JP2831918B2 - Superconducting element manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a superconducting element using an oxide superconducting thin film, and more particularly to a method for manufacturing a superconducting element having a Josephson junction formed on an end face of the thin film.

[0002]

2. Description of the Related Art Conventionally, as a superconducting element, a tunnel-type Josephson junction element having a laminated structure using a metal superconductor such as Pb or Nb and sandwiching a thin insulating layer capable of tunneling a superconducting pair has been used. Are known. Such a conventional tunnel type Josephson device needs to operate at an extremely low temperature close to the temperature of liquid helium. In addition, there is a problem that the circuit configuration becomes complicated due to the current-voltage characteristic having a hysteresis peculiar to the tunnel-type Josephson junction, and it has not been widely used.

On the other hand, as a Josephson junction element using a metal superconductor and having no hysteresis characteristics, a so-called bridge type junction in which a main electrode made of a metal superconductor is connected by a thin metal laminated thereon has been developed. Is underway. However, such a bridge-type junction requires operation at an extremely low temperature close to the liquid helium temperature, as in the case of the tunnel-type junction described above, and has a low resistance of the bridge portion and a superconductivity of the metal superconductor. Since the gap itself was small, it was difficult to obtain a large output voltage.

Under these circumstances, an oxide superconducting material which exhibits superconducting properties at a temperature higher than the temperature of liquid nitrogen has been discovered and has attracted much attention. Oxide superconductors have superconducting gaps that are about an order of magnitude larger than conventional metal superconductors, and have the potential to be laminated with oxide materials that exhibit a wide range of properties from metallic conduction to insulators. doing. Using such an oxide superconductor, a tunnel Josephson junction or an SNS having a large output voltage and no hysteresis (superconductor / normal conductor / superconductor)
If it becomes possible to produce a mold junction, at least there is no need for cryogenic operation and a large output voltage can be obtained, as compared with the above-described Josephson junction using a conventional metal superconductor. Application is expected.

However, the oxide superconductor has a large anisotropy in its characteristics. In a laminated structure in which the c-axis direction having a short coherence length is perpendicular to the substrate, a practically sufficient superconducting current can flow through the laminated interface. Have the essential problem of not being able to. On the other hand, in a laminated structure having the c-axis of the oxide superconductor parallel to the substrate and an interface parallel to the substrate, the oxide superconductor has a characteristic that the thermal expansion coefficient in the c-axis direction which is unique to the oxide superconductor is abnormally large, so that it is manufactured at high temperature. At the time of cooling after the film, there is a problem that a large strain is generated between the substrate and the substrate crystal, and the oxide superconductor thin film is easily cracked. In order to prevent this, if a twin structure film in which the c-axis is dispersed in a plurality of crystal orientations in the plane of the substrate is used, the effective London penetration distance increases, and unless an extremely fine junction is produced. However, there is a problem that good characteristics cannot be obtained.

As a means for solving the above-mentioned problem, an oxide superconductor thin film formed so that the c-axis is perpendicular to the substrate surface is etched so as to form an angle with the substrate surface. A method of forming a bond on the end face is being studied. Such a joint is called an edge joint,
It is expected that there is a possibility that the problem of anisotropy of the oxide superconductor can be avoided.

[0007]

However, in the above-described edge junction using the conventional oxide superconductor thin film, the following problems occur due to the manufacturing process. That is, in the conventional manufacturing process of the edge junction, as shown in FIG. 4, a first oxide superconductor thin film 2 and an insulator film 3 are sequentially formed on an oxide substrate 1 (FIG. 4A). This laminated film is etched so as to form a desired angle with the substrate surface (FIG. 4B), exposing the end surface of the first oxide superconductor thin film 2. During this etching, part of the exposed oxide substrate 1 is also removed,
The substrate 1 has a step 1a. Then, a junction barrier layer 4 made of an insulator or a normal conductor and a second oxide superconductor thin film 5 are sequentially formed on the substrate 1 on which such a step 1a is formed, and edge joining is performed. (FIG. 4-c).

Here, even when an oxide substrate 1 on which an oxide superconductor can be epitaxially grown is used, since the lattice matching with the oxide superconductor is generally not sufficient, a step 1a is formed on the substrate 1. If it occurs, for example, the portion 5a of the second oxide superconductor thin film 5 growing on the main surface of the substrate 1 and the portion 5b growing on the inclined surface, unless the inclination of the step portion with respect to the substrate surface is sufficiently small. Are different from each other in the growth direction, and the second oxide superconductor thin film 5 has a crystal grain boundary 6.
Will occur. Since such a crystal grain boundary 6 behaves as a Josephson junction, in the case of an edge junction having a large inclination angle, a grain boundary junction is further inserted in series at the edge portion junction, and sufficient characteristics can be obtained. Disappears.

For this reason, in the conventional edge joining, it was necessary to manufacture such that the inclination of the edge portion became sufficiently gentle. This increases the effective area of the junction, which hinders the realization of a Josephson junction having a high critical current density and a high junction resistance required for high-temperature operation. In other words, the development of an edge-type Josephson junction using an oxide superconductor having a high critical temperature is expected to make a significant contribution to the industry, but the production of a stable edge portion for practical use is expected. The development of methods is a challenge.

SUMMARY OF THE INVENTION The present invention has been made to address such a problem, and an edge-type Josephson junction using an oxide superconductor thin film can be realized after setting the inclination of the edge portion freely. As a result, it is an object of the present invention to provide a method for manufacturing a superconducting element that enables a variety of superconducting elements including a Josephson junction having a large critical current density and a large output voltage to be manufactured with good reproducibility.

[0011]

Method of manufacturing a superconducting device of the present invention, in order to solve the problem] has, on a _substrate, the thin film composition in PrBa 2 _Cu 3 _{O 7} is substantially _{represented, REBa 2 Cu 3 O 7 (} RE is A first oxide superconductor thin film whose composition is substantially represented by at least one element selected from rare earth elements excluding Pr). After forming an insulator film, the PrBa ₂ Cu ₃ O
_The laminated film is partially removed at an angle to the substrate surface until a portion of the thin film ₇ is removed, and the end surface of the first oxide superconductor thin film and the PrBa ₂ Cu ₃ O ₇ thin film are removed. Exposing the surface of the first oxide superconductor thin film and at least the end surface of the first oxide superconductor thin film and the PrBa ₂ Cu ₃ O ₇ thin film
A thin film of an insulator or a normal conductor and a second oxide superconductor thin film whose composition is substantially represented by REBa ₂ Cu ₃ O ₇ are sequentially laminated so as to cover the exposed surface of And a forming step.

In the method of manufacturing a superconducting element according to the present invention, first, a PrBa ₂ Cu ₃ O ₇ film and a REBa ₂ Cu ₃ O ₇ film (a first oxide superconducting thin film) serving as a first superconducting electrode are formed on a substrate. ). The composition (molar ratio) of these thin films does not have to strictly satisfy the ratio, and a slight variation is allowed as long as desired characteristics, for example, in the oxide superconductor thin film, the superconducting characteristics can be obtained. Is done. The amount of oxygen in the oxide superconductor is usually adjusted so as to obtain optimal superconducting characteristics. As the above laminated film, on the REBa ₂ Cu ₃ O ₇ film, PrBa ₂ Cu ₃ O ₇
It is preferable to form a film by lamination.

The substrate temperature, substrate material, and the like during the production of the laminated film are selected so that the c-axis of the laminated film is perpendicular to the substrate surface. Lamination is desirably performed in the same manufacturing apparatus without breaking vacuum in order to obtain a high-quality superconducting film, but is not necessarily limited to this. Next, an insulator film is formed on the laminated film. This insulating film is formed on the entire surface of the laminated film when a later etching step for exposing the end face of the laminated film is performed using a photoresist as a mask. Also,
In the case where an insulator film is used as a mask used in the etching step, an insulator may be selectively formed over the stacked film by a lift-off step using a photoresist. In addition, a method in which both are used in combination and a first insulating film is formed on the entire surface of the laminated film, and then a second insulating film is selectively formed thereon may also be employed.

These insulator films can be used in the same manner whether they are crystallized or amorphous. However, in the case where a lift-off process using a photoresist is employed, it is necessary to suppress the substrate temperature at the time of forming the insulator film. Therefore, it is preferable to use an amorphous insulator film.

The etching step for forming the junction is performed, for example, by ion milling using a photoresist or the above-mentioned selectively formed insulator film as a mask. The angle between the end surface of the laminated film exposed by etching and the substrate surface can be accurately controlled by, for example, appropriately selecting the irradiation angle of ions for etching.

In the method of manufacturing a superconducting element according to the present invention, the above-mentioned etching is performed by using PrBa ₂ Cu ₃ O ₇
This is performed so that at least the surface of the film is exposed. In the actual process, it is necessary to reliably etch the REBa ₂ Cu ₃ O ₇ film, was allowed to proceed for etching until a portion of PrBa ₂ Cu ₃ O ₇ film, it is preferable to stop etching. Therefore, at the lower end of the exposed end face of the laminated film,
Although a difference in PrBa ₂ Cu ₃ O ₇ will occur, the substrate surface itself is not exposed.

Next, a thin film of an insulator or a normal conductor serving as a junction barrier, and REBa ₂ C serving as a _second superconducting electrode.
u ₃ O ₇ and a film, the end face of at least the first oxide superconductor thin film was exposed by etching, practice laminated film edge,
And it is formed so as to cover the surface of the PrBa ₂ Cu ₃ O ₇ thin film . At this time, it is preferable to perform cleaning with low-energy ions in order to remove the damaged layer on the etching end surface in advance in order to obtain a junction with good reproducibility. When the exposed end face of the laminated film is steep, the deposition of the material serving as the junction barrier layer and the REBa ₂ Cu ₃ O ₇ serving as the _second superconducting electrode is made oblique according to the inclination angle. It is preferable to perform from the direction.

The junction barrier layer must be formed of a material having good lattice matching with the oxide superconductor, and it is desirable to use an oxide having a perovskite type or a crystal structure similar thereto. In particular, when obtaining an SNS type junction, by using a normal conductor having the same crystal structure of the oxide superconductor, for example, a PrBa ₂ Cu ₃ O ₇ film, the effect of the present invention can be sufficiently exhibited. Can be.

The superconducting element according to the manufacturing method of the present invention is a second superconducting electrode using a photoresist as a mask.
This is completed by processing the REBa ₂ Cu ₃ O ₇ film by etching to a width that allows a predetermined bonding area to be obtained.

[0020]

In the production of the conventional edge type junction, the end surface of the oxide superconductor directly formed on the substrate is exposed by being inclined from the substrate surface by an etching process, and the junction is formed on the exposed end surface. In this case, when exposing the substrate surface by etching, a part of the substrate is also removed, and as a result, a gap made of the substrate material is formed at the lower end of the joint. SrTiO ₃ MgO, LaAlO _{3 and the} like are widely used as substrate materials, but the lattice matching between these materials and the oxide superconductor is generally not perfect, and the second superconducting electrode is placed on such a step. When an oxide superconductor thin film is formed, a crystal grain boundary is formed at a step portion, and a structure including a grain boundary junction in series with an edge junction is obtained. The formation of grain boundary junctions can be prevented by making the slope of the edge part extremely small.However, if such a means is used, the junction area increases and it becomes difficult to control the etching process. I have no choice.

On the other hand, in the manufacturing method of the superconducting element of the present invention, the oxide superconductor is not formed directly on the substrate, but has the same crystal structure as the oxide superconductor of REBa ₂ Cu ₃ O ₇ composition. And behaves as a normal conductor or insulator
First, a PrBa ₂ Cu ₃ O ₇ film is formed. Then, when exposing the end face of the oxide superconductor thin film laminated thereon, at least a portion of the PrBa ₂ Cu ₃ O ₇ film on the substrate is left without removing it. As a result, a step formed at the lower end of the edge portion exists in the PrBa ₂ Cu ₃ O ₇ film.

PrBa ₂ Cu ₃ O ₇ has extremely high lattice matching with REBa ₂ Cu ₃ O ₇ and, since the crystal structure itself is the same, unless the step portion is formed extremely high, the exposed end face is not exposed. Has a steep slope, but PrBa ₂ Cu ₃
O ₇ and REBa ₂ Cu ₃ O ₇ that grow on the flat surface portion of the film, the crystal orientation the REBa ₂ Cu ₃ O ₇ to grow can be the same on the inclined portion, thus REBa ₂ Cu ₃ O ₇ film Generation of grain boundaries can be prevented. In particular, when a barrier material having high compatibility with an oxide superconductor such as PrBa ₂ Cu ₃ O ₇ which is highly useful as an edge junction is used, this effect can be remarkably obtained. For this reason, in the manufacturing method of the present invention, the inclination of the edge portion can be steeper than in the conventional method, and as a result, a Josephson element or the like having a small area can be easily realized, and the control of the manufacturing process Can be enhanced.

In the method for manufacturing a superconducting element of the present invention, PrBa is also preliminarily formed on the oxide superconductor thin film as a laminated film.
By forming a ₂ Cu ₃ O ₇ film, the upper PrBa ₂ Cu
The generation of grain boundaries in the oxide superconductor thin film covering the edge of the ₃ O ₇ film can be prevented. However, although the oxide superconductor thin film on the upper PrBa ₂ Cu ₃ insulator on O ₇ film layer and the insulating film upper surface occurs grain boundary, these grain boundary portion upper PrBa ₂ Since it is separated from the edge junction by the thickness of the Cu ₃ O ₇ film and is not used as a path of superconducting current, it does not affect the characteristics of the edge junction. That is, the upper PrBa ₂ Cu ₃ O ₇ film formed on the first oxide superconductor thin film forms an edge junction portion, which is an active region of the element, with a part of the oxide superconductor thin film serving as the second superconducting electrode. And acts to isolate a certain distance from the grain boundary part inevitably formed. The upper PrBa ₂ Cu
_{Even when the 3} O ₇ film is not formed, no grain boundary is formed in the path of the direct superconducting current, so that the characteristics of the edge junction are not directly affected.

[0024] In order to minimize the occurrence of grain boundary in the second superconducting electrodes is in the upper PrBa ₂ Cu ₃ O ₇ film it is desirable not to form the insulating film, PrBa ₂ Cu ₃ O ₇ films Is an insulator containing a large number of localized levels, and at high temperatures, leakage current through this localized level poses a problem. Above all,
In the edge junction of a small area manufactured by the method for manufacturing a superconducting element of the present invention, since it is essential to prevent the formation of a parasitic current path other than at the edge, the upper PrBa ₂ Cu
It is preferable to form an insulator film on the ₃ O ₇ film to prevent leakage current.

[0025]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a view showing a manufacturing process of an embodiment to which a method of manufacturing a superconducting element according to the present invention is applied. The figure shows
SrTiO ₃ crystal polished parallel to the (100) plane as a substrate material, YBa ₂ Cu ₃ O ₇ as a superconducting electrode material, and PrBa ₂ Cu ₃ O ₇ as a barrier layer material at the edge portion,
4 shows a manufacturing process of an SNS type edge junction.

[0027] First, as shown in FIG. 1 (a), SrTiO ₃
(100) On a substrate 11, a lower PrBa ₂ Cu ₃ O ₇ film 12 and a YBa serving as a first superconducting electrode are formed by a multi-reactive sputtering method.
_{The 2} Cu ₃ O ₇ film 13 and the upper PrBa ₂ Cu ₃ O ₇ film 14 were continuously laminated. At this time, the substrate temperature during film formation is
The c-axis of these stacked films is kept constant at 780 ° C, which is oriented perpendicular to the substrate surface, and the thickness of each layer is the lower PrBa ₂ Cu ₃ O
_{7 The} film 12 is 100 nm and the first YBa ₂ Cu ₃ O ₇ film 13 is 300 nm.
nm, and the upper PrBa ₂ Cu ₃ O ₇ film 14 was set to 200 nm.

Next, on the upper PrBa ₂ Cu ₃ O ₇ film 14,
A MgO thin layer 15 as an insulator film was grown to 20 nm in the same vacuum vessel. YBa ₂ Cu in the laminated film thus obtained
The superconducting transition temperature of the ₃ O ₇ film 13 was 89K.

After forming the laminated film, a pattern of a positive resist is formed at a predetermined position by a normal optical exposure method, and 500 nm thick MgO is deposited at a low temperature by an evaporation method using an electron gun. 1 by dissolving and removing
As shown in (b), the island-like pattern 16 of the amorphous MgO layer
Was formed.

Using the island-shaped pattern 16 of the amorphous MgO layer as a mask, the MgO thin layer 15, the upper PrBa ₂ Cu ₃ O ₇ film 14, and the first YBa ₂ Cu ₃ O ₇ film 13 are formed by ion milling.
Was continuously etched until the lower PrBa ₂ Cu ₃ O ₇ film 12 was exposed to form an edge end (FIG. 1C).
The inclination angle of the edge portion with respect to the substrate surface was controlled by the irradiation angle of the ions to be milled. In this embodiment, the inclination angle of the edge portion is set to be 45 degrees.

Next, after irradiating Ar ions accelerated at a low voltage of 60 V for several minutes to remove a strain region generated at the time of etching, the device is immediately mounted on a multi-source sputtering apparatus and heated in an oxygen atmosphere. The residual strain at the edge end was recovered.

Thereafter, PrBa ₂ Cu ₃ O to be a junction barrier layer
_{7 The} film 17 has a thickness of 5 nm and the second superconducting electrode YBa ₂
A Cu ₃ O ₇ film 18 was laminated to a thickness of 300 nm. For processing the upper superconducting electrode, a normal lithography process and etching with Ar ions were used to finally form a junction having a width of 5 μm at the edge (FIG. 1D). Therefore, the junction area in this embodiment is 0.4 × 5 μm.

FIG. 2 shows the current-voltage characteristics of the thus obtained superconducting element at liquid helium temperature. Since the PrBa ₂ Cu ₃ O ₇ film 17 as a barrier layer has a normal conductivity property, it exhibits characteristics as an SNS type Josephson junction without hysteresis. It was confirmed that it did not exist. The critical current of the Josephson junction is 2 mA,
About 5mV as I _c · R _n product, which is the output voltage of the device is obtained.

FIG. 3 is a diagram showing the dependence of the critical current on the applied magnetic field, which was performed to confirm that the edge junction according to this embodiment exhibited uniform Josephson characteristics. FIG.
As can be seen from the above, the fabricated device exhibited an ideal Fraunhofer pattern. Such Josephson characteristics was confirmed even at a temperature of 50K, 100 .mu.A as Josephson current at this temperature, I _c · R _n product is 250μV is obtained. This verifies that a superconducting element manufactured by using the manufacturing method of the present invention can operate at a high temperature.

As described above, by applying the manufacturing method of the present invention, it is possible to stably form an edge junction having excellent characteristics even on a laminated film end face having an inclination angle of 45 degrees.

[0036]

As described above, according to the present invention, various superconducting elements including a small-area Josephson junction element which can operate at a high temperature using an oxide high-temperature superconductor can be stably reproduced. It can be manufactured with good efficiency and is expected to make a great contribution to the industry.

[Brief description of the drawings]

FIG. 1 is a sectional view schematically showing a manufacturing process of a superconducting element according to one embodiment of the present invention.

FIG. 2 is a diagram showing current-voltage characteristics of a superconducting element obtained according to one embodiment of the present invention.

FIG. 3 is a diagram showing the applied magnetic field dependence of the critical current of the superconducting device obtained according to one embodiment of the present invention.

FIG. 4 is a cross-sectional view schematically showing a manufacturing process of a conventional edge bonding.

[Explanation of symbols]

11 SrTiO ₃ (100) substrate 12 Lower PrBa ₂ Cu ₃ O ₇ film 13 First YBa ₂ Cu ₃ O ₇ film 14 Upper PrBa ₂ Cu ₃ O ₇ film 15 MgO thin layer 17: PrBa ₂ Cu ₃ O ₇ film serving as a junction barrier layer 18: Second YBa ₂ Cu ₃ O ₇ film

Claims (1)

(57) [Claims] To 1. A _substrate, a thin film having a composition by PrBa 2 _Cu 3 _{O 7} is substantially _represented, REBa 2 _Cu 3 _{O 7}
(RE represents at least one element selected from rare earth elements other than Pr), a step of sequentially laminating and forming a first oxide superconductor thin film whose composition is substantially represented by: After forming an insulator film on the film, the PrBa ₂
Until a part of the Cu ₃ O ₇ thin film is removed, the laminated film is partially removed at an angle to the substrate surface, and the end surface of the first oxide superconductor thin film and the PrBa ₂ Cu ₃ O ₇
Exposing the surface of the thin film, and at least the end face of the first oxide superconductor thin film and
An insulating or normal conductor thin film and a REBa thin film so as to cover the exposed surface of the PrBa ₂ Cu ₃ O ₇ thin film.
₂₎ forming a second oxide superconductor thin film, whose composition is substantially represented by Cu ₃ O ₇ , in order, and laminating the second oxide superconductor thin film.