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IEEE Transactions on Applied Superconductivity

Influence of the Superconducting Ground Plane on the Performance of RSFQ Cells

Ruben van Staden, Kyle Jackman, Coenrad Johann Fourie, Pascal Febvre

Published Jan 17, 2017• 5 citations
Publisher

Abstract

Single-flux-quantum (SFQ) digital circuits are mostly based on cells that rely on reliable foundry processes that make use of a superconducting ground plane as a reference for the active elements and the microstrip line interconnects. The quantum of magnetic flux h/2e, associated with the binary information, corresponds to a magnetic field energy density that needs to be localized in space to limit interactions between adjacent cells. In other words, mutual inductances can harm the proper behaviour of circuits unless they are taken carefully into account during the design phase. We studied extensively the Josephson transmission line cell with different geometrical configurations of the ground plane and bias pads. We found with the use of InductEx that the return current sometimes follows paths that are far from what intuition tells, which can lead to nonoptimized designs. In this paper, we emphasize the limitations due to the presence of external or internal magnetic fields. Then, we compare obtained performances with the ones with optimized geometries for which the presence of the magnetic field is taken into account from the design phase.

DOI
10.1109/tasc.2017.2654345

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Single-flux-quantum (SFQ) digital circuits are mostly based on cells that rely on reliable foundry processes that make use of a superconducting ground plane as a reference for the active elements and the microstrip line interconnects. The quantum of magnetic flux h/2e, associated with the binary information, corresponds to a magnetic field energy density that needs to be localized in space to limit interactions between adjacent cells. In other words, mutual inductances can harm the proper behaviour of circuits unless they are taken carefully into account during the design phase. We studied extensively the Josephson transmission line cell with different geometrical configurations of the ground plane and bias pads. We found with the use of InductEx that the return current sometimes follows paths that are far from what intuition tells, which can lead to nonoptimized designs. In this paper, we emphasize the limitations due to the presence of external or internal magnetic fields. Then, we compare obtained performances with the ones with optimized geometries for which the presence of the magnetic field is taken into account from the design phase.
The design of single flux-quantum (SFQ) circuits requires well-defined inductances. In many cases these are created by irregular geometries and a computer-aided design tool is necessary for inductance calculations. This becomes a challenging task in cases where no ground plane is used and thus three-dimensional calculations are required. For this purpose, InductEx was developed and verified successfully for simple structures. In this work we take the next step and demonstrate the applicability of InductEx to the design of rapid single flux-quantum (RSFQ) cells. We use InductEx for inductance modeling as an integral part of the design cycle for an RSFQ cell with flux quantum transfer via mutual inductance and for a DC-reprogrammable logic circuit with similar inductive coupling. The relation between coupling factor and self-inductance is optimized in the circuit layout. All coupling structures are duplicated in SQUID modulation test circuits to measure and verify calculation accuracy. We find excellent agreement between the calculated and measured inductance values. Furthermore, the operational area of the basic cell is determined and compared with similar investigations reported earlier. Based on this comparison we conclude that InductEx is a valuable inductance design tool, and that mutual coupling in structures using ground plane holes and RSFQ-compatible self-inductances can be modeled and designed reliably.
Two effects of the bias supply line on the performance of RSFQ circuits have been studied: inductive coupling between the bias supply line and circuit inductances; effect of the mirrored current in the ground plane. The following results of the study are presented: 3D calculations and experimental measurements of inductive coupling between parallel and perpendicular microstrips in presence of one and two ground planes; calculated margins dependence on the external magnetic field in RSFQ cell. Bias current density extraction method has been developed and implemented in RSFQ circuit design flow.
Large-scale SFQ integrated circuits require a large amount of dc-bias current. The magnetic fields induced by the dc-bias currents and the return currents flowing in a ground plane have a great influence on the performance of SFQ circuits. As an approach to reduce the influence, the dc-power (DCP) layer placed under a ground plane is newly introduced in SRL advanced Nb process. We have experimentally evaluated the effects of the DCP layer using SQUIDs and large-scale Josephson transmission line (JTL) circuits with more than 10,000 junctions. The SQUIDs showed that there was small magnetic flux coupling between the DCP lines and themselves and that the coupled flux was enhanced by the return current. The coupled flux decreased with increasing line width of the DCP lines and with increasing distance from the centerlines of them. In the large-scale JTL tests, the cells with DCP patterns showed wider operating margins than the CONNECT OPEN cells with partial magnetic shielding of bias current lines. The test results also showed that a line shape along the periphery of a cell was more effective than a plane shape within a cell as a DCP layer pattern for bias current feeds.
ENGLISH ABSTRACT: Rapid Single Flux Quantum (RSFQ) cells are key in the design of complex and applicable RSFQ electronic circuits. These cells are low-level circuit elements that are used repeatedly to build larger, applicable RSFQ circuitry. Making these cells simple to layout and manufacture, but reliable for extensive use demands a careful development process for RSFQ cells. Cell functionality is verified through simulations, thereafter the cell is laid out in special software packages. Inductance of on-chip superconductor structures is extracted through careful modelling with numerical field solver software. A cell library has been developed by incorporating existing or published cells after further analysis and optimization, as well as developing new cells. Cells that have been adapted into the library include the Josephson transmission line (JTL), Splitter, Merger, D-Flip Flop (DFF), T-Flip Flop (TFF), NOT, AND, OR and XOR, DC-SFQ and SFQ-DC and PTL Driver and Receivers. New cells include NOR, NAND and XNOR. The cells were designed for the IPHT’s RSFQ1D 1kA/cmª and Hypres’ 4.5kA/cmª processes. The cells in the library have good bias current operating margins obtained through simulations (> ±26%). All cells have all the parameters listed in the thesis including extracted inductance values. In order to have a complete and verified RSFQ cell library, cells have been sent for fabrication at IPHT and Hypres facilities. These cells can now be tested on-chip, in the laboratory, to establish functionality and practical bias current margins. All test signal patterns and bias currents required for testing are defined to allow co-workers or collaborators to test the cells.
The Rapid Single Flux Quantum (RSFQ) technique is regarded as one of the most promising electronics due to its extremely high operating speed and low power consumption. Middle-scale RSFQ applications operating at multigigahertz frequency have recently been reported on. At such high operating frequencies, the on-chip interconnects start to play a limiting role for the performance of the densely packaged digital circuits. Our previous studies have theoretically investigated the parasitic coupling effects between superconductive microstrip transmission lines and conclusions have been drawn about the critical coupling levels of several typical microstrip geometries. Here, basic RSFQ structures are investigated experimentally, which contain a Josephson transmission line stage with large inductance and can either trap or pass SFQ pulses. These structures are incorporated in critical microstrip layouts, thus being subjected to parasitic coupling. The bias levels are monitored at which the structures start to pass the SFQ pulses with and without coupling influences. Thus, the conclusions of our previous theoretical investigations are verified experimentally.
We have developed a cell library for superconducting large scale integrated (LSI) digital circuits based on Single Flux Quantum (SFQ) devices. The circuits were designed for a 6-kA/cm2Nb/AlOx/Nb junction process with 10 mask levels including one ground plane and two Nb wiring layers. The initial design and optimization of the circuit parameters were achieved by means of the optimization functions in a circuit simulator, PSCAN2, to ensure that important circuit parameters, Josephson critical current (IC), bias current (Ibias), and inductance satisfied minimal margin requirements. Critical margins of ICand Ibiaswere then further improved manually. To compensate the scattering in the circuit parameters from fabrication by design as much as possible, the critical junction parameters were optimized to further centralize the IC. The library cells were laid out following our design rules determined by systematic experiments on process control monitors (PCM). Basic cells including Josephson transmission lines, splitters, confluence buffers, D flip-flops, T flip-flops, and XOR and NDRO gates were designed, fabricated, and successfully tested at low frequencies. Wide overlaps of the operating regions for the common bias voltages were confirmed.