Polishing agent for gallium oxide substrates, and polishing method

Abstract

[Problem] The present invention addresses the problem of providing: a polishing composition for polishing a gallium oxide substrate at a high polishing rate; and a polishing method for the same. [Solution] The present invention provides a polishing composition for gallium oxide substrates, the polishing composition containing silica particles, water, and polyvalent anions. The content of the polyvalent anions is 0.002 mole / liter or less in the polishing composition. The polyvalent anions include at least one kind of polyvalent anions selected from the group consisting of sulfate ions, oxalate ions, citrate ions, and phosphate ions. The polishing composition for gallium oxide substrates additionally contains monovalent anions, and the content of the monovalent anions is 0.04 mole / liter or less in the polishing composition. The monovalent anions include at least one kind of monovalent anions selected from the group consisting of nitrate ions, chloride ions, bromide ions, chlorate ions, bromide ions, acetate ions, formate ions, propionate ions, and butyrate ions. The polishing composition for gallium oxide substrates has a pH of 5.0 to 9.5 inclusive.

Description

Abrasive for gallium oxide substrate and polishing method

[0001] The present invention relates to a polishing composition for oxide substrates such as gallium oxide.

[0002] Compound semiconductor substrates such as silicon carbide, gallium nitride, and gallium oxide have been proposed as alternatives to silicon substrates for power devices. Among these, gallium oxide is superior in terms of the magnitude of the dielectric breakdown field and the cost of crystal growth. Compound semiconductor substrates are manufactured through processes that include polishing. In the polishing process, there is a demand for improving productivity by increasing the polishing speed. Because gallium oxide has a strong cleavage tendency, it is also required to polish it with low frictional resistance to prevent breakage.

[0003] Patent Document 1 describes a polishing composition containing abrasive grains and water for gallium oxide. In the examples, a polishing composition containing colloidal silica and deionized water is described. Many commercially available colloidal silicas contain anions, but the document does not specifically describe the presence or absence, amount, or type of the anions.

[0004] Patent Document 2 describes a polishing slurry for a gallium oxide substrate that contains manganese dioxide particles and water.

[0005] WO2020 / 67057WO2020 / 194944

[0006] The present invention provides a polishing composition capable of polishing a gallium oxide substrate at a high removal rate, and a polishing method using the same.

[0007] The present invention provides, as a first aspect, a polishing composition for a gallium oxide substrate, comprising silica particles, water, and polyvalent anions, wherein the content of the polyvalent anions in the polishing composition is 0.002 mol / L or less; as a second aspect, the polishing composition according to the first aspect, wherein the polyvalent anion is at least one polyvalent anion selected from the group consisting of sulfate ion, oxalate ion, citrate ion, and phosphate ion; as a third aspect, the polishing composition according to the first aspect, further comprising a monovalent anion, wherein the content of the monovalent anion in the polishing composition is 0.04 mol / L or less; as a fourth aspect, the polishing composition according to any one of the third aspect, wherein the monovalent anion is at least one monovalent anion selected from the group consisting of nitrate ion, chloride ion, bromide ion, chlorate ion, bromate ion, acetate ion, formate ion, propionate ion, and butyrate ion; According to a fifth aspect, the polishing composition according to any one of the third aspect, wherein the monovalent anion is at least one monovalent anion selected from the group consisting of nitrate ion, chloride ion, and acetate ion; according to a sixth aspect, the polishing composition according to the first aspect, which has a pH of 5.0 or more and 9.5 or less; according to a seventh aspect, the polishing composition according to the first aspect, which has an average primary particle diameter of 5 to 100 nm; according to an eighth aspect, the polishing composition according to the first aspect, which has an average secondary particle diameter of 30 nm to 150 nm; according to a ninth aspect, ３ ~2.30 g / cm ３as a tenth aspect, the polishing composition according to the first aspect, wherein the silica particles have a heating loss rate of 3.0% or less when heated at 200°C to 700°C; as an eleventh aspect, a method for producing a polishing composition according to any one of the first to tenth aspects, comprising the following steps (A) to (C): step (A): preparing an aqueous silica sol; step (B): contacting the aqueous silica sol obtained in step (A) with a strongly acidic ion exchange resin to obtain an acidic silica sol; and step (C): adjusting the pH of the acidic silica sol obtained in step (B) with an alkali metal hydroxide or ammonia; as a twelfth aspect, a method for producing a polishing composition according to the eleventh aspect, wherein the aqueous silica sol in step (A) is produced by heating an active silicic acid aqueous solution obtained by removing the alkali metal from an aqueous alkali silicate solution; and as a thirteenth aspect, a method for producing a polishing composition according to the eleventh aspect, further comprising step (D): step (D): performing anion exchange on the polishing composition obtained in step (C). As a fourteenth aspect, there is provided a method for producing a polishing composition according to the twelfth aspect, further comprising the step (D): step (D): performing anion exchange on the polishing composition obtained in step (C). As a fifteenth aspect, there is provided a method for producing a polishing composition according to any one of the first to tenth aspects, comprising the following steps (A), (B), (E), and (F): step (A): preparing an aqueous silica sol; step (B): contacting the aqueous silica sol obtained in step (A) with a strongly acidic ion exchange resin to obtain an acidic silica sol; step (E): performing anion exchange on the acidic silica sol obtained in step (B) to obtain an alkaline silica sol; and step (F): adding a monovalent acid to the alkaline silica sol obtained in step (E) to adjust the pH. As a sixteenth aspect, there is provided a method for producing a polishing composition according to the fifteenth aspect, in which the aqueous silica sol in step (A) is produced by heating an active silicic acid aqueous solution obtained by removing the alkali metal from an aqueous alkali silicate solution. According to a seventeenth aspect, there is provided a method for polishing a gallium oxide substrate, the method comprising the step of polishing a gallium oxide substrate using the polishing composition according to any one of the first to tenth aspects; andAccording to an eighteenth aspect, there is provided a method for polishing a gallium oxide substrate according to the seventeenth aspect, in which a polishing rate for the gallium oxide substrate is 3.0 μm / hour to 10.0 μm / hour.

[0008] To use gallium oxide as a power device substrate, precision polishing is required, and it is necessary to polish it without causing surface irregularities or atomic-level defects in the compound. Furthermore, to remove processing damage from the previous process, a certain thickness must be removed by polishing. To shorten the polishing time and improve processing damage and productivity, polishing must be performed at a high polishing rate.

[0009] A state in which silica particles are dispersed in a solvent such as water is called colloidal silica, also called silica sol. The present invention has discovered that in a polishing composition containing water, silica particles, and polyvalent anions, the polishing rate can be improved by adjusting the amount of the specific polyvalent anions to a certain level or less. Furthermore, it has been discovered that the polishing rate can be further improved by adjusting the amount of the specific monovalent anions to a certain level or less.

[0010] Since polyvalent anions and monovalent anions are often contained in colloidal silica, which is one of the raw materials for polishing compositions, it is important to not only limit the amount of acid or salt added to adjust the pH of the polishing composition to a certain amount or less, but also to keep the amount of anions contained in the polishing composition to a certain amount or less by using colloidal silica whose anion content is a certain amount or less, or by using an anion exchange resin or the like to reduce the amount of anions contained in the colloidal silica.

[0011] In the present invention, it has been found that specific polyvalent anions and specific monovalent anions affect the polishing rate when polishing a gallium oxide substrate, and it has been found that a high polishing rate can be obtained by reducing the amount of these ions to a specific amount or less.

[0012] The present invention is a polishing composition for a gallium oxide substrate, which comprises silica particles, water, and polyvalent anions, wherein the content of the polyvalent anions in the polishing composition is 0.002 mol / L or less.

[0013] When the silica particles are dispersed in water and are colloidal silica particles, the water in the silica sol can be used in the polishing composition. Alternatively, water can be added to the silica sol and used as the water in the polishing composition.

[0014] The silica particles in the polishing composition can be contained in a range of 0.1 to 50% by mass, but when used to polish a gallium oxide substrate, they can be used in a range of 1 to 40% by mass, 5 to 40% by mass, 5 to 35% by mass, 5 to 30% by mass, 10 to 30% by mass, or 10 to 25% by mass. Examples of polyvalent anions include sulfate ions, oxalate ions, citrate ions, and at least one polyvalent anion selected from the group consisting of phosphate ions, sulfite ions, carbonate ions, phosphite ions, borate ions, malonate ions, and tartrate ions. It has been found that sulfate ions, oxalate ions, citrate ions, and phosphate ions affect the polishing rate, and it is therefore preferable to reduce the content of these polyvalent anions in the polishing composition to a certain amount or less.

[0015] The content of the polyvalent anion is preferably 0.002 mol / L or less in order to obtain a high polishing rate. The upper limit of the content of the polyvalent anion can be in the range of 0.001 mol / L or less, 0.0008 mol / L or less, or 0.0006 mol / L or less. Furthermore, in order to obtain stability of the silica sol, the content of the polyvalent anion is preferably 0.000001 mol / L or more, and the lower limit can be in the range of 0.000002 mol / L or more, 0.000003 mol / L or more, 0.000005 mol / L or more, or 0.000007 mol / L or more, or 0.00001 mol / L or more.

[0016] Furthermore, the polishing composition preferably contains a monovalent anion, and the content of the monovalent anion in the polishing composition is preferably 0.04 mol / L or less.

[0017] The monovalent anion may be at least one selected from the group consisting of nitrate, chloride, bromide, chlorate, bromate, acetate, formate, propionate, butyrate, hydrogen sulfate, and hydrogen carbonate.Furthermore, the monovalent anion may be at least one selected from the group consisting of nitrate, chloride, and acetate.

[0018] The content of the monovalent anion is preferably 0.04 mol / L or less. To obtain a high polishing rate, the upper limit of the content of the monovalent anion may be 0.03 mol / L or less, 0.02 mol / L or less, or 0.01 mol / L or less. To obtain stability of the silica sol, the content of the monovalent anion may be 0.00001 mol / L or more, 0.0001 mol / L or more, 0.001 mol / L or more, or 0.002 mol / L or more.

[0019] The pH of the polishing composition is preferably 5.0 or more and 9.5 or less to obtain a high polishing rate. The upper limit of the pH can be in the range of 9.1, 9.0, 8.9, 8.8, or 8.7. The lower limit of the pH can be in the range of 6.0, 6.2, 6.5, 6.8, or 7.0.

[0020] In order to obtain a high polishing rate, it is preferable that the average secondary particle diameter of the silica particles is 30 nm or more. The lower limit of the average secondary particle diameter can be in the range of 50 nm, 55 nm, or 60 nm. In order to obtain high polishing quality, the average secondary particle diameter of the silica particles is preferably less than 500 nm, and the upper limit can be in the range of 300 nm, 200 nm, or 150 nm.

[0021] The average secondary particle diameter (D ＤＬＳ The particle size distribution of silica particles can be determined by dynamic light scattering. Dynamic light scattering is a method in which silica particles are irradiated with laser light, and the scattering intensity of the resulting scattered light varies depending on the particle size of the silica particles. This method determines the particle size distribution from the scattering intensity.

[0022] An example of a measuring device used to measure the particle size distribution of silica fine particles by dynamic light scattering is a dynamic light scattering particle size distribution measuring device (manufactured by Malvern Panalytical).

[0023] The average primary particle diameter (D ＢＥＴ ) can be measured by calculating the sphere-equivalent particle diameter (nm) from the surface area (or specific surface area) measured using a nitrogen gas adsorption method (BET method), and the average primary particle diameter can be measured.

[0024] An example of an apparatus used to measure the average primary particle size of silica particles by the nitrogen gas adsorption method (BET method) is a nitrogen gas adsorption specific surface area measuring apparatus (Monosorb manufactured by Quantachrome). Specifically, the specific surface area S (m ) measured by the nitrogen adsorption method obtained by the above apparatus is ２ / g) and the spherical equivalent particle diameter formula ((D ＢＥＴ ) = 2720 / S) is used as the average primary particle diameter (D ＢＥＴ )

[0025] The content of the polyvalent anions and monovalent anions can be determined by ion chromatography.

[0026] The present invention further provides the silica particles having a true density of 2.15 g / cm ３ In order to obtain a high polishing rate, the lower limit of the true density of the silica particles is preferably 2.16 g / cm or more. ３ , 2.17 g / cm ３ , 2.18 g / cm ３ , or 2.19 g / cm ３ The upper limit is 2.30 g / cm ３ , 2.28 g / cm ３ , 2.26 g / cm ３ , or 2.25 g / cm ３ The range may be:

[0027] The true density of the silica particles can be calculated using a measuring device capable of measuring the volume and mass of an equivalent amount of silica fine particles.

[0028] An example of a density measuring device used to measure the volume and mass of silica particles and calculate the true density from the measured values ​​is a dry automatic density measuring device (AccuPyc II TEC, manufactured by Micromeritics). When using this density meter, a predetermined amount of silica particles can be placed in the density meter to automatically measure the true density.

[0029] In the present invention, it is further preferred that the silica particles have a weight loss rate of 3.0% or less when heated to 200° C. to 700° C. To obtain a high polishing rate, the upper limit of the weight loss rate when heated to 700° C. of the silica particles may be 2.0% or 1.8%, and the lower limit may be 0.6%, 0.8%, 1.0%, 1.2%, or 1.4%.

[0030] The heat loss rate of the silica particles can be measured by simultaneous thermogravimetry and differential thermal analysis (TGDTA).

[0031] An example of a measuring device used to measure the heat loss of silica particles by the simultaneous differential thermal and thermogravimetric measurement method is a thermogravimetric differential thermal analyzer (product name TG-DTA2000SA, manufactured by Bruker).

[0032] The above-mentioned true density and heat loss can be measured using silica particles (silica powder) extracted from the colloidal silica dispersion contained in the polishing composition of the present invention. The silica particles (silica powder) can be obtained by drying the colloidal silica. The drying method is not particularly limited. Specific drying methods include hot air drying, air drying, far-infrared heating drying, dehumidified air drying, natural drying (including sun drying), vacuum reduced pressure drying, indirect heating drying, microwave heating drying, and vacuum freeze drying. One or more drying methods can be combined.

[0033] Furthermore, in the drying step described above, impurities such as metal cations can be removed from the colloidal silica dispersion by contacting the colloidal silica dispersion with a strong acid cation exchange resin and / or a strong base anion exchange resin, and further with a weak acid (carboxylic acid) chelating resin and / or a weak base (amine) chelating resin as a pretreatment.

[0034] Anion exchange resins converted to hydroxyl group form by passing a strong alkaline aqueous solution through them can be used, and examples of strong acid cation exchange resins include Amberlite IR-120B, Amberjet 1020, and DOWEX MARATHONGH, both manufactured by The Dow Chemical Company; Diaion SK104 and Diaion PK208, both manufactured by Mitsubishi Chemical Holdings Corporation; and Duolite C20J, both manufactured by Sumika Chemtech Co., Ltd. Examples of strong base anion exchange resins include Amberlite IRA400J, Amberlite IRA410J, and Amberjet 4400, both manufactured by The Dow Chemical Company; Diaion SA10A and Diaion SA20A, both manufactured by Mitsubishi Chemical Holdings Corporation; and Duolite UBA120, both manufactured by Sumika Chemtech Co., Ltd.

[0035] The colloidal silica dispersion contains SiO ２ In order to obtain a high polishing rate, it is preferable that the solid content concentration of the SiO ２ The lower limit of the solid content concentration of the SiO 2 may be in the range of 2 mass %, 5 mass %, 10 mass %, 15 mass %, or 20 mass %. ２ The upper limit of the solid content concentration may be in the range of 40 mass %, 35 mass %, 30 mass %, or 25 mass %.

[0036] The smaller the shape parameter SF1, which indicates the irregularity of the particle shape of the silica particles, the smaller the coefficient of friction between the substrate and the polishing pad during polishing, reducing the risk of cracking the substrate. From the viewpoint of reducing the coefficient of friction, SF1 is preferably less than 1.60, and the upper limit of SF1 can be 1.53, 1.35, 1.30, 1.25, 1.22, 1.19, or 1.18.

[0037] A shape parameter SF1 indicating the irregularity of the particle shape of the silica particles ＞２５ The larger the value of SF1, the higher the polishing rate. ＞２５ is preferably 1.16 or more, more preferably 1.23 or more, or 1.25 or more, even more preferably 1.30 or more, and even more preferably 1.40 or more.

[0038] (Measurement of particle size and shape factor by transmission electron microscope image analysis) The shape factor SF1 is calculated by (area of ​​a circle having the maximum diameter of the particle as its diameter) / (projected area). That is,

[0039] SF1 = (DL ２ ×π / 4) / S Equation (1).

[0040] DL is the maximum particle diameter (nm), and is the maximum length (nm) of a silica particle obtained from a transmission electron microscope photograph, which is the maximum length between any two points on the periphery of the image. S is the projected area (nm) of the silica particle. ２ Specifically, transmission electron microscope photographs taken at 10,000 or 30,000 magnifications are input as electronic data into an image analyzer (manufactured by Nireco Corporation, trade name LUZEX) at a resolution of 146 dpi, and the number of pixels occupied by the silica particles is converted into an area, which is the projected area. For example, in a 10,000 magnification photograph, the length of one side of one pixel is 17.8 nm, and therefore the area per pixel is 319 nm. ２ In addition, in a 30,000-fold magnification photograph, the length of one side of one pixel is 6.0 nm, and therefore the area per pixel is 36 nm. ２ SF1 was calculated by determining the maximum length DL and projected area S for each of 2000 particles recognized by an image analyzer, calculating the values ​​for each particle using the above formula, and averaging these values ​​to obtain SF1.

[0041] The shape factor SF1 ＞２５ is the average value of SF1 of particles with a circle-equivalent diameter of 25 nm or more. That is, it can be determined by extracting only particles with a circle-equivalent diameter of 25 nm or more, which is determined by image analysis of the transmission electron microscope photograph, and calculating the average value of SF1 of those particles. The circle-equivalent diameter is the projected area S (nm ２) is the diameter (nm) of a perfect circle having the same projected area as the particle.

[0042] D = 2 × (S / π) ０．５ Formula (2) is given.

[0043] As the colloidal silica dispersion used in the polishing composition of the present application, a silica sol produced by an alkoxide method can be used, but a silica sol containing silica particles obtained by heating an aqueous silicic acid solution obtained by removing alkali metal ions from an aqueous alkali silicate solution can be preferably used. Silica sol produced by an alkoxide method typically has a true density of 2.15 g / cm. ３ and the loss on heating rate is 2.0% or more.

[0044] A pH adjuster can be added to the polishing composition of the present invention to adjust the pH by adding an alkaline substance to the acidic aqueous silica sol to 5.0 to 9.5, or 6.0 to 9.5, or 6.0 to 9.1, or 6.0 to 9.0, or 6.0 to 8.9, or 6.5 to 8.8, or 7.0 to 8.7.

[0045] The alkaline substance is an alkali metal hydroxide or ammonia, and sodium hydroxide, potassium hydroxide, or ammonia is preferably used, and can be added, for example, as a 0.1 to 30% by mass aqueous solution.

[0046] The polishing composition of the present invention can use a water-soluble compound. Examples of water-soluble compounds 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 compounds that can be used include alginic acid, pectinic acid, carboxymethyl cellulose, polyaspartic acid, polyglutamic acid, polyamic acid, ammonium polyamic acid, polyvinylpyrrolidone, hydroxyethyl cellulose, glycerin, polyglycerin, polyvinyl alcohol, and carboxyl- or sulfonic-acid-modified polyvinyl alcohol. The water-soluble compound can be contained in an amount of 0.01 to 10% by mass relative to the silica particles.

[0047] The silica particles used in the polishing composition of the present invention can be used as a silica sol. The polishing composition can be produced by a production method including the following steps (A) to (C): step (A): preparing an aqueous silica sol; step (B): contacting the aqueous silica sol obtained in step (A) with a strongly acidic ion exchange resin to obtain an acidic silica sol; and step (C): adjusting the pH of the acidic silica sol obtained in step (B) with an alkali metal hydroxide or ammonia.

[0048] The aqueous silica sol used in step (A) can be obtained, for example, by any of the following methods (a1) to (a4): step (a1): a silica sol obtained by ion-exchanging an aqueous sodium silicate solution and removing alkali metal ions to obtain an active silicic acid solution and then heat-treating the resulting solution; step (a2): a silica sol obtained by hydrolysis of an alkoxysilane; step (a3): a silica sol obtained by wet-pulverizing silica powder in an aqueous medium; step (a4): a silica sol obtained by dispersing silica powder obtained by combustion hydrolysis of silicon tetrachloride in a flame in an aqueous medium.

[0049] The method of step (a1) can be roughly divided into a step (a-I) of obtaining active silicic acid, a step (a-II) of heating and sizing the active silicic acid, and a step (a-III) of adjusting the concentration of the obtained silica sol.

[0050] The step (a-I) for obtaining active silicic acid is a step of converting a water-soluble alkali metal silicate containing a metal oxide other than silica in a ratio of 300 to 500,000 ppm relative to silica into SiO 2 derived from the silicate. ２ The aqueous solution of alkali metal silicate dissolved in the solution at a concentration of 0.5 to 10.0 mass % or 1 to 6 mass % is brought into contact with a hydrogen-type strongly acidic cation exchange resin to obtain SiO ２ This is a step of producing an aqueous solution of active silicic acid having a concentration of 1 to 6% by mass, and recovering the produced liquid.

[0051] The step (a-II) of heating and sieving the activated silicic acid includes the following steps (a-II-I) and (a-II-II): Step (a-II-I): To the aqueous solution of the activated silicic acid recovered in step (a-I), an aqueous solution containing at least one selected from the group consisting of alkali metal hydroxides, ammonia, and organic bases is added as a stabilizer, to obtain an aqueous solution of activated silicic acid containing 0.5 to 10.0 mass % or 1 to 6 mass % of SiO ２ or a step of adding at least one kind selected from the group consisting of inorganic acids and organic acids as a stabilizer to produce an aqueous solution of stabilized active silicic acid having a concentration of 0.5 to 10.0 mass %, or 1 to 6 mass % of SiO ２ The process involves producing an aqueous solution of stabilized activated silicic acid having a concentration and a pH of 1 to 4.

[0052] Step (a-II-II): An aqueous solution containing a silicate compound obtained by adding an aqueous solution containing at least one selected from the group consisting of alkali metal hydroxides, ammonia, and organic bases to the aqueous solution of active silicic acid recovered in Step (a-II-I), or an aqueous solution containing a silicate compound having a pH of 10 to 12.5 and SiO ２ The method includes the step of forming an aqueous solution containing a silicate compound having a concentration of 0.1 to 8% by weight.

[0053] Further, the method may include a step of supplying an aqueous solution of active silicic acid obtained in the same manner as in step (a-I) or (a-II-I) to an aqueous solution containing a silicate compound produced by the above method, while maintaining the temperature of the resulting mixed solution at 110 to less than 150°C, or 110 to 145°C, or 110 to 140°C, or 110 to 135°C, or 110 to 130°C, and stirring sufficiently, for 1 to 30 hours until the pH of the mixed solution reaches 9 to 12.

[0054] The step (a-III) of adjusting the concentration of the obtained silica sol is a step of concentrating the silica sol to 10 to 50 mass %, and impurities can be removed before or after the concentration. This step (a-III) is not essential but is performed if desired.

[0055] In step (B), the aqueous silica sol can be contacted with a hydrogen-form strongly acidic cation exchange resin.

[0056] In step (C), an alkali metal hydroxide or ammonia is added to the acidic silica sol obtained in step (B) so that the pH of the acidic silica sol becomes 6.0 to 9.5, or 6.0 to 9.1, or 6.0 to 9.0, thereby obtaining an acidic silica sol containing 10 to 50 mass % or 30 to 50 mass % of SiO ２ The present invention also includes a step of producing a stable aqueous silica sol having a concentration of 0.1 to 50% by mass, substantially free of polyvalent metal oxides other than silica, and having an average primary particle size of colloidal silica of 5 to 100 nm. These silica sols can be contained in a range of 0.1 to 50% by mass to prepare a polishing composition, but when used to polish a gallium oxide substrate by adding water to adjust the silica concentration, it can be used in a range of 0.1 to 40% by mass, 1 to 40% by mass, 5 to 40% by mass, 5 to 35% by mass, 5 to 30% by mass, 10 to 30% by mass, or 10 to 25% by mass.

[0057] Furthermore, the polishing composition obtained in step (C) can be subjected to an additional step (D) of anion exchange.

[0058] The polishing composition can be produced by a production method including the following steps (A), (B), (E), and (F): step (A): preparing an aqueous silica sol, step (B): contacting the aqueous silica sol obtained in step (A) with a strongly acidic ion exchange resin to obtain an acidic silica sol, step (E): performing anion exchange on the acidic silica sol obtained in step (B) to obtain an alkaline silica sol, and step (F): adding a monovalent acid to the alkaline silica sol obtained in step (E) to adjust the pH.

[0059] The present invention is a method for polishing a gallium oxide substrate, which comprises a step of polishing a gallium oxide substrate with the polishing composition.

[0060] In the polishing method, the polishing rate of the gallium oxide substrate can be 3.0 μm / hour to 10.0 μm / hour, or 4.0 μm / hour to 10.0 μm / hour.

[0061] (Average secondary particle diameter measured by DLS (dynamic light scattering): also referred to as DLS particle diameter) The average secondary particle diameter of silica sol was determined by dynamic light scattering using a particle diameter measuring device, Zetasizer Nano (manufactured by Malvern Panalytical Co., Ltd.) The average secondary particle diameter was measured as the Z-average particle diameter measured by dynamic light scattering.

[0062] (Preparation of Heat-Dried Powder Sample) Silica sol (SiO ２ 75.0 g of pure water was added to 25.0 g of SiO ２ A silica sol sample with a concentration adjusted to 10% by mass was prepared. The resulting silica sol sample was then subjected to hydrogen-type strongly acidic cation exchange resin Amberlite (trade name) IR-120B and hydroxyl-type strongly basic anion exchange resin Amberlite (trade name) IRA-410 to remove as many cations and anions as possible, and then heated and dried in an electric furnace (trade name DX302, manufactured by Yamato Scientific Co., Ltd.) at 290°C for 1 hour in an air atmosphere. The dried powder was thoroughly ground in an agate mortar to obtain a heat-dried powder sample.

[0063] (Preparation of freeze-dried powder sample) Silica sol (SiO ２75.0 g of pure water was added to 25.0 g of SiO ２ A silica sol sample with a concentration adjusted to 10% by mass was prepared. The resulting silica sol sample was then subjected to hydrogen-type strongly acidic cation exchange resin Amberlite (trade name) IR-120B and hydroxyl-type strongly basic anion exchange resin Amberlite (trade name) IRA-410 to remove as many cations and anions as possible, followed by freeze-drying under a vacuum pressure of 5 Pa or less using a freeze-drying apparatus (trade name FDU-2100, manufactured by Tokyo Rikakikai Co., Ltd.). The dried powder was thoroughly ground in an agate mortar to obtain a freeze-dried powder sample.

[0064] (Specific surface area diameter by BET method (nitrogen gas adsorption method): average primary particle diameter by BET method, also referred to as BET particle diameter) The above heat-dried powder of each silica sol was used as a measurement sample. The specific surface area value of the measurement sample was measured by the nitrogen adsorption method (BET method) using a specific surface area measurement device (trade name Monosorb) (manufactured by Quantachrome Instruments Japan LLC), and the average primary particle diameter was calculated from the obtained specific surface area value.

[0065] (pH Measurement) This indicates the value obtained from the pH measurement result, and was measured using a pH meter (product name: Multi Water Quality Meter MM-60R, product name: pH Electrode GST-5741C, manufactured by DKK-TOA Corporation).

[0066] (Measurement of True Density) The above-mentioned heat-dried powder of each silica sol was used as a measurement sample. The sample was measured by a dry automatic density measuring device (trade name AccuPyc II TEC, manufactured by Micromeritics Co., Ltd.) ３ The cell was filled to 80% and its true density was measured.

[0067] (Measurement of Heat Loss Rate) Using a thermogravimetric differential thermal analyzer (product name TG-DTA2000SA, manufactured by Bruker), the heat loss (%) was measured when the silica powder was heated from room temperature to 700°C as follows.

[0068] A predetermined amount of the freeze-dried powder of each silica sol was placed in a platinum container and heated in a nitrogen atmosphere from room temperature to 700°C at a heating rate of 10°C / min. The flow rate of nitrogen gas was 100 cc / min. The heat loss rate of the silica powder was calculated using the following formula from the values ​​of the mass M1 of the sample when it reached 200°C and the mass M2 of the sample when it reached 700°C.

[0069] Heating loss rate (%) = (M1-M2) ÷ M1 x 100

[0070] (Production of Activated Silica Aqueous Solution (a1)) As a raw material water-soluble alkali metal silicate, a JIS No. 3 aqueous sodium silicate solution was prepared. The main components of this sodium silicate aqueous solution other than water were SiO ２ is 28.8 mass%, Na ２ The O content was 9.47% by mass. 478 g of the above sodium silicate aqueous solution was dissolved in 2992 g of pure water to prepare 3500 g of a sodium silicate aqueous solution. Next, the above sodium silicate aqueous solution was passed through a column packed with hydrogen-type strongly acidic cation exchange resin Amberlite IR-120B at a space velocity of 4.5 per hour, and then 3000 g of the resulting activated silicic acid aqueous solution (a1) was recovered in a container.

[0071] (Synthesis Example A) A reaction apparatus was used, which was a 3 L stainless steel pressure vessel equipped with a stirrer, a heater, etc. An aqueous solution of activated silicic acid (a1) (SiO ２ The pH was adjusted to 11.1 and the electrical conductivity to 14.8 mS / cm using a 10% by mass aqueous potassium hydroxide solution and pure water, and then the temperature of the liquid in the container was adjusted to 100 to 120°C by heating. After the temperature in the container reached 100 to 120°C, the active silicic acid aqueous solution (a1) (SiO ２ A feed solution of 3.5% by mass per minute was continuously supplied until the pH of the reaction solution reached 11.0 and the electrical conductivity reached 3.6 mS / cm. Subsequently, the reaction solution was heated for 1 hour while maintaining the temperature at 100 to 120°C, to obtain colloidal silica.

[0072] The obtained colloidal silica was filtered at room temperature using a commercially available ultrafiltration device equipped with a tubular ultrafiltration membrane made of polysulfone with a pore size of about 5 nm. ２ The solution was concentrated to a concentration of 40% by mass, and then used as colloidal silica A (pH 10.8, sulfate ion concentration 2 ppm, D ＤＬＳ 83 nm, D ＢＥＴ 28nm, SF1 is 1.50, SF1 ＞２５ was set to 1.50).

[0073] (Synthesis Example B) A reaction apparatus was used, which was a 3 L stainless steel pressure vessel equipped with a stirrer, a heater, and the like. Using colloidal silica (trade name ST-50T) manufactured by Nissan Chemical Industries, Ltd., a 10 mass % aqueous potassium hydroxide solution, and pure water, the pH was adjusted to 11.2 and the electrical conductivity to 3.7 mS / cm, and then the liquid temperature in the vessel was adjusted to 100 to 120°C by heating. After the temperature in the vessel reached 100 to 120°C, an aqueous activated silicic acid solution (a1) (SiO ２ The feed solution was continuously supplied with 3.5% by mass per minute until the pH of the reaction solution reached 10.5 and the electrical conductivity reached 1.1 mS / cm.

[0074] Subsequently, the resulting reaction solution was heated for 1 hour while being kept at 100 to 120° C. to obtain colloidal silica.

[0075] The obtained colloidal silica was filtered at room temperature using a commercially available ultrafiltration device equipped with a tubular ultrafiltration membrane made of polysulfone with a pore size of about 5 nm. ２ The solution was concentrated to a concentration of 40% by mass, and then used as colloidal silica B (pH 10.5, sulfate ion concentration 46 ppm, D ＤＬＳ 52 nm, D ＢＥＴ 33nm, SF1 is 1.20, SF1 ＞２５ was set to 1.20).

[0076] (Synthesis Example C) A reaction apparatus was used, which was a 3 L stainless steel pressure vessel equipped with a stirrer, a heater, and the like. Using colloidal silica B obtained in Synthesis Example B, a 10 mass % aqueous potassium hydroxide solution, and pure water, the pH was adjusted to 11.4 and the electrical conductivity to 8.9 mS / cm, and then the liquid temperature in the vessel was adjusted to 100 to 120°C by heating. After the temperature in the vessel reached 100 to 120°C, an aqueous activated silicic acid solution (a1) (SiO ２ Aqueous solutions of potassium hydroxide (3.0 to 4.0 mass % as a percentage) and 10 mass % were continuously fed as feed solutions until the pH of the reaction solution reached 10.7 and the electrical conductivity reached 1.7 mS / cm.

[0077] Subsequently, the resulting reaction solution was heated for 1 hour while being kept at 100 to 120° C. to obtain colloidal silica.

[0078] The obtained colloidal silica was filtered at room temperature using a commercially available ultrafiltration device equipped with a tubular ultrafiltration membrane made of polysulfone with a pore size of about 5 nm. ２ The solution was concentrated to a concentration of 40% by mass, and then used as colloidal silica C (pH 10.8, sulfate ion concentration 2 ppm, D ＤＬＳ 102 nm, D ＢＥＴ 62nm, SF1 is 1.16, SF1 ＞２５ was set to 1.16).

[0079] (Synthesis Example D) A reaction apparatus was used, which was a 3 L stainless steel pressure vessel equipped with a stirrer, a heater, etc. An aqueous solution of activated silicic acid (a1) (SiO ２ The pH was adjusted to 11.1 and the electrical conductivity to 11.3 mS / cm using a 10% by mass aqueous solution of potassium hydroxide and pure water, and then the temperature of the liquid in the container was adjusted to 100 to 120°C by heating. After the temperature in the container reached 100 to 120°C, the active silicic acid aqueous solution (a1) (SiO ２ The feed solution was continuously supplied with 3.5% by mass per minute until the pH of the reaction solution reached 11.0 and the electrical conductivity reached 3.3 mS / cm.

[0080] Subsequently, the resulting reaction solution was heated for 1 hour while being kept at 100 to 120° C. to obtain colloidal silica.

[0081] The obtained colloidal silica was filtered at room temperature using a commercially available ultrafiltration device equipped with a tubular ultrafiltration membrane made of polysulfone with a pore size of about 5 nm. ２ The solution was concentrated to a concentration of 40% by mass, and colloidal silica D (pH 10.8, sulfate ion concentration 6 ppm, D ＤＬＳ 45 nm, D ＢＥＴ 19nm, SF1 is 1.51, SF1 ＞２５ was set to 1.64).

[0082] (Production of active silicic acid aqueous solution (a2)) Active silicic acid aqueous solution (a1) (SiO ２ The content of sulfuric acid as a stabilizer was 0.313 mass% / SiO ２ An 8% aqueous solution of sulfuric acid was added so that the resulting solution was a stabilized aqueous solution of activated silicic acid (a2).

[0083] (Synthesis Example E) A reaction apparatus was used, which was a 3 L stainless steel pressure vessel equipped with a stirrer, a heater, etc. The stabilized activated silicic acid aqueous solution (a2) (SiO ２ The pH was adjusted to 11.0 and the electrical conductivity to 10.1 mS / cm using a 10% by mass aqueous solution of potassium hydroxide and pure water, and the liquid temperature in the container was then adjusted to 110 to 130°C by heating. After the temperature in the container reached 100 to 130°C, the container was kept at 110 to 130°C while further adding the stabilized aqueous solution of activated silicic acid (a2) (SiO ２ The reaction mixture was continuously fed as a feed solution (3.2 mass % per minute) until the pH of the reaction mixture reached 10.9 and the electrical conductivity reached 3.5 mS / cm.

[0084] Subsequently, the resulting reaction solution was heated for 2 hours while being kept at 110 to 130° C. to obtain colloidal silica.

[0085] The obtained colloidal silica was filtered at room temperature using a commercially available ultrafiltration device equipped with a tubular ultrafiltration membrane made of polysulfone with a pore size of about 5 nm. ２The solution was concentrated to a concentration of 40% by mass, and then used as colloidal silica E (pH 9.8, sulfate ion concentration 1100 ppm, D ＤＬＳ 56 nm, D ＢＥＴ 20nm, SF1 is 1.52, SF1 ＞２５ was 1.57).

[0086] Experimental Example 1 Colloidal silica A prepared in Synthesis Example A was passed through a column packed with hydrogen-type strongly acidic cation exchange resin Amberlite IR-120B at a space velocity of 4.5 per hour, and the resulting acidic colloidal silica A(+) was then recovered in a container.

[0087] (Experimental Example 2) Colloidal silica B prepared in Synthesis Example B was passed through a column packed with hydrogen-form strongly acidic cation exchange resin Amberlite IR-120B at a space velocity of 4.5 per hour, and the resulting acidic colloidal silica B(+) was recovered in a container. Hydroxyl-form strongly basic anion exchange resin Amberlite (trade name) IRA-410 was added to the container containing the acidic colloidal silica B(+), and after stirring for 30 minutes, the IRA-410 was removed using a #100 nylon mesh, yielding colloidal silica B(+-) from which anions had been removed.

[0088] Experimental Example 3 Colloidal silica C prepared in Synthesis Example C was passed through a column packed with hydrogen-type strongly acidic cation exchange resin Amberlite IR-120B at a space velocity of 4.5 per hour, and the resulting acidic colloidal silica C(+) was then recovered in a container.

[0089] Experimental Example 4 Colloidal silica D prepared in Synthesis Example D was passed through a column packed with hydrogen-type strongly acidic cation exchange resin Amberlite IR-120B at a space velocity of 4.5 per hour, and the resulting acidic colloidal silica D(+) was then recovered in a container.

[0090] (Experimental Example 5) Colloidal silica E prepared in Synthesis Example E was passed through a column packed with hydrogen-form strongly acidic cation exchange resin Amberlite IR-120B at a space velocity of 4.5 per hour, and the resulting acidic colloidal silica E(+) was recovered in a container. Hydroxyl-form strongly basic anion exchange resin Amberlite (trade name) IRA-410 was added to the container containing the acidic colloidal silica E(+), and after stirring for 30 minutes, the IRA-410 was removed using a #100 nylon mesh, yielding colloidal silica E(+-) from which anions had been removed.

[0091] (Experimental Example 6) Nissan Chemical Co., Ltd., product name ST-YL (product name Snowtex YL, pH 9.6, silica concentration 40 mass%, sulfate ion concentration 1000 ppm, D ＤＬＳ 98 nm, D ＢＥＴ 60nm, SF1 1.25, SF1 ＞２５ The resulting acidic colloidal silica YL(+) was collected in a container. Amberlite (trade name) IRA-410, a hydroxyl-form strongly basic anion-exchange resin, was added to the container containing the colloidal silica YL(+). The mixture was stirred for 30 minutes, and the IRA-410 was then removed using a #100 nylon mesh, yielding colloidal silica YL(+-) from which anions had been removed.

[0092] (Experimental Example 7) Colloidal silica C(+) prepared in Experimental Example 3 and Nissan Chemical Co., Ltd.'s product name ST-O (product name Snowtex O, pH 2.6, silica concentration 20 mass%, sulfate ion concentration less than 1 ppm, D ＤＬＳ 19 nm, D ＢＥＴ 12 nm, SF1 1.39) (colloidal silica O) was mixed with colloidal silica C(+) in a silica solids weight ratio of 7:3 to obtain a mixture C(+)+O of colloidal silica C(+) and colloidal silica O.

[0093] (Example 1) The colloidal silica A(+) prepared in Experimental Example 1 was mixed and stirred with pure water to adjust the silica concentration to 22% by mass, and then an aqueous potassium hydroxide solution in an amount to adjust the pH to 8.4 was added under stirring, and pure water was further added to adjust the silica concentration to 20% by mass, thereby producing the polishing composition of Example 1.

[0094] (Example 2) The colloidal silica A(+) prepared in Experimental Example 1 was mixed with pure water and stirred to a silica concentration of 22% by mass, and then sulfuric acid (concentration 8% by mass) was added to a sulfate ion concentration of 0.28 × 10 －３ M was added under stirring, and then an aqueous potassium hydroxide solution (concentration: 10% by mass) was added under stirring to adjust the pH to 8.3, and pure water was further added to adjust the silica concentration to 20% by mass and the sulfate ion concentration to 0.25×10 －３ The polishing composition of Example 1 was prepared by adjusting the temperature to M.

[0095] Example 3 A polishing composition of Example 3 was produced in the same manner as in Example 2, except that the amount of sulfuric acid added was changed so as to give the concentration shown in Table 1.

[0096] Example 4 A polishing composition of Example 4 was produced in the same manner as in Example 2, except that an aqueous oxalic acid solution (concentration: 10% by mass) was added in place of sulfuric acid so as to have the concentration shown in Table 1.

[0097] For Examples 5 to 8, each polishing composition was prepared in the same manner as Example 2, except that the type and amount of acid added instead of sulfuric acid (sulfuric acid (concentration 8% by mass), oxalic acid aqueous solution (concentration 10% by mass), hydrochloric acid (concentration 3.7% by mass), or acetic acid (concentration 6% by mass)) was changed as shown in Table 2.

[0098] Comparative Example 1 A polishing composition of Comparative Example 1 was produced in the same manner as in Example 2, except that the amount of sulfuric acid added was changed so as to give the concentration shown in Table 1.

[0099] (Comparative Example 2) Colloidal silica A prepared in Synthesis Example A was mixed with pure water and stirred to a silica concentration of 22% by mass, and then sulfuric acid (concentration 8% by mass) was added in an amount to give a pH of 8.4 while stirring, and pure water was further added to adjust the silica concentration to 20% by mass, thereby producing the polishing composition of Comparative Example 2.

[0100] For Comparative Examples 3 to 5, each polishing composition was prepared in the same manner as in Comparative Example 2, except that the type and amount of acid added instead of sulfuric acid (oxalic acid aqueous solution (concentration 10% by mass), citric acid aqueous solution (concentration 10% by mass), phosphoric acid (concentration 9.8% by mass)) were changed as shown in Table 1.

[0101] In Examples 9 to 11, each polishing composition was prepared in the same manner as in Comparative Example 2, except that the type and amount of acid added instead of sulfuric acid were as shown in Table 2.

[0102] For Examples 12 to 15, each polishing composition was prepared in the same manner as in Example 1, except that the amount of aqueous potassium hydroxide solution added was changed to an amount that would give a pH as shown in Table 3.

[0103] (Example 16) The colloidal silica A(+) prepared in Experimental Example 1 was mixed and stirred with pure water to make the silica concentration 12% by mass, and then an aqueous potassium hydroxide solution was added under stirring to adjust the pH to 8.4, and pure water was further added to adjust the silica concentration to 10% by mass, thereby producing the polishing composition of Example 1.

[0104] Example 17 Colloidal silica A(+) prepared in Experimental Example 1 and pure water were mixed and stirred to a silica concentration of 32 mass %, and then nitric acid was added to a nitrate ion concentration of 0.38×10 －３ Aqueous potassium hydroxide solution was added under stirring to adjust the pH to 8.1, and then pure water was added to adjust the silica concentration to 30 mass % to produce the polishing composition of Example 17.

[0105] Example 18 Colloidal silica E(+-) prepared in Experimental Example 5 was mixed with pure water and stirred to a silica concentration of 22 mass %, and then nitric acid was added to a nitrate ion concentration of 0.38 × 10 －３ A potassium hydroxide solution was added under stirring in an amount to give a pH of 8.3, and then pure water was added to give a silica concentration of 20 mass % and a nitrate ion concentration of 0.36×10 －３ The polishing composition of Example 18 was prepared by the above procedure.

[0106] Example 19 The colloidal silica YL(+-) prepared in Experimental Example 6 was mixed with pure water and stirred to a silica concentration of 22 mass %, and then nitric acid was added to a nitrate ion concentration of 0.51 × 10 －３ Aqueous potassium hydroxide solution was added under stirring to a pH of 8.6, and pure water was added to a silica concentration of 20 mass % and a nitrate ion concentration of 0.46×10 －３ The polishing composition of Example 19 was prepared by the procedure described above.

[0107] Example 20 Colloidal silica B(+-) prepared in Experimental Example 2 was mixed with pure water and stirred to a silica concentration of 22 mass %, and then nitric acid was added to a nitrate ion concentration of 0.20 × 10 －３ Aqueous potassium hydroxide solution was added under stirring to a pH of 8.4, and pure water was added to a silica concentration of 20 mass % and a nitrate ion concentration of 0.18×10 －３ The polishing composition of Example 19 was prepared by the procedure described above.

[0108] For Examples 21 to 24, each polishing composition was prepared in the same manner as in Example 1, except that the type of colloidal silica and the pH after addition of the aqueous potassium hydroxide solution were changed as shown in Table 4.

[0109] In addition, the trace amount of sulfate ions contained in Examples 1 and 5 to 23 was 0.01 × 10 －３ All M's are derived from the raw material colloidal silica A and are not additionally added. In Table 4, PL-3 represents a commercially available alkoxide-process colloidal silica, trade name PL-3 (manufactured by Fuso Chemical Co., Ltd.), and C(+)+O represents a mixture of colloidal silica C(+) and colloidal silica O.

[0110]

[0111]

[0112]

[0113]

[0114]

[0115] A commercially available gallium oxide single crystal substrate was polished by the following method. Polishing machine: TriboLab CMP (manufactured by Bruker) Pressure: 250 g / cm ２ Platen rotation speed: 80 rpm Substrate rotation speed: 80 rpm Polishing pad: SUBA600 (manufactured by Nitta DuPont) Amount of polishing composition used: 0.9 L Supply rate of polishing composition: 100 mL / min (circulated use) Polishing time: 1 hour Substrate: β-gallium oxide, diameter 2 inches Number of substrates: 1

[0116] (Method for Measuring Polishing Rate) The polishing rate was determined from the weight loss of the substrate before and after polishing, using the density of the substrate as 5.88 g / cm ３ In the following Tables 6 to 9, the polishing rate μm / h indicates micrometers / hour.

[0117]

[0118]

[0119]

[0120]

[0121] As in the above examples, in a polishing composition for a gallium oxide substrate containing silica particles and water, by setting the content of polyvalent anions to 0.002 mol / L or less, a high polishing rate can be obtained, as shown in the polishing rates in Tables 6 to 9. Furthermore, a higher polishing rate can be obtained by balancing with other parameters.

[0122] By setting the content of polyvalent anions to 0.002 mol / L or less and the content of monovalent anions to 0.04 mol / L or less, an even higher polishing rate can be obtained, as shown by the polishing rates in Table 7.

[0123] As shown by the polishing rates in Table 8, by adjusting the content of polyvalent anions to 0.002 mol / L or less and further adjusting the pH to 6.0 to 9.1, an even higher polishing rate can be obtained.

[0124] Furthermore, the true density of silica particles, the rate of weight loss at 200°C to 700°C, the average secondary particle diameter, SF1 ＞２５For example, in the polishing compositions shown in Examples 18 to 24, when the true density of the silica particles is further increased to 2.15 g / cm ３ By doing so, as shown in the polishing rates in Table 9, a higher polishing rate can be obtained.

[0125] Furthermore, by setting the weight loss rate of silica particles on heating at 200° C. to 700° C. to 2.0% or less, as shown by the polishing rates in Table 9, an even higher polishing rate can be obtained.

[0126] Furthermore, by making the average secondary particle diameter of the silica particles 50 nm or more, as shown by the polishing rates in Table 9, an even higher polishing rate can be obtained.

[0127] Furthermore, silica particles SF1 ＞２５ By making the SF1 of the silica particles 1.23 or more, a higher polishing rate can be obtained, as shown by the polishing rate in Table 9. Furthermore, by making the SF1 of the silica particles less than 1.35, the frictional resistance can be kept low, as shown by the friction coefficient in Table 9.

[0128] It is possible to provide a polishing composition capable of polishing a gallium oxide substrate at a high removal rate, and a polishing method using the same.

Claims

1. A polishing composition for a gallium oxide substrate, comprising silica particles, water and a polyvalent anion, wherein the content of the polyvalent anion in the polishing composition is 0.002 mol / L or less.

2. The polishing composition according to claim 1, wherein the polyvalent anion is at least one type of polyvalent anion selected from the group consisting of sulfate ion, oxalate ion, citrate ion, and phosphate ion.

3. The polishing composition according to claim 1, further comprising a monovalent anion, the content of said monovalent anion in said polishing composition being 0.04 mol / L or less.

4. The polishing composition according to claim 3, wherein the monovalent anion is at least one type of monovalent anion selected from the group consisting of nitrate ion, chloride ion, bromide ion, chlorate ion, bromate ion, acetate ion, formate ion, propionate ion, and butyrate ion.

5. The polishing composition according to claim 3, wherein the monovalent anion is at least one type of monovalent anion selected from the group consisting of nitrate ion, chloride ion, and acetate ion.

6. The polishing composition according to claim 1, having a pH of 5.0 or more and 9.5 or less.

7. The polishing composition according to claim 1, wherein the average primary particle size of the silica particles is 5 to 100 nm.

8. The polishing composition according to claim 1, wherein the average secondary particle size of the silica particles is 30 nm to 150 nm.

9. The true density of the silica particles is 2.15 g / cm ３ ~2.30g / cm ３ 2. The polishing composition according to claim 1, wherein 10. The polishing composition according to claim 1, wherein the silica particles have a heating loss rate of 3.0% or less when heated at 200°C to 700°C.

11. A method for producing a polishing composition according to any one of claims 1 to 10, comprising the following steps (A) to (C): step (A): preparing an aqueous silica sol; step (B): contacting the aqueous silica sol obtained in step (A) with a strongly acidic ion exchange resin to obtain an acidic silica sol; and step (C): adjusting the pH of the acidic silica sol obtained in step (B) with an alkali metal hydroxide or ammonia.

12. A method for producing a polishing composition according to claim 11, wherein the aqueous silica sol in step (A) is produced by heating an active aqueous silicic acid solution obtained by removing the alkali metal from an aqueous alkali silicate solution.

13. The method for producing a polishing composition according to claim 11, further comprising the step of: (D) performing anion exchange on the polishing composition obtained in (C).

14. The method for producing a polishing composition according to claim 12, further comprising the step of: (D) performing anion exchange on the polishing composition obtained in (C).

15. A method for producing a polishing composition according to any one of claims 1 to 10, comprising the following steps (A), (B), (E) and (F): step (A): preparing an aqueous silica sol; step (B): contacting the aqueous silica sol obtained in step (A) with a strongly acidic ion exchange resin to obtain an acidic silica sol; step (E): performing anion exchange on the acidic silica sol obtained in step (B) to obtain an alkaline silica sol; step (F): adding a monovalent acid to the alkaline silica sol obtained in step (E) to adjust the pH.

16. A method for producing a polishing composition according to claim 15, wherein the aqueous silica sol in step (A) is produced by heating an active aqueous silicic acid solution obtained by removing the alkali metal from an aqueous alkali silicate solution.

17. A method for polishing a gallium oxide substrate, comprising the step of polishing a gallium oxide substrate with the polishing composition according to any one of claims 1 to 10.

18. The method for polishing a gallium oxide substrate according to claim 17, wherein the polishing rate of the gallium oxide substrate is 3.0 μm / hour to 10.0 μm / hour.

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