Photothermal Measurements of Thermal Conductivity of Thin Amorphous Films

A nondestructive method for the determination of thermal conductivity of transparent, thermally thin coating on opaque, thermally thick substrates is proposed. The sample is heated by intensity modulated, unfocused laser beam. The photodeflection detection is used for signal registration and the "mirage" signal dependence on modulation frequency is measured. The method is based on 1-D model of heat propagation in layered structures. The light interference in coating layer is taken into account. Two geometries of photothermal experiments are considered. The modulated light may illuminate the coated surface of sample or the opposite one. It is shown that the ratio of signals measured in these geometries does not depend on parameters of the probe beam. The proposed method is verified experimentally for silicon wafers coated with DLC. These results are very promising.