JPH0669919B2 - Manufacturing method of superconducting ceramic thin film - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a superconducting ceramic thin film. In particular, it relates to a method for producing a superconducting thin film having grains whose C axis grows perpendicularly to the substrate surface.

[Prior Art] Conventionally, the flatness of the thin film surface has been achieved while producing and applying a Y-Ba-Cu-O-based or La-Ba-Cu-O-based superconducting ceramic thin film. In consideration of epitaxial film formation, which has excellent properties and thin film crystal homogeneity, the production of polycrystalline thin films by heat treatment was also important.

When using the heat treatment method, it is important to minimize the reaction between the substrate and the thin film interface. Especially, when the film thickness is less than several thousand Å, the reactivity with Y ₁ Ba ₂ Cu ₃ O _7-Z is generally observed. Even if a SrTiO ₃ single crystal substrate, which is said to have a low content, is used, an electrically insulating second phase appears, which hinders the superconducting property of the film.

Therefore, it is important to lower and shorten the temperature and time for heat treatment. However, if the temperature is simply lowered, the crystal growth and the particle growth are insufficient and the superconducting property cannot be obtained. When a Y ₁ Ba ₂ Cu ₃ O _7-Z thin film is formed, it is generally 900 ° C when performed in an O ₂ atmosphere.
Before and after, it must be performed in air at around 850-900 ℃.

[Problems to be Solved by the Invention] In order to solve the above technical problems, the present invention provides
That is, in order to produce a superconducting thin film with excellent physical properties such as surface flatness and crystal homogeneity
A heat treatment for growing the Y ₁ Ba ₂ Cu ₃ O _7-Z crystal phase,
You have to do it at a lower temperature. It is necessary to carry out heat treatment at a lower temperature and to prepare a thin film with good orientation in which crystal grains have grown sufficiently. By heat treatment at a lower temperature, the interaction between the substrate and the thin film interface can be reduced as much as possible, and further, Y ₁ Ba ₂ Cu ₃ O _7-Z having a superconducting property can be obtained by the heat treatment at a lower temperature. It is an object to provide a production method capable of producing a thin film. As a result, it is an object of the present invention to provide a method for thinning a Y ₁ Ba ₂ Cu ₃ O _7-Z oxide superconducting material. Then, the present invention provides an oxide superconducting thin film having grains in which the C axis of the thin film crystal grows perpendicular to the substrate surface by heat treatment, and provides a device utilizing weak bondability of grain boundaries. be able to.

[Structure of the Invention] [Means for Solving Problems] In order to solve the above technical problems, the present invention provides Y-Ba.
-Cu-O system and _{(Y 1 Ba 2 Cu 3 O} 7-Z) thin film produced on a substrate, the resulting amorphous thin film of this composition by heat treatment in an N ₂ stream, the thin film was allowed to crystallize, next,
Provided is a method for producing a superconducting ceramic thin film, which is characterized by imparting superconductivity to a crystallized thin film produced by performing a cooling process of heat treatment in an atmosphere of O ₂ gas only. In that case, heat treatment in a N ₂ gas stream is carried out by maintaining the temperature range of 780 to 820 ° C. for 5 minutes or more, and the temperature rise to a predetermined temperature range is also carried out in a N ₂ gas stream. Switch the N ₂ gas to O ₂ gas and keep the O ₂
It is preferable to cool in a gas stream for 3 hours or more.

According to one of the methods for producing an oxide superconducting thin film according to the present invention, the composition of cations is Y: Ba: C by the sputtering method.
An amorphous thin film with u = 1: 2: 3 is formed on a substrate (for example, SrTiO ₃ , MgO), and heat treatment is performed.

That is, according to the present invention, the heat treatment finally leads to Y ₁ Ba ₂ Cu ₃
In order to obtain an O _{7 -Z} crystalline thin film and obtain the thin film with sufficient superconductivity, a starting thin film is prepared by a method other than the sputtering method as long as it can be obtained with a predetermined cation composition ratio. Can also be used. Also, the target thin film to be heat treated according to the present invention may be amorphous.

The “heat treatment” according to the present invention is to change the growth of Y ₁ Ba ₂ Cu ₃ O _7-Z crystal at 780 to 820 ° C. from pure N ₂ gas into a pure O ₂ gas stream until the temperature becomes 400 ° C. or less. It is possible to impart excellent superconductivity to the produced thin film by incorporating oxygen into the produced Y ₁ Ba ₂ Cu ₃ O _7-Z thin film by cooling for 4 hours.

By growing the Y ₁ Ba ₂ Cu ₃ O _7-Z thin film crystal in N ₂ gas flow, the growth is about 100% compared to that in O ₂ gas.
The temperature of the heat treatment for crystallization can be lowered by about 50 ° C. as compared with the case of performing in air.

Crystallization of Y ₁ Ba ₂ Cu ₃ O _7-Z at a heat treatment temperature of 780 to 820 ° C. naturally leads to grain growth as the heat treatment time becomes longer, but in the case of a thin film with a thickness of 1 μm or less, it takes 30 minutes. The holding time is sufficient to obtain the effect. Further, the cooling process for incorporating oxygen into the thin film crystal is preferably performed for 3 to 4 hours. However, the heat treatment time depends largely on the thickness, area, etc. of the thin film, so it is necessary to select it appropriately, and it is not necessary to specify the thickness, area.

On the other hand, according to the method of the present invention, it is important that the holding at the maximum temperature during the heat treatment process is performed in a non-oxidizing atmosphere, and a substrate, for example, a silicon substrate, in which the interaction with the thin film is particularly promoted in the oxidizing atmosphere. It can also be applied to a heat treatment method when an oxide superconducting thin film is formed on a substrate, a metal substrate or the like.

Further, the heat treatment time needs to be further shortened in order to minimize interlayer reaction during the heat treatment process at the interface between the superconducting thin film and the substrate.

As the substrate used in the present invention, a single crystal of Si, MgO, SrTiO ₃ or ZrO ₂ or a metal substrate can be used, and a substrate having good compatibility with the crystal characteristics of the superconducting substance is preferable. A polycrystalline body or a single crystal can be used for this substrate.

The initial formation method of the superconducting composition thin film on the substrate can be performed on the substrate surface by physical vapor deposition such as MBE or sputtering.

[Operation] The heat treatment used according to the present invention is performed in an N ₂ gas stream, and is cooled in the O ₂ gas atmosphere when shifting to the cooling step. ~ 100 ° C
The superconducting thin film having a good C-axis orientation was obtained at a low heat treatment temperature.

The method for producing an oxide superconducting thin film of the present invention can be further used for forming a superconducting crystal thin film for products such as Josephson junctions, infrared detectors, and optical switches.

Next, the method for producing the superconducting ceramic thin film of the present invention will be specifically described by way of examples, but the present invention is not limited thereto.

Example 1 A Y ₁ Ba ₂ Cu ₃ O _7-Z thin film was formed on a (100) plane of SrTiO ₃ by a high frequency sputtering method using a Y ₁ Ba ₂ Cu ₃ O _7-Z sintered body as a sputtering target. Was formed to a thickness of 1 μm. The sputtering gas pressure at this time was 20 mTorr, and a mixed gas of 90% Ar + 10% O ₂ was used. The substrate was not particularly heated, and the formed thin film was in an amorphous state and was an insulator.

The heat treatment is performed in an N ₂ gas stream atmosphere, i.e., pre-N ₂
The thin film prepared as described above was placed in a tubular furnace heated to 850 ° C. while flowing a gas. At this time,
The temperature of the tubular furnace was raised to 850 ° C in about 15 minutes. The holding time at 850 ° C. was 1 hour, and N ₂ gas was kept flowing during this time. Immediately after holding, the gas to be introduced into the furnace was changed from N ₂ gas to O ₂ gas.
Switch to gas and in an O ₂ gas stream, 400 for about 4 hours
It was cooled to below ℃. After that, the heater switch was turned off, the furnace was left to cool to 200 ° C., and then taken out.
The gas flow rate was 250 cc / min for both N ₂ and O ₂ gas.

Next, as a comparative example, the above heat treatment was performed in air in the same heat treatment cycle.

FIG. 1 shows the result of X-ray diffraction of the thin film after heat treatment.
Among them, the upper diffractogram is heat treated in N ₂ gas, and the lower diffractogram is heat treated in air of the comparative example.

Y ₁ Ba ₂ C with C axis oriented perpendicular to the substrate surface by heat treatment
It can be seen that an unknown phase, which is considered to be C-axis oriented, has grown like the u ₃ O _7-Z phase. The unknown phase is considered to be the phase generated by the reaction with the substrate SrTiO ₃ . This reaction phase is
It disappeared by lowering the heat treatment temperature and by using the MgO substrate.

From FIG. 1, comparing the thin films of N ₂ gas flow heat treatment and air heat treatment, Y ₁ Ba ₂ Cu ₃ O _7-Z was found in the N ₂ gas air flow.
The diffraction peak of the crystal becomes very remarkable, and according to the present invention,
It can be seen that the crystallization of the superconducting thin film is promoted by changing the heat treatment atmosphere from that in the air to that in the N ₂ gas stream.

On the other hand, in the O ₂ gas stream heat treatment, under the same temperature condition, the growth of Y ₁ Ba ₂ Cu ₃ O _7-Z crystal was further insufficient, and the heat treatment was performed at a higher temperature (900 ° C or higher). There is a need to.

FIG. 2 shows the temperature dependence of the electrical resistivity of each thin film subjected to heat treatment in N ₂ gas stream and air stream. That is, the air-heat treated sample thin film did not exhibit superconducting properties above the liquid helium temperature. The sample thin film heat-treated in N ₂ gas flow showed superconducting properties at 58K. Low Tc means
It may be due to the reaction product with the substrate.

[Example 2] A thin film of a superconducting composition and heat treatment similar to those in Example 1 are performed, but the timing of introducing O ₂ gas after heat treatment in a N ₂ gas stream was examined and compared.

The amorphous thin film was formed on the (100) plane of the SrTiO ₃ crystal by the same method as in Example 1. Heat treatment up to 800 ℃ 1
Heat up in 5 minutes, hold at 800 ° C for 30 minutes, then raise to 500 ° C 6
Lower the temperature at 0 minutes, hold at 500 ℃ for 30 minutes, and then increase to 200 ℃ 1
The temperature was lowered over 20 minutes, and the heat-treated crystalline thin film was taken out.

As for the heat treatment atmosphere of II, N ₂
It was carried out in a gas stream and was held in an O ₂ gas stream after the temperature was kept at 500 ° C. I: Temperature increase → 800 ° C hold in a N ₂ gas stream, temperature decrease → 500 ° C hold → temperature decrease by O ₂ It was carried out in a gas stream, and the two were compared and examined.

Y ₁ Ba ₂ C
u ₃ O _7-Z phase was grown. From the result of X-ray diffraction observation, no difference was observed between the two.

Table 1 shows the electrical properties of the thin films obtained in each case.
That is, I indicates a treatment in which O ₂ gas was introduced immediately after holding at 800 ° C., and II indicates a treatment in which O ₂ gas was introduced during holding at 500 ° C.

In the table, Tc and onset mean the temperature at which the electric resistivity begins to fall, and Tc and zero mean the temperature at which the electric resistivity becomes 1 × 10 ⁻⁴ mΩ · cm or less.

Table 1 I II Tc, onset About 90K About 90K Tc, zero 41K No superconductivity Room temperature resistance value 9mΩ ・ cm 50mΩ ・ cm From Table 1, immediately after 800 ℃, that is, at the maximum temperature for crystallization. It was found that the superconducting property can be obtained by introducing O ₂ gas immediately after the end of holding.

[Example 3] Next, the heat treatment temperature for crystallization was set to 750 ° C, 800 ° C, 85 ° C.
The temperature was changed to 0 ° C. for comparison.

The superconducting composition thin film was formed on the (100) plane of the SrTiO ₃ crystal in the same manner as in Example 1 and heat-treated at the above-mentioned three heat treatment temperatures.

The heat treatment was carried out in the same cycle as in Example 1 by raising the temperature to each holding temperature in about 15 minutes in a N ₂ gas stream and holding at the maximum temperature (heat treatment temperature) for 30 minutes, then converting the N ₂ gas to O ₂ gas. switching,
The temperature was lowered to 400 ° C. or lower in an O ₂ gas stream over 3 to 4 hours, further cooled to 200 ° C. or lower, and the heat-treated superconducting thin film was taken out.

Table 2 shows the thin film characteristics after the heat treatment.

However, * 1 is the slope of the electrical resistivity in the normal conduction state with respect to temperature change, and indicates that negative corresponds to the so-called semiconducting property and positive corresponds to the metallic property.

* 2 is a result determined from the diffraction peak intensity of X-ray diffraction, and shows that the production amount is large in the order of ×, Δ, ◯, ⊚.

From the above results, it was clarified that 800 ° C is an appropriate heat treatment temperature. The reason why the heat treatment at 850 ° C. becomes insulating is that a reaction product with the SrTiO ₃ substrate is generated, and by changing the substrate to MgO, the same heat treatment at 850 ° C.
It was possible to obtain a thin film in which Tc and zero were 47K.

Example 4 Using a (100) plane of MgO crystal as a substrate, heat treatment temperatures of 750 and 7
Heat treatment was carried out at 80, 800 and 850 ° C. according to the present invention. The obtained results were compared. That is, an amorphous thin film was formed on the (100) plane of the MgO crystal so as to have a thickness of 5000 Å by the same production method as in Example 3.

Next, in the heat treatment, as in Example 3, the temperature was raised to the respective heat treatment temperatures in the N ₂ gas stream for about 15 minutes and kept for 30 to 60 minutes, and then the N ₂ gas was changed to the O ₂ gas, 40 in O ₂ gas stream
Decrease the temperature over 3 to 4 hours until the temperature drops below 0 ℃, and then add 200
After cooling to below ℃, it was taken out.

The electric characteristics of the obtained thin film are shown in Table 3.

From the above results, it was clarified that a crystalline thin film having superconductivity at 70K can be obtained by heat treatment at a heat treatment temperature of 800 ℃ and a holding time of 30 to 60 minutes. The Tc and onset were about 90 K for both thin films, and by observation of X-ray diffraction lines, a film in which the C axis was oriented substantially perpendicular to the substrate surface was obtained. The superconducting thin film particles were in the form of plates of 2 to 3 μm.

Example 5 As a sputtering target, BaCu alloy, metal Y,
By the multi-source rf magnetron sputtering method using metallic Cu, the ratio of cations on the (100) plane of SrTiO ₃ crystal is
A thin film of a composition of Y: Ba: Cu = 1: 2: 3 was formed. The substrate is not heated. Sputtering gas pressure is Ar4Pa
Therefore, O ₂ was not forcibly incorporated into the prepared superconducting thin film. The obtained thin film has a black color,
It was an insulator. The film thickness was 3000Å.

Superconducting properties were obtained by subjecting this thin film to the heat treatment according to the present invention. Heat treatment up to 800 ℃ in N ₂ gas flow 15
After heating for 60 minutes and holding for 60 minutes, the atmospheric gas is switched to O ₂ gas, the temperature is lowered to 400 ° C in an O ₂ gas stream over 3 to 4 hours, and then the sample is allowed to cool to 200 ° C. The thin film was taken out.

The thin film characteristics after heat treatment are room temperature resistivity 1.8 mΩ ・ cm,
It was Tc, onset, 85K, and Tc, zero 73K.

[Example 6] The MgO crystal (1
An alloy thin film with a thickness of 5000Å was prepared on the (00) surface.

The heat treatment was the same cycle as in Example 5, and the holding time at 800 ° C. was changed to 15, 30, and 60 minutes to form thin films. In addition, as a comparative example, a thin film obtained by performing a heat treatment in a N ₂ gas stream until the end without introducing O ₂ gas in a cycle with a holding time of 30 minutes was produced.

The results are shown in Table 4.

However, the unit of * 1 is mΩ · cm, and * 2 is calculated from the (005) diffraction line peak in the X-ray diffraction diagram.

From Table 4, a good T can be obtained by heat treatment at 800 ° C for 30 to 60 minutes.
It was revealed that a superconducting thin film of c and zero was obtained. The reason why the superconducting thin film heat-treated without introducing O ₂ gas has poor superconducting properties may be due to lack of oxygen in the crystal.

[Effects of the Invention] The following remarkable technical effects were obtained by the method for producing a ceramics superconducting thin film of the present invention.

First, in the conventional heat treatment method, a high heat treatment temperature of 850 to 900 ° C. is generally used, but in the heat treatment according to the present invention, the heat treatment is performed in an N ₂ gas flow, and then by switching to an O ₂ gas flow and cooling. , Heat treatment temperature of 800 ℃, which is 50-100 ℃ lower than before
By heat treatment before and after, a superconducting thin film having good superconducting properties and good C-axis orientation was obtained.

Secondly, the interaction (chemical reaction) between the substrate and the thin film could be reduced by lowering the heat treatment temperature.

Third, the heat treatment process was improved and the crystallinity of the obtained thin film was improved only by using a very general and inexpensive gas of N ₂ and O ₂ for the heat treatment.

[Brief description of drawings]

FIG. 1 shows X-ray diffraction patterns of a superconducting thin film heat-treated according to the present invention and a thin film heat-treated in air, respectively. FIG. 2 shows the temperature dependence of the electrical resistivity of each thin film heat-treated in the same manner as in FIG.

Claims (4)

[Claims] 1. A Y-Ba-Cu-O system _{(Y 1 Ba 2 Cu 3 O} 7-Z) oxide superconductor thin film was produced on a substrate, the generated this composition amorphous thin film N ₂ stream by heat treatment in, allowed crystallize the film, then, the cooling process of the heat treatment O ₂
A method for producing a superconducting ceramic thin film, which is characterized by imparting superconductivity to a crystallized thin film produced by performing only in an atmosphere of air flow. 2. A process according the one of claims performs by holding 5 minutes or more N ₂ gas stream heat treated at a temperature in the range of seven hundred eighty to eight hundred twenty ° C., again be heating to a predetermined temperature range, N _{2 2. The method according} to claim 1, wherein the N ₂ gas is switched to O ₂ gas after the heat treatment is completed in an air stream and the temperature is cooled to 400 ° C. or lower in the O ₂ gas stream for 3 hours or more. Manufacturing method of conductive ceramic thin film. 3. The method for producing a superconducting ceramic thin film according to claim 1, wherein the heat treatment cycle is performed once or more. 4. The method for producing a thin film according to claim ₃ , wherein the substrate is a single crystal of Si, MgO, SrTiO ₃ or ZrO ₂ .