JP4604239B2 - Meander line type Josephson junction array - Google Patents

Description

  The present invention relates to a voltage standard device composed of a Josephson junction array in which a large number of Josephson junctions that generate a constant voltage by applying a bias current and a microwave are connected in series.

  The Josephson voltage standard is based on the physical law of AC Josephson effect that can accurately convert frequency to voltage. A microwave transmission line is used to supply microwaves to a Josephson junction on a chip made of silicon or the like. As a microwave transmission line, a quasi-planar waveguide (CPW) has been used (see Non-Patent Document 1 below).

  FIG. 1 shows a quasi-planar waveguide having a wiring structure in which a signal conductor wiring 2 and a ground conductor (external conductor) 3 are provided on one surface of a dielectric substrate 1. Since the signal wiring has a structure sandwiched between ground conductors and signal transmission is possible using only one surface of the dielectric, circuit characteristics evaluation and connection with other circuits are easy.

  The Josephson junction array used in the voltage standard is a series of a large number of Josephson junctions composed of a superconductor lower electrode 4, a normal conductor intermediate layer 5, and a superconductor upper electrode 6. is there.

As shown in FIG. 2, in order to uniformly apply microwaves to all junctions, a Josephson junction 7 array is also known in which ground conductors 3 are arranged on both sides of a junction to form a quasi-planar transmission line. (See Non-Patent Document 2).
Yoshihiro Konishi, “Basics and Applications of Microwave Circuits”, General Electronic Publishing Company, August 20, 1992 (1st edition), pages 70-71 SPBenz: Appl. Phys. Lett. 67 (18), 30 October 1995 “Superconductor−normal −superconductor junctions for programmable voltage standards”

  The problem here is that when a large output voltage is to be obtained, it is necessary to connect many Josephson junctions in series, and the Josephson junction array becomes very long, leading to an increase in chip size. That is. Further, if the chip size is increased, the manufacturing cost is increased.

  By making the Josephson junction array into a meander line (meander) type, twice as many Josephson junctions can be arranged as long as the array has the same length. However, if the meander line type is used, the inductance of the array increases. Therefore, if the characteristic impedance of the array is kept at, for example, 50 ohms, it is necessary to increase the capacitance between the array and the external conductor. Need to be very small. However, reducing the gap between the array and the outer conductor can lead to short circuit failures due to particles. Therefore, if only the lower electrode is widened and the ground conductor and capacitance are secured via the insulating film (dielectric film), the lower electrode and the ground conductor are insulated by the insulating film (dielectric film). The risk of a short circuit failure of the ground conductor can be reduced.

  By making the Josephson junction array a meander-line type, double Josephson junctions can be integrated, and by sandwiching an insulating film between the lower electrode of the array and the outer conductor of the microwave transmission line, the array and the outer conductor are electrically connected. Therefore, the possibility of a deterioration in insulation performance or a short-circuit failure due to dust or impurities is reduced. The above effects are effective for reducing the chip size or obtaining a large output voltage without increasing the chip size, improving the production yield, and reducing the manufacturing cost of the voltage standard chip. .

  The best mode for carrying out the invention will be described below.

  As shown in FIG. 3, in the conventional Josephson junction array, two Josephson junctions 7 are arranged on the lower electrode 4, two Josephson junctions are connected by the upper electrode 6, and both sides are grounded. Conductors 3 are arranged to form an array.

  When the inductance of the array is L and the capacitance of the array and the ground conductor 3 is C, the characteristic impedance Z of the array is given by Z = √ (L / C). Since the characteristic impedance must always be a constant value, typically 50 ohms, it is necessary to adjust the spacing between the array and the ground conductor to match the impedance.

  As shown in FIG. 4, in order to double the number of junctions with the same length as the Josephson junction shown in FIG. 3, the lower electrode 4 and the upper electrode 6 are arranged in a meander line (meander) shape. If the Son junctions 7 are arranged in two rows, a double Josephson junction can be arranged. Since the inductance L of the array increases significantly, the size of the lower electrode 4 is increased and brought closer to the ground conductor 3.

  FIG. 5 is a cross-sectional view taken along the plane AA of FIG. It is configured. In order to facilitate the production, the upper electrode 6 and the ground conductor 3 can also use a conductor film of the same layer.

The Josephson junction array structure according to the present invention can be used for a digital-analog converter. Moreover, it can be set as the programmable Josephson voltage standard junction array using this digital-analog converter.
Further, the programmable Josephson voltage standard junction array can be formed into a chip shape to form a Josephson voltage standard chip, and the Josephson voltage standard chip can be used as a Josephson voltage generator.

Quasiplanar waveguide Sectional view of a conventional Josephson junction array Plan view of a conventional Josephson junction array The top view which shows one Example of this invention AA sectional view of FIG.

Explanation of symbols

1: Board
2: Signal wiring conductor
3: Ground conductor
4: Bottom electrode
5: Middle layer
6: Upper electrode
7: Josephson junction
8: Insulating film

Claims (9)

A Josephson junction array structure in which a Josephson junction array on a substrate and a quasi-planar microwave transmission line in which outer conductors are arranged on both sides of the array are formed ,
The Josephson junction array is arranged in a meander line shape and includes a lower electrode, an intermediate layer, and an upper electrode.
The lower electrode has a capacitance with the outer conductor through an insulating film ,
A Josephson junction array structure characterized in that a characteristic impedance given by the inductance and the capacitance of the Josephson junction array is constant . 2. The Josephson junction array structure according to claim 1, wherein the upper electrode and the outer conductor are conductor films of the same layer .   The Josephson junction array structure according to claim 1, wherein the substrate is a dielectric or a semiconductor.   2. The Josephson junction array structure according to claim 1, wherein the outer conductor is a superconductor.   The Josephson junction array structure according to claim 1, wherein the insulating film is a dielectric. Digital - In analog converter, digital, characterized in that the Josephson junction array structure is used according to any one of claims 1 to 5 - analog converter. A programmable Josephson voltage standard junction array, wherein the digital-analog converter according to claim 6 is used. A Josephson voltage standard chip, wherein the programmable voltage standard junction array according to claim 7 is formed on a substrate and cut into a chip shape. . 10. A Josephson voltage generator, wherein the programmable voltage standard chip according to claim 8 is used.

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