JP7516763B2 - Method for producing silica particles, method for producing silica sol, method for removing intermediate product, and method for polishing - Google Patents

Description

The present invention relates to a method for producing silica particles, a method for producing silica sol, a method for removing intermediate products, and a polishing method.

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

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

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

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

By the way, intermediate products may be generated during or after the production of silica sol by hydrolysis and condensation of alkoxysilane. These intermediate products are considered to be silica that remains as a solid with insufficient growth or silica that precipitates from dissolved silicic acid after production. In addition, since such silica is in a reactive state, the intermediate products may be the starting point to generate fine particles of about 10 nm or to grow the particles more than expected. Therefore, silica sol containing intermediate products causes an increase in viscosity and gelation, and deteriorates its storage stability. Since such intermediate products and fine particles are considered to be silica with a lower degree of condensation than the desired colloidal silica, they deteriorate the mechanical properties of colloidal silica in the obtained silica sol, reduce the polishing speed, and have an adverse effect on the polishing properties of the obtained polishing liquid. In addition, when a polishing liquid containing fine particles is used for polishing, the fine particles cover the polished object, and the colloidal silica in the polishing liquid that should originally contribute to polishing does not contribute to polishing. Furthermore, when a polishing liquid containing silica with a low degree of condensation is used for polishing, the polishing liquid is poorly removable from the polished object after polishing.

The methods of producing silica sol by hydrolysis and condensation reactions of alkoxysilanes disclosed in Patent Documents 1 to 3 do not disclose any method for dealing with such intermediate products and fine particles, and depending on the production conditions, a silica sol containing a large amount of intermediate products and fine particles may be obtained. In particular, since the intermediate products are presumed to remain in a reactive state, chemical changes progress during storage of the silica sol, causing gelation and generation of fine particles. As a result, the storage stability of the silica sol is poor, which not only adversely affects the polishing properties of the resulting polishing liquid, but also makes it difficult to remove from the object to be polished.

The present invention was made in consideration of these problems, and an object of the present invention is to provide a method for producing a silica sol with fewer intermediate products. Another object of the present invention is to provide a method for removing intermediate products in a silica sol that can easily reduce the amount of intermediate products in the silica sol.

Conventionally, silica sols containing intermediate products and fine particles have existed and have been used as polishing liquids without removing the intermediate products and fine particles, and the polishing properties and storage stability of the resulting polishing liquids have not been sufficient. However, after extensive research, the inventors have discovered that by ultrafiltering silica sols using ultrafiltration membranes with a specific range of molecular weight cutoff, intermediate products that cause fine particles can be removed and a silica sol with excellent storage stability can be obtained, thus completing the present invention.

That is, the gist of the present invention is as follows.
[1] A method for producing silica particles, comprising the following steps (1) and (2):
Step (1): A step of subjecting tetraalkoxysilane to a hydrolysis reaction and a condensation reaction to obtain a silica sol.
Step (2): A step of ultrafiltering the silica sol using an ultrafiltration membrane having a molecular weight cutoff of 5,000 to 80,000.
[2] The method for producing silica particles according to [1], wherein step (2) is carried out after step (1).
[3] The method for producing silica particles according to [1] or [2], wherein the content of silica particles in the silica sol is 10% by mass to 50% by mass, based on a total amount of the silica sol (100% by mass).
[4] The method for producing silica particles according to any one of [1] to [3], wherein the average secondary particle diameter of the silica particles measured by a DLS method is 20 nm to 100 nm.
[5] The method for producing silica particles according to any one of [1] to [4], which does not include a step of heating to 100° C. or higher.
[6] A method for producing a silica sol, comprising the method for producing silica particles according to any one of [1] to [5].
[7] A method for removing an intermediate product in a silica sol, comprising removing the intermediate product in the silica sol by the method for producing a silica sol according to [6].
[8] A polishing method, comprising the steps of: polishing with a polishing composition containing silica particles obtained by the method for producing silica particles according to any one of [1] to [5].

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

FIG. 2 is a diagram showing a secondary electron image of a silica sol containing an intermediate product observed with a field emission scanning electron microscope. FIG. 2 is a diagram showing a secondary electron image of the silica sol obtained in Comparative Example 1 observed with a field emission scanning electron microscope. FIG. 2 is a diagram showing a secondary electron image of the silica sol obtained in Example 1 observed with a field emission scanning electron microscope. FIG. 2 is a diagram showing a secondary electron image of the silica sol obtained in Example 2 observed with a field emission scanning electron microscope. FIG. 2 is a diagram showing a secondary electron image of the silica sol obtained in Comparative Example 2 observed with a field emission scanning electron microscope. FIG. 1 is a diagram showing a secondary electron image of the silica sol obtained in Comparative Example 3 observed with a field emission scanning electron microscope. FIG. 2 is a diagram showing a secondary electron image of the silica sol used in Reference Example 1 observed with a field emission scanning electron microscope. FIG. 2 is a diagram showing a secondary electron image of the silica sol obtained in Reference Example 2 observed with a field emission scanning electron microscope.

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

(Method for producing silica particles)
The method for producing silica particles of the present invention includes the following steps (1) and (2).
Step (1): A step of subjecting tetraalkoxysilane to a hydrolysis reaction and a condensation reaction to obtain a silica sol.
Step (2): A step of ultrafiltering the silica sol using an ultrafiltration membrane having a molecular weight cutoff of 5,000 to 80,000.

(Step (1))
The step (1) is a step of obtaining a silica sol by subjecting a tetraalkoxysilane to a hydrolysis reaction and a condensation reaction.
The method for producing silica particles of the present invention includes the step (1), and thus the metal impurity content of the obtained silica sol can be reduced.

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

The raw materials constituting the silica particles may be raw materials other than tetraalkoxysilane, such as low condensates of tetraalkoxysilane, but because of their excellent reactivity, it is preferable that, out of 100% by mass of all the raw materials constituting the silica particles, tetraalkoxysilane is 50% by mass or more and raw materials other than tetraalkoxysilane are 50% by mass or less, and it is even more preferable that tetraalkoxysilane is 90% by mass or more and raw materials other than tetraalkoxysilane are 10% by mass or less.

Examples of the solvents and dispersion media used in the hydrolysis and condensation reactions include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents and dispersion media may be used alone or in combination of two or more. Among these solvents and dispersion media, water and alcohol are preferred, and water and methanol are more preferred, because the solvents and by-products used in the hydrolysis and condensation reactions are the same as those used in the hydrolysis and condensation reactions, and because they are convenient to manufacture.

The hydrolysis reaction and the condensation reaction may be carried out in the presence or absence of a catalyst, but are preferably carried out in the presence of a catalyst, since this can accelerate the hydrolysis reaction and the condensation reaction.
Examples of the catalyst include acid catalysts such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid, formic acid, and citric acid, and alkali catalysts such as ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. Among these catalysts, alkali catalysts are preferred because they have excellent catalytic action and are easy to control the particle shape, and alkali catalysts are preferred because they can suppress the inclusion of metal impurities, are highly volatile, and are easy to remove after the condensation reaction, with ammonia being more preferred.

(Relationship between step (1) and step (2))
Step (2) may be carried out during or after step (1). However, it is preferable to carry out step (2) after step (1) since intermediate products such as insufficiently grown silica remaining as a solid and silica precipitated from dissolved silicic acid after production can be efficiently removed.

(Step (2))
The step (2) is a step of ultrafiltering the silica sol using an ultrafiltration membrane having a molecular weight cutoff of 5,000 to 80,000.
The method for producing silica particles of the present invention includes step (2), which makes it possible to efficiently remove intermediate products such as silica that remains as a solid due to insufficient growth and silica that precipitates from dissolved silicic acid after production, thereby suppressing deterioration such as gelation and improving the storage stability of the silica sol.

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

The molecular weight cutoff of the ultrafiltration membrane is 5,000 to 80,000, preferably 6,000 to 60,000, and more preferably 7,000 to 40,000. When the molecular weight cutoff of the ultrafiltration membrane is 5,000 or more, the membrane has excellent permeability. When the molecular weight cutoff of the ultrafiltration membrane is 80,000 or less, the membrane has excellent selectivity.

Ultrafiltration is characterized by its ability to separate components by molecular weight that would normally pass through the pores of filter paper or filters with pore sizes on the order of microns. Therefore, to increase the selectivity of fractionation by molecular weight using ultrafiltration, a method using centrifugation is preferred.

The centrifugal force of ultrafiltration by centrifugation is preferably 500 g to 10,000 g, and more preferably 800 g to 5,000 g. When the centrifugal force of ultrafiltration is 500 g or more, the filtration speed is sufficient and the productivity of silica particles and silica sol is excellent. Furthermore, when the centrifugal force of ultrafiltration is 10,000 g or less, the selectivity of ultrafiltration is excellent.

The rotation speed of ultrafiltration by centrifugation is preferably 1,000 rpm to 20,000 rpm, and more preferably 2,000 to 10,000 rpm. When the rotation speed of ultrafiltration is 1,000 rpm or more, the filtration speed is sufficient and the productivity of silica particles and silica sol is excellent. When the rotation speed of ultrafiltration is 20,000 rpm or less, the selectivity of ultrafiltration is excellent.

The centrifugation time for ultrafiltration by centrifugation is preferably 10 to 300 minutes, and more preferably 20 to 180 minutes. If the centrifugation time for ultrafiltration is 10 minutes or more, intermediate products in the silica sol can be suppressed. Furthermore, if the centrifugation time for ultrafiltration is 300 minutes or less, the productivity of silica particles and silica sol is excellent.

(Steps other than steps (1) and (2))
The method for producing silica particles of the present invention may include steps other than step (1) and step (2) as long as the performance of the silica particles is not impaired.
An example of a process other than the process (1) and the process (2) is a pressurized heat treatment process. However, since the effect expected from the pressurized heat treatment is reduced by suppressing the intermediate product, it is preferable not to include a process of heating to 100° C. or higher, and it is more preferable not to include a process of heating to 150° C. or higher.

(Physical properties of silica particles)
The average primary particle size of the 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. Furthermore, when the average primary particle size of the silica particles is 100 nm or less, the surface roughness and scratches of the polished object, such as a silicon wafer, can be reduced, and sedimentation of the silica particles can be suppressed.

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

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

The average secondary particle diameter of the silica particles is preferably 10 nm to 200 nm, and 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 during cleaning after polishing is excellent, and the storage stability of the silica sol is excellent. When the average secondary particle diameter of the silica particles is 200 nm or less, the surface roughness and scratches on the polished object, such as a silicon wafer, can be reduced during polishing, particles and the like can be removed easily during cleaning after polishing, and sedimentation of the silica particles can be suppressed.

The average secondary particle size of silica particles is measured by the DLS 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 the desired range using known conditions and methods.

The cv value of the silica particles is preferably 15 to 50, more preferably 20 to 40, and even more preferably 25 to 35. When the cv value of the silica particles is 15 or more, the polishing rate for the workpiece, such as a silicon wafer, is excellent, and the productivity of the silicon wafer is excellent. Furthermore, when the cv value of the silica particles is 50 or less, the surface roughness and scratches on the workpiece, such as a silicon wafer, during polishing can be reduced, and the removal of particles and the like during cleaning after polishing is excellent.

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

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

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

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 even more preferably 2.0/nm ² to 7.0/nm ^2. When the surface silanol group density of the silica particles is 0.1/nm ² or more, the silica particles have a suitable surface repulsion, and the dispersion stability of the silica sol is excellent. In addition, when the surface silanol group density of the silica particles is 10/nm ² or less, the silica particles have a suitable surface repulsion, and the aggregation of the silica particles can be suppressed.

The surface silanol group density of the silica particles is measured by the Sears method, specifically, under the following conditions.
A silica sol equivalent to 1.5 g of silica particles is collected, and pure water is added to make the liquid volume 90 mL. In an environment of 25° C., a 0.1 mol/L aqueous hydrochloric acid solution is added until the pH becomes 3.6, 30 g of sodium chloride is added, and pure water is gradually added to completely dissolve the sodium chloride. Finally, pure water is added until the total volume of the test liquid becomes 150 mL to obtain a test liquid.
The obtained test solution is placed in an automatic titrator, and a 0.1 mol/L aqueous solution of sodium hydroxide is added dropwise to measure the titration amount A (mL) of the 0.1 mol/L aqueous solution of sodium hydroxide required to change the pH from 4.0 to 9.0.
The amount V (mL) of 0.1 mol/L aqueous sodium hydroxide solution required to change the pH from 4.0 to 9.0 per 1.5 g of silica particles is calculated using the following formula (4), and the surface silanol group density ρ (number/ ^nm2 ) of the silica particles is calculated using the following formula (5).
V=(A×f×100×1.5)/(W×C)... (4)
A: Titration amount (mL) of 0.1 mol/L sodium hydroxide aqueous solution required to change the pH from 4.0 to 9.0 per 1.5 g of silica particles
f: Titer of 0.1 mol/L aqueous sodium hydroxide solution used C: Concentration of silica particles in silica sol (mass%)
W: Amount of silica sol collected (g)
ρ=(B× _NA )/(10 ¹⁸ ×M×S _BET )...(5)
B: The amount (mol) of sodium hydroxide required to change the pH per 1.5 g of silica particles from 4.0 to 9.0 calculated from V
N _A : Avogadro's number (pieces/mol)
M: Amount of silica particles (1.5 g)
S _BET : specific surface area (m ² /g) of silica particles measured when calculating the average primary particle diameter

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

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

The metal impurity content of the silica particles is preferably 5 ppm or less, and more preferably 2 ppm or less.

When polishing silicon wafers for semiconductor devices, metallic impurities adhere to and contaminate the surface of the workpiece being polished, adversely affecting the characteristics of the wafer and diffusing into the interior of the wafer, degrading its quality, resulting in a significant decrease in the performance of semiconductor devices manufactured from such wafers.
Furthermore, when metal impurities are present on silica particles, coordination interactions occur between the acidic surface silanol groups and the metal impurities, changing the chemical properties (acidity, etc.) of the surface silanol groups and the three-dimensional environment of the silica particle surfaces (e.g. ease of aggregation of silica particles), thereby affecting the polishing rate.

The metal impurity content of silica particles 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 so that the total amount is exactly 10 g to create a test solution, which is then measured using an inductively coupled plasma mass spectrometer. The target metals are sodium, potassium, iron, aluminum, calcium, magnesium, zinc, cobalt, chromium, copper, manganese, lead, titanium, silver, and nickel, and the sum of the contents of these metals is the metal impurity content.

The metal impurity content of the silica particles can be reduced to 5 ppm or less by obtaining the silica particles through hydrolysis and condensation reactions using alkoxysilane as a main raw material.
In the method of deionizing alkali silicate such as water glass, sodium and the like derived from the raw material remain, so it is extremely difficult to reduce the metal impurity content of the silica particles to 5 ppm or less.

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

Silica particles preferably have no pores because they have excellent mechanical strength and storage stability.
The presence or absence of pores in the silica particles is confirmed by a multipoint BET analysis using an adsorption isotherm with nitrogen as the adsorption gas.

The silica particles preferably contain an alkoxysilane condensate as a main component, and more preferably a tetraalkoxysilane condensate as a main component, because they have excellent mechanical strength and storage stability. The main component means that the silica particles contain 50% by mass or more of the total components constituting the silica particles (100% by mass).
In order to obtain silica particles mainly composed of alkoxysilane condensates, it is preferable to use alkoxysilane as the main raw material. In order to obtain silica particles mainly composed of tetraalkoxysilane condensates, it is preferable to use tetraalkoxysilane as the main raw material. The main raw material means that the main raw material is 50% by mass or more out of 100% by mass of all raw materials constituting the silica particles.

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

Silica sol may be produced by using the dispersion of silica particles obtained by the method for producing silica particles of the present invention as is, or by removing unnecessary components from the obtained dispersion and adding necessary components.

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

The content of silica particles in the silica sol 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 the total amount of the silica sol (100% by mass). When the content of silica particles in the silica sol is 3% by mass or more, the polishing rate for a workpiece such as a silicon wafer is excellent. Furthermore, when the content of silica particles in the silica sol is 50% by mass or less, the 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.

The content of the solvent/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, out of a total amount of 100% by mass of the silica sol. When the content of the solvent/dispersion medium in the silica sol is 50% by mass or more, the 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. Furthermore, when the content of the solvent/dispersion medium in the silica sol is 97% by mass or less, the polishing rate for the object to be polished, such as a silicon wafer, is excellent.

The content of silica particles and solvent/dispersion medium in the silica sol can be set to the desired range by removing unnecessary components from the silica particle dispersion and adding necessary components.

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

Examples of antibacterial and biocide agents include hydrogen peroxide, ammonia, quaternary ammonium hydroxide, quaternary ammonium salt, ethylenediamine, glutaraldehyde, hydrogen peroxide, methyl p-hydroxybenzoate, sodium chlorite, etc. These antibacterial and biocide agents may be used alone or in combination of two or more. Among these antibacterial and biocide agents, hydrogen peroxide is preferred because of its excellent affinity with silica sol.
Biocides also include those commonly referred to as bactericides.

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

The pH of the silica sol is preferably 6.0 to 8.0, more preferably 6.5 to 7.8. When the pH of the silica sol is 6.0 or more, the dispersion stability is excellent and the aggregation of the silica particles can be suppressed. 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 adjusted to a desired range by adding a pH adjuster.

(Method for Removing Intermediate Products from Silica Sol)
The method for removing intermediate products in a silica sol of the present invention is a method according to the method for producing a silica sol of the present invention, and the specific production method is as described above.

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

The water-soluble polymer increases the wettability of the polishing composition to the object to be polished, such as a silicon wafer. The water-soluble polymer is preferably a polymer having a functional group with high water affinity, and this functional group with high water affinity has a high affinity with the surface silanol groups of the silica particles, so that the silica particles and the water-soluble polymer are stably dispersed in close proximity in the polishing composition. Therefore, when polishing an object to be polished, such as a silicon wafer, the effects of the silica particles and the water-soluble polymer function synergistically.

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 hydroxyethyl cellulose, hydrolyzed hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, ethyl cellulose, ethyl hydroxyethyl cellulose, and carboxymethyl cellulose.
An example of the copolymer having a polyvinylpyrrolidone skeleton is a graft copolymer of polyvinyl alcohol and polyvinylpyrrolidone.
Examples of polymers having a polyoxyalkylene structure include polyoxyethylene, polyoxypropylene, and copolymers of ethylene oxide and propylene oxide.

These water-soluble polymers may be used alone or in combination of two or more. Among these water-soluble polymers, cellulose derivatives are preferred, and hydroxyethyl cellulose is more preferred, because they have a high affinity with the surface silanol groups of the silica particles and act synergistically to impart good hydrophilicity to the surface of the object to be polished.

The mass average molecular weight of the water-soluble polymer is preferably 1,000 to 3,000,000, more preferably 5,000 to 2,000,000, and even more preferably 10,000 to 1,000,000. When the mass average molecular weight of the water-soluble polymer is 1,000 or more, the hydrophilicity of the polishing composition is improved. Furthermore, when the mass average molecular weight of the water-soluble polymer is 3,000,000 or less, the affinity with silica sol is excellent, and the polishing rate for the object to be polished, such as a silicon wafer, is excellent.

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

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

In addition to the silica sol and the water-soluble polymer, the polishing composition may contain other components, such as a basic compound, a polishing accelerator, a surfactant, a hydrophilic compound, a preservative, an antifungal agent, a pH adjuster, a pH buffer, a surfactant, a chelating agent, an antibacterial agent, or a biocide, as necessary, within the range that does not impair the performance of the polishing composition.
In particular, it is preferable to include a basic compound in the polishing composition, since it is possible to perform chemical polishing (chemical etching) by applying a chemical action to the surface of an object to be polished, such as a silicon wafer, and because a synergistic effect with the surface silanol groups of the silica particles can improve the polishing rate of an object to be polished, such as a silicon wafer.

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

The content of the basic compound in the polishing composition is preferably 0.001% by mass to 5% by mass, and more preferably 0.01% by mass to 3% by mass, based on 100% by mass of the total amount of the polishing composition. If the content of the basic compound in the polishing composition is 0.001% by mass or more, the polishing speed of the object to be polished, such as a silicon wafer, can be improved. Furthermore, if 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, the aggregation of silica particles in the polishing composition can be suppressed, and the dispersion stability of the polishing composition is excellent. When the pH of the polishing composition is 12.0 or less, the dissolution of silica particles can be suppressed, and the stability of the polishing composition is excellent.
The pH of the polishing composition can be adjusted to a desired range by adding a pH adjuster.

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

(Polishing method)
The polishing method of the present invention is a polishing method using a polishing composition containing silica particles obtained by the method for producing silica particles of the present invention.
As the polishing composition, it is preferable to use the polishing composition described above.
A specific example of the polishing method is a method in which the surface of a silicon wafer is pressed against a polishing pad, the polishing composition of the present invention is dropped onto the polishing pad, and the surface of the silicon wafer is polished.

(Application)
The silica particles obtained by the method for producing silica particles of the present invention and the silica sol obtained by the method for producing a silica sol of the present invention can be suitably used for polishing purposes, for example, polishing semiconductor materials such as silicon wafers, polishing electronic materials such as hard disk substrates, polishing in the planarization process in the production of integrated circuits (chemical mechanical polishing), polishing synthetic quartz glass substrates used for photomasks and liquid crystals, polishing magnetic disk substrates, and the like, and among these, they can be particularly suitably used for polishing silicon wafers and chemical mechanical polishing.

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

(Measurement of average primary particle size)
The silica sol obtained in Comparative Example 1 and Reference Example 1 was dried at 150°C, and the specific surface area of the silica particles was measured using an automatic specific surface area measuring device "BELSORP-MR1" (model name, Microtrack BEL Co., Ltd.). The average primary particle size was calculated using the following formula (1), assuming the density to be 2.2 g/ ^cm3 .
Average primary particle diameter (nm) = 6000/(specific surface area (m ² /g) x density (g/cm ³ )) ... (1)

(Measurement of average secondary particle size and cv value)
The average secondary particle diameter of the silica particles of the silica sols obtained in Comparative Example 1 and Reference Example 1 was measured using a dynamic light scattering particle size measurement device "Zetersizer Nano ZS" (model name, manufactured by Malvern Instruments), and the cv value was calculated using the following formula (2).
cv value=(standard deviation (nm)/average secondary particle diameter (nm))×100 (2)

(Calculation of Association Ratio)
The association ratio was calculated from the measured average primary particle size and average secondary particle size using the following formula (3).
Association ratio = average secondary particle diameter / average primary particle diameter ... (3)

(Measurement of surface silanol group density)
The silica sol obtained in Comparative Example 1 and Reference Example 1 was placed in a 200 mL tall beaker in an amount equivalent to 1.5 g of silica particles, and purified water was added to make the liquid volume 90 mL.
In an environment of 25°C, a pH electrode was inserted into the tall beaker, and the test solution was stirred for 5 minutes with a magnetic stirrer. With stirring continued with the magnetic stirrer, 0.1 mol/L aqueous hydrochloric acid solution was added until the pH reached 3.6. The pH electrode was removed from the tall beaker, and with stirring continued with the magnetic stirrer, 30 g of sodium chloride was added, and pure water was gradually added to completely dissolve the sodium chloride. Finally, pure water was added until the total amount of the test solution reached 150 mL, and the test solution was stirred for 5 minutes with a magnetic stirrer to obtain a test solution.

The tall beaker containing the obtained test solution was set in an automatic titration device "COM-1600" (manufactured by Hiranuma Sangyo Co., Ltd.), and the pH electrode and burette included with the device were inserted into the tall beaker. While stirring the test solution with a magnetic stirrer, 0.1 mol/L aqueous sodium hydroxide solution was dropped through the burette, and the titration amount A (mL) of 0.1 mol/L aqueous sodium hydroxide solution required to change the pH from 4.0 to 9.0 was measured.
The consumption amount 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 using the following formula (6), and the surface silanol group density ρ (pieces/ ^nm2 ) of the silica particles was calculated using the following formula (7).
V=(A×f×100×1.5)/(W×C)... (6)
A: Titration amount (mL) of 0.1 mol/L sodium hydroxide aqueous solution required to change the pH from 4.0 to 9.0 per 1.5 g of silica particles
f: Titer of 0.1 mol/L aqueous sodium hydroxide solution used C: Concentration of silica particles in silica sol (mass%)
W: Amount of silica sol collected (g)
ρ=(B× _NA )/(10 ¹⁸ ×M×S _BET )...(7)
B: The amount (mol) of sodium hydroxide required to change the pH per 1.5 g of silica particles from 4.0 to 9.0 calculated from V
N _A : Avogadro's number (pieces/mol)
M: Amount of silica particles (1.5 g)
S _BET : specific surface area (m ² /g) of silica particles measured when calculating the average primary particle diameter

(Measurement of Metal Impurity Content)
The silica sol containing 0.4 g of silica particles obtained in Comparative Example 1 was accurately weighed out, 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 so that the total amount was exactly 10 g to prepare a test solution, and the metal impurity content was measured using a high-frequency inductively coupled plasma mass spectrometer "ELEMENT2" (model name, manufactured by Thermo Fisher Scientific).

(Evaluation of intermediate products in silica sol)
For the silica sols obtained in the Examples, Comparative Examples, and Reference Examples, the amount of intermediate products was evaluated by the following procedure.
First, the silica sol was taken and diluted 5,000 times with ultrapure water. 5 μL of the 5,000-fold diluted solution was taken and dropped onto a mirror silicon wafer (manufactured by Electronics and Materials Corporation), and dried at 50° C. for 10 minutes. This was attached to a field emission scanning electron microscope (FE-SEM, model name "S-5200", manufactured by Hitachi High-Technologies Corporation), and 100 to 200 colloidal silica particles were observed at an acceleration voltage of 5 kV and a magnification of 150,000 times, and images were taken.
From the photographed images, the amount of intermediate products in the silica sol was evaluated according to the following indexes. The intermediate products refer to the areas that look like the areas surrounded by the black line in FIG.
A: No intermediate product was observed or only a small amount of intermediate product was observed. B: Intermediate product was observed. C: A large amount of intermediate product was observed.

[Comparative Example 1]
A raw material solution in which tetramethoxysilane and methanol were mixed at a volume ratio of 2.3:1 and a 3% by mass aqueous ammonia solution as an auxiliary solvent were prepared. A reaction solvent in which methanol, pure water, and ammonia were mixed in advance was charged into a reaction tank equipped with a thermometer, a stirrer, a supply pipe, and a distillation line. The concentration of water in the reaction solvent was 13% by mass, and the concentration of ammonia in the reaction solvent was 0.9% by mass.
While maintaining the temperature of the reaction solvent at 34° C., the reaction solvent, raw material solution, and auxiliary solvent were adjusted to 0.77:1:0.31 (volume ratio), and the raw material solution and auxiliary solvent were dropped into the reaction tank at a uniform rate for 150 minutes to obtain a silica sol. The temperature of the obtained silica sol was raised to remove methanol and ammonia while adjusting the liquid amount by adding pure water so that the content of silica particles was about 20% by mass, and a silica sol with a content of silica particles of about 20% by mass was obtained.

The obtained silica particles had an average primary particle diameter of 30.7 nm, an average secondary particle diameter of 65.0 nm, a cv value of 30.6, an association ratio of 2.12, and a surface silanol group density of 6.2 particles/ ^nm2 .
The metal impurity contents in the obtained silica particles were 1.1 ppm for sodium, 0.140 ppm for potassium, 0.015 ppm for iron, 0.135 ppm for aluminum, 0.075 ppm for calcium, 0.07 ppm for zinc, and less than 0.005 ppm for magnesium, cobalt, chromium, copper, manganese, lead, titanium, silver, and nickel.

Figure 2 shows a secondary electron image of the obtained silica sol observed with a field emission scanning electron microscope. Table 1 shows the evaluation results of the amount of intermediate products in the obtained silica sol.

[Example 1]
4.51g of the silica sol obtained in Comparative Example 1 was subjected to ultrafiltration by centrifugation using an ultrafiltration membrane with a molecular weight cutoff of 10,000 (product name "Microsep Advance with 10K Omega MCP010C41", manufactured by Pall Corporation) under conditions of a centrifugal force of 1,075g, a rotation speed of 3,100 rpm, and a centrifugation time of 1 hour, to obtain a silica sol. The amount of liquid that permeated the ultrafiltration membrane was 2.36g, its transmittance was 52.3 mass%, and the silica concentration in the permeated liquid was 260 μg/g.
A secondary electron image of the obtained silica sol observed with a field emission scanning electron microscope is shown in Fig. 3. The evaluation results of the obtained silica sol are shown in Table 1.

[Example 2]
A silica sol was obtained in the same manner as in Example 1, except that the molecular weight cutoff of the ultrafiltration membrane (product name "Microsep Advance with 30K Omega MCP010C41", manufactured by Pall Corporation) and the conditions of centrifugation were changed as shown in Table 1.
A secondary electron image of the obtained silica sol observed with a field emission scanning electron microscope is shown in Fig. 4. The evaluation results of the obtained silica sol are shown in Table 1.

[Comparative Example 2]
A silica sol was obtained in the same manner as in Example 1, except that the molecular weight cutoff of the ultrafiltration membrane (product name "Microsep Advance with 3K Omega MCP010C41", manufactured by Pall Corporation) and the conditions of centrifugation were changed as shown in Table 1.
A secondary electron image of the obtained silica sol observed with a field emission scanning electron microscope is shown in Fig. 5. The evaluation results of the obtained silica sol are shown in Table 1.

[Comparative Example 3]
A silica sol was obtained in the same manner as in Example 1, except that the molecular weight cutoff of the ultrafiltration membrane (product name "Microsep Advance with 100K Omega MCP010C41", manufactured by Pall Corporation) and the conditions of centrifugation were changed as shown in Table 1.
A secondary electron image of the obtained silica sol observed with a field emission scanning electron microscope is shown in Fig. 6. The evaluation results of the obtained silica sol are shown in Table 1.

[Reference Example 1]
A commercially available silica sol (product name "PL-3", manufactured by Fuso Chemical Co., Ltd.) was used as it was.
The silica particles used had an average primary particle diameter of 36.1 nm, an average secondary particle diameter of 71.2 nm, a cv value of 28.6, an association ratio of 1.97, and a surface silanol group density of 5.5/ ^nm2 .
A secondary electron image of the silica sol used, observed with a field emission scanning electron microscope, is shown in Fig. 7. The evaluation results of the silica sol used are shown in Table 1.

[Reference Example 2]
A silica sol was obtained in the same manner as in Example 1, except that a commercially available silica sol (product name "PL-3", manufactured by Fuso Chemical Co., Ltd.) was used and the centrifugation conditions were changed as shown in Table 1.
A secondary electron image of the obtained silica sol observed with a field emission scanning electron microscope is shown in Fig. 8. The evaluation results of the obtained silica sol are shown in Table 1.

A large amount of intermediate products was confirmed in the silica sol obtained in Comparative Example 1, in which ultrafiltration was not performed. On the other hand, almost no intermediate products were confirmed in the silica sol obtained in Examples 1 and 2, in which ultrafiltration was performed using an ultrafiltration membrane with a molecular weight cutoff within a specific range. Furthermore, the silica sol obtained in Comparative Examples 2 and 3, in which ultrafiltration was performed using an ultrafiltration membrane with a molecular weight cutoff outside the specific range, had low transmittance, and the amount of intermediate products removed by ultrafiltration was small, and intermediate products were confirmed.
Incidentally, it is understood that commercially available silica sol (product name "PL-3", manufactured by Fuso Chemical Co., Ltd.) does not have the problem of the present invention since it contains a small amount of intermediate products to begin with.

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

Claims (6)

A method for producing a silica sol containing silica particles having a cv value of 15 to 50, comprising the following steps (1) and (2):
Step (1): A step of subjecting tetraalkoxysilane to hydrolysis and condensation reaction using an alkali catalyst to obtain a silica sol. Step (2): A step of ultrafiltering the silica sol using an ultrafiltration membrane with a molecular weight cutoff of 7,000 to 40,000. The method for producing silica sol according to claim 1, wherein step (2) is carried out after step (1). The method for producing a silica sol according to claim 1 or 2, wherein the content of silica particles in the silica sol is 3% by mass to 50% by mass, based on 100% by mass of the total amount of the silica sol. The method for producing a silica sol according to any one of claims 1 to 3, comprising silica particles having an average secondary particle diameter of 20 nm to 100 nm as measured by DLS. The method for producing silica sol according to any one of claims 1 to 4, which does not include a step of heating to 100°C or higher as a step other than steps (1) and (2). A polishing method using a polishing composition containing a silica sol obtained by the method for producing a silica sol according to any one of claims 1 to 5.

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