NbN-Based Overdamped Josephson Junctions for Quantum Voltage Standards

524,288 NbN-based Josephson junctions were integrated to produce a programmable Josephson voltage standard (PJVS) on a die of 15 mm × 15 mm, and the PJVS circuit was cooled to 10 K using a cryocooler and operated with a current margin of about 1.0 mA. Although an output voltage of 10 V was required for a voltage standard, the circuit was designed to generate the maximum output voltage of 17 V because it was difficult to avoid a reduction of the output voltage due to defects. Although a perfect chip without any defect was rarely fabricated, the high voltage chip that generated at least 10 V was fabricated with the fabrication yield of larger than 30%. The fabrication yield was also improved by optimizing the film growth conditions to reduce the film stress and the number of particles. Applications for a secondary voltage standard and an ac Josephson voltage standard are also described. key words: NbN, TiN, SNS, voltage standard, cryocooler