Effects of residual stress on surface
acoustic wave spectroscopy measurements of thin-film
R. E. Kumon, D. C. Hurley, and V. K. Tewary (National Institute of Standards and Technology, Boulder, CO 80305-3328).
We have developed a method to examine the effects of residual stress on the effective elastic constants of films measured by surface acoustic wave (SAW) spectroscopy. Using optical techniques, we excite and detect SAW pulses to determine the phase velocity dispersion. We then employ a Green's function approach to invert the dispersion relation for two effective second-order elastic constants and the thickness of the film. The five test samples consist of TiN films deposited on crystalline Si, with each sample having a different thickness (287 to 3330 nm) and equibiaxial compressive stress (5.4 to 0.5 GPa). With increasing thickness and decreasing stress, the effective values of C11 increase and C13 decrease. Because the effective Cij are a function of the natural (unstressed) cij and cijk and the residual stress, we are able to determine c11 and c13 uniquely and c111, c112, and c123 nonuniquely by fitting the model equations to the measured data.