JPH04362015A - Formation of barium titanate thin film - Google Patents

Abstract

PURPOSE:To produce the 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:The soln. of a barium salt or a barium alkoxide in a carboxylic acid or in the mixture of a carboxylic acid and an alcohol and the soln. of a titanium alkoxide in a carboxylic acid or in an alcohol contg. a carboxylic acid are mixed so that the molar ratio of Ba to Ti is conrolled to 1:1. The mixture is uniformly mixed with a hydrolysis inhibitor such as alcoholamines, glycols and beta-diketones while being heated and refluxed, water is added to adjust its viscosity, and a coating soln. is prepared. 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 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, in the former method, even if barium alkoxide solution and titanium alkoxide solution are mixed together with a hydrolysis inhibitor in dry nitrogen, hydrolysis cannot be completed completely due to the large amount of alkoxide compound blended. It is difficult to suppress the oxidation, and the long-term stability of the coating solution in the atmosphere after mixing is insufficient. In addition, in the latter method, glycerin, which is a high boiling point solvent, is a very viscous solvent, and the film tends to become powdery during baking, and the solubility of barium salt is not as high as alcohol. The amount of methanol must be increased. 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. In addition, 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 forming a barium titanate thin film that provides excellent stability of the coating solution and allows easy removal of the solvent of the coating solution. Another object of the present invention is to provide a method capable of forming 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. Also, one or two selected from the group consisting of barium inorganic salts and barium alkoxides.
Prepare more than one type of barium compound, alcohol amine,
One or more hydrolysis inhibitors for barium alkoxide and titanium alkoxide selected from the group consisting of glycols and β-diketones are prepared. In the first production method, a titanium alkoxide solution is prepared by dissolving titanium alkoxide in the alcohol, and in this solution, the barium compound is added to the barium compound dissolved in the carboxylic acid or an organic solution containing the carboxylic acid and the alcohol. A coating solution is prepared by uniformly mixing the solution with the hydrolysis inhibitor while heating and refluxing the solution so that the molar ratio of Ba and Ti is 1:1. 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 compound is added to this solution in moles of Ba and Ti. A coating solution is prepared by uniformly mixing with the hydrolysis inhibitor at a ratio of 1:1 while heating and refluxing. 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. A barium titanate thin film is obtained by producing and finally firing this precursor at a temperature of 700-1000°C.

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 this alcohol include one or more alcohols selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol, and ethoxyethanol. The selection will be made based on the following. Alcohols having 2 to 4 carbon atoms are preferred 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. A transparent and uniform titanium 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 A barium solution is prepared by dissolving a barium compound in an organic solution containing the carboxylic acid or a mixture of this carboxylic acid and alcohol. The barium compound is one or more compounds selected from the group consisting of barium inorganic salts and barium alkoxides. Inorganic salts of barium include barium acetate, barium hydroxide, barium carbonate, barium chloride, barium nitrate, etc., and barium alkoxides include methoxide, ethoxide, isopropoxide, butoxide, methoxyethoxide, ethoxyethoxide, etc. can be mentioned. When the barium compound is barium alkoxide, it is dissolved in alcohol containing carboxylic acid, or dissolved only in alcohol that has been dehydrated. A clear, homogeneous barium solution is prepared by adding acid. The concentration of this barium solution is preferably 0.1 to 2.0 mol/L. If it is less than 0.1 mol/L, it will be too dilute and difficult to form a film, and if it exceeds 2.0 mol/L, there will be problems with solubility. Among carboxylic acids, it is preferable to dissolve barium acetate or barium hydroxide in acetic acid because the solubility of barium acetate or barium hydroxide increases.

(c) Addition of hydrolysis inhibitor Even when titanium alkoxide or barium alkoxide is dissolved in alcohol, carboxylic acid, or alcohol containing carboxylic acid, they are hydrolyzed by moisture in the air to form a white precipitate. For ease of use, a hydrolysis inhibitor is added to each solution of titanium alkoxide and barium alkoxide in a molar ratio of 0.1<[hydrolysis inhibitor]/[Ba+Ti]<3. The addition timing may be when the barium solution and the titanium alkoxide solution are mixed, which will be described below, but it is preferable for operational reasons to add it when or before each alkoxide is dissolved in alcohol or the like. 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 or barium ions of the titanium or barium alkoxide to form a chelate compound, 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 to Ti is 1:1. If the titanium alkoxide solution uses alcohol as a solvent, mix the barium solution,
Further, when a carboxylic acid is used as a solvent, a barium compound that has not been made into a solution may be directly mixed. If no hydrolysis inhibitor is added during the preparation of titanium alkoxide solution or barium alkoxide solution,
Here, a hydrolysis inhibitor is added in the same proportion as in (c) above to prepare a transparent and viscous mixed solution. In addition to the hydrolysis inhibitor, the carboxylic acid mentioned above acts as a stabilizer for this mixed solution. This mixed solution was heated to reflux, and then water was added to this mixed solution in a molar ratio of 5<[H2O]/[Ba+
Ti] is added in a relatively large amount within the range of <50 to prepare a coating solution. By heating under reflux, the reaction of the mixed solution is promoted, and the mixed solution becomes more stable against moisture and oxygen. In addition, by adding water, the unreacted alkoxide compound in the mixed solution whose viscosity has decreased due to heating and refluxing is actively hydrolyzed, and the coating solution is increased to a desired viscosity according to the coating method described below. . For example, when coating by dip coating or spin coating, the concentration of the coating solution is preferably about 0.1 to 2.0 mol/L. The final addition of water to the prepared coating solution completely suppresses precipitation of particulate hydroxides and oxides even when exposed to moisture for a long period of time.

(e) Formation of coating film The coating solution is applied to the surface of a heat-resistant substrate. The substrate may be of 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,
Ceramics such as silica, alumina, magnesia, zirconia, titania, strontium titanate, boron nitride, silicon nitride, boron carbide, and silicon carbide 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, even if the conventional method mixes alkoxide solutions of barium and titanium together with a hydrolysis inhibitor in dry nitrogen, hydrolysis does not occur due to the large amount of alkoxide compounds blended. In the present invention, it has been difficult to completely suppress these substances by (1) dissolving titanium alkoxide and barium compounds in alcohol or carboxylic acid, and (2)
By heating both solutions and the hydrolysis inhibitor under reflux, then adding water and mixing them uniformly, chelation of the mixed solution is much more accelerated than with conventional methods, and the coating solution remains stable even when exposed to moisture. Since hydrolysis reaction no longer occurs, it has excellent long-term stability. Another advantage is that the viscosity of the coating solution can be adjusted by adding water. 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 is possible to uniformly and uniformly form a barium titanate thin film with a thickness of 1 μm or less on a 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> 10.22 g of barium acetate was dissolved in 150 mL of acetic acid. Separately, 11.37 g of titanium isopropoxide and 2.10 g of diethanolamine, which is a hydrolysis inhibitor for this alkoxide, were dissolved in 100 mL of isoprobanol. The above barium solution and titanium isopropoxide solution were mixed under heating under reflux so that the molar ratio of Ba to Ti was 1:1. For this mixed solution, [H2O]/
Water was added so that the molar ratio of [Ba] was 30 to prepare a coating solution. This coating solution was left in a constant temperature and humidity room at a room temperature of 25° C. and a relative humidity of 65% for one month, but there was no precipitation of particulate hydroxides or oxides, and the viscosity of the solution did not change before and after being left.

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.

Claims (4)

[Claims] Claim 1: 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. Prepare one or more carboxylic acids, dissolve titanium alkoxide in the alcohol to prepare a titanium alkoxide solution, and prepare one or two carboxylic acids selected from the group consisting of an inorganic salt of barium and barium alkoxide. A barium solution is prepared by dissolving the above barium compound in the above carboxylic acid or an organic solution containing the above carboxylic acid and the above alcohol.
A species or two or more of the barium alkoxide and titanium alkoxide hydrolysis inhibitors are prepared, the barium solution and the titanium alkoxide solution are prepared such that the molar ratio of Ba and Ti is 1:1, and the molar ratio of Ba and Ti is 1:1. A mixed solution is prepared by uniformly mixing the barium solution, the titanium alkoxide solution, and the hydrolysis inhibitor at a ratio of A coating solution is applied to the surface of a heat-resistant substrate to form a coating film, the substrate on which the coating film is formed is subjected to dry heat treatment to produce a barium titanate precursor, and the precursor is heated at 700 to 1000°C. A method for forming barium titanate thin films by firing at high temperatures. 2. The barium solution is an organic solution of barium alkoxide, which suppresses the hydrolysis of one or more barium alkoxides and titanium alkoxides selected from the group consisting of alcohol amines, glycols, and β-diketones. After separately adding the agent to the barium alkoxide organic solution and the titanium alkoxide solution,
The method for forming a barium titanate thin film according to claim 1, wherein both solutions are mixed to prepare a mixed solution. [Claim 3] 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. 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 of a mixture of the carboxylic acid and the alcohol, and then adding an inorganic salt of barium and a titanium alkoxide solution. One or more barium compounds selected from the group consisting of barium alkoxides are prepared, one or more barium alkoxides selected from the group consisting of alcohol amines, glycols, and β-diketones and titanium. Prepare an alkoxide hydrolysis inhibitor,
The barium compound and the titanium alkoxide solution are prepared so that the molar ratio of Ba to Ti is 1:1, and the barium compound, the titanium alkoxide solution, and the hydrolysis inhibitor in the molar ratio are heated to reflux. preparing a mixed solution by adding water to the mixed solution, preparing the coating solution by applying the coating solution to the surface of a heat-resistant substrate to form a coating film,
The substrate on which the coating film was formed was subjected to dry heat treatment to produce a barium titanate precursor, and the precursor was heated at 700 to 1000°C.
A method for forming a barium titanate thin film by firing at a temperature of . 4. The method for forming a barium titanate thin film according to claim 3, wherein the coating solution is prepared by mixing a hydrolysis inhibitor into the titanium alkoxide solution and then adding a barium compound to the mixed solution.