Design and Fabrication of a MEMS-Array Pressure Sensor System for Passive Underwater ...

241 Pages·2012·9.09 MB·English

Design and Fabrication of a MEMS-Array Pressure Sensor
System for Passive Underwater Navigation Inspired by the
Lateral Line
by
Stephen Ming-Chang Hou
M.Eng., Electrical Engineering and Computer Science
Massachusetts Institute of Technology, 2004
S.B., Electrical Science and Engineering
Massachusetts Institute of Technology, 2003
S.B., Physics
Massachusetts Institute of Technology, 2003
Submitted to the Department of Electrical Engineering and Computer Science
in partial fulfillment of the requirements for the degree of
Electrical Engineer
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2012
(cid:13)c Massachusetts Institute of Technology 2012. All rights reserved.
Author............................................................................
Department of Electrical Engineering and Computer Science
March 14, 2012
Certified by........................................................................
Jeffrey H. Lang
Professor of Electrical Engineering
Thesis Supervisor
Accepted by.......................................................................
Leslie A. Kolodziejski
Chair, Department Committee on Graduate Theses
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Design and Fabrication of a MEMS-Array Pressure Sensor System for
Passive Underwater Navigation Inspired by the Lateral Line
by
Stephen Ming-Chang Hou
Submitted to the Department of Electrical Engineering and Computer Science
on March 14, 2012, in partial fulfillment of the
requirements for the degree of
Electrical Engineer
Abstract
An object within a fluid flow generates local pressure variations that are unique and char-
acteristic to the object’s shape and size. For example, a three-dimensional object or a
wall-like obstacle obstructs flow and creates sharp pressure gradients nearby. Similarly, un-
steadyflowcontainsvorticalpatternswithassociateduniquepressuresignatures. Detection
of obstacles, as well as identification of unsteady flow features, is required for autonomous
undersea vehicle (AUV) navigation. An array of passive underwater pressure sensors, with
their ability to “touch at a distance” with minimal power consumption, would be able to
resolve the pressure signatures of obstacles in the near field and the wake of objects in the
intermediate field. As an additional benefit, with proper design, pressure sensors can also
be used to sample acoustic signals as well.
Fishalreadyhaveabiologicalversionofsuchapressuresensorsystem,namelythelateral
line organ, a spatially-distributed set of sensors over a fish’s body that allows the fish to
monitor its hydrodynamic environment, influenced by the external disturbances. Through
its ability to resolve the pressure signature of objects, the fish obtains “hydrodynamic
pictures”.
Inspired by the fish lateral line, this thesis describes the development of a high-density
array of microelectromechanical systems (MEMS) pressure sensors built in KOH-etched
silicon and HF-etched Pyrex wafers. A novel strain-gauge resistor design is discussed,
and standard CMOS/MEMS fabrication techniques were used to build sensors based on
the strain-gauge resistors and thin silicon diphragms. Measurements of the diaphragm
deflection and strain-gauge resistance changes in response to changes in applied external
pressure confirm that the devices can be reliably calibrated for use as pressure sensors to
enable passive navigation by AUVs. A set of sensors with millimeter-scale spacing, 2.1 to
2.5 µV/Pa sensitivity, sub-pascal pressure resolution, and −2000 Pa to 2000 Pa pressure
range has been demonstrated. Finally, an integrated circuit for array processing and signal
amplification and to be fabricated with the pressure sensors is proposed.
Thesis Supervisor: Jeffrey H. Lang
Title: Professor of Electrical Engineering
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Acknowledgments
This thesis would not have been possible without the efforts of many people. First, I would
like to express gratitude to my advisor, Prof. Jeff Lang, for the opportunity to work under
his mentorship throughout my graduate studies. His deep intuition and focus on important
questions will always influence me. I thank the other faculty under whom I have worked
on various projects over the years, Profs. Franz Hover, Marty Schmidt, Alex Slocum and
Michael Triantafyllou, for their creative insights and guidance. Their boundless supply of
energy, enthusiasm and knowledge make MIT such an incredible place to study.
I would like to thank the other members of the Underwater Sensors Group and the
Precision Engineering Research Group. I am grateful to my officemate Dr. Vicente Fer-
nandez, who worked on complementary aspects of our overall project, for our conversations
and mutual encouragement. I am especially thankful to Drs. Hong Ma, Alexis Weber and
James White for their many hours showing me the ropes in the lab when I first started, and
generously providing many pieces of advice throughout, even when they were busy prepar-
ing to graduate. Drs. Jian Li, Joachim Sihler and Xue’en Yang also extended invaluable
tips on fabrication techniques.
I would like to thank other members of the MEMS community at MIT. In particular,
I thank Prof. Joel Voldman for sharing his experience with HF etching. I would like to
thank Kevin Nagy and Dr. Nikolay Zaborenko for their bonding recipes, and countless
other fabrication laboratory users for sharing bits of advice, which has motivated me to
contribute to our collective knowledge.
InthespiritofthecollaborativenatureofMIT,IthankDrs.SteveBathurst,JerryChen,
Ivan Nausieda and Vanessa Wood for sharing their laboratory equipment for experimenting
with printable electronics.
I would like to express great appreciation for the MIT Microsystems Technology Lab-
oratories (MTL), i.e. my second home, where my fabrication work was carried out. In
particular, I am grateful to MTL staff members Bernard Alamariu, Bob Bicchieri, Kurt
Broderick, Eric Lim, Kris Payer, Dave Terry, Paul Tierney, Tim Turner and Dennis Ward,
who patiently trained me on each of the machines, maintained tools around the clock,
answered my calls and e-mails when I needed help, and offered advice.
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I thank Pierce Hayward for training me to use the Zygo profilometer. I am also grateful
to Wayne Ryan and Steve Turschmann for building various equipment that my research
required.
I would like to thank the National Science Foundation, the NOAA MIT Sea Grant
College Program (project grant R/RT-2/RCM-17), the MIT-Singapore Alliance, the MIT
Deshpande Center and the MIT Department of Electrical Engineering and Computer Sci-
ence (EECS) for their funding of my research.
I would like to thank Sam Chang, Dr. Shahriar Khushrushahi, Adam Reynolds, Yasu
Shirasaki and my family for taking the time to read my thesis and offer improvements,
especially on short notice. However, any outstanding errors are mine alone.
Of course, I am grateful to those at MIT who have provided me with a life outside the
lab. I will always remember my friends from the Graduate Student Council, the EECS
Graduate Students Association, the Resources for Easing Friction and Stress and Ashdown
House for inspiring a selfless sense of community and belonging. I also appreciate the EECS
department, the Educational Studies Program and BLOSSOMS for the distinct privilege of
teaching bright young people on behalf of MIT.
Finally, I would like to thank my family for their unwavering support and encourage-
ment. I am indebted to my parents’ dedicated efforts in providing me the opportunities
I’ve had throughout my life. Their eternal love and faith in me will be carried to future
generations. I dedicate this thesis to them.
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Contents
1 Introduction 21
1.1 The Fish Lateral Line and Biomimetics . . . . . . . . . . . . . . . . . . . . 24
1.2 MEMS Pressure Sensors: A Broader Context . . . . . . . . . . . . . . . . . 27
1.3 Where This Work Fits In . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1.4 Project Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
1.5 Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2 Device Design 39
2.1 Overview of Device Features . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.2 Wafer-Level Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.2.1 Air channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.2.2 Round 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.2.3 Round 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.2.4 Round 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.2.5 Round 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.2.6 Round 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.3 Properties of Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.3.1 Crystalline structure of silicon . . . . . . . . . . . . . . . . . . . . . 52
2.3.2 The elastic properties of silicon . . . . . . . . . . . . . . . . . . . . . 53
2.3.3 The elastic properties of thin silicon diaphragms . . . . . . . . . . . 55
2.4 Wheatstone Bridge Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
2.5 Piezoresistive Pressure Sensors . . . . . . . . . . . . . . . . . . . . . . . . . 60
2.6 Mechanical Model of the Diaphragm . . . . . . . . . . . . . . . . . . . . . . 64
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8 CONTENTS
2.6.1 Diaphragm assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 64
2.6.2 Governing equations . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
2.6.3 The nature of the deflection function . . . . . . . . . . . . . . . . . . 67
2.6.4 Deflection as a function of pressure . . . . . . . . . . . . . . . . . . . 68
2.6.5 Strain as a function of pressure . . . . . . . . . . . . . . . . . . . . . 70
2.6.6 Stress as a function of pressure . . . . . . . . . . . . . . . . . . . . . 71
2.7 Strain-Gauge Resistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
2.7.1 Relative change in resistance of an infinitesimally small resistor . . . 73
2.7.2 Simulating resistance change . . . . . . . . . . . . . . . . . . . . . . 74
2.7.3 Strain-gauge resistor pattern . . . . . . . . . . . . . . . . . . . . . . 78
2.8 Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
2.9 Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
2.10 Summary of Expected Performance . . . . . . . . . . . . . . . . . . . . . . . 84
3 Fabrication 85
3.1 Overview of Fabrication of Round 4 Devices . . . . . . . . . . . . . . . . . . 86
3.2 Variations on Fabrication for Rounds 1, 2, 3a and 3b . . . . . . . . . . . . . 90
3.3 Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.4 Potassium Hydroxide Etching of (100) Silicon . . . . . . . . . . . . . . . . . 95
3.4.1 Silicon nitride mask and silicon oxide etch stop . . . . . . . . . . . . 96
3.4.2 Finding the crystalline plane . . . . . . . . . . . . . . . . . . . . . . 98
3.4.3 Mechanical wafer protection . . . . . . . . . . . . . . . . . . . . . . . 99
3.5 Pyrex Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
3.5.1 Substrate material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
3.5.2 Process flow for etching Pyrex . . . . . . . . . . . . . . . . . . . . . 102
3.5.3 Process design for etching Pyrex . . . . . . . . . . . . . . . . . . . . 107
3.5.4 Masking materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
3.5.5 Pyrex etchants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
3.5.6 Importance of retaining photoresist on gold . . . . . . . . . . . . . . 109
3.6 Anodic Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
CONTENTS 9
3.7 Process Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
3.7.1 Order of key steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
3.7.2 Placement of insulating oxide growth step . . . . . . . . . . . . . . . 115
3.7.3 Open access to cavities. . . . . . . . . . . . . . . . . . . . . . . . . . 115
3.7.4 Wet processing bonded wafers . . . . . . . . . . . . . . . . . . . . . . 116
3.8 Mask Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
3.8.1 List of masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
3.8.2 Mask alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
4 Measurements 121
4.1 Photographs of Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
4.2 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
4.2.1 Manometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
4.2.2 Zygo profilometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
4.2.3 Off-wafer differential amplifiers . . . . . . . . . . . . . . . . . . . . . 129
4.2.4 Tech-Etch cable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
4.2.5 Water-level probe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4.3 Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
4.4 Drifting Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
4.4.1 Experimental set up . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
4.4.2 Dummy bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
4.4.3 E&M interference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
4.4.4 Fluctuating pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
4.4.5 Unstable contact resistance . . . . . . . . . . . . . . . . . . . . . . . 144
4.4.6 Stabilizing the contact resistance . . . . . . . . . . . . . . . . . . . . 144
4.4.7 Gold on chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
4.5 External Bridge Measurements from Isolated Sensors . . . . . . . . . . . . . 148
4.6 Underwater Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
4.6.1 Out-of-water tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
4.6.2 Bucket tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
10 CONTENTS
4.7 Summary of Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
5 Concluding Remarks 165
5.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
5.2 Contributions and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 168
5.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
5.3.1 Further testing of Round 4 devices . . . . . . . . . . . . . . . . . . . 170
5.3.2 Improved strain-gauge design . . . . . . . . . . . . . . . . . . . . . . 170
5.3.3 Integrated amplifier and array read-out circuitry . . . . . . . . . . . 172
5.3.4 Flexible substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
A Chemical Formulas 181
B Standard Operating Procedures 183
B.1 Mask-Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
B.2 TRL Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
B.2.1 Double-sided PR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
B.2.2 With paint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
B.3 KOH Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
B.4 Post-KOH Clean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
B.5 Hot Phosphoric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
B.5.1 ICL hot phosphoric. . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
B.5.2 TRL acid hood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
B.6 EV501 and Anodic Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
C Process Flows 197
C.1 Standard Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
C.2 Finding the Crystalline Plane for (100) Silicon . . . . . . . . . . . . . . . . 199
C.3 Strain-Gauge Silicon Sensors Without Transistors . . . . . . . . . . . . . . . 200
C.4 Strain-Gauge Silicon Sensors With Transistors . . . . . . . . . . . . . . . . 204