JPH04170305A - Production of thin film superconductor - Google Patents

Abstract

PURPOSE:To improve the crystallinity, to optimize the oxidation state of superconductor and to improve superconductive characteristics by forming a metal oxide thin film, subjecting to heat treatment and further irradiating the film with UV ray or an electromagnetic wave having shorter wavelength than UV ray. CONSTITUTION:A multiple metal oxide thin film expressed by A-B-Cu-0, Bi-Sr- Ca-Cu-O or Tl-Ba-Ca-Cu-0 is heated at 800-1000 deg.C in oxygen gas for <=30min. In the formula, A is at least one element selected from Y, La and lanthanum series elements, B is at least one element selected from alkaline earth metal elements, and concns. of A, B and Cu satisfy the relation of. 0.5<=(A+B)/Cu<=2.5. Then this film is irradiated with UV ray or an electromagnetic wave having shorter wavelength than UV ray under reduced pressure or in an inert gas atmosphere. Thus, the superconductive thin film having about 90 K critical temp. is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing a superconductor. In particular, it relates to a method for manufacturing thin film superconductors.

[Prior art] Although the details of the superconducting mechanism of oxide superconducting materials represented by the Y-Ba-Cu-0 system are not clear, their transition temperature is as high as the liquid nitrogen temperature or higher, and they are used in various devices such as quantum interference devices. Application to the electronics field is expected.

These materials exhibit a change from insulator (semiconductor) to superconductor depending on the amount of oxygen atoms contained in the crystal, that is, the oxidation state. In order to obtain a good superconducting material, it is necessary to improve the crystallinity and control the oxidation state. Furthermore, when these materials are actually used in various electronic devices, it is essential to obtain thin films of good quality. The thinning of a superconductor is achieved by decomposing the superconductor material into ultrafine particles in an atomic state and then depositing the superconductor material on a substrate as a thin composite oxide film. A more homogeneous film with good crystallinity can be obtained compared to a sintered body.

However, the crystallinity of the thin film obtained in the process of forming a composite oxide thin film and the amount of oxygen incorporated into it are not necessarily sufficient to obtain good superconducting properties. It is necessary to carry out treatments to improve the properties and optimize the oxygen content.

[Problems to be Solved by the Invention] It is known that in an oxide superconducting thin film, its crystal structure and oxygen atom content, that is, its oxidation state, have an effect on superconducting properties. These crystallinities and oxidation states can be controlled to a certain extent by controlling the atmosphere and substrate temperature during thin film production.

However, because it interacts with various other factors during fabrication (e.g. collision of accelerated ions with the membrane surface, release of oxygen from the membrane surface due to being placed in a reduced pressure state, etc.), it depends only on the fabrication conditions. There are limits to improving the crystallinity of the film and optimizing the oxidation state. Therefore, with conventional thin film forming techniques, there have been problems in that crystallinity is low, oxidation state, etc. cannot be optimized, and it is difficult to obtain a good thin film.

In order to solve the problems of the prior art described above, the method of the present invention aims to provide a thin film manufacturing method that can improve crystallinity and optimize the oxidation state.

[Means for Solving the Problems] In order to achieve the above object, the method for manufacturing a thin film superconductor of the present invention is a method for manufacturing a thin film superconductor including a metal oxide thin film on a substrate, the method comprising: a metal oxide thin film on a substrate; After the metal oxide thin film is formed, the metal oxide thin film is subjected to heat treatment, and the surface of the metal oxide thin film is further irradiated with ultraviolet rays or electromagnetic waves having a shorter wavelength than the ultraviolet rays.

In the configuration of the present invention, as the metal oxide thin film,
Preferably, an AB-Cu-0 composite compound is used.

[Here, A is at least one element selected from Y, La, and La series elements (atomic numbers 57.59 to 60.62 to 71), and B is at least one element selected from alkaline earth metals (group IIa) elements. 1 type of element, or 1. 8. The concentrations of B element and Cu element are within the range of the following formula.

0.5≦(A+B)/Cu≦2.5] Furthermore, in the structure of the present invention, as the metal oxide thin film, Bi-3r-Ca-Cu-O) or Tl-B
It is preferable to use an a-Ca-Cu-0 composite compound.

Further, in the configuration of the present invention, the heat treatment conditions are preferably such that heating is performed in oxygen gas to a temperature in the range of 800 to 1000°C.

Further, in the configuration of the present invention, the heat treatment time is 3
Preferably, the time is within 0 minutes.

Furthermore, in the configuration of the present invention, it is preferable that the irradiation with ultraviolet rays or electromagnetic waves having a shorter wavelength than ultraviolet rays be performed under reduced pressure or in an inert gas atmosphere.

[Function] According to the configuration of the present invention described above, by performing heat treatment in oxygen after depositing the thin film superconductor, and then irradiating it with ultraviolet rays or electromagnetic waves having a shorter wavelength than ultraviolet rays,
Improved crystallinity and optimized oxidation state have been achieved. By using this method, it is possible to improve the crystallinity and optimize the oxidation state, which was extremely difficult to achieve by controlling only thin film manufacturing conditions, which has been conventionally performed.

In addition, the effect can be exerted through extremely simple and short-time processing that only requires short-time heat treatment and electromagnetic wave irradiation as a process after forming the thin film.

Further, according to a preferred configuration of the method of the present invention in which an A-B-Cu-○ composite compound is used as the metal oxide thin film,
A metal oxide thin film with excellent superconducting properties can be obtained.

In addition, as a metal oxide thin film, Bi-Sr-Ca-Cu-
According to a preferable configuration of the method of the present invention in which a composite compound of O) or Tl-Ba-Ca-Cu, -0 is used, a metal oxide thin film having excellent superconducting properties as described above can be obtained.

In addition, the heat treatment conditions are 800 to 1000 in oxygen gas.
According to a preferred configuration of the method of the present invention in which heating is performed to a temperature in the range of .degree. C., it becomes an effective means for further improving crystallinity and optimizing the oxidation state.

Further, according to a preferable configuration of the method of the present invention in which the heat treatment time is 30 minutes or less, it becomes an effective means for improving crystallinity and optimizing the oxidation state as described above.

Furthermore, according to a preferred configuration of the present invention in which irradiation with ultraviolet rays or electromagnetic waves with a shorter wavelength than ultraviolet rays is performed under reduced pressure or in an inert gas atmosphere, oxygen elements, especially around copper ions in the crystal, are can be generated to optimize the oxidation state.

[Example 1] An example of the present invention will be described in more detail below.

As the method for forming the metal oxide thin film, for example, high frequency magnetron sputtering or the like is utilized to form an oxide thin film 12 on a substrate 11, as shown in FIG.

Next, after heat-treating this oxide thin film 12, it is irradiated with ultraviolet rays or electromagnetic waves 13 having a shorter wavelength than ultraviolet rays under reduced pressure or in an inert gas. In this case, substrate 1
1, a single crystal substrate is effective in order to form a quaternary oxide thin film 12 with high crystallinity, and Mg0XLaA10
, LaGa0, SrTiO3 and the like are particularly effective.

Next, this thin film was heated at 800°C to 1000°C in an oxygen atmosphere.
℃ to perform heat treatment, and then irradiate it with ultraviolet light or electromagnetic waves with a shorter wavelength than ultraviolet light. At this time, there is no need to heat the sample.

Ultraviolet rays or electromagnetic waves with wavelengths shorter than ultraviolet rays include ordinary W, Mo, Rh, Cu, and Fe.

X-ray tube such as Co, Cr, At, Mg, Zr or H,
Radiant light from an ultraviolet source such as He or Ne or a mercury lamp, or gamma rays emitted by gamma decay from a radioactive element, etc., can also exhibit similar effects. The irradiation process is performed by irradiating the superconducting thin film with this electromagnetic wave.

Specific examples will be described below.

Example 1 Bi and Sr were sintered by high-frequency planar magnetron sputtering using a (100)-plane MgO single crystal as a model shown in FIG. 1 as the substrate 11.

A target made of an oxide containing the elements Ca and Cu was sputter-deposited in a mixed gas atmosphere of Ar and 02 to deposit BiSrC with C-axis orientation on the substrate.
aCu2O□ was deposited as a thin film. The crystal structure of this thin film is Cu-0 in the unit crystal lattice.
It has a structure called a low-temperature layer that includes two surfaces.

In this case, the gas pressure was 0.5 Pa, the sputtering power was 150 W, the sputtering time was 10 minutes, and the thickness of the thin film was 1
The thickness was 10 nm, and the substrate temperature was 580°C.

The thin film thus obtained exhibited superconductivity, and among its superconducting transition temperatures, the zero resistance temperature was 60 and the onset temperature was 70.

Next, this thin film was heat-treated at 920° C. for 20 minutes in 1 atm oxygen. In this treatment, the thin film is placed in a quartz tube with a diameter of 5 cm, and oxygen gas is supplied at a flow rate of 100 per unit time.
It was heated from the surrounding area with an electric heater while flowing at a rate of m1/min. The temperature change rate is 2000℃/h when the temperature rises, and natural cooling when the temperature decreases (approximately -2000℃/h at around 900℃)
And so.

Furthermore, the thin film subjected to this heat treatment was placed in a vacuum (1Q-2P
a) and irradiated with ultraviolet rays for 30 minutes at room temperature. A low-pressure mercury lamp (maximum intensity emission wavelength: 250 nm) was used as the ultraviolet light source. The UV intensity at the sample surface is approximately 3
mW/cnf).

FIG. 2 shows the temperature change in resistivity of the thin film immediately after its formation (a) and after heat treatment and ultraviolet irradiation (b).

As can be seen from FIG. 2, the onset temperature was about 70K immediately after formation, but rose to about 20K, or about 90K, after treatment. However, it has been confirmed that if a thin film is subjected to only heat treatment or electromagnetic wave irradiation treatment immediately after formation, the superconducting properties deteriorate rather than before the treatment.

Therefore, the reason for the improvement in superconducting properties as shown in Figure 2 is that during the thin film formation process, the impact of ions on the thin film and changes in various conditions associated with the amount of film deposited (for example, changes in the amount of radiant heat absorbed by the film) The crystallinity was incomplete due to changes in the effective substrate temperature, etc., but the crystallinity was restored by heat treatment, and the oxidation state was further optimized by subsequent ultraviolet irradiation. Conceivable.

The present inventors have discovered that there is an optimal temperature and treatment time for heat treatment, which is a process for improving crystallinity.

If the temperature is too high, recrystallization of the film material progresses and at the same time some of the atoms within the film evaporate, resulting in deterioration of the surface condition of the film and, in fact, deterioration of its superconducting properties. Furthermore, if the temperature is low, the effect of improving crystallinity tends to disappear.

The treatment temperature is within an effective range of 800°C to 1000°C, with temperatures around 950°C being particularly effective. Further, the heat treatment needs to be carried out in an oxidizing atmosphere, and in particular, it is most suitable to carry out the heat treatment in an oxygen atmosphere. Regarding the processing time, if the processing time is too long, the effects of extreme recrystallization of the film material and evaporation of atoms will appear, which will actually have the opposite effect on improving the superconducting properties, so a processing time of 30 minutes or less is effective. I discovered that.

This heat treatment process alone improves crystallinity, but
The oxidation state of the oxide thin film becomes uncertain.

Metal oxide superconductors are known to have moderate oxygen vacancies, which play an important role in their superconducting properties.

This means that the oxidation state of the superconductor, in other words, either too much or too little oxygen can cause deterioration of the superconducting properties.

Therefore, if the oxidation state can be controlled to an optimal value after the heat treatment process, it is possible to further improve the superconducting properties. The present inventors have already confirmed that when a metal oxide superconductor is irradiated with ultraviolet rays or electromagnetic waves with a shorter wavelength than ultraviolet rays, oxygen vacancies are efficiently generated in the oxygen sites in the crystal, especially around copper ions. ing.

Therefore, in the method for producing a thin film superconductor of the present invention, this technique is applied to achieve optimization of the amount of oxygen vacancies, that is, the oxidation state, with very good controllability. Furthermore, irradiation with ultraviolet rays or electromagnetic waves with a shorter wavelength than ultraviolet rays has the effect of further improving crystallinity to some extent, making the application of this method extremely effective.

Optimization of the oxidation state can also be achieved by heating in an appropriate atmosphere or by irradiating reducing ions (for example, hydrogen ions). However, in the irradiation of electromagnetic waves with a shorter wavelength than the ultraviolet rays of the present invention,
It is a very effective method, as it does not require heating the sample, the oxidation state can be controlled with high precision by adjusting the irradiation time, and it does not cause deterioration of crystallinity.

In the above examples, the irradiation with ultraviolet rays or electromagnetic waves with a shorter wavelength than ultraviolet rays was carried out in a vacuum, or the irradiation was not limited to a vacuum, but under a reduced pressure of 1 atmosphere or less, or
It has been confirmed that similar effects can be obtained even in inert gases such as r and He.

In addition, the metal oxide thin film 12 includes Bi-Sr-Ca-C
In addition to u-0, A-B-Cu-0 or T l-B
The effectiveness of the present invention can also be achieved by using a composite compound of a-Ca-Cu-0 deposited on a substrate by a physical vapor deposition method such as thermal evaporation, e.g., electron beam evaporation or laser beam evaporation. We have confirmed that this will be done. Although the compositional formula of these metal oxide superconductors has not yet been clearly determined,
-B-Cu-0, oxygen-deficient perovskite (A
, B) 3Cua 07-x, and regarding the Kono type material, the present inventors have determined that A is Y and La.

and La series elements (atomic number 57.59-60.62
~71), B is Ba, Sr, etc.
If the element ratio of at least one group A element and the produced thin film is within the range of 0.5≦(A+B)/Cu≦2.5, superconductivity will occur even if there is a slight difference in critical temperature. It was confirmed that the phenomenon was observed.

Also, B i-3r-Ca-Cu-0, T I-Ba
-Ca-Cu-0 superconductors can have critical temperatures exceeding 100 K and are extremely useful in practice, but they contain a large number of elements, making it relatively difficult to form excellent superconducting films. The present inventors believe that if the manufacturing conditions are strictly controlled,
We confirmed that these materials can also be made into thin films with good reproducibility. Thin film formation methods are not limited to physical vapor deposition, but include chemical vapor deposition, such as atmospheric or reduced pressure chemical vapor deposition, plasma chemical vapor deposition, and photochemical vapor deposition. It was also confirmed that it is effective if the ratio of the component elements is matched.

The mechanism by which this type of metal oxide thin film develops superconductivity and the details of changes in superconducting properties due to differences in constituent elements are not clear. However, there is a consensus in previous studies that improving crystallinity and controlling the oxidation state have a significant impact on superconducting properties.

The present invention establishes a process for improving crystallinity and optimizing the oxidation state for forming a thin film superconductor.

As explained above, according to the embodiments of the present invention, a process for ensuring the reliability and long-term stability of an element using an oxide high temperature superconductor is provided, which has extremely great industrial value. Since the superconductor used is more homogeneous and thin-film single crystal than conventional sintered bodies, the present invention can realize a superconducting element with extremely high precision. The major feature of this work is that it has found an efficient and simple process for improving crystallinity and optimizing the oxidation state.

[Effects of the Invention] As explained above, according to the present invention, a thin film superconductor is heat-treated in oxygen after being deposited, and then crystalline is improvement and optimization of oxidation state can be achieved.

Further, as a metal oxide thin film, A-B-Cu-0 composite compound, B i-Sr-Ca-Cu-0, or T l-B
According to a preferred configuration of the method of the present invention in which the a-Ca-Cu-0 composite compound is used, a metal oxide thin film with even better superconducting properties can be obtained.

In addition, the heat treatment conditions are 800 to 1000 in oxygen gas.
According to a preferred configuration of the method of the present invention in which heating is performed to a temperature in the range of .degree. C., it becomes an effective means for further improving crystallinity and optimizing the oxidation state.

Further, according to a preferable configuration of the method of the present invention in which the heat treatment time is 30 minutes or less, it becomes an effective means for improving crystallinity and optimizing the oxidation state as described above.

Furthermore, according to a preferred configuration of the present invention in which the irradiation with ultraviolet rays or electromagnetic waves with a shorter wavelength than ultraviolet rays is performed under reduced pressure or in an inert gas atmosphere, the loss of oxygen elements in the crystal, especially around copper ions, can be eliminated. can be generated and the oxidation state can be optimized.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing a method for manufacturing a thin film superconductor according to an embodiment of the present invention, and FIG. 2(a) shows a temperature change curve of resistivity immediately after the thin film superconductor is deposited on a substrate. Figure, Figure 2 (b
) is a diagram showing a temperature change curve of resistivity after heat treatment and irradiation with ultraviolet rays or electromagnetic waves having a shorter wavelength than ultraviolet rays. 11... Substrate, 12... Metal oxide thin film, 13...
- Ultraviolet light or electromagnetic waves with a shorter wavelength than ultraviolet light.

Claims (1)

[Scope of Claims] (1) A method for manufacturing a thin film superconductor including a metal oxide thin film on a substrate, which comprises forming a metal oxide thin film and then subjecting the metal oxide thin film to heat treatment, The method for producing a thin film superconductor further comprises irradiating the surface of the metal oxide thin film with ultraviolet rays or electromagnetic waves having a shorter wavelength than ultraviolet rays. (2) The method for producing a thin film superconductor according to claim 1, wherein an AB-Cu-O composite compound is used as the metal oxide thin film. [Here, A is at least one element selected from Y, La, and La series elements (atomic numbers 57, 59-60, 62-71), and B is at least one element selected from alkaline earth metals (group IIa) elements. One type of element, and A, B elements, and C
The concentration of the u element is within the range of the following formula. 0.5≦(A+B)/Cu≦2.5] (3) As the metal oxide thin film, Bi-Sr-Ca-Cu
-O) or a Tl-Ba-Ca-Cu-O composite compound. (4) Heat treatment conditions are 800-1000℃ in oxygen gas
2. The method for producing a thin film superconductor according to claim 1, wherein the heating is performed to a temperature in the range of . (5) The method for producing a thin film superconductor according to claim 1, wherein the heat treatment time is 30 minutes or less. (6) The method for producing a thin film superconductor according to claim 1, wherein the irradiation with ultraviolet rays or electromagnetic waves having a shorter wavelength than ultraviolet rays is performed under reduced pressure or in an inert gas atmosphere.