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1
Introduction
Microelectromechanical systems (MEMS) is a loosely defined term for man-
made mechanical components that are characterized by small size. Translated
literally, MEMS should have dimensions in the micron-scale and have both
electrical and mechanical components that form a system. Many MEMS devices
do not meet these requirements. For example, microfluidic channels may not
have any electrical components. In Europe, MEMS is often called microsystems.
This term may be more accurate but MEMS is more catchy and is used in United
States, Asia, and increasingly in Europe.
While the exact definition of MEMS is difficult to formalize, most peo-
ple agree on the idea of MEMS. Typically, the MEMS device introduces a
paradigm shift in manufacturing and/or application. For example, miniature
silicon accelerometers have largely replaced the costly macroscopic piezoelectric
accelerometers. The MEMS accelerometers are smaller, but their real advantage
is the manufacturing process that utilizes the batch fabrication processes origi-
nally developed for the integrated circuit technology. Batch fabrication enables
simultaneous processing of thousands of identical devices on a single wafer.
This is in contrast to the traditional series manufacturing one device at the
time. Batch fabrication has made the accelerometers economical, and with the
lower cost of silicon accelerometers, the use of inertial sensors has widened first
in the automotive industry and more recently in the consumer market.
In addition to providing a cheaper and/or better alternative to the existing
technology, MEMS has enabled completely new devices: Inkjet print heads have
made low-cost color printing a reality. Micromirror arrays containing more than
one million individual mirrors were developed for high definition television, and
are used in data projectors in offices, classrooms, auditoriums, and in homes
for video games and home theaters.
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2
INTRODUCTION
1.2
This chapter gives an overview of the MEMS industry, the history, the type
of devices on market, and the fabrication methods used to make them. The
overview will pave the way for the detailed device studies in the subsequent
chapters.
1.1 History of MEMS
The history of micromachining is tied to the development of integrated circuit
(IC) technology. Starting in the 1960s, researchers experimented with using IC
fabrication technologies (for example lithography, silicon etching, and thin film
growth) to make mechanical structures. Some of the early devices such as the
resonant gate transistor [1] were not commercially successful but the work lead
to a commercial adoption of pressure sensors and accelerometers in the 1970s.
Many of the early processes and applications are documented in the classical
paper by Petersen [2].
The significant research and development effort in the 1980s and 1990s have
lead to new fabrication technologies, devices, and markets for MEMS. Notably,
surface micromachining has enabled integration of mechanical components with
the integrated circuits leading to low cost accelerometers and micromirror ar-
rays. Commercialization of the MEMS technologies has finally started to impact
society on a larger scale. Today, most consumers have, knowingly or unknow-
ingly, encountered MEMS based products.
Building on the technological advancements, MEMS market is currently
growing by all measures. The number of MEMS devices sold is increasing and
new products are coming out every year. Devices such as microphones that once
looked too expensive to implement with microfabrication technologies are sold
by millions [3]. While the future for the MEMS is bright, it remains to be seen
whether the technology will saturate or if new manufacturing processes and
applications will be invented to further drive the development.
1.2 MEMS applications are diverse
The MEMS market size is currently over $6B per year, but as shown in Fig-
ure 1.1, only a few devices are genuinely mass produced. The oldest application,
pressure sensors, commands revenue of over $500M per year. As the price and
size of pressure sensors keep decreasing, new applications emerge. For example,
integration of pressure sensors inside hypothermic needles [4] or sport watch is
possible due to minute size of MEMS pressure sensors.
The other major sensor market, the inertial sensors, has historically been
dominated by the automotive industry. Recently, the reduction in price has en-
abled adoption of MEMS accelerometers in consumer devices such as orientation
sensors in digital cameras and game console user interfaces. Gyroscopes have
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1.2
MEMS APPLICATIONS ARE DIVERSE
3
Emerging applications:
• RF resonators
• RF switches
• Lab-on-a-chip
• Drug delivery systems
• Optical switches
• Microspectrometers
Figure 1.1: The MEMS market is dominated by few applications all generating rev-
enues over $500M/year. Emerging device fields currently generate revenue less than
$100M/year but show huge growth potentials.
also entered into mass production and show double-digit growth in revenue.
The main markets for the gyroscopes are the automotive industry, inertial nav-
igation to aid GPS, and image stabilization for digital cameras. It is expected
that the gyroscopes will also be utilized to enhance the human-computer user
interface.
For average consumers, the inkjet print heads may be the most familiar mi-
crodevice. Each replacement inkjet cartridge has a micromachined inkjet nozzle
head. The inkjet print heads are frequently regarded as the largest MEMS mar-
ket in terms of revenue. However, as the inkjet manufacturers sell the MEMS
component as a part of the printer system, it is difficult to attach an accurate
dollar value to the MEMS component portion of the inkjet market. Regardless,
the inkjet revenue is measured in hundreds of millions.
The lucrative digital microdisplay (DMD) market is dominated by a single
manufacturer, Texas Instruments, who holds the key patents in the field. In
projection displays, the high contrast ratio of mechanically actuated mirrors
enables the micromirrors to compete against the more common LCD technology.
The MEMS displays are a unique MEMS product in that they contain millions
of moving structures. Fabrication on this scale would not be possible without
batch fabrication methods.
Another more recent display product is the reflection based display for
portable devices introduced by Qualcomm. The device is based on interfer-
ometry, and unlike LCD displays, it does not require any back light. Other
optical MEMS devices, such as switches for fiber optical communications, hold
promise but may take several years to gain acceptance.
Silicon microphones are the latest entry to the mass market. The growth is
driven by cell phone industry that demands solderability – a characteristic not
met by otherwise excellent traditional electret microphones. The microphones
are an encouraging example of a MEMS product that only a few years ago was
deemed too expensive but is now rapidly gaining market share.
Beyond these established markets, a number of MEMS devices hold promise.
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4
INTRODUCTION
1.3
The optical switches were regarded the next killer application but the collapse
of several network companies in 2001-2002 has dampened the interest in optical
networking. Radio frequency (RF) switches is an interesting application as mi-
cromechanical switches offer performance advantages over solid-state devices.
Currently, the MEMS switches are considered for radars and test equipment
where the high performance is needed. Adoption by the cell phone industry
appears likely but lifetime and cost issues remain. The low cost and the small
size of RF microresonators have also raised interest. This market is attractive
as the revenue is high, but in terms of power handling and signal-to-noise ratio,
the miniature resonators are not as robust as the established macroscopic res-
onators. Biomedical devices are appealing, as the small size naturally interfaces
well with biological systems. It remains to be seen whether these or some other
application will break the $100M barrier.
1.3 MEMS fabrication is based on batch processing
Microfabrication has historically been tied to integrated circuit (IC) fabrication
and most of the processing tools and terminology have been adopted directly
from IC manufacturing. The parallels are so deep that sometimes a retiring IC
manufacturing plant is converted to MEMS fabrication that does not require
the latest fabrication technology. But the MEMS fabrication technology is not
easy to master. The MEMS specific challenges include packaging of movable
mechanical structures, manufacturing of thick structures, and obtaining good
absolute dimensional control.
The focus of this book is on device design but some exposure to fabrication
technologies is necessary in order to understand the limitations of the tech-
nology. In other words, the understanding of the fabrication is not required to
explain how a particular MEMS device operates but it explains why the device
looks the way it does. The overview given here is enough to explain the general
fabrication steps for the devices covered in this book. To supplement the fab-
rication overview in this book, there are a number of good introductory [5–8]
and advanced [9,10] books about microfabrication. In addition, microfabrication
has been reviewed in several journal papers that provide concise introduction
to the field [2, 3, 11, 12]. These books and review papers combined with the
large choice of IC fabrication textbooks [7, 13, 14], provide a solid foundation
for micromanufacturing and process integration.
The batch fabrication is a radical departure from the traditional series man-
ufacturing, and is well suited for making relatively simple, mechanical compo-
nents on a large scale. As illustrated in Figure 1.2, individual devices are pho-
tolithographically defined onto a wafer using a photomask and ultraviolet light.
The batch fabrication process, specifically the use of photolithography, allows
the defining of any shape on the surface of the wafer but it is difficult to fab-
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1.3
MEMS FABRICATION IS BASED ON BATCH PROCESSING
5
Figure 1.2: An illustration of the batch fabrication. Thousands of components can
simultaneously be defined on a single wafer using photolithography.
ricate truly three-dimensional shapes. The process can be compared to carving
shapes out of cardboard. Due to fabrication limitations, the MEMS components
often look flat or two-dimensional.
The cost for processing the MEMS wafer does not depend on the number
of devices on it and the batch fabrication is an economical way to make a large
number of devices. On a typical wafer, there can be thousands of devices and
even a small MEMS foundry can fabricate components by millions. Once all the
processing steps have been completed, the wafer is diced into individual pieces
or dies. Finally, the dies are packaged, often together with an IC.
Numerous MEMS fabrication processes have been developed. Traditionally,
the MEMS processes have been divided into surface micromachining and bulk
micromachining. We will review these two technologies in the following sections.
1.3.1 Surface micromachining makes thin structures
Surface micromachining is based on patterning thin-films on top of a substrate
wafer [11]. The surface micromachined structures are relatively flat which sim-
plifies the subsequent wafer processing. A typical fabrication process is illus-
trated in Figure 1.3 where steps of thin-film deposition followed by selective
etching are repeated to form semi-3D structures. The thickness of each layer
may vary but it is typically less than 5 µm. The simplest structures, such as
accelerometers, have two structural layers and one sacrificial layer as shown
in Figure 1.3. The record in complexity is the five structural layer process by
Sandia National Laboratories that was developed for complex moving devices
(microengines and gears) [15].
The surface micromachining resembles the traditional IC manufacturing
that is also based on processing thin-films on a silicon wafer. The compatibility
with the IC processing is one of the main advantages of surface micromachining,
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6
INTRODUCTION
1.3
(a) Surface micromachin-
ing starts with a substrate
wafer, typically silicon with
diameter of 100-200 mm and
thickness of 500-700 µm.
(b) A sacrificial layer is de-
posited or grown on the
wafer. Silicon dioxide with
thickness of 1-2 µm is com-
monly used.
(c) A hole is made by lithog-
raphy and etching of the
sacrificial layer. The small-
est dimensions are usually 2-
3 µm.
(d) A structural layer, typi-
cally 1-5 µm polycrystalline
silicon is deposited.
(e) The structural
layer is
defined using lithography
and etching.
(f) The structure is released
by removing the sacrificial
layer by etching.
Figure 1.3: Typical surface micromachining process involves combinations of layer
depositions, optical lithography, and etches to fabricate thin microstructures.
as it is relatively easy to integrate surface micromachining with IC:s to combine
mechanical and electrical components on the same chip. The single-chip inte-
gration may lead to better performance and reduced packaging cost especially
if a large number of connections between the mechanical and electrical parts
are needed. For example, the realization of micromirror arrays with more than
a million individually controlled mirrors would not be possible without on-chip
control for the individual mirrors.
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1.4
MEMS FABRICATION IS BASED ON BATCH PROCESSING
7
1.3.2 Bulk micromachining makes thick structures
Unlike surface micromachining, which is based on thin-film deposition on top of
a substrate wafer, bulk micromachining defines structures by selectively etching
the substrate [12]. This can result in relatively thick structures; the typical
wafer thickness of 500-700 µm is about 100 times the typical thickness for
surface micromachines. The large thickness is useful, for example, in inertial
sensors that benefit from a large mass. In addition to the thickness, the bulk
micromachined structures can be made of single-crystal silicon as opposed to
amorphous or polycrystalline thin-films. The predictable and stable material
parameters of crystalline silicon are desirable for mechanical sensors.
Bulk micromachined accelerometers and pressure sensors were the first com-
mercialized MEMS products. These devices have been hugely successful, for
example, MEMS pressure sensors represent over 90% of all sold pressure sensor
units [3]. Early bulk micromachined devices were made by wet etching [16] and
several exotic processes have been developed. For example, thin membranes for
pressure sensors have been defined using epitaxial growth of silicon combined
with electrochemical etch stop [2]. The wet processes are still used but ad-
vances in plasma processing have made dry etching the mainstream. Especially
the combination of deep reactive ion etching (DRIE) and silicon on insulator
(SOI) technology has simplified the bulk manufacturing and reduced the device
size [17–19].
DRIE enables etching narrow channels through the entire wafer and results
in almost vertical sidewalls. It is possible to make channels with aspect ratio
of 50 to 1 or better, meaning that a 500 µm deep trench can be only 10 µm
wide. In contrast to the wet etching where the depth and width of a trench
are typically equal, the reduction of device size can be significant and more
devices are obtained from a single wafer. As the cost of processing one wafer
is approximately constant, the smaller device size directly reduces the device
cost.
Figure 1.4 shows a possible process to make accelerometers using SOI wafers,
DRIE etching, and wafer bonding. The final structure is hermetically sealed at
the wafer level, which greatly reduces the cost of final assembly and packaging.
The manufacturing process is quite straightforward and results in a compact
structure with well-defined features.
The SOI wafers used in MEMS are manufactured by bonding two silicon
wafers together with a 1-2 µm layer of silicon dioxide in between them. The
silicon dioxide acts as a natural etch stop and all etched structures have the
desired thickness determined by the SOI thickness. The silicon dioxide can also
be used as a sacrificial layer for making free structures of single-crystal silicon.
As two silicon wafers are used for making one SOI wafer, the SOI wafers are
more expensive than bare silicon wafers. However, the material cost increase is
compensated by the processing costs savings.
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8
INTRODUCTION
1.4
(a) The process is started
with a SOI wafer with
substrate thickness of 550-
650 µm and SOI thickness
of 10-20 µm.
(b) A 2 µm recess is etched
on front and back to define
where movable structure is.
(c) Trenches are etched us-
ing DRIE on the back of the
wafer to define the sensing
mass. The oxide stops the
etch.
(d) Final DRIE on the front
of the wafer defines the sup-
porting beams that hold the
mass.
(e) The device is completed by bonding wafers on
front and back to make a hermetically shielded ac-
celerometer. After dicing, the dies are ready for final
assembly.
Figure 1.4: Bulk micromachining process for making a silicon accelerometer.
1.4 Introduction to the Practical MEMS book
This book is focused on in-depth analysis of microdevice operation. The em-
phasis sets the Practical MEMS book apart from other MEMS books that cover
both the fabrication and device operation. The integrated approach of includ-
ing both fabrication and analysis has merits and an integrated textbook is a
good first introduction to the microsystems. However, the depth of analysis in
textbooks that cover both fabrication and applications is limited to describing
the device operation.
This book goes further into exploring why certain devices are successful and
others have failed. The first part of this book covers the traditional microsensors
(accelerometers and pressure sensors) that are a major and growing commercial
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1.4
INTRODUCTION TO THE PRACTICAL MEMS BOOK
9
microsystem application. The emphasis is on measuring small signals that is
a fundamental challenge when making small sensor systems. Since ability to
do simplified analytical design analysis is invaluable in the early stage of any
sensor design, the physical principles behind the sensor operation are illustrated
by numerous calculated examples. These examples are carefully chosen to both
illuminate the device operation and to quantify the performance trade-offs in
the microsensors.
The second half of the book introduces actuators. The merits of different
actuation schemes are illustrated by developing scaling laws for different actu-
ation schemes. Capacitive, thermal, piezoelectric actuation theory is developed
and illustrated with examples. Applications ranging from optical, RF, and sens-
ing (gyroscopes) are explored with emphasis on critical evaluation of whether
MEMS has a competitive advantage to replace the current technologies. Again,
the physical challenges of miniaturization are illustrated with several calculated
examples. For example, the effect of mirror size is studied in optical MEMS ap-
plications such as optical displays and microscanners.
Finally, the book is concluded with an introduction to MEMS fabrication
economics. The cost, yield, and profits in batch fabrication are investigated.
Several case studies are used to illustrate the challenges of making a profit with
microfabrication.
Key concepts
• MEMS stands for microelectromechanical system. Europeans prefer the
shorter and often more accurate word microsystem.
• Microdevices have been developed since the 1960s. Since the 1990s, the
field has been growing rapidly.
• Pressure sensors, accelerometers, optical displays, and inkjet printers are
established commercial MEMS applications.
• Optical networking and RF MEMS hold commercial promise but have
not yet become significant industry.
• Batch fabrication enables fabrication of thousands or even millions of
identical mechanical components on a single wafer
• Optical lithography is used to define the shape of the structures. Large
number of devices can be made using the same optical mask.
• Surface micromachining is based on processing and patterning thin-films
on a wafer.
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10
INTRODUCTION
1.4
• Bulk micromachining is based on etching the wafer to make relatively
thick structures.
Exercises
Exercise 1.1
Download or copy from the library the classical review paper from 1982 by
Petersen [2] and recent review by Bryzek et al. [3] and answer to the following
questions: 1. How does the MEMS fabrication processes in the papers differ? 2.
List applications that are highlighted in papers. How does applications in the
two papers differ and is this difference reflected in manufacturing processes?
Exercise 1.2
Using your favorite search engine, find at least three estimates for the world
wide MEMS market size. Comment on how reliable you feel the sources are and
whether there is discrepancy between the estimates.
Exercise 1.3
Using your favorite search engine, find an estimate of the world wide market
size for integrated circuits (ICs) and compare it to the MEMS market size.
Noting that silicon microcircuits and silicon microsensors were invented around
the same time in the 1960s, think of reasons that could explain the difference
in the market size.
Exercise 1.4
Why surface micromachining is more compatible with integrated circuit fabri-
cation than bulk micromachining?
Exercise 1.5
List MEMS applications that you have personally encountered.
Exercise 1.6
Why optical lithography is important in microfacrication?
Exercise 1.7
The number of citations is a relatively objective way to judge the importance
of academic publications. Most publications are cited less than ten times, the
papers that resonate well with the academic community will be cited more than
30 times, and seminal papers receive over a hundred citations.
Google Scholar (scholar.google.com) is a free tool to search scholarly
literature. Google Scholar will also give an estimate of the number of citations
for each search result has received. The more cited articles will have a higher
ranking and will appear first. For example, search “RF MEMS” will display
the article by C. Goldsmith, et al. titled “Performance of low-loss RF MEMS
capacitive switches,” that has been cited more than 200 times.
Try out Google Scholar to find a highly cited journal paper on: i. MEMS
accelerometer, ii. MEMS pressure sensor, iii. MEMS gyroscope, iv. RF MEMS
Copyright Ville Kaajakari (ville@kaajakari.net)
Downloaded from: www.kaajakari.net/PracticalMEMS
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1.4
INTRODUCTION TO THE PRACTICAL MEMS BOOK
11
switch, v. optical MEMS, vi. MEMS microphone, and vii. MEMS inkjet print
head. Note the number of citations manuscripts in different applications have
received and compare the search results. How does the importance to the aca-
demic community correlate with the commercial interest? Note that in old ap-
plications such as accelerometers the word MEMS may not appear in the article.
Other relevant keywords include silicon and solid-state.
Copyright Ville Kaajakari (ville@kaajakari.net)
Downloaded from: www.kaajakari.net/PracticalMEMS
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