Thickness measurement for submicron metallic coatings on a transparent substrate by laser optoacoustic technique

A new nondestructive technique for determining the thicknesses of submicron metallic coatings on transparent substrates is developed. The technique is based on measuring the frequency dependence of the efficiency of thermooptical conversion on the thickness of a metallic film in the case of its contact with a transparent fluid. Experiments were conducted with three chromium coatings of different thicknesses (0.2, 0.3, and 0.6 μm) on quartz substrates. Two different experimental schemes were used: a direct scheme (laser radiation hits the film from the side of the substrate) and an indirect one (the laser action upon the film occurs from the side of the fluid). The film thickness is determined by approximating the experimental frequency dependences of thermooptical conversion efficiency by theoretical curves with the use of the least-squares method. The optoacoustic method can be used for determination of coating thicknesses in the range from 50 nm to 5 μm with an error of about 50 nm.