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IEEE Transactions on Instrumentation and Measurement

Development of Nb/Al-oxide/Nb Josephson junction array at KRISS (voltage standard)

Se II Park, Kyu-Tae Kim, Yong Ho Lee, Rae Duk Lee

Published Apr 1, 1993• 5 citations
Publisher

Abstract

By using the selective niobium anodization process (SNAP) an all refractory 210-junction array was fabricated at KRISS. The array consists of Nb/Al-oxide/Nb junctions that have A=20 mu m*40 mu m. V/sub g/=2.8 mV, an average critical current approximately=650 mu A, and the spread of critical currents approximately=+or-5%. When operated at 94 GHz, the array generated zero-crossing steps up to 240 mV with stability times of more than 1 h.>

DOI
10.1109/19.278632

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Japanese Journal of Applied Physics•Jul 1, 1988•11 citations
By using the selective niobium anodization process (SNAP) an all refractory 210-junction array was fabricated at KRISS. The array consists of Nb/Al-oxide/Nb junctions that have A=20 mu m*40 mu m. V/sub g/=2.8 mV, an average critical current approximately=650 mu A, and the spread of critical currents approximately=+or-5%. When operated at 94 GHz, the array generated zero-crossing steps up to 240 mV with stability times of more than 1 h.>
We have developed a new fabrication process for integrated Nb/Al/sub 2/O/sub 3//Nb Josephson series arrays using the selective niobium anodization process (SNAP), the image reversal technique (IRT) and lift-off. To avoid chaotic behavior, the critical current of the array was decreased by post-fabrication annealing or the in situ multilayer oxidation process. In the frequency range 70-100 GHz, the array containing 2520 junctions produced stable quantized voltage steps up to 2 V with step widths of 40-100 /spl mu/A and stability times of more than 4 h.>
Niobium/aluminum-oxide/niobium Josephson tunnel junctions have been fabricated using electron beam evaporation to form all-metal films in a high vacuum system. A quality factor, V m (the product of the maximum dc Josephson current and the subgap resistance defined at 2 mV) between 36–37 mV, has been obtained for the best junctions of a 6 nm-thick Al layer by cooling down the surface of the base electrode for 100 minutes under a high background pressure, before the deposition of aluminum, after the formation of the base niobium electrode. An array of 80 series-connected junctions has also been fabricated using the selective niobium anodization process and the planarization process. The current-voltage characteristics of the array showed a standard deviation of the maximum dc Josephson current of 3.3%.