JPH0769788A - Method for forming oxide superconductor thin film - Google Patents

Abstract

PURPOSE:To provide a new method capable of expecting formation of a thin film of a YBaCuO oxide superconductor having C axis inclined to substrate face and excellent in crystal quality in forming the thin film on a substrate. CONSTITUTION:Stepped parts 17 constituted of the first inclined surface 17a and the second inclined surface perpendicular to the first inclined surface 17a are successively formed on an SrTiO3 substrate having a crystal face inclined by a prescribed angle theta from (001) face to a direction of [100] or [010] as a main face to provide a substrate 19 for formation of thin film. A YBaCuO superconductor thin film 21 is formed on this substrate 19 so that the first inclined face 17a of each stepped part 17 becomes (001) face and the length L is integral times longer than that of lattice interval of (a) axis or (b) axis of YBaCuO and the length (d) of the second inclined surface 17b is interlaminar distance in laminar structure of YBaCuO or dimensions close to the interlaminar distance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an oxide superconductor thin film.

[0002]

2. Description of the Related Art As a method for forming an oxide superconductor thin film on a substrate, there is known a method using various film forming methods such as a vacuum vapor deposition method, a CVD method and a sputtering method. In each case, the good high temperature superconductor thin film had the c-axis perpendicular to the substrate surface. Regarding this point, for example, Reference I (Japanese Journal of Applied Physics (JJAP), Vol. 27, No.
1, (1988), pp. L91-L93) show the results when the vacuum evaporation method was used.

By the way, it is considered that in the oxide superconductor thin film having a layered structure, the electric conductivity in the c-axis direction of the thin film is lost as the crystallinity of the thin film becomes complete. Therefore, when the thin film of the oxide superconductor is formed such that its c-axis is perpendicular to the substrate, the more excellent the crystallinity, the more the electric conductivity in the thickness direction of the substrate is lost. This can be a problem when using oxide superconductor thin films. This is because when an element is formed using an oxide superconducting thin film formed on a substrate, it is considered that an electrode is formed on the thin film and an electric signal is passed in the thickness direction of the thin film in many cases. In such a case, if the electrical conductivity in the thickness direction of the thin film cannot be obtained or is extremely low, a desired element cannot be formed.

Therefore, in order to secure the electric conductivity in the thickness direction of the oxide superconductor thin film, for example, reference II (Applied Physics Letters (Appl. Phys. Le.
tt. 53 (22), (1988) 2232), Y
A method of forming a BaCuO-based high-temperature superconductor thin film so that the C-axis is inclined or parallel to the substrate surface is disclosed. In addition, in Document III (IEE, Magn., 27 (1991) 100.
9) discloses a study on the orientation of a YBaCuO-based high-temperature superconductor thin film when the plane orientation of the substrate used is different or the thin film growth conditions (substrate temperature, etc.) are different. And in this document III, Sr is used as a substrate.
When a TiO ₃ (100) substrate is used, the thin film formed when the substrate temperature is relatively low (475 ° C. to 550 ° C.) has a-axis orientation, and when the substrate temperature is further increased, a / c
It is disclosed that mixed orientation is achieved and c-axis orientation is achieved at a substrate temperature of about 700 ° C.

[0005]

However, the method disclosed in the above-mentioned Document III has a problem that a high-quality thin film cannot be obtained due to a low substrate temperature and the like.
The method disclosed in Document II has a problem that a high-quality thin film cannot be obtained because the crystal orientation becomes complicated even when the substrate temperature is high.

The present invention has been made in view of the above circumstances. Therefore, the object of the present invention is to form a thin film of an oxide superconductor on a substrate by using a thin film having a c-axis tilted with respect to the substrate surface. Another object of the present invention is to provide a novel method which can be expected to form a thin film having excellent crystal quality.

[0007]

To achieve this object, according to the present invention, when a thin film of an oxide superconductor having a layered structure is formed on a substrate, a first inclined surface and An oxide superconductor thin film is formed by forming a substrate which has a step formed of a second inclined surface in contact with a first inclined surface and which continuously has a step and which satisfies the following (a) and (b). It is characterized in that it is used as a substrate.

(A) As the substrate, a substrate having a crystal plane having a lattice constant equal to or close to the lattice constant in the layer in the layered structure of the oxide superconductor is used, and the first inclined plane is the crystal plane. With.

(B) The length d of the second inclined surface is set to an interlayer distance in the layered structure of the oxide superconductor or a value close thereto.

In carrying out the present invention, the first substrate
It is preferable to set the length L of the inclined surface to a value that is an integer multiple (including 1) of the lattice constant in the layer in the layered structure of the oxide superconductor or a value close thereto.

A specific example of such a substrate for forming an oxide superconductor thin film is not limited to this, but is an oxide substrate having a perovskite structure and having the above-mentioned step on the surface. As a more specific example, as a SrTiO ₃ substrate, from the (001) plane to [1
An example is a SrTiO ₃ substrate having a crystal plane tilted by a predetermined angle θ in the [00] or [010] direction as a main surface and the first tilted surface as a (001) plane.

The oxide superconductor having a layered structure, which is the subject of the present invention, can be various oxide superconductors having such a structure. As an example, for example, YB
aCuO type, BiSrCaCuO type and TlBaCaC
Examples thereof include those selected from uO type.

[0013]

According to the structure of the present invention, since the thin film of the oxide superconductor grows so that the direction perpendicular to the first inclined surface becomes the c-axis, the c-axis of the obtained oxide superconductor thin film has the c-axis. The inclination angle of the inclined surface is 1 with respect to the vertical line of the substrate surface. The term "substrate surface" as used herein means a flat substrate surface when there is no step, and for example, the main surface 15 of the preliminary substrate 11 shown in the embodiment (see FIG. 1A).
Is. Hereinafter, for convenience of description, such a substrate surface is referred to as a main surface of the substrate. Further, since the length d of the second inclined surface is set in consideration of the interlayer distance in the layered structure of the oxide superconductor, the thin film growing on one certain first inclined surface and the other first thin film. Since the lattice matching between the two at the boundary with the thin film growing on the inclined surface (the lattice matching in the thickness direction of the thin film) is better than when the length of the second inclined surface is not considered, a good quality thin film is obtained. can get.

Further, in the structure in which the length L of the first inclined surface is set to a value that is an integer multiple (including 1) of the lattice constant in the layer of the layered structure of the oxide superconductor or a value close thereto, Good lattice matching can be achieved between the thin film growing on one of the first inclined surfaces and the thin film growing on the other first inclined surface in the direction orthogonal to the thickness direction.

[0015]

EXAMPLES Examples of the method for forming an oxide superconductor thin film of the present invention will be described below. 1 and 2 are diagrams used for the description. In particular, FIGS. 1A and 1B are sectional views for explaining the substrate used in the method of the present invention.
(C) is a cross-sectional view for explaining a state after forming a thin film of an oxide superconductor on the substrate, and FIG. 2 is a view for explaining a structure of a YBaCuO-based high-temperature oxide superconductor.

In this embodiment, as shown in FIG. 1A, the SrTiO ₃ substrate 11 has a predetermined crystal plane, and in this example, the (001) plane (shown by an imaginary line 13 in the figure) is used. A SrTiO ₃ substrate 11 (hereinafter, referred to as a “preliminary substrate”) having a crystal plane 15 tilted by an angle θ in the [100] or [010] direction is prepared. Here, the angle θ
The preliminary substrate 11 whose angle is 11.3 ° is used. Next, on the main surface 15 of the preliminary substrate 11, a first inclined surface 17 having a length L is formed.
a and the second inclined surface 17 that is in contact with the first inclined surface 17a
A step 17 composed of b and b is continuously formed to form a substrate 19 for forming an oxide superconductor thin film. However, each step 1
7 is the next step 1 on the second inclined surface 17b of the step 17.
The first slanted surfaces 17a of No. 7 are connected to each other so as to be continuous. Further, in this embodiment, each step 1
In No. 7, the first inclined surface 17a is made of SrTiO ₃ (00
1) Formed so as to be composed of faces (the reason will be described later). Further, each step 17 is formed so that the first inclined surface 17a and the second inclined surface 17b are orthogonal to each other (the reason will be described later). The term “perpendicular” here means that the angles formed by the two inclined surfaces are right angles, and may be angles in the vicinity thereof within the scope of the object of the present invention. Further, the length d of the second inclined surface of each step 17 is about 3.9 angstroms (hereinafter referred to as "A °") in this embodiment, and the length of the first inclined surface 17a is set. In this embodiment, the length L is about 19.5 A ° (the reason will be described later).

The substrate 19 having a surface where the steps 17 are continuous is first of all the above-mentioned SrTiO ₃ (00
1) For example, by polishing and processing the substrate (001)
It is obtained by forming the preliminary substrate 11 whose main surface is the surface 15 inclined by θ from the surface, and then forming the step 17 by appropriately performing cleaning treatment, etching treatment, and heat treatment on the preliminary substrate 11. it is conceivable that. By doing so, the possibility of obtaining the substrate 19 is SrTiO ₃ (1
This can also be inferred from the fact that a Ti surface usually appears on the (00) surface.

Next, the first inclined surface 17a is covered with SrTiO ₃
(001) plane, the reason why each step is formed so that the first inclined surface 17a and the second inclined surface are orthogonal to each other, and the respective lengths L and d of the inclined surfaces 17a and 17b.
The reason why is set to each of the above values will be described.

The reason is as follows: When the interlayer distance of an oxide superconductor having a layered structure is represented by c and the lattice constant in the layer of the oxide superconductor is represented by a, The length d of the second inclined surface 17b of the step 17 on the surface of the oxide superconductor thin film forming substrate 19 used in the present invention is ideally d = c, and the length L of the first inclined surface 17a is L = na is ideal (where n is a positive integer): When YBaCuO is used as an oxide superconductor, one example that can substantially satisfy the above conditions of d = c and L = na is a lattice. Sr whose constant is 3.905 A °
This is because it is preferable to use TiO ₃ as in this embodiment. The details will be described below.

FIG. 2 is a diagram schematically showing the structure of YBa ₂ Cu ₃ O ₇ . In the structure of YBa ₂ Cu ₃ O ₇ , the distance c between a Cu layer sandwiched between two Ba layers and another Cu layer above or below the Cu layer is 3.9 A °.
And the distance between two adjacent Ba layers is also close to 3.9 A °, for example, in Document IV (JJ
AP, 26, L1144 (1987)). That is, in YBa ₂ Cu ₃ O ₇ having a layered structure, the interlayer distance is about 3.9 A °. Therefore, the second inclined surface 17b of each step 17 of the substrate 19 is
When the length d of the first inclined surface 1 is about 3.9 A °,
And YBa ₂ Cu ₃ O ₇ films grown in 7a, and the other of the YBa ₂ Cu ₃ O ₇ films grown on the first inclined surface 17a next to the first inclined surface 17a, when good lattice matching considered (It is considered that lattice matching in the thickness direction of the thin film can be secured.).

In the layered structure of the YBaCuO-based high-temperature oxide superconductor, the lattice constant in the layer, that is, the lattice spacing in the a-axis direction or the lattice spacing in the b-axis direction is 3.892 A ° in the former case, and the latter. Then, it is 3.826 A °, and it is known that all are values close to 3.9 A °. Therefore, the fact that the length L of the first inclined surface 17a is 19.5 A ° as described above means 19.5 A °.
Since /3.9=5, the length L of the first inclined surface 17a is L.
Is the Y-axis direction of the YBaCuO-based oxide high-temperature superconductor or b
This means that it is an integral multiple (five times in this case) of the lattice spacing in the axial direction. By doing so, it is considered that the lattice matching in the direction orthogonal to the thickness direction of the YBa ₂ Cu ₃ O ₇ thin films formed on the first inclined surface 17a of each step can be secured.

Further, the first inclined surface 17a of each step is Sr.
If the (001) plane of TiO ₃ is used, SrTi
Since the lattice constant of the O ₃ (001) plane is 3.9 A °,
It is considered that the lattice matching between this crystal plane and the YBaCuO-based thin film formed on the crystal plane is ensured, so that a good YBaCO thin film that is a c-axis oriented film can be obtained. For this reason, the condition of the step 17 is set as described above in this embodiment.

In the above description, the first inclined surface 17
The length L of a is set to 5 times the lattice constant (lattice spacing) of the a-axis or the b-axis of YBaCuO, but the multiple n (however, an integer) may be another value. In this case, a predetermined crystal plane of the SrTiO ₃ substrate, in this example, a crystal plane 15 tilted at an angle θ from the (001) plane (shown by the phantom line 13 in FIG. 1) to the [100] or [010] direction is obtained. Angle θ for
It may be selected so as to satisfy tan ⁻¹ (a _S / na _S ).
Here, n is the above multiple and a _S is the lattice constant of the SrTiO ₃ crystal.

On the oxide superconductor thin film forming substrate 19 obtained as described above, as shown in FIG. 1C, YBaC is formed.
The uO thin film 21 is grown under the conditions capable of forming a c-axis oriented film. Then, in this embodiment, the thin film 21 is c
The axis is perpendicular to the first inclined surface, that is, the substrate 19
The main surface (main surface 15 of FIG. 1A) is inclined by θ. Here, as a method for growing the YBaCuO thin film 21, a known method using a vacuum vapor deposition method, a sputtering method, a CVD method or the like may be used.
In that case, it is considered that there is no need to lower the substrate heating temperature in the method of the present invention, and therefore it can be considered that there is no fear of deterioration of crystal quality due to the substrate heating temperature. In the thin film 21 formed in this way, the crystal orientation orthogonal to the surface thereof is tilted by θ with respect to the c-axis of the thin film, so that electric conductivity in the direction orthogonal to the surface of the thin film 21 is secured.

Although the embodiment of the method for forming an oxide superconductor thin film of the present invention has been described above, the present invention is not limited to the above embodiment. For example, the SrTiO ₃ substrate used in the above-mentioned embodiment may be an Nb-doped SrTiO ₃ substrate. This is preferable because the electrical conductivity of the substrate is enhanced. Further, instead of the SrTiO ₃ substrate, another substrate having a lattice constant close to that of the YBaCuO oxide superconductor may be used. If another oxide superconductor having a layered structure is used, YBaCu
The present invention can also be applied to the formation of thin films other than those based on O.
As another oxide superconductor having a layered structure, for example, B
Examples thereof include iSrCaCuO type and TlBaCaCuO type.

[0026]

As is apparent from the above description, according to the method for forming an oxide superconductor thin film of the present invention, a predetermined substrate having a predetermined step continuously on the substrate surface is used as the thin film forming substrate. Since it is used, a thin film in which the c-axis is slightly inclined with respect to the vertical line of the main surface of the substrate can be obtained. Therefore, the formed thin film exhibits conductivity even in the direction perpendicular to the main surface of the substrate (thin film surface). Therefore, it is useful in device fabrication and the like. Furthermore, for example, since another type of thin film can be continuously formed on the oxide superconductor thin film, the bonding between the other type thin film and the oxide superconductor thin film also has conductivity in the direction perpendicular to the thin film. It can be used as a joint to have. Therefore, for example, it is considered that the utilization range of the oxide superconductor thin film can be expanded.

[Brief description of drawings]

FIG. 1A to FIG. 1C are diagrams for explaining an embodiment, in particular, FIGS. 1A and 1B are explanatory views of a substrate for forming an oxide superconductor thin film, and FIG. 1C is a thin film formation. It is an explanatory view of the situation after.

FIG. 2 is a diagram for explaining an example, showing YBaCuO.
It is a figure with which a structure explanation of a system oxide superconductor is carried out.

[Description of Reference Signs] 11: Preliminary substrate 13: Predetermined crystal plane (for example, (001) plane 15: Surface in which a predetermined crystal plane is inclined by θ (main surface) 17: Step 17a: First inclined surface 17b: Second Inclined surface 19: Substrate for forming oxide superconductor thin film 21: Oxide superconductor thin film

Claims (5)

[Claims] 1. When forming a thin film of an oxide superconductor having a layered structure on a substrate, a step formed on the surface by a first inclined surface and a second inclined surface in contact with the first inclined surface. A method for forming an oxide superconducting thin film, characterized in that a substrate having the following (a) and (b) is satisfied as a substrate for forming an oxide superconducting thin film. (A) As the substrate, a substrate having a crystal plane having a lattice constant equal to or close to the lattice constant in the layer in the layered structure of the oxide superconductor is used, and the first inclined plane is constituted by the crystal plane. . (B) The length d of the second inclined surface is set to an interlayer distance in the layered structure of the oxide superconductor or a value close thereto. 2. The method for forming an oxide superconductor thin film according to claim 1, wherein the length L of the first inclined surface of the substrate is L.
Is a value that is an integer multiple (including 1) of the lattice constant in the layer in the layered structure of the oxide superconductor, or a value close thereto. 3. The method for forming an oxide superconductor thin film according to claim 1, wherein the substrate is an oxide substrate having a perovskite structure and having the step on the surface. A method for forming an oxide superconductor thin film. 4. The method for forming an oxide superconductor thin film according to claim 1, wherein the substrate is a SrTiO ₃ substrate (001).
An SrTiO ₃ substrate having a crystal plane tilted at a predetermined angle from the plane to a [100] or [010] direction as a main surface and the first tilted surface being a (001) plane is used. Method of forming thin film. 5. The method for forming an oxide superconductor thin film according to claim 1, wherein the oxide superconductor is YBaCuO-based or BiSrCa.
A method for forming an oxide superconductor thin film, which is selected from CuO-based and TlBaCaCuO-based.