Fabrication and Characterization of Flexible Thin-Film Superconducting Microwave Cables
Type of DegreePhD Dissertation
Electrical and Computer Engineering
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The future of superconducting and cryogenic electronic systems depends on 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 detector arrays. Flexible superconducting cables, based on Nb and polyimide (PI), in a microstrip and stripline transmission line configuration is demonstrated in this work. In addition to providing useful mechanical and electrical properties, polyimide also offers low thermal leakage and is amenable to multi-layer fabrication, which are important considerations for electrical interconnects in densely-integrated cryogenic electronics systems. In order to maintain superconductivity in niobium (Nb) thin films, film stress and degradation must be minimized. In a stripline configuration with embedded traces, the superconductor material will be subjected to subsequent fabrication steps; these must not degrade the properties of the superconductor. We observed degradation in the superconducting properties of Nb, such as reduction of both superconducting transition temperature and critical current, as a result of curing a polyimide passivation layer at supplier recommended curing temperature (350 ◦C). This deterioration in the superconducting properties may be due to the diffusion of hydrogen or oxygen into Nb during the curing process We have investigated multiple material stack-ups to protect Nb-based superconducting thin film flexible cables. We show that curing polymers above a certain temperature on top of a Nb layer can adversely affect the superconducting properties including superconducting transition temperature (Tc) and critical current (Ic). Multiple barrier materials have been explored: metals such as Al, Cr and Ta, alternative polymer layers such as Asahi Glass AL-X2010. Metal barrier layers may be viable options for potential use in superconducting flexible cables for high frequency use, provided they do not unduly degrade the high frequency signal propagation due to microwave skin effects or proximity effects. We used nominal curing temperatures of 350 ◦C for PI-2611 and 190 ◦C for AL-X2010. Different thicknesses of metal barrier layers (in the range of 10’s of nm) and multiple metal stack-ups have been fabricated and tested. Atomic layer deposition (ALD) deposited Al2O3 was also investigated as an alternate barrier layer to protect Nb superconductivity at elevated temperatures. By varying the Ar pressure and applied power during sputter deposition, we have produced both tensile and compressive films in order to find the pressure that yields a near zero stress Nb and Nb/Al thin film. 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 on both sides (top and bottom) to preserve Nb superconductivity during the curing step in order to overcome the observed degradation effects. We have designed, fabricated and characterized a fully-shielded stripline structure, with bottom ground – middle signal – top ground configuration, and we report on these efforts here. The stripline fabrication process incorporates thin layers of Al between the Nb and PI, which serves as barrier layers, to protect the Nb superconductivity during the PI curing step. 20 µm thick layers of photo-definable PI (HD-4110), 250 nm thick layers of Nb, 20 nm thick Al layers and an electroplated Cu via process, were used for this work. 25 cm long, 3-metal layer stack-up (i.e., ground-signal-ground) stripline transmission lines were fabricated with a line width of 24 µm, yielding a characteristic impedance of ∼ 50 Ω. We also fabricated stripline resonators, with a length of 13 cm, following the same fabrication process as stripline transmission line, in order to provide a sensitive measurement of the loss of the stripline structures. Both transmission lines and resonators were encapsulated with 4 µm thick HD-4100 to enhance the mechanical reliability and robustness of the cables. In order to better design and optimize various types of low loss superconducting flexible transmission line cables, we have explored multiple techniques to separate the conductor and dielectric losses using weakly-coupled SC embedded microstrip transmission line resonators. The high-quality factor resonators provide sensitive measurements of the aggregate loss properties of the conductor and dielectric, as functions of frequency and temperature. Different resonator structures were investigated such as embedded, non-embedded and embedded with a barrier layer of Al2O3. One of the techniques included the measurement of quality factor (Ql) for each resonator case after being exposed to elevated temperatures: 225 ◦C, 250 ◦C and 275 ◦C. The impact of temperature on the overall resonator internal loss was studied, in order to better characterize and separate the dielectric loss and conductor loss. The other technique involved the use of external magnetic field to suppress the superconductivity and thereby reducing the conductor loss. Different intensities of magnetic field were applied to find the the reduction in Tc of the resontor. Surface resistance analysis was performed on the different resonator structures mentioned previously. Residual resistance and BCS resistance were extracted using the temperature dependence of surface resistance for each resonator. The results of this work are important for understanding loss and transmission properties of similarly designed and fabricated transmission line interconnects. The data presented in this paper provides design guidance for constructing low-loss, flexible thin-film superconducting interconnects using transmission line configurations. It also provides a solution to robust, multi-layer interconnects, with enhanced shielding and low cross-talk for use in future cryogenic electronics systems.