JP2013133249A - Method for producing glass substrate for hdd, and glass blank for hdd and glass substrate for hdd obtained by the production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a glass substrate for HDD having little strain inside glass blanks, and hardly generating a crack in a high-temperature heat treatment step in a production step of the glass substrate, and to provide glass blanks and a glass substrate obtained by the production method.SOLUTION: This method for producing a glass substrate for HDD has a blank molding step of cutting molten glass drooping from an outflow nozzle by a cutting means to obtain a glass gob, and press-molding the glass gob to obtain glass blanks; a heating/heat retaining step of heating the glass gob cut by the cutting means, by a heating means or the like provided below the cutting means, or the like; and a press molding step of press-molding the glass gob. Glass blanks and a glass substrate obtained by the production method are also provided.

Description

  The present invention relates to a method for manufacturing a glass substrate for HDD, a glass blank for HDD obtained by the manufacturing method, and a glass substrate for HDD. More specifically, a method for producing a glass substrate for HDD that is less distorted in the glass blanks and is less susceptible to cracking in the process of applying high heat in the production process of the glass substrate, and a glass blank for HDD obtained by the production method, and The present invention relates to a glass substrate for HDD.

  In recent years, with the improvement in performance of disk devices (for example, hard disk drives and HDDs) equipped with information recording media, the quality level required for the media used has increased. The medium refers to a disk in which a magnetic material is formed on a glass substrate for HDD (hereinafter sometimes simply referred to as a glass substrate).

  In order to dramatically increase the storage capacity of the HDD, fine magnetic particles are used. However, since the fine particles of the Co-based magnetic material used in the past have a relatively weak coercive force, there is a possibility of causing so-called “thermal fluctuation” in which recorded data disappears due to the influence of ambient heat. is there. Therefore, it has been studied to use a magnetic material having a high coercive force. For magnetic materials with high coercivity, a high-temperature heat treatment after film formation (assist recording, heat assist method) can be used to form a highly heat-resistant magnetic film, eliminating "thermal fluctuation". . Thus, in the magnetic film forming process (especially when the heat assist method is employed), the glass substrate is subjected to a high temperature heat treatment.

  In order to withstand such a high temperature heat treatment, it is necessary to increase the heat resistance temperature (glass transition temperature, Tg) of the glass substrate. One means for this is to increase the content ratio of alkaline earth metal oxides as the glass material constituting the glass substrate, but the glass material with a high heat resistance temperature is a general glass substrate material. As compared with, the slope of the viscosity curve of glass tends to be steep. That is, in press molding, after the molten glass is discharged from the melting nozzle, the viscosity rapidly increases in the process of being pressed. Therefore, glass with a steep slope in the viscosity curve of glass has a rapid solidification of the glass melt that has flowed out of the melting nozzle, making it difficult to control the shape of the glass blanks during press molding. Met.

  On the other hand, as a technique for examining the molding conditions (especially heating conditions) in the press molding of glass materials, the tip of the melting nozzle is covered in order to reduce the devitrification of the glass material when discharging the molten glass from the melting nozzle. A technique for heating the glass melt indirectly by heating with a burner is proposed (Patent Document 1). In the apparatus of Patent Document 1, the tip of the nozzle is uniformly heated by the radiant heat from the cover, the devitrification of the glass melt is suppressed, and the flow rate is adjusted.

  However, even when an apparatus such as that described in Patent Document 1 is used, if a glass material with a short heat resistance and a short foot (that is, a glass material with a steep viscosity curve) as described above is used, the temperature is high. In the case of forming a magnetic material, the slight deformation may occur or the glass material may break in some cases. In the glass substrate for HDD, such a slight deformation causes a problem in recording and reproduction, which causes a problem in that the yield rate is reduced. Moreover, it becomes clear that such a deformation | transformation and a crack problem generate | occur | produce also when performing a chemical strengthening process with respect to a glass substrate not only in the film-forming process of a magnetic layer, but the distortion (( It was found that this was caused by residual strain.

Japanese Patent Laid-Open No. 10-1318

  The present invention has been made in view of the above-described conventional problems. For HDDs, there is little residual strain inside the glass blanks, and deformation and cracking are less likely to occur in the process of applying high heat in the glass substrate manufacturing process. It aims at providing the manufacturing method of a glass substrate, the glass blanks for HDD obtained by this manufacturing method, and the glass substrate for HDD.

  As a result of further studies by the present inventor, such residual strain is mostly generated during press molding, and annealing processing (residual strain due to heating is reduced as usual) in glass with extremely high heat resistance. It has become clear that even if the process is performed, the problem cannot be solved sufficiently. Conventionally, such residual distortion has been difficult to detect and has not been identified as a problem. However, by detecting the residual distortion by detecting the amount of retardation (phase difference) in transmitted light, the problem becomes apparent. It came to do.

  As one aspect of the present invention for solving the above problems, the molten glass suspended from the outflow nozzle is cut by a cutting means into a glass gob, and the glass gob is supported by a table and then press-molded to form a glass blank. A method for manufacturing an HDD glass substrate having a blank forming step, wherein the glass gob cut by the cutting means is heated or kept warm by a heating means or a heat retaining means provided below the cutting means. It is a manufacturing method of the glass substrate for HDD which has a heating heat retention process and the press molding process of press-molding the said glass gob.

  According to this aspect, by having such a configuration, it is possible to adjust the temperature of the glass gob before press molding to be high, and it is difficult for internal strain to remain in the glass blanks obtained by less molding. As a result, in the high heat-resistant material, the residual strain generated by the conventional method can be sufficiently eliminated, and a glass substrate that is less likely to be deformed or cracked even after high-temperature heat treatment in a later process can be obtained.

  The heating means is preferably at least one of a gas burner or a heater.

  By using at least one of the gas burner and the heater, the glass gob cut by the cutting means is heated to a high temperature and is not easily distorted inside during press molding.

  The heat retaining means is preferably a heat insulating material.

  By using the heat insulating material, the glass gob cut by the cutting means is kept at a high temperature and is less likely to be distorted inside during press molding.

  The difference Δ between the viscosity log ηa at 1100 ° C. and the viscosity log ηb at 1500 ° C. of the glass gob is preferably 2.0 dPa · s or more.

  The glass gob having such a configuration has a large rate of increase in viscosity before press molding, and the viscosity becomes too high during press molding, and the press tends to cause distortion inside. Since the present invention can be adjusted to a high temperature before press molding even for such a glass gob, the glass blanks obtained are less likely to be distorted.

  The glass transition temperature (Tg) of the molten glass is preferably Tg> 600 ° C.

  A glass substrate obtained by using a molten glass having such a configuration is preferably used as a material for manufacturing a disk having a large storage capacity from the viewpoint of being able to withstand high-temperature heat treatment. In the case where such molten glass is used, the present invention can suppress the occurrence of internal distortion of the obtained glass blanks, so that the resulting glass substrate has few defects and can improve the production yield. Moreover, the glass substrate obtained from such a molten glass can perform the chemical strengthening process which is a high temperature heat treatment process, and can obtain the glass substrate which was more excellent in various physical properties, such as impact resistance and heat resistance.

  It is preferable that the manufacturing method of the glass substrate of this invention is a manufacturing method of the glass substrate for HDD for assist recording.

  The assist recording method is an indispensable recording method for obtaining a disk having a large storage capacity. The assist recording method requires a glass substrate provided with a magnetic film having a high coercive force. A high coercivity magnetic film is formed on a glass substrate and then heat-treated at a high temperature. The glass substrate obtained by the method for producing a glass substrate of the present invention has few internal strains, so that there are few cracks in high-temperature heat treatment, and a glass substrate having a high coercive force magnetic film can be efficiently produced by a heat assist method. Therefore, it is particularly useful when manufacturing an HDD glass substrate for assist recording.

  It is preferable that the manufacturing method of the glass substrate of this invention further has a chemical strengthening process.

  By chemically strengthening glass blanks with little internal distortion, the surface of the glass substrate obtained is fully strengthened. As a result, the physical properties of the glass substrate such as heat resistance and impact resistance are improved.

  It is preferable that the glass raw material which comprises the said molten glass contains 15-30 mol% alkaline-earth metal oxide.

  When the glass material contains 15 to 30 mol% of an alkaline earth metal oxide, Tg is improved and the meltability of the molten glass is improved. As a result, it is easy to produce glass blanks with low viscosity and less internal distortion during press molding.

The glass blank for HDD of the present invention is a glass blank for HDD manufactured by the above-described method for manufacturing a glass substrate for HDD, and has a center hole, the radius of the center hole being r ₀ , and the radius of the glass blank being r. _{When 1} , the maximum value of the retardation amount when linearly polarized light is incident in the direction perpendicular to the recording surface is 20 nm / mm or less over the entire circumference at the position of the radius r defined by the following formula (I). It is preferable.

r = r ₀ + (r ₁ −r ₀ ) / 2 (I)
Glass blanks have a maximum retardation amount, so that the shape is stable, and deformation and cracking are less likely to occur even when subjected to high-temperature heat treatment.

The glass substrate for HDD of the present invention is a glass substrate for HDD manufactured by the above method for manufacturing a glass substrate for HDD, and has a central hole, the radius of the central hole being r ₀ ′, and the radius of the glass substrate being When r ₁ ′ is set, the maximum value of the retardation amount when linearly polarized light is incident in the direction perpendicular to the recording surface is 1 nm / mm over the entire circumference at the position of the radius r ′ defined by the following formula (II). The following is preferable.

r ′ = r ₀ ′ + (r ₁ ′ −r ₀ ′) / 2 (II)
Since the glass substrate has the maximum value of the retardation amount, the shape is stable, and deformation and cracking are less likely to occur when an impact or the like is applied.

  The retardation amount (nm) can be used as an index representing the strain (internal strain) of glass blanks and is represented by the following formula.

Re = (nx−ny) × d
However, nx represents the refractive index of the direction x with the largest refractive index in the plane direction of glass blanks (or glass substrate), ny represents the refractive index in the y direction orthogonal to the x direction, and d represents the glass blanks. (Or glass substrate) thickness (nm unit).

As a measuring method, the amount of retardation in the entire circumference at the position of radius r = r ₀ + (r ₁ −r ₀ ) / 2 of the glass blank is linearly polarized on the glass blank using PA-100 (Photonics Lattice). It is possible to measure by observing the change of the polarization state after passing.

  ADVANTAGE OF THE INVENTION According to this invention, there is little distortion inside a glass blank, and the manufacturing method of the glass substrate for HDD which hardly produces a deformation | transformation and a crack in the process in which the high heat | fever in the manufacturing process of a glass substrate is given, For HDD obtained by this manufacturing method Glass blanks and glass substrates for HDDs can be provided.

FIG. 1 is an explanatory diagram of a blank forming step in the method for manufacturing a glass substrate of one embodiment (Embodiment 1) of the present invention. FIG. 2 is an explanatory diagram of a press used in the method for manufacturing a glass substrate according to one embodiment (Embodiment 1) of the present invention. FIG. 3 is an explanatory diagram of a press used in the method of manufacturing a glass substrate according to one embodiment (Embodiment 1) of the present invention. FIG. 4 is an explanatory diagram of a blank forming step in the method for manufacturing a glass substrate according to one embodiment (Embodiment 1) of the present invention. FIG. 5 is an explanatory view of a glass gob press-molded in a press machine in the method for manufacturing a glass substrate according to one embodiment (Embodiment 1) of the present invention. FIG. 6 is an explanatory view for explaining the position at which the retardation of the glass blank or glass substrate of one embodiment (Embodiment 1) of the present invention is measured. FIG. 7 is a viscosity curve of the molten glass produced by the method for producing a glass substrate of Example 1 of the present invention.

(Embodiment 1)
Hereinafter, the manufacturing method of the glass substrate of this embodiment is demonstrated in detail. In the present embodiment, as shown in FIG. 1, when the molten glass 2 suspended from the melting nozzle 1 is cut by the blade 3, it is heated by a gas burner 4 (heating means) provided below the blade 3. And a blank forming process of press-molding the heated glass gob 5 with a press 6. In addition to the blanks molding school specification, the glass substrate manufacturing method of the present embodiment has, for example, a glass melting step before the blanks molding step, and after the blanks molding step, a heat treatment step, a coring step, a grinding step, It has a rough polishing process, a mirror polishing process, and a final cleaning process. Hereinafter, the blanks molding process which is a characteristic part of the present embodiment will be described in detail. In the present invention, the glass gob refers to a molten glass lump obtained by cutting molten glass with a blade. Moreover, a glass blank means the intermediate molded object of a glass substrate.

<Blanks molding process>
In the blank forming process, the molten glass melted by the glass melting process described later is suspended from the melting nozzle, and the molten glass is heated by a gas burner provided below the position at which the molten glass is cut by a blade (cutting means). It has a process (heating heat retention process) and a press molding process for press molding the obtained glass gob.

It does not specifically limit as a material of the glass raw material which comprises molten glass. For example, as the material of the glass material constituting the molten glass, soda lime glass, aluminosilicate glass, borosilicate glass, Li ₂ O—SiO ₂ glass, Li ₂ O—Al ₂ O ₃ —SiO ₂ glass, R 'O-Al ₂ O ₃ -SiO ₂ -based glass (R' = Mg, Ca, Sr, Ba) or the like can be employed.

  Especially, it is preferable that the glass raw material which comprises molten glass contains 15-30 mol% alkaline-earth metal oxide. For example, it is preferable to contain 5-15 mol% MgO, 5-15 mol% CaO, 0-5 mol% SrO, 0-5 mol% BaO. Moreover, although it is not an alkaline-earth metal oxide, ZnO may be contained as necessary because the same effect can be obtained. Hereinafter, in the present invention, ZnO is treated similarly as an alkaline earth metal oxide without particular notice.

  When the glass raw material which comprises a molten glass contains the alkaline-earth metal of the said range, Tg of the molten glass obtained improves and the meltability of a molten glass improves. As a result, it is easy to produce glass blanks with low viscosity and less internal distortion during press molding. When the content of the alkaline earth metal oxide is less than 15 mol%, there is a tendency that the effect of improving Tg and improving the meltability cannot be obtained sufficiently. On the other hand, when the content of the alkaline earth metal oxide exceeds 30 mol%, the tendency of devitrification is increased and the glass tends to be unstable.

  Moreover, it does not specifically limit as a melting method of glass, Usually, the method of fuse | melting the said glass raw material at high temperature by well-known temperature and time is employable.

  It is preferable that the glass transition temperature (Tg) of the molten glass used with the manufacturing method of the glass substrate of this embodiment exceeds 600 degreeC. When a glass substrate is manufactured using a molten glass having a Tg exceeding 600 ° C., the obtained glass substrate can withstand high-temperature heat treatment. Therefore, the glass substrate can be preferably used as a material for manufacturing a disk having a large storage capacity. In the manufacturing method of the glass substrate of this embodiment, when such molten glass is used, generation | occurrence | production of the internal distortion of the glass blanks obtained can be suppressed. Therefore, the obtained glass substrate has few defects such as deformation and cracking. As a result, the glass substrate manufacturing method of the present invention can improve the yield during manufacturing. Moreover, the glass substrate obtained from such a molten glass can perform the chemical strengthening process which is a high temperature heat treatment process mentioned later, and is more excellent in various physical properties, such as impact resistance and heat resistance.

  An example of the process of obtaining glass blanks from molten glass will be described with reference to FIGS. 2 to 4 illustrate a press that can be used in the direct press method. The direct press method is a method in which molten glass is supplied and pressure molded.

  First, as shown in FIG. 2, the molten glass is press-molded by a press machine 6 having a rotary table 6a. The molten glass 2 is suspended from the melting nozzle 1 on the lower mold 6c of the press machine lower part 6b arranged on the rotary table 6a. The turntable 6a is controlled to be rotatable in the direction of arrow A1 by a control mechanism (not shown). As shown in FIG. 3, the press machine lower part 6b is provided on the rotary table 6a. Lower molds 6c are respectively provided on the upper surface of the press machine lower part 6b.

  As shown in FIG. 4A and FIG. 4B, the blade 3 cuts the molten glass 2 suspended from the melting nozzle 1 from both sides. The cut molten glass 2 becomes a glass gob 5 and is placed on the lower mold 6c. The timing at which the blade 3 cuts the molten glass 2 is not particularly limited. The blade 3 is driven at predetermined time intervals in accordance with the speed and amount of the molten glass 2 to be hung, and continuously cuts the molten glass 2 to produce the glass gob 5. The shape of the glass gob 5 supplied onto the lower mold 6c is preferably controlled so as to have axial symmetry. In such a state, the disk-shaped glass blanks 7 having excellent flatness are easily obtained by press molding. The turntable 6a rotates after confirming that the glass gob 5 is placed on the lower mold 6c.

  As shown in FIGS. 1 and 4, in the present embodiment, the molten glass 1 is heated by the gas burner 4 below the blade 3 (heating process). The position where the gas burner 4 is provided is not particularly limited as long as the glass gob 5 cut by the blade 3 can be heated. For example, the gas burner 4 may be provided and heated only below the blade 3, or may be provided and heated only at a location where the glass gob 5 moves after the turntable 6 a rotates, and provided at both positions. You may heat. The number of gas burners 4 installed is not particularly limited, and may be one or more. The gas burner 4 may be disposed at a position where the gas burner 4 can be heated until immediately before being press-molded by the press machine 6 (see the gas burner 4a), and may be accommodated so as not to contact the press machine at the time of press molding.

  The molten glass 2 of 1300 to 1350 ° C. drooped from the melting nozzle 1 is heated by the gas burner 4 and maintained at about 1250 to 1300 ° C. The temperature at the outlet of the gas burner 4 is about 1500 to 1600 ° C. The mold including the lower mold 6c is kept at a temperature of about 550 to 650 ° C., for example. Therefore, at the time of press molding (see FIG. 5), the temperature of the glass gob is maintained at about 1150 to 1250 ° C. As shown in FIG. 5, the glass gob is pressed with an upper mold 6e of the upper press 6d and a lower mold of the lower press on which the glass gob is placed, and is molded into glass blanks (press molding process).

  In the press molding step, it is preferable to perform butt molding in which the thermal conductivity of the mold is 120 W / m · K or more and the press time is 0.05 to 0.20 seconds. The pressing time at this time is preferably 0.05 to 0.20 seconds, more preferably 0.08 to 0.17 seconds.

  As a pressurizing means for applying pressure to the lower mold and the upper mold to pressurize the molten glass, a known pressurizing means can be appropriately employed. For example, an air cylinder, a hydraulic cylinder, an electric cylinder using a servo motor, or the like can be employed.

  If a glass substrate is produced using the manufacturing method of the glass substrate of this embodiment, the temperature of the glass gob before press molding can be adjusted to the said temperature. Therefore, the glass gob having a steep gradient of the viscosity curve of the glass such that the difference Δ between the viscosity log ηa at 1100 ° C. and the viscosity log ηb at 1500 ° C. is 2.0 dPa · s or more, more preferably 2.2 dPa · s or more. Even so, it is possible to reduce the distortion generated inside during press molding. Therefore, according to the glass substrate manufacturing method of the present embodiment, the magnetic film can be formed using a high coercive force material, which can contribute to an increase in the capacity of the disk.

  Moreover, the obtained glass substrate can be used as a glass substrate for HDD for assist recording. Here, the assist recording method is an essential recording method for obtaining a disk having a large storage capacity. The assist recording method requires a glass substrate provided with a magnetic film having a high coercive force. A high coercivity magnetic film is formed on a glass substrate and then heat-treated at a high temperature. Since the glass substrate obtained by the method for producing a glass substrate of the present invention has less internal strain, it is less likely to be deformed or cracked during high-temperature heat treatment, and a glass substrate having a high coercive force magnetic film is efficiently produced by a heat assist method. obtain. Therefore, it is particularly useful when manufacturing an HDD glass substrate for assist recording. The heat assist method will be described in detail in the magnetic film forming step described later.

  Next, other steps that can be adopted by the present invention will be described. In addition, the manufacturing method of the glass substrate of this invention should just have the above-mentioned blanks shaping | molding process, Although there are preferable conditions in each process about other processes, it is not specifically limited. For this reason, the other steps described below are examples, and the design can be changed as appropriate.

<Glass melting process>
The glass melting step is a step of melting a glass material. The glass melting method is not particularly limited, and a method of melting the above glass material at a high temperature at a known temperature and time can be usually employed.

<Blanks molding process>
The blank molding process is as described above. According to the manufacturing method of the glass substrate of this embodiment, since the temperature of the glass gob is kept high at the time of press molding, internal distortion is hardly caused by press molding.

<Heat treatment process>
The heat treatment step is a step aimed at correcting the flatness of glass blanks and removing internal strain. Although it does not specifically limit as a method of heat processing, For example, the method of using a setter (alumina, zirconia, etc.) and stacking alternately with glass blanks, putting into a heat processing furnace, and applying heat can be employ | adopted.

  The conditions for the heat treatment are not particularly limited. For example, the temperature during the heat treatment can be performed in a temperature range from Tg to Tg + 100 (° C.) of the glass blank. By performing the heat treatment within the temperature range, the flatness of the glass blanks can be sufficiently corrected, the deterioration of the shape of the glass blanks is reduced, and the adhesion marks caused by the fusion with the setter are further reduced. The possibility of occurring can be reduced.

<Coring process>
A coring process is a process of forming an inner hole (center hole) in the center part of the surface of the obtained glass blanks using a diamond core drill. The center of the glass blanks is determined by this coring process. In addition, in this embodiment, a glass blank means the glass molding before finishing the coring process and performing the grinding process of a main plane.

The glass blank of this embodiment is a glass blank with few internal distortions through a blanks shaping | molding process as above-mentioned. Specifically, as shown in FIG. 6A, the glass blank of the present embodiment has the following formula (I) when the radius of the center hole P is r ₀ and the radius of the glass blank is r _1. The maximum value of the retardation amount when linearly polarized light is incident in the direction perpendicular to the main surface (recording surface) is 20 nm / mm or less over the entire circumference at the position of the defined radius r. More preferably, the maximum value of the retardation amount is 15 nm / mm or less.

r = r ₀ + (r ₁ −r ₀ ) / 2 (I)
The retardation amount (Re) is an index representing strain (internal strain) of glass blanks or glass substrates, and is represented by the following formula.

Re = (nx−ny) × d
(Where nx represents the refractive index in the direction x having the largest refractive index in the plane direction of the glass blank (or glass substrate), ny represents the refractive index in the y direction orthogonal to the x direction, and d represents the glass The thickness (nm) of blanks (or glass substrate).

  If there is distortion (stress variation) inside the glass blank or the glass substrate, the refractive index varies depending on the direction in the glass blank or the glass substrate. Therefore, the internal strain can be measured by measuring the amount of retardation.

  As a measuring method, the amount of retardation in the entire circumference of the position of radius r of the glass blank is passed through the glass blank using PA-100 (Photonics Lattice), and the change in the polarization state after passing is observed. The method of doing can be adopted.

  In addition, by suppressing the amount of retardation to be small, distortion in the recording medium generated by the clamp can be reduced in the clamp method that presses linearly in the circumferential direction as is generally used in a normal HDD, and impact resistance is improved. This can be further improved.

  As described above, if the retardation amount is produced within a predetermined range at the time of producing the glass blanks, the retardation amount of the HDD glass substrate produced using the retardation is naturally reduced. As a result, since the internal distortion of the glass blank is reduced, the impact resistance of the glass blank is improved.

<Grinding process>
The grinding process is a process of lapping the both main surfaces of the obtained glass substrate in the same manner as the shape measurement process. By performing this grinding process, it is possible to remove in advance the fine irregularities formed on the main surface in the coring process and the shape measurement process, which are the previous processes, and the subsequent polishing process on both main surfaces can be performed in a short time. Can be completed with.

<Rough polishing process>
The rough polishing step is a step of polishing both main surfaces of the glass substrate using an abrasive slurry so that the surface roughness finally required in the subsequent mirror polishing step can be efficiently obtained. It does not specifically limit as a grinding | polishing method employ | adopted at this process, It is possible to grind | polish using a double-side polisher.

  As the polishing pad to be used, when the hardness of the polishing pad decreases due to heat generated by polishing, the shape change of the polishing surface becomes large. Therefore, it is preferable to use a hard pad, for example, urethane foam. The polishing liquid preferably uses cerium oxide having an average primary particle diameter of 0.6 to 2.5 μm, and cerium oxide is dispersed in a solvent to form a slurry. It does not specifically limit as a solvent, Neutral water and acidic / alkaline aqueous solution are employable. Moreover, a dispersing agent can be added to these solvents. The mixing ratio of the solvent and cerium oxide is about 1: 9 to 3: 7. When the average primary particle diameter is less than 0.6 μm, the polishing pad tends to fail to polish both main surfaces well. On the other hand, when the average primary particle diameter exceeds 2.5 μm, the polishing pad may deteriorate the flatness of the end face or generate scratches.

  It does not specifically limit as supply_amount | feed_rate of abrasive slurry, For example, it is 5-10L / min.

  The amount of polishing of the glass substrate in the rough polishing step is preferably about 25 to 40 μm. When the polishing amount of the glass substrate is less than 25 μm, scratches and defects tend not to be sufficiently removed. On the other hand, when the polishing amount of the glass substrate exceeds 40 μm, the glass substrate is polished more than necessary, and the production efficiency tends to decrease.

  The glass substrate after the rough polishing step is preferably washed with a neutral detergent, pure water, IPA or the like.

<Mirror polishing process>
The mirror polishing process is a process of polishing both main surfaces of the glass substrate more precisely. In the mirror polishing process, a double-side polishing machine similar to the double-side polishing machine used in the rough polishing process can be used.

  The polishing pad is preferably a soft pad having a lower hardness than the polishing pad used in the rough polishing step. For example, urethane foam or suede is preferably used.

  As the abrasive slurry, a slurry containing cerium oxide or the like similar to the coarse polishing step can be used. However, in order to make the surface of the glass substrate smoother, it is preferable to use an abrasive slurry having a finer grain size and less variation. For example, it is preferable to use a slurry obtained by dispersing colloidal silica having an average particle size of 40 to 70 nm in a solvent to form a slurry. It does not specifically limit as a solvent, Neutral water and acidic / alkaline aqueous solution are employable. Moreover, a dispersing agent can be added to these solvents. The mixing ratio of the solvent and colloidal silica is preferably about 1: 9 to 3: 7.

  It does not specifically limit as supply_amount | feed_rate of abrasive slurry, For example, it is 0.5-1L / min.

  The polishing amount in the mirror polishing step is preferably about 2 to 5 μm. By setting the polishing amount in such a range, the obtained glass substrate can remove fine defects such as minute roughness and waviness generated on the surface of the glass substrate, or minute scratches generated in the previous process. Is done. As a result, the glass substrate manufacturing method of the present invention can improve the flatness of the obtained glass substrate, and can produce a glass substrate on which the magnetic head can float more stably in the end region.

  In this step, by appropriately adjusting the polishing conditions in the mirror polishing step, the flatness of both main surfaces of the glass substrate is reduced to 3 μm or less, and the surface roughness Ra of both main surfaces of the glass substrate is reduced to 0.1 nm. be able to.

<Chemical strengthening process>
The chemical strengthening step is a step of immersing the glass substrate in a strengthening treatment liquid to improve the impact resistance, vibration resistance, heat resistance, and the like of the glass substrate.

  The chemical strengthening step is a step of chemically strengthening the glass substrate. Examples of the strengthening treatment liquid used for chemical strengthening include a mixed solution of potassium nitrate (60%) and sodium nitrate (40%). In the chemical strengthening, the strengthening treatment liquid can be heated to 300 ° C. to 400 ° C., the glass substrate is preheated to 200 to 300 ° C., and immersed in the strengthening treatment liquid for 3 to 4 hours. In this immersion, it is preferable that the immersion is performed in a state of being housed in a holder that holds the end faces of the plurality of glass substrates so that both main surfaces of the glass substrate are chemically strengthened.

  In addition, after the chemical strengthening process, a standby process for waiting the glass substrate in the air and a water immersion process are adopted to remove the strengthening treatment liquid adhering to the surface of the glass substrate and to homogenize the surface of the glass substrate. It is preferable. By adopting such a process, the chemically strengthened layer is formed uniformly, the compressive strain is uniform, deformation is difficult to occur, the flatness is good, and the mechanical strength is also good. The waiting time and the water temperature in the water immersion process are not particularly limited. For example, the standby time may be waited for 1 to 60 seconds in the air and immersed in water at about 35 to 100 ° C., and may be appropriately determined in consideration of manufacturing efficiency.

<Final cleaning process>
The final cleaning step is a step of cleaning and cleaning the glass substrate. It does not specifically limit as a washing | cleaning method, What is necessary is just the washing | cleaning method which can clean the surface of the glass substrate after a mirror polishing process. In the present invention, scrub cleaning is employed.

  The cleaned glass substrate is subjected to ultrasonic cleaning and drying steps as necessary. The drying step is a step of drying the surface of the glass substrate after removing the cleaning liquid remaining on the surface of the glass substrate with isopropyl alcohol (IPA) or the like. For example, a water rinse cleaning process is performed on the glass substrate after scrub cleaning for 2 minutes to remove the cleaning liquid residue. Next, an IPA cleaning process is performed for 2 minutes, and water remaining on the surface of the glass substrate is removed by IPA. Finally, the IPA vapor drying step is performed for 2 minutes, and the liquid IPA adhering to the surface of the glass substrate is dried while being removed by the IPA vapor. The drying process of the glass substrate is not particularly limited, and for example, a known drying method such as spin drying or air knife drying can be employed.

<Inspection process>
The inspection step is a step of inspecting the glass substrate that has undergone the above-described steps for the presence or absence of scratches, cracks, foreign matters, and the like. The inspection is performed visually or using an optical surface analyzer (for example, “OSA6100” manufactured by KLA-TENCOL). After the inspection, the glass substrate is stored in a dedicated storage cassette and vacuum-packed in a clean environment so that foreign matter or the like does not adhere to the surface, and then shipped.

As mentioned above, the glass substrate of this embodiment obtained through the said process is a glass substrate obtained by grinding, grinding | polishing etc. the glass blank with few internal distortions as above-mentioned. Therefore, as shown in FIG. 6B, when the radius of the center hole P ′ is r ′ ₀ and the radius of the glass substrate is r ′ ₁ , the glass substrate of the present embodiment is represented by the following formula (II). In the entire circumference at the position of the defined radius r ′, the maximum value of the retardation amount when linearly polarized light is incident in the direction perpendicular to the main surface (recording surface) is 1 nm / mm or less. More preferably, the maximum value of the retardation amount is 0.8 nm / mm or less.

r ′ = r ₀ ′ + (r ₁ ′ −r ₀ ′) / 2 (II)
The definition of the retardation amount (Re) and the measuring method are as described above.

  As described above, if the retardation amount is produced within a predetermined range during the production of the glass substrate, the internal distortion is reduced. Therefore, the impact resistance of the glass substrate is improved, and deformation and cracking are less likely to occur.

<Magnetic film formation process>
A magnetic film formation process is a process of forming a magnetic film on a glass substrate using a vapor deposition apparatus. The method for forming the magnetic film is not particularly limited, and a conventionally known method can be employed. For example, a method of forming a thermosetting resin in which magnetic particles are dispersed by spin coating on a substrate, or a method of forming by sputtering or electroless plating can be employed. The film thickness by spin coating is about 0.3 to 1.2 μm, the film thickness by sputtering is about 0.04 to 0.08 μm, and the film thickness by electroless plating is 0.05 to 0.1 μm. Degree. When forming a magnetic film by these forming methods, a glass substrate is hold | maintained at about 250-350 degreeC.

  The magnetic material used for the magnetic film is not particularly limited, and a conventionally known magnetic material can be used. In order to obtain a high coercive force, it is possible to use a Co-based alloy based on Co having a high crystal anisotropy and added with Ni or Cr for the purpose of adjusting the residual magnetic flux density.

  In the case of producing a medium for assist recording, an alloy composed of a transition metal element and a noble metal element, such as a Co—Pt alloy, which includes a transition metal element (Co) and a noble metal element (Pt). An alloy having substantially the same atomic content, or a Co— in which the atomic content of the transition metal element (Co) and the noble metal element (Pt) is substantially equal, and the atomic content of Ni is 0.1% to 50%. Ni—Pt alloy, Co—Ni—Pt alloy, Fe—Pt—Ag alloy, Fe—Pt alloy, Cu—which have substantially the same atomic content of transition metal elements (Co and Ni) and noble metal element (Pt), Cu It is preferable to form a thin film containing an oxide. In this case, a soft magnetic layer (a material having a small coercive force, a Co-based amorphous material, etc.) is preferably laminated below the thin film. Hereinafter, for example, a case where an Fe—Pt alloy is employed will be described.

  In the thin film containing the Fe—Pt alloy and the Cu oxide, the content ratio of Fe and Pt in the thin film is preferably about Fe: Pt = 45 to 55:55 to 45 in terms of molar ratio.

  Moreover, when forming the thin film containing a Fe-Pt alloy and Cu oxide by sputtering method, it can produce by co-sputtering film formation which respectively made Fe, Pt, and Cu oxide a target, for example. At this time, instead of Fe and Pt, a sputtering film may be formed using an Fe—Pt alloy as a target. When sputtering film formation is performed using an Fe—Pt alloy as a target, the composition ratio of Fe—Pt is easily fixed.

The Cu oxide is not particularly limited as long as it is an oxide of Cu, and CuO, CuO ₂ , or a mixture thereof can be used. By adding Cu oxide to the Fe—Pt alloy, the crystal grain size during film formation can be reduced. Furthermore, high orientation and high ordering by rapid heating can be promoted by suppressing the crystal grain size during film formation and inducing tensile stress acting between the crystal grains. The method of adding Cu oxide is not particularly limited, and for example, sputtering film formation may be performed using a material mixed with Cu oxide in advance as a target, or simultaneous sputtering film formation with another target using Cu oxide as a target. May be.

  Among the materials constituting the thin film, the Cu content is preferably 10% by volume or more and 20% by volume or less in terms of CuO with respect to the total amount of the Fe—Pt alloy and the Cu oxide.

  When producing a medium for assist recording, the glass substrate after film formation is heated. By heating, the thin film can function as a magnetic recording layer.

  The heating rate in the heating step is preferably 30 ° C./second or more, more preferably 50 ° C./second or more. When the heating rate is increased, the coercive force tends to decrease.

  The heating method in a heating process is not specifically limited. An example of the heating method is infrared heating.

  In the heating step, the glass substrate is preferably held for a predetermined time after being heated to, for example, 550 to 650 ° C. The holding time is, for example, 15 minutes to 1 hour.

  By the above method, a magnetic film can be formed on a glass substrate.

  Further, in order to improve the sliding of the magnetic head, a lubricant may be coated on the surface of the magnetic film.

  Further, if necessary, the magnetic film may be provided with an underlayer or a protective layer. The underlayer and the protective layer are selected according to the type of the magnetic film.

  The present invention is not limited to the manufacturing method of the HDD, and can be used as a manufacturing method of, for example, a magneto-optical disk or an optical disk.

  In addition, according to the present invention, if necessary, the grinding process is sequentially performed in two steps, or the chemical strengthening process is performed after any of the coring process, the grinding process, the rough polishing process, and the mirror polishing process. The design can be changed as appropriate.

  Furthermore, in the present invention, the glass substrate may be subjected to a hydrogen fluoride immersion treatment as an edge relaxation treatment for scratches generated on the glass substrate.

As mentioned above, according to the manufacturing method of the glass substrate of this embodiment, there is little distortion inside the glass blanks, and the manufacturing method of the glass substrate that hardly causes deformation and cracking in the process of applying high heat in the manufacturing process of the glass substrate is provided. can do.
(Embodiment 2)
The manufacturing method of the glass substrate of this embodiment is that the molten glass is heated by the heater provided below the position where the molten glass is cut by the blade in the heating step (heating heat retention step) of the blank forming step. This is the same as the manufacturing method of the glass substrate of the first embodiment. Therefore, description of the overlapping steps is omitted.

  It does not specifically limit as a heater, It can select suitably from well-known various heaters, and can use it. For example, a cartridge heater that is used by being embedded in a member provided around a molten glass melted from a melting nozzle, or a sheet heater that is used while being in contact with the outside of the member can be used. In addition to various thermocouples, known sensors such as platinum resistance thermometers and various thermistors can be used as the temperature sensor. Specifically, a molybdenum disilicide heater or the like can be used.

The heater can be provided at the same position as the gas burner of the first embodiment. When a heater is provided below the blade, the heater set temperature is preferably about 1500 to 1600 ° C., for example. The number of heaters is not particularly limited, and may be one or more. You may provide only in the downward direction of a braid | blade, may provide only around the metal mold | die which carried out rotation, and may provide in those both positions. The molten glass suspended from the melting nozzle and having a temperature of 1300 to 1350 ° C. is cut into blades and then heated by the heater, so that it is maintained at about 1150 to 1250 ° C. at the time of press molding.
(Embodiment 3)
The manufacturing method of the glass substrate of this embodiment is the heating process (heating heat retention process) of the blanks molding process, except that the molten glass is kept warm by a heat insulating material provided below the position where the molten glass is cut by the blade. These are the same as the manufacturing method of the glass substrate of Embodiment 1. Therefore, description of the overlapping steps is omitted. As described above, in the present embodiment, the glass gob is not heated but kept warm. Therefore, it is not a heating process but a heat insulation process (heating heat insulation process).

  It does not specifically limit as a heat insulating material, The thing which can embrace many gaseous phases and can reduce heat conductivity is employable. For example, glass wool, various fibers, alumina silica, porous ceramics, polyurethane foam and the like can be employed. In addition, as the heat insulating material, a material having a good thermal barrier property can be adopted. For example, as long as it is a synthetic resin, polyimide, polyamideimide, polyamide, polyphenylene sulfite, polybutylene terephthalate, phenol resin, and the like can be employed.

  The heat insulating material can be provided at the same position as the gas burner of Embodiment 1. That is, only the lower part of the blade may be covered with a heat insulating material, only the periphery of the rotationally moved mold may be covered, or the glass gob may be covered at both positions. In addition, it can be provided so as to cover the periphery of the mold. In this case, it is preferable to provide a mechanism for removing the heat insulating material before press molding or to provide a position that does not interfere with the press molding.

  The molten glass suspended from the melting nozzle and having a temperature of 1300 to 1350 ° C. is cut into blades and then kept warm by a heat insulating material, so that it is maintained at about 1130 to 1200 ° C. at the time of press molding.

  EXAMPLES Hereinafter, the manufacturing method of the glass substrate of this invention is explained in full detail by an Example. In addition, the manufacturing method of the glass substrate of this invention is not limited to the Example shown below at all.

<Example 1>
A glass substrate was prepared by the following method.

[Glass melting process]
As glass materials, 65 mol% SiO ₂ , 2 mol% Al ₂ O ₃ , 3 mol% Na ₂ O, 5 mol% K ₂ O, 6 mol% MgO, 14 mol% CaO, 5 mol An aluminosilicate glass containing% ZrO ₂ was melted to produce a molten glass (Tg: 670 ° C.). The viscosity curve of this molten glass is shown in FIG. The viscosity curve was created by the platinum ball pulling method.

[Blanks molding process]
The molten glass was discharged from the melting nozzle at 1300 ° C. The viscosity of this molten glass was 2.7 dPa · s. Glass blanks were obtained by cutting the molten glass every 10 g with a pair of blades. A blade having a V shape in plan view was selected, and the inner angle of the V shape was set to 80 °. An arc shape was used as a planar view shape of the portion where the V-shaped crosses. The glass gob was heated so as to be sandwiched from both ends by a pair of gas burners (setting temperature of the jet outlet: 1500 ° C.) provided below the blade. The viscosity of the obtained glass gob immediately before press molding was 3.5 dPa · s. From FIG. 7, the temperature of the glass gob at this time was about 1180 ° C. The temperature of the glass gob was measured using a radiation thermometer (Konica Minolta Co., Ltd., IR-630).

  In press molding, an upper mold that opposes the lower mold with a glass gob supplied to the center of the lower mold forming surface is used, and a tungsten-based material having a thermal conductivity of 150 W / m · K is used for the upper mold and the lower mold. It was. The pressing time was 1 second, and butt molding was performed so that the thickness of the blanks after molding was uniform.

[Heat treatment process]
The obtained glass blanks were heat-treated at 670 ° C. for 3 hours in order to remove internal strain.

[Coring process]
Using a core drill equipped with a cylindrical diamond grindstone, a circular center hole having a diameter of about 19.6 mm was formed in the center of the blank. Using a drum-shaped diamond grindstone, the outer peripheral end surface and the inner peripheral end surface of the blanks were processed to have an inner diameter and an outer diameter of 65 mm in outer diameter and 20 mm in inner diameter.

  About the obtained glass blank, the retardation in the position of r from a center hole was measured. The method for measuring retardation is as described above. The retardation of the glass blanks was 16 nm.

[Grinding process]
Both surfaces of the obtained glass substrate were ground using a double-side grinding machine (Hamai Sangyo Co., Ltd., 16B type). As grinding conditions, diamond pellets of # 1700 mesh were used, the load was 100 g / cm ² , the upper platen was rotated at 20 rpm, and the lower platen was rotated at 30 rpm.

[Rough polishing process]
Both main surfaces of the glass substrate were roughly polished using a double-side polishing machine (manufactured by Hamai Sangyo Co., Ltd., 16B type). The polishing pad was a urethane foam pad, the abrasive grains were cerium oxide abrasive grains having an average primary particle size of 1 μm, and the mixing ratio of water and cerium oxide was 80:20. Furthermore, pH was adjusted with the adjustment liquid containing a sulfuric acid. The load was 100 g / cm ² . The supply amount of the abrasive slurry was 5 to 10 L / min.

[Mirror polishing process]
Both main surfaces of the glass substrate were polished more precisely using a double-side polishing machine (Hamai Sangyo Co., Ltd., 16B type). As the abrasive slurry, colloidal silica having an average primary particle diameter of 20 nm was dispersed in water as abrasive grains to form a slurry, and the mixing ratio of water and colloidal silica was 80:20. Furthermore, pH was adjusted with the adjustment liquid containing a sulfuric acid. The load was 120 g / cm ² . The supply amount of the abrasive slurry was 0.5 to 1 L / min. In this step, 100 batches of glass substrates were processed into 5 batches. Ra of the obtained glass substrate was 2 or less.

[Chemical strengthening process]
Potassium nitrate was melted at 500 ° C., and the glass substrate was immersed for 1 hour.

[Final cleaning process]
The glass substrate was scrubbed. As a cleaning liquid, a liquid obtained by diluting KOH and NaOH mixed at a mass ratio of 1: 1 with ultrapure water (DI water) and adding a nonionic surfactant to enhance the cleaning performance is obtained. Using. The cleaning liquid was supplied by spraying. After scrub cleaning, in order to remove the cleaning liquid remaining on the surface of the glass substrate, a water rinse cleaning process is performed in an ultrasonic bath for 2 minutes, an IPA cleaning process is performed in an ultrasonic bath for 2 minutes, and finally the glass substrate is cleaned with IPA vapor. The surface of was dried.

  About the obtained glass substrate, the retardation in the position of r 'from a center hole was measured. The method for measuring retardation is as described above. The retardation of the glass substrate was 0.6 nm.

[Magnetic film forming process]
A Fe—Pt alloy film was formed by sputtering, and then heat treatment (600 ° C., 1 hour) was performed to form a magnetic film.

<Example 2>
A glass substrate was produced in the same manner as in Example 1 except that the heating method in the blank forming step in Example 1 was changed to a molybdenum disilicide heater (heater setting temperature: 1500 ° C.). The viscosity of the obtained glass gob immediately before press molding was 3.4 dPa · s. From FIG. 7, the temperature of the glass gob at this time was about 1200 ° C. The retardation of the glass blanks was 15 nm. The retardation of the glass substrate was 0.2 nm.

<Example 3>
A glass substrate was produced in the same manner as in Example 1 except that the heating method in the blank forming step in Example 1 was changed to an insulating material of alumina silica. The viscosity of the obtained glass gob immediately before press molding was 3.9 dPa · s. From FIG. 7, the temperature of the glass gob at this time was about 1140 ° C. The retardation of the glass blanks was 18 nm. The retardation of the glass substrate was 0.9 nm.

<Example 4>
A glass substrate was produced in the same manner as in Example 1 except that, in the heating method in the blank forming step among the steps described in Example 1, the heating position by the gas burner was set to the left and right of the press. The viscosity of the obtained glass gob immediately before press molding was 3.3 dPa · s. From FIG. 7, the temperature of the glass gob at this time was about 1210 ° C. The retardation of the glass blanks was 12 nm. The retardation of the glass substrate was 0.1 nm.

<Example 5>
In the heating method in the blank forming step among the steps described above in Example 1, a glass substrate was produced by the same method as in Example 1 except that gas burners were further installed on the left and right sides of the press machine and heated. The viscosity of the obtained glass gob immediately before press molding was 3.1 dPa · s. From FIG. 7, the temperature of the glass gob at this time was about 1240 ° C. The retardation of the glass blanks was 7 nm. The retardation of the glass substrate was 0.1 nm.

<Comparative Example 1>
In the heating method in the blank forming step among the steps described above in Example 1, a glass substrate was produced by the same method as in Example 1 except that the heating position by the gas burner was set near the outlet of the melting nozzle above the blade. . The viscosity of the obtained glass gob immediately before press molding was 4.1 dPa · s. From FIG. 7, the temperature of the glass gob at this time was about 1120 ° C. The retardation of the glass blanks was 21 nm. The retardation of the glass substrate was 1.2 nm.

<Comparative example 2>
In the heating method in the blank forming step among the steps described above in Example 1, a glass substrate is obtained by the same method as in Example 1 except that an insulating material of alumina silica is installed in the vicinity of the outlet of the melting nozzle above the blade. Was made. The viscosity of the obtained glass gob immediately before press molding was 4.4 dPa · s. From FIG. 7, the temperature of the glass gob at this time was about 1080 ° C. The retardation of the glass blanks was 30 nm. The retardation of the glass substrate was 3.7 nm.

<Comparative Example 3>
A glass substrate was produced in the same manner as in Example 1 except that heating and heat retention were not performed in the heating method in the blank forming step among the steps described above in Example 1. The viscosity of the obtained glass gob immediately before press molding was 5.0 dPa · s. From FIG. 7, the temperature of the glass gob at this time was about 1030 ° C. The retardation of the glass blank was 53 nm. The retardation of the glass substrate was 10.6 nm.

  About the glass substrate obtained in Examples 1-5 and Comparative Examples 1-3, the quality rate of the chemical strengthening process and the quality rate of the film-forming process were measured. The test method is shown below, and the results are shown in Table 1.

[Non-defective rate of chemical strengthening process]
The presence or absence of deformation of the glass substrate after the chemical strengthening process and the occurrence of defects such as cracks and chips were inspected. As for the presence or absence of deformation, micro waviness was measured using an optical measuring instrument Newview manufactured by Zygo, and flatness was measured using a flatness measuring instrument Optiflat manufactured by Phase Shift Technology, and compared with a glass substrate before chemical strengthening. Those with no difference in shape were counted as good products. The cracks and chips were visually inspected. The non-defective product rate was inspected with 100 sheets as one batch, and the number of non-defective products in all samples was expressed as the good product rate.

[Non-defective rate of film formation efficiency]
The presence or absence of deformation of the glass substrate after the film forming process and the occurrence of defects such as cracks and chips were inspected. About a deformation | transformation, it measured by the method similar to the above, and counted what was not a shape difference compared with the glass substrate before film-forming as a good product. The cracks and chips were visually inspected. The non-defective product rate was inspected with 100 sheets as one batch, and the number of non-defective products in all samples was expressed as the good product rate.

  As shown in Table 1, the glass blanks and glass substrates produced by the methods of Examples 1 to 4 have a small amount of retardation, and the yield rate is 98% or more in the chemical strengthening process and the film forming process, which are high-temperature heat treatments. there were. On the other hand, the glass blanks and the glass substrate produced by the method of Comparative Example 1 or Comparative Example 2 heated or kept warm above the blade have a large amount of retardation, and the yield rate is high in the chemical strengthening process and the film forming process, which are high-temperature heat treatments. It became low with 52 to 87%. The glass blanks and the glass substrate produced by the method of Comparative Example 3 in which no heating / heating process is provided have a large amount of retardation, and the yield rate is 3% and 41% respectively in the chemical strengthening process and the film forming process, which are high-temperature heat treatments. It became low.

DESCRIPTION OF SYMBOLS 1 Melting nozzle 2 Molten glass 3 Blade 4 Gas burner 5 Glass gob 6 Press machine 6a Rotary table 6b Lower press machine 6c Lower mold 6d Upper press machine 6e Upper mold 7 Glass blanks

Claims (10)

A method for producing a glass substrate for HDD having a blank forming process in which molten glass suspended from an outflow nozzle is cut by a cutting means into a glass gob, and the glass gob is supported on a table and then press-molded to obtain glass blanks. ,
A heating and keeping step of heating or keeping the glass gob by a heating means or a heat retaining means provided below the cutting means, the glass gob cut by the cutting means;
And a press molding step of press molding the glass gob.   2. The method for manufacturing a glass substrate for HDD according to claim 1, wherein the heating means is at least one of a gas burner and a heater.   The method for producing a glass substrate for HDD according to claim 1, wherein the heat retaining means is a heat insulating material.   The manufacturing method of the glass substrate for HDD of any one of Claims 1-3 whose difference (DELTA) of the viscosity log (eta) a in 1100 degreeC of the said glass gob and the viscosity log (eta) b in 1500 degreeC is 2.0 dPa * s or more.   The manufacturing method of the glass substrate for HDD of any one of Claims 1-4 whose glass transition temperature (Tg) of the said molten glass is Tg> 600 degreeC.   It is a manufacturing method of the glass substrate for HDD for assist recording, The manufacturing method of the glass substrate for HDD of any one of Claims 1-5.   The manufacturing method of the glass substrate for HDD of any one of Claims 1-6 which further has a chemical strengthening process.   The manufacturing method of the glass substrate for HDD of any one of Claims 1-7 in which the glass raw material which comprises the said molten glass contains 15-30 mol% alkaline-earth metal oxide. It is the glass blanks for HDD manufactured by the manufacturing method of the glass substrate for HDD of any one of Claims 1-8,
It has a center hole, and when the radius of the center hole is r ₀ and the radius of the glass blank is r ₁ , it is perpendicular to the recording surface at the entire circumference at the position of the radius r defined by the following formula (I). HDD glass blanks having a maximum retardation amount of 20 nm / mm or less when linearly polarized light is incident in the direction.
r = r ₀ + (r ₁ −r ₀ ) / 2 (I) It is the glass substrate for HDD manufactured by the manufacturing method of the glass substrate for HDD of any one of Claims 1-8,
When the center hole has a radius of r ₀ ′ and the radius of the glass substrate is r ₁ ′, the recording surface has a radius r ′ defined by the following formula (II). On the other hand, a glass substrate for HDD, wherein the maximum amount of retardation when linearly polarized light is incident in the vertical direction is 1 nm / mm or less.
r ′ = r ₀ ′ + (r ₁ ′ −r ₀ ′) / 2 (II)