Argon milling induced loss mechanisms in superconducting quantum circuits*

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 on niobium and aluminum as a potential coherence limiting process. We find that niobium microwave resonators show an order of magnitude decrease in quality-factors after surface argon milling, while aluminum resonators are resilient to the same process. We characterize the Nb surface with XPS, AFM, and STEM. The argon milled niobium surface regrows a layered oxide structure of primarily Nb₂O₅ causing an increase in both two-level-system defect losses and residual losses. Two-tone spectroscopy measurements reveal increased two-level-system electrical dipole moments of the average tunneling defect at the argon milled niobium surface. A carefully timed etch fully removes the induced losses and shows a path towards state-of-the-art overlap Josephson junction based qubits on niobium.