JP2684745B2 - Quasi-planar Josephson junction device - Google Patents

Description

The present invention relates to a device having a Josephson junction, and more particularly to a quasi-planar Josephson junction device.

The present invention can be applied to any device using the Josephson effect, such as SQUID for ultra-sensitive magnetic field measurement and ultra-sensitive electromagnetic wave detector.

<Prior Art> In order to obtain a Josephson junction with good performance, the bridge length (the length of the weak junction) is ideally set to about 3 to 5 times the coherent length of the bridge material used. It is known that this should be done, and even if this ideal bridge length cannot be obtained, the bridge length should be shortened so as to come as close as possible to this.

In the Josephson junction using Nb, the ideal bridge length is about several hundred Å. In order to obtain such an extremely short bridge length with good reproducibility, the quasi-plane type Josephson junction is advantageous.

The quasi-planar type Josephson junction is a superconducting thin film formed on a flat substrate surface, and another superconducting thin film is laminated on the upper surface of the superconducting thin film via an insulating layer. It is a structure in which a bridge spans over the entire length, and the bridge length can be determined by the film thickness dimension that can be controlled relatively easily, which is extremely advantageous compared to the planar Josephson junction that relies on microfabrication on a plane. .

<Problems to be Solved by the Invention> By the way, when a Josephson junction element is formed using an oxide high-temperature superconductor represented by YBCO, a conventional superconductor such as Nb is used for the following reason. It is difficult to obtain a good quasi-plane type Josephson junction such as.

First, a high temperature superconducting thin film such as YBCO is generally unstable and easily diffused, and it is not easy to form a laminate in a good state.

In addition, the high temperature superconducting thin film obtained by the current film forming technology is
The surface flatness is as bad as about 100 Å, and pin contacts are likely to occur when they are laminated with an insulating layer in between.

Furthermore, the oxide high-temperature superconductor is subject to processing restrictions such as deterioration with water, and the process cannot be selected relatively freely, unlike the Nb-based superconductor.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a quasi-planar Josephson junction device having high performance even when an oxide high-temperature superconductor is used, and having good manufacturing reproducibility and yield. I am trying.

<Means for Solving the Problems> In order to achieve the above-mentioned object, in the present invention, as shown in FIG. 1 corresponding to the embodiment, a step is provided on the surface of the substrate 1 and the upper plane thereof is A groove 1b is formed, one end of which reaches the end face of the stepped portion 1a and is opened laterally. Then, on the surface of the substrate 1, superconducting thin films 2 and 3 are formed on the upper and lower planes sandwiching the stepped portion 1a, respectively, and the superconducting thin films 2 and 3 penetrate the superconductors invading the groove 1a. It is configured to be joined to each other through.

<Operation> By forming the groove 1b on the upper surface of the substrate 1, the superconductor enters the groove 1b when the superconducting thin film 2 is formed.

If one end of the groove 1b is extended to the end face of the step portion 1a and the superconducting thin films 2 and 3 are formed so that the superconductor in the groove 1b communicates with the superconducting thin film 3 in the lower side, The superconductor is a weak junction J connecting the superconducting thin films 2 and 3, and a quasi-planar Josephson junction can be obtained without stacking.

<Embodiment> FIG. 1 is a perspective view of an embodiment of the present invention. A step is provided on the surface of a MgO substrate 1 and a step portion 1 is formed.
It has upper and lower planes with a as the boundary. A groove 1b, which is shallower than the step size and whose one end is open laterally at the step portion 1a, is formed on the plane on the upper step side.

Then, the superconducting thin film 2 having a uniform film thickness smaller than the step size is formed on both the upper and lower planes of the substrate 1.
And 3 are formed. A part of the superconducting thin film 2 on the upper side penetrates into the groove 1b, and the upper surface of the superconducting thin film 3 on the lower side reaches a higher position than the bottom surface of the groove 1b. It is weakly joined by a superconductor penetrating into 1b. That is, the Josephson junction element in which the superconducting thin films 2 and 3 are joined using the superconductor in the groove 1b as the weak junction J is obtained.

Next, the manufacturing method of this embodiment will be described. 2 to 5 are explanatory views of the manufacturing procedure.

First, as shown in the side view in FIG.
A gO (100) substrate 1 was prepared, and a step of 500 nm as shown in FIG. 3 was formed on the surface thereof. This step is formed by, for example, uniformly applying an AZ-based resist on the surface of the substrate 1, and then photolithographically removing one side with the substantially central portion as a boundary and the other side, and then leaving the remaining resist film. A method of performing Ar ion milling using as a mask can be adopted.

Next, as shown in a perspective view in FIG. 4, a groove 1b having a depth of 250 nm and a width of 1 μm is extended to a length of about 20 μm in the upper plane so that one end is opened laterally at the step 1a. Formed. As a method for forming it, EB exposure positive resist was used to
A method of forming an m-width mask and performing Ar ion milling can be adopted.

And finally, a uniform thickness of 350 nm on the entire surface of the substrate 1.
Was formed into a superconducting thin film. This superconducting thin film is, for example, YBCO
It is a thin film and can be formed by sputtering. The sputtering conditions were as follows: substrate temperature 620 ° C., atmospheric gas Ar / O ₂ = 40/60, pressure 20 mTorr, target composition Y: Ba: Cu = 1: 6.7: 18.3, film formation at a rate of about 30 Å / min. it can.

As shown in FIG. 5 which is an end view as seen from the direction V in FIG. 4 after the film formation, the superconductor enters the groove 1b during the film formation process and, due to the presence of the step, the superconducting thin film is formed on the upper side thin film 2 And the thin film 3 on the lower side, which are separated by the groove 1b.
Only the superconductor that has entered the inside makes it in a state of being weakly joined, and an element having the structure shown in FIG. 1 can be obtained immediately after film formation.

It should be particularly noted that in the device obtained by this manufacturing method, the weak junction J is integrally formed with the superconducting thin films 2 and 3 during the film forming process. That is, in the conventional quasi-planar Josephson junction element, the stacked two
Although a bridge is separately formed on the surface of the two superconducting thin films to obtain a weak junction, the oxide high-temperature superconducting thin film such as YBCO generally has a so-called surface deterioration layer that does not show superconductivity on the surface and the bridge part is formed. While it is difficult to make the superconducting film on the superconducting thin film that has been previously formed in a superconducting manner, the above-mentioned manufacturing method does not have such a fear.

In addition, this manufacturing method does not require any step such as patterning after the formation of the superconducting thin films 2 and 3, and is particularly effective when using an oxide high temperature superconductor which is easily deteriorated with respect to water.

Then, in the element of the present invention, the weak junction length is determined by using the step of the substrate 1, the depth and width of the groove 1b, and the film thickness of the superconducting thin film 3 as parameters. can get.

As shown in FIG. 6, the stepped portion 1a of the substrate 1 has an overhanging shape so that the superconducting thin films 2 and 3 are separated from each other and the superconducting thin film 3 and the superconducting thin film 3 which have entered the groove 1b are separated. It is desirable because it can be more easily joined to. Such an overhang can be achieved by making an angle in the direction of irradiation of Ar ions during ion milling.

Further, as the material of the superconducting thin films 2 and 3, it is needless to say that a high temperature oxide superconductor other than YBCO or a metal-based superconductor such as Nb can be used, and the material of the substrate 1 is ZrO ₂
Any material having good compatibility with the superconductor used, such as YSZ or YSZ, can be used.

<Effects of the Invention> As described above, according to the present invention, a quasi-plane type Josephson junction can be formed by a simple process without stacking superconducting thin films and without requiring fine processing of the thin films. Even if an oxide high-temperature superconductor is used, a high-performance element can be obtained with good reproducibility and high yield.

[Brief description of the drawings]

FIG. 1 is a perspective view of an embodiment of the present invention, FIGS. 2 to 5 are explanatory views of a manufacturing method thereof, and FIG. 6 is an explanatory view of a stepped shape of a substrate of another embodiment of the present invention. 1 ... Substrate 1a ... Step 1b ... Groove 2,3 ... Superconducting thin film J ... Weak junction

Claims (1)

(57) [Claims] 1. A substrate having a step formed on the surface thereof, and a groove having one end reaching the step and being opened to the side on a plane on the upper step side, the upper step side and the lower step side sandwiching the step section. A quasi-planar type Josephson junction device in which superconducting thin films are formed on respective planes, and the upper and lower superconducting thin films are joined to each other through the superconductors that enter the grooves.