A new technique of post-oxidation annealing has been introduced to improve the properties and long term reliability of ultrathin (<100 Å) MOS gate dielectrics. In this technique, after oxidation, nitridation is done in NH3 followed by a light reoxidation in O2 and then an inert anneal in Ar or N2. Using this technique both optimum performance as well as reliability can be obtained without sacrificing either. NH3 anneal of SiO2 improved the hot electron immunity, but degraded the interface quality. Good properties could be obtained by a strong reoxidation of the nitrided films, however, at the expense of a substantial increase in the film thickness. Nitrogen and argon ambients were found to be equally effective at improving film properties. By annealing the film in an inert ambient following reoxidation of the nitroxide, fixed charge can be further decreased with little oxide grown, electron mobility in NMOS FETs increases further, and the hot electron lifetime is much longer than that of the starting oxide.