Ultrathin SiON films formed by thermal nitridation of SiO2 films with NH3 gas are analyzed by high-resolution angle-resolved photoelectron spectroscopy to investigate the interfacial chemistry, the chemical state in-depth distribution of nitrogen atoms, and the band offsets. Although nitrogen atoms are mostly homogeneously distributed in the NH3-treated SiON films, the [Si-Si3-xOx](3)N or [Si-N3-xOx](3)N component exists closer to the surface than the [Si-Si3-xNx](3)N or Si3N4 component. The valence-band offsets of HN3-treated SiON films are determined by a substrate subtraction method to be 4.4 eV. This is the same as for SiO2 film. Valence-band offsets for NO-treated SiON film are measured to be 4.4 eV for SiO2/Si and 3.3 eV for SiON/Si. Furthermore, O 1 s loss spectra reveal that SiON films consist mainly of a SiO2 region with a band gap of 8.9 eV, and a smaller region of probably Si3N4 with a band gap of about 5.3 eV. (C) 2004 American Vacuum Society.