JPH07112927B2 - Method for manufacturing oxide superconducting thin film - Google Patents

Description

The present invention relates to a method for producing an oxide superconducting thin film.

[Prior Art] The superconducting phenomenon is a phenomenon in which electric resistance becomes zero at a certain temperature or lower. Figure 3 shows typical resistance-temperature characteristics. The temperature at which the resistance begins to decrease is set to onset Superconducting transition temperature T _con ,
The temperature at which it becomes completely zero is called the zero-resistance superconducting transition temperature T _cend . By the way, an oxide with a composition formula of Ln ₁ Ba ₂ Cu ₃ O _7-d (Ln is Y (yttrium) or a lanthanoid element) has a superconducting transition temperature higher than that of materials known until then. It was discovered that the substance is a conductor, and the possibility of its application is being further highlighted as its superconducting transition temperature T _cend exceeds the boiling point of liquid nitrogen, 77K. In order to apply this oxide superconductor to electronic devices, superconducting wiring, etc., a thin film forming technique is essential.

Several methods have been reported as techniques for forming these oxide superconductor thin films. These can be roughly divided into a sputtering method and a vapor deposition method.

As a typical example of the sputtering method, reference (1) “Y. Enomoto et
al.; Japanese Journal of Applied Physics, vol.26, N
o.7, p.L 1248 ”. The manufacturing method according to this document will be described below.

As the target material, a sintered body made of Y-Ba-Cu-O is used. First, a target is sputtered in a mixed gas of oxygen and argon to form a thin film of Y-Ba-Cu-O on a SrTiO ₃ substrate. Oxide superconductors are extremely sensitive to composition, and even a slight deviation causes a decrease in superconducting transition temperature, loss of superconductivity, and so on. :
The composition of the target is adjusted to an appropriate atomic ratio so that Ba: Cu = 1: 2: 3. Since it does not become a superconductor as it is, the superconducting transition temperature T _cend defined by the temperature at which the electric resistance becomes zero is taken out from the sputtering apparatus after forming the thin film and subjected to an appropriate heat treatment in oxygen gas at 77K.
The thin film as described above is produced. However, since the ratio of sputtering from the target is different for each element,
As the sputtering progresses, the composition ratio of the target surface changes, so that the atomic ratio of the peritoneum formed on the substrate deviates from Y: Ba: Cu = 1: 2: 3. Therefore, T _cend is 7
It has a serious drawback that it is greatly reduced from 7K and has no reproducibility.

As a typical example of the vapor deposition method, reference (2) “RB Laibowit
z.; Physical Review B, vol.35, No.16, p.8821 ”. The manufacturing method according to this document will be described below.

The three metals of Y, Ba, and Cu are loaded into different hearths, and the three metals are simultaneously deposited by using three independent electron beam sources. At that time, an oxygen atmosphere of about 10 ⁻³ Torr was introduced into the vacuum apparatus, and vapor deposition was carried out in this background gas atmosphere. Furthermore, by heating the substrate to about 450 ° C., oxidation is promoted during vapor deposition. Since the amount of oxygen contained in the film is insufficient even if the film is co-deposited in an oxygen atmosphere, after the deposition is completed, heat treatment is performed at a temperature of 900 to 950 ° C in an electric furnace in which oxygen gas is flown. . However, in this method, three metals are vapor-deposited simultaneously in an oxygen atmosphere, and the temperature of the substrate and the film thickness monitor are different, so that the atomic ratio of Y, Ba and Cu is 1: 2 as in the former sputtering method. It is difficult to control: 3 accurately and reproducibly, 77K
It has a major drawback that it is difficult to form a thin film having the above superconducting transition temperature with good reproducibility.

[Problems to be Solved by the Invention] Both of the above two methods for forming an oxide superconducting thin film have a fatal drawback that accurate control of the atomic ratio, which has the greatest effect on the superconducting properties of the thin film, cannot be reproducibly performed. is doing. As a method of forming a superconducting thin film that can solve this drawback, a method of vapor-depositing three metals in a stack is described in reference (3) “BYTsaur et.
al.; Applied Physics Letters, vol.51, No.11, p.858 ''
And is proposed in Reference (4) “Fumi Yamamoto et al .; Japanese Patent Application No. 62-268343”.

In the method in Reference (3), first, three metals of Y, Ba and Cu are laminated and vapor-deposited in the order of Cu → Ba → Y so that the composition ratio of Y: Ba: Cu = 1: 2: 3. There is. Using this laminated structure as one unit, laminated vapor deposition is performed 6 times. After the completion of vapor deposition, heat treatment is performed in an electric furnace in which oxygen gas is flown to promote oxidation,
Obtaining oxide superconducting thin film. The top data of the superconducting transition temperature T _cend of this oxide superconducting thin film is 72 when yttrium-stabilized zirconia (YSZ) is used as the substrate.
When K and sapphire are used as the substrate, 40K is reported.
In either case, T _cend is 77K or less.

In Reference (4), Yb (ytterbium) is used in place of Y in order to form a solid solution with Ba.
An oxide superconducting thin film is obtained by heat-treating a laminated vapor deposition film composed of three metals, Cu and Cu, in oxygen. As a result, the superconducting transition temperature T _cend rises, and 75 K is obtained when MgO is used as the substrate as top data. But,
This value does not exceed the boiling point of liquid nitrogen temperature of 77K, which is two criteria for application.

FIG. 4 shows T _con and T _cend of 10 samples which were deposited in the same lot and annealed in oxygen. T _con does not vary, but T _cend has a large variation from 50K to 75K, and has the drawback of poor reproducibility. The cause is considered as follows.

When an oxide superconducting thin film is produced by heat-treating the above-mentioned metal-deposited vapor-deposited film in oxygen gas, mutual diffusion of metals and oxidation proceed simultaneously during heat treatment. Therefore,
Because only Ba, which is easily oxidized, moves toward the surface with high oxygen concentration and has uneven Ba concentration, or the diffusion constant of the oxide is extremely small, diffusion does not proceed sufficiently. The atomic ratio of Ba and Cu is the composition ratio 1:
A part that deviates from 2: 3 is formed. Therefore, T
_It is thought that _cend will decrease. Since the diffusion phenomenon of the mixture of oxide and metal is slightly different for each sample, T _cend variation and poor reproducibility between samples in the same lot are also due to the same cause.

The object of the present invention is to solve the above-mentioned problems, and with the superconducting transition temperature T _cend of 77 K or more, the superconducting transition temperature between the samples manufactured in the same lot as well as between the samples manufactured in different lots. It is to provide a method for manufacturing an oxide superconducting thin film that can suppress variations.

[Means for Solving the Problems] In order to achieve such an object, the present invention separates each metal such that the atomic ratio of Y or lanthanoid metal, Ba and Cu is 1: 2: 3. Using the three-layer deposition layer formed by sequentially depositing on the substrate as one unit, the lamination unit is laminated once or a plurality of times on the substrate, and is deposited in an atmosphere or a vacuum in which a chemical reaction with each metal does not occur. The film is characterized in that the film is heated to cause mutual diffusion in the laminated deposition film to form a uniform alloy, and is heat-treated in oxygen gas.

[Operation] In the present invention, a composition ratio of 1: 2: 3 is obtained by heat treatment at a temperature at which the deposited films are sufficiently interdiffused with each other in an atmosphere that does not react with the laminated metal to form an alloy. Since the heat treatment is carried out in oxygen gas after making both of them uniform in the depth direction, uniform Ln ₁ Ba ₂ Cu ₃ O _7-d (Ln is Y or a lanthanoid element) can be formed with good reproducibility. Therefore,
It is possible to manufacture an oxide superconducting thin film having a high superconducting transition temperature and less variation in superconducting properties.

[Examples] Hereinafter, the present invention will be described in detail with reference to Examples. The oxide superconducting thin film is manufactured in steps.

Y (or lanthanoid metal), Ba and Cu are respectively loaded into independent hearths of electron beam evaporation.

Three metals are sequentially stacked and deposited by electron beam evaporation on a substrate formed of MgO, SrTiO ₃ , ZrO, YSZ or Al ₂ O ₃ . Each film thickness is set so that the atomic ratio of Y (or lanthanoid metal), Ba, and Cu is 1: 2: 3.
In this embodiment, the total thickness of the three-layer laminated film is 10 to 200 nm.
I chose a range of degrees. The stacking order was tried for all the permutations, but there was no change in the superconducting properties. Therefore, any stacking order may be used.

One unit is the above-mentioned three-layer laminated structure, and one unit is
Laminate deposition on the substrate once or multiple times. When this unit is laminated on the substrate only once, this step is completed in the step.

After vapor deposition, heat treatment is performed in a gas (inert gas, nitrogen gas, carbon dioxide gas, etc.) that does not react with the laminated vapor deposition film or in vacuum to form a uniform alloy. The heating rate and the cooling rate are
It is 10 ° C / h to 1000 ° C / h. If the heat treatment temperature is high, the required treatment time is short, and conversely, if the heat treatment temperature is low, the homogenization takes a long time. From a practical point of view, the heat treatment temperature is preferably 400 ° C. or higher, and the heat treatment conditions of 400 ° C. to 800 ° C. for 10 minutes to several hours can be selected.

After that, heat treatment is performed in an electric furnace in which oxygen gas is passed. The rate of temperature rise is 10 ℃ / h to 1000 ℃ / h, and the rate of temperature decrease is 10 ℃ / h to 5
It is 00 ° C / h. The lower limit of the heat treatment temperature is 700 ° C. From a practical viewpoint, the heat treatment temperature is preferably 750 ° C or higher,
It is possible to select heat treatment conditions of ℃ to 950 ℃, 10 minutes to several hours.

FIG. 1 shows experimental results of resistance and temperature of the typical oxide superconducting thin film obtained in this example. Yb as lanthanide metal
The substrate used was MgO. The stacking order was Yb → Ba → Cu, the film thickness d ₀ of the stacking unit of the three metals was 200 nm, and the stacking number n of the stacking unit was 3 times. Therefore, the film thickness d of the entire laminated film is d = d ₀ n = 600 nm. Nitrogen gas was used in this embodiment as a gas that does not react with Yb, Ba and Cu. The heat treatment temperature and time in nitrogen gas were 800 ° C and 2 hours, respectively. The temperature raising / lowering rate was 500 ° C / h.

As shown in Fig. 1, the superconducting transition temperature T at which the resistance becomes zero
_{The cend} is 79K, which is above the nitrogen boiling point of 77K.

Fig. 2 shows the same lot or different lots.
The T _con and T _cend of each sample are shown. The variation of the data is very small and T _cend exceeds 77K for all samples. This indicates that there is extremely small variation in data not only between samples prepared in the same lot but also between samples of different lots. It is obvious that the effect of introducing the process of the above embodiment is great.

The above obtained results show that the lanthanoid metal is Yb and the substrate is Mg.
Although the conditions are specified and described such as O, similar results were obtained not only for this combination but also for the contents described in the examples. Further, in the present embodiment, in the heat treatment in a gas that does not react with the deposited metal vapor, it is described that the heat treatment in oxygen gas is performed after lowering the temperature to room temperature.
Of course, after performing the heat treatment at the predetermined heat treatment temperature for the predetermined time in the former heat treatment, the gas may be switched to the oxygen gas and the heat treatment in the oxygen gas may be started without lowering the temperature.

In addition, although the method of using the electron beam as the metal deposition method has been described in the present embodiment, it is needless to say that the oxide superconducting thin film can be manufactured by resistance heating vapor deposition, high frequency heating vapor deposition, or sputter vapor deposition. .

[Effects of the Invention] As described above, according to the present invention, an oxide superconducting thin film having a zero resistance superconducting transition temperature of a boiling point of 77K or higher at a nitrogen temperature can be produced with good reproducibility and small variation. There is an effect that it can open up a wide range of applications to electronic devices and superconducting wires that operate at a nitrogen temperature.

[Brief description of drawings]

FIG. 1 is a diagram showing resistance / temperature characteristics of an oxide superconducting thin film according to an embodiment of the present invention, FIG. 2 is a temperature characteristic diagram of an oxide superconducting thin film according to an embodiment of the present invention, and FIG. 3 is an oxide. FIG. 4 is a resistance / temperature characteristic diagram of a superconductor, and FIG. 4 is a temperature characteristic diagram of an oxide superconducting thin film produced by a conventional manufacturing method.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masashi Mukata 1-1-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (72) Inventor Koji Takaragawa 1-1-6 Uchisai-cho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) References Japanese Patent Laid-Open No. 64-72425 (JP, A)

Claims (1)

[Claims] 1. Y or lanthanoid metal, Ba and Cu
As a unit, a three-layer deposition layer formed by sequentially depositing each of the metals so that the atomic ratio of is 1: 2: 3 is defined as one unit, and the lamination unit is formed on the substrate once or a plurality of times. Laminated layers are deposited, and the deposited film is heated in an atmosphere or a vacuum in which a chemical reaction with each of the metals does not occur, causing mutual diffusion in the deposited film to form a uniform alloy. A method for producing an oxide superconducting thin film, characterized by heat treatment.