JP2883464B2 - Method of laminating thin films of different materials on oxide superconducting thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for laminating thin films of different materials on an oxide superconducting thin film. More specifically, the present invention relates to a method of laminating thin films made of different materials on an oxide superconducting thin film made of an oxide superconductor.

[0002]

2. Description of the Related Art An oxide superconductor has a higher critical temperature than conventional metal-based superconductors, and is considered to be more practical. For example, the critical temperature of a Y-Ba-Cu-O-based oxide superconductor is 80 K or more, and a Bi-Sr-Ca-Cu-O-based oxide superconductor and a Tl-Ba-Ca-Cu-O-based oxide It is reported that the critical temperature of superconductor is 100K or more.

When an oxide superconductor is applied to various electronic devices, it is necessary to make the oxide superconductor thinner and stack it. For example, when a superconducting junction called a tunnel-type Josephson junction is realized using an oxide superconductor, a first oxide superconducting thin film, a non-superconducting thin film, and a second oxide superconducting thin film are sequentially laminated. Must.

The above-mentioned Josephson element utilizing the Josephson junction is a two-terminal element, and if a logical circuit is to be constructed, the circuit becomes complicated. For this reason, various three-terminal superconducting elements have been considered. Among the three-terminal superconducting elements, various types of superconducting transistors combining a superconductor and a semiconductor have been disclosed. When an oxide superconductor is used for the superconducting transistor, it may be necessary to stack a semiconductor thin film on the oxide superconducting thin film. Further, a superconducting element combining a superconductor and a normal conductor has been considered, but when an oxide superconductor is used for this superconducting element, a thin film of a normal conductor such as a metal is formed on the oxide superconducting thin film. May be stacked.

[0005] Each of the above-described various superconducting elements operates by a superconducting current flowing through a non-superconductor between superconductors arranged close to each other. The distance between the superconductors is determined by the coherence length of the superconductor. Since the oxide superconductor has a very short coherence length, in the above-described superconducting element using the oxide superconductor, the distance between the superconductors must be about several nm. On the other hand, considering the characteristics of the element, each thin film of the superconducting element must have good crystallinity. That is, it is preferable that all the thin films are single crystals or polycrystals having an orientation close to that of a single crystal.If there is a polycrystalline or amorphous thin film in which the crystal directions are not aligned at all, the performance of the superconducting element is generally low. Not stable.

[0006]

Generally, when a plurality of thin films are stacked, the surface of the lower thin film is cleaned before growing the upper thin film. If the surface of the lower thin film is not cleaned, the interface will not be uniform due to contaminants deposited on the surface of the lower thin film and oxides formed on the surface of the lower thin film, and the upper thin film and the lower thin film will not be uniform. A part of the interface with is discontinuous. Therefore, the performance of the element or the integrated circuit may not reach a predetermined value or may not operate.

In particular, the oxide superconductor has a very short coherence length and therefore tends to lose superconductivity. When an oxide superconducting thin film is used as a lower layer and a thin film is further laminated thereon, the oxide superconducting thin film has Particular attention must be paid to the surface condition. That is, the surface of the oxide superconducting thin film used as the lower thin film is required to be clean, excellent in crystallinity and superconductivity.

In conventional devices and integrated circuits using semiconductors, methods such as pure water cleaning, chemical cleaning, dry etching, and wet etching have been used to clean the surface of the underlying thin film. Since the oxide superconductor has high reactivity, the above methods cannot be applied. When the surface of the oxide superconducting thin film is treated by the above method, a reaction occurs on the surface of the oxide superconducting thin film, and on the contrary, the cleanliness of the thin film surface,
Crystallinity and superconductivity are lost.

Immediately after the formation of the oxide superconducting thin film, a method of continuously laminating an upper thin film using the same film forming apparatus has been adopted. There is a problem that materials that can be used are limited.

It is an object of the present invention to provide a method of solving the above-mentioned problems of the prior art and further stacking a thin film on an oxide superconducting thin film without deteriorating various characteristics of the oxide superconducting thin film. It is in.

[0011]

According to the present invention, there is provided a method for laminating a second thin film made of a material different from that of an oxide superconductor on an oxide superconductive thin film made of an oxide superconductor. The temperature of the oxide superconducting thin film in an ultra-high vacuum of 10 ^-9 Torr or more is set to a state in which oxygen peculiar to the oxide superconductor constituting the oxide superconducting thin film is easily taken in.
In becomes a temperature below after the heat treatment the difference is heated to a temperature within 100 ° C. with temperature, by forming the second thin film oxide superconductor thin film, the excellent crystallinity 2
A method of laminating thin films of different materials on an oxide superconducting thin film, characterized by laminating the above thin films.

In the present invention, the above-mentioned oxide superconducting thin film is formed of Y
_When composed of ₁ Ba ₂ Cu ₃ O _7-X oxide superconductor, the heating temperature is preferably 350 to 400 ° C.

[0013]

According to the method of the present invention, when a thin film of a different material is laminated on an oxide superconducting thin film, the lower oxide superconducting thin film is heat-treated in an ultra-high vacuum to clean the surface and then the upper thin film. The main feature is where the layers are stacked. The method of the present invention can be applied, for example, to the case where an insulating thin film or a normal conducting thin film is laminated on an oxide superconducting thin film to produce a superconducting element. In the method of the present invention, the above-described heat treatment is performed immediately before forming the upper oxide superconducting thin film on the lower oxide superconducting thin film. This heat treatment removes contaminants (hydrocarbons, metal carbides) deposited on the surface of the underlying oxide superconducting thin film. In addition, the crystallinity of the portion where the crystallinity of the surface of the lower oxide superconducting thin film is disordered is recovered, and the superconductivity is improved.

In the method of the present invention, the pressure at the time of the heat treatment is preferably set to 1 × 10 ⁻⁹ Torr or less. 1 × 10 ^-9
If the pressure is higher than Torr, the above-mentioned heat treatment will not provide any effect. Further, when the oxide superconducting thin film of the lower layer in the method of the present invention uses the _{Y 1 Ba 2 Cu 3 O 7} -x oxide superconductor, the heating temperature during the heat treatment to 350 to 400 ° C..
When the heating temperature is lower than 350 ° C, contaminants on the surface are not removed, and when the heating temperature is higher than 400 ° C, oxygen in the oxide superconductor crystal constituting the thin film is easily moved, and the ultra-high true
Under an air atmosphere, oxygen in the oxide superconductor crystal is lost. In the method of the present invention, the heat treatment time is
Although it is necessary to adjust the pressure and the heating temperature, it is generally in the range of several minutes to several hours.

According to the method of the present invention, for example, it is possible to clean the surface of an oxide superconducting thin film whose surface has been deteriorated by, for example, contacting with air, and to stack a thin film of a different material thereon. Therefore, it is possible to form an oxide superconducting thin film and a thin film of another material under optimum conditions using different film forming apparatuses, and to produce a laminated film having better characteristics than before. Can be.

The present invention can be applied to any oxide superconductor, but in particular, Y-Ba-Cu-O-based oxide superconductor, Bi-Sr-Ca-Cu-O-based oxide superconductor Body, Tl-Ba
It is preferably applied to a -Ca-Cu-O-based oxide superconductor. This is because these oxide superconductors are currently most excellent in various superconducting properties including the critical temperature.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following disclosure is merely an example of the present invention, and does not limit the technical scope of the present invention.

[0018]

[Example 1] Y ₁ Ba ₂ Cu was prepared by the method of the present invention.
_An MgO thin film was laminated on the ₃ O _7-X oxide superconducting thin film. FIG.
The procedure for laminating an oxide superconducting thin film by the method of the present invention will be described with reference to FIG.

First, as shown in FIG.
0) A 300 nm thick Y ₁ Ba ₂ Cu ₃ O _{7 -X} oxide superconducting thin film 1 having a thickness of 300 nm was formed on the surface of the substrate 3 by off-axis sputtering, laser ablation, and reaction. The film is formed by a method such as a reactive evaporation method, an MBE method, or a CVD method.
The conditions for forming the oxide superconducting thin film by the off-axis sputtering method are described below.

When the oxide superconducting thin film 1 is taken out of the chamber after film formation, the surface of the oxide superconducting thin film 1 reacts with moisture, carbon dioxide and the like in the air, and a deteriorated portion 10 is formed. In addition, it is contaminated with hydrocarbons, BaCO ₃ , BaCuO _{2 and the} like. After housed in an ultra-high vacuum chamber and evacuating to a high vacuum of 1 × 10 ^-9 Torr or less, monitor with LEED (slow electron diffraction), QMS (quadrupole mass spectrometer), and XPS (X-ray photoelectron spectroscopy) While performing the following heat treatment, contaminants were removed, the deteriorated portion 10 disappeared as shown in FIG. 1 (c), and a crystalline surface appeared. Pressure 1 × 10 ^-9 Torr or less Heating temperature 350 to 400 ° C (substrate temperature) Processing time 10 minutes

The oxide superconducting thin film 1 whose surface has been cleaned by the above heat treatment is transported to a film forming apparatus while maintaining an ultra-high vacuum, and an MgO thin film 2 having a thickness of 2 nm is deposited as shown in FIG. The film is formed by an evaporation method. The film forming conditions are shown below. Pressure 10Pa Substrate Temperature 200 ° C The laminated film produced by the method of the present invention has excellent crystallinity in both the lower and upper thin films,
It was found that the consistency at the interface was good.

Example 2 An Ag thin film was laminated on a Y ₁ Ba ₂ Cu ₃ O _7-X oxide superconducting thin film by the method of the present invention in the same manner as in Example 1. First, Mg equal to that used in Example 1
A 300 nm thick c-axis oriented Y ₁ B
a ₂ Cu ₃ O _7-X oxide superconducting thin film is formed by off-axis sputtering, laser ablation, reactive evaporation, MB
The film is formed by a method such as an E method or a CVD method. After film formation, the oxide superconducting thin film whose surface has been taken out of the chamber and degraded is housed in an ultra-high vacuum chamber, evacuated to a high vacuum of 1 × 10 ⁻⁹ Torr or less, and then subjected to LEED (slow electron diffraction), QMS (4 The following heat treatment was performed while monitoring with a quadrupole mass spectrometer) and XPS (X-ray photoelectron spectroscopy). Pressure 1 × 10 ^-9 Torr or less Heating temperature 350 to 400 ° C (substrate temperature) Processing time 10 minutes

An Ag thin film having a thickness of 200 nm is formed by an evaporation method on the oxide superconducting thin film whose surface has been cleaned by the above heat treatment. The film forming conditions are shown below. Pressure 10Pa Substrate Temperature 200 ° C The laminated film produced by the method of the present invention has excellent crystallinity in both the lower and upper thin films,
It was found that the consistency at the interface was good.

[0024]

As described above, according to the present invention,
A method for laminating thin films of different materials on an oxide superconducting thin film without deteriorating properties is provided. The laminated film produced by the method of the present invention has not only excellent superconducting properties but also excellent matching at the interface. By applying the present invention to the production of a superconducting element and a superconducting integrated circuit, it is possible to produce a high-performance superconducting device which has not been obtained conventionally.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a step of laminating thin films of different materials on an oxide superconducting thin film in the method of the present invention.

[Explanation of symbols]

 Reference Signs List 1 oxide superconducting thin film 2 MgO thin film 3 substrate

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-133416 (JP, A) JP-A-63-308979 (JP, A) JP-A-2-296723 (JP, A)

Claims (2)

(57) [Claims] To 1. A composed of an oxide superconductor oxide superconductor thin film, a method of laminating a second thin film made of a material different from the oxide superconductor, 10 ^-9 Torr or more ultrahigh In a vacuum, the temperature of the oxide superconducting thin film is increased by an oxidation characteristic of the oxide superconductor constituting the oxide superconducting thin film.
After performing a heat treatment at a temperature not higher than the temperature at which oxygen in the material superconductor is likely to be lost and a temperature difference from the temperature within 100 ° C., the second thin film is formed on the oxide superconducting thin film. A method of laminating thin films of different materials on an oxide superconducting thin film, comprising laminating a second thin film having excellent crystallinity by forming a film. 2. The method according to claim 1, wherein the oxide superconducting thin film is made of Y ₁ Ba ₂ Cu ₃ O.
It is composed of _7-x oxide superconductor and the heating temperature is 350 ~ 4
The method for laminating an oxide superconducting thin film according to claim 1, wherein the temperature is 00 ° C.