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IEEE Transactions on Applied Superconductivity

The effects of Dc bias current in large-scale SFQ circuits

Hirotaka Terai, Y. Kameda, Shinichi Yorozu, Akira Fujimaki, Zhen Wang

Published Jun 1, 2003• 54 citations
Publisher

Abstract

The supply of bias current is one of the key problems to be overcome for fully operational large-scale SFQ circuits. Large currents cause various undesirable effects that degrade circuit operation. The magnetic field induced by a large DC bias current affects the operation of several SFQ cells, even with a microstrip-line structure applied for the bias lines. In this paper, experimental results using SQUIDs show that bias-line shielding is the most effective way to reduce the effect of the DC bias current. The operating margins of SFQ cells were also found to be sensitive to the current in the ground plane. The location of a ground bonding should be close to the point of current injection if we wish to avoid undesirable diffusion of current through the ground plane. We confirmed the effectiveness of bias-line shielding and of the location of ground bonding with a circuit composed of about 500 junctions.

DOI
10.1109/tasc.2003.813917

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IEEE Transactions on Applied Superconductivity•Jun 1, 2007•13 citations
The supply of bias current is one of the key problems to be overcome for fully operational large-scale SFQ circuits. Large currents cause various undesirable effects that degrade circuit operation. The magnetic field induced by a large DC bias current affects the operation of several SFQ cells, even with a microstrip-line structure applied for the bias lines. In this paper, experimental results using SQUIDs show that bias-line shielding is the most effective way to reduce the effect of the DC bias current. The operating margins of SFQ cells were also found to be sensitive to the current in the ground plane. The location of a ground bonding should be close to the point of current injection if we wish to avoid undesirable diffusion of current through the ground plane. We confirmed the effectiveness of bias-line shielding and of the location of ground bonding with a circuit composed of about 500 junctions.
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.
In this study we have investigated the influence of an externally applied ground current on large-scale single-flux-quantum (SFQ) circuits. A Josephson transmission line containing 10 4 junctions with a total bias current of 1.8 A was examined for this purpose. The test results show that the ground current affects the circuit operation remarkably strongly. In the worst case, the ground current of about 10 mA for a 5 mm die chip disturbs the circuit operation. The results also show that DC/SFQ and SFQ/DC converters are very sensitive to the ground current. To eliminate the effect of the ground current, we have made SFQ circuits completely shielded from the external magnetic field by using upper and lower ground planes. It was found that the shielded SFQ circuits are much less sensitive to the ground current. These results show that the magnetic shielding structure is one solution for reducing the effects of the ground current and hence the external magnetic field.
We have evaluated the effects of a DC power (DCP) layer in a 10-Nb-layer device using SQUIDs and large-scale Josephson transmission lines (LS-JTLs). The 10-Nb-layer device has recently been developed for SFQ LSIs, with an active layer including Josephson junctions (JJs) at the top, two passive transmission line layers in the middle, and the DCP layer for bias current feeds at the bottom. The evaluation with SQUIDs revealed that the 10-Nb-layer device structure drastically reduced the magnetic flux induced by DC currents flowing through the DCP line, in comparison with the previous advanced process device structure. A major factor for this reduction was an increase in the number of ground layers between the DCP layer and the active layer. In the test of the LS-JTLs containing about 12000 JJs, we obtained the operating margins as wide as numerically simulated ones. We also observed no difference in the margins between a method for extracting ground return currents through ground layers and that through the dedicated layer. These results demonstrated that the 10-Nb-layer device structure is suitable for SFQ-LSIs.
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.
The practical implementation of RSFQ technology in most digital electronics application areas requires much more complexity than the presently developed circuits. There are two important issues in building large-scale RSFQ circuits: 1) the recycling of the bias currents and 2) the transfer of SFQ pulses between circuits located far apart. RSFQ circuits are well known to operate with DC current bias. Even though the DC current biasing is more forgiving than the problematic AC biasing, it can still be a big concern when the circuit size becomes large. Dramatic reduction of the total bias current can be achieved by biasing several RSFQ circuits in series, where each circuit is positioned on a separate ground plane. In this work, we have used magnetically coupled Josephson transmission lines as inputs and outputs of an isolated shift register to show the feasibility of using the concept of serial biasing in current recycling. The circuit was simulated, fabricated with Nb technology, and tested at a temperature of 4.2 K. Test results show that SFQ pulses were transferred into the shift register built on a separate ground plane, clocked through it, and sent out back to the circuit on the original ground plane. We also studied on how to transfer SFQ pulses over an extended length, an important issue in building large RSFQ circuits. We have designed the circuits to test our microstrip line and multichip module approaches. We designed, optimized, fabricated and tested the circuits. Test results show that SFQ pulses can be successfully transmitted over an extensive distance in a chip and between chips.
We diagnosed circuits to identify of margin reduction mechanisms in large-scale SFQ circuits. Two types of shift registers with partial or full bias line shields were designed and tested. We investigated the locations and origins of circuit errors by measuring bias margins and output waveforms. The results indicated that bias line currents can degrade the bias margin around the periphery in large-scale circuits with partial bias line shields. On the other hand, no apparent reduction of bias margins was observed in any part of the circuit area in the circuits with full bias line shields. However, we observed a reduction of the bias margin due to flux trapping even when we used a completely non-magnetic He dewar surrounded by a dual-layer permalloy metal shield. We found that the probability of finding a defect in a circuit with 10,000 Josephson junctions is less than 60%.