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International Conference Radioelektronika

Superconducting RSFQ logic: Towards 100GHz digital electronics

Vratislav Michal, Emanuele Baggetta, Mario Aurino, Sophie Bouat, Jean-Claude Villégier

Published Apr 1, 2011• 10 citations
Publisher

Abstract

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.

DOI
10.1109/radioelek.2011.5936486

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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.
Rapid-Single-Flux-Quantum (RSFQ) digital circuits based on shunted Josephson junctions have been shown to be able to digitally process signals up to several hundreds GHz. The principle of operation is based on the generation, transport and processing of picosecond voltage pulses of 2.07 mV.ps quantized area, which corresponds to one quantum flux h/2e. Operation of RSFQ circuits can be performed with clock frequencies of the order of 20 to 50 GHz with present technology. We will present current developments and experimental results obtained so far, based on circuits developed in the JeSEF RSFQ process of IPHT Jena, using a 1 kA/cm2 shunted Nb/Al-AlOx/Nb junctions. Some particular potential applications for space will be emphasized
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Superconductive Rapid Single Flux Quantum (RSFQ) logic and memory, in which ones and zeros are represented by the presence or absence within a timing window of quantized picosecond voltage pulse (/spl int/v(t)dt=h/2e=2.07 mV/spl middot/ps), corresponding to one SFQ, can be integrated into a digital computing system with an operating rate of several tens of GHz, based on the present Nb Josephson junction integrated circuit technology. It is the most promising technology beyond semiconductor transistors for low-power high-end computation. However, as the operating speed of circuits and systems increase, timing uncertainty from fabrication process variations makes global synchronization very hard. In this paper, we present a globally asynchronous, locally synchronous timing methodology for RSFQ digital design, which can solve the global synchronization problem. We also demonstrate the recent experimental results of some asynchronous circuits and systems implemented in RSFQ technology. Several key components such as a self-timed shift register, a self-timed demultiplexor, a Muller-C element, a completion detector, and a clock generator have been designed and tested. High speed operation has been confirmed up to 20 Gb/s for a prototype data buffer system, which consists two self-timed shift registers and an on-chip 5-38 GHz clock generator.
A new family of dc-powered Josephson junction digital devices, the Rapid Single Flux Quantum (RSFQ) logic, is described. The devices use overdamped Josephson junctions and two-junction interferometers to store, pass and process the digital information presented in form of single flux quanta. We have carried out extensive numerical simulation of the dynamics of the RSFQ logic gates and of some more complex circuits including serial full adder and reversible shift register, within the standard microscopic-theory ("Werthamer") description of Josephson junctions. The minimum clock cycles of the basic RSFQ circuits turn out to be as small as 2.5 ps. The most promising ways to use the RSFQ logic circuits at the present stage of development of the Josephson junction digital technology are discussed.
In the future generations of telecommunication satellites, it will be necessary to reduce the analog part in reception systems and achieve RF front-end at higher carrier frequencies with higher sensitivity and higher bandwidth. RSFQ (Rapid Single Flux Quantum) superconducting logic is suitable to process very high speed digital data processing with very low power dissipation and with performances well beyond what should be possible with CMOS technology in the next decades. The RSFQ circuit technology based on superconducting niobium nitride (NbN) presently developed at CEA-G involves NbN/Ta(sub X)N/NbN internally shunted Josephson junctions with high critical current density and high maximum switching frequency close to 1THz, as required by ultra-fast RSFQ electronics. Very low power dissipation and very low noise level of fast NbN RSFQ gates designed and fabricated at CEA-G are presented. The RSFQ circuit for a sampling comparator has been studied operating in a sigma-delta modulator with a 30 GHz sinusoidal input carrier signal and high sampling clock frequency of about 200 GHz. We will present the thermal noise and how it influences the phase fluctuations. Such a nitride technology has been recently demonstrated to be fully compatible with large scale integration on 200 mm diameter silicon wafer at reasonable fabrication cost in a micro-nano-electronic platform at CEA-Leti. The applicability of NbN RSFQ digital functions in the next decade Space Telecoms will finally be possible thanks to the development of reliable, low power consuming, space qualified cryocoolers operating at about 10K. Moreover semiconductor system design and integration techniques can be simply adapted to superconductivity, to fit the rules of any RSFQ library-foundry network, such as "CONNECT" for Japan and "FLUXONICS" for Europe.