In this work, we investigated how different under- and capping layers on patterned Nb films impacted the RF losses of flexible thin-film superconducting microstrip transmission line resonators measured in a frequency range from 2 to 20 GHz. We studied how different thicknesses of Ti(10 and 50 nm) under-layer, used for adhesion, impacts conductor losses. We also studied Cu(20, 50, 100, and 200 nm) capping layers and how they affect conductor loss. These studies were carried out on 20-μm-thick spin-on polyimide (PI-2611) thin films and characterized at various cryogenic temperatures between 1.2 and 4.2 K. The results indicate normal-superconductor (Ti/Nb) and superconductor-normal (Nb/Cu) bilayer structures have increased surface resistance, which leads to an increase in microwave loss when compared to Nb-only signal traces. We quantified this additional loss by extracting resonator quality factors for weakly coupled resonators with various conductor stack-ups. Our experimental results can help inform decisions regarding material stack-ups when designing multiconductor multilayer superconducting flexible cables intended for use with ultralow-temperature electronic systems.
In this work, we investigated how different under- and capping layers on patterned Nb films impacted the RF losses of flexible thin-film superconducting microstrip transmission line resonators measured in a frequency range from 2 to 20 GHz. We studied how different thicknesses of Ti(10 and 50 nm) under-layer, used for adhesion, impacts conductor losses. We also studied Cu(20, 50, 100, and 200 nm) capping layers and how they affect conductor loss. These studies were carried out on 20-μm-thick spin-on polyimide (PI-2611) thin films and characterized at various cryogenic temperatures between 1.2 and 4.2 K. The results indicate normal-superconductor (Ti/Nb) and superconductor-normal (Nb/Cu) bilayer structures have increased surface resistance, which leads to an increase in microwave loss when compared to Nb-only signal traces. We quantified this additional loss by extracting resonator quality factors for weakly coupled resonators with various conductor stack-ups. Our experimental results can help inform decisions regarding material stack-ups when designing multiconductor multilayer superconducting flexible cables intended for use with ultralow-temperature electronic systems.
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.
Flexible interconnects with superconducting (SC) traces and polyimide dielectric can provide low signal loss and low thermal conductivity at cryogenic temperatures, and are expected to find use in densely integrated cryogenic electronics systems. The ultra-low loss of SC interconnects at microwave frequencies can be accurately characterized through the use of weakly coupled SC resonators. The high-quality factor resonators provide sensitive measurements of the aggregate loss properties of the conductor and dielectric, as functions of frequency, temperature, and signal power. In this work, resonators with different linewidths were fabricated and tested to study how the linewidth impacts the overall internal resonator loss, in order to better characterize and separate the individual loss components (i.e., dielectric loss vs. conductor loss). The results of this work are important for understanding loss and transmission properties of similarly designed and fabricated transmission line interconnects.
We present recent work on design, fabrication, and characterization of thin-film, flexible superconducting stripline cables. Fabrication details, dc, and microwave performance (up to 14 GHz) of cables with Al/Nb/Al conductors and polyimide (PI) dielectric are discussed. Thin Al was used as a barrier layer between Nb and polyimide to protect the Nb superconductivity during subsequent PI curing steps, which were performed at a reduced temperature. The dc performance, including critical transition temperature and critical current, of the stripline transmission lines is presented. We observed Tcvalues of ~8.8 and ~8.6 K for signal traces and ground planes, respectively. Microwave characterization, including cross-talk, was performed on stripline transmission lines and resonators. We observed reasonable impedance match (S11better than -15 dB), low insertion loss (S21better than 0.04 dB/cm) and cross-talk below -60 dB for stripline transmission lines measured up to 10 GHz at 4.2 K. Furthermore, similarly fabricated stripline resonators exhibited quality factors ~7 000 at 10 GHz and 1.2 K for stripline resonators. The results of these experiments provide insight into material stack-ups for robust, multi-layer superconducting flexible cables that can find use in future cryogenic electronics systems.
Abstract The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over 10 6 at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and indicate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.
We describe progress and initial results achieved towards the goal of developing integrated multi-conductor arrays of shielded controlled-impedance flexible superconducting transmission lines with ultra-miniature cross sections and wide bandwidths (dc to >10 GHz) over meter-scale lengths. Intended primarily for use in future scaled-up quantum computing systems, such flexible thin-film niobium/polyimide ribbon cables could provide a physically compact and ultra-low thermal conductance alternative to the rapidly increasing number of discrete coaxial cables that are currently used by quantum computing experimentalists to transmit signals between the several low-temperature stages (from ∼4 K down to ∼20 mK) of a dilution refrigerator. We have concluded that these structures are technically feasible to fabricate, and so far they have exhibited acceptable thermo-mechanical reliability. S -parameter results are presented for individual 2-metal layer Nb microstrip structures having 50 Ω characteristic impedance; lengths ranging from 50 to 550 mm were successfully fabricated. Solderable pads at the end terminations allowed testing using conventional rf connectors. Weakly coupled open-circuit microstrip resonators provided a sensitive measure of the overall transmission line loss as a function of frequency, temperature, and power. Two common microelectronic-grade polyimide dielectrics, one conventional and the other photo-definable (PI-2611 and HD-4100, respectively) were compared. Our most striking result, not previously reported to our knowledge, was that the dielectric loss tangents of both polyimides, over frequencies from 1 to 20 GHz, are remarkably low at deep cryogenic temperatures, typically 100 × smaller than corresponding room temperature values. This enables fairly long-distance (meter-scale) transmission of microwave signals without excessive attenuation, and also permits usefully high rf power levels to be transmitted without creating excessive dielectric heating. We observed loss tangents as low as 2.2 × 10 −5 at 20 mK, although losses increased somewhat at very low rf power levels, similar to the well-known behavior of amorphous inorganic dielectrics such as SiO 2 . Our fabrication techniques could be extended to more complex structures such as multiconductor cables, embedded microstrip, 3-metal layer stripline or rectangular coax, and integrated attenuators and thermalization structures.
Abstract We describe a superconducting (SC) cable-to-cable connection method for thin-film flexible (Nb/polyimide) parallel transmission line cables that exhibits low loss from DC to microwave frequencies. Design, fabrication procedures, assembly methods and characterization results are reported. The connector assembly incorporates a monolithic array of short superconductive bridging traces that function as a ganged electrical splice between two ribbon cables. The assembly is micro-mechanically self-aligned and uses pressure to make simultaneous electrical contacts between multiple 50 μ m wide microstrip signal traces, as well as between ground planes. Each contacting surface comprises thin SC Nb overcoated with ultra-thin Au, providing nearly zero DC resistance and very good RF performance. A simulation model of the microwave performance of microstrip-to-microstrip interconnections at 4.2 K was successfully matched to measurement results. This new cable-to-cable connection approach exhibited an insertion loss <0.12 dB and a return loss >15 dB up to 14 GHz. The connections performed reproducibly throughout multiple thermal cycles and multiple disassembly/reassembly cycles.