Frequency dependent AC breakdown and pre-breakdown current of thin polyimide films prepared by vapor deposition polymerization

Dielectric breakdown of thin polyimide films prepared by vapor deposition polymerization has been investigated for various AC voltages. Real time measurements were made automatically with a system composed of a digital oscilloscope controllable signal generator and microcomputer The AC electric strength decreases with temperature, and it increases with frequency in the range of 10 Hz - 3 kHz, The pre-breakdown current was also measured using the bridge circuit. The very large conductive current and the abnormal current peak due to the polarity reversal were also observed. The frequency dependence of the electric strength has been discussed in terms of the probability of the establishment of the conductive path and the distortion of electric field leading to the breakdown.