Thermocompression bonding of a superconductive flexible cable and a non-superconductive flexible cable was performed to a single multi-chip module (MCM) test vehicle using a unique bonding approach. Custom tooling was designed and machined to retrofit an existing flip chip bonder to perform the atypical assembly demonstration. 59 of 61 connections were achieved of the non-superconductive Cu cable. Of the measurable connections down to 5.5 K, 11 of 11 connections were maintained. The superconductive Nb cable demonstrated full connectivity at room temperature. Of the measured connections down to 5 K, an average onset superconducting transition temperature of 8.80 K was observed. Techniques demonstrated provide options beyond conventional bonding approaches for cryogenic interconnects.

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.

We describe a superconducting multi-chip module (S-MCM) technology using Mo as a robust substrate on which to construct multi-layer superconducting redistribution layers for chip-to-chip signal transmission for densely-integrated cryogenic and quantum electronics. The mechanical robustness and ductile nature of Mo can allow for the integration of chips on a larger scale S-MCM substrate compared to currently available technologies. We demonstrate this integration technology by flip-chip bonding Si chips to Mo substrates using In bumps and epoxy underfill. Superconducting daisy-chain test structures were formed by Mo substrates with polyimide dielectric and superconducting Nb traces connected to Si chips with varying numbers of transitions and bump array densities. Resistance and superconducting transition temperatures of the various daisy-chain configurations were measured from room temperature to 4.2 K. To explore CTE-related challenges, assemblies using Si chips with dimensions up to 27 mm x 22 mm (In bump array size of 20 mm x 20 mm) were found to survive the multiple thermal cycles from room temperature to cryogenic temperatures.

Gamma radiation effects on superconducting microwave transmission line structures, which may find use in future radiation challenging environments, such as satellites or accelerators have been investigated. Two versions of weakly coupled through-type Nb microstrip transmission line resonator material stack-ups were explored. In one version, the Nb signal trace was encapsulated with 20 µm of HD-4110 and in the other version the top Nb was not encapsulated. Exposure of the resonators to gamma radiation was performed for 28 days at room temperature in a sealed vacuum chamber using Co-60 as the gamma radiation source. The quality factors of the resonators were extracted at various cryogenic temperatures below the critical temperature of Nb and resonant frequencies up to 20 GHz. A large dose of gamma radiation used in this work showed a small change in the Nb superconducting properties.

Interconnect properties position superconducting digital circuits to build large, high performance, power efficient digital systems. We report a board-to-board communication data link, which is a critical technological component that has not yet been addressed. Synchronous communication on chip and between chips mounted on a common board is enabled by the superconducting resonant clock/power network for Reciprocal Quantum Logic circuits. The data link is extended to board-to-board communication using isochronous communication, where there is a common frequency between boards but the relative phase is unknown. Our link uses over-sampling and configurable delay at the receiver to synchronize to the local clock phase. A single-bit isochronous data link has been demonstrated on-chip through a transmission line, and on a multi-chip module through a superconducting tape between driver and receiver with variable phase offset. Measured results demonstrated correct functionality with a clock margin of 3 dB at 3.6 GHz, and with 5 fJ bit−1 at 4.2 K.

Abstract Design, fabrication, and testing of cryogenic cables with thermal performance considerations is presented in this work. Cables were designed with commercial off-the-shelf materials for low-GHz operation in a small form factor. Thermal and microwave simulations were performed during the design of cables. Fabrication processes were developed to create features in Cu-cladding and to successfully deposit thin-film metal in large step-height regions. Post-fabrication processes such as laser machining and connector assembly were used to cutout and interface with fabricated cables. Microwave performance was measured at room temperature and at 4.2 K with comparable results to simulations performed. Cables designed and fabricated in this work provide a solution to thermal and spatial limitations while achieving targeted microwave performance at cryogenic temperatures.

We present reliability characterization of a polyimide/copper-based flexible interconnect designed for cryogenic and quantum computing applications. This interconnect design uses commercial fabrication processes and off-the-shelf parts. Experiments were performed on production flexible interconnect parts. Samples underwent a series of tests including rapid thermal cycling by liquid helium submersion, cold bias stress, room temperature fatigue bending, connect-disconnect of the interconnect connector and in-plane shear stress. The results found that the samples are remarkably robust. All tested flexible interconnect tapes showed no failures in the entire study post reliability stress. Failures were observed on the connector solution chosen for this engineering design. This paper will present the experimental data and results from the flexible interconnects bonded to a commercial off-the-shelf connector part. A model is described to estimate a system failure rate due to the flexible interconnect and connector subassembly failures. The results of this study conclude that the flexible interconnect tapes are well suited for cryogenic temperature applications.

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.

We have investigated Al 2 O 3 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 ~20 nm thick layers of Al 2 O 3 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 T', and I', 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.

We describe our initial approach and progress towards a reliable and useful technique to connect multiple flexible superconducting cables using a cable-to-cable connector. The test structure comprises multiple components, including flexible thin-film superconducting cables, a bridging connector, a silicon well chip, and a polyimide membranes with Cu pillars. The thin-film superconducting cables used in this work contain 12 parallel Nb lines. A bridging connector which was used to connect two superconducting cables had a stripline structure with 12 parallel superconducting Nb transmission lines with 300 $\mu$ m pitch among them. A silicon well chip and a polyimide membrane with Cu pillars have been used in the assembly for high-precision micro-mechanical alignment and to apply uniform pressure on the contact points of cables and the bridging connector. DC resistances of signal traces of connected flexible cables were measured at temperature ranging from 293 K to 4.2 K in an evacuated cryogenic environment. We present the details of the design, fabrication, assembly, and test results, which show that this is a promising cable-to-cable connection approach.

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 μm 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.

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/AI 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 de performance, including critical transition temperature and critical current, of the stripline transmission lines is presented. We observed T-c values of similar to 8.8 and similar to 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 (S-11 better than -15 dB), low insertion loss (S-21 better 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 similar to 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.

The future of superconducting and cryogenic electronic systems can significantly benefit from densely integrated superconducting multi-layer and multi-signal flexible cables due to the massive number of electrical interconnects needed in systems such as superconducting quantum computers and cryogenic detector arrays. In order to maintain superconductivity in niobium (Nb) thin films, film stress and degradation must be minimized. We are working towards configurations with embedded traces, where it is expected that the superconductor material will be subjected to subsequent fabrication steps that must not degrade the properties of the superconductor. We previously observed degradation of the superconducting properties of Nb, such as reduction of both transition temperature and critical current, as a result of curing a polyimide passivation layer at supplier recommended curing temperature (350 oC). The deterioration in the superconducting properties may be due to mechanical stress in the film or diffusion of impurities into the Nb during the curing process Film stress plays a vital role in the superconducting properties of Nb. Previous research by other groups has focused on in situ ion bombardment, substrate fixturing and wafer preparation in order to minimize film stress. In this work, we discuss the role of argon (Ar) pressure and power during Nb sputtering on the quality of Nb and Nb/Al thin films. By varying the Ar pressure and applied power during sputter deposition, we have produced both tensile and compressive films on flexible substrates in order to find the pressure that yields a near zero stress Nb and Nb/Al thin film at room temperature. A low stress Nb film was tested with a thin Al barrier layer (of the order of 10's of nm) between Nb and polyimide to protect the Nb superconductivity during the PI curing step. Nb traces with a thickness of roughly 250nm and a width of 50um were used for this work. Nb films deposited at different Ar pressures and power levels were tested for critical transition temperature (Tc), critical current (Ic), and sheet resistance (Ω/□), to compare the superconducting behavior of different Nb films. Details of the fabrication processes, experimental procedures and performance results will be presented. This work will help determine materials stacks-ups that may be useful for future multi-layer Nb-based flexible superconducting cables. Acknowledgment: We gratefully acknowledge financial support and technical guidance from Microsoft Research for this work.

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.

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.

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.