Dependence of space charge formation in polyimide film on applied voltage wave form

It is found in our previous work that hetero space charges always observed before breakdown occurs in polyimide film under high DC electric field. We have also found that the time-to-breakdown and the accumulated amount of hetero space charges depend on the applied DC electric field. In this report, we have investigated whether the time-to-breakdown and the space charge accumulation depend on the applied voltage wave form or not. By measuring the space charge accumulation using PEA method, it is found that the very close space charge behavior in polyimide film is observed under rectangular voltage application to that observed under DC voltage application. Under high DC electric field, homo charges accumulate at first and they decrease. Following the decrease of homo space charges, the hetero charges appear near electrodes and finally the breakdown occurs. Such process to breakdown was also observed under the application of rectangular voltage wave form. Net voltage application time in rectangular voltage is sometimes close to the time-to-breakdown under DC electric field. However, the time-to-breakdown seems to strongly depend on the humidity of the experimental condition.