The role played by the chemical species present in HF/water-ethanol solutions as last cleaning steps of the [100] silicon surface has been analyzed. The concentrations of these species as a function of the ethanol content have been calculated from measured equilibrium constants for the dissociation and homoconjugation reactions of HF in water-ethanol solutions. The correlation of these data with the measured etching rates of silicon oxide in these solutions suggests that etching in KF/pure ethanol is controlled almost completely by the ion HF2-. The contribution of the H2F2 dimers to the etching process diminishes as ethanol content increases. This indicates an increasing activation energy of the dimer etching reaction when the ethanol content is raised. This characteristic behavior is associated with the slower and more homogeneous etching action of HF/ethanol solutions as well as with the absence of faceting and lower microroughness generation in [100] silicon Surfaces. The effects of overetching are not significant, and moreover, the characteristic surfactant properties of alcohols contribute to the effects of these cleaning solutions.