A highly selective dry etching process for the removal of silicon nitride (Si3N4) layers from silicon and silicon dioxide (SiO2) is described and its mechanism examined. This new process employs a remote O-2/N-2 discharge with much smaller flows of CF4 or NF3 as a fluorine source as compared to conventional Si3N4 removal processes, Etch rates of Si3N4 of more than 30 nm/min were achieved for CF4 as a source of fluorine, while simultaneously the etch rate ratio of Si3N4 to polycrystalline silicon was as high as 40, and SiO2 was not etched at all. For NF3 as a fluorine source, Si3N4 etch rates of 50 nm/min were achieved, while the etch rate ratios to polycrystalline silicon and SiO2 were approximately 100 and 70,respectively. In situ ellipsometry shows the formation of an approximately 10-nm-thick reactive layer on top of the polycrystalline silicon. This oxidized reactive layer suppresses etching reactions of the reactive gas phase species with the silicon.