JPH042615A - Production of superconductive thin film - Google Patents

Abstract

PURPOSE:To improve superconductivity by periodically depositing a Bi- containing layer on a basal plate in an oxidative atmosphere and another layer containing Cu and one or more alkalline earth metal selected from Ca, Sr and Ba thereon in a more oxidative atmosphere. CONSTITUTION:A basal plate 1 of MgO, etc., is heated with a heater 3 at 600-700 deg.C and a Bi target 4a, an SrCu target 4b and a Ca2Cu target 4c are sputtered in an Ar/O2 mixture gas atmosphere. A shutter 6 is initially rotated to stop an aperture 5 at a position above the Bi target 4a and sputtered atoms are oxidized to form a Bi2O2 layer on the basal plate 1. The mixture ratio of Ar/O2 is then changed or O3 or N2O is used instead of O2 to change the atmosphere more oxidative. The aperture 5 is stopped at a position above the SrCu target 4b and then at a position above the Ca2Cu target 4c to deposit an Sr2Ca2Cu3Ox layer on the Bi2O2 layer. The above-mentioned process is repeated to deposit a Bi-containing oxide and another oxide containing an alkalline earth metal and Cu. Heat treatment is subsequently carried out, thus obtaining the objective Bi-based oxide superconductive thin film.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a thin film superconductor, and particularly to a method for producing a Bi-based oxide superconductor having a critical temperature of 100 or higher in a stable manner in the form of a thin film. It is about the method.

Conventional technology It was discovered that B1-5r-Ca Cu-0-based materials exhibit a transition temperature of 100 or higher as high-temperature superconductors [H,
Maeda+ Y, Tanaka, M, Fuku
tomi and t.

Asano, Japanese Journal of Applied Physics
of Applied Physics) Vol.
27. L209-210 (1988)].

Although the details of the superconducting mechanism of this type of material are not clear,
Niobium nitride (NbN) and germanium niobium (NbN) are used as high-temperature superconductors because they have a transition temperature higher than that of liquid nitrogen.
It is expected to have more promising properties than binary compounds such as baGe). It is thought that if this material can be made into a thin film, the possibility of realizing a superconducting device will increase, and various methods are currently being used to make this kind of material into a thin film.

Problems to be Solved by the Invention It is known that this B1-5r-Ca-Cu-0 type material has crystal structures having different superconducting transition temperatures. Since these crystal structures are very similar and phases with different crystal structures are easily formed, it is difficult to obtain a single phase in the form of a thin film that exhibits a superconducting transition temperature of 100 or higher, for example.

In addition, in the conventional technology, after forming a thin film, it is heated at 800° in an oxygen atmosphere.
Since heat treatment at a high temperature of C or higher is required, heat treatment at such high temperatures tends to damage the film surface.In order to apply high-temperature superconducting thin films as devices, a low-temperature process that does not damage the thin film surface is required. development is desired.

Means for Solving the Problem: A layer containing bismuth and a layer containing at least one alkaline earth metal selected from calcium, strontium, and barium and copper are periodically deposited on a heated substrate in an oxidizing atmosphere. By depositing a layer containing an alkaline earth metal and copper in a more oxidizing atmosphere than when depositing a layer containing bismuth, a Bi-based oxide superconductor that exhibits high-temperature superconductivity at temperatures below 800°C can be produced. It is characterized by stably obtaining 100 or more phases. Further, it is preferable in terms of reproducibility to heat-treat the deposited layer after the deposition.

Effect The present inventors formed a thin film superconductor on an oxide superconductor containing Bi by sputtering using, for example, three different targets, and investigated the substrate temperature, oxygen gas pressure, crystal structure of the thin film, and superconducting properties (electric The relationship between resistance and temperature characteristics was investigated in detail. As a result, an oxide containing bismuth and an oxide containing an alkaline earth metal and copper are deposited periodically on a heated substrate in an oxidizing atmosphere, and a layer containing an alkaline earth metal and copper is deposited. By depositing B1-5r- in a more oxidizing atmosphere than when depositing a bismuth-containing layer to form a periodic structure and adjusting the thin film composition, B1-5r- Ca-Cu-0
It was possible to stably obtain a thin film having an arbitrary crystal structure of the system, for example, a phase having a zero resistance temperature of 100 or more.

By depositing each layer with varying oxidizing strength,
The following may be considered as the reason why a phase of 0 or more is likely to be formed. In the same oxidizing atmosphere, Bi, Sr,
The oxidation rates of Ca and Cu are different, and the oxidation rates are in the order of Sr>Ca>Bi>Cu. Bi
When it is sufficiently oxidized, it becomes Bi2O3 and becomes an insulator.

Therefore, when depositing a layer containing bismuth, it is necessary to stop oxidation when the layer becomes Bi2O□. On the other hand, as copper is oxidized, Cu-+Cu20 → Cu O-Cu0
2, but the oxidation rate is slow. Therefore, the deposition of layers containing alkaline earth metals and copper must be sufficiently oxidized to the extent that CuO2 is formed.

When producing an oxide thin film with a complex composition such as an oxide thin film superconductor, conventional techniques may involve sputtering using a single target synthesized in advance to have the desired composition.

However, with this method, it is impossible to change the amount of oxidation for each required element. The manufacturing method of the present invention is characterized in that the amount of oxidation of required elements can be varied.

Embodiment Although various methods are conceivable for periodically depositing different substances, vacuum processes are limited to methods for periodically depositing materials while controlling the film thickness on the order of the size of atoms. For example, the sputtering method uses multiple sputtering targets to periodically deposit on the substrate while controlling the shutter, and the vacuum evaporation method uses multiple evaporation sources to periodically deposit on the substrate while controlling the shutter. MBE method,
(including EB evaporation method), vapor phase growth method that uses multiple gas sources to grow while periodically causing chemical reactions on the substrate while controlling the gas flow rate, and vapor phase growth method that uses multiple ion sources to grow while controlling the shutter. Possible methods include ion beam evaporation, which involves periodic deposition on a substrate.

- As an example, using sputtering method, Bi stutter and 5rz
Two kinds of targets, Ca2Cu3 target, were subjected to subinterisolation in a mixed gas of argon (Ar) and oxygen (O□) by repeating alternately in the order of Bi target-+Sr, Ca2Cu3 target → Bi target, and 550 °C
were deposited periodically on Mg0(100) substrates heated from 800°C to 800°C.

At this time, it was found that by appropriately adjusting the sputtering rate of each target material, a crystal structure corresponding to a phase of B1-5r-Ca-CuO having a superconducting transition temperature of 100 K or higher was formed. Moreover, when the deposition was carried out simultaneously rather than periodically, only a phase having a superconducting transition temperature of 80°C could be formed. When deposition is performed periodically, the zero resistance temperature can be raised by changing the oxygen concentration between sputtering Bi targets and sputtering 5r2Ca2Cu3 targets, and the superconducting properties can be improved. .

A specific embodiment of the present invention will be explained based on FIGS. 1 to 3.

In the thin film superconductor manufacturing apparatus of this embodiment shown in FIG. 1, 1 is a substrate, 2 is a substrate holder, 3 is a heater, and 4a is a Bi
Tutat, 4b is 5rcu target, 4c is Ca2
It is a Cu target. The temperature of the base 1 is maintained at a high temperature of 600°C to 700°C, and all of the targets 48 to 4c are discharged. At this time, each target 4a~
Regarding 4c, the amount of atoms flying in such a way that one atomic layer is formed within a sufficiently controllable time is optimally adjusted by the electric discharge power. Note that the discharge power can be either direct current or high frequency. Direct current sputtering can only sputter conductive targets, but high frequency sputtering can sputter even insulating targets. In this example, the base 1
Using Mg0 (100), this substrate 1 and target 4
The distance between a to 4c is 75 mm, and typical discharge power in DC magnetron sputtering is 7 W for the Bi target 4a and 7 W for the 5rCu target 4b in an argon/oxygen (5:1) mixed atmosphere of 3 Pa. 14W
, CazCu target 4c was set to 60W. Further, in this embodiment, a rotatable shutter 6 provided with an aperture 5 is used. Since this shutter 6 can completely control the position of the aperture 5 and its stop time by external control, it is possible to stack atomic layers on the substrate 11 in an orderly manner and adjust the amount of evaporation.

When Mg0 (100) was used as the substrate 1, the substrate 1 was heated to about 680''C, and sputtering of each target layer (4a to 4c) was performed in an argon/oxygen mixed atmosphere.The mixed gas pressure was 3 Pa and the gas flow rate was The ratio is argon:oxygen=5=
It is 1. First, the aperture 5 is stopped above the Bi target 4a so that only the atoms sputtered by this target 4a fly onto the substrate 1. The sputtered atoms are oxidized by oxygen in the sputtering atmosphere, and Bi is deposited on the substrate 1.
A zO□ layer (layer containing bismuth) 7 is formed. after that,
The shutter 6 was sequentially rotated to obtain the desired laminated structure.

For example, in order to form a crystal structure exhibiting a superconducting transition temperature of 100K or more, as shown in FIG. 2, on the Bi202 layer 7, a 5r-0 layer 8a, a Cu-0 layer 8b, a Ca layer 8c,
Cu-0 layer 8b, Ca layer 8c, Cu-0 layer 8b, S
The r0 layer 8a is sequentially deposited. For convenience, the bottom layer 5r-0 layer 8a to the top layer 5r-0 layer 8a are collectively called 5rz.
cazcu, , O, , layer (layer containing alkaline earth metal and copper) 8. A BizOz layer 7 is deposited thereon again. At this time, the composition ratio of each layer is controlled by the open time of the shutter 6 and the input power to the target cameras 4a to 4c. For example, in this embodiment, the typical value of the open time of the shank 6 is 50 seconds to 8 seconds for the old target 4a.
0 seconds (input chamber cuff W), 5 at 5rCu target 4b
0 seconds to 90 seconds (14 W), and 80 seconds to 150 seconds (60 W) for Ca2Cu target 4c.

2, the stoichiometric ratio (Bi:Sr:C
A thin film was formed in accordance with a:Cu=2:2:2:3).

First, when forming the BizOz layer 7, 5r2Ca2C+
go, when forming a thin film in an argon/oxygen (5:1) mixed atmosphere of 3 Pa with a constant oxygen concentration when forming layer 8, a thin film exhibiting a superconducting on-cent temperature of 100 or higher was obtained. However, the zero resistance temperature was not very high (about 60K).

Furthermore, the reproducibility was not very good.

Next, the Bi2O□ layer 7 is deposited 5r2Ca2Cu30) (
This was done in a more oxidizing atmosphere than when layer 8 was deposited.

In this example, the argon/oxygen mixed gas was used, and the flow rate of argon was kept constant, and the ratio of Ar and oxygen during the deposition of the Bi2O□ layer 7 was 5:1, the gas pressure was 3 Pa, and the gas pressure was 3 Pa, 5r2Ca2Cu30.
The ratio of argon and oxygen during the deposition of the X layer 8 was 2:1.

As a result, as shown in FIG. 3(a), a thin film having a superconducting onset temperature of 100 or higher was obtained, and the zero resistance temperature was also 100 or higher.

On the other hand, the ratio of argon and oxygen is fixed at 221, and B
When the i202 layer 7 and Sr, Ca2Cu:+OxOsO4 are deposited alternately, Bi2O3 layers are formed in places in the thin film, and as shown in Figure 3(b), the electrical resistance at room temperature is extremely high and superconducting. did not show.

This experimental result shows that the manufacturing method of this example is effective. Note that when changing the mixing ratio of argon and oxygen, the amount of evaporation changes and the composition becomes different under the same sputtering conditions, so it is necessary to adjust the sputtering rate as appropriate by adjusting the opening and closing time of the shank.

Note that as a method of creating an oxidizing atmosphere, ozone (O3) or nitrous oxide (N20) can be used instead of oxygen (O□). Ozone and nitrous oxide have stronger oxidizing power than oxygen, so they are more effective. The mixed gas ratio of argon and oxygen was fixed at 5:1, and B was used without introducing ozone.
When the izOz layer 7 is deposited and ozone is introduced at 5% of the amount of oxygen during the deposition of the Sr, Ca2Cu, and OX layer 8, the superconducting on-cent temperature of the obtained thin film is 110K, and the zero resistance temperature is 100 or more. Ta. Similarly, when nitrous oxide is introduced, the zero resistance temperature is 100.
A thin film exhibiting the above properties was obtained.

The present inventors have also confirmed that the oxidizing atmosphere can be adjusted by exposing the atmosphere to light having a wavelength of 400 nm or less and changing its intensity (for example, luminous intensity). When oxygen is exposed to light with a wavelength of 400 nm or less, 3th →
The chemical reaction of 203 is promoted and ozone is generated. Since the amount of ozone generated changes depending on the intensity of light, the strength of the oxidizing atmosphere can be controlled by changing the intensity of light.

Furthermore, the reproducibility (in order to obtain superconducting properties, it was confirmed that it is effective to continue heat treatment in an oxidizing atmosphere after the completion of deposition. When the superconducting properties were investigated by varying the heat treatment temperature, the heat treatment temperature was 300℃. The appropriate treatment time was approximately 1 hour at a temperature between °C and 700''C.

In the above example, 100% of the Bi-based oxide thin film superconductor
Although the above-mentioned manufacturing method was limited to the above-mentioned phase, the present invention
80 phase (stoichiometric ratio is Bi
:Sr:Ca:Cu=2:2:1:2).

Effects of the Invention According to the present invention, it is possible to stably obtain a thin film of a Bi-based oxide superconductor having a superconducting transition temperature of 100 or higher at a low temperature of 800° or lower. damage can be avoided, and can contribute to the realization of superconducting devices with more promising properties than binary compounds.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the thin film superconductor manufacturing apparatus used in the example of the present invention, Figure 2 is a crystal structure diagram of the thin film superconductor of this example, and Figure 3 is a diagram of the temperature dependence of its electrical resistance. It is. 1--1111 Substrate 3--Heater...contains bismuth: JLi 8--layer containing alkaline earth metal and copper

Claims (4)

[Claims] (1) A layer containing bismuth and a layer containing at least one alkaline earth metal selected from calcium, strontium, and barium and copper are deposited periodically on a heated substrate in an oxidizing atmosphere, 1. A method for producing a thin film superconductor, the method comprising obtaining a thin film superconductor by depositing a layer containing similar metals and copper in an atmosphere that is more oxidizing than the layer containing bismuth. (2) Oxidizing strength is determined by oxygen (O_2) or ozone (O
2. The method for producing a thin film superconductor according to claim 1, wherein the control is performed by adjusting the concentration of nitrous oxide (N_2O) or nitrous oxide (N_2O) in the atmosphere. (3) The wavelength of 400nm that exposes the atmosphere to the strength of oxidizing properties
2. The method for manufacturing a thin film superconductor according to claim 1, wherein the method is controlled by changing the intensity of light. (4) The method for producing a thin film superconductor according to claim 1, characterized in that, after depositing the oxide containing bismuth, the alkaline earth metal, and the oxide containing copper, the deposited layer is subsequently heat-treated.