with thanks to all references

1. 1
Abstract
1. MEMS, an acronym that originated in the United States, also referred to as
Microsystems Technology (MST) in Europe and Micromachines in Japan is a process,
technology used to create tiny integrated devices or systems that combine mechanical
and electrical components. These devices (or systems) have the ability to sense, control
and actuate on the micro scale, and generate effects on the macro scale.
2. They are fabricated using integrated circuit (IC) batch processing techniques i:e
by combining silicon-based microelectronics with micromachining technology. They have
a magical range of size from a few micrometers to millimetres. This is a rapidly emerging
and promising technology for the 21st Century which combine electrical, electronic,
mechanical, optical, material, chemical, and fluids engineering disciplines and has the
potential to revolutionize both industrial and consumer products.
3. 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 have varied use in systems ranging across automotive, medical, electronic,
communication and defence applications. Its techniques and microsystem-based devices
have the potential to effect all of our lives and the way we live.

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Introduction
4. Microelectromechanical systems (MEMS) are small integrated devices or systems
that combine electrical and mechanical components. They range in size from the sub
micrometer level to the millimeter level and there can be any number, ranging from a few
to millions, in a particular system. 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. MEMS are not only about miniaturization of
mechanical systems; they are also a new paradigm for designing mechanical devices
and systems.
5. The interdisciplinary nature of MEMS relies on design, engineering and
manufacturing expertise from a wide and diverse range of technical areas including
integrated circuit fabrication technology, mechanical engineering, materials science,
electrical engineering, chemistry and chemical engineering, as well as fluid engineering,
optics, instrumentation and packaging. Current example of MEMS devices include
accelerometers for airbag sensors, microphones, projection display chips, blood and tire
pressure sensors, optical switches, analytical components such as lab-on-chip,
biosensors ,locks inertial sensors micro transmissions, micro mirrors, micro actuator
(Mechanisms for activating process control equipment by use of pneumatic, hydraulic, or
electronic signals) optical scanners, fluid pumps, transducer, pressure and flow sensors
and other products.
Fig 1 MEMS silicon motor together with a strand of human hair and (b) the legs of a
spider mite standing on gears from a micro-engine.

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What is MEMS Technology?
6. Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical
elements, sensors, actuators, and electronics on a common silicon substrate through
microfabrication technology. While the electronics are fabricated using integrated circuit
(IC) process sequences, the micromechanical components are fabricated using
compatible "micromachining" processes that selectively etch away parts of the silicon
wafer or add new structural layers to form the mechanical and electromechanical devices.
7. MEMS generally consist of mechanical microstructures, microsensors,
microactuators and microelectronics, all integrated onto the same silicon chip.The work
of various components is described as under:
a) Microelectronics acts as brain of the system . It receives data/info, process
this information and signal the microactuators to react and create some form of
changes to the environment.
b) Microsensors acts as arms ,eyes, nose etc. They collect data and detect
changes in the system’s environment by measuring mechanical, thermal,
magnetic, chemical phenomena or electromagnetic information and pass this
information to microelectronics for processing.
c) A microactuator acts as a switch or a trigger to activate an external device.
As the processed data is received. It takes decisions based on this data,
sometimes activating an external device.
d) Microstructure tiny structures built through micromachachining right into the
silicon of the MEMS. These microstructures can be used as valves to control the
flow of a substance or as very small filters.

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Fig 2 Block diagram of MEMS
8. MEMS technology is a paradigm shift in designing and creating complex
mechanical devices, systems and their integrated electronics using batch fabrication
techniques. MEMS are attractive for diverse applications from display technologies to
sensor systems to optical networks because of their small size and weight, which allow
systems to be miniaturized and less cost.
Fig 3 MEMS devices
Historical Background
8. The invention of the telephone at Bell Telephone Laboratories in 1947 sparked a
fast-growing microelectronic technology. Jack Kilby of Texas Instruments built the first

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Integrated circuit in 1958 using germanium (Ge) devices. It consisted of one transistor,
three Resistors, and one Capacitor. Later that same year Robert Noyce of Fairchild
Semiconductor announced the development of a planar double-diffused Si IC. IN 1959
Richard Feynman gave a milestone presentation at California Institute of Technology
“There’s Plenty of Room at the Bottom” there he issued a public challenge by offering
$1000 to the first person to create an electrical motor smaller than 1/64th of an inch.
9. The following list summarizes some of the key MEMS milestones:-
a) 1961: First silicon pressure sensor demonstrated.
b) 1967: Invention of surface micromachining. Description of use of sacrificial
material to free micromechanical devices from the silicon substrate.
c) 1970: First silicon accelerometer demonstrated.
d) 1979: First micromachined inkjet nozzle.
e) 1980: First experiments in surface micromachined silicon.
f) 1982: Disposable blood pressure transducer.
g) 1982: LIGA Process.
h) 1988: First MEMS conference held and the term MEMS was coined.
j) 1990: Methods of micromachining aimed towards improving sensors.
k) 1992: Multi-User MEMS Process (MUMPS) sponsored by Defence
Advanced Research Projects Agency (DARPA).
l) 2001: Tri-axis accelerometers appears in the market.
m) 2004: TI’s DLP chip sales rose to nearly $900 million
n) 2007: MEMS industry group (MEMS-IG) with founding members including
Xerox, Corning, Honeywell, Intel and JDS Uniphase formed.

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MEMS Material Description
11. MEMS technology can be implemented using a number of materials the choice of
which will depend on the device being created and the market sector in which it has to
operate. Some of the important materials used in MEMS fabrication are given below:
a) Silicon. The economies of scale, ready availability of cheap high-
quality materials and ability to incorporate electronic functionality make silicon
attractive for a wide variety of MEMS applications. Silicon also has significant
advantages engendered through its material properties. In single crystal form,
silicon is an almost perfect Hookean material, meaning that when it is flexed there
is virtually no hysteresis and hence almost no energy dissipation.
Fig 4 Crystallographic index planes of silicon
b) Polymers. Due to complexity and relative expense of crystalline silicon.
Polymers are also used as they can be produced in huge volumes, with a great
variety of material characteristics. MEMS devices can be made from polymers by
processes such as injection moulding, embossing or stereolithography and used
in microfluidic applications such as disposable blood testing cartridges.
c) Metals. Metals like Gold, Ni, Al, Cr, Pl, and Ag can also be used to
create MEMS elements. While not being as advantageous as silicon in terms of
mechanical properties. Still when used within limitations can exhibit very high
degrees of reliability. Metals can be deposited by electroplating, evaporation, and
sputtering processes.

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MEMS Fabrication Methods
12. A MEMS process is usually a structured sequence of following operations to form
actual devices.
Fig 5 MEMS fabrication block diagram
13. Some of the important and specific MEMS processes are discussed below:-
a) Photolithography. Photolithography is the photographic technique
to transfer copies of a master pattern, usually a circuit layout in IC applications,
onto the surface of a substrate (usually a silicon wafer). The substrate is covered
with a thin film of oxide material, usually silicon dioxide (SiO2), in the case of silicon
wafers, on which a pattern of holes will be formed. A thin layer of an organic
polymer called a photoresist, which is sensitive to ultraviolet radiation, is then
deposited on the oxide layer. A photomask is then placed in contact with the
photoresist coated surface. The wafer is exposed to the ultraviolet radiation which
enable the transferring of desired pattern to the photoresist which is then
developed in a way very similar to the process used for developing photographic
films. Photoresist material is of two types; positive and negative. (Fig 6). Positive
photoresist is weakened by UV radiation whereas negative photoresist is
strengthened. On developing, the rinsing solution removes either the exposed
areas or the unexposed areas of photoresist leaving a pattern of bare and
photoresist-coated oxides on the wafer surface.

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Fig 6 Photoresist and silicon dioxide in photolithography.
The resulting photoresist pattern is either the positive or negative image of the
original pattern of the photomask. A special chemical is used to attack and remove
the uncovered oxide from the exposed areas of the photoresist.
b) Etching. Etching is a process for creating the base structures like
trenches, cavities, final release of the membranes, cantilevers or free hanging
masses in surface micromachining. Etching or sacrificial etching involves the
undercutting by etching of a structure. This is of two types:-
i) Wet Etching. Wet etching describes the removal of material
through the immersion of a material (typically a silicon wafer) in a liquid bath
of a chemical etchant. These etchants can be isotropic or anisotropic.
Isotropic etchants etch the material at the same rate in all directions, and
consequently remove material under the etch masks at the same rate as
they etch through the material; this is known as undercutting. Isotropic
etchants are limited by the geometry of the structure to be etched.
Anisotropic etchants etch faster in a preferred direction. Potassium
hydroxide (KOH) is the most common anisotropic etchant as it is relatively
safe to use.

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Fig 7 Isotropic etching and anisotropic wet etching
(ii) Dry Etching. Dry etching relies on vapour phase or plasma-
based methods of etching using suitably reactive gases or vapours usually
at high temperatures. The two most common special dry MEMS etches are
xenon difluoride (XeF2) etching. It is an isotropic silicon etch process and
has a strong selectivity for silicon above Al, SiO2, Si3N4 and photoresist.
The typical etch rates are 1 to 3 μm/min and it is commonly used for release
etch. The other is Reactive Ion Etching (RIE) which utilizes additional
energy in the form of radio frequency (RF) power to drive the chemical
reaction. Energetic ions are accelerated towards the material to be etched
supplying the additional energy needed for the reaction; as a result the
etching can occur at much lower temperatures (typically 150º - 250ºC,
sometimes room temperature or even lower) than those usually needed
(above 500ºC).
Fig 8 Protective polymer deposition
(c) Powder blasting. Powder blasting is a flexible, cost-effective and
accurate technique for making fluidic channels and interconnections. A resist film
is laminated on the glass wafer and illuminated with UV light through a mask. After
development, Al2O3 particles are powder blasted on the substrate through a

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moving nozzle and the areas not covered by film are etched. Shaped wells can be
round or rectangular.
e) Physical Vapour Deposition Physical Vapour Deposition (PVD) is a
technique used to deposit thin films one atom (or molecule) at a time onto various
surfaces (e.g. onto semiconductor wafers). The coating source is physical (i.e.
solid or liquid) rather than chemical as in chemical vapour deposition. Evaporation
and sputtering are commonly used PVD process, for instance for the deposition of
aluminium or gold conductors.
f) Chemical Vapour Deposition Chemical Vapour Deposition (CVD) is a
chemical process used to produce high-purity, high-performance solid materials.
In a typical CVD process, the substrate is exposed to one or more volatile
precursors, which react and/or decompose on the substrate surface to produce
the desired deposit. Frequently, volatile by-products are also produced, which are
removed by gas flow through the reaction chamber.
g) Surface Micromachining Surface micromachining involves
processing above or in the top layers of the substrate, the substrate only using as
a carrier on which to build. Material is added to the substrate in the form of layers
of thin films. The process usually involves films of two different materials: a
structural material out of which the free standing structure is made (generally
polycrystalline silicon or polysilicon, silicon nitride or aluminium) and a sacrificial
material, deposited wherever either an open area or a free standing mechanical
structure is required (usually an oxide, but also resist or metals are used. A
sacrificial layer of oxide is deposited on the silicon substrate surface using a
pattern and photolithography. A polysilicon layer is then deposited and patterned
using RIE processes to form a cantilever beam with an anchor pad. The wafer is
then wet etched to remove the oxide (sacrificial) layer releasing the beam. More
complex MEMS structures can be made using several structural polysilicon and
sacrificial silicon dioxide layers, including sliding structures, actuators and free
moving mechanical gears.

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Fig 9 Surface micromachining using a sacrificial layer
h) Bulk Micromachining Bulk micromachining starts with the deposition
of a masking layer on both sides of the wafer, mostly LPVCD low stress silicon
nitride. In the most simple process, this mask is then structured and the wafer is
subsequently etched in KOH etch. Depending on the mask pattern cantilevers of
free hanging silicon nitride layers, cavities, membranes and wafer through holes
are formed.
j) High Aspect Ratio Micromachining. High-aspect-ratio micromachining
(HARM) is a process that involves micromachining as a tooling step followed by
injection moulding or embossing and, if required, by electroforming to replicate
microstructures in metal from moulded parts. It is one of the most attractive
technologies for replicating microstructures at a high performance-to-cost ratio.
Products micromachined with this technique include high aspect ratio fluidic
structures such as moulded nozzle plates for inkjet printing and microchannel
plates for disposable micro titre plates in medical diagnostic applications.
(k) LIGA. LIGA is German acronym lithographie, galvanoformung, abformung
(Lithography, Electroplating, and Molding) that describes a fabrication technology
used to create high-aspect-ratio microstructures. LIGA is an important tooling and
replication method for high-aspect-ratio microstructures. The technique employs
X-ray radiation to expose thick acrylic resist of PMMA under a lithographic mask.
The exposed areas are chemically dissolved and, in areas where the resist is
removed, metal is electroformed, thereby defining the final product or the tool
insert for the succeeding moulding step.

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Fig 10 LIGA Process
LIGA is capable of creating very finely defined microstructures up to 1000 μm high.
LIGA provides a radically new way to produce small precise micromachined parts
at relatively low cost. LIGA is an important tooling and replication method for high-
aspect-ratio microstructures. A compromise which combines some features of LIGA
with surface micromachining eliminating the need for exposure to X-rays has been
developed and is known as SLIGA (Sacrificial LIGA). SLIGA enable the production
of MEMS components with much lower manufacturing infrastructures in terms of
investment, facilities and access to advanced materials and technology.
Packaging
14. The proper operation of MEMS devices depends critically upon the ‘clean’
environment provided by the package and is considered an enabler for the
commercialisation of MEMS. Packaging of microsensors presents special problems as
part of the sensor requires environmental access while the rest may require protection
from environmental conditions and handling. MEMS package should:
a) Provide protection & be robust enough to withstand its operating
environment.
b) Allow for environmental access and connections to physical domain (optical
fibres, fluid feed lines etc).
c) Minimize electrical interference effects from inside and outside the device.

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d) Dissipate generated heat and withstand high operating temperatures.
e) Minimize stress from external loading.
f) Handle power from electrical connection leads without signal disruption.
Fig 11 MEMS packaging
Applications
15. From a very early vision in the early 1950’s, MEMS has gradually made its way
out of research laboratories and into everyday products. MEMS components have begun
appearing in numerous commercial products and applications in day to day life a brief is
as follows:
a) Automotive. Automotive has been the first mass market for
MEMS products and is the main driving force for the MEMS industry. There are
currently over 100 sensors in each modern, high end, car of which about 30 % is
MEMS products, mainly accelerometers, gyros, inclinometers, pressure- and
flow sensors (engine management: air intake, oil and coolant pressure, particle
and NOx emission). The increasing complexity of the cars, due to demands on
safety, driver and passenger comfort and environmental restrictions is aiding
MEMS market to grow for the coming years. Expected growth areas are: IR
sensors for air quality, accelerometers for motor maintenance, microscanners for
displays, energy scavengers for tire pressure management etc.

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Fig 12 (a) MEMS application
b) IT Peripherals. The major products within the IT peripherals market
are read/write heads and inkjet print heads. But are under pressure due to
alternative technologies offered respectively by solid state memories and laser
printing. New MEMS applications in this field include microphones,
accelerometers and RF MEMS products.
Fig 12 (b) MEMS in Ink jet printer
c) Telecommunication. The optical telecom market is growing steadily
over the coming years and MOEMS is playing an important role in this growth.
There are currently many MOEMS based concepts and technologies which are
being proposed and tested. The wireless market in general is becoming an
interesting sector with many new functionalities on offer by RF MEMS
components. Currently, MEMS resonators in particular are increasingly replacing
conventional quartz resonators.

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d) Consumer Electronics and Life Style Products Consumer electronics
is currently the most interesting area for suppliers of accelerometers,
microphones and other MEMS products. Apple’s I-Phone and Nintendo’s Wii
console are interesting examples; both use accelerometers for image
stabilisation and gaming control. Other interesting opportunities include:
microphones and zoom lenses in mobile phones and oscillators in watches. High
end mobile phones also employ inertial sensors such as accelerometers and
gyroscopes for applications such as scrolling, character recognition, gaming and
image stabilization.
Fig 12 (c) MEMS application in smart phones
e) Medical and Life Science Applications. There is a paradigm shift in
the present healthcare model. One of the enablers behind this is microfluidic
based Point of Care (PoC) instruments and other is Lab on Chip (LoC) devices.
The result is more effective, personalized, safe and cost-effective therapy, better
diagnosis and treatment; and, most importantly, increased patient satisfaction.
Microfluidics and LoC technologies offer advantages such as increased
sensitivity, mobility, and efficiency in assays as well as helping to multiply the
number of tests performed per day in laboratories.

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Fig 12(d) MEMS application in medical
f) Military areas. The major area were MEMS are used are Inertial
navigation units on a chip for munitions guidance and personal navigation,
Electromechanical signal processing for ultra-small and ultra-low-power wireless
communications, Distributed unattended sensors for asset tracking, environmental
monitoring, and security surveillance. Integrated fluidic systems for miniature
analytical instruments, propellant, and combustion control. Weapons safing, arming,
and fuzing. Embedded sensors and actuators for condition-based maintenance.
Mass data storage devices for high density and low power. Integrated micro-opto-
mechanical components for identify-friend-or-foe systems, displays, and fiber-optic
switches.
Fig 12 (e) MEMS application in military
Advantages of MEMS
17. 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

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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.
18 These can be classified into four main points.
(a) Ease of production.
(b) MEMS can be mass-produced and are inexpensive to make.
(c) Ease of parts alteration.
(d) Higher reliability than their macro scale counterparts.
Disadvantages of MEMS
19. Due to their size, it is physically impossible for MEMS to transfer any significant
power. MEMS are made up of Poly-Si (a brittle material), so they cannot be loaded with
large forces. MEMS is also a disruptive technology in that it differs significantly from
existing technology, requiring a completely different set of capabilities and competencies
to implement it. MEMS involves major scaling, packaging and testing issues, and, as a
disruptive technology, faces challenges associated with developing manufacturing
processes that no longer fit established methods. For the true commercialisation of
MEMS, foundries must overcome the critical technological bottlenecks, the economic
feasibility of integrating MEMS-based components, as well as the market uncertainty for
such devices and applications. Cost reduction is critical and will ultimately result from
better availability of infrastructure, more reliable manufacturing processes and technical
information as well as new standards on interfacing.

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The Future of MEMS
20. Some of the major challenges the MEMS industry is facing includes:
a) Access to Foundries. MEMS companies today have very limited
access to MEMS fabrication facilities, or foundries, for prototype and device
manufacture. In addition, the majority of the organizations expected to benefit from
this technology currently do not have the required capabilities and competencies
to support MEMS fabrication. Affordable and receptive access to MEMS
fabrication facilities is crucial for the commercialisation of MEMS.
b) Design, Simulation and Modelling. Due to the highly integrated and
interdisciplinary nature of MEMS, it is difficult to separate device design from the
complexities of fabrication. Consequently, a high level of manufacturing and
fabrication knowledge is necessary to design a MEMS device. Furthermore,
considerable time and expense is spent during this development and subsequent
proto type stage. It is important that MEMS designers have access to adequate
analytical tools. Currently, MEMS devices use older design tools and are
fabricated on a ‘trial and error’ basis. Therefore, more powerful and advanced
simulation and modelling tools are necessary for accurate prediction of MEMS
device behaviour.
c) Packaging and Testing. The packaging and testing of devices is
probably the greatest challenge facing the MEMS industry. MEMS package
typically must provide protection from an operating environment as well as enable
access to it. Currently, there is no generic MEMS packaging solution, with each
device requiring a specialized format. Consequently, packaging is the most
expensive fabrication step and often makes up 90% (or more) of the final cost of
a MEMS device.
d) Standardization. Due to the relatively low number of commercial MEMS
devices and the pace at which the current technology is developing,
standardization has been very difficult. To date, high quality control and basic
forms of standardization are generally only found at multi-million dollar (or billion
dollar) investment facilities. The networking of smaller companies and

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organizations on a global scale is extremely important and necessary to lay the
foundation for a formal standardization system.
e) Education and Training. The complexity and interdisciplinary nature of
MEMS require educated and well-trained scientists and engineers from a diversity
of fields and backgrounds. The current numbers of qualified MEMS-specific
personnel is relatively small and certainly lower than present industry demand.
Therefore, in order to match the projected need for these MEMS scientists and
engineers, an efficient and lower cost education methodology is necessary.
MEMS in India
21. In India first privately funded MEMS research lab was set up in Jul 2002 at
Bengaluru with collaboration between Indian Institute of Science (IISc) and Cranes
Software International Ltd. The lab’s primary objective was to conduct research in MEMS
and develop designs for MEMS-based devices. India’s fabrication facilities are located at
the Central Electronics Research Institute in Pilani, the Indian Technical Institute in
Bangalore, Bharat Engineering Co. Ltd. in Bangalore and Semiconductor Complex Ltd.
in Chandigarh. MEMS has been one of the thrust areas for most of the Microelectronics
laboratories for the last ten years. Various IIT laboratories are working in close interaction
with Indian industry such as BEL, Bangalore and have carried out several sponsored
research projects for DRDO, ISRO and DST. Many processes developed in this
laboratory were transferred to industry for commercialization. Projects are undergoing to
include MEMS sensors for acoustic applications and ultrasound sensors, besides
development of analysis tools and software for engineers working in the area.
Conclusion
21. The market for MEMS devices is still being developed but does not have the
explosive growth. Despite MEMS being an enabling technology for the development and
production of many new industrial and consumer products
22. The automotive industry, motivated by the need for more efficient safety systems
and the desire for enhanced performance, is the largest consumer of MEMS-based

20. 20
technology. In addition to accelerometers and gyroscopes, micro-sized tire pressure
systems are now standard issues in new vehicles, putting MEMS pressure sensors in
high demand. Such micro-sized pressure sensors can be used by physicians and
surgeons in a telemetry system to measure blood pressure at an early stage, allowing
early detection of hypertension and restenosis. Medical applications include the detection
of DNA sequences and metabolites. MEMS biosensors can also monitor several
chemicals simultaneously, making them perfect for detecting toxins in the environment.
23. Lastly, the dynamic range of MEMS based silicon ultrasonic sensors have many
advantages over existing piezoelectric sensors in non-destructive evaluation, proximity
sensing and gas flow measurement. Silicon ultrasonic sensors are also very effective
immersion sensors and provide improved performance in the areas of medical imaging
and liquid level detection.
References
24. The following were referred for the gathering of data and figures:-
a) “An Introduction to MEMS” published in 2002 by Prime Faraday
Partnership.
b) www.studymafia.org.
c) www.google.com.
d) www.wikipedia.com.
e) Introduction to Micro engineering http://www.dbanks.demon.co.uk/ueng/.
f) Technology watch.