METHOD OF MANUFACTURING MAGNETIC RECORDING MEDIUM GLASS SUBSTRATE

Abstract

Provided is a magnetic recording medium glass substrate manufacturing method capable of manufacturing a magnetic recording medium glass substrate having high surface smoothness and low surface waviness with productivity. In a primary lapping process and a secondary lapping process of the manufacturing method, diamond pads (20A and 20B) in which diamond abrasive grains are fixed by a binder are used. A plurality of convex portions (21) with a flat top are arranged in a tile shape on a lapping surface (20a) of each of the diamond pads (20A and 20B). In the diamond pad (20A) used in the primary lapping process, the average diameter of the diamond abrasive grains is in the range of 4 μm to 12 μm and the content of the diamond abrasive grains in the convex portion (20A) is in the range of 5 vol % to 70 vol %. In the diamond pad (20B) used in the secondary lapping process, the average diameter of the diamond abrasive grains is in the range of 1 μm to 5 μm and the content of the diamond abrasive grains in the convex portion (20B) is in the range of 5 vol % to 80 vol %.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of manufacturing a magnetic recording medium glass substrate.

[0002] Priority is claimed on Japanese Patent Application No. 2009-257112, filed Nov. 10, 2009, and Japanese Patent Application No. 2010-182326, filed Aug. 17, 2010, the content of which is incorporated herein by reference.

BACKGROUND ART

[0003] The recording density of a magnetic recording medium used in a hard disk drive (HDD) has been significantly improved. In particular, with the introduction of an MR head or a PRML technique, the surface recording density of the magnetic recording medium has significantly increased. In recent years, for example, a GMR head or a TMR head has been introduced and the surface recording density has increased at a rate of 1.5 times per year. However, there is a demand for an improvement in recording density.

[0004] With the increase in the recording density of the magnetic recording medium, there is an increasing demand for a substrate for the magnetic recording medium. An aluminum alloy substrate and a glass substrate have been used as the substrate for the magnetic recording medium. Of the two kinds of substrates, in general, the hardness, surface smoothness, rigidity, and impact resistance of the glass substrate are better than those of the aluminum alloy substrate. Therefore, a magnetic recording medium glass substrate capable of improving the recording density has received increased attention.

[0005] When the magnetic recording medium glass substrate is manufactured, a large glass plate is cut into disk-shaped glass substrates, or a disk-shaped glass substrate is directly formed from molten glass by press-molding using a mold. Then, a lapping (grinding) process and a polishing process are performed on the main surface and end surface of the obtained glass substrate.

[0006] In a method of manufacturing a magnetic recording medium glass substrate according to the related art, a primary lapping process (grinding), a secondary lapping process (grinding), a primary polishing process (polishing), and a secondary polishing process (polishing) are performed on the main surface of the glass substrate in this order. Then, a lapping process and a polishing process for the inner and outer circumferential end surfaces of the glass substrate are performed between the processes.

[0007] Regarding the related art of the invention, for example, the following PTL 1 is known. PTL 1 discloses a technique which performs a primary lapping process using a diamond pellet, such as a resin, metal, or vitrified grinding wheel, and a secondary lapping process using a diamond pad to process a substrate in a short time without incurring defects, such as low surface smoothness, scratches, grinding marks, or suction marks.

CITATION LIST

Patent Literature

[0008] [PTL 1] Japanese Patent No. 4049510

SUMMARY OF INVENTION

Technical Problem

[0009] However, in recent years, with a reduction in the floating height of a magnetic head, there is a demand for a magnetic recording medium glass substrate with a surface waviness or a surface roughness lower than that in the related art. The inventors found that the grinding allowance of one surface was in the range of 100 μm to 300 μm in the primary lapping process, and when the glass substrate was damaged in the primary lapping process, processing strain occurred in the glass substrate, which caused long-period waviness on the surface of the end-product magnetic recording medium.

[0010] The invention has been made in view of the above-mentioned problems, and an object of the invention is to provide a magnetic recording medium glass substrate manufacturing method capable of manufacturing a magnetic recording medium glass substrate having high surface smoothness and low surface waviness with high productivity.

Solution to Problem

[0011] The invention provides the following means.

[0012] According to a first aspect of the invention, there is provided a method of manufacturing a magnetic recording medium glass substrate. The method includes a step of performing a primary lapping process on a surface of the glass substrate except for at least an end surface and a step of performing a secondary lapping process on the surface of the glass substrate. The primary lapping process and the secondary lapping process use diamond pads in which diamond abrasive grains are fixed by a binder. A plurality of convex portions with a flat top are arranged in a tile shape on a lapping surface of the diamond pad. In the diamond pad used in the primary lapping process, the average diameter of the diamond abrasive grains is in the range of 4 μm to 12 μm and the content of the diamond abrasive grains in the convex portion is in the range of 5 vol % to 70 vol %. In the diamond pad used in the secondary lapping process, the average diameter of the diamond abrasive grains is in the range of 1 μm to 5 μm and the content of the diamond abrasive grains in the convex portion is in the range of 5 vol % to 80 vol %.

[0013] According to a second aspect of the invention, in the method of manufacturing a magnetic recording medium glass substrate according to the first aspect, in the diamond pads used in the primary lapping process and the secondary lapping process, the convex portion may have outside dimensions including a size of 1.5 mm to 5 mm×1.5 mm to 5 mm and a height of 0.2 mm to 3 mm, and a gap between adjacent convex portions may be in the range of 0.5 mm to 3 mm.

Advantageous Effects of Invention

[0014] As described above, in the invention, the diamond pads are used in the primary lapping process and the secondary lapping process, and the diamond pads used in the primary lapping process and the secondary lapping process are optimized. Therefore, it is possible to manufacture a magnetic recording medium glass substrate having high surface smoothness and low surface waviness with high productivity.

BRIEF DESCRIPTION OF DRAWINGS

[0015] FIG. 1 is a diagram illustrating a process of manufacturing a magnetic recording medium glass substrate according to the invention and is a perspective view illustrating a main surface lapping process.

[0016] FIG. 2A is an enlarged plan view illustrating a pad surface of a diamond pad used in the main surface lapping process.

[0017] FIG. 2B is an enlarged cross-sectional view illustrating the diamond pad used in the main surface lapping process taken along the line A-A'.

[0018] FIG. 3 is a diagram illustrating the process of manufacturing the magnetic recording medium glass substrate according to the invention and is a perspective view illustrating an inner/outer circumferential end surface lapping process.

[0019] FIG. 4 is a diagram illustrating the process of manufacturing the magnetic recording medium glass substrate according to the invention and is a perspective view illustrating an inner circumferential end surface polishing process.

[0020] FIG. 5 is a diagram illustrating the process of manufacturing the magnetic recording medium glass substrate according to the invention and is a perspective view illustrating an outer circumferential end surface polishing process.

[0021] FIG. 6 is a diagram illustrating the process of manufacturing the magnetic recording medium glass substrate according to the invention and is a perspective view illustrating a main surface polishing process.

[0022] FIG. 7 is a perspective view illustrating another example of the structure of a lapping machine or a polishing machine used in the invention.

DESCRIPTION OF EMBODIMENTS

[0023] Hereinafter, a method of manufacturing a magnetic recording medium glass substrate according to the invention will be described in detail with reference to the accompanying drawings.

[0024] The magnetic recording medium glass substrate manufactured by the invention is a disk-shaped glass substrate having a central hole. A magnetic recording medium is obtained by sequentially forming, for example, a magnetic layer, a protective layer, and a lubrication film on the surface of the glass substrate. In a magnetic recording/reproducing device (HDD), the center of the magnetic recording medium is attached to a rotating shaft of a spindle motor and a magnetic head writes or reads information to or from the magnetic recording medium while floating and traveling on the surface of the magnetic recording medium rotated by the spindle motor. The magnetic recording medium glass substrate may be made of, for example, SiO₂--Al₂O₃--R₂O-based chemical strengthening glass (R indicates at least one kind of element selected from alkali metal elements), SiO₂--Al₂O₃--Li₂O-based glass ceramics, or SiO₂--Al₂O₃--MgO--TiO₂-based glass ceramics. Among them, in particular, for example, SiO₂--Al₂O₃--MgO--CaO--Li₂O--Na₂O--ZrO₂--Y.- sub.2O₃--TiO₂--As₂O₃-based chemical strengthening glass, SiO₂--Al₂O₃--Li₂O--Na₂O--ZrO₂--As.su- b.2O₃-based chemical strengthening glass, SiO₂--Al₂O₃--MgO--ZnO--Li₂O--P₂O₅--ZrO₂--K₂O--Sb₂O₃-based glass ceramics, SiO₂--Al₂O₃--MgO--CaO--BaO--TiO₂--P₂O₅--As.- sub.2O₃-based glass ceramics, and SiO₂--Al₂O₃--MgO--CaO--SrO--BaO--TiO₂--ZrO₂--Bi.- sub.2O₃--Sb₂O₃-based glass ceramics may be appropriately used. In addition, for example, the magnetic recording medium glass substrate is made of lithium disilicate, a SiO₂-based crystal (for example, quartz, cristobalite, tridymite), cordierite, enstatite, aluminum magnesium titanate, spinel-based crystal ([Mg and/or Zn]Al₂O₄, [Mg and/or Zn]₂TiO₄, and a solid fluid between the two crystals), forsterite, spodumene, and glass ceramics having, for example, solid fluids of these crystals as crystal phases.

[0025] When the magnetic recording medium glass substrate is manufactured, first, a large flat glass plate is cut into glass substrates, or a glass substrate is directly formed from molten glass by press-molding using a mold. In this way, the disk-shaped glass substrate having a central hole is obtained.

[0026] Then, a lapping (grinding) process and a polishing process are performed on the surface (the main surface) of the obtained glass substrate except for an end surface. In addition, a process of performing lapping and polishing on the inner and outer circumferential end surfaces of the glass substrate is preformed between the two processes. In the invention, a chamfering process may be performed on the inner and outer circumferential end surfaces of the glass substrate, simultaneously with the lapping process.

[0027] Specifically, in this embodiment, a primary main surface lapping process, an inner/outer circumferential end surface lapping process, an inner circumferential end surface polishing process, a secondary main surface lapping process, an outer circumferential end surface polishing process, a primary main surface polishing process, and a secondary main surface polishing process are performed in this order.

[0028] Among them, in the primary main surface lapping process, the primary lapping process is performed on two main surfaces (finally, a recording surface of the magnetic recording medium) of a glass substrate W using a lapping machine 10 shown in FIG. 1. That is, the lapping machine 10 includes a pair of upper and lower plates 11 and 12. A plurality of glass substrates W are interposed between the plates 11 and 12 which are rotated in the opposite direction. Then, the two main surfaces of each glass substrate W are polished by grinding pads which are provided on the plates 11 and 12.

[0029] As shown in FIGS. 2A and 2B, the grinding pad used in the primary lapping process is a diamond pad 20A in which diamond abrasive grains are fixed by a binder (bond). A plurality of convex portions 21 with a flat top are arranged in a tile shape on a lapping surface 20a of the diamond pad 20A. In addition, the diamond pad 20A is formed by arranging a plurality of convex portions 21 in which the diamond abrasive grains are fixed by the binder on the surface of a base material 22.

[0030] In the diamond pad 20A used in the primary lapping process, it is preferable that the convex portion 21 have the following outside dimensions S: a size of 1.5 mm to 5 mm×1.5 mm to 5 mm; a height T of 0.2 mm to 3 mm; and a gap G of 0.5 mm to 3 mm between adjacent convex portions 21. In the invention, when the diamond pad 20A satisfying the above-mentioned ranges is used, it is possible to uniformly spread, for example, a coolant or a grinding liquid and smoothly discharge grinding swarf between the convex portions 21 of the lapping surface 20a.

[0031] In the diamond pad 20A used in the primary lapping process, the average diameter of the diamond abrasive grains is in the range of 4 μm to 12 μm and preferably in the range of 6 μm to 12 μm, and the content of the diamond abrasive grains in the convex portion 21 is preferably in the range of 5 vol % to 70 vol %, more preferably in the range of 5 vol % to 40 vol %, and most preferably in the range of 20 vol % to 30 vol %. When the diameter and content of the diamond abrasive grains are less than the above-mentioned ranges, the processing time increases, which results in an increase in costs. On the other hand, when the diameter and content of the diamond abrasive grains are more than the above-mentioned ranges, it is difficult to obtain a desired surface roughness. The binder of the diamond pad 20A may be made of a resin, such as a polyurethane-based resin, a phenol-based resin, a melamine-based resin, or an acryl-based resin.

[0032] In the inner/outer circumferential end surface lapping process, a lapping process is performed on the inner circumferential end surface of the central hole of the glass substrate W and the outer circumferential end surface of the glass substrate W using a lapping machine 30 shown in FIG. 3. That is, the lapping machine 30 includes an inner circumferential whetstone 31 and an outer circumferential whetstone 32. A laminate X obtained by laminating a plurality of glass substrates W with a spacer S therebetween and the central holes aligned with each other is interposed between the inner circumferential whetstone 31 which is inserted into the central holes of the glass substrates W and the outer circumferential whetstone 32 which is arranged on the outer circumferential sides of the glass substrates W in the diametric direction of each glass substrate W while being rotated on its own axis, and the inner circumferential whetstone 31 and the outer circumferential whetstone 32 are rotated in a direction opposite to the rotational direction of the laminate X. Then, the inner circumferential end surface of each glass substrate W is ground by the inner circumferential whetstone 31. At the same time, the outer circumferential end surface of each glass substrate W is ground by the outer circumferential whetstone 32.

[0033] The surface of each of the inner circumferential whetstone 31 and the outer circumferential whetstone 32 has a corrugated shape in which convex portions and concave portions are alternately arranged in the axial direction. Therefore, the inner circumferential whetstone 31 and the outer circumferential whetstone 32 can grind the inner circumferential end surface and the outer circumferential end surface of each glass substrate W and chamfer edge portions (chamfered surfaces) between the two main surfaces and the inner and outer circumferential end surfaces of each glass substrate W. In this embodiment, the lapping process is performed on the inner and outer circumferential end surfaces of the glass substrate W in one stage, but the invention is not limited thereto. The lapping process may be performed in two stages (primary and secondary lapping processes).

[0034] In the inner circumferential whetstone 31 and the outer circumferential whetstone 32, the diamond abrasive grains are fixed by a binder. The binder may be made of a metal material, such as copper, copper alloy, cobalt, tungsten carbide, or nickel. It is preferable that the average diameter of the diamond abrasive grains in the inner circumferential whetstone 31 and the outer circumferential whetstone 32 be equal to or greater than 4 μm and equal to or less than 12 μm. The content of the diamond abrasive grains in the inner circumferential whetstone 31 and the outer circumferential whetstone 32 is preferably in the range of 5 vol % to 95 vol % and more preferably in the range of 20 vol % to 90 vol %. When the diameter and content of the diamond abrasive grains are less than the above-mentioned ranges, the processing time increases, which results in an increase in costs. On the other hand, when the diameter and content of the diamond abrasive grains are more than the above-mentioned ranges, it is difficult to obtain a desired surface roughness.

[0035] In the inner circumferential end surface polishing process, a polishing process is performed on the inner circumferential end surface of the central hole of each glass substrate W using a polishing machine 40 shown in FIG. 4. That is, the polishing machine 40 includes an inner circumference polishing brush 41. The laminate X is rotated on its own axis and the inner circumference polishing brush 41 inserted into the central hole of each glass substrate W is moved in the vertical direction while being rotated in a direction opposite to the rotational direction of the glass substrate W. In this case, a polishing liquid is dropped onto the inner circumference polishing brush 41. Then, the inner circumferential end surface of each glass substrate W is polished by the inner circumference polishing brush 41. At the same time, the edge portion (chamfered surface) of the inner circumferential end surface chamfered by the inner/outer circumferential end surface lapping process is also polished. For example, slurry obtained by dispersing cerium oxide abrasive grains in water may be used as the polishing liquid.

[0036] In the secondary main surface lapping process, similarly to the primary main surface lapping process, a secondary lapping process is performed on two main surfaces of the glass substrate W using the lapping machine 10 shown in FIG. 1. That is, a plurality of glass substrates W are interposed between a pair of upper and lower plates 11 and 12 which are rotated in the opposite direction and the two main surfaces of each glass substrate W are ground by the grinding pads which are provided on the plates 11 and 12.

[0037] Similarly to the grinding pad 20A shown in FIGS. 2A and 2B, the grinding pad used in the secondary lapping process is a diamond pad 20B in which diamond abrasive grains are fixed by a binder (bond). A plurality of convex portions 21 with a flat top are arranged in a tile shape on a lapping surface 20a of the diamond pad 20B. In addition, the diamond pad 20B is formed by arranging a plurality of convex portions 21 in which the diamond abrasive grains are fixed by the binder on the surface of a base material 22.

[0038] Similarly to the diamond pad 20A shown in FIGS. 2A and 2B, in the diamond pad 20B used in the second lapping process, it is preferable that the convex portion 21 have the following outside dimensions S: a size of 1.5 mm to 5 mm×1.5 mm to 5 mm; a height T of 0.2 mm to 3 mm; and a gap G of 0.5 mm to 3 mm between adjacent convex portions 21. In the invention, when the diamond pad 20B satisfying the above-mentioned ranges is used, it is possible to uniformly spread, for example, a coolant or a grinding liquid and smoothly discharge grinding swarf between the convex portions 21 of the lapping surface 20a.

[0039] In the diamond pad 20B used in the secondary lapping process, the average diameter of the diamond abrasive grains is in the range of 1 μm to 5 μm and preferably in the range of 2 μm to 5 μm and the content of the diamond abrasive grains in the convex portion 21 is preferably in the range of 5 vol % to 80 vol % and more preferably in the range of 50 vol % to 70 vol %. When the diameter and content of the diamond abrasive grains are less than the above-mentioned ranges, the processing time increases, which results in an increase in costs. On the other hand, when the diameter and content of the diamond abrasive grains are more than the above-mentioned ranges, it is difficult to obtain a desired surface roughness. The binder of the diamond pad 20B may be made of a resin, such as a polyurethane-based resin, a phenol-based resin, a melamine-based resin, or an acryl-based resin.

[0040] In the outer circumferential end surface polishing process, a polishing process is performed on the outer circumferential end surface of the glass substrate W using a polishing machine 50 shown in FIG. 5. That is, the polishing machine 50 includes a rotating shaft 51 and an outer circumference polishing brush 52. A laminate X obtained by laminating a plurality of glass substrates W with a spacer S therebetween and the central holes aligned with each other is rotated on its own axis by the rotating shaft 51 which is inserted into the central hole of each glass substrate W, and the outer circumference polishing brush 52 contacted with the outer circumferential end surface of each glass substrate W is moved in the vertical direction while being rotated in a direction opposite to the rotational direction of the laminate X. In this case, a polishing liquid is dropped onto the outer circumference polishing brush 52. Then, the outer circumferential end surface of each glass substrate W is polished by the outer circumference polishing brush 52. At the same time, the edge portion (chamfered surface) of the outer circumferential end surface chamfered in the inner/outer circumferential end surface lapping process is also polished. For example, slurry obtained by dispersing cerium oxide abrasive grains in water may be used as the polishing liquid.

[0041] In the primary main surface polishing process, a primary polishing process is performed on two main surfaces of each glass substrate W using a polishing machine 60 shown in FIG. 6. That is, the polishing machine 60 includes a pair of upper and lower plates 61 and 62. A plurality of glass substrates W are interposed between the plates 61 and 62 which are rotated in the opposite direction, and two main surfaces of each glass substrate W are polished by grinding pads which are provided on the plates 61 and 62.

[0042] The polishing pad used in the primary polishing process is, for example, a hard abrasive cloth made of urethane. When two main surfaces of the glass substrate W are polished (ground) by the polishing pad, a polishing liquid is dropped onto the glass substrate W. For example, slurry obtained by dispersing cerium oxide abrasive grains in water may be used as the polishing liquid.

[0043] Similarly to the primary main surface polishing process, in the secondary main surface polishing process, a secondary polishing process is performed on two main surfaces of each glass substrate W using the polishing machine 60 shown in FIG. 6. That is, a plurality of glass substrates W are interposed between the plates 61 and 62 which are rotated in the opposite direction, and two main surfaces of each glass substrate W are polished by the grinding pads which are provided on the plates 61 and 62.

[0044] The polishing pad used in the secondary polishing process is, for example, a soft abrasive cloth, such as suede. When two main surfaces of the glass substrate W are polished by the polishing pad, a polishing liquid is dropped onto the glass substrate W. For example, slurry obtained by dispersing cerium oxide abrasive grains or colloidal silica in a solvent, such as water, may be used as the polishing liquid.

[0045] Then, the glass substrate W subjected to the lapping process and the polishing process is sent to a final cleaning process and a test process. In the final cleaning process, the glass substrate W is cleaned by, for example, a chemical cleaning method using a detergent (chemical) and ultrasonic wave and the polishing agent used in the above-mentioned processes is removed. In the test process, for example, an optical tester using a laser is used to examine whether the surface (the main surface, the end surface, and the chamfered surface) of the glass substrate W is scratched or twisted.

[0046] In the method of manufacturing the magnetic recording medium glass substrate according to the invention, in the primary lapping process and the secondary lapping process, the use of the diamond pads 20A and 20B shown in FIGS. 2A and 2B makes it possible to smooth the surface of the glass substrate W except for the end surface in a short time while smoothly discharging grinding swarf between the convex portions 21 of the lapping surface 20a. In addition, it is possible to reduce the processing time in the subsequent primary main surface polishing process and secondary main surface polishing process. Therefore, according to the invention, it is possible to manufacture a magnetic recording medium glass substrate having high surface smoothness and low surface waviness with high productivity.

[0047] In the invention, grinding liquids on the market may be used in the primary lapping process and the secondary lapping process. The grinding liquids are mainly classified into aqueous grinding liquids and oil grinding liquids. The aqueous grinding liquid includes, for example, pure water, an appropriate amount of alcohol, polyethyleneglycol serving as a viscosity modifier, amine, and a surface-active agent. The oil grinding liquid includes oil and a stearic acid serving as an extreme-pressure additive. For example, aqueous Sabrelube 9016 (manufactured by Chemetall) may be used as the grinding liquid on the market.

[0048] In the invention, a polishing aid or an anticorrosive may be added to the grinding liquids used in the primary lapping process and the secondary lapping process, and the polishing liquids used in the primary polishing process and the secondary polishing process.

[0049] Specifically, the polishing aid includes organic polymers having at least a sulfonic acid group or a carboxylic acid group. Among them, it is preferable to use an organic polymer including sodium sulfonate or sodium carboxylate and having an average molecular weight of 4000 to 10000. In this way, it is possible to further improve the smoothness of the surface of the glass substrate W in the above-mentioned processes.

[0050] Examples of the organic polymer including sodium sulfonate or sodium carboxylate includes GEROPON SC/213 (trade name/Rhodia), GEROPON T/36 (trade name/Rhodia), GEROPON TA/10 (trade name/Rhodia), GEROPON TA/72 (trade name/Rhodia), Newcalgen WG-5 (trade name/Takemoto Oil & Fat Co., Ltd.), Agrisol G-200 (trade name/Kao Corporation), Demol EP powder (trade name/Kao Corporation), Demol RNL (trade name/Kao Corporation), Isoban 600-SF35 (trade name/Kuraray Co., Ltd.), Polystar OM (trade name/NOF Corporation), Sokalan CP9 (trade name/BASF Japan Ltd.), Sokalan PA-15 (trade name/BASF Japan Ltd.), Toxanon GR-31A (trade name/Sanyo Chemical Industries, Ltd.), Solpol 7248 (trade name/Toho Chemical Industry Co., Ltd.), Sharoll AN-103P (trade name/Dai-Ichi Kogyo Seiyaku Co., Ltd.), Aron T-40 (trade name/Toagosei Co., Ltd.), Panakayaku CP (trade name/Nippon Kayaku Co., Ltd.), and Disrol H12C (trade name/Nippon Nyukazai Co., Ltd.). Among these, in particular, it is desirable to use Demol RNL (trade name/Kao Corporation) or Polystar OM (trade name/by NOF Corporation) as the polishing aid.

[0051] In the magnetic recording medium which is manufactured using the glass substrate W, in general, the magnetic layer includes a corrosion-prone material, such as Co, Ni, or Fe. Therefore, an anticorrosive is added to the grinding liquid or the polishing liquid to prevent the corrosion of the magnetic layer. In this way, it is possible to obtain a magnetic recording medium with good electromagnetic conversion properties.

[0052] It is preferable that benzotriazole or derivatives thereof be used as the anticorrosive. For example, a compound obtained by replacing one or two or more hydrogen atoms of benzotriazole with a carboxyl group, a methyl group, an amino group, or a hydroxyl group may be used as the benzotriazole derivative. Examples of the benzotriazole derivative include 4-carboxybenzotriazole or its salt, 7-carboxybenzotriazole or its salt, benzotriazole butyl ester, 1-hydroxymethylbenzotriazole, and 1-hydroxybenzotriazole. The amount of anticorrosive added is preferably equal to or less than 1 mass % and more preferably in the range of 0.001 mass % to 0.1 mass % with respect to the total amount when the diamond slurry is used.

[0053] The invention is not necessarily limited to the above-described embodiment, but various modifications and changes of the invention can be made without departing from the scope and spirit of the invention.

[0054] For example, as shown in FIG. 7, the lapping machine 10 used in the primary main surface lapping process and the secondary main surface lapping process and the polishing machine 50 used in the primary main surface polishing process and the secondary main surface polishing process may have the following structure: each of the lapping machine 10 and the polishing machine 50 includes a pair of upper and lower plates 71 and 72 and a plurality of carriers 73 which are provided on one surface of the lower plate 71 facing the upper plate 72; glass substrates'(not shown) are set in a plurality of openings 74 (in this embodiment, 35 openings) provided in each of the carriers 73; and two main surfaces of each of the plurality of glass substrates are ground or polished by the grinding pads or the polishing pads provided on the lower plate 71 and the upper plate 72.

[0055] Specifically, the rotating shafts 71a and 72a provided at the centers of the lower plate 71 and the upper plate 72 are rotated by a motor (not shown) such that the lower plate 71 and the upper plate 72 can be rotated in the opposite direction with the central axes aligned with each other. In addition, a concave portion 75 for arranging a plurality of carriers 73 (in this embodiment, five carriers) is provided in the surface of the lower plate 71 facing the upper plate 72.

[0056] The plurality of carriers 73 are obtained by forming a reinforced epoxy resin having aramid fiber or glass fiber mixed therewith in a disk shape. The plurality of carriers 73 are arranged around the rotating shaft 71a in the concave portion 75. A planetary gear unit 76 is provided over the entire outer circumferential portion of each carrier 73. A sun gear unit 77 which is engaged with the planetary gear 76 of each carrier 73 and is rotated together with the rotating shaft 71a is provided in the inner circumferential portion of the concave portion 75, and a fixed gear unit 78 which is engaged with the planetary gear unit 76 of each carrier 73 is provided in the outer circumferential portion of the concave portion 75.

[0057] In this way, when the sun gear unit 77 is rotated together with the rotating shaft 71a, the plurality of carriers 73 make a so-called sun-and-planet motion in which they are rotated on their own axes in a direction opposite to the rotational direction of the rotating shaft 71a while being rotated around the rotating shaft 71a in the concave portion 75 in the same direction as that in which the rotating shaft 71a is rotated, by the engagement between the sun gear unit 77 and the fixed gear unit 78, and the planetary gear units 76.

[0058] Therefore, the lapping machine 10 and the polishing machine 50 having the above-mentioned structure can grind or polish two main surfaces of each of the plurality of glass substrates held in the openings 75 of each carrier 73 with the grinding pads or the polishing pads provided on the lower plate 71 and the upper plate 72 while moving the plurality of glass substrates as in the sun and planet system. In this structure, it is possible to rapidly grind or polish the glass substrates with high accuracy.

EXAMPLES

[0059] Next, the effect of the invention will be clearly described with reference to examples. The invention is not limited to the following examples, but various modifications and changes to the invention can be made without departing from the scope and spirit of the invention.

Example 1

[0060] In Example 1, first, a glass substrate (TS-10SX manufactured by Ohara, Inc.) with an outside diameter of 48 mm, a central hole diameter of 12 mm, and a thickness of 0.560 mm was used.

[0061] The primary main surface lapping process, the inner/outer circumferential end surface lapping process, the inner circumferential end surface polishing process, the secondary main surface lapping process, the outer circumferential end surface polishing process, the primary main surface polishing process, and the secondary main surface polishing process were performed on the glass substrate in this order.

[0062] Specifically, in the primary main surface lapping process, a plurality of glass substrates were interposed between a pair of upper and lower plates, which were rotated in the opposite direction, of the lapping machine and the two main surfaces of each of the glass substrates were ground by the grinding pads which are provided on the plates. In this case, a diamond pad (Trizact (trade name) manufactured by Sumitomo 3M Limited) was used as the grinding pad in the primary lapping process. In the diamond pad, the outside dimensions of a convex portion were as follows: the convex portion had a size of 2.6 mm×2.6 mm and a height of 2 mm; the gap between the convex portions was 1 mm; the average diameter of diamond abrasive grains was 9 μm; the content of the diamond abrasive grains in the convex portion was about 20 vol %; and an acryl-based resin was used as a binder. A 4-way type double-sided polishing machine (16B manufactured by Hamai Co., Ltd.) was used as the lapping machine and grinding was performed under the following conditions: the number of rotations of the plate: 25 rpm; pressing pressure: 120 g/cm²; and the processing time: 15 minutes. The grinding liquid obtained by diluting Sabrelube 9016 (manufactured by Chemetall) 1 to 10 with water was used and the amount of grinding of one surface of each glass substrate was about 100 μm.

[0063] In the inner/outer circumferential end surface lapping process, a lapping machine including an inner circumferential whetstone and an outer circumferential whetstone was used. A laminate obtained by laminating a plurality of glass substrates with a spacer therebetween and the central holes aligned with each other was interposed between the inner circumferential whetstone which was inserted into the central holes of the glass substrates and the outer circumferential whetstone which was arranged on the outer circumferential sides of the glass substrates in the diametric direction of each glass substrate while being rotated on its own axis. The inner circumferential whetstone and the outer circumferential whetstone were rotated in a direction opposite to the rotational direction of the laminate. In this way, the inner and outer circumferential end surfaces of each glass substrate were respectively ground by the inner circumferential whetstone and the outer circumferential whetstone at the same time. In this case, as the inner circumferential whetstone and the outer circumferential whetstone, a whetstone was used in which the content of diamond abrasive grains with an average grain diameter of 10 μm was 80 vol % and a copper alloy was used as the binder. Grinding was performed under the following conditions: the number of rotations of the inner circumferential whetstone: 1200 rpm; the number of rotations of the outer circumferential whetstone: 600 rpm; and the processing time: 30 seconds.

[0064] In the inner circumferential end surface polishing process, a polishing machine including an inner circumference polishing brush was used. The laminate was rotated on its own axis, and the inner circumference polishing brush inserted into the central hole of each glass substrate was moved in the vertical direction while was being rotated in a direction opposite to the rotational direction of the glass substrate. During the operation, a polishing liquid was dropped onto the inner circumference polishing brush. In this way, the inner circumferential end surface of each glass substrate was polished. In this case, a nylon brush was used as the inner circumference polishing brush and ceria slurry was used as the polishing liquid. Polishing was performed under the following conditions: the number of rotations of the inner circumference polishing brush: 300 rpm and the processing time: 10 minutes.

[0065] In the secondary main surface lapping process, a lapping machine including a pair of upper and lower plates was used. A plurality of glass substrates were interposed between the plates which were rotated in the opposite direction and two main surfaces of each of the glass substrates were ground by the grinding pads which are provided on the plates. In this case, a diamond pad (Trizact (trade name) manufactured by Sumitomo 3M Limited) was used as the grinding pad in the secondary lapping process. In the diamond pad, the outside dimensions of a convex portion were as follows: the convex portion had a size of 2.6 mm×2.6 mm and a height of 2 mm; the gap between the convex portions was 1 mm; the average diameter of diamond abrasive grains was 4 μm; the content of the diamond abrasive grains in the convex portion was about 50 vol %; and an acryl-based resin was used as a binder. A 4-way type double-sided polishing machine (16B manufactured by Hamai Co., Ltd.) was used as the lapping machine and grinding was performed under the following conditions: the number of rotations of the plate: 25 rpm; pressing pressure: 120 g/cm²; and the processing time: 10 minutes. The grinding liquid obtained by diluting Sabrelube 9016 (manufactured by Chemetall) 1 to 10 with water was used and the amount of grinding of one surface of each glass substrate was about 30 μm.

[0066] In the outer circumferential end surface polishing process, a polishing machine including an outer circumference polishing brush was used. A laminate obtained by laminating a plurality of glass substrates with a spacer therebetween and the central holes aligned with each other was rotated on its own axis by a rotating shaft which was inserted into the central hole of each glass substrate, and the outer circumference polishing brush contacted with the outer circumferential end surface of each glass substrate was moved in the vertical direction while being rotated in a direction opposite to the rotational direction of the laminate. During the operation, a polishing liquid was dropped onto the outer circumference polishing brush. In this way, the outer circumferential end surface of each glass substrate was polished. In this case, a nylon brush was used as the outer circumference polishing brush and ceria slurry was used as the polishing liquid. The polishing was performed under the following conditions: the number of rotations of the outer circumference polishing brush: 300 rpm; and the processing time: 10 minutes.

[0067] In the primary main surface polishing process, a polishing machine including a pair of upper and lower plates was used. A plurality of glass substrates were interposed between the plates which were rotated in the opposite direction, and two main surfaces of each of the glass substrates were polished by the polishing pads which are provided on the plates while a polishing liquid was dropped onto the glass substrates. In this case, a suede-type pad (manufactured by Filwel Co., Ltd.) was used as the polishing pad in the primary polishing process, and polishing slurry obtained by adding a ceria-based polishing material (SHOROX manufactured by Tohoku Metal Chemical Co., Ltd.; and grain diameter: 1.0 μm) on the market to water such that the content of ceria was 0.6 mass % was used as the polishing liquid. A 4-way double-sided polishing machine (16B manufactured by Hamai Co., Ltd.) was used as the lapping machine and grinding was performed under the following conditions while a grinding liquid was supplied at a rate of 8 liters/minute: the number of rotations of the plate: 30 rpm; pressing pressure: 110 g/cm²; and the processing time: 40 minutes. The amount of grinding of one surface of each glass substrate was about 15 μm.

[0068] In the secondary main surface polishing process, a polishing machine including a pair of upper and lower plates was used. A plurality of glass substrates were interposed between the plates which were rotated in the opposite direction, and two main surfaces of each of the glass substrates were polished by the polishing pads provided on the plates while a polishing liquid was dropped onto the glass substrates. In this case, a suede-type pad (manufactured by Filwel Co., Ltd.) was used as the polishing pad in the secondary polishing process, and polishing slurry obtained by adding a ceria abrasive-containing solution (average grain diameter: 0.5 μm; and SHOROX manufactured by Showa Denko K.K.) including 12 mass % of solid and a silica abrasive-containing solution (average grain diameter: 0.08 μm; and Compol manufactured by Fujimi Incorporated) including 40 mass % of solid to water such that the content of ceria was 0.6 mass % and the content of silica was 0.2 mass % was used as the polishing liquid. A 4-way double-sided polishing machine (16B manufactured by Hamai Co., Ltd.) was used as the lapping machine and grinding was performed under the following conditions while a grinding liquid was supplied at a rate of 7 liters/minute: the number of rotations of the plate: 25 rpm; pressing pressure: 110 g/cm²; and the processing time: 30 minutes. The amount of grinding of one surface of each glass substrate was about 2 μm.

[0069] Then, a chemical cleaning process using ultrasonic waves and an anionic surface-active agent and a cleaning process using pure water were performed on the obtained glass substrate. In this way, a magnetic recording medium glass substrate according to Example 1 was obtained.

Comparative Example 1

[0070] In Comparative example 1, in the primary main surface lapping process, a lapping machine including a pair of upper and lower plates was used. A plurality of glass substrates were interposed between the plates which were rotated in the opposite direction, and two main surfaces of each of the glass substrates were ground by the grinding pads provided on the plates while a grinding liquid was dropped onto the glass substrates. In this case, a resin bond diamond lapping plate using diamond abrasive grains with an average grain diameter of 9 μm was used as the grinding pad in the primary lapping process, and water was used as the grinding liquid. The lapping plate used in the primary lapping process included diamond grains bonded to the entire surface of a grinding surface by a binder and had a flat surface. In addition, the content of diamond abrasive grains was about 20 vol % and a polyurethane-based resin was used as the binder.

[0071] A 4-way double-sided polishing machine (16B manufactured by Hamai Co., Ltd.) was used as the lapping machine and grinding was performed under the following conditions while the grinding liquid was supplied to the press plates: the number of rotations of the plate: 25 rpm; pressing pressure: 120 g/cm²; and the processing time: 15 minutes. A magnetic recording medium glass substrate was manufactured by the same processes as those in Example 1 except for the above-mentioned processes.

Example 2

[0072] In Example 2, in the primary main surface lapping process according to Example 1, a metal bond diamond pad was used as the grinding pad. The diamond pad was obtained by fixing diamond abrasive grains to a substrate made of a polyurethane-based resin using electroless nickel plating. In the diamond pad, the outside dimensions of a convex portion were as follows: the convex portion had a size of 2.6 mm×2.6 mm and a height of 2 mm; the gap between the convex portions was 1 mm; the average diameter of diamond abrasive grains was 9 μm; and the content of the diamond abrasive grains in the convex portion was about 20 vol %. Manufacturing conditions other than the above were the same as those in Example 1.

Example 3

[0073] In Example 3, in the secondary main surface lapping process according to Example 1, a metal bond diamond pad was used as the grinding pad. The diamond pad was obtained by fixing diamond abrasive grains to a substrate made of a polyurethane-based resin using electroless nickel plating. In the diamond pad, the outside dimensions of a convex portion were as follows: the convex portion had a size of 2.6 mm×2.6 mm and a height of 2 mm; the gap between the convex portions was 1 mm; the average diameter of diamond abrasive grains was 4 μm; and the content of the diamond abrasive grains in the convex portion was about 50 vol %. Manufacturing conditions other than the above were the same as those in Example 1.

[0074] Then, the surface roughness Ra of each of the magnetic recording medium glass substrates according to Examples 1 to 3 and Comparative example 1 was measured. An atomic force microscope (D3000 manufactured by Digital Instruments Inc.) was used to measure the surface roughness Ra.

[0075] As a result, the surface roughness Ra of the magnetic recording medium glass substrate according to Example 1 was 0.3 nm, the surface roughness Ra of the magnetic recording medium glass substrate according to Example 2 was 0.4 nm, and the surface roughness Ra of the magnetic recording medium glass substrate according to Example 3 was 0.6 nm. The surface roughness Ra of the magnetic recording medium glass substrate according to Comparative example 1 was 0.9 nm.

[0076] The waviness of the magnetic recording medium glass substrate according to Example 2 was 5% less than that of the magnetic recording medium glass substrate according to Example 1, and the waviness of the magnetic recording medium glass substrate according to Example 3 is equal to that of the magnetic recording medium glass substrate according to Example 1, and the waviness of all of the magnetic recording medium glass substrate according to Examples 1 to 3 was less than that of the magnetic recording medium glass substrate according to Comparative example 1.

[0077] As described above, in Examples 1 to 3, it was possible to manufacture a glass substrate (magnetic recording medium glass substrate) with a greater surface smoothness than that in Comparative example 1.

[0078] In Examples 2 and 3, the metal bond diamond pad formed using electroless nickel plating as the binder was used as the grinding pad for the primary or secondary lapping process. In the metal bond diamond pad, the holding force of the diamond abrasive grains was improved and it is expected that the lifespan of the pad will increase. However, in Example 2, since the metal bond diamond pad was used in the primary lapping process, the holding force of the diamond abrasive grains was improved, but the breaking of the diamond abrasive grains occurred. As a result, the polishing capability of the pad according to Example 2 was slightly less than that of the pad according to Example 1. In Example 3, the metal bond diamond pad was used in the secondary lapping process. However, since the content of the diamond abrasive grains was large, the holding force of the diamond abrasive grains was not very strong. As a result, the polishing capability of the pad according to Example 3 was slightly less than that of the pad according to Example 1. In contrast, in Example 1, the diamond pad using the resin as the binder was used and had an elastic force stronger than the metal bond diamond pad. Therefore, in Example 1, the glass substrate with a surface roughness Ra higher than that of the glass substrate according to Examples 2 and 3 was obtained.

INDUSTRIAL APPLICABILITY

[0079] The invention can be applied to manufacture a magnetic recording medium glass substrate.

REFERENCE SIGNS LIST

[0080] 10: LAPPING MACHINE

[0081] 11, 12: PLATE

[0082] 20A, 20B: DIAMOND PAD

[0083] 20a: LAPPING SURFACE

[0084] 21: CONVEX PORTION

[0085] 22: BASE MATERIAL

[0086] 30: LAPPING MACHINE

[0087] 31: INNER CIRCUMFERENCE WHETSTONE

[0088] 32: OUTER CIRCUMFERENCE WHETSTONE

[0089] 40: POLISHING MACHINE

[0090] 41: INNER CIRCUMFERENCE POLISHING BRUSH

[0091] 50: POLISHING MACHINE

[0092] 51: ROTATING SHAFT

[0093] 52: OUTER CIRCUMFERENCE POLISHING BRUSH

[0094] 60: POLISHING MACHINE

[0095] 61, 62: PLATE

[0096] 71: LOWER PLATE

[0097] 72: UPPER PLATE

[0098] 73: CARRIER

[0099] 74: OPENING

[0100] 75: CONCAVE PORTION

[0101] 76: PLANETARY GEAR UNIT

[0102] 77: SUN GEAR UNIT

[0103] 78: FIXED GEAR UNIT

[0104] W: GLASS SUBSTRATE

[0105] X: LAMINATE

[0106] S: SPACER

Claims

1. A method of manufacturing a magnetic recording medium glass substrate comprising: a step of performing a primary lapping process on a surface of the glass substrate except for at least an end surface; and a step of performing a secondary lapping process on the surface of the glass substrate, wherein the primary lapping process and the secondary lapping process use diamond pads in which diamond abrasive grains are fixed by a binder, a plurality of convex portions with a flat top are arranged in a tile shape on a lapping surface of the diamond pad, in the diamond pad used in the primary lapping process, the average diameter of the diamond abrasive grains is in the range of 4 μm to 12 μm and the content of the diamond abrasive grains in the convex portion is in the range of 5 vol % to 70 vol %, and in the diamond pad used in the secondary lapping process, the average diameter of the diamond abrasive grains is in the range of 1 μm to 5 μm and the content of the diamond abrasive grains in the convex portion is in the range of 5 vol % to 80 vol %.

2. The method of manufacturing a magnetic recording medium glass substrate according to claim 1, wherein, in the diamond pads used in the primary lapping process and the secondary lapping process, the convex portion has outside dimensions including a size of 1.5 mm to 5 mm×1.5 mm to 5 mm and a height of 0.2 mm to 3 mm, and a gap between adjacent convex portions is in the range of 0.5 mm to 3 mm.