To enhance the reliability of ultrathin silicon oxide/Si(100) films and clarify the effect of fluorine on it, in situ pyrolytic-gas passivation (PGP) using NF3 was simultaneously performed with the previously proposed PGP using N2O. As a result, the following synergistic effects of F and N passivation for the films were confirmed: The electrical characteristics, such as the time- dependent dielectric breakdown lifetime, potential barrier height energy of the oxide, and interface state density, were significantly improved. Quantitative analyses of F and N indicated that this is probably caused by microscopic structural changes in the oxide near the oxide-Si (100) substrate interface. It is, therefore, believed that F passivation effectively contributes to compensate the inconsistent-state bonding sites near the interface that remain with N passivation. (c) 2006 American Institute of Physics.