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IEEE Transactions on Applied Superconductivity

SNS junction on Nb-Ti base for microwave circuits

M. Schubert, Ludwig Fritzsch, G. Wende, H.‐G. Meyer

Published Mar 1, 2001• 12 citations
Publisher

Abstract

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.

DOI
10.1109/77.919531

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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.
Several applications of Josephson junctions (JJs) in low-temperature superconducting circuits such as programmable voltage standards and SFQ logical circuits or SQUID sensors require JJs with sub-µm dimensions. A method for preparing superconductor-isolator-superconductor (SIS) JJs as well as superconductor-normal metal-superconductor (SNS) JJs was developed which allows the preparation of both types of junctions by nearly the same technological process. Titanium is used as the normal metal for the SNS JJ. Ti can be anodically oxidized and a technology similar to the well-tried Nb-Al technology for SIS JJs can be applied to the SNS system, too. By electron-beam exposure of a negative deep UV photoresist (ma-N 2400) a stable resist mask is generated which has to withstand the following three self-aligned process steps: etching of the Nb counterelectrode by RIE, isolation by anodization and lifting of a SiO planarization layer. By this process SNS (Nb-Ti-Nb) and SIS (Nb-Al/AlO x -Nb) Josephson junctions with smallest junction areas of 0.49 µm 2 were prepared and characterized by dc measurements and under the influence of microwave irradiation. The Nb-Ti-Nb junctions have I c R n products of 87 µV. They can be used for programmable Josephson voltage standard circuits at frequencies of 70 GHz.
Single nonhysteretic superconductor-metal-superconductor Josephson junctions consisting of Nb-Ti-Nb were prepared using the well-known anodization process for the isolation of the junctions. The 4×4 μm2 junctions show overdamped Josephson current-voltage characteristics at 4.2 K. The characteristic voltage IcRn is about 8.0 μV at a critical current density jc of about 29 kA/cm2. Under the influence of 10.5 GHz microwave irradiation the constant-voltage steps of n=0 and ±1 are observed with current amplitudes of about 3 mA.
Nonhysteretic Josephson junctions with a superconductor-insulator-normal metal-insulator-superconductor (SINIS) structure fabricated using the reliable Nb-AlO/sub x/ technology have been used for the design and fabrication of a 1-V inherently stable Josephson voltage standard. The superconducting circuit can be driven at the same microwave frequencies as conventional SIS circuits. The microwave-induced 1-V step is suitable for calibration purposes. The successful operation of the 1-V circuit is due to an internal, phase-locked oscillation of all SINIS junctions which are embedded in a low-impedance strip-line. The microwave power required for exciting the coherent oscillation is very low and allows the integration of low power sources. Even operation without any external microwave source appears to be possible.
We present a microwave circuit for Josephson voltage standards. Here, the Josephson junctions are integrated in a microwave transmission line designed as coplanar strips (CPS). The new layout offers the possibility of achieving a higher scale of integration and to considerably simplify the fabrication technology. The characteristic impedance of the CPS is about 50 Ω, and this should be of interest for programmable Josephson voltage standard circuits with SNS or SINIS junctions. To demonstrate the function of the microwave circuit design, conventional 10 V Josephson voltage standard circuits with 17 000 Nb/AlOx/Nb junctions were prepared and tested. Stable Shapiro steps at the 10 V level were generated. Furthermore, arrays of 1400 SINIS junctions in this microwave layout exhibited first-order Shapiro steps.
NIST is using and developing superconductor-normal metal-superconductor (SNS) Josephson arrays for both programmable DC and AC voltage standards. Increasing the output voltage is difficult because the output voltage per junction is small; hence series arrays with large numbers of junctions are needed. The best way to generate higher voltages and achieve the best operating margins for the broadband drive signals is by densely packing the junctions into shorter arrays. NIST has been working on stacked SNS junctions to achieve this goal. By stacking junctions in the array, more junctions may be placed per length, while preserving a lumped microwave element. In this paper we introduce our results on stacked SNS junctions using MoSi/sub 2/ and Ti as barrier materials. These barriers were chosen because they can be reactive-ion etched (RIE) in contrast to our standard PdAu barriers, which must be wet etched. Using RIE, alternating layers of barrier material and Nb may be etched in a single step. We indirectly quantify the junction uniformity in the arrays by measuring the current range of the constant-voltage steps when the arrays are biased with a microwave drive.
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.