Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2000-05-12
2002-08-27
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S426000, C428S543000, C428S690000
Reexamination Certificate
active
06440531
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a glass substrate for an information recording medium such as a magnetic disk or the like, and more particularly to a highly acid-resistant glass substrate having an excellent level of surface smoothness.
2. Description of the Related Art
Generally, aluminum substrates are used as substrates for use in information recording mediums. However, since it is difficult for aluminum substrates to increase their surface smoothness, the aluminum substrates are not suitable for use in disks for high-density recording. Instead, glass substrates which have a high level of surface hardness and which can be polished to a high level of surface smoothness are used in disks for high-density recording.
Glass substrates for use in information recording mediums are fabricated by forming a molten material according to a float process, a rod process, and a press process, cutting the formed material to a desired substrate shape, if necessary, and thereafter polishing the substrate to adjust its thickness and surface smoothness to a predetermined range. If desired, the polished substrate is strengthened by an ion exchange or the like. Consequently, fine particles of foreign matter, primarily a polishing compound, remain attached to the surface of the substrate. While those fine particles of foreign matter cannot completely be removed from the substrate by an ordinary cleaning process, they have heretofore posed no significant problems. However, the present trend toward high-density information recording mediums inevitably demands a smaller gap between the information recording medium and the head for reading information from and writing information in the information recording medium. The fine particles of foreign matter need to be removed from the substrate as completely as possible because they would tend to hit the head and cause an error or crash because of the small gap between the information recording medium and the head. Though complete removal of fine particles of foreign matter from the substrate cannot easily be performed, they can efficiently be removed when they are dissolved by an acid such as sulfuric acid, hydrofluoric acid, etc., or the surface of the substrate is etched.
Japanese patent publication No. 46-4271 discloses a chemically strengthened glass composed of 1 to 5 weight % of MgO, 0 to 5 weight % of K
2
O, 5 to 25 weight % of Na
2
O, 5 to 25 weight % of Al
2
O
3
+ZrO
2
, and SiO
2
, the sum of which is 80% or more of the entire composition. However, the disclosed glass is poor in acid resistance and hence cannot be treated by an acid for the removal of foreign matter therefrom.
Japanese laid-open patent publication No. 5-32431 discloses another chemically strengthened glass of improved acid resistance which is composed of 62 to 75 weight % of SiO
2
, 5 to 15 weight % of Al
2
O
3
, 4 to 10 weight % of Li
2
O, 4 to 12 weight % of Na
2
O, and 5.5 to 15 weight % of ZrO
2
, with the weight ratio of Na
2
O/ZrO
2
being in the range from 0.5 to 2.0 and the weight ratio of Al
2
O
3
/ZrO
2
being in the range from 0.4 to 2.5. The publication, however, fails to show a causal dependence of the acid resistance on surface irregularities. Furthermore, since the glass revealed in the publication contains a large amount of ZrO
2
, ZrO
2
tends to be separated out as fine crystals in the glass, which project on the surface after it is polished and are liable to cause an error and crash.
Japanese laid-open patent publication No. 62-187140 reveals still another chemically strengthened glass composed of 64 to 70 weight % of SiO
2
, 14 to 20 weight % of Al
2
O
3
, 4 to 6 weight % of Li
2
O, 7 to 10 weight % of Na
2
O, 0 to 4 weight % MgO, and 0 to 1.5 weight % of ZrO
2
. However, the publication also fails to show a causal dependence of the acid resistance on surface irregularities.
Japanese laid-open patent publication No. 9-22525 discloses a process of cleaning a glass substrate pulled up out of a strengthening liquid with a solution containing an acid, in the fabrication of a glass substrate for use in a magnetic disk. The disclosed process is aimed at only the removal of a chemically strengthening salt. The publication is silent as to the acid resistance which the glass substrate is required to have, and does not suggest or describe the level of surface smoothness in question.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a glass substrate for an information recording medium which can easily be treated by an acid to achieve high cleanliness and high smoothness.
To achieve the above object, there is provided in accordance with the present invention a glass substrate for an information recording medium having at least one surface as a recording surface, the glass substrate having an acid resistance represented by an etching rate of at most 45 nm/min. upon contact with a hydrofluoric acid having a temperature of 50° C. and a concentration of 0.1 weight %, at least the recording surface having an average surface roughness Ra smaller than 0.3 nm.
When glass is etched by an acid, the components of the glass are not uniformly dissolved, but those components which are weaker on the acid are dissolved at first, and then those stronger on the acid are dissolved gradually. The components which are weaker on the acid include alkali, alkaline earth group, alumina, etc., and those stronger on the acid include silica, zirconia, titania, etc. The surface of the glass treated with the acid, as microscopically observed, has a porous layer of network structure mainly composed of silica, zirconia, titania, etc. When the substrate thus produced is cleaned by an alkali for the purposes of removing the porous layer and fats and oils, since the porous layer is thick and unevenly dissolved by the alkali if the substrate is low in acid resistance, pores appear on the surface, producing surface irregularities which make it impossible to achieve a level of surface smoothness that is required by the information recording medium. It has been found according to the present invention that in order to keep the surface smoothness (average surface roughness) Ra of the substrate in the range of Ra<0.3 nm when cleaned by the alkali after having been treated with the acid, the etching rate of the substrate glass with an aqueous solution of 0.1 weight % of hydrofluoric acid at a temperature of 50° C., as an indication of acid resistance, needs to be 45 nm/min. or less.
An acid liquid used in the present invention serves to dissolve a polishing compound on the glass substrate or etch the surface of the glass substrate to remove a polishing compound therefrom. The acid liquid may include an inorganic acid such as hydrofluoric acid, a mixture of hydrofluoric acid and ammonium fluoride, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, etc., or an organic acid such as sulfamic acid, formic acid, oxalic acid, citric acid, malic acid, hydroxyacetic acid, gluconic acid, etc.
A polishing compound mainly composed of cerium oxide, which is used most generally, can be removed most efficiently when it is dissolved in an aqueous solution of sulfuric acid. If the concentration of the sulfuric acid used to remove the polishing compound were less than 0.01 weight %, then the removing ability would be insufficient, and if the concentration of the sulfuric acid exceeded 5 weight %, then fine defects of the glass substrate would appear on the surface. Therefore, the concentration of the sulfuric acid should preferably in the range from 0.01 to 5 weight %. In order to keep a desired level of surface smoothness under this condition, the etching rate of the substrate glass with an aqueous solution of 0.1 weight % of hydrofluoric acid at a temperature of 50° C. should preferably be 45 nm/min. or less.
Even if a polishing compound other than cerium oxide is used, the attached polishing compound can be removed irrespective of its type by etching the substrate surface with an acid having a high etching effect
Ataka Koichi
Koyama Akihiro
Kurachi Junji
Matsuno Yoshihiro
Mitani Kazuishi
Nippon Sheet Glass Co., Ltd
Resan Stevan A.
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