Method of polishing glass substrate for information...

Abrading – Abrading process – Glass or stone abrading

Reexamination Certificate

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C451S057000

Reexamination Certificate

active

06749487

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of polishing a glass substrate for information recording media, and a glass substrate for information recording media.
2. Description of the Related Art
Conventionally, glass substrates for information recording media used in hard disk drives (hereinafter referred to as “HDDs”) or the like have been manufactured through a series of steps such as the following.
1. Disk processing step in which a glass plate is processed into a donut-shaped glass substrate.
2. Chamfering step in which chamfered surfaces are formed at edge parts at the inner and outer peripheries of the donut-shaped glass substrate.
3. EP (edge polishing) step in which the edge surfaces at the inner and outer peripheries of the glass substrate are polished.
4. Surface polishing step in which the major surfaces of the glass substrate to form recording surfaces are polished.
5. Chemical strengthening step in which sodium ions and potassium ions are introduced into the surfaces of the glass substrate, thus strengthening the glass substrate.
6. Inspection step in which the glass substrate that has been subjected to all of the steps up to the chemical strengthening step is inspected with regard to whether or not certain predetermined criteria are satisfied.
The surface polishing step, which is the fourth step, is comprised of a lapping step for reducing the thickness of the glass substrate to a certain predetermined value, and a precision polishing step for giving the major surfaces of the glass substrate a precision finish.
In the lapping step, rough grinding is carried out using abrasive grains of alumina or the like until the thickness of the glass substrate becomes the predetermined value. The precision polishing step following the lapping step is comprised of a first polishing step and a second polishing step. In the first polishing step, the major surfaces of the glass substrate are polished using abrasive grains of cerium oxide or the like, to remove minute cracks in the major surfaces that have arisen through the lapping step and to make the major surfaces into mirror surfaces. In the second polishing step, the major surfaces are polished using abrasive grains having a mean grain diameter lower than the mean grain diameter of the abrasive grains used in the first polishing step, thus finishing the major surfaces.
Of these steps in the surface polishing step, the product quality of the glass substrate is affected in particular by the first polishing step and the second polishing step. Here, the product quality of the glass substrate relates to the shape of the edge parts of the glass substrate, the roughness of the surfaces, and the minute waviness of the surfaces.
FIG. 6
is a sectioned perspective view of a glass substrate.
FIGS. 7A and 7B
are sectional views showing the cross-sectional shape around an edge part of a glass substrate;
FIG. 7A
shows the edge part shape in the case that roll-off has not occurred, and
FIG. 7B
shows the edge part shape in the case that roll-off has occurred.
The product quality with regard to the edge part shape of the glass substrate refers to the extent of so-called “roll-off” in which a peripheral part of the recording surface is shaved off more than a central part thereof (see FIG.
7
B). If the extent of roll-off becomes larger, then the difference in the so-called “flying height”, which is the distance from the recording surface to the magnetic head, between the central part and the peripheral part of the recording surface becomes larger, and hence the traveling stability of the magnetic head and the accuracy of reading and writing drop, and thus errors occur more frequently. Consequently, the lower the extent of roll-off, the better the product quality.
The product quality with regard to the roughness of the surfaces of the glass substrate refers to the height of projections in particular out of undulations formed on the glass substrate surfaces due to compressive stress generated during the precision polishing step. If the projections are high, then head crashes may occur in which the magnetic head of the HDD collides with the projections, or thermal asperity may occur in which heat generated through such collisions causes malfunctioning in which the magnetic head detects abnormal signals. Consequently, the lower the projections, the better the product quality with regard to the roughness.
The product quality with regard to the minute waviness of the surfaces of the glass substrate refers to the form of appearance of undulations larger than the “roughness”.
The following is an explanation of the minute waviness. The “minute waviness” is one aspect of the shape of a substrate surface, and refers to a wave-like shape of wavelength of the order of several hundred microns to millimeters and amplitude of the order of nanometers. Undulations of wavelength shorter than this constitute the “roughness” described above, whereas undulations of wavelength longer than this come under “flatness”. There are no precise criteria for classifying into “roughness”, “minute waviness” and “flatness”. On an actual glass substrate surface, undulations having a wavelength and an amplitude both of the order of nanometers (hereinafter referred to as “ultra-small undulations”) are present in random fashion.
The form of appearance of the ultra-small undulations as captured at a span of the order of nanometers is the “roughness”. Within the roughness, the form of appearance of the ultra-small undulations is random, but if one looks with a relatively long span, then a regularity to the wavelength can be seen. This regularity of the wavelength in the form of appearance of the ultra-small undulations corresponds to the “minute waviness”. The minute waviness can be measured using an Optiflat (product name; made by Phase Shift Technology) optical measuring apparatus. To increase the recording density of a magnetic recording medium, it is necessary to reduce the flying height, and hence it is necessary to make the minute waviness small. Consequently, the smaller the minute waviness, the better the product quality.
In recent years, with increases in the recording density of magnetic recording media, demands on the product quality as described above of glass substrates have become increasingly strict.
Conventionally, as shown in
FIG. 8
, in the precision polishing step, polishing has been carried out for a time period t1, by controlling the polishing load P1 applied to the glass substrate and the rotational speed R1 (the number of revolutions per unit time) of the plates on which the polishing members that polish the glass substrate are mounted. To meet the required product quality as described above, the polishing has been carried out with the polishing load P1 reduced and the rotational speed R1 of the plates reduced.
However, with such a glass substrate polishing method, there is a problem that the polishing rate (the polishing amount per unit time) drops, and hence the time t1 required for the precision polishing step becomes long, and thus the productivity becomes poor.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of polishing a glass substrate for information recording media, and a glass substrate for information recording media, which enable polishing to be carried out such that the product quality is improved, without the productivity dropping, and without the processing cost increasing.
To attain the above object, the present invention provides a method of polishing a glass substrate for information recording media, comprising a precision polishing step having a first polishing step and a second polishing step carried out in this order, in which, after carrying out rough polishing on major surfaces of a glass substrate, precision polishing is carried out on the major surfaces by feeding abrasive grains onto the major surfaces, pushing polishing members against the major surfaces, and rotating the major surfaces and the polishing members relative to one anothe

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