Silica particles, silica sol, polishing composition, polishing method, semiconductor wafer manufacturing method, and semiconductor device manufacturing method

Abstract

The present invention addresses the problem of providing: silica particles in which secondary aggregation is suppressed, and which have superior dispersion stability and are suitable for polishing; a silica sol containing the silica particles; and a polishing composition containing the silica sol. The present invention pertains to silica particles that have an average roundness coefficient of 0.90 or higher as measured via field-emission scanning electron microscopy, the standard deviation of the roundness coefficient being 0.05 or less.

Description

Technical Field

[0001] The present invention relates to silica particles, silica sol, polishing compositions, polishing methods, methods for manufacturing semiconductor wafers, and methods for manufacturing semiconductor devices.

Background Art

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

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

[0004] Regarding the method for manufacturing silica particles, many studies have been made so far. For example, Patent Documents 1 to 4 and Non-Patent Documents 1 to 2 disclose methods for manufacturing silica particles by hydrolysis reaction and condensation reaction of alkoxysilane.

[0005] By the way, generally, the silica particles in the silica sol obtained by hydrolysis reaction and condensation reaction of alkoxysilane show various shapes such as spherical, cocoon-shaped, and irregular-shaped immediately after synthesis, as disclosed in Non-Patent Document 1.

[0006] Patent Document 1 discloses cocoon-shaped silica particles. Patent Document 2 discloses silica particles of an irregular shape (elongated shape).

[0007] However, when cocoon-shaped or irregularly shaped silica particles, such as those disclosed in Patent Documents 1 and 2, are used as an abrasive composition, while the abrasive composition exhibits excellent polishing power, it can cause scratches on the workpiece, resulting in the problem of not being able to obtain a workpiece with a smooth surface. Furthermore, when cocoon-shaped or irregularly shaped silica particles are used as an abrasive composition, the physical properties of the silica particles tend to change, such as secondary aggregation, during storage or use, resulting in poor stability of the abrasive performance. To address these challenges, more perfectly spherical silica particles are needed.

[0008] Even when silica particles with a large particle size are used as an abrasive composition, while they exhibit excellent abrasive power, they can scratch the workpiece, resulting in the inability to obtain a smooth surface. Furthermore, during storage and use, the silica particles are prone to changes in physical properties, such as secondary aggregation and sedimentation, leading to poor stability in abrasive performance. To address these challenges, silica particles with smaller particle sizes are needed.

[0009] While Patent Documents 3-4 and Non-Patent Document 2 disclose silica particles that are close to perfect spheres or have small particle sizes, they do not disclose silica particles that are both close to perfect spheres and have small particle sizes. Therefore, they are not sufficient to solve the aforementioned problems of not being able to obtain a polished workpiece with a smooth surface or the problem of poor stability in polishing performance. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 11-60232 [Patent Document 2] Japanese Patent Publication No. 4-187512 [Patent Document 3] International Publication No. 2013 / 073025 [Patent Document 4] Japanese Patent Publication No. 61-209910 [Non-patent literature]

[0011] [Non-Patent Document 1] "Technology and Properties of High-Purity Colloidal Silica," Shinichi Sugita, JETI, Vol. 61, No. 3, pp. 58-61, (2013). [Non-Patent Document 2] "Controlled growth of monodisperse silica spheres in the micron size range," Staver, Journal of Colloid and Interface Science, Vol. 26, pp. 62-69, (1968). [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] This invention has been made in view of the above circumstances, and aims to provide silica particles that suppress secondary aggregation, have excellent dispersion stability and are suitable for polishing, a silica sol containing the silica particles, and a polishing composition containing the silica sol. Another objective of this invention is to provide a polishing method suitable for polishing, a method for manufacturing a semiconductor wafer including the polishing method, and a method for manufacturing a semiconductor device including the polishing method. [Means for solving the problem]

[0013] As a result of diligent research, the inventors discovered that the aforementioned problems could be solved by increasing the circularity coefficient of the silica particles, and thus completed the present invention.

[0014] In other words, the gist of the present invention is as follows: [1] Silica particles having a mean value of the circularity coefficient measured by a field emission scanning electron microscope of 0.90 or higher, and a standard deviation of the circularity coefficient of 0.05 or lower. [2] The average value of the circularity coefficient measured by a field emission scanning electron microscope is 0.90 or more, and the average value of the Heywood diameter measured by a field emission scanning electron microscope is 30 nm or less, silica particles. [3] The average value of the circularity coefficient measured by a field emission scanning electron microscope is 0.90 or more, and the average value of the aspect ratio measured by a field emission scanning electron microscope is 1.20 or less, silica particles. [4] The average value of the Heywood diameter measured by a field emission scanning electron microscope is 20 nm or less, the silica particles according to any one of [1] to [3]. [5] The average value of the aspect ratio measured by a field emission scanning electron microscope is 1.20 or less, the silica particles according to [1], [2] or [4]. [6] The standard deviation of the Heywood diameter is 3.00 nm or less, the silica particles according to [4]. [7] The standard deviation of the aspect ratio is 0.15 or less, the silica particles according to [5]. [8] The metal impurity content rate is 5 ppm or less, the silica particles according to any one of [1] to [7]. [9] The silica particles according to any one of [1] to [8], which contain a tetraalkoxysilane condensate as a main component.

[10] A silica sol containing the silica particles according to any one of [1] to [9].

[11] A polishing composition containing the silica sol according to

[10] .

[12] A polishing method including a step of polishing using the polishing composition according to

[11] .

[13] A method for manufacturing a semiconductor wafer including a step of polishing using the polishing composition according to

[11] .

[14] A method for manufacturing a semiconductor device including a step of polishing using the polishing composition according to

[11] . [Effect of the Invention]

[0015] The silica particles of the present invention suppress secondary aggregation, have excellent dispersion stability, and when used as a polishing composition, a polished object with a smooth surface can be obtained, and the stability of the polishing performance is excellent. Further, the silica sol of the present invention suppresses secondary aggregation of silica particles, has excellent dispersion stability, and when used as a polishing composition, a polished object with a smooth surface can be obtained, and the stability of the polishing performance is excellent. Furthermore, the polishing composition of the present invention can obtain a polished object with a smooth surface, and has excellent stability of polishing performance.

[0016] The polishing method of the present invention is suitable for polishing. Further, since the method for manufacturing a semiconductor wafer and the method for manufacturing a semiconductor device of the present invention include the polishing method of the present invention, they have excellent production stability of the polished object.

Embodiments for Carrying Out the Invention

[0017] The present invention will be described in detail below. However, the present invention is not limited to the following embodiments, and can be variously modified and implemented 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.

[0018] (Silica particles) The average value of the circularity coefficient of the silica particles of the present invention measured by a field emission scanning electron microscope is 0.90 or more. When the average value of the circularity coefficient of the silica particles of the present invention is 0.90 or more, secondary aggregation of the silica particles is suppressed, the dispersion stability of the silica particles in the dispersion liquid is excellent, and when the silica particles are used as a polishing composition, a polished object with a smooth surface can be obtained, and the silica particles have excellent stability of polishing performance.

[0019] The average value of the circularity coefficient of the silica particles of the present invention is preferably 0.90 to 0.99, and more preferably 0.92 to 0.98. When the average value of the circularity coefficient of the silica particles of the present invention is 0.90 or higher, as described above, secondary aggregation of the silica particles is suppressed, the dispersion stability of the silica particles in the dispersion is excellent, a smooth surface can be obtained when the silica particles are used as an abrasive composition, and the silica particles exhibit excellent abrasive performance stability. Furthermore, when the average value of the circularity coefficient of the silica particles is 0.99 or lower, the silica particles can be easily manufactured.

[0020] The standard deviation of the circularity coefficient of the silica particles of the present invention is preferably 0.05 or less, more preferably 0.01 to 0.05, and even more preferably 0.02 to 0.04. When the standard deviation of the circularity coefficient of the silica particles of the present invention is 0.05 or less, secondary aggregation of the silica particles is suppressed, the dispersion stability of the silica particles in the dispersion is excellent, a smooth surface can be obtained when the silica particles are used as an abrasive composition, and the silica particles exhibit excellent abrasive performance stability. Furthermore, when the standard deviation of the circularity coefficient of the silica particles of the present invention is 0.01 or more, the silica particles can be easily manufactured.

[0021] The average Heywood diameter of the silica particles of the present invention is preferably 30 nm or less, more preferably 5 nm to 20 nm, and even more preferably 6 nm to 19 nm. When the average Heywood diameter of the silica particles of the present invention is 30 nm or less, a smooth surface can be obtained when the silica particles are used as an abrasive composition, and the silica particles exhibit excellent stability in abrasive performance. Furthermore, when the average Heywood diameter of the silica particles of the present invention is 5 nm or more, the storage stability of the silica sol is excellent.

[0022] The standard deviation of the Heywood diameter of the silica particles of the present invention is preferably 3.00 nm or less, more preferably 0.10 nm to 3.00 nm, and even more preferably 0.50 nm to 2.50 nm. When the standard deviation of the Heywood diameter of the silica particles of the present invention is 3.00 nm or less, secondary aggregation of the silica particles is suppressed, the dispersion stability of the silica particles in the dispersion is excellent, a smooth surface can be obtained when the silica particles are used as an abrasive composition, and the silica particles exhibit excellent abrasive performance stability. Furthermore, when the standard deviation of the Heywood diameter of the silica particles of the present invention is 0.10 nm or more, the silica particles can be easily manufactured.

[0023] The average major axis of the silica particles of the present invention is preferably 31 nm or less, more preferably 6 nm to 21 nm, and even more preferably 7 nm to 20 nm. When the average major axis of the silica particles of the present invention is 6 nm or more, the storage stability of the silica sol is excellent. Furthermore, when the average major axis of the silica particles of the present invention is 31 nm or less, when the silica particles are used as an abrasive composition, a workpiece with a smooth surface can be obtained, and the silica particles exhibit excellent stability in abrasive performance.

[0024] The average value of the short diameter of the silica particles of the present invention is preferably 29 nm or less, more preferably 4 nm to 19 nm, and even more preferably 5 nm to 18 nm. When the average value of the short diameter of the silica particles of the present invention is 4 nm or more, the storage stability of the silica sol is excellent. Furthermore, when the average value of the short diameter of the silica particles of the present invention is 29 nm or less, when the silica particles are used as an abrasive composition, a workpiece with a smooth surface can be obtained, and the silica particles exhibit excellent stability in abrasive performance.

[0025] The average aspect ratio of the silica particles of the present invention is preferably 1.20 or less, more preferably 1.01 to 1.20, and even more preferably 1.02 to 1.18. When the average aspect ratio of the silica particles of the present invention is 1.20 or less, secondary aggregation of the silica particles is suppressed, the dispersion stability of the silica particles in the dispersion is excellent, a smooth surface can be obtained when the silica particles are used as an abrasive composition, and the silica particles exhibit excellent abrasive performance stability. Furthermore, when the average aspect ratio of the silica particles of the present invention is 1.01 or more, the silica particles can be easily manufactured.

[0026] The standard deviation of the aspect ratio of the silica particles of the present invention is preferably 0.15 or less, more preferably 0.01 to 0.15, and even more preferably 0.02 to 0.14. When the standard deviation of the aspect ratio of the silica particles of the present invention is 0.15 or less, secondary aggregation of the silica particles is suppressed, the dispersion stability of the silica particles in the dispersion is excellent, a smooth surface can be obtained when the silica particles are used as an abrasive composition, and the silica particles exhibit excellent abrasive performance stability. Furthermore, when the standard deviation of the aspect ratio of the silica particles of the present invention is 0.01 or more, the silica particles can be easily manufactured.

[0027] The mean values ​​and standard deviations of the circularity coefficient, Heywood diameter, major axis, minor axis, and aspect ratio of the silica particles of the present invention are measured using a field emission scanning electron microscope. Specifically, the measurements and calculations are performed under the conditions shown below.

[0028] A dispersion of silica particles is dropped onto a silicon substrate and dried. The silicon substrate is then irradiated with an electron beam at an acceleration voltage of 5kV using a field emission scanning electron microscope. Secondary electron images are then taken at a magnification of 50,000 to 200,000x so that the total number of silica particles is 80 or more. The Heywood diameter, major axis, minor axis, circularity coefficient, and aspect ratio of all silica particles are measured, and their mean values ​​and standard deviations are calculated. The concentration of silica particles in the dispersion may be adjusted as appropriate.

[0029] The shape of silica particles will be determined as described below. To determine whether two or more silica particles are bonded together, if a black line is visible between the silica particles, they are determined to be individual, unbonded silica particles. If no black line is visible between the silica particles, they are determined to be a single bonded silica particle. Furthermore, if three or more silica particles are aggregated, determination is difficult, and they will be excluded from the measurement.

[0030] The reason for setting the field emission scanning electron microscope's magnification to 50,000 to 200,000 times is to enable the identification of the individual shapes of silica particles smaller than 30 nm.

[0031] The circularity coefficient is 4πS / L, where S is the area of ​​the secondary electron image of silica particles captured by the field emission scanning electron microscope described above, and L is the perimeter of the said secondary electron image. 2 The value will be calculated using the formula. The Heywood diameter is defined as the diameter of the circle that has the same area as the secondary electron image. The major axis is defined as the longest side of the quadrilateral that circumscribes the secondary electron image, minimizing its area. The minor axis is defined as the shorter side of the quadrilateral that circumscribes the secondary electron image, minimizing its area. The aspect ratio is determined by the length of the longest side of the rectangle that circumscribes the secondary electron image with the smallest area, which corresponds to D. L The shorter side of the quadrilateral that circumscribes the secondary electron image and minimizes its area is D. S In that case, D L / D S The value will be calculated using the formula.

[0032] The metal impurity content of the silica particles of the present invention is preferably 5 ppm or less, and more preferably 2 ppm or less.

[0033] During the polishing of silicon wafers for semiconductor devices, metal impurities adhere to and contaminate the surface of the wafer being polished, adversely affecting its properties and diffusing into the wafer's interior, leading to a degradation of its quality. As a result, the performance of semiconductor devices manufactured using such wafers is significantly reduced.

[0034] Furthermore, if metal impurities are present in the silica particles of the present invention, coordination interactions occur between the acidic surface silanol groups and the metal impurities, which can change the chemical properties (such as acidity) of the surface silanol groups, alter the steric environment of the silica particle surface (such as the ease with which the silica particles aggregate), and affect the polishing rate.

[0035] The metal impurity content of the silica particles of this invention 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. Then, pure water is added to the remaining sulfuric acid droplets to make a total volume of exactly 10 g to create a test solution, which is then measured using an inductively coupled plasma mass spectrometer. The target metals are sodium, potassium, iron, aluminum, calcium, magnesium, zinc, cobalt, chromium, copper, manganese, lead, titanium, silver, and nickel, and the sum of the content of these metals is defined as the metal impurity content.

[0036] The metal impurity content of the silica particles of the present invention can be reduced to 5 ppm or less by performing hydrolysis and condensation reactions using alkoxysilane as the main raw material to obtain silica particles. In methods involving the deionization of alkali silicates such as water glass, residual sodium and other substances from the raw materials remain, making it extremely difficult to reduce the metal impurity content of silica particles to 5 ppm or less.

[0037] The silica particles of the present invention preferably have no pores, as they exhibit excellent mechanical strength and storage stability. The presence or absence of pores in the silica particles of this invention is confirmed by BET multipoint analysis using adsorption isotherms with nitrogen as the adsorbed gas.

[0038] The silica particles of the present invention have excellent mechanical strength and storage stability, and therefore it is preferable that they mainly consist of alkoxysilane condensates, and more preferably tetraalkoxysilane condensates. The main component refers to a component whose content is 50% or more by mass in 100% by mass of all components constituting the silica particles.

[0039] To obtain silica particles mainly composed of alkoxysilane condensates, it is preferable to use alkoxysilane as the main raw material. To obtain silica particles mainly composed of tetraalkoxysilane condensates, it is preferable to use tetraalkoxysilane as the main raw material. The main raw material is defined as making up 50% by mass or more of the total raw materials constituting the silica particles (100% by mass).

[0040] (Method for manufacturing silica particles) The silica particles of the present invention can be obtained, for example, by adding a solution (B) containing tetraalkoxysilane and a solution (C) containing an alkaline catalyst to a solution (A) containing water, and causing the tetraalkoxysilane to undergo hydrolysis and condensation reactions.

[0041] Solution (A) contains water. Solution (A) is preferable to contain a solvent other than water because it exhibits excellent dispersibility in the reaction solution of tetraalkoxysilane.

[0042] Other solvents in solution (A) include, for example, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents may be used individually or in combination of two or more. Among these solvents, monohydric alcohols are preferred because they readily dissolve tetraalkoxysilane, produce the same by-products as those used in the hydrolysis and condensation reactions, and offer excellent manufacturing convenience. Methanol and ethanol are more preferred, and methanol is even more preferred.

[0043] Solution (A) preferably contains an alkaline catalyst because it can increase the reaction rates of the hydrolysis and condensation reactions of tetraalkoxysilanes.

[0044] Examples of alkaline catalysts in solution (A) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. These alkaline catalysts may be used individually or in combination of two or more. Among these alkaline catalysts, ammonia is preferred because it exhibits excellent catalytic activity, allows for easy control of particle shape, suppresses the inclusion of metal impurities, and has high volatility, resulting in excellent removal after hydrolysis and condensation reactions.

[0045] The concentration of water in solution (A) is preferably 3% to 30% by mass, and more preferably 5% to 25% by mass, based on 100% by mass of solution (A). When the concentration of water in solution (A) is 3% by mass or higher, it is easier to control the hydrolysis reaction rate of tetraalkoxysilane. Furthermore, when the concentration of water in solution (A) is 30% by mass or lower, the reaction balance between the hydrolysis reaction and the condensation reaction is good, making it easier to control the particle shape.

[0046] The concentration of the alkaline catalyst in solution (A) is preferably 0.5% to 2.0% by mass, and more preferably 0.6% to 1.5% by mass, based on 100% by mass of solution (A). When the concentration of the alkaline catalyst in solution (A) is 0.5% by mass or higher, aggregation of silica particles is suppressed, and the dispersion stability of silica particles in the dispersion is excellent. Furthermore, when the concentration of the alkaline catalyst in solution (A) is 2.0% by mass or lower, the reaction does not proceed excessively quickly, resulting in excellent reaction control.

[0047] The concentration of solvents other than water in solution (A) is preferably the remainder of the water and alkaline catalyst.

[0048] Examples of tetraalkoxysilanes in solution (B) include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetraisopropoxysilane. These tetraalkoxysilanes may be used individually or in combination of two or more. Among these tetraalkoxysilanes, tetramethoxysilane and tetraethoxysilane are preferred, and tetramethoxysilane is more preferred, because they undergo rapid hydrolysis, leave little unreacted residue, have excellent productivity, and allow for easy acquisition of a stable silica sol.

[0049] The raw materials for the silica particles of the present invention may include raw materials other than tetraalkoxysilane, such as low condensates of tetraalkoxysilane. However, due to their excellent reactivity, it is preferable that tetraalkoxysilane accounts for 50% or more by mass and other raw materials account for 50% or less by mass of the total raw materials constituting the silica particles, and more preferably that tetraalkoxysilane accounts for 90% or more by mass and other raw materials account for 10% or less by mass.

[0050] Solution (B) may contain only tetraalkoxysilane without a solvent, but it is preferable to include a solvent because it provides excellent dispersibility of the tetraalkoxysilane in the reaction solution.

[0051] Examples of solvents in solution (B) include methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents may be used individually or in combination of two or more. Among these solvents, monohydric alcohols are preferred, methanol and ethanol are more preferred, and methanol is even more preferred, because the by-products used in the hydrolysis and condensation reactions are the same, and they offer excellent manufacturing convenience.

[0052] The concentration of tetraalkoxysilane in solution (B) is preferably 60% to 95% by mass, and more preferably 70% to 90% by mass, based on 100% by mass of solution (B). When the concentration of tetraalkoxysilane in solution (B) is 60% by mass or higher, the reaction solution tends to become more homogeneous. Furthermore, when the concentration of tetraalkoxysilane in solution (B) is 95% by mass or lower, the formation of gel-like substances can be suppressed.

[0053] The concentration of the solvent in solution (B) is preferably 5% to 40% by mass, and more preferably 10% to 30% by mass, based on 100% by mass of solution (B). If the concentration of the solvent in solution (B) is 5% by mass or higher, the formation of gel-like substances can be suppressed. Also, if the concentration of the solvent in solution (B) is 40% by mass or lower, the reaction solution tends to become more homogeneous.

[0054] The rate at which solution (B) is added per unit time relative to the volume of solution (A) is preferably 0.05 kg / hour / L to 1.3 kg / hour / L, and more preferably 0.1 kg / hour / L to 0.8 kg / hour / L. When the rate at which solution (B) is added is 0.05 kg / hour / L or higher, the productivity of silica particles is excellent. Furthermore, when the rate at which solution (B) is added is 1.3 kg / hour / L or lower, the formation of gel-like substances can be suppressed.

[0055] Examples of alkaline catalysts in solution (C) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. These alkaline catalysts may be used individually or in combination of two or more. Among these alkaline catalysts, ammonia is preferred because it exhibits excellent catalytic activity, allows for easy control of particle shape, suppresses the inclusion of metal impurities, and has high volatility, resulting in excellent removal after hydrolysis and condensation reactions.

[0056] Solution (C) preferably contains a solvent because it can reduce fluctuations in the concentration of the alkaline catalyst in the reaction mixture.

[0057] Examples of solvents in solution (C) include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents may be used individually or in combination of two or more. Among these solvents, water and monohydric alcohols are preferred, and water is more preferred, because the by-products used in the hydrolysis and condensation reactions are the same, and they offer excellent manufacturing convenience.

[0058] The concentration of the alkaline catalyst in solution (C) is preferably 0.5% to 10% by mass, and more preferably 1% to 6% by mass, based on 100% by mass of solution (C). When the concentration of the alkaline catalyst in solution (C) is 0.5% by mass or higher, it is easy to adjust the concentration of the alkaline catalyst in the reaction solution from the start to the end of the reaction. Furthermore, when the concentration of the alkaline catalyst in solution (C) is 10% by mass or lower, fluctuations in the concentration of the alkaline catalyst in the reaction solution can be reduced.

[0059] The concentration of the solvent in solution (C) is preferably 90% to 99.5% by mass, and more preferably 94% to 99% by mass, based on 100% by mass of solution (C). When the concentration of the solvent in solution (C) is 90% by mass or higher, fluctuations in the concentration of the alkaline catalyst in the reaction solution can be minimized. Furthermore, when the concentration of the solvent in solution (C) is 99.5% by mass or lower, it is easier to adjust the concentration of the alkaline catalyst in the reaction solution from the start to the end of the reaction.

[0060] The rate at which solution (C) is added per unit time relative to the volume of solution (A) is preferably 0.02 kg / hour / L to 0.5 kg / hour / L, and more preferably 0.04 kg / hour / L to 0.3 kg / hour / L. When the rate at which solution (C) is added is 0.02 kg / hour / L or higher, the productivity of silica particles is excellent. Furthermore, when the rate at which solution (C) is added is 0.5 kg / hour / L or lower, the formation of gel-like substances can be suppressed.

[0061] It is preferable to add solutions (B) and (C) into solution (A). Adding solutions (B) and (C) into solution (A) improves the mixability of each component in the reaction solution, suppresses abnormal reactions in the air, and makes it easier to control the circularity coefficient and particle size, especially when using highly volatile alkaline catalysts such as ammonia and when proceeding with hydrolysis and condensation reactions at high reaction temperatures. Adding into a solution means adding below the liquid surface, and by positioning the supply outlets for solution (B) and solution (C) below the liquid surface of solution (A), solutions (B) and (C) can be added into solution (A).

[0062] The timing of adding solution (B) and solution (C) may be the same or alternate, but it is preferable that they be added at the same time, as this minimizes variations in the reaction composition and avoids complicating the procedure.

[0063] The reaction temperature for the hydrolysis and condensation reactions is preferably 50°C to 80°C, and more preferably 55°C to 75°C. A reaction temperature of 50°C or higher facilitates control of particle shape, such as Heywood diameter and circularity coefficient. Furthermore, a reaction temperature of 80°C or lower suppresses bumping and solvent volatilization, reducing fluctuations in the reaction mixture.

[0064] The concentration of water in the reaction system for the hydrolysis and condensation reactions is preferably maintained at 3% to 30% by mass, and more preferably at 5% to 25% by mass, out of 100% by mass of the total amount in the reaction system. When the water concentration in the reaction system is 3% by mass or higher, it is easier to control the hydrolysis reaction rate of tetraalkoxysilane. Furthermore, when the water concentration in the reaction system is 30% by mass or lower, the reaction balance between the hydrolysis reaction and the condensation reaction is good, and it is easier to control the particle shape.

[0065] The concentration of the alkaline catalyst in the reaction system for hydrolysis and condensation reactions is preferably maintained at 0.5% to 2.0% by mass, and more preferably at 0.6% to 1.5% by mass, out of 100% by mass of the total amount in the reaction system. When the concentration of the alkaline catalyst in the reaction system is 0.5% by mass or higher, the aggregation of silica particles is suppressed, and the dispersion stability of silica particles in the dispersion is excellent. Furthermore, when the concentration of the alkaline catalyst in the reaction system is 2.0% by mass or lower, the reaction does not proceed excessively quickly, and the reaction controllability is excellent.

[0066] The method for producing silica particles of the present invention is preferable to further include the following step (1), since it is possible to remove unnecessary components and add necessary components. Step (1): Concentrating the resulting silica particle dispersion and adding a dispersion medium.

[0067] Examples of dispersion media include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These dispersion media may be used individually or in combination of two or more. Among these dispersion media, water and monohydric alcohols are preferred, with water being more preferred, due to their excellent affinity for silica particles.

[0068] Since the method for producing silica particles of the present invention can increase the degree of condensation of the silica particles, it is preferable to further include the following step (2). Step (2): A process of pressurizing and heating the dispersion of silica particles obtained in step (1).

[0069] The pressure for the pressurized heat treatment is preferably 0.10 MPa to 2.3 MPa, and more preferably 0.14 MPa to 1.0 MPa. When the pressurized heat treatment pressure is 0.10 MPa or higher, the degree of condensation of silica particles can be increased. Furthermore, when the pressurized heat treatment pressure is 2.3 MPa or lower, silica particles can be produced without significantly changing the average primary particle diameter, average secondary particle diameter, cv value, or association ratio, and the dispersion stability of the silica sol is excellent.

[0070] Pressurization can be achieved by heating a silica particle dispersion to a temperature above the boiling point of the dispersion medium while it is sealed. When a silica particle aqueous dispersion is heated to 100°C or higher while sealed, the pressure will be equal to the saturated water vapor pressure at that temperature.

[0071] The temperature for the pressurized heat treatment is preferably 100°C to 220°C, and more preferably 110°C to 180°C. If the pressurized heat treatment temperature is 100°C or higher, the degree of condensation of silica particles can be increased. If the pressurized heat treatment temperature is 220°C or lower, silica particles can be produced without significantly changing the average primary particle diameter, average secondary particle diameter, CV value, or association ratio, and the dispersion stability of the silica sol is excellent.

[0072] The pressurized heat treatment time is preferably 0.25 to 10 hours, and more preferably 0.5 to 8 hours. If the pressurized heat treatment time is 0.25 hours or longer, the degree of condensation of silica particles can be increased. If the pressurized heat treatment time is 10 hours or less, silica particles can be produced without significantly changing the average primary particle diameter, average secondary particle diameter, CV value, or association ratio, and the dispersion stability of the silica sol is excellent.

[0073] Pressurized heat treatment is more preferably performed in an aqueous dispersion because it can increase the degree of condensation of silica particles without significantly changing the average primary particle diameter, average secondary particle diameter, cv value, or association ratio.

[0074] When performing pressurized heat treatment in an aqueous dispersion, the pH of the aqueous dispersion is preferably 6.0 to 8.0, and more preferably 6.5 to 7.8. If the pH of the aqueous dispersion is 6.0 or higher when performing pressurized heat treatment in an aqueous dispersion, gelation of silica sol can be suppressed. Furthermore, if the pH of the aqueous dispersion is 8.0 or lower when performing pressurized heat treatment in an aqueous dispersion, the degree of condensation of silica particles can be increased without significantly changing the average primary particle diameter, average secondary particle diameter, cv value, or association ratio.

[0075] (Silica sol) The silica sol of the present invention contains the silica particles of the present invention. The silica sol may be produced by using the silica particle dispersion of the present invention as is, or by removing unnecessary components and adding necessary components from the silica particle dispersion of the present invention.

[0076] The silica sol of the present invention preferably contains the silica particles and dispersion medium of the present invention. Examples of dispersion media in the silica sol of the present invention include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These dispersion media in the silica sol may be used individually or in combination of two or more. Among these dispersion media in the silica sol, water and monohydric alcohols are preferred, and water is more preferred, due to their excellent affinity with silica particles.

[0077] The silica particle content in the silica sol of the present invention is preferably 3% to 50% by mass, more preferably 4% to 40% by mass, and even more preferably 5% to 30% by mass, based on 100% by mass of the total amount of silica sol. When the silica particle content in the silica sol of the present invention is 3% by mass or more, the polishing rate for the workpiece, such as silicon wafers, is excellent. Furthermore, when the silica particle content in the silica sol of the present invention is 50% by mass or less, aggregation of silica particles in the silica sol and polishing composition can be suppressed, resulting in excellent storage stability of the silica sol and polishing composition.

[0078] The content of the dispersion medium in the silica sol of the present invention is preferably 50% to 97% by mass, more preferably 60% to 96% by mass, and even more preferably 70% to 95% by mass, based on 100% by mass of the total amount of silica sol. When the content of the dispersion medium in the silica sol of the present invention is 50% by mass or more, aggregation of silica particles in the silica sol and polishing composition can be suppressed, resulting in excellent storage stability of the silica sol and polishing composition. Furthermore, when the content of the dispersion medium in the silica sol of the present invention is 97% by mass or less, the polishing rate for the workpiece, such as silicon wafers, is excellent.

[0079] The content of silica particles and dispersion medium in the silica sol of the present invention can be set to a desired range by removing unnecessary components and adding necessary components from the components of the resulting silica particle dispersion.

[0080] In addition to silica particles and a dispersion medium, the silica sol of the present invention may optionally contain other components such as oxidizing agents, preservatives, fungicides, pH adjusters, pH buffers, surfactants, chelating agents, and antimicrobial biocides, to the extent that it does not impair its performance. In particular, since silica sol has excellent storage stability, it is preferable to include an antimicrobial biocide in the silica sol.

[0081] Examples of antimicrobial biocides include hydrogen peroxide, ammonia, quaternary ammonium hydroxide, quaternary ammonium salt, ethylenediamine, glutaraldehyde, hydrogen peroxide, methyl p-hydroxybenzoate, and sodium chlorite. These antimicrobial biocides may be used individually or in combination of two or more. Among these antimicrobial biocides, hydrogen peroxide is preferred due to its excellent affinity for silica sol. Biocides include what are commonly known as fungicides.

[0082] The content of the antimicrobial biocide in the silica sol of the present invention is preferably 0.0001% to 10% by mass, and more preferably 0.001% to 1% by mass, based on 100% by mass of the total amount of silica sol. When the content of the antimicrobial biocide in the silica sol of the present invention is 0.0001% by mass or more, the storage stability of the silica sol is excellent. When the content of the antimicrobial biocide in the silica sol of the present invention is 10% by mass or less, the original performance of the silica sol is not impaired.

[0083] The pH of the silica sol of the present invention is preferably 6.0 to 8.0, and more preferably 6.5 to 7.8. When the pH of the silica sol of the present invention is 6.0 or higher, it exhibits excellent dispersion stability and can suppress the aggregation of silica particles. Furthermore, when the pH of the silica sol of the present invention is 8.0 or lower, it prevents the dissolution of silica particles and exhibits excellent long-term storage stability. The pH of the silica sol of the present invention can be set to a desired range by adding a pH adjusting agent.

[0084] (polishing composition) The polishing composition of the present invention contains the silica sol of the present invention. The polishing composition of the present invention preferably contains the silica sol and water-soluble polymer of the present invention.

[0085] Water-soluble polymers enhance the wettability of the polishing composition to the workpiece, such as silicon wafers. Preferably, the water-soluble polymer has functional groups with high water affinity. These water-affinity functional groups have a high affinity for the surface silanol groups of silica particles, resulting in a more stable dispersion of silica particles and water-soluble polymers in close proximity within the polishing composition. Therefore, during polishing of workpieces such as silicon wafers, the effects of silica particles and water-soluble polymers function synergistically.

[0086] Examples of water-soluble polymers include cellulose derivatives, polyvinyl alcohol, polyvinylpyrrolidone, copolymers having a polyvinylpyrrolidone skeleton, and polymers having a polyoxyalkylene structure.

[0087] Examples of cellulose derivatives include hydroxyethylcellulose, hydrolyzed hydroxyethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose, methylcellulose, ethylcellulose, ethylhydroxyethylcellulose, and carboxymethylcellulose.

[0088] Examples of copolymers having a polyvinylpyrrolidone skeleton include graft copolymers of polyvinyl alcohol and polyvinylpyrrolidone.

[0089] Examples of polymers having a polyoxyalkylene structure include polyoxyethylene, polyoxypropylene, and copolymers of ethylene oxide and propylene oxide.

[0090] These water-soluble polymers may be used individually or in combination of two or more. Among these water-soluble polymers, cellulose derivatives are preferred, and hydroxyethylcellulose is more preferred, because they have a high affinity for the silanol groups on the surface of silica particles and act synergistically to provide good hydrophilicity to the surface of the object to be polished.

[0091] 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, and 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. Furthermore, when the weight-average molecular weight of the water-soluble polymer is 3,000,000 or less, it exhibits excellent affinity with silica sol and provides a superior polishing rate for materials to be polished, such as silicon wafers.

[0092] The weight-average molecular weight of water-soluble polymers is measured by size exclusion chromatography using a 0.1 mol / L NaCl solution as the mobile phase, based on polyethylene oxide.

[0093] The content of water-soluble polymers in the abrasive composition is preferably 0.02% to 10% by mass, and more preferably 0.05% to 5% by mass, based on 100% by mass of the total amount of the abrasive composition. When the content of water-soluble polymers in the abrasive composition is 0.02% by mass or more, the hydrophilicity of the abrasive composition is improved. Furthermore, when the content of water-soluble polymers in the abrasive composition is 10% by mass or less, aggregation of silica particles during the preparation of the abrasive composition can be suppressed.

[0094] In addition to silica sol and water-soluble polymer, the polishing composition of the present invention may optionally contain other components such as basic compounds, polishing accelerators, surfactants, hydrophilic compounds, preservatives, fungicides, pH adjusters, pH buffers, surfactants, chelating agents, and antimicrobial biocides, to the extent that it does not impair its performance.

[0095] In particular, it is preferable to include a basic compound in the polishing composition because it allows for chemical polishing (chemical etching) by applying a chemical action to the surface of the workpiece, such as a silicon wafer, and the polishing speed of the workpiece, such as a silicon wafer, can be improved due to the synergistic effect with the silanol groups on the surface of the silica particles.

[0096] Examples of basic compounds include organic basic compounds, alkali metal hydroxides, alkali metal bicarbonates, alkali metal carbonates, and ammonia. These basic compounds may be used individually or in combination of two or more. Among these basic compounds, ammonia, tetramethylammonium hydroxide, tetraethylammonium hydroxide, ammonium bicarbonate, and ammonium carbonate are preferred due to their high water solubility and excellent affinity with silica particles and water-soluble polymers. Ammonia, tetramethylammonium hydroxide, and tetraethylammonium hydroxide are more preferred, and ammonia is even more preferred.

[0097] The content of the basic compound in the polishing composition of the present invention is preferably 0.001% to 5% by mass, and more preferably 0.01% to 3% by mass, based on 100% by mass of the total amount of the polishing composition. When the content of the basic compound in the polishing composition is 0.001% by mass or more, the polishing speed of the workpiece, such as silicon wafers, can be improved. Furthermore, when the content of the basic compound in the polishing composition is 5% by mass or less, the stability of the polishing composition is excellent.

[0098] The pH of the polishing composition of the present invention is preferably 8.0 to 12.0, and more preferably 9.0 to 11.0. When the pH of the polishing composition is 8.0 or higher, aggregation of silica particles in the polishing composition can be suppressed, and the dispersion stability of the polishing composition is excellent. Furthermore, when the pH of the polishing composition of the present invention is 12.0 or lower, dissolution of silica particles can be suppressed, and the stability of the polishing composition is excellent. The pH of the polishing composition of the present invention can be set to a desired range by adding a pH adjusting agent.

[0099] The polishing composition of the present invention can be obtained by mixing the silica sol of the present invention with, if necessary, a water-soluble polymer and other components. However, considering storage and transportation, it may be prepared at a high concentration first and then diluted with water or the like immediately before polishing.

[0100] (polishing method) The polishing method of the present invention is a method that includes the step of polishing using a polishing composition containing silica sol obtained by the silica sol production method of the present invention.

[0101] It is preferable to use the polishing composition of the present invention described above. Specific polishing methods include, for example, pressing the surface of a silicon wafer against a polishing pad, dropping the polishing composition of the present invention onto the polishing pad, and polishing the surface of the silicon wafer.

[0102] (Method of manufacturing semiconductor wafers) The semiconductor wafer manufacturing method of the present invention includes the polishing method of the present invention, and the specific polishing method is as described above. Examples of semiconductor wafers include silicon wafers and compound semiconductor wafers.

[0103] (Manufacturing methods for semiconductor devices) The method for manufacturing a semiconductor device according to the present invention includes the polishing method of the present invention, and the specific polishing method is as described above.

[0104] (Application) The silica particles and silica sol of the present invention can be suitably used for polishing applications. For example, the silica particles and silica sol of the present invention can be used for polishing semiconductor materials such as silicon wafers, polishing electronic materials such as hard disk substrates, polishing (chemical mechanical polishing) in the planarization process when manufacturing integrated circuits, polishing synthetic quartz glass substrates used in photomasks and liquid crystals, polishing magnetic disk substrates, etc., and can be used particularly suitably for polishing silicon wafers and chemical mechanical polishing.

[0105] The silica particles of the present invention have a high circularity coefficient, and when used as a composite material with resins, they impart transparency and water repellency to the resins, while also improving dispersibility and mixability, thus improving the processability of the composite material. For this reason, the silica particles of the present invention can be suitably used as hard coat agents, paints, and binders for ceramics. [Examples]

[0106] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples without departing from its essence.

[0107] (Measurement of silica particle shape) The silica particle dispersions obtained in the examples and comparative examples were diluted 5,000 times with ultrapure water, and 5 μL of the diluted silica particle dispersion was dropped onto a silicon substrate and dried. Next, using a field emission scanning electron microscope (model "S-5200", manufactured by Hitachi High-Technologies Corporation, FE-SEM), the silicon substrate was irradiated with an electron beam at an acceleration voltage of 5 kV, and secondary electron images were taken at a magnification of 50,000 to 200,000 times so that the total number of silica particles was 80 or more. The Heywood diameter, major axis, minor axis, circularity coefficient, and aspect ratio of all silica particles were measured, and their mean and standard deviation were measured.

[0108] For measuring the shape of silica particles, the captured field emission scanning electron microscope images were imported into image analysis-based particle size distribution measurement software (software name "Mac-View Ver.4", manufactured by Mountec Co., Ltd.).

[0109] The shape of the silica particles was determined as described below. Whether two or more silica particles were joined together was determined by whether a black line was visible between them (indicating they were individual, unjoined silica particles) or whether a black line was not visible (indicating a single joined silica particle). Furthermore, aggregated silica particles of three or more were excluded from the measurement due to the difficulty in determining their shape.

[0110] [Example 1] Solution (A1) was obtained by mixing 34.2 parts by mass of pure water, 188.3 parts by mass of methanol, and 8.8 parts by mass of 29% by mass aqueous ammonia. To solution (A1), solution (B1), prepared by mixing 100 parts by mass of tetramethoxysilane and 33.3 parts by mass of methanol, and solution (C1), prepared by mixing 25.0 parts by mass of pure water and 2.2 parts by mass of 29% by mass aqueous ammonia, were added at a constant rate over 103 minutes. During the addition, the reaction mixture was stirred while maintaining its temperature at 70°C. After the addition was completed, the reaction mixture was stirred for a further 30 minutes while maintaining its temperature at 70°C.

[0111] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass. The evaluation results of the obtained silica particles are shown in Table 1.

[0112] [Example 2] Except for setting the reaction solution temperature to 60°C, the procedure was carried out in the same manner as in Example 1 to obtain a dispersion of silica particles with a silica particle content of approximately 20% by mass. The evaluation results of the obtained silica particles are shown in Table 1.

[0113] [Example 3] Solution (A3) was obtained by mixing 40.6 parts by mass of pure water, 207.6 parts by mass of methanol, and 5.2 parts by mass of 29% by mass aqueous ammonia. To solution (A3), a solution (B3) prepared by mixing 100 parts by mass of tetramethoxysilane and 17.6 parts by mass of methanol, and a solution (C3) prepared by mixing 21.5 parts by mass of pure water and 2.1 parts by mass of 29% by mass aqueous ammonia were added at a constant rate over 93 minutes. During the addition, the reaction mixture was stirred while maintaining its temperature at 50°C. After the addition was completed, the reaction mixture was stirred for a further 30 minutes while maintaining its temperature at 50°C.

[0114] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass. The evaluation results of the obtained silica particles are shown in Table 1.

[0115] [Example 4] Solution (A4) was obtained by mixing 34.9 parts by mass of pure water, 204.4 parts by mass of methanol, and 13.1 parts by mass of 29% by mass aqueous ammonia. To solution (A4), a solution (B4) prepared by mixing 100 parts by mass of tetramethoxysilane and 17.6 parts by mass of methanol, and a solution (C4) prepared by mixing 19.4 parts by mass of pure water and 5.3 parts by mass of 29% by mass aqueous ammonia were added at a constant rate over 94 minutes. During the addition, the reaction mixture was stirred while maintaining its temperature at 70°C. After the addition was completed, the reaction mixture was stirred for a further 30 minutes while maintaining its temperature at 70°C.

[0116] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass. The evaluation results of the obtained silica particles are shown in Table 1.

[0117] [Comparative Example 1] A commercially available silica particle dispersion (product name "PL-1", manufactured by Fuso Chemical Industries, Ltd.) was used as is. The evaluation results of the silica particles used are shown in Table 1.

[0118] [Comparative Example 2] A commercially available silica particle dispersion (product name "PL-2", manufactured by Fuso Chemical Industries, Ltd.) was used as is. The evaluation results of the silica particles used are shown in Table 1.

[0119] [Comparative Example 3] A commercially available silica particle dispersion (product name "PL-2L," manufactured by Fuso Chemical Industries, Ltd.) was used as is. The evaluation results of the silica particles used are shown in Table 1.

[0120] [Comparative Example 4] Solution (A'4) was obtained by mixing 21.4 parts by mass of pure water, 95.2 parts by mass of methanol, and 3.7 parts by mass of 29% by mass aqueous ammonia. To solution (A'4), solution (B'4), which was a mixture of 100 parts by mass of tetramethoxysilane and 17.6 parts by mass of methanol, and solution (C'4), which was a mixture of 21.9 parts by mass of pure water and 3.2 parts by mass of 29% by mass aqueous ammonia, were added at a constant rate over 94 minutes. During the addition, the reaction mixture was stirred while maintaining the temperature at 42°C. After the addition was completed, the reaction mixture was stirred for a further 30 minutes while maintaining the temperature at 42°C.

[0121] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass. The evaluation results of the obtained silica particles are shown in Table 1.

[0122] [Comparative Example 5] The procedure was the same as in Example 1, except that solutions (B1) and (C1) were added dropwise from above the liquid surface of solution (A1) rather than into the solution (A1). However, a large amount of silica particles adhered to the inner wall of the reaction vessel and the stirring blades, making it difficult to continue the operation.

[0123] [Table 1]

[0124] As can be seen from Table 1, the silica particles obtained in Examples 1 to 4 have a larger average value of circularity coefficient, smaller average values ​​of Heywood diameter, major axis, minor axis, and aspect ratio compared to the commercially available silica particles in Comparative Examples 1 to 3 and the silica particles obtained in Comparative Example 4. These values ​​also have smaller standard deviations, indicating that the silica particles have less variation in particle shape and smaller particle size.

[0125] The silica particles obtained in Examples 1-4 are expected to exhibit excellent dispersion stability due to their small particle size and low variation in particle shape, thereby suppressing secondary aggregation. Furthermore, because the silica particles obtained in Examples 1-4 have small particle size and low variation in particle shape, when used as an abrasive composition, they are expected to produce a smooth surface on the workpiece and exhibit excellent abrasive performance stability.

[0126] Furthermore, the silica particle dispersion obtained in Example 1 was evaluated in the same manner 9 months after production, and showed almost no change, with virtually no secondary aggregation observed, indicating excellent dispersion stability.

[0127] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on Japanese Patent Application No. 2020-11471, filed on 28 January 2020, the contents of which are incorporated herein by reference. [Industrial applicability]

[0128] The silica particles and silica sol of the present invention can be suitably used for polishing applications. For example, the silica particles and silica sol of the present invention can be used for polishing semiconductor materials such as silicon wafers, polishing electronic materials such as hard disk substrates, polishing (chemical mechanical polishing) in the planarization process when manufacturing integrated circuits, polishing synthetic quartz glass substrates used in photomasks and liquid crystals, polishing magnetic disk substrates, etc., and can be used particularly suitably for polishing silicon wafers and chemical mechanical polishing.

Claims

1. Silica particles having an average circularity coefficient of 0.90 or higher as measured by a field emission scanning electron microscope, an average Heywood diameter of 30 nm or less as measured by a field emission scanning electron microscope, and a metal impurity content of 5 ppm or less.

2. Silica particles according to claim 1, wherein the average value of the Heywood diameter measured by a field emission scanning electron microscope is 20 nm or less.

3. Silica particles according to claim 1 or 2, wherein the average value of the aspect ratio measured by a field emission scanning electron microscope is 1.20 or less.

4. The silica particles according to claim 2, wherein the standard deviation of the Heywood diameter is 3.00 nm or less.

5. The silica particles according to claim 3, wherein the standard deviation of the aspect ratio is 0.15 or less.

6. A silica sol comprising silica particles according to any one of claims 1 to 5.

7. An abrasive composition comprising the silica sol described in Claim 6.

8. A polishing method comprising the step of polishing using the polishing composition described in Claim 7.

9. A method for manufacturing a semiconductor wafer, comprising the step of polishing using the polishing composition described in Claim 7.

10. A method for manufacturing a semiconductor device, comprising the step of polishing using the polishing composition described in Claim 7.

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