2. Books
1.G. K. Ananthasuresh, K. J. Vinoy, S. Gopalkrishnan K. N. Bhat, V. K. Aatre, Micro
and
Smart Systems, Wiley India, 2012.
2. S. E.Lyshevski, Nano-and Micro-Electromechanical systems: Fundamentals of
Nano-and
Microengineering (Vol. 8). CRC press, (2005).
3. S. D. Senturia, Microsystem Design, Kluwer Academic Publishers, 2001.
4. M. Madou, Fundamentals of Microfabrication, CRC Press, 1997.
5. G. Kovacs, Micromachined Transducers Sourcebook, McGraw-Hill, Boston,
1998.
6. M.H. Bao, Micromechanical Transducers: Pressure sensors, accelerometers,
and
Gyroscopes, Elsevier, New York, 2000.
5. Course Objective
To introduce :-
Basic terms and definitions of MEMS.
MEMS processes and transducer
mechanisms.
Course Outcomes
The students will be able to :-
List and define basic terminology used in
MEMS processes and transducers.
Describe transducer working mechanism.
10. Micro-electromechanical systems (MEMS)
is a process technology used to create tiny
integrated devices or systems that
combine mechanical and electrical
components.
They are fabricated using integrated circuit
(IC) batch processing techniques and can
range in size from a few micrometers to
millimetres.
11. MEMS devices (or systems) have the ability
to sense, control and actuate on the micro
scale, and generate effects on the macro
scale.
MEMS, an acronym, originated in the United
States
Microsystems Technology (MST) - Europe
and Micromachines in - Japan.
Regardless of terminology, the uniting factor
of a MEMS device is in the way it is made.
12. The device electronics are fabricated using
‘computer chip’ IC technology.
The micromechanical components are
fabricated by sophisticated manipulations of
silicon and other substrates using
micromachining processes.
Processes such as bulk and surface
micromachining, as well as high-aspect-ratio
micromachining (HARM) selectively remove
parts of the silicon or add additional structural
layers to form the mechanical and
electromechanical components.
13. While integrated circuits are designed
to exploit the electrical properties of
silicon.
MEMS takes advantage of either
silicon’s mechanical properties or both
its electrical and mechanical
properties.
14. MEMS can be found in systems ranging
across automotive, medical, electronic,
Communication and defence applications.
Current MEMS devices include
accelerometers for airbag sensors, inkjet
printer heads, computer disk drive read/write
heads, projection display chips, blood
pressure sensors, optical
switches,microvalves, biosensors.
16. MEMS devices are very small; their
components are usually microscopic.
Levers, gears, pistons, as well as
motors and even steam engines have
all been fabricated by MEMS
17. (a) A 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
[Reference: Sandia National Labs, SUMMiT *Technology,
http://mems.sandia.gov].
18. The DENSO Micro-Car is a miniature version of Toyota’s first passenger car. Fabricated
using MEMS, at 1/1000th the size of the original, it consists of a 0.67 mm magnetic-type working
motor and when supplied with 3 V 20 mA of alternating current through a 18 μm copper wire,
the
engine runs at 600 rpm equivalent to 5-6 mm/s
21. History
The history of MEMS is useful to illustrate
its diversity, challenges and applications.
The following list summarizes some of the
key MEMS milestones
22. 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.
He issues a public challenge by offering
$1000 to the first person to create an
electrical motor smaller than 1/64th of an
inch.
23. 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.
24. 1970’s
1970 First silicon accelerometer
demonstrated
1979 First micromachined inkjet
nozzle
25. 1980’s
Early 1980’s: first experiments in surface micromachined
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”. Instrumental
paper to entice the scientific community – reference for
material properties and etching data for silicon.
1982 LIGA Process
1988 First MEMS conference
26. 1990’s
Methods of micromachining aimed towards
improving sensors.
1992 MCNC starts the Multi-User MEMS Process
(MUMPS) sponsored by Defense Advanced
Research Projects Agency (DARPA)
1992 First micromachined hinge
1993 First surface micromachined accelerometer
sold (Analog Devices, ADXL50)
1994 Deep Reactive Ion Etching is patented
1995 BioMEMS rapidly develops
2000 MEMS optical-networking components
become big business
27. Principles
MEMS - mechanical microstructures,
microsensors, microactuators and
microelectronics, all integrated onto the
same silicon chip.
The designer applies the
Classical Lagrangian
Newtonian mechanics
Electromagnetics (Maxwell’s equations) to
study conventional electromechanical
systems and MEMS.
28. Microsensors detect changes in the
system’s environment by measuring
mechanical
thermal
magnetic
chemical or
electromagnetic information or
phenomena.
Microelectronics process this information
and signal the microactuators to react
and create some form of changes to the
Environment.
29. However, MEMS is not just about the
miniaturization of mechanical
components or making things out of
silicon.
MEMS is a manufacturing technology
A paradigm(way of seeing things) for
designing and creating complex
mechanical devices and systems as
well as their integrated electronics
using batch fabrication techniques.
30. Micro-optoelectromechanical systems
(MOEMS) is also a subset of MST and
together with MEMS forms the
specialized technology fields using
miniaturized combinations of optics,
electronics and mechanics.
32. MEMS has several distinct advantages as a
manufacturing technology.
Interdisciplinary nature of MEMS
technology and its micromachining
techniques
Batch fabrication
increased performance
reliability,
reduced physical size,
volume,
weight and
cost.
33. (a)The first commercial accelerometer from Analog
Devices (1990);its size is less than 1 cm2 (left) ,
and (b)
capacitive sense plates, 60 microns deep (right) .
34.
35. The accelerometer is essentially a
capacitive or piezoresistive device
consisting of a suspended pendulum proof
mass/plate assembly.
As acceleration acts on the proof mass,
micro machined capacitive or
piezoresistive plates sense a change in
acceleration from deflection of the plates.
36. Scaling Properties/Issues
Scaling theory is a valuable guide to
what may work and what will not.
By understanding how phenomena
behave and change as their scale size
changes.
We can gain some insight and better
understand the profitable approaches.
37. Why are we now interested in miniature
versions of devices and systems?
Reductions in cost, weight, and Power
consumption might be some of the reasons for
miniaturization.
But this might not be the case always.
Sometimes, a particular principle would not
even work if the mircosystems.
Certain micro-opto-mechanical devices do not
work at the macro level. In most other cases,
miniaturization is preferred because scaling
leads to several advantages
38. When miniaturizing any device or system, it is
critical to have a good understanding of the
scaling properties of the transduction
mechanism, the overall design, and the
material and the fabrication processes
involved.
The scaling properties of any one of these
components could present a great challenge.
Since MEMS devices can be thousands of
times smaller than their macroscale
counterparts, we cannot expect that the
macroscale phenomena and designs will
transfer directly to the microscale.
39. MEMS performance is inversely related to
size
The raw sensitivity of most sensors
decreases, however the frequency response
improves..
The fundamental limit of most MEMS sensor
system is thermal noise.
Temperature, the vibration of molecules,
causes all mechanical and electrical devices
to jitter around with an average kinetic energy
of a few thousands of a billionth of a billionth
of a Joule.
While objects on the macroscale are virtually
unaffected by this small amount of energy,
MEMS devices that are built on the
40. SCALING IN THE MECHANICAL DOMAIN
A fixed–fixed beam deformed under its
own weight.
41. Inertial effects are due to various accelerations
experienced by a body. These include
acceleration due to gravity, centrifugal and
centripetal accelerations, etc.
We cannot always neglect inertial effects. This is
because things move faster at small scales and,
hence, have significantly large inertial forces.
We note that the natural frequency of a beam is
inversely proportional to its size.
So, as the size decreases, the natural frequency
of free vibration goes up, indicating that small
beams vibrate at a much greater rate than large
beams of same proportions.
42. SCALING IN THE ELECTROSTATIC DOMAIN
Electrostatic force is widely used in
microsystems, but at the macroscale we
hardly use it.
What makes electrostatic force so
attractive at the microscale? The answer
is the scaling effect
43. SCALING IN THE THERMAL DOMAIN
Did you ever wonder why elephants have large ears ?
It has to do with scaling effects.
Metabolic activity in living creatures produces heat, and warm-
blooded animals must maintain a certain temperature.
The heat produced is proportional to the volume (cube of the
size) of the animal.
This heat is generally lost through the skin, that is, the surface of
the animal.
Because of scaling, we can say that large animals produce
much more heat than their surfaces can lose by convection as
compared with small animals.
This is because large animals have relatively more volume than
surface area.
In order not to overheat, large animals need special
appendages for increasing surface area. Large ears on
elephants are such appendages.
44. SCALING IN DIFFUSION
Diffusion is the phenomenon of spreading of
a species due to its concentration gradients
without the help of an external force.
Heat and electric currents flow because of
diffusion.
Diffusion happens when two different gases,
liquids, or solids come into contact with
each other.
The rate of diffusion depends on the diffusion
coefficient
46. 1. BULK MICROMACHINING
Bulk micromachining involves the removal of
part of the bulk substrate.
It is a subtractive process that uses wet
anisotropic etching or a dry etching method
such as reactive ion etching (RIE), to create
large pits, grooves and channels.
Materials typically used for wet etching
include silicon and quartz
while dry etching is typically used with silicon,
metals, plastics and ceramics.
47. 2.SURFACE MICROMACHINING
Surface micromachining involves processing above the
substrate, mainly using it as a foundation layer on which to
build.
It was initiated in the 1980’s
Material is added to the substrate in the form of layers of thin
films on the surface of the substrate (typically a silicon wafer).
These layers can either by structural layers or act as spacers,
later to be removed, when they are known as sacrificial layers.
Hence the process usually involves films of two different
materials:
A) A structural material out of which the free standing structure is
made (generally polycrystalline silicon or polysilicon, silicon
nitride and aluminium) and
B) A sacrificial material, deposited wherever either an open area
or a free standing mechanical structure is required (usually an
oxide).
48.
49. (HARM), which includes technology such as LIGA (a German
acronym from Lithographie, Galvanoformung, Abformung translated
as lithography, electroforming and moulding).
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 and includes
techniques known as LIGA.
Products micromachined with this technique include highaspect-
ratio fluidic structures such as moulded nozzle plates for inkjet
printing and microchannel plates for disposable microtitreplates in
medical diagnostic applications.
The materials that can be used are electroformable metals and
plastics, including acrylate, polycarbonate, polyimide and styrene.
51. Substrates
The most common substrate material for
micromachining is
SILICON
Other crystalline semiconductors
germanium (Ge) and
gallium arsenide (GaAs)
52. successful in the microelectronics industry and will
continue to be in areas of miniaturization for several
reasons:
i) silicon is abundant,
ii)inexpensive, and
iii)can be processed to unparalleled purity
silicon’s ability to be deposited in thin films is very
amenable to MEMS
high definition and reproduction of silicon device shapes
using photolithography are perfect for high levels of
MEMS precision
silicon microelectronics circuits are batch fabricated (a
silicon wafer contains hundreds of identical chips not just
one)
53. Lithography
There are processes for creating
material layers required in the fabrication
of microsystems.
The successful development of
microsystems involves successive steps
of deposition and patterning of various
material layers.
One of the key steps in patterning is the
process of transferring a geometrical
pattern on a mask to a radiation sensitive
material called a resist.
This process is known as lithography.
54. Wet/Dry etching processes
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.
55. 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 also
known as undercutting
The most common form of isotropic
silicon etch is HNA, which comprises a
mixture of hydrofluoric acid (HF), nitric
acid (HNO3) and acetic acid
56. Anisotropic etchants etch faster in a preferred direction.
Potassium hydroxide (KOH) -most common anisotropic
etchant -relatively safe to use.
Structures formed in the substrate are dependent on the
crystal orientation of the substrate or wafer.
Most such anisotropic etchants progress rapidly in the
crystal direction perpendicular to the (110) plane and
less rapidly in the direction perpendicular to the (100)
plane.
The direction perpendicular to the (111) plane etches
very slowly if at all. Figures 19c and 19d shows
examples of anisotropic etching in (100) and (110)
silicon.
Silicon wafers, originally cut from a large ingot of silicon
grown from single seed silicon, are cut according to the
crystallographic plane. They can be supplied in terms of
the orientation of the surface plane.
57. Deposition Processes
Evaporation, sputtering, CVD,
etc
One of the basic building blocks in
MEMS processing is the ability to
deposit thin films of material. In this
text we assume a thin film to have a
thickness anywhere between a few
nanometer to about 100 micrometer.
MEMS deposition technology can be
classified in two groups:
58. Depositions that happen because of
a chemical reaction:
◦ Chemical Vapour Deposition (CVD)
◦ Electrodeposition
◦ Epitaxy
◦ Thermal oxidation
Depositions that happen because of
a physical reaction:
◦ Physical Vapor Deposition (PVD)
◦ Casting
59. Film stress, exotic processes
Stress is defined microscopically as the force per unit area acting on
the surface of a differential volume element of a solid body.
Mechanical stress in thin films is an important reliability issue in
microelectronic devices and systems.
The presence of large stresses can lead to the formation of defects
that can cause device failure. The ability to control the magnitude of
stress during film formation is, therefore, crucial to the fabrication of
defect-free and reliable electronic devices and systems.
However, the origin of stress in thin films is still a subject of intense
debate. The development of a detailed understanding of the origin of
stress hinges on our ability to make accurate stress measurements
during and after film deposition.
To this end, two novel MEMS structures were developed to measure
the stress of thin films deposited using chemical vapor deposition
(CVD).
60. Deposited films will often be under mechanical stress.
A further factor limiting film thickness and the structures
that can be created is mechanical stress in the
deposited films.
Too much stress will lead to the structure buckling or
the films’ wrinkling or cracking.
(Buckling is the sudden change in shape of a
structural component under load such as the
bowing of a column under compression or the
wrinkling of a plate under shear)
High-temperature nitride films have a particular
problem: they exhibit high tensile stresses within the film
and cannot be deposited directly onto silicon.
A stress-relieving layer of oxide is required.
62. It is possible to control the stress in films by altering the
deposition parameters or the composition of the resulting
film (using PECVD to deposit a hydrogen-contaminated
silicon oxynitride layer, for instance).
The mechanical, electrical, and chemical (etching) properties
of the film will all be affected by the deposition parameters
used, so it is necessary to carefully characterize and monitor
each process (a demanding and time-consuming job) or seek
out a foundry that has experience with the processes required
for the device under development.
Nitride films have higher stress but are mechanically harder
and chemically more resilient (to attach to and with respect to
diffusion of ions or moisture).
Signs of Stress: Stress in films may cause one of the following
several problems:
Cracking or wrinkling of the film
Strings peeling off from sharp corners
Twisting or buckling of structures (particularly, cantilever
beams)
Buckling of the silicon wafer (in extreme cases)
63. Mechanical Transducers
There is a tremendous variety of direct
mechanical sensors that have been or
could be micromachined depending
on their sensing mechanism (usually
piezoresistive, piezoelectric or
capacitive) and the parameters
sensed (typically strain, force and
displacement).
64. Piezoresistive sensors
As a result of the piezoresistive effect
(defined as the change in resistivity of
the material with applied strain),
changes in gauge dimension result in
proportional changes in resistance in the
sensor.
The piezoresistive effect in
semiconductors is considerably higher
than in traditional metals, making silicon
an excellent strain sensor. MEMS
piezoresistors are readily manufactured
using bulk silicon doped with p-type or
n-type impurities.
65. Piezoelectric sensors utilize the piezoelectric effect in
which an applied strain (or force) on a piezoelectric
crystal results in a potential difference across the
crystal. Similarly, if the crystal is subjected to a
potential difference, a displacement, or strain, is
produced.
The effect can be used to sense mechanical stress (i.e.
displacement) and as an actuation mechanism,
although displacements are small even for large
voltages. Common piezoelectric materials used for
MEMS applications include quartz, lead zirconate
titanate (PZT), polyvinylidene fluoride (PVDF) and ZnO,
PVDF and ZnO being the most common. Silicon is not
piezoelectric; hence a thin film of a suitable material
must be deposited on the devices.
66. Capacitive sensors
Capacitive (or electrostatic) sensing is
one of the most important (and widely
used) precision sensing mechanisms
and includes one or more fixed
conducting plates with one or more
moving conducting plates. Capacitive
sensing relies on the basic parallel-
plate capacitor equation shown
below. As capacitance is inversely
proportional to the distance between
the plates, sensing of very small
displacements is extremely accurate.
67. Resonant sensors
MEMS resonant sensors consist of
micromachined beams or bridges which
are driven to vibrate at their resonant
frequency. They can be attached to
membranes or designed to adhere to a
particular substance (as in the case of a
biosensor).
Movement of the membrane or
increased build-up of the binding
substance will affect the resonant
frequency and can be monitored
using implanted piezoresistors.
68. a) Strain gauge - a strain gauge is a conductor or
semiconductor that is fabricated on or bonded
directly to the surface to be measured.
An example of a polysilicon strain sensor unable to
be fabricated by any other method than MEMS is
an implantable piezoresistive strain gauge to
measure forces in heart and brain tissue.
b) Accelerometer - accelerometers sense
acceleration by using a suspended proof mass on
which external acceleration can act (Figure 7).
Upon acceleration (or deceleration), a force
(F=ma) is generated on the proof mass resulting
in displacement. The force or displacement is
usually measured by piezoresistive and capacitive
methods.
69. b) Accelerometer - accelerometers
sense acceleration by using a suspended
proof mass on which external
acceleration can act (Figure 7). Upon
acceleration (or deceleration), a force
(F=ma) is generated on the proof mass
resulting in displacement. The force or
displacement is usually measured by
piezoresistive and capacitive methods.
71. c) Gyroscope – a gyroscope is a device
that measures the rotation rate and
detects inertial angular motion. As a
result it can be found, for example, in
transportation, navigation and missile
guidance applications. It relies on
measuring the influence of the Coriolis
force on a body in a rotating frame. MEMS
gyroscopes typically use vibrating
structures because of the difficulty of
micromachining rotating parts with
sufficient useful mass.
72. d) Pressure sensor - MEMS pressure
sensors are usually based around thin
membranes with sealed gas or
vacuum-filled cavities on one side of
the membrane and the pressure to be
measured on the other side.
Piezoresistive and capacitive membrane
deflection measurement techniques are
most commonly used in commercial
pressure sensors.
73. Transduction methods
The process of converting one form of
energy to another is known
as transduction.
A transducer is a device
that converts energy from one form to
another.
74. Accelerometers
An accelerometer is a device that
measures proper acceleration. Proper
acceleration, being
the acceleration (or rate of
change of velocity) of a body in its
own instantaneous rest frame.
75. Highly sensitive accelerometers are
components of inertial navigation systems for
aircraft and missiles. Accelerometers are
used to detect and monitor vibration in
rotating machinery. Accelerometers are used
in tablet computers and digital cameras so
that images on screens are always displayed
upright. Accelerometers are used in drones
for flight stabilization. Coordinated
accelerometers can be used to measure
differences in proper acceleration, particularly
gravity, over their separation in space; i.e.,
gradient of the gravitational field.
76. Conceptually, an accelerometer
behaves as a damped mass on a
spring.
When the accelerometer experiences
an acceleration, the mass is displaced
to the point that the spring is able to
accelerate the mass at the same rate
as the casing.
The displacement is then measured to
give the acceleration.
77. Gyroscope
Gyroscope is a device that measures the
rotation rate and detects inertial angular
motion.
As a result it can be found, for example,
in transportation, navigation and missile
guidance applications.
It relies on measuring the influence of
the Coriolis force on a body in a rotating
frame. MEMS gyroscopes typically use
vibrating structures because of the
difficulty of micromachining rotating parts
with sufficient useful mass.
78. Pressure sensor - MEMS pressure
sensors are usually based around thin
membranes with sealed gas or
vacuum-filled cavities on one side of
the membrane and the pressure to be
measured on the other side.
Piezoresistive and capacitive
membrane deflection measurement
techniques are most commonly used
in commercial pressure sensors.
79. MEMS microphones
The application of MEMS (microelectro-
mechanical systems) technology to
microphones has led to the development
of small microphones with very high
performance.
MEMS microphones offer high SNR, low
power consumption, good sensitivity, and
are available in very small packages that
are fully compatible with surface mount
assembly processes.
MEMS microphones exhibit almost no
change in performance after reflow
soldering and have excellent
temperature characteristics.
80. Mechanical structures:
These are non-moving structures, such as
microbeams and microchannels.
Miniature mechanical structures showing
(a) polymer mesopump; (b) silicon nano
tip fabricated using bulk micromachining.
81. Actuators
Sensors are transducers that convert
mechanical, thermal, or other forms of
energy into electrical energy,
actuators do the exact opposite.
82. An actuator is a device that is responsible for
moving or controlling a mechanism or
system.
It is controlled by a signal from a control
system or manual control. It is operated by a
source of energy, which can be mechanical
force, electrical current, hydraulic fluid
pressure, or pneumatic pressure, and
converts that energy into motion.
An actuator is the mechanism by which a
control system acts upon an environment.
The control system can be simple (a fixed
mechanical or electronic system), software-
based (e.g. a printer driver, robot control
system), a human, or any other input.
