JP3507883B2 - Substrate for film production produced by the same method as the substrate surface treatment method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment method for a ceramic single crystal substrate and a substrate obtained by the method. More specifically, the present invention relates to a method for producing an epitaxial thin film as a base for oxide electronics. The present invention relates to a substrate having a required crystal lattice plane with a smoothness of sub-nanometer dimension and a surface treatment method for obtaining the substrate.

[0002]

_2. Description of the Related Art Conventional techniques relating to the surface treatment of single crystal substrates such as strontium titanate and titanium oxide (TiO ₂ ) include, for example, a heating method in a vacuum using an electric heater and an etching method using a hydrofluoric acid buffer solution. It has been known. The former is to obtain a flat surface by heating the substrate to evaporate or diffuse the surface atoms, and the latter to soak the substrate in a liquid such as acid or alkali to dissolve the surface layer. There is also surface treatment by light heating, but this surface treatment by light heating is applied to the substrate material with a color that shows a large absorption for light in the visible to far infrared region such as a silicon semiconductor substrate. Has been done.

Among the above-mentioned conventional techniques, in the heating method using an electric heater, a flat surface is obtained by raising the substrate temperature to about 1000 ° C. for a substrate material having a low melting point such as a silicon semiconductor. In order to obtain a flat crystal lattice plane for a substrate material having a higher melting point such as the ceramic single crystal substrate, in a clean atmosphere,
For a metal oxide substrate, 1000 in a clean oxygen atmosphere.
It is necessary to raise the substrate temperature to a high temperature of -1200 ° C or higher. However, the substrate temperature is reduced to 10 by using the above conventional technique.
It is quite difficult to raise the temperature to 00-1200 ° C or higher in an oxygen atmosphere, and in addition to the problem of the life of the heater, it is difficult to maintain good thermal contact between the substrate and the heater with good reproducibility. For practical reasons, it is not practical for industrial application.

In the etching method, selective etching easily occurs due to crystal lattice defects of the substrate and polishing scratches on the substrate surface. Therefore, there is a problem that irregularities called pits are likely to occur. Furthermore, since this technology is a wet method, it is an in-situ process in which a series of steps from substrate surface treatment to thin film production / element processing, which is an important basic technology in oxide electronics, is performed in a clean vacuum.
(In-situ) There is a problem that it cannot be applied to the process.

In the substrate surface treatment method by light heating, since the ceramic substrate does not absorb a large amount of light in the range from visible rays to far infrared rays in a normal state, a crystal having a sub-nanometer dimension smoothness is obtained. I could not get the lattice plane.

[0006]

Under these circumstances, the inventors of the present invention have, in view of the above-mentioned prior art, propose a ceramic single crystal substrate having a flat crystal lattice plane in the sub-nanometer dimension. As a result of intensive research aimed at developing a new substrate surface treatment method that makes it possible to obtain the substrate, the substrate is preheated to generate a light absorption center (color center) due to oxygen deficiency and further irradiated with light energy. By doing so, they have found that the intended purpose can be achieved, and have completed the present invention. The purpose of the present invention is to
An object of the present invention is to provide a simple substrate processing method for solving the above-mentioned problems of the prior art and obtaining a substrate surface having a flat crystal lattice plane in the sub-nanometer dimension, and a film-forming substrate obtained by this method.

[0007]

The present invention for solving the above-mentioned problems comprises the following technical means. (1) A method of treating the surface of a ceramic single crystal substrate by photoheating, which comprises the following steps; 1) preheating the substrate in a reducing atmosphere to generate light absorption centers (color centers) due to oxygen vacancies. 2) Subsequently, light irradiation is performed, and high-temperature heat treatment is efficiently performed by absorption of light energy by the light absorption centers to obtain a substrate having a flat crystal lattice plane in the sub-nanometer dimension. 3) Further And a step of removing the light absorption center by heating the substrate in an oxygen atmosphere. (2) The surface treatment method as described in (1) above, wherein the wavelength of the light energy is in a range from far infrared rays to visible rays. (3) In the normal state, the substrate has a small absorption rate of light energy, but by performing preheating in a reducing atmosphere of vacuum, hydrogen or a rare gas, the light absorption center due to oxygen deficiency is generated and The surface treatment method according to (1) above, which is a substance having an increased absorptance. (4) A film-forming substrate having a sub-nanometer-dimensional flat crystal lattice plane, which is produced by the surface treatment method according to any one of (1) to (3). (5) A method for producing an epitaxial thin film on a ceramic single crystal substrate, which comprises the following steps: 1) A substrate having a flat crystal lattice plane in the sub-nanometer dimension is produced by the method described in (1) above. Step 2) a step of producing an epitaxial multilayer laminated thin film while maintaining a clean surface without exposing the substrate surface to the atmosphere,
A method characterized in that a series of operations according to step 1 is performed in the same chamber.

Next, the present invention will be described in more detail.
It According to the present invention, the purpose is to reduce the atmosphere such as vacuum.
After preheating the substrate in the container,
It is achieved by irradiating with energy. In the present invention
As a ceramic single crystal substrate,
Light energy absorption rate is low, but vacuum, hydrogen, rare gas
By preheating in a reducing atmosphere such as
Light absorption center (color center) is generated, and the absorption rate of light energy
If the substance increases, the appropriate substance is used. This
Substrates such as, for example, strontium titanate
Mu (SrTiO₃ Barium titanate (BaTiO₃ ),
Titanium oxide (TiO₂), Tin oxide (SnO)₂ ), Oxidation
Indium (In₂ O₃ ) Etc. are illustrated, but in these
The material is not limited to the above, and any equivalent material can be used as appropriate.

In the present invention, light energy is applied to the substrate through a substrate holder (fixing table) in a vacuum or a reducing atmosphere such as hydrogen and a rare gas, heater heating, or high-power light energy irradiation. Preheat by the method
It produces (blackens) a light absorption center due to oxygen deficiency. In this case, the heating means uses a method in which the light of a high-power infrared lamp is condensed using a confocal mirror and is irradiated onto the surface of the substrate in the vacuum container through a quartz window for introducing light. The substrate is fixed on a platinum plate and is preheated by applying light energy by direct irradiation light and reflected light from the platinum plate and heat conduction by contact with the platinum plate heated by the light irradiation. The heating condition is that the output of the infrared lamp is 1k.
W, the inside of the container is kept vacuum and the degree of vacuum is 8 × 10 ⁻⁶ Pa (6 ×
10 ^-8 Torr) . Subsequently, when this is irradiated with light, light energy is efficiently absorbed by the substrate by the light absorption center and the substrate is heated. That is, the light absorption center generated by oxygen deficiency in the substrate absorbs light energy, and the substrate itself generates heat. In this case, since the preheating and the heating of the substrate surface by light irradiation occur almost at the same time, there is no clear distinction between the two processes. The means and conditions for light irradiation are the same as the conditions for preheating, and light irradiation including preheating is performed for 1 hour. When light irradiation is continued, the number of light absorption centers is further increased by heating and the light energy is absorbed at an accelerated rate, and the substrate temperature can be raised to a sufficiently high temperature for surface treatment of the ceramic single crystal substrate having a high melting point. as a result,
A surface having a crystal lattice plane with sub-nanometer smoothness is obtained.

The light absorption centers in the blackened substrate after the treatment can be removed by heat treatment in an oxygen atmosphere even at a low temperature of 700 to 800 ° C. or less.
It can be returned to a transparent substrate with no light absorption center. In addition, since a series of operations of the substrate treatment of the present invention can be continuously performed in the same container such as a vacuum chamber, a clean surface can be maintained without exposing the substrate surface after the treatment to the atmosphere. It is possible to produce an epitaxial multilayer laminated thin film as it is.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a substrate surface treatment method according to the present invention and a thin film deposited on a film-forming substrate obtained by the method will be described in detail below with reference to the drawings.
As one embodiment of the present invention, an example in which the surface treatment is performed on a strontium titanate single crystal substrate is shown. A series of treatments are performed in the same container, which is kept in a vacuum that is a reducing atmosphere.
Done in. The substrate 5 is placed on a substrate fixing base 6 such as a platinum plate for preheating, and fixed so as to face the light introduction window 4 (FIG. 1).

Light energy 2 from a high-power infrared lamp 1 of about 1 kW is irradiated onto a substrate 5 through a light introduction window 4 by using a condensing device 3 such as a confocal mirror. The substrate is transparent as shown in FIG. 2 (a), and a large absorption of light energy does not occur from the visible light region to the far infrared region. Therefore, the light energy 2 passes through the substrate 5 and mainly heats the platinum plate 6 under the substrate.

The substrate is preheated by thermal contact with the heated platinum plate 6. By being heated in a vacuum atmosphere, oxygen deficiency occurs in the substrate, and as shown in FIG. 2B, generation (blackening) of light absorption centers starts. When the substrate begins to turn black, the substrate itself absorbs light energy and generates heat. As a result, the substrate is heated to the high temperature necessary for the substrate surface treatment, and a surface having a crystal lattice plane with sub-nanometer smoothness is obtained.

A blackened substrate (FIG.
(B)) can be returned to a transparent substrate having no light absorption center by heating again in an oxidizing atmosphere.
FIG. 2 (c) is a photograph of the substrate of the present invention obtained by heating with light energy to obtain a crystal lattice plane having a sub-nanometer dimensional smoothness, and then heating again in an oxygen atmosphere to restore the transparency. is there. On the substrate surface-treated by the above method, a Y-based oxide superconductor (YBa ₂ Cu ₃
An O ₇ ) thin film was formed and an epitaxial thin film was formed on the ceramic single crystal substrate. Examples of the thin film that can be formed by the present invention include YBa ₂ Cu ₃ O ₇ and Bi ₂ Sr ₂ C.
Cu-based oxides such as a ₂ Cu ₃ O _y High temperature superconductors and La
Examples thereof include _1-x Pb _x MnO ₃ and La _1-x Sr _x MnO ₃ , Mn-based oxides exhibiting a magnetoresistive effect or a giant magnetoresistive effect.

The results regarding the substrate surface treatment and the film produced on the treated substrate will be described below. FIG. 3 is an observation result of the surface morphology of the substrate by an atomic force microscope (AFM). FIG. 3A shows the surface morphology of the strontium titanate single crystal substrate before the surface treatment. FIG. 3B shows the surface morphology of the substrate heat-treated at 800 ° C. in a vacuum container by a conventional resistance heater. FIG. 3A and FIG.
There is almost no difference between (b). That is, it is understood that the surface modification is hardly performed and the substrate surface having the crystal lattice plane cannot be obtained at the substrate temperature of about 800 ° C. On the other hand, the surface of the substrate treated by the present invention (see FIG.
In (c), steps are seen at regular intervals. The height of this step is the height of one lattice of the single crystal (0.3905n
Since it is almost the same as that of m), it indicates that a single crystal substrate having a terrace-shaped crystal lattice plane, that is, a smoothness of sub-nanometer dimension is obtained. The step occurs because the substrate surface actually cut out and polished does not completely match the (001) crystal lattice plane.

Next, FIG. 4 shows the result of surface observation by RHEED (reflection high-speed electron beam diffraction). If the surface is almost completely flat in the sub-nanometer dimension, the diffraction image of the electron beam by RHEED appears as spots (points) on the arc. FIG. 4A is an RHEED diffraction image of the surface of the substrate that has been surface-treated according to the present invention. Diffraction points are lined up on an arc. If the surface has nanometer-dimensional irregularities with respect to the lattice plane, the diffraction point becomes a linear shape called a streak extending in the longitudinal direction of the paper. Since the streak shape is not included in the diffraction points in FIG. 4A, it can be seen that a nearly perfect flat crystal lattice plane is obtained. On the other hand, the diffraction image shown in FIG. 4B is a diffraction image of the surface of the substrate processed at 800 ° C. A streak pattern appears in the longitudinal direction of the paper, which indicates that the surface still has nanometer-dimensional irregularities with respect to the crystal lattice plane. That is, similar to the result of AFM, the surface modification hardly proceeds even when the surface treatment is performed at the substrate temperature of 800 ° C. However, when the substrate surface treatment method according to the present invention is used, a crystal lattice plane having a sub-nanometer smoothness is obtained. You can see that

FIG. 5 shows the surface-treated substrate of the present invention, and FIG. 6 shows the Y-based oxide superconductor (YBa ₂ Cu ₃ O) prepared by the pulse laser deposition method on the substrate before the treatment.
₇ ) A scanning electron microscope (SEM) photograph of the surface of the thin film is shown. It can be seen that many surface particles are generated on the film (FIG. 6) formed on the substrate before the treatment. It is considered that these particles are grown by using the irregularities on the substrate as nuclei, and are particularly prominent in the case of a perovskite-based oxide composed of multiple elements. On the other hand, almost no particles are found on the surface of the film formed on the substrate according to the present invention under the same conditions, which shows that a high-quality film can be formed. The thin film is formed by exposing the surface-treated substrate once to the atmosphere from the vacuum container and observing it with an AFM, then returning it to the vacuum container again and then forming the film by the pulse laser deposition method. It is possible to obtain a film having a quality equal to or higher than that of a film obtained by continuously performing the surface treatment and the film formation without producing the film.

[0018]

EXAMPLES Next, the present invention will be specifically described based on examples, but the present invention is not limited to the examples. Example A strontium titanate single crystal substrate was placed on a platinum plate of a substrate fixing base in a vacuum container, and 1 kW infrared rays were irradiated onto the substrate through a quartz light introducing window using a light concentrating device. The vacuum vessel should be heated to 4 × 10 ^-7 Pa (3 × 10 ^-9 Tor) before heating.
Vacuum is applied up to r) , and 8 × 10 ^-6 P even during light irradiation heating
The vacuum degree of a (6 × 10 ⁻⁸ Torr ) was maintained. The substrate was preheated by the application of light energy by the direct irradiation light and the reflected light from the platinum plate and the thermal contact with the platinum plate heated by the light irradiation to generate the light absorption center by the oxygen deficiency. Subsequently, this was irradiated with light for 1 hour under the same conditions as the preheating, and the substrate temperature was heated to a high temperature to form a surface having a crystal lattice plane having a smoothness of sub-nanometer dimension. When only the small sample portion is heated to a high temperature by light heating in vacuum and the temperature is measured, a thermocouple or a radiation thermometer is used. In the method using the thermocouple, the temperature cannot be accurately measured due to the problems such as the small heat capacity of the sample and the thermal contact with the thermocouple. In addition, the heating light is scattered / absorbed in a complicated manner in the vacuum container, and the scattered light interferes with the temperature measurement. In addition, since the substrate is transparent, its accurate emissivity cannot be obtained. For this reason, the surface temperature of the substrate during heating cannot be measured even by using a radiation thermometer. Therefore, in order to estimate the substrate temperature, a tubular electric furnace (capable of heating up to 1350 ° C.) is used, and the atmospheric temperature inside the tube is 1350 ° C. and the degree of vacuum is 1.3 × 10 ⁻¹ Pa (1 ×
A substrate heating experiment was conducted under the condition of 10 ⁻³ Torr) . However, since the generation of the light absorption center did not occur even at this temperature, the temperature of the substrate reached by the light heating was 1350.
It was estimated to be higher than ° C. Next, the substrate which has been surface-treated and blackened by the above-mentioned light heating method is subjected to 80 ° C. in an oxygen atmosphere.
A heat treatment was carried out at 0 ° C. to remove the light absorption center, and the transparent substrate was returned. It was confirmed by AFM measurement that the smoothness of sub-nanometer was not changed even after this treatment. On this substrate, a Y-based high-temperature oxide superconductor (YBa ₂ Cu ₃ ) was formed by a pulse laser deposition method using the fourth harmonic of a YAG solid-state laser.
A thin film of O ₇ ) was prepared. Manufacturing conditions are substrate temperature 740
C., the oxygen pressure during vapor deposition is 50 Pa. The film thickness of the vapor-deposited thin film is about 30 nm.

[0019]

As described above, strontium titanate (SrTiO ₃ ) and barium titanate (BaTi) are used.
O ₃ ), titanium oxide (TiO ₂ ), tin oxide (SnO)
₂ ), Indium Oxide (In ₂ O ₃ ) and other substrates with low light absorption and high melting point can generate high output light energy if they generate light absorption centers by heating in a reducing atmosphere. A substrate surface having a crystal lattice plane with a sub-nanometer dimension of smoothness can be obtained by a simple method of giving. When the substrate of the present invention is used, generation of impurity surface particles can be significantly suppressed when forming a film such as an oxide. The generation of surface particles is a serious problem in forming a laminated element of a film which is a basic technology of oxide electronics, and the substrate according to the present invention breaks through this problem.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a surface treatment method for a ceramic single crystal substrate according to an embodiment of the present invention.

FIG. 2 (a) is a photograph of a strontium titanate substrate before surface treatment. (B) is a photograph of a substrate on which pre-heating was performed and light absorption centers were introduced (blackened) in the process of surface treatment according to an embodiment of the present invention. (C) is the substrate of the present invention in which the light absorption center is removed by heating in an oxygen atmosphere after the surface treatment by light heating.

FIG. 3A is a view before performing a surface treatment, FIG. 3B is a resistance heater heating method to 800 ° C., and FIG. 3C is an interatomic surface of a substrate surface-treated according to one embodiment of the present invention. It is an image observed by a force microscope (AFM).

FIG. 4A is a reflection type high-energy electron diffraction pattern of a substrate surface which is surface-treated according to one embodiment of the present invention, and FIG. 4B is a substrate surface heated to 800 ° C. in vacuum by a resistance heater heating method ( It is a diffraction image by RHEED).

FIG. 5 is a scanning electron microscope (SEM) photograph of the surface of the Y-based oxide superconductor (YBa ₂ Cu ₃ O ₇ ) thin film deposited on the substrate after the surface treatment according to the present invention.

FIG. 6 is a scanning electron microscope (SEM) photograph of the surface of the Y-based oxide superconductor (YBa ₂ Cu ₃ O ₇ ) thin film deposited on the substrate before the surface treatment according to the present invention.

[Explanation of symbols]

1 Light energy source (high-power infrared lamp, etc.) 2 Light energy 3 Focusing device (confocal mirror, etc.) 4 Light introduction window (quartz, etc.) 5 Ceramic single crystal substrate (strontium titanate, etc.) 6 Substrate fixing table (platinum plate) Etc.) 7 containers (containers such as vacuum chambers kept in a reducing atmosphere such as vacuum)

Continuation of the front page (56) Reference JP-A-64-20689 (JP, A) JP-A-2-263799 (JP, A) JP-A-4-83714 (JP, A) JP-A-4-260695 (JP , A) JP-A-6-183894 (JP, A) JP-A-6-234595 (JP, A) JP-A-8-83802 (JP, A) JP-A-8-102560 (JP, A) Kusumori Tsuyoshi Others, Surface treatment technology for strontium titanate single crystal substrate, Report of Nagoya Institute of Technology, August 25, 2000, Vol. 49, No. 1, pp. 87-90 (58) Fields investigated (Int.Cl. ⁷ , DB name) C30B 1/00-35/00 WPI (DIALOG) JSTPlus (JOIS)

Claims (5)

(57) [Claims] 1. A method for treating the surface of a ceramic single crystal substrate by photoheating, comprising the steps of: 1) preheating the substrate in a reducing atmosphere to obtain light absorption centers (color centers) due to oxygen vacancies. 2) Subsequently, light irradiation is performed, and high-temperature heat treatment is efficiently performed by absorption of light energy by the light absorption centers to obtain a substrate having a flat crystal lattice plane in the sub-nanometer dimension. ) Further, a step of removing the light absorption center by heating the substrate in an oxygen atmosphere, and a substrate surface treatment method. 2. The wavelength of the light energy is in the region from far infrared rays to visible light.
The surface treatment method described in. 3. The substrate has a small absorption rate of light energy in a normal state, but the substrate is preheated in a reducing atmosphere of vacuum, hydrogen or a rare gas to absorb the light energy due to oxygen deficiency. 2. The surface treatment method according to claim 1, wherein the substance is a substance that has a center and an absorption rate of light energy increases. 4. A film-forming substrate having a sub-nanometer-dimensional flat crystal lattice plane, which is produced by the surface treatment method according to any one of claims 1 to 3. 5. A method for producing an epitaxial thin film on a ceramic single crystal substrate, which comprises the following steps: 1) A substrate having a flat crystal lattice plane in the sub-nanometer dimension by the method according to claim 1. And 2) a step of producing an epitaxial multilayer laminated thin film while keeping a clean surface without exposing the surface of the substrate to the atmosphere, and performing a series of operations in the same chamber.