WO2017038201A1 - Method for producing glass substrate for information recording medium, method for producing information recording medium, information recording medium glass substrate, and magnetic recording medium - Google Patents

Abstract

[Problem] The present invention provides an information recording medium glass substrate production method, the glass substrate having a very low Ra and being able to sufficiently support higher recording capacity and higher accuracy, which will be sought in the future. [Solution] By means of a rough polishing step for polishing the main surface of a glass substrate, the glass substrate is produced, the main surface of which has an arithmetic average roughness Ra of 0.2 nm or less if measured using an atomic force microscope at a resolution of 256×256 pixels and a scan size of 1×1 μm, and thereafter the glass substrate is polished by means of a finish polishing step for polishing using a polishing slurry including at least one type of abrasive grain selected from the group consisting of diamond, aluminum oxide, and cerium oxide having an average particle size of 15 nm or less.

Description

Method for manufacturing glass substrate for information recording medium, method for manufacturing information recording medium, glass substrate for information recording medium, and magnetic recording medium

The present invention relates to a method for manufacturing a glass substrate for information recording medium, a method for manufacturing an information recording medium, a glass substrate for information recording medium, and a magnetic recording medium.

In recent years, the increase in recording capacity of hard disks (information recording media) has become remarkable. Glass substrates such as chemically tempered glass and crystallized glass, which are excellent in impact resistance and easily obtain flatness (smoothness), have been used for hard disk substrates. In order to increase the capacity of a hard disk using a glass substrate, it is important to improve the performance of a magnetic film formed on the glass substrate.

In order to improve the performance of the magnetic film, it is required to reduce the Ra (arithmetic mean roughness) of the main surface of the glass substrate. By reducing Ra, the unevenness of the magnetic film is reduced, and the distance between the read head and the glass substrate is shortened, so that the density of the memory element can be increased.

In order to reduce Ra, the surface of the glass substrate may be polished using fine abrasive grains. Generally, as polishing abrasive grains having a diameter of 30 nm or less, a final polishing is performed using a colloidal silica slurry. When colloidal silica has a particle diameter of 10 nm or less, when used as an abrasive, colloidal silica has a high affinity with a glass substrate, and has a problem that Ra adheres firmly to the glass substrate and raises Ra.

From the above background, it has been proposed to improve the flatness by using organic particles, for example, particles of styrene resin, acrylic resin, urethane resin, or the like as an abrasive instead of colloidal silica ( For example, see Patent Document 1).

International Patent Application No. 2014/067290

However, the organic particles of Patent Document 1 have a problem that the improvement range of Ra is small. In addition, since the particles are large, it has been confirmed that there are cases where the particles do not sufficiently enter between the glass substrate and the polishing pad at the initial stage of the polishing machine, the polishing load increases, and the glass substrate may crack. Yes.

In order to increase the recording capacity and the accuracy, it is preferable that Ra on the main surface of the glass substrate is 0.05 nm or less, but it is difficult to perform final polishing with organic particles. Further, in other polishing slurries, a glass substrate having a main surface Ra of 0.05 nm or less has not been realized. Therefore, there is a possibility that the conventional technology cannot cope with the further increase in recording capacity and accuracy required in the future.

One object of the present invention is to solve the above-mentioned problems, and to provide a method for producing a glass substrate for a minute information recording medium of Ra that can sufficiently cope with further higher recording capacity and higher accuracy required in the future. It is to provide.

The above issues
In the method for producing a glass substrate for information recording medium for polishing a glass substrate,
By the rough polishing step of polishing the main surface of the glass substrate, the arithmetic average roughness Ra on the main surface when measured with a resolution of 256 × 256 pixels at 1 μm × 1 μm square using an atomic force microscope is 0.2 nm. The following glass substrates are manufactured,
Thereafter, the glass substrate is polished by a final polishing step in which polishing is performed using a polishing slurry containing at least one abrasive selected from the group consisting of diamond, aluminum oxide and cerium oxide having an average particle size of 15 nm or less. This can be solved by the method for producing a glass substrate for information recording media.

According to the present invention, it is possible to provide a method of manufacturing a glass substrate for a minute information recording medium of Ra that can sufficiently cope with further higher recording capacity and higher accuracy required in the future.

Hereinafter, modes for carrying out the present invention will be described. The present embodiment will be described in detail by taking as an example the production of a glass substrate for a magnetic disk (glass substrate for hard disk; hereinafter simply referred to as “glass substrate”).

As the glass used for the glass substrate of the present invention, any glass that can be used for a normal glass substrate for a magnetic disk can be used. For example, there are glass plates having the following glass composition in terms of mol%.
(1) 55 to 75% of SiO ₂ , 5 to 17% of Al ₂ O ₃ , 4 to 27% of Li ₂ O + Na ₂ O + K ₂ O, 0 to 20% of MgO + CaO + SrO + BaO, the total content of these components Is 90% or more, ZrO ₂ may be contained in an amount of 0 to 6%, TiO ₂ may be contained in an amount of 0 to 6%, or SiO _{2 in an amount} of 62 to 74% and Al ₂ O _{3 in an amount} of 7 to 18 in terms of mol%. %, B ₂ O ₃ 2 to 15%, MgO, CaO, SrO and BaO at least one component in a total content of 8 to 21%, the total content of the seven components is 95% or more, Li Glass which may contain any one or more of ₂ O, Na ₂ O and K ₂ O in total less than 4%.
(2) 60 to 75% of SiO ₂ , 7 to 17% of Al ₂ O ₃ , 0 to 2% of B ₂ O ₃ , and any one or more of MgO, CaO, SrO and BaO in total 8 to 26 %, The total content of the seven components is 95% or more, and any one or more of Li ₂ O, Na ₂ O and K ₂ O may be contained in total in less than 4%.
(3) 55 to 75% of SiO ₂ , 3 to 17% of Al ₂ O ₃ , 0 to 10% of B ₂ O ₃ , and one or more components of MgO, CaO, SrO and BaO in total from 0 to 8 The total content of these components is 90% or more, and any one or more of Li ₂ O, Na ₂ O and K ₂ O is 0 to less than 4% in total, and ZrO ₂ is 0 to 6 %, Glass containing 0 to 6% TiO ₂ .

In this embodiment, a glass substrate made of, for example, Al ₂ O ₃ —SiO ₂ glass (aluminosilicate glass) is used as a starting material. And usually a glass substrate is manufactured through the following steps. That is, a circular hole is formed in the center of the glass substrate, and chamfering, main surface lapping, and end mirror polishing are sequentially performed. Then, the glass substrate in which the above-mentioned processing was performed may be laminated, and an inner peripheral end face may be etched. Then, the main surface of the glass substrate is polished to a flat and smooth surface (hereinafter referred to as a main surface polishing step), and a magnetic recording medium is applied to the main surface (film formation of a magnetic film) to form a glass substrate. To manufacture.

Incidentally, the main surface lapping process may be divided into a rough lapping process and a fine lapping process, and a shape processing process (drilling in the center of the glass plate, chamfering, end mirror polishing) may be provided between them. A chemical strengthening step may be provided after the polishing step. In addition, when manufacturing the glass substrate which does not have a circular hole in the center, the punching of the glass substrate center is unnecessary.

The main surface lapping is performed using aluminum oxide abrasive grains having an average particle diameter of 6 to 8 μm or aluminum oxide abrasive grains in a typical example. The reduction amount (polishing amount) of the main surface of the glass substrate due to lapping is usually 100 to 400 μm. However, as long as the arithmetic average roughness Ra on the main surface of the glass substrate can be finally set to 0.05 nm or less, it may be out of the numerical range.

(Main surface polishing process)
Next, the polishing process of the main surface including the features of the present invention will be specifically described. The main surface polishing step includes a rough polishing step and a final polishing step, and may further include, for example, three polishing steps in which the rough polishing step is divided into a plurality of steps. Next, the rough polishing step is divided into two steps, a first polishing step and a second polishing step, and the finish polishing step will be described as an example of one step of the third polishing step. May be divided into three or more steps, and the finish polishing step may be divided into two or more steps.

The first polishing step is a rough polishing step, in which the main surface of the glass substrate is polished with abrasive grains of cerium oxide, aluminum oxide, or zirconium oxide. In particular, it is preferable to use cerium oxide abrasive grains because it is easy to achieve both a polishing rate and a polished surface quality.

In the first polishing step, the main surface of the substrate is polished with a polishing slurry containing abrasive grains having an average particle size of usually 0.5 to 2.0 μm. At this time, polishing may be performed using the polishing slurry and a urethane polishing pad. A suede pad is preferable as the polishing pad. The suede pad is typically a polishing pad in which a foamed urethane sheet having a foam layer and a base layer are laminated, and the foamed hole is long in the thickness direction of the sheet. is there. In addition, the average particle diameter of this embodiment shall be based on the volume-based average particle diameter (dispersion diameter) measured using the laser diffraction / scattering method.

The first polishing step may be divided into two or more polishing steps using different polishing conditions and polishing apparatuses such as different abrasive grain size, polishing slurry composition, and polishing pad.

At that time, by using a three-dimensional surface structure analysis microscope (for example, NV200 manufactured by Zygo), a fine waviness (Wa) measured in a wavelength region of 1 mm × 0.7 mm under the condition of λ ≦ 0.25 mm, for example, Polish to 1 nm or less.

The polishing amount of the glass substrate in this polishing is approximately 20 to 50 μm, but is appropriately set so that the arithmetic average roughness Ra on the main surface of the glass substrate after the second polishing step is 0.2 nm or less. That's fine.

Next, the second polishing process will be described. The second polishing step is also a rough polishing step, which is a step of polishing the main surface of the glass substrate with a colloidal silica slurry (hereinafter also referred to as a colloidal silica polishing step). In the colloidal silica polishing step, the main surface of the glass substrate is polished using a colloidal silica slurry and a polishing pad. The colloidal silica abrasive has an average particle size of 30 nm or less, more preferably 20 nm or less. The polishing pad is preferably a suede pad. The glass substrate polished with the colloidal silica slurry is polished so that the Ra of the main surface measured by AFM is 0.2 nm or less.

As described above, this second polishing step is also divided into two or more polishing steps using different polishing conditions and polishing apparatuses such as colloidal silica abrasive grain size, colloidal silica slurry composition, and different polishing pads. It may be polished so that Ra of the main surface of the glass substrate finally measured by AFM is 0.2 nm or less.

Next, the third polishing process will be described. The third polishing process is a process that characterizes the present invention and is a finish polishing process.

As the polishing slurry used in the final polishing process, the present applicants conducted the following examination. That is, as described above, colloidal silica has a high affinity with a glass substrate when the particle diameter is 10 nm or less, and even if it adheres firmly to the glass substrate and is polished, Ra (arithmetic mean roughness) Will rise. When a glass substrate is polished with particles of colloidal silica having a particle diameter smaller than 10 nm, it is considered that Ra of the substrate can be lowered if the glass substrate can be cleaned appropriately. However, there is a high possibility that the abrasive grains are firmly attached to the glass substrate by a dehydration condensation reaction between the colloidal silica and the hydroxyl group on the surface of the glass substrate. Therefore, the present applicants considered that the adhesion to the glass substrate can be changed by the material of the particles, and investigated particles having an average particle diameter of 15 nm or less. As a result, as compared with colloidal silica smaller than 10 nm, it was found that Ra was reduced by using particles (inactive) having low reactivity with the glass substrate, and the target Ra of 0.05 nm or less could be realized. .

Therefore, the final polishing step of the present embodiment is a polishing that includes at least one type of abrasive selected from the group consisting of diamond, aluminum oxide, and cerium oxide having an average particle diameter of 15 nm or less on the main surface of the glass substrate. Polishing is performed using the slurry.

Regarding the measurement of the average particle diameter of the polishing slurry, it is preferable to use the BET diameter calculated from the BET specific surface area. Further, the particle diameter may be measured from a transmission electron microscope image.

Specifically, the polishing slurry using diamond preferably has an average particle diameter of 10 nm or less. More preferably, it is 4 nm or less. Further, the maximum diameter is more preferably 30 nm or less. Incidentally, the polishing slurry using aluminum oxide or cerium oxide preferably has an average particle diameter of 20 nm or less. More preferably, it is 15 nm or less.

The above-described polishing slurry is a so-called aqueous medium, and the slurry contains water. Further, it may contain a water-soluble polymer, oligomer or monomer. Specifically, (A) polyacrylic acid, polyacrylic acid ammonium salt, polyacrylic acid sodium salt, polymethacrylic acid, polymethacrylic acid ammonium salt, polymethacrylic acid sodium salt, polyamide, polyamic acid ammonium salt, polyamic acid sodium salt Polycarboxylic acids such as salts, salts of polycarboxylic acid salts and copolymers thereof, (B) polyvinyl polymers such as polyvinyl alcohol and polyvinylpyrrolidone, (C) organic acid salts such as citrate and oleate, Diphosphate and the like are used.

The pH of the polishing slurry is preferably 4-9. If the pH is 4 or less, a leaching layer is generated on the glass surface, and the glass surface becomes rough when cleaning with an alkaline cleaning agent after polishing. On the other hand, when the pH is 9 or more, deterioration of the urethane resin contained in the suede pad, the nonwoven fabric pad, and the hard urethane pad used as the polishing pad is not preferable.

In order to stabilize the dispersibility of the particles in the polishing slurry, various dispersants may be appropriately added.
For example, an organic acid salt such as sodium citrate and sodium oleate, an anionic polymer such as a polyacrylate and polysulfonate, or a salt of a copolymer of polyacrylic acid and polymaleic acid can be used.

The solid content concentration of the polishing slurry is preferably 0.0001 to 0.1% by weight. If it is less than 0.001% by weight, there is no polishing ability, and if it is 0.1% by weight or more, scratches occur on the glass substrate due to agglomeration of abrasive grains, which is not preferable. More preferably, the content is 0.001 to 0.01% by weight.

Since the solid content concentration of the polishing slurry of this embodiment is very low as described above, an increase in cost can be suppressed even if a relatively expensive diamond slurry is used. Incidentally, the solid content concentration of the diamond slurry may be 0.01%.

As the polishing pad to be used, a suede pad, a nonwoven fabric pad, or a hard urethane pad used in conventional colloidal silica polishing can be used. In this embodiment, a suede pad is preferable. The suede pad of this embodiment is a polishing pad in which a foamed urethane sheet having a foam layer and a base layer are laminated as a representative, and the foamed holes are long in the thickness direction of the sheet. There is a polishing pad. As a foundation layer of this polishing pad, for example, one using a PET film can be used.

By polishing with a polishing slurry containing any one of diamond, aluminum oxide, and cerium oxide having an average particle diameter of 15 nm or less, the main surface of the glass substrate is subjected to atomic force microscopy ( Ra measured by AFM) has a flatness of 0.05 nm or less.

研磨 After polishing, wash to remove abrasive grains. And a glass substrate is dried after washing | cleaning. As a drying method, a drying method using isopropyl alcohol vapor, spin drying, vacuum drying, or the like is used.

The glass substrate of the present invention is obtained by the above series of steps. Further, the magnetic recording medium (magnetic film) is coated on the main surface to obtain the magnetic recording medium of the present invention.

As described above, since the glass substrate of the present embodiment has excellent flatness (smoothness) with an Ra of 0.05 nm or less, further higher recording capacity and higher accuracy required for magnetic recording media in the future. It can respond enough.

In addition, since the present embodiment is configured to perform the final polishing process using an inert and fine polishing slurry having a small average particle diameter, the orientation of the magnetic film formed on the glass substrate is improved, and further memory is obtained. Higher density can be expected. Further, in a heat-assisted magnetic recording type hard disk, the distance between the head that oscillates near-field light that heats the magnetic film and the substrate is 10 nm or less, and the unevenness of the irradiation spot is made smaller, thereby making the substrate more uniform. Can be heated. Then, the effect of improving the stability of writing can be expected.

Hereinafter, the present invention will be further described with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

(Production of glass substrate)
Aluminosilicate glass A formed by the float process (mol% display content is SiO ₂ : 67.7%, Al ₂ O ₃ : 4.9%, MgO: 10.9%, TiO ₂ : 4%, Na ₂ O: 4.9%, K ₂ O: 7.6% Al ₂ O ₃ —SiO ₂ glass plate) and aluminosilicate glass B (in terms of mol%, SiO ₂ : 67%, Al ₂ O ₃ : 13%, B ₂ O ₃ : 1%, MgO: 9%, CaO: 5.5%, SrO ;: 4.5% Al ₂ O ₃ —SiO ₂ glass plate) with an outer diameter of 65 mm The glass substrate A and the glass substrate B (glass substrate having a circular hole in the center) were obtained so that a glass substrate having an inner diameter of 20 mm and a plate thickness of 0.635 mm was obtained. The inner peripheral surface and the outer peripheral surface were ground using a diamond grindstone, and the upper and lower surfaces of the glass substrate were lapped using aluminum oxide abrasive grains.

Next, the end surfaces of the inner and outer circumferences were chamfered so that the chamfering width was 0.15 mm and the chamfering angle was 45 °. After the inner and outer peripheral processing, a cerium oxide slurry was used as an abrasive, a brush was used as a polishing tool, and the end face was mirror-finished by brush polishing. The removal amount in the radial direction was 30 μm.

Thereafter, a cerium oxide polishing step (first polishing step: rough polishing step) was performed. That is, the upper and lower main surfaces of the glass substrate were polished by a double-side polishing apparatus using a cerium oxide slurry (cerium oxide average particle size: about 1.1 μm) as an abrasive and a suede pad as a polishing tool. The amount of polishing was 35 μm in total in the thickness direction of the upper and lower main surfaces.

Thereafter, a colloidal silica polishing step (second polishing step: rough polishing step) was performed. That is, the upper and lower main surfaces of the glass substrate were polished by a double-side polishing apparatus using a colloidal silica slurry (colloidal silica average particle size: about 20 nm) as an abrasive and a suede pad as a polishing tool. The amount of polishing was 2.6 μm in total in the thickness direction of the upper and lower main surfaces.

Also, the glass substrate was polished so that the arithmetic average roughness Ra on the main surface was 0.15 to 0.2 nm.

Next, a final polishing step (third polishing step) was performed using the following test slurries A to D and the following polishing pads F and G.

(Test slurry A)
While 4.9 L of distilled water was irradiated with ultrasonic waves, 0.1 L of diamond slurry (product name μ Diamond Andante) manufactured by Air Brown Co., Ltd. was added to prepare a test slurry A. The slurry had a pH of 4.5 and a solid content concentration of 0.01% by weight. A diamond abrasive having an average particle diameter of 4 nm was used.

(Test slurry B)
0.2 g of sodium oleate was added to 0.1 L of distilled water and stirred. While stirring, 2 g of aluminum oxide particles (average particle size 15 nm) manufactured by NANAREAN NANOPRODUCT TECHNOLOGY were added, and bead mill dispersion was performed to prepare dispersion B. 2 g of dispersion B and 500 g of pure water were mixed to prepare test slurry B. The slurry had a pH of 4.2 and a solid content concentration of 0.008% by weight.

(Test slurry C)
0.2 g of sodium oleate was added to 0.1 L of distilled water and stirred. While stirring, 2 g of cerium oxide particles (average particle size: 15 nm) manufactured by NANAREAN NANOPRODUCT TECHNOLOGY were added, and bead mill dispersion was performed to prepare dispersion C. Test slurry C was prepared by mixing 2 g of dispersion C, 500 g of water, and 0.0014 g of citric acid. The slurry had a pH of 3.9 and a solid content concentration of 0.008% by weight.

(Test slurry D)
10.0 mL of nitric acid was added to 1.52 L of distilled water and stirred. While stirring, 1.48 L of colloidal silica (product name ST-XL) manufactured by Nissan Chemical Industries, Ltd. was added to prepare a test slurry D. The pH of the slurry was 1.9. The average particle diameter was 7 nm.

(Polishing pad F)
The suede pad was made of urethane foam, and the physical properties were a Shore A hardness of 65.0 °, a compressibility of 2.3%, and a density of 0.68 g / cm ³ .

(Polishing pad G)
The physical properties of the nonwoven fabric pad were as follows: Shore A hardness 51.5 °, compression rate 11.7%, and density 0.28 g / cm ³ .

[Examples 1 and 5]
The main surfaces of the glass substrate A (Example 1) and the glass substrate B (Example 5) after the colloidal silica polishing step are subjected to a speed fam 9B double-side polishing machine using the test slurry A and the polishing pad F. Polished for 5 minutes.

[Examples 2 and 6]
The main surfaces of the glass substrate A (Example 2) and the glass substrate B (Example 6) after the colloidal silica polishing step are subjected to 5 by a speed fam 9B double-side polishing machine using the test slurry A and the polishing pad G. Polished for a minute.

[Examples 3 and 7]
The main surfaces of the glass substrate A (Example 3) and the glass substrate B (Example 7) after the colloidal silica polishing step are subjected to 5 by a speed fam 9B double-side polishing machine using the test slurry B and the polishing pad F. Polished for a minute.

[Examples 4 and 8]
The main surfaces of the glass substrate A (Example 4) and the glass substrate B (Example 8) after the colloidal silica polishing step were subjected to 5 by a speed fam 9B double-side polishing machine using the test slurry C and the polishing pad F. Polished for a minute.

[Comparative Examples 1 and 2]
The main surfaces of the glass substrate A (Comparative Example 1) and the glass substrate B (Comparative Example 2) after the colloidal silica polishing step are subjected to 5 by a speed fam 9B double-side polishing machine using the test slurry D and the polishing pad F. Polished for a minute.

In Examples 1 to 8 and Comparative Examples 1 and 2, after each of the above-described finish polishing steps, pure water shower cleaning, scrub cleaning with Berglin and water, scrub cleaning with Berglin and an alkaline detergent, scrub cleaning with Berglin and water Cleaning and pure water shower cleaning were sequentially performed, and air blowing was performed.

Thereafter, AFM measurement was performed using AFM (trade name Cypher) manufactured by Allaisam, and Ra on the main surface was determined. The measurement area was 1 × 1 μm, and the number of measurement points was measured at 256 points (resolution of 256 × 256 pixels) in both X and Y directions. The results are shown in Table 1.

From Table 1, using glass substrate A, when diamond particles (test slurry A), aluminum oxide particles (test slurry B), and cerium oxide particles (test slurry C) are used (Examples 1 to 4), Ra is It was confirmed to be 0.05 nm or less. On the other hand, when colloidal silica particles (test slurry D) are used (Comparative Example 1), Ra is 0.68 nm, and it is presumed that Ra has increased due to affinity and the like.

Further, when test slurry A to test slurry C were used on glass substrate B (Examples 5 to 8), it was confirmed that Ra was 0.05 nm or less. On the other hand, when colloidal silica particles (test slurry D) are used (Comparative Example 2), Ra is 0.72 nm, and it is presumed that Ra has increased due to affinity and the like.

Although the embodiments and examples of the glass substrate and the manufacturing method thereof, and the magnetic recording medium have been described above, the present invention is not limited to the above-described embodiments and examples, and the present invention described in the claims. Various modifications and improvements can be made within the scope of the present invention. For example, although the example in which the polishing process of the main surface of the glass substrate has three polishing processes has been shown, this is not restrictive. In short, other methods may be performed for the pre-process before the finish polishing process.

In addition, this application claims priority based on basic application No. 2015-172727 filed on September 2, 2015 in Japan, and the entire contents of these Japanese patent applications are incorporated herein by reference. To do.

Claims (9)

1. In the method for producing a glass substrate for information recording medium for polishing a glass substrate,
   By the rough polishing step of polishing the main surface of the glass substrate, the arithmetic average roughness Ra on the main surface when measured with a resolution of 256 × 256 pixels at 1 μm × 1 μm square using an atomic force microscope is 0.2 nm. The following glass substrates are manufactured,
   Thereafter, the glass substrate is polished by a final polishing step in which polishing is performed using a polishing slurry containing at least one abrasive selected from the group consisting of diamond, aluminum oxide and cerium oxide having an average particle size of 15 nm or less. A method for producing a glass substrate for an information recording medium.
2. The method for producing a glass substrate for an information recording medium according to claim 1, wherein the abrasive grains of the polishing slurry used in the final polishing step are diamond having an average particle diameter of 10 nm or less.
3. 3. The method for producing a glass substrate for an information recording medium according to claim 1, wherein the solid content concentration of the polishing slurry used in the finish polishing step is 0.0001 to 0.1% by weight.
4. 4. The method for producing a glass substrate for an information recording medium according to claim 1, wherein the arithmetic average roughness Ra on the main surface of the glass substrate is set to 0.05 nm or less by the finish polishing step.
5. The method for producing a glass substrate for an information recording medium according to any one of claims 1 to 4, wherein the rough polishing step is polishing with a polishing slurry containing abrasive grains of aluminum oxide or cerium oxide.
6. 6. The glass substrate for an information recording medium according to claim 5, wherein the rough polishing step comprises a polishing step with a polishing slurry containing cerium oxide abrasive grains, and a subsequent polishing step with a polishing slurry containing colloidal silica abrasive grains. Manufacturing method.
7. A method for producing an information recording medium, wherein a magnetic recording layer is formed on the main surface of the glass substrate for information recording medium produced by the method for producing a glass substrate for information recording medium according to any one of claims 1 to 6.
8. A disk-shaped information recording medium glass substrate having a circular hole in the center,
   A glass substrate for an information recording medium, wherein the arithmetic average roughness Ra on the main surface of the glass substrate measured at a resolution of 256 × 256 pixels at 1 μm × 1 μm square using an atomic force microscope is 0.05 nm or less .
9. A magnetic recording medium, wherein a magnetic recording layer is provided on the main surface of the glass substrate for information recording medium according to claim 8.