Superconducting resonators are central to superconducting quantum information technologies and essential for bosonic qubit architectures, where long-lived storage modes enable hardware-efficient error correction. Achieving ultra-high quality factors in scalable planar circuits is challenging because multiple dissipation channels contribute to the total loss. Here we report planar
α-Ta resonators fabricated on 300 mm ultra-high-resistivity (
>10 k
Ω cm) intrinsic silicon using industrial processes, achieving median internal Q factors exceeding 40 million and maxima above 60 million. Energy-participation-ratio analysis identifies a dominant participation-controlled interface loss mechanism and places conservative upper bounds on substrate-associated dissipation. For the best-performing substrate, the inferred substrate loss tangent is below
1.0×10−8, establishing industrial MCZ silicon among the lowest-loss substrate platforms reported for superconducting resonators. At the same time, the exceptionally low losses show no clear correlation with commonly cited silicon substrate metrics such as room-temperature resistivity or impurity concentrations. More broadly, these studies establish industrial 300 mm processing, careful interface engineering, and 300 mm MCZ silicon substrates as a promising platform for resonator-heavy superconducting quantum architectures with ultra-high quality factors.
An important figure of merit in microwave superconducting quantum interference device multiplexer (MW-MUX) is the quality factor of superconducting resonators. In advanced industrial science and technology (AIST), the intrinsic quality factor Qi of the resonators in the MW-MUX chips degraded significantly compared to those in the resonator-only chips owing to something during the fabrication process. In this study, we have investigated the damage process by conducting controlled experiments and an SEM-EDX analysis. As a result, we have confirmed that the metallic layers (Pd and Nb) were not removed completely and remained along the edge of the resonator, which resulted in the degradation of Qi. Further, we have invented a new process to overcome this imperfection during the removal of Pd layer using so called “caldera planarization”. The metal line that remained along the edge was not observed in the MW-MUX chips fabricated using this improved process. Finally, we have achieved Qi as high as that of resonator-only chips using the improved process.