Dual-frequency plasma using electron cyclotron resonance(ECR) and radio-frequency (rf) discharges was used for the nitridation of silicon dioxide (SiO2) films, which were grown by conventional thermal oxidation. Nitridation was performed under ion densities less than 10(11) ions/cm(3) and substrate temperatures of 300+/-20 degreesC. The effects of plasma mode (ECR/dc, ECR/rf, ECR, and rf) and the principal process variables (N-2 pressure, ECR power, rf applied voltage, and nitridation time) on formation and growth of the nitrided layer were studied by spectroscopic ellipsometry, Auger electron spectroscopy, and atomic force microscopy. It was observed that the nitrided films were composed of an upper layer of silicon oxynitride (SiOxNy) and an inner layer of SiO2. The plasma mode, N-2 pressure, and rf bias affected the reactions and concentration of particles in the gas phase, and therefore impacted growth of the nitrided layer. Plasma parameters and plasma modes were optimized to produce a dense upper nitride layer, while minimizing the effect on the inner SiO2 layer, with the goal of preserving the electronic integrity of the Si-SiO2 interface.