Dependence of the O2diffusion rate on oxide thickness during silicon oxidation

Abstract

We address the rate of O2 diffusion through the oxide layer at Si–SiO2 interfacesusing an atomic-scale approach. In particular, we investigate the combined effectof a percolative diffusion mechanism and of a dense oxide layer located close tothe silicon substrate. We first extend our atomic-scale description of O2 diffusionin amorphous SiO2 to the case of a densified oxide. This yields an activationenergy which compares well with the experimental result. Next, we investigatethe dependence of the O2 diffusion rate on oxide thickness at Si–SiO2interfaces using Monte Carlo simulations. We consider both homogeneous andnonhomogeneous oxide layers. The nonhomogeneous oxide is composed oftwo layers, a normal and a densified one. The thickness and the massdensity of the densified layer are taken from experiment. In the case of anormal oxide, we find that the O2 diffusion rate increases with decreasingthickness, as a result of the percolative nature of the diffusion mechanism.When a densified layer is inserted, the diffusion coefficient drops below itsvalue for bulk amorphous SiO2, for oxide thicknesses larger than 2 nm.This result is consistent with the experimental behaviour of the oxidationkinetics.