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Superconductor Science and Technology

The influence of the ground current on large-scale single-flux-quantum circuits

K. Fujiwara, T Hikida, Nobuyuki Yoshikawa, Akira Fujimaki, Shinichi Yorozu, Hirotaka Terai

Published Mar 16, 2006• 11 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/19/5/s41

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IEICE Transactions on Electronics•Jan 1, 2014•6 citations
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.
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.
We have investigated the effect of a dc-power (DCP) layer on the performance of single flux quantum (SFQ) circuits by changing the film thickness of a ground plane (GP). The DCP layer was placed below the GP and was used as bias current feeds. Two measurements were conducted. One was direct detection of the magnetic flux above the GP by using SQUIDs. The results indicated that the GPs with film thicknesses of more than 400 nm were effective to completely suppress the flux under the condition that the return current was extracted from the ground plane. The other was evaluation of the operating margins of the 408 bit shift register circuits (SRs) with 8032 Josephson junctions and a total designed bias current of 1.05 A. The results showed that a 500 nm thick GP had an advantage over a 300 nm thick GP in the dependence of the upper margin on the bias current port of the SR.
An increase in bias currents supplied to single-flux-quantum (SFQ) circuits is a rising problem in building large-scale integrated SFQ circuits, because the magnetic fields generated by bias currents and ground return currents affect the circuit operation. Current recycling technique, which can drastically reduce the total bias current of SFQ circuits, is expected to be a very effective solution for large-scale SFQ circuits. To realize the current recycling technique, signal transmission between SFQ logic circuits located on different ground planes is required. Two types of inductively coupled pulse transfer circuits were examined in this study. One is composed of a simple JTL driver and a DC-SQUID receiver whose junctions are slightly under-damped to enhance the magnetic sensitivity of the SQUID. The other circuit is composed of an SFQ driver based on a delay flip-flop (DFF) and a SQUID receiver. The advantage of the DFF-based circuit is that the coupling time between the driver and the receiver can be increased to ensure appropriate switching in the receiver junction. Two types of transfer circuits were implemented using the SRL 2.5 kA/cm2Nb process and their correct operation was experimentally confirmed. Circuit structures for effective coupling of the SFQ pulse transfer were also investigated. It was shown that a ground plane boundary should be separated from a ground plane hole underneath coupling inductors to obtain high coupling factors.
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.
A promising application of a single-flux quantum (SFQ) circuit is read-out circuitry for a multi-channel superconductive sensor array. In such applications, the SFQ read-out circuit is expected to operate outside a magnetic shield. We investigated an SFQ circuit structure, which is tolerant to an external magnetic field, using the AIST 2.5kA/cm2 Nb standard 2 process, which has four Nb wiring layers including the ground plane. By covering the entire circuit using an upper Nb wiring layer called the control (CTL) layer, the influences of the external magnetic field on the SFQ circuit operation can be avoided. We experimentally evaluated the sheet inductance of the wiring layer underneath the CTL shielding layer to design a magnetic-field-tolerant SFQ circuit. We implemented and measured test circuits comprising toggle flip-flops (TFFs) to evaluate their magnetic field tolerances. The operating margin and maximum operating frequency of the designed TFF did not deteriorate with increases in the magnetic field applied to the test circuit, whereas the operating margin of the conventional TFF was reduced by applying the magnetic field. We have also demonstrated the high-speed operation of the designed TFF operated in an unshielded environment at a frequency of up to 120GHz with a wide operating margin.