In situ Al$_2$O$_3$ passivation of epitaxial tantalum and aluminum films enables long-term stability in superconducting microwave resonators

TL;DR

This study demonstrates that in situ Al₂O₃ passivation of epitaxial tantalum and aluminum films significantly enhances the long-term stability of superconducting microwave resonators, maintaining high quality factors over more than a year.

Contribution

The paper introduces an in situ Al₂O₃ passivation technique that prevents oxidation and preserves device performance, advancing scalable superconducting quantum circuit fabrication.

Findings

Resonators with Al₂O₃ maintain Qi > 1 million after 14 months.

Native oxide devices show substantial Qi decline over time.

X-ray photoelectron spectroscopy confirms preserved chemical integrity with Al₂O₃ passivation.

Abstract

Long-term stability of superconducting microwave resonators is essential for scalable quantum technologies; however, surface and interface degradation continue to limit device stability. Here, we demonstrate exceptional stability in microstrip resonators fabricated from epitaxial tantalum and aluminum films, protected by in situ deposited Al$_2$O$_3$ under ultra-high vacuum. These resonators initially exhibit internal quality factors (Qi) exceeding one million and maintain high performance with minimal degradation after up to fourteen months of air exposure. In contrast, devices relying on native surface oxides show substantial declines in Qi over time, indicating increased microwave losses. X-ray photoelectron spectroscopy reveals that the in situ Al$_2$O$_3$ effectively suppresses interfacial oxidation and preserves the chemical integrity of the underlying superconducting films, whereas native oxides permit progressive oxidation, leading to device degradation. These findings establish a robust, scalable passivation strategy that addresses a longstanding materials challenge in the development of superconducting quantum circuits.