JP2013112573A - Method for making porous silica precursor coating solution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a precursor coating solution which, when applied to a substrate surface and fired, can give a porous silica film excellent in resistance to chemical solutions.SOLUTION: There is provided a method for making a precursor coating solution used in the formation of a porous silica film, which method comprises a first step of dissolving a functional group-containing alkoxysilane together with a surfactant in a solvent and subjecting the dissolved alkoxysilane to hydrolysis and dehydration condensation in the presence of a catalyst to obtain first sol and a second step of adding a tetraalkoxysilane or a polymer thereof to the first sol and subjecting the dehydration condensate contained in the first sol and the tetraalkoxysilane or the polymer thereof to hydrolysis and dehydration condensation to obtain second sol.

Description

  The present invention relates to a method for producing a porous silica precursor coating solution used for forming a porous silica film.

  In recent years, for example, a porous silica film having a relative dielectric constant of 2 or less is used as an interlayer insulating film of a semiconductor device, for example. A CVD method or a coating method is used to form the porous silica film, but a coating method that can be carried out with simple equipment is promising. For example, Patent Document 1 discloses a method for forming a porous silica film using a coating method. In this, a precursor coating solution is prepared by hydrolyzing and dehydrating and condensing an alkoxysilane solution to which a surfactant is added in the presence of a catalyst, and the prepared precursor coating solution is applied to the surface of a substrate. The applied one is baked. As a result, the surfactant contained in the precursor coating solution is removed, and the removed portion becomes pores, thereby forming a porous silica film.

  In the porous silica film thus formed, recesses such as trenches and holes are formed by dry etching. Then, after the etching residue is removed with an acid (for example, hydrofluoric acid), Cu, for example, is embedded as a wiring material in the recess, and the wiring structure is obtained by planarization by CMP using slurry. Here, if the porous silica film has low resistance to chemicals such as acids and alkalis (CMP slurry) used for forming the wiring structure, the size of the pores in contact with the chemicals increases, and the porous silica film There arises a problem that the relative dielectric constant increases as the strength decreases.

  In order to improve the chemical resistance of the porous silica membrane, in Patent Document 1, the precursor is obtained by mixing an alkoxysilane having a methyl group with the alkoxysilane and hydrolyzing and dehydrating and condensing the resulting solution. It has been proposed to produce a body coating solution.

  However, as a result of intensive studies by the present inventors, as in the conventional example, after mixing the alkoxysilane and the alkoxysilane having a functional group and then hydrolyzing and dehydrating condensation to obtain a precursor coating liquid, It was found that the porous silica film does not exhibit sufficient chemical resistance. This is because the alkoxysilane having a functional group is less likely to proceed with dehydration condensation than the alkoxysilane, and the molecular weight of the dehydration condensate having a functional group is relatively small. It is thought that this is because the functional group does not easily exist on the surface of the porous silica film as a result of flowing together with the solvent during coating.

JP 2004-143029 A

  This invention makes it the subject to provide the porous silica precursor coating liquid which can obtain the porous silica membrane excellent in chemical-solution resistance in view of the above point.

  In order to solve the above problems, the present invention is a method for preparing a precursor coating solution used for forming a porous silica film, in which an alkoxysilane having a functional group is dissolved in a solvent together with a surfactant. A first step of obtaining a first sol by hydrolyzing and dehydrating and condensing the resulting alkoxysilane in the presence of a catalyst; and adding a tetraalkoxysilane or a polymer thereof to the first sol, and being contained in the first sol And a second step of obtaining a second sol by hydrolysis and dehydration condensation of the dehydration condensate and tetraalkoxysilane or a polymer thereof.

  According to the present invention, in the first step, a first sol containing a dehydration condensate of an alkoxysilane having a functional group is obtained. That is, a dehydration condensate of an alkoxysilane having a functional group, which is difficult to dehydrate and condense compared to tetraalkoxysilane. This dehydration condensate contains a surfactant that becomes pores during firing. Next, in the second step, a second sol containing a dehydration condensate of the dehydration condensate contained in the first sol and tetraalkoxysilane or a polymer thereof is obtained. The dehydration condensate contained in the second sol thus obtained is obtained by mixing and hydrolyzing and dehydrating and condensing an alkoxysilane having a functional group and a tetraalkoxysilane as in the conventional example. It contains more functional groups than the condensate. And if the precursor coating liquid of this invention is apply | coated to the base-material surface, even if a part of dehydration condensate flows out with the solvent from the base-material surface, the functionality contained in the dehydration-condensation remaining on the base-material surface More groups. Thereby, the porous silica film obtained by baking what apply | coated the precursor coating liquid to the base-material surface shall have many functional groups in the surface. As a result, if a chemical group having high chemical resistance is selected as the functional group, a porous silica film having excellent chemical resistance can be formed.

  In the present invention, the alkoxysilane having a functional group is preferably a trialkoxysilane having a functional group. Since trialkoxysilane has only one bond with a functional group, the dehydration condensation rate is higher than that of dialkoxysilane or monoalkoxysilane having two or three bonds with a functional group, and it is porous. The advantage that the skeleton of the porous silica film can be strengthened is obtained.

The figure which shows the chemical resistance of the porous silica film | membrane formed using the porous silica precursor coating liquid obtained in the Example of this invention.

  Hereinafter, a method for producing a porous silica precursor coating solution according to an embodiment of the present invention will be described. In the method of preparing the porous silica precursor coating liquid of the present embodiment, first, an alkoxysilane having a functional group is dissolved in a solvent together with a surfactant in a tank, and a catalyst and water are added to the solution obtained by the dissolution. Add water for decomposition and stir. Thereby, the alkoxysilane having a functional group is hydrolyzed and dehydrated and condensed to obtain a first sol (first step). The dehydration condensate contained in the first sol incorporates a surfactant that becomes pores during firing.

  Here, as the alkoxysilane having a functional group, for example, trialkoxysilane having a functional group such as methyltrimethoxysilane (MTMS) or phenyltrimethoxysilane (PhTMS) is used alone or in combination of two or more. Can be used. Since the trialkoxysilane has only one bond to the functional group, the dehydration condensation rate is higher than that of dialkoxysilane or monoalkoxysilane having two or three bonds to the functional group, There is an advantage that the skeleton of the porous silica film can be strengthened. As the functional group, at least one kind selected from an alkyl group such as a methyl group or an ethyl group; and an aryl group such as a phenyl group, a tolyl group, or a xylyl group, depending on the properties required for the porous silica membrane as well as the chemical resistance. can do. In order to obtain a highly hydrophobic porous silica film, it is preferable to select an alkyl group, while in order to obtain a porous silica film having excellent dry etching resistance, it is preferable to select an aryl group. .

  As the surfactant, a nonionic surfactant such as a polyoxyethylene-polyoxypropylene condensate can be suitably used. As the solvent, at least one selected from alcohols such as methanol, ethanol and 2-propanol; ketones such as acetone; ethers such as ethyl ether; and nitriles such as acetonitrile is used alone or in combination. Can be used.

  As the catalyst, at least one selected from an acid catalyst and an alkali catalyst can be used. Examples of the acid catalyst include inorganic acids such as nitric acid, hydrochloric acid and sulfuric acid, and organic acids such as formic acid and acetic acid. Examples of the alkali catalyst include ammonium salts such as tetramethylammonium hydroxide.

  The hydrolysis and dehydration condensation time in the first step can be set to at least 30 minutes or more. When the time is shorter than 30 minutes, the dehydration condensation is insufficient, resulting in a problem that a dehydration condensation product having a relatively large molecular weight cannot be obtained. In this case, the functional group contained in the dehydration condensate obtained in the second step described later is reduced. Moreover, what is necessary is just to set the temperature of the hydrolysis of the 1st process and dehydration condensation to the temperature lower than the vaporization temperature of a solvent, for example, setting to room temperature is preferable.

  A tetraalkoxysilane for forming a silica skeleton or a polymer thereof is added to the first sol obtained in the first step and stirred. Here, the first sol contains a catalyst and water for hydrolysis. For this reason, the dehydration condensate contained in the first sol and the tetraalkoxysilane or a polymer thereof are hydrolyzed and dehydrated to obtain a second sol (second step).

  Here, as the tetraalkoxysilane, at least one selected from tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) can be used. As the polymer of tetraalkoxysilane, at least one selected from methyl silicate (MS) and ethyl silicate (ES) can be used.

  The hydrolysis and dehydration condensation time in the second step can be set to at least 30 minutes or more. If it is shorter than 30 minutes, there is a problem that the dehydration condensation becomes insufficient. In addition, the temperature of the hydrolysis and dehydration condensation in the second step is preferably set to room temperature, for example, as in the first step.

  As described above, according to this embodiment, after the dehydration condensation of the alkoxysilane containing the functional group to obtain the first sol, the dehydration condensation product and the tetraalkoxysilane or the polymer thereof are further dehydration condensation. Thus, a second sol was obtained. The dehydration condensate contained in the second sol thus obtained is compared with the dehydration condensate obtained by dehydration condensation of an alkoxysilane containing a functional group and tetraalkoxysilane or a polymer thereof as in the conventional example. , Comes to contain many functional groups. Then, when the precursor coating liquid composed of the second sol is applied to the surface of the silicon substrate as a base material, even if a part of the dehydration condensate flows out together with the solvent from the silicon substrate surface, the dehydration remaining on the silicon substrate surface The functional group contained in the condensate increases. Thereby, the porous silica film obtained by baking the thing which apply | coated the precursor coating liquid on the silicon substrate surface in a vacuum atmosphere can have many functional groups in the surface. As a result, if a chemical group having high chemical resistance is selected, a porous silica film having excellent chemical resistance can be formed. The precursor coating solution may be obtained by adjusting the viscosity by dissolving the solvent in the second sol. Moreover, since a well-known thing can be used about the coating method and baking method of a precursor coating liquid, detailed description is abbreviate | omitted here.

Examples of the present invention will be described below.
Example 1
In Example 1, 0.1 mol of methyltrimethoxysilane (MTMS) and a nonionic surfactant (trade name “P450” manufactured by Daiichi Kogyo Seiyaku Co., Ltd., HO (CH ₂ CH ₂ O) ₁₃ (CH (CH ₃ ) CH ₂ O) ₂₀ (CH ₂ CH ₂ O) ₁₃ H.) 0.01 mol is dissolved in ethanol, and the resulting solution is dissolved in nitric acid. 0.01 mol and 1 mol of water were further added, and these nitric acid and water added were stirred at 25 ° C. for 3 hours to obtain a first sol. Next, 0.0013 mol of methyl silicate (MS) (trade name “Methyl silicate 51” manufactured by Fuso Chemical Industry Co., Ltd.) was added to the first sol, and the mixture was further stirred at 25 ° C. for 2 hours. A sol was obtained. A transparent and uniform precursor coating solution was prepared by adding ethanol to the second sol to adjust the viscosity to be suitable for coating.

  The precursor coating solution obtained in Example 1 was applied to the silicon substrate surface by a spin coating method at a rotational speed of 1200 rpm, and the body coating solution applied was baked at 350 ° C. for 1 hour in a vacuum atmosphere. . The temperature raising time up to 350 ° C. was 15 minutes. The dielectric constant k of the porous silica film obtained by firing was 2.0, the refractive index was 1.23, the elastic modulus was 4.0 GPa, and the film thickness was about 150 nm. The silicon substrate on which this porous silica film was formed was cut into 30 mm square, and the cut piece was immersed in a 1 wt% KOH aqueous solution heated at 50 ° C. for 10 minutes. As a result, the film thickness reduction amount (wet etching amount) was 35 nm. Moreover, as a result of immersing the cut piece in a 0.5 wt% HF aqueous solution heated at 23 ° C. for 10 minutes, it was confirmed that the film thickness reduction amount was 23 nm.

(Example 2)
In Example 2, 0.1 mol of phenyltrimethoxysilane (PhTMS) was used instead of MTMS, and 0.011 mol of tetraethoxysilane (TEOS) was added instead of MS. A precursor coating solution was prepared by the method described above. As in Example 1 above, the relative dielectric constant k of the porous silica film obtained by applying the precursor coating solution obtained in Example 2 to the surface of the silicon substrate and baking it is 2.5, The refractive index was 1.29, the elastic modulus was 4.5 GPa, and the film thickness was 150 nm. The silicon substrate on which the porous silica film was formed was cut into 30 mm squares, and the cut piece was immersed in a 1 wt% KOH aqueous solution heated at 50 ° C. for 10 minutes. As a result, the thickness reduction amount was 10 nm, As a result of immersing the cut piece in a 0.5 wt% HF aqueous solution heated at 23 ° C. for 10 minutes, it was confirmed that the film thickness reduction amount was 5 nm.

(Example 3)
In Example 3, a precursor coating solution was prepared in the same manner as in Example 1 except that 0.01 mol of MTMS and 0.01 mol of PhTMS were used, and the addition amount of MS was 0.0025 mol. As in Example 1 above, the relative dielectric constant k of the porous silica film obtained by applying the precursor coating solution obtained in Example 3 to the surface of the silicon substrate and baking it is 2.0. The refractive index was 1.23, the elastic modulus was 3.8 GPa, and the film thickness was 150 nm. The silicon substrate on which the porous silica film was formed was cut into 30 mm squares, and the cut piece was immersed in a 1 wt% KOH aqueous solution heated at 50 ° C. for 10 minutes. As a result, the thickness reduction amount was 7 nm. As a result of immersing the cut piece in a 0.5 wt% HF aqueous solution heated at 23 ° C. for 10 minutes, it was confirmed that the film thickness reduction amount was 3 nm.

  As described above, it was found that a porous silica film having excellent chemical resistance was obtained when the precursor coating liquids of Examples 1 to 3 were applied to the substrate surface and baked.

Hereinafter, a comparative example with respect to the above embodiment will be described.
(Comparative Example 1)
In this comparative example 1, 0.1 mol of MTMS, 0.0013 mol of MS, and 0.01 mol of nonionic surfactant were dissolved in ethanol in a tank, and nitric acid was added to the solution obtained by this dissolution. 0.01 mol and 1 mol of water were further added, and those added with nitric acid and water were stirred at 25 ° C. for 3 hours to obtain a sol. Ethanol was added to this sol to adjust the viscosity to be suitable for coating to obtain a transparent and uniform porous silica precursor coating solution. As in Example 1 above, the relative dielectric constant k of the porous silica film obtained by applying the precursor coating solution obtained in Comparative Example 1 to the surface of the silicon substrate and baking it is 2.0. The refractive index was 1.23, the elastic modulus was 4.0 GPa, and the film thickness was 150 nm. The silicon substrate on which the porous silica film was formed was cut into 30 mm square, and the cut piece was immersed in a 1 wt% KOH aqueous solution heated at 50 ° C. for 10 minutes. As a result, the film thickness reduction amount was 72 nm, As a result of immersing the cut piece in a 0.5 wt% HF aqueous solution heated at 23 ° C. for 10 minutes, it was confirmed that the film thickness reduction amount was 41 nm. From this, it was found that the chemical resistance was lower than that of Example 1.

(Comparative Example 2)
In Comparative Example 2, a precursor coating solution was prepared in the same manner as in Comparative Example 1 except that PhTMS 0.1 mol was used instead of MTMS and TEOS 0.011 mol was used instead of MS. As in Example 1 above, the relative dielectric constant k of the porous silica film obtained by applying the precursor coating solution obtained in Comparative Example 2 to the surface of the silicon substrate and baking it is 2.5, The refractive index was 1.28, the elastic modulus was 4.5 GPa, and the film thickness was 150 nm. The silicon substrate on which the porous silica film was formed was cut into 30 mm square, and the cut piece was immersed in a 1 wt% KOH aqueous solution heated at 50 ° C. for 10 minutes. As a result, the film thickness reduction amount was 43 nm. As a result of immersing the cut piece in a 0.5 wt% HF aqueous solution heated at 23 ° C. for 10 minutes, it was confirmed that the film thickness reduction amount was 25 nm. From this, it was found that the chemical resistance was lower than that in Example 2.

(Comparative Example 3)
In Comparative Example 3, a precursor coating solution was prepared in the same manner as in Comparative Example 1 except that 0.01 mol of MTMS and 0.01 mol of PhTMS were used, and the addition amount of MS was 0.0025 mol. As in Example 1 above, the dielectric constant k of the porous silica film obtained by applying the precursor coating solution obtained in Comparative Example 3 to the surface of the silicon substrate and baking the surface is 2.0. The refractive index was 1.23, the elastic modulus was 3.6 GPa, and the film thickness was 150 nm. The silicon substrate on which the porous silica film was formed was cut into 30 mm squares, and the cut piece was immersed in a 1 wt% KOH aqueous solution heated at 50 ° C. for 10 minutes. As a result, the film thickness reduction amount was 20 nm. As a result of immersing the cut piece in a 0.5 wt% HF aqueous solution heated at 23 ° C. for 10 minutes, it was confirmed that the film thickness reduction amount was 11 nm. From this, it was found that the chemical resistance was lower than that in Example 3.

(Comparative Example 4)
In Comparative Example 4, 0.1 mol of MTMS and 0.01 mol of a nonionic surfactant were dissolved in ethanol in a tank, and 0.01 mol of nitric acid and water were added to the solution obtained by this dissolution. 1 mol was further added, and these nitric acid and water added were stirred at 25 ° C. for 3 hours to obtain a sol. Ethanol was added to this sol to adjust the viscosity to be suitable for coating to obtain a transparent and uniform porous silica precursor coating solution. As in Example 1 above, the relative dielectric constant k of the porous silica film obtained by applying the precursor coating solution obtained in Comparative Example 4 to the surface of the silicon substrate and baking it is 2.3. The refractive index was 1.28, the elastic modulus was 5.0 GPa, and the film thickness was 150 nm. The silicon substrate on which the porous silica film was formed was cut into 30 mm square, and the cut piece was immersed in a 1 wt% KOH aqueous solution heated at 50 ° C. for 10 minutes. As a result, the film thickness reduction amount was 42 nm. As a result of immersing the cut piece in a 0.5 wt% HF aqueous solution heated at 23 ° C. for 10 minutes, it was confirmed that the film thickness reduction amount was 17 nm.

  As described above, it was found that the chemical resistance of the porous silica film obtained by applying the precursor coating liquids of Comparative Examples 1 to 3 on the surface of the base material and baking it was insufficient.

In addition, this invention is not limited to the said embodiment and Example. For example, in the second step, hydrolysis water and a catalyst may be further added. Also in the second step, a pore-forming surfactant may be added.

Claims (2)

A method for producing a precursor coating solution used for forming a porous silica film,
A step of dissolving an alkoxysilane having a functional group in a solvent together with a surfactant, and hydrolyzing and dehydrating the dissolved alkoxysilane in the presence of a catalyst to obtain a first sol;
Adding a tetraalkoxysilane or a polymer thereof to the first sol, hydrolyzing and dehydrating the dehydration condensate contained in the first sol and the tetraalkoxysilane or a polymer thereof to obtain a second sol; A method for producing a porous silica precursor coating liquid, comprising: The method for producing a porous silica precursor coating solution according to claim 1, wherein the alkoxysilane having a functional group is a trialkoxysilane having a functional group.

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