Fabrication and surface treatment of electron-beam evaporated niobium for low-loss coplanar waveguide resonators

TL;DR

This study demonstrates that ultra-high vacuum electron-beam evaporated niobium films can achieve very low microwave loss and high quality factors in coplanar waveguide resonators, suitable for quantum applications.

Contribution

It introduces a fabrication process yielding low-loss niobium films with high superconducting transition temperature and minimal two-level-system loss for quantum microwave devices.

Findings

Superconducting transition temperature of 9.20 K.

Two-level-system loss tangent as low as 1.5 x 10^-7.

Internal quality factors exceeding one million in single-photon regime.

Abstract

We characterize low-loss electron-beam evaporated niobium thin films deposited under ultra-high vacuum conditions. Slow deposition yields films with a high superconducting transition temperature ($9.20 \pm 0.06 \rm ~K$) as well as a residual resistivity ratio of $4.8$. We fabricate the films into coplanar waveguide resonators to extract the intrinsic loss due to the presence of two-level-system fluctuators using microwave measurements. For a coplanar waveguide resonator gap of $2~\mu \rm m$, the films exhibit filling-factor-adjusted two-level-system loss tangents as low as $1.5 \times 10^{-7}$ with single-photon regime internal quality factors in excess of one million after removing native surface oxides of the niobium.