FLUOROCARBON HIGH-DENSITY PLASMAS .2. SILICON DIOXIDE AND SILICON ETCHING USING CF4 AND CHF3

We report a study of the application of CF4 and CHF3 electron cyclotro n resonance (ECR) discharges to selective etching of SiO2 over Si. Due to significant fluorocarbon film deposition for plasma operation with out 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 etc hing is 55 V for CHF3 and 35 V for CF4 at a pressure of 1 mTorr. At 10 0 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 pressu re. A plot of the oxide etch rate vs rf bias exhibits a fluorocarbon f ilm suppression regime at low rf voltages and an oxide sputtering regi me at higher rf voltages. In the fluorocarbon suppression regime, the etch rate is primarily deter-mined by fluorocarbon deposition which re sults in a thin fluorocarbon film being present on the SiO2 surface du ring steady-state etching. In the oxide sputtering regime, the oxide e tch rate increases linearly with the ion current to the wafer and the square root of the ion energy. The etch yields decrease with increasin g microwave power and decreasing pressure and are in the range 0.5-2 a toms per incoming ion. The silicon etch rate is much lower in CHF3 tha n 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 p resent on the silicon surface during steady-state etching. The fluoroc arbon film thickness is congruent-to 5.5 nm in CHF3 as compared to con gruent-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 etc h depth of congruent-to 2.5 mum deep contact holes etched using 1 mTor r CHF3 plasmas into photoresist patterned SiO2 was measured by scannin g electron microscopy as a function of the feature width. The etch dep th decreased by congruent-to 10% as the feature size was reduced from 1.3 to 0.6 mum.