Nitrogen Plasma Passivated Niobium Resonators for Superconducting Quantum Circuits

Abstract

Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over a timescale of minutes when exposed to an ambient environment. In this work, we show that nitrogen plasma treatment forms a niobium nitride layer at the metal-air interface which prevents such oxidation. X-ray photoelectron spectroscopy confirms the doping of nitrogen more than 5 nm into the surface and a suppressed oxygen presence. This passivation remains stable after aging for 15 days in an ambient environment. Cryogenic microwave characterization shows an average filling factor adjusted two-level-system loss tangent $\mathrm{F}\hat{\mathrm{I}}{´}_{\mathrm{TLS}}$ of $(2.9\pm 0.5)\cdot 10^{-7}$ for resonators with 3 $\hat{\mathrm{I}}¼$m center strip and $(1.0\pm 0.3)\cdot 10^{-7}$ for 20 $\hat{\mathrm{I}}¼$m center strip, exceeding the performance of unpassivated samples by a factor of four.

7 pages, 5 figures