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IEEE Spectrum

Superconductor ICs: the 100-GHz second generation

DARREN K. BROCK, Elie K. Track, J. M. Rowell

Published Dec 1, 2000• 65 citations
Publisher

Abstract

In the shadow of high-temperature superconductivity, 35 years of research in low-temperature superconductor ICs quietly comes to fruition, yielding the world's fastest circuits. They are made with a superconducting metal, niobium, rather than a compound semiconductor. Their exotic technology is based on Josephson junction devices and the transmission of single quanta of magnetic flux along superconductor interconnects. This paper describes these ICs and also a logic family called rapid single flux quantum (RSFQ) logic, which is based on the storage and transmission of quanta of magnetic flux.

DOI
10.1109/6.887595

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In the shadow of high-temperature superconductivity, 35 years of research in low-temperature superconductor ICs quietly comes to fruition, yielding the world's fastest circuits. They are made with a superconducting metal, niobium, rather than a compound semiconductor. Their exotic technology is based on Josephson junction devices and the transmission of single quanta of magnetic flux along superconductor interconnects. This paper describes these ICs and also a logic family called rapid single flux quantum (RSFQ) logic, which is based on the storage and transmission of quanta of magnetic flux.
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The Rapid Single-Flux-Quantum logic (RSFQ) presents an efficient way to achieve ultra fast digital operations. This logic uses the macroscopic effects which can be found in nano-structured superconductor devices such as the Josephson AC and DC effects, and the magnetic flux quantization. As the typical time of the Josephson junction quantized phase leap 2π is in the order of picoseconds, the RSFQ circuits can operate with clock frequencies in the order of hundreds of GHz. This invited paper focuses on basic principles of superconducting RSFQ logic as well as on typical applications, such as the signal conversion or petaflops computing systems. In addition we point out the benefits of the semi-metallic NbN-TaxN-NbN Josephson junction technology, which has been developed at CEA-INAC in Grenoble.
Key digital circuits for signal processing with tens of gigahertz speed and a few microwatts power dissipation have been recently demonstrated using a simple, 2 /spl mu/m linewidth process. These are the fastest digital circuits reported to date. These circuits use novel rapid single flux quantum (RSFQ) technology that takes advantage of the fundamental properties of superconductivity. In this technology, data and clock are picosecond-long quantized voltage pulses corresponding to the transfer of single magnetic flux quanta, /spl Phisub 0/=h/2e=2 mVps. These SFQ pulses are regenerated at each gate and are processed by circuits comprised of overdamped Josephson junctions interconnected via microstrip inductances. The inherent internal memory of all RSFQ gates (primarily flip-flops), combined with high switching speed, makes serial, bit-pipelined circuit design advantageous. This paper presents experiments in RSFQ digital circuits, including an N/spl times/8b serial multiplier, and on-chip test systems based on RSFQ shift registers and on SFQ samplers. They enable test and study of RSFQ circuits at full GHz speed while communicating with semiconductor electronics at low speed.>
This paper presents the history of superconductor digital circuits starting from several years after the discovery of the Josephson junction in 1962. The first two decades were mainly devoted to developing voltage-state logic, which is similar to semiconductor logic. Research on circuits employing the manipulation of single magnetic flux quanta resulted in a form called RSFQ in the mid-1980s; this is the basis of superconductor logic systems of today. The more difficult problem of random access memory is reviewed. We analyze the present status of the field and outline the work that lies ahead to realize a successful superconductor digital technology.