A method for forming a dielectric film on a substrate by plasma-assisted deposition, includes: introducing a Si-containing process gas to a reaction space wherein a substrate having a surface with patterned recesses is placed; and applying RF power to the process gas in the reaction space to form a dielectric film on the surface by plasma reaction. The RF power is comprised of pulses of high-frequency RF power and pulses of low-frequency RF power, which overlap and are synchronized.