Localization and mitigation of two-level system (TLS) and non-TLS losses at interfaces of niobium superconducting quantum resonators

We have characterized the metal-air (MA), substrate-air (SA) and metal-substrate (MS) interfaces of niobium-on-silicon resonators with cross-sectional scanning transmission electron microscopy (STEM) and x-ray photoemission spectroscopy (XPS). These resonators have a median internal loss tangent δ_int of 1.07 ppm at the single-photon level, with 71% due to two-level systems (TLS). We removed the SA process oxide by selective chemical etching, and reduced δ_int to 0.48 ppm, now 47% TLS. Next we reduced the MA oxide thickness from 4.8 to 1.6 nm in several steps, and further reducing δ_int to 0.19 ppm. We found that both TLS and non-TLS losses were uniformly distributed throughout the niobium process oxide. 70% of TLS losses were associated with the SA silicon oxide, 24% with the MA niobium oxide, and 6% other locations including the MS interface. In contrast only 17% of non-TLS losses came from SA, 68% from MA, and 15% from other locations. Together, the MA and SA interfaces accounted for at least 92% of loss. Interfacial materials analysis correlated with these observations gives some insight into host materials for TLS and non-TLS losses. Post-fabrication etching improved median internal quality factors from 0.93 million to 5.26 million.