Surface Engineering of Superconducting Coplanar Waveguide Resonators via Integrated Atomic Layer Etching and Deposition

Superconducting microwave resonators are the key components that determine qubit coherence in quantum processors. Loss channels at the metal–air interface and within non-stoichiometric native oxides are major sources of two-level systems that degrade the resonator quality factor. While capping high-quality dielectric layers can protect the top surface, the patterned sidewalls remain exposed to air, leading to regrowth of lossy oxides. To overcome this limitation, we developed an integrated atomic layer etching (ALE) and atomic layer deposition (ALD) process for surface remodification of niobium and aluminum coplanar waveguide resonators. The ALE step gently removes native oxides, followed by in situ ALD of conformal Al₂O₃, HfO₂, or SiO₂ protection layers. We compared the Al-to-O ratio of ALD-grown Al₂O₃ films with that of native AlO_x. XPS analysis revealed that the ALD film exhibits a fully oxidized Al-to-O ratio of 0.67, while the native oxide shows a sub-stoichiometric ratio of 0.52. AFM results indicate that the ALE treatment reduces the surface roughness of the ALD-grown Al₂O₃ from 0.166 nm to 0.127 nm—about 13% smoother than after gentle Ar ion milling. Cross-sectional TEM confirms interface quality and conformality. Cryogenic microwave measurements show improved internal quality factors and reduced aging. This method offers a scalable pathway for reliable superconducting device fabrication and packaging.