JPH03197320A - Manufacturing method of thin film superconductor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing a thin film superconductor of an oxide superconductor having a high critical temperature, and particularly to a method for producing a thin film of an oxide superconductor having a N d2CuOa type crystal structure. be.

Conventional technology Ba is an oxide superconductor with a high superconducting transition temperature.
-La-Cu-0 system superconductor discovered [C, C, Hednorf? Sito, K., Müller, J., G., Bedno-Rz and A.
, Muller) w Buhuitoshi 1 lift fu 1? ZeitshriFtfur Physi
k B)-Condensed Matter, v
ol, 64°189-193 (+986)]. Since then, many new oxide superconductors have been discovered.

By the way, recently, N d-Ce-C, which has a different charge transport carrier in the normal conduction state from these conventional oxide superconductors, has been developed.
A new oxide superconductor with the N d2 C110A type crystal structure represented by u-0 has been discovered.
ra, H, Takagi and S, Uchida)
, 2i f + - (Nature) vol, 337, 34
5-347 (1989)]. Although the details of the superconducting mechanism of this type of material are not clear, the transition temperature may be even higher, and promising applications such as the realization of new devices are expected.

Problems to be Solved by the Invention However, with current technology, Nd-Ce-Cu-0 based materials can only be formed mainly through the process of sintering, and therefore can only be obtained in the form of ceramic powder or blocks. On the other hand, when putting this type of material into practical use,
Although there is a strong demand for processing into a thin film, it is considered extremely difficult to fabricate a thin film with good superconducting properties using conventional techniques.The present invention aims to solve the problems of the conventional techniques. This is the purpose.

Means for Solving the Problems The present invention provides a method in which the main component is (A
1- *B By introducing this, the oxygen content is brought to an optimum value lower than the stoichiometric ratio. Here, A is Nd, Ss.

B represents at least one element of Ce and Th. X represents 0.06≦
It is a numerical value in the range of X≦0.08.

Effects The present invention, with the above-described configuration, has made it possible to obtain a high-quality, high-performance thin film superconductor having an Nd2CLIOa type crystal structure with good reproducibility.

EXAMPLE The present inventors fabricated a thin film of this oxide superconductor having the N d2Cuo a-type crystal structure by sputter deposition, and investigated in detail the relationship between the fabrication conditions and the superconductivity of the thin film. As a sputtering target, a sintered body obtained by heat-treating an oxide containing element A, element B, and Cu at high temperature in the atmosphere was used. However, A is Nd, S
m, at least one of Pr, B is Ce, Th
Indicates at least one element of The result was 400
For example, Nd
+, 5sCe@, and +5Cuba thin films were formed by sputtering a target containing Nd, Ce, and Cu, and crystallized r4 films were successfully obtained. It was also confirmed that the use of a single crystal substrate has the effect of improving the crystallinity of the Nd+, 5sCeu, +5CIJOJ thin film compared to the case where an amorphous material such as alumina is used as the substrate. M8
When a 0 single crystal is used as a substrate, (001),
Alternatively, it is oriented in the (110) crystal axis direction. Especially 5rTi03. BaTio3. LaAlO3, LaGa
When a single crystal with a perovskite crystal structure such as O3 is used as a substrate, N
The inventors have discovered that the crystal axes of IL, 5sCee, and +5CuOa are oriented, and a thin film with good crystallinity can be obtained.

Furthermore, in the case of producing an oxide thin film, a mixture of approximately equal amounts of an inert gas such as argon and oxygen or an oxidizing gas is used as a sputtering gas. However, N To CuO
In an oxide superconductor with an a-type crystal structure, when the film is formed with an extremely low partial pressure of oxygen or oxidizing gas in the sputtering gas, surprisingly good superconductivity can be obtained, that is, almost the same as that of ceramic materials. The present inventors have discovered that the following can be obtained with good reproducibility.

The cause of this is not clear at present, but it is said that a reducing atmosphere is good for sintering ceramics made of this type of material, and by lowering the oxygen partial pressure during sputter deposition, unnecessary oxygen is removed from the thin film. or the composition of the thin film is Nd+, 5sCes, +5
It seems that good results are obtained because CuOa-u and an appropriate amount of oxygen vacancies are introduced. Furthermore, it has been found that good superconducting properties can be obtained even when using only an inert gas that does not contain any oxygen or oxidizing gas. It was confirmed that argon is relatively easy to use as an inert gas and gives good results. The reason for these is that although a certain amount of oxygen is required to create the Nd2CuOn type crystal structure, it is thought that the oxygen can be sufficiently supplied from the target.

Sputter-deposited films, especially when the substrate temperature is 750°C to 10°C.
Although the crystallinity of the film at 00°C is sufficient to obtain superconducting properties, it is necessary to heat it for a certain period of time in the temperature range of 900°C to 1100°C in the air or in an atmosphere containing oxygen at 10-3 atmospheres or more. It was confirmed that by doing so, it was possible to improve crystallinity and obtain even better superconducting properties. Additionally, it has been found that rapid cooling to room temperature or lower after heat treatment reduces the oxygen content and facilitates subsequent reduction treatment.

In addition, the present inventors have discovered that optimal superconducting properties can be obtained by performing appropriate reduction treatment and reducing the oxygen content to an optimal value lower than the stoichiometric ratio by introducing oxygen vacancies. .

The method of reduction treatment is in a vacuum or at an oxygen partial pressure of 10
It has been found that heating in an inert gas atmosphere such as argon at a pressure of -3 atmospheres or less is effective. At this time, the amount of oxygen desorbed or the amount of defects strongly depends on the oxygen diffusion process. In other words, the processing temperature when heating! ! The appropriate temperature range is 500°C to 900°C, but the required processing time is also affected by the film thickness and surface condition of the superconductor, but it has been found that the higher the processing temperature, the shorter the processing time. . However, if the processing time is too long, the superconductivity will be impaired, and it has been found that there is an optimum processing time for each processing temperature.

As another reduction treatment method, we have discovered that superconducting properties can be obtained by replacing oxygen with fluorine by heating in an atmosphere containing at least fluorine gas.

More specific examples are shown below.

Using an oxide ceramic sintered body containing Nd, Ce, and Cu as a target, strontium titanate (100
) A thin film was fabricated on the surface of the substrate by high-frequency planar magnetron scattering. This target is Nd2
A sintered body obtained by subjecting O3, CeO2, and CuO to heat treatment in the air at 1050° C. for 8 hours was used. Although pure argon gas was used as the sputtering gas, a thin film with good crystallinity could be formed. The reason for this seems to be that a certain amount of oxygen is required to create the N d2Cuo a type crystal structure, and that oxygen is most suitably supplied from the target.

When the temperature of the substrate during vapor deposition was 400°C to 1000°C, signs of superconductivity were observed in the electrical resistance of the thin film at low temperatures, but especially in thin films formed at 750°C to 1000°C, zero resistance was observed. was confirmed to be about 20%, and the crystallinity was also good and the reproducibility was excellent.

More specific examples will be shown below in order to better understand the content of the present invention.

Using an oxide ceramic sintered body of Nd+, 5sCei, +sCu'50x as a target, Mgo,
Alternatively, a thin film was formed on a (100)- or (110)-plane single crystal substrate of SrT 1o3.

Sputtering power 160W, sputtering gas pressure 3X10-3
By performing sputter deposition in about 1 hour under the condition of Torr, about 0. 5-0. A thin film with a thickness of 8 μm was obtained. The sputtering gas was pure argon gas, and the substrate temperature was varied to investigate the relationship between crystallinity and superconducting properties. After the above process, we investigated the composition of the thin film and found that the ratio of metal elements was Nd:Ce:Cu=1.
85: 0.15: 1. It was in a nearly stoichiometric ratio with O. The crystal structure of the thin film was also investigated by X-ray diffraction.

As a result, it was found that when a 5rTiO3 (100) substrate was used as the substrate, the formed tilllH,tc axis was oriented perpendicularly to the substrate to have an N d2CuOa type crystal structure. When a 5rT1C)3(110) substrate is used, a thin film of the (103) plane grows, and Mg0(1
The (110)ii and (100) planes grew on the 00) plane and the (110) plane, respectively. Substrate temperature to 7
The crystallinity of the film at 50°C to 1000°C is sufficient to obtain superconducting properties, but even in the atmosphere or lo-3
900°C to 1100°C in an atmosphere containing oxygen above atmospheric pressure
It was confirmed that crystallinity could be improved by heating in the temperature range of 1 to 2 hours, and even better superconducting properties could be obtained. It has been found that a thin film crystal structure in which the C-axis is oriented perpendicular to the substrate is suitable for providing excellent superconducting properties. In that sense, as a substrate, in addition to 5rTio3 (100) substrate, BaTio3
．． It was confirmed that single crystal (100) substrates having a perovskite crystal structure such as LaAlO3 and LaGaO3 are excellent.

Some of the obtained thin films exhibited superconducting properties after being deposited or after the heat treatment described above to promote crystallization; It has been found that it is possible to further improve the superconducting properties, and even for those that did not exhibit superconducting properties, it is possible to obtain optimal superconducting properties. It has also been found that rapid cooling to room temperature or lower after heat treatment reduces the oxygen content and facilitates subsequent reduction treatment. It has been found that heating in a vacuum or in an inert gas atmosphere such as argon with an oxygen partial pressure of 1 o-a atmosphere or less is effective for the reduction treatment. The figure shows the temperature dependence of electrical resistance of a typical thin film with a thickness of 5000 A when heated in vacuum while varying the processing temperature and processing time. Curve 11 is before reduction treatment, curves 12, 13, and 14 are 600
Curve 15 was treated at 800°C for 2 hours.

The treatment time required to obtain excellent superconducting properties with a transition temperature of 22 or higher is 8 to 30 hours at 600°C and 1 hour at 800°C.
The treatment was for ~3 hours. From this, the amount of oxygen desorption or loss strongly depends on the oxygen diffusion process, and the appropriate temperature range for heating is 500°C to 900°C, but the optimal processing time is Although it is influenced by the film thickness and surface condition of the body, the higher the processing temperature, the shorter the processing time, but the optimum time range is narrow. It was confirmed that utilizing the substitution effect with fluorine is an effective way to reduce the amount.

For example, it was confirmed that if a thin film is exposed to active fluorine by causing a discharge in an atmosphere of fluorine-containing CFC gas, the amount of oxygen can be reduced due to the substitution effect with fluorine, and a good superconducting thin film can be formed. did.

It was confirmed that this result was the same for combinations containing S+w, Pr, or at least one of these instead of Nd, and for combinations containing Th or at least one of these instead of Ce.

Effects of the Invention The present invention has made it possible to obtain a high-quality, high-performance thin film superconductor with an N d2Cuo a-type crystal structure with good reproducibility. It is essential for the application of this invention, and the industrial value of the present invention is great.

[Brief explanation of drawings]

The figure is a diagram showing the temperature dependence of electrical resistance of a thin film superconductor manufactured in an example of the present invention. 1.l...Before reduction processing, 12.13.14...60
Treatment at 0°C for 2, 16, and 52 hours, 2 hours at 15...800°C.

Claims (5)

[Claims] (1) A reduction treatment is performed on a sputter-deposited film of a composite oxide whose main component is (A_1_-_xB_x)_2CuO_4 having an Nd_2CuO_4 type crystal structure (here, A is Nd, Sm, Pr B represents at least one element of Ce and Th; x is a numerical value in the range of 0.06≦x≦0.08). manufacturing method. (2) The method for manufacturing a thin film superconductor according to claim 1, characterized in that a single crystal substrate having a perovskite crystal structure is used as the substrate. (3) After sputter deposition, further in the atmosphere or 10^
-^ In an atmosphere containing oxygen of 3 atmospheres or more, 900℃~1
2. The method for producing a thin film superconductor according to claim 1, wherein a vapor deposited film obtained by heating in a temperature range of 100° C. for a certain period of time is used. (4) As a method of reduction treatment, in a vacuum or in an inert gas atmosphere with an oxygen partial pressure of 10^-^3 atm or less at 500°C.
2. The method for producing a thin film superconductor according to claim 1, wherein the heating is performed at a temperature range of -900[deg.] C. for a certain period of time. (5) The method for manufacturing a thin film superconductor according to claim 1, wherein the reduction treatment includes exposing the thin film superconductor to an atmosphere containing at least fluorine gas.