Focused Ion Beam Fabricated Non-equilibrium Superconducting Devices
The developments over the last decade in Focused Ion Beam (FIB) instrument technology have reached a point where there is sufficient control of an ion beam to make cuts, trenches, and other shapes in a sample on a scale of tens of nanometers. This work concentrates on the use of an FIB instrument for making superconducting devices. It is shown for the first time that planar-bridge (Nb/Cu/Nb) Superconductor/Normalmetal/ Superconductor (SNS) junctions can be reliably fabricated using a standard FIB instrument. This is demonstrated by the responses of junctions to microwaves and magnetic fields; the junctions display the appropriate Josephson behaviour demanded by current technological applications. In addition, the reproducibility in junction behaviour (the variation of critical current is approximately 10%) is the best so far observed for this type of junction. The SNS junction fabrication method has been successfully extended for making high-density SNS junction arrays, dc-SQUIDs, and related devices. A simple model is devised to explain the normal-state resistance and critical current of a junction. The model is based on the geometry of a junction as defined by the FIB instrument and the film deposition. The model is mostly successful in qualitatively explaining many of the geometrical factors that affect the electrical properties of the junction. Nb/Cu/Nb junction series arrays, made using an FIB instrument, are also successfully fabricated. The yield of the junctions forming small arrays is found to be similar to the yield of single junctions. For the series arrays studied here, new observations have been made: the electrical properties of an array have been found to be dependent on the spacing of the junctions and the number of junctions in the array. This work also investigates the thermal properties of SNS and micron-scale superconductor/insulator/normal-metal junction based devices for use in bolometer device based applications. It is shown that self-heating raises the temperature of the junctions significantly above their operating temperatures. For a device sitting on a low thermally conductive membrane, it is found that the effects of heating, or cooling, in the junctions are exaggerated.
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