Superconducting Resonators on Thin Film Flexible Substrates

Abstract

In cryogenic electronics applications and experiments, i.e., for cold detectors and low temperature quantum computing, the base temperature is typically below 100 milli-Kelvin. In order to reach this temperature range, a significant effort is undertaken to reduce the thermal load of the system by using microwave and dc cables with a low thermal conductivity. As the number of connections increase in superconducting cryogenic computers, the more difficult it becomes to cool the system down. One practical solution to address this issue is to reduce the cross-sectional area of the cables used in the cryogenic system. For cryogenic cables and interconnects that deliver microwave signals, we want low loss dielectric materials to interface different kinds of microwave devices, especially superconductor devices. There- fore, we want to minimize losses in the substrate by selectively choosing dielectrics that are low-loss and mechanically compliant when used as free-standing flex cables. Half-wavelength, capacitively coupled superconducting microstrip resonators have been constructed on various flexible substrates including 50.8 μm (2 mil) thick free-standing Kap- ton polyimide and 20 μm thick spin-on polyimide. Cu normal conducting and Nb super- conducting resonators were designed and fabricated on these substrates. We extracted the complex dielectric permittivity of the films over a wide frequency range and at cryogenic temperatures below 9 K. In order to better characterize these thin and low loss dielectric films, we require high-quality factor resonators to allow extraction of the dielectric properties. We designed these resonators to be weakly coupled to reduce the coupling loss, this involved several iterative designs and measurements at cryogenic temperatures. The metal stack-up of these superconducting resonators mainly consisted of a 250 nm thick superconducting Nb film to limit conductor loss. The fabricated resonators exhibited high-quality factors (Q) at a temperature of ∼1.2 K of 7000 for E-series Kapton film, 13 000 for a spin-on PI-2611 and 17 000 for a spin-on HD-4100 in the 2-10 GHz frequency range. These results imply the loss tangent of all the materials measured is less than 0.00016 at 2 GHz. These high-Q measurement results provide evidence that spin-on polyimide film, as well as commercially available Kapton can be potentially useful as a microwave substrate material for flexible superconducting interconnects or cables, which are of great interest for use in cryogenic electronics systems. In this work, we also investigated how different under layers and capping layers of the Nb films impacted the conductor losses in the GHz frequency range. We mainly studied how the different thickness of a Ti (10 and 50 nm) under-layer for adhesion impacted the superconductor losses. This is important, as Ti is a commonly used metal for adhesion to various substrate materials. We also studied Cu (10, 50, 100, and 200 nm) capping layers and how they affect the conductor loss. Conductor loss studies were carried out on 20 μm thick spin on polyimide (PI-2611) films and were characterized at different cryogenic temperatures. The results indicate normal-to-superconductor (Ti/Nb) and superconductor- normal (Nb/Cu) bi-layer structures have more surface resistance, which leads to an increase of RF loss when compared to “bare” Nb conductor traces. We quantified this additional loss by measuring Q-factors at different cryogenic temperatures (4.2 K, 3.6 K, 3.0 K, and 1.2 K). This work provides experimental support for the fabrication and performance of bi-layer normal-superconducting structures on these types of thin dielectric films. Lastly, we investigated embedded superconducting resonators on thin polyimide (PI- 2611 and HD-4100). We built a series of embedded superconducting edge coupled half- wavelength microstrip transmission line resonators (MTLR) with a top layer dielectric that was half of the thickness of the bottom dielectric. These resonators provided useful information for building other multi-layer polyimide based microwave structures such as stripline transmission lines and stripline resonators that are expected to be fabricated in future ef- forts. From these measurement results, we determined superconducting Nb film quality can be degraded during the top polyimide layer curing process, even at a reduced curing temperature of 225◦C. We observed that an Al/Nb/Al (20 nm/250 nm/20 nm) metal stack-up can effectively prevent this degradation from occurring. We expect that the degradation is related to oxidation of the thin Nb due to exposure to H2O or O2, though determination of the exact mechanism was not the focus of this work. These experiments were studied on both types of polyimide PI-2611 (low-stress) and HD-4100 (photo-definable). Both of these films exhibited similar results, which leads to the conclusion that Al/Nb/Al metal-stackup may be useful for more complex multi-layer structures, such as stripline.