Direct thermal nitridation of silicon in a nitrogen plasma has been performed above 1000 °C. From Auger electron spectroscopy analysis, the formed films contain some oxygen and are identified as oxynitride films. Refractive indices of these films varied from 1.5 to 1.9 with nitridation time. The film thickness is about 40 A˚ after nitridation of 10 h at 1145 °C and the film growth is saturated at this value. Capacitance‐voltage characteristics of Al gate metal‐nitride‐semiconductor capacitors show a stable behavior. The fixed charge densityQssis estimated to be on the order of 1012cm−2.