JPH01166413A - Complex oxide superconductive thin film and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a thin film of a complex oxide superconductive material with a high critical current density by making the substantial part of the surface of a complex oxide superconductor thin film smooth. CONSTITUTION:The substantial part of the surface of a complex oxide superconductor thin film including mainly a complex oxide shown as the formula I is formed to be smooth. In the formula I, element alpha is Ba or Sr, and x is a number to satisfy 0.01<=x<=0. In this case, the substantial part means the major part of the surface except a partial void, a detect, and the like inevitable when the oxide is evaporated physically in a large area generally. In such a composition, a membrane of a complex oxide superconductive material with a high critical current density Jc can be obtained.

Description

[Detailed description of the invention] Slyo pμ rimin! The present invention relates to a superconducting thin film and a method for forming the same, and more particularly to a composite oxide superconducting thin film with significantly improved critical current density and a method for producing the same. The superconducting thin film obtained by the present invention has a high critical current, a high superconducting critical temperature, and other excellent properties such as smoothness, and is used in various electronic devices such as integrated circuits. It is particularly useful as a wiring material for components.

BACKGROUND OF THE INVENTION Superconductivity, which is said to be a phase transition of electrons, is a phenomenon in which the electrical resistance of a conductor becomes zero under certain conditions and exhibits complete diamagnetic properties.

In the field of electronics, which is a typical application field of superconductivity, various superconducting elements have been proposed and developed. A typical example is an element that utilizes the Josephson effect, in which a quantum effect appears macroscopically due to an applied current when superconducting materials are weakly bonded together. Further, tunnel junction type Josephson devices are expected to be extremely high-speed switching devices with low power consumption because the energy gap of the superconducting material is small. Furthermore, since the Josephson effect on electromagnetic waves and magnetic fields appears as a precise quantum phenomenon, it is expected that Josephson elements will be used as ultrasensitive sensors for magnetic fields, microwaves, radiation, etc.

In ultra-high-speed electronic computers, the power consumption per unit area has reached the limit of cooling capacity, so there is a demand for the development of superconducting elements. There is also a desire to use superconducting materials as wiring materials.

However, despite various efforts, the superconducting critical temperature Tc of superconducting materials has not been able to exceed 23K for Nb and Ge for a long period of time, but since last year, [La, Ba
] Sintered materials of oxides such as 5cuoa or (La, Sr)2CuO1 have been discovered as superconducting materials with high Tc, and the possibility of realizing non-low temperature superconductivity has greatly increased. In these materials, dramatically higher Tc than 30 to 50 was observed, and Tc of 70 or more was observed.
has also been observed.

Problems to be Solved by the Invention Conventionally, when producing the above composite oxide superconductor thin film,
Physical vapor deposition, such as sputtering, was performed using an oxide produced by sintering or the like as a vapor deposition source.

However, the conventional superconductor thin film manufactured in this way has a small critical current density Jc, and therefore cannot be put to practical use as an actual electronic circuit even if the critical temperature Tc is high.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art described above and to provide a thin film of a composite oxide superconducting material having a high critical current density Jc, and a method for producing the same.

Means for Solving the Problems According to the present invention, the following formula is expressed: There is provided a composite oxide superconductor thin film mainly containing a composite oxide, characterized in that a substantial portion of the surface of the composite oxide superconductor thin film is smooth. .

The composite oxide superconducting thin film of the present thin film mainly contains a composite oxide represented by the above general formula; it is conceivable that. Above element α
is selected from 13a or Sr.

The atomic ratio of La5Ba, Sr, and Cu is preferably within a range that satisfies the above formula, but is not necessarily strictly limited to this ratio, and ±
Even compositions with atomic ratios that deviate within a range of 50%, more preferably within a range of ±20%, may exhibit effective superconducting properties. That is, in the claims, the expression "mainly contains" means that the atomic ratios other than those defined by the above formula are also included.

Furthermore, the above definition also means that the product may contain elements other than the above elements, that is, unavoidable impurities that mix in on the order of pμm, and third components that are added for the purpose of improving other properties of the product. It means.

Elements that can be added as the third component include elements in the periodic table II.
Sr, Ca, Mg, Be of group a elements, elements of group IJa of the periodic table other than the above, Ib, nb, mb of the periodic table.

■ Elements selected from group a and group a, for example, Ti,
I can list V.

A feature of the present invention is that a substantial portion of the surface of the superconducting thin film is smooth. In this case, "substantial part"
This means that the majority of the surface, for example, 80% or more of the surface, is smooth, excluding partial voids, defects, etc., which are generally unavoidable when performing physical vapor deposition over a large area.

The surface smoothness of the superconducting thin film mentioned above is determined by the surface roughness Rmax (reference length =
1,000 μm) is 0.2 μm or less. This value allows the obtained thin film to be examined under a microscope, especially in an SEM.
It can be observed and confirmed. According to the experimental results of the present inventors, the roughness of the thin film surface Rmax is 0.2 μm.
If it exceeds , the critical current density Jc decreases significantly.

The composite oxide superconducting thin film of the present invention is generally formed on a substrate. This substrate is preferably an oxide single crystal substrate having a lattice spacing close to the lattice spacing of the composite oxide crystal, and for example, a MgO single crystal, 5rTiO* single crystal, or ZrO7 single crystal substrate can be used. . The (001) plane or (110) plane of the above-mentioned MgO single crystal or SrTiO3 single crystal substrate can be used as the film formation surface. Furthermore, a metal substrate or a semiconductor substrate having the above-mentioned single crystal phase can also be used.

Another object of the present invention is to provide a method for manufacturing a composite oxide superconducting thin film having a smooth surface as described above.

The method for manufacturing a composite oxide superconducting thin film according to the present invention is performed using the following formula' (La+-x αj 2 Cu 04 (where the element α is Ba or Sr, and X is 0.01≦x
≦0.2), a method for producing a composite oxide superconductor thin film by physical vapor deposition, which mainly contains a composite oxide represented by It is characterized by being carried out under conditions such that the parts are smooth.

As the physical vapor deposition, sputtering, ion blating, vacuum evaporation, etc. can be used, but sputtering is generally preferred, particularly RF magnetron sputtering.

It is preferable to heat the substrate during the physical vapor deposition, and the substrate temperature is 200 to 950°C, more preferably 500°C.
to 920°C. When the substrate temperature is less than 200°C, the crystallinity of the composite oxide is poor and it becomes amorphous.
A superconducting thin film cannot be obtained. Furthermore, if the substrate temperature exceeds 950° C., the crystal structure changes and the above-mentioned composite oxide does not become a superconductor.

As the substrate, it is preferable to use an oxide single crystal substrate having a lattice spacing close to that of the composite oxide crystal, and for example, MgO single crystal, SrTiO3 single crystal, or ZrO2 single crystal can be used.

As the film formation surface, it is preferable to use the (0013 plane or (110) plane) of an MgO single crystal or SrTiO3 single crystal substrate.

In a preferred embodiment of the present invention, the film formation rate during the physical vapor deposition described above is 0.05 to 1 Å/sec, more preferably 0.05 to 1 Å/sec.
It is set in the range of 1 to 0.8 Å/sec.

According to the experimental results of the present inventors, when the deposition rate during physical vapor deposition exceeds 1 Å/sec, the critical current density of the obtained superconducting thin film decreases significantly, making it impossible to obtain a practical thin film. Also,
If the film formation rate is less than 0.05 Å/sec, the film formation rate becomes extremely slow and is not industrially practical.

In another preferred embodiment of the present invention, the atmosphere during the physical vapor deposition is a mixed gas of an inert gas and oxygen, and the ratio of oxygen in this mixed gas is 5 to 95%, more preferably 10 to 80%. %.

A sputtering method can be used for the above-mentioned physical vapor deposition, and in that case, the gas pressure during sputtering is set to 0.0.
0.01 to 0.5 Torr, more preferably 0.01 to 0.5 Torr.
It is preferable to set it in the range of 01 to Q, 3 Torr, and add 0□ to 10 in the sputtering gas during sputtering.
It is preferable to conduct the reaction in an atmosphere containing 80 molecule % of .

Other sputtering gases that can be used together with 02 are preferably inert gases, particularly argon.

In a preferred embodiment of the invention, the sputtering method is RF sputtering, in particular magnetron sputtering. R preferably used in the present invention
In the case of F magnetron sputtering, for example, lQ
For a cmφ target, the high frequency power during sputtering was increased from the conventional 1.9 W/cd to 5 to 100 W.
, that is, 0.0. '064=1,27
W/cut, more preferably 10 to 60 W1, that is, 0.127 to 0.76 W/cm per unit cross-sectional area!
It is preferable to apply it.

After film formation, it is preferable to perform an annealing heat treatment in which the obtained thin film is heated and slowly cooled in an oxygen-containing atmosphere. This heat treatment is preferably carried out at a heating temperature in the range of 800 to 960°C, and the cooling rate during the heat treatment is preferably 10°C/min or less. During this heat treatment, the oxygen partial pressure is set at 0.1 to 10
It is preferable to use atmospheric pressure.

Function Conventionally, when producing thin films of composite oxide superconductors, physical vapor deposition, generally sputtering, was performed using a composite oxide sintered body as a target, but superconducting thin films obtained by conventional methods The current density Jc was low and it was not practical.

The above type of composite oxide superconductor has crystal anisotropy in its critical current density. That is, current tends to flow in a direction parallel to the plane determined by the a-axis and b-axis of the crystal. Therefore, for the purpose of aligning the crystal directions, a specific surface of a single crystal such as MgO1SrTiO3 or YSZ, which has a lattice spacing close to that of a composite oxide superconductor crystal, has been used as a substrate for film formation. . However, in the composite oxide superconducting thin films produced so far, the critical current density Jc could only rise to about 100,000 A/cm2 at most, due to reasons such as the inability to align the crystal directions sufficiently. .

The present invention improves the surface smoothness of the composite oxide superconducting thin film, thereby increasing the critical current density Jc by two orders of magnitude.
This has been improved to the order of 1,000,000 A/cm.

The reason why the critical current density Jc is greatly improved by improving the surface smoothness of the composite oxide superconducting thin film cannot be explained at present, but the composite oxide superconductor of the present invention has crystallization in its electrical resistance. A composite oxide superconducting thin film that has anisotropy and is formed on the film-forming surface of a substrate has a C-axis of its crystal that is perpendicular or at an angle close to perpendicular to the film-forming surface of the substrate, and in particular, the critical current density Jc is It is thought that it will become larger. Therefore, it is preferable to use the (001) plane of the MgO single crystal substrate or the 5rTiOa single crystal substrate as the film forming surface. Alternatively, the C-axis can be made parallel to the substrate using the (110) plane, and a direction perpendicular to the C-axis can be specified and used. Furthermore, since Mg0SSrTiO has a coefficient of thermal expansion close to that of the above composite oxide superconductor, unnecessary stress is not applied to the thin film during heating and cooling processes, and there is no risk of damaging the thin film.

According to an aspect of the present invention, the thin film after being formed has an oxygen partial pressure of 0.1.
It is preferable to perform an annealing treatment in which heat treatment is performed by heating to 800 to 960°C, more preferably 850 to 950°C, and cooling at a cooling rate of 10°C/min or less in an oxygen-containing atmosphere of ~10 atm. This treatment is to adjust oxygen defects in the above-mentioned composite oxide, and a thin film that does not undergo this treatment will have poor superconducting properties, and may not exhibit superconductivity. Therefore, it is preferable to perform the above heat treatment.

EXAMPLES The method for producing a composite oxide superconducting thin film with a smooth surface according to the present invention will be explained below using examples, but it goes without saying that the technical scope of the present invention is not limited to the following disclosure. be.

In the following examples of the present invention, the method for producing a superconducting thin film of the present invention described above was carried out by RF magnetron sputtering.

The target used was the element α, which is La or Sr, and the atomic ratio of La:α:Cu of 1.8=
This is a composite oxide sintered body made by sintering raw material powder with a ratio of 0.2:1 according to a conventional method. The target has a diameter of 100
A disk of mmφ was used. The film forming conditions in each case were the same, and the film forming conditions were as follows.

Substrate Mgo (001) surface laminate temperature 700°C Pressure 0. ITorr Sputtering gas 02 (20%)/Ar (80%) High frequency power 40W (0.51W/cI! Time)
6 hours Film thickness: 0.88 μm Film formation rate: 0.35 A/sec Annealing: 900°C/3 hours (cooling at 5°C/min) Other film forming conditions are the same, and the film forming rate is 1.5 Å/sec. Comparative examples were created for each target.

The critical temperature Tc in Table 1 was measured by the direct current four-terminal method according to a conventional method. In addition, the critical current density Jc is 77,
Indicates the amount of current when the electrical resistance is detected by increasing the amount of current while measuring the electrical resistance in the direction parallel to the surface of the sample in OK liquid nitrogen, converted to the unit area of the current path. Ta. Further, the surface roughness R□8 of the thin film was calculated from a SEM (scanning electron microscope) photograph.

Table 1 As shown above, the superconducting thin film according to the present invention has a significantly improved critical current compared to the comparative example.

It should be noted that voids on the order of several microns were observed on the surface of the thin film formed by the method of the present invention, although only a few (approximately 1% of the total surface area) were observed.
When observed under 10,000 times magnification, no irregularities were observed over most of the surface area. On the other hand, on the surface of the composite oxide superconducting thin film of the comparative example produced by a method outside the scope of the method of the present invention, a large number of daleins of several microns were present.

Effects of the Invention As detailed above, the superconducting thin film according to the present invention exhibits a much higher Jc than that produced by the conventional method.

Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (28)

[Claims] (1) Formula: (La_1_−_xα_x)_2CuO_4
(However, element α is Ba or Sr, and x is 0.0
1≦x≦0.2) In a composite oxide superconductor thin film mainly containing a composite oxide represented by A composite oxide superconducting thin film characterized by: (2) Surface roughness R_ of the above composite oxide superconductor thin film
m_a_x (reference length = 1,000 μm) is 0.2 μm
The composite oxide superconducting thin film according to claim 1, which is as follows. (3) The composite oxide superconductor is (La_1_-_x
Ba_x)_2CuO_4 (where x is 0.01≦x≦
The superconducting thin film according to claim 1 or 2, characterized in that the superconducting thin film contains a composite oxide represented by a number satisfying 0.2. (4) The composite oxide superconductor is (La_1_-_x
Sr_x)_2CuO_4 (where x is 0.01≦x≦
The superconducting thin film according to claim 1 or 2, characterized in that the superconducting thin film contains a composite oxide represented by a number satisfying 0.2. (5) the composite oxide thin film is formed on a substrate,
Claims 1 to 4, wherein the substrate is an oxide single crystal substrate having a lattice spacing close to that of the composite oxide crystal. Superconducting thin film. (6) The superconducting thin film according to claim 5, wherein the substrate is MgO single crystal, SrTiO_3 single crystal, or ZrO_2 single crystal. (7) The superconducting thin film according to claim 5 or 6, wherein the {001} plane or {110} plane of the MgO single crystal or SrTiO_3 single crystal substrate is the film-forming surface. (8) Formula: (La_1_−_xα_x)_2CuO_4
(However, element α is Ba or Sr, and x is 0.0
1≦x≦0.2) In a method for producing a composite oxide superconductor thin film mainly containing a composite oxide expressed by the following by physical vapor deposition, the surface of the thin film obtained by physical vapor deposition 1. A method for producing a composite oxide superconducting thin film, which is carried out under conditions such that a substantial portion thereof is smooth. (9) Surface roughness R_ of the above composite oxide superconductor thin film
m_a_x (reference length = 1,000μam) is 0.2μ
9. The method according to claim 8, characterized in that it is less than or equal to m. (10) The film formation rate during the above physical vapor deposition is 0.05 to 1 Å.
Claim 8, characterized in that the range is /second.
or the method described in paragraph 9. (11) The atmosphere during the above physical vapor deposition is a mixed gas of inert gas and oxygen, and the ratio of oxygen in this mixed gas is 5 to 95.
%. The method according to any one of claims 8 to 10. (12) The atmosphere during the physical vapor deposition is a mixed gas of inert gas and oxygen, and the ratio of oxygen in this mixed gas is 10 to 8.
12. The method according to claim 11, characterized in that the percentage is 0%. (13) The method according to any one of claims 8 to 12, characterized in that the substrate is heated during the physical vapor deposition. (14) The substrate temperature during the physical vapor deposition is from 200 to 95
14. The method according to claim 13, characterized in that the temperature is 0°C. (15) The substrate temperature during the physical vapor deposition is from 500 to 92
15. The method according to claim 14, characterized in that the temperature is 0°C. (16) Any one of claims 8 to 15, characterized in that the substrate is an oxide single crystal substrate having a lattice spacing close to the lattice spacing of the composite oxide crystal. The method described in. (17) As the substrate, MgO single crystal, SrTiO_
17. The method according to claim 16, characterized in that a ZrO_2 single crystal or a ZrO_2 single crystal is used. (18) The method for producing a superconducting thin film according to claim 17, wherein the {001} plane or {110} plane of the MgO single crystal or SrTiO_3 single crystal substrate is used as the film formation surface. (19) According to any one of claims 8 to 18, wherein the physical vapor deposition is sputtering, and the gas pressure during sputtering is in the range of 0.001 to 0.5 Torr. Method described. (20) The gas pressure during the sputtering is 0.01
20. A method according to claim 19, characterized in that it is within the range of ~0.3 Torr. (21) The method according to any one of claims 8 to 20, wherein the ratio of O_2 in the sputtering gas during the sputtering is 10 to 80 mol%. (22) The above physical vapor deposition is performed by RF sputtering, and the high frequency power during this RF sputtering is 0.06
The method according to any one of claims 8 to 21, characterized in that the power is in the range of 4 to 1.27 W/cm^2. (23) The above high frequency power is 0.064 to 1.27 W/c
23. The method according to claim 22, characterized in that it is within the range of m^2. (24) The method according to any one of claims 19 to 23, wherein the sputtering is magnetron sputtering. (25) The method according to any one of claims 8 to 24, wherein the thin film is subjected to heat treatment in which the thin film is heated and slowly cooled in an oxygen-containing atmosphere after the film formation. (26) The heating temperature during the above heat treatment is 800 to 960°C
26. The method according to claim 25, characterized in that the method is within the range of . (27) Claim 25 or 2, characterized in that the cooling temperature during the heat treatment is 10°C/min or less.
The method for producing a superconducting thin film according to item 6. (28) Claims 25 to 2, characterized in that the oxygen partial pressure during the heat treatment is 0.1 to 10 atm.
The method described in any one of Section 7.