Thermal properties of thin films measured by photothermal methods

Abstract

Determination of thermal properties of thin films deposited on thick substrates is a very complicated measuring problem. The influence of such films on the average thermal properties of the sample is small, which is why quite sophisticated experimental techniques must be used to solve the problem. The authors present two photothermal measuring methods allowing estimation of thermal properties of the film in two directions - parallel and perpendicular to the sample surface. The first method allows determination of the thermal conductivity of the film in the direction parallel to the sample surface. The method is based on analysis of thermal wave propagation from a point source. Signals are measured for a sample coated by a thin film and second sample without a coating are compared. This comparison allows an estimation of the influence of the thin film on heat transport and a determination of its thermal conductivity. The second method is based on the analysis of a thermal wave transition through an interface between the sample and its surroundings. This transition depends on the thermal properties of contacting media and may be modified by the film deposited on the sample surface. It is convenient to describe the influence of the film by its thermal resistance. This parameter may be interpreted as a ratio of film thickness to its effective thermal conductivity. In this case it is thermal conductivity in the direction perpendicular to the sample surface. As in the previous method signals are measured for a coated sample and a sample without coating are compared. Thermal properties of the thin films are measured by photothermal methods.