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 …