Process Optimization for Superconducting Resonators via Identification and Mitigation of Interface Loss Mechanisms*

CPW resonators plays a vital role for many quantum circuits. They also serve as a platform to characterize loss mechanisms in superconducting materials in the microwave frequency range. We report on the characterization and iterative process development of niobium-based CPW resonators. Using a set of post-fabrication chemical etching techniques, we have systematically reduced the metal-air and substrate-air oxide thickness, improving the device quality factor beyond 5 million at single-photon-excitation power (measured at 100 mK). Following these low temperature electrical measurements, resonator samples have been probed using a suite of structural characterization tools (XPS, TEM and AFM) in order to determine the thickness, chemical composition and location of parasitic interface layers and their contribution towards quality factor.