HUMIDITY-DEPENDENT MECHANICAL-PROPERTIES OF POLYIMIDE FILMS AND THEIRUSE FOR IC-COMPATIBLE HUMIDITY SENSORS

Thin polyimide layers have been investigated to determine their volume expansion as a result of humidity absorption and desorption, respecti vely. Furthermore, we have used the experiments in order to clarify th e physical and chemical processes of polyimide films in a wet environm ent as well as their humido-mechanical properties. It can be shown tha t the humidity-induced material extension is fairly linear in a wide r ange of humidity. The observed material behaviour strongly depends on the conditions of the polyimide fabrication process. Analogous to the well-known temperature extension coefficient, a humidity extension val ue of about 60 to 80 ppm/%RH is observed. This physical effect can be used to create piezoresistive humidity sensors working similarly to bi metal elements influenced by temperature.