Reaction kinetics in the Ti/SiO2 system and Ti thickness dependence on reaction rate

Abstract

The reaction kinetics of Ti films on SiO2 were investigated using Rutherford backscattering spectrometry, x-ray diffraction, Auger electron spectroscopy, and transmission electron microscopy. Consistent with earlier studies, the reaction results in the formation of a TiOw/Ti 5Si3/SiO2 stack at temperatures in the range 700-820°C. As the silicide layer grows, the concentration of O in TiO w increases, with the reaction ceasing at w∼1.2. In addition, the reaction rate depends on the initial Ti thickness, as thicker Ti films possess faster reaction rates. Applying current diffusion-controlled kinetic growth models, we find nominal agreement with our data at each thickness and predict activation energies in the range 3.0-3.4 eV. However, such a model is unable to account for either the Ti thickness dependence or the slowing and eventual cessation of silicide formation as the oxide composition approaches its limiting value. We implement a model which takes into account the reduction in the thermodynamic driving force for Ti5Si3 formation due to the incorporation of oxygen into the overlying Ti. This model predicts a silicide growth relationship of the form kt= (1/2)x2+ax 2f∑∞n=3(1/n)(x/x f)n, with k independent of Ti thickness and given by k=k0 exp(-ΔE/kBT). The final Ti5Si 3 thickness, xf, is determined by the initial Ti thickness, the stoichiometries of each phase formed and the final oxide composition. This model yields a more accurate fit to our data than if we assume parabolic growth since it predicts the eventual cessation of the reaction as x approaches xf. We find ΔE=2.9±0.1 eV. Our model also seems to explain the dependence on initial Ti thickness.