Degradation of Quality Factor of Superconducting Resonators by Remaining Metallic Film and Improved Fabrication Process Using Caldera Planarization

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 ${\boldsymbol{Q}_{{\mathbf i}}}$ 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 ${\boldsymbol{Q}_{{\mathbf i}}}$ . 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 ${\boldsymbol{Q}_{{\mathbf i}}}$ as high as that of resonator-only chips using the improved process.