JPH07221354A - Josephson junction element and its manufacture - Google Patents

Abstract

PURPOSE:To reduce the fluctuation of the characteristic of a Josephson junction element so as to uniformize and stabilize the characteristic. CONSTITUTION:A YBCO single-crystal thin film (YBa2Cu3Oy ceramic superconducting single-crystal thin film) having a prescribed thickness d1 is deposited on a substrate composed of an MgO single crystal and a pair of left and right electrode sections 2a and 2b is formed of the YBCO thin film. Both sections 2a and 2b are connected to each other through a bridge section 3 composed of a YBCO single-crystal thin film having a prescribed thickness d2. At the boundary sections between the electrode sections 2a and 2b and bridge section 3, both single-crystal thin films are weakly coupled with each other and the critical current of this Josephson junction element is controlled in accordance with the cross-sectional areas of the boundary sections which vary depending upon the width W and thickness d2 of the bridge section 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Josephson junction device having a planar structure, which is a so-called composite Josephson junction device manufactured by using a ceramic superconducting thin film (oxide superconducting thin film) and the same. The present invention relates to a manufacturing method.

[0002]

2. Description of the Related Art With the progress of thin film forming technology in recent years, attempts have been made to form various sensors by forming a thin film of a superconducting ceramic on a substrate, and in particular, in order to realize a highly sensitive magnetic sensor. Needs to create a Josephson junction in the superconducting thin film.

Conventionally, as this type of Josephson junction, there is a polycrystalline Josephson junction utilizing naturally occurring crystal grain boundaries, and as a manufacturing method thereof, there is a method of recrystallizing an amorphous thin film by annealing or superconductivity Attempts have been made to form a crystal thin film by CVD or the like.

[0004]

However, when the above-mentioned polycrystalline Josephson junction is formed, since the crystal grain boundaries that are accidentally formed are used, the characteristics caused by the crystal grain boundaries are caused. There was a problem that there was a variation in. As a result, not only is it difficult to control the critical current of the Josephson junction device, but there is also the problem that a homogeneous and stable Josephson junction device cannot be manufactured.

The present invention has been made in view of the above problems, and an object of the invention is to reduce variations in Josephson junction characteristics with a novel structure to achieve homogenization and stabilization. It is an object of the present invention to provide a Josephson junction element capable of manufacturing and a manufacturing method thereof.

[0006]

In order to achieve the above object, the invention of a Josephson junction element described in claim 1 is a ceramic superconducting Josephson junction element having a planar structure, in which a superconducting single crystal thin film is used. The gist of the invention is that it is composed of a pair of formed electrode portions and a bridge portion which is also formed of a superconducting single crystal thin film and joins the both electrode portions in a bridge shape.

The invention of the Josephson junction element described in claim 2 is the Josephson junction element according to claim 1, wherein an interface portion between the superconducting single crystal thin film of the electrode portion and the superconducting single crystal thin film of the bridge portion is provided. Has a cross-sectional area of 0.5-2.
It is configured to be 5 μm ² .

The invention of the Josephson junction device described in claim 3 is the Josephson junction device according to claim 2, wherein the film thickness of the superconducting single crystal thin film forming the electrode part forms the bridge part. It is configured to be twice or more the thickness of the superconducting single crystal thin film.

According to a fourth aspect of the present invention, there is provided a method for manufacturing a Josephson junction device, wherein a ceramic superconducting single crystal thin film is deposited on a substrate and a predetermined region of the ceramic superconducting single crystal thin film is removed to remove a pair of electrodes. And forming a bridge portion that joins the two electrode portions in a bridge shape by forming a ceramic superconducting single crystal thin film on the substrate.

[0010]

According to the invention described in claims 1 to 3, the bonding is achieved by utilizing the crystal grain boundary at the interface between the superconducting single crystal thin film of the electrode portion and the superconducting single crystal thin film of the bridge portion as the weakly coupled portion. Josephson current flows through. In this case, the bridge portion is made of a superconducting single crystal thin film similarly to the electrode portion, so that variations in Josephson junction characteristics due to grain boundaries are reduced.

According to the invention described in claim 4, the ceramic superconducting single crystal thin film is deposited on the substrate, and
A pair of electrode portions is formed by removing a predetermined region of the ceramic superconducting single crystal thin film. Then, by forming a ceramic superconducting single crystal thin film on the substrate, both electrode parts are joined in a bridge type. As a result, claim 1
Josephson junction device is manufactured.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIGS. 1A and 1B are a plan view and A showing a composite Josephson junction element according to this embodiment.
FIG. In FIGS. 1A and 1B, Mg
A pair of left and right electrode portions 2a and 2b made of a ceramic superconducting single crystal thin film YBa ₂ Cu ₃ O _y (hereinafter abbreviated as YBCO single crystal thin film) are formed on a substrate 1 made of O single crystal so as to face each other. There is. Also, the electrode portion 2 on the substrate 1
Between the electrodes a and 2b, the same YBC as the electrodes 2a and 2b is provided.
The bridge portion 3 formed of the O single crystal thin film is provided, and the interface portion between the YBCO single crystal thin film of the electrode portions 2a and 2b and the YBCO single crystal thin film of the bridge portion 3 is weakly coupled. In FIG. 1, d ₁ is the film thickness of the electrode portions 2a and 2b,
d ₂ is the film thickness of the bridge portion 3 (the same is the film thickness of the YBCO single crystal thin film on the electrode portion), and W is the bridge width. Then, in the same element, the Josephson current flows through the crystal grain boundary formed at the interface portion as a weak coupling portion.

A method of manufacturing the Josephson junction element will be described below with reference to FIGS. In each figure,
(A) is a plan view of the device, and (b) is a cross-sectional view taken along the line AA of the device.

First, as shown in FIGS. 2A and 2B, a YBCO single crystal thin film 20 is deposited on the entire surface of the substrate (MgO single crystal) 1 with a uniform film thickness d ₁ (for example, d ₁ = 500 nm). . As a method of forming a thin film, a YBCO sintered body is
An RF magnetron sputtering method in which r ions are sputtered and deposited on the substrate 1 is used, and the conditions are shown below.

[Conditions for producing single crystal thin film] Target: YBa ₂ Cu _3.5 O _y sintered body Substrate temperature: 720 ° C. Ar flow rate: 20 sccm O ₂ flow rate: 20 sccm Sputtering pressure: 0.15 Torr Sputtering power: 150 W Distance between target substrates: 80 mm Deposition rate: 0.6 nm / min Under the above conditions, the sputtering pressure is the normal pressure (7.
The value is set to a value higher than 5 mTorr), and as a result, it is possible to manufacture a single crystal thin film having a high quality and a thin film thickness.

After that, as shown in FIGS. 3 (a) and 3 (b),
The YBCO single crystal thin film 20 is divided into two parts to form a pair of left and right electrode parts 2a and 2b. That is, the electrode part 2
a and 2b are formed in a substantially triangular shape and their tops face each other. Alternatively, both electrode portions 2a and 2b may be formed separately by grooves having a constant interval. The electrode portions 2a and 2b are formed by using a photolithography method and a dry etching method using an Ar ion beam. The processing conditions are shown below.

[Fine processing conditions by photolithography method] Photoresist: S1400-25 resist manufactured by Shipley Co., Ltd. Resist application: Spin coater 4000 rpm Prebaking: 90 ° C.-15 min Exposure: UV-10 sec Developer: MF-312 Afterbaking manufactured by Shipley : 140 ° C.-15 min [Ar ion beam etching] Beam angle: 45 ° Ar pressure: 4 × 10 ⁻⁵ Torr Accelerating voltage: 550 V Etching rate: 15 nm / min Then, as shown in FIGS. 4 (a) and 4 (b). , The YBCO single crystal thin film 30 is again formed on the entire surface of the substrate 1 to have a uniform film thickness d.
₂ (eg, d ₂ = 100 nm) is deposited. This thin film is formed by using the RF magnetron sputtering method under the same conditions as in the production of the YBCO single crystal thin film 20.

Then, as shown in FIGS. 5 (a) and 5 (b),
The YBCO single crystal thin film 30 other than the necessary portion is removed by using the photolithography method and the dry etching method, and the bridge portion 3 made of the YBCO single crystal thin film 30 is formed between the top portions of the left and right triangular electrode portions 2a and 2b. . The bridge portion 3 is formed under the same conditions as when forming the electrode portions 2a and 2b. At this time, YB of the electrode portions 2a and 2b
At the interface between the CO single crystal thin film 20 and the YBCO single crystal thin film 30 of the bridge portion 3, the single crystal thin films are weakly bonded to each other. Then, the critical current of the element is controlled according to the cross-sectional area (= d ₂ × W) of the interface portion which is determined by the bridge width W (for example, W = 10 μm) and the film thickness d ₂ of the bridge portion 3. .

Next, the evaluation results of the characteristics of the Josephson junction device manufactured as described above will be described with reference to the following table.

[0020]

[Table 1]

Table 1 shows the film thickness d ₁ of the electrode portions 2a and 2b as 5
00 nm, the film thickness d ₂ of the bridge portion 3 is ₂₀ to 300 nm
(When the bridge width W = 10 μm, the cross-sectional area of the interface =
It shows the evaluation result of the Josephson junction when 0.2 to 3.0 μm ² ). In Table 1, the Shapiro step is a constant voltage step obtained when a Josephson junction is irradiated with microwaves of a predetermined frequency,
In this evaluation, the quality of the Josephson junction was judged by confirming the presence or absence of the Shapiro step.

According to Table 1, the cross-sectional area of the interface is 0.5 to
The Shapiro step was confirmed for all the devices of 2.5 μm ² (d ₂ = 50 to 250 nm), and good judgment as a Josephson junction device was obtained. On the other hand, regarding the elements having the cross-sectional areas of 0.2 μm ² (d ₂ = 20 nm) and 3.0 μm ² (d ₂ = 300 nm),
I could not confirm the Shapiro step. or,
According to the results in Table 1, it was confirmed that the critical current changes depending on the film thickness d ₂ of the bridge portion 3. Then, in the device having a cross-sectional area of 0.2 μm ² , the critical current becomes approximately 0, and in the device having a cross-sectional area of 3.0 μm ² , the critical current greatly exceeds the limit value as the Josephson junction device.

Table 1 shows the evaluation results when the bridge width W = 10 μm. However, it is of course possible to change the bridge width W, and the cross-sectional area of the junction is 0.5. It was confirmed that a good evaluation as a Josephson junction element can always be obtained even if the bridge width W is not specified within a range of up to 2.5 μm ² . Also, the film thickness d ₁ is 5
Even when set to 00 nm or more, the cross-sectional area = 0.5 to
It was confirmed that the same evaluation could be obtained with 2.5 μm ² .

On the other hand, the following Table 2 shows the evaluation results of the Josephson junction when the film thickness d ₁ of the electrode portions 2a and 2b is 250 nm.

[0025]

[Table 2]

According to Table 2, the cross-sectional area of the interface is 0.5 to
Shapiro step was confirmed for the device of 1.0 μm ² (d ₂ = 50 to 100 nm), and a good evaluation result as a Josephson junction device was obtained. In contrast,
The cross-sectional area is 0.2 μm ² (d ₂ = 20 nm) and 2.0 to
With respect to the device having a thickness of 3.0 μm ² (d ₂ = 200 to 300 nm), there was no Shapiro step or the presence / absence of the Shapiro step was uncertain, and a good evaluation could not be obtained.

For a device having a cross-sectional area of 2.0 to 2.5 μm ² , when d ₁ = 500 nm (Table 1)
Although a good evaluation was obtained for d ₁ = 250n
When it was m, a good evaluation could not be obtained. That is,
In the case of Table _{2, d 1: d 2 = 250} : 100 Shapiro step in the _{presence, d 1: d 2 = 250} : no 100 Shapiro step in. In this way, the electrode parts 2a, 2
It was found that when the film thickness d _{1 of} b is smaller than twice the film thickness d ₂ of the bridge portion 3 (d ₁ <2d ₂ ), a good evaluation as a Josephson junction device cannot be obtained. That is, as shown in FIG. 6, when the film thickness d ₁ of the electrode portions 2a and 2b is reduced, the YBCO single crystal thin film 30 ′ and the YBCO single crystal thin film 30 ″ of the bridge portion 3 deposited on the electrode portions 2a and 2b are separated from each other. They are continuously connected, and this continuous portion causes an excessive increase in the critical current.

As described in detail above, in the Josephson junction device of this embodiment, the crystal grain boundary formed at the interface between the electrode portions 2a and 2b and the bridge portion 3 is used as the weakly coupled portion, The desired Josephson current can flow through the Josephson junction. In this case, the critical current of the device can be controlled to an appropriate value for the Josephson junction device by adjusting the film thickness d ₂ of the bridge portion 3. Further, unlike the conventional case, since the naturally occurring crystal grain boundaries are not used, the variation of the crystal grain boundaries in the bridge portion 3 can be reduced, and a uniform and stable Josephson junction element is manufactured and provided. be able to. In addition, magnetic noise and electrical noise can be reduced as the crystal grain boundaries are stabilized.

Furthermore, the cross-sectional area of the interface is 0.5 to 2.5.
μm ² and the film thickness d of the electrode portions 2a and 2b
By satisfying both the condition that ₁ is at least twice the film thickness d ₂ of the bridge portion 3 and regardless of the film thickness d ₁ of the electrode portions 2a and 2b and the bridge width W, a good Josephson characteristic is obtained. be able to.

The present invention is not limited to the above embodiment, but can be embodied in the following modes. In the Josephson junction device of the above embodiment, the same YBCO as the bridge portion 3 is provided on the upper surfaces of the electrode portions 2a and 2b as shown in FIG.
Although the single crystal thin film is deposited, the YBCO single crystal thin film on the upper surfaces of the electrode portions 2a and 2b may be removed as shown in FIG. In this case, even if the film thickness d ₁ of the electrode portions 2a and 2b is thin, the YBCO single crystal thin film on the upper surfaces of the electrode portions 2a and 2b and the YBCO single crystal thin film on the bridge portion 3 are not continuously connected. Therefore, regardless of the condition (d ₁ ≧ 2d ₂ ) that regulates the film thickness d ₁ of the electrode portions 2a and 2b, it is always good if the cross-sectional area of the interface portion is within the range of 0.5 to 2.5 μm ^2. Josephson junction characteristics can be obtained.

In the above embodiment, the YBCO single crystal thin film 2
The RF magnetron sputtering method was used as a manufacturing method for 0 and 30, and this was used in vacuum deposition method, ion cluster (ICB) method, ion plating method, chemical vapor deposition (C
It is also possible to change to the VD) method. Further, in the above embodiment, the dry etching method using Ar ion beam was used as the method for forming the electrodes 2a and 2b and the bridge portion 3, but it may be changed to the wet etching method.

In the above embodiment, the MgO single bond is used as the substrate 1.
Crystal was used, but SrTiO₃A single crystal or the like may be used.
Also, a YBCO single crystal thin film is used as a ceramic superconductor.
Then, a Josephson junction device was manufactured._XSimple
Layered structure, CuO_X-CuO _yComposite layered structure, AO_X−
CuO_yThe following ceramic superconductor with composite layered structure
You can also use the body to make Josephson junction devices
It [CuO_XSimple layered structure] (La, M)₂CuO_Four M = Ba, Sr, Ca, K LaCuO_Four (Ln, M)₂CuO_Four Ln = Nd, Pr, Sm M
= Ce, Th (Nd, Sr, Ce)₂CuO_Four [CuO_X-CuO_yComposite Layered Structure] Ba₂LnCu₃O₇ Ln = Y and rare earth Ba₂LnCu_FourO₈ Ba₂Ln₂Cu₇O₁₅ (Ln, M)₂(Ln, M¹)₂Cu₃O_y Ln = Nd, Sm, Eu M = Ce M¹= Ba [AO_X-CuO_yComposite Layered Structure] Bi₂Sr₂Ca_n-1Cu_nO_{2n + 4} n = 1 to 5 Tl₁Ba₂Ca_n-1Cu_nO_{2n + 3} n = 1 to 5 Tl₁Sr₂Ca_n-1Cu_nO_{2n + 3} n = 2,3 Pb₂Sr₂(Ln, Ca, Sr) Cu₃O₈ Ln =
Y and rare earth (Tl, Pb) Sr₂(Ln, Ca) Cu₂O_y

[0033]

According to the present invention, it is possible to reduce variations in the Josephson junction characteristics with a novel structure, and to achieve the homogenization and stabilization.

[Brief description of drawings]

FIG. 1 shows a Josephson junction device according to an embodiment,
(A) is a top view and (b) is an AA sectional view.

2A and 2B are views for explaining the method of manufacturing the Josephson junction element in the example, where FIG. 2A is a plan view and FIG.
FIG. 6 is a sectional view taken along line AA.

3A and 3B are views for explaining the method for manufacturing the Josephson junction element in the example, where FIG. 3A is a plan view and FIG.
FIG. 6 is a sectional view taken along line AA.

4A and 4B are views for explaining the method for manufacturing the Josephson junction element in the example, where FIG. 4A is a plan view and FIG.
FIG. 6 is a sectional view taken along line AA.

5A and 5B are views for explaining the method for manufacturing the Josephson junction element in the example, where FIG. 5A is a plan view and FIG.
FIG. 6 is a sectional view taken along line AA.

FIG. 6 is a cross-sectional view showing a state in which the YBCO single crystal thin film on the upper surface of the electrode portion and the YBCO single crystal thin film on the bridge portion are continuously connected.

FIG. 7 is a cross-sectional view of a Josephson junction element according to another embodiment.

[Explanation of symbols]

1 ... Substrate, 2a, 2b ... Electrode part, 3 ... Bridge part, 20
... YBCO single crystal thin film, 30 ... YBCO single crystal thin film.

Claims (4)

[Claims] 1. A ceramic superconducting Josephson junction device having a planar structure, wherein a pair of electrode portions formed of a superconducting single crystal thin film and both electrode portions formed of the same superconducting single crystal thin film are formed into a bridge type. A Josephson junction device comprising a bridge portion to be joined. 2. The cross-sectional area of the interface between the superconducting single crystal thin film of the electrode portion and the superconducting single crystal thin film of the bridge portion is 0.
The Josephson junction element according to claim 1, which has a thickness of ⁵ to 2.5 μm ² . 3. The Josephson junction device according to claim 2, wherein the film thickness of the superconducting single crystal thin film forming the electrode portion is at least twice the film thickness of the superconducting single crystal thin film forming the bridge portion. 4. A ceramic superconducting single crystal thin film is deposited on a substrate, and a predetermined region of the ceramic superconducting single crystal thin film is removed to form a pair of electrode portions, and then a ceramic superconducting single crystal is formed on the substrate. A method for manufacturing a Josephson junction element, characterized in that a bridge portion for joining the two electrode portions in a bridge shape is formed by forming a thin film.