A PIEZORESISTIVE HUMIDITY SENSOR

This paper deals with a new type of humidity sensor. Whereas conventio nal humidity sensors utilize the change of the specific resistivity or the dielectric properties here the humidity induced volume change of a polyimide layer leads to a deformation of a silicon membrane. The be nding stresses will be transformed into an output voltage by a piezore sistive Wheatstone bridge. This principle offers many advantages: (i) a separation of electrical transducing elements (piezoresistors) from measured humidity value, (ii) an advanced electrical long-term stabili ty owing to qualified passivation concepts for the piezoresistors, (ii i) the well-known opportunities to miniaturize humidity sensors using semiconductor technologies and micromechanics, (iv) use of only conven tional IC processes. The fabrication process of the developed humidity sensor is nearly like that of a common pressure sensor. The first fab ricated and tested piezoresistive humidity sensors showed a comparable behaviour to capacitive thin-film humidity sensors, that means fast t ransient response (T90 almost-equal-to 20-25 s) and high accuracy (dow n to 1% r.h.).