Reactive ion etching of silicon oxide and silicon nitride was conducted by the injection of nitrogen trifluoride (NF3) and nitrogen oxide trifluoride gas (F3NO). The etching process was studied using a residual gas analyzer (RGA) and optical emission spectroscopy (OES); this included confirming and comparing the characteristics of the F3NO plasma to that of the NF3 plasma by discharging and measuring the pure NF3 plasma and F3NO plasma. Furthermore, silicon oxide and silicon nitride etching were performed using a process gas (NF3, F3NO) and an argon mixture. The plasma etching process was similarly diagnosed by RGA and OES, and the etch rate was calculated by measuring the reflection. The etch rate of silicon oxide during F3NO/Ar plasma etching is approximately 94% of that for NF3/Ar plasma etching and the etch rate of silicon nitride is approximately 76% of that for NF3/Ar plasma etching under the same conditions. The RGA and OES measurements confirmed that more O+, NO+, and O2+ ions were generated in the F3NO plasma than in the NF3 plasma. This difference makes it possible to confirm the variation in etch rates between silicon oxide and silicon nitride.