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In situFourier transform infrared (FTIR) absorption spectroscopy has been used to detect reaction products resulting from the etching of Si in Cl2plasmas. Silicon tetrachloride SiCl4was the only gas-phase product species detected during etching. Unsaturated silicon chlorides SiClx(x=1-3) were not observed in the plasma within the present level of detection. By comparing the absorbances of SiCl4in Cl2plasma etching of Si and in pure SiCl4gases, it is suggested that the concentrations of SiCl4or product species during etching are comparable to the feedstock Cl2gas densities, e.g., [SiCl4] ?1 ×1013cm-3at a pressure of 0.5 mTorr. In contrast, on the surface-etched Si, unsaturated silicon chlorides SiClx(x=1-3) as well as SiCl4were found to occur by FTIR reflection absorption spectroscopy (RAS). Moreover, absorption features of silicon oxides were observed both in the gas phase and on the surface, presumably arising from reactions between Si produced from etching and oxygen included in the reactor chamber owing to a small leak.