JP5814064B2 - Manufacturing method of glass substrate for magnetic disk and manufacturing method of magnetic disk - Google Patents

Description

  The present invention relates to a method for manufacturing a glass substrate for a magnetic disk used as a recording medium for a computer or the like, and a method for manufacturing a magnetic disk.

  With the advancement of information technology in recent years, information recording technology, in particular magnetic recording technology, has made remarkable progress. 2. Description of the Related Art A magnetic disk used for an HDD (Hard Disk Drive) or the like, which is one of magnetic recording media, has been rapidly reduced in size, thinned, increased in recording density, and increased in access speed. In an HDD, a magnetic disk having a magnetic layer on a disk-shaped substrate is rotated at high speed, and recording and reproduction are performed while a magnetic head is flying over the magnetic disk.

  As the access speed is increased, the rotational speed of the magnetic disk is also increased. Therefore, a higher substrate strength is required for the magnetic disk. As the recording density increases, the magnetic head is also changing from a thin film head to a magnetoresistive head (MR head) and a large magnetoresistive head (GMR head), and DFH (Dynamic Flying Height). With the introduction of the control mechanism, the flying height of the magnetic head from the magnetic disk (the narrowest distance among the gaps between the magnetic head and the magnetic disk) has been reduced to about 2 nm. For this reason, if there are irregularities on the surface of the magnetic disk, there may occur a crash failure that the magnetic head collides, adiabatic compression of air, or a thermal asperity failure that causes a read error when heated by contact. In order to suppress such troubles in the magnetic head, it is important to finish the main surface of the magnetic disk as a very smooth surface.

  Therefore, at present, a glass substrate has been used instead of a conventional aluminum substrate as a substrate for a magnetic disk. This is because a glass substrate made of glass which is a hard material is superior in flatness of the substrate surface compared to an aluminum substrate made of a metal which is a soft material. In addition, since the glass substrate is harder than the aluminum substrate, it is possible to suppress distortion and fluttering of the substrate during high-speed rotation. Thereby, the risk of collision with the head can be reduced.

  On the other hand, even when a glass substrate is used, since the flying height of the head decreases as the recording density increases, it is more important to smooth the surface of the magnetic disk glass substrate and remove particles (contamination). It becomes. In particular, in next-generation bit-patterned media and discrete track media, the magnetic particles are classified, so the presence of fine irregularities on the glass substrate surface and fine particles adhering to the glass substrate surface may become serious. is expected.

  Therefore, as described in Patent Document 1, in order to improve the smoothness of the glass substrate and remove particles on the surface of the glass substrate, a polishing process and a cleaning process using ultrasonic waves are performed on the glass substrate. Yes. As described in Patent Document 2, as a cleaning process using ultrasonic waves, cleaning is performed only by ultrasonic treatment using high frequency for the purpose of suppressing damage to the surface of the glass substrate. Further, as described in Patent Document 3, cleaning by ultrasonic treatment using a low frequency capable of forming intense vacuum bubbles is also performed.

JP 2004-335081 A JP 2002-167240 A JP 2001-046991 A Japanese Patent Laid-Open No. 11-033561 JP 05-079882 A

  In recent years, in order to improve the smoothness of the glass substrate surface, the particle diameter of the abrasive grains used for polishing the glass substrate has been steadily reduced. On the other hand, in order to realize a further reduction in the flying height, an extremely minute convex shape is not allowed on the magnetic disk surface. Therefore, the removal of the fine abrasive grains used for polishing the glass substrate is an important issue.

  By the way, in the ultrasonic cleaning process performed after the polishing process for the purpose of removing particles on the surface of the glass substrate, the frequency band to be applied to the target particle diameter is determined. Therefore, the frequency is increased as the particle diameter of the cleaning target is reduced. There is a need. For example, in Patent Document 1, 50 kHz ultrasonic cleaning is performed after polishing using 0.8 μm polishing abrasive grains.

  Therefore, when the abrasive grains are smaller than this, it is considered that the ultrasonic frequency needs to be set higher. Even if particles having a particle size of about 10 nm to 40 nm are irradiated with ultrasonic waves having a relatively low frequency (for example, 80 kHz), they cannot be removed sufficiently.

  However, it has been found that when the ultrasonic frequency is increased, the abrasive grains cannot be sufficiently removed. This is presumed to be due to the fact that the movement of fine particles other than the particle size to be cleaned becomes active as the ultrasonic frequency increases, and collides with larger particles.

  For example, it has been found that by irradiating ultrasonic waves with a relatively high frequency (120 kHz to 950 kHz), particles having a particle size of about 10 nm to 40 nm are aggregated without being removed. When the ultrasonic frequency is increased, particles having a target particle diameter that can be removed are aggregated, so that the particles present on the surface of the glass substrate remain in the aggregate after being changed in shape.

  In addition, Patent Document 4 and Patent Document 5 describe a factory waste liquid treatment method that applies the bond strengthening and aggregation promotion of an agglomerate by an aggregating agent and the aggregating action by ultrasonic waves.

  Therefore, the present invention is proposed in view of the above circumstances, and uses abrasive grains having a small particle size in the glass substrate polishing step, and at a high frequency in the ultrasonic cleaning step after the polishing step. It is an object of the present invention to provide a method for manufacturing a glass substrate for a magnetic disk and a method for manufacturing a magnetic disk that can effectively remove particles on the surface of the glass substrate even when ultrasonic treatment is performed.

  In order to solve the above-described problems and achieve the above object, a method for manufacturing a glass substrate for a magnetic disk according to the present invention has any one of the following configurations.

[Configuration 1]
A polishing process for polishing a glass substrate using a surface plate provided with a slurry containing a colloidal silica abrasive having a predetermined particle size and a polishing pad, and an ultrasonic cleaning for ultrasonic cleaning of the glass substrate after the polishing process A method of manufacturing a glass substrate for a magnetic disk comprising: a step of contacting a glass substrate that has undergone a polishing step with an aqueous solution to which a flocculant has been added, and then having a predetermined particle size. Ultrasonic treatment is performed at a first frequency for agglomerating particles made of colloidal silica abrasive grains, and then ultrasonic cleaning is performed at a second frequency lower than the first frequency to form a coagulation formed by the ultrasonic treatment. The aggregate is removed.

[Configuration 2]
In the method for producing a glass substrate for magnetic disk having Configuration 1, the flocculant is any one or more of L-lactic acid, DL-lactic acid, salicylic acid, L-malic acid, DL-malic acid, and acrylamide. The concentration of the flocculant in the aqueous solution to which the flocculant is added is 10 ppm to 1000 ppm.

[Configuration 3]
In the method for manufacturing a magnetic disk glass substrate having Configuration 1 or Configuration 2, the particle size of the colloidal silica abrasive grains is 10 nm to 40 nm, the first frequency is 500 kHz or more, and the second circumference Several waves are characterized by being 200 kHz or less.

[Configuration 4]
In the method for manufacturing a glass substrate for a magnetic disk having any one of Configurations 1 to 3, the glass substrate is made of amorphous aluminosilicate glass.

  The magnetic disk manufacturing method according to the present invention has the following configuration.

[Configuration 5]
At least a magnetic layer is formed on the main surface of the magnetic disk glass substrate obtained by the method for producing a magnetic disk glass substrate having any one of Structures 1 to 4.

  In the method for manufacturing a glass substrate for a magnetic disk according to the present invention, in the ultrasonic cleaning step, the glass substrate that has been subjected to the polishing step is brought into contact with an aqueous solution to which a flocculant has been added, and then a colloidal having a predetermined particle size. Sonication is performed with a first frequency that causes the particles of silica abrasive particles to agglomerate. At this time, the fine colloidal silica adhering to the surface of the glass substrate is strongly agglomerated and grown to a diameter that can be treated by ultrasonic cleaning with the second frequency lower than the first frequency. Next, ultrasonic cleaning is performed at a second frequency lower than the first frequency to remove aggregates formed by the ultrasonic treatment. That is, the colloidal silica abrasive grains are removed from the surface of the glass substrate together with the flocculant by ultrasonic cleaning with the second frequency.

  In the method for producing a glass substrate for a magnetic disk according to the present invention, the flocculant is at least one of L-lactic acid, DL-lactic acid, salicylic acid, L-malic acid, DL-malic acid, and acrylamide. The concentration of the flocculant in the aqueous solution to which the flocculant is added is preferably 10 ppm to 1000 ppm, and particularly preferably in the range of 20 ppm to 200 ppm. Since the flocculant itself is agglomerated by ultrasonic waves of the first frequency, the concentration of the flocculant is preferably as low as possible. However, when the concentration is less than 10 ppm, the aggregating action cannot be sufficiently obtained. On the other hand, if the concentration of the flocculant exceeds 1000 ppm, the flocculant itself is promoted to aggregate, which may cause contamination of the surface of the glass substrate.

  As a method of bringing the glass substrate into contact with the aqueous solution to which the flocculant is added, in addition to a method of immersing the glass substrate in the aqueous solution for a predetermined time, the aqueous solution may be applied to the surface of the glass substrate.

  In the method for manufacturing a glass substrate for a magnetic disk according to the present invention, the first frequency is set to 500 kHz or more, and the second frequency wave is set to 200 kHz or less, in particular, colloidal silica abrasive grains having a particle size of 10 nm to 40 nm. In contrast, a suitable removal effect can be obtained.

  According to the present invention, in the ultrasonic cleaning step performed after the polishing step for the glass substrate, even when the colloidal silica abrasive particles having a small particle size are used in the polishing step, the fine colloidal silica abrasive particles are applied to the glass substrate surface. Can be easily removed, and particles on the surface of the glass substrate can be effectively removed, causing a crash failure that the magnetic head collides, adiabatic compression of air, or overheating due to contact, resulting in read errors. It is possible to suppress the thermal asperity failure that occurs.

  That is, the present invention uses a glass substrate surface even when a polishing abrasive grain having a small particle size is used in the polishing step of the glass substrate and ultrasonic treatment is performed at a high frequency in the ultrasonic cleaning step after the polishing step. It is possible to provide a method for manufacturing a glass substrate for a magnetic disk and a method for manufacturing a magnetic disk that can effectively remove the particles.

  Embodiments for carrying out the present invention will be described below.

  The present inventor, for the purpose of further improving the smoothness of the surface of the glass substrate, polishes the glass substrate using a surface plate provided with a slurry containing a colloidal silica abrasive having a predetermined particle size and a polishing pad. In the polishing step, the particle size of the abrasive grains used is reduced, and in the ultrasonic cleaning step after polishing, ultrasonic cleaning is performed using a relatively high frequency for the purpose of removing fine particles. As a result, there has been a problem that particles (particle diameter, for example, 10 nm to 40 nm) caused by abrasive grains that have not been a problem in the past aggregate and remain on the surface of the glass substrate.

  That is, it has been found that as the abrasive grains become smaller, foreign matter remains on the glass substrate surface even if the ultrasonic frequency is increased. When this foreign material was analyzed, it was an aggregate of fine abrasive grains. From this, it is considered that the high frequency of ultrasonic waves aggregated and increased in size due to foreign particles as nuclei and the initial high frequency became inappropriate for cleaning and removal.

  Therefore, as a result of intensive studies to solve this problem, in the ultrasonic cleaning step performed after the polishing step, the glass substrate that has undergone the polishing step is brought into contact with an aqueous solution to which a flocculant is added, and then the first frequency (for example, , 500 kHz or more) is irradiated with ultrasonic waves (ultrasonic treatment), particles caused by the abrasive grains are aggregated, and then switched to a second frequency lower than the first frequency (for example, 200 kHz or less). It has been found that by irradiating with ultrasonic waves (ultrasonic cleaning), particles on the surface of the glass substrate can be effectively removed, and concave defects due to ultrasonic waves are not generated in the glass substrate.

[Method of Manufacturing Glass Substrate for Magnetic Disk According to the Present Invention]
Hereinafter, the manufacturing method of the glass substrate for magnetic disks which concerns on this invention is demonstrated concretely.

  The method for manufacturing a glass substrate for a magnetic disk shown in the present embodiment includes a polishing step of polishing a glass substrate using a slurry including a colloidal silica abrasive having a predetermined particle size and a surface plate provided with a polishing pad. And an ultrasonic cleaning step of ultrasonically cleaning the glass substrate after the polishing step.

  In the ultrasonic cleaning step, the glass substrate that has undergone the polishing step is brought into contact with an aqueous solution to which a flocculant has been added, and then ultrasonic waves are generated with a first frequency that causes the particles made of colloidal silica abrasive grains having a predetermined particle size to aggregate. After the treatment, ultrasonic cleaning is performed at a second frequency lower than the first frequency to remove aggregates formed by the ultrasonic treatment.

  In the ultrasonic cleaning process, the ultrasonic treatment using a relatively high first frequency removes particles having a particle size to be cleaned and particles having a particle size (for example, 10 nm to 40 nm) that is not to be cleaned (for example, 10 nm to 40 nm). It is intended to form an aggregate by agglomerating abrasive grains. The second ultrasonic treatment using the relatively low second frequency aims to remove aggregates formed by the first ultrasonic treatment without causing a concave defect in the glass substrate. To do.

  When the particle diameter of the colloidal silica abrasive used in the polishing step is 10 nm to 40 nm, ultrasonic treatment is performed at a first frequency of 500 kHz or more, and ultrasonic cleaning is performed at a second frequency of 200 kHz or less. Can do. However, the second frequency needs to be 40 kHz or higher, which is a frequency that does not cause a concave defect on the glass substrate surface.

The relationship between the frequency of ultrasonic cleaning and the particle size of the particles to be cleaned is obtained by the following (Expression 1) showing the relationship of the cleaning target diameter in the ultrasonic frequency band using the sound velocity of the amplitude. Can do.
δ _ac = (2ν / ω) ^0.5 (in water) (Expression 1)
(∵σ _ac : thickness of sound pressure boundary layer, ν: speed of sound, ω: frequency (Hz))

  When a glass substrate is polished using fine colloidal silica abrasive grains having a particle diameter of 10 nm to 40 nm, a frequency of 100000 kHz or more is required to remove particles made of remaining abrasive grains by ultrasonic cleaning. However, in ultrasonic cleaning, the frequency band that can be stably oscillated is limited to about 2000 kHz.

  By the way, when a microsphere exists in the sound wave propagation medium, when the sound wave is propagated in the sound wave propagation medium, the microsphere moves in the standing wave by the work of translational motion (sound pressure antinode). (Maximum at position) and work by pressure vibration (minimum at node position). Since the work due to pressure oscillation is larger than the work due to translational movement, the work of the entire system is minimized at the position of the node, and as a result, the microspheres move in the direction of the node existing in the sound pressure of the standing wave. Collect with power. That is, as the oscillation frequency increases, the frequency of aggregate formation increases.

  Therefore, particles made of fine colloidal silica abrasive grains can be removed by utilizing the relationship with the diameter of an object to be cleaned by ultrasonic waves and the property of agglomerating fine particles.

  Specifically, the colloidal silica abrasive grains are aggregated, and the aggregate diameter is grown to a range that can be removed by ultrasonic waves in a specific frequency band, and then removed by ultrasonic waves in a specific frequency band. In the case of this means, it is efficient to perform high-frequency ultrasonic treatment in which the number of nodes of sound waves increases, and the formation of aggregates is possible in a short time.

  However, when the diameter of the colloidal silica abrasive grains is less than a certain diameter, the aggregate breakage occurs when low frequency ultrasonic treatment is performed by repulsive force (repulsive force) acting on the fine abrasive grains. Occurred and broken pieces of aggregates are scattered on the surface of the glass substrate. Therefore, it is necessary to reinforce the aggregated state so as to withstand low-frequency ultrasonic treatment.

  In order to reinforce the bonding state of the agglomerates, a means for forming the agglomerates after strengthening the agglomerate bonding by bringing a flocculant into contact with the colloidal silica abrasive before aggregation is effective. Because the flocculant used is easy to entangle the flocculants due to their molecular structure, if the colloidal silica abrasive grains with the flocculent adhered to the surface contact each other, the binding state of the colloidal silica abrasive aggregates is strong Become.

  The flocculant is, for example, one or more of L-lactic acid, DL-lactic acid, salicylic acid, L-malic acid, DL-malic acid, and acrylamide. The concentration of the flocculant in the aqueous solution to which the flocculant is added is preferably 10 ppm to 1000 ppm, and particularly preferably in the range of 20 ppm to 200 ppm. Since the flocculant itself is agglomerated by ultrasonic waves of the first frequency, the concentration of the flocculant is preferably as low as possible. However, when the concentration is less than 10 ppm, the aggregating action cannot be sufficiently obtained. On the other hand, if the concentration of the flocculant exceeds 1000 ppm, the flocculant itself is promoted to aggregate, which may cause contamination of the surface of the glass substrate.

  As a method of bringing the glass substrate into contact with the aqueous solution to which the flocculant is added, in addition to a method in which the glass substrate is immersed in the aqueous solution for a predetermined time (for example, about 130 seconds), an aqueous solution is applied to the surface of the glass substrate. Also good.

  Here, when the ultrasonic treatment (aggregate formation) by high frequency is performed simultaneously with the process of bringing the flocculant into contact with the colloidal silica abrasive grains, the aggregate of the flocculant itself is formed and adheres to the glass substrate. Will occur. Therefore, it is necessary to carry out the treatment with the flocculant before the aggregate formation treatment with ultrasonic waves. When performing ultrasonic treatment, the concentration of the flocculant on the glass substrate is preferably less than 10 ppm.

  The ultrasonic treatment and the ultrasonic cleaning can be performed separately in two different cleaning tanks, but can also be performed continuously by switching the frequency in the middle of one cleaning tank. The timing of switching from the frequency of ultrasonic treatment to the frequency of ultrasonic cleaning can be measured in advance by measuring the time during which aggregates are formed by irradiating the frequency used in ultrasonic treatment and feeding back the conditions. preferable. Thereby, ultrasonic cleaning can be performed after sufficiently forming an aggregate having a particle size to be subjected to ultrasonic cleaning by ultrasonic treatment.

  In this way, by continuously ultrasonically cleaning the glass substrate after polishing at different frequencies, agglomerating particles caused by the abrasive grains to form an aggregate, and then removing the aggregate Even when using a fine abrasive grain (particle size 10 nm to 40 nm) in the polishing step and performing ultrasonic treatment at a relatively high frequency (500 kHz or more) in the ultrasonic cleaning step after the polishing step, Particles on the glass substrate surface can be effectively removed.

  The ultrasonic treatment is preferably performed in a liquid adjusted to alkaline (for example, in a KOH solution), and the ultrasonic cleaning is performed in water or a liquid adjusted to alkaline (for example, in a KOH solution). Preferably, it is done. According to the study of the present inventor, it is difficult to obtain an aggregate by agglomerating fine abrasive grains under acidic conditions. Preferably, the pH of the cleaning solution for ultrasonic treatment and ultrasonic cleaning is preferably in the range of 11 to 14, and more preferably in the range of 13 to 14. Of course, it is desirable to adjust the surface roughness of the substrate so as not to deteriorate.

[Manufacturing process of glass substrate for magnetic disk]
Below, the manufacturing process of the glass substrate for magnetic discs which has the ultrasonic cleaning process mentioned above is demonstrated in detail. In addition, the order of each process is not limited to the following description, It can replace suitably.

(1) Material processing step In the material processing step, plate-like glass can be used. As the glass, aluminosilicate glass, soda lime glass, borosilicate glass, or the like can be used. In particular, it is preferable to use an aluminosilicate glass in that a glass substrate for a magnetic disk excellent in flatness of the main surface and substrate strength can be provided. The plate-like glass can be manufactured by using a known manufacturing method such as a press method, a float method, a downdraw method, a redraw method, or a fusion method using these glasses as materials. Of these methods, if a press method is used, a sheet glass can be produced at a low cost.

(2) 1st grinding (lapping) process In a 1st lapping process, the main surface of a disk-shaped glass substrate is lapped and the shape of a glass substrate is adjusted. The first lapping step can be performed using alumina loose abrasive grains by a double-side grinding apparatus using a planetary gear mechanism. Specifically, the lapping platen is pressed on both sides of the disk-shaped glass substrate from above and below, and a grinding liquid containing loose abrasive grains is supplied onto the main surface of the glass substrate, and these are moved relatively to perform lapping. I do. By this lapping process, a glass substrate having a flat main surface can be obtained.

(3) Shape processing step (coring step for forming a hole, chamfering step for forming a chamfered surface at the end (outer peripheral end and inner peripheral end) (chamfered surface forming step))
In the coring step, for example, an inner hole can be formed in the central portion of the disk-shaped glass substrate using a cylindrical diamond drill to obtain an annular glass substrate. In the chamfering step, the inner peripheral end surface and the outer peripheral end surface are ground with a diamond grindstone, and a predetermined chamfering process is performed on the glass substrate.

(4) Second lapping step In the second lapping step, a second lapping process is performed on both main surfaces of the obtained glass substrate. By performing the second lapping step, the fine uneven shape formed on the main surface of the glass substrate in the shape processing step, which is the previous step, can be removed, and the polishing step for the subsequent main surface is completed in a short time. It becomes possible.

  In the second lapping step, grinding can be performed using a fixed abrasive polishing pad made of a diamond sheet by a double-side grinding apparatus using a planetary gear mechanism. The diamond sheet only needs to have diamond particles as abrasive grains. For example, a diamond sheet in which diamond particles are adhered to a base material made of PET can be used.

(5) End surface polishing step In the end surface polishing step, the outer peripheral end surface and the inner peripheral end surface of the glass substrate are mirror-polished by a brush polishing method. At this time, as the abrasive grains, for example, a slurry containing cerium oxide abrasive grains can be used. By this end surface polishing step, the end surface of the glass substrate becomes a mirror surface.

(6) Main surface polishing step (first polishing step)
As the main surface polishing step, first, a first polishing step is performed. The first polishing process is a process whose main purpose is to remove scratches and distortions remaining on both main surfaces in the lapping process described above. In the first polishing step, both main surfaces are polished using a hard resin polisher by a double-side polishing apparatus having a planetary gear mechanism. As the abrasive, cerium oxide abrasive grains can be used. Moreover, it is preferable to wash | clean the glass substrate which finished the 1st grinding | polishing process with neutral detergent, pure water, IPA, etc.

  In addition, as a double-side polishing apparatus, a pair of polishing cloth (a polishing pad of a hard resin polisher) can be attached to the main surface portion of the upper and lower side surface plates. In this double-side polishing apparatus, a glass substrate can be installed between polishing cloths attached to the upper and lower surface plates, and one or both of the upper and lower surface plates can be moved to polish both main surfaces of the glass substrate. .

(7) Chemical strengthening process In a chemical strengthening process, a glass substrate is immersed in a chemical strengthening liquid and a chemical strengthening process is performed. As the chemical strengthening solution used for the chemical strengthening treatment, for example, a mixed solution of potassium nitrate (60%) and sodium nitrate (40%) can be used. In the chemical strengthening treatment, the chemical strengthening solution is heated to 300 ° C. to 400 ° C., the glass substrate is preheated to 200 ° C. to 300 ° C., and immersed in the chemical strengthening solution for 3 hours to 4 hours. . In soaking, in order to chemically strengthen both surfaces of the glass substrate, it is preferable to perform the immersion in a state of being accommodated in a holder so that the plurality of glass substrates are held at the end surfaces.

  Thus, by immersing in the chemical strengthening solution, the lithium ions and sodium ions in the surface layer of the glass substrate are respectively replaced with sodium ions and potassium ions having a relatively large ion radius in the chemical strengthening solution. Will be strengthened. Note that the chemically strengthened glass substrate may be cleaned with sulfuric acid and then with pure water, IPA, or the like.

(8) Main surface polishing step (final polishing step)
As the final polishing step, a second polishing step is performed. The second polishing step is a step aimed at finishing both main surfaces into a mirror shape. In the second polishing step, both main surfaces are mirror-polished using a soft foam resin polisher by a double-side polishing apparatus having a planetary gear mechanism. As the polishing abrasive grains, a slurry having colloidal silica having a particle diameter of 10 nm to 40 nm finer than the cerium oxide abrasive grains used in the first polishing step can be used. In this final polishing step, a double-side polishing apparatus using a planetary gear mechanism can be used in the same manner as in the first polishing step.

(9) Ultrasonic cleaning process The glass substrate is subjected to a cleaning process using ultrasonic waves after the final polishing process. The ultrasonic cleaning process is a process aimed at removing particles adhering to the surface of the glass substrate after the final polishing process using two or more types of ultrasonic frequency bands.

  In the ultrasonic cleaning process, first, the glass substrate subjected to the final polishing process is brought into contact with the aqueous solution to which the flocculant is added, so that the aqueous solution to which the flocculant is added is in contact with the aqueous solution for a predetermined time (for example, about 130 seconds). Immerse the glass substrate. An aqueous solution may be applied to the surface of the glass substrate. The flocculant is at least one of L-lactic acid, DL-lactic acid, salicylic acid, L-malic acid, DL-malic acid, and acrylamide. The concentration of the flocculant in the aqueous solution to which the flocculant is added is preferably 10 ppm to 1000 ppm, and particularly preferably in the range of 20 ppm to 200 ppm.

  Next, this glass substrate is immersed in pure water, a KOH aqueous solution, or the like, and then irradiated with ultrasonic waves. Specifically, first, ultrasonic treatment is performed at a relatively high first frequency (500 kHz or more) to form an aggregate, and subsequently, a relatively low second frequency (40 kHz or more and 200 kHz or less). By performing ultrasonic cleaning in step (1), particles containing aggregates aggregated by ultrasonic treatment from the glass substrate surface are removed.

  The ultrasonic treatment and the ultrasonic cleaning can be performed by switching the frequency in one ultrasonic cleaning process. The timing of switching from the frequency of ultrasonic treatment to the frequency of ultrasonic cleaning is the size of aggregates (for example, 1000 nm to 3000 nm) formed when ultrasonic waves are irradiated at a relatively high frequency in advance and ultrasonic cleaning. It is preferable to define a time relationship and feed back the conditions. Thereby, ultrasonic cleaning can be performed after sufficiently forming an aggregate by ultrasonic treatment.

[Magnetic disk manufacturing process (recording layer formation process)]
For example, a perpendicular magnetic layer is formed by sequentially forming, for example, an adhesion layer, a soft magnetic layer, a nonmagnetic underlayer, a perpendicular magnetic recording layer, a protective layer, and a lubricating layer on the main surface of the glass substrate obtained through the above-described steps. A recording disk can be manufactured. Examples of the material constituting the adhesion layer include a Cr alloy. Examples of the material constituting the soft magnetic layer include a CoTaZr-based alloy. Examples of the nonmagnetic underlayer include a granular nonmagnetic layer. An example of the perpendicular magnetic recording layer is a granular magnetic layer. Examples of the material constituting the protective layer include hydrogenated carbon. Examples of the material constituting the lubrication layer include a fluororesin. For example, these recording layers and the like are more specifically formed by using an in-line sputtering apparatus on a glass substrate, a CrTi adhesion layer, a CoTaZr / Ru / CoTaZr soft magnetic layer, and a CoCrSiO ₂ nonmagnetic granular material. An underlayer, a CoCrPt—SiO ₂ · TiO ₂ granular magnetic layer, and a hydrogenated carbon protective film can be sequentially formed, and a perfluoropolyether lubricating layer can be formed by a dipping method.

A Ru underlayer may be used in place of the CoCrSiO ₂ nonmagnetic granular underlayer. Further, a NiW seed layer may be added between the soft magnetic layer and the underlayer. Further, a CoCrPtB magnetic layer may be added between the granular magnetic layer and the protective layer.

  Next, examples and comparative examples performed for clarifying the effects of the present invention will be described.

  The size of particles per unit area remaining on the surface of the glass substrate after performing the ultrasonic cleaning process on the glass substrate subjected to the first polishing process and the second polishing process (final polishing process) And the number were evaluated using an optical measuring machine (OSA).

As the glass substrate, SiO ₂ : 58 wt% to 75 wt%, Al ₂ O ₃ : 5 wt% to 23 wt%, Li ₂ O: 3 wt% to 10 wt%, Na ₂ O: 4 wt% to 13 wt% Aluminosilicate glass containing wt% as the main component was used. Li ₂ O may be greater than 0% by weight and 7% by weight or less.

(1) Main surface polishing step (first polishing step)
As a main surface polishing step, first, a first polishing step was performed. In the first polishing step, the main surface was polished using a hard resin polisher by a double-side polishing apparatus having a planetary gear mechanism. As the abrasive, a slurry containing cerium oxide having a particle diameter of 0.2 nm to 4.5 nm was used.

  The glass substrate which finished this 1st grinding | polishing process was immersed in each washing tank of neutral detergent, a pure water, and IPA (isopropyl alcohol) one by one, and was wash | cleaned.

(2) Main surface polishing step (final polishing step)
Next, a second polishing step was performed as the main surface polishing step. The purpose of this second polishing step is to finish the main surface into a mirror surface. In the second polishing step, mirror polishing of the main surface was performed using a soft foamed resin polisher by a double-side polishing apparatus having a planetary gear mechanism. As the abrasive, a slurry containing colloidal silica abrasive grains (average particle diameter of 10 nm to 40 nm) finer than the cerium oxide abrasive grains used in the first polishing step was used.

  In this example and comparative example, polishing was performed with the pH of the slurry set to 2. At this time, polishing is performed by adding an additive containing acetic acid and acetate to the slurry. This is for controlling the pH of the slurry constant during the polishing process. As the slurry (polishing liquid), a mixed liquid obtained by adding the above colloidal silica particles to ultrapure water, and 0.5% by weight of citric acid as an additive was used.

(3) Ultrasonic cleaning step The glass substrate after the final polishing step was immersed in an aqueous flocculant solution having a concentration of 50 ppm for 130 seconds, and then immersed in an aqueous KOH solution having a concentration of 2% by weight. Ultrasonic cleaning was performed under the conditions. Thereafter, the unit area of particles remaining on the surface of the glass substrate using an optical measuring machine after being sequentially immersed in each washing tank of neutral detergent, pure water, pure water, IPA, and IPA (steam drying). The number of hits and the presence or absence of cracks on the substrate surface were evaluated.

  In Comparative Examples 1 to 4, immersion in the flocculant aqueous solution was not performed, and in Examples 1 to 8, immersion in the flocculant aqueous solution was performed.

〔Evaluation results〕
The evaluation results are shown in [Table 1].

  From [Table 1], in Comparative Examples 1 to 4, the number of particles having a particle diameter of 15 nm to 40 nm was all 51 or more, and it was determined to be defective (×).

Here, the particle size is the cumulative average particle size (50% diameter) in the particle size distribution measured by the light scattering method using a particle size / particle size distribution measuring device (Nikkiso Co., Ltd., Nanotrac UPA-EX150). is there.
In Examples 1 to 4, treatment with an aqueous flocculant solution was performed, and the number of particles having a particle diameter of 15 to 25 nm was 0 to 5, excellent (◎), and the number of particles having a particle diameter of 30 to 40 nm was 6 -10, it was determined to be good (◯).

  In Examples 1 to 4, the number of sound wave nodes increases as the frequency increases, so that the frequency of aggregate formation increases and aggregates can be formed more efficiently. It is thought that the silica abrasive grains could be removed.

  In Examples 5 to 8, the treatment with the flocculant aqueous solution was performed, and the number of particles having a particle diameter of 15 nm to 40 nm was determined to be good (◯) or acceptable (Δ) (11 to 50).

  In Examples 5 to 8, the frequency of the ultrasonic treatment is changed. As described above, since the particle size of the object to be removed by the ultrasonic wave is limited, the flocculant treatment and the ultrasonic treatment are performed. It is considered that the particle size of the aggregate formed by the above and the particle size of the object to be removed by the second frequency are insufficiently matched.

  In addition, generation | occurrence | production of the crack by the impact added to a glass substrate was not discovered.

[DFH touchdown test]
Next, a magnetic disk is manufactured using a glass substrate that has been subjected to the cleaning process under the conditions shown in Table 1 above, and the DFH head element section is manufactured using a Kubota Comps HDF tester (Head / Disk Flyability Tester). A touchdown test was performed. This test evaluates the distance when the head element unit contacts the magnetic disk surface by gradually protruding the element unit by the DFH mechanism and detecting contact with the magnetic disk surface by the AE sensor. . The head used was a DFH head for a 320 GB / P magnetic disk (2.5 inch size). The flying height when there is no protrusion of the element portion is 10 nm. Other conditions were set as follows.

Magnetic disk: 2.5 inch (inner diameter 20 mm, outer diameter 65 mm, plate thickness 0.8 mm) glass substrate is manufactured, and a recording layer or the like is formed on the glass substrate. Evaluation radius: 22 mm
Magnetic disk rotation speed: 5400 RPM
Temperature: 25 ° C
Humidity: 60%
From the results of the DFH touchdown test, when the substrate ( Example 1 to Example 8 ) in which the generation of cracks in the glass substrate is suppressed and the number of remaining particles can be effectively reduced is used, the head element portion and the magnetic field The distance contacted by the disk could be reduced to 1.0 nm or less.

  Note that the materials, sizes, processing procedures, inspection methods, and the like in the above embodiment are merely examples, and various modifications can be made within the scope of the effects of the present invention. In addition, various modifications can be made without departing from the scope of the object of the present invention.

  The present invention is applied to a method of manufacturing a glass substrate for a magnetic disk used as a recording medium for a computer or the like.

Claims (5)

A polishing step of polishing the glass substrate using a surface plate provided with a slurry containing a colloidal silica abrasive having a predetermined particle size and a polishing pad, and an ultrasonic cleaning of the glass substrate that has undergone the polishing step A method of manufacturing a glass substrate for a magnetic disk having a sonic cleaning step,
In the ultrasonic cleaning step,
The glass substrate that has undergone the polishing step is brought into contact with an aqueous solution to which a flocculant is added,
Next, ultrasonic treatment is performed at a first frequency for aggregating particles made of colloidal silica abrasive grains having the predetermined particle diameter,
Next, ultrasonic cleaning is performed at a second frequency lower than the first frequency to remove aggregates formed by the ultrasonic treatment. A method of manufacturing a glass substrate for a magnetic disk, comprising: The flocculant is at least one of L-lactic acid, DL-lactic acid, salicylic acid, L-malic acid, DL-malic acid and acrylamide,
The method for producing a glass substrate for a magnetic disk according to claim 1, wherein the concentration of the flocculant in the aqueous solution to which the flocculant is added is 10 ppm to 1000 ppm. The colloidal silica abrasive has a particle size of 10 nm to 40 nm,
The first frequency is 500 kHz or more;
The method for manufacturing a glass substrate for a magnetic disk according to claim 1, wherein the second frequency wave is 200 kHz or less. The said glass substrate consists of amorphous aluminosilicate glass. The manufacturing method of the glass substrate for magnetic discs as described in any one of the Claims 1 thru | or 3 characterized by the above-mentioned. 5. A magnetic disk comprising: at least a magnetic layer formed on a main surface of the glass substrate for a magnetic disk obtained by the method for manufacturing a glass substrate for a magnetic disk according to claim 1. Manufacturing method.