An experimental study of the etching of Si with SF6/halogen and halogen mixtures has been performed in a multipolar electron cyclotron resonance plasma with independent rf biasing. At constant total pressures and at a constant ion energy (75 eV), the anisotropy and the etch rate of silicon have been measured as a function of halogen percentages in the mixtures. The results are explained in terms of a recently developed diffusion model for plasma etching. Fluorine, chlorine, and bromine mixtures, contrary to iodine, etch Si, and SiO2. The bromine and chlorine adsorption on silicon appears to be monolayerlike, whereas fluorine adsorption is clearly of the multilayer type. Experimental results on etching anisotropy and selectivity with respect to SiO2are also analyzed with emphasis on the peculiar behavior of bromine.