Patent Publication Number: US-2012045971-A1

Title: Method of manufacturing glass substrate for magnetic recording media

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Japanese Patent Application No. 2010-182330 filed Aug. 17, 2010, the content of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to a method of manufacturing a glass substrate for magnetic recording media. 
     2. Background Art 
     The magnetic recording media used for a hard disk drive (HDD) have significantly improved in recording density. Since the introduction of MR heads and of PRML technology, the areal recording density has further increased. Moreover, since the introduction of GMR heads or TMR heads, the areal recording density has continued to increase at a rate of about 1.5 times a year. However, in the future a higher lever of areal recording density will be required. 
     In addition, with the improvement in recording density of such magnetic recording media, there are growing demands for a substrate used for the magnetic recording media. As a substrate for magnetic recording media, an aluminum substrate and a glass substrate are used conventionally. Of these, generally a glass substrate can be superior to aluminum substrate in hardness, surface smoothness, rigidity, and impact resistance. Therefore, a glass substrate has attracted growing attention as a substrate for magnetic recording media because it is expected to achieve a high recording density. 
     When manufacturing a glass substrate for magnetic recording media, a disk-shaped glass substrate can be obtained by cutting a large plate-shaped glass plate to a disk-shaped substrate or by press-molding molten glass directly on to the disk-shaped glass substrate using a mold. For surfaces and end faces of the obtained glass substrate, a lapping (grinding) process and polishing process are carried out. 
     In addition, as a conventional method of manufacturing a glass substrate for magnetic recording media, a primary lapping process, secondary lapping process, primary polishing process, and secondary polishing process of a surface (data side) of a glass substrate are performed in this order. During these processes, a lapping process and polishing process on the inner and outer peripheral end faces of the substrate were performed. 
     On the data side of the glass substrate, in the primary lapping process, a diamond grindstone was used, in the second lapping process, a diamond grindstone having a smaller grain size than that used in the primary lapping process was used, in the primary polishing process, cerium oxide slurry was used, and in the secondary polishing process, a cerium oxide slurry having a particle size smaller than that in the primary polishing process was used. 
     In addition, regarding prior art documents relating to the present invention, for example, in Patent Literature 1, it is disclosed that when the primary lapping process using diamond pellets such as resin, metal and vitrified, and then the secondary lapping process using a diamond pad were performed, a substrate having surface smoothness and no defects such as scratches or grinding traces or aspiration traces can be manufactured, and it is also possible to decrease the process time.
     Patent literature 1: JP Patent No. 4,049,510.   

     SUMMARY OF THE INVENTION 
     However, with the recent development of low magnetic head flying height, it is required to improve waviness of the glass substrate of magnetic recording media and surface roughness. Studies have revealed that although the primary lapping process has a grinding allowance of 100 μm to 300 μm per-side, the glass substrate could become damaged in this primary lapping process. As a result, the surface of the magnetic recording medium in the final product has a long-term waviness. 
     In addition, polishing the glass substrate using cerium oxide chemical mechanical polishing (CMP) has become a general technology. However, cerium oxide is expensive; thus a technology requiring no, or very little, cerium oxide is preferable. 
     In view of such conventional circumstances, the object of the present invention is to provide a method of manufacturing a glass substrate for magnetic recording media which can produce glass substrates for magnetic recording media with high productivity, wherein the manufactured substrate has high surface smoothness and little waviness. 
     The present invention provides the following: 
     (1) A method of manufacturing a glass substrate for magnetic recording media, comprising: 
     a primary lapping process, 
     a secondary lapping process, 
     a tertiary lapping process, and 
     a polishing process, for a surface except for an end face of a glass substrate, 
     in this order; 
     in the primary, secondary and tertiary lapping process, the diamond pad on which a diamond abrasive was fixed using a binder, the surface of the diamond pad has a structure in which plural tile-like projections having a flat top were provided in line; 
     wherein 
     the diamond pad used in the primary lapping process has an average diamond grain size of 4 to 12 μm, and a content of diamond grains in the projection of 5 to 70% by volume, 
     the diamond pad used in the secondary lapping process has an average diamond grain size of 1 to 5 μm, and a content of diamond grains in the projection of 5 to 80% by volume, 
     the diamond pad used in the tertiary lapping process has an average diamond grain size of 0.2 to 2 μm, and a content of diamond grains in the projection of 5 to 80% by volume, and 
     in the polishing process, the silicon oxide is used as abrasive; 
     (2) The method of manufacturing a glass substrate for magnetic recording media according to (1), 
     wherein the size of the projections of the diamond pad used in the primary, secondary and tertiary lapping process, is 1.5 to 5 mm square with a height 0.2 to 3 mm, and the space between adjacent projections is 0.5 to 3 mm; 
     (3) A method of manufacturing a glass substrate for magnetic recording media, comprising: 
     a primary lapping process, 
     a secondary lapping process, and 
     a polishing process, for a surface except for an end face of a glass substrate, 
     in this order; 
     in the primary and secondary lapping process, the diamond pad on which diamond abrasive grain was fixed using binder is used, and the surface of the diamond pad has a structure in which plural tile-like projections having a flat top were provided in line; 
     wherein 
     the diamond pad used in the primary lapping process has an average diamond grain size of 3 to 12 μm, and a content of diamond grains in the projection of 5 to 70% by volume, 
     the diamond pad used in the secondary lapping process has an average diamond grain size of 0.2 to 5 μm, and a content of diamond grains in the projection of 5 to 80% by volume, and 
     in the polishing process, the silicon oxide is used as abrasive; 
     (4) The method of manufacturing a glass substrate for magnetic recording media according to (3), 
     wherein the size of the projections of the diamond pad used in primary, secondary and tertiary lapping process, is 1.5 to 5 mm square with a height 0.2 to 3 mm, and the space between adjacent projections is 0.5 to 3 mm; 
     (5) The method of manufacturing a glass substrate for magnetic recording media according to any one of (1) to (4), in the polishing process, the silicon oxide is used as abrasive without using cerium oxide. 
     As described above, according to the present invention, a glass substrates for magnetic recording media, which has high surface smoothness and little waviness can be manufactured with high productivity. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing lapping operation for explaining the method of manufacturing a glass substrate for magnetic recording media according to the present invention. 
         FIG. 2A  is a plan view showing an enlarged pad surface of diamond pad used in the lapping process. 
         FIG. 2B  is a cross-section view showing an enlarged pad surface of diamond pad used in the lapping process. 
         FIG. 3  is a perspective view showing the inner and outer peripheral lapping process for explaining the method of manufacturing a glass substrate for magnetic recording media according to the present invention. 
         FIG. 4  is a perspective view showing the inner peripheral polishing process for explaining the method of manufacturing a glass substrate for magnetic recording media according to the present invention. 
         FIG. 5  is a perspective view showing the outer peripheral polishing process for explaining the method of manufacturing a glass substrate for magnetic recording media according to the present invention. 
         FIG. 6  is a perspective view showing a polishing process for explaining the method of manufacturing a glass substrate for magnetic recording media according to the present invention. 
         FIG. 7  is a perspective view showing another configuration example of a lapping or polishing machine used in the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following method for manufacturing a glass substrate for magnetic recording media according to the present invention will be described in detail with reference to the drawings. 
     A glass substrate for magnetic recording media manufactured by the method of the present invention is a disk-shaped glass substrate having a central aperture. A magnetic recording media is formed by laminating a magnetic layer, protective layer and lubricant layer sequentially on the surface of the glass substrate. In addition, in a magnetic recording and reproducing apparatus (an HDD), writing or reading of information is performed for magnetic recording media by attaching a center of the magnetic recording media to an axis of a spindle motor, and making a magnetic head float and travel over the surfaces of the magnetic recording media which is rotated by the spindle motor. 
     As for the glass substrate for magnetic recording media, for example, SiO 2 —Al 2 O 3 —R 2 O-based chemical glass (R represents at least one member selected from alkali metals.), SiO 2 —Al 2 O 3 —Li 2 O-based ceramic glass, SiO 2 —Al 2 O 3 —MgO—TiO 2 -based glass ceramics can be used. Among them, SiO 2 —Al 2 O 3 —MgO—CaO—Li 2 O—Na 2 O—ZrO 2 —Y 2 O 3 —TiO 2 —As 2 O 3 -based strengthened chemical glass, SiO 2 —Al 2 O 3 —Li 2 O—Na 2 O—ZrO 2 —As 2 O 3 -based strengthened chemical glass, SiO 2 —Al 2 O 3 —MgO—ZnO—Li 2 O—P 2 O 5 —ZrO 2 —K 2 O—Sb 2 O 3 -based strengthened chemical glass, SiO 2 —Al 2 O 3 —MgO—CaO—BaO—TiO 2 —P 2 O 5 —As 2 O 3 -based glass ceramics, SiO 2 —Al 2 O 3 —MgO—CaO—SrO—BaO—TiO 2 —ZrO 2 —Bi 2 O 3 —Sb 2 O 3 -basedglass ceramics can be suitably used. Further, for example, lithium silicate, SiO 2 —-based crystal (quartz, cristobalite, and tridymite), cordierite, enstatite, aluminum magnesium titanate, spinel-based crystal ([Zn, and/or Mg] Al 2 O 4 , [Mg and/or Zn] 2 TiO 4 , as well as a solid solution between these two crystals), forsterite, Spodumene, a glass ceramic which includes solid solution crystals thereof as crystal phase can be suitably used as a glass substrate for magnetic recording medium. 
     In the process of producing glass substrates for this magnetic recording media, firstly, a disk-shaped glass substrate which has a central aperture is obtained by cutting a glass substrate from a big glass plate, or directly pressing a glass substrate from melting glass by using a mold. 
     Then, lapping (grinding) process and polishing process are carried out for surfaces and end faces of the resulting glass substrate. In addition, between these processes, the method may further include a lapping process and polishing process for an inner and outer peripheral end face of the glass substrates. 
     As the method of manufacturing a glass substrate for magnetic recording media according to the present invention, when the lapping process is carried out for surfaces except end face of a glass substrate, a diamond pad to which diamond abrasive grain was fixed with binder is used. The diamond pad is described below. Therefore, according to the present invention, with no use or less use of expensive cerium oxide slurry used in conventional polishing process, a polishing surface having little waviness and high surface smoothness can be obtained. 
     In other words, the polishing process using conventional cerium oxide slurry becomes unnecessary, and, as a result the two-step conventional polishing process can be changed to a one-step polishing process with use of silicon oxide slurry. Therefore, the method of the present invention reduces cost for polishing a glass substrate for magnetic recording media, and obtains high productivity. 
     The method of manufacturing a glass substrate for magnetic recording media according to the present invention is described in detail as following, referring to examples of the first and the second embodiments. 
     Example of First Embodiment 
     As example of a first embodiment, primary lapping process, inner and outer peripheral lapping process, inner peripheral polishing process, second lapping process, tertiary lapping process, outer peripheral polishing process and primary polishing process are performed in this order. 
     As the primary lapping process, a lapping machine  10  as shown in  FIG. 1  is used, and a primary lapping process is carried out for surfaces except end face of the glass substrate W. In other words, this lapping machine  10  includes a pair of upper and lower faceplates  11  and  12 , and several sheets of glass substrates are sandwiched between the faceplates  11  and  12  which are rotating in directions opposite to each other, and then both sides of the glass substrates W are lapped by grinding pads installed in the faceplates  11  and  12 . 
     The grinding pad used for the primary lapping process is a diamond pad  20 A to which diamond abrasive grain was fixed with binder (bond) as shown in  FIGS. 2A and 2B . In addition, the lapping surface  20   a  has plural tile-like projections having a flat top which are installed in line. 
     It is preferable that the size S of the projections  21  of the diamond pad  20 A used in primary lapping process be 1.5 to 5 mm square with a height T 0.2 to 3 mm, and the space G between adjacent projections be 0.5 to 3 mm. According to the present invention, by using the diamond pad  20 A satisfying the above range, liquid coolant or lapping fluid can reach everywhere equally, and grinding dust can be discharged smoothly from a space interval of the projection  21  of the lap surface  20   a.    
     In addition, it is preferable that the diamond pad  20 A used in the primary lapping process have an average diamond grain size of 4 to 12 μm, and a content of diamond grains in the projection  21  of 5 to 70% by volume. It is more preferable that the diamond pad  20 A have a content of diamond grains of 20 to 30% by volume. This increases cost due to an increase in the process time when the diamond abrasive grain size and content are less than the above range. On the other hand, it becomes more difficult to obtain desired surface roughness when the diamond abrasive grain size and the content exceed the above range. In addition, as a binder of diamond pad  20 A, for example, polyurethane resin, phenol-based resin, melamine-based resin, etc. can be used. 
     As the inner and outer peripheral lapping process, lapping machine  30  as shown in  FIG. 3  is used, and lapping process is carried out for inner peripheral end surface of a central aperture of glass substrate W and outer peripheral end face of the glass substrate W. The lapping machine  30  includes inner peripheral grindstone  31  and outer peripheral grindstone  32 . The laminated substrates X is obtained by laminating plural pieces of glass substrates W in the state that the central apertures is matched each other wherein spacers (not shown) are sandwiched. While rotating the laminated substrates X around axis, and inserting an inner peripheral grindstone  31  into a central aperture of glass substrates and putting glass substrates W in a radial direction between outer peripheral grindstone  32  which is placed on the periphery of each glass substrate W, the inner peripheral grindstone  31  and outer peripheral grindstone  32  are rotated in the direction opposite to that of laminated substrates X. And at the same time as lapping the inner peripheral end face of the glass substrates W by the inner peripheral grindstone  31 , outer peripheral end face of the glass substrates W is lapped by the peripheral grindstone  32 . 
     In addition, since there are waveforms having projection and recess lines alternately in an axial direction on the surface of the inner peripheral grindstone  31  and the outer peripheral grindstone  32 , in addition to grinding the inner peripheral end face and the outer peripheral end face of glass substrates W, it is possible to chamfer the edge portion between the two principal surfaces and inner and outer peripheral end face of glass substrates W. 
     As the inner peripheral polishing process, a polishing machine  40  as shown in  FIG. 4  is used, and the polishing process is carried out for the inner peripheral end face of a central aperture of glass substrate W. That is, the polishing machine  40  has an inner peripheral polishing brush  41 , while rotating laminated substrates X around the axis, and the inner peripheral polishing brush  41  inserted in a central aperture of each glass substrate W is rotated in the direction opposite to that of the glass substrates W and moved vertically. At this time, polishing fluid is dropped to the inner peripheral polishing brush  41 . And then, the inner peripheral end face of glass substrates W is polished by the inner peripheral polishing brush  41 . At the same time, the edge portion of the inner and outer peripheral end face, which was chamfered in the inner and outer peripheral lapping process, is polished. In addition, as the polishing fluid, for example, the slurried fluid obtained by dispersing silicon oxide (colloidal silica) or cerium oxide abrasive grain into water can be used. 
     As the secondary lapping process, like the primary lapping process, a lapping machine  10  as shown in  FIG. 1  is used, and, the secondary lapping process is carried out for surfaces except end face of glass substrate W. That is, while sandwiching plural pieces of glass substrates W between a pair of upper and lower faceplates  11  and  12 , which are rotating in directions opposite to each other, both sides of these glass substrate W are lapped by a grinding pad installed in the faceplate  11 ,  12 . 
     The grinding pad used for the secondary lapping process, like the diamond pad  20 A as shown in  FIGS. 2A and 2B , is a diamond pad  20 B to which diamond abrasive grain was fixed with binder (bond). In addition, the lapping surface  20   a  has plural tile-like projections having a flat top which are installed in line. 
     As the same as the diamond pad  20 A shown in  FIGS. 2A and 2B , it is preferable that the size S of the projections  21  of the diamond pad  20 B used in secondary lapping process be 1.5 to 5 mm square with a height T 0.2 to 3 mm, and the space G between adjacent projections be 0.5 to 3 mm. According to the present invention, by using the diamond pad  20 B to satisfy the above range, liquid coolant or lapping fluid can reach everywhere equally, and grinding dust can be discharged smoothly from an interval of the projection  21  of the lap surface  20   a.    
     In addition, it is preferable that the diamond pad  20 B used in the secondary lapping process have an average diamond grain size of 1 to 5 μm, and a content of diamond grains in the projection  21  of 5 to 80% by volume. It is more preferable that the content of diamond grains be 50 to 70% by volume. This increases cost due to an increase in the process time when the diamond abrasive grain size and content are less than the above range. On the other hand, it becomes more difficult to obtain desired surface roughness when the diamond abrasive grain size and content exceed the above range. In addition, as a binder of diamond pad  20 B, for example, polyurethane resin, phenol-based resin, melamine-based resin, etc. can be used. 
     As the tertiary lapping process, like the primary and secondary lapping processes, the lapping machine  10  as shown in  FIG. 1  is used, and the tertiary lapping process is carried out for surfaces except for the end face of the glass substrate W. In other words, this lapping machine  10  includes a pair of upper and lower faceplates  11  and  12 , several sheets of glass substrates sandwiched between the faceplates  11  and  12  which are rotating in directions opposite to each other, and both sides of the glass substrates W are lapped by grinding pads installed in the faceplates  11  and  12 . 
     The same as the diamond pad  20 A as shown in  FIGS. 2A and 2B , the grinding pad used for the tertiary lapping process is a diamond pad  20 C to which diamond abrasive grain was fixed with binder (bond). In addition, the lapping surface  20   a  has plural tile-like projections having a flat top which are installed in line. 
     The same as the diamond pad  20 A shown in  FIGS. 2A and 2B , it is preferable that the size S of the projections  21  of the diamond pad  20 A used in tertiary lapping process be 1.5 to 5 mm square with a height T 0.2 to 3 mm, and the space G between adjacent projections be 0.5 to 3 mm. According to the present invention, by using the diamond pad  20 C to satisfy the above range, liquid coolant or lapping fluid can reach everywhere equally, and grinding dust can be discharged smoothly from an interval of the projection  21  of the lap surface  20   a.    
     In addition, it is preferable that the diamond pad  20 A used in the tertiary lapping process have an average diamond grain size of 0.2 to 2 μm, and a content of diamond grains in the projection  21  of 5 to 80% by volume. It is more preferable that it have a content of diamond grains of 50 to 80% by volume. This increases cost due to increase the process time when diamond abrasive grain size and content are less than the above range. On the other hand, it becomes more difficult to obtain desired surface roughness when the diamond abrasive grain size and content exceed the above range. In addition, as a binder of diamond pad  20 C, for example, polyurethane resin, phenol-based resin, melamine-based resin, etc. can be used. 
     As the outer peripheral polishing process, a polishing machine  50  as shown in  FIG. 5  is used, and the polishing process is carried out for the outer peripheral end face of the glass substrate W. The polishing machine  50  has a rotating shaft  51  and outer peripheral polishing brush  52 . The laminated substrates X is obtained by laminating plural pieces of glass substrates W in the state that the central apertures match each other wherein spacers (not shown) are sandwiched. While rotating laminated substrates X around the axis by the rotating shaft  51  inserted in a central aperture of each glass substrate W, the outer peripheral polishing brush  52  in contact with the surface of the outer peripheral end face of the glass substrates, is rotated in the direction opposite to that of the glass substrates W and moved vertically. At this time, polishing fluid is dropped to the outer peripheral polishing brush  51 . And then, the outer peripheral end face of glass substrates W is polished by the outer peripheral polishing brush  51 . At the same time, the edge portion of the inner and outer peripheral end face, which was chamfered in the inner and outer peripheral lapping process, is polished. In addition, as the polishing fluid, for example, the slurried fluid obtained by dispersing silicon oxide (colloidal silica) or cerium oxide abrasive grain into water can be used. 
     As primary polishing process, polishing machine  60  as shown in  FIG. 6  is used, and primary polishing process is carried out for surfaces except for the end face of the glass substrate W. That is, while sandwiching plural pieces of glass substrates W between a pair of upper and lower faceplates  61  and  62 , which are rotating in directions opposite to each other, both sides of these glass substrates W are polished by the grinding pad installed in the faceplates  61  and  62 . 
     The grinding pad used for the primary polishing process is a hard polishing cloth made of urethane, for example. When the polishing process is carried out for surfaces except for the end face of glass substrate W, polishing fluid is dropped to the glass substrates W. As the polishing fluid, for example, the slurried fluid obtained by dispersing silicon oxide (colloidal silica) abrasive grain into water can be used. 
     In addition, the glass substrate W subjected to the lapping and polishing process is fed to the cleaning process and final inspection process. And in the final cleaning process, for example, a chemical cleaning method using cleaner (chemical) combined with an ultrasonic method is used to clean the glass substrate W and remove the abrasive, for example, used in the above step. In addition, with the inspection process, by using laser optical inspection equipment, for example, examination of the presence or absence of distortion or scratches on the surface of the glass substrate W can be performed. 
     As the method of manufacturing a glass substrate for magnetic recording media according to the present invention, diamond pads  20 A,  20 B,  20 C to which diamond abrasive grains as shown in  FIGS. 2A and 2B  were fixed to with binder in the primary, secondary and tertiary lapping process are used. In addition, the lapping surface  20   a  of the diamond pads  20 A,  20 B,  20 C has plural tile-like projections having a flat top which are installed in line. 
     According to the present invention, by using such diamond pads  20 A,  20 B,  20 C, it is possible that while grinding dust is discharged smoothly from an interval of projection  21  of lapping surface  20   a , the surfaces except for the end face of the glass substrate W can be polished to smoothness. In addition, since a two-step conventional polishing process (primary and secondary polishing processes) can be changed to a one-step polishing process, the application of expensive cerium oxide abrasive grain can be reduced. In addition, since the polishing process has a long process time in comparison with a lapping process, process time can be reduced. Thus, according to the present invention, it is possible to manufacture the glass substrate for magnetic recording media, wherein the substrate has a high surface smoothness and little waviness with high productivity. 
     Example of the Second Embodiment 
     As an example of a second embodiment, primary lapping process, inner and outer peripheral lapping process, inner peripheral polishing process, second lapping process, outer peripheral polishing process and primary polishing process are performed in this order. 
     As the primary lapping process, the lapping machine  10  as shown in  FIG. 1  is used, and a primary lapping process is carried out for surfaces except for the end face of the glass substrate W. In other words, this lapping machine  10  includes a pair of upper and lower faceplates  11  and  12 , several sheets of glass substrates sandwiched between the faceplates  11  and  12  which are rotating in directions opposite to each other, both sides of the glass substrates W are lapped by grinding pads installed in the faceplates  11  and  12 . 
     The grinding pad used for the primary lapping process, the same as the diamond pad  20 A as shown in  FIGS. 2A and 2B , is a diamond pad  20 D to which diamond abrasive grain was fixed with binder (bond). In addition, the lapping surface  20   a  has plural tile-like projections having a flat top which are installed in line. 
     It is preferable that the size S of the projections  21  of the diamond pad  20 D used in primary lapping process be 1.5 to 5 mm square with a height T 0.2 to 3 mm, and the space G between adjacent projections be 0.5 to 3 mm. According to the present invention, by using the diamond pad  20 D to satisfy the above range, liquid coolant or lapping fluid can reach everywhere equally, and grinding dust can be discharged smoothly from an interval of the projection  21  of the lap surface  20   a.    
     In addition, it is preferable that the diamond pad  20 D used in the primary lapping process have an average diamond grain size of 3 to 10 μm, and a content of diamond grains in the projection  21  of 5 to 70% by volume. It is more preferable that it have a content of diamond grains of 20 to 30% by volume. This increases cost due to increase the process time when diamond abrasive grain size and content are less than the above range. On the other hand, it becomes more difficult to obtain desired surface roughness when the diamond abrasive grain size and content exceed the above range. In addition, as a binder of diamond pad  20 D, for example, polyurethane resin, phenol-based resin, melamine-based resin, etc. can be used. 
     As the inner and outer peripheral lapping process, lapping machine  30  as shown in  FIG. 3  is used, and lapping process is carried out for inner peripheral end surface of a central aperture of glass substrate W and outer peripheral end face of the glass substrate W. The lapping machine  30  includes the inner peripheral grindstone  31  and outer peripheral grindstone  32 . The laminated substrates X is obtained by laminating plural pieces of glass substrates W in the state that the central apertures match each other wherein spacers (not shown) is sandwiched. While rotating the laminated substrates X around axis, and inserting the inner peripheral grindstone  31  into a central aperture of glass substrates and putting glass substrates W in a radial direction between the outer peripheral grindstone  32  which is placed on the periphery of each glass substrate W, the inner peripheral grindstone  31  and outer peripheral grindstone  32  are rotated in the direction opposite to that of laminated substrates X. And at the same time as grinding the inner peripheral end face of the glass substrates W by the inner peripheral grindstone  31 , the outer peripheral end face of the glass substrates W is lapped by peripheral grindstone  32 . 
     In addition, since there are waveforms having a projection and recess line alternately in an axial direction on the surface of the inner peripheral grindstone  31  and the outer peripheral grindstone  32 , in addition to grinding inner peripheral end face and the outer peripheral end face of glass substrates W, it is possible to chamfer the edge portion between the two principal surfaces and inner and outer peripheral end face of glass substrates W. 
     As the inner peripheral polishing process, using the polishing machine  40  as shown in  FIG. 4 , and polishing process is carried out for the inner peripheral end face of a central aperture of the glass substrate W. That is, the polishing machine  40  has an inner peripheral polishing brush  41 , while rotating laminated substrates X around axis, and the inner peripheral polishing brush  41  inserted in a central aperture of each glass substrate W is rotated in the direction opposite to that of the glass substrates W and moved vertically. At this time, polishing fluid is dropped to inner peripheral polishing brush  41 . And then, the inner peripheral end face of glass substrates W is polished by the inner peripheral polishing brush  41 . At the same time, the edge portion of the inner and outer peripheral end face, which was chamfered in the inner and outer peripheral lapping process, is polished. In addition, as the polishing fluid, for example, the slurried fluid obtained by dispersing silicon oxide (colloidal silica) or cerium oxide abrasive grain into water can be used. 
     As the secondary lapping process, like the primary lapping process, the lapping machine  10  as shown in  FIG. 1  is used, and, the secondary lapping process is carried out for surfaces except end face of glass substrate W. That is, while sandwiching plural pieces of glass substrates W between a pair of upper and lower faceplates  11  and  12 , which are rotating in directions opposite to each other, both sides of these glass substrate W are lapped by grinding pad installed in the faceplate  11 ,  12 . 
     The grinding pad used for the secondary lapping process, like the diamond pad  20 A as shown in  FIGS. 2A and 2B , is a diamond pad  20 E to which diamond abrasive grain was fixed with binder (bond). In addition, the lapping surface  20   a  has plural tile-like projections having a flat top which are installed in line. 
     The same as the diamond pad  20 A as shown in  FIGS. 2A and 2B , it is preferable that the size S of the projections  21  of the diamond pad  20 E used in the secondary lapping process be 1.5 to 5 mm square with a height T 0.2 to 3 mm, and the space G between adjacent projections be 0.5 to 3 mm. According to the present invention, by using the diamond pad  20 E to satisfy the above range, liquid coolant or lapping fluid can reach everywhere equally, and grinding dust can be discharged smoothly from an interval of projection  21  of lap surface  20   a.    
     In addition, it is preferable that the diamond pad  20 E used in the secondary lapping process have an average diamond grain size of 0.2 to 2 μm, and a content of diamond grains in the projection  21  of 5 to 80% by volume. It is more preferable that it have a content of diamond grains of 50 to 70% by volume. This increases cost due to an increase in the process time when diamond abrasive grain size and content are less than the above range. On the other hand, it becomes more difficult to obtain desired surface roughness when the diamond abrasive grain size and content exceed the above range. In addition, as a binder of diamond pad  20 E, for example, polyurethane resin, phenol-based resin, melamine-based resin, etc. can be used. 
     As the outer peripheral polishing process, using the polishing machine  50  as shown in  FIG. 5 , the polishing process is carried out for outer peripheral end face of glass substrate W. The polishing machine  50  has a rotating shaft  51  and outer peripheral polishing brush  52 . The laminated substrates X is obtained by laminating plural pieces of glass substrates W in the state that the central apertures match each other wherein spacers (not shown) are sandwiched. While rotating laminated substrates X around the axis by the rotating shaft  51  inserted in a central aperture of each glass substrate W, the outer peripheral polishing brush  52  in contact with the surface of the outer peripheral end face of the glass substrates, is rotated in the direction opposite to that of the glass substrates W and moved vertically. At this time, polishing fluid is dropped to outer peripheral polishing brush  51 . And then, the outer peripheral end face of glass substrates W is polished by the outer peripheral polishing brush  51 . At the same time, the edge portion of the inner and outer peripheral end face, which was chamfered in the inner and outer peripheral lapping process, is polished. In addition, as the polishing fluid, for example, the slurried fluid obtained by dispersing silicon oxide (colloidal silica) or cerium oxide abrasive grain into water can be used. 
     As the primary polishing process, the polishing machine  60  as shown in  FIG. 6  is used, and the primary polishing process is carried out for surfaces except end face of glass substrate W. That is, while sandwiching plural pieces of glass substrates W between a pair of upper and lower faceplates  61 ,  62 , which are rotating in directions opposite to each other, both sides of these glass substrate W are polished by the grinding pad installed in the faceplates  61 ,  62 . 
     The grinding pad used for the primary polishing process is a hard polishing cloth made of urethane, for example. When the polishing process is carried out for surfaces except for the end face of glass substrate W, polishing fluid is dropped to the glass substrates W. As the polishing fluid, for example, the slurried fluid obtained by dispersing silicon oxide (colloidal silica) abrasive grain into water can be used. 
     In addition, the glass substrate W subjected to the lapping and polishing process is fed to the cleaning process and final inspection process. And in the final cleaning process, for example, a chemical cleaning method using cleaner (chemical) combined with an ultrasonic method is used to clean the glass substrate W and remove the abrasive, for example, used in the above step. In addition, the inspection process, by using laser optical inspection equipment, for example, examination of the presence or absence of distortion or scratches on the surface of the glass substrate W. 
     As the method of manufacturing a glass substrate for magnetic recording media according to the present invention, diamond pads  20 D,  20 E to which diamond abrasive grain as shown in  FIGS. 2A and 2B  were fixed to with binder in the primary and secondary lapping process are used. In addition, the lapping surface  20   a  of the diamond pads  20 D,  20 E has plural tile-like projections having a flat top which are installed in line. 
     According to the present invention, by using such diamond pads  20 D,  20 E, it is possible that while grinding dust is discharged smoothly from an interval of projection  21  of lap surface  20   a , the surfaces except of end face of the glass substrate W can be polished to smoothness. In addition, since two-steps conventional polishing process (primary and secondary polishing processes) can be changed to one-step polishing process, the application of expensive cerium oxide abrasive grain can be reduced. In addition, since the polishing process has a long process time in comparison with a lapping process, the process time can be reduced. Thus, according to the present invention, it is possible to manufacture the glass substrate for magnetic recording media, wherein the substrate has a high surface smoothness and little waviness with high productivity. 
     According to the present invention, as a lapping fluid used in each lapping process of the first and second embodiments, a commercial source can be used. Lapping fluid, by classifying roughly, includes water-based lapping fluid and oil-based lapping fluid. The water-based lapping fluid contains pure water and a suitable amount of alcohol with addition of polyethylene glycol as a viscosity modifier, amine, and surfactant etc. On the other hand, the oil-based lapping fluid contains oil with addition of an appropriate amount of stearic acid as an extreme pressure additive. For example, as commercial lapping fluid, water-based Sabrelube 9016 (manufactured by Chemetall) can be used. 
     In addition, according to the present invention, polishing auxiliaries and anticorrosive may be added to the lapping fluids used in each lapping process and polishing fluids used in polishing process in the first and the second embodiments. 
     Specifically, polishing auxiliaries include an organic polymer having sulfonate group or carboxylic acid group at least. And an organic polymers containing sodium sulphonate or sodium carboxylate, having average molecular weight of 4000-10000, is preferable. Thus, it is possible to smooth surfaces of the glass substrate W in the above processes. 
     As an organic polymer containing a carboxylic acid or sodium sulfonate, for example, GEROPON SC/213 (Product Name/Rhodia), GEROPON T/36 (Product Name/Rhodia), GEROPON TA/10 (Product Name/Rhodia), GEROPON TA/72 (Product Name/Rhodia), New Karugen WG-5 (Product Name/Takemoto Oil &amp; Fat Corporation), Agurizoru G-200 (Product Name/Kao Corporation), Demoru EP Powder (Product Name/Kao Corporation), Demoru RNL (Product Name/Kao Corporation), Isoban 600-SF35 (Product name/Kuraray Ltd.), Polystar OM (Product Name/NOF Corporation), Sokalan CP9 (Product Name/BASF Japan (Ltd.)), Sokalan PA-15 (Product Name/BA SF Japan Ltd.), Tokisanon GR-31A (Product Name/Sanyo Chemical Industries, Corporation), 7248 Sorpol (Product Name/Toho Chemical Industries, Ltd.), Sharoru AN-103P (Product Name/Dai-ichi Kogyo Seiyaku Co., Ltd. Corporation), Aron T-40 (Product Name/Toagosei Chemical Industry Co., Ltd.), Panakayaku CP (product Name/Nippon Kayaku Inc.), roll disk H12C (Product Name/Nippon Nyukazai Co., Ltd.) can be used. As the polishing auxiliaries, Demoru RNL (Product Name/Kao Corporation) and Polystar OM (Product Name/NOF Corporation) is preferably used. 
     In addition, the magnetic recording media manufactured using the glass substrate W generally includes corrosion-prone material such as Co, Ni and Fe in magnetic layer. Thus, by adding a corrosion inhibitor in the lapping and polishing solution described above, it is possible to prevent the magnetic layer from corrosion and to obtain a magnetic recording medium excellent in electromagnetic conversion characteristics. 
     As a corrosion inhibitor, it is preferable to use benzotriazole or a derivative thereof. As derivatives of benzotriazole, for example, the compound obtained by substituting one, two or more hydrogen atoms of benzotriazole by a carboxyl group, methyl group, amino group, hydroxyl group. In addition, as derivatives of benzotriazole, 4-carboxy benzotriazole or a salt thereof, 7-carboxymethyl benzotriazole or a salt thereof, benzotriazole butyl ester, 1-hydroxymethyl benzotriazole, 1-hydroxybenzotriazole etc. can be used. As the addition amount of corrosion inhibitor, the total amount of diamond slurry during use, it is preferable to use less than 1% by mass concerning to the total amount of diamond slurry during using, and 0.001% to 0.1% by mass is more preferable. 
     In addition, the present invention is not intended to be limited to those of the embodiments. It is possible to make changes in various ways without departing from the spirit of the invention. 
     For example, regarding the lapping machine used in each lapping process and polishing machine used in polishing process of the first and second embodiments, it is possible that the polishing or lapping machine include a pair of lower faceplate  71  and upper faceplate  72  and plural carriers  73  which, for example, were placed at the surface of lower faceplate  71  which is opposed to the faceplate  72  as shown in  FIG. 7 . Glass substrates (not shown) are placed into a plural number of opening  74  (for example, 35 in the present embodiment) installed in each carrier  73 , and then the glass substrates are lapped by lapping pad or polished by polishing pad which are installed in the lower faceplate  71  and upper faceplate  72 . 
     Specifically, the lower faceplate  71  and upper faceplate  72  can be rotated at the same central axis and in directions opposite to each other with the rotation movement of the rotation shafts  71   a  and  72   a  which are rotated by drive motor (not shown), wherein the rotation shafts  71   a  and  72   a  are placed at the center of the lower faceplate  71  and upper faceplate  72 . Also, a recess  75  for placing the plurality carriers  73  (for example, five in this embodiment) is provide on the surface of lower faceplate  71  wherein the surface is facing upper faceplate  72 . 
     As a plural carrier  73 , for example, it can be obtained by forming a disk made from epoxy resin which is strengthened by mixing aramid fiber and fiberglass. And these plural carriers  73  are placed around the axis  71   a  in the inner wall of recess  75 . In addition, a planet gears part  76  is installed in the periphery of each carrier  73  over the entire circumference. Meanwhile, in the inner periphery of the recess  75 , a sun gear portion  77  which rotates with the rotation shaft  71   a  while being intermeshed with the planetary gear carrier  76  of each  73  is provided. In the periphery of the recess  75 , the fixed gear portion  78  which is intermeshed with planetary gear  76  of each carrier  73  is provided. 
     When the sun gear portion  77  rotates with the rotating shaft  71   a , since sun gear portion  77  and fixed gear portion  78  are intermeshed with the planetary gear  76 , these plural carriers  73  move (revolution) in the same direction as that of rotation shaft  71   a  around the periphery of the rotation shaft  71   a  in a recess  75 . At the same time, each of these plural carriers  73  rotates (rotation) in the direction opposite to that of rotation shaft  71   a  around each central axis. The above motion is so-called planetary motion. 
     Therefore, when the above configuration is adopted by the lapping machine used in each lapping process, the plural glass substrates held at the opening  75  of each carrier  73  can be lapped by the lapping pad or polished by the polishing pad placed at the lower faceplate  71  and upper faceplate  72  while being moved as planetary motion. In the case of this configuration, the glass substrate can be lapped or polished not only more accurately, but also quickly. 
     EXAMPLE 
     The effects of the present invention become clear by the following Examples. It is apparent that the present invention is not limited to the Examples, but may be modified and changed without departing from the scope and spirit of the invention. 
     Example 1 
     In Example 1, firstly, a glass substrate (manufactured by Ohara, TS-10SX) having an outside diameter of 48 mm, center hole of 12 mm, thickness of 0.560 mm was used. 
     And, for the glass substrate, primary lapping process, inner and outer peripheral lapping process, inner peripheral polishing process, secondary lapping process, the tertiary lapping process, outer peripheral polishing process and primary polishing process were performed in this order. 
     In detail, the primary lapping process is carried out by using a lapping machine including a pair of upper and lower faceplates. While several sheets of glass substrates are sandwiched between the faceplates which are rotating in directions opposite to each other, both sides of the glass substrates are lapped by lapping pads installed in the faceplates. At this time, Toraizakuto (Product Name/Manufactured by Sumitomo 3M) was used as the lapping pad of the primary lapping process. The size of the projections of the lapping pad is 2.6 mm square with a height 2 mm, and the space between adjacent projections is 1 mm, the average particle size of the diamond grains is 9 μm, a content of diamond grains in the projection of 20%. As the lapping machine, the four-way double-sided lapping machine (16B, manufactured by Hamai Co., Ltd) was used. The lapping process is carried out for 15 minutes, under the condition that rotational speed of the faceplate is 25 rpm, the processing pressure is 120 g/cm 2 . As a lapping fluid, Sabrelube 9016 (Manufactured by Chmetall) is used after diluted 1:10 in water, lapping amount per one side of the glass substrate was about 100 μm. 
     In the inner and outer peripheral lapping process, lapping machine including inner peripheral grindstone and outer peripheral grindstone was used. The laminated substrates are obtained by laminating plural pieces of glass substrates in the state that the central apertures match each other wherein spacers are sandwiched. While rotating the laminated substrates around the axis, and inserting inner peripheral grindstone into a central aperture of glass substrates and putting glass substrates in a radial direction between outer peripheral grindstones which is placed on the periphery of each glass substrate, the inner peripheral grindstone and outer peripheral grindstone are rotated in the direction opposite to that of laminated substrates. And at the same time as grinding inner peripheral end face of the glass substrates by inner peripheral grindstone, outer peripheral end face of the glass substrates is lapped by peripheral grindstone. The inner peripheral grindstone and outer peripheral grindstone use diamond grains with the average particle size of 10 μm. The rotation speed of the inner peripheral grindstone and outer peripheral grindstone is 1200 rpm and 600 rpm, respectively, and processing was carried out for 30 seconds. 
     As the inner peripheral polishing process, using a polishing machine having an inner peripheral polishing brush, while laminated substrates are rotated around the axis, and the inner peripheral polishing brush inserted in a central aperture of each glass substrate is rotated in the direction opposite to that of the glass substrates and moved vertically, at the same time polishing fluid is dropped to inner peripheral polishing brush, the inner peripheral end face of glass substrates is polished by the inner peripheral polishing brush. In this case, a nylon brush was used as the inner peripheral polishing brush. The rotation speed of inner peripheral polishing brush is 300 rpm, and processing was carried out for 10 minutes 
     The secondary lapping process is carried out by using a lapping machine including a pair of upper and lower faceplates. While several sheets of glass substrates is sandwiched between the faceplates which are rotating in directions opposite to each other, both sides of the glass substrates are lapped by lapping pads installed in the faceplates. At this time, Toraizakuto (Product Name) manufactured by Sumitomo 3M was used as the lapping pad of the secondary lapping process. The size of the projections  21  of the lapping pad is 2.6 mm square with a height 2 mm, and the space between adjacent projections is 1 mm, the average particle size of the diamond grains is 3 μm, a content of diamond grains in the projection of 50%. As the lapping machine, the four-way double-sided lapping machine (16B, manufactured by Hamai Co., Ltd) was used. Lapping process is carried out for 10 minutes, under the condition that rotational speed of the faceplate is 25 rpm, the processing pressure is 120 g/cm 2 . As a lapping fluid, Sabrelube 9016 (Manufactured by Chmetall) is used after diluted 1:10 in water, lapping amount per one side of the glass substrate was about 30 μm. 
     The tertiary lapping process is carried out by using a lapping machine including a pair of upper and lower faceplates. While several sheets of glass substrates is sandwiched between the faceplates which are rotating in directions opposite to each other, both sides of the glass substrates are lapped by lapping pads installed in the faceplates. At this time, Toraizakuto (Product Name) manufactured by Sumitomo 3M was used as the lapping pad of the primary lapping process. The size of the projections  21  of the lapping pad is 2.6 mm square with a height 2 mm, and the space between adjacent projections is 1 mm, the average particle size of the diamond grains is 0.5 μm, a content of diamond grains in the projection of 60%. As the lapping machine, the four-way double-sided lapping machine (16B, manufactured by Hamai Co., Ltd) was used. Lapping process is carried out for 10 minutes, under the condition that rotational speed of the faceplate is 25 rpm, the processing pressure is 120 g/cm 2 . As a lapping fluid, Sabrelube 9016 (Manufactured by Chmetall) is used after diluted 1:10 in water, lapping amount per one side of the glass substrate was about 10 μm. 
     In the outer peripheral polishing process, a polishing machine having an outer peripheral polishing brush was used. The laminated substrates were obtained by laminating plural pieces of glass substrates in the state that the central apertures match each other wherein spacers are sandwiched. While rotating laminated substrates around the axis by the rotating shaft inserted in a central aperture of each glass substrate, the outer peripheral polishing brush in contact with the surface of the outer peripheral end face of the glass substrates, is rotated in the direction opposite to that of the glass substrates and moved vertically. At the same time polishing fluid is dropped to outer peripheral polishing brush, outer peripheral end face of glass substrates was polished by the outer peripheral polishing brush. In the case, a nylon brush was used as the outer peripheral polishing brush. The rotation speed of outer peripheral polishing brush is 300 rpm, and processing was carried out for 10 minutes 
     In the primary polishing process, the polishing machine including a pair of upper and lower faceplates is used. While sandwiching plural pieces of glass substrates between a pair of upper and lower faceplates, which are rotating in directions opposite to each other, both sides of these glass substrate are polished by a polishing pad installed in the faceplates. 
     In this case, as the primary polishing pad, Smoke Aid type (manufactured by Filwel) was used. The polishing fluid is polishing slurry having silica content is 0.5 wt % by adding water to a silica abrasive polishing fluid having 40 wt % silica as solids content (0.08 μm average particle size, Compol, manufactured by Fujimi). As the polishing machine, the four-way double-sided polishing machine (16B, manufactured by Hamai Co., Ltd) was used. Polishing process is carried out for 30 minutes, under the condition that rotational speed of the faceplate is 25 rpm, the processing pressure is 110 g/cm 2 , while the polishing liquid is supplied in a rate of 7 L/minutes. Polishing amount per one side of the glass substrate was about 2 μm. 
     And the glass substrates obtained after the above processes were treated using chemical cleaning by the anionic surfactant combined with ultrasound treatment. As a result, the glass substrates for magnetic recording media of Example 1 were obtained. 
     Example 2 
     In Example 2, the lapping processes of the glass substrates in Example 1 were changed to two-steps. As the lapping pad used in the primary lapping process, lap of glass substrate, Toraizakuto (Product Name, Manufactured by Sumitomo 3M) was used. The size of the projections of the lapping pad is 2.6 mm square with a height 2 mm, and the space between adjacent projections is 1 mm, the average particle size of the diamond grains is 4 μm, a content of diamond grains in the projection of 50%. As the lapping machine, the four-way double-sided lapping machine (16B, manufactured by Hamai Co., Ltd) was used. Lapping process is carried out for 10 minutes, under the condition that rotational speed of the faceplate is 25 rpm, the processing pressure is 120 g/cm 2 . As a lapping fluid, Sabrelube 9016 (Manufactured by Chmetall) is used after diluted 1:10 in water, lapping amount per one side of the glass substrate was about 30 μm. 
     The secondary lapping process, the primary polishing process, and others were carried out as the same as in Example 1. 
     Comparative Example 1 
     In Comparative Example 1, the lapping process of glass substrates in Example 1 is changed to two-steps, and the polishing process was changed to two-steps. In detail, the tertiary lapping process in Comparative Example 1 was not carried out, and two-step polishing processes were carried out under the following conditions 
     In the primary polishing process, the polishing machine including a pair of upper and lower faceplates is used. While sandwiching plural pieces of glass substrates between a pair of upper and lower faceplates, which are rotating in directions opposite to each other, both sides of these glass substrate are polished by polishing pad installed in the faceplates. 
     In this case, the primary polishing pad, Smoke Aid type (manufactured Filwel) was used. The polishing fluid is polishing slurry having ceria content of 0.6 wt % obtained by adding water to ceria abrasive polishing fluid (SHOROX Tohoku Metal Chemical Co., Ltd., particle size 1.0 microns). As the polishing machine, the four-way double-sided polishing machine (16B, manufactured by Hamai Co., Ltd) was used. Polishing process is carried out for 40 minutes, under the condition that rotational speed of the faceplate is 30 rpm, the processing pressure is 110 g/cm 2 , while the polishing liquid is supplied in a rate of 8 L/minutes. Polishing amount per one side of the glass substrate was about 15 μm. 
     In the secondary polishing process, the polishing machine including a pair of upper and lower faceplates is used. While sandwiching plural pieces of glass substrates between a pair of upper and lower faceplates, which are rotating in directions opposite to each other, both sides of these glass substrate are polished by polishing pad installed in the faceplates. 
     In this case, the primary polishing pad, Smoke Aid type (manufactured Filwel) was used. The polishing fluid is polishing slurry having ceria content of 0.6 wt % obtained by adding water to a commercially available ceria abrasive polishing fluid having ceria content of 12 wt % (SHOROX Tohoku Metal Chemical Co., Ltd., particle size 1.0 microns). As the polishing machine, the four-way double-sided polishing machine (16B, manufactured by Hamai Co., Ltd) was used. Polishing process is carried out for 30 minutes, under the condition that rotational speed of the faceplate is 25 rpm, the processing pressure is 110 g/cm 2 , while the polishing liquid is supplied in a rate of 8 L/minutes. Polishing amount per one side of the glass substrate was about 2 
     Then, surface roughness Ra and little waviness of the glass substrate obtained in Examples 1 and 2, and Comparative Example 1 were measured. As the measurement of surface roughness Ra and little waviness, Atomic Force Microscope (D3000 Manufactured by Digital Instruments) was used. 
     As a result, surface roughness Ra of glass substrates for magnetic recording medium of Example 1 was 0.3 nm, micro-waviness Wa is 0.2 nm, surface roughness Ra of glass substrates for magnetic recording medium of Example 2 is 0.3 nm, micro-waviness Wa is 0.25 nm, surface roughness Ra of glass substrates for magnetic recording medium of Comparative Example 1 is 0.5 nm, micro-waviness Wa was 0.3 nm. Therefore, in Examples 1 and 2, a glass substrate (glass substrate for magnetic recording media) having high surface smoothness and little swell can be manufactured, in comparison with Comparative Example 1. 
     DENOTATION OF REFERENCE NUMERALS 
     
         
           10  . . . lapping machine 
           11 , 12  . . . faceplate 
           20 A,  20 B . . . diamond pad 
           20   a  . . . lap surface 
           21  . . . projection 
           30  . . . lapping machine 
           31  . . . inner peripheral grindstone 
           32  . . . outer peripheral grindstone 
           40  . . . polishing machine 
           41  . . . inner peripheral polishing brush 
           50  . . . polishing machine 
           51  . . . rotating shaft 
           52  . . . outer peripheral polishing brush 
           60  . . . polishing machine 
           61 , 62  . . . faceplate 
           71  . . . bottom faceplate 
           72  . . . upper faceplate 
           73  . . . carrier 
           74  . . . opening 
           75  . . . recess 
           76  . . . planet gears part 
           77  . . . sun gears part 
           78  . . . fixed gear part 
         W . . . glass substrate 
         X . . . laminated substrates