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Journal of Physics Conference Series

Magnetic shielding effect of grounded superconducting niobium layers

Yoshinao Mizugaki, Ryuta Kashiwa

Published Feb 1, 2008• 5 citations
Publisher

Abstract

Magnetic isolation is an important issue in the realization of superconducting LSIs including more than ten thousands Josephson junctions. In this paper, we present mutual inductances of two superconducting strip lines coupled through a grounded shield layer. A conventional dc-superconducting quantum interference device (dc-SQUID) method on Nb/AlO x /Nb Josephson IC chips is employed. We have tested one dc-SQUID layout with a floating shield layer for reference, and ten dc-SQUID layouts with grounded shield layers. Four superconducting niobium layers are used as a ground plane, a SQUID line, a shield layer, and a control line from bottom to top. We have confirmed that the number, positions, and dimensions of ground contacts make differences in the mutual inductances. Ground contacts should be placed to enhance the shielding current for effective magnetic isolation.

DOI
10.1088/1742-6596/97/1/012056

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Magnetic isolation is an important issue in the realization of superconducting LSIs including more than ten thousands Josephson junctions. In this paper, we present mutual inductances of two superconducting strip lines coupled through a grounded shield layer. A conventional dc-superconducting quantum interference device (dc-SQUID) method on Nb/AlO x /Nb Josephson IC chips is employed. We have tested one dc-SQUID layout with a floating shield layer for reference, and ten dc-SQUID layouts with grounded shield layers. Four superconducting niobium layers are used as a ground plane, a SQUID line, a shield layer, and a control line from bottom to top. We have confirmed that the number, positions, and dimensions of ground contacts make differences in the mutual inductances. Ground contacts should be placed to enhance the shielding current for effective magnetic isolation.
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.
Mutual inductance between two superconducting strip lines coupled through a grounded shield layer is evaluated by both experiments and numerical calculation. A conventional superconducting quantum interference device (SQUID) method on a Nb Josephson IC chip is employed for experiments, whereas a three-dimensional inductance extraction program "FastHenry" is used in numerical calculation. Five test circuits are designed to investigate the effects of two ground contacts placed parallel to the strip lines with different spacing. Both the experimental and numerical results confirm that the mutual inductance is at its minimum when the contacts are placed close to the Josephson junctions, that is, the edges of the SQUID line under evaluation. In the experiments, the minimum value of the mutual inductance is 26% of the value for the reference circuit with a floating shield layer. A model including the Meissner effect in a superconducting loop is found to provide a quasi-quantitative explanation for the dependence on the spacing between two contacts.
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.
We evaluate mutual inductance between two horizontally-placed strip lines in niobium integrated chips, by means of both experiments and numerical calculations. In the experiments, one strip line is used as a part of a superconducting quantum interference device (SQUID) loop, and the other is used as a control line for applying external magnetic flux to the SQUID loop. The mutual inductance is obtained from the modulation period of the SQUID. In the numerical calculations, we employ a three-dimensional inductance extraction software (FastHenry). Three types of strip line structures on a lower ground plane (GP) are examined: two horizontally-placed microstrip lines without an upper GP (MSLs), two horizontally-placed strip lines with a floating upper GP (FLSLs), and two strip lines with an upper GP connected to the lower GP (COSLs). We have found that each strip line structure has different dependence of mutual inductance on the line center distance. The mutual inductance between the MSLs exhibits strong dependence on the line center distance, whereas that between FLSLs is almost independent on the distance. Negligible mutual inductance is obtained for the COSLs.
We present numerical and experimental investigation on mutual inductance coupled through a superconducting Nb thin film. The inductance extraction program FastHenry with superconductor support is used for numerical calculation. The self- and mutual inductances of two superconducting strip lines are calculated as functions of the width of the shield layer inserted between them. The results demonstrate that the mutual coupling is reduced with increasing shield layer width. A superconducting quantum interference method is employed to measure the mutual inductance on Nb IC chips. The experimental results of the mutual inductance agree with the calculated results.
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.