AMBIENT HUMIDITY AND MOISTURE - A DECISIVE FAILURE SOURCE IN PIEZORESISTIVE SENSORS

Piezoresistive sensors have found widespread applications (e.g. in the car industry, chemical processes, industrial measurement techniques, etc.). In many cases a hermetic encapsulation of the sensor elements i s impossible, so that humidity, moisture or other forms of pollution i n disturbing quantities can attack the sensor chip besides the well-kn own influence of the ambient temperature. In this paper we present the results of metrological investigations that deal with the influence o f humidity and moisture on the stability of the output voltage of piez oresistive sensors. The main conclusion is that the influence of humid ity and moisture on the sensor behaviour cannot be neglected. The form ing of condensed water on the sensor surface causes changes in the sen sor offset voltage up to the nominal output voltage during time ranges of 0.1-0.5 h. Disturbing processes induced by condensation did not ca use any irreversible change in the zero offset voltage of the sensors during the investigation time range. The observed humidity-induced ins tabilities of sensor output voltage are caused by very complex physica l and chemical mechanisms, which are described in this paper.