JP5650522B2 - Method for manufacturing glass substrate for magnetic recording medium - Google Patents

Description

  The present invention relates to a method for producing a glass substrate for use in a magnetic recording medium.

  For example, a glass substrate for use in a magnetic recording medium such as a hard disk (HD) incorporated in a hard disk drive (HDD) is a glass melting process for melting a glass material, and by pressing the molten glass material with a mold. Press forming process for producing a disk-shaped glass substrate, coring process for forming a circular hole at the center of the obtained glass substrate, and grinding the main surface (recording surface) of the obtained annular glass substrate to obtain a glass substrate First lapping step to preliminarily adjust the thickness (plate thickness), flatness, etc., end surface grinding to finely adjust the outer diameter size, roundness, etc. of the glass substrate by grinding the inner and outer peripheral end surfaces of the glass substrate Process, end surface polishing process for polishing and smoothing the inner and outer peripheral end surfaces of the glass substrate, and grinding the main surface of the glass substrate again to fine-tune the thickness and flatness of the glass substrate. 2 lapping step, first polishing step (rough polishing step) for roughing and smoothing the main surface of the glass substrate, chemical strengthening step for strengthening the surface of the glass substrate, plate thickness measuring step for measuring the plate thickness of the glass substrate , A second polishing step (precise polishing step) in which the main surface of the glass substrate is precisely polished and further smoothed, a final cleaning step in which the glass substrate is cleaned, and an inspection step in which the thickness and flatness of the glass substrate are inspected And so on.

  In addition, in order to suppress chipping (chipping) or the like between corners between the main surface and the end surface of the glass substrate, the inner peripheral end surface and the outer peripheral end surface of the glass substrate may be chamfered, for example, in an end surface grinding process. There is (the surface formed by chamfering is called chamfer surface). The chemical strengthening step and the plate thickness measuring step may be reversed in order.

  In recent years, magnetic recording media such as HD are required to have very high smoothness (good flatness and small surface roughness) as the density increases. Therefore, very high smoothness is also required for a glass substrate for a magnetic recording medium. The smoothness of the glass substrate finally obtained is determined in the second polishing step. The second polishing process is generally performed using a double-side polishing apparatus. The double-side polishing apparatus includes a cylindrical upper surface plate and a lower surface plate whose opposing surfaces are parallel to each other and whose rotation directions are opposite to each other. A polishing pad for polishing the main surface of the glass substrate is attached to the opposing surface of each surface plate. The glass substrate is sandwiched between upper and lower polishing plates, and in this state, the polishing pad and the glass substrate rotate and move relative to each other in the plane direction, whereby the glass substrate is polished.

  In the second polishing step, it is usual to polish a plurality of (for example, 100) glass substrates at a time. At this time, for example, the polishing amount (polishing allowance) is set to 0.5 to 2 μm or the like. Therefore, it is necessary that the variation in the thickness of the plurality of glass substrates is within this polishing amount (0.5 to 2 μm). Otherwise, among the plurality of glass substrates, only the glass substrate having a relatively large thickness is sandwiched between the upper and lower polishing pads and polished, and the glass substrate having a relatively smaller thickness is sandwiched between the upper and lower polishing pads. It is because it is not attached and is not polished. Since the glass substrate that has not been polished or is not sufficiently polished in the second polishing step has poor smoothness, the production yield of the glass substrate is reduced. Therefore, as described above, a plate thickness measuring step for measuring the plate thickness of the plurality of glass substrates is performed before the second polishing step. And based on the measurement result of this plate thickness measurement process, a plurality of glass substrates are ranked, and the glass substrates in which the variation in the thickness of the plurality of glass substrates is less than the polishing amount of the second polishing step are the same. A plurality of glass substrates divided into groups (for example, 100 sheets or the like) are precisely polished at once for each group.

  Therefore, the measurement accuracy of the plate thickness measurement process greatly affects the yield. For example, if the polishing amount in the second polishing step is 1 μm, it is necessary to manage the thickness of the glass substrate at a level of 0.1 μm. If the plate thickness measurement accuracy is low, glass substrates with a plate thickness that is too thick or too thin than the others will be mixed in the group, and a glass substrate with poor smoothness that is not sufficiently polished in the second polishing step will be created. End up.

  Conventionally, the thickness of a glass substrate has been measured using a contact-type measuring instrument such as a micrometer. However, the measurement level is about 10 μm, and it has become impossible to cope with the recent precise thickness management. Moreover, the glass substrates must be extracted from the manufacturing process one by one for measurement, and the glass substrates may be dried to become defective products. Furthermore, since the meter head contacts the glass substrate, the glass substrate may be damaged.

  Patent Document 1 discloses a technique for measuring the thickness of a glass substrate in a non-contact manner using light while the glass substrate is held in water. According to this, drying and scratching of the glass substrate at the time of measuring the plate thickness can be avoided. However, this measurement principle is that the glass substrate immersed in the water tank is irradiated with laser light obliquely from the light emitting part, and the light reflected obliquely from the surface of the glass substrate and the light reflected obliquely from the back surface are received by the light receiving part. Since the plate thickness is obtained based on the difference between the light receiving positions, the light irradiation angle to the glass substrate and the light reflection angle from the glass substrate greatly affect the measurement result. For this reason, measurement errors are likely to occur due to mechanical factors. For example, if the arrangement of the light receiving elements arranged in the light receiving portion is slightly disturbed, it is amplified and becomes a large error and appears in the measurement result. Therefore, the measurement accuracy is still not sufficient to cope with the recent precise plate thickness management. In addition, in order to make the light irradiation angle to the glass substrate constant, the glass substrate must be extracted from the manufacturing process and fixed to the water tank, and productivity is greatly improved when the thickness of all glass substrates is measured. There is also a problem that it drops. Furthermore, the plate thickness measurement disclosed in Patent Document 1 is performed after the completion of precision polishing, and is not performed based on the plate thickness measurement result.

JP 2009-59427 A (paragraphs 0013, 0034 to 0037, 0041, 0042)

  An object of the present invention is to provide a method for manufacturing a glass substrate for a magnetic recording medium, in which a plurality of glass substrates are precisely polished at once based on the thickness measurement results of a plurality of glass substrates. It is to measure the thickness of the substrate accurately without contact.

  The present invention relates to a plate thickness measurement step for measuring the plate thickness of a plurality of glass substrates, a plurality of glass substrates divided into groups based on the measurement results of the plate thickness measurement step, and a plurality of glass substrates divided into groups for each group. A method of manufacturing a glass substrate for a magnetic recording medium including a precision polishing step of precisely polishing at a time, and in the plate thickness measurement step, the glass substrate is irradiated with light having a predetermined wavelength band in the plate thickness direction, The substrate of the glass substrate irradiated with light by receiving the interference light between the light reflected in the thickness direction on the surface of the substrate and the light reflected in the thickness direction on the back surface, and analyzing the received interference light for each wavelength The thickness is measured by spectral interference.

  According to this configuration, the glass substrate is irradiated with light in the thickness direction, and the interference light between the reflected light in the thickness direction on the surface of the glass substrate and the reflected light in the thickness direction on the back surface is received and received. Since the thickness of the glass substrate is measured by spectral interference based on the interference light, the irradiation angle of light with respect to the glass substrate and the reflection angle of light from the glass substrate have no influence on the measurement result. Therefore, a measurement error does not occur due to mechanical factors, and the thickness of the glass substrate can be measured with an accuracy sufficient to cope with the recent precise thickness management. As a result, all glass substrates are sufficiently precisely polished in the precision polishing step and smoothness is improved, so that the production yield of the glass substrate is not lowered. In addition, since it is not necessary to move the glass substrate to a specific location in order to measure the plate thickness, the plate thickness can be measured while the glass substrate is left in the manufacturing process, and the glass substrate is dried or productivity is lowered. There is nothing. Further, since the thickness of the glass substrate is measured in a non-contact manner using light, the glass substrate is not damaged during the thickness measurement.

  In the present invention, in the plate thickness measurement step, the carrier carrying a plurality of glass substrates is moved in the plate thickness direction of the glass substrate, so that the plate thickness measurement can be performed as determined by the configuration of the optical system for spectral interference. It is preferable to continuously measure the thickness of the plurality of glass substrates sequentially by moving the plurality of glass substrates sequentially. This is because the plate thickness measurement is performed while performing a normal production operation, so that the problem of a decrease in productivity due to the plate thickness measurement is more reliably suppressed. In addition, for example, if a device that irradiates light on a glass substrate or a device that receives interference light from the glass substrate is disposed downstream in the moving direction of the glass substrate, these devices need not be moved. The glass substrate approaching by movement in the thickness direction is irradiated with light in the thickness direction, and the interference light reflected in the thickness direction is received to measure the thickness of each glass substrate one by one. There are also advantages.

  In the present invention, a chemical strengthening step for strengthening the surface of the glass substrate is performed before the precision polishing step, and in the precision polishing step, it is preferable to perform polishing so that the chemical strengthening layer remains on the main surface of the glass substrate. This is because the chemically strengthened layer improves the impact resistance, vibration resistance, heat resistance, and the like of the finally obtained glass substrate. In addition, the glass substrate that has not been sufficiently polished in the precision polishing step not only has poor smoothness, but also the thickness of the chemically strengthened layer remains in spots. And since a compressive stress is applied to the chemically strengthened layer, if such a chemically strengthened layer remains in spots, there arises a problem that the glass substrate is distorted. Therefore, when all the glass substrates are sufficiently precisely polished in the precision polishing step, there is an advantage that such a problem of distortion is also eliminated.

  In this invention, it is preferable that the grinding | polishing amount of a precision grinding | polishing process is 0.5-1 micrometer. This is because the thickness of the glass substrate can be precisely controlled, and the amount of polishing in the precision polishing step can be set to a small value to this extent.

  In this invention, it is preferable to arrange | position a glass substrate in a liquid and measure the plate | board thickness of a glass substrate at a plate | board thickness measurement process. This is because it is possible to prevent the glass substrate from drying and soiling.

  According to the present invention, in a method for manufacturing a glass substrate for a magnetic recording medium in which a plurality of glass substrates are precisely polished at once based on the thickness measurement results of a plurality of glass substrates, the glass substrate is not extracted from the manufacturing process. It becomes possible to accurately measure the thickness of the substrate without contact. Therefore, since all the glass substrates are sufficiently precisely polished in the precision polishing step, the production yield of the glass substrate does not decrease. And since a glass substrate with high smoothness is obtained, it can contribute to density increase of a magnetic recording medium.

It is a manufacturing-process figure of the glass substrate for magnetic recording media which concerns on embodiment of this invention. It is a cross-sectional perspective view of the glass substrate for magnetic recording media which concerns on embodiment of this invention. It is explanatory drawing of the principle of the plate | board thickness measurement of the glass substrate performed at the plate | board thickness measurement process which concerns on embodiment of this invention. It is explanatory drawing of the specific condition in which the plate | board thickness measurement process which concerns on embodiment of this invention is performed. It is explanatory drawing of another specific situation where the plate | board thickness measurement process which concerns on embodiment of this invention is performed. It is a partial side view which shows the structure of the principal part of the double-side polish apparatus which can be used at the 2nd polishing process which concerns on embodiment of this invention. It is an arrow line view which follows the AA line of FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a manufacturing process diagram of a glass substrate for a magnetic recording medium according to this embodiment, and FIG. 2 is a cross-sectional perspective view of the glass substrate for a magnetic recording medium according to this embodiment.

  In this embodiment, the glass substrate 10 for magnetic recording media includes a glass melting step (Step S1), a press forming step (Step S2), a coring step (Step S3), a first lapping step (Step S4), and an end surface grinding step. (Step S5), end face polishing step (Step S6), second lapping step (Step S7), first polishing step (Step S8), chemical strengthening step (Step S9), plate thickness measurement step (Step S10), second It is manufactured through a polishing process (step S11), a final cleaning process (step S12), and an inspection process (step S13).

In the glass melting step (S1), the glass material is melted. Glass material is composed of a glass composition whose main component is silicon dioxide (SiO _2). The glass composition may or may not contain magnesium, calcium and / or cerium. Exemplary glass compositions include, for _{example, SiO 2, Al 2 O 3} , B 2 O 3, Li 2 O, Na 2 O, K 2 O, MgO, CaO, BaO, SrO, and ZnO, and the like.

  In the press molding step (S2), a molten glass material is poured into a mold and press molded to produce a disk-shaped glass substrate. As the size of the glass substrate at this time, for example, the outer diameter is 2.5 inches, 1.8 inches, 1.0 inches, 0.8 inches, and the plate thickness is 2 mm, 1 mm, 0.63 mm, etc. is there.

  In the coring step (S3), a circular hole is formed in the center of the obtained glass substrate using, for example, a diamond core drill. In the first lapping step (S4), the main surfaces (recording surfaces) 11 and 12 of the obtained annular glass substrate 10 are ground to preliminarily adjust the plate thickness, parallelism, flatness, and the like of the glass substrate 10. For the grinding process in the first lapping step, for example, a double-sided grinding apparatus including a grinding plate on which diamond pellets are attached is used.

  In the end surface grinding step (S5), the inner peripheral end surface 13 and the outer peripheral end surface 14 of the glass substrate 10 are ground to finely adjust the outer diameter size, roundness, etc. of the glass substrate 10. In the end surface grinding step, the inner peripheral end surface 13 and the outer peripheral end surface 14 of the glass substrate 10 are chamfered using, for example, a diamond grindstone to form a chamfer surface 16. In the inner peripheral end surface 13 and the outer peripheral end surface 14, a portion sandwiched between the chamfer surfaces 16 and 16 is referred to as a side wall surface 15.

  In the end surface polishing step (S6), the inner peripheral end surface 13 and the outer peripheral end surface 14 of the glass substrate 10 are polished and smoothed. In the second lapping step (S7), the main surfaces 11 and 12 of the glass substrate 10 are again ground to finely adjust the plate thickness, parallelism, flatness and the like of the glass substrate 10. For the grinding process in the second lapping step, for example, a double-sided grinding device including a grinding plate on which diamond pellets are attached is used.

  In the first polishing step (rough polishing step: S8), the main surfaces 11 and 12 of the glass substrate 10 are rough polished and smoothed. For polishing in the first polishing step, for example, a double-side polishing apparatus having a pair of upper and lower surface plates with a foamed urethane pad attached as a polishing pad is used, and a polishing liquid containing cerium oxide as abrasive grains is used as the polishing liquid. .

  In the chemical strengthening step (S9), a chemical strengthening layer is formed on the surface of the glass substrate 10. For example, by immersing the glass substrate 10 in a solution containing sodium ions and potassium ions, lithium ions and sodium ions present on the surface layer of the glass substrate 10 are replaced with sodium ions and potassium ions in the solution. The surface layer is a chemically strengthened layer. A compressive stress is applied to the chemically strengthened layer. By forming such a chemically strengthened layer, the impact resistance, vibration resistance, heat resistance, and the like of the finally obtained glass substrate 10 are improved.

  In the plate thickness measurement step (S10), the plate thicknesses of the plurality of glass substrates 10 are measured. Based on the measurement results, the glass substrates 10 are ranked, and the glass substrates 10 in which the variation in the thickness of the glass substrates 10 is less than the polishing amount of the next second polishing step are grouped together (for example, one batch 100). Etc.).

  In the second polishing step (precision polishing step: S11), the main surfaces 11 and 12 of the glass substrate 10 are precisely polished and further smoothed. For polishing in the second polishing step, for example, a double-side polishing apparatus including a pair of upper and lower surface plates to which a polyurethane suede pad is attached as a polishing pad is used, and silica (colloidal silica) is included as abrasive grains as polishing liquid. A polishing liquid is used. This precise polishing is performed at once for each group divided based on the measurement result of the plate thickness measurement process. The polishing amount in the second polishing process is, for example, 0.5 to 1 μm. In the second polishing process, precision polishing is performed so that the chemical strengthening layer formed in the chemical strengthening process remains on the main surfaces 11 and 12 of the glass substrate 10.

  In the final cleaning step (S12), for example, the filtered foreign water, ion-exchanged water, ultrapure water, acidic detergent, neutral detergent, alkaline detergent, organic solvent, and surfactant are removed from the foreign substances adhering to the glass substrate 10. Wash and remove using various cleaning agents containing

  In the inspection step (S13), the plate thickness and flatness of the finally obtained glass substrate 10 are inspected. Only the glass substrate 10 that has passed the inspection is used for manufacturing a magnetic recording medium such as a hard disk (HD), and a magnetic layer is formed on the main surfaces 11 and 12.

  Next, the plate thickness measurement step (S10) which is a characteristic part of the present embodiment will be described in detail. FIG. 3 is an explanatory diagram of the principle of measuring the thickness of the glass substrate 10 performed in the thickness measuring step according to the present embodiment. A spectroscopic unit 20 is provided in the production line of the glass substrate 10. The spectroscopic unit 20 includes a light source, a spectroscope, and an FFT processing unit.

  The light source is an SLD (Super Luminescent Diode) light source, which is an infrared light L in a wide wavelength band, for example, near red having a wavelength band of about 0.70 to 1.0 μm and a center wavelength of about 0.83 μm. Generate outside light. The generated infrared light L passes through the polarization maintaining fiber 21 and is irradiated from the sensor head 22 to the glass substrate 10. At this time, the infrared light L is applied to the glass substrate 10 in the thickness direction.

  A part of the irradiated infrared light L is reflected in the thickness direction on the surface of the glass substrate 10 and returns to the sensor head 22 as reflected light a. The remaining part is reflected in the thickness direction on the back surface of the glass substrate 10 and returns to the sensor head 22 as reflected light b. The two reflected lights a and b interfere with each other for each wavelength. The interference light passes through the polarization maintaining fiber 21 and returns to the spectroscopic unit 20. That is, the interference light is received.

  The interference light returned to the spectroscopic unit 20 is analyzed for each wavelength by the spectroscope. That is, the light intensity is detected for each wavelength from the received light waveform of a CCD (Charge Coupled Device) after being dispersed by a diffraction grating, and a light intensity spectrum distribution is obtained.

  The obtained light intensity spectrum distribution is subjected to waveform analysis such as FFT (Fast Fourier Transform) processing in the FFT processing unit, and the thickness of the glass substrate 10 irradiated with the infrared light L is measured by spectral interference. Is done.

  As described above, the plate thicknesses of the plurality of glass substrates 10 are measured in the plate thickness measurement step.

  Examples of the plate thickness measuring device including the spectroscopic unit 20, the polarization maintaining fiber 21, and the sensor head 22 as described above are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2009-270939 and 2010-121977. A plate thickness measuring device (sensor head “SI-F80”, spectroscopic unit “SI-F80U”) manufactured by Keyence Corporation can be employed.

  In this embodiment, the said plate | board thickness measuring apparatus is provided in the production line of the glass substrate 10 between the place where a chemical strengthening process is performed, and the place where a 2nd polishing process is performed, for example. The plate thickness may be measured one sheet at a time, but in this embodiment, as shown in FIG. 4, a plurality of glass substrates 10... By stacking on top and measuring the plate thickness sequentially, the number of handling is reduced and the plate thickness is measured efficiently.

  In the plate thickness measuring apparatus using the SLD light source, the measurement range (see FIG. 4) in which the plate thickness can be measured from the sensor head 22 to the object to be measured is determined by the configuration of the optical system for spectral interference. The object to be measured must be installed within the measurement range. On the other hand, outside the measurement range, for example, even when another glass substrate 10 exists between the sensor head 22 and the object to be measured, the other glass substrate 10 has excellent light transmittance, and the surface of the other glass substrate 10. However, it is possible to measure the thickness of the object to be measured without any problem on the condition that it has excellent smoothness and little light scattering.

  In this embodiment, by utilizing this, glass existing in a measurement range in which the plate thickness measurement determined by the distance from the sensor head 22 can be performed by moving the plate thickness measurement carrier C1 on which a plurality of glass substrates 10 are loaded. The substrate 10 is configured so that it can be sequentially replaced. By comprising in this way, the plate | board thickness of many glass substrates 10 can be measured efficiently by repeating the movement of carrier C1, and plate | board thickness measurement. Here, in order to prevent the glass substrate 10 from further drying and sticking of dirt, the glass substrate 10 is placed in the liquid X and immersed in the liquid X (liquid tank 50) to measure the plate thickness. In order to suppress the adhesion of impurities in the liquid X, the liquid X to be immersed is preferably pure water with few impurities, and it is also effective to circulate it through a filter or to contain a detergent. The example shown in FIG. 4 is an example in which plate thickness measurement is performed during storage of the glass substrate 10 between processes.

  In addition, as a value of the plate thickness measured in the plate thickness measuring step, a value obtained by measuring only one predetermined position of the glass substrate 10 may be obtained, or obtained by measuring a plurality of locations of the glass substrate 10. It may be an average value. When measuring a plurality of locations, the glass substrate 10 to be measured is efficiently rotated by providing a mechanism for rotating the glass substrate 10 around the direction parallel to the plate thickness direction or moving it in a direction perpendicular to the plate thickness direction. The location can be measured.

  Based on the result of the plate thickness measurement of the plurality of glass substrates 10 performed in the plate thickness measurement step (S10) as described above, the glass substrates 10 are ranked, and the variation in the plate thickness of the plurality of glass substrates 10 is found. The glass substrates 10 that are less than the polishing amount in the second polishing step (0.5 to 1 μm in the present embodiment) are divided into the same group (for example, 100 batches).

  Further, instead of performing the plate thickness measurement during the storage of the glass substrate 10 between the processes, as shown in FIG. 5, the conveyance of the glass substrate 10 from the chemical strengthening process to the second polishing process is automated, and the process is automatically performed. The plate thickness may be measured during conveyance. A plurality of glass substrates 10 that have completed the chemical strengthening process are stacked and transported (moved) on the transport carrier C2, and the sensor head 22 is disposed, for example, downstream in the transport direction, and approaches by transport in the thickness direction. The thickness of the glass substrate 10 can be sequentially measured within the measurement range.

  Next, the second polishing step (S11) performed after the plate thickness measurement step will be described in detail. In the present embodiment, the polishing process is substantially the same except for the type of polishing pad and the type of polishing liquid, whether it is the first polishing process or the second polishing process.

  FIG. 6 is a partial side view showing the configuration of the main part of the double-side polishing apparatus that can be used in the second polishing step according to the present embodiment, and FIG. 7 is an arrow view along the line AA in FIG. It is a top view of a lower surface plate.

  As shown in FIG. 6, the double-side polishing apparatus 30 includes a pair of upper and lower upper surface plates 31 and a lower surface plate 32. Each of the surface plates 31 and 32 has a cylindrical shape (outer diameter: about 1000 mm), the opposing surfaces are parallel to each other, and the rotation directions are opposite to each other. Polishing pads (polyurethane suede pads in this embodiment) 33 and 34 for polishing the main surface of the glass substrate are attached to the opposing surfaces of the surface plates 31 and 32, respectively.

  As shown in FIG. 7, a plurality of disc-shaped carriers 37 (four in the illustrated example) are installed on the polishing pad 34 of the lower surface plate 32. A sun gear 35 is provided at the center of the lower surface plate 32, and an internal gear 36 is provided at the periphery of the lower surface plate 32. Gear teeth are formed on the peripheral portion of the carrier 37 and mesh with the sun gear 35 and the internal gear 36. When the sun gear 35 rotates by such a planetary gear mechanism, the carrier 37 revolves around the sun gear 35 while rotating.

  A plurality of circular holes 38... 38 are formed in the carrier 37, and the glass substrates 10 are loosely fitted into the circular holes 38 one by one. Since the upper surface plate 31 and the lower surface plate 32 are in contact with each other, the carriers 37... 37 and the glass substrate 10... 10 are sandwiched between the upper and lower polishing pads 33 and 34. By rotating 37, the polishing pads 33 and 34 and the glass substrate 10 move relative to each other in the surface direction. At this time, the polishing liquid (colloidal silica in the present embodiment) is supplied between the polishing pads 33 and 34 and the glass substrate 10 to polish the glass substrate 10.

  In the present embodiment, the polishing pads 23 and 24 are foamed urethane pads in the first polishing step (S8), and polyurethane suede pads are used in the second polishing step (S11). However, it is not limited to this.

  In the present embodiment, the polishing liquid is a slurry containing abrasive grains (free abrasive grains). The abrasive grains are not particularly limited, and those conventionally used generally in the field of glass polishing can be used. For example, cerium oxide, silicon carbide, silica (colloidal silica), zirconia, alumina and the like can be preferably used. Among these, cerium oxide is more preferable from the viewpoints of cost, smoothness obtained, and the like. The particle diameter of the abrasive grains is preferably from 1 to 100 nm, more preferably from 1 to 80 nm, further preferably from 1 to 50 nm, and more preferably from 1 to 20 nm from the viewpoint of the smoothness obtained. Those are particularly preferred.

  In the present embodiment, a polishing liquid containing cerium oxide as abrasive grains is used in the first polishing step (S8), and a polishing liquid containing silica (colloidal silica) as abrasive grains in the second polishing step (S11). Is used. However, it is not limited to this.

  In the second polishing step (S11), precise polishing is performed for each group (for example, 100 batches) divided based on the measurement result of the plate thickness measurement step. The polishing amount at that time is, for example, 0.5 to 1 μm. In addition, precision polishing is performed so that the chemically strengthened layer formed in the chemical strengthening process remains on the main surfaces 11 and 12 of the glass substrate 10.

  In this embodiment, after the first polishing step (S8), the chemical strengthening step, the plate thickness measuring step, and the second polishing step (S11) are performed in this order. The order may be reversed, and after the first polishing step (S8), the plate thickness measurement step, the chemical strengthening step, and the second polishing step (S11) may be followed. This is because the thickness of the glass substrate 10 does not change even when chemical strengthening is performed.

  As described above in detail with specific examples, in this embodiment, a plurality of thickness measurement steps (S10) for measuring the thickness of the plurality of glass substrates 10 and a plurality of measurement results based on the measurement results of the thickness measurement steps. There is provided a method of manufacturing a glass substrate for a magnetic recording medium, which includes a second polishing step (S11) in which the glass substrates 10 are divided into groups and the plurality of glass substrates 10 divided into groups are precisely polished at once for each group. Then, in the plate thickness measurement step (S10), the glass substrate 10 is irradiated with light L having a predetermined wavelength band in the plate thickness direction, and the light a reflected on the surface of the glass substrate 10 in the plate thickness direction and the plate thickness on the back surface. The thickness of the glass substrate 10 irradiated with the light L is obtained by receiving the interference light with the light b reflected in the direction and analyzing the received interference light for each wavelength.

  Therefore, the irradiation angle of the light L to the glass substrate 10 and the reflection angles of the light a and b from the glass substrate 10 have no influence on the measurement result. Therefore, a measurement error does not occur due to mechanical factors, and the thickness of the glass substrate 10 can be measured with an accuracy sufficient to cope with recent precise thickness management. As a result, in the second polishing step (S11), all the glass substrates 10 ... 10 (for example, 100 batches) are sufficiently precisely polished to improve the smoothness, so that the production yield of the glass substrate 10 is reduced. There is nothing to do. Moreover, since it is not necessary to move the glass substrate 10 to a specific place in order to measure the plate thickness, the plate thickness can be measured while the glass substrate 10 is placed in the manufacturing process, and the glass substrate 10 can be dried or productivity can be measured. Will not drop. Moreover, since the plate | board thickness of the glass substrate 10 is measured non-contactly using the light L, the glass substrate 10 is not damaged at the time of plate | board thickness measurement.

  In the present embodiment, the plate thickness measurement step is continuously performed while arranging a plurality of glass substrates 10... 10 in the plate thickness direction and moving in the plate thickness direction (see FIG. 4). As a result, the plate thickness measurement is performed without taking out the glass substrate from the transport carrier in a normal production operation, so that the problem of a decrease in productivity due to the plate thickness measurement is further reliably suppressed. Since it is possible to irradiate infrared light L in the plate thickness direction to the glass substrate 10 approaching by conveyance in the plate thickness direction and to receive the interference light reflected in the plate thickness direction, the sensor head Even if 22 is not moved, it is possible to measure the thickness of each glass substrate 10 one by one. However, the present invention is not limited to this, and the plate thickness may be measured by moving only the sensor head 22 or moving the sensor head 22 and the glass substrate 10 depending on the situation. Further, instead of performing the plate thickness measurement during storage of the glass substrate 10 during the process, the conveyance of the glass substrate 10 from the chemical strengthening step to the second polishing step may be automated and the plate thickness measurement may be performed during the automatic transfer. Good (see FIG. 5). A plurality of glass substrates 10 on which the chemical strengthening process has been completed are stacked and moved on the carrier C2, and the sensor head 22 is arranged on the downstream side in the conveyance direction, and the plate of the glass substrate 10 approaching by conveyance in the plate thickness direction. The thickness can be measured sequentially within the measurement range.

  In the present embodiment, a chemical strengthening step (S9) for strengthening the surface of the glass substrate 10 is performed before the second polishing step (S11). In the second polishing step, the main surfaces 11 and 12 of the glass substrate 10 are formed. Polish to leave a chemically strengthened layer. Thereby, the impact resistance, vibration resistance, heat resistance, and the like of the finally obtained glass substrate 10 are improved by the chemical strengthening layer. There are also the following advantages. That is, the glass substrate 10 that was not sufficiently polished in the second polishing step not only has poor smoothness, but also the thickness of the chemically strengthened layer remains in spots. And since a compressive stress is applied to the chemically strengthened layer, if such a chemically strengthened layer remains in spots, there arises a problem that the glass substrate 10 is distorted. However, in this embodiment, since all the glass substrates 10... 10 are sufficiently precisely polished in the second polishing step, such a problem of distortion is also eliminated.

  In the present embodiment, the polishing amount in the second polishing process is 0.5 to 1 μm. In the present embodiment, the plate thickness of the glass substrate 10 can be precisely managed, and therefore the polishing amount in the second polishing step can be set to a small value to this extent.

  In the present embodiment, in the plate thickness measurement step, the glass substrate 10 is placed in the liquid X and the plate thickness of the glass substrate 10 is measured (see FIG. 4). Thereby, it can prevent that the glass substrate 10 dries and a dirt adheres.

  Hereinafter, the present invention will be described in more detail through examples, but the present invention is not limited to these examples.

(Production of glass substrate)
According to the manufacturing process shown in FIG. 1, using a glass material having the following composition (mass%), the glass melting step (S1) to the inspection step (S13) are performed, and the outer diameter is about 65 mm (2.5 inches). An annular aluminosilicate glass substrate having an inner diameter (circular hole diameter) of about 20 mm and a plate thickness of 1 mm was produced. The chamfa surface was not formed.

(Composition of glass material)
・ SiO ₂ : 50 to 70%
· _Al 2 _O 3: 0.1~20%
・ B ₂ O ₃ : 0 to 5%
_{However, SiO 2 + Al 2 O 3} + B 2 O 3 = 50~85%, _{also, Li 2 O + Na 2 O} + K 2 O = 0.1~20%, also a MgO + CaO + BaO + SrO + ZnO = 2~20%.

(Example)
In the examples, the plate thickness measuring step (S10) was performed using the plate thickness measuring device (sensor head “SI-F80”, spectroscopic unit “SI-F80U” manufactured by Keyence Corporation) shown in FIG. In addition, the second polishing step (S11) was performed using the double-side polishing apparatus 30 shown in FIGS. A glass substrate was produced with 100 batches, and the flatness of the obtained glass substrate was evaluated.

  The flatness of the glass substrate is evaluated by obtaining TIR using “OptiFlat” (optical interference surface shape measuring device) manufactured by Phase Shift Technology, and determining that the TIR is 2 μm or more as non-defective products. Rate). Here, TIR is an index representing the flatness of the glass substrate, and is a total value of the distance from the least square plane of the evaluation surface to the highest point and the distance from the least square plane to the lowest point. The highest point and the lowest point are obtained from the measured values for one round in the circumferential direction at a radius of 25 mm on the evaluation surface.

  As a result, in the example, the non-defective product rate of flatness was 95% (20 sampling evaluation). From this, it was found that the glass substrate having high smoothness was obtained in high yield in the examples. It is considered that this is because the thickness measurement accuracy of the glass substrate before the second polishing step is good and the thickness management of the glass substrate is precisely performed.

(Comparative Example 1)
In Comparative Example 1, the thickness measurement step (S10) was performed in the same manner as in the example except that the thickness measurement device disclosed in Patent Document 1 ("LK-G15" manufactured by Keyence Corporation) was used. Then, a glass substrate was produced, and the flatness of the obtained glass substrate was evaluated. As a result, in Comparative Example 1, the non-defective product ratio of flatness was 65% (20 sheets sampling evaluation). From this, it turned out that the comparative example 1 has a low yield of a glass substrate with high smoothness compared with an Example. This is considered because the thickness measurement accuracy of the glass substrate before the second polishing step is not sufficient, and the thickness management of the glass substrate has not been performed accurately.

(Comparative Example 2)
In Comparative Example 2, a glass substrate was prepared in the same manner as in Example except that the plate thickness measurement step (S10) was performed using a micrometer, and the flatness of the obtained glass substrate was evaluated. . As a result, in Comparative Example 2, the non-defective product rate of flatness was 25% (20 sheets sampling evaluation). From this, it was found that in Comparative Example 2, the yield of the glass substrate having high smoothness was extremely low. It is considered that this is because the thickness measurement accuracy of the glass substrate before the second polishing step is low, and it was not possible to cope with precise thickness management of the glass substrate.

10 Glass substrates for magnetic recording media 11, 12 Main surface (recording surface)
13 inner peripheral end surface 14 outer peripheral end surface 15 side wall surface 16 chamfer surface 20 spectroscopic unit 21 polarization maintaining fiber 22 sensor head 30 double-side polishing apparatus 31, 32 surface plate 33, 34 polishing pad 37 carrier 50 liquid tank C1 plate thickness measurement carrier C2 Carrier L for transportation Infrared light X Liquid a, b Reflected light

Claims (4)

Thickness measurement process for measuring the thickness of multiple glass substrates, and multiple glass substrates are divided into groups based on the measurement results of the thickness measurement process. A method for producing a glass substrate for a magnetic recording medium, comprising a precision polishing step for polishing,
In the plate thickness measurement process,
Move the carrier loaded with multiple glass substrates in the thickness direction of the glass substrate,
By sequentially moving a plurality of glass substrates to a measurement range capable of measuring the plate thickness determined by the configuration of the optical system of spectral interference,
Measure the thickness of multiple glass substrates sequentially and sequentially,
In each measurement when sequentially measuring the thickness of the plurality of glass substrates sequentially,
Irradiate the glass substrate with light having a predetermined wavelength band in the thickness direction,
Receives interference light between the light reflected in the thickness direction on the surface of the glass substrate and the light reflected in the thickness direction on the back surface,
By analyzing the received interference light for each wavelength,
A method for producing a glass substrate for a magnetic recording medium, comprising measuring the thickness of a glass substrate irradiated with light by spectral interference. Before the precision polishing process, a chemical strengthening process to strengthen the surface of the glass substrate is performed,
2. The method for producing a glass substrate for a magnetic recording medium according to claim 1, wherein in the precision polishing step, polishing is performed so that the chemically strengthened layer remains on the main surface of the glass substrate. The method for producing a glass substrate for a magnetic recording medium according to claim 1 or 2 , wherein the polishing amount in the precision polishing step is 0.5 to 1 µm. The method for producing a glass substrate for a magnetic recording medium according to any one of claims 1 to 3 , wherein in the plate thickness measurement step, the glass substrate is placed in a liquid and the plate thickness of the glass substrate is measured. .

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