High-duality silicon dioxide films have been deposited by plasma-enhanced chemical vapour deposition and plasma oxidation using a single magnet electron cyclotron resonance plasma generator with both oxygen and nitrous oxide as the oxygen source. Langmuir probe measurements were used to characterise both molecular oxygen and nitrous oxide discharges. Low electron temperatures, resulting in low sheath potential drops, coupled with the shape of the field lines results in low sputtering from the chamber walls between the source region and substrate. The resulting buildup of an insulating layer of oxide on the chamber walls results in a very clean process. In situ thermal desorption and ellipsometry measurements coupled with X-ray photoelectron spectroscopy have allowed us to characterise the quality of the silicon surface prior to deposition and just after initiation of the plasma. Plasma oxidation dominates over chemical vapour deposition during the early stages of oxide film growth using either oxygen or nitrous oxide as the oxygen source gas. Extensive ex-situ spectroscopic ellipsometry (SE) indicates that the bulk properties of the films are comparable to those of high-temperature thermal oxides. There is no interface layer measurable by SE for any of the films produced by plasma oxidation or chemical vapour deposition with silane using either molecular oxygen or nitrous oxide as the oxygen source gas even though the fatter results in approximately a monolayer of nitrogen at the Si/SiO2 interface. Chemical analysis by the total reflected X-ray fluorescence technique and by vapour-phase deposition coupled with graphite furnace atomic absorption spectroscopy found no impurities in the films made with silane and molecular oxygen. The high quality of the Si/SiO2 interface made with these two gases was confirmed by capacitance-voltage measurements on Al-gate capacitors. After a 1 min anneal at 950 degrees C interface state densities below 3 x 10(10) eV(-1) cm(-2) were obtained.