2. What are MEMS?
o MEMS is an acronym for Micro-Electro-Mechanical Systems.
o The first M (micro) indicates the small size of MEMS devices.
Any engineering system that performs electrical and
mechanical functions with components in micrometers is a
MEMS. (1 µm = 1/10 of human hair)
o The E (electro) refers to electricity, often in the form of
electrostatic forces.
o The second M (mechanical) refers to the fact that these tiny
devices have moving parts.
o Lastly, S (systems) indicates that “electro” and “mechanical”
go together, that the electricity and moving parts are
integrated into a single system on a MEMS device.
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3. What are MEMS?
o Micro Electro Mechanical Systems (MEMS ) are devices
that have static or movable components with some
dimensions on the scale of microns.
o MEMS combine microelectronics and micromechanics,
and sometimes micro-optics
They are referred by different names in different countries
o MEMS : USA
o MicroSystemsTechnology (MST): EUROPE
o Micromachines : JAPAN
o Smart materials and Smart Structures: India
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4. What are MEMS?
o Micro-electromechanical systems (MEMS) is a process
technology used to create tiny integrated devices or
systems that combine mechanical and electrical
components.
o They are fabricated using integrated circuit (IC) batch
processing techniques and can range in size from a few
micrometers to millimetres.
o These devices (or systems) have the ability to sense,
control and actuate on the micro scale, and generate
effects on the macro scale.
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5. How are MEMS made?
• Let us first consider creating a
thin, flexible diaphragm that may
ultimately be used as part of a
MEMS pressure sensor.
• We start with a thin silicon
substrate, called a wafer, typically
measuring 200-400 µm thick fig
a.
• A thin layer of silicon dioxide
(SiO2) is then “grown” on the
wafer by placing it in a furnace at
an elevated temperature fig b.
• Next, a thin layer of
photosensitive material called
photoresist, or simply resist, is
deposited on the SiO2 layer in a
process called spinning Fig c.
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6. • A transparent plate with selective
opaque regions called a mask is then
brought in close proximity to the
wafer Fig.d.
• Ultraviolet light is shown through
the mask Fig e.
• On the regions of the photoresist
that make contact with the UV light,
the resist
• undergoes a photochemical process
in which it hardens and becomes
less soluble. (This is true for a
negative resist. If a positive resist
were used, then the exposed regions
would become more soluble.) The
unexposed resist is removed by
using a chemical called a developer,
leaving a portion of the SiO2 layer
exposed Fig.f.
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9. MEMS and Microsystems
MEMS as a Microsensor
Microsensors (To sense and detect certain physical,
chemical, biological and optical quantity and convert it
into electrical output signal)
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11. MEMS and Microsystems
Microactuators (to operate a device component, e.g.,
valves, pumps, electrical and optical relays and switches;
grippers, tweezers and tongs; linear and rotary motors;
micro gyroscopes, etc.
MEMS as a Microatuator -Motor
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14. MEMS and Microsystem Products
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Micromotors
• All three components Rotor Stator andTorque transmission gear made
with Nickel.
• The toothed rotor,Which has diameter 700µm.
• Gear wheel 250µm.
• The gap between the rotor and the Axle and between Rotor and the
Stator is 4µm.
• The Height of the unit 120µm
16. Microturbines
• The turbine is made by Nickel.
• The rotor has a diameter of 130µm.
• A gap between the axle and the rotor is 5µm.
• The turbine height is 150µm.
• The maximum rpm 150,000perminute with lifetime up to 100
million rotations.
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17. Micro- optical components
• These components are used for high speed signal
transmission in the telecommunication industry.
• It is silicon based manufacturing process.
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21. Evaluation of Microfabrication
• The origin of modern Micro fabrication to the
invention of transistors by W. Schockley, J. Bardeen, and
W. H. Brattain in 1947.
• The IC concept first evolved from the production of a
monolithic circuit at RCA in 1955 after the invention of
transistors.
• The first IC was produced 3 years later by Jack Kilby of
TI.
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22. Microelectronics
1. Stationary structures.
2. Transmit electricity for specific
electrical functions.
3. IC die is protected from
contacting media
4. Use single crystal silicon dies,
silicon compounds, ceramics
and plastic materials
5. Fewer components to be
assembled
Microsystems
1. May involve moving
components
2. Perform a great variety of
specific biological, chemical,
electromechanical and optical
functions
3. Delicate components are
interfaced with working media
4. Use single crystal silicon dies
and few other materials, e.g.
GaAs, quartz, polymers, ceramics
and metals
5. Many more components to be
assembled
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23. Microelectronics
1. Complex patterns with high
density of electrical circuitry
over substrate
2. Mature IC design
methodologies.
3. Large number of electrical
feed-through and leads
4. Industrial standards available
5. Mass production.
6. Fabrication techniques are
proven and well documented
7. Manufacturing techniques are
proven and well documented
Microsystems
1. Lack of engineering design
methodology and standard.
2. Simpler patterns over substrates
with simpler electrical circuitry.
3. Fewer electrical feed-through and
leads.
4. No industrial standard to follow in
design, material selections,
fabrication processes and packaging.
5. Batch production, or on customer-
need basis.
6. Many microfabrication techniques
are used for production, but with
no standard procedures.
7. Distinct manufacturing techniques.
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28. Miniaturization Makes Engineering Sense!!!
• Small systems tend to move or stop more quickly due to low
mechanical inertia. It is thus ideal for precision movements and for
rapid actuation.
• Miniaturized systems encounter less thermal distortion and
mechanical vibration due to low mass.
• Miniaturized devices are particularly suited for biomedical and
aerospace applications due to their minute sizes and weight.
• Small systems have higher dimensional stability at high
temperature due to low thermal expansion.
• Smaller size of the systems means less space requirements. This
allows the packaging of more functional components in a single
device.
• Less material requirements mean low cost of production and
transportation.
• Ready mass production in batches.
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29. Applications of Microsystems in the
Aautomotive Industry
• 52 million vehicles produced worldwide in 1996There will be 65
million vehicle produced in 2005.
• Principal areas of application of MEMS and microsystems.
• Safety
• Engine and power train
• Comfort and convenience
• Vehicle diagnostics and health monitoring
• Telematics, e.g. GPS, etc
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32. Application of MEMS and Microsystems in
Biomedical Industry
• Disposable blood pressure transducers: Lifetime 24 to 72
hours; annual production 20 million units/year, unit price
$10
• Catheter tip pressure sensors
• Sphygmomanometers
• Respirators
• Lung capacity meters
• Barometric correction instrumentation
• Medical process monitoring
• Kidney dialysis equipment
• Micro bio-analytic systems: bio-chips, capillary
electrophoresis, etc.
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33. Application of MEMS and Microsystems in
Aerospace Industry
• Cockpit instrumentation.
• Sensors and actuators for safety -e.g. seat ejection
• Wind tunnel instrumentation
• Sensors for fuel efficiency and safety
• Microsattellites
• Command and control systems with MEMtronics
• Inertial guidance systems with microgyroscopes, accelerometers
and fiber optic gyroscope.
• Attitude determination and control systems with micro sun and
Earth sensors.
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34. Application of MEMS and Microsystems in
Aerospace Industry
• Power systems with MEMtronicswitches for active solar cell
array reconfiguration, and electric generators.
• Propulsion systems with micro pressure sensors, chemical
sensors for leak detection, arrays of single-shot thrustors,
continuous microthrusters and pulsed microthrousters.
• Thermal control systems with micro heat pipes, radiators and
thermal switches.
• Communications and radar systems with very high bandwidth,
low-resistance radio-frequency switches, micromirrorsand
optics for laser communications, and micro variable capacitors,
inductors and oscillators
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35. Application of MEMS and Microsystems in
Consumer Products
• Scuba diving watches and computers
• Bicycle computers
• Sensors for fitness gears
• Washers with water level controls
• Sport shoes with automatic cushioning control
• Digital tire pressure gages
• Vacuum cleaning with automatic adjustment of brush beaters
• Smart toys
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36. Application of MEMS and Microsystems in
theTelecommunication Industry
• Optical switching and fiber optic couplings
• RF relays and switches
• Tunable resonators
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38. Substrates and Wafers
▪ The frequently used term substrate in
microelectronics means a flat macroscopic object on
which micro-fabrication.
▪ Substrate serves an additional purpose: It acts a
signal transducer besides supporting other
transducer that convert mechanical action to
electrical out puts or vice versa.
▪ In semiconductor, the substrate is a single crystal cut
in slices from a larger piece called wafer.
▪ Wafers can be of silicon or other single crystalline
materiel such as a Quartz, or Gallium Arsenide.
39. Over view Materials for MEMS and
Microsystems
▪ This chapter will cover the materials used in “silicon-
based” MEMS and Microsystems. As such, silicon will be
the principal material to be studied.
▪ Other materials to be dealt with are silicon compounds
such as: SiO2, SiC, Si3N4 and polysilicon.
▪ Also will be covered are electrically conducting of
silicon piezoresistors and piezoelectric crystals for
electromechanical actuations and signal transductions.
▪ An overview of polymers, which are the “rising stars” to
be used as MEMS and Microsystems substrate
materials, will be studied too.
40. Why silicon is the most important in IC
Industry?
▪ Silicon (Si) is the most abundant material on earth. It
almost always exists in compounds with other
elements.
▪ Single crystal silicon is the most widely used substrate
material for MEMS and microsystems.
▪ The popularity of silicon for such application is
primarily for the following reasons.
41. Why silicon is the most important in IC Industry?
I. It is mechanically stable and it is feasible to be integrated into
electronics on the same substrate (b/c it is a semiconducting
material).
II. Electronics for signal transduction such as the p or n-type
piezoresistive can be readily integrated with the Sisubstrate-ideal
for transistors.
III. Silicon is almost an ideal structure material. It has about the
same Young’s modulus as steel (∼2x105MPa), but is as light as
aluminum with a density of about 2.3 g/cm3.
IV. It has a melting point at 1400oC, which is about twice higher than
that of aluminum. This high melting point makes silicon
dimensionally stable even at elevated temperature.
42. V. Its thermal expansion coefficient is about 8 times smaller than
that of steel, and is more than 10 times smaller than that of
aluminium.
VI. Silicon shows virtually no mechanical hysteresis. It is thus an ideal
candidate material for sensors and actuators.
VII. Silicon wafers are extremely flat for coatings and additional thin
film layers for either being integral structural parts, or performing
precise electromechanical functions.
VIII.There is a greater flexibility in design and manufacture with silicon
than with other substrate materials. Treatments and fabrication
processes for silicon substrates are well established and
documented.
43. Crystal Growing and Theory
❑ How the single crystal can be grown in practice.
❑ The starting point in any IC fabrication is we must have single crystal
silicon wafer so must get the substrate material.
❑ In order to get substrate material you must grow single crystal silicon.
❑ If its found nature as SiO2, so first a silicon dioxide is reduced in order to
obtain silicon its purified in order to get very high purity semiconductor
grade silicon i.e. 99.9999% purity.
❑ Crystal growth can be broadly classified as 2 types
1. Bridgman Technique ( NOT IN SYLLABUS)
2. Czochralski Technique
44. Czochralski Method
❑ Czochralski (CZ) is also known as liquid solid mono component growth
system.
❑ It has basically 4 subsystems in this CZ crystal growth system i.e.
1. Furnace
2. Crystal pulling mechanism
3. Ambient control and
4. Control system
Scheme of the Bridgman technique.
1 crucible, 2 growing crystal, 3 seed,
4 furnace.
45. ❑ The most important part in this furnace is the crucible i.e. a cup in which
charge is going to be placed and this is usually made up of quartz.
❑ Usually the crucible is made of quartz and it is a single, you can use it only
once because after the crystal growth when ever cooling down the system
thermal mismatch usually the quartz crucible is going to crack so you can
not reuse the crucible.
❑ The quartz crucible is usually placed inside a graphite susceptor. A susceptor
is you can view it outer jacket i.e. I have bigger cup of graphite in which I
am going to place the quartz cup.
❑ Heating is done by usually by RF.
• So inside the furnace
✓ quartz crucible
✓ graphite susceptor
✓ Heater
✓ Cooling for the outer quartz chamber
46. Crystal pulling mechanism
❑ A pull rod is pull up during the crystal growth but beware of Oxygen.
❑ I have the melt inside the crucible but you can remember the seed crystal
was not in contact with the melt, it was held some ware up while the charge
was below.
❑ After the charge is molten its uniformly in the liquid state then pull rod is
gradually load.
❑ Very slowly pull rod is pull up so what will happen the melt is contact with
the seed crystal will get solidified and pull up the solid crystal.
❑ But accurate control is necessary and the pull rate you should carefully
adjusted.
47. Seed
Single crystal silicon
Quartz crucible
Water cooled chamber
Heat shield
Carbon heater
Graphite crucible
Fig: Schematic of Crystal pulling mechanism
59. Silicon Compounds
▪ There are 3principal silicon compounds used in MEMS and
microsystems:
1. Silicon dioxide (SiO2)
2. Silicon carbide (SiC) and
3. Silicon nitride (Si3N4)
▪ Each Has distinct characteristic and unique applications.
60. Silicon Dioxide(SiO2)
▪ It is least expensive material to offer good thermal and
electrical insulation.
▪ Also used a low-cost material for “masks” in micro
fabrication processes such as etching, deposition and
diffusion.
▪ Used as sacrificial material in “surface micromachining”.
▪ Above all, it is very easy to produce:
✓ by dry heating of silicon: Si + O2→SiO2
✓ by oxide silicon in wet steam: Si + 2H2O →SiO2+ 2H2
62. Silicon Carbide (SiC)
❑ The principle applications of SiC in Microsystems is its
dimensional and chemical stability at high temperature.
❑ It has very strong resistance to oxidation even at very high
temperature.
❑ Thin films of silicon carbide are often deposited over MEMS
components to protect them from extreme temperature.
❑ Its very high melting point and resistance to chemical
reactions make it ideal candidate material for being masks
in micro fabrication processes.
❑ Using SiC in MEMS is that Dry etching with aluminium
masks can easily pattern the thin SiC film.
63. Silicon Nitride (Si3N4)
▪ Produced by chemical reaction:
3SiCl2H2+ 4NH3→Si3N4+ 6HCL + 6H2
▪ Used as excellent barrier to diffusion to water and ions.
▪ Its ultra strong resistance to oxidation and many etchants
make it a superior material for masks in deep etching.
▪ Applications of silicon nitride include optical waveguides,
encapsulants to prevent diffusion of water and other toxic
fluids into the substrate.
▪ Also used as high strength electric insulators.
64. Polycrystalline Silicon
▪ It is usually called “Polysilicon”.
▪ It is an aggregation of pure silicon crystals with randomly
orientations deposited on the top of silicon substrates:
66. Polycrystalline silicon –cont’d
• These polysilicon usually are highly doped silicon.
• They are deposited to the substrate surfaces to
produce localized “resistors” and “gates for
transistors”.
• Being randomly oriented, polysilicon is even stronger
than single silicon crystals.
77. Quartz-Cont’d
▪ Quartz is ideal material for sensors because of its extreme
dimensional stability.
▪ It is used as piezoelectric material in many devices.
▪ It is also excellent material for microfluidics systems used in
biomedical applications.
▪ It offers excellent electric insulation in microsystems.
▪ A major disadvantage is its hard in machining. It is usually
etched in HF/NH4F into desired shapes.
▪ Quartz wafers up to 75 mm diameter by 100 µm thick are
available commercially.
87. Polymers
❑ What is polymer?
Polymers include: Plastics, adhesives, Plexi glass and Lucite.
❑ Principal applications of polymers in MEMS:
Currently in biomedical applications and adhesive bonding.
New applications involve using polymers as substrates with
electric conductivity made possible by doping.
❑ Molecular structure of polymers:
It is made up of long chains of organic (hydrocarbon)
molecules.
The molecules can be as long as a few hundred nm.
❑ Characteristics of polymers:
Low melting point; Poor electric conductivity
Thermoplastics and thermoset sare common industrial
products
Thermoplastics are easier to form into shapes.
Thermosets have higher mechanical strength even at
temperature up to 350oC
88. Polymers as industrial materials
❑ Polymers are popular materials used for many
industrial products for the following advantages:
✓ Light weight
✓ Ease in processing
✓ Low cost of raw materials and processes for producing
polymers
✓ High corrosion resistance
✓ High electrical resistance
✓ High flexibility in structures
✓ High dimensional stability
89. Polymers for MEMS and microsystems
1) Photo-resist polymers are used to produce masks for creating desired patterns on
substrates by photolithography technique.
2) The same photoresistpolymers are used to produce the prime mold with
desirable geometry of the MEMS components in a LIGA processin micro
manufacturing.
3) Conductive polymers are used as “organic” substrates for MEMS and
microsystems.
4) The ferroelectric polymersthat behave like piezoelectric crystals can be used as
the source of actuation in micro devices such as in micro pumping.
5) The thin Langmuir-Blodgett (LB) film scan be used to produce multilayer
microstructures.
6) Polymers with unique characteristics are used as coating substance to capillary
tubes to facilitate effective electro-osmotic flow in microfluidics.
7) Thin polymer films are used as electric insulatorsin micro devices, and as dielectric
substancein micro capacitors.
8) They are widely used for electromagnetic interference (EMI) and radio frequency
interference (RFI) shielding in microsystems.
9) Polymers are ideal materials for encapsulation of micro sensors and the packaging
of other microsystems.
90. Conductive Polymers
❑ Polymers are poor electric conducting materials by
nature.
❑ A comparison of electric conductivity of selected
materials are:
92. Langmuir-Blodgett (LB) films
▪ The process was first introduced by Langmuir in 1917
and was later refined by Blodgett. That was why it is
called Langmuir-Blodgett process, or LB films.
▪ The processin volves the spreading volatile solvent over
the surface-active substrate materials.
▪ The LB process can produce more than one single
monolayer by depositing films of various compositions
onto a substrate to produce a multilayer structure.
▪ LB films are good candidate materials for exhibiting
ferro(iron)-, pyro(heat)and piezoelectric properties. LB
films may also be produced with controlled optical
properties such as refractive index and anti reflections.
▪ They are thus ideal materials for micro sensors and
optoelectronic devices.
93. Langmuir-Blodgett (LB) films –Cont’
❑ Following are a few examples of LB film applications in
microsystems:
❑ Langmuir-Blodgett (LB) films –Cont’d
(1)Ferroelectric (magnetic) polymer thin films:
▪ The one in particular is the Poly-vinylidenefluoride (PVDF).
▪ Applications of this type of films include:
- Sound transducers in air and water,
- Tactile sensors,
- Biomedical applications such as tissue compatibility, cardio-
pulmonary sensors and implantable transducers and sensors
for prosthetics and rehabilitation devices.
▪ As a piezoelectric source. The piezoelectric coefficient of PVDF is
given in Table 7-14.
(2) Coating materials with controllable optical properties:
Broadband optical fibers that transmit light at various wavelengths.
94. Langmuir-Blodgett (LB) films –Cont’d
(3) Microsensors:
• Many electrically conducting polymeric materials are sensitive to
the exposed gas and other environmental conditions. So they are
suitable materials for micro sensors.
• Its ability of detecting specific substances relies on the reversible
and specific absorption of species of interest on the surface of
thepolymer layer and the subsequent measurable change of
conductivity of the polymer.
