JPH02192774A - Quasi-planar type josephson junction element - Google Patents

Abstract

PURPOSE:To obtain an excellent Josephson element by using oxide high temperature superconductor by a method wherein a superconducting thin film of almost uniform thickness is formed on the surface of a substrate, on which a step- difference is formed, so as to interpose the step-difference and to be positioned on both sides thereof, and both of the film surfaces are joined by a bridge of superconductor. CONSTITUTION:On the surface of an MgO substrate 1, a step-difference is formed. High temperature superconducting films 2, 3 made of YBCO are formed on the surface of the upper step side and the lower step side, respectively, so as to interpose the step part 1a. The surfaces of the thin films 2, 3 are joined by a bridge 4 made of YBCO. This manufacturing method is as follows. Resist is spread uniformly on a substrate; one side half thereof is eliminated by photolithography; Ar ion milling is performed by using the left resist as a mask, thereby manufacturing the substrate 1; a YBCO thin film is uniformly sputtered on the substrate; thereon negative resist is spread uniformly, and eliminated by exposure and development so as to leave the bridge 4 part; by using the remaining resist film as a mask, the YBCO thin film is etched, thereby obtaining the bridge 4 coming into contact with the thin films 2, 3.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an element having a Josephson junction, and more particularly to a quasi-planar Josephson junction element.

In addition, the present invention includes 5QUID for ultra-high sensitivity magnetic field measurement,
It can be applied to all devices that utilize the Josephson effect, such as ultra-sensitive electromagnetic wave detectors.

<Conventional technology> In order to obtain a Josephson junction with good performance, the bridge length (length of the weak junction) should ideally be about 3 to 5 times the coherent length of the bridging material used. It is also known that even if this ideal bridge length cannot be obtained, the bridge length should be shortened so as to get as close to it as possible.

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

In a quasi-planar Josephson junction, another superconducting thin film is layered on top of a superconducting thin film formed on a flat substrate surface, with an insulating layer interposed between the two superconducting thin films. The bridge length can be determined by the film thickness dimension, which can be controlled relatively easily, which is extremely advantageous compared to the planar Josephson junction, which is suitable for microfabrication on a plane. be.

<Problem to be solved by the invention> By the way, when making a Josephson junction element using an oxide high-temperature superconductor represented by YBCO, for the following reasons, when using a conventional superconductor such as Nb-based superconductor, It is difficult to obtain a good quasi-planar Josephson junction like this.

First, high-temperature superconducting thin films such as YBCO are generally unstable and easily diffused, and it is not easy to stack them in good condition.

Furthermore, high-temperature superconducting thin films obtained using current film-forming techniques have a surface flatness that is approximately 100% poor, and pin contact is likely to occur when they are stacked on each other with an insulating layer interposed between them.

Furthermore, oxide high-temperature superconductors are subject to processing limitations such as being susceptible to deterioration due to water, and processes cannot be selected relatively freely as in the case of Nb-based superconductors.

The present invention was made in view of these points, and aims to provide a quasi-planar Josephson junction element that has high performance even when using an oxide high-temperature superconductor and has good manufacturing reproducibility. There is.

Means for Solving the Problems> In order to achieve the above object, in the present invention, as shown in FIG. A superconducting thin film 2.3 having a substantially uniform thickness is formed on both sides of the superconducting thin film 2.3, and the superconducting thin films 2 and 3 are formed by a bridge 4 of a superconductor or a normal conductor formed across the surfaces of both films. It is joined.

Function> A quasi-planar Josephson junction with substantially the same structure as the conventional one is obtained by providing a drop between the surfaces of superconductor 4-thin 1ff2 and 3 without stacking them with an insulating layer interposed therebetween. The bridge length is determined by the difference between the step dimension of the substrate 1 and the film thickness dimension of the superconducting thin film 3 on the lower stage side, and is similar to the conventional method in that it does not depend on the accuracy of microfabrication on a plane.

<Example> FIG. 1 is a perspective view of an embodiment of the present invention.

A step is provided on the surface of the MgO substrate 1, and YB is formed on the upper and lower surfaces with the step 1a in between.
High temperature superconducting thin films 2 and 3 made of CO are formed.

The film thicknesses of the high-temperature superconducting thin films 2 and 3 are approximately equal to each other, and therefore the surfaces of the high-temperature superconducting thin films 2 and 3 have a height difference that is approximately equal to the step of the substrate 1.

4, these high temperature superconducting thin films 2 and 3 are joined by a YBCO bridge 4 formed across both of these surfaces.

In this example, the height difference of the substrate 1 is 500 r+m, the film thickness of the high temperature superconducting thin films 2 and 3 is 300 nm, and the bridge 4 is in complete superconducting contact with the end face of the high temperature superconducting thin film 2 and the surface of the high temperature superconducting thin film 3. and
This results in a bridge length of 200 nm.

The manufacturing method of this example will be explained below. FIGS. 2 to 9 are explanatory diagrams of the manufacturing procedure.

First, as shown in Fig. 2, a flat plate-shaped Mg of an appropriate thickness is
An O(100) substrate 10 was prepared, a resist was uniformly applied to its surface, and then one side was left and the other side was removed by photolithography with the approximate center as a boundary (Figure 3).
.

Next, using the remaining resist film 5 as a mask, the surface of the substrate 100 is etched by 500 nm by Ar ion milling (Fig. 4), and by removing the remaining resist film 5, a stepped portion 1a as shown in Fig. 5 is formed. A substrate 1 was obtained.

Then, on the surface of the substrate 1 having a step of 500 nm,
As shown in FIG. 6 (al), a YBCo thin film 20 was formed by sputtering as shown in FIG.
60, the pressure was 20 m Torr, the composition of the target was Y:Ba:Cu=1:6.7:18.3, and the Y film thickness was 5QQnm at a rate of 30 people/min.
A BCQ thin film 20 was obtained. At this time, the obtained YBCo thin film 20 was thinner in the upper part of the stepped part 1a of the substrate 1 than in other parts, as shown in an enlarged view of the main part in FIG.

Next, as shown in FIG. 7, a negative resist 60 (for example, CMS-E manufactured by Toyo Soda Industries Ltd.
After applying X-(R)) in a -like manner, electron beam exposure and development using an organic solvent resulted in a 1.3 μm thick film, as shown in the side view in FIG. 8(a) and the top view in FIG. 8(b). Step 1 in width
A resist film 6 having a strip-shaped pattern was formed to straddle above a.

Then, using this resist film 6 as a mask, the surface of the YBCo thin film 20 is approximately 200 nm thick by Ar ion milling.
m-etched. As a result, as shown in Fig. 9,
The YBCO thin film 20 was separated into an upper thin film 2 and a lower thin film 3 except for the portion protected by the resist film 6 in the thin portion above the stepped portion 1a. The thickness of the thin films 2 and 3 is about 300 nm each, but only the part protected by the resist film 6 remains with a film thickness of 500 nm, and this part has a width of 1 m and a thickness of about 200 nm. The bridge 4 is in contact with the thin film 2.3. Finally, the resist film 6 was removed to obtain an element having the structure shown in FIG. Note that even if some YBCO thin film remains in the step portion 1a other than the bridge, it is effective as long as the film does not deteriorate and exhibit superconductivity.

What is particularly noteworthy about the device obtained by this manufacturing method is that the bridge 4 is not formed separately on the high-temperature superconducting thin films 2 and 3, but is formed integrally with YB.
The point is that the upper and lower high temperature superconducting thin films 2 and 3 and the bridge 4 are obtained by ion milling the CO thin film 20. Thereby, the bridge 4 is reliably brought into superconducting contact with the end face of the upper high temperature superconducting thin film 2 and the surface of the lower high temperature superconducting thin film 3. In high-temperature superconducting thin films made of oxides such as YBCO, a layer that does not exhibit superconductivity (surface deterioration layer) generally occurs on its surface, and when the bridge 4 is laminated later by film formation, it contacts the thin films 2 and 3 in a superconducting manner. Although this is difficult, this problem can be solved by the above-described manufacturing method.

This manufacturing method also has the advantage that no water is used at all, and when an oxide superconductor is used, deterioration during the manufacturing process is less likely to occur.

Furthermore, in this manufacturing method, the thin film 2.3 and the bridge 4 are formed from an integrated thin film by ion milling, and
In this element structure, the bridge length is determined by the difference between the step difference in the substrate 10 and the thickness of the lower thin film 3, so the bridge length can be adjusted by adjusting the depth of ion milling. Compared to the fact that it is impossible to adjust the bridge length at the time of bridge formation,
This is also advantageous in terms of uniformity of device yield and performance.

In the above embodiments, the materials of the superconducting thin films 2 and 3 and the bridge 4 are not limited to YBCO, but other oxide high-temperature superconductors or metal-based superconductors such as Nb can be used. Also, regarding the material of the substrate 1, M
In addition to gO, any material having good compatibility with the superconductor used can be used, such as ZrO or ysz.

The structure of the present invention can also be applied to a case where the material of the fillet nozzle 4 is not the same as the material of the superconducting thin films 2 and 3. Of course, in this case, the above manufacturing method cannot be adopted, but below we will explain an example of a method for manufacturing a quasi-planar Josephson junction element with a so-called SNS structure in which the bridge is formed of a normal conductor such as Au. . FIG. 10 to FIG. 13 are explanatory diagrams thereof.

First, in exactly the same manner as in the previous manufacturing method, a YBCO thin film 20 is uniformly formed on the upper surface of the substrate 1 having a step as shown in FIG.

Next, the YBCOi film 20 is subjected to Ar ion milling and separated into upper and lower thin films 2 and 3 as shown in FIG. Then, as shown in FIG. 11, after applying a positive resist 70 on the upper surface, a first resist 70 is applied by exposure and development.
As shown in FIG. 2(a) as a central vertical cross-sectional view and as shown in FIG.

Next, Au is deposited from above the resist film 7, and -
A similar Au film 0 is formed. Finally, by removing the excess Au film together with the resist film 7 by a lift-off method, the Au bridge 4 remains in the window 7a.

In this manufacturing method, in order to obtain the structure shown in FIG. 10, a YBCO thin film thinner than the step of the substrate 1 is formed from the state shown in FIG. 5 without using ion milling. You can also get In this case, as shown in FIG. 14, by slightly overhanging the stepped portion 1a of the substrate 1, the thin films 2 and 3 can be easily separated.

In addition to etching the substrate 1 as a method for forming a step on the substrate 1 in the manufacturing method of each of the embodiments described above, a substrate material may be epitaxially grown on the upper surface of a flat substrate.

<Effects of the Invention> As explained above, according to the present invention, a quasi-planar Josephson junction can be obtained without stacking superconducting thin films, making it possible to reproduce high-performance devices even using oxide high-temperature superconductors. You can get it easily.

Further, since only one layer of the superconducting thin film needs to be formed, there is an advantage that the manufacturing process is simplified.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of an embodiment of the present invention; FIGS. 2 to 9 are explanatory diagrams of a manufacturing method thereof; FIGS. 10 to 13 are explanatory diagrams of a manufacturing method of another embodiment of the present invention; FIG. 14 is an explanatory diagram of the step shape of a substrate according to still another embodiment of the present invention.・Substrates, steps, superconducting thin films, bridges

Claims (1)

[Claims] A superconducting thin film with a substantially uniform thickness is formed on both sides of the step on the surface of a substrate provided with a step, and the superconducting thin film on both sides of the step is formed across the surfaces of both films. A quasi-planar Josephson junction device that is connected by a bridge of superconductors or normal conductors.