JP2544760B2 - Preparation method of superconducting thin film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a superconducting thin film, and more particularly, to significantly improve the critical current of a complex oxide superconducting thin film having a high superconducting critical temperature. It also relates to a method for producing a superconducting thin film.

The superconducting thin film obtained by the present invention has a high critical current and at the same time has excellent properties in other properties such as smoothness, and is particularly useful as a wiring material for various electronic components including integrated circuits. is there.

2. Description of the Related Art The superconducting phenomenon, which is said to be a phase transition of electrons, is a phenomenon in which the electric resistance of a conductor becomes zero under certain conditions and shows perfect diamagnetism.

Various superconducting elements are known in the field of electronics. A typical example is an element utilizing the Josephson effect in which quantum effects appear macroscopically by an applied current when superconducting materials are weakly joined.

The tunnel junction type Josephson device is expected as a switching technology with extremely high speed and low power consumption because the energy gap of the superconducting material is small. Also,
Since the Josephson effect for electromagnetic waves and magnetic fields appears as an accurate quantum phenomenon, it is expected that the Josephson device will be used as an ultrasensitive sensor for magnetic fields, microwaves, radiation, etc. Furthermore, the development of superconducting elements is required for ultra-high speed computers whose power consumption per unit area reaches the limit of cooling capacity.

Further, it has been proposed to use a superconducting material with no current loss as a wiring material for various electronic components as the degree of integration of electronic circuits increases.

On the other hand, despite various efforts, the superconducting critical temperature Tc of superconducting materials could not exceed 23K of Nb ₃ Ge for a long time, but since the end of last year, [La, Ba] ₂ CuO ₄ or Sintered oxides such as [La, Sr] ₂ CuO ₄ have been discovered as superconducting materials with high Tc, and the possibility of realizing non-low temperature superconductivity is increasing greatly. For these substances, 30-50K
A dramatically higher T _C is observed compared to the conventional one.

Further, it has been announced that a composite oxide represented by Y ₁ Ba ₂ Cu ₃ O _7-X called YBCO is a 90K-class superconductor. Oxygen defects in the crystal play a major role in the superconducting properties of these complex oxide superconductors. That is,
If oxygen deficiency in the crystal is not proper, Tc will be low, and the difference between the onset temperature and the temperature at which the resistance becomes zero will be large.

Problems to be Solved by the Invention Conventionally, when the above-mentioned composite oxide superconductor thin film was prepared, physical vapor deposition was performed using an oxide generated by sintering or the like as a vapor deposition source.

As the physical vapor deposition method, a sputtering method is generally used. However, since the above-mentioned superconductor has a small critical current density Jc, its practicality is low even if the critical temperature Tc is high. This characteristic does not change even when it is formed into a thin film, which has been a serious problem in practical application of the composite oxide superconductor.

Therefore, an object of the present invention is to provide a method for solving the above-mentioned problems of the prior art and producing a thin film of a complex oxide superconducting material having a high critical current Jc.

Means for Solving the Problems According to the present invention, the following formula: Ln ₁ Ba ₂ Cu ₃ O _7-X (where Ln is La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er,
A composite oxide superconducting thin film mainly containing a composite oxide represented by: represents at least one lanthanoid element selected from Yb, and x is a number satisfying 0 ≦ x <1. In the method, there is provided a method for producing a superconducting thin film, characterized in that the high frequency power is in the range of 0.064 to 1.27 [W / cm ² ] and more preferably in the range of 0.127 to 0.76 [W / cm ² ]. This high-frequency power is 5 when the target size is a disk with a diameter of 10 cm.
To 100 W, more preferably 10 to 60 W. Incidentally, it is also advantageous to adopt a magnetron sputtering method at the time of sputtering.

The composite oxide superconducting thin film produced by the method of the present invention,
It contains complex oxides represented by the above general formula: Ln ₁ Ba ₂ Cu ₃ O _7-X , and these complex oxides are considered to be mainly composed of perovskite type or pseudo perovskite type oxides.

The lanthanoid element Ln is La, Nd, Sm, Eu, Gd,
It is selected from Dy, Ho, Y, Er and Yb.
Ho, Y, Er and Dy, which are superconducting thin films having high critical current density Jc, high critical temperature Tc and excellent surface smoothness, are particularly preferable.

The lanthanoid element Ln, Ba, and the atomic ratio of Cu is preferably 1: 2: 3 as in the above formula, but is not necessarily strictly limited to this ratio, and from these ratios It will be understood that a composition having an atomic ratio deviated within a range of ± 50%, more preferably within a range of ± 20%, is also within the scope of the present invention. That is, in the claims, the expression (mainly including the complex oxide represented by the above formula) means that the atomic ratio of Ln: Ba: Cu defined by the above formula is 1: 2: 3. It is meant to include things other than those.

Furthermore, the above definition includes elements other than the above Ln, Ba, Cu and O, that is, impurities that are practically unavoidable mixed in in the ppm order and a third component added for the purpose of improving other characteristics. It means that you can do it.

The elements that can be added as the third component are the periodic table II.
Sr, Ca, Mg, Be of group a elements, periodic table other than the above IIIa
Group elements, Periodic Table Ib, IIb, IIIb, IVa and VIIIa
An element selected from the group, for example, Ti or V can be mentioned.

In one embodiment of the present invention, the film formation rate is 0.05 to 1Å /
The sputtering is carried out for a second, more preferably 0.1 to 0.8 Å / second. Further, the film is formed so that the film thickness is in the range of 0.1 to 10 μm, more preferably 0.5 to 2 μm.

Further, the sputtering is a pressure of 0.001 ~ 0.5 Torr, more preferably under a pressure of 0.01 ~ 0.3 Torr and O ₂
Is preferably carried out in an atmosphere containing 5 to 95 molecule%, more preferably 10 to 80 molecule%. Argon, which is an inert gas, is preferable as the other sputtering gas that can be used together with the gas other than O ₂ . Also, the substrate is 200 ~ 950 ℃,
More preferably, the sputtering is performed while heating at 500 to 920 ° C.

According to the aspect of the present invention, as the substrate for forming the above-mentioned complex oxide superconducting thin film, a perovskite-type crystal substrate, an oxide substrate, or a metal substrate or semiconductor in which the perovskite-type crystal or oxide is formed as a buffer layer It is possible to use a substrate. Preferably, the substrate is MgO single crystal, SrTiO ₃ single crystal, ZrO ₂ single crystal, Y
SZ single crystal, Al ₂ O ₃ single crystal, or polycrystalline Al ₂ O ₃ , further,
A metal substrate or a semiconductor substrate having a film formation surface formed of these substances is preferable. In particular, the MgO single crystal or SrTiO ₃ single crystal substrate preferably has a {001} plane or a {110} plane as a film formation surface.

Further, in the aspect of the present invention, the thin film after film formation is subjected to oxygen partial pressure.
In an oxygen-containing atmosphere of 0.1 to 10 atm at 800 to 960 ° C for 0.5 to 20 hours, more preferably at 850 to 950 ° C for 1 to 10 hours,
It is preferable to perform annealing by cooling at a cooling rate of 10 ° C./minute or less.

The method for producing the superconducting thin film of the present invention is 0.064 to 1.27 (W / c
m ² ], more preferably 0.127 to 0.76 [W / cm ² ] in the range of applying high frequency power while performing the sputtering.

For example, when preparing a thin film of a complex oxide superconductor represented by Y ₁ Ba ₂ Cu ₃ O _7-X called YBCO, the conventional Y ₁ Ba ₂ C
Sputtering was performed using a sintered body such as u ₃ O ₇ as a target. However, the superconducting thin film obtained by the conventional method has a particularly low critical current density Jc and is not practical.

This is because the above complex oxide superconductor has crystal anisotropy in its critical current density, that is, a
This is because current easily flows in a direction parallel to the plane determined by the axis and the b-axis, but the crystal directions could not be sufficiently aligned by the conventional method. Conventionally, in order to align the crystal directions, a specific surface of a single crystal such as MgO, SrTiO ₃ and YSZ having a lattice spacing close to that of a complex oxide superconductor crystal was used as a film-forming surface as a substrate. .

In the method of the present invention, in addition to the conventional method, the high frequency power applied at the time of sputtering, for example, to a target of 10 cmφ is about 1.9 W / cm ^{2 in} the related art, and the total is 5
~ 100W, i.e. 0.064 ~ 1.27W / c per unit cross section
m ² and more preferably 10 to 60 W as a whole, that is, 0.127 to 0.76 W / cm ² per unit cross-sectional area, so that the crystal directions of the composite oxide are aligned and the structure is densified. As a result, a superconducting thin film having a significantly improved Jc as compared with the conventional method can be obtained.

Here, according to the experiments by the present inventors, when the applied high frequency power exceeds the above range, no significant difference in characteristics from the thin film produced by the conventional method was found. On the other hand, when the sputtering is carried out under the conditions that do not reach the above range,
The film formation rate was extremely slow, and a thin film having an effective film thickness could not be formed.

In the method of the present invention, film formation is carried out by sputtering under the above-mentioned conditions, and it is preferable to further heat the substrate temperature at the time of sputtering to 200 to 950 ° C, more preferably 500 to 920 ° C for sputtering. Substrate temperature is 200 ℃
If it is less than the above range, the crystallinity of the composite oxide is poor and the composite oxide becomes amorphous, so that a superconducting thin film cannot be obtained. Further, when the substrate temperature exceeds 950 ° C., the crystal structure changes, and the above composite oxide does not become a superconductor.

According to the embodiment of the present invention, a MgO single crystal, SrTiO ₃ single crystal or ZrO ₂ single crystal substrate is preferable as the substrate for forming the above-mentioned composite oxide superconducting thin film. In particular, it is preferable to use the {001} plane or the {110} plane of the MgO single crystal substrate or the SrTiO ₃ single crystal substrate as the film formation surface.

This is because the complex oxide superconductor of the present invention has crystal anisotropy in its electric resistance as described above. Therefore, the complex oxide superconducting thin film formed on the film forming surface of the substrate is It is considered that the c-axis of the crystal is perpendicular or nearly perpendicular to the film formation surface of the substrate, and the critical current density Jc becomes particularly large. Therefore, MgO single crystal substrate or SrT
It is preferable to use the {001} plane of the iO ₃ single crystal substrate as the film formation surface. It is also possible to use the {110} plane to make the c-axis parallel to the substrate and specify the direction perpendicular to the c-axis for use. Furthermore, since MgO and SrTiO ₃ have a coefficient of thermal expansion close to that of the above-mentioned composite oxide superconductor, unnecessary stress is not applied to the thin film during heating and cooling, and there is no risk of damage to the thin film.

According to an embodiment of the present invention, the thin film after deposition has an oxygen partial pressure of 0.1.
It is preferable to perform an annealing treatment in which a heat treatment of heating to 800 to 960 ° C., more preferably 850 to 950 ° C., and cooling at a cooling rate of 10 ° C./min or less is performed in an oxygen-containing atmosphere at −10 atm. This treatment is for adjusting oxygen defects in the above-mentioned composite oxide, and the superconducting property of the thin film that does not undergo this treatment is poor and the superconducting property may not be exhibited in some cases. Therefore, it is preferable to perform the above heat treatment.

EXAMPLES The present invention will be described below with reference to examples, but it goes without saying that the technical scope of the present invention is not limited to the following disclosure.

A superconducting thin film was produced by the RF magnetron sputtering method by the method of the present invention described above. The target used was the lanthanoid element Ln shown in Table 1 below.
And Ba and Cu atomic ratio Ln: Ba: Cu is a composite oxide Ln-Ba-Cu-O ceramic having a ratio of Ln: Ba: Cu of 1: 2.24: 4.35, and the target has a circular shape with a diameter of 100 mmφ. The film forming conditions in each case were the same, and the film forming conditions were as follows.

Substrate MgO (001) surface O ₂ / (O ₂ + Ar) 20% Pressure 0.1Torr Substrate temperature 700 ℃ High frequency power 40W (0.51W / cm ² ) Time 6 hours Film thickness 0.88μm (deposition rate 0.35Å / sec) Composition After the film formation, the temperature was kept at 900 ° C. for 3 hours under atmospheric pressure (O ₂ partial pressure of about 0.2 Torr), and then cooled at a cooling rate of 5 ° C./min. For comparison, Table 1 also shows the results when a complex oxide superconducting thin film containing Ho was prepared under exactly the same conditions except that the high frequency power was 150 W (1.9 W / cm ² ). .

The critical temperature Tc was measured by the four-terminal method according to a conventional method. In addition, the critical current density Jc is 77.0K, and the amount of current is increased while measuring the electrical resistance of the data, and the amount of current when the electrical resistance is detected in the sample is converted to the unit area of the current path. There is.

As described above, the superconducting thin film produced by the method of the present invention has a significantly improved critical current as compared with the comparative example.

Further, the fact that the structure of the composite oxide superconducting thin film produced by the method of the present invention is uniform and dense means that the surface of the composite oxide superconducting thin film of the comparative example produced by the conventional method has a few micron of grains. On the other hand, in the case of the method of the present invention, it can be inferred from the fact that no unevenness is observed even when the surface is magnified 10,000 times with SEM and observed.

EFFECTS OF THE INVENTION As described in detail above, the superconducting thin film obtained by the method of the present invention exhibits a higher critical current density Jc than that produced by the conventional method.

The method of the present invention merely reduces the high frequency power of sputtering as compared with the conventional method, and does not use a special device. According to the present invention, it is possible to more stably supply a high-performance superconducting thin film.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication H01B 12/00 ZAA H01B 12/00 ZAA H01L 39/24 ZAA H01L 39/24 ZAAB (72) Inventor Shuji Yazu 1-1-1 Kunyokita, Itami-shi, Hyogo Sumitomo Electric Industries, Ltd. Itami Works (72) Inventor Tetsuji Kamishiro 1-1-1 Kunyo-kita, Itami-shi, Hyogo Itami Ltd. In-house (56) References JP 64-35819 (JP, A) JP 64-14814 (JP, A)

Claims (24)

(57) [Claims] 1. A formula: Ln ₁ Ba ₂ Cu ₃ O _7-X (where Ln is La, Nd,
At least one lanthanoid element selected from Sm, Eu, Gd, Dy, Ho, Y, Er, and Yb is represented, and x is 0 ≦ x.
<A number that satisfies <1) In the method for producing a composite oxide superconductor thin film mainly containing a composite oxide by a sputtering method, MgO single crystal, SrTiO ₃ single crystal, Z
Using a rO ₂ single crystal, YSZ single crystal or Al ₂ O ₃ single crystal substrate, the high frequency power applied during film formation was 0.064 to 1.27 (W / c
m ² ]. The method for producing a superconducting thin film, characterized in that 2. The high frequency power applied during film formation is 0.127.
The method for producing a superconducting thin film according to claim 1, wherein the superconducting thin film is in the range of 0.76 [W / cm ² ]. 3. The composite oxide superconductor is Y ₁ Ba ₂ Cu ₃ O _7-X.
The method for producing a superconducting thin film according to claim 1 or 2, further comprising a complex oxide represented by (where x is a number satisfying 0 ≦ x <1). 4. The composite oxide superconductor is Er ₁ Ba ₂ Cu ₃ O.
_7-X (where x is a number satisfying 0 ≦ x <1) is included, and the production of the superconducting thin film according to claim 1 or 2 is included. Method. 5. The composite oxide superconductor is Ho ₁ Ba ₂ Cu ₃ O.
_7-X (where x is a number satisfying 0 ≦ x <1) is included, and the production of the superconducting thin film according to claim 1 or 2 is included. Method. 6. The composite oxide superconductor is Dy ₁ Ba ₂ Cu ₃ O.
_7-X (where x is a number satisfying 0 ≦ x <1) is included, and the production of the superconducting thin film according to claim 1 or 2 is included. Method. 7. The gas pressure during the sputtering is 0.00
The method for producing a superconducting thin film according to any one of claims 1 to 6, wherein the superconducting thin film is in the range of 1 to 0.5 Torr. 8. The gas pressure during the sputtering is 0.01
To 0.3 Torr. 7. The method for producing a superconducting thin film according to claim 1, wherein the superconducting thin film is in the range of 0.3 to 0.3 Torr. 9. The method according to claim 1, wherein the ratio of O _{2 in} the sputtering gas at the time of the sputtering is 5 to 95 molecule%. Manufacturing method of superconducting thin film. 10. The sputtering method according to claim 1, wherein the ratio of O _{2 in} the sputtering gas during the sputtering is 10 to 80 molecule%. Manufacturing method of superconducting thin film. 11. The method for producing a superconducting thin film according to claim 1, wherein the substrate is heated during the sputtering. 12. The substrate temperature during the sputtering is 20
Claim 11 characterized in that it is 0 to 950 ° C
A method for producing a superconducting thin film according to item. 13. The substrate temperature during the sputtering is 50.
Claim 11 characterized in that it is 0 to 920 ℃
A method for producing a superconducting thin film according to item. 14. An MgO single crystal substrate or SrTiO _{3 is used as the} substrate.
The single crystal substrate is used, and its {001} plane or {110} plane is used as a film formation surface.
The method for producing a superconducting thin film according to any one of items 13 to 13. 15. The method for producing a superconducting thin film according to claim 1, wherein the film is formed at a film forming rate in the range of 0.05 to 1 Å / second. 16. The method for producing a superconducting thin film according to claim 1, wherein the film is formed at a film forming rate in the range of 0.1 to 0.8 Å / sec. . 17. The method for producing a superconducting thin film according to any one of claims 1 to 16, wherein the film is formed to a film thickness in the range of 0.1 to 10 μm. 18. The method for producing a superconducting thin film according to claim 1, wherein the superconducting thin film is formed to a film thickness of 0.5 to 2 μm. 19. The superconducting thin film according to claim 1, wherein after the film formation, the thin film is heat-treated by heating and gradually cooling in an oxygen-containing atmosphere. Of manufacturing. 20. The method for producing a superconducting thin film according to claim 19, wherein the heat treatment is carried out at a heating temperature in the range of 800 to 960 ° C. for a time in the range of 0.5 to 20 hours. 21. The method for producing a superconducting thin film according to claim 19, wherein the heat treatment is performed at a heating temperature in the range of 850 to 950 ° C. for a time in the range of 1 to 10 hours. 22. The cooling rate at the time of the heat treatment is 10 ° C./minute or less
21. A method for producing a superconducting thin film according to any one of items 21 to 21. 23. The oxygen partial pressure at the time of the heat treatment is 0.1 to 10 atmospheres.
Item 9. A method for producing a superconducting thin film according to any one of items. 24. The method for producing a superconducting thin film according to any one of claims 1 to 23, wherein the sputtering method is magnetron sputtering.