Impact of Surface Roughness on the Performance of Superconducting Niobium Resonators

Superconducting circuit-based quantum processors, a leading platform in quantum computing, rely heavily on high-quality, defect-free niobium (Nb) resonators for qubit storage and coherence [1]. Loss mechanisms in these resonators, including two-level systems (TLS), quasi-particles, and grain boundary impurities, are influenced by surface morphology and contribute to microwave energy dissipation at cryogenic temperatures [2]. This study investigates the effect of surface roughness on the quality factor (Qi) of Nb resonators. By controlling oxygen plasma exposure, we reduced the RMS surface roughness from 0.98 nm in ozone-treated Nb samples to 0.31 nm in plasma-treated Nb samples. Analysis using AFM, XPS, and TEM revealed that smoother surfaces correlate with reduced pinhole depth. We observed an area-dependent quality factor, attributed to changes in surface resistivity. This approach minimized variations in quality factors across resonators, enhancing uniformity in Nb-based devices and yielding more consistent qubit readout performance. Detailed results from this study will be presented at the conference.



References:

1. Kjaergaard, M. et al., Materials loss measurements using superconducting microwave resonators, Rev. Sci. Instrum. 91, 091101 (2023).

2. Crowley, K. D. et al., Disentangling losses in tantalum superconducting circuits, Phys. Rev. X 13, 041005 (2023).