JPS62122288A - superconducting weak coupling device - Google Patents

Abstract

PURPOSE:To obtain a cross sectional area, a length and the like at a weak coupling part with high reproducibility, by using the intersection of two linear regions, which are crossed at a right angle, as the weak coupling part. CONSTITUTION:A three-layer structure comprising a superconductor film 1, an insulating film 2, and a superconductor film 1 is formed. A minute opening part, which is separated by less than 1mum from an insulating film 2, is made to be a junction part of the superconductor on both sides, i.e., a weak coupling part. Thus a superconductive weak coupling element having a longitudinal microbridge structure is formed. In forming the opening parts in the insulating film, two linear grooves are formed so that they do not reach the basis superconductor film and they are crossed each other. At the intersecting part, the groove parts reach the superconductor film part. Namely, the intersecting part of the linear grooves become the weak coupling part.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a superconducting switching circuit that operates at an extremely low temperature near the temperature of liquid helium, and particularly relates to the structure and manufacturing method of a superconducting weakly coupled element suitable for high-speed switching operation. .

[Background of the invention]

Conventional superconducting weak coupling elements create a thin constriction in a part of the superconducting film, and by limiting the superconducting critical current at the constriction, a phase difference is created between the superconductors on both sides of the constriction, producing superconducting tunneling characteristics. method was used. In such a superconducting weakly coupled device, ie, a microbridge device, the superconducting critical current is determined by the width and length of the constricted portion and the film thickness. However, in a planar microbridge element, it is difficult to accurately reproduce the length and width of the constricted portion due to its shape.

On the other hand, a so-called vertical microbridge, which forms a layered structure of a superconducting electrode film and an insulating film, and forms a fine opening in a part of the insulating film to form a superconducting weakly coupled element through the opening, is a ``stable Joseph'' microbridge. "Manufacture of Son junction devices and their application to logic circuits" by Susumu Namba.

It is proposed in Superconducting Quantum Electronics, page 369, etc. Even in vertical microbridges, with the conventional method of forming a cylindrical hole in an insulating film to form a weak coupling part of a superconductor, it is difficult to obtain reproducibility in the cross-sectional area of the weak coupling part. there were.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method and structure for manufacturing a superconducting weakly coupled device that can obtain the cross-sectional area, length, etc. of a weakly coupled portion with high reproducibility in a superconducting weakly coupled device. As a result, a superconducting weakly coupled device having a superconducting critical current with reproducibility and uniformity is obtained.

[Summary of the invention]

In the present invention, a vertical microstructure is formed in which a three-layer structure of a superconducting film, an insulating film, and a superconducting film is formed, and a minute opening within the dimension of the insulating film is used as a joining part of superconductors on both sides, that is, a weak coupling part. A bridge-structured superconducting weak coupling element is constructed. In forming the opening in the insulating film, two straight grooves having the desired weak coupling width and not reaching the underlying superconducting film are formed so as to intersect with each other, and the groove portion is formed at the intersection. so that it reaches the superconducting film part.

In other words, the portion where the linear grooves intersect becomes a weakly coupled portion. This is the first weak bond forming method. Second
The method for forming a weak bond is as follows. That is, the underlying superconducting film is etched to a predetermined depth, leaving a linear portion having the width of the weak bond to be formed. The etched portion is backfilled with an insulating film.

- At a predetermined depth, leaving a linear portion having the width of the weak bond to be formed, including an insulating film on both sides, at a position intersecting the superconducting film linear portion formed in the second step. Similarly to the sixth etching, the etched portion is backfilled with an insulating film.

An upper layer superconducting film is formed on this to obtain a desired superconducting weakly coupled element.

[Embodiments of the invention]

The present invention will be explained based on examples.

(1) Example 1 A Nb film layer was formed on a thermally oxidized Si wafer by DC sputtering. The thickness of the Nb film was 1100 nm.

After forming the Nb film, a resist pattern for the electrode film is formed,
The Nb film was patterned by reactive ion etching using CF 4 gas. A resist pattern for the glabellar insulating film was formed using an optical exposure method. Film thickness 1
After forming a 100n SiO interlayer insulating film 2 by a vacuum evaporation method using resistance heating, a SiO film 2 is formed by a lift-off method.
A pattern was formed. Note that in this SiO film 2 pattern forming step, the pattern of the weak Josephson junction was excluded. Next, an electron beam exposure device was used to form a Josephson junction weak-coupling portion. Specifically, after applying a positive type resist layer, the first linear region including the 1-weak bond portion was exposed by electron beam exposure. The line width was 0.15 nm. S is etched to a depth of 60 nm by reactive ion etching using CHF s gas.
iO#Itf! : Formed. Again, the second linear region including the weak coupling portion was exposed by electron beam exposure. The second linear region was perpendicular to the first linear region and had a line width of 1.5 μm. A SiO groove with a thickness of 60 nm was formed by a reactive ion etching method using CHF gas. In the area where the first SiO trench and the second SiO trench intersect, the SiO film is etched to the bottom, and the underlying N
b The film layer was exposed. This region was defined as a superconducting weak coupling region.

Next, the SiO film and the Nb film surface layer were cleaned by high frequency discharge in Ar gas, and then the Nb film layer was formed by DC sputtering. The thickness of the Nb film layer was 200 nm, and this Nb1ml was used as a weakly coupled upper electrode. After forming a resist pattern on the upper part, a pattern of the Nb film 1 was formed by reactive ion etching using CF 4 gas. Through the above steps, the fabrication of the vertical microbridge was completed.

The vertical microbridge fabricated by the above method exhibited typical superconducting weakly coupled device characteristics in terms of DC voltage-current characteristics and magnetic field dependence of superconducting current. Furthermore, ten vertical microbridges having the same dimensions were fabricated on the same chip, and the superconducting ti values of these vertical microbridges were within the range of ±20%.

(2) Example 2 A Nb film layer was formed on a thermally oxidized Si wafer by DC sputtering. The thickness of the i'J b film layer is 200 n
It was set as m. After forming the Nb film shape, a resist pattern for an electrode film was formed, and a pattern of the Nb film 1 was formed by reactive ion etching using cF and gas. Next, using a negative resist film, the first linear region including the weak bonding portion was exposed to light using an electron beam. Line width is 0.1
It was set to 5 nm. The portions excluding the linear regions were etched by reactive ion etching using CF4 gas. The etching depth was lQQnm.

With the resist film remaining, a SiO film 2 was formed to a thickness of 1001 m by vacuum evaporation. By the seven-off method,
The resist and SiO film on the linear region were removed. Next, a second linear region including a weak coupling portion was formed at a position orthogonal to the first linear region. That is, after applying a negative resist, a linear resist pattern was formed by performing electron beam exposure. The line width of the straight line pattern was 0.15 nm. The portions excluding the linear regions were etched by reactive ion etching using CF4 gas. The etching depth was 1QQnm. With the resist remaining, a SiO film 2 was formed to a thickness of 1100 nm by vacuum evaporation. The resist and SiO film 2 on the linear region are removed by the lift-off method.
was removed.

Next, after cleaning the 5iOPIX and Nb film surface layers by high frequency discharge in /yr gas, the Nb film 1 was formed by direct current sputtering. The thickness of the Nb film is 200n.
m, and this Nb film 1 was used as a weakly coupled upper electrode. After forming the resist pattern for the upper electrode, the Nb film 1 was patterned by reactive ion etching using CFa gas. Through the above steps, a vertical microbridge with a cross section of 0.15 μm square and a length of 0.1 μm was completed.

Like the microbridge described in Example 1, the vertical microbridge produced by the above method exhibited typical superconducting weakly coupled device characteristics in terms of DC voltage-current characteristics and magnetic field dependence of superconducting current. Ten vertical microbridges having the same dimensions were fabricated on the same chip, and the superconducting current distribution of these vertical microbridges was within the range of ±20% as in Example 1. When samples were prepared five times under the same conditions, the distribution of the average critical current value of the device was within ±30 inches.

〔Effect of the invention〕

As described in the implementation column, the structure and manufacturing method of the superconducting weakly coupled device according to the present invention had the following effects.

(1) The 1Ifr area and length of the superconducting weak coupling portion can be made uniform. Therefore, it is possible to provide reproducibility and uniformity to the superconducting critical current. For example, the criticality t in the circuit
The tIL distribution width could be set to ±20%.

(2) The capacitance of the present superconducting weakly coupled device, which affects switching speed performance, is significantly smaller than that of the conventionally used sandwich-type Josephson junction. For example, compared to the capacitance of a 25 μm square Sand Inch Josephson junction, the capacitance of the present superconducting weakly coupled element is about 1150.

As described above, the superconducting weakly coupled device according to the present invention having the two effects is suitable for application to integrated logic circuits and the like.

[Brief explanation of drawings]

FIG. 1(a) is a top view of the superconducting weak coupling element shown in Example 1, and FIG. 1(b) is a sectional view taken along line bb/ in FIG. 1(a).
FIG. 3(C) is a sectional view taken along the ccZ line in FIG. 1(a). FIG. 2(a) is a top view of the superconducting weak coupling element shown in Example 2, and FIG. 2(b) is a cross-sectional view taken along the line bb' in FIG. 2(a).

Claims (1)

[Claims] 1. In a three-layer film structure of a superconducting film, an insulating film, and a superconducting film, an opening through which a current can flow between the superconducting films on both sides is formed in a part of the insulating film, and the insulating film opening is formed in the insulating film. 1. A weakly coupled superconducting element, characterized in that the superconducting films are located at the intersections of the linear grooves, and the superconducting films on both sides are directly connected to each other at the insulating film openings.