JPH0829938B2 - Composite oxide superconducting thin film and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconducting thin film and a film forming method thereof, and more specifically, to a complex oxide superconducting thin film having a significantly improved critical current density, and a method thereof. The present invention relates to a manufacturing method. The superconducting thin film obtained by the present invention has a high critical current, a high superconducting critical temperature, and other excellent characteristics such as smoothness, and is suitable for various electronic devices such as integrated circuits. It is especially useful as a wiring material for parts.

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 device 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, in the field of ultra-high speed computers, etc., where the 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 low loss wiring materials.

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 Tc 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 superconductor having a critical temperature in the order of 90K. It is said that oxygen defects in the crystal play a major role in the superconducting properties of these complex oxide superconductors, and Tc is low unless proper oxygen defects are formed in the crystal. And the temperature at which the resistance is completely zero becomes large.

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

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 formula: Ln ₁ Ba ₂ Cu ₃ O _7-x (where Ln is Tm
And / or Lu, and x is a number satisfying 0 ≦ x <1), and a complex oxide superconductor thin film mainly containing a complex oxide represented by Surface roughness R _max (standard length 10
A composite oxide superconducting thin film is provided, characterized in that 00 μm) is less than 0.06 μm.

Further, 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 these Atomic ratio compositions that deviate from the ratio by ± 50%, more preferably ± 20%, can be included in the scope of the present invention. That is, in the claims, the expression "mainly contains the complex oxide represented by the above formula" means that the superconducting thin film produced by the method of the present invention is Ln: Ba: Cu defined by the above formula. It is meant to include those with atomic ratios other than 1: 2: 3.

Furthermore, the above definition is added for the purpose of improving the elements other than the above Ln, Ba, Cu and O, that is, unavoidable impurities mixed in the ppm order and other properties of the obtained sintered body or thin film. It means that the third component may be contained.

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
Examples include elements selected from the group, such as Ti and V.

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. Preferred substrate materials include 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.

The main feature of the present invention is that a substantial part of the surface of the superconducting thin film is smooth. In this case, "substantial part" generally means that most of the surface excluding partial voids and defects that cannot be avoided when physical vapor deposition is applied to a large area, for example, 80% or more of the surface is smooth. It means that there is.

The smoothness of the surface of the above-mentioned superconducting thin film is the roughness of the surface of the thin film R _max (reference length = 1,
000 μm) is less than 0.06 μm. This value can be confirmed by observing the obtained thin film with a microscope, particularly with SEM. According to the experimental results of the present invention,
When the roughness of the surface of the thin film is R _max of 0.06 μm or more, the critical current density Jc is significantly reduced.

Another object of the present invention is to provide a method for producing a complex oxide superconducting thin film having the above-mentioned smooth surface.

Further, as a method for producing the above-described composite oxide superconducting thin film according to the present invention, according to the present invention, the formula: Ln ₁ Ba ₂ Cu ₃ O _7-x (where Ln represents Tm and / or Lu, and x is 0 ≦ x <1
A method for producing a composite oxide superconductor thin film mainly containing a composite oxide represented by the formula (1), the oxidation having a lattice spacing close to that of the crystal of the complex oxide. 0.064-1.2 using a single crystal substrate
While applying a high frequency power of 7 W / cm ² , the film is formed by the RF sputtering method at a film formation rate of 0.05 to 1 Å / sec. There is provided a method for producing a composite oxide superconducting thin film, characterized in that the roughness R _max (reference length 1000 μm) is less than 0.06 μm.

As the physical vapor deposition, sputtering, ion plating, vacuum vapor deposition and the like can be used, but sputtering is generally preferable, and RF magnetron sputtering is particularly preferable.

The substrate is preferably heated during the physical vapor deposition, and the substrate temperature is 200 to 950 ° C., more preferably 500 to 920.
℃. If the substrate temperature is lower than 200 ° C., the crystallinity of the composite oxide is poor and the composite oxide becomes amorphous, so that a superconducting thin film cannot be obtained. Moreover, when the substrate temperature exceeds 950 ° C., the crystal structure changes, and the above complex oxide does not become a superconductor.

As the substrate, an oxide single crystal substrate having a lattice spacing close to that of the complex oxide crystal is preferably used, and for example, MgO single crystal, SrTiO ₃ single crystal or ZrO ₂ single crystal can be used. The {001} plane or {110} plane of the MgO single crystal or SrTiO ₃ single crystal substrate is preferably used as the film formation surface.

In a preferred embodiment of the present invention, the film formation rate during the above physical vapor deposition is 0.05 to 1Å / sec, and more preferably 0.1 to 1.
It is set in the range of 0.8Å / sec. According to the experimental results of the present inventors, when the deposition rate during physical vapor deposition exceeds 1Å / sec,
The critical current density of the obtained superconducting thin film is drastically reduced, and a practical thin film cannot be obtained. Further, if the film forming rate is less than 0.05 Å / sec, the film forming rate becomes extremely slow, which is not industrial.

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,
The proportion of oxygen in this mixed gas is 5 to 95%, more preferably 10 to 80%.

A sputtering method can be used as the physical vapor deposition, in which case the gas pressure during sputtering is 0.00
The range of 1 to 0.5 Torr is more preferable, and the range of 0.01 to 0.3 Torr is more preferable, and it is preferable to perform the sputtering in an atmosphere containing 10 to 80 molecule% of O ₂ in the sputtering gas. As the other sputtering gas that can be used together with the gas other than O ₂ , an inert gas, particularly argon is preferable.

In a preferred embodiment of the invention, the sputtering method used is RF sputtering, in particular magnetron sputtering. RF preferably used in the present invention
In the case of magnetron sputtering, for example, 10 cmφ
For the target of the above, the high frequency power at the time of sputtering from about 1.9 W / cm ² to 5 to 100 W, that is, 0.064 to 1.27 W / cm ² , more preferably 10 to 60 per unit cross-sectional area.
It is preferable to apply W, that is, 0.127 to 0.76 W / cm ² per unit cross-sectional area.

After film formation, the obtained thin film is heated in an oxygen-containing atmosphere.
It is preferable to perform annealing heat treatment for slow cooling. The heat treatment is preferably performed at a heating temperature in the range of 800 to 960 ° C., and the cooling rate during the heat treatment is preferably 10 ° C./minute or less. During this heat treatment, the oxygen partial pressure is preferably 0.1 to 10 atm.

In the past, when a thin film of a complex oxide superconductor was produced, physical vapor deposition, generally sputtering was performed using a target mainly composed of a complex oxide sintered body of the same system. The superconducting thin film obtained in step 1 had a low critical current density Jc and was not practical.

The above-mentioned type of complex oxide superconductor has a crystal anisotropy in its critical current density. That is, current easily flows in a direction parallel to a plane determined by the a-axis and the b-axis of the crystal.
Therefore, so far, for the purpose of aligning the crystal directions, as a substrate, a specific surface of a single crystal such as MgO, SrTiO ₃ and YSZ having a lattice spacing close to the lattice spacing of the complex oxide superconductor crystal is used as a film-forming surface. Was used. However, in the complex oxide superconducting thin films that have been produced so far, the critical current density Jc can only be increased to about 100,000 A / cm ^{2 at the} maximum because the crystal directions could not be sufficiently aligned. It was

The present invention improves the critical current density Jc by two orders of magnitude by improving the surface smoothness of the composite oxide superconducting thin film.
It has been improved to the order of 0,000 A / cm ² .

The reason why the critical current density Jc is significantly improved in this way by improving the surface smoothness of the composite oxide superconducting thin film cannot be explained at present.However, the composite oxide superconductor of the present invention has a crystalline electric resistance. The complex oxide superconducting thin film having anisotropy formed on the film forming surface of the substrate has a c-axis of its crystal which is perpendicular or nearly perpendicular to the film forming surface of the substrate, and particularly has a critical current density Jc. It is expected to grow. Therefore, it is preferable to use the {001} plane of the MgO single crystal substrate or the SrTiO 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. In addition, Mg
O and SrTiO ₃ have a coefficient of thermal expansion close to that of the above-mentioned composite oxide superconductor, so that unnecessary stress is not applied to the thin film during the heating and cooling processes, and there is no risk of damaging 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 Hereinafter, a method for producing a complex oxide superconducting thin film having a smooth surface according to the present invention will be described with reference to Examples, but the technical scope of the present invention is not limited to the following disclosure. is there.

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

The target used was an Ln-Ba-Cu-based composite oxide prepared by sintering a raw material powder having an atomic ratio Ln: Ba: Cu of Tm or Lu and Ba and Cu of 1: 2.24: 4.35 according to a conventional method. It is a ceramic. A disk with a diameter of 100 mmφ was used as the target.
The film forming conditions in each case were the same, and the film forming conditions were as follows.

Substrate MgO (001) surface Substrate temperature 700 ℃ Pressure 0.1Torr Sputtering gas O ₂ (20%) / Ar (80%) High frequency power 40W (0.51W / cm ² ) Time 6 hours Film thickness 0.88μm Film deposition rate 0.35Å / Second annealing 900 ° C / 3 hours (cooling at 5 ° C / min) For comparison, the complex oxide was prepared under the same conditions as above, except that the same target was used and the film formation rate was set to 1.5Å / sec. First result of superconducting thin film
It is shown in the table as a comparative example.

The critical temperature Tc in Table 1 was measured by the four probe method according to the standard rod. Also, the critical current density Jc is 77.0K, the current amount when the electric resistance is detected by increasing the amount of current while measuring the electric resistance in the direction parallel to the surface of the sample, the unit area of the current path. The converted value is shown. The surface roughness R _max of the film formation was calculated from SEM (scanning electron microscope) photographs.

As described above, the superconducting thin film according to the present invention has a significantly improved critical current as compared with the comparative example.

Although a few micro-order voids were observed on the surface of the thin film formed by the method of the present invention (about 1% of the total surface area), the surface was enlarged 10,000 times by SEM. When observed, no ruggedness was observed at 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 range of the method of the present invention, many grains of several microns were present.

EFFECTS OF THE INVENTION As described in detail above, the superconducting thin film according to the present invention exhibits a much higher Jc than that produced by the conventional method.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H01L 39/12 ZAA C 39/24 ZAA B // C30B 29/22 ZAA Z 9261-4G (72) Inventor Shuji Yazu 1-1-1 Kunyokita, Itami City, Hyogo Prefecture Sumitomo Electric Industries, Ltd. Itami Works (72) Inventor Tetsuji Kamishiro 1-1-1 Kunyokita, Itami City, Hyogo Prefecture Itami Co., Ltd. In-house (56) References Jpn. J. Appl. Phys. 26 (9) (1987) p. L1484-6 Jpn. J. Appl. Phys. 26 (9) (1987) p. L1489-91 Jpn. J. Appl. Phys. 26 (11) (1987) p. L1837-8

Claims (2)

[Claims] 1. A formula: Ln ₁ Ba ₂ Cu ₃ O _7-x (where Ln represents Tm and / or Lu, and x is 0 ≦ x
<A number satisfying 1> In a composite oxide superconductor thin film mainly containing a composite oxide, the surface roughness R _max (reference length) of a substantial part of the surface of the above composite oxide superconductor thin film 1000 μm) is less than 0.06 μm, a composite oxide superconducting thin film. 2. A formula: Ln ₁ Ba ₂ Cu ₃ O _7-x (where Ln represents Tm and / or Lu, and x is 0 ≦ x
A method for producing a composite oxide superconducting thin film mainly containing a composite oxide represented by <1> by physical vapor deposition, wherein the lattice spacing is close to the lattice spacing of the crystals of the composite oxide. Using the oxide single crystal substrate, the complex oxide superconductor obtained by applying a high frequency power of 0.064 to 1.27 W / cm ² and forming a film by the RF sputtering method at a film forming rate of 0.05 to 1 Å / sec. A method for producing a composite oxide superconducting thin film, characterized in that the surface roughness R _max (reference length 1000 μm) of a substantial part of the surface of the thin film is less than 0.06 μm.