A kinetic model for the thermal nitridation of SiO2/Si

A kinetic model has been developed in order to understand the underlying reasons for observed nitrogen distributions in SiO₂ films on Si which have been thermally nitrided in NH₃. The calculations simulate the nitridation process, considering first-order chemical kinetics and Arrhenius dependence of the diffusion and reaction rates on temperature. The calculations show that as the substrate reacts with diffusing species, which initially consist primarily of nitrogen, a nitrogen-rich oxynitride forms at the interface. For nitridation temperature of 1000 °C and above, an oxygen-rich oxynitride subsequently forms at the interface due to reaction of the substrate with an increasing concentration of diffusion oxygen which has been displaced by the slower nitridation of the SiO₂. This sequence of events results in a nitrogen distribution in which the peak in the interfacial nitrogen concentration occurs away from the the interface. The results of the calculations are compared with observed nitrogen distributions. The calculations correctly predict that, (i) for a nitridation temperature of 800 °C, the peak of the interfacial nitrogen concentration remains at the interface, while for nitridation temperatures≥1000 °C it moves away from the interface, and (ii) for a nitridation temperature of 1150 °C, the peak interfacial nitrogen concentration is lower than that which occurs at 1000 °C, even though the position of the peak is essentially the same. The effect of interfacial strain is included in the simulations, and is found to be necessary to account for the observed width of the interfacial nitrogen distribution.