Surface-enhanced thermal pulse measurements of the thermal and electrical properties of polyimide films

Polyimide films formed on silicon by using spin coating and curing techniques, or commercial polyimide foil glued to a copper substrate, have shown a sharp surface-bound negative charge layer extending to a depth of 30 nm as revealed by the thermal pulse method. The origin of this charge layer, which is insensitive to applied voltages, is believed to be environmental electrochemistry involving moisture, but other mechanisms are possible. Under an applied voltage, additional charge is observed to penetrate the dielectric deeper in the bulk, consistent with injection from the substrate. Separate analysis of the two contributions to the thermal pulse signals yields both bulk and near-surface thermal properties. Preliminary measurements acid analysis suggest that the near-surface diffusivity is a factor of two smaller than the bulk value.