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Methoden zur Verbesserung der Funktionsstabilität digitaler Elektronik mit niedriger Schaltenergie

Olaf Wetzstein

Published Sep 8, 2011• 0 citations
OpenAlex

Abstract

This thesis is a contribution to automate electronic circuit design for \ntechnologies dealing with low switching energy.The intention is to increase \nthe stability of Rapid Single Flux Quantum (RSFQ) circuits. In order to \nachieve this goal, new design tools are introduced and innovative circuit \ntopologies are implemented.The concepts that have been elaborated are \ndemonstrated using RSFQ circuits, but they are valid for any other digital \nelectronics.Future developments with main focus on power efficiency will \ntake advantage of these new concepts and design tools.The aim of this work \nis to analyze the influences of thermal noise on the reliability of \nsensitive electronics.By means of an analytic description, the influence of \nthermal noise is classified. Based on this estimation the stability of \ncircuits is evaluated. This approach of correlating the topology of a \ncircuit with its immunity to thermal noise is unique so far.The results of \nthe analysis demonstrate that using a symmetric circuit topology \nsignificantly improves the circuit's robustness against both the influence \nof thermal noise and parameter spread caused by the fabrication process. \nIn order to realize symmetric circuits, phase-shifting-elements such as \npi-junctions become important.The technology that permits the fabrication \nof phase-shifting-elements has been developed during the recent years.The \nessential work flow which is necessary to implement these elements into \nRSFQ electronics is presented in detail in this work. The predicted \nimprovements are experimentally proved.

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This thesis is a contribution to automate electronic circuit design for \ntechnologies dealing with low switching energy.The intention is to increase \nthe stability of Rapid Single Flux Quantum (RSFQ) circuits. In order to \nachieve this goal, new design tools are introduced and innovative circuit \ntopologies are implemented.The concepts that have been elaborated are \ndemonstrated using RSFQ circuits, but they are valid for any other digital \nelectronics.Future developments with main focus on power efficiency will \ntake advantage of these new concepts and design tools.The aim of this work \nis to analyze the influences of thermal noise on the reliability of \nsensitive electronics.By means of an analytic description, the influence of \nthermal noise is classified. Based on this estimation the stability of \ncircuits is evaluated. This approach of correlating the topology of a \ncircuit with its immunity to thermal noise is unique so far.The results of \nthe analysis demonstrate that using a symmetric circuit topology \nsignificantly improves the circuit's robustness against both the influence \nof thermal noise and parameter spread caused by the fabrication process. \nIn order to realize symmetric circuits, phase-shifting-elements such as \npi-junctions become important.The technology that permits the fabrication \nof phase-shifting-elements has been developed during the recent years.The \nessential work flow which is necessary to implement these elements into \nRSFQ electronics is presented in detail in this work. The predicted \nimprovements are experimentally proved.
The rapid single-flux quantum electronics (RSFQ) is a superconducting, naturally digital circuit family which is currently close to being commercially applied. RSFQ is outstanding because of its very low switching energy resulting in very low power consumption. This advantage causes, however, a significant influence of thermal noise. For industrial applications, a certain noise immunity is required which is still a challenge, especially for circuits of higher complexity. Integrating phase-shifting elements is a new concept for further improvements concerning stability against the influence of thermal noise. We have already shown that the implementation of phase-shifting elements significantly reduces the influence of thermal noise on circuit behavior by experimentally analyzing the bit-error rate (Mielke et al 2009 IEEE Trans. Appl. Supercond. 19 621–5). Concepts which are easily implementable in standard niobium technology are especially promising. The π-phaseshifter consists of a superconducting loop which is able to store a single flux quantum. The loop current related to the stored flux creates a well-defined phase shift. To achieve the correct functionality of complex circuits it is essential to store exactly one flux quantum in each π-phaseshifter during the cooling down of the chip. Thus, for studying the feasibility of this new approach, the initialization reliability of the π-phaseshifter needs to be verified. We present an experimental investigation of this reliability to obtain a general assessment for the application of the π-phaseshifter in niobium technology. Furthermore, we compare the configuration shielded by a solid ground plane with a configuration with a ground-plane hole below the π-phaseshifter. Justified by the experimental results we suggest programmable RSFQ circuits based on π-phaseshifters. The characteristics of these devices can be influenced by a controlled initialization of the π-phaseshifter. The fabrication was performed by FLUXONICS Foundry.
The Rapid Single Flux Quantum electronics is a superconducting, naturally digital circuit family which is currently close to getting commercially applied.RSFQ is outstanding by its very low switching energy resulting in a very low power consumption which is the reason for a significant influence on thermal noise.For industrial applications a certain noise immunity is required which is still a challenge especially for circuits of higher complexity.In superconducting electronics resistors are the only on-chip noise sources.We analyzed the influence of each individual resistor in a toggle Flip-Flop circuit to the overall bit-error rate.The shunt resistors of comparator structures are the dominant noise sources and bias resistors have almost no influence.We analyzed in detail the types of thermal introduced switching errors of an toggle Flip-Flop which was optimized with respect to the fabrication yield.Simulation studies were accomplished to investigate the influence of circuit parameters on quality and on quantity of switching errors.By identifying and manipulating the parameters with the most significant influence on the noise susceptibility an enhanced noise immunity of the device can be achieved.The goal is to improve the robustness of RSFQ circuits against the influence of thermal noise for an specific topology by changing the circuit parameters of the device.
The article describes the activities aimed at bringing superconductive high-speed electronics towards maturity.Integrated circuits based on the effects in superconducting materials have become feasible for some years already.The main focus lies on the so-called Rapid Single-Flux-Quantum (RSFQ) electronics which allows the combination of high operation frequencies (well above 20 GHz) with very low energy consumption when logical states are changed.Thus, this circuit technology exhibits a promising potential for realising energy-efficient modules for signal processing and computing.Recently, a dedicated design infrastructure has been developed in Europe.Closely associated is a foundry for digital RSFQ integrated circuits so that the fabrication of integrated circuits is viable.In the contribution, the basic features of the RSFQ circuit design are addressed with special emphasis on implementation on the wafer scale.
The influence of thermal noise gives a major restriction in the development of Rapid Single Flux Quantum (RSFQ) logic circuits with high-T/sub c/ superconductors. In the past, we have demonstrated a general way for determining the digital bit error rate and for predicting operation ranges with good noise immunity. The method is based on a single Fokker-Planck equation which includes the full system dynamics. However, the complexity of this equation increases dramatically with the number of Josephson junctions. Our new approach describes a method to divide an RSFQ logic cell into several basic primitives. They can be characterized quickly in terms of switching probability for a lot of different parameters. From these analyses, look-up tables are built-up. We describe how to combine these data in order to achieve the bit-error rate for the whole cell. While saving more than 90% of computation time, the comparison with our previous results for full system analyses shows less than 4% difference. This approach provides for a powerful tool to include the influence of thermal noise into the optimization process of RSFQ logic circuits as well.
Abstract\nThis diploma work is about Rapid Single Flux Quantum (RSFQ) circuits. This\nsupercoducting digital circuitry has the most promising properties in terms of speed and\npower consumption. The active elements of the RSFQ circuits are Josephson junctions (JJ).\nFor their fabrication low temperature superconducting niobium films are used. Today the\ndesign and fabrication of RSFQ circuits are done in a standardized way with well-defined\ndesign rules. Nevertheless, to design complicated circuits with more than several hundred\njunctions a lot of practical problems must be solved.\nDifferent aspects of the RSFQ circuits design process were investigated in this work.\nIn the theoretical part we propose a new, more realistic model of the Josephson junction for\nphysical simulation (which is necessarily used for design of every RSFQ gate) in order to\nimprove its quality and precision. The new model for the JJ includes additional inductances,\nwhich must reflect a specific environment of the junction layout. The influence of parameter\ndeviations was investigated in the simulation. As sources of the deviation we chose the\nspread of c (the Stewart-McCumber parameter) and that of the inductances. Then the\noperational margins of the basic elements of the RSFQ logic SR flip-flop were determined\nand compared with the prediction of the previously used model. To prove the results of\nsimulations we designed a test structure of the SR flip-flop, with c=1 and c=2 for the wellestablished\nHYPRES technology (USA).\nAnother part of the work is practical measurements. A test chip, containing chains of 100\nJosephson junctions was designed and measured in order to estimate the spread of critical\ncurrents in real technology.
Rapid single flux quantum (RSFQ) electronics is characterized by a very low switching energy. This advantage leads to a noise susceptibility, which becomes a challenge for large-scale circuits as well as for circuits using Josephson junctions with reduced critical current density. We demonstrate an improved operation range and advanced noise immunity of basic cells resulting from an implemented phase shifting element. One of those elements is the π-phase shifter. It consists of a single flux quantum trapped in a superconducting loop. The π-phase shifter can be easily produced in standard niobium technology without any process modifications. Utilizing a mature process brings advantages concerning the reliable fabrication of complex circuits.