Measurement of the near-surface elastic properties of solids and thin supported films

This paper provides an overview of the principal dynamical methods for measuring the elastic properties of the near-surface regions of solids and thin supported solid films. It concentrates on the most widely used surface/guided wave methods, namely surface Brillouin scattering (SBS), laser-generated surface acoustic waves (LSAW) and acoustic microscopy (AM), and emphasizes the common underlying principles shared by these three methods. It is shown how these methods obtain elastic properties and other information such as film thickness and density, by coupling into the various guided modes that exist at surfaces and in thin supported films, including Rayleigh surface waves, pseudo-surface waves, Sezawa modes and lateral waves. At the outset, a brief introduction is provided of surface and thin-film dynamics in the context of surface dynamic response functions and the reflectivity of sound incident on the surface through a fluid couplant. The main characteristics of the various guided modes and the features they give rise to in the surface response and reflectivity are discussed. Following this, there are individual sections devoted to each of the three principal methods. This article is aimed at providing the reader with a broad appreciation and basic understanding of SBS, LSAW and AM and the fundamental underlying principles they share. Numerous examples are given of recent applications of these methods to the elastic characterization of solid surfaces and thin supported films. Extensive references are provided to guide the reader to the literature on experimental details and methods of data interpretation and analysis, and to recent reported results.