Bulk micromachined piezoresistive devices

• The majority of commercially available micromachined pressure sensors
• These devices are etched from single-crystal silicon wafers,
  which have relatively well-controlled mechanical properties.
• The piezoresistors are fashioned by selectively doping portions of the diaphragm
  to form junction-isolated resistors.
• This form of isolation permits the designer to exploit the substantial piezoresistive coefficient of silicon
  and locate the resistors at the points of maximum stress on the diaphragm.

Surface micromachined piezoresistive devices

• Surface micromachined piezoresistive pressure sensors have also been reported.
• The structural material for the diaphragm may be either silicon nitride or polysilicon.
• This fabrication approach permits small devices with high packing density to be fabricated.
• The maximum deflection of the diaphragm is limited to the thickness of the sacrificial layer.

Temperature Sensitivity of Piezoresistive Pressure Sensors

Piezoresistance effect is inherently temperature dependent

Temperature sensitivity is a major concern for piezoresistive sensors

Piezoresistive sensors often require temperature-compensation circuitry

Different ways can be employed for the temperature compensation:

Wheatstone bridge configuration was shown to be effective for reducing the temperature coefficient
of offset (TCO)

Constant current source supply for the bridge is the easiest solution to compensate TCS.

Passive temperature compensation using simple resistor networks
can be used for the temperature ranges from 0oC to 85oC.

The disadvantage of this method is the reduced output signal caused by voltage dividers.

For temperature ranges from - 40oC to 125oC more extensive compensation networks are necessary.
The solution may be active temperature compensation that works with active devices
which supply a temperature dependent voltage supply of the bridge
or temperature dependent signal amplification.

More details

Related Reading

Carr J.J. Brown J.M., Introduction to Biomedical Equipment Technology, Third Ed., Prentice Hall Upper Saddle River, New Jersey, 1998.

Gad-el-Hak M. (ed.), The MEMS Handbook, CRC Press, 2002.

Middelhoek, S., Audet, S. A., Silicon sensors, Academic Press, 1989.

Rai-Choudhury P., Handbook of Microlithography, Micromachining, and Microfabrication, vol. 2, London, SPIE, 1997.

Rufer L., Les microsystemes electromecaniques. in Mir, S. (Ed.), Les applications des microsystemes sur silicium. Traite EGEM, Hermes Science Publications, pp. 19-64, 2002.

Senturia S. D., Microsystem Design, Kluwer Academic Publishers, 2001.