THE METROLOGICAL BEHAVIOR OF BIMORPHIC PIEZORESISTIVE HUMIDITY SENSORS

The paper describes some design aspects of newly developed bimorphic p iezoresistive humidity sensors. Their operation relies on the bimorphi c bending of a polymer-coated diaphragm as a result of the humidity-in duced swelling of the polymer, The mechanical bending is evaluated by means of piezoresistors located on the diaphragm, but physically separ ated 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 th e benefits and limitations of the piezoresistive humidity sensor, we h ave 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 ass uming a zero-humidity offset voltage and a linear relationship between the voltage and the humidity. The transfer functions were validated b y measurements on prototypes. The measurement accuracy and long-term s tability of the sensors were found to be limited by the temperature se nsitivity of the polymer's response function and by an initial voltage drift due to the relaxation of technologically induced mechanical str ess. These undesired effects can, however, be reduced by means of elec tronic temperature compensation and controlled artificial ageing.