mems based pressure sensors
TRANSCRIPT
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MEMS Based
Pressure Sensing
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What are MEMS?
Micro Electro Mechanical System is a highly miniaturized device or an
devices combining electrical and mechanical components that is fabrica
using integrated circuit (IC) batch processing techniques and can range
from micrometers to millimeters.
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Applications of MEMS
Accelerometers
Gyroscopes
Microphones
Pressure Sensors
Displays Bio-MEMS
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Types of MEMS Pressure Sensors
Piezoresistive Pressure Sensors
Capacitive Pressure Sensors
Resonant Pressure Sensors
Microphones
Optical Pressure Sensors
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Fabrication of Micro Machined Silicon
Diaphragms MEMS pressure sensors typically employ a diaphragm as the sensor
The most common fabrication method is anisotropic wet silicon etch
Diaphragm thickness can be controlled either by timing etch duratio
boron doping or by electrochemical etch stops. The diaphragms are generally rectangular.
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Piezoresistive Pressure Sensors
The Piezoresistive effect describes change in the electrical resistivity
semiconductor when mechanical stress is applied.
In Piezoresistive pressure sensors, piezoresistors (made of semicond
are diffused into the membrane.
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Piezoresistive Pressure Sensors
Piezoresistors are placed at four edges.
They are all oriented in the same direction.
The change In resistance is calculated from
= +
The two pairs of piezoresistors are placed on opposite sides of a full bridg The galvanometer reading in the
bridge circuit can be calibrated to
give pressure reading directly.
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Piezoresistive Pressure Sensors
The temperature sensitivity of piezoresistor is a drawback.
In the shown arrangement, since all the piezoresistors are at same
temperature, the temperature effects should cancel out. But due to
manufacturing tolerances, temperature coefficients will be different a
hence error arises.
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Overcoming the Drawbacks
A temperature sensor can be placed on board and an algorithm can b
generated to nullify temperature effects.
Inclusion of a dummy bridge on the sensor chip in addition to the p
sensitive bridge.
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Capacitive Pressure Sensors
Capacitive pressure sensors are typically based upon a parallel plate
arrangement whereby one electrode is fixed and the other flexible, th
diaphragm.
As the flexible electrode deflects under applied pressure, the gap bet
electrodes decreases and the capacitance increases. The change in capacitance can be measured using a bridge impedenc
and the pressure reading can be obtained.
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Capacitive Pressure Sensors
Capacitive sensors are highly sensitive to pressure, have low power consum
low temperature sensitivity.
The fixed electrode is another diaphragm that is free to deflect but does n
any pressure, and hence is acceleration insensitive.
Drawbacks associated with the capacitive approach are the inherently non
of the sensor.
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Overcoming the Drawbacks
Use of bossed diaphragms.
Sensing capacitance is measured from a particular part of the diaphr
(non linearity reduced at the expense of sensitivity).
Operating the sensor in touch mode, where the diaphragm is touchinfixed electrode.
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Resonant Pressure Sensors
This sensor works on the principle that the measurand typically alter
stiffness, mass, or shape of the resonator, hence causing a change in
resonant frequency.
Resonant pressure sensors typically use a resonating mechanical struc
a strain gauge to sense the deflection of the pressure-sensitive diaph
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Resonant Pressure Sensors
As the pressure is applied on the diaphragm, it is pushed in.
The nitride seal bends the mechanical member and subjects it to tens
Hence the resonating frequency of the member changes.
The resonating frequency can be measured and calibrated to give dirpressure readings.
A vacuum is trapped around the resonator, to prevent damping of
oscillations.
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Performance features of different types
sensing