Study of waveguide and anisotropy effects upon surface acoustic wave velocities in thin films

Abstract

Impulsive Stimulated Thermal Scattering (ISTS) is an in-situ non-destructive characterization technique based on laser excitation and detection of surface acoustic waves (SAWs) for mechanical properties of thin metal films in the microelectronic industry. In this work, the effect of elastic anisotropy from silicon wafers on ISTS measurements has been studied. It has been shown that for films of submicron thickness, the anisotropic effects on acoustic velocities are substantial. Moreover, at a particular orientation of propagating SAWs, two surface acoustic modes are observed rather than only one acoustic mode as in an isotropic model. With regard to dispersive behavior, a previously unknown result has been established that the pseudo-surface wave branch of an uncoated substrate is a qd->O limit for the second-order acoustic waveguide mode of a film-substrate system. Theoretical analysis for calculating acoustic modes of a supported film for arbitrary orientation, along with an experimental technique featuring optically heterodyned signal detection, provide an adequate basis for accurate characterization of thickness and elastic properties of thin films on anisotropic substrates. Furthermore, in the preliminary study of the waveguide effect on Damascene Structure, it has been considered that the widths of the bar structures as well as the spaces between the bar structures should be included in the scaling parameters for dispersion curves.