The reactive ion etching of silicon(100) in Cl2/CCl4plasmas at 13.56 MHz is described.Insituellipsometry is used to determine the extent and nature of silicon surface roughening under a variety of plasma conditions. Quadrupole mass spectrometry yields complementary information regarding the composition of the plasma. In contrast to our previous experiments using pure Cl2, we find that no significant roughening of silicon occurs in Cl2/10%CCl4plasmas at 100 mTorr, irrespective of rf power in the range 50–300 W (0.3–1.8 W cm−2); indeed smoothing of previously roughened surfaces takes place at 100 mTorr and 100 W. When the plasma pressure is reduced to 50 mTorr, considerable roughening occurs at rf powers ≥100 W and a marked dependence on power is recorded. Water vapor has a far less dramatic effect on the etching and roughening of silicon(100) in Cl2/10%CCl4plasmas, compared to the effects observed in pure Cl2. This suggests that hydroxide micromasks are efficiently removed by reactive species that derive from CCl4. The mass spectrometric data support the hypothesis that it is CCl2and CCl3radicals that probably interact with>Si–OH to form COCl2and expose reactive silicon sites.