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Introduction to Refractory Josephson Junctions

E. L. Wolf

Published Aug 22, 2017• 2 citations
Publisher

Abstract

This chapter concerns the underlying refractory Josephson junctions that have dominated the voltage standard field since perhaps 1990. Before going into the issues in the technical development of refractory tunnel junctions, the chapter describes that a large effort was made at the IBM Corporation to develop a computer technology using Josephson junctions based on Pb alloys, including Bi and Au. The chapter focuses on the tunnel junction technology needed for the refractory metals, of which the most important is Nb, having the highest transition temperature, 9.26 K, among the elements. Fabrication of non-hysteretic shunted Josephson junctions is of interest to exploit the rapid single flux quantum (RSFQ) computing circuitry as a successor to the latching circuitry investigated earlier at IBM. A general feature of the Josephson arrays is design such that the linear array of junctions also serves as a stripline for the microwave propagation to the junctions.

DOI
10.1201/9781315364520-2

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This chapter concerns the underlying refractory Josephson junctions that have dominated the voltage standard field since perhaps 1990. Before going into the issues in the technical development of refractory tunnel junctions, the chapter describes that a large effort was made at the IBM Corporation to develop a computer technology using Josephson junctions based on Pb alloys, including Bi and Au. The chapter focuses on the tunnel junction technology needed for the refractory metals, of which the most important is Nb, having the highest transition temperature, 9.26 K, among the elements. Fabrication of non-hysteretic shunted Josephson junctions is of interest to exploit the rapid single flux quantum (RSFQ) computing circuitry as a successor to the latching circuitry investigated earlier at IBM. A general feature of the Josephson arrays is design such that the linear array of junctions also serves as a stripline for the microwave propagation to the junctions.
We have developed a novel process for fabricating all refractory Josephson tunnel junctions whose base and counter electrodes are composed of (Nb-NbN) double-layered films. In this process, tunnel junctions have been isolated using a reactive ion etching from a junction sandwich which had been formed on an entire silicon wafer, and also the reactive ion etching has been used for patterning electrodes and insulation layers in devices. Fabricated junctions have yielded good tunneling characteristics with excellent uniformity and reproducibility. The maximum critical current density was widely changed from 300 A/cm2to 8 kA/cm2. All refractory single flux quantum(SFQ) memory cells in a 32×32 arrangement have been integrated with a 2.5 μm minimum linewidth on a chip using this process, and an SFQ cell in the memory array has been successfully operated. The maximum deviation of critical currents in the memory array was measured to be ±6.5%.
A procedure is described for fabricating thin film Nb/Nb oxide/Pb-alloy Josephson tunnel junctions that satisfies the principal requirements for integrated circuit design and fabrication. A deposited Nb film, evaporated from an e-gun heated source, was patterned by chemical etching to form a base electrode. A junction was completed by plasma etching and plasma oxidizing the Nb junction area to form a tunnel barrier and by depositing a Pb alloy counterelectrode. Josephson tunnel junctions with Nb/Nb oxide/Pb-Au-In structures were prepared with low excess subgap currents in the current-voltage (I-V) curve and with reproducible and stable I-V characteristics. Variations in junction current density from run-run were ± 15%. Seven 3-junction interferomeeters out of a population of 50,000 were shorted (99.99% yield) for causes not immediately attributable to photoresist-related defects. No changes in Josephson current were detected after thermal cycling 17,000 interferometers 1,800 × between room temperature and 4.2°K, after storing them for 2 years at 5°C, or after annealing 5,000 interferometers for 4.5 hours at 70°C.
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.
An interesting feature of Superconductor Normal metal Superconductor - Josephson junctions for digital applications is due to their non-hysteretic current-voltage characteristics in a broad temperature range below Tc. This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor Normal metal Insulator-Superconductor Nb/Al-AlOx/Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100 nm. They exhibit non-hysteretic current-voltage characteristics with RnIc values higher than 0.5 mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2 K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.
A new generation of the Josephson digital circuits-rapid single flux quantum (RSFQ) devices-based on overdamped externally shunted high quality Nb-AlO x -Nb tunnel junctions have been designed, fabricated and experimentally investigated. All-refractory material 11-layer 5 mu m integrated technology has been successfully employed for the preparation of the RSFQ microcircuits. The T flip-flop has been demonstrated to operate properly up to clock frequencies of 100 GHz at 25% margins. The proper operation of the buffer stage for a one-directional SFQ pulse transmission at clock frequencies up to 125 GHz has been achieved. The possibilities to convert the information from the usual DC form to SFQ and vice versa have been demonstrated. An output signal in an SFQ/DC converter as high as 220 mu V has been measured. A new counter type A/D converter which includes a comparator and reversible binary counter with DC outputs has been fabricated and tested. The proposed ADC seems to be very promising for high-accuracy measurement of medium-bandwidth signals. The possibilities of creating a number of SFQ devices with outstanding parameters are discussed.