Silicon dioxide thin films have been deposited at low substrate temperatures using a microwave plasma. A new type of microwave excitation, the distributed electron cyclotron resonance (DECR), which provides high density plasma (≈ 1011 cm−3) of low‐energy ions, has been used. Pure and are mixed in the discharge. At constant pressure (0.1 pa), the ratio of flow to flow was varied from 1–9. We have studied the effects of the gas phase composition at two different microwave powers (800 and 1200 W) on the refractive index, atomic composition, infrared absorption bands, etch rate, and electrical properties of the films. For ratio larger than 4, near‐stoichiometric films are obtained, with N and H atomic contaminations below 2 and 5 atomic percent (a/o), respectively, even at low temperature and without post deposition annealing. For , the films have physical and chemical properties similar to those obtained by RF plasma enhanced chemical vapor deposition at 350°C, in terms of refractive index in the range of 1.47–1.48, etch rate in “P‐etch” (6 Å/s), and Si‐O‐Si stretching mode vibration (1058 cm−1). These films exhibit, at and 1200 W, electrical resistivity of 1015 Ω cm and critical field larger than 3.5 MV cm−1, without post‐deposition annealing.