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Journal of Physics Conference Series

Experimental and simulation results of a symmetrical pad to reduce a stray ground current in superconducting integrated circuits

Hideo Suzuki, T Ono, Nobuyuki Yoshikawa

Published Jul 1, 2017• 3 citations
Publisher

Abstract

The method of extracting bias current is widely used in large scale SFQ circuits to avoid harmful effect by spreading the bias return current on a ground plane, which causes malfunction or degrading the operating margin by the stray magnetic field. Although extracting the bias current is effective, we found that the extraction using a normal pad structure with a bias pad and a current extracting pad still produces a large stray ground current (magnetic field) for its asymmetric structure. We proposed a bilaterally symmetrical pad structure, which can reduce the stray ground current significantly in superconducting circuits. The bias current is extracted from a nearby ground pad of a bias pad in usual method, whereas the bias current is extracted from two of the symmetrical ground pads which are arranged both sides of the bias pad in the symmetrical structure. Then, the bias current is extracted on halves from each of the two pads symmetrically, reducing the stray ground current. First, we have simulated the ground current by using FastHenry program. The results showed that the symmetrical pad structure outside of ground plane reduced spread of the ground return current significantly. Next, we have designed a test chip and confirmed the effectiveness of the symmetrical structure experimentally.

DOI
10.1088/1742-6596/871/1/012067

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The method of extracting bias current is widely used in large scale SFQ circuits to avoid harmful effect by spreading the bias return current on a ground plane, which causes malfunction or degrading the operating margin by the stray magnetic field. Although extracting the bias current is effective, we found that the extraction using a normal pad structure with a bias pad and a current extracting pad still produces a large stray ground current (magnetic field) for its asymmetric structure. We proposed a bilaterally symmetrical pad structure, which can reduce the stray ground current significantly in superconducting circuits. The bias current is extracted from a nearby ground pad of a bias pad in usual method, whereas the bias current is extracted from two of the symmetrical ground pads which are arranged both sides of the bias pad in the symmetrical structure. Then, the bias current is extracted on halves from each of the two pads symmetrically, reducing the stray ground current. First, we have simulated the ground current by using FastHenry program. The results showed that the symmetrical pad structure outside of ground plane reduced spread of the ground return current significantly. Next, we have designed a test chip and confirmed the effectiveness of the symmetrical structure experimentally.
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.
Although larger scale integration enhances the practicability of superconducting Josephson circuits, several technical problems begin to emerge during its progress. One of the problems is the increase of current through a ground plane (ground current). Excess ground current produces additional magnetic field and reduces operation margins of the circuits, because superconducting Josephson devices are very sensitive to magnetic field. In this paper, we evaluate current distribution in a superconducting ground plane by means of both experiments and numerical calculation. We also verify two methods for suppressing the ground current. One is a slot structure in the ground plane, and the other is alignment of the current-extraction point. Suppression of the ground current is quantitatively evaluated.
Complex RSFQ circuits are typically dc-biased with one or more current bias trees, with current returning through a superconducting ground plane. As the bias currents become larger in more complex circuits, it is increasingly critical to pay attention to the distribution of these return currents, and the effects of the resulting magnetic fields on the performance of the RSFQ circuits. We have modeled the current and field distributions, and found that magnetic field and flux are indeed significant. This has been confirmed by direct measurement using a distribution of SQUIDs. Furthermore, we have measured the performance of several RSFQ circuits, and have found that currents in the ground plane can significantly affect performance margins. Approaches to circuit and system designs that can reduce these problems are discussed.
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.
Mutual inductances between two superconducting strip lines coupled through a grounded shield layer are evaluated by both experiments and numerical calculation. A conventional superconducting quantum interference device method on a Nb Josephson integrated circuit chip is employed for experiments. Four test circuits are designed to investigate the effects of ground contacts. Grounding the shield layer at one point or at two points located perpendicular to the line direction does not improve the shielding effect, whereas grounding at two points located parallel to the line direction reduced the mutual inductance by 67%. Mutual inductances calculated using an inductance extraction program, FASTHENRY, agree with the experimental results. Numerical results of current distributions in the shield layers demonstrate that the enhanced shielding current improves the magnetic isolation.