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 responsible for oxidation at the Si(100)SiO2 interface are addressed. First we focus on the properties of negative oxygen species in amorphous SiO2. We initially focus on the relative energetics of neutral and negative oxygen species in both the atomic and the molecular state. The energy landscape and the dissociation properties of the O2- and O22- are also considered. We find that, in amorphous SiO2, the negative oxygen species incorporate in the oxide network and are accommodated by significant network distortions. The investigation of the dissociation processes of the negatively charged molecular oxygen species in amorphous SiO2 highlights the dependence of the dissociation energies on the disordered nature of the network and on the charge carried by the oxygen molecule. In a second stage, we address the atomistic processes occurring at the Si(100)SiO2 interface. We consider the neutral O2 molecule and the negatively charged molecular species. In the case of the neutral O2 molecule, different spin states are investigated. For all these species we study sequentially the network incorporation and the dissociation process. We find that at the Si(100)-SiO2 interface all these species incorporate in the network either spontaneously or by crossing small energy barriers. In the vicinity of the interface, the neutral O2 molecule attacks and penetrates into Si-Si bonds. This process is favored by the charge transfer from the Si-Si bonds to the molecular species. The triplet spin state of the O2 molecule narrows the channels for network incorporation. Indeed, molecules in the triplet spin state preferentially incorporate in the network in correspondence of the upper layer of the Si substrate. Negatively charged species spontaneously incorporate in any Si-Si bond of the oxide, regardless from distance from the interface. Their incorporation is often associated to the formation of network defects, confirming their ability to modify the network. After the incorporation process, the dissociation and hence the oxidation of Si-Si bonds is spontaneous. We find either vanishing or very small dissociation energies. The energy required for crossing the small barriers is provided by the energy gained during the incorporation process. These results are consistent with a variety of experimental observations.