Method for the production of glass substrates for magnetic recording mediums

Abstract

A method for the production of a glass substrate for magnetic recording mediums comprises disk-processing, grinding, polishing and cleaning steps and the method is characterized in that the grinding step is carried out using a cooling liquid and a both side-grinding device for grinding a glass substrate for magnetic recording mediums, in which thin plate-like nonferrous metal-bonded grinding wheels, resin-bonded grinding wheels or vitrified bonded grinding wheels, containing abrasive grains are adhered to the surfaces of the upper and lower surface tables of the device. The method for the production of a glass substrate for magnetic recording mediums permits the easy and efficient production of a glass substrate for magnetic recording mediums, the method does not require the use of any multistage cleaning stage under the application of ultrasonics and any acid-cleaning stage and it can thus eliminate the possibility of any contamination.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for the production of a glass substrate for magnetic recording mediums and more specifically to a method for the production of a glass substrate for magnetic recording mediums, which comprises disk-processing, grinding, polishing and cleaning steps and which permits the inhibition of any contamination of the glass substrate with ions such as Fe ions and/or abrasive grains during the grinding step to thus easily and efficiently form a glass substrate for magnetic recording mediums whose contamination is well-controlled.

[0003] 2. Description of the Prior Art

[0004] There has been desired for the reduction of the magnetic head's flying height of a magnetic disk-recording device in order to improve the recording density of a magnetic recording disk, as the storage capacity of the magnetic disk-recording device has increasingly been high. To reduce the flying height of the head, there has been desired for the development of a substrate for magnetic recording mediums, which is excellent in the surface smoothness, has only a small amount of deposits on the surface thereof and has a substantially small quantity of surface defects.

[0005] As the conventional substrates for magnetic recording mediums excellent in the surface smoothness, there have mainly been used those each prepared by plating an aluminum alloy plate with Ni-P and then polishing the plated main surface of the plate in a multiple step process.

[0006] However, the magnetic disk-recording device has recently been adopted even in the portable personal computer such as notebook-sized personal computers and the magnetic recording medium should be rotated at a high speed on the order of not less than 10,000 rpm in order to improve the response speed of the magnetic disk-recording device. For this reason, there has been desired for the development of a substrate for magnetic disk-recording mediums having high strength capable of withstanding such severe conditions. As such a substrate, which can satisfy the foregoing requirements, there has been adopted a glass substrate.

[0007] Such a mainly adopted glass substrate for magnetic recording mediums includes, for instance, a chemically strengthened glass substrate whose strength is improved by a chemical strengthening treatment or a crystallized glass substrate prepared by melting and molding glass to give a glass substrate, maintaining the glass substrate at a high temperature ranging from 600 to 800° C. over a long period of time to thus partially separate out crystalline phases in the substrate.

[0008] The chemically strengthened glass substrate is, for instance, one obtained by melting a glass material and forming the melt into a glass substrate for chemically strengthened glass substrates, then subjecting the glass substrate to grinding and polishing treatments, and immersing it into a molten salt of, for instance, sodium nitrate or potassium nitrate to form a compression stressed layer on the surface layer thereof and to thus improve the breaking strength thereof. The crystallized glass substrate is one, which comprises 40 to 80% of crystalline glass phases and 20 to 60% of an amorphous glass phase and whose strength is improved by the action of the crystalline phase.

[0009] Conventionally, in the method for the production of a glass substrate for magnetic recording mediums, which comprises disk-processing, grinding, polishing and cleaning steps, the foregoing glass substrate is exposed to a variety of contaminants in these processing steps. In particular, in the step (grinding step) for reducing the thickness of a glass material (blank), the step is carried out using an abrasive liquid containing abrasive grains such as carborundum, alumina or zirconia while circulating the abrasive liquid and therefore, some of the abrasive grains remain on the surface of the glass substrate. In addition, a serious problem arises such that the glass surface is contaminated with, for instance, Fe since the surface table used for the grinding step is made of cast iron and the abrasive liquid is circulated during the grinding operations.

[0010] If abrasive grains remain on a glass substrate, scratch marks are formed on the glass substrate in the subsequent polishing step and if the scratch marks thus generated are not eliminated during the polishing step, they remain thereon as surface defects.

[0011] Moreover, if contaminants containing Fe still remain even on the magnetic disk provided thereon with a magnetic recording medium as a final product, the magnetic disk would suffer from such a serious problem that blisters are formed on the contaminated portions under the conditions of accelerated test for inspecting the disk for the reliability in a high temperature and humidity environment.

[0012] To remove these contaminants, the polishing step should be carried out in multiple stages while applying ultrasonics and the glass substrate should sufficiently be cleaned by immersing it in an inorganic acid such as hydrochloric acid, sulfuric acid, nitric acid or phosphoric acid or an organic acid such as formic acid, oxalic acid, citric acid, tartaric acid or hydroxyacetic acid as a measure for the prevention of contamination with Fe, after the completion of the grinding step. This results in the use of a large-scaled device. In addition, effective acid cleaning requires the use of quite serious conditions such as a high temperature and a high acid concentration. For this reason, such an acid cleaning step require the use of an expensive device having high resistance to acids and sufficient care should be taken to ensure safety and sanitation.

SUMMARY OF THE INVENTION

[0013] Accordingly, it is an object of the present invention to provide a method for easily and efficiently producing a glass substrate for magnetic recording mediums, which does not require the use of any multistage cleaning stage under the application of ultrasonics and any acid-cleaning stage and which can thus eliminate the possibility of any contamination.

[0014] The inventors of this invention have conducted various studies to achieve the foregoing object, have recognized that it is most effective to inhibit the contamination of the glass substrate with ions such as Fe ions and/or abrasive grains during the grinding step, rather than to improve the method for removing contaminants, have thus found that the foregoing object can be accomplished by the use of a both side-grinding device, in which thin plate-like nonferrous grindstones containing abrasive grains are adhered to the surfaces of the upper and lower surface tables of the device and a cooling liquid, in place of the conventionally used surface tables of cast iron for grinding operations and an abrasive liquid containing abrasive grains and have thus completed the present invention.

[0015] According to an aspect of the present invention, there is provided a method for the production of a glass substrate for magnetic recording mediums, which comprises disk-processing, grinding, polishing and cleaning steps, the method being characterized in that the grinding step is carried out using a both side-grinding device, in which thin plate-like nonferrous metal-bonded grinding wheels, resin-bonded grinding wheels or vitrified bonded grinding wheels, containing abrasive grains are adhered to the surfaces of the upper and lower surface tables of the device and a cooling liquid.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The method for the production of a glass substrate for magnetic recording mediums according to the present invention will hereunder be described in more detail.

[0017] The production method of the present invention makes use of a thin plate-like nonferrous metal-bonded grinding wheel, resin-bonded grinding wheel or vitrified bonded grinding wheel, containing abrasive grains. In this respect, the abrasive grains are not restricted to any specific one, but preferably used are, for instance, those comprising diamond grains, cubic boron nitride grains, boron carbide grains, silicon carbide grains, zirconia grains or alumina grains, with the abrasive grains containing diamond being particularly preferred.

[0018] The nonferrous metal-bonded grinding wheel usable in the production method according to the present invention may be, for instance, nickel-base material-bonded, tungsten-base material-bonded, cobalt-base material-bonded and bronze-base material-bonded as well as mixtures thereof-bonded ones. Moreover, the resin-bonded grinding wheel usable herein may be, for instance, phenol resin-bonded and polyimide resin-bonded ones. In addition, the vitrified bonded grinding wheel may be, for instance, glassy material-bonded and ceramic material-bonded ones.

[0019] In the production method according to the present invention, the grain size of the abrasive grains is preferably on the order of 5 to 30 &mgr;m and the content of the abrasive grains in the grinding wheel preferably ranges from 0.5 to 3% by volume. In addition, the grinding wheel has a thin plate-like shape and, for instance, it is practical that the grinding wheel has a circular disk-like shape whose thickness ranges from 2 to 7 mm and whose diameter ranges from 5 to 30 mm.

[0020] Particularly preferably used in the production method of the present invention are metal-bonded diamond grain-containing grinding wheels disclosed in, for instance, Japanese Examined Patent Publication Nos. Sho 60-21942, Sho 61-33890, Sho 61-33891 and Hei 1-33309.

[0021] In the production method of the present invention, the thin plate-like grinding wheels such as those discussed above are used by adhering them to the surfaces of the upper and lower surface tables of a both side-grinding device for grinding a glass substrate for magnetic recording mediums. In this connection, the thin plate-like grinding wheel is arranged on and adhered to the surface tables as disclosed in, for instance, Registered Design No. 770,021 and Japanese Examined Patent Publication No. Hei 6-22790 and the grinding step is carried out while carrying out dressing.

[0022] In the production method of the present invention, the glass components scraped off from the glass substrate during the grinding operations are accumulated in the circulated cooling liquid and a small amount of the grinding wheel components are likewise accumulated therein to thus contaminate the glass substrate. Accordingly, it is preferred to continuously remove these components accumulated in the circulated cooling liquid in the course of the circulation or to intermittently remove the components present in the circulated cooling liquid in predetermined intervals.

[0023] Therefore, in the production method according to the present invention, it is preferred to remove solid contents present in the cooling liquid used in the grinding step and to then recycle it in the step.

[0024] As means for the removal of the solid contents present in the cooling liquid used in the grinding step, there may be listed, for instance, separation by filtration through a filtering medium such as diatomaceous earth, paper or cloth; centrifugation; and separation by sedimentation. The efficiency of the separation by sedimentation is considerably low as compared with the centrifugation. The separation by filtration through paper or cloth as a filtering medium has a low ability of separating fine particles as compared with the separation by filtration through diatomaceous earth. On the other hand, the separation by filtration through diatomaceous earth has a considerably low filtration rate if a large amount of the solid contents should be removed. Accordingly, it is preferred to remove most of particles having a large particle size, in advance, through the centrifugation and to then remove fine particles by filtration through diatomaceous earth in high efficiency, while taking into consideration the foregoing facts. If the solid contents present in the cooling liquid used in the grinding step are removed using a separation system comprising a centrifugal separator and a diatomaceous earth-filtering device arranged in series, the amount of solid contents present in the cooling liquid thus treated can be reduced to a level of not more than 0.2 g/L.

[0025] The glass substrate subjected to the foregoing grinding operations is, for instance, immersed in an ultrasonic-cleaning bath for 2 minutes and then dried while devising a measure to keep the substrate clean. If a glass substrate is subjected to the foregoing grinding operations, the contamination of the glass substrate can sufficiently be inhibited by only a single-stage ultrasonic-cleaning step. In other words, a glass substrate, whose contamination is sufficiently controlled, can easily be produced without causing any trouble in the subsequent steps (polishing step).

[0026] A glass substrate for magnetic recording medium can be produced by subjecting a glass material to the foregoing grinding step, followed by cleaning and then subjecting the glass material to polishing and cleaning steps according to the usual method for the production of such a glass substrate.

[0027] The present invention will hereunder be described in more detail with reference to the following Examples and Comparative Examples, but the present invention is not restricted to these specific Examples.

EXAMPLE 1

[0028] According to the usual procedures for the production of a glass substrate for magnetic recording mediums, a lithium silicate crystallized glass plate (TS-10SX available from K. K. OHARA; comprising 70 to 80% of quartz-cristobalite and the balance of amorphous glass phases) was subjected to inner and outer diameter processing to give a large number of doughnut-like substrates each having an outer diameter of 65 mm, an inner diameter of 20 mm and a thickness of 1.2 mm.

[0029] A plurality of circular disk-like metal-bonded diamond grain-containing grinding wheels, in which the abrasive grains were #1000 diamond grains, the composition of the metal bond was Ni-15Cu-15Sn-0.5P, whose thickness was 5 mm and whose diameter was 15 mm, were produced. 3000 Pieces of the resulting metal-bonded diamond grain-containing grinding wheels were adhered to each surface of the upper and lower surface tables of a both side-grinding device (16B-5L-III available from SPEEDFAM Co., Ltd.). The upper and lower surface tables were rotated in the opposite directions, while using the foregoing both side-grinding device and Noritake Cool CG-250MD (available from Noritake Co., Ltd.) as a cooling liquid while supplying cooling water to thus grind the foregoing doughnut-like substrates having a thickness of 1.2 mm. The grinding step was carried out under the following conditions: a grinding pressure of 150 g/cm2; a rotational number of the upper and lower surface tables of 30 rpm; a rinding time of 15 minutes and a flow rate of the cooling liquid of 10 L/min. This primary grinding provided doughnut-like substrates each having a thickness of 0.85 mm. Incidentally, the cooling liquid was filtered, outside the grinding device, using a centrifugal separator CF-150 (available from Toto Separator Co., Ltd.) and a diatomaceous earth-filtering device RRF-20TA (available from Mitaka Industry Co., Ltd.) and the filtered cooling liquid was recycled.

[0030] A large number of circular disk-like metal-bonded diamond grain-containing grinding wheels were produced, in which the abrasive grains were #1500 diamond grains, the metal bond composition was Ni-15Cu-15Sn-0.5P, a thickness was 5 mm and the diameter was 15 mm. 3000 Pieces of the resulting metal-bonded diamond grain-containing grinding wheels were adhered to each surface of the upper and lower surface tables of a both side-grinding device (16B-5L-III available from SPEEDFAM Co., Ltd.). The upper and lower surface tables were rotated in the opposite directions, while using the foregoing both side-grinding device and Noritake Cool CG-250MD (available from Noritake Co., Ltd.) as a cooling liquid while supplying cooling water to thus grind the foregoing doughnut-like substrates having a thickness of 0.85 mm. The grinding step was carried out under the following conditions: a grinding pressure of 100 g/cm2; a rotational number of the upper and lower surface tables of 30 rpm; a grinding time of 12 minutes and a flow rate of the cooling liquid of 10 L/min. This secondary grinding provided doughnut-like substrates each having a thickness of 0.67 mm. Incidentally, the cooling liquid was filtered, outside the grinding device, using a centrifugal separator CF-150 (available from Toto Separator Co., Ltd.) and a diatomaceous earth-filtering device RRF-20TA (available from Mitaka Industry Co., Ltd.) and the filtered cooling liquid was recycled.

[0031] The doughnut-like glass substrates having a thickness of 0.67 mm and obtained after the foregoing secondary grinding were cleaned by immersing in an ultrasonic-cleaning bath for 2 minutes, while applying ultrasonics of 28 KHz, at a flow rate of 2 L/min. Thereafter, these substrates were dried under conditions, which could inhibit any contamination of the substrates.

[0032] Then the doughnut-like substrates (100 sheets) obtained after the foregoing secondary grinding were fitted to a 16B both side-polishing device (available from Hamai Co., Ltd.) to thus polish these glass substrates using Mirek 801 (CeO2-containing abrasive material; average grain size D5 0=1.5 &mgr;m; available from Mitsui Mining & Smelting Co., Ltd.) as an abrasive material and MHC15A (foamed urethane; available from Rodel Nitta Co., Ltd.) as an abrasion cloth so that the reduced thickness of the glass substrate reached 15 &mgr;m per side (primary polishing step).

[0033] The glass substrates (100 sheets) obtained after the foregoing primary polishing step were likewise fitted to a 16B both side polishing machine available from Hamai Co., Ltd. and the both sides of the substrates were then polished using an abrasive liquid containing 0.5% by mass of CEP available from Mitsui Mining and Smelting Co., Ltd. (a solid solution comprising 100 parts by mass of cerium oxide and one part by mass of silicon oxide; average particle size D5 0=0.2 &mgr;m) as an abrasive material and MHC 14E (foamed urethane) available from Rodel Nitta Co., Ltd. as an abrasion cloth, under the conditions of an abrasion pressure of 60 g/cm2, a rotational number of 30 rpm and an abrasion time of 20 minutes (second polishing stage). Doughnut-like glass substrates having a thickness of 0.638 mm were produced as a result of the secondary polishing step.

[0034] The glass substrates polished above were scrub-cleaned over 3 steps (for 3 seconds per step) using a cleaning device for glass substrates available from SPEEDFAM Co., Ltd. and a sponge disk available from Kanebo Ltd. Then, these glass substrates were subjected to dip-cleaning using SPC 397 (weak alkaline cleaning agent) available from Kyodo Fats & Oils Co., Ltd. as a cleaning agent, followed by rinsing, in order, in 5 baths under the application of ultrasonics using ultra-pure water. Then the glass substrates were immersed in isopropyl alcohol and thereafter dried in the isopropyl alcohol vapor.

[0035] Regarding the scratch marks formed on the surface of the glass substrates during the grinding step and remained thereon, shallow ones were disappeared through the polishing step, but deep scratch marks were left even after the polishing step in the form of recesses (pits). Moreover, these recesses had a tendency that they were connected into dotted lines. The surfaces of twenty glass substrates randomly selected from the foregoing 100 pieces of glass substrates obtained after the foregoing single polishing step were observed using a differential interference microscope with a magnification of ×125. However, there was not observed any defect in this Example.

[0036] With regard to the fine deposits remaining on the surface of the dried glass substrate, the number of deposits having a diameter of 1 to 2 &mgr;m was determined using a laser type surface defect-detector RZ-3000 (available from Hitachi Electronic Engineering Co., Ltd.). The detection was carried out using 20 pieces of glass substrates randomly selected from 100 pieces of glass substrates obtained after the foregoing single polishing step. The number of deposits having a diameter ranging from 1 to 2 &mgr;m was found to be 68, in total, (3.4 per glass substrate on the average). In addition, these 68 deposits were also analyzed by SEM-EDS and as a result, it was found that any Fe ion could not be detected.

[0037] The glass substrate produced according to the foregoing method was free of any contamination with ions such as Fe ions and the degree of contamination with the abrasive grains was also found to be extremely low. Accordingly, the glass substrate was found to be quite useful as a glass substrate for magnetic recording mediums.

Comparative EXAMPLE 1

[0038] According to the usual procedures for the production of a glass substrate for magnetic recording mediums, a lithium silicate crystallized glass plate (TS-10SX available from K. K. OHARA; comprising 70 to 80% of quartz-cristobalite and the balance of amorphous glass phases) was subjected to inner and outer diameter processing to give a large number of doughnut-like substrates each having an outer diameter of 65 mm, an inner diameter of 20 mm and a thickness of 1.2 mm.

[0039] In this Comparative Example, there were used a both side-grinding device 16B-5L-III provided with upper and lower surface tables of cast iron (available from SPEEDFAM Co., Ltd.) as a grinding machine and a liquid containing 150 g/L of #400 SiC abrasive grains as an abrasive liquid. The foregoing doughnut-like glass substrates having a thickness of 1.2 mm were subjected to grinding by oppositely rotating the upper and lower surface tables while supplying the foregoing abrasive liquid. The grinding step was carried out under the following conditions: a grinding pressure of 200 g/cm2; a rotational number of the upper and lower surface tables of 30 rpm; a grinding time of 25 minutes and a flow rate of the abrasive liquid of 5 L/min (primary grinding step). This primary grinding step thus provided doughnut-like glass substrates each having a thickness of 0.85 mm.

[0040] Then there were used a both side-grinding device 16B-5L-III provided with upper and lower surface tables of cast iron (available from SPEEDFAM Co., Ltd.) as a grinding machine and a liquid containing 100 g/L of #1500 Al2 O3 abrasive grains as an abrasive liquid. The foregoing doughnut-like glass substrates having a thickness of 0.85 mm were subjected to grinding by oppositely rotating the upper and lower surface tables while supplying the foregoing abrasive liquid. The grinding step was carried out under the following conditions: a grinding pressure of 100 g/cm2; a rotational number of the upper and lower surface tables of 30 rpm; a grinding time of 30 minutes and a flow rate of the cooling liquid of 5 L/min (secondary grinding step). This secondary grinding step thus provided doughnut-like glass substrates each having a thickness of 0.67 mm.

[0041] The doughnut-like glass substrates having a thickness of 0.67 mm and obtained after the foregoing secondary grinding were cleaned by immersing in an ultrasonic-cleaning bath for 2 minutes, while applying ultrasonics of 28 KHz, at a flow rate of 2 L/min. Thereafter, these substrates were dried under conditions, which could inhibit any contamination of the substrates.

[0042] Then the doughnut-like substrates having a thickness of 0.67 mm and obtained after the foregoing secondary grinding were subjected to primary polishing, secondary polishing, cleaning and drying according to the same procedures used in Example 1. The surfaces of 20 pieces of glass substrates randomly selected from the foregoing 100 pieces of glass substrates obtained after the foregoing single polishing step were observed using a differential interference microscope with a magnification of ×125. As a result, there were observed the generation of defects originated from scratch marks formed and remaining on the surfaces of the glass substrates during the grinding step, in 8 pieces of glass substrates out of the selected 20 pieces of glass substrates.

[0043] With regard to the fine deposits remaining on the surface of the dried glass substrate, the number of deposits having a diameter ranging from 1 to 2 &mgr;m was determined by the same procedures used in Example 1. The detection was carried out using 20 pieces of glass substrates randomly selected from 100 pieces of glass substrates obtained after the foregoing single polishing step. The number of deposits having a diameter ranging from 1 to 2 &mgr;m was found to be 104, in total, (5.2 per glass substrate on the average). In addition, these 104 deposits were also analyzed by SEM-EDS and as a result, it was found that Fe ions could be detected in 32 deposits out of the 104 deposits.

[0044] The glass substrate produced according to the foregoing method was contaminated with both ions such as Fe ions and the abrasive grains. Thus, the resulting glass substrates for magnetic recording mediums are not necessarily useful as glass substrates for magnetic recording mediums.

[0045] As has been described above in detail, the method for the production of a glass substrate for magnetic recording mediums according to the present invention permits the easy and efficient production of a glass substrate for magnetic recording mediums, the method does not require the use of any multistage cleaning stage under the application of ultrasonics and any acid-cleaning stage and it can thus eliminate the possibility of any contamination.

Claims

1. A method for the production of a glass substrate for magnetic recording mediums, which comprises disk-processing, grinding, polishing and cleaning steps, the method being characterized in that the grinding step is carried out using a cooling liquid and a both side-grinding device for grinding a glass substrate for magnetic recording mediums, in which thin plate-like nonferrous metal-bonded grinding wheels, resin-bonded grinding wheels or vitrified bonded grinding wheels, containing abrasive grains are adhered to the surfaces of the upper and lower surface tables of the device.

2. The method for the production of a glass substrate for magnetic recording mediums as set forth in

claim 1, wherein the abrasive grains are diamond, cubic boron nitride, boron carbide, silicon carbide, zirconia or alumina grains.

3. The method for the production of a glass substrate for magnetic recording mediums as set forth in

claim 2, wherein the abrasive grains are diamond grains.

4. The method for the production of a glass substrate for magnetic recording mediums as set forth in

claim 1, wherein solid contents present in the cooling liquid used in the grinding step are removed and then recycled to the device.

5. The method for the production of a glass substrate for magnetic recording mediums as set forth in

claim 2, wherein solid contents present in the cooling liquid used in the grinding step are removed and then recycled to the device.

6. The method for the production of a glass substrate for magnetic recording mediums as set forth in

claim 3, wherein solid contents present in the cooling liquid used in the grinding step are removed and then recycled to the device.

7. The method for the production of a glass substrate for magnetic recording mediums as set forth in

claim 4, wherein the solid contents present in the cooling liquid are removed by filtration through diatomaceous earth as a filtering medium.

8. The method for the production of a glass substrate for magnetic recording mediums as set forth in

claim 5, wherein the solid contents present in the cooling liquid are removed by filtration through diatomaceous earth as a filtering medium.

9. The method for the production of a glass substrate for magnetic recording mediums as set forth in

claim 6, wherein the solid contents present in the cooling liquid are removed by filtration through diatomaceous earth as a filtering medium.