JPH069216A - Production of superconducting thin film of oxide - Google Patents

Abstract

PURPOSE:To prepare a superconducting thin film of oxide having excellent clarity of surface, crystallinity and superconductivity only by film formation without carrying out after-treatment. CONSTITUTION:A chamber 2 is evacuated to <=1X10<-9>Torr, O2 containing 5-70% O3 and pure N2O or pure NO2 are fed to the vicinity of a substrate 4 and the pressure is made to be 6X10<-6> to 8X10<-5>Torr. Three cells having set metal Y, metal Ba and metal Cu, respectively, are used as K cells 3 and heated to about 1,150-1,350 deg.C, about 570-640 deg.C and about 950-1,090 deg.C. The substrate temperature is adjusted to 650-730 deg.C, a film is formed at 5-20Angstrom /minute. When the thin film reaches given thickness, the temperature is dropped in the atmosphere of the film formation as it is.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an oxide superconducting thin film. More specifically, the present invention relates to a method for producing an oxide superconducting thin film having excellent surface cleanability, crystallinity, and superconducting properties by MBE without performing heat treatment after film formation.

[0002]

2. Description of the Related Art Oxide superconductors are considered to have a higher critical temperature and higher practicability than conventional metal-based superconductors. For example, the critical temperature of the Y-Ba-Cu-O-based oxide superconductor is 80 K or higher, and the Bi-Sr-Ca-Cu-O-based oxide superconductor and the Tl-Ba-Ca-Cu-O-based oxide are included. It has been announced that the critical temperature of superconductors is over 100K. The superconducting property of the oxide superconductor has a crystal anisotropy, and the critical current density is the largest in the direction perpendicular to the c-axis of the crystal. Therefore, when using an oxide superconductor, it is necessary to pay attention to the crystal orientation.

When the oxide superconductor is applied to so-called superconducting electronics technology such as superconducting elements and superconducting integrated circuits, it is generally necessary to use the oxide superconductor in a thin film. When the oxide superconductor is thinned,
The problem of the crystal anisotropy of the superconducting property becomes more remarkable. Further, in order to realize a high-performance superconducting element and a superconducting integrated circuit, a superconducting conductive channel through which a current flows in a direction parallel to the substrate and a superconducting conductive channel through which a current flows in a direction perpendicular to the substrate are required. For example, a current flows in a direction parallel to the substrate through the electrodes and the like, and a current flows in a direction perpendicular to the substrate through the interlayer wiring. Therefore, when the oxide superconductor is used in a superconducting device or a superconducting integrated circuit, the c-axis oriented oxide superconducting thin film having a high critical current density in the direction parallel to the substrate and the critical current density in a direction perpendicular to the substrate are high. An oxide superconducting thin film having an a-axis orientation (or a b-axis orientation, which will be hereinafter represented by a more general a-axis orientation in the present specification) must be produced.

When manufacturing a superconducting device or a superconducting integrated circuit having a multilayer structure, the above-mentioned c-axis oriented oxide superconducting thin film and a
It is necessary to stack an axially oriented oxide superconducting thin film. The orientation of the oxide superconducting thin film is controlled by the film forming temperature (generally the substrate temperature during film forming). That is, when forming an a-axis oriented oxide superconducting thin film, the substrate temperature is about 50 to 100% higher than the substrate temperature when forming a c-axis oriented oxide superconducting thin film.
The film formation may be performed at a substrate temperature lower by ° C.

[0005]

Generally, when laminating a plurality of thin films, the cleanliness and crystallinity of the surface of the lower thin film become problems. If the surface of the lower thin film is not clean, the contaminants deposited on the surface of the lower thin film and the oxides formed on the surface of the lower thin film may cause the upper and lower thin films to physically or electrically interact at the interface. It becomes discontinuous.
Moreover, when superconducting thin films are stacked, unnecessary weak bonds are formed. Therefore, the performance of the element or integrated circuit may not reach a predetermined value or may not operate.

In particular, oxide superconductors have a very short coherence length. Therefore, when an oxide superconducting thin film is used as a lower layer and a thin film is further laminated thereon, particular attention should be paid to the surface state of the oxide superconducting thin film. I have to pay. Further, oxygen in the oxide superconductor crystal is relatively unstable and may easily escape from the crystal. When oxygen is lost, the superconducting property of the oxide superconductor is extremely deteriorated, and in extreme cases, the superconducting property may not be exhibited. That is, the surface of the oxide superconducting thin film used as the lower thin film is clean,
Excellent crystallinity and superconductivity are required.

Conventionally, when laminating another thin film on the oxide superconducting thin film, the lower layer oxide superconducting thin film was formed and then heat-treated in a high vacuum to clean the surface. However, in this method, oxygen in the oxide superconductor forming the oxide superconducting thin film may be lost, and the superconductivity of the oxide superconducting thin film may be lost.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing an oxide superconducting thin film having excellent surface states without post-treatment, which solves the above-mentioned problems of the prior art.

[0009]

According to the present invention, in a method for producing an oxide superconducting thin film on a substrate by the MBE method, the exhaust gas is introduced into a vacuum chamber evacuated to a pressure of 1 × 10 ^-9 Torr or less. O containing ₅ to 70% by volume of O ₃ while performing
₂ , pure N ₂ O or pure NO ₂ is supplied to maintain the pressure in the vacuum chamber at 6 × 10 ⁻⁶ to 8 × 10 ⁻⁵ Torr, and the film is formed while heating the substrate. A method for producing an oxide superconducting thin film is provided.

In the present invention, it is preferable that after the film formation, the substrate temperature is lowered to room temperature in the atmosphere during the film formation.

[0011] In the present invention, the 1 × 10 ^-9 vacuum chamber Torr was evacuated to a pressure of less than, O ₂ including O ₃ 5 to 70 vol% while the exhaust, pure N ₂ O or Pure N
O ₂ is supplied to adjust the pressure in the vicinity of the substrate in the vacuum chamber to 6
On the substrate heated to 650 to 730 ° C., which was maintained at × 10 ⁻⁶ to 8 × 10 ⁻⁵ Torr, Y was transferred from the Knudsen cell evaporation source at 1150 to 1350 ° C. where the metal Y was set to the metal Ba. 570 set
Ba is supplied from the Knudsen cell evaporation source at ~ 640 ° C and Cu is supplied from the Knudsen cell evaporation source at 950 to 1090 ° C in which metallic Cu is set to form a film. There is provided a method for producing an oxide superconducting thin film, which comprises lowering the substrate temperature to room temperature below.

In the above method of the present invention, 1 × 10 ^-9 To
O ₂ containing 8% by volume of O ₃ is supplied to the vacuum chamber evacuated to a pressure of rr or less to maintain the pressure in the vicinity of the substrate in the vacuum chamber at 5 × 10 ^-5 Torr. , 70
1220 ℃ with metal Y set on the substrate heated to 0 ℃
Y from the Knudsen cell evaporation source, and Ba from the Knudsen cell evaporation source at 620 ° C in which metal Ba is set, and metal Cu.
Cu from the Knudsen cell evaporation source set to 1000 ℃
Is preferably supplied to perform film formation, and after the film formation, the substrate temperature is lowered to room temperature in the atmosphere during film formation.

[0013]

According to the method of the present invention, ₅ to 70% by volume of O ₃ is contained in a vacuum chamber having a background pressure of 1 × 10 ^-9 Torr or less.
The main characteristic is that the film formation by the MBE method is carried out under the pressure of 6 × 10 ⁻⁶ to 8 × 10 ⁻⁵ Torr lower than the conventional one by supplying containing O ₂ , pure N ₂ O or pure NO _2. is there. The background pressure in the vacuum chamber is very low, less than 1 × 10 ^-9 Torr, and there is no leak or gas generation in the chamber.
No contaminants (hydrocarbons, metal carbides) are deposited on the oxide superconducting thin film produced by the method of the present invention.

In the method of the present invention, it is preferable that after the above film formation, the substrate temperature is lowered to room temperature in the atmosphere during film formation. When the temperature is lowered, the oxide superconductor crystal forming the oxide superconducting thin film transforms from a tetragonal system to an orthorhombic system.

The method of the present invention more specifically comprises 1 × 10 5.
In a vacuum chamber which is evacuated to a pressure of less than ^-9 Torr, O ₂ including O ₃ 5 to 70 vol% while the exhaust, pure N ₂ O
Alternatively, pure NO ₂ is supplied to increase the pressure in the vacuum chamber to 6
X is about 570 from a Knudsen cell evaporation source in which metal Y of about 1150 to 1350 ° C. is set on a substrate heated to 650 to 730 ° C. while maintaining at × 10 ⁻⁶ to 8 × 10 ⁻⁵ Torr. Approximately 950 Ba from the Knudsen cell evaporation source with metal Ba set at ~ 640 ° C.
A method can be used in which Cu is supplied from a Knudsen cell evaporation source in which metallic Cu of ~ 1090 ° C is set to form a film, and then the substrate temperature is lowered to room temperature in the atmosphere during film formation. .

In the method of the present invention, the substrate temperature is 650 ° C.
If it is less than the above, a polycrystalline oxide superconducting thin film in which a-axis oriented portions and c-axis oriented portions are mixed is obtained. Also,
When the substrate temperature exceeds 730 ℃, Cu is not oxidized and Y ₁ Ba ₂ Cu ₃
No O _7-X oxide superconductor is generated. When forming an oxide superconducting thin film by the MBE method, the temperature of the Knudsen cell is extremely important, but the temperature of each Knudsen cell is not limited to the material used for the evaporation source, but also the temperature of each part of the MBE apparatus. The value varies depending on the geometrical arrangement and the material of the Knudsen cell. In the method of the present invention, by setting the temperature of each Knudsen cell within the above range, an oxide superconducting thin film having excellent characteristics can be formed. A typical value in the method of the present invention is a substrate temperature of 700 ° C.,
Examples include Knudsen cell 1220 ° C. for metal Y, Knudsen cell 620 ° C. for metal Ba, and Knudsen cell 1000 ° C. for metal Cu.

In the method of the present invention, it is preferable to use O ₂ containing ₅ to 70% by volume of O ₃ , pure N ₂ O or pure NO ₂ as the oxidizing gas. These gases have a stronger oxidizing power than pure O ₂ , and are particularly preferable when an oxide superconducting thin film is prepared by using a metal as an evaporation source by the MBE method in which the pressure in the chamber is low. However, when O ₂ containing O ₃ is used, if O ₃ exceeds 70% by volume, an oxide superconducting thin film having good characteristics may not be obtained. Therefore, when using the O ₂ containing O ₃ to the oxidizing gas in the process of the invention, it is preferred to use O ₂ including O ₃ 5 to 70% by volume, in particular 8% by volume is preferred.

Further, in the method of the present invention, it is preferable that the growth rate of the oxide superconducting thin film is 5 to 20 Å / min.
The growth rate of the oxide superconducting thin film is affected by the substrate temperature, the temperature of each Knudsen cell, the geometrical arrangement of each part of the MBE device, the material of the Knudsen cell, the pressure in the chamber, the atmosphere in the chamber, etc. . According to the method of the present invention, a high quality thin film can be obtained when the growth rate of the oxide superconducting thin film is within the above range.

Since the oxide superconducting thin film produced by the method of the present invention has a clean surface and is excellent in crystallinity and superconducting property, a thin film of another material and an oxide superconducting thin film having a different orientation are formed on it. Suitable for stacking etc. In particular, in the method of the present invention, the film forming step alone does not require any post-treatment,
Since an oxide superconducting thin film having an excellent surface state can be produced, another thin film can be continuously grown on it, which also contributes to improvement in production efficiency.

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

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

[0022]

EXAMPLE A Y ₁ Ba ₂ Cu ₃ O _7-X oxide superconducting thin film was prepared by the method of the present invention. The configuration of the MBE device for carrying out the method of the present invention will be described with reference to FIG. M shown in FIG.
The BE apparatus includes a vacuum chamber 2 having a main exhaust system 1 and a Knudsen cell (hereinafter referred to as K cell) 3 for accommodating an evaporation source mounted on the bottom of the vacuum chamber 2.
And a substrate holder 5 for holding the substrate 4 mounted on the top of the vacuum chamber 2. The substrate holder 5 is provided with a heater 5a for heating the substrate 4. Furthermore, the sample introduction unit 10, the liquefied nitrogen shroud 6 for forming a cold trap around the evaporation source, and a reflection high-energy electron diffraction device (hereinafter, referred to as RHEED) for observing a thin film during film formation Eight etc. are equipped. Further, immediately before the substrate 4, a shutter 9 for controlling the on / off of the film forming process is provided. Further, if necessary, a plurality of K cells 3 may be provided, or an electron gun 11 may be attached instead of the K cells 3.

The MBE apparatus of FIG. 4 is particularly characteristic in that it includes a throttle means 21 and a gate valve 22 provided near the center of the vacuum chamber 2 and an auxiliary exhaust system 20 mounted on the substrate side of the gate valve 22. It is a point to have. Aperture means 21
Is composed of a plate-shaped member having a through hole in the center. This through hole is provided so as not to interfere with the molecular beam emitted from the K cell 3 and the electron gun 11 toward the substrate 4. Such a diaphragm means 21 is provided in the vacuum chamber 2
By providing it inside, the effective diameter between the substrate 4 side and the evaporation source side in the vacuum chamber 2 becomes small, and it becomes easy to generate a differential pressure between the substrate 4 side and the evaporation source side. The structure of the throttle means 21 is not limited to this, and any structure can be used as long as it can block the flow of gas between the substrate side and the evaporation source side in the vacuum chamber. it can.

Further, the gate valve 22 is constructed so as to be able to close the through hole 21 provided in the throttle means 21, and in the closed state, it connects the substrate side of the vacuum chamber 2 and the evaporation source side. It is configured so that it can be shut off in an airtight manner. Although not shown, the gate valve 22 is constructed so that its opening and closing can be operated from outside the vacuum chamber 2.

Further, the sub-evacuation system 20 is provided on the substrate 4 side with respect to the diaphragm means 21 and the gate valve 22 and can exhaust the substrate side of the vacuum chamber 2. A turbo pump, a cryopump or the like is preferable as the auxiliary exhaust system 20. A diffusion pump or the like is preferably used as the main exhaust system 1 provided on the evaporation source side with respect to the throttle means 21 and the gate valve 22.

Using the above MBE apparatus, a c-axis oriented Y ₁ Ba ₂ Cu ₃ O _{7 -X} oxide superconducting thin film was prepared by the method of the present invention. The procedure will be described below. First, the substrate 4 is set in the substrate holder 5 and the evaporation source is set in the K cell 3, and then the inside of the vacuum chamber 2 is evacuated to a high vacuum of 1 × 10 ⁻⁹ Torr or less. In this example, MgO was set on the substrate 4, and metal Y, metal Ba, and metal Cu were set in different K cells as evaporation sources. Subsequently, while the vacuum chamber 2 is being evacuated, 8% by volume of O ₃ is supplied from the air supply system 7, and the pressure around the substrate 4 is adjusted to 5 × 10 ⁻⁵ Torr. This MB
Since the E unit is provided with the throttling means 21, the pressure on the evaporation source side is lower than the pressure around the substrate by one digit or more. Further, by configuring the apparatus so that the oxygen gas ejected from the air supply system 7 hits the film forming surface of the substrate, the substantial oxygen pressure on the film forming surface can be further increased.

Simultaneously with the above-described supply of O ₂ , the heater 5a and the K cell 3 heat the substrate and the evaporation source to predetermined temperatures. In this embodiment, the substrate temperature is 700
℃, the temperature of each K cell is 1220 ℃ Y cell, 620 Ba cell
℃, Cu cell was 1000 ℃. When the molecular beam is stably generated from the evaporation source, the shutter 9 is opened and the film formation is started. At this time, the growth state of the thin film can be observed by RHEED8. In this example, an oxide superconducting thin film having a thickness of 900Å was grown at a film forming rate of 10Å / min.

When the thin film on the substrate 4 reaches a predetermined film thickness of 900Å by the above film formation, the heater 5a is turned off while maintaining the atmosphere during film formation, and the substrate temperature is lowered. The c-axis oriented Y ₁ Ba ₂ Cu ₃ O _{7 -X} oxide superconducting thin film thus prepared was evaluated by XPS and LEED without exposing the surface thereof to the atmosphere. The results are shown in FIGS.

FIG. 1 is a LEED image of an oxide superconducting thin film produced by the method of the present invention. Since spots are seen in the LEED image of FIG. 1, the surface of the oxide superconducting thin film produced by the method of the present invention is a crystalline surface. 2 and 3 are XPS spectra of the oxide superconducting thin film produced by the method of the present invention. FIG. 2 is a spectrum of a region where the 3p peak of Y and the peak of C are seen.
As can be seen from FIG. 2, no C peak is observed on the surface of the oxide superconducting thin film produced by the method of the present invention. Therefore, the surface of the oxide superconducting thin film produced by the method of the present invention is clean because no C compound as a contaminant is present. FIG. 3 is a spectrum of a region where a Cu peak is seen. As can be seen from FIG. 3, the surface of the oxide superconducting thin film produced by the method of the present invention has a strong Cu satellite peak. This indicates that the surface has good superconductivity.

[0030]

As described above, according to the present invention,
Provided is a manufacturing method in which an oxide superconducting thin film having a good surface condition is formed and obtained without post-treatment. Since the oxide superconducting thin film produced by the method of the present invention has excellent surface cleanliness, crystallinity, and superconducting properties, when it is used as the lower layer of the laminated film, the interface consistency becomes good. By applying the present invention to the production of superconducting elements and superconducting integrated circuits, it is possible to produce a high-performance superconducting device which has never been obtained before.

[Brief description of drawings]

FIG. 1 is a photograph showing the crystalline state of an oxide superconducting thin film produced by the method of the present invention.

FIG. 2 is an XPS spectrum showing the state of the surface of an oxide superconducting thin film produced by the method of the present invention.

FIG. 3 is an XPS spectrum showing the state of the surface of an oxide superconducting thin film produced by the method of the present invention.

FIG. 4 is a schematic diagram of an example of an MBE apparatus that implements the method of the present invention.

[Explanation of symbols]

1 ... Main exhaust system, 2 ... Vacuum chamber, 3 ... Knudsen cell (K cell), 4 ... Substrate, 5 ... Substrate holder, 5a ... Heater, 6 ... Liquid nitrogen shroud, 7 ...・ Air supply system, 8 ・ ・ Reflection high-speed electron beam diffractometer (RHEED),
9 ・ ・ Shutter, 10 ・ ・ Sample introduction part, 11 ・ ・ Electron gun, 20 ・ ・ Secondary exhaust system, 21 ・ ・ Throttle means, 22 ・ ・ Gate valve

Claims (4)

[Claims] 1. A method for producing an oxide superconducting thin film on a substrate by the MBE method, in which a vacuum chamber evacuated to a pressure of 1 × 10 ⁻⁹ Torr or less is used while O ₃ is being evacuated.
Of 5 to 70% by volume of O ₂ , pure N ₂ O or pure NO ₂ is supplied to adjust the pressure in the vacuum chamber to 6 × 10 ⁻⁶ to 8 × 10 ⁻⁵ Torr.
The method for producing an oxide superconducting thin film, characterized in that the film formation is carried out while maintaining the temperature of the substrate while heating the substrate. 2. The method for producing an oxide superconducting thin film according to claim 1, wherein after the film formation, the substrate temperature is lowered to room temperature in the atmosphere during the film formation. 3. A method for producing an oxide superconducting thin film on a substrate by the MBE method, wherein O ₃ is supplied to a vacuum chamber evacuated to a pressure of 1 × 10 ⁻⁹ Torr or less while the gas is evacuated.
5 to 70% by volume of O ₂ , pure N ₂ O or pure NO ₂ is supplied to adjust the pressure in the vicinity of the substrate in the vacuum chamber to 6 × 10 ⁻⁶ to 8
While maintaining at × 10 ^-5 Torr and heating to 650-730 ℃,
Metal Y was set from the Knudsen cell evaporation source of 1150 to 1350 ° C, Ba was set from the Knudsen cell evaporation source of 570 to 640 ° C in which the metal Ba was set, and metal Cu was set.
Fabrication of oxide superconducting thin film characterized in that Cu is supplied from a Knudsen cell evaporation source at 950 to 1090 ° C to form a film, and then the substrate temperature is lowered to room temperature in the atmosphere during film formation Method. 4. A vacuum chamber evacuated to a pressure of 1 × 10 ^-9 Torr or less is supplied with O ₂ containing 8% by volume of O ₃ while the gas is evacuated to obtain a pressure near the substrate in the vacuum chamber. Is maintained at 5 × 10 ^-5 Torr, and Y is supplied from a Knudsen cell evaporation source of 1220 ° C. with metal Y set on a substrate heated to 700 ° C. Ba is supplied from the sencel evaporation source and Cu is supplied from the Knudsen cel evaporation source at which metal Cu is set at 1000 ° C. to form a film, and then the substrate temperature is lowered to room temperature in the atmosphere during the film formation. The method for producing an oxide superconducting thin film according to claim 3, wherein