Thermally grown silicon dioxide (SiO2) films were nitrided using dual-frequency electron cyclotron resonance (ECR) and rf discharges. The structure and composition of the layers were studied by atomic force microscopy, spectroscopic ellipsometry, Fourier transform infrared spectroscopy, and Auger electron spectroscopy. A two-layer structure consisting of a silicon oxynitride (SiOxNy) outer layer and a SiO2 inner layer was found. The Si-SiO2 interface was found to be a suboxide (SiOx, x<2). The ECR/rf plasma mode enabled production of a more thermally stable structure and composition, when compared to rf plasma alone, due to incorporation of more bonded nitrogen.