Reactive ion etching of silicon nitride with CHF3/O2 plasmas has been studied in a hexode reactor and compared to silicon dioxide etching. Measurements of etch rates as a function of gas composition and pressure were combined with Langmuir probe data for the ion flux to the substrate to give etch yields (number of substrate atoms removed per bombarding ion). At low oxygen content, the etch yields of both materials are about 2 atoms/ion and are essentially independent of pressure between 10 and 60 mTorr. At higher oxygen content, the etch yield is considerably larger for silicon nitride, and for both materials the etch yields increase with increasing pressure. From these results it can be concluded that in a CHF3/O2 plasma at low oxygen content, the etch mechanism is mostly direct reactive ion etching for both silicon nitride and silicon oxide. On the other hand, at higher oxygen content the etching is ion enhanced for both materials, but to a much greater extent in the case of silicon nitride.