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IEEE Transactions on Magnetics

High quality Nb-AlO/sub x/-Nb junctions for microwave receivers and SFQ logic device

V. P. Koshelets, Sergey Kovtonyuk, I. L. Serpuchenko, Lyudmila V. Filippenko, А. В. Щукин

Published Mar 1, 1991• 36 citations
Publisher

Abstract

The specific capacitance of high-quality (V/sub m/>50 mV) Nb-AlO/sub x/-Nb tunnel junctions is determined by the following methods: the measurement of zero field step resonances in specially prepared long Josephson junctions, and the definition of the resonant voltages in two-junction interferometers based on Nb-AlO/sub x/-Nb junctions. The results obtained by these methods were compared with each other and with the figures calculated from the measurements of tunnel barrier parameters. The application of the procedure for the fabrication of single flux quantum (SFQ) logic devices is discussed.

DOI
10.1109/20.133877

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The specific capacitance of high-quality (V/sub m/>50 mV) Nb-AlO/sub x/-Nb tunnel junctions is determined by the following methods: the measurement of zero field step resonances in specially prepared long Josephson junctions, and the definition of the resonant voltages in two-junction interferometers based on Nb-AlO/sub x/-Nb junctions. The results obtained by these methods were compared with each other and with the figures calculated from the measurements of tunnel barrier parameters. The application of the procedure for the fabrication of single flux quantum (SFQ) logic devices is discussed.
Nb/Al–AlO x /Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlO x /Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V m = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm 2 , which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.
Measurement of Specific Capacitance for Nb/Al–AlO x_{x}x​/Nb Josephson Junctions in Single-Flux Quantum Circuit Applications
In this study, we investigate different technologies to measure the specific capacitances C8of Nb/Al-AlOx/Nb Josephson junctions. They involve measuring Fiske mode resonance in long Josephson junctions, superconducting quantum interference device (SQUID) resonance, and Josephson plasma resonance. We find a logarithmic relationship between the junction C8and its critical current density Jcin the range 0.1-2 kA/cm2using the Fiske mode resonance method. Our measured C8values using SQUID resonance and Josephson plasma resonance are compared with the results obtained by other groups via analyzing the major errors in the C8calculation. We discuss the accuracy in evaluating C8for all three techniques and suggest using Fiske mode resonance as a relatively stable method for C8extraction for the future design of our single-flux quantum circuit.
Abstract Josephson tunnel junctions form the basis for various superconductor electronic devices. For this reason, enormous efforts are routinely taken to establish and later on maintain a scalable and reproducible wafer-scale manufacturing process for high-quality Josephson junctions. Here, we present an anodization-free fabrication process for Nb/Al-AlO x /Nb cross-type Josephson junctions that requires only a small number of process steps and that is in general intrinsically compatible with wafer-scale fabrication. We show that the fabricated junctions are of very high quality and, compared to other junction types, exhibit not only a significantly reduced capacitance but also an almost rectangular critical current density profile. Our process hence enables the usage of low capacitance Josephson junctions for superconductor electronic devices such as ultra-low noise dc-superconducting quantum interference devices (SQUIDs), microwave SQUID multiplexers based on non-hysteretic rf-SQUIDs and RFSQ circuits.
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.
A modified SNIP process was used to fabricate high-quality 0.5, 2, and 4 mu m/sup 2/ small-area Nb/AlO/sub x//Nb trilayer tunnel junctions with current densities as large as 5000 A/cm/sup 2/. The average junction quality factors for the junctions at 4.4 K were V/sub m/(2 mV)=39 mV for J/sub c/=3000 A/cm/sup 2/ and V/sub m/ (2 mV)=27 mV for J/sub c/=5000 A/cm/sup 2/. The best values of V/sub m/ obtained were 50 mV for J/sub c/=3000 A/cm/sup 2/ and 41 mV for J/sub c/=5000 A/cm/sup 2/. These devices were designed and fabricated for use in a W-band mixer receiver. The substrate was 50- mu m-thick fused or crystal quartz. Special methods were developed for handling such thin insulating substrates and patterning films. The fabrication process was self-aligned and used SiO/sub 2/ instead of anodized Nb as the thick insulator. SiO/sub 2/ isolated the junction area and defined the opening for contact to the Nb wiring layer. The authors have fabricated series arrays of up to 12 junctions, with individual junction areas of 0.5 mu m/sup 2/. The array I-V quality was not degraded compared to that of an individual junction.