Effects of Ultraviolet Photon Irradiation on the Dielectric Properties of Polyimide

Ultraviolet photons were irradiated to the surface of all-aromatic polyimide films using a KrF excimer laser or a KrCl excimer lamp, and electrical properties such as permittivity, dielectric loss factor, conductivity, and space charge distribution were measured. In the case of the intense laser irradiation, the photo-induced change is drastic. The irradiated side of the sample is carbonized and becomes conductive. The apparent thickness of the remaining insulating region estimated by the space charge distribution measurements agrees with that observed by an optical microscope. The permittivity of the remaining apparently insulating region decreases as the irradiation proceeds, which is due to a decrease in the number of carbonyl groups. The dielectric loss factor and conductivity in this region increase since the leakage current increases even in this apparently insulating region. The activation energy of conduction also increases. In the case of the weak lamp irradiation, the permittivity decreases and the conductivity increases, although clear surface change is hardly observable. It is indicated from space charge distribution measurements that carrier traps are formed by the irradiation.