JP7206695B2 - Silica sol, polishing composition, method for polishing silicon wafer, method for producing silicon wafer, chemical-mechanical polishing composition, and method for producing semiconductor device - Google Patents

Description

The present invention relates to colloidal silica. It also relates to silica sol. The present invention also relates to polishing compositions. The present invention also relates to a method for polishing a silicon wafer. The present invention also relates to a method for manufacturing silicon wafers. The present invention also relates to chemical-mechanical polishing compositions. Furthermore, the present invention relates to a method of manufacturing a semiconductor device.

A polishing method using a polishing liquid is known as a method for polishing the surface of materials such as metals and inorganic compounds. Among them, the final polishing of prime silicon wafers for semiconductors and their reclaimed silicon wafers, and chemical mechanical polishing (CMP ), the surface state greatly affects the semiconductor characteristics, so the surfaces and end faces of these parts are required to be polished with extremely high precision.
In such precision polishing, a polishing composition containing silica sol is employed, and colloidal silica is widely used as abrasive grains which are the main component of the composition. Colloidal silica is produced by thermal decomposition of silicon tetrachloride (fumed silica, etc.), by deionization of alkali silicate such as water glass, and by hydrolysis reaction of alkoxysilane (generally called "sol-gel called "law"), etc. are known.

It is known that the physical properties of colloidal silica affect its performance as a polishing liquid, and many studies have been made. In particular, the influence of silanol groups on the surface of colloidal silica particles on the polishing performance has been extensively studied not only in the polishing of silicon wafers but also in chemical and mechanical polishing during the manufacture of semiconductor devices.

For example, Patent Document 1 discloses colloidal silica for use in polishing silicon wafers. Further, Patent Document 2 discloses colloidal silica for use in polishing insulating films of semiconductor wafers. Further, Patent Document 3 discloses colloidal silica for use in polishing metal films, barrier metal films and insulating films. Furthermore, Patent Document 4 discloses colloidal silica for use in selective polishing of multilayer wiring portions.

JP-A-02-158684 JP-A-2006-231436 JP 2009-224767 A International publication 2014/007063 pamphlet

However, it is unclear whether the colloidal silica disclosed in Patent Document 1 uses tetraalkoxysilane as a main raw material, and it is unclear whether the content of metal impurities is low. Even if a silicon wafer is polished using silica, it is highly likely that it does not have a sufficient polishing rate.
In addition, since the colloidal silica disclosed in Patent Documents 2 to 4 is not a silicon wafer to be polished, even if a silicon wafer is polished using the colloidal silica disclosed in Patent Documents 2 to 4, sufficient polishing can be achieved. Likely to have no rate.

By the way, in the polishing of silicon wafers for semiconductors and the chemical mechanical polishing in the manufacture of semiconductor devices, transition metals, alkali metals, alkaline earth metals, etc. adhere to the surface of the object to be polished as impurities and contaminate the surface. , adversely affect semiconductor characteristics. Therefore, in these polishing processes, there is a demand for high-purity colloidal silica in which contamination with metal impurities is minimized, that is, colloidal silica produced by a hydrolysis reaction using tetraalkoxysilane as a main raw material.
In addition, in the polishing of silicon wafers using such high-purity colloidal silica, in order to improve the productivity of silicon wafers, it is required to improve the polishability of silicon wafers, that is, to improve the polishing rate. .

The present invention has been made in view of such problems, and an object of the present invention is to provide colloidal silica, silica sol, and polishing composition having a low metal impurity content and an excellent polishing rate. Another object of the present invention is to provide a method for polishing a silicon wafer and a method for manufacturing a silicon wafer, which are excellent in silicon wafer productivity.

When a silicon wafer is polished using conventional high-purity colloidal silica, the polishing rate is not necessarily sufficient. However, as a result of extensive studies, the present inventors found that optimizing the surface silanol group density of high-purity colloidal silica improves the polishing rate in polishing silicon wafers using high-purity colloidal silica. , have completed the present invention.

That is, the gist of the present invention is as follows.
[1] Colloidal silica containing tetraalkoxysilane as a main raw material and having a surface silanol group density of 4/nm ² or more as measured by the Sears method.
[2] The colloidal silica according to [1], which has an average primary particle size of 15 nm to 100 nm as measured by the BET method.
[3] The colloidal silica according to [1] or [2], which has an average secondary particle size of 40 nm to 200 nm as measured by the DLS method.
[4] The colloidal silica according to any one of [1] to [3], wherein the tetraalkoxysilane is tetramethoxysilane.
[5] A silica sol containing the colloidal silica according to any one of [1] to [4] and water.
[6] The silica sol according to [5], wherein the content of colloidal silica is 3% by mass to 50% by mass based on 100% by mass of the total silica sol.
[7] The silica sol according to [5] or [6], which has a metal impurity content of 1 ppm or less.
[8] A polishing composition comprising the silica sol according to any one of [5] to [7] and a water-soluble polymer.
[9] A method for polishing a silicon wafer, comprising polishing a silicon wafer using the polishing composition according to [8].
[10] A method for manufacturing a silicon wafer, including the method for polishing a silicon wafer according to [9].
[11] A chemical mechanical polishing composition comprising the silica sol according to any one of [5] to [7].
[12] A method for manufacturing a semiconductor device, comprising the step of performing chemical mechanical polishing using the chemical mechanical polishing composition according to [11].

The colloidal silica of the present invention is excellent in polishing rate. Moreover, the silica sol of the present invention is excellent in polishing rate. Moreover, the polishing composition of the present invention is excellent in polishing rate. Further, the method for polishing a silicon wafer of the present invention is excellent in silicon wafer productivity. Furthermore, the silicon wafer manufacturing method of the present invention is excellent in silicon wafer productivity.

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

(colloidal silica)
The colloidal silica of the present invention has a surface silanol group density of 4/nm ² or more as measured by the Sears method. By setting the surface silanol group density of colloidal silica to 4/nm ² or more, it has excellent adhesion to silicon wafers and excellent affinity with water-soluble polymers, resulting in excellent polishing rate and silicon wafer productivity. Excellent for

(Surface silanol group density)
In this specification, the surface silanol group density of colloidal silica shall be measured by the Sears method. Specifically, it shall be measured and calculated under the conditions shown below.
A silica sol corresponding to 1.5 g of silica is collected, and pure water is added to adjust the liquid volume to 90 mL. Add 0.1 mol / L hydrochloric acid aqueous solution until the pH reaches 3.6 in an environment of 25 ° C., add 30 g of sodium chloride, gradually add pure water to completely dissolve sodium chloride, and finally test Pure water is added until the total volume of the liquid reaches 150 mL to obtain a test liquid.
Put the obtained test solution in an automatic titrator, add dropwise 0.1 mol / L sodium hydroxide aqueous solution, pH 4.0 to 9.0 0.1 mol / L sodium hydroxide required Measure the titer A (mL) of the aqueous solution.
Using the following formula (1), the consumption V (mL) of 0.1 mol/L sodium hydroxide aqueous solution required for pH to change from 4.0 to 9.0 per 1.5 g of colloidal silica was calculated. Then, the surface silanol group density ρ (number/nm ² ) of colloidal silica is calculated using the following formula (2).
V=(A×f×100×1.5)/(W×C) (1)
A: Titration volume (mL) of 0.1 mol/L sodium hydroxide aqueous solution required for pH to change from 4.0 to 9.0 per 1.5 g of colloidal silica
f: titer of 0.1 mol / L sodium hydroxide aqueous solution used C: silica concentration in silica sol (% by mass)
W: amount of silica sol collected (g)
ρ=(B×N _A )/(10 ¹⁸ ×M×S _BET ) (2)
B: Amount of sodium hydroxide (mol) required for pH to change from 4.0 to 9.0 per 1.5 g of silica calculated from V
N _A : Avogadro's number (number/mol)
M: amount of silica (1.5 g)
S _BET : Specific surface area (m ² /g) of colloidal silica measured when calculating average primary particle size

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

The surface silanol group density of colloidal silica is preferably 4/nm ² to 15/nm ² , more preferably 5/nm ² to 14/nm ² , and 5.5/nm ² to 13/nm. ² is more preferred. When the surface silanol group density of colloidal silica is 4/nm ² or more, it has excellent adhesion to silicon wafers and excellent affinity with water-soluble polymers, so the polishing rate is excellent and productivity of silicon wafers is improved. Excellent. In addition, when the surface silanol group density of colloidal silica is 15/nm ² or less, the strength of colloidal silica is excellent for an object to be polished typified by a silicon wafer, so the polishing rate is excellent, and the particles in cleaning after polishing are excellent. It is excellent in removability such as.

The surface silanol group density of colloidal silica can be set within a desired range by adjusting the conditions for the hydrolysis reaction/condensation reaction of tetraalkoxysilane. Specifically, the following method can be used.
The so-called sol-gel method, which uses tetraalkoxysilane as the main raw material, hydrolyzes tetraalkoxysilane in the presence of a catalyst such as an acid or base, and dehydrates and condenses the resulting silanol groups to form siloxane bonds while growing particles. to obtain colloidal silica. At that time, the silanol group disappears as the condensation reaction progresses. can increase the density of silanol groups inside and on the surface of colloidal silica. The surface silanol group density can also be increased by subjecting colloidal silica to hydrothermal treatment to hydrate siloxane bonds. On the other hand, the silanol group density inside or on the surface can be lowered by subjecting colloidal silica to a treatment such as firing in a dry gas atmosphere at 600°C to 1250°C.

(Average primary particle size)
The colloidal silica of the present invention preferably has an average primary particle size of 30 nm to 70 nm as measured by the BET method.

In this specification, the average primary particle size of colloidal silica is measured by the BET method. Specifically, the specific surface area of colloidal silica is measured using an automatic specific surface area measuring device, and the density is set to 2.2 g / cm ³ using the following formula (3), and the average primary particle size is calculated. do.
Average primary particle size (nm) = 6000/(specific surface area (m ² /g) x density (g/cm ³ )) (3)

The average primary particle size of colloidal silica is preferably 15 nm to 100 nm, more preferably 20 nm to 90 nm, even more preferably 30 nm to 70 nm. When the average primary particle size of the colloidal silica is 15 nm or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent, and the storage stability of the silica sol is excellent. Further, when the average primary particle size of the colloidal silica is 100 nm or less, the surface roughness and flaws of the object to be polished typified by a silicon wafer during polishing can be reduced, and the dispersion stability of the silica sol is excellent.

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

(Average secondary particle size)
The colloidal silica of the present invention preferably has an average secondary particle size of 60 nm to 120 nm as measured by the DLS method.

In this specification, the average secondary particle size of colloidal silica is measured by the DLS method. Specifically, it is measured using a dynamic light scattering particle size measuring device.

The colloidal silica has an average secondary particle size of preferably 40 nm to 200 nm, more preferably 50 nm to 160 nm, even more preferably 60 nm to 120 nm. When the average secondary particle size of colloidal silica is 40 nm or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent, the removability of particles and the like in cleaning after polishing is excellent, and the storage stability of silica sol is excellent. . In addition, when the average secondary particle diameter of colloidal silica is 200 nm or less, the surface roughness and scratches of the object to be polished typified by a silicon wafer during polishing can be reduced, and the removability of particles and the like in cleaning after polishing is excellent. , excellent dispersion stability of silica sol.

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

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

In this specification, the cv value of colloidal silica is obtained by measuring the average secondary particle size of colloidal silica using a dynamic light scattering particle size measuring device, and calculating the cv value using the following formula (4). do.
cv value = (standard deviation (nm)/average secondary particle size (nm)) x 100 (4)

The association ratio of colloidal silica is preferably 1.2 to 2.5, more preferably 1.5 to 2.2. When the association ratio of colloidal silica is 1.2 or more, the polishing rate for an object to be polished represented by a silicon wafer is excellent, and the productivity of silicon wafers is excellent. In addition, when the association ratio of colloidal silica is 2.5 or less, surface roughness and scratches on the object to be polished, typified by silicon wafers, during polishing can be reduced, and aggregation of colloidal silica in the silica sol and polishing composition can be suppressed. and excellent storage stability of silica sol and polishing composition.

In this specification, the association ratio of colloidal silica is calculated using the following formula (5) from the average primary particle size measured by the above-described measuring method and the average secondary particle size measured by the above-described measuring method. Calculate the association ratio.
Association ratio = average secondary particle size/average primary particle size (5)

The particle shape of colloidal silica is not particularly limited as long as the surface silanol group density is 4/nm ² or more. For example, wart-shaped, curved, branched, etc.) and the like. Among these colloidal silica particle shapes, a spherical shape is preferable when it is desired to reduce the surface roughness and scratches of an object to be polished such as a silicon wafer during polishing, and the polishing rate for an object to be polished such as a silicon wafer. When it is desired to further increase , the irregular shape is preferable.

The colloidal silica of the present invention uses tetraalkoxysilane as a main raw material.
In the present specification, the main raw material means 50% by mass or more of the raw materials constituting colloidal silica.

(Method for producing colloidal silica)
The colloidal silica of the present invention is obtained by hydrolyzing/condensing tetraalkoxysilane as a main raw material. A known production method may be used for the production method of the colloidal silica of the present invention.

Examples of tetraalkoxysilanes include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetraisopropoxysilane, and the like. These tetraalkoxysilanes may be used alone or in combination of two or more. Among these tetraalkoxysilanes, tetramethoxysilane and tetraethoxysilane are preferred because they undergo rapid hydrolysis, do not easily leave unreacted substances, are excellent in productivity, and can easily yield a stable silica sol. Methoxysilane is more preferred.

As a raw material constituting colloidal silica, a low condensate obtained by partially hydrolyzing tetraalkoxysilane may be used in addition to tetraalkoxysilane.

Examples of the solvent/dispersion medium used in the hydrolysis reaction/condensation reaction include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. One of these solvents and dispersion media may be used alone, or two or more thereof may be used in combination. Among these solvents/dispersion media, water and alcohol are preferred, and water and methanol are more preferred, since the one used in the hydrolysis reaction/condensation reaction and the by-product are the same and are excellent in terms of convenience in production. .

The hydrolysis/condensation reaction may be carried out in the presence of a catalyst or in the absence of a catalyst, but the presence of a catalyst is preferred because the hydrolysis/condensation reaction can be promoted.
Examples of catalysts include acid catalysts such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid, formic acid, and citric acid, and alkalis such as ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. A catalyst etc. are mentioned. Among these catalysts, alkali catalysts are preferable because they have excellent catalytic action and are easy to control the particle shape, they can suppress the contamination of metal impurities, and they are highly volatile and excellent in removability after the condensation reaction. , alkali catalysts are preferred, and ammonia is more preferred.

(silica sol)
The silica sol of the invention contains the colloidal silica of the invention and water.

The content of colloidal silica is preferably 3% by mass to 50% by mass, more preferably 4% by mass to 40% by mass, and even more preferably 5% by mass to 30% by mass, based on 100% by mass of the total silica sol. When the content of colloidal silica is 3% by mass or more, the polishing rate for an object to be polished typified by a silicon wafer is excellent. Further, when the colloidal silica content is 50% by mass or less, aggregation of the colloidal silica in the silica sol or the polishing composition can be suppressed, and the storage stability of the silica sol or the polishing composition is excellent.

The content of water is preferably 50% by mass to 97% by mass, more preferably 60% by mass to 96% by mass, and even more preferably 70% by mass to 95% by mass, based on 100% by mass of the total amount of silica sol. When the water content is 50% by mass or more, aggregation of colloidal silica in the silica sol or polishing composition can be suppressed, and the storage stability of the silica sol or polishing composition is excellent. Further, when the water content is 97% by mass or less, the polishing rate for an object to be polished represented by a silicon wafer is excellent.

Examples of water include ion-exchanged water, distilled water, pure water, and ultrapure water.

In addition to colloidal silica and water, the silica sol of the present invention optionally contains oxidizing agents, preservatives, antifungal agents, pH adjusters, pH buffers, surfactants, and chelating agents, as long as the performance of the silica sol is not impaired. , other ingredients such as antibacterial and biocidal agents.
In particular, since the silica sol has excellent storage stability, it is preferable to include an antibacterial/biocide in the silica sol.

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

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

The silica sol of the present invention preferably has a metal impurity content of 1 ppm or less for the following reasons.
In the polishing of silicon wafers for semiconductor devices, metallic impurities adhering to and contaminating the surface of the object to be polished adversely affect the characteristics of the wafer and diffuse into the inside of the wafer, degrading quality. The performance of the manufactured semiconductor device is significantly degraded.
In addition, the silica sol of the present invention can achieve an excellent polishing rate for an object to be polished typified by a silicon wafer by optimizing the surface silanol group density of colloidal silica. When metal impurities are present in the silica sol, coordinated interactions occur between the surface silanol groups showing acidity and the metal impurities, changing the chemical properties (acidity, etc.) of the surface silanol groups, changes the three-dimensional environment (e.g., the ease with which colloidal silica agglomerates) and affects the polishing rate.

In this specification, the content of metal impurities in silica sol is measured by high frequency inductively coupled plasma mass spectrometry (ICP-MS). Specifically, 2 g of silica sol is accurately weighed, sulfuric acid and hydrofluoric acid are added, heated, dissolved and evaporated, and pure water is added to the remaining sulfuric acid droplets so that the total amount is exactly 10 g to create a test solution. shall be measured using a high frequency inductively coupled plasma mass spectrometer. The target metals are sodium, potassium, iron, aluminum, calcium, magnesium, zinc, cobalt, chromium, copper, manganese, lead, titanium, and silver.

The metal impurity content of the silica sol can be reduced to 1 ppm or less by performing a hydrolysis reaction/condensation reaction (sol-gel method) using tetraalkoxysilane as a main raw material.
In the method of deionizing alkali silicate such as water glass, it is difficult to reduce the metal impurity content of the silica sol to 1 ppm or less because sodium and the like derived from raw materials remain.

(Method for producing silica sol)
The silica sol of the present invention can be obtained by removing unnecessary components and adding necessary components from among the components in the reaction solution immediately after colloidal silica production. A known production method may be used as the method for producing the silica sol of the present invention.

In the silica sol of the present invention, components other than colloidal silica and water may remain in the reaction solution immediately after production of colloidal silica as long as the performance is not impaired. Examples thereof include solvents/dispersion media such as alcohols, catalysts such as ammonia, and the like.

The content of colloidal silica and the content of water in the silica sol can be adjusted by adding water after removing unnecessary components in the reaction solution immediately after production of colloidal silica.

In the production of silica sol, a filtration step may be included in order to remove coarse particles or avoid aggregation due to fine particles.
Filtration methods include, for example, natural filtration under normal pressure, reduced pressure filtration, pressure filtration, centrifugal filtration and the like.
Filtration may be performed at any time and any number of times in the production of silica sol, but is preferably performed immediately before preparation of the polishing composition because the polishing composition has excellent storage stability and polishing performance.

The pH of the silica sol is preferably 6.0-9.0, more preferably 7.0-8.0. When the pH of the silica sol is 6.0 or more, the long-term storage stability of the silica sol is excellent. Further, when the pH of the silica sol is 9.0 or less, aggregation of colloidal silica can be suppressed, and the silica sol has excellent dispersion stability.

(Polishing composition)
The polishing composition of the invention contains the silica sol of the invention and a water-soluble polymer.
A water-soluble polymer enhances the wettability of the polishing composition to an object to be polished, typified by a silicon wafer. The water-soluble polymer is preferably a polymer having a functional group with high water affinity. Colloidal silica and water-soluble polymer are stably dispersed in the vicinity of each other. Therefore, the effects of colloidal silica and the water-soluble polymer work synergistically when polishing an object to be polished, typified by a silicon wafer.

Examples of water-soluble polymers include cellulose derivatives, polyvinyl alcohol, polyvinylpyrrolidone, copolymers having a polyvinylpyrrolidone skeleton, and polymers having a polyoxyalkylene structure.
Examples of cellulose derivatives include hydroxyethylcellulose, hydrolyzed hydroxyethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose, methylcellulose, ethylcellulose, ethylhydroxyethylcellulose, carboxymethylcellulose and the like.
Examples of copolymers having a polyvinylpyrrolidone skeleton include graft copolymers of polyvinyl alcohol and polyvinylpyrrolidone.
Examples of polymers having a polyoxyalkylene structure include polyoxyethylene, polyoxypropylene, copolymers of ethylene oxide and propylene oxide, and the like.
These water-soluble polymers may be used singly or in combination of two or more. Among these water-soluble polymers, cellulose derivatives are preferred because they have a high affinity with the surface silanol groups of colloidal silica and act synergistically to impart good hydrophilicity to the surface of the object to be polished, and hydroxyethyl cellulose. is more preferred.

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

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

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

The pH of the polishing composition is preferably 8.0 to 12.0, more preferably 9.0 to 11.0. When the pH of the polishing composition is 8.0 or more, aggregation of colloidal silica in the polishing composition can be suppressed, and the dispersion stability of the polishing composition is excellent. Moreover, when the pH of the polishing composition is 12.0 or less, dissolution of colloidal silica can be suppressed, and the stability of the polishing composition is excellent.
The pH of the polishing composition can be set within a desired range by adding a pH adjuster, which will be described later.

In addition to the silica sol and the water-soluble polymer, the polishing composition of the present invention may optionally contain a basic compound, a polishing accelerator, a surfactant, a hydrophilic compound, a preservative, and an antiseptic within a range that does not impair its performance. Other ingredients such as fungicides, pH adjusters, pH buffers, surfactants, chelating agents, antibacterials and biocides may also be included.
In particular, chemical polishing (chemical etching) can be performed by giving a chemical action to the surface of an object to be polished, typified by a silicon wafer. It is preferable to include a basic compound in the polishing composition because it can improve the polishing rate of the polishing body.

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

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

The polishing composition of the present invention is obtained by mixing the silica sol of the present invention, a water-soluble polymer, and, if necessary, other components. , may be diluted with water or the like immediately before polishing.

(Application)
The colloidal silica, silica sol, and polishing composition of the present invention can be suitably used for polishing purposes. It can be used for polishing (chemical-mechanical polishing) in the planarization process, polishing synthetic quartz glass substrates used for photomasks and liquid crystals, polishing magnetic disk substrates, etc. Since it has an excellent affinity for silicon surfaces, silicon It can be used particularly preferably for polishing wafers.

By polishing a silicon wafer using the colloidal silica, silica sol, and polishing composition of the present invention, adhesion and contamination of the silicon wafer by metal impurities can be suppressed, and the production time can be shortened. Excellent productivity of pure silicon wafers.

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 unless it departs from the gist thereof.

(Measurement of average primary particle size)
The silica sol obtained in Examples and Comparative Examples was freeze-dried, and the specific surface area of colloidal silica was measured using an automatic specific surface area measuring device "Flowsorb II" (model name, manufactured by Shimadzu Corporation), and the following formula ( 3) was used to calculate the average primary particle size with a density of 2.2 g/cm ³ .
Average primary particle size (nm) = 6000/(specific surface area (m ² /g) x density (g/cm ³ )) (3)

(Measurement of average secondary particle diameter and cv value)
For the silica sol obtained in Examples and Comparative Examples, the average secondary particle size of colloidal silica was measured using a dynamic light scattering particle size measuring device "Zetasizer Nano ZS" (model name, manufactured by Malvern), The cv value was calculated using the following formula (4).
cv value = (standard deviation (nm)/average secondary particle size (nm)) x 100 (4)

(Calculation of association ratio)
From the measured average primary particle size and average secondary particle size, the association ratio was calculated using the following formula (5).
Association ratio = average secondary particle size/average primary particle size (5)

(Surface silanol group density)
An amount corresponding to 1.5 g of silica of the silica sol obtained in Examples and Comparative Examples was collected in a 200 mL tall beaker, and pure water was added to adjust the liquid volume to 90 mL.
In an environment of 25° C., the pH electrode was inserted into a tall beaker, and the test solution was stirred for 5 minutes with a magnetic stirrer. While stirring with a magnetic stirrer was continued, a 0.1 mol/L hydrochloric acid aqueous solution was added until the pH reached 3.6. Remove the pH electrode from the tall beaker, add 30 g of sodium chloride while continuing to stir with a magnetic stirrer, and gradually add pure water to completely dissolve the sodium chloride until the total volume of the test solution reaches 150 mL. Pure water was added to the temperature, 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 is set in an automatic titrator "COM-1600" (manufactured by Hiranuma Sangyo Co., Ltd.), the pH electrode and burette attached to the device are inserted into the tall beaker, and the test is performed with a magnetic stirrer. While stirring the liquid, 0.1 mol / L sodium hydroxide aqueous solution is added dropwise through a burette, and the titration amount of 0.1 mol / L sodium hydroxide aqueous solution required to change the pH from 4.0 to 9.0 A (mL) was measured.
Using the following formula (1), the consumption V (mL) of 0.1 mol/L sodium hydroxide aqueous solution required for pH to change from 4.0 to 9.0 per 1.5 g of colloidal silica was calculated. Then, the surface silanol group density ρ (number/nm ² ) of colloidal silica was calculated using the following formula (2).
V=(A×f×100×1.5)/(W×C) (1)
A: Titration volume (mL) of 0.1 mol/L sodium hydroxide aqueous solution required for pH to change from 4.0 to 9.0 per 1.5 g of colloidal silica
f: titer of 0.1 mol / L sodium hydroxide aqueous solution used C: colloidal silica concentration in silica sol (% by mass)
W: amount of silica sol collected (g)
ρ=(B×N _A )/(10 ¹⁸ ×M×S _BET ) (2)
B: Amount of sodium hydroxide (mol) required for pH to change from 4.0 to 9.0 per 1.5 g of silica calculated from V
N _A : Avogadro's number (number/mol)
M: amount of silica (1.5 g)
S _BET : Specific surface area (m ² /g) of colloidal silica measured when calculating average primary particle size

(polishing rate)
To 97.5 g of the silica sol obtained in Examples and Comparative Examples, 3 g of 29% by mass aqueous ammonia was added and mixed to obtain a silica sol having a pH of 10.7. To the resulting silica sol having a pH of 10.7, 40 g of a 1.4% by mass hydroxyethyl cellulose aqueous solution (“SE550” (trade name), manufactured by Daicel Corporation, mass average molecular weight of about 800,000) was added and mixed, and then pure water was added. 62.5 g was added and mixed, and filtered through a 1 μm membrane filter to obtain a polishing composition with a pH of 10.5.

The resulting polishing composition was diluted 30 times with pure water, and a silicon chip (p-type, crystal orientation <100>) cut into a 3 cm square was polished under the following polishing conditions.
Polishing device: Doctor lap "ML-180" (model name, manufactured by Maruto Co., Ltd.) and simple drive arm "MLK-250" (model name, manufactured by Maruto Co., Ltd.)
Load: 200g/ ^cm2
Surface plate rotation speed: 150 rpm
Wafer holder rotation speed: 150 rpm
Polishing pad: Suede type Supply rate of polishing composition: 30 mL/min Polishing time: 1 hr
The relative polishing rate of silica sol obtained in Examples was calculated using the following formula (6).
Relative polishing rate = (difference in mass before and after polishing of silicon chip of Example (g)/polishing time (hour))/(difference in mass before and after polishing of silicon chip of Comparative Example 1 (g)/time of polishing (hour)) ... (6)

(Metal impurity content)
Accurately weigh 2 g of the silica sol obtained in Example 2, add sulfuric acid and hydrofluoric acid, heat, dissolve and evaporate, add pure water to the remaining sulfuric acid droplets so that the total amount is exactly 10 g, and prepare a test solution. was prepared, and the metal impurity content was measured using a high-frequency inductively coupled plasma mass spectrometer "ELEMENT2" (model name, manufactured by Thermo Fisher Scientific).
The metal impurity content is 0.290 ppm sodium, 0.020 ppm potassium, 0.002 ppm iron, 0.013 ppm aluminum, 0.024 ppm calcium, 0.001 ppm magnesium, 0.026 ppm zinc, cobalt, All of chromium, copper, manganese, lead, titanium and silver were less than 0.001 ppm.

[Example 1]
A raw material solution was prepared by mixing tetramethoxysilane and methanol at a volume ratio of 3:1. A reaction solvent prepared by mixing methanol, pure water, and ammonia in advance was charged in 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 32% by mass, and the concentration of ammonia in the reaction solvent was 2% by mass.
While maintaining the temperature of the reaction solvent at 30° C., the ratio of the reaction solvent to the raw material solution was adjusted to 2.3:1 (volume ratio), and the raw material solution was added dropwise to the reaction tank at a uniform rate for 180 minutes to obtain silica sol. The resulting silica sol was heated to remove methanol and ammonia while adjusting the liquid volume by adding pure water so that the content of colloidal silica was about 20% by mass. A silica sol of about 20% by weight was obtained.
Table 1 shows the evaluation results of the obtained silica sol.

[Example 2]
Example 1 except that the concentration of water in the reaction solvent was 26% by mass, the concentration of ammonia in the reaction solvent was 1.9% by mass, and the ratio of the reaction solvent to the raw material solution was 2.5:1 (volume ratio). A silica sol was obtained by performing the same operation.
Table 1 shows the evaluation results of the obtained silica sol.

[Example 3]
The concentration of water in the reaction solvent was 26% by mass, the concentration of ammonia in the reaction solvent was 1.7% by mass, and the temperature of the reaction solvent was maintained at 28°C. A silica sol was obtained in the same manner as in Example 1 except that (volume ratio) was used.
Table 1 shows the evaluation results of the obtained silica sol.

[Example 4]
Example 1 except that the concentration of water in the reaction solvent was 26% by mass, the concentration of ammonia in the reaction solvent was 1.7% by mass, and the ratio of the reaction solvent to the raw material solution was 2.5:1 (volume ratio). A silica sol was obtained by performing the same operation.
Table 1 shows the evaluation results of the obtained silica sol.

[Example 5]
The concentration of water in the reaction solvent was 32% by mass, the concentration of ammonia in the reaction solvent was 1.5% by mass, and the temperature of the reaction solvent was maintained at 33°C. A silica sol was obtained in the same manner as in Example 1 except that (volume ratio) was used.
Table 1 shows the evaluation results of the obtained silica sol.

[Example 6]
Example 1 except that the concentration of water in the reaction solvent was 35% by mass, the concentration of ammonia in the reaction solvent was 1.9% by mass, and the ratio of the reaction solvent to the raw material solution was 2.4:1 (volume ratio). A silica sol was obtained by performing the same operation.
Table 1 shows the evaluation results of the obtained silica sol.

[Example 7]
Example 1 except that the concentration of water in the reaction solvent was 26% by mass, the concentration of ammonia in the reaction solvent was 1.8% by mass, and the ratio of the reaction solvent to the raw material solution was 2.5:1 (volume ratio). A silica sol was obtained by performing the same operation.
Table 1 shows the evaluation results of the obtained silica sol.

[Comparative Example 1]
Ultra-pure colloidal silica “PL-7” (trade name, manufactured by Fuso Chemical Industry Co., Ltd.) was used as silica sol.
Table 1 shows the evaluation results of the obtained silica sol.

As can be seen from Table 1, the silica sols obtained in Examples 1 to 7 having a high surface silanol group density had excellent polishing rates compared to the silica sol obtained in Comparative Example 1 having a low surface silanol group density.

The colloidal silica, silica sol, and polishing composition of the present invention can be suitably used for polishing purposes. It can be used for polishing (chemical-mechanical polishing) in the planarization process, polishing synthetic quartz glass substrates used for photomasks and liquid crystals, polishing magnetic disk substrates, etc. Since it has an excellent affinity for silicon surfaces, silicon It can be used particularly preferably for polishing wafers.

Claims (9)

A silica sol for polishing silicon wafers,
containing colloidal silica and water,
The metal impurity content is 1 ppm or less,
The surface silanol group density of the colloidal silica measured by the Sears method is 5/nm ² to 14/nm ² ,
A silica sol , wherein the colloidal silica has a cv value of 15-50. The silica sol according to claim 1, wherein the colloidal silica has an average primary particle size of 15 nm to 100 nm as measured by the BET method. 3. The silica sol according to claim 1, wherein the colloidal silica has an average secondary particle size of 40 nm to 200 nm as measured by the DLS method. The silica sol according to any one of claims 1 to 3 , wherein the colloidal silica content is 3% by mass to 50% by mass in 100% by mass of the total amount of the silica sol. A polishing composition comprising the silica sol according to any one of claims 1 to 4 and a water-soluble polymer. A method for polishing a silicon wafer, comprising polishing a silicon wafer using the polishing composition according to claim 5 . A method for manufacturing a silicon wafer, comprising the method for polishing a silicon wafer according to claim 6 . A chemical mechanical polishing composition comprising the silica sol according to any one of claims 1 to 4 . A method for manufacturing a semiconductor device, comprising the step of performing chemical mechanical polishing using the chemical mechanical polishing composition according to claim 8 .