A PIEZORESISTIVE HUMIDITY SENSOR

This paper deals with a new type of humidity sensor. Whereas conventional 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 bending stresses will be transformed into an output voltage by a piezoresistive 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 stability owing to qualified passivation concepts for the piezoresistors, (iii) the well-known opportunities to miniaturize humidity sensors using semiconductor technologies and micromechanics, (iv) use of only conventional IC processes. The fabrication process of the developed humidity sensor is nearly like that of a common pressure sensor. The first fabricated and tested piezoresistive humidity sensors showed a comparable behaviour to capacitive thin-film humidity sensors, that means fast transient response (T90 almost-equal-to 20-25 s) and high accuracy (down to 1% r.h.).