AC electrical breakdown and conduction in PMMA thin films and the influence of LiClO4 as an ionic impurity

Poly(methyl methacrylate) ( PMMA ) has been widely used in electrical insulator and optical device applications. It is known to be polar material, especially above the glass transition temperature T_g (∼110°C). In this work we have measured the breakdown strength of PMMA thin films of 300nm thickness between Al electrodes under alternating voltage at frequencies between 10Hz and 3kHz. Below T_g, the breakdown strength increases gradually from 2 MV/cm to 3 MV/cm as the frequency is increased. On the other hand, the breakdown strength decreases drastically at a temperature near T_g. Above T_g, the breakdown strength decreases with the frequency. The decrease in the breakdown strength at high temperature is considered in terms of heating due to the rotation of polar side chains in PMMA. We have also investigated the influence of the addition of ionic impurities (LiClO₄) on the ac breakdown strength and also on the electrical conduction in PMMA films. While the breakdown strength decreases considerably with the concentration of LiClO₄ at low temperature, it changes little at high temperature although there is a considerable increase in the conduction current.