Thermal-pulse investigation of thermally grown silicon dioxide electrets

The thermal-pulse method has been applied to samples of thermally grown silicon dioxide on silicon wafers. During this nondestructive experiment the surface of the electret sample has been excited by means of a sub-nanosecond laser pulse. The electrical potential across the electret layer changes due to the diffusion of thermal energy through the sample and due to the internal electrical field. The investigated samples were charged by a corona method. The charge centroid was estimated by a modified CV-method which will be described briefly. In the next step the samples were excited by means of a short laser pulse. Since the charge centroid for corona charged samples is close to the irradiated surface, the field distribution in the bulk SiO₂ is nearly homogeneous. Therefore parameters like the thermal diffusivity can be derived from the observed thermal-pulse response. Knowing the thermal properties of the layers, an approximation to the field distribution and thus the charge profile in the same electret layers was gained from the experimental data