The silicon oxide films were grown by Electron Cyclotron Resonance(ECR) diffusion and CVD method at low temperature. The flatband voltage(V_FB) was minimum at 200W, and reached a steady value at microwave powers higher than 400W. Also The flatband voltage(V_FB) was proportional to interface oxide charge density(Q_it+Q_f). For high quality SiO₂ film, consequently, it was desirable to grow SiO₂ films at lower microwave power.