Surface and interface roughness of 40-Angstrom oxynitride films grown on preoxidized (100) silicon surfaces in a nitric oxide (NO) ambient at 800 degrees C have been investigated using atomic force microscopy with power spectral density, and cross-sectional transmission electron microscopy measurements. The results showed that the NO oxynitride surface is smoother and has less interfacial roughness compared to the thermal oxide (without NO anneal). These results are important given the current technological interest in oxynitrides for ultrathin gate dielectric applications.