An Introduction to
Microelectromechanical
Systems Engineering
Second Edition
For a listing of recent titles in the Artech House Microelectromechanical
Systems (MEMS) Series, turn to the back of this book.
An Introduction to
Microelectromechanical
Systems Engineering
Second Edition
Nadim Maluf
Kirt Williams
Artech House, Inc.
Boston • London
www.artechhouse.com
Library of Congress Cataloging-in-Publication Data
A catalog record for this book is available from the U.S. Library of Congress.
British Library Cataloguing in Publication Data
Maluf, Nadim.
An Introduction to microelectromechanical systems engineering–2nd ed. –(Artech House
microelectromechanical library)
1. Microelectromechanical systems
I. Title II. Williams, Kirt
621.3’81
ISBN 1-58053-590-9
Cover design by Igor Valdman
© 2004 ARTECH HOUSE, INC.
685 Canton Street
Norwood, MA 02062
All rights reserved. Printed and bound in the United States of America. No part of this book

List of Conferences and Meetings 10
Summary 11
References 11
Selected Bibliography 12
CHAPTER 2
Materials for MEMS 13
Silicon-Compatible Material System 13
Silicon 13
Silicon Oxide and Nitride 19
Thin Metal Films 20
Polymers 21
Other Materials and Substrates 21
Glass and Fused Quartz Substrates 21
Silicon Carbide and Diamond 22
Gallium Arsenide and Other Group III-V Compound Semiconductors 22
Polymers 23
Shape-Memory Alloys 23
Important Material Properties and Physical Effects 24
vii
Piezoresistivity 24
Piezoelectricity 26
Thermoelectricity 29
Summary 31
References 31
Selected Bibliography 32
CHAPTER 3
Processes for Micromachining 33
Basic Process Tools 34
Epitaxy 34
Oxidation 35

Single Crystal Reactive Etching and Metallization 72
Summary 74
References 75
Selected Bibliography 77
viii Contents
CHAPTER 4
MEM Structures and Systems in Industrial and Automotive Applications 79
General Design Methodology 79
Techniques for Sensing and Actuation 81
Common Sensing Methods 81
Common Actuation Methods 82
Passive Micromachined Mechanical Structures 85
Fluid Nozzles 85
Hinge Mechanisms 88
Sensors and Analysis Systems 89
Pressure Sensors 89
High-Temperature Pressure Sensors 93
Mass Flow Sensors 94
Acceleration Sensors 96
Angular Rate Sensors and Gyroscopes 104
Carbon Monoxide Gas Sensor 114
Actuators and Actuated Microsystems 116
Thermal Inkjet Heads 116
Micromachined Valves 119
Micropumps 126
Summary 128
References 129
Selected Bibliography 131
CHAPTER 5
MEM Structures and Systems in Photonic Applications 133

Summary 185
References 186
Selected Bibliography 187
CHAPTER 7
MEM Structures and Systems in RF Applications 189
Signal Integrity in RF MEMS 189
Passive Electrical Components: Capacitors and Inductors 190
Quality Factor and Parasitics in Passive Components 190
Surface-Micromachined Variable Capacitors 192
Bulk-Micromachined Variable Capacitors 195
Micromachined Inductors 197
Microelectromechanical Resonators 200
Comb-Drive Resonators 201
Beam Resonators 203
Coupled-Resonator Bandpass Filters 206
Film Bulk Acoustic Resonators 208
Microelectromechanical Switches 211
Membrane Shunt Switch 213
Cantilever Series Switch 213
Summary 214
References 214
Selected Bibliography 216
CHAPTER 8
Packaging and Reliability Considerations for MEMS 217
Key Design and Packaging Considerations 218
Wafer or Wafer-Stack Thickness 219
Wafer Dicing Concerns 219
Thermal Management 220
Stress Isolation 221
Protective Coatings and Media Isolation 222

ing in Salt Lake City in 1989 called the Micro Tele-Operated Robotics Workshop. I
was there to present an invited paper that claimed MEMS should be used to fabri
-
cate resonant structures for the purposes of timekeeping, and I was privileged to be
part of this group of visionaries for one and a half exciting days. The proceedings
may not be in print any longer. However, I recall that they were given an Institute of
Electrical and Electronic Engineers (IEEE) catalog number of 89TH0249-3. Discus
-
sion at the workshop about the name of this new field of research raged for over an
hour, and several acronyms were offered, debated, and rejected. When the dust set
-
tled, I recall that Professor Roger Howe of the University of California at Berkeley
stood up and announced, “Well, then, the name is MEMS.” In this way, the group
came to consensus. The research they conducted, unique to any currently being con-
ducted in the United States (or the world for that matter) would hereafter be known
as “MEMS.”
In those early, heady, exciting, and terribly uncertain days, many issues faced
those in the nascent field that researchers today would find hard to remember. For
example, our hearty band constantly worried if any scholarly journal would publish
the papers we wrote. Sources of research funding were hard to find and difficult to
maintain. MEMS fabrication was itself a major issue. Topics of conversation were
frequently about the nature, properties, and standardization of the polysilicon that
the pioneering band of researchers was using to demonstrate the early, elementary
structures of the day. Even the most daring and idealistic of students occasionally
turned down the offer to work with the faculty of that era: the work sometimes
appeared too farfetched for the taste of even the green-eyed zealots among the
graduate student population.
In the 10 years since the momentous events of that watershed workshop, the
National Science Foundation (NSF) funded a set of MEMS projects under its
“Emerging Technologies Initiative,” headed at the time by George Hazelrigg. NSF

-
trial career has been focused both on bringing MEMS products successfully to
market and on defending his company’s market share against encroachment by
other technologies. Because this book is written from Dr. Maluf’s practical perspec-
tive, this volume is sure to have lasting value to the myriad of engineers and execu-
tives who are struggling to find a way into the field of MEMS. This book also will
serve as a useful resource for those already in the field who wish to broaden their
expertise in MEMS fabrication. When I reviewed the manuscript, I was ready to
offer Dr. Maluf a great deal of suggestions and corrections. I was quite humbled to
realize that, instead, I was eager to have a copy of the new book on my own shelf. It
will serve as a reference for not only myself, but also the students and engineers who
frequently ask me, “What book should I buy to learn how to make MEMS?”
Albert (“Al”) P. Pisano, Ph.D.
MEMS Program Manager
DARPA
xiv Foreword
Preface
The past few years have witnessed an increasing maturity of the MEMS industry
and a rapid introduction of new products addressing applications ranging from bio
-
chemical analysis to fiber-optic telecommunications. The market size for MEMS
products has doubled in the past 5 years and is projected to grow at this fast rate for
the foreseeable future. The corresponding technology has enjoyed a fast pace of
development and has rapidly spread to institutions and companies on all inhabited
continents. A search of the keyword MEMS in all granted patents in the United
States since 1998 returns nearly 4,000 patents and references. Many devices have
left universities to go into commercial development, and several have reached the
stage of becoming products. It is therefore appropriate to extensively revise the text
to incorporate advances in the field, new products, as well as suggestions from the
readers.

the material in Chapter 8 on packaging to include packaging of optical MEMS prod
-
ucts and added an entirely new section on reliability and quality assurance. We
added several references to each chapter to direct the advanced reader to the source
of the material. We also expanded the glossary to assist the novice in understanding
and relating to a new terminology.
Many people provided us technical information and materials specifically for
the second edition of this book. We thank Bardia Pezeshki of Santur Corporation;
John (Hal) Jerman of Iolon; Asif Godil of Lightconnect; Greg Ortiz of Surface Tech
-
nology Systems; Bonnie Gray; Greg Jepson of Bullen Ultrasonics; Chris Bang and
Den Feinberg of Microfabrica; Malcom Gower of Exitech; Amy Wang; Brian Paegel
of The Scripps Research Institute; Carol Schembri and John Larson of Agilent Tech
-
nologies; Didier Lacroix and Ken Cioffi of Discera; Michael Cohn of MicroAssem
-
bly; Nelson Fuller of Alumina Micro; and Stephen Durant and Christopher Eide of
Morrison and Foerster. Evan Green and Carter Hand of New Focus were kind
enough to review portions of the manuscript. Thanks go to our editor, Mark Walsh,
for his unwavering support. Kirt Williams further thanks his former graduate advi
-
sor, Professor Richard S. Muller, for having such a profound effect on his life for
introducing him to MEMS.
xvi Preface
“It was the best of times, it was the worst of times, it was the age of wisdom, it was
the age of foolishness…” from A Tale of Two Cities by Charles Dickens, engraved
on a thin silicon nitride membrane. The entire page measures a mere 5.9 µmona
side, sufficiently small that 60,000 pages—equivalent to the Encyclopedia Britan
-
nica—can fit on a pinhead. The work, by T. Newman and R. F. W. Pease of Stan

Chapter 1—MEMS: A Technology from Lilliput. This introductory chapter
defines the scope of the technology and the applications it addresses. A short analy
-
sis of existing markets and future opportunities is also included.
Chapter 2—The Sandbox: Materials for MEMS. This chapter reviews the
properties of materials common in micromachining. The emphasis is on silicon
and materials that can be readily deposited as thin films on silicon substrates.
Three physical effects—piezoresistivity, piezoelectricity, and thermoelectricity—are
described in some detail.
Chapter 3—The Toolbox: Processes for Micromachining. Various fabrication
techniques used in semiconductor manufacturing and micromachining are intro
-
duced. These include a number of deposition and etch methods, as well as lithogra
-
phy. The discussion on etch methods covers the topics of anisotropic etching,
dependence on crystallographic planes, and deep reactive ion etching. Three com
-
plete manufacturing process flows are described at the end.
xix
Chapter 4—The Gearbox: Commercial MEM Structures and Systems. This
chapter includes descriptions of a select list of commercially available
micromachined sensors and actuators. The discussion includes the basic principle of
operation and a corresponding fabrication process for each device. Among the
devices are pressure and inertial sensors, a microphone, a gas sensor, valves, an
infrared imager, and a projection display system.
Chapter 5—The New Gearbox: A Peek into the Future. The discussion in this
chapter centers on devices and systems still under development but with significant
potential for the future. These include biochemical and genetic analysis systems,
high-frequency components, display elements, pumps, and optical switches.
Chapter 6—The Box: Packaging for MEMS. The diverse packaging require

“ And I think to myself, what a wonderful world oh yeah!”
—Louis Armstrong
The Promise of Technology
The ambulance sped down the Denver highway carrying Mr. Rosnes Avon to the
hospital. The flashing lights illuminated the darkness of the night, and the siren
alerted those drivers who braved the icy cold weather. Mrs. Avon’s voice was clearly
shaken as she placed the emergency telephone call a few minutes earlier. Her hus
-
band was complaining of severe palpitations in his heart and shortness of breath.
She sat next to him in the rear of the ambulance and held his hand in silence, but her
eyes could not hide her concern and fear. The attending paramedic clipped onto the
patient’s left arm a small device from which a flexible cable wire led to a digital dis-
play that was showing the irregular cardiac waveform. A warning sign in the upper
right-hand corner of the display was flashing next to the low blood-pressure read-
ing. In a completely mechanical manner reflecting years of experience, the para-
medic removed an adhesive patch from a plastic bag and attached it to Mr. Avon’s
right arm. The label on the discarded plastic package read “sterile microneedles.”
Then, with her right hand, the paramedic inserted into the patch a narrow plastic
tube, while the fingers of her left hand proceeded to magically play the soft keys on
the horizontal face of an electronic instrument. She dialed in an appropriate dosage
of a new drug called Nocilis™. Within minutes, the display was showing a recover
-
ing cardiac waveform, and the blood pressure warning faded in the dark green color
of the screen. The paramedic looked with a smile at Mrs. Avon, who acknowledged
with a deep sigh of relief.
Lying in his hospital bed the next morning, Mr. Avon was slowly recovering
from the disturbing events of the prior night. He knew that his youthful days were
behind him, but the news from his physician that he needed a pacemaker could only
cause him anguish. With an electronic stylus in his hand, he continued to record his
thoughts and feelings on what appeared to be a synthetic white pad. The pen recog

into text. Another small accelerometer embedded in his pacemaker would enable him
to play tennis again. He also could write and draw at will because the storage capac-
ity of his disk drive was enormous, thanks to miniature read and write heads. And
finally, as the patient went to sleep, an array of micromirrors projected a pleasant
high-definition television image onto a suspended screen.
Many of the miniature devices listed in this story, in particular the pressure,
acceleration, and yaw-rate microsensors and the micromirror display, already exist
as commercial products. Ongoing efforts at many companies and laboratories
throughout the world promise to deliver, in the not-too-distant future, new and
sophisticated miniature components and microsystems. It is not surprising, then,
that there is widespread belief in the technology’s potential to penetrate in the future
far-reaching applications and markets.
What Are MEMS—or MST?
In the United States, the technology is known as microelectromechanical systems
(MEMS); in Europe, it is called microsystems technology (MST). A question asking
for a more specific definition is certain to generate a broad collection of replies with
few common characteristics other than “miniature.” But such apparent divergence
in the responses merely reflects the diversity of applications this technology enables,
rather than a lack of commonality. MEMS is simultaneously a toolbox, a physical
product, and a methodology, all in one:
•
It is a portfolio of techniques and processes to design and create miniature
systems.
•
It is a physical product often specialized and unique to a final application—
one can seldom buy a generic MEMS product at the neighborhood electronics
store.
2 MEMS: A Technology from Lilliput
•
“MEMS is a way of making things,” reports the Microsystems Technology

to provide self diagnostics and a digital output. It is anticipated that the next
generation of devices will also incorporate the entire airbag deployment circuitry
that decides whether to inflate the airbag. As the technology matures, the airbag
crash sensor may be integrated one day with micromachined yaw-rate and other
inertial sensors to form a complete microsystem responsible for passenger safety
and vehicle stability.
Examples of future microsystems are not limited to automotive applications
(see Table 1.1). Efforts to develop micromachined components for the control of
fluids are just beginning to bear fruit. These could one day lead to the integration
of micropumps with microvalves and reservoirs to build new miniature drug
delivery systems.
What Is Micromachining?
Micromachining is the set of design and fabrication tools that precisely machine and
form structures and elements at a scale well below the limits of our human percep
-
tive faculties—the microscale. Micromachining is the underlying foundation of
MEMS fabrication; it is the toolbox of MEMS.
What Is Micromachining? 3
Arguably, the birth of the first micromachined components dates back many
decades, but it was the well-established integrated circuit industry that indirectly
played an indispensable role in fostering an environment suitable for the develop
-
ment and growth of micromachining technologies. As the following chapters will
show, many tools used in the design and manufacturing of MEMS products are
“borrowed” from the integrated circuit industry. It should not then be surprising
that micromachining relies on silicon as a primary material, even though the tech
-
nology has certainly been demonstrated using other materials.
Applications and Markets
Present markets are primarily in pressure and inertial sensors, inkjet print heads

Low-power, high-density mass data storage devices
Embedded sensors and actuators for condition-based maintenance
Integrated fluidic systems for miniature propellant and combustion control
Miniature fluidic systems for early detection of threats from biological and
chemical agents
Electromechanical signal processing for small and low-power wireless
communication
Active, conformable surfaces for distributed aerodynamic control of aircraft