Ultradry oxidation assisted by an in situ passivation method that uses a little pyrolytic N2O gas was investigated to improve the reliability of ultrathin silicon oxide films less than 5 nm thick as gate insulators. The passivation was found to significantly improve the potential barrier height energy and time-dependent dielectric breakdown lifetimes without increasing the interface state density. It was also found that these remarkable improvements of the main electrical properties can be obtained only with a moisture content less than 1 ppb. Although the apparent existence of the N-related inclusions and the change in the binding states of the oxide cannot be confirmed, it appears that a small microscopic structural change may be brought about near, the oxide-silicon substrate interface.