SILICON CAPACITIVE ACCELEROMETER.docx

MEMS UNIT 2

1. SILICON CAPACITIVE
ACCELEROMETER
PIEZORESISTIVE PRESSURE SENSORCONDUCTOMETRIC GAS SENSOR ELECTROSTATIC COMB-DRIVE


Category

Sensor

Sensor

Sensor

Actuator and sensor.

PurposeMeasures the acceleration of the
body on which the sensor is
mounted
Measures the pressure, typically of
gases or liquids
It detects and quantifies the presence
of a gas, that is, its concentration
As an actuator: sets something into
motion or applies force on something.
As a sensor: measures displacements by
quantifying capacitance
change

Key wordsProof mass, suspension,
capacitance

Piezoresistivity, diaphragm

Adsorption, desorption
Linear electrostatic actuator, suspension,
fixed and moving comb
fingers

Principle of

operation
Converts displacement caused by
the inertial force on the proof-mass
to
a voltage signal via a change in
capacitance between movable and
fixed parts.
External pressure loading causes
deflection, strain, and stress on the
membrane. The strain causes a
change in the resistance of the
material,which is measured using
the Wheatstone bridge
configuration.
The principle is that a suitable
catalyst, when heated to an
appropriate temperature, either
promotes or reduces the oxidation of
the combustible gases. The additional
heat released by the oxidation
reaction can be detected.
By applying external voltage between
fixed and moving combs,
an electrostatic force is generated that
provides the actuation
along the length of the comb fingers


Application
Automotive, aerospace, machine
tools, biomedical, consumer
products, etc
Automotive, aerospace, appliances,
biomedical, etc
Environmental monitoring,
automotive application, air
conditioning inairplanes, spacecraft,
living spaces,sensor networks, breath
analyzers, food control applications,
etc.
1. In closed-loop capacitive sensors
where a feedback force is to be
provided.
2. In opto-mechanical systems to move
optical elements such as mirrors and
lenses.

2. 3. In resonant sensors and gyroscopes to
set a component into motion.


Material
used
1. Single-crystal silicon to form the
physical structure.
2. Silicon dioxide sandwiched in a
silicon-on-insulator (SOI) wafer
gives electrical
isolation.
3. Handle-layer of the SOI wafer is
the substrate.
4. Gold for electrodes.
The material used to form this
device is single-crystal silicon
The materials used for making these
are films of metal oxide such as SnO2
and TiO2.
1. Polysilicon and single-crystal silicon to
form the physical structure.
2. Silicon dioxide as a sacrificial layer
material.
3. Silicon nitride for electrical insulation.
4. Gold for electrodes.
Fabrication
process
This is a bulk micromachining. In
general, almost any
microfabrication process can be
used to make an accelerometer.
Bulk micromachining with bipolar
circuitry plus glass frit wafer-
bonding is the fabrication
process.
Gas sensors are fabricated using
single-crystalline SnO2 nanobelts.
Nanobelts are synthesized by thermal
evaporation of oxide powders under
controlled conditions in the absence
of a catalyst.
Fabrication is mainly by surface
micromachining. Sandia National
Laboratories uses a very sophisticated
surface micromachining process called
SuMMiT (Sandia Ultra Multilayer
Microfabrication Technology) that has
five movable and released polysilicon
structural layers with four gap layers
created by the sacrificial oxide

Advantages
1. Very low sensitivity to
temperature-induced drift.
2. Higher output levels than other
types.
3. Amenability for force-balancing
and hence for closed-loop
operation.
4. High linearity.
1. Compact size, making them
suitable for a variety of applications,
including those
that use an array of such sensors to
measure pressure distribution.
2. Good thermal stability, since
thermal compensation can be built
into the sensor.
3. Good market potential due to low
Micromachining simplifies this
through miniaturization and also
reduces the cost

3. cost.
Typical
Applications
1. Consumer: airbag deployment
systems in cars, active suspensions,
adaptive brakes,
alarm systems, shipping recorders,
home appliances, mobile phone,
toys, etc.
2. Industrial: crash-testing robotics,
machine control, vibration
monitoring, etc.
3. High-end applications:
military/space/aircraft industry
navigation and inertial
guidance, impact detection, tilt
measurement, high-shock
environments, cardiac
pacemaker, etc
1. Direct pressure-sensing
applications: such as weather
instrumentation, combustion
pressure in an engine cylinder or a
gasturbine, appliances such as
washing machines, etc.
2. Altitude-sensing applications:
such as in aircraft, rockets, satellites,
weather
balloons, where the measured
pressure is converted using an
appropriate formula.
3. Flow-sensing applications,
manifold pressure sensing: in
automobiles, etc.
1. Environmental monitoring.
2. Exhaust gas sensing in automobiles.
3. Air conditioning in airplanes,
spacecrafts, houses, and sensor
networks.
4. Ethanol for breath analyzers.
5. Odor sensing in food-control
applications, etc
Actuators: resonators, microgrippers,
force balancing
accelerometers and microscanners.
Sensors: automotive, aerospace,
machine tools, biomedical, space
applications, etc.
Principle of
Operation
1. Proof mass: the inertial mass
used in the accelerometer whose
displacement
Diaphragm: a thin sheet of a flexible
material, anchored at its
circumference, over which
1. Conductivity: a material property
that quantifies the material’s ability to
conduct
1. Electrostatics: the phenomena arising
from stationary or slowly moving electric
charges. Electrostatic phenomena arise

4. Key
Definitions
relative to a rigid frame is a
measure of the influence of
external acceleration.
2. Suspension: the compliant
structure by which the proof mass
is suspended fromthe frame.
3. Capacitance:the capacity of a
body to hold an electrical charge.
Capacitance is also a
measure of the amount of electric
charge stored for a given electric
potential.
Differential capacitance
arrangement: in this arrangement,
there are three plateswith a
movable middle plate. As the plate
moves, the capacitance between
one of the pairs will increase while
that of the other decreases. This
gives a signal that is
linearlyproportional to the applied
acceleration.
Quality factor: a system’s quality
factor, Q, describes the sharpness
of the system’sdynamic response.
differential pressure is applied.
Piezoresistivity: the dependence of
electrical resistivity on mechanical
strain. Polysilicon shows substantive
piezoresistivity
electric current when an electric
potential (difference) is applied. It
depends on the
number of free electrons available.
2. Adsorption: the process of
collection and adherence of ions,
atoms or molecules on a
surface. This is different from
absorption, a much more familiar
term. In absorption, the species
enterinto the bulk, that is,the
volume;in adsorption, they stay put
on the surface.
3. Desorption: the reverse of
adsorption; species (ions, atoms or
molecules) are given
out by the surface.
4. Combustion: a technical term for
burning: a heat-generating chemical
reaction
between a fuel (combustible
substance) and an oxidizing agent. It
can also result in
light (e.g. a flame).
from the forces that electric charges
exert on one another, as described by
Coulomb’s law.
2. Parallel-plate capacitor: a capacitor
consists of two conductors separated by
a
nonconductive region that may be
dielectric medium or air gap. The
conductors contain equal and opposite
charges on their facing surfaces, and the
medium
contains an electric field.
3. Folded-beam suspension: a planar
suspension to suspend a body so that it
can
move freely with low stiffness in one
planar direction but is stiff in the other
planar direction as well as in the out-of-
plane direction. This is a compliant-
design equivalent of a sliding joint.