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Laser optoacoustic measurement of thickness of submicron metal coatings

The direct problem of optoacoustics for the system, consisted of a metal film coating on a quartz substrate and immersed into a liquid is solved. The influence of a film coating thickness on an efficiency of optoacoustic (OA) excitation for two schemes of OA signal detection is investigated. In the forward mode a metal film is irradiated by laser pulses through the quartz substrate. In the backward mode (Fig. 1b) laser pulses irradiate the film through a liquid. The acoustic response of the system is determined in a liquid for both detection schemes. Experiments are carried out with the OA method in the forward mode using chrome films of different thickness and two immersion liquids: ethanol and acetone. Results of experiments coincide well with theoretical calculations, that allows one to determine the thickness of metal film coatings by the measured spectral transfer function of OA excitation. This method can be used also for measurements of thermophysical parameters of metal films.