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IEEE Transactions on Applied Superconductivity

Comparison of RSFQ Logic Cells With and Without Phase Shifting Elements by Means of BER Measurements

Olaf Wetzstein, Thomas Ortlepp, Ronny Stolz, J. Kunert, H.‐G. Meyer, Hannes Toepfer

Published Feb 3, 2011• 13 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2010.2102998

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IEEE Transactions on Applied Superconductivity•Mar 1, 1993•7 citations
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.
We have found that the size of some rapid single-flux quantum (RSFQ) logic cells based on conventional 0-type Josephson junctions can be significantly reduced by using a π-type junction as a phase shifter in passive (nonswitching) mode. In comparison with the recently suggested active (switching) π-junctions mode, the passive mode offers much greater operation margins for their critical current Icπ. This gives π-junctions a chance to be implemented in RSFQ designs in the near future. As an example, we have simulated the operation of a toggle flip flop with zero-geometrical inductance of the fluxon storage loop. Simulations show that the parametric inductance of the π-junction and its normal resistance Rn form a low-pass filter, which sets the low limit for π-junctions IcπRn product, but offers a wide range of variations of the other parameters. The possible reduction of RSFQ cell size by using π-junctions opens the way to scale superconducting logic circuits down to the submicron dimensions.
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.
We present a concept of an advanced rapid single-flux-quantum (RSFQ) logic circuit family using the combination of 0-shifted and π-shifted Josephson junctions. A π-shift in the current-phase relationship can be obtained in several types of Josephson junctions, such as Josephson junctions containing a ferromagnet barrier layer, depending on its thickness and temperature. We use a superconducting quantum interference devices composed of a pair of 0- and π-shifted Josephson junctions (0-π SQUIDs) as a basic circuit element. Unlike the conventional RSFQ logic, bistability is obtained by spontaneous circular currents without using a large superconductor loop, and the state can be flipped by smaller driving currents. These features lead to energy- and/or space-efficient logic gates. In this paper, we show several example circuits where we represent signals by flips of the states of a 0-π SQUID. We obtained successful operation of the circuits from numerical simulation.
A vital precondition for the realization of rapid single-flux quantum (RSFQ) digital circuits with reduced critical currents of the Josephson junctions is the implementation of an efficient technique for superconductive phase dropping. In this paper, we present a novel phase shifting element consisting of a miniature superconductive ring located over a ground plane hole. Contrary to the solutions reported up to now, this topology can be simply integrated within complex digital RSFQ circuits realized with conventional fabrication technology.
The reduction of the critical current density in rapid single-flux quantum (RSFQ) circuits enables new application fields, like quantum computing and photonic detector readout. The low current density fabrication process creates new design challenges, such as lower stability against thermal fluctuations, violation of the lumped elements condition for microstrip inductances and increased sensitivity to the technological spread. To overcome these issues, we suggest a passive phase shifter as a promising alternative technique for superconductive phase dropping in the RSFQ electronics. Here, we study experimentally their applicability in high-speed RSFQ digital circuits. Conclusions are drawn about the impact of the passive phase shifters on the complexity, the speed and the bit error rate of the investigated RSFQ circuits. We demonstrate the successful operation of different circuits with implemented passive phase shifters at low and high speeds.
Some basic cells in the rapid single-flux quantum (RSFQ) family of superconductive logic circuits were studied. An input stage, a buffer, and an OR gate were simulated and laid out for a standard niobium-based fabrication process. The resulting circuits perform properly for clock speeds up to 1 GHz. For the simpler circuits tested, the measured margins are wide, consistent with simulations, and not very dependent on clock speed. However, margin decreases with increased circuit complexity. The input stage converts a rising edge into an SFQ pulse, which has a small amplitude and narrow width in time. On the present circuits, these pulses are about 200 mu V in amplitude and 10 ps in width. To facilitate measurement, the authors have chosen as the output an asymmetrical superconducting quantum interference device (SQUID) consisting of two Josephson junctions. It converts SFQ pulses into 2.5 mV latching output levels on chip. By inductively coupling SFQ pulses into the SQUID loop, DC current isolation between the RSFQ circuit-under-test and the output stage is provided. The SQUID was optimized for 2 GHz operations.>