Model predictions of feature-size-dependent step coverages by PVD aluminum: surface diffusion

The three-dimensional ballistic transport and reaction model (BTRM), a flux-based model which predicts the evolution of film profiles during low pressure deposition processes, is discussed and the governing equations for finite trenches are presented. The equations governing the evolution of deposited films are material balances on species adsorbed on the surface and include surface diffusion and non-cosine flux distributions. Simulations of aluminum physical vapor deposition (PVD) indicate that surface diffusion can result in feature-size-dependent step coverages. A single parameter in the governing equation for aluminum PVD the ratio of a characteristic rate of surface diffusion to a characteristic deposition rate, governs the effect of diffusion on step coverage in this process. Step coverage can be increased for a given process and given trench dimensions by increasing the surface diffusion or decreasing the deposition rate. According to the BTRM, the diffusivity should be scared by the square of the feature size to maintain a target step coverage for given operating conditions. The assumed forms for the angular distributions of fluxes from the source to the feature do not cause feature-size-dependent step coverages in the absence of surface diffusion, although they do impact step coverage for specified feature geometry and dimensions.