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Journal of Applied Physics

Grounding positions of superconducting layer for effective magnetic isolation in Josephson integrated circuits

Yoshinao Mizugaki, Ryuta Kashiwa, Masataka Moriya, Kouichi Usami, Tadayuki Kobayashi

Published Jun 1, 2007• 10 citations
Publisher

Abstract

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.

DOI
10.1063/1.2739221

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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.
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.
We evaluated mutual inductances between two superconducting striplines coupled through a superconducting intermediate layer in Nb Josephson IC.In order to measure the mutual inductances we designed eight test circuits based on a superconducting quantum interference device (SQUID), which included intermediate layers of two different thicknesses and four different widths. The inductance extraction program FastHenry with superconductor support was used for numerical calculation. The experimental results agreed with the calculated results, both of which confirmed that a thicker and wider intermediate layer reduced the mutual coupling more efficiently.
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.
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.
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.