Evolution of MEMS

➢As microelectronics ruled 20th century, MEMS will be in 21th

century.

➢Evolution of microelectronics with 1st transistor in 1948.

➢Is growing rapidly in the past 50 years as given by Moore’s Law

➢Miniaturization of signal conditioning

➢Silicon sensors, first silicon strain gauge in 1958.

➢ 1959-A talk by Richard Feynman, a physicist on “There’s plenty

of room at the bottom “ issued a challenge and subsequently
challenge was met.
 What is MEMS?
Micro-Electro-Mechanical Systems (MEMS) is the integration of
mechanical elements, sensors, actuators, and electronics on a common
silicon substrate through micro fabrication technology.
A technology to create any integrated devices or systems that combine
mechanical and electrical components. These devices have the ability
to sense, control and actuate on a micro scale and generate effects on
macro scale.
A device that consists of micro machines and microelectronics where
micro machines are controlled by microelectronics.
A system or a device that has static and movable components with some
dimension on the scale of micron.
 Advantages
• Smaller systems tend to move quickly than larger systems because of lower inertia of
mass.

• Minute size of small devices encounter fewer problems in thermal distortion and
vibration. Smaller systems with lower mass has much higher natural frequency than
those expected from most machines and devices in operations.
• Reduced Manufacturing time and Cost
• Low power consumption
• Miniaturisation

portability
Easily and massively deployed
Easily maintained and replaced
For applications in medicine and surgery.
Satellite and spacecraft engineering.

• High accuracy in motion and dimensional stability make them suitable for
telecommunication systems.
 Why do we make things small?

– Cost Reduced
– Compatibility
• Batch Fabrication • Integration with IC/
• Larger wafer in diameter electronics
– Speed Increased • Capability of Arrays
• Shorter distance between • Avoidable Drawbacks
elements – Noise Amplification
• Reduce RC delay – High Developing Cost
– Rigidity Enhanced – Fundamental Limitations
• Very High Resonant
Frequency
• Mostly Single Crystal
Silicon. No Fatigue!
Evolution of MEMS
1950’s
• 1958 Silicon strain gauges commercially available
• 1959 “There’s Plenty of Room at the Bottom” – Richard Feynman gives a milestone presentation at California Institute of
Technology. Issues a public challenge byoffering $1000 to the first person to create an electrical motor smaller than 1/64 th of
an inch.

1960’s
• 1961 First silicon pressure sensor demonstrated
• 1967 Invention of surface micromachining. Westinghouse creates the Resonant Gate Field Effect Transistor, (RGT).
Description of use of sacrificial material to free micromechanical devices from the silicon substrate.

1970’s
• 1970 First silicon accelerometer demonstrated
• 1979 First micro machined inkjet nozzle

1980’s
• Early 1980’s first experiments in surface micro machined silicon. Late 1980’s micromachining leverages microelectronics
industry and widespread experimentation and documentation increases public interest.
• 1982 Disposable blood pressure transducer
• 1982 “Silicon as a Mechanical Material” [9]. Instrumental paper to entice the scientific community – reference for material
properties and etching data for silicon.
• 1982 LIGA Process
• 1988 First MEMS Conference

1990’s
• Methods of micromachining aimed toward improving sensors.
• 1992 MCNC starts the Multi-User MEMS Process (MUMPS) sponsored by Defense Advanced Research Projects Agency
(DARPA)
• 1992 First micro machined hinge
• 1993 First surface micro machined accelerometer sold (Analog Devices, ADXL50)
• 1994 Deep Reactive Ion Etching is patented
• 1995 BioMEMS rapidly develop
• 2000 MEMS Optical-networking components become big business
 Comparison of ME & MST
Microelectronics Micro system Technology

Uses SCS die, Si compounds. Uses SCS die and GaAs, quartz, polymers and
metals

Stationary structures May involve moving components

Primary 2D structures Complex 3D structures

Fewer components in assembly Many components to be assembled.

Mature IC design methodology Lack of engineering design methodology and

standards

Manufacturing techniques are proved and well Distinct Manufacturing techniques

documented

Packaging technology is well established Packaging Technology is at the infant stage.

 Applications
• Automobile Industry • Aerospace Industry
Tire pressure sensor Cockpit Instrumentation
Engine oil sensor Micro gyroscope
Combustion sensor Micro satellite
Fuel rail pressure sensor
• Safety • Industrial Products
Air Bag Deployment system Water level controls
Antilock braking systems Refrigeration systems
Navigation (micro gyroscope ) Manufacturing process sensor
• Engine and power train
Airflow control • Consumer products
Fuel pump pressure and fuel injection control Smart Toys
Crankshaft positioning Sport shoes with automatic cushioning control
Washers with water level controls
• Health care Industry Vacuum cleaning
Disposable blood pressure transducer (DPT)
Intrauterine pressure sensor (IUP)
Angioplasty pressure sensor • Telecommunications
Infusion pump pressure sensor Optical switching and fiber-optic couplings
Sphygmomanometer RF switches
Lung capacity meters Tunable resonators
Kidney dialysis equipment
 Application cont..

• There are plenty of applications for MEMS. As a breakthrough

technology, MEMS is building synergy between previously unrelated
fields such as biology and microelectronics, many new MEMS and
Nanotechnology applications will emerge, expanding beyond that
which is currently identified or known.
• The commercial applications include:
• 1. Inkjet printers, which use piezo-electrics or thermal bubble ejection
to deposit ink on paper.
2. Accelerometers in modern cars for a large number of purposes
including airbag deployment in collisions.
3. Accelerometers in consumer electronics devices such as game
controllers, personal media players / cell phones and a number of
Digital Cameras.
4. In PCs to park the hard disk head when free-fall is detected, to
prevent damage and data loss.
 Application cont..

• 5. MEMS gyroscopes used in modern cars and other applications to

detect yaw; e.g. to deploy a roll over bar or trigger dynamic stability
control.
6. Silicon pressure sensors e.g. car tire pressure sensors, and disposable
blood pressure sensors.
7. Displays e.g. the DMD chip in a projector based on DLP technology
has on its surface several hundred thousand micromirrors.
8. Optical switching technology, which is, used for switching
technology and alignment for data communications.
9. Interferometric modulator display (IMOD) applications in consumer
electronics (primarily displays for mobile devices).
10. Improved performance from inductors and capacitors due the
advent of the RF-MEMS technology
Micro Accelerometers
Micro Accelerometers
Micro Flying Robot
World’s Smallest Car
World’s Smallest Guitar
Blood Pressure Sensors
 Pressure sensors

www.evgroup.com
Digital Micro Mirrors
Poly silicon Electrostatic Micro
motor
MedicalApplication
 Multidisciplinary Nature of Micro
system Design and Manufacture
Areas Involved Engineering Disciplines Involved

➢ Electro chemistry ➢Mechanical

➢ Electro hydrodynamics ➢Electrical
➢ Molecular Biology ➢Chemical
➢ Plasma Physics ➢Materials
➢ Scaling Laws ➢Industrial
➢ Quantum Physics
➢ Molecular Physics
SILICON AS MATERIAL FOR
MEMS
 Si as a ideal substrate material
• Mechanically stable and can be integrated into electronics on the same
substrate.
• Has the same young’s modulus as steel, but is as light as Al .
• M.P. of about 1400 Celsius, which is twice as high as Al.
Dimensionally stable even at high temperatures.
• Thermal expansion coefficient is about 8 times smaller than that of
steel, and is more than 10 times smaller than that of Al.
• Virtually no mechanical hysteresis. Si wafers are extremely flat and
accept coatings and additional thin-film layers for building micro
structural geometry or conducting electricity.
• Treatments and fabrication processes for Si substrates are well-
established and documented
 Si- Compatible Material system
• Single crystal silicon
• Polysilicon
• Silicon nitride
• Silicon dioxide
Atomic Order of a Crystal Structure
Amorphous Atomic Structure
Crystal Structure
 Unit Cell in 3-D Structure

Unit cell
Faced-centered Cubic (FCC) Unit Cell
 Silicon structure
• In an FCC lattice, each atom is bonded to
12 nearest neighbor atoms.

• The lattice structure Si, however is more

A complex. It can be considered the result of
two inter-penetrating FCC crystals.

• A Si unit cell thus has 18 atoms with 8

B atoms at the corners plus 6 atoms on the
faces and another 4 interior atoms.

• Many perceive the crystal structure of Si

Fig: Merger of 2 FCC crystals to be a diamond lattice at a cubic lattice
spacing of 0.543nm.

•Spacing between the diamond subcell is

0.235 nm.
Silicon has the basic diamond crystal structure –
two merged FCC cells offset by a/4 in x, y and z.
 Diamond lattice
of Si

Si atom has 18 atoms with _ 8 @ corners

_ 6 on faces
_ and another 4 interior atoms
 Crystal planes

➢ [111] direction is defined by a vector of 1 unit in x, y and z.

➢ A popular method of designating crystal planes and orientation is the

Miller indice.These indices are effectively used to designate planes of
materials in cubic crystal families
Designation of cubic crystal
(0,1,0) (0,0,1)

(1,0,1)
Silicon atoms on three designated planes

0.543 nm 0.768 nm 0. 768nm

0.768 nm
(100) plane (110) plane
(111) plane
Atoms at corners of cube

Atoms at the centre of faces

Atoms at interior of unit cell

 Plane (1 1 1)

Lattice distances between adjacent atoms

are shortest for (1 1 1) plane
This makes attractive forces between the
atoms stronger on (1 1 1).
(1 1 1) contains 3 of the 4 atoms that are
situated at the centre of faces of the unit
cell.
Hence growth of the crystal is slowest in
this plane,and fabrication process
proceeds slowest….eg. etching
 Silicon wafers

Because of the orientation dependant machinability of

Si substrates…Wafers supplied indicate which direction
the cuts have been made.

Primary flats: used to indicate the crystal orientation of

wafer structure

Secondary flats: used to indicate the dopent type of the

substrate
 The diverse young’s moduli and
shear moduli of elasticity of silicon crystals

Miller index for Young’s Shear modulus

orientation modulus G, GPa
E, GPa
<100> 129.5 79.0

<110> 168.0 61.7

<111> 186.5 57.5

Silicon Production
Czochralski (CZ)
crystal growing
 CZ Crystal Puller

Crystal puller
and rotation
mechanism Crystal seed

Single crystal Molten

silicon polysilicon

Quartz Heat shield

crucible
Carbon heating
element Water jacket
• All Si wafers come
from “Czochralski”
grown crystals.
• Polysilicon is melted,
then held just below
1417 °C, and a single
crystal seed starts the
growth.
• Pull rate, melt
temperature and
rotation rate control
the growth
 Mechanical properties of Si

• Si is an elastic material with no plasticity or creep below

800 Celsius.
• Virtually no fatigue failure under all circumstances.
• It is brittle material. This, undesirable brittle fracture
behavior with weak resistance to impact loads need to be
considered in the design of micro system.
• The Si structure is anisotropic. This makes accurate stress
analysis of Si struc tedious ,since directional mechanical
property must be included.
 Silicon Dioxide
• Uses:-
1. as a thermal and electric insulator.
2. as a mask in the etching of Si substrates.
3. as a sacrificial layer in micromachining
• It has much stronger resistance to most etchants than Si.
• It can be produced by heating Si in an oxidant such as
oxygen with or without steam.
 Silicon Carbide and silicon nitride

• Principal application is its dimensional and chemical stability at high

temperatures.
• Thin films of SiC are often deposited over MEMS components to protect them
from extreme temperatures.
• Dry etching with Al masks can easily pattern the thin SiC film.
• Silicon nitride
It provides an excellent barrier to diffusion of water and ions such as sodium.
• Its ultra strong resistance to oxidation and many etchants makes it suitable for
masks for deep etching.
• Applications include optical wave guides, encapsulants to prevent diffusion of
water and other toxic fluids into the substrate.
• It is also used as high strength electric insulators and ion implantation masks.
 Polysilicon

• By chemical vapor deposition. It has become a Si in

polycrystalline form can be deposited onto Si substrates
principal material in surface micromachining.
• It is widely used in the IC industry for resistors, gates for
transistors, thin–film transistors.
• Highly doped Polysilicon can drastically reduce the
resistivity of Polysilicon to produce conductors and control
switches. They are the ideal materials for micro resistors
as well as easy ohmic contacts.
 Polymers and Ceramics

• It is widely used in MEMS.

1. as a photo resist.
2. as a functional materials.
3. as a conductive polymers.
4. as a electrical insulators.
5. as a EMI and RFI shielding.
6. And Encapsulation of micro sensors and packaging of micro devices.

CERAMICS
• It is a Functional materials in MEMS.
• To make a Micro components.
• Encapsulation to prevent it from high temperature.
 Scaling Issues
• Two types of scaling.
1. Scaling of Geometry.
- That is size
2. Scaling of Phenomenological behavior.
- Size and Material property is involved in scaling law.

• Volume related phenomena decreases much more rapidly than surface

related phenomena Which in turn decreases much more rapidly than length
related phenomena.

Length 10 L 0.1, Surface 100 L² 0.01, Volume 1000 L³ 0.001

-Dragan Fly
Surface /Volume Ratio= High
-Elephant
Surface /Volume Ratio= Low
 Scaling In Microfludics
Q=(πа4∆P)/8μL, Where а -Radius of the tube and
∆P-Pressure drop over the length tube L
Vav = Q/πа²
∆P/L = 8μV av/а²
Scaling laws for Q α а4 (fluid flow in micro channels)
∆P/L α а‫־‬²

Ex: If the radius is reduced by a factor of 10, Q= а4, the volumetric flow reduces by 10,000 times.
∆P/L α а‫־‬² - Pressure drop per unit length increases by 100 times
So , the volumetric flow by pressuring in micro domain is unfavourable.
• The surface force ‘f’, which is proportional to the surface area of the inner wall of the tube,
scales much favorably, than the pressuring means.
Surface Area=2π а L
Equivalent volume of the fluid= π а² L
Surface Area/Volume ratio=2 / а
Surface force α a ‫־‬¹

.
Basic Steps in Micro fabrication
There are only 3 basic steps for building
microstructures

»Deposition
»Pattern Definition (lithography)
»Etching
 Micro-Fabrication Overview
For typical processes
Wafers Needed for previous
Devices
2D example
Sacrificial Etch

Deposition Lithography Etch

•Oxidation or •Add resist •Wet isotropic or
•Deposition •Transfer pattern •Wet anisotropic or
•Remove resist •RIE

Repeat as Necessary
 Deposition
Depositions that happen because of a chemical
reaction:
– Chemical Vapor Deposition– AP, LP, PE (CVD)
– Electro deposition
– Epitaxy
– Thermal oxidation
Depositions that happen because of a physical
reaction:
– Physical Vapor Deposition (PVD)
 Evaporation
• Evaporation is a common method of thin film
deposition. The source material is evaporated in a
vacuum. The vacuum allows vapor particles to
travel directly to the target object (substrate),
where they condense back to a solid state.
Evaporation is used in micro fabrication, and to
make macro-scale products such as plastic film.
• Thermal evaporation is based on sublimating of a
heated material onto a substrate in a vacuum.
 Evaporation

Thermal Evaporation Electron Beam Evaporation

 Sputtering
• A process carried out by plasma.

• Plasma is a gas that carries electrical

charges.

• It contains equal number of electrons

and positively charged ions with
extremely high kinetic energy that can
be used to perform plasma enhanced
vapor deposition, etching, ion
implantation and sputtering.

• It is preferred due to a wider choice of

materials to work with, better step
coverage and better adhesion to
substrate
Chemical Vapor Deposition (CVD)
 Oxidation
• Forming Silicon Dioxide by oxidation
– O2 chamber @ 800-1200 oC (steam optional)
– Silicon reacts with O2 to grow oxide
– 2 micron max film thickness

O2 chamber O2 chamber
3hr, 1000 ºC 1μm
Oxide

Si Wafer Si Wafer
 Oxidation

• Forming Silicon Dioxide by oxidation

– Uses: insulation, masking, sacrificial layer
– Easy to grow and etch (HF)
– Good insulator/ diffusion barrier (Ex. B, P, As)
– High dielecytic breakdown field (500 V/mm); ρ= 1016 ohm-cm
– Good Ge/ GaAs => higher mobility/direct bandgap, but no stable
oxide => Can’t make MOS device

O2 chamber O2 chamber
3hr, 1000 ºC 1μm
Oxide

Si Wafer Si Wafer
 Chemical Vapor Deposition
• Gases react to deposit film on surfaces
• Example: Polysilicon at 580-650C
Si H4 → Si + 2H2
Typical CVD Furnace:

ICL, MIT
 Chemical Vapor Deposition

• Vapour Phase • Liquid phase

➢ Low pressure CVD (LPCVD) ➢ Electrolysis
➢ Atmospheric pressure CVD (APCVD) ➢ Electroplating
➢ Plasma Enhanced CVD (PECVD) ➢ Electrode less plating
➢ Photo enhanced CVD
➢ Laser enhanced CVD
➢ Epitaxy method
Materials are introduced in a heated furnace, chemical reaction occurs
on the surface of the wafer, resulting in deposition
Chemical Vapour Deposition

LPCVD PECVD
 Low Pressure Chemical Vapor
Deposition
• CVD furnace consists of a heated
quartz tube, a sample holder, a pump
and a set of gas injectors.
• The furnace is heated in an inert gas
until it reaches the deposition
temperature.
• Next gas is evacuated and reactive
species are introduced through injectors
at the deposition pressure.
• Many materials including
Polycrystalline silicon, silicon nitride,
silicon dioxide and refractory materials
can be deposited by LPCVD.
• LPCVD films are the highest quality
films available yielding the most
controllable mechanical characteristics.
• LPCVD can deposit films conformally
on the sample, this is highly desirable
for refitting and scaling cantilever
SiO2 can be deposited by several methods
• By reaction of silane and oxygen
500 c
SiH 4 ⎯⎯⎯ → SiO2 + 2H 2
• Dichlorosilane and water
900c
Sicl2 H2 + 2H2O ⎯⎯⎯
→ SiO2 + 2H2 + 2HCl

Silicon nitride
• Dichlorosilane and ammonia
800c
3Sicl2 H2 + 4NH3 ⎯⎯⎯ → Si3 N4 + 6H2 + 6HCl
 Pattern Transfer
2 ways
1. Mechanical Mask – stencil( brass, Phosphor Bronze)
2. Lithography – is a process of imprinting a geometric process from a
mask onto a thin layer of material called resist which is radiation
sensitive which in turn transfers the pattern to the underlying films
or substrates through etching process.

PROCESS STEPS

• Photo resist film coating

• Pre baking
• Alignment of mask
• Exposed to radiation
• Developing
• Post baking
 Lithography
• Positive Resist:
Exposed resist is removed in
developer
Ex: PMMA

• Negative Resist:
Non-Exposed resist is removed
in developer
Ex: Kodak KTFR

Types of Radiation:
1. UV-optical
2. X-ray
3. Electron beam
• The pattern transfer process is accomplished
by using a lithographic exposure tool that
emits radiation.
• The performance of the tool is determined
by 3 properties
➢ Resolution
➢ Registration
➢ Throughput
Resolution: The minimum feature size that
can be transferred with high fidelity to a
resist film on the surface of the film.

Registration: A measure of how accurately

patterns of successive masks can be aligned
with respect to the previously defined
patterns on a wafer.

Throughput: The number of wafers that can

be exposed per hour for a given mask level.
 Photolithography
• “Photoresist” (PR) used to transfer 7mm Positive Thick
patterns Resist
• Viscous PR spun onto wafer 1. Dispense, Spin
– Dispense, then spin
3500rpm
– Speed controls thickness
• Thickness: 1 – 10mm 2. Pre-bake 90ºC
• Requires baking (~100ºC) 60min
3. UV mask expose 15
sec
PR Dispenser
4. Develop in aqueous
Wafer solution
Chuck 5. Post-bake 90ºC
Spin Xrpm 30min
 Photolithography
• Expose PR to UV light thru mask
– Properties change in exposed regions
– Masks: laser etched or print-transferred chrome
• Positive resist – destroy bonds, soluble
• Negative resist – crosslinking, less soluble

We use: Positive Negative

Mask UV light
PR Expose
Substrate Substrate

PR Reaction
 Etching
• Wet etching where the material is dissolved when immersed
in a chemical solution
• Dry etching where the material is sputtered or dissolved
using reactive ions or a vapor phase etchant
Types of Dry etching
 Micromachining Techniques
• Bulk Micromachining
• Surface Micromachining
• LIGA
• Micro stereo lithography
Bulk Micromachining
(Additive Process)
Basic Sacrificial Layer Processing
 Surface Micromachining
(Subtractive Process)

Devices obtained by
1. Sacrificial layer
technique
2. Plasma etching and
Sacrificial layer
technique
3. Wet anisotropic
etching with IC
technology
 LIGA
German words for lithography, electroplating, and molding
High Aspect Ratio Micromachining Technique
Low cost coplanar waveguide
LIGA - Process
Major Fabrication Steps in LIGA
Process
 LIGA pros & cons
Advantages:
• High aspect ratio micro-structures can be built.
• Allows fabrication in polymers and other materials
Disadvantages:
• Requires synchrotron radiation for X-rays;
masks expensive
• Mostly only single mask structures; complex 3D is difficult
• Integration difficult
• Good for small parts, but most useful devices require
assembly
 Stereo lithography

❖ SL is suitable for fabricating complex 3D

microstructures with high aspect ratios.
❖ These Type were fabricated “freeform” directly from
computer aided design (CAD) files without the use of
molds. These prototypes were used for visualization
of complex shapes not easily seen or understood on
conventional drawings.
 Stereolithography

X-Y Scanner

Laser Beam forming Computer

Head optics
Iw

Power
Software
supply

2
w
0
Photo Polymer
Resin
 Micro Stereo Lithography
• SL and MSL are same but Except the resolution
of MSL is low..
• MSL widely used to fabricate High Aspect ratio
and complex 3D structures.
Classification of MSL:
➢ Scanning MSL.
➢ Projection MSL.
➢ Broad Spectrum of materials to create MEMS
devices.
Examples
An overview of mechanical design of Microsystems
Definition of Computer Aided Design in
Microsystems Technology
In MEMS technology, CAD is defined as a
tightly organized set of cooperating computer
programs that enable the simulation of
manufacturing processes, device operation and
packaged Microsystems behavior in a
continuous sequence, by a Microsystems
engineer.

MEMS simulation:
1. System level simulation
2. Process level simulation
Top down / Bottom up
 MEMS CAD Motivation
• Match system specifications
– Optimize device performance
– Design package
– Validate fabrication process
• Shorten development cycle
• Reduce development cost
 Some tools
• Device design:
Cadence, LEdit, Spice, MATLAB, …
• Process design:
TSuprem (fabrication crosssection)
IntelliSuite, AnisE (bulk silicon etching)
• Analysis:
FEM systems, analytic tools,
MEMCAD, IntelliSuite, ANSYS, Coventorware
 Coventor ware
• The coventorware has four modules namely
• Designer - Designer is a front end tool for MEMS device construction;
generates 2 dimensional (2D) device layout and 3 dimensional (3D)
model, includes layout editor, process designer and Material Data Base
(MDB).
• Analyzer is a group of solvers based on finite element analysis, for
detailed device analysis requiring mechanical, electromechanical,
thermo electro mechanics, optics, fluidics, piezoresistive, piezoelectric,
etc..
• System builder is a tool to extract detailed design specific behavioral
model from the analyzer and provides a system model evaluation
environment for the designs built from the bottom up approach.
• Architect is a system level simulation tool for MEMS devices and sub
systems.
Reference:MEMS and Micro systems :Design
and manufacturing, by Tai Ran Hsu

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