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An introduction to mems
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Prime Faraday Technology Watch
ISBN 1-84402-020-7 An Introduction to MEMS January 2002
An Introduction to MEMS (Micro-electromechanical Systems)
MEMS has been identified as one of the most promising technologies for
the 21
st
Century and has the potential to revolutionize both industrial and
consumer products by combining silicon-based microelectronics with
micromachining technology. Its techniques and microsystem-based
devices have the potential to dramatically affect of all of our lives and the
way we live.
This report presents a general introduction to the field of MEMS, with
emphasis on its commercial applications and device fabrication methods.
It also describes the range of MEMS sensors and actuators, the
phenomena that can be sensed or acted upon with MEMS devices, and
outlines the major challenges facing the industry.
PRIME Faraday Partnership
PRIME Faraday Partnership
This title is for sale in paperback at Amazon.co amazon.co/exec/obidos/ASIN/ Technology Watch titles are written for managers, especially in small and medium-sized manufacturing companies. They offer a practical introduction to cutting-edge developments that affect – or likely soon will affect – the design, development, manufacture and marketing of PRIME products – products with interdependent mechanical and electronic (and possibly software) parts. All Technology Watch titles can be downloaded free of charge from the Prime Faraday Partnership’s Technology Watch website primetechnologywatch.org/. Selected titles can be purchased in paperback from Amazon.co. In addition to market and technology reviews, the Technology Watch website also provides news cuttings, case studies, an events diary and details of funding opportunities. The service is sponsored by the DTI and managed by the PRIME Faraday Partnership, which marries the academic strengths of Loughborough University and the University of Nottingham to the technology-transfer expertise of Pera.
Prime Faraday Technology Watch – January 2002 iii
Contents
- Introduction...................................................................................... Page
- Micro-electromechanical Systems (MEMS)................................................
- 2 What is MEMS?..............................................................................
- 2 Definitions and Classifications............................................................
- 2 History.......................................................................................
- 2 Applications..................................................................................
- 2.4 Established MEMS Applications...........................................................
- 2.4 New MEMS Applications..................................................................
- 2 MEMS Market..............................................................................
- 2 Miniaturization Issues.....................................................................
- MEMS Fabrication Methods.................................................................
- 3 Photolithography...........................................................................
- 3 Materials for Micromachining............................................................
- 3.2 Substrates....................................................................................
- 3.2 Additive Films and Materials..............................................................
- 3 Bulk Micromachining......................................................................
- 3.3 Wet Etching..................................................................................
- 3.3 Dry Etching..................................................................................
- 3 Surface Micromachining..................................................................
- 3.4 Fusion Bonding..............................................................................
- 3 High-Aspect-Ratio-Micromachining....................................................
- 3.5 LIGA..........................................................................................
- 3.5 Laser Micromachining.....................................................................
- 3 Computer Aided Design...................................................................
- 3 Assembly and System Integration........................................................
- 3 Packaging....................................................................................
- 3.8 Multi-Chip Modules........................................................................
- 3.8 Passivation and Encapsulation............................................................
- 3 Foundry Services...........................................................................
- MEMS Transducers...........................................................................
- 4 Mechanical Transducers...................................................................
- 4.1 Mechanical Sensors.........................................................................
- 4.1 Mechanical Actuators......................................................................
- 4 Radiation Transducers.....................................................................
- 4.2 Radiation Sensors...........................................................................
- 4.2 Radiation (Optical) Actuators.............................................................
- 4 Thermal Transducers.......................................................................
- 4.3 Thermal Sensors.............................................................................
- 4.3 Thermal Actuators...........................................................................
- 4 Magnetic Transducers.....................................................................
- 4.4 Magnetic Sensors...........................................................................
- 4.4 Magnetic Actuators.........................................................................
- 4 Chemical and Biological Transducers...................................................
- 4.5 Chemical and Biological Sensors.........................................................
- 4.5 Chemical Actuators.........................................................................
- 4 Microfluidic Devices......................................................................
- Future of MEMS............................................................................... Prime Faraday Technology Watch – January 2002 iv
- 5 Industry Challenges.......................................................................
- 5 The Way Ahead...........................................................................
- References..............................................................................................
- Appendix A Glossary of Terms.................................................................
- Appendix B Sources of MEMS Information and Advice.................................
MEMS, an acronym that originated in the United States, is also referred to as Microsystems
Technology (MST) in Europe and Micromachines in Japan. Regardless of terminology, the
uniting factor of a MEMS device is in the way it is made. While the device electronics are
fabricated using ‘computer chip’ IC technology, the micromechanical components are
fabricated by sophisticated manipulations of silicon and other substrates using
micromachining processes. Processes such as bulk and surface micromachining, as well as
high-aspect-ratio micromachining (HARM) selectively remove parts of the silicon or add
additional structural layers to form the mechanical and electromechanical components. While
integrated circuits are designed to exploit the electrical properties of silicon, MEMS takes
advantage of either silicon’s mechanical properties or both its electrical and mechanical
properties.
In the most general form, MEMS consist of
mechanical microstructures, microsensors,
microactuators and microelectronics, all integrated
onto the same silicon chip. This is shown
schematically in Figure 1.
Microsensors detect changes in the system’s
environment by measuring mechanical, thermal,
magnetic, chemical or electromagnetic information
or phenomena. Microelectronics process this
information and signal the microactuators to react
and create some form of changes to the environment.
MEMS devices are very small; their components are usually microscopic. Levers, gears,
pistons, as well as motors and even steam engines have all been fabricated by MEMS (Figure
2). However, MEMS is not just about the miniaturization of mechanical components or
making things out of silicon (in fact, the term MEMS is actually misleading as many
micromachined devices are not mechanical in any sense). MEMS is a manufacturing
technology; a paradigm for designing and creating complex mechanical devices and systems
as well as their integrated electronics using batch fabrication techniques.
Figure 1. Schematic illustration of
MEMS components.
Figure 2. (a) A MEMS silicon motor together with a strand of human hair [1], and (b)
the legs of a spider mite standing on gears from a micro-engine [2 - Sandia National
Labs, SUMMiT *Technology, mems.sandia].
From a very early vision in the early 1950’s, MEMS has gradually made its way out of
research laboratories and into everyday products. In the mid-1990’s, MEMS components
began appearing in numerous commercial products and applications including accelerometers
used to control airbag deployment in vehicles, pressure sensors for medical applications, and
inkjet printer heads. Today, MEMS devices are also found in projection displays and for
micropositioners in data storage systems. However, the greatest potential for MEMS devices
lies in new applications within telecommunications (optical and wireless), biomedical and
process control areas.
MEMS has several distinct advantages as a manufacturing technology. In the first place, the
interdisciplinary nature of MEMS technology and its micromachining techniques, as well as
its diversity of applications has resulted in an unprecedented range of devices and synergies
across previously unrelated fields (for example biology and microelectronics). Secondly,
MEMS with its batch fabrication techniques enables components and devices to be
manufactured with increased performance and reliability, combined with the obvious
advantages of reduced physical size, volume, weight and cost. Thirdly, MEMS provides the
basis for the manufacture of products that cannot be made by other methods. These factors
make MEMS potentially a far more pervasive technology than integrated circuit microchips.
However, there are many challenges and technological obstacles associated with
miniaturization that need to be addressed and overcome before MEMS can realize its
overwhelming potential.
2 Definitions and Classifications............................................................
This section defines some of the key terminology and classifications associated with MEMS.
It is intended to help the reader and newcomers to the field of micromachining become
familiar with some of the more common terms. A more detailed glossary of terms has been
included in Appendix A.
Figure 3 illustrates the classifications of microsystems technology (MST). Although MEMS
is also referred to as MST, strictly speaking, MEMS is a process technology used to create
these tiny mechanical devices or systems, and as a result, it is a subset of MST.
Figure 3. Classifications of microsystems technology [3].
1960’s
1961 First silicon pressure sensor demonstrated
1967 Invention of surface micromachining. Westinghouse creates the Resonant Gate
Field Effect Transistor, (RGT). Description of use of sacrificial material to free
micromechanical devices from the silicon substrate.
1970’s
1970 First silicon accelerometer demonstrated
1979 First micromachined inkjet nozzle
1980’s
Early 1980’s: first experiments in surface micromachined silicon. Late 1980’s:
micromachining leverages microelectronics industry and widespread
experimentation and documentation increases public interest.
1982 Disposable blood pressure transducer
1982 “Silicon as a Mechanical Material” [9]. Instrumental paper to entice the scientific
community – reference for material properties and etching data for silicon.
1982 LIGA Process
1988 First MEMS conference
1990’s
Methods of micromachining aimed towards improving sensors.
1992 MCNC starts the Multi-User MEMS Process (MUMPS) sponsored by Defense
Advanced Research Projects Agency (DARPA)
1992 First micromachined hinge
1993 First surface micromachined accelerometer sold (Analog Devices, ADXL50)
1994 Deep Reactive Ion Etching is patented
1995 BioMEMS rapidly develops
2000 MEMS optical-networking components become big business
2 Applications..................................................................................
Today, high volume MEMS can be found in a diversity of applications across multiple
markets (Table 1).
Table 1. Applications of MEMS [10].
Automotive Electronics Medical Communications Defence
Internal
navigation
sensors
Disk drive heads
Blood pressure
sensor
Fibre-optic
network
components
Munitions
guidance
Air conditioning
compressor
sensor
Inkjet printer
heads
Muscle
stimulators & drug
delivery systems
RF Relays,
switches and
filters
Surveillance
Brake force
sensors &
suspension
control
accelerometers
Projection
screen
televisions
Implanted
pressure sensors
Projection
displays in
portable
communications
devices and
instrumentation
Arming systems
Fuel level and
vapour pressure
sensors
Earthquake
sensors
Prosthetics Voltage controlled
oscillators (VCOs)
Embedded
sensors
Airbag sensors
Avionics
pressure
sensors
Miniature
analytical
instruments
Splitters and
couplers
Data storage
"Intelligent" tyres
Mass data
storage systems Pacemakers Tuneable lasers Aircraft control
As an emerging technology MEMS products are centred around technology-product
paradigms rather than product-market paradigms. Consequently, a MEMS device may find
numerous applications across a diversity of industries. For example, the MEMS inkjet printer
head nozzle in widespread use today has developed from a nozzle originally used in nuclear
separation. The commercialisation of selected MEMS devices is illustrated in Table 2.
Table 2. Commercialisation of selected MEMS devices [11].
Product Discovery Evolution
Cost Reduction/
Application
Expansion
Full
Commercialisation
Pressure sensors 1954-1960 1960-1975 1975-1990 1990-present
Accelerometers 1974-1985 1985-1990 1990-1998 1998
Gas sensors 1986-1994 1994-1998 1998-2005 2005
Valves 1980-1988 1988-1996 1996-2002 2002
Nozzles 1972-1984 1984-1990 1990-1998 1998
Photonics/displays 1980-1986 1986-1998 1998-2004 2004
Bio/Chemical sensors 1980-1994 1994-1999 1999-2004 2004
RF switches 1994-1998 1998-2001 2001-2005 2005
Rate (rotation) sensors 1982-1990 1990-1996 1996-2002 2002
Micro relays 1977-1982 1993-1998 1998-2006 2006
It is not within the scope of this report to detail all the current and potential applications
within each market segment. Instead, a selection of the most established MEMS devices is
detailed along with the most potentially significant future applications.
and navigation control systems, vibration monitoring, fuel sensors, noise reduction, rollover
detection, seatbelt restraint and tensioning etc. As a result, the automotive industry has
become one of the main drivers for the development of MEMS for other equally demanding
environments. Some of the leading airbag accelerometer manufacturers include Analog
Devices, Motorola, SensorNor and Nippondenso.
Accelerometers are not just limited to automotive applications. Earthquake detection, virtual
reality video games and joysticks, pacemakers, high performance disk drives and weapon
systems arming are some of the many potential uses for accelerometers.
ii) Medical pressure sensor
Another example of an extremely successful MEMS application is the miniature disposable
pressure sensor used to monitor blood pressure in hospitals. These sensors connect to a
patients intravenous (IV) line and monitor the blood pressure through the IV solution. For a
fraction of their cost ($10), they replace the early external blood pressure sensors that cost
over $600 and had to be sterilized and recalibrated for reuse. These expensive devices
measure blood pressure with a saline-filled tube and diaphragm arrangement that has to be
connected to an artery with a needle.
Figure 6. Schematic illustration of a piezoresistive pressure sensor.
The disposable sensor consists of a silicon substrate which is etched to produce a membrane
and is bonded to a substrate (Figure 6). A piezoresistive layer is applied on the membrane
surface near the edges to convert the mechanical stress into an electrical voltage. Pressure
corresponds to deflection of the membrane. The sensing element is mounted on a plastic or
ceramic base with a plastic cap over it, designed to fit into a manufacturer’s housing (Figure
7). A gel is used to separate the saline solution from the sensing element.
As in the case of the MEMS airbag sensor, the disposable blood pressure sensor has been one
of the strongest MEMS success stories to date. The principal manufacturers being Lucas
Novasensor, EG & G IC Sensors and Motorola with over 17 millions units per year. More
recently, the technology from the blood pressure sensor has been taken a step further in the
development of the catheter-tip pressure sensor. This considerably smaller MEMS device is
designed to fit on the tip of a catheter and measure intravascular pressure (its size being only
0 mm x 0 mm x 0 mm).
Pressure sensors are the biggest medical MEMS application to date with the accelerometer
MEMS a distant second. Although the majority of these accelerometer applications remain
under development, advanced pacemaker designs include a MEMS accelerometer device that
measures the patient’s activity. The technology, similar to that found in the airbag sensor,
enables the patient’s motion and activity to be monitored and signals the pacemaker to adjust
its rate accordingly.
iii) Inkjet printer head
One of the most successful MEMS applications is the inkjet printer head, superseding even
automotive and medical pressure sensors. Inkjet printers use a series of nozzles to spray
drops of ink directly on to a printing medium. Depending on the type of inkjet printer the
droplets of ink are formed in different ways; thermally or piezoelectrically.
Invented in 1979 by Hewlett-Packard, MEMS thermal inkjet printer head technology uses
thermal expansion of ink vapour. Within the printer head there is an array of tiny resistors
known as heaters. These resistors can be fired under microprocessor control with electronic
pulses of a few milliseconds (usually less than 3 microseconds). Ink flows over each resistor,
which when fired, heat up at 100 million ºC per second, vaporizing the ink to form a bubble.
As the bubble expands, some of the ink is pushed out of a nozzle within a nozzle plate,
landing on the paper and solidifying almost instantaneously. When the bubble collapses, a
vacuum is created which pulls more ink into the print head from the reservoir in the cartridge
(Figure 8). It is worth noting there are no moving parts in this system (apart from the ink
itself) illustrating that not all MEMS devices are mechanical.
Figure 8. Thermal inkjet print technology [16].
Figure 7. (a) Disposable blood pressure sensor connected to an IV line [14],
(b) disposable blood pressure sensors (as shipped) [15], and (c) intracardial
catheter-tip sensors for monitoring blood pressure during cardiac
catheterisation, shown on the head of a pin [13].
2.4 New MEMS Applications
The experience gained from these early MEMS applications has made it an enabling
technology for new biomedical applications (often referred to as bioMEMS) and wireless
communications comprised of both optical, also referred to as micro-optoelectromechanical
systems (MOEMS), and radio frequency (RF) MEMS.
i) BioMEMS
Over the past few years some highly innovative products have emerged from bioMEMS
companies for revolutionary applications that support major societal issues including DNA
sequencing, drug discovery, and water and environmental monitoring. The technology
focuses on microfluidic systems as well as chemical testing and processing and has enabled
devices and applications such as ‘lab-on-a-chip’, chemical sensors, flow controllers,
micronozzles and microvalves to be produced. Although many devices are still under
development, microfluidic systems typically contain silicon micromachined pumps, flow
sensors and chemical sensors. They enable fast and relatively convenient manipulation and
analysis of small volumes of liquids, an area of particular interest in home-based medical
applications where patients can use devices to monitor their own conditions, such as blood
and urine analysis.
One example of a new bioMEMS device is the microtitreplate on which a number of cavities
can be simultaneously filled accurately and repeatably by capillary force (Figure 11a). This is
a relatively simple MEMS product in the form of a piece of plastic with high-aspect-ratio
micromachined microchannels and is classified as a ‘lab-on-a-chip’ product. Its dimensions
are only 20 mm x 37 mm x 3 mm and enables automatic filling of 96 microwells by the use
of capillary action.
Figure 10. The MEMS Digital Micromirror Device
(DMD) [17].
Future lab-on-a-chip technology may include implantable ‘pharmacy-on-a-chip’ devices to
carefully release drugs into the body from tiny chambers embedded in a MEMS device,
eliminating the need for needles or injections. The delivery of insulin is one such application,
as is the delivery of hormones, chemotherapy drugs and painkillers. First generation devices
are being developed which release their medication upon signals from an outside source,
wired through the skin. Proposed second generation devices may be wireless and third
generation MEMS chips could interact with MEMS sensors embedded in the body to respond
to the body’s own internal signals.
One of the most recent MEMS microfluidic devices to emerge from development laboratories
incorporates a ‘Pac-Man’-like microstructure that interacts with red blood cells (Figure 11b).
The device from Sandia National Laboratories, U.S, contains silicon microteeth that open
and close like jaws trapping and releasing a single red blood cell unharmed as it is pumped
through a 20 μm channel. The ultimate goal of this device is to puncture cells and inject them
with DNA, proteins, or pharmaceuticals to counter biological or chemical attacks, gene
imbalances and natural bacterial or viral infections.
ii) MOEMS
Optical communications has emerged as the only practical means to address the network
scaling issues created by the tremendous growth in data traffic caused by the rapid rise of the
Internet. Current routing technology slows the information (or bit) flow by transforming
optical signals into electronic information and then back into light before redirecting it. All
optical networks offer far superior throughput capabilities and performance over traditional
electronic systems.
The most significant MOEMS device products include waveguides, optical switches, cross
connects, multiplexers, filters, modulators, detectors, attenuators and equalizers. Their small
size, low cost, low power consumption, mechanical durability, high accuracy, high switching
density and low cost batch processing of these MEMS-based devices make them a perfect
solution to the problems of the control and switching of optical signals in telephone networks.
An example of a MEMS optical connect is shown in Figure 12. Here a network of 256
MEMS micromirrors route information in the form of photons (the elementary particle that
corresponds to an electromagnetic wave) to and from any of 256 input/output optical fibres.
Figure 11. (a) Micromachined microtitreplate with 96 cavities filled by capillary
force [18,19], and (b) a bioMEMS device actuated with ‘microteeth’ to trap,
hold and release single red blood cells (unharmed). The little balls in the
channels are red blood cells [2].
2 MEMS Market..............................................................................
The three most well known market studies are the Network of Excellence in Multifunctional
Microsystems (NEXUS) study (1998), the System Planning Corporation (SPC) study (1999)
and the Battelle study (1990) and there is discrepancy between each study [23, 24, 25
respectively]. The size of the MEMS market (M
3
) is contingent on how MEMS is defined
(M 3 is shorthand for MEMS, Microsystems and Micromachining and although it is not yet
common, it is used as a reference for the entire MEMS market. Smaller M 3 figures are
obtained if MEMS is considered as just micromachining, which is more elemental and at the
device level. Alternatively, much larger M
3
figures arise if MEMS is examined at the system
or subsystem level (as in the case of NEXUS). Depending on the study under review, the M
3
market today ranges from $4 billion to $14 billion. Much of the current market centres on
read/write heads for computer disk drives, pressure sensors, inkjet printer heads and
accelerometers. Table 3 provides the NEXUS worldwide M
3
market size in 1996 and
forecasts for 2002 for existing MEMS product types.
Product Types
1996
Units
(millions)
$
(millions)
2002 Units
(millions)
$
(millions)
HDD heads 530 4500 1500 12000
Inkjet print heads 100 4400 500 10000
Heart pacemakers 0 1000 0 3700
In vitro diagnostics 700 450 4000 2800
Hearing aids 4 1150 7 2000
Pressure sensors 115 600 309 1300
Chemical sensors 100 300 400 800
Infrared imagers 0 220 0 800
Accelerometers 24 240 90 430
Gyroscopes 6 150 30 360
Magnetoresistive sensors 15 20 60 60
Microspectrometers 0 3 0 40
TOTAL 1595 $13,033 6807 $34,
In the area of emerging MEMS products, Table 4 provides the NEXUS worldwide M
3
market
size in 1996 and forecasts for 2002. Drug delivery systems (microfluidic microdosing
systems), lab-on-a-chip devices and MEMS-based optical switches are predicted to reach
billion dollar market segments by 2002.
Table 3. Worldwide M 3 market size in 1996 and 2002 for existing MEMS product types
in $US millions [23].
Product Types
1996
Units
(millions)
$
(millions)
2002 Units
(millions)
$ (millions)
Drug delivery systems 1 10 100 1000
Optical switches 1 50 40 1000
Lab on ship 0 0 100 1000
Magneto optical heads 0 1 100 500
Projection valves 0 10 1 300
Coil on chip 20 10 600 100
Micro relays 0 50 100
Micromotors 0 5 2 80
Inclinometers 1 10 20 70
Injection nozzles 10 10 30 30
Anti-collision sensors 0 0 2 20
Electronic noses 0 0 0 5
TOTAL 33 $107 1045 $4,
A more recent market study by NEXUS/Roger Grace Associates, shown in Table 5, estimated
the M 3 market to be $14 billion in 2000, increasing to $30 billion by 2004. This
corresponds to a compounded annual growth rate (CAGR) of 21%. Telecommunications is
forecast to be the major growth area, comprised of both optical MEMS and RF MEMS-based
devices.
Application Sector 2000 2004 CAGR(%)
IT/Peripheral $ 8,700 $13,400 11.
Medical/Biochemical 2,400 7,400 32.
Industrial/Automation 1,190 1,850 11.
Telecommunications 130 3,650 128.
Automotive 1,260 2,350 16.
Environmental Monitoring 520 1,750 35.
TOTAL $14,200 $30,400 21%
2 Miniaturization Issues.....................................................................
As previously stated, MEMS is not about miniaturization; it is a manufacturing technology
used to create tiny integrated microdevices and systems using IC batch fabrication techniques.
Similarly, miniaturization is not just about shrinking down existing devices (although there
have been some classic examples, namely the DENSO Micro-Car as shown in Figure 14); it’s
about completely rethinking the structure of a microsystem.
Table 4. Worldwide M 3 market size in 1996 and 2002 for emerging MEMS product types
in $US millions [23].
Table 5. Worldwide shipment of M 3 products by application sector for
2000-2004 in $US millions [23,26].
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An introduction to mems
University: United College of Engineering and Research
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Prime Faraday Technology Watch
ISBN 1-84402-020-7 An Introduction to MEMS January 2002
An Introduction to MEMS (Micro-electromechanical Systems)
MEMS has been identified as one of the most promising technologies for
the 21st Century and has the potential to revolutionize both industrial and
consumer products by combining silicon-based microelectronics with
micromachining technology. Its techniques and microsystem-based
devices have the potential to dramatically affect of all of our lives and the
way we live.
This report presents a general introduction to the field of MEMS, with
emphasis on its commercial applications and device fabrication methods.
It also describes the range of MEMS sensors and actuators, the
phenomena that can be sensed or acted upon with MEMS devices, and
outlines the major challenges facing the industry.
PRIME Faraday Partnership
PRIME Faraday Partnership
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