The chemical dry etching of silicon nitride (Si 3 N 4 )and silicon nitride (SiO 2 ) in a downstream plasma reactor using CF 4 , O 2 , and N 2 has been investigated. A comparison of the Si 3 N 4 and SiO 2 etch rates with that of polycrystalline silicon shows that the etch rates of Si 3 N 4 and SiO 2 are not limited by the amount of fluorine arriving on the surface only. Adding N 2 in small amounts to a CF 4 /O 2 microwave discharge increases the Si 3 N 4 etch rate by a factor of 7, but leaves

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... SiO 2 etch rate unchanged. This enables etch rate ratios of Si 3 N 4 over SiO 2 of 10 and greater. The Si 3 N 4 etch rate was investigated with respect to dependence of tube length, tube geometry, and lining materials. Argon actinometry has shown that the production of F atoms in the plasma is not influenced by the addition of N 2 to the discharge. Mass spectrometry shows a strong correlation between the Si 3 N 4 etch rate and the NO concentration. X-ray photoelectron spectra of the silicon nitride samples obtained immediately after the etching process show that F atoms are the dominant foreign species in the reaction layer, and that N 2 addition to the feed gas enhances the O atom incorporation. Based on these data, we propose a mechanism for the etch rate enhancement of N 2 addition to a CF 4 /O 2 discharge.