Investigation of thin film shear modulus from spatially resolved measurement of Love wave velocity

Abstract

The investigation of transversely polarized surface acoustic waves (SAWs) is strongly hindered by their missing out-of-plane oscillation component because commonly applied measurement techniques are based on access to that component. Nevertheless, these waves are very interesting for accurate extraction of thin film elastic properties as well as for SAW devices and sensor applications. In this contribution we report a new development of spatially resolved SAW analysis. The scanning acoustic force microscope (SAFM) has been extended in its sensitivity to in-plane polarized waves. A nonlinear mechanism could be found that couples the in-plane surface oscillations to torsional cantilever vibrations. This mechanism is used to transfer the rf SAW signals to kHz frequencies and to measure the SAW phase velocity. The new technique has been applied to study the Love wave propagation in the layered system fused silica on ST-cut quartz. The phase velocity could be derived from measurements of the phase shift along a propagation distance in the micron range. The obtained velocities show a clear spatial variation at elastic changes within the investigated surface. This is used to investigate the shear modulus of the layer material by numerically solving the inverse problem of SAW propagation.