JPH01167223A - Production of superconducting thin film - Google Patents

Abstract

PURPOSE:To obtain the title thin film giving high critical current density, by producing, under specified film-forming gas pressure through the physical deposition technique, composite oxide superconducting this film comprised mainly of specific composite oxide. CONSTITUTION:In the process for production, by means of physical deposition, of composite oxide superconducting thin film comprised mainly of a composite oxide of the formula (alpha is Ba or Sr; 0.01<=x<=0.2), the film-forming gas pressure in said physical deposition is brought to between 0.001 and 0.6Torr. If said gas pressure exceeds 0.6Torr, significant reduction in the critical current density of the superconducting thin film obtained will result, thus practicable thin film cannot be produced. On the other hands, if said pressure is lower than 0.001Torr, reduction in both the critical current density and critical temperature will result. In general, the film-forming gas to be used is a mixture of oxygen gas and an inert gas, said inert gas being argon.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a superconducting thin film, and more particularly, to a method for producing a superconducting thin film that significantly improves the critical current of a composite oxide superconducting thin film having a high superconducting critical temperature. This invention relates to a method for producing a thin film.

The superconducting thin film having a high critical current obtained by the present invention is particularly useful as a wiring material for various electronic components including integrated circuits.

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 are known. 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 addition, in ultra-high-speed electronic computers, the power consumption per unit area is reaching the limit of cooling capacity, so there is a demand for the development of superconducting elements, and as the degree of integration of electronic circuits increases, There is 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 of Nb3Ge for a long time, but since last year, [:La, B
a]2cuo4 or [La, Sr:]2cu04
Sintered materials of oxides such as oxides have been discovered as superconducting materials with high Tc, greatly increasing the possibility of realizing non-low temperature superconductivity. In these materials, a T value of 30 to 50, which is significantly higher than conventional materials, has been observed, and a Tc of 70 or more 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 oxides produced by sintering etc. 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 method for producing a thin film of a composite oxide superconducting material having a high critical current density Jc.

Means for Solving the Problems According to the invention, the formula: (La+-x αM)2
Cu04 (however, element α is Ba or Sr, and
is a number that satisfies 0.01≦x≦0.2) In a method for producing a composite oxide superconductor thin film mainly containing a composite oxide represented by 0.01≦x≦0.2 by physical vapor deposition, Pressure 0001~0.6 Torr
Provided is a method for producing a superconducting thin film characterized in that r is within a range.

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

The composite oxide superconducting thin film produced by the method of the present invention mainly contains a composite oxide represented by the above general formula: (La+-Mα, )2 ClO2, and these composite oxides are perovskite-type or pseudo-type. It is thought to be mainly composed of perovskite-type oxides. The above element α is Ba
or Sr, and preferably has a composition ratio that satisfies the above formula, but is not necessarily strictly limited to this ratio, and is within a range of ±50%, more preferably a range of ±20% from these ratios. A composition with a different atomic ratio may also 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 applies to the above α, La, Cu and ■
This means that it may contain elements other than the above, that is, unavoidable impurities mixed in on the order of ppm, and a third component added for the purpose of improving other properties of the product.

Examples of elements that can be added as the third component include Sr, Ca, Mg, Be of group IIa elements of the periodic table, elements of group 11Ia of the periodic table other than the above, Ib, nb of the periodic table,
Mention may be made of elements selected from the mb, rVa, and a groups, such as T1 and .

The feature of the present invention is that the film forming gas pressure during the physical vapor deposition is set to 0.0.
01 to 0.6 Torr, more preferably 0,0
0.05 to 0.4 Torr.

According to the experimental results of the present inventors, when the film-forming gas pressure during physical vapor deposition exceeds 0.6 Torr, the critical current density of the obtained superconducting thin film decreases significantly, making it impossible to obtain a practical thin film. Further, when the film forming gas pressure becomes less than 0.001 Torr, both the critical current density Jc and the critical temperature Tc decrease.

In this case, the film-forming gas pressure means the gas pressure in the chamber where physical vapor deposition is performed, and in the case of vacuum evaporation or ion blating, it refers to the atmospheric gas pressure in the chamber, and in the case of sputter link, it refers to the gas pressure in the chamber where physical vapor deposition is performed. means gas pressure. In either case, a mixed gas of oxygen gas and an inert gas is generally used as the film forming gas, and argon is used as the inert gas. The ratio of oxygen gas to inert gas is preferably 5 to 95 mol %, more preferably 10 to 80 mol % of 02.

At the time of the above-mentioned physical vapor deposition, preferably sputtering, the film formation rate is 0.05 to 1 person/second, more preferably 0.1 to 1 person/second.
It is preferable to form the film at a rate of 0.8 people/second. In addition, in the case of RF magnetron sputtering which is preferably used in the present invention, for example, for a target of 10 cmφ,
During sputtering, high frequency power is reduced to 1.9 W/C compared to conventional
% to 5 to 100 W, i.e. 0 per unit cross-sectional area.
．． 064-1.27W/cffl.

More preferably, it is preferable to apply 10 to 60 W, that is, 0127 to 0.76 W/cffl per unit cross-sectional area.

In addition, the substrate is heated at 200 to 950°C, more preferably at 50°C.
It is preferable to carry out heating at 0 to 920°C.

If the substrate temperature is less than 200° C., the composite oxide has poor crystallinity and becomes amorphous, making it impossible to obtain a superconducting thin film. Furthermore, if the substrate temperature exceeds 950° C., the crystal structure changes and the above-mentioned composite oxide does not become a superconductor.

According to an aspect of the present invention, the substrate on which the above-mentioned composite oxide superconducting thin film is formed is preferably an MgO single crystal, SrTiO3 single crystal, or ZrO2 single crystal substrate;
O single crystal substrate or SrT i○3 single crystal substrate (00
It is preferable to use the 1) plane or the (110) plane as the film-forming plane. Furthermore, a metal substrate or a semiconductor substrate having the above-mentioned single crystal phase can also be used.

Further, according to an aspect of the present invention, the thin film after film formation is heated in an oxygen-containing atmosphere with an oxygen partial pressure of 01 to 10 atm.
℃, more preferably 850 to 950℃,
It is preferable to perform annealing by cooling at a cooling rate of 0° C./min or less. 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.

Last monthConventionally, when producing a thin film of a composite oxide superconductor,
Physical vapor deposition, generally sputtering, has been performed using a composite oxide sintered body as a target, but superconducting thin films obtained by conventional methods have low critical current densities Jc and are not practical.

This is because the composite oxide superconductor made by the conventional method has crystal anisotropy in its critical current density. That is,
Although current tends to flow in a direction parallel to the plane determined by the a-axis and b-axis of the crystal, conventional methods have not been able to align the crystal directions sufficiently. Therefore, conventionally, in order to align the crystal directions, MgO1SrTiO, which has a lattice spacing close to that of the composite oxide superconductor crystal, has been used as a substrate.
Specific surfaces of single crystals such as + and YSZ were used as film-forming surfaces.

In the method of the present invention, the conventional method is improved and the film forming gas pressure during the above physical vapor deposition is set to 0. f) 01~0.6To
rr range, more preferably 0.005 to 0.4T
By setting it in the range of orr, the crystal directions of the composite oxide are aligned, and as a result, a superconducting thin film with significantly improved Jc compared to the conventional method is obtained.

The composite oxide superconductor of the present invention has crystal anisotropy in its electrical resistance, and the composite oxide superconducting thin film formed on the film-forming surface of the substrate has a C-axis of the crystal on the film-forming surface of the substrate. The angle is perpendicular or nearly perpendicular to the critical current density Jc.
is expected to increase. Therefore, it is preferable to use the ([101) plane of the MgO single crystal substrate or the 5rTi○3 single crystal substrate as the film forming surface. Also, (110)
It is also possible to use a surface to make the C-axis parallel to the substrate and specify a direction perpendicular to the C-axis. Furthermore, MgO1SrT
Since iCh has a coefficient of thermal expansion close to that of the composite oxide superconductor described above, unnecessary stress is not applied to the thin film during heating and cooling processes, and there is no risk of damaging the thin film.

EXAMPLES 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.

The method for producing a superconducting thin film of the present invention described above was carried out by RF magnetron sputtering. The target used had an atomic ratio of La:α:Cu of Shia, element α which is Ba or Sr, and Cu of 1.8:0.
．． A composite oxide sintered body made by sintering 2 + 1 raw material powder according to a conventional method was used. The target was a disk with a diameter of 100 mm, and the film forming conditions were the same in each case, and the film forming conditions were as follows.

Substrate MgO (001) surface plate temperature 690℃ High frequency power 150W (1.9Wlon time
6 hours Film thickness: 0.88 μm Film formation rate: 0.35 people/second Film formation gas pressure: 0.15 Torr Film formation gas composition: 02/Ar (20:80) After film formation, the temperature was increased to 910 °C under atmospheric pressure. After holding for a period of time, it was cooled at a cooling rate of 5° C./min. For comparison, Table 1 shows comparative examples of the results when composite oxide superconducting thin films were fabricated under the same conditions as above, except that the film-forming gas pressure was 0.0008 Torr and 0.7 Torr. Shown as 1 and 2.

The critical temperature Tc shown in Table 1 was measured by a four-terminal method according to a conventional method. In addition, the critical current density Jc was determined by increasing the amount of current while measuring the electrical resistance of the sample in 4.2, and converting the amount of current when the electrical resistance of the sample was detected into the unit area of the current path. writing things down.

Table 1 As shown above, the superconducting thin film produced by the method of the present invention has significantly improved critical current compared to the comparative example.

In addition, even when observing the surface of the composite oxide superconducting thin film produced by the method of the present invention under SEM magnification of 10,000 times, no irregularities were observed over approximately 98% or more of the surface area. Ta. On the other hand, many grains of several microns were present on the surface of the composite oxide superconducting thin film of the comparative example produced by a method outside the scope of the present invention.

Effects of the Invention As detailed above, the superconducting thin film obtained by the method of the present invention has a higher J than that produced by the conventional method.
c.

Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (19)

[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 method for producing a composite oxide superconductor thin film mainly containing a composite oxide represented by .001~0.6To
A method for producing a superconducting thin film, characterized in that the temperature is within the range of rr. (2) The film-forming gas pressure during the physical vapor deposition is 0.005 to 0.0.
A method according to claim 1, characterized in that the temperature is in the range of 4 Torr. (3) The composite oxide superconductor is (La_1_-_x
Ba_x)_2CuO_4 (where x is 0.01≦x≦
The method according to claim 1 or 2, characterized in that the method comprises 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 method according to claim 1 or 2, characterized in that the method comprises a composite oxide represented by a number satisfying 0.2. (5) The method according to any one of claims 1 to 4, characterized in that the substrate is heated during the physical vapor deposition. (6) The substrate temperature during the physical vapor deposition is between 200 and 950.
The method according to claim 5, characterized in that the temperature is .degree. (7) The substrate temperature during the physical vapor deposition is between 500 and 920.
7. The method according to claim 6, characterized in that the temperature is .degree. (8) Any one of claims 1 to 7, 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. (9) As the substrate, MgO single crystal, SrTiO_3
9. The method according to claim 8, characterized in that a single crystal or a ZrO_2 single crystal is used. (10) The method for producing a superconducting thin film according to claim 9, characterized in that the {001} plane or {110} plane of the MgO single crystal or SrTiO_3 single crystal substrate is used as the film-forming surface. (11) The method according to any one of claims 1 to 10, wherein the physical vapor deposition is sputtering. (12) The method according to claim 11, wherein the sputtering is magnetron sputtering. (13) The method according to any one of claims 11 and 12, wherein the ratio of O_2 in the sputtering gas during the sputtering is 5 to 95 mol%. (14) The method according to claim 13, wherein the ratio of O_2 in the sputtering gas during the sputtering is 10 to 80 mol%. (15) Any one of claims 11 to 14, characterized in that the sputtering is performed by RF sputtering, and the high frequency power is within the range of 0.064 to 1.27 W/cm^2. The method described in. (16) The method according to any one of claims 1 to 15, characterized in that after the film formation, a heat treatment of heating and slowly cooling the thin film in an oxygen-containing atmosphere is performed. (17) The heating temperature during the above heat treatment is 800 to 960°C
17. The method according to claim 16, characterized in that the method is within the range of . (18) Claim 16 or 1, characterized in that the cooling temperature during the heat treatment is 10°C/min or less.
The method described in Section 7. (19) Claims 16 to 1, 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 8.