1. Field of the Invention
The present invention relates to a method of manufacturing a glass substrate for information recording media, a glass substrate for information recording media manufactured using the method, and an information recording medium using the glass substrate.
2. Prior Art
Information recording media include magnetic disks, optical magnetic disks and optical disks. A magnetic disk, for example, generally has a donut shape with a hole in the center thereof, and a magnetic film that acts as a data recording medium is covered onto one or both major surfaces of the disk. Data is recorded onto such a magnetic disk by means of differences in strength of magnetism.
A glass substrate for such a magnetic disk is usually manufactured through a manufacturing process described below.
FIG. 9 is a flow chart of a conventional manufacturing process for a glass substrate for a magnetic disk.
First, a starting material glass plate of thickness 1.0 mm is prepared (step P1). Next, a donut-shaped glass disk having an outside diameter of 84 mm and an inside diameter of 25 mm is cut out from the starting material glass plate (step P2). The method of doing this is to irradiate with a laser beam along cutting lines to cause strain or insert cutting lines using a wheel cutter, and then cut along the cutting lines. The cutting lines run along the outer periphery and the inner periphery of the glass disk; the outer cutting line is set 0.5 to 1.0 mm outside the outer periphery, and the inner cutting line 0.5 to 1.0 mm inside the inner periphery.
The next step is to grind the cut-out surfaces at the outer and inner peripheries of the glass disk, thus adjusting the outside and inside diameters and also chamfering the cut-out surfaces (step P3). After the grinding, the average roughness Ra is, for example, 0.3 to 0.4 μm, and the maximum roughness Rmax, for example, 3 to 4 μm. The grinding is carried out on one glass disk at a time, and is comprised of first stage grinding using a diamond grindstone of #324 roughness (coarse), and second stage grinding using a diamond grindstone of #500 roughness (fine). The chamfering is carried out at an angle of 45°, and 0.15 mm is chamfered at a time.
Next, 30 or so of the glass disks are stacked on top of one another, and the outer peripheral edge surfaces of the glass disks are polished using an outer peripheral edge surface polishing machine (step P4), and then 100 or so of the glass disks are stacked on top of one another, and the inner peripheral edge surfaces of the glass disks are polished using an inner peripheral edge surface polishing machine (step P5). Each of these two polishing steps is carried out by pushing a rotating brush against the surfaces to be polished of the rotating stacked glass disks while spraying a cerium oxide slurry on the surfaces. After the polishing, the average roughness Ra is, for example, 0.05 to 0.4 μm, and the maximum roughness Rmax, for example, 0.3 to 2.5 μm.
The major surfaces of the glass disks are then polished by sandwiching the glass disks at the major surfaces between two pads impregnated with a cerium oxide slurry and rotating (step P6). Next, abrasive grains and the like attached to the glass disks are washed off using warm water, alkaline washing water or pure water (step P7), and then the glass disks are strengthened using chemical strengthening treatment (step P8). Finally, salt from the chemical strengthening treatment, foreign matter and the like attached to the glass disks are once again washed off using warm water, alkaline washing water or pure water (step P9).
In general, the inside and outside peripheral edge surfaces of a magnetic disk are not used as data recording surfaces. Nevertheless, the inner and outer peripheral edge surfaces of the glass disk are ground and polished in steps P3 to P5 of the manufacturing process described above, because cracks and undulations inevitably remain on the cut-out surfaces at the inside and outside peripheries of the glass disk after the glass disk is cut out in step P2, and such cracks may lead to fracture of the glass disk, and moreover foreign matter such as abrasive grains that appear during the manufacturing process may collect in the recessed parts of such undulations, and this foreign matter may fly out when the glass disk is used as a magnetic disk and rotated at high speed, adversely affecting the recording surfaces.
The conventional method of manufacturing a glass substrate for information recording media described above, however, suffers from the following problems:
(1) The grinding/chamfering step P3 and the polishing steps P4 and p5 are time-consuming and costly, and moreover variation in surface roughness between glass disks and variation in surface roughness from place to place on a single glass disk occur.
(2) Setting of glass disks must be carried out separately for the inner peripheral edge surface polishing machine and the outer peripheral edge surface polishing machine, and hence many operators are required, and the cost is high.
(3) To improve the processing efficiency, polishing is carried out simultaneously on 100 or so glass disks stacked on top of one another using the inner peripheral edge surface polishing machine, and 30 or so glass disks stacked on top of one another using the outer peripheral edge surface polishing machine, and hence the major surfaces of the glass disks rub against one another and may become scratched.
(4) Even though the inner and outer peripheral edge surfaces of each glass disk are smoothed by chamfering and polishing in steps P3 to P5, fine cracks of depth, for example, 1 to 60 μm may remain, resulting in the strength of the glass dropping. Moreover, if there are cracks or the like on the inner and outer peripheral edge surfaces of the glass disk, then the strength will not increase sufficiently even if chemical strengthening treatment is carried out.
(5) Large-grained glass powder generated during the grinding in step P3 may adhere to the major surfaces of the glass disk, resulting in scratches.
(6) In step P5 in particular, uneven polishing may occur due to the rotating brush not contacting the stacked glass disks uniformly, resulting in the problem of the extent of polishing varying between the glass disks.