JPH02170308A - 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 Industrial Application The present invention relates to superconductors. In particular, it relates to a thin film superconductor formed from a Bi layered composite oxide.

Niobium nitride (NbN) and germanium niobium (Nb3Ge), which are A15 type binary compounds, have been known as conventional superconductors, but the superconducting transition temperature of these materials was at most 24°C. On the other hand, perovskite-based ternary compounds are expected to have even higher transition temperatures, and Ba-La-C
A u-0-based high-temperature superconductor has been proposed [J, G, Den
dorz and K, A, Mulier
t fur physik B)-Condens
ed Matter B4.189-193 (198B
)].

Furthermore, some of the oxide superconductors discovered in recent years have superconducting transition temperatures exceeding the liquid nitrogen temperature (77.3 Kelvin), which is expected to greatly expand the field of application of superconductors. In particular, a Bi-8r-Ca-Cu-0 superconductor formed from a Bi layered composite oxide,
It was also discovered that they exhibit superconducting transition temperatures above 100°C. [H, Maeda, Y, Tanaka, M.
Japanese Jour
nal of Applied Physlcs)
Vol, 27. L209-210 (1988) The details of the superconducting mechanism of this type of material are not clear, but the transition temperature may be higher than room temperature, making it a more promising property than conventional binary compounds as a high-temperature superconductor. There is expected. It is thought that if this material can be made into a thin film, there will be a greater possibility of realizing superconducting devices, and various methods are currently being attempted to make this kind of material into a thin film. High-temperature heat treatment is required.

Problems to be solved by the invention For example, 5r2Ca2C using Bi25g target
By alternately sputtering with a ui target in a mixed gas of argon and oxygen and periodically layering on a Mg0 (100) substrate, a typical Bi layered structure composite oxide, 1-5r-Ca-Cu-0, is produced. However, for the appearance of a phase with a transition temperature of 100 K or higher, the substrate temperature during thin film formation must be 700°C or higher, and due to the high temperature, diffusion is affected and reproduction is difficult. There was a problem with sexuality. Furthermore, B i -8r-Ca-Cu
-〇It was found that when a high temperature heat treatment is performed to obtain superconducting properties after forming a thin film, the composition ratio becomes different from that before the heat treatment. Therefore, it was necessary to adjust the composition ratio during thin film formation so that the composition ratio would be optimal after heat treatment. However, it is difficult to optimize the composition using such a method.

This is because changes in composition are very sensitive to temperature during high-temperature heat treatment.

The present inventors have discovered that such difficulties can be avoided when making this type of material into a thin film by forming other types of thin films on the interface with the substrate and on the surface of the thin film superconductor. Ta.

Means for Solving the Problems At least two Bi-0 layers formed on a substrate;
In a thin film superconductor made of a Bi layered composite oxide thin film having a layered structure having a plurality of Cu-0 layers between the two Bi-0 layers, the Bi-0 layer is parallel to the substrate surface. The present invention attempts to solve the above problem by forming the Bi layered composite oxide thin film and further forming Bi oxide thin films in contact with two surfaces of the Bi layered composite oxide thin film. In particular, as shown in FIG. 1, BizOa and BI4TfsO+z are applied to two surfaces of the Bi layered composite oxide thin film, namely, the interface with the substrate on which the thin film superconductor is formed and the surface of the thin film superconductor.

Alternatively, the problem is attempted to be solved by forming a thin film of Bi oxide material, which is not a superconducting material, such as Bi2Ti40++.

Effects The present inventors have developed the following for this Bi-containing oxide superconductor:
For example, we investigated in detail the composition of thin films obtained by magnetron sputtering and the relationship between crystallinity and superconducting properties. As a result, by using Bi oxide superconductor as a thin film superconductor material and applying the structure of the present invention to the interface or surface, it is possible to prevent diffusion at the interface or surface due to high temperature treatment, and after forming the thin film. It was found that a critical temperature of 100 or higher was stably obtained without any change in composition, and the crystallinity was improved and reproducibility was excellent.
It was also found that the latitude in processing conditions increases. As described above, the present invention makes it possible to obtain a high-quality, high-performance thin film superconductor with good reproducibility.

Example Bi layered structure composite oxide, Bi-8r-Ca-
Cu-0, B i-8r-Ba-Cu-OzB 1-C
a-Ba-Cu-0, or B i -Pb-8r-C
a-Cu-01Bi-Pb-8r-Ba-OlBi-P
b-Ca-Ba-0 etc. are considered as superconductors, but here - as an example, Bi -8r-Ca-Cu-
An example of consideration for the 0 series will be described. In Figure 1, 12
A and 12B indicate Bi oxide thin films.

At present, the Bin-like structure composite oxide thin film 13 shown in FIG. It is obtained by alternately sputtering the 5rTiOa (100) and laminating it on the 5rTiOa (100) substrate 11.

In this sputtering, BiQss and 5r-Ca-
It was found that when a thin film having a layered structure is formed by adjusting the sputtering rate of the Cu compound target and selecting sputtered particles using a shutter, a phase of 100 or higher appears at a critical temperature corresponding to the layering period. Moreover, when the lamination was carried out simultaneously rather than periodically, only a phase having a critical temperature of 80°C could be formed. In particular, one can start the thin film formation on the substrate by depositing atoms from the Bi2C)3 target or end the thin film formation with the Bi2C)3 target.
It has been found that when the attachment of atoms from the O3 target ends, thin films exhibiting good superconducting properties are obtained.

The thin film superconductor having the structure of the present invention shown in FIG. 1 was devised based on these results. It has also been discovered that when the substrate temperature is particularly between 500 and 900[deg.] C., a phase with a critical temperature of 100 or higher and having very good crystallinity can be formed. Although the formed thin film showed superconducting transition even in its original state, it more reliably showed a critical temperature of 100 or more when heat treated in oxygen at about 850''C.

There is a method for forming the Bi layered composite oxide thin film 13 in which a Bi-0 layer and a layer containing the lla group and Cu elements are arranged periodically. Periodic stacking can be achieved by controlling the opening/closing shutter in front of the evaporation source or by switching the type of gas during formation by vapor phase growth.

It is also possible to form a good laminated film by the sputtering method, but in this case, the high oxygen gas pressure and sputter discharge during sputtering are considered to be convenient for the formation of a Bi-based phase with a critical temperature of 100 K or higher. .

(Example 1) The present invention will be described below using the drawings. In the above-mentioned thin film forming method, one target 21 is set to B as shown in FIG.
i253 and sputtering is started from Bi2O3. First, Ar gas is introduced into a vacuum apparatus and discharged to start sputtering. 5r-Ca- by the shutter 24
After blocking sputtered atoms from the Cu compound target 22 and forming a Bi25s thin film 12A as a Bi compound thin film on the substrate surface set at 200°C, the substrate 11 was heated to 700″C with a heater 25 and oxygen was introduced. Then, Bi25s and 5r-Ca-
Sputter a Cu compound target, Bi, lla
Group and Cu elements are deposited on the surface.

In this case, a thin film is formed by adjusting the composition of the thin film to the stoichiometric ratio of the Bi-based oxide superconductor by setting sputtering conditions and using a shutter. In other words, this is achieved by adjusting the shutter rotation time. After this thin film is formed, the introduction of oxygen gas is finally stopped, and sputtering is performed using only Ar to coat the surface of the Bi-based oxide superconducting thin film 13 again with the Bi20i thin film 12B.
form.

By subjecting this to high-temperature heat treatment at 850° C. for 5 hours in an oxygen atmosphere, a thin film exhibiting stable superconducting properties was obtained. The layered structure of this thin film superconductor is as shown in FIG. 1(b). In other words, BI-0 layer 130 and 5r-Ca
-Cu layer 131 forms Bi-based oxide superconducting thin film 13 .

The crystallinity of this thin film was examined by X-ray diffraction.

In the absence of B120s, the diffraction pattern exhibited by a superconducting phase exhibiting a superconducting transition above 100 Kelvin is
Even though the thin film forming process was exactly the same, the results showed that the crystallinity was inferior to that in the case where the BizO3 thin film was formed on the substrate surface. Furthermore, it was found that in the composition of the Bi layered composite oxide, Ca was abnormally reduced in the absence of the Bi2O3 thin film.

(Example 2) By adding one more target 23 to the two targets of Example 1, it became possible to form a Q thin film superconducting pair having the same configuration as in Example 1. In other words, the second
As shown in the figure, B i is another target 23.
A Ti-containing B i -T t -0 compound such as JT 130121 or Bi2Ti-40 was used. At this time, Ar gas and oxygen gas are first introduced into the vacuum apparatus to cause discharge, and the opening of the shutter is
It was arranged so that flying atoms from the i-oxide target 23 were guided onto the substrate 11. After forming the Bi-Ti-0 thin film 12A with a film thickness of 20-30 angstroms, the opening of the shutter 24 was opened so as to alternately guide atoms from the Bi25s and 5r-Ca-Cu compound targets 22 onto the substrate. I made it back and forth. After forming the thin film 13 with a thickness of about 500 angstroms in this way, the position of the shutter opening is controlled again to guide the flying atoms from the Bi oxide containing Ti onto the substrate. 12B was formed. The thin film obtained as described above was placed in a heat treatment furnace and heated at 850° C. for 5 hours while oxygen was flowing, resulting in a film exhibiting superconducting transition at temperatures of 100 Kelvin or more.

This thin film exhibited similar characteristics even when heated at 800° C., confirming that it has a large tolerance to annealing. However, the critical current was smaller and the superconducting portion relative to the total volume was influenced by the annealing temperature.

Effects of the Invention As described above, the thin film superconductor of the present invention provides a structure with good reproducibility of a Bi layered composite compound thin film exhibiting a superconducting transition at 100 Kelvin or higher. According to the present invention, it is possible to realize a stable composition and a large margin for processing conditions, which has great industrial value.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the structure of a thin film superconductor according to an embodiment of the present invention, and FIG. 2 is a thin film forming apparatus composed of a plurality of targets and shutters used to form the superconductor of the present invention. FIG. 11...Base, 12...Bi oxide thin film, 13...Bi layered structure composite oxide thin film, 2
3...Bi oxide target, 24...
・Shutter, 25... Heater. Name of agent Patent attorney Shigetaka Awano 1 person 1 Figure (α) Figure 12 Biz03 target

Claims (3)

[Claims] (1) At least two layers of Bi-O formed on the substrate
In the thin film superconductor, the Bi-O layer is parallel to the substrate surface. A thin film superconductor characterized in that the Bi layered composite oxide thin film is formed as follows, and further Bi oxide thin films are formed in contact with two surfaces of the Bi layered composite oxide thin film. (2) The surface of the thin film superconductor is a Bi-O layer or Bi_
The thin film superconductor according to claim 1, which is a 2O_3 layer or a Bi-Ti-O layer. (3) Bi layered structure The interface between the composite oxide thin film and the substrate is made of Bi
-O layer, or Bi_2O_3 layer, or Bi-Ti-
The thin film superconductor according to claim 1, which is an O layer.