Heat treatment with H2O vapor at a pressure of 5.4 x 10(6) Pa at 270 degrees C was applied to annealing thin thermally grown SiO2 and plasma oxidized lavers to improve the Si-O bonding network near the SiO2/Si interfaces. The heat treatment increased the Si-O-Si average bonding angle from 140.7 +/- 0.3 degrees to 142.5 +/- 0.3 degrees C near the interfaces for thermally grown SiO2 formed in dry oxygen at 100 degrees C and reduced the distribution of the angle. The increase of the Si-O-Si average bonding angle from 140.7 +/- 0.3 degrees to 143.8 +/- 0.3 degrees and the narrowing of the angle distribution were also observed for the 3-nm-thick plasma oxidized films. These results show that strain relaxation at the SiO2/Si interfaces was achieved at low temperature by the heat treatment.