JP2005116421A - Dielectric thin film forming agent, manufacturing method for dielectric thin film forming agent, dielectric thin film and method for forming dielectric thin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mild acidic dielectric thin film forming agent, a manufacturing method for the dielectric thin film forming agent, a dielectric thin film and a method for forming the dielectric thin film. <P>SOLUTION: This dielectric thin film forming agent is a mild acidic solution containing alkaline earth metal salt, ammonium titanium peroxocitrate, and hydroxycarboxylic acid and forming a titanium compound. Since the dielectric thin film forming agent does not contain hydrochloric acid, hydrogen chloride gas is not generated even if it is heat-treated, and safe film forming work can be done without requiring a special device. By performing two step heat-treatment, the dielectric thin film having performance comparable to other methods can be formed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a dielectric thin film forming agent and a dielectric thin film forming method, and more particularly to a dielectric thin film forming agent and a dielectric thin film forming method used for, for example, a dielectric thin film capacitor.

  Dielectric thin films such as barium titanate, strontium titanate, and barium titanate are drawing attention as being useful for downsizing electronic components.

Non-Patent Document 1 discloses a method for producing these dielectric thin films, for example, a method for producing a dielectric thin film forming agent for forming a barium titanate thin film.
In Non-Patent Document 1, titanium tetrachloride and oxalic acid are mixed in an aqueous solution to produce titanium oxalate, and barium chloride is added to the aqueous solution containing titanium oxalate to precipitate titanium barium oxalate. Thereafter, the pH of the solution is adjusted with 0.1 N hydrochloric acid, and an aqueous hydrogen peroxide solution is added to form a dielectric thin film forming aqueous solution.
And the barium titanate thin film is obtained by depositing the said aqueous solution on the glass substrate heated at 400 degreeC by the spray pyrolysis method.

Non-patent document 2 discloses a method for obtaining titanium peroxocitrate ammonium.
In Non-Patent Document 2, when metal titanium powder is put into an aqueous ammonia solution of hydrogen peroxide, titanium reacts with hydrogen peroxide to produce peroxotitanic acid. When excess hydrogen peroxide decomposes and disappears, the solution gels, so add citric acid so that the molar ratio with titanium is 1: 1 before gelation, and use a stable water-soluble titanium compound. It is described to obtain certain aqueous titanium peroxocitrate solutions.
A titanium oxide thin film is obtained by applying this aqueous solution of ammonium peroxocitrate on a glass substrate and subjecting it to heat treatment.

The Murata Science Foundation Annual Report No.16 (2002), 28 Modern Chemistry March 2000, page 25

  However, in the method according to Non-Patent Document 1, hydrochloric acid must be used in order to obtain a stable aqueous solution of barium titanium oxalate. Corrosion, etc.) and worker safety.

In addition, when an aqueous solution containing an alkaline earth metal element such as barium or strontium is added to an aqueous titanium peroxocitrate solution obtained by the method according to Non-Patent Document 2, an alkaline earth metal element in the mixed aqueous solution, for example, In the case of barium, when the molar ratio of barium to titanium is 1: 2 or more, poorly water-soluble precipitation occurs, and an aqueous solution containing such aggregates is applied on a glass substrate and heat-treated. If a thin film is formed, there is a problem that aggregates remain in the thin film and a uniform thin film cannot be obtained.
In order to prevent precipitation, the solution must be a strongly acidic aqueous solution, and safety must be taken into account when handling the solution.
Therefore, a main object of the present invention is to provide a dielectric thin film forming agent and a method for manufacturing the dielectric thin film forming agent that do not cause these problems, and to provide a dielectric thin film and a dielectric thin film forming method. is there.

The invention according to claim 1 is a dielectric thin film forming agent characterized by being a weakly acidic solution containing an alkaline earth metal salt, ammonium peroxocitrate ammonium and hydroxycarboxylic acid, and forming a titanium compound. .
The invention according to claim 2 is the dielectric thin film forming agent according to claim 1, wherein the molar ratio of the alkaline earth metal element and the hydroxycarboxylic acid is 1:15 or more.
The invention according to claim 3 includes a step of obtaining a solution containing an alkaline earth metal salt and a step of obtaining a solution containing titanium peroxocitrate ammonium and hydroxycarboxylic acid, and comprising the steps of mixing the solutions. This is a method for producing a dielectric thin film forming agent.
The invention according to claim 4 is characterized in that it is formed of a dielectric thin film forming agent which is a weakly acidic solution containing an alkaline earth metal salt, titanium peroxocitrate ammonium and hydroxycarboxylic acid, and forming a titanium compound. It is a dielectric thin film.
The invention according to claim 5 is the dielectric thin film according to claim 4, wherein the thickness of the dielectric thin film is 0.01 to 0.4 μm.
According to a sixth aspect of the present invention, there is provided a heat treatment step of applying a first heat treatment after applying a mixed solution containing an alkaline earth metal salt, hydroxycarboxylic acid, and ammonium peroxocitrate ammonium to a substrate and drying the mixture. A method of forming a dielectric thin film comprising a heat treatment step of performing a second heat treatment at a temperature higher than the heat treatment temperature of 1.

According to this invention, since hydrochloric acid is not used, the equipment is not corroded by corrosive gas, and since it is a weakly acidic solution, there is an effect that the safety of work due to the weakly acidic solution can be achieved.
In addition, when the dielectric thin film is formed using the dielectric thin film forming agent of the present invention, no uniform precipitation occurs in the solution of the forming agent, so that a uniform dielectric thin film can be formed.
And since organic substance can be excluded by carrying out heat processing of multiple times to high temperature processing sequentially, generation | occurrence | production of the defect product of a dielectric thin film can be reduced.

  The above object, other objects, features, and advantages of the present invention will become more apparent from the following description of the best mode for carrying out the invention.

  In the present invention, when an aqueous solution containing an alkaline earth metal element is mixed with an aqueous ammonium peroxocitrate solution, a slightly water-soluble precipitate may be formed when the molar ratio of the alkaline earth metal element to titanium is 1: 2 or more. It has been confirmed that, in the mixed solution that does not use a strong acid, it is difficult to obtain a stable mixed aqueous solution in which the molar ratio of alkaline earth metal to titanium is 1: 1.

The alkaline earth metal referred to in the present invention refers to calcium, strontium, barium, and radium among the Group 2 elements of the periodic table, but strontium and barium are preferably used.
Barium titanate, strontium titanate, and solid solutions thereof are high dielectric constant materials and are suitable as capacitor materials.

  Hydroxycarboxylic acid is an organic compound having a carboxyl group and a hydroxy group in one molecule, and examples thereof include lactic acid, glyceric acid, tartaric acid, citric acid, tropic acid, and benzylic acid, with citric acid being preferred. This is because it is presumed that changes in physical properties due to ligand substitution in an aqueous solution can be suppressed.

The titanium compound refers to a compound of titanium and an alkaline earth metal, and corresponds to so-called barium titanate, strontium titanate, strontium barium titanate, and the like.
The solution for forming the titanium compound is weakly acidic and has a pH of 3 or higher. Since the pH of the solution is 3 or more, there is no danger of handling a strongly acidic solution, and the safety of workers can be ensured.
In addition, although the organic solvent can also be used for the solvent for setting it as a solution, when safety | security, cost, etc. are considered, it is preferable to use water as a solvent.

  The dielectric thin film forming agent has a molar ratio of alkaline earth metal element to hydroxycarboxylic acid of 1:15 or more. This is because if the molar ratio between the alkaline earth metal element and the hydroxycarboxylic acid is less than 1:15, a slightly water-soluble precipitate is formed. If the molar ratio between the alkaline earth metal element and the hydroxycarboxylic acid is 1:15 or more, the alkaline earth metal salt of hydroxycarboxylic acid, for example, the normal salt of barium citrate is a poorly water-soluble compound. The presence of excess citric acid will form a water-soluble citrate complex, thus avoiding the formation of a coprecipitate of barium and titanium ammonium peroxocitrate.

  In the step of obtaining a solution containing an alkaline earth metal salt for producing the present dielectric thin film forming agent, for example, barium acetate or strontium acetate is used as the alkaline earth metal salt.

The step of obtaining a solution containing titanium peroxocitrate ammonium and hydroxycarboxylic acid for producing the dielectric thin film forming agent is performed by introducing metal titanium powder into an aqueous hydrogen peroxide solution and reacting titanium with hydrogen peroxide. In this step, citric acid is added so that the molar ratio with titanium is 1: 1 after peroxotitanic acid is generated, and an aqueous solution of ammonium peroxocitrate, which is a stable water-soluble titanium compound, can be obtained. .
Titanium peroxocitrate may be dissolved in water.
Thereafter, addition is performed so that the amount of hydroxycarboxylic acid becomes excessive.
By using an excessive amount of hydroxycarboxylic acid, it is possible to prevent the formation of a poorly water-soluble precipitate.

  In order to produce the dielectric thin film forming agent, the respective solutions are mixed. Since an excessive amount of hydroxycarboxylic acid is used, it is possible to prevent the formation of a hardly water-soluble precipitate, and a dielectric thin film forming agent can be obtained.

A dielectric thin film formed of a dielectric thin film forming agent, which is a weakly acidic solution containing an alkaline earth metal salt, titanium ammonium peroxocitrate and hydroxycarboxylic acid, which is a weakly acidic solution forming a titanium compound, is a dielectric free of aggregates Since it is formed of a thin film forming agent, the composition distribution is uniform.
Note that a thickener may be added to facilitate the formation of a thin film on the substrate. For example, polyvinyl alcohol corresponds to the thickener.

The above-mentioned dielectric thin film has a thickness of 0.01 to 0.4 μm, more preferably 0.1 μm or more and 0.3 μm or less.
If the thickness is 0.01 μm or less, the function as a dielectric cannot be exhibited. If the thickness is 0.4 μm or more, the content of carbon derived from citric acid or polyvinyl alcohol in the thin film is increased, This is because the dielectric constant may be lowered.

As the substrate for forming the dielectric thin film, a silicon substrate is used. Since the silicon substrate usually has a silicon oxide layer on the surface, the substrate has a two-layer structure of a silicon layer and a silicon oxide layer. ing.
Then, a platinum layer serving as an electrode layer is provided in close contact with the silicon oxide. The reason why the electrode layer is formed of platinum is that it is necessary to be a substance that is conductive and hardly oxidized because heat treatment is performed in a later step.

  A raw material solution containing an alkaline earth metal salt, titanium peroxocitrate ammonium, hydroxycarboxylic acid, and a dielectric thin film forming agent, which is a weakly acidic solution for forming a titanium compound, with a thickener such as polyvinyl alcohol added Is applied onto the platinum layer by spin coating and dried. This coating / drying step may be repeated a plurality of times in order to obtain a dielectric thin film having a desired thickness.

The substrate on which the raw material solution has been applied and dried is subjected to heat treatment in the first heat treatment step.
In this heat treatment, the heat treatment is performed at a temperature at which crystal grains having a relatively small grain size grow, that is, in a range of 300 ° C to 550 ° C. More preferably, it is 400 degreeC or more and 500 degrees C or less. If the temperature is 300 ° C. or lower, carbon derived from hydroxycarboxylic acid or the like cannot be sufficiently removed, and the dielectric constant may be lowered. If the temperature is 550 ° C. or higher, large crystal grains are generated, and the flatness of the surface is impaired. Because.
In addition, a rapid thermal annealing apparatus, an electric furnace, etc. can be used for heat processing.

After the first heat treatment, the second heat treatment is performed at a temperature higher than the first heat treatment temperature, that is, in a range of 600 ° C. to 850 ° C., more preferably 800 ° C. or less, and 650 It is above ℃. If the temperature is 600 ° C. or lower, a sufficient dielectric constant of the dielectric layer cannot be obtained, and if it is 850 ° C. or higher, an initial short circuit at the time of capacitor formation is increased.
By this second heat treatment, the film formation can be made more reliable, the carbon content derived from hydroxycarboxylic acid or the like can be further reduced, and the dielectric constant can be improved.

  A dielectric thin film is formed by the above-described process. Such a dielectric thin film (dielectric layer) is a ferroelectric material such as barium titanate, strontium titanate, or barium strontium titanate.

  Hereinafter, more specific examples of the present invention will be described, but the present invention is not limited to these examples.

(Example 1)
An aqueous titanium peroxocitrate solution containing titanium at a concentration of 0.1 mol / L was prepared by stirring the titanium raw material and citric acid in the air at room temperature. To this solution, a 0.1 mol / L barium acetate aqueous solution was mixed so that the molar ratio of barium to titanium was 1: 1.
It should be noted that the concentration of citric acid charged into the aqueous titanium peroxocitrate solution was adjusted in advance to be 5, 10, 15, 20 times the barium concentration in barium acetate.
As a result, depending on the concentration of citric acid, a slightly water-soluble precipitate may or may not be precipitated.
That is,
When the molar ratio of citric acid to barium was 5: 1, a slightly water-soluble precipitate was formed.
When the molar ratio of citric acid to barium was 10: 1, a slightly water-soluble precipitate was formed.
When the molar ratio of citric acid to barium was 15: 1, hardly water-soluble precipitates did not occur.
When the molar ratio of citric acid to barium was 20: 1, poorly water-soluble precipitate did not occur.
Therefore, by adding citric acid, which is a weak acid, at a molar ratio of 15 times or more with respect to barium, a stable aqueous solution having a molar ratio of barium to titanium of 1: 1 is prepared at a pH of 3 or more. I was able to.
A solution having a molar ratio of citric acid to barium of 15: 1 or more is used as a barium titanate thin film forming agent.

(Comparative Example 1)
As a comparative example, 0.1 mol / L of barium acetate aqueous solution and 0.1 mol / L of titanium peroxocitrate aqueous solution were mixed with each other so that the molar ratio of barium to titanium was 1: 1. Precipitation occurred.
The same was true when the molar ratio of barium to titanium was 1: 2, but when the molar ratio was 1: 4, hardly water-soluble precipitates did not occur.

(Example 2)
Barium acetate was dissolved by adding 0.179 g of barium acetate dihydrate to 5.000 g of ion-exchanged water and stirring in the air at room temperature. Furthermore, 0.064 g of strontium acetate hemihydrate was added and stirred in the air at room temperature to dissolve strontium acetate, and a mixed solution containing strontium and barium was prepared.
On the other hand, 0.239 g of ammonium peroxocitrate was added to 5.000 g of ion-exchanged water, and dissolved by stirring in the air at room temperature to obtain an aqueous solution of titanium peroxocitrate ammonium. 4.203 g of citric acid was added to the aqueous solution and dissolved by stirring in the air at room temperature to prepare an aqueous solution of titanium peroxocitrate ammonium containing excess citric acid.
By mixing the two solutions, a dielectric thin film forming agent for forming a strontium barium titanate thin film having a molar ratio of barium, strontium, titanium and citric acid of 7: 3: 10: 200 is obtained. It was.

(Example 3)
A dielectric thin film forming solution was prepared by mixing 0.5 g of 5 wt% polyvinyl alcohol aqueous solution with 10 g of the dielectric thin film forming agent prepared in Example 2.
This solution was applied onto platinum of a substrate (platinum / silicon oxide / silicon) using spin coating, and dried at 500 ° C. This process was repeated 10 times.
This substrate (dielectric thin film forming agent / platinum / silicon oxide / silicon) is heat-treated at 450 ° C. for 3 hours in the atmosphere using an electric furnace, and then continuously heat-treated at 650 ° C. for 3 hours in the air. I did it.
Through these steps, a thin film of strontium barium titanate having a film pressure of 0.26 μm and a relative dielectric constant of 340 could be formed on the substrate.
Since the dielectric thin film forming agent does not contain hydrochloric acid, it does not generate corrosive hydrogen chloride gas even after heat treatment, and does not require any special equipment, and performs safe film formation I was able to.
Moreover, the performance of the obtained dielectric thin film has a relative dielectric constant comparable to that of a dielectric thin film obtained by a sol-gel method or the like.

  The dielectric thin film forming agent according to the present invention can be applied to, for example, a dielectric thin film capacitor.

Claims (6)

  A dielectric thin film forming agent comprising a weakly acidic solution containing an alkaline earth metal salt, ammonium peroxocitrate ammonium and hydroxycarboxylic acid, and forming a titanium compound.   The dielectric thin film forming agent according to claim 1, wherein the molar ratio of the alkaline earth metal element and the hydroxycarboxylic acid is 1:15 or more. Obtaining a solution containing an alkaline earth metal salt;
A step of obtaining a solution containing titanium peroxocitrate ammonium and hydroxycarboxylic acid,
A method for producing a dielectric thin film forming agent, comprising the step of mixing the solutions.   A dielectric thin film comprising an alkaline earth metal salt, ammonium peroxocitrate ammonium and hydroxycarboxylic acid, and formed of a dielectric thin film forming agent which is a weakly acidic solution for forming a titanium compound.   5. The dielectric thin film according to claim 4, wherein the thickness of the dielectric thin film is 0.01 to 0.4 [mu] m. After applying a mixed solution containing an alkaline earth metal salt, hydroxycarboxylic acid, and ammonium peroxocitrate ammonium to the substrate and drying,
A heat treatment step for performing the first heat treatment;
A method for forming a dielectric thin film, comprising a heat treatment step of performing a second heat treatment at a temperature higher than the first heat treatment temperature.