JP2010080030A - Method for manufacturing glass substrate for magnetic disk, and method for manufacturing magnetic disk - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a glass substrate for a magnetic disk which includes a polishing step capable of appropriately controlling an edge shape without deteriorating the quality of a principal surface. <P>SOLUTION: In the method for manufacturing the glass substrate for the magnetic disk, which includes the polishing step for polishing the glass substrate by using a polishing liquid including abrasive grains, the polishing is conducted by using the polishing liquid containing a nonionic surfactant added thereto so that the frictional coefficient of the solvent of the polishing liquid is equal to or more than that of water. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a magnetic disk mounted on a magnetic disk device such as a hard disk drive (HDD) and a method for manufacturing a glass substrate for a magnetic disk.

There is a magnetic disk as one of information recording media mounted on a magnetic disk device such as a hard disk drive (HDD). A magnetic disk is configured by forming a thin film such as a magnetic layer on a substrate, and an aluminum substrate has been conventionally used as the substrate. However, recently, in response to the pursuit of higher recording density, the ratio of the glass substrate capable of narrowing the distance between the magnetic head and the magnetic disk as compared with the aluminum substrate is gradually increasing. Further, the surface of the glass substrate is polished with high accuracy so as to increase the recording density so that the flying height of the magnetic head can be reduced as much as possible.

As described above, high smoothness on the surface of the magnetic disk is indispensable for reducing the flying height (flying height) necessary for increasing the recording density. In order to obtain a high smoothness on the surface of the magnetic disk, a substrate surface with a high smoothness is required in the end. Therefore, it is necessary to polish the glass substrate surface with high accuracy.

A conventional glass substrate polishing method is performed using a polishing pad of a polisher such as polyurethane while supplying a slurry (polishing liquid) containing a metal oxide abrasive such as cerium oxide or colloidal silica. A glass substrate having high smoothness can be obtained by polishing using, for example, a cerium oxide-based abrasive and then finishing polishing (mirror polishing) using colloidal silica abrasive grains. Here, for example, it has been proposed to use a colloidal silica slurry whose pH is adjusted to be acidic for polishing a magnetic disk substrate (see Patent Document 1 below). In addition, it has also been proposed to use a colloidal silica slurry adjusted to have a pH of more than 10.2 and 12 or less by containing an alkali in the polishing liquid for polishing a glass substrate for magnetic disks (the following patents). Reference 2).

By the way, the slurry in which a metal oxide abrasive such as cerium oxide or colloidal silica is made turbid and the glass substrate quality after polishing are strongly correlated, and the quality of the main surface of the glass substrate is controlled by controlling the properties of the slurry. Although it is well known that the improvement is effective, according to the study of the present inventors, it has been found that not only the main surface but also the shape of the end surface is affected.

JP-A-7-240025 JP 2003-173518 A

In recent years, information recording density of 60 Gbit / inch ² or higher has been required for hard disk drives (HDD). Also, in order to effectively use the limited disk area, an HDD of LUL (Load Unload) method has been used instead of the conventional CSS (Contact Start and Stop) method. In the LUL method, when the HDD is stopped, the magnetic head is retracted to an inclined table called a ramp located outside the magnetic disk, and during the start-up operation, after the magnetic disk starts to rotate, the magnetic head is moved from the ramp onto the magnetic disk. Slide and fly to record and playback. During the stop operation, the magnetic head is retracted to the ramp outside the magnetic disk, and then the rotation of the magnetic disk is stopped. This series of operations is called LUL operation. The magnetic disk for LUL HDDs does not require a contact sliding area (CSS area) with the magnetic head as in the CSS system, and the recording / playback area can be expanded, which is preferable for high information capacity. It is.

In order to improve the information recording density under such circumstances, it is necessary to reduce the spacing loss as much as possible by reducing the flying height of the magnetic head. In order to achieve an information recording density of 60 gigabits or more per square inch, the flying height of the magnetic head needs to be 10 nm or less. Unlike the CSS method, the LUL method does not require a concave / convex shape for CSS on the magnetic disk surface, and the magnetic disk surface can be extremely smoothed. Therefore, in the magnetic disk for LUL method, the flying height of the magnetic head can be further reduced compared to the CSS method, so that the S / N ratio of the recording signal can be increased and the recording capacity of the magnetic disk device can be increased. There is also an advantage of being able to contribute.

With the recent demand for higher recording density with the introduction of the LUL method, there is a need for the extension (guarantee) of the head flying area to the outermost periphery of the disk. Therefore, on the surface of the glass substrate for magnetic disks, the smooth surface of the main surface is expanded by moving the sag start position of the end shape to the outermost peripheral part, and it is an indispensable technical problem to expand the guaranteed area. ing.

Conventionally, when controlling the shape of the end face of a substrate, it has been carried out by improving the surface hardness of the polishing pad. A soft pad with low surface hardness, that is, a soft pad, sinks the pad surface that contacts the glass substrate edge under high pressure during polishing, and increases the area of contact with the glass substrate edge. As a result, the end shape is rounded (see FIG. 3A). On the other hand, by using a hard surface, that is, a hard pad, the sinking of the pad surface in contact with the glass substrate edge is reduced, and the area in contact with the glass substrate edge is reduced to reduce the glass substrate edge. By reducing the amount of grinding of the part, curling of the end shape has been suppressed, and flattening of the end part has been realized. However, when a hard pad having a high surface hardness is used, there is a problem in that the quality of the main surface is deteriorated because roughness, waviness and scratches occur. That is, in the improvement by changing the hardness of the polishing pad, there is always a trade-off relationship between the end shape and the main surface quality, and it is extremely difficult to satisfy both.

The present invention has been made to solve these problems, and an object of the present invention is to provide a method for producing a glass substrate for a magnetic disk having a polishing step capable of suitably controlling the end shape without deteriorating the main surface quality. And a method of manufacturing a magnetic disk using a glass substrate manufactured by the manufacturing method.

In order to solve the above-mentioned problems, the present inventor examined the possibility of improvement by using a slurry instead of a pad. Assuming that the edge shape is rounded due to the sinking of the low hardness pad and the grinding of the edge shape due to the concentration of slurry on the outer periphery of the substrate due to the centrifugal force during polishing, In order to maintain high quality, it is difficult to increase the hardness of the pad, and suppression of concentration of the slurry on the outer peripheral portion of the substrate due to centrifugal force during polishing is an improvement item. In conventional slurries, abrasives made of metal oxides such as cerium oxide and colloidal silica are made turbid in water and used only in the preparation of a small amount of dispersant, but the edge shape is mainly affected by the polishing pad. The influence of the properties of the slurry on the shape of the end is hardly regarded as a problem and has not been studied.

Therefore, the present inventors have focused on the improvement of the end shape by changing the physical properties of the slurry, and have conducted further earnest studies, and as a result, completed the present invention.
That is, the present invention has the following configuration.
(Configuration 1) A method of manufacturing a glass substrate for a magnetic disk having a polishing step of polishing a glass substrate using a polishing liquid containing polishing abrasive grains, wherein the friction coefficient of a solvent of the polishing liquid is contained in the polishing liquid A method for producing a glass substrate for a magnetic disk, wherein a nonionic surfactant is added so that is equal to or greater than that of water.

(Configuration 2) A method of manufacturing a glass substrate for a magnetic disk according to Configuration 1, wherein the friction coefficient of the solvent of the polishing liquid is 0.7 or more.
(Structure 3) The method for manufacturing a glass substrate for a magnetic disk according to Structure 1 or 2, wherein the glass substrate is a glass substrate used for a magnetic disk mounted in a load / unload magnetic recording apparatus.

(Configuration 4) A method of manufacturing a magnetic disk, comprising forming at least a magnetic layer on a glass substrate for a magnetic disk obtained by the manufacturing method according to any one of configurations 1 to 3.

According to the present invention, a method for manufacturing a glass substrate for a magnetic disk having a polishing step capable of suitably controlling the end shape without deteriorating the main surface quality, and a magnetic field using the glass substrate manufactured by the manufacturing method. A method for manufacturing a disk can be provided.

Hereinafter, embodiments of the present invention will be described in detail.
The method for manufacturing a glass substrate for a magnetic disk according to the present invention includes a polishing step of polishing a glass substrate using a polishing liquid containing polishing abrasive grains, and the friction coefficient of the solvent of the polishing liquid in the polishing liquid. Is characterized in that a nonionic surfactant is added so that the solvent is equal to or higher than water.

  Conventionally, the polishing liquid used for polishing is basically a combination of the abrasive and water, which is a solvent. Therefore, the polishing liquid has good permeability due to the high water permeability. In the rotation processing of the glass substrate, it penetrates to the outermost peripheral portion of the glass substrate by centrifugal force, and the concentration of the polishing liquid on the outer peripheral portion of the substrate increases the grinding of the end portion of the substrate, resulting in the rounded shape of the end portion of the substrate ( (See FIG. 3 (a)). As described above, with the recent demand for higher recording density, there is a need for the extension (guarantee) of the head floating area to the outermost periphery of the disk. For this purpose, it is necessary to expand the smooth surface of the main surface by suppressing the rounding of the conventional end shape and moving the start position of the end shape to the outermost periphery.

In the present invention, by adding a specific additive to the polishing liquid used in the polishing step, the physical properties of the polishing liquid are changed, and the polishing liquid concentrated on the outer peripheral portion of the glass substrate by centrifugal force during the rotational processing during polishing. Since it can be suppressed, grinding at the edge of the substrate can be suppressed, and the smooth surface of the main surface can be extended to the outer peripheral side accordingly. That is, according to the present invention, for example, as shown in FIG. 3B, the end shape of the substrate can be suitably controlled.

In the present invention, specifically, a nonionic surfactant is added to the polishing liquid so that the friction coefficient of the solvent of the polishing liquid is equal to or higher than that when water is used as the solvent. By making the friction coefficient of the solvent of the polishing liquid equal to or higher than when the solvent is water (that is, the conventional polishing liquid), the physical properties of the polishing liquid are changed and the permeability of the polishing liquid is suppressed. Thus, the penetration to the outer periphery of the substrate is suppressed even under high pressure during polishing.

In the present invention, the additive added to the polishing liquid is such that the friction coefficient of the solvent of the polishing liquid is equal to or higher than that when the solvent is water (that is, the conventional polishing liquid), preferably larger than water. If it is a thing, it will not restrict | limit in particular, However, In order to demonstrate the effect of this invention preferably, the thing with a high slip effect is not preferable, for example, a surface active agent, a thing with a high surface tension, a high coefficient of friction, and a low permeability Is preferred. From such a viewpoint, the additive is specifically preferably a nonionic surfactant, for example, an ethanol-based nonionic surfactant having a low molecular weight is preferable.
Since the friction coefficient is usually 0.7 or less when the solvent of the polishing liquid is water, in the present invention, the friction coefficient of the solvent of the polishing liquid to which the surfactant is added is 0.7 or more (preferably 0). Greater than .7). The friction coefficient of the solvent can also be adjusted by the addition amount of the surfactant added to the polishing liquid. However, if the friction coefficient is large, as shown in FIG. However, it tends to be formed in a slightly raised shape. However, even in this case, the smooth surface of the main surface can be expanded to the outer peripheral side, and in some cases, the end face shape can be finally adjusted by the polishing process of the end face.

The friction coefficient of the solvent of the polishing liquid can be measured by, for example, the following Bowden friction coefficient measurement method.
When a substance with mass is moving due to the force acting in the direction of the contact surface when two objects are in contact, the force acting in the opposite direction of the direction of the substance is called frictional force, and the load is If P, the proportionality constant is μ, and the frictional force is F, then F = μP, and μ is the friction coefficient.
Specifically, as shown in FIG. 5, an appropriate amount of a solvent to be measured is dropped on a plate-like glass 21 of a test material, the following steel ball 22 is placed, and a load (P) is applied by a Bowden tester. When sliding in the direction of the arrow shown in the figure, stick slip occurs when the fluctuation of the dynamic friction coefficient increases, and the waveform can be confirmed in the chart as shown in FIG. 6, so the frictional force (F) is measured, The coefficient of friction can be calculated more. The measurement conditions at this time are as follows.
Test material: Soda glass Steel ball: φ5mm glass ball Load (P): 100g
Sliding distance: 25mm
Sliding speed: 4.0mm / sec
Test temperature: Room temperature Evaluation method: Adopting the average value of the friction coefficient of 4 to 20 times out of 20 sliding times The Bowden friction coefficient measuring method can measure the dynamic friction coefficient by a point contact sliding test. It is possible to match the material, surface pressure and speed with the actual machine.

  In the present invention, in order to compose a polishing liquid (also referred to as a slurry) containing colloidal silica abrasive grains, pure water, for example, RO water is used, and one or more of the above nonionic surfactants are used. A polishing liquid may be added by mixing. Here, RO water is pure water that has been subjected to RO (Reverse Osmotic Pressure Membrane) treatment. It is particularly preferable to use RO-DI water that has been subjected to RO treatment and DI treatment (deionization treatment). This is because RO water or RO-DI water has a very small content of impurities, such as alkali metals, and also has a small ion content and is neutral.

From the viewpoint of polishing efficiency, it is preferable to use abrasive grains such as colloidal silica contained in the polishing liquid having an average particle diameter of about 20 to 500 nm.

The polishing method in the present invention is not particularly limited. For example, the polishing cloth and the glass substrate are relatively moved while contacting the glass substrate and the polishing cloth and supplying the polishing liquid containing the abrasive. The surface of the glass substrate may be polished into a mirror surface.
For example, FIG. 4 is a longitudinal sectional view showing a schematic configuration of a planetary gear type double-side polishing apparatus that can be used in a mirror polishing process of a glass substrate. The double-side polishing apparatus shown in FIG. 4 meshes with the sun gear 2, the internal gear 3 arranged concentrically on the outer side, the sun gear 2 and the internal gear 3, and the sun gear 2 and the internal gear 3. An upper surface plate 5 and a lower surface plate 6 on which a carrier 4 that revolves and rotates according to rotation, and a polishing cloth 7 that can hold the workpiece 1 held by the carrier 4 are attached, respectively. A polishing liquid supply unit (not shown) for supplying a polishing liquid is provided between 5 and the lower surface plate 6.

  By such a double-side polishing apparatus, during polishing, the workpiece 1 held by the carrier 4, that is, the glass substrate is sandwiched between the upper surface plate 5 and the lower surface plate 6, and the upper and lower surface plates 5, 6 are polished. While supplying the polishing liquid between the cloth 7 and the workpiece 1, the carrier 4 revolves and rotates according to the rotation of the sun gear 2 and the internal gear 3, and the upper and lower surfaces of the workpiece 1 are moved. Polished.

A polishing pad can be used as the polishing cloth. The polishing pad is preferably a polishing pad of a soft polisher. The polishing pad preferably has an Asker C hardness of 60 to 80. The contact surface of the polishing pad with the glass substrate is preferably made of a foamed resin with an open foam pore, particularly foamed polyurethane. When polishing is performed in this manner, the surface of the glass substrate can be polished into a smooth mirror surface.

In the present invention, the glass constituting the glass substrate is preferably an amorphous aluminosilicate glass. Such a glass substrate can be finished to a smooth mirror surface by polishing the surface. As such an aluminosilicate glass, SiO ₂ is 58 wt% to 75 wt%, Al ₂ O ₃ is 5 wt% to 23 wt%, Li ₂ O is 3 wt% to 10 wt%, Na ₂ An aluminosilicate glass containing O as a main component in an amount of 4 wt% or more and 13 wt% or less (however, an aluminosilicate glass containing no phosphorus oxide) can be used. For example, SiO ₂ is 62 wt% to 75 wt%, Al ₂ O ₃ is 5 wt% to 15 wt%, Li ₂ O is 4 wt% to 10 wt%, and Na ₂ O is 4 wt% to 12 wt%. Wt% or less, ZrO ₂ is contained in an amount of 5.5 wt% or more and 15 wt% or less as a main component, and the weight ratio of Na ₂ O / ZrO ₂ is 0.5 or more and 2.0 or less, Al ₂ O ₃ / ZrO _An amorphous aluminosilicate glass containing no phosphorous oxide having a weight ratio of ₂ of 0.4 to 2.5 can be obtained. In addition, it is desirable that the glass does not contain an alkaline earth metal oxide such as CaO or MgO. Examples of such glass include N5 glass (trade name) manufactured by HOYA Corporation.

In the present invention, the surface of the glass substrate after mirror polishing is preferably a mirror surface having an arithmetic average surface roughness Ra of 0.6 nm or less. Further, it is preferable that the mirror surface has a maximum roughness Rmax of 6 nm or less. In the present invention, Ra and Rmax are roughnesses calculated in accordance with Japanese Industrial Standard (JIS) B0601.
In the present invention, the surface roughness (for example, the maximum roughness Rmax, the arithmetic average roughness Ra) is preferably the surface roughness of the surface shape obtained when measured with an atomic force microscope (AFM).

In this invention, it is preferable to perform a chemical strengthening process after the grinding | polishing process by this invention. As a method of chemical strengthening treatment, for example, a low-temperature ion exchange method in which ion exchange is performed in a temperature range that does not exceed the temperature of the glass transition point, for example, a temperature of 300 degrees Celsius or more and 400 degrees Celsius or less is preferable. The chemical strengthening treatment is a process in which a molten chemical strengthening salt is brought into contact with a glass substrate, whereby an alkali metal element having a relatively large atomic radius in the chemical strengthening salt and a relatively small atomic radius in the glass substrate. This is a treatment in which an alkali metal element is ion-exchanged, an alkali metal element having a large ion radius is permeated into the surface layer of the glass substrate, and compressive stress is generated on the surface of the glass substrate. Since the chemically strengthened glass substrate is excellent in impact resistance, it is particularly preferable for mounting on a HDD for mobile use, for example. As the chemical strengthening salt, alkali metal nitric acid such as potassium nitrate or sodium nitrate can be preferably used.

Recently, in order to improve the information recording density of a magnetic disk, magnetic anisotropy along the circumferential direction of the disk may be imparted to the magnetic layer of the magnetic disk. This is because the circumferential direction of the disk, that is, the moving direction of the magnetic head, contributes to higher recording density if magnetic anisotropy is given along this direction. By performing a tape polishing process on the surface of the disk-shaped glass substrate, it is possible to form a texture composed of streaks that are oriented in the circumferential direction of the disk. When a magnetic layer is formed on the textured glass substrate, magnetic anisotropy can be generated in the circumferential direction of the disk. This texturing process uses a diamond abrasive as an abrasive in the tape polishing process. A texture is formed on the surface of the glass substrate by bringing the polishing tape into contact with the glass substrate, supplying a diamond abrasive, and relatively moving the polishing tape and the glass substrate.

As shown in FIG. 1 and FIG. 2, the disk-shaped glass substrate having both main surfaces 11, 11 and an outer peripheral side end surface 12 and an inner peripheral side end surface 13 therebetween, according to the method for manufacturing a magnetic disk glass substrate of the present invention. 1 is obtained. The outer peripheral side end surface 12 includes chamfered surfaces 12b and 12b between the side wall surface 12a and the main surfaces on both sides thereof. The inner peripheral side end face 13 has the same shape.

In the present invention, the magnetic disk is manufactured by forming at least a magnetic layer on the magnetic disk glass substrate according to the present invention. As the magnetic layer material, a hexagonal CoPt ferromagnetic alloy having a large anisotropic magnetic field can be used. As a method for forming the magnetic layer, a method of forming a magnetic layer on a glass substrate by sputtering, for example, DC magnetron sputtering can be used. Further, by interposing an underlayer between the glass substrate and the magnetic layer, the orientation direction of the magnetic grains of the magnetic layer and the size of the magnetic grains can be controlled. For example, by using a cubic base layer such as a Cr-based alloy, for example, the magnetization easy direction of the magnetic layer can be oriented along the magnetic disk surface. In this case, a magnetic disk of the in-plane magnetic recording system is manufactured. Further, for example, by using a hexagonal underlayer containing Ru or Ti, for example, the easy magnetization direction of the magnetic layer can be oriented along the normal of the magnetic disk surface. In this case, a perpendicular magnetic recording type magnetic disk is manufactured. The underlayer can be formed by sputtering as with the magnetic layer.

In the present invention, a protective layer and a lubricating layer are preferably formed in this order on the magnetic layer. As the protective layer, an amorphous hydrogenated carbon-based protective layer is suitable. For example, the protective layer can be formed by a plasma CVD method. The thickness of the protective layer is preferably 30 angstroms to 80 angstroms. Further, as the lubricating layer, a lubricant having a functional group at the end of the main chain of the perfluoropolyether compound can be used. In particular, it is preferable that the main component is a perfluoropolyether compound having a terminal hydroxyl group as a polar functional group. The film thickness of the lubricating layer is preferably 5 angstroms to 15 angstroms. The lubricating layer can be applied and formed by a dip method.

Hereinafter, embodiments of the present invention will be specifically described with reference to examples. In addition, this invention is not limited to a following example.
Example 1
(1) Rough lapping step (rough grinding step), (2) Shape processing step, (3) Fine lapping step (fine grinding step), (4) End surface polishing step, (5) Main surface polishing step, (6 ) A glass substrate for a magnetic disk of this example was manufactured through a chemical strengthening step.

(1) Coarse lapping process First, a glass substrate made of aluminosilicate glass having a diameter of 66 mmφ and a thickness of 1.5 mm was obtained from molten glass by direct pressing using an upper mold, a lower mold, and a body mold. In this case, in addition to the direct press, a glass substrate may be obtained by cutting out from a sheet glass formed by a downdraw method or a float method with a grinding wheel. As the aluminosilicate _glass, SiO 2: 58 to 75 _{wt%, Al} 2 O 3: _{5~23 wt%,} Li 2 O: _{3~10 wt%,} Na 2 O: _4~13 chemical containing wt% Tempered glass was used. Next, a lapping process was performed on the glass substrate in order to improve dimensional accuracy and shape accuracy. This lapping process was performed using a double-sided lapping machine and using abrasive grains having a particle size of # 400. Specifically, first, using alumina abrasive grains of particle size # 400, setting the load to about 100 kg and rotating the sun gear and the internal gear of the lapping device, both surfaces of the glass substrate housed in the carrier are faced. Lapping was performed with an accuracy of 0 to 1 μm and a surface roughness (Rmax) of about 6 μm.

(2) Shape processing step Next, a cylindrical grindstone is used to make a hole in the center portion of the glass substrate, and the outer peripheral end face is ground to a diameter of 65 mmφ. Chamfered. The surface roughness of the end face of the glass substrate at this time was about 4 μm in Rmax. In general, a 2.5-inch HDD (hard disk drive) uses a magnetic disk having an outer diameter of 65 mm.
(3) Fine lapping step Next, the grain size of the abrasive grains was changed to # 1000 and the surface of the glass substrate was lapped so that the surface roughness was about 2 μm in Rmax and about 0.2 μm in Ra. The glass substrate after the lapping process was immersed in each washing tank (applied with ultrasonic waves) in a neutral detergent and water in order to perform ultrasonic cleaning.
(4) End face polishing step Next, the surface roughness of the end face (inner periphery, outer periphery) of the glass substrate was polished by brush polishing to about 1 μm at Rmax and about 0.3 μm at Ra while rotating the glass substrate. And the surface of the glass substrate which finished the said end surface grinding | polishing was water-washed.

(5) Main surface polishing step Next, a first polishing step for removing scratches and distortions remaining in the lapping step described above was performed using the double-side polishing apparatus shown in FIG. In the double-side polishing apparatus, a glass substrate held by a carrier 4 is brought into close contact between upper and lower polishing surface plates 5 and 6 to which a polishing pad is attached as a polishing cloth 7, and the carrier 4 is connected to the sun gear 2 and the internal gear 3. And the glass substrate is clamped by the upper and lower surface plates 5 and 6. Thereafter, the polishing liquid is supplied and rotated between the polishing pad and the polishing surface of the glass substrate, so that the glass substrate revolves while rotating on the surface plates 5 and 6, and both surfaces are polished simultaneously. . Specifically, a hard polisher (hard foamed urethane) was used as the polisher, and the first polishing step was performed. As a polishing liquid, ethanol-based low molecular weight nonionic surfactant was further added in an amount of 0.1 mol / liter in RO water in which 10% by weight of cerium oxide as an abrasive was dispersed. The friction coefficient of the solvent of the polishing liquid was 0.8. In addition, the friction coefficient of the solvent when this nonionic surfactant is not added is 0.7. The glass substrate after the first polishing step was sequentially immersed in each cleaning bath of neutral detergent, pure water, pure water, IPA (isopropyl alcohol), and IPA (steam drying), ultrasonically cleaned, and dried. .

Next, using the same double-side polishing apparatus as used in the first polishing step, the polishing was changed to a polishing pad of soft polisher (suede) (foamed polyurethane with Asker C hardness of 72), and the second polishing step was performed. This second polishing step is, for example, mirror polishing for finishing the surface roughness of the glass substrate main surface to a smooth mirror surface with an Rmax of about 6 nm or less while maintaining the flat surface obtained in the first polishing step. It is processing. The polishing liquid used was adjusted to neutrality by adding 0.1 mol / liter of an ethanol-based low molecular weight nonionic surfactant to RO water in which colloidal silica (average particle size 80 nm) was dispersed. The friction coefficient of the solvent of the polishing liquid was 0.8. The glass substrate after the second polishing step was sequentially immersed in each cleaning bath of neutral detergent, pure water, pure water, IPA, and IPA (steam drying), ultrasonically cleaned, and dried.
When the surface of the glass substrate after mirror polishing was analyzed with an atomic force microscope (AFM) and an electron microscope, it was specular and no convex surface defects were observed. And the edge part shape was able to be controlled to the favorable shape, without rounding conventionally.

(6) Chemical strengthening step Next, chemical strengthening was performed on the glass substrate after the cleaning. For chemical strengthening, a chemical strengthening solution in which potassium nitrate and sodium nitrate were mixed was prepared, the chemical strengthening solution was heated to 380 ° C., and the cleaned and dried glass substrate was immersed for about 4 hours to perform chemical strengthening treatment. The glass substrate after chemical strengthening was sequentially immersed in each of washing tanks of sulfuric acid, neutral detergent, pure water, pure water, IPA, and IPA (steam drying), ultrasonically cleaned, and dried.
Further, when the surface roughness of the main surface of the glass substrate obtained through the above steps was measured with an atomic force microscope (AFM), it had an ultra-smooth surface with Rmax = 2.13 nm and Ra = 0.20 nm. A glass substrate was obtained. Further, when the surface of the glass substrate was analyzed with an atomic force microscope (AFM) and an electron microscope, it was mirror-like and no convex surface defects were observed.
The obtained glass substrate had an outer diameter of 65 mm, an inner diameter of 20 mm, and a plate thickness of 0.635 mm.
Thus, a glass substrate for magnetic disk of this example was obtained.

Next, the following film formation process was performed on the magnetic disk glass substrate obtained in this example to obtain a magnetic disk.
On the glass substrate, an adhesion layer made of a Cr alloy, a soft magnetic layer made of a CoTaZr alloy, an underlayer made of Ru, a perpendicular magnetic recording layer made of a CoCrPt alloy, a protective layer, and a lubricating layer were sequentially formed.
The protective layer is for preventing the magnetic recording layer from deteriorating due to contact with the magnetic head, and is made of hydrogenated carbon having a thickness of 5 nm, and wear resistance is obtained. The lubricating layer is formed by perfluoropolyether liquid lubricant by dipping and has a thickness of 0.9 nm.

In the magnetic disk thus obtained, the flying area of the magnetic head can be expanded by several hundred μm compared to the conventional case (comparative example described later), and the guaranteed area can be expanded.

Hereafter, the comparative example with respect to the above-mentioned Example is given.
(Comparative example)
In the first polishing and the second polishing in the (5) main surface polishing step of Example 1, polishing treatment was performed using a polishing liquid to which a nonionic surfactant was not added. Except for this point, a magnetic disk glass substrate was produced in the same manner as in Example 1. The substrate end surface after the polishing treatment was rounded as shown in FIG.
Further, a magnetic disk was manufactured in the same manner as in Example 1 using the glass substrate according to this comparative example. In the obtained magnetic disk, the flying area of the magnetic head can only be assured up to the area inside the above-mentioned embodiment because of the shape of the edge of the substrate.

It is sectional drawing of the glass substrate for magnetic discs. It is a whole perspective view of the glass substrate for magnetic discs. It is sectional drawing for demonstrating the board | substrate edge part shape by grinding | polishing process. It is a longitudinal cross-sectional view which shows schematic structure of a double-side polish apparatus. It is the schematic for demonstrating the measuring method of the friction coefficient of the solvent of polishing liquid. It is a wave form diagram of the chart obtained in said measurement.

Explanation of symbols

1 glass substrate 4 carrier 5 upper surface plate 6 lower surface plate 7 polishing cloth (polishing pad)
11 Main surface of substrate 12, 13 End surface of substrate

Claims (4)

A method for producing a glass substrate for a magnetic disk having a polishing step of polishing a glass substrate using a polishing liquid containing abrasive grains,
A method for producing a glass substrate for a magnetic disk, comprising adding a nonionic surfactant to the polishing liquid so that a friction coefficient of a solvent of the polishing liquid is equal to or higher than that of water.   2. The method of manufacturing a glass substrate for a magnetic disk according to claim 1, wherein the friction coefficient of the solvent of the polishing liquid is 0.7 or more.   3. The method for manufacturing a glass substrate for a magnetic disk according to claim 1, wherein the glass substrate is a glass substrate used for a magnetic disk mounted on a load / unload type magnetic recording apparatus. A method for manufacturing a magnetic disk, comprising forming at least a magnetic layer on a glass substrate for a magnetic disk obtained by the manufacturing method according to claim 1.

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