Fabrication and characterization of physically-linked large-scale josephson junction test arrays

The efforts of National Institute of Metrology (NIM) towards large scale Nb/ Nb
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Si1-
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/Nb Josephson junction arrays for the quantum voltage standards are presented in this paper. Physically-linked arrays with 10,000 and 20,000 junctions (corresponding to 1 and 2 junctions vertically) are fabricated. DC I-V characteristics of different stacked arrays are measured and experimental results are discussed. Focusing ion beam (FIB) together with scanning electron microscope (SEM) are used to examine the cross-section of three types of junction stacks which vertically contains 2, 3, and 4 junctions respectively. It is shown that grain boundary defects in the niobium wiring layer exist for all of these junction stacks. A method which potentially could solve this problem is proposed.