High-Quality Factor Superconducting Flexible Resonators Embedded in Thin-Film Polyimide HD-4110

We present the design, fabrication procedure, and measurement results of Nb superconducting microstrip transmission line resonators fabricated using thin-film polyimide HD-4110, including both nonembedded and embedded versions. These resonators were used to characterize the microwave dielectric loss tangent of 20 μm thick polyimide HD-4110 at deep cryogenic temperatures. We observed high-quality factors (Q) up to 21 040 at 1.2 K in the frequency range of 2-21 GHz for nonembedded resonators, indicating that the dielectric loss tangent can be less than 5×10-5. Embedded resonators with an additional 20 μm thick encapsulation layer also exhibited Q values as high as ~19 200. Dielectric and conductor (quasiparticle) loss have been compared between the two types of resonators. This study provides information applicable to the design of future high-density, flexible, multilayer superconducting cables, which are of great interest for potential applications in cryogenic electronics systems, including quantum computers.