Si masked by patterned SiO2 was etched using an electron cyclotron resonance hydrogen plasma. The dependence of the etch rate and the etched profile on substrate temperature and dc bias potential has been investigated, and an etch mechanism has been considered. The substrate temperature was varied between room temperature and 400-degrees-C, and a bias potential between - 100 and + 100 V was applied. The etch rate of Si had the highest value at the lowest substrate temperature and decreased with increasing temperature. For the bias potential, the etch rate of Si had its peak value at 0 V. It decreased with increase of the potential which was either positive or negative. Si was also etched laterally when the substrate temperature was low. For the negative bias potential, the etched Si had a terrace on the border of a SiO2 mask. It is considered that the temperature dependence of the etch rate and the etched profile is caused by the action of hydrogen atoms, and the bias potential dependence is caused by the action of hydrogen ions. Thus, we conclude that both hydrogen ions with moderate energy and hydrogen atoms play an important role in Si surface etching.