The Si etch rate dependences of microwave plasma etching using a or mixture on the etching parameters, such as wafer temperature, microwave power, mixing ratio of , impinging ion energy, and coverage of the Si surface with C atoms or C‐F compounds, are investigated under the low gas pressure conditions which make anisotropic etching possible. The results reveal the following: (i) The Si etch rate shows negligible dependence on the wafer temperature (50°–300°C) or microwave power greater than 80W. (ii) The Si etch rate depends heavily on the surface coverage by C atoms or C‐F compounds. Then, (carbon free etching gas) is tested and the results reveal that the Si etch rate and the selectivity to in reach 110 nm/min and 4.4–30, respectively. These values are 5.5 and 4.4–30 times larger than those for gas.