  Superconducting Nb thin films have recently attracted significant attention
due to their utility for quantum information technologies. In the processing of
Nb thin films, fluoride-based chemical etchants are commonly used to remove
surface oxides that are known to affect superconducting quantum devices
adversely. However, these same etchants can also introduce hydrogen to form Nb
hydrides, potentially negatively impacting microwave loss performance. Here, we
present comprehensive materials characterization of Nb hydrides formed in Nb
thin films as a function of fluoride chemical treatments. In particular,
secondary-ion mass spectrometry, X-ray scattering, and transmission electron
microscopy reveal the spatial distribution and phase transformation of Nb
hydrides. The rate of hydride formation is determined by the fluoride solution
acidity and the etch rate of Nb2O5, which acts as a diffusion barrier for
hydrogen into Nb. The resulting Nb hydrides are detrimental to Nb
superconducting properties and lead to increased power-independent microwave
loss in coplanar waveguide resonators. However, Nb hydrides do not correlate
with two-level system loss or device aging mechanisms. Overall, this work
provides insight into the formation of Nb hydrides and their role in microwave
loss, thus guiding ongoing efforts to maximize coherence time in
superconducting quantum devices.
