Effect of film thickness and electrode material on space charge formation and conductivity in polyimide films

Polyimides (PI) are well-known materials used as passivation and insulating layers in microelectronics or power electronics. Though the electric field and temperature withstanding of polyimides have been investigated for long, little information is available on the space charge behavior in relatively thin polyimide films. In this work, the space charge behavior was investigated with the Laser Intensity Modulation Method (LIMM) which is suited for films of several µm in thickness. It is complemented by DC conductivity measurements. The analysis is made on 12 and 18 µm thick PI-layers deposited on Si-substrates with using Al or Au top electrodes. A build-up of negative charges can be observed, irrespective of the polarity of the applied voltage, as the external field is increasing in the range 25–125 kV/mm. With decreasing film thickness, the DC conductivity increases and a diminution of the internal electric field distortion occurs. The native alumina formed between the aluminium electrode and the PI could act as a barrier to electrons injection from the top electrode.