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Progress in Superconductivity and Cryogenics

Technical comparison between superconductive RSFQ logic circuits and silicon CMOS digital logics

Wanju Cho, Gunhwi Moon

Published Jan 1, 2006• 0 citations
OpenAlex

Abstract

The development technique of digital logic using CMOS device is close reached several limitations These make technical needs that are ultra high speed superconductive systems based on CMOS silicon digital computing technique. Comparing digital logic based on silicon CMOS, the computing technique based on ultra high speed superconductive systems has many advantages which are ultra low power consumption, ultra high operation speed. etc. It is estimated that features like these increasingly improve the possibility of ultra low power and ultra superconductive systems. In this paper digital logics of current CMOS technique and RSFQ superconductive technique are compared with and analyzed.

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The development technique of digital logic using CMOS device is close reached several limitations These make technical needs that are ultra high speed superconductive systems based on CMOS silicon digital computing technique. Comparing digital logic based on silicon CMOS, the computing technique based on ultra high speed superconductive systems has many advantages which are ultra low power consumption, ultra high operation speed. etc. It is estimated that features like these increasingly improve the possibility of ultra low power and ultra superconductive systems. In this paper digital logics of current CMOS technique and RSFQ superconductive technique are compared with and analyzed.
The rapid-pace growing demand for high-performance computation and big-data manipulation entails substantial increase in global power consumption, and challenging thermal management. Thus, there is a need in allocating competitive alternatives for complementary metal-oxide-semiconductor (CMOS) technologies. Superconducting platforms, such as rapid single flux quantum (RSFQ) lack electric resistance and excel in power efficiency and time performance. However, traditional RSFQs require 3D geometry for their Josephson junctions (JJs) imposing a large footprint, and hence preventing device miniaturization and increasing processing time. Here, we demonstrate that RSFQ logic circuits of planar geometry with weak-link bridges are scalable, relatively easy to process and are CMOS-compatible on a Si chip. Universal logic gates, as well as combinational arithmetic circuiting that are based on these devices are demonstrated. The power consumption and processing time of these logic circuits were as low as 0.8 nW and 13 ps, an order of magnitude improvement with respect to the equivalent traditional-RSFQ logic circuits and two orders of magnitude with respect to CMOS. The competitive performance of planar RSFQ logic circuits renders them for promising CMOS substitutes, especially in the supercomputational realm.
Superconducting digital technology based on Rapid Single Flux Quantum logic (RSFQ) is a digital technology characterized by high speed, low power consumption and low operational temperatures. RSFQ is attractive for many applications and in particular for high frequency Digital Signal Processing (DSP) and for digital control of mK devices like superconducting qubits. Due to the 30 GHz clock speed, RSFQ DSP could be used to implement a Successive Interference Canceller for multi-user detection in a 3G wireless systems. The being developed DSP consists of a digital core, an RSFQ Multiply Accumulate unit (MAC), RSFQ fast cache memory and external semiconductor memory.The first part of this thesis is focusing on architecture and implementation of the hierarchical CMOS-RSFQ hybrid memory for the RSFQ DSP. The architecture ofthe RSFQ cache is optimized in order to reach the highest speed and reliable data communication with CMOS memory. Aspects of block synchronization betweenthe RSFQ cache and the RSFQ digital core are discussed and solved using a new developed RSFQ gate, C3. For design of different components of the DSP, a designapproach based on validated VHDL models is proposed and experimentally verified.The second part of this thesis is focusing on the design oflow-dissipative RSFQ circuits for interface to the superconducting qubits. Power dissipation ofRSFQ circuits is minimized. A low-temperature thermal model of resistively shunted Josephsonjunctions is developed and applied to optimize the shape of the shunt resistors in order to reduce their electron temperature. Depending on the available fabricationprocess the lowest noise temperature of RSFQ circuit can be in the range from 90 mK to 180 mK.
The authors argue that superconducting digital electronics is the future trend for high-speed and low-power applications. Low-T/sub c/ technology based on stable Nb-AlO/sub x/-Nb tunneling junctions has been successful in constructing most of the digital circuit logic elements. A four-stage shift register constructed with latching logic operated up to 4 GHz with a power consumption of about 60 mu W per gate. A 12-b analog-digital coverter constructed with a single flux quantum (SFQ) logic showed proper operation with the total power dissipated being only about 100 mu W. Fabrication of Josephson tunneling junctions with high-T/sub c/ materials has been tougher than with low-T/sub c/ materials. However, recently developed very high speed rapid single flux quantum (RSFQ) logic circuits can be applied to weak link type junctions fabricated with YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. Simulations of the various circuits with weak link type junctions, e.g., shift registers, analog-digital converters, etc., show fast operation and reasonable margins. Applications of similar concepts can be applied to NbN circuits operating at above 10 K.>
Design and manufacturing of superconductive electronics have been evolving over the past three decades with significant progress made in related fields. Rapid single flux quantum (RSFQ) logic circuits have become popular among superconductive logic families and its energy-efficient variants (ERSFQ and eSFQ) have shown promise as an ultra lowpower and high-speed circuit fabric. SFQ circuits have been demonstrated at tens of GHz with an energy consumption of an attojoule per gate. There are many differences between SFQ and conventional CMOS circuits. SFQ logic circuits are based on the manipulation of the quantized magnetic flux pulses. Most of the logic gates are sequential in nature requiring the clock to be distributed to every logic gate. SFQ logic gates have no gain and hence splitters are needed to drive multiple fanouts. Design and successful demonstration of a controllable superconducting switch and a compact reliable memory element have evaded researchers so far. This paper starts by describing key differences between SFQ logic and conventional CMOS and concludes by listing key challenges that must be overcome to achieve the very large scale integration of SFQ circuits and make the demonstration of a superconductive CPU a reality.
Abstract. While human progress in the world of electronics, computers and computing is undeniable, new researches show that we can achieve greater speed and lower power consumption based on the principles of quantum physics. Changing from the digital world of today, which is based on CMOS transistors and the electric current, to quantum with no electric current and based on magnetic flux, is now in the middle of its way. Clearly any research in this area or even a comprehensive scientific attention is laudable from the experts’ viewpoint. This paper tries to show quantum computers more tangible by describing and analyzing some basic principles and explaining the entrance to this technology. In this article it is explained how the spintronics, superconductivity, and Josephson junction open the ways to the next generation of faster, more compact, and more energy-efficient computers in future; computers that bring about several hundred times the speed and negligible power consumption, along with the revolution of basic principles of electronics. We'll see how the spin transistors transfer data in lower speed and volume, using the electron spin direction to be high or low, or how the superconductivity, which is the key to the future supercomputers, solve the problem of supercomputers power consumption using technologies such as RSFQ.