SPACE-CHARGE DISTRIBUTIONS IN ELECTRON-BEAM CHARGED MYLAR AND KAPTON FILMS

Electron-beam charging and polarization of polymer films is one method used to make electrets for microphones and other devices. Insight into charge location, magnitude, and polarity can lead to a better understanding of storage and transport mechanisms in polymer films. The laser-induced pressure pulse (LIPP) method was used to determine space-charge distributions in 22-μm Mylar and Kapton films charged with 10-to-40 keV electron beams. For relatively low deposited-charge densities, corresponding to surface potentials below 300 V, two charge layers develop, one at the surface of the samples and one at a depth corresponding approximately to the electron range. For higher deposited-charge densities, when breakdown in the surrounding air occurs during pressurization, the surface-charge layer is positive. For deposited-charge densities in excess of 1 μC/cm2, the space-charge layer is broader and, in the case of Kapton, is located deeper in the material than for smaller charge densities. Heating of the samples to 110°C causes a decrease of the charge density without broadening of the charge layers.