Vertically integrated Josephson circuits with stacked Nb/Alox/Nb junctions

Since Josephson circuit has a multi-layered wiring structure, planarization is necessary for fabricating integrated Josephson circuits. We used bias-sputtered SiO2films as the insulators in Josephson circuits for planarization. By using with bias-sputtered SiO2films, the surface on the junctions becomes smooth. The planarized junctions show the good I-V characteristics such as a gap voltage of 2.9 mV and a Vm of over 50 mV. In addition, we stacked another Josephson junction above the planarized junction. We obtained a high quality junction with the same gap voltage and Vm as the lower junctions. Bias-sputtered SiO2films are effective for the planarization of Josephson circuits, and makes it possible to fabricate three-dimensional integrated circuits.