The Preparation and Properties of Niobium Superconducting Structures Prepared by Electron Beam Evaporation in Uhv

Available from UMI in association with The British Library. This work has achieved the development of a fabrication method for the realisation of superconductor-insulator -superconductor (SIS) junctions based on niobium thin films with an artificial barrier layer. Such devices are likely to have advantages over lead alloy devices because of the enhanced mechanical and chemical stability. A principal objective was to attempt to exploit the offset mask technology developed by Dolan that has been successfully employed by Davies et.al. in the realisation of lead alloy SIS junction heterodyne mixers. In order to transfer this technology to niobium based devices it is essential that an evaporation method is used to allow shadow formation beneath the offset mask. As a result of the chemical reactivity and the low vapour pressure of niobium, a UHV system has been developed which incorporates an electrostatically focussed electron beam evaporation source. This is capable of providing deposition rates of close to 10A/sec. with a source to substrate distance of 120mm. During deposition the system pressure is below 5 times 10 ^{-9} mbar. These process parameters have been recorded, for a number of depositions, on a specially built data acquisition system controlled by a BBC microcomputer. Such recording allows detailed comparison of conditions which helps in the understanding of differences between the superconducting behaviour of various films. A further extremely important aspect of these real time measurements is that it helps to ensure optimum settings of the focus conditions of the electron beam source. The results of the depositions are extremely encouraging with critical temperatures of between 9.1 and 9.3K being achieved. These results compare well with expected values for bulk niobium. An all-metal offset mask technology has been developed to replace the photo-resist technique pioneered by Dolan. This new method is needed because the photo-resist is not compatible with a UHV environment and in any case, will not withstand the high source temperature (<3000K) required for niobium deposition. The offset mask used in this work consisted of a 50A chromium seed layer followed by a 1 mu m copper film on top of which was deposited a 0.2mu m aluminium mask layer. The copper and aluminium have been selectively etched to produce a offset beam of the required dimensions. This has been used to fabricate all niobium junction structures which have been observed to have dimensions of the order of 1mu m^2 which is essential for applications at sub-millimeter wavelengths. It is too early to report final results on those junctions but it can be seen as very encouraging for future development.