mems based pressure sensors

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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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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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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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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 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 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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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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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

mems based pressure sensors

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