Scaling effects on grain boundary diffusivity; Au in Cu

This work quantifies the effect of grain size on grain boundary diffusivity. Diffusion of Au into Cu thin films of varying grain size is measured using secondary ion mass spectroscopy depth profiles. Appropriate models for fitting the composition profiles to analytical solutions to diffusion equations are identified by characterizing the microstructure both before and after the diffusion anneal. Finite-element numerical simulations are also employed to extract the diffusion coefficients directly from fits to experimental data. The results indicate that factors such as non-columnar grains, migrating grain boundaries, dislocations and stress gradients must be appropriately accounted for in order to obtain meaningful results. Overall, a strong scaling effect is not observed, which suggests that the atomic structure of boundaries in nanocrystalline thin-film materials probably resembles that of their coarse-grained counterparts.