Simulation of atomistic processes during silicon oxidation

Silicon dioxide (SiO2) films grown on silicon monocrystal (Si) substrates form the gate oxides in current Si-based microelectronics devices. The understanding at the atomic scale of both the silicon oxidation process and the properties of the Si(100)-SiO2 interface is of significant importance in state-of-the-art silicon microelectronics manufacturing. These two topics are intimately coupled and are both addressed in this theoretical investigation mainly through first-principles calculations. We first address the atomic structure at the interface. We construct atomistic models of the Si(100)-SiO2 interface accounting for the density of coordination defects, the amount and location of partially oxidized Si atoms, and the mass density profile, as measured in electron-spin-resonance, photoemission, and X-ray reflectivity experiments, respectively. A variety of model interfaces are obtained, differing by the degree of complexity in the transition region. The nature of the transition structure at Si(100)-SiO2 interface is investigated by addressing the inverse ion-scattering problem. In particular, we refer to a new set of ion-scattering measurements carried out in the channeling geometry to achieve sensitivity to Si displacements at the interface. To interpret these experimental data, we perform ion-scattering simulations on a selected set of model interfaces presenting different atomic-scale features in the transition region at the Si(100)-SiO2 interface. Silicon displacements larger than 0.09 Å are found to propagate for three layers into the Si substrate. Transition structures consistent with these distortions include Si-Si in-plane dimers, O-protrusion, and/or disordered bond patterns at the Si(100)-SiO2 interface. A transition structure with regularly ordered O bridges results inconsistent with experiments. We then address the silicon oxidation process. We first focus on the diffusion limited regime occurring during the growth of thick oxide films. In particular, we provide an atomic-scale description of the long-range oxygen migration through the disordered SiO2 oxide. The O2 molecule is firmly identified as the transported oxygen species and is found to percolate through interstices without exchanging oxygen atoms with the network. The associated activation energy is found in agreement with experimental values. Then, we address the O2 diffusion rate through the thin oxide layer at the Si-SiO2 interface. In particular, we investigate the combined effect of a percolative diffusion mechanism and of a dense oxide layer located close to the silicon substrate. We find that when a thin densified layer is present at the Si(100)-SiO2 interface, the O2 diffusion rate drops below its value for bulk amorphous SiO2 for oxide thicknesses larger than 2 nm. These results support the blocking layer model introduced to explain the failure of the Deal and Grove model in describing the oxidation kinetics of thin oxide films. Finally, the atomic scale mechanisms r