Optoacoustic technique for thickness measurement of submicron metal coatings

Abstract

The new nondestructive method for thickness measurement of submicron metal coatings on transparent substrate is developed. The method is based on the optoacoustic (OA) transformation in the system, where the coating is covered by an optically transparent liquid. Theoretical treatment of the problem consists of two steps. At the first step laser-induced thermal field in the system is calculated, taking into account the large thermal conductivity of the metal film and partial heat diffusion into the liquid. At the second step the system of wave equations for scalar potential of vibration velocities is solved. Heat sources, determined at the first step, are free form of wave equations. Three chrome coatings of different thickness (approximately 0.2, 0.3, and 0.6 μm) deposited on the quartz substrate are tested experimentally. Two different organic liquids (acetone and ethanol) are used to cover chrome coatings. Nanosecond diode-pumped Nd:YAG laser operated at the main harmonic is used to perform OA transformation (laser pulse duration is τ L = 12 ns, the laser energy is about 0.2 mJ). Two detection modes are used. In forward mode laser pulse irradiates the film from the side of the substrate and in backward mode—from the side of the liquid. Detection of induced ultrasonic pulses is performed by the wide-band piezoelectric transducer in the liquid in both cases. The thickness of the coatings is determined by the least squares fitting of the theoretical dependencies of spectral transfer functions of OA transformation to experimental data. It is demonstrated, that the developed technique can be used for measurement of metal coatings thickness within the range from 50 nm to 5 μm with the error about 50 nm.