Nitrogen plasma passivated niobium resonators for superconducting quantum circuits
Kaiwen Zheng, Daria Kowsari, N. J. Thobaben, Xinyi Du, Xingrui Song, Sheng Ran, Erik Henriksen, David Wisbey, Kater Murch
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 FδTLS of (2.9±0.5)·10−7 for resonators with a 3 μm center strip and (1.0±0.3)·10−7 for a 20 μm center strip, exceeding the performance of unpassivated samples by a factor of four.