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Applied Physics Letters

All-high-T c superconductor rapid-single-flux-quantum circuit operating at ∼30 K

S.L. Shokhor, Boris E. Nadgorny, Michael Gurvitch, Vasili K. Semenov, Yu. A. Polyakov, Konstantin K. Likharev, S. Y. Hou, Julia M. Phillips

Published Nov 6, 1995• 60 citations
Publisher

Abstract

We have implemented a simple circuit of the rapid single-flux-quantum (RSFQ) logic family using a single-layer YBa2Cu3O7−x thin-film structure with 14 in-plane Josephson junctions formed by direct electron beam writing. The circuit includes two dc/SFQ converters, two Josephson transmission lines, a complete RS SFQ flip-flop, and an SFQ/dc converter (readout SQUID). Low-frequency testing has shown that the dc-current-biased circuit operates correctly and reliably at T∼30 K, a few degrees below the effective critical temperature of the junctions. Prospects for a further increase of the operation temperature and implementation of more complex RSFQ circuits are discussed in brief.

DOI
10.1063/1.114812

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We have implemented a simple circuit of the rapid single-flux-quantum (RSFQ) logic family using a single-layer YBa2Cu3O7−x thin-film structure with 14 in-plane Josephson junctions formed by direct electron beam writing. The circuit includes two dc/SFQ converters, two Josephson transmission lines, a complete RS SFQ flip-flop, and an SFQ/dc converter (readout SQUID). Low-frequency testing has shown that the dc-current-biased circuit operates correctly and reliably at T∼30 K, a few degrees below the effective critical temperature of the junctions. Prospects for a further increase of the operation temperature and implementation of more complex RSFQ circuits are discussed in brief.
A simple test circuit of the rapid single-flux-quantum (RSFQ) logic family has been implemented in a planarized all-refractory technology using only two superconducting layers and small Nb/AlOx/Nb Josephson junctions (nominally 1.5×1.5 μm2) with critical current density jc≊6 kA/cm2. A special layout design of the integrated circuit (including a SFQ pulse generator, Josephson transmission line, and T flip–flop) was used to reduce 3D inductances resulting from absence of the ground plane. For samples with various junction parameters, the maximum frequency of the SFQ pulse train, which could be divided by 2 by the flip–flop, ranged from 200 to 370 GHz.
We present a concept of an advanced rapid single-flux-quantum (RSFQ) logic circuit family using the combination of 0-shifted and π-shifted Josephson junctions. A π-shift in the current-phase relationship can be obtained in several types of Josephson junctions, such as Josephson junctions containing a ferromagnet barrier layer, depending on its thickness and temperature. We use a superconducting quantum interference devices composed of a pair of 0- and π-shifted Josephson junctions (0-π SQUIDs) as a basic circuit element. Unlike the conventional RSFQ logic, bistability is obtained by spontaneous circular currents without using a large superconductor loop, and the state can be flipped by smaller driving currents. These features lead to energy- and/or space-efficient logic gates. In this paper, we show several example circuits where we represent signals by flips of the states of a 0-π SQUID. We obtained successful operation of the circuits from numerical simulation.
A rapid single-flux quantum (RSFQ) set-reset trigger circuit has been implemented using YBaCuO grain boundary junctions on an asymmetric 32 degrees Y-ZrO 2 bicrystal. The circuit contains 18 Josephson junctions aligned along the grain boundary and required six film layers: a YBaCuO base electrode, two amorphous SrTiO 3 insulating layers, a superconducting Pb/Au/Cr ground plane and Au/Cr wiring. It consists of two DC-to-SFQ converters, two Josephson transmission lines and an SFQ-to-DC converter. Two pulse trains, 'signal' and 'clock', were applied to change the state of the trigger from '0' to '1' and vice versa, and registered at the SFQ-to-DC: output. All experiments were performed at 4.2 K.
A 3 bit single flux quantum (SFQ) shift register based on high-Tc bicrystal Josephson junctions has been designed, fabricated, and experimentally tested. The circuit consists of 26 bicrystal Josephson junctions and includes the shift register itself, two dc-SFQ converters, one readout superconducting quantum interference device, serving as a SFQ-dc converter, and three Josephson transmission lines. The correct operation of all circuit components has been demonstrated by low frequency testing at a temperature of 50 K.
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.
Single flux quantum (SFQ) circuit elements have been designed and fabricated using the YBa 2 Cu 3 O 7-δ ramp-edge junction technology. Logic operations of SFQ circuit elements, such as a toggle flip-flop (T-FF), a set-reset flip-flop (RS-FF), and a 96-junction Josephson transmission line (JTL), were successfully demonstrated, and dc supply current margins were confirmed up to temperatures higher than 30 K. The circuit layout was improved in order to suppress the critical current (I c ) spread that appears during the junction fabrication procedure. By employing the new circuit layout rule, correct operations at temperatures from 27 K to 34 K with dc supply current margins wider than ±7% were confirmed for the T-FF with a single output. Moreover, the maximum operating frequencies of T-FFs were measured to be 360GHz at 4.2K and 210GHz at 41K, which are substantially higher than the values for the circuits with the conventional layout. According to the simulation result, the maximum operating frequency at 40 K was expected to be approximately 50% of the characteristic frequency at a bit error rate (BER) less than 10 -6 .