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Applied Physics Letters

Superconductor-normal-superconductor junctions for programmable voltage standards

S. P. Benz

Published Oct 30, 1995• 133 citations
Publisher

Abstract

Series arrays of Nb–PdAu–Nb Josephson junctions were fabricated with characteristics ideally suited for application in programmable voltage standards and D/A converters with fundamental accuracy. Large arrays of junctions with applied microwave power showed constant voltage steps with current amplitudes as large as 7 mA. A novel coplanar waveguide design enabled uniform microwave power coupling to a five-segment array of 8192 junctions, so each segment had constant voltage steps over the same bias range. The 8192-junction device generated 1.1 mA steps at 186 mV with 11 GHz power and a maximum constant voltage step of 260 mV at 15.34 GHz.

DOI
10.1063/1.114302

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Series arrays of Nb–PdAu–Nb Josephson junctions were fabricated with characteristics ideally suited for application in programmable voltage standards and D/A converters with fundamental accuracy. Large arrays of junctions with applied microwave power showed constant voltage steps with current amplitudes as large as 7 mA. A novel coplanar waveguide design enabled uniform microwave power coupling to a five-segment array of 8192 junctions, so each segment had constant voltage steps over the same bias range. The 8192-junction device generated 1.1 mA steps at 186 mV with 11 GHz power and a maximum constant voltage step of 260 mV at 15.34 GHz.
Design and fabrication of Nb-PdAu-Nb trilayer Josephson junctions are described. The microwave response of an array of 1000 of these junctions was measured, and constant-voltage step heights mere characterized as a function of the microwave amplitude and frequency. Experimental results fit well to point-junction simulations at the 3 GHz design frequency of the microwave distribution network. The observed step height of 3.8 mA shows that the array and microwave distribution are sufficiently uniform for application in programmable Josephson voltage standards.
Superconductor-Normal-Superconductor (SNS) junctions have been used in the design and fabrication of a 1-V rapidly programmable voltage standard. The superconducting circuit is a series array of 32 768 Nb-PdAu-Nb junctions with taps that divide the array into a binary sequence of smaller array segments with a minimum segment size of 128 junctions. The 16-GHz drive frequency is set by the characteristic frequency of the junctions. A computer-controlled 8-channel bias system controls the current in each segment and allows the rapid selection of any one of 513 discrete voltage levels. The system is designed for fast dc measurements and the synthesis of precise ac waveforms.
A programmable dc voltage standard for output voltages of up to 10 V has been realized. The series arrays, consisting of about 69120 overdamped superconductor/insulator/normal metal/insulator/superconductor (SINIS) Josephson junctions, have been fabricated using the reliable Nb-Al/Al/sub 2/O/sub 3/ technology. The arrays can be operated in conventional Josephson voltage standards at microwave frequencies from 70 GHz to 75 GHz. Steps of constant voltage are observed at very low microwave power levels, since the major part of the microwave power is generated by the junctions themselves. The operation of the arrays and the formation of Shapiro steps are discussed.
New microwave circuits for voltage standard application containing 256 shunted bicrystal junctions were designed and investigated. The series array follows a meander line, which prevents the uniform distribution of microwave current along the array. To overcome this drawback, we are placing the meander parallel to a coplanar waveguide line. This provides parallel feeding of microwave power to series connected junctions. A circuit divided in 8 sub-arrays represents a seven-bit digital-to-analog converter. At a frequency f=27332 MHz and for a constant power each of the segments of the array demonstrated the first current step at 78 K.
Superconductor-normal metal-superconductor (SNS) junctions are well-suited for large-scale integrated superconducting circuits. Potential applications are programmable voltage standards, digital-to-analog converters and large-scale RSFQ circuits. The application of SNS junctions provides some advantages, e.g. a simplified fabrication technology and reduced parasitic inductances. But it requires a technological process with dimensions down to the submicrometer region to achieve sufficiently high characteristic voltages I/sub C/R/sub N/. In our process the normal metal interlayer is titanium. Therefore the sidewall insulation of the junctions can be made by anodization, as it is known from the Nb-Al technology. Nb-Ti-Nb Josephson junctions and junction arrays were fabricated down to the submicrometer range with I/sub C/R/sub N/ values up to 87 microvolt depending on the titanium thickness. Their measured critical currents have a small spread of about 10 percent. Shapiro steps are observed under the influence of microwave irradiation. For the application of such junctions in a programmable Josephson voltage standard a new type of microwave circuit using coplanar strips is proposed and successfully tested in 10 V SIS circuits.
Five-bit all-NbN digital-to-analog converters (DACs) for a programmable voltage standard have been fabricated using NbN/TiN x /NbN Josephson junctions and their operation has been demonstrated. The DAC consists of six arrays of 128, 128, 256, 512, 1024 and 2048 junctions per array. We have measured the current–voltage characteristics for the junction arrays driven with a 16 GHz microwave signal at 4.2 and 10 K, resulting in constant-voltage steps with heights larger than 2 mA. The maximum output voltage for the DAC was 136 mV, indicating that all 4096 junctions in the DAC yielded a voltage corresponding to the microwave frequency through the ac Josephson effect.