The metrological behaviour of bimorphic piezoresistive humidity sensors

The paper describes some design aspects of newly developed bimorphic piezoresistive humidity sensors. Their operation relies on the bimorphic bending of a polymer-coated diaphragm as a result of the humidity-induced swelling of the polymer, The mechanical bending is evaluated by means of piezoresistors located on the diaphragm, but physically separated from the polymer. This physical separation between the mechanical and the electrical transducer is expected to improve the reliability and long-term stability of the sensor devices. In order to evaluate the benefits and limitations of the piezoresistive humidity sensor, we have developed empirical models based on consistent material parameters to describe the sensor behaviour. Transfer functions correlating the sensor output voltage to the relative humidity have been developed assuming a zero-humidity offset voltage and a linear relationship between the voltage and the humidity. The transfer functions were validated by measurements on prototypes. The measurement accuracy and long-term stability of the sensors were found to be limited by the temperature sensitivity of the polymer's response function and by an initial voltage drift due to the relaxation of technologically induced mechanical stress. These undesired effects can, however, be reduced by means of electronic temperature compensation and controlled artificial ageing.