Flexible Superconducting Microwave Interconnects and Connectors
Type of DegreePhD Dissertation
Electrical and Computer Engineering
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One of the major limitations to constructing densely-integrated cryogenic electronic systems is the electrical interconnect technology. Superconducting cables with multiple signals, high signal density, low loss, low thermal leakage and small cross-sections are desirable. The superconducting characteristics of thin-film niobium (Nb) make it a viable material for realizing low-temperature (4 K or below) superconducting cables, such as high-density DC cables and RF cables including microstrip and stripline. Thin-film flexible superconducting ribbon cables incorporating polymer dielectrics are particularly useful for making multiple interconnections between different substrates and/or different temperature zones. Half-wavelength, capacitively coupled superconducting microstrip transmission line resonators and stripline transmission line resonators were used to study the RF/microwave properties (such as loss tangent) of various flexible substrates and compare the performance of transmission lines with similar structure. These high-quality factor resonators were used to characterize substrate’ dielectric loss and superconductor loss at multiple frequencies from 10 MHz to 21 GHz and temperatures from ∼ 1.2 K to 4.2 K. As a transition structure, embedded microstrip transmission line resonators were used to investigate impact of encapsulation layer thickness, and related fabrication processes, on resonator performance, which is related to materials properties (i.e. superconductor loss and dielectric loss). Nb-based superconducting thin-film microstrip transmission lines and microwave connectors were also designed, fabricated and characterized. To investigate the feasibility of a superconducting microwave connector, Ti/Cu/Au DC cables and RF cables were used and tested at different temperatures (room temperature, 77 K and 4.2 K) with this type of connector. Fabrication procedures, assembly and characterization results are demonstrated. The microwave performance of Nb microstrip exhibited significantly low transmission loss of less than 0.85 dB/m up to 14 GHz at 4.2 K. Microwave connectors were used to connect microstrip cables end-to-end. The connectors were designed to be easily aligned for narrow trace widths and use pressure to make electrical connection, providing sufficiently low resistance of ∼15 mΩ and excellent microwave performance. A distributed element model was used to model the microstrip-to-microstrip interconnection of a reconnected structure. The proposed microwave connection methods exhibited a return loss better than 15 dB and an insertion loss less than 0.12 dB up to 14 GHz. Moreover, the microwave connector showed both encouraging thermal and re-assembly reliability and performance. To aid in the design of the superconducting flexible cables using Nb, it is important to evaluate not only the electrical performance, but also mechanical reliability performance, since these cables should be robust when flexed. In this study, fatigue and bending tests on Nb-only and Ti/Nb/Cu multilayer signal lines on flexible Kapton substrates were performed. Critical current (Ic) of these wires during fatigue and bending testing were measured. From the fatigue test, Ic decreased with increasing number of bending-flattening cycles. Ic 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 Ic degradation of ∼ 1.2 % and 0 % in tensile and compressive cases, respectively. The Ic degradation values for the Nb-only cables were ∼ 32.8 % and ∼ 27.2 % for tensile and compression, respectively. From bending tests, where the sample was held in an intentionally curved configuration during testing, Ic degradation of the Nb-only cables was more severe than that of the Ti/Nb/Cu cables during tensile bending. When the cables were subjected to compressive bending tests, the Ic of Ti/Nb/Cu cables was minimally affected, even for the smallest bending radius tested (5 mm), while the Ic of the Nb-only cables exhibited a degradation of ∼ 27.9 %. These results demonstrate that Ti adhesion layer and Cu capping layer provide reliability enhancement for superconducting Nb flex cables fabricated on Kapton. Superconducting properties (Tc and Ic) and mechanical reliability performance (fatigue behavior) for embedded and non-embedded Nb DC cables on PI-2611 polyimide were investigated. A slightly lower Tc value of ∼ 8.2 K was found for Nb embedded cables compared to non-embedded Nb DC testing lines with a Tc of ∼8.8 K, which was explained as due to subsequent fabrication steps causing a degradation of the superconducting properties of Nb. Ic of embedded DC cables was comparable with that for non-embedded ones, with values of ∼ 0.2 A at 4.2 K. We found that the use of a properly-cured PI-2611 encapsulation layer can effectively enhance the mechanical reliability performance (i.e. bending fatigue behavior) of Nb superconducting cables. Embedded Nb DC cables showed degradation of Ic less than 5 %, while that of non-embedded DC cables was more than 26.5 % after 500 fatigue cycles. This projects and work presented in this dissertation not only provide design guidance for constructing ultralow-loss flexible thin-film superconducting interconnects and connectors, but also discuss methods to build interconnects with enhanced mechanical reliability. The results presented here are critical for future high-performance interconnects that are expected to find great use in cryogenic electronics systems, including systems for superconducting quantum computing and related technologies.