JPH04362014A - Production of barium titanate thin film - Google Patents

Abstract

PURPOSE:To inexpensively produce a homogeneous high-purity thin film having 1mum thickness on a base plate by using a highly stable coating soln. from which the solvent is easily removed. CONSTITUTION:A titanium alkoxide soln. adjusted by using an alcohol or a carboxylic acid as the solvent and the soln. of a barium salt such as barium acetate in a carboxylic acid such as acetic acid or the barium salt itself are mixed so that the molar ratio of Ba to Ti is controlled to 1:1. The mixture is uniformly mixed with a hydrolysis inhibitor such as alcoholamines, glycols and beta-diketones to prepare a coating soln. The soln. is applied on the surface of a heat-resistant base plate to form a coating film, the coated base plate is dried and heat-treated to form a precursor for barium titanate, and the precursor is baked at 700-1000 deg.C to obtain a barium titanate thin film.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing 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, piezoelectric film, or semiconductor film on a substrate.

0002

2. Description of the Related Art Conventionally, as a method for 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 alcohol amine is added to prevent the hydrolysis reaction of the alkoxide compound, which is the starting material, so the precipitation of particulate hydroxides and oxides is suppressed in the coating solution, resulting in uniformity. It has the advantage of producing excellent thin films. Another method for forming a barium titanate thin film is a method using a coating solution in which titanium alkoxide and barium salt are dissolved in a mixed solvent of glycerin and methanol (Tsuchiya, Kono, Kiyoshi, Hatano, Ceramics Journal, 98 [8 ] 743 pages (1990
)) is proposed. This method increases the viscosity of the coating solution by using glycerin, a high boiling point solvent, in addition to using only a lower alcohol such as methanol as a solvent for titanium alkoxide and barium salt. It has the advantage that a uniform film can be obtained without agglomeration.

0003

[Problems to be Solved by the Invention] However, the former method uses barium alkoxide, which is expensive and difficult to handle, which increases the production cost. Even when mixed, it is difficult to completely suppress hydrolysis due to the large amount of the alkoxide compound, and the long-term stability of the coating solution in the atmosphere after mixing is insufficient. In addition, in the latter method, the high boiling point solvent glycerin is a very viscous solvent, and the film tends to become powdery during baking, so it is necessary to increase the amount of methanol blended to compensate for this property. For this reason, the concentration of inorganic substances dissolved in this mixed solvent is relatively reduced, and furthermore, there is a drawback that it becomes difficult to remove organic substances in the subsequent drying and firing steps. Also, unlike the former method, this method does not add a hydrolysis inhibitor, so
If the prepared coating solution is left in the air for a long time, it tends to gel, resulting in poor storage stability. An object of the present invention is to provide a method for manufacturing a barium titanate thin film on a substrate at low cost, with good stability of the coating solution and easy removal of the solvent of the coating solution. Another object of the present invention is to provide a method capable of producing a homogeneous barium titanate thin film on a substrate with high purity and a uniform thickness of 1 μm or less.

0004

[Means for Solving the Problems] In order to achieve the above object, in the production method of the present invention, first, one or two types selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol and ethoxyethanol are used. The above alcohols are prepared, and one or more carboxylic acids selected from the group consisting of formic acid, acetic acid, propionic acid, and butyric acid are prepared. In addition, one or more barium salts selected from the group consisting of barium acetate, barium hydroxide, barium carbonate, barium chloride, and barium nitrate are prepared, and one or more barium salts selected from the group consisting of alcohol amines, glycols, and β-diketones are prepared. One or more types of titanium alkoxide hydrolysis inhibitors are prepared. In the first production method, a titanium alkoxide solution is prepared by dissolving titanium alkoxide in the alcohol, and a barium solution in which the barium salt is dissolved in the carboxylic acid is added to the solution so that the molar ratio of Ba and Ti is 1:1. A coating solution is prepared by uniformly mixing the hydrolysis inhibitor with the hydrolysis inhibitor. In the second production method, a titanium alkoxide solution is prepared by dissolving titanium alkoxide in the carboxylic acid or an organic solution containing the carboxylic acid and the alcohol, and the barium salt is added to this solution with Ba and T.
A coating solution is prepared by uniformly mixing i with the hydrolysis inhibitor at a molar ratio of 1:1. Next, 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 on which this coating film is formed is subjected to dry heat treatment to form a barium titanate precursor. Finally, this precursor is 700 to 1000
A barium titanate thin film is obtained by firing at a temperature of ℃.

The present invention will now be described. (a) Preparation of titanium alkoxide solution Examples of titanium alkoxides include methoxide, ethoxide, isopropoxide, butoxide, methoxyethoxide, and ethoxyethoxide. The solvent for the titanium alkoxide solution is alcohol or carboxylic acid. Examples of the alcohol include one or more compounds selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol, and ethoxyethanol. 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. A transparent and uniform alkoxide solution is prepared by dissolving titanium alkoxide in a dehydrated alcohol or carboxylic acid or an organic solution of a mixture of carboxylic acid and alcohol in a dry nitrogen atmosphere. The purpose of dissolving in a dry nitrogen atmosphere is to prevent 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 will be too dilute and difficult to form a film, and if it exceeds 5.0 mol/L, there will be problems with solubility.

(b) Preparation of barium solution One or more barium salts selected from the group consisting of barium acetate, barium hydroxide, barium carbonate, barium chloride and barium nitrate are dissolved in the carboxylic acid. The concentration of this solution is preferably 0.1 to 2.0 mol/L. 0.1 mol/
If it is less than 2.0 mol/L, it will be too dilute and difficult to form a film.
If it exceeds L, there will be problems with solubility. Among carboxylic acids,
When barium acetate or barium hydroxide is dissolved in acetic acid,
Barium acetate or barium hydroxide is preferred because it has higher solubility.

(c) Addition of hydrolysis inhibitor Even when titanium alkoxide is dissolved in alcohol or carboxylic acid, it is easily hydrolyzed by moisture in the air to form a white precipitate. Therefore, titanium alkoxide is added as a hydrolysis inhibitor. Approximately 1 to 3 moles are added per mole. This addition time may be when the barium solution or barium salt is mixed with the titanium alkoxide solution as described below, but it is preferable for operation to add it when or before dissolving titanium alkoxide in alcohol or carboxylic acid. . Examples of the hydrolysis inhibitor include one or more compounds selected from the group consisting of alcohol amines, glycols, and β-diketones. Monoethanolamine, diethanolamine, triethanolamine, mono-2-propanolamine, di-2-propanolamine, etc. are used as alcohol amines, and ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, etc. are used as glycols. It will be done. When a hydrolysis inhibitor is added, the inhibitor reacts with the titanium ions of the titanium alkoxide to form a chelate compound, making the titanium alkoxide solution stable against moisture and oxygen.

(d) Preparation of coating solution A coating solution is prepared by uniformly mixing a barium compound with a titanium alkoxide solution so that the molar ratio of Ba to Ti is 1:1. When the titanium alkoxide solution uses alcohol as a solvent, the above-mentioned barium solution may be mixed, and when the titanium alkoxide solution uses carboxylic acid as a solvent, the unsolvated barium salt may be directly mixed. If a hydrolysis inhibitor is not added during the preparation of the titanium alkoxide solution, the hydrolysis inhibitor is added at the same rate as in (c) above during this mixing to prepare a transparent and viscous coating solution. In addition to hydrolysis inhibitors, carboxylic acids act as stabilizers for the coating solution. Here, if the mixed solution of the barium compound and the titanium alkoxide solution is heated under reflux, the reaction of the solution will be promoted. In particular, the hydrolysis inhibitor sufficiently reacts with the alkoxide compound in the titanium alkoxide solution to reliably suppress the formation of hydroxides and oxides, and as a result,
The coating solution is more stable against moisture and oxygen, which is preferred. If the viscosity of the coating solution is too low, water is added to actively hydrolyze unreacted alkoxide compounds in the solution, increasing the coating solution to the desired viscosity depending on the coating method described below. You can also do it. For example, when coating by dip coating or spin coating, the concentration of the coating solution is preferably about 0.05 to 2.0 mol/L.

(e) Formation of coating film The coating solution is applied to the surface of a heat-resistant substrate. The substrate may be any material as long as it can withstand the firing temperature described below, and may include metals such as gold, silver, and platinum, alloys containing at least one of these metals as a main component, glass, carbon, silicon, silica, and alumina. , magnesia, zirconia, titania, strontium titanate, boron nitride, silicon nitride, boron carbide, silicon carbide, and other ceramics can be used. The shape of the substrate may be any shape such as fibrous, film, plate, or bulk shape. It is desirable to polish the surface of the substrate to make it smooth and to wash it to remove oil and the like before coating. In addition to applying the coating solution to the substrate using screen printing, there are also spray coating methods in which the coating solution is sprayed, dip coating methods in which the substrate is immersed in the coating solution and then pulled up, and There is a spin coating method that aims to make the thickness of the coating film uniform and thin by spinning it. A spin coating method is preferable from the viewpoint of uniformity of film thickness.

(f) Formation of barium titanate thin film The coating film formed on the surface of the substrate is dried at a temperature of room temperature to 200°C. The dried coating on the 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 forming the coating film to heat treatment, and by adjusting the number of repetitions, a barium titanate thin film with a desired thickness can be obtained. The concentration of the barium solution and titanium alkoxide solution, the viscosity of the coating solution,
Although it varies depending on the pulling speed of the substrate, the amount of spraying, etc., by performing the process from coating film formation to heat treatment in one step, it is possible to form a thin film with a submicron thickness after firing, and it is possible to increase the number of layers of the above coating film. A thin film several microns thick can also be obtained by this method.

[0011]

[Effects of the Invention] As described above, the conventional method uses barium alkoxide as a starting material in addition to titanium alkoxide, resulting in high manufacturing costs and the amount of alkoxide compound blended, making it impossible to completely hydrolyze it. However, according to the present invention, by using an easy-to-handle and inexpensive barium salt, the amount of alkoxide compounds is reduced, and due to the action of the hydrolysis inhibitor and carboxylic acid, moisture or oxygen in the coating solution is reduced. As a result, a homogeneous barium titanate thin film can be easily produced at low cost. In addition, unlike conventional methods, the present invention does not use glycerin, which has a high boiling point and high viscosity, as a solvent, so there is no need to mix a large amount of alcohol to dilute the solvent, which reduces the concentration of inorganic substances in the coating solution. It becomes possible to uniformly form a barium titanate thin film with a thickness of 1 μm or less 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 examples described below do not limit the technical scope of the present invention. <Example 1> 0.05 mol of titanium isopropoxide was dissolved in 100 mL of isopropanol, and then 0.05 mol of titanium isopropoxide was dissolved in 100 mL of isopropanol, followed by 0.05 mol of titanium isopropoxide.
0.5 mol was added as a hydrolysis inhibitor for this alkoxide. Separately, 0.05 mol of barium acetate was dissolved in 100 mL of acetic acid. This barium solution and the titanium isopropoxide solution were mixed in such a manner that the molar ratio of Ba and Ti was 1:1.
mixed so that For this mixed solution, [H2O
A coating solution was prepared by adding water such that the molar ratio of ]/[Ba] was 3.

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 still.
It was pulled up 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,
The solvent in the coating film was removed by heat treatment at 0°C. After repeating the above coating process, drying process, and heat treatment process 6 times, and then baking at 800°C for 2 hours under atmospheric pressure, a submicron-thick barium titanate (BaT
A thin film of iO3) was obtained.

<Example 2> 0.05 mol of titanium isopropoxide was added to 100 mL of acetic acid.
Then 0.05 mol of diethanolamine was added as a hydrolysis inhibitor for the alkoxide. Furthermore, after adding 0.05 mol of barium hydroxide to this solution,
A coating solution was prepared by uniformly mixing. Thereafter, a submicron-thick barium titanate (BaTiO3) thin film was obtained on the surface of the alumina substrate by processing in the same manner as in Example 1.

Claims (6)

[Claims] Claim 1: One or more alcohols selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol and ethoxyethanol are prepared, and one or more alcohols selected from the group consisting of formic acid, acetic acid, propionic acid and butyric acid are prepared. Prepare one or more carboxylic acids, dissolve titanium alkoxide in the alcohol to prepare a titanium alkoxide solution, and prepare a titanium alkoxide solution from the group consisting of barium acetate, barium hydroxide, barium carbonate, barium chloride, and barium nitrate. A barium solution is prepared by dissolving one or more selected barium salts in the carboxylic acid, and one or more selected barium salts selected from the group consisting of alcohol amines, glycols, and β-diketones are added to the carboxylic acid. A titanium alkoxide hydrolysis inhibitor is prepared, the barium solution and the titanium alkoxide solution are prepared so that the molar ratio of Ba to Ti is 1:1, and the barium solution and the titanium alkoxide solution are prepared in the molar ratio. and the hydrolysis inhibitor are uniformly mixed 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 subjected to dry heat treatment. A method for producing a barium titanate thin film, comprising: producing a barium titanate precursor, and firing the precursor at a temperature of 700 to 1000°C. 2. The method for producing a barium titanate thin film according to claim 1, wherein the coating solution is prepared by mixing a hydrolysis inhibitor into the titanium alkoxide solution and then adding a barium solution to the mixed solution. 3. The method for producing barium titanate according to claim 1, wherein water is added to the coating solution to adjust the viscosity of the coating solution. Claim 4: One or more alcohols selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol and ethoxyethanol are prepared, and the alcohol selected from the group consisting of formic acid, acetic acid, propionic acid and butyric acid. A titanium alkoxide solution is prepared by preparing one or more kinds of carboxylic acids, dissolving titanium alkoxide in the carboxylic acid or an organic solution containing the carboxylic acid and the alcohol, and adding barium acetate and hydroxide. One or more barium salts selected from the group consisting of barium, barium carbonate, barium chloride, and barium nitrate are prepared, and one or more barium salts selected from the group consisting of alcohol amines, glycols, and β-diketones are prepared. Prepare at least one hydrolysis inhibitor for the titanium alkoxide, prepare the barium salt and the titanium alkoxide solution so that the molar ratio of Ba to Ti is 1:1, and prepare the barium salt and the titanium alkoxide solution in the molar ratio of Ba and Ti. A coating solution was prepared by uniformly mixing the titanium alkoxide solution and the hydrolysis inhibitor, and the coating solution was applied to the surface of a heat-resistant substrate to form a coating film. A method for producing a barium titanate thin film, which comprises subjecting a substrate to a dry heat treatment to produce a barium titanate precursor, and firing the precursor at a temperature of 700 to 1000°C. 5. The method for producing a barium titanate thin film according to claim 4, wherein the coating solution is prepared by mixing a hydrolysis inhibitor into the titanium alkoxide solution and then adding a barium salt to the mixed solution. 6. The method for producing barium titanate according to claim 4, wherein water is added to the coating solution to adjust the viscosity of the coating solution.