Highly selective etching of silicon nitride over silicon and silicon dioxide

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 wit h much smaller flows of CF4 or NF3 as a fluorine source as compared to conv entional Si3N4 removal processes, Etch rates of Si3N4 of more than 30 nm/mi n were achieved for CF4 as a source of fluorine, while simultaneously the e tch rate ratio of Si3N4 to polycrystalline silicon was as high as 40, and S iO2 was not etched at all. For NF3 as a fluorine source, Si3N4 etch rates o f 50 nm/min were achieved, while the etch rate ratios to polycrystalline si licon and SiO2 were approximately 100 and 70,respectively. In situ ellipsom etry 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. (C) 1999 American. Vacuum Society. [S0734-2101(99)05706-1].