Silica sol manufacturing method and polishing method

Abstract

To provide a method for producing a silica sol with a low metal content that facilitates temperature control in a step of replacing a dispersion medium in a dispersion liquid of silica particles and has high productivity.SOLUTION: A method for producing a silica sol has a step of replacing a dispersion medium in a dispersion liquid of silica particles in a container having a glass layer on its surface. Preferably, it further has a step of adding, to a solution (A) containing an alkali catalyst, a solution (B) containing tetraalkoxysilane and a solution (C) containing water for hydrolysis and condensation of the tetraalkoxysilane, yielding a dispersion liquid of silica particles. There is also provided a method for polishing using a polishing composition containing the silica sol obtained by the method for producing a silica sol.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing silica sol and a polishing method. [Background technology]

[0002] Polishing methods using polishing liquids are known as methods for polishing the surfaces of materials such as metals and inorganic compounds. In particular, in the final polishing of silicon wafers and chemical mechanical polishing (CMP), the surface condition significantly affects the properties of the final product, so the surfaces and edge faces of these parts must be polished with extremely high precision.

[0003] In such precision polishing, polishing compositions containing silica particles are used, and silica sols such as colloidal silica are widely used as the abrasive grains that are the main component of the polishing compositions. Colloidal silica is known to be produced by different methods, such as by thermal decomposition of silicon tetrachloride (fumed silica, etc.), by deionization of alkali silicate such as water glass, or by hydrolysis and condensation reaction of alkoxysilane (generally referred to as the "sol-gel method").

[0004] Many studies have been conducted on methods for producing silica sol. For example, Patent Document 1 discloses a method for producing a silica sol by producing a dispersion of silica particles through a hydrolysis reaction and a condensation reaction of an alkoxysilane, and then replacing the dispersion medium in the obtained dispersion of silica particles with water. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-108924 Summary of the Invention [Problem to be solved by the invention]

[0006] In general, industrial-scale synthesis of compounds is often carried out in a reaction vessel made of metal such as stainless steel, etc. However, when a metal reaction vessel is used, there is a problem that metal components such as Fe are eluted from the metal constituting the reaction vessel during the production process, and the produced silica sol is contaminated with the metal. However, in precision polishing, metal impurities adhere to the surface of the workpiece, contaminating it and adversely affecting the properties of the workpiece and the final product. Therefore, it is necessary to prevent the inclusion of metal impurities in the silica sol manufacturing process.

[0007] One possible solution to this problem is to provide a resin layer on the surface of a container used to produce silica sol. However, since resin has a lower heat transfer coefficient than metal, covering the surface of a metal container with a resin layer reduces the thermal efficiency when heating or cooling from the outside, which causes a problem of impairing the productivity of silica sol. In particular, since the process of replacing the dispersion medium in a dispersion of silica particles requires heating, using a container with a resin layer on its surface causes a problem of poor heating efficiency and difficulty in controlling the temperature for replacing the dispersion medium.

[0008] To address this issue, Patent Document 1 does not disclose any ideas regarding the container used in the replacement step.

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing a silica sol having a low metal content, which is highly productive and facilitates temperature control in the step of replacing the dispersion medium in a dispersion of silica particles, and a polishing method using this silica sol. [Means for solving the problem]

[0010] As described above, in conventional methods for producing silica sol, when a container having a resin layer on its surface is used in the process of replacing the dispersion medium in a dispersion liquid of silica particles to prevent the incorporation of metal impurities, it becomes difficult to control the temperature in the process of replacing the dispersion medium, resulting in poor productivity. As a result of extensive research, the inventors discovered that by using a container having a glass layer on its surface in the process of replacing the dispersion medium in a dispersion liquid of silica particles, it is possible to easily control the temperature during the replacement of the dispersion medium while suppressing the incorporation of metal impurities and maintaining a low metal content, and thus completed the present invention.

[0011] That is, the gist of the present invention is as follows. [1] A method for producing a silica sol, comprising a step of replacing a dispersion medium in a dispersion of silica particles in a container having a glass layer on the surface. [2] The method for producing a silica sol according to [1], wherein the temperature in the step of replacing the dispersion medium in the dispersion of silica particles is 50°C to 150°C. [3] The method for producing a silica sol according to [1] or [2], further comprising the step of adding a solution (B) containing a tetraalkoxysilane and a solution (C) containing water to a solution (A) containing an alkali catalyst to carry out a hydrolysis reaction and a condensation reaction of the tetraalkoxysilane, thereby obtaining a dispersion of silica particles. [4] The method for producing a silica sol according to any one of [1] to [3], wherein the alcohol is removed in the step of replacing the dispersion medium in the dispersion of silica particles. [5] The method for producing a silica sol according to any one of [1] to [4], wherein the alkali catalyst is removed in the step of replacing the dispersion medium in the dispersion of silica particles. [6] The method for producing a silica sol according to any one of [1] to [5], wherein water is added in the step of replacing the dispersion medium in the dispersion of silica particles. [7] The method for producing silica particles according to any one of [1] to [6], wherein the glass layer is two layers. [8] The method for producing a silica sol according to any one of [1] to [7], wherein the metal content in the silica sol is 1 ppm or less. [9] A polishing method, which comprises polishing using a polishing composition containing a silica sol obtained by the method for producing a silica sol according to any one of [1] to [8]. [Effects of the Invention]

[0012] According to the method for producing silica sol of the present invention, temperature control can be easily performed in the process of replacing the dispersing medium in a dispersion of silica particles, and a silica sol with excellent productivity and a low metal content can be produced.By using a polishing composition containing such silica sol, metal contamination of the polished object can be prevented and high-quality polished products can be obtained. DETAILED DESCRIPTION OF THE INVENTION

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

[0014] (Method of producing silica sol) The method for producing a silica sol of the present invention includes a step of replacing a dispersion medium in a dispersion of silica particles in a container having a glass layer on its surface. Here, the "surface" refers to the liquid-contacting surface of the container (the surface that contacts the retained liquid in the container), which usually corresponds to the inner surface of the container.

[0015] Containers with a glass layer on the surface are preferred because they can be designed on an industrial scale and have excellent mechanical strength, and stainless steel containers with a glass layer on the surface are more preferred because they have excellent corrosion resistance.

[0016] The method of providing a glass layer on the surface of the container is preferably a method of lining the surface of the container with glass, since this method is easy to manufacture.

[0017] The glass layer is preferably two-layered, since this allows for both adhesion to metal and corrosion resistance. That is, it is preferable to provide an inner glass layer having excellent adhesion to metal and an outer glass layer having excellent corrosion resistance.

[0018] The inner glass layer having excellent adhesion to metal is preferably a glass layer formed of glass containing silicon oxide as the main component and containing calcium oxide, sodium oxide, potassium oxide, etc., and the thickness of this glass layer is preferably 0.1 mm to 0.3 mm. The outer corrosion-resistant glass layer is preferably a glass layer formed of glass containing silicon oxide as the main component and containing lithium oxide, sodium oxide, etc., and the thickness of this glass layer is preferably 1.2 mm to 1.5 mm. The total thickness of the glass layers, including the case of two glass layers, is preferably 1.3 mm to 1.8 mm.

[0019] The step of replacing the dispersion medium in the dispersion of silica particles in the method for producing a silica sol of the present invention is a step of removing unnecessary components from the dispersion of silica particles and adding necessary components to give the silica sol the desired composition.

[0020] The method for producing a silica sol of the present invention can efficiently produce silica particles having a low metal content, and can produce a silica sol having a low metal content from a dispersion of such silica particles according to the present invention, so it preferably includes the following step (1), and it is preferable to produce a silica sol from the dispersion of silica particles obtained in step (1) by replacing the dispersion medium according to the present invention. It is also preferable to subject the dispersion of silica particles obtained in this replacement step to a pressurized and heated treatment to increase the degree of condensation of the silica particles.

[0021] That is, the method for producing a silica sol of the present invention preferably comprises the following steps (1) to (3). In addition, from the viewpoint of preventing metal contamination of the obtained silica particles and silica sol, it is preferable to use a reaction vessel having a glass layer on its surface in step (1), and it is also preferable to use a container having a glass layer on its surface in step (3). Step (1): A reaction step of carrying out a hydrolysis reaction and a condensation reaction of tetraalkoxysilane to obtain a dispersion of silica particles. Step (2): A replacement step of replacing the dispersion medium in the dispersion of silica particles obtained in step (1) in a container having a glass layer on the surface. Step (3): A pressure-heating step of pressurizing and heating the dispersion of silica particles obtained in step (2).

[0022] Hereinafter, a method for producing a silica sol by the method for producing a silica sol of the present invention will be described in accordance with the steps (1) to (3), but the present invention is not limited to the following method in any way.

[0023] (Step (1): Reaction step) The reaction vessel used in the reaction step preferably has a glass layer on its surface (the surface of the reaction vessel that comes into contact with the reaction solution) in the same manner as the vessel used in the substitution step, from the viewpoint of preventing metal contamination originating from the reaction vessel during the reaction.

[0024] Containers with a glass layer on the surface are preferred because they can be designed on an industrial scale and have excellent mechanical strength, and stainless steel containers with a glass layer on the surface are more preferred because they have excellent corrosion resistance.

[0025] The method of providing a glass layer on the surface of the container is preferably a method of lining the surface of the container with glass, since this method is easy to manufacture.

[0026] The glass layer is preferably two-layered, since this allows both adhesion to metal and corrosion resistance of the container to be achieved. It is preferable to provide an inner glass layer having excellent adhesion to metal and an outer glass layer having excellent corrosion resistance. The type and thickness of the glass constituting the inner layer, the type and thickness of the glass constituting the outer layer, and the total thickness of the glass layer can be the same as those of the glass layer provided on the surface of the container used in the replacement step described above.

[0027] The volume of the reactor is 0.5 m 3 ~20m 3 is preferred, 1m 3 ~10m 3is more preferable, and 2m 3 ~5m 3 More preferably, the volume of the reaction vessel is 0.5 m 3 When the volume of the reaction vessel is 20 m or more, the productivity of the silica particles is excellent. 3 When the temperature is not more than 100° C., it is easy to control the temperature in the hydrolysis reaction and condensation reaction of the alkoxysilane.

[0028] Examples of tetraalkoxysilanes include tetraalkoxysilanes in which the alkoxy group has 1 to 12 carbon atoms, such as 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, with tetramethoxysilane being more preferred, because they undergo a fast hydrolysis reaction, are less likely to leave unreacted substances, are highly productive, and allow stable silica sol to be easily obtained.

[0029] The hydrolysis reaction and condensation reaction of tetraalkoxysilane are preferably carried out by adding a solution (B) containing tetraalkoxysilane and a solution (C) containing water to a solution (A) containing an alkali catalyst, because the hydrolysis reaction and condensation reaction are easily controlled, the reaction rates of the hydrolysis reaction and condensation reaction can be increased, gelation of the silica particle dispersion can be prevented, and silica particles with a uniform particle size can be obtained. In this case, it is preferable to add the solution (B) containing the tetraalkoxysilane and the solution (C) containing water to the solution (A) containing the alkali catalyst while stirring, and to carry out the hydrolysis reaction and condensation reaction of the tetraalkoxysilane while stirring. Note that it is preferable to add the solutions (B) and (C) to the solution (A) at approximately the same time and for approximately the same amount of time.

[0030] The solution (A) contains an alkali catalyst.

[0031] Examples of alkali catalysts in solution (A) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. These alkali catalysts may be used alone or in combination of two or more. Among these alkali catalysts, ammonia is preferred because it has excellent catalytic activity, is easy to control the particle shape, can suppress metal contamination, is highly volatile, and is easily removable after the hydrolysis reaction and the condensation reaction.

[0032] The solution (A) preferably contains water, since this can promote the hydrolysis of the alkoxysilane.

[0033] The solution (A) preferably contains a solvent other than water, since this provides excellent dispersibility of the tetraalkoxysilane in the reaction liquid.

[0034] Examples of solvents other than water in solution (A) include alcohols such as methanol, ethanol, propanol, isopropanol, and ethylene glycol; ketones such as acetone and methyl ethyl ketone; and esters such as ethyl acetate. These solvents may be used alone or in combination of two or more. Among these solvents, alcohols are preferred because they easily dissolve tetraalkoxysilane, the solvents used in the hydrolysis reaction and the condensation reaction are the same as the by-products, and they are convenient for production. Methanol and ethanol are more preferred, and methanol is even more preferred.

[0035] The concentration of the alkali catalyst in solution (A) is preferably 0.5% by mass to 2.0% by mass, and more preferably 0.6% by mass to 1.5% by mass, based on 100% by mass of solution (A). When the concentration of the alkali catalyst in solution (A) is 0.5% by mass or more, aggregation of silica particles is suppressed, and the dispersion stability of silica particles in the resulting silica particle dispersion is excellent. Furthermore, when the concentration of the alkali catalyst in solution (A) is 2.0% by mass or less, the reaction does not proceed excessively quickly, and reaction controllability is excellent.

[0036] The concentration of water in solution (A) is preferably 3% by mass to 30% by mass, more preferably 5% by mass to 25% by mass, based on 100% by mass of solution (A). When the concentration of water in solution (A) is 3% by mass or more, the dispersibility of silicic acid produced by the hydrolysis reaction in the reaction liquid is excellent. Furthermore, when the concentration of water in solution (A) is 30% by mass or less, the dispersibility of tetraalkoxysilane in the reaction liquid is excellent.

[0037] The concentration of the solvent other than water in the solution (A) is preferably the balance of the alkali catalyst and water.

[0038] The solution (B) contains a tetraalkoxysilane.

[0039] The solution (B) preferably contains a solvent, as this provides excellent dispersibility of the tetraalkoxysilane in the reaction liquid.

[0040] Examples of the solvent in solution (B) include alcohols such as methanol, ethanol, propanol, isopropanol, and ethylene glycol; ketones such as acetone and methyl ethyl ketone; and esters such as ethyl acetate. These solvents may be used alone or in combination of two or more. Among these solvents, alcohols are preferred, more preferably methanol and ethanol, and even more preferably methanol, because the solvents used in the hydrolysis reaction and the condensation reaction are the same as the by-products, and therefore are convenient for production.

[0041] The concentration of tetraalkoxysilane in solution (B) is preferably 76% by mass to 89% by mass, and more preferably 77% by mass to 88% by mass, based on 100% by mass of solution (B). When the concentration of tetraalkoxysilane in solution (B) is 76% by mass or more, the amount of solvent used can be reduced, resulting in excellent productivity of silica particles. Furthermore, when the concentration of tetraalkoxysilane in solution (B) is 89% by mass or less, the dispersibility of tetraalkoxysilane in the reaction liquid is excellent.

[0042] The concentration of the solvent in solution (B) is preferably 11% by mass to 24% by mass, more preferably 12% by mass to 23% by mass, based on 100% by mass of solution (B). When the concentration of the solvent in solution (B) is 11% by mass or more, the dispersibility of the tetraalkoxysilane in the reaction liquid is excellent. Furthermore, when the concentration of the solvent in solution (B) is 24% by mass or less, the amount of solvent used can be reduced, resulting in excellent productivity of silica particles. The concentration of the solvent in solution (B) is preferably the remainder of the tetraalkoxysilane in solution (B).

[0043] The rate of addition of solution (B) to solution (A) is preferably 10 g silica / hour to 300 g silica / hour / kg solution, and more preferably 40 g silica / hour / kg solution to 200 g silica / hour / kg solution. When the rate of addition of solution (B) is 10 g silica / hour / kg solution or more, the reaction time is shortened and productivity is excellent. Furthermore, when the rate of addition of solution (B) is 300 g silica / hour / kg solution or less, the dispersibility of the tetraalkoxysilane in the reaction solution is excellent. Here, "g silica / hour / kg solution" represents the mass of tetraalkoxysilane in solution (B) added per hour to 1 kg of solution (A) converted into the mass of silica (g).

[0044] The solution (C) contains water.

[0045] The solution (C) may contain an alkali catalyst, since this allows the hydrolysis reaction and condensation reaction to proceed efficiently.

[0046] Examples of alkali catalysts in solution (C) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. These alkali catalysts may be used alone or in combination of two or more. Among these alkali catalysts, ammonia is preferred because it has excellent catalytic activity, is easy to control particle shape, can suppress metal contamination, is highly volatile, and is easily removable after the hydrolysis reaction and condensation reaction.

[0047] The solution (C) may contain a solvent other than water.

[0048] Examples of the solvent other than water in the solution (C) include alcohols such as methanol, ethanol, propanol, isopropanol, ethylene glycol, etc. These solvents may be used alone or in combination of two or more.

[0049] The concentration of water in solution (C) is preferably 95% by mass to 100% by mass, more preferably 96% by mass to 99% by mass, based on 100% by mass of solution (C). When the concentration of water in solution (C) is 95% by mass or more, the dispersibility of silicic acid produced by the hydrolysis reaction in the reaction liquid is excellent.

[0050] The concentration of the alkali catalyst in solution (C) is preferably 0 to 5% by mass, more preferably 1 to 4% by mass, based on 100% by mass of solution (C). When the concentration of the alkali catalyst in solution (C) is 5% by mass or less, the reaction does not proceed excessively quickly, resulting in excellent reaction controllability.

[0051] The concentration of the solvent other than water in the solution (C) is preferably the balance of water and the alkali catalyst.

[0052] For example, when Solution (C) contains an alkali catalyst, the addition rate of Solution (C) to Solution (A) is preferably 0.4 g alkali catalyst / hour / kg solution to 12 g alkali catalyst / hour / kg solution, and more preferably 1.7 g alkali catalyst / hour / kg solution to 7.7 g alkali catalyst / hour / kg solution. Addition of Solution (C) at a rate of 0.4 g alkali catalyst / hour / kg solution or more suppresses aggregation of silica particles, resulting in excellent dispersion stability of silica particles in the resulting dispersion. Addition of Solution (C) at a rate of 12 g alkali catalyst / hour / kg solution or less prevents the reaction from proceeding excessively quickly, resulting in excellent reaction controllability. Here, "g alkali catalyst / hour / kg solution" represents the mass (g) of alkali catalyst in solution (C) added per hour per 1 kg of solution (A).

[0053] The water concentration in the reaction system for the hydrolysis and condensation reactions is preferably maintained at 3% to 30% by mass, and more preferably 5% to 25% by mass, based on 100% by mass of the reaction solution in the reaction vessel. When the water concentration in the reaction system is 3% by mass or more, the dispersibility of the intermediate product, silicic acid, in the reaction solution is excellent. Furthermore, when the water concentration in the reaction system is 30% by mass or less, the dispersibility of the tetraalkoxysilane in the reaction solution is excellent. Here, the concentration of water in the reaction system refers to the total amount of water in the total amount of liquid and substances dissolved in the liquid in the reaction system during the hydrolysis and condensation reactions. The total amount of liquid and substances dissolved in the liquid in the reaction system is only solution (A) at the start of the reaction, and during the reaction it is the total amount of solution (A), solution (B), solution (C), and the alcohol produced in the reaction. The liquid and substances dissolved in the liquid in the reaction system do not include silica particles dispersed in the liquid.

[0054] The concentration of the alkali catalyst in the reaction system for the hydrolysis and condensation reactions is preferably maintained at 0.5% to 2.0% by mass, more preferably 0.6% to 1.5% by mass, based on 100% by mass of the reaction solution in the reaction system. When the concentration of the alkali catalyst in the reaction system is 0.5% by mass or more, aggregation of silica particles is suppressed, resulting in excellent dispersion stability of the silica particles in the resulting silica particle dispersion. Furthermore, when the concentration of the alkali catalyst in the reaction system is 2.0% by mass or less, the reaction does not proceed excessively quickly, resulting in excellent reaction controllability. Here, the concentration of the alkali catalyst in the reaction system refers to the total amount of the alkali catalyst in the total amount of the liquid and the substances dissolved in the liquid in the reaction system in the hydrolysis reaction and the condensation reaction.

[0055] The reaction temperature (temperature of the reaction solution in the reaction system) for the hydrolysis and condensation reactions of tetraalkoxysilane is preferably 15°C to 50°C, more preferably 20°C to 45°C. When the reaction temperature is 15°C or higher, the reaction does not proceed too slowly, resulting in excellent controllability. Furthermore, when the reaction temperature is 50°C or lower, an excellent balance between the hydrolysis reaction rate and the condensation reaction rate is achieved.

[0056] The hydrolysis reaction and condensation reaction of tetraalkoxysilane are exothermic reactions, and therefore, in order to control the reaction temperature, it is preferable to carry out the reaction while cooling the reaction vessel or the reaction liquid. The cooling method is not particularly limited, but the following methods can be mentioned. (1) A cooling jacket is installed around the outer periphery of the reaction vessel, and the entire reaction vessel is cooled with a cooling medium such as water. (2) A circulation pipe for a cooling medium such as water is installed in the reaction liquid in the reaction tank to cool the reaction liquid. The above methods (1) and (2) may be used alone or in combination. In particular, when the cooling method (1) above is adopted, the present invention in which a glass layer is provided on the surface of the reaction vessel can improve cooling efficiency and productivity compared to when a resin layer is provided on the surface of the reaction vessel.

[0057] (Physical properties of silica particles) The preferred physical properties of the silica particles produced in the above step (1) will be described below.

[0058] The average primary particle diameter of the silica particles is preferably 5 nm to 100 nm, more preferably 10 nm to 60 nm. When the average primary particle diameter of the silica particles is 5 nm or more, the storage stability of the silica sol is excellent. Furthermore, when the average primary particle diameter of the silica particles is 100 nm or less, the surface roughness and scratches on the polished object, such as a silicon wafer, can be reduced, and sedimentation of the silica particles can be suppressed.

[0059] The average primary particle diameter of the silica particles is measured by the BET method. Specifically, the specific surface area of ​​the silica particles 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) x density (g / cm 3 )) ··· (1)

[0060] The average primary particle size of the silica particles can be set within a desired range using known conditions and methods.

[0061] The average secondary particle diameter of the silica particles is preferably 10 nm to 200 nm, more preferably 20 nm to 100 nm. When the average secondary particle diameter of the silica particles is 10 nm or more, the removal of particles and the like during cleaning after polishing is excellent, and the storage stability of the silica sol is excellent. Furthermore, when the average secondary particle diameter of the silica particles is 200 nm or less, the surface roughness and scratches on the polished object, typified by a silicon wafer, can be reduced during polishing, the removal of particles and the like during cleaning after polishing is excellent, and sedimentation of the silica particles can be suppressed.

[0062] The average secondary particle size of the silica particles is measured by the DLS method (dynamic light scattering method), specifically, using a dynamic light scattering particle size measuring device.

[0063] The average secondary particle size of the silica particles can be set within a desired range using known conditions and methods.

[0064] The cv value of the silica particles is preferably 10 to 50, more preferably 15 to 40, and even more preferably 20 to 35. When the cv value of the silica particles is 10 or more, the polishing rate for a workpiece such as a silicon wafer is excellent, and the productivity of the silicon wafer is excellent. Furthermore, when the cv value of the silica particles 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.

[0065] The cv value of the silica particles is calculated using the following formula (2) after measuring the average secondary particle diameter of the silica particles using a dynamic light scattering particle diameter measuring device. cv value = (standard deviation (nm) / average secondary particle size (nm)) × 100 (2)

[0066] The association ratio of silica particles is preferably 1.0 to 4.0, more preferably 1.1 to 3.0. When the association ratio of silica particles is 1.0 or more, the polishing rate for a workpiece, such as a silicon wafer, is excellent, resulting in excellent silicon wafer productivity. Furthermore, when the association ratio of silica particles is 4.0 or less, the surface roughness and scratches on a workpiece, such as a silicon wafer, during polishing can be reduced, and aggregation of silica particles can be suppressed.

[0067] The association ratio of silica particles 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)

[0068] The surface silanol group density of silica particles is 0.1 / nm 2 ~10 pieces / nm 2 is preferable, and 0.5 particles / nm 2 ~7.5 pieces / nm 2 More preferably, 2.0 particles / nm 2 ~7.0 pieces / nm 2 It is more preferable that the surface silanol group density of the silica particles is 0.1 / nm 2 When the surface silanol group density of the silica particles is 10 / nm or more, the silica particles have a moderate surface repulsion, and the dispersion stability of the silica sol is excellent. 2 When the particle size is equal to or less than this, the silica particles have an appropriate surface repulsion, and aggregation of the silica particles can be suppressed.

[0069] The surface silanol group density of silica particles is measured by the Sears method, specifically, under the following conditions. Take a silica sol equivalent to 1.5 g of silica particles and add pure water to make the liquid volume 90 mL. In an environment of 25°C, add 0.1 mol / L hydrochloric acid aqueous solution until the pH reaches 3.6, add 30 g of sodium chloride, gradually add pure water to completely dissolve the sodium chloride, and finally add pure water until the total volume of the test liquid is 150 mL to obtain the test liquid. The obtained test solution is placed in an automatic titrator, and 0.1 mol / L aqueous sodium hydroxide solution is added dropwise to measure the titer A (mL) of 0.1 mol / L aqueous sodium hydroxide solution required to change the pH from 4.0 to 9.0. The amount of 0.1 mol / L sodium hydroxide solution consumed per 1.5 g of silica particles, V (mL), required to change the pH from 4.0 to 9.0 was calculated using the following equation (4), and the surface silanol group density ρ (number / nm 2 ) is calculated. V=(A×f×100×1.5) / (W×C) ··· (4) A: The amount (mL) of 0.1 mol / L sodium hydroxide solution required to change the pH from 4.0 to 9.0 per 1.5 g of silica particles f: Potency of the 0.1 mol / L sodium hydroxide solution used C: Concentration of silica particles in silica sol (mass%) W: Amount of silica sol collected (g) ρ=(B×N A ) / (10 18 ×M×S BET ) (5) B: The amount of sodium hydroxide (mol) required to change the pH from 4.0 to 9.0 per 1.5 g of silica particles calculated from V N A : Avogadro's number (units / mol) M: Silica particle amount (1.5g) S BET : The specific surface area (m) of silica particles measured when calculating the average primary particle diameter 2 / g)

[0070] The method for measuring and calculating the surface silanol group density of the silica particles was based on "GW Ears, Jr., Analytical Chemistry, Vol. 28, No. 12, pp. 1981-1983 (1956)," "Haba Shinichi, Development of Abrasives for Semiconductor Integrated Circuit Processing, Doctoral Dissertation, Kochi University of Technology, pp. 39-45, March 2004," "Japanese Patent No. 5967118," and "Japanese Patent No. 6047395."

[0071] The surface silanol group density of the silica particles can be set within a desired range by adjusting the conditions for the hydrolysis reaction and condensation reaction of the alkoxysilane.

[0072] Examples of the shape of silica particles include spherical, chain-like, cocoon-like (also called knob-like or peanut-like), irregular shapes (for example, wart-like, bent, branched, etc.), etc. Among these shapes of silica particles, spherical shapes are preferred when it is desired to reduce the surface roughness and scratches on a polished object, such as a silicon wafer, during polishing, and irregular shapes are preferred when it is desired to further increase the polishing rate for a polished object, such as a silicon wafer.

[0073] (Step (2): Substitution step) In the substitution step, it is preferable to remove alcohol and alkali catalysts such as ammonia from the dispersion liquid of silica particles by substituting the dispersion medium. As the dispersion medium to be added to replace the dispersion medium in the dispersion liquid of silica particles in the substitution step, a desired dispersion medium can be selected.

[0074] In the substitution step, the dispersion medium in the dispersion liquid of silica particles is preferably removed by heating the dispersion medium in the dispersion liquid of silica particles under normal pressure or reduced pressure to evaporate it. The heating temperature in this case is set to the boiling point of the dispersion medium under the pressure conditions during substitution or a temperature slightly higher than the boiling point, preferably 50°C to 150°C.

[0075] Examples of the dispersion medium to be added to the dispersion liquid of silica particles in the substitution step include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These dispersion media may be used alone or in combination of two or more. Among these dispersion media, water and alcohol are preferred, and water is more preferred, because they have excellent affinity with silica particles.

[0076] In the substitution step, removal of the dispersion medium from the dispersion liquid of silica particles and addition of a new dispersion medium may be carried out simultaneously, or one of them may be carried out first and the other may be added later.

[0077] In the method for producing silica sol of the present invention, the substitution step is carried out using a container having a glass layer on its surface (liquid-contacting surface), because this allows for efficient heating during substitution, excellent productivity, and a low metal content to be maintained.

[0078] The glass layer of the container having a glass layer on its surface is as described above.

[0079] The volume of the container is 0.1 m 3 ~20m 3 is preferable, and 0.2 m 3 ~10m 3 is more preferable, and 0.5m 3 ~5m 3 More preferably, the volume of the container is 0.1 m 3 When the volume of the container is 20 m or more, the productivity of the silica particles is excellent. 3 If the temperature is equal to or less than this, heating during replacement can be carried out efficiently.

[0080] (Pressure and heating process) The pressure when the silica particle dispersion obtained in the substitution step is pressurized and heated is preferably 0.10 MPa to 2.3 MPa, more preferably 0.14 MPa to 1.0 MPa. When the pressure of the pressurized and heated treatment is 0.10 MPa or more, the degree of condensation of the silica particles can be increased. When the pressure of the pressurized and heated treatment is 2.3 MPa or less, a silica sol can be produced without significantly changing the average primary particle size, average secondary particle size, cv value, or association ratio of the silica particles, and the dispersion stability of the silica sol is excellent. Pressurization can be achieved by heating the silica particle dispersion in a sealed state to a temperature above the boiling point of the dispersion medium. When the silica particle aqueous dispersion is heated to 100°C or higher in a sealed state, the pressure becomes the saturated water vapor pressure at that temperature.

[0081] The temperature for the pressure and heat treatment is preferably 100°C to 220°C, and more preferably 110°C to 180°C. When the temperature for the pressure and heat treatment is 100°C or higher, the degree of condensation of the silica particles can be increased. When the temperature for the pressure and heat treatment is 220°C or lower, a silica sol can be produced without significant changes in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles, and the dispersion stability of the silica sol is excellent.

[0082] The time for the pressure and heat treatment is preferably 0.25 to 10 hours, more preferably 0.5 to 8 hours. When the time for the pressure and heat treatment is 0.25 hours or more, the degree of condensation of the silica particles can be increased. When the time for the pressure and heat treatment is 10 hours or less, the silica sol can be produced without significantly changing the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles, and the dispersion stability of the silica sol is excellent.

[0083] The pressure and heat treatment is preferably carried out in an aqueous dispersion, since it can increase the degree of condensation of silica particles without significantly changing the average primary particle size, average secondary particle size, cv value, or association ratio.

[0084] The pH of the aqueous dispersion during the heat and pressure treatment is preferably 6.0 to 8.0, more preferably 6.5 to 7.8. When the pH of the aqueous dispersion during the heat and pressure treatment is 6.0 or higher, gelation of the silica sol can be suppressed. When the pH of the aqueous dispersion during the heat and pressure treatment is 8.0 or lower, the degree of condensation of the silica particles can be increased without significantly changing the average primary particle size, average secondary particle size, cv value, or association ratio of the silica particles.

[0085] In the pressurizing and heating step, it is preferable to use a container having a glass layer on the surface, since this allows for efficient pressurizing and heating, excellent productivity, and a low metal content to be maintained.

[0086] Containers having a glass layer on the surface are preferred because they can be designed on an industrial scale and have excellent mechanical strength, and containers having a glass layer on the surface of a metal container are more preferred because they have excellent corrosion resistance.

[0087] The method of providing a glass layer on the surface of the container is preferably a method of lining the surface of the container with glass, since this method is easy to manufacture.

[0088] The glass layer is preferably two-layered, since this allows both adhesion to metal and corrosion resistance of the container to be achieved. It is preferable to provide an inner glass layer having excellent adhesion to metal and an outer glass layer having excellent corrosion resistance. The type and thickness of the glass constituting the inner layer, the type and thickness of the glass constituting the outer layer, and the total thickness of the glass layer can be the same as those of the glass layer provided on the surface of the container used in the replacement step described above.

[0089] The volume of the container is 0.1 m 3 ~20m 3 is preferable, and 0.2 m 3 ~10m 3 is more preferable, and 0.5m 3 ~5m 3 More preferably, the volume of the container is 0.1 m 3 When the volume of the container is 20 m or more, the productivity of the silica sol is excellent. 3 If the temperature is below this, the pressurization and heating can be carried out efficiently.

[0090] (Silica sol properties, ingredients, etc.) The content of silica particles in the silica sol produced from the silica sol of the present invention is preferably 3% by mass to 50% by mass, more preferably 4% by mass to 40% by mass, and even more preferably 5% by mass to 30% by mass, based on 100% by mass of the total amount of silica sol. When the content of silica particles in the silica sol is 3% by mass or more, the polishing rate for a polished object, such as a silicon wafer, is excellent. Furthermore, when the content of silica particles in the silica sol is 50% by mass or less, aggregation of silica particles in the silica sol or polishing composition can be suppressed, and the storage stability of the silica sol or polishing composition is excellent.

[0091] The content of the dispersion medium in the silica sol is preferably 50% by mass to 97% by mass, more preferably 60% by mass to 96% by mass, and even more preferably 70% by mass to 95% by mass, based on the total amount of the silica sol (100% by mass). When the content of the dispersion medium in the silica sol is 50% by mass or more, aggregation of silica particles in the silica sol or polishing composition can be suppressed, resulting in excellent storage stability of the silica sol or polishing composition. Furthermore, when the content of the dispersion medium in the silica sol is 97% by mass or less, the polishing rate for a workpiece, typically a silicon wafer, is excellent.

[0092] The content of silica particles and dispersion medium in the silica sol can be set within a desired range in the substitution step.

[0093] In addition to silica particles and a dispersion medium, the silica sol may contain other components such as an oxidizing agent, an antiseptic, an antifungal agent, a pH adjuster, a pH buffer, a surfactant, a chelating agent, and an antibacterial / biocide, as needed, within a range that does not impair the performance of the silica sol. In particular, it is preferable to include an antibacterial biocide in the silica sol, since this gives the silica sol excellent storage stability.

[0094] Examples of antibacterial biocides include hydrogen peroxide, ammonia, quaternary ammonium hydroxides, quaternary ammonium salts, ethylenediamine, glutaraldehyde, methyl p-hydroxybenzoate, and sodium chlorite. These antibacterial biocides may be used alone or in combination of two or more. Among these antibacterial biocides, hydrogen peroxide is preferred because of its excellent affinity with silica sol. Antimicrobial biocides also include those commonly referred to as disinfectants.

[0095] The content of the antibacterial biocide in the silica sol is preferably 0.0001% by mass to 10% by mass, and more preferably 0.001% by mass to 1% by mass, based on the total amount of the silica sol (100% by mass). When the content of the antibacterial biocide in the silica sol is 0.0001% by mass or more, the storage stability of the silica sol is excellent. When the content of the antibacterial biocide in the silica sol is 10% by mass or less, the original performance of the silica sol is not impaired.

[0096] The pH of the silica sol is preferably 6.0 to 8.0, more preferably 6.5 to 7.8. When the pH of the silica sol is 6.0 or higher, the dispersion stability is excellent and aggregation of the silica particles can be suppressed. Furthermore, when the pH of the silica sol is 8.0 or lower, dissolution of the silica particles is prevented and long-term storage stability is excellent. The pH of the silica sol can be adjusted to a desired range by adding a pH adjuster.

[0097] The content of metals mixed as impurities in the silica sol (metal impurity content) is preferably 1 ppm or less, more preferably 0.2 ppm or less.

[0098] When polishing silicon wafers, metal impurities adhere to the surface of the object to be polished, contaminating the object and adversely affecting the properties of the object and the final product. Furthermore, if metal impurities are present in the silica sol, coordination interactions occur between the acidic surface silanol groups and the metal impurities, which changes the chemical properties (acidity, etc.) of the surface silanol groups and the three-dimensional environment of the silica particle surfaces (e.g., the tendency of silica particles to aggregate), thereby affecting the polishing rate.

[0099] The metal content of silica sol is measured using inductively coupled plasma mass spectrometry (ICP-MS). Specifically, a silica sol containing 0.4 g of silica particles is accurately weighed, 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 to make a total of exactly 10 g to create a test solution, which is then measured using an 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, and the total content of these metals is taken as the metal content.

[0100] The metal content of the silica sol can be reduced to 1 ppm or less by performing the substitution step using a container having a glass layer on the surface. As described above, the silica particles are preferably produced 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 other elements derived from the raw materials remain, making it difficult to reduce the metal content of the silica sol to 1 ppm or less.

[0101] (polishing composition) The silica sol obtained by the method for producing a silica sol of the present invention can be suitably used as a polishing composition. The polishing composition preferably contains a silica sol produced by the method for producing a silica sol of the present invention and a water-soluble polymer.

[0102] The water-soluble polymer enhances the wettability of the polishing composition 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 groups of the silica particles, so that the silica particles and the water-soluble polymer are stably dispersed in close proximity in the polishing composition. Therefore, when polishing an object to be polished, such as a silicon wafer, the effects of the silica particles and the water-soluble polymer function synergistically.

[0103] Examples of water-soluble polymers include cellulose derivatives, polyvinyl alcohol, polyvinylpyrrolidone, copolymers having a polyvinylpyrrolidone skeleton, and polymers having a polyoxyalkylene structure.

[0104] 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. Examples of copolymers having a polyvinylpyrrolidone skeleton include graft copolymers of polyvinyl alcohol and polyvinylpyrrolidone. Examples of polymers having a polyoxyalkylene structure include polyoxyethylene, polyoxypropylene, and copolymers of ethylene oxide and propylene oxide.

[0105] 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, because they have high affinity with the surface silanol groups of silica particles and act synergistically to impart good hydrophilicity to the surface of the object to be polished.

[0106] 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 composition is improved. Furthermore, when 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 a workpiece, such as a silicon wafer, is excellent.

[0107] 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.

[0108] The content of the water-soluble polymer in the polishing composition is preferably 0.02% by mass to 10% by mass, more preferably 0.05% by mass to 5% by mass, based on 100% by mass of the total amount of the polishing composition. When the content of the water-soluble polymer in the polishing composition is 0.02% by mass or more, the hydrophilicity of the polishing composition is improved. Furthermore, when the content of the water-soluble polymer in the polishing composition is 10% by mass or less, aggregation of silica particles during preparation of the polishing composition can be suppressed.

[0109] In addition to the silica sol and the water-soluble polymer, the polishing composition may contain other components, such as basic compounds, polishing accelerators, surfactants, hydrophilic compounds, preservatives, antifungal agents, pH adjusters, pH buffers, surfactants, chelating agents, and antibacterial and biocide agents, as needed, provided that the performance of the polishing composition is not impaired. In particular, it is preferable to include a basic compound in the polishing composition, since it can exert a chemical action on the surface of the object to be polished, such as a silicon wafer, thereby performing chemical polishing (chemical etching), and the synergistic effect with the surface silanol groups of the silica particles can improve the polishing rate of the object to be polished, such as a silicon wafer.

[0110] Examples of basic compounds include organic basic compounds, alkali metal hydroxides, alkali metal hydrogencarbonates, alkali metal carbonates, and ammonia. These basic compounds may be used alone or in combination of two or more. Among these basic compounds, ammonia, tetramethylammonium hydroxide, tetraethylammonium hydroxide, ammonium hydrogencarbonate, and ammonium carbonate are preferred because they have high water solubility and excellent affinity with silica particles and water-soluble polymers, with ammonia, tetramethylammonium hydroxide, and tetraethylammonium hydroxide being more preferred, and ammonia being even more preferred.

[0111] The content of the basic compound in the polishing composition is preferably 0.001% by mass to 5% by mass, more preferably 0.01% by mass to 3% by mass, based on 100% by mass of the total amount of the polishing composition. When the content of the basic compound in the polishing composition is 0.001% by mass or more, the polishing rate of a polished object, typically a silicon wafer, can be improved. Furthermore, when the content of the basic compound in the polishing composition is 5% by mass or less, the stability of the polishing composition is excellent.

[0112] The pH of the polishing composition is preferably 8.0 to 12.0, more preferably 9.0 to 11.0. When the pH of the polishing composition is 8.0 or higher, aggregation of silica particles in the polishing composition can be suppressed, and the polishing composition has excellent dispersion stability. When the pH of the polishing composition is 12.0 or lower, dissolution of silica particles can be suppressed, and the polishing composition has excellent stability. The pH of the polishing composition can be adjusted to a desired range by adding a pH adjuster.

[0113] The polishing composition can be obtained by mixing the silica sol obtained by the silica sol manufacturing method of the present invention, the water-soluble polymer, and, if necessary, other components. However, in consideration of storage and transportation, the polishing composition can be prepared at a high concentration first and then diluted with water or the like immediately before polishing.

[0114] (polishing method) The polishing method of the present invention is a polishing method using a polishing composition containing the silica sol obtained by the method for producing silica sol of the present invention. The polishing composition used is preferably the polishing composition described above. A specific polishing method includes, for example, a method in which the surface of a silicon wafer is pressed against a polishing pad, a polishing composition is dropped onto the polishing pad, and the surface of the silicon wafer is polished.

[0115] (Application) The silica sol obtained by the method for producing a silica sol of the present invention can be suitably used for polishing purposes, and in particular can be suitably used for polishing silicon wafers and chemical mechanical polishing. [Example]

[0116] The present invention will be explained in more detail below using experimental examples and comparative experimental examples in place of the examples, but the present invention is not limited to the descriptions of the following examples as long as it does not deviate from the gist of the invention.

[0117] (sample) A silica sol with a silica particle / ammonia / water ratio of 19 / 1.4 / 79.6 (mass %) was used as a sample. The silica particles are produced by subjecting tetramethoxysilane to a hydrolysis reaction and a condensation reaction.

[0118] (Measurement of the content of each metal) A sample containing 0.4 g of silica particles was accurately weighed, sulfuric acid and hydrofluoric acid were added, and the mixture was heated, dissolved, and evaporated. Pure water was added to the remaining sulfuric acid droplets to make a total volume of exactly 10 g to create a test solution, and the content of each metal shown in Table 1 was measured using a high-frequency inductively coupled plasma mass spectrometer "ELEMENT2" (model name, manufactured by Thermo Fisher Scientific).

[0119] [Example 1] 40 mL of the sample was placed in a stainless steel container (product number "9000", manufactured by Kobelco Eco-Solutions Co., Ltd.) with a glass lining on the surface (a cylindrical container with a bottom, an inner diameter of 2.5 cm, total glass layer thickness: 1.5 mm) and stirred at 90°C for 24 hours, after which the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0120] [Example 2] The same procedure as in Example 1 was carried out except that the stirring time was changed to 48 hours, and the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0121] [Comparative Example 1] 40 mL of the sample was placed in a stainless steel container (a cylindrical container with a bottom and an inner diameter of 2.5 cm) and stirred at 90°C for 24 hours, after which the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0122] Comparative Example 2 The same procedure as in Comparative Example 1 was carried out except that the stirring time was changed to 48 hours, and the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0123] Comparative Example 3 40 mL of the sample was placed in a stainless steel container whose surface was coated with a fluororesin (a cylindrical container with a bottom, an inner diameter of 2.5 cm, and a fluororesin layer thickness of 300 μm), and after stirring at 90°C for 24 hours, the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0124] Comparative Example 4 The same procedure as in Comparative Example 3 was carried out except that the stirring time was changed to 48 hours, and the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0125] [Reference example 1] The content of each metal in the sample itself was measured. The evaluation results are shown in Table 1.

[0126] Table 1 also lists the heat transfer coefficients calculated from the temperature rise rates inside the container for the glass or fluororesin formed on the surface of the container.

[0127] [Table 1]

[0128] From Comparative Examples 1 and 2, it can be seen that when a stainless steel container is used as a container for the replacement step, a large amount of iron is generated, and therefore it is unsuitable for applications requiring high purity. Furthermore, from Comparative Examples 3 and 4, it can be seen that although a low metal content can be maintained when a container having a resin layer on the surface is used, the heat transfer coefficient is low, making it difficult to control the temperature in the dispersion medium replacement process, which requires heating, and therefore is unsuitable. On the other hand, when a container having a glass layer with a high heat transfer coefficient on the surface is used, as in Examples 1 and 2, it is possible to achieve both temperature control in the replacement process and maintenance of a low metal content, and it is found to be suitable for producing silica sol. [Industrial Applicability]

[0129] The silica sol obtained by the method for producing a silica sol of the present invention can be suitably used for polishing purposes, and in particular can be suitably used for polishing silicon wafers and chemical mechanical polishing.

Claims

1. a step of replacing a dispersion medium in a dispersion liquid of silica particles in a container having a glass layer on the surface, the step of substituting the dispersion medium in the dispersion of silica particles includes a step of heating and evaporating the dispersion medium in the dispersion, The method for producing a silica sol further comprises a step of adding a solution (B) containing a tetraalkoxysilane and a solution (C) containing water to a solution (A) containing an alkali catalyst to carry out a hydrolysis reaction and a condensation reaction of the tetraalkoxysilane, thereby obtaining a dispersion of silica particles.

2. 2. The method for producing a silica sol according to claim 1, wherein the temperature in the step of replacing the dispersion medium in the dispersion of silica particles is 50°C to 150°C.

3. The method for producing a silica sol according to claim 1 or 2, wherein the alcohol is removed in the step of replacing the dispersion medium in the dispersion of silica particles.

4. The method for producing a silica sol according to any one of claims 1 to 3, wherein the alkali catalyst is removed in the step of replacing the dispersion medium in the dispersion of silica particles.

5. The method for producing a silica sol according to any one of claims 1 to 4, wherein water is added in the step of replacing the dispersion medium in the dispersion of silica particles.

6. The method for producing a silica sol according to any one of claims 1 to 5, wherein the glass layer is two layers.

7. The method for producing a silica sol according to any one of claims 1 to 6, wherein the metal content in the silica sol is 1 ppm or less.

8. A polishing method, comprising polishing using a polishing composition containing the silica sol obtained by the method for producing a silica sol according to any one of claims 1 to 7.

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