Method for producing silica sol and polishing method

Abstract

To provide a method for producing a silica sol with a low metal content by preventing metal pollution derived from a container used in a step of replacing a dispersion medium in a dispersion liquid of silica particles.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 fluororesin layer on its surface. Preferably, the method 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 method for polishing. [Background technology]

[0002] Polishing methods using polishing solutions are known for polishing the surfaces of materials such as metals and inorganic compounds. In particular, in the final finishing polishing of silicon wafers and chemical mechanical polishing (CMP), the surface condition greatly affects the characteristics of the final product, so the surfaces and edges of these parts are required to be polished with extremely high precision.

[0003] In such precision polishing, polishing compositions containing silica particles are employed, and silica sols such as colloidal silica are widely used as the main abrasive grains. Depending on the manufacturing method, colloidal silica is known to be produced by thermal decomposition of silicon tetrachloride (fumed silica, etc.), deionization of alkali silicates such as water glass, and hydrolysis and condensation reactions of alkoxysilanes (generally called 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 silica sol by producing a dispersion of silica particles by hydrolysis and condensation reactions of alkoxysilane, and then replacing the dispersion medium in the resulting silica particle dispersion with water. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-108924 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The synthesis of compounds on an industrial scale is generally often carried out in a reaction vessel made of metal such as stainless steel. However, when using a metal reaction vessel, there is a problem that metal components such as Fe elute from the metal constituting the reaction vessel during the manufacturing process, and the produced silica sol is contaminated with the metal. On the other hand, in precision polishing, metal impurities adhere to the surface of the workpiece to be polished, contaminating the workpiece to be polished, which has an adverse effect on the characteristics of the workpiece to be polished and the final product. Therefore, it is necessary to suppress the mixing of metal impurities in the manufacturing process of silica sol.

[0007] Regarding this problem, Patent Document 1 does not describe any ingenuity regarding the container used in the substitution step.

[0008] The present invention has been made in view of such problems, and an object of the present invention is to prevent metal contamination derived from the container used in the step of replacing the dispersion medium in the dispersion of silica particles, and to provide a method for producing a silica sol with a low metal content and a polishing method using this silica sol.

Means for Solving the Problems

[0009] As a result of intensive studies, the present inventors have found that by using a container having a fluororesin layer on the surface in the step of replacing the dispersion medium in the dispersion of silica particles, it is possible to suppress the mixing of metal impurities and obtain a silica sol with a low metal content, and thus have completed the present invention.

[0010] That is, the gist of the present invention is as follows. [1] A method for producing a silica sol, comprising a step of replacing the dispersion medium in a dispersion of silica particles in a container having a fluororesin layer on the surface. [2] The method for producing a silica sol according to [1], wherein the temperature of the step of replacing the dispersion medium in the dispersion of silica particles is 50°C to 150°C. [3] A method for producing the silica sol according to [1] or [2], further comprising a step of adding a solution (B) containing 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 tetraalkoxysilane to obtain a dispersion of silica particles. [4] The method for producing a silica sol according to any one of [1] to [3], wherein in the step of replacing the dispersion medium in the dispersion of silica particles, alcohol is removed. [5] The method for producing a silica sol according to any one of [1] to [4], wherein in the step of replacing the dispersion medium in the dispersion of silica particles, the alkali catalyst is removed. [6] The method for producing a silica sol according to any one of [1] to [5], wherein in the step of replacing the dispersion medium in the dispersion of silica particles, water is added. [7] The method for producing a silica sol according to any one of [1] to [6], wherein the fluororesin contains tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin. [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, wherein polishing is carried out using a polishing composition containing the silica sol obtained by the method for producing a silica sol according to any one of [1] to [8]. [Advantages of the Invention]

[0011] According to the method for producing a silica sol of the present invention, in the step of replacing the dispersion medium in the dispersion of silica particles, it is possible to prevent metal contamination derived from the used container and to produce a silica sol having a low metal content. Using a polishing composition containing such a silica sol, it is possible to prevent metal contamination of the polished object and to obtain a high-quality polished product. [Embodiments for Carrying Out the Invention]

[0012] The present invention will be described in detail below, but the present invention is not limited to the embodiments described below and can be implemented with various modifications within the scope of its gist. In this specification, when the expression "~" is used, it is used to include the numerical value or physical property value before and after it.

[0013] (Method for producing silica sol) The present invention provides a method for producing silica sol, comprising the step of replacing the dispersion medium in a dispersion of silica particles within a container having a fluororesin layer on its surface. Here, "surface" refers to the liquid-contacting surface of the container (the surface that contacts the liquid held inside the container), which usually corresponds to the inner surface of the container.

[0014] Containers having a fluororesin layer on their surface are preferable because they allow for industrial-scale design and have excellent mechanical strength, and stainless steel containers having a fluororesin layer on their surface are even preferable because they have excellent corrosion resistance.

[0015] As fluororesins used to form the fluororesin layer, polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether ternary copolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), trifluorochloroethylene-ethylene copolymer (ECTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), etc., can be used, and any two or more of these mixed resins can also be used. Among these fluororesins, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA) is preferred because it can be melt-molded and has excellent moldability.

[0016] Because these fluororesins have excellent corrosion resistance, chemical resistance (acid and alkali resistance), and heat resistance, applying a fluororesin layer to the surface of the reaction vessel can reliably prevent metal contamination originating from the reaction vessel. Furthermore, in the exothermic hydrolysis and condensation reactions of alkoxysilanes, it is necessary to proceed with the reaction while cooling. However, since resins have a lower heat transfer coefficient compared to metals, covering the surface of a metal reaction vessel with a resin layer reduces the thermal efficiency when heating or cooling from the outside, raising concerns that the productivity of silica particles may be impaired. Nevertheless, fluororesins have a relatively high heat transfer coefficient among highly durable resins, and the problems of cooling efficiency and temperature control are relatively small.

[0017] A method for applying a fluororesin layer to the surface of a reaction vessel is preferable because it is easy to manufacture and process. This method involves pre-treating the surface of the reaction vessel as needed, and then baking the fluororesin onto it using electrostatic coating. However, other coating methods, such as heat-bonding press coating, can also be employed.

[0018] The thickness of the fluororesin layer is preferably 100 μm to 1000 μm, and more preferably 200 μm to 500 μm. When the thickness of the fluororesin layer is 100 μm or more, the effect of preventing metal contamination by providing the fluororesin layer can be sufficiently obtained. On the other hand, when the thickness of the fluororesin layer is 1000 μm or less, the decrease in thermal efficiency due to providing the fluororesin layer can be suppressed, and the cost of forming the fluororesin layer can be reduced.

[0019] The step of replacing the dispersion medium in the silica particle dispersion in the silica sol production method of the present invention is a step of removing unnecessary components from the silica particle dispersion and adding necessary components to make the silica sol a desired component.

[0020] The present invention provides a method for producing silica sol that efficiently produces silica particles with a low metal content, and from a dispersion of such silica particles, can produce silica sol with a low metal content according to the present invention. Therefore, it is preferable to have the following step (1), and it is preferable to produce silica sol by replacing the dispersion medium from the dispersion of silica particles obtained in step (1) according to the present invention. Furthermore, it is preferable to subject the dispersion of silica particles obtained in this replacement step to pressurized heat treatment to increase the degree of condensation of the silica particles.

[0021] In other words, the method for producing silica sol according to the present invention preferably comprises the following steps (1) to (3). Furthermore, from the viewpoint of preventing metal contamination of the resulting silica particles and silica sol, it is preferable to use a reaction vessel having a fluororesin layer on its surface in step (1), and it is also preferable to use a container having a fluororesin layer on its surface in step (3). Step (1): A reaction step in which a dispersion of silica particles is obtained by carrying out hydrolysis and condensation reactions of tetraalkoxysilane. Step (2): A replacement step in which the dispersion medium in the dispersion of silica particles obtained in step (1) is replaced in a container having a fluororesin layer on its surface. Step (3): Pressurized heating process in which the dispersion of silica particles obtained in step (2) is subjected to pressurized heating.

[0022] The following describes a method for producing silica sol according to the silica sol production method of the present invention, following steps (1) to (3). However, the present invention is not limited to the following method.

[0023] (Process (1): Reaction Process) From the viewpoint of preventing metal contamination originating from the reaction vessel during the reaction, it is preferable that the reaction vessel used in the reaction step has a fluororesin layer on its surface (the surface of the reaction vessel that comes into contact with the reaction liquid), similar to the container used in the displacement step.

[0024] Containers having a fluororesin layer on their surface are preferable because they allow for industrial-scale design and have excellent mechanical strength, and stainless steel containers having a fluororesin layer on their surface are even preferable because they have excellent corrosion resistance.

[0025] A preferred method for applying a fluororesin layer to the surface of a container is one in which the fluororesin is baked onto the surface of the container by electrostatic coating after pre-treatment as necessary, because it is easy to manufacture and process. However, other coating methods such as heat-bonding press coating can also be employed.

[0026] The type of fluororesin and its thickness that constitute the fluororesin layer can be the same as those used for the fluororesin layer provided on the surface of the container used in the substitution process described above.

[0027] The volume of the reaction vessel is 0.5 m³. 3 ~20m 3 Preferably, 1m 3 ~10m 3 More preferably, 2m 3 ~5m 3 This is even more preferable. The volume of the reaction vessel is 0.5 m³. 3 The above conditions result in excellent silica particle production. Furthermore, the reaction vessel volume is 20 m³. 3 The following conditions facilitate temperature control in the hydrolysis and condensation reactions of alkoxysilanes.

[0028] Examples of tetraalkoxysilanes include tetraalkoxysilanes, tetraethoxysilanes, tetrapropoxysilanes, and tetraisopropoxysilanes, which have 1 to 12 carbon atoms in the alkoxy group. These tetraalkoxysilanes may be used individually or in combination of two or more. Among these tetraalkoxysilanes, tetramethoxysilane and tetraethoxysilane are preferred, and tetramethoxysilane is more preferred, because they undergo rapid hydrolysis, leave little unreacted residue, have excellent productivity, and allow for easy acquisition of a stable silica sol.

[0029] The hydrolysis and condensation reactions of tetraalkoxysilane are easily controllable, the reaction rates of the hydrolysis and condensation reactions can be increased, gelation of the silica particle dispersion can be prevented, and silica particles of uniform particle size can be obtained. Therefore, a method of adding a solution (B) containing tetraalkoxysilane and a solution (C) containing water to a solution (A) containing an alkaline catalyst is preferred. In this case, it is preferable to add a solution (B) containing tetraalkoxysilane and a solution (C) containing water to a solution (A) containing an alkaline catalyst while stirring, and to allow the tetraalkoxysilane to undergo hydrolysis and condensation reactions while stirring. It is preferable to add solutions (B) and (C) to solution (A) almost simultaneously and for approximately the same amount of time.

[0030] Solution (A) contains an alkaline catalyst.

[0031] Examples of alkaline catalysts in solution (A) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. These alkaline catalysts may be used individually or in combination of two or more. Among these alkaline catalysts, ammonia is preferred because it exhibits excellent catalytic activity, allows for easy control of particle shape, suppresses metal contamination, and has high volatility, resulting in excellent removal after hydrolysis and condensation reactions.

[0032] Solution (A) preferably contains water because it can promote the hydrolysis of alkoxysilane.

[0033] Solution (A) is preferable to contain a solvent other than water because it exhibits excellent dispersibility of tetraalkoxysilane in the reaction solution.

[0034] Other solvents in solution (A) include, for example, 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 individually or in combination of two or more. Among these solvents, alcohols are preferred because they readily dissolve tetraalkoxysilane, produce the same by-products as those used in the hydrolysis and condensation reactions, and offer excellent manufacturing convenience. Methanol and ethanol are more preferred, and methanol is even more preferred.

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

[0036] The concentration of water in solution (A) is preferably 3% to 30% by mass, and more preferably 5% 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 higher, the dispersibility of silicic acid produced by the hydrolysis reaction in the reaction solution is excellent. Furthermore, when the concentration of water in solution (A) is 30% by mass or lower, the dispersibility of tetraalkoxysilane in the reaction solution is excellent.

[0037] The concentration of solvents other than water in solution (A) is preferably the remainder of the alkaline catalyst and water.

[0038] Solution (B) contains tetraalkoxysilane.

[0039] Solution (B) is preferably a solvent because it exhibits excellent dispersibility of tetraalkoxysilane in the reaction mixture.

[0040] Examples of solvents 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 individually or in combination of two or more. Among these solvents, alcohols are preferred, methanol and ethanol are more preferred, and methanol is even more preferred, because the by-products used in the hydrolysis and condensation reactions are the same, and they offer excellent manufacturing convenience.

[0041] The concentration of tetraalkoxysilane in solution (B) is preferably 76% to 89% by mass, and more preferably 77% 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 higher, the amount of solvent used can be reduced, resulting in excellent silica particle production. Furthermore, when the concentration of tetraalkoxysilane in solution (B) is 89% by mass or lower, the dispersibility of tetraalkoxysilane in the reaction solution is excellent.

[0042] The concentration of the solvent in solution (B) is preferably 11% to 24% by mass, and more preferably 12% 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 higher, the dispersibility of the tetraalkoxysilane in the reaction solution is excellent. Furthermore, when the concentration of the solvent in solution (B) is 24% by mass or lower, the amount of solvent used can be reduced, resulting in excellent silica particle production. The concentration of the solvent in solution (B) is preferably the remainder of the tetraalkoxysilane in solution (B).

[0043] The rate at which solution (B) is added to solution (A) is preferably 10 g silica / hour / kg to 300 g silica / hour / kg, and more preferably 40 g silica / hour / kg to 200 g silica / hour / kg. When the rate at which solution (B) is added is 10 g silica / hour / kg or higher, the reaction time is shortened and productivity is excellent. Furthermore, when the rate at which solution (B) is added is 300 g silica / hour / kg or lower, the dispersibility of tetraalkoxysilane in the reaction solution is excellent. Here, "g silica / hour / kg solution" refers to the value (g) obtained by converting the mass of tetraalkoxysilane in solution (B) added per hour to 1 kg of solution (A) into the mass of silica.

[0044] Solution (C) contains water.

[0045] Solution (C) may contain an alkaline catalyst because it can efficiently carry out hydrolysis and condensation reactions.

[0046] Examples of alkaline catalysts in solution (C) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. These alkaline catalysts may be used individually or in combination of two or more. Among these alkaline catalysts, ammonia is preferred because it exhibits excellent catalytic activity, allows for easy control of particle shape, suppresses metal contamination, and has high volatility, resulting in excellent removal after hydrolysis and condensation reactions.

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

[0048] Other solvents in solution (C) besides water include, for example, alcohols such as methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents may be used individually or in combination of two or more.

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

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

[0051] The concentration of solvents other than water in solution (C) is preferably the remainder of the water and alkaline catalyst.

[0052] The rate at which solution (C) is added to solution (A) is, for example, preferably 0.4 g alkali catalyst / hour / kg to 12 g alkali catalyst / hour / kg, and more preferably 1.7 g alkali catalyst / hour / kg to 7.7 g alkali catalyst / hour / kg, when solution (C) contains an alkaline catalyst. If the rate at which solution (C) is added is 0.4 g alkali catalyst / hour / kg or higher, aggregation of silica particles is suppressed, and the dispersion stability of silica particles in the resulting dispersion is excellent. If the rate at which solution (C) is added is 12 g alkali catalyst / hour / kg or lower, the reaction does not proceed excessively quickly, and the reaction is well controllable. Here, "g alkali catalyst / hour / kg solution" represents the mass (g) of alkali catalyst in solution (C) added per hour to 1 kg of solution (A).

[0053] The concentration of water in the reaction system for hydrolysis and condensation reactions is preferably maintained at 3% to 30% by mass, and more preferably at 5% to 25% by mass, of the reaction solution in the reaction vessel. When the water concentration in the reaction system is 3% by mass or higher, 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 lower, the dispersibility of 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 liquids and substances dissolved in the liquids in the reaction system during hydrolysis and condensation reactions. At the start of the reaction, the total amount of liquids and substances dissolved in the liquids is only solution (A), and during the reaction, it becomes the total amount of solution (A), solution (B), solution (C), and the alcohol produced by the reaction. Silica particles dispersed in the liquids are not included in the definition of liquids and substances dissolved in the liquids in the reaction system.

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

[0055] The reaction temperature (temperature of the reaction solution in the reaction system) for the hydrolysis and condensation reactions of tetraalkoxysilanes is preferably 15°C to 50°C, and more preferably 20°C to 45°C. A reaction temperature of 15°C or higher prevents the reaction from proceeding excessively slowly, resulting in excellent controllability. Furthermore, a reaction temperature of 50°C or lower provides an excellent balance between the hydrolysis and condensation reaction rates.

[0056] Since the hydrolysis and condensation reactions of tetraalkoxysilanes are exothermic reactions, it is preferable to carry out the reaction while cooling the reaction vessel or reaction solution in order to control the reaction temperature. There are no particular restrictions on the cooling method, but the following methods are possible. (1) A cooling jacket is installed around the outer circumference of the reaction vessel, and the entire reaction vessel is cooled with a cooling medium such as water. (2) Install circulating pipes for a cooling medium such as water in the reaction vessel to cool the reaction liquid. The methods described in (1) and (2) above may be used individually or in combination.

[0057] (Physical properties of silica particles) The preferred physical properties of the silica particles produced in the aforementioned process (1) are 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. Further, when the average primary particle diameter of the silica particles is 100 nm or less, the surface roughness and scratches of the polished object typified by the silicon wafer can be reduced, and the 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 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)

[0060] The average primary particle diameter of the silica particles can be set within a desired range by 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 removability of particles and the like in the cleaning after polishing is excellent, and the storage stability of the silica sol is excellent. Further, when the average secondary particle diameter of the silica particles is 200 nm or less, the surface roughness and scratches of the polished object typified by the silicon wafer during polishing can be reduced, the removability of particles and the like in the cleaning after polishing is excellent, and the sedimentation of the silica particles can be suppressed.

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

[0063] The average secondary particle diameter of the silica particles can be set within a desired range by known conditions and methods. <6000281> 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 higher, the polishing rate on the workpiece, such as silicon wafers, is excellent, resulting in superior silicon wafer productivity. Furthermore, when the CV value of the silica particles is 50 or lower, the surface roughness and scratches on the workpiece, such as silicon wafers, during polishing can be reduced, and the removal of particles and other contaminants during cleaning after polishing is excellent.

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

[0066] The association ratio of silica particles is preferably 1.0 to 4.0, and more preferably 1.1 to 3.0. When the association ratio of silica particles is 1.0 or higher, the polishing rate for the workpiece, such as silicon wafers, is excellent, and the productivity of silicon wafers is excellent. Furthermore, when the association ratio of silica particles is 4.0 or lower, the surface roughness and scratches of the workpiece, such as silicon wafers, during polishing can be reduced, and the 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 and the average secondary particle diameter measured using the aforementioned 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 groups / nm. 2 ~10 pieces / nm 2 Preferably, 0.5 particles / nm 2 ~7.5 pieces / nm 2 More preferably, 2.0 pieces / nm 2 ~7.0 pieces / nm 2 A more preferable result is a surface silanol group density of 0.1 particles / nm of silica particles. 2As a result, the silica particles have appropriate surface resilience, and the dispersion stability of the silica sol is excellent. Furthermore, the surface silanol group density of the silica particles is 10 / nm. 2 The following conditions allow the silica particles to have appropriate surface resilience, thereby suppressing the aggregation of silica particles.

[0069] The surface silanol group density of silica particles is measured by the Sears method. Specifically, it is measured and calculated under the conditions shown below. Take a silica sol equivalent to 1.5 g of silica particles and add pure water to make a volume of 90 mL. At 25°C, add 0.1 mol / L hydrochloric acid solution until the pH reaches 3.6, add 30 g of sodium chloride, and gradually add pure water to completely dissolve the sodium chloride. Finally, add pure water until the total volume of the test solution is 150 mL to obtain the test solution. The obtained test solution is placed in an automatic titrator, and a 0.1 mol / L sodium hydroxide solution is added dropwise to measure the titration volume A (mL) of the 0.1 mol / L sodium hydroxide solution required to change the pH from 4.0 to 9.0. Using the following formula (4), calculate the amount of 0.1 mol / L sodium hydroxide solution (V) consumed to change the pH of 1.5 g of silica particles from 4.0 to 9.0. Using the following formula (5), calculate the surface silanol group density ρ (particles / nm) of the silica particles. 2 Calculate ). V=(A×f×100×1.5) / (W×C) ··· (4) A: The amount of 0.1 mol / L sodium hydroxide solution required to change the pH of 1.5 g of silica particles from 4.0 to 9.0 (mL) f: Titer 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) The amount of sodium hydroxide (mol) required to change the pH of 1.5g of silica particles from 4.0 to 9.0, calculated from B:V. N A Avogadro's number (particles / mol) M: Silica particle amount (1.5g) S BET :Specific surface area (m²) of silica particles measured when calculating the average primary particle diameter 2 / g)

[0070] The methods for measuring and calculating the surface silanol group density of the silica particles were based on the following references: "GWSears, Jr., Analytical Chemistry, Vol.28, No.12, pp.1981-1983 (1956)," "Shinichi Haba, Development of Polishing Agents for Semiconductor Integrated Circuit Processes, Doctoral Dissertation, Kochi University of Technology, pp.39-45, March 2004," "Patent Publication No. 5967118," and "Patent Publication No. 6047395."

[0071] The surface silanol group density of silica particles can be set to a desired range by adjusting the conditions of the hydrolysis and condensation reactions of alkoxysilanes.

[0072] Examples of silica particle shapes include spherical, chain-like, cocoon-like (also referred to as knob-like or peanut-like), and irregular shapes (e.g., warty, bent, branched, etc.). Among these silica particle shapes, spherical is preferred when the goal is to reduce surface roughness and scratches on the workpiece, such as a silicon wafer, during polishing, while irregular shapes are preferred when the goal is to increase the polishing rate on the workpiece, such as a silicon wafer.

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

[0074] In the displacement step, to remove the dispersion medium from the silica particle dispersion, it is preferable to heat and evaporate the dispersion medium in the silica particle dispersion under normal pressure or reduced pressure. The heating temperature at this time should be the boiling point of the dispersion medium under the pressure conditions during displacement, or slightly higher, and is preferably 50°C to 150°C.

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

[0076] In the substitution process, the removal of the dispersion medium from the silica particle dispersion and the addition of a new dispersion medium may be performed simultaneously, or one may be performed first and the other later.

[0077] In the silica sol production method of the present invention, the substitution step is carried out using a container having a fluororesin layer on its surface (wetted surface) because it allows for efficient heating during substitution, offers excellent productivity, and maintains a low metal content.

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

[0079] The volume of the container is 0.1 m³. 3 ~20m 3 Preferably, 0.2m 3 ~10m 3 More preferably, 0.5m 3 ~5m 3 This is even more preferable. The container volume is 0.1 m³. 3 The above conditions result in excellent silica particle production. Also, the container volume is 20 m³. 3 The following conditions allow for efficient heating during substitution.

[0080] (Pressurized heating process) The pressure used when pressurizing and heating the dispersion of silica particles obtained in the substitution step is preferably 0.10 MPa to 2.3 MPa, and more preferably 0.14 MPa to 1.0 MPa. A pressurizing and heating pressure of 0.10 MPa or higher can increase the degree of condensation of the silica particles. Furthermore, a pressurizing and heating pressure of 2.3 MPa or lower allows for the production of silica sol without significantly changing the average primary particle diameter, average secondary particle diameter, cv value, or association ratio of the silica particles, resulting in excellent dispersion stability of the silica sol. Pressurization can be achieved by heating the silica particle dispersion to above the boiling point of the dispersion medium while it is sealed. When the silica particle aqueous dispersion is heated to above 100°C while sealed, the pressure will be equal to the saturated water vapor pressure at that temperature.

[0081] The temperature for the pressurized heat treatment is preferably 100°C to 220°C, and more preferably 110°C to 180°C. A pressurized heat treatment temperature of 100°C or higher can increase the degree of condensation of silica particles. Furthermore, a pressurized heat treatment temperature of 220°C or lower allows for the production of silica sol without significantly changing the average primary particle diameter, average secondary particle diameter, cv value, or association ratio of the silica particles, resulting in excellent dispersion stability of the silica sol.

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

[0083] Pressurized heat treatment is more preferably performed in an aqueous dispersion because it can increase the degree of condensation of silica particles without significantly changing the average primary particle diameter, average secondary particle diameter, cv value, or association ratio.

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

[0085] In the pressurized heating process, it is preferable to use a container having a fluororesin layer on its surface, as this allows for the maintenance of a low metal content.

[0086] Containers having a fluororesin layer on their surface are preferable because they allow for industrial-scale design and have excellent mechanical strength, and containers having a fluororesin layer on the surface of stainless steel are even preferable because they have excellent corrosion resistance. A preferred method for applying a fluororesin layer to the surface of a container is one in which the fluororesin is baked onto the surface of the container by electrostatic coating after pre-treatment as necessary, because it is easy to manufacture and process. However, other coating methods such as heat-bonding press coating can also be employed.

[0087] The type of fluororesin and its thickness that constitute the fluororesin layer can be the same as those used for the fluororesin layer provided on the surface of the container used in the substitution process described above.

[0088] The volume of the container is 0.1 m³. 3 ~20m 3 Preferably, 0.2m 3 ~10m 3 More preferably, 0.5m 3 ~5m 3 This is even more preferable. The container volume is 0.1 m³. 3 The above conditions result in excellent silica sol productivity. Furthermore, the container volume is 20 m³. 3 The following conditions allow for efficient pressurized heating.

[0089] (Physical properties and components of silica sol) The silica particle content in the silica sol produced by the present invention is preferably 3% to 50% by mass, more preferably 4% to 40% by mass, and even more preferably 5% to 30% by mass, based on 100% by mass of the total silica sol. When the silica particle content in the silica sol is 3% by mass or more, the polishing rate for the workpiece, such as silicon wafers, is excellent. Furthermore, when the silica particle content in the silica sol is 50% by mass or less, aggregation of silica particles in the silica sol and polishing composition can be suppressed, resulting in excellent storage stability for the silica sol and polishing composition.

[0090] The content of the dispersion medium in the silica sol is preferably 50% to 97% by mass, more preferably 60% to 96% by mass, and even more preferably 70% to 95% by mass, based on 100% by mass of the total silica sol. 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 and polishing composition can be suppressed, resulting in excellent storage stability of the silica sol and polishing composition. Furthermore, when the content of the dispersion medium in the silica sol is 97% by mass or less, the polishing rate for the workpiece, such as silicon wafers, is excellent.

[0091] The content of silica particles and dispersion medium in the silica sol can be set to a desired range during the substitution process.

[0092] In addition to silica particles and dispersion medium, silica sol may contain other components as needed, such as oxidizing agents, preservatives, fungicides, pH adjusters, pH buffers, surfactants, chelating agents, and antimicrobial biocides, to the extent that it does not impair its performance. In particular, since silica sol has excellent storage stability, it is preferable to include an antimicrobial biocide in the silica sol.

[0093] Examples of antimicrobial biocides include hydrogen peroxide, ammonia, quaternary ammonium hydroxide, quaternary ammonium salt, ethylenediamine, glutaraldehyde, methyl p-hydroxybenzoate, and sodium chlorite. These antimicrobial biocides may be used individually or in combination of two or more. Among these antimicrobial biocides, hydrogen peroxide is preferred due to its excellent affinity for silica sol. Antimicrobial biocides include what are commonly known as disinfectants.

[0094] The content of the antimicrobial biocide in the silica sol is preferably 0.0001% to 10% by mass, and more preferably 0.001% to 1% by mass, based on 100% by mass of the total silica sol. When the content of the antimicrobial biocide in the silica sol is 0.0001% by mass or more, the silica sol exhibits excellent storage stability. When the content of the antimicrobial biocide in the silica sol is 10% by mass or less, the original performance of the silica sol is not impaired.

[0095] The pH of the silica sol is preferably 6.0 to 8.0, and more preferably 6.5 to 7.8. When the pH of the silica sol is 6.0 or higher, it exhibits excellent dispersion stability and can suppress the aggregation of silica particles. Furthermore, when the pH of the silica sol is 8.0 or lower, it prevents the dissolution of silica particles and exhibits excellent long-term storage stability. The pH of silica sol can be set to a desired range by adding a pH adjusting agent.

[0096] The metal content (metal impurity content) of the silica sol as an impurity is preferably 1 ppm or less, and more preferably 0.2 ppm or less.

[0097] In the polishing of silicon wafers, metallic impurities adhere to the surface of the object being polished, contaminating it and negatively affecting the properties of both the object and the final product. Furthermore, the presence of metal impurities in silica sol can lead to coordination interactions between the acidic surface silanol groups and the metal impurities, altering the chemical properties of the surface silanol groups (such as acidity), the steric environment of the silica particle surface (such as the ease with which silica particles aggregate), and ultimately affecting the polishing rate.

[0098] The metal content of silica sol is measured by 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 create a test solution with a total volume of exactly 10 g, and the result is 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 sum of the content of these metals is considered the metal content.

[0099] The metal content of silica sol can be reduced to 1 ppm or less by performing a substitution process using a container with a fluororesin layer on its surface. As mentioned above, silica particles are preferably produced by hydrolysis and condensation reactions using tetraalkoxysilane as the main raw material. In methods involving the deionization of alkali silicates such as water glass, residual sodium and other substances from the raw materials remain, making it difficult to reduce the metal content of the silica sol to 1 ppm or less.

[0100] (polishing composition) The silica sol obtained by the silica sol production method of the present invention can be suitably used as an abrasive composition. The polishing composition preferably contains silica sol produced by the silica sol production method of the present invention and a water-soluble polymer.

[0101] Water-soluble polymers enhance the wettability of the polishing composition to the workpiece, such as silicon wafers. Preferably, the water-soluble polymer has functional groups with high water affinity. These water-affinity functional groups have a high affinity for the surface silanol groups of silica particles, resulting in a more stable dispersion of silica particles and water-soluble polymers in close proximity within the polishing composition. Therefore, during polishing of workpieces such as silicon wafers, the effects of silica particles and water-soluble polymers function synergistically.

[0102] Examples of water-soluble polymers include cellulose derivatives, polyvinyl alcohol, polyvinylpyrrolidone, copolymers having a polyvinylpyrrolidone skeleton, and polymers having a polyoxyalkylene structure.

[0103] Examples of cellulose derivatives include hydroxyethylcellulose, hydrolyzed hydroxyethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose, methylcellulose, ethylcellulose, ethylhydroxyethylcellulose, and carboxymethylcellulose. 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.

[0104] These water-soluble polymers may be used individually or in combination of two or more. Among these water-soluble polymers, cellulose derivatives are preferred, and hydroxyethylcellulose is more preferred, because they have a high affinity for the silanol groups on the surface of silica particles and act synergistically to provide good hydrophilicity to the surface of the object to be polished.

[0105] 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, it exhibits excellent affinity with silica sol and provides excellent polishing rate for materials to be polished, such as silicon wafers.

[0106] The mass-average molecular weight of water-soluble polymers is measured by size exclusion chromatography using a 0.1 mol / L NaCl solution as the mobile phase, based on polyethylene oxide.

[0107] The content of water-soluble polymers in the abrasive composition is preferably 0.02% to 10% by mass, and more preferably 0.05% to 5% by mass, based on 100% by mass of the total amount of the abrasive composition. When the content of water-soluble polymers in the abrasive composition is 0.02% by mass or more, the hydrophilicity of the abrasive composition is improved. Furthermore, when the content of water-soluble polymers in the abrasive composition is 10% by mass or less, aggregation of silica particles during the preparation of the abrasive composition can be suppressed.

[0108] In addition to silica sol and water-soluble polymers, the abrasive composition may contain other components as needed, to the extent that it does not impair its performance, such as basic compounds, polishing accelerators, surfactants, hydrophilic compounds, preservatives, fungicides, pH adjusters, pH buffers, surfactants, chelating agents, and antimicrobial biocides. In particular, it is preferable to include a basic compound in the polishing composition because it allows for chemical polishing (chemical etching) by applying a chemical action to the surface of the workpiece, such as a silicon wafer, and the polishing speed of the workpiece, such as a silicon wafer, can be improved due to the synergistic effect with the silanol groups on the surface of the silica particles.

[0109] Examples of basic compounds include organic basic compounds, alkali metal hydroxides, alkali metal bicarbonates, alkali metal carbonates, and ammonia. These basic compounds may be used individually or in combination of two or more. Among these basic compounds, ammonia, tetramethylammonium hydroxide, tetraethylammonium hydroxide, ammonium bicarbonate, and ammonium carbonate are preferred due to their high water solubility and excellent affinity with silica particles and water-soluble polymers. Ammonia, tetramethylammonium hydroxide, and tetraethylammonium hydroxide are more preferred, and ammonia is even more preferred.

[0110] The content of basic compounds in the polishing composition is preferably 0.001% to 5% by mass, and more preferably 0.01% to 3% by mass, based on 100% by mass of the total amount of the polishing composition. When the content of basic compounds in the polishing composition is 0.001% by mass or more, the polishing speed of the workpiece, such as silicon wafers, can be improved. Furthermore, when the content of basic compounds in the polishing composition is 5% by mass or less, the stability of the polishing composition is excellent.

[0111] The pH of the abrasive composition is preferably 8.0 to 12.0, and more preferably 9.0 to 11.0. When the pH of the abrasive composition is 8.0 or higher, aggregation of silica particles in the abrasive composition can be suppressed, and the dispersion stability of the abrasive composition is excellent. Furthermore, when the pH of the abrasive composition is 12.0 or lower, dissolution of silica particles can be suppressed, and the stability of the abrasive composition is excellent. The pH of the abrasive composition can be set to a desired range by adding a pH adjusting agent.

[0112] The polishing composition can be obtained by mixing silica sol obtained by the silica sol production method of the present invention, a water-soluble polymer, and other components as needed. However, considering storage and transportation, it may be prepared at a high concentration first and then diluted with water or the like immediately before polishing.

[0113] (polishing method) The polishing method of the present invention is a method of polishing using a polishing composition containing silica sol obtained by the silica sol production method of the present invention. It is preferable to use the polishing composition described above. Specific polishing methods include, for example, pressing the surface of a silicon wafer against a polishing pad, dropping a polishing composition onto the polishing pad, and polishing the surface of the silicon wafer.

[0114] (Application) The silica sol obtained by the silica sol production method of the present invention can be suitably used for polishing applications, and is particularly suitable for polishing silicon wafers and chemical mechanical polishing. [Examples]

[0115] The present invention will be described in more detail below using alternative experimental examples and comparative experimental examples, but the present invention is not limited to the descriptions of the following examples without departing from its essence.

[0116] (sample) A silica sol with a silica particle / ammonia / water ratio of 19 / 1.4 / 79.6 (mass%) was used as a sample. Silica particles are produced by hydrolysis and condensation reactions of tetramethoxysilane.

[0117] (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 create a test solution with a total volume of exactly 10 g. 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).

[0118] [Example 1] A 40 mL sample was supplied to a container (a bottomed cylindrical container with an inner diameter of 2.5 cm and a fluororesin layer thickness of 300 μm) in which the surface of a stainless steel container was coated with fluororesin (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin). 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.

[0119] [Example 2] Except for a stirring time of 48 hours, the procedure was carried out in the same manner as in Example 1, and the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0120] [Comparative Example 1] 40 mL of the sample was supplied to a stainless steel container (a bottomed cylindrical container with an inner diameter of 2.5 cm), stirred at 90°C for 24 hours, and then the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0121] [Comparative Example 2] Except for a stirring time of 48 hours, the procedure was carried out in the same manner as in Comparative Example 1, and the content of each metal in the sample was measured. The evaluation results are shown in Table 1.

[0122] [Reference example 1] The content of each metal in the sample itself was measured. The evaluation results are shown in Table 1.

[0123] [Table 1]

[0124] Comparative Examples 1 and 2 show that using stainless steel containers as containers in the displacement process results in the generation of a large amount of iron, making them unsuitable for applications requiring high purity. On the other hand, as shown in Examples 1 and 2, it can be seen that a low metal content can be maintained when using a container having a fluororesin layer on its surface. [Industrial applicability]

[0125] The silica sol obtained by the silica sol production method of the present invention can be suitably used for polishing applications, and is particularly suitable for polishing silicon wafers and chemical mechanical polishing.

Claims

1. A method for producing silica sol, comprising the step of replacing the dispersion medium in a dispersion of silica particles in a container having a fluororesin layer on its surface, wherein the volume of the container is 0.1 m³ or more.

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

3. A method for producing silica sol according to claim 1 or 2, further comprising the step of adding a solution (B) containing tetraalkoxysilane and a solution (C) containing water to a solution (A) containing an alkaline catalyst to carry out a hydrolysis reaction and a condensation reaction of tetraalkoxysilane to obtain a dispersion of silica particles.

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

5. A method for producing silica sol according to any one of claims 1 to 4, comprising the step of replacing the dispersion medium in a dispersion of silica particles, wherein an alkaline catalyst is removed.

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

7. A method for producing silica sol according to any one of claims 1 to 6, wherein the fluororesin comprises a tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin.

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

9. A polishing method comprising polishing using an abrasive composition containing a silica sol obtained by a method for producing a silica sol according to any one of claims 1 to 8.

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