JPH02298257A - Thin film of oxide superconductor and its production - Google Patents

Abstract

PURPOSE:To provide a structure raising the superconducting critical temp. of a thin film of Bi oxide superconductor by alternately laminating a thin lamellar film of oxide superconductor containing Bi, Cu, and alkaline earths as principal components and a thin lamellar oxide film containing Bi and Ti. CONSTITUTION:A thin lamellar film 21 of oxide superconductor and a thin lamellar oxide film 22 are alternately laminated on a base material 15. The above thin lamellar film 21 of oxide superconductor contains at least bismuth (Bi), copper (Cu), and alkaline earths (group IIa) as principal components, and the above thin lamellar oxide film 22 contains at least bismuth (Bi) and titanium (Ti) as principal components. At this time, the above alkaline earths mean at least one kind among the group IIa elements. By this method, the periodical lamination of Bi oxide can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an oxide superconductor thin film containing bismuth, which is expected to have a high critical temperature of 100 or higher, and a method for producing the same.

Conventional technology Niobium nitride (NbN) and germanium niobium (Nb3Ge) are used as A15 type binary compounds as high-temperature superconductors.
were known, but the superconducting transition temperature of these materials was at most 23°C. On the other hand, perovskite compounds are expected to have even higher transition temperatures, and Ba-La-
A Cu-0-based high-temperature superconductor was proposed [(K. I. Mikoller, Phytonlift Future Physics)
(J, G.

Bednorz and K. A. Muller;
Zetshrlft Fur PhyslkB)-
Condensed Matter vol.64
, 189-193 (198G).

In addition, DI-5r-Ca-Cu-0 material is 100%
It was also discovered that H. Maeta, Y. Tana, M. Fukutomi Ant.
1゛Light ° Physifukus) (H, Naeda,
Y, Tanaka, M, Fukutoml
and T, Asano+ (Japanese
Journal of Applied Physics
s) 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, and it is expected that it will have more promising properties as a high-temperature superconductor than conventional binary compounds.

Furthermore, it was previously expected that higher superconducting transition temperatures could be achieved by alternately layering superconductors and insulators.
[M.H. Co., Ltd.

Russ, Jr. (Physical Training Letters°) (M, H, Cohen and D, H, Doug
lass, Jr., (Physlcal R
(view Letters) Yol, +9.11
8-121 (19B?)) Ko.

Problems to be Solved by the Invention However, with current technology, B1-5r-Ca-Cu-0 material can only be formed through the process of sintering, so it can only be obtained in the form of ceramic powder or blocks. . On the other hand, when this type of material is to be put to practical use, there is a strong demand for processing it into a thin film, but it has been difficult to fabricate a thin film with good superconducting properties using conventional techniques. That is, old-5r-Ca-Cu-
It is known that the 0 system has several phases with different superconducting transition temperatures, but it has been extremely difficult to achieve a phase with a transition temperature of 100 or higher in the form of a thin film. .

In addition, conventionally, in order to form a thin film exhibiting good superconducting properties in this Bl system, heat treatment at at least 700°C or higher or heating during formation is required, and therefore, the period with the insulating film, which is expected to have a high superconducting transition temperature, is It was considered extremely difficult to obtain a typical laminated structure, and it was also considered extremely difficult to construct an integrated device using this structure.

The present invention aims to solve the problems of the prior art.

Means for Solving the Problems The first oxide superconducting thin film of the present invention comprises a layered oxide superconducting thin film whose main components include at least bismuth (stL), copper (Cu), and alkaline earth (group IIa); is at least bismuth (81) and titanium (TI)
This is an oxide superconducting thin film characterized by having a structure in which layered oxide thin films containing .

Furthermore, the second method of manufacturing an oxide superconducting thin film of the present invention includes:
An oxide thin film formed by periodically stacking an oxide containing at least B1 and an oxide containing at least copper and alkaline earth (group IIa) on a substrate;
This is a method for producing an oxide superconducting thin film, characterized in that an oxide thin film formed by periodically stacking an oxide containing TI and an oxide containing at least TI are further alternately stacked.

Here, alkaline earth represents at least one or two or more elements of group IIa elements. In the first invention, the alkaline earth is a stable metal. 0□A superconducting thin film of a different type, which has a crystal structure covered with an oxide film layer or a layer mainly composed of this, and an insulating thin film with an oxide layered structure containing old and T1 are alternately By adopting a stacked structure, it is possible to stack the superconducting film and the insulating film with less mutual diffusion, and as a result, an increase in the superconducting transition temperature in the Bl-based superconducting thin film is realized.

Furthermore, in the second aspect of the present invention, in order to achieve the above structure, an oxide containing at least old and an oxide containing at least copper and alkaline earth (group IIa) or an oxide containing at least T1 are periodically mixed. By manufacturing thin films by laminating them and controlling them at the molecular level, it is possible to obtain a laminated layer of a Bl-based superconducting thin film and an insulating film with good reproducibility.

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

First, the present inventors studied the interaction between the old thread superconducting thin film and various insulating films in order to realize a periodic laminated structure of the old thread superconducting thin film and the insulating film.

Usually, the BI-based superconducting thin film is obtained by vapor deposition on a substrate heated to 600 to 700°C. After vapor deposition, the thin film exhibits superconducting properties as it is, but it is then subjected to heat treatment at 850 to 950°C to improve its superconducting properties.

However, if the insulating film is laminated next to the old superconducting thin film when the substrate temperature is high, or if heat treatment is performed after forming the insulating film, interdiffusion of elements occurs between the superconducting film and the insulating film, resulting in superconducting properties. was found to be significantly degraded. In order to prevent mutual diffusion, the superconducting film and insulating film must have excellent crystallinity, the lattice matching between the superconducting film and the insulating film must be excellent, and the insulating film must be heat-treated at 850 to 950°C. It is considered essential to be stable against

As a result of various studies, the present inventors have determined that at least T
It has been found that a thin film having a layered structure of Bl oxide containing I is suitable as an insulating film. The reason for this is that the 81-layered oxide containing TI is known to exhibit a layered perovskite in which the BI20° oxide layer sandwiches a structure consisting of elements such as T1 and oxygen). It is very stable even in high-temperature heat treatment at the interface between the materials in the structure, and it is also very stable between the Bl-based superconductor and the
It is considered that the lattice matching with II oxide is extremely excellent.

Furthermore, the present inventors have discovered that when the old thread superconducting thin film and the B1-Tl based oxide thin film are laminated periodically, the superconducting transition temperature inherent to the Bl based superconducting thin film increases.

In order to further understand the contents of the first invention by the present inventors, a specific example will be shown using FIG.

(Example 1) FIG. 1 is a perspective view schematically showing the inside of the binary magnetron sputtering apparatus used in this example, and 11 is an old-5r-
Ca-Cu-0 target, 12 is El-TiO target, 13 is shutter, 14 is aperture, 15
16 indicates a substrate, and 16 indicates a heater for heating the substrate. 11.12 Two pieces of targera made by press forming a sintered body
was used and arranged as shown in FIG. That is, M
Each target is installed at an angle of approximately 30° so as to focus on the g0 (100) substrate 15. There is a rotating shutter 13 in front of the target, and by controlling the rotation of the aperture 14 provided therein with a pulse motor, old-5r-Ca-Cu-0 → old-Tl-0
4 Old-5r-Ca-Cu-0+BI-TI-0+B1-
Snof ivy deposition can be performed with a cycle of 5r-Ca-Cu-0. Old-5r-Ca-Cu-01i, B
FIG. 2 conceptually shows how the 1-Tl-0 films are stacked.

In Fig. 2, 21 is a former-5r-Ca-Cu-0 film;
22 indicates the old-T1-0 film. Input power to target 11.12, old-5r-Ca-Cu-0 and old-TI
B1-5r-Ca-Cu-0 film 21.111 deposited on the substrate 15 by controlling the sputtering time of -0
The thickness of the 1-Sr-Ca-Cu-0 film 22 can be changed.

The substrate 16 was heated to about TOOO° C. by the heater 16, and each target was sputtered in a mixed atmosphere of argon and oxygen (1:1) at 0.5 Pa. After the thin film was prepared, heat treatment was performed at 1850° C. for 5 hours in an oxygen atmosphere. In this example, the sputtering power of each target is set to B
1-5r-Ca-Cu-0: 150f, B1-T1
-0: 100 f, and sputtering time of target 11.12 was controlled. Old-5r-Ca-Cu-0 film 2
The composition ratio of element 1 is 1111:Sr:Ca:Cu:2
:2:2:3.8l-The composition ratio of the elements of TAGERA) 11 and 12 was adjusted so that the composition ratio of the elements of the TIO film 22 was old:TI:4:3.

When the 8l-Sr-Ca-Cu-0 film 21 is formed on the base 15 without being laminated with the B1-Tl-0 film 22, that is, the characteristics of the B1-5'r-Ca-Cu-0 film 21 itself are as follows. , a superconducting transition occurs at 115, and the resistance becomes zero at 97. Furthermore, according to the present inventors, the limit of the film thickness that can be reduced while maintaining crystallinity is the
-0 film 22 was about 200A. It is preferable for the insulating film to be as thin as possible, so the old-TI with a film thickness of 20G
-0 film 22, old -8r-Ca-Cu-0 film 21
By changing the film thickness of (8l-Sr-Ca-
A stacked structure (Cu-0 film→BI-TI-0 film) was fabricated for 20 cycles. The temperature characteristics of the resistance of the superconducting thin film at that time are shown in FIG. In FIG. 3, B1-5r-Ca-Cu
-ON 21 film thickness is 100A1300A.

The characteristics at 500A are 31.32.
33. In characteristic 31, the zero resistance temperature is approximately 30
It was found that the characteristics of N and B1-5r-Da-Cu-0 films "21 deteriorated.The reason for this was that N B1-5
It is considered that the crystallinity of the film 2L22 is destroyed due to mutual diffusion of elements between the r-Ca-Cu-0 film 21 and the B[-TIO film 22. Furthermore, in characteristic 33, B1-Tl-0
The original superconducting properties of the old -5r-Ca-Cu-0 film 21 when deposited on the substrate 15 without periodic lamination with the film 22 are almost the same, and the superconducting properties of the old -5r-Ca-Cu-0 film 21 are almost the same as those of the old -T1-0 film 22 when laminated with the insulating film 22. No effect was confirmed. However, the present inventors found that in property 32, both the superconducting transition temperature and the zero resistance temperature rise to about 5. Although the detailed reason for this effect is still unknown, B1-5r-C
The effect of mutual diffusion of elements at the laminated interface between the a-Cu-0 film 21 and the old-T1-0 film 22 is small, and the thin B1-T
Multiple old-5r-Ca-Cu-0 through the l-0 film 22
By stacking the film 21, B1-5r-Ca-Cu-
It is conceivable that some change was caused in the superconducting mechanism in the 0 film 21.

Note that the effect of increasing the superconducting transition temperature is
The present inventors have confirmed that a film thickness of the a-Cu-0 film 21 in a range of 200 to 400 A is effective.

Note that when the present inventors performed sputtering by adding lead (Pb) to target 11 or process 2, even if the temperature of base 15 was approximately 100°C lower than in the above embodiment, results similar to those in the above embodiment were obtained. I found that it was possible to obtain

In addition, the present inventors used 8l instead of the old -TiO film 22.
-TI-Nb-0, B1-Tl-Ta-0, BITI-
Ca-0,Bl-Tj-Sr-0,8l-T1-Ba-
It was confirmed that the first invention is also effective when a -0 film is used for 0, old-Tl-Na-0, B1-Tl-.

Furthermore, the present inventors have discovered that the oxide of B1 and 5r1Ca, C
The oxide of
When layered periodically in the order of u-0-+ 5r-Cu-〇→B1-0, Bl oxide and TI oxide are evaporated separately in vacuum from different evaporation sources to form BIO. →
When laminated periodically in the order of T1-0 → old-0, (
Old-5r-Ca-Cu-0 superconducting thin film and B1-Tl0 with extremely good controllability, stable film quality, and extremely flat film surface compared to the layered structure manufacturing method shown in Example 1)
It was discovered that an insulating film can be obtained.

Furthermore, the present inventors have discovered that former-0,5r-Cu-0,Ca-
Cu-01TIO was evaporated from separate sources and old-5
When the r-Ca-Cu-0 superconducting thin film and the B1-Tl-0 insulating film were laminated periodically, extremely good controllability < m (Bl
It has been found that a thin film having a periodic structure of -5r-Ca-Cu-0) .n(Bl-TI-0) can be formed. Here, mln represents a positive integer.

Furthermore, this m(Bi-5r-Ca-Cu-0) ・n
(Bl-TI-0) thin film is a superconducting thin film obtained by simultaneously depositing 8l-5r-Ca-Cu-0 shown in (Example 1) and an oxide obtained by simultaneously depositing B1-71-0. It was also discovered that the crystallinity is far superior to thin films obtained by periodically laminating insulating films, and the properties such as superconducting transition temperature and critical current density are superior. Furthermore, the present inventors discovered that the former-5r-Ca-Cu-
It was found that the thin film surfaces of both the 0 superconducting thin film and the B1-Tl-0 insulating film were extremely flat.

These matters can be explained using the diagram shown in FIG. 4, which shows the concept of lamination. In other words, by sequentially stacking different elements constituting a layered structure, the stacked elements move only in a plane parallel to the substrate surface, and the elements perpendicular to the substrate surface move. This is thought to be due to the lack of movement. Furthermore, the a-axis length of the crystal with the layered oxide perovskite structure containing old and TI is almost equal to that of B1-5r-Ca-Cu-0, which is thought to be due to the fact that continuous epitaxial growth is possible. .

Furthermore, it was surprisingly discovered that the temperature of the substrate and the heat treatment temperature required to obtain good superconducting properties are lower than conventional ones.

There are several possible methods for periodically stacking old-0,5r-Cu-0, Ca-Cu-0, and TiO.

Generally, by controlling an opening/closing shutter in front of the evaporation source in an MBE device or a multi-source EB evaporation device, or by switching the type of gas when producing by vapor phase growth method,
Periodic stacking can be achieved.

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, when the present inventors laminated different thin layers using sputtering on this old-type oxide superconductor, they surprisingly discovered that it was possible to produce a good laminated film.

This is probably because the high oxygen gas pressure and sputter discharge during sputtering are favorable for the formation of a phase with a critical temperature of 100 K or higher in the old yarn and for the formation of the old -TIO insulating film.

The method of layering different materials using sputter deposition is as follows:
There is a method of periodically controlling the discharge position of one sputtering target with a composition distribution, but this can be achieved relatively easily by using a method of sputtering multiple targets with different compositions. 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. Alternatively, it can 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 second invention by the present inventors, specific examples will be shown below.

(Example 2) FIG. 5 shows a schematic diagram of a quaternary magnetron sputtering apparatus used in this example. In FIG. 5, 51 is a Bl target, 52 is a 5rCu alloy target, and 53 is a CaCu
An alloy target, 54 a T1 target, 55 a shutter, 56 a slit, 57 a substrate, and 58 a heater for heating the substrate. Total of 4 targets 51.52.53
．． 54 were arranged as shown in FIG. That is, Mg0
(1,00) Each target is installed at an angle of about 30" so as to focus on the base 57. In front of the target is a rotating shutter 55, and the shutter 55 is driven by a pulse motor. The rotation of the slit 56 is controlled, and the cycle and sputtering time for each target can be set. The substrate 57 is heated to about 600"C with a heater 58, and heated with argon and oxygen (5:1).
Sputtering of each target was performed in a gas mixture atmosphere of 3 Pa. The sputtering current of each target is Bl
:30 mA, 5rCu:80 mA, CaCu:
The experiment was conducted at 300 mA and Tl: 40G mA. Sputtering was performed with a cycle of 81→5rCu-+CaCu4Bl, and each target was sputtered so that the elemental composition ratio of the B1-5r-Ca-Cu-0 film was BI:Sr:Ca:Cu=2:2:2:3. As a result of adjusting the sputtering time and repeating the above cycle 20 times, it was possible to produce a phase having a critical temperature of 100 or higher. Even in this state, this old-5r-Ca-Cu-0 thin film showed a superconducting transition of over 100K, but furthermore, in oxygen at 850"G,
After 1 hour of heat treatment, reproducibility is very good.
The superconducting transition temperature was 120 K, and the temperature at which resistance became zero was 100 K. In the phase whose superconducting transition temperature exceeds 100 K, the metal element is B1-5r-Cu-Ca-Cu-Ca.
It is said that it is made up of oxide layers arranged in the order -Cu-S+"BI, and it is thought that the manufacturing method of the present invention is extremely useful in creating this structure. , Similarly, in the old → T1 → old cycle, B1-
Even if the sputtering time of each target is adjusted so that the elemental composition ratio of the Ti0 film is Bl:Tl:4:3, the number of cycles is reduced to 4, and the thickness of the B1-Tl0 film is made thinner. A Bl-Ti0 film with extremely excellent crystallinity was obtained.

Furthermore, the present inventors have determined that mX(Bi+ 5rCu−+ Ca
Cu-+ 5rCu→old)→nX(Bi→T1→81)
Sputter each target with a cycle of m (Bl-
A 5r7Ca-Cu-0) on (Bl-TI-0) thin film was fabricated on the substrate 57. Here, mT n represents a positive integer. The present inventors investigated the superconducting properties of films obtained by periodically stacking films while changing m when n=4. Figure 6 shows the temperature changes in the resistance of the film obtained when m = 2, 8, and 16, respectively, as characteristics 61, 62, and 83. In Figure 6, when m = 6, the highest superconducting transition temperature and zero resistance temperature, that is, characteristic 62.The superconducting transition temperature and zero resistance temperature of characteristic 62 are B1-5r-Ca-Cu
The values were about 8 higher than those of the -O film. Although the detailed reason for this effect is still unknown,
By the method shown in this example, B l-5r-Ca-Cu-0
By periodically stacking the film and the old-T1-0 film, the 8l-5r-Ca-Cu-0 film and the B1-Tl-0 film are epitaxially grown through the old 202 film. There is no effect of interdiffusion of elements at the interface,
By layering B1-5r-Ca-Cu-OFlX, which also has excellent crystallinity, through a thin B1-Tl-〇 film, which has excellent crystallinity, a superconducting mechanism can be achieved in the old -5r-Ca-Cu-0 film. It is possible that some change has occurred.

Note that the effect of increasing the superconducting transition temperature is 1111→5
The inventors have confirmed that rCu-+ CaCu+ 81 cycles are effective in the range of 4-10.

In addition, when the present inventors added lead (Pb) to the target 51 or 54 and performed sputtering, the temperature of the base 57 was approximately 100°C lower than that of the above embodiment, but the result was the same as that of the above embodiment. We have found that results can be obtained.

In addition, the present inventors used B1 instead of the 8l-TI-0 model.
Tl-Nb-0, B1-Tl-Ta-0, B1-Tl-
Ca-0,B1-Tl-5r-0゜DI-TI-Ba-
It was confirmed that the second invention is also effective when a -0 film is used for 0,B1-Tl-Na-0,B1-Tl-.

Effects of the Invention As described above, the oxide superconducting thin film of the first invention has Bl
The present invention provides a structure that increases the superconducting transition temperature of a superconducting oxide superconducting thin film, and the second method of manufacturing an oxide superconducting thin film of the present invention realizes the first invention more effectively and is suitable for applications such as devices. The industrial value of the present invention is great because a low-temperature process, which is essential for this purpose, has been established.

[Brief explanation of the drawing]

FIG. 1 is a perspective view schematically showing an apparatus for producing an oxide superconducting thin film according to an embodiment of the first invention, FIG. 2 is a cross-sectional view showing the structure of the thin film, and FIG. 3 is the apparatus shown in FIG. 1. Graph 1 showing the temperature characteristics of resistance in the thin film obtained by Fig. 4 is a cross-sectional view showing the concept of the structure of the thin film of the second invention, and Fig. 5 is the manufacturing of the thin film in one embodiment of the second invention. FIG. 6, which is a schematic perspective view of the apparatus, is a graph showing the temperature characteristics of the resistance in the thin film obtained by the apparatus of FIG. 11, 12.5L52, 53.54... Sputtering target, 13.55... Shutter, 14.
...Aperture, 56...Slit, 15.57.
...MgO base, 16, 58... heater, 21
・--81-Sr-Ca-Cu-0 film, 22-
--Bl-TI-0 membrane, 31.32.33.81.8
2.83...Temperature characteristics of thin film resistance. Name of agent: Patent attorney Shigetaka Awano (1 person) Figure 1 Figure 2 Figure 3 Figure 5 lω Humidity C
) Figure 4 ooooooo. ooooooo. ■■■O■■■ ・ ・ ・ ・ ・ ・ ・
ooooooooo.・ ・ ・ ・ ・ ・ ・
1oooooo■-MuO ......-Mu0 O0■■■■■-3r-0 O000O00-Ti 0 0(E)(E)0(E)(E)0 1 o o o o o ,○01 ooooooo. Figure 5

Claims (4)

[Claims] (1) The main components are at least bismuth (Bi) and copper (C).
u), and a layered oxide superconducting thin film containing an alkaline earth (group IIa), and a main component of which is at least bismuth (Bi).
and titanium (Ti) (here, alkaline earth represents at least one or two or more elements of group IIa elements). Superconducting thin film. (2) An oxide thin film formed by periodically stacking an oxide containing at least bismuth (Bi) and an oxide containing at least copper and alkaline earth (group IIa) on a substrate; ) and oxide thin films formed by periodically stacking oxides containing at least titanium (Ti) (here, the alkaline earth is at least one type of group IIa element). or two or more elements.) A method for producing an oxide superconducting thin film. (3) The method for producing an oxide superconducting thin film according to claim 2, characterized in that the evaporation of the laminated material is performed using at least two types of evaporation sources. (4) The method for producing an oxide superconducting thin film according to claim 2, wherein the evaporation of the laminated material is performed by sputtering.