Particle dispersion liquid filtration method, and method for manufacturing polishing liquid for polishing magnetic disk substrate

Abstract

To provide a particle dispersion liquid filtration method by which a particle dispersion liquid suitable for improvement in quality of a polishing liquid for a magnetic disk substrate can be obtained, and further, a service life of a filter can be elongated when obtaining the particle dispersion liquid.SOLUTION: In one embodiment, disclosed is a particle dispersion liquid filtration method including a process (process 1) in which a particle dispersion liquid is filtered by a membrane filter. When the number of particles of 0.1 μm or more, which are obtained by particle size distribution measurement using the number counting method in the particle dispersion liquid of 1 ml filtered in the process 1, is represented by NPC, the average pore number of the membrane filter used in the process 1 is represented by NP, and the average pore diameter of the membrane filter used in the process 1 is represented by APD, a value represented by the formula 1 of [(NP) / [(NPC)×(APD)]]×1000000 is 3.0 or more and 50 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a method for filtering a particle dispersion and a method for producing a polishing liquid for polishing magnetic disk substrates. [Background technology]

[0002] In recent years, magnetic disk drives have become smaller and have larger capacities, and there is a demand for even higher recording densities. To achieve this, it is becoming increasingly important to secure a sufficient recording area, and there are stricter demands for reducing defects, such as scratches.

[0003] One of the causes of scratches, which are one type of defect on the substrate surface of a magnetic disk drive, is coarse particles contained in the polishing solution. Typically, coarse particles in the polishing solution are removed by filtration during the manufacturing process. For example, Patent Document 1 proposes a method for separating target inorganic nanoparticles and impurity inorganic nanoparticles by cross-flow filtration of a solution containing target inorganic nanoparticles with particle diameters of 1 nm to 500 nm. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-46621 Summary of the Invention [Problem to be solved by the invention]

[0005] In filtration, there is a trade-off between the quality of the filtrate obtained (e.g., fewer coarse particles) and the lifespan of the filter used for filtration. In other words, increasing the quality of the filtrate tends to shorten the clogging (lifespan) of the filter.

[0006] Therefore, in one aspect, the present disclosure provides a filtration method capable of extending the life of a filter used in filtration to obtain a particle dispersion that can improve the quality of an abrasive liquid for polishing magnetic disk substrates. The disclosure also provides a method for producing an abrasive liquid for polishing magnetic disk substrates, which includes a step of mixing the particle dispersion obtained by the filtration method with various compounds used in the abrasive liquid, and a method for producing a magnetic disk substrate, which includes polishing a surface to be polished with the abrasive liquid for polishing magnetic disks obtained by the production method. [Means for solving the problem]

[0007] In one aspect, the present disclosure relates to a method for filtering a particle dispersion, comprising a step (step 1) of filtering the particle dispersion through a membrane filter, wherein the value represented by the following formula 1 is 3.0 or more and 50 or less, where NPC is the number of particles of 0.1 μm or more obtained by particle size distribution measurement using a number counting method in 1 ml of the particle dispersion filtered in step 1, NP is the average number of pores in the membrane filter used in step 1, and APD is the average pore diameter of the membrane filter used in step 1: Equation 1: [(NP) / [(NPC)×(APD)]]×1000000

[0008] In one aspect, the present disclosure relates to a method for producing a polishing slurry for polishing magnetic disk substrates, the method comprising the step of mixing a particle dispersion obtained by the method for filtering a particle dispersion of the present disclosure with at least one compound selected from the group consisting of an acid, an oxidizing agent, a heterocyclic aromatic compound, an amine compound, and a water-soluble polymer.

[0009] In one aspect, the present disclosure relates to a method for manufacturing a magnetic disk substrate, comprising supplying a polishing liquid for polishing magnetic disk substrates, obtained by the method for manufacturing a polishing liquid for polishing magnetic disk substrates of the present disclosure, to a surface of a substrate to be polished, and polishing the surface to be polished by bringing a polishing pad into contact with the surface to be polished and moving at least one of the polishing pad and the substrate to be polished. [Effects of the Invention]

[0010] According to the present disclosure, in one aspect, it is possible to achieve both an improvement in the quality of a polishing liquid containing a particle dispersion obtained by the method for filtering a particle dispersion of the present disclosure and an extension of the life of a filter used in the method for filtering a particle dispersion of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present disclosure is based on the finding that in a method for filtering a particle dispersion, which includes a step (step 1) of filtering a particle dispersion through a membrane filter, when the number of particles of 0.1 μm or larger in 1 ml of the particle dispersion filtered in step 1, obtained by particle size distribution measurement using a number counting method, is defined as NPC, the average pore number of the membrane filter used in step 1 is defined as NP, and the average pore diameter of the membrane filter used in step 1 is defined as APD, if the filtration is performed using a combination of a particle dispersion and a membrane filter that satisfies the condition expressed by the following formula 1, which is between 3.0 and 50, it is possible to achieve both high quality of a polishing liquid for magnetic disk substrates that contains the particle dispersion obtained by the filtration and a long life of the filter used for the filtration. Equation 1: [(NP) / [(NPC)×(APD)]]×1000000

[0012] That is, in one aspect, the present disclosure relates to a method for filtering a particle dispersion, comprising a step (step 1) of filtering a particle dispersion through a membrane filter, in which, when NPC is the number of particles of 0.1 μm or more obtained by particle size distribution measurement using a number counting method in 1 ml of the particle dispersion filtered in step 1, NP is the average number of pores in the membrane filter used in step 1, and APD is the average pore diameter of the membrane filter used in step 1, the value represented by the following formula 1 is 3.0 or more and 50 or less (hereinafter also referred to as the "filtration method of the present disclosure"). Equation 1: [(NP) / [(NPC)×(APD)]]×1000000 According to the filtration method of the present disclosure, it is possible to improve the quality of a polishing liquid containing a particle dispersion obtained by the filtration method (hereinafter also referred to as "filtered particle dispersion") and extend the life of the filter used for the filtration.

[0013] Although the details of the mechanism by which the filtration method of the present disclosure exhibits its effects are not clear, it is presumed as follows. When a particle dispersion containing coarse particles (particles with extremely large diameters that should not exist) is filtered through a membrane filter, the greater the number of pores per given area of ​​the membrane filter, the greater the number of flow paths for the particle dispersion, resulting in a longer filter life. On the other hand, when removing coarse particles from a particle dispersion containing a low content of coarse particles through a membrane filter, even a filter with a small number of pores per given area is less likely to clog the filter because it contains fewer coarse particles that cause pore clogging. Conversely, even a filter with a large number of pores per given area, if its average pore diameter is small, is more likely to clog due to particles that are the main component of the particle dispersion. In other words, the filter life can be affected by the number of coarse particles in the particle dispersion and the average number and average pore diameter of the filter. Therefore, in the present disclosure, by performing filtration using a combination of a particle dispersion and a membrane filter in which the value represented by the above formula 1 falls within a predetermined range, it is believed that both improved quality of the polishing liquid containing the filtered particle dispersion and extended filter life can be achieved. However, the present disclosure need not be construed as being limited to these mechanisms.

[0014] In this disclosure, the term "coarse particles" refers to particles having a particle size larger than the particle size originally expected for a particle dispersion. In this disclosure, coarse particles are defined as particles having a particle size of 0.1 μm or more as determined by particle size distribution measurement using a number counting method.

[0015] In the present disclosure, a "scratch" is a physical property that is important for achieving high density or high integration, particularly in magnetic disk substrates or substrates for semiconductor devices, and refers to a minute flaw on the substrate surface that is 1 nm or more but less than 100 nm deep, 5 nm or more but less than 500 nm wide, and 100 μm or more long. These scratches can be quantitatively evaluated as the number of scratches using an optical full-surface defect inspection machine described in the examples below.

[0016] [Membrane filter filtration process (Process 1)] In one or a plurality of embodiments, the filtration method of the present disclosure is a filtration method for a particle dispersion, comprising a step (step 1) of filtering a particle dispersion with a membrane filter, wherein, when NPC is the number of particles of 0.1 μm or larger in 1 ml of the particle dispersion filtered in step 1, as obtained by particle size distribution measurement using a number counting method, NP is the average number of pores in the membrane filter used in step 1, and APD is the average pore diameter of the membrane filter used in step 1, the value represented by the following formula 1 is 3.0 or more and 50 or less: Equation 1: [(NP) / [(NPC)×(APD)]]×1000000 Hereinafter, a filtration process performed using a combination of a membrane filter and a particle dispersion to be filtered through the membrane filter, in which the value calculated by the formula 1 satisfies the range, will also be simply referred to as the "filtration process of the present disclosure."

[0017] (Processed particle dispersion) <Large particles in the dispersion of particles to be treated> In the filtration process of the present disclosure, the number of coarse particles present in the particle dispersion before it is subjected to the step (step 1) of filtering the particle dispersion through a membrane filter (i.e., the particle dispersion filtered in step 1; hereinafter, also referred to as the "particle dispersion to be processed") can be calculated by number count particle size distribution measurement. In the present disclosure, number count particle size distribution measurement refers to a method in which particles in the particle dispersion are introduced one by one into a small measurement cell, and laser light is irradiated onto the measurement cell to detect light scattering and light blocking by the particles, and the particle size distribution is calculated based on this. A suitable example of a measurement device for number count particle size distribution measurement is the Accusizer A7000 series from Entegris.

[0018] In the present disclosure, when the number of particles of 0.1 μm or more in 1 mL of the particle dispersion to be processed obtained by particle size distribution measurement using a number counting method is defined as NPC, from the viewpoint of efficiently performing the filtration step of the present disclosure and from the viewpoint of improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, the NPC is preferably 50 million particles / mL or less, more preferably 45 million particles / mL or less, even more preferably 20 million particles / mL or less, even more preferably 15 million particles / mL or less, even more preferably 10 million particles / mL or less, and even more preferably 7.5 million particles / mL or less. From the same viewpoint, the NPC is preferably 1 million particles / mL or more, more preferably 3 million particles / mL or more, even more preferably 5 million particles / mL or more, and even more preferably 6 million particles / mL or more. When the NPC is 50 million particles / mL or less and exceeds 10 million particles / mL, the value represented by Equation 1 in the present disclosure is 3.0 or more, preferably 3.5 or more, more preferably 4.0 or more, and even more preferably 4.5 or more, from the viewpoint of efficiently performing the filtration process of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure; and from the same viewpoint, it is preferably 9.0 or less, more preferably 6.0 or less, and even more preferably 5.5 or less. When the NPC is 10 million cells / mL or less and more than 7.5 million cells / mL, the value represented by the formula 1 in the present disclosure is, from the same viewpoint, preferably 9.0 or more, more preferably 10.0 or more, and even more preferably 11.0 or more, and from the same viewpoint, preferably 15.0 or less, more preferably 14.0 or less, and even more preferably 12.5 or less. When the NPCs are 7.5 million / mL or less, the value represented by the formula 1 in the present disclosure is, from the same viewpoint, preferably 15.0 or more, more preferably 20.0 or more, even more preferably 25.0 or more, and from the same viewpoint, 50 or less, preferably 40.0 or less, more preferably 37.0 or less, even more preferably 35.0 or less, and even more preferably 30.0 or less.

[0019] <Particles in the dispersion of particles to be treated> In one or more embodiments, the particle dispersion liquid to be processed that is subjected to the filtration step of the present disclosure contains, in addition to the coarse particles, particles that are abrasive grains for a polishing liquid for magnetic disk substrates as a main component. In the present disclosure, "containing particles that are abrasive grains for a polishing liquid for magnetic disk substrates as a main component" means that the particles that are abrasive grains for a polishing liquid for magnetic disk substrates account for 95 mass % or more of the solid content (coarse particles + abrasive grains) in the particle dispersion liquid to be processed. Suitable examples of the particles that are abrasive grains for a polishing liquid for magnetic disk substrates contained in the particle dispersion liquid to be processed (hereinafter simply referred to as "particles contained in the particle dispersion liquid to be processed") include one or more particles selected from silica particles, alumina particles, ceria particles, zirconia particles, and silicon carbide particles. Among these, from the viewpoint of efficiently carrying out the filtration step of the present disclosure and from the viewpoint of improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, the particles contained in the particle dispersion to be treated are more preferably one or more types selected from silica particles, alumina particles, and ceria particles, even more preferably one or more types selected from silica particles and ceria particles, and even more preferably silica particles. That is, in the filtration step of the present disclosure, it is even more preferable that the particles contained in the particle dispersion to be filtered with the membrane filter include silica particles.

[0020] When the particles contained in the particle dispersion liquid to be treated that is subjected to the filtration process of the present disclosure include silica particles (hereinafter, the silica particles contained in the particle dispersion liquid to be treated are also referred to as "Component A"), from the viewpoint of efficiently performing the filtration process of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, one or more types selected from colloidal silica, fumed silica, and pulverized silica are preferred, with colloidal silica being more preferred among these. Colloidal silica can be obtained, for example, by a production method in which it is produced from an aqueous silicic acid solution. Furthermore, these silica particles may be surface-modified or surface-reformed with functional groups, or may be composite particles formed with surfactants or other inorganic compounds. Component A may be one type or a combination of two or more types. The shape of component A may be spherical or non-spherical. The preferred form of use of component A is a silica slurry in which component A is dispersed in a medium containing water.

[0021] In the present disclosure, when the particles contained in the particle dispersion to be treated include silica particles (component A), the proportion of component A to the total particles contained in the particle dispersion to be treated is preferably 50% by mass or more, more preferably 75% by mass or more, even more preferably 90% by mass or more, still more preferably 98% by mass or more, and even more preferably substantially 100% by mass. In the present disclosure, the proportion of silica particles to the total particles contained in the particle dispersion to be treated being substantially 100% by mass means that the particles intended to be contained in the particle dispersion to be treated are silica particles only, but the presence of particles other than silica particles mixed in as impurities is permitted.

[0022] In the present disclosure, when the particles contained in the particle dispersion to be treated include silica particles (component A), the average particle size of the primary particles of component A is preferably 1 nm or more, more preferably 3 nm or more, even more preferably 5 nm or more, still more preferably 10 nm or more, still more preferably 12 nm or more, from the viewpoint of efficiently performing the filtration step of the present disclosure and from the viewpoint of improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, and from the same viewpoints is 100 nm or less, more preferably 80 nm or less, still more preferably 50 nm or less, still more preferably 30 nm or less, still more preferably 25 nm or less, still more preferably 20 nm or less. In the present disclosure, when the particles contained in the dispersion liquid of particles to be treated include silica particles, the average particle size of the primary particles of the silica particles is calculated from the amount of silanol calculated by potentiometric titration, and specifically, can be measured by the method described in the Examples.

[0023] In the filtration step of the present disclosure, the content of particles contained in the particle dispersion to be treated is preferably 1% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and even more preferably 15% by mass or more, from the viewpoint of efficiently performing the filtration step of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, and from the same viewpoint, is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less. When the particles are a combination of two or more types, the content of the particles is the total content of those particles.

[0024] When the particles contained in the particle dispersion to be treated include silica particles (component A), the content of component A in the particle dispersion to be treated is preferably 1 mass % or more, more preferably 5 mass % or more, even more preferably 10 mass % or more, and even more preferably 15 mass % or more, from the viewpoint of efficiently performing the filtration step of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, and is preferably 50 mass % or less, more preferably 45 mass % or less, and even more preferably 40 mass % or less, from the viewpoint of reducing scratches in the resulting polishing liquid. When component A is a combination of two or more types, the content of component A is the total content of those.

[0025] <Water contained in the dispersion of particles to be treated> In one or more embodiments, the particle dispersion to be processed that is subjected to the filtration step of the present disclosure contains water. Examples of water contained in the particle dispersion include ion-exchanged water, distilled water, and ultrapure water. The content of water in the particle dispersion can be the remainder obtained by subtracting the particles and other components contained as necessary from 100% by mass.

[0026] <Other components in the dispersion of particles to be treated> In one or more embodiments of the present disclosure, the dispersion of particles to be treated may further contain, in addition to particles and water, at least one selected from the group consisting of an acid (component B), an oxidizing agent (component C), a heterocyclic aromatic compound (component D), an amine compound (component E), and a water-soluble polymer (component F), which are suitable examples of other components that can be incorporated into the polishing liquid for polishing magnetic disk substrates described below. Details of components B to F will be described later.

[0027] <pH of the dispersion of particles to be treated> In one or more embodiments, from the viewpoint of efficiently performing the filtration step of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, the pH of the particle dispersion to be treated is preferably 8.5 or more, more preferably 8.8 or more, even more preferably 9.0 or more, and is preferably 11 or less, more preferably 10.8 or less, even more preferably 10.5 or less. Even when the particles contained in the dispersion of particles to be treated include silica particles, the suitable pH is the same as the suitable pH of the dispersion of particles to be treated. The pH of the particle dispersion liquid to be treated can be adjusted using a known pH adjuster. Preferred pH adjusters include sodium hydroxide, potassium hydroxide, ammonia, and tetramethylammonium hydroxide. In this disclosure, the pH is the value measured 1 minute after immersing a sample at 25°C in a pH meter. Specifically, the pH can be measured by the method described in the Examples.

[0028] (membrane filter) The membrane filter used in the filtration process of the present disclosure is a so-called surface type filter in which filtration occurs on the surface of the filter, and means that the filter material is in the form of a membrane and has pores of a certain diameter. The filter material of the membrane filter used in the filtration step of the present disclosure is preferably at least one selected from polyethersulfone (PES), polytetrafluoroethylene (PTFE), nylon, polysulfone (PS), polyvinylidene fluoride, cellulose derivatives, and polycarbonate. Among these, from the viewpoint of efficiently performing the filtration step of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, at least one selected from polyethersulfone (PES) and polysulfone (PS) is more preferred, and polyethersulfone (PES) is even more preferred.

[0029] The thickness of the membrane filter layer used in the filtration process of the present disclosure is preferably 0.05 μm or more, and more preferably 0.10 μm or more, from the viewpoint of efficiently performing the filtration process of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, and from the same viewpoint, is preferably 0.20 μm or less, and more preferably 0.15 μm or less.

[0030] In one or more embodiments, the membrane filter used in the filtration step of the present disclosure satisfies the value represented by the following formula 1 of 3.0 or more and 50 or less, where NP is the average pore number, APD is the average pore diameter, and NPC is the number of particles of 0.1 μm or more in 1 ml of the particle dispersion liquid to be processed, as obtained by particle size distribution measurement using a number counting method. Equation 1: [(NP) / [(NPC)×(APD)]]×1000000 In the present disclosure, the "average pore number NP" refers to the number of pores per 3000 μm when the membrane filter used in the filtration process of the present disclosure is observed at 20,000 magnifications under a scanning electron microscope. 2 The photograph of the field of view was scanned into a personal computer as image data, and the projection image of the openings (pores) of the membrane filter was analyzed using image analysis software. 2 The number of openings per photograph was counted, and the average value was calculated as the average pore number NP (unit: pores / 3000 μm 2) In other words, in the present disclosure, the average pore number NP is the number of pores per 3000 μm of the membrane filter used in the filtration step of the present disclosure. 2 The number of winning openings. In the present disclosure, the term "average pore diameter APD" refers to the average value of the pore diameter of the membrane filter used in the filtration process of the present disclosure, and is ... 2 The sum of the individual pore diameters obtained from the image analysis of 10 photographs of the same field of view was divided by the total number of pores to obtain the average pore diameter APD (unit: μm). In the present disclosure, the average pore number NP and average pore diameter APD refer to those on the side (primary side) into which the liquid to be filtered flows into the membrane filter.

[0031] The average pore diameter (APD, unit: μm) of the membrane filter of the present disclosure is preferably 0.1 μm or more, more preferably 0.3 μm or more, even more preferably 0.5 μm or more, even more preferably 0.8 μm or more, even more preferably 0.9 μm or more, and even more preferably 1.0 μm or more, from the viewpoint of efficiently performing the filtration process of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure. From the same viewpoint, it is preferably 2 μm or less, more preferably 1.5 μm or less, and even more preferably 1.2 μm or less. The average pore diameter of the membrane filter of the present disclosure can be adjusted by selecting a membrane filter that satisfies the above average pore diameter from existing products, or by adjusting the conditions during membrane filter preparation.

[0032] The average number of pores (NP, unit: (pores / 3000 μm) of the membrane filter of the present disclosure 2 )) is preferably 10 particles / 3000 μm from the viewpoint of efficiently performing the filtration step of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure. 2 More than 40 pieces / 3000μm is more preferable. 2 More preferably, 60 pieces / 3000 μm 2 More preferably, 100 pieces / 3000 μm 2 More preferably, 200 pieces / 3000 μm2 From the same viewpoint, it is preferably 1000 pieces / 3000 μm 2 Less than 500 / 3000μm, preferably 2 Less than 400 / 3000 μm, more preferably 2 More preferably, 300 pieces / 3000 μm or less 2 More preferably, 250 pieces / 3000 μm or less 2 The following is the result. The average pore number of the membrane filter of the present disclosure can be adjusted by selecting a membrane filter that satisfies the above average pore number from existing products, or by adjusting the conditions during membrane filter preparation.

[0033] The membrane filter of the present disclosure may be a pleated membrane filter (hereinafter simply referred to as a "pleated filter") from the viewpoint of efficiently performing the filtration process of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure. In the present disclosure, the pleated filter may be a hollow cylindrical cartridge type filter formed by molding a membrane into a pleated shape.

[0034] The membrane filters may be used in one stage or in multiple stages (for example, in a series arrangement).

[0035] The filtration pressure in the filtration step of the present disclosure is preferably 0.16 MPa or more, more preferably 0.18 MPa or more, and even more preferably 0.20 MPa or more, from the viewpoint of efficiently performing the filtration step of the present disclosure and improving the quality of the polishing liquid containing the particle dispersion obtained by the filtration method of the present disclosure, and from the same viewpoint, is preferably 0.49 MPa or less, more preferably 0.45 MPa or less, even more preferably 0.40 MPa or less, and even more preferably 0.30 MPa or less. In the present disclosure, the "filtration pressure in the filtration process of the present disclosure" refers to the difference between the pressure on the side (primary side) where the particle dispersion to be treated flows in and the pressure on the side (secondary side) where the filtrate flows out of a membrane filter used in the filtration process of the present disclosure, and in one or more embodiments, the filtration pressure in the filtration process of the present disclosure can be adjusted by adjusting the pressure applied to the primary side.

[0036] The filtration method in the filtration step of the present disclosure may be a circulation method in which filtration is repeated, or a single-pass method. Alternatively, a batch method in which single-pass methods are repeated may be used. A pump may be used to apply pressure to the liquid. In the single-pass method, in addition to using a pump, a pressure filtration method in which air pressure or the like is introduced into a tank to reduce fluctuations in the filter inlet pressure may also be used.

[0037] [Pre-filtration process] In one or more embodiments, the filtration method of the present disclosure preferably includes a step of filtering using a depth filter prior to the step 1, ie, a pre-filtration step. Therefore, in one or more other embodiments, the filtration method of the present disclosure preferably includes the following step 0 before step 1. Step 0: Filtration of particle dispersion through a depth filter Step 1: A step of filtering the particle dispersion obtained in step 0 through a membrane filter Here, when the number of particles of 0.1 μm or more obtained by particle size distribution measurement using a number counting method in 1 ml of the particle dispersion obtained in step 0, i.e., the particle dispersion filtered in step 1, is defined as NPC, the average number of pores in the membrane filter is defined as NP, and the average pore diameter of the membrane filter is defined as APD, it is preferable that the value represented by the following formula 1 is 3.0 or more and 50 or less. Equation 1: [(NP) / [(NPC)×(APD)]]×1000000

[0038] In this disclosure, the term "depth filter" refers to a type of filter in which filtration occurs internally. More specifically, it refers to a filter characterized by the fact that the mesh size of the filter material is coarse at the inlet side where fluid flows into the depth filter and fine at the outlet side where liquid flows out, and that the mesh size becomes finer continuously or stepwise from the inlet side to the outlet side. Therefore, among coarse particles, larger particles are captured near the inlet side, and smaller particles are captured near the outlet side. Depth filters come in a variety of shapes, including bag-type and hollow cylindrical cartridge-type filters. Furthermore, a filter material having the above characteristics that is pleated is also classified as a depth filter because it functions as a depth filter. Commercially available depth filters include, for example, bag-type filters (3M's Bag 500 series) and cartridge-type filters (Advantech Toyo's TCPD series, Nippon Pall's Profile II series, 3M's GPJ series, Daiwabo's Wavestar II series, and Roki Techno's SHP type).

[0039] The depth filter may be used in a single stage, or two or more identical filters may be combined in series in multiple stages, or non-depth filters with different pore sizes may be combined so that the liquid to be filtered passes through the filters with the largest pores first. Furthermore, these may be used in combination with bag-type and cartridge-type filters.

[0040] The mesh size of the depth filter is preferably 0.1 μm or more from the viewpoints of improving productivity and extending the life of the filter, and is preferably 5.0 μm or less, more preferably 3.0 μm or less, even more preferably 2.0 μm or less, even more preferably 1.0 μm or less, and even more preferably 0.5 μm or less from the viewpoints of maintaining the quality of the particle dispersion obtained by the filtration method of the present disclosure and reducing coarse particles. In the present disclosure, the mesh size is an index indicating the size of the openings of the depth filter, and is also referred to as the nominal filtration accuracy.

[0041] In one or more embodiments, the filtration method of the present disclosure may include a general dispersion step or particle removal step in addition to the filtration step (step 1) of the present disclosure and, if necessary, a pre-filtration step using a depth filter (step 0). Examples of such steps include a dispersion step using a high-pressure dispersion device such as a high-speed dispersion device or a high-pressure homogenizer, and a coarse particle settling step using a centrifugal separator. When using these methods for treatment, they may be used alone or in combination with two or more other methods, with no particular restriction on the order of treatment. Furthermore, the treatment conditions and number of treatments may be appropriately selected and used. In one or more embodiments, the filtration method of the present disclosure may include, before step 1, a step of confirming that the value represented by Equation 1 for the particle dispersion and membrane filter to be filtered is 3.0 or more and 50 or less. In one or more embodiments, the filtration method of the present disclosure may further include, prior to step 1, a step of adjusting the value of the particle dispersion and membrane filter to be filtered, as expressed by Equation 1, to be 3.0 or more and 50 or less.

[0042] In one or more embodiments, the filtration method of the present disclosure is preferably a method of (a) subjecting a processed particle dispersion containing particles and water, and, if necessary, other components (component B, component C, component D, component E, component F) that can be incorporated into a polishing liquid for magnetic disk substrates, to the filtration step of the present disclosure to obtain a particle dispersion obtained by the filtration method of the present disclosure; or (b) subjecting a processed particle dispersion containing particles and water to the filtration step of the present disclosure, and then mixing the filtered particle dispersion obtained in the filtration step with other components (component B, component C, component D, component E, component F) that can be incorporated into a polishing liquid for magnetic disk substrates, to obtain a particle dispersion obtained by the filtration method of the present disclosure. Among these, from the viewpoint of improving the quality of the resulting polishing liquid for magnetic disk substrates, a more preferred method is (b) subjecting a processed particle dispersion containing particles and water to the filtration step of the present disclosure, and then mixing the filtered particle dispersion obtained in the filtration step with other components (component B, component C, component D, component E, component F) that can be incorporated into a polishing liquid for magnetic disk substrates, to obtain a particle dispersion obtained by the filtration method of the present disclosure.

[0043] [Particle dispersion obtained by the filtration method of the present disclosure (particle dispersion after filtration)] The particle dispersion obtained by the filtration method of the present disclosure is a particle dispersion obtained by subjecting the treated particle dispersion to the filtration step of the present disclosure, i.e., a particle dispersion after filtration in the filtration step of the present disclosure. By adding the particle dispersion obtained by the filtration method of the present disclosure to a polishing liquid for magnetic disks, it is possible to obtain a polishing liquid for magnetic disks that can produce substrates with reduced scratches.

[0044] The suitable pH of the particle dispersion obtained by the filtration method of the present disclosure is, in one or more embodiments, the same as the pH of the particle dispersion to be treated described above, preferably 8.5 or more, more preferably 8.8 or more, even more preferably 9.0 or more, and is preferably 11 or less, more preferably 10.8 or less, even more preferably 10.5 or less. In the present disclosure, when the particles contained in the particle dispersion to be treated include silica particles, the suitable pH of the particle dispersion obtained by the filtration method of the present disclosure is the same as the suitable pH of the particle dispersion obtained by the above-mentioned filtration method of the present disclosure that does not specify that the particles contained in the particle dispersion to be treated include silica particles. In the present disclosure, the pH of the particle dispersion obtained by the filtration method of the present disclosure when the particles contained in the particle dispersion to be treated include silica particles is also a value measured 1 minute after immersing a sample at 25°C in a pH meter.

[0045] The content and pH of each component in the particle dispersion obtained by the filtration process of the present disclosure are substantially the same as those of the treated particle dispersion, because the mass of the coarse particles removed by the filtration process of the present disclosure is small. In the present disclosure, "each component" refers to non-coarse particles, water, and, if present, the acid (component B), oxidizing agent (component C), heterocyclic aromatic compound (component D), amine compound (component E), and water-soluble polymer (component F). "Substantially the same" means that the composition of the treated particle dispersion and the particle dispersion after filtration obtained by the filtration process of the present disclosure are the same except for the number of coarse particles.

[0046] The particle dispersion after filtration, i.e., the particle dispersion obtained by the filtration method of the present disclosure, can be applied to various fields such as abrasives, coating agents, fillers, ceramic binders, catalyst carriers, adsorbents, etc. Furthermore, the particle dispersion after filtration, i.e., the particle dispersion obtained by the filtration method of the present disclosure, can be suitably used as abrasive grains in a polishing liquid used for polishing substrates such as semiconductor substrates and magnetic disk substrates, and can be more suitably used as abrasive grains in a polishing liquid used for polishing magnetic disk substrates.

[0047] Generally, a polishing liquid is a mixture of abrasive grains, water, and, if necessary, additives. Therefore, in one or more embodiments, the filtration method of the present disclosure may include, after the filtration step of the present disclosure, a step of mixing the filtered particle dispersion obtained in the filtration step of the present disclosure with water and additives. In the present disclosure, the term "blending" includes mixing the filtered particle dispersion, water, and, if necessary, additives simultaneously or in any order. The blending can be performed using a mixer such as a homomixer, a homogenizer, an ultrasonic disperser, or a wet ball mill. That is, the method for producing the polishing slurry for polishing magnetic disk substrates of the present disclosure preferably includes a step of mixing the particle dispersion obtained by the filtration method of the present disclosure with at least one selected from an acid, an oxidizing agent, a heterocyclic aromatic compound, an amine compound, and a water-soluble polymer. In the present disclosure, the additive refers to a component other than abrasive grains and water that can be blended into the polishing liquid used for polishing a substrate to be polished. Preferred examples of the component other than abrasive grains and water include an acid (component B), an oxidizing agent (component C), a heterocyclic aromatic compound (component D), an amine compound (component E), and a water-soluble polymer (component F), which will be described later.

[0048] [Polishing solution for magnetic disk substrates] In one aspect, the present disclosure relates to a polishing liquid for magnetic disk substrates obtained by the method for producing a polishing liquid for polishing magnetic disk substrates of the present disclosure (hereinafter, the polishing liquid may also be referred to as the "polishing liquid of the present disclosure"). In one or more embodiments, the polishing liquid of the present disclosure may further contain, in addition to the particle dispersion obtained by the filtration method of the present disclosure, one or more compounds selected from an acid (component B), an oxidizing agent (component C), a heterocyclic aromatic compound (component D), an amine compound (component E), and a water-soluble polymer (component F). In one or more embodiments, the abrasive grains in the polishing liquid of the present disclosure are particles in a particle dispersion obtained by the filtration method of the present disclosure. The polishing liquid of the present disclosure refers to both a polishing liquid for magnetic disk substrates obtained by further blending other components that can be blended into a polishing liquid for magnetic disk substrates with the particle dispersion obtained by the filtration method of the present disclosure, and a polishing liquid for magnetic disk substrates obtained by filtering a particle dispersion to be treated and some or all of the other components that can be blended into a polishing liquid for magnetic disk substrates in the filtration step of the present disclosure to obtain a particle dispersion after filtration, and further blending the remaining other components that can be blended into a polishing liquid for magnetic disk substrates.

[0049] <Abrasive grains contained in the polishing liquid of the present disclosure> The abrasive grains contained in the polishing liquid of the present disclosure are derived from the particles contained in the dispersion of particles to be treated, that is, they are derived from the particle dispersion after filtration obtained in the filtration step of the present disclosure. The content of abrasive grains in the polishing liquid of the present disclosure is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.2% by mass or more, from the viewpoint of improving the polishing rate of the polishing liquid of the present disclosure, and is preferably 10% by mass or less, more preferably 7.5% by mass or less, and even more preferably 5% by mass or less, from the viewpoint of improving dispersibility and storage stability. When the abrasive grains contained in the polishing liquid of the present disclosure include silica particles (component A), the content of component A in the polishing liquid of the present disclosure is preferably 0.05 mass% or more, more preferably 0.1 mass% or more, and even more preferably 0.2 mass% or more, from the viewpoint of improving the polishing rate of the polishing liquid of the present disclosure, and is preferably 10 mass% or less, more preferably 7.5 mass% or less, and even more preferably 5 mass% or less, from the viewpoint of improving dispersibility and storage stability. When the abrasive grains contained in the polishing liquid of the present disclosure include silica particles, the proportion of component A in the abrasive grains of the polishing liquid of the present disclosure is preferably 50 mass% or more, more preferably 75 mass% or more, even more preferably 90 mass% or more, still more preferably 98 mass% or more, and even more preferably substantially 100 mass%. The proportion of component A in the abrasive grains of the polishing liquid of the present disclosure being substantially 100% by mass means that the particles contained are silica particles, but the presence of particles other than silica particles mixed in as impurities is permitted.

[0050] <Water contained in the polishing liquid of the present disclosure> The water contained in the polishing liquid of the present disclosure originates from the water contained in the particle dispersion after filtration obtained in the filtration step of the present disclosure, but may be added separately during preparation of the polishing liquid of the present disclosure, or a portion of the water may be distilled off. Furthermore, when the polishing liquid of the present disclosure further contains one or more optional components selected from the group consisting of an acid (component B), an oxidizing agent (component C), a heterocyclic aromatic compound (component D), an amine compound (component E), and a water-soluble polymer (component F), the water may be introduced along with these components. The water content in the polishing liquid of the present disclosure corresponds to the remainder obtained by subtracting from 100% by mass the particles contained in the particle dispersion obtained by the filtration method of the present disclosure, the acid, oxidizing agent, heterocyclic aromatic compound, amine compound, water-soluble polymer, and other components described below, and is preferably 60% by mass or more, more preferably 80% by mass or more, and is preferably 99% by mass or less, more preferably 97% by mass or less.

[0051] In one or more embodiments, the polishing liquid of the present disclosure preferably further contains at least one selected from an acid (component B), an oxidizing agent (component C), a heterocyclic aromatic compound (component D), an amine compound (component E), and a water-soluble polymer (component F). Components B to F are described below.

[0052] <Acid (component B)> The polishing liquid of the present disclosure preferably contains an acid (hereinafter also referred to as "component B"). In the present disclosure, component B includes an acid or a salt thereof. Component B may be one type or a combination of two or more types.

[0053] Examples of Component B include inorganic acids such as nitric acid, sulfuric acid, sulfurous acid, persulfuric acid, hydrochloric acid, perchloric acid, phosphoric acid, phosphonic acid, phosphinic acid, pyrophosphoric acid, tripolyphosphoric acid, and amidosulfuric acid; and organic acids such as organic phosphoric acid, organic phosphonic acid, and carboxylic acid. Among these, Component B preferably contains an inorganic acid and an organic phosphonic acid, and more preferably contains an inorganic acid, from the viewpoints of improving the polishing rate and reducing scratches in the polishing liquid. The inorganic acid is preferably at least one selected from nitric acid, sulfuric acid, hydrochloric acid, perchloric acid and phosphoric acid, and more preferably phosphoric acid. The organic phosphonic acid is preferably at least one selected from 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), aminotri(methylenephosphonic acid), ethylenediaminetetra(methylenephosphonic acid), and diethylenetriaminepenta(methylenephosphonic acid), with HEDP being more preferred. Examples of salts of these acids include salts of the above acids with at least one selected from metals, ammonia, and alkylamines. Examples of the metals include metals belonging to Groups 1 to 11 of the periodic table.

[0054] When the polishing liquid of the present disclosure contains component B, the content of component B in the polishing liquid of the present disclosure is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.5% by mass or more, from the viewpoints of improving the polishing rate and reducing scratches in the polishing liquid, and from the same viewpoints, is preferably 5% by mass or less, more preferably 4% by mass or less, even more preferably 3% by mass or less, and even more preferably 2% by mass or less. When component B is a combination of two or more types, the content of component B is the total content thereof.

[0055] <Oxidizing agent (ingredient C)> From the viewpoint of improving the polishing rate and further reducing scratches during polishing with the polishing liquid of the present disclosure, it is preferable that the polishing liquid of the present disclosure further contains an oxidizing agent (hereinafter also referred to as "component C"). Component C may be one type or a combination of two or more types.

[0056] From the viewpoint of improving the polishing rate and further reducing scratches during polishing with the polishing liquid of the present disclosure, examples of component C include peroxides, permanganic acid or its salts, chromic acid or its salts, peroxoacids or their salts, oxyacids or their salts, metal salts, nitric acids, sulfuric acids, etc. Among these, at least one selected from hydrogen peroxide, iron(III) nitrate, peracetic acid, ammonium peroxodisulfate, iron(III) sulfate, and ammonium iron(III) sulfate is preferred, and hydrogen peroxide is more preferred from the viewpoint of improving the polishing rate, preventing metal ions from adhering to the surface of the substrate to be polished, and being easily available.

[0057] When the polishing liquid of the present disclosure contains component C, the content of component C in the polishing liquid of the present disclosure is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more, and is preferably 4% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less, from the viewpoint of further improving the polishing rate and further reducing scratches during polishing with the polishing liquid of the present disclosure. When component C is a combination of two or more types, the content of component C is the total content thereof.

[0058] <Heterocyclic aromatic compounds (component D)> In one or more embodiments, the polishing liquid of the present disclosure preferably further contains a heterocyclic aromatic compound (including a salt thereof) (hereinafter also referred to as "Component D") from the viewpoint of further reducing scratches during polishing with the polishing liquid of the present disclosure. Component D may be one type or a combination of two or more types.

[0059] From the viewpoint of further reducing scratches during polishing with the polishing liquid of the present disclosure, component D is preferably a heterocyclic aromatic compound containing two or more nitrogen atoms in the heterocycle, more preferably three or more nitrogen atoms in the heterocycle, and preferably nine or fewer nitrogen atoms in the heterocycle, more preferably five or fewer nitrogen atoms in the heterocycle, and even more preferably four or fewer nitrogen atoms in the heterocycle.

[0060] In one or more embodiments, Component D is preferably at least one selected from 1,2,4-triazole, 3-amino-1,2,4-triazole, 5-amino-1,2,4-triazole, 3-mercapto-1,2,4-triazole, 1H-tetrazole, 5-aminotetrazole, 1H-benzotriazole (BTA), 1H-tolyltriazole, 2-aminobenzotriazole, 3-aminobenzotriazole, and alkyl- or amine-substituted derivatives thereof. Examples of the alkyl group in the alkyl-substituted derivative include lower alkyl groups having 1 to 4 carbon atoms, and in one or more embodiments, examples include methyl and ethyl groups. In one or more embodiments, examples of the amine-substituted derivative include 1-[N,N-bis(hydroxyethylene)aminomethyl]benzotriazole and 1-[N,N-bis(hydroxyethylene)aminomethyl]tolyltriazole. Among these, from the viewpoint of further reducing scratches, component D is more preferably at least one selected from 1H-benzotriazole (BTA), 1H-tolyltriazole, 2-aminobenzotriazole, and 3-aminobenzotriazole, and 1H-benzotriazole (BTA) is even more preferred.

[0061] When the polishing liquid of the present disclosure contains component D, the content of component D in the polishing liquid of the present disclosure is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, and even more preferably 0.02% by mass or more from the viewpoint of further reducing scratches, and is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 1% by mass or less, and even more preferably 0.2% by mass or less from the viewpoint of improving the polishing rate. When component D is a combination of two or more types, the content of component D refers to the total content thereof.

[0062] <Amine compound (ingredient E)> In one or more embodiments, the polishing liquid of the present disclosure preferably further contains an amine compound (hereinafter also referred to as "component E") from the viewpoint of further reducing scratches during polishing with the polishing liquid of the present disclosure. From the viewpoint of further reducing scratches with the polishing liquid of the present disclosure, the number of amino groups in the molecule of component E is preferably 2 or more and 4 or less. Component E may be one type or a combination of two or more types.

[0063] In the present disclosure, suitable examples of the amine compound (component E) include an aliphatic amine compound and an alicyclic amine compound. In one or more embodiments, from the viewpoint of further reducing scratches, the aliphatic amine compound is preferably at least one selected from ethylenediamine, N,N,N',N'-tetramethylethylenediamine, 1,2-diaminopropane, 1,3-diaminopropane, 1,4-diaminobutane, hexamethylenediamine, 3-(diethylamino)propylamine, 3-(dibutylamino)propylamine, 3-(methylamino)propylamine, 3-(dimethylamino)propylamine, N-aminoethylethanolamine, N-aminoethylisopropanolamine, and N-aminoethyl-N-methylethanolamine, more preferably at least one selected from N-aminoethylethanolamine, N-aminoethylisopropanolamine, and N-aminoethyl-N-methylethanolamine, and even more preferably N-aminoethylethanolamine (AEEA). In one or more embodiments, from the viewpoint of further reducing scratches, the alicyclic amine compound is preferably at least one selected from piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, 1-amino-4-methylpiperazine, N-methylpiperazine, and hydroxyethylpiperazine (HEP), and more preferably hydroxyethylpiperazine (HEP).

[0064] When the polishing liquid of the present disclosure contains component E, the content of component E in the polishing liquid of the present disclosure is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, and even more preferably 0.02% by mass or more, from the viewpoint of further reducing scratches during polishing with the polishing liquid of the present disclosure, and is preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, from the viewpoint of improving the polishing rate. When component E is a combination of two or more types, the content of component E refers to the total content thereof.

[0065] <Water-soluble polymer (component F)> In one or more embodiments, the polishing liquid of the present disclosure preferably further contains a water-soluble polymer (hereinafter also referred to as "component F") from the viewpoint of further reducing scratches during polishing with the polishing liquid of the present disclosure. In the present disclosure, "water-soluble" means having a solubility in water (20°C) of 0.5 g / 100 mL or more, preferably 2 g / 100 mL or more. Component F may be one type or a combination of two or more types.

[0066] The water-soluble polymer (component F) is more preferably an anionic water-soluble polymer having an anionic group in the molecule. In one or more embodiments, a suitable example of a monomer having an anionic group in the molecule is a vinyl monomer having an anionic group in the molecule. Preferred examples of the anionic group of a vinyl monomer having an anionic group in the molecule include a carboxylic acid group and a sulfonic acid group. Specific examples of vinyl monomers having a carboxylic acid group in the molecule include at least one selected from acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, and salts thereof. Specific examples of vinyl monomers having a sulfonic acid group in the molecule include at least one selected from 2-acrylamido-2-methylpropanesulfonic acid, styrenesulfonic acid, and salts thereof. The water-soluble polymer having, as a constituent unit, a vinyl monomer having an anionic group in the molecule may also have a vinyl monomer other than the vinyl monomer having an anionic group in the molecule. When component F is a water-soluble polymer having, as a constituent unit, a vinyl monomer having an anionic group in the molecule, suitable specific examples of component F include acrylic acid / 2-acrylamido-2-methylpropanesulfonic acid copolymer (AA / AMPS), polystyrenesulfonic acid, and salts thereof. In one or more embodiments, a suitable example of a monomer having an anionic group in its molecule is an aromatic compound monomer containing a sulfonic acid group or a salt thereof. The aromatic compound monomer containing a sulfonic acid group or a salt thereof is preferably a compound having a structure in which at least one hydrogen atom of an aromatic ring is substituted with a sulfonic acid group, or a salt thereof, and more preferably at least one selected from phenolsulfonic acid, naphthalenesulfonic acid, and salts thereof. Preferred examples of the salt include alkali metal salts, ammonium salts, and organic amine salts. The water-soluble polymer having an aromatic compound monomer containing a sulfonic acid group or a salt thereof in its molecule as a structural unit as component F preferably contains a structural unit other than the aromatic compound monomer containing a sulfonic acid group or a salt thereof in its molecule, and a specific example of such a structural unit is preferably one or more structural units selected from a methylene group and a bis(4-hydroxyphenyl)sulfone (BisS) group. When component F is a water-soluble polymer having, as a constituent unit, an aromatic compound monomer containing a sulfonic acid group or a salt thereof in the molecule, suitable examples of component F include at least one selected from the group consisting of a formalin condensate of phenolsulfonic acid (PhS), a formalin condensate of naphthalenesulfonic acid (NaS), a formalin condensate of bis(4-hydroxyphenyl)sulfone (BisS) and phenolsulfonic acid (PhS), and salts thereof.

[0067] From the viewpoint of reducing scratches during polishing with the polishing liquid of the present disclosure, the weight-average molecular weight of component F is preferably 500 or more, more preferably 1,000 or more, and even more preferably 1,500 or more, and is preferably 50,000 or less, more preferably 30,000 or less, even more preferably 20,000 or less, even more preferably 10,000 or less, and even more preferably 5,000 or less. In the present disclosure, the weight-average molecular weight of component F can be a value measured by a conventional method. Alternatively, a value listed in a catalog can be used.

[0068] When the polishing liquid of the present disclosure contains component F, the content of component F in the polishing liquid of the present disclosure is, from the viewpoint of reducing scratches during polishing with the polishing liquid of the present disclosure, preferably 0.001% by mass or more, more preferably 0.005% by mass or more, even more preferably 0.01% by mass or more, and preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less. When component F is a combination of two or more types, the content of component F refers to the total content thereof.

[0069] <Other ingredients> The polishing liquid according to the present disclosure may further contain other components as needed. In one or more embodiments, the other components may include a thickener, a surfactant, and the like.

[0070] The contents of each component described above are the contents at the time of use in the polishing process, and the polishing liquid of the present disclosure may be stored and supplied in a concentrated state to the extent that its storage stability is not impaired. The concentrated state of the polishing liquid of the present disclosure, i.e., the concentration rate of the concentrate, is preferably 1.5 times or more, more preferably 10 times or more, even more preferably 30 times or more, and even more preferably 50 times or more from the viewpoint of production and transportation costs, and is preferably 300 times or less, more preferably 200 times or less, even more preferably 150 times or less, and even more preferably 100 times or less from the viewpoint of storage stability. The concentration ratio of the polishing liquid concentrate in the present disclosure means [solids concentration of the polishing liquid concentrate / solids concentration of the polishing liquid at the time of use]. The solids concentration of the polishing liquid concentrate and the solids concentration of the polishing liquid at the time of use are the ratio of the mass of the components other than water in the polishing liquid concentrate to the mass of the polishing liquid concentrate, and the ratio of the mass of the components other than water in the polishing liquid at the time of use to the mass of the polishing liquid at the time of use, respectively.

[0071] The pH of the polishing liquid of the present disclosure at 25°C is preferably 0.1 or more, more preferably 0.5 or more, even more preferably 0.7 or more, and even more preferably 1.0 or more, from the viewpoint of reducing scratches during polishing with the polishing liquid of the present disclosure, and is preferably 4.0 or less, more preferably 3.0 or less, even more preferably 2.5 or less, and even more preferably 2.0 or less, from the viewpoint of ensuring a high polishing rate during polishing with the polishing liquid of the present disclosure. In the present disclosure, even when the abrasive grains contained in the polishing liquid include silica particles, the suitable pH is the same as the suitable pH of the polishing liquid. In the present disclosure, the pH of the polishing liquid can be measured by the same method as that for the above-described dispersion of particles to be treated and the particle dispersion after filtration obtained in the filtration step of the present disclosure.

[0072] The polishing liquid according to the present disclosure may be a so-called one-component type, in which all components are premixed and supplied to the market, or may be a so-called two-component type, in which components are mixed at the time of use.

[0073] [Method of manufacturing magnetic disk substrate] In one aspect, the present disclosure relates to a method for manufacturing a magnetic disk substrate (hereinafter also referred to as the "substrate manufacturing method of the present disclosure"), which includes supplying a polishing liquid for a magnetic disk substrate obtained by the manufacturing method of the present disclosure to a surface of a substrate to be polished, and bringing a polishing pad into contact with the surface to be polished and moving at least one of the polishing pad and the substrate to be polished to polish the surface to be polished.

[0074] In the substrate manufacturing method of the present disclosure, suitable materials for the substrate to be polished include metals or semimetals such as silicon, aluminum, nickel, tungsten, copper, tantalum, titanium, etc., or alloys thereof, glassy substances such as glass, glassy carbon, amorphous carbon, etc., ceramic materials such as alumina, silicon dioxide, silicon nitride, tantalum nitride, titanium carbide, etc., resins such as polyimide resin, etc. Among these, metals such as aluminum, nickel, tungsten, copper, etc., and alloys containing these metals as the main component are preferred, and for example, one selected from Ni-P plated aluminum alloy substrates, crystallized glass, tempered glass, etc., is more preferred, and Ni-P plated aluminum alloy substrates are even more preferred.

[0075] When the substrate manufacturing method of the present disclosure includes multiple polishing steps, the polishing liquid obtained by the method for manufacturing a polishing liquid for polishing magnetic disk substrates of the present disclosure is preferably used in the second or subsequent steps, and more preferably in the final polishing step. The final polishing step refers to the final polishing step when multiple polishing steps are included.

[0076] When there are multiple polishing steps, a separate polishing machine may be used for each step to avoid contamination with abrasives or polishing solutions from the previous step, and when separate polishing machines are used, it is preferable to clean the substrate after each step. Note that the polishing machine is not particularly limited. According to the substrate manufacturing method of the present disclosure, it is possible to efficiently manufacture magnetic disk substrates with reduced scratches on the substrate surface. [Example]

[0077] Hereinafter, the present disclosure will be described with reference to examples, but the present disclosure is not limited thereto.

[0078] [Processed particle dispersion] In Examples 1 to 3 and Comparative Example 2, the following dispersion of particles to be treated was used. Colloidal silica slurry (pH 9.0, manufactured by JGC Catalysts and Chemicals, average primary particle diameter 18.0 nm, silica particle concentration 40% by mass) In Examples 4 to 7, the following dispersion of particles to be treated was used. Colloidal silica slurry (pH 10.1, manufactured by JGC Catalysts and Chemicals, average primary particle diameter 6.0 nm, silica particle concentration 20% by mass) In Examples 8 to 10 and Comparative Example 1, the following dispersion of particles to be treated was used. Colloidal silica slurry (pH 9.0, manufactured by JGC Catalysts and Chemicals, average primary particle diameter 25.0 nm, silica particle concentration 48% by mass)

[0079] [Measurement of the average particle size of primary particles of silica particles] The average particle size of the primary particles of the silica particles in the colloidal silica slurry is measured as follows. 1.5 g of solids of colloidal silica slurry was weighed into a 200 mL beaker, and 100 mL of ion-exchanged water was added and mixed with a stirrer. Next, using a potentiometric titrator, the pH of the sample solution was adjusted to 3.0 with 0.1 mol / L hydrochloric acid standard solution. 30.0 g of sodium chloride was added and dissolved with a stirrer. Immerse the sample in a thermostatic water bath (20 ± 2°C) for approximately 30 minutes. Using a potentiometric titrator, titration was performed with 0.1 mol / L sodium hydroxide standard solution. The amount (g) of sodium hydroxide standard solution used when the pH changed from 4.0 to 9.0 was recorded (A). A blank test was also performed, and the amount (g) of sodium hydroxide standard solution required for the blank titration (B) was recorded. The average primary particle size (nm) was then calculated using the following formula: Average particle size of primary particles of silica particles (nm) = 3100 ÷ 26.5 × (AB) ÷ precisely weighed weight of colloidal silica slurry (g)

[0080] [Measurement of pH of the particle dispersion to be treated, the particle dispersion after filtration obtained by the filtration step of the present disclosure, and the polishing liquid] The pH of the particle dispersion to be treated, the particle dispersion after filtration obtained in the filtration step of the present disclosure, and the polishing liquid was measured using a pH meter (HM-30G, Toa Dempa Kogyo Co., Ltd.) The measurement temperature was 25°C, and the values ​​were read one minute after the electrodes of the pH meter were immersed in the particle dispersion to be treated, the particle dispersion after filtration obtained in the filtration step of the present disclosure, and the polishing liquid.

[0081] [Method for measuring NPC, the number of particles of 0.1 μm or larger obtained by particle size distribution measurement using the number count method in 1 ml of particle dispersion to be processed] A colloidal silica slurry with a solids content of 40% by mass was collected in a 100 mL beaker as a particle dispersion to be treated, diluted with ion-exchanged water to a solids concentration of 0.25% by mass, and measured using an Accusizer FXnano (manufactured by Entegris Japan, flow rate 60 mL / min, injection volume 5 mL, temperature 25°C). The result was converted to the number of coarse particles of 0.1 μm or larger contained in 1 mL of 40% solids content, and the number of particles of 0.1 μm or larger NPC (unit: particles / 1 mL) obtained by particle size distribution measurement using a number counting method in 1 mL of the particle dispersion to be treated according to the present disclosure was obtained. Note that colloidal silica slurries with different numbers of coarse particles of 0.1 μm or larger were adjusted by pre-filtration using a Profile II filter manufactured by Pall Corporation.

[0082] [Membrane filter pore size and number] The membrane filter to be measured was observed under a scanning electron microscope (Hitachi High-Technologies Corporation, FE-4800, 30 kV, 20,000 magnification), and the 3000 μm 2 Ten photographs of the field of view were scanned into a personal computer as image data, and analyzed using image analysis software (Mitani Shoji "WinROOF2017") to calculate the diameter of each pore. The average value of all the pores observed was taken as the average pore diameter APD (unit: μm). The number of filter pores per photograph was also counted using the photographs, and the average of the 10 photographs was taken as the average pore number NP (unit: pores / 3000 μm 2 ) was decided.

[0083] [Preparation of membrane filters] The membrane filters used in Examples 1, 5, and 8 are prepared according to the following procedure. Polysulfone (Udel P3500, molecular weight 59,000, manufactured by Solvay) was dissolved in a solvent (dimethylacetamide, manufactured by Fujifilm Wako Pure Chemical Industries) to a concentration of 20% by mass and thoroughly stirred to prepare a uniform membrane-forming solution. The membrane-forming solution was uniformly applied to a 10 cm square metal plate to a thickness of 0.12 mm. Immediately thereafter, the metal plate was immersed in 1 L of ion-exchanged water adjusted to 15°C for 10 minutes. The polymer coagulated in the ion-exchanged water was removed and dried overnight at 25°C, and this was used as the membrane filter of Preparation Example 1. The side in contact with the metal plate was designated the primary side (the side into which the particle dispersion to be treated flows). The membrane filters used in Examples 2, 6, and 9 are prepared according to the following procedure. Polyethersulfone (Sumikaexcel 5003PS, molecular weight 50,000, manufactured by Sumitomo Chemical) was dissolved in a solvent (dimethylacetamide, manufactured by Fujifilm Wako Pure Chemical Industries) to a concentration of 30% by mass, and a membrane filter was obtained in the same manner as in Preparation Example 1. In Examples 3, 7, and 10, the following commercially available filters were used. Tocell TCS002 (manufactured by Advantech, material: polyethersulfone, filtration accuracy 0.2 μm, film thickness 0.12 mm) In Example 4, Comparative Examples 1 and 2, the following commercially available filters were used. K020A025A (Advantech, Material: Polycarbonate, Filtration accuracy 0.20 μm, Film thickness 0.12 mm)

[0084] [Filtration process] Examples 1 to 10, Comparative Examples 1 to 2 The colloidal silica slurry was pre-filtered using a 20-inch Pall Profile II filter (nominal filtration accuracy: 1.0 μm, depth filter) to obtain a treated particle dispersion according to the present disclosure. The number of particles 0.1 μm or larger (NPC) per ml of the treated particle dispersion obtained by pre-filtration, as determined by particle size distribution measurement using a number counting method, is shown in Table 1. The prepared membrane filters and commercially available membrane filters having the average pore numbers NP and average pore diameters APD shown in Table 1 were cut to a diameter of 25 mm and placed in a plastic holder (PP-25) manufactured by Advantec Corporation. Filtering was performed under conditions of a pressure of 0.2 MPa and a filtration rate of 50 g / min (Examples 1 to 10 are the filtration process of the present disclosure). The particle dispersions obtained in the filtration process (Examples 1 to 10 are the filtered particle dispersions according to the present disclosure) all had a pH of 9.0.

[0085] [Filter life] In the above filtration process, the total amount of liquid passing through was calculated when the filtration rate dropped to 17 g / min (1 / 3 of the initial rate). The filter life equivalent to that of an actual production scale was then calculated by multiplying the amount of liquid passing through by the ratio of the effective filtration area of ​​a 25 mm diameter filter to the total effective filtration area assumed for use in actual production. The results are shown in Table 1.

[0086] [Preparation of polishing solution] Polishing solutions containing filtered particle dispersions obtained by the filtration process of the present disclosure (Examples 1-10) and polishing solutions containing filtered particle dispersions obtained by a filtration process other than the present disclosure (Comparative Examples 1-2) were prepared by adding and mixing 0.1% by mass of 1H-benzotriazole sodium salt (Component D), 0.03% by mass of N-aminoethylethanolamine (Component E), 0.02% by mass of acrylic acid / acrylamido-2-methylpropanesulfonic acid copolymer sodium salt (molar ratio AA / AMPS: 90 / 10, weight-average molecular weight 2000, manufactured by Toagosei Co., Ltd.) (Component F), 1.0% by mass of phosphoric acid (Component B), 0.4% by mass of hydrogen peroxide (Component C), and 5% by mass of the filtered particle dispersions (Component A) obtained in Examples 1-10 and Comparative Examples 1-2. The pH of the resulting polishing solutions was all 1.8.

[0087] Using the polishing solutions prepared as described above in Examples 1 to 10 and Comparative Examples 1 and 2, finish polishing was carried out on the substrates to be polished under the following polishing conditions. The number of scratches on the substrates after each polishing was evaluated. The evaluation results of the finish polishing are shown in Table 1 below. [Substrate to be polished] The substrate to be polished was a Ni-P plated aluminum alloy substrate that had been roughly polished in advance with a polishing solution containing silica abrasive grains. The substrate had a thickness of 0.6 mm, an outer diameter of 97 mm, and an inner diameter of 25 mm. The centerline average roughness (Ra) measured using an AFM (Digital Instrument NanoScope IIIa Multimode AFM) was 1 nm. The mass ratio of Ni to P in the Ni-P plating was 88:12. [Polishing conditions] Polishing machine: Double-sided polishing machine (9B type double-sided polishing machine, manufactured by Speedfam) Number of substrates to be polished: 10 Polishing liquid: Polishing liquids of Examples 1 to 10 and Comparative Examples 1 and 2 Polishing pad: Suede type (foam layer: polyurethane elastomer, thickness 0.9 mm, average pore size 10 μm, manufactured by Fujibo Co., Ltd.) Plate rotation speed: 32.5 rpm Polishing load: 10.5 kPa (set value) Polishing liquid supply amount: 100mL / min Substrate to be polished 1cm 2 Feed rate per: 0.076 mL / min Substrate to be polished 1cm 2 Polishing amount per piece: 0.23mg Polishing time: 6 minutes

[0088] [Scratch evaluation] Optical full surface defect inspection machine: OSA6100 (manufactured by KLA-Tencor) Evaluation: Four substrates were randomly selected from the substrates inserted into the inspection machine, and each substrate was irradiated with a laser at 10,000 rpm to measure the number of scratches. The total number of scratches on both sides of each of the four substrates was divided by 8 to calculate the number of scratches per substrate surface. The evaluation results for the number of scratches are shown in Table 1 as a relative value, with Comparative Example 1 being set at 100.

[0089] [Table 1]

[0090] As shown in Table 1, when the number of particles of 0.1 μm or larger in 1 ml of the particle dispersion to be processed obtained by particle size distribution measurement using a number counting method is defined as NPC, the average number of pores of the membrane filter used in the filtration step is defined as NP, and the average pore diameter of the membrane filter used in the filtration step is defined as APD, in Examples 1 to 10, in which the filtration step was carried out using a combination of the particle dispersion and the membrane filter such that the value represented by the following formula 1 was between 3.0 and 50, the filter life was improved compared to Comparative Examples 1 and 2, in which the value represented by the following formula 1 did not satisfy the predetermined range, and the quality of the polishing liquid containing the filtered particle dispersion obtained in the filtration step was maintained or improved, thereby reducing scratches. Equation 1: [(NP) / [(NPC)×(APD)]]×1000000 Furthermore, when comparing Examples 1 and 2, even if the average pore diameter was the same, a decrease in the average pore number suppressed the number of scratches and significantly extended the filter life, which was the opposite of the behavior normally expected. Furthermore, in Examples 5 to 7, which have fewer coarse particles, a similar behavior was observed, which is opposite to the behavior normally expected, in that even if the average pore diameter is the same, as the average number of pores decreases, the number of scratches is suppressed and the filter life is significantly extended. [Industrial Applicability]

[0091] The filtration method of the present disclosure can extend the life of the filter, and the polishing liquid for magnetic disk substrates containing the particle dispersion obtained by the filtration method of the present disclosure can reduce the occurrence of scratches.

Claims

1. A method for filtering a particle dispersion, comprising a step (step 1) of filtering a particle dispersion through a membrane filter, a method for filtering a particle dispersion, wherein the value represented by the following formula 1 is 3.0 or more and 50 or less, where NPC is the number of particles of 0.1 μm or more obtained by particle size distribution measurement using a number counting method in 1 ml of the particle dispersion filtered in step 1, NP is the average pore number of the membrane filter used in step 1, and APD is the average pore diameter of the membrane filter used in step 1: Formula 1: [(NP) / [(NPC)×(APD)]]×1000000

2. The method for filtering a particle dispersion according to claim 1 , wherein the NPC is 50 million particles / mL or less.

3. The method for filtering a particle dispersion according to claim 1 , wherein the APD is 0.1 μm or more and 2 μm or less.

4. 2. The method for filtering a particle dispersion according to claim 1, further comprising a step of filtering with a depth filter before step 1.

5. 2. The method for filtering a particle dispersion according to claim 1, further comprising, before step 1, a step of confirming that the value expressed by Equation 1 of the particle dispersion to be filtered and the membrane filter is 3.0 or more and 50 or less.

6. 2. The method for filtering a particle dispersion according to claim 1, further comprising, before step 1, a step of adjusting the value of the particle dispersion to be filtered and the membrane filter expressed by Equation 1 to be 3.0 or more and 50 or less.

7. The method for filtering a particle dispersion according to claim 1 , wherein the particles contained in the particle dispersion include silica particles.

8. 8. The method for filtering a particle dispersion according to claim 7, wherein the average particle size of the primary particles of the silica particles is 1 nm or more and 100 nm or less.

9. 9. A method for producing an abrasive slurry for polishing magnetic disk substrates, comprising the step of mixing a particle dispersion obtained by the method for filtering a particle dispersion according to claim 1 with at least one compound selected from the group consisting of an acid, an oxidizing agent, a heterocyclic aromatic compound, an amine compound, and a water-soluble polymer.

10. 10. A method for manufacturing a magnetic disk substrate, comprising: supplying the polishing liquid for polishing magnetic disk substrates obtained in claim 9 to a surface of a substrate to be polished; and polishing the surface to be polished by bringing a polishing pad into contact with the surface to be polished and moving at least one of the polishing pad and the substrate to be polished.

Images