Ultrathin SiO2 films were grown by Si surface irradiation with an O2+Ar electron cyclotron resonance (ECR) microwave plasma at low temperatures. These films without hydrogen and silicon deposition were easily grown in a few minutes at low temperatures and at relatively low working pressures (1.0–3.9×10-1 Pa). The promoting growth process of an ultrathin oxidized layer (1–9 nm) was analyzed by in situ Fourier transform infrared reflective absorption spectroscopy. By using the sputter erosion technique, the dependence of x-ray photoelectron spectroscopy on the depth profiling of the films was obtained. The compositional deviations of the films from the stoichiometric SiO2 were also discussed in the interface region of Si/SiO2. This study indicated that an O2 ECR microwave plasma was efficient to form controlled ultrathin SiO2 dielectric films. The dependence of the film quality on the working pressure and gas flow rate ratios of the O2 to Ar was discussed. © 1999 American Institute of Physics.