The simultaneous application ofinsituellipsometry and quadrupole mass spectrometry to the reactive ion etching of polysilicon/SiO2/Si(100) multilayers in pure Cl2plasmas at 13.56 MHz is described. At 100 mTorr and 50 W the surface texture of the unetched polysilicon is retained, but significant roughening is induced when the power is raised to 100 W. Water vapor is shown to play a critical role in the roughening process and is probably due to micromasking by hydroxide groups at the polysilicon surface. Detailed models of the ellipsometric results for roughened surfaces are derived and correlated with their scanning electron microscopy images. When CClx(x=1–4) species are present in the plasma, due to the erosion of a carbon‐containing resist, roughening of polysilicon is prevented at 100 W and the etch rate of the underlying SiO2is increased. Accompanying this increase is a shift in the product distribution for SiO2towards higher SiCl4but lower O2. In contrast the etch rate and product distribution for polysilicon remain almost unaffected by the presence of resist.