In situFourier-transform infrared (FTIR) ellipsometry has been performed on silicon substrates processed in a rf-biased transformer coupled plasma reactor. Plasmas inCHF3,CF4,C2F6,andC4F8have been used. The reaction layer, which is present on the surface of the silicon wafer during the plasma process, has been analyzed in detail, addressing both chemical composition and thickness. The absolute reliability (expressed in terms of thickness) of the results is of the order of 0.01 nm, which corresponds to 3% of a monolayer. The instabilities of a silicon surface, which have been observed under specific conditions, can be of the order of tens of percents of a monolayer, which clearly illustrates the advantage of using a realin situtechnique like FTIR ellipsometry over quasiin situtechniques like x-ray photoemission spectroscopy and Auger electron spectroscopy. ForCHF3plasmas it has been found that, if the bias increases to moderate levels (30–50 V), the fluorocarbon film deposition rate decreases and the silicon etching reaction rate increases. The reaction layer changes from a thick, predominantlyCFxpolymerlike film to a thin, carbon dominated layer of plasma and etching products showing vibrational absorptions ofSiFx,C–C, andCF2.Increasing the bias voltage in aCHF3plasma has a similar effect as increasing the F/C ratio of the feed gas.