JPH05279025A - Oxide superconducting thin film - Google Patents

Abstract

PURPOSE:To increase the critical current density by arranging a thin film consisting of an oxide superconducting material shown by a specified formula in a fixed direction so that the ab face of a crystal is parallel to a substrate. CONSTITUTION:The raw powders of a Y compd. such as Y2O3, a Ba compd. such as BaCO3 and a Cu compd. such as CuO are mixed in a specified molar ratio, the mixture is formed, and the formed body is heated and oxidized at 300-1000 deg.C in an oxidizing atmosphere, then crushed and calcined. A film is formed on a substrate by sputtering, etc., with the calcined material as a target. While the film is formed, a magnetic field of 0.3T is applied in parallel or in almost parallel to the ab face perpendicular to the c axis of the thin film to provide an oxide superconducting thin film having a high critical current density and shown by the formula (x=0 to 1, y=2.0 to 4.0, z=1.0 to 4.0, w=4.0 to 10.0, A is Y, Gd, Yb, Eu, Tb, Dy, Ho, Er, Tm, Lu, Sc and other lanthanides, and B is Ra, Ba, Sr, Ca, Mg and Be).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film used for a device or the like using the crystal anisotropy of an oxide superconducting material.
In this specification, the ab plane of a crystal being substantially parallel to the substrate (generally referred to as c-axis orientation) means
In the X-ray diffraction pattern, the relative intensity of the peak due to the c-axis orientation is 10 times or more higher than that due to the ab-axis orientation.

[0002]

2. Description of the Related Art Conventionally, superelectronic materials have been elements such as mercury and lead, alloys such as NbN, Nb ₃ Ge and Nb ₃ Ga, or Nb ₃ (Al _0.8 Ge _0.2 ).
A metal material made of a three-element compound such as is used.
However, these Tc (superconducting critical temperature) onset is 25K
It was up to.

On the other hand, in recent years, attention has been paid to ceramic-based superconducting materials. This material was first reported by the Zurich Research Laboratories of IBM as Ba-La-Cu-O (oxide) high-temperature oxide superconductor, and was further known as LSCO (cuprate-lanthanum-strontium). These are (A _1-x B
Since the oxides in (x) yCuOz were only mixed and fired, the Tc onset was only 30K.

[0004]

The possibility of superconductivity of these oxide ceramics utilizes a single-layer perovskite type structure, and there are many crystals in the structure.
The crystal directions were also different. Since the contact area at the grain boundary was also small, the critical current density was also small. Therefore, Tco of oxide superconducting material (temperature at which resistance becomes zero)
It has been strongly demanded that the critical current density be improved while operating the liquid nitrogen at a temperature higher than the liquid nitrogen temperature (77K) or higher if desired.

For this purpose, the inventors of the present invention have proposed a "method for producing a superconducting material" (March 27, 1987, Japanese Patent Application No. 62-
75205).

The present invention is a further development of the invention and is applied to a thin film of an oxide superconducting material.

[0007]

The present invention is designed to exhibit superconductivity at a high temperature close to room temperature, and in order to obtain a high critical current density, it is deformed when a thin film of an oxide superconducting material is produced by a heating step. The magnetic field direction is added in parallel or substantially parallel to the direction in which the C axis of the crystal having the perovskite structure should have, and the crystal generation surface is arranged in a fixed direction. As a result, by applying a magnetic field of 0.3 T or more at the same time during or after the formation of the thin film, preferably by simultaneously heating to 300 to 1000 ° C. to facilitate rearrangement, the c-plane (plane parallel to the ab axis) ), The critical current density can be increased to 1 × 10 ⁴ A / cm ² or more.

A typical superconducting material used in the present invention is an oxide using an element of Group 3a and 2a of the periodic table of elements and copper.

The superconducting material of the present invention is (A _1-x Bx) yCuzOw
x = 0.1 to 1, y = 2.0 to 4.0, preferably 2.5 to 3.5, z =
1.0 to 4.0, preferably 1.5 to 3.5, w = 4.0 to 10.0, preferably 6 to 8, can generally be shown. A uses one kind or a plurality of kinds of elements selected from the yttrium group and other lanthanoids. The yttrium group is a physics and chemistry dictionary (Iwanami Shoten
1st April 1963) Y (yttrium), Gd (gadolinium), Yb (ytterbium), Eu (europium),
Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Lu (lutetium),
Uses Sc (scandium) and other lanthanoids.

B is Ra (radium), Ba (barium), Sr
One or more of the elements selected from (strontium), Ca (calcium), Mg (magnesium), and Be (beryllium) are used.

The oxide superconducting material according to the present invention has a modified perovskite structure, whose crystal structure is shown in FIG. And, it has copper (2), other copper (3), and oxygen (6) and oxygen vacancy (7) located around them. Periodic Table of Elements 3a Group elements (1) For example Y, Periodic Table of Elements 2
It has a group a element such as Ba (4). In addition, the periodic table of elements in this specification is a dictionary of physics and chemistry (Iwanami Shoten, April 1963 1
Issued daily). Then, as a mechanism for generating superconductivity, a pair of electrons (electron pair) is generated by the interaction between oxygen (5) having a layered structure and copper (2) at the center of the plane (that is, the plane formed by the ab axis, that is, It is said to move the c-axis). Furthermore, as a reason for producing the paired electron, it has been considered to be a phonon based on the BCS theory until now. However, the inventors of the present invention have explained that the reason for this is that the rare earth element (1), which is a screw-magnetic material, or an oxygen bacanic acid
The model is that a pair of electrons with opposite spins can be formed by using a quasi-particle called magnon as an intermediary by interaction with (7). It can be considered that the magnon acts like a shadow warrior to move the electron pair on the surface having the layer structure.

For this reason, an external magnetic field is applied during or after film formation to interact with the magnetic substance or magnon, and it is particularly preferable to add it while heating to make the crystal axes of the polycrystals coincide with each other or substantially coincide with each other. Therefore, a single crystal can be manufactured more easily.

Then, a single crystal of the oxide superconducting material of the present invention can be produced at a lower temperature. It is easy for a current to flow in the C plane (parallel to the ab axis) of FIG. For this reason, it is extremely important to dispose a polycrystal having different crystal orientations in the negative direction in order to obtain a high critical current density even if the polycrystal is a polycrystal.

According to the present invention, oxide ceramics obtained by temporarily firing an oxide material containing such an element or its starting material is subjected to a reduced pressure such as a sputtering method, an electron beam vapor deposition method, an ion plating method, a chemical vapor deposition method or the like. Method of forming a film by spraying
It is effective for a thin film forming method of forming a film under atmospheric pressure, such as a method and a screen printing method. The present invention applies a magnetic field, preferably 0.3 Tesla (T) or more, during the film formation or during the thermal anneal after the film formation, so that the magnetic field is in the same direction as the direction of the magnetic field or near the same direction. It has been found that crystals can be grown while arranging most or all of the crystals or polycrystals in the direction. Then, it was found that the crystal easily aligns with the magnetic field in the c-axis direction.

By doing so, one crystal grain exhibiting a polycrystal can be enlarged, and by extension, the barrier at the crystal grain boundary can be further disappeared to form a single crystal. Then, it becomes possible to match all the crystals to the ab plane (plane perpendicular to the C axis). As a result, the critical current density was 10 ² A / cm ² when the crystal orientations were different.
From (77K), it was possible to increase to 10 ⁴ to 10 ⁵ A / cm ² (measured at 77K) by the method of the present invention, and to approach to about 1/5 of a single crystal. And we made it easier to make a large-area single crystal structure, which is ideal for oxide superconducting materials.

[0016]

In the present invention, a magnetic field is applied vertically or horizontally to the surface on which this thin film is to be formed in order to form such an axially arranged superconducting thin film. Applying an electric field in the direction is effective for improving Tco. Therefore, in the present invention, a magnetic field may be formed at a position sufficiently distant therefrom, and a means for inducing the magnetic field in the vicinity of the oxide superconducting material heated by a magnetic material such as nickel or iron may be provided, which is particularly expensive. It also has the other feature of not using equipment.

Further, for example, it is effective to align the crystal axis of the substrate forming the surface to be formed in accordance with the crystal array direction by the magnetic field. For example, MgO (magnesium oxide), Sr
In TiO ₃ (strontium titanate) and YSZ (yttrium stabilized zircon), the (100) plane is used, and a magnetic field is applied to the surface to be formed in the vertical direction to perform film formation or annealing after film formation. The ab plane can be parallel to the formation surface. Also, when used for a crystal substrate having a (110) plane and further applying a magnetic field parallel to the formation surface, the ab plane is formed in the direction perpendicular to the formation surface or obtained by an annealing after the formation. You can Then, by using the crystal orientation of the formation surface and the magnetic field together, a thin film closer to a single crystal can be obtained.

The present invention will be described below with reference to Examples.

[0019]

【Example】

[Example 1] As an example of the present invention, A was used as Y and B was used as Ba.

The starting materials are yttrium oxide (Y ₂ O ₃ ) as a Y compound, BaCO ₃ as a Ba compound, and CuO as a copper compound.
Was used. These are obtained from Kojundo Kagaku Kogyo Co., Ltd., and fine powder having a purity of 99.95% or higher is used. After film formation, for example, x = 0.67, y = 3, z = 3, w = 6 to 9 (YBa ₂ ) Cu
₃ chose as much as possible with the O ₆ ~ _8.

These are thoroughly mixed in a mortar and encapsulated, and a tablet is formed by applying a load of 100 Kg / cm ² (outer diameter 25 mm
φ, thickness 3mm cylindrical). Further, it was heated and oxidized at 500 to 1400 ° C., for example, 950 ° C. for 8 hours in an oxidizing atmosphere, for example, and then gradually cooled (gradual heating rate of 4 ° C./min) at 400 ° C. for 1 hour. This process was pre-baked.

At the time of this oxidation anneal, a magnetic field was applied from the outside. This magnetic field is a direct current magnetic field in which the end faces of the metal led out from the magnet are arranged in the vertical or horizontal direction of the tablet, and one is N and the other is S, and the strength is 0.3T or more, for example 1.
5T. It goes without saying that the stronger the magnetic field, the better.

FIG. 2 shows an outline of a sputtering apparatus for producing the superconducting thin film of the present invention.

In FIG. 2, the target (10), the reaction chamber (4
1), Doping system (40), Exhaust system (25).

The doping system consists of argon (32), oxygen (33)
And a gas containing a halogen element (here, nitrogen fluoride (NF
₃ ) is used) (34). The exhaust system (25) comprises a turbo molecular pump (21), a pressure regulating valve (22), and a rotary pump (23). The substrate (30) is arranged on a holder (31) having a heater (29) and is heated from room temperature to a maximum temperature of 950 ° C. During film formation, the temperature was set to 400 to 900 ° C, for example 750 ° C. The distance between the target (10) and the formation surface of the substrate (30) is 2 to 15 cm.

The target is a tablet target (1) having the same crystal orientation as that of the thin film to be formed as described above.
2) was used. This is because by setting the crystal orientation of the target in the same direction as the direction in which the thin film is to be formed, a thin film closer to a single crystal can be obtained.

On the target side, the packing plate (1
3), magnet (16), cooling water inlet (15), cooling water outlet
(15 ') and shield plate (17). These are electrically separated from the sputtering apparatus main body (41) by the Teflon insulator (18). Then, a high negative voltage is applied to the current introducing terminal (20) with respect to the target (12).

Further, by using the magnetic field generator (26), the substrate holder
A magnetic pole (27) having a magnetic circuit end is provided inside (31), and a magnetic pole (14) showing the magnetic circuit end is provided on the other side of the target (12) on the other side. A magnetic field (28) is generated during this period. This magnetic field (28) has the surface on which the substrate (30) is formed in the vertical direction.

In the case of performing the DC (direct current) sputtering method, this target is negatively applied and the substrate (30) is at the ground potential.

When the AC (alternating current) sputtering method is performed, the substrate is used as an electrically floating surface.

[Experimental Example 1] YBa ₂ Cu was used as the target (12).
Using a _3.8 O ₆ ~ _8. The distance between the target and the substrate is 3 cm
And The pressure of argon was 4 × 10 ⁻¹ Pa, and the amount of oxygen was 20% with respect to Ar. The output of DC sputter was set to 500W. A magnetic field strength of 0.2 T was generated on the substrate surface. This target had a diameter of 25 mm. The substrate (2) was heated to 750 ° C. and the holder (3) was rotated so as to have a uniform thickness. Under these conditions, a thin film (thickness 0.5 to 3 μm) (50) was formed at a rate of 5 to 100 A / min, for example, 50 A / min, and this was gradually cooled.
After that, while applying a magnetic field to the film formation, the inside of the film was made to contain only oxygen to atmospheric pressure, and oxygen annealing was performed at 850 ° C. for 3 hours, followed by slow cooling. In particular, hold at 350-500 ℃ for 2 hours after this,
The crystal structure of all the crystals in the film was transformed to the orthorhombic deformed perovskite. Tc as an oxide superconducting material
I was able to make 98K as o.

In the drawing, the target (12) and the thin film (5
The thin line of (0) is the surface made of copper (2) and oxygen (5) in Fig. 1, that is, a
It is a symbol of the b side.

The critical current density was obtained by measuring 3 × 10 ⁴ A / cm ² in the direction parallel to the substrate surface.

In this experiment, when no magnetic field was applied, the Tco was 60 K and the critical current density was 600 A / cm ² . That is, the crystal structure as shown in FIG. 1 was sufficiently formed by thermal annealing during film formation and thereafter, and the c-axis direction was also formed in the direction parallel to the magnetic field, that is, in the direction perpendicular to the surface to be formed. It became clear from the results of line diffraction.

"Experimental Example 2" A target (12) having a surface perpendicular to the ab plane was used, and Y _0.5 Yb _0.5 BaS was used as its constituent material.
using rCu _3.6 O ₆ ~ _8. The direction of applying the magnetic field (28) was as shown in FIG. That is, the substrate (30), the holder (31) and the heater (29) in the apparatus of FIG. 2 are the same. Thus, the thin film (50) could be formed on the surface to be formed with the ab plane in the vertical direction. The parts not shown are the same as those in FIG. Magnetic field source (2
It has magnetic poles (27) and (14) derived from 6) through a magnetic circuit. The magnetic field (28) was provided parallel to the formation surface of the substrate (30). The value is the same as in Experimental Example 1. As a result, platinum (prepared by the sputtering method at 500 ° C) was provided on the substrate, such as glass, alumina, ZrO ₂ etc., as the surface to be formed, and for the superconducting material thin film formed on it, further to the surface. Gold was provided as an electrode, and the current density produced between this gold and platinum on the back surface was 2.4 × 10 ⁴ A / cm ² . However, the direction parallel to the surface to be formed was only 1 × 10 ³ A / cm ² . We got 93K as Tco.

[Embodiment 2] As an embodiment, an example using an electron beam vapor deposition apparatus as shown in FIG. 4 is shown. In the drawing, the substrate (30), the holder (31), and the heater (29) are attached to the vacuum container (41).
Have inside. The exhaust system (25) has a turbo molecular pump (21), a valve (22), and a rotary pump (23). Crucible (55)
Yttrium (51), Copper (32), Barium (33) respectively
And an electron beam (not shown) was used to deposit these on the formation surface of the substrate (30). Further, a magnetic circuit is provided from the magnetic field generation source (26) to have magnetic pole portions (27) and (27 ') thereof to form a magnetic field (28). Thus, Y: Ba: Cu was set to 1: 2: 3, and this was annealed in oxygen while applying a magnetic field as in the experimental example. Then Tc onset 97K, Tco 94K
I was able to get The critical current density of the resulting thin film was also 1.5 × 10 ⁴ A / cm ² .

When no magnetic field was applied, only an order of 10 ² A / cm ² was obtained.

Example 3 In Experimental Example 1 of Example 1, the substrate was MgO (100) or SrTiO ₃ (100). Then, a magnetic field was applied during film formation so as to be 1.5 T on the surface to be formed.
Then, a single crystal thin film with a thickness of 1 cm ² or more was formed on this substrate with a thickness of 1.
It could be obtained at 5 μm.

Example 4 In Experimental Example 2 of Example 1, the substrate was MgO (110), SrTiO ₃ (110). Then, a magnetic field was applied so as to be 1.5 T on the formation surface. Then, a single crystal thin film having a thickness of 3 μm could be obtained on this substrate with a thickness of about 5 mm ² .

[Embodiment 5] In this embodiment, thermal annealing is performed at 700 to 950 ° C. on a substrate obtained by a sputtering method (Embodiments 1, 3, 4) and an electron beam evaporation method (Embodiment 2). -When doing
The magnetic field was applied so that the magnetic field would be in the c-axis direction in accordance with the crystal plane that was previously made. In addition, an electric field of 10
³ was added ~5 × 10 ⁴ V / cm. Then, in addition to the crystal arrangement, oxygen (6) in FIG. 1 is also removed, and many oxygen vacancy (7) is formed. As a result, Tco could be further improved by about 100K, and 230-280K was obtained. The magnetic field current density is also 4 × 10.
⁵ A / cm ² was obtained. In the present invention, the molded product has a thin film shape. However, this shape is 3 ~ depending on the needs of the market.
It can be modified into a film structure, a band structure, and a line structure having a thickness of 30 μm.

[0041]

According to the present invention, it has become possible to form a thin film of an oxide superconducting material which operates at a temperature above the liquid nitrogen temperature, which has been impossible at all, with its crystal axes aligned.

Further, in order to more easily match the layer structures between the polycrystalline structures in the compound of the ultimate material, a plurality of elements 2a and 3a in the periodic table of elements can be mixed. As shown in the present invention, by applying a magnetic field during heating to more unify the molecular arrangement, the presence of voids and lowering of grain boundary barriers in the final finished compound is further eliminated. And, by extension, Tc onset,
It is estimated that Tco can be heated to a higher temperature.

The embodiment of the present invention is a thin film. However, instead of being vaporized to form a thin film, the tablet is pre-baked or main-baked, then powdered again, the powder is dissolved in a solvent, and the solution is coated on a substrate or the like by a printing method or a spraying method. It is also possible to obtain a superconducting film in which the crystal orientation of the thin film is uniform by drying it, firing it in an oxidizing atmosphere while applying a magnetic field, and then firing it in a reducing atmosphere. It is also effective to repeat this to form a multilayer film.

According to the present invention, a Josephson element or the like having a desired shape has been completed, and a thermal anneal is conducted again at 300 ° C.
It is also effective to perform the above, and at the same time, apply a magnetic field in the direction perpendicular to the direction in which an electric current should flow to align the crystal orientation in the negative direction.

[Brief description of drawings]

FIG. 1 is an example of a crystal structure of an oxide superconducting material used in the present invention.

FIG. 2 is a schematic view of a sputtering apparatus used in the present invention.

FIG. 3 is a main part of a sputtering apparatus used in the present invention

FIG. 4 is an electron beam vapor deposition apparatus used in the present invention.

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location H01L 39/24 ZAA B 8728-4M

Claims (2)

[Claims] 1. A thin film made of an oxide superconducting material, which is arranged in a certain direction so that the ab plane of the crystal of the thin film is substantially parallel to the substrate, and as a result has a high critical current density. Oxide superconducting thin film. 2. The oxide superconducting material according to claim 1, wherein
_1-x Bx) yCuOz, x = 0 to 1, y = 2.0 to 4.0, z = 1.0 to 4.
0, w = 4.0 to 10.0, A is Y (yttrium), Gd (gadolinium), Yb (ytterbium), Eu (europium),
Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Lu (lutetium),
Consists of one or more elements selected from Sc (scandium) and other lanthanoids, and B is Ra (radium), Ba (barium), Sr (strontium), Ca (calcium), Mg (magnesium), Be. An oxide superconducting thin film characterized by comprising an element selected from (beryllium).