Argon milling induced decoherence mechanisms in superconducting quantum circuits

TL;DR

This study investigates how argon milling affects superconducting circuits, revealing that niobium is significantly degraded while aluminum remains resilient, and suggests a recovery method for niobium-based qubits.

Contribution

It provides detailed analysis of argon milling effects on niobium and aluminum superconducting resonators, identifying defect mechanisms and a recovery process for niobium.

Findings

Niobium resonators' quality factors decrease by an order of magnitude after argon milling.

Aluminum resonators are unaffected by argon milling.

A short dry etch can fully recover niobium resonator losses.

Abstract

The fabrication of superconducting circuits requires multiple deposition, etch and cleaning steps, each possibly introducing material property changes and microscopic defects. In this work, we specifically investigate the process of argon milling, a potentially coherence limiting step, using niobium and aluminum superconducting resonators as a proxy for surface-limited behavior of qubits. We find that niobium microwave resonators exhibit an order of magnitude decrease in quality-factors after surface argon milling, while aluminum resonators are resilient to the same process. Extensive analysis of the niobium surface shows no change in the suboxide composition due to argon milling, while two-tone spectroscopy measurements reveal an increase in two-level system electrical dipole moments, indicating a structurally altered niobium oxide hosting larger two-level system defects. However, a short dry etch can fully recover the argon milling induced losses on niobium, offering a potential route towards state-of-the-art overlap Josephson junction qubits with niobium circuitry.