Thin oxides thermally grown in reactive-ion-etched (RIE) silicon substrates in N2O and diluted O2 ambient have been studied. The microroughness of the oxide-silicon interface was investigated using a spectrophotometer, atomic force microscopy (AFM), and cross-sectional high-resolution electron microscopy (HRTEM). The microroughness is strongly dependent on the RIE conditions and the post etching treatments. Furthermore, oxidation-enhanced interface microroughness has been observed. As compared with the pure oxides grown in diluted oxygen, N2O-grown oxides exhibit stronger immunity to RIE-induced damage. N2O oxidation of the etched specimens treated with an after-treatment-chamber (ATC) process result in the best electrical properties.