Flexible superconducting Nb microstrip transmission line cables, with lengths of similar to 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 similar to 0.02 dB/cm) compared to the Cu-based microstrip measured at 4.2 K to have a loss of similar to 0.57 dB/cm.

Dense, controlled-impedance, superconducting cables with small cross-sections are desirable, especially for quantum computing applications. In this study, superconductivity properties, rf microwave response and mechanical reliability performance of embedded Nb dc cables and Nb microstrip transmission line resonators with different thicknesses of polyimide PI-2611 encapsulation layers (0, 4 and 8 µm) have been investigated. Critical temperature (T_c) and critical current (I_c) of embedded Nb dc cables are ~ 8.2 K and ~ 0.2 A, respectively. Embedded Nb resonators yield high loaded quality factor (Q_L), with values as high as 14481 at ~ 1.2 K and at a fundamental resonance of ~ 2 GHz. From mechanical fatigue testing, we have observed that a polyimide encapsulation layer can effectively enhance the mechanical reliability of superconducting Nb flexible cables.

Half-wavelength, capacitively-coupled superconducting microstrip resonators have been constructed on 50.8 μm (2 mil) thick flexible Kapton polyimide substrates. The metal stack-up on each side was a 50 nm Ti adhesion layer followed by a 250 nm Nb layer. These resonators yield high quality factors (loaded Q as high as 4110) at 1.2 K in the 2-10 GHz frequency range, implying a loss tangent of less than 0.000275 at 2 GHz. This work provides complex dielectric permittivity information for Kapton materials that have not previously been reported for this temperature (1-6 K) and frequency range. Furthermore it provides confidence that commercially available flexible Kapton is potentially useful as a substrate material for flexible superconducting interconnects or cables, which are of great interest for use in cryogenic electronics systems.

Similar to observations from other groups, we have observed degradation of the superconducting properties of Nb thin films that have been subjected to subsequent high temperature fabrication processes, such as curing of a passivating polyimide layer at 350°C. This degradation may be caused by interaction with material that diffuses into the Nb during the subsequent processes, and is the subject of ongoing research. To counteract these degradation effects, we have explored multiple materials as barrier layers to attempt to isolate and protect the Nb. The effectiveness of the barrier layer depends on the use of an appropriate layer stack that minimizes degradation in the superconducting thin film, is compatible with subsequent fabrication steps, and is stable and repeatable. We have investigated multiple material stack-ups to protect Nb-based superconducting thin film in flexible structures. We show that curing polymers above a certain temperature on top of a Nb layer can adversely affect the superconducting properties including critical transition temperature (Tc) and critical current (Ic). DC electrical characterization of patterned film test structures were carried out using a closed-cycle cryostat to determine Tc and Ic for the samples. Details of the fabrication processes, experimental procedures and performance results will be presented. Results of these experiments are expected to provide insight into possible materials stack-ups for packaging and interconnect structures for future cryogenic electronics 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.

Niobium is a viable material for thin-film superconducting flexible microwave cables. To aid in the design of superconducting flexible cables using Nb, it is important to evaluate not only the superconductor electrical performance, but also mechanical reliability performance since these cables should be reasonably robust when flexed. In this paper, we performed fatigue and bending tests on Nb-only and Ti/Nb/Cu multilayer signal lines on flexible Kapton substrates and measured the change in critical current (I-c) of these wires. From the fatigue tests, I-c degradation of Nb-only cables occurred at a lower number of cycles than the Ti/Nb/Cu cables. After 250 fatigue cycles, Ti/Nb/Cu wires with the thickest Ti adhesion layer and Cu capping layer exhibited the lowest I-c degradation of similar to 1.2% and 0% in tensile and compressive cases, respectively. From bending tests, where the sample was held in an intentionally curved configuration during testing, I-c degradation of the Nb-only cables was more severe than that of the Ti/Nb/Cu cables during tensile bending and the I-c of Ti/Nb/Cu cables was minimally affected during compressive bending. These results demonstrate that a Ti adhesion layer and Cu capping layer provide reliability enhancement for superconducting Nb flex cables fabricated on Kapton.

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 SiO2. 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.

In this work carbon nanotubes are studied in order to develop cold cathode materials with efficient field emission characteristics. Randomly aligned Multi-Walled carbon nanotubes (MWCNTs) are fabricated using chemical vapor deposition (CVD) under different growth conditions. These nanotubes are then tested for their field emission characteristics at different pressures ranging from 2×10-7 Torr to 20×10-3 Torr in Helium gas. Effects of different gases at different pressure on the field emission properties of carbon nanotubes are studied and results are presented. Fowler-Norheim (FN) plots revealed nonlinear slopes for all the samples tested in different background pressures. It is known that the slope of the FN plots gives information about the field enhancement factor of the surface. Therefore, the nonlinearity is attributed to a nonlinear field enhancement factor. These data are used in determining a proper cold cathode material to be used as a trigger electrode for pseudospark switches.