Introduction to MEMS

[ MEMS ] Micro – electromechanical systems By : [ AwaisHusain]
Chapter I
Introduction to Micro-electromechanical System and
MEMS design
MEMS
(Micro-electromechanical System)

What is MEMS Technology?
Micro-Electro-Mechanical Systems, or MEMS, is a technology that in its
most general form can be defined as miniaturized mechanical and electro-
mechanical elements (i.e., devices and structures) that are made using the
techniques of microfabrication. The critical physical dimensions of MEMS devices
can vary from well below one micron on the lower end of the dimensional
spectrum, all the way to several millimeters. Likewise, the types of MEMS devices
can vary from relatively simple structures having no moving elements, to
extremely complex electromechanical systems with multiple moving elements
under the control of integrated microelectronics. The one main criterion of MEMS
is that there are at least some elements having some sort of mechanical
functionality whether or not these elements can move. The term used to define
MEMS varies in different parts of the world. In the United States they are
predominantly called MEMS, while in some other parts of the world they are
called “Microsystems Technology” or “micromachined devices”.
While the functional elements of MEMS are miniaturized structures, sensors,
actuators, and microelectronics, the most notable (and perhaps most interesting)
elements are the microsensors and microactuators. Microsensors and
microactuators are appropriately categorized as “transducers”, which are defined as
devices that convert energy from one form to another. In the case of microsensors,
the device typically converts a measured mechanical signal into an electrical
signal.

History of MEMS
MEMS word introduced in 1986 i.e. in proposalsubmitted to DARPA
(Defense Advanced research project agency) by the center for engineering design
university of UTAH. Thomas Edison’s first successful light bulb model done in
December1879 at Menlo park.
In 1904, British scientist John Ambrose Fleming first showed his device famous
as“Fleming Diode” to convert an alternating current signal in to direct current
signal. The “Fleming Diode” was base on an effect that Thomas Edison used in
light bulb model i.e. “vacuum tube”.
From 1904 to 1960 many other inventors tried to improve the “Fleming
Diode”,the only one who succeeded was New York inventor Lee De Forest.
In 16 December 1947, first time a Solid State Electronic Transistor known as
“Point Contact Transistor” developed by John Bardeen and Walter Brattain
at bell laboratories led by physicist William Shockly. This group has been working
together on experiments and theories of electric field effects in solid state
materials, with the aim of replacing “Vacuum Tubes” with a smaller and less
power consuming devices. And Silicon oxidation is demonstrated in 1953 in Bell
Telephone Laboratories & with this monolithic transistors are implemented. Got
Nobel prize in 1956.
 1954: Piezoresistive effect in Germanium and Silicon (C.S. Smith), this
discovery showed that siliconand germanium could sense air or waterpressure
better than metal. Many MEMS devices such as strain gauges, pressure
sensors, and accelerometers utilize the Piezoresistive Effect in silicon.

 1988 Batch fabricated pressure sensors via wafer bonding (Nova Sensor) 1988
Rotary electrostatic side drive motors (Fan, Tai, Muller)
 1991 Polysilicon hinge (Pister, Judy, Burgett, Fearing)
 1991 The carbonnanotube is discovered
 1992 Grating light modulator (Solgaard, Sandejas, Bloom)
 1992 Bulk micromachining (SCREAM process, Cornell)
 1993 Digital mirror display (Texas Instruments)
 1993 MCNC creates MUMPS foundry service
 1993 First surface micromachined accelerometer in high volume production
(Analog Devices)
 1994 Bosch process forDeep Reactive Ion Etching is patented
 1996 Richard Smalley develops a technique for producing carbon nanotubes of
uniform diameter
 1999 Optical network switch (Lucent)
 2000s Optical MEMS boom
 2000s Bio-MEMS proliferate

Micro - electromechanical systems
MEMS, also written as micro-electro-mechanical, MicroElectroMechanical or
microelectronic and MicroElectroMechanical systems and the related
micromechatronics) is the technology of microscopic devices, particularly those
with moving parts. It merges at the nano-scale into nano-electromechanical
systems (NEMS) and nanotechnology. MEMS are also referred to as
micromachines in Japan, or micro systems technology (MST) in Europe.
MEMS are made up of components between 1 and 100 micro metres in size
(i.e., 0.001 to 0.1 mm), and MEMS devices generally range in size from 20 micro
metres to a millimetre (i.e., 0.02 to 1.0 mm), although components arranged in
arrays (e.g., digital micromirror devices) can be more than 1000 mm2. They usually
consist of a central unit that processes data (the microprocessor) and several
components that interact with the surroundings such as microsensors. Because of
the large surface area to volume ratio of MEMS, forces produced by ambient
electromagnetism (e.g., electrostatic charges and magnetic moments), and fluid
dynamics (e.g., surface tension and viscosity) are more important design
considerations than with larger scale mechanical devices. MEMS technology is
distinguished from molecular nanotechnology or molecular electronics in that the
latter must also consider surface chemistry.
The potential of very small machines was appreciated before the technology
existed that could make them (see, for example, Richard Feynman's famous 1959
lecture There's Plenty of Room at the Bottom). MEMS became practical once they
could be fabricated using modified semiconductor device fabrication technologies,
normally used to make electronics. These include molding and plating, wet etching
(KOH, TMAH) and dry etching (RIE and DRIE), electro discharge machining
(EDM), and other technologies capable of manufacturing small devices. An early
example of a MEMS device is the resonistor, an electromechanical monolithic
resonator patented by Raymond J. Wilfinger, and the resonant gate transistor
developed by Harvey C. Nathanson.

MEMS manufacturing technologies :
Bulk micromachining
Bulk micromachining is the oldest paradigm of silicon based MEMS. The
whole thickness of a silicon wafer is used for building the micro-mechanical
structures. Silicon is machined using various etching processes. Anodic bonding of
glass plates or additional silicon wafers is used for adding features in the third
dimension and for hermetic encapsulation. Bulk micromachining has been
essential in enabling high performance pressure sensors and accelerometers that
changed the sensor industry in the 1980s and 90's.
In detail :
Bulk micromachining is a process used to produce micromachinery or
micro - electromechanical systems (MEMS).Unlike surface micromachining,
which uses a succession of thin film deposition and selective etching, bulk
micromachining defines structures by selectively etching inside a substrate.
Whereas surface micromachining creates structures on top of a substrate, bulk
micromachining produces structures inside a substrate.
Usually, silicon wafers are used as substrates for bulk micromachining, as
they can be anisotropically wet etched, forming highly regular structures. Wet
etching typically uses alkaline liquid solvents, such as potassium hydroxide (KOH)
or tetramethylammonium hydroxide (TMAH) to dissolve silicon which has been
left exposed by the photolithography masking step. These alkali solvents dissolve
the silicon in a highly anisotropic way, with some crystallographic orientations
dissolving up to 1000 times faster than others. Such an approach is often used with
very specific crystallographic orientations in the raw silicon to produce V-shaped
grooves. The surface of these grooves can be atomically smooth if the etch is
carried out correctly, and the dimensions and angles can be precisely defined.
Pressure sensors are usually created by bulk micromachining technique.
Bulk micromachining starts with a silicon wafer or other substrates which is
selectively etched, using photolithography to transfer a pattern from a mask to the
surface. Like surface micromachining, bulk micromachining can be performed
with wet or dry etches, although the most common etch in silicon is the anisotropic
wet etch. This etch takes advantage of the fact that silicon has a crystal structure,

which means its atoms are all arranged periodically in lines and planes. Certain
planes have weaker bonds and are more susceptible to etching. The etch results in
pits that have angled walls, with the angle being a function of the crystal
orientation of the substrate. This type of etching is inexpensive and is generally
used in early, low-budget research.
Surface micromachining
Surface micromachining uses layers deposited on the surface of a substrate
as the structural materials, rather than using the substrate itself. Surface
micromachining was created in the late 1980s to render micromachining of silicon
more compatible with planar integrated circuit technology, with the goal of
combining MEMS and integrated circuits on the same silicon wafer. The original
surface micromachining concept was based on thin polycrystalline silicon layers
patterned as movable mechanical structures and released by sacrificial etching of
the underlying oxide layer. Inter digital comb electrodes were used to produce in-
plane forces and to detect in-plane movement capacitively. This MEMS paradigm
has enabled the manufacturing of low cost accelerometers for e.g. automotive air-
bag systems and other applications where low performance and/or high g-ranges

are sufficient. Analog Devices has pioneered the industrialization of surface
micromachining and has realized the co-integration of MEMS and integrated
circuits.
In detail :
Unlike Bulk micromachining, where a silicon substrate (wafer) is selectively
etched to produce structures, surface micromachining builds microstructures by
deposition and etching of different structural layers on top of the substrate.[1]
Generally polysilicon is commonly used as one of the layers and silicon dioxide is
used as a sacrificial layer which is removed or etched out to create the necessary
void in the thickness direction. Added layers are generally very thin with their size
varying from 2-5 Micro metres. The main advantage of this machining process is
the possibility of realizing monolithic microsystems in which the electronic and the
mechanical components(functions) are built in on the same substrate. The surface
micromachined components are smaller compared to their counterparts, the bulk
micromachined ones.
As the structures are built on top of the substrate and not inside it, the
substrate's properties are not as important as in bulk micromachining, and the
expensive silicon wafers can be replaced by cheaper substrates, such as glass or
plastic. The size of the substrates can also be much larger than a silicon wafer, and
surface micromachining is used to produce TFTs on large area glass substrates for
flat panel displays. This technology can also be used for the manufacture of thin
film solar cells, which can be deposited on glass, but also on PET substrates or
other non-rigid materials .

DIFFERENCES: ICs Vs MEMS
Though most Si-based MEMS and ICs are fabricated using same microfabrication
processes, the two are different on many aspects, some of which are listed in the
following table:
S. No MEMS ICs
1 3D complex structures 2D structures
2 Doesn’t have any basic building block Transistor is basic building block of ICs
3 May have moving parts No moving parts
4 May have interface with external media. Totally isolated with media
5
Functions include biological, chemical,
optical
Only electrical
6 Packaging is very complex
Packaging techniques are well
developed.
WHY MEMS ?
1. MEMS allow miniaturization of existing devices
2. MEMS offer solutions which cannot be attained by macro-machined products,
e.g., capacitive pressure sensor capable of sensing pressure of the order of
1 mTorr is not possible with macromachined capacitive diaphragm.
3. 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- microelectronics, optics-microelectronics).
4. MEMS allows the complex electromechanical systems to be manufactured
using batch fabrication techniques, decreasing the cost and increasing the
reliability.
5. It allows integrated systems, viz., sensors, actuators, circuits, etc. in a single
package and offers advantages of reliability, performance, cost, ease of use,
etc.

Applications :
 Inkjet printers, which use piezoelectric or thermal bubble ejection to deposit
ink on paper.
 Accelerometers in modern cars for a large number of purposes including
airbag deployment and electronic stability control.
 Inertial Measurement Units (IMUs): MEMS Accelerometers and MEMS
gyroscopes in remote controlled, or autonomous, helicopters, planes and
multi - rotors (also known as drones), used for automatically sensing and
balancing flying characteristics of roll, pitch and yaw. MEMS magnetic field
sensor (magnetometer) may also be incorporated in such devices to provide
directional heading. MEMS are also used in Inertial navigation systems
(INSs) of modern cars, airplanes, submarines and other vehicles to detect
yaw, pitch, and roll; for example, the autopilot of an airplane.
 Accelerometers in consumer electronics devices such as game controllers ,
personal media players / cell phones (virtually all Smartphone’s, various
HTC PDA models) and a number of Digital Cameras (various Canon Digital
IXUS models). Also used in PCs to park the hard disk head when free-fall is
detected, to prevent damage and data loss.
 MEMS microphones in portable devices, e.g., mobile phones, head sets and
laptops. The market for smart microphones includes smart phones, wearable
devices, smart home and automotive applications.
 Silicon pressure sensors e.g., car tire pressure sensors, and disposable blood
pressure sensors
 Displays e.g., the digital micromirror device (DMD) chip in a projector
based on DLP technology, which has a surface with several hundred
thousand micromirrors or single micro-scanning-mirrors also called
microscanners
 Optical switching technology, which is used for switching technology and
alignment for data communications
 Bio-MEMS applications in medical and health related technologies from
Lab-On-Chip to MicroTotalAnalysis (biosensor, chemosensor), or
embedded in medical devices e.g. stents.

 Interferometric modulator display (IMOD) applications in consumer
electronics (primarily displays for mobile devices), used to create
interferometric modulation − reflective display technology as found in
mirasol displays
 Fluid acceleration such as for micro-cooling
 Micro-scale energy harvesting including piezoelectric, electrostatic and
electromagnetic micro harvesters.
 Micromachined ultrasound transducers.
References
"WhatIs MEMS Technology?" WhatIs MEMS Technology? N.p., n.d. Web.
28 Apr. 2014.
"Fabricating MEMS and Nanotechnology." Fabricating MEMS and
Nanotechnology. N.p., n.d. Web. 28Apr. 2014.
D. J. Nagel and M. E. Zaghloul,“MEMS: Micro Technology, MegaImpact,”
IEEECircuits Devices Mag.,pp. 14-25, Mar. 2001.
K. W. Markus and K. J. Gabriel,“MEMS: The Systems Function Revolution,”
IEEEComputer, pp. 25-31, Oct. 1990.
K. W. Markus, “Developing Infrastructureto Mass-ProduceMEMS,” IEEE
Comput. Sci. Eng., Mag., pp. 49-54, Jan. 1997.
M. E. Motamedi, "Merging Micro-optics with Micromechanics: Micro-Opto-
Electro-Mechanical (MOEM) devices", Critical Reviews of Optical Science
and Technology, V. CR49, SPIEAnnualMeeting, Proceeding of Diffractive
and Miniaturized Optics, page 302-328, July, 1993
http://seor.gmu.edu/student_project/syst101_00b/team07/components.h
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