The effects of gas additives in tetrafluoromethane plasma on the etching characteristics with or without the assistance of ion bombardment were studied. Both mass spectrometry and optical emission spectroscopy techniques were applied to analyze the plasma species. The concentrations of F, O, and 02 in CF4 plasma with some percentage of N2O, O2, and N2were measured. Interpretation of the etch rate variation with gas additives is given based on these measurements. By adding some percentage of N2O to CF4, higher silicon etch rate and lower photoresist etch rate were achieved compared with adding O2 to CF4. The silicon etch rate with CF4 + N2O also outperforms that with CF4 +N2. The competition of O and O2 with F in reaching the silicon etch sites is believed to be responsible for the result that the CF4 +O2 plasma etches silicon even slower than the CF4 + N2O plasma, which has lower F, much lower O, and very little 02 concentrations in the plasma compared to those in CF4 +O2 plasma. The effect of oxygen in retarding silicon etching was observed to be reduced by bombarding the silicon surface with energetic ions. When ion bombardment with sufficient ion energy was added to the etching process, the silicon etch rate with CF4 +O2 plasma exceeded that with CF4 + N2O plasma due to higher F concentration existing in the CF4 +O2 plasma. In contrast to silicon etching, CF4 + O2 plasma etches photoresist faster than CF4 + N2O plasma due to higher oxygen concentration existing in the CF4 +O2 plasma. Therefore, CF4 + N2O plasma can achieve better silicon-to-photoresist etching selectivity when the effect of energetic ion bombardment is not significant. As a comparison, silicon dioxide etch rate was also found to be slightly higher when N2O was used to replace 02 as a gas additive in CF4 plasma. This also results in higher silicon-to-silicon dioxide etching selectivity.
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