The chemical dry etching of silicon nitride (Si3N4) and silicon nitride (SiO2) in a downstream plasma reactor using CF4, O-2, and N-2 has been investigated. A comparison of the Si3N4 and SiO2 etch rates with that of polycrystalline silicon shows that the etch rates of Si3N4 and SiO2 are not limited by the amount of fluorine arriving on the surface only. Adding N-2 in small amounts to a CF4/O-2 microwave discharge increases the Si3N4 etch rate by a factor of 7, but leaves the SiO2 etch rate unchanged. This enables etch rate ratios of Si3N4 over SiO2 of 10 and greater. The Si3N4 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 Si3N4 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 CF4/O-2 discharge.