We report a study of the application of CF4 and CHF3 electron cyclotron resonance (ECR) discharges to selective etching of SiO2 over Si. Due to significant fluorocarbon film deposition for plasma operation without rf sample bias in the pressure range below 10 mTorr, rf biasing is required for etching of SiO2 and Si. The rf threshold voltage for etching is 55 V for CHF3 and 35 V for CF4 at a pressure of 1 mTorr. At 100 V rf bias, silicon dioxide etch rates were greater than congruent-to 600 nm/min in CF4 and 450 nm/min for 1000 W plasmas at 1 mTorr pressure. A plot of the oxide etch rate vs rf bias exhibits a fluorocarbon film suppression regime at low rf voltages and an oxide sputtering regime at higher rf voltages. In the fluorocarbon suppression regime, the etch rate is primarily deter-mined by fluorocarbon deposition which results in a thin fluorocarbon film being present on the SiO2 surface during steady-state etching. In the oxide sputtering regime, the oxide etch rate increases linearly with the ion current to the wafer and the square root of the ion energy. The etch yields decrease with increasing microwave power and decreasing pressure and are in the range 0.5-2 atoms per incoming ion. The silicon etch rate is much lower in CHF3 than in CF4, which translates into better SiO2-to-Si etch selectivity in CHF3 (congruent-to 15) than in CF4 (congruent-to 5). The lower Si etch rate in CHF3 is due to a greater thickness of the fluorocarbon film present on the silicon surface during steady-state etching. The fluorocarbon film thickness is congruent-to 5.5 nm in CHF3 as compared to congruent-to 2.5 nm in a CF4 discharge (at a rf bias of 100 V). The oxide surface is free of fluorocarbon film for the same conditions. The etch depth of congruent-to 2.5 mum deep contact holes etched using 1 mTorr CHF3 plasmas into photoresist patterned SiO2 was measured by scanning electron microscopy as a function of the feature width. The etch depth decreased by congruent-to 10% as the feature size was reduced from 1.3 to 0.6 mum.