THIN-FILM CHARACTERIZATION BY ACOUSTIC MICROSCOPY

In this paper a brief review of the recent applications of acoustic mi croscopes for material property determination of thin films is given. Acoustic microscopes generate Rayleigh waves near the surface (up to o ne or two wave length depth) of the specimen under inspection. Since t he Rayleigh wave speed is sensitive to thin film properties such as Yo ung's modulus, Poission's ratio, longitudinal and shear wave speeds, e lastic wave attenuation coefficient and density, these properties, in principle, can be extracted from the acoustic microscope generated sig nals. In this paper it is discussed how one can extract these properti es fi om the V(z) curve generated by an acoustic microscope using the simplex algorithm V(z) curve is obtained by vertically moving the micr oscope lens from the specimen surface, thus varying the distance (z) b etween the focal point of the lens and the reflecting surface of the s pecimen and recording the corresponding variation of the voltage (V) w ith z. Extraction of material properties of a thin film specimen from its V(z) curve requires solving the inversion problem, that has not be en done by many investigators. The author does it by the simplex algor ithm technique which is an optimization technique for solving one or m ore unknowns in linear or nonlinear equations. Some theoretical and ex perimental results involving thin (a few micron thick) metal films on a substrate and thin biological cells are also presented in this paper .