JPH02296731A - Manufacture of thin film superconductor - Google Patents

Abstract

PURPOSE:To transform only a stable phase having excellent surface smoothness and having a critical temp. of 80K into a thin film at a low temp. with good reproducibility by periodically laminating an oxide contg. B, an oxide contg. Sr and Ca and an oxide contg. Cu on a substrate. CONSTITUTION:A shutter 32 having a slit 33 is disposed between a substrate 31 and a target constituted of Bi, Sr/Ca and Cu arranged in such a manner that focus is connected on the substrate. Next, the substrate 31 is heated by a heater 34 as well as the shutter 32 is rotated to execute the sputtering of each target in an oxygen-contg. atmosphere, by which 2 layers of oxide at least contg. Bi, 3 layers of oxide contg. Sr and Ca and 2 layers of oxide contg. Cu are periodically laminated.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to the use of copper and bismuth (strontium calcium), which is expected to have a high critical temperature higher than the astigmatism point of liquid nitrogen.
:3:2 molar ratio of oxide thin film superconductor.

Conventional technology Niobium nitride (NbN) and germanium niobium (NbaGe) are used as A15 type binary compounds as high-temperature superconductors.
were known, but the critical temperature of these materials was at most 24°C. On the other hand, perovskite-based ternary compounds are expected to have even higher critical temperatures, and Ba-La-C
U-0 series high-temperature superconductors were proposed.
Z and K, A, Muller, (Zetsh
rlftFurphyslk B)-Condense
dMatter 11i4, 189-193 (198
G)) Co.

Furthermore, it has been discovered that materials of the BI-8r-Ca-Cu-0 series exhibit critical temperatures of 77 °C or higher. B1-8r-
It is known that superconducting phases of 80 and 105 exist in the Ca-Cu-0 system [H.
Tanariki, Imu Fukutomi and T.A.), (
SEARCH BASE・SYNCHRONAL OFFICE・APPLIED TOML AND T. Asano+(Japan)
ese Journal of App-+1e
d Physics) Vol. 27, L209-210
(+988)]. The details of the superconducting mechanism of this type of material are not clear, but the critical temperature may be higher than room temperature, and it is more likely to be used as a high-temperature superconductor than conventional binary compounds.
More promising properties are expected. Bl-Sr-Ca-Cu
The superconducting phase at 80 in the -0 series is more stable than the superconducting phase at 105, and has a smoother surface when formed into a thin film, so it is extremely promising for creating thin film devices using superconductivity. It is considered to be a material.

Problems to be Solved by the Invention However, with current technology, Bl-Sr-Ca-Cu-0 based materials can only be formed through the process of sintering, so they can only be obtained in the form of ceramic powder or blocks. . On the other hand, if this type of material is to be put into practical use, there is a strong desire to process it into a thin film, but with conventional techniques, it is difficult to fabricate a thin film with good surface smoothness and stable superconducting properties. Ta. That is, Bi
- It is known that there are several phases with different critical temperatures in the Sr-Ca-Cu-0 system. It has been considered extremely difficult to fabricate individual superconductors in the form of thin films with good reproducibility and achieve stable superconducting properties. In addition, in order to form a thin film that exhibits good superconducting properties in conventional BI systems, heat treatment at at least 700°C or heating during formation is required, and the surface smoothness is not good, making it difficult to integrate devices. It was considered extremely difficult to construct. In the production of thin film devices using superconductivity, it is necessary that the surface condition be smooth and the superconducting properties stable.

The purpose of the present invention is to fabricate a superconducting thin film with good surface smoothness and stable superconducting properties using a low-temperature process. The film is selectively thinned.

Means for Solving the Problems The method for manufacturing a superconductor of the present invention includes forming, on a substrate, at least two layers of an oxide containing bismuth, three layers of an oxide containing strontium and calcium, and two layers of an oxide containing copper. It is obtained by periodically laminating layers, and achieves high reproducibility in a low-temperature process by substituting some of the strontium with calcium and/or some of the calcium with strontium in the laminated structure. It is.

Several methods of creating new thin films by periodically layering different materials have been tried for metal thin films and oxide thin films.
It has been common knowledge that when the substrate temperature is raised, the period + tlt structure disappears due to interlayer diffusion. For this reason, when creating a periodic structure, the substrate is usually cooled.

The present inventors replaced part of the strontium with calcium and/or part of the calcium with strontium by, for example, sputtering using two or more different targets on the oxide superconductor containing B1, We discovered that the critical temperature of a superconducting thin film is related to the atomic arrangement in the periodic crystal structure of the thin film. When strontium was replaced with calcium and calcium was replaced with strontium, an increase in the critical temperature was observed by gradually increasing the replacement amount from zero. When 40% of strontium was replaced with calcium and 40% of calcium was replaced with strontium, a critical temperature of 80°C was obtained with good reproducibility.

If the substitution exceeds 50%, the crystallinity of the thin film decreases and the critical temperature also decreases. The present invention has made it possible to obtain high-quality, high-performance thin film superconductors using low-temperature processes with good reproducibility.

Examples Examples of the present invention will be described below with reference to the drawings.

First, an example of the study conducted by the present inventors will be described.

That is, for example, a bismuth target, a first alloy target of strontium and calcium with an appropriately changed composition ratio, a copper target, and a second alloy target of calcium and strontium with an appropriately changed composition ratio are mixed with argon and oxygen. They were alternately sputtered in a mixed gas and deposited periodically on a Mg0 (100) substrate. If 40% of strontium is replaced with calcium and 40% of calcium is replaced with strontium, 6o
It has been discovered that a phase with a critical temperature of 80° C. can be stably produced even at a substrate temperature below o'c.

Figure 1 shows the X-ray diffraction pattern of the obtained thin film, which matches the crystal system and lattice constant of a phase with a critical temperature of 80°C that has been obtained in ceramic superconductors so far. FIGS. 2(a) and 2(b) are X-ray photoelectron spectra of the obtained thin film. The binding energy of a photoelectron excited by X-rays and ejected from a solid reflects the electronic state of the atom in which the electron existed in the solid. For example, even if electrons are in the same core level of strontium, if the position of the strontium atom in which the electron exists differs in the crystal, the difference in the magnitude of the electric field in which each electron was placed will be reflected in the binding energy. . In other words, the distance, number, and type of ligands in the crystal can be determined from this electronic spectrum. The greater the distance to the ligand and the greater the number, the smaller the binding energy. Here, the 3d121) electron spectra of strontium and calcium are shown.

Spectral separation was performed on the electronic spectrum of 36.2p. If strontium and calcium are localized in the crystal, each core electron spectrum consists of a single component.

It can be seen from this figure that each spectrum consists of two components. In other words, 40% of strontium and calcium in the crystal replace each other's atomic positions. This means that 20% of calcium and 40% of calcium are substituted for each other.

). When a conventional single-element target was used, a phase of 80 could not be achieved unless the substrate temperature was 700°C or higher. This method utilizes high temperatures and uses diffusion, so it is sensitive to temperature changes, and the reproducibility of characteristics has been outstanding. For example, when 20% of strontium is replaced with calcium and 40% of calcium is replaced with strontium, the inventors set the sputtering rate of bismuth and strontium alloy target and strontium and calcium alloy target appropriately at a substrate temperature of 400-BoooC. When adjusted, a phase of 80° appeared corresponding to the stacking period, a critical temperature of 80° was obtained in a thin film, and good reproducibility of superconductivity was also obtained.

In addition, if the lamination is done simultaneously rather than periodically, the
The reproducibility of phases with critical temperatures of was not good. Although the produced thin film showed a critical temperature in its original state, it more reliably showed a critical temperature of 80°C when it was heat-treated in oxygen at about 500-700''C.

There are several possible methods for periodically stacking bismuth and oxides containing strontium, calcium, and copper. In general, periodic stacking can be achieved by controlling an opening/closing shutter in front of the evaporation source in an MBE device or multi-source EB evaporation device, or by switching the type of gas during production using the vapor phase growth method. can. However, sputtering deposition has traditionally been considered unsuitable for this type of extremely thin layer stacking. The reason for this is thought to be the incorporation of impurities due to the high gas pressure during film formation and damage caused by high energy particles.

However, the present inventors discovered that it is possible to produce a good laminated film in other ways by laminating different thin layers on this Bi-based oxide superconductor by sputtering. This may be because the high oxygen gas pressure and sputter discharge during sputtering are favorable for the formation of a Bt-based phase with a critical temperature of 80 K. In particular, when 20% of the strontium in the strontium layer (layer A in Figure 4) is replaced with calcium and 40% of the calcium in the calcium layer (ffB in Figure 4) is replaced with strontium, it is stable at low temperatures. It was found that the membrane could be obtained with good reproducibility.

Another method of stacking different materials by sputter deposition is to periodically control the discharge position of one sputtering target with a composition distribution, but there is also a method of sputtering multiple targets with different compositions. This can be achieved relatively easily using . In this case, a periodic laminated film can be produced by periodically controlling the amount of sputtering for each of a plurality of targets, or by providing a shutter in front of the target and opening and closing it periodically. It can also be manufactured by a method in which the substrate is moved periodically and moved over each target. When laser sputtering or ion beam sputtering is used, periodic laminated films can be realized by periodically moving a plurality of targets and periodically changing the targets irradiated with the beam.

In this way, by sputtering using a plurality of targets, a periodic stack of Bl-based oxides can be produced relatively easily.

In order to further understand the content of the present invention, specific examples will be shown below.

(Example 1) A total of four targets of B1 and 5rCa alloy, Cu, and CaSr alloy were used and arranged as shown in FIG. 3. That is, each target is installed at an angle of approximately 30 degrees so as to focus on the Mg0 (j, oo) substrate 31. There is a rotating shutter 32 in front of the target, and the rotation of the slit 33 provided in the shutter allows the BI+
Sputter deposition is performed in a cycle of 5rCa alloy-ThCu+CaSr alloy→Cu-8r alloy gold CuBl. The substrate 31 was heated to about 600° C. with a heater 34, and each target was sputtered in a 3 Pa mixed atmosphere of argon:oxygen (5:1). The sputtering current of each target is Bi: 30mA + 5
rCa alloy: 50 m AmCu: 250 m
A+ CaSr' Alloy 50 Alloy 50 A Shatter 32
When periodic lamination was performed with a rotation period of 10 minutes, it was possible to produce a phase with a critical temperature of 80°C at a substrate temperature of 600°C. 1100n by evaporation for about 10 hours
A thin film of about 100% was produced. The composition is Bl: Sr:
Ca:Cu=2:2:1:2. Even in this state, this thin film showed a critical temperature of 80°C, but when it was further heat-treated in oxygen at 650°C for 1 hour, the reproducibility became very good. The structure of a phase with a critical temperature of 80°C, including the results of this study, is summarized as shown in Figure 4.

Basically, the metal element is Bl+Sr+Cu→Ca→Cu
→S r4B It is composed of oxide layers arranged in the order of Sr, which is large in number, replaces some Ca.

Table 1 The Sr oxide layer is partially replaced by Ca.

This can be thought of as helping to maintain the electrical neutrality of the crystal as a whole and the periodicity of the structure. The manufacturing method of the present invention is considered to be very effective in manufacturing this structure. Also, it is now formed at a lower temperature than before. This differs from the conventional method in that it does not use diffusion between elements during or after vapor deposition, but is an effect of element substitution from the time of vapor deposition. Moreover, although the case where an alloy is used as an example of the target is shown here, it is also possible to use a mixed oxide by using the high frequency sputtering method.

Effects of the Invention As described above, the method for producing a thin film superconductor of the present invention has a high reproducibility of a Bl-based oxide superconducting thin film having a critical temperature of 80°C, which is the most promising superconducting device material due to its stability and surface smoothness. It provides a good low temperature process,
This established a low-temperature process, which is essential for device applications. Therefore, the industrial value of the present invention is great.

[Brief explanation of drawings]

Figure 1 is an X-ray diffraction pattern graph of a thin film in an example of the present invention, Figure 2 is an X-ray photoelectron spectroscopy spectrum graph of a thin film in an example of the present invention, and Figure 3 is an example of the present invention. FIG. 4 is a perspective view of the thin film manufacturing apparatus in the embodiment of the present invention, and FIG. 4 is a structural diagram showing the atomic arrangement in the crystal of the thin film in the embodiment of the present invention. 31... MgO base, 32... Shutter 33.
...Slit, 34 ...Name of Heater's agent Patent attorney Shigetaka Awano Haka1○O○Oσ8i ■○○○

Claims (5)

[Claims] (1) Two oxides containing at least bismuth are placed on the substrate.
3 layers, oxides containing strontium and calcium
1. A method for producing a thin film superconductor, comprising periodically stacking two layers of an oxide containing copper. (2) The method for manufacturing a thin film superconductor according to claim 1, wherein the oxide containing strontium and calcium is formed by replacing a portion of the strontium oxide with calcium oxide. (3) The method for manufacturing a thin film superconductor according to claim 1, wherein the oxide containing strontium and calcium is formed by replacing a portion of calcium oxide with strontium oxide. (4) The method for manufacturing a thin film superconductor according to claim 1, characterized in that the evaporation of the laminated material is performed by sputtering. (5) The method for manufacturing a thin film superconductor according to claim 1, wherein the evaporation of the laminated material is performed by sputtering a plurality of targets having at least two types of compositions.