WO2016060169A1

WO2016060169A1 - Method for manufacturing substrate for magnetic disc

Info

Publication number: WO2016060169A1
Application number: PCT/JP2015/079066
Authority: WO
Inventors: 健太岩間; 亮太郎竹内
Original assignee: Hoya株式会社
Priority date: 2014-10-14
Filing date: 2015-10-14
Publication date: 2016-04-21
Also published as: CN106716532A; SG11201702400TA; MY181185A; JPWO2016060169A1; CN106716532B; JP6374522B2

Abstract

　To improve yield after polishing of a substrate for a magnetic disc. In the present invention, a slurry comprising a hydrophobic macromolecular material is passed through a filter having openings 100 nm or less in diameter, the slurry being the raw material for a polishing solution used in polishing the main surface of a substrate. Polishing is carried out by using the polishing solution after removing, using the filter, particles contained in the slurry that have a grain diameter greater than the average grain diameter of an abrasive grain.

Description

Manufacturing method of magnetic disk substrate

The present invention relates to a method for manufacturing a magnetic disk substrate having a polishing process.

Today, a personal computer, a notebook personal computer, a DVD (Digital Versatile Disc) recording device and the like have a built-in hard disk device for data recording. In particular, in a hard disk device used in a portable computer such as a notebook personal computer, a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and the magnetic head slightly floats above the surface of the magnetic disk. Magnetic recording information is recorded on or read from the magnetic layer by a (DFH (Dynamic Flying Height) head). As the substrate of this magnetic disk, a glass substrate is preferably used because it has a property that it is less likely to undergo plastic deformation than a metal substrate or the like. In order to stably read and write magnetic recording information by the magnetic head, it is required to make the surface irregularities of the magnetic disk glass substrate as small as possible.

In order to reduce the surface unevenness of the magnetic disk glass substrate, the glass substrate is subjected to a polishing process. An abrasive containing fine abrasive grains such as silica (SiO ₂ ) is used for precise polishing for making a glass substrate into a final product. In order to improve the surface quality of the glass substrate after the polishing treatment, such an abrasive is used as a polishing agent in a predetermined size by performing a filtering treatment or centrifugal separation. Moreover, when using for grinding | polishing, circulating the slurry containing a silica abrasive grain at the time of a grinding | polishing process, after filtering the slurry used for grinding | polishing, it reuses for grinding | polishing.
For example, in the final process of the polishing process, a mirror polishing process using silica abrasive grains on the main surface of the glass substrate is performed. In this mirror polishing process, a method of manufacturing a glass substrate for a magnetic disk using a polishing liquid (including silica abrasive grains) after filtering using a filter having a minimum trapped particle diameter of 1 μm or less is known (Patent Literature). 1). The glass substrate after the mirror polishing process is cleaned with a cleaning liquid in order to remove foreign matters such as abrasive grains adhering to the surface (final cleaning process).

Also, it has been attempted to remove foreign matters from the slurry by filtering the slurry containing silica abrasive grains before the polishing treatment. For example, Patent Document 2 attempts to filter the slurry with a filter using polyethersulfone (PES).

JP 2010-079948 A JP 2013-170119 A

In the final cleaning process after the polishing process, when a cleaning liquid having a high etching power is used to remove foreign substances such as abrasive grains from the surface of the magnetic disk glass substrate, the magnetic disk glass substrate is etched by the cleaning liquid, Slight irregularities are formed. This slight unevenness was sufficiently smaller than the flying distance of the conventional magnetic head and was once negligible.

However, in recent years, as the recording density of the magnetic disk increases, in order to reliably read and record a weak magnetic field, the flying distance of the magnetic head from the magnetic disk surface has been extremely reduced. For this reason, slight unevenness due to etching cannot be ignored. Therefore, it has been attempted to perform a final cleaning process on the magnetic disk substrate by using a cleaning liquid having a lower etching power than conventional ones.

On the other hand, foreign matters derived from the slurry containing silica abrasive grains used for the polishing process may adhere to the main surface of the magnetic disk substrate after the mirror polishing process. Among these foreign substances, there are plate-shaped foreign substances (hereinafter referred to as plate-like foreign substances) having an extremely flat shape. When the magnetic layer is formed on the main surface with the plate-like foreign material remaining on the main surface of the magnetic disk substrate, surface irregularities are formed on the surface of the magnetic disk. If the magnetic recording information on the magnetic disk is read / written by a magnetic head having a very short flying distance, the magnetic head may collide with the surface irregularities. Since the plate-like foreign matter has a large adhesion area with the magnetic disk substrate, it cannot be easily removed with a cleaning solution having a low etching power.

The plate-like foreign matter is a foreign matter having an irregular shape larger than the average particle diameter (d50) of the substantially spherical silica abrasive grains, and is considered to be removed by a filter. Here, the average particle diameter indicates a median diameter measured based on a volume distribution using a laser diffraction / scattering method.
However, when the slurry was filtered with a filter using polyethersulfone, the filter was clogged with silica abrasive grains, and foreign matters could not be efficiently removed from the slurry.

Therefore, an object of the present invention is to provide a method for manufacturing a magnetic disk substrate that can improve the yield after polishing of the magnetic disk substrate by removing foreign substances contained in the polishing liquid.

The present inventor examined the cause of the silica abrasive grains being easily clogged in a filter using polyethersulfone. As a result, since the surface of the silica particles is hydrophilic, it is found that the silica particles are likely to adhere to the hydrophilic polyethersulfone, and it is also found that the smaller the silica particles are, the more difficult it is to leave after the adhesion. Was invented.

In order to solve the above problems, a first aspect of the present invention is a method of manufacturing a magnetic disk substrate,
By sandwiching the substrate between a pair of polishing pads, supplying a polishing liquid containing abrasive grains between the polishing pad and the substrate, and sliding the polishing pad and the glass substrate relatively, the magnetic disk Including a polishing process for polishing both main surfaces of the substrate;
The abrasive is colloidal silica having an average particle size of 40 nm or less,
The polishing liquid is obtained by subjecting the abrasive grains and a slurry containing large diameter particles having a larger particle diameter than the average particle diameter of the abrasive grains to a removal treatment to remove the large diameter particles,
The removal treatment is characterized in that the slurry is passed through a filter made of a hydrophobic polymer material and having an opening diameter of 100 nm or less.

Since a filter made of a hydrophobic polymer material is hard to adhere abrasive grains contained in the slurry and particles larger than the average particle diameter of the abrasive grains, the filter is not easily clogged even when the slurry is passed through the filter. For this reason, particles larger than the average particle diameter of the abrasive grains can be efficiently removed as a filtration residue.

The colloidal silica concentration in the slurry is preferably 30% by weight or less and passed through the filter.

It is preferable that the pH of the slurry is adjusted to an alkaline range of 8 to 13 and passed through the filter.

The hydrophobic polymer material is preferably polyvinylidene fluoride (PVDF).

The filter is preferably a hydrophobic polymer film formed into a porous film by a phase transition method.

The above manufacturing method is optimal when polishing is performed using colloidal silica obtained using water glass and an ion exchange resin.

It is preferable to adjust the pH of the polishing liquid obtained by passing the slurry through the filter to acidity before performing the polishing treatment.

According to the method for manufacturing a magnetic disk substrate described above, foreign matters such as plate-like foreign matters can be removed from the silica abrasive grains used in the polishing process. For this reason, plate-like foreign substances do not adhere to the main surface of the magnetic disk substrate, and the yield after the polishing process of the magnetic disk substrate can be improved.

Hereinafter, a method for manufacturing a magnetic disk substrate according to an embodiment of the present invention will be described.
(Magnetic disk substrate)
First, the magnetic disk substrate will be described. The magnetic disk substrate has a disk shape and a ring shape in which a circular center hole concentric with the outer periphery is cut out. A magnetic disk is formed by forming magnetic layers (recording areas) in the annular areas on both sides of the magnetic disk substrate. As the magnetic disk substrate, a glass substrate, an aluminum alloy substrate having a NiP alloy film formed on the surface, or the like can be used.

In this embodiment, a mirror polishing process is performed before the magnetic layer is formed. In the mirror polishing process, the main surface of the magnetic disk substrate is polished using a double-side polishing apparatus equipped with a planetary gear mechanism. Specifically, the main surface on both sides of the magnetic disk substrate is polished while holding the outer peripheral side end face of the magnetic disk substrate in the holding hole provided in the holding member of the double-side polishing apparatus. The double-side polishing apparatus has a pair of upper and lower surface plates (upper surface plate and lower surface plate), and an annular plate-shaped polishing pad (for example, as a whole on the upper surface of the lower surface plate and the bottom surface of the upper surface plate) Resin polisher) is attached. While supplying the polishing liquid between the main surface of the magnetic disk substrate and the polishing pad, either or both of the upper surface plate and the lower surface plate are moved, so that the magnetic disk substrate and the polishing pad are moved. The two main surfaces of the magnetic disk substrate are polished relative to each other.

In the present embodiment, a polishing liquid that contains colloidal silica (silica abrasive grains) as free abrasive grains is preferable as the polishing liquid used for the mirror polishing process.
Colloidal silica contained in the polishing liquid used for the mirror polishing treatment can be produced by a sol-gel method using tetramethyl orthosilicate, tetraethyl orthosilicate or the like as a raw material, or an ion exchange method using water glass as a raw material. Among these, it is preferable to manufacture by an ion exchange method from a cost viewpoint.
Specifically, silica sand and an alkali agent (for example, Na ₂ CO ₃ , NaHCO ₃ , NaOH, K ₂ CO ₃ , KHCO ₃ , KOH, etc.) are mixed and heated to melt to produce silicate. To do. Next, the obtained silicate is cooled as necessary, and then dissolved in water to produce an aqueous silicate solution (water glass). The water glass is mixed with a proton-type cation exchange resin to lower the pH of the aqueous silicate solution. Thereafter, by performing a heat treatment at a predetermined temperature for a predetermined time, the condensation polymerization of silanol groups in the silicate aqueous solution is promoted, colloidal silica is generated, and a slurry containing colloidal silica as abrasive grains is obtained. .

The slurry containing colloidal silica thus generated may contain large-sized particles (coarse particles, plate-like substances, etc.) having a large particle size that are inappropriate for use as abrasive grains. Specifically, the average particle diameter of colloidal silica suitable as abrasive grains is 60 nm or less, preferably 10 to 60 nm, more preferably 20 to 50 nm, whereas large diameters inappropriate for use as abrasive grains. The particle diameter of the particles is 2 times or more of the average particle diameter, and more inappropriate is 5 times or more. For example, the particle size of large particles is 200 nm to 1 μm.
Moreover, the slurry containing colloidal silica produced in this way may contain a plate-like substance derived from raw material silica sand. This plate-like substance is a silicate crystal containing aluminum, and this crystal is a layered layered silicate (for example, a layered clay mineral such as montmorillonite, saponite, and kaolinite). This plate-like material has a very flat shape. When such a plate-like substance adheres to a precisely polished surface, it is easy to adhere, and thus it becomes difficult to clean. Therefore, this plate-like substance is a foreign substance to the slurry containing colloidal silica (hereinafter referred to as a plate-like foreign substance). Called).
This plate-like foreign material remains without melting even when silica sand and an alkali agent are mixed and melted, and contains colloidal silica produced from water glass in water glass obtained by dissolving the melt in water. It remains in the slurry.

As for the plate-like foreign matter, among the large-diameter particles, particles having a maximum length of 5 times or more the thickness are plate-like foreign matters. For example, the maximum length of the plate-like foreign material is 50 nm to 1 μm, preferably 70 nm to 300 nm, more preferably 130 to 240 nm, and the thickness is 1 to 25 nm, preferably 1 to 5 nm.
Here, the maximum length of the plate-like foreign material refers to the length of the longest side of the cuboid frame that circumscribes the plate-like foreign material, and the thickness of the plate-like foreign material refers to the length of the shortest side of the cuboid frame. When observing a plate-like foreign substance attached to a glass substrate with SEM or AFM, the length of the long side of the rectangle circumscribing the outline of the plate-like foreign substance is the maximum length, and the maximum height of the plate-like foreign substance is the thickness. Can do.
In this embodiment, the removal process described below is performed in advance.

(Removal process)
A removal process is a process which removes the particle | grains larger than the average particle diameter of an abrasive grain contained in a slurry as a filtration residue by letting a slurry pass a filter.
As the filter, a microfiltration membrane made of a hydrophobic polymer material and having an opening diameter of 100 nm or less can be used. Here, the hydrophobic polymer material is a material made of a polymer having a small polarity and hardly forming a hydrogen bond with water. Since a filter made of a hydrophobic polymer material is hard to adhere abrasive grains contained in the slurry or particles larger than the average particle diameter of the abrasive grains, the filter is not easily clogged even when the slurry is passed through the filter. For this reason, particles larger than the average particle diameter of the abrasive grains can be efficiently removed as a filtration residue.
As such a hydrophobic polymer material, for example, a material having a water contact angle (85 to 125 °) can be used.
Here, the opening diameter is a diameter of a circle inscribed in the opening. For example, the shape of the opening of the filter can be measured with a scanning electron microscope (SEM) to obtain the opening diameter of the opening.

As the hydrophobic polymer material used for the microfiltration membrane, for example, polypropylene, polyethylene, fluororesin, or the like can be used. Examples of the fluororesin include polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), perfluoroethylene propene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), polyvinyl fluoride (PVF), and polyfluoride. Vinylidene (PVDF), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), or the like can be used. Among these, it is preferable to use a fluororesin having high water repellency. By using a highly water-repellent fluororesin, the contact area between the filter and the slurry is reduced, so that clogging of the filter can be further reduced.

As a filtration method, any method such as natural filtration by gravity, vacuum filtration (suction filtration), pressure filtration, centrifugal filtration, or cross flow ultrafiltration can be used.

A microfiltration membrane made of a hydrophobic polymer material may be used as a porous membrane filter, or a non-woven filter membrane made of a hydrophobic polymer fiber. Moreover, it is good also as a plate-shaped filtration board by compression-molding a film-form filter material. Moreover, it is good also as a hollow fiber membrane shape, a spiral membrane shape, and a tubular membrane shape, and it can select suitably according to the filtration method.

In the present embodiment, the flatness of the opening provided in the hydrophobic polymer film is preferably smaller than 0.3, and the opening is most preferably a perfect circle (the flatness is 0). A hydrophobic polymer film having a small aperture flatness can be produced by, for example, a phase transition method. A hydrophobic polymer film formed into a porous film by a phase transition method has an opening with an extremely small flatness.
Specifically, in the phase transition method, first, a solution in which a polymer material that is a material of the hydrophobic polymer film is dissolved in a solvent is applied to the support plate. Next, the support plate coated with the solution is immersed in a non-solvent such as water. Then, the polymer in the solution applied to the support plate is deposited on the support plate, and a solid-phase porous film is formed on the support plate.

The solvent can be appropriately selected from those that dissolve the polymer material that is the material of the hydrophobic polymer membrane. The non-solvent can be appropriately selected from those in which the polymer material used as the material of the hydrophobic polymer film is hardly soluble or insoluble and does not mix with the solvent.
For example, PVDF can be selected as the material for the hydrophobic polymer membrane, triethyl phosphate (TEP) can be selected as the solvent, and water (deionized water) can be selected as the non-solvent.

By passing the slurry through the filter, particles having a particle size larger than the average particle size of the abrasive grains contained in the slurry can be removed as a filtration residue. Here, in order to efficiently filter the slurry, it is preferable that the concentration of colloidal silica in the slurry is 30 wt% or less, preferably 5 to 25 wt%, and passed through the filter. When the colloidal silica concentration was adjusted and passed through the filter, when the colloidal silica concentration was 20% by weight, the filter passage amount per unit time was 800 liters, whereas at 25% by weight 430 liters, It is confirmed that 30% by weight is 150 liters, 35% by weight is 30 liters, and 40% by weight is 10 liters. As the colloidal silica concentration increases, the amount of filter passage decreases rapidly, causing clogging of the filter. It was. For this reason, it is preferable to perform the filter treatment in a state where the concentration of colloidal silica in the slurry is diluted as much as possible within the range of the concentration of colloidal silica used for the second polishing treatment. Specifically, when the concentration of colloidal silica used for the second polishing treatment is 20% by weight or less, the filter treatment is performed with the concentration of colloidal silica in the slurry diluted to about 20 to 25% by weight.

In order to improve the dispersibility of the abrasive grains in the slurry, it is preferable to adjust the pH of the slurry to an alkaline range of 8 to 13 and pass it through a filter. By adjusting the pH of the slurry to the alkaline side, the filter passage amount is relatively increased, and by adjusting the pH to the acidic side, the filter permeation amount is relatively decreased. For this reason, it is preferable to adjust the pH of the slurry to an alkaline range of 8 to 13 in order to prevent clogging of the filter and to suitably remove the plate-like foreign matters from the polishing slurry of colloidal silica abrasive grains.

After performing the above removal treatment, it is preferable to perform a treatment for reducing the surface charge of the colloidal silica in order to agglomerate the colloidal silica in the obtained polishing liquid. By aggregating colloidal silica, it is possible to increase the polishing rate and reduce the surface roughness of the glass substrate after the polishing treatment.
As a method of reducing the surface charge of colloidal silica, there is a method of adjusting the pH of the polishing liquid to an acidity of 1 or more and 5 or less.
Alternatively, an additive for reducing the surface charge of colloidal silica in the polishing liquid (for example, sulfuric acid compounds such as K ₂ SO ₄ and Na ₂ SO ₄ , phosphoric acid compounds such as K ₃ PO ₄ and Na ₃ PO ₄ , NaNO _{3 and the} like) Of nitric acid compound) is preferably added. If the surface charge of the colloidal silica is reduced before the removal treatment, the adsorbent with a positive surface charge is less likely to adhere to coarse particles and plate-like foreign matter, and the coarse particles and plate-like foreign matter can be removed from the slurry. It becomes difficult.

In particular, when the plate-like foreign material adheres to the main surface of the glass substrate, it is difficult to remove it by a subsequent cleaning process or the like. For this reason, the mirror polishing process performed using colloidal silica from which the plate-like foreign material has been removed in advance as free abrasive grains is suitable for the mirror polishing process of the glass substrate. Specific examples of the glass used for the magnetic disk glass substrate include aluminosilicate glass, soda lime glass, and borosilicate glass. In particular, aluminosilicate glass can be suitably used in that it can be chemically strengthened and a glass substrate for a magnetic disk excellent in the flatness of the main surface and the strength of the substrate can be produced.
Here, the manufacturing method of the glass substrate for magnetic discs is demonstrated.

(Method for producing glass substrate for magnetic disk)
First, a glass blank for a magnetic disk is produced by press molding. A magnetic disk glass blank (hereinafter simply referred to as a glass blank) is a material for a disk-shaped magnetic disk glass substrate having a pair of main surfaces, and is a form before a center hole is cut out.
Next, a hole is made in the central portion of the produced glass blank to produce a ring-shaped (annular) glass substrate. Next, shape processing is performed on the glass substrate with holes. Next, end face polishing is performed on the glass substrate that has been processed into a shape. Next, grinding with fixed abrasive is performed on the glass substrate on which the end face has been polished. Next, the first polishing is performed on the main surface of the glass substrate. Next, chemical strengthening is performed on the glass substrate as necessary. Thereafter, second polishing (mirror polishing) is performed on the glass substrate. After the second polishing, a glass substrate for magnetic disk is obtained through a cleaning process.
Hereinafter, each process will be further described.

(A) Press molding process The lump of the molten glass which cut | disconnected the front-end | tip part of the molten glass flow is pinched | interposed between the press molding surfaces of a pair of metal molds, and is pressed to form a glass blank. After pressing for a predetermined time, the mold is opened and the glass blank is taken out.

(B) Circular hole formation treatment A glass substrate having a circular central hole can be obtained by forming a circular hole on a glass blank using a core drill or the like.

(C) Shape processing In the shape processing, chamfering is performed on the edge of the glass substrate after the circular hole is formed.

(D) End surface polishing process In the end surface polishing process, mirror finishing is performed on the inner end face and the outer peripheral end face of the glass substrate by brush polishing. At this time, an abrasive slurry containing particles such as cerium oxide as free abrasive grains is used.

(E) Grinding process In the grinding process using the fixed abrasive grains, the main surface of the glass substrate is ground using a double-side grinding apparatus having a planetary gear mechanism. Specifically, the main surface on both sides of the glass substrate is ground while holding the outer peripheral side end face of the glass substrate generated from the glass blank in the holding hole provided in the holding member of the double-side grinding apparatus. The double-sided grinding apparatus has a pair of upper and lower surface plates (upper surface plate and lower surface plate), and a glass substrate is sandwiched between the upper surface plate and the lower surface plate. Then, by moving one or both of the upper surface plate and the lower surface plate and relatively moving the glass substrate and each surface plate, both main surfaces of the glass substrate can be ground.

(F) First polishing treatment In the first polishing, for example, when grinding with fixed abrasive grains is performed, scratches and distortions remaining on the main surface are removed, or fine surface irregularities (microwaveness, roughness) are adjusted. Objective.

In the first polishing process, a glass substrate is polished using a double-side polishing apparatus having a configuration similar to that of a double-side grinding apparatus while applying a polishing liquid containing loose abrasive grains to the double-side polishing apparatus. As the free abrasive grains, for example, cerium oxide abrasive grains, aluminum oxide abrasive grains, zirconia abrasive grains, etc. (particle size: diameter of about 1 to 2 μm) are used. In the double-side polishing apparatus, similarly to the double-side grinding apparatus, the glass substrate is sandwiched between a pair of upper and lower surface plates. An annular flat polishing pad (for example, a resin polisher) is attached to the upper surface of the lower surface plate and the bottom surface of the upper surface plate as a whole. While supplying the polishing liquid between the main surface of the glass substrate and the polishing pad, the glass substrate and the polishing pad move relatively by moving either the upper surface plate, the lower surface plate, or both. Then, both main surfaces of the glass substrate are polished.

(G) Chemical strengthening treatment In the chemical strengthening treatment, the glass substrate is chemically strengthened by immersing the glass substrate in a chemical strengthening solution. As the chemical strengthening liquid, for example, a mixed melt of potassium nitrate and sodium nitrate can be used. The chemical strengthening process may not be performed.

(H) Second polishing (mirror polishing) treatment The second polishing treatment is intended for mirror polishing of the main surface. Also in the second polishing, a double-side polishing apparatus having the same configuration as the double-side polishing apparatus used for the first polishing is used. The machining allowance by the second polishing is, for example, about 1 μm. The second polishing process is different from the first polishing process in that the type and particle size of the free abrasive grains are different and the hardness of the resin polisher is different.

In the second polishing process, a polishing liquid obtained by performing the above-described removal process and containing colloidal silica as free abrasive grains is used.
By performing the second polishing treatment, the roughness (Ra) of the main surface can be set to 0.15 nm or less and the micro waveness of the main surface can be set to 0.1 nm or less.
In the second polishing process, a multi-stage polishing process may be performed using a plurality of types of polishing abrasive grains. For example, before performing the polishing process using a polishing liquid containing colloidal silica as free abrasive grains, the polishing process may be performed using a polishing liquid containing cerium oxide (CeO ₂ ) as free abrasive grains. Moreover, after performing the polishing process using a polishing liquid containing colloidal silica as free abrasive grains, a final polishing process may be performed using a polishing liquid containing colloidal silica as free abrasive grains.

(I) Cleaning Process After the second polishing process, the glass substrate becomes a glass substrate for a magnetic disk before the surface of the glass substrate is cleaned using an alkaline cleaning liquid and the magnetic layer is formed.
At this time, in the cleaning process, it is preferable to use an alkaline cleaning liquid in which the difference in surface roughness Ra between the glass substrate before and after the cleaning process is 0.05 nm or less. Since plate-like foreign substances adhering to the glass substrate are difficult to remove, an alkaline cleaning liquid having a high cleaning power has been conventionally used. For this reason, the alkaline cleaning liquid having a strong cleaning power is likely to act on the main surface of the glass substrate having no plate-like foreign matter and roughen the main surface. However, in this embodiment, since the polishing process is performed using the silica abrasive grains subjected to the above-described removal process, no plate-like foreign matter adheres to the glass substrate. For this reason, in the present embodiment, an alkaline cleaning liquid having a weaker cleaning power than that of the prior art, that is, an alkaline cleaning liquid that makes the difference in the surface roughness Ra of the glass substrate before and after the cleaning process 0.05 nm or less can be used. Note that Ra is the surface roughness specified in JIS B0601. This surface roughness is obtained based on data obtained by measuring a range of 1 μm × 1 μm with a resolution of 256 × 256 pixels using an atomic force microscope (AFM).

In addition, the cleaning treatment is preferably non-scrub cleaning in which the glass substrate is immersed in or brought into contact with the cleaning liquid in terms of not causing scratches on the glass substrate. In the conventional cleaning process, scrub cleaning is performed to remove the plate-like foreign matter by rubbing the glass substrate with a brush or a cleaning pad in order to remove the plate-like foreign matter firmly attached to the glass substrate. However, this scrub cleaning tends to damage the main surface of the glass substrate. In this embodiment, since it polishes using the slurry containing the silica abrasive grain which performed the removal process mentioned above, a plate-shaped foreign material does not adhere to a glass substrate. For this reason, it is not necessary to perform scrub cleaning as in the past. For this reason, in this embodiment, unnecessary scratches are not applied to the main surface of the glass substrate by performing non-scrub cleaning in which the glass substrate is immersed in or brought into contact with the cleaning liquid.

As described above, the glass substrate is used as the magnetic disk substrate, but the present invention can also be applied to an aluminum alloy substrate. In the case of an aluminum alloy substrate, an aluminum alloy substrate obtained by rolling an aluminum alloy and forming a NiP plating film on the surface of the aluminum alloy substrate cut into a disk shape is used, and the NiP plating film surface is polished using a polishing pad. Will be. A magnetic disk using an aluminum alloy substrate is obtained by laminating a soft magnetic layer, a nonmagnetic underlayer, a perpendicular magnetic recording layer, a protective layer, a lubricating layer, and the like on an aluminum alloy substrate.

Specifically, when manufactured through the following respective processing steps, the same polishing pad used in the polishing step of the glass substrate can be used as the polishing pad used in the polishing step.
The molten aluminum alloy is cast, rolled, cut out as a disk-shaped aluminum alloy base plate, and processed to a predetermined size by grinding the main surface and the end face. After that, the surface of the aluminum alloy base plate is subjected to NiP plating film formation with a thickness of 5 to 30 μm to obtain an aluminum alloy substrate. Subsequently, the main surface of the aluminum alloy substrate subjected to NiP plating is polished using a polishing pad to reduce microwaviness. The polishing process is usually performed in two stages by changing the type and particle size of the abrasive grains. In the first polishing process, a slurry containing aluminum oxide abrasive grains having an average particle diameter of 0.3 to 3 μm is used. In the polishing treatment, an aluminum alloy substrate is sandwiched between the polishing pads subjected to the opening treatment using a slurry containing colloidal silica abrasive grains having an average particle size of 40 nm or less, preferably 5 to 40 nm, and relatively slid. Reduces scratches and undulations on the NiP plating surface. Further, after the first polishing process and after the second polishing process, a cleaning process is performed in order to remove particles such as abrasive grains and polishing residue adhering to the substrate surface after the polishing process. In this second polishing treatment, a polishing liquid containing colloidal silica as free abrasive grains obtained by performing the above-described removal treatment can be used.

Examples of the present invention and comparative examples will be described below.
(Creation of colloidal silica)
A slurry containing colloidal silica was obtained by an ion exchange method using silica sand and sodium carbonate as raw materials. The average particle diameter of colloidal silica is as shown in Table 1.

(Removal process)
In Examples, a porous film of polyvinylidene fluoride (PVDF) prepared by a phase transition method was prepared and used as a filter. The opening diameter of the prepared porous membrane is as shown in Table 1.
The concentration of colloidal silica in the slurry was adjusted to 30% by weight or less, and the pH of the slurry was adjusted to 10 and passed through a filter.

In the comparative example, a porous film of polyethersulfone (PES) prepared by a phase transition method was prepared and used as a filter. The opening diameter of the prepared porous membrane is as shown in Table 1.
Also in the comparative example, the colloidal silica concentration in the slurry was adjusted to 30% by weight or less, and the pH of the slurry was adjusted to 10 to pass through the filter. However, the filter was clogged and could not pass through the filter.
For this reason, the subsequent processing cannot be performed in the comparative example, and the subsequent processing is performed only in the example.

(Glass substrate polishing process)
Next, the pH of the filtrate that passed through the filter in the separation process was adjusted to 3, and this was used as a polishing liquid to perform mirror polishing of the glass substrate. While supplying the above polishing liquid between the main surface of the glass substrate and the polyurethane polishing pad, the main surface of the glass substrate was polished by moving the polishing pad relative to the main surface of the glass substrate.
After the above-described polishing treatment, the main surface of the cleaned and dried glass substrate was subjected to detection of foreign matters or defects using a laser type surface inspection apparatus. The laser threshold is set to a range in which the number of detected foreign matters or defects is 20 or less, and the detected foreign matter or defects are measured using an SEM (scanning electron microscope) or AFM (atomic force microscope). The number of foreign matters that were 100 nm or more was counted. Next, using a cross-sectional TEM (transmission electron microscope), it was confirmed whether or not a foreign material having a major axis of 100 nm or more is a layered flat foreign material (a plate-like foreign material made of layered silicate). In a glass substrate polished with a slurry containing silica by an ion exchange method that has not been subjected to removal treatment by a filter, it was confirmed that a plurality of layered flat foreign matters were attached to the main surfaces of the front and back surfaces of the glass substrate (two pieces). / More than one layered flat foreign material is attached). On the other hand, when the substrate was polished after removing the foreign matter using a filter for the slurry containing silica by the ion exchange method, the layered flat foreign matter adhering to the front and back main surfaces of the glass substrate was not detected (substrate There is no adhesion of layered flat foreign material on the main surface).

[Evaluation of foreign substances on the main surface of the glass substrate]
After the polishing treatment, the main surface of the cleaned and dried glass substrate was subjected to detection and identification of foreign matter using a laser type surface inspection apparatus, SEM, and AFM. For 100 glass substrates manufactured under the same conditions, 1 point of foreign matter is detected and identified for each piece, A for the case where the total number of foreign matters per 100 points is 0, B for 1 to 2 cases, The case of 3 to 10 was evaluated as C, and the case of 11 or more was evaluated as D. If the evaluation is A, B or C, it can be used as a polishing liquid.
The results are shown in Table 1.

Regarding the glass substrates of Examples 1 to 6, the number of foreign matters per 100 glass substrates is 2 or less, and the filtrate obtained by passing the slurry through a filter made of a hydrophobic polymer material is suitable as a polishing liquid. I understand.

As mentioned above, although the manufacturing method of the substrate for magnetic disks of this invention was demonstrated in detail, this invention is not limited to the said embodiment, You may make various improvement and change in the range which does not deviate from the main point of this invention. Of course.

Claims

A method for manufacturing a magnetic disk substrate, comprising:
By sandwiching the substrate between a pair of polishing pads, supplying a polishing liquid containing abrasive grains between the polishing pad and the substrate, and sliding the polishing pad and the glass substrate relatively, the magnetic disk Including a polishing process for polishing both main surfaces of the substrate;
The abrasive is colloidal silica having an average particle size of 40 nm or less,
The polishing liquid is obtained by subjecting the abrasive grains and a slurry containing large diameter particles having a larger particle diameter than the average particle diameter of the abrasive grains to a removal treatment to remove the large diameter particles,
The method for producing a substrate for a magnetic disk, wherein the removing process is a process of passing the slurry through a filter made of a hydrophobic polymer material and having an opening diameter of 100 nm or less.
2. The method of manufacturing a magnetic disk substrate according to claim 1, wherein the concentration of colloidal silica in the slurry is set to 30% by weight or less and passed through the filter.
3. The method of manufacturing a magnetic disk substrate according to claim 1, wherein the pH of the slurry is adjusted to an alkaline range of 8 to 13 and passed through the filter.
The method for manufacturing a magnetic disk substrate according to any one of claims 1 to 3, wherein the hydrophobic polymer material is polyvinylidene fluoride (PVDF).
The method for manufacturing a magnetic disk substrate according to any one of claims 1 to 4, wherein the filter is a hydrophobic polymer film formed into a porous film by a phase transition method.
6. The method for producing a magnetic disk substrate according to claim 1, wherein the colloidal silica is obtained using water glass and an ion exchange resin.
The method for producing a magnetic disk substrate according to any one of claims 1 to 6, wherein the pH of the polishing liquid obtained by passing the slurry through the filter is adjusted to be acidic before performing the polishing treatment.