Method for producing silica sol and method for suppressing intermediate product in silica sol

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

Technical Field

[0001] The present invention relates to a method for producing silica sol. Further, the present invention relates to a method for suppressing intermediate products in silica sol.

Background Art

[0002] As a method for polishing the surface of materials such as metals and inorganic compounds, a polishing method using a polishing liquid is known. Among them, in the final finishing polishing of prime silicon wafers for semiconductors and these recycled silicon wafers, and in chemical mechanical polishing (CMP) such as planarization of interlayer insulating films, formation of metal plugs, and formation of embedded wirings during semiconductor device manufacturing, since the surface state greatly affects semiconductor characteristics, the surfaces and end faces of these parts are required to be polished with extremely high precision.

[0003] In such precision polishing, a polishing composition containing silica sol is adopted, and colloidal silica is widely used as the abrasive grains that are the main component. Colloidal silica is known to be obtained by thermal decomposition of silicon tetrachloride (such as fumed silica), by deionization of alkali silicates such as water glass, by hydrolysis reaction and condensation reaction of alkoxysilane (generally referred to as the "sol-gel method"), etc., depending on the manufacturing method.

[0004] Regarding the method for producing silica sol containing colloidal silica, many studies have been made so far. For example, Patent Documents 1 to 3 disclose methods for producing silica sol by hydrolysis reaction and condensation reaction of alkoxysilane.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, intermediate products may be generated during or after the production of silica sol by the hydrolysis reaction and condensation reaction of alkoxysilane. This intermediate product is considered to be silica that remained as a solid with insufficient growth, silica precipitated from dissolved silicic acid after production, etc. Since such an intermediate product is considered to be silica with a lower degree of condensation than the desired colloidal silica, it deteriorates the mechanical properties of the colloidal silica in the obtained silica sol, reduces the polishing rate, etc., and has an adverse effect on the polishing properties of the obtained polishing liquid. In addition, since the obtained polishing liquid contains silica with a low degree of condensation, the removability from the polished object after polishing of the obtained polishing liquid is poor. Furthermore, problems such as aggregation, sedimentation, thickening, and gelation of silica in the silica sol and the polishing liquid are likely to occur, and the obtained silica sol and the obtained polishing liquid show unstable behavior and are poor in storage stability.

[0007] The methods for producing silica sol by the hydrolysis reaction and condensation reaction of alkoxysilane disclosed in Patent Documents 1 to 3 do not describe anything about 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, it has an adverse effect on the polishing properties of the obtained polishing liquid, is poor in removability from the polished object after polishing of the obtained polishing liquid, and is poor in storage stability of the obtained silica sol and the obtained polishing liquid.

[0008] 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 with few intermediate products. Another object of the present invention is to provide a method for suppressing intermediate products in a silica sol that reduces intermediate products.

Means for Solving the Problems

[0009] Conventionally, there have been many silica sols containing intermediate products, and since they have been used as polishing liquids without removing the intermediate products as they are, the polishing characteristics and storage stability of the obtained polishing liquids have not been sufficient. However, as a result of intensive studies by the present inventors, it has been found that by adding an oxidizing agent typified by a neutral oxidizing agent to the silica sol, a silica sol with few intermediate products can be obtained, and the present invention has been completed.

[0010] 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) A step of hydrolyzing and condensing tetraalkoxysilane to obtain a silica sol reaction solution. Step (2) A step of adding an oxidizing agent to the silica sol reaction solution. [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 parts by mass to 5 parts by mass of an oxidizing agent is added with respect to 100 parts by mass of the silica conversion content of tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction. [4] The method for producing a silica sol according to any one of [1] to [3], including the following step (3) after step (2). Step (3) A step of heating the silica sol reaction solution to which the oxidizing agent has been added. [5] A method for suppressing intermediate products in a silica sol, wherein an oxidizing agent is added to a silica sol reaction solution obtained by hydrolyzing and condensing 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 0.005 parts by mass to 5 parts by mass of an oxidizing agent is added with respect to 100 parts by mass of the silica conversion content of tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction. [8] The method for suppressing intermediate products in a silica sol according to any one of [5] to [7], wherein the silica sol reaction solution to which the oxidizing agent has been added is heated.

Effects of the Invention

[0011] The method for producing the silica sol of the present invention can obtain a silica sol with few intermediate products, and the obtained polishing liquid has excellent polishing characteristics, excellent removability from the polished object after polishing, and excellent storage stability of the obtained silica sol and the obtained polishing liquid. Further, the method for suppressing intermediate products in the silica sol of the present invention can easily reduce the intermediate products in the silica sol, and the obtained polishing liquid has excellent polishing characteristics, excellent removability from the polished object after polishing, and excellent storage stability of the obtained silica sol and the obtained polishing liquid.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0013] The present invention will be described in detail below. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist. In the present specification, when the expression "~" is used, it is used as an expression including the numerical values or physical property values before and after it.

[0014] (Method for Producing Silica Sol) The method for producing the silica sol of the present invention includes the following steps (1) and (2). Step (1) A step of hydrolyzing and condensing tetraalkoxysilane to obtain a silica sol reaction solution. Step (2) A step of adding an oxidizing agent to the silica sol reaction solution.

[0015] (Step (1)) Step (1) is a step of hydrolyzing and condensing tetraalkoxysilane to obtain a silica sol reaction solution. As a method for hydrolyzing and condensing tetraalkoxysilane to obtain a silica sol reaction solution, a known production method may be used.

[0016] Examples of the tetraalkoxysilane include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetraisopropoxysilane, and the like. These tetraalkoxysilanes may be used alone or in combination of two or more. Among these tetraalkoxysilanes, tetramethoxysilane and tetraethoxysilane are preferred because they have a fast hydrolysis reaction and condensation reaction, are less likely to leave unreacted substances, are excellent in productivity, and can easily obtain a stable silica sol, and tetramethoxysilane is more preferred.

[0017] As the raw material constituting the colloidal silica, in addition to the tetraalkoxysilane, a low condensate obtained by partially hydrolyzing and condensing the tetraalkoxysilane may be used.

[0018] Examples of the solvent / dispersion medium used in the reaction for performing the hydrolysis reaction and condensation reaction include water, methanol, ethanol, propanol, isopropanol, ethylene glycol, and the like. These solvent / dispersion media may be used alone or in combination of two or more. Among these solvent / dispersion media, water and alcohol are preferred because the one used in the hydrolysis reaction and condensation reaction is the same as the by-product, and it is excellent in production convenience, and water and methanol are more preferred.

[0019] When performing the hydrolysis reaction and condensation reaction, it may be carried out in the presence of a catalyst or without a catalyst, but the presence of a catalyst is preferred because the hydrolysis reaction and 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 catalysts such as ethylenediamine, diethylenetriamine, triethylenetetramine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. Among these catalysts, an alkali catalyst is preferred because it has excellent catalytic activity and is easy to control the particle shape, and ammonia is more preferred because it can suppress the mixing of metal impurities and has high volatility and excellent removability after the condensation reaction.

[0020] Other steps may be included between step (1) and step (2). It is preferable to include a step of removing unnecessary components and adding necessary components among the components in the silica sol reaction solution between step (1) and step (2). For example, removing solvents and dispersion media such as alcohol, and catalysts such as ammonia, and adding water so as to achieve a desired content rate can be mentioned.

[0021] (Step (2)) Step (2) is a step of adding an oxidizing agent to the silica sol reaction solution.

[0022] The oxidizing agent has an effect of suppressing intermediate products in the silica sol. For example, neutral oxidizing agents such as hydrogen peroxide and benzoyl peroxide; acidic oxidizing agents such as sulfuric acid and nitric acid, etc. can be mentioned. These oxidizing agents may be used alone or in combination of two or more. Among these oxidizing agents, since the physical properties of the colloidal silica in the silica sol can be maintained while efficiently suppressing intermediate products, and the pH of the silica sol can be maintained near neutrality, neutral oxidizing agents are preferable, and hydrogen peroxide is more preferable.

[0023] In this specification, the intermediate product refers to a portion that appears as a portion surrounded by a black line in FIGS. 1 and 2 in the FE-SEM image taken at a magnification of 100,000 to 200,000 times using a field emission scanning electron microscope (FE-SEM). This intermediate product is considered to be silica that remained as a solid with insufficient growth during or after the production of silica sol by the hydrolysis reaction and condensation reaction of alkoxysilane, or silica precipitated from dissolved silicic acid after production, etc.

[0024] The addition amount of the oxidizing agent to the silica sol reaction solution is preferably 0.005 to 5 parts by mass, more preferably 0.01 to 1 part by mass, based on 100 parts by mass of the silica equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction. When the addition amount of the oxidizing agent is 0.005 parts by mass or more, the intermediate product can be efficiently suppressed. Further, when the addition amount of the oxidizing agent 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 total amount of the tetraalkoxysilane subjected to the hydrolysis reaction and the condensation reaction becomes silica through the hydrolysis reaction and the condensation reaction.

[0025] After step (2), other steps may be included. Since the intermediate product can be efficiently suppressed, it is preferable to include step (3) of heating the silica sol reaction solution to which the oxidizing agent has been added after step (2).

[0026] The temperature for heating in step (3) is preferably 30°C to 100°C, more preferably 40°C to 95°C. When the temperature for heating in step (3) is 30°C or higher, the intermediate product can be efficiently suppressed. Further, when the temperature for heating in step (3) is 100°C or lower, the volatilization of the dispersion medium of the silica sol and the oxidizing agent can be suppressed.

[0027] (Method for suppressing intermediate product 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 solution obtained by hydrolyzing and condensing tetraalkoxysilane, and steps (1) and (2) described above may be performed, and if necessary, step (3) described above may be performed.

[0028] (Preferred properties of silica sol) The content rate of colloidal silica in the silica sol is preferably 3 to 50% by mass, more preferably 4 to 40% by mass, and still more preferably 5 to 30% by mass in 100% by mass of the silica sol. When the content rate of colloidal silica is 3% by mass or more, the polishing rate with respect to the object to be polished typified by a silicon wafer is excellent. Further, when the content rate of colloidal silica is 50% by mass or less, aggregation and sedimentation of colloidal silica can be suppressed, and unevenness in the concentration of colloidal silica in the silica sol can be suppressed.

[0029] The content rate of the solvent / dispersion medium in the silica sol is preferably 50 to 97% by mass, more preferably 60 to 96% by mass, and still more preferably 70 to 95% by mass in 100% by mass of the silica sol. When the content rate of the solvent / dispersion medium is 50% by mass or more, aggregation and sedimentation of colloidal silica can be suppressed, and unevenness in the concentration of colloidal silica in the silica sol can be suppressed. Further, when the content rate of the solvent / dispersion medium is 97% by mass or less, the polishing rate with respect to the object to be polished typified by a silicon wafer is excellent.

[0030] The content rate of colloidal silica and the solvent / dispersion medium in the silica sol can be set to a desired range by a step of removing unnecessary components and adding necessary components among the components in the above-described silica sol reaction solution.

[0031] Examples of the solvent / dispersion medium in the silica sol include water, methanol, ethanol, propanol, isopropanol, ethylene glycol, and the like. These solvent / dispersion media may be used alone or in combination of two or more. Among these solvent / dispersion media, water and alcohol are preferable, and water is more preferable because of excellent affinity with colloidal silica.

[0032] 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 long-term storage stability of the silica sol is excellent. Further, when the pH of the silica sol is 9 or less, aggregation and sedimentation of colloidal silica can be suppressed, and unevenness in the concentration of colloidal silica in the silica sol can be suppressed.

[0033] 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.

[0034] In the polishing of a silicon wafer for a semiconductor device, when metal impurities adhere to and contaminate the surface of the object to be polished, it not only adversely affects the wafer characteristics but also diffuses into the wafer interior, deteriorating the quality. As a result, the performance of the semiconductor device manufactured using such a wafer is significantly reduced. In addition, when metal impurities are present in the silica sol, a coordination interaction occurs between the surface silanol groups showing acidity and the metal impurities, changing the chemical properties (such as acidity) of the surface silanol groups, changing the steric environment on the colloidal silica surface (such as the ease of aggregation of colloidal silica), and affecting the polishing rate.

[0035] The metal impurity content in the silica sol shall be measured by high-frequency inductively coupled plasma mass spectrometry (ICP-MS). Specifically, accurately weigh 2 g of the silica sol, 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 to prepare a test solution, and measure it 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.

[0036] The metal impurity content in the silica sol can be made 1 ppm or less by obtaining the silica sol through a hydrolysis reaction and a condensation reaction using tetraalkoxysilane as the main raw material. In the method by deionization of alkali silicate such as water glass, since sodium and the like derived from the raw material remain, it is difficult to make the metal impurity content in the silica sol 1 ppm or less.

[0037] In order to reduce the metal impurity content in the silica sol, it is preferable to use a dispersant, additive, etc. used in the production of the silica sol that has an extremely low metal content or no metal at all, use a reaction vessel, etc. that has extremely low metal contamination, and furthermore, to have equipment that keeps the production site in an environment with extremely low metal contamination.

[0038] 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, it has excellent polishing rate with respect to the workpiece to be polished typified by a silicon wafer and excellent storage stability of the silica sol. Also, when the average primary particle diameter of the colloidal silica is 200 nm or less, it is possible to reduce the surface roughness and scratches of the workpiece to be polished typified by a silicon wafer during polishing, and it is possible to suppress the sedimentation of the colloidal silica. The average primary particle diameter of the colloidal silica shall be measured by the BET method. Specifically, the specific surface area of the colloidal silica is measured using a specific surface area automatic 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)

[0039] The average primary particle diameter of the colloidal silica can be set within a desired range by known conditions and methods.

[0040] 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, it has excellent polishing rate with respect to the workpiece to be polished typified by a silicon wafer, excellent removability of particles, etc. in cleaning after polishing, and excellent storage stability of the silica sol. When the average secondary particle diameter of the colloidal silica is 300 nm or less, it is possible to reduce the surface roughness and scratches of the workpiece to be polished typified by a silicon wafer during polishing, excellent removability of particles, etc. in cleaning after polishing, and it is possible to suppress the sedimentation of the colloidal silica. The average secondary particle diameter of the colloidal silica shall be measured by the DLS method. Specifically, it shall be measured using a dynamic light scattering particle size measuring device.

[0041] The average secondary particle diameter of the colloidal silica can be set within a desired range by known conditions and methods.

[0042] The cv value of the colloidal silica in the silica sol is preferably from 15 to 50, more preferably from 20 to 40. When the cv value of the colloidal silica is 15 or more, the polishing rate with respect to the object to be polished typified by a silicon wafer is excellent, and the productivity of the silicon wafer is excellent. Further, when the cv value of the colloidal silica is 50 or less, the surface roughness and scratches of the object to be polished typified by a silicon wafer during polishing can be reduced, and the removability of particles and the like in the cleaning after polishing is excellent. The cv value of the colloidal silica is measured by measuring the average secondary particle diameter of the colloidal silica 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)

[0043] The aggregation ratio of the colloidal silica in the silica sol is preferably from 1.2 to 2.5, more preferably from 1.5 to 2.2. When the aggregation ratio of the colloidal silica is 1.2 or more, the polishing rate with respect to the object to be polished typified by a silicon wafer is excellent, and the productivity of the silicon wafer is excellent. Further, when the aggregation ratio of the colloidal silica is 2.5 or less, the surface roughness and scratches of the object to be polished typified by a silicon wafer during polishing can be reduced, and the aggregation of the colloidal silica can be suppressed. The aggregation ratio of the colloidal silica is calculated using the following formula (3) from the average primary particle diameter measured by the above-described measurement method and the average secondary particle diameter measured by the above-described measurement method. Aggregation ratio = average secondary particle diameter / average primary particle diameter ··· (3)

[0044] The viscosity of the silica sol is preferably from 1 mPa·s to 50 mPa·s, more preferably from 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 workpiece typified by a silicon wafer is excellent. Further, when the viscosity of the silica sol is 50 mPa·s or less, the intermediate product can be sufficiently removed, the removability of particles and the like in the cleaning after polishing of a workpiece typified by a silicon wafer is excellent, and the storage stability of the silica sol is excellent. The viscosity of the silica sol is the value measured using an E-type viscometer under the conditions of 25 °C and a shear rate of 150 / second.

[0045] (Polishing liquid) A polishing liquid can be obtained by dissolving a water-soluble polymer in the silica sol. The water-soluble polymer enhances the wettability of the polishing liquid with respect to a workpiece typified by a silicon wafer. The water-soluble polymer is preferably a polymer having a highly hydrophilic functional group, and the affinity between this highly hydrophilic functional group and the surface silanol group of colloidal silica is high, and colloidal silica and the water-soluble polymer are stably dispersed in the vicinity of each other in the polishing liquid. Therefore, when polishing a workpiece typified by a silicon wafer, the effects of colloidal silica and the water-soluble polymer function synergistically.

[0046] Examples of the water-soluble polymer include cellulose derivatives, polyvinyl alcohol, polyvinyl pyrrolidone, copolymers having a polyvinyl pyrrolidone skeleton, polymers having a polyoxyalkylene structure, and the like. Examples of the cellulose derivative include hydroxyethyl cellulose, hydrolyzed hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, ethyl cellulose, ethyl hydroxyethyl cellulose, carboxymethyl cellulose, and the like. Examples of the copolymer having a polyvinyl pyrrolidone skeleton include a graft copolymer of polyvinyl alcohol and polyvinyl pyrrolidone. 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 and hydroxyethyl cellulose is more preferable because it has a high affinity for the surface silanol groups of colloidal silica and acts synergistically to impart good hydrophilicity to the surface of the object to be polished.

[0047] The mass average molecular weight of the water-soluble polymer is preferably from 1,000 to 3,000,000, more preferably from 5,000 to 2,000,000, and still more preferably from 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. Further, when the mass average molecular weight of the water-soluble polymer is 3,000,000 or less, it has excellent affinity with the silica sol and excellent polishing rate with respect to the object to be polished typified by a silicon wafer.

[0048] The mass average molecular weight of the water-soluble polymer shall be measured by size exclusion chromatography under the condition that a 0.1 mol / L NaCl solution is used as the mobile phase in terms of polyethylene oxide.

[0049] The content of the water-soluble polymer is preferably from 0.02 to 10% by mass, more preferably from 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 and sedimentation of colloidal silica during preparation of the polishing liquid can be suppressed.

[0050] The pH of the polishing liquid is preferably from 8 to 12, more preferably from 9 to 11. When the pH of the polishing liquid is 8 or more, aggregation and sedimentation of colloidal silica in the polishing liquid can be suppressed. 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 within a desired range by adding a pH adjuster.

[0051] (Use) The silica sol produced by the method for producing a silica sol of the present invention and the polishing liquid containing the same can be suitably used for polishing applications. For example, polishing of semiconductor materials such as silicon wafers, polishing of electronic materials such as hard disk substrates, polishing in a planarization process when manufacturing integrated circuits (chemical mechanical polishing), polishing of synthetic quartz glass substrates used for photomasks and liquid crystals, polishing of magnetic disk substrates, etc. It can be used, and among them, it can be particularly suitably used for polishing silicon wafers and chemical mechanical polishing. When using the silica sol and the polishing liquid for polishing, it may be carried out in the same manner as known polishing. For example, when polishing a silicon wafer, after adjusting the concentration and adding additives, it may be dropped onto a polishing pad set on a turntable of a polishing machine and polished.

Examples

[0052] Hereinafter, the present invention will be described more specifically using examples, but the present invention is not limited to the description of the following examples as long as the gist thereof is not deviated from.

[0053] (Metal impurity content) 2 g of the silica sol reaction solution obtained in the production example was accurately weighed, sulfuric acid and hydrofluoric acid were added, heated, dissolved, and evaporated, and pure water was added to the remaining sulfuric acid droplets so that the total amount was exactly 10 g to prepare a test solution. The metal impurity content was measured using an inductively coupled plasma mass spectrometer (ICP-MS, model name "ELEMENT2", manufactured by Thermo Fisher Scientific). The metal impurity content was 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, and all of magnesium, cobalt, chromium, copper, manganese, lead, titanium, silver, and nickel were less than 0.001 ppm.

[0054] (Measurement of Intermediate Product in Silica Sol) For the silica sols obtained in Example 2 and Comparative Example 1, the ratio of the area of the intermediate product was calculated by the following procedure. First, an aliquot of the silica sol was taken and diluted 5,000-fold with ultrapure water. Then, 5 μL of the 5,000-fold diluted solution was taken and dropped onto a mirror silicon wafer (manufactured by Electronics End Materials Corporation), and dried at 50 °C for 10 minutes. It was then mounted on 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. The identification between the colloidal silica particles and the intermediate product was carried out by importing the taken images 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 was a and the area of the colloidal silica particles was b, the ratio of the area of the intermediate product was calculated using the following formula (4). Ratio of area of intermediate product (%) = {a / (a + b)} × 100 ··· (4) The FE-SEM images of the silica sol obtained in Example 2 are shown in Fig. 1, and the FE-SEM images of the silica sol obtained in Comparative Example 1 are shown in Fig. 2. The intermediate product refers to a portion that appears as a part surrounded by a black line in Figs. 1 and 2 in the FE-SEM image.

[0055] (Measurement of Viscosity of Silica Sol) 2 mL of the silica sols obtained in the examples and comparative examples were loaded into 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.) under the conditions of 25 °C and a shear rate of 150 / sec.

[0056] (Measurement of Average Primary Particle Size) The silica sols obtained in the examples and comparative examples were freeze-dried, and the specific surface area of the colloidal silica was measured using a specific surface area automatic measurement device (model name "FlowSorb II", manufactured by Shimadzu Corporation). Using the following formula (1), with a density of 2.2 g / cm 3As such, the average primary particle diameter was calculated. Average primary particle diameter (nm) = 6000 / (specific surface area (m 2 / g) × density (g / cm 3 )) ··· (1)

[0057] (Measurement of average secondary particle diameter and cv value) The silica sols obtained in the examples and comparative examples were used to measure the average secondary particle diameter of colloidal silica using a dynamic light scattering particle size analyzer (DLS, model name "Zetasizer Nano ZS", manufactured by Malvern), and the cv value was calculated using the following formula (2). cv value = (standard deviation (nm) / average secondary particle diameter (nm)) × 100 ··· (2)

[0058] (Calculation of aggregation ratio) The aggregation ratio was calculated using the following formula (3) from the measured average primary particle diameter and average secondary particle diameter. Aggregation ratio = average secondary particle diameter / average primary particle diameter ··· (3)

[0059] [Production example] Tetramethoxysilane and methanol were mixed at a volume ratio of 3:1 to prepare a raw material solution. A reaction solvent obtained by previously mixing methanol, pure water, and ammonia was charged into a reaction vessel 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 set at a volume ratio of 2.3:1, and the raw material solution was dropped into the reaction vessel at a uniform rate for 180 minutes to obtain a silica sol reaction solution. The obtained silica sol reaction solution was adjusted in liquid volume by adding pure water so that the content of colloidal silica became about 20% by mass, and the temperature was raised to remove methanol and ammonia, thereby obtaining a silica sol reaction solution with a colloidal silica content of about 20% by mass.

[0060] [Example 1] To the silica sol reaction solution obtained in the production example, 35% hydrogen peroxide solution was added so that the hydrogen peroxide content was 0.5 parts by mass with respect to 100 parts by mass of the silica equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and condensation reaction, and a silica sol was obtained. The evaluation results of the obtained silica sol are shown in Table 1.

[0061] [Example 2] To the silica sol reaction solution obtained in the production example, 35% hydrogen peroxide solution was added so that the hydrogen peroxide content was 0.5 parts by mass with respect to 100 parts by mass of the silica equivalent content of the tetraalkoxysilane subjected to the hydrolysis reaction and condensation reaction, heated with a heater, held at 50 °C for 1 hour, and further held at 80 °C for 1 hour to obtain a silica sol. The evaluation results of the obtained silica sol are shown in Table 1.

[0062] [Comparative Example 1] The silica sol reaction solution obtained in the production example was used as the silica sol as it was. The evaluation results of the obtained silica sol are shown in Table 1.

[0063]

Table 1

[0064] As can be seen from Table 1, Figure 1 and Figure 2, the silica sol obtained by the production method of Example 2 including the step of adding an oxidizing agent has a lower ratio of the area of the intermediate product and a suppressed amount of the intermediate product compared with the silica sol obtained by the production method of Comparative Example 1 not including the step of adding an oxidizing agent. Also, as can be seen from Table 1, although the physical properties of the colloidal silica such as the average primary particle diameter, average secondary particle diameter, cv value, and aggregation ratio are almost the same between the silica sol obtained by the production method of the example including the step of adding an oxidizing agent and the silica sol obtained by the production method of the comparative example not including the step of adding an oxidizing agent, since there is a viscosity difference of the silica sol caused by the generation of the intermediate product, by including the step of adding an oxidizing agent, it is possible to efficiently suppress the intermediate product while maintaining the physical properties of the colloidal silica in the silica sol.

Industrial Applicability

[0065] 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 applications. For example, polishing of semiconductor materials such as silicon wafers, polishing of electronic materials such as hard disk substrates, polishing in a planarization step when manufacturing integrated circuits (chemical mechanical polishing), polishing of synthetic quartz glass substrates used for photomasks and liquid crystals, polishing of magnetic disk substrates, etc. Among them, it can be particularly suitably used for polishing of silicon wafers and chemical mechanical polishing.

Claims

1. A method for producing a silica sol in which the cv value of the average secondary particle diameter of colloidal silica in the silica sol is 15 to 50, including the following steps (1) and (2). Step (1): A step of hydrolyzing and condensing tetraalkoxysilane in the presence of ammonia to obtain a silica sol reaction solution. Step (2): A step of adding hydrogen peroxide to the silica sol reaction solution, wherein the addition amount of the hydrogen peroxide is 5 parts by mass or less with respect to 100 parts by mass of the silica-converted content of the tetraalkoxysilane subjected to the hydrolysis and condensation reaction.

2. The method for producing a silica sol according to claim 1, wherein 0.005 parts by mass to 5 parts by mass of hydrogen peroxide is added with respect to 100 parts by mass of the silica-converted content of the tetraalkoxysilane subjected to the hydrolysis reaction and condensation reaction.

3. The method for producing a silica sol according to claim 1 or 2, including the following step (3) after step (2). Step (3): A step of heating the silica sol reaction solution to which the hydrogen peroxide has been added to 30°C to 100°C.

4. A method for suppressing intermediate products in a silica sol, in which hydrogen peroxide is added to a silica sol reaction solution obtained by hydrolyzing and condensing tetraalkoxysilane in the presence of ammonia, wherein the addition amount of the hydrogen peroxide is 5 parts by mass or less with respect to 100 parts by mass of the silica-converted content of the tetraalkoxysilane subjected to the hydrolysis reaction and condensation reaction, and the cv value of the average secondary particle diameter of colloidal silica in the silica sol is 15 to 50.

5. The method for suppressing intermediate products in a silica sol according to claim 4, wherein 0.005 parts by mass to 5 parts by mass of hydrogen peroxide is added with respect to 100 parts by mass of the silica-converted content of the tetraalkoxysilane subjected to the hydrolysis reaction and condensation reaction.

6. The method for suppressing intermediate products in a silica sol according to claim 4 or 5, wherein the silica sol reaction solution to which hydrogen peroxide has been added is heated to 30°C to 100°C.

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