Selective silicon processing at 775 and 850-degrees-C using an SiH2Cl2/HCl/H-2 based chemistry was studied. The selectivity of each experimental condition was quantified by measuring the silicon nuclei density/cm2 on large blanket areas of SiO2. The morphology of the selective silicon films was examined for texture and hillocks. The roles of water vapor and atomic hydrogen on the microchemistry of an SiO2 surface was investigated. Thermodynamic modeling of the effect of different atmospheric leak rates on the water vapor and atomic hydrogen concentrations was carried out with the aid of the SOLGAS program. A new understanding of selectivity is proposed. An SiO2 surface is an interruption of the bulk continuous random network structure and, as such, has a characteristic density of def ect sites. The degree of selectivity observed is determined by the type of species which terminate these defect sites, i.e., by the number of defect sites which are not chemically passivated against the adsorption of silicon species. A universal selective silicon processing regime is predicted and good agreement with a wide range of published selective silicon processes is shown.