JP2009064489A - Manufacturing method of glass substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a glass substrate to obtain a glass substrate in which the size and number of surface defect are managed and defects are reduced when used as a magnetic disk. <P>SOLUTION: In a method of manufacturing a glass substrate, a glass blank material is prepared; the glass substrate which has a nearly circular shape using the glass blank material is obtained; the glass substrate is irradiated with light having a directivity from circumference and radial direction of the glass substrate to the glass substrate; a defect of 1 μm or less is detected by light reflected from the glass substrate; and defect of the glass substrate is removed if the number of defects is more than a predetermined number. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a glass substrate for a magnetic disk mounted on a hard disk drive device.

  There is a magnetic disk as a magnetic recording medium mounted on a hard disk drive device (HDD device). The magnetic disk is manufactured by depositing a NiP film on a metal substrate made of an aluminum-magnesium alloy or the like, or laminating a magnetic layer or a protective layer on a glass substrate or a ceramic substrate. Conventionally, an aluminum alloy substrate has been widely used as a substrate for a magnetic disk. However, with recent downsizing, thinning, and high-density recording of magnetic disks, surface flatness and A glass substrate having excellent strength with a thin plate has been used.

For magnetic disks formed by forming at least a magnetic layer on a glass substrate, defects present on the surface are inspected. For example, Patent Document 1 discloses a light projecting system that irradiates a light beam having a width in a direction perpendicular to the main scanning direction and relatively scans the transmissive substrate, and transmits scattered light from the transmissive substrate. Having a high-magnification objective lens that receives light in a direction perpendicular to the surface of the conductive substrate and a light receiver that receives light from the objective lens arranged along the direction of imaging corresponding to a direction perpendicular to the surface. It is disclosed that defect detection of a magnetic disk substrate is performed by a defect detection optical system including a light receiving system that forms an image of a scanning position of a magnetic substrate on a light receiving element.
JP 2002-55061 A

  In recent years, in HDD devices, the distance (flying height) between the magnetic head and the magnetic disk has become smaller, and as the flying height becomes smaller, as a magnetic disk, in order to suppress defects when used as a magnetic disk, Acceptable substrate surface defects are also becoming smaller. Correspondingly, the requirements for the size and number of surface defects of the magnetic disk substrate are becoming stricter.

  The present invention has been made in view of the above points, and provides a glass substrate manufacturing method capable of obtaining a glass substrate in which the size and number of surface defects are controlled to suppress defects when used as a magnetic disk. The purpose is to do.

  The method for producing a glass substrate of the present invention includes a step of preparing a glass blank material, a step of obtaining a glass substrate having a substantially circular shape using the glass blank material, and a circumference of the glass substrate with respect to the glass substrate. Irradiating the glass substrate with light having directivity from a direction and a radial direction, detecting a defect of 1 μm or less by reflected light from the glass substrate, and when the number of defects is a predetermined number or more, the glass And a step of performing defect removal processing on the substrate.

  According to this method, since the size and number of surface defects are managed, a glass substrate that can suppress defects when used as a magnetic disk and can suppress occurrence of defects when mounted on an HDD device is obtained. be able to.

  In the manufacturing method of the glass substrate of this invention, it is preferable that the process of detecting the said defect is performed after the mirror polishing process of mirror-polishing the main surface of the said glass substrate.

  In the method for producing a glass substrate of the present invention, in the case of including a chemical strengthening step for chemically strengthening the glass constituting the glass substrate, the step of detecting the defect includes mirror polishing the main surface of the glass substrate. It is preferable to be performed after the chemical strengthening step.

  In the manufacturing method of the glass substrate of this invention, it is preferable that the said defect removal process is a process which carries out the process which mirror-polishes the said glass substrate, or the process which wash | cleans the said glass substrate. In this case, it is preferable to change the conditions according to the number and size of the defects in the process of cleaning the glass substrate.

  In the method for producing a glass substrate of the present invention, the predetermined number of defects is a number at which a TA ratio by a thermal asperity (TA) test of a magnetic disk formed with at least a magnetic layer on the glass substrate is not more than a predetermined value. It is preferable that

  The glass substrate for a magnetic disk of the present invention is a glass substrate for a magnetic disk having at least a substantially flat main surface and having a substantially circular shape, wherein the glass substrate is formed on the main surface with respect to the glass substrate. Magnetism formed by irradiating the glass substrate with light having directivity from the circumferential direction and the radial direction, and defects of 1 μm or less detected by reflected light from the glass substrate forming at least a magnetic layer on the glass substrate. It is characterized in that there are a number in which the TA rate by the thermal asperity (TA) test of the disk is a predetermined value or less.

  According to this configuration, since the size and number of surface defects are managed, it is possible to suppress defects when the magnetic disk is used, and it is possible to suppress the occurrence of problems when mounted on the HDD device.

  The method for manufacturing a substrate according to the present invention includes a step of preparing a blank material, a step of obtaining a substrate having a substantially circular shape using the blank material, and a direction from a circumferential direction and a radial direction of the substrate with respect to the substrate. Irradiating the substrate with light having the property, detecting a defect of 1 μm or less by reflected light from the substrate, and performing defect removal processing on the substrate when the number of defects is a predetermined number or more And a process.

  According to the method for producing a glass substrate of the present invention, a glass blank material is prepared, a glass substrate having a substantially circular shape is obtained using the glass blank material, and the circumferential direction of the glass substrate with respect to the glass substrate When the glass substrate is irradiated with light having directivity from the radial direction, a defect of 1 μm or less is detected by reflected light from the glass substrate, and when the number of defects is a predetermined number or more, Since the defect removal process is performed, the size and number of surface defects can be managed, and a glass substrate that can suppress defects when used as a magnetic disk can be obtained.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
As an apparatus for inspecting defects on the surface of a magnetic disk substrate, for example, a glass substrate, there are an apparatus having the configuration shown in FIG. 1 and an apparatus having the configuration shown in FIG. In the present specification, the defect is mainly caused by residual materials used in various processes for manufacturing a substrate, dust generation during the manufacturing apparatus itself or substrate processing, and it adheres to the substrate surface. Convex defects: Concave defects such as scratches generated during substrate processing and damages generated during substrate flow / transfer; uneven defects due to blank materials, etc.

  The apparatus shown in FIG. 1 is an optical defect inspection apparatus (Optical Surface Analyzer), and includes two lasers 11 and 12 and a detector 13 that detects reflected light of the laser light. The laser 11 irradiates the glass substrate S, which is a measurement object, with directivity from the radial direction, and the laser 12 irradiates the glass substrate S, which is a measurement object, with directivity from the circumferential direction. . In such an apparatus, it is possible to detect a defect having a length in the radial direction and a defect having a length in the circumferential direction. Further, each laser can split the laser beam, that is, it can be divided into a laser beam perpendicular to the glass substrate S and a laser beam horizontal to the glass substrate S. As for the defects, the direction of the laser beam is more suitable depending on the type of the defect. Therefore, it becomes possible to accurately detect various defects by dispersing the laser beam as described above. Furthermore, in the apparatus shown in FIG. 1, since the laser diameter is small, for example, about 4 μm × 5 μm, the laser wavelength is short, and the power is large, the defect detection sensitivity is high.

  Since the apparatus shown in FIG. 1 has the above-described characteristics, it is possible to accurately detect various defect types. That is, it is possible to detect a defect of 1 μm or less by reflected light from the glass substrate S. The apparatus shown in FIG. 1 is an example, and is an apparatus of a type that irradiates a glass substrate with light having directivity from the circumferential direction and the radial direction, and detects defects by reflected light from the glass substrate. If present, it can be used in the present invention as well.

  The apparatus shown in FIG. 2 changes the optical path of the laser 21, the laser beam to the glass substrate S that is the object to be measured, and transmits the beam splitter 22 that passes through the mirror 23 disposed on the opposite side of the laser 21, and the glass substrate. And an APD (Avalanche Photo Diode) detector 24 for detecting S reflected light. In this apparatus, since laser light is not dispersed, it is difficult to accurately detect various defects. Further, in this apparatus, the laser diameter is larger than that of the apparatus shown in FIG. 1 (for example, 2 μm × 64 μm), and the ratio between the signal from the defect and the signal from a place other than the defect (SNR: Signal-to- Noise ratio is poor and the laser wavelength is long, so the defect detection sensitivity is low.

  Thus, the apparatus shown in FIG. 1 can detect even finer defects, for example, defects of 1 μm or less, compared to the apparatus shown in FIG. Here, the correlation between the defect detected by the apparatus shown in FIG. 1 and the defect detected by the apparatus shown in FIG. 2 was examined. The results are shown in FIGS. FIG. 3 is a diagram showing the results of inspecting the glass substrate obtained by grinding, shaping, and polishing the glass blank material with the apparatus shown in FIG. 1 and the apparatus shown in FIG. 3 (a) and 3 (b) show the results of inspecting defects having a size of less than 0.3 μm, and FIGS. 3 (c) and 3 (d) show the inspection of defects having a size of 0.3 μm or more and less than 1 μm. 3 (e) and 3 (f) show the results of inspecting defects having a size of 1 μm or more. 3A, 3C, and 3E show the results of inspecting the concave defects, and FIGS. 3B, 3D, and 3F show the results of inspecting the convex defects.

  As can be seen from FIGS. 3 (e) and 3 (f), for defects having a size exceeding 1 μm, both the concave defect and the convex defect have a relatively good correlation between the count numbers of both apparatuses. On the other hand, as can be seen from FIGS. 3 (a) to 3 (d), regarding the defect having a size of 1 μm or less, there is no correlation between the count numbers of both devices for both the concave defect and the convex defect. As described above, the apparatus shown in FIG. 1 irradiates the glass substrate with light having a small laser diameter and high power from the circumferential direction and the radial direction of the glass substrate. Although it can be detected, in the apparatus shown in FIG. 2, it is considered that a very small defect cannot be detected because the laser is irradiated onto the glass substrate in a situation where the laser diameter is large and the SNR is poor.

  Further, the results detected by the apparatus shown in FIG. 1 and the results detected by the apparatus shown in FIG. 2 with respect to defects on the glass substrate before the cleaning before forming the magnetic layer are expressed as SEM (scanning electron microscope) / EDX (energy dispersion). Type X-ray fluorescence analyzer). The results are shown in FIGS. 4 (a) and 4 (b). FIG. 4A shows the analysis results for the defects detected by the apparatus shown in FIG. 2, and FIG. 4B shows the analysis results for the defects detected by the apparatus shown in FIG. As shown in FIG. 4 (b), the apparatus shown in FIG. 1 detects defects that are difficult to wash away by cleaning before forming the magnetic layer, such as abrasives, metals, and fine defects. As shown in FIG. 4A, the apparatus shown in FIG. 2 does not accurately detect fine defects such as abrasives, metals, and fine defects. As described above, this is considered to be because the detection accuracy of defects having a size of 1 μm or less differs greatly between the two apparatuses.

  Thus, by performing defect detection on a glass substrate as shown in FIG. 1 using a type of device that irradiates the glass substrate with light having directivity from the circumferential direction and the radial direction of the glass substrate, It is possible to accurately detect defects having a size of 1 μm or less. As a result of intensive studies on the influence of the defect having a size of 1 μm or less on the HDD device, the present inventors have found that the defect having a size of 1 μm or less and the collision of the magnetic head with the magnetic disk. And found that there is a relationship. Then, the present inventors have found that by managing the number of defects having a size of 1 μm or less, defects in the HDD can be suppressed and defects in the HDD device can be suppressed. It came to.

  Here, the thermal asperity (TA (Phenomenon that occurs when a magnetic head collides with a magnetic disk)) test is conducted on the relationship between a defect of 1 μm or less and the collision of a magnetic head with a magnetic disk. It can be expressed by the TA rate. FIG. 5 shows the result of the TA test performed by changing the number of defects having a size of 1 μm or less on the glass substrate. In the TA test, a magnetic disk is mounted on an HDD device equipped with a GMR (Giant MagnetoResistance) head, the disk peripheral speed is set to 8 m / s, and the signal output from the GMR head is amplified and observed with a digital oscilloscope. The TA waveform observed there was counted. As can be seen from FIG. 5, the TA ratio increases as the number of defects having a size of 1 μm or less on the glass substrate increases. From this result, the TA rate can be reduced by reducing the number of defects having a size of 1 μm or less on the glass substrate.

  Therefore, a relationship between the TA rate and the number of defects having a size of 1 μm or less is obtained in advance, and a TA rate value is determined based on a request from the HDD device type, magnetic head type, and the like. The number of defects having a size of 1 μm or less corresponding to is obtained from the above relationship, and the glass substrate is processed so as to be the number. That is, the manufacturing process is managed so that the number of defects obtained from the TA ratio value through the above relationship is obtained. For example, after a predetermined processing step, the number of defects of 1 μm or less on the glass substrate is obtained, and when the number exceeds a predetermined value, a defect removal process is performed on the glass substrate. Note that the TA ratio value is not limited, and is appropriately determined from the type of the HDD device, the type of the magnetic head, and the like.

  The glass substrate thus managed is a glass substrate for a magnetic disk having at least a substantially flat main surface and having a substantially circular shape, and the glass substrate has a substantially circular shape on the main surface of the glass substrate. Magnetism formed by irradiating the glass substrate with light having directivity from the circumferential direction and the radial direction, and defects of 1 μm or less detected by reflected light from the glass substrate forming at least a magnetic layer on the glass substrate. This means that there are a number in which the TA rate by the TA test of the disk is a predetermined value or less. Since the size and number of surface defects of this glass substrate are controlled, defects when used as a magnetic disk can be suppressed, and occurrence of problems when mounted on an HDD device can be suppressed.

  In this way, by controlling the number of defects of 1 μm or less on the glass substrate to be equal to or less than the number obtained from the glide test, the number of defective products that cause the magnetic head to collide with the magnetic disk is reduced. can do. The magnetic disk is formed by forming at least a magnetic layer on the glass substrate described above. That is, a magnetic disk is usually manufactured by sequentially laminating an underlayer, a magnetic layer, a protective layer, and a lubricating layer on a glass substrate that has been subjected to chemical strengthening treatment on the surface as necessary. The underlayer in the magnetic disk is appropriately selected according to the magnetic layer.

  The defect removal process is, for example, a process of mirror polishing a glass substrate or a process of cleaning the glass substrate. Note that the glass substrate cleaning process includes not only a cleaning process using a liquid but also various cleaning processes such as a cleaning process using a gas and a cleaning process using laser irradiation. Further, when obtaining the number and size of defects in the step of detecting defects, it is preferable to change processing conditions for cleaning the glass substrate in accordance with the number and size of defects. Thus, by changing the conditions of the cleaning process according to the number and size of defects, defects of 1 μm or less can be effectively removed from the glass substrate, and the number corresponding to a predetermined value or less of the glide test. Can be.

  In the production of a glass substrate, (1) shape processing step and first lapping step, (2) end shape step (coring step for forming a hole, chamfering at the end (outer peripheral end and inner peripheral end) Chamfering step for forming a surface (chamfered surface forming step)), (3) end surface polishing step (outer peripheral end and inner peripheral end), (4) second lapping step, (5) main surface polishing step (first And a second polishing step), (6) a chemical strengthening step, and (7) a texture treatment step. Note that the end polishing step and the second lapping step may be mixed. Further, the texture processing step is not required for a magnetic disk substrate used in a perpendicular magnetic recording type HDD apparatus.

  In the manufacturing method of the glass substrate of this invention, a glass blank material is prepared, the glass substrate which has a substantially circular shape is obtained using the said glass blank material, The circumferential direction of the said glass substrate with respect to the said glass substrate When the glass substrate is irradiated with light having directivity from the radial direction, a defect of 1 μm or less is detected by reflected light from the glass substrate, and when the number of defects is a predetermined number or more, The defect removal process is performed. According to such a method, since the size and number of surface defects are managed, a glass substrate that can suppress defects when it is used as a magnetic disk and can suppress occurrence of defects when mounted on an HDD device. Can be obtained.

  In such a manufacturing process, the step of detecting a defect of 1 μm or less is preferably performed after the mirror polishing step of mirror polishing the main surface of the glass substrate. In addition, the step of detecting defects of 1 μm or less is performed after the chemical strengthening step of mirror-polishing the main surface of the glass substrate in the case of including a chemical strengthening step of chemically strengthening the glass constituting the glass substrate. It is preferable.

Next, examples performed for clarifying the effects of the present invention will be described.
(Example)
First, the melted aluminosilicate glass was molded into a disk shape by direct pressing using an upper mold, a lower mold, and a barrel mold to obtain an amorphous plate glass. Next, both main surfaces of the plate glass were lapped to form a disk-shaped glass base material. Next, the glass base material was cut using a diamond cutter, and a glass substrate having a diameter of 29 mm was cut out from the glass base material. The diameter of the glass base material was 96 mm, and six glass substrates could be collected from one glass base material. Next, using a cylindrical core drill, a hole was formed in the center of the glass substrate to obtain an annular glass substrate (coring).

  Next, the end surface of the glass substrate was subjected to mirror polishing by a brush polishing method. At this time, as the abrasive grains, a slurry (free abrasive grains) containing cerium oxide abrasive grains was used. Further, the inner peripheral end portion was mirror polished by a magnetic polishing method. And the glass substrate which finished the mirror polishing process was washed with water. As a result, the diameter of the glass substrate was 27.4 mm, and the substrate used for the 1-inch magnetic disk could be obtained.

  Next, lapping processing was performed on both main surfaces of the obtained glass substrate in the same manner as the above lapping. Next, a first polishing step was first performed as a main surface polishing step. As the abrasive, cerium oxide abrasive grains were used. And the glass substrate which finished this 1st grinding | polishing process is immersed in each washing tank of neutral detergent, pure water (1), pure water (2), IPA (isopropyl alcohol), and IPA (steam drying) sequentially. , Washed.

  Next, a second polishing step for finishing the main surface into a mirror surface was performed on both main surfaces of the glass substrate. As the abrasive, cerium oxide abrasive grains finer than the cerium oxide abrasive grains used in the first polishing step were used. And the glass substrate which finished this 2nd grinding | polishing process is made into neutral detergent (1), neutral detergent (2), pure water (1), pure water (2), IPA (isopropyl alcohol), IPA (vapor drying) ) Were sequentially immersed in each washing tank and washed. Note that ultrasonic waves were applied to each cleaning tank.

  Next, chemical strengthening was performed on the glass substrate after the lapping step and the polishing step described above. For chemical strengthening, a chemical strengthening solution in which potassium nitrate (60%) and sodium nitrate (40%) are mixed is prepared, this chemical strengthening solution is heated to 380 ° C., and the cleaned glass substrate is placed therein for about 4 hours. This was done by dipping.

  Subsequently, the glass substrate thus chemically strengthened was inspected for defects using the OSA apparatus shown in FIG. At this time, an optical defect inspection apparatus OSA6100 (trade name, manufactured by KLA) was used as the inspection apparatus, and the laser wavelength was 405 nm and the laser diameter was 5 μm × 4 μm. At this time, the number (threshold value) of defects of 1 μm or less corresponding to a predetermined value of the TA ratio of the TA test was set to one.

  About 100 glass substrates, a glass substrate is produced in the process as described above, and defect inspection is performed. For a glass substrate in which the number of defects of 1 μm or less exceeds a threshold value, as the defect removal process, in the second polishing process. Polishing treatment was performed. In this way, a glass substrate in which the number of defects of 1 μm or less was controlled to be equal to or less than the threshold value was produced.

  Such a glass substrate was immersed in a 20 ° C. water bath to be rapidly cooled and maintained for about 10 minutes. And the glass substrate which finished quenching was immersed in the concentrated sulfuric acid heated at about 40 degreeC, and was wash | cleaned. Further, the glass substrate after the sulfuric acid cleaning was cleaned by immersing in a cleaning tank of pure water (1), pure water (2), IPA (isopropyl alcohol), and IPA (steam drying). In addition, ultrasonic waves were applied to each cleaning tank.

  After performing texture treatment and precision cleaning on these glass substrates, a magnetic disk is manufactured by sequentially laminating an underlayer, a magnetic layer, a protective layer, and a lubricating layer, and this magnetic disk is loaded / unloaded. Durability test was performed by mounting on HDD device. In the durability test, a magnetic head using a GMR element was used, and the flying height of the magnetic head was 10 nm, and the load / unload operation was repeated. as a result. The magnetic disk of this example was durable without failure in 1 million LUL (load / unload) operations.

(Comparative example)
Without controlling the number of defects of 1 μm or less on the glass substrate, glass substrates were produced in the same manner as in the examples, and the underlayer, magnetic layer, protective layer, and lubrication were applied to these glass substrates in the same manner as in the examples. The layers were sequentially laminated to produce a magnetic disk, and this magnetic disk was mounted on a load / unload type HDD device and the durability test was conducted. As a result, for some of the magnetic disks manufactured from the glass substrate, problems such as a head crash occurred after 600,000 LUL operations. Since the number of defects of 1 μm or less is not managed, it is considered that such fine defects exist at a high rate and cause a head crash or the like.

  The present invention is not limited to the above embodiment, and can be implemented with appropriate modifications. For example, in the above embodiment, the management of the number of defects of 1 μm or less in the glass substrate is described, but the present invention is not limited to this, and in other magnetic disk substrates such as an aluminum alloy substrate. The present invention can also be applied when managing the number of defects of 1 μm or less. Although the types of defects differ between the glass substrate and other substrates such as an aluminum alloy substrate, the correlation between the number of defects of 1 μm or less and the TA rate of the TA test is also obtained in other substrates such as the aluminum alloy substrate. Therefore, by managing the number of defects of 1 μm or less using the correlation, and by using the correlation, a magnetic disk substrate that can suppress defects when a magnetic disk is used Can be obtained.

  In the above embodiment, the case where the TA test is used for the collision of the magnetic head with the magnetic disk has been described. However, the present invention is not limited to this, and the glide for the collision of the magnetic head with the magnetic disk is described. A test or a certification test may be used. In addition, the number, size, processing procedure, and the like of the members in the above embodiment are merely examples, and various changes can be made within the range where the effects of the present invention are exhibited. In addition, various modifications can be made without departing from the scope of the object of the present invention.

It is a figure which shows schematic structure of the apparatus which detects the defect on a glass substrate. It is a figure which shows the other example of schematic structure of the apparatus which detects the defect on a glass substrate. (A)-(f) is a figure which shows the correlation between the apparatus shown in FIG. 1, and the apparatus shown in FIG. 2 about the count of the defect of various magnitude | sizes. (A) is a figure which shows the analysis result of the defect detected with the apparatus shown in FIG. 2, (b) is a figure which shows the analysis result of the defect detected with the apparatus shown in FIG. It is a figure which shows the relationship between TA rate and the number of the defects of a magnitude | size of 1 micrometer or less.

Explanation of symbols

11, 12, 21 Laser 13 Detector 23 Mirror 24 APD detector S Glass substrate

Claims (8)

  A step of preparing a glass blank, a step of obtaining a glass substrate having a substantially circular shape using the glass blank, and light having directivity from the circumferential direction and the radial direction of the glass substrate with respect to the glass substrate Irradiating the glass substrate, detecting a defect of 1 μm or less by reflected light from the glass substrate, and performing a defect removing process on the glass substrate when the number of defects is a predetermined number or more And a method of manufacturing a glass substrate.   The method for producing a glass substrate according to claim 1, wherein the step of detecting the defect is performed after a mirror polishing step of mirror polishing the main surface of the glass substrate.   In the case of including a chemical strengthening step of chemically strengthening the glass constituting the glass substrate, the step of detecting the defect is performed after the chemical strengthening step of mirror-polishing the main surface of the glass substrate. The method for producing a glass substrate according to claim 1 or 2, wherein the glass substrate is a glass substrate.   The method for manufacturing a glass substrate according to any one of claims 1 to 3, wherein the defect removing process is a process of mirror polishing the glass substrate or a process of cleaning the glass substrate.   The method for producing a glass substrate according to claim 4, wherein in the process of cleaning the glass substrate, conditions are changed according to the number and size of the defects.   2. The predetermined number of the defects is a number in which a TA ratio according to a thermal asperity (TA) test of a magnetic disk formed by forming at least a magnetic layer on the glass substrate is a predetermined value or less. The manufacturing method of the glass substrate in any one of Claim 5.   A glass substrate for a magnetic disk having at least a substantially flat main surface and having a substantially circular shape, and having directivity from a circumferential direction and a radial direction of the glass substrate on the main surface with respect to the glass substrate. When the glass substrate is irradiated with light having a defect of 1 μm or less detected by reflected light from the glass substrate, a TA by a thermal asperity (TA) test of a magnetic disk in which at least a magnetic layer is formed on the glass substrate. A glass substrate for a magnetic disk, characterized in that there are a number in which the rate is a predetermined value or less.   A step of preparing a blank material, a step of obtaining a substrate having a substantially circular shape using the blank material, and irradiating the substrate with light having directivity from the circumferential direction and the radial direction of the substrate And a step of detecting a defect of 1 μm or less by reflected light from the substrate and a step of performing a defect removal process on the substrate when the number of defects is a predetermined number or more. A method for manufacturing a substrate.

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