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Proceedings of the IEEE

Superconducting digital electronics

H. Hayakawa, Nobuyuki Yoshikawa, Shinichi Yorozu, Akira Fujimaki

Published Sep 20, 2004• 72 citations
Publisher

Abstract

Single-flux quantum logic (SFQ) circuits, in which a flux quantum is used as an information carrier, have the possibility for opening the door to a new digital system operated at over 100-GHz clock frequency at extremely low power dissipation. The SFQ logic system is a so-called pulse logic, which is completely different from the level logic for semiconductors like CMOS, so circuit design technologies for SFQ logic circuits have to be newly developed. Recently, much progress in basic technologies for designing SFQ circuits and operating circuits at high speeds has been made. With advances in these design tools, large-scale circuits including more than several thousand junctions can be easily operated with the clock frequency of more than several tens of gigahertz. High-end routers and high-end computers are possible applications of SFQ logic circuits because of their high throughput nature and the low power dissipation of SFQ logic. In this paper, recent advances of SFQ circuit design technologies and recent developments of switches for high-end routers and microprocessors for high-end computers that are considered possible applications for SFQ logic will be described.

DOI
10.1109/jproc.2004.833658

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•Nov 5, 2018•9 citations
Single-flux quantum logic (SFQ) circuits, in which a flux quantum is used as an information carrier, have the possibility for opening the door to a new digital system operated at over 100-GHz clock frequency at extremely low power dissipation. The SFQ logic system is a so-called pulse logic, which is completely different from the level logic for semiconductors like CMOS, so circuit design technologies for SFQ logic circuits have to be newly developed. Recently, much progress in basic technologies for designing SFQ circuits and operating circuits at high speeds has been made. With advances in these design tools, large-scale circuits including more than several thousand junctions can be easily operated with the clock frequency of more than several tens of gigahertz. High-end routers and high-end computers are possible applications of SFQ logic circuits because of their high throughput nature and the low power dissipation of SFQ logic. In this paper, recent advances of SFQ circuit design technologies and recent developments of switches for high-end routers and microprocessors for high-end computers that are considered possible applications for SFQ logic will be described.
Single flux quantum (SFQ) logic circuits, in which a single flux quantum is used for an information carrier, have a possibility to open a new digital system operated with over 100 GHz clock frequencies at extremely low power dissipations. Recently, much progress in basic technologies for designing SFQ circuits and operating circuits with high speeds has been done. By taking advancements in these designing tools, circuits including more than several thousands junctions easily operated with the clock frequency of more than several tens GHz. Applications of SFQ circuits such as high performance computers, high end router and high end servers are being developed in the world taking advantages of the high through-put nature and the low power dissipation of the SFQ logic.
I describe the fundamental technologies concerning the single flux quantum (SFQ) logic circuits. Recent remarkable progress in the computer-aided-design (CAD) tool, which is based on the cell-based design technique, enables us to make SFQ integrated circuits. Although the circuit demonstrated so far has 2, 300 Josephson junctions at most, this number will increase because the CAD tool is advancing. The on-chip test system has also been developed to test the high-speed nature of the SFQ circuit. For example, the shift-register employed as a circuit under test operates correctly at 55GHz. I also describe the potential applications of the SFQ circuits. An analog-to-digital converter utilizing the high-speed nature and high sensitivity in the SFQ circuits will be applicable to the front end of the future base stations, which require broad band width, high sensitivity, and flexibility. Packet switches used in the future high-end router need high throughput and functionality. Only SFQ-based packet switches can meet these requirements. Another potential application is a high-end server. We have studied micro-architectures of a processor executing the Java Byte Codes directly and a multiprocessor system constructed by connecting the processors and memories via the ultra-broad-band SFQ network switches. Throughout the study, we employ the concept of the complexity-reduction (CORE), in which the high-speed nature of the SFQ circuits is used for the reduction of the circuit scale or simplification of the micro-architecture; the multiprocessor system has high performances. These clear applications promote rapid technological progress in the SFQ technology, though we must make every endeavor to find new application fields.
Single flux quantum (SFQ) electronics is extremely fast and has very low on-chip power dissipation. SFQ VLSI is an excellent candidate for high-performance computing and other applications requiring extremely high-speed signal processing. Despite this, SFQ technology has generally not been accepted for system implementation. We argue that this is due, at least in part, to the use of outdated tools to produce SFQ circuits and chips. Assuming the use of tools equivalent to those employed in the semiconductor industry, we estimate the density of Josephson junctions, circuit speed, and power dissipation that could be achieved with SFQ technology. Today, CMOS lithography is at 90?65?nm with about 20 layers. Assuming equivalent technology, aggressively increasing the current density above 100?kA?cm?2 to achieve junction speeds approximately 1000?GHz, and reducing device footprints by converting device profiles from planar to vertical, one could expect to integrate about 250?M Josephson junctions cm?2 into SFQ digital circuits. This should enable circuit operation with clock frequencies above 200?GHz and place approximately 20?K gates within a radius of one clock period. As a result, complete microprocessors, including integrated memory registers, could be fabricated on a single chip.
Superconducting devices have intrinsically superior characteristics to those of semiconductor devices. Presently, we can fabricate more than twenty thousand junctions on one chip using niobium technology. We have demonstrated the operation of a network system with a superconducting interconnection chip using voltage-state logic. Single flux quantum devices are promising for future superconducting applications because the clock frequency of SFQ logic is higher than that of voltage-state. We have proposed a high-end switch based on hybrid architecture using optical devices, semiconductors and SFQ devices. To demonstrate the high-speed operation of SFQ circuits, we developed an arbiter circuit that uses SFQ components and the arbiter circuit operates at 60 GHz. We also have developed a high Tc superconducting (HTS) SFQ sampler system for observing ultra-fast signal waveforms. In addition, we will discuss the prospects of future superconducting devices based on the fabrication technologies we developed.
Single Flux Quantum (SFQ) electronic circuits originated with the advent of Rapid Single Flux Quantum (RSFQ) logic in 1985 and have since evolved to include more energy-efficient technologies such as ERSFQ and eSFQ. SFQ logic circuits, based on the manipulation of quantized flux pulses, have been demonstrated to run at clock speeds in excess of 120 GHz, and with bit-switch energy below 1 aJ. Small SFQ microprocessors have been developed, but characteristics inherent to SFQ circuits and the lack of circuit design tools have hampered the development of large SFQ systems. SFQ circuit characteristics include fan-out of one and the subsequent demand for pulse splitters, gate-level clocking, susceptibility to magnetic fields and sensitivity to intra-gate and inter-gate inductance. Superconducting interconnects propagate data pulses at the speed of light, but suffer from reflections at vias that attenuate transmitted pulses. The recently started IARPA SuperTools program aims to deliver SFQ Computer-Aided Design (CAD) tools that can enable the successful design of 64 bit RISC processors given the characteristics of SFQ circuits. A discussion on the technology of SFQ circuits and the most modern SFQ fabrication processes is presented, with a focus on the unique electronic design automation CAD requirements for the design, layout and verification of SFQ circuits.