Oxynitride films have been formed by rapid thermal processing of N-implanted wafers. The formation mechanism, the chemical composition, and the physical thickness of the oxynitride films were studied by x-ray photoelectron spectroscopy (XPS). Segregation of nitrogen to the surface was performed on nitrogen ion-implant wafers under nitrogen gas. The: outcome of the segregation is the formation of an ultrathin oxynitride layer ranging in thickness from 4 to 8.5 Angstrom. Oxidation of nitrogen ion-implanted wafers. where the nitrogen is segregated beforehand and nitrogen ion-implanted wafers with no segregation prior to oxidation, is the focus of this study. XPS results showed that simultaneous segregation and oxidation forms an oxynitride film consisting of two layers where the interface is rich in nitrogen and the surface in oxygen. In the case of nitrogen segregation prior to oxidation, the nitrogen atoms in the oxynitride film, formed at oxidation temperatures less than 1000 degrees C, are uniformly distributed throughout the Blm in the form of SiOxNy. At high temperatures (similar to 1100 degrees C), the composition of the oxynitride formed by simultaneous segregation and oxidation becomes similar to that where N-segregation is performed prior to oxidation. The presence of nitrogen atoms retards significantly the diffusion of oxygen to the substrate surface thus producing uniform ultrathin films ranging in thickness from 13 to 40 Angstrom.