Description:

NIST has led the way in the development of programmable DC and AC voltage standards for use in calibration laboratories. The standards have been constructed of superconductor-normal-metal-superconductor Josephson junctions because of their reproducibility, stable voltage steps and their immunity from noise. Now, to improve the quality of the standards, NIST is packing the junctions as densely as possible in three dimensions with junctions separated by a distance of only 20 nanometers. NIST researchers have succeeded in making the first 2- and 3-junction stacks using molybdenum di-silicide (MoSi2) as the normal metal and niobium as the superconductor for superconductor-normal-metal-superconductor (SNS) Josephson junctions. Precise three-dimensional control of the junctions during fabrication is critical for achieving uniformity of the electrical characteristics for the junction stacks and large high-density arrays. These new MoSi2 circuits have demonstrated sufficient uniformity for thousands of junctions to display large quantized-voltage steps at frequencies up to 20 GHz. For the past few years the project has been searching for a practical barrier material to allow them to vertically stack junctions in order to make three-dimensional arrays. Higher junction density is required to increase the output voltage as well as the operating bandwidth of both programmable and ac Josephson array circuits. Nanometer control of the barrier thickness, typically 20-30 nanometers, is essential because junction electrical characteristics depend exponentially on barrier thickness. Reproducibility and uniformity of the fabrication process make molybdenum di-silicide the leading candidate for future lumped-array Josephson voltage standard circuits and systems. The best result NIST researchers have demonstrated is a 1 V step with only 8200 stacks (3 junctions in each stack).
 

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