1. Field of the Invention
The present invention relates to a sheet glass flattening method, a method of manufacturing products using a glass substrate produced using the flattening method, and the products. The present invention relates more particularly to a method of manufacturing glass substrate for an information recording disk used for hard disks, optical recording medium, and similar things, and a glass substrate technique applicable advantageously to magnetic recording disks and the like manufactured by the above-mentioned manufacturing method.
2. Description of the Related Art
Conventionally, thin glass substrates have been used as substrates used for magnetic recording disks (hard disks), optical disks, liquid crystal displays, and the like. Recently, glass substrates have been noted because they have excellent characteristics such as higher flatness and sheet thinning capability, compared with other resin substrates and metal substrates.
Usually, the thin glass substrate is prepared by processing a sheet glass blank obtained by fabrication such as the down-drawing method (fusion method) or floating method to a predetermined dimension and then abrading the surface of the result.
The down-drawing method is generally a method of manufacturing a sheet glass by drawing down molten glass and then pulling down vertically the same. In more detail, for example, the method (an example of the down-drawing method) is known by which molten glass is run down along the front and back surfaces of a molding body with a wedged cross section and then joined at the lower portion of the molding body in a sheet form, and the sheet is cooled and solidified by pulling down it with a pulling roller to produce a sheet glass (Japanese Unexamined Patent publication (Tokkai-Hei) No. 5-163032 or U.S. Pat. No. 3,338,696).
A sheet glass produced by the down-drawing method is thinner than that produced by other methods and is used as a thin glass substrate for magnetic recording disks and a thin glass substrate for liquid crystal displays. However, because of the characteristic of the forming method, the flatness of the sheet is degraded compared with the sheet glass produced by the floating method. For that reason, where the thin film glass produced by the down-drawing method is used for the above-mentioned applications, heat treatment (heating and annealing) is needed to correct and improve the flatness.
In the method, as shown in concrete in FIGS. 1(a) and 1(b), the heating and annealing are carried out with thin glass substrates 1 sandwiched between cut and polished densified sheets (thick flatness correcting aluminum sheet) 10 and with a paper or carbon paper 12 acting as a lubricant inserted between either surface of the thin glass substrates 11 laminated and the densified sheet 10. Numeral 13 represents a fin for preventing the densified sheet to be deformed.
With plural thin glass sheets laminated to increase the efficiency, when the heat treatment is treated without any consideration, the plural glass sheets are thermally bonded together, thus completely being damaged in use. Hence it is needed to prevent the thin glass sheets from being bonded together. To overcome this problem, there is the method which-inserts a paper or carbon paper between thin glass sheets and utilizes paper cinders, or method which utilizes a carbon paper acting as a lubricant (Japanese Unexamined Patent Publication (Tokkai-hei) No. 6-247730).
In the method in which a paper is sandwiched between thin glass sheets, as the number of laminated thin glass layers increases, the accuracy of flatness cannot be accomplished to a desired value because of the paper's thickness and elasticity, and labor is needed in handling and cleaning cinders. In more detail explanation, in the paper sandwiched between thin glass sheets, its peripheral portion is burnt out but its center portion remains carbonized due to oxygen shortage so that a difference in thickness occurs between the peripheral portion and the center portion of the thin glass sheet. The thickness is increased by the laminated value, and the flatness becomes poor due to the increased thickness transferred onto the thin glass sheet. This results in insufficient satisfaction to a required accuracy. Moreover, when the thin sheet glass is taken out of a heating furnace after the thermal treatment, paper cinders may be scattered around, thus worsening the working environment. Thus the handling is troublesome. Moreover, a tar component from the paper adhered on the surface of the thin glass sheet causes troublesome and labor for cleaning.
In the method in which a carbon paper is sandwiched between thin glass sheets, the thickness and elasticity of a carbon paper make it difficult to set the accuracy of flatness to a desired value and the costly carbon paper causes an increase in cost. In more detail, since the carbon paper with a thickness of 0.5 mm or more is thicker than paper, the elasticity absorbs the flatness of the sheet glass. As a result, the insufficient correction of the flatness causes the improved flatness accuracy. The carbon paper also leads to higher costs because of its insufficient strength and its brittleness.
In order to avoid the above-mentioned problems, it may be considered that fine powders of an inorganic material is used as the lubricant. In this case, it is difficult to scatter (coat) uniformly the fine powders over the surface of a thin glass sheet, and the fine powders are scattered, thus worsening the working environment. Moreover, since the fine powders fuse on the surface of a thin glass sheet or hurt the surface thereof, they deteriorate the abrasion property of the thin glass sheet to an unusable state.
In addition, when paper or carbon paper is inserted between thin glass sheets, dusts in air are involved. This causes flaws due to rubbing, thus resulting in a decrease in yield.
Because of such problems, it has been difficult to treat efficiently and thermally plural thin glass sheets laminated with the flatness of a required accuracy, without producing flaws and foreign matter bonding.
For that reason, at the cost of the efficiency, a heat treatment is performed with thin glass sheets respectively sandwiched between densified sheets with good flatness to secure the flatness of a required accuracy, or at the cost of the accuracy, a heat treatment is performed with paper or carbon paper sandwiched between thin glass sheets. Hence the cinders is troublesome while cleaning is unavoidable.
In the conventional sheet glass flattening method, it has been difficult to prevent thin glass sheets laminated from being bonded and to obtain the flatness with a required accuracy by treating efficiently and thermally with plural thin glass sheets laminated, without producing flaws and foreign matter bonding.
There has been a problem in that a conventional glass substrate is costly because the flatness with an accuracy required at the cost of the efficiency must be secured, or because a thin glass sheet prepared at the cost of the accuracy requires a considerable polishing to secure the flatness of a required accuracy uses.
Many aluminum substrate have been used as the substrate for magnetic recording disks. However, with the demands for small-sized and thinned magnetic recording disks and low-floating of a magnetic head, glass substrates have been used at an increasing use rate because small-sizing and thinning, high flatness, and low floating over a magnetic recording disk are easily realized, compared with aluminum substrate.
However, a magnetic recording disk glass substrate of that kind is costly and the surface condition of the substrate is poor because flaws unremovable in the polishing step and foreign matter bonding are left on the substrate. Hence, there has been a problem in that the head crush occurs when the substrate is used for a magnetic recording disk, or defects occur in a film such as a magnetic layer, thus causing an error.
Where a glass substrate is used as a magnetic recording disk substrate, the surface of the glass substrate is generally subjected to a chemical strengthening process by the low-temperature ion exchanging method to improve the shock resistance and Vibration resistance.
This chemical strengthen is made after cutting and polishing the glass substrate. There is a substrate cleaning step as the step prior to the chemical reinforcement process. This cleaning step is carried out with pure water, as described in Japanese Unexamined Patent Publication (Tokkai-Hei) No. 2-285508.
Conventionally, the polishing is made to the front and back surfaces of a glass substrate, but is not made to the outer peripheral end surface and inner peripheral end surface. Since these ends have rough surfaces, dusts may occur from the ends unintentionally rubbed when the glass substrate is carried, or stored into or taken out of a housing case.
Recently, the slight dusting causes problems with the high density magnetic recording disks
It is possible to polish the end surfaces. However, this leads to a high cost because it is difficult to polish the end surface to the surface roughness with which dusting can be prevented. In addition, it is possible to polish to etch chemically the ends. However, the strong etching effect may make it difficult to obtain end surfaces with good surface roughness. Particularly, in the case of the glass substrate used for magnetic recording disks, the roundness is impaired through the etching process so that the center deviation results in rejected products. The strong etching effect may reversely deteriorate the surface roughness of the end surface.
Moreover, where the glass substrate is used as a magnetic recording disk substrate, the surface of the glass substrate is generally subjected to a chemical strengthening process by the low-temperature ion exchanging method to prevent the substrate breakage due to shock or vibration to improve the shock resistance and vibration resistance.
The method disclosed in Japanese Unexamined Patent Publication (Tokkai-Hei) No. 5-32431 is known as a method of manufacturing a magnetic recording disk using that kind of chemical reinforced glass substrate. According to the method described in the publication, the glass substrate is reinforced by immersing into a mixed solution of potassium nitrate and sodium nitrate at 400.degree. C., and an underlayer and a magnetic layer are successively formed on the chemical reinforced glass substrate.
However, as described in the conventional method, in the magnetic recording disk which is manufactured by pulling up a glass substrate from the chemical reinforcement solution, cleaning it, and then successively forming an underlayer and a magnetic layer on the glass substrate, there is a disadvantage in that abnormal protrusions sometimes occur on the surface of a magnetic recording disk. As described above, the abnormal protrusions formed on the surface of a magnetic recording disk may cause the head crush to a magnetic head so that the magnetic recording disk cannot be used as a product.
Since molten salt is adhered with the glass substrate after the chemical strengthening process, cleaning is required. Conventionally, Japanese Unexamined Patent Publication (Tokkai-Hei) No. 2-285508, for example, discloses a magnetic recording disk glass substrate after the chemical reinforcement process is cleaned using alkaline cleaning agent, pure water, and organic cleaning agent.
With the high recording density of a magnetic recording disk, it has been required to decrease the distance (spacing) between the magnetic recording disk and the magnetic head. Hence, the current problem is to remove completely the foreign matters on a glass substrate causing protrusions formed on the surface of a magnetic recording disk.
However, the above-described conventional cleaning method can provide a cleaning effect to some extent. Particularly, it has been difficult to remove completely molten salt left on the glass substrate even if the glass substrate is cleaned after being pulled up from the chemical reinforcement solution.