Analysis of the effect of thermal nitridation of silicon dioxide on silicon interstitial concentration

The thermal nitridation of SiO2 has been analyzed in order to understand how the process results in large interstitial supersaturations in the silicon substrate as manifested by the greatly enhanced diffusivity of substitutional impurities such as phosphorus and boron and the growth of stacking faults. It is postulated that the interstitial injection is due to the growth of a thin oxygen-rich layer at the dielectric/silicon interface and is magnified by the presence of a nitrogen-rich layer near the interface which constrains the interstitials to the interface region. Because of the slow rate of growth of the oxide-rich layer, an initial transient period exists during which many of the interstitials created at the interface are injected into the substrate, raising the concentration in the silicon. For long times, a steady-state analysis shows that almost all of the interstitials generated at the interface diffuse back into the oxide. By extending a previous analysis for standard oxidation of silicon, a quantitative model was developed for interstitial supersaturation in the silicon which incorporates the transient regime and successfully predicts the diffusion enhancement of phosphorus with time. In addition, nitrogen incorporation in the bulk of the oxynitride and oxidation at the dielectric/silicon interface were modeled by an exponential decay to an equilibrium structure with a common time constant.