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USB-Flash-Ausg.:#R#<br/>Information technology and electrical engineering - devices and systems, materials and technologies for the future : 54. IWK, Internationales Wissenschaftliches Kolloquium ; proceedings ; 07 - 10 September 2009 / Faculty of Electrical and Information Technology, Technische Universität Ilmenau. - Ilmenau : Verl. ISLE, 2009. - ISBN 978-3-938843-45-1

Design of integrated rapid single flux quantum circuits for fabrication in a niobium-based foundry process

Hannes Töpfer, Thomas Ortlepp

Published Nov 25, 2009• 0 citations
OpenAlex

Abstract

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.

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•Jan 1, 2006•0 citations
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.
Rapid Single-Flux-Quantum Circuits Fabricated Using 20-kA/cm2\hbox{20}\hbox{-}\hbox{kA}/\hbox{cm}^{2}20-kA/cm2 Nb/AlOx/Nb\hbox{Nb}/\hbox{AlO}_{x}/\hbox{Nb}Nb/AlOx​/Nb Process
We designed and fabricated rapid single-flux-quantum (RSFQ) circuits using a new 20-kA/cm2process based on conventional Nb/AlOx/Nb Josephson junction (JJ) technology. Circular JJs were fabricated to improve their uniformity. They had sufficiently low spread of the critical currents for large-scale RSFQ circuits. We selected the circuit design parameters carefully, including the bias voltage and McCumber-Stewart parameter, balancing the energy consumption, switching speeds, margins, and integration density. We demonstrated several logic gates and shift registers at a clock frequency of 154 GHz with the designed bias voltage of 2.5 mV, and demonstrated energy-efficient shift registers using the low-voltage RSFQ (LV-RSFQ) technique at 83 GHz.
Superconducting Rapid Single Flux Quantum (RSFQ) circuits have the ability to perform digital operation with clock signals in the microwave frequency range. The clock signal can be either generated on-chip via Josephson oscillators, or injected externally through microwave lines. We have investigated several configurations to digitize and process on-chip microwave signals for space-based applications, based on the European FLUXONICS Foundry with a niobium-based technology that can opearate at clock frequencies in the 30-40 GHz range. This presentation will focus on the main features of the technology, the expected performances for specific applications and the current status of experimental results of several circuits measured at 4K. It will also feature prospects and expectations, based on circuits already designed and in preparation.
Advancement of superconductor digital electronics, especially Rapid Single Flux Quantum (RSFQ) logic circuit is described. Ultra short pulse of a voltage generated across a Josephson junction and release from charging/discharging process for signal transmission in RSFQ circuits enable us to reduce power consumption and gate delay. The power-delay products (PDPs) of RSFQ integrated circuits (ICs) are 4 or 5 orders of magnitude smaller than those of semiconductor ICs. The fabrication process technology and the related designing technology have been advanced, and RSFQ ICs have been applied to software-defined radio receivers, superconductor detector array systems, and supercomputers. Recently, several kinds of energy-efficient single flux quantum circuit have been proposed to obtain further advantage to semiconductor devices. The PDPs of these circuits become at least 1 order smaller than those of conventional RSFQ circuits.
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.
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.
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.