Polishing composition having excellent storage stability and method for producing same

Abstract

[Problem] To provide a polishing composition for use in the polishing of silicon wafers and with which it is possible to achieve a high polishing rate and improve storage stability. [Solution] This polishing composition contains an aqueous silica particle dispersion, wherein the following Rsp value derived from the relaxation times obtained by pulsed NMR measurement of the aqueous silica particle dispersion and pure water is at least 0.01 but less than 0.15: Rsp=(Rav-Rb) / (Rb). Rsp is an indicator that indicates the affinity of the silica particles with water, Rav is the reciprocal of the relaxation time of an aqueous silica particle dispersion having a silica concentration of 5 mass%, and Rb is the reciprocal of the relaxation time of the pure water.

Description

Polishing composition with excellent storage stability and method for producing the same

[0001] The present invention relates to a polishing composition using silica-based abrasive grains and a method for producing the same.

[0002] Polishing compositions using silica-based abrasive grains are used in polishing silicon wafers. Polishing compositions generally contain silica-based abrasive grains in an aqueous medium such as water, and optionally contain alkaline compounds, water-soluble compounds, chelating agents, oxidizing agents, metal corrosion inhibitors, etc. Aiming to shorten polishing time by achieving high polishing rates, polishing compositions have been disclosed that are defined using a function indicating water affinity calculated from the relationship between the inverse of the pulse NMR relaxation time in a dispersed state of silica abrasive grains and the total surface area of ​​the silica abrasive grains (see Patent Document 1). Also disclosed is a polishing composition that defines the relationship between the BET specific surface area of ​​silica particles contained as abrasive grains and the specific surface area measured by pulse NMR (see Patent Document 2). Furthermore, polishing compositions have been disclosed in which the NMR relaxation time is evaluated using the solvent affinity of the abrasive grains (see Patent Documents 3 and 4). A polishing composition has been disclosed in which the pulse NMR measurement value Rsp=(Rav-Rb) / (Rb) of a dispersion of silica particles is 0.15 to 0.7, and the shape factor SF1 of the silica particles is 1.20 to 1.80, whereby the polishing rate is improved and the polished surface is improved (see Patent Document 5).

[0003] International Publication No. WO 2018 / 116890 International Publication No. WO 2015 / 152151 JP 2017-117894 Publication No. WO 2018 / 012174 International Publication No. WO 2020 / 091000

[0004] The present invention addresses the problem of providing a polishing composition that achieves a high removal rate when used to polish silicon wafers and has improved storage stability during storage or transportation. The inventors focused on the amount of water molecules bound to the silica particle surface, using the Rsp value derived from the relaxation time obtained by pulse NMR measurement as an indicator, and found that by using silica particles with an Rsp value within a predetermined range as abrasive grains, the storage stability of the polishing composition can be improved while maintaining high polishing performance.

[0005] The present invention provides, as a first aspect, a polishing composition containing an aqueous dispersion of silica particles, wherein the polishing composition has the following Rsp value derived from relaxation times obtained by pulse NMR measurement of the aqueous dispersion of silica particles and pure water: Rsp=(Rav-Rb) / (Rb), where Rav is the reciprocal of the relaxation time of an aqueous dispersion of silica particles having a silica concentration of 5 mass %, and Rb is the reciprocal of the relaxation time of pure water. The present invention provides, as a second aspect, the polishing composition according to the first aspect, wherein the silica particles have an average secondary particle diameter of 40 to 200 nm as measured by a dynamic light scattering method and an average primary particle diameter of 20 to 100 nm as measured by a nitrogen gas adsorption method. As a third aspect, the polishing composition according to the first aspect, wherein a treatment liquid obtained by contacting the silica particle aqueous dispersion with a cation exchange resin, an anion exchange resin, and a cation exchange resin in this order to perform ion exchange is frozen at −70° C. to −80° C., and the treatment liquid is further freeze-dried at room temperature under a pressure of 5 Pa or less to remove the dispersing medium, and the silica particle powder obtained has a silanol group amount of 0.1 to 1.5 mmol / g as calculated by the following formula from the mass loss when heated from room temperature to 700° C. and the molecular weight of water molecules in thermogravimetric analysis: silanol group amount (mmol / g)=2×(M2−M1)÷M Ｈ２Ｏ ÷ (M0-M1) × 1000 where M0 is the mass of the silica particle powder subjected to thermogravimetric analysis before heating, M1 is the mass loss amount when the temperature reached 200°C, M2 is the mass loss amount when the temperature reached 700°C, and M Ｈ２Ｏ and represent the molecular weight of a water molecule, respectively. As a fourth aspect, a treatment liquid obtained by contacting the silica particles with a cation exchange resin, an anion exchange resin, and a cation exchange resin in this order to perform ion exchange is frozen at −70° C. to −80° C., and then freeze-dried at room temperature under a pressure of 5 Pa or less to remove the dispersant. The resulting silica particle powder has a density of 2.20 to 2.35 g / cm as measured by dry density measurement. ３a polishing composition according to the first aspect, wherein the silica particle aqueous dispersion has been subjected to a thermal history at a temperature of 140° C. or higher and lower than 260° C.; a polishing composition according to any one of the first to fourth aspects, wherein the silica particle aqueous dispersion has a concentration of 1 to 40 mass %; a polishing composition according to any one of the first to fourth aspects, wherein the silica particle aqueous dispersion has a concentration of 1 to 40 mass %; a polishing composition according to any one of the first to sixth aspects, wherein the polishing composition further contains at least one additive selected from the group consisting of an acidic compound, a basic compound, a water-soluble compound, a chelating agent, an oxidizing agent, and a metal corrosion inhibitor; an eighth aspect, the polishing composition according to the seventh aspect, wherein the basic compound is at least one basic compound selected from the group consisting of an alkali metal hydroxide, an alkali metal carbonate, an alkali metal hydrogencarbonate, and a nitrogen-containing basic compound; and a ninth aspect, the polishing composition according to the seventh aspect, wherein the chelating agent is an aminocarboxylic acid chelating agent or a phosphonic acid chelating agent. As a tenth aspect, there is provided a polishing composition according to any one of the first to ninth aspects, wherein the polishing composition has a pH of 1 to 12. As an eleventh aspect, there is provided a polishing composition according to any one of the first to tenth aspects, wherein, in a test in which the polishing composition is heated and stored at 50°C for 28 days, the rate of increase in the average secondary particle diameter of the polishing composition after heat storage at 50°C for 28 days, as measured by dynamic light scattering, is less than 5% relative to the average secondary particle diameter of the polishing composition before heat storage at 50°C for 28 days, as measured by dynamic light scattering. As a twelfth aspect, there is provided a polishing composition according to any one of the first to eleventh aspects, wherein the polishing composition is used for polishing a silicon wafer or a device wafer. As a thirteenth aspect, there is provided a method for producing a polishing composition according to any one of the first to twelfth aspects, comprising: preparing a colloidal silica dispersion as a precursor; and heating the precursor at a temperature of 140°C or higher but lower than 260°C to obtain the aqueous dispersion of silica particles.

[0006] The present invention focuses on the amount of water molecules bound to the silica particle surface, using the Rsp value derived from the relaxation time obtained by pulse NMR measurement of a dispersion of silica particles dispersed in an aqueous medium used in a polishing composition and pure water, as an indicator, and has discovered that a specific Rsp value can achieve a high polishing rate and improve the storage stability of the polishing composition. Since a high polishing rate (speed) leads to improved wafer production efficiency, a polishing composition is required to achieve a high polishing rate. Furthermore, if the storage stability of the silica particles contained in the polishing composition is poor, the silica particles will aggregate during storage or transportation, generating micro-foreign matter, which can cause a decrease in the polishing rate and the occurrence or increase of defects during polishing, so high storage stability is also required.

[0007] Polishing compositions contain silica particles as abrasive grains. The interaction between the particle surface and the aqueous medium in the polishing composition changes depending on the surface condition of the silica particles, and this change in interaction affects the removal rate and storage stability of the polishing composition. Water present on or around the surface of silica particles used as abrasive grains in polishing compositions is classified into bound water and free water depending on its state. Free water refers to water that exists around the silica particles but exists in a free state without interacting with the silica particles. On the other hand, bound water refers to water that is hydrogen-bonded to the silica particles via silanol groups on the surface of the silica particles. This bound water plays an important role in ensuring good contact between the silica particles and water.

[0008] The state (behavior) of water molecules can be determined from the relaxation time of water molecule protons measured by pulsed NMR. The relaxation mechanisms in nuclear magnetic resonance (NMR) include the release of absorbed energy and the dephasing of the precession of aligned nuclear spins. The former is called spin-lattice relaxation (longitudinal relaxation), with its relaxation time T1, while the latter is called spin-spin relaxation (transverse relaxation), with its relaxation time T2. T1 relaxation is most likely to occur when the speed of molecular motion and the resonant frequency are similar. For example, free water molecules not in contact with silica particles (free water) and water molecules in contact with silica particles (bound water) have different response times to magnetic field changes, i.e., different relaxation times. In this case, a shorter relaxation time indicates a larger particle surface area in contact with water and higher particle dispersibility. Furthermore, T2 relaxation occurs due to magnetic interactions. Measurement of relaxation time by pulsed NMR can be considered a measurement method that utilizes the difference in T2 relaxation. The presence of bound water within a specific range effectively improves polishing speed and storage stability during polishing.

[0009] In the present invention, by highly controlling the affinity between silica particles as abrasive grains and the aqueous medium in the polishing composition, it is possible to achieve both improved polishing rate and improved storage stability, particularly in polishing silicon wafers.

[0010] The present invention will be described in detail below. In the present invention, the aqueous dispersion of silica particles is sometimes referred to as a colloidal silica dispersion or a silica sol, and the silica particles are sometimes referred to as colloidal silica.

[0011] <Rsp Value> The present invention relates to a polishing composition containing an aqueous dispersion of silica particles, wherein the polishing composition has the following Rsp value, derived from the relaxation times obtained by pulse NMR measurement of the aqueous dispersion of silica particles and pure water, of 0.01 or more and less than 0.15: Rsp = (Rav - Rb) / (Rb). The Rsp value is an index indicating the affinity of silica particles for an aqueous medium (water), where Rav is the reciprocal of the relaxation time of an aqueous dispersion of silica particles having a silica concentration of 5% by mass, and Rb is the reciprocal of the relaxation time of pure water. The Rsp value can be, for example, 0.03 to less than 0.15, 0.05 to less than 0.15, 0.01 to 0.14, 0.03 to 0.14, 0.05 to 0.14, or 0.07 to 0.14. When the Rsp value is 0.01 or more, the silica particles have water affinity, allowing them to maintain dispersibility in the polishing composition. In addition, when Rsp is less than 0.15, the hydroxyl groups (OH) of water molecules bound near the surface of the silica particles can be easily removed. － ) and can suppress excessive dissolution of the silica particle surfaces due to the hydroxy groups when the polishing composition is prepared. As a result, aggregation of the silica particles in an aqueous medium (water) is suppressed, and when a polishing composition containing the silica particles is used for polishing a silicon wafer, the polishing rate is improved and the storage stability of the polishing composition is also improved.

[0012] The measurement principle of this technique (pulse NMR) is based on the fact that solvent molecules in contact with or adsorbed to particle surfaces and solvent molecules in the solvent bulk (solvent molecules in a free state not in contact with the particle surface) respond differently to changes in magnetic field. When a change in magnetic field is applied to liquid molecules, energy transfer occurs as they transition from an excited state to a ground state. Generally, energy transfer occurs via the particles for liquid molecules (solvent molecules) adsorbed to particle surfaces, while energy transfer is less likely to occur for liquid molecules (solvent molecules) in the bulk liquid because there is no intermediary between them. As a result, the NMR relaxation time of liquid molecules (solvent molecules) adsorbed to particle surfaces is shorter than the relaxation time of molecules in the bulk liquid.

[0013] Furthermore, the relaxation time measured for a silica particle aqueous dispersion is the average value of two relaxation times reflecting the liquid volume concentration on the silica particle surface and the liquid volume concentration in the free state (liquid in the bulk liquid but not adsorbed to the particle surface). Here, the relaxation time constant R is the reciprocal of the relaxation time T, and is calculated by Rav = (Ps * Rs) + (Pb * Rb). Rav: average relaxation time constant, i.e., the reciprocal of the relaxation time of the silica aqueous dispersion. Ps: volume concentration of the liquid on the particle surface, i.e., the volume concentration of the silica aqueous dispersion. Rs: relaxation time constant of the liquid molecules adsorbed to the particle surface, i.e., the reciprocal of the relaxation time of the liquid molecules adsorbed to the particle surface. Pb: volume concentration of the bulk liquid, i.e., the volume concentration of a blank aqueous solution (i.e., pure water) excluding silica particles in the silica aqueous dispersion. Rb: relaxation time constant of the bulk liquid molecules, i.e., the reciprocal of the relaxation time of pure water.

[0014] The pulse NMR measurement value of the silica particle aqueous dispersion is calculated as Rsp = (Rav - Rb) / (Rb). Rav and Rb are the reciprocals of the relaxation times (transverse relaxation time T2, specifically, the NMR relaxation time after dispersing the silica abrasive grains, and the NMR relaxation time of pure water) measured using a pulse NMR device (trade name) Acorn Area manufactured by Xigo nanotools. The measurement conditions are: magnetic field: 0.3 T, measurement frequency: 13 MHz, measurement nucleus: １ H NMR, measurement method: CPMG pulse sequence method, sample volume: 0.77 mL, temperature: 30°C. Rsp is an index of the affinity of the particle surface. Note that, when the specific surface area is the same, the larger this value (Rsp) is, the higher the hydrophilicity is.

[0015] <Particle diameter (average primary particle diameter, average secondary particle diameter)> The average primary particle diameter of the colloidal silica dispersion refers to the average primary particle diameter of silica particles, which are the dispersoid. In the present invention, the average primary particle diameter of the colloidal silica dispersion (silica particles) refers to the particle diameter converted from the specific surface area measured by the nitrogen gas adsorption method (BET method), unless otherwise specified. The specific surface area diameter (average particle diameter (specific surface area diameter) D (nm)) measured by the nitrogen gas adsorption method (BET method) is calculated based on the specific surface area S (m２ / g), the average primary particle diameter is given by the formula D(nm) = 2720 / S. In the present invention, the average primary particle diameter of the silica particles is, for example, 20 to 100 nm, and can also be, for example, 20 to 85 nm, 30 to 70 nm, or 35 to 60 nm. When the average primary particle diameter of the silica particles is 20 nm or more, the silica particles come into contact with both the silicon wafer and the polishing pad when used to polish a silicon wafer, thereby improving the polishing rate. Furthermore, the movement distance of the silica particles due to Brownian motion can be prevented from becoming excessively large, thereby reducing the frequency of contact between silica particles in the polishing composition and improving storage stability. When the average primary particle diameter of the silica particles is 100 nm or less, the occurrence of defects due to scratches caused by rubbing the silica particles against the silicon wafer surface when used to polish a silicon wafer can be reduced. Furthermore, the silica particles settle in the polishing composition, preventing adhesion of the silica particles to each other at the bottom of the storage container, thereby improving the storage stability of the polishing composition.

[0016] The silica particles in the silica particle aqueous dispersion or the polishing composition can be measured by dynamic light scattering to determine their average secondary particle diameter and their dispersion state (whether the silica particles are dispersed or aggregated). The average secondary particle diameter represents the average value of secondary particle diameters (dispersed particle diameters), and the average secondary particle diameter in a completely dispersed state is said to be approximately 1.5 to 2 times the average primary particle diameter (a specific surface area diameter obtained by nitrogen gas adsorption (BET) method) or Sears method, and represents the average value of primary particle diameters). The larger the value of the average secondary particle diameter / average primary particle diameter, the more likely it is that the silica particles in the medium are in an aggregated state. In the present invention, the average secondary particle diameter of the silica particles, i.e., for example, colloidal silica dispersion (colloidal silica particles), measured by dynamic light scattering can be, for example, 40 to 200 nm, 40 to 150 nm, 40 to 100 nm, or 50 to 80 nm. By having the average secondary particle diameter within the above range, aggregation of the silica particles in the polishing composition can be suppressed. As a result, the polishing rate when used for polishing silicon wafers can be further improved, and the storage stability of the polishing composition can be improved. In the present invention, the average secondary particle size of the silica particles measured by dynamic light scattering method is in accordance with ISO 22412:2017, and is measured using a Zetasizer Nano (trade name, manufactured by Malvern Panalytical).

[0017] <Polyvalent Metal Impurities> The polishing composition containing the silica particle aqueous dispersion of the present invention has a polyvalent metal impurity content of 1 to 100 ppm / SiO ２ , 1~50ppm / SiO ２ , or 1 to 40 ppm / SiO ２When the amount of polyvalent metal impurities contained in the polishing composition is within the above range, it is possible to suppress changes in the pH or electrolyte concentration of the polishing composition due to elution of the polyvalent metal impurities, or deterioration in the dispersion stability of the silica particles due to the cationized polyvalent metal in the polishing composition acting as an aggregating agent for the silica particles. Furthermore, when the amount of polyvalent metal impurities contained in the silica particles is within the above range, the network of Si—O—Si bonds that constitutes the skeleton of the silica particles becomes dense, and excessive dissolution of the silica particle surface by hydroxy groups can be suppressed. As a result, it is possible to further improve the removal rate when used to polish silicon wafers, and to improve the storage stability of the polishing composition.

[0018] Examples of the polyvalent metal impurities include iron, aluminum, calcium, magnesium, titanium, zirconium, copper, nickel, chromium, zinc, and lead. The amount of polyvalent metal impurities can be analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectrometry (AA), or the like after pretreating the polishing composition and completely dissolving the silica particles contained in the composition. The pretreatment for dissolving the silica particles can be carried out, for example, by the following steps: 36 mL of a 38% by weight aqueous solution of hydrofluoric acid, 6 mL of a 61% by weight nitric acid, and 0.3 mL of a 96% by weight sulfuric acid are added to 1 g of silica particles contained in the polishing composition, and the resulting mixed solution is treated at 210°C for 15 minutes using a microwave sample pretreatment device to dissolve the silica particles, and the decomposed solution is then recovered. The recovered decomposed solution is heated at 120°C under atmospheric pressure until the solvent evaporates and the mixture is dried to obtain a dried product. To the obtained dried product, 0.3 mL of 61% by mass nitric acid is added, and then pure water is added so that the total mass of the dried product, nitric acid, and pure water becomes 3 g, to obtain a sample for metal impurity analysis.

[0019] <Silanol Group Amount> The silica particles contained in the polishing composition of the present invention can be made to have a predetermined silanol group amount. That is, a colloidal silica dispersion containing the silica particles can be freeze-dried at room temperature under a pressure of 5 Pa or less to remove the dispersing medium, and the resulting silica particle powder can have a silanol group amount calculated by the following formula from the mass loss upon heating from room temperature to 700°C in thermogravimetric analysis and the molecular weight of water molecules, of, for example, 0.1 to 1.5 mmol / g, 0.3 to 1.5 mmol / g, 0.5 to 1.5 mmol / g, 0.3 to 1.4 mmol / g, 0.5 to 1.4 mmol / g, 0.3 to 1.3 mmol / g, or 0.3 to 1.3 mmol / g. Silanol group amount (mmol / g) = 2 × (M2 - M1) ÷ M Ｈ２Ｏ ÷ (M0-M1) × 1000 where M0 is the mass of the silica particle powder subjected to thermogravimetric analysis before heating, M1 is the mass loss amount when the temperature reached 200°C, M2 is the mass loss amount when the temperature reached 700°C, and M Ｈ２Ｏ represents the molecular weight of a water molecule.

[0020] To obtain a silica particle powder by the freeze-drying method, the target silica particle aqueous dispersion is first contacted with a cation exchange resin, an anion exchange resin, and a cation exchange resin in that order to obtain an ion-exchanged solution, which is then frozen at −70° C. to −80° C. and freeze-dried at room temperature under a pressure of 5 Pa or less to remove the dispersing medium, thereby obtaining the desired silica particle powder.

[0021] Silica particles have silanol groups on their surface, and these silanol groups are easily adsorbed with water by hydrogen bonding.Therefore, in order to quantify the amount of silanol groups in silica particles from the mass loss due to heating, it is preferable to heat the silica particles to about 200 ° C, at which the adsorbed water is desorbed, as described above, to remove the silanol groups.In addition, if the amount of silanol groups in silica particles is greater than the above, for example, 1.5 mmol / g, when two silica particles come into contact, they are more likely to bond with each other, and the stability of the polishing composition is deteriorated.If the amount of silanol groups in silica particles is less than the above, for example, 0.1 mmol / g, the surface of the silica particles is not sufficiently solvated in polar solvent, so that they cannot maintain dispersibility and are subject to aggregation and precipitation, and the stability of the polishing composition is deteriorated.

[0022] <Density> The silica particles contained in the polishing composition of the present invention can have a predetermined density. That is, the silica particle powder obtained by freeze-drying a colloidal silica dispersion containing the silica particles at room temperature under a pressure of 5 Pa or less to remove the dispersion medium has a density measured by dry density measurement of, for example, 2.20 to 2.35 g / cm. ３ , 2.22-2.35g / cm ３ , 2.25-2.35g / cm ３ , 2.27-2.35g / cm ３ , 2.25-2.33g / cm ３ , 2.25-2.30g / cm ３ , or 2.27 to 2.30 g / cm ３ The procedure for obtaining silica particle powder by freeze-drying is the same as that described for the determination of the amount of silanol groups.

[0023] The density of the silica particles contained in the polishing composition is 2.20 g / cm ３ or more, or 2.25 g / cm ３ By satisfying the above conditions, the network of Si-O-Si bonds constituting the skeleton of the silica particles becomes dense, and adsorption and desorption of impurities such as metal ions is suppressed, thereby preventing deterioration of dispersibility of the silica particles due to fluctuations in the pH, electrolyte concentration, or composition balance of the polishing composition. ３By satisfying the following conditions, it is possible to prevent the silica particles from settling in the polishing composition, bonding to each other, and adhesion to the container, which in turn can further improve the polishing rate when used to polish silicon wafers, and also improve the storage stability of the polishing composition.

[0024] <Production of aqueous silica dispersion> The method for producing the colloidal silica dispersion (silica sol) used in the present invention can be roughly divided into the steps of (I) obtaining active silicic acid, (II) preparing a precursor silica sol, (III) further heating the precursor silica sol, and (IV) concentrating the obtained silica sol.

[0025] <<Step (I)>> Step (I) for obtaining activated silicic acid is divided into step (a1) for obtaining activated silicic acid and step (a2) for purifying the activated silicic acid. Of the above, step (a1) is essential, and step (a2) is not essential but may be performed as desired.

[0026] For example, steps (a1) and (a2) can be carried out as follows: Step (a1): An aqueous solution of alkali metal silicate containing a metal oxide other than silica in a ratio of 300 to 10,000 ppm relative to silica is added to the silicate-derived SiO ２ The solution is diluted with pure water to a concentration of 1 to 6 mass % as a SiO ion exchanger, and then brought into contact with a hydrogen-type strongly acidic cation exchange resin to obtain a SiO ion exchanger. ２ An aqueous solution of activated silicic acid having a concentration of 1 to 6 mass % is obtained. Step (a2): An acid selected from mineral acids such as hydrochloric acid, sulfuric acid, nitric acid, or phosphoric acid is added to the aqueous activated silicic acid solution obtained in step (a1) to adjust the pH to 1 to 2, and the solution is left to stand for 12 hours or more. This dissolves and ionizes metal oxides other than silica, and the solution is then contacted with a hydrogen-type strongly acidic cation exchange resin to remove the dissolved metal ions, and then contacted with a hydroxide-type strongly basic anion exchange resin to remove the anions, thereby obtaining SiO 2 . ２ A purified active silicic acid having a concentration of 1 to 6% by weight is obtained.

[0027] In the step (a1), an inexpensive industrial product, SiO, is used as the alkali metal silicate. ２ / Na ２It is preferable to use sodium water glass having a SiO molar ratio of about 2 to 4. The main impurities contained in the water glass are polyvalent metals, such as aluminum, iron, calcium, and magnesium, which are contained in relatively large amounts. The alkali metal silicate aqueous solution is preferably brought into contact with the hydrogen-type strongly acidic cation exchange resin by passing the alkali metal silicate aqueous solution through a column packed with the ion exchange resin. The column-passed liquid contains SiO ２ The recovered solution is an aqueous solution of activated silicic acid with a concentration of 1 to 6% by mass, preferably 2 to 6% by mass. The amount of hydrogen ion exchange resin used may be an amount sufficient to exchange all of the alkali metal ions in the aqueous alkali metal silicate solution with hydrogen ions; specifically, an amount such that the exchange capacity of the hydrogen ion exchange resin is 1 to 5 times, 1 to 3 times, or 1 to 2 times the total amount of alkali metal ions contained in the aqueous alkali metal silicate solution, on an equivalent basis, is preferred. The speed at which the resin is passed through the column is preferably a space velocity of about 1 to 10 per hour.

[0028] Step (II) The step (II) of preparing a precursor silica sol is, for example, a step of carrying out the following steps (b) and (c), or a step of preparing a commercially available silica sol. Step (b): Add an aqueous solution of sodium or potassium hydroxide to the aqueous solution of activated silicic acid obtained in step (I) to form a precursor silica sol. ２ A stabilized aqueous solution of active silicic acid is obtained, the concentration of which is adjusted to 1 to 6 mass % and the pH to 7 to 10. Step (c): The aqueous solution of active silicic acid recovered in step (b) is heated with sufficient stirring for 0.5 to 30 hours while maintaining the temperature at 110 to less than 160°C, 110 to 150°C, 120 to 150°C, or 130 to 150°C, to obtain a precursor silica sol.

[0029] The aqueous solution of sodium hydroxide or potassium hydroxide used in step (b) is preferably obtained by dissolving commercially available industrially produced sodium hydroxide or potassium hydroxide having a purity of 95% by mass or more in industrial water or ion-exchanged water from which cations have been removed, to a concentration of preferably 2 to 20% by mass.

[0030] The equipment used in step (c) may be a normal acid-resistant, alkali-resistant, and pressure-resistant vessel equipped with a stirrer, a temperature control device, a liquid level sensor, a pressure reducing device, a liquid supply device, the above-mentioned cooling device, etc. In step (c), the liquid temperature in the vessel is maintained at 110 to less than 160°C.

[0031] Step (III) The step (III) of further heating the precursor silica sol includes the following steps (d), (e), and (f): Of the above steps, step (f) is essential, and steps (d) and (e) are not essential but may be performed as desired.

[0032] Step (d) or step (e) can be carried out before step (f). Either step (d) or step (e) may be carried out, or both may be carried out, or neither may be carried out. When both are carried out, either step (d) or step (e) may be carried out first. Step (d): The precursor silica sol obtained in step (II) or the precursor silica sol with the pH adjusted obtained in the following step (e) is concentrated to SiO using a known concentrating device such as a concentrator equipped with an ultrafiltration membrane or a vacuum concentrator. ２ The precursor silica sol obtained in step (II) or the SiO sol obtained in step (d) is concentrated to a concentration of 10 to 40% by mass. ２ The precursor silica sol having the adjusted concentration is brought into contact with a hydrogen-type strongly acidic cation exchange resin to obtain a product liquid, and a basic compound is added to the product liquid to adjust the pH to 7-10.

[0033] Step (f): The precursor silica sol obtained in step (II) and the SiO obtained in step (d) are mixed together to form a mixture. ２ The precursor silica sol with the adjusted concentration or the precursor silica sol with the adjusted pH obtained in step (e) is heated for 0.5 to 30 hours under sufficient stirring at a temperature of, for example, 140 to less than 300°C, 160 to 280°C, 160 to 260°C, 160 to 240°C, 170 to 240°C, 180 to 240°C, or 190 to 240°C, to obtain a silica sol. In step (f), the liquid temperature in the container is maintained at 160 to less than 300°C.

[0034] <<Step (IV)>> In step (IV) of further concentrating the obtained silica sol, the silica sol obtained in step (III) is concentrated to 10 to 50 mass %. This step (IV) is not essential but is performed as desired. For the concentration, a known concentrating device such as a concentrator equipped with an ultrafiltration membrane or a vacuum concentrator can be used.

[0035] <Additives> In addition to silica particles as abrasive grains and water as an aqueous medium, the polishing composition of the present invention can further contain at least one additive selected from the group consisting of acidic compounds, basic compounds, water-soluble compounds, chelating agents, oxidizing agents, and metal corrosion inhibitors.

[0036] The mass percentage of the solids content of the polishing composition excluding water as the aqueous medium can be, for example, 0.01 to 40 mass%, 0.01 to 30 mass%, 0.01 to 20 mass%, 0.1 to 40 mass%, 0.1 to 20 mass%, or 0.1 to 15 mass%, and the mass percentage of silica particles in the solids content can be 80 to 100 mass%, or 85 to 99.9 mass%. The mass percentage of silica particles in the polishing composition can be 1 to 40 mass%, 1 to 30 mass%, 1 to 20 mass%, or 3 to 30 mass%. By setting the solids content of the polishing composition excluding the aqueous medium to 0.01 mass% or more, excessive dissolution of the silica particle surfaces by hydroxy groups can be suppressed. By setting the solids content of the polishing composition excluding the aqueous medium to 40 mass% or less, an increase in the number of silica particles in the polishing composition and an increase in the frequency of contact between silica particles due to a decrease in the amount of solvent can be prevented, thereby suppressing aggregation. In addition, the viscosity of the polishing composition can be prevented from becoming excessively high, making it easier to handle. As a result, the storage stability of the polishing composition is improved, making it easier to store or transport. The amount of solids in the polishing composition can be measured, for example, by heating the composition in an electric furnace at 1000°C for 1 hour and weighing the residue.

[0037] Examples of the acidic compound include mineral acids such as hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, and organic acids such as formic acid, oxalic acid, citric acid, acetic acid, lactic acid, malic acid, malonic acid, succinic acid, tartaric acid, butyric acid, fumaric acid, benzoic acid, glycolic acid, propionic acid, and ascorbic acid. The acidic compound may be used alone or in combination of two or more thereof.

[0038] Examples of the basic compound include alkali metal hydroxides, alkali metal carbonates, alkali metal bicarbonates, and nitrogen-containing basic compounds. The basic compound may be used alone or in combination. Among the basic compounds, sodium hydroxide or potassium hydroxide is preferred as the alkali metal hydroxide. Sodium bicarbonate or potassium bicarbonate is preferred as the alkali metal bicarbonate. Sodium carbonate or potassium carbonate is preferred as the alkali metal carbonate. Ammonia or a quaternary ammonium salt is preferred as the nitrogen-containing basic compound, and a quaternary ammonium compound is more preferred from the viewpoint of achieving a high polishing rate. Examples of the quaternary ammonium compound include tetramethylammonium hydroxide, ethyltrimethylammonium hydroxide, diethyldimethylammonium hydroxide, tetraethylammonium hydroxide, tetramethylammonium hydroxide, or salts thereof. Among these, tetraethylammonium hydroxide or a salt thereof is preferred, and tetramethylammonium hydroxide or a salt thereof is more preferred. The basic compound can be blended into the polishing composition in an amount of, for example, 0.1 to 50 mass%, 0.1 to 30 mass%, 0.1 to 10 mass%, 1 to 50 mass%, 1 to 30 mass%, or 1 to 10 mass%, based on the silica particles. By blending the amount of the basic compound within the above range, the pH of the polishing composition can be prevented from excessively increasing or decreasing, thereby improving the stability of the polishing composition.

[0039] By adding these acidic or basic compounds, the pH of the polishing composition according to the present invention can be adjusted to within the range of 1 to 12, 1 to 6, 3 to 6, 7 to 12, 7 to 10, 8 to 11, or 10 to 12. By adjusting the pH of the polishing composition to within the above range, when the polishing composition is used to polish a silicon wafer, the Si bonds on the silicon wafer surface are cleaved by the acid or base, making it easier for the silica particles to scrape the silicon wafer surface, thereby improving the removal rate.

[0040] Any water-soluble compound can be used as the water-soluble compound. Examples include monomers having a carboxylic acid group, such as acrylic acid, methacrylic acid, and maleic acid, and their polymers, such as polyacrylic acid and polymethacrylic acid, and their salts, such as ammonium polyacrylate, potassium polyacrylate, ammonium polymethacrylate, and potassium polymethacrylate. Other examples include alginic acid, pectinic acid, carboxymethyl cellulose, polyaspartic acid, polyglutamic acid, polyamic acid, ammonium polyamic acid, polyvinylpyrrolidone, hydroxyethyl cellulose, hydroxypropyl cellulose, glycerin, polyglycerin, polyvinyl alcohol, polyacrylamide and its derivatives, polymethacrylamide and its derivatives, and carboxyl- or sulfonic-acid-modified polyvinyl alcohol. The water-soluble compound can be blended into the polishing composition in an amount of, for example, 0.01 to 10 mass%, 0.01 to 5 mass%, 0.01 to 3 mass%, 0.05 to 10 mass%, 0.1 to 10 mass%, or 0.05 to 5 mass% relative to the silica particles. By blending the amount of the water-soluble compound within the above range, the dispersion stability of the silica particles contained in the polishing composition can be improved, while the flatness of the silicon wafer can be improved by protecting the silicon wafer surface other than the convex portions on which the load is concentrated when the silicon wafer is polished.

[0041] The chelating agent may be an aminocarboxylic acid chelating agent or a phosphonic acid chelating agent. Examples of the aminocarboxylic acid chelating agent include ethylenediaminetetraacetic acid, nitrilotriacetic acid, diethylenetriaminepentaacetic acid, hydroxyethylethylenediaminetriacetic acid, triethylenetetraminehexaacetic acid, 1,3-propanediaminetetraacetic acid, 1,3-diamine-2-hydroxypropanetetraacetic acid, hydroxyethyliminodiacetic acid, dihydroxyethylglycine, glycol ether diaminetetraacetic acid, dicarboxymethylglutamic acid, and ethylenediamine-N,N'-disuccinic acid. Examples of the phosphonic acid chelating agent include hydroxyethylidene diphosphonic acid, nitrilotris(methylenephosphonic acid), phosphonobutanetricarboxylic acid, and ethylenediaminetetra(methylenephosphonic acid). The chelating agent can be blended into the polishing composition in an amount of, for example, 0.01 to 10 mass%, 0.01 to 5 mass%, 0.01 to 3 mass%, 0.05 to 10 mass%, 0.1 to 10 mass%, or 0.05 to 5 mass% relative to the silica particles. By blending the amount of the chelating agent within the above range, it is possible to reduce contamination of silicon wafers by metal impurities while improving the dispersion stability of the silica particles contained in the polishing composition.

[0042] Examples of the oxidizing agent include hydrogen peroxide, potassium permanganate, potassium periodate, hypochlorous acid, and ozone water. The oxidizing agent can be blended into the polishing composition at a ratio of, for example, 0.01 to 10 mass%, 0.01 to 5 mass%, 0.01 to 3 mass%, 0.05 to 10 mass%, 0.1 to 10 mass%, or 0.05 to 5 mass% relative to the silica particles. By blending the amount of oxidizing agent within the above range, when the polishing composition is used to polish a silicon wafer, the silicon wafer surface is oxidized, making it easier to scrape off the surface with the silica particles, thereby improving the removal rate.

[0043] Examples of the metal corrosion inhibitor include triazole compounds, pyridine compounds, pyrazole compounds, pyrimidine compounds, imidazole compounds, guanidine compounds, thiazole compounds, tetrazole compounds, triazine compounds, and hexamethylenetetramine.

[0044] Examples of the triazole compound include 1,2,3-triazole, 1,2,4-triazole, 3-amino-1H-1,2,4-triazole, benzotriazole (BTA), 1-hydroxybenzotriazole, 1-hydroxypropylbenzotriazole, 2,3-dicarboxypropylbenzotriazole, 4-hydroxybenzotriazole, 4-carboxy-1H-benzotriazole, 4-carboxy-1H-benzotriazole methyl ester (methyl 1H-benzotriazole-4-carboxylate), 4-carboxy-1H-benzotriazole butyl ester (butyl 1H-benzotriazole-4-carboxylate), 4-carboxy-1H-benzotriazole octyl ester (octyl 1H-benzotriazole-4-carboxylate), 5-hexylbenzotriazole, (1,2,3-benzotriazolyl-1-methyl) (1,2,4-triazole), (1-benzotriazolyl)methyl]phosphonic acid, 3H-1,2,3-triazolo[4,5-b]pyridin-3-ol, 1H-1,2,3-triazolo[4,5-b]pyridine, 1-acetyl-1H-1,2,3-triazolo[4,5-b]pyridine, 3-hydroxypyridine, 1,2,4-triazolo[1,5-a]pyridin pyrimidine, 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, 2-methyl-5,7-diphenyl-[1,2,4]triazolo[1,5-a]pyrimidine, 2-methylsulfanyl-5,7-diphenyl-[1,2,4]triazolo[1,5-a]pyrimidine, 2-methylsulfanyl-5,7-diphenyl-[1,2,4]triazolo[1,5-a]pyrimidine, and 2-methylsulfanyl-5,7-diphenyl-4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine.

[0045] Examples of the pyridine compound include pyridine, 8-hydroxyquinoline, prothionamide, 2-nitropyridin-3-ol, pyridoxamine, nicotinamide, iproniazid, isonicotinic acid, benzo[f]quinoline, 2,5-pyridinedicarboxylic acid, 4-styrylpyridine, anabasine, 4-nitropyridine-1-oxide, ethyl pyridine-3-acetate, quinoline, 2-ethylpyridine, quinolinic acid, arecoline, citrazinic acid, pyridine-3-methanol, 2-methyl-5-ethylpyridine, 2-fluoropyridine, pentafluoropyridine, 6-methylpyridin-3-ol, and ethyl pyridine-2-acetate.

[0046] Examples of the pyrazole compound include pyrazole, 1-allyl-3,5-dimethylpyrazole, 3,5-di(2-pyridyl)pyrazole, 3,5-diisopropylpyrazole, 3,5-dimethyl-1-hydroxymethylpyrazole, 3,5-dimethyl-1-phenylpyrazole, 3,5-dimethylpyrazole, 3-amino-5-hydroxypyrazole, 4-methylpyrazole, N-methylpyrazole, 3-aminopyrazole, and 3-aminopyrazole.

[0047] Examples of the pyrimidine compound include pyrimidine, 1,3-diphenyl-pyrimidine-2,4,6-trione, 1,4,5,6-tetrahydropyrimidine, 2,4,5,6-tetraaminopyrimidine sulfate, 2,4,5-trihydroxypyrimidine, 2,4,6-triaminopyrimidine, 2,4,6-trichloropyrimidine, 2,4,6-trimethoxypyrimidine, 2,4,6-triphenylpyrimidine, 2,4-diamino-6-hydroxylpyrimidine, 2,4-diaminopyrimidine, 2-acetamidopyrimidine, 2-aminopyrimidine, and 4-aminopyrazolo[3,4-d]pyrimidine.

[0048] Examples of the imidazole compound include imidazole, 1,1'-carbonylbis-1H-imidazole, 1,1'-oxalyldiimidazole, 1,2,4,5-tetramethylimidazole, 1,2-dimethyl-5-nitroimidazole, 1,2-dimethylimidazole, 1-(3-aminopropyl)imidazole, 1-butylimidazole, 1-ethylimidazole, 1-methylimidazole, and benzimidazole.

[0049] Examples of the guanidine compound include guanidine, 1,1,3,3-tetramethylguanidine, 1,2,3-triphenylguanidine, 1,3-di-o-tolylguanidine, and 1,3-diphenylguanidine.

[0050] Examples of the thiazole compound include thiazole, 2-mercaptobenzothiazole, and 2,4-dimethylthiazole.

[0051] Examples of the tetrazole compound include tetrazole, 5-methyltetrazole, 5-amino-1H-tetrazole, and 1-(2-dimethylaminoethyl)-5-mercaptotetrazole.

[0052] Examples of the triazine compounds include triazine and 3,4-dihydro-3-hydroxy-4-oxo-1,2,4-triazine.

[0053] The metal corrosion inhibitor can be blended in the polishing composition in an amount of, for example, 0.0001 to 10 mass % relative to the silica particles. By blending the amount of the metal corrosion inhibitor in the above range, the stability of the polishing composition can be improved while maintaining the corrosion inhibitor effect.

[0054] <Stability of Polishing Composition: Heated Storage Test> In a test in which the polishing composition of the present invention is heated and stored at 50°C for 28 days, it is desirable that the rate of increase in the average secondary particle diameter of the polishing composition after 28 days of heated storage at 50°C, as measured by dynamic light scattering, relative to the average secondary particle diameter of the polishing composition before heated storage, as measured by dynamic light scattering, is low (particle diameter remains unchanged), and the rate of increase can be, for example, less than 5%, or can be 4% or less, 0 to less than 5%, 0 to 4%, or 0 to 3%. A polishing composition that satisfies this rate of increase shows almost no change in the particle size or shape of the silica particles contained in the polishing composition even after transportation or storage, and therefore does not experience the generation of coarse particles due to aggregation of silica particles or a decrease in the amount of active ingredient due to sedimentation of silica particles, thereby achieving the desired polishing performance.

[0055] The polishing composition of the present invention can be produced, stored, and transported in the form of a concentrated liquid. The silica concentration in the form of the concentrated liquid is not particularly limited as long as it satisfies the increase rate of the average secondary particle diameter in the test of heating and storing at 50°C for 28 days, but can be, for example, 1 to 30 mass%, 5 to 30 mass%, 5 to 25 mass%, or 10 to 25 mass%. By producing, storing, and transporting the polishing composition in the form of a concentrated liquid, it is possible to reduce transportation costs and save space during storage. The concentrated liquid is diluted with any solvent such as pure water immediately before use, and the desired SiO ２ The concentration can be adjusted to, for example, 0.1 to 10% by mass before being subjected to polishing. ２ By setting the concentration within a range that satisfies the above-mentioned increase rate, it is possible to suppress a decrease in polishing performance due to aggregation of silica particles contained in the concentrated liquid, etc. The pH of the concentrated liquid can be adjusted to a range of 1 to 12, 1 to 6, 3 to 6, 7 to 12, 7 to 10, 8 to 12, 9 to 12, or 10 to 12.

[0056] The polishing composition of the present invention can be used for polishing silicon wafers or device wafers. Polishing machines are available for single-side polishing and double-side polishing, and silicon wafers can be polished using the polishing composition of the present invention in either type of machine. The polishing process for silicon wafers typically involves multiple polishing stages, including a primary polishing step performed at the beginning of the polishing process and a finish polishing step performed after the primary polishing step. The primary polishing and finish polishing may each be performed in two stages. The polishing composition of the present invention may be used for both the primary polishing and the finish polishing, or the polishing composition of the present invention may be used for only one of the primary polishing and the finish polishing. The polishing composition of the present invention may be circulated within the polishing machine during each polishing step, or may be used as a single-use flow-through solution. The polishing composition of the present invention can also be used for CMP polishing of device wafers.

[0057] The present invention will be described in more detail below with reference to examples, although the present invention is not limited thereto.

[0058] 1) Synthesis of colloidal silica dispersion (Synthesis Example 1) Synthesis of colloidal silica dispersion A As a raw material, a water-soluble alkali metal silicate, a JIS No. 3 aqueous solution of sodium silicate was prepared. The main components of this sodium silicate aqueous solution other than water were SiO ２ The concentration is 28.8% by mass, Na ２ The O concentration was 9.47% by mass.

[0059] The sodium silicate aqueous solution was diluted with pure water to obtain SiO ２ A sodium silicate aqueous solution (a) having a concentration of 4% by mass was prepared. Next, the sodium silicate aqueous solution (a) was passed through a column packed with a hydrogen-type strongly acidic cation exchange resin (trade name) Amberlite IR-120B at a space velocity of 4.5 per hour to remove cations, thereby producing an activated silicic acid aqueous solution.

[0060] A 10% by mass aqueous solution of sodium hydroxide was added to the obtained aqueous solution of activated silicic acid to adjust the pH to 8.5 to 9.5, thereby obtaining a stabilized aqueous solution of activated silicic acid.２ The concentration was 3.2% by mass. 2400 g of the stabilized activated silicic acid aqueous solution obtained above was placed in a reaction apparatus comprising a 3 L SUS pressure vessel equipped with a stirrer, a heater, etc., and the liquid temperature in the vessel was adjusted to 130 to 150°C by heating. After the temperature in the vessel reached 130 to 150°C, the vessel was heated for 2 hours and 30 minutes while maintaining the temperature at 130 to 150°C, thereby obtaining a colloidal silica dispersion (precursor silica sol) having an average primary particle size of 10 to 15 nm.

[0061] The obtained colloidal silica dispersion was filtered at room temperature using a commercially available ultrafiltration device equipped with a polysulfone ultrafiltration membrane (manufactured by Advantec Co., Ltd. (trade name) Q2000 150E) with a molecular weight cutoff of 200,000. ２ By concentrating the mixture to a concentration of 33 mass %, SiO ２ A precursor silica sol with an adjusted concentration was obtained. ２ The precursor silica sol with the adjusted concentration was passed through a column packed with a hydrogen-type strongly acidic cation exchange resin (trade name) Amberlite IR-120B at a space velocity of 10 per hour to remove cations, and a 10% by mass aqueous solution of sodium hydroxide was added to the resulting dispersion to adjust the pH to 7 to 8, thereby obtaining a precursor silica sol with an adjusted pH. ２ The precursor silica sol with the adjusted concentration and pH was placed in a reaction apparatus equipped with a stirrer, a heater, etc., in a 3 L SUS pressure vessel, and the liquid temperature in the vessel was adjusted to 220 to 240° C. by heating. After the temperature in the vessel reached 220 to 240° C., the vessel was heated for 2 hours and 20 minutes while maintaining the temperature at 220 to 240° C., thereby obtaining colloidal silica dispersion A.

[0062] (Synthesis Example 2) Synthesis of colloidal silica dispersion B SiO ２ Colloidal silica dispersion B was obtained in the same manner as in Synthesis Example 1, except that the precursor silica sol whose concentration and pH had been adjusted was heated at 210 to 230° C. in an SUS pressure vessel.

[0063] Synthesis Example 3 Synthesis of Colloidal Silica Dispersion C Colloidal silica dispersion B obtained in Synthesis Example 2 was passed through a column packed with a hydrogen-type strongly acidic cation exchange resin (trade name) Amberlite IR-120B at a space velocity of 10 per hour to remove cations, and a 10% by mass aqueous sodium hydroxide solution was added to the resulting dispersion to adjust the pH to 9 to 10. 2400 g of colloidal silica dispersion B adjusted to a pH of 9 to 10 was placed in a 3 L stainless steel pressure-resistant vessel equipped with a stirrer, a heater, and the like, and the liquid temperature in the vessel was adjusted to 190 to 210°C by heating. After the temperature in the vessel reached 190 to 210°C, the vessel was heated for 12 hours while maintaining the temperature in the vessel at 190 to 210°C, thereby obtaining colloidal silica dispersion C.

[0064] Synthesis Example 4 Synthesis of Colloidal Silica Dispersion D Colloidal silica dispersion D was obtained in the same manner as in Synthesis Example 3, except that colloidal silica dispersion B, the pH of which had been adjusted, was heated at 170 to 190°C for 72 hours using an SUS pressure vessel.

[0065] (Synthesis Example 5) Synthesis of Colloidal Silica Dispersion E Colloidal silica dispersion E was synthesized in the same manner as in Synthesis Example 4 of WO 2020 / 091000. Specifically, 8% by mass of sulfuric acid was added to the aqueous active silicic acid solution obtained by the same procedure as in Synthesis Example 1 above to adjust the pH to 2 to 3, thereby obtaining an aqueous active silicic acid solution stabilized with sulfuric acid. Pure water was added to the aqueous active silicic acid solution stabilized with sulfuric acid obtained above under stirring, and then a 10% by mass aqueous potassium hydroxide solution was added to obtain SiO ２After adjusting the concentration to 3.2% by mass and the pH to 12.0, the liquid temperature in the vessel was adjusted to 110-130 ° C. using a reactor equipped with a stirrer, a heating device, etc., in a 3 L SUS pressure vessel. After the temperature in the vessel reached 110-130 ° C., the vessel was maintained at 110-130 ° C., and the aqueous solution of activated silicic acid stabilized with sulfuric acid obtained above was continuously supplied as a feed solution until the pH of the reaction solution reached 11.4. After supplying the aqueous solution of activated silicic acid stabilized with sulfuric acid, the temperature in the vessel was kept at 110 ° C. and the reaction was allowed to proceed for 1 hour. After supplying the aqueous solution of activated silicic acid stabilized with sulfuric acid, the temperature in the vessel was maintained at 110 ° C., and the reaction solution was continuously supplied as a feed solution until the pH of the reaction solution reached 11.1. After supplying the aqueous solution of activated silicic acid stabilized with sulfuric acid, the temperature in the vessel was maintained at 110-130 ° C., and the reaction solution was further heated for 2 hours to obtain a reaction solution.

[0066] The obtained reaction solution was concentrated at room temperature using a commercially available ultrafiltration device equipped with a polysulfone ultrafiltration membrane (manufactured by Advantec Co., Ltd., product name: Q2000 150E) having a molecular weight cutoff of 200,000, to a SiO concentration of 40 mass %, thereby obtaining a colloidal silica dispersion E.

[0067] (Synthesis Example 6) Synthesis of Colloidal Silica Dispersion F 2,400 g of colloidal silica dispersion (trade name PL-3) manufactured by Fuso Chemical Co., Ltd. was placed in a reaction apparatus equipped with a stirrer, a heater, etc. in a 3 L stainless steel pressure vessel, and the temperature was adjusted to 190 to 210° C. After the temperature inside the vessel reached 190 to 210° C., the vessel was heated for 1 hour while maintaining the temperature inside the vessel at 190 to 210° C., thereby obtaining Colloidal Silica Dispersion F.

[0068] (Synthesis Example 7) Synthesis of colloidal silica dispersion G Colloidal silica dispersion G was obtained in the same manner as in Synthesis Example 6, except that the heating time was changed to 5 hours while the temperature inside the vessel was maintained at 190 to 210°C.

[0069] 2) Analysis of colloidal silica dispersions and silica particles The colloidal silica dispersions A to G prepared above, as well as a colloidal silica dispersion manufactured by Nissan Chemical Industries, Ltd. (trade name: Snowtex XL) and a colloidal silica dispersion manufactured by Fuso Chemical Co., Ltd. (trade name: PL-3) were analyzed as follows.

[0070] 2-1) Particle size measurement method The average primary particle size of silica particles was measured by nitrogen gas adsorption method. Pure water was added to each colloidal silica dispersion, and SiO ２ A sample with a concentration adjusted to 10% by mass was prepared. Next, 10 mL of hydrogen-type strongly acidic cation exchange resin Amberlite (trade name) IR-120B was added to 5 g of the obtained sample and stirred for 30 minutes to obtain a sample from which cations had been removed. The obtained sample was filtered through a nylon mesh to remove the cation exchange resin, and then heated at 290°C for 1 hour in an air atmosphere using an electric furnace (trade name DX302, manufactured by Yamato Scientific Co., Ltd.) to remove the solvent and obtain a dried product. The dried product was ground in an agate mortar for 10 minutes to obtain a heat-dried powder. The heat-dried powder was used as a measurement sample, and the specific surface area value of the measurement sample was measured by nitrogen gas adsorption method (BET method) using a specific surface area measurement device (trade name) Monosorb (manufactured by Quantachrome Instruments Japan LLC). The average primary particle diameter was calculated from the obtained specific surface area value. The average secondary particle size of silica particles was measured by dynamic light scattering (DLS) using a Zetasizer Nano (trade name, manufactured by Malvern Panalytical) as follows: 0.1 g of colloidal silica dispersion was dispensed into a polystyrene cell with an optical path length of 10 mm, and a 0.15 mass % aqueous sodium chloride solution was added to obtain a colloidal silica dispersion with a silica concentration adjusted so that the count rate at an attenuator setting of 7 was 200 to 400 kcps. The amount of the prepared colloidal silica dispersion introduced into the cell was adjusted so that the height of the liquid surface from the bottom of the cell was approximately 1 cm, and the average secondary particle size of the silica was measured under conditions of an attenuator setting of 7 and a temperature of 22.0°C.

[0071] 2-2) Pulse NMR analysis method Pure water was added to each colloidal silica dispersion, and SiO ２ A sample with a concentration adjusted to 5% by mass was prepared. 0.77 mL of the sample was placed in a glass NMR tube, and the relaxation time of the sample was measured using a pulsed NMR device (trade name) Acorn Area manufactured by Xigo nanotools under the following conditions: Magnetic field: 0.3 T Measurement frequency: 13 MHz Measurement nuclei:１ H NMR measurement method: CPMG pulse sequence method Temperature: 30°C The reciprocal of the relaxation time of the colloidal silica dispersion obtained by measurement under the above conditions was defined as Rav, and the reciprocal of the relaxation time of pure water obtained by measurement under the same conditions was defined as Rb. The Rsp value of each colloidal silica dispersion was calculated using the following formula: Rsp = (Rav - Rb) / (Rb).

[0072] 2-3) Method for quantifying the amount of silanol groups Pure water was added to each colloidal silica dispersion to determine the amount of SiO ２ A sample with a concentration adjusted to 10% by mass was prepared. Next, 20 mL of hydrogen-type strongly acidic cation exchange resin Amberlite (trade name) IR-120B was added to 100 g of the obtained sample, and the cations were removed by stirring for 30 minutes. The obtained sample was filtered through a nylon mesh to remove the cation exchange resin, and then 20 mL of hydroxyl-type strongly basic anion exchange resin Amberlite (trade name) IRA-410 was added and stirred for 30 minutes to remove the anions. The obtained sample was filtered through a nylon mesh to remove the anion exchange resin, and then 20 mL of hydrogen-type strongly acidic cation exchange resin Amberlite (trade name) IR-120B was added again and stirred for 30 minutes to remove the cations. The obtained sample was filtered through a nylon mesh to remove the cation exchange resin, and a dispersion from which the cations and anions had been removed was obtained. The dispersion from which the ions had been removed was placed in a 300 mL recovery flask. A frozen dispersion was obtained by immersing the recovery flask containing the dispersion for 10 minutes in ethanol cooled to -70°C to -80°C using dry ice. The frozen dispersion was left to stand at room temperature under a vacuum of 5 Pa or less using a freeze-drying apparatus (product name FDU-2100, manufactured by Tokyo Rikakikai Co., Ltd.), causing the frozen water to sublimate and obtaining a freeze-dried sample. The freeze-dried sample was ground in an agate mortar for 10 minutes to obtain a freeze-dried powder.

[0073] The freeze-dried powder was heated from room temperature to 700°C using a thermogravimetric differential thermal analyzer (product name TG-DTA2000SA, manufactured by Bruker) to measure the mass loss. Specifically, 5 to 10 mg of the freeze-dried powder was placed in a platinum container, and the initial sample mass was designated M0. The platinum container containing the freeze-dried powder was placed in the thermogravimetric differential thermal analyzer and heated in a nitrogen gas atmosphere from room temperature to 700°C at a heating rate of 10°C / min. The flow rate of the nitrogen gas was 100 cc / min. The mass loss M1 when the temperature inside the apparatus reached 200°C was taken as the mass loss due to the removal of water adsorbed on the surface of the silica particles contained in the freeze-dried powder, and the mass loss M2 when the temperature inside the apparatus reached 700°C minus M1 was taken as the mass loss due to the removal of water generated by dehydration condensation of silanol groups. The amount of silanol groups in the silica particles was calculated using the following formula: Amount of silanol groups (mmol / g) = 2 × (M2 - M1) ÷ 18 ÷ (M0 - M1) × 1000

[0074] 2-4) Method for Measuring Dry Density The freeze-dried powder of the colloidal silica dispersion prepared in the same manner as in 2-3) was used as a sample for measuring dry density. The measurement sample was measured in 1 cm 2 using a dry automatic density measuring device (trade name AccuPyc II TEC, manufactured by Micromeritics). ３ The aluminum cell was filled to 80% with the gas, and the dry density of the silica particles was measured at 25° C. by a gas substitution method using He gas.

[0075] 3) Preparation of Polishing Composition (Example 1) Using the colloidal silica dispersion A obtained in Synthesis Example 1, the following reagents and pure water were added so that the silica particles derived from the colloidal silica dispersion A were 20.5 mass %, the tetrasodium salt of ethylenediaminetetraacetic acid was 0.65 mass %, the tetramethylammonium hydroxide (TMAH) was 1.0 mass %, and the potassium carbonate was 1.5 mass %, and the mixture was stirred for 30 minutes to obtain the polishing composition of Example 1. (Example 2) The polishing composition of Example 2 was prepared in the same manner as in Example 1, except that the colloidal silica dispersion B obtained in Synthesis Example 2 was used. (Example 3) The polishing composition of Example 3 was prepared in the same manner as in Example 1, except that the colloidal silica dispersion C obtained in Synthesis Example 3 was used. (Example 4) The polishing composition of Example 4 was prepared in the same manner as in Example 1, except that the colloidal silica dispersion D obtained in Synthesis Example 4 was used.

[0076] (Comparative Example 1) A polishing composition of Comparative Example 1 was prepared in the same manner as in Example 1, except that a colloidal silica dispersion (trade name: Snowtex XL) manufactured by Nissan Chemical Industries, Ltd. was used as the colloidal silica dispersion. (Comparative Example 2) A polishing composition of Comparative Example 2 was prepared in the same manner as in Example 1, except that a colloidal silica dispersion E manufactured by Synthesis Example 5 was used. (Comparative Example 3) A polishing composition of Comparative Example 3 was prepared in the same manner as in Example 1, except that a colloidal silica dispersion (trade name: PL-3) manufactured by Fuso Chemical Co., Ltd. was used as the colloidal silica dispersion. (Comparative Example 4) A polishing composition of Comparative Example 4 was prepared in the same manner as in Example 1, except that a colloidal silica dispersion F manufactured by Synthesis Example 6 was used. (Comparative Example 5) A polishing composition of Comparative Example 5 was prepared in the same manner as in Example 1, except that a colloidal silica dispersion G manufactured by Synthesis Example 7 was used.

[0077] <Evaluation> (Storage Stability Test) The average secondary particle diameter of silica particles was measured immediately after preparation of the polishing compositions of Examples 1 to 4 and Comparative Examples 1 to 5. Each polishing composition was stored in a commercially available thermostatic chamber set at 50°C, and the average secondary particle diameter was measured after 28 days. The average secondary particle diameter was measured according to the procedure described in 2-1) Particle Size Measurement Method in the above-mentioned 2) Analysis of Colloidal Silica Dispersion and Silica Particles. When the rate of change (rate of increase) of the average secondary particle diameter after storage at 50°C was less than 5% compared to the average secondary particle diameter immediately after preparation (before storage at 50°C) of each polishing composition, the storage stability was determined to be good.

[0078] (Polishing Test) The polishing compositions of Examples 1 to 4 and Comparative Examples 1 to 5 were diluted 20 times by mass with pure water to obtain polishing test compositions. A polishing test was performed using the above polishing test compositions under the polishing conditions shown below. The wafer to be polished was a single crystal silicon wafer with a diameter of 200 mm, a P-type conductivity, a Miller index <100> crystal orientation, and a resistivity of 100 Ω·cm or less. The polishing test composition was supplied by a circulation system, and the liquid temperature was 23 to 25°C. The polishing time per batch was 60 minutes, and three batches of polishing were performed using the same polishing pad. The amount of polishing liquid was 25 kg, and no replenishment of new polishing liquid or pH adjustment was performed between batches.

[0079] (Polishing conditions) Polishing machine: Double-sided polishing machine 13BF, manufactured by Hamai Sangyo Co., Ltd. Polishing pad: LP-57, manufactured by JH-ROHDES Co., Ltd., groove width 2 mm, groove pitch 20 mm Polishing load: 150 g / cm ２ Lower surface plate rotation speed: 6.6 rpm Upper surface plate rotation speed: 20 rpm Rotation ratio: 3.3 Number of wafers polished: 3 sets of 1 wafer / carrier were prepared, and a total of 3 wafers were polished simultaneously Supply amount of polishing test composition: 6.4 L / min Polishing time: 60 min Carrier: Epoxy glass carrier (thickness 0.70 mm)

[0080] (Evaluation conditions for polished wafers) The polishing rate was calculated by measuring the wafer thickness before and after polishing using a laser displacement meter manufactured by Keyence Corporation (Keyence Corporation, product name SI-F1000), subtracting the wafer thickness after polishing from the wafer thickness before polishing, and dividing the result by the polishing time of 60 minutes. Surface roughness was measured using an optical interference microscope system (product name) BW-M7000 manufactured by Nikon Solutions Corporation, using a 100x objective lens, and measuring the root mean square height in a 111 μm square area at the center of the wafer.

[0081] (Results) Table 1 shows the physical properties of the colloidal silica dispersions used in Examples 1 to 4 and Comparative Examples 1 to 5, and Table 2 shows the results of the polishing test and the storage stability test of each polishing composition.

[0082]

[0083]

[0084] As shown in Table 2, the polishing rates were 0.33 to 0.34 μm / min in Examples 1 to 4 and 0.32 to 0.34 μm / min in Comparative Examples 1 to 5, which were comparable in performance. The surface roughness was 1.09 to 1.16 nm in Examples 1 to 4 and 1.13 to 1.17 nm in Comparative Examples 1 to 5, which were also comparable in performance. This is thought to be because the silica particles based on colloidal silica dispersions A to D, each having an Rsp value of 0.07 to 0.14, have sufficient hydrophilicity to maintain the dispersibility of the silica particles in the polishing composition, and therefore were able to uniformly and efficiently polish away silicon wafer surfaces altered by alkaline components such as TMAH and potassium carbonate in the polishing test, achieving the desired polishing rates and surface roughness.

[0085] On the other hand, in the storage stability test, in Examples 1 to 4, where the Rsp value of the colloidal silica dispersion used to prepare the polishing composition was within a certain range, the rate of change in average secondary particle diameter was 0 to 2%, which was a good result. This is because, in silica particles based on a colloidal silica dispersion with an Rsp value of 0.07 to 0.14, hydroxy groups (OH －) and excessive dissolution of the silica particle surfaces by hydroxy groups was suppressed, which is thought to have suppressed aggregation of the silica particles and resulted in excellent storage stability. On the other hand, in the polishing compositions of Comparative Examples 1 to 5, which used silica particles based on colloidal silica dispersions whose Rsp values ​​were not within a certain range, it is thought that many hydroxy groups were present near the silica particle surfaces. This likely led to dissolution of the silica particles, which then became active by cleaving the Si—O—Si bonds on the silica particle surfaces, which likely led to aggregation of the silica particles and resulted in poor storage stability.

[0086] According to the examples of the present invention, it has been confirmed that by using silica particles having an Rsp value in a specific range as determined by pulse NMR measurement in a silica sol used in a polishing composition, when the silica particles are used as a polishing composition for polishing silicon wafers, a high removal rate can be obtained and storage stability can be improved. Therefore, the polishing composition of the present invention can be used not only as an abrasive for silicon wafers, but also as an abrasive for CMP of device wafers because of its high removal rate and storage stability.

[0087] In the silica sol used in the polishing composition, by using silica particles having a specific Rsp value obtained by pulse NMR, a high polishing rate can be obtained when used to polish silicon wafers, and the storage stability of the polishing composition can be improved.

Claims

1. A polishing composition containing an aqueous dispersion of silica particles, the polishing composition having the following Rsp value derived from relaxation times obtained by pulse NMR measurement of the aqueous dispersion of silica particles and pure water: Rsp = (Rav - Rb) / (Rb), where Rav is the reciprocal of the relaxation time of an aqueous dispersion of silica particles having a silica concentration of 5 mass%, and Rb is the reciprocal of the relaxation time of pure water.

2. The polishing composition according to claim 1, wherein the silica particles have an average secondary particle size of 40 to 200 nm as measured by dynamic light scattering and an average primary particle size of 20 to 100 nm as measured by nitrogen gas adsorption.

3. The polishing composition according to claim 1, wherein the silica particle aqueous dispersion is brought into contact with a cation exchange resin, anion exchange resin, and cation exchange resin in this order to effect ion exchange, the treatment liquid is frozen at -70°C to -80°C, and the dispersion medium is removed by freeze-drying the liquid at room temperature under a pressure of 5 Pa or less to obtain a silica particle powder, which has a silanol group amount of 0.1 to 1.5 mmol / g as calculated by the following formula from the mass loss when heated from room temperature to 700°C in thermogravimetric analysis and the molecular weight of water molecules: silanol group amount (mmol / g)=2×(M2-M1)÷M Ｈ２Ｏ ÷ (M0-M1) x 1000, where M0 is the mass of the silica particle powder subjected to thermogravimetric analysis before heating, M1 is the mass loss amount when the temperature reached 200°C, M2 is the mass loss amount when the temperature reached 700°C, and M Ｈ２Ｏ represents the molecular weight of a water molecule.

4. The silica particle dispersion is brought into contact with a cation exchange resin, an anion exchange resin, and a cation exchange resin in this order to perform ion exchange. The resulting treated liquid is frozen at -70°C to -80°C, and then freeze-dried at room temperature under a pressure of 5 Pa or less to remove the dispersion medium. The resulting silica particle powder has a density of 2.20 to 2.35 g / cm as measured by dry density measurement. ３ The polishing composition according to claim 1 , 5. The polishing composition according to any one of claims 1 to 4, wherein the aqueous dispersion of silica particles has been subjected to a thermal history at a temperature of 140°C or higher and lower than 260°C.

6. The polishing composition according to any one of claims 1 to 4, wherein the concentration of the silica particles is 1 to 40 mass %.

7. A polishing composition according to any one of claims 1 to 6, further comprising at least one additive selected from the group consisting of acidic compounds, basic compounds, water-soluble compounds, chelating agents, oxidizing agents, and metal corrosion inhibitors.

8. The polishing composition according to claim 7, wherein the basic compound is at least one basic compound selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, alkali metal hydrogencarbonates, and nitrogen-containing basic compounds.

9. The polishing composition according to claim 7, wherein the chelating agent is an aminocarboxylic acid chelating agent or a phosphonic acid chelating agent.

10. The polishing composition according to any one of claims 1 to 9, wherein the polishing composition has a pH of 1 to 12.

11. A polishing composition according to any one of claims 1 to 10, wherein in a test in which the polishing composition is heated and stored at 50°C for 28 days, the increase rate of the average secondary particle diameter measured by dynamic light scattering of the polishing composition after heating and storage at 50°C for 28 days is less than 5% compared to the average secondary particle diameter measured by dynamic light scattering of the polishing composition before heating and storage at 50°C for 28 days.

12. The polishing composition according to any one of claims 1 to 11, which is used for polishing silicon wafers or device wafers.

13. A method for producing a polishing composition according to any one of claims 1 to 12, comprising the steps of: preparing a colloidal silica dispersion as a precursor; and heating the precursor at a temperature of 140°C or higher and lower than 260°C to obtain the aqueous dispersion of silica particles.

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