The effect of etch products on the Si etching characteristics in electron cyclotron resonance plasmas with Cl2 was investigated by using both plasma diagnostics and a numerical simulation. The etch products both in the gas phase and on the etched surface were observed by in situ Fourier transform infrared (FTIR) absorption spectroscopy. The concentration range of silicon tetrachloride SiCl4 detected as the gas-phase etch product during Si etching was about 1012 molecules/cm3 within the pressure range of 0.1–10 mTorr, while that of silicon oxide was 1010–1012 molecules/cm3. On the etched surface, unsaturated silicon chlorides SiClx (x=1–3) as well as SiCl4 were observed. Moreover, the effect of the etch products was numerically investigated by using a kinetic etching model. The calculated etch rate showed good agreement with the measured one within the pressure range of 0.1–10 mTorr even though the sticking coefficient of silicon oxide was varied from 0.001 to 0.5. The coverage of the SiClx layer saturated about one monolayer beyond pressure of 1 mTorr. The bare Si surface area and the actual coverage of the oxide layer decreased by increasing the gas pressure. However, the actual coverage of oxide layer at ≪1 mTorr varied according to the sticking coefficient of silicon oxide. The calculated concentration of the etch products was the same order of magnitude as the concentration obtained by in situ FTIR diagnostics. It is expected that the effect of etch products on the etching characteristic,- - especially etched profiles, becomes more and more important at pressure of less than 1 mTorr. © 1999 American Vacuum Society.