An atomic-scale investigation was made of oxygen diffusion during silicon oxidation. The energetics of various oxygen species in the oxide were analyzed using density functional calculations. The results confirmed that the interstitial O2 molecule was the most stable oxygen species. A classical scheme was used to describe the energetic and topological properties of the percolative diffusion of O2 molecules through the interstitial network of the oxide. By studying a large set of disordered oxide structures, distributions of energy minima, transition barriers and the number of connections between nearest-neighbor minima were derived. These distributions were mapped onto a lattice model in order to study long-range O2 diffusion using Monte Carlo simulations. The resultant activation energy for diffusion was found to be in agreement with experimental values. The atomic-scale approach was extended to an oxide of higher density, revealing a significant decrease in the diffusivity. To treat O2 diffusion at the Si/SiO2 interface directly, a lattice model of the interface was constructed which incorporated appropriate energetic and connectivity properties in a statistical manner. This lattice model revealed a thin oxide layer of higher density at the interface; in accord with X-ray reflectivity data. Monte Carlo simulations of O2 diffusion were carried out for this model and the dependence of the diffusion rate upon the oxide thickness was obtained. For oxide thicknesses down to about 2nm, it was found that the presence of an oxide layer of higher density at the Si/SiO2 interface caused a decrease in the O2 diffusion rate with respect to its value in bulk SiO2; in qualitative agreement with observed oxidation kinetics.

Multiscale Modeling of Oxygen Diffusion through the Oxide During Silicon Oxidation. A.Bongiorno, A.Pasquarello: Physical Review B, 2004, 70[19], 195312. See also: Journal of Physics - Condensed Matter, 2003, 15[16], S1553-60