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Planar Josephson junctions and arrays by electron beam lithography and ion damage


In the years to come, the size and cost of cryo-coolers will get smaller and the demand for a VLSI high- temperature superconducting (HTS) Josephson junction technology will increase. One possible candidate to fill this need is the 'ion-damage' Josephson junction. These junctions are fabricated by using ion bombardment to create localized narrow regions of defects in the plane of a thin film superconductor. These regions have a superconducting transition temperature lower than that of the bulk film and act as non-hysteretic Josephson junctions. The advantage of these junctions over other technologies is that they have no interfaces between different materials, and can be placed over 10 times closer to each other in comparison to competing techniques. Individual junctions and multi-junction serial arrays were fabricated and characterized. Current-voltage characteristics were well described by the restively shunted junction model at temperatures close to the critical temperature of the weak link. At lower temperatures the junctions became strongly coupled and exhibit flux flow characteristics. It is suggested that this is due to the barrier changing from non- superconducting to superconducting, which allows Abrikosov vortices to enter the junction. Junction arrays have flat giant Shapiro steps at N times the voltage predicted by the ac Josephson relation, where N is the number of junctions in the array. Modulation of the critical current was measured to observe the dc Josephson effect. Unlike other planar junction types these junctions do not exhibit flux-focusing effects in magnetic field measurements and are more comparable with classical sandwich type Josephson junctions. Low temperature (100̕C) anneals substantially increased the critical current and operating temperature possibly from the recombination of vacancy-interstitial pairs formed during implantation. Junctions were also fabricated from the low temperature superconductor MgB₂ for comparisons with the HTS junctions and to show that this technology is not limited to HTS. Overall it has been shown that this technique can be used to reproduce junctions with uniform resistances and critical currents. With a few innovations in materials and/or lithography, ion damage Josephson junctions may become common place in the medical, communications, and defense industries

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