Self Heating in Piezoresistive Pressure Sensors at Cryogenic Temperatures

Abstract

Piezoresistive pressure transducers are being used in cryogenic environments for measuring dynamic and absolute pressures without the use of capillary tubes. A common type of piezoresistive pressure transducer is constructed on a micromachined silicon diaphragm that has an implanted resistance bridge on its surface. It is well known that these transducers have temperature dependent properties that are regular and reproducible and which can be accommodated via standard calibration techniques. However, we have found that at temperatures below 30 K some of these sensors display secondary effects caused by self heating. Self heating in a piezoresistive pressure sensor operating at cryogenic temperatures causes calibration errors due to the thermally dependent sensitivity changes of the transducer (about 1%/K). A similar calibration problem occurs when the sensors are used to measure pressure fluctuations in a cryogenic gas that is also changing temperature, especially below 30 K. Although the self heating problem can be avoided by the use of very low excitation currents, the problem of measuring pressure fluctuations in a gas undergoing temperature changes is much more difficult and less tractable.