JP2020075830A - Method for producing silica sol and method for suppressing intermediate products in silica sol - Google Patents

Abstract

To provide a method for producing silica sol with few intermediate products, and provide a method for suppressing intermediate products in silica sol by which few intermediate products are produced.SOLUTION: A method for producing silica sol comprises a step (1) and a step (2). The step (1) is a step of obtaining a silica sol reaction liquid by hydrolyzing and condensing a tetraalkoxysilane. The step (2) is a step of adding an oxidizing agent to the silica sol reaction liquid. A method for suppressing intermediate products in the silica sol comprises adding an oxidizing agent to a silica sol reaction liquid obtained by hydrolyzing and condensing a tetraalkoxysilane.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing silica sol. The present invention also relates to a method for suppressing intermediate products in silica sol.

  As a method for polishing the surface of a material such as a metal or an inorganic compound, a polishing method using a polishing liquid is known. Above all, final finish polishing of prime silicon wafers for semiconductors and these recycled silicon wafers, and chemical mechanical polishing (CMP) such as flattening of interlayer insulating film at the time of manufacturing semiconductor devices, formation of metal plugs, and formation of embedded wiring. ), The surface condition greatly affects the semiconductor characteristics, and therefore the surfaces and end faces of these components are required to be polished with extremely high precision.

  In such precision polishing, a polishing composition containing silica sol is adopted, and colloidal silica is widely used as the abrasive grains as the main component thereof. Colloidal silica is produced by thermal decomposition of silicon tetrachloride (fumed silica, etc.), by deionization of alkali silicates such as water glass, and by hydrolysis / condensation reaction of alkoxysilane ( Generally called "sol-gel method") and the like are known.

  Many studies have been made so far regarding a method for producing a silica sol containing colloidal silica. For example, Patent Documents 1 to 3 disclose a method of producing a silica sol by a hydrolysis reaction / condensation reaction of an alkoxysilane.

JP, 11-60232, A International Publication No. 2008/123373 International Publication No. 2004/000922

  By the way, an intermediate product may be generated during or after the production of the silica sol by the hydrolysis reaction / condensation reaction of the alkoxysilane. This intermediate product is considered to be silica that remains as a solid with insufficient growth, silica precipitated from dissolved silicic acid after production, or the like. Since such an intermediate product is considered to be a silica having a lower degree of condensation than the desired colloidal silica, it deteriorates the mechanical properties of the colloidal silica in the obtained silica sol, lowers the polishing rate, and the like. Adversely affect the polishing characteristics of. Further, since the obtained polishing liquid contains silica having a low degree of condensation, the removability of the obtained polishing liquid from the object to be polished after polishing is poor. Further, problems such as aggregation, sedimentation, thickening, and gelation of silica in the silica sol or the polishing liquid are likely to occur, and the obtained silica sol or the obtained polishing liquid exhibits unstable behavior, resulting in poor storage stability.

  The method for producing a silica sol by hydrolysis reaction / condensation reaction of alkoxysilane disclosed in Patent Documents 1 to 3 does not describe anything about dealing with such an intermediate product, and the intermediate product may be produced depending on the production conditions. A silica sol containing a large amount of is obtained. As a result, the polishing characteristics of the obtained polishing liquid are adversely affected, the removability of the obtained polishing liquid from the object to be polished after polishing is poor, and the storage stability of the obtained silica sol and the obtained polishing liquid is poor.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a method for producing a silica sol containing few intermediate products. Another object of the present invention is to provide a method for suppressing intermediate products in a silica sol that reduces the amount of intermediate products.

  Conventionally, a large amount of silica sol containing an intermediate product has been used as it is as a polishing liquid without removing the intermediate product as it is. Therefore, it cannot be said that the polishing liquid obtained has sufficient polishing characteristics and storage stability. However, as a result of intensive studies, the present inventors have found that by adding an oxidizing agent represented by a neutral oxidizing agent to the silica sol, a silica sol with a small amount of intermediate products can be obtained, and the present invention has been completed. Came to do.

That is, the gist of the present invention is as follows.
[1] A method for producing silica sol, which includes the following step (1) and step (2).
Step (1) A step of obtaining a silica sol reaction liquid by hydrolyzing and condensing tetraalkoxysilane.
Step (2) A step of adding an oxidizing agent to the silica sol reaction liquid.
[2] The method for producing a silica sol according to [1], wherein the oxidizing agent is a neutral oxidizing agent.
[3] Add 0.005 to 5 parts by mass of an oxidizing agent to 100 parts by mass of the silica-containing content of the tetraalkoxysilane used in the hydrolysis reaction / condensation reaction, in [1] or [2] A method for producing the described silica sol.
[4] The method for producing a silica sol according to any one of [1] to [3], which includes the following step (3) after the step (2).
Step (3) A step of heating the silica sol reaction liquid to which the oxidizing agent is added.
[5] A method for suppressing an intermediate product in a silica sol, which comprises adding an oxidizing agent to a silica sol reaction liquid obtained by hydrolyzing and condensing a tetraalkoxysilane.
[6] The method for suppressing an intermediate product in a silica sol according to [5], wherein the oxidizing agent is a neutral oxidizing agent.
[7] Add 0.005 to 5 parts by mass of an oxidizing agent to [5] or [6] with respect to 100 parts by mass of silica content of tetraalkoxysilane subjected to hydrolysis reaction / condensation reaction. A method for suppressing intermediate products in the described silica sol.
[8] The method for suppressing an intermediate product in a silica sol according to any one of [5] to [7], wherein the silica sol reaction solution containing an oxidizing agent is heated.

  The method for producing a silica sol of the present invention makes it possible to obtain a silica sol with a small amount of an intermediate product, excellent polishing properties of the obtained polishing liquid, and excellent removability from the object to be polished after polishing the obtained polishing liquid, The storage stability of the obtained silica sol and the obtained polishing liquid is excellent. Further, the method for suppressing the intermediate product in the silica sol of the present invention can easily reduce the amount of the intermediate product in the silica sol, the polishing liquid obtained has excellent polishing characteristics, and the obtained polishing liquid can be easily removed after polishing. It has excellent removability from the polishing body and excellent storage stability of the obtained silica sol and the obtained polishing liquid.

3 is an FE-SEM image of the silica sol obtained in Example 2. 3 is an FE-SEM image of the silica sol obtained in Comparative Example 1.

  The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be variously modified and implemented within the scope of the gist thereof. In this specification, when the expression "to" is used, it is used as an expression including numerical values or physical values before and after the expression.

(Method for producing silica sol)
The method for producing a silica sol of the present invention includes the following step (1) and step (2).
Step (1) A step of obtaining a silica sol reaction liquid by hydrolyzing and condensing tetraalkoxysilane.
Step (2) A step of adding an oxidizing agent to the silica sol reaction liquid.

(Process (1))
The step (1) is a step of hydrolyzing and condensing the tetraalkoxysilane to obtain a silica sol reaction liquid. As a method of hydrolyzing and condensing the tetraalkoxysilane to obtain a silica sol reaction solution, a known manufacturing method may be used.

  Examples of the tetraalkoxysilane include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetraisopropoxysilane. These tetraalkoxysilanes may be used alone or in combination of two or more. Among these tetraalkoxysilanes, hydrolysis reaction / condensation reaction is fast, unreacted substances are unlikely to remain, excellent productivity, and stable silica sol can be easily obtained. Tetramethoxysilane is more preferable.

  As the raw material forming the colloidal silica, a low-condensation product obtained by partially hydrolyzing and condensing the tetraalkoxysilane may be used in addition to the tetraalkoxysilane.

  Examples of the solvent / dispersion medium used in the reaction during the hydrolysis / condensation reaction include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents and dispersion media may be used alone or in combination of two or more. Among these solvents / dispersion media, the one used in the hydrolysis reaction / condensation reaction and the one produced as a by-product are the same, and water and alcohol are preferable, and water and methanol are more preferable, since they are excellent in convenience in production. ..

The hydrolysis reaction / condensation reaction may be carried out in the presence of a catalyst or in the absence of a catalyst, but the presence of a catalyst is preferred because the hydrolysis reaction / condensation reaction can be promoted.
Examples of the catalyst include acid catalysts such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid, formic acid and citric acid, and alkali metals such as ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine and tetramethylammonium hydroxide. Examples thereof include catalysts. Among these catalysts, an alkaline catalyst is preferable because it is excellent in catalytic action and the particle shape can be easily controlled, and it is possible to suppress the mixing of metal impurities, and since the volatility is high and the removability after the condensation reaction is excellent. , Ammonia is more preferred.

Other steps may be included between the step (1) and the step (2).
It is preferable to include a step of removing unnecessary components from the components in the silica sol reaction liquid and adding necessary components between the steps (1) and (2). For example, a solvent / dispersion medium such as alcohol or a catalyst such as ammonia may be removed and water may be added so as to obtain a desired content.

(Process (2))
Step (2) is a step of adding an oxidizing agent to the silica sol reaction liquid.

  The oxidizing agent has an effect of suppressing an intermediate product in the silica sol, and examples thereof include neutral oxidizing agents such as hydrogen peroxide and benzoyl peroxide; and acidic oxidizing agents such as sulfuric acid and nitric acid. These oxidizing agents may be used alone or in combination of two or more. Among these oxidizing agents, while maintaining the physical properties of colloidal silica in the silica sol, the intermediate product can be efficiently suppressed, and the pH of the silica sol can be maintained in the vicinity of neutral. Is preferred, and hydrogen peroxide is more preferred.

In the present specification, the intermediate product is a black image in FIGS. 1 and 2 in an FE-SEM image taken at a magnification of 100,000 to 200,000 times using a field emission scanning electron microscope (FE-SEM). A part that looks like a part surrounded by a line.
This intermediate product is considered to be silica that remains as a solid with insufficient growth during production of silica sol by hydrolysis reaction / condensation reaction of alkoxysilane or after production, silica precipitated from dissolved silicic acid after production, and the like.

The addition amount of the oxidizing agent to the silica sol reaction liquid is preferably 0.005 parts by mass to 5 parts by mass with respect to 100 parts by mass of the silica-containing content of the tetraalkoxysilane subjected to the hydrolysis reaction / condensation reaction, and is preferably 0. More preferably from 01 part by mass to 1 part by mass. When the addition amount of the oxidizing agent is 0.005 parts by mass or more, the intermediate product can be efficiently suppressed. When the amount of the oxidizing agent added is 5 parts by mass or less, the physical properties of colloidal silica in the silica sol can be maintained.
The silica-equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction / condensation reaction means the silica when the total amount of the tetraalkoxysilane subjected to the hydrolysis reaction / condensation reaction becomes silica through the hydrolysis reaction / condensation reaction. The theoretical amount of.

Other steps may be included after the step (2).
Since the intermediate product can be efficiently suppressed, it is preferable to include a step (3) of heating the silica sol reaction liquid to which the oxidizing agent is added, after the step (2).

  The temperature to be heated in step (3) is preferably 30 ° C to 100 ° C, more preferably 40 ° C to 95 ° C. If the heating temperature in step (3) is 30 ° C. or higher, the intermediate product can be efficiently suppressed. Further, when the heating temperature in the step (3) is 100 ° C. or lower, volatilization of the dispersion medium of the silica sol and the oxidizing agent can be suppressed.

(Method of suppressing intermediate products in silica sol)
The method for suppressing the intermediate product in the silica sol of the present invention is a method of adding an oxidizing agent to the silica sol reaction liquid obtained by hydrolyzing and condensing tetraalkoxysilane, and the above-mentioned step (1) and step (2 ) May be performed, and the above-described step (3) may be performed as necessary.

(Preferable characteristics of silica sol)
The content of colloidal silica in the silica sol is preferably 3 to 50% by mass, more preferably 4 to 40% by mass, and even more preferably 5 to 30% by mass in 100% by mass of the silica sol. When the content of colloidal silica is 3% by mass or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent. Further, when the content of colloidal silica is 50% by mass or less, aggregation / sedimentation of colloidal silica can be suppressed and uneven concentration of colloidal silica in the silica sol can be suppressed.

  The content of the solvent / dispersion medium in the silica sol is preferably 50 to 97 mass%, more preferably 60 to 96 mass%, and even more preferably 70 to 95 mass% in 100 mass% of the silica sol. When the content of the solvent / dispersion medium is 50% by mass or more, aggregation / sedimentation of colloidal silica can be suppressed and uneven concentration of colloidal silica in the silica sol can be suppressed. When the content of the solvent / dispersion medium is 97% by mass or less, the polishing rate for an object to be polished represented by a silicon wafer is excellent.

  The content of the colloidal silica and the solvent / dispersion medium in the silica sol should be set in a desired range by the step of removing unnecessary components from the components in the silica sol reaction solution described above and adding the necessary components. You can

  Examples of the solvent / dispersion medium in the silica sol include water, methanol, ethanol, propanol, isopropanol, ethylene glycol and the like. These solvents and dispersion media may be used alone or in combination of two or more. Among these solvents / dispersion media, water and alcohol are preferable, and water is more preferable, because they have excellent affinity with colloidal silica.

  6-9 are preferable and, as for pH of silica sol, 7-8 are more preferable. When the pH of the silica sol is 6 or more, the long-term storage stability of the silica sol is excellent. Further, when the pH of the silica sol is 9 or less, aggregation / sedimentation of the colloidal silica can be suppressed and uneven concentration of the colloidal silica in the silica sol can be suppressed.

  The content of metal impurities in the silica sol is preferably 1 ppm or less, more preferably 0.5 ppm or less, still more preferably 0.1 ppm or less.

When polishing a silicon wafer of a semiconductor device, metal impurities adhere to and contaminate the surface of the object to be polished, which adversely affects the wafer characteristics and diffuses inside the wafer to deteriorate the quality. The performance of the manufactured semiconductor device is significantly reduced.
When metal impurities are present in the silica sol, a coordinative interaction occurs between the surface silanol groups exhibiting acidity and the metal impurities, and the chemical properties (acidity etc.) of the surface silanol groups are changed, or colloidal It changes the three-dimensional environment of the silica surface (e.g., the ease of aggregation of colloidal silica) and affects the polishing rate.

  The content rate of metal impurities in silica sol shall be measured by high frequency inductively coupled plasma mass spectrometry (ICP-MS). Specifically, weigh accurately 2 g of silica sol, add sulfuric acid and hydrofluoric acid, heat, dissolve and evaporate, and add pure water to the remaining sulfuric acid drop so that the total amount becomes exactly 10 g to prepare a test solution. However, it shall be measured using a high frequency inductively coupled plasma mass spectrometer. The target metals are sodium, potassium, iron, aluminum, calcium, magnesium, zinc, cobalt, chromium, copper, manganese, lead, titanium, silver and nickel.

The content of metal impurities in the silica sol can be set to 1 ppm or less by performing a hydrolysis reaction / condensation reaction using tetraalkoxysilane as a main raw material to obtain silica sol.
In the method of deionizing alkali silicate such as water glass, it is difficult to keep the content of metal impurities in the silica sol at 1 ppm or less, since sodium and the like derived from the raw materials remain.

  Further, in order to reduce the metal impurity content in the silica sol, a dispersant or an additive used in the production of the silica sol having a very low metal content or no metal content is used, and the metal contamination of the reaction vessel is low. It is preferable to use an extremely low-priced equipment, and further to provide a facility for keeping the manufacturing place in an environment where the metal contamination is extremely low.

The average primary particle diameter of the colloidal silica in the silica sol is preferably 10 nm to 200 nm, more preferably 15 nm to 100 nm. When the average primary particle diameter of the colloidal silica is 10 nm or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent, and the storage stability of silica sol is excellent. Further, when the average primary particle diameter of the colloidal silica is 200 nm or less, the surface roughness and scratches of the object to be polished represented by a silicon wafer during polishing can be reduced, and the sedimentation of the colloidal silica can be suppressed.
The average primary particle size of the colloidal silica is measured by the BET method. Specifically, the specific surface area of colloidal silica is measured using an automatic specific surface area measuring device, and the average primary particle diameter is calculated using the following formula (1).
Average primary particle diameter (nm) = 6000 / (specific surface area (m ² / g) × density (g / cm ³ )) (1)

  The average primary particle diameter of colloidal silica can be set in a desired range by known conditions and methods.

The average secondary particle diameter of the colloidal silica in the silica sol is preferably 20 nm to 300 nm, more preferably 30 nm to 200 nm. When the average secondary particle diameter of the colloidal silica is 20 nm or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent, the removal property of particles and the like in the cleaning after polishing is excellent, and the storage stability of silica sol is excellent. .. When the average secondary particle diameter of the colloidal silica is 300 nm or less, the surface roughness and scratches of the object to be polished, which is represented by a silicon wafer during polishing, can be reduced, the removability of particles and the like in cleaning after polishing is excellent, and colloidal silica can be obtained. The sedimentation of silica can be suppressed.
The average secondary particle size of colloidal silica is measured by the DLS method. Specifically, it shall be measured using a dynamic light scattering particle size measuring device.

  The average secondary particle diameter of colloidal silica can be set in a desired range by known conditions and methods.

The cv value of the colloidal silica in the silica sol is preferably 15 to 50, more preferably 20 to 40. When the cv value of colloidal silica is 15 or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent, and the productivity of the silicon wafer is excellent. When the cv value of colloidal silica is 50 or less, the surface roughness and scratches of the object to be polished represented by a silicon wafer during polishing can be reduced, and the removability of particles and the like in cleaning after polishing is excellent.
As for the cv value of colloidal silica, the average secondary particle size of colloidal silica is measured using a dynamic light scattering particle size measuring device, and the cv value is calculated using the following formula (2).
cv value = (standard deviation (nm) / average secondary particle diameter (nm)) × 100 (2)

The association ratio of colloidal silica in the silica sol is preferably 1.2 to 2.5, more preferably 1.5 to 2.2. When the association ratio of colloidal silica is 1.2 or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent, and the productivity of the silicon wafer is excellent. When the association ratio of colloidal silica is 2.5 or less, surface roughness and scratches on the object to be polished represented by a silicon wafer during polishing can be reduced, and aggregation of colloidal silica can be suppressed.
The association ratio of colloidal silica is calculated from the average primary particle diameter measured by the above-mentioned measurement method and the average secondary particle diameter measured by the above-mentioned measurement method using the following formula (3). I shall.
Association ratio = average secondary particle diameter / average primary particle diameter (3)

The viscosity of the silica sol is preferably 1 mPa · s to 50 mPa · s, more preferably 2 mPa · s to 40 mPa · s. When the viscosity of the silica sol is 1 mPa · s or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent. When the viscosity of the silica sol is 50 mPa · s or less, the intermediate products can be sufficiently removed, and the removal of particles and the like in the cleaning of the object to be polished represented by a silicon wafer after polishing is excellent, and the silica sol is excellent. It has excellent storage stability.
The viscosity of the silica sol is a value measured using an E-type viscometer under the conditions of 25 ° C. and a shear rate of 150 / sec.

(Polishing liquid)
The silica sol can obtain a polishing liquid by dissolving a water-soluble polymer.
The water-soluble polymer enhances the wettability of the polishing liquid with respect to the object to be polished represented by a silicon wafer. The water-soluble polymer is preferably a polymer having a functional group with high water affinity, and the functional group with high water affinity and the surface silanol group of colloidal silica have high affinity and are Colloidal silica and water-soluble polymer are stably dispersed in the vicinity. Therefore, the effects of the colloidal silica and the water-soluble polymer function synergistically when polishing an object such as a silicon wafer.

Examples of the water-soluble polymer include cellulose derivatives, polyvinyl alcohol, polyvinylpyrrolidone, copolymers having a polyvinylpyrrolidone skeleton, polymers having a polyoxyalkylene structure, and the like.
Examples of the cellulose derivative include hydroxyethyl cellulose, hydrolyzed hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethylmethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, ethyl cellulose, ethyl hydroxyethyl cellulose, carboxymethyl cellulose and the like.
Examples of the copolymer having a polyvinylpyrrolidone skeleton include a graft copolymer of polyvinyl alcohol and polyvinylpyrrolidone.
Examples of the polymer having a polyoxyalkylene structure include polyoxyethylene, polyoxypropylene, and a copolymer of ethylene oxide and propylene oxide.
These water-soluble polymers may be used alone or in combination of two or more. Among these water-soluble polymers, a cellulose derivative is preferable because it has a high affinity with the surface silanol groups of colloidal silica and acts synergistically to give good hydrophilicity to the surface of the object to be polished, and thus hydroxyethyl cellulose is preferable. Is more preferable.

  The mass average molecular weight of the water-soluble polymer is preferably 1,000 to 3,000,000, more preferably 5,000 to 2,000,000, and even more preferably 10,000 to 1,000,000. When the mass average molecular weight of the water-soluble polymer is 1,000 or more, the hydrophilicity of the polishing liquid is improved. When the mass average molecular weight of the water-soluble polymer is 3,000,000 or less, the affinity for silica sol is excellent, and the polishing rate for an object to be polished represented by a silicon wafer is excellent.

  The mass average molecular weight of the water-soluble polymer is to be measured by size exclusion chromatography under the condition of using a 0.1 mol / L NaCl solution as a mobile phase in terms of polyethylene oxide.

  The content of the water-soluble polymer is preferably 0.02 to 10% by mass, and more preferably 0.05 to 5% by mass in 100% by mass of the polishing liquid. When the content of the water-soluble polymer is 0.02% by mass or more, the hydrophilicity of the polishing liquid is improved. Further, when the content of the water-soluble polymer is 10% by mass or less, aggregation / sedimentation of colloidal silica at the time of preparing the polishing liquid can be suppressed.

The pH of the polishing liquid is preferably 8-12, more preferably 9-11. When the pH of the polishing liquid is 8 or more, it is possible to suppress the aggregation and precipitation of colloidal silica in the polishing liquid. Further, when the pH of the polishing liquid is 12 or less, dissolution of colloidal silica can be suppressed.
The pH of the polishing liquid can be set in a desired range by adding a pH adjuster.

(Use)
Silica sol produced by the method for producing silica sol of the present invention and a polishing liquid containing the same can be preferably used for polishing applications, for example, polishing of semiconductor materials such as silicon wafers, polishing of electronic materials such as hard disk substrates, It can be used for polishing (chemical mechanical polishing) in the flattening process when manufacturing integrated circuits, polishing of synthetic quartz glass substrates used for photomasks and liquid crystals, polishing of magnetic disk substrates, etc., especially polishing of silicon wafers. And particularly preferably used for chemical mechanical polishing.
When the silica sol and the polishing liquid are used for polishing, they may be used in the same manner as known polishing. For example, when polishing a silicon wafer, the concentration may be adjusted, an additive may be added, and then the silicon wafer may be dropped onto a polishing pad set on a regular plate of a polishing machine and polished.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the description of the following examples without departing from the scope of the invention.

(Metal impurity content rate)
Accurately weigh 2 g of the silica sol reaction solution obtained in the production example, add sulfuric acid and hydrofluoric acid, heat, dissolve, and evaporate, and add pure water to the remaining sulfuric acid drop so that the total amount becomes exactly 10 g and test. A liquid was prepared and the metal impurity content rate was measured using a high frequency inductively coupled plasma mass spectrometer (ICP-MS, model name "ELEMENT2", manufactured by Thermo Fisher Scientific).
The content of metal impurities is 0.220 ppm for sodium, 0.028 ppm for potassium, 0.003 ppm for iron, 0.027 ppm for aluminum, 0.015 ppm for calcium, 0.014 ppm for zinc, magnesium, cobalt, chromium, copper, Manganese, lead, titanium, silver, and nickel were all less than 0.001 ppm.

(Measurement of intermediate products in silica sol)
With respect to the silica sol obtained in Example 2 and Comparative Example 1, the area ratio of the intermediate product was calculated by the following operation.
First, the silica sol was separated and diluted with ultrapure water 5,000 times. 5 μL of the diluted solution diluted 5,000 times was taken, dropped on a mirror silicon wafer (manufactured by Electronics End Materials Co., Ltd.), and dried at 50 ° C. for 10 minutes. It was attached to a field emission scanning electron microscope (FE-SEM, model name "S-5200 type", manufactured by Hitachi High-Technologies Corporation), with an acceleration voltage of 5 kV and a magnification of 150,000 times, 100 to 200 units. Individual particles of colloidal silica were observed and an image was taken.
The identification of the colloidal silica particles and the intermediate product was carried out by incorporating the photographed image into image analysis type particle size distribution measurement software (software name "Mac-View Ver.4", manufactured by Mountech Co., Ltd.). When the area of the intermediate product is a and the area of the particles of the colloidal silica is b, the area ratio of the intermediate product was calculated using the following formula (4).
Area ratio of intermediate product (%) = {a / (a + b)} × 100 (4)
An FE-SEM image of the silica sol obtained in Example 2 is shown in FIG. 1, and an FE-SEM image of the silica sol obtained in Comparative Example 1 is shown in FIG. The intermediate product is a portion that looks like a portion surrounded by a black line in FIGS. 1 and 2 in the FE-SEM image.

(Measurement of viscosity of silica sol)
2 mL of the silica sol obtained in Examples and Comparative Examples was loaded into a sample cup, and an E-type viscometer (model name “TVE-25”, manufactured by Toki Sangyo Co., Ltd.) under the conditions of 25 ° C. and shear rate of 150 / sec. Was used to measure the viscosity of silica sol.

(Measurement of average primary particle size)
The silica sol obtained in Examples and Comparative Examples was freeze-dried, and the specific surface area of colloidal silica was measured using an automatic specific surface area measuring device (model name “Flowsorb II”, manufactured by Shimadzu Corporation), and the following formula ( Using 1), the average primary particle diameter was calculated at a density of 2.2 g / cm ³ .
Average primary particle diameter (nm) = 6000 / (specific surface area (m ² / g) × density (g / cm ³ )) (1)

(Measurement of average secondary particle size and cv value)
The average secondary particle diameter of the colloidal silica was measured using the dynamic light scattering particle size measuring device (DLS, model name "Zetasizer Nano ZS", manufactured by Malvern Instruments) for the silica sols obtained in the examples and comparative examples. Then, the cv value was calculated using the following formula (2).
cv value = (standard deviation (nm) / average secondary particle diameter (nm)) × 100 (2)

(Calculation of meeting ratio)
From the measured average primary particle diameter and average secondary particle diameter, the association ratio was calculated using the following formula (3).
Association ratio = average secondary particle diameter / average primary particle diameter (3)

[Production example]
Tetramethoxysilane and methanol were mixed at a ratio of 3: 1 (volume ratio) to prepare a raw material solution. A reaction solvent equipped with a thermometer, a stirrer, a supply pipe and a distillation line was charged with a reaction solvent prepared by mixing methanol, pure water and ammonia in advance. The concentration of water in the reaction solvent was 32% by mass, and the concentration of ammonia in the reaction solvent was 1.5% by mass.
While maintaining the temperature of the reaction solvent at 33 ° C., the reaction solvent and the raw material solution were set to 2.3: 1 (volume ratio), and the raw material solution was dropped into the reaction tank at a uniform rate for 180 minutes to obtain a silica sol reaction liquid. It was The obtained silica sol reaction liquid was heated to remove methanol and ammonia while adjusting the amount of liquid by adding pure water so that the content of colloidal silica was about 20% by mass. A silica sol reaction solution having a rate of about 20% by mass was obtained.

[Example 1]
35 parts by mass of hydrogen peroxide was added to the silica sol reaction solution obtained in Production Example so that the hydrogen peroxide was 0.5 parts by mass with respect to 100 parts by mass of the silica-containing content of tetraalkoxysilane subjected to the hydrolysis / condensation reaction. % Hydrogen peroxide solution was added to obtain silica sol.
Table 1 shows the evaluation results of the obtained silica sol.

[Example 2]
35 parts by mass of hydrogen peroxide was added to the silica sol reaction solution obtained in Production Example so that the hydrogen peroxide was 0.5 parts by mass with respect to 100 parts by mass of the silica-containing content of tetraalkoxysilane subjected to the hydrolysis / condensation reaction. % Hydrogen peroxide solution was added, the mixture was heated with a heater, kept at 50 ° C. for 1 hour, and further kept at 80 ° C. for 1 hour to obtain a silica sol.
Table 1 shows the evaluation results of the obtained silica sol.

[Comparative Example 1]
The silica sol reaction liquid obtained in Production Example was directly used as silica sol.
Table 1 shows the evaluation results of the obtained silica sol.

As can be seen from Table 1, FIG. 1 and FIG. 2, the silica sol obtained by the manufacturing method of Example 2 including the step of adding an oxidizing agent is the manufacturing method of Comparative Example 1 not including the step of adding an oxidizing agent. It can be seen that the area ratio of the intermediate product is low and the amount of the intermediate product is suppressed as compared with the silica sol obtained in 1.
Further, as can be seen from Table 1, the silica sol obtained by the production method of the example including the step of adding the oxidizing agent and the silica sol obtained by the production method of the comparative example not including the step of adding the oxidizing agent Since the average primary particle diameter, the average secondary particle diameter, the cv value, and the association ratio have substantially the same physical properties, the silica sol has a viscosity difference due to the generation of the intermediate product. By including the step of adding the oxidizing agent, the intermediate product can be efficiently suppressed while maintaining the physical properties of the colloidal silica in the silica sol.

  Silica sol produced by the method for producing silica sol of the present invention and a polishing liquid containing the same can be preferably used for polishing applications, for example, polishing of semiconductor materials such as silicon wafers, polishing of electronic materials such as hard disk substrates, It can be used for polishing (chemical mechanical polishing) in the flattening process when manufacturing integrated circuits, polishing of synthetic quartz glass substrates used for photomasks and liquid crystals, polishing of magnetic disk substrates, etc., especially polishing of silicon wafers. And particularly preferably used for chemical mechanical polishing.

Claims (8)

A method for producing a silica sol, which comprises the following step (1) and step (2).
Step (1) A step of obtaining a silica sol reaction liquid by hydrolyzing and condensing tetraalkoxysilane.
Step (2) A step of adding an oxidizing agent to the silica sol reaction liquid.   The method for producing a silica sol according to claim 1, wherein the oxidizing agent is a neutral oxidizing agent.   The production of the silica sol according to claim 1 or 2, wherein 0.005 parts by mass to 5 parts by mass of the oxidizing agent is added to 100 parts by mass of the silica-containing content of tetraalkoxysilane subjected to the hydrolysis reaction / condensation reaction. Method. The method for producing a silica sol according to claim 1, further comprising the following step (3) after step (2).
Step (3) A step of heating the silica sol reaction liquid to which the oxidizing agent is added.   A method for suppressing an intermediate product in a silica sol, which comprises adding an oxidizing agent to a silica sol reaction solution obtained by hydrolyzing and condensing a tetraalkoxysilane.   The method for suppressing an intermediate product in a silica sol according to claim 5, wherein the oxidizing agent is a neutral oxidizing agent.   The silica sol according to claim 5 or 6, wherein 0.005 parts by mass to 5 parts by mass of an oxidizing agent is added to 100 parts by mass of the silica-containing content of tetraalkoxysilane subjected to the hydrolysis reaction / condensation reaction. Method of suppressing intermediate products.   The method for suppressing an intermediate product in a silica sol according to any one of claims 5 to 7, wherein the silica sol reaction solution containing an oxidizing agent is heated.

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