JP2639510B2 - 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,
More specifically, the present invention relates to a novel method for producing a superconducting thin film having a uniform composition and a high superconducting critical temperature.

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 electrical resistance of a conductor becomes zero under specific conditions, indicating complete diamagnetism.

That is, under the superconductivity, even if a current flows through the superconductor, there is no power loss, and a high-density current continues to flow forever.
Therefore, for example, if superconductivity technology is applied to power transmission,
The power transmission loss of about 7%, which is currently caused by power transmission, can be significantly reduced. In addition, the application as an electromagnet for generating a high magnetic field is, for example, a technology that advantageously promotes the realization of a nuclear fusion reaction that is said to consume more power than the amount of power generated in the development together with MHD power generation, electric motors, etc. Expected. It is also expected to be used as power for magnetic levitation trains, electromagnetic propulsion vessels, etc. in the fields of measurement and medical treatment, such as NMR, pion therapy, and high energy physics experiments.

Apart from the use in the large-sized apparatus as described above, another use of the superconducting material includes production of various superconducting elements. A typical example is an element using the Josephson effect in which a quantum effect macroscopically appears by an applied current when weakly joining superconducting materials. Since the energy gap of the superconducting material is small, the tunnel junction type Josephson device is expected as a switching device with extremely high speed and low power consumption. In addition, since the Josephson effect on electromagnetic waves and magnetic fields appears as an accurate quantum phenomenon, it is expected that the Josephson element is used as an ultrasensitive sensor for magnetic fields, microwaves, radiation, and the like. Furthermore, as the degree of integration of electronic circuits increases, the power consumption per unit area reaches the limit of the cooling capacity. Therefore, the development of superconducting elements has been demanded for ultra-high-speed computers.

On the other hand, despite various efforts, the superconducting critical temperature Tc of the superconducting material could not exceed 23 K of Nb ₃ Ge for a long time, but since late 1986 (La, Ba) ₂ CuO ₄ or A sintered material of K ₂ NiF ₄ type oxide such as [La, Sr] ₂ CuO ₄ has been discovered as a long conducting material having a high Tc, and the possibility of realizing non-low temperature superconductivity has been greatly increased. For these substances, 30
Dramatically higher Tc of ~ 50K than before is observed, and Tc of 70K or more is also observed. However, these superconducting materials are sintered materials, and unreacted particle portions are present microscopically, and the composition and the structure tend to be nonuniform, so that they cannot be directly applied to electronic devices. Also,
In order to apply to various electronic devices, a thin film structure and detailed composition and control of the structure are required. Further, it is expected that a superconducting material is produced by depositing a superconducting material on a metal or other wire or tape-like material, and a technique for depositing a superconducting material is also required for the production.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for producing a thin film of a superconducting material having a high critical temperature Tc and a uniform composition and structure. It is in.

Means for Solving the Problems That is, according to the present invention, La, Ba and physical vapor deposition is performed using a composite oxide containing all elements of Cu as a target, La,
A method for producing a superconducting thin film, characterized by forming a thin film of a perovskite oxide or a pseudo perovskite oxide containing both Ba and Cu, is provided.

Here, according to a preferred embodiment of the present invention, it is advantageous that the target contains a perovskite oxide or a pseudo perovskite oxide.

A powder mixture of oxides, carbonates, nitrates or sulfates of Ba; oxides, carbonates, nitrates or sulfates of La; and oxides, carbonates, nitrates or sulfates of Cu It is also advantageous that the target is pre-sintered at a temperature of 250 to 1200 ° C., and the target is an oxide of Ba, an oxide of carbonate, nitrate or sulfate; an oxide of La,
Carbonate, nitrate or sulfate: After pre-sintering a powder mixture of each oxide, carbonate, nitrate or sulfate of Cu, the mixture is heated at a temperature in the range of 700 to 1500 ° C, preferably 700 to 1300 ° C. Advantageously, it is sintered.

The atomic ratio Ba / (Ba + La) in the target is preferably in the range of 0.04 to 0.96, more preferably 0.1 to 0.7. Also, the atomic ratio Cu / (Ba + La) in the target
Is preferably in the range of 0.5 to 0.7. The atomic ratio of Ba, La and Cu of the target is determined by the Ba, La
Advantageously, it is adjusted according to the vapor deposition efficiency of Ba, La and Cu, based on the atomic ratio of La and Cu.

The deposition atmosphere contains Ar and O _2, and the partial pressure of Ar is in the range of 2.0 × 10 ⁻³ to 9 × 10 ⁻¹ Torr, preferably 6.0 × 10 ⁻³ to 9 × 1 Torr.
Advantageously, it is in the range of 0 ^-2 Torr. Further, it is advantageous that the partial pressure of O _{2 in the} above vapor deposition atmosphere is in the range of 0.7 × 10 ⁻³ to 9 × 10 ⁻² Torr, preferably in the range of 1.0 × ⁻³ to 8 × 10 ⁻² Torr.

The physical vapor deposition may be RF sputtering, the high-frequency power during sputtering is 100 W / cm ² or less, preferably 15 W / cm
Advantageously it is less than cm ² . Further, the vapor deposition can be performed by magnetron sputtering.

During sputtering, it is advantageous to heat the substrate with a heater. Here, the heating temperature of the substrate is 185
11480 ° C., preferably 285-1000 ° C., and the distance between the substrate and the target is 3 to 300 n
m, preferably in the range from 15 to 300 mm.

The substrate may be one selected from the group consisting of glass, quartz, Si, stainless steel, and ceramics.

Further, the physical vapor deposition may be any one of ion plating, vacuum vapor deposition, ion beam vapor deposition, and molecular beam vapor deposition.

Further, according to one embodiment of the present invention, it is advantageous to heat-treat the formed thin film.

Here, the heating temperature in the heat treatment is 280 to 1480
Advantageously, it is in the range of from 285 to 1200 ° C.

The heat treatment is advantageously performed in an atmosphere having an oxygen partial pressure of 5 to 10 ^-1 Torr or less.

Action The method for producing a superconducting thin film provided by the present invention is characterized in that La and Ba
And physical vapor deposition using a composite oxide containing Cu and Cu as a target to form a thin film of a perovskite oxide or a pseudo-provovskite oxide.

Here, the physical vapor deposition can be any of various sputtering methods, ion plating methods, vacuum vapor deposition methods, ion beam vapor deposition methods, and molecular beam vapor deposition methods, and the details will be described later. In the method of the present invention, since a sintered material is used as a target, it does not scatter even when irradiated with an electron beam or the like, and is advantageous for sputtering or ion plating while adjusting components.

In the method of the present invention, it is advantageous that the target itself also has a perovskite-type or pseudo-perovskite-type structure, and in this case, the target thin film easily adopts a desired perovskite or pseudo-perovskite structure.

According to a preferred embodiment of the present invention, Ba, La, and Cu mixed powder of each oxide, carbonate, nitrate or sulfate is calcined at 250 to 1200 ° C, more preferably 250 to 1100 ° C. It is advantageous to use, as a target, a sintered product or a product obtained by sintering the product at a temperature in the range of 700 to 1500 ° C., more preferably 700 to 1300 ° C. The sintered body obtained through such a process easily has the above-mentioned crystal structure, and particularly when the sintering is performed in the above-described preferable range, this crystal structure is easily formed. Here, the term “temporary sintering” refers to a process of temporarily firing a powder material to form a composite oxide.

The target may be only temporarily sintered or fully sintered. Further, a powder obtained by pulverizing the sintered body or a sintered body block may be used. In the case of a sintered body powder, the evaporation efficiency is good and a high film formation rate can be obtained, so the particle size is 0.06 to 3 m
It is advantageous to use powders in the range of m.

In the target used in the method of the present invention, the atomic ratio Ba / (Ba + La) is preferably in the range of 0.04 to 0.96, and more preferably 0.1 to 0.4. Further, the atomic ratio Cu / (Ba + La) in the target is preferably in the range of 0.5 to 0.7.

When the atomic ratio Ba / (Ba + La) of the target is less than 0.04 or exceeds 0.96, the superconducting critical temperature of the deposited film does not become a desired value. In addition, considering the vapor deposition efficiency of Ba, La and Cu, the atomic ratio of Ba, La and Cu of the target
It is advantageous to determine based on the atomic ratio of Ba, La and Cu. This is because the constituents of the thin film of the invention, Ba and La
This is because the melting points and the like of the oxides of Cu and Cu are different from each other, and thus the vapor deposition efficiencies are different from each other. That is,
Unless the element ratio of the target is properly selected, the thin film will not have the desired element ratio. In the case of sputtering, the atomic ratio of the target can also be determined from the sputtering coefficient of each metal oxide.

According to still another preferred embodiment of the present invention, when the method of the present invention is performed by sputtering, the substrate is heated to 285 to 1,480 ° C. by a heater or the like, and more preferably to 285 to 1480 ° C.
It is advantageous to heat to a temperature in the range of 000 ° C. When the substrate is heated, the thin film undergoes a function similar to sintering, and turns into a suitable provskite-type oxide or pseudo-provovskite-type oxide. However, if the substrate temperature is too high, it becomes difficult to control the composition of the deposited film, and the desired perovskite-type oxide or pseudo-perovskite-type oxide cannot be obtained.

The above physical vapor deposition can be performed by, for example, sputtering. Here, the sputtering atmosphere is Ar
O ₂ and the partial pressure of Ar is 2.0 × 10 ^{−3 to} 9 × 10 ⁻¹ Torr, more preferably 6.0 × 10 ^{−3 to} 9 × 10 ⁻² Torr, and the O ₂ partial pressure is
0.7 × 10 ^{-3 to} 9 × 10 ^-2 Torr, more preferably 1.0 to 10 ^-3
Advantageously, ８8 × 10 ⁻² Torr. That is, when the Ar partial pressure does not reach the above range, the deposition rate is low, whereas when the Ar partial pressure exceeds the above range, glow discharge is unlikely to occur, and oxide having desired superconducting properties cannot be deposited. When the O ₂ partial pressure does not reach the above range, the crystallinity of the deposited film is poor, and it is difficult to obtain a perovskite oxide or a pseudo perovskite oxide. On the other hand, the higher the O ₂ partial pressure, the better the crystallinity. However, if the O ₂ partial pressure is exceeded, the deposition rate is significantly reduced.

Further, the method of the present invention can be carried out by high frequency sputtering (RF sputtering).

In this case, the high frequency power applied during sputtering is
It is 100 W / cm ² or less, more preferably 15 W / cm ² or less. R
In the case of F sputtering, the deposition rate increases as the high-frequency power increases, but if it exceeds 15 W / cm ² , the plasma is not stable, and if it exceeds 100 W / cm ² , arc discharge or abnormal discharge tends to occur. Therefore, a high frequency power within the above range is used.

Further, the distance between the substrate and the target is advantageously in the range of 3 to 300 mm, more preferably 15 to 300 mm. That is, when the distance between the substrate and the target is small, it is difficult to generate plasma, so that a certain distance or more is required. In particular, in the case of high-frequency magnetron sputtering, the plasma on the target is converged near the magnet, and if the distance between the substrate and the target is too small, the film formation becomes uneven. On the other hand, when the distance between the substrate and the target is large, the deposition rate increases and the efficiency decreases.

Examples of the substrate that can be advantageously used in the method of the present invention include glass, quartz, Si, stainless steel, and ceramics such as MgO, BaTiO ₃ , SiO ₂ , sapphire, and YSZ.

Further, according to a preferred embodiment of the present invention, the thin film formed by the physical vapor deposition is further heat-treated to produce a high-quality thin film having a small difference between a superconducting start temperature and a temperature at which the resistance becomes completely zero. can do. That is, by this heat treatment, the superconducting characteristics of the deposited film can be improved by controlling the oxygen atom ratio of the deposited film. As described above, the purpose of the heat treatment is to homogenize the composition of the deposited film into a perovskite oxide or a pseudo perovskite oxide. For this reason, at a heating temperature that does not reach the following temperature range, it is difficult to obtain a target perovskite-type oxide or pseudo-perovskite-type oxide, and a desired superconducting critical temperature cannot be obtained, or an extremely long processing time is required. And
On the other hand, if the processing temperature exceeds the following range, the target perovskite oxide or pseudo perovskite oxide disappears, and the superconducting critical temperature is remarkably lowered. The preferred temperature range of this heat treatment is 280 to 1480 ° C, more preferably 280 to 1100 ° C.

As described above, the thin film produced according to the method of the present invention generally has a composition represented by the formula: Ba _A La _B Cu _C O _D (A, B, C, and D each represent an atomic ratio). If so, it is considered to contain any of the following compositions or a mixture thereof.

(Ba, La) ₂ CuO _4-X (x is a number of 1 or less), (Ba, La) CuO _3-X (x is a number of 1 or less), (Ba, La) ₃ Cu ₂ O _7-y ( (y is a number of 1 or less) Further, it is considered that the composition further contains any of the following compositions or a mixed phase thereof, but at present, the composition is not limited to this.

Hereinafter, the present invention will be described with reference to Examples, but it is needless to say that the technical scope of the present invention is not limited to these Examples.

First, an apparatus used to carry out the method of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of a high-frequency sputtering apparatus used for producing a superconducting oxide thin film of the present invention. The apparatus shown in FIG. 1 is provided in opposition to a raw material target 2, a chamber 1, a target 2 disposed in the chamber 1, a sputter power source 3 juxtaposed to sputter this target with sputter atoms. And a substrate 4 on which a thin film is to be formed on the surface.
The chamber 1 is connected to a vacuum pump (not shown) via a port 7 so that the inside can be evacuated.

A bias voltage is applied to the substrate 4 using a high-voltage power supply 5. The substrate 4 is further provided with a heater 6 for heating so that the substrate temperature can be adjusted. Further, chamber 1
Is provided with a gas introduction hole 8.

FIG. 2 is a schematic view of a high-frequency excitation type ion plating apparatus used for producing a superconducting oxide thin film of the present invention.

The apparatus shown in FIG. 2 includes a chamber 11, a raw material target 12 disposed in the chamber 11, an electron gun 13 juxtaposed to melt and vaporize the raw material target with an electron beam, and a raw material target 12. Provided
It mainly comprises a substrate 14 on which a thin film is formed on the surface. The chamber 11 is connected to a vacuum pump (not shown) so that the inside can be evacuated.

A bias voltage is applied to the substrate 14 using a high-voltage power supply 15. The substrate 14 is further provided with a heating heater 16 so that the substrate temperature can be adjusted. In the chamber 11, a high-frequency coil 17 is further provided between the raw material target 12 and the substrate 14 so as to cover the evaporated particles. Further, the chamber 11 is provided with a gas introduction hole 18 for introducing oxygen.

Production Example 1 A superconducting thin film was produced using the sputtering apparatus shown in FIG.

Raw material target 2 includes BaCO ₃ , La ₂ (CO ₃ ) ₃ , C
The uO powder is mixed and molded so that the atomic ratio Ba / (Ba + La) becomes 0.2, pre-sintered at 810 ° C., pulverized, molded, and further molded.
A sintered block obtained by main sintering at 0 ° C. was used, and a Si crystal was used for the substrate 4. The film forming conditions are as follows.

Oxygen partial pressure 1 × 10 ^-2 Torr, Ar partial pressure 1 × 10 ^-2 Torr, Substrate temperature 690 ° C, Substrate bias voltage -150V, High frequency power 20W / cm ² , Distance between substrate and target 50mm Deposition rate 5Å The film was formed to a thickness of about 1 μm at / sec.
For comparison, other conditions were the same, and the same operation was performed in an atmosphere containing no O ₂ to form a thin film.

Next, a sample was prepared to measure the resistance of each of the obtained thin films. The sample for resistance measurement is the substrate 4
A pair of Al electrodes was further formed on both ends of the thin film formed thereon by vacuum evaporation, and lead wires were soldered to the Al electrodes.

The thin film prepared by the method of the present invention prepared at a partial pressure of oxygen of 1 × 10 ⁻² Torr inside the chamber has a temperature at which the superconducting phenomenon starts at about 60K, and becomes a complete superconductor below 51K. In contrast, thin films made without oxygen in the chamber
At about the same temperature, the resistance began to decrease, but the decrease was gentle, and the superconductor became a complete superconductor for the first time at around several K. This indicates that the oxygen in the thin film can be appropriately controlled by introducing oxygen into the chamber during film formation, and a superconducting thin film having a desired composition can be formed.

Production Example 2 The sintered body of Production Example 1 was pulverized to a viscosity of 0.2 mm, and this was used as a raw material target, and sputtered under the same conditions as in Production Example 1 to produce a superconducting thin film.

The film formation rate was 9 D / sec, and it was confirmed that a significantly higher film formation rate was obtained as compared with Production Example 1. In addition, the temperature at which the superconductivity phenomenon of the thin film produced using the sintered compact powder target of this production example started was about 55K, and a complete superconductor was obtained below 49K. As described above, the use of the sintered powder target enabled efficient film formation, and the obtained thin film exhibited excellent characteristics as a superconductor.

Production Example 3 The same operation as in Production Example 1 was repeated, but in this Production Example,
The raw material powders BaCO ₃ , La ₂ (CO ₃ ) ₃ and CuO are mixed so that the atomic ratio of Ba: La: Cu becomes 0.7: 0.85: 1, and the preliminary sintering is performed.
A temporary sintered material obtained at 400 ° C. for 12 hours was used as a target.

The superconducting onset temperature of the obtained thin film was 58K. From this experiment, it was confirmed that a superconducting thin film could be produced even if only the temporary sintering was used as the target.

Production Example 4 The thin film sample obtained in Production Example 1 was heat-treated at 610 ° C. for 20 minutes in an oxygen atmosphere (oxygen partial pressure: 1.3 × 10 ⁻¹ Torr).

When the sample after the heat treatment was measured by the same method as in the previous production example, the resistance decreased more steeply than in the case of production example 1, and the difference between the superconducting start temperature and the zero temperature of complete resistance was 1 ° C. became.

Production Example 5 The same operation as in Production Example 1 was repeated, but the main sintering temperature was 93.
The temperature was changed to 0 ° C., the high frequency power was changed to 1.8 W / cm ² , and the substrate was changed to magnesia.

The temperature at which the superconducting phenomenon started measured by the same measurement method as in Preparation Example 1 was 62K, and at 35K or less, a complete superconductor was obtained.

Effect of the Invention As described above, according to the present invention, it is possible to obtain a superconducting oxide thin film having a much higher Tc than a conventional superconductor.

Therefore, the superconducting thin film obtained by the present invention is used in a field applied as a thin film element, for example, a switching element of Matisoo called a Josephson element, a memory element of Anacker, and a superconducting quantum interferometer (SQUID). Can be used advantageously.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of an example of a sputtering apparatus that can be used to carry out the method of the present invention, and FIG. 2 is a diagram of an ion plating apparatus that can be used to carry out the method of the present invention. It is an outline sectional view of an example. [Main reference numbers] 1... Chamber 2... Raw material target 3... High-frequency power supply 4... Substrate 5... High-voltage power supply 6... Heater 7. , 11 ... chamber, 12 ... raw material target, 13 ... electron gun, 14 ... substrate, 15 ... high voltage power supply, 16 ... heater, 17 ... high frequency coil, 18 ... gas introduction hole

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Ito ▲ Saki ▼ Hideo 1-1-1, Koyokita, Itami-shi, Hyogo Sumitomo Electric Industries, Ltd. Itami Works (72) Inventor Saburo Tanaka, Itami-shi, Hyogo 1-1-1, Kita Sumitomo Electric Industries, Ltd. Itami Works (72) Inventor Keizo Harada 1-1-1, Koyokita, Itami-shi, Hyogo Prefecture Sumitomo Electric Industries, Ltd. Itami Works (72) Inventor Chief Tetsuji 1-1-1, Kunyokita, Itami-shi, Hyogo Sumitomo Electric Industries, Ltd. Itami Works (56) References JP-A-63-207009 (JP, A) JP-A-63-224116 (JP, A) Kaisho 63-224375 (JP, A)

Claims (1)

(57) [Claims] 1. Physical vapor deposition at a substrate temperature of 285 to 1480 ° C. under a partial pressure of oxygen of 0.7 × 10 ^{−3 to} 9 × 10 ⁻² Torr using a composite oxide containing all elements of La, Ba and Cu as a target. And forming a thin film of a perovskite-type oxide or pseudo-perovskite-type oxide containing all of La, Ba and Cu.