Bolometers: Theory, Types and Applications : Theory, Types, and Applications [1 ed.] 9781617287350, 9781617282898

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Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Bolometers: Theory, Types and Applications : Theory, Types, and Applications, Nova Science Publishers, Incorporated, 2010. ProQuest Ebook Central,

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Bolometers: Theory, Types and Applications : Theory, Types, and Applications, Nova Science Publishers, Incorporated, 2010. ProQuest Ebook Central,

PHYSICS RESEARCH AND TECHNOLOGY

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BOLOMETERS: THEORY, TYPES AND APPLICATIONS

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PHYSICS RESEARCH AND TECHNOLOGY

BOLOMETERS: THEORY, TYPES AND APPLICATIONS

TORRENCE M. WALCOTT

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EDITOR

Nova Science Publishers, Inc. New York

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Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS.

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Additional color graphics may be available in the e-book version of this book. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Bolometers : theory, types, and applications / [edited by] Torrence M. Walcott. p. cm. Includes index. ISBN 978-1-61728-735-0 (e-book) 1. Bolometer. 2. Bolometer--Industrial applications. 3. Detectors. 4. Electromagnetic devices. 5. Terahertz technology. I. Walcott, Torrence M. TK7872.D48B65 2009 539.7'7--dc22 2010017741

Published by Nova Science Publishers, Inc. © New York

Bolometers: Theory, Types and Applications : Theory, Types, and Applications, Nova Science Publishers, Incorporated, 2010. ProQuest Ebook

CONTENTS Preface Chapter 1

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Chapter 2

vii  Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films Deposited from Plasma Discharge A. Kosarev, A. Torres and M. Moreno  Investigations of Properties of High Temperature Superconducting Bolometers I. A. Khrebtov 

1 

59 

Chapter 3

Operating Uncooled Resistive Bolometers in a Closed-Loop Mode Denoual Matthieu and Allègre Gilles 

91 

Chapter 4

A Security Camera as an Example for a THz Imaging Application S. Anders, T. May, E. Heinz, G. Zieger and H. G. Meyer 

109 

Chapter 5

Noise Properties of High-Tc Superconducting Transition Edge Bolometers with Electrothermal Feedback Igor A. Khrebtov, Konstantin V. Ivanov and Valery G. Malyarov 

Chapter 6

Chapter 7

Chapter 8

Comparative Investigation of Passive and Active Operating Modes for High-Tc Superconducting Transition Edge Bolometers with Electrothermal Feedback for Infrared Waves S. V. Baryshev, A. V. Bobyl, K. V. Ivanov, I. A. Khrebtov, V. G. Malyarov and V. U. Zerov  Experimental Modelling Active Strong Electrothermal Feedback Mode for High-Tc Superconducting Bolometer on Silicon Nitride Membrane I. A. Khrebtov and A. D.Tkachenko  YBCO Films on SrTiO3 Substrates with Recordly Low 1/f Noise for Bolometer Applications B. Dam, F.C. Klaassen, J. M. Huijbregtse, I.A. Khrebtov, K.V. Ivanov and S.V. Baryshev

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125 

145 

163 

173 

vi Chapter 9

Contents Absolute High-Tc Superconducting Radiometer with ElectricalSubstitution For X-Rays Measurements I.A Khrebtov, V.G. Malyarov, K.V. Ivanov, A.D. Nikolenko and V.F. Pindyurin 

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Index

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183 

197 

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PREFACE Chapter 1 - This chapter is a review of thin film micro-bolometers with Si-Ge thermo-sensing films deposited from plasma discharge. The text is organized into 11 sections. It starts with an introduction. Section 2 describes principles of performance of a microbolometer and its characteristics. In section 3 author analyze requirements of the materials for configuration design and configuration. Section 4 describes the material used for thermo sensing in micro-bolometers (Si-Ge:H), which is deposited by plasma. In section 5 modeling of the devices is presented. Different novel configurations of micro-bolometers and their fabrication process, including surface micro machining, are considered in section 6. The experimental techniques used for the characterization of micro-bolometers and characteristics are discussed in section 7. In section 8 the application of Si-Ge micro-bolometers for detection in the THz range is described. Section 9 shows non-resistive configuration of the SiGe:H micro-bolometers. The characteristics of the commercially available devices are compiled and analyzed in section 10. Section 11 is a summary of this contribution. Chapter 2 - The review describes the noise properties of the high temperature superconducting (HTSС) bolometers developed for the applications in the optical electronic devices of infrared and submillimeter wave-lengths. The principle of high-Tc transition edge bolometer operation and bolometer noise theory are considered, taking into account the peculiarities of constant bias current and constant bias voltage modes. The published results of bolometer noise modeling are discussed. Various sources of the excess 1/f-noise in HTSС films as temperature sensitive element for bolometer are reviewed, including the experimental data and modem noise models. Comparative analysis of noise characteristics of the most developed HTSС bolometers for application (antenna-coupled microbolometers and bolometers based on silicon micromachining technology) is reported. Chapter 3 - When feedback configurations are available, operating sensors in a closed-loop mode has advantages over the open-loop mode, including improved linearity and wider dynamic range. This chapter proposes feedback configurations referred to as external electronic feedback (EEF) to improve the performance of uncooled resistive bolometers. EEF is an effective technique to speed up bolometers without significantly complicating the system. For resistive bolometers in which the temperature increase due to the incident radiation is measured by a resistance change, the feedback principle is based on heat dissipation in the resistive temperature sensor, i.e., the thermometer, or at its vicinity. The possibility of working at a controlled constant temperature through EEF closed-loop mode, is also

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viii

Torrence M. Walcott

interesting when materials with high temperature coefficient of resistance near metal-insulator transition are used for the thermometer. The existing EEF configurations differ depending on how and where the heat is applied, mainly whether an external heater is used or not. Three feedback configurations exist: the first two configurations are classical and have been known for years, whereas the third configuration is a new one, developed recently. The first configuration, commonly used for an anemometer, is simple and directly coupled to the thermometer, but an oscillations issue can limit its dynamic range because of the dependency between the electrical and the thermal working points. The second configuration, based on the electrical substitution principle, requires an external resistive heater which limits its use to the bolometer specifically designed for it. The third configuration implements the electrical substitution principle through a capacitive coupling to the thermometer. This type of coupling, at the expense of more complicated electronics, allows a separate control of the electrical and thermal working points. By operating bolometers in closed-loop mode, not only can performance be improved, but new functions can be facilitated, such as self-test and self-calibration allowing for complete smart bolometers. Chapter 4 - Transition edge sensors (TES) are among the most sensitive detectors of electromagnetic radiation in the lower part of the terahertz band (0.1 THz to 1 THz). These cryocooled detectors have been originally developed for the field of astrophysics. The working principle is to detect the temperature increase that indicates a radiation signal. This increase is measured by a thermistor made of a superconducting thin film that is heated to its transition temperature. The steep part of the resistance versus temperature curve yields a large and linear response. The signal can be read out by biasing the device with a constant voltage and measuring the resulting current with a superconducting quantum interference device (SQUID). To achieve a high sensitivity, it is necessary to thermally decouple the absorber and thermistor from the temperature bath, so that absorbed photons cause a sufficiently large temperature increase. The decoupling can be done by fabricating the device as a microelectromechanical system (MEMS) where absorber and thermistor are placed on a thin silicon nitride membrane. Author present the fabrication and operation of such transition edge sensors as an example of detectors for THz radiation. Further, author show their integration into a complete imaging system for security screening of passengers in airports or visitors of public events. The author security camera is meant to complement other screening devices by scanning people as they pass the camera at a distance of several meters. For that purpose fast scanning that approaches video rate is required. The camera uses an opto-mechanical scanner to image the field of view with an array of 20 individual sensors. Because of their high sensitivity and speed, the author bolometers can detect the natural (thermal) THz radiation and in doing so are capable to work at frame rates up to 10 Hz. Chapter 5 - Numerical and experimental modeling of the characteristics (sensitivity, constant time, noise properties) of high-Tc transition edge superconducting bolometers, operating in the various modes with an electrothermal feedback, are carried out: the mode with constant bias current, i.e. passive positive electrothermal feedback, the mode with constant bias voltage, i.e. passive negative electrothermal feedback and the mode with active electronic negative electrothermal feedback . It is shown, that in the modes with negative electrothermal feedback it is possible essentially to reduce constant time of bolometers till 515 of times in at some prize of the noise equivalent power on high frequencies. The

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Preface

ix

estimation of influence of various noise components on a performance of the bolometers, operating with positive or negative electrothermal feedbacks, is carried out at the variation of bolometer parameters. Chapter 6 - Numerical and experimental modeling of the characteristics of constant bias current mode, constant bias voltage mode, and active electronic negative electrothermal feedback mode of high-Tc superconducting transition edge GdBa2Cu3O7-x bolometers on Si/Si3N4 membrane for infrared waves are carried out. Comparative analysis of mentioned modes and estimation on how noise components effect on bolometers operation are also performed. Here author conclude, that active electronic negative electrothermal feedback mode is the most favorable one the bolometer can operate in due to its (i) thermal stability (25-fold increase in comparison with constant bias current mode) and (ii) attainable performance characteristics such as effective time constant (experiences 15-fold drop in comparison with that in constant bias current mode) and detectivity/noise equivalent power (detectivity ≥109 сm×Hz1/2/W for up to 300-Hz modulated optical beams). Chapter 7 - The comparative investigations of the properties of high-Tc bolometer in two operating modes – traditional and with strong active negative feedback – were carried out. In constant current mode GdBaCuO bolometer on Si/Si3N4 membrane has the maximum detectivity is of 6×109 cmHz1/2/W at λ=7.2 μm, response time 6 ms, the detectivity was due to mainly the phonon noise in frequency range of 3-20 Hz. Investigations showed that response time of bolometer could be decreased on more than order, using active negative and positive electrothermal feedback modes. Electrothermal feedback loop effects on noise behavior of bolometer as well. The estimation for present concrete variant with using parameter D*/τ1/2 shows some advantage of mode with feedback, when it is necessary high rate of bolometer response. Chapter 8 - The excess 1/f noise investigations of YBCO films on SrTiO3 substrates are reported. The epitaxial films produced by the laser ablation are characterized by the high perfect structure, sharp superconducting transition, strong pinning and very low excess 1/f noise. It is observed that the level at normal state depends on the dislocation density. The best noise properties are observed for films with the dislocation density of (8-12) disl/μm2. These films have noise Hooge-parameters of around (1.6-3)×10-6, which is 3 order less than published values. The calculation of the bolometer based on investigated films, shows the possibility to reach the noise equivalent power limited by only the phonon noise in the modulating frequency range beginning above 0.1 Hz. Chapter 9 - The present work considers the practical possibility of the construction of an absolute radiometer with electrical substitution for power based on the high-Tc superconducting YBaCuO film bolometer cooled with liquid nitrogen to measure the power of radiation of the X-ray range circa 1μW with an accuracy of 1%. This accuracy is provided with high sensitivity of the bolometers, having the noise equivalent power about 3.8×10-12 W/ Hz1/2 (with modulation) and 2.6×10-9 W (without modulation). The main sources affecting on an accuracy of the absolute measurements such as external reflection, the passage of radiation through the substrate, photo-stimulated electron emission from the receiving surface, the stability of synchrotron radiation source and instability of cryostat temperature are analysed. The radiometer can be applied to measure absolute power of “white” and monochromatic synchrotron radiation flows in the spectral range from 80 to 2000eV.

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In: Bolometers: Theory, Types and Applications Editor: T. M. Walcott, pp. 1-57

ISBN: 978-1-61728-289-8 © 2011 Nova Science Publishers, Inc.

Chapter 1

THIN FILM MICRO-BOLOMETERS WITH SI-GE THERMO-SENSING FILMS DEPOSITED FROM PLASMA DISCHARGE A. Kosarev∗, A. Torres and M. Moreno Electronics department, Institute for Astrophysics, Optics and Electronics, Puebla, 72000, Mexico

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ABSTRACT This chapter is a review of thin film micro-bolometers with Si-Ge thermo-sensing films deposited from plasma discharge. The text is organized into 11 sections. It starts with an introduction. Section 2 describes principles of performance of a micro-bolometer and its characteristics. In section 3 we analyze requirements of the materials for configuration design and configuration. Section 4 describes the material used for thermo sensing in micro-bolometers (Si-Ge:H), which is deposited by plasma. In section 5 modeling of the devices is presented. Different novel configurations of micro-bolometers and their fabrication process, including surface micro machining, are considered in section 6. The experimental techniques used for the characterization of micro-bolometers and characteristics are discussed in section 7. In section 8 the application of Si-Ge microbolometers for detection in the THz range is described. Section 9 shows non-resistive configuration of the Si-Ge:H micro-bolometers. The characteristics of the commercially available devices are compiled and analyzed in section 10. Section 11 is a summary of this contribution.

1. INTRODUCTION Differing from photo-detectors, a bolometer senses radiation by raising the temperature of the thermo-sensing layer due to radiation absorption. As a result, there are changes of the electronic properties, i.e., conductivity. Photo-detectors have spectral limitations because of ∗

E-mail: [email protected]

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A. Kosarev, A. Torres and M. Moreno

the structure of the electron energy levels, while bolometers do not have this kind of limitation. Recent interest in bolometers is related to both, new materials [non-crystalline semiconductors with high temperature coefficient of resistance (TCR)], and micromachining technology that provides efficient thermo-isolation. The conjunction of these factors and their compatibility with the silicon CMOS technology for integral circuit (IC), have made possible the fabrication of the bolometer and the read-out circuitry on the same chip. A micro-bolometer is formed by three components: a supporting structure or microbridge (deposited over a sacrificial layer), which provides thermo isolation to the thermosensing film, the thermo-sensing film itself, and an IR absorbing film. Among the materials that have been used as thermo-sensing layer in micro-bolometers are vanadium oxide, amorphous and poly-crystalline semiconductors and some metals. Vanadium oxide has a high value of TCR, but it is not a standard material in the IC technology. Metals are compatible with the IC technology but have low TCR values. Non-crystalline semiconductors deposited by plasma, e.g., amorphous silicon (a-Si:H) has shown a high TCR value. It is deposited at low temperature and is fully compatible with the silicon technology. Intrinsic amorphous semiconductors have a very high resistance and Boron doping has been reported to reduce its high resistance. But doping reduces the TCR and consequently the responsivity, therefore there is a trade off between responsivity and the resistance of the cell. Polycrystalline Si-Ge films reported as thermo-sensing films require higher deposition temperature (Tdep ≈ 600 ˚C) and demonstrate lower TCR values (TCR ≈ 2%/K) resulting in lower voltage responsivity (RU ≈ 104 V/W) and detectivity (D* ≈ 3x108 cmHz1/2/W) in comparison with plasma deposited films: Tdep = 300 ˚C, TCR = 4%/K, RU =105 V/W and D* = [4-7]x109 cmHz1/2/W presented in this work.

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2. PRINCIPLE OF PERFORMANCE The micro-bolometer is a thin layer of a material with a high TCR (thermal coefficient of resistance), with electric contacts on it and thermally insulated from its surroundings. The active material should absorb radiation in the required spectral range, otherwise an additional absorption coating should be used [2.1]. Figure 2.1.(a) shows the basic structure of a bolometer.

Figure 2.1. Structure of a bolometer (a) and its equivalent diagram (b). (Reproduced from [5.11] with permission).

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

The absorber has a heat capacitance Cth and it is linked to a heat sink by a thermal conductance Gth . Incident optical power causes a rise in temperature above the heat sink, this results in a change of resistance of the thermo-sensitive layer, which is measured as signal. The active element, the resistor, must be thermally insulated in order to obtain large temperature variations, even with small incident power.

2.1. Bolometer Operation

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A bolometer is a device that detects incoming radiation by producing a change in electrical resistance proportional to the amount of radiation received. Incoming radiation is absorbed by the bolometer, which causes an increase in its temperature, which in turn causes a change in its electrical resistance. It employs a temperature-sensitive electrical resistor. The change in temperature causes a change in electrical resistance, which is measured by an external circuit. When the radiation is removed, the temperature of the bolometer returns to its initial value, which is determined by the ambient surroundings in which it is immersed. If the resistance increases with increasing temperature, such as is found with metals, the bolometer is said to have a positive TCR; if it decreases with increasing temperature, as is found in amorphous semiconductors under most operating conditions and crystalline semiconductors at room temperature, it is said to have a negative TCR [2.2, 2.3]. The resistor should have a large thermal TCR in such a way that for such a small temperature increase, there is a result in a large change in the resistance of the thermo-sensitive layer. The bolometer performance is described by a thermal capacitance Cth and it is connected to an infinite heat sink by a support having a thermal conductance Gth as shown by the equivalent diagram in Figure 2.1.(b). In absence of external radiation and of applied bias, the temperature of the bolometer is the same as that of the heat sink T0. Both, the incident radiation and the bias voltage set the current temperature on the bolometer.

2.2. Characteristics of the Bolometer Many features are used to describe the performance of a bolometer, but the following are the most useful in analyzing the performance of uncooled detectors: the responsivity R, noise ν, detectivity D∗ and thermal response time τth .

a) Responsivity Responsivity is defined as the signal generated per unit of incident power. In the case of bolometers, the generated signal (output signal) is an electrical signal, voltage or current, and the input signal is the incident radiant power on the detector. (Pinc )

R=

output signal input signal

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(2.1)

4

A. Kosarev, A. Torres and M. Moreno

Depending on the output signal, voltage or current, the responsivity can be voltage responsivity (RV = dV /Pinc ) or current responsivity (RI = dI /Pinc ). Solution of the 1D balance equation with incident power described by a sine function with frequency ω allows us to write responsivity as a function of α (symbol for TCR) and the operation conditions as shown in Eq. 2.2, [2.1, 2.4].

RV =

εαVβ 1

Gth (1+ ω 2τ th2 )2

(2.2)

where ε is the emissivity of the absorbing surface, α is the temperature coefficient of resistance, V is applied voltage, ω is the angular frequency of signal modulation, Gth is the thermal conductance coupling the detector to its surroundings and τth is the thermal time constant. β is the bridge factor of the biasing circuit of a bolometer:

β=

RL RL + R

(2.3)

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where R and RL are the detector and bias resistor resistances, respectively.

b) Noise Noise is an important parameter of bolometers because it determines the minimum power that can be detected. The major noise sources associated with the bolometer are [2.1, 2.5]: a) thermal conductance noise, b) the Johnson-Nyquist noise and c) the low-frequency (1/f ) noise. The temperature fluctuation noise is due to fluctuations of the detector temperature caused by the statistical nature of heat exchange between the detector and its surroundings. The mean-squared fluctuations of temperature [2.3] are

ΔT 2 =

4kT 2Δf Rth 1+ ω 2τ th2

(2.4)

where ∆f is the frequency band. The spectral noise voltage due to temperature fluctuations is

4kT 2Δf 2 2 V = α V ΔT = α V Rth 1+ ω 2τ th2 2 th

2

2

2

(2.5)

Johnson noise is found in all resistive materials, the random motion of the free carriers within the material produces it, and Eq. 2.6 gives it. VJ2 = 4kTRB Δf (2.6)

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

1/f noise is characterized by a spectrum in which the noise power depends approximately inversely upon frequency. Its origin is still a matter of debate, but it is described by the empirical expression given by Eq. 2.7

V1/2 f = k1/ f

V2 Δf f

(2.7)

where the value of k1/f depends on the material and technology of the detector. The total noise of bolometer is given by the sum of these components given by Eq. 2.8.

VN2 = Vth2 + VJ2 + V1/2 f

(2.8)

The characteristics of the bolometer such as material properties, frequency and biasing determine which one of these components will be dominant.

c) Detectivity Detectivity is the normalized signal-to-noise ratio and is expressed as Eq. 2.9 [2.1, 2.6].

R (A Δf ) D = V d VN

1/ 2

*

(2.9)

by including the total noise, D∗ results as:

εαVB Rth (Ad Δf )

1/ 2

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*

D* =

(1 + ω τ ) (V 2

2 1/ 2

2 th

+ VJ2 + V1/2 f )

1/ 2

εαVB A1/d 2 Rth 2 ⎡ ⎤1/ 2 2 2 2 V 1/ 2 4kT α V R (1+ ω 2τ th2 ) ⎢⎢4kTR + k1/ f 1/f f + 1+ ω 2τ 2 th ⎥⎥ ( th ) ⎦ ⎣

(2.10)

(2.11)

From the expression for D∗, Eq.2.11, some conditions are extracted to get a high performance of the micro-bolometer: an efficient absorption of radiation, high α, high thermal insulation of the active element (high Rth ), low Cth and low noise.

d) Thermal Response Time Thermal response time τth is the time required for the temperature of a pixel to decrease to 1/e of its value when thermal radiation from a steady state source falling on the pixel is instantaneously removed. Thermal capacitance Cth and thermal conductance Gth determine the τth as it is shown in Eq.2.12 [2.2].

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A. Kosarev, A. Torres and M. Moreno

τ th =

Cth Gth

(2.12)

A low thermal capacitance results in a short time response, but low thermal conductance results in large responsivity.

3. REQUIREMENTS FOR DESIGN AND MATERIALS From the micro-bolometer operation described in the precedent section, the material to be chosen for the fabrication of such a device must have a high TCR, to absorb energy in the wavelength of interest and generate low noise. In addition to that, if the device is going to be used for image detection, the material and the device fabrication process must be compatible with standard CMOS processes, in order to take advantage of the very low cost of the mass fabrication of ICs which when integrated with micro-bolometers, will form the monolithic imaging system.

3.1. Properties of Bolometer Materials a) Temperature Coefficient of Resistance The temperature coefficient of resistance is the parameter used to measure the temperature dependence of the resistance of a material. It is defined as the change in resistance per unit change in temperature.

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α≈

1 dR R dT

(3.1)

The electrical conduction in amorphous semiconductors is a thermally activated process and it is in general described in the following way:

⎛ Ea ⎞ ⎟ ⎝ kT ⎠

ρ(T ) = ρ 0 exp⎜

(3.2)

Then, from this description and neglecting ρ0 for obtaining an expression for α we get:

α ≈−

Ea kT 2

(3.3)

Therefore from equation (2.2) it follows that for obtaining a high responsivity a material with a large value of α should be used. The last means that a material with large activation energy is useful for obtaining a bolometer with large sensitivity. But large activation energy values are associated with either intrinsic or low-doped materials, which will result in a very high resistance of a device fabricated with such a material.

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

In view of the aforementioned, there always be a trade-off between a large TCR sensitivity material and a device with a intrinsic high resistance, which will result in a complicated driving circuitry. Fortunately, this trade-off may be solved by the introduction of new thermo-sensitive materials such as a-SiGe:H which have shown large TCR values and low resistivity or new bolometer configuration for reducing the resistance of them, both issues are going to be discussed in the following sections.

b) Thermal Conductance The property that depicts the ability of a material to transfer heat by conduction is the thermal conductivity (gth ). Thermal conductance Gth is the intrinsic property of a body relating its ability to conduct heat. It is the quantity of heat that passes through a unit area in a unit time, when there is a unit temperature difference between the two sides of a surface. For a plate of thermal conductivity gth , area A and thickness L, the thermal conductance in WK−1 is defined in Eq.3.4. Its reciprocal property is the thermal resistance Rth measured in KW−1.

Gth = gth

A L

(3.4)

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The thermal conductance between a bolometer and its supporting structure is the sum of the thermal conductivity of the supporting legs, plus the thermal conductivity due to any surrounding gas and the radiation transfer. Therefore, a proper design of the supporting legs/structure, the choice of a material with supporting material with a very low gth and the proper packaging of the system, will allow obtaining a large responsivity in the devices. Again a trade-off appears, as large is the value of Gth, larger the responsivity but longer response time, which may be a problem when designing a high resolution IR video camera. It is pertinent to note that Gth also depends on the bias of the device [3.1]

c) Thermal Capacitance Also called heat capacity, is a measure of the ability of a material to absorb and store heat. The thermal capacitance of a certain amount of matter is the quantity of heat required to raise its temperature by one degree Kelvin. It is defined as Cth (equation 3.5), where dQ/dT is the change in heat (Q) with temperature (T) and it has units of energy per degree [3.2, 3.3].

Cth =

dQ dT

(3.5)

The heat capacity is an “extensive variable” because it is sensitive to the size of the object, thus a large quantity of matter will have a proportionally large heat capacity. A more useful quantity is the specific heat cp (also called specific heat capacity), which is the amount of heat required to change the temperature of one unit of mass of a substance by one degree. Equation 3.6 relates the thermal capacitance and the specific heat capacity cp, where ν is the volume and ρ is the density.

Cth = c pνρ

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(3.6)

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A. Kosarev, A. Torres and M. Moreno

The specific heat is therefore an “intensive variable”, which means, it is no dependent on the amount of material, but it is dependent on the type of material, as well as the physical conditions of heating. Therefore, the proper design of the supporting legs/structure is of vital importance in determining the response time of the device without reducing its performance.

4. SILICON-GERMANIUM AS THERMO-SENSING MATERIAL DEPOSITED BY PLASMA

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4.1. Different Thermo Sensing Materials The thermo-sensing material is perhaps the most important element in a micro-bolometer. The increment in temperature in the sensing material causes a change in some temperaturedependent parameter. In the case of a micro-bolometer that parameter is the resistance. The thermo-sensing material should have a large temperature coefficient of resistance, TCR (α(T)), which is defined by Equation 3.3, where Ea is the activation energy, K is the Boltzman constant and T is temperature. A large TCR means that a small change in temperature in the sensing material will result in a large change in resistance. Equation 3.3 shows that the TCR and Ea are directly related, thus a high Ea in the material is desired. For un-cooled micro-bolometers vanadium oxide, VOx, was the first thermo-sensing element employed [4.1.-4.3.], since it has a relatively high TCR, α(T) ≈ 0.021 K-1, however it is not a standard material in silicon CMOS technology. Some metals have been employed also, which are compatible with Si-CMOS technology, however they have low values of TCR (Pt, α(T)≈0.0015 K-1). Hydrogenated amorphous silicon (a-Si:H) prepared by plasma is a material commonly used in micro-bolometers as thermo-sensing film, for room temperature operation [4.4]. It is compatible with the IC technology, has a very high activation energy, Ea ≈ 1 eV and high value of TCR, α(T) ≈ 0.13 K1, however it also has a very high undesirable resistivity (~1x10-9 Ω cm), which causes a mismatch with the input impedance of the read-out circuits. In order to reduce the a-Si:H high resistance, boron doping has been employed. The B doped a-Si:H films present a significant reduction in its resistivity (to 5x10-3 Ω cm), however a reduction in Ea and TCR is obtained also, Ea ≈ 0.22 eV and TCR ≈ 0.028 K-1 [4.4]. We have studied and used amorphous silicon – germanium, a-SixGey:H [4.5-4.6], deposited by PECVD as thermosensing films in un-cooled micro-bolometer, obtaining high activation energy, Ea= 0.34 eV, consequently a high value of TCR = 0.043 K-1 and improved resistivity (1x10-6 Ω cm). Table 4.1 shows the most common materials employed as thermo-sensing films in microbolometers. As can be seen in Table 4.1, there are available several materials which can be used as thermo-sensing films in micro-bolometers. Intrinsic amorphous silicon, a-Si:H and amorphous silicon –germanium, a-SiGe:H, show the largest TCR values and are fully compatible with the silicon CMOS technology, however they have also the smallest values of room temperature conductivity, σRT.

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Table 4.1. Common materials employed as thermo-sensing films in micro-bolometers Material VOx a-Si:H a-Si:H,B (plasma) a-SiGe:H (plasma) Poly-SiGe GexSi1-xOy YBaCuO

TCR (K-1) 0.021 0.13 0.028

Ea (eV) 0.16 1 0.22

σRT (Ω cm)-1 2x10-1 ~ 1x10-9 5x10-3

Reference [2.1-2.3] [2.7] [2.4]

0.043

0.34

1x10-6

[2.5-2.6]

0.024 0.048 0.033

0.18 0.32 0.26

9x10-2 2.6x10-5 1x10-3

[2.8-2.9] [2.10-2.12] [2.13-2.16]

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4.2. Study of Silicon-Germanium Thin Films Deposited by Plasma The study and applications of hydrogenated amorphous silicon germanium films (aSiGe:H) deposited by plasma (RF frequency discharge from SiH4, GeH4 and GeF4 mixtures) has gained interest because of the ability to adjust the alloy composition and to tailor the optical and electrical properties to the requirements of specific electronic device applications [4.17], and also the possibility to achieve high uniformity in large areas, high deposition rate (Vd) and low deposition temperatures. GeH4 is conventionally used as germane source gas and only few works have reported the properties of a-SiGe:H films prepared from a mixture of SiH4 and GeF4 [4.18,4.19]. We have investigated the structural, and electrical properties of a-SiGe:H,F films deposited by plasma enhanced chemical vapor deposition (PECVD) method, from the mixture of SiH4 +GeF4, H2 and Ar for dilution, details of which have been described in Ref. [4.20]. In our study, 100% SiH4 and GeF4 were adopted as the gas sources and H2 and Ar were used for dilution at a deposition temperature of 300 oC. Germanium content in gas mixture (x), defined as the gas flow ratio of x =100*(GeF4)/(SiH4 + GeF4)%, was varied from 0 to 1 (see Table 4.2.).

a) Deposition rate (Vd) The deposition rate, Vd as a function of x is presented in Table 4.2 and the behavior is also shown in Figure 4.1. Vd increases reaching a maximum value at x = 0.2, and then decreases to x = 1. The deposition rate of the samples deposited with Ar dilution was changed with x from 3.5 to 0.2 A/s while less Vd variations in the range of 1.9–1.2 A/s for the samples deposited with H2 dilution were observed. Similar behavior of Vd(x) has been reported in Refs. [4.21,4.22] for the deposition with GeH4.

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Figure 4.1. The growth rate of a-SiGe:H,F as a function of x. The solid lines are guides to the eye. ((reproduced from [4.5] with permission).

An increase of Vd with addition of GeF4 could be related to the large cross-section of these molecules in comparison with SiH4, and therefore a higher probability of dissociation under electron impacts, resulting in higher concentration of Ge related precursors in the discharge and higher growth rate. Alternatively or additionally Ge–F and Ge–H precursors could have higher sticking coefficient in comparison with those for Si–H when Si content is relatively low and the growing surface consists of mostly Si atoms terminated by hydrogen. These factors could be responsible for the increase of deposition rate with x increased from 0 to 0.2. Figure 4.2 shows the Ge content determined by SIMS in solid phase Xs as a function of x, and Figure 4.3 shows the Si–Ge bond content determined from the Raman spectra, as a function of x. From these graphs, it is clear that there is a preferential incorporation of germanium to silicon. Both SIMS and Raman spectra data demonstrate that incorporation of Ge in solid phase reaches saturation at x ≈ 0.5. The Vd reduction at large x-values was explained by lower sticking coefficient for Ge atoms, when the growing film surface consists of mainly Ge atoms. It is interesting and important to note that H2 dilution provided practically the same Vd up to x =1 in contrast to deposition with Ar dilution, suggesting an important role of H coverage of the growing film.

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Table 4.2. Properties of a-SiGe:H,F deposited by PECVD at different GeF4 gas content The deposition rate and some properties of a-SiGe:H,F deposited by PECVD at different GeF4 Ea (eV) Sample Dilution Gex Thickness σRT x 10-5 Deposition (Ω-1 cm-1) d (μm) Rate Vd (A/s) 127 Without 0 1.3 0.91 0.0001 0.86 129 H 0.1 1.6 0.57 25 0.46 131 Ar 0.1 2.9 1.00 1.70 0.48 134 H 0.2 1.9 0.68 2.20 0.46 139 Ar 0.2 3.5 2.50 4.80 0.41 142 H 0.5 0.7 0.25 2.70 0.40 144 Ar 0.5 1.1 0.41 2.10 0.49 146 H 1 1.2 0.60 21 0.39

contents in gas mixture Ef γ x 10-4 σ0 x 102 (eV) (eV/K) (Ω-1 cm-1) 2.60 0.78 41 0.27 0.46 2.7 1.10 0.48 7.1 0.65 0.46 4.2 0.60 0.39 4.0 0.44 0.4 )1.40 2.00 0.43 20 2.50 0.32 36

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Figure 4.2. The Ge solid phase composition Xs vs gas phase composition, x, for the films deposited. The solid lines are guides to the eye. ((reproduced from [4.5] with permission).

Figure 4.3. The Si–Ge bond content vs gas phase composition, x, determined from the Raman spectra. ((reproduced from [4.5] with permission).

b) Composition The IR spectra of the a-SiGe:H,F samples for different GeF4 contents are shown in Figure 4.4 (a) and (b). It can be observed that as x increases, the peaks at k = 1880 cm-1, corresponding to the stretching mode of Ge–H [4.23] and at k = 560 cm-1 assigned to Ge–H wagging and rocking modes [4.23], gradually increase. Figure 4.5 (a) and (b) display the Raman spectra of a-Si-Ge:H,F deposited with H2 and Ar dilution, and with different content of x, respectively. For the samples with H2 and Ar dilution with x = 0, it can be seen that a main peak is localized at 480 cm-1 which corresponds

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to the TO phonon of a-Si [4.23] and a peak located at 317 cm-1, which corresponds to LA phonon. As x increases from 0 to 0.2 the intensity of Si–TO phonon peak at 470 cm-1 decreases and disappear to zero for x = 0.2 in both dilutions. However, it is relevant to point out that in the sample with Ar dilution, as x increases from 0 to 0.1 the peak at 470 cm-1 decreases rapidly, with its center shifting from 470 cm to 490 cm-1. The results are similar to those reported in Ref. [4.22] for the samples deposited by RF glow discharge. The Ge content in the films, x, determined from secondary ion mass spectroscopy (SIMS) analysis as a function of GeF4 content in the gas mixture, is shown in Figure 4.2.

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Figure 4.4. Infrared spectra of a-SiGe:H,F films deposited at LF PECVD, (a) with H dilution, (b) with Ar dilution, for different germanium content in gas phase x. ((reproduced from [4.5] with permission).

As it is observed from comparison of the curves in Figures 4.1 and 4.2, the increase of Ge content in the films is approximately proportional to Vd, which means that an effective incorporation of Ge atoms into the films is obtained and at x = 20% of GeF4 in gas phase, the films practically contain 100% of Ge in solid phase.

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Figure 4.5. Raman spectra of a-SiGe:H,F films deposited at LF PECVD, (a) with H dilution, (b) with Ar dilution, for different germanium content in gas phase x. ((reproduced from [4.5] with permission).

c) Electrical Properties Figure 4.6 shows the thermal dependence of the conductivity σ(T). The conductivity is defined as [4.24] σ(T) = σmin eγ/k exp(-Ea/KT) = σ0 exp (-Ea/KT) where σmin is minimal metallic conductivity [4.24], γ is the temperature coefficient of Fermi level, k is the Boltzmann constant, T is temperature, Ea is the activation energy, and σ0 = eγ/k is the pre-

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factor. As shown in Figure 4.6, the conductivity at room temperature of a-Si:H is σRT = 1.0 x 10-9 Ω-1 cm-1. The temperature dependence of conductivity σ(T), shown in Figure 4.6, demonstrates a drastic change of about four orders of magnitude as result of the 10% GeF4 incorporation in gas mixture. Further increase of GeF4 in gas mixture from x = 0.1 to 0.5 causes a formation of a-Ge:H,F film, x = 1 with relatively small changes in conductivity. It is interesting to note that in the films deposited with H2 dilution and x = 1, the highest conductivity values practically did not change with an increase of x, from 0.5 to 1, which means no change in Fermi level position (see Table 4.2).

Figure 4.6. The temperature dependence conductivity of a-SiGe:H,F for the films with different germanium content in gas phase x. (reproduced from [4.5] with permission).

4.3. Study of Silicon-Germanium-Boron Alloys as Thermo-Sensing Films In this section, we present a study of germanium-boron-silicon alloys (a-GexBySiz:H) deposited by PECVD at low temperature Ts≈ 300 oC and compatible with the silicon IC technology [4.25]. We varied the germanium gas content Gex during the films deposition process in order to observe a reduction in its conductivity and the effect on the activation energy Ea. We compared these results with those of an intrinsic reference film (a-GexSiy:H).

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Also it is discussed the effect on the electrical properties of the films studied, when they are patterned (µm scale) and deposited over a-SiN micro-bridge structure.

a) Samples Preparation The germanium-boron-silicon alloys (a-GexBySiz:H) were deposited by plasma at substrate temperature of 300 oC. Three sets of films were deposited from SiH4, GeH4, B2H6 and H2 gas mixture, with a fixed SiH4, B2H6 and H2 gas flows, while the GeH4 gas flow was varied resulting in a Ge gas content Xg = 0.3, 0.45, 0.55 and a B gas content Yg = 0.11, 0.09, 0.07 in the samples labeled as process number 478, 479 and 480, respectively. An intrinsic film (a-GexSiy:H) was deposited in order to compare its characteristics with that of the boron alloys, labeled as process number 443. This film was deposited under the same conditions as used for the boron alloys, with a Ge gas content Xg = 0.5. Since those films are studied for applications as low resistance thermo-sensing films for micro-bolometers, we studied the films electrical properties after patterning them with photolithography to one cell dimensions (70 x 66 μm2). Assuming that stress arisen in the film deposited over a SiN micro-bridge could have an effect on the film conductivity, we also studied the films deposited on a micro-bridge. For that propose, we prepared three different kinds of samples for each type of the four thermo-sensing films (three boron alloys with different Gex content and the intrinsic reference film). The first kind of sample is commonly used for Ea and σ measurements, it consist of the film deposited over a corning glass, over the sample are deposited aluminum stripes for contacts. Figure 4.7 a) shows that sample labeled as “stripes sample”. The second sample consists of a film deposited over a corning glass and patterned on small dimensions. The dimensions of the thermo-sensing film are 70 x 66 μm2. Figure 4.7 b shows this sample labeled as “patterned sample”. This sample allows us to observe dimensions and “fabrication” effect on the conductivity. The third sample consists of a SiN micro-bridge fabricated with surface micro-machining techniques and over it is deposited the thermo-sensing film and contacts. The dimensions of the film in this configuration are 70 x 66 μm2. Figure 4.7 c shows this kind of sample labeled as “micro-bridge sample”. The different samples fabricated for characterization are listed in Table 4.3.

Figure 4.7. Three different samples for electrical characterization: a) stripes, b) patterned and c) microbridges. ((reproduced from [7.1] with permission).

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Table 4.3. Gas and solid content in the different thermo-sensing films and the samples available for characterization

Gas Mixture

Ge x Gas content Bx Deposition

Process 478 SiH4: 50 ssccm. GeH4: 25 ssccm. B2H6: 5ssccm. 0.3 0.11 6

Thermo-sensing films Process 479 Process 480 SiH4: 50 SiH4: 50 ssccm. ssccm. GeH4: 50 GeH4: 75 ssccm. ssccm. B2H6: B2H6:5ssccm 5ssccm. 0.45 0.55 0.09 0.07 7 9.5

Process 443 SiH4: 25 ssccm. GeH4: 25 ssccm. H2: 1000 ssccm. 0.5 0 2.8

0.59 0.078 0.32

0.67 0.05 0.26

0.71 0.04 0.23

0.88 0.11 2.0x10-5

#1: Stripes #5: Patterned Not available

#2: Stripes #6: Patterned #9: Micro-bridge

#3: Stripes #7: Patterned #10: Micro-bridge

#4: Stripes #8: Patterned

O

rate ( A /s) Solid Ge y content Si y obtained By from SIMS

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Samples

#11: Micro-bridge

b) Results of Films Characterization We performed measurements of temperature dependence of conductivity σ(T) in the above samples, in the range of T= 300 – 400 K. These measurements allowed us to obtain σ(T) dependence and then to determine the Ea, the TCR and the room temperature conductivity, σRT. The composition in solid phase of the different films was characterized by secondary ion mass spectroscopy (SIMS). Figure 4.8 shows σ(T) curves for four different thermo-sensing films (three boron alloys with different Gex gas content, Gex = 0.3, 0.45, 0.55 and the intrinsic film with Gex = 0.5), fabricated in three different sample configurations (stripes, patterns and micro-bridges). The boron alloys (a-GexBySiy:H) have a significantly larger conductivity (by about 2-3 orders) in comparison with that of the intrinsic reference film (a-GexSiy:H). An increment in the Gex content in gas phase in the boron alloys results in an increase of the room temperature conductivity, from σRT = 2.8 x10-3 (Ωcm)-1 (for Gex = 0.3) to σRT = 1 x10-2 (Ωcm)-1 (for Gex = 0.45) and σRT = 2.5 x10-2 (Ωcm)-1 (for Gex = 0.55), while for the intrinsic film the room temperature conductivity is σRT = 6 x10-5 (Ωcm)-1 (for Gex = 0.5). The increment in the σ is accompanied with a reduction in the Ea. We obtained an Ea= 0.22 eV (for Gex = 0.3), Ea= 0.21 eV (for Gex = 0.45) and Ea= 0.18 eV (for Gex = 0.55), while in the intrinsic film is Ea= 0.345 eV (for Gex = 0.5). The Ea as a function of Gex is shown in Figure 4.9. The reduction in the thermo-sensing films dimensions, from the stripes samples (10 x1.5 mm2) to the patterned samples (70 x 66 μm2), has no significant effect on the Ea, however it has on the σ. We observed a reduction by 50 – 80 % of the σ value in the patterned samples in comparison with that of the stripes samples.

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Figure 4.8. Conductivity dependence with temperature for the different thermo-sensing films (process: 443, 478, 479 and 480). ((reproduced from [7.1] with permission).

A slight increase in the Ea of the thermo-sensing films deposited over a SiN micro-bridge was observed, in comparison with that of the stripes and patterned samples; however the micro-bolometer samples showed a larger reduction in the σ values, around 60 – 90 %. Table 4.4 shows the values of Ea, TCR, σRT and σ0 of the different samples. The Gex content and the sample structure dependence of σ are shown in Figure 4.10. From SIMS we obtained the solid composition in the thermo-sensing films. For the film with gas content: Gex=0.3 and Bx=0.11 (process 478), we observed an increase in the solid content: Gey=0.59 and By=0.32 respectively. For the film with Gex=0.45 and Bx=0.09 (process 479), we observed Gey=0.67 and By=0.26, respectively and for the film with Gex=0.55 and Bx=0.07 (process 480), we observed Gey=0.71 and By=0.23, respectively.

Figure 4.9. Ea as function of Ge gas content, Gex, in the boron alloys, a-GexBySiz:H. ((reproduced from [7.1] with permission).

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Figure 4.10. Conductivity dependence on the Ge gas content, Gex. ((reproduced from [7.1] with permission).

Table 4.4. Comparison of Ea, TCR, σRT and σ0 in stripes, patterned and micro-bridges samples for the different thermo-sensing films

Stripes Samples

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Patterned samples

Microbridge samples

Process 478 0.22 -0.028 2.8x10-3 12.02 0.225 -0.029 1.4x10-3 7.27

Thermo-sensing films Process Process 479 480 0.21 0.18 -0.027 -0.023 1x10-2 2.5x10-2 36.46 24.55 0.20 0.20 -0.025 -0.025 4x10-3 1.2x10-2 8.23 28.26 0.22 0.20

Process 443 0.345 -0.044 6x10-5 34.85 0.36 -0.046 1.08x10-5 11.13 0.37

Not available

-0.028

-0.025

-0.047

σRT (Ωcm)-1

1.2x10-3

7x10-3

2.2x10-5

σ0 (Ωcm)-1

5.94

15.58

32.8

Ea (eV) TCR (K-1) σRT (Ωcm)-1 σ0 (Ωcm)-1 Ea (eV) TCR (K-1) σRT (Ωcm)-1 σ0 (Ωcm)-1 Ea (eV) TCR (K-1)

These results suggested a strong preferential B and Ge incorporation from gas phase during the film deposition process. The By solid content demonstrated values about 3 times larger than the content in gas phase Bx, while the Gey solid content increased by a factor of 1.3 – 2 from the Gex gas content. Those results are shown in Table 4.3. As conclusion, we can state that the boron alloys (a-GexBySiy:H) demonstrated an increment in their conductivity (between 2 and 3 orders) in comparison of that of the intrinsic film (a-GexSiy:H). The increment in σ was accompanied by a reduction in Ea, from Ea = 0.345 eV for the intrinsic film to Ea = 0.22, 0.21 and 0.18 eV for Gex = 0.3, 0.45 and 0.55, respectively, in the boron alloys. From SIMS we obtained the solid composition in the

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thermo-sensing films. We observed a significant increase in the B and Ge solid content from the gas content, suggesting strong preferential B and Ge incorporation from gas phase.

5. MODELING

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5.1. Introduction Modeling is an important tool in design micro-bolometers and in understanding their performance and its relationship to material characteristics and construction. Various approaches have been employed in modeling such as analytical models enabling analytical solutions; analytical models and numerical solutions, empirical relations between variables, previously measured or calculated; or combination of them. Formulas obtained for analytical 1D (one dimensional) model discussed in Sect. 2 are commonly used in most of publications. 1D modeling does not provide, however, any instrument in order to develop understanding temperature distribution in micro-bolometer cell, its relationship with material parameters, construction and consequently signal and noise parameters. Temperature field T(x,y) on surface of micro-bolometer can be calculated analytically or numerically and both methods rely on the solution of the heat-transport equation under proper boundary conditions. Unfortunately there is a little information on 2D (two dimensional) modeling reported in literature. It is also relevant to note that such important issues on modeling as calculation finally some of performance characteristics (e.g. responsivity) and experimental verification of the calculations can be found even rarely in the reported studies. Dilner et al [5.1] reported on 3D analytical model for two cells and solving the problem by FEM ANSYS, no data on experimental verification. In ref. [5.2] analytical 2D model was used, calculated with FEM ANSYS and mean temperature difference have been obtained without experimental verification. SPICE simulation [5.3] was used for 1D model for metal film micro-bolometer and voltage responsivity has been calculated as a function of frequency, the results of calculations have been compared with R(f) measurements. Analytical 2D model was analyzed in ref [5.4] with 2D finite difference algorithm for non steady state conditions and temperature field T(x,y) has been calculated and then compared with that experimentally measured. Parameterized ideal bolometer 1D analytical model was studied by Sudiwala et al. [5.5] and Woodcraft [5.6] and DC responsivity has been calculated and further compared with that experimentally measured. Analysis of temperature and stress of the manufactured structure was performed with ANSYS in ref. [5.5] to find an optimal point between thermal insulation and robustness of the structure as a function of the dimensions of supporting legs. Thus to our knowledge there is not sufficient modeling study and especially 2D modeling for thin film micro-bolometers, which are of much promise and have already, attracted much attention of researchers. This work describes 2 dimensional (2D) modeling of micro-bolometer starting with temperature distribution over active area and its dependence on geometrical factors and thermo-conductivity conditions for some concrete configurations experimentally realized.

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5.2. 2D Modeling Our approach of modeling is based on analytical description of the task with differential equations with appropriate boundary conditions for some concrete configurations realized and characterized experimentally; their configurations fabrication and characteristics (both material employed and device performance) have been published in refs. [5.7, 5.8]. Then the analytical problem is numerically solved using software “Flex PDE” from “PDE Solutions Inc.” with variation of both geometrical factors and IR illumination conditions. Material parameters for thermo-sensing layer were used from experimental data in refs. [5.7 - 5.10]. The configuration modeled is thin film micro-bolometer fabricated in two leg “bridge” configuration with thermo-sensing layer of Si-Ge deposited by plasma its temperature dependence of conductivity is described in ref. [5.11]. Two configurations of the electrodes: planar when current flow is parallel to the layer and sandwich when current flow is perpendicular to the thermo-sensing film are considered. We verified our modeling by comparing the results of calculations in the form of some graphics with experimental data.

5.3. Results of Modeling

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Diagram of the configurations used in the modeling is shown in Figure 6.1a for planar electrodes and in Figure 6.1b for sandwich electrodes. Characteristic dimensions and notifications used are shown in Figure 5.1.

Figure 5.1. Diagram of microbolometer (2 leg bridge, planar and sandwich configuration) structure used in 2D modeling. (reproduced from [5.11] with permission).

We start consideration with the case when the micro-bolometer is uniformly IR illuminated that provides energy coming in the structure and thermo-conductivity through the legs provides coming out energy. In this case on the surface of the thermo-sensing layer a 2D temperature distribution T(x,y) is formed and energy balance equation can be written for temperature in the form

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~

d 2 T ( x, y ) d 2 T ( x, y ) Q( x, y ) aT 4 ( x, y ) + = − dx 2 dy 2 D D

(5.1)

where: T(x,y) is the temperature distribution on the surface of the thermo-sensing layer, D is the temperature diffusion( conductivity) coefficient, Q(x,y) is the intensity of IR light, generally is non-uniform, aT 4(x,y) is component describing radiation losses, a is the StephanBoltzman constant for thermal radiation. Temperature diffusion coefficient is related to material parameters by the equation:

D=

g th cp ρ

(5.2)

where: g th is the thermo-conductivity coefficient, c p is specific heat, and

ρ is the density

of the film (material of supporting bridge is silicon-nitride deposited by plasma). In a steady state conditions

dT ( x, y ) = 0 under uniform illumination Q(x,y)=Q0 and dt

neglecting radiation losses Eq.[5.1] can be rewritten as

⎡ d 2T ( x, y ) d 2T ( x, y ) ⎤ + D⎢ = Q0 2 dy 2 ⎥⎦ ⎣ dx

(5.3)

Temperature on the surface of the TS film T(x,y) can be described as sum of ambient temperature (i.e. without IR illumination) equal also to the temperature of the substrate, T0 ~

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Tsubs = T0 and temperature changed due to IR illumination T ( x, y ) , which is equal to zero at ~

the substrate T ( x, y ) substr = 0 , i.e.: ~

T ( x, y ) = T0 + T ( x, y )

(5.4)

Taking into account the above notations Eq.(5.2) can be rewritten as ~

~

Q d 2 T ( x , y ) d 2 T ( x, y ) + =− 0 2 2 dx dy D

(5.5)

Boundary conditions are described temperature at the supporting legs (2 conditions), which is equal to substrate (ambient) temperature:

Tx = 0 = Tx = Lx = T0

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(5.6)

23

Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

Two perpendicular sides are thermally isolated from the substrate that prevents a change of their temperature:

∂T ( x, y ) =0 ∂y y =0, y = Ly

(5.7)

Thus we have second order differential equation Eq.(5.4) with 4 boundary conditions ~

eqs.5.6 and 5.7 that is sufficient to obtain unambiguous solution for T ( x, y ) in steady state condition. The following step is calculation of conductivity distribution using equation

⎛ − Ea ⎞ ⎛ − Ea ⎞ ⎛ς ⎞ ⎟⎟ ⎟⎟ = σ min exp⎜ ⎟ exp⎜⎜ ⎝k⎠ ⎝ kT ( x, y ) ⎠ ⎝ kT ( x, y ) ⎠

σ ( x, y ) = σ 0 exp⎜⎜

(5.8)

where: σmin= 200 Ω-1cm-1 is the minimum metal conductivity, ζ is the temperature coefficient of the Fermi level position, Ea is the activation energy equal to the Fermi level position at T= 0K. From our previous measurements [5.11]: ζ = 1.9x10-4eVK-1, Ea= 0.39 eV, then σ0= 1771 Ω-1cm-1. The potential distribution V(x,y) and electric field E ( x, y ) = −∇U ( x, y ) can be determined from the Poisson equation, which for charge density ρ = 0 reduces to the Laplace equation

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∇ 2V ( x, y ) = 0

(5.9)

The continuity equation in steady state condition relates the current density as →

∇ J ( x, y ) = 0

(5.10)

also we can write for current density →

→

J ( x , y ) = σ ( x, y ) ∗ E ( x , y )

(5.11)

Then substituting (5.11) in (5.10) we obtain →

∇((σ ( x, y ) ∗ E ( x, y ) = 0

(5.12)

and taking into account that →

E ( x, y ) = −∇U ( x, y )

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(5.13)

24

A. Kosarev, A. Torres and M. Moreno

we obtain equation for potential distribution as

− ∇(σ ( x, y ) ∗ ∇U ( x, y ) ) = 0

(5.14)

This equation is numerically solved with the following boundary conditions:

U ( y = y 0 ) = 0 , and U ( y = y 0 + a y ) = U b (in this work we fix Ub=1 V) and U ( x, y ) distribution was obtained. Then we use eq.(5.13) to find electric field E(x,y) and get all to write equation for current density →

→

J ( x, y ) = σ ( x, y ) ∗ E ( x, y )

(5.15)

Finally we calculate integral current from the cell under IR illumination as →

I = ∫ J ( x, y ) ∗ dS S

(5.16)

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The above stages of the calculations are shown graphically: in Figure 5.2 (a) temperature distribution T(x,y), in Figure 5.2 (b) corresponding conductivity distribution σ(x,y), in Figure 5.2 (c) a potential distribution U(x,y) and in Figure 5.2 (d) electric field E(x,y) and in Figure5.2 (e) current density distribution.

5.2. a) Figure 5.2 (Continued)

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25

5.2.b)

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5.2.c)

5.2.d)

5.2.e) Figure 5.2. 2D distribution of ΔT (x,y) temperature difference due to IR radiation (a), and corresponding conductivity σ(x,y) (b), potential distribution U(x,y) (c), electric field distribution E(x,y) (d) and density of current j(x,y) (e). (from [5.11] reproduced with permission).

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A. Kosarev, A. Torres and M. Moreno ~

Effect of cell dimensions and Q0/D ratio on temperature distribution T ( x, y ) is demonstrated. As can be seen reducing of cell dimensions results in both reduction of temperature maximum in cell center and changes in the form of temperature distribution: it becomes sharper. Thus reducing cell dimensions should be accompanied by improvement of thermo-isolation (characterized by factor D) in order to avoid loss in responsivity. The above modeling allows us to study an influence of relative area of thermo-sensing layer (ratio of this area to area of the bridge) on responsivity. As it is shown in Figure 5.3 (a) for planar structure and in Figure 5.3 (b) for sandwich structure responsivity reaches its maximum value at ratio ax/lb ≈ 0.8 for both planar and sandwich structures. Signal of microbolometer cell depends on both IR intensity Q0 and termo-conductivity described by D coefficient. In equations these factors appear as ratio Q0/D. It is of interest to study signal as a function of this ratio.

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5.3.a)

5.3.b) Figure 5.3. Relative signal ΔI/Idark as a function of ratio (active area width)/bridge width i.e. relative with for planar (a) and sandwich (b) configurations. (from [5.11] reproduced with permission).

Finally integral current is calculated for both planar and sandwich electrodes. The calculated signal current ΔI = f(Q0/D) for different values of a/D ratio is shown in Figure 5.4.

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Figure 5.4. Signal current ΔI as a function of Q0/D parameter for different a/D values. (from [5.11] reproduced with permission).

We can see that three regions exist: 1) low Q0/D region where ΔI ~ (Q0/D)γ and γ ≈ 1 as in the previous case (thermo-conductivity controls signal), 2) mid range Q0/D region, where γ ≈ 3 (IR illumination controls signal), and 3) high Q0/D region, where ΔI = f(Q0/D) becomes weak and reveal clear trend to saturation ( thermal radiation controls signal). In order to reveal the above regions clearer three asymptotic lines l1, l2 and l3 are shown in Figure 5.4. Thus in this case as in previous one we can observe region with very strong dependence of signal (γ ≈ 3) on IR intensity at fixed (by construction and material used for supporting legs) thermo-conductivity, although this case describes the behavior more adequate from point of view physics. Existence of the region 2) is very interesting because of two aspects: a) in this region with strong dependence on IR intensity micro-bolometer will have significantly larger value of differential responsivity ∂R / ∂Q that is an important advantage for performance, and b) this region allows us experimentally evaluate micro-bolometer construction from point of view its thermo-sensing layer thermo-isolation from relatively simple measurements of signal as a function of IR intensity. It is worthy to note that to authors’ knowledge such strong dependence of signal on intensity has not been reported in literature.

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Table 5.1. Summary on the reported models for microbolometers Ref.

Model

Solution method Numerical FEM (ANSYS)

Device model Linear thermopile array (2cells)

Variables

[5.1]

Analytical 3D (2cells)

[5.2]

Analytical 2D

Numerical FEM (ANSYS)

4-element thermoelectric IR sensor

2D steady state T(x,y), τth in center and periphery

[5.3]

Equivalent circuit 1D

SPICE simulation

Metal fim microbolometer

[5.4]

Analytical 2D

Ni thin film bolometer

[5.5]

Parameterised ideal thermal bolometer 1D analytical model

2D finite difference algorithm(non steady state conditions) Derivation of parameters by fitting the I(V,T) characteristics

Dimensions, number of elements, position of junctions, electrical resistance VCVS(ΔT), Gth, and Cth are extracted from fabricated structure Thickness, density, Cth, Gth

β, n, Tg, resistance, Gso

Cooled bolometer (100 mK)

Design geometry (pixel separation)

Results of modeling 3D steady state and transient temperature distribution

Calculated parameters 1D signal gradient ΔT, τ, crosstalk parameter ΔT2/ ΔT1 for 3 configuraions separations, for 3 ambients. Mean temperature difference ΔTmean, sensitivity

Verification method Not reported

Effects of optical input and ambient temperature T(x,y) cross section of heated sensor

Voltage signal ΔV(T), responsivity vs ferquency Τ, final T, relative change of resistance, responsivity

Measurements of responsivity vs frequency

DC, responsivity vs frequency, Ge

DC responsivity, NEP

Not reported

Measurements of T(x,y) distribution by a thermo -graphic camera Responsivity measurements at different bias current

Thin Film Micro-Bolometers with Si-Ge Themo-Sensing Films…

29

5.4. Experimental Results Relevant to Modeling

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The behavior ΔI ~ (Q0/D)γ predicted by the calculations above can be verified experimentally. If the construction of the micro-bolometer is determined D value is fixed and D = const therefore the above equation is reduced to ΔI ~ (Q0)γ that is IR intensity dependence of current signal. The measurements of ΔI ~ (Q0)γ that is IR intensity dependence of current signal was performed in the open circuit cryostat (“LakeShore”, model MTD-150) with temperature controller (“LakeShore”, DR-93CA) and a Si diode as temperature detector for the encapsulated samples.For the samples located on wafer without encapsulation we used the system with microprobes in vacuum (“MMR Technologies Inc.”, model LTMP-2). Both systems provide temperature control and measurements in controlled vacuum. For impedance matching we used circuit based on LMC 6001 operation amplifier. Electrical measurements were conducted with electrometer (“Keithley”, model 6517A) controlled with computer.

Figure 5.5. Experimental data on responsivity of microbolometer versus IR intensity. (from [5.11] reproduced with permission).

IR illumination was obtained from the source with Globar (“Jobin Yvon”, model Triax320) through ZnSe window in the cryostat providing integral IR intensity (with maximum at λ ≈ 10 µm) and IR intensity Q0 ≈ 10-7 W/cm2. The results of the measurements are shown in Figure 5.5. The experimental points can be fitted with ΔI ~ (Q0)γ dependence with γ ≈ 0.9 and correlation coefficient. This experimental value is close to the calculated γ = 1 in the thermo-conductivity controlled region. Unfortunately our experimental facilities allowed us to carry out IR intensity dependence in very limited region of illumination intensity. To our knowledge 2D modeling of both temporal response and noise has not been reported in literature so far. It is pity because such modeling would be not only useful for device design but would reveal some correlations not observed yet experimentally.

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6. MICRO-BOLOMETERS CONFIGURATIONS AND FABRICATION The majority of micro-bolometers reported in the literature have a “planar” configuration, that is, the current flows parallel to the surface of the film and the resistance of the device is set by the inter-electrode distance (which is in the range of 17 – 60 μm). An alternative microbolometer configuration is labeled “sandwich”, in which the current flows perpendicular to the surface of the film, in this case the contact electrodes are separated by the thickness of the thermo-sensing film (which is in the range of 0.5 – 1 μm), resulting in a considerable reduction of the resistance of the device. In this section we describe planar and sandwich bridge configurations of un-cooled micro-bolometers. The structures were fabricated by surface micro-machining techniques based on thin films deposited by PECVD, emphasis is made on the fabrication process, and the key issues in order to have a large yield at the end of the fabrication of the devices are discussed.

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6.1. Micro-Bolometer Configurations The device structures are fabricated on a (001) silicon wafer with resistivity ≈ 2000 Ohmcm, as it is depicted in Figure 6.1 [6.1], the fabrication process starts with the deposition of a 0.3 µm thick oxide layer by CVD at a substrate temperature Tdep= 350 oC, followed by the deposition of an Al film (thickness of 2.5 µm), by e-beam evaporation. Wet etching patterns the Al film in order to form a sacrificial structure. In this step, a solution based on H3PO4 - CH3COOH – HNO3 is used for obtaining a well-controlled sidewall angle on the Al sacrificial structure. A supporting SiNx -1 film 0.8 µm thick is deposited over the Al structure by LF PECVD at a discharge frequency f=110 kHz and Tdep = 350 oC. The depositing conditions for this film have been optimized for achieving high resistance to the subsequent chemical etchings and good mechanical and thermal properties. Reactive Ion Etching - RIE, patterns the SiNx -1 film in order to form the SiNx microbridge on which the micro-bolometer will be built. Up to this point, the fabrication process steps for both planar and sandwich structures are the same.

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

Figure 6.1. Fabrication process flow of the two micro-bolometers structures (reproduced from [7.1] with permission).

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A. Kosarev, A. Torres and M. Moreno

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Figure 6.2. Micro-bolometer configurations. A. Planar structure, B. Sandwich structure (reproduced from [7.1] with permission).

For the planar configuration the fabrication process continues as follows: a 0.2 µm thick Ti layer is deposited by e-beam evaporation over the SiNx-1 film and patterned in order to form the electrodes (stripes), contact lines and bonding pads. Then above the patterned metal the thermo-sensing a-SixGe1-x:H film (thickness of 0.5 μm) and an IR absorber SiNx-2 (thickness of 0.2 μm) are deposited consequently by LF PECVD at Tdep= 300 and 350 ˚C, respectively. The active area is patterned by RIE and finally the Al – sacrificial film is etched with Al-etch solution, in order to form the micro-bridge. For the sandwich configuration the fabrication process is as follows: A Ti layer (0.2 µm thick), is deposited by e-beam evaporation and patterned forming the bottom electrode. Then the thermo-sensing a-SixGe1x:H film (thickness of 0.5 µm) and a IR absorber SiNx-2 film (thickness of 0.2 µm), are consequently deposited by LF PECVD, at Ts = 300 oC and 350 oC, respectively. A window is opened in the top SiNx-2 film by RIE and a 10 nm thick Ti layer is deposited, in order to form the top electrode. A second thicker Ti layer (thickness of 0.2 µm) is deposited in order to contact the thin top electrode with the contact pad. The active area is patterned by RIE and finally the Al sacrificial layer is removed by wet Al-etch solution. Figure 6.2 depicts both structures illustrating their difference. In Figure 6.3 it is shown a fabricated micro-bolometer. The active area of the micro-bolometer is AB = 70 x 66 μm2.

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

A. SEM top view of a planar structure micro-bolometer

33

B. SEM cross section view of a planar structure micro-bolometer

Figure 6.3. SEM images of one cell of a planar structure micro-bolometer with a-SixGey:H thermosensing film (reproduced from [7.1] with permission).

In Table 6.1 it is shown the main performance characteristics of the fabricated microbolometers, and these are compared with data reported in literature. Because of the data published do no cover all the figures of merit considered in this work; our discussion is limited by the comparison of the available data and our two structures: planar and sandwich.

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References

cmHz1/2W-1

Detecti vity, D*

RI, AW-1

Current responsivity,

RU, VW-1

Voltage responsivity,

Ohm ll

R

Cell resistance,

TCR, K-1

Ea, eV

Layer

Thermo sensing

Table 6.1. Performance characteristics of the fabricated micro-bolometers and compared with data from literature

VOX

0.16

0.021

-

2.5 x 107

-

-

Planar [6.7]

a-Si:H,B

0.22

0.028

3 x107

106

-

-

Planar [6.1]

a-SixGey:H

0.34

0.043

5x108

7.2x105

2x10-3

7.9x109

Planar [6.3-6.6]

a-SixGey:H

0.34

0.043

1x105

2.2x105

0.3 – 14

4x109

Sandwich [6.3,6.6]

The micro-bolometers here studied demonstrated the highest activation energy Ea and, consequently a higher TCR value however; their resistances were about one order of magnitude higher than those in a-SiH,B [6.2-6.3]. The sandwich structure showed the lowest cell resistance, which implies that will be well matched to input of a standard IC. The voltage responsivity in the fabricated samples resulted to be about one order of magnitude less than those reported.

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A. Kosarev, A. Torres and M. Moreno

We have not found any published data about current responsivity, which is a more practical figure of merit for high resistive samples. Our study showed very a high current responsivity and also a higher noise in the sandwich configuration [6.4-6.7] (Table 6.1), when compared with those devices found in literature. However, the detectivity of both planar and sandwich configurations were similar and close to the theoretical limit determined by the background photon noise.

7. CHARACTERIZATION OF MICRO-BOLOMETERS 7.1. Characterization of Temperature Dependence of Conductivity in the Films and Estimation of Thermal Coefficient of Resistance, TCR

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Figure 7.1 shows the installation used for the characterization of the thermo-sensing films, it is a microprobe system, which consist of a vacuum chamber, with a pump and microprobes for characterization directly on the wafers. A temperature controller is used in order to vary the temperature in the wafer. The micro-probes are connected to an electrometer. We employed this system for the estimation of the temperature dependence of conductivity in the thermo-sensing films.

Figure 7.1. Installation for characterization of the thermo-sensing films (reproduced from [7.1] with permission).

The characterization consists of the measurement of current voltage I(U) characteristics in dark condition, in the thermo-sensing films at different temperatures, employing an electrometer controlled by a PC and a temperature controller. The measurements were performed from T = 300 to 400 K. In planar structures used for conductivity measurements we observed linear I(U) characteristics, suggesting that current is controlled by bulk properties rather than contacts. This allows us to use Ohm law to calculate resistivity. From I(U) measurements in the films we can calculate its conductivity, σ, by knowing its dimensions, L, W and thickness th, with Equation 7.1.

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I

σ=

A J = Transversa l V E L

I W ⋅ th I L = = . V V W ⋅ th L

35

(7.1)

The conductivity is related to the activation energy Ea, by Eq. 7.2, from this equation we can extract the Ea as is shown in Eq. 7.3, which represents a line of the form y = a + bx. If the lnσ of the thermo-sensing film is plotted at different temperature values (T=300 – 400 K), then the Ea can be extracted by fitting a straight line and calculating its slope, as is shown in Figure 7.2.

⎛ − Ea ⎞ σ = σ o exp⎜ ⎟ ⎝ KT ⎠ 1 KT

(7.3)

ln Conductivity

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-1

-1

(Ohm cm )

ln σ = ln σ o − E a

(7.2)

0.00091 +

b =Ea=0.35- 0.02 eV 0.00034

y=a+bx 0.00012

0.00005

28

30

32

34

1/KT

36

38

40

Figure 7.2. Conductivity dependence with temperature of an a-SixGey:H thermo-sensing film in a temperature range T= 300 - 400 K (reproduced from [7.1] with permission).

The temperature coefficient of resistance is related with the Ea by equation (obtained from Eq.7.2).

TCR = α ≈ −

Ea KT 2

(7.4)

For the Ea = 0.35 eV obtained in the graph of Figure 7.2 the TCR value is α ≈ -0.045 (% K-1).

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7.2. I(U) Measurements in Dark and under Infrared (IR) Radiation In this section is described the procedure performed in order to obtain the current voltage I(U) characteristics of the micro-bolometers, from this measurement it is possible to determine the micro-bolometer electrical resistance and responsivity. Figure 7.3 shows the installation used for the current voltage I(U) measurement in dark and under IR illumination conditions.

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Figure 7.3. Installation employed for I(U) measurements in micro-bolometers. (reproduced from [7.1] with permission).

The source of IR light is a SiC globar source, which provides IR illumination in the range of λ=1 – 20 µm. The samples were placed in a vacuum chamber at pressure P≈20 mTorr, at room temperature and illuminated through a zinc selenide window (ZnSe). The window has a 70% transmission in the range of λ=0.6 – 20 µm. The source of IR light is a SiC globar source, which provides intensity I0=5.3x10-2 W/cm2 in the range of λ=1 – 20 µm. The current was measured with an electrometer controlled by a PC in dark and under IR illumination, from U=0 to 7 V. The micro-bolometer electrical resistance can be calculated form Eq. 7.5, where γ is the slope in logarithmic scale of the micro-bolometer I(U) characteristics, since the micro-bolometer it is not always an ohmic resistor resistor.

log I = γ log(V ) + const and

I=

1 V R

for γ = 1

(7.5)

Figure 7.4 shows an I(U) curve obtained from an a-SixGey:H thermo-sensing film microbolometer. The insert in Figure 7.4 shows the same curve when plotted in a Log I (Log U) scale. Here we can see clearly the linear behavior of the micro-bolometer I(U) characteristics.

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1.5x10

-8

1.0x10

-8

5.0x10

-9

37

1E -8

I(A)

Current (A)

Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

γ = 1.02

1E -9

1

U (V )

10

Current in dark condition (0 to 7V)

0.0 0

1

2

3 4 5 Voltage (V)

6

7

Figure 7.4. I(U) characteristics of a micro-bolometer (with an an a-SixGey:H). (reproduced from [7.1] with permission).

7.3. Calculation of Responsivity The responsivity, R, is a figure of merit in detectors; it was defined in section 2.2 as the ratio between the output signal and the input signal. Under bias voltage, the output signal is the increment in current from dark condition to IR illumination and the input signal is the incident radiant power on the detector surface. The set of Eqs. 7.6 – 7.8 describe how the current responsivity is calculated.

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RI =

ΔI PIncident

(7.6)

where:

ΔI = I IR − I Dark

(7.7)

PIncident = Acell ⋅ I 0

(7.8)

Eq. 7.8 shows that the radiant power incident, Pincident, is equal to product of the microbolometer cell area, Acell, by the IR source intensity, I0. I0 was measured with a thermopile, which provides a transmission of 95% in the range of 0.13 to 11 μm. Eq. 7.9 shows how I0 was obtained, where R is the thermopile responsivity, R= 2.6x10-4 AW-1 and Athermopile is the thermopile area, Athermopile = 2.8 x10-3 cm-2, data was taken from the thermopile data sheet. Ithermopile is the thermopile current measured at a distance of 7cm form the IR source, Ithermopile = 39x10-9 A, therefore I0 =5.3x10-2 Wcm-2.

I0 =

I thermopile R ⋅ Athermopile

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(7.9)

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A. Kosarev, A. Torres and M. Moreno

When the micro-bolometer operates under current bias, the output signal is the increment in voltage from dark condition to IR illumination, while the input signal is the incident radiant power on the detector surface. Eqs. 7.10 and 7.11 show the voltage responsivity, Ru.

RU =

ΔU PIncident

(7.10)

ΔU = U IR − U Dark

(7.11)

Figure 7.5 shows the I(U) characteristics in dark and under IR illumination in one microbolometer (with an a-SixGey:H film), where we can see the increment in current, ΔI. ΔI=5 nA

-8

Current (A)

2.0x10

at (7 V)

-8

1.5x10

-8

1.0x10

-9

5.0x10

Current in dark condition Current under IR radiation

0.0 0

1

2

3 4 5 Voltage (V)

6

7

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Figure 7.5. I(U) characteristics of a micro-bolometer (with an a-SixGey:H) in darkness and under IR illumination. (reproduced from [7.1] with permission).

7.4. Noise Measurements In this section are presented the noise spectral density (NSD) measurements performed in the micro-bolometers. The NSD is defined as the noise per unit of bandwidth. Figure 7.6 shows the installation employed for noise measurements in the micro-bolometers. A lock-in amplifier was used for this kind of measurements. The amplifier in the regime of noise measurements operates as true quadratic detector. This equipment is used for measuring the voltage noise levels at different frequencies, for that purpose it has a reference input, where it is connected a sinusoidal function generator and also it contains a signal input, where the micro-bolometer is connected. Since the microbolometer has a high resistance (Rbol ≈ 500 MΩ), it was necessary the use of an impedance matcher, which is built of an operational amplifier (ultra low input current amplifier) in a follower configuration (voltage gain ≈ 1). The impedance matcher circuit has an ultra low input current noise of 0.13 fA/Hz1/2, it can provide almost noiseless amplification of high resistance signal sources. It has a very high input impedance ( Zin ≈ 1 TΩ) and a small output

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39

impedance, Zout, compatible with the lock-in amplifier input impedance, Zin. The microbolometer is in series with a load resistor RL and is biased with a U=1.5 V battery.

Spectral density of current noise, Inoise (A / Hz1/2)

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Figure 7.6. Installation for noise measurements in micro-bolometers. (reproduced from [7.1] with permission).

Isystem noise Icell+system noise Icell noise

-14

10

-15

10

-16

10

1

10 100 Frequency, f (Hz)

Figure 7.7. Spectral density of noise of one micro-bolometer (with an a-SixGey:H film) (reproduced from [7.1] with permission).

The NSD of the system and the total NSD (system + cell) were measured separately, and a subtraction of both was made in order to obtain the micro-bolometer noise. Figure 7.7 shows the current noise spectral density in the system, Isystem noise(f); in the system + the microbolometer (with an a-SixGey:H film), Isystem + bolometer noise(f) and in the bolometer, Ibolometer noise (f), which is obtained with Equation 5.12, taking into account the energetic addition of noise.

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( I bolometer

noise

( f ))2 = ( I system+

bolometer noise

( f ))2 − ( I system

noise

( f ))2 (7.12)

The spectral density of current noise will be employed for the determination of detectivity.

7.5. Calculation of Detectivity Another figure of merit is the detecivity, D*, which is shown in Equation 7.13, where RI is the micro-bolometer current responsvity, Acell is the micro-bolometer area, Inoise is the NSD in the micro-bolometer and Δf is the lock-in system bandwidth (set at Δf = 1). For the calculations the value of NSD used is at a frequency around 100 Hz.

D* =

RI ⋅ Acell I noise / Δf

(7.13)

The detectivity expresses one of the most important characteristics in detectors, which is related to the signal to noise ratio.

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7.6. Thermal Response Time Characterization The thermal time constant, τth of the micro-bolometer was determined employing the installation showed in Figure 7.8. A voltage pulse train is applied to the micro-bolometer by a function generator, through a load resistor, RL, and the response current is measured by an oscilloscope. The impedance matcher circuit described in section 7.4 is employed in order to match the high micro-bolometer resistance with the oscilloscope input impedance.

Figure 7.8. Installation for the micro-bolometer thermal time constant characterization (reproduced from [7.1] with permission).

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

41

Figure 7.9 shows a curve obtained from the thermal time measurements, we can observe a voltage pulse train, which is applied to the micro-bolometer and its response current, which has an increasing exponential behavior. Voltage step applied (V= 10 V ) M icro-bolom eter response current

12

-8

8.0x10

-9

6.0x10

-9

4.0x10

-9

4 2

2.0x10

-9

Voltage (V)

10 8 6

0

Current (A)

1.0x10

0.0 -0.4

-0.2

0.0

0.2

0.4

Tim e (sec.)

Figure 7.9. Micro-bolometer thermal time constant (reproduced from [7.1] with permission).

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The time necessary for achieve a steady state is the thermal time constant, in Figure 7.9 the thermal constant is around τth ≈ 200 msec. The thermal time should be larger than the electrical time, τelec, which is the product of the coaxial cable capacitance (Ccable ≈ 10 pF) used in the measurements, and the resistance of the micro-bolometer (Rcell ≈ 500 MΩ). Therefore τelec ≈ 5 msec, which is 2 orders smaller than the micro-bolometer thermal time, τth. The thermal time constant, τth, is related with the thermal capacitance, Cth and the thermal conductance, Gth, by equation 7.14.

τ th =

C th = C th Rth Gth

(7.14)

In order to calculate Cth and Gth, the method described in the following section was used.

7.7. Temperature Dependence of Thermal Resistance and Calibration Curve In this section is presented the procedure followed in order to estimate the temperature dependence of thermal resistance in the micro-bolometers, this procedure is based on that performed in [7.1-7.3]. In the micro-bolometer, I(U) measurements are performed at different temperatures, as is shown in Figure 7.10. The linear part of these curves is used in order to obtain the electrical resistance in the micro-bolometers. With the data obtained is possible to graph the calibration curve of the device (Figure 7.11), which is a curve of the micro-bolometer resistance vs. temperature. At high values of voltage bias, the increment of current in the micro-bolometer will result in a self heating of the device. From I(U) characteristics it is possible to extract the power, P

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42

A. Kosarev, A. Torres and M. Moreno

(W), dissipated by the micro-bolometer. By combining the data of the I(U) characteristics (Figure 7.10) and the calibration curve (Figure 7.11), it is possible to extract the increment in temperature (ΔT) by the power (P=U*I) applied to the micro-bolometer, for each temperature, as is shown in Figure 7.12. The thermal resistance, Rth, is obtained as the slope of the ΔT vs. P curves, for each temperature. Figure 7.13 shows the temperature dependence of the Rth of the micro-bolometer, Rth decreases as the temperature of the micro-bolometer increases. 110

I(U)

100

V(T=260K) V(T=280K) V(T=300K) V(T=320K) V(T=340K) V(T=360K) V(T=380K)

90

Voltage (V)

80 70 60 50 40 30 20 10 0 0.0

1.0x10

-8

2.0x10

-8

3.0x10

-8

4.0x10

-8

5.0x10

-8

Current (A)

R (ohms)

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Figure 7.10. U(I) curves of one micro-bolometer at different temperatures (reproduced from [7.1 ] with permission).

6.0x10

9

4.0x10

9

2.0x10

9

R vs Tem perature

0.0 260

280

300

320

340

360

380

T (K ) Figure 7.11. Calibration curve of one micro-bolometer (reproduced from [7.1] with permission).

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films … T= T= T= T= T= T=

ΔT (K)

15

260 280 300 320 340 360

K K K K K K

43

R th = Δ T / P o w e r

10

5

.

0 -6

0 .0

1 .0 x 1 0 P o w e r (W )

2 .0 x 1 0

-6

3 .0 x 1 0

-6

-1

Rth (K W )

Figure 7.12. Thermal resistance extracted from ΔT vs Power curve (reproduced from [7.1] with permission).

6x10

6

5x10

6

4x10

6

3x10

6

2x10

6

1x10

6

Therm al Resistance, R th

0 Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

260

280

300

320 T (K)

340

360

Figure 7.13. Thermal resistance of one micro-bolometer (reproduced from [7.1] with permission).

8. MICRO-BOLOMETERS IN THZ REGION The frequency range from a few hundred gigahertz to a few tens of terahertz is commonly related to terahertz range. Many applications for this range including biology, medicine, non-destructive control of materials, imaging, home-land security, etc. are rapidly developing. In this section we shall describe our results on the study of Si_ge microbolometer in terahertz range of frequency following ref. [8.1]. The review of recent achievements, applications and principles of terahertz technology can be found in [8.2-8.5]. Because of the low energy of quanta in this spectral region and low output power of the majority of sources, the detection in terahertz spectral range is always a challenge especially if un-cooled detector is desired. . Among different types of detectors for the terahertz frequency range known and already used in the practical systems are Schottky diodes, Golay cells, filed effect transistors, cooled and un-coolled micro-bolometers. For the applications the

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44

A. Kosarev, A. Torres and M. Moreno

selection of the particular detector is a trade-of between sensitivity, speed, size and weight, and price. Un-cooled micro-bolometers have an advantage of relatively low price; compatibility with the CMOS technology, room temperature operation and the possibility of fabrication the imaging devices. Un-cooled micro-bolometers are already used for the infrared (IR) spectral region [8.68.8] and for the detection and imaging at the terahertz frequency range [8.9-8.13]. Different materials as thermo-sensing layer are discussed in Sect.4 As noted amorphous semiconductors have an advantage of high temperature coefficient of resistance (TCR), compatibility with CMOS technology and possibility of building a detector matrix with the read-out circuit on the same chip. One of the possible disadvantages of bolometer with thermo-sensing layer made of amorphous semiconductor is the high noise level and consequently low sensitivity. Noise, however, is poorly studied in amorphous materials including those deposited by plasma and further research may reveal ways for reducing noise level. In this section we shall describe experiments on detection signal in terahertz region with un-cooled micro-bolometer with Si-Ge thermo-sensing layer.

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8.1. Experimental Details The configuration of micro-bolometers and the fabrication process flow are described in Section 6 (planar structure was used in these experiments). It should be noted that these micro-bolometers have been designed and optimized for IR detection rather than for THz range. Backward Wave Oscillator (BWO) generated radiation at frequency f=0.934 and provided power P≈50 μW. The radiation was modulated by the mechanical chopper with the frequency of 30-300Hz. The samples were placed in front of the BWO output cone (see inset in Figure 8.1). The sample was connected in series with the load resistor (RL=10kΩ) biased from the battery in order to reduce the setup noise. The signal from the load resistor was amplified by the low noise amplifier and analyzed by the spectrum analyzer. That allowed us to measure the micro-bolometers response to the terahertz radiation and its noise simultaneously.

8.2 Results Figure 8.1 shows the typical current – voltage characteristic of the sample. The bolometer resistance varies from sample to sample within the range Rb=5-10MΩ. The weak superlinearity of the characteristic seen in Figure 8.1 is probably due to the self heating of the bolometer. The inset in Figure 8.1 shows the schematic view of the setup. The inset in Figure 8.2 shows the background noise spectra and two examples of the asmeasured noise spectra of the bolometer. The background noise was measured with the metal resistor replacing the bolometer. The scheme was biased for 1-6V and the value of the replacing resistor was the same as that of the bolometer.

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

45

-6

D=17 mm

2x10

-6

Bolometer Cone

15 mm

1x10

-6

1x10

BWO

-6

1x10

I ~ V

I, A

(b) -7

8x10

-7

6x10

-7

4x10

-7

2x10

0 0

1

2

3

4

5

6

V, V

Figure 8.1. Typical current-voltage characteristic of the SixGye:H bolometer. Inset shows the setup configuration (reproduced from [8.1] with permission).

10

SI ~ I

-22

-13

2

S, A/Hz

10

-14

V=4V

10

V

2

S, V/Hz

I

10

-23

V=1V

-15

10

4kTR -16

10

1

-7

10

10

10 Frerquency f, Hz

100

-6

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

I, A

Figure 8.2. Dependence of noise SI on the current for several bolometers. Inset shows the background noise of the system and two examples of as-measured noise spectra (reproduced from [8.1] with permission).

Dashed line in the inset shows the level of the thermal noise. As seen, at the frequency f>20Hz the noise level of the setup is very close to the level of the thermal noise, i.e. the noise figure of the amplifier is very close to unity. At low frequencies f 2.5 x107 VW-1. However, the larger array is already in development and is not available commercially. The other two companies which develop IRFPAs based on VOx films, “BAe systems” and “DRS technologies” do not show completely information in their devices. Table 10.2 shows some characteristics in the IRFPAs fabricated by “BAe systems” [10.3], while Table 10.3 shows the information obtained from “DRS technologies” [10.4]. Table 10.4 shows the main characteristics of the IRFPAs based on amorphous silicon (aSi:H,B) technology developed by “L-3 communications”. Table 10.1. Performance characteristics of Raytheon VOx IRFPAs [10.1, 10.2] Performance parameter Array dimensions Pixel size (μm2) Spectral response (μm) Responsivity (V/W) NETD at f/1 (mK) Pixel operability (%) Operating temperature (oC)

Capability (f/1 and 300 K scene) 320 x 240 320 x 240 640 x 480 50 x 50 25 x 25 25 x 25 8 – 14 8 – 14 8 – 14 > 2.5 x 107 > 2.5 x 107 > 2.5 x 107 98 25 25 25

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50

A. Kosarev, A. Torres and M. Moreno Table 10.2. Performance characteristics of BAe VOx IRFPAs [10.3] Performance parameter Array dimensions Pixel size (μm2) Spectral response (μm) Responsivity (V/W) NETD at f/1 (mK) Frame rate (Hz) Operating temperature (oC)

Capability (f/1 and 300 K scene) 320 x 240 640 x 480 640 x 480 28 x 28 28 x 28 28 x 28 7.5 – 14 7.5 – 14 7.5 – 14 60 30 30 -46 to +49 -46 to +49 -46 to +52

Table 10.3. Performance characteristics of DRS VOx IRFPAs [10.4] Performance parameter Array dimensions Pixel size (μm2) Spectral response (μm) Responsivity (V/W) NETD at f/1 (mK) Frame rate (Hz) Operating temperature (oC)

Capability (f/1 and 300 K scene) 320 x 240 640 x 480 320 x 240 25.4 x 25.4 25.4 x 25.4 51 x 51 8 – 12 8 – 12 8 – 12 60 30 60 -20 to +49 -20 to +49 -10 to +45

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Table 10.4. Performance characteristics of L-3 com. a-Si:H,B IRFPAs [10.5-10.6] Performance parameter Information year Array dimensions Pixel size (μm2) Pixel resistance (MΩ) Pixel thermal resistance (KW-1) Spectral response (μm) Responsivity (V/W) NETD at f/1 (mK) Thermal time constant (ms)

Capability (f/1 and 300 K scene) 2000 2007 2007 160 x 120 160 x 120 320 x 240 48 x 48 30 4 x 107 5 - 14 7 - 14 7 - 14 1 x 106 99.9 > 99.9 > 99.9 -30 - +60 -30 - +60 -30 - +60

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Table 10.6. Performance characteristics in the main micro-bolometer IRFPAs commercially available [10.1-10.2, 10.5-10.9] Performance parameter Manufacturer

Capability (f/1 and 300 K scene) Raytheon L-3 Communications ULIS

Thermo-sensing material Array dimensions Pixel pitch (μm2) Pixel resistance (MΩ) Spectral response (μm) Responsivity (V/W) NETD at f/1 (mK) Frame Rate (Hz) Thermal time constant (ms) Pixel operability (%) Operating temperature (oC)

VOx

a-Si:H,B

a-Si:H

640 x 480 50 x 50 8 – 14 > 2.5 x 107 50 -

320 x 240 48 x 48 30 5 - 14 1 x 106 50 11

640 x 480 25 8 – 14 85 60 7

> 98 25

-

> 99.9 -30 - +60

As was discussed at the beginning of the section, “ULIS (CEA/LETI)” in France is developing IRFPAs based on amorphous silicon, they are working on large arrays, with a maximum size of 640 x 480 pixels and a pixel size or pitch (distance between the centers of adjacent micro-bolometers within an array) of 25 μm2 [10.7-10.9]. “ULIS” micro-bolometers have showed the shorter thermal time constant, which is in the range of 7 ms. Table 10.5 shows the main characteristics of “ULIS” micro-bolometers. Finally Table 10.6 shows the performance characteristics for the three main microbolometer IRFPAs which available commercially. Information about D* and ROIC is not showed by these companies. As it can bee seen in Table 10.6, there is an important advance in the development of uncooled micro-bolometer IRFPAs. The tendency is clear, an increment in the number of pixels, a reduction in the pixel pitch and an improvement in the pixel performance characteristics,

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A. Kosarev, A. Torres and M. Moreno

such as the reduction in the thermal time constant, a reduction in the NETD and an increment in detectivity, D*.

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11. SUMMARY A thermal detector (bolometer) senses radiation by changes of electronic properties, e.g., conductivity resulting from rising of temperature due to radiation absorption, differing from photo-detectors. The bolometer has no limitations on spectral sensitivity related to optical gap. The bolometer has a long history starting as a single-cell device and later being converted into two- dimensional imaging devices. In recent developments, plasma deposited materials are used for both supporting and thermo- sensing layers. A commonly used thermo-sensing material is hydrogenated silicon deposited by plasma. In this contribution, we have discussed micro-bolometers with Ge-Si:H films deposited by low frequency plasma. We demonstrated some advantages of this material such as high value of temperature resistor coefficient and moderate (without doping) resistance. The latter allows easier matching output resistance of device cell with read-out circuitry. The bolometers studied have simple two-leg configuration providing rather effective thermoisolation and were fabricated with surface micro-machining technique on silicon wafers by means of a process completely compatible with standard CMOS technology. Microbolometer structure with current flow along thermo-sensing film (planar structure) is the most reported structure. We have discussed both planar and sandwich structures, i.e., with current flow perpendicular to thermo-sensing film surface. The latter demonstrated significantly higher current responsivity, faster response and, unfortunately, higher noise, resulting in detectivity values similar to those for planar ones. 2D modeling shows interesting results predicting both linear and then cubic dependence of responsivity on IR intensity for reduced thermoconductance. Experimental data show only linear dependence suggesting that a modern level of technology did not allow reducing temperature losses to a sufficient level in order to observe cubic dependence at low radiation level. Cubic dependence being realized could increase significantly the response of the micro-bolometers. Most of the reported micro-bolometers based on plasma deposited materials deal with temperature change of resistive properties of semiconductor thermo-sensing film. It is of much interest, however, to study thin film junction (e.g., Schottky barrier or p-i-n) as thermo-sensing element. The junction can be fabricated by the same plasma deposition method, which is successfully applied for fabrication p-i-n thin film solar cells. As estimates show, micro-bolometer with such a junction promises higher responsivity, low noise and some additional functionality related with possibility to control responsivity by variation of bias voltage. The micro-bolometers studied were designed and optimized for sensing IR radiation, but preliminary experiments with these devices showed their high sensitivity and low noise for radiation in teraherz range. This is of much promise for twodimensional imaging devices in the teraherz range.

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Thus, concluding, we see many promises of thin film micro-bolometers based on plasma deposited materials and potential ways to improve their performance characteristics. We have no doubt that further systematical work would bring new achievements in this area.

ACKNOWLEDGMENT The authors would like to acknowledge Dr. R. Ambrosio, Technology and Engineering Institute, UACJ, Mexico, Dr. M.Garcia, Benemerita Universidad de Puebla, Mexico, Prof. S. Rumiantsev, Rensselaer Polytechnic Institute, NJ, USA for materials provided and discussions.

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Section 2 [2.1] L. Dobrzanski, E. Nossarzewska-Orlowska, Z. Nowak, and J. Piotrowski, Sens. Actuators, A, 1997, vol. 60, 154–159. [2.2] P. W. Kruse and D. D. Skatrud, Eds., Uncooled Infrared Imaging Arrays And Systems, ser. Semiconductors and Semimetals. USA: Academic Press, 1997, vol. 47. [2.3] P. W. Kruse, Uncooled Thermal Imaging: Arrays, Systems and Applications. USA: SPIE Press, 2001. [2.4] K. C. Liddiard, Infrared Phys., vol. 24, no. 1, pp. 57–64, Jan. 1984. [2.5] S. Sedky, P. Fiorini, K. Baert, L. Hermans, and R. Mertens, IEEE Trans. Electron Devices, 1999,vol. 46, no. 4, 675–682. [2.6] J. D. Vincent, Fundamentals of Infrared Detector Operation and Testing. USA: Wiley, 1989.

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[4.3] Yong-Hee Han, In-Hoon Choi, Ho-Kwan Kang, Jong-Yeon Park, Kun-Tae Kim, HyunJoon Shin, Sung Moon. Journal of Thin Solid Films, 2003, vol. 425, 260-264. [4.4] A. J. Syllaios, T. R. Schimert, R. W. Gooch, W. L. Mc.Cardel, B. A. Ritchey, J. H. Tregilgas. Mat. Res. Soc. Symp. Proc. 2000, vol.609 A14.4.1. [4.5] R. Ambrosio, A. Torres, A. Kosarev, A. Illinski, C. Zúñiga, A.S. Abramov. J. NonCryst. Solids, 2004, vol. 338-340, 91-96. [4.6] A. Torres, A. Kosarev, M. L. García Cruz, R. Ambrosio, J. Non-cryst. Solids, 2003, vol. 329, no.1-3, 179 – 183. [4.7] Andri Schaufelbühl, N. Schneeberger, Ulrich Münch, Marc Waelti, Oliver Paul, Oliver Brand, Henry Baltes, Christian Menolfi, J. of Microelectromechanical systems, 2001,vol. 10, No. 4, 503-510. [4.8] S. Sedky, P. Fiorini, M. Caymax, C. Baert, L. Hermans and R. Mertens, IEEE Electron Device Letters, 1998,vol. 19, No. 10, 376- 378. [4.9] Sherif Sedky, Paolo Fiorini, Kris Baert, Lou Hermans and Robert Mertens. IEEE Transactions on Electron Devices, 1999, vol. 46, No. 4 , 675 - 682. [4.10] Enrique Iborra, Marta Clement, Lucía Vergara Herrero and Jesús Sangrador, J.of Microelectromechanical Systems, 2002, vol. 11, No. 4, 322- 329. [4.11] A.H.Z. Ahmed and R. Niall Tait. IEEE Transactions on Electron Devices, 2005, vol. 52, No. 8, 1900-1906. [4.12] A. Ahmed, R.N. Tait, J. of Infrared Physics and technology, 2005, vol. 46, 468-472. [4.13] Agha Jahanzeb, Christine M. Travers, Zeynep Celik Butler, Donald P Butler and Stephen G. Tan, IEEE transactions on Electron devices, 1997, vol. 44. No. 10, 17951801. [4.14] Christine M. Travers, Agha Jahanzeb, Donald P Butler and Zeynep Celik Butler. J.of Microelectromechanical Systems, 1997, vol. 6, No. 3, 271- 276. [4.15] J. Delerue, A. Gaugue, P. Testé, E. Caristan, G. Klisnick, M. redon and A.Kreisler, IEEE Transactions on Applied Superconductivity, 2003, vol. 13, No. 2, 176-179. [4.16] Ali Yildiz, Zeynep Celik-Butler, and Donal P. Butler, IEEE Sensors Journal, 2004, vol. 4, No. 1, 112-117. [4.17] J. Kanicki, Amorphous and Microcrystalline Semiconductor Devices: Optoelectronic Devices, Artech House, Boston, MA, 1991. [4.18] D. Slobodin, S. Aljishi, Y. Okada, S. Wagner, Mater. Res. Soc. Symp. Proc. 1986, vol. 70 275. [4.19] S. Aljishi, D. Slobodin, J. Kolodezey, V. Chu, S. Wagner, Mater. Res. Soc.Symp.Proc. 1986, vol.70, 269. [4.20] R. Ambrosio, A. Torres, A. Kosarev, C. Zuniga, A. Abramov. VII Int. Workshop on Non Crystalline Solids, Mexico, 2003, p. 64. [4.21] K.D. Mackenzie, J.R. Egger, D.J. Leopold, Y. Li, S. Lin, W. Paul, Phys. Rev. B, 1985, vol.31, 2198. [4.22] Y. Chou, L. Chen, J. Appl. Phys. 1998, vol.83, 4111. [4.23] D. Bermejo, M. Cardona, J. Non-Cryst. Solids 32 (1979) 421. [4.24] N.F. Mott, J. Non-Cryst. Solids, 1985, vol.77and78, 115. [4.25] A. Torres, M. Moreno, A. Kosarev, A. Heredia, J. Non Crystalline Solids, 2008, vol. 354, 2556-2560.

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Section 5

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[5.1] Dillner U., Riesenberg R. Proc. 7th Int. Conf. on Infrared Sensors and Sytems, Erfurt 2002. May 14-16, 151-156. [5.2] U. Dillner. V. Baier, E. Kessler, J. Muller, A. Berger, D. Behrendt, and H.-A. Preller. Proc. IPS2 2004 AMA Service GmbH, Wunstorf, 2004, 149-153. [5.3] Lambkin Lane B., O’Heifearnan I., Gillham J., Watton R. IEEE/LEOS Int. Conf.on Optical MEMS, 2000, Kauai, HI, USA, Aug.21-24, 99-100. [5.4] W. Lang, K. Kuhl, and E. Obermeier. Sens. Actuators, A, 1990, vol. A21-A23, 473-477. [5.5] R. V. Sudiwala, M. J. Griffin, and A. L. Woodcraft. Int. J. of Inf. And Mm. Waves, 2002, vol. 23, no. 4, 545-573. [5.6] A. L. Woodcraft, R. V. Sudiwala, M. J. Griffin, E. Wakui, B. Maffei, C. Tucker, C. V. Haynes, F. Gannaway, P. A. R. Ade, J. J. Bock, A. D.Turner, S. Sethuraman, and J. W. Beeman. Int. J. of Inf. and Mm. Waves, 2002, vol. 23, no. 4, 575-595. [5.7] A.Kosarev, A.Torres, Y.Hernandez, R.Ambrosio, C.Zuniga, R.Asomoza, Y.Kudriavtsev, R.Silva-Gonzalea, E.Gomes-Barojas, A.Ilinski, A.S.Abramov. J. Mater. Res. 2006, vol. 21, 1, 88-104. [5.8] M.Moreno, A.Kosarev, A.Torres, R.Ambrosio. Thin Solid Films, 2007, vol. 515, 76077610. [5.9] R.Ambrosio, A.Torres, A.Kosarev, A.S.Abramov, A.Heredia, M.Landa, M.Garcia. Mat. Res. Soc. Symp.Proc. 2004, vol. 808, A4.29.1-6. [5.10] A.Torres, M.Moreno, A.Kosarev, A.Heredia. J.Non-Crystalline Solids, 2008, vol.354, 2556-2560. [5.11] Maria de la Luz Garcia Cruz.. PhD Thesis, Institute National for Astrophysics, Optics and Electronics, 2006, Puebla, Mexico. [5.12] M.Garcia-Cruz, R.Ambrosio-Lazaro, A.Torres-Jacome, A.Kosarev.“Eurosensors XIX”, Barcelona, Spain, Proceedings, 2005, vol. I(M,T), MP37-MP-38.

Section 6 [6.1] A. J. Syllaios, T. R. Schimert, R. W. Gooch, W. L. Mc.Cardel, B. A. Ritchey, J. H. Tregilgas. Proc. Mat. Res. Soc. Symp., 2000, vol. 609, A14.4.1. [6.2] T. Schimert, C. Hanson , J. Brady, T. Fagan, M. Taylor, W. McCardel, R. Gooch, M. Gohlke, and A.J. Syllaios, Infrared Technology and Applications XXXV, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton, Proc. of SPIE, 2009, vol. 7298, 72980T,1-5 . [6.3] M. Moreno, A. Kosarev, A. Torres, R. Ambrosio. J. Non Crystalline Solids, 2008, vol. 354, 2598 – 2602. [6.4] M. Moreno, A. Kosarev, A. Torres, I. Juarez. J. Non Crystalline Solids, 2008, vol. 354, 2552 – 2555. [6.5] M.Moreno, A.Kosarev, A.Torres, R. Ambrosio. Int. Journal of high speed electronics and systems, 2008, vol. 18, issue 4, 1045 – 1054. [6.6] M. Moreno, A. Kosarev, A. Torres, R. Ambrosio. Thin solid films, 2007, vol. 515, 7607-7610. [6.7] B. E. Cole, R.E. Higashi and R. A. Wood, in Proc. to the Int., IEDM Technical Digest , 1998, 459–462.

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A. Kosarev, A. Torres and M. Moreno

Section 7 [7.1] Mario Moreno, Ph.D. Thesis, INAOE, Puebla, Mexico, 2008, 70-75. [7.2] Sherif Sedky, Ph.D. Thesis, Katholieke Universiteit Leuven, Belgium 1998, 22-25. [7.3] Mario Moreno, Roberto Ambrosio, Alfonso Torres, Andrey Kosarev, Maria García, and Jose Mireles, , Physica Status Solidi C, 1-4, DOI 10.1002/pssc.200982739, 2010

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Section 8 [8.1] A.Kosarev, S.Rumyantsev, M.Moreno, A.Torres, S. Boubanga, W.Knap. Solid State Electronics, 2010, in press. [8.2] M. Tonouchi, Nature photonics, 2007, vol.1, 97. [8.3] P.H.Siegel. IEEE Trans. Microwave Theory Tech. 2004, vol.52, 2438–2446. [8.4] M.S. Shur, V. Ryzhii. International Journal of High Speed Electronics and Systems, 2003, vol. 13, 2, 575-600. [8.5] W. Knap, M. Dyakonov, D. Coquillat, F. Teppe, N. Dyakonova, Journal of Infrared, Millimeter, and Terahertz Waves, 2009, in press. [8.6] M. Moreno, A. Kosarev, A. Torres, R. Ambrosio. Int. Journal of High Speed Electronics and Systems, 2008, vol. 18, 4, 1045–1054. [8.7] L. Sun, B. Chang, J. Zhang, Y. Qian, and Y. Qiu. Proc. of SPIE, 2007, vol. 6423, 64232D-1. [8.8] A. Ahmed, R.N. Tait. Infrared Physics and Technology, 2005, vol. 46, 6, 468-72. [8.9] B.A. Knyazev, M.A. Dem'yanenko,D.G. Esaev. 2007 Joint 32nd International Conference on Infrared and Millimeter Waves and the 15th International Conference on Terahertz Electronics (IRMMW-THz), 2008, 360-1. [8.10] A.L. Aseev, D.G. Esaev, M.A. Dem’yanenko, I.V. Marchishin, B.A. Knyazev, G.N. Kulipanov, N.A. Vinokurov. Proc.of FEL, 2007, Novosibirsk, Russia, 83. [8.11] B.N. Behnken, G. Karunasiri,D.R. Chamberlin, P.R. Robrish, J. Faist.. Optics Letters, 2008, vol. 33, 5, 440-442. [8.12] A.J. Miller,A. Luukanen,E.N.Grossman. Proceedings of the SPIE - The International Society for Optical Engineering, 2004, vol. 5411, 1, 18-24. [8.13] E.Peytavit,P.Agnese,J.L.Ouvrier Buffet, A. Beguin,F. Simoens. The Joint 30th International Conference on Infrared and Millimeter Waves (IEEE Cat. No. 05EX1150), 2005 vol. 1, 257-258. [8.14] F. N. Hooge, T. G. M. Kleinpenning, and L. K. J. Vandamme, Rep. Prog. Phys. 1981, vol.44, 479. [8.15] R.E.Johanson, M. Gunes, S.O. Kasap. IEE Proceedings-Circuits, Devices and Systems, 2002, vol. 149, 1, 68-74. [8.16] “Properties of amorphous silicon and its alloys”, ed. By T. Searle, 1999, INSPEC. [8.17] http://tydex.ru/en/products/thz_optics/golay_cell. [8.18] www.SpectrumDetector.com, www.goodrich.com.

Section 9 [9.1] Kanicki Jerzy „ Amorphous and Microcrystalline Semiconductor devices”, Artech House Inc., MA, 1992 vol.II , 303.

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Thin Film Micro-Bolometers with Si-Ge Thermo-Sensing Films …

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[9.2] Sze S.M. Physics of semiconductor devices, John Wiley and Sons, 1981, New York. [9.3] Kanicki Jerzy „ Amorphous and Microc rystalline Semiconductor devices”, Artech House Inc., MA, 1992, vol.II, 232. [9.4] Kim J.-K., Han C.-Hi. Sensors and Actrs, 2001, vol. A89, 22-27. [9.5] Ishikawa T. et al. Proc. SPIE, 1999, vol. 36598 Infrared Technology and Applications XXV, 556. [9.6] Szajda K.S., Sodini C.G., Bowman H.F. IEEE J. Solid State Circuits, 1996, vol. 31, 1308.

Section 10 [10.1] [10.2] [10.3] [10.4] [10.5] [10.6] [10.7] [10.8]

Rogalski Antoni, Progress in Quantum Electronics, 2003, vol. 27, 59-210. http://www.raytheon.com/products/320ufe/ (November 2007). www.baesystems.com/ (November 2007). http://www.drsinfrared.com/ (November 2007). http://www.l-3com.com/ (November 2007). http://www.thermal-eye.com/ (November 2007). http://www.ulis-ir.com/ (November 2007). Tissot J.L., Fièque B., Trouilleau C., Robert P., Crastes A., Minassian C., Legras O., “Infrared Technology and Application XXXII”, edited by Bjorn F. Andresen, Gabor F. Fulop, SPIE Proc. 2006,vol. 6206. [10.9] Mottin Eric, Bain Astrid, Martin Jean-Luc, Ouvrier-Buffet Jean-Louis, Bisotto Sylvette, Yon Jean-Jacques, Tissot Jean-Luc, Infrared Technology and Applications XXVIII, SPIE Proc., 2003, vol. 4820, 200-207.

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This work was supported by CONACYT projects 42367 and 48454.

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Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Bolometers: Theory, Types and Applications : Theory, Types, and Applications, Nova Science Publishers, Incorporated, 2010. ProQuest Ebook

In: Bolometers: Theory, Types and Applications Editor: T. M. Walcott, pp. 59-84

ISBN: 978-1-61728-289-8 © 2011 Nova Science Publishers, Inc.

Chapter 2

INVESTIGATIONS OF PROPERTIES OF HIGH TEMPERATURE SUPERCONDUCTING BOLOMETERS I. A. Khrebtov∗ S. I. Vavilvv State Optical Institute, S. Petersburg, Russia

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ABSTRACT The review describes the noise properties of the high temperature superconducting (HTSС) bolometers developed for the applications in the optical electronic devices of infrared and submillimeter wave-lengths. The principle of high-Tc transition edge bolometer operation and bolometer noise theory are considered, taking into account the peculiarities of constant bias current and constant bias voltage modes. The published results of bolometer noise modeling are discussed. Various sources of the excess 1/fnoise in HTSС films as temperature sensitive element for bolometer are reviewed, including the experimental data and modem noise models. Comparative analysis of noise characteristics of the most developed HTSС bolometers for application (antenna-coupled microbolometers and bolometers based on silicon micromachining technology) is reported.

Keywords: Noise; bolometer; film; high temperature superconductivity.

1. INTRODUCTION Bolometers operating in infrared and submillimeter wavelength region have wide and successful applications [1]. After the discovery of high-TC superconductivity (HTSС) it has become obvious, that one of the most promising applications of HTSС materials is the development of the high sensitive infrared (IR) bolometers [2-4]. The HTSС bolometers based on the use of sharp temperature dependencies of the resistance, critical current and ∗

E-mail: [email protected]

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kinetic inductance were considered in reviews [ I , 6-7]. In the following years it was shown that HTSС bolometers were among the successfully developing applications in the area of high- TC superconductivity. The operating temperature close to nitrogen liquid temperature 77 K, thin film technology, sharp temperature dependence of resistance in the superconducting transition region provides the high responsivity and small response time. The estimation showed the possibility of close approach to a level of the detectivity D* ~ 2×10 10 cmHz1/2/W, limited by the background fluctuations in IR-region [3, 5- 8]. Recently the detectivity D* = 3.8×109 cmHz1/2/W at the time constant of 0.4 ms [9] and D* = 1.8×1010 cmHz1/2/W at a modulating frequency of 4 Hz were obtained [10]. As any thermal detector, HTSС bolometers have more wide spectral range of sensitivity. At the wavelengths longer than 20 µm they have no competitors among quantum photodetectors with the same cooling temperature. Eventually, microbolometers manufactured by silicon micromachining technology can be arranged in two dimensional staring arrays [11—13]. HTSС bolometers have good prospects in various applications, including IR-spectroscopy and radiometry, space observation, thermal imaging and so on. The sensitivity of the HTSС bolometer is limited by various noise sources due to the thermodynamic origin, the nature of superconductivity and HTSС film fabrication technology. The excess noise with its characteristic 1/f-dependence limits the noise equivalent power (NEP) of HTSС bolometers at a low frequency region of its operation. It should be noted, that in spite of the evident progress of the manufacture of HTSС bolometers with the detectivity D* near to theoretical threshold, however, the excess l/f-noise problem arising at the development of HTSС devices, including detectors ready for wide application, is of primary importance [14, 15]. In this review the effect of various noise components on noise equivalent power of socalled HTSС transition edge bolometers of various designs, operating at the midpoint of the superconducting transition, are considered, the results of HTSС film excess noise study on various substrates and the correlation of noise with film technology are analyzed and experimental noise characteristics of the bolometers are discussed.

2. BOLOMETER NOISE THEORY AND MODELING 2.1. Principle of Bolometer Operation and Theory of Noise in Bolometers The theoretical estimations of HTSС bolometers as new sensitive IR-detectors have already indicated the potentially high sensitivity of these broadband detectors, determined primarily through their degree of thermal isolation from the surroundings [3, 5-13, 16, 17], or high speed of response due to antenna-coupled microbolometer detection [1, 8, 17]. Below we shall consider some theoretical questions concerning the main bolometer characteristics including responsivity, response time and, mainly, noise parameters, which were discussed earlier in [3, 5-8, 10, 11, 16-19]. The operating principle of the HTSС transition edge bolometer is illustrated by Figure 1 [12]. The radiation is absorbed by a thin HTSС film on a substrate with the heat capacity C [J/K], which is weakly coupled to a temperature-stabilized heat sink through thermal conductance G [W/K]. The absorbed IR-radiation causes the heating of HTSС film, operating

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Investigations of Properties of High Temperature…

61

at the temperature Tb, corresponding to the middle of the superconducting transition and changes its resistance Rh proportionally to the temperature coefficient of resistance β = 1/Rb×dR/dT [1/K]. The constant bias current Ib or voltage Vb are applied to the bolometer and an output voltage or current signals are measured with readout electronics. In recent years, a number of the works were published concerning the effect of electrothermal feedback on the noise properties of cryogenic bolometers, especially, of the superconducting transition edge bolometers. It is explained by using the constant bias voltage mode (CVM) with current readout, thus making it possible to decrease the response time and expand the dynamical range of bolometers. Below is the review of HTSС bolometer theory, taking into account the influence of feedback on the noise [10]. Firstly, the attention was drawn to this problem in [20].

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Figure 1. Principle of operation of HTSС transition edge bolometer [12]. a) Schematic design of bolometer; b) Change of the resistance of Rb in the superconducting transition region at absorbing IRradiation.

Later it was shown the negative electrothermal feedback (ETF) mode, realized at constant voltage bias, is substantial for superconducting transition edge bolometers, operating at liquid helium temperature [21]. Modeling the positive and negative ETF modes for HTSС bolometers was carried out and discussed in [10, 21-23].

2.2. Bolometer Noise Modeling Let us discuss some results of noise parameters modeling of HTSC bolometers of various designs, that were reported in [1, 3, 10, 11, 18, 25-27]. The conditions of the achievement of the NEP, limited by the statistical photon noise, depending on the wavelength of optical radiation and thermal conductance, were analyzed and are shown in Figure 2(a) [3].

2.2.1. Passive Operational Modes with Constant Current Bias (CCM) and Constant Voltage Bias (CVM) In the present article the theory of HTSC bolometer operation has received further development in view of influence of electro-thermal feedback to basic performances of the bolometers, which fundamentaly were developed in [6, 10-13, 15].

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The bolometers on the basis of HTSC films work more often in a mode with constant bias current (CCM). In this case bolometer is included in the scheme with resistance of load RL > > Rb.

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Figure 2. The HTSC bolometer noise equivalent power NEP and the detectivity D* as the function of the infrared wavelength, power loading PIR and the thermal conductance G. a) The PIR and G for an ideal thermal detector, plotted as a function of the cut-off frequency fc of the cold low-pass filter [3]. The NEPs of the ideal detectors are shown along with the separate contributions from phonon and photon noises. b) The detectivity as a function of wavelength for diffraction-limited microbolometer pixels with the field view Ώ = 0.02 sr and τ = 10 ms are shown for a comparison with the photoconductive detectors, operating at 77 K and a pyroelectric one at 300 K [11]. The thick lines show the predicted D* for YBaCuO bolometers on Si and Si3N4 membranes. These lines were calculated using the estimates for the minimum achievable heat capacity and thermal conductance and the measured voltage noise in YBaCuO films. The PC and PV lines show the photon noise limits for photoconductive (PC) and photovoltaic (PV) detectors.

At operation in CCM mode the radiation, absorbed bolometer, at a positive temperature coefficient of resistance of YBaCuO film β=dR/dT×R-1 > 0 conducts to rising heat of sensitive element from YBaCuO film, to thermal instability and transition of YBaCuO film in a normal state with loss of bolometer sensitivity. This effect name as positive electrothermal feedback, which at increase of a bias current can result in thermal instability. The alternate solution is the application of a mode with active (electronic) negative electro-thermal feedback (ANETF) [6, 10-13]. In this case the bolometer is included in an equal-arm bridge. At reception of radiation there is a heating of the bolometer and imbalance of the bridge, the signal of an imbalance is amplified by the amplifier K and through the resistor Rf in a closed loop of negative feedback effects on voltage of power supply of the bridge and, accordingly, on working parameters of the bolometer (responsivity, noise and constant of time) [6, 10-13, 15-17]. The authors of article [10] described the functional behavior of bolometer as an electrical system with closed feedback loop and got new expressions for bolometric characteristics. According to their model, the voltage responsivity Sv of the bolometer operating at constant bias current (CCM), i.e., in positive ETF mode, is given by:

Sv =

ε ⋅ L0

Ib ⋅ (1 − L0 ) ⋅ (1 + ω2 ⋅τ e2 )

1/2

, [V/W]

(1)

where έ is the optical absorption, L0 = βPb/G is the loop gain coefficient, β = dR/dT×1/R – temperarure coefficient of resistance, Pb = Ib2Rb is the bias power, Ib – current of bais, G –

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Investigations of Properties of High Temperature…

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thermal conductance of bolometer, ω = 2πf is the circle frequency, f is the operating frequency, τo is the intrinsic thermal time constant, τ e = τo /(1-L0) is the effective thermal time constant. For stable operation Lo should be smaller 1. In literature Lo = 0.3 is selected as close to the optimum. If the bolometer works in a passive mode with constant bias voltage (CVM), i.e., in a mode with negative electro-thermal feedback, the current sensitivity is equal:

SI =

εL0

Vb (1 + L0 )(1 + ω 2τ e2 )

where: τe =

1/2

, [A/W]

(2)

/(1+L0), L0 = Vb2β/RbG and Vb – bias voltage. In this case there is no limit for

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increase of a bias, as HTSC film has a positive β. And from expression (1.2) follows, that with increase of a factor L0 the stability of work is not upset. Thus, if the bolometer works in CVM mode, effective time constant can be reduced in (1 + L0) due to the increase of bias power J. At L0>>1 value of effective time constant: τe=С/Jβ. For reaching maximum response it is necessary to select YBaCuO film with the greatest coefficient β and to reduce a heat power in the bolometer. The reduction of a value of time constant can be received, utilizing the increase of a bias voltage. However, it is necessary to note, that the increase of speed under operation of negative electro-thermal feedback is accompanied by reduction of current reponsivity is proportionaly to the same value (1 + L0). In a counterbalance to CCM mode in CVM mode there is no direct limitation on value of a feedback ratio L0. Nevertheless, the limitation of L0 exists. To keep resistance of the bolometer in an operating point at increase of Joule power because of increase of a bias voltage, it is necessary to supply sufficient cooling. At selected β and Rb (or Tb) the maximum value of L0 will be determined in temperature of cooling T0 [10]: L0max = β (Tb – T0) = β ∆T

(3)

In practice for HTSC bolometers the minimum temperature of a bath cooling by liquid nitrogen (LN2) T0 ≥ 77 K. Receiving typical temperature of transition Tc for YBaCuO of a film, close to 87 K, we obtain, that in a passive mode CVM at ΔT ≈ 10 K and β = (0,5-2) K-1 the maximum factor of negative feedback can reach L0max ≈ 10 − 30.

2.2.2. Operational Mode of HTSC Bolometer with Active Negative Electrothermal Feedback (АNETF) The functional behaviour of HTSC bolometer as electrical systems with the made closed loop of negative electro-thermal feedback, that was discussed earlier in [6, 15] at analysis of HTSC bolometer operation in a mode of АNETF is considered. Following expressions are obtained for basic performances of the bolometer (voltage responsivity Sv, time constant and overall noise voltage:

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I. A. Khrebtov

Sv =

0,5 L 0 1 1 ⋅ ⋅ , [V/W] 2 2 1/ 2 I b 1 + 0,5 L0 ⋅ F ⋅ K (1 + ω τ e )

(4)

where: L0 =Jβ/G – factor of feedback, J - power of Joule, К – coefficient of gain, τ0 and τe – values of thermal and effective time constants: τe=

/(1+0,5L0 FK)

(5)

The factor of negative electro-thermal feedback in active mode is equal: Laf = 0,5L0⋅F⋅K

(6)

The frequency dependence of a noise voltage, reduced to an output of the bridge, is written as:

V n = V n* ×

1 + (ωτ 0 ) 2 (1 + 0 . 5 L 0 FK ) + (ωτ 0 ) 2

2

1/2 , [V/Hz ]

(7)

where: F - transmission factor of a signal from an output of the bridge on the bolometer for an equal-arm bridge, K - gain of the amplifier, Vn* - total noise voltage of the bolometer and noise voltage of the scheme, no taking into account electro-thermal feedback, which is equal:

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Vn* = (VR ) 2 + (0,5 V1 / f ) 2 + (0,5 V ph ) 2

(8)

where: VR - thermal noise of the bridge, Vph - phonon noise, V1/f - current flicker - noise, accordingly, reduced to an output of the bridge without the account of АNETF [16]. From expression (1.7) follows, what on low frequencies on f > 1/(2πτe) the influence of АNETF mode on a noise should not be observed practically. Noise equivalent power NEP and detectivity D* in a band ∆f =1 Hz is possible to estimate, utilizing equations (1.9), (1.10): NEP

=

V n , [W/Hz1/2] (1.9), A1 / 2 , [сmHz1/2/W] D* = NEP Sv

(10)

where: Vn [V/Hz1/2] - overall noise voltage of different independent noise sources, Sv [V/W] voltage responsivity, A [cm2] - area of a receiving unit. The overall equivalent noise power can be recorded as: NEPΣ = (NEP2ph+ NEP2R + NEP21/f )1/2 , [Вт/Гц1/2] NEPph =

(4kT G ) 2 b

ε

1

2

,

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(11)

Investigations of Properties of High Temperature…

NEPR = V R / S V =

(4kT

b

) (1 + ω τ ) 12

J

2

ε ⋅ L0 12

G⎛ α ⎞ NEP1 f = ⎜⎜ n a ⎟⎟ 1 + ω 2τ 2 εβ ⎝ NAtf ⎠

(

65

2 12

,

)

12

(11a)

where: J - power of Joule, αn - noise parameter of Hooge [18], N = 1021см-3 – density of charge carriers in HTSC film [19], A - area and t - thickness of a film, ε - an absorption coefficient of radiation, which at calculations received equal ~ 1. In publication [19] was shown, what overall NEPΣ, stipulated by different noise sources, is identical to passive modes CCM and CVM. Thus, the overall noise equivalent power NEPΣ can be recorded as:

[

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2 4κTb G ⎛ 4κTb G 2 G 2α n ⎞ ⎜ ⎟ 1 + ω 2τ 0 2 NEPΣ = + + 2 2 2 2 2 a ⎟ ⎜ ε ⎝ ε β J ε β NAtf ⎠

]

1/ 2

(12)

In expression (12) first components (NEPphon) is stipulated by a phonon noise, second (NEPR) - thermal noise of resistance (the noise of a Johnson) and third component (NEP1/f) depends on a excess low-frequency current 1/f-noise. It is necessary to note, what NEPR and NEP1/f depend on a thermal of time constant τ0, but not from an effective time constant τe, as it takes place for voltage responsivity SV. It is possible to explain this fact, taking into consideration influence of electro-thermal feedback to a thermal noise of a Johnson and current flicker 1/f -noise, which normally neglected earlier. In case of operation in CVM mode the bias power J can be more, than in CCM mode, therefore according to (12) component of a thermal noise is supressed with increase J. The conditions of the achievement of the NEP, limited by the statistical photon noise, depending on the wavelength of optical radiation and thermal conductance, were analyzed and are shown in Figure 2(a) [3]. It is seen that for a NEP decrease the bolometer should operate in the far IR-region, where infrared power loading (PIR) is low. So by decreasing thermal conductance G, i.e. the phonon noise, the NEP could be achieved near to the photon limit. Figure 2(b) shows the calculated feasibility of the HTSC microbolometer arrays to improve the detectivity in comparison with quantum photodetectors on wavelength range more, than 20 µm [11]. The authors of work [18], using the experimental data, obtained during the development of Si membrane GdBaCuO bolometer [9], made the calculated analysis of the noise behavior on the frequencies in the range of f = 1-10 kHz, using the experimentally determined parameters: the thermal conductance in range G = 4.5-450 µW/K and the noise Hoogeparameter άN = 0.043-430. Figure 3(a) shows, that the phonon noise approaches the total noise magnitude within the frequency range 20 Hz < f < 200 Hz. At f < 4 Hz the flicker noise will add a reasonable contribution. At f > 1 kHz the Johnson noise will the main noise source. At temperature T = 300 K, where aN > 103 is very high, the flicker noise will dominate. Figure 3(b) shows that with increasing parameter aN and, thus, degrading film quality, D*-value decreases. In the maximum, the achievable D* will be about 80% of the possible phonon noise limit of 5×109

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cmHz1/2/W for the Hooge-parameter of HTSC film at the transition midpoint of aN = 1-2, and the optical absorption έ -0,26 [9].

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Figure 3. Calculated noise performance of GdBaCuO bolometer on YSZ/Si membrane in CCM operation [9, 18]. a) Noise density Vn2 as a function of frequency for T = 85 K and 300 K and time constants of 0.5 ms and 1.5 ms. b) Detectivity D* as a function of frequency for various Hoogeparameter aN.

Figure 4. The calculated NEP of bolometers of various designs in CCM operation as a function of the thermal сonductance and noise parameters of YBaCuO films. a) The estimated NEP of the bolometer on Si3N4 membrane with A = 1 mm2 operated at 84 K as function of the thermal conductance G [25]. The time constants τ is set equal to 0.1 s for all G. b) The NEP dependencies on frequency at the variation of the Hooge-parameter aN (a1 = 2×10 -3 , a2 = 2 × 10 -6 ) for bolometers on SrTiO 3 substrates: 1-membrane bolometer, A - 50×50 µm2: , R b -200Ώ, G = 4×10 -5 W/K, τ = 62 µs; 2microbolometer on solid substrate, A = 1×10 µm2 , R b = 25 Ώ, G = 1.6 ×10 -4 W/K, τ = 0.25 µs [26].

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The estimation of the possibility of achieving the phonon noise limit for membrane bolometers with large area A=1 mm2 illustrated in Figure 4(a) [25]. It is seen that the requirement NEP < 3 × 10-12 W/Hz1/2 puts an limit to G of 5 × 10-6 W/K, at which the membrane bolometer is phonon limited. The manufacture of bolometers with calculated performance had been carried out by combining the existing Si3N4 micromachining technology [10]. The efficiency of using YBaCuO films on SrTiO3 substrates with an extremely low measured Hooge-parameter close to aN = 10-6 was estimated for various bolometer designs in [26]. Figure 4(b) shows the NEP for slow membrane bolometer, limited only by the phonon noise, that can be achieved at low modulating frequencies f < 0.01 Hz (aN = 2×10-6). At high f > 104 Hz the increase of the NEP is explained by the effect of the Johnson noise. The fast microbolometer on solid substrate with a higher G operates at a higher bias current, therefore the excess l/f-noise begins to dominate and to increase the NEP at a higher f < 1 kHz (aN = 2×10-3) and f < 0.1 Hz (aN = 2×10-6). At aN = 2×10-6 the microbolometer will operate at the phonon limit in frequency region of 0.1 Hz - 10 MHz. Above shown results of modeling relates to the bolometers operating in constant bias current, i.e., with the positive electrothermal feedback (CCM operation). The estimation of CVM operation with the negative electrothermal feedback for the composite YBaCuO bolometer on a sapphire substrate (5×5×0.05 mm3), suspended on Au wires, is shown in Table 1 [27]. The bolometer is being developed for the absolute X-rays measurement. The calculation was carried out assuming aN < 10-3 and the sink temperature in CVM Ts=79.5 K. In CCM operation the bolometer functions without the modulation of radiation and the NEP is mainly limited by the temperature instability in the cryostat. In CVM operation the negative ETF decreases time constant down to 46 ms, so the bolometer can operate with modulation, and now the NEP is limited by the phonon noise.

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3. EXCESS 1/F NOISE IN HTSC FILMS FOR BOLOMETERS As it was shown above, the excess low frequency noise of high-TC superconducting films is one of important noise contributions to the overall of HTSC bolometer noise. The first attempts to do HTSC film bolometers have shown that the excess 1/f-noise was the main reason of the restriction of their sensitivity [1-8, 11, 28]. Firstly, the problem of the noise in the HTSC material prospective for new microelectronic devices, operating at liquid nitrogen temperatures have been discussed in review of 1994 year [43]. The best value of the Hoogeparameter aN = 5×10-4 for YBaCuO (c-axis oriented) film on SrTiO3 was reported in this review. Here we consider the results of 1/f-noise study in HTSC films, prepared on substrates of various materials suitable for bolomelric applications [29-42]. The parameters and noise characteristics of the test HTSC film samples of meander-type (m) and bridge-type (b), fabricated by magnetron sputtering (M) or laser ablation (L) and standard photolithography technique, are presented in Table 2.

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Table 1. Calculated parameters of YBaCuO composite bolometer in CVM and CCM operations

Table 2. Parameters and noise characteristics of high-Tc superconducting films for bolometers No., Ref.

Substrate, deposition method

Film size, mm×mm×µm

RN, Ώ

∆ω, deg.

β, K-1

a 300

aN at RN

1[39] 2[39] 3[39]

SrTiO3, L NdGaO3, L MgO, L

0.31×0.02×0.2, b 0.33×0.010×0.2, b 0.51×0.047×0.2, b

55 86 21

0.3 0.2 0.6

0.7 1.5 0.7

8×10-4 5×10-3

3×10-4 2×10-4 0.014

4[39]

YSZ, L

0.52×0.014×0.2, b

103

0.6

1.8

0.3

0.4

5[39] 6[34] 7[42]

Al2O3, L ZrO2/Si, M LaAl03, M

0.52×0.037×0.2, b 0.5×0.035×0.2, b 0.001×0.0007×0.2, b

64 570 35

1.7

0.7 0.5 1.0

0.6 1.4 1×10-2

0.09 0.06 1.4×10-4

8[41] 9[41] 10[18]

SrTiO3, L SrTiO3, L *YSZ/Si, M

0.5×0.045×0.2, b 0.5×0.006×0.2, b 10×0.05×0.05, m

79 252 9700

0.17 0.1

2.6 5.8 0.6

2×10-2 7×10-4 1.4×103

1×10-3 2×10-6 0.01

11[13] 12[11] 13[10] 14[35]

YSZ/Si , L YSZ/Si, L *YSZ/Si/SiN, M CeO2/Al2O3, L

1.4×0.003×0.07, m 1×3×0.04, b 17×0.025×0.06, m 1×1×0.17, b

16000 7 20000 6

0.6 2.6 5.8 0.5

3,6 1.1 1.0 4.2

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The film samples (No. 1-5) on various substrates were prepared with the use of same laser ablation, that gives a possibility to make the comparision of the obtained results more correctly. The procedure of film deposition do not vary significantly when the substrate material is changed so all film structure changes should be attributed only to the two main sources: (i) the change of the substrate material, (ii) the difference, quality (mosaic spread and surface rouhness). The last source is obvious – the better the mosaic spread of substrate, the better epitaxial HTSC film may be fabricated. This effect is especially distinct for substrates with good coincidence site lattice conditions (SrTiO3, NdGaO3) with ∆ω smaller than 0.3 deg. for which its mosaic spread is negligible comparing to a small coherent scattering domain size in (ab) plane. Samples on MgO, Al2O3, YSZ, “bad” SrTiO3, YSZ/Si with ∆ω in the range more than 0.5 deg. have larger lattice mismatch. In Table: *GdBaCuO film, others – YBaCuO films, RN is the resistance in the normal state, the carrier density per unit volume N = 1021 cm-3 was used for calculation of the Hoogeparameters aN for all films, ∆ω is the half of film (005) rocking curve from X-ray measurements. The films had c-axis, oriented perpendicular to the substrate plane. The critical temperatures of superconducting transition were in the region of 85-92 K. Note, the film data are taken mainly from the publications, concerning the transition edge bolometer applications. The temperature dependencies of the critical current jc for YBaCuO films on different substrates, obtained from magneto-physical measurements [37-40], showed both films on NdGaO3 and SrTiO3 had the steepest jc(T) curves and higher jc, showing the jc = (0.8 – 4) × 106 A/cm2 at T= 80 K. Another group of films on Al2O3, YSZ and MgO substrates has the weakest dependence jc(T) and , for example, for films Al2O3 substrate the jc = 6×104 A/cm2. Thus , the higher degree of structural perfection the higher the jc. Certain correlation between the structural properties, critical current and noise behavior have been observed. Noise intensities for all HTSC films from T = 300 K to Tc are close to a Vn2 ~ 1/f behavior. At the super-conducting transition region coefficient a varied in the interval a = 0.8-2. The typical T-dependence of noise spectra density within T = 300 K to Tc has the form of Vn2/(IR)2 ~ Ti, where i = 1.3 - 3 [31, 34]. Typical behavior of the excess noise for epitaxial YBaCuO films of high structural quality, fabricated by laser ablation, is shown on Figure 5 [37-39] The similar noise peaks and the increase of l/f-noise at T< Tc were also observed in [28, 30, 31, 33, 34, 36-40, 45-50]. The investigation of the critical current and noise behavior showed that the films with high structural perfection had the most and sharpest critical current density jc(T), low l/f-noise in normal state and the absence of magneto-dependent peak noise in the transition "tail" region [37-40]. Measurement of the influence of a small magnetic field on a less granular sample [34] showed that the magnetic field noticeably increase the noise in maximum, indicating the motion due to thermally activated two-level or diffusive vortex [34, 36, 45, 47, 49]. A few models were proposed to interpret the origin of the noise peak. These include the vortex motion [34, 45, 46, 51 ], thermodynamic phase transition of the vortex state [50] and critical current fluctuations of weak links [34, 52]. Investigation of critical current behavior in YBaCuO films allows one to think that the factor, due to vortex motion, dominates in structural perfect films. The factor of weak links dominates in films of granular type. In most cases the experimenters deal with films of average quality and then both factors affect simultaneously.

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There is another peculiarity of noise behavior of HTSC films at T < Tc and R = (0.050.2) RN similar to one for the low- Tc superconducting films [40, 53]. The additional temperature-narrow noise peaks were detected, mainly, for narrow YBaCuO bridges [46] or meander [54]. Note, these structures are important for bolometer applications. Figure 6a shows that the intensity of narrow peak noise in the range of T 10 kHz, allows one to operate near to thermodynamic phonon fluctuations [39, 53, 64]. However, because of element contact with massive substrate having high thermal conductivity, high bias current density is used in these microbolometers. Thus, 1/f-noise has a stronger effect on their noise equivalent power NEP at low frequencies of operation [see Figure 4(b)]. In addition to their very small size, the local defects can effect on noise properties of such detectors, therefore, in microbolometers very narrow temperature-dependent noise peaks at transition tail would be observed. Noise behavior was sensitive to current and magnetic field (see Figure 6) [46, 69]. Note, that for transition edge microbolometers flicker noise at T < Tc is less dangerous than the noise peak proportional to dR/dT in the transition middle which has been observed for YBaCuO microbolometer on YSZ substrate [46, 53]. On others substrates (NdGaO3, A12O3, MgO) the excess noise of microbolometers was due to the fluctuations of normal phase resistance and the percolation of transport current in the vicinity of Tc. Table 3 and Figures 8, 9 show the results of experimental investigation of antennacouple YBaCuO microbolometers on various substrates [39, 53, 42, 64, 66-68], Figure 8 shows the noise characteristics of the microbolometer on a NdGaO3 substrate (No. 3, Table 3) that was patterned and provided by an Au bow-tie antenna. It is seen from Figure 8a the noise and microbolometer response in a high frequency range have similar temperature dependencies in the transition region. The noise voltage in the maximum is close to that calculated from a phonon noise, furthermore, its coincidence with the signal dependence confirms that the observed noise is a phonon one. Figure 8b shows the sharp drop of 1/f-noise is observed at the transition, so at the operating point 1/f-noise substantially decreases. Nevertheless, the excess noise restricts the sensitivity of a detector up to some kHz, which is seen on Figurte 9 [39].

Figure 8. a) Temperature dependencies of the resistance R, signal V, and noise voltage Vn at f = 100 kHz for e for the microbolometer on a NdGaO3 substrate (No. 3, Table 3). The thermal conductance G, used in the calculation, is due to due the thermal boundary resistance between YBaCuO film and substrate. b) Temperature dependencies of the resistance (solid) and the 1/f-noise voltage Vn (dotted) at 10 Hz fora similar microbolometer.

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It is obvious that frequency dependence of the experimental noise and calculated phonon noise beyond 10 kHz also agree. From Table 3 and Figure 8, 9 it is seen that the best antenna microbolometers on NdGaO3 have electrical NEPe , limited only by the phonon noise, within the frequency range of (0.001-3) MHz the optical NEP and D* on the efficiency of film antenna and wavelength of radiation [63].

Figure 9. a) Frequency dependence of the noise voltage Vn for theYBuCuO microbolometer on a NdGaO3 substrate (No. 3, Tabl. 3). Solid line is the calculated phonon noise. b) Experimental frequency dependencies of the electrical responsivity Se and noise equivalent power NEPe for the same sample.

Recently, developed techniques of magnetron sputtering, chemical and ion etching permitted the fabrication of microbolometers with size down to l×l µm2 [42].The concentration of 1/f-noise sources, sources i.e., the fluctuator density in the energy range 0.20.6 eV, was reduced approximately 3-times compared to the known results. This allowed the excess noise in YBaCuO microbridges to be considerablyreduced achieving the Hoogeparameter aN of about 10-4, that had been obtained earlier on macrobridges only [37-39, 41, 59]. In this case the electrical NEPe of 1.5 × 10-12 W/Hz1/2 (phonon noise limit) would beachieved in the frequency range from 100 Hz to 30 MHz.

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Table 3. Parameters and noise chracteristics of antenna-coupled YBaCuO microbolometers in CCM operation No., Ref.

Substrate materials

YBaCuO film size, µm2

Rb, Ώ

a N.

G, mW/ K

τ, ns

Se, kW/ Hz1/2

NEPe, pW/ Hz1/2

1[64]

YSZ

13×6

40

0.4

0.036

2×104

0.48

4,5

2[66,67]

YSZ membrane

10×5

75

0.13

0.003

104

*2.9

3[53]

NdGaO3

14×2

24

2×10-2

0.3

3×102

0.24

4[42]

LaAlO3

1×0.7

20

4

0.005

30

1.7

5[68]

Al2O3

5×1.5

318

0.12

0.035

19

0.4

1.4×10-

*9 16 1.5 14

In Table: * - measured optical S and NEP ; the carrier density per unit volume N = 1021 cm-3 was used for calculation of the Hooge-parameters aN.

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In applications of microbolometers, for example, in satellite instruments or thermonuclear plasma it is necessary to have information about the effect of various radiation sources on the on characteristics of diagnostics, detectors. The effect of ion 84Kr radiation with energy E = 230 MeV and 16O2 with E = 2 MeV on YBaCuO microbolometers on YSZ substrate have been investigated in works [70,71]. Also, the information about the noise of composite bolometer on sapphire substrate after exposure to ionising radiation was reported in [35]. As it was expected, t he irradiation of heavy ions increased the structure disorder of HTSC films and it led to the increase of 1/f-noise in normal and superconducting phases and also critical temperature and resistance. The abnormal behavior of flicker noise in normal phase after of the irradiation may be due not only to structural failure but with arising elastic strains in system of HTSC film- substrate [61].

4.2. Noise of HTSC Bolometers Based on Silicon Micromachining Technology

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At present, HTSC bolometers fabricated by silicon membrane micromachining technology are considered as the most prospective high performance infrared detectors [1, 6, 9-13, 18, 19, 72, 73], However, in spite of large advantages of the silicon technology there is a problem of the excess flicker noise in HTSC films on silicon substrates. [11, 13, 18, 38, 39, 74-76]. The structural quality of films is affected by some mechanisms: (i) through the large lattice mismatch between the buffer layer and the superconducting film on top, (ii) through the large difference of the temperature dependent expansion coefficients within the layer system, (iii) through the interdiffusion along grain boundaries and the resulting chemical reactions. Thus, during thermal cycling, microcracks formation is observed in the system. It may the result in the effects on the bolometers, especially, noise.

Figure 10. a) SEM image of silicon membrane illustrates the microcrack formation in a rectangular crack pattern, along the low index crystallographic directions due to thermal cycling between 77 K and 300 K [18]. b) Temperature dependent resistance Rb and the noise voltage Vn of YGdCuO bolometer on an YSZ/Si membrane recorded at f=12.5 Hz, ● — R (T) curve, x— virgin sample, □-aged sample after thermal cycling.

Figure 10(а) illustrates the formation of microcracks in GdBaCuO film on Si-membrane and Figure 10(b) shows the effect of it on noise behavior of one from bolometers (as No. 1, Tabl. 4) [9, 18]. Note the emergence of the isolated noise spikes for the aged sample at 81 K and 83 K. However, typically, a noise maximum is observed in the center of the transition region, where the slope dR/dT is peaking, and the intensity of this noise is the same for both

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Investigations of Properties of High Temperature…

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the virgin and aged samples. This noise maximum is due to the phonon noise . Also this confirms the spectral investigations in Figure 11(a).

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Figure 11. a) Noise spectrum Vn2(f) of GdBaCuO bolometers on YSZ/Si membrane in CCM operation [18]. Virgin sample No. 1: 1 - Ib = 0.24 mA, T – 300K, 2 - Ib = 1.1 mA, Rb = 0.001RN, virgin sample No. 2 (data of No. 2 are shown in Tabl. 3): 4- Ib= 0.24 mA, Rb = 0.24 mA, Rb=0.5RN; aged sample No. 2:3- Ib = 0.24 mA, Rb =0.5RN. b) Noise spectrum of one from GdBaCuO bolometers on YSZ/Si/Si3N4 membrane in CCM operation at T= 89.5 K [10]. The phonon noise was calculated using Eqs. (1, 8) at following parameters: Ib= 17.8 µA, Rh = 1.89 kΏ, β = 5.8 1/K, G = 18 µW/K, Lo = 0.19, τe = 95 ms, Sv= 8.4 kV/W (electrical one at έ = 1) at f = l/2π τe.

Figure 11(a) 11 (a) shows that at f < 10 Hz flicker noise is dominant and the Vn2 strictly follows the 1/f-dependence, with Vn2 ~ f (0.8-1.0). The Vn2 measured at operating T, where the bolometric signal has a maximum, typically revealed a plateau at frequencies 10 < f < 200 Hz due to phonon noise. At frequencies > 10 Hz l/f-noise decreases, while the phonon noise is invariable up to 200 Hz. Further the phonon noise begins to decrease as Vn2 ~ 1/f 2, i.e., more than flicker noise. As result, the noise at high frequencies is due to the Johnson noise. Figure 11(b) shows a typical voltage noise spectrum Vn (f) of GdBaCuO bolometer on an YSZ/Si/Si3N4 membrane at constant bias current. The bolometef differs from the previous Simembrane bolometer by a low thermal conductance and, accordingly, low response time, but higher responsivity (see Tabl. 4, No. 3). The calculated spectrum of the phonon noise, the Johnson noise, fitted 1/f-noise and the noise measurement setup, and their orthogonal sum are also shown. It can be seen that between 0.2 and 3 Hz the measured spectrum is also fully due to the phonon noise. Attention should be paid to sample No.4 in Table 4 with large area, that is an electricalsubstitution bolometer, developed for absolute measurement in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum [72, 75, 76]. The application of this bolometer requires a combination of a high thermal conductance G with low noise. So, the NEP of this bolometer was limited by the preamplifier noise. For absolute measurement it is important to have the time stability of bolometric characteristics including the noise ones. During the development of these bolometers it was shown that the passivation with Au or AgAu of YBaCuO films on CeO/YSZ/Si membranes does not degrade the performance of the bolometers, but imptoves them. Note, the authors paid attention to the fact that thoigh the noise does decrease with increasing HTSC film volume, the excess noise spectral density is not inversely proportional to the sample volume.

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Table 4. Parameters and noise characteristics of HTSC bolometers based on silicon micromachining technology in CCM operation No Ref

Membrane

A/t, µm2/ µm µm2/µm

Rb, Ώ

aN

G, mW/K

τ, ms

Sv, kV/W

NEP, W/Hz1/2×10-12

D*, cmHz1/2/W

1[9,18]

*YSZ/Si

72×104/1

6400

0.01

0.3

0.58

0.85

22.0

3.8×109

2[13]

YSZ

8000/0.7

8000

3.6

8.5×10-5

110

32

1.5

8×109

3[10]

*YSZ/Si/SiN

95×104/1

3600

1.0

1.5×10-2

115

4.76

5.5

1.8×1010

4[72]

CeO/YSZ/Si

16×106/2.8

0.4

6.0

1.9×10-3

16.8

87.0

4.1×109

5[19]

CeO2/YSZ

104/0.08

-

-

6.2×10-4

0.56

12

4.0

2.5×109

6[73]

CeO2/YSZ

2500/0.16

50

15.0

8×10-4

1.2

5.6

1.2

4.2×109

In Table: * - GdBaCuO films, others-YBaCuO film; t is the thickness of the membrane; Sv, NEP and D* are obtained from optical measurements and their values depend on the optical absorption.

I. A. Khrebtov

79

CONCLUSION The analysis of a short history of active HTSС development for various applications and, namely, bolometers for infrared and submillimeter regions shows noticeable progress as in the understanding a origin of excess 1/f -noise of HTSС films as in the improvement of the technology which permits to obtain high perfect YBaCuO films with noise Hooge-parameter in the normal state close to 2×10-6. Note, in first publications concerning 1/f-noise in HTSС materials with aN ~ 105 had been only reported. Also, the detectivity of first bolometers was about D* ~ 107 cmHz1/2/W which had been limited by low frequency flicker noise. The detectivity of the best modern HTSС bolometers is close to theoretical values due to the phonon noise achieving D* = 1.8×1010 cmHz1/2/W. It is important to note that these results were obtained at the use of modern silicon micromachining technology which has the good prospects of applications such as IR and submillimeter arrays. There are main problems, which should be resolved. They related to temporal stability of the bolometric performance including noise characteristics. The study of connections of 1/f-noise with degraded processes in HTSС films is the most important task on the way of successful bolometer applications.

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[3]

[4]

[5] [6] [7] [8] [9]

P. L. Richards, Bolometers for infrared and millimeter waves, J. Appl. Phys. 76 (1994) 1-24. S. V. Gaponov, M. A. Kalyagin, L. V. Malysheva, D. V. Paveljev, A. D. Tkachenko and I. A. Khrebtov, Investigation of bolometric properties of YBa2Cu307.x films, Sov. Tech. Phys. Lett. (USA) 15 (1988) 482-484. P. L. Richards, P. L. Clarke, R. Leoni, Ph. Lerch, S. Verghese, R. Beasley, T. H. Geballe, R. H. Hammond, P. Rosenthal and S. R. Spilman, Feasibility of the high-Tc superconducting bolometer, Appl. Phys. Lett. 54 (1989) 283-285. S. Verghese, P. L. Richards, S. A. Sachtjen and K. Char, Sensitive bolometers using high-Tc superconducting thermometer for wavelengths 20-300 µm, J. Appl. Phys. 74 (1993) 4251-4253. P. W. Kruse, Physics and applications of high-Tc superconductors for infrared detectors, Semicond. Science and Technol. 5 (1990) S229-S240. Z. M. Zhang and A. Frenkel, Thermal and nonequilibrium responses of superconductors for radiation detectors, J. Supercond. 7 (1994) 871-884. I. A. Khrebtov, Superconducting infrared and submillimeter radiation receivers, Sov. J. Opt. Techn. (USA) 58 (1991) 261-270. I. A. Khrebtov, Theoretical analysis of the properties ofhigh-Tc superconductor bolometers, Superconductivity: Phys. Chem. Techn. (USA) 5 (1992) 558-567. H. Neff, J. Laukemper, I. A. Khrebtov, A. D. Tkachenko, E. Steinbeiss, W. Michalke, M. Burnus, T. Heidenblut, G. Hefle and B. Schwierzi, Fast, high performance, composite type high-Tc transition edge bolometer on micromachined silicon membrane, Appl. Phys. Lett. 66 (1995) 2421-2423.

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[10] de M. J. M. E. Nivelle, M. P. Bruijn, de R. Vries, J. J. Wijnbergen, de P. A. J. Korte, S. Sanchez, M. Elwenspoek, T. Heidenblut, B. Schwierzi, W. Michalke and E. Steinbeiss, Low noise high-Tc superconducting bolometers on silicon nitride membranes for farinfrared detection, J. Appl. Phys. 82 (1997) 4719-4726. [11] S. Verghese, P. L. Richards, K. Char, D. K. Fork and T. H. Geballe, Feasibility of infrared imaging arrays using high-Tc superconducting bolometers, J. Appl. Phys. 71 (1992) 2491-2498. [12] B. R. Johnson and P. W. Kruse, Silicon micro structure superconducting microbolometer infrared arrays, Proc. SPIE 2020 (1993) 2-11. [13] B. R. Johnson, M. C. Foote, H. A. Marsh and B. D. Hunt, Epitaxial YBa2Cu3O7 superconducting infrared microbolometers on silicon, Proc. SPIE 2261 (1994) 24-30. [14] L. B. Kiss and P. Svedlindh, New noise exponents in random conductor-superconductor and conductor-isolator mixtures, Phys. Rev. Lett. 71 (1993) 2817-2821. [15] Sh. Kogan, Electronic Noise and Fluctuations in Solids, Cambridge University Press, Cambridge UK (1996). [16] H. Neff, Modelling and optimisation of high-Tc superconducting bolometers: The effect of film thickness, J. Appl. Phys. 69 (1991) 8375-8379. [17] Q. Hu and P. L. Richards, Design analysis of a high-Tc superconducting microbolometer, Appl. Phys. Lett. 55 (1989) 2444-2446. [18] H. Neff, I. A. Khrebtov, A. D. Tkachenko, E. Steinbeiss, W. Michalke, O. K. Semchinova, T. Heidenblut, J. Laukemper, Noise bolometric performance and aging of thin high Tc superconducting films on silicon membranes. Thin Solid Films 324 (1998) 230-238. [19] L. Mechin, J. Villegier and D. Bloyet, Suspended epitaxial YBaCuO microbolometers fabricated by silicon micromachining: Modeling and measurements, J. Appl. Phys. 81 (1997) 7039-7047. [20] J. C. Mather, Bolometer noise: nonequilibrium theory, Appl. Opt. 21 (1982) 1125-1129. [21] A. T. Lee, P. L. Richards, S.-W. Nam, B. Cabera and K. D. Irwin, Superconducting bolometer with strong electrothermal feedback, Appl. Phys. Lett. 69 (1996) 1801-1803. [22] A. T. Lee, J. M. Gildemeister, S-F. Lee and P. L. Richards, Voltage-biased high-Tc superconducting infrared bolometers with strong electrothermal feedback, IEEE Trans. Appl. Super-cond. 7 (1997) 2378-2381. [23] H. Neff, A. M. N. Lima, G. S. Deep, R. C. S. Feire, E. Melcher, I. A. Khrebtov and A. D. Tkachenko, Nonlinearity and electrothermal feedback of high Tc transition edge bolometers, Appl. Phys. Lett. 76 (2000) 640-642. [24] F. N. Hooge, T. G. Kleinpenning and L. K. Vandamme, Experimental studies of 1/f noise,Rep. Prog. Phys. 44 (1981) 479-532. [25] P. A. J. de Korte, M. J. M. E. de Nivelle and J. J. Winjbergen, Bolometeric detector for OH-observation, Proc. SPIE 2578 (1995) 294-303. [26] I. A. Khrebtov, B. Dam, A. D. Tkachenko, F. C. Klaassen, J. M. Huijbregtse and K. V. Ivanov, YBCO films on SrTiO3 substrates with recordly low 1/f noise for bolometer applications, Proc. of 4th European Workshop on Low Temperature Electronics, ESA WPP-171 (2000) 335-339. [27] A. Khrebtov, A. D. Tkachenko, K. I. Ivanov, A. D. Nikolenko and V. F. Pindyurin, Absolute high-Tc superconducting radiometer with electrical-substitution for X-rays measurements, Phys. IV France 12 (2002), Pr3-137-Pr3-140.

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[28] S. V. Gaponov, M. A. Kalyagin, M. B. Kraykhin, L. V. Malysheva, P. A. Pavlov, D. G. Paveljev, A. D. Tkachenko, I. A. Khrebtov and A. Yu. Churin, Noise properties of YBaCuO films, Sov. Tech. Phys. Lett. (USA) 15 (1989) 482-485. [29] R. D. Black, L. G. Turner, A. Mogro-Campero, T. C. McGee and A. L. Robinson, Thermal fluctuation and 1/f noise in oriented and unoriented YBa2Cu3Oj.x films, Appl. Phys. Lett. 55 (1989) 2233-2235. [30] P. Rosenthal, R. Hammond, M. R. Beasley, R. Leoni, Ph. Lerch and J. Clarke, Low frequency resistance fluctuations in films of high temperature superconductors, IEEE Trans, on Magn. 25 (1989) 973-975. [31] R. C. Lacoe, J. P. Hurell, K. Springer, I. D. Raistrick, R. Hu, J. F. Burch and R. S. Simon, Low frequency 1/f noise measurements in YBa2Cu3O7 thin films and the implication for HTS IR detectors, IEEE Trans. Magn. MAG-28 (1991) 2832-2835. [32] J. Hall, H. Hickman and T. M. Chen, Resistance drift noise in high Tc superconducting transition region bolometers, Solid State Communication 76 (1990) 921-922. [33] I. A. Khrebtov, M. B. Krayuhin, V. N. Leonov, A. D. Tkachenko, A. Yu. Klimov, D. G. Paveljev, A. A. Ivanov, Bolometric characteristics of YBaCuO and LaSrCuO films, Cryogenics 32 ICEC Suppl. (1992) 533-535. [34] I. A. Khrebtov, V. N. Leonov, A. D. Tkachenko, A. V. Bobyl, V. Yu. Davydov and V. I. Kozub, Comparative low-frequency noise studies of YBaCuO films, in Noise in Physical Systems and 1/f Fluctuations, Proc. 12th Int. Conf., eds. P. H. Handel and A. L. Chung, AIP 285 (1993) 123-126. [35] J. C. Brasunas and B. Lakew, Transition edge noise in YBa2Cu3O7.^ thin films before and after exposure to ionizing radiation, J. Appl. Phys. 775 (1994) 7565-7566. [36] M. Fardmanesh, A. Rothwarf and K. J. Scoles, Noise characteristics and detectivity ofYBa2Cu307 superconducting bolometers, J. Appl. Phys. 79 (1996) 2006-2011. [37] I. A. Khrebtov, V. N. Leonov, A. D. Tkachenko, P. V. Bratukhin, A. A. Ivanov and A. V. Kuznetsov, 1/f noise and critical current density of high-Tc superconducting films for bolometers, in Noise in Physical Systems and 1/f Fluctuations, Proc. 14th Int. Conf., eds. C. Clayes and E. Simoen, Leuven (1997) 313-316. [38] I.A. Khrebtov, V. N. Leonov, A. D. Tkachenko, P. V. Bratukhin, A. A. Ivanov, A. V. Kuznetsov, H. Neff and E. Steinbeiss, W. Michalke, T. Heidenblut and J. K. Laukemper, Noise of high-Tc superconducting films and bolometers, J. Phys. IV France 8 Pr3 (1998)293-296. [39] I.A. Khrebtov, V. N. Leonov, A. D. Tkachenko, A. A. Ivanov, P. V. Bratukhin, A. V. Kuznetsov, E. Steinbeiss, Noise of high-Tc superconducting bolometers, Proc. SPIE 3287 (1998) 288-299. [40] I. A. Khrebtov, V. N. Leonov, A. D. Tkachenko, P. V. Bratukhin, A. A. Ivanov and A. V. Kuznetsov, Magnetic flux noise and pinning in low and high-Tc superconducting films, in Physics and Materials Science of Vortex States, Flux Pinning and Dynamics, ed. R. Kossowsky, Klumer Academic Publishers (1999) 307-320. [41] A. Khrebtov, A. D. Tkachenko, K. V. Ivanov, B. Dam, F. C. Klaassen and J. M. Huijbregtse, The Noise Characteristics of YBCO Films with Strong Pinning, Techn. Phys. Lett. (USA) 26 (2000) 1078-2000. [42] S. F. Karmanenko, A. A. Semenov, I. A. Khrebtov, V. N. Leonov, T. H. Johansen, Yu. M. Galperin, A. V. Bobyl, A. I. Dedoboretz, M. E. Gaevski, A. V. Lunev and R. A.

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I. A. Khrebtov Suris, Fabrication process and noise properties of antenna-coupled microbolometers based on superconducting YBCO films, Supercond. Sci. Technol.13 (2000) 273-286. L. B. Kiss and P. Svedlindh, Noise in high Tc superconductors, IEEETrans. Electron.Devices 41 (1994) 2112- 2122. A. V. Bobyl, M. E. Gaevski, I. A. Khrebtov, S. V. Konnikov, D. V. Shantsev, V. A. Solvev, R. A. Suris and A. D. Tkachenko, Resistance flicker noise and current percolation in c-oriented YBa2Cu3O7.x films in the vicinity of Tс Physica C247 (1995) 7-33. G. Jung, M. Bonaldi, S. Vitale and J. Konopka, One the origin of low frequency noise in HTSC thin films, Physica C 180 (1991) 276-279. V. N. Leonov and I. A. Khrebtov, Noise of YBaCuO microbolometers, Superconductivity: Phys. Chem. Techn. (USA) 4 (1991) 1260-1266. Z. Celik-Butler, W. Yang and D. P. Butler, Measurements of noise and temperature coefficient of resistance on YBa2Cu307x thin films in magnetic field, Appl. Phys. Let. 60 (1992) 246-248. V. Palenskis, A. Stadalnicas and J. Joudvirsis, Low-frequency noise, electric and magnetic characteristics of films in the superconducting temperature region, ibid., as [34], 131-134. D. H. Kim, W. N. Kang, Y. H. Kim, J. H. Park, J. J. Lee, G. H. Yi, T. S. Hahn and S. S. Choi, Voltage noise and vortex states in YBa2Cu3O7 films, Physica C 246 (1995) 235240. D. G. Steel, D. H. Kim, K. E. Gray, S. E. Pfanstiel, J. H. Kang and J. Talvacchio, Electrical-noise signatures of possible vortex transitions in epitaxial YBa2Cu3O7 thin films, Physica C 248 (1995) 55-60. J. H. Lee, S. C. Lee and Z. G. Kim, Noise measurement near the transition region in YBa2Cu3O7.x thin-film superconductor, Phys. Rev. B 40 (1989) 806-6809. D. V. Shantsev, A. V. Bobyl, M. E. Gaevskii, O. L. Shalaev and R. A. Suris, Noise properties of inhomogeneous non-linear medium: application to high-Tc superconductors, ibid., as [37] 321-324. I. A. Khrebtov, V. N. Leonov, A. D. Tkachenko, M. B. Krajuhin, A. V. Bobyl and V. Yu. Davydov, Comparative noise study of bolometers based on conventional and highTc superconductors, in 6th AIAA/ASME Thermophys. and Heat Transfer Conf., ASME HTD 277 (1994) 69-76. A. V. Bobyl, M. E. Gaevski, S. F. Karmanenko, I. A. Khrebtov, V. N. Leonov, D. V. Shantsev, V. A. Solvev and R. A. Suris, Magneto-depending noise of a single latent weak link in YBa2Cu3O7.xfilm, Physica C266 (1996) 32-40. S. Jiang, P. Hallemeir, C. Surya and G. M. Phillips, Low-frequency excess noise in YBaCuO thin films near the temperature, ibid., as [34], 119—122. R. F. Voss and J. Clarke, Flicker (1/f) noise: Equlibrium temperature and resistance fluctuations, Phys. Rev. B 13 (1976) 556-573. V. I. Kozub, Influence of structural relaxation on the parameters of a superconductor, Phys. Rev. B 49 (1994) 6895-6902. B. Dam, and J. M. Huijbregtse, F. C. Klaassen, R. C. F. van der Geest, G. Doornbos, J. H. Rector, A. M. Testa, S. Freisem, J. C. Martinez, B. Stauble-Pumpin and R. Griessen, Origin of high critical currents in YBaiCu3O7.x superconducting thin films, Nature 399 (1999) 439-442.

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[59] A. Khrebtov, A. D. Tkachenko, K. V. Ivanov, B. Dam, F. C. Klaassen and J. M. Huijbregtse, Noise, transport and structural properties of high-Tc YBa2Cu307.x films with noise Hooge-parameter in normal state near to 10-6', in Noise in Physical Systems and 1/f Fluctuations, Proc. 16th Int. Conf., ed. G. Bosman, World Scientific Co. Pte. Ltd. (2001) 43-46. [60] Yu. M. Galperin, V. L. Gurevich and V. L. Kozub, Low-frequency noise in high temperature superconductors, Fizika tverdogo tela 31 (1989) 155-164. [61] A. V. Bobyl, M. E. Gaevskii, S. F. Karmanenko, R. N. Kutt, R. A. Suris, I. A. Khrebtov, A. D. Tkachenko and A. I. Morosov, Intrinsic microstrains and normal-phase flicker noise in YBa2CujO7 epitaxial films grown on various substrates, J. Appl. Phys. 82 (1997) 1274—1280. [62] A. V. Bobyl, I. A. Khrebtov, A. D. Tkachenko, K. V. Ivanov, B. Dam, F. C. Klaassen and J. M. Huijbregtse, Nature of sharp temperature dependency of normal phase flicker noise of epitaxial YBa2Cu3O7.x films, ibid., as [59], 43-46. [63] V. N. Leonov and I. A. Khrebtov, Antenna-coupled thermal radiation detectors (Review), Instr. Exp. Techn. (USA) 36 (1993) 501-520. [64] M. Nahum, Q. Hu, P. L. Richards, S. A. Sachtjen, N. Newman and B. F. Cole, Fabrication and measurement of a high-Tc superconducting microbolometer, IEEE Trans. Magn. MAG-27(1991)3081-3085. [65] P. Langolis, D. Robbes, M. L. Chok. Sing, C. Gunter, D.Bloet, J. F. Hamet, R. Desfeux and H. Murray, Superconducting fast microbolometers operating below their critical temperature, J. Appl. Phys. 76 (1994) 3858-3868. [66] I. R. Rice, E. N. Grossman, L. J. Borcherdt and D. A. Rudman, High-Tc superconducting antenna-coupled microbolometer on silicon, Proc. SPIE 2159 (1994) 98-109. [67] I. R. Rice, E. N. Grossman and D. A. Rudman, Antenna-coupled high-Tc air bridge microbolometer on silicon, Appl. Phys. Lett. 65 (1994) 773-775. [68] V. N. Leonov and I. A. Khrebtov, Stability and noise of fast YBaCuO antenna microbolometers on sapphire substrates, Tech. Phys. Lett. (USA) 20 (1994) 951-953. [69] I. A. Khrebtov, V. N. Leonov, V. I. Kozub, V. Yu. Davydov, N. N. Faleev, M. V. Belousov, S. F. Karmanenko and R. Chakalov, Structural diagnostics and investigation of the low-frequency current noise in the anisotropic YBaCuO films on the LaAlO3 substrates, Superconductivity: Phys. Chem. Techn. (USA) 6 (1993) 623-632. [70] D. V. Akinshin, A. A. Astaspov, L. N. Zaitsev, A. Yu. Klimov, V. N. Leonov, S. P. Molodni-akov, D. G. Paveliev, V. K. Pankratov, V. A. Skuratov and I. A. Khrebtov, Effect ofmKrand 16O ion beams on noise characteristics of YBaCuO microbolometers, Sov. Techn. Phys. Lett. (USA) 17(1) (1991) 40-42. [71] A. A. Astaspov, Yu. Klimov, V. N. Leonov, D. G. Paveliev, V. A. Skuratov and I. A. Khrebtov, Anomalous behavior of current noise in YBaCuO microbolometers above Ta Sov. Techn. Phys. Lett. (USA) 17(4) (1991) 278-280. [72] L. R. Vale, R. H. Ono, D. G. McDonald and R. J. Phelan, Large area YBa2Cu3O7.x bolometers on Si subsrates, Supercond. Sci. Technol. 12 (1999) 856-858. [73] S. J. Berkowitz, A. S. Hirahara, K. Char and E. N. Grossman, Low noise hightemperature superconducting bolometers for infrared imaging, Appl. Phys. Lett. 69 (1996) 2125-2127.

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[74] H Neff., W. Schauble, J Laukemper, M. Burnus, T. Heidenblut, G. Hefle and B. Schwierzi, W. Michalke and E. Steinbeiss, Excess noise, structural properties and their effects on bolo-metric performance of thin superconducting films on silicon membranes, J. Appl. Phys. 77 (1995) 4580-4583. [75] D. G. McDonald, R. J. Phelan, Jr., L. R. Vale, R. H. Ono, J. P. Rice, L. Borcherdt, D. A. Rudman, J. Cosgrove and P. Rosenthal, Noise from YBaCuO films: size and substrate dependence, IEEE Trans, on Appl. Superconductivity 7 (1997) 3091-3095. [76] D. G. McDonald, R. J. Phelan, Jr., L. R. Vale, R. H. Ono and D. A. Rudman, Passivation, transition width, and noise for YBCO bolometers on silicon, IEEE Trans, on Appl. Superconductivity 9 (1999) 4471-1474.

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Chapter 3

OPERATING UNCOOLED RESISTIVE BOLOMETERS IN A CLOSED-LOOP MODE Denoual Matthieu∗ and Allègre Gilles GREYC-electronique, ENSICAEN UCBN, Caen, France

ABSTRACT

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When feedback configurations are available, operating sensors in a closed-loop mode has advantages over the open-loop mode, including improved linearity and wider dynamic range. This chapter proposes feedback configurations referred to as external electronic feedback (EEF) to improve the performance of uncooled resistive bolometers. EEF is an effective technique to speed up bolometers without significantly complicating the system. For resistive bolometers in which the temperature increase due to the incident radiation is measured by a resistance change, the feedback principle is based on heat dissipation in the resistive temperature sensor, i.e., the thermometer, or at its vicinity. The possibility of working at a controlled constant temperature through EEF closed-loop mode, is also interesting when materials with high temperature coefficient of resistance near metal-insulator transition are used for the thermometer. The existing EEF configurations differ depending on how and where the heat is applied, mainly whether an external heater is used or not. Three feedback configurations exist: the first two configurations are classical and have been known for years, whereas the third configuration is a new one, developed recently. The first configuration, commonly used for an anemometer, is simple and directly coupled to the thermometer, but an oscillations issue can limit its dynamic range because of the dependency between the electrical and the thermal working points. The second configuration, based on the electrical substitution principle, requires an external resistive heater which limits its use to the bolometer specifically designed for it. The third configuration implements the electrical substitution principle through a capacitive coupling to the thermometer. This type of coupling, at the expense of more complicated electronics, allows a separate control of the electrical and thermal working points.

∗

E-mail: [email protected]

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Denoual Matthieu and Allègre Gilles By operating bolometers in closed-loop mode, not only can performance be improved, but new functions can be facilitated, such as self-test and self-calibration allowing for complete smart bolometers.

1. INTRODUCTION Notation Conventions The notation for the variables follows the common electronic convention with lower and upper case characters to denote time dependent and time independent qualities respectively. Lower and upper case subscripts are used to represent the small signal component, i.e., the variations about a quiescent working point, and the total value respectively. The following example applied to the current flowing through the resistive thermometer of a bolometer illustrates this notation convention. iBIAS (t ) = I BIAS + ibias (t ) where iBIAS (t ) is the bias current flowing through the resistive thermometer, I BIAS is the quiescent working point, ibias (t ) is small signal component of the bias current. An exception is made for electrical signal analog to temperature, in that case the subscript is always in upper case and a second subscript level, following the expressed convention, is added as illustrated by VTBIAS , VTbolometer which represent the temperature quiescent working

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point voltage and the monitored bolometer temperature small signal voltage respectively. Lower case symbols are also used to represent the Laplace transform of the timedependent quantities, for example ibias ( s ) = L(ibias (t )) is the Laplace transform of the small variations of the bias current around a quiescent working point. The block diagram formalism of control theory [1, 2] commonly used in electrical engineering is used to exhibit the performance of open and closed-loop configurations. This formalism enables compact representation of the system while keeping the physical model transparent enough.

Bolometer Principle and Model All bolometers are thermal systems mainly constituted of a thermal absorbing mass coupled to a thermostat at T0 as illustrated in Figure 1.

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Figure 1. Schematic of a resistive bolometer. The incident radiation power is absorbed and converted into heat. The heat rises the temperature of the sensing element part of a thermal system coupled to a heat sink at constant temperature T0. The resistance change of the resistive sensing element caused by temperature change is monitored through current biasing here. In steady state the power loss to the heat sink balances the absorbed radiation power and the Joule effect power produced by biasing, i.e. pRADIATION+pBIAS=pSINK.

The principle of operation of a resistive bolometer is based on sensing the temperature rise through the resistance change induced in a resistive thermometer or thermistor [3, 4]. Any incoming radiation onto the absorbing element will have its radiation power converted into heat, causing a temperature rise of the temperature sensing element and a heat flow through the link. The two ways to recover an electrical signal from R(T) are current biasing and voltage biasing. Both of them induce (ETF) Electro Thermal Feedback that shall be taken into account. Current or voltage biasing are chosen depending on the sign of the ETF phenomenon and its impact on stability so as to prevent thermal runway [5]. In this chapter, only current biasing will be considered as it is widely and commonly used, however all the considerations presented can be applied with minor changes when voltage biasing is considered. The current biasing is an easy way as T becomes a voltage that can be amplified with well known instrumentation amplifiers, the output is then a voltage ( vTBOLOMETER ). The performance of the bolometer is characterized mainly by three figures of merit : the responsivity ( R ) which characterizes the intrinsic transfer function of the bolometer, the ∗

specific detectivity ( D ) which enables the comparison of bolometers upon their noise equivalent power normalized to unit area and unit bandwidth, and the intrinsic thermal time constant ( τ th ). The specific detectivity is derived from the responsivity, the geometrical dimensions and the noise equivalent power of the sensor. The responsivitiy is therefore a key criterion for bolometer comparison. Under the assumption of heat conduction mechanisms

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Denoual Matthieu and Allègre Gilles

predominance, verified when operating under vacuum and for small temperature fluctuations, the responsivity is expressed as follows:

R (ω ) [V / W ] =

vTbolometer pradiation

=

α η I BIAS RB (Geff2 + ω 2Cth2 )

with vTbolometer the output voltage, pradiation the power of the incoming radiation, RB the resistance of the resistive thermometer,

α=

1 ⎛ dRB ⎞ ⎜ ⎟ the temperature coefficient of RB ⎝ dT ⎠

resistance (TCR) of RB , η the absorption coefficient of the absorbing layer, I BIAS the bias current, Cth the thermal capacity. Geff , the effective thermal conductance, is expressed to take into account the electro thermal effect [2, 3, 5] as follows :

⎛ dR ⎞ 2 Geff = Gth − ⎜ B ⎟ I bias = Gth − α Pbias ⎝ dT ⎠ with Gth the conductance of the thermal link between the bolometer and the substrate. Expressed with block diagram and transfer function formalism, illustrated in Figure 2, the relation between the bolometer output voltage and the incoming radiation power is expressed by:

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vTbolometer ( s ) = R ( s ) ⋅ G ( s ) ⋅ pradiation ( s ) where R( s ) =

A

1 + τ eff s

is the transfer function of the bolometer with A =

η α RB I bias Geff

.

The effective time constant, taking into account the ETF phenomenon, is expressed by

τ eff =

Cth C and is slightly different from the intrinsic thermal time constant τ th = th that Geff Gth

only depends on the thermal properties of the bolometer. The voltage after amplification by the readout electronics is denoted vTB as illustrated in Figure 3 to distinguish it from the voltage at the very output of the bolometer and corresponds to: vTb ( s ) = R ( s ) ⋅ G ( s ) ⋅ pradiation ( s ) . The design and the materials used for the fabrication have major impact on the responsivity of the bolometer and then on the time constant. In order to increase the responsivity of the bolometer, the TCR of the resistive thermometer should be high and possibly maximized by an appropriate choice of the thermal working point in the case of transition materials. Similarly, the absorption coefficient, η , of the absorbing layer should be as as close to 1 as possible for the range of wavelength covered

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by the application. On the contrary, the thermal conductance, Gth , should be as low as possible.

Figure 2. Block diagram representation of the bolometer on the left side and equivalent transfer function on the right side.

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Figure 3. Schematic representation of a bolometer and its associated electronics with R(s) and G(s) the Laplace transforms of the responsivity and of the gain of the readout electronics respectively.

The considerations on TCR have driven the choice of vanadium oxide with high TCR values between 2.10-2 and 5.10-2 K-1 for the resistive thermometers material in most cases and for commercial applications [4, 6, 7]. The absorbing material with the highest absorption coefficient, η, at the infrared wavelength target range for the absorption layer used are dark metal -black gold [8, 9]- or dark polymer paint [3] and latterly nanostructured materials [10] are considered. Specific calibration procedure with optical sources are required to characterize the absorption coefficient of the absorbing layer. Thermal conduction reduction can be achieved by an relevant choice of the material used, but in that case the design of the bolometer plays a crucial role. Indeed, thermal isolation of the resistive thermometer of the bolometer from the substrate through the design of membrane type bolometers suspended by hinges reduces the thermal conduction [4]. It should be recalled here that thermal exchange is limited to conduction mechanism because of operation under vacuum condition and small temperature difference with the environment for experimentation. In order to decrease the time constant of the bolometer, the thermal capacity has to be as low as possible whereas the thermal conductance has to be as high as possible. These goals can be achieved through the design of the bolometer, the material choice and technology but the constraint for the thermal conductance is in that case opposite to the constraint related to responsivity. In summary, to increase the responsivity, the bolometer should have a high TCR material for the resistive thermometer and a low thermal conductance ( Gth ) ; to be fast (small τ th ), the bolometer should have a small heat capacity ( Cth ) and a high thermal conductance ( Gth ). The non-consistent constraints on Gth prevent the simultaneous maximization of both responsivity and time constant. Consequently, the choice of Gth will result from a trade-off between responsivity and time constant ; in the optimization processes responsivity is usually

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preferred, leading to slow bolometers. This latter consideration is not relevant for applications where time is not an issue as in the metrology domain for example. It should be noted that increasing the responsivity directly affects the measurement dynamic range. As the voltage swing of the readout electronics is limited in practice the higher the responsivity is the smaller the radiation power input range is. The design and practical realization of the bolometer impact on the performance of the bolometer have been sketched. Now, once the bolometer has been designed and fabricated some performance enhancement remain achievable through its implementation. Especially, similarly to every physical system that can be operated in a closed-loop configuration, the performance of a bolometer and its associated electronics can be improved through closed-loop operation.

2. CLOSED-LOOP OPERATION FOR BOLOMETERS The closed-loop configuration has advantages over the open-loop configuration, including wider frequency bandwidth and improvements in linearity. Independently of the implementation, a closed-loop system has a forward part, a feedback path and a comparison node for a set point versus an input from a sensor regarding the status of the process that is being controlled [11]. On the basis of the comparison, control action is taken to increase, reduce or maintain the variable controlled. In the resistive bolometer case, the primary physical quantity is temperature, as a consequence the quiescent working point is also a temperature ( TBIAS ). The control variable shall allow control action on the sensor and is

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power type ( p FEEDBACK ). To produce the feedback power, electrical or optical means can be considered, through various implementations. The implementations of the electrical feedback power are the subject of the section 2.2. Although they may present advantages in terms of metrology and especially by potentially avoiding the calibration procedure required for the characterisation of the absorbing layer, the optical feedback power means are not currently used for bolometric applications.

2.1. Advantages of Closed-Loop Operation for Bolometers For a given bolometer, like for any sensor, the performance in terms of bandwidth, dynamics and even linearity can be improved when closed-loop operation is made possible. In the case of resistive bolometers for which the resistive thermometer temperature is a primary measure of the incoming radiation power, the physical feedback quantity is heat produced by optical or electrical means. The performance improvement described in this section are independent of the heat production mean and independent of the feedback implementation that are presented in section 2.2. A large part of them are common to all kinds of sensors when operated in closed-loop. The description of the performance improvement is mainly shown through block diagram formalism illustrated in Figure 4. The summation node symbolizes the physical addition of power. The comparison node that produces the difference between the status of the process

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Operating Uncooled Resistive Bolometers in a Closed-Loop Mode

91

and the set point also symbolizes the voltage shift done in practice to work within the electronics dynamic range.

2.1.1. Linearization and Wider Dynamic Range The set point for the quiescent thermal working point VTBIAS is a constant, it consequently does not appear into the expressions using the Laplace formalism below that only consider variations around a working point. The closed-loop transfer function is given by:

vTb ( s ) =

R( s ) ⋅ G ( s) pradiation ( s) 1 + K ⋅ R( s ) ⋅ G ( s)

for high values of K , the temperature of the resistive thermometer doesn't anymore depend on the incoming radiation power.

vTb ( s) ≈

1 pradiation ( s ) K

Temperature variations of the bolometer caused by radiation power are reduced by the feedback path gain of the controller. The small temperature deviation assumption stated earlier to somehow linearize the expression of the responsivity of the bolometer is particularly relevant in closed-loop configuration with the control of temperature. The incoming radiation power is treated as a disturbance and rejected by the system. In closed-loop configuration the output of the system is the feedback power p feedback . Its

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knowledge can give an estimate of the radiation power and is expressed in Laplace formalism by:

p feedback ( s) = −

K ⋅ R( s) ⋅ G ( s ) pradiation ( s ) 1 + K ⋅ R( s) ⋅ G ( s)

for high electronics gains of the controller and readout electronics, the transfer function can be simplified, showing that the feedback power p feedback , is both proportional to the radiative power pradiation and independent of the thermal parameters of the bolometer:

p feedback ( s ) ≈ − pradiation ( s ) for K ⋅ R ( s ) ⋅ G ( s ) >> 1 Therefore, a direct linear power reading is enabled. It should be noted that in practice the feedback power is generated by the Joule effect of a control voltage in a resistor and therefore the relation between this control voltage and the feedback power is usually non-linear except when digital techniques are used. Those issues are addressed in the implementation section 2.2.

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Figure 4. Schematic representation of a bolometer and its associated readout and control electronics in closed-loop configuration. The quiescent thermal working point is defined by the set point voltage VT at the input of the comparison node. BIAS

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With a proper system design, all of the thermal parameters and variables, are reduced to second-order terms, thus both greatly improving the linearity of the system and making the sensitivity, and the bandwidth dependant upon the signal-conditioning electronics. A work at NIST from [12] illustrates this linearity improvement with a two orders of magnitude linearity improvement from 10 µW to 1 mW linear range increase with a doped silicon resistive thermometer due to closed-loop operation. The reduced impact of the thermal parameters also reduces the sensitivity to their evolution over time and therefore to the effect of aging of the sensor thereby providing greater stability of the system over time. In open-loop the dynamic measurement range is limited by the responsivity of the bolometer, the gain of the conditioning electronics and the supply voltage. The higher the responsivity of the bolometer and the gain of the conditioning electronics are, the smaller the dynamic measurement range is. The more the sensor is sensitive, the less the dynamic range is:

(PRADIATION )MAX


> Rb

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(Figure 1a). The incident radiation, absorbed by bolometer, results in the heating of sensitive element and change of its resistance. Positive coefficient β leads to the heating, increase of the bolometer resistance and additional growth of Joule heat, which in its turn results in an additional increase of temperature and Rb. This effect is due to the positive ETF., which at increase of bias current leads to the heat instability and jump of HTSC film in normal state with the loss of sensitivity. Figure 1b shows the electrical circuit for bolometer in the CVM mode with passive negative ETF {10-12]. Bias voltage is applied to the SQUID input coil, which is in series with Rb of HTSC bolometer. To obtain the CVM mode the resistance of shunt resistor should be RSh 1 the effective time constant will look like: τe=С/Jβ. The proceeding for achievement of the maximal speed it is necessary to choose HTSC films with the greatest β and to reduce heat power of bolometer. The decrease of time constant is possible to obtain using the increase of bias voltage. However, it is necessary to note that the increase in speed under the influence of negative ETF is accompanied by reduction of the current sensitivity in proportion to the same amount

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130

of (1+L0). As against the CCM mode in the CVM one there is no direct restriction on the value of feedback coefficient L0. Nevertheless, restriction of L0 is existed. To keep bolometer resistance in operating point at increase of the Joule power due to increase of bias voltage it is necessary to provide sufficient cooling. At selected values β and Rb (or Tb) the maximal value of L0 will be determined by temperature of cooling bath T0 [12]: Lmax=β⋅(Tb−T0)=βΔT

(10)

However in practice, for HTSC bolometers the minimal temperature of bath is equal to temperature of cooling liquid nitrogen 77 K. Having in a view that typical Tc of YBaCuO -1 film may be near to 87 K, so ΔT ≈ 10 K. At β =(0.5−2 K ) the maximal coefficient of

feedback about Lmax = 10−30 times would be obtained in passive CVM mode [12]. influence of the electrotermal feedback on noise characteristics is important having in a view the prospects of applied use of HTSC bolometers. Usually the ultimate sensitivity of bolometer is characterized by the noise equivalent power (NEP) or the detectivity (D*) at bandwidth Δ f =1Hz as: =

NEP

A1 / 2 , [cmHz1/2/W] V n , [W/Hz1/2] (11), and D* = NEP Sv

(12)

where: Vn [V/Hz1/2] is the total noise as composed of a sum of various independent noise sources, S v [V/W] is the voltage sensitivity, A is the area of sensitive element. Taking into account the sum of the noise contributions, the total equivalent noise power is written as:

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NEPΣ = (NEP2ph+ NEP2R + NEP21/f )1/2

(13)

The most important noise components for HTSC bolometers are as follows. The first term is the phonon noise caused by the random energy exchange between the bolometer element and the heat sink through the thermal conductance G:

(

V ph = SV 4kTb2G

NEPph

(4kT G ) = 2 b

)

12

1

(14)

2

ε

(15)

In case of the constant bias current in passive CCM mode with voltage readout, the Johnson noise voltage, taking into account the effect of ETF, can be written as [8]:

VR =

(4kb ⋅Tb ⋅ Rb )1 2 1− L

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(16)

Noise Properties of High-Tc Superconducting Transition Edge…

131

Factor L, written with Eq.6, is used in Eq.16, which shows that the Johnson noise voltage is increased at the output by a factor (1-L) due to positive ETF. Thus NEPR due to the Johnson noise of HTSC film resistance is written as:

(4kT J ) (1 + ω τ ) = 12

NEPR = V R / SV

2

2 12

b

(17)

ε ⋅ L0

A similar method is used for calculation of the flicker noise V1/f and NEP1/f due to HTSC film:

V1 f

2 1 ⎛⎜ H ⋅ U b = ⋅⎜ 1 − L ⎝ NAtf a

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NEP1 f

G ⎛ H ⎜ = εβ ⎜⎝ NAtf

a

⎞ ⎟ ⎟ ⎠

12

⎞ ⎟⎟ ⎠

(18) 12

(1 + ω τ ) 2

2 12

(19)

Eq.19 is obtained using the empirical formula of Hooge for 1/f-noise[14], where H is the noise Hooge-parameter due to HTSC film quality, N=1021 cm-3 is the density of charge carriers in the film [15], A is the area and t is the thickness of the film, the coefficient a at f is close to 1. For the estimation of NEP1/f for a concrete case it is possible to use parameter H, taking into account the value of Rb and H in operating point. In the CVM mode with negative ETF the same as in the CCM mode, the electrothermal interaction renders influence on noise of HTSC bolometer. The model of ETF (Figure 2) is used as in the previous case to estimate influence of ETF on noise. Coefficient of transformation A will be current sensitivity of bolometer S*I leaving out of account ETF (Eq.8), bolometer voltage Ub is the coefficient of transfer k. Using the Eq.2 the expressions for noise voltage for these components with account of the CVM mode will be written as:

Voltage of Johnson noise:

VR

12 ( 4k b ⋅ Tb ⋅ Rb ) =

Voltage of excess current noise: Voltage of phonon noise:

(20)

1+ L

V1 f

(

2 ⎞ 1 ⎛⎜ H ⋅ U b ⎟ = ⋅⎜ a ⎟ 1+ L ⎝ N ⋅ A⋅ t ⋅ f ⎠

V ph = S I ⋅Rb 4kb ⋅ Tb2 ⋅ G

)

12

12

(21) (22)

Operation with passive negative CVM mode leads to the decrease of noise voltage in (1+L), where the coefficient of electrothermal feedback L is determined with Eq.10 and can be L>>1. It was shown [9] that exactly the same results for NEP, due to different sources of noise, were obtained in both passive CCM and CVM modes. Thus, the resulting NEPΣ can be rewritten as:

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4κTb G

1/ 2

⎞ ⎛ 4κT G 2 G2H (23) ⎟ 1 + ω 2τ 2 + ⎜⎜ 2 b 2 + 2 2 NEP Σ= 2 a ⎟ ε ⎝ ε β J ε β NAtf ⎠ It should be also noted that according to Eqs.14 -23 NEPR and NEP1/f depend on the thermal time constant τ but no the effective time constant τe as for SV or SI in Eqs.7, 9. This fact is explained by taking into account the effect of ETF on the Johnson and flicker noise contributions. In case of the CVM mode the bias power J can be taken larger than for the CCM mode, so according to Eq.23 only the Johnson noise contribution of NEPR (2-d term) would be suppressed with an increasing of the Joule power J. Other contributions of NEPΣ would be reduced with decrease of bolometric parameter G or using more quality HTSС films with smaller H and larger β. 2

[

]

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2.3. Characteristics of HTSС Bolometers in Active ANETF Mode Passive negative ETF reduces effective constant time τe by the value of (1+L0). However, at increase of current Ib the 1/f-noise increases as well and, thus, detectivity D* reduces. The influence of negative ETF can be increased without deterioration of D* using the external loop of feedback with the electronic amplifierх [13]. The circuit with equal-arm bridge and differential amplifier (Figure 1) is used for operation in the ANETF mode. The change of bolometer resistance, arising under the action of heating by optical radiation, results in an unbalance of bridge. The voltage of unbalance Uout from bridge is transferred to the differential amplifier K, and further the amplified signal through resistor Rf acts on a diagonal of the bridge feed of Um. The amplifier is connected in such a manner that the increase of bolometer resistance results in reduction of a feed voltage Um and, hence, in reduction of Joule power and resistance of bolometer. Thus, the operation with an active negative the ANETF mode is realized. The block-diagram in Figure 3 explains the mechanism of the ANETF mode. The power of optical radiation Popt, absorbed with bolometer on an input of the block-diagram, increases its resistance and thus is transformed to change of a voltage ΔUb through the voltage sensitivity SV* leaving out of account ETF (Eq.4). Resistance Rf and total resistance of the bridge Rm form a voltage divider (see Figure 1). In view of a divider the change of a voltage in a diagonal of abridge feed is equal to product F ×Uout, where F =Rm/(Rm + Rf) is the voltage divider. For equal-arm bridge: Rm ≈ Rl. The change of a voltage of a feed bridge under action Uout is transfered to bolometer through a divider M2 =Rb / (R1+Rb). At small signals the change of Rb is much less than resistance R1, therefore, for the equal-arm bridge, Rb=R1 and M2=0.5.The change of a voltage on bolometer under the influence of negative feedback conducts to change of the Joule power ΔJ, that partially compensates the action of the absorbed optical power, thus Rb→R1 and Uout decreases. With the account of a fore said the increase of resistance ΔRb and, hence, Uout in the ANETF mode will be defined by no compensated absorbed Popt , i.e.(Popt -ΔJ).

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Figure 3. Block-diagram of active negative electrothermal feedback mode (ANETF).

Analysis of block-diagram on Figure 3 allows one to get the expression for Sv of electronic circuit with bolometer at ε=1 as:

Sv =

0.5 L 0 1 1 ⋅ ⋅ 2 I b 1 + 0.5L0 ⋅ F ⋅ K ( 1 +ω τ e 2

)1 / 2

,

where: L0 =Jβ/G is the coefficient of feedback, K is the amplifier gain and

(24)

is the effective

time constant:

τe =

τ 1 + 0.5 L0 ⋅ F ⋅ K

.

(25)

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In this case the coefficient of feedback in active mode Laf is written as: Laf = 0.5L0⋅F⋅K

(26)

Substituting noise power for the optical radiation power in the block-diagram on Figure 3 and performing transformations, frequency dependence of noise voltage reduced to the bridge output:

Vn = V

* n

(

0.5 1 + ω 2τ 2

((1 + 0.5L

)

12

2 2 0 ⋅ F ⋅ K) +ω τ 2

)

12

Vn* = (VR ) 2 + (0.5 V1 / f ) 2 + (0.5 V ph ) 2

(27) (28)

where: VR is the Johnson noise voltage, 0.5 V1/f and 0.5 Vph are the voltages due to the 1/fnoise and the phonon noise, respectively, reduced to the bridge output without ANEFT. From Eq.27 it follows that on low frequencies at f1/ (2πτ) influence of the ANETF mode should be not observed practically. The noise equivalent power NEP and the detectivity D* are estimated using Eq.11, 12.

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3. NUMERICAL AND EXPERIMENTAL MODELLING 3.1. Comparative Numerical Modeling in Passive CCM and CVM Modes The comparative calculation and experimental research were carried out using GdBaCuO bolometers on Si/SiN membrane. The technology of manufacturing bolometer was described in the work [8]. The bolometer had an area A = 0.85×0.85 mm2, the thermal conductance G=1.4×10-4 -1.3×10-5 W/K, the heat capacity C =6.5×10-7 J/K, the thermal constant time τ≈50 ms, the coefficient of absorption ε≈0.7. Other parameters are submitted in Table 1. Figure 4 shows characteristics of typical superconducting transition of GdBaCuO bolometer, the results of investigation of which are reported below. Formulas obtained in the sections 2.2 and 2.3 are used for numerical modeling bolometer characteristics. The results of calculation on Figure 5 shows that at increase of bias voltage in the CVM mode the voltage sensitivity Sv becomes less on low frequencies in comparison with the CCM mode. However, at the expense of increase of bolometer speed the meaning of Sv is kept up to more high frequencies and already at frequencies more than 12 Hz, Sv in the CVM mode will exceed Sv, received in the CCM mode.

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Table 1. Characteristics of GdBaCuO bolometers Number Mode T0, K Rb, кΩ β, K-1 G, W/K Ib, μА τe, ms fcut-off, Hz Su, кV/W Vn ×10-9, V/Hz1/2 NEP ×10-11, W/Hz1/2 D* ×109, сm Hz1/2/W D*/τe½, cm Hz/W

592 CCM 85.0 4.70 1.5 1.1×10-4 65 6 25 1.4 52

АNETF 85.0

756 CCM 84.3

АNETF 83.6

79.2

72 1.2 132 0.11 49

124 0.4 400 0.07 16

0.9 1.3×10-5 40 23 7 3.5 32

60 2.7 60 0.179 3.6

150 1.6 100 0.086 2.2

4

44.5

22.8

0.9

2.0

2.5

2.3

0.19

0.37

9.4

4.25

3.4

3.0×1010

5.5×109

1.85×1010

6.3×1010

8.2×1010

8.5×1010

In Table: T0 is the temperature of cooling bath. An absolute black body with T=373 K (λmax=7 μm) was used at measurement of voltage sensitivity. The gain amplification of amplifier is 500-1000.

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Rb

4 4,1

α, kΩ/К

3

0,88

2

2,0

20

1,8

18

1,6

16

1,4

14

1,2

12

1,0

10

0,8 4,7

-1

α

β, К

β

5

0,6

1

0,4 0,2

0

0,0

8 6

135

Rb , k Ω

Noise Properties of High-Tc Superconducting Transition Edge…

4 2 0 -2

80

82

84

86 84,42

88

90

T b, K

Figure 4. Temperature dependencies of resistance Rb, (■), slope of superconducting transition α=dR/dT (○) and temperature coefficient of resistance β (∆) of GdBaCuO bolometer on Si/Si3N4 membrane.

In the CVM mode at bias voltage Ub =0.24 V the effective constant time τe will decrease in comparison with the thermal constant time τ in 5 of times, thus, Sv will increase in comparison by one in the CCM mode as in ~5 of times as well. CCM Ib=40 μА

4

CVM Ub=0.24 V

10

CVM Ub=0.18 V

S, V/W

3

10

2

10

1

10

1

10

100

1000

Figure 5. Calculated frequency dependences of voltage sensitivity for bolometer in the CCM and the CVM modes. 1/f-noise (Ub=0.24 V) phonon noise (Ub=0.24 V) Johnson noise (Ub=0.24 V)

-1/2

Vn, V Hz

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f, Hz

-8

10

-9

10

1/f-noise (Ub=0.18 V) phonon noise (Ub=0.18 V)

-10

10

Johnson noise (Ub=0.18 V)

1

10

100

1000

f, Hz

Figure 6. Calculated frequency dependences of the noise voltages for bolometer in the CCM and the CVM modes.in the CVM mode at two values of bias voltage.

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Figure 6 shows the influence of negative ETF in passive CVM mode on the noise components at bias voltage 0.24 V, at which β is maximum and resistance of bolometer is 0.92 kΩ (see Figure 4). The suppression of the Johnson noise and the 1/f-noise are increased on low frequencies at the expense of amplification action of the negative ETF. According to Eq.14 the reduction of the phonon noise on low frequencies is connected to reduction of Sv, under action of negative ETF, caused by decrease of bolometer temperature. On frequencies more than 12 Hz the value of Vph in the CVM mode is more, than in the CCM mode because Sv in the CVM mode at these frequencies is higher. The frequency dependences of a total noise voltage of in the CCM and the CVM modes are submitted in Figure 7. The influence ETF on noise voltage on low frequencies is well visible at the same temperatures in the CCM and the CVM modes. The suppression of the noise in the CVM mode rather than in the CCM mode at same temperature of bolometer does not give a prize in threshold sensitivity. Frequency dependences of the components of NEPΣ, received in the CVM mode (at Ub=0.18 V), practically coincide with similar dependences of NEP, received in the CCM mode (at Ib=40 μA), in all considered range. It is explained to that in the CVM mode the factor of suppression of Vn and Sv is same. Note that in CCM mode the factor of increase of Vn and Sv has one meaning. -7

CCM Ib=40 μ A CVM Ub=0.24 V

1/2

10

Vn, V/ Hz

CVM Ub=0.18 V

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10

-8

1

10

100

1000

f, Hz

Figure 7. Calculated frequency dependences of total noise in the CCM and the CVM modes.

In the CVM mode at the greater bias voltage Ub=0.24 V rather noise voltage for Ub=0.18 V happens the decrease of two components of NEPΣ, caused by the Johnson noise and the 1/fnoise (at f > 10 Hz), however the component of NEPph, determined by phonon noise remains constant (Figure 8).

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NEP1/f (Ub=0.24 V) NEPph (Ub=0.24 V) NEPR (Ub=0.24 V)

-10

10

NEP1/f (Ub=0.18 V)

NEP, W/Hz

1/2

NEPph (Ub=0.18 V) NEPR (Ub=0.18 V)

-11

10

-12

10

1

10

100

1000

f, Hz

Figure 8. Calculated frequency dependences of NEP for various noise voltage components in the CVM mode at various bias voltage.

CCM Ib=40 μ A CVM U b=0.24 V

NEPΣ, W/Hz

1/2

CVM U b=0.18 V

10

10

-10

-11

1

10

100

1000

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f, Hz

Figure 9. Calculated frequency dependences of total NEPΣ in the CCM and the CVM modes.

The reduction of the Johnson noise at Ub = 0.24 V first of all is determined by the decrease of working resistance of bolometer. On the frequencies more than 12 Hz NEPR decreases due to the Johnson noise as at the expense of greater, than the meaning of sensitivity in the CCM mode (Figure 5). It is for the same reason the decrease of NEP1/f is observed. The growth of NEP1/f at Ub = 0.24 V up to frequency 12 Hz is caused by the large current through bolometer. The total meaning of the NEPΣ for bolometer in the CCM and the CVM modes are submitted in Figure 9.

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10

1/2

D*, cmHz /W

10

CCM Ib=40 μA CVM Ub=0.24 V

10

CVM Ub=0.18 V

9

1

10

100

1000

f, Hz

Figure 10. Calculated frequency dependences of total detectivity D* in the CCM and the CVM modes.

It is visible, that for one working temperature the frequency dependences of NEPΣ are coincided. Displacement of a minimum of NEPΣ in the party of the large frequencies in the CVM mode at Ub = 0.24 V occurs because of displacement of a minimum of NEP1/f (Figure 8), which in this case determines total NEPΣ.. A maximum of the detectivity D* in the CVM mode at Ub = 0.24 V (Figure 10) is displaced in the party of the large frequencies. It is the important result, taking into account, that in the given mode the speed is increased and there is an opportunity to use bolometer on higher frequencies of modulation with greater detectivity.

The current (65-150 μA) and frequency (1-2000 Hz) dependences of voltage sensitivity and noise voltage in superconducting transition at Tb = 80-90 K, calculated and measured in the CCM and the ANETF modes at Rb = 4.7 кΩ, are shown in Figures 11, 12 and in Table 1. Experiment: CCM (Ib=65 μA) ANETF (Ib=72 μA) ANETF (Ib=124 μA) Calculation: CCM (Ib=65 μA)

1000

SV, V/W

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3.2. Comparative Numerical and Experimental Modeling in CCM and ANETF Modes

ANETF (Ib=72 μA) ANETF (Ib=124 μA)

100

10 1

10

100

1000

f, Hz

Figure 11. Frequency dependences of voltage sensitivity of bolometer 592 in the CCM and the ANETF modes.

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Experiment: Calculation: ANETF Ib=150 μA ANETF Ib=150 μA ANETF Ib=60 μA ANETF Ib=60 μA CCM Ib=60 μA

-7

CCM Ib=60 μA

Vn, V/Hz

1/2

10

-9

4.7x10 -8

10

1/2

1/f -9

10

1

10

100

1000

f, Hz

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Figure 12. Frequency dependences of noise voltages of bolometer 756 in the CCM and the ANETF modes.

The measurements shows, that with growth of a bias current at the expense of increase of a negative feedback the Sv and τe decreases (Figure 11 and Table 1). The effective time constantτe has decreased in 15 of times (with 6 up to 0.4 ms) for bolometer 592 in the ANETF mode and in 14.4 of times (with 23 ms up to 1.6 ms) for bolometer 756 at comparison with the CCM mode. Thus, the relative reduction of τe with growth of a current Ib is less than Sv, i.e. τe ≈1/Ib2, and back Sv ≈ 1/Ib2.8 (see Table 1). As against to action of bias current, the increase of factor of amplification K and resistance Rf in feedback loop leads to reduction of τe and Sv is equally, as both parameters influence on only size of the gain factor of feedback and do not influence on meaning of own Sv or time constant of bolometer. Suppression of the noise voltage on size (1+Laf) is observed experimentally in the ANETF mode (Figure 12). Note, all three noise components considered, including Johnson noise of bolometer and resistors of bridge, and also noise of the circuit experienced this suppression. The greatest suppression is observed on low frequencies up to frequency of cutoff and it depends on size of gain factor of active feedback. Thus, the change of bias current will render stronger the influence on noise voltage, than the change of factor K or Rf, if the phonon or Johnson noise will be dominant. Note that since the identical suppression tests not only noise voltage, but also SV , then reduction of NEPΣ in the ANETF mode occurs only at the expense of decrease of the Johnson noise component NEPR. According to the theory, considered by us, NEPR of bolometer in the ANETF mode back is proportional to size of bias current and as against noise voltage does not depend on gain amplification K and meaning of resistor Rf in feedback loop.

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140

10

D*, cmHz

1/2

/W

10

9

10

ANETF (Ib=150 μA) ANETF (Ib=60 μA) 8

CCM (Ib=60 μA)

10

1

10

f, Hz

100

1000

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Figure 13. Experimental frequency dependences of detectivity D*. for bolometer 756 in the CCM and the ANETF modes.

The discrepancy between calculated and experimentally measured dependences on Figure 12 can be explained by a stronger dependence of the 1/f-noise voltage on the frequency and current for the bias currents above the thermal instability threshold. At high frequencies, where the active ANETF action is not as pronounced, the measured noise is close to the Johnson noise in cooled resistor of the bridge at T0= 80 K (level of Vn=4.7×10-9 V/Hz1/2 on Figure 12). On the basis of the received results it is possible to approve, that using the ANETF mode at the expense of high own responsivity of bolometer caused larger bias current, than in the CCM mode. The increase of the D* is achieved in such frequency range, where the Johnson noise contribution in total noise is appreciable, that shows Figure 13. It should be noted that bolometers are frequently characterized by the ratio D*/τe½ at fcutoff. In the ANETF mode, this parameter for the bolometer 756 amounts to 8.5×1010 cm Hz/W, which is better as compared to 6.3×1010 cm Hz/W for the CCM mode. For example, as a result of that is seen on Figure 13. In the ANETF mode the D* = 109 cm Hz1/2/W at f = 300 Hz, but D* = 2×108 cm Hz1/2/W only at the same frequency in the CCM mode. In addition, it was experimentally established that a bolometer operating in the ANETF mode retains the linearity of conversion to at least tenfold greater maximum values of the input optical power as compared to that for the CCM mode.

CONCLUSION −

−

The greatest efficiency of a negative feedback in the passive CVM mode is achieved in a maximum of temperature coefficient of resistance β. Necessary speed of HTSC bolometer in this case is provided at the least level of the 1/f-noise. By results of the carried out modeling of the CCM mode and the CVM mode it was shown, that in the CCM mode in a maximum of sensitivity of the effective constant time τe grows twice in comparison with thermal constant time τ. In the CVM mode the negative ETF allows to reduce effective constant time in ~5 of times and in the ANETF mode up to 15 times and more.

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−

−

−

−

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−

−

141

It is shown, that the constant time in the CCM mode has stronger dependence on a bias current, and in the CVM mode- from a bias voltage, than sensitivity of bolometer. The maximum of detectivity D* is displaced in the party of the large frequencies in the CVM mode at the expense of displacement of a minimum NEP1/f caused by the 1/f-noise at increase of the bias voltage. It is an important result, taking into account, that in the given mode the speed is increased and there is an opportunity to use bolometer on higher frequencies of modulation with greater D*. In the ANETF mode the maximal speed, as well as in the CVM mode is achieved in a maximum of factor of a feedback, which, as well as in the CCM mode practically coincides with a maximum of the superconducting transition steepness dR/dT, but not with a maximum of temperature coefficient of resistance. The suppression of all noise components of the bolometer and also the noise of the measuring circuit is observed in the ANETF mode. The greatest suppression occurs up to cut-off frequency and depends on the size of factor of a feedback. However, it is not only noise voltage but also the sensitivity bolometer are tested by the identical suppression, and so the reduction of NEP in the ANETF mode occurs basically at the expense of decrease of Johnson noise component of NEPR. Thus according to the considered theory NEPR of bolometer back is proportional to size of bias current and as against noise voltage does not depend on factor of amplification K and resistance of feedback Rf. On the basis of the received results it is possible to approve, that, using the ANETF mode the increase of detectivity of HTSC bolometer may be achieved in that frequency range, where we feel the contribution of the Johnson noise in total noise. Note also presence of a minimum of a noise voltage of bolometer in the ANETF mode. Its occurrence is associated with the growth of noise at the expense of increase of bias current. The comparison with the settlement dependences confirms that at the large bias currents growth of the 1/f-noise stronger, than it was supposed. It can be caused by strengthening frequency dependences of a noise at the large bias currents and is due to transition of bolometer in nonisothermal mode. Using low noise differential preamplifier and having cooled resistors of the bridge up to 80 K, least noise level of the measuring circuit determined by thermal noise of resistors of the bridge and HTSС bolometer on a basis of GdBaCuO film was achieved. In result, in the ANETF mode the meaning of D*=1.0×109 сm Hz1/2/W on f=300 Hz in the ANETF mode and only D*=2.0×108 сmHz1/2/W in the CCM mode are received. Taking into account that in the ANETF mode the constant time decreases, to use this mode there is most favorably on high frequencies more of cutoff frequency. The D* on low frequencies decreases at the expense of growth of the 1/f-noise at the greater bias current in the ANETF mode. However, the increase of speed allows operating on higher frequencies, where the Johnson noise dominates and criterion, describing the totality of bolometer parameters: responsivity and speed, achieves D*/τe½ = 8.5×1010 cm Hz/W.

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Igor A. Khrebtov, Konstantin V. Ivanov and Valery G. Malyarov

ACKNOWLEDGMENTS This work is supported by the ISTC program, project № 2920. Authors thank Prof. E. Steinbeiss and W. Michalke for the preparing the bolometers for experimental investigations and M.A. Timofeeva for the help in preparing of the manuscript.

REFERENCES [1] [2]

[3] [4]

[5] [6]

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[7]

[8]

[9]

[10] [11]

[12]

[13]

P. W. Kruse, “Physics and application of high-Tc superconductors for infrared detectors“, Semicond. Sci. and Technol., 5, pp. S229- S240, 1990. S. Verghese, P. L. Richards, K. Char, D. K. Fork and T.H. Geballe, “Feasibility of infrared imaging arrays using high Tc superconducting bolometers”, J. Appl. Phys., 71, pp. 2491-2498, 1992. P. L. Richards, “Bolometers for infrared and millimetre waves”, J. Appl. Phys., 76, pp. 1-24, 1994. J. Kreisler and A. Gaugue, “Recent progress in high-temperature superconductor bolometric detectors: from the mid-infrared to the far-infrared (THz) range”, Supercond. Sci. Technol., 13, pp. 1235-1245, 2000. A. Khrebtov, “Noise properties of high temperature superconducting bolometers”, Fluctuation and Noise Letters, 2, No. 2, pp. R51-R70, 2002. R. Johnson and P. W. Kruse, “Silicon microstructure superconducting microbolometer infrared arrays”, Proc. SPIE, 2020, pp. 2-11, 1993. H. Neff, J. Laukemper, I. A. Khrebtov, A. D. Tkachenko, E. Steinbeiss, W. Michalke, M. Burnas, T. Heidenblut, G. Hefle and B. Schwierzi, “Sensitive high-Tc transition edge bolometer on a micromachined silicon membrane”, Appl. Phys. Lett., 66, pp. 2421-2423, 1995. M.J.M.E. de Nivelle, M.P. Bruijn, R.de Vries, J.J. Wijnbergen, P.A.de Korte, S. Sanchez, M. Elwenspoek, T. Heidenblut, B. Schwierzi, W. Michalke and E. Steinbeiss, “Low noise high-Tc superconducting bolometers on silicon nitride membranes for farinfrared detection”, J.Appl.Phys., 82, pp.4719-4726, 1997. L. Mechin, J. Villegier and D. Bloyet, “Suspended epitaxial YBaCuO microbolometers fabricated by silicon micromachining: Modeling and measurements”, J. Appl. Phys., 81, pp. 7039-7047, 1997. K. D. Irwin. “An application of electrothermal feedback for resolution cryogenic particle detection”, Appl. Phys. Lett., 66, pp. 1998-2000, 1995. T. Lee, P. L. Richards, S. W. Nam, B. Cabrera and K. D. Irwin, “A superconducting bolometer with strong electrothermal feedback”, Appl. Phys. Lett., 69, pp. 1801-1803, 1996. T. Lee, J. M. Gildemeister, S-F. Lee and P. L. Richards, “Voltage-biased high-Tc superconducting infrared bolometers with strong electrothermal feedback”, IEEE Trans. Appl. Supercond., 7, pp. 2378-2381, 1997. I.A. Khrebtov, A.D.Tkachenko, K.V. Ivanov and E. Steinbeiss, “Study of a mode of an active electrothermal negative feedback for high-temperature superconducting bolometers”, J. Opt. Technol., 68, pp.63-67, 2001.

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[14] F.N. Hooge, T.G. Kleinpenning and L.K. Vandamme, “Experimental studies of 1/f noise”, Rep. Prog. Phys. 44, pp. 479-532, 1981. [15] L. B. Kiss and P. Svedlinh, “Noise in high-Tc superconductors”, IEEE Trans. Electron. Devices, 41, pp. 2112-2122, 1994.

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In: Bolometers: Theory, Types and Applications Editor: T. M. Walcott, pp. 145-162

ISBN: 978-1-61728-289-8 © 2011 Nova Science Publishers, Inc.

Chapter 6

COMPARATIVE INVESTIGATION OF PASSIVE AND ACTIVE OPERATING MODES FOR HIGH-TC SUPERCONDUCTING TRANSITION EDGE BOLOMETERS WITH ELECTROTHERMAL FEEDBACK FOR INFRARED WAVES S. V. Baryshev1, A. V. Bobyl1, K. V. Ivanov2, I. A. Khrebtov2, V. G. Malyarov2 and V. U. Zerov2

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1

Ioffe Physico-Technical Institute, 26 Polytekhnicheskaya, St.-Petersburg 194021, Russia 2 Vavilov State Optical Institute, 12 Birzhevaya line, St. Petersburg 199034, Russia

ABSTRACT Numerical and experimental modeling of the characteristics of constant bias current mode, constant bias voltage mode, and active electronic negative electrothermal feedback mode of high-Tc superconducting transition edge GdBa2Cu3O7-x bolometers on Si/Si3N4 membrane for infrared waves are carried out. Comparative analysis of mentioned modes and estimation on how noise components effect on bolometers operation are also performed. Here we conclude, that active electronic negative electrothermal feedback mode is the most favorable one the bolometer can operate in due to its (i) thermal stability (25-fold increase in comparison with constant bias current mode) and (ii) attainable performance characteristics such as effective time constant (experiences 15fold drop in comparison with that in constant bias current mode) and detectivity/noise equivalent power (detectivity ≥109 сm×Hz1/2/W for up to 300-Hz modulated optical beams).

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1. INTRODUCTION The discovery of high-temperature superconducting (HTSC) materials has inspired the development of transition edge bolometers acting at liquid nitrogen (LN2) temperatures [1-7]. Its inherent coefficient of resistance β in superconducting transition vicinity, thin film technology development and 80 to 90 K operating temperature range permit to design bolometers with high responsivity and speed. Besides, cheapness and simplicity of the LN2 cooling systems in comparison with liquid helium ones are the most essential features for creation of space-base optical devices. Decrease in heat conductivity and specific heat at LN2 temperatures results in reduction of noises of thermodynamic origin and improvement in detectivity D*. Thus, the best achieved detectivity values of HTSC transition edge bolometers on different substrates (up to 2×1010 cm×Hz1/2×W-1 in wide spectral range) are comparable with those of other detector types for infrared applications [1-3, 7-28]. To fabricate a complete measuring instrument, to avoid drawbacks due to intrinsic HTSC film properties and to achieve mentioned detectivity/responsivity values, Joule heating must be used to provide electrothermal feedback (ETF) [8-14, 21, 22, 24-28] and to converse optical radiation absorbed with bolometer into electrical signal. The ETF begins to act in thermosensitive element (thermometer: HTSC film + absorber) of bolometer when absorbed optical power changes the resistance, current and Joule power and effects on main features of bolometer such as responsivity, time constant, dynamic range, and noise properties. In early papers [8-10, 12, 14] two passive operating ETF modes were proposed and examined mainly: (i) constant bias current mode/positive ETF (CCM) and (ii) constant bias voltage/negative ETF mode (CVM). In this work, the performance of transition edge bolometer based on GdBa2Cu3O7-x thermometer with Si/Si3N4 membrane is simulated theoretically and experimentally. The data concerning comparison of the basic passive ETF modes (CCM and CVM) and active electronic negative ETF mode (ANETF) (this mode was first investigated in Ref. 12) are presented. According to results of theoretical and experimental simulation, active electronic negative electrothermal feedback mode is considered to be the most favorable one for transition edge bolometer performance due to thermal stability and attainable trade-off between speed and responsivity. The pronounced decrease in the bolometer time constant value gives an additional chance in the competition between HTSC bolometers and quantum photo-detectors [1].

2. BOLOMETER THEORY 2.1. Modes of HTSC Bolometer Operation HTSC bolometers commonly operate in CCM. Here bolometer with resistance Rb is included in the circuit with the load resistor RL>>Rb (fig.1a). Incident radiation absorbed with a sensitive element of a bolometer results in heating of HTSC thermometer and changes its resistance. The most disadvantage of CCM is the thermal instability of thermometer due to positive coefficient β, that increases resistance Rb, Joule heat, and operating temperature and can push the thermometer into normal (not superconducting) state, eventually. Figure 1b

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shows the principal scheme of CVM traditionally used for low-temperature bolometers [810].

Figure 1. Scheme of bolometer connection: a) in the CCM; b) in the CVM; c) in ANETF mode. The notation for main circuit parameters is pointed out: supply voltage +U, voltmeter V, loading resistor RL, bolometer resistance Rb, shunting resistor Rsh, SQUID is the superconducting quantum interference device, K is differential amplifier, feedback resistor Rf, R1 ≈ Rb is an arm resistor of the bridge

Bias voltage is applied to the SQUID input coil connected in series with Rb (HTSC bolometer). Here, the shunting resistor rate should be as following Rsh1. Thus, to achieve the maximum speed, HTSC film with largest β should be used. It is important, that the larger speed, the lower current responsivity of bolometer (Eq.6). Contrary to CCM, CVM has no direct feedback coefficient L0 restriction. Nevertheless, this restriction exists. In order to maintain value of Rb at operating point, it is necessary to increase bias voltage and, consequently, provide reasonable cooling to compensate Joule power rising. For given β and Rb (or Tb) the maximum L0 value is determined by cooling bath temperature T0 [10]:

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Comparative Investigation of Passive and Active Operating Modes…

Lmax = β ⋅ (Tb − T0 ) = β ⋅ ΔT .

149

(7)

In the case of HTSC bolometers the lowest temperature of cooling bath equals 77 K (LN2 temperature). Then taking into account, that typical Tc = 87 K (it corresponds to ΔT = (Tb–T0) = 10 K) and β ~ 0.5–2 K , one can estimate the maximum feedback coefficient Lmax -1

to be 10 to 30 in CVM. Now it’s necessary to consider ETF effect on noise characteristics, since they have critical importance for bolometer performance. The ultimate responsivity of bolometer is characterized with resulting (sum) noise equivalent power NEP or detectivity D* within bandwidth Δf = 1Hz

NEP =

D* =

Vn* [W/Hz1/2], Sv

(8)

A1/2 [cm×Hz1/2/W], NEP

(9)

where Vn* [V/Hz1/2] is total/sum voltage of various independent noise sources, A is sensitive area of detector, and Sv [V/W] is the voltage responsivity. The most important noise components for HTSC bolometers are phonon noise, Johnson noise, and flicker or 1/f-noise. Phonon noise is caused by the random energy exchange between the bolometer and the heat sink. In CCM and CVM it can be written as:

Vph = S v ⋅ (4 ⋅ k ⋅ Tb2 ⋅ G)

12

(for CCM),

Vph =S I ⋅Rb ⋅ (4 ⋅ k b ⋅ Tb2 ⋅ G )

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12

NEPph

(4 ⋅ k ⋅ T =

2 b

⋅ G) 2

(for CVM).

(10)

1

ε

.

(11)

Voltage noise and resulting NEP due to Johnson noise in CCM and CVM are given by

VR

( 4 ⋅ k ⋅Tb ⋅ Rb )1 2 =

VR = NEPR

(for CCM),

1−L

(4 ⋅ k b ⋅ Tb ⋅ Rb )1 2 1+L

(4 ⋅ k ⋅ T =

2 b

⋅ J)

(for CVM).

⋅ (1 + ω 2 ⋅ τ 2 ) ε ⋅ L0

12

(12)

12

.

(13)

One can calculate 1/f-noise (the noise caused by film quality) voltage V1/f and NEP1/f as

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150

12

V1 f

2 1 ⎛ H ⋅Ub = ⋅ ⎜⎜ 1 − L ⎝ N ⋅ A⋅t ⋅ f a

⎞ ⎟ ⎟ ⎠

V1 f

2 1 ⎛ H ⋅Ub = ⋅ ⎜⎜ 1+ L ⎝N ⋅ A⋅t⋅ f a

⎞ ⎟ ⎟ ⎠

NEP1 f

G = ε ⋅β

(for CCM), 12

⎛ H ⋅ ⎜⎜ ⎝ N ⋅ A⋅t ⋅ f

(for CVM).

a

⎞ ⎟ ⎟ ⎠

12

(14)

⋅ (1 + ω 2 ⋅ τ 2 ) . 12

(15)

Expressions are obtained with empirical Hooge formula [28], where H is noise Hooge parameter, N ~ 1021 cm-3 is density of charge carriers of a film [29], A is area and t is thickness of the film, the coefficient a is closed to 1. In order to estimate NEP1/f, operating point Rb and H should be valued in each specific case. It was shown [9, 30], that resulting NEPΣ due to all noise sources in both CCM and CVM can be rewritten as: NEP Σ =

4 ⋅ k ⋅ Tb 2 ⋅ G

ε2

⎛ 4 ⋅ k ⋅T ⋅ G2 ⎞ G2 ⋅ H ⎟[1 + ω 2 ⋅ τ 2 ] + ⎜⎜ 2 b2 + 2 a ⎟ 2 ε ⋅ β ⋅ ε ⋅ β ⋅ ⋅ ⋅ ⋅ J N A t f ⎝ ⎠

1/2

.(16)

Eq.16 demonstrates also that in the case of CVM, the increase in Joule power J (without a risk of thermal instability) can results in suppression of the Johnson noise component (2nd term of the expression). Other components of NEPΣ can be reduced only by means of decrease in bolometric parameters such as G or/and utilization of high-quality HTSC films with low H and large β parameters.

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2.3. Characteristics of HTSC Bolometers in ANETF Mode It was shown, that passive negative ETF reduces effective time constant τe by a factor of (1+L0). However, increase in current Ib and 1/f-noise voltage leads to decrease in detectivity D*. The influence of negative ETF can be enhanced distinctly without deterioration of D*, if external feedback loop with the electronic amplifier is used [12]. The circuit that includes equal-arm bridge coupled with differential amplifier is used for operation in ANETF mode (see figure 1c and figure 2). Changes in bolometer resistance caused by absorbed optical power result in offset of the bridge. The offset voltage of the bridge is transferred to differential amplifier K. Further, amplified signal through resistor Rf acts on bridge feeding diagonal Um. The amplifier is connected in such a way that an increase in bolometer resistance results in reduction of feeding voltage Um and, consequently, in reduction of Joule power with finally reduction of bolometer resistance. The block-diagram in figure 2 explains the details of ANETF mode. Optical power Popt absorbed with bolometer is at the input of block-diagram. It increases bolometer resistance (resistance of HTSC thermometer, in fact) and convert it into voltage ΔUb via the voltage responsivity Sv* (Sv* is responsivity without ETF) (see Eq.1). Resulting offset voltage equals M1×ΔUb, where M1 – transmission coefficient, that equals 0.5 in case of equal-arm bridge.

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Next offset voltage is transferred to differential amplifier K, so as Uout = K×M1×ΔUb. Resistance Rf and total resistance of the bridge Rm form voltage divider F = Rm/(Rm+Rf) (see figure 1c). Thus, due to divider F, the change in voltage of bridge feeding diagonal is F×Uout. The change in feeding voltage of the bridge is transferred to bolometer through divider M2 = Rb/(R1+Rb). For the equal-arm bridge at small signal values the change in Rb is much less than R1 and, consequently, Rb ≈ R1 and thus M2 = 0.5. The change in bolometer voltage under negative feedback leads to change in Joule power ΔJ and to partially compensation of absorbed optical power. As a result, Rb tends to R1 and Uout decreases (i.e. bridge offset decreases). Thus, increment of resistance Rb and voltage Uout in ANETF mode is defined by uncompensated value of absorbed optical power (Popt–ΔJ).

Figure 2. Block-diagram of active electronic negative electrothermal feedback mode (ANETF). The notation in the figure is given in the text.

Analysis of block-diagram in figure 2 allows one to get the expression for Sv in ANETF mode with ε = 1 as:

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Sv =

0.5 ⋅L 0 1 1 , ⋅ ⋅ 2 I b 1 + 0.5 ⋅ L 0 ⋅ F ⋅ K ( 1 +ω ⋅τ e 2 ) 1 / 2

(17)

where K is amplifier gain and τe is the effective time constant given by

τe =

τ

1 + 0.5 ⋅ L 0 ⋅ F ⋅ K

.

(18)

In active ETF mode the coefficient of feedback Laf should be written as:

L af = 0.5 ⋅ L 0 ⋅ F ⋅ K .

(19)

Supplying the noise power as input signal (see figure 2 block-diagram) instead of the optical power and performing some mathematical transformations, one can get frequency dependence of noise voltage Vn* normalized to bridge output signal as

0.5 ⋅ (1 + ω2 ⋅ τ2 )

12

V = Vn ⋅ * n

((1 + 0.5 ⋅ L

0

⋅ F ⋅ K )2 + ω2 ⋅ τ2 )

12

,

Vn = (VR )2 + (0.5 ⋅ V1 / f )2 + (0.5 ⋅ Vph )2 ,

(20) (21)

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where VR is Johnson noise voltage, 0.5 ⋅V1/f and 0.5⋅Vph are the voltages due to 1/f-noise and phonon noise, respectively, reduced to the bridge output signal without ANEFT. From Eq.20 the fact of noise voltage suppression by (1+0.5⋅L0⋅F⋅K) factor at low frequencies ( f >

1 2 ⋅ π ⋅τ

) effect of ANETF mode is negligible.

The noise equivalent power NEP and the detectivity D* should be estimated with Eq.8, 9, using Eq. 17 and 20 coupled with 21 in there.

3. SAMPLES

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Epitaxial GdBa2Cu3O7-x (GBCO) film thermometers were grown on micromachined Si/Si3N4 membrane by magnetron sputtering (for details see Ref. [9]). The real model of the bolometer is shown in figure 3, where figure 3a is the schematic bolometer draw for ease of understanding, figure 3b and figure 3c are the top and side views of real construction with external connector used in measurements. The temperature of the superconducting transition found by means of dc measurements was 85.0 K and 84.4 K for bolometers #592 and #756, respectively.

Figure 3. Design of membrane type bolometer: a) the schematic draw of bolometer with 1 – GBCO film meander, 2 – Au infrared absorbing coating, 3 – Pt contact pads, 4 – YSZ buffer layer, 5 – Si3N4 membrane, 6 – Si substrate, 7 – Au wires; b) top and c) side view of real construction used in measurements.

The bolometer had a receiving area of 0.85×0.85 mm2, the thermal conductance G = (1.4–13)×10-4 W/K, the heat capacity C = 6.5×10-7 J/K, the thermal time constant τ ≈ 50 ms, and the absorption coefficient ε ≈ 0.7. Other measured parameters are submitted in Table 1. Figure 4 shows typical characteristics оf HTSC GBCO bolometer #756.

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Table 1. Characteristics of GBCO bolometers Sample number Mode Ta, K Rb, kΩ β, K-1 G, W/K Ib, μА τe, ms fcut-off, Hz Sv, kV/W Vn*, 10-9 V/Hz1/2 NEP∑, 10-11 W/Hz1/2 D*, 109 сm×Hz1/2/W D*/τe½, cm×Hz/W

592 CCM 85.0

756 АNETF 85.0

CCM 84.4

83.6

АNETF 79.2

40 23 7 3.5 32 0.9 9.4 6.3×1010

0.88 1.3×10-5 60 2.7 60 0.179 3.6 2.0 4.25 8.2×1010

150 1.6 100 0.086 2.2 2.5 3.4 8.5×1010

4.7

65 6 25 1.4 52 4 2.3 3.0×1010

1.5 1.1×10-4 72 1.2 132 0.11 49 44.5 0.19 5.5×109

124 0.4 400 0.07 16 22.8 0.37 1.85×1010

In Table: fcut-off = 1/(2⋅π⋅τe), Ta is the temperature of the base. An absolute black body with T = 373 K (λmax = 7 μm) was used for voltage responsivity measurements. The amplifier gain was 500–1000.

4. NUMERICAL AND EXPERIMENTAL MODELING 4.1. Comparative Numerical Modeling in Passive CCM and CVM Modes

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Formulas obtained in the sections 2.2 and 2.3 for numerical bolometer characteristics modeling were used. The results of calculation (figure 5a) show, that as bias signal (voltage in CVM or current in CCM) increases the responsivity S in CVM reduces greater at low frequencies in comparison with that in CCM. However, in CVM response speed of bolometer is greater and S begins to exceed CCM responsivity already at frequencies higher than 12 Hz.

Figure 4. Temperature dependencies of resistance Rb (■), slope of superconducting transition α = dR/dT (○), and temperature coefficient of resistance β (∆) of GBCO bolometer #756 on Si/Si3N4 membrane.

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In addition, in CVM at bias voltage Ub = 0.24 V effective time constant τe is tenfold lower than thermal time constant τ and so does SI in comparison with responsivity calculated without ETF at the same Ub and Rb. Figure 5b demonstrates τe dependencies on absorbed optical power Popt in CCM at Ib = 40 μA and CVM at Ub = 0.24 V. Such dependence exists due to heating of bolometer caused by absorbed optical radiation. It’s clearly seen, that at operating power values (in the case of CCM for given simulation parameters the nonlinear duty occurs due to thermal instability) effective time constant in CVM is 10 to 20 times less than τe in CCM. Figure 6a shows frequency dependencies of noise voltages in CVM under negative ETF: bias voltage is 0.18 V and 0.24 V, β is 0.88 K-1 and bolometer resistance is 4.7 kΩ at T = 84.4 K (see figure 4). The suppression of all noise components at low frequencies in CVM is observed. Besides, even at bias voltage of 0.18 V the voltage of each noise component was lower in comparison with the corresponding voltages in CCM (figure 6b and 7a). According to Eq.10, the phonon noise reduction here is due to decrease in current responsivity SI under negative ETF.

Figure 5. (a) Calculated frequency dependences of bolometer responsivity S in CCM and CVM; (b) simulation of effective time constant dependencies on absorbed optical power Popt in CCM (Ib = 40 μA) and CVM (Ub = 0.24 V), here, left scale is for τe of CCM and right one is for τe of CVM (see arrows).

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Figure 6. Calculated frequency dependences of (a) noise voltages in CVM at two bias voltage values, 0.18 V and 0.24 V, and (b) noise voltages in CCM (at 40 μA operating current) and CVM (at 0.18 V operating voltage).

The frequency dependences of total noise voltage in CCM and CVM are illustrated in figure 6 and 7a. The influence of ETF on noise voltage at low frequencies is obvious in CCM and CVM. The suppression of the noise in CVM mode rather than in CCM at the same bolometer temperature does not lead to strong gain in threshold responsivity. Frequency dependence of Vn* received in CVM at Ub = 0.18 V almost coincides with similar dependence of Vn* received in CCM at Ib = 40 μA (figure 7a) in spectral range under review, since the factor of Vn* and SI suppression in CVM is the same, just as the factor of increase in Vn* and Sv in CCM.

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Figure 7. Calculated frequency dependences of (a) total noise voltage in CCM and CVM and (b) NEP of noise components in CVM at various Ub.

Figure 7b shows, that at frequencies f>12 Hz in CVM the decrease in NEP (obtained at Ub = 0.24 V) due to Johnson and 1/f-noise components is observed in comparison with NEP obtained at Ub = 0.18 V. NEPph component due to phonon noise, however, remains the same. The reduction of Johnson noise at Ub = 0.24 V first of all is determined by decrease in resistance of bolometer at operating point. At frequencies higher than 12 Hz, NEPR and NEP1/f decrease due to higher responsivity value in comparison with that in CCM (figure 5a). The growth of NEP1/f at Ub = 0.24 V up to 12 Hz is caused by larger current through bolometer. The resulting NEPΣ for investigated bolometers in CCM and CVM are shown in figure 8a. It is seen, that at Ib = 40 μA and Ub = 0.18 V the frequency dependences of NEPΣ coincide. Frequency shift of NEPΣ minimum towards the higher frequencies in CVM at Ub = 0.24 V occurs because of NEP1/f minimum shift (figure 7b). Thus, 1/f-noise plays the major role and sets the resulting NEPΣ.

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Figure 8. Calculated frequency dependences of resulting NEPΣ (a) and detectivity D* (b) in CCM (at 40 μA operating current) and CVM (at 0.18 V and 0.24 V operating voltages).

Since detectivity D* is reciprocal value to NEP, the same frequency shift of D* in CVM at Ub = 0.24 V (figure 8b) is predictable. It’s a very important result indicating the simultaneous possibility to get high response speed and to use the bolometer at higher modulation frequencies with greater detectivity in CVM.

4.2. Comparative Numerical and Experimental Modeling in CCM and ANETF Modes Calculated and measured dependences of voltage responsivity and noise voltage on current (60–150 μA) and frequency (1–2000 Hz) at Tb = 84.4 K and Rb = 4.7 kΩ in CCM and ANETF mode are shown in figures 9a and 9b, some main parameters are cited in Table 1.

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Figure 9. Frequency dependence of (a) voltage responsivity of bolometer #592 in CCM (at Ib = 65 μA) and ANETF mode (at Ib = 72 and 124 μA) and (b) noise voltage of bolometer #756 in CCM (at Ib = 60 μA) and ANETF mode (at Ib = 60 and 150 μA).

The measurements show, as bias current grows (due to increase in effect of ANETF) Sv and τe decrease (see figure 9a and Table 1). It’s seen from the table, that more than 10-fold decrease in effective time constant τe occurs: from 6 ms in CCM to 0.4 ms in ANETF mode for bolometer #592 and from 23 ms in CCM to 1.6 ms in ANETF mode for bolometer #756. Besides, the relative reduction of τe, as Ib current grows, is less than that of Sv: τe ≈ 1/Ib2 and Sv ≈ 1/Ib2.8. It was experimentally revealed, that noise voltage Vn* suppresses by factor of (1+Laf) in ANETF mode (figure 9b). Note, all the noise components considered, including Johnson noise of bridge resistors and noise of the used amplifier microchip, suppress as well. The greatest suppression is observed at low frequencies up to cut-off frequency (fcut-off = 1/2⋅π⋅τe). It all depends on value of active feedback gain factor. At the same time, if phonon or Johnson noise dominates, any bias current change will effect on noise voltage stronger than on K or Rf parameters. It’s necessary to note, since noise voltage Vn* and responsivity Sv have the

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identical suppression, the reduction of NEPΣ in ANETF mode can occur only due to decrease in Johnson noise component NEPR. According to considered theory, NEPR of bolometer in ANETF mode is inversely proportional to bias current and doesn’t depend on gain coefficient K and feedback loop resistor Rf in contrast to noise voltage. The discrepancy between calculated and measured dependences in figure 9b can be explained taking into account a stronger dependence of 1/f-noise voltage on frequency and bias current as compared to its standard behavior. Such a stronger dependence can be due to thermal instability of bolometer, when overheating of HTSC meander caused by bias current leads to formation of spatially distributed normal-conductivity regions. At high frequencies, where effect of ANETF action is not so pronounced, the measured noise is close to Johnson noise of cooled bridge resistor at T0 = 80 K (Vn* = 4.7×10-9 V/Hz1/2 marker in figure 9b).

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Figure 10. (a) measured frequency dependences of detectivity D* for bolometer #756 in CCM and ANETF modes, (b) voltage responsivity dependence on base temperature in CCM and ANETF modes; 83.8 K, 83.1 K and 84.42 K are operating point temperatures of the base at each specific bias current (see text below).

Figure 10a shows frequency dependence of detectivity D*. It’s seen, that in ANETF mode increase in bias current leads to D* peak shift towards the higher frequencies, and also increases the value of cut-off frequency. This shift of D* peak is determined by rise in feedback coefficient Laf (which depends on Ib2 and β) and by rise in current 1/f-noise. Such a peculiarity should be taken into account, when choosing bias current in ANETF mode for given operating frequency range of a receiver. Since time constant in ANETF mode reduces, application of this mode is the most preferable when operating at frequencies near fcut-off (for bolometer #756 at Ib = 60 μА the operating spectrum is 14 Hz to fcut-off = 60 Hz). The higher bias current, the lower detectivity in ANETF mode at low frequencies due to rise in 1/f-noise, but increase in response speed gives detectivity gain at higher frequencies, where Johnson noise prevails. It’s more demonstrably often to characterize a bolometer with the ratio D*/τe½ at fcut-off. In ANETF mode this parameter of bolometer #756 amounts to 8.5×1010 cm×Hz/W and 6.3×1010 cm×Hz/W in CCM. As one can see in figure 10a, D* = 109 cm×Hz1/2/W at f = 300 Hz in ANETF mode and only 2×108 cm×Hz1/2/W in CCM (see also Table 1). In addition, it was experimentally revealed, that a bolometer operating in ANETF mode keeps on linear conversion at input optical power Popt that is order-of-magnitude larger than that in CCM (Popt max = 2.1×10-6 W at Ib = 45 μA in CCM, Popt max = 9.1×10-6 W at Ib = 45 μA

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and 2.24×10-5 W at Ib = 130 μA in ANETF mode, where Popt max – maximum absorbed optical power a bolometer keeps on linear conversion of optical radiation into electrical signal).

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4.3. Temperature Stabilization of Bolometer Operating Point In order to obtain high threshold responsivity, the temperature of sensor should be constant and independent on fluctuations of environment temperature, when operating. To determine the stability of operating point in CCM and ANETF mode, the temperature of bolometer substrate was changed with additional filament heater, bolometer was fed with weak modulated optical signal, and the changes of responsivity were measured. Thus, changes in responsivity allow one to determine operating point fluctuations, when temperature of the base (environment) varies. Figure 10b presents dependence of bolometer voltage responsivity on base temperature in ANETF mode at Ib = 40 μA (Ta = 83.8 K) and Ib = 60 μA (Ta = 83.1 K) and in CCM at Ib = 40 μA (Ta = 84.42 K), where 83.8 K, 83.1 K and 84.42 K are operating point temperatures of the base for each case. The experimental results presented in figure 10b let us conclude, that thermal stability of bolometer responsivity in ANETF mode is not so high as compared to stability of temperature and resistance of bolometer estimated in CCM or CVM just as ΔTb/ΔTa (ΔTb and ΔTa – the temperature change of bolometer and base, respectively), since in ANETF mode stability of the temperature is controlled with simultaneous changes in voltage and current resulting in changes in responsivity. Here, the allowed responsivity change is 30% with respect to maximum value. In ANETF mode at Ib = 60 μA (in CCM at 40 μA) the change in the base temperature by 2.3 K (by 1 K), when cooling, and by 1.4 K (by 1.3 K), when heating, meets this 30-% condition. Thus, application of ANETF mode extends the operating temperature range, where the responsivity value conserves at given accuracy, by factor of 1.5. It is important to note, that in ANETF mode the responsivity dependence on base temperature Ta is nearly linear over the whole temperature range contrary to CCM, in which the same dependence is not only nonlinear, but also is non-monotone. Let us add here, that in CVM SI(T) (this plot isn’t shown here) is almost constant, but only at temperatures below LN2-point (approximately 40 K to 70 K). Thus, in actual temperature range of bolometer performance CVM responsivity experiences the temperature decay and approaches Sv(T) of CCM.

CONCLUSION Numerical and experimental modeling of the characteristics of high-temperature transition edge superconducting GBCO bolometers on Si/SiN membrane for infrared waves are carried out. It’s shown, that 1. At maximum responsivity in CCM effective time constant τe grows twice in comparison with thermal time constant τ contrary to 5-fold and 15-fold (or even more) reduction of τe in CVM and ANETF mode, respectively;

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2. The maximum of detectivity D* shifts towards the higher frequencies both in CVM and in ANETF mode in comparison with D*(f) of CCM. The former is due to shift of NEP1/f minimum at high bias voltages (here, higher than 0.18 V) while the latter is due to rise in feedback coefficient Laf. It is an important result, since it allows one to detect high-frequency (more than cut-off frequency in ANETF case) modulated optical beams with greater D*; 3. The suppression of all bolometer noise components and also of the noise of measuring circuit is observed in ANETF mode at frequencies up to fcut-off. It depends on value of active feedback gain factor. However, suppression of resulting NEPΣ is only due to decrease in Johnson noise component NEPR, that gives corresponding rise in detectivity in modulation frequency range, where NEPR prevails; 4. In CVM and ANETF mode the temperature stability of bolometer on base temperature Ta has 25-fold gain in comparison with that in CCM. Besides, the temperature stability of responsivity on base temperature in ANETF mode is estimated as 1.5-fold better than one can obtain in CCM/CVM, that extends the operating temperature range of bolometer pronouncedly; 5. Bolometer #756 has D* = 1.0×109 сm×Hz1/2/W at f = 300 Hz in ANETF mode and D* = 2.0×108 сm×Hz1/2/W in CCM. Thus, the value of D*/τe½ parameter in ANETF mode amounts to 8.5×1010 cm×Hz/W at Ib = 150 μA and at Ta = 79.2 K with lownoise differential preamplifier and LN2-cooled bridge resistors of measuring circuit (for details see also Table 1).

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ACKNOWLEDGMENTS This work is supported by the ISTC program, project #2920. Authors thank Prof. E. Steinbeiss and W. Michalke for the preparing the bolometers for experimental investigations and M.A. Timofeeva for the help in preparing of the manuscript.

REFERENCES [1] [2] [3] [4] [5] [6]

[7] [8]

Kruse P W 1990. Semicond. Sci. and Technol. 5 S229. Verghese S, Richards P L, Char K, Fork D K and Geballe T H 1992. J. Appl. Phys. 71 2491. Richards P L 1994. J. Appl. Phys. 76 1. Neff H, Laukemper J, Khrebtov I A, Tkachenko A D, Steinbeiss E, Michalke W, Burnas M, Heidenblut T, Hefle G, and Schwierzi B 1995. Appl. Phys. Lett. 66 2421. Mechin L, Villegier J and Bloyet D J 1997. Appl. Phys. 81 7039. de Nivelle M J M E, Bruijn M P, de Vries R, Wijnbergen J J, de Korte P A, Sanchez S, Elwenspoek M, Heidenblut T, Schwierzi B, Michalke W, and Steinbeiss E 1997. J. Appl. Phys. 82 4719. Kreisler A J and Gaugue A 2000. Supercond. Sci. Technol. 13 1235. Irwin K D 1995. Appl. Phys. Lett. 66 1998.

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162 [9] [10] [11] [12] [13]

[14] [15] [16] [17] [18]

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[22] [23] [24]

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S. V. Baryshev, A. V. Bobyl, K. V. Ivanov et al. Lee T, Richards P L, Nam S W, Cabrera B, and Irwin K D 1996. Appl. Phys. Lett. 69 1801. Lee T, Gildemeister J M, Lee S-F, and Richards P L 1997. IEEE Trans. Appl. Supercond. 7 2378. Neff H, Lima A M N, Deep G S, Freire R C S, Melcher E, Khrebtov I A, and Tkachenko A D 2000. Appl. Phys. Lett. 76 640. Khrebtov I A, Tkachenko A D, Ivanov K V, and Steinbeiss E 2001. J. Opt. Technol. 68 290. Khrebtov I A, Tkachenko A D, Ivanov K V, Nikolenko A D, and Pindyurin V F 2002. Proc. 5th European Workshop on Low temperature Electronics (J. Phys. IV Vol. 12 No. 3) p 137. Ivanov K V, Khrebtov I A, and Stepanov A I 2004. J. Opt. Technol. 71 51. Rice J P 2000. Metrologia 37 433. Turner A D, Bock J J, Beeman J W, Glenn J, Hargrave P C, Hristov V V, Nguyen H T, Rahman F, Sethuraman S, and Woodcraft A L 2001. Appl. Opt. 40 4921. Kruse P W 2001. Uncooled Thermal Imaging Arrays, Systems, and Applications (SPEI Press) p 89. Guillet B, Mechin L, Yang F, Routoure J M, Le Dem G, Gunther C, Chakalov R A, and Robbes D 2003. NATO Advanced Research Workshop—Advanced Experimental Methods for Noise Research in Nanoscale Electronic Devices (Brno, August 2003) (Abstract Book) p 73. Mahmood A, Butler D P, and Celik-Butler Z 2006. Sensors Actuators A 132 452. Yang F, Mechin L, Routoure J-M, Guillet B, and Chakalov R A 2006. J. Appl. Phys. 99 024903. Allegre G, Guillet B, Robbes D, Mechin L, Lebargy S, and Nicoletti S 2007. Meas. Sci. Technol. 18 183. Fieque B, Tissot J L, Trouilleau C, Crastes A, and Legras O 2007. Infrared Phys. Technol. 49 187. Khokhlov D A, Khrebtov I A, Baryshev S V, Bobyl A V, Ivanov A A, and Nikolaev D A 2007. Techn. Phys. Lett. 33 548. Ivanov K V, Khokhlov D A, Khrebtov I A, Kulikov Yu V, Malyarov V G, Nikolenko A D, Pindyurin V F, and Zerov V Yu 2007. Nuclear Instruments and Methods in Physics Research A 575 272. Khrebtov I A, Malyarov V G, Ivanov K V, Khokhlov D A, Nikolenko A D, and Pindyurin V F 2007. J. Opt. Technol. 74 479. Denoual M, Delaunay S, Allegre G, and Robbes D 2009. Meas. Sci. Technol. 20 015105. Hooge F N, Kleinpenning T G, and Vandamme L K 1981. Rep. Prog. Phys. 44 479. Kiss L B and Svedlinh P 1994. IEEE Trans. Electron. Devices 41 2112. Khrebtov I A 2002. Fluctuation and Noise Letters 2 R51.

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Chapter 7

EXPERIMENTAL MODELLING ACTIVE STRONG ELECTROTHERMAL FEEDBACK MODE FOR HIGH-TC SUPERCONDUCTING BOLOMETER ON SILICON NITRIDE MEMBRANE I. A. Khrebtov1and A. D.Tkachenko2 1

Vavilov State Optical Institute, 12 Birzhevaya line, St. Petersburg 199034, Russia 2 E.Steinbeiss, W.Michalke Institut fur Physicalishe Hochtechnologie, Helmholzweg 4, 07743 Jena, Germany

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ABSTRACT The comparative investigations of the properties of high-Tc bolometer in two operating modes – traditional and with strong active negative feedback – were carried out. In constant current mode GdBaCuO bolometer on Si/Si3N4 membrane has the maximum detectivity is of 6×109 cmHz1/2/W at λ=7.2 μm, response time 6 ms, the detectivity was due to mainly the phonon noise in frequency range of 3-20 Hz. Investigations showed that response time of bolometer could be decreased on more than order, using active negative and positive electrothermal feedback modes. Electrothermal feedback loop effects on noise behavior of bolometer as well. The estimation for present concrete variant with using parameter D*/τ1/2 shows some advantage of mode with feedback, when it is necessary high rate of bolometer response.

1. INTRODUCTION High-Tc superconducting (HTSC) bolometers are among the successful developing applications of HTSC (Refs. [1, 2]). Their calculated detectivity for the middle IR is comparable with the sensitivity of photoelectric detectors that operate at liquid-nitrogen temperatures, but the bolometers possess a wider spectral range. The best prospects the HTSC

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bolometers have in the far IR range. At wavelengths longer than 20 μm the HTSC bolometers have no competitors among the photoconductive detectors with the same cooling temperature. The cost of a bolometer materials is an order of magnitude less than in case of photodetectors, for example, those of the HgCdTe type. Eventually, microbolometers manufactured by silicon micromachining technology can be arranged in 2D-arrays (Refs. [3, 4]). With the very broad spectral bandwidth of sensitivity HTSС bolometers have good prospectives for the application in infrared Fourier-spectroscopy and space observations. However, in these applications HTSС bolometer performance is limited by a trade off between speed and sensitivity. Nevertheless, heat mechanism of bolometer operation allows one to solve this problem using electrothermal feedback mode as in passive as in active variants. The results of these investigations for high-Tc GdBaCuO bolometer on Si3N4 membrane are reported in the present work.

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2. BOLOMETER THEORY Bolometers are the thermal detectors and make use of the steep change in the resistance under absorbed radiation at the superconducting transition edge. A thin HTSC film with a working temperature corresponding to the middle of the transition serves as the sensitive element. The temperature coefficient of resistance given by β = R-1⋅dR/dT characterise the slope of the film temperature dependence of resistance. For superconducting bolometers β > 0. The bolometers temperature change with absorbing radiation is determined by the optical absorption ε, and the thermal conductance G by which it is coupled to a heat sink with T0. The intrinsic thermal time constant of the bolometers τ0 is equal to C/G, where C is the bolometer heat capacity. The superconducting bolometers are operated with a constant current bias (Ib = const) – Constant Current Mode, CCM, or a constant voltage bias (Vb = const) – Constant Voltage Mode, CVM. Recently, superconducting bolometers operated in CVM or a Voltage Biased Bolometers, that maintains itself in the transition region through the use of strong negative electrothermal feedback (ETF), has been reported (Ref. [5]). For HTSC bolometers this mode of operation has been discussed in works (Refs. [2, 6, 8]). The P = V⋅I is the power dissipated in the bolometer by the bias and it is the power which adds up to the absorbing radiation power. This power changes of the bolometers temperature and determines the electrothermal feedback (ETF). The ETF will be positive if P increases with the rise of the bolometer T and R, and negative if P decreases with the rise of the bolometer T and R. The first case is realized in the CCM, when P = I2⋅R. This mode is more traditional for operating bolometers (Ref. 7). The Second case is realized in the CVM, when P = V2/R (Refs.[2, 6, 8]). The effect of the ETF on the bolometer parameters may be take into account by replacement the thermal conductance G (heat-loss factor) on effective thermal conductance Ge = G⋅(1±β⋅P/G). The factor β⋅P/G = V2⋅β/(R⋅G) = L is a gain of the ETF loop. The effect of the ETF on the bolometer parameters may be take into account by replacement the thermal conductance G (heat-loss factor) on effective thermal conductance Ge = G⋅(1±β⋅P/G). The factor β⋅P/G = V2⋅β/(R⋅G) = L is a gain of the ETF loop. For a constant current biased bolometer Ge = G⋅(1-L), and L = I2⋅R⋅β/G must be τ0 G ⋅ (1 − L ) 1 − L

(1)

(2)

Here ω is the modulation frequency of the input signal. Thus, in CCM the responsivity and the response time increase with increasing L. For a voltage biased bolometer Ge = G⋅(1+L) and as a L = V2⋅β/(R⋅G) > 0 (β>0 for superconducting bolometer) as there is no limit for increasing bias voltage. In this mode current responsivity SI and τe are equal [see (3) and (4)]

SI = ε ⋅

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τe =

L 1 ⋅ V ((1 + L )2 + ω2 ⋅ τ0 2 )1/ 2

τ C = 0 < τ0 G ⋅ (1 + L ) 1 + L

(3)

(4)

In this case SI and τe decease with increasing L, and response time of the bolometer can be decreased with a factor (1 + L). First, the influence of the effect of ETF on the noise properties of the superconducting bolometers was take into account in (Ref. [9]). For HTSC bolometers these questions have been discussed in (Ref. [2, 6]). Noise equivalent power (NEP) for HTSC bolometer in CVM operation is due to mainly three noise contributions NEP2 =

2 4⋅k ⋅T⋅P (1 + ω2 ⋅ τ02 ) + γ ⋅ 4 ⋅ k ⋅ T2 ⋅ G + αH ⋅ G2 ⋅ (1 + ω2 ⋅ τ02 ) 2 L n ⋅ v ⋅f β

(5)

where k is Boltzman constant, T is the bolometer temperature, γ describes the reduction of the thermal fluctuation noise due to the temperature difference between the sensitive element and the heat sink (Ref. [9]), αH is the noise Hooge-parameter (Ref. [10]), n is the carrier density per unit volume of the HTSС film, v is the volume of HTSС film, f is the frequency. The first contribution of Eq. (5) is due to Johnson’s noise of the bolometer resistance, second term is caused by the random heat exchange between the sensitive element and the surroundings through the thermal conductance G, third contribution is due to excess flicker 1/f noise. It should be noted that Eqs. (5) for the NEP is the same for both CCM and CVM operations. Practically coefficient γ near to 1. Thus, at CVM operation only the Johnson contribution can be decreased essentially since the value L = P⋅β/G can be made >>1. Note, limited value of P and, accordingly L, is due to temperature difference ΔT between T of bolometer element and T0 of the heat sink. HTSC bolometer is cooled with liquid nitrogen so

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this ΔT can be about 10-15 K. Estimation made in (Ref. [6]) for published HTSC bolometers showed that decreasing the time constant could reach about 30 times. On other hand, the NEP could be decreased by 3-4 times at holding the response time. For practical realisation of the characteristics calculated for passive CVM it is necessary: (i) to use cooling SQUID readout circuit and (ii) to use large bias voltage for obtaining large loop gain L that increases 1/f noise contribution of the NEP. In contrast to passive CVM operation the improvements in the sensor time response can be obtained using analogue electronic feedback control (Ref. [11]).

3. EXPERIMENTAL TECHNIQUE AND RESULTS

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The production process of membrane GdBaCuO bolometer (Figure 1) has been described in (Ref. [2]). A membrane Si3N4/Si 0.6 μm thick with an area 1.2×1.2 mm2 was formed in a (100) silicon substrate. A 40 nm epitaxial YSZ/CeO2 was deposited on the membrane as a buffer layer. On the buffer layer a 60 nm GdBaCuO HTSC film was deposited by magnetron sputtering. The bolometer area A = 0.85×0.85 mm2, Tb=90.5 K, the resistance in normal state Rn = 25 kOhm, G = 1.1×10-4 K/W. The bolometric characteristics in constant current mode (CCM) and in active CVM mode were investigated. The samples were mounted in liquid nitrogen optical cryostat. Operating temperature was controlled by wire heater in range 78-100 K. Measuring IR-radiation was modulated in range 1-1000 Hz, In the same frequency range the responsivity and noise were measured.

Figure 1. A design of the membrane bolometer. 1 – GdBaCuO film meander, 2 – absorptive Au coating, 3 – Pt contact pads, 4 – YSZ buffer layer, 5 – Si3N4 membrane, 6 – Si substrate, 7 – Au wire contacts.

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Figure 2. The scheme of a difference amplifier for operation with GdBaCuO bolometer in a mode with active negative (electronic) electro-thermal feedback. U+ – power source of the bridge (+12 V); RE, RL – load resistances; Ri - input resistances; R1 – resistors of the bridge; Rf – resistor of feedback; Uout – voltage on an output of the amplifier; Rb – resistance of the bolometer; the dashed line leads round a microcircuit OP270FZ.

The bolometer in CCM operation has the following characteristics: Rb = 7.4 kOhm, τ0 = 6 ms, S = 1730 V/W at Ib = 0.035 mA, frequency modulation fm = 25 Hz and radiation wavelength of 7.2 μm, D* = 5.6×109 cmHz1/2/W at fm=25 Hz. Noise voltage increases with increasing in transition range and has maximum which coincides with maximum of the responsivity (see Figure 3). It was shown the detectivity in CCN operation was due to mainly the phonon noise in the frequency range of 3-20 Hz. Excess 1/f noise limited the D* on fm < 3 Hz. In active CVM operation we investigated the dependences of the responsivity SV at fm = 12.5 Hz on the temperature of bolometer base, bias voltage of bolometer at constant amplifier gain of 1000. The bolometer under investigation was arranged within a bridge circuit connected to analogue electronic feedback loop. The signal bridge put into differencial two cascade amplifier based on TL082 with feedback loop to bias voltage of bridge (see Figure 2). The figure 4 shows the responsivity S changes very less in T0 range of 78.5-87 K and then begins to increase more sharply in the transition range.

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Figure 3. The dependencies of resistance (R), responsivity (S) and noise voltage (Vn) on temperature for bolometer at current Ib = 0.025 mA in CCM.

S (arb. un.)

2

Vb (V) 1

1,5

1 0,8 0,6 0,4 0,2 0

0,5

0

S(Tb) arb S(Vb), arb

78

80

82

84

86

88

90

92

Base temperature (K)

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Figure 4. The dependencies of responsivity (S) on base temperature and, accordingly, at various bias voltage and L for bolometer operating in active CVM. The resistance of bolometer in operating point is 4.7 kOhm, frequency 12.5 Hz.

Noise and responsivity measurements of active CVM operation showed complex dependences on bias voltages and currents, base temperatures and frequencies. It can be explained by the change of loop gain of active electrothermal feedback with the change of base temperature. Figure 5 shows the behaviour of voltage responsivity at different values of bias voltage. It is seen that the responsivity decreases with increasing electrothermal feedback when bolometer voltage and current increase. At the increasing bias voltage in active CVM in 5.6 times the responsivity decreased in about of 5 times, i.e. the responsivity is versus proportional to bolometer voltage in CVM. On other hand it is seen the response time decreases also. In comparison with constant current mode the frequency cutoff increases from 27 Hz (τ = 6⋅10-3 s) to 800 Hz (τ = 2⋅10-4 s) at the most strong feedback (Vb = 1905 mV, T0 of base is 78.5 K). Taking into account data of figure 5 for CVM and responsivity dependence on current for CCM, we obtained, that at almost near bias currents (CVM, Ib = 0.072 mA and CCM, Ib = 0.065 mA) the responsivity in CVM decreased in ~ 30 times, as response time. Thus, the decreasing the responsivity and response time is the same in CVM. However, in these conditions the excess 1/f noise increases very strongly and the detectivity becomes worse. Figure 6 shows noise dependencies of bolometer operating in

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Experimental Modelling Active Strong Electrothermal Feedback …

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CVM. It should be noted that input microscheme TL082 used as preamplifier has high excess noise. 336 mV 1347mV 1905 mV 584 mV CCM, 0.035mA

S (arb. un.) 1

0,1 1

10

100 Frequency (Hz)

1000

10000

Figure 5. The dependencies of responsivity (S) in CCM and active CVM on frequency. -1/2

Vn (VHz

)

1907mV 584mV 194mV Vn, amp

1,E-06

1349mV 336mV CCM, 0.065mA

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1,E-07

1,E-08

1,E-09 1

10

Frequency (Hz)

100

1000

Figure 6. The dependencies of noise voltage (Vn) on frequency in CCM and active CVM at various voltage bias, Rb = 4.7 kOhm.

It is seen that amplifier noise is very high about 10-7 V/Hz1/2 in range of 1-300 Hz and probably was due to resistors on input as well. Active feedback mode influences on behaviour of bolometer and preamplifier noise by complex way. The most depression of noise in CVM was observed at Rb = 4.7 kOhm when Vb = 584 mV. At f = 5 Hz this depression was about 40 times. Note, that depressed noise at Vb = 584 mV (Ib = 0.125 mA) was less than intrinsic noise of bolometer in CCM at Ib = 0.065 mA. At the increasing bias in CVM up to Vb = 1905 mV

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1,E+10

D*, 1907mV D*,CCM, 0.035mA S, 584mV

D*,584mV S, 1907mV S, CCM 0.035mA 10000

1,E+09

1000

1,E+08 100

1,E+07 1,E+06 1

10

100

S (V/W)

-1

D* (cmW Hz

1/2

)

(0.409 mA), i.e. in ~ 3.3-4 times, value of noise and its frequency dependence are the same one’s as for in CCM at Ib = 0.065 mA. Noise spectra in CVM at Vb = 1905 mV and Vb = 1347 mV have frequency dependencies near to Vn2 ~ 1/f in frequency range of 1-80 Hz. Noise behaviour in CVM at Vb = 194mV-584 mV at high frequencies can be explained that in this frequency range coefficient of electrothermal feedback loop becomes small and the contribution of amplifier noise is more essential and namely it is due to sum noise. Correct analysis of noise characteristics showed on figure 6 is difficult because of high amplifier noise. Summary of comparing bolometric characteristics measured in CCM and active CVM is showed on figure 7 and table.

10 1000

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Frequency (Hz) Figure 7. The dependencies of responsivity (S) and detectivity (D*) in CCM active CVM at various voltage bias.

Table 1. Parameters of high-Tc GdBaCuO bolometer in various operating modes

Parameter D*(max), cmHz1/2W-1 D*(cutoff), cmHz1/2W-1 D*(400 Hz), cmHz1/2W-1 τ, s D*max/τ1/2, cmHzW-1 D*cutoff/τ1/2, cmHzW-1 D*400Hz/τ1/2, cmHzW-1

Constant current mode

Mode of bolometer operating Active constant voltage mode Vb=584 mV Vb=1905 mV

6×109 (10 Hz)

2×109 (10 Hz)

2.5×108 (10 Hz)

6×109 (27 Hz)

3.5×108 (400 Hz)

1.5×108 (800 Hz)

5×108 (400 Hz)

3.5×108 (400 Hz)

2.4×108 (400 Hz)

6×10-3

4×10-4

2×10-4

7.8×1010

1×1011

1.7×1010

7.8×1010

1.75×1010

1×1010

6.5×109

1.75×1010

1×1010

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It shows three variants of operating regimes. In CCM the best detectivity about 6×109 cmHz1/2W-1 was obtained at Ib = 0.035 mA (Rb = 7.4 kOhm) in frequency range of 5-80 Hz, response time was about τ = 6 ms. The most optimal active CVM operation is observed at Vb = 584 mV (Ib = 0.125 mA), and Tb = 90 K, when the bolometer has time constant of 0.4 ms and the detectivity D* = 2×109 cmHz1/2W-1 at fm = 10 Hz. Note, that in this case often used parameter D*/τ1/2, equal to 1011 cmHzW-1, is better than in CCM operation (7.8×1010 cmHzW1 ). This is estimation of CVM characteristics has preliminary character. It is necessary to improve noise parameters of electronics to optimise feedback loop and to optimise operating point of bolometer for CVM.

CONCLUSION

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It is shown, using analogue electronic feedback control allows to increase the loop gain of the HTSC-bolometer electrothermal negative feedback. Intrinsic loop gain of bolometer L = V2⋅β/R⋅G was near to 1 and active CVM allowed one to increase it in 15 times in the optimal operating point. Obtained characteristics of active CVM operation concerning the increase of bolometer speed are in accordance with estimation of passive ETF (Ref. [6]). The parameter D*/τ1/2, equal to 1011 cmHz/W in active CVM, is better than in CCM operation 7.8×1010 cmHz/W (see Table 1). Noise parameters would be improved using low noise electronics, optimising feedback loop and operating point of bolometer. This is estimation of CVM characteristics has preliminary character. It is necessary to improve noise parameters of electronics, to optimise feedback loop and to optimise operating point of bolometer for CVM.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

Neff H and al 1995, Sensitive high-Tc transition edge bolometer on a micromachined silicon membrane, Appl. Phys. Lett., 66, 2421-2423. De Nivelle M J M E and al 1997, Low noise high-Tc superconducting bolometers on silicon nitride membranes for far-infrared detection, J. Appl. Phys., 82, 10, 4719-4726. Verghese S and al 1992, Feasibility of infrared imaging arrays using high Tc superconducting bolometers, J. Appl. Phys., 71, 6, 2491-2498. Johnson R B and al 1995, High performance linear arrays of YBa2Cu3O7 superconducting infrared microbolometers on silicon, Proc. SPIE, 2475, 56-61. Lee A T and al 1996, A superconducting bolometer with strong electrothermal feedback, Appl. Phys. Lett., 12, 69, 1801-1803. Lee A T and al 1997, Voltage-biased high-Tc superconducting infrared bolometers with strong electrothermal feedback, IEEE Trans. Appl.Superconductivity, 7, 2378-2381. Khrebtov I A and Tkachenko A D 1999, High-temperature superconductor bolometers for IR region, J. Opt. Technol.,66, 8, 735-740. Neff H and al 2000, Non-linearity and electrothermal feedback of high-Tc-transition edge bolometers, Appl. Phys. Lett., 76, 5, 640-642.

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Mather C J 1982, Bolometer noise: nonequilibrium theory, Applied Optics, 21, 6, 11251129. [10] Hooge F H and al 1981, Experimental studies of 1/f noise, Rep. Prog. Phys., 44, 479532. [11] Van Oudheusden B W 1997, Effect of operating conditions on the dynamic response of thermal sensors with and without analog feedback, Sensors and Actuators A: Physical, 58, 2, 129-135.

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[9]

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In: Bolometers: Theory, Types and Applications Editor: T. M. Walcott, pp. 173-182

ISBN: 978-1-61728-289-8 © 2011 Nova Science Publishers, Inc.

Chapter 8

YBCO FILMS ON SRTIO3 SUBSTRATES WITH RECORDLY LOW 1/F NOISE FOR BOLOMETER APPLICATIONS B. Dam, F.C. Klaassen1, J. M. Huijbregtse1, I.A. Khrebtov2, K.V. Ivanov2 and S.V. Baryshev3 1

Institute COMPAS and Faculty of Scieces, Division of Physics and Astronomy, Vrije Universiteit, Amsterdam, the Netherlands 2 Vavilov State Optical Institute, St. Petersburg, Russia 3 Ioffe Physico-Technical Institute, St.-Petersburg, Russia

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ABSTRACT The excess 1/f noise investigations of YBCO films on SrTiO3 substrates are reported. The epitaxial films produced by the laser ablation are characterized by the high perfect structure, sharp superconducting transition, strong pinning and very low excess 1/f noise. It is observed that the level at normal state depends on the dislocation density. The best noise properties are observed for films with the dislocation density of (8-12) disl/μm2. These films have noise Hooge-parameters of around (1.6-3)×10-6, which is 3 order less than published values. The calculation of the bolometer based on investigated films, shows the possibility to reach the noise equivalent power limited by only the phonon noise in the modulating frequency range beginning above 0.1 Hz.

1. INTRODUCTION New device of the cryoelectronics, based on high-Tc superconducting films, such as SQUIDs, IR-bolometers, commutators, filters, analogue to digital converters and so on, are in progress. To achieve bolometric detection near to the thermodynamical limit, it is necessary the HTSC films with a low 1/f noise and a high slope of the superconducting transition. This is especially important for bolometers, operating in space devices, where small fluctuations in the background radiation have to be detected. So electrical noise studies of high-Tc thin films

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are an important step towards further improvement of the HTSC bolometer performance (Refs. 1, 2). It should be noted that, owing to the technological progress after the discovery of HTSC, the noise Hooge-parameter N has decreased by six orders of magnitude to 2×10-4 (Ref. 2). Nevertheless, the main origins of excess 1/f-noise and also the technological ways of decreasing it are not understood. Optimization of the laser ablation process and improvement of substrate quality allowed us to manufacture YBCO films with recordly low Hooge-parameters. Measured film noise parameters are used for the estimation of HTSC bolometers, based on these films.

2. NOISE EQUIVALENT POWER AND NOISE VOLTAGE OF HTSC BOLOMETER Taking into account the sum of the noise contributions, the noise equivalent power (NEP) can be written as (Ref. 3.)

NEP = ( NEP

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2 photon

2 4 ⋅ k ⋅ T 2 ⋅ G 4 ⋅ k ⋅ T ⋅ R V1/ f 1/ 2 + + 2 ) + S2 S ε2

(1)

where: T is the temperature of the bolometer, k is the Boltzmann constant and ε is the optical absorption. The first noise contribution is a photon noise. However, in practice, the NEP for the HTSC bolometers is limited by the other terms of Eq. 1. The second term is the phonon noise, caused by the random energy exchange between the bolometer element and the heat sink through the thermal conductance G. The third term is the Johnson’s noise of the bolometer resistance R. The fourth noise contribution is due to the excess 1/f-noise, which has a noise spectral power density (Ref. 4):

V12/ f =

α ⋅ ( I ⋅ R )2 N ⋅ A ⋅d ⋅f a

(2)

where: α is the noise Hooge-parameter, I is the bias current, N is the carrier per unit volume of HTSC film (for which often the approximate value of N = 1021 is used (Ref. 1), A is the film area, d is the film thickness, f is the frequency and a is the constant near to 1. The optical sensitivity of the bolometer as a system with distributed parameters yields (Ref. 5):

⎤ ⎡ b ⋅β⋅ R S = ε⋅⎢ ⎥ 2 ⎣ G ⋅ ((1 − b ) + 2 ⋅ π ⋅ f ⋅ τ ) ⎦

1/ 2

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(3)

YBCO Films on SrTiO3 Substrates with Recordly Low 1/f Noise …

175

where: β is the temperature coefficient of resistance, b = I2⋅β⋅R/G is the thermal stability coefficient, τ = C/G is the thermal time constant and C is the heat capacity of the bolometer element. Actually, the measured output noise spectral power density of the HTSC bolometer consists of the following components (4): Vn2 = VG2 + VR2 + V12/ f =

4 ⋅ k ⋅ T2 ⋅ b ⋅β ⋅ R α ⋅b⋅G ⋅R + 4 ⋅k ⋅ T⋅R + (1 − b ) 2 + 2 ⋅ π ⋅ f ⋅ τ β⋅ N ⋅ A ⋅d ⋅f a

(4)

The best situation is when the NEP is limited by the thermo-dynamical noise only. This necessitates both a small α and G and a high slope of the superconducting transition dR/dT. When G is small, β is high, the optimum bias current decreases and the effect of the excess current noise on the NEP and Vn becomes negligible.

3. SAMPLES AND EXPERIMENTAL TECHNIQUE

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Pulsed excimer laser deposition is employed to produce epitaxial c-oriented YBa2Cu3O7-x films with a thickness of about 130-200 nm on (100) SrTiO3 substrates measuring 5×5×1 mm. More details on the deposition procedure can be found in (Refs. 6, 7). The current density is measured by means of torque magnetometry (Ref. 8). The test samples have a geometry of two or four-probe bridges (with typically 50 or 500 nm length and 3-50 nm width) and they are patterned by conventional photolithography and wet chemical etching. As electrical current leads we attach gold wires with a bonding machine to evaporated silver pads.

Figure l. Scheme for noise measurements. 1 – bridge, 2 – resonance transformer, 3-preamplifier, 4spectrum analyzer

The samples are glued to the cold holder, placed in the vacuum cavity of a optical metallic LN2 cryostat with regulative temperature. The microstructure is analyzed by means of X-ray diffraction. The dislocation density of films can be controlled by changing the substrate temperature during deposition. Chemical etching in combination with AFM allows us to determine the density of dislocations (Ref. 8). Noise spectra are measured by a DC four or two probe method with load resistor (see Fig.1). The instrumental noise level Vno at f = 12.5 Hz is about 0.2 nV/Hz1/2 at R(bridge) < 1

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Ω and nV/Hz1/2 at R > 30 Ω. Such a dependence of Vno on R is due to the properties of the transformer. Low noise batteries are used as source of bias current. The measured current densities for samples at the normal state have to be about 106 A/cm2 to detect an excess current noise higher, than the Vno level. The transformer is used in the resonance mode at f = 12.5 Hz, so noise frequency dependencies in the range of 1-80 Hz are measured using only the preamplifier and only for samples, operating at high currents in the normal state. The dependence of noise on the resistance and current are measured at temperatures between 78300 K, while varing the dislocation density (DD) between 8-100 disl/µm2 and using substrates with a miscut in the range of 0.07-0.37 degree. The temperature dependence of dR/dT is obtained from the bridge response to the modulation of the IR-laser radiation. The excess 1/f - noise Hooge-parameter formula (2).

N is used to compare the various films in accordance to

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4. MAIN RESULTS OBTAINED AND DISCUSSION

Figure2. Noise spectral power density Vn2 (f) of sample 613A at T = 92 K (Rb = 79 Ω, I = 29 mA) and at T = 300K (Rb = 222 Ω, I = 13 mA)

Table 1 and figures 2-4 show main results of the investigations of the structural, the noise properties of the films. It should be noted, the films are characterized by a transition temperature Tc at about 90.5 K and sharp transition with ∆Tc < 0.5 K. Moreover, they have very high critical current densities jc up to 108 A/cm2 at T = 4.2 K (Ref. 8). All measured bridges with low and high DD stay in the superconducting state (R = 0, T = 78 K) at a current density. Note, that at these conditions no excess noise higher, than the Vno = 0.2 nV/Hz1/2 (f = 12.5 Hz) is observed. In the normal state (T = 300 K and T = 100 K) the excess noise voltage is proportional to current. Figure 2 shows, that the spectral behaviour of the noise for the sample 613A with a high DD = 99 disl/μm2 near to typical Vn2(f) ~ 1/fa dependence with a = 1.1. However, more weak frequency dependence of noise is observed in the normal state for the frequencies < 10 Hz, where Vn2(f) ~ 1/f0.48.

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Figure 3. Temperature dependence of R (curve 1) and noise voltage Vn at f = 12.5 Hz (curve 2) of sample 611B, I = 8 mA, Rn = 100 Ω

The lowest excess noise in the normal state is observed for films with a low DD = (8.312.5) disl/μm2. The αN(100 K) in the normal state are about (1.6-3)×10-6 for these samples (611A, 611B, 667A). For film 613A with DD = 99 disl/μm2 a noise parameter αN(100 K) = 10-3. Bridge 613PA made from the same film had undergone an additional treatment (30 minutes at 800 °C in flowing Ar + O2) in order to improve the overall film crystallinity without annihilating dislocations. As result the noise properties improved essentially: αN(300 K) decreased in 47 times and αN (100 K) – in 23 times. It is shown this treatment decreases the defect density and, thus, leads to decreasing 1/f noise both at room temperature and at 100 K. Note, that at T = 300 K a noise parameter became hear to one’s of the best samples with low dislocation density. It allows one to conclude, that at T = 300 K the excess noise does not depend on the dislocation density. The main origins of the excess noise at these temperatures are due to other factors, may be, oxygen transition between the lattice positions 01 and 05 in CuO planes (Refs. 9, 10). Nevertheless, at normal state the excess noise depends on the value of dislocation density. The change of frequency dependence of the noise at normal state (see Fig.2) direct on the change mechanism of noise, as well. Table 1 shows that αN(100 K) of post-annealing film 613A1 in ~20 times more, than αN(100 K) of sample 611B with low DD = 12.5 disl/μm2. The films deposited on well-oriented SrTiO3 substrates (substrate miscut Mis. = 0.0780.37 deg.) have very high perfect stucture, as confirmed by a FWHM of the 005 rocking curve of ∆ω = 0.1 deg. Note, that films on SrTiO3 with ∆ω = 0.3 deg., reported earlier (Ref.

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2), had αN(100 K) = 3×10-4 only. It should be noted, that 4.7-fold increasing in miscut did not deteriorate the noise properties of films in the normal state essentially. Noise behaviour in the transition region is more complicated. So we report only preliminary results which are interesting to the very low excess 1/f-noise in normal state. Figure 3, 4 shows the noise peaks, observed in the transition region. It is seen the noise peaks have maximum, coinciding with maximum of dR/dT. At present, we may give only preliminary suggestions about its origin. Firstly, we do not exclude arising this noise contribution from equilibrium or local temperature fluctuations (Refs. 11, 12). Second possible cause of noise is the conductance fluctuation due to variations in the fraction of the superconducting phase in the current path through the film (Refs. 13, 14). Finally, noise peak in the middle of transition may be due to phonon noise of HTSC bridge coupled with surroundings through the thermal conductance G. In this case bridge can operate as usual bolometer, characterized by this phonon or temperature noise (see Eqs. 1, 4). Note, that G is due to mainly thermal contact between SrTiO3 substrate and cooper holder through silver past and has the values of (5-9)×10-3 W/K. The thermal conductance is obtained measuring the change of sample resistance in the transition at various bias currents. Taking into account G = 9×10-3 W/K for sample 611B (Fig. 3) and thermal capacity of substrate, calculated Vn = 3.7×10-8 V/Hz1/2 for f 2 (1/K) will allow one to improve the detection properties of bolometers in low-frequency region. However, for membrane bolometer the influence of 1/f - noise can be negligible even at αN = 10-3. Table 1. Samples characteristics Sample d (nm) A (μm2) Rn (Ω) 611A 611B 613A 613PA1 613PA2 667A

202 202 158 158 158 154

22×500 6×500 45×500 17×500 25×500 17×50

117 252 79 150 100 32

∆ω (deg.) 0.1 01 0.17 0.17 0.17 0.14

Mis. (deg.) DD (disl/μm2) α(300 K) ×10-4 α(100 K) ×10-6 0.14 0.14 0.078 0.078 0.078 0.37

12.5 12.5 99 99 99 8.3

3.7 7.0 220 4.7 8.4 0.78

1.6 2.0 1000 43 160 3.0

where: Rn is the resistance in normal state, ∆ω is the FWMH of 005 rocking curve, Mis. is the substrate miscut, DD is the dislocation density, αN (300 K) is Hooge-parameter at T = 300 K,

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αN (100 K) is Hooge-parameter at T = 100 K.

Figure 6. a)The dependencies of the sensitivity and b) the NEP on frequency for the bolometers of different types at variation of noise Hooge-parameter αN (100 K) (αN1 = 2×10-3, αN2 = 2×10-6); 1 – bolometer on massive substrate, R = 203.5 Ω, G = 2.5×10-3 W/K, τ = 62.5 μs; 2 – antenna-couple microbolometer, R=25Ω, G=1.6×10-4 W/K, τ = 250 ns;3 – membrane bolometer, R = 203.5 Ω, G = 4×10-5 W/K, τ = 62.5 μs.

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CONCLUSION It is shown experimentally, that structural perfect YBa2Cu3O7-x films on SrTiO3 substrates, with a high critical current density due to strong dislocations pinning, simultaneously have a very low excess 1/f-noise in the normal state. The value of this noise depends on the dislocation density and the best noise Hooge-parameter αN(100 K) = (1.63)×10-6 is observed for films with a DD = 12.5 disl/μm2. At this value of (100 K) the excess noise voltage exceeds the Johnson resistance noise only 1.5-2 times. Note, that these noise parameters are, as far we know, the best ever reported. Also we observed noise peaks, coinciding with a maximum of the slope in the superconducting resistive range. This noise is probably phonon noise due to random thermal phonon exchange between the HTSC film and the heat sink. In future, more detailed research is necessary to understand the source of that noise. Theoretical modeling of the different bolometer types, based on the noise parameters, we found, that our films shows the possibility to reach the NEP, limited only by the phonon noise, in the frequency region beginning above 0.1 Hz. The relation between the temperature coefficient of the resistance to noise Hoogeparameter reaches values of the order of 2.5×106, which is 3 orders better, than published earlier (Ref. 2).

ACKNOWLEDGMENTS

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This work was support by the reseach program of FOM (stichting fundamental Onderzoek der Materie), which is supported by NWO (stichting Nederlands Wetenschappelijk Onderzoek).

REFERENCES [1] Kiss L. B., Svedlindh P. 1994. Noise in High-Tc Superconductors, IEEE Trans. on Electron Devices, 41, 2112-2122. [2] Khrebtov I.A. et al 1998, Noise of high-Tc superconducting bolometers, Proc. SPIE, 3287, 288-299. [3] Richards P.L. et al 1989, Feasibility of the high-Tc superconducting bolometer, Appl. Phys. Lett., 54, 283-285. [4] Hooge F.H. et al 1981, Experimental studies of 1/f noise, Rep.Prog. Phys., 44, 479-532. [5] Khrebtov I.A. 1992, Theoretical analysis of the properties of high-Tc-superconductor bolometers, Supercondutivity: Phys. Chem. Techn., 5, 558-567. [6] Dam B et al 1994, Laser ablation threshold of YBa2Cu3O6+x, Appl.Phys.Lett., 65, 15811583. [7] Huijbregtse J.M. et al 1999, High-quality off-stoichiometric YBa2Cu3O7-b films produced by diffusion-assisted preferential laser ablation, J.Appl. Phys., 86, 6528-6537. [8] Huijbregtse J.M. et al 1999, Origin of high critical currents in YBa2Cu3O7-∆ superconducting thin films., Nature, 399, 439-442.

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B. Dam, F.C. Klaassen, J. M. Huijbregtse et al.

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[9] Bobyl A.V. et al 1997, Intrinsic microstrains and norma-phase flicker noise in YBa2Cu3O7 epitaxial films grown on various substrates, J. Appl. Phys., 82, 1274-1280. [10] Chen N.Y. et al 1995, Low 1/f normal-state resistance noise in high – resistivity YBa2Cu3Oy, Appl. Phys. Lett., B13, 556-573. [11] Voss R.F. and Clarke J. 1976, Flicker (1/f) noise: Equilibrium temperature and resistance fluctuations, Phys. Rev., B49, 6895-6902. [12] KozubV.I. 1994, Influence of structural relaxation on the parameters of a superconductor, Phys, Rev. B13, 556-573. [13] Song Yi et al 1992, Anisotropic 1/f noise and motion of magnetic vortices in YBa2Cu3O7∆, Phys.Rev., B45, 7574-7576. [14] Bobyl A.V et al 1995, Resistance flicker noise and current percolation in c-oriented YBa2Cu3O7-x films in the vicinity of Tc, Physica C, 247, 7-33.

Bolometers: Theory, Types and Applications : Theory, Types, and Applications, Nova Science Publishers, Incorporated, 2010. ProQuest Ebook

In: Bolometers: Theory, Types and Applications Editor: T. M. Walcott, pp. 183-195

ISBN: 978-1-61728-289-8 © 2011 Nova Science Publishers, Inc.

Chapter 9

ABSOLUTE HIGH-TC SUPERCONDUCTING RADIOMETER WITH ELECTRICAL-SUBSTITUTION FOR X-RAYS MEASUREMENTS I.A Khrebtov1, V.G. Malyarov1, K.V. Ivanov1, A.D. Nikolenko2 and V.F. Pindyurin2 1

S.I. Vavilov State Optical Institute, St. Petersburg, Russia Budker Institute of Nuclear Physics, Novosibirsk, Russia

2

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

ABSTRACT The present work considers the practical possibility of the construction of an absolute radiometer with electrical substitution for power based on the high-Tc superconducting YBaCuO film bolometer cooled with liquid nitrogen to measure the power of radiation of the X-ray range circa 1μW with an accuracy of 1%. This accuracy is provided with high sensitivity of the bolometers, having the noise equivalent power about 3.8×10-12 W/ Hz1/2 (with modulation) and 2.6×10-9 W (without modulation). The main sources affecting on an accuracy of the absolute measurements such as external reflection, the passage of radiation through the substrate, photo-stimulated electron emission from the receiving surface, the stability of synchrotron radiation source and instability of cryostat temperature are analysed. The radiometer can be applied to measure absolute power of “white” and monochromatic synchrotron radiation flows in the spectral range from 80 to 2000eV.

1. INTRODUCTION Synchrotron radiation (SR) is a tool useful to conduct metrological measurements in the broad spectral range from the infrared radiation to the hard X-radiation [1]. The possibility of exact computation of an absolute spectral flow and its time and geometrical characteristics makes it possible to use an SR source as a primary standard of accuracy as high as 10-3÷10-4 [2].

Bolometers: Theory, Types and Applications : Theory, Types, and Applications, Nova Science Publishers, Incorporated, 2010. ProQuest Ebook

184

I.A Khrebtov, V.G. Malyarov, K.V. Ivanov et al.

At the same time, the standard detector problem keeps being very topical for SR metrological measurements. The reason is that the installation of selective spectral elements (monochromators, filters etc) in the way of the SR beam in most cases leads to appearance of significant uncertainty and to a loss of accuracy of the metrological measurements. This loss can make up to units, tens and more percents due to the uncertainty in the characteristics of the selective elements used. That is essentially significant in the soft X-ray range of 80÷2000 eV, where optical parameters of the selective elements can vary significantly even in time because of the variation of their surface properties, in particular, because of the deposition of thin carbon or other films on their surfaces. One of the most common approaches at metrological measurements on SR is the application reference detectors of radiation. So, in the range of 50-1500 eV, the heliumcooled radiometer with electrical-substitution based on semiconductor temperature sensor allows one to measure power of the order 1-10 μW with accuracy up to 0.2% [3]. In spite of individual successful examples, it should be noted the range of 80÷2000 eV is not provided with X-radiation detectors satisfactorily, and the task of the development of absolute detectors as primary standards to conduct metrological measurements both on SR and other radiation sources keeps being topical. The present work considers the practical possibility of an absolute radiometer construction with electrical-substitution for power on the base of high-Tc superconducting (HTS) bolometer cooled with liquid nitrogen to measure radiation power of the X-ray range circa 1μW with an accuracy of 1%.

2. DESIGN, PRINCIPLE AND CHARACTERISTICS OF RADIOMETER

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2.1. Design and Principle of Operation The design and operation principle of the absolute electrical-substitution radiometer (AESR) based on HTSC bolometer are discussed using Fig.1, 2. The ultrahigh-vacuum system of the monochromator beam line is connected to the radiometer vacuum system, as for example in [3]. The system of the radiation shields and diaphragms (4) at liquid N2 temperature (Fig.1) limits the influence of 300 K background irradiation. The incident SR is absorbed with NiCr film heater (1), deposited on sapphire (or SrTiO3) substrate (2) with size of 5×5×0.05 mm3. The superconducting YBCO film meander thermometer (3) is deposited on back side of substrate. Such thermosensitive element is suspended on four thin Au (or W) wires (10) to heat sink (7). They are used for current bias of YBCO and NiCr films and for thermal isolation of bolometer, providing needful thermal conductance Gb, in the dependence on diameter (20-30 μm) and length (3-12 mm). Teflon gasket (6) with Gf and heat Cu sink (7) with heat capacity Cs form the thermal filter with constant time τf= Cs/Gf, that permits to decrease temperature fluctuations of N2 cryostat. Note, it is necessary to carry out conditions: Gb