Thermal characterization of thin films by photothermally induced laser beam deflection

The photothermal deflection technique, also known as the "mirage effect", is a powerful non-destructive means of evaluating the thermal properties of both bulk materials and thin films. In this experiment, the sample is heated by a modulated laser beam, and the deflection of a second laser beam passing through the heated region above the sample (parallel to the surface of the sample) is measured as a function of distance between the probe and heating beams. The temperature profile above the sample and hence the beam deflections, are determined by the sample properties. These measurements can be analyzed to determine the thermal diffusivity of the sample. For multilayered samples it is often possible to find the thermal constants for a single layer if the properties of the other materials in the sample are previously measured or otherwise known. We have non-linear regression software for the analysis for samples that may have any number of layers, based on a Green's function formalism with no approximations, for the calculation of beam deflections. We have applied this technique to a number of bulk and thin film systems, from which representative results are presented.