Calculations of effect of anisotropic stress/strain on dopant diffusion in silicon under equilibrium and nonequilibrium conditions

Understanding changes in dopant diffusion under strain is critical for controlling junction profiles in current and future very large scale integrated technology due to expanding use of large strains to enhance channel mobility. We use density functional theory calculations to investigate the stress dependence of boron (B) and arsenic (As) diffusion including vacancy (V) and interstitial (I) mechanisms under arbitrary stress/strain states. We have also analyzed the effects of stress on I and V diffusion with resulting impact on transient enhanced diffusion and coupled diffusion. For B diffusion, which is primarily mediated by I, we find greatly enhanced diffusion under tensile stress. Due to low symmetry of calculated transition state, we predict strongly anisotropic diffusion under anisotropic strain, with the strongest effects in direction of strain. This has a major impact on control of lateral junction abruptness as seen in two-dimensional simulations. The predicted behavior is consistent with combined analysis of vertical diffusion under biaxial [P. Kuo, J. L. Hoyt, J. F. Gibbons, J. E. Turner, and D. Lefforge, Appl. Phys. Lett. 66, 580 (1995)] and hydrostatic [Zhao et al.] stress. in contrast, we find isotropic As diffusion for both I and V mediated processes. We predict As diffusivity to increase substantially under compressive strain, but to show little change under tensile strain, consistent with experimental observations [N. Sugh, J. Appl. Phys. 96, 261 (2004)]. (c) 2006 American Vacuum Society.