JP2502743B2 - Method of manufacturing thin film superconductor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a thin film superconductor containing at least bismuth, which is expected to have a high critical temperature of 100 K or higher.

Conventional technology As high-temperature superconductors, niobium nitride (NbN) and germanium niobium (Nb ₃ Ge) were known as A15 type binary compounds, but the critical temperature of these materials was at most 24K. On the other hand, perovskite-based ternary compounds are expected to have even higher critical temperatures, and Ba-La-Cu-O-based high-temperature superconductors have been proposed [J.G.
K. A. Muller, (Zeit Schrift Fair Physik Bäh) -Condensed Matter (JGDe
ndorz and KAMuller, (Zetshrift Furphysik B) -Co
ndensed Matter 64,189-193 (1986)]]. further,
It was also discovered that Bi-Sr-Ca-Cu-O-based materials exhibit a critical temperature of 100K or higher [H Maeda, Wai Tanaka,
M. Fuktomi and T. Asano, Japanese Journal of Applied Physics (H.
Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japanese Jou
rnal of Applied Physics Vol.27, L209-210 (198
8)].

Although the details of the superconducting mechanism of this kind of material are not clear, the critical temperature may become higher than room temperature, and more promising properties are expected as a high temperature superconductor than conventional binary compounds.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, the Bi—Sr—Ca—Cu—O-based material can be formed only in the process of sintering mainly by the present technology, and thus can be obtained only in the form of ceramic powder or block. . On the other hand, when putting this type of material into practical use, it is strongly demanded to process it into a thin film, but it has been difficult to produce a thin film having good superconducting properties by the conventional techniques. That is, it is known that there are several phases having different critical temperatures in the Bi-Sr-Ca-Cu-O system. In particular, a phase having a critical temperature of 100 K or higher is achieved in the form of a thin film. Was very difficult.

Further, in the past, in order to form a thin film showing good superconducting properties in this Bi system, it is necessary to perform heat treatment at least 700 ° C. or heating during formation, and it is considered very difficult to construct an integrated device. It was

The present invention aims to solve such problems of the conventional technology.

Means for Solving the Problems The method for producing a superconductor according to the present invention comprises an oxide containing at least bismuth, an oxide containing at least strontium, an oxide containing at least calcium and an oxide containing at least copper on a substrate. Is obtained by periodically laminating the strontium, and in the laminated structure, a part of strontium is replaced with calcium and / or a part of calcium is replaced with strontium.

Action In the present invention, several methods of periodically laminating different substances to form a new thin film have been tried with a metal thin film and an oxide thin film, but when the substrate temperature is raised, the periodic structure disappears due to interlayer diffusion. It was common sense to lose. For this reason, when forming a periodic structure, the substrate may be cooled. The present inventors have proposed the oxide superconductor containing Bi,
For example, a part of strontium is calcium by sputtering using two or more different targets,
And / or part of calcium was replaced with strontium, and it was discovered that the critical temperature of the 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, the critical temperature was increased by gradually increasing the replacement amount from zero. When 40% of strontium was replaced with calcium and 40% of calcium was replaced with strontium, the critical temperature was high and the reproducibility of superconducting properties was excellent. However, when strontium is replaced with calcium or calcium is replaced with strontium by 50% or more, the crystallinity of the thin film is reduced and the critical temperature is also lowered. According to the present invention, it is possible to obtain a high quality and high performance thin film superconductor with good reproducibility.

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

First, a study example by the present inventors will be described. That is, for example, a bismuth target, a first alloy target of strontium and calcium whose composition ratio is appropriately changed, a copper target, and a second alloy target of calcium and strontium whose composition ratio is appropriately changed are argon and oxygen. Alternately sputtered in mixed gas to produce MgO (10
0) Periodically laminated on the substrate. Strontium 40
It has been discovered that, when% is replaced by calcium and 40% of calcium is replaced by strontium, a phase having a critical temperature of 100 K or more can be stably prepared even at a substrate temperature of 650 ° C. or less. FIG. 1 is an X-ray diffraction pattern of the obtained thin film, which coincides with the crystal system and lattice constant of the phase having a critical temperature of 100 K or higher, which has been obtained so far in ceramic superconductors.
2 (a) and 2 (b) are X-ray photoelectron spectra of the obtained thin film. The binding energy of the photoelectrons excited by the X-rays and ejected from the solid reflects the electronic state of the atoms existing in the solid. For example, if the electron in the same core level of strontium has a different position in the crystal of the strontium atom in which the electron existed, the difference in the magnitude of the electric field in which each electron was placed is reflected in the binding energy. . In other words, the distance and number of ligands in the crystal and the difference of several kinds can be known from this electronic spectrum. The larger the distance to the ligand and the larger the number, the smaller the binding energy. Here we show the 3d and 2p electron spectra of strontium and calcium.
Since the total orbital angular momentum of the electron is preserved, the spectra are separated only for the 3d5 / 2 and 2p3 / 2 electron spectra. If strontium and calcium are localized in the crystal, their core electron spectra consist of a single component. From this figure, it can be seen that each spectrum consists of two components. In other words, 40% of strontium and calcium in the crystal are substituting atomic positions for each other (this is the strontium layer: layer A in FIG. 4 and the calcium layer: layer B in FIG. 4). At 20% of strontium and calcium will be replacing each other). When the conventional single element target was used, the phase of 100K or more could not be realized unless the substrate temperature was 700 ° C or more. In this method, since high temperature is used and diffusion is used, it is sensitive to temperature change and there is a problem in reproducibility of characteristics. The inventors have, for example, 40 strontium
% Is replaced with calcium and 40% of calcium is replaced with strontium, the substrate cycle temperature is 400-650 ℃ Then, it was found that a phase of 100K or more appeared, a critical temperature of 110K was obtained in the thin film, and the critical temperature could be increased. Moreover, when the stacking was performed not periodically but at the same time, only the phase having the critical temperature of 80K could be produced. The prepared thin film showed a critical temperature in the as-is state, but when it was heat-treated in oxygen at about 500-700 ° C, it showed a critical temperature of 100K or more more reliably.

There are several possible methods for periodically stacking bismuth and an oxide containing strontium, calcium, and copper. In general, periodic stacking can be achieved by controlling the opening and closing shutters in front of the evaporation source with an MBE device or a multi-source EB vapor deposition device, and by switching the gas type when manufacturing by the vapor phase growth method. it can. However, sputtering deposition has hitherto been unsuitable for stacking very thin layers of this type. The reason for this is considered to be contamination of impurities due to high gas pressure during film formation and damage by particles having high energy. However,
The present inventors performed a lamination of different thin layers by sputtering on this Bi-based oxide superconductor,
Other than that, it was discovered that a good laminated film can be produced. It is thought that the high oxygen gas pressure during sputtering and the sputter discharge favor the formation of a Bi-based phase having a critical temperature of 100 K or higher. Particularly, when 20% of strontium in the strontium layer (: FIG. 4 layer A) is replaced with calcium and 20% of calcium in the calcium layer (: FIG. 4 layer B) is replaced with strontium, stable at particularly low temperature. It was found that various films could be obtained with good reproducibility.

As another method of stacking different substances by sputter deposition, there is a method of periodically controlling the discharge position of one sputtering target having a composition distribution.
This can be achieved relatively easily by using the method of sputtering a plurality of targets having different compositions.
In this case, the sputtering amount of each of the plurality of targets can be periodically controlled, or a shutter can be provided in front of the target to periodically open and close the target to form a periodic laminated film. It can also be manufactured by a method of periodically moving the substrate to move it on each target. When laser sputtering or ion beam sputtering is used, a periodic laminated film is realized by periodically moving a plurality of targets to periodically change the targets irradiated by the beams. As described above, the periodic stacking of Bi-based oxides can be relatively easily prepared by sputtering using a plurality of targets.

Specific examples will be shown below in order to further understand the contents of the present invention.

(Example 1) A total of four targets of Bi and SrCa alloys and Cu, CaSr alloys were used and arranged as shown in FIG. That is, Mg
Each target is about 30 so as to focus on the O (100) substrate 31.
It is installed at an angle. There is a rotating shutter 32 in front of the target, and by the rotation of the slit 33 provided therein, sputter deposition is performed in a cycle of Bi → SrCa alloy → Cu → CaSr alloy → Cu → CaSr alloy → Cu → SrCa alloy → Bi. Done. The substrate 31 was heated to about 600 ° C. by the heater 34, and each target was sputtered in the dregs in an argon: oxygen (5: 1) mixed atmosphere 3 Pa. Sputtering current of each target was Bi: 30mA, SrCa alloy: 50mA, Cu: 250mA, CaSr alloy 50mA, and the periodic cycle of lamination was performed with the rotation cycle of the shutter 32 being 10 minutes. A layer with a critical temperature could be prepared. A thin film of about 100 nm was formed by vapor deposition for about 10 hours. The composition is Bi: S
It was r: Ca: Cu = 2: 2: 2: 3. Even in this state, this thin film showed a critical temperature of 100K or higher,
When the heat treatment was performed at 0 ° C. for 1 hour, the reproducibility was very good. The structure of the phase having a critical temperature of 100K or higher is summarized in Fig. 4 including the results of this study. Basically, the metal element consists of an oxide layer arranged in the order of Bi → Sr → Cu → Ca → Cu → Ca → Cu → Sr → Bi, and holes are supplied to the Cu oxide layer. For convenience, as shown in Table 1, some Sr whose valence is +1 more than Ca is larger than Ca.
It replaces Ca.

The oxide layer of Sr is partially replaced by Ca. This is considered to be useful for maintaining the electrical neutrality of the crystal as a whole and for maintaining the periodicity of the structure. The manufacturing method of the present invention is considered to be very effective in producing this structure. In addition, it is now formed at a lower temperature than before. This is an effect of element substitution from the time of vapor deposition, rather than using diffusion between elements during or after vapor deposition, which is different from the conventional technique. Further, although the case where an alloy is used as an example of the target is shown here, it is possible to use a high frequency sputtering method even for a mixed oxide.

Effects of the Invention As described above, the method for producing a thin film superconductor of the present invention is
It provides a reproducible low-temperature process for Bi-based oxide superconducting thin films with a superconducting critical temperature of 0 K or higher, and can establish a low-temperature process that is essential for device applications.

[Brief description of drawings]

FIG. 1 is a graph showing an X-ray diffraction pattern of a thin film superconductor according to an embodiment of the present invention, and FIG. 2 is a graph showing an X-ray photoelectron spectroscopy spectrum of a thin film superconductor according to an embodiment of the present invention. FIG. 4 is a perspective view showing an apparatus used in a method for manufacturing a thin film superconductor in an embodiment of the present invention, and FIG. 4 is a constitution showing an atomic arrangement in a crystal of the thin film superconductor in an embodiment of the present invention. It is a figure. 31 …… MgO substrate, 32 …… Shutter, 3 …… Slit, 3
4 ... heater.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H01L 39/24 ZAA H01L 39/24 ZAAB // C04B 41/87 ZAA C04B 41/87 ZAAF H01B 12 / 06 ZAA H01B 12/06 ZAA (56) Reference JP-A 63-169375 (JP, A) JP-A 2-120229 (JP, A) JP-A 1-157406 (JP, A) JP-A 2- 120232 (JP, A) JP-A-2-59403 (JP, A) Nikkei Superconducting No. 9 (Sho 63-5-16) P. 7-8

Claims (3)

(57) [Claims] 1. A two-layer oxide containing at least bismuth, two-layer oxide containing strontium, an oxide layer containing calcium, and three-layer oxide containing copper are periodically laminated on a substrate. In the characteristic thin film superconductor, x% (0 <x ≦ 40) of the oxide containing strontium is calcium oxide, or y of the calcium oxide is
% (0 <y ≦ 40) is replaced with strontium oxide to form a thin film superconductor. 2. The method for producing a thin film superconductor according to claim 1, wherein the evaporation of the laminated material is performed by spattering. 3. The method for producing 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 kinds of compositions.