JP2023004284A - Method for producing silica particles, method for producing silica sol and polishing method - Google Patents

Abstract

Kind Code: A1 A method for producing silica particles having a low metal content by preventing metal contamination originating from a reaction vessel during hydrolysis and condensation reactions of alkoxysilane is provided. Kind Code: A1 A method for producing silica particles, comprising a step of hydrolyzing and condensing tetraalkoxysilane in a reaction tank having a fluororesin layer on its surface. As the fluorine resin, a tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin is preferable. A method for producing silica sol, including the method for producing silica particles. A polishing method comprising polishing with a polishing composition containing the silica sol obtained by the method for producing a silica sol. [Selection figure] None

Description

TECHNICAL FIELD The present invention relates to a method for producing silica particles, a method for producing silica sol, and a polishing method.

A polishing method using a polishing liquid is known as a method for polishing the surface of materials such as metals and inorganic compounds. In particular, in the final polishing of silicon wafers and chemical mechanical polishing (CMP), the surface conditions greatly affect the characteristics of the final product, so the surfaces and end faces of these parts are polished with extremely high precision. is required.

In such precision polishing, a polishing composition containing silica particles is employed, and colloidal silica is widely used as abrasive grains, which are the main component of the composition. Colloidal silica is produced by thermal decomposition of silicon tetrachloride (fumed silica, etc.), deionized alkali silicate such as water glass, hydrolysis reaction and condensation reaction of alkoxysilane (generally " known as "sol-gel method").

Many studies have been made so far on methods for producing silica particles. For example, Patent Document 1 discloses a method for producing a dispersion of silica particles by hydrolysis reaction and condensation reaction of alkoxysilane.

JP 2018-108924 A

Synthesis of compounds on an industrial scale is generally carried out in a reaction vessel made of metal such as stainless steel. However, when a metal reaction vessel is used, metal components such as Fe are eluted from the metal constituting the reaction vessel during the hydrolysis reaction and condensation reaction of the alkoxysilane, and the produced dispersion of silica particles is contaminated with the metal. there is a problem
On the other hand, in precision polishing, metal impurities adhere to the surface of the object to be polished, contaminating the object to be polished, and adversely affecting the characteristics of the object to be polished and the final product. Therefore, it is necessary to suppress the contamination of metal impurities in the hydrolysis reaction and condensation reaction of alkoxysilane.

With respect to this problem, Patent Document 1 does not describe any device related to the reaction tank used for the hydrolysis reaction and condensation reaction of alkoxysilane.

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 reaction tank during the hydrolysis reaction and condensation reaction of alkoxysilane, and to reduce the metal content. An object of the present invention is to provide a method for producing silica particles, a method for producing silica sol, and a polishing method using this silica sol.

As a result of intensive studies, the present inventors have found that in the hydrolysis reaction and condensation reaction of alkoxysilane, by using a reaction vessel having a fluororesin layer on the surface, the contamination of metal impurities is suppressed and the metal content is low. The inventors have found that it is possible to produce silica particles and silica sol with a low molecular weight, and have completed the present invention.

That is, the gist of the present invention is as follows.
[1] A method for producing silica particles, comprising a step of hydrolyzing and condensing tetraalkoxysilane in a reaction vessel having a fluororesin layer on its surface.
[2] The method for producing silica particles according to [1], wherein the hydrolysis reaction and condensation reaction of the tetraalkoxysilane are carried out at a reaction temperature of 15°C to 50°C.
[3] The method for producing silica particles according to [1] or [2], wherein the hydrolysis reaction and condensation reaction of the tetraalkoxysilane are performed while cooling.
[4] The method for producing silica particles according to any one of [1] to [3], wherein the tetraalkoxysilane contains tetramethoxysilane.
[5] Hydrolysis reaction and condensation reaction of tetraalkoxysilane are performed by adding solution (B) containing tetraalkoxysilane and solution (C) containing water to solution (A) containing alkali catalyst. ] A method for producing silica particles according to any one of [4].
[6] The method for producing silica particles according to any one of [1] to [5], wherein the fluororesin contains a tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin.
[7] A method for producing silica sol, comprising the method for producing silica particles according to any one of [1] to [6].
[8] The method for producing a silica sol according to [7], wherein the silica sol has a metal content of 1 ppm or less.
[9] A polishing method comprising polishing with a polishing composition containing the silica sol obtained by the method for producing a silica sol according to [7] or [8].

According to the method for producing silica particles and the method for producing silica sol of the present invention, silica particles and silica sol having a low metal content are produced by preventing metal contamination from the reaction tank during the hydrolysis reaction and condensation reaction of alkoxysilane. A polishing composition containing such a silica sol can be used to prevent metallic contamination of an object to be polished and to obtain a high-quality polishing product.

Although the present invention will be described in detail below, the present invention is not limited to the following embodiments, and can be implemented with various modifications within the scope of the gist thereof. In addition, when the expression "-" is used in this specification, it is used as an expression including numerical values or physical property values before and after it.

(Method for producing silica particles)
The method for producing silica particles of the present invention comprises a step of hydrolyzing and condensing tetraalkoxysilane in a reaction vessel having a fluororesin layer on its surface. Here, the "surface" is the wetted surface of the reaction vessel (the surface in contact with the reaction liquid), and usually corresponds to the inner surface of the reaction vessel.

A reactor with a fluororesin layer on the surface can be designed on an industrial scale and has excellent mechanical strength. A reaction tank having a fluororesin layer on the surface of a stainless steel container is more preferable because it is excellent.

Examples of the fluororesin used for forming the fluororesin layer include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene Fluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether terpolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), trifluorochloroethylene-ethylene copolymer (ECTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), etc. can be used, and a mixed resin of any two or more of these can also be used.
Among these fluororesins, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA) is preferable because it can be melt-molded and has excellent moldability.

These fluororesins have excellent corrosion resistance, chemical resistance (acid resistance, alkali resistance), and heat resistance. be able to.
In the hydrolysis and condensation reactions of alkoxysilane, which are exothermic reactions, it is necessary to proceed with the reaction while cooling. By covering with a resin layer, there is a concern that the thermal efficiency at the time of heating or cooling from the outside is lowered, and the productivity of silica particles is impaired. However, the fluororesin has a relatively high heat transfer coefficient among the highly durable resins, and the problem of cooling efficiency and temperature control is relatively small.

The method of providing a fluororesin layer on the surface of the reaction tank is preferably a method of baking the fluororesin by electrostatic coating after the surface of the reaction tank is subjected to pretreatment as necessary, since manufacturing and processing are easy. However, other methods such as a coating method using a hot press can also be employed.

The thickness of the fluororesin layer is preferably 100 μm to 1000 μm, 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, it is possible to suppress a decrease in thermal efficiency due to the provision of the fluororesin layer, and to suppress the cost of forming the fluororesin layer.

The volume of the reaction tank is preferably 0.5 m ³ to 20 m ³ , more preferably 1 m ³ to 10 m ³ , still more preferably 2 m ³ to 5 m ³ . When the volume of the reaction tank is 0.5 m ³ or more, the productivity of silica particles is excellent. Further, when the volume of the reaction vessel is 20 m ³ or less, it is easy to control the temperature in the hydrolysis reaction and condensation reaction of the alkoxysilane.

Tetraalkoxysilanes include, for example, tetraalkoxysilanes having 1 to 12 carbon atoms in the alkoxy group 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 preferable because they have a fast hydrolysis reaction, do not easily leave unreacted substances, are excellent in productivity, and can easily obtain a stable silica sol. Methoxysilane is more preferred.

The hydrolysis reaction and condensation reaction of tetraalkoxysilane are easy to control the hydrolysis reaction and condensation reaction, can increase the reaction rate of the hydrolysis reaction and condensation reaction, prevent the silica particle dispersion from gelling, and the particles A method of adding a solution (B) containing a tetraalkoxysilane and a solution (C) containing water to a solution (A) containing an alkali catalyst is preferable because silica particles having a uniform diameter can be obtained.
In this case, the solution (B) containing the tetraalkoxysilane and the solution (C) containing water are added to the solution (A) containing the alkali catalyst with stirring, and the tetraalkoxysilane is hydrolyzed and reacted with stirring. A condensation reaction is preferred. In addition, it is preferable to add the solution (B) and the solution (C) to the solution (A) almost at the same time over about the same amount of time.

Solution (A) contains an alkali catalyst.

Examples of the alkali catalyst in the solution (A) include ethylenediamine, diethylenetriamine, triethylenetetramine, ammonia, urea, ethanolamine, tetramethylammonium hydroxide and the like. One of these alkali catalysts may be used alone, or two or more thereof may be used in combination. Among these alkali catalysts, ammonia is excellent in catalytic action, easy to control particle shape, can suppress metal contamination, has high volatility and is excellent in removability after hydrolysis reaction and condensation reaction. preferable.

The solution (A) preferably contains water because it can promote hydrolysis of the alkoxysilane.

The solution (A) preferably contains a solvent other than water because the tetraalkoxysilane has excellent dispersibility in the reaction solution.

Examples of the solvent other than water in the solution (A) include alcohols such as methanol, ethanol, propanol, isopropanol and ethylene glycol; ketones such as acetone and methyl ethyl ketone; 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 one used in the hydrolysis reaction and the condensation reaction is the same as the by-product, and the convenience in production is excellent. Alcohols, such as methanol and ethanol. is more preferred, and methanol is even more preferred.

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

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 the solution (A) is 3% by mass or more, the silicic acid produced by the hydrolysis reaction is excellent in dispersibility in the reaction liquid. Moreover, when the concentration of water in the solution (A) is 30% by mass or less, the dispersibility of the tetraalkoxysilane in the reaction liquid is excellent.

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

Solution (B) contains a tetraalkoxysilane.

The solution (B) preferably contains a solvent because the tetraalkoxysilane is excellent in dispersibility in the reaction solution.

Examples of the solvent in the 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, methanol and ethanol are more preferred, and methanol is even more preferred, since the one used in the hydrolysis reaction and the condensation reaction and the by-product are the same and are excellent in terms of convenience in production. .

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

The concentration of the solvent in the 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 the solution (B). When the concentration of the solvent in the solution (B) is 11% by mass or more, the tetraalkoxysilane is excellent in dispersibility in the reaction liquid. Moreover, when the concentration of the solvent in the solution (B) is 24% by mass or less, the amount of the solvent to be used can be reduced, and the productivity of the silica particles is excellent. The concentration of solvent in solution (B) is preferably the balance of the tetraalkoxysilane in solution (B).

The rate of addition of solution (B) to solution (A) is preferably 10 g silica/h/kg solution to 300 g silica/h/kg solution, more preferably 40 g silica/h/kg solution to 200 g silica/h/kg solution. preferable. When the addition rate of the solution (B) is 10 g silica/hour/kg solution or more, the reaction time is shortened and the productivity is excellent. Further, when the addition rate of the solution (B) is 300 g silica/hour/kg solution or less, the tetraalkoxysilane is excellent in dispersibility in the reaction liquid.
Here, "g silica/hour/kg solution" is a value obtained by converting the mass of tetraalkoxysilane in solution (B) added per hour to 1 kg of solution (A) into mass of silica ( g).

Solution (C) contains water.

The solution (C) may contain an alkali catalyst since it allows the hydrolysis reaction and condensation reaction to proceed efficiently.

Examples of the alkali catalyst in the solution (C) include ethylenediamine, diethylenetriamine, triethylenetetramine, ammonia, urea, ethanolamine, tetramethylammonium hydroxide and the like. One of these alkali catalysts may be used alone, or two or more thereof may be used in combination. Among these alkali catalysts, ammonia is excellent in catalytic action, easy to control particle shape, can suppress metal contamination, has high volatility and is excellent in removability after hydrolysis reaction and condensation reaction. preferable.

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

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

The concentration of water in the 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 the solution (C). When the concentration of water in the solution (C) is 95% by mass or more, the silicic acid produced by the hydrolysis reaction is excellent in dispersibility in the reaction liquid.

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

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

The rate of addition of the solution (C) to the solution (A) is, for example, when the solution (C) contains an alkali catalyst, preferably 0.4 g alkali catalyst/hour/kg solution to 12 g alkali catalyst/hour/kg solution, 1.7 g alkaline catalyst/hour/kg solution to 7.7 g alkaline catalyst/hour/kg solution is more preferred. When the addition rate of the solution (C) is 0.4 g alkali catalyst/hour/kg solution or more, aggregation of the silica particles is suppressed, and the dispersion stability of the silica particles in the resulting dispersion is excellent. Further, when the addition rate of the solution (C) is 12 g alkali catalyst/hour/kg of solution or less, the reaction does not proceed excessively fast, and the reaction controllability is excellent.
Here, "g alkali catalyst/hour/kg solution" represents the mass (g) of the alkali catalyst in solution (C) added per hour to 1 kg of solution (A).

The concentration of water in the reaction system of the hydrolysis reaction and condensation reaction is preferably maintained at 3% to 30% by mass, preferably 5% to 25% by mass, in 100% by mass of the reaction liquid in the reaction tank. is more preferable. When the concentration of water in the reaction system is 3% by mass or more, the dispersibility of the intermediate silicic acid in the reaction solution is excellent. Moreover, when the concentration of water in the reaction system is 30% by mass or less, the tetraalkoxysilane is excellent in dispersibility in the reaction liquid.
Here, the concentration of water in the reaction system means the total amount of water in the total amount of liquid and substances dissolved in the liquid in the reaction system in the hydrolysis reaction and condensation reaction. The total amount of the liquid and the substances dissolved in the liquid in the reaction system is only the solution (A) at the start of the reaction, and during the reaction, the solution (A), the solution (B), the solution (C) and the alcohol produced in the reaction total amount. The liquid in the reaction system and the substance dissolved in the liquid do not include silica particles dispersed in the liquid.

The concentration of the alkali catalyst in the reaction system of the hydrolysis reaction and condensation reaction is preferably maintained at 0.5% by mass to 2.0% by mass in 100% by mass of the reaction liquid in the reaction system, and 0.6% by mass. % to 1.5% by weight is more preferred. When the concentration of the alkali catalyst in the reaction system is 0.5% by mass or more, aggregation of the silica particles is suppressed, and the dispersion stability of the silica particles in the resulting dispersion of silica particles is excellent. Further, when the concentration of the alkali catalyst in the reaction system is 2.0% by mass or less, the reaction does not proceed excessively fast, and the reaction controllability is excellent.
Here, the concentration of the alkali catalyst in the reaction system means 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.

The reaction temperature (the temperature of the reaction liquid in the reaction system) for the hydrolysis reaction and condensation reaction 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 excessively slowly, resulting in excellent controllability. Moreover, when the reaction temperature is 50° C. or lower, the balance between the hydrolysis reaction rate and the condensation reaction rate is excellent.

Since the hydrolysis reaction and condensation reaction of tetraalkoxysilane are exothermic reactions, they are preferably carried out while cooling the reaction vessel or the reaction solution in order to control the reaction temperature.
The cooling method is not particularly limited, but includes the following methods.
(1) A cooling jacket is provided around the outer periphery of the reaction vessel, and the entire reaction vessel is cooled with a cooling medium such as water.
(2) 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 singly or in combination of two.

(Physical properties of silica particles)
Preferred physical properties and the like of silica particles produced by the method for producing silica particles of the present invention are described below.

The average primary particle size of silica particles is preferably 5 nm to 100 nm, more preferably 10 nm to 60 nm. When the average primary particle size 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 flaws of the object to be polished typified by a silicon wafer can be reduced, and the sedimentation of the silica particles can be suppressed.

The average primary particle size of silica particles is measured by the BET method. Specifically, the specific surface area of silica particles is measured using an automatic specific surface area measuring device, and the average primary particle size is calculated using the following formula (1).
Average primary particle size (nm) = 6000/(specific surface area (m ² /g) x density (g/cm ³ ))
... (1)

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

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

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

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

The cv value of the silica particles is preferably 10-50, more preferably 15-40, even more preferably 20-35. When the cv value of the silica particles is 10 or more, the polishing rate for an object to be polished typified by a silicon wafer is excellent, and the productivity of silicon wafers is excellent. Further, when the cv value of the silica particles is 50 or less, the surface roughness and flaws of the object to be polished typified by a silicon wafer during polishing can be reduced, and the removability of particles and the like in cleaning after polishing is excellent.

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

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 an object to be polished represented by a silicon wafer is excellent, and the productivity of silicon wafers is excellent. Further, when the association ratio of silica particles is 4.0 or less, surface roughness and flaws of an object to be polished, typified by a silicon wafer, during polishing can be reduced, and aggregation of silica particles can be suppressed.

The association ratio of silica particles is calculated using the following formula (3) from the average primary particle size measured by the above-described measuring method and the average secondary particle size measured by the above-described measuring method.
Association ratio = average secondary particle size/average primary particle size (3)

The surface silanol group density of the silica particles is preferably 0.1/nm ² to 10/nm ² , more preferably 0.5/nm ² to 7.5/nm ² , and 2.0/nm. ² to 7.0/nm ² is more preferable. When the surface silanol group density of the silica particles is 0.1/nm ² or more, the silica particles have an appropriate surface repulsion and the silica sol has excellent dispersion stability. Further, when the surface silanol group density of the silica particles is 10/nm ² or less, the silica particles have an appropriate surface repulsion, and aggregation of the silica particles can be suppressed.

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.
Silica sol corresponding to 1.5 g of silica particles is collected, and pure water is added to adjust the liquid volume to 90 mL. Add 0.1 mol / L hydrochloric acid aqueous solution until the pH reaches 3.6 in an environment of 25 ° C., add 30 g of sodium chloride, gradually add pure water to completely dissolve sodium chloride, and finally test Pure water is added until the total volume of the liquid reaches 150 mL to obtain a test liquid.
Put the obtained test solution in an automatic titrator, add dropwise 0.1 mol / L sodium hydroxide aqueous solution, pH 4.0 to 9.0 0.1 mol / L sodium hydroxide required Measure the titer A (mL) of the aqueous solution.
Using the following formula (4), the consumption V (mL) of 0.1 mol/L sodium hydroxide aqueous solution required for the pH to change from 4.0 to 9.0 per 1.5 g of silica particles was calculated. Then, the surface silanol group density ρ (number/nm ² ) of the silica particles is calculated using the following formula (5).
V=(A×f×100×1.5)/(W×C) (4)
A: Titration volume (mL) of 0.1 mol/L sodium hydroxide aqueous solution required for pH to change from 4.0 to 9.0 per 1.5 g of silica particles
f: titer of 0.1 mol / L sodium hydroxide aqueous solution used C: concentration of silica particles in silica sol (% by mass)
W: amount of silica sol collected (g)
ρ=(B×N _A )/(10 ¹⁸ ×M×S _BET ) (5)
B: Amount of sodium hydroxide (mol) required for pH to be from 4.0 to 9.0 per 1.5 g of silica particles calculated from V
N _A : Avogadro's number (number/mol)
M: amount of silica particles (1.5 g)
S _BET : Specific surface area (m ² /g) of silica particles measured when calculating average primary particle size

The method for measuring and calculating the surface silanol group density of the silica particles is described in "GW Sears, Jr., Analytical Chemistry, Vol.28, No.12, pp.1981-1983 (1956).", "Shinichi Haba , Development of Abrasive for Semiconductor Integrated Circuit Process, Kochi University of Technology Doctoral Dissertation, pp.39-45, March 2004”, “Patent No. 5967118” and “Patent No. 6047395”.

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.

Examples of the shape of silica particles include spherical, chain, cocoon-like (also referred to as knob-like and peanut-like), and irregular shapes (eg, wart-like, curved, branched, etc.). Among these silica particles, when it is desired to reduce the surface roughness and scratches of an object to be polished such as a silicon wafer during polishing, a spherical shape is preferable. If you want to raise it more, the irregular shape is preferable.

(Method for producing silica sol)
The method for producing silica sol of the present invention includes the method for producing silica particles of the present invention.

The method for producing a silica sol of the present invention removes unnecessary components and adds necessary components to make the silica sol a desired component. Therefore, the silica particles obtained by the method for producing silica particles of the present invention It is preferable to have a replacement step of replacing the dispersion medium in the dispersion of.
In addition, since the method for producing a silica sol of the present invention can increase the degree of condensation of silica particles, it is preferable to include a pressure heating step of pressurizing and heating the dispersion of silica particles obtained in the substitution step.
Each step will be described below.

(Replacement step)
In the replacement step, it is preferable to remove the alcohol and the alkali catalyst such as ammonia in the silica particle dispersion liquid by replacing the dispersion medium.
A desired dispersion medium can be selected as the dispersion medium added to replace the dispersion medium in the dispersion liquid of the silica particles in the replacement step.

In order to remove the dispersion medium in the dispersion of silica particles in the replacement step, a method of heating and evaporating the dispersion medium in the dispersion of silica particles under normal pressure or reduced pressure is preferred. The heating temperature at this time may be the boiling point of the dispersion medium or a temperature slightly higher than the boiling point under the pressure conditions at the time of replacement, and is preferably 50°C to 150°C.

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

In the replacement step, the removal of the dispersion medium in the silica particle dispersion and the addition of the new dispersion medium may be performed simultaneously, or one of them may be performed first and the other may be performed later.

The replacement step can be carried out using a container having a fluororesin layer on the surface (wetted surface), since the heating at the time of replacement can be efficiently advanced, the productivity is excellent, and a low metal content can be maintained. preferable.

Containers with a fluororesin layer on the surface can be designed on an industrial scale and are superior in mechanical strength. A container having a fluororesin layer on the surface of the container is more preferable.

As for the method of providing a fluororesin layer on the surface of the container, it is preferable to bake the fluororesin on the surface of the container by electrostatic coating, as described above, because it is easy to manufacture and process. is.
As for the type and thickness of the fluororesin layer, the same ones as those for the fluororesin layer provided on the surface of the reaction vessel can be employed.

The volume of the container is preferably 0.1 m ³ to 20 m ³ , more preferably 0.2 m ³ to 10 m ³ , still more preferably 0.5 m ³ to 5 m ³ . When the volume of the container is 0.1 m ³ or more, the productivity of silica particles is excellent. Further, when the volume of the container is 20 m ³ or less, heating during replacement can proceed efficiently.

(Pressure heating step)
The pressure at which the dispersion of silica particles obtained in the substitution step is subjected to pressure and heat treatment is preferably 0.10 MPa to 2.3 MPa, more preferably 0.14 MPa to 1.0 MPa. When the pressure of the pressurized heat treatment is 0.10 MPa or more, the degree of condensation of the silica particles can be increased. Further, when the pressure of the pressurized heat treatment is 2.3 MPa 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. , excellent dispersion stability of silica sol.
Pressurization may be carried out by heating the dispersion liquid of silica particles to the boiling point or higher of the dispersion medium in a closed state. When the aqueous dispersion of silica particles is heated to 100° C. or higher in a sealed state, the pressure becomes the saturated water vapor pressure at that temperature.

The temperature of the pressurized heat treatment is preferably 100°C to 220°C, more preferably 110°C to 180°C. When the temperature of the pressurized heat treatment is 100° C. or higher, the degree of condensation of the silica particles can be increased. Further, when the temperature of the pressurized heat treatment is 220 ° C. 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. Excellent dispersion stability of silica sol.

The time for the pressurized heat treatment is preferably 0.25 hours to 10 hours, more preferably 0.5 hours to 8 hours. The degree of condensation of the silica particles can be increased when the time of the pressurized heat treatment is 0.25 hours or longer. In addition, when the time of the pressure heating 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. Excellent dispersion stability of silica sol.

Since the pressurized heat treatment can increase the degree of condensation of silica particles without significantly changing the average primary particle size, average secondary particle size, cv value, and association ratio, it is more preferable to perform it in an aqueous dispersion. preferable.

The pH at which the pressure and heat treatment is performed in the aqueous dispersion is preferably 6.0 to 8.0, more preferably 6.5 to 7.8. Gelation of the silica sol can be suppressed when the pH is 6.0 or more when performing the pressurized heat treatment in the aqueous dispersion. In addition, if the pH is 8.0 or less when the pressure and heat treatment is performed in the aqueous dispersion, the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles are greatly changed. The degree of condensation of silica particles can be increased without

In the pressurizing and heating step, it is preferable to use a container having a fluororesin layer on the surface, because the pressurizing and heating can be performed efficiently, the productivity is excellent, and a low metal content can be maintained.

Containers with a fluororesin layer on the surface can be designed on an industrial scale and are superior in mechanical strength. A container having a fluororesin layer on the surface of the container is more preferable.

As for the method of providing a fluororesin layer on the surface of the container, it is preferable to bake the fluororesin on the surface of the container by electrostatic coating, as described above, because it is easy to manufacture and process. is.
As for the type and thickness of the fluororesin layer, the same ones as those for the fluororesin layer provided on the surface of the reaction vessel can be employed.

The volume of the container is preferably 0.1 m ³ to 20 m ³ , more preferably 0.2 m ³ to 10 m ³ , still more preferably 0.5 m ³ to 5 m ³ . When the volume of the container is 0.1 m ³ or more, the silica sol productivity is excellent. Further, when the volume of the container is 20 m ³ or less, the pressurization and heating can proceed efficiently.

The content of silica particles in the silica sol 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 silica sol. When the content of silica particles in the silica sol is 3% by mass or more, the polishing rate for an object to be polished typified by a silicon wafer is excellent. Further, when the content of silica particles in the silica sol is 50% by mass or less, aggregation of the 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.

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 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 or polishing composition can be suppressed, and the storage stability of the silica sol or polishing composition is excellent. Moreover, when the content of the dispersion medium in the silica sol is 97% by mass or less, the polishing rate for an object to be polished represented by a silicon wafer is excellent.

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

Silica sol, in addition to silica particles and a dispersion medium, optionally contains oxidizing agents, preservatives, antifungal agents, pH adjusters, pH buffers, surfactants, chelating agents, antibacterial Other ingredients such as biocides may be included.
In particular, it is preferable to incorporate an antibacterial and biocidal agent into the silica sol, since the silica sol has excellent storage stability.

Antimicrobial biocides include, for example, hydrogen peroxide, ammonia, quaternary ammonium hydroxides, quaternary ammonium salts, ethylenediamine, glutaraldehyde, methyl p-hydroxybenzoate, sodium chlorite, and the like. One of these antibacterial and biocidal agents may be used alone, or two or more thereof may be used in combination. Among these antibacterial and biocidal agents, hydrogen peroxide is preferred because of its excellent affinity with silica sol.
Antimicrobial biocides also include what is commonly referred to as fungicides.

The content of the antibacterial biocidal agent in the silica sol is preferably 0.0001% by mass to 10% by mass, more preferably 0.001% by mass to 1% by mass, based on 100% by mass of the total silica sol. When the content of the antimicrobial biocidal agent in the silica sol is 0.0001% by mass or more, the storage stability of the silica sol is excellent. If the content of the antibacterial biocidal agent in the silica sol is 10% by mass or less, the original performance of the silica sol is not impaired.

The pH of the silica sol is preferably 6.0-8.0, more preferably 6.5-7.8. When the pH of the silica sol is 6.0 or higher, the dispersion stability is excellent and aggregation of silica particles can be suppressed. Moreover, when the pH of the silica sol is 8.0 or less, dissolution of the silica particles is prevented, and the long-term storage stability is excellent.
The pH of the silica sol can be set within a desired range by adding a pH adjuster.

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

In the polishing of silicon wafers, metal impurities adhere to the surface of the object to be polished and contaminate the object to be polished, thereby adversely affecting the characteristics of the object to be polished and the final product.
In addition, when metal impurities are present in the silica sol, coordinated interaction occurs between the surface silanol groups showing acidity and the metal impurities, which changes the chemical properties (acidity, etc.) of the surface silanol groups, It changes the three-dimensional environment of the surface (easiness of aggregation of silica particles, etc.) and affects the polishing rate.

The metal content of silica sol is measured by high frequency inductively coupled plasma mass spectrometry (ICP-MS). Specifically, silica sol containing 0.4 g of silica particles is accurately weighed, sulfuric acid and hydrofluoric acid are added, heated, dissolved and evaporated, and pure water is added to the remaining sulfuric acid droplets so that the total amount is exactly 10 g. Prepare a test solution by using a high-frequency inductively coupled plasma mass spectrometer. The target metals are sodium, potassium, iron, aluminum, calcium, magnesium, zinc, cobalt, chromium, copper, manganese, lead, titanium, silver, and nickel, and the total content of these metals is the metal content. .

According to the present invention, the metal content of the silica sol is 1 ppm or less by obtaining silica particles by performing hydrolysis reaction and condensation reaction using tetraalkoxysilane as the main raw material in a reaction tank having a fluororesin layer on the surface. can be done.
In the method of deionizing alkali silicate such as water glass, it is extremely difficult to reduce the metal content of the silica sol to 1 ppm or less because sodium and the like derived from raw materials remain.

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

A water-soluble polymer enhances the wettability of the polishing composition to an object to be polished, typified by a silicon wafer. The water-soluble polymer is preferably a polymer having a functional group with high water affinity, and the affinity between the functional group with high water affinity and the surface silanol group of the silica particles is high, and the polishing composition contains The silica particles and the water-soluble polymer are stably dispersed in closer proximity. Therefore, the effects of the silica particles and the water-soluble polymer work synergistically when polishing an object to be polished, typified by a silicon wafer.

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

Examples of cellulose derivatives include hydroxyethylcellulose, hydrolyzed hydroxyethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose, methylcellulose, ethylcellulose, ethylhydroxyethylcellulose, carboxymethylcellulose and the like.
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, copolymers of ethylene oxide and propylene oxide, and the like.

These water-soluble polymers may be used singly or in combination of two or more. Among these water-soluble polymers, cellulose derivatives are preferred because they have a 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, and hydroxyethyl cellulose. is more preferred.

The weight average molecular weight of the water-soluble polymer is preferably 1,000 to 3,000,000, more preferably 5,000 to 2,000,000, even more preferably 10,000 to 1,000,000. When the weight average molecular weight of the water-soluble polymer is 1,000 or more, the hydrophilicity of the polishing composition is improved. Further, when the weight 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 an object to be polished such as a silicon wafer is excellent.

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

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. Further, 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.

In addition to the silica sol and the water-soluble polymer, the polishing composition optionally contains a basic compound, a polishing accelerator, a surfactant, a hydrophilic compound, an antiseptic, an anti-mold agent, as long as the performance of the polishing composition is not impaired. Other ingredients such as pH adjusters, pH buffers, surfactants, chelating agents, antimicrobial biocides, etc. may be included.
In particular, chemical polishing (chemical etching) can be performed by giving a chemical action to the surface of an object to be polished, typified by a silicon wafer. It is preferable to include a basic compound in the polishing composition because it can improve the polishing rate of the polishing body.

Basic compounds include, for example, organic basic compounds, alkali metal hydroxides, alkali metal hydrogencarbonates, alkali metal carbonates, and ammonia. These basic compounds may be used individually by 1 type, and may use 2 or more types together. Among these basic compounds, ammonia, tetramethylammonium hydroxide, tetraethylammonium hydroxide, ammonium hydrogencarbonate, and ammonium carbonate are preferred because they are highly water-soluble and have excellent affinity with silica particles and water-soluble polymers. , ammonia, tetramethylammonium hydroxide, and tetraethylammonium hydroxide are more preferred, and ammonia is even more preferred.

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 an object to be polished typified by a silicon wafer can be improved. Moreover, 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.

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 more, aggregation of silica particles in the polishing composition can be suppressed, and the dispersion stability of the polishing composition is excellent. Moreover, when the pH of the polishing composition is 12.0 or less, dissolution of silica particles can be suppressed, and the stability of the polishing composition is excellent.
The pH of the polishing composition can be set within a desired range by adding a pH adjuster.

The polishing composition can be obtained by mixing the silica sol obtained by the silica sol production method of the present invention, a water-soluble polymer, and, if necessary, other components. It may be prepared at a high concentration and diluted with water or the like immediately before polishing.

(polishing method)
The polishing method of the present invention is a method of polishing using a polishing composition containing the silica sol obtained by the silica sol production method of the present invention.
As the polishing composition, it is preferable to use the polishing composition described above.
A specific polishing method includes, for example, a method of 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.

(Application)
The silica sol obtained by the method for producing a silica sol of the present invention can be suitably used for polishing, and can be particularly suitably used for silicon wafer polishing and chemical mechanical polishing.

Hereinafter, the present invention will be described in more detail using experimental examples and comparative experimental examples in place of examples, but the present invention is not limited to the description of the following examples unless it departs from the gist thereof. Absent.

(sample)
A silica sol of silica particles/methanol/ammonia/water=6/58/3.4/32.6 (% by mass) was used as a sample.
Silica particles are produced by hydrolyzing and condensing tetramethoxysilane.

(Measurement of content of each metal)
Accurately weigh a sample containing 0.4 g of silica particles, add sulfuric acid and hydrofluoric acid, heat, dissolve and evaporate, add pure water to the remaining sulfuric acid droplets so that the total amount is exactly 10 g to create a test solution. Then, 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).

[Example 1]
A sample was placed in a stainless steel container coated with fluororesin (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) (bottomed cylindrical container with an inner diameter of 2.5 cm, fluororesin layer thickness: 300 μm). After supplying 40 mL and stirring at 50° C. for 24 hours, the content of each metal in the sample was measured.
Table 1 shows the evaluation results.

[Example 2]
The content of each metal in the sample was measured in the same manner as in Example 1, except that the stirring time was 48 hours.
Table 1 shows the evaluation results.

[Comparative Example 1]
40 mL of a sample was supplied to a stainless steel container (bottomed cylindrical container with an inner diameter of 2.5 cm), stirred at 50° C. for 24 hours, and then the content of each metal in the sample was measured.
Table 1 shows the evaluation results.

[Comparative Example 2]
The content of each metal in the sample was measured in the same manner as in Comparative Example 1 except that the stirring time was 48 hours.
Table 1 shows the evaluation results.

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

From Comparative Examples 1 and 2, it can be seen that the use of a stainless steel container is not suitable for applications requiring high purity because a large amount of iron is generated.
On the other hand, from Examples 1 and 2, it can be seen that a low metal content can be maintained by using a container having a fluororesin layer on its surface.

The silica sol obtained by the method for producing a silica sol of the present invention can be suitably used for polishing, and can be particularly suitably used for silicon wafer polishing and chemical mechanical polishing.

Claims (9)

A method for producing silica particles, comprising a step of hydrolyzing and condensing tetraalkoxysilane in a reaction vessel having a fluororesin layer on the surface thereof. The method for producing silica particles according to claim 1, wherein the hydrolysis reaction and condensation reaction of tetraalkoxysilane are performed at a reaction temperature of 15°C to 50°C. 3. The method for producing silica particles according to claim 1, wherein the hydrolysis reaction and condensation reaction of the tetraalkoxysilane are performed while cooling. The method for producing silica particles according to any one of claims 1 to 3, wherein the tetraalkoxysilane contains tetramethoxysilane. Claims 1 to 4, wherein the hydrolysis reaction and condensation reaction of tetraalkoxysilane are carried out by adding a solution (B) containing tetraalkoxysilane and a solution (C) containing water to a solution (A) containing an alkali catalyst. A method for producing silica particles according to any one of the above. The method for producing silica particles according to any one of claims 1 to 5, wherein the fluororesin comprises a tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin. A method for producing silica sol, comprising the method for producing silica particles according to any one of claims 1 to 6. The method for producing a silica sol according to claim 7, wherein the silica sol has a metal content of 1 ppm or less. A polishing method comprising polishing with a polishing composition containing the silica sol obtained by the method for producing a silica sol according to claim 7 or 8.