Szymanski, Marek Artur; (2001) Modelling of silicon oxidation processes. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The oxide on silicon is a major factor in silicon's domination of microelectronics. Yet, there is still no comprehensive validated atomistic picture of the oxidation process. This work concentrates on the properties and diffusion mechanisms of different oxygen species (the potential oxidising species in the context of silicon oxidation) in the amorphous oxide. My periodic-boundary DFT calculations for a representative model of amorphous silica emphasise the influence of static disorder on defect properties, the importance of a statistical approach, and the use of appropriate techniques. They show it is important to go beyond models based on quartz. The calculations include atomic and molecular oxygen, in neutral and negative charge states, and ozone. The comparison of defect properties in quartz and SiO2 shows three types of difference: different average energetics, a big spread of values in the amorphous sample for some of the species, and the possibility of different stable configurations in the amorphous and crystalline host materials. I conclude that quartz can be a poor mimic of amorphous silica for some of the properties. For example, the significant site-to-site variations in incorporation energies of atomic oxygen originate mostly from the intrinsic medium-range strain fields of amorphous structure, and to a lesser extent from the different structures of the defect local environments. Neither of the factors can be simulated using quartz. I establish, in agreement with the accepted picture, that the neutral oxygen molecule is the species responsible for oxygen transport from the gas phase to Si/oxide interface for the thick oxides. The molecule travels interstitially through the bigger voids of the amorphous structure with the 7-membered and bigger rings providing the preferable passages between the voids. The estimated activation energy for this process agrees well with the experimental results. My results imply, however, a new picture of silicon oxidation arising from charge transfer from silicon to the diffusing molecular oxygen with its possible dissociation into atomic species near the Si/oxide interface. The model explains anomalies like layer-by-layer growth at terraces, roughness oscillations, the distribution of oxygen isotope incorporation, the effects of excitation, and deviations from Deal-Grove (diffusion-reaction) kinetics. Based on this concept, new approaches to oxidation control and optimisation of oxide properties are suggested.

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