JPH08204245A - Josephson element - Google Patents

Abstract

PURPOSE: To obtain a Josephson element which can be fabricated as easily as the quasi-planar Josephson junction without locating the junction at the outermost layer while suppressing deterioration of characteristics due to aging. CONSTITUTION: Two thin films 1, 2 of superconductor are laminated through an insulation film 3 of oxide or nitride of a superconductor. A superconductor region 4, not oxidized nor nitrided, is formed partially on the insulation film 3 and the two thin films 1, 2 of superconductor are mutually bonded through the superconductor region 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Josephson element, for example, SQUID used for measuring a weak magnetic field such as biomagnetism, a millimeter wave detecting element, and any element having a Josephson junction. Applicable Josephson device.

[0002]

2. Description of the Related Art Conventionally, a Josephson junction is roughly divided into a tunnel type and a microbridge type (weak coupling type).
Two types are known. Of these, the tunnel type is the most commonly used type and has a structure in which two superconducting thin films are tunnel-joined by a thin insulating film (barrier). On the other hand, the microbridge type has a structure in which two superconducting thin films are joined by a minute bridge (weak coupling portion), and two superconducting thin films provided on a plane are joined by a bridge formed on the same plane. In addition to the planar type, two superconducting thin films are partially laminated via an insulating film, and the superconducting thin films above and below are formed in the laminated part to form a bridge across both surfaces. There is a quasi-planar type.

[0003]

By the way, of the above types of Josephson junctions, the tunnel type requires an extremely thin insulating film (barrier) of about several tens of liters, and requires advanced film forming technology. Therefore, it is said that it is difficult to make it, and it is difficult to reduce the noise because the size of the joint cannot be reduced.

On the other hand, the microbridge type is advantageous in reducing noise because the size of the joint is small in principle, but it is necessary to make the cross-sectional area and length of the bridge extremely small, so that it is on a plane. It is extremely difficult to create a joint with a flat type that must rely on the microfabrication described above.

On the other hand, the quasi-planar type among the microbridge types has a bridge length (weak link length).
Is determined by the thickness of the insulating film, and when Nb or the like is used as the superconductor thin film, the insulating film can be several hundred liters, which is easier to create than a tunnel-type insulating film, and has a thickness of that level. Since it is relatively easy to control the film thickness of the saano film, it is advantageous in that the weak link length can be shortened easily and reproducibly as compared with the planar microbridge.

However, in the quasi-planar type Josephson junction, the Josephson junction is the outermost layer of the device because it forms a bridge that extends over both of the two superconducting thin films laminated with the insulating film interposed therebetween. Therefore, there is a drawback that the bonding characteristics are likely to deteriorate with time due to the influence of the external environment.

An object of the present invention is to make a Josephson device which is relatively easy to make similarly to the quasi-planar type Josephson junction, and in which the junction does not reach the outermost layer, and thus the junction characteristics are less deteriorated with time. To provide.

[0008]

A structure for achieving the above object will be described with reference to FIG. 1 which is an embodiment drawing, and a Josephson element of the present invention has two superconductor thin films 1 and 2. Are laminated via an insulating film 3 made of oxide or nitride of a superconductor, and a superconductor region 4 which is not partially oxidized or nitrided is formed in the insulating film 3 and its insulation It is characterized in that the two superconductor thin films 1 and 2 are joined to each other by the superconductor region 4 of the membrane 3.

Here, in the present invention, both ends of the superconductor region 4 in the insulating film 3 are in direct contact with the superconductor thin films 1 and 2, respectively, and also due to the manufacturing process described later. A minute piece of a substance exhibiting the proximity effect may be interposed between one end of the superconductor region 4 and any one of the superconductor thin films 1 or 2.

[0010]

The superconductor region 4 in the insulating film 3 constitutes a microbridge that joins the two superconducting thin films 1 and 2 existing above and below it. The weak link length of this Josephson junction is determined by the thickness of the insulating film 3 similarly to the quasi-planar microbridge, and the weak link length may be about 100 to several hundred Å. The controllability and reproducibility of the film thickness are good. Since such a microbridge exists inside the two superconductor thin films 1 and 2, the microbridge is not easily affected by the external atmosphere or the like, and is not easily deteriorated with time.

[0011]

1 is a schematic sectional view showing the structure of a basic embodiment of the present invention. The lower electrode 1 is formed on the substrate 10, and the upper electrode 2 is stacked thereabove with the insulating film 3 interposed therebetween. In this example, the lower electrode 1 and the upper electrode 2
Are Nb superconducting thin films.

The insulating film 3 is an oxide of Nb superconductor, such as Nb.
₂ O ₅ , the film thickness is about 4 to 500 Å, the substantially central portion thereof is not locally oxidized, and thus this non-oxidized region is the superconductor region 4 made of Nb. The superconductor region 4 in the insulating film 3 forms a weakly coupled Josephson junction that joins the lower electrode 1 and the upper electrode 2.

According to such a structure, the superconductor region 4 forming the Josephson junction is covered with the lower and upper electrodes 1 and 2 at the upper and lower sides thereof, and with the insulating film 3 at the periphery thereof. Therefore, the characteristics are not deteriorated with time due to the influence of the atmosphere or the like.

Next, the manufacturing method of the above embodiment of the present invention will be described. 2 to 4 are explanatory views of the manufacturing procedure thereof, in which (A) is a plan view and (B) is a cross-sectional view taken along the line BB.

First, a uniform Nb superconducting thin film is formed on the substrate 10, and the lower electrode 1 is patterned by using a photolithography technique or the like. Then, as shown in FIG. A minute dot-shaped resist pattern 20 is formed by using an EB exposure device or the like at a position where a Josephson junction is desired to be formed.

Next, as shown in FIG. 3, the surface of the lower electrode 1 is anodized with the resist pattern 20 interposed. As a result, the surface of the lower electrode 1 is entirely oxidized to form the insulating film 3 made of Nb ₂ O ₅ except for the portion where the resist pattern 20 is formed, while the portion directly below the resist pattern 20 is not oxidized and exceeds Nb. It remains as a conductor and becomes the superconductor region 4 formed locally in the insulating film 3.

After removing the resist pattern 20, the Nb superconductor thin film is formed from above the insulating film 3 and patterned to form the upper electrode 2 laminated on the lower electrode 1 and the insulating film 3. . As a result, the Josephson device having the structure shown in FIG. 1 is obtained.

Here, the Josephson junction of the present invention corresponds to a kind of bridge type Josephson junction, and therefore, the bridge length needs to be shortened to several hundred Å, but according to the element structure of the present invention, the bridge is formed. The length depends on the film thickness of the insulating film 3. The insulating film 3 has a film thickness of 100Å to 10
In the range of 00Å, control can be performed easily with good reproducibility. Further, in the bridge type Josephson junction, it is necessary to reduce the bridge width (cross-sectional area) in addition to the bridge length. However, according to the above-described manufacturing method using the anodic oxidation process, the bridge pattern is covered with the resist pattern 20. The area that is not oxidized becomes smaller than the actual size of the resist pattern 20 due to the entry of oxygen from the exposed surface and the like around the area, and the size (cross-sectional area) of the superconductor area 4 is reduced by that amount, and the EB exposure is performed. A superconductor region 4 having a size smaller than the limit of forming the resist pattern 20 is obtained, and a microbridge type Josephson junction having a bridge width exceeding the limit of fine processing on a plane is obtained.

Next, the above-described embodiment of the present invention is applied to DC-
A specific example applied to the SQUID ring will be shown. FIG. 5 is a plan view (A) showing the overall structure and an enlarged view of the B portion (B).
Is.

In this example, the lower electrode 1 has a pattern in which a part of the ring is notched, and both ends of the notched ring extend to the outside of the ring to form lead portions 1a and 1b. Each lead-out portion 1 so that it extends over the two lead-out portions 1a and 1b of the lower electrode 1.
The upper electrode 2 is formed via an insulating film 3 (not shown) formed by oxidizing the surfaces of a and 1b. The lower electrode 1 and the upper electrode 2 are Nb superconductor thin films, respectively, and the insulating film 3 between them is Nb ₂ O ₅ as in the above-mentioned embodiment. Then, the superconductor region 4 having the structure shown in FIG. 1 is formed in each insulating film 3 in the portion where the upper electrode 2 and the lead-out portions 1a and 1b of the lower electrode 1 are laminated. ing. With such a structure, the lower electrode 1 and the upper electrode 2 are joined at two locations by the fine superconductor regions 4 and 4 which are locally formed in the insulating film 3 which is interposed therebetween, so that the super electrode as a whole is superposed. DC-S with two Josephson junctions in the conduction loop
A QUID ring is obtained.

Here, in the Josephson element such as the DC-SQUID ring, the junction capacitance of the element sometimes poses a problem. According to the structure of the Josephson device of the present invention, the area of the superconductor region 4 which is the Josephson junction can be made considerably small, and the distance between the lower and upper electrodes 1 and 2 can be 5
Since it can be made considerably thicker (about several hundred liters to about 1,000 liters) than that of 0 liters, there is also an advantage that the junction capacitance can be made extremely small.

In the above-mentioned manufacturing method, the anodic oxidation method is used to form the insulating film 3, but in addition, thermal oxidation or UV-
A method such as O ₃ oxidation or plasma oxidation can also be used. The insulating film is not limited to the oxide of a superconductor, and may be a nitride when a superconductor thin film that is insulated by nitriding is used. In the case of a nitride, the insulating film is formed by plasma nitriding. can do. Further, the superconductor thin film is not limited to Nb, and it goes without saying that any superconductor thin film that is insulated by oxidation or nitridation can be used.

Further, when the insulating film 3 is formed, the non-oxidized region is drastically reduced due to oxygen flowing from around the resist pattern 20, and the superconductor region 4 connecting the lower and upper electrodes 1 and 2 is formed. When it is difficult to form the oxides, the oxidation process may be performed by using, in addition to the resist pattern 20, a fine pattern of a substance that exhibits a proximity effect, such as Au, as a mask. That is, in the above-described manufacturing process, after patterning the lower electrode 1, Au is uniformly formed on the lower electrode 1.
A thin film of a substance that exhibits a proximity effect, such as a thin film, is formed. Then, a resist pattern 20 equivalent to that shown in FIG. 2 is formed thereon, and an Au thin film is patterned by sputter etching or the like using this as a mask to form a minute Au pattern on the lower electrode 1. And that small Au
By oxidizing the surface of the lower electrode 1 by anodic oxidation or the like with the pattern interposed, or with the resist pattern 20 further interposed thereon, the lower electrode 1
An insulating film 3 having a locally non-oxidized region, that is, a superconductor region 4 is formed on the surface of the. Then, the upper electrode 2 on top of it
When forming the resist pattern 20, if the resist pattern 20 remains, it is removed. For the minute Au pattern, this may be removed, but without removing this, the upper electrode 2 may be formed on the resist pattern 20. Good. When the fine Au pattern is not removed, as shown in the schematic cross-sectional view of FIG. 6, the lower electrode 1 and the upper electrode 2 have a structure in which they are connected via the superconductor region 4 and the fine Au pattern 60. Since Au exerts a proximity effect on Nb superconductors,
The lower and upper electrodes 1 and 2 are weakly coupled to each other via the superconductor region 4 and the minute Au pattern 60, and the same effect as that of the above-described embodiment can be obtained.

[0024]

As described above, according to the present invention,
Two superconductor thin films are laminated via an insulating film made of oxide or nitride of the superconductor thin film, and the two superconductor thin films are joined by a superconductor region locally provided on the insulating film. Since the micro-bridge type Josephson junction is formed by the above, the bridge can be isolated from the outside by the superconducting thin film and the insulating film above and below, and the influence of the outside can be obtained like a quasi-plane type bridge. This leads to an advantage that the junction characteristics are not deteriorated with time and the degree of freedom of the subsequent process can be increased particularly when a device such as SQUID is manufactured.

The bridge length of the superconductor region formed in the insulating film is determined by the thickness of the insulating film, and the bridge length required when Nb or the like is used as the superconductor thin film. Is a few hundred Å, which is a film thickness that can be created with good reproducibility under good controllability, so it is possible to obtain a high-precision Josephson element with the desired bridge length with good reproducibility. Compared to the junction-type Josephson junction, it is easier to fabricate the element, and the junction capacitance can be reduced by increasing the thickness of the insulating film. In this respect, application to devices such as SQUID is also possible. This is a great advantage.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing the configuration of a basic embodiment of the present invention.

2A and 2B are explanatory views of the procedure of the manufacturing method, in which FIG. 2A is a plan view and FIG.

FIG. 3 is a plan view (A) and a cross-sectional view (B) taken along line BB of FIG.
Illustration of the procedure of the manufacturing method of the basic embodiment of the present invention shown in

FIG. 4 is a plan view (A) and a cross-sectional view (B) taken along line BB of FIG.
Step explanatory drawing of the manufacturing method of the basic embodiment of the present invention shown in

FIG. 5 is a plan view (A) showing an overall configuration of an embodiment in which the present invention is applied to a DC-SQUID ring and an enlarged view of a portion B thereof (B).

FIG. 6 is a schematic cross-sectional view showing the configuration of another embodiment of the present invention.

[Explanation of symbols]

1 Lower electrode (Nb superconductor thin film) 2 Upper electrode (Nb superconductor thin film) 3 Insulating film (Nb ₂ O ₅ ) 4 Superconductor region (Nb) 10 Substrate 20 Resist pattern 60 Small Au pattern

Claims (1)

[Claims] 1. A superconducting thin film, wherein two superconducting thin films are laminated via an insulating film made of an oxide or a nitride of the superconducting thin film, and the insulating film is not partially oxidized or nitrided. A Josephson device in which a body region is formed, and the two superconductor thin films are joined to each other by the superconductor region of the insulating film.