Characterization of Fabrication-Induced Noise Sources in Superconducting Niobium for Quantum Information Science Applications

Superconducting niobium (Nb) has garnered significant attention for its application in quantum information technologies. In the production of two-dimensional superconducting qubits, current fabrication practices commonly employ fluoride-based chemical etchants, such as buffered oxide etch (BOE), to minimize surface contaminants and native oxides from Nb thin films, which can otherwise negatively impact quantum circuitry. However, these etchants also have the potential to introduce hydrogen into Nb thin films, leading to the formation of Nb hydrides, which may adversely affect microwave loss performance. In this study, we have conducted a comprehensive materials characterization of Nb hydrides formed in Nb thin films under various etchant treatments including hydrofluoric acid, BOE, and ammonium fluoride. Leveraging an array of techniques, including secondary-ion mass spectrometry, X-ray scattering, and transmission electron microscopy, we examined the spatial distribution and phase transformation of the resulting Nb hydrides. Our data reveal that the rate of hydride formation is dependent on both the hydrogen concentration in the solution and the etch rate of the native surface oxide, Nb₂O₅, which acts as a diffusion barrier for hydrogen incorporation into Nb. Through correlative device measurements, we demonstrate that Nb hydrides introduced by wet chemical etching are detrimental to the superconducting properties of Nb and result in increased power-independent microwave loss in coplanar waveguide resonators. In addition, our resonator results did not show a correlation between Nb hydrides and two-level system loss nor device aging mechanisms. Overall, this work provides valuable insights into the formation of Nb hydrides and their impact on microwave loss in superconducting quantum circuitry. It also suggests fabrication strategies to mitigate these effects, thus supporting ongoing efforts to enhance coherence time in superconducting quantum devices.