JPH01188662A - Production of superconducting thin film - Google Patents

Abstract

PURPOSE:To increase the critical current density of a thin film of a superconductor of a compd. oxide represented by a specified formula when the thin film is formed by physical vapor deposition by carrying out the physical vapor deposition at a prescribed rate of film formation. CONSTITUTION:A thin film of a compd. oxide superconductor is formed on a substrate by physical vapor deposition. The superconductor is based on a compd. oxide represented by the formula (where Ln is Tm and/or Lu and x=0-1) and the physical vapor deposition is carried out at 0.05-1Angstrom /sec rate of film formation.

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 obtained by the present invention has a high critical current and also has excellent properties such as smoothness, making it particularly useful as a wiring material for various electronic components including integrated circuits. be.

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.

Various types of superconducting elements are known in the field of electronics. 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.

Tunnel junction type Josephson devices are expected to be extremely high-speed switching devices with low power consumption because of the small energy gap of superconducting materials. 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 ultra-sensitive sensors for magnetic fields, microwaves, radiation, etc. Furthermore, in fields such as ultra-high-speed computers where power consumption per unit area has already reached the limit of cooling capacity, there is a demand for the development of superconducting elements as ultra-high-speed arithmetic elements or as low-loss wiring materials.

On the other hand, despite various efforts, the superconducting critical temperature Tc of superconducting materials could not exceed 23K of Nb3Ge for a long period of time, but last year, in the future, [La, Ba
]2CIJO4 or [:La, Sr:] 2cu○
Sintered materials of oxides such as No. 4 have been discovered as superconducting materials with high TC, greatly increasing the possibility of realizing non-low temperature superconductivity. These materials have a T of 30 to 50, which is dramatically higher than that of conventional materials. has been observed. Further, it has been announced that a composite oxide represented by Y1Ba2CLI307-X, called YBCO, is a superconductor having a critical temperature of about 90°C. Oxygen defects in the crystal are said to play a major role in the superconducting properties of these composite oxide superconductors, and if appropriate oxygen defects are not formed in the crystal, the Tc will be low, and the onset temperature will be low. The difference between the temperature and the temperature at which the resistance becomes completely zero also becomes large.

Problems to be Solved by the Invention As a method for producing a composite oxide superconductor thin film as described above, physical vapor deposition is widely practiced using a composite oxide produced by sintering or the like as a deposition source.

As a physical vapor deposition method, a sputtering method is particularly common. However, since the above superconductor has a small critical current density Jc, it has low practicality even if the critical temperature Tc is high. This property does not change even when it is made into a thin film, and has been a major problem when putting composite oxide superconductors into practical use.

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 Jc.

Means for Solving the Problems According to the present invention, in a method for producing a composite oxide superconductor thin film mainly containing a composite oxide represented by the following formula; % formula % by physical vapor deposition,
There is provided a method for producing a superconducting thin film, characterized in that the film formation rate during the physical vapor deposition is in the range of 0.05 to 1 Å/sec.

As the above-mentioned physical vapor deposition, other vapor deposition methods under reduced pressure such as sputtering, ion blating, and vacuum vapor deposition can be used, but sputtering is generally preferred, particularly RF magnetron sputtering.

The composite oxide superconducting thin film produced by the method of the present invention contains a composite oxide represented by the above general formula: n+Ba2cusC)+-8, and these composite oxides are perovskite-type or pseudo-perovskite-type oxides. It is thought that objects are the main subject.

The lanthanoid element Ln is preferably Tm and/or Lu.

Further, the atomic ratio of the lanthanoid elements Ln, Ba, and Cu is preferably 1:2:3 as shown in the above formula, but it is not necessarily strictly limited to this ratio; Within a range of ±50% from the ratio, more preferably ±
Compositions with atomic ratios that deviate within a range of 20% can also be included within the scope of the present invention. That is, in the claims, "mainly includes a complex oxide represented by the above formula"
This expression means that the superconducting thin film produced by the method of the present invention is Ln:Ba:Cu defined by the above formula.
This means that the atomic ratio of is also inclusive of those other than 1:2.3.

Furthermore, the above definition refers to elements other than Ln, Ba, Cu, and ○ mentioned above, that is, unavoidable impurities mixed in on the order of ppm, and elements added for the purpose of improving other properties of the obtained sintered body or thin film. This means that it may contain a third component.

Elements that can be added as the third component are listed in Periodic Table 11.
Group A elements Sr, Ca, Mg, Be, Group NLA elements of the periodic table other than the above, Ib, nb, ■b, ■ of the periodic table
Elements selected from groups a and ■a, for example, Ti,
V, etc. can be exemplified.

According to an aspect of the present invention, in the substrate for forming the above composite oxide superconducting thin film, a perovskite crystal substrate,
It is possible to use an oxide substrate, or a metal substrate or semiconductor substrate on which a perovskite crystal or oxide is formed as a buffer layer. Preferred substrate materials include MgO single crystal, 5rTi03 single crystal, ZrO2
A single crystal, a ysz single crystal, an A I 203 single crystal, or a polycrystalline Al2O3, as well as a metal substrate or a semiconductor substrate having a film-forming surface formed of these materials, are preferable. In particular, MgO
The film formation surface of the single crystal or 5rTiO+ single crystal substrate is set to (0
01) plane or (110) plane is preferable.

The main feature of the present invention is that the film formation rate during the physical vapor deposition is reduced to 0.
．． 05-1 Å/sec, more preferably 0.1-0.8
The point is Å/second.

According to the experimental results of the present inventors, when the film formation 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. Furthermore, if the film formation rate is less than 0.05 Å/sec, the film formation rate will be extremely slow, which is not industrially practical.

When the above-mentioned physical vapor deposition is performed by sputtering, the sputtering is performed under a pressure of 0.001 to 0.5 Torr, more preferably under a pressure of 0.01 to 0.3 Torr, and at a pressure of 0.01 to 0.3 Torr.
It is preferable to carry out the reaction in an atmosphere containing 5 to 95 mol % of 2, more preferably 10 to 80 mol %. Argon, which is an inert gas, is preferably used as a sputtering gas other than 02.

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

When physical vapor deposition is performed by sputtering as described above, factors for controlling the film formation rate include the film formation gas pressure, the ratio of 02 + Ar), and in the case of RF sputtering, the RF power.

In either case, the proportion of 02 in the sputtering gas is preferably 5 to 95 mol%.

When sputtering is performed by RF sputtering, the high frequency power is preferably within the range of 0.064 to 1.27 W/cm, particularly within the range of 0.127 to 0.76 W/c++f. Furthermore, the ratio of 02 in the sputtering gas is in the range of 10 to 80 mol%, and the film forming gas pressure is 0.001-0.5 Torr, especially 0.
．． It is preferable to set it within the range of 0.01-0.3 Torr.

When controlling the film forming gas pressure or the ratio of 02/(Ch+Ar), when the above sputtering is performed by RF sputtering, the high frequency power is 1.27 to 2.55 W /
Within the range of cnt, especially 1.53 to 2.29W/cut
It is preferable to set it within the range of. And 02/ (0
When controlling the ratio of 02+Ar), the ratio of 02 in the sputtering gas is preferably within the range of 30 to 95 mol%, particularly within the range of 40 to 80 mol%, and the deposition gas pressure at a pressure of 0.001 to 0.5 Torr, especially 0.00 Torr. It is preferable to set it within the range of 0.3 Torr to 0.3 Torr.

On the other hand, when controlling the film-forming gas pressure, the ratio of 02 in the sputtering gas is preferably within the range of 10 to 80 mol%, and the film-forming gas pressure is set to 0.00.
1-0.5 Tor, r pressure, especially 0.05-0.5T
It is preferable to set it within the range of orr.

According to an aspect of the present invention, the substrate on which the above composite oxide superconducting thin film is formed includes a perovskite crystal substrate,
It is possible to use an oxide substrate, or a metal substrate or semiconductor substrate on which a perovskite crystal or oxide is formed as a buffer layer. Preferably, the substrate is MgO single crystal, SrTiO3 single crystal, ZrO2
Single crystal, ysz single crystal, Al2O3 single crystal, or polycrystalline Al2O3, as well as metal substrates and semiconductor substrates whose film-forming surfaces are formed of these materials, are preferable. In particular, the film formation surface of the MgO single crystal or 5rTi○3 single crystal substrate is (001
) plane or (110) plane is preferable.

Furthermore, in an aspect of the present invention, the thin film after deposition is heated to an oxygen partial pressure [
1, 800-960℃ in an oxygen-containing atmosphere of 1-10 atm.
for 0.5 to 20 hours, more preferably 850 to 950
It is preferable to perform annealing by heating at a temperature of 1 to 10 hours and cooling at a cooling rate of 10° C./min or less.

Further, the film is formed to have a thickness in the range of 01 to 10 μm, more preferably in the range of 0.5 to 2 μm.

Last month, the method for producing a superconducting thin film of the present invention was developed by performing the above-mentioned physical vapor deposition at 0.
05-1 Å/sec, more preferably 0.1-0.8 Å/sec
Its main feature is that it can be formed at a deposition rate of seconds.

Conventionally, when producing a thin film of composite oxide superconductor, physical vapor deposition, generally sputtering, was performed using a target made of a composite oxide sintered body of the same type. The resulting superconducting thin film has a critical current density J
c was low and could not be put to practical use.

This is because the composite oxide superconductor 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, the conventional methods have not been able to align the crystal directions sufficiently.

Therefore, conventionally, in order to align the crystal directions, a specific surface of a single crystal such as MgO1SrTi○3 or YSZ, which has a lattice spacing close to that of a composite oxide superconductor crystal, has been used as a film-forming surface. Ta.

In the method of the present invention, by improving the conventional method and setting the film formation rate during the physical vapor deposition to 0.05 to 1 Å/sec, more preferably 0.1 to 0.8 Å/sec, composite oxide align the crystal directions. As a result, a superconducting thin film with significantly improved Jc compared to conventional methods can be obtained.

In the method of the present invention, film formation is performed by physical vapor deposition, preferably sputtering, under the above conditions.
Preferably, the substrate temperature during sputtering is 200-95
It is preferable to perform physical vapor deposition, preferably sputtering, by heating to 0°C, more preferably 500 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.

In the case of RF magnetron sputtering, which is preferably used in the present invention, for example, for a target of IG cmφ, the high frequency power is 1.9
W/cffl to 5 to 100 W, that is, 0.064 to 1.27 W/cd per unit cross-sectional area, more preferably 10 to 60 W1, that is, 0 per unit cross-sectional area.
．． It is preferable to apply 127 to 0.76 W/clIt.

According to a preferred embodiment of the present invention, the substrate on which the composite oxide superconducting thin film is formed is MgO single crystal, 5rT
An i○3 single crystal substrate or a 1rO2 single crystal substrate is preferable, and it is particularly preferable to use the (001) plane or (110) plane of an MgO single crystal substrate or a 5rTi○3 single crystal substrate as the film forming surface. Furthermore, a metal substrate or a semiconductor substrate having the above-mentioned single crystal phase can also be used.

This is because, as already explained, the composite oxide superconductor of the present invention has crystal anisotropy in its electrical resistance, and the composite oxide superconductor thin film formed on the film-forming surface of the substrate is It is thought that the C-axis of the crystal is perpendicular or nearly perpendicular to the substrate film-forming surface, and that the critical current density Jc becomes particularly large. Therefore, it is preferable to use the (001) plane of the MgO single crystal substrate or the 5rTi○3 single crystal substrate as the film forming surface. Also, using the (110) plane, C
It is also possible to make the axis parallel to the substrate and specify a direction perpendicular to the C-axis. Furthermore, Mg0SSrTi○3 has a coefficient of thermal expansion close to that of the above-mentioned composite oxide superconductor, so when heated,
No unnecessary stress is applied to the thin film during the cooling process.
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 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.

Example 1 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 Ln-Ba, which was made by sintering raw material powder with Tm or U, Ba, and Cu in an atomic ratio Ln:Ba:Cu of 1:2.24:4.35 according to a conventional method.
-Cu-based composite oxide ceramic. The target was a disk with a diameter of 100 mmφ. The film forming conditions were the same for each sample, and the film forming conditions were as follows.

Substrate MgO (001) plane 02/(02+Ar) 20% Substrate temperature 700°C Pressure Q, 1 Torr High frequency power 40 W (0.51 W/cffl) Time 6 hours Film thickness 0.88 μm (film formation rate 0.35 A/sec) After the film was formed, the temperature was maintained at 900° C. for 1 hour at atmospheric pressure at 02° C., and then the film was cooled at a cooling rate of 5° C./min.

For comparison, Table 1 shows the results when a composite oxide superconducting thin film was fabricated under exactly the same conditions, except that the same target was used and the deposition rate was 1.5 Å/sec. This is an indication.

The critical temperature Tc was measured by a four-terminal method according to a conventional method. In addition, the critical current density Jc is calculated by increasing the amount of current while measuring the electrical resistance of the sample with 77, OK, and converting the amount of current when electrical resistance is detected in the sample into the unit area of the current path. It is written down.

Table 1 Example 2 A composite oxide superconducting thin film was produced under the same conditions as in Example 1 except that the film forming conditions were as follows. The results are shown in Table 2.

Substrate MgO (001) plane 02/(o□10Ar) 50% Substrate temperature 700℃ Pressure Q, l Torr High frequency power 150W (1,9W/cnt) Time
6 hours Film thickness: 0.88 μm (Film forming rate: 0.35 A/sec) Table 2 Example 3 A composite oxide superconducting thin film was formed under the same conditions as in Example 1 except that the film forming conditions were as follows. Created. The results are shown in Table 3.

Substrate MgO (001) surface Oh/(CL+Ar) 20% Substrate temperature 700°C Pressure 0.15 Torr High frequency power 150 W (1.9 W/cffI) Time 6 hours Film thickness 0.88 μm (Deposition rate 0.35 Å/sec) Table 3 As described above, the superconducting thin film produced by the method of the present invention has a significantly improved critical current compared to the comparative example. Also,
The structure of the composite oxide superconducting thin film produced by the method of the present invention is uniform, whereas the surface of the composite oxide superconducting thin film of the comparative example produced by the conventional method has dalein of several microns. The method of the present invention can also be inferred from the fact that when the surface is observed with an SEM at a magnification of 10,000 times, no irregularities are observed over most of the surface area.

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.
Indicates C.

According to the method of the present invention, compared to conventional methods, it is possible to stably supply a high-performance superconducting thin film simply by lowering the film formation rate during physical vapor deposition.

Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (1)

[Claims] Formula: Ln_1Ba_2Cu_3O_7_-_x (wherein, Ln represents Tm and/or Lu, and x is 0≦x
< 1) In a method for producing a composite oxide superconductor thin film mainly containing a composite oxide represented by A method for producing a superconducting thin film, the method comprising performing vapor deposition.