WO2016043288A1 - Method for manufacturing magnetic disk substrate - Google Patents

Abstract

　The present invention is a method for manufacturing a magnetic disk substrate that includes a polishing process for polishing the main surface of the substrate by sandwiching a disk-shaped substrate with a pair of polishing pads, supplying slurry including abrasive gains to between the polishing pads and the substrate, and sliding the polishing pads and the substrate relatively to each other. The abrasive gains and plate-shaped substances having a grain size larger than the average grain size of the abrasive grains, both included in the slurry, have a difference in the amount of surface charge, and an adsorption process for causing a solid adsorbent, which is adsorbed to at least the plate-shaped substances and has a surface charge the sign of which differs from the surface charge of the plate-shaped substances, to be adsorbed to the shaped-like substances in the slurry by mixing the adsorbent into the slurry is executed before the polishing process is executed.

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 polishing step of the polishing step using silica abrasive grains on the main surface of the glass substrate, polishing slurry (including silica abrasive grains) after filtering using a filter having a minimum trapped particle diameter of 1 μm or less A method of manufacturing a magnetic disk glass substrate to be used is known (Patent Document 1). The glass substrate after the final polishing treatment is washed with a cleaning liquid in order to remove foreign matters such as abrasive grains adhering to the surface (final washing treatment).

JP 2010-079948 A

In the final cleaning process after the polishing process, if a cleaning liquid having a high cleaning 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 using a cleaning liquid having a lower cleaning 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 final cleaning process. In recent years, it has been clarified that there is a plate-like foreign material (hereinafter referred to as a plate-like foreign material) having a very flat shape due to the improvement in inspection accuracy of the surface inspection apparatus. 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, which makes it difficult to read / write stable magnetic recording information. The plate-like foreign matter has a large adhesion area with the magnetic disk substrate, and therefore cannot be easily removed with a cleaning liquid having a low cleaning power. On the other hand, it is not preferable to use a cleaning solution having a high cleaning power for the glass substrate in order to remove the plate-like foreign matter, because irregularities due to etching are formed on the main surface.
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 is filtered with a filter, the filter is easily clogged with the silica abrasive grains, and thus foreign substances cannot 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 replaces a filter that is easily clogged and cannot sufficiently remove the above-mentioned plate-like foreign matters or a centrifugal separator that cannot sufficiently remove the above-mentioned plate-like foreign matters before the polishing treatment. A new method that can remove the plate-like foreign material was examined. As a result, the surface of the silica particles has a negative surface potential, and the surface potential of the silica abrasive grains of the large plate-like foreign material is larger than the surface potential of the roughly spherical silica abrasive grains of a small size, Paying attention to the large absolute value, the following method was invented.

In order to solve the above-described problem, a first aspect of the present invention is a method in which a disk-shaped substrate is sandwiched between a pair of polishing pads, a slurry containing abrasive grains is supplied between the polishing pad and the substrate, and the polishing is performed. A method for manufacturing a magnetic disk substrate including a polishing process for polishing a main surface of the substrate by sliding the pad and the substrate relatively,
The polishing abrasive grains contained in the slurry and a plate-like substance having a particle size larger than the average particle diameter of the polishing abrasive grains have a difference in the amount of surface charge,
Before performing the polishing treatment, a solid adsorbent adsorbed on at least the plate-like substance and having a surface charge different in sign from the surface charge of the plate-like substance is mixed with the slurry. An adsorption process for adsorbing an adsorbent to the plate-like substance is performed.

According to a second aspect of the present invention, a disk-shaped substrate is sandwiched between a pair of polishing pads, a slurry containing abrasive grains is supplied between the polishing pad and the substrate, and the polishing pad and the substrate are relative to each other. A method of manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of the substrate by sliding on the substrate,
Abrasive grains contained in the slurry stock solution and a plate-like substance having a particle size larger than the average particle diameter of the abrasive grains have a difference in the amount of surface charge, and are adsorbed on at least the plate-like substance, and The plate-like substance that adsorbs the adsorbent after the adsorbent is adsorbed to the plate-like substance by mixing a solid adsorbent having a surface charge different in sign from the surface charge of the plate-like substance into the slurry stock solution. Is used as the slurry used for the polishing treatment.

The adsorbent is preferably an organic polymer.

According to a third aspect of the present invention, a disk-shaped substrate is sandwiched between a pair of polishing pads, a slurry containing abrasive grains is supplied between the polishing pad and the substrate, and the polishing pad and the substrate are relative to each other. A method of manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of the substrate by sliding on the substrate,
Before performing the polishing treatment, among the abrasive grains contained in the slurry and an organic polymer adsorbed on at least the plate-like substance among the plate-like substances having a particle diameter larger than the average particle diameter of the abrasive grains An adsorbing process is performed in which the adsorbent is adsorbed to the plate-like substance in the slurry by preparing the adsorbent in the slurry.

According to a fourth aspect of the present invention, a disk-shaped substrate is sandwiched between a pair of polishing pads, a slurry containing abrasive grains is supplied between the polishing pad and the substrate, and the polishing pad and the substrate are relative to each other. A method of manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of the substrate by sliding on the substrate,
An adsorbent made of at least an organic polymer adsorbed on the plate-like substance is prepared in a solvent among the abrasive grains contained in the slurry and a plate-like substance having a particle size larger than the average particle size of the abrasive grains. And
It is characterized in that an adsorption process for adsorbing the adsorbent on a plate-like substance in the slurry is performed by introducing a solvent in which the adsorbent is prepared into a slurry before performing the polishing process.

The abrasive grains are preferably silica particles having an average particle size of 10 nm or more and 60 nm or less.

After the adsorption treatment, a separation treatment for separating the plate-like substance adsorbed on the adsorbent from the slurry is performed,
The polishing treatment is preferably performed using the slurry from which the plate-like substance has been removed by the separation treatment.

The abrasive grains are preferably silica abrasive grains obtained using water glass and an ion exchange resin.

After the polishing process, a cleaning process for cleaning the main surface of the substrate is performed, and in the cleaning process, an alkali cleaning liquid is used that makes the difference in arithmetic mean roughness Ra of the substrate surface before and after the cleaning process 0.05 nm or less. Is preferred.

It is preferable that an additive for reducing the absolute value of the surface charge of the abrasive grains is added to the slurry before the polishing treatment after the adsorption treatment.

The content of alkaline earth metal ions in the slurry before the adsorption treatment is preferably 200 ppm or less.

In the adsorption treatment, it is preferable to remove a plate-like substance whose maximum length is 5 times or more of the thickness among the particles contained in the slurry.

The adsorbent is an organic polymer, and after the polishing process, at least one of (1) bringing an organic solvent into contact with the adsorbent remaining on the surface of the magnetic disk substrate and (2) oxidizing. It is preferable to remove by performing one. In particular, when the adsorbent cannot be completely decomposed by oxidation, it is effective to dissolve the adsorbent in an organic solvent.

The arithmetic average roughness Ra of the surface of the substrate after the polishing treatment is preferably 0.15 nm or less.

According to a fifth aspect of the present invention, a disk-shaped substrate is sandwiched between a pair of polishing pads, a slurry containing abrasive grains is supplied between the polishing pad and the substrate, and the polishing pad and the substrate are relative to each other. A method of manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of the substrate by sliding on the substrate,
Before performing the polishing treatment, due to the difference in the amount of surface charge each of the polishing abrasive grains and large particles having a particle diameter larger than the average particle diameter of the polishing abrasive grains contained in the slurry has, By mixing the slurry with a solid adsorbent that is more easily adsorbed by large-diameter particles than particles having an average particle diameter of abrasive grains and has a surface charge that is different in sign from the surface charge of the large-diameter particles. The adsorbent is adsorbed on the large-diameter particles in the slurry.

According to a sixth aspect of the present invention, a disk-shaped substrate is sandwiched between a pair of polishing pads, a slurry containing abrasive grains is supplied between the polishing pad and the substrate, and the polishing pad and the substrate are relative to each other. A method of manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of the substrate by sliding on the substrate,
Before performing the polishing treatment, the polishing abrasive grains contained in the slurry and the difference in surface charge amount of the large-diameter particles having a particle diameter larger than the average particle diameter of the polishing abrasive grains, respectively, Adsorption that adsorbs the adsorbent to the large-diameter particles in the slurry by creating an adsorbent in the slurry that is made of an organic polymer that is easily adsorbed to the large-diameter particles compared to particles having an average particle diameter. It is characterized by performing processing.

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 substrate, or the like can be used.

In this embodiment, a final polishing process is performed before the magnetic layer is formed. In the final 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 containing colloidal silica (silica abrasive grains) as free abrasive grains is used as the polishing liquid used in the final polishing process.
Colloidal silica contained in the polishing liquid used in the final polishing treatment can be produced by a sol-gel method using tetramethyl orthosilicate, tetraethyl orthosilicate, or the like, 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 polycondensation of silanol groups in the silicate aqueous solution is promoted, silica particles are generated, and a slurry containing colloidal silica 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 to it, making it difficult to clean. Therefore, this plate-like substance is a foreign substance (hereinafter referred to as a plate-like foreign substance) against a slurry containing colloidal silica. 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.

Here, the maximum length, which is the particle size of the plate-like foreign matter, refers to the maximum length of the long side of the rectangular frame that circumscribes the outline of the plate-like foreign matter, for example, when a two-dimensional image of the plate-like foreign matter is obtained. In addition, the maximum length of the longest side of the rectangular parallelepiped frame that circumscribes the three-dimensional image of the plate-like foreign material is referred to, and the length of the shortest side of the rectangular parallelepiped frame at this time is referred to as the thickness. Particles having a maximum length of 5 times or more the thickness are plate-like foreign substances. For example, the maximum length of the plate-like foreign material is 200 nm to 1 μm and the thickness is 10 to 25 nm.
In the present embodiment, an adsorption process and a separation process described below are performed in advance.

(Adsorption treatment)
In the adsorption process, the surface charge having different values for the particles having the average particle diameter of the abrasive grains contained in the slurry and the large particles (coarse particles, plate-like substances, etc.) having a particle diameter larger than the average particle diameter. By mixing the slurry with an adsorbent having a surface charge different from the surface charge of the large-diameter particles, which is easily adsorbed on the large-diameter particles, particularly the plate-like foreign matters, compared to the abrasive grains. This is a treatment for adsorbing large-diameter particles, particularly plate-like foreign matters, on the adsorbent in the slurry.
Specifically, an adsorbent having a positive surface charge is introduced into the slurry containing colloidal silica, thereby adsorbing the plate-like foreign material to the adsorbent.

Silica-based particles such as colloidal silica and large-diameter particles in the slurry have a negative surface charge. This surface charge depends on the surface area of the silica-based particle, and the larger the particle, the larger the absolute amount of the negative surface charge. For this reason, when an adsorbent is introduced into the slurry, the adsorbent is attracted to the silica-based particles with a force proportional to the surface charge of the silica-based particles.
Here, the silica-based particles are dispersed in the slurry by repelling each other due to negative surface charges. This repulsive force suppresses the movement distance of the silica-based particles in the slurry. On the other hand, since the amount of adsorbent charged into the slurry is small, there is little repulsion between the adsorbents. For this reason, the adsorbent in the slurry moves toward the silica-based particles having a larger absolute amount of surface charge and is adsorbed by the silica-based particles having a larger absolute amount of surface charge.

In addition, since the plate-like foreign material is flat and has a large surface area, the probability of the plate-like foreign material coming into contact with the surface when the adsorbent moves increases as the silica-based particles having a large particle diameter are flat. Furthermore, larger adsorbents are adsorbed on large-diameter particles having a large surface area, particularly plate-shaped particles.
Further, since the plate-like substance having a larger particle diameter has a larger mass, the moving speed is lower. For this reason, the time for which the adsorbent comes into contact with the plate-like substance having a larger particle diameter becomes longer, and more adsorbent is adsorbed.

Therefore, among the particles present in the slurry, a small number of plate-like substances contained as foreign matters with respect to a large number of abrasive grains are reliably adsorbed by the adsorbent. The particle size of the adsorbent is larger than the average particle size of the abrasive grains and smaller than the particle size of the large particles.

The silica-based particles that have adsorbed the adsorbent are less likely to adhere to the substrate even when used for polishing. Even if the silica-based particles adsorbing the adsorbent are attached to the substrate, the silica-based particles adsorbing the adsorbent can be easily removed from the substrate by the cleaning treatment. It is preferable that the large-sized particles adsorbing the adsorbent are separated and removed from the slurry by the following separation treatment.

(Separation process)
A separation process is a process which isolate | separates the large diameter particle which adsorb | sucked the adsorbent from a slurry after an adsorption process. By separating the large-diameter particles adsorbing the adsorbent together with the adsorbent, the large-diameter particles in the slurry can be reduced. In this way, large particles are removed from the slurry.
In the silica-based particles to which the adsorbent having a positive surface charge is attached, the negative surface charge is neutralized. At this time, since the silica-based particles are stably dispersed in the slurry by repelling each other due to the surface charge, the silica-based particles with the neutralized surface charge lose repulsive force and may aggregate and precipitate. . For this reason, by putting an appropriate amount of adsorbent having a positive surface charge into the slurry, large particles can be precipitated, and the precipitate can be separated by filtration or the like. Supernatant may be used. As described above, large-diameter particles such as plate-like foreign matters and coarse particles can be removed from the slurry.

The amount of adsorbent added to the slurry is preferably greater than the number of large particles present in the slurry in order to ensure that the adsorbent is adsorbed on the large particles. For this reason, it is preferable to adjust so that the density | concentration of an adsorbent with respect to the whole quantity of the slurry after adding adsorbent may be 0.01 wt% or more. Moreover, when there is too much addition amount of an adsorbent, there exists a possibility that the quantity which an adsorbent adsorb | sucks abrasive grain may increase, and production efficiency may fall. For this reason, it is preferable that the number of adsorbents is sufficiently smaller than the number of abrasive grains so that sufficient polishing can be performed even if the adsorbent adsorbs abrasive grains. For this reason, it is more preferable to adjust the addition amount so that the concentration of the adsorbent with respect to the total amount of the slurry after the adsorbent is added is 5 wt% or less.

As the adsorbent having a positive surface charge, for example, fine particles (polymer fine particles) made of an organic polymer and having a particle diameter of 50 nm to 100 nm can be used. As the organic polymer, a polymer obtained by polymerizing a monomer that is insoluble or hardly soluble in water is preferably used. Specifically, a vinyl polymer, an acrylic polymer, or the like can be used.
Examples of vinyl polymers include styrene, α-methylstyrene, divinylbenzene, methyl methacrylate, methyl acrylate, t-butyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, 2-ethylhexyl acrylate, n-butyl acrylate, and ethylene glycol. Dimethacrylate or the like can be used. Also, carboxyl group-containing vinyl monomers such as methacrylic acid, acrylic acid, and vinyl acetate or salts thereof; sulfonic acid group-containing vinyl monomers such as styrenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, and sodium styrenesulfonate. Alternatively, a salt thereof; fine particles composed of a vinyl polymer may be produced by using one or more monomers such as a hydroxyl group-containing vinyl monomer such as hydroxyethyl methacrylate.

Examples of the acrylic polymer that can be used include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, 2-dimethylaminoethyl acrylate, 2-hydroxyethyl acrylate, and the like. Among these, it is preferable to use polymethyl methacrylate resin (PMMA).

In order to make the surface charge of the obtained polymer fine particles positive, a cationic polymerization initiator can be used. As the cationic polymerization initiator, for example, 2,2′-azobis (2-methylpropionamidine) dihydrochloride and the like can be used.

As the polymerization method, any polymerization method such as a suspension polymerization method or an emulsion polymerization method can be used. Among these, it is preferable to use a soap-free emulsion polymerization method in order to make the particle diameter of the polymer fine particles serving as the adsorbent uniform. Specifically, the monomer is emulsified in a solvent in which the monomer is insoluble or hardly soluble, and a polymerization initiator soluble in the solvent is added. At this time, by adjusting the addition amount of the polymerization initiator so that the polymerization initiator per monomer droplet in the solvent is 1 molecule or less, the progress of the polymerization reaction in the monomer droplet can be controlled by the reaction time. It can be controlled, and the particle diameter of the polymer fine particles growing in each droplet can be made uniform.

As the polymerization reaction proceeds, the particle size of the polymer particles increases and the polymer particles become harder. Further, the shape of the polymer fine particles changes from an irregular shape to a spherical shape as the polymerization reaction proceeds. As the polymer fine particles are irregular and soft, the adhesion area to the silica-based particles increases, and as the polymer fine particles are spherical and hard, the adhesion area to the silica-based particles tends to decrease. By setting the average particle size of the polymer fine particles to 200 nm or less, preferably 50 nm to 100 nm, amorphous and soft polymer fine particles can be obtained.

When the adsorbent having a positive surface charge is put into a slurry containing colloidal silica and stirred, the adsorbent adheres to the large diameter particles in the slurry and neutralizes the negative surface charge of the large diameter particles. As a result, large-diameter particles are precipitated, so that foreign matters can be removed by filtration, centrifugation, or the like.

Note that the particle diameter of the silica-based particles to be removed can be adjusted by adjusting the particle diameter of the adsorbent having a positive surface charge. Specifically, the particle diameter of the adsorbent is preferably equal to or greater than the average particle diameter of the abrasive grains so that the abrasive grains to be left in the slurry are difficult to adsorb. On the other hand, the particle size of the adsorbent is preferably smaller than the average particle size of the large particles so that the adsorbent is easily adsorbed by the large particles to be removed, and is less than half the average particle size of the large particles. Preferably there is.
From the above viewpoint, the average particle diameter of the adsorbent is more preferably 50 nm to 100 nm.

Also, by adjusting the pH of the slurry containing colloidal silica, the surface charge of the silica particles in the slurry can be varied, and the particle diameter of the silica particles removed by the adsorbent can be adjusted.

By growing polymer fine particles by a polymerization reaction in an appropriate solvent, when the polymer fine particles having a preferable particle size and hardness are produced, the polymerization reaction is stopped, and by introducing this solvent into a slurry containing colloidal silica, The polymer fine particles in the solvent adhere to the large-sized particles in the slurry as an adsorbent, and neutralize the negative surface charge of the large-sized particles. As a result, large-diameter particles are precipitated, so that foreign matters can be removed by filtration, centrifugation, or the like.
In this case, in order to facilitate mixing with the slurry, it is preferable to grow the polymer fine particles in a hydrophilic solvent. As a polymerization method for growing polymer fine particles, any polymerization method such as a suspension polymerization method or an emulsion polymerization method can be used. In particular, it is preferable to use the soap-free emulsion polymerization method.

An adsorbent made of an organic polymer having a surface charge having a sign different from that of the surface charge of the large particle may be prepared in the slurry. In this case, a monomer that becomes polymer fine particles may be charged into a slurry containing colloidal silica, and a polymerization reaction may be performed in the slurry.
Since colloidal silica is a hydrocolloid and does not affect the polymerization reaction, the polymerization reaction can be carried out by suspending or emulsifying the monomer in the slurry. When the polymerization reaction is performed in the slurry, polymer fine particles having a particle diameter smaller than that of the large particles and before being completely cured can be prepared and adsorbed on the large particles.
Even when the polymerization reaction is performed in the slurry, polymer fine particles having the same composition as described above can be produced.
The polymerization reaction in the slurry can be stopped, for example, by cooling the slurry.
The amount of monomer added to the slurry is preferably adjusted so that the monomer concentration relative to the total amount of slurry after the monomer addition is 0.01 wt% or more. In addition, if the amount of monomer added is too large, the amount of adsorbent composed of the organic polymer to be produced increases in the amount adsorbed on the abrasive grains, which may reduce production efficiency. It is more preferable to adjust the amount of monomer added so that the monomer concentration relative to the total amount is 5 wt% or less.

When the polymerization reaction is carried out in a slurry, the larger the silica particle having a larger particle diameter, the larger the absolute amount of the negative surface charge. Therefore, more monomers having a positive charge are collected around the silica particle having a larger particle diameter. For this reason, the polymer fine particles grow around the large-diameter particles and the adsorbent is easily formed. The prepared adsorbent is adsorbed by the large-sized particles, and the large-sized particles are precipitated by neutralizing the negative surface charge of the large-sized particles. For this reason, foreign substances can be removed by filtration, centrifugation, or the like.

Most of the adsorbent is precipitated and removed with the large particles. Even if the adsorbent is not removed and remains in the slurry slightly, it does not affect the polishing process.
In preparation for the case where the adsorbent remains in the slurry and the adsorbent adheres to the substrate after the final polishing process, it is preferable to perform a process of cleaning the substrate and removing the adsorbent from the substrate after the final polishing process. . Any method can be used for cleaning. For example, it can be removed by ashing polymer fine particles that are adsorbents. In order to incinerate the polymer fine particles, for example, the polymer fine particles are decomposed by irradiating the substrate after the final polishing process in the air with ultraviolet rays, and the polymer fine particles are oxidized by ozone generated from oxygen in the air. The polymer fine particles can be ashed. Alternatively, the fine polymer particles may be ashed by placing the substrate after the final polishing treatment in an ozone atmosphere.
Further, for example, by bringing the polymer fine particles into contact with a solvent containing an organic solvent or an anionic surfactant, at least a part of the polymer fine particles can be dissolved in the solvent, and the polymer fine particles can be removed. When the polymer fine particles cannot be sufficiently removed by ashing, it is particularly effective to dissolve the polymer fine particles in a solvent. In this case, it is more preferable to perform a treatment for ashing the polymer fine particles and then a treatment for dissolving the remaining polymer fine particles.
If the adsorbent does not remain on the substrate after the final polishing process, or if there is no problem in using the substrate even if it remains, the cleaning step of removing the adsorbent can be omitted.
In the following description, the adsorption process and the separation process performed as necessary after the adsorption process are collectively referred to as a removal process.

After the above removal treatment, it is preferable to perform a treatment for reducing the surface charge of the colloidal silica in order to aggregate the colloidal silica. 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 for reducing the surface charge of colloidal silica, specifically, an additive for reducing the surface charge of colloidal silica in the slurry (for example, sulfate compounds such as K ₂ SO ₄ and Na ₂ SO ₄ , K ₃ PO ₄ , Na ₃ PO _{4 and} other phosphoric acid compounds, NaNO _{3 and} other nitric acid compounds) are preferably added. If the surface charge of the colloidal silica is reduced before the removal treatment, the adsorbent having a positive surface charge is less likely to adhere to the large particles, and it is difficult to remove the large particles from the slurry.

Further, the alkaline earth metal ion content of the slurry before the removal treatment in the present embodiment is preferably 200 ppm or less. When the content of alkaline earth metal ions exceeds 200 ppm, the surface charge of the silica abrasive grains is reduced, and the above-described removal treatment makes it difficult to obtain a sufficient removal effect. The amount of alkaline earth metal in the slurry can be reduced, for example, by making the raw material highly pure when preparing the slurry, or by bringing an ion exchange resin or the like into contact with the slurry stock solution.

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 final polishing process performed using colloidal silica from which the plate-like foreign material has been removed in advance as the free abrasive grains is suitable for the final 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 (final 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 while applying a polishing slurry containing loose abrasive grains to the double-side polishing apparatus using a double-side polishing apparatus having the same configuration as the double-side grinding apparatus. As the free abrasive grains, for example, cerium oxide abrasive grains or zirconia abrasive grains (particle size: diameter of about 1 to 2 μm) is 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.

(H) Second polishing (final polishing) treatment The second polishing treatment aims at 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 containing colloidal silica as the free abrasive grains subjected to the above-described removal process is used.
By performing the second polishing treatment, the main surface roughness (arithmetic mean roughness Ra) can be 0.15 nm or less and the main surface micro-waveness can be 0.1 nm or less.

(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 the arithmetic average roughness Ra of the surface of 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 this embodiment, it is necessary to use an alkaline cleaning liquid that has a weaker cleaning power than the prior art, that is, an alkaline cleaning liquid that makes the difference in arithmetic mean roughness Ra of the surface of the glass substrate before and after the cleaning treatment 0.05 nm or less. it can. Note that Ra is an arithmetic average roughness specified in JIS B0601. This arithmetic average roughness is obtained based on data obtained by measuring a range of 1 μm × 1 μm with a resolution of 512 × 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.

In the cleaning process after the second polishing process, it is preferable to perform a process of cleaning the polymer fine particles attached to the glass substrate after the final polishing process. Specifically, the polymer fine particles can be ashed by placing the glass substrate after the second polishing treatment in an ozone atmosphere. Further, the fine polymer particles may be ashed by ozone generated by irradiating the glass substrate after the second polishing treatment with ultraviolet rays in the air. When the polymer fine particles are not completely removed by ashing, the polymer fine particles may be removed by washing the glass substrate with a cleaning agent containing an organic solvent or an anionic surfactant that dissolves the polymer fine particles.
When the polymer fine particles do not remain on the glass substrate after the second polishing treatment, or when there is no problem in using the glass substrate even if the polymer fine particles remain, the cleaning step for removing the polymer fine particles can be omitted.

Examples of the present invention and comparative examples will be described below.
[Example 1]
(Creation of colloidal silica)
A slurry containing colloidal silica having an average particle size of 20 nm was obtained by ion exchange using silica sand and sodium carbonate as raw materials.

(Adsorption treatment)
An adsorbent was added to and mixed with the slurry containing the colloidal silica. As the adsorbent, polystyrene fine particles having a particle diameter of 50 nm were used. The amount of adsorbent added was adjusted so that the concentration of the adsorbent relative to the total amount of slurry was 1 wt%.

(Separation process)
After the adsorption treatment, polystyrene fine particles precipitated by adsorbing silica-based particles in the slurry were filtered and separated by a filter.

(Glass substrate polishing process)
Next, the final polishing process of the glass substrate was performed using the filtrate which passed the filter by the separation process as a polishing liquid. 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.

[Example 2]
(Adsorption treatment)
A slurry containing colloidal silica obtained in the same manner as in Example 1 was charged with styrene monomer and suspended in the slurry by stirring, and then a polymerization initiator was added to the resulting suspension. By stirring at 70 ° C. for 20 minutes, polystyrene fine particles having a particle diameter of 50 nm were generated. The amount of styrene monomer charged was adjusted so that the concentration relative to the total amount of the suspension was 1 wt%.

(Separation process)
After the adsorption treatment, silica-based particles precipitated by adsorbing polystyrene fine particles in the slurry were filtered and separated.

(Glass substrate polishing process)
Next, the final polishing process of the glass substrate was performed using the filtrate which passed the filter by the separation process as a polishing liquid. 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.

[Comparative Example]
The slurry containing colloidal silica obtained in the same manner as in Example 1 was used as a polishing liquid without performing adsorption treatment and separation treatment, and the final polishing treatment of the glass substrate was performed in the same manner as in the example.

[Detection of plate-like foreign material on the main surface of the glass substrate]
After polishing, the main surface of the cleaned and dried glass substrate was subjected to detection and identification of foreign matter using an optical surface inspection device and a scanning electron microscope (SEM).

As a result, no plate-like foreign matter was detected for the glass substrates of Examples 1 and 2, but a plate-like foreign matter was detected for the glass substrate of the comparative example. In Examples 1 and 2, it is considered that the plate-like foreign matter did not adhere to the glass substrate because the plate-like foreign matter in the slurry was adsorbed to the adsorbent by the adsorption treatment.

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.
In the above embodiment, the case where the polishing process is performed using the silica abrasive grains has been described. However, the present invention is not limited to this, and the present invention may be applied to the case where the polishing process is performed using other abrasive grains. Good.

Claims (16)

1. By sandwiching a disk-shaped substrate between a pair of polishing pads, supplying a slurry containing abrasive grains between the polishing pad and the substrate, and sliding the polishing pad and the substrate relatively, the substrate A method for manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of
   The polishing abrasive grains contained in the slurry and a plate-like substance having a particle size larger than the average particle diameter of the polishing abrasive grains have a difference in the amount of surface charge,
   Before performing the polishing treatment, a solid adsorbent adsorbed on at least the plate-like substance and having a surface charge different in sign from the surface charge of the plate-like substance is mixed with the slurry. A method for manufacturing a magnetic disk substrate, comprising performing an adsorption process for adsorbing an adsorbent on the plate-like substance.
2. By sandwiching a disk-shaped substrate between a pair of polishing pads, supplying a slurry containing abrasive grains between the polishing pad and the substrate, and sliding the polishing pad and the substrate relatively, the substrate A method for manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of
   The abrasive grains contained in the slurry stock solution and the plate-like substance having a particle size larger than the average particle diameter of the abrasive grains have a difference in the amount of surface charge, and are adsorbed to at least the plate-like substance, and A solid adsorbent having a surface charge different in sign from the surface charge of the plate-like substance is mixed with the slurry stock solution to adsorb the adsorbent to the plate-like substance, and then the plate-like substance adsorbed to the adsorbent is added to the plate-like substance. A method for producing a magnetic disk substrate, wherein a slurry removed from a slurry stock solution is used as a slurry used for the polishing treatment.
3. 3. The method of manufacturing a magnetic disk substrate according to claim 1, wherein the adsorbent is an organic polymer.
4. By sandwiching a disk-shaped substrate between a pair of polishing pads, supplying a slurry containing abrasive grains between the polishing pad and the substrate, and sliding the polishing pad and the substrate relatively, the substrate A method for manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of
   Before performing the polishing treatment, it is composed of an abrasive polymer contained in the slurry and an organic polymer adsorbed on at least the plate-like substance among the plate-like substances having a particle diameter larger than the average particle diameter of the polishing abrasive grains. A method of manufacturing a magnetic disk substrate, comprising: performing an adsorption process for adsorbing the adsorbent on a plate-like substance in the slurry by preparing the adsorbent in the slurry.
5. By sandwiching a disk-shaped substrate between a pair of polishing pads, supplying a slurry containing abrasive grains between the polishing pad and the substrate, and sliding the polishing pad and the substrate relatively, the substrate A method for manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of
   An adsorbent made of an organic polymer adsorbed on at least the plate-like substance is prepared in a solvent among the abrasive grains contained in the slurry and the plate-like substance having a particle size larger than the average particle size of the abrasive grains. ,
   A magnetic disk substrate characterized by performing an adsorption process for adsorbing the adsorbent to a plate-like substance in the slurry by introducing a solvent in which the adsorbent is prepared into a slurry before performing a polishing process. Production method.
6. 6. The method for manufacturing a magnetic disk substrate according to claim 1, wherein the abrasive grains are silica particles having an average particle diameter of 10 nm to 60 nm.
7. After the adsorption treatment, a separation treatment for separating the plate-like substance that adsorbs the adsorbent from the slurry is performed,
   7. The method for manufacturing a magnetic disk substrate according to claim 1, wherein the polishing treatment is performed using the slurry from which the plate-like substance has been removed by the separation treatment.
8. The method for producing a magnetic disk substrate according to any one of claims 1 to 7, wherein the polishing abrasive grains are silica abrasive grains obtained using water glass and an ion exchange resin.
9. After the polishing process, a cleaning process for cleaning the main surface of the substrate is performed, and in the cleaning process, an alkali cleaning liquid is used that sets the difference in arithmetic mean roughness Ra of the surface of the substrate before and after the cleaning process to 0.05 nm or less. The method for producing a magnetic disk substrate according to any one of claims 1 to 8.
10. The magnetic disk substrate production according to any one of claims 1 to 9, wherein an additive for reducing the absolute value of the surface charge of the abrasive grains is added to the slurry before the polishing treatment after the adsorption treatment. Method.
11. The method for manufacturing a magnetic disk substrate according to any one of claims 1 to 10, wherein the content of alkaline earth metal ions in the slurry before the adsorption treatment is 200 ppm or less.
12. The magnetic disk substrate according to any one of claims 1 to 11, wherein in the adsorption treatment, among the particles contained in the slurry, a plate-like substance having a maximum length of 5 times or more the thickness is adsorbed. Production method.
13. The adsorbent is an organic polymer, and after the polishing process, at least one of (1) bringing an organic solvent into contact with the adsorbent remaining on the surface of the magnetic disk substrate and (2) oxidizing. 13. The method for manufacturing a magnetic disk substrate according to claim 1, wherein the magnetic disk substrate is removed by performing one of the steps.
14. 14. The method for manufacturing a magnetic disk substrate according to claim 1, wherein the arithmetic average roughness Ra of the surface of the substrate after the polishing treatment is 0.15 nm or less.
15. By sandwiching a disk-shaped substrate between a pair of polishing pads, supplying a slurry containing abrasive grains between the polishing pad and the substrate, and sliding the polishing pad and the substrate relatively, the substrate A method for manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of
    Before performing the polishing treatment, due to the difference in the amount of surface charge each of the polishing abrasive grains and large particles having a particle diameter larger than the average particle diameter of the polishing abrasive grains contained in the slurry has, By mixing the slurry with a solid adsorbent that is more easily adsorbed by large-diameter particles than particles having an average particle diameter of abrasive grains and has a surface charge that is different in sign from the surface charge of the large-diameter particles. A method for producing a substrate for a magnetic disk, comprising performing an adsorption treatment for adsorbing an adsorbent on the large-diameter particles in the slurry.
16. By sandwiching a disk-shaped substrate between a pair of polishing pads, supplying a slurry containing abrasive grains between the polishing pad and the substrate, and sliding the polishing pad and the substrate relatively, the substrate A method for manufacturing a magnetic disk substrate including a polishing process for polishing the main surface of
    Before performing the polishing treatment, the polishing abrasive grains contained in the slurry and the difference in surface charge amount of the large-diameter particles having a particle diameter larger than the average particle diameter of the polishing abrasive grains, respectively, Adsorption that adsorbs the adsorbent to the large-diameter particles in the slurry by creating an adsorbent in the slurry that is made of an organic polymer that is easily adsorbed to the large-diameter particles compared to particles having an average particle diameter. A method of manufacturing a magnetic disk substrate, comprising performing a process.