Nitrogen Plasma Passivated Niobium Resonators for Superconducting Quantum Circuits

TL;DR

This study demonstrates that nitrogen plasma treatment creates a stable niobium nitride surface layer that significantly reduces microwave loss in superconducting resonators, enhancing their performance for quantum circuits.

Contribution

The paper introduces a nitrogen plasma passivation method that forms a stable niobium nitride layer, reducing surface oxidation and microwave loss in superconducting niobium resonators.

Findings

Nitrogen plasma creates a niobium nitride surface layer.

Passivation reduces surface oxidation and loss tangent.

Enhanced resonator performance with a fourfold reduction in microwave loss.

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 $\rm{F\delta_{TLS}}$ of $(2.9\pm0.5)\cdot10^{-7}$ for resonators with 3 $\rm{\mu}$m center strip and $(1.0\pm0.3)\cdot10^{-7}$ for 20 $\rm{\mu}$m center strip, exceeding the performance of unpassivated samples by a factor of four.