A new procedure for thermal nitridation of silicon is reported. Active nitrogen generated using a microwave discharge is passed over silicon wafers in an externally heated quartz tube at low pressure (0.3 Torr). Nitridation is shown to be significantly enhanced by the addition of small amounts (20–200 ppm) of SF6to the nitrogen before it enters the discharge. In this way films about 100 A˚ thick with a refractive index of 2.0±0.06 can be grown at 1050 °C in 4 h. The films have etch rates of 10–15 A˚/min in buffered HF (NH4F:HF=7:1). Auger analysis shows [N]/[O] atomic ratios of about 2. A 60‐A˚ film grown at 1000 °C inhibited substrate oxidation for over 16 h in dry oxygen at 1000 °C.