Rate differences in synchrotron radiation‐stimulated etching among several kinds of SiO2 and Si materials have been studied. The SiO2 etching rate is almost independent of the crystallinity. On the other hand, the Si etching rate decreases with improved crystallinity and increased activated dopant density. Impurity doping into polycrystalline (poly‐) Si leads to etching rate reduction irrespective of the conduction type. These material selectivities are explained by a reaction model that is described in terms of reaction centers produced by electronic excitation of the material surface.