Thin films of silicon oxide and oxynitride were prepared by rapid thermal processing. Under optimum conditions (e.g., nitridation at 1100 °C for 4 s followed by reoxidation at 1150 °C for 60 s), their electrical properties (charge trapping, low‐field leakage, and charge to breakdown) were significantly improved over those of the starting oxide. Values ofQbdas high as 260 C/cm2for a positive current density of 200 mA/cm2were obtained. Yet, the improvement was much smaller for negative than positive stress. This difference is ascribed to the asymmetrical role of nitridation on the prevention of interface trap generation at high fields and is consistent with models of the generation of interface traps described in the literature.