Thermally oxidized SiO2films on silicon wafers were directly thermally nitrided by heating in an anhydrous ammonia gas ambient. The nitrided film was studied by Auger electron spectroscopy, infrared spectroscopy, and etching rate measurements. It was found by AES measurement that nitrogen permeates through SiO2films even as thick as 1000 Å, if nitrided at 900 to 1100 °C, and that it distributes throughout the film. At the silicon interface, a nitrogen pileup was observed. Results of infrared spectroscopy and etching rate measurements confirmed the AES measurements.