Temperature-dependent reaction pathways of fluorination and nitridation in native Nb oxide under NF3/H-radical exposure

Native niobium forms Nb2O5 and NbOx upon air exposure, and these suboxide components are known to introduce two-level systems that limit the performance of superconducting quantum circuits. Reliable low-temperature modification of the native oxide is, therefore, of considerable interest. In this study, we systematically investigated the temperature-dependent reactions of native Nb oxides exposed to NF3 and H radicals from 25 to 250 °C. X-ray photoelectron spectroscopy revealed three distinct regimes: fluorination at 25 °C forming NbF5- and NH4F-like species; stabilization of NbO2F and fluorine-mediated nitridation at 100–150 °C; and a mechanistic transition at 200 °C, leading to the direct nitridation of Nb–O bonds and the formation of NbON. At 250 °C, deep nitridation penetrated into the underlying Nb metal, resulting in a thick NbON layer and a loss of selectivity. These results indicate that temperatures below 200 °C allow selective fluorination and nitridation of the native oxide while suppressing excessive modification of the underlying metallic Nb. The mechanistic insights obtained here provide a basis for understanding fluorination and nitridation reactions in Nb oxide under low-temperature radical-processing environments.