An investigation of the roughening of silicon(100) surfaces in Cl2 reactive ion etching plasmas by i n s i t u ellipsometry and quadrupole mass spectrometry

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Abstract
The conditions which cause silicon roughening in Cl2 RIE plasmas are investigated. In situ ellipsometry provides a quantitative interpretation of the extent and nature of the roughening process, while mass spectrometry yields complementary information regarding the composition of the plasma. The degree of roughening is reproducible when base pressures are ≊10−5 Torr but is strongly dependent on the rf power and process pressure. Careful selection of these parameters (50 W, 100 mTorr) retains a smooth silicon surface and gradually smooths those which have been roughened. Water vapor has a very significant effect on the etching reactions. In low concentrations it induces roughening and in high concentrations it prevents any etching of silicon. We suggest that silicon oxide micromasks are not responsible for roughening. Instead we propose that hydroxyl (SiOH) groups are the more likely masking species. Roughening of silicon is efficiently prevented when the wafer is patterned with positive photoresist. It is possible that CClx (x=1–4) species can remove the micromasks to retain a smooth surface.

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