Quadruple-stacked Nb/Nb_xSi_1-x/Nb Josephson junctions for large-scale array application

Large-scale Josephson junction (JJ) arrays are essential in many applications, especially quantum voltage standards application for which hundreds of thousands of junctions are required to realize a high quantum voltage. For almost all applications, high-quality JJ arrays must be realized in a small chip area. This study proposes vertically quadruple-stacked Nb/(Nb_xSi_1-x/Nb)₄ JJs to increase the integration density of junctions in an array. The current-voltage (I-V) characteristics of a single stack of Nb/(Nb_xSi_1-x/Nb)₄ JJs have been measured at 4.2 K. The uniformity of junctions in one stack and the uniformity of several stacks over the entire 2 inches wafer have been analyzed. By optimizing the fabrication parameters, a large-scale quadruple-stacked Nb/(Nb_xSi_1-x/Nb)₄ array consisting of 400000 junctions is realized. Good DC I-V characteristics are obtained, indicating the good uniformity of the large-scale array.