Influence of mass density and mechanical properties on the surface acoustic wave velocity dispersion

The surface acoustic wave technique is a non-contact, non-destructive opto-acoustic technique used to measure film thickness and material properties, such as elastic properties and density. Simulation of acoustic wave velocity dispersion revealed that the contribution density makes is larger in the first mode, whereas the modulus makes a larger contribution in the second mode. The effect of thickness does not depend on the wavelengths and oscillation modes as the density and modulus. In fact, the dispersion curves measured experimentally in the electro-deposited Cu films show more variation in second mode during self-anneal, while the difference between two samples, which contain different impurity level, is more significant in first mode. The velocity change during self-anneal is mainly due to the modulus change, which result from the texture change accompanied with grain growth. In contrast, a different impurity concentration causes density variation, which is considered to be related to the change of formation energy of voids or vacancy-type defect. These results coincide with the simulation. The understanding obtained in the present investigation suggests the possibility to improve the precision of thickness or material properties determination and the possibility to determine density and modulus simultaneously by analyzing the acoustic wave velocity dispersion.