Heat Transfer in Multilayered Thin Film Materials Applied to Photothermal Deflection Spectroscopy

Abstract

Heat transfer from a modulated laser incident on a multilayered thin film material is theoretically analyzed as a continuum system accounting for thermal contact resistance between the layers and anisotropy of thermal conductivity of the films. Formulations are obtained for the intensity-averaged deflection of a probe beam (PB) laser bounced on the surface as it passes through the thermally excited region of the gas. The PB formulations are the basis for nonintrusively measuring material properties. An algorithm is used to determine property values by matching measured and simulated PB deflections with selected properties as fitting parameters. Experiments are reported to provide data for validating the PB formulations for bulk standard reference and film/substrate materials. For the latter, a virtual film bonded to a substrate of the same material was used. Results show that predicted and measured PB deflections are in good agreement for thin films when the modulation frequency positions the thermal penetration depth inside the film. It is also shown that for increasing thermal contact resistance, anisotropy of thermal conductivity of the film materials, and modulated beam radius and frequency, the PB deflections are reduced through the effect of these variables on heat flow in the solid and gas.