Thermal oxidation of silicon nitride and silicon oxynitride films

Abstract

The oxidation behavior of low-pressure chemical vapor deposition silicon oxynitrides was investigated for layer compositions ranging from that of pure nitride to oxynitride with an atomic ratio of O/N=1. Oxidations were performed at 850–1000 °C in ambients having different H2O/O2 flow ratios. Rutherford backscattering spectrometry and elastic recoil detection were employed to quantify the increase in oxygen content of the films upon oxidation. Hydrogen profiles in oxidized samples were measured using nuclear reaction analysis and elastic recoil detection. We observed that the oxidation rate for either oxynitride composition is nearly two orders of magnitude smaller than that of silicon. Linear oxidation kinetics were measured for the conditions applied in this study, which are indicative of a reaction-controlled process. The activation energy of this process was determined to be 2 eV for silicon nitride and appeared to decrease with growing oxygen content in oxynitrides. The hydrogen profiles measured in oxidized samples showed a peak at the oxide/oxynitride interface. The size of this peak varies with oxidation conditions and film composition. A reaction mechanism for the oxidation of (oxy)-nitride is proposed that relates the observed hydrogen accumulation to Si–NH and Si–OH groups at the oxidizing interface. The measured acceleration of the oxidation rate induced by adding small amounts of HCl to the ambient is interpreted in terms of enhanced transport of NH species to the surface. This effect is greatest for pure nitride and diminishes with increasing oxygen content in oxynitride material.