Ultrathin silicon oxide films 5-6 nm thick have been grown in a double-walled furnace and annealed in N2 and Ar at temperatures varying between 850 and 1100°C. The breakdown field distribution obtained is very tight and centered above 11 MV/cm for as-grown oxides at 850°C. The oxides that received a post-oxidation anneal (POA) at 1000°C show a consistent improvement in breakdown field distribution and breakdown charge density as compared to the oxides annealed at lower temperatures. Furthermore, under high field current stress, oxides with a POA at 1000°C show a positive voltage flatband Vfb shift, while oxides with POA at a temperature T<1000°C show a negative Vfb shift. These results point out the efficacy of a high-temperature POA of 5-6 nm oxides on breakdown strength and on the reduction of some defects responsible for the positive charge trapping.