Effects of aging on polyimide: a model for dielectric behavior (of relative humidity sensors)

Quantifying the effect of moisture on the electrical properties of thin dielectric films requires the use of a model that takes into account the geometry of the test structure and the dielectric properties of the air layer above the device. Using this approach, it was found that the electrical properties of dry PI (polyimide) do not change substantially after six weeks of aging at 85 degrees C/85% RH (relative humidity). The dielectric permittivity of PI as a function of %RH increased with aging for excitation frequencies between 12 Hz and 100 kHz. This increase is correlated with observed changes in the surface chemistry of the aged PI film. Shifts in ESCA peak intensities indicate oxidation of the PI surface and deimidization with aging. Possible mechanisms for the observed changes in dielectric properties with aging include a change in the morphology of the bulk PI, resulting in a decreased surface to volume ratio, increased water absorption, a change in the interaction of absorbed water with the PI matrix, and dielectric interfacial effects.<>