Dispersion of surface acoustic waves on rough anisotropic materials

The effect of surface roughness on adhesion and tribological properties of films and interfaces is of key importance. Therefore it is of the utmost importance to be able to measure this quantity and to predict the perturbing effects different roughness levels may cause. Roughness is known to affect the propagation of surface acoustic waves on a material but there is little useful quantitative data on the topic. This work investigates the dispersive effect of roughness on surface acoustic wavepackets (30-200 MHz frequency range) for different degrees of nanometer roughness on silicon (001) and (111) surfaces. We show that the dispersion effect is significant, and that although available theory agrees qualitatively with the results, the theory is not adequate to predict the real SAW dispersion. These experimental results have considerable implications for design of SAW devices, for accuracy of Brillouin spectroscopy measurements and for possible applications to non destructive testing. Previously unknown dispersive effects on anisotropic crystal surfaces are also demonstrated and an empirical modelling approach is discussed