Native SiO2 passivating films grown thermally on flat Si(100) surfaces exhibit many ideal properties that justify regarding them as well-defined (albeit noncrystalline) phases with a continuous and almost defect-free interface with the Si substrate in spite of a large difference in density. Global constraint theory explains this remarkable situation without recourse to elaborate mathematical or geometrical local models, and it correctly predicts the scaling dependence of defect densities in films doped with N and H with essentially no adjustable parameters.