HUMIDITY SENSING PROPERTIES AND ELECTRICAL PERMITTIVITY OF NEW PHOTOSENSITIVE POLYIMIDES

We present experimental and theoretical results for the electrical permittivity of new photosensitive as well as humidity-sensitive polyimides, of potential use in capacitive humidity microsensors. Our designs for the latter are based on interdigitated comb-like microelectrodes for which we have developed a fully three-dimensional simulation program to calculate the capacitance. We solve the Laplace equation, subject to appropriate boundary conditions, for the electric potential and field distributions outside the electrodes using the isoparametric finite-element method. The behaviour of the permittivity of these materials, as a function of relative humidity (RH), is studied at different temperatures, frequencies and thicknesses. Their response to humidity is linear for a wide range of RH. The permittivity changes are modelled using an effective-medium theory (ODEEM). Our modelling work is relevant for gas sensors in general, since gases are known to affect the permittivity of polymeric materials.