NIST researchers demonstrate stacked Josephson junction arrays using [MoSi.sub.2] barriers. (General Developments).Researchers at NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. have demonstrated that molybdenum molybdenum (məlĭb`dənəm) [Gr.,=leadlike], metallic chemical element; symbol Mo; at. no. 42; at. wt. 95.94; m.p. about 2,617°C;; b.p. about 4,612°C;; sp. gr. 10.22 at 20°C;; valence +2, +3, +4, +5, or +6. di-silicide is an excellent normal-metal material for junction barriers in high-density arrays of superconductor-normal-metal-superconductor (SNS SNS sympathetic nervous system. ) Josephson junctions. 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. A NIST researcher and a guest researcher from South Korea, have succeeded in making the first two- and three-junction stacks using molybdenum di-silicide as the normal metal and niobium niobium (nīō`bēəm), metallic chemical element; symbol Nb; at. no. 41; at. wt. 92.9064; m.p. about 2,468°C;; b.p. 4,742°C;; sp. gr. 8.57 at 20°C;; valence +2, +3, +4, or +5. as the superconductor A material that has little resistance to the flow of electricity. Traditional superconductors operate at absolute zero (-459.67 degrees Fahrenheit or -273.15 degrees Celsius). Experiments in the 1980s raised the temperature to -321 degrees Fahrenheit. . Precise three-dimensional control of the junctions during fabrication fabrication (fab´rikā´sh  n),n the construction or making of a restoration. is critical for achieving uniformity of the electrical characteristics for the junction stacks and large high-density arrays. These new Mo[Si.sub.2] circuits have demonstrated sufficient uniformity for thousands of junctions to display large quantized-voltage steps at frequencies up to 20 GHz. The barrier material has similar etch properties to niobium so that it is possible to reactively etch uniform vertical stacks and three-dimensional structures. This was not the case for previously attempted barrier materials such as palladium-gold and titanium. A contributing factor to this success is that the arrays were fabricated using a recently installed fully automated multilayer sputtering A popular method for adhering thin films onto a substrate. Sputtering is done by bombarding a target material with a charged gas (typically argon) which releases atoms in the target that coats the nearby substrate. It all takes place inside a magnetron vacuum chamber under low pressure. system that allows sub-nanometer control of vertic al film thicknesses. Precise control of the barrier thickness, typically 20 nm to 30 nm, 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. CONTACT: Paul Dresselhaus, (303) 497-5211; haus@boulder.nist.gov. |
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