Microelectromechanical systems (MEMS) are small integrated devices or
systems that combine electrical and mechanical components. They range in
size from the sub micrometer
(or sub micron) level to the millimeter level, and there can be any
number, from a few to millions, in a particular system. MEMS extend the
fabrication techniques developed for the integrated circuit industry to
add mechanical elements such as beams, gears, diaphragms, and springs to
Examples of MEMS device applications include inkjet-printer cartridges,
miniature robots, microengines, locks, inertial
sensors, microtransmissions, micromirrors, micro actuators,
optical scanners, fluid pumps, transducers,
and chemical, pressure and flow sensors. New applications are emerging as
the existing technology is applied to the miniaturization and integration
of conventional devices.
These systems can sense, control, and activate mechanical processes on
the micro scale, and function individually or in arrays to generate
effects on the macro scale. The micro fabrication technology enables
fabrication of large arrays of devices, which individually perform simple
tasks, but in combination can accomplish complicated functions.
MEMS are not about any one application or device, nor are they defined
by a single fabrication process or limited to a few materials. They are a
fabrication approach that conveys the advantages of miniaturization,
multiple components, and microelectronics to the design and construction
of integrated electromechanical
systems. MEMS are not only about miniaturization of mechanical systems;
they are also a new paradigm for designing mechanical devices and
The MEMS industry has an estimated $10 billion market, and with a
projected 10-20% annual growth rate, it is estimated to have a $34 billion
market in 2002 . Because of the significant impact that MEMS can have
on the commercial and defense markets, industry and the federal
government have both taken a special interest in their development.
II. Historical Background
The invention of the transistor
at Bell Telephone Laboratories in 1947 sparked a fast-growing
microelectronic technology. Jack Kilby of Texas Instruments built the
circuit (IC) in 1958 using germanium (Ge) devices. It consisted of one
and one capacitor.
The IC was implemented on a sliver of Ge that was glued on a glass slide.
Later that same year Robert Noyce of Fairchild Semiconductor announced the
development of a planar double-diffused
Si IC. The complete transition from the original Ge transistors with grown
and alloyed junctions to silicon (Si) planar double-diffused devices took
about 10 years. The success of Si as an electronic material was due
partly to its wide availability from silicon dioxide (SiO2) (sand),
resulting in potentially lower material costs relative to other
Since 1970, the complexity of ICs has doubled every two to three years.
The minimum dimension of manufactured devices and ICs has decreased from
20 microns to the sub micron levels of today. Current
ultra-large-scale-integration (ULSI) technology enables the fabrication of
more than 10 million transistors and capacitors on a typical chip.
IC fabrication is dependent upon sensors to provide input from the
surrounding environment, just as control systems need actuators (also
referred to as transducers) in order to carry out their desired functions.
Due to the availability of sand as a material, much effort was put into
developing Si processing and characterization tools. These tools are now
being used to advance transducer technology. Today's IC technology far
outstrips the original sensors and actuators in performance, size, and
Attention in this area was first focused on microsensor (i.e.,
microfabricated sensor) development. The first microsensor, which has also
been the most successful, was the Si pressure sensor. In 1954 it was
discovered that the piezoresistive
effect in Ge and Si had the potential to produce Ge and Si strain
gauges with a gauge factor (i.e., instrument sensitivity) 10 to 20
times greater than those based on metal films. As a result, Si strain
gauges began to be developed commercially in 1958. The first high-volume
pressure sensor was marketed by National Semiconductor in 1974. This
sensor included a temperature controller for constant-temperature
operation. Improvements in this technology since then have included the
utilization of ion
implantation for improved control of the piezoresistor
fabrication. Si pressure sensors are now a billion-dollar industry
Around 1982, the term micromachining
came into use to designate the fabrication of micromechanical parts (such
as pressure-sensor diaphragms or accelerometer suspension beams) for Si
microsensors. The micromechanical parts were fabricated by selectively etching
areas of the Si substrate
away in order to leave behind the desired geometries. Isotropic
etching of Si was developed in the early 1960s for transistor
etching of Si then came about in 1967. Various etch-stop techniques
were subsequently developed to provide further process flexibility.
These techniques also form the basis of the bulk micromachining
processing techniques. Bulk micromachining designates the point at which
the bulk of the Si substrate is etched away to leave behind the desired
micromechanical elements . Bulk micromachining has remained a powerful
technique for the fabrication of micromechanical elements. However, the
need for flexibility in device design and performance improvement has
motivated the development of new concepts and techniques for
Among these is the sacrificial layer technique, first demonstrated in
1965 by Nathanson and Wickstrom , in which a layer of material is
deposited between structural layers for mechanical separation and
isolation. This layer is removed during the release etch to free the
structural layers and to allow mechanical devices to move relative to the
substrate. A layer is releasable when a sacrificial layer separates it
from the substrate. The application of the sacrificial layer technique to
micromachining in 1985 gave rise to surface micromachining, in which the
Si substrate is primarily used as a mechanical support upon which the
micromechanical elements are fabricated.
Prior to 1987, these micromechanical structures were limited in motion.
During 1987-1988, a turning point was reached in micromachining when, for
the first time, techniques for integrated fabrication of mechanisms (i.e.
rigid bodies connected by joints for transmitting, controlling, or
constraining relative movement) on Si were demonstrated. During a series
of three separate workshops on microdynamics held in 1987, the term MEMS
was coined. Equivalent terms for MEMS are microsystems (preferred in
Europe) and micromachines (preferred in Japan).
III. Fabrication Technologies
The three characteristic features of MEMS fabrication technologies are
miniaturization, multiplicity, and microelectronics. Miniaturization
enables the production of compact, quick-response devices. Multiplicity
refers to the batch fabrication inherent in semiconductor processing,
which allows thousands or millions of components to be easily
and concurrently fabricated. Microelectronics provides the intelligence to
MEMS and allows the monolithic
merger of sensors, actuators, and logic to build closed-loop
feedback components and systems. The successful miniaturization and
multiplicity of traditional electronics systems would not have been
possible without IC fabrication technology. Therefore, IC fabrication
technology, or microfabrication, has so far been the primary enabling
technology for the development of MEMS. Microfabrication provides a
powerful tool for batch processing and miniaturization of mechanical
systems into a dimensional domain not accessible by conventional
(machining) techniques. Furthermore, microfabrication provides an
opportunity for integration of mechanical systems with electronics to
develop high-performance closed-loop-controlled MEMS.
Advances in IC technology in the last decade have brought about
corresponding progress in MEMS fabrication processes. Manufacturing
processes allow for the monolithic integration of microelectromechanical
structures with driving, controlling, and signal-processing electronics.
This integration promises to improve the performance of micromechanical
devices as well as reduce the cost of manufacturing, packaging, and
instrumenting these devices .
A. IC Fabrication
Any discussion of MEMS requires a basic understanding of IC fabrication
technology, or microfabrication, the primary enabling technology for the
development of MEMS. The major steps in IC fabrication technology are film
growth, doping, lithography, etching, dicing, and packaging.
Film growth: Usually, a polished Si wafer is used as the substrate, on
which a thin film is grown. The film, which may be epitaxial
Si, SiO2, silicon nitride (Si3N4), polycrystalline
Si (polysilicon), or metal, is used to build both active or passive
components and interconnections between circuits.
Doping: To modulate the properties of the device layer, a low and
controllable level of an atomic impurity may be introduced into the layer
diffusion or ion implantation.
Lithography: A pattern on a mask is then transferred to the film by
means of a photosensitive (i.e., light sensitive) chemical known as a
photoresist. The process of pattern generation and transfer is called
photolithography. A typical mask consists of a glass plate coated with a
patterned chromium (Cr) film.
Etching: Next is the selective removal of unwanted regions of a film or
substrate for pattern delineation. Wet chemical etching or dry etching may
be used. Etch-mask materials are used at various stages in the removal
process to selectively prevent those portions of the material from being
etched. These materials include SiO2, Si3N4, and hard-baked
Dicing: The finished wafer is sawed or machined into small squares, or
dice, from which electronic components can be made.
Packaging: The individual sections are then packaged, a process that
involves physically locating, connecting, and protecting a device or
component. MEMS design is strongly coupled to the packaging requirements,
which in turn are dictated by the application environment.
B. Bulk Micromachining and Wafer Bonding
Bulk micromachining is an extension of IC technology for the
fabrication of 3D structures. Bulk micromachining of Si uses wet- and
dry-etching techniques in conjunction with etch masks and etch stops to
sculpt micromechanical devices from the Si substrate. The two key
capabilities that make bulk micromachining a viable technology are:
1) Anisotropic etchants of Si, such as ethylene-diamine and
pyrocatechol (EDP), potassium hydroxide (KOH), and hydrazine (N2H4). These
preferentially etch single crystal Si along given crystal planes.
2) Etch masks and etch-stop techniques that can be used with Si
anisotropic etchants to selectively prevent regions of Si from being
etched. Good etch masks are provided by SiO2 and Si3N4, and some metallic
thin films such as Cr and Au (gold).
A drawback of wet anisotropic etching is that the microstructure
geometry is defined by the internal crystalline structure of the
substrate. Consequently, fabricating multiple, interconnected
micromechanical structures of free-form geometry is often difficult or
impossible. Two additional processing techniques have extended the range
of traditional bulk micromachining technology: deep anisotropic dry
etching and wafer bonding. Reactive gas plasmas can perform deep
anisotropic dry etching of Si wafers, up to a depth of a few hundred
microns, while maintaining smooth vertical sidewall profiles. The other
technology, wafer bonding, permits a Si substrate to be attached to
another substrate, typically Si or glass. Used in combination, anisotropic
etching and wafer bonding techniques can construct 3D complex
microstructures such as microvalves and micropumps .
C. Surface Micromachining
Surface micromachining enables the fabrication of complex
multicomponent integrated micromechanical structures that would not be
possible with traditional bulk micromachining. This technique encases
specific structural parts of a device in layers of a sacrificial material
during the fabrication process. The substrate wafer is used primarily as a
mechanical support on which multiple alternating layers of structural and
sacrificial material are deposited and patterned to realize
micromechanical structures. The sacrificial material is then dissolved in
a chemical etchant that does not attack the structural parts. The most
widely used surface micromachining technique, polysilicon surface
micromachining, uses SiO2 as the sacrificial material and polysilicon as
the structural material.
At the University of Wisconsin at Madison, polysilicon surface
micromachining research started in the early 1980s in an effort to create
high-precision micro pressure sensors. The control of the internal
stresses of a thin film is important for the fabrication of
microelectromechanical structures. The microelectronic fabrication
industry typically grows polysilicon, silicon nitride, and silicon dioxide
films using recipes that minimize time. Unfortunately, a deposition
process that is optimized to speed does not always create a low internal
stress film. In fact, most of these films have internal stresses that are
highly compressive (tending to contract). A freestanding plate of highly
compressive polysilicon that is held at all its edges will buckle (i.e.,
collapse or give way). This is highly undesirable. The solution is to
modify the film deposition process to control the internal stress by
making it stress-free or slightly tensile.
One way to do this is to dope the film with boron, phosphorus, or
arsenic. However, a doped polysilicon film is conductive, and this
property may interfere with the mechanical devices incorporated
electronics. Another problem with doped polysilicon is that it is
roughened by hydrofluoric acid (HF), which is commonly used to free
sections of the final mechanical device from the substrate. Rough
polysilicon has different mechanical properties than smooth polysilicon.
Therefore, the amount of roughening must be taken into account when
designing the mechanical parts of the micro device.
A better way to control the stress in polysilicon is through post annealing,
which involves the deposition of pure, fine-grained, compressive (i.e.,
can be compressed) polysilicon. Annealing the polysilicon after deposition
at elevated temperatures can change the film to be stress-free or tensile.
The annealing temperature sets the film's final stress. After this,
electronics can then be incorporated into polysilicon films through
selective doping, and hydrofluoric acid will not change the mechanical
properties of the material .
Deposition temperature and the film's silicon to nitride ratio can
control the stress of a silicon nitride (Si3N4) film. The films can be
deposited in compression, stress-free, or in tension .
Deposition temperature and post annealing can control silicon dioxide
(SiO2) film stress. Because it is difficult to control the stress of SiO2
accurately, SiO2 is typically not used as a mechanical material by itself,
but as electronic isolation or as a sacrificial layer under
In the micromolding process, microstructures are fabricated using molds
to define the deposition of the structural layer. The structural material
is deposited only in those areas constituting the microdevice structure,
in contrast to bulk and surface micromachining, which feature blanket
deposition of the structural material followed by etching to realize the
final device geometry. After the structural layer deposition, the mold is
dissolved in a chemical etchant that does not attack the structural
material. One of the most prominent micromolding processes is the LIGA
process. LIGA is a German acronym standing for lithographie,
galvanoformung, und abformung (lithography, electroplating, and molding).
This process can be used for the manufacture of high-aspect-ratio
3D microstructures in a wide variety of materials, such as metals,
polymers, ceramics, and glasses. Photosensitive polyimides
are also used for fabricating plating molds. The photolithography process
is similar to conventional photolithography, except that polyimide works
as a negative resist .
IV. Applications of MEMS
Here are some examples of MEMS technology:
A. Pressure Sensors
MEMS pressure microsensors typically have a flexible diaphragm that
deforms in the presence of a pressure difference. The deformation is
converted to an electrical signal appearing at the sensor output. A
pressure sensor can be used to sense the absolute air pressure within the
intake manifold of an automobile engine, so that the amount of fuel
required for each engine cylinder can be computed. In this example,
piezoresistors are patterned across the edges of a region where a silicon
diaphragm will be micromachined. The substrate is etched to create the
diaphragm. The sensor die is then bonded to a glass substrate, creating a
sealed vacuum cavity under the diaphragm. The die is mounted on a package,
where the topside of the diaphragm is exposed to the environment. The
change in ambient pressure forces the downward deformation of the
diaphragm, resulting in a change of resistance of the piezoresistors.
On-chip electronics measure the resistance change, which causes a
corresponding voltage signal to appear at the output pin of the sensor
Accelerometers are acceleration sensors. An inertial mass suspended by
springs is acted upon by
acceleration forces that cause the mass to be deflected from its initial
position. This deflection is converted to an electrical signal, which
appears at the sensor output. The application of MEMS technology to
accelerometers is a relatively new development.
One such accelerometer design is discussed by DeVoe and Pisano (2001)
. It is a surface micromachined piezoelectric
accelerometer employing a zinc oxide (ZnO) active piezoelectric film. The
design is a simple cantilever structure, in which the cantilever beam
serves simultaneously as proof mass
and sensing element. One of the fabrication approaches developed is a
sacrificial oxide process based on polysilicon surface micromachining,
with the addition of a piezoelectric layer atop the polysilicon film. In
the sacrificial oxide process, a passivation
layer of silicon dioxide and low-stress silicon nitride is deposited on a
bare silicon wafer, followed by 0.5 micron of liquid phase chemical vapor
deposited (LPCVD) phosphorous-doped polysilicon. Then, a 2.0-micron
layer of phosphosilicate glass (PSG) is deposited by LPCVD and patterned
to define regions where the accelerometer structure will be anchored to
the substrate. The PSG film acts as a sacrificial layer that is
selectively etched at the end to free the mechanical structures. A second
layer of 2.0-micron-thick phosphorus-doped polysilicon is deposited via
LPCVD on top of the PSG, and patterned by plasma etching to define the
mechanical accelerometer structure. This layer also acts as the lower
electrode for the sensing film. A thin layer of silicon nitride is next
deposited by LPCVD, and acts as a stress-compensation layer for balancing
the highly compressive residual stresses in the ZnO film. By varying the
thickness of the Si3N4 layer, the accelerometer structure may be tuned to
control bending effects resulting from the stress gradient through the
device thickness. A ZnO layer is then deposited on the order of 0.5
micron, followed by sputtering
of a 0.2-micron layer of platinum (Pt) deposited to form the upper
electrode. A rapid thermal anneal is performed to reduce residual stresses
in the sensing film. Afterwards, the Pt, Si3N4, and ZnO layers are
patterned in a single ion milling etch step, and the devices are then
released by passivating the ZnO film with photoresist, and immersing the
wafer in buffered
hydrofluoric acid, which removes the sacrificial PSG layer .
C. Inertial Sensors
Inertial sensors are a type of accelerometer and are one of the
principal commercial products that utilize surface micromachining. They
are used as airbag-deployment sensors in automobiles, and as tilt or shock
sensors. The application of these accelerometers to inertial
measurement units (IMUs) is limited by the need to manually align and
assemble them into three-axis systems, and by the resulting alignment
tolerances, their lack of in-chip analog-to-digital conversion circuitry,
and their lower limit of sensitivity. A three-axis force-balanced
accelerometer has been designed at the University of California, Berkeley,
 to overcome some of these limitations. The accelerometer was designed
for the integrated MEMS/CMOS
technology. This technology involves a manufacturing technique where a
single-level (plus a second electrical interconnect level) polysilicon
micromachining process is integrated with 1.25-micron CMOS.
A three-level polysilicon micromachining process [10,11] has enabled
the fabrication of devices with increased degrees of complexity. The
process includes three movable levels of polysilicon, each separated by a
sacrificial oxide layer, plus a stationary level. Operation of the small
gears at rotational speeds greater than 300,000 rpm has been demonstrated.
Microengines can be used to drive the wheels of microcombination locks.
They can also be used in combination with a microtransmission to drive a
pop-up mirror out of a plane. This device is known as a micromirror.
E. Some other applications
MEMS IC fabrication technologies have also allowed the manufacture of
microtransmissions using sets of small and large gears interlocking with
other sets of gears to transfer power.
A recently developed MicroStar cross-connect fabric developed by Bell
Labs , a micro-optoelectromechanical system device, is based on MEMS
technology. The most pervasive bottlenecks for communications carriers are
the switching and cross-connect fabrics that switch, route, multiplex,
and restore traffic in optical networks. The optical transmission systems
move information as photons,
but switching and cross-connect fabrics until now have been largely
electronic, requiring costly and time-consuming bandwidth-limiting
optical-to-electronic-to-optical conversions at every network connection
and cross point. MicroStar is composed of 256 mirrors, each one 0.5 mm in
diameter, spaced 1 mm apart, and covering less than 1 square inch of
silicon. The mirrors sit within the router so that only one wavelength can
illuminate any one mirror. Each mirror can tilt independently to pass its
wavelength to any of 256 input and output fibers. The mirror arrays are
made using a self-assembly process that causes a frame around each mirror
to lift from the silicon surface and lock in place, positioning the
mirrors high enough to allow a range of movement. MicroStar is part of
Lucent Technology's Lambda Router cross-connect system aimed at helping
carriers deliver vast amounts of data unimpeded by conventional
As a final example, MEMS technology has been used in fabricating
vaporization microchambers for vaporizing liquid microthrusters for nanosatellites
. The chamber is part of a microchannel with a height of 2-10
made using silicon and glass substrates. The nozzle is fabricated in the
silicon substrate just above a thin-film indium tin oxide heater deposited
V. The Future
Each of the three basic microsystems technology processes we have seen,
bulk micromachining, sacrificial surface micromachining, and
micromolding/LIGA, employs a different set of capital and intellectual
resources. MEMS manufacturing firms must choose which specific
microsystems manufacturing techniques to invest in .
MEMS technology has the potential to change our daily lives as much as
the computer has. However, the material needs of the MEMS field are at a
preliminary stage. A thorough understanding of the properties of existing
MEMS materials is just as important as the development of new MEMS
Future MEMS applications will be driven by processes enabling greater
functionality through higher levels of electronic-mechanical integration
and greater numbers of mechanical components working alone or together to
enable a complex action. Future MEMS products will demand higher levels of
electrical-mechanical integration and more intimate interaction with the
physical world. The high up-front investment costs for large-volume
commercialization of MEMS will likely limit the initial involvement to
larger companies in the IC industry. Advancing from their success as
sensors, MEMS products will be embedded in larger non-MEMS systems, such
as printers, automobiles, and biomedical diagnostic equipment, and will
enable new and improved systems .
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