JP2525852B2 - 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 electric resistance of a conductor becomes zero under certain conditions and shows perfect diamagnetism.

That is, under superconductivity, there is no power loss even when a current is passed through the superconductor, and a high-density current continues to flow forever.
Therefore, for example, if superconductivity technology is applied to power transmission,
It is possible to greatly reduce the transmission loss of about 7%, which is said to be caused by the current transmission. 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 utilizing the Josephson effect in which quantum effects appear macroscopically by an applied current when superconducting materials are weakly joined. 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 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 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 materials of K ₂ NiF ₄ type 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 greatly increasing. 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, it is necessary to have a thin film structure and control the fine composition and structure. 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 Problem That is, according to the present invention, a target of a mixture of oxides and / or complex oxides containing at least one selected from the group consisting of Y, Ba and Cu or all contained in the element group By using the target of the complex oxide containing the element of the above, physical vapor deposition is performed in an atmosphere in which the oxygen partial pressure is in the range of 0.7 × 10 ^-3 Torr to 8 × 10 ^-2 Torr. There is provided a method for producing a superconducting thin film, which comprises forming an oxide thin film.

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.

The target is a powder mixture of Ba oxide, carbonate, nitrate or sulfate; Y oxide, carbonate, nitrate or sulfate; Cu oxide, carbonate, nitrate or sulfate. It is also advantageous that the calcination is carried out at a temperature of 250 to 1200 ° C. Further, the target is an oxide, carbonate, nitrate or sulfate of Ba; Y oxide, carbonate, nitrate. Or sulphate; and 700 after sintering a powder mixture of Cu oxides, carbonates, nitrates or sulphates
Advantageously, the main sintering is carried out at a temperature in the range of -1500 ° C, preferably 700-1500 ° C.

The target can also use multiple targets, in which case each target is a Ba oxide,
It may be an oxide of Y or an oxide of Cu. Also, (Ba, Y) O _x
(However, x is a real number of 1 or more) and CuO may be used.

The atomic ratio Ba / (Ba + Y) in the target is preferably in the range of 0.05 to 0.95, and more preferably 0.2 to 0.6. Also, the atomic ratio Cu / (Ba + Y) in the target
Is preferably in the range of 0.5 to 0.7. The atomic ratio of Ba, Y and Cu of the target is B of the thin film to be formed.
Ba, Y and Cu based on the atomic ratio of a, Y and Cu
Is adjusted according to the vapor deposition efficiency of

The vapor deposition atmosphere contains Ar and O ₂ , and the Ar partial pressure is in the range of 1.0 × 10 ^-3 to 1 × 10 ^-1 Torr, preferably 5.0 × 10 ^-3 to 1 × 1.
It is advantageous to be in the range of 0 ^-1 Torr. Further, it is advantageous that the partial pressure of O _{2 in the} vapor deposition atmosphere is in the range of 0.7 × 10 ⁻³ to 8 × 10 ⁻² Torr, preferably in the range of 1.0 × 10 ⁻³ to 1 × 10 ⁻¹ Torr. .

The physical vapor deposition may be RF sputtering, the high frequency power during sputtering is 95 W / cm ² or less, preferably 15 W / c.
It is advantageously less than or equal to m 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
~ 1450 ℃, preferably in the range of 184 ~ 1000 ℃ is advantageous, the distance between the substrate and the target is 4 ~ 230m
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 250 to 1700.
Advantageously, it is in the range of 250 ° C, more preferably 250-1200 ° C.

Further, it is advantageous to perform the heat treatment in an atmosphere having an oxygen partial pressure of 5 × 10 ^-1 Torr or less.

Action The method for producing the superconducting thin film provided by the present invention is:
And a complex oxide containing Cu as a target are subjected to physical vapor deposition to form a thin film of a perovskite type oxide or a pseudo perovskite type 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 the sintered material is used as a target, it does not scatter even when irradiated with an electron beam or the like, which is advantageous for sputtering or ion plating while adjusting the components.

Further, in the method of the present invention, it is advantageous that the target itself also has a perovskite type structure or a pseudo perovskite type structure. 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, a mixed powder of Ba, Y, and Cu oxides, carbonates, nitrates or sulfates is temporarily stored at 250 to 1200 ° C, more preferably 250 to 1100 ° C. It is advantageous to use, as a target, a sintered product, or a main sintered product thereof 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 pre-sintered or may be main-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.

Further, in consideration of composition control during film formation, it is also advantageous to perform physical vapor deposition using a plurality of targets.
For example, if three targets are used, it is possible to exemplify that Ba, Y, and Cu oxides are simultaneously used as the targets. Further, it is also possible to use two targets of (Ba, Y) O _x (where x is a real number of 1 or more) and CuO.

In the target used in the method of the present invention, the atomic ratio Ba / (Ba + Y) is preferably in the range of 0.05 to 0.95, and more preferably 0.2 to 0.6. The atomic ratio Cu / (Ba + Y) is preferably within the range of 0.5 to 0.7.

When the atomic ratio Ba / (Ba + Y) of the target is less than 0.04 or exceeds 0.97, the superconducting critical temperature of the deposited film does not reach the desired value. Also, considering the deposition efficiency of Ba, Y and Cu, the atomic ratio of Ba, Y and Cu of the target should be
It is advantageous to determine based on the atomic ratio of Ba, Y and Cu. This is Ba, Y which is a constituent of the thin film of the invention.
This is because the melting points and the like of the Cu oxide and Cu oxide 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 another preferred embodiment of the present invention, when the method of the present invention is carried out by sputtering, the substrate is heated to 184-1450 by a heater or the like, more preferably 184-975.
It is advantageous to heat to a temperature in the range of ° C. The heating of the substrate causes the thin film to act in a manner similar to that of sintering, and becomes an appropriate perovskite type oxide or pseudo perovskite 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 an Ar partial pressure of 1.0 × 10 ^-3 to 1 × 10 ^-1 Torr, more preferably 5.0 × 10 ^-3 to 1 × 10 ^-1 Torr, and an O ₂ partial pressure of
0.7 × 10 ^-3 to 8 × 10 ^-2 Torr, more preferably 1.0 × 10 ^-3
Advantageously, ８8 × 10 ⁻² Torr. That is, when the Ar partial pressure does not reach the above range, the deposition rate is slow. On the other hand, when the Ar partial pressure exceeds the above range, glow discharge is difficult to occur, and the deposition of oxide having desired superconducting properties cannot be obtained. Further, if the O ₂ partial pressure does not reach the above range, the crystallinity of the deposited film is poor, it is difficult to obtain a perovskite type oxide or a pseudo perovskite type oxide, and the higher the O ₂ partial pressure, the better the crystallinity. However, if it exceeds the above range, the deposition rate remarkably decreases.

The method of the present invention can also be carried out by radio frequency sputtering (RF sputtering).

In this case, the high frequency power applied during sputtering is
It is 95 W / cm ² or less, more preferably 15 W / cm ² or less. RF
In the case of sputtering, the higher the high frequency power is, the higher the deposition rate is. However, if it exceeds 15 W / cm ² , the plasma is not stable, and if it exceeds 95 W / cm ² , arc discharge or abnormal discharge tends to occur. Therefore, 95W / cm ² or less, more preferably 15W
It was decided to use high-frequency power in the range of / cm ² or less.

Further, it is advantageous that the distance between the substrate and the target is in the range of 4-230 mm, more preferably 15-230 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. Particularly, in the case of high frequency magnetron sputtering, the plasma on the target is converged near the magnet, so that if the distance between the substrate and the target is too small, the film formation becomes non-uniform. On the other hand, when the distance between the substrate and the target is large, the deposition rate is low and the efficiency is low.

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.

Furthermore, according to a preferred embodiment of the present invention, by further heat-treating the thin film formed by physical vapor deposition, a high quality thin film with a small difference between the superconducting start temperature and the temperature at which the resistance becomes completely zero is obtained. Can be made. 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 type oxide or a pseudo perovskite type 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, at a treatment temperature exceeding the following range, the target perovskite type oxide or pseudo perovskite type oxide disappears, and the superconducting critical temperature is significantly lowered. The preferable temperature range of this heat treatment is 270 to 1450 ° C, more preferably 270 to 1090 ° C.

As described above, the thin film prepared according to the method of the present invention generally has a composition represented by the formula: Ba _A Y _B Cu _C O _D (A, B, C and D each represent an atomic ratio). In particular, (Ba, Y) ₂ Cu O _4-x (x is a number of 1 or less), (Ba, Y) Cu O _3-x (x is a number of 1 or less), (Ba, Y) _It is considered to include any of the composite oxides having a composition of ₃ Cu ₂ O _7-y (y is a number of 1 or less) or a mixed phase thereof.

Further, it may be possible to further include any one of the following compositions or a mixed phase thereof, but it is not limited to these at present.

Ba _0.6 Y _0.4 Cu O ₃ Ba Y _0.3 Cu _0.7 O _{3 The} present invention will be described below with reference to examples, but 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. Furthermore, 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 the 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.

As the raw material target 2, BaCO ₃ , Y ₂ (CO ₃ ) ₃ , Cu
The powder of O is adjusted so that the atomic ratio Ba / (Ba + Y) becomes 0.4.
In addition, after mixing so that the atomic ratio Cu / (Ba + Y) becomes 0.6, it is molded, calcinated at 820 ° C, pulverized, and molded for further 1080
The sintered body block obtained by main sintering at ℃ was used for the substrate 4.
Si crystal was used. The film forming conditions are as follows.

Oxygen partial pressure 4 × 10 ^-2 Torr, Ar partial pressure 3 × 10 ^-2 Torr, substrate temperature 700 ° C, substrate bias voltage -60V, high frequency power 25W / cm ² , distance between substrate and target 40mm, deposition rate 3Å A film having a thickness of about 1 μm was formed at a time of / sec.
For comparison, the 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 produced by the method of the present invention in which the oxygen partial pressure inside the chamber is 4 × 10 ^-2 Torr has a temperature at which the superconducting phenomenon starts is about 98K, and becomes a perfect superconductor at 94K or less. In contrast, thin films made without oxygen in the chamber
The resistance started to decrease at almost the same temperature, but the decrease was gentle, and the superconductivity became complete for the first time in the vicinity of several K. From this, it is understood that by introducing oxygen into the chamber during film formation, the oxygen in the thin film can be appropriately controlled to form a superconducting thin film having a desired composition.

Preparation Example 2 The sintered body of Preparation Example 1 was crushed to a grain size of 0.3 mm, and this was used as a target material for sputtering under the same conditions as in Preparation Example 1 to prepare a superconducting thin film.

It was confirmed that the film forming rate was 14 Å / sec, and that the film forming rate was significantly higher than that of the preparation example 1. Further, the temperature at which the superconducting phenomenon of the thin film produced by using the target of the sintered body powder of this production example was about 97K, and at 92K or less, it became a complete superconductor. When a target of such a sintered body powder was used, a film could be formed efficiently, and the obtained thin film showed 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 powders BaCO ₃ , Y ₂ (CO ₃ ) ₃ and CuO were mixed so that the atomic ratio of Ba: Y: Cu was 0.9: 0.25: 1, and the preliminary sintering was performed to 4
The temporary sintered material obtained by performing the operation at 00 ° C. for 12 hours was used as a target.

The superconducting starting temperature of the obtained thin film was 95K. From this experiment, it was confirmed that a superconducting thin film can be produced even when an oxide obtained only by preliminary sintering is used as a target.

Preparation Example 4 The thin film sample obtained in Preparation Example 1 was heat-treated at a temperature of 550 ° C. for 20 minutes in an oxygen atmosphere (oxygen partial pressure: 0.9 × 10 ⁻¹ Torr).

When the sample after the heat treatment was measured in the same manner as in the previous preparation example in this way, the decrease in resistance was more rapid than in the case of preparation example 1, and the difference between the superconducting start temperature and the zero resistance zero temperature was 2 ° C. became.

Preparation Example 5 The same operation as in Example 1 was performed to prepare a superconducting thin film.
However, the target was prepared by setting the main sintering temperature to 920 ° C.
Film formation was performed using magnesia as a substrate with a high frequency power of 2.0 W / cm ² .

The temperature at which the superconducting phenomenon started was 98K measured by the same measurement method as in Preparation Example 1, and a perfect superconductor was obtained at 95K or less.

Effects of the Invention As described above, according to the present invention, it becomes possible to obtain a superconducting oxide thin film having a Tc much higher than that of a conventional superconductor. Therefore, the superconducting thin film obtained by the present invention is applied to a thin film device, for example, a switching device of Ma-tisoo called Josephson device, a memory device of Anacker, and a superconducting quantum interferometer (SQUID). ) Etc. can be used advantageously.

[Brief description of 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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Saburo Tanaka 1-1-1 Kunyokita, Itami City, Hyogo Prefecture Sumitomo Electric Industries, Ltd. Itami Works (72) Inventor Keizo Harada 1-chome, Koyokita, Itami City, Hyogo Prefecture No. 1-1 Sumitomo Electric Industries, Ltd. Itami Works (72) Inventor Tetsuji Kamishiro 1-1-1 Kunyokita, Itami City, Hyogo Prefecture Sumitomo Electric Industries Itami Works (56) References JP 63 -225414 (JP, A) JP-A-63-224116 (JP, A)

Claims (1)

(57) [Claims] 1. A target of a mixture of an oxide and / or a complex oxide containing at least one selected from the group consisting of Y, Ba and Cu, or a complex oxide containing all the elements contained in the element group. Form a thin film of perovskite-type oxide or pseudo-perovskite-type oxide by performing physical vapor deposition using a target in an atmosphere with an oxygen partial pressure of 0.7 × 10 ^-3 Torr to 8 × 10 ^-2 Torr. And a method for producing a superconducting thin film.