JP7565671B2 - Method for producing silica sol and method for suppressing intermediate products in silica sol - Google Patents

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.

Polishing methods using polishing liquids are known as a method for polishing the surfaces of materials such as metals and inorganic compounds. In particular, in the final polishing of prime silicon wafers for semiconductors and reclaimed silicon wafers, as well as in chemical mechanical polishing (CMP) for planarizing interlayer insulating films during semiconductor device manufacturing, forming metal plugs, forming embedded wiring, and the like, the surface condition has a significant effect on the semiconductor characteristics, so the surfaces and end faces of these components must be polished with extremely high precision.

In such precision polishing, polishing compositions containing silica sol are used, and colloidal silica is widely used as the abrasive grain, which is the main component. Colloidal silica is known to be produced by different manufacturing methods, such as by thermal decomposition of silicon tetrachloride (fumed silica, etc.), by deionization of alkali silicate such as water glass, and by hydrolysis and condensation reactions of alkoxysilanes (commonly known as the "sol-gel method").

Many studies have been conducted on methods for producing silica sols containing colloidal silica. For example, Patent Documents 1 to 3 disclose methods for producing silica sols by hydrolysis and condensation reactions of alkoxysilanes.

Japanese Patent Application Publication No. 11-60232 International Publication No. 2008/123373 International Publication No. 2004/000922

However, intermediate products may be generated during or after the production of silica sol by hydrolysis and condensation reactions of alkoxysilane. These intermediate products are thought to be silica that remains as a solid without being fully grown, or silica that precipitates from dissolved silicic acid after production. Such intermediate products are thought to be silica with a lower degree of condensation than the desired colloidal silica, and therefore have a negative effect on the polishing properties of the resulting polishing liquid, such as worsening the mechanical properties of the colloidal silica in the resulting silica sol and reducing the polishing speed. In addition, since the resulting polishing liquid contains silica with a low degree of condensation, the resulting polishing liquid is poorly removable from the polished object after polishing. Furthermore, problems such as aggregation, precipitation, thickening, and gelation of silica in the silica sol and polishing liquid are likely to occur, and the resulting silica sol and polishing liquid exhibit unstable behavior and poor storage stability.

The methods of producing silica sol by hydrolysis and condensation reactions of alkoxysilanes disclosed in Patent Documents 1 to 3 do not disclose any method for dealing with such intermediate products, and depending on the production conditions, a silica sol containing a large amount of intermediate products may be obtained. As a result, the polishing properties of the resulting polishing liquid are adversely affected, the resulting polishing liquid is poorly removable from the polished object after polishing, and the resulting silica sol and polishing liquid have poor storage stability.

The present invention was made in consideration of these problems, and an object of the present invention is to provide a method for producing silica sol with fewer intermediate products. Another object of the present invention is to provide a method for suppressing intermediate products in silica sol, thereby reducing the amount of intermediate products.

Conventionally, there exists a lot of silica sol containing intermediate products, and because it has been used as a polishing liquid without removing the intermediate products, the polishing properties and storage stability of the resulting polishing liquid were not sufficient. However, as a result of extensive research, the inventors have discovered that by adding an oxidizing agent, such as a neutral oxidizing agent, to silica sol, a silica sol with fewer intermediate products can be obtained, and have completed the present invention.

That is, the gist of the present invention is as follows.
[1] A method for producing a silica sol, comprising the following steps (1) and (2):
Step (1) is a step of hydrolyzing and condensing tetraalkoxysilane to obtain a silica sol reaction liquid.
Step (2): 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] The method for producing a silica sol according to [1] or [2], wherein 0.005 to 5 parts by mass of an oxidizing agent is added per 100 parts by mass of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction, calculated as silica.
[4] The method for producing a silica sol according to any one of [1] to [3], comprising the following step (3) after step (2):
Step (3) A step of heating the silica sol reaction liquid to which the oxidizing agent has been added.
[5] A method for suppressing intermediate products in a silica sol, comprising adding an oxidizing agent to a silica sol reaction liquid obtained by hydrolysis and condensation of a tetraalkoxysilane.
[6] The method for suppressing intermediate products in a silica sol according to [5], wherein the oxidizing agent is a neutral oxidizing agent.
[7] The method for suppressing intermediate products in a silica sol according to [5] or [6], wherein an oxidizing agent is added in an amount of 0.005 parts by mass to 5 parts by mass per 100 parts by mass of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction, calculated as silica.
[8] The method for suppressing intermediate products in a silica sol according to any one of [5] to [7], wherein the silica sol reaction liquid to which an oxidizing agent has been added is heated.

The method for producing silica sol of the present invention can obtain a silica sol with a small amount of intermediate products, and the polishing properties of the resulting polishing liquid are excellent, the removability of the resulting polishing liquid from the polished object after polishing is excellent, and the storage stability of the resulting silica sol and the resulting polishing liquid is excellent. Furthermore, the method for suppressing intermediate products in silica sol of the present invention can easily reduce the amount of intermediate products in the silica sol, and the polishing properties of the resulting polishing liquid are excellent, the removability of the resulting polishing liquid from the polished object after polishing is excellent, and the storage stability of the resulting silica sol and the resulting polishing liquid is excellent.

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

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

(Method for producing silica sol)
The method for producing a silica sol of the present invention includes the following steps (1) and (2).
Step (1) is a step of hydrolyzing and condensing tetraalkoxysilane to obtain a silica sol reaction liquid.
Step (2): Adding an oxidizing agent to the silica sol reaction liquid.

(Step (1))
Step (1) is a step of hydrolyzing and condensing tetraalkoxysilane to obtain a silica sol reaction liquid. The method of hydrolyzing and condensing tetraalkoxysilane to obtain a silica sol reaction liquid may be a known production method.

Examples of tetraalkoxysilanes include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetraisopropoxysilane. These tetraalkoxysilanes may be used alone or in combination of two or more. Among these tetraalkoxysilanes, tetramethoxysilane and tetraethoxysilane are preferred, and tetramethoxysilane is more preferred, because they undergo rapid hydrolysis and condensation reactions, are less likely to leave unreacted materials, are highly productive, and can easily produce a stable silica sol.

In addition to tetraalkoxysilane, the raw material for colloidal silica may also be a low condensate obtained by partially hydrolyzing and condensing tetraalkoxysilane.

Examples of the solvents and dispersion media used in the hydrolysis and condensation reactions 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 and dispersion media, water and alcohol are preferred, and water and methanol are more preferred, because the solvents used in the hydrolysis and condensation reactions are the same as the by-products, and they are convenient to manufacture.

The hydrolysis reaction and the condensation reaction may be carried out in the presence or absence of a catalyst, but is preferably carried out in the presence of a catalyst since this can accelerate the hydrolysis reaction and the condensation reaction.
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 catalysts such as ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. Among these catalysts, alkali catalysts are preferred because they have excellent catalytic action and are easy to control the particle shape, and ammonia is more preferred because it can suppress the inclusion of metal impurities, is highly volatile, and is easily removable after the condensation reaction.

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

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

The oxidizing agent has the effect of suppressing intermediate products 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, neutral oxidizing agents are preferred, and hydrogen peroxide is more preferred, since they can efficiently suppress intermediate products while maintaining the physical properties of the colloidal silica in the silica sol, and can maintain the pH of the silica sol near neutral.

In this specification, the intermediate product refers to a portion that looks like the portion surrounded by the black line in FIG. 1 or FIG. 2 in a field emission scanning electron microscope (FE-SEM) image taken at a magnification of 100,000 to 200,000 using an FE-SEM.
This intermediate product is thought to be silica that remains as a solid due to insufficient growth during or after the production of silica sol by hydrolysis and condensation reaction of alkoxysilane, or silica that precipitates from dissolved silicic acid after production.

The amount of the oxidizing agent added to the silica sol reaction liquid is preferably 0.005 parts by mass to 5 parts by mass, and more preferably 0.01 parts by mass to 1 part by mass, relative to 100 parts by mass of the silica-equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and condensation reaction. When the amount of the oxidizing agent added is 0.005 parts by mass or more, intermediate products can be efficiently suppressed. Furthermore, when the amount of the oxidizing agent added is 5 parts by mass or less, the physical properties of the colloidal silica in the silica sol can be maintained.
The silica-equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction refers to the theoretical amount of silica when the entire amount of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction is converted into silica through the hydrolysis reaction and the condensation reaction.

After step (2), other steps may be included.
It is preferable to include a step (3) of heating the silica sol reaction liquid to which the oxidizing agent has been added after the step (2), since this allows efficient suppression of intermediate products.

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

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

(Favourable properties of silica sol)
The content of colloidal silica in the silica sol is preferably 3 to 50 mass%, more preferably 4 to 40 mass%, and even more preferably 5 to 30 mass% in 100 mass% of the silica sol. When the content of colloidal silica is 3 mass% or more, the polishing rate for a polished object such as a silicon wafer is excellent. When the content of colloidal silica is 50 mass% or less, aggregation and precipitation of colloidal silica are suppressed, and unevenness in the concentration of colloidal silica in the silica sol can be suppressed.

The content of the solvent/dispersion medium in the silica sol is preferably 50-97% by mass, more preferably 60-96% by mass, and even more preferably 70-95% by mass, based on 100% by mass of the silica sol. If the content of the solvent/dispersion medium is 50% by mass or more, aggregation and precipitation of colloidal silica can be suppressed, and unevenness in the concentration of colloidal silica in the silica sol can be suppressed. Furthermore, if the content of the solvent/dispersion medium is 97% by mass or less, the polishing rate for the object to be polished, such as a silicon wafer, is excellent.

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

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

The pH of the silica sol is preferably 6 to 9, and more preferably 7 to 8. When the pH of the silica sol is 6 or more, the silica sol has excellent long-term storage stability. Furthermore, when the pH of the silica sol is 9 or less, aggregation and precipitation of colloidal silica can be suppressed, and uneven concentration of colloidal silica in the silica sol can be suppressed.

The metal impurity content in the silica sol is preferably 1 ppm or less, more preferably 0.5 ppm or less, and even more preferably 0.1 ppm or less.

When polishing silicon wafers for semiconductor devices, metallic impurities adhere to and contaminate the surface of the workpiece being polished, adversely affecting the characteristics of the wafer and diffusing into the interior of the wafer, degrading its quality, resulting in a significant decrease in the performance of semiconductor devices manufactured from such wafers.
Furthermore, when metal impurities are present in the silica sol, a coordinate interaction occurs between the surface silanol groups, which exhibit acidity, and the metal impurities, changing the chemical properties (acidity, etc.) of the surface silanol groups and the three-dimensional environment of the colloidal silica surface (ease of aggregation of colloidal silica, etc.), thereby affecting the polishing rate.

The metal impurity content in silica sol is measured by inductively coupled plasma mass spectrometry (ICP-MS). Specifically, 2 g of silica sol is accurately weighed out, sulfuric acid and hydrofluoric acid are added, and the mixture is heated, dissolved, and evaporated. Pure water is added to the remaining sulfuric acid droplets so that the total amount is exactly 10 g to create a test solution, which is then measured using an inductively coupled plasma mass spectrometry device. 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 reduced to 1 ppm or less by obtaining the silica sol by carrying out a hydrolysis reaction and a condensation reaction using tetraalkoxysilane as a main raw material.
In the method of deionizing alkali silicate such as water glass, sodium and the like derived from the raw material remain, so it is difficult to reduce the metal impurity content in the silica sol to 1 ppm or less.

In order to reduce the metal impurity content in the silica sol, it is preferable to use dispersants and additives with extremely low or no metal content in the production of the silica sol, to use reaction vessels and the like that are extremely low in metal contamination, and to use equipment that maintains the production site in an environment with extremely low metal contamination.

The average primary particle size of 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 size of colloidal silica is 10 nm or more, the polishing rate for a workpiece such as a silicon wafer is excellent, and the storage stability of the silica sol is excellent. Furthermore, when the average primary particle size of colloidal silica is 200 nm or less, the surface roughness and scratches of a workpiece such as a silicon wafer during polishing can be reduced, and sedimentation of 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 the colloidal silica is measured using an automatic specific surface area measuring device, and the average primary particle size is calculated using the following formula (1).
Average primary particle diameter (nm) = 6000/(specific surface area (m ² /g) x density (g/cm ³ )) ... (1)

The average primary particle size of colloidal silica can be set within the desired range using 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 a polished object such as a silicon wafer is excellent, the removability of particles and the like during cleaning after polishing is excellent, and the storage stability of the 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 a polished object such as a silicon wafer during polishing can be reduced, the removability of particles and the like during cleaning after polishing can be excellent, and sedimentation of the colloidal silica can be suppressed.
The average secondary particle size of colloidal silica is measured by DLS method, specifically, using a dynamic light scattering particle size measuring device.

The average secondary particle size of colloidal silica can be set within the desired range using 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 the colloidal silica is 15 or more, the polishing rate for a workpiece such as a silicon wafer is excellent, and the productivity of the silicon wafer is excellent. When the cv value of the colloidal silica is 50 or less, the surface roughness and scratches on a workpiece such as a silicon wafer during polishing can be reduced, and the removal of particles and the like during cleaning after polishing is excellent.
The cv value of colloidal silica is calculated by measuring the average secondary particle size of colloidal silica using a dynamic light scattering particle size measurement device and 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 a polished object such as a silicon wafer is excellent, and the productivity of silicon wafers is excellent. When the association ratio of colloidal silica is 2.5 or less, the surface roughness and scratches of a polished object such as a silicon wafer during polishing can be reduced, and the aggregation of colloidal silica can be suppressed.
The association ratio of colloidal silica is calculated using the following formula (3) 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.
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, and 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 a polished object such as a silicon wafer is excellent. When the viscosity of the silica sol is 50 mPa·s or less, intermediate products are sufficiently removed, and the removal of particles and the like during cleaning after polishing of a polished object such as a silicon wafer is excellent, and the storage stability of the silica sol is excellent.
The viscosity of the silica sol is a value measured using an E-type viscometer under conditions of 25° C. and a shear rate of 150/sec.

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

Examples of the water-soluble polymer include cellulose derivatives, polyvinyl alcohol, polyvinylpyrrolidone, copolymers having a polyvinylpyrrolidone skeleton, and polymers having a polyoxyalkylene structure.
Examples of cellulose derivatives include hydroxyethyl cellulose, hydrolyzed hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, ethyl cellulose, ethyl hydroxyethyl cellulose, and carboxymethyl cellulose.
An example of the copolymer having a polyvinylpyrrolidone skeleton is a graft copolymer of polyvinyl alcohol and polyvinylpyrrolidone.
Examples of polymers having a polyoxyalkylene structure include polyoxyethylene, polyoxypropylene, and copolymers 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, cellulose derivatives are preferred, and hydroxyethyl cellulose is more preferred, since they have high affinity with the surface silanol groups of colloidal silica and act synergistically to impart good hydrophilicity to the surface of the polished object.

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. If the mass average molecular weight of the water-soluble polymer is 1,000 or more, the hydrophilicity of the polishing liquid is improved. Furthermore, if the mass average molecular weight of the water-soluble polymer is 3,000,000 or less, the affinity with silica sol is excellent, and the polishing rate for the object to be polished, such as a silicon wafer, is excellent.

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

The content of the water-soluble polymer is preferably 0.02 to 10 mass% and more preferably 0.05 to 5 mass% in 100 mass% of the polishing liquid. If the content of the water-soluble polymer is 0.02 mass% or more, the hydrophilicity of the polishing liquid is improved. Furthermore, if the content of the water-soluble polymer is 10 mass% or less, aggregation and precipitation of colloidal silica during preparation of the polishing liquid can be suppressed.

The pH of the polishing liquid is preferably 8 to 12, and more preferably 9 to 11. When the pH of the polishing liquid is 8 or more, aggregation and precipitation of colloidal silica in the polishing liquid can be suppressed. Furthermore, 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 adjusted to a desired range by adding a pH adjuster.

(Application)
The silica sol produced by the method for producing a silica sol of the present invention and a polishing liquid containing the same can be suitably used for polishing purposes, for example, polishing semiconductor materials such as silicon wafers, polishing electronic materials such as hard disk substrates, polishing in the planarization step in the production of integrated circuits (chemical mechanical polishing), polishing synthetic quartz glass substrates used for photomasks and liquid crystals, polishing magnetic disk substrates, and the like, and among these, they can be particularly suitably used for polishing silicon wafers and 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 the solution may be dropped onto a polishing pad set on the platen of a polishing machine for polishing.

The present invention will be described in more detail below using examples, but the present invention is not limited to the description of the following examples as long as it does not deviate from the gist of the invention.

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

(Measurement of intermediate products in silica sol)
For the silica sols obtained in Example 2 and Comparative Example 1, the area ratio of the intermediate product was calculated by the following procedure.
First, the silica sol was taken and diluted 5,000 times with ultrapure water. 5 μL of the 5,000-fold diluted solution was taken and dropped onto a mirror silicon wafer (manufactured by Electronics and Materials Corporation), and dried at 50° C. for 10 minutes. This was attached to a field emission scanning electron microscope (FE-SEM, model name "S-5200", manufactured by Hitachi High-Technologies Corporation), and 100 to 200 colloidal silica particles were observed at an acceleration voltage of 5 kV and a magnification of 150,000 times, and images were taken.
Colloidal silica particles and the intermediate product were distinguished by importing the captured image into image analysis particle size distribution measurement software (software name "Mac-View Ver. 4", manufactured by Mountec Co., Ltd.). When the area of the intermediate product is a and the area of the colloidal silica particles is b, the ratio of the area 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 Figure 1, and an FE-SEM image of the silica sol obtained in Comparative Example 1 is shown in Figure 2. The intermediate product is the area in the FE-SEM image that looks like the area surrounded by the black line in Figures 1 and 2.

(Measurement of Viscosity of Silica Sol)
2 mL of the silica sol obtained in each of the Examples and Comparative Examples was placed in a sample cup, and the viscosity of the silica sol was measured using an E-type viscometer (model name "TVE-25", manufactured by Toki Sangyo Co., Ltd.) at 25°C and a shear rate of 150/sec.

(Measurement of average primary particle size)
The silica sol obtained in each of the Examples and Comparative Examples was freeze-dried, and the specific surface area of the colloidal silica was measured using an automatic specific surface area measuring device (model name "Flowsorb II", manufactured by Shimadzu Corporation). The average primary particle size was calculated using the following formula (1), assuming a density of 2.2 g/ ^cm3 .
Average primary particle diameter (nm) = 6000/(specific surface area (m ² /g) x density (g/cm ³ )) ... (1)

(Measurement of average secondary particle size and cv value)
For the silica sols obtained in the Examples and Comparative Examples, the average secondary particle size of the colloidal silica was measured using a dynamic light scattering particle size measurement device (DLS, model name "Zetersizer Nano ZS", manufactured by Malvern Instruments), and the cv value was calculated using the following formula (2).
cv value=(standard deviation (nm)/average secondary particle diameter (nm))×100 (2)

(Calculation of Association Ratio)
The association ratio was calculated from the measured average primary particle size and average secondary particle size 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 volume ratio of 3:1 to prepare a raw material solution. A reaction solvent prepared by mixing methanol, pure water, and ammonia in advance was charged into a reaction tank equipped with a thermometer, a stirrer, a supply pipe, and a distillation line. 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 adjusted 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. The temperature of the obtained silica sol reaction liquid was raised to remove methanol and ammonia while adjusting the liquid volume by adding pure water so that the colloidal silica content was about 20 mass%, and a silica sol reaction liquid with a colloidal silica content of about 20 mass% was obtained.

[Example 1]
To the silica sol reaction liquid obtained in the Production Example, 35% by mass of hydrogen peroxide was added so that the amount of hydrogen peroxide was 0.5 parts by mass per 100 parts by mass of the silica-equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and condensation reaction, thereby obtaining a silica sol.
The evaluation results of the obtained silica sol are shown in Table 1.

[Example 2]
To the silica sol reaction liquid obtained in the Production Example, 35% by mass of hydrogen peroxide solution was added so that the hydrogen peroxide was 0.5 parts by mass per 100 parts by mass of the silica-equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and condensation reaction, and the mixture was heated with a heater and maintained at 50°C for 1 hour, and further maintained at 80°C for 1 hour to obtain a silica sol.
The evaluation results of the obtained silica sol are shown in Table 1.

[Comparative Example 1]
The silica sol reaction liquid obtained in Production Example was used as a silica sol as it was.
The evaluation results of the obtained silica sol are shown in Table 1.

As can be seen from Table 1 and FIGS. 1 and 2, the silica sol obtained by the production method of Example 2, which includes a step of adding an oxidizing agent, has a lower area ratio of the intermediate product and the amount of the intermediate product is suppressed, compared to the silica sol obtained by the production method of Comparative Example 1, which does not include a step of adding an oxidizing agent.
Furthermore, as can be seen from Table 1, although the silica sol obtained by the production method of the Example including a step of adding an oxidizing agent and the silica sol obtained by the production method of the Comparative Example not including a step of adding an oxidizing agent have almost the same physical properties of colloidal silica, namely, average primary particle size, average secondary particle size, cv value, and association ratio, there is a difference in the viscosity of the silica sol due to the generation of intermediate products. Therefore, by including a step of adding an oxidizing agent, it is possible to efficiently suppress the intermediate products while maintaining the physical properties of the colloidal silica in the silica sol.

The silica sol produced by the silica sol production method of the present invention and the polishing liquid containing it can be suitably used for polishing purposes, such as polishing semiconductor materials such as silicon wafers, polishing electronic materials such as hard disk substrates, polishing in the planarization process when manufacturing integrated circuits (chemical mechanical polishing), polishing synthetic quartz glass substrates used in photomasks and liquid crystals, polishing magnetic disk substrates, etc., and can be particularly suitably used for polishing silicon wafers and chemical mechanical polishing.

Claims (8)

A method for producing a silica sol, comprising the following steps (1) and (2), wherein the method does not include a step of adding a dispersant, does not include a step of adding an anionic compound having one or more double bonds, and does not include a step of converting into a sulfonic acid group:
Step (1) A step of hydrolyzing and condensing tetraalkoxysilane in the presence of ammonia to obtain a silica sol reaction liquid.
Step (2) is a step of adding an oxidizing agent containing hydrogen peroxide to the silica sol reaction liquid, wherein the amount of the oxidizing agent added is 5 parts by mass or less per 100 parts by mass of the silica-equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction. The method for producing a silica sol according to claim 1 , wherein the oxidizing agent is added in an amount of 0.005 parts by mass to 5 parts by mass based on 100 parts by mass of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction calculated as silica. The method for producing a silica sol according to claim 1 or 2 , comprising the following step (3) after step (2):
Step (3) A step of heating the silica sol reaction liquid to which the oxidizing agent has been added. A method for suppressing intermediate products in a silica sol, comprising adding an oxidizing agent containing hydrogen peroxide to a silica sol reaction liquid obtained by hydrolyzing and condensing a tetraalkoxysilane in the presence of ammonia, wherein the amount of the oxidizing agent added is 5 parts by mass or less per 100 parts by mass of the silica-equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction, and the method does not include a step of adding a dispersing agent, a step of adding an anionic compound having one or more double bonds, or a step of converting into a sulfonic acid group. The method for suppressing intermediate products in silica sol according to claim 4 , wherein 0.005 to 5 parts by mass of an oxidizing agent is added per 100 parts by mass of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction, calculated as silica. 6. The method for suppressing intermediate products in silica sol according to claim 4 or 5 , wherein the silica sol reaction liquid to which the oxidizing agent has been added is heated. The method for producing a silica sol according to any one of claims 1 to 3 , which does not include a step of adding a metal nitrate or a metal sulfate. 7. The method for inhibiting intermediate products in a silica sol according to claim 4 , which does not include a step of adding a metal nitrate or a metal sulfate.

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