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 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.