JP2024080083A - Polishing composition for magnetic disk substrates - Google Patents

Abstract

[Problem] In one aspect, the present disclosure provides a polishing liquid composition that can improve the polishing rate and reduce scratches and waviness. [Solution] In one aspect, the present disclosure relates to a polishing liquid composition for magnetic disk substrates, comprising silica particles (component A), an acid (component B), an oxidizing agent (component C) and water, where component A is silica particles such that when 1 g of an aqueous silica solution containing 5% by mass of silica particles is placed in a container with a side length of 1 cm and placed at 25° C., the water evaporation rate is 11 mg/h or less. [Selected Figures] None

Description

This disclosure relates to a polishing composition for magnetic disk substrates, and a method for manufacturing and polishing magnetic disk substrates using the same.

In recent years, magnetic disk drives have become smaller and have larger capacities, and there is a demand for higher recording density. To achieve higher recording density, technological developments are underway to reduce the unit recording area and lower the flying height of the magnetic head in order to improve the detection sensitivity of weakened magnetic signals. To accommodate the lower flying height of the magnetic head and ensure the recording area, there are increasingly strict requirements for magnetic disk substrates in terms of improved smoothness and flatness, as typified by reduced surface roughness, waviness, and edge sagging (roll-off), as well as reduced defects, as typified by reduced scratches, protrusions, pits, etc.

In response to such demands, for example, Patent Document 1 discloses a polishing composition comprising an abrasive grain, an additive, and a water-soluble polymer, in which the ratio of D50 ₍ after ₎ to D50 ₍ before ₎ is less than 2.0, where D50 ₍ before ₎ is the _D50 value of the particles of the polishing composition measured before leaving the polishing composition at 80°C for 5 days, and _D50( after ₎ is the _D50 value of the particles of the polishing composition measured after leaving the polishing composition at 80°C for 5 days.
Patent Document 2 discloses a container-containing abrasive dispersion including an abrasive dispersion containing water and abrasive grains dispersed in the water, and a container filled with the abrasive dispersion in a sealed state, in which a void increase rate calculated from an increase in void volume ΔV _A in the container and a volume _VD of the abrasive dispersion by the formula: void increase rate (%)=(ΔV _A /V _D )×100; is 0.5% or less after a temperature cycle test in which the temperature is raised and lowered five times between 25° C. and 43° C.

International Publication No. 2017/130749 JP 2018-53147 A

With the increase in capacity of magnetic disk drives, the required characteristics for the surface quality of substrates are becoming more severe, and there is a demand for a polishing composition that can improve the polishing rate and reduce scratches and waviness on the substrate surface. In general, there is a trade-off between the polishing rate and scratches and waviness, and for example, there is a problem that if the polishing rate is improved, scratches and waviness on the substrate surface after polishing become worse.
Furthermore, during polishing, the silica slurry solidifies in the polishing machine or in a slurry tank that supplies the polishing liquid, and the solidified coarse particles become mixed into the polishing liquid, causing a problem of worsening scratches.

Therefore, the present disclosure provides a polishing composition for magnetic disk substrates that can improve the polishing rate and reduce scratches and waviness, as well as a method for polishing a substrate using the same and a method for manufacturing a magnetic disk substrate.

In one aspect, the present disclosure relates to a polishing liquid composition for magnetic disk substrates, comprising silica particles (component A), an acid (component B), an oxidizing agent (component C), and water, where component A is silica particles that provide an aqueous silica solution with a silica particle content of 5% by mass, with a water evaporation rate of 11 mg/h or less in a 25°C environment.

In one aspect, the present disclosure relates to a method for producing a polishing liquid composition, which includes a mixing step of mixing two types of silica particles having different average primary particle sizes so that the water evaporation rate of an aqueous silica solution containing 5% by mass of silica particles in an environment of 25°C is 11 mg/h or less.

In one aspect, the present disclosure relates to a method for polishing a substrate, the method comprising polishing a substrate to be polished with the polishing composition of the present disclosure, the substrate to be polished being a substrate used in the manufacture of magnetic disk substrates.

In one aspect, the present disclosure relates to a method for manufacturing a magnetic disk substrate, which includes a polishing step of polishing a substrate to be polished using the polishing composition of the present disclosure.

According to one or more embodiments of the present disclosure, it is possible to produce a silica dispersion that can improve the polishing speed and reduce scratches and waviness.

This disclosure is based on the finding that by using a polishing liquid composition containing, as abrasive grains, silica particles such that the water evaporation rate of an aqueous silica solution with a silica concentration of 5% by mass in a 25°C environment is equal to or less than a predetermined value, the polishing rate can be improved and scratches and waviness on the substrate surface after polishing can be reduced.

That is, in one aspect, the present disclosure relates to a polishing liquid composition for magnetic disk substrates (hereinafter also referred to as the "polishing liquid composition of the present disclosure") that contains silica particles (component A), an acid (component B), an oxidizing agent (component C), and water, where component A is silica particles such that the water evaporation rate in an environment of 25°C is 11 mg/h or less when 1 g of an aqueous silica solution containing 5% by mass of silica particles is placed in a container with a side length of 1 cm.

Although the details of the mechanism by which the effects of the present disclosure are manifested are not clear, it is presumed as follows.
Generally, silica particles are stable in dispersion under strong acidity, regardless of their isoelectric point. This is because water molecules are attracted to the silica surface, and the resulting hydration repulsion stabilizes the dispersion. In silica particles with a water evaporation rate of more than 11 mg/h, the density of the hydrated area per unit volume is small. Therefore, the hydration repulsion force is smaller than the strong external energy such as centrifugation, and the silica particles aggregate. However, in silica particles with a water evaporation rate of 11 mg/h or less, the hydration density per unit volume increases, and the hydration repulsion force is greater than the external energy. Therefore, it is considered that the aggregation of silica particles is suppressed, and scratches and undulations can be reduced while improving the polishing speed.
Furthermore, an increase in hydration density per unit volume leads to an increase in the binding force of silica acting on each water molecule, and it is presumed that the increased effect of retaining water molecules on the silica surface can also slow the rate at which water evaporates.
However, the present disclosure need not be construed as being limited to these mechanisms.

In the present disclosure, the "waviness" of a substrate refers to unevenness on the substrate surface having a longer wavelength than the roughness. In the present disclosure, for example, waviness observed at a wavelength of 60 to 160 μm is referred to as "short wavelength waviness", and for example, waviness observed at a wavelength of 500 to 5000 μm is referred to as "long wavelength waviness". By reducing the waviness (short wavelength waviness, long wavelength waviness) of the substrate surface after polishing, the flying height of the magnetic head in a magnetic disk drive can be reduced, and the recording density of the magnetic disk can be improved. The waviness (short wavelength waviness, long wavelength waviness) of the substrate surface can be measured, for example, by the method described in the Examples. In the present disclosure, "reduction of waviness" refers to reduction of at least one of the short wavelength waviness and the long wavelength waviness.
In the present disclosure, scratches on the substrate surface can be detected, for example, by an optical defect inspection device and can be quantitatively evaluated as the number of scratches. The number of scratches can be specifically evaluated by the method described in the Examples.

[Silica particles (component A)]
The silica particles (hereinafter also referred to as "Component A") contained in the polishing liquid composition of the present disclosure are silica particles such that the water evaporation rate of 11 mg/h or less is obtained when 1 g of an aqueous silica solution containing 5 mass % of silica particles is placed in a container having a side length of 1 cm and placed at 25° C. Component A may be one type or a combination of two or more types.
In the present disclosure, a "container with one side measuring 1 cm" refers to a container whose inside bottom is a regular rectangle with one side measuring 1 cm. The height of the container can be 1 to 5 cm or 2 to 4 cm, preferably 3 cm.
The water evaporation rate is 11 mg/h or less, preferably 10.6 mg/h or less, more preferably 10.2 mg/h or less, from the viewpoint of improving the polishing rate and reducing scratches and waviness, and is preferably 6.0 mg/h or more, more preferably 7.0 mg/h or more, from the viewpoint of improving the wettability and spread of the polishing liquid composition on the substrate and the polishing pad. In the present disclosure, the water evaporation rate can be specifically determined by the method described in the Examples. In brief, the water evaporation rate can be determined from the weight loss of the silica aqueous solution before and after storage when 1 g of the silica aqueous solution containing 5 mass% silica particles is weighed into a container with a side length of 1 cm and stored at 25° C. for a predetermined time (for example, 30 hours).
Component A that satisfies the above-mentioned range of water evaporation rate can be obtained, for example, by mixing two types of silica particles having different average primary particle sizes, as described below.

Component A can be, from the viewpoint of improving the polishing speed and reducing scratches, colloidal silica, fumed silica, pulverized silica, silica obtained by surface modification of these, etc., and among these, colloidal silica is preferred.

From the viewpoint of improving the polishing rate and reducing waviness, the average primary particle diameter of component A is preferably 5 nm or more, more preferably 7.5 nm or more, and even more preferably 10 nm or more, and from the viewpoint of reducing scratches, it is preferably 40 nm or less, more preferably 35 nm or less, and even more preferably 30 nm or less. More specifically, the average primary particle diameter of component A is preferably 5 nm or more and 40 nm or less, more preferably 7.5 nm or more and 35 nm or less, and even more preferably 10 nm or more and 30 nm or less. The average primary particle diameter of silica particles (component A) can be determined specifically by the method described in the examples.

In one or more embodiments, component A is preferably two types of silica particles having different average primary particle diameters, that is, a mixed silica in which two types of silica particles having different average primary particle diameters are mixed. In a mixed silica in which three or more types of silica particles having different average primary particle diameters are mixed, the particle density becomes too high, and the silica particles are likely to aggregate by centrifugation, so a mixed silica in which two types of silica particles having different average primary particle diameters are mixed may be preferable. As component A which is two types of silica particles having different average primary particle diameters, from the viewpoint of suppressing silica particle aggregation, a combination of silica particles a having an average primary particle diameter of 25 nm or more and 70 nm or less (hereinafter also simply referred to as "silica particles a") or silica particles b having an average primary particle diameter of 10 nm or more and 24 nm or less (hereinafter also simply referred to as "silica particles b") and silica particles c having an average primary particle diameter of 1 nm or more and 9 nm or less (hereinafter also simply referred to as "silica particles c"). Each of silica particles a, b, and c may be one type or a combination of two or more types.
The average primary particle diameter of the silica particles a is preferably 25 nm or more, more preferably 26 nm or more, and even more preferably 27 nm or more, from the viewpoint of improving the polishing rate and reducing waviness, and is preferably 70 nm or less, more preferably 50 nm or less, and even more preferably 30 nm or less, from the viewpoint of reducing scratches.
The average primary particle diameter of the silica particles b is preferably 10 nm or more, more preferably 14 nm or more, and even more preferably 18 nm or more, from the viewpoint of improving the polishing rate and reducing waviness, and is preferably 24 nm or less, more preferably 23 nm or less, and even more preferably 22 nm or less, from the viewpoint of reducing scratches.
The average primary particle diameter of the silica particles c is preferably 1 nm or more, more preferably 2 nm or more, and even more preferably 3 nm or more, from the viewpoints of improving the polishing rate and reducing scratches and waviness, and from the same viewpoints, is preferably 9 nm or less, more preferably 8 nm or less, and even more preferably 7 nm or less.

From the viewpoint of reducing scratches, component A is preferably silica particles having a particle size change ratio D50'/D50 of 0.85 or more and less than 1.15 when an aqueous silica solution containing 5% by mass of silica particles is centrifuged at a rotation speed of 25,000 rpm for 60 minutes. From the same viewpoint, the particle size change ratio D50'/D50 is preferably 0.85 or more and less than 1.15, more preferably 0.9 or more and 1.12 or less, and even more preferably 0.95 or more and 1.09 or less.
Here, D50 indicates the particle size before centrifugation at which the cumulative volume frequency from the small diameter side in the scattering intensity distribution measured by dynamic light scattering is 50%, and D50' indicates the particle size after centrifugation at which the cumulative volume frequency from the small diameter side in the scattering intensity distribution measured by dynamic light scattering is 50%.
In the present disclosure, the particle size change rate D50'/D50 can be specifically determined by the method described in the Examples.
Component A satisfying the above-mentioned range of the rate of change in particle diameter D50'/D50 can be obtained, for example, by mixing two types of silica particles having different average primary particle diameters.

From the viewpoint of reducing scratches, component A is preferably silica particles having a particle size change ratio D90'/D90 of 0.8 or more and 1.25 or less when an aqueous silica solution containing 5% by mass of silica particles is centrifuged at a rotation speed of 25,000 rpm for 60 minutes. From the same viewpoint, the particle size change ratio D90'/D90 is preferably 0.85 or more and 1.20 or less, more preferably 0.90 or more and 1.15 or less, and even more preferably 0.95 or more and 1.10 or less.
Here, D90 refers to the particle size before centrifugation at which the cumulative volume frequency from the small diameter side in the scattering intensity distribution measured by dynamic light scattering is 90%, and D90' refers to the particle size after centrifugation at which the cumulative volume frequency from the small diameter side in the scattering intensity distribution measured by dynamic light scattering is 90%.
In the present disclosure, the particle size change rate D90'/D90 can be specifically determined by the method described in the Examples.
Component A satisfying the above-mentioned range of the particle size change rate D90'/D90 can be obtained, for example, by mixing two types of silica particles having different average primary particle sizes.

From the viewpoint of suppressing silica particle aggregation, the pore size of component A is preferably 10 nm or more, more preferably 12 nm or more, and even more preferably 14 nm or more, and from the viewpoint of improving the polishing rate and reducing waviness, the pore size is preferably 22 nm or less, more preferably 21 nm or less, and even more preferably 20 nm or less. In the present disclosure, the pore size can be measured by the method described in the Examples.
The pore size of component A that satisfies the above range can be obtained, for example, by mixing two types of silica particles having different average primary particle sizes.

The content of component A in the polishing composition of the present disclosure is preferably 0.1 mass% or more, more preferably 1 mass% or more, and even more preferably 3 mass% or more, calculated as _SiO2 , from the viewpoint of improving the polishing rate, and is preferably 20 mass% or less, more preferably 15 mass% or less, and even more preferably 10 mass% or less, calculated as _SiO2 , from the viewpoint of reducing scratches. More specifically, the content of component A in the polishing composition of the present disclosure is preferably _0.1 mass% or more and 20 mass% or less, more preferably 1 mass% or more and 15 mass% or less, and even more preferably 3 mass% or more and 10 mass% or less, calculated as SiO2. When component A is composed of two or more types of silica particles, the content of component A refers to the total content thereof.

When component A is a combination of silica particles a and silica particles c (mixed silica), the mass ratio a/c of the content of silica particles a to the content of silica particles c in the polishing liquid composition of the present disclosure is, from the viewpoints of improving the polishing rate and reducing scratches, preferably from 90/10 to 20/80, more preferably from 85/15 to 30/70, and even more preferably from 80/20 to 40/60.
When component A is a combination of silica particles b and silica particles c (mixed silica), the mass ratio b/c of the content of silica particles b to the content of silica particles c in the polishing liquid composition of the present disclosure is, from the viewpoints of improving the polishing rate and reducing scratches, preferably from 90/10 to 40/60, more preferably from 85/15 to 50/50, and even more preferably from 80/20 to 60/40.
When the silica particles a are a combination of two or more types, the content of the silica particles a refers to the total content thereof. When the silica particles b are a combination of two or more types, the content of the silica particles b refers to the total content thereof. When the silica particles c are a combination of two or more types, the content of the silica particles c refers to the total content thereof.

[Acid (Component B)]
The polishing composition of the present disclosure contains an acid (component B). In the present disclosure, the use of an acid includes the use of an acid or a salt thereof. Component B may be one type or a combination of two or more types.

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, amidosulfuric acid, and the like; organic acids such as organic phosphoric acid, organic phosphonic acid, and carboxylic acid; and the like. Among them, from the viewpoint of improving the polishing rate and reducing scratches, it is preferable to contain inorganic acid and organic phosphonic acid, and it is preferable to contain inorganic acid. From the same viewpoint, the content of inorganic acid in component B is preferably 0.5 mass% or more, more preferably 0.7 mass% or more, and even more preferably 0.8 mass% or more.
From the same viewpoint, the inorganic acid is preferably at least one selected from nitric acid, sulfuric acid, hydrochloric acid, perchloric acid and phosphoric acid, more preferably at least one selected from sulfuric acid and phosphoric acid, and even more preferably phosphoric acid.
From the same viewpoint, 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), and HEDP is more preferable. Examples of salts of these acids include salts of the above acids and at least one selected from metals, ammonia, and alkylamines. Examples of the above metals include metals belonging to Groups 1 to 11 of the periodic table. Among these, from the viewpoints of improving the polishing rate and reducing scratches, salts of the above acids and metals belonging to Group 1A or ammonia are preferable.

From the viewpoint of improving the polishing rate and reducing scratches, the content of component B in the polishing composition of the present disclosure is preferably 0.01 mass% or more, more preferably 0.1 mass% or more, and even more preferably 0.5 mass% or more, and from the viewpoint of reducing scratches, it is preferably 3 mass% or less, more preferably 2 mass% or less, and even more preferably 1.5 mass% or less. From the same viewpoint, the content of component B in the polishing composition of the present disclosure is preferably 0.01 mass% or more and 3 mass% or less, more preferably 0.1 mass% or more and 2 mass% or less, and even more preferably 0.5 mass% or more and 1.5 mass% or less. When component B is a combination of two or more types, the content of component B refers to the total content thereof.

[Oxidizing agent (component C)]
The polishing composition of the present disclosure contains an oxidizing agent (hereinafter also referred to as "component C") from the viewpoints of improving the polishing rate and reducing scratches. In one or more embodiments, component C is an oxidizing agent that does not contain a halogen atom. Component C may be one type or a combination of two or more types.

Component C, from the viewpoint of improving the polishing rate and reducing scratches, may be, for example, 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 ease of availability.

From the viewpoint of improving the polishing rate, the content of component C in the polishing liquid composition of the present disclosure is preferably 0.01 mass% or more, more preferably 0.05 mass% or more, and even more preferably 0.1 mass% or more, and from the viewpoint of reducing scratches, it is preferably 4 mass% or less, more preferably 2 mass% or less, even more preferably 1 mass% or less, and even more preferably 0.5 mass% or less. From the same viewpoint, the content of component C in the polishing liquid composition of the present disclosure is preferably 0.01 mass% or more and 4 mass% or less, more preferably 0.05 mass% or more and 2 mass% or less, even more preferably 0.1 mass% or more and 1 mass% or less, and even more preferably 0.1 mass% or more and 0.5 mass% or less. When component C is a combination of two or more types, the content of component C refers to the total content thereof.

[water]
The polishing composition of the present disclosure contains water as a medium. Examples of water include distilled water, ion-exchanged water, pure water, and ultrapure water. The content of water in the polishing composition of the present disclosure can be the remainder excluding component A, component B, component C, and any optional components described below.

[Other ingredients]
In one or more embodiments, the polishing liquid composition of the present disclosure may contain other components as necessary within a range that does not impair the effects of the present disclosure. Examples of other components include heterocyclic aromatic compounds, aliphatic amine compounds, alicyclic amine compounds, thickeners, dispersants, rust inhibitors, basic substances, polishing rate enhancers, surfactants, water-soluble polymers, etc.

[pH of polishing composition]
The pH of the polishing composition of the present disclosure is preferably 0.1 or more, more preferably 0.5 or more, and even more preferably 1 or more, from the viewpoint of reducing scratches, and is preferably 6 or less, more preferably 4 or less, and even more preferably 2 or less, from the viewpoint of improving the polishing rate. More specifically, the pH of the polishing composition of the present disclosure is preferably 0.1 or more and 6 or less, more preferably 0.5 or more and 4 or less, and even more preferably 1 or more and 2 or less. The pH can be adjusted using the above-mentioned acid (component B) or a known pH adjuster. In the present disclosure, the above pH is the pH of the polishing composition at 25°C, and can be measured using a pH meter, and can be, for example, the value after 2 minutes of immersing the electrode of the pH meter in the polishing composition.

[Method of producing the polishing composition]
The polishing liquid composition of the present disclosure can be produced, for example, by blending component A, component B, component C, and water, and, if desired, optional components (other components) by a known method. For example, in one or more embodiments, the polishing liquid composition of the present disclosure can be produced by blending at least component A, component B, component C, and water. Thus, in one aspect, the present disclosure relates to a method for producing a polishing liquid composition, which includes a step of blending at least component A, component B, component C, and water. In the present disclosure, "blending" includes mixing component A, component B, component C, and water, and optional components (other components) as necessary, simultaneously or in any order. Silica particles (component A) may be mixed in the form of a concentrated slurry, or may be mixed after diluting with water or the like. When component A is made of multiple types of silica particles, the multiple types of silica particles can be blended simultaneously or separately. When component B is made of multiple types of acids, the multiple types of acids can be blended simultaneously or separately. When component C is made of multiple types of oxidizing agents, the multiple types of oxidizing agents can be blended simultaneously or separately. The blending can be carried out using a mixer such as a homomixer, a homogenizer, an ultrasonic disperser, a wet ball mill, etc. The preferred blending amount of each component in the method for producing the polishing liquid composition can be the same as the preferred content of each component in the polishing liquid composition of the present disclosure described above.

Embodiments of the polishing liquid composition disclosed herein may be of the so-called one-component type, in which all components are premixed and supplied to the market, or of the so-called two-component type, in which components are mixed at the time of use.

In the present disclosure, "the content of each component in the polishing liquid composition" refers to the content of each component at the time of use, that is, at the time when the use of the polishing liquid composition for polishing is started.
In one or more embodiments, the content of each component in the polishing composition of the present disclosure can be considered as the blending amount of each component.

The polishing composition of the present disclosure may be stored and supplied in a concentrated state to the extent that its storage stability is not impaired. This is preferable in that production and transportation costs can be further reduced. The concentrated polishing composition of the present disclosure may be appropriately diluted with the above-mentioned water as necessary when used. The dilution ratio is not particularly limited as long as the content (at the time of use) of each of the above-mentioned components can be secured after dilution, and may be, for example, 10 to 100 times.

[Method of producing the polishing composition]
In one aspect, the present disclosure relates to a method for producing a polishing liquid composition (hereinafter also referred to as the "polishing liquid composition production method of the present disclosure"), which includes a mixing step of mixing two types of silica particles having an average primary particle size such that the water evaporation rate of 1 g of an aqueous silica solution containing 5% by mass of silica particles in a container having a side length of 1 cm and at 25° C. is 11 mg/h or less. In one or more embodiments, the mixing step is a step of obtaining silica particles (component A) having a water evaporation rate of 11 mg/h or less in an environment of 25° C. when 1 g of an aqueous silica solution containing 5% by mass of silica particles in a container having a side length of 1 cm is placed. According to the polishing liquid composition production method of the present disclosure, a polishing liquid composition that can achieve an improvement in the polishing rate and a reduction in scratches and waviness can be provided.

From the viewpoint of reducing waviness and scratches, in one or more embodiments, the mixing step preferably includes mixing two types of silica particles having different average primary particle sizes such that the particle size change ratio D50'/D50 before and after centrifugation when an aqueous silica solution containing 5% by mass of silica particles is centrifuged at a rotation speed of 25,000 rpm for 60 minutes is 0.85 or more and less than 1.15.

The mixing step preferably includes mixing silica particles a having an average primary particle diameter of 25 nm to 70 nm or silica particles b having an average primary particle diameter of 10 nm to 24 nm with silica particles c having an average primary particle diameter of 1 nm to 9 nm. In one or more embodiments, mixing silica particles a or silica particles b with silica particles c includes mixing silica particles a or silica particles b with silica particles c and, if necessary, water, component B, component C, and other components. Here, "mixing" includes mixing silica particles a or b with silica particles c and, if necessary, water, component B, component C, and other components simultaneously or in any order. Silica particles a, b, and c may each be one type or a combination of two or more types.

In the mixing step, the mixing ratio (a/c) of the amount of the silica particles a to the amount of the silica particles c to be mixed is preferably 90/10 to 20/80, more preferably 85/15 to 30/70, and further preferably 60/40, from the viewpoint of achieving an improvement in the polishing rate and a reduction in waviness.
In the mixing step, the mixing ratio (b/c) of the amount of silica particles b to the amount of silica particles c to be mixed is preferably 90/10 to 40/60, more preferably 85/15 to 50/50, and even more preferably 80/20, from the viewpoint of achieving an improvement in the polishing rate and a reduction in waviness.
By mixing the silica particles so that the blending ratio satisfies the above range, the water evaporation rate, the particle size change rate D50'/D50, the particle size change rate D90'/D90 and the void size tend to fall within the above-mentioned preferred ranges.
When the silica particles a are a combination of two or more types, the blending amount of the silica particles a is the total blending amount thereof.When the silica particles b are a combination of two or more types, the blending amount of the silica particles b is the total blending amount thereof.When the silica particles c are a combination of two or more types, the blending amount of the silica particles c is the total blending amount thereof.

[Polishing liquid kit]
In one aspect, the present disclosure relates to a kit for preparing the polishing liquid composition of the present disclosure, the kit including a silica dispersion (first liquid) containing component A and water, and an additive aqueous solution (second liquid) containing components B and C, which are not mixed with each other (hereinafter, also referred to as the "polishing liquid kit of the present disclosure").
The first liquid and the second liquid may be mixed at the time of use and diluted with water as necessary. The water contained in the first liquid may be the entire amount of water used in preparing the polishing composition, or may be a part of the amount. The second liquid may contain a part of the water used in preparing the polishing composition. The first liquid and the second liquid may each contain the above-mentioned optional components (other components) as necessary. When the first liquid and the second liquid are mixed, the above-mentioned optional components (other components) may be further mixed.
According to the present disclosure, it is possible to obtain a polishing composition that can achieve an improvement in the polishing rate and a reduction in scratches and waviness.

[Polished substrate]
In one or more embodiments, the substrate to be polished is a substrate used for manufacturing a magnetic disk substrate. In one or more embodiments, the surface of the substrate to be polished is polished with the polishing composition of the present disclosure, and then a magnetic layer is formed on the substrate surface by sputtering or the like, thereby manufacturing a magnetic disk substrate.

Materials of the substrate to be polished that are preferably used in the present disclosure include, for example, metals or semimetals such as silicon, aluminum, nickel, tungsten, copper, tantalum, and titanium, or alloys thereof, glassy materials such as glass, glassy carbon, and amorphous carbon, ceramic materials such as alumina, silicon dioxide, silicon nitride, tantalum nitride, and titanium carbide, and resins such as polyimide resins. In particular, the substrate to be polished is preferably one containing metals such as aluminum, nickel, tungsten, and copper, and alloys mainly composed of these metals. As the substrate to be polished, for example, Ni-P plated aluminum alloy substrates and glass substrates such as crystallized glass, reinforced glass, aluminosilicate glass, and aluminoborosilicate glass are more suitable, and Ni-P plated aluminum alloy substrates are even more suitable. In the present disclosure, the term "Ni-P plated aluminum alloy substrate" refers to an aluminum alloy substrate whose surface is ground and then electrolessly plated with Ni-P.

The shape of the substrate to be polished can be, for example, a disk, plate, slab, prism, or other shape with a flat surface, or a lens or other shape with a curved surface. Of these, a disk-shaped substrate to be polished is suitable. In the case of a disk-shaped substrate to be polished, the outer diameter is, for example, about 2 to 100 mm, and the thickness is, for example, about 0.4 to 2 mm.

In one or more embodiments, the polishing liquid composition of the present disclosure can be suitably used for polishing a substrate after rough polishing. The substrate to be polished can be a substrate after rough polishing. Examples of abrasive grains used for rough polishing include alumina, silica, and the like.

[Substrate Polishing Method]
In one aspect, the present disclosure relates to a method for polishing a substrate (hereinafter also referred to as the "polishing method of the present disclosure"), which comprises polishing a substrate to be polished using the polishing liquid composition of the present disclosure, and the substrate to be polished is a substrate used in the manufacture of a magnetic disk substrate. By using the polishing method of the present disclosure, it is possible to produce a high-quality magnetic disk substrate with reduced scratches and waviness on the substrate surface after polishing with high yield and good productivity. As described above, the substrate to be polished in the polishing method of the present disclosure includes those used in the manufacture of magnetic disk substrates, and among them, substrates used in the manufacture of magnetic disk substrates for perpendicular magnetic recording are preferred. The polishing method and conditions can be the same as those of the substrate manufacturing method of the present disclosure described below.

In one or more embodiments, polishing a substrate to be polished using the polishing liquid composition of the present disclosure means supplying the polishing liquid composition of the present disclosure to the surface of the substrate to be polished, contacting the surface to be polished with a polishing pad, and moving at least one of the polishing pad and the substrate to be polished to perform polishing, or sandwiching the substrate to be polished between plates to which a polishing pad such as a nonwoven organic polymer-based polishing cloth is attached, and polishing the substrate to be polished by moving the platen and the substrate to be polished while supplying the polishing liquid composition of the present disclosure to a polishing machine.

[Method of manufacturing magnetic disk substrate]
In general, a magnetic disk is manufactured by polishing a substrate to be polished after a grinding process through a rough polishing process and a finish polishing process, and then forming the substrate into a magnetic disk in a recording part forming process. The polishing composition of the present disclosure can be used in a polishing process, preferably a finish polishing process, for polishing a substrate to be polished in a method for manufacturing a magnetic disk substrate. That is, 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) including a process for polishing a substrate to be polished using the polishing composition of the present disclosure (hereinafter also referred to as the "polishing process"). The substrate manufacturing method of the present disclosure is particularly suitable for a method for manufacturing a magnetic disk substrate for a perpendicular magnetic recording system.

In one or more embodiments, the polishing step in the substrate manufacturing method of the present disclosure is a step of supplying the polishing liquid composition of the present disclosure to the surface of the substrate to be polished, contacting the polishing pad with the surface to be polished, and moving at least one of the polishing pad and the substrate to be polished to perform polishing. In one or more embodiments, the polishing step in the substrate manufacturing method of the present disclosure is a step of sandwiching the substrate to be polished between a platen to which a polishing pad such as a nonwoven organic polymer-based polishing cloth is attached, and polishing the substrate to be polished by moving the platen and the substrate to be polished while supplying the polishing liquid composition of the present disclosure to the polishing machine.

When the polishing process of the substrate to be polished is performed in multiple stages, the polishing process in the substrate manufacturing method of the present disclosure is preferably performed in the second stage or later, and more preferably in the final polishing process or finish polishing process. In this case, a separate polishing machine may be used for each step to avoid contamination with abrasive grains or polishing liquid composition from the previous step, and when separate polishing machines are used, it is preferable to clean the substrate to be polished after each polishing step. Furthermore, the polishing liquid composition of the present disclosure can also be used in circulating polishing in which the used polishing liquid is reused. There are no particular limitations on the polishing machine, and any known polishing machine for substrate polishing can be used.

The polishing pad used in the present disclosure is not particularly limited, and may be, for example, a suede type, a nonwoven fabric type, a polyurethane independent foam type, or a two-layer type in which these are laminated, and from the viewpoint of improving the polishing speed, a suede type polishing pad is preferred.

The polishing load in the polishing step of the substrate manufacturing method of the present disclosure is preferably 5.9 kPa or more, more preferably 6.9 kPa or more, and even more preferably 7.5 kPa or more, from the viewpoint of improving the polishing rate, and is preferably 20 kPa or less, more preferably 18 kPa or less, and even more preferably 16 kPa or less, from the viewpoint of reducing scratches. In the substrate manufacturing method of the present disclosure, the polishing load refers to the pressure of the platen applied to the polishing surface of the substrate to be polished during polishing. The polishing load can be adjusted by applying air pressure or a weight to at least one of the platen and the substrate to be polished.

The supply rate of the polishing liquid composition of the present disclosure in the polishing step of the substrate manufacturing method of the present disclosure, from the viewpoint of reducing scratches, is preferably ^0.05 mL/min or more and 15 mL/min or less, more preferably 0.06 mL/min or more and 10 mL/min or less, even more preferably 0.07 mL/min or more and 1 mL/min or less, and even more preferably 0.07 mL/min or more and 0.5 mL/min or less, per 1 cm2 of the substrate to be polished.

The polishing composition of the present disclosure can be supplied to the polishing machine, for example, by continuously supplying the composition using a pump or the like. When supplying the polishing composition to the polishing machine, in addition to supplying the composition as a single liquid containing all the components, the composition can be divided into multiple blending component liquids and supplied as two or more liquids, taking into consideration the stability of the polishing composition. In the latter case, the multiple blending component liquids are mixed, for example, in the supply pipe or on the substrate to be polished, to produce the polishing composition of the present disclosure.

According to the substrate manufacturing method of the present disclosure, by using the polishing composition of the present disclosure, it is possible to produce high-quality magnetic disk substrates with reduced scratches on the substrate surface after polishing with high yield and good productivity.

The present disclosure will be explained in more detail below with reference to examples, but these are merely illustrative and the present disclosure is not limited to these examples.

1. Preparation of Silica Particles (Examples 1 to 6 and Comparative Examples 1 to 3)
(Examples 1 to 6)
The silica particles of Examples 1 to 6 were prepared by mixing two types (a+c or a+b) of the silica particles shown in Table 1 so as to obtain the mass ratio shown in Table 1.
In preparing the silica particles, the following silica particles a, b and c were used.
Silica particle a: Colloidal silica aqueous dispersion (Cataloid SI-50W, 40% by mass, pH 9.1), manufactured by JGC Catalysts and Chemicals. Silica particle b: Colloidal silica aqueous dispersion (Cataloid SI-40W, 40% by mass, pH 9.1), manufactured by JGC Catalysts and Chemicals. Silica particle c: Colloidal silica aqueous dispersion (Cataloid SI-550W, 20% by mass, pH 10.6), manufactured by JGC Catalysts and Chemicals (Comparative Examples 1 to 3).
The silica particles in Comparative Examples 1, 2 and 3 were the silica particles a, b and c described above, respectively.

2. Preparation of Polishing Liquid Composition The polishing liquid compositions of Examples 1 to 6 and Comparative Examples 1 to 3 were prepared by mixing the silica particles (component A or non-component A), acid (component B), oxidizing agent (component C) and water shown in Table 1. The content (effective amount) of each component in each polishing liquid composition was 6 mass% for silica particles (component A or non-component A), 1.6 mass% for acid (component B), and 1.0 mass% for oxidizing agent (component C). The content of water is the remainder excluding the silica particles (component A or non-component A), acid (component B), and oxidizing agent (component C). The pH of the polishing liquid compositions of Examples 1 to 6 and Comparative Examples 1 to 3 was 1.6.
The following components B and C were used in the preparation of the polishing composition.
(Component B)
Phosphoric acid [75% phosphoric acid made by Nippon Chemical Industry Co., Ltd.]
(Component C)
Hydrogen peroxide solution [ADEKA 35% hydrogen peroxide]

3. Measurement of each parameter [Measurement of average primary particle size of silica particles]
Using a field emission scanning electron microscope (S-4800) manufactured by Hitachi High-Technologies Corporation, images were taken at random from 10 fields of view at a scanning voltage of 10 kV and a magnification of 200 K. As the object to be photographed, copper grit on which a carbon film was vapor-deposited was subjected to hydrophilization treatment, and several drops of an aqueous solution in which the aqueous dispersion of each silica particle was diluted to 0.1 mass % were dropped onto the copper grit. The aqueous solution was then dried overnight at room temperature to obtain a sample.
The image data obtained by the above method was used to digitize the particle shapes and calculate the circle-equivalent diameter for 1,000 or more particles as a parameter using image analysis software (WinROOF2013) manufactured by Mitani Shoji Co., Ltd. Based on this value, the D50 value was determined as the average primary particle diameter.

[pH Measurement]
The pH of the polishing composition was measured at 25° C. using a pH meter (manufactured by DKK-TOA Corporation), and the value measured 2 minutes after immersing the electrode in the polishing composition was recorded.

[Measurement of void diameter]
The silica dispersion was left to stand in a dryer at 110°C for three days to evaporate most of the water. The resulting silica powder was then crushed using an agate mortar. The crushed silica powder was pre-baked at 300°C for 90 minutes using a pore analyzer (ASAP-2020) to completely evaporate the water, after which the pore distribution was calculated using physical adsorption using nitrogen gas. The following equation holds during the adsorption isothermal process.
Ln(p/p0)=-2VLγcosθ/rRT
VL is the molar volume of gas molecules liquefied by capillary condensation, γ is the surface tension, θ is the contact angle, and r is the radius of the pores. Based on the above formula, the pore volume and pore radius r at a certain relative pressure p/p0 are analyzed from the amount of gas adsorbed. The pore diameter indicates the value when the maximum pore volume is given, and the total pore volume was calculated from the value obtained by integrating the spectrum obtained.

[Measurement of water evaporation rate in a 25°C environment]
The water evaporation rate in a 25° C. environment was calculated as follows.
(1) Prepare a 5% by mass aqueous silica solution.
(2) Using a precision electronic balance, the aqueous silica solution prepared in (1) above is precisely weighed to 1.00 g into a plastic container having a square shape with a base length of 1 cm and a height of 3 cm. The weight of each plastic container is also precisely weighed in advance.
(3) Each of the aqueous silica solutions weighed in (2) above is placed in a thermostatic chamber at 25° C. The humidity in the thermostatic chamber is adjusted to 35-40%. The humidity during the experiment was 38%.
(4) After 30 hours, each sample is removed from the thermostatic chamber and precisely weighed using the same precision electronic balance as in (2) above.
(5) The weight loss of the aqueous silica solution is calculated from the weight after 30 hours, the weight of the plastic container measured beforehand, and the weight of the aqueous silica solution placed in the plastic container.
(6) The weight loss of the aqueous silica solution per unit time is calculated as the water evaporation rate (mg/h).

[Method for measuring particle diameters D50 and D90 of silica particles by DLS measurement]
Silica particles were mixed with ion-exchanged water to prepare a 5% by mass silica particle dispersion. In Examples 1 to 6, silica particles a or silica particles b were mixed with silica particles c in a mass ratio as shown in Table 1. The prepared 5% by mass silica particle dispersion was diluted to 0.5% by mass with ion-exchanged water, and then placed in the measuring device described below and measured under the conditions described below. In the obtained particle size distribution, the particle sizes at which the cumulative volume frequency from the small diameter side was 50% and 90% were defined as D50 and D90, respectively.
<Measurement conditions>
Measurement equipment: Malvern Zetasizer Nano "Nano S"
Sample volume: 1.5mL
Laser: He-Ne, 3.0 mW, 633 nm
Scattered light detection angle: 173°

[Measurement of D50'/D50 and D90'/D90]
(1) Prepare a 5% by mass silica aqueous solution.
(2) The aqueous silica solution prepared in (1) above is placed in a centrifuge tube, and centrifuged at 25,000 rpm for 60 minutes using an ultracentrifuge.
(3) After centrifugation, the silica aqueous solution is subjected to a dispersion treatment using ultrasonic waves for 30 minutes.
(4) The dispersed silica aqueous solution is diluted 10 times and the above DLS measurement is carried out.
(5) The particle diameter D50 value obtained by the DLS measurement was compared between those having and having not been subjected to ultracentrifugation, and the particle diameter change rate D50'/D50 was calculated by taking the value not subjected to ultracentrifugation as D50 and the value subjected to ultracentrifugation as D50'.
Furthermore, the particle size change rate D90'/D90 was calculated from the particle size D90 obtained by the DLS measurement and the particle size D90' obtained when ultracentrifugal separation was performed.

4. Polishing Method The following substrates were polished under the polishing conditions shown below using the polishing compositions of Examples 1 to 6 and Comparative Examples 1 to 3. Then, the polishing rate, waviness, and number of scratches were measured by the measurement methods described below, and the results are shown in Table 1.

[Polished substrate]
The substrate to be polished was a Ni-P plated aluminum alloy substrate that had been roughly polished with a polishing composition containing silica abrasive grains. The substrate had a thickness of 0.6-0.8 mm, an outer diameter of 95-100 mm, an inner diameter of 25 mm, and a center line average roughness (Ra) of 1 nm as measured by an AFM (Digital Instrument NanoScope IIIa Multi Mode AFM).

[Polishing conditions]
Polishing test machine: Double-sided polishing machine (manufactured by SpeedFam)
Polishing pad: Fujibo CF4303 (suede type, foam layer: polyurethane elastomer, thickness 0.69 mm)
Carrier: Aramid, thickness 0.7 nm (made by Sagami PCI)
Polishing composition supply amount: 100 mL/min Lower platen rotation speed: 31.4 rpm
Polishing load: 140 g/ ^cm2
Polishing time: 290 seconds Number of substrates: 10 After polishing, the substrates were removed from the double-sided polisher, and the surfaces of the substrates were cleaned with an automatic cleaner.

5. Evaluation [Evaluation of polishing rate]
The weight of each substrate before and after polishing was measured using a Sartorius BP-210S, and the mass loss was calculated from the change in mass of each substrate. The polishing rate was calculated by dividing the average mass loss of all 10 substrates by the polishing time using the following formula. The polishing rate measurement results for Examples 1 to 6 and Comparative Examples 1 to 3 are shown in Table 1 as relative values with Comparative Example 1 set to 100.
Mass loss (mg) = {mass before polishing (mg) - mass after polishing (mg)}
Polishing rate (mg/min) = mass reduction (mg) / polishing time (min)

[Evaluation of short wavelength waviness and long wavelength waviness]
Two substrates were randomly selected from the ten polished substrates, and measurements were made on four random points (total of 16 points) on both sides of each selected substrate under the following conditions. The average values of the measurements at the 16 points were calculated as the short wavelength waviness and long wavelength waviness of the substrate. Relative values were calculated with Comparative Example 1 set to 100, and the results are shown in Table 1.
<Measurement conditions>
Measuring instrument: New View 7300 (Zygo)
Lens: 2.5x Zoom: 0.5x Short wavelength region: 60-160μm
Long wavelength region: 50 to 500 μm
Analysis software: Zygo Metro Pro (Zygo)

[Scratch Evaluation]
Measuring instrument: Candela OSA7100,6110 (KLA Tencor)
Measurement recipe: 95T Substrate rotation speed: 10000 rpm Measurement mode: PSc, PScR
Laser voltage: Circumferential laser voltage 650mV, radial laser voltage 600mV
Evaluation: Four of the substrates were randomly selected from those placed in the polishing tester, 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 measurement results of the number of scratches in Examples 1 to 6 and Comparative Examples 1 to 3 are shown in Table 1 as relative values with Comparative Example 1 being set to 100.

As shown in Table 1 above, the polishing compositions of Examples 1 to 6, which use silica particles with a water evaporation rate of 11 mg/h or less in an environment at 25°C, showed improved polishing speed and reduced short-wavelength waviness, long-wavelength waviness, and scratches compared to the polishing compositions of Comparative Examples 1 to 3, which use silica particles with a water evaporation rate of more than 11 mg/h in an environment at 25°C.

This disclosure makes it possible to provide, for example, a magnetic disk substrate suitable for high recording density.

Claims (11)

The composition comprises silica particles (component A), an acid (component B), an oxidizing agent (component C) and water,
Component A is a polishing liquid composition for magnetic disk substrates, which is composed of silica particles such that the water evaporation rate of 11 mg/h or less when 1 g of an aqueous silica solution containing 5% by mass of silica particles is placed in a container having a side length of 1 cm and placed at 25°C. 2. The polishing liquid composition according to claim 1, wherein component A is silica particles such that, when an aqueous silica solution containing 5% by mass of silica particles is centrifuged at a rotation speed of 25,000 rpm for 60 minutes, the particle size change rate D50'/D50 before and after centrifugation is 0.85 or more and less than 1.15.
Here, D50 indicates the particle size before centrifugation at which the cumulative volume frequency from the small diameter side becomes 50% in the scattering intensity distribution measured by the dynamic light scattering method,
D50' indicates the particle size after centrifugation at which the cumulative volume frequency from the small diameter side becomes 50% in the scattering intensity distribution measured by dynamic light scattering. The polishing composition according to claim 1 or 2, having a pH of 0.1 or more and 6 or less. The polishing composition according to any one of claims 1 to 3, wherein the average primary particle size of component A is 5 nm or more and 40 nm or less. The polishing composition according to any one of claims 1 to 4, wherein component A is colloidal silica. The polishing composition according to any one of claims 1 to 5, wherein component A is two types of silica particles having different average primary particle sizes. The polishing composition according to any one of claims 1 to 6, wherein the magnetic disk substrate is an aluminum alloy substrate plated with Ni-P. A method for producing a polishing liquid composition, comprising a mixing step of mixing two types of silica particles having different average primary particle sizes so that the water evaporation rate of an aqueous silica solution containing 5% by mass of silica particles in an environment of 25°C is 11 mg/h or less. The method for producing a polishing liquid composition according to claim 8, wherein the mixing step includes mixing two types of silica particles having different average primary particle sizes such that the particle size change ratio D50'/D50 before and after centrifugation when an aqueous silica solution containing 5% by mass of silica particles is centrifuged at a rotation speed of 25,000 rpm for 60 minutes is 0.85 or more and less than 1.15. A method for polishing a substrate, comprising polishing the substrate with the polishing composition according to any one of claims 1 to 7, the substrate being a substrate used in the manufacture of magnetic disk substrates. A method for manufacturing a magnetic disk substrate, comprising a polishing step of polishing a substrate to be polished using the polishing composition according to any one of claims 1 to 7.