An investigation was made, at the atomic scale, of the O diffusion process occurring during Si oxidation. Firstly, the energetics of several O species in the oxide were considered by using density-functional calculations. The results confirmed that the interstitial O2 molecule was the most stable O species. A classic scheme was then 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 then mapped onto a lattice model in order to study the long-range O2 diffusion process via Monte Carlo simulations. The resultant activation energy for diffusion was found to be in agreement with experimental values. The atomic-scale approach was also extended to an oxide of higher density; revealing an appreciable decrease in the diffusivity. In order to explore O2 diffusion at the Si/SiO2 interface directly, a lattice model of the interface was constructed which incorporated the 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 the O2 diffusion for this model were carried out and the dependence of the diffusion rate upon 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 drop of 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 (14pp)