

C 63 SmC 74 N 91 I

59

C 37 SmC 49 N 57 I

60

C 49 SmC 121 SmA 128 N 164.5 I

61

C 85.0 (SmF 43) SmC 112 SmA 148 N 151 I

62

C 35 SmC 57 SmA 64 N 69.5 I

63

C 49 SmC 77 SmA 93 N 108 I

64

C 56 SmC 105.5 SmA 131 N 136 I

C8H17O OC8H17

O

F

F O

C8H17O O

OC8H17

O

F

C7H15O O

OC8H17

CN C8H17

C7H15O

N OC8H17

C8H17 N

F

F

C9H19

C7H15

F C5H11

F C7H15

1339

1340

7.1.3

Liquid Crystal Materials for Devices

with appropriate effects on phase transition temperatures, spontaneous polarization, and on the switching time of the SmC∗-mixture are needed [32, 33]. Molecular tilting requires the presence of a lateral dipole from a polar moiety. Therefore, esters (e.g., > 58–60, > Table 7) or compounds with an axial nitrile-group like > 61 are common SmC-materials, but often show high viscosities [34]. The use of heteroatoms like, e.g., pyrimidines, like > 62 leads to ferroelectric host materials with lower viscosity [33]. Beneficial are again lateral F-substituents showing high polarity facilitating molecular tilting and hence the SmC-phase is often exhibited over a wide temperature range and the mixture has additionally low viscosity. One of the major advantages of Ortho-difluor-terphenyls like > 63 and > 64 is that the SmC-phase is exhibited over a wide range to high temperatures through the use of two alkyl chains which facilitates the formulation of ferroelectric host mixtures with low melting points and low viscosity. Chiral dopants for practically applicable FLC-mixtures should incorporate cheap and easily accessible chiral groups. With an appropriate induced spontaneous polarization, they should lead to fast switching times without significantly increasing the rotational viscosity of the mixture and without depressing the SmC∗-SmA∗-transition.

6

Toxicological Investigations on Liquid Crystals

Apart from the physical properties, toxicological data have to be investigated before introduction of new compounds into the market in order to guarantee a save handling of the displays. The actual environmental concentration of a substance depends on the amount emitted, but this is greatly influenced by the physical and chemical properties of the substance and the particular transport media involved (air, water, soil). Physicochemical properties, such as volatility and solubility, determine the distribution among the different environmental compartments. In addition, factors such as chemical stability, adsorption and biodegradability are decisive and may influence and reduce the initial environmental concentration. Degradability of LCs by bacteria is assessed in ready biodegradability tests (OECD 301), which are stringent tests and provide limited opportunity for biodegradation. Since most of the pollutants are found in water, aquatic organisms are used for a first indication of environmental effects of chemicals. Organisms used are primary producers (algae), primary consumers (daphnia), secondary consumers (fish), and destruents (bacteria). According to OECD guidelines (201– 203), the organisms are exposed to the test compound dissolved in the test medium. The measured endpoints are death (fish), immobilization (daphnia), growth inhibition (algae), and respiration inhibition (bacteria). Most commonly, the results are expressed as median lethal concentration (LC50), which is estimated to kill 50% of the test organisms, or as median effect concentration (EC50), which is estimated to cause the specified effect, e.g., respiration inhibition. Depending on their chemical stability, some liquid crystals are not easily biodegradable, whereas others have shown degradation rates up to 55% within 28 days. Also, no toxic effects were observed to fish, algae, daphnia, and bacteria in commercially available LCs [35].

Acknowledgments The authors thank their colleagues and collaborators in LC R + D of Merck KGaA for their valuable contributions and technical assistance.

Liquid Crystal Materials for Devices

7.1.3

References 1. Stewart RG (1996) Active matrix LCD. In: SID seminar lecture notes, May 13, 1996, vol 1. San Diego, CA, M5, pp 1–35 2. Yamauchi S, Aizawa M, Clerc JF, Uchida T, Duchene J (1989) SID Symp Dig 89:378–381 3. Hirai H, Kinoshita Y, Shohara K, Murayama A, Hatoh H, Matsumoto S (1989) Optimization of cell condition and driving method in a VAN LCD for color video display. Japan Display, pp 184–187 4. Vertogen G, de Jeu WH (1988) Thermotropic liquid crystals, fundamentals. Springer, Berlin 5. Tarumi K, Finkenzeller U, Schuler B (1992) Dynamic behaviour of twisted nematic liquid crystals. Jpn J Appl Phys 31:2829–2836 6. Berreman DW (1975) Liquid-crystal twist cell dynamics with backflow. J Appl Phys 46(9):3746–3751 7. Iwata Y, Naito H, Inoue M, Ichinose H, KlasenMemmer M, Tarumi K (2004) Transient current of nematic liquid crystals with negative dielectric anisotropy induced by step-voltage excitation. Jpn J Appl Phys 43:L1588–L1591 8. Sasaki A, Uchida T, Miyagami S (1986) Active addressing for flat panel display. In: Japan Display, 62 9. Bremer M, Tarumi K (1993) Gas phase molecular modeling of liquid crystals: electro-optical anisotropies. Adv Mater 5:842–848 10. Klasen M, Bremer M, Go¨tz A, Manabe A, Naemura S, Tarumi K (1998) Calculation of optical and dielectric anisotropy of nematic LCs. Jpn J Appl Phys 37: L945–L948 11. Kuwajima S, Manabe A (2000) Computing the rotational viscosity of nematic liquid crystals by an atomistic molecular dynamics simulation. Chem Phys Lett 332:105–109 12. Gooch CH, Tarry HA (1975) The optical properties of twisted nematic liquid crystal structures with twist angles  90 degrees. J Phys D Appl Phys 8:1575–1584 13. Gooch CH, Tarry HA (1974) Optical characteristics of twisted nematic liquid-crystal films. Electron Lett 10(1):2–4 14. Alt PM, Pleshko P (1974) Scanning limitations of liquid-crystal displays. IEEE Trans Electron Devices 21(2):146–155 15. Schadt M, Buchecker R, Villinger A (1990) Synergisms, structure-material relations and display performance of novel fluorinated alkenyl liquid crystals. Liq Cryst 7(4):519–536 16. Poetsch E, Reiffenrath V, Rieger B (1995) 1,2-Di-(4alkenylcyclohexyl)-ethenes - A new LC material for application in STN displays. Proceedings of the 24th Arbeitstagung Flu¨ssigkristalle, Freiburg, Germany, Poster P-1

17. Petrzilka M, Schadt M (1987) Liquid crystal components having an alkenyl chain. European Patent Application EP 0122389 B, Roche AG 18. Gray GW, Hird M, Lacey D, Toyne KJ, Reiffenrath V, Wa¨chtler A, Krause J, Finkenzeller U, Geelhaar T (1987) Fluorinated oligophenyls and their use in liquid crystal materials. European Patent Application EP 329 752 B1, Merck GmbH 19. Kirsch P, Reiffenrath V, Bremer M (1999) Flu¨ssigkristalle mit negativer dielektrischer Anisotropie. Synlett 4:389–396 20. Klasen M, Bremer M, Tarumi K (2000) New liquidcrystal materials for active matrix displays with negative dielectric anisotropy and low rotational viscosity. Jpn J Appl Phys 39:L1180–L1182 21. Bremer M, Lietzau L (2005) 1, 1, 6, 7-Tetrafluoroindanes: improved liquid crystals for LCD-TV application. New J Chem 29:72–74 22. Bremer M, Klasen-Memmer M, Pauluth D, Tarumi K (2006) Novel liquid crystal materials with negative dielectric anisotropy for TV application. J Soc Info Disp 14(6):517–521 23. Hanaoka K, Nakanishi Y, Inoue Y, Tanuma S, Koike Y, Okamoto K (2004) A new MVA-LCD by polymer sustained alignment technology. SID Symp Dig 04:1200–1202 24. Kim SG, Kim SM, Youn SK, Lee HK, Lee SH, Lee GD, Lyu J-J, Kim KH (2007) Stabilization of the liquid crystal director in the patterned vertical alignment mode though formation of pretilt angle by reactive mesogen. Appl Phys Lett 90:261910–261913 25. Chen TS, Huang CW, Hsieh CC, Huang BH, Jian JH, Chan TW, Tsao CH, Chen CW, Chen CY, Chiu CH, Pai CH, Jaw JH, Lin HC, Chiu CY, Su JJ, Norio S, Chang TJ, Liau WL, Lien A (2009) Advanced MVA III technology for high-quality LCD TVs. SID Symp Dig 09:776–779 26. Go¨tz A, Klasen-Memmer M, Bremer M, Taugerbeck A, Tarumi K, Pauluth D (2008) Advanced LC materials for fast switching display modes. In: Proceedings IDW 08, Niigata, Japan, pp 1527–1530 27. Go¨tz A, Taugerbeck A, Bernatz G, Tarumi K (2010) Advanced liquid-crystal materials for the polymersustained vertically aligned (PS-VA) mode. SID Digest 10 28. Kikuchi H, Yokota M, Hisakado Y, Yang H, Kajiyama T (2002) Polymer-stabilized liquid crystal blue phases. Nat Mater 1:64–69 29. Kikuchi H (2008) Liquid crystalline blue phases. Struct Bond 128:99–117 30. Meyer RB, Liebert L, Strzelecki L, Keller P (1975) Ferroelectric liquid crystals. J Phys Lett 36:69–71

1341

1342

7.1.3

Liquid Crystal Materials for Devices

31. Clark NA, Lagerwall ST (1980) Submicrosecond bistable electro-optic switching in liquid crystals. Appl Phys Lett 36(11):899–901 32. Kuczynski W, Stegemeyer H (1980) Ferroelectric properties of smectic C liquid crystals with induced helical structure. Chem Phys Lett 70(1):123–126 33. Geelhaar T (1988) Ferroelectric mixtures and their physico-chemical properties. Ferroelectrics 85:329–349 34. Hird M, Goodby JW, Toyne KJ (2001) The development of materials, mixtures and gels for ferroelectric displays. Mol Cryst Liq Cryst 360:1–15 35. Simon-Hettich B, Broschard TH, Becker W, Takeuchi H, Saito H, Ohnishi H, Takatsu H, Naemura S, Kobayashi K (2001) Ecotoxicological properties of liquid-crystal compounds. J Soc Info Disp 9(4):307–312 36. Gray GW, Harrison KJ, Nash JA (1973) New family of nematic liquid crystals for displays. Electron Lett 9:130–131 37. Eidenschink R, Erdmann D, Krause J, Pohl L (1977) Substituted phenylcyclohexanes – A new class of liquid-crystalline compounds. Angew Chem Int Ed Engl 16(2):100 38. Boller A, Scherrer H (1974) Flu¨ssigkristalline mischungen. German Patent DE-2 415 929, Roche AG 39. Kelly SM (1984) The synthesis and transitiontemperatures of benzoate ester derivatives of 2fluoro-4-hydroxybenzonitriles and 3-fluoro-4hydroxybenzo-nitriles. Helv Chim Acta 67:1572–1579 40. Kelly SM, Schadt M (1984) The synthesis and transition-temperatures of ester derivatives of 2-fluoro4-hydroxybenzonitriles and 3-fluoro-4hydroxybenzo-nitriles also incorporating aliphatic ring-systems. Helv Chim Acta 67:1580–1587

41. Eidenschink R, Ro¨mer M, Weber G, Gray GW, Toyne KJ (1983) Verfahren zur Herstellung von acrylamid-polymerisaten. German Patent DE-3 321 373, Merck GmbH 42. Eidenschink R (1983) Low viscous compounds of highly nematic character. Mol Cryst Liq Cryst 94:119–125 43. Eidenschink R, Krause J, Pohl L (1978) Cyclohexanderivate. German Patent DE-2 636 684, Merck GmbH 44. Osman MA, Revesz L (1980) trans, trans-Cyclohexyl Cyclohexanoates – a new class of aliphatic liquid crystals. Mol Cryst Liq Cryst 56:157–161 45. Steinstra¨sser R (1972) Nematische p, p’disubstituierte Benzoesa¨urephenylester und niedrig schmelzende eutektische Gemische. Z Naturforsch B27:774–779 46. Eidenschink R, Krause J, Weber G (1983) Bicyclohexylderivate. German Patent DE 3 206 269, Merck GmbH 47. Eidenschink R, Pohl L, Ro¨mer M, del Pino F (1979) Partiell hydrierte Oligo-1,4-phenylene, diese enthaltende dielektrika und elektrooptisches anzeigeelement. German Patent DE- 2 948 836, Merck GmbH 48. De Jeu WH, Eidenschink R (1983) Smectic behaviour and molecular structure: on the mesomorphism of a series of hydrocarbons. J Chem Phys 78:4637–4640 49. Malthete J, Canceill J, Gabard J, Jacques J (1981) Recherches sur les substances mesogenes - VIII: preparation et proprietes mesomorphes de series isometriques. Tetrahedron 37(16):2815–2821 50. Scheuble B, Weber G, Wa¨chtler AEF, Reiffenrath V, Krause J, Hittich R (1988) German Patent DE 3 711 306, Merck GmbH

Further Reading Hirschmann H, Reiffenrath V (1998) Applications: TN, STN displays. In: Handbook of liquid crystals, vol 2A. Wiley, Weinham, pp 199–229 Kirsch P, Bremer M (2000) Nematic liquid crystals for active matrix displays: molecular design and synthesis. Angew Chem Int Ed Engl 39:4216–4235 Pauluth D, Tarumi K (2004) Advanced liquid crystals for television. J Mater Chem 14:1219–1227 Pauluth D, Tarumi K (2005) Optimization of liquid crystals for television. J Soc Info Disp 13(8):693–702 Plach HJ, Weber G, Rieger B (1990) Liquid crystal mixtures for active matrix displays using new

terminally fluorinated compounds. SID Symp Dig 90:91–94 Tarumi K, Bremer M, Geelhaar T (1997) Recent liquid crystal material development for active matrix displays. Annu Rev Mater Sci 27:423–441 Tarumi K, Bremer M, Schuler B (1996) Development of new liquid crystal materials for TFT LCDs. IEICE Trans Electron E79-C:1035–1039 Tarumi K, Heckmeier M, Klasen-Memmer M (2002) Advanced liquid crystal materials for TFT monitor and TV applications. J Soc Info Disp 10(2):127–132

7.1.4 Physical Properties of Nematic Liquid Crystals Carl V. Brown 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1344

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7

Bulk Properties of Nematic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345 Ordering in Nematic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345 Refractive Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1346 Dielectric Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1348 Elastic Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1351 Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1352 Flexoelectricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1353 Temperature Dependence of the Bulk Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1353

3 3.1 3.2 3.3 3.4

Electro-optical Switching in the Planar Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1355 The Fre´edericksz Transition in a Planar Nematic Liquid Crystal Layer . . . . . . . . . . . . 1355 Capacitance as a Function of Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356 Optical Transmission as a Function of Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1357 Dynamic Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1358

4

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1359

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_7.1.4, # Springer-Verlag Berlin Heidelberg 2012

1344

7.1.4

Physical Properties of Nematic Liquid Crystals

Abstract: The physical parameters of nematic liquid crystal materials that are relevant to the operation and optimization of nematic display modes will be presented in this entry. These parameters include the anisotropic refractive indices, dielectric permittivities, elastic constants and viscosities. The a.c. voltage Fre´edericksz transition in a planar nematic layer is presented as a case study to illustrate the roˆles that these different physical parameters perform in the operation of an electro-optical device. List of Abbreviations: 5CB, 4-Pentyl-40 -Cyanobiphenyl; TNI, Nematic to Isotropic Phase Transition Temperature; Dn, Birefringence; De, Dielectric Anisotropy

1

Introduction

In this entry, calamitic (rod-shaped) nematic liquid crystals will be considered. There are many excellent reviews and books at different levels that describe the physical properties of nematic liquid crystal materials and explain the underlying physical and chemical origins of these properties [1–6] (please also see the ‘‘Further Reading’’ list). Reference [7] reviews the values of these parameters in a range of nematic liquid crystal materials and the trends in these values as a function of temperature (and wavelength dispersion in the case of refractive index). The current entry is intended to be helpful for the display engineer who is looking for a concise introductory overview of the scientific background that will be relevant for the specific types of nematic liquid crystal displays discussed in this book. A model of one of the rod-shaped molecules is shown in > Fig. 1. This idealized structure is based on a cyanobiphenyl molecule; such molecules were constituents of the first commercially available room temperature nematic liquid crystal mixtures [8, 9]. There is a fairly rigid core region within which mobile charge can respond to an applied electric field. At one end of the core there is a flexible terminal group, for example, a hydrocarbon chain. At the other end of the core there is a terminal permanent dipole group. Now consider applying an external electric field to the molecule as shown in > Fig. 2. The orientation of the molecule will be kept fixed. The free charge in the core of the molecule responds to the electric field with the positive charge moving in the direction of the electric field and negative charge moving in the opposite direction. The core of the molecule has therefore become polarized. This polarization, unlike the fixed permanent terminal dipole, is induced by the applied electric field. The electric dipole moment is defined as p = QL, where Q is the magnitude of the charge (Q = +Q = |Q|) and L is the vector displacement from Q to +Q where L = |L| is the distance between the separated positive and negative charges [10]. This illustrates that the rod shape of the polarizable core molecule gives rise to a larger induced dipole moment p|| parallel to the molecular axis and a smaller dipole moment p⊥ perpendicular to the molecular axis. Corresponding polarizabilities, a|| and a⊥, are the induced dipole moments per unit electric field parallel and perpendicular to the molecular axis, respectively. Rigid core Terminal permanent dipole

Terminal flexible chain

. Fig. 1 An idealized model of a molecule of a material that exhibits a nematic liquid crystal phase

7.1.4

Physical Properties of Nematic Liquid Crystals

Electric field

Electric field

Q p Q

a

Q

Q

p

b

. Fig. 2 An external electric field is applied either (a) perpendicular to or (b) parallel to the core of the idealized molecule from > Fig. 1. This results in the polarization of the free charge in the core

n

a

n

S=1

b

θ

S = 0.6

c

S=0

. Fig. 3 An ensemble of rod-shaped molecules with no positional ordering showing: (a) perfect orientational ordering; (b) a high degree of orientational order for example as found in a nematic liquid crystal; (c) with no order for example as found in an isotropic liquid

This is an intuitive approach to discussing molecular polarizability, the induced dipole moment per unit field, but it does illustrate how polarizability anisotropy can arise in a single molecule. In the cyanobiphenyls, two connected phenyl rings with delocalized electrons provide the mobile charge and the asymmetry in the core of the molecule. Clearly, in a core with different chemical ring structures (e.g., cyclohexane) the charge can be less mobile giving lower polarizability.

2

Bulk Properties of Nematic Liquid Crystals

2.1

Ordering in Nematic Liquid Crystals

In nematic liquid crystals an anisotropic response to an externally applied electric (or magnetic) field can arise from the shape and chemical properties of the individual molecules, as discussed in > Sect. 1 above. In the nematic liquid crystal phase the ensemble of molecules takes up an ordered arrangement in which the molecules all tend to point in a particular direction. This direction is called the n-director, which is illustrated in > Fig. 3b. The nematic liquid crystal

1345

1346

7.1.4

Physical Properties of Nematic Liquid Crystals

molecules have thermal energy and so are mobile, and their orientation fluctuates as a function of time. To find the direction of the n-director in a given region of liquid crystal material an average of the molecular orientations in > Fig. 3b is taken. This average can be taken by looking at hundreds or thousands of molecules in a region at a snapshot in time. Alternatively, an average can be taken over time of the orientation of a particular molecule. > Figure 3b illustrates the molecular alignment in a nematic liquid crystal at an instant in time with the average molecular alignment direction denoted by the n-director. The molecules are shown in this case by filled ellipses. On each molecule the end at which the terminal dipole is attached is not shown. It can be assumed that the end with the dipole is equally likely, on average, to point along the n-director as it is to point the opposite direction to the n-director. This means that the bulk liquid crystal as an ensemble of molecules has no net electric polarization in the absence of an applied electric field and so the properties of an ensemble of molecules in the nematic phase are the same parallel to the n-director as they are antiparallel to the n-director. There is n  n symmetry: the arrow indicating the direction of the n-director in > Fig. 3a, b points upward, but it could equally well point downward. It is also assumed that each molecule can freely rotate around its long axis, that is, its rotation about this axis is unhindered. In > Fig. 3a all the molecules point in the direction of the n-director, and this is indicative of perfect orientational order. In > Fig. 3c the situation shown is, when any orientation angle y is equally probable, there is no orientational order and this is indicative of an isotropic liquid. The ordering in a nematic liquid crystal in > Fig. 3b lies between these two extremes. The nematic order parameter S, defined in > Eq. 1, is used as a quantitative measure of the amount of order [5]. R  3 f ðyÞcos2 ðyÞ dO 1 1
2 R ð1Þ  S ¼ 3cos y  1 ¼ 2 2 2 f ðyÞ dO The distribution function f(y, f) dO expresses the probability of finding a molecule pointing in a direction within the small solid angle range dO = sin(y)dydf centered around the direction described by the zenithal and azimuthal spherical polar coordinate components y and f. The zenithal angle y is defined relative to the nematic n-director. It is assumed that all azimuthal angles f are equally likely – imagine rotating each molecule’s axis about the n-director to form a cone with half angle y. As shown in > Fig. 1, S = 1 corresponds to perfect order, S = 0 to having no preferred molecular direction, and in a nematic liquid crystal the value of S lies between these extremes (see also > Chap. 7.1.1).

2.2

Refractive Index

At the frequencies of visible light (around 1014 Hz) the value of the refractive index of a nematic liquid crystal material is typically determined by the polarization of electrons in the molecules and from the polarization of the electrons in the constituent atoms. The refractive index is a bulk property and so both the ordering of the molecules and the polarizabilities of the individual molecules contribute to the observed optical anisotropy (or birefringence). In > Fig. 4a the polarizabilities a|| and a⊥ are shown for one of the molecules in an ensemble of rod-shaped molecules. For the idealized molecule from > Fig. 2, it is assumed that the induced dipole moment is bigger parallel to the molecular axis than it is perpendicular to the molecular axis and so there is an anisotropy in the molecular polarizability with a|| > a⊥.

7.1.4

Physical Properties of Nematic Liquid Crystals

α||

n ||

n

n α⊥

n⊥

a

z x

b

y

. Fig. 4 (a) An ensemble of rod-shaped molecules showing the molecular polarizabilities a|| and a⊥ for one of the molecules. (b) The average molecular orientation of the ensemble of molecules is indicated by an open ellipse where the long axis coincides with the n-director

In > Fig. 4b the open ellipse indicates the average molecular direction, the n-director, of an ensemble of the molecules in a particular region of space. Consider polarized visible light incident on this region of space. If the electric field vector of the electromagnetic wave is in the vertical direction (parallel to the z-axis) then the light wave will experience the refractive index of the nematic liquid crystal material parallel to the n-director, n||. If, instead, the electric field vector is in the horizontal direction (parallel to either the x-axis or the y-axis) then the light wave will experience the refractive index of the nematic liquid crystal material perpendicular to the n-director, n⊥. The optic axis is parallel to the n-director and so the extraordinary refractive index ne is equal to n||, the ordinary refractive index no is equal to n⊥, and the birefringence of this optically uniaxial medium is given by > Eq. 2. Dn ¼ ne  no ¼ nk  n?

ð2Þ

Now consider the influence of the ordering of the molecules. As stated above the electronic polarizability plays a dominant role in determining the value of the refractive index. If the molecules were perfectly ordered with all the molecules aligned parallel to the n-director, as depicted for S = 1 in > Fig. 3a, then just a|| would contribute to n||, and just a⊥ would contribute to n⊥. If there was no molecular order and the molecules were randomly oriented, as depicted for S = 0 in > Fig. 3c, then the polarizability of the medium would be the same in all directions and equal to the average  a ¼ 1=3ðax þ ay þ az Þ ¼ 1=3ðak þ 2a? Þ. The refractive indices would then be equal, n|| = n⊥, and the material would be optically isotropic with Dn = 0. This illustrates that for the case where there is no molecular order a material with anisotropic molecules can still possess isotropic physical properties. The nematic liquid crystal material shows partial orientational ordering of the molecules, as depicted in > Fig. 3b, and so there are contributions from both a|| and a⊥ to n||, and from both a|| and a⊥ to n⊥. The explicit dependences of these contributions on the S order parameter, combined with the Lorenz–Lorentz equation [11], lead to the Vuks expression [12] in > Eq. 3. SDa njj  n? ¼ 2   1 n a 2

2

ð3Þ

In the Haller extrapolation technique [13] it is assumed that S order parameter has a temperature dependence given by the functional form in > Eq. 4 and that the polarizabilities vary weakly with the temperature.

1347

1348

7.1.4

Physical Properties of Nematic Liquid Crystals

S¼

  T b 1 TNI

ð4Þ

The value of the exponent b and the ratio Da= a can be found from the gradient and intercept respectively of a plot of Log½ðn2jj  n2? Þ=ð n2  1Þ against Logð1  T =TNI Þ. > Equation 4 predicts that S = 0 at the nematic to isotropic transition temperature TNI but from measurements in nematic materials the value is typically in the range S = 0.3–0.4 at the transition, which is weakly first order [1]. The extrapolation is adequate for materials with a wide nematic range away from the transition [14]. Commercial nematic liquid crystal mixtures have physical properties tailored for particular display applications, and mixtures are typically available with birefringence in the range Dn = 0.06–0.26 [15]. The values for the alkylcyanobiphenyls 5CB and 8CB, with their highly polarizable and anisotropic core structure, are toward the upper end of this range at room temperature [16, 17]. Different ring systems in the core of the molecules have an important roˆle in determining the birefringence values. For example, the alkylcyanobiphenyl material 5CB has a birefringence of 0.194 [16, 17] but the corresponding alkylcyanophenylcyclohexane material, with the same alkyl chain length, has a significantly lower birefringence of 0.125 (both at reduced temperature of 0.95 and wavelength of 589 nm) [18]. A discussion of the effect of the core structure, including a comparison with an alkylbicyclohexane material with very low birefringence, can be found in Chap. 3.3 of reference [6]. A change in the refractive index with wavelength, that is, dispersion, occurs in nematic materials that have strong electronic absorptions. Taking the nematic mesogen 5CB again as an example, at a temperature of 25.2 C the principal refractive indices are n|| = 1.59 and n⊥ = 1.91 at a wavelength of 400 nm, and n|| = 1.53 and n⊥ = 1.72 at a wavelength of 700 nm [19].

2.3

Dielectric Constants

In the absence of a magnetic response the electric permittivity e for a material is given by the square of its refractive index. Thus, the permittivity e (also called the dielectric constant) is also determined by the induced polarization of the material when an electric field is applied. The refractive index is the more relevant parameter at optical frequencies around 1014 Hz since it can be used to calculate the speed and wavelength of light in the material and thus the optical path length introduced by a layer of the material. Nematic liquid crystal displays are voltage addressed with a.c. waveforms with frequencies generally of the order kHz, and at such frequencies the permittivity is the most appropriate parameter to use. In liquid crystal displays the permittivity is important for two reasons: (1) its role in electro-optical switching, and (2) since its value determines the (variable) capacitance of a pixel in the display. In a nematic liquid crystal material the permittivity e|| parallel to the n-director is generally different from the permittivity e⊥ perpendicular to the n-director; thus the dielectric anisotropy De = e|| e⊥ is nonzero. In a material with molecules that have permanent dipoles (e.g., as in > Fig. 1) there is a contribution to the polarizability of the material from the field-induced partial reorientation of the molecular dipoles in an applied electric field. As an example, in the isotropic liquid water the molecules have a strong permanent dipole moment given by p = |p| = 6.0  1030 cm, which leads to a very high isotropic dielectric constant of e = 78.5 at 20 C [10]. The Langevin theory of orientational polarization describes how the aligning force on the molecular dipoles

Physical Properties of Nematic Liquid Crystals

7.1.4

from the applied electric field is frustrated by the thermal energy and the associated thermal motion of the molecules. At typical temperatures T and feasible electric field strengths, only small rotation angles occur, which gives rise to the bulk polarization P given by > Eq. 5 [10, 20], which contributes to the permittivity. P¼

Np2 ELoc 3kB T

ð5Þ

In > Eq. 5, N is the number of molecular dipoles per unit volume, ELoc is the electric field experienced at each of the dipoles, and kB is the Boltzmann constant. Lorentz calculated that the local field was related by > Eq. 6 to the externally applied electric field E and the bulk polarization P by considering a permanent dipole at the center of a polarizable spherical cavity [10, 20]. P ELoc ¼ E þ ð6Þ 3eO For the idealized rod-shaped molecules shown in > Fig. 1, the ‘‘Langevin’’ contribution to the polarization is stronger when the electric field is parallel to the n-director than when the electric field is applied perpendicular to the n-director. This is because the permanent dipole is a terminal group at the end of the molecule and so the component of the permanent dipole moment along the molecule (and the n-director) is much greater than perpendicular to it. This can lead to a material showing a large positive dielectric anisotropy, De = e||  e⊥. For the rigid core shown in > Fig. 2, both the electronic molecular polarization and the Langevin contribution to the polarizability of the material give an enhancement of the value of e||. The Maier and Meier equations for the permittivity components e|| and e⊥ of the nematic phase are given in > Eqs. 7 and > 8, respectively [7, 21, 22].   NFh 2 Fp2   1  ð1  3cos2 bÞS a þ DaS þ eO 3 3kB T 

 NFh 1 Fp2 1  a  DaS þ 1 þ ð1  3cos2 bÞS ðe?  1Þ ¼ eO 3 2 3kB T ðejj  1Þ ¼

ð7Þ ð8Þ

The angle b is the angle between the direction of the permanent molecular dipole p and the molecular axis, and parameters F and h account for the anisotropic interactions that lead to the local electric field. The degree of order of the molecules, expressed by the value of the order parameter S, has a similar roˆle in determining the amount of directional anisotropy arising from the Langevin type contribution (terms in p2) as it does for the electronic molecular polarizability (terms in Da), which was discussed in > Sect. 2.2 above. In the limit of no dipole, p = 0, the Maier and Meier equations predict the values of the refractive indices at optical frequencies, see > Eq. 3. For materials with a high component of the permanent dipole moment parallel to the molecular axis, accompanied by a high dielectric anisotropy, the Maier and Meier equations lead to the relationship shown in > Eq. 9 between S, the average permittivity, the dielectric anisotropy, and the average refractive index [23]. S¼

De 2 Þ 3ðe  n

ð9Þ

The Langevin contribution to the polarization and the permittivity is not present at optical frequencies but makes a significant contribution to the permittivities of typical nematic liquid

1349

7.1.4

Physical Properties of Nematic Liquid Crystals

crystals up to frequencies of around 105 Hz for e|| and 107 Hz for e⊥ [22, 24]. This is because the friction, that is, viscous drag, between the molecules limits the speed with which the molecules can rotate. For an electric field applied parallel to the n-director (e||) when the frequency is increased above 105 Hz the degree of molecular rotation about the short molecular axis that can be achieved in each half period before the change in polarity of the a.c. electric field is not sufficient to produce a significant change in the observed polarization of the bulk fluid. The frequency is often higher when the electric field is applied perpendicular to the n-director (e⊥) since there is generally less resistance or hindrance to molecules rotating around their long axis [7, 22]. The energy of an anisotropic dielectric in an electric field is lowest when the largest component of the permittivity tensor is aligned along the direction of the applied field. In > Fig. 5 an intuitive description of the action of an applied electric field on a region of nematic liquid crystal is shown. In each case, the open ellipse indicates a polarizable region of liquid crystal material comprising of many molecules and the average molecular orientation angle, the n-director, is along the long axis of the ellipse. In > Fig. 5a, b a material with positive dielectric anisotropy is illustrated for which the dielectric constant parallel to the n-director, e||, is the largest. The liquid crystal responds to the applied electric field by polarizing, predominantly parallel to the n-director. The separated charge then exerts a torque that acts to rotate the n-director to reorient along the direction of the electric field, the lowest energy configuration. This torque, and the resultant rotation direction, is in the same direction regardless of the polarity of the electric field as shown in > Fig. 5a, b.

n

Electric field

Electric field

n

a

b

Reorients

Reorients

n

n

Reorients

Electric field

Reorients

Electric field

1350

c

z

d

x

y

. Fig. 5 The electric field-induced reorientation of the n-director: (a) and (b) in a nematic liquid crystal material with positive dielectric anisotropy (D« > 0) and; (c) and (d) in a nematic liquid crystal material with negative dielectric anisotropy (D« < 0)

Physical Properties of Nematic Liquid Crystals

7.1.4

> Figure 5c, d illustrate the situation for a material with negative dielectric anisotropy for which the dielectric constant perpendicular to the n-director, e⊥, is the largest. Here the polarization in response to the applied electric field is predominantly perpendicular to the n-director and the separated charge then exerts a torque that acts to rotate the n-director to reorient to being perpendicular to the direction of the electric field. The polarity independence of the reorientation effect is consistent with the expression for the energy in a dielectric being of the form ½DE where jEj is the magnitude of the electric field. Note that this is the Gibbs electric energy of the system that is minimized assuming an electric field generated by a constant externally applied voltage. This leads to an electric energy of the form ½eO(e⊥ + Decos2y)E2, which is a minimum when y = 0 (n-director parallel to E) for De > 0, and is a minimum when y = 90 (n-director perpendicular to E) for De < 0. Commercial nematic liquid crystal mixtures are typically available with dielectric anisotropies in the range De = 6 to 30 [15], but nematic liquid crystal mixtures have been developed with values in the range De = 10 to 50 [7] at room temperature.

2.4

Elastic Constants

When a nematic liquid crystal is in equilibrium and is not subject to any external forces the minimum energy configuration is where the n-director aligns parallel in neighboring regions of the material. In this undistorted state, depicted in > Fig. 6a, there is no spatial variation in the orientation of the n-director. A distortion, in which the orientation of the n-director changes with distance, is opposed by elastic torques and has an associated elastic energy cost associated with it. In nematic continuum theory any type of deformation in a nematic liquid crystal is described in terms of the contributions from three principle spatial distortions of the n-director [25, 26]. Representations of ‘‘pure’’ splay, twist, and bend distortions are depicted in > Fig. 6b–d, respectively. In > Fig. 6 the orientation of the n-director in a particular region of the material is indicated by the long axes of the open ellipses. A mark is placed at one end of the ellipses in > Fig. 6c only to emphasize that the twist distortion involves the n-director rotating out of the page. Because of the n  n symmetry, the structure repeats itself when the n-director has rotated through 180 , which is half of the single period of rotation shown in > Fig. 6c.

a

c

b

d

. Fig. 6 The spatial variation of the n-director is shown by the open ellipses (a) in undistorted nematic liquid crystal, (b) when there is a splay distortion, (c) when there is a twist distortion, and (d) when there is a bend distortion

1351

1352

7.1.4

Physical Properties of Nematic Liquid Crystals

Nematic continuum theory describes distortions for which spatial variation of the n-director changes smoothly and continuously over a length scale that is much greater than the molecular length. It is assumed that these long-range distortions have no affect on the value of the nematic S order parameter. The bulk elastic deformation energy in nematic continuum theory, WB, is given by the Frank-Oseen [25, 26] free energy in > Eq. 10 (see also > Chap. 7.2.3). 2WB ¼ K11 ðr  nÞ2 þ K22 ðn  r  nÞ2 þ K33 ðn  r  nÞ2

ð10Þ

In the theory the elastic energy associated with each of the splay, twist, and bend distortions are quantified by the K11, K22, and K33 elastic constants, respectively. A survey of the literature on a range of single component and binary mixture nematic liquid crystal materials in Chap. 11.3 of reference [7] found that K11 was between 5.0 and 9.0  1012 N, K22 was between 2.8 and 5.3  1012 N, and K33 was between 4.7 and 18.5  1012 N. Measurements performed by the current author and coworkers on five commercially available liquid crystal mixtures found that K11 was between 7.3 and 12.7  1012 N, the ratio K22/K11 was between 0.4 and 1.0, and the ratio K33/K11 was between 0.8 and 2.1 [27].

2.5

Viscosity

The viscosity is the internal friction in a fluid and governs the rate of flow of the fluid. One method for measuring the shear bulk viscosity of a liquid involves measuring the force required to slide a moving plate across a stationary plate with the fluid contained in the space between the plates [28]. The viscosity is defined by the ratio of the shear stress to the shear strain, where the shear stress is defined as the force Fx per unit cross sectional area and the shear strain is defined as the velocity vx divided by the size of the spacing between the plates. The dynamic properties of nematic liquid crystals are anisotropic, and the shear (or Miesowicz) viscosities Z1, Z2, and Z3 measured using the geometries shown in > Fig. 7a–c, respectively, will in general take very different values [29, 30]. In a liquid crystal display, the fluid is held within a gap between two fixed substrates and so there is no bulk fluid flow. It is the internal microscopic motion of the fluid that is important for the electro-optic switching, and in display engineering applications an important viscosity is g1. This is the viscosity associated with rotational motion of the nematic n-director and Plate velocity vx

Plate velocity vx

Fx

n Fx

a

Fx

b

Plate velocity vx

Fx

n

Fx

Fx

n

c

. Fig. 7 The moving plate geometry for measuring the shear viscosity. The upper plate moves in the x-direction at a velocity vx in response to a shear force Fx, and the lower plate is stationary. The alignment of the nematic n-director is homeotropic and parallel to the z-direction in (a), planar and parallel to the x-direction in (b), and planar and parallel to the y-direction in (c) [29, 30]

Physical Properties of Nematic Liquid Crystals

7.1.4

it plays a role in limiting the angular speed of reorientation of the nematic n-director. To fully describe the dynamic motion in a nematic liquid crystal material requires six viscosities, the six Leslie coefficients from dynamic nematic continuum theory [31, 32]. The value of g1 is directly related to two of the six Leslie coefficients and the shear viscosities Z1, Z2, and Z3 described above are given by combinations of these viscosities. Commercial nematic liquid crystal mixtures are typically available with rotational viscosities in the range g1 = 0.05–0.45 Pa s [7, 15].

2.6

Flexoelectricity

Meyer [33] proposed that an ensemble of nematic molecules that possess a shape polarity (e.g., are wedge shaped) and that also possess a permanent electric dipole moment will also show a flexoelectric effect. In > Sect. 2.1 it was discussed that for an undistorted nematic liquid crystal consisting of molecules that possess a terminal permanent dipole moment it can be assumed that the end with the dipole is equally likely, on average, to point along the n-director as it is to point the opposite direction to the n-director. With the dipoles as likely to point parallel to n as they are antiparallel to the n-director the material then possesses no net bulk polarization in the absence of externally applied electric or magnetic fields. For wedge-shaped molecules the presence of a distortion, that is, when there are gradients in the n-director field, creates a bias so that more of the molecular dipoles point in one direction leading to a net polarization Pf. In these nematic materials a splay or bend deformation will induce a polarization Pf in the material given by > Eq. 11 [1, 33] where e11 and e33 are the flexoelectric coefficients corresponding to splay and bend distortions, respectively. Pf ¼ e11 nðr  nÞ þ e33 ðr  nÞ  n

ð11Þ

Flexoelectricity has been observed and quantified for a number of different nematic and cholesteric materials including for materials that are polar and for those that are nonpolar [7, 34–38]. The coupling between the flexoelectric polarization and an applied electric field can be used in switching. This provides the switching mechanism in a tight-pitch cholesteric display mode based on the flexoelectroptic effect [39–41] and it has also been suggested that flexoelectricity plays a key roˆle in switching in certain zenithal bistable nematic display modes [42–47] (see also > Chaps. 7.3.5 and > 7.3.6).

2.7

Temperature Dependence of the Bulk Properties

The temperature variations of a number of physical parameters for the nematic material 5CB are indicated in > Fig. 8. The nematic to isotropic phase transition temperature (TNI) is taken to be 35.2 C. As discussed in > Sect. 2.2, the temperature dependence of the S order parameter, shown in > Fig. 8a [48] can be approximated by > Eq. 4 at temperatures somewhat below TNI. The fitting > Eq. 12 has been used in the literature (e.g., V.V. Belyaev, Chap. 8.4 in reference [7]) in which T∗ = 36.4 C. The parameters SO and T∗, which appear in > Eq. 12 but not in > Eq. 4, account for the fact that S = 0.4 at TNI.   T b S ¼ SO 1   T

ð12Þ

1353

7.1.4 a

0 15

S

0.4 0.2

0.20 Δn 0.15

10

Δε

0.10

5

0.05

b 100

0.00 K33

8 K11

6 K22

4 2 0

c

Optical birefringence

Dielectric anisotropy

S Order parameter

Physical Properties of Nematic Liquid Crystals

0.6

Elastic constant (pN)

1354

20

25

30

35

Temperature (°C)

. Fig. 8 The variation with temperature of (a) the S order parameter [7, 48], (b) the dielectric anisotropy D« [49] and the optical birefringence Dn at a wavelength of 589 nm [50], and (c) the K11, K22, and K33 elastic constants [51, 52], for the material 5CB [6, 7]

The dielectric anisotropy De (left hand y-axis) [49] and the optical birefringence Dn at a wavelength of 589 nm (right hand y-axis) [50] are shown in > Fig. 8b. At temperatures above TNI the nematic material is in its isotropic phase and so both De and Dn are zero. The principle refractive indices n|| and n⊥ depend on the S order parameter and on the molecular polarizabilities. The principle dielectric permittivities e|| and e⊥ also depend on the S order parameter and the molecular polarizabilities. Immediately below the transition temperature TNI the values of both of these parameters show an abrupt increase that is consistent with the weakly first order transition in the S order parameter. Both De and Dn show an increase in value as the temperature is decreased. This corresponds with the increase in S at lower temperatures. The ‘‘Langevin’’ contribution term in > Eqs. 7 and > 8 predicts a reciprocal temperature dependence of the dielectric anisotropy De. The values of K11 and K33 elastic constants from electric and magnetic Fre´edericksz transition measurements [51] (see > Sect. 3.1) and the K22 elastic constants from dynamic light scattering measurements [52] are shown in > Fig. 8c. For 5CB it is found that the magnitudes of the elastic constant are in the order K33 (largest) > K11 > K22 (smallest). The magnitudes of the elastic constants all show an increase as the temperature is decreased. It has been predicted using mean field theory that the elastic constants scale with the square of the

7.1.4

Physical Properties of Nematic Liquid Crystals

S order parameter [1, 2, 53, 54] and this dependence is followed for some ‘‘simple’’ nematic materials, for example, single component materials [55]. The rotational viscosity g1 generally shows an exponential behavior on the reciprocal of the temperature, which for the material 5CB can be written in the simplified form of > Eq. 13 [7, 56] where A and B are experimentally determined constants.   B ð13Þ g1 ¼ AS exp kB T

3

Electro-optical Switching in the Planar Geometry

3.1

The Fre´edericksz Transition in a Planar Nematic Liquid Crystal Layer

> Figure 9 shows a planar (i.e., homogeneous) layer of nematic liquid crystal confined between two conducting plates in the x–y plane (see also > Chap. 7.2.2 for details of planar surface alignment). The thickness of the layer is given by d and the bounding plates are at z = 0 and at z = d. The open ellipses show the orientation of the nematic n-director, n(z), at different positions z within the nematic layer. The n-director can be assumed to be fixed so that it always remains parallel to the boundaries in the regions close to the boundaries, a situation referred to as infinite (i.e., infinitely strong) surface anchoring. These boundary conditions can be expressed as y(0) = 0 and y(d) = 0 where y(z) is the angle between the director and the y-axis at a particular position z in the nematic layer, as illustrated in > Fig. 9b. An a.c. voltage is applied across the nematic layer via the conducting plates so that the layer is subject to an electric field in the z-direction. When the voltage is below a critical voltage VC, the Fre´edericksz threshold voltage [57], the layer remains in the undistorted configuration as shown in > Fig. 9a. When a voltage is applied that is above the threshold voltage the nematic n-director begins to distort within the layer, as shown in > Fig. 9b, c. The response of the nematic material to the applied voltage depends on its dielectric anisotropy. The material in this example has positive dielectric anisotropy, De > 0,

V < Vc

d

V > Vc

V

θ

V >> Vc

V

V

z x

y

a

b

c

. Fig. 9 The orientation of the n-director n(z) across a planar layer of nematic liquid crystal in response to an applied a.c. voltage. The voltage VC is the Freedericksz threshold voltage and the nematic liquid crystal has a positive dielectric anisotropy, D« > 0

1355

1356

7.1.4

Physical Properties of Nematic Liquid Crystals

and so an electric field in the z-direction will exert a torque that acts to reorient the nematic n-director so that it is parallel with the electric field. This dielectric torque is resisted by an elastic torque since any distortion in the nematic layer has an elastic energy associated with it. The n-director at the boundaries has a fixed orientation in the y-direction and cannot rotate. As a result of the elasticity of the medium the director orientation varies smoothly from a maximum value ym = y(d/2) in the center of the layer to y = 0 at the boundaries. When the voltage is further increased above the threshold value, the dielectric torque increases in relation to the elastic torque and the n-director switches toward being vertical for a greater proportion of the width of the layer. The n-director is still fixed to lie planar at the boundaries and so the regions of distortion become more concentrated toward the cell boundaries as depicted in > Fig. 9c. A typical plot of the variation of the maximum tilt angle ym as a function of the r.m.s. magnitude of the applied a.c. voltage is shown in > Fig. 10a. There is no change in the value of ym when the voltage is below the voltage VC. When the voltage is raised above the critical voltage there is a steep rise in maximum tilt angle with an onset that occurs with an abrupt threshold at V = VC. The abrupt nature of the threshold results from the symmetry of the system. Before the voltage is applied the orientation angle of the n-director is exactly y = 0 throughout the layer. When the director starts to reorient there is a bifurcation. The director can reorient in either direction: the tilt of the director in the center of the cell could equally rotate in the positive (+ym) direction as in the negative (ym) direction. The n-director profile just above the threshold voltage was illustrated in > Fig. 9b. For an applied voltage that is only just greater than the threshold voltage VC, the distortion of the director is purely splay in nature and so it is the splay elastic constant that appears in the equation for the Fre´edericksz threshold voltage, > Eq. 14 below [1, 2, 57, 58]. sffiffiffiffiffiffiffiffiffiffiffiffi p2 K11 ð14Þ VC ¼ eO De The other physical parameter that appears in > Eq. 14 is the dielectric anisotropy De. It is intuitively clear that if the value of De is increased then the dielectric reorienting torque at a given voltage will be higher and so the threshold voltage will be lower. Typical threshold values for commercial nematic liquid crystal materials are of the order of 1 V.

3.2

Capacitance as a Function of Voltage

The permittivity, and thus the capacitance, of a planar nematic layer changes as a function of the voltage applied across the layer as illustrated in > Fig. 10b. The geometry is equivalent to that of a parallel plate capacitor and so a suitable integral of the reciprocal of the permittivity as a function of the z-direction determines the capacitance. For an applied voltage below the threshold voltage VC the permittivity of the layer is given by e⊥ because the n-director at all z-positions in the layer is aligned along the y-direction and therefore the permittivity perpendicular to the n-director is parallel with the z-direction. Above the threshold voltage, the n-director central region of the cell starts to abruptly reorient toward the z-direction and this is accompanied by a sharp increase in the permittivity since for De > 0 the component of the permittivity parallel to the n-director e|| is larger than e⊥. As the voltage is further raised an increasing proportion of the nematic layer becomes aligned with the director parallel to the n-direction. In the case of infinite planar surface

Physical Properties of Nematic Liquid Crystals

7.1.4

90° Mid-layer tilt angle θm 0°

a

Voltage

ε|| Permittivity of the layer ε⊥

b

Voltage 1 Optical transmission

c

To

0 VC

Voltage

. Fig. 10 The variation of (a) tilt of the n-director in the center of the layer, (b) the permittivity of the layer, and (c) the optical transmission through the layer between crossed polarizers oriented as shown in > Fig. 11 for a planar nematic liquid crystal layer as a function of the voltage applied across the layer

anchoring there is always a region of distortion near to the surfaces and so the value of the permittivity never reaches e|| but it asymptotes toward e|| at higher voltages. Measuring the capacitance voltage curve for a planar nematic layer in a parallel plate capacitor yields the value of e⊥ when V < VC, the value of e|| for V >> VC (e.g., from the intercept of a plot of e versus 1/V [59]), and the value of the K11 elastic constant from VC (once De is known). The gradient of the e versus V curve immediately above the threshold voltage depends on the ratio of the elastic constants, K33/K11. This ratio can be found from this gradient or by fitting the whole e versus V curve using nematic continuum theory [60, 61]. It has been suggested that the presence of a significant flexoelectric polarization may have the effect of reducing the gradient in this area of the e versus V curve for a nonionic nematic material [62, 63].

3.3

Optical Transmission as a Function of Voltage

A planar nematic layer acts as a uniform birefringent slab in the absence of an applied voltage, and the n-director defines the optic axis of layer. The optical transmission Tout/Tin of a birefringent slab with thickness d and birefringence Dn under normal illumination with monochromatic light of wavelength l polarized at an f angle to the optical axis is given in > Eq. 15 [64] (see also > Chap. 7.2.1).

1357

1358

7.1.4 Tin

Physical Properties of Nematic Liquid Crystals

45°

45°

Linear polariser

Nematic liquid crystal layer

Tout

Crossed analyser

. Fig. 11 An arrangement for modulating the intensity of light transmitted through a planar nematic layer, which is positioned between crossed polarizers that are oriented at +45 and 45 to the direction of the n-director

  Tout pDnd ¼ sin2 ð2’Þsin2 l Tin

ð15Þ

A typical curve is shown in > Fig. 10c of the optical transmission Tout/Tin through a planar nematic layer in the geometry shown in > Fig. 11 as a function of the voltage applied across the layer. Below the threshold voltage the layer acts as a fixed birefringent slab and the transmission is given by > Eq. 15 with f = 45 and Dn = ne  no = n||  n⊥ (see > Sect. 2.2). Above the transition voltage the nematic n-director in the layer deforms. The refractive index experienced by the vertical component of the incident light wave at a position z in the nematic layer is given by neff in > Eq. 16 when the n-director is tilted relative to the plane of the layer by an angle y(z). 1 n2eff ðzÞ

¼

cos2 yðzÞ sin2 yðzÞ þ n2jj n2?

ð16Þ

The value of neff reduces from n|| toward the value of n⊥ as the n-director reorients toward higher angles y(z) at higher voltages. The effective birefringence of the layer, Dn(V ) = neff(V )  no, which can be calculated using a suitable integral across the layer, is a voltage-dependent value. For a thick layer, defined as one for which the retardation at zero voltage fulfills the condition 2pDn(V = 0)d/l >> 2p, the optical transmission oscillates between zero and unity as a function of voltage as the birefringence goes through full and half wave plate conditions [64]. At the highest voltages the birefringence of the layer tends to zero and so the optical transmission between crossed polarizers also tends to zero.

3.4

Dynamic Switching

The graph in > Fig. 10a of the tilt of the n-director in the center of the layer as a function of voltage applies under conditions of static equilibrium. If a voltage is suddenly applied to the nematic layer the value of ym increases toward the equilibrium value. The initial increase is exponentially dependent on the time, and the time constant ton depends on the physical parameters of the nematic liquid crystal and on the cell thickness according to > Eq. 17 [5, 58, 65, 66].

Physical Properties of Nematic Liquid Crystals

ton /

g d2

1  eO De V 2  VC2

7.1.4 ð17Þ

The nematic elastic constants do not appear in this expression, but the switch-on time is strongly dependent on the applied voltage. The time constant diverges to infinity at the threshold voltage, and very long values of the time constant are found for voltages that are close to the Fre´edericksz threshold voltage VC. In > Sect. 3.2, the use of the capacitance voltage curve to measure a number of physical properties of the nematic liquid crystal was discussed [60, 61]. Great care must be taken with this method to allow sufficient settling time, and sufficient resolution in the voltage steps, when measuring the capacitance in the region close to the threshold voltage. The critical slowing down also has implications for the operation of displays such as STN, where the nematic material properties and the twist geometry are both optimized to produce a sharp electro-optic switching threshold for passive multiplex addressing (see > Chap. 7.3.1). When a voltage that is applied to the nematic layer is suddenly removed, the value of ym decreases toward zero. The initial decrease is exponentially dependent on the time, and the relaxation time constant toff depends on the ratio of the nematic viscosity g1 to a combination of elastic constants K, as well as on the cell thickness according to > Eq. 18 [5, 58, 65, 66]. toff /

g1 d 2 K

ð18Þ

The relaxation back to the undistorted medium is a viscoelastic process and it is not dependent on the voltage. The elasticity provides the restoring force toward the equilibrium state and the viscosity limits the speed of reorientation. The discussion in this section is simplified in order to emphasize the influences on dynamic switching of the different physical parameters that are discussed in this chapter. A detailed and mathematically rigorous discussion of dynamic reorientation in nematic liquid crystal layers can be found in reference [58].

4

Summary

In this entry it has been shown how the bulk anisotropic physical properties of nematic liquid crystal materials are determined both by the chemical structure of the individual molecules and by the degree of order of the molecules in the nematic phase. For example, the birefringence Dn and the dielectric anisotropy De depend on the anisotropic molecular properties in two directions and on the S order parameter. The discussion has been limited to ‘‘conventional’’ single component thermotropic nematic materials for which their nematic phase has uniaxial symmetry.

References 1. de Gennes PG, Prost J (1995) The physics of liquid crystals, 2nd edn. Oxford University Press/ Clarendon, Oxford. ISBN 0198517858 2. Chandrasekhar S (1992) Liquid crystals, 2nd edn. Cambridge University Press, Cambridge. ISBN 052142741X

3. Collings PJ (2001) Liquid crystals nature’s delicate phase of matter, 2nd edn. Princeton University Press, Princeton. ISBN 9780691086729 4. Khoo I-C (2007) Liquid crystals, 2nd edn. Wiley, New York. ISBN 0471751537

1359

1360

7.1.4

Physical Properties of Nematic Liquid Crystals

5. Blinov LM (1994) Electrooptic effects in liquid crystal materials. Springer, New York. ISBN 0387940308 6. Demus D, Goodby J, Gray GW, Spiess H-W, Vill V (eds) (1998) Handbook of liquid crystals: fundamentals, vol 1. Wiley-VCH, Weinheim. ISBN 3527292969 7. Dunmur DA, Fukuda A, Luckhurst GR (2001) Physical properties of liquid crystals: nematics, EMIS data review series. Institution of Engineering and Technology, London. ISBN 0852967845 8. Gray GW, Harrison KJ, Nash JA (1973) New family of nematic liquid-crystals for displays. Electron Lett 9:130–130 9. Gray GW, Harrison KJ, Nash JA, Constant J, Hulme JS, Kirton J, Raynes EP (1973) In: Porteer RS, Johnson JF (eds) Liquid crystals and ordered fluid, vol 2. Plenum, New York 10. Duffin WJ (1980) Electricity and magnetism, 3rd edn. McGraw-Hill, London. ISBN 07084111X 11. Born M, Wolf E (1999) Principles of optics electromagnetic theory of propagation, interference and diffraction of light, 7th edn. Cambridge University Press, Cambridge. ISBN 9780521642224 12. Vuks MF (1966) Determination of the optical anisotropy of aromatic molecules from double refraction of liquids. Opt Spectrosc 20:361–364 13. Haller I (1975) Thermodynamic and static properties of liquid crystals. Prog Solid State Chem 10(2): 63–118 14. Bradshaw MJ (1984) Physical properties of nematic liquid crystals. PhD thesis, Exeter University, UK 15. Licristal Sales range, LC Marketing and Sales, Merck KGaA, Darmstadt, Germany 16. Horn RG (1978) Refractive indices and order parameters of two liquid crystals. J Phys (France) 39(1):105 17. Wu ST, Wu CS, Warenghem M, Ismaili M (1993) Refractive index dispersions of liquid crystals. Opt Eng 32(8):1775–1780 18. Abdoh MMM, Srinivasa SNC, Prasad JS (1982) Orientational order in the nematogenic homologous series Trans-4-alkyl (4-cyanophenyl)-cyclohexane. J Chem Phys 77(5):2570–2576 19. Kuball H-G, Bru¨nning H (1997) Helical twisting power and circular dichroism as chirality observations: the intramolecular and intermolecular chirality transfer. Chirality (USA) 9:407 20. Lorrain P, Corson D (1970) Electromagnetic fields and waves, 2nd edn. W.H. Freeman, New York. ISBN 0716703300 21. Maier W, Saupe A (1960) Eine einfache molekularstatistische theorie der nematischen kristallinflussigen phase. Z Naturforsch A 15:287 22. Maier W, Meier G (1961) Eine einfache theorie der dielektrischeneigenschaften homogen orientierter kristallinflussiger phasen des nematischen typs. Z Naturforsch A 16:262

23. Raynes EP (1986) The chemistry and physics of thermotropic liquid crystals. In: Gunter P (ed) Electrooptic and photorefractive materials. Springer, Berlin, pp 80–89 24. Haase W, Wro´bel S (2003) Relaxation phenomena: liquid crystals, magnetic systems, polymers, high-TC superconductors, metallic glasses. Springer, Berlin 25. Frank FC (1958) On the theory of liquid crystals. Discuss Faraday Soc 25:19 26. Oseen CW (1933) On the theory of liquid crystals. Trans Faraday Soc 29:883 27. Stro¨mer JF, Brown CV, Raynes EP (2006) Study of elastic constant ratios in nematic liquid crystals. Appl Phys Lett 88:051915 28. Faber TE (1995) Fluid dynamics for physicists. Cambridge University Press, Cambridge. ISBN 0521429692 29. Miesowicz M (1936) Der Einfluβ des magnetischen Feldes auf die Viskosita¨t der Flu¨ssigkeiten in der nematischen Phase. Bull Acad Pol A 228–247 30. Miesowicz M (1946) The three coefficients of viscosity of anisotropic liquids. Nature 158:27 31. Ericksen JL (1960) Anisotropic fluids. Arch Ration Mech Anal 4:231–237 32. Leslie FM (1968) Some constitutive equations for liquid crystals. Arch Ration Mech Anal 28:265–283 33. Meyer RB (1969) Piezoelectric effects in liquid crystals. Phys Rev Lett 22:918 34. Prost J, Pershan PS (1976) Flexolectricity in nematic and smectic A liquid crystals. J Appl Phys 47:2298 35. Dozov I, Martinot-Lagarde P, Durand G (1982) Flexoelectrically controlled twist of texture in a nematic liquid crystal. J Phys Lett (France) 43:L365 36. Madhusudana NV, Durand G (1985) Linear flexoelectro-optic effect in a hybrid aligned nematic liquid crystal cell. J Phys Lett (France) 46:L195 37. Valenti B, Bertoni C, Barbero G, Taverna-Valabrega P, Bartolino R (1987) Flexoelectricity in the hybrid aligned nematic cell. Mol Cryst Liq Cryst 146:307 38. Blinov LM, Durand G, Yablonsky SV (1992) Curvature oscillations and linear electrooptical effect in a surface layer of a nemtic liquid crystal. J Phys II 2:1287 39. Patel JS, Meyer RB (1987) Flexoelectric electrooptics of a cholesteric liquid crystal. Phys Rev Lett 58(15):1358–1540 40. Rudquist P, Komitov L, Lagerwall ST (1994) Linear electro-optic effect in cholesteric liquid crystal. Phys Rev E 50(6):4743 41. Coles HJ, Clarke MJ, Morris SM, Broughton BJ, Blatch AE (2006) Strong flexoelectric behaviour in bimesogenic liquid crystals. J Appl Phys 99:34104– 34108 42. Bryan-Brown GP, Brown CV, Jones JC (1995) Bistable nematic liquid crystal device. UK Patent GB2318422, 16 Oct 1995

Physical Properties of Nematic Liquid Crystals 43. Brown CV, Towler MJ, Hui VC, Bryan-Brown GP (2000) Numerical analysis of nematic liquid crystal alignment on asymmetric surface grating structures. Liq Cryst 27(2):233 44. Davidson AJ, Mottram NJ (2002) Flexoelectric switching in a bistable nematic device. Phys Rev E 65(5):51710 45. Denniston C, Yeomans JM (2001) Flexoelectric surface switching of bistable nematic devices. Phys Rev Lett 87:275505 46. Parry-Jones LA, Elston SJ (2005) Flexoelectric switching in a zenithally bistable nematic device. J Appl Phys 97(9):93515 47. Spencer TJ, Care CM (2006) Lattice Boltzmann scheme for modeling liquid-crystal dynamics: Zenithal bistable device in the presence of defect motion. Phys Rev 74(6):61708 48. Dalmolen LGP, Picken SJ, de Jong AF, de Jeu WH (1985) The order parameters (P2) and (P4) in nematic p-alkyl-p’-cyano-biphenyls – polarized raman measurements and the influence of molecular association. J Phys (France) 46:1443 49. Ratna BR, Shashidhar R (1976) Dielectric properties of 4’-normal-alkyl-4-cyanobiphenyls in their nematic phases. Pramana 6:278 50. Baran JW, Borowski F, Kedzierski J, Raszewski Z, Zmija J, Sadowska K (1978) Optical anisotropy and molecular parameters of liquid crystalline 4,4’normal-pentylcyanobiphenyl (PCB). Bull Pol Acad Sci (Poland) 26:117 51. Bradshaw MJ, Raynes EP, Bunning JD, Faber TE (1985) The Frank constants of some nematic liquid crystals. J Phys 46:1513 52. Coles HJ (1998) In: Elston SJ, Sambles R (eds) Optics of thermotropic liquid crystals. Taylor & Francis, London 53. Saupe A (1960) Temperaturabhangigkeit and grosse der deformationskonstanten nematischer flussigkeiten. Z Naturforsch 15a:810

7.1.4

54. Saupe A (1960) Biegungselastizitat der nematischen phase von azoxyanisol. Z Naturforsch 15a:815 55. Haller I, Lister JD (1972) In: Brown GH, Labes MM (eds) Liquid crystals, vol 3. Gordon & Breach, New York, p 85 56. Belyaev VV (2008) Viscosity of nematic liquid crystals. Cambridge International Science, Cambridge. ISBN 9781904602088 57. Fre´edericksz V, Zolina V (1933) Forces causing the orientation of an anisotropic fluid. Trans Faraday Soc 29:919–930 58. Stewart IW (2004) The static and dynamic continuum theory of liquid crystals a mathematical introduction. Taylor & Francis, London 59. Clark MG, Raynes EP, Smith RA, Tough RJA (1980) Measurement of the permittivity of nematic liquid crystals in magnetic and electric fields using extrapolation process. J Phys D Appl Phys 13:2151–2164 60. Bradshaw MJ, Raynes EP (1983) The elastic constants and electric permittivities of mixtures containing terminally cyano substituted nematogens. Mol Cryst Liq Cryst 91:145 61. Bradshaw MJ, Raynes EP, Bunning JD, Faber TE (1985) The Frank constants of some nematic liquid crystals. J Phys (France) 46(9):1513 62. Deuling HJ (1974) Method to measure flexoelectric coefficients of nematic liquid crystals. Solid State Commun 14:1073 63. Brown CV, Mottram NJ (2003) Influence of flexoelectricity above the nematic Freedericksz transition. Phys Rev E 68(3):31702–31706 64. Yeh P, Gu C (2009) Optics of liquid crystal displays, 2nd edn. Wiley/Wiley-Blackwell, Hoboken. ISBN 9780470181768 65. Jakeman E, Raynes EP (1972) Electro-optic response times in liquid crystals. Phys Lett A 39(1):69 66. Pieranski P, Brochard F, Guyon E (1973) Static and dynamic behaviour of a nematic liquid crystal in a magnetic field. J Phys 34:35–48

Further Reading Dunmur D, Sluckin T (2010) Soap, science, and flatscreen TVs. Oxford University Press, Oxford. ISBN 9780199549405 Khoo I-C (2007) Liquid crystals, 2nd edn. Wiley/WileyBlackwell, Chichester. ISBN 9780471751533 Oswald P, Pieranski P (2005) Nematic and cholesteric liquid crystals concepts and physical properties

illustrated by experiments. Taylor & Francis, London. ISBN 9780415321402 Prost J, Wyart J, Brochard F (2009) P-G de Gennes’ impact in science volume I: solid state and liquid crystals. World Scientific, Singapore. ISBN 9789814273800

1361

Part 7.2

Liquid Crystal Material Physics

7.2.1 Optics of Liquid Crystals and Liquid Crystal Displays Philip W. Benzie . Steve J. Elston 1

Origin of Optical Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366

2

Birefringence in Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1367

3

Understanding the Optics of Common Liquid Crystal Devices . . . . . . . . . . . . . . . . . . 1367

4

Optical Modeling of Liquid Crystal Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1369

5

Jones 2
2 Matrix Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1370

6 Extended Jones 2
2 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1374 6.1 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1375 7

Normally White Twisted Nematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1377

8

VAN Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1378

9

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1384

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_7.2.1, # Springer-Verlag Berlin Heidelberg 2012

1366

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

Abstract: The basic elements of a liquid crystal device or display incorporate a switchable anisotropic material that is sandwiched between two glass plates coated with transparent electrodes (e.g., Indium Tin Oxide) on their surfaces. The optical characteristics of the device are dependent on the polarizing optics that sandwich it and the director structure of the liquid crystal within the bulk of the device. Usually, the glass plates have polarizers attached whose axes are appropriately orientated depending on the geometry of the display. In order to model the optical properties of a liquid crystal device, we present a variety of optical methods. We introduce the basic concepts of optical anisotropy and describe the advantages/ disadvantages of different device geometries (TN, VAN, and IPS). The conventional Jones 2
2 matrix method and the extended Jones Method are discussed in the context of modeling TN devices. We show how the Jones matrix methods can be applied to the calculation of the optical transmission characteristics of TN devices at normal incidence. A schematic for implementing the extended Jones method within software is presented and we show how this can be applied to model biaxial and uniaxial devices to obtain isocontrast and isotransmission characteristics for typical device structures. List of Abbreviations: IPS, In-Plane Switching; NB TN, Normally Black Twisted Nematic; NW TN, Normally White Twisted Nematic; VAN, Vertically Aligned Nematic

1

Origin of Optical Anisotropy

An electromagnetic wave propagating through a medium (in say the z-direction) can in general be represented by the wave’s electric field, written as E ¼ E0 e iðotkz Þ where E0 is a vector representing the magnitude and polarization of the wave, o is the angular frequency of the wave, and k is the propagation constant. The phase shift incurred by traveling a distance, z, through the medium can be defined as kz. The propagation constant can be expanded into real and imaginary components, k = k0 + ik00 , where k0 relates to the wavelength (in free-space it is given by k0 = 2p / l where l is the wavelength) and k00 relates to the attenuation of the wave as it propagates (in free-space, and loss-free media it is zero). As a wave propagates through a medium its electric field interacts with charges in the material (generally electrons), and it is this interaction that determines how the propagation constant is modified from its free-space value. Importantly, the modification to the real part of the propagation constant is represented by the refractive index of the material, n, leading to k 0 ¼ 2pn=lfreespace . The refractive index is directly related to the relative dielectric permitpffiffiffi tivity at the frequency of the wave through n ¼ e, that is, it depends on the polarizability of the material. However, it is important to remember that for light-waves the frequency is 5
1014 Hz, and at these frequencies the relative dielectric permittivity can differ substantially from its low-frequency value (especially in polar materials). This is the case for liquid crystals, where the low-frequency values are typically 10, and the values for light-wave frequencies are typically 2.5 (giving n 1.6). For many molecules and solid materials, the polarizability is different in different directions. This means that for propagating electromagnetic waves the effective refractive index depends on the orientation of the electric field. For example, in quartz (SiO2), for a wavelength of 590 nm, the refractive index varies between 1.544 and 1.553 depending on the electric field direction. This phenomenon is known as optical anisotropy.

Optics of Liquid Crystals and Liquid Crystal Displays

7.2.1

These effects are not generally present in liquids. Even if the individual molecules have directionally dependent polarizabilities, the random orientation and fluctuation of molecules means that the bulk refractive index is independent of electric field orientation. However, in liquid crystal materials the ordering of the molecules means that bulk optical anisotropy can be manifest in the liquid state. Additionally, it turns out that for the rather complex liquid crystal molecules the molecular polarizability can be substantially different along the molecule and perpendicular to it. This together with the typical uniaxial ordering present in liquid crystal phases leads to significant amounts of optical anisotropy. For example, for the common liquid crystal material E7, at a wavelength of 550 nm the refractive index varies between 1.53 and 1.76 depending on the electric field direction.

2

Birefringence in Liquid Crystals

For most liquid crystal materials, the largest refractive index is along the director, which is also the optic axis, and the smallest refractive index is perpendicular to this direction. Most materials are uniaxial, and the refractive index along the optic axis is designated ne and that perpendicular to the optic axis is designated no. The difference between them is then the optical anisotropy, Dn ¼ ne  no , resulting in birefringence. If we consider a simple uniform layer of liquid crystal material aligned homogeneously, that is, with its director parallel to the substrate surfaces, then light entering this layer (perpendicular to the surfaces) can be considered (in general) to have electric field components along and perpendicular to the director. The component with its electric field polarized along the director will ‘‘see’’ ne and the component with its electric field vector polarized perpendicular to the director will ‘‘see’’ no. The result is that these two components will travel with different propagation constants, one being k 0 ¼ 2pne =lfreespace and the other k 0 ¼ 2pno =lfreespace. They will consequently become out of phase with one another as they propagate, with a phase difference Df ¼ 2pne z=lfreespace  2pno z=lfreespace ¼ 2p

Dnz ; lfreespace

where z is the distance propagated through the liquid crystal layer. This means that if a liquid crystal layer of thickness d is placed between crossed polarizers, with its director (or optic axis) set at an angle w to the incident polarization, the overall transmission at normal incidence can be derived using conventional Jones matrix methods as shown in > Sect. 5 and is written   pDnz : I ¼ Io sin2 ð2wÞsin2 lfreespace This is illustrated in > Fig. 1 for a range of liquid crystal layers. An electro-optic modulation can then be achieved (at least in principle) by varying either the orientation of the director w (as in IPS), or the effective value of the optical anisotropy Dn (as in VAN).

3

Understanding the Optics of Common Liquid Crystal Devices

The above analysis can be used to gain some qualitative understanding of the operation of some liquid crystal displays. For example, in the vertically aligned nematic (VAN) mode the

1367

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

1

Transmission

1368

0.8 0.6 0.4 0.2 0 400

1.3 um 1.8 um 2.6 um 4.0 um

500 600 Wavelength (nm)

700

. Fig. 1 Transmission as a function of wavelength for liquid crystal layers placed between crossed polarizers. The light is at normal incidence and polarized in the plane of the incident polarizer, and the director (optic axis) is at 45 to the axis of polarization. The optical anisotropy is Δn = 0.2 and transmissions are shown for layer thicknesses of 1.3, 1.8, 2.6, and 4.0 mm

liquid crystal director is arranged initially to be perpendicular to the display surfaces. For normally incident light this means that no birefringence is observed (i.e., the effective refractive index is independent of the polarization direction). Therefore, the effective value of Δn is zero and there is no transmission. When an electric field is applied, the director tilts toward a planar state, leading to a nonzero value of effective Δn and hence increase in transmission. Alternatively, in the in-plane switching (IPS) mode the director is parallel to the display surfaces, and with no field applied the structure is uniform, leading to an effective value of Δn equal to its maximum. The field then causes reorientation of the director within the plane of the device, effectively changing the angle w in the above equation, and hence resulting in electro-optic modulation of the transmission. Unfortunately, the internal structures and operation of liquid crystal display operating modes means that they cannot be understood fully in terms of the above simple analysis. This is principally for two reasons: ● The complex internal director structures present, especially when electric fields are applied, mean that the device cannot be treated as a uniform structure with a single optic axis. ● We are not interested in only normally incident light of single wavelength, but off-axis light and color (in order to understand the viewing properties of real displays). Therefore, to understand fully the behavior of twisted nematic (TN), vertically aligned nematic (VAN), and in-plane switching (IPS) technology more sophisticated numerical modeling approaches are necessary. The TN liquid crystal display-operating mode was the first to achieve significant commercial success in the 1970s, and it is still very common. Its principle is very easy to understand, but

Optics of Liquid Crystals and Liquid Crystal Displays

7.2.1

the details of its optical properties are somewhat more complicated. With no electric field applied, the liquid crystal layer is arranged to be twisted by 90 across the thickness of the device. In this state the polarization state of light passing through the layer is twisted with the liquid crystal director and therefore also rotated by 90 . When a field is applied the liquid crystal reorientates to be perpendicular to the device surfaces and the polarization state of the light is no longer rotated. Therefore, by placing the structure between polarizers, it is possible to engineer a simple electro-optic device. However, there are some issues: When a field is applied to a TN layer, only partial reorientation takes place. While a large field (several volts across a typical device) does result in substantial reorientation of the director there do remain regions near the device surfaces that are not switched. Further, when applying moderate fields in order to partially switch the layer (e.g., to achieve a gray state on a display) a complex director profile occurs, involving both twist and tilt across the device. This results in substantial nonuniformity in viewing properties. If a device is optimized for light traveling through the LC layer at normal incidence then for light traveling at an angle to normal incidence the transmission (especially for partially switched gray states) can vary substantially. Therefore, the display contrast for a gray levels is a strong function of viewing angle. The simple concept that the polarization state of the light follows the twisted liquid crystal layer is only true if the pitch of the twisted structure satisfies the so-called Mauguin limit [1]: pitch


Dn >> 1: l

For typical liquid crystal optical anisotropies this implies, for devices operating at visible wavelengths, a pitch of 40 mm, leading to a TN device thickness of 10 mm. This works well for simple display devices (watches, calculators, etc.) but the switching speed of liquid crystal layers of such thickness is rather slow and this makes Mauguin-limit devices unsuitable for use in video monitor applications. It is therefore necessary to use thinner twisted liquid crystal layers where the Mauguin limit is not well satisfied. In this case, a simple rotation of the polarization by 90 cannot be guaranteed, and the polarization state of light passing through the layer becomes dependent of wavelength. Correction of the wavelength dependence on the transmission characteristics, can to some degree, be overcome by the addition of compensating films to the display. However, the fundamental limitations of the TN operating mode mean that for high quality displays other modes have been developed. The most common of these are the vertically aligned nematic (VAN) and in-plane switching (IPS) modes.

4

Optical Modeling of Liquid Crystal Devices

Ideally, a liquid crystal display should provide wide-angle viewing with even optical isotransmission characteristics at all wavelengths and angles. The contrast ratio is also particularly important, since this limits the achievable dynamic range of the display. Contrast ratio is a measure of the ratio of the transmission in the bright state and the dark state (including the light leakage when switched off). Contrast ratio is dependent on viewing angle, hence isocontrast plots are useful to determine a display’s characteristics. To maximize the dynamic range it is necessary to ensure that the display has a good even extinction, that is, low

1369

1370

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

transmission at all viewing angles in its dark state and high transmission at all angles in its bright state. These viewing properties are dependent on the director orientation whether switched or without an applied field. Therefore, if we wish to simulate and optimize the isocontrast, isotransmission and chrominance characteristics of a display, it is necessary to simulate the propagation of electromagnetic waves within the anisotropic material. A panoply of methods are available however the two most common methods can be split into the following categories: ● Stratified methods (Jones Method 2
2; Extended Jones 2
2, Berreman 4
4) [2–6] ● Mesh-based methods (FDTD, BPM) [7, 8] Stratified methods normally use matrix type solvers to calculate the solution of the wave equation and are most appropriate for one-dimensional modeling. Alternatively, mesh-based optical methods such as finite difference time domain (FDTD) and beam propagation mode (BPM) are better suited to two- or three-dimensional modeling. FDTD and BPM enable simulation of diffraction and scattering effects which the extended Jones method cannot model. When using stratified methods, the optical device is split into strata of defined dielectrics constants and crystallographic orientation (i.e., orientation of the principal director). The two most common matrix type solvers are the extended Jones 2
2 method and the Berreman 4
4 method. The extended Jones 2
2 method is relatively easy to implement and is a computationally efficient way to determine the optical transmission characteristics of device for various viewing angles. In the derivation of the extended Jones method, reflections are usually neglected and it is thus not appropriate (without adaptation) for cholesterics [9]. The stratified approach also means that the extended Jones method cannot be applied to threedimensional geometries and is not appropriate for scenarios in which scattering or diffraction effects are relevant. The 4
4 Berreman method is an alternative to the Jones 2
2 method that incorporates reflections and is well-suited to the simulation of cholesterics. The mesh-based method FDTD provides a direct numerical solution of Maxwell’s equations and is a fully explicit approach suited to 3D device structures. However, FDTD is computationally expensive for large structures. If scatter, reflection, and diffraction effects are to be included, the FDTD method is preferable; however, the computation time for large device structures (>10 mm) makes it unfeasible for simulating large structures. Alternatively, the beam propagation method utilizes a partial differential solution to the Helmholtz equation in the frequency domain that offers a good compromise between the inclusion of scattering and diffraction effects and computational efficiency. We refer the interested reader to [10], which provides a good discussion of BPM and FDTD methods that are beyond the scope of this chapter. In the following sections, we shall focus on Jones matrix methods which are appropriate for a wide range of optical device technologies and show how they can be applied to typical liquid crystal device geometries.

5

Jones 2
2 Matrix Method

For optical modeling within the paraxial approximation, the transmission of polarized light through an inhomogeneous layered birefringent material can be simulated for uniaxial media using the Jones matrix method [6, 11–13]. Here we describe the basic concepts of the Jones matrix and its application to LC devices.

Optics of Liquid Crystals and Liquid Crystal Displays

7.2.1

If we consider a completely polarized monochromatic plane wave propagating in freespace along the z-axis of our coordinate system, we can utilize a time-independent complex column vector to describe the electric field in a plane perpendicular to the direction of propagation:     Ex Ex0 e i’x ¼ E¼ i’y Ey Ey0 e where Ex0 and Ey0 are the amplitude and ’x ; ’y represents the phase difference of the electric field. The Jones vector can hence be used to uniquely describe linear and right- or left-handed circular/elliptical polarization states using complex notation. A birefringent layer can be represented as a complex 2
2 matrix that produces a linear transformation of the incident polarization state (Jones vector) to determine the output polarization state. This matrix can describe media with absorption, retardation, transmission, or single reflections at the interface between layers in the case of the extended Jones method (discussed in > Sect. 6). The simplest example of a Jones matrix is an ideal linear polarizer, we will now describe the formalization of the Jones matrix for polarized light propagating through a horizontal polarizer (x-axis). By multiplication of the incident polarization state with the 2
2 Jones matrix, Jpol, we can determine the output polarization state for any incident polarization state:    0    Ex Ex Ex 1 0 ¼ Jpol ¼ Ey 0 Ey Ey 0 0 To determine the output polarization state for an arbitrary azimuthal angle ’ of the polarizer with respect to the x-axis, we can employ rotation matrices to perform a linear transformation of the Jones vector and determine the output state. Two coordinate transformations are required: the first decomposes the incident polarization states into the fast axis and slow axes of the medium; the second rotation returns the emerging polarizing states to the laboratory frame:  0        Ex Ex Ex cos f  sin f 1 0 cos f sin f ¼ R ðfÞ Jpol R ðfÞ ¼ Ey 0 Ey Ey sin f cos f 0 0  sin f cos f To simulate a uniaxial anisotropic slab of arbitrary azimuthal orientation, we replace our Jones matrix for the polarizer with a matrix that describes the phase retardation introduced along the slow and fast axis by a birefringent media:    0      Ex Ex 0 cos f  sin f cos f sin f exp ino 2p l d   ¼ Ey Ey 0 0 exp ine 2p d sin f cos f  sin f cos f l Typically, the absolute phase is considerably larger than the phase retardation since for a given thickness the phase may be offset by many periods. Hence, if we are only interested in the relative phase retardation, we may factorize our equation to remove the mean absolute phase change, Ф, and only consider the phase retardation, G ¼ 2pDnd=l, to give        0   Ex Ex cos f sin f cos f  sin f iF exp i G2 0  ¼ e Ey Ey 0  sin f cos f sin f cos f 0 exp i G2 The phase factor e iF can usually be neglected if interference effects due to multiple reflections are not present.

1371

1372

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

One of the most useful properties of the Jones matrix formulation is that a series of optical elements can be handled by multiplying out the matrices in series. In a TN, the azimuthal orientation of the director reorients between the boundary conditions. We can thus represent the optical properties of twisted liquid crystal layers by splitting the device into a series of thin planar layers, JW, whose azimuthal orientation match the twist angle through the bulk of the liquid crystal. Each layer then has a phase retardation of G=N where N is the total number of layers:    N   Y rftot rftot Jw R þ M¼ R  N N m¼1 It can be shown that as the number of layers tends to infinity the matrix M that represents the rotation of the director within the TN reduces to [14, 15] sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !  2ffi   t cos wt  i G2 sinw wt atwist sinw G cos atwist sin atwist wt t ; where wt ¼ atwist 2 þ M¼ sinatwist cosatwist 2 atwist sinw wt coswt þ i G2 sinw wt t

t

Thus, the transmission Jones matrix for twisted devices (NW TN, NB TN, etc.) of arbitrary total twist angle when placed between polarizers with the analyzer at an arbitrary angle can be simulated: !      Ex 0 cosfa sinfa 10 cosfa sinfa cosf0 sinf0 ¼ sinfa cosfa sinfa cosfa sinf0 cosf0 00 Ey 0 0 1 !   sinw sinw   cosw i G t atwist w t cosf0 sinf0 Ex 10 cosatwist sinatwist t 2 wt t @ A sinw sinw G t t Ey sinatwist cosatwist 00 atwist coswt þi sinf0 cosf0 wt

2 wt

Here, fa is the angle the output analyzer makes with the x-axis and f0 is the initial position of the director within the liquid crystal at the input before twist is imposed. For a normally black twisted nemetic (NB TN), the azimuthal orientations of the polarizers and analyzer that sandwich the liquid crystal are parallel to each other and the director orientation at the input polarizer is parallel to the incident polarizer. At the output, after the director has orientated through a 90 twist within the bulk, the director is perpendicular to the output polarizer. Hence pffiffiffi to specify these conditions we can define that, f0 ¼ 0, f paffiffiffi¼ 0,ftwist ¼ p=2, Ex ¼ 1 2, Ey ¼ 0. Note, that the incident Jones Vector is scaled by 1 2 to represent half the intensity of unpolarized light passing through the incident polarizer. By series multiplication of the above matrices, the resulting matrix is a column vector. We can obtain the transmission by multiplying the output Jones vector with its Hermitian conjugate (i.e., I ¼ E  E) to determine the transmission characteristics. This results in the Gooch–Tarry equation [16]: pffiffiffiffiffiffiffiffiffiffiffiffiffi

1 2 sin2 wt 1 sin2 ftwist 1 þ u2 Dn ¼ ; u ¼ 2d T ¼ ftwist 2 2 2 2 l wt 1þu Where u is the Mauguin parameter and ftwist ¼ p=2. It should be clear that the resulting equation produces a series of minima in transmission depending on the thickness, wavelength, and birefringence of the material. Thus, topobtain extinction in the NW mode, the ffiffiffi pffiffiffiffiffimaximum pffiffiffiffiffi Mauguin parameter must take a value of 3; 15; 35; ::: Most TN devices are designed so that they operate in the first or second minimum of the Gooch–Tarry curve, thus an appropriate device thickness and optical anisotropy is selected to ensure this condition.

Optics of Liquid Crystals and Liquid Crystal Displays

7.2.1

0.5 NB−TN NW−TN

0.45 0.4

Transmission

0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

0

2

4

6 8 10 u (Mauguin parameter)

12

14

. Fig. 2 Transmission as a function of the Mauguin parameter for an NB TN and NW TN device at normal incidence for randomly polarized light at the input to the device

To derive the Gooch–Tarry equation for a NW TN, we can use the same procedure; however, we must rotate the analyzer by setting fa ¼ p=2. The transmission characteristics for a NW TN are complementary to that of the NB TN mode (> Fig. 2). An expression for the general case of a structure with an arbitrary total twist between polarizers of arbitrary orientation can be derived [17], .pffiffiffiffiffiffiffiffiffiffiffiffiffi

2 T ¼ cos b cosðatwist  ’a Þ þ sinðbÞ 1 þ a2 sinðatwist  fa Þ þ where,

a2 sin ðbÞ2 cos ðatwist þ 2f0  fa Þ2 1 þ a2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi b ¼ atwist 1 þ atwist 2   Dnd p : a¼ l atwist 2

Providing that the total twist is much less or greater than ðno þ ne Þd =ð2lo Þ where d is the device thickness and lo is the illuminating wavelength, this expression is valid for a wide range of twist angles. If this criteria is not met, Bragg diffraction must be accounted for and other optical methods such as the Berreman 4
4 method are required. Nonetheless, the formulas are useful for determining the normal incidence optical properties of super twisted nematic structures (STN) or for example highly chiral uniform standing helix ‘‘Grandjean’’ devices. In this section we have assumed normally incident light in our examples; to model off-axis optics, we require more sophisticated techniques discussed in the next section. The characteristics of the NW TN discussed here are expanded to demonstrate how the off-axis viewing properties can be modeled in > Sects. 6 and > 7.

1373

1374

7.2.1 6

Optics of Liquid Crystals and Liquid Crystal Displays

Extended Jones 2
2 Method

In this section we will discuss the extended Jones method as outlined by Gu and Yeh [4–6]. The extended Jones method is not limited to the paraxial approximation and is appropriate for simulating the off-axis properties of LC devices. This is because unlike the conventional Jones Matrix method we are able to simulate the Fresnel refraction of birefringent crystals and the single reflection at the interface between medium. Furthermore, the extended Jones method may be applied to biaxial materials which is not possible with the conventional Jones matrix method. To simulate the propagation of an incident wave vector through an anisotropic medium, Maxwell’s equations are solved in momentum space. We select the following two expressions for the electric field and the magnetic field (> Eqs. 1 and > 2). The charge density and current density are assumed to be zero within the medium, hence the current density J is neglected from > Eq. 2. We apply the constitutive equations and assume that the material is magnetically isotropic, so that the electric displacement is related to the electric field via the dielectric tensor (> Eq. 3). The optical dielectric tensor, e, may be real or complex depending whether we wish to include absorption, however we shall only consider reciprocal medium. In the proceeding discussion we consider planar homogenous waves (> Eqs. 5 and > 6) and their propagation through a general anisotropic medium. r
E þ

@B ¼0 @t

ð1Þ

r
H 

@D ¼0 @t

ð2Þ

D ¼ eE

ð3Þ

B ¼ mH

ð4Þ

Eðr; tÞ ¼ E o e iðotkrÞ

ð5Þ

Hðr; tÞ ¼ H o e iðotkrÞ

ð6Þ

By substitution of > Eqs. 5 and > 6 into > Eqs. 1 and > 2 and taking into account the curl operator, in k wave space we find the expression (> Eq. 10) relating the magnetic field and electric field by using the constitutive > Eq. 4.  ik
E ¼ ioB

ð7Þ

k
E ¼ oB

ð8Þ

k
E ¼ omH

ð9Þ

H¼

k
E om

ð10Þ

The same procedure is repeated for magnetic field in > Eq. 2 to form > Eqs. 11–12. Applying the constitutive > Eqs. 3–12 gives us > Eq. 13. Thereafter, substitution of > Eq. 10 into > Eq. 13 produces an expression relating only the incident wave vector, electric field, and

Optics of Liquid Crystals and Liquid Crystal Displays

7.2.1

dielectric tensor. This expression is a 3
3 matrix from which the determinant can be taken to find nontrivial solutions if we know two of the components of the wave vector k.  ik
H ¼ ioD

ð11Þ

k
H ¼ oD

ð12Þ

k
H ¼ oeE   k
E ¼ oeE k
om

ð13Þ ð14Þ

k
ðk
E Þ þ o2 meE ¼ 0

ð15Þ

jk
ðk
EÞ þ o2 meEj ¼ 0

ð16Þ

From > Eq. 16, provided we know the angle of incidence (yi, fi), and the projection of those components with respect to the x- and y-axis (a, b), from the previous layer, we can calculate the z-component, g, of the k vector.

6.1

Implementation

When applying the extended Jones method to the modeling of a display, we split our device into stratum and include layers to model the incident polarizer and output analyzer (> Fig. 3). In > Fig. 3, the incident polarizer stratum Lp has a specified refractive index indices (n1, n2, and n3) and polar/azimuthal (ycn, fcn, ccn), orientation. The refractive indices are converted into dielectric constants. We usually assume there is no optical dispersion and rotate the dielectric tensor using Euler angles. The azimuthal orientation of the director at the incident polarizer and analyzer specify whether the analyzers are crossed or parallel polarized. In the bulk of the liquid crystal (the inter-dynamic layers), we have m layers of equal thickness (L1, L2, . . . Lm); each layer has a specified refractive index and orientation of the principal axis. In the following flow diagram (> Fig. 4) we show how the extended Jones method can easily be implemented in software. In our discussion, we focus on how the wave equation is solved for propagation of a wave vector within the liquid crystal. However, to fully simulate a device, we must also take in to account the refraction of the incident beam, between the isotropic medium (air) and device structure when modeling the polarizers or glass interfaces. The glass interfaces are simple isotropic/uniaxial media and are neglected (Do; Pa ; Da ; Ppol ; Di ) from our description. Initially, an incident wave vector, k, enters the interdynamical layers with polarization states s and p. This wave vector can be orientated at any angle to calculate the transmission characteristics for multiple azimuthal and polar angles (yi, fi) at a given wavelength. It is thus possible to calculate isotransmission characteristics that are analogous to what is seen when viewing a device with a conoscopic illumination (i.e., viewing the transmission characteristics of a device placed between two positive lenses). On the left-hand side (> Fig. 4), the parameters of the layer stack are specified for the anisotropy and orientation of the liquid crystal device. These parameters can be stored in a look-up table specifying the tilt, twist, pitch, and anisotropy for the material split into, m,

1375

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

k = [a,b,g]

z

θc

niso = 1.0 (air) Lp L1 L2 L3 ... Lm La

z

l = 633 nm

qi

s

p fi

c ε3 ψc

x

x Inter-dynamic layers

1376

ε1

polariser

y ε2

L4

y Crystallographic axis of each stratum defined by dielectric constants and Euler angles e0n12

analyser

niso = 1.0 (air)

fc

e=R 0

Incident illumination onto stratified layers of a LC device

0 0 e0n22 0 R -1 0 e0n32

exx exy exz = eyx eyy eyz ezx ezz ezz

. Fig. 3 Schematic showing method of stratification of an LC device to model the optics of the polarizer/ analyzer, crystallographic axis for each layer, and off-axis illumination

Layer stack parameters n = 1:m

Propagation matrix e–gadn 0 –g d 0 e bn

dn

(qcn,fcn,ycn) constants

Angle of incidence (qi,fi)

e1, e2, e3 eo c,l mo l niso Incident Polarisation

o0 = s, e0 = p

Rotate dielectric e´ tensor

x5g 4 + x4g 3 + x3g 2 + x2g + x1 |(k × (k × E)+w2 meE)|

(gs,s = 1,2,3,4)>0 = γa,γb

M (q , f ) m

= DoPaDa

Calculate incident wave vector

a, b

Calculate polarisation components (on, en.) en–1 · en

on–1 · en

en–1 · on

on–1 · on

P PnDn Ppol Di

n=1

T (q , f ) = Ttr [MM†] Transmission

Dynamic matrix

. Fig. 4 Flow diagram of the extended Jones matrix 2
2 stratified approach for the interdynamical layers

Optics of Liquid Crystals and Liquid Crystal Displays

7.2.1

layers of equal thickness, dn. We calculate the wave vector and the angle of its refraction entering the liquid crystal using Snell’s law by using the average refractive index of the medium, this provides the a, b of the incident wave vector, the g component is initially unknown. The determinant of |(k
(k
E) + o 2meE)|, produces a quartic in g whose coefficients are a function of (exx, exy, exz, eyx, eyy, eyz, ezx, ezy, ezz, a, b, m, l, c) . The wavelength, l, magnetic permeability, m and c the speed of light are known and the dielectric constants are determined from rotation of the dielectric tensor. The coefficients of this quartic may be directly implemented into code and an exact solution for the quartic can be calculated to provide solutions in g for the four roots of the equation. Negative solutions (the reflected components) are neglected and the two remaining solutions ga, gb are used to determine the propagation matrix. These values tend to be numerically large and therefore the modulus with respect to 2p is taken to determine the phase difference. The polarization of the extraordinary and ordinary vector components within the medium are calculated for the positive solutions of the wave equation by determining the dot product of polarization states for the current and previous layers. The dynamic matrix relates the transmission coefficients of the incident and refracted components of each layer. It is dynamic in the sense that at each layer the matrix is updated to determine the transmission coefficients from the result of the previous polarization state and the current/calculated polarization state. In the following sections we will look at the optical characteristics of the VAN and TN devices using this approach.

7

Normally White Twisted Nematic

In > Fig. 5, we show the optical properties of a normally white twisted nematic (NW TN) calculated using the extended Jones method. As previously mentioned, in the normally white mode, crossed polarizers are used and the rubbing direction at the substrates is parallel to the analyzer and polarizer. An electric field is applied across the electrodes. Without an applied field, the long axis of the director is planar and twists around the z-axis. This produces a reasonably uniform bright state of less than 50% transmission for unpolarized light incident upon the device (> Fig. 5a) without applied fields. In the on-state, an applied field switches the device to an homeotropic state with reduced transmission at normal incidence; however, at wide angles (30 from normal) there is still significant transmission (> Fig. 5b). The field-on state is driven at 4.8 V and the device has a Freedericksz threshold of approximately 1 V. The driving voltage during the field-on state presented in > Fig. 5a is reasonably large. As can be seen in > Fig. 5c, as the electric field increases past the Freedericksz threshold, the transmission decreases for increasing voltage as the device orientates to a homeotropic state [18]. During this orientation, the off-axis viewing properties are asymmetrical and angularly dependent. The contrast ratio of the device can be determined by dividing the transmission in the off-state by the transmission in the on-state. The normally white TN has high maximum contrast at normal incidence and at off-axis angles the contrast ratio is reduced by poor wide-angle extinction in the homeotropic state. Contrast ratios of greater than 50:1 are achievable within a 30 field of view without compensation (> Fig. 5d). The highest contrast ratio achievable is limited by the ability to achieve a uniform dark state of low transmission. To improve the extinction during the dark state and hence the contrast ratio of the device, it is possible to include optical compensation layers [6, 19]. In the proceeding section, we shall see how compensation can be used to improve the contrast ratio in VAN devices.

1377

7.2.1 8

Optics of Liquid Crystals and Liquid Crystal Displays

VAN Display

The operating principle of the VAN mode was discussed earlier. In practice, to use it in a highquality display, two points should be noted: ● In the field-off state, the liquid crystal layer is a uniformly aligned anisotropic structure with its optic axis perpendicular to the device surfaces. ● When placed between crossed polarizers, this structure is naturally dark at normal incidence. This ensures that at all wavelengths good extinction will occur at normal incidence for a given thickness of device. For wide-angle viewing along an axis at 45 to the cross polarizers, a small amount of transmission occurs thus reducing the extinction in the dark state (> Fig. 6a). A liquid crystal material with negative dielectric anisotropy is selected so that transverse electric fields orientate the director structure toward a planar state (> Fig. 6b). When

Pol

aris

Pola

er

rise

r

–

E

er

er

Analys

Analys

Isotransmission

Isotransmission 0.0

0.36

0.3

1 1

0.0

6

01

60°

0.

0.41 0. 06

40° 0. 41

1

0.36

0.4

0. 46

6

0.11

0.4

20°

1 0.1

0.3 6

06

0.

0.

6

0.0

6

0.3

0.36

. Fig. 5 (Continued)

270°

b

0.0 1

0.06

1 0.0

0.41

a

0.01 01

0.46

0.11 6 0.1 0.21

270°

0.1

1

06 0.

01

36

41

0.

0.

0.

0.41

0° 180°

0.46

180°

0.01

0. 36

1378

0°

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

0.5 Normal incidence

0.45 0.4

Transmission

0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

0

0.5

1

1.5

2 2.5 3 Voltage [Volts]

c

3.5

4

4.5

5

Isocontrast 10

20

10

1000

2000

1000

0

00 10

500

50

20

0

0

10

20 20

10

00

10 50

00

50 20

10

20 0 1 50

20

50

0

0° 20

500

20

00 10 500 0 0 2 100

0 10

0

20

00 20 0 50

100 200

500

10

0

50

50

20

00

00 20 0 0

50

10

0

50

20

180°

d

20

10 00 10 0

50020

2010 10 0 0 00

00 20 0 20 10 00 0 20500 00 50 10

50

10

0

20

20

10

10

270°

. Fig. 5 Transmission characteristics for a twisted nematic uniaxial device with a 90 twist angle (normally white mode) without compensation. Elastic constants: K1 = 1.07 e – 011 N, K2 = 6.5e – 012 N, K3 = 1.62 e – 011 N, D« = 13.7. Device thickness = 2.41 mm, l = 633 nm, n1 = 1.519, n2 = 1.519, n3 =1.738, cross polarizers at 45

1379

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

Isotransmission, 0 V 0.11

0.0

6

0.0

1

60°

0.01

01

0.

40° 0.

0.06

0.11

180°

0.11

01

20°

0°

01 0.

0.0 6

01

0.

0.01 1

0.0

0.0

6

6

0.0

0.11

270°

a

Director profile through device

A

Isotransmission, 5 V 0.16

0.16

0.21

0.21

0.26

0.26

0.21

0.31 0.26

0.31

0.36

0.36

0.3

1

0.41

180°

0°

0.

41

0.4

1

6

0.3

0.4

0.36

0.4

6

6

0.41

1380

0.41

0.36

. Fig. 6 (Continued)

270°

A

b

0.36

Director profile through section A-A

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

10 1500 50 50 20 0 0 00 10 1000 0

5

5

20

10

500 0 1020 0 0 50 50 00 10

20

50

0 0

10

10 20

10

5

c

5

5

10

20

20

2000 1000 500 20 0 100 50

10

5

0°

50 0 10200

00 0 20 20 0 0 0 0 005 0 510 1 0 0 2010 50

20

500

50

00

50 100 20 0 500 1000

00 00 10 500

20

20

180°

20 10 5000

20

00

2000 1000

5

10 20

2 10 0

Isocontrast

1000 2021000 5000 0 50 10 00 500

20

5

270°

0.5 Normal incidence

0.45

Vertical +45 degrees Vertical −45 degrees

0.4

Horizontal ±45 degrees

Transmission

0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

d

0

0.5

1

1.5

2

2.5 3 Voltage [Volts]

3.5

4

4.5

5

. Fig. 6 Transmission voltage characteristics for a single domain vertically aligned nematic (VAN) device without compensation. Elastic constants: K1 = 1.6 e – 011 N, K2 = 6.7 e – 012 N, K3 = 1.5 e – 011 N, D« =3.66. Device thickness = 2.5 mm, l = 540 nm, n1 = 1.487, n2 = 1.487, n3 = 1.611, cross polarizers at  45

1381

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

Isotransmission, 0 V 1 0.0

00

02

LC

01

1 0 .0

00

01

1

0 0. 02

0.0

00

0. 0 0 001 2

0.

00

2

0.0

0 .0 0 1 05 0.00 0.0002 0.0 00 1

0.

00

0.

05

02

0.

00

0.0

1

0.0

0.

05

0.

05 0 0.0. 0. 1 0 .0 0 00 000 02 010 205 0.0

5

0.0

00

0 .0

180°

0°

05

2

00

Compensator

0.0 00 5

2

05

00

05

0.

00 0.

01

1

00

00

0.

0.0 0 0 0.001 5

0.0

0.

0.0

2 00 0.0 0.0 00 1 0.0 00 2

02 00 00. 01 1 0 0 0.0 0. 0102 05 02 0 000 0 0 0.0 0.0. 0. 1 0 0 . 0 5 0.00

0.

0.0

02

0.0

05

0.01

a

Director profile through device

270°

Isotransmission, 5 V 0.16 0.21

0 .2 1

0.26

0.26

0.26

0.31

0.3

0.3 6

1

LC

0.31 0.36

0.3

0.41 41

6

0.

0.26

180° 0.4 1

0.3

1

0.41 0.36

0.36 0.31

0.31

0.2 6

0.2 6

0.2 1

0°

0.2 0.2 1

6

Compensator

0.3 6

0 .3 1

0 .4 1

1382

0.16

b . Fig. 7 (Continued)

270°

Director profile through device

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

Isocontrast 25

25 50 100 50 150 2

15

0 100 25 0

00 10 0 25 0 15 100

10 10 0

150 25 0

2000 1000 250 100

00

00 20000 1

0

15

15

0 250

50

50

0

2 10000 00

25

25

c

10

0°

100 150 00 10000 250 2

00

25

2000

25

200 100 0 0

50

1000 2000

180°

20

00

20

50

1 20000 00

25 100 150 0

50

270°

0.5 Normal incidence

0.45

Vertical ±45 degrees Horizontal ±45 degrees

0.4

Transmission

0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

d

0

0.5

1

1.5

2 2.5 3 Voltage [Volts]

3.5

4

4.5

5

. Fig. 7 Transmission voltage characteristics for a dual domain VAN device with compensation. Elastic constants: K1 = 1.6 e – 011 N, K2 = 6.7 e –012 N, K3 = 1.5 e – 011 N, D« = 3.66. Device thickness = 2.5 mm, l = 540 nm, n1 = 1.487, n2=1.487, n3=1.611, cross polarizers at 45

1383

1384

7.2.1

Optics of Liquid Crystals and Liquid Crystal Displays

the critical field is exceeded, a Freedericksz transition occurs and ideally tilt of the primary axis occurs in a direction at 45 to the crossed polarizers thus inducing transmission (> Fig. 6d). For large fields, the director will lie parallel to the substrate and the optical characteristics will be equivalent to a wave-plate. During this reorientation, the effective birefringence changes as a function of tilt, hence the transmission characteristics are dependent on the device thickness and the effective birefringence. For the moderate fields that can typically be achieved with lowcost TFT driving circuits (approx. 5 V), the director does not fully orientate into a homogenous state. Hence, the resulting asymmetry in the tilt optically manifests itself as asymmetry in viewing properties in the vertical and horizontal directions (> Figs. 6b and d). A contrast ratio of approximately 500:1 can be achieved for a viewing angle of 10–15 . However, in this operating mode due to the poor extinction at wide angles in the dark state, a contrast ratio of only 10:1 is achievable at an angle of 40 . It is possible to improve the extinction in the dark state by including an optical compensator. For example, a discotic film of negative optical anisotropy can be placed at the output of the device. If the liquid crystal has an anisotropy of Δn and a thickness of d, then ideally the compensating film should be a uniaxial layer with its optic axis perpendicular to the device surfaces but with negative optical anisotropy, such that DnLC dLC þ Dncomp:film dcomp:film ¼ 0. In > Fig. 7a, we show that the addition of a compensator has reduced the wide-angle transmission by a factor of 10, thus significantly improving the isocontrast characteristics. It should be noted that for the single domain structure described in > Fig. 6, when a field is applied, the direction of tilt change is, in principle, degenerate. This would lead to a very patchy appearance in a display. Hence, it is not sufficient to simply break this degeneracy because if all of the tilt change takes place in one direction of reorientation then the aforementioned highly asymmetric viewing properties would result (> Fig. 6d). Therefore, it is necessary to engineer the display such that tilt change takes place in at least two directions – we have simulated this property by optically averaging the transmission provided by the two geometries. Optically this geometry results in an even and symmetric bright state at wide angles (> Fig. 7b) and high-contrast characteristics at wide angles (> Fig. 7c). The voltage-transmission characteristics which are now symmetric in the horizontal and vertical viewing direction are shown in > Fig. 7d. The operating principle of the IPS mode was also outlined above. However, in this mode, optical modeling of the field-applied state is actually rather complicated. The in-plane electric field does not cause uniform reorientation across the whole thickness of the liquid crystal layer. Surface anchoring means that the reorientation is concentrated in the center of the LC layer, and that near the surfaces there is less rotation of the director. Additionally, the complex electrode patterns necessary to achieve the in-plane electric fields lead to significant field nonuniformity, and hence further influence the director reorientation. The result is that the field-applied director structure is a complex three-dimensional profile, with consequently difficult to understand optical properties.

9

Conclusion

We have presented and described some of the most commonly used optical methods when dealing with polarizing optics in anisotropic optical systems. We have shown how the Jones matrix method can be used to find analytical solutions for the transmission characteristics of display modes including normally white and normally black TN displays. The Jones matrix

Optics of Liquid Crystals and Liquid Crystal Displays

7.2.1

method that we have presented is a powerful technique that can be applied to various device geometries when the simulation of normal incidence optical transmission is required. For more complex situations when off-axis illumination is necessitated, we have shown how the extended Jones method can be implemented in software. The extended Jones method is well suited to determining the isocontrast and isotransmission characteristics of displays and can aid the understanding and interpretation of conoscopic imaging. With the advent of biaxial liquid crystal materials, choosing the optimal optical geometries for future displays from a vast parameter space of possibilities will require optical simulation.

References 1. Maugin C (1911) Bull Soc Franc Miner 34:17 2. Berreman DW (1972) Optics in stratified and anisotropic media: 4
4-matrix formulation. J Opt Soc Am 62(4):502–510 3. Azzam RMA, Bashara NM (1977) Ellipsometry and polarized light. North-Holland, Amsterdam 4. Clair Gu, Pochi Yeh (1993) J Opt Soc Am 10:966–973 5. Pochi Yeh (1982) J Opt Soc Am 72(4):507–513 6. Pochi Yeh, Clair Gu (1999) Optics of liquid crystal displays. Wiley Interscience, New York 7. Kriezis EE, Elston SJ (1999) Finite-difference time domain method for light wave propagation within liquid crystal devices. Opt Commun 165:99–105 8. Kriezis EE, Elston SJ (2000) Wide-angle beam propagation method for liquid-crystal device calculations. Appl Opt 39:5707–5714 9. Yang Deng-Ke et al (2000) Modeling of the reflection of cholesteric liquid crystals using jones matrix. J Phys D Appl Phys 33:672–676 10. Obayya S (2010) Computation photonics. Wiley, Chichester 11. Jones RC (1941) A new calculus for the treatment of optical systems, III The Sohncke Theory of optical activity. J Opt Soc Am 31(7):500–503

12. Jones RC (1942) A new calculus for the treatment of optical systems, IV. J Opt Soc Am 32(8):486–493 13. Scharf T (2006) Polarized light in liquid crystals and polymers. Wiley, Hoboken 14. Azzam RMA, Bashara NM (1972) Simplified approach to the propagation of polarized light in anisotropic media – application to liquid crystals. J Opt Soc Am 62(11):1252–1257 15. Chandrasekhar S, Rao KNS (1968) Optical rotatory power of liquid crystals. Acta Cryst A 24:445 16. Gooch CH, Tarry HA (1975) The optical properties of twisted nematic liquid crystal structures with twist angles 90 degrees. J Phys D Appl Phys 8:1575 17. Raynes EP (1987) The optical properties of supertwisted liquid crystal layers. Mol Cryst Liq Lett 4(3–4):69–75 18. Chigrinov VG (1999) Liquid crystal devices: physics and applications. Artech House, Boston, p 101 19. Mori H et al (1997) Performance of a novel optical compensation film based on negative birefringence of discotic compound for wide-viewing-angle twisted-nematic liquid-crystal displays. Jpn J Appl Phys 36:143–147, Part 1 No. 1A

1385

7.2.2 Alignment Properties of Liquid Crystals Lesley Parry Jones 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1388

2

Types of Surface Alignment and Common Device Geometries . . . . . . . . . . . . . . . . 1389

3 3.1 3.2 3.2.1 3.2.2 3.2.3 3.3 3.4

Methods of Obtaining Uniform Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390 Homeotropic Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390 Homogeneous Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1391 Rubbing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1391 Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1392 Photo-Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1392 Pretilted Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1393 Intermediate Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1393

4

Multi-Domain Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395

5

Bistable Alignment and Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1396

6 6.1 6.2 6.3 6.4 6.5 6.6

Other Types of Liquid Crystal Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398 Cholesteric Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398 Smectic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1399 Electric and Magnetic Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1400 Shear Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1400 Polymer Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1400 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1401

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_7.2.2, # Springer-Verlag Berlin Heidelberg 2012

1388

7.2.2

Alignment Properties of Liquid Crystals

Abstract: This entry begins by outlining the main types of liquid crystal surface alignment, and common device geometries, and then goes on to describe how both uniform and patterned surface alignment can be achieved via conventional techniques such as rubbing and photoalignment. Finally, the entry concludes with information on bistable alignment, bulk alignment techniques (such as field alignment and polymer networks), and how to align non-nematic phases such as cholesterics and smectics. List of Abbreviations: ECB, Electrically Controlled Birefringence; HAN, Hybrid Aligned Nematic; IPS, In-Plane Switching; LCD, Liquid Crystal Display; OCB, Optically Compensated Bend; TFTs, Thin-Film-Transistors; TN, Twisted Nematic; ULH, Uniform Lying Helix; VAN, Vertically Aligned Nematic

1

Introduction

As described in > Chaps. 7.1.4 and > 7.2.1, many of the interesting and useful properties of liquid crystals arise from the fact that the molecules are anisotropic, and because of the orientational order present, so too is the material itself. However, in a bulk sample of a liquid crystal, the director (see > Chap. 7.1.4) is generally not constant, but fluctuates in space on a length scale of the order of tens of microns, as can be seen in the picture of the nematic liquid crystal in > Fig. 1. Such a sample will not display any macroscopic anisotropy, because on average, the director is not well defined. In order to observe the anisotropy of a liquid crystal, the degeneracy in the director orientation needs to be broken. One way of doing this is to sandwich the liquid crystal between two parallel glass plates, separated by a very small gap (of the order of microns), whose inside surfaces have been treated in such a way as to constrain the director to be in a particular direction. This type of surface alignment, applied to calamitic, nematic liquid crystals is used almost exclusively in liquid crystal displays (LCDs), and will be the subject of the greater part of this entry. Alternative forms of alignment and other types of liquid crystals will be discussed in > Sect. 6.

. Fig. 1 Photograph of an unaligned nematic liquid crystal viewed under a polarizing microscope

Alignment Properties of Liquid Crystals

2

7.2.2

Types of Surface Alignment and Common Device Geometries

We begin by summarizing the common terminology that is often used to describe the most usual kinds of surface alignment, which are depicted in > Fig. 2. When the surface director is parallel to the local surface normal, the alignment is usually referred to as being ‘‘homeotropic’’ or ‘‘vertical,’’ as shown in > Fig. 2a. The other main type of surface alignment is known as ‘‘planar’’ or ‘‘homogeneous’’ alignment, and refers to the case where the surface director is coplanar with the local surface, as shown in > Fig. 2b. Strictly, the term planar alignment refers to the case where the alignment of the director in the plane of the substrate is degenerate (and hence there is still no single director), whereas homogeneous alignment corresponds to the case where there is a preferred alignment direction within the plane of the substrate. The case of ‘‘intermediate’’ alignment refers to all situations between homeotropic and homogeneous alignment. If we define y as the angle between the director and the tangent to the local surface, as shown in > Fig. 2c, then homogeneous alignment is broadly defined as 0 < y < 10
, homeotropic alignment as 80
< y < 90
, and intermediate alignment as 10
< y < 80
. y is generally known as the polar angle of the director, and f as the azimuthal angle (defining the angle within the plane of the substrate). When y is close to but not equal to 0
, or close to but not equal to 90
, this is referred to as pretilted alignment, and will be discussed further in > Sect. 3. The strength of alignment can be expressed as an anchoring energy per unit surface area, which is usually assumed to take the following form: W ¼ W0 þ Wy sin2 ðy  y0 Þ þ Wf sin2 ðf  f0 Þ where y0 is the preferred polar angle, and f0 is the preferred azimuthal angle [1]. The measurement of the polar and azimuthal anchoring energy coefficients Wy and Wf is a research area in its own right. However, simple measurements can usually be made by monitoring a single parameter of a cell (such as optical retardation or capacitance) as a function of applied electric or magnetic field (out-of-plane for polar anchoring, and in-plane for azimuthal anchoring). By comparing the experimental results with those predicted by a theoretical model with different anchoring strengths, the measured anchoring strength is assumed to be that which provides the best match between experiment and theory. The polar anchoring energy is typically an order of magnitude greater than the azimuthal anchoring energy, for example, Wy  103 Jm2 and Wf  104 Jm2 [2]. So far we have described the possible alignment of a liquid crystal adjacent to a single surface. However, the phrase ‘‘liquid crystal alignment’’ can sometimes refer to both surfaces of a liquid crystal cell, and the resulting director structure in the bulk. Some examples of typical cell structures are shown in > Fig. 3, with their commonly used names. > Figure 3a shows a liquid crystal cell that is homeotropically aligned on both surfaces, which is often termed a VAN (vertically aligned nematic) cell. This type of alignment is used in one of the most Homeotropic alignment

a

Homogeneous alignment

b

. Fig. 2 Basic types of liquid crystal surface alignment

Intermediate alignment

c

θ

1389

1390

7.2.2

Alignment Properties of Liquid Crystals

Alignment direction VAN cell

a

b

c

π-cell (bend state) x

d

π-cell (splay state)

Fréedericksz cell

HAN cell

TN cell x x

e

f

. Fig. 3 Common types of liquid crystal device geometry

commonly used liquid crystal modes amongst LCD manufacturers, for example, Samsung, Sharp, and Sony. > Figures 3b–e illustrate cells where both surfaces are homogeneously aligned, although not necessarily in the same direction. In > Fig. 3b and c, the alignment directions are antiparallel and parallel, respectively (these are degenerate in the case of zero pretilt). These types of cells are sometimes referred to as Fre´edericksz and p-cells, respectively, and are the basis for electrically controlled birefringence (ECB) and in-plane switching (IPS) modes used by other LCD manufacturers such as Hitachi and LG. When nondegenerate, these two cases behave slightly differently under applied voltage. The case of parallel alignment (p-cell, > Fig. 3c) has a greater symmetry than the antiparallel case (Freedericksz cell, > Fig. 3b) and can form an alternative state illustrated in > Fig. 3d known as a ‘‘bend’’ state, in contrast with the ‘‘splay’’ state of > Fig. 3c. The terms ‘‘bend’’ and ‘‘splay’’ refer to the eigenmodes of elastic deformation, as explained in > Chap. 7.1.4. The bend state forms the basis for another type of LCD which is of particular interest due to its fast response speed [3] and is now being developed by Samsung as the ‘‘optically compensated bend’’ (OCB) mode. > Figure 3e shows the situation where the two rubbing directions are mutually perpendicular: this type of cell is known as a twisted nematic, or TN. The TN was one of the first liquid crystal modes to be developed, and is still used today in many displays. Finally, > Fig. 3f shows what is referred to as hybrid alignment, where one surface is homeotropic, and the other is planar. This type of cell is generally referred to as a HAN (hybrid-aligned nematic) cell.

3

Methods of Obtaining Uniform Alignment

3.1

Homeotropic Alignment

Homeotropic (or vertical) alignment can be achieved in a number of different ways, the first of which is very simply to coat the surface with a surfactant such as lecithin. Lecithin (which can

Alignment Properties of Liquid Crystals

7.2.2

be derived from egg yolk) can be purchased in solid form and dissolved in a solvent such as ethanol or IPA. A layer of lecithin can be created simply by wiping a tissue soaked in such a solution over the surface of the glass. Lecithin has both hydrophilic and hydrophobic portions (> Fig. 4a), and it is the hydrophilic polar head which adheres to the glass, leaving the hydrophobic tails perpendicular to the surface. As shown in > Fig. 4b, when the liquid crystal material is introduced close to this surface, the liquid crystal molecules align parallel to the hydrophobic tails, thereby aligning homeotropically with the surface [4]. However, the method primarily used in the production of VAN mode LCDs is the deposition of a hydrophobic polymer layer which promotes homeotropic alignment. In manufacture, the polymer will generally be printed in order to avoid material wastage, but for the production of smaller research samples in the lab, conventional coating methods such as spin or dip coating are perfectly adequate. An example of a suitable alignment polymer is JALS2017, which is available from JSR Corporation.

3.2

Homogeneous Alignment

3.2.1

Rubbing

The most common method used to create planar alignment is also to use a polymer coating technique, although of course a different polymer is used, for example, polyvinyl alcohol (PVA, > Fig. 5a) or a polyimide such as PI2555 (> Fig. 5b) from Dupont. A liquid crystal in contact with such a surface will adopt planar alignment (i.e., the azimuthal angle of the director is degenerate within the plane of the substrate). If, however, the polymer surface is treated in such a way as to create a preferred azimuthal direction, then the liquid crystal will adopt homogeneous alignment. This preference is most often created using a ‘‘rubbing’’ technique [5] in which a cloth is rotated across the surface in a uniform fashion, as illustrated in > Fig. 5c. The way in which a rubbed polymer can homogeneously align a liquid crystal is not completely understood, but is generally visualized to be mechanical in origin. The motion of the fibers of the rubbing cloth across the surface of the polymer is thought to reorient the long chains of the polymer so that they lie substantially along the rubbing direction, creating a series of ‘‘nano-grooves.’’ The depth and width of the grooves have been observed and measured by many groups (e.g., [6]) via STM, SEM, and AFM techniques to be on the scale of a few nm to a few tens of nm. When the liquid crystal is in contact with this surface, it costs less elastic

Chemical structure of lecithin

Homeotropic alignment with lecithin

O O

O

O O

a

O

O P O

N+

b

. Fig. 4 Illustration of homeotropic alignment using a lecithin surfactant

1391

1392

7.2.2

Alignment Properties of Liquid Crystals

Chemical structure of PVA

Chemical structure of PI2555 O

O

O

N OH

a

n

b

N O

Rubbing technique

Roller Substrate

Chuck

O

O

n

Liquid crystal alignment on nanogrooves Roller direction Chuck direction

d

c . Fig. 5 Illustration of rubbed homogeneous alignment using a polymer layer

energy for the molecules to align parallel to the nano-grooves than perpendicular to them (see > Fig. 5d), and hence homogeneous planar alignment is achieved [4, 7].

3.2.2

Evaporation

Although widely used in mass manufacture, using a rubbed polymer as an alignment layer is not ideal as it is a contact process which can introduce small particles and static charges onto the substrates, reducing LCD production yields through damage to thin-film-transistors (TFTs) (see also > Chap. 7.6.1). There are a number of noncontact alternatives that have been investigated to a greater or lesser success. The earliest technique used was the oblique evaporation of silicon oxide (at around 50
to the surface normal) which can be used to create homogeneous alignment [8]. Rubbing is not necessary as the preferred alignment direction is defined by the azimuthal angle of the evaporation direction. In order to create a uniform alignment direction, therefore, it is necessary to place the boats containing the silicon oxide at a distance from the substrate which is large compared with the substrate size. This technique is, therefore, impractical in the mass production of LCDs [7] due to the large size of the substrates used, which would mean that very large evaporation chambers would be needed.

3.2.3

Photo-Alignment

Another alternative that has been investigated more in recent years is that of photo-alignment [9–11]. In this technique, special polymer alignment films are used, which are sensitive to UV radiation. Depending on the type of polymer used, the UV light can cause either cis–trans isomerization, photo-degradation, or cross-linking to occur. These transformations can be

Alignment Properties of Liquid Crystals

7.2.2

highly polarization sensitive, so that polymer films exposed to polarized UV light become anisotropic and hence can induce homogeneous liquid crystal alignment. As well as providing a noncontact alternative to rubbed alignment, photo-alignment also offers great flexibility in terms of patterned alignment, which will be discussed in greater detail below. Until recently, photo-alignment had not been taken into mass manufacture, however, in 2009 Sharp announced that its 10th Generation LCD factory in Sakai, Japan would be employing photoalignment technology, becoming the first company in the world to do so.

3.3

Pretilted Alignment

Homogeneous alignment created by rubbing, such as described above, is usually accompanied by what is known as a surface pretilt [12]. As illustrated in > Fig. 6a, the tilt angle of the liquid crystal director relative to the substrate surface depends on the sense of rubbing, that is, there is no 180
azimuthal angle degeneracy in the rubbing process. The value of the pretilt angle depends on the polymer used for the alignment layer, the rubbing conditions and the liquid crystal used, but is generally between 0
and 10
.

3.4

Intermediate Alignment

In the previous section, it has been stated that homogeneous alignment created by rubbing tends to result in surface pretilt angles of between 0
and 10
to the surface. Likewise, rubbed homeotropic alignment tends to result in small pretilts, again between 0
and 10
to the surface normal (see > Fig. 6b). How is it possible to achieve intermediate pretilt angles of between 10
and 80
to the surface? One method is again to use the oblique evaporation of silicon oxide, but this time at much greater angles to the surface normal of around 80
–90
[7, 13]. For a long time, this was the only way in which pretilt angles greater than about 20
could be achieved.

Effect of rubbing direction on planar surface pretilt Rubbing direction

No rubbing

Side view

Top view

a Effect of rubbing direction on homeotropic surface pretilt Rubbing direction

b Side view . Fig. 6 Effect of rubbing direction on surface pretilt

No rubbing

1393

1394

7.2.2

Alignment Properties of Liquid Crystals

However, more recently, such intermediate pretilts can be achieved by what is essentially a spatial averaging of local homeotropic and planar alignment. One method is to make a solution of two polymers which individually can be used to create homeotropic and planar alignment [14]. When the solution is spun onto a substrate, the solvent evaporates and the two polymers come out of solution and form small phase-separated regions on the substrate in what might appear to be a haphazard fashion. The key to the success of this method is in choosing polymers which have different solubilities in the solvent, so that they come out of solution at different times. By carefully controlling this process, it is possible to create ‘‘nanostructured’’ surfaces where the patches of homeotropic and planar alignment materials are less than a micron in extent. Of course, this surface must be rubbed in order to give a preferential azimuthal alignment just as a standard planar alignment layer would. A cartoon illustrating the effect of this type of surface on a neighboring liquid crystal layer is illustrated in > Fig. 7. Although the regions of homeotropic and planar alignment are very small, they are still orders of magnitude larger than the size of a liquid crystal molecule (which is generally a few nm to a few tens of nm), and therefore very close to the surface, the liquid crystal molecules will align to the local surface alignment (either homeotropic or planar, depending on position). However, as we move away from the surface, the director changes gradually until at some point, it becomes practically independent of position along the substrate. Not only that, but the angle of the director is in the region of interest between 10
and 80
. Thus, a certain distance in from the real surface of the substrate, there is an imaginary surface (indicated by the dotted line in > Fig. 7) at which the effective surface pretilt is at this intermediate ‘‘average’’ value. The value of the pretilt is largely determined by the area fractions of homeotropic and planar regions, although it will also depend on other factors like their individual pretilts, the anchoring strengths at the true surface, and the anisotropic elastic constants governing the cost in energy to distortions in the liquid crystal director. The distance of the imaginary surface from the true surface is of the order of the length scale of the individual regions of homeotropic and planar alignment material. Therefore, if the imaginary surface is to have any useful meaning in a liquid crystal cell of a few microns thickness, it is preferable for this length scale to be submicron. If this is not the case, and the regions of differing alignment are large compared with the cell thickness, then the concept of an ‘‘effective’’ pretilt angle is meaningless, and the cell simply forms differently aligned regions in different parts of the cell: these are normally called domains, and will be discussed further in > Sect. 4. These two extreme cases are illustrated in > Fig. 8.

. Fig. 7 Illustration of the length scale over which a uniform director is formed as a result of mixed or patterned alignment

Alignment Properties of Liquid Crystals

Intermediate alignment

a

7.2.2

Multi-domain alignment

b

. Fig. 8 Illustration of the way in which the ratio of the cell gap to the pitch of mixed or patterned alignment affects the type of alignment achieved

Another way in which to create regions of homeotropic and planar alignment (whether on a small or large length scale compared with the cell gap), is to use what is generally termed ‘‘patterned’’ alignment. Patterned alignment refers to any kind of nonuniform surface alignment where the structure has been controlled to give a prescribed pattern. It is possible to create patterns of different azimuthal alignment directions within either a homeotropic or planar alignment layer. The latter of these two cases will be discussed later in the bistable alignment > Sect. 5. Here, we restrict ourselves to the case of patterned homeotropic and planar regions, which are created with a view to creating alignment with intermediate pretilt angle. Due to the difficulty of creating an alignment layer patterned with well-defined regions of two different polymers promoting homeotropic and homogeneous alignment, patterned alignment is most conveniently achieved using photo-alignment. The basic process for creating a patterned photo-alignment layer is exactly the same as a uniform one, except that parts of the surface are masked off during the UV exposure step, so that these areas of the alignment layer do not undergo whichever process it is (cis–trans isomerization, photo-degradation, or crosslinking, depending on the material) that occurs in the regions that are exposed to the UV radiation. What therefore results is an alignment layer which is patterned between homeotropic and homogeneous regions. In the same way that the work of Yeung et al. describes, by varying the area fraction of homeotropic and homogeneous regions, the effective surface pretilt angle at an imaginary surface can be tuned between 0
and 90
[15].

4

Multi-Domain Alignment

As mentioned above, if the length scale of the spatial variation of the surface alignment is of the order of or larger than the cell gap, then the concept of an average surface alignment is meaningless because the cell is effectively divided into multiple domains, each of which has its own separate alignment, as illustrated in > Fig. 8b. This concept of multi-domaining is often used to improve the azimuthal viewing angle characteristics of certain LC modes, particularly TN, IPS, and VAN modes. Each of these modes, when used as a single domain has a lack of azimuthal symmetry in either the ‘‘on’’ or ‘‘off ’’ states that leads to a high dependence of the display contrast ratio or brightness on azimuthal viewing angle. Some of this dependence is unavoidable because the orientation of the polarizers is the same across the whole display. However, great improvements can be made by patterning the alignment of the liquid crystal, so that within each pixel of the display, there are two or sometimes four different domains each with a different alignment.

1395

1396

7.2.2 5

Alignment Properties of Liquid Crystals

Bistable Alignment and Devices

Bistability in liquid crystal devices can be divided into two types: surface bistability and bulk bistability, and it is the former of these which will be described principally here. Bulk bistability can arise from the types of surface alignment previously described, as there are often a number of ways in which the liquid crystal director structure can form between two surfaces. For example, in a twisted nematic device, which has two homogeneously aligned surfaces with mutually orthogonal alignment directions, the direction of twist of the nematic director from one surface to the other can either be clockwise or anticlockwise. In the case of an achiral nematic liquid crystal and zero pretilt, the two states are energetically degenerate, which could lead to a bistable system. However, in practice the degeneracy is usually broken by a small amount of chirality, which will favor one twist direction over the other. Alternatively, if a planar aligned device is filled with a chiral liquid crystal which has a natural pitch equal to four times the device thickness, there will be a bulk bistability in which one state has zero twist between the two surfaces and another state has a p twist between the two surfaces. This is the basis of the bistable liquid crystal mode BiNem pursued by Nemoptic [16]. The term surface bistability, by contrast, refers to the case where the director at the surface (or very close to the surface) can occupy more than one orientation, and this of course will then impact on the bistability of the entire device. Surface bistability is generally classified as ‘‘zenithal’’ or ‘‘azimuthal,’’ with these terms referring to the possible angles of the director on the surface. An example of zenithal or polar bistability is that which can be produced by ‘‘grating alignment’’ [17], and is illustrated in > Fig. 9. Here, there is a one-dimensional grating structure on one cell surface with a pitch of the order of a micron. If the surface of the grating were left untreated, the liquid crystal molecules would tend to align along the grooves, as illustrated in > Fig. 5d. However, in this case, the surface is treated with a homeotropic surfactant such as lecithin, which causes the director to align perpendicular to the local surface normal. If the grating is sufficiently deep in comparison with the pitch, there are two possible stable states of the director, as illustrated in > Fig. 9a and b. The first state (> Fig. 9a) is known as the defect-free state, and has a continuous and smooth variation of the director, which results in an effective alignment at the dotted line which is roughly vertical. The second state (> Fig. 9b) is known as the defect state, and has a periodic array of defects at the peaks and troughs of the grating structure. The alignment of the director just above the grating is roughly horizontal. There are, therefore, two states with completely different ‘‘effective’’ surface alignment, which will be of roughly equal energy, for a certain aspect ratio of the grating structure. It is possible to switch between the two states by coupling to the flexoelectric polarization of the liquid crystal (discussed further in > Chap. 7.7.2), and therefore create a bistable liquid crystal device. The second type of surface bistability is known as azimuthal bistability, where the ‘‘effective’’ surface alignment (in practice some distance from the true surface) differs in terms of azimuthal angle (angle in the plane of the surface), rather than the polar or zenithal angle, as in the previous example. Such alignment can be created either by creating a two-dimensional grating structure [18], patterned photo-alignment (as previously described), or patterned rubbed alignment. This latter method can be achieved either by conventional rubbing (using patterned photoresist to cover the areas which should not be rubbed during a particular step), or by micro-patterning using an AFM tip [19]. This is illustrated in > Fig. 10, which shows a chequerboard design created by the patterned rubbing technique. This results in two possible

Alignment Properties of Liquid Crystals

7.2.2

a

b . Fig. 9 Zenithal bistability via grating alignment allows either (a) a defect-free state or (b) a defect state. With a homeotropic surface opposite the grating surface, the defect-free state resembles a VAN cell (a), and the defect state resembles a HAN cell (b)

Patterned rubbing

Two possible bulk alignment directions

. Fig. 10 Azimuthal bistability via patterned alignment

1397

1398

7.2.2

Alignment Properties of Liquid Crystals

azimuthal alignment directions, at 45
to the rubbing axes. The liquid crystal can then be switched between one preferred alignment direction and the other using in-plane electrodes placed underneath the rubbed surfaces.

6

Other Types of Liquid Crystal Alignment

So far, the discussion in this entry on liquid crystal alignment has focused on achiral and weakly chiral nematic liquid crystals, which are relatively easy to align compared with other liquid crystal phases. Provided that good alignment layers are used, and that the cell is clean, the intended alignment should be obtained with relative ease, especially if the cell is filled in the isotropic phase and cooled slowly across the phase transition to the nematic phase. However, more highly chiral nematic liquid crystals (cholesterics) and smectic (layered) liquid crystals are more difficult to align, and it is often necessary to employ some tricks to obtain satisfactory alignment.

6.1

Cholesteric Liquid Crystals

In cholesteric liquid crystals, the nematic director is not constant but rotates uniformly in space, resulting in a helical structure. If a cholesteric liquid crystal is introduced into a cell with two planar surfaces, generally the ‘‘planar’’ or ‘‘Grandjean’’ texture is formed in which the axis of the helix is normal to the cell surfaces, so that the director at the surfaces can be consistent with the alignment direction, as illustrated in > Fig. 11a. Aligning the helical axis parallel to the surfaces (> Fig. 11b) to form what is known as the uniform-lying-helix (ULH) texture is much more difficult because the bulk of the cell is helical whereas the surfaces are not. The preferred azimuthal direction of the cholesteric helix relative to a nearby rubbed planar surface is neither perpendicular nor parallel to the rubbing direction, but at an angle, the sense of which depends on whether the helix is left or right handed. This is because the preferred helical direction is determined by the minimization of overall elastic energy, and since a cholesteric liquid crystal has minimal elastic energy when the director twists at a certain rate, it follows that the preferred helical direction is at a nontrivial angle to the rubbing direction. Because of this, parallel homogeneous alignment is not ideal for forming ULH textures. The texture can be improved by using skewed alignment and/or applying an external electric field [20]. An alternative approach is to use microchannels to force the direction of the helical axis [21].

Grandjean / planar texture

a

Uniform Lying Helix (ULH) texture

b

. Fig. 11 Alignment textures formed by cholesteric liquid crystals

Alignment Properties of Liquid Crystals

6.2

7.2.2

Smectic Liquid Crystals

Smectic liquid crystals are often difficult to align because of their layered structure. Once the layer structure is formed after cooling through a phase transition, it is very difficult to improve (although it can be destroyed). It is therefore essential to obtain a good layer structure when the first layered (smectic) phase is formed, for example, in a liquid crystal which has the following phase sequence: crystalline solid – smectic – nematic – isotropic liquid, it is relatively easy to obtain good alignment in the smectic phase because the orientation of the director can be formed in the nematic phase, and then the layer structure forms on cooling from nematic to smectic (see > Fig. 12a, which is illustrated for the case of a smectic A phase). If, however, there is no nematic phase (as is often the case for anti-ferroelectric liquid crystals) and the transition is directly from isotropic to smectic, this is more tricky as both the orientation of the director and the layer structure are formed simultaneously. Usually, better alignment is obtained by cooling slowly across the phase transition, because this results in fewer seeding points for the growth of the lower temperature phase, and hence fewer domains. The application of an external electric field can also sometimes be useful, as explained further in > section 6.3. The following phase sequence is typical for materials which have a smectic C phase: crystalline solid – smectic C – smectic A – nematic – isotropic liquid. Here, the alignment of the layers in the smectic A phase can be quite good, because of the upper-lying nematic phase, as explained above. This is only the case if the material is achiral, or if the pitch in the nematic phase is greater than four times the device thickness, that is, so that the director is untwisted in the nematic phase before the phase transition from nematic to smectic A. However, when cooling from the smectic A phase to the smectic C phase, the molecules tilt with respect to the layer normal, that is, the layers shrink. However, the layer pitch along the surfaces has already been well defined in the smectic A phase, and so what happens is that the layers kink to form a chevron structure (> Fig. 12b), which preserves the layer pitch whilst allowing layer shrinkage to occur [22]. This chevron structure complicates (but does not prevent) the use of chiral smectic C materials as bistable ferrorelectric devices (see > Chap. 7.3.5).

Smectic A layer structure

a

Smectic C "chevron structure"

b

. Fig. 12 Common alignments of smectic A and C liquid crystals

1399

1400

7.2.2 6.3

Alignment Properties of Liquid Crystals

Electric and Magnetic Fields

It has been mentioned above that external electric fields can be applied to aid the alignment of some liquid crystal phases. In > Chap. 7.1.4, it was described how an electric field can reorient the liquid crystal so that the director becomes either parallel or perpendicular to the direction of the applied electric field. In the same way that this effect can be used to reorient a liquid crystal that is already well aligned (for example by surface alignment), it can also be used to align a previously unaligned sample of liquid crystal. In the case of a simple nematic liquid crystal, however, the alignment achieved will only persist so long as the field is applied, and when it is removed, the multi-domain texture will reappear. The same effect can also be achieved with a magnetic field, as liquid crystals have anisotropy in their magnetic susceptibility as well as their electric susceptibility. However, in many cases, an electric field can be very useful to help create an initial alignment direction which might then perhaps be frozen in via some other bulk influence, for example, a polymer network (see below), or smectic layers (as described above). This type of effect can be invaluable in assisting the alignment of liquid crystals which do not align well by simple surface alignment.

6.4

Shear Alignment

A further way to align liquid crystals is by ‘‘shearing.’’ The action of sliding one of the cell substrates over the other can break the degeneracy in the director orientation, so that the molecules tend to align parallel to the direction of motion (or ‘‘shear’’). This type of alignment is useful for liquid crystals which are difficult to otherwise align, and conventional surface alignment is insufficient, for example, smectic liquid crystals [23].

6.5

Polymer Networks

Polymer networks can be used to stabilize the alignment of a liquid crystal cell, and would usually only be employed in the case where the alignment has been particularly difficult to obtain, or is at risk of being disturbed during later use, although the use of polymer networks to increase the ‘‘switch-off ’’ speed of nematic liquid crystals has been considered [24]. For example, in the case where an applied electric field is necessary during cooling through a phase transition in order to obtain good alignment, it may be that the alignment does not persist once the field is removed. One way to ‘‘freeze-in’’ the good alignment is to dissolve in the liquid crystal a small percentage of a UV-activated monomer, (known as a reactive mesogen, an example of which, RM257, is shown in > Fig. 13), whose molecules align with the local

O O

. Fig. 13 Chemical structure of RM257

O

O

O

O

O

O O

O

Alignment Properties of Liquid Crystals

7.2.2

director. When a UV light is shone onto the liquid crystal cell (with the field applied), the monomers cross-link to form a polymer network, which then acts as a weak ‘‘internal surface’’ to help the liquid crystal alignment to persist when the field is removed. This technique of polymer stabilization is often used to assist the alignment of cholesteric and smectic liquid crystals. Polymer networks can also be used in completely the opposite way to make a polarizer-free liquid crystal mode, known as a polymer network LC, PNLC [25], which works on the principle of a switchable scatterer. In this case, the monomer is cured whilst the liquid crystal is in the isotropic phase, so that it forms a random structure. When the liquid crystal is cooled into the nematic phase, the random structure persists, and the liquid crystal aligns locally with this random structure, creating a scattering element. An applied electric field can reorient the liquid crystal molecules so that the element becomes transparent, but the scattering state always returns when the field is removed. Hence, the mode works by switching between a scattering and a transparent state, and as such is polarizer-free and therefore particularly suited to reflective displays [26].

6.6

Summary

In this entry, we hope to have provided the reader with a basic yet practical guide to liquid crystal alignment. Various methods have been described for producing both uniform and patterned alignment, suitable both for laboratory experiments and mass production. For a more comprehensive review of the alignment properties of liquid crystals, the reader is referred to the excellent works of Cognard, Sonin [4], and Takatoh [7].

References 1. Rapini A, Papoular MJ (1969) J Phys (France) Colloq 30:C4–C54 2. Tomlin MG (1997) J Opt Technol 64:458 3. Bos P, Koehler K, Beran R (1984) Mol Cryst Liq Cryst 113:329 4. Sonin AA (1995) The surface physics of liquid crystals. Gordon and Breach, Amsterdam 5. Mauguin C (1911) Bull Soc Fr Min 34:71 6. Pidduck AJ, Bryan-Brown GP, Haslam SD, Bannister R (1996) Liq Cryst 21:759 7. Takatoh K, Hasegawa M, Koden M, Itoh N, Hasegawa R, Sakamoto M (2005) Alignment technologies and applications of liquid crystal devices. Taylor & Francis, London 8. Janning J (1979) Appl Phys Lett 21:173 9. Gibbons W, Shannon P, Sun S, Swetlin B (1991) Nature 351:49 10. Schadt M, Schmitt K, Hozinkov V, Chigrinov V (1992) Jpn J Appl Phys 31:2155 11. O’Neill M, Kelly SM (2000) J Phys D Appl Phys 33: R67–R84

12. Raynes EP (1974) EI Lett 10:141–142 13. Guyon E, Pieranski P, Boix M (1973) Appl Eng Sci Lett 1:19 14. Sze-Yan Yeung F, Xie F-C, Kwok H-S, Wan J, Tsui O, Sheng P (May 2005) 23.2: High pretilt angles by nano-structured surfaces and their applications. SID international symposium digest of technical papers, vol 36, pp 1080 15. Paul Gass, Heather Stevenson, Richard Bray, Harry Walton, Nathan Smith, Shinichi Terashita, Martin Tillin (2003) Sharp Tech J 85:24 16. Dozov I, Nobili M, Durand G (1997) Appl Phys Lett 70(9):1179 17. Bryan-Brown G, Brown CV, Jones JC, Wood EL, Sage IC, Brett P, Rudin J (1997) 5.3: Grating aligned bistable nematic device. In: Proceedings of society for information display international symposium. Digest of technical papers, Vol XXVIII, Boston MA, pp 37–40 18. Gwag JS, Fukuda J, Yoneya M, Yokoyama H (2007) Appl Phys Lett 91:073504

1401

1402

7.2.2

Alignment Properties of Liquid Crystals

19. Kim J-H, Yoneye M, Yamamoto J, Yokoyama H (2001) Appl Phys Lett 78:3055 20. Salter PS, Elston SJ, Raynes EP, Parry-Jones LA (2009) Jpn J Appl Phys 48:101302 21. Carbone G, Salter P, Elston SJ, Raynes EP, de Sio L, Ferjani S, Strangi G, Umeton C, Bartolino R (2009) Appl Phys Lett 95:011102 22. Rieker TP, Clark NA, Smith GS, Parmar DS, Sirota EB, Safinya CR (1987) Phys Rev Lett 59:2658 23. Hachiya S, Tomoike K, Yuasa K, Togawa S, Sekiya T, Takahashi K, Kawasaki K (1993) J Soc Inf Disp 1:295

Further Reading Cognard J (1982) Molecular crystals and liquid crystals. Gordon and Breach, New York, Supplement 1, p 1

24. Dessaud N, Raynes EP (2001) Proceedings of international display workshops IDW, pp 41–44, Nagoya 25. Drzaic PS (1995) Liquid crystal dispersions. World Scientific Publishing, Singapore, pp 392–399 26. Minoura K, Asaoka Y, Satoh E, Deguchi K, Satoh T, Ihara I, Fujiwara S, Miyata A, Itoh Y, Gyoten S, Matsuda N, Kubota Y (2009) Making a mobile display using polarizer-free reflective LCDs and ultralow-power driving technology. Inf Dis 25:12–16

7.2.3 Liquid Crystal Theory and Modelling N. J. Mottram . C. J. P. Newton 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1404

2 2.1 2.2 2.3 2.4

Fundamentals of Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Length and Timescales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuum Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dependent Variables, Independent Variables, and Parameters . . . . . . . . . . . . . . . . .

1404 1404 1405 1405 1406

3 3.1 3.1.1 3.1.2 3.1.3 3.1.4

Nematic Liquid Crystal Theories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Director-Based Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Additional Physical Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Example: The p-Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1407 1408 1408 1412 1412 1413

4 4.1 4.2 4.3 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.4.7 4.4.8 4.4.9 4.4.10

Q-Tensor Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Uniaxial Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biaxial Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Tensor Order Parameter Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phenomenological Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dynamic Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Landau-de Gennes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrostatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Energy: Alignment Direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strong Anchoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Energy: Planar Degenerate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The p-Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1414 1414 1416 1417 1418 1419 1419 1420 1421 1422 1424 1425 1425 1425 1427

5

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1428

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_7.2.3, # Springer-Verlag Berlin Heidelberg 2012

1404

7.2.3

Liquid Crystal Theory and Modelling

Abstract: In this chapter we explain the rationale behind the theoretical modeling of liquid crystals and explain the important steps to construct a realistic and accurate model for a particular physical system. We then summarize two commonly used theories of nematics: one based on using the director as a dependent variable and one based on using the tensor order parameter. Using an example problem, the p-cell, we show the advantages and disadvantages of these two theoretical approaches, demonstrating the importance of carefully considering the choice of model before embarking on simulations.

1

Introduction

The theory and modeling of liquid crystalline materials has been an area of research for over 100 years, when the first fundamental theories of the state were proposed. Various theories have fallen by the wayside, such as the ‘‘swarm’’ theory of Bose [1], and others have grown in popularity, such as the Oseen-Frank [2, 3] theory for nematics (see [4] and [5] for a survey of the history of the area). However, the topic remains one of intense activity and debate. The use of mathematical models and underlying theory can be hugely rewarding, leading to insights into the fundamental behavior of a system and providing the opportunity to optimize the performance of liquid crystalline devices, but it is not without risks. While one model of a liquid crystalline system may be useful for a particular application, it may also lead to erroneous conclusions in a different setting. Care must be taken when developing a model and selecting a theoretical basis for the model. The misuse of mathematical models can have considerable and unpredictable consequences with far-reaching impact. This chapter introduces the motivation and fundamentals of the use of theory and models in liquid crystal systems and briefly considers two specific theories which have previously been used successfully. An example of a liquid crystal device (the p-cell) is examined using these two theories in order to demonstrate when, in one instance, simple models can fail dramatically and, for another instance, an unnecessarily complicated model leads to a waste of computational resources.

2

Fundamentals of Modeling

2.1

Motivation

The motivations for developing mathematical theories or using models to describe liquid crystals and liquid crystal devices are varied. On the most fundamental level there is the desire to understand the behavior of the liquid crystal phase, how the fluid orders orientationally and positionally, and which molecular interactions exist and give rise to larger scale phenomena. In this case it is natural to build a theoretical framework for liquid crystals from other, wellunderstood, physical effects or the inherent symmetries contained within the system and, if necessary, to introduce additional elements in an attempt to understand other phenomena. This process of developing fundamental theories has value in itself but is usually of particular benefit when used to explain experimental evidence or to predict behavior. The cycle of theoretical development, experimental comparison, and theory refinement is often used and rarely reaches a final conclusion. However, insight into the fundamental physics and the performance of physical devices is often gained along the way.

Liquid Crystal Theory and Modelling

2.2

7.2.3

Length and Timescales

As mentioned above, the selection of a theory or model to use in any given instance is of prime importance and is not always a simple undertaking. One approach to selecting the most appropriate theory is to consider the important length and timescales involved in the situation to be modeled. The range of relevant scales in the area of liquid crystal physics is large. The orientational and positional self-organization of molecules to form a liquid crystal phase is influenced by the atomic and molecular structure which exists at the sub-nanometer lengthscale and the picosecond timescale. However, large liquid crystal devices can have dimensions of meters (although the relevant length-scale is probably either the liquid crystal layer thickness, typically of the order of micrometers, or the interpixel distance, typically hundreds of micrometers) and timescales of milliseconds. The range of length and timescales therefore covers at least six orders of magnitude and a useable theory that accurately models such disparate scales does not exist. However, in many cases the problem at hand does not require such an all encompassing theory and we may concentrate on a smaller section of the complete behavior, assuming that the influence of phenomena at other length and timescales is either negligible or can be averaged. For instance, in the continuum modeling described below, we assume that molecular properties can be averaged to create macroscopic quantities such as the director n, the average orientation of a group of molecules at a point in space and at a specific time.

2.3

Continuum Modeling

With the necessary restriction of length and timescales mentioned above in mind, the main focus of this chapter will be on continuum theories of liquid crystalline materials. While models based on atomistic and molecular considerations have considerable value in understanding the small-scale behavior of these classes of molecules, and their interactions with other effects such as surfaces, the computational resource necessary to enable even the smallest liquid crystal device to be simulated is beyond present capabilities. For the reader interested in such models, a review and discussion can be found in the following reference and the references therein [6]. For most scientists and engineers who wish to simulate the operation of a liquid crystal device it is therefore to a continuum theory that they look. However, there are many types of continuum theory and it can be difficult to distinguish between them and to decide on the most appropriate theory, given a particular experiment or device. Here we provide a brief summary of two of the most commonly used theories for nematic liquid crystals, and provide references to other, more specialized, theories. We will first briefly summarize the commonly used Ericksen–Leslie theory, sometimes called the Ericksen–Leslie– Parodi theory, which can be extremely useful when modeling most ‘‘standard’’ liquid crystal devices. This theory is now over 40 years old and there have been many reviews and research papers written on this subject (see [7] for a complete description and many examples of the use of Ericksen–Leslie theory). For this reason we will only summarize the main points of the model, including some additional physical phenomena (flexoelectricity, ionic motion) which may be useful for more accurate modeling of liquid crystal devices, and point the reader to references where this theory is extensively discussed. We will also concentrate on a macroscopic theory, the so-called Landau-de Gennes or Q-tensor theory, which allows the modeling of

1405

1406

7.2.3

Liquid Crystal Theory and Modelling

regions of low order, such as defects and surface regions. Because of the inclusion of order as a dependent variable, such theories are sometimes called mesoscopic theories, since they contain information about the molecular ordering which is more commonly addressed using smaller length-scale approaches. In this chapter we will review the main aspects of the simplest form of this type of continuum model, discussing various additional physical phenomena through additional references to other work in the area.

2.4

Dependent Variables, Independent Variables, and Parameters

Assuming that the length- and timescales of the system being considered mean that it is appropriate to use a continuum theory, one of the key considerations when embarking on the simulation of a device is the classification of relevant dependent and independent variables and the system parameters. The classification used in this Chapter is as follows: ● Parameters are constants of the system which will not vary in space or time and are (at least potentially) experimentally measurable quantities. It is possible to use derived parameters which are themselves functions of other parameters but these quantities will also be constants of the system. ● Independent variables are quantities which vary in the system (i.e., spatial coordinates or time) and do not depend on the parameters in the system. By changing an independent variable (i.e., considering a different time or location during the simulation) the observation of the dependent variables will change. ● Dependent variables are quantities that describe the state of the system and will depend on one or more of the independent variables and may also depend on the system parameters. Dependent variables may be predetermined (i.e., the electric potential at an electrode may be assumed to be a specific function of time) or to be determined through the simulation of the system. Each of the dependent variables which are not predetermined will have associated with it an equation which governs how it depends on the independent variables, the predetermined dependant variables, and parameters. An example system would be a nematic liquid crystalline material in a simple Freedericksz cell [7, 8], where the order parameter S is assumed to be constant. For this case we might choose the director n, fluid velocity v, fluid pressure p, and the electric field E as the dependent variables. These variables would depend on the independent variables which might be the position in the cell x = (x, y, z) and time t. The dependent variables would also depend on the material and cell parameters such as: the elastic constants, fluid viscosities, density, and the relative permittivities of the liquid crystalline material; the dimensions of the liquid crystal layer; details of the anchoring at the substrates; the constant applied voltages at the substrate electrodes; etc. With these definitions we then use various physically derived equations, for instance the balance equations for mass, linear, and angular momentum or Maxwell’s equations, in order to produce governing equations for the spatial and temporal form of the dependent variables. In order to determine these unknown dependent variables it is necessary to have the same number of governing equations. For instance, for the example given above the dependent variables are the director n (a vector in 3 so there are three unknowns), the velocity v (again three unknowns), the pressure p (a scalar, so just one unknown), and the electric field

Liquid Crystal Theory and Modelling

7.2.3

E (three unknowns) so we have 11 unknowns and require 11 governing equations. As we will see later, the appropriate equations are the three components of the balance of linear momentum (which govern the fluid velocities), the three components of the balance of angular momentum (governing the director), the scalar mass balance equation (sometimes called the continuity equation, which effectively governs the pressure), and the three components of the relevant Maxwell equation (Gauss’s law) for the electric field. In fact, because the director is a unit vector, one more equation is needed, n2 = 1, but this extra equation is balanced by an unknown Lagrange multiplier. The number of unknowns (dependent variables and the Lagrange multiplier) is therefore 12 and the number of governing equations is 12. Because the governing equations are often in the form of differential equations, additional unknowns, namely, the integration constants, are often introduced once these equations are solved. Conditions on the dependent variables must therefore be specified at the boundaries of the region of interest, and possibly any internal boundaries where material properties change abruptly. For dynamic problems there should also be an initial condition specified, which prescribes the initial form of the dependent variables at the start of the simulation. Any specific modeling problem is specified by the appropriate set of governing equations, which govern the spatial and temporal behavior of the dependent variables, together with the correct boundary and initial conditions. This set of equations must then be solved (exactly or approximately, analytically or numerically).

3

Nematic Liquid Crystal Theories

Although there are many existing and developmental electro-optic devices which contain smectic liquid crystals, we shall here concentrate on theories for nematic liquid crystals. For the reader who is interested in smectic devices there we refer them to the discussions in the following references [7, 8]. We have split this section into two parts: the first concerns a very commonly used directorbased approach, and the second additionally considers the ordering of molecules as a dependent variable. We illustrate these two approaches using the liquid crystal p-cell. The p-cell is a liquid crystal cell where a sample of nematic liquid crystal is confined between two flat surfaces which are treated so that the molecules close to the surfaces have a preferred direction. This preferential direction manifests itself in the form of a ‘‘pretilt’’ of the director, a fixed direction which the director must take at the surface. In the p-cell the surfaces are arranged in the so-called parallel alignment where the pretilt angle (from the horizontal) takes values of opposite sign at the two surfaces (see > Fig. 1). This type of alignment often gives rise to two stable states: the splay state and the bend state (see > Fig. 1). Given a liquid crystal with a positive dielectric anisotropy the cell can be switched from the splay to the bend state by the application of an electric field across the cell [9]. At first sight we might assume that this device could be modeled using a theory which models the director without the need to consider the possibility of small-scale phenomena such as defects. However, we shall see later that the obvious length-scale (the cell thickness, of the order of microns) is not the only one in the system. As the electric field strength increases we will observe a second smaller length-scale, an internal reorientation layer which, eventually, necessitates the use of a more complex theory. In the sections below we will therefore consider this device using two different theories, showing the advantages and disadvantages of each.

1407

1408

7.2.3

Liquid Crystal Theory and Modelling

z θ = θp

E

θ = −θp

a

b

. Fig. 1 The stable states of the p-cell: (a) the splay state and (b) the bend state

3.1

Director-Based Model

For most devices based on the electro-optic effects of nematics, the most important macroscopic variable of concern to the scientist or engineer is the director n(x, t), the average orientation of the liquid crystalline molecules, which may depend on the spatial and temporal variables, position x = (x, y, z) and t. By definition, the director is a unit vector and we must ensure that any theory we develop satisfies this constraint. Other important dependent variables may be the electric field E(x, t), the magnetic field H(x, t), the fluid velocity v(x, t) and pressure p(x, t), and the concentration of positive or negative ion species nh(x, t), ne(x, t). In this section we summarize the standard governing equations for these variables which we hope may act as a relatively complete reference for those attempting to model nematic liquid crystal devices where there are no regions of reduced order (i.e., no regions of very high distortion such as close to defects). A number of examples of the use of these equations may be found in Stewart [7] and de Gennes [8].

3.1.1

Governing Equations

The governing equations for the fluid velocity and pressure are given by the balances of mass and linear momentum r  v ¼ 0;

r

dv ¼ rF þ r  t dt

ð1Þ

where v is the fluid velocity, r ¼ ð@=@x; @=@y; @=@zÞ is shorthand for the gradient vector, the derivatives in each direction, r is the (constant) fluid density, F is the external body force per unit mass, and t is the stress tensor. In the last term of the second equation of (> 1), the term r  t is a vector with elements (t11;1 þ t12;2 þ t13;3; t21;1 þ t22;2 þ t23;3; þ t23;3; t31;1 þ t32;2 þ t33;3 ) where tij;j ¼ @tij =@xj , the derivative of the ij th element of the tensor t with respect to the j th independent spatial variable xj , and x = (x, y, z) = (x1, x2, x3). In this form > Eq. 1 are the same as the Navier–Stokes equations for an incompressible fluid. However, it is in the form of the stress tensor that liquid crystals differ from Newtonian fluids, as we will see later.

Liquid Crystal Theory and Modelling

7.2.3

The governing equations for the director are the constraint that n is a unit vector, and the balance of angular momentum which can be written as n  n ¼ 1;

G þ g þ r  s ¼ 0;

ð2Þ

where the function s is defined through the couple stress tensor l = n  s, g is the intrinsic body force, and G is related to the external body couple through K = n  G. In order to complete these equations we need to ‘‘close’’ the system by specifying various quantities mentioned in > Eqs. 1 and > 2, which will be functions of the dependent variables and material parameters: the stress tensor t, the couple stress s, the intrinsic body force g, and the external forces and couples F and K, if present. The equations for the stress, couple stress, and intrinsic body force, the ‘‘constitutive equations,’’ can be shown [10] to be as follows: tij ¼ pdij  gi ¼ lni 

@w @w np;j þ ~t ij ; sij ¼ þ ~s ij ; @np;j @ni;j

ð3Þ

@w þ g~i ; @ni

ð4Þ

w ¼ wðni ; ni;j Þ;

where p is the pressure arising from the fact that the fluid is assumed to be incompressible, l is a Lagrange multiplier deriving from the n  n = 1 condition, ~t, ~s, and ~ g (which is actually related to the dynamic part of the stress tensor ~t through eijk nj ~gk ¼ eijk~tkj , where the summation of repeated indices has been used and eijk is the alternator, see [7]) are dynamic contributions, and w is the elastic energy density. These last four quantities will be specific to the type of liquid crystal we are considering and must obey certain constraints which derive from the symmetry of the phase and from basic thermodynamic considerations. Further details can be found in Stewart [7] and are summarized below. For the elastic energy density w of a nematic liquid crystal we assume that the director is ‘‘headless’’ so that n and –n are equivalent, therefore replacing n with –n in w must leave the elastic energy unchanged; any translation of the system leaves the energy unchanged; any rigid rotation of the system leaves the energy unchanged; and there are only small distortions present so that we may neglect terms smaller than ð@n=@xÞ2 . These assumptions lead to the general form of the elastic energy density 1 1 1 w ¼ K1 ðr  nÞ2 þ K2 ðn  r  nqÞ2 þ K3 ðn  r  nÞ2 2 2 2 1 þ ðK2 þ K4 Þr  ððn  rÞn ðr  nÞnÞ: 2

ð5Þ

It is relatively straightforward [8] to see that the first distortion term rn is associated with splaying of the director configuration, the second term nr  n is associated with twisting of the configuration, the third term n  r  n is associated with bending of the director configuration, and the fourth term is associated with a distortion in two directions (a saddlesplay term). In > Eq. 5 the Ki are elastic constants, specific to each liquid crystal material, are temperature dependent, and are usually called the Frank elastic constants. The constant q is the natural twist of the material which is nonzero only in chiral nematics. It can also be shown that (K2 + K4) = 0 in a chiral nematic [7]. It is common (but rarely accurate) to use the ‘‘one constant approximation’’ which asserts that K1 = K2 = K3 = K and K4 = 0 (some authors write K2 + K4 = 0 instead) which can greatly simplify the governing equations and may allow further

1409

1410

7.2.3

Liquid Crystal Theory and Modelling

mathematical analysis than would normally be possible. However, if the governing equations are to be solved numerically, there is little to be gained by using such an approximation and the specific values of each constant can be used. For the liquid crystal material 5CB (at 26 C) the first three elastic constants take the values K1 = 6.2  1012 N, K2 = 3.9  1012 N, and K3 = 8.2  1012 N [11]. The dynamic contributions ~t, ~s, and ~ g are also assumed to vanish in a rigid rotation so that they are functions of velocity gradients and director rotation through the rate of strain tensor D, the vorticity tensor W, and a vector N, where

 @vj @vj @vi 1 @vi i ; W Ni ¼ dn þ ¼  ð6Þ Dij ¼ 12 @x ij @xi @xi dt  Wik nk ; 2 @xj j The dynamic contributions must also be invariant to translations, rotations, and the transformation n ! –n and can therefore be shown to be ~tij ¼ a1 nk np Dkp ni nj þ a2 Ni nj þ a3 Nj ni þ a4 Dij þ a5 Dik nk nj þ a6 Djk nk ni ;

ð7Þ

g~i ¼ ða2  a3 ÞNi þ ða5  a6 ÞDik nk ;

ð8Þ

~sij ¼ 0;

ð9Þ

where the ai are the Leslie viscosities and can be related to the (experimentally more useful) Miesowicz viscosities [12, 13]. The equations above therefore specify the Ericksen–Leslie equations for a nematic liquid crystal. Parodi later added a restriction to the Leslie coefficients (ai) through an Onsager relation, which showed that a2 + a3 = a6 – a5 [14]. With this assumption there are therefore five independent viscosities and, augmented with the Parodi relationship, the governing equations are often called the Ericksen–Leslie–Parodi equations. In 5CB (at 26 C) the five viscosity parameters take the values a1 = 0.0060 Pas, a2 = 0.0812 Pas, a3 = 0.0036 Pas, a4 = 0.0652 Pas, and a5 = 0.0640 Pas [11]. All that remains is to consider possible external forces and couples that occur within liquid crystal samples. The most common body force (per unit mass) F, in > Eq. 1, will be a gravity force F = (0, 0, – g), although, because of the small-scale nature of most liquid crystalline flows, this term will usually be negligible. With regard to the body torque, K, the usual terms will be those derived from electromagnetic forces. We will treat applied magnetic and electric fields separately in the following discussion. In both cases, however, the application of a field can be either seen as an externally applied torque or, more usually, it is seen as a change in the free energy density. The most usual way to include such fields is therefore to augment the elastic energy with an additional magnetic or electric term. This is how we will proceed below. We first consider the simpler magnetic field case, followed by the electric field case. A liquid crystal molecule placed in a magnetic field H will distort that field, inducing a magnetization M and increasing the magnetic energy. It will distort the magnetic field by different amounts depending of the orientation of the molecule and, for a general director, the magnetization can be written as M ¼ wm? H þ wma (nH)n. The magnetic susceptibilities wmjj and wm? are usually negative while the diamagnetic anisotropy wma ¼ wmjj  wm? positive. The magnetic induction is B ¼ m0 ðH þ MÞ ¼ m0 ðð1 þ wm? ÞH þ wma ðnHÞnÞ;

Liquid Crystal Theory and Modelling

7.2.3

where m0 is the permeability of free space. The magnetic energy density is ð 1 1 wm ¼  BdH ¼  m0 m? H2  m0 ma ðn  HÞ2 ; 2 2

ð10Þ

where we have defined m? ¼ 1 þ wm? ; mjj ¼ 1 þ wmjj and ma ¼ mjj  m? ¼ wma This energy density should be added to the elastic energy density w in > Eq. 5 and, when integrated over the region, leads to the total free energy. To be accurate, for a specific director configuration, the magnetic field H should be calculated from Maxwell’s equations for the field and the magnetic induction B, i.e., r  H = 0 and rB = 0. However, since ma is usually very small (approximately 10–6, unitless if SI units are considered) compared with m? then we are usually safe to assume that B is directly proportional to H. In that case Maxwell’s equations become r  H = 0 and rH = 0, which are solved by setting H to be constant and prescribed by the external source of the magnetic field. It is therefore usual to assume that the magnetic field is constant, although the magnetic energy density in > Eq. 10 will still depend on the director orientation. A similar approach can be used for the electric field situation. The polarization P (the equivalent to the magnetization M) is made up of an induced (dielectric) component and other components, for instance if a flexoelectric polarization is present. The dielectric polarization is related to the applied electric field through the susceptibility tensor, so that the total polarization can be written as the sum of the dielectric polarization and other components Ps thus: P ¼ we? Eþwea ðn  EÞn þ Ps The electric displacement D (equivalent to the magnetic induction B) is then D = e0(E + P) so that D ¼ e0 ðð1 þ we? ÞE þ wea ðn  EÞEÞ þ e0 Ps ¼ e0 ðe? Eþea ðn  EÞEÞ þ e0 Ps ;

ð11Þ

where e? ¼ 1 þ we? ;ejj ¼ 1 þ wejj and ea ¼ wejj  wejj  we? are the dielectric anisotropy (sometimes denoted by De). The most commonly used form of the additional polarization term in nematics is the flexoelectric polarization, e0 Ps ¼ e11 ðr  nÞn þ e33 n  r  n. There are unfortunately two conflicting conventions for the sign of the e33 term and here we have opted for the original form suggested by Meyer [15]. The factor of e0 in the definition of the flexoelectric polarization acts only as a scaling factor and is very often incorporated into the definitions of e11 and e33. It is more usual therefore not to see the factor of e0 in the final term in > Eq. 11. Using this latter definition of the flexoelectric polarization, the electrostatic energy density is (similar to the magnetic case) then ð 1 1 ð12Þ we ¼  D  dE ¼ e0 e? E2  e0 ea ðn  EÞ2  e0 Ps  E: 2 2 However, because the electric susceptibilities are much larger than the magnetic susceptibilities, the appropriate Maxwell’s equations must be solved in order to obtain the (nonconstant) form of the electric field E throughout the liquid crystal sample. Maxwell’s equations for the electric field and the electric displacement are r  E = 0 and rD = 0. The first of these equations implies that the electric field may be written in terms of an electric potential U, such that E = rU, and the second equation may be solved, together with the equations for the director, fluid velocity, and pressure, to provide the solution for the electric potential U(x, t).

1411

1412

7.2.3 3.1.2

Liquid Crystal Theory and Modelling

Boundary Conditions

In most liquid crystal devices the glass or plastic substrates within which the liquid crystal material is sandwiched are treated to provide some form of alignment of the director. The main forms of alignment are: ● Strong (or infinite) anchoring: the alignment layer at the substrate is strong enough to fix the liquid crystal molecules there. The director is therefore fixed to be a specific value n = ns at the substrate. ● Weak anchoring: the alignment method defines a fixed direction, a preferred direction, ns. The director at the surface may take a different value from ns but this will cost energy. Mathematically, strong anchoring gives a fixed, ‘‘Dirichlet’’ boundary condition, n = ns. For weak anchoring we specify a surface energy, such as the commonly used Rapini–Papoular energy [7, 16] ws = Ws(1 – (nns)2), which is added to the bulk energy. A solution for n should then minimize the total energy. The inclusions of surface energy terms means that the boundary conditions change from Dirichlet conditions to Neumann conditions, where a balance of torques at the surface lead to the gradient of the free energy, normal to the substrate, being specified at the boundary (see [7] for more details). Boundary conditions for the fluid velocity and pressure are dependent on the problem considered. However, in most liquid crystal display situations, the appropriate boundary conditions will be that the fluid is stationary at a solid boundary, i.e., v = 0 at the substrate, and that pressure at either end of the device is equal, i.e., p = pa at x =  Lx and y =  Ly, where Lx, Ly is the xy extent of the device under consideration. The usual boundary conditions for the electric potential are that, at electrodes, the potential takes a constant or time-varying value, i.e., U = 0 at one electrode and U = V(t) at the other electrode.

3.1.3

Additional Physical Phenomena

As well as the relatively standard consideration of the director, flow velocity, electric field, etc., it is sometimes appropriate to consider additional dependent variables in order to accurately model liquid crystal devices. One of the most common examples of this is the inclusion of mobile ionic species in the system. It is often the case that the complete removal of ionic contaminants is nearly impossible and, in any case, the repeated application of electric fields can cause the introduction of ions (from surface layers and/or the liquid crystal molecules decomposing). Such ionic species often shield the applied voltages and can influence the behavior of the director and thus the electro-optic response of the device. As an example of the type of governing equations that result, we here consider two ionic species which may move within the fluid due to the application of an electric field as well as through convection of the ions with the fluid flow and natural diffusion. The governing equations for the number densities of the two species ne and nh (negative and positive ions) is   @ne De þ ðv  rÞne ¼ me r  ne rU  rne þ Re ; ð13Þ @t me   @nh Dh þ ðv  rÞnh ¼ mh r  nh rU þ rnh þ Rh ; ð14Þ @t mh

Liquid Crystal Theory and Modelling

7.2.3

where me and mh are the negative and positive ion mobilities and De and Dh are the negative and positive ion diffusivities. The quantities Re and Rh model any possible effects associated with the production or destruction of ions within the device, which may be functions of parameters such as the applied voltage or even of the ionic densities. For such a system it is usual to consider the boundaries to be insulated, i.e., that no current can flow into or out of the cell. Mathematically, these conditions can be expressed in the following forms: De Dh rnh 0 ¼ ne rU  rne ; 0 ¼ nh rU þ ð15Þ me mh The only change to the previously stated governing equations is that the Maxwell’s equation must be altered to be r  D ¼ q h nh  q e ne ;

ð16Þ

where qe and qh are the charges on the negative and positive ions, respectively. This set of equations, together with appropriate initial condition for ne and nh at t = 0 should then be solved at the same time as the governing equations for the director, flow velocity, electric field potential, etc.

3.1.4

Example: The p-Cell

We illustrate the use of these equations using a simple model of the p-cell [17] (see > Fig. 1). To allow easy comparisons with the Q-tensor model we describe later, we will ignore flow effects and ionic contaminants, and assume strong anchoring at the boundaries. The only equation that remains is the angular momentum balance (> 2) and substituting from (> 3–4) and (> 7–9) gives a set of equations which govern the components of the director. For static solutions @n=@t ¼ 0, and these equations are equivalent to the Euler–Lagrange equations for the minimization of the total energy in the system subject to the constraint that n is a unit vector. However, in this example we consider the dynamic equations. If we assume that the director always lies in a single plane (the xz-plane in our example) and that the director does not vary in the x and y directions, then the director can be written as n = (cos y(z, t), 0, sin y(z, t)) where y, the tilt angle, is the angle that the director makes with the positive x-axis and is a function of z and t only. For this example we use the ‘‘one constant approximation’’ and we neglect any additional polarization terms, such as Ps. As mentioned above, we write the electric field as the gradient of the electric potential, E = rU, and then, after some relatively simple but long-winded manipulations of the equations (see, e.g., [7]) we obtain governing equations for the y(z, t) and U(z, t): @y ¼ K yzz  e0 ea sin y cos yUz2 : @t  e0 ðe? þ ea sin2 yÞUzz  e0 ea sin 2y yz Uz ¼ 0; g1

ð17Þ ð18Þ

where the subscript z denotes differentiation with respect to z and g1 = a3 – a2 is the rotational viscosity of the director. > Equations 17 and > 18 must be solved subject to appropriate boundary conditions. y at the boundaries is given by the pretilts at the surfaces (see below) and for the electric potential we set U = 0 at the bottom surface and U = V, the applied voltage, at the top.

1413

1414

7.2.3

Liquid Crystal Theory and Modelling

In the p-cell we have equal and opposite pretilts at the boundaries and this allows us to set the boundary conditions for the problem. However we do need to be careful when assigning values at the boundary to the director angle y. To see this we consider static solutions @y=@t ¼ 0, in the zero field case, when V = 0. For this situation, > Eq. 18, with the boundary conditions, leads to U(z)  0. > Equation 17 then leads to yzz = 0, and solutions are therefore linear in z. For the splay state, > Fig. 1a, the situation is straightforward, the director tilt changes linearly, from yp to yp, and the tilt in the center of the cell is zero. To set the boundary conditions for the bend state, > Fig. 1b, we first note that n and –n are equivalent and so y and y – p are equivalent. We can therefore consider the tilt varying linearly from –yp to yp – p. The tilt in the center of the cell is then –p/2. There is a third possible state: the twist state. This state is an important one since the bend state will, once the electric field is removed, normally relax into the twist state since it is of a lower energy. However, since we will be concerned with the transition between states when the field is applied we have restricted the director to the xz-plane and will not consider the relaxation to the twist state. > Figure 2 shows the tilt profiles obtained when we apply a number of different voltages to the splay and the bend states. For this situation, where the liquid crystal has a positive dielectric anisotropy, the electrostatic energy is a minimum when the director is aligned with the field (i.e., when y = p/2, or equivalently y = p/2). Aligning the director with the field increases the distortion, and the associated elastic energy, and the final configuration results from a balance between the electric field and elastic effects, subject of course to the boundary conditions. > Figure 2 shows the solutions to > Eqs. 17 and > 18 for voltages V = 0, 2, 5, 30. (Given our comments above about the twist state, the solution for the bend state at 0 V is only the transient solution, before relaxation to the twist state.) The parameters used for this simulation were typical for a nematic liquid crystal: g1 = 0.02 Pas, K = 14.7  1012 N, e0 = 8.854  1012 F m1, ea ¼ 14; e? ¼ 5:1; yp = p/30 rads. The extent of the liquid crystal region (the cell gap) was taken to be 2  106 m. In > Fig. 2a we see that as the voltage increases, for the splay state, the director becomes increasingly vertical (y = p/2 or –p/2) in the majority of the cell, with regions of distortion at the boundaries and in the center of the cell as well. For the splay state the tilt at the center of the cell remains at zero. We see that the increasing electric field is producing a region of relatively small length-scale. For the bend state, in > Fig. 2b, the bulk of the cell, including the center of the cell, can align vertically, with distortion regions only at the boundaries. For both states, when the voltage is removed the director configuration reverts back to the initial state, splay, or bend, respectively. For these range of voltages switching between the two states does not occur. In fact, as will be discussed later, the fact that this model does not lead to switching between the two states is a consequence, and a problem, with the chosen model. We will see later that a more realistic model does in fact allow switching between states.

4

Q-Tensor Models

4.1

Uniaxial Order

In addition to the director n, the average molecular orientation direction, we could also compute the standard deviation of the distribution or orientations of molecules. This would provide additional information about the state of matter at that point in space. However, rather

7.2.3

Liquid Crystal Theory and Modelling

2 θ, director angle (radians)

1.5 1 0.5 0 −0.5 −1 −1.5 −2

a

0

0.5

1

z, distance through cell

1.5

2

(x10−6m)

θ, director angle (radians)

0

b

0V 2V 5V 30V

−0.5 −1 −1.5 −2 −2.5 −3 −3.5

0

0.5

1

z, distance through cell

1.5

2

(x10−6m)

. Fig. 2 Plots of the director angle for the p-cell in the (a) splay state and (b) the bend state

than the standard deviation of this distribution, the usual measure of this amount of order is the scalar order parameter, usually denoted by S, which is a weighted average of the molecular orientation angles ym between the long molecular axes and the director S¼

1 < 3cos2 ym  1 >; 2

where < > denotes the thermal or statistical average, i.e., ð 1 S¼ ð3cos2 ym  1Þ f ðym Þ dV ; 2 B

ð19Þ

ð20Þ

where f(ym) is the probability distribution function of the molecular angle ym. When the material is crystalline all molecules align exactly with the director and so ym = 0 for all molecules, which means that < cos2 ym > = 1 and S = 1. When all molecules lie in the plane perpendicular to the director, but randomly oriented in that plane, the average still leads to the same director but ym = p/2 for all molecules so that < cos2 ym > = 0 and S = 1/2. In the

1415

1416

7.2.3

Liquid Crystal Theory and Modelling

isotropic liquid phase the molecules are randomly oriented and so f(ym) is constant and equal R to 1/(2p) (this can be derived from the property of probability distributions that f(ym)dA = 1). Therefore, performing the integration in > Eq. 20 in spherical coordinates, where ym is the angle between the molecule and the director and fm is the azimuthal angle, i.e., the angle between a fixed direction in the plane perpendicular to the director and the projection of the director onto that plane, we obtain (using the substitution X = cos(ym)) ð ð ð 1 2p p=2 1 2 ð21Þ cos ym f ðym ÞdA ¼ cos2 ym sin ym dym dfm ¼ 2p 3 0 0 dBþ and so < cos2 ym > = 1/3 and from > Eq. 20, S = 0. Although it is possible to achieve a molecular configuration for which S is negative, i.e., –1/2 < S < 0, it is more usual that, in the equilibrium liquid crystal state, the scalar order parameter is positive, 0 < S < 1. As the temperature of the material changes the scalar order parameter will change from S = 0 in the isotropic state, at high temperature, to S > 0 in the liquid crystalline state, at low temperature. A typical scalar order parameter in the middle of the phase region, for this type of liquid crystal, might be S = 0.6. It is possible to now construct a theory, similar to the Ericksen–Leslie theory discussed in the previous section, which treats S(x, t) as well as n(x, t) as dependent variables. Ericksen developed just such a theory [18] and it is possible to consider regions of low order, and to model defects, with this type of model. However, a single order parameter theory does not describe the most general form of a nematic liquid crystal consisting of rod-like, uniaxial molecules. For more general cases we must consider biaxiality.

4.2

Biaxial Order

The fundamental principle of any biaxial system is that there is no axis of complete rotational symmetry (i.e., no axis about which a rotation of any angle leaves the system unchanged), unlike a uniaxial system which has an axes of rotational symmetry (such as the director n in uniaxial liquid crystals). However, there can be defined a set of perpendicular axes (only two need to be defined as the third is then specified as perpendicular to the other two, n  m) for each of which there is a reflection symmetry. In liquid crystals the two axes, or directors, n and m are therefore defined and the symmetries are the reflections n ! – n and m ! – m. The most general nematic state for uniaxial molecules is then the biaxial state described by two vectorvalued variables and two scalar-valued variables, the directors n(x, t) and m(x, t) and the scalar order parameters S1(x, t) and S2(x, t), the measure of order with respect to the two directors, all of which may depend on the spatial coordinates x and time. We assume, without loss of generality, that the directors are of unit length, |n| = 1 and |m| = 1. These directors can be represented in terms of the standard Euler rotation angles. Since the director n is of unit length it can be written as n ¼ ðcosy cosf; cosysinf; sin yÞ:

ð22Þ

The director m is perpendicular to n and therefore has one remaining degree of freedom, the angle c, so that m ¼ ðsinfcosc  cosfsincsiny;  sinfsinc siny  cosfcosc; sinccosyÞ:

ð23Þ

Liquid Crystal Theory and Modelling

7.2.3

The angle c is the angle from m to the direction (sin f, – cos f, 0) in the xy-plane, which is also perpendicular to n. A theory can now be constructed using the five dependent variables, y(x, t), f (x, t), c(x, t), S1(x, t), and S2(x, t). However, there may be problems with a theory such as this based on Euler angles. When the zenithal angle y equals p/2 the azimuthal angle f is undefined and, as with all such angle variables, there may be a problem with multivaluedness since f = 0 is equivalent to f = 2p. Care must therefore be taken when solving the governing differential equations if the dependent variables y, f, and c are to be used. This problem of degeneracy of rotation angles can however be avoided if the directors are not described in terms of these angles, but left in terms of the six components of the directors n = (nx, ny, nz) and m = (mx, my, mz). However, in this case three Lagrange multipliers (similar to l used in the Ericksen–Leslie theory) must be used to ensure that the constraints that n, m are unit vectors and perpendicular. This approach can be simplified if an alternative approach is used, using the so-called order tensor for the system.

4.3

The Tensor Order Parameter Q

We now define an alternative approach which removes the problems of the angle representation and the complexities of the Lagrange multipliers described above. Instead of defining the nematic state in terms of the five separate variables mentioned above, we construct a 33 matrix which contains all the information about the nematic state, i.e., the information contained in the five variables. The problems of solving the Euler angle governing equations will be removed with this approach. Consider the 3  3 matrix (or tensor), M = S1 (n  n) + S2 (m  m), where for any vector h the ij th element of the product h  h is hi hj, the ith element of h multiplied by the j th element of h. This matrix will be symmetric, since ni nj = nj ni and mi mj = mj mi, and the trace of M will be S1 + S2 since |n| = 1 and |m| = 1. The tensor M contains the same information as the five separate variables. However, if we construct a theory using M there will be no problems with the degeneracies of Euler angles. In fact the tensor M is the second moment tensor for the molecular distribution and therefore is directly related to the orientational distribution function [19]. As will be indicated later, it is actually more useful to use the tensor: 1 ð24Þ Q ¼ S1 ðn  nÞ þ S2 ðm  mÞ  ðS1 þ S2 Þ I; 3 where I is the identity matrix so that the trace of Q is zero. The tensor order parameter Q is therefore a symmetric traceless matrix and can be written as 0 1 q2 q3 q1 B C q4 q5 A; ð25Þ Q ¼ @ q2 q3

q5 q1  q4

and from the definitions of n and m and > Eqs. 24 and > 25 we see that the elements qi can be written in terms of the variables y, f, c, S1, and S2 thus: q1 ¼ S1 cos2 y cos2 f þ S2 ðsin f cos c  cos f sin c sin yÞ2 

1 ðS1 þ S2 Þ; 3

q2 ¼ S1 cos2 y sin f cos f S2 ðcos fcos c þ sin f sin c sin yÞðsin f cos c  cos f sin c sin yÞ;

ð26Þ ð27Þ

1417

1418

7.2.3

Liquid Crystal Theory and Modelling

q3 ¼ S1 sin y cos y cos f þ S2 sin c cos y ðsin f cos c  cos f sin c sin yÞ; q4 ¼ S1 cos2 y sin2 f þ S2 ðcos f cos c þ sin f sin c sin yÞ2 

1 ðS1 þ S2 Þ; 3

q5 ¼ S1 cos y sin y sin f  S2 sin c cos y ðcos f cos c þ sin f sin c sin yÞ:

ð28Þ ð29Þ ð30Þ

The eigenvalues of the matrix Q, described by > Eqs. 26–30, are l1 ¼ ð2S1  S2 Þ=3; l2 ¼  ðS1 þ S2 Þ=3; l3 ¼ ð2S2  S1 Þ=3 :

ð31Þ

Uniaxial states exist when two of these eigenvalues are the same, i.e., when l1 = l2 so that S1 = 0 or when l2 = l3 so S2 = 0 or when l1 = l3 so S1 = S2. When all the eigenvalues are the same we have an isotropic system and we have S1 = 0 and S2 = 0 so that Q = 0. We shall see later that the use of Q rather than M as our dependent tensor variable was in fact dictated by the property that the isotropic state is described by Q = 0. There will be no problems when solving the governing equations for the five dependent variables q1, q2, q3, q4, and q5 because the Euler angles only appear in sin and cos functions and the problem with the multivaluedness of the angles is removed.

4.4

Phenomenological Theory

As was seen for the director-based theory, it is necessary to construct certain quantities such as Ð the total free energy F ¼ wdV, the integral of the free energy density over the volume of the region under consideration. This energy may include terms such as: the elastic energy of any distortion to the structure of the material; thermotropic energy which dictates the preferred phase of the material; electric and/or magnetic energy from an externally applied electric or magnetic field, and, in polar materials, the internal self-interaction energy of the polar molecules; and surface energy terms representing the interaction energy between the bounding surface and the liquid crystal molecules at the surface. The total energy is therefore F ¼ F distortion þ F thermotropic þ F electromagnetic þ F surface ð ð ¼ ðwd þ wt þ we Þ du þ ðws Þ ds; V

ð32Þ

S

where the energy densities, wd, etc., depend on the dependent variables, the tensor order parameter elements. In static situations the governing equations may be derived from the minimization of this energy, using the calculus of variations, leading to sets of differential equations in the bulk of the material and at the surface, for each of the dependent variables. The solution of these bulk equations subject to the surface boundary conditions gives the equilibrium configuration of the dependent variables through the sample. A version of such a theory which considers fluid flow as well as variations in the order tensor is beyond the scope of this summary article and we refer the interested reader to the following reference [20]. The free energy density is assumed to depend on the tensor order parameter Q and all firstorder differentials of Q. It is assumed that distortions of Q are small and therefore higher order differentials and high powers of first-order differentials will be negligible. The bulk free energy

Liquid Crystal Theory and Modelling

7.2.3

density, wb = wd + wt + we, is therefore the integral of a function dependent on the elements of Q and all derivatives of the elements whereas the surface free energy density is assumed to be a function of the elements of Q only: ð ð wb ðqi ; r qi Þdu þ ws ðqi Þ ds: ð33Þ F ¼ V

S

We write down the different terms that might contribute to the free energy density and assign model parameters to them. Knowing what values to use for these parameters when we come to solve the resulting equations can be difficult. However, we can often relate these model parameters to measured quantities by assuming that we have a uniaxial state and writing down the energy terms on that basis. We then relate the energy terms that we obtain to those in the corresponding director model assigning the parameters appropriately.

4.4.1

Dynamic Equations

When the dynamic evolution of the Q-tensor is required a simple dissipation principle can be used to show that the governing equations will be @D ¼ r:Gi  f i ; ð34Þ @ q_ i 1 where D is the dissipation function D ¼ tr ðð@Q=@tÞ2 Þ; q_ i ¼ @qi =@t, and the viscosity g is 2 related to the standard nematic viscosity g1 (which is equal to a3 – a2 in Leslie viscosities) by 2 g = g1/ð2Sexp Þ, and Sexp is the uniaxial order parameter of the liquid crystal when the experimental measurement of g1 was taken. The other quantities in > Eq. 34 are gradients of the free energy, given by Gij ¼ @wb =@qi;j and f i ¼ @wb =@qi . In fact a similar dissipation principle approach can be used to derive the Ericksen–Leslie equations (see [7]). g

4.4.2

Boundary Conditions

Similar to the director-based theory, there are two types of boundary conditions which will be considered: strong (infinite) or weak anchoring. ● For strong, or infinite, anchoring we will assume the Dirichlet conditions. That is, the order tensor will be fixed at a specified value at the domain boundary that has been dictated by some substrate alignment technique. In this case the boundary condition is Q = Qs, where Qs is the prescribed order tensor at the boundary. In this case there is no surface energy and ws = 0 in > Eq. 32. ● For a weak anchoring effect there exists a surface energy which is added to the free energy and must be minimized at the same time as the bulk free energy. This minimization leads to the condition that, on the boundary, the liquid crystal variables qi satisfy X  @wb  @ws nj ; ð35Þ ¼ @qi;j @qi j¼1;2;3 or equivalently nGi = Gi where Gi = ∂ws/∂qi and n is the normal at the substrate pointing into the liquid crystal regions. Usually the surface is planar or circular so that the surface

1419

1420

7.2.3

Liquid Crystal Theory and Modelling

normal is relatively simple, e.g., n = (0, 0, 1) if the surface is a plane parallel to the xy-plane. However, in general the substrate normal can be position dependent n(x). It is now necessary to specify each of the components of the free energy. The following sections describe the thermotropic, elastic, electrostatic, and surface energies.

4.4.3

Landau-de Gennes

The thermotropic energy, wt, is a potential function which dictates which state the liquid crystal would prefer to be in, i.e., a uniaxial state, a biaxial state, or the isotropic state. At high temperature this potential should have a minimum energy in the isotropic state, i.e., Q = 0 whereas at low temperatures there should be minima at three uniaxial states, i.e., the states where any two of the eigenvalues of Q are equal. We will not consider a situation where a biaxial state is a local minima. The simplest form of such a function is a Taylor expansion about Q = 0: wt ¼ a tr ðQ2 Þ þ

2b c tr ðQ3 Þ þ ðtr ðQ2 ÞÞ2 ; 3 2

ð36Þ

which is a quartic function of the qis. This energy is sometimes written as wt ¼ a tr ðQ2 Þ þ

2b tr ðQ3 Þ þ c tr ðQ4 Þ: 3

ð37Þ

> Equations

36 and > 37 are equivalent only if the factor of two in the c term is included. The coefficients a, b, and c will in general be temperature dependent although it is usual to approximate this dependency by assuming that b and c are independent of temperature while a = a(T – T ∗) = aDT where a > 0 and T ∗ is the fixed temperature at which the isotropic state becomes unstable. By writing this energy density in terms of the order parameters and directors mentioned above, it is relatively straightforward to show that this term has stationary points when S1 ¼ 0;

ð38Þ

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1
b þ b2  24ac ; 4c pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1
S1 ¼ b  b2  24ac : 4c

ð39Þ

S1 ¼

ð40Þ

By calculating d 2 wt =dS12 and comparing the energies of each solution we find that 2

2

b b ● S1 = 0, the isotropic state, is globally stable for a > 27c , metastable for 0 < a < 27c , and unstable
for a