Reducing two-level-system microwave loss in niobium resonators with nitrogen plasma passivation

Microwave loss in niobium metallic structures used in superconducting quantum circuits is limited by a native oxide layer that grows back within hours after removal. This regrowth behavior prevents the incorporation of low loss niobium components in state-of-the-art superconducting quantum processors. We show that a low temperature nitrogen plasma treatment forms a 5 nm thick niobium nitride passivation layer which suppresses the presence of surface oxide. X-ray photoelectron spectroscopy measurements confirm the presence of nitrogen atoms and a suppressed oxygen concentration, which remain stable after 15 days of aging in an ambient environment. Cryogenic microwave transmission measurements of passivated niobium coplanar waveguide resonators reveal a filling factor adjusted two-level-system loss tangent that is 4 times lower than unpassivated devices.