JPH0556282B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a technique for forming a thin film of an oxide ceramic superconducting material.

"Prior Art" In recent years, oxide ceramic-based superconducting materials (hereinafter referred to as oxide superconducting materials) have attracted attention as high-temperature superconductors. The Ba-La-Cu-O type of this material was first reported by IBM's Zurich Research Institute, and the YBCO (YBa ₂ CuO ₆ - ₈ ) type was also reported as a similar high-temperature superconductor.

The currently known oxide superconducting materials listed above are
It is manufactured by mixing powders of oxide superconducting materials and baking them to form tablets.

``Problems to be Solved by the Invention'' Since the conventional oxide superconducting materials described above are formed in a tablet shape, it is difficult to apply the oxide superconducting materials even if an attempt is made to make an element using them, for example.

In addition, in order to utilize oxide superconducting materials in superconducting elements, it is possible to form thin films of oxide superconducting materials, but thin films of oxide superconducting materials with high superconducting properties (hereinafter referred to as oxide superconducting thin films) are considered. There was no known method to form it.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for forming an oxide superconducting thin film having high superconductivity.

"Means for Solving the Problems" The present invention is a method for forming a thin film of an oxide superconducting material on a heated surface to be formed, which method orients the crystal axis of the oxide superconducting thin film in a predetermined direction. The gist of the present invention is that, during film formation, a magnetic field is applied to the surface to be formed in the predetermined direction.

Typical superconducting materials used in the present invention are oxides using elements of groups A and A of the periodic table of elements and copper.

As an example of the oxide superconducting material used in the present invention, (A _1-X B _x ) _y Cu _z O _w , X=0.1 to 1, y
Materials having the composition shown by =2.0 to 4.0, z = 1.0 to 4.0, and w = 4.0 to 10.0 can be mentioned. In the above composition formula, A is Y (yttrium), Gd
(gadolinium), Yb (yzterbium), Eu (europium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm
(thulium), Lu (lutetium), Sc (scandium) and other lanthanoids
Indicates a species or multiple kinds of elements, B is (barium),
It represents one or more elements selected from Sr (strontium) and Ca (calcium).
The periodic table of elements in this specification is based on the Physical and Chemistry Dictionary (Iwanami Shoten, published on April 1, 1963).

The present invention was made based on the fact that a polycrystalline oxide superconducting thin film can have a high critical current density by aligning its crystal orientation.

In view of the above circumstances, a configuration has been developed in which a highly oriented oxide superconducting thin film with uniform crystal orientation can be formed by forming the film while applying a magnetic field.

That is, the present invention is characterized by forming an oxide superconducting thin film by controlling the orientation direction by controlling the direction of the magnetic field during film formation.

"Operation" By applying a magnetic field in accordance with the desired orientation direction during film formation, a superconducting thin film with good orientation in which the directions of crystal axes are aligned can be formed.

Example 1 This example is an example in which an oxide superconducting thin film having a polycrystalline or single crystal structure is formed on a substrate at a temperature of 200°C to 500°C.

Considering the degree of freedom in selecting the type of substrate that will be the surface to be formed, and the difference in thermal expansion coefficient between the oxide superconducting material and the substrate, it is difficult to determine the substrate temperature during film formation and the temperature during thermal annealing after film formation. It is important to keep the temperature below 500°C.

In addition, in this example, a film formation method using microwaves is used as a film formation method, and at the same time, by applying a magnetic field during film formation, the crystal axis is oriented in a predetermined direction. be.

As a result, an oxide superconducting thin film having a high critical current density can be obtained.

In this example, a film formation method using microwaves is adopted because active oxygen such as ozone can be obtained with high efficiency by microwave plasma, and in addition, it does not cause spatter (damage) to the surface on which it is formed. It is from. The microwave frequency is
Typically, a frequency of 2.45 GHz can be used from 500 MHz to 60 GHz.

In this example, a magnetic field is applied to the substrate (hereinafter simply referred to as the substrate), which is the surface on which the film is formed, at the same time as the film is formed. parallel or approximately parallel to the direction that should be aligned with
The magnetic field is applied at a strength of 0.1 T or more, and by applying this magnetic field, the crystal formation surface to be formed on the substrate is arranged in a certain direction.

By applying a magnetic field to this substrate at the same time as film formation, magnetic axial growth or, in the case of growing a single crystal, magnetic epitaxial growth could be performed.

As above, for example, the c plane (plane parallel to the ab axis)
It was possible to improve the critical current density to 1×10 ⁻⁴ A/cm ² or more (in this case, an a-plane is formed in a direction parallel to the surface of the substrate).

The oxide superconducting material shown in this example is the first
As shown in the figure, it has a deformed pervskite structure.

What is shown in FIG. 1 are copper 2, other copper 3, oxygen 6 located around it, oxygen vacancy 7, oxygen 5 having a layered structure, and element 1 of group a of the periodic table. For example, it includes Y and an element of group a of the periodic table of elements, such as Ba4.

In the crystal structure shown in FIG. 1, current flows more than two orders of magnitude more easily in the C-plane direction (plane parallel to the a-b axis) than in the direction perpendicular thereto (c-axis direction). Therefore, in order to obtain a high critical current density, it is more important to align the crystal orientations in one direction than to obtain polycrystals with disparate crystal orientations.

In this example, the interaction between microwaves and a magnetic field is generated at a high pressure of 1 to 800 torr, and the substrate is placed parallel or perpendicular to the magnetic field in a region with strong magnetic field strength, and all crystals are placed in a predetermined position. It was placed in the direction.

Furthermore, in order to improve the orientation of the oxide superconducting thin film, it is effective to use a substrate having a crystal orientation such that the plane on which the oxide superconducting thin film is grown coincides with its orientation axis.

The oxide superconducting material produced in this example can be made into a single crystal at a lower temperature.

In this example, an organic reactive gas of an element or an oxide material using such an element is calcined,
The film was formed by spraying finely powdered particles or elemental salt particles into a plasma filled with active oxygen.

Furthermore, in this example, a magnetic field was applied as necessary during thermal annealing in the same reactor after film formation. This magnetic field is preferably 0.1 Tesla (T)
It was applied at the above strength in the direction of orienting the crystal axes or in a direction closer to it. By applying a magnetic field during this thermal annealing, it was possible to grow crystals while aligning most or all of the crystals, that is, polycrystals, in the orientation direction. This takes advantage of the phenomenon that the c-axis direction of the crystal tends to align with the direction of the applied magnetic field during film formation.

By doing so, one polycrystalline grain can be made larger, and the barrier at the grain boundary can be further eliminated, resulting in a structure that can be made into a single crystal. and each crystal
It is now possible to align with the a-b plane (the plane perpendicular to the C-axis). Then, the critical current density was changed from 10 ² A/cm ² (77K) in the case where the crystal orientations were different.
It has become possible to increase the current to 10 ⁴ to 10 ⁵ A/cm ² (measured at 77K), which is approximately 1/5 of that of a single crystal. This also made it easier to create a large-area single-crystal structure, which is the ideal oxide superconducting material.

Example 2 This example is an example in which an oxide superconducting thin film was formed using a microwave plasma CVD apparatus capable of applying a magnetic field, the outline of which is shown in FIG.

This device includes a plasma generation space 31 that can be maintained at atmospheric pressure or a reduced pressure state, an auxiliary space 12, electromagnets 15 and 15' that generate a magnetic field, and a power source 35 thereof,
Microwave oscillator 14, vacuum pump 26 constituting the exhaust system, rotary pump 24, pressure adjustment valve 19, substrate holder 10', substrate (film forming object) 10, microwave introduction window 39, gas introduction system 1
6, 17, water cooling system 28, 28', halogen lamp 20 for heating, reflector 21, halogen light introduction port 2
2, halogen lamp power supply 20, halogen light 27,
Heating space 13, gate valve 11, valves 25, 2
3. A liquid 32 mixed with fine particles, a bubbler 33, an oxygen introduction system 17, a gas introduction system 16, a space 30 divided into four parts, and a gas introduction port 34 are provided.

An example of how to use the apparatus shown in FIG. 2 will be described below.

First, the substrate 10 is placed on the substrate holder 10',
The gate valve 11 is disposed in the plasma generation space 31 . In this example, the substrate is a MgO or SrTiO ₃ or YSZ substrate with 100 or 110 planes or
A silicon wafer with an insulating film formed on a portion of its upper surface was used for use as an IC component. Also, this board holder 1
0' was made of stainless steel in order to avoid disturbing microwaves and electromagnetic fields as much as possible.

Further, when operating at atmospheric pressure, the valve 25 may be closed and the valve 23 may be opened.

A liquid 32 mixed with a reactive gas or fine particles, which is a starting material for an oxide superconducting thin film, is mixed and sealed in a bubbler 33 .

When a vapor phase method is used for film formation, the liquid 32 may be bubbled with oxygen introduced from the oxygen introduction system 17 and released together with oxygen into the reaction space from the gas introduction port 34. When using the spray method, the gas introduction port 34 is used as a spray nozzle, oxygen or atmospheric air is introduced at high pressure from the gas introduction system 16, and the liquid 32 mixed with fine particles is bubbled with the oxygen introduced from the gas introduction system 17 to create a gas. What is necessary is to send it out to the introduction port 34.

In the manufacturing process, first, the whole is pumped by the turbo molecular pump 26 and the rotary pump 14.
Evacuate to 1×10 ^-6 torr or less. Next, a non-product gas (a gas that does not form a solid after the decomposition reaction), for example, 200 SCCM of oxygen, is introduced into the plasma generation region 31,
Let this pressure be 30 torr. And 500M from the outside
Microwaves with a frequency of Hz or higher, for example, 2.4 GHz, are applied at an intensity of 0.5 to 5 KW, for example 1 KW, to cause a gas phase reaction. At this time, the water cooling system 28, 28'
A magnetic field with a strength of about 0.3 T is applied perpendicularly to the substrate surface by the cooled magnets 15 and 15'.

In this way, high-density plasma of non-product gas is generated in the plasma generation space 31. Then, the surface of the object 10 on the substrate holder 10' is cleaned with ozone or active oxygen ions having high energy from this high-density plasma region.

Next, this reaction system is filled with oxygen or an oxide gas, e.g.
Organic solutions of N ₂ O, NO, NO ₂ and elements constituting superconducting materials. Alkyl compounds or halogen compounds such as Y(OC ₂ H ₅ ) ₃ (triethoxyyttrium), CuBr ₂ (cupric bromide), etc. After forming a film of Y:Ba:Cu in an organic solution such as benzene or an aqueous solution, 1:2:
A liquid 32 that has been melted to a concentration of 3 is bubbled with oxygen and is introduced into the plasma from 34 along with the oxygen.

Also, as other methods, YBr ₃ , BaBr ₂ , CuBr ₂
It is also possible to use a spray method in which a salt such as the like is dissolved in water or an organic solution and the water is sprayed with air at high pressure.

In this example, the oxygen/reactive gas ratio of the carrier gas was set to 3000 to 1 (100 in this case).

Further, as the fine particles serving as the starting material, a finely powdered oxygen superconducting material consisting of an element of Group A of the Periodic Table of Elements, element A, and copper, or a salt or an organic compound of the element may be used.

In this way, the interaction between microwave energy and magnetic field caused hybrid resonance, generating high-density plasma.

The plasma temperature of this high-density plasma is 1150℃
(The melting temperature of oxide superconducting materials) is 3,000 to 10,000 degrees Celsius, so reactive atoms excited by such high energy can be sufficiently activated. As a result, an oxide superconducting thin film having the original crystal structure can be formed on the substrate.

In addition, in this embodiment, the substrate 10 is heated to 150 to 500°C.
heated to. This heating was accomplished by heating through the substrate holder 10' by the plasma itself or in addition to a halogen lamp heater 20.

Furthermore, in this example, annealing was performed at 400° C. after forming the oxide superconducting thin film. and
We were able to form an oxide superconducting thin film having a modified orthorhombic pervskite structure with a thickness of 1 μm to 1 mm.

In the apparatus shown in FIG. 2, a Helmholtz coil system using two ring-shaped electromagnets 15 and 15' was used for the magnetic field. Here, the results of examining the strength of the electric field and magnetic field for the space 30 divided into four parts are
As shown in the figure.

In FIG. 3A, the horizontal axis (X-axis) is the horizontal direction of the space 30 (reactive gas release direction), and the vertical axis (R-axis) is the diametrical direction of the magnet. The curves in the drawings indicate equipotential surfaces of the magnetic field. The number shown on the line indicates the strength of the magnetic field obtained when the magnet 15 is about 2000 Gauss.

By adjusting the strength of the magnet 15, the space 100 (defined within 875 Gauss ± 185 Gauss) having the interaction between the electrodes and the magnetic field can be made approximately uniform over a wide area on the substrate.

In particular, line 36 in the drawing shows the isomagnetic scene which gives rise to the condition of a hybrid resonance of 875 Gauss.

The horizontal axis in FIG. 3B indicates the flow direction of the reactive gas, as in FIG. 3A, and the vertical axis indicates the strength of the electric field (electric field strength).

The space 100 that produces this resonance condition is designed to be a region where the electric field is maximum, as shown in FIG. 3B.

The material for producing the oxide superconducting thin film undergoes an active separation reaction in this hybrid resonance region, and is formed as a thin film along the magnetic field (perpendicular to the magnetic field) on the substrate 10 to which the magnetic field is applied.

In this embodiment, the area 100 includes a substrate or a substrate 10.
may be installed. In this case, the board installation position is the area where the strength of the magnetic field is maximum, that is, the coils 15 and 1.
The central part of the Helmholtz coil consisting of 5' is preferred.

Further, the region where the electric field strength is maximum shown in FIG. 3B can be moved by adjusting the munching of the microwave power source.

Note that the critical current density of the oxide superconducting thin film formed in this example was 3 × 10 ⁴ A/cm ² , but this value was measured in a direction parallel to the substrate surface (forming surface). This was obtained by

In addition, the X-ray diffraction results show that the crystal structure shown in Figure 1 is sufficiently formed, and the c-axis direction is also parallel to the magnetic field, that is, perpendicular to the substrate (forming surface). confirmed.

Example 3 In this example, Y _0.5 Yb _0.5 BaSrCu ₃ O ₆ to ₈ was used as the component material in Example 1, and the substrate 10 and holder 10' were changed to the state shown in FIG. 2 in the apparatus shown in FIG. 2. This is an example in which film formation was performed by installing the device at right angles to the surface. That is, in this embodiment, a magnetic field is applied parallel to the substrate.

Note that in this example, film formation was performed at a substrate temperature of 500°C.

In this way, the film could be formed such that the substrate surface (forming surface) and the a-b plane of the oxide superconducting thin film were perpendicular.

In this example, the critical current density is 2.4
×10 ⁴ A/cm ² and Tco of 93K could be obtained.

In this example, the substrate is glass, alumina,
ZrO ₂ etc. can be used.

Example 4 In this example, in the case of Example 2, the substrate is
This is an example of MgO (100) or SrTiO ₃ (100). In this example, a magnetic field of 0.2 T was applied to the substrate during film formation, and the substrate temperature was set to 450°C.

In this example, a single crystal thin film of 1 cm ² or more and a thickness of 1.5 μm could be formed on this substrate.

Example 5 In this example, in the case of Example 1, the substrate is
Examples are MgO (110) and SrTiO ₃ (110). In this example, the strength of the magnetic field was set to 0.2T on the substrate surface, and the temperature of the substrate was set to 450°C.

In this example, a 5 mm ² single crystal thin film with a thickness of 3 μm could be obtained on the substrate surface.

Example 6 In this example, the oxide superconducting thin film produced in Example 1 was further thermally annealed at a temperature of 300 to 550°C after film formation. During this thermal annealing, a magnetic field is applied at the same time so that the direction of the magnetic field is aligned with the c-axis direction in accordance with the pre-formed crystal plane, and an electric field of 10 ³ to 5 × 10 is applied in the direction perpendicular to this magnetic field. It was applied at a strength of ⁴ V/cm.

Tco of the oxide superconducting thin film produced in this example
could be improved by about 10K compared to the case of Example 1. Also, the magnetic field current density is 4×
It was possible to obtain 10 ⁵ A/cm ² .

In Examples 1 to 6 above, the object to be formed was formed into a thin film. However, this shape is based on the needs of the market, such as a membrane structure with a thickness of 3 to 30 μm, a band structure,
It can be modified and improved into a linear structure.

Furthermore, in the above embodiments, a magnetic field was applied vertically or horizontally to the surface on which the oxide superconducting thin film was to be formed, but in addition to this, it is also possible to apply a microwave electric field that generates active oxygen to the surface of the substrate. This is effective for lowering the film forming temperature.

For example, it is also effective to align the crystal axis of the substrate in accordance with the direction in which the crystals are arranged by this magnetic field. For example MgO (magnesium oxide), SrTiO ₃
(strotium titanate), YSZ (yztrium
The a-b plane is formed by applying a magnetic field perpendicular to the substrate using (100) stabilized zircon), and if necessary, thermal annealing in the magnetic field after film formation. It is possible to form an oxide superconducting thin film by making the surface parallel to the substrate surface.

Furthermore, if a crystal substrate having (110) is used and a magnetic field is applied parallel to the surface of the substrate, a film can be formed with the a-b plane perpendicular to the substrate.
Of course, by performing thermal annealing in a magnetic field after film formation, a thin film that is closer to a single crystal than one containing a single crystal or polycrystal can be obtained.

"Effects" According to the present invention, it has become possible to create a thin film of an oxide superconducting material that operates at a temperature higher than the liquid nitrogen temperature by aligning its crystal axes, which was previously thought to be impossible.

It has also become possible to produce oriented polycrystalline oxide superconducting thin films on substrates with amorphous surfaces such as glass, silicon oxide, and silicon nitride.

Furthermore, in the present invention, since an oxide superconducting thin film having a high critical current density can be formed, it has become possible to use the thin film as a lead wire for a semiconductor integrated circuit.

In the present invention, in order to make it easier to match the layer structure between the polycrystalline structures of the compound of the target material, a plurality of elements a and a in the periodic table of elements can be mixed.

In the present invention, a thin film with high superconductivity can be formed by applying a magnetic field during heating, which makes the molecular arrangement more uniform and eliminates voids and grain boundary barriers in the final compound. As a result, Tc
It is estimated that the onset and Tco can be made higher.

Furthermore, by utilizing the present invention, it has become possible to manufacture devices using the crystal anisotropy of oxide superconducting materials.

[Brief explanation of the drawing]

FIG. 1 shows an example of the crystal structure of the oxide superconducting material formed in the example. FIG. 2 shows an outline of the magnetic field application microwave plasma reactor used in the examples. FIG. 3 shows the magnetic and electric fields of the device of FIG.

Claims (1)

[Claims] 1. A method for forming a thin film of an oxide superconducting material on a heated surface to be formed, which method comprises: A method for forming an oxide superconducting thin film, comprising applying a magnetic field to the surface to be formed in the predetermined direction. 2 In claim 1, the superconducting material _is ( _{A1- XBx} ) _yCuzOw , X=0.1~1, y=2.0 _~
4.0, z=1.0~4.0, w=4.0~10.0, A is Y
(yztrium), Gd (gadolinium), Yb (yzterbium), Eu (europium), Tb (terbium), Dy (dysprosium), Ho (holmium),
Consisting of one or more elements selected from Er (erbium), Tm (thulium), Lu (lutetium), Sc (scandium) and other lanthanoids, B is barium (barium), Sr (strontium),
A method for forming an oxide superconducting thin film characterized by containing one or more elements selected from Ca (calcium). 3. A method for forming an oxide superconducting thin film according to claim 1, wherein the surface on which the film is formed is arranged perpendicularly or horizontally to the magnetic field.