JP6034636B2 - Method for manufacturing substrate for magnetic recording medium - Google Patents

Description

  The present invention relates to a method for manufacturing a glass substrate for a magnetic recording medium.

  In recent years, with the improvement of the storage capacity of an information recording apparatus (for example, a hard disk drive HDD) equipped with an information recording medium, the quality level required for the information recording medium used is increasing. As one of the quality standards required for information recording media, higher smoothness is required for information recording media. In order to increase the storage capacity of the information recording medium, it is necessary to improve the recording density. For this reason, it is necessary to reduce the flying height of the read head. In recent years, the flying height has been reduced to several nanometers, and a head crush occurs where the read head collides with the defect due to a slight uneven defect, leading to a read error. For this reason, strict smoothness is demanded for glass substrates for information recording media (hereinafter sometimes simply referred to as glass substrates) used for information recording media. In order to increase the storage capacity of the information recording medium, it is required to further expand the recordable area. In recent years, hard disk drives having a storage capacity of 500 GB on both sides have been developed, and in order to produce such a large-capacity information recording medium, an end area exceeding 97% of the radius from the center of the information recording medium Therefore, it is necessary to extend the traveling area (that is, the recordable area) of the magnetic head until the quality level required for the glass substrate for the information recording medium can be recorded to such an end. Such high-precision thickness uniformity has been demanded.

  On the other hand, the main surface of the glass substrate is generally polished by a two-stage polishing process to be a flat surface with very high smoothness. Among these, in the first stage polishing process (first polishing process (rough polishing)), conventionally, cerium oxide, zirconium oxide, etc., which are abrasive grains having a relatively high processing rate, are used, and a hard urethane pad (generally Asker- Hard polishing pads such as C hardness exceeding 90) have been used. Such a first polishing step is performed in order to smooth the surface to a certain extent by polishing the main surface roughened in the grinding step performed in the preceding stage in a short time. Further, in the subsequent second stage polishing process (second polishing process (precision polishing)), the main surface of the glass substrate is made extremely smooth, so that polishing with a relatively small particle size such as colloidal silica is performed. Abrasive grains are used, and a soft polishing pad such as a soft suede pad (generally Asker-C hardness of about 60 to 90) is used. By providing such two polishing steps, the smoothness required for the glass substrate of the information recording medium could be obtained in a short time.

  However, as described above, since the required level of smoothness as the quality level of the glass substrate is increased, the information recording medium using the glass substrate obtained by the above-described method has a fineness that has not been a problem in the past. A reading error due to unevenness sometimes occurred. As a result of scrutinizing such a problem, when polishing is performed using a hard polishing pad in the first polishing process, the abrasive grains are strongly pressed against the glass substrate, and as a result, relatively deep processing marks may remain. There was a possibility that the pit (fine irregularities) sometimes remained without being completely removed in the second polishing step where such processing marks were small.

  For such a problem, it is conceivable to use a soft polishing pad that is softer than the hard polishing pad in the first polishing step. For example, a technique described in Patent Document 1 has been proposed as a manufacturing method using a soft polishing pad in the first polishing step.

JP 2012-18739 A

  However, when the glass substrate is manufactured by using the soft polishing pad softer than the conventional hard polishing pad in the first polishing step and performing the final polishing in the second polishing step as in the method described in Patent Document 1. When an information recording medium having a large storage capacity is manufactured using such a glass substrate, the occurrence of reading errors on the main surface can be greatly reduced, but in some information recording media, information reading in the outermost peripheral area is possible. Sometimes it turns out that there is a problem with reading errors.

  As a result of scrutinizing such a problem, it has been found that a reading error in the outermost peripheral region is caused by a sag of the end face generated in some glass substrates. It has also been clarified that such a problem does not occur when a hard polishing pad is used in the first polishing process. As a result of further investigation of the cause, it became clear that the following phenomenon was the cause.

  First, in the glass substrate polishing process, a plurality of glass substrates are generally placed in the holding holes of a disk-shaped member called a carrier provided with a plurality of holding holes, and polishing pads are attached to the upper and lower surfaces of the glass substrates. The glass substrate is polished by rotating the carrier while sandwiched by the attached polishing surface plate. At this time, when the soft polishing pad is used in the first polishing step, the glass substrate is removed depending on the position of the holding hole of the carrier on which the glass substrate is placed, as compared with the case where the hard polishing pad is used. It has been found that there is a tendency for large differences to occur. In particular, in the first polishing process, since a relatively large margin is polished using polishing abrasive grains having a high processing rate, the tendency increases.

  Thereafter, the glass substrate that has finished the first polishing step is polished again by the soft polishing pad in the second polishing step. At this time, even if the pressing force of the polishing surface plate against the glass substrate is constant, the glass substrate has a relatively small thickness. The glass substrate having a relatively large thickness than the glass substrate having a large pressure is intensively applied, and the soft polishing pad is pushed into the glass substrate, resulting in the end of the glass substrate having a large thickness. It became clear that the end face sagging progresses in the part.

  Furthermore, with the increase in the storage capacity of information recording media in recent years, it is considered that the outermost peripheral edge, which has not been conventionally used as a recording area, is also used as a recording surface, and the above-described problems have become apparent. In this specification, end face sagging means that the main surface in the vicinity of the end face is gradually displaced from the substrate surface toward the inside of the substrate as it goes toward the edge of the substrate.

  The present invention has been made in view of such a conventional problem, and reduces the generation of fine pits (concave portions) that affect magnetic recording characteristics, and has an excellent end face shape and an expanded recordable area. Another object of the present invention is to provide a method for producing a glass substrate for a magnetic recording medium.

  In view of the above problems, the present inventor sorts the glass substrate that has undergone the first polishing process based on the plate thickness, and provides the sorted glass substrate to the second polishing process, thereby generating end face sagging in the glass substrate. In particular, the present invention has been completed by paying attention to the point that the occurrence of reading errors can be suppressed even in information recording in the outermost peripheral portion.

  That is, the method for producing a glass substrate for a magnetic recording medium of the present invention is a method for producing a glass substrate for a magnetic recording medium having a polishing step, and the polishing step comprises a polishing pad having an Asker-C hardness of 60 or more and 90 or less. A first polishing step for polishing a glass substrate using a glass, a selection step for selecting a glass substrate based on a plate thickness after the rough polishing step, and an Asker-C hardness of 60 to 90 after the selection step And a second polishing step of polishing the glass substrate using a polishing pad.

  According to the method for producing a glass substrate of the present invention, in the first polishing step, the glass substrate is polished using a polishing pad having an Asker-C hardness of 60 or more and 90 or less. Generation of (concave portions) can be reduced. In addition, since a sorting step for sorting the glass substrates based on the plate thickness is adopted after the first polishing step, in the subsequent second polishing step, the glass substrate is pushed to a glass substrate that is thicker than other glass substrates. It is possible to prevent the pressure from being concentrated and to suppress the occurrence of end face sagging.

Furthermore, also in the second polishing step, the glass substrate is polished using a polishing pad having an Asker-C hardness of 60 or more and 90 or less, so that the final glass substrate can have a good surface state. As a result, a glass substrate for a magnetic recording medium having excellent smoothness and end face shape can be produced.

  In the first polishing step, using a carrier in which a plurality of holding holes for holding the glass substrate is formed, and in the sorting step, the glass substrate is sorted by sorting based on the position of the holding hole. preferable.

  In the first polishing step, the degree of polishing of the glass substrate held in the holding hole varies depending on the position of the holding hole formed in the carrier. By grasping the tendency of the difference in the thickness of the glass substrate to be obtained and sorting by classifying the glass substrate based on the position of the holding hole, the physical properties of the glass substrate that has undergone the first polishing process (for example, the thickness of the plate) The glass substrate can be sorted based on the plate thickness without directly measuring the thickness). As a result, the selected glass substrate is subjected to the subsequent second polishing step, thereby preventing the pressing of the polishing pad from concentrating on the glass substrate having a thickness greater than that of the other glass substrate, and suppressing the occurrence of end face sagging. can do.

  In the first polishing step, a carrier in which the holding holes are concentrically formed is used, and in the sorting step, sorting is performed by classifying each glass substrate held in the holding holes formed in concentric circles. It is preferable.

  In the first polishing step, the glass substrate held in the concentric holding holes is polished in the same manner as other glass substrates held in the same concentric holding holes, and has substantially the same thickness. Is done. Therefore, by classifying each glass substrate held in a plurality of concentrically formed holding holes, the physical properties (for example, plate thickness) of the glass substrate that has undergone the first polishing step are measured indirectly. In addition, glass substrates having substantially the same thickness can be selected. As a result, by subjecting the selected glass substrate to the subsequent second polishing step, it is possible to prevent the glass substrate having a plate thickness thicker than that of the other glass substrate from being selectively polished and to suppress the occurrence of end face sagging. be able to.

  In the sorting step, it is preferable to sort the glass substrate by measuring the physical properties of the glass substrate polished in the first polishing step and classifying the glass substrate based on the physical properties.

  By directly measuring the physical properties (for example, plate thickness) of the glass substrate that has undergone the first polishing process, glass substrates having substantially the same plate thickness can be accurately selected. As a result, the selected glass substrate is subjected to the subsequent second polishing step, thereby preventing the pressing of the polishing pad from concentrating on the glass substrate having a thickness greater than that of the other glass substrate, and suppressing the occurrence of end face sagging. can do.

  According to the present invention, there is provided a method for manufacturing a glass substrate for a magnetic recording medium having an excellent end face shape and an expanded recordable area while reducing generation of fine pits (concave portions) that affect magnetic recording characteristics. be able to.

FIG. 1 is a flowchart of each step in the method for producing a glass substrate according to one embodiment of the present invention. FIG. 2 is a schematic diagram of a double-side polishing apparatus used in the method for manufacturing a glass substrate according to an embodiment of the present invention. FIG. 3 is a schematic diagram of a carrier used in the method for producing a glass substrate according to an embodiment of the present invention. FIG. 4 is a schematic diagram of a carrier used in the method for producing a glass substrate according to one embodiment of the present invention. FIG. 5 is a schematic diagram of a carrier used in the method for producing a glass substrate according to an embodiment of the present invention.

  Hereinafter, embodiments of the method for producing a glass substrate of the present invention will be described in detail. FIG. 1 is a flowchart of each step in the glass substrate manufacturing method of the present embodiment. The glass substrate is, for example, a glass blanks preparation step (glass material melting step, press molding step), glass substrate formation / grinding step (first lapping step, coring step, second lapping step), polishing step (first polishing step, It is manufactured through a selection process, a second polishing process), a chemical strengthening process, a cleaning process, and a magnetic thin film forming process. First, the polishing process which is a characteristic part of this embodiment will be described in detail.

<Polishing process>
A grinding | polishing process is a process performed with respect to the glass substrate which passed through the glass substrate formation / grinding process mentioned later. The polishing process includes a first polishing process (rough polishing process), a sorting process, and a second polishing process (precision polishing process).

(First polishing process)
The first polishing step is a step of polishing both main surfaces of the glass substrate so that the surface roughness finally required in the subsequent second polishing step can be efficiently obtained. In the first polishing process, a double-side polishing apparatus is used.

  FIG. 2 is a schematic diagram of a double-side polishing apparatus used in the glass substrate manufacturing method of the present embodiment. The double-side polishing apparatus 1 includes a pair of metal surface plates composed of an upper surface plate 2 and a lower surface plate 3, and a carrier 4 that holds a glass substrate 5. The upper surface plate 2 includes a polishing pad 2a, and the lower surface plate 3 includes a polishing pad 3a. The thickness of the carrier 4 is smaller than the thickness of the glass substrate 5. Therefore, both main surfaces of the glass substrate 5 are partly exposed from the carrier 4 while being held in the holding holes 4 a of the carrier 4. As a result, the glass substrate 5 is sandwiched from above and below by the upper surface plate 2 and the lower surface plate 3 while being held in the holding holes 4a. In this state, both the main surfaces of the glass substrate 5 are polished by rotating the upper surface plate 2 and the lower surface plate 3 and the carrier 4 so as to slide with each other.

  The polishing pad is a processing tool for polishing both main surfaces of the glass substrate 5. As the polishing pad, a polishing pad having an Asker-C hardness of 60 to 90, preferably 70 to 87, more preferably 75 to 84 is used. Specifically, a suede pad made of polyurethane is used, for example, NP178 (Asker-C hardness 82) made by Filwel is used. In the present embodiment, since such a soft polishing pad is used instead of a hard urethane pad as in the prior art, a relatively deep processing mark is not formed on the glass substrate, and the subsequent second polishing step continues. Both main surfaces of the glass substrate can be polished to such an extent that excellent smoothness can be realized.

  As the polishing liquid, a cerium oxide having an average primary particle diameter of 0.6 to 2.5 μm is used as abrasive grains, and the abrasive grains are dispersed in water to form a slurry. The mixing ratio of water and cerium oxide can be about 1: 9 to 3: 7.

  It does not specifically limit as a load added to the glass substrate 5 at the time of grinding | polishing, For example, it can be set as 4-12 kPa.

  The polishing amount of the glass substrate 5 in the first polishing step is preferably about 25 to 40 μm. When the polishing amount of the glass substrate 5 is less than 25 μm, there is a tendency that scratches and defects cannot be removed sufficiently. On the other hand, when the polishing amount of the glass substrate 5 exceeds 40 μm, polishing is performed more than necessary, and the production efficiency tends to decrease.

(Selection process)
The sorting step is a step of sorting the glass substrate 5 that has undergone the first polishing step based on the thickness of the glass substrate 5. The method for selecting the glass substrate 5 based on the plate thickness is not particularly limited, and may be determined by measuring (or estimating) the plate thickness by an indirect method, or measuring the plate thickness by a direct method ( (Or estimation).

  As an example of a method for measuring (or estimating) and selecting the plate thickness by an indirect method, an example of a method for classifying the glass substrate 5 based on the position of the holding hole 4a formed in the carrier 4 will be described with reference to FIG. While explaining. FIG. 3 is a schematic diagram of the carrier 4 used in the method for manufacturing the glass substrate 5 of the present embodiment. The carrier 4 is formed with a plurality of holding holes 4a in an annular shape (concentric shape). Reference sign C1 indicates a concentric circle on the outer peripheral side, and reference sign C2 indicates a concentric circle on the inner peripheral side with respect to C1. Fourteen holding holes 4a are formed on the concentric circle C1, and eight holding holes 4a are formed on the concentric circle C2. The holding holes 4 a are arranged such that the centers of the holding holes 4 a are concentrically arranged, and the plurality of holding holes 4 a arranged on the same concentric circle have the same distance from the center of the carrier 4.

  Here, in the first polishing step, the glass substrate 5 held in the holding hole 4a of the carrier 4 is sandwiched between upper and lower surface plates and polished by rotating the surface plate. At this time, the rotation of the carrier 4 is controlled so as to revolve while rotating. Since the holding hole 4a formed on the concentric circle C1 is larger in distance from the center of the carrier 4 than the holding hole 4a formed on the concentric circle C2, when the carrier 4 rotates, the holding hole 4a Rotate and move in a wider range in the radial direction. Therefore, for example, the glass substrate 5 held in the holding hole 4a formed on the concentric circle C1 is polished while moving in a relatively wide range from the vicinity of the outer periphery of the surface plate to the vicinity of the inner periphery as the carrier 4 rotates. Is done. On the other hand, when the carrier 4 rotates, the glass substrate 5 held in the holding hole 4a formed on the concentric circle C2 provided on the inner periphery of the concentric circle C1 is compared between the outer periphery and inner periphery of the surface plate. Polishing while moving in a narrow range.

  As a result, even when polished under the same polishing conditions, the glass substrate 5 held in the holding hole 4a formed on the concentric circle C1 and the glass held in the holding hole 4a formed on the concentric circle C2. The polishing history (traveling position, traveling distance, etc. of each glass substrate 5 at the time of polishing) does not match with the substrate 5, and the plate thickness tends to be different. Such a tendency is caused by a soft polishing pad as a polishing pad. It occurs remarkably when used.

  Here, as described above, in the glass substrate 5 held in the holding holes 4a formed on the same concentric circle, the polishing history is the same. Therefore, in this embodiment, by classifying the glass substrate 5 based on the position of the holding hole 4a, the glass substrate 5 can be selected based on the plate thickness without directly measuring physical properties such as the plate thickness. . Specifically, the glass substrates 5 held and polished by the holding holes 4a formed on a certain concentric circle (for example, on the concentric circle C1) are collected as one group, and are collected on another concentric circle (for example on the concentric circle C2). The glass substrate 5 held and polished in the formed holding hole 4a is collected as another group. In the same group, the glass substrates 5 are polished based on the same polishing history, so that the plate thickness is substantially the same. Therefore, in the 2nd polish process mentioned later, it can prevent that the glass substrate 5 whose plate | board thickness is thicker than the other glass substrate 5 is selectively grind | polished, and can suppress generation | occurrence | production of an end surface sagging.

  Moreover, although the case where the number of concentric circles is 2 is illustrated in FIG. 3, the glass substrate 5 can be classified indirectly based on the plate thickness similarly when the number of concentric circles is 3 or more. FIG. 4 is a schematic diagram of the carrier 4 used in the method for manufacturing the glass substrate 5 of the present embodiment. In the carrier 4, in addition to the holding holes 4 a formed on the concentric circle C <b> 1 and the concentric circle C <b> 2, holding holes 4 a are also formed on a concentric circle C <b> 3 further provided on the inner peripheral side of the concentric circle C <b> 2. Four holding holes 4a are formed on the concentric circle C3.

  The holding hole 4a formed on the concentric circle C3 has a smaller distance from the center of the carrier 4 than the holding hole 4a formed on the concentric circle C2. Therefore, when the carrier 4 rotates, the outer periphery and inner periphery of the surface plate It is polished while being moved in a narrower range near the middle. Further, when the carrier 4 rotates, the movement distance of the glass substrate 5 held in the holding hole 4a formed on the concentric circle C3 is the movement distance of the glass substrate 5 held in the holding hole 4a formed on the concentric circle C2. Even smaller than the distance. Therefore, even when polished under similar polishing conditions, the glass substrate 5 held in the holding hole 4a formed on the concentric circle C2 and the glass substrate held in the holding hole 4a formed on the concentric circle C3 The polishing history (traveling position, traveling distance, etc. of each glass substrate 5 at the time of polishing) does not match that of 5, and the plate thickness may be different.

  Therefore, the thickness of the glass substrate 5 held and polished by the holding hole 4a formed on the concentric circle C2 and the thickness of the glass substrate 5 held and polished by the holding hole 4a formed on the concentric circle C3 are as follows. If they are different, they can be collected as another group and subjected to the second polishing step. On the other hand, the thickness of the glass substrate 5 held and polished in the holding hole 4a formed on the concentric circle C2 and the thickness of the glass substrate 5 held and polished in the holding hole 4a formed on the concentric circle C3 are as follows. When they are substantially the same, for example, the glass substrate 5 held and polished in the holding hole 4a formed on the concentric circle C2, and the glass substrate held and polished in the holding hole 4a formed on the concentric circle C3. 5 may be combined into one group.

  It should be noted that a concentric circle and another concentric circle (for example, concentric circles) can be obtained by examining in advance how thick the glass substrate 5 is to be held and polished in holding holes 4a formed on which concentric circles. The glass substrate 5 held in the holding hole 4a formed in C2 and the concentric circle C3) is polished to have the same thickness, but formed in one concentric circle and another concentric circle (for example, the concentric circle C1 and the concentric circle C2). It is possible to know information such as that the glass substrate 5 held in the holding hole 4a is polished so that the plate thickness is different, and the conditions of the selection process can be further optimized.

  Another example of the method for classifying the glass substrate 5 based on the position of the holding hole 4a will be described with reference to FIG. FIG. 5 is a schematic diagram of the carrier 4 used in the method of manufacturing the glass substrate 5 of the present embodiment, and FIG. 5A is a schematic diagram illustrating the arrangement of the holding holes 4a. FIG. 5B is a schematic diagram illustrating the region of the carrier 4.

  As shown in FIG. 5A, the carrier 4 has a holding hole 4a formed in a zigzag shape and in an annular shape. Thus, even when the centers of the holding holes 4a constituting the same ring are not formed concentrically, as shown in FIG. 5B, the region of the carrier 4 is changed to the outer region R1. And the inner peripheral region R2, the glass substrate 5 held and polished by the holding hole 4a to which the center of the holding hole 4a belongs in the region R1, and the holding hole 4a to which the center of the holding hole 4a belongs to the region R2. And the glass substrate 5 that is held and polished. The number of regions is not particularly limited as long as it is 2 or more. Further, the shape of the region is not particularly limited, but the shape of the region is preferably circular so that the polishing history is substantially the same. In addition, arrangement | positioning of the holding hole 4a which comprises a ring is not specifically limited, Arrangement | positioning of a spiral shape etc. may be sufficient besides zigzag shape. In addition, when it is necessary to irregularly arrange the holding holes 4a for reasons such as arranging more holding holes 4a, the holding holes 4a are within the range in which the carrier can stably rotate. It may be arranged irregularly.

  Thus, by classifying the glass substrate 5 held in the holding hole 4a to which the center of the holding hole 4a belongs in the same region as one group, the glass held in the holding hole 4a formed in the same region. By considering that the polishing history of the substrate 5 is substantially the same, the glass substrate 5 can be selected based on the thickness of the glass substrate. Therefore, in the second polishing step to be described later, the glass substrate 5 that is thicker than the other glass substrate 5 is selected. Polishing can be prevented, and the occurrence of end face sagging can be suppressed.

  Next, a method for measuring (or estimating) the plate thickness by a direct method and selecting it will be described. The method for measuring (or estimating) the plate thickness by a direct method is not particularly limited. For example, the plate thickness may be measured for a glass substrate that has undergone the first polishing step, and by measuring the weight. The plate thickness may be estimated, the plate thickness may be estimated by measuring the volume, or the plate thickness may be estimated by measuring the resonance frequency.

  It does not specifically limit as a method to measure plate | board thickness with respect to the glass substrate which passed through the 1st polish process, For example, it can measure using spectral interference laser displacement meter SI-80F (made by Keyence Corporation). The selection criteria for the plate thickness are not particularly limited, and for example, glass substrates having a plate thickness with an error of 0.1 to 1.0 μm can be selected as one group.

  The method for measuring the weight of the glass substrate that has undergone the first polishing step is not particularly limited. For example, the weight can be measured by measuring the weight using an electronic balance in an environment with few disturbance factors. Since the areas of the main surfaces of the glass substrate are substantially the same, the difference in plate thickness can be estimated from the difference in weight. The selection criteria for the plate thickness are not particularly limited. For example, glass substrates having a weight of 1 to 8 mg error can be selected as one group.

  Thus, if the glass substrate is selected by measuring (or estimating) the plate thickness by directly measuring the physical properties of the glass substrate that has undergone the first polishing step, the plate thickness may be changed in the second polishing step described later. It is possible to prevent the glass substrate thicker than the glass substrate from being selectively polished and to suppress the occurrence of end face sagging.

(Second polishing step)
The second polishing process is a polishing process performed on the glass substrate selected through the selection process. In the second polishing step, as in the first polishing step, for example, a double-side polishing apparatus (for example, 16B type manufactured by Hamai Sangyo Co., Ltd.) shown in FIG. 2 is used.

  The glass substrate used for the second polishing step is sorted based on the plate thickness in advance through the sorting step described above. Therefore, the plurality of glass substrates that are simultaneously polished in the second polishing step have no difference in plate thickness, and all the glass substrates are polished substantially uniformly. As a result, it is possible to suppress edge sagging that occurs due to selective polishing of a glass substrate having a thickness greater than that of other glass substrates, and a glass substrate having an excellent end surface shape can be obtained. It is done.

  As a polishing pad attached to the surface plate of the double-side polishing apparatus, Asker-C hardness is 60 or more and 90 or less, preferably 70 or more and 87 or less, from the viewpoint of coexistence of the surface state such as surface roughness and micro waviness and the end face shape. More preferably, a polishing pad of 75 to 84 is used. Specifically, a polishing pad such as a suede pad made of polyurethane or a nonwoven fabric pad is used, and for example, NP385 (Asker-C hardness 77) made by Filwel is used. By using such a polishing pad, it is possible to achieve both a surface state such as surface roughness and fine waviness and a good end face shape.

  As the polishing liquid, in order to make the surface of the glass substrate smoother, an abrasive slurry having a finer grain size and less variation can be used. For example, a slurry obtained by dispersing colloidal silica having an average particle size of 20 to 80 nm in a solvent can be used as an abrasive slurry. It does not specifically limit as a solvent, Water can be employ | adopted. In addition, a surfactant or a dispersant can be added to these solvents. The mixing ratio of the solvent and colloidal silica can be about 1: 9 to 3: 7.

  It does not specifically limit as a load added to a glass substrate at the time of grinding | polishing, For example, it can be set as 4-12 kPa.

  The polishing amount of the glass substrate in the second 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 undulation generated on the surface of the glass substrate, or minute processing marks generated in the past process. Is done. Also, by appropriately adjusting the polishing conditions in the second polishing step, the flatness of both main surfaces of the glass substrate can be reduced to 3 μm or less, and the surface roughness Ra of both main surfaces of the glass substrate can be reduced to about 0.1 nm. it can.

  Next, other steps that can be adopted by the present embodiment will be described with reference to FIG. In addition, the manufacturing method of the glass substrate of this embodiment should just have the above-mentioned grinding | polishing process (especially selection process), and it does not specifically limit about other processes. For this reason, the other steps described below are examples, and the design can be changed as appropriate.

<Glass blanks preparation process>
The glass blanks preparation step includes a glass material melting step for melting a glass material and a press molding step for obtaining a glass substrate (blanks) from the molten glass material.

As the material of the glass material, 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 glass (R ′ = Mg, Ca, Sr, Ba) or the like can be used.

  The method for melting the glass in the glass material melting step is not particularly limited, and usually a method of melting the glass material at a known temperature and time at a high temperature can be employed.

  The method for obtaining blanks in the press molding step is not particularly limited. For example, a method of pouring a molten glass material into a lower mold and press molding with an upper mold to obtain a disk-shaped glass substrate (blanks) may be employed. it can. Blanks are not limited to press molding. For example, sheet glass formed by a downdraw method, a float method, or the like may be cut out with a grinding wheel. In these cases, the press molding process may be performed by other processes (for example, It is replaced with a process of cutting out glass. In this molding process, foreign matter and bubbles are mixed in the vicinity of the surface of the blank, or scratches are generated, resulting in defects.

  The size of the blanks is not particularly limited, and for example, blanks having various sizes such as 2.5 inches, 1.8 inches, 1 inch, and 0.8 inches in outer diameter can be manufactured. It does not specifically limit about the thickness of a glass substrate, For example, blanks of various thickness, such as 2 mm, 1 mm, 0.8 mm, 0.63 mm, can be produced.

  Blanks produced by press molding or cutting can be alternately stacked with heat-setter setters and passed through a high-temperature electric furnace to promote reduction of warpage and crystallization of glass.

<Glass substrate formation / grinding process>
The glass substrate forming / grinding process includes a first lapping process, a coring (inner and outer periphery cutting) process, and a second lapping process.

  The first lapping step is a step of preliminarily adjusting the parallelism, flatness and thickness of the glass substrate by grinding both main surfaces of the blank. The lapping process in the first lapping process can be performed using alumina-based loose abrasive grains by a double-sided lapping apparatus using a planetary gear mechanism. Specifically, in the lapping process, both main surfaces of the blanks are pressed against the lapping platen from above and below, a grinding liquid containing free abrasive grains is supplied onto the main surface of the plate glass, and these are relatively moved. Can be done. By this lapping process, a glass substrate having a flat main surface is obtained.

  The coring (inner and outer periphery cutting) step is a step of opening a circular hole (center hole) in the center of the glass substrate. Specifically, the coring (inner and outer periphery cutting) step is a step of forming an annular glass substrate by forming an inner hole at the center of the glass substrate using a cylindrical diamond drill. The inner peripheral end surface and the outer peripheral end surface of the obtained glass substrate are subjected to predetermined chamfering by grinding with a drum-shaped grinding wheel using diamond or the like. Further, the inner peripheral surface of the glass substrate is polished by an inner peripheral end surface polishing machine, and the outer peripheral surface of the glass substrate is polished by an outer peripheral end surface polishing machine.

The second lapping step is a step of grinding both main surfaces of the glass substrate to remove large scratches and improve flatness. In the second lapping step, both main surfaces of the glass substrate are lapped with a lapping machine (manufactured by Hamai Sangyo Co., Ltd.). The wrapping conditions are not particularly limited. For example, a # 1500 mesh diamond pellet is used, the load is 70 g / cm ² , the upper surface plate rotation speed is 30 rpm, and the lower surface plate rotation speed is 10 rpm. As for the surface roughness of the glass substrate obtained through the second lapping step, for example, Rmax is about 3 μm and Ra is about 0.3 μm. The glass substrate that has undergone the second lapping step is substantially free from defects such as large swells, chips and cracks.

  In addition, there is a possibility that the grinding liquid or glass powder remains on the surface of the glass substrate that has undergone the glass substrate formation / grinding process. Therefore, in this embodiment, it is preferable to provide a cleaning process. In the cleaning process, various cleaning methods can be employed. For example, the glass substrate may be subjected only to alkali cleaning, may be subjected to acid cleaning after acid cleaning, or may be only subjected to acid cleaning.

<Polishing process>
The polishing process is as described above. The polishing process includes a first polishing process, a sorting process, and a second polishing process.

  In the glass substrate manufacturing method of the present embodiment, since the glass substrate is polished using a polishing pad having an Asker-C hardness of 60 or more and 90 or less in the first polishing step, fine pits that affect magnetic recording characteristics ( The occurrence of recesses can be reduced. In addition, since a sorting step for sorting the glass substrate based on the plate thickness is employed after the rough polishing step, in the subsequent second polishing step, a polishing pad is formed on the glass substrate having a plate thickness that is thicker than other glass substrates. It is possible to prevent the pressing pressure from being concentrated, and to suppress the occurrence of end face sagging. As a result, a glass substrate for a magnetic recording medium having excellent smoothness and end face shape can be produced.

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

  The chemical strengthening method employed in the chemical strengthening step is not particularly limited, but usually, a glass substrate is immersed in a heated strengthening treatment solution, and alkali ions (for example, lithium ions) contained in the glass substrate have a relatively small ion radius. An ion exchange method is employed in which ions are replaced with alkali ions having a larger ion radius (for example, potassium ions and sodium ions). By adopting the chemical strengthening step, a reinforcing layer (ion exchange layer and compressive stress layer) can be formed on the main surface, outer peripheral end surface and inner peripheral end surface of the glass substrate.

  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 obtained glass substrate has a uniform chemical strengthening layer, a uniform compressive strain, hardly deforms, has good flatness, and is excellent in mechanical strength. The water temperature of the standby time and the water immersion process is not particularly limited. For example, it is preferable to wait for 1 to 60 seconds in the air and to immerse in water at about 35 to 100 ° C., and the user may determine appropriately in consideration of manufacturing efficiency. .

<Washing process>
The cleaning process is a process of cleaning and cleaning the glass substrate. It does not specifically limit as a cleaning method, What is necessary is just the cleaning method which can clean the surface of a glass substrate.

  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, the cleaning step may be a step of performing a water rinse cleaning step on the glass substrate after scrub cleaning for 2 minutes to remove a residue of the cleaning solution. Next, as the cleaning process, an IPA cleaning process can be performed for 2 minutes, and a process of removing water remaining on the surface of the glass substrate by IPA can be employed. Finally, the washing step can be performed by performing an IPA vapor drying step for 2 minutes and drying the liquid IPA adhering to the surface of the glass substrate while removing the IPA vapor with 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. After the glass substrate that has undergone these steps is inspected for scratches, cracks, adhesion of foreign matter, etc. by visual inspection or using an optical surface analyzer (for example, “OSA6100” manufactured by KLA-TENCOL), the foreign matter is exposed on the surface. It can be shipped after being stored in a dedicated storage cassette and vacuum packed in a clean environment.

<Magnetic thin film formation process>
A magnetic thin film formation process is a process of forming a magnetic thin film (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 the magnetic film is formed by these forming methods, the glass substrate is held at about 100 to 500 ° C. depending on the type of the magnetic film.

  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.

  When a recording medium is manufactured, an alloy composed of a transition metal element and a noble metal element, such as a Co—Pt alloy, and an atom of the transition metal element (Co) and the noble metal element (Pt). Alloys with almost the same content, or Co—Ni in which the atomic content of the transition metal element (Co) and the noble metal element (Pt) is almost equal and the atomic content of Ni is 0.1% or more and 50% or less -Pt alloy, Co-Ni-Pt alloy, Co-Cr-Pt alloy, Fe-Pt alloy, Cu-Cu alloy with almost the same atomic content of transition metal elements (Co and Ni) and noble metal element (Pt) 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, or the like) can be stacked below the thin film.

  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.

  As described above, according to the method for manufacturing a glass substrate of the present embodiment, a magnetic recording medium having an excellent end face shape and an expanded recordable area while reducing generation of fine pits (concave portions) affecting magnetic recording characteristics. A method for producing a glass substrate can be provided.

  Note that the glass substrate manufacturing method of the present embodiment can be changed in design, such as omitting the first polishing step or performing the chemical strengthening step before the second polishing step, if necessary.

  Furthermore, in the present embodiment, as a measure against the drop strength, a step of chemically strengthening the outer peripheral end surface and the inner peripheral end surface other than the main surface of the glass substrate may be employed, or an edge mitigation process for scratches generated on the glass substrate. As such, a step of subjecting the glass substrate to a hydrogen fluoride immersion treatment may be employed.

  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 blanks preparation process]
As the glass material, an aluminosilicate glass mainly composed of SiO ₂ , Al ₂ O ₃ , R ₂ O (R = K, Na, Li) is used, and the molten glass material is press-molded, and the outer diameter is 67 mm. Disk-shaped blanks were produced. The thickness of the blanks was 1.0 mm.

[Glass substrate formation / grinding process]
Both main surfaces of the blanks were ground using a double-side grinding machine (Hamai Sangyo Co., Ltd., 16B type). As grinding conditions, alumina powder having a particle size of # 600 was used, the load was 50 g / cm ² , the upper surface plate rotation speed was 30 rpm, and the lower surface plate rotation speed was 20 rpm.

  Next, 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.

  Next, 100 blanks were piled up, and in this state, the outer peripheral end face and the inner peripheral end face of the blanks were polished using an end face polishing machine (TKV-1 manufactured by Hadano Machinery Co., Ltd.). Nylon fiber having a diameter of 0.2 mm was used as the brush hair of the polishing machine. As the polishing liquid, a slurry containing cerium oxide having an average primary particle diameter of 3 μm as abrasive grains (polishing liquid component) was used.

Thereafter, both surfaces of the blank were ground again 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 70 g / cm ² , the upper platen was rotated at 20 rpm, and the lower platen was rotated at 30 rpm.

[Polishing process]
(First polishing process)
Both surfaces of the blanks were subjected to rough polishing using a double-side polishing apparatus (manufactured by Hamai Sangyo Co., Ltd., 16B type). A suede pad (manufactured by Filwel, trade name: NP178, Asker-C hardness: 82) was used as a polishing pad. As the abrasive grains, cerium oxide abrasive grains having an average primary particle diameter of 1 μm were used. The maximum load during processing was set to 9.0 kPa. As the carrier, as shown in FIG. 3, a carrier 4 was used in which holding holes 4a (total of 22) were formed on double concentric circles (concentric circle C1 and concentric circle C2). As shown in FIG. 2, a total of five carriers 4 were arranged. The processing amount was 20 μm.

(Selection process)
The glass substrate 5 held in the holding hole 4a formed on the concentric circle C1 is classified as one group, and the glass substrate 5 held in the holding hole 4a formed on the concentric circle C2 is classified as another group. Thus, the glass substrate 5 was selected.

(Second polishing step)
Each glass substrate 5 grouped was subjected to precision polishing using a double-side polishing apparatus (manufactured by Hamai Sangyo Co., Ltd., 16B type, see FIG. 2) in the same manner as in the first polishing step. A suede pad (manufactured by Filwel, trade name: NP385, Asker-C hardness: 77) was used as a polishing pad. As the abrasive slurry, colloidal silica abrasive grains (polishing liquid component) having an average primary particle diameter of 20 nm were dispersed in water to form a slurry. The mixing ratio of water and abrasive grains was 80:20. Furthermore, the pH of the slurry was adjusted with an adjusting solution containing sulfuric acid. The maximum load during processing was 10.5 kPa. The processing amount was 3 μm.

[Chemical strengthening process]
Subsequently, the obtained glass substrate was chemically strengthened. As the chemical strengthening treatment liquid, an aqueous solution of a mixed molten salt of potassium nitrate (KNO ₃ ) and sodium nitrate (NaNO ₃ ) was used. The mixing ratio was 1: 1 by mass ratio. The temperature of the chemical strengthening treatment liquid was 380 ° C., and the immersion time was 25 minutes.

[Washing 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.

[Magnetic thin film formation process]
A glass substrate was formed from an Fe—Pt alloy by a sputtering method, and then a heat treatment (600 ° C., 1 hour) was performed to form a magnetic thin film, thereby producing a glass substrate for a magnetic recording medium.

<Example 2>
In the screening process, the thickness of the glass substrate after the first polishing process is measured using a spectral interference laser displacement meter SI-80F (manufactured by Keyence Co., Ltd.), and the thickness error is within 0.3 μm. A glass substrate was produced by the same method as in Example 1 except that the glass substrate was sorted by classifying the two as a group.

<Example 3>
In the first polishing step, the carrier 4 divided into the region R1 and the region R2 shown in FIG. 5 is used, and in the sorting step, the glass substrates 5 held in the holding holes 4a formed in the region R1 are grouped into one group. (14 sheets in total), and the glass substrate 5 held in the holding hole 4a formed in the region R2 was classified as another group (8 sheets in total), except that the glass substrate 5 was selected. A glass substrate was produced in the same manner as in Example 1.

<Example 4>
In the sorting step, the weights of the glass substrates that have undergone the first polishing step are measured, and the glass substrate is sorted by classifying those having a weight error within 2 mg as one group. A glass substrate was produced by the same method.

<Comparative Example 1>
A glass substrate was produced in the same manner as in Example 1, except that the glass substrate that had undergone the first polishing step was subjected to the second polishing step without performing the sorting step.

  The following evaluation was performed about the glass substrate obtained by Examples 1-4 and the comparative example 1. FIG. The results are shown in Table 1.

(End face shape)
The shape of the end surface of the glass substrate which passed through the 2nd polish process was measured using NewView by Zygo, and the standard deviation (Å) of the end surface shape was computed. The measurement range is 30.0 mm to 31.7 mm from the center of the glass substrate (radius 32.5 mm), the shape profile in the radial direction is obtained, low pass filtering is performed with 110 μm as a reference, and then the measurement range The maximum value of the height displacement from the line connecting both ends was obtained.

(Recordable area evaluation)
About each glass substrate obtained by the manufacturing method of Examples 1-4 and the comparative example 1, it converted into media and mounted in the magnetic drive, and the in-plane area | region which can be recorded was confirmed. The percentage of substrates in which the recordable area satisfies the non-defective standards was evaluated. The evaluation criteria are shown below.
◎: 97% or more of evaluated drives ○: 94% or more of evaluated drives to less than 97% △: Less than 94% of evaluated drives

  As shown in Table 1, in Examples 1 to 4 based on the manufacturing method of the present invention, compared to Comparative Example 1, since the plate thickness was arranged substantially the same by the selection process, the variation in the end face shape was small, The result of evaluation of the recordable area was also good, and a glass substrate with an excellent end face shape and an expanded recordable area was obtained.

DESCRIPTION OF SYMBOLS 1 Double-side polish apparatus 2 Upper surface plate 2a, 3a Polishing pad 3 Lower surface plate 4 Carrier 4a Holding hole 5 Glass substrate

Claims (3)

A method of manufacturing a magnetic recording medium base plate having a polishing process,
The polishing step includes
A first polishing step of polishing a substrate held in a plurality of holding holes formed in a carrier using a polishing pad having Asker-C hardness of 60 or more and 90 or less;
After the first polishing step, a sorting step of sorting by classifying the board based on the position of the holding hole,
After該選based process, and a second polishing step of the substrate that has been selected in該選based process Asker-C hardness is Migaku Ken with a polishing pad 60 and 90, inclusive,
Method for producing a board for a magnetic recording medium. A method of manufacturing a magnetic recording medium substrate having a polishing step,
The polishing step includes
A first polishing step of polishing a substrate held in a plurality of concentric holes formed in a carrier using a polishing pad having Asker-C hardness of 60 to 90;
A sorting step of sorting by after the first polishing step, classifying each substrate held by the plurality of holding holes formed in the concentric,
A second polishing step of polishing the substrate selected in the selection step with a polishing pad having an Asker-C hardness of 60 or more and 90 or less after the selection step;
A method for manufacturing a substrate for a magnetic recording medium. A method of manufacturing a magnetic recording medium substrate having a polishing step,
The polishing step includes
A first polishing step of polishing a substrate held in a plurality of holding holes formed in a carrier using a polishing pad having Asker-C hardness of 60 or more and 90 or less;
After the first polishing step, a selection step of the weight of the substrate were measured respectively, sorted by classifying the substrate on a weight basis,
A second polishing step of polishing the substrate selected in the selection step with a polishing pad having an Asker-C hardness of 60 or more and 90 or less after the selection step;
A method for manufacturing a substrate for a magnetic recording medium.

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