JP3161472B2 - Method of forming barium titanate thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a thin film made of barium titanate. More specifically, the present invention relates to a method for forming a barium titanate thin film that can be used as a dielectric film, a piezoelectric film, or a semiconductor film on a base.

[0002]

2. Description of the Related Art Conventionally, as a method of forming a barium titanate thin film on a substrate, for example, a method using a coating solution prepared by dissolving at least barium alkoxide and titanium alkoxide in alcohol and further adding a hydrolysis inhibitor ( JP-A-1-286922) is disclosed. In this method, a hydrolysis inhibitor such as alcoholamine is added to prevent the hydrolysis reaction of the alkoxide compound as a starting material, so that precipitation of fine-particle hydroxides and oxides in the coating solution is suppressed, and the homogeneity is reduced. There is a feature that a thin film excellent in quality can be obtained. As another method for forming a barium titanate thin film, a method using a coating solution in which a titanium alkoxide and a barium salt are dissolved in a mixed solvent of glycerin and methanol (Tsuchiya, Kawano, Kiyoshi, Hatano, Journal of Ceramics, 98 [8] 743 (199)
0)) has been proposed. This method increases the viscosity of the coating solution by using glycerin, which is a high-boiling solvent, without using only a lower alcohol such as methanol as a solvent for the titanium alkoxide and the barium salt. There is an advantage that a uniform film can be obtained without aggregation.

[0003]

However, in the former method, even when a barium alkoxide solution and a titanium alkoxide solution are mixed together with a hydrolysis inhibitor in dry nitrogen, the hydrolysis is completely completed due to the large amount of the alkoxide compound. And the long-term stability of the coating solution in the air after mixing is not sufficient. In the latter method, glycerin, a high-boiling solvent, is a solvent having a very high viscosity, and the film tends to be powdery at the time of firing, and the solubility in barium salts is not as high as that of alcohol. The amount of methanol must be increased. For this reason, there has been a defect that the concentration of the inorganic substance dissolved in the mixed solvent is relatively reduced, and it is difficult to remove the organic substance in the subsequent drying step and baking step. Also, since this method does not add a hydrolysis inhibitor as in the former case, when the prepared coating solution is left in the air for a long time, there is a problem that the coating solution easily gels and lacks preservability. An object of the present invention is to provide a method for forming a barium titanate thin film which is excellent in stability of a coating solution and can easily remove a solvent of the coating solution. Another object of the present invention is to provide a method capable of forming a uniform barium titanate thin film with high purity and a uniform thickness of 1 μm or less on a substrate.

[0004]

In order to achieve the above object, in the production method of the present invention, first, one or two kinds selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol and ethoxyethanol are provided. The above alcohol is prepared, and one or more carboxylic acids selected from the group consisting of formic acid, acetic acid, propionic acid and butyric acid are prepared. One or two selected from the group consisting of barium inorganic salts and barium alkoxides;
Prepare more than one kind of barium compound, alcohol amine,
One or more barium alkoxide and titanium alkoxide hydrolysis inhibitors selected from the group consisting of glycols and β-diketones are prepared. In the first production method, a titanium alkoxide is dissolved in the alcohol to prepare a titanium alkoxide solution, and the barium compound is dissolved in the carboxylic acid or an organic solution in which the carboxylic acid and the alcohol are mixed. The solution is uniformly mixed with the hydrolysis inhibitor while heating and refluxing so that the molar ratio of Ba and Ti becomes 1: 1 to prepare a coating solution. Further, in the second production method, a titanium alkoxide is dissolved in an organic solution obtained by mixing the carboxylic acid or the carboxylic acid and the alcohol to prepare a titanium alkoxide solution. A coating solution is prepared by uniformly mixing with the hydrolysis inhibitor while heating and refluxing so that the ratio becomes 1: 1. Subsequently, in both the first and second production methods, a coating solution is applied to the surface of a heat-resistant substrate to form a coating film, and the substrate formed with the coating film is dried and heat-treated to form a barium titanate precursor. A barium titanate thin film is produced and finally this precursor is fired at a temperature of 700 to 1000 ° C.

Hereinafter, the present invention will be described. (a) Preparation of titanium alkoxide solution Examples of titanium alkoxide include methoxide, ethoxide, isopropoxide, butoxide, methoxyethoxide, ethoxyethoxide and the like. The solvent of the titanium alkoxide solution is an alcohol or a carboxylic acid.
Examples of the alcohol include one or more alcohols selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol, and ethoxyethanol, in consideration of compatibility with carboxylic acids and barium compounds described below. Selected. Alcohols having 2 to 4 carbon atoms are preferable in order to facilitate film formation. Examples of the carboxylic acid include one or more compounds selected from the group consisting of formic acid, acetic acid, propionic acid, and butyric acid. The titanium alkoxide is dissolved in an organic solution of alcohol or carboxylic acid or a mixture of carboxylic acid and alcohol dehydrated in a dry nitrogen atmosphere to prepare a transparent and uniform titanium alkoxide solution.
The dissolution in a dry nitrogen atmosphere is to prevent a reaction with moisture and oxygen. The concentration of this solution is 0.1 ~
5.0 mol / L is preferred. If it is less than 0.1 mol / L, it is too dilute to form a film, and if it exceeds 5.0 mol / L, there is a problem in solubility.

(B) Preparation of a barium solution A barium solution is prepared by dissolving a barium compound in the carboxylic acid or an organic solution obtained by mixing the carboxylic acid and an alcohol. The barium compound is one or more compounds selected from the group consisting of barium inorganic salts and barium alkoxides. Barium inorganic salts include barium acetate, barium hydroxide, barium carbonate, barium chloride, barium nitrate, and the like.Barium alkoxides include methoxide, ethoxide, isopropoxide, butoxide, methoxy ethoxide, ethoxy ethoxide, and the like. Is mentioned. When the barium compound is a barium alkoxide, the barium compound is dissolved in an alcohol containing a carboxylic acid, or the barium alkoxide is dissolved only in an alcohol that has been subjected to a dehydration treatment. A clear and uniform barium solution is prepared by adding the acid. The concentration of this barium solution is 0.1 to 2.0
Mol / L is preferred. If it is less than 0.1 mol / L, it is too thin to form a film, and if it exceeds 2.0 mol / L, there is a problem in solubility. Dissolving barium acetate or barium hydroxide with acetic acid among carboxylic acids is preferable because solubility of barium acetate or barium hydroxide increases.

(C) Addition of hydrolysis inhibitor Even if titanium alkoxide or barium alkoxide is dissolved in alcohol, carboxylic acid or alcohol containing carboxylic acid, respectively, it is hydrolyzed by moisture in the air to form a white precipitate. Therefore, the molar ratio of the hydrolysis inhibitor is 0.1 <[hydrolysis inhibitor] / [Ba + T
i] is added to each solution of titanium alkoxide and barium alkoxide in the range of <3. This addition may be made at the time of mixing the barium solution and the titanium alkoxide solution described below, but it is preferable from an operational point of view to add each alkoxide when or before dissolving it in alcohol or the like. As the hydrolysis inhibitor, alcohol amine,
One or more compounds selected from the group consisting of glycols and β-diketones. Monoethanolamine, diethanolamine, triethanolamine, mono-2-propanolamine, di-2-propanolamine, and the like are used as alcohol amines, and ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, and the like are used as glycols. Can be When the hydrolysis inhibitor is added, the inhibitor reacts with the titanium ion or barium ion of the alkoxide of titanium or barium to generate a chelate compound, thereby making each alkoxide solution stable to moisture and oxygen.

(D) Preparation of Coating Solution A coating solution is prepared by uniformly mixing a barium compound with the titanium alkoxide solution so that the molar ratio of Ba and Ti becomes 1: 1. When the titanium alkoxide solution uses an alcohol as a solvent, the barium solution may be mixed. When the carboxylic acid is used as a solvent, an unsolutioned barium compound may be directly mixed.
When a hydrolysis inhibitor is not added when preparing a titanium alkoxide solution or a barium alkoxide solution,
Here, the hydrolysis inhibitor was added in the same ratio as in (c) above,
Prepare a clear, viscous mixed solution. In addition to the hydrolysis inhibitor, the above-mentioned carboxylic acid acts as a stabilizer for the mixed solution. The mixed solution is heated to reflux, and then water is added to the mixed solution in a molar ratio of 5 <[H ₂ O] / [Ba + T
i] Add a relatively large amount in the range of <50 to prepare a coating solution. By heating and refluxing, the reaction of the mixed solution is promoted, and the mixed solution is further stabilized against moisture and oxygen. Further, by adding water, the unreacted alkoxide compound in the mixed solution whose viscosity has decreased by heating and refluxing is positively hydrolyzed to raise the coating solution to a desired viscosity according to a coating method described below. .
For example, when coating by dip coating or spin coating, the concentration of the coating solution is desirably about 0.1 to 2.0 mol / L. Even if the prepared coating solution is exposed to moisture for a long time by the last addition of water, precipitation of particulate hydroxides and oxides is completely suppressed.

(E) Formation of coating film The coating solution is applied to the surface of a heat-resistant substrate. The substrate only needs to withstand the sintering temperature described below, and includes metals such as gold, silver, and platinum, alloys containing at least one of these metals as a main component, glass, carbon, silicon, and the like.
Silica, alumina, magnesia, zirconia, titania, strontium titanate, boron nitride, silicon nitride,
Ceramics such as boron carbide and silicon carbide can be used. The shape of the substrate is fibrous, film, plate,
Any shape such as a bulk shape may be used. It is desirable that the surface of the substrate is polished and smoothed before coating, and further washed to remove oil and the like. As a method of application, in addition to a method of applying a coating solution to a substrate by a screen printing method, a spray coating method of spraying a coating solution, a dip coating method of dipping a substrate after dipping in a coating solution, Is applied to make the thickness of the coating film uniform and thinner. The spin coating method is desirable from the viewpoint of uniformity of the film thickness.

(F) Formation of Barium Titanate Thin Film The coating film formed on the substrate surface is dried at a temperature of room temperature to 200 ° C. The coating on the dried substrate is heat treated at 300-500 ° C. to remove the solvent. As a result, a composite oxide precursor of barium and titanium is generated. When this precursor is fired at 700 to 1000 ° C. under atmospheric pressure, a barium titanate thin film is formed on the substrate. The thickness can be increased by repeating the steps from the formation of the coating film to the heat treatment, and a barium titanate thin film having a desired thickness can be obtained by adjusting the number of repetitions. The concentration of the barium solution and the titanium alkoxide solution, the viscosity of the coating solution, the pulling rate of the substrate, the spray amount, etc., vary. Can be formed,
By increasing the number of layers of the coating film, a thin film having a thickness of several microns can be obtained.

[0011]

As described above, even if the conventional method mixes each alkoxide solution of barium and titanium with a hydrolysis inhibitor in dry nitrogen, the hydrolysis is prevented because of the large amount of the alkoxide compound. Although it was difficult to completely suppress the titanium alkoxide and barium compound in alcohol or carboxylic acid in the present invention,
By heating and refluxing both solutions and the hydrolysis inhibitor, and further adding water and mixing uniformly, chelation of the mixed solution is remarkably accelerated as compared with the conventional method, and the coating solution is hydrolyzed even when exposed to moisture. Since the decomposition reaction no longer occurs, it has an effect of excellent long-term stability. Another advantage is that the viscosity of the coating solution can be adjusted by adding water. Further, in the present invention, since high-viscosity, high-viscosity glycerin is not used for the solvent as in the conventional method, it is not necessary to mix a large amount of alcohol for diluting the solvent, and thereby the concentration of the inorganic substance in the coating solution is reduced. And a barium titanate thin film having a thickness of 1 μm or less can be uniformly and uniformly formed on the substrate.

[0012]

EXAMPLES Next, examples of the present invention will be described in order to show specific embodiments of the present invention. The embodiments described below do not limit the technical scope of the present invention. <Example 1> 10.22 g of barium acetate was dissolved in 150 mL of acetic acid. Separately, titanium isopropoxide 11.37 g
And 2.10 g of diethanolamine as a hydrolysis inhibitor of this alkoxide were dissolved in 100 mL of isopropanol. The barium solution and the titanium isopropoxide solution were mixed while heating and refluxing so that the molar ratio of Ba and Ti became 1: 1. [H ₂ O]
Water was added so that the molar ratio of / [Ba] became 30 to prepare a coating solution. The coating solution was allowed to stand in a constant temperature and humidity room at room temperature of 25 ° C. and a relative humidity of 65% for one month, but no hydroxide or oxide in the form of fine particles was precipitated, and the solution viscosity did not change before and after the standing.

The above coating solution was applied to the surface of an alumina substrate by dip coating. That is, the substrate was immersed in the above-mentioned coating solution that had been allowed to stand,
Pulled vertically at a speed of seconds. After drying the coating film formed on the substrate surface together with the substrate at a temperature of 100 ° C.,
Heat treatment was performed at 0 ° C. to remove the solvent of the coating film. After repeating the above-mentioned coating step, drying step and heat treatment step six times, further baking at 800 ° C. under atmospheric pressure for 2 hours,
Sub-micron thick barium titanate (Ba)
A thin film of TiO ₃ ) was obtained.

Continuation of front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C01G 23/00 C04B 35/46

Claims (4)

(57) [Claims] 1. A method comprising preparing one or more alcohols selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol and ethoxyethanol, and selecting from the group consisting of formic acid, acetic acid, propionic acid and butyric acid. Preparing one or more carboxylic acids obtained, dissolving a titanium alkoxide in the alcohol to prepare a titanium alkoxide solution, one or two selected from the group consisting of barium inorganic salts and barium alkoxides The barium compound is dissolved in an organic solution obtained by mixing the carboxylic acid or the carboxylic acid and the alcohol to prepare a barium solution. One or more selected from the group consisting of alcohol amines, glycols and β-diketones Hydrolysis of two or more barium alkoxides and titanium alkoxides A decomposition inhibitor is prepared, and the barium solution and the titanium alkoxide solution are mixed with B
Prepare a mixed solution by preparing the molar ratio of a to Ti so as to be 1: 1 and uniformly mixing the barium solution, the titanium alkoxide solution and the hydrolysis inhibitor in the molar ratio while heating and refluxing. Water is added to the mixed solution to prepare a coating solution, the coating solution is applied to the surface of a heat-resistant substrate to form a coating film, and the substrate on which the coating film is formed is dried and heat-treated. A method for forming a barium titanate thin film, comprising: generating a barium titanate precursor; and firing the precursor at a temperature of 700 to 1000 ° C. 2. The barium solution is an organic solution of barium alkoxide, and inhibits hydrolysis of one or more of barium alkoxide and titanium alkoxide selected from the group consisting of alcoholamines, glycols and β-diketones. 2. The method for forming a barium titanate thin film according to claim 1, wherein an agent is separately added to the barium alkoxide organic solution and the titanium alkoxide solution, and then the two solutions are mixed to prepare a mixed solution. 3. A method comprising preparing one or more alcohols selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol and ethoxyethanol, and selecting from the group consisting of formic acid, acetic acid, propionic acid and butyric acid. Preparing one or more carboxylic acids obtained, dissolving a titanium alkoxide in an organic solution obtained by mixing the carboxylic acid or the carboxylic acid and the alcohol to prepare a titanium alkoxide solution; One or more barium compounds selected from the group consisting of barium alkoxides are prepared, and one or more barium alkoxides and titanium selected from the group consisting of alcoholamines, glycols and β-diketones Prepare an alkoxide hydrolysis inhibitor, and the barium compound And the titanium alkoxide solution are prepared such that the molar ratio of Ba and Ti is 1: 1. The barium compound, the titanium alkoxide solution and the hydrolysis inhibitor in the molar ratio are uniformly heated and refluxed. Mixing to prepare a mixed solution, adding water to the mixed solution to prepare a coating solution, applying the coating solution to the surface of a heat-resistant substrate to form a coating film, and forming the coating film A method of forming a barium titanate thin film, wherein the precursor is dried and heat-treated to produce a barium titanate precursor, and the precursor is fired at a temperature of 700 to 1000 ° C. 4. The method for forming a barium titanate thin film according to claim 3, wherein a hydrolysis inhibitor is mixed with the titanium alkoxide solution, and a barium compound is added to the mixture to prepare a coating solution.