Using NbN / TiN x / NbN / TiN x / NbN double-junction stack technology we have demonstrated a programmable Josephson voltage standard chip that operates up to 10.16 V output voltage cooled with a two-stage Gifford-McMahon cryocooler. The circuit uses double-junction stacks, where two junctions are fabricated in each stack, in order to integrate 327 680 junctions into a 15.3 mm ϫ 15.3 mm chip. A 1-to-32 microwave distribution circuit is also integrated on the chip. The chip is divided into 22 cells, which perform as an 11-bit digital-to-analog converter. The 21 working cells include 307 200 junctions biased with 16 GHz microwaves at 10.2 K that generated flat voltage steps with current margins greater than 1 mA, which indicates good uniformity of the stacked junctions.

A refrigeration system was designed and constructed for realizing a liquid-He-free programmable Josephson voltage standard. The system is equipped with a two-stage Gifford-McMahon cooler, a thermal-radiation shield, a magnetic-field shield and semi-rigid coaxial cables to supply microwave power to a chip. The performance of the system was examined by use of a NbN-based 8-bit digital-to-analog converter (DAC) chip designed as a 1 V programmable voltage standard. When operated at 8.5 K on the cryocooler, constant-voltage steps with amplitudes greater than 1 mA were observed for every segment of junction arrays on the chip.

A series array of NbN/TiNx/NbN/TiNx/NbN double-barrier junctions was fabricated on Si wafers, and their current–voltage (I–V) characteristics were measured with and without microwave power in order to investigate their performance for programmable voltage standards. By adjusting the thickness of the TiNx barrier, nearly identical critical currents were obtained for 128 lower and 128 upper junctions in an array. When applying 8 GHz microwave power to an array, a large constant-voltage step (∼3 mA) appeared on the I–V curve. The zero-voltage critical current and the height of the first (n=1) constant-voltage step showed microwave-power dependences coincident with theoretical prediction.

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