JPH0818913B2 - Method of manufacturing thin film superconductor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a superconductor. In particular, it relates to a method for producing a compound thin film superconductor.

Conventional technology As high-temperature superconductors, niobium nitride (NbN) and germanium niobium (Nb ₃ Ge) were known as A15 type binary compounds, but the superconducting transition temperature of these materials is at most 24 ° K. It was On the other hand, perovskite ternary compounds are expected to have higher transition temperatures, and Ba-La-Cu-O high-temperature superconductors have been proposed [JGBendorz and KAMu.
ller, Zeit Schrift Fair Physik (Ze ts
hrift frphysik B) -Condensed Matter 64,189-193
(1986)]. Furthermore, it has been recently proposed that the Y-Ba-Cu-O system is a higher temperature superconducting material. (Reference) [M.
K.Wu et al., Physical Review Letters (Physical R
eview Letters) Vol, 58 No9,908-910 (1987)] The details of the superconducting mechanism of Y-Ba-Cu-O-based materials are not clear, but the transition temperature may rise above the liquid nitrogen temperature. As a high-temperature superconductor, more promising properties are expected than conventional binary compounds.

Problems to be Solved by the Invention Although the above-mentioned composite compound-based material can be thinned by an ion process to form a high-temperature superconductor, heat treatment is required to realize good superconducting properties. This heat treatment is performed to control the crystallinity of the composite compound film and the amount of oxygen, but these optimization conditions are not always obtained at the same temperature, and long-term treatment is required, and the temperature rise ,
There is a problem that the condition setting such as cooling is complicated and the reproducibility is poor.

Means for Solving Problems The basic structure of the thin film superconductor formed by the manufacturing method of the present invention is an oxide containing at least A, B, and Cu on the surface of the substrate, and the molar ratio of the elements is The ternary compound film 12 is attached. The inventors of the present invention, by superimposing this type of superconductor on the heated substrate, for example, by depositing the above-described composite compound film by a process such as vapor deposition, and subsequently or after heat treatment, irradiating with hydrogen ions, The present invention has been found out by the fact that it is formed. Here, A is at least one of Sc, Y and lanthanum series elements (atomic number 57-71), and B is at least one element of Group IIa elements such as Ba, Sr, Ca, Be and Mg.

Action The method of manufacturing a thin film superconductor according to the present invention has a great feature in that the superconductor is made into a thin film. That is, thinning is performed by decomposing the material of the superconductor into ultrafine particles in the atomic state and then depositing it on the substrate, and subsequently, or after heat treatment and oxygen treatment, irradiating hydrogen ions to form a superconductor. The composition is essentially homogeneous compared to conventional sintered bodies. Therefore, a very high precision superconductor is realized using the method of the present invention.

Embodiment An embodiment of the present invention will be described with reference to the drawings.

In FIG. 1, the ternary compound film 12 is formed by, for example, a sputtering method. In this case, the base 11 is intended to hold the ternary compound coating 12 exhibiting superconductivity. This coating 12 is usually formed at a high temperature of several thousand degrees Celsius, and since superconductivity is operated at a low temperature such as liquid nitrogen temperature (-195 degrees Celsius), the adhesion between the substrate 11 and the coating 12 is particularly deteriorated and the coating 12 is often damaged. The present inventors have confirmed that this is the case. Furthermore, as a result of investigating detailed thermal characteristics of the substrate with respect to various materials, the inventors found that if the linear thermal expansion coefficient of the substrate α> 10 ^{−6 /} ° C.
It was confirmed that there was no breakage of the film and that the film was put to practical use. For example, if quartz glass with α <10 ^{−6 /} ° C is used for the substrate, the film
The present inventors have confirmed that 12 is a discontinuous coating with numerous cracks and is difficult to put into practical use.

Furthermore, the present inventors have found that the base material 11 of FIG. 1 has an optimum material in terms of functionality.

That is, in order to form the ternary compound film 12 having high crystallinity on the surface 13 of the substrate 11, a single crystal substrate is effective. As a result of detailed investigation of the optimum substrate material, the present inventors have found that magnesium oxide, sapphire (α-Al ₂ O
₃ ), spinel, strontium titanate, silicon,
It was confirmed that a single crystal such as gallium arsenide is effective.
However, this is effectively a highly crystalline coating 12 on the surface 13.
Therefore, at least the substrate surface 13 may be a single crystal.

The present inventors have confirmed that when processing this type of superconductor into an arbitrary shape, for example, a cylindrical shape, the contract sintered porcelain is more effective than the single crystal as the base, and the optimum porcelain material is used. Found out. That is, as the porcelain substrate, the inventors of the present invention have found that the adhesion of the superconducting coating 12 to the substrate 11 is optimal, such as alumina, magnesium oxide, zirconium oxide, steatite, forsterite, beryllia, and spinel. Confirmed. Also in this case, like the single crystal, even the surface of the substrate is preferably made of this kind of porcelain.

To form a thin film superconductor, first, a composite compound film of AB-Cu-O is deposited on a substrate by a physical vapor deposition method such as sputtering deposition or thermal deposition such as electron beam deposition or laser beam deposition. In this case, the superconductor A-B-
Although the crystal structure and composition formula of Cu-O have not been clearly determined, it is also called oxygen-deficient perovskite (A, B) ₆ Cu ₆ O ₁₄ . The inventors have found that the element ratio in the produced coating is It was confirmed that the superconducting phenomenon was found even if there was some difference in the critical temperature within the range of. The method of forming the composite compound film is not limited to the physical vapor deposition method, but may be a chemical vapor deposition method such as atmospheric pressure or reduced pressure chemical vapor deposition method, plasma chemical vapor deposition method, photochemical method. The present inventors have confirmed that the vapor phase growth method is also effective if the ratios of the components A, B and Cu are matched.

The inventors have determined that when depositing the composite compound coating on the surface 13 of the substrate 11, there is an optimum temperature range for the substrate. That is, the optimum substrate temperature range is 100
~ 1000 ℃. If the temperature is 100 ° C. or lower, the adhesion of the composite oxide film to the surface of the substrate will be poor. Further, at 1000 ° C. or higher, the deviation from the structural ratio of the components A, B and Cu in the composite oxide coating becomes large.

Furthermore, the inventor of the present invention has found that the temperature of the substrate for depositing the complex compound coating is most preferably in the range of 500 to 900 ° C. in view of the function of this type of vapor deposition apparatus and the reproducibility of the characteristics of the complex oxide coating. Confirmed. In this case, the formed composite compound film is composed of amorphous, microcrystalline or single crystal.

Surprisingly, however, this type of coating exhibits semiconducting properties, and superconductivity may not be seen at liquid He temperatures (4 ° K). Moreover, it was confirmed that the film had a high resistance when discharged in the air and was extremely unstable and not reliable.

The present inventors further heat-treat this type of composite compound film in an atmospheric atmosphere, a mixed gas of argon and oxygen, or an oxide atmosphere such as pure oxygen, thereby causing superconductivity and greatly improving long-term stability. I found that In this case, the optimum heat treatment temperature is 700-1000 ℃, and the heat treatment time is
0.1 to 10 hours. In addition, when the time is 10 hours or more, the resistivity becomes high, the characteristics of the coating become unstable, and steep superconductivity is not exhibited.

On the other hand, the present inventors have found that the superconducting property can be improved by irradiating the coating film with hydrogen ions. In this case, it was found that the coating had a great effect when heated to 300 to 600 ° C. At 600 ° C or higher, hydrogen ions do not react with the coating compound and the effect cannot be obtained.

(Specific Example) Y ₂ Ba ₄ Cu ₆ O sintered by high frequency planar magnetron sputtering using a sapphire single crystal R surface as a substrate 11.
_{A 14} target was sputter-deposited in a mixed gas atmosphere of Ar and O ₂ and deposited as a crystalline Y ₂ Ba ₄ Cu ₆ O ₁₄ coating on the substrate to form a layered structure.

In this case, the gas pressure is 0.5 Pa and the sputtering power is 150.
W, sputtering time 1 hour, film thickness of 0.5 μm, substrate temperature 800 ° C. The formed film as it is, or further in air or oxygen atmosphere, heat treated at 900 ℃ for 2 hours and then cooled in 3 to 4 hours, then heat the film to 450 ℃ and accelerate hydrogen ions at a voltage of 2 KV. Irradiated.

FIG. 2 shows a ternary compound film 1 whose main component is Y ₂ Ba ₄ Cu ₆ O ₁₄ by sputtering vapor deposition using a sapphire R surface as the substrate 11.
2 shows an X-ray diffraction spectrum of the ternary compound film 12 in the example when 2 was attached. In FIG. 2, spectrum a is obtained from the coating 12 treated by the method of the present invention, and spectrum b is obtained from the structure exhibiting superconductivity. As shown in the figure, the film spectrum a was similar to the spectrum b, and superconductivity occurred.

 The superconducting transition temperature of the coating was 90 ° K.

In this embodiment, the film thickness of the coating film 12 is 0.5 μm, but it was confirmed that superconductivity is generated even when the film thickness is as thin as 0.1 μm or less, or as thick as 10 μm or more.

The inventors have experimentally investigated in detail the effectiveness with respect to crystalline substrates other than sapphire. Magnesium oxide,
A film of Y ₂ Ba ₄ Cu ₆ O ₁₄ structure was formed on the spinel single crystal substrate.
It was confirmed that all of them exhibited superconductivity by depositing them by the sputtering deposition method as in the case of sapphire single crystal and subjecting these coatings to the hydrogen ion treatment of the present invention. Similar results were obtained with strontium titanate, silicon, and gallium arsenide single crystals.

The crystal structure of the superconductor of the present invention is complicated, and it is not well understood. If the temperature of the single crystal substrate is raised above the epitaxial temperature to increase the single crystallinity, a tetragonal perovskite structure is likely to be formed, and a superconductor cannot be obtained with good reproducibility in many cases. Therefore, as described in the embodiments of the present invention, the substrate temperature is selected in a rather low range, a complex compound film containing an orthorhombic perovskite or a microcrystalline structure is formed and then crystallized by heat treatment to remove oxygen and crystal defects by hydrogen treatment. The present inventors have experimentally confirmed that a superconductor can be obtained with better reproducibility by optimizing.

In this case, the substrate having the single crystal structure is effective in assisting the solid phase epitaxial growth of the coating when heat-treated. In particular, when a film in an amorphous state is previously formed on a substrate and this is heat-treated, the crystalline film is effectively solid-phase-epitaxially deposited on the surface of the crystalline substrate, and then the hydrogen of the present invention is continuously applied without breaking the vacuum of the film forming apparatus. The present inventors have confirmed that the ion treatment is effective for forming a thin film having excellent superconducting properties. When the crystallinity of the superconducting film is not particularly required (when a steep superconducting dislocation is not required), a polycrystalline porcelain substrate is effective.

As a method of forming a composite compound film on the surface of a substrate, a metal main component is deposited on the substrate by physical vapor deposition, and further an oxygen beam or oxygen ions are applied to the film during the film formation so that the metal on the surface of the substrate is irradiated. It is also possible to oxidize the main component. However, although the thus obtained coating film containing more oxygen than necessary shows good crystallinity, the superconducting property may not be optimum.

The inventors have found that heat treatment at 800 ° C. or higher improves the crystallinity of the thin film composite compound film. Therefore, the hydrogen treatment of the present invention is particularly effective when applied to coatings heat treated above 800 ° C.
The above hydrogen treatment may be carried out by introducing a desired gas into a vacuum chamber that is usually used, applying a high frequency to this gas from a parallel electrode to cause discharge, and disposing a composite compound film in this discharge plasma. It was confirmed that this method improves the superconducting properties. However, in this method, the coating is irradiated with particles other than ions to change the surface condition of the coating, so it is desirable to separate the ion source and the ion irradiation part.

FIG. 3 shows a method for realizing this condition. Hydrogen gas or a gas containing hydrogen is introduced into the ion source 31, and a high frequency voltage is applied to the electrodes 32 and 33 facing each other with this gas interposed therebetween to generate plasma. A substrate on which a magnetic field generation source 34 for forming a magnetic field is arranged in this plasma and hydrogen ions efficiently generated are formed into a composite compound film
By applying a voltage between the substrate table 36 on which the 35 is arranged and the plasma of the ion source, oxygen ions are extracted from the ion source and irradiated on the composite compound film on the substrate table. At this time, the inventors have found that by heating the substrate to 300 to 600 ° C. by the heater 37, the efficiency of hydrogen ion treatment can be increased, the treatment time can be shortened, and the superconducting property of the coating film can be improved.

Also, the voltage applied between the plasma and the characteristic sample stand is 5KV.
In the following cases, the surface of the coating film was sputtered, but it was confirmed that hydrogen ion treatment can be effectively performed on the inside of the coating film.

FIG. 4 shows that hydrogen gas or a gas containing hydrogen is introduced into the vacuum chamber 41, the gas is irradiated with microwaves to cause discharge, plasma is generated, and a magnetic field 42 is applied to the plasma to ionize hydrogen ions. Is used as an ion source. In this case, the normal microwave source 43
When a microwave of 2.45 GHz is used and the magnetic field strength is set to about 875 Gauss, cyclotron resonance of electrons occurs, so that the efficiency of hydrogen ionization increases. The structure is such that the hydrogen ion extracted from the ion source 44 is irradiated on the composite compound film arranged on the sample table. In this case, it was found that the high-energy hydrogen ions efficiently ionized by microwaves efficiently hydrogenate the composite compound film and improve the superconducting property.

In the hydrogen treatment method as described above, by irradiating the film with hydrogen ions and simultaneously irradiating a short-wavelength light of 500 nm or less, the crystal defects due to hydrogenation are efficiently compensated, and in particular, ultraviolet irradiation is effective. confirmed.

As for the detailed characteristics such as the crystal structure of this type of coating, since the coating is constrained on the substrate, there are large strains or defects in the coating that do not exist in ordinary sintered bodies. For this reason, it is not possible to infer the manufacturing method of the coating from the manufacturing method of the sintered body. The physical meaning of hydrogen treatment of the coating is not clear, but it can be considered as follows. That is, the compound compound film deposited on the substrate by sputtering deposition or the like does not form the compound (A, B) ₆ Cu ₆ O ₁₄ . In this case, for example, a complex oxide in which the oxide of the element A is dispersed in the network of BCuO ₃ tetragonal perovskite structure is formed. The generation of structures exhibiting superconductivity is associated with heat treatment. That is, the heat treatment time is 1
The reason why the superconductivity is not obtained within the time period is considered to be due to insufficient formation of this structure. Further, it is necessary to properly contain oxygen in order to obtain good superconducting properties, and if it is too large, it becomes an insulator, and if it is too small, it becomes unstable and decomposes due to moisture or the like. When the hydrogen treatment according to the present invention is performed, the oxygen content can be easily controlled to an appropriate value, and the treatment can be performed in a short time, which is convenient for producing a superconducting thin film.

Although the heat treatment was performed in a normal heater heating furnace,
An engineering heat treatment method using laser light, infrared rays, or a heating method using an electron beam can be applied.

Details of changes in superconducting properties due to changes in constituent elements A and B of this type of ternary compound superconductor (A, B) ₆ Cu ₆ O ₁₄ are not clear. However, it is true that A is trivalent and B is bivalent. Although Y has been described as an example of the A element, Sc, La, and even lanthanum series elements (atomic numbers 57 to 71) do not change the essential characteristics of the invention to the extent that the superconducting transition temperature changes. .

Also in the B element, the change of the group IIa element such as Sr, Ca, Ba changes the superconducting transition temperature by about 10 ° K, but does not essentially change the characteristics of the present invention.

EFFECTS OF THE INVENTION In particular, the superconductor according to the present invention has a great feature in that the superconductor is thinned and treated with hydrogen ions. That is, thinning decomposes the material of the superconductor into ultrafine particles called atomic state and then deposits it on the substrate,
The composition of the formed superconductor is essentially homogeneous compared to conventional sintered bodies. Further, the hydrogen ion treatment according to the present invention has good controllability and can be treated for a short time as compared with the heat treatment which is usually performed, and can compensate a crystal defect to realize a desired crystal structure. Therefore, a very high precision superconductor is realized by the present invention.

As described above, according to the method for manufacturing a thin film superconductor of the present invention, since it is formed in a thin film shape on a crystalline substrate, for example, the accuracy is essentially higher than that of a sintered body and Si or GaAs is used.
It can be integrated with other devices, and is used in the manufacture of various superconducting devices such as Josephson devices. In particular, the transition temperature of this type of compound superconductor may reach room temperature, the range of conventional practical use is wide, and the industrial value of the present invention is high.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of a thin film superconductor formed by a method for manufacturing a thin film superconductor according to an embodiment of the present invention, and FIG. 2 is a basic characteristic diagram of the thin film superconductor of the present invention, FIG. 3, and FIG. FIG. 1 is a schematic configuration diagram of an apparatus for a hydrogen treatment apparatus of the present invention. 11 …… Substrate, 12 …… Ternary compound coating, 31.34 …… Ion source, 32,33 …… High frequency electrode, 34,42 …… Magnetic field generator, 35…
… Substrate, 36… Substrate stand, 37… Heater.

Claims (14)

[Claims] 1. A method for producing a thin film superconductor, characterized in that, in a composite compound coating film composed of an oxide containing at least Cu, the heated coating film is irradiated with hydrogen ions. 2. The method for producing a thin film superconductor according to claim 1, wherein plasma generated by discharging a gas containing at least hydrogen in a vacuum chamber is used as the hydrogen ion source. 3. The method for producing a thin film superconductor according to claim 2, wherein a plasma generated by applying a high frequency voltage containing microwaves to the gas is used as the hydrogen ion source device. 4. A method for producing a thin film superconductor according to claim 3, wherein hydrogen ions generated by applying a magnetic field to the plasma are irradiated. 5. A composite compound coating for hydrogen ion irradiation is applied to 300
The method for producing a thin film superconductor according to claim 1, characterized in that heating is performed at a temperature of from ℃ to 600 ℃ or less. 6. A method of irradiating hydrogen ions generated by discharge of gas in a vacuum chamber by applying a voltage between plasma in the vacuum chamber and a sample stage on which a complex compound coating is installed. A method of manufacturing a thin film superconductor according to claim 2. 7. A hydrogen ion is irradiated by arranging an electrode placed at a predetermined potential between a plasma generated by applying a high-frequency voltage to hydrogen gas and a sample stage. 6. The method for producing a thin film superconductor according to item 6. 8. The method for producing a thin film superconductor according to claim 6, wherein a direct current voltage of 5 KV or less is applied between the plasma and the sample stage. 9. The method for producing a thin film superconductor according to claim 3, wherein the composite compound film is provided in the plasma generated by applying a high frequency voltage. 10. The method for producing a thin film superconductor according to claim 2, wherein light rays are irradiated simultaneously with hydrogen ions. 11. The method for producing a thin film superconductor according to claim 1, wherein after forming the composite compound coating film, hydrogen ion irradiation is continuously performed by the same apparatus. 12. The method for producing a thin film superconductor according to claim 1, wherein after forming the composite compound coating film, heat treatment is performed in an atmosphere containing oxygen, and then hydrogen ions are irradiated. 13. The heat treatment is performed at 800 ° C. or higher, and the hydrogen ion irradiation is performed at 6
13. The method for producing a thin film superconductor according to claim 12, which is performed on a coating film at a temperature of 00 ° C. or less. 14. The method for producing a thin film superconductor according to claim 1, wherein the composite compound film formed while irradiating hydrogen ions is irradiated with hydrogen ions.