Thin film characterization by acoustic microscopy

Abstract

In this paper a brief review of the recent applications of acoustic microscopes for material property determination of thin films is given. Acoustic microscopes generate Rayleigh waves near the surface (up to one or two wave length depth) of the specimen under inspection. Since the Rayleigh wave speed is sensitive to thin film properties such as Young's modulus, Poission's ratio, longitudinal and shear wave speeds, elastic wave attenuation coefficient and density, these properties, in principle, can be extracted from the acoustic microscope generated signals. In this paper it is discussed how one can extract these properties from the V(z) curve generated by an acoustic microscope using the simplex algorithm. V(z) curve is obtained by vertically moving the microscope lens from the specimen surface, thus varying the distance (z) between the focal point of the lens and the reflecting surface of the specimen and recording the corresponding variation of the voltage (V) with z. Extraction of material properties of a thin film specimen from its V(z) curve requires solving the inversion problem, that has not been done by many investigators. The author does it by the simplex algorithm technique which is an optimization technique for solving one or more unknowns in linear or nonlinear equations. Some theoretical and experimental results involving thin (a few micron thick) metal films on a substrate and thin biological cells are also presented in this paper.