Niobium Hydride Formation in Superconducting Thin Films during Wet Chemical Processing

Wet etching procedures are used at different stages in the nanofabrication of superconducting qubits. However, the use of acidic solutions has the potential to introduce hydrogen into the bulk by diffusion via interstitial sites in the Nb lattice. Niobium hydrides have been previously found to play a key role in the quality factor (coherence) of 3D SRF niobium cavities, and more recently similar hydrides have been discovered in niobium films of 2D superconducting qubits. [*] Here we investigate how chemical processes used in Nb film fabrication could lead to different hydrogen loading amounts and precipitation states. The exposure times and pH of various etchant solutions were varied to determine the formation of Nb hydrides as detected by X-ray reflectivity and X-ray diffraction in Nb (110) / Si (001) thin films. Specifically, changes in the Nb film density, thickness, roughness, crystallinity, and d-spacing have been characterized. Additionally, atomic force microscopy was utilized to observe the etching effects on the surface roughness of Nb thin films and the silicon substrate. This study finds that low pH etching solutions have a propensity to convert Nb metal films to NbH_x, which is detrimental for superconducting device performance. [*] J. Lee et al, arXiv:2108.10385