Non-contact thickness gauging of a thin film using surface waves and a void effect on their propagation

In the first part of this paper, laser ultrasonics has been performed in an original way to evaluate the thickness of a gold thin film deposited on a silicon substrate. To this end, the influence of the film thickness and the elastic parameters of both materials on the first Rayleigh mode propagation has been theoretically studied. Then, using simultaneously the head wave and the Rayleigh wave propagating on the substrate, its elastic parameters were determined. These parameters allowed us to deduce the film thickness by matching the experimental group velocity dispersion curve with the theoretical one in a frequency range up to 25 MHz. In the second part, the effect of a void, consisting of a circular area without layer, on the first Rayleigh mode propagation has been investigated. The experimental results show clearly time of flight shifts proportionally to the size of the area and depending on the considered frequency component. A good agreement was obtained with finite-element modeling. This effect could be the basis for a frequency-selective surface acoustic lens.