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IEEE Transactions on Applied Superconductivity

Fabrication and Logic Operation of Oxide SFQ-Circuit-Components

Y. Tarutani, Yoshihiro Ishimaru, H. Wakana, Seiji Adachi, Ken‐ichi Nakayama, Yasuo Oshikubo, Osami Horibe, Y. Morimoto, K. Tanabe

Published Jun 1, 2005• 21 citations
Publisher

Abstract

Basic SFQ circuit components, such as SFQ-dc converter, set-reset flip-flop (RS-FF), confluence buffer and splitter, have been fabricated by the YBa/sub 2/Cu/sub 3/O/sub 7-x/-ramp-edge-junction technology and their logic operations were investigated. Circuit parameters were optimized in order to widen the margins of these oxide SFQ circuit components. The SFQ-dc converter that comprised double SQUIDs connected in series showed the output voltage as much as 0.3 mV at 22 K owing to high I/sub c/R/sub n/ value of the ramp-edge junction. Logic operations of the RS-FF, the confluence buffer and the splitter were correctly performed by using the SFQ-dc converter. Finite operating margins were obtainable for these circuits up to the temperature near 30 K.

DOI
10.1109/tasc.2005.849854

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IEEE Transactions on Applied Superconductivity•Jun 1, 2003•8 citations
Basic SFQ circuit components, such as SFQ-dc converter, set-reset flip-flop (RS-FF), confluence buffer and splitter, have been fabricated by the YBa/sub 2/Cu/sub 3/O/sub 7-x/-ramp-edge-junction technology and their logic operations were investigated. Circuit parameters were optimized in order to widen the margins of these oxide SFQ circuit components. The SFQ-dc converter that comprised double SQUIDs connected in series showed the output voltage as much as 0.3 mV at 22 K owing to high I/sub c/R/sub n/ value of the ramp-edge junction. Logic operations of the RS-FF, the confluence buffer and the splitter were correctly performed by using the SFQ-dc converter. Finite operating margins were obtainable for these circuits up to the temperature near 30 K.
Single flux quantum (SFQ) circuit components such as an SFQ-dc converter and a confluence buffer have been fabricated by using an YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// ramp-edge junction technology and their logic operations at temperatures up to near 60 K were investigated. The SFQ-dc converter was correctly operated in a wide temperature range from 4.2 K to 56 K and found to be useful for detecting output signals from other SFQ circuit components at any operating temperatures. The basic function that a signal from either of two input Josephson transmission lines (JTLs) was transmitted to an output JTL was confirmed for the confluence buffer and finite operating margins were obtained at temperatures from 42 K to 61 K. The narrowest margin of dc supply current obtained at temperatures from 55 K to 60 K was /spl plusmn/20% and was consistent with the simulation. Margin reduction due to thermal noise was also evaluated. According to the analytical calculation, the operating margin to keep the bit-error rate less than 10/sup -5/ was as large as /spl plusmn/20% even at 50 K when the value of junction critical-current I/sub c/ was kept near 0.4 mA.
YBa2Cu3O7-x(YBCO) ramp-edge junction technologies have been developed for fabricating high-operating-temperature SFQ(Single Flux Quantum) circuits. The Ic(critical current) spread that is the key parameter for integrating junctions could be decreased to 8% for a 1000-junction array that was formed on a groundplane. The background technologies that enabled this uniform junction characteristic are precise control of the counter-electrode deposition-conditions and deposition of the extremely smooth multiplayer that showed the roughness as small as 2 nm. Based upon these ramp-edge junction technologies, oxide SFQ-circuit-elements were fabricated and their logic operations were investigated. SFQ circuit elements such as a confluence buffer, a set-reset flipflop, and a toggle flipflop have been fabricated and their logic operations were measured by using an SFQ-dc converter. Both toggle flipflops that have one output and two outputs have been fabricated. These circuits could be operated up to the temperature of near 60 K under finite operating margins. These experimental results enable us to further develop fuctional and much more integrated SFQ circuits.
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 .
RSFQ circuit elements were fabricated based on the YBa/sub 2/Cu/sub 3/O/sub 7-x/ ramp-edge junctions and their characteristics were investigated. In particular, two types of interface circuits were designed, fabricated and their amplification functions were evaluated. The interface circuits comprised series array of SQUID's that shared a common inductor each other. Each SQUID was switched between zero voltage state and voltage state by current injection. Input signal was AC Josephson current for the type-one interface circuit. The input signal was an SFQ for the type-two interface circuit. Both interface circuits could successfully be operated. In particular, an output voltage more than 1 mV was obtained for the type-one interface circuit that comprised the stack of five SQUID's.
We fabricated single flux quantum RS flip-flop circuits by using ramp-edge Josephson junctions and tested their performance. The junctions were constructed with two Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) electrodes and a barrier layer. As barrier materials we used cobalt-doped YBCO. To insulate between YBCO layers we used Sr/sub 2/AlTaO/sub 6/ (SAT) thin films. We also fabricated RS flip-flop circuits by using YBCO ground planes to reduce the inductance of YBCO film. The fabricated Josephson junctions showed typical RSJ-like current-voltage (I-V) characteristics above 50 K. We could successfully demonstrate the correct operation of the RS flip-flop circuits at the temperatures near 50 K. We used a computer controlled digital measurement set-up to test the circuits. The RS flip-flop circuit fabricated on a ground plane showed more definite set and reset states on the voltage-flux (V-/spl phi/) modulation curves of the read SQUID than those of the circuit made without ground plane. This may be attributed to the shielding effect by a YBCO ground plane.
Latching-type driver circuits integrated with high-temperature superconductivity (HTS) junctions and two types of resistor were fabricated. These circuits could successfully perform latching operation by means of a single-flux-quantum (SFQ) signal input. Ramp-edge-type HTS junctions were fabricated by interface engineering and showed a hysteretic current-voltage characteristics under a temperature of 50 K. The critical current (I/sub c/) of these 5-micron-wide junctions was about 0.2 mA and the I/sub c/R/sub n/ product was about 1.7 mV at 4.2 K. The circuits included two types of resistors, which were made of RF-magnetron-sputtered indium-tin-oxide (ITO) and Au films. A driver with a parallel 2-junction-stack could convert an SFQ pulse, which was transformed through several Josephson transmission lines (JTL's) from a DC/SFQ circuit, to a 2.3-mV level signal.