Case Studies in Micromechatronics: From Systems to Processes 3662613190, 9783662613191

Table of contents :
Preface
Contents
1 Introduction
1.1 Mechatronic Systems
1.2 A Brief History of Micromechatronics
1.2.1 Integrated Circuits
1.2.2 Piezoresistivity
1.2.3 Silicon Bulk Micromachining
1.2.4 LIGA
1.2.5 Surface Micromachining
1.2.6 Soft Lithography and Lab-on-Chip Systems
1.2.7 Direct Micromachining
1.2.8 The Future
1.3 Overview of Microfabrication Methods
1.4 Applications and Markets
1.5 Topics of the Addressed Case Studies
1.6 Summary
References
2 Piezoresistive Pressure Sensors
2.1 Applications of Pressure Sensors
2.1.1 Atmospheric Pressure Measurement
2.1.2 Automotive Industry
2.1.3 Flow Measurements in Aircraft Research
2.2 Design and Fabrication of Piezoresistive Pressure Sensors
2.2.1 Basics
2.2.2 Functional Principle of Piezoresistive Pressure Sensors
2.2.3 Silicon Wafer Fabrication and Crystallography of Single-Crystal Silicon
2.2.4 Electrical Properties of Single-Crystal Silicon and Doping
2.2.5 Piezoresistive Effect of Doped Single-Crystal Silicon
2.2.6 Sensor Design
2.3 Exemplary Fabrication Process of the Silicon Membrane Device as Part of a Piezoresistive Pressure Sensor
2.4 Excursus: Lithography
2.5 Summary
References
3 Surface Micromachined Acceleration Sensors
3.1 Accelerometer Applications
3.1.1 Inertial Navigation Systems
3.2 Design and Functional Principles of Acceleration Sensors
3.2.1 Accelerometer Basics
3.2.2 Acceleration Sensor Designs
3.2.3 Multi-Axis Designs
3.2.4 Capacitance Detection Circuits
3.2.5 Closed-Loop Accelerometers
3.3 Surface Micromachining
3.3.1 Basic Sacrificial Layer Processing
3.3.2 Surface Micromachined Devices
3.4 Deep Reactive Ion Etching (DRIE)
3.5 Process Chain for the Fabrication of a Surface Micromachined Accelerometer
3.6 Thin Film Deposition
3.6.1 Physical Vapor Deposition (PVD)
3.6.2 Chemical Vapor Deposition (CVD)
3.6.3 Non Vacuum Deposition Techniques
3.7 Excursus: Further Sensor Principles and Characteristics
3.7.1 Sensor Classification Schemes
3.7.2 Static and Dynamic Sensor Characteristics
3.8 Summary
References
4 Mechanical Microgrippers
4.1 Applications of Microgrippers
4.1.1 Handling Technology and 3D Micropart Assembly
4.1.2 In-Liquid Living Cell Handling and Manipulation
4.1.3 Minimally Invasive Surgeries
4.1.4 Investigation of Multi-Axial Stress States of Skeletal Muscle Tissue
4.2 Design and Functional Principles of Mechanical Microgripers
4.3 Materials
4.3.1 Silicon
4.3.2 Su-8
4.3.3 Cyclic Olefin Copolymers (COC)
4.3.4 Further Suitable Materials
4.4 Fabrication Methods
4.4.1 Silicon Bulk Technique: ICP-DRIE
4.4.2 UV Depth Lithography
4.4.3 Microinjection Molding MIM
4.4.4 Further Fabrication Methods
4.5 Actuation Principles
4.5.1 Definition of an Actuator
4.5.2 Classification of Actuation Principles
4.5.3 Scaling Laws
4.5.4 Comparison of Microactuation Principles
4.5.5 Shape Memory Actuation Principle
4.5.6 Pneumatic Actuation Principle
4.5.7 Further Microactuation Principles
4.6 Further Microgripper Components and All-in System
4.6.1 Design of the Microgripper Gear
4.6.2 Gripping Force Sensing Principle
4.6.3 Mechanical Microgrippers Actuated by SMA Actuator Elements and Pneumatically
4.7 Exemplary Fabrication Process of the Pneumatically Actuated SU-8 Microgripper
4.8 Excursus: Further Microactuator Systems
4.8.1 Electromagnetic Micropump for Biomedical Microfluidic Systems
4.8.2 Bistable Microvalve
4.8.3 Membrane-Type Micropump with Passive Silicon Microvalve
4.9 Summary
References
5 Point-of-Care Diagnostic Systems
5.1 Applications of Point-of-Care Diagnostic (POCD) Systems
5.1.1 Antigen-Based POCD Systems
5.1.2 POCD Against Antibiotic Resistance
5.1.3 Application of POCDs in the Developing World—Malaria
5.1.4 Working Principle of Antigen-Based Lateral Flow Immunoassays
5.1.5 Application of POCDs in Self-Diagnosis—the Pregnancy Test
5.1.6 Application of POCDs in Self-Testing and Disease Management—Glucose Meter
5.1.7 Application of POCDs in Primary Care—Streptococcal Pharyngitis
5.1.8 Molecular POCD Systems
5.1.9 Application of POCDs in Personalized Medicine—C-Reactive Protein (CRP)
5.2 Microfluidics—The Basics
5.2.1 Brief Introduction
5.2.2 What is a Fluid?
5.2.3 Viscosity
5.2.4 Reynolds Number and Flow Regimes
5.2.5 Laminar and Turbulent Flow
5.2.6 Other Dimensionless Parameters
5.2.7 More about Diffusion
5.2.8 Surface Tension and Wetting
5.2.9 Resistance to Flow
5.2.10 Stokes Flow
5.3 Microfluidic Components for Active Flow Control
5.3.1 Microchannels
5.3.2 Microfluidic Valves
5.3.3 Micropumps
5.3.4 Microfluidic Mixers
5.3.5 Microfluidic Sample Separation
5.4 Soft Lithography
5.4.1 Micro-Contact Printing
5.4.2 Microtransfer Molding
5.4.3 Micromolding in Capillaries
5.4.4 Micro-Replica Molding
5.4.5 Nanoimprint Lithography
5.4.6 Solvent-Assisted Micromolding
5.5 Quartz Technology
5.5.1 Quartz Crystal
5.5.2 Production of Synthetic Quartz
5.5.3 Properties of Single Crystal Quartz—Anisotropy
5.5.4 Piezoelectric Properties of Quartz
5.5.5 Crystal Oscillators—Quartz Crystal Microbalance
5.5.6 Miniaturization of QCMs
5.6 Laser Micromachining
5.6.1 Lasing
5.6.2 Stimulated Emission
5.6.3 Population Inversion
5.6.4 Laser Operation
5.6.5 Types of Lasers
5.6.6 Pulsed Laser Ablation (PLA)
5.6.7 Two-Photon Polymerization (2PP)
5.7 Fabrication Chain of a POCD System for the Detection of CRP
5.7.1 Fabrication of QCM
5.7.2 Master Wafer Fabrication and the Production of the Microfluidic Channels
5.7.3 Assembly of QCM Sensor for the Detection of CRP
5.8 Excursus: Microbioreactors
5.9 Summary
References
6 Flexible and Stretchable Sensor Arrays
6.1 Applications of Flexible and Stretchable Sensor Arrays
6.2 Design, Materials and Fabrication Strategies
6.3 Process Chain for the Fabrication of a Flexible Sensor Array
6.4 Excursus: Cleanroom Technology
6.4.1 Defects and Yield
6.4.2 Cleanroom Design
6.4.3 The Human Factor in Cleanrooms
6.4.4 Process Liquids and Gases
6.4.5 Advanced Cleanroom Concepts
6.5 Summary
References
Index

Citation preview

Stephanus Büttgenbach Iordania Constantinou Andreas Dietzel Monika Leester-Schädel

Case Studies in Micromechatronics From Systems to Processes

Case Studies in Micromechatronics

Stephanus Büttgenbach · Iordania Constantinou · Andreas Dietzel · Monika Leester-Schädel

Case Studies in Micromechatronics From Systems to Processes

Stephanus Büttgenbach Technische Universität Braunschweig Braunschweig, Germany

Iordania Constantinou Technische Universität Braunschweig Braunschweig, Germany

Andreas Dietzel Technische Universität Braunschweig Braunschweig, Germany

Monika Leester-Schädel Technische Universität Braunschweig Braunschweig, Germany

ISBN 978-3-662-61319-1 ISBN 978-3-662-61320-7  (eBook) https://doi.org/10.1007/978-3-662-61320-7 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, 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. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Responsible Editor: Michael Kottusch This Springer imprint is published by the registered company Springer-Verlag GmbH, DE part of Springer Nature. The registered company address is: Heidelberger Platz 3, 14197 Berlin, Germany

Preface

This book was developed on the basis of the material that was made available to the students at the lectures Principles of Microtechnology and Applications of Microtechnology at Technische Universität Braunschweig, which has been supplemented and adapted over the years. These courses were initiated by Stephanus Büttgenbach and later continued by Andreas Dietzel and Monika ­Leester-Schädel with some adjustments. Additionally, some material from courses on actuators, Lab-on-Chip and biosensors has been included in this book. Since the emergence of the field of micromechatronics and the entry of this subject into academic teaching, numerous textbooks have been written focusing on microfabrication techniques. Some books on microsystem applications that pick out certain ­sub-areas such as sensors, micro-robotics or Lab-on-Chip and treat them in depth are also available. It was important to the authors to not just add another textbook to this topic, but to open up a different approach to teaching. The readers should quickly get an idea of the exciting applications of micromechatronic systems before they can decide how far to dive into the details of the physical and chemical processes of microfabrication. The idea underlying this book is to give a vivid illustration of the topic through selected case studies covering industrial applications and systems of current research. Using these examples, the reader should be given the opportunity to go deeper and, in particular, to also get to know important aspects of microfabrication. The book is aimed at interested students, but also at engineers in industry and research who want to get a vivid and quick introduction to micromechatronics. The book was written in the course of the year 2019 on the basis of an initial concept from 2018. Iordania Constantinou joined the author team when she started working as a junior professor at the Institute of Microtechnology of Technische Universität Braunschweig in 2018. With this book, we wanted to deviate from the typical view on our field of expertise by allowing a helicopter view, without letting it lack a well-founded presentation of the case studies. The book can thus be read in various ways. On the one hand, readers can get a brief introduction and overview of the various applications of micromechatronics.

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On the other hand, one can also dive deeper by studying the technical details also offered in this book which can further be deepened by the given literature references. Many people have contributed to making this book possible. First of all, we would like to thank Ann-Kathrin Klein who has read all chapters very carefully from the perspective of future users and has made many helpful suggestions. Bettina Thürmann, Anke Vierheller, and Barbara Matheis contributed their technological expertise through valuable comments. Many thanks go to the numerous former and current collaborators whose research results are included here. In particular, we thank Benjamin Gursky, Jan Niklas Haus, Eugen Koch, Foelke Purr, Martin Schwerter, and Jan Thies for helpful discussions. Thanks are due to Nathan Shewmon for his linguistic support. Sabine Kral-Aulich has contributed many valuable hints regarding publication rights. Finally, we would like to acknowledge financial support from the SMART BIOTECS framework, an alliance between Technische Universität Braunschweig and Leibniz Universität Hannover, supported by the Ministry of Science and Culture (MWK) of Lower Saxony, Germany. Braunschweig January 2020

Stephanus Büttgenbach Iordania Constantinou Andreas Dietzel Monika Leester-Schädel

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 Piezoresistive Pressure Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3 Surface Micromachined Acceleration Sensors . . . . . . . . . . . . . . . . . . . . 87 4 Mechanical Microgrippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 5 Point-of-Care Diagnostic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 6 Flexible and Stretchable Sensor Arrays . . . . . . . . . . . . . . . . . . . . . . . . . 275 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

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Contents 1.1 Mechatronic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 A Brief History of Micromechatronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.1 Integrated Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.2 Piezoresistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.3 Silicon Bulk Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2.4 LIGA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.2.5 Surface Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.2.6 Soft Lithography and Lab-on-Chip Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2.7 Direct Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.2.8 The Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.3 Overview of Microfabrication Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.4 Applications and Markets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.5 Topics of the Addressed Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.1 Mechatronic Systems Miniaturization is one of the most striking developments in engineering. The reduction in dimensions and weight of technical devices and components aims at the improvement of functionality, user-friendliness, portability, and efficiency. Miniaturized systems consume less energy, and for their production less material is used. Therefore, miniaturization contributes to sustainability as well. An early example is the miniaturization of mechanical chronographs starting in the 18th century. With their sophisticated technology they hold great fascination until today. The end of the 19th century saw the invention of electron tubes, which serve for generating, rectifying, amplifying and modulating electrical signals.

© Springer-Verlag GmbH Germany, part of Springer Nature 2020 S. Büttgenbach et al., Case Studies in Micromechatronics, https://doi.org/10.1007/978-3-662-61320-7_1

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They are also highly developed precision-engineered devices: delicate structure elements control the electric current between anode and cathode. The invention of the semiconductor transistor in 1947 caused a paradigm shift in the miniaturization of technical systems. This breakthrough resulted in the development of integrated circuits, which—since their launch in 1961—follow Moore’s law: the number of transistors per chip doubles about every two years. Highly integrated circuits allow not only for today’s consumer electronics products such as smartphones, tablet computers and digital cameras, but also led to the emergence of mechatronics which stands for the functional and spatial integration of mechanical, electronic, and informatics components. During the course of the 20th century mechanical elements of technical systems were increasingly replaced by electronic and informatics components, thereby improving product features such as energy consumption and reliability. Concurrently, the economic efficiency increased because the expenses during the production as well as during the use phase continuously decreased: a persistent trend even today. A general mechatronic system (Fig. 1.1) includes: • Local control, sensor and actuator modules. Signals are received from the technical process and a local control generates a feedback signal to automatically regulate the process. Sensors and actuators are connectors between the process and the local control. • Communication modules. The connection to interacting mechatronic systems and to the central control is made by LAN or WLAN modules. • Human machine interface. Displays and operating elements allow external control of the process by a human operator.

Fig. 1.1  Schematic diagram of a general mechatronic system

1.1  Mechatronic Systems

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Fig. 1.2  Typical implementations of mechatronic systems

Of course, not every mechatronic system contains all of these elements. Figure 1.2 shows some typical implementations [1]. Current examples of high performance mechatronic products are electronic stability programs in motor vehicles, robots for application in automated manufacturing, and video endoscopes for diagnostics and therapy. In many cases, the extended functionality results in a lack of installation space, requiring miniaturization of the functional components. Also, the general trend towards mobile devices requires miniaturization techniques. These demands gave rise to the development of the field of micromechatronics, which utilizes microstructure technologies to miniaturize mechatronic systems. Since the highly sophisticated manufacturing technologies of integrated circuits had proven to be very suitable for fabricating tiniest structures, it seemed obvious to start with these technologies for integrating micromechanical, microelectronic, microoptical, microfluidic, and—where applicable—(bio)chemical functions in a small space. In addition to the integrated circuit technologies, additional techniques mainly

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Fig. 1.3  Characteristic dimensions in the micro world

originating from precision machining extended the technological basis in the course of the development of micromechatronic systems. Micromechatronic systems have dimensions of up to approximately 10 mm, whereby typical functional parts have a size between about 10 nm and 100 µm. In general, they consist of various components such as sensors for measuring physical or chemical quantities, actuators for conversion of electrical, thermal, or chemical energy into mechanical work, and signal and information processing microelectronic components. Frequently used equivalent terms are: microsystems, micromachines, and micro-electro-mechanical systems (MEMS). Figure 1.3 illustrates characteristic dimensions in the micro world.

1.2 A Brief History of Micromechatronics 1.2.1 Integrated Circuits As pointed out above, the rapid development of micromechatronics started in 1947 with the invention of the transistor by William Shockley, John Bardeen, and Walter Brattain [2]. The first commercial transistors had dimensions of about 1 cm. This enabled a significant reduction in the size of current and voltage control elements compared to the electron tubes used until then. Although transistors could be further scaled down, miniaturization of electronic circuits was limited because the transistors and other electronic components had to be wired together. The final step towards microelectronics was the development of the planar technology allowing the fabrication of all components including wiring on a semiconductor chip with edge length in the range of a few millimeters and thickness of approximately 400 µm. However, it has to be noted, that the structural layer has a

1.2  A Brief History of Micromechatronics

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Fig. 1.4  Cross-sectional view of a field effect transistor as part of an integrated circuit

thickness of only some microns (Fig. 1.4). The first integrated circuits (ICs) were realized by Jack Kilby [3] and Robert Noyce [4] at the end of the 1950s. Initially, germanium (Ge) was used due to its simpler production process. However, by the 1960s single crystalline silicon (Si) became the dominant material in microelectronics because of its high availability and the excellent electrical and chemical properties of silicon dioxide (SiO2), which can be simply prepared by thermal oxidation. During IC production, high-quality SiO2 is used for several purposes: to provide surface passivation, to ensure electrical isolation, and to serve as a mask during doping processes. Doping means the introduction of a low and controllable level of impurity atoms into the tetravalent silicon in order to modulate its conductivity. Dopants frequently used are the trivalent boron (p-type dopant) and the pentavalent phosphorus (n-type dopant). The first commercial IC launched in 1961 comprised of four transistors and five resistors [5], but the complexity of ICs grew rapidly. In 1971 the first microprocessor (Intel 4004) comprising 2300 transistors came onto the market. The minimum structural size was 10 µm. Today’s ICs consist of some 109 transistors and have finest structures in the range of 10 nm. This development, which is largely based on increasingly advanced lithographic methods, is well described by Moore’s law. In 1965 Gordon Moore predicted a doubling of the number of transistors per chip about every two years [6]. Figure 1.5 illustrates Moore’s law using the example of the development of microprocessors. The manufacturing of ICs is carried out in three phases. In the first stage, the discs of single crystalline silicon (wafers) are fabricated. The wafers are the raw material for the second phase, the so-called front-end manufacturing. Using sophisticated semiconductor technologies, transistors and further electronic components are being created. In the third phase, the so-called back-end manufacturing, the wafers are separated into single chips which are encapsulated into a semiconductor package. IC fabrication technologies also form the basis for the manufacturing of micromechatronic systems. They will later be discussed in detail in connection with individual case studies.

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Fig. 1.5  Moore’s law illustrated by the example of the development of microprocessors. The minimal structure size is displayed in brackets after the processor name. (The diagram is based on the data given in Wikipedia, The Free Encyclopedia, Transistor Count [7] and references therein)

1.2.2 Piezoresistivity A development of great importance for the progress of micromechatronics was the discovery of the piezoresistive effect. In 1953 Charles Smith investigated the influence of mechanical load on semiconductor materials. He found that there is a far greater impact of mechanical strain on the electrical resistance of silicon than that of conventional strain gages (twenty- to 50-fold increased [8]). It was soon realized that this so-called piezoresistive effect offers great benefits when used for the measurement of mechanical quantities. This led to the development of the first key product of micromechatronics: piezoresistive pressure sensors for biomedical and automotive applications. Initially, discrete silicon strain gauges were mounted to pressure sensor plates. In the early 1960s, integrated pressure transducers were first realized consisting of a single-crystal silicon membrane with stress sensitive piezoresistive regions formed by the localized diffusion of impurities [9].

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Subsequently, the basic piezoresistive pressure sensor concept was extended to the measurement of further physical quantities. The first piezoresistive accelerometer was presented in 1979 [10]. These developments led to a rapid evolution of the US sensor industry in the 1970s and 1980s [11].

1.2.3 Silicon Bulk Micromachining Another major development resulted from the need to reduce the thickness of silicon wafers in order to fabricate sensor membranes. The manufacturing technology for silicon transistors utilizes the technique of isotropic etching, which removes material from the substrate using a chemical process. The etch rate is independent of the crystal orientation. Therefore, the material is removed equally in all directions. In 1970 H. A. Waggener published a paper on electrochemically controlled thinning of silicon, in which the technique of anisotropic wet etching of silicon was illustrated [12]. The etch rate depends on the crystal orientation [13]. In addition, methods to selectively modify the etch rates can be applied. These include etch stop techniques [14] as well as applying a pre-etch step, which does not depend on the crystal orientation [15, 16]. An overview of the broad variety of structures, including membranes, cantilevers, and grooves (Fig. 1.6), which can be created using these processes, is given in the famous paper by Kurt Petersen [17]: “Silicon as a mechanical material.” Additional important techniques of bulk micromachining are methods to create complex multilayer systems using wafer bonding processes. Silicon and borosilicate glass wafers can be assembled into a hermetically tight composite using

Fig. 1.6  Basic structures of silicon bulk micromachining. The notations  —, k, and l are Miller indices (see e.g. [20])—represent the directions in single-crystal silicon. (Adapted from Büttgenbach and Dietzel [21])

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electric fields by so-called anodic bonding, also known as field-assisted bonding. Anodic bonding was first published in 1969 [18]. Using a thin intermediate glass layer, two silicon wafers can also be connected. In 1986 the process of silicon direct bonding, which allows the combination of silicon wafers without the use of intermediate layers, was developed [19]. This method does not need high electric fields; however, the surfaces of the wafers have to be made hydrophilic. The combination of anisotropic wet etching and wafer bonding techniques can be used to construct complex three-dimensional microstructures such as microvalves, micropumps, print heads, pressure sensors, accelerometers, and many others.

1.2.4 LIGA In the early 1980s, in the context of the development of uranium enrichment by the separation nozzle method [22], a process was invented for producing microstructures with high aspect ratios from metallic materials or plastics [23]. This ­so-called LIGA technique consists of three steps. First, a thick resist layer (PMMA) of up to some 100 µm thickness is lithographically patterned using radiation from a synchrotron X-ray source. Second, metallic material is deposited by electroplating into the resist structures. After removal of the resist, the metallic microstructure can be used—in a third step—in a plastic molding process. The resulting “second generation” polymer microstructures can in turn be utilized for galvanoforming. The acronym LIGA is composed from the first letters of the German designations for the three steps—Lithographie, Galvanoformung, and Abformung. For technical and cost reasons, the X-ray LIGA technique remained a niche process [24]. However, in the 1990s new UV sensitive resists emerged of which thick layers (some 10 µm to some 100 µm) can be deposited and exposed to UV light [25]. Very often the epoxy-based negative photoresist SU-8 is used [26]. Although the achievable aspect ratio and the steepness of the structures are inferior to X-ray LIGA, the UV LIGA process has the advantage of using optical lithography equipment. Major areas of application of UV LIGA are electromagnetic microsystems (MagMEMS), for example, micromotors [27] and inductive microsensors [28]. The LIGA processes together with additional techniques such as the deep reactive ion etching process (DRIE, see below) represent the group of high aspect ratio microstructure technologies (HARMST).

1.2.5 Surface Micromachining In the 1980s technological approaches to fabricate free-standing and freely movable microstructures were investigated. As a result, the sacrificial layer technology, already introduced in 1967 for integrating a cantilever into a transistor [29] was

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rediscovered. This led to the surface micromachining technique. In contrast to bulk micromachining, the substrate wafer primarily serves as a mechanical support, on which alternating structural and sacrificial layers are deposited. After patterning of the structural layers, the sacrificial layers are etched away releasing free-standing or freely movable micromechanical structures. The preferred material combination utilizes polycrystalline silicon as structural and silicon dioxide as sacrificial layers. First polysilicon surface micromachined devices included freestanding cantilevers [30], electrostatic comb drive actuators [31] and micromotors [32], as well as microswitches [33]. In the 1990s, commercial products were launched on the market for the first time, accelerometers [34], gyroscopes [35], the famous digital mirror device (DMD) [36], and micromachined optical switches [37]. A major breakthrough was the deep reactive ion etching (DRIE) process— also called advanced silicon etching (ASE)—developed by Robert Bosch GmbH in 1992 [38]. In order to achieve high silicon etching rates and to fabricate high aspect ratio silicon microstructures, a special plasma source is used to generate highly dense plasmas. The designed deep etch process consists of two alternating steps: an etch step, and a step which passivates the side walls of the structure and protects them during the next etch step. This technique enables the use of thick poly-silicon structural layers, thus improving the sensitivity of surface micromachined inertial sensors. The DRIE process also represents an important addition to bulk micromachining techniques because it enables the fabrication of silicon microstructures whose geometry is no longer defined by the crystalline structure of the substrate.

1.2.6 Soft Lithography and Lab-on-Chip Systems Soft lithography was invented at the end of the 1990s [39]. It comprises a set of processes for the manufacturing of high-resolution structures in the size range between about 30 nm and 500 µm. Soft lithography utilizes relief structures— stamps or molds—usually made of elastomer. They are molded using a rigid original form. The most frequently used elastomer is PDMS (polydimethylsiloxane). The invention of soft lithography opened up the possibility to fabricate microfluidic devices from PDMS [40] initiating the rapid development of so-called ­lab-on-chip (LOC) systems. LOC systems integrate one or more laboratory processes on a chip with an area of just a few square centimeters [41]. Extensive studies of LOC systems started in the early 1990s. A. Manz, N. Graber, and H. M. Widmer developed the concept of the total micro analysis system (µTAS) [42], a micro device that performs all steps necessary for chemical analysis automatically. Today the term LOC is more familiar. Initially, LOC systems were made of silicon and glass. After the invention of soft lithography, PDMS was also widely used.

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The first LOC device ever presented was a gas chromatograph on a two-inch silicon wafer. It was developed in 1979 [43] and consisted of microvalves, a detector, and a separation channel of 1.5 m length, 200 µm width, and 40 µm depth. It was fabricated by silicon bulk micromachining. The fabrication of the chromatograph demonstrated outstanding technological performance; however, it was an economic failure. Conventional gas chromatographs were much cheaper at that time [44].

1.2.7 Direct Micromachining The micromachining methods discussed so far are based on the use of masks. First, the microstructures are created in auxiliary material, for example in a photoresist layer. Subsequently, they are transferred into the final material, for instance via etching techniques. But maskless techniques have also been developed. Scaling of precision machining methods resulted in micromachining technologies, where a tool is directly applied to the material to be structured. The most relevant of these technologies are ultra-precision machining [45], direct laser writing [46], micro electro discharge machining [47], and printing processes [48].

1.2.8 The Future Currently, the number of micromechatronic devices and applications continually increases. Considerable efforts are being made, for example, in the areas of LOC systems [41], optical MEMS [49], which are also referred to as MOEMS (micro-opto-electro-mechanical systems), MagMEMS [27, 50], and RF MEMS (radio frequency systems) [51]. There is a clear trend towards higher integration of sensors, actuators, and signal conditioning components on a single substrate. This development is called “Smart System Integration” or “System-in-Package.” It is also known as ­“More-than-Moore” [52], while the term “More Moore” stands for the development of further miniaturized microelectronic systems on a single chip. Figure 1.7 shows a graphical history of micromechatronics. These advances enable the rapid development of the internet of things. The term “Internet of Things” stands for the network of physical devices, vehicles with driver assistance systems, sensors monitoring vital functions or environmental parameters, home appliances, and other objects which are able to exchange data. The 1991 paper of Mark Weiser, “The Computer for the 21st Century” [53], already described such a vision and called it ubiquitous computing.

1.2  A Brief History of Micromechatronics

Fig. 1.7  Advances in micromechatronics since the invention of the transistor

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1 Introduction

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1.3 Overview of Microfabrication Methods The historical summary in the previous paragraph shows three different origins of micromechatronic fabrication technologies. First, there are the processes used for the production of integrated circuits. These include photolithography, physical and chemical vapor deposition, epitaxy, diffusion, ion implantation, and wet and dry etching techniques. Second, there are processes that evolved from microelectronics technology in order to enable the manufacturing of three-dimensional, free standing, and movable micro devices. These are, for example, deep UV and X-ray lithography, anisotropic wet chemical etching, deep dry etching, electroplating, and wafer bonding. Third, scaling and further development of precision machining methods resulted in micromachining technologies, where a tool is directly applied to the material to be structured. These comprise, amongst others, laser micromachining, microcutting, micro electro discharge machining, micromolding, and printing techniques. Micromachining processes may also be grouped into the categories: additive, material modifying, and subtractive. Table 1.1 summarizes the major processes used to fabricate micromechatronic devices. Table 1.2 presents important parameters of particularly relevant micromachining technologies. Details of the individual processes will be discussed in the following chapters. Table 1.1  Classification of microfabrication techniques Additive processes

Material modifying processes

Subtractive processes

Thin film deposition • Physical vapor deposition • Molecular beam epitaxy • Chemical vapor deposition • Chemical solution deposition

Radiative processing • Resist exposure • Polymer hardening • Two-photon polymerization

Etching processes • Wet chemical etching • Reactive ion etching • Ion beam etching

Printing techniques • Ink-jet printing • Microcontact printing • Microstereolithography • Fused deposition modeling • Selective laser printing • Powder based 3D printing

Thermal processing • Recrystallization • Diffusion • Oxidation • Phase transition

Radiative and thermal processing • Laser ablation • Electric discharge machining

Self-assembling mechanisms Ion beam processing • Ion implantation • Molecular self-assembly • Amorphization • Cell growth • DNA origami

Assembly techniques • Wafer bonding • Die bonding • Wire bonding

Mechanical modification • Plastic forming and shaping • Scanning probe manipulation

Tool-assisted material removal • Chemical-mechanical polishing • Drilling • Milling • Sawing

1.3  Overview of Microfabrication Methods

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Table 1.2  Important parameters of selected micromachining technologies. (Adapted from Büttgenbach and Dietzel [21]) Design freedom

Minimal Aspect structure ratio size

Process steps

2,5 D

6

2,5 D 2,5 D