JP2000086400A - Production of oxide single crystal substrate nd electronic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide both a method for production by which an oxide single crystal substrate having a substrate surface flat on the atomic level and good in crystallinity can efficiently and stably be produced at a low cost and an electronic device having an oxide superconductor thin film or an oxide insulator thin film good in crystallinity. SOLUTION: This method for producing an oxide single crystal substrate comprises (a) a polishing step for polishing the surface of the oxide single crystal substrate, (b) a surface removing step for applying ultrasonic vibration to the oxide single crystal substrate in a liquid and removing the surface part in which the stress due to the polishing remains and (c) a heat-treating step for heat-treating the oxide single crystal substrate after removing the surface. An oxide superconductor thin film or an oxide ferroelectric thin film is epitaxially grown on the oxide single crystal substrate after passing through the steps. Thereby, the oxide single crystal superconductor thin film or the oxide insulator thin film good in crystallinity can be formed on the surface of the oxide single crystal substrate. As a result the high-quality electronic device using the oxide single crystal superconductor thin film or the oxide insulator thin film can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for manufacturing an oxide single crystal substrate on which an oxide superconductor thin film or an oxide ferroelectric thin film is epitaxially grown, and particularly to an oxide having good crystallinity on the substrate surface. Single crystal substrate manufacturing method, and
The present invention relates to an electronic device using an oxide ferroelectric thin film or an oxide single crystal substrate having an oxide ferroelectric thin film.

[0002]

2. Description of the Related Art An oxide superconductor is attracting attention as a material exhibiting high-temperature superconductivity, and an element utilizing the property is being developed. Typical examples are a Josephson element that realizes high-speed switching by using superconducting characteristics between superconductors with an insulator interposed, and a brain generated by using the magnetic flux sensing characteristics of a Josephson junction. Superconducting quantum interference device (SQUI)
D), and superconducting transistors and electromagnetic wave sensors. In addition, devices utilizing the ferroelectric characteristics of oxide ferroelectrics include non-volatile memories and infrared sensors, which are being put to practical use and commercialized. Many of these devices are researched, developed, and manufactured using oxide superconductor thin films or oxide ferroelectric thin films formed on oxide single crystal substrates. There is an increasing demand for substrates for epitaxial growth of superconductor thin films and oxide ferroelectric thin films.

[0003] Since the electrical properties of oxide superconductors and oxide ferroelectrics have anisotropy, that is, a property that strongly depends on the crystal orientation, it is necessary to obtain a high-quality functional element having stable properties. It is necessary to use an oxide superconductor thin film and an oxide ferroelectric thin film having an aligned crystal axis direction. Therefore, it is necessary to use an oxide single crystal substrate which is close to the film in lattice constant and coefficient of thermal expansion, does not react with the film, and does not cause a phase transition within the operating temperature range of the film, as the substrate for epitaxial growth. SrTiO ₃ (strontium titanate), LaAIO ₃ (aluminum lanthanate), Lr
aGaO ₃ (lanthanum gallate), NdGaO ₃ (neodymium gallate), MgO (magnesium oxide) and the like have been widely used in research and development.

However, when the oxide single crystal substrate is polished with hard abrasive grains such as diamond used for the surface finishing of a normal single crystal substrate, the surface portion deformed due to polishing (work-affected layer) Reaches a depth of several μm,
The crystallinity and flatness of the substrate surface are significantly impaired.
Therefore, conventionally, a surface treatment method has been adopted in which a polishing treatment is performed by mechanochemical polishing using soft abrasive grains such as colloidal silica or a chemical polishing using a chemical agent, or a substrate surface is sputter-etched before film formation. . However, the crystallinity and flatness of the surface of the oxide single crystal substrate treated by such a surface treatment method can be improved by epitaxially growing a thin film having high-quality crystal characteristics, or by using it for an artificial lattice or a laminated device. It was not enough to apply.

Therefore, research and development of a surface finishing method for a high-quality single-crystal substrate in which an atomic plane having high crystallinity is exposed has been made for SrTiO ₃ single crystal. One of the results of this research is that crystals have a unique crystal structure in which they are regularly arranged in a lattice pattern due to the bonding force between atoms and molecules. Focusing on the arrangement, SrTiO
_{3 A} single crystal (crystal plane (100)) is heat-treated at 950 ° C. in air to rearrange the atoms and molecules on the surface portion distorted by various processings, thereby forming one crystal lattice unit (a-axis length: 3.9).
Å) A method for obtaining a substrate surface having a table-like atomic step surface having a step (R. Sum;
Physica C, 235-240 (1994) 621-622). Also, Sr
A TiO ₃ single crystal substrate (crystal face (100)) is alternately immersed in an acid solution (for example, a mixed solution of hydrofluoric acid and ammonium fluoride) and water to repeatedly etch the substrate surface, thereby obtaining a single crystal substrate. Crystal lattice unit (a-axis length: 3.9
(Ii) A method of obtaining a substrate surface having a step surface having a step has also been proposed (Japanese Patent Application Nos. 07-267800 and 08-920).
No. 00).

[0006]

However, since stress due to polishing remains on the polished surface of the substrate, the crystallinity of the substrate surface obtained by simply heat-treating it is not very good. As an example, FIG. 5A shows a surface shape image of the polished strontium titanate substrate (crystal face (100)) by an atomic force microscope. As shown in this surface profile image, at the stage of polishing, although there is flatness of 1 nm or less, atomic steps are not observed and crystallinity cannot be said to be good. If the high-speed reflection electron diffraction image also has good crystallinity, a spot-like bright spot should be observed on the circumference, but the result is streak-like and the crystallinity cannot be said to be good. In addition, even if heat treatment is performed in this state to rearrange the crystals on the surface, as shown in the surface shape image by the atomic force microscope in FIG. But not enough. In addition, the method of repeatedly immersing the substrate in an acid solution and water and repeatedly etching the substrate surface can obtain a substrate surface with good crystallinity, but the work is complicated because the etching process is repeated. In addition, since the temperature and pH of an acid solution used for etching must be strictly controlled, it is not suitable as a method for efficiently and stably producing a large number of substrates in a factory or the like. The problem to be solved by the present invention is to provide an oxide single crystal substrate which can efficiently and inexpensively and stably produce an oxide single crystal substrate having a substrate surface which is flat at the atomic level and has good crystallinity. Another object of the present invention is to provide an electronic device formed by forming an oxide superconductor thin film or an oxide insulator thin film having good crystallinity on the surface of an oxide single crystal substrate.

[0007]

In order to solve the above problems, in a method for manufacturing an oxide single crystal substrate according to the present invention, an ultrasonic vibration is applied to a polished oxide single crystal substrate in a liquid. Thus, while the surface of the substrate remains flat, the surface portion where the stress due to polishing remains (processed layer) is forcibly removed by ultrasonic vibration, and then the oxide single crystal substrate is subjected to heat treatment (annealing). The crystals on the surface were rearranged. In this manufacturing method, it is desirable that the surface portion forcibly removed by the ultrasonic vibration be a portion within 10 nm in the thickness direction from the substrate surface. In this case, it is desirable to appropriately use pure water or an organic solvent such as ethanol, acetone, or isopropyl alcohol depending on the chemical properties of the substrate as the liquid in which the oxide single crystal substrate is immersed. Further, it is preferable that the heat treatment after the surface treatment be performed at a temperature that is approximately half the melting point of the oxide single crystal substrate in absolute temperature. The heat treatment after the surface treatment is desirably performed in air or oxygen. Further, an electronic device according to the present invention is obtained by epitaxially growing an oxide superconductor thin film or an oxide ferroelectric thin film on the surface of an oxide single crystal substrate manufactured by the method of the present invention. Sr is used as a material of the oxide single crystal substrate of this electronic device.
TiO ₃ (strontium titanate), LaAIO ₃
(Aluminum lanthanate), LaGaO ₃ (lanthanum gallate), NdGaO ₃ (neodymium gallate), or MgO (magnesium oxide) is preferably used.

[0008]

Embodiments of the present invention will be described below. FIG. 1 is a process explanatory view showing an embodiment of a manufacturing method according to the present invention. As shown, the manufacturing method according to the present invention includes (1) a polishing step, (2) a surface removing step, and (3) a heat treatment step. (1) In the polishing step, an oxide single crystal substrate sliced in a plate shape along a desired crystal orientation is subjected to polishing using hard abrasive grains such as diamond or soft abrasive grains such as colloidal silica. (2) In the surface removing step, the polished oxide single crystal substrate is placed in a container containing a liquid (eg, pure water, an organic solvent, or the like) and subjected to ultrasonic vibration using a known ultrasonic cleaning device. Is applied, the substrate surface and the vicinity of the surface where the polishing stress remains are forcibly removed while leaving the substrate surface flat. However, the surface portion of the oxide single crystal substrate to be removed in this step is 10n from the substrate surface in the thickness direction.
m. If the thickness exceeds 10 nm, the flatness of the substrate surface may be impaired. (3) In the heat treatment step, the oxide single crystal substrate from which the surface has been removed is transferred into an electric furnace and subjected to heat treatment, whereby the crystal on the substrate surface is rearranged. This heat treatment is desirably performed at a temperature that is almost half the melting point of the oxide single crystal substrate in absolute temperature. If the temperature of the heat treatment is too high, the crystallinity of the substrate may be impaired, and if the temperature is too low, the crystal rearrangement cannot be favorably promoted. This heat treatment is desirably performed in oxygen. This is because when heat treatment is performed in an atmosphere including a large amount of a substance other than oxygen, the crystal component of the oxide single crystal substrate changes.

As described above, by subjecting the polished oxide single crystal substrate to ultrasonic vibration in a liquid, the surface portion where the polishing stress remains remains while maintaining the substrate surface flatness. To produce an oxide single-crystal substrate that is flat at the atomic level and has good surface crystallinity by heat-treating the oxide single-crystal substrate and rearranging the crystals on the substrate surface after forcible removal by vibration. Can be. Therefore, the oxide superconductor thin film or oxide ferroelectric thin film is epitaxially grown on the oxide single crystal substrate manufactured by the above method, so that the oxide single crystal substrate has good crystallinity on the surface thereof. A conductor thin film or an oxide insulator thin film can be formed, whereby a high-quality electronic device using the oxide superconductor thin film or the oxide insulator thin film can be obtained. In addition, the above method can significantly reduce the number of processing steps and reduce the acid used for etching as compared with the conventional method in which the substrate surface is repeatedly etched by immersing the substrate in an acid solution and water alternately. Since the method does not include a process that requires more rigor than control of the temperature and pH of the solution, it is effective as a method for efficiently and stably producing a large amount of substrates in a factory or the like.

[0010]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

Example 1 An SrTiO ₃ substrate (crystal face (1
00)), and c-axis-oriented YBa ₂ C
A u ₃ O ₇ single crystal thin film was prepared. First, SrTi after polishing
After removing the organic and inorganic contaminants on the surface of the O ₃ substrate (crystal face (100)), the SrTiO ₃ substrate is placed in a liquid tank of an ultrasonic cleaning apparatus, and ultrasonic vibration is applied in the liquid to obtain the SrTiO ₃ substrate. The surface portion of the SrTiO ₃ substrate was mechanically removed. Acetone was used as the liquid, and a commercially available ultrasonic cleaning device (40 kHz-150 W) was used. On the surface of the SrTiO ₃ substrate after the surface treatment, as shown in the surface shape image by the atomic force microscope in FIG. 1A, the surface portion where the stress due to polishing remained was uniformly removed, and the unevenness was maintained at 1 nm. ing. Thereafter, heat treatment was performed in air. The temperature was raised at a rate of 10 ° C./min and kept at 1000 ° C. for 2 hours, and then lowered to room temperature at a rate of 10 ° C./min. S after the heat treatment
_On the surface of the rTiO ₃ substrate, clear atomic steps were observed as shown in the surface shape image by the atomic force microscope in FIG. 1B, and the height was about 4 ° corresponding to the a-axis length of SrTiO ₃ . FIG. 1 (c) is an enlarged view of a part of FIG. 1 (b), and a table-like atomic step plane having a step of one crystal lattice unit is clearly observed. The atomic steps are very clear compared to the surface shape image of the SrTiO ₃ substrate heat-treated without performing the ultrasonic cleaning treatment (FIG. 5B), and the heat treatment is performed after performing the ultrasonic cleaning treatment. It can be seen that the crystallinity of the SrTiO ₃ substrate surface was significantly improved. As shown in FIG. 2, a spot-like bright spot was also observed on the high-speed reflection electron diffraction image in which an electron beam was incident from the (100) direction, indicating that the crystallinity of the surface was very good. ing.

On the SrTiO ₃ substrate (crystal plane (100)) thus prepared, YBa is applied by molecular beam epitaxy.
The ₂ Cu ₃ O ₇ films were grown. As a result of observing the crystallinity of the thin film with a high-speed reflection electron diffraction image, the YBa ₂ Cu ₃ O ₇ thin film was epitaxially grown in a c-axis orientation matching the crystallinity of the SrTiO ₃ substrate. SrT manufactured by conventional method
When a YBa ₂ Cu ₃ O ₇ thin film is grown on an iO ₃ substrate, the film thickness increases and the crystallinity of the thin film deteriorates, but the YBa ₂ Cu ₃ O ₇ thin film deteriorates on the SrTiO ₃ substrate manufactured by the method of the present invention.
When a Ba ₂ Cu ₃ O ₇ thin film was grown, the crystallinity of the thin film did not deteriorate even when the film thickness was increased, and a uniform and high quality thin film could be formed.

[Example 2] MgO substrate (crystal face (100))
The surface-treated, zirconium titanate c-axis oriented on treated surface (PZT: Pb (Zr0.52Ti0.48) O 3) created the single crystal thin film. First, the polished MgO substrate (crystal face (10
0)) After removing organic and inorganic stains on the surface,
The surface portion of the MgO substrate was mechanically removed by placing the MgO substrate in a liquid tank of an ultrasonic cleaning device and applying ultrasonic vibration in the liquid. Since MgO is slightly soluble in water, MgO is used here in isopropyl alcohol.
The substrate was subjected to ultrasonic cleaning. A commercially available ultrasonic cleaning device (40 kHz-150 W) was used. M after the above surface treatment
On the surface of the gO substrate, as shown in the surface shape image by the atomic force microscope in FIG. 4A, the surface portion where the stress due to polishing remained remains uniformly removed, and the unevenness is maintained at 1 nm. Thereafter, heat treatment was performed in air. The temperature was raised at a rate of 5 ° C./min, kept at 1100 ° C. for 2 hours, and then lowered to a room temperature at a rate of 5 ° C./min. On the surface of the MgO substrate after the heat treatment, clear atomic steps were observed as shown by the surface shape image by the atomic force microscope in FIG. 4B, and the height was about 4.2 ° corresponding to the a-axis length of MgO. Met. Atomic steps were very clear and a crystal plane flat at the atomic level was observed, as compared with the case where the heat treatment was performed without performing the ultrasonic cleaning treatment. It can be seen that the heat treatment after the ultrasonic cleaning treatment significantly improved the crystallinity of the MgO substrate surface.

A platinum (Pt) thin film (crystal face (100)) is epitaxially grown on the MgO substrate (crystal face (100)) thus prepared at 600 ° C. by a high-frequency sputtering method. Similarly, a PZT thin film was grown at 600 ° C. by a high frequency sputtering method to a thickness of 200 nm. As a result of analyzing the PZT thin film by the high-speed reflection electron diffraction image,
Epitaxial growth was performed in a c-axis orientation matching the crystallinity of the gO ₃ substrate. A gold (Au) electrode having a diameter of 0.25 mm is formed on a PZT thin film, and a P electrode between the Au electrode and the Pt thin film is formed.
The dielectric and ferroelectric properties of the ZT thin film were measured. As a result, relative dielectric constant 100, dielectric loss 0.04, remanent polarization 2
0 μC / cm ² and coercive electric field of 100 kV / cm, showing good electrical characteristics.

[0015]

As described above, according to the method for manufacturing an oxide single crystal substrate according to the present invention, an oxide single crystal substrate having a substrate surface that is flat at the atomic level and has good crystallinity can be efficiently formed. It can be manufactured stably at low cost. In addition, the electronic device of the present invention, in which an oxide superconductor thin film or an oxide insulator thin film is epitaxially grown on the surface of the oxide single crystal substrate manufactured by this manufacturing method, has an excellent oxide superconductivity. Since the semiconductor device has a body thin film or an oxide insulator thin film, various devices using these thin films can be manufactured with high quality.

[Brief description of the drawings]

FIG. 1 is a process explanatory view showing an embodiment of a manufacturing method according to the present invention.

FIG. 2A is a view showing a surface shape image of a SrTiO ₃ substrate after surface treatment in a manufacturing method according to the present invention, and FIG.
Is SrTiO after heat treatment in the manufacturing method according to the present invention.
₃ shows a surface shape image of the substrate, (c) are diagrams showing a surface shape image of a partially enlarged in (b).

FIG. 3 shows Sr after heat treatment in the manufacturing method according to the present invention.
FIG. ₃ is a diagram showing a high-speed reflection electron diffraction image of a TiO ₃ substrate.

4A is a diagram showing a surface shape image of an MgO substrate after a surface treatment in the manufacturing method according to the present invention, and FIG. 4B is a diagram showing a surface shape image of the MgO substrate after the heat treatment in the manufacturing method according to the present invention. FIG.

5A is a diagram showing a surface shape image of a polished SrTiO ₃ substrate, and FIG. 5B is a diagram showing a surface shape image of a SrTiO ₃ substrate after a heat treatment in a conventional manufacturing method.

Claims (8)

[Claims] A polishing step of polishing a surface of the oxide single crystal substrate; and a surface removing step of applying ultrasonic vibration to the oxide single crystal substrate in a liquid to remove a surface portion where stress due to polishing remains. And a heat treatment step of heat-treating the oxide single crystal substrate after surface removal. 2. The method for manufacturing an oxide single crystal substrate according to claim 1, wherein a surface portion to be removed in the surface removing step is a portion having a thickness of 10 nm or less from a substrate surface. 3. The heat treatment temperature in the heat treatment step is as follows:
3. The method for producing an oxide single crystal substrate according to claim 1, wherein the melting point of the oxide single crystal substrate is substantially half of the melting point of the oxide single crystal substrate. 4. The method for manufacturing an oxide single crystal substrate according to claim 1, wherein the heat treatment in the heat treatment step is performed in air or oxygen. 5. The method according to claim 1, wherein the liquid in the surface removing step is an organic solvent. 6. An electronic device, comprising an oxide superconductor thin film epitaxially grown on a surface of an oxide single crystal substrate manufactured by the manufacturing method according to claim 1. 7. An electronic device, wherein an oxide ferroelectric thin film is epitaxially grown on a surface of an oxide single crystal substrate manufactured by the manufacturing method according to claim 1. 8. The oxide single crystal substrate according to claim 6, wherein one of strontium titanate, aluminum lanthanate, lanthanum gallate, neodymium gallate, and magnesium oxide is used as the material of the oxide single crystal substrate. Electronic devices.