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To address the challenge of dense connectors for cryogenic systems, we describe a face-to-face cable connection approach for connecting microwave superconducting stripline cables. The approach provides a space-efficient and reliable interconnect technology for future cryogenic and quantum technology applications at microwave frequencies. A self-aligned assembly was used and was comprised of bottom and top alignment molybdenum pieces and flexible superconducting stripline cables. SU-8, a photodefinable epoxy-based photoresist, was used to form structures defined on the bottom and top Mo pieces to provide alignment and distribute mechanical pressure in the contact regions of the cables. We measured critical transition temperatures in the range of 8.6-9.0 K and an average critical current of 10.2 mA. Time-domain reflectometry measurements at 4.2 K showed a relatively small impedance discontinuity at the connection point and minimal microwave power dependency was observed. This cable-to-cable connection approach for thin, flexible superconducting cables shows promising results as a solution for dense signal integration for microwave and DC interconnects at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

We have studied the effects of gamma radiation on flexible superconducting microstrip transmission line structures, to explore their suitability for use in harsh environments where they may be subjected to radiation. In this work, we used two conductor variants: one with only Nb and another with a conductor stack of Al/Nb/Al. Spin-on polyimide HD-4110 was used as the dielectric substrate and both the variants were encapsulated with a layer of HD-4110 to increase the robustness of the structure by protecting the superconducting trace from potential mechanical damage. The resonators were irradiated at room temperature in a vacuum sealed chamber at a pressure of 1E-6 Torr using cobalt-60 as a source of gamma radiation. Samples were exposed to doses up to 60.8 Mrad (608 kGy). Quality factors of the resonators were extracted in the frequency range from 2 to 20 GHz, at various cryogenic temperatures between 1.2 K and 4.2 K, using a closed-cycle cryostat. We observed a small increase in microwave loss for higher temperature ranges, which points to a small change in the superconducting properties of the conductors. The results of this work show that low mass, flexible superconducting cables may be suitable alternatives to bulky coaxial cables for signal transmission at cryogenic temperatures in extreme (radiation) environments. [ABSTRACT FROM AUTHOR]

We have investigated Al2O3 as a thin interface material between Nb and polyimide, deposited using a relatively low temperature thermal atomic layer deposition (ALD) process to preserve the superconducting properties of the Nb layer. 250 nm thick Nb traces, with $\sim$ 20 nm thick layers of Al2O3 were used for this work. HD-4110 spin-on polyimide by HD MicroSystems with a thickness of ~ 20 μm was used with different curing temperatures of 225 °C and 375 °C. DC electrical characterization of patterned Nb lines were carried out using a pulse-tube based cryostat to determine Tc and Ic for the samples at different steps in the fabrication process. Details of the fabrication processes, experimental procedures and performance results are included. Results of these experiments provide insight into materials stack-ups and fabrication process options for robust, multi-layer superconducting flexible cables that can be used for signal transmission in future densely-integrated cryogenic electronics systems. [ABSTRACT FROM AUTHOR]

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  $\mu$m 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 $\times$10−5. Embedded resonators with an additional 20 $\mu$m thick encapsulation layer also exhibited Q values as high as $\sim 19$ 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. [ABSTRACT FROM PUBLISHER]

Flexible superconducting Nb microstrip transmission line cables, with lengths of $\sim$5 cm, were fabricated using 2 mil (50.8  $\mu$m) thick E-series Kapton films. A 50 nm Ti layer was used to improve the adhesion of sputter-deposited 250-nm-thick Nb signal line and ground plane metallization. The signal traces of the microstrip were directly interfaced with edge-launch SMA connectors and measured up to 14 GHz in liquid He. For comparison, we also fabricated normal conductor (Ti/Cu) microstrip transmission lines with a similar design. The microwave performance of the Nb-based transmission lines indicates significantly lower intrinsic insertion loss (less than $\sim$ 0.02 dB/cm) compared to the Cu-based microstrip measured at 4.2 K to have a loss of $\sim$0.57 dB/cm. [ABSTRACT FROM PUBLISHER]