JP4423835B2 - Method for manufacturing dielectric oxide film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a dielectric oxide film, which is formed by applying a voltage to a substrate immersed in a solution containing metal ions.
[0002]
[Prior art]
Conventionally, as a method for forming a dielectric oxide film on a substrate, a dry film forming method represented by a sputtering method or a CVD method, and a wet film forming method represented by a sol-gel method or an electrophoretic film forming method are used. An example is a membrane method. The wet film-forming method has a feature that a film can be formed in a large area with simple equipment, and the composition variation is extremely small when an oxide composed of a plurality of metal ions is formed.
[0003]
In the sol-gel method, since a chemical reaction between metal ions and oxygen is used in a solution, an oxide can be formed at a relatively low temperature, and a complex oxide film that is difficult to produce is formed. There are advantages such as being able to. Furthermore, in the high-concentration sol-gel method (H.Simooka and M.Kuwabara, J. Am. Ceram. Soc., 78, 2849 (1995)) in which the metal ion concentration proposed by Kuwahara et al. Reduces the shrinkage strain, which is a disadvantage of the sol-gel method, and enables crystallization at about 100 ° C. by controlling the hydrolysis reaction.
[0004]
On the other hand, the electrophoretic film forming method is a method in which particles such as oxides are dispersed in a liquid, and charged particles are deposited on a substrate to which a voltage is applied to form a film. By reducing the particle diameter to 1 μm or less, a thin, dense and smooth film can be produced. However, it is difficult to make the particle size smoother and denser than the particle size, and heat treatment at 1000 ° C. or higher is necessary after film formation in order to achieve this.
[0005]
Therefore, a method is considered in which fine particles are precipitated from a solution and used instead of adding particles from the outside. Japanese Patent Application Laid-Open No. 5-163005 “Oxide Superconducting Current-Limiting Conductor Manufacturing Method” and Japanese Patent Application Laid-Open No. 8-53776 “Metal Oxide Coating Method and Metal Oxide Coating” In the membrane method, a precursor solution mainly composed of cations constituting oxides is hydrolyzed, sol particles containing these cations are generated from the liquid, and the solution in which these particles are dispersed is used for electricity. Film formation is performed by the electrophoresis method. In this way, fine particles of 1 μm or less are obtained, and a dense film is produced by electrophoresis.
[0006]
[Problems to be solved by the invention]
The dielectric oxide film uses its dielectric properties or properties as an insulator, and those properties are also used for materials having piezoelectricity and ferroelectricity. At that time, there has been no particular problem even if the dielectric oxide film is opaque.
[0007]
However, in recent years, with the spread of optical communication, the demand for new optical elements that process optical signals has increased, and among them, optical elements such as optical waveguides using dielectric oxide films with electro-optical properties have been proposed. Has been. These elements are formed by forming a dielectric oxide film having a desired shape such as a planar shape or a strip shape on a substrate, and the dielectric oxide film is transparent with a thickness of several μm to transmit light. It is desirable.
[0008]
The conventional dry film formation method and sol-gel method are suitable for producing a transparent oxide film. However, in order to obtain a crystalline film, a heat treatment of at least 500 ° C. is required. Further, in the former, it is difficult to form a uniform film over a large area with a composition variation problem, and in the latter, it is difficult to make an arbitrary shape and to increase the film thickness because contraction strain occurs. Furthermore, if the above-described high-concentration sol-gel method is used, a thick and transparent oxide film can be produced, but it is difficult to produce a desired shape on the substrate.
[0009]
On the other hand, in the case of the electrophoretic film forming method, it is easy to form a thick film, and an oxide film can be formed into an arbitrary shape by changing the electrode shape. Furthermore, if crystallized particles are used, heat treatment after film formation can be eliminated.
[0010]
However, a method of forming a film by electrophoresis using a solution in which sol particles obtained by hydrolyzing a precursor compound are dispersed as disclosed in JP-A-5-163005 and JP-A-8-53776. Is a method of forming a film using particles that are visible even though it is 1 μm or less, it is difficult to obtain an optically transparent film and to produce a fine film structure of 1 μm or less.
[0011]
The precursor compound that has undergone hydrolysis is subsequently subjected to polycondensation to increase the molecular weight to form sol particles, increasing the viscosity of the solution and forming a broad sol state. The electrophoretic electrodeposition method in the prior art described above uses this sol-state solution. Thus, when particles that can be seen even though they are particles of 1 μm or less are not transparent, it is difficult to make them transparent after heat treatment at a high temperature at which a liquid phase sintering reaction occurs.
[0012]
Accordingly, an object of the present invention is to improve a conventional high-concentration sol-gel method, and to produce a transparent dielectric oxide film having an arbitrary shape such as a planar shape or a belt shape without heat treatment at a high temperature after the film formation. Is to provide.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, a method for producing a dielectric oxide film of the present invention comprises:
Providing a precursor solution containing metal ions constituting the dielectric oxide, the precursor solution having a concentration from substantially saturated to supersaturated,
Hydrolyzing the precursor solution at a temperature equal to or lower than a temperature at which gelation is performed by hydrolysis and subsequent condensation polymerization reaction to obtain a transparent sol solution;
Forming a gel film equal to the solute composition of the sol solution on the cathode side electrode by applying a voltage to a pair of electrodes immersed in the sol solution;
Heat-treating the gel film to form an oxide film;
It is characterized by providing.
[0014]
And the precursor solution containing the metal ion is an organic alkoxide, a metal acetylacetonate, an inorganic salt, and at least one of those obtained by substituting a part of the functional groups of these compounds with other functional groups. It is characterized by being a transparent solution dissolved in a solvent.
[0015]
In addition, the step of forming the gel film includes a step of applying a direct current voltage to the pair of electrodes to form a gel film on the cathode side electrode, or a step of applying a voltage in which a direct current is superimposed on the pair of electrodes. And a step of forming a gel film on the low voltage side electrode.
[0016]
The cathode-side electrode is a plate, rod, or wire made of a conductive material, and the conductive material includes a surface of an insulating material in which a conductive material is formed entirely or partially. And
[0017]
Further, the precursor solution is a solution in which an alkoxide of Ba and Ti is dissolved in an organic solvent, and the dielectric oxide film is BaTiO ₃ .
[0018]
DETAILED DESCRIPTION OF THE INVENTION
The method for producing a dielectric oxide film of the present invention includes:
(1) preparing a precursor solution containing a metal ion constituting the dielectric oxide, the precursor solution having a concentration from the saturation to the supersaturation,
(2) a step of hydrolyzing the precursor solution at a temperature equal to or lower than a temperature at which gelation is performed by hydrolysis and subsequent condensation polymerization reaction to obtain a transparent sol solution;
(3) forming a gel film equal to the solute composition of the sol solution on the cathode side electrode by applying a voltage to a pair of electrodes immersed in the sol solution;
(4) a step of heat-treating the gel film to form an oxide film;
It has.
[0019]
The dielectric oxide film manufacturing method of the present invention is a method of forming a film by electrophoresis using a sol solution obtained by hydrolyzing a precursor solution. Then, when the precursor solution is hydrolyzed to form a sol solution, an optically transparent sol solution is prepared by using the sol containing cluster particles having a small molecular weight by suppressing the condensation polymerization reaction.
[0020]
As a method for suppressing the polycondensation reaction, a method of hydrolyzing at a temperature equal to or lower than the temperature at which gelation occurs by hydrolysis and subsequent polycondensation reaction is employed. This temperature varies depending on the raw materials and solvent used and the amount of water added. For example, Ba ethoxide and Ti isopropoxide are dissolved at a concentration of 1 mol / L in a solvent in which methanol and ethylene glycol monomethyl ether are mixed at a ratio of 3: 2, and then hydrolyzed by adding water 8 times the amount of Ba ions. When it is, it is about 0 ° C. When the amount of water is 10 times, it is about -10 ° C.
[0021]
Since the sol containing cluster particles generated by hydrolysis is positively charged, when a direct current electric field is applied to the pair of electrodes, the sol moves to the cathode side and adheres to the substrate used for the cathode side electrode. When the voltage is further applied, the sol containing cluster particles starts to aggregate on the substrate, changes from the sol state to the gel state, and forms a gel film equal to the solute composition of the sol solution on the substrate. A transparent dielectric oxide film can be obtained by heat-treating this gel film.
[0022]
If particles having a diameter of about 0.1 μm are formed in the sol solution, the particles are present with the grain boundaries left in the dry state. However, such a grain boundary is not observed in the film obtained by the present invention. Even if the particles are finer than that, grain boundaries are confirmed by grain growth if heat treatment is performed at a high temperature. In the case of a film obtained by heat-treating the film prepared according to the present invention at 700 ° C. and strongly crystallized, the observed particle size is as small as about several tens of nm, which is about the same as the film manufactured by the vapor phase growth method. That is, in the present invention, the size of the particles that move by electrophoresis is very fine and is determined to be about the size of cluster particles, and it is considered that the sol solution is very close to the liquid phase.
[0023]
In the present invention, a high-concentration precursor solution whose concentration ranges from substantially saturated to supersaturated is used. For this reason, shrinkage | contraction distortion can be reduced and the gel and oxide without a crack can be obtained.
[0024]
In this high-concentration precursor solution, by applying a film forming method in which an electric field is applied to the sol solution containing the cluster particles, a planar shape having a sufficient thickness on the substrate by a low-temperature heat treatment of 500 ° C. or lower. Alternatively, a transparent dielectric oxide film having an arbitrary shape such as a belt shape can be formed, and an optical waveguide or the like can be manufactured.
[0025]
Examples of the dielectric oxide include perovskite compounds represented by the general formula ABO ₃ (where A is Ba, Sr, Ca, Pb, etc., B is Ti, Zr, Sn, etc.), specifically ( Ba, Sr) TiO ₃ , PbZrO ₃ , (Pb, Ca) TiO _{3 and the} like.
[0026]
As the solute of the precursor solution, inorganic acid salts such as metal alkoxides, metal acetylacetonates, and acetates containing the metal constituting the dielectric oxide are used. In addition, a compound in which a part of the functional group of these compounds is substituted with another functional group, for example, a part of the alkyl group is substituted with a carboxyl group can be used.
[0027]
Examples of the organic solvent include various solvents such as alcohols such as methanol and ethanol, and ethers such as ethylene glycol monomethyl ether and ethylene glycol monoethyl ether, which are used appropriately in combination with the precursor to be dissolved.
[0028]
A DC voltage is used as the voltage applied when forming the gel film. It is also possible to control the adhesion of the gel film by superimposing alternating current on direct current and changing the waveform, period and amplitude of this alternating current.
[0029]
As the electrode on the cathode side for forming the gel film, a plate, rod, wire, or insulating material made of a conductive material and a conductive material formed entirely or partially on the surface is used. At this time, the gel film can be selectively formed by making the electrode to be formed into an arbitrary shape. It is also possible to form a gel film with an electrode having an arbitrary shape by applying a mask using a photoresist or the like over the conductive film formed on the entire surface of the conductive substrate or the insulator substrate.
[0030]
【Example】
(Example 1)
As an embodiment of the present invention, a method for producing a BaTiO ₃ film as a dielectric oxide film, that is, a method for producing a BaTiO ₃ film on a Pt film formed on a Si wafer will be described with reference to FIG. FIG. 1 is a cross-sectional view schematically showing an apparatus for producing a dielectric oxide film by the method of the present invention.
[0031]
First, a precursor solution was prepared by dissolving Ba ethoxide and Ti isopropoxide in a solvent so as to have the same molar ratio as the metal ion composition constituting the BaTiO ₃ film as the dielectric oxide film. As the solvent, a liquid obtained by mixing methanol and ethylene glycol monomethyl ether containing no water in a ratio of 3: 2 was used. The solute concentration was 1.1 mol / L close to the saturation concentration.
[0032]
Next, this precursor solution is transferred to a glass container 1 containing dry nitrogen gas, and further, the glass container is placed in a bath at −30 ° C., which is equal to or lower than the temperature at which gelation occurs by hydrolysis and subsequent condensation polymerization reaction. Soaked. In a cooled state, the precursor solution was hydrolyzed by adding 8 times the molar amount of water to the molar amount of Ba ions. Then, it was made -10 degreeC over 1 hour, moved to a 0 degreeC thermostat, and performed aging, and the sol solution 2 containing a cluster-form sol was produced.
[0033]
Next, the substrate 3 and the anode electrode 4 were inserted into the glass container 1 containing the sol solution 2. The substrate 3 is obtained by forming a Pt film on a Si wafer subjected to surface oxidation treatment by a sputtering method. The anode electrode 4 is a stainless steel rod. These insertions were performed under a dry nitrogen atmosphere so that the sol solution 2 was not exposed to the air.
[0034]
Next, after the glass container 1 is moved to the bath 5 set to −20 ° C. with the refrigerant 6, the substrate 3 and the anode electrode 4 are connected to a DC power source (not shown) so that the substrate 3 becomes a cathode. Was applied. As the voltage was increased, a transparent gel film 7 was deposited on the substrate 3. FIG. 2 shows the adhesion amount per unit area of the gel film to the cathode-side electrode when the voltage is changed with the application time kept constant for 10 minutes. When the voltage is low, it does not adhere much, but the amount of adhesion increases at 10V or more.
[0035]
Next, the substrate 3 with the gel film 7 is taken out and a transparent dielectric oxide film crystallized into BaTiO ₃ is obtained by aging in the atmosphere at 50 ° C. for 2 days and further at 150 ° C. for 1 day. Obtained. FIG. 3 shows an X-ray diffraction (XRD) chart showing that the obtained dielectric oxide film is crystallized into BaTiO ₃ .
[0036]
(Example 2)
Photoresist was applied to the entire surface of the substrate 3 on which the Pt film shown in Example 1 was formed to a thickness of 5 μm, and then a strip-shaped figure having a width of 2 μm was exposed and developed. As a result, a band-shaped groove having a width of 2 μm with the Pt film exposed at the bottom was formed on the surface of the substrate 3.
[0037]
Next, after removing a part of the resist film at the end of the substrate 3 so that the Pt film can be energized, it is immersed in the same sol solution as in Example 1 and a DC voltage is applied between the anode electrode and the anode electrode. Applied. As a result, the gel film selectively adhered in the groove of the resist provided on the substrate 3. Thereafter, aging was performed in the same manner as in Example 1, and then the resist on the substrate was stripped with a stripping solution. As a result, a gel band structure having a width of about 1.2 μm and a height of 4 μm was obtained. Further, in the same manner as in Example 1, a band-shaped transparent dielectric film was obtained by heat treatment. This method of manufacturing a strip-shaped transparent dielectric film is a useful method as a method of manufacturing an optical waveguide.
[0038]
(Example 3)
An electron beam resist is applied to the surface of the substrate 3 on which the Pt film shown in Example 1 is formed to a thickness of 2 μm, and then a figure in which circles of 1 μm diameter are arranged in a triangular lattice at a pitch of 4 μm is drawn with an electron beam. Developed. As a result, an array of 1 μm diameter holes with the Pt film exposed on the bottom was formed on the surface of the substrate 3.
[0039]
Next, after removing a part of the resist film at the end of the substrate 3 so that the Pt film can be energized, it is immersed in a sol solution similar to that in Example 1, and a DC electric field is applied between the anode electrode and the anode electrode. Applied. As a result, the gel film selectively adhered in the holes of the resist provided on the substrate 3. Thereafter, aging was performed in the same manner as in Example 1, and then the resist on the substrate was peeled off with a stripping solution. As a result, a cylindrical group of gel cylinders having a diameter of about 0.6 μm arranged in a triangular lattice at a pitch of 4 μm was obtained. . Further, in the same manner as in Example 1, a transparent dielectric film composed of a column group was obtained by heat treatment. This method for producing a transparent dielectric film comprising a group of cylinders is a useful method for producing a two-dimensional photonic crystal.
[0040]
【The invention's effect】
As is apparent from the above description, according to the method for producing a dielectric oxide film of the present invention, a film forming method by an electrophoresis method using a sol solution containing cluster particles based on a high concentration sol-gel method. By adopting, a transparent gel film can be formed in an arbitrary shape and crystallized at a low temperature, so that a transparent dielectric oxide film can be obtained while suppressing thermal influence on the surroundings.
[0041]
Then, as a precursor solution for obtaining a sol solution, at least one of metal alkoxide, metal acetylacetonate, inorganic salt, and those obtained by substituting some of the functional groups of these compounds with other functional groups, organic By using a transparent solution dissolved in a solvent, the effects of the present invention can be obtained more reliably.
[0042]
In addition, as a step of forming the gel film, a step of applying a voltage in which a direct current is superimposed on a pair of electrodes is applied instead of a step of applying a direct current voltage to the pair of electrodes. Control becomes easier and the effects of the present invention can be obtained more reliably.
[0043]
In addition, a two-dimensional photonic for an optical waveguide can be obtained by using a plate, bar, wire, or insulating material formed entirely or partially on the surface of a conductive material as an electrode on the cathode side. A dielectric oxide film having an arbitrary shape can be obtained in accordance with a use such as a crystal.
[0044]
Furthermore, by using a solution in which an alkoxide of Ba and Ti is dissolved in an organic solvent as a precursor solution, a BaTiO ₃ dielectric oxide film suitable for applications such as an optical waveguide and a two-dimensional photonic crystal can be obtained. it can.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing an apparatus for producing a dielectric oxide film by the method of the present invention.
2 is a graph showing the relationship between the adhesion amount of the gel film obtained by the production method of the present invention shown in Example 1 and the applied voltage. FIG.
3 is an X-ray diffraction (XRD) chart showing the crystallinity of a BaTiO ₃ film obtained by the manufacturing method of the present invention shown in Example 1. FIG.
[Explanation of symbols]
1 Glass container 2 Sol solution 3 Substrate 4 Anode electrode 5 Bath 6 Refrigerant 7 Gel film

Claims (5)

Preparing a precursor solution containing metal ions constituting the dielectric oxide, the precursor solution having a concentration from substantially saturated to supersaturated,
Hydrolyzing the precursor solution at a temperature equal to or lower than a temperature at which gelation occurs by hydrolysis and subsequent condensation polymerization reaction to obtain a transparent sol solution;
Forming a gel film equal to the solute composition of the sol solution on the cathode side electrode by applying a voltage to a pair of electrodes immersed in the sol solution;
Heat treating the gel film to form an oxide film;
A method for producing a dielectric oxide film, comprising: The precursor solution containing the metal ion is a metal alkoxide, a metal acetylacetonate, an inorganic salt, and at least one of those obtained by substituting some of the functional groups of these compounds with other functional groups in an organic solvent. 2. The method for producing a dielectric oxide film according to claim 1, wherein the dielectric oxide film is a dissolved transparent solution. In the step of forming the gel film, a direct current voltage is applied to the pair of electrodes to form a gel film on the cathode side electrode, or a direct current is applied to the pair of electrodes. The method for producing a dielectric oxide film according to claim 1, wherein the method is a step of forming a gel film on an electrode on a low voltage side. The cathode-side electrode is a plate, rod, or wire made of a conductive material, and the conductive material includes a surface of an insulating material formed entirely or partially with a conductive material, The manufacturing method of the dielectric oxide film in any one of Claims 1-3. 5. The dielectric according to claim 1, wherein the precursor solution is a solution in which an alkoxide of Ba and Ti is dissolved in an organic solvent, and the dielectric oxide film is BaTiO _3. Manufacturing method of body oxide film.

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