Optical Microscanners and Microspectrometers using Thermal Bimorph Actuators

276 Pages·2002·9.82 MB·English

by  Gerhard Lammel

OPTICAL MICROSCANNERS AND MICROSPECTROMETERS
USING THERMAL BIMORPH ACTUATORS
MICROSYSTEMS
Volume 8
Series Editor
Stephen D. Senturia
Massachusetts Institute of Technology
Editorial Board
Roger T. Howe, University of California, Berkeley
D. Jed Harrison, University ofA lberta
Hiroyuki Fujita, University of Tokyo
Jan-Ak:e Schweitz, Uppsala University
OTHER BOOKS IN THE SERIES:
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Bartlomiej F. Romanowicz
Hardbound, ISBN 0-7923-8306-0, October 1998
Microcantilevers for Atomic Force Microscope Data Storage
Benjamin W. Chui
Hardbound, ISBN 0-7923-8358, October 1998
Bringing Scanning Probe Microscopy Up to Speed
Stephen C. Mione, Scott R. Manalis, Calvin F. Quate
Hardbound, ISBN 0-7923-8466-0, February 1999
Micromachined Ultrasound-Based Proximity Sensors
Mark R. Hornung, Oliver Brand
Hardbound, ISBN 0-7923-8508-X, April1999
Microfabrication in Tissue Engineering and Bioartiticial Organs
Sangeeta Bhatia
Hardbound, ISBN 0-7923-8566-7, August 1999
Microscale Heat Conduction in Integrated Circuits and Their Constituent Films
Y. Sungtaek Ju, Kenneth E. Goodson
Hardbound, ISBN 0-7923-8591-8, August 1999
Scanning Probe Lithography
Hyongsok T. Soh, Kathryn Wilder Guarini, Calvin F. Quate
Hardbound, ISBN 0-7923-7361-8, June 2001
Optical Microscanners and
Microspectrometers using
Thermal Bimorph Actuators
Gerhard Lammel
Sandra Schweizer
and
Philippe Renaud
EPFL, Swiss Federal Institute of Technology Lausanne
SPRINGER SCIENCE+BUSINESS MEDIA, LLC
A C.I.P. Catalogue record for this book is available from the Library of Congress.
ISBN 978-1-4419-4946-2 ISBN 978-1-4757-6083-5 (eBook)
DOI 10.1007/978-1-4757-6083-5
ABOUT THE COVER
Left: Scanning electron microscope (SEM) picture of a wafer with an array of 2D micromirrOIs
with "L" -shaped actuator beams, developed by Sandra Schweizer. The dry-release process leads to
a high production yield.
Right (series of four pictures): Tunable optical tilter of porous silicon in motion, developed by
Gerhard Larnmel. Under a voltage of O to 2.1 voit the plate turns by 90° and changes the filter
wavelength.
Printed an acid-free paper
AU Rights Reserved
© 2002 Springer Science+Business Media New York
Originally published by Kluwer Academic Publishers, Boston in 2002
No part of the material protected by this copyright notice may be reproduced or
utilized in any form OI by any means, electronic OI mechanical,
including photocopying, recording or by any information storage and
retrieval system, without written permis sion from the copyright owner.
TABLE OF CONTENTS
Preface ........................................................ xi
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 MEMS ..................................................... 2
1.3 MOEMS .................................................... 3
1.3.1 Display technologies ..................................... 3
1.3.2 Printers and industrial machining ............................ 9
1.3.3 Imaging scanners ........................................ 9
1.3.4 Telecommunication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.5 Adaptive or corrective optics .............................. 12
1.3.6 Spectroscopy applications ................................ 13
1.3.7 Medical applications ..................................... 15
1.4 MOEMS actuation principles ................................... 16
1.4.1 Electrostatic actuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.4.2 Piezoelectric actuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.4.3 Magnetic actuation ...................................... 23
1.4.4 Thermal actuation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2 Basics for a thermally actuated micromirror . . . . . . . . . . . . . . . . . . . . 27
2.1 Microactuator specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.2 Principle of the presented microscanner . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3 The thermal bimorph actuator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.1 Thermal expansion ...................................... 29
2.3.2 Stress in bimorph cantilevers and initial rest position ........... 29
2.3.3 Thermal bimorph actuator design ........................... 41
2.4 Static temperature distribution in the microscanner. . . . . . . . . . . . . . . . . . 46
2.4.1 Analytical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.4.2 Determination of constants ................................ 51
2.4.3 Influence of the different constants on the temperature distribution 55
v
2.4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
2.5 Response time of the bimorph beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
2.5.1 Thermal cut-off. ........................................ 59
2.5.2 Measurements of the cut-off frequency of the bimorph actuator ... 60
2.5.3 Conclusion ............................................ 61
2.5.4 Dynamic temperature distribution in the bimorph beam ......... 62
2.6 General conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3 Microscanner technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.1 Fabrication process ........................................... 67
3.2 Process improvements ........................................ 70
3.2.1 Dry mirror release .................................... 70
3.2.2 Mirror stiffness and flatness improvement. ................... 70
3.2.3 Mirror reflectivity ....................................... 76
3.3 Conclusions ................................................ 76
4 One-dimensional microscanner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.1 Test Set-up ................................................. 77
4.2 Static characterization of Chromium based actuators . . . . . . . . . . . . . . . . 78
4.2.1 Single device characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.2.2 Comparing angular deflection among various devices . . . . . . . . 80
4.2.3 Resistance variation of Cr layer with temperature . . . . . . . . . . . . . . 84
4.3 Static characterization of Nickel based actuators. . . . . . . . . . . . . . . . . . . . 86
4.3.1 Angle vs. temperature .................................... 86
4.3.2 Resistance vs. temperature ................................ 86
4.3.3 Angle vs. power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.3.4 Summary ............................................. 88
4.4 Dynamic characterization ...................................... 88
4.4.1 Fundamental resonance frequency calculation. . . . . . . . . . . . . . . . . 89
4.4.2 Experimental comparison of the resonance frequency. . . . . . . . . . . 94
4.4.3 Influence of damping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.4.4 Thermal cut-off ........................................ 98
4.4.5 Dynamic microscanner performances . . . . . . . . . . . . . . . . . . . . . . . 98
4.4.6 Lateral suspension mirror ................................ 100
4.4. 7 Dynamics of the tunable optical filter with Ni based actuator. . . . 102
4.4.8 Summary ............................................ 104
vi
4.5 1D scanner applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
4.5.1 Barcode reader ........................................ 105
4.5.2 Micromechanical detector for molecular beams .............. 109
4.6 Conclusions ................................................ 111
5 Two-dimensional microscanner ............................... 113
5.1 Principle ................................................... 113
5.2 Design and modelling of the raster natural frequency ................ 113
5.2.1 Analytical model ....................................... 113
5.2.2 Simulations ........................................... 119
5.3 Dynamic measurements ...................................... 120
5.3.1 2D-microscanner type 1 ................................. 120
5.3.2 2D-microscanner type 2 ................................ 122
5.3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.4 Microprojector application .................................... 123
5.4.1 Scanner requirements ................................... 123
5.4.2 Pixel resolution ........................................ 124
5.4.3 Static and dynamic mirror deformations .................... 126
5.4.4 Experimental results .................................... 127
5.5 Conclusions ............................................... 133
6 Advanced Optical Filters of Porous Silicon. . . . . . . . . . . . . . . . . . . . . 135
6.1 Principle .................................................. 135
6.2 History of porous silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
6.3 Fabrication of porous silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
6.3.1 Single etch cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
6.3.2 Double etch cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
6.4 Parameters determining the structure of porous silicon .............. 139
6.4.1 Why porous silicon is porous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6.4.2 Substrate doping ....................................... 140
6.4.3 Illumination .......................................... 141
6.4.4 Electrolyte concentration ................................ 141
6.4.5 Current densities ....................................... 142
6.5 Electropolishing ............................................ 144
6.6 Porous silicon as sacrificial layer ............................... 145
6.7 Calculation of optical interference filters ......................... 145
vii
6. 7.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
6.7.2 General theory for simulation of optical multilayer filters ....... 146
6. 7.3 Bragg band reflectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6.7.4 Fabry-Perot bandpass filters .............................. 150
6.7.5 Multi band reflectors ................................... 151
6.7.6 Edge filters ........................................... 152
6.7.7 Angular dependence .................................... 153
6.8 Fabrication of optical filter of porous silicon ...................... 153
6.8.1 Effective media theory .................................. 153
6.8.2 Lateral homogeneity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
6.8.3 Depth homogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
6.8.4 Oxidation and aging .................................... 161
6.9 Summary ................................................. 163
7 Micromachining using porous Silicon ......................... 165
7.1 Goals for the technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
7.2 Metal masks ............................................... 166
7.3 Nitride masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
7.4 Free-standing porous silicon films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
7.5 Mask removal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
7.6 Thermal actuator design ...................................... 169
7 .6.1 Calculation of optimum layer thicknesses . . . . . . . . . . . . . . . . . . . 173
7.6.2 Calculation of beam length ............................... 180
7.6.3 Electrical heater resistance ............................... 180
7. 7 Mechanical filter plate suspension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
7. 7.1 Shape and size of filter plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
7.7.2 Ways of suspension .................................... 181
7.8 Homogeneity of the optical filter. .............................. 184
7.8.1 Frontside mask only .................................... 185
7.8.2 Frontside and backside mask ............................. 186
7.8.3 Experimental comparison ................................ 187
7.8.4 Electropolished well .................................... 189
7.9 Process flow ............................................... 190
7.1 OSummary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
viii
8 Thnable Optical Filter and IR Gas Spectroscopy . • • . • • • • • • • • • • • . 195
8.1 Overview of devices ........................................ 195
8.2 Optical characterization ...................................... 198
8.2.1 Visible light .......................................... 198
8.2.2 Infrared light .......................................... 200
8.3 Chip separation and packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
8.3.1 Cleaving ............................................. 202
8.3.2 Bonding on PCB ....................................... 204
8.3.3 Encapsulation ......................................... 204
8.3.4 Protection by a fusible link ............................... 205
8.4 System integration for gas sensing .............................. 205
8.4.1 Principle of infrared gas absorption spectroscopy . . . . . . . . . . . . . 205
8.4.2 Set-up ............................................... 208
8.4.3 Experimental results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
8.5 Summary ................................................. 210
9 Conclusions and outlook .....................................2 11
9.1 Conclusions ................................................2 11
9.1.1 Micromirror ...........................................2 11
9.1.2 Tunable optical filter ................................... 213
9.2 Outlook ................................................... 215
Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
A.1 Complement to the curvature calculation due to residual stress .... 217
A.2 Complement to the static temperature distribution calculation . . . . 221
A.3 Large deflections . . • • . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • • . . 225
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
Symbols and Abbreviations. • . . • . . . • • . . • • . • • . • • . . • • . • • • • • • . • . • . • 255
Glossary of terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
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