Number densities of Si and 0 atoms for 3.5-6.5 nm thick silicon oxide films on Si(100), which were oxidized at 700-900 degrees C using an in situ pyrolytic-gas passivation (PGP) method, were determined by Rutherford backscattering spectrometry. PGP was recently proposed to passivate the Si dangling bonds with a little pyrolytic N2O. It was found for all films that excess Si atoms relative to the stoichiometric SiO2 composition exist near the silicon oxide-Si(100) interface, but the number of excess Si atoms is less than that of normal oxidation films. In particular, the number of excess Si atoms of the 700 degrees C grown PGP-oxidized films is almost equivalent to that of the 850 degrees C grown normal oxidation ones. This suppression of the excess Si atom generation might cause a strong retention of high electrical reliability for low-temperature PGP-oxidized films. (c) 2005 American Vacuum Society.