Glass manufacturing – Processes – With chemically reactive treatment of glass preform
Reexamination Certificate
2002-08-06
2003-04-22
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
With chemically reactive treatment of glass preform
C065S031000
Reexamination Certificate
active
06550278
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a glass substrate for use as an information recording medium such as a magnetic disk or the like which can be used as a hard disk, and a method of manufacturing such a glass substrate.
2. Description of the Related Art
Hard disk drives has magnetic heads which are slightly lifted off corresponding magnetic surfaces of hard disks as they rotate in reading data from and storing data in the hard disks.
If a hard disk has a perfectly flat surface, then when a magnetic head is to be lifted off the hard disk surfaces from a CSS (Contact Start-Stop) mode, the magnetic head tends to adhere to the hard disk surface. Therefore, it has been customary for hard disk surfaces to have minute surface roughness referred to as texture.
Conventional texturing techniques for roughening hard disk surfaces include a film texturing process for growing a film with a rough surface on the surface of a glass substrate and a laser texturing process for applying a laser beam to form surface roughness directly on a glass substrate.
Recent higher-density recording hard disk designs require hard disk drives to reduce the height of lifted magnetic heads while operating in a seek mode.
The inventor has found that the surface roughness of a glass substrate for use as a hard disk has to satisfy certain conditions in order to avoid adhesion or sticking of the magnetic head which is lifted to a reduced height.
However, the conventional texturing processes including the film texturing process and the laser texturing process fail to produce the level of fine surface roughness which satisfies those conditions.
SUMMARY OF THE INVENTION
According to the present invention, a glass substrate for use as an information recording medium has a finely roughened surface on which a magnetic film is to be formed, said finely roughened surface having an average surface roughness (Ra) in the range of 0.3 nm≦Ra<3.0 nm and including surface irregularities shaped and distributed isotropically and arranged substantially in succession, the surface irregularities including 5 to 50,000 peaks or convexities having a height of at least 3 nm and no convexities having a height of at least 15 nm within an area of 50 &mgr;m×50 &mgr;m. The glass substrate has an acid-resistant criterion in terms of an etching rate of at least 16 nm/min. upon contact with hydrofluoric acid at a temperature of 50° C. and a concentration of 0.1 weight %.
The average surface roughness (Ra) is extended three-dimensionally such that the central-line average roughness defined by JIS B0601 is applicable to a measured surface (10 &mgr;m×10 &mgr;m), and is defined as follows:
Ra
=
(
1
/
n
)
∑
i
=
1
n
abs
(
Zi
-
Z0
)
where n represents the number of data points of a scanning probe microscope, abs an absolute value, Zi an ith data value of the scanning probe microscope, and
Z0
=
1
/
n
∑
i
=
1
n
Zi
When the surface of the glass substrate is chemically strengthened by an ion exchange to produce a surface compressive stress, the glass substrate is made suitable for hard disks.
If the glass substrate contains SiO
2
and Al
2
O
3
, then the difference (SiO
2
—Al
2
O
3
) between their molar fractions (molar %) is preferably at most 59.5 molar %.
FIG. 1
of the accompanying drawings shows the relationship between the difference (SiO
2
—Al
2
O
3
) and the average surface roughness (Ra). It can be seen from
FIG. 1
that if the difference (SiO
2
—Al
2
O
3
) between the molar fractions exceeded 59.5 molar %, the surface roughness (Ra) of the roughened surface could not exceed 0.3 nm even when the concentration of the hydrofluoric acid or sulfuric acid used in an acid treatment process.
FIG. 2
of the accompanying drawings shows the relationship between the difference (SiO
2
—Al
2
O
3
) between the molar fractions and the number of convexities having a height of at least 3 nm in the area of 50 &mgr;m×50 &mgr;m. A study of
FIG. 2
reveals that if the difference (SiO
2
—Al
2
O
3
) between the molar fractions exceeded 59.5 molar %, the number of convexities having a height of at least 3 nm would be at most 5.
FIG. 3
of the accompanying drawings shows the relationship between the difference (SiO
2
—Al
2
O
3
) between the molar fractions and the acid resistance of the glass substrate (the etching rate (nm/min.) upon contact with hydrofluoric acid at a temperature of 50° C. and a concentration of 0.1 weight %). A review of
FIG. 3
indicates that if the difference (SiO
2
—Al
2
O
3
) between the molar fractions exceeded 59.5 molar %, the acid resistance of the glass substrate would be less than 16 nm/min.
Therefore, the difference between the molar fractions of SiO
2
and Al
2
O
3
should preferably be at most 59.5 molar %.
From the composition of the glass, the difference between the molar fractions of SiO
2
and Al
2
O
3
has a lower limit of 42.5 molar %. The acid resistance at the time the difference between the molar fractions of SiO
2
and Al
2
O
3
is 42.5 molar % (the etching rate (nm/min.) upon contact with hydrofluoric acid at a temperature of 50° C. and a concentration of 0.1 weight %) is 2000 nm/min.
Preferable constituent proportions (molar fractions) of the glass substrate which include other constituents may be in the following ranges:
SiO
2
: 55-70 molar %
Al
2
O
3
: 1-12.5 molar %
Li
2
O: 5-20 molar %
Na
2
O: 0-12 molar %
K
2
O: 0-2 molar %
MgO: 0-8 molar %
CaO: 0-10 molar %
SrO: 0-6 molar %
BaO: 0-2 molar %
TiO
2
: 0-8 molar %
ZrO
2
: 0-4 molar %
The glass substrate according to the present invention may contain, in addition to the above constituents, colorants of Fe
2
O
3
, MnO, NiO, Cr
2
O
3
, CoO, etc., and clarifiers of SO
3
, AS
2
O
3
, Sb
2
O
3
, etc. insofar as they do not impair the characteristics of the glass substrate.
Of the above constituents, SiO
2
is a major constituent of the glass. If the proportion of SiO
2
were less than 55 molar %, then the durability of the glass would be lowered, and if the proportion of SiO
2
exceeded 70 molar %, then the viscosity of the glass would be increased and the glass would not easily be melted. Therefore, the proportion of SiO
2
should preferably be in the range from 55 to 70 molar %.
Al
2
O
3
serves to increase the rate of an ion exchange and also to increase the durability of the glass. If the proportion of Al
2
O
3
were less than 1 molar %, then rate of an ion exchange and the durability of the glass would not be increased. If the proportion of Al
2
O
3
were in excess of 12.5 molar %, then the viscosity of the glass would be increased, the devitrification resistance of the glass would be lowered, and the glass would not easily be melted. Therefore, the proportion of Al
2
O
3
should preferably be in the range from 1 to 12.5 molar %.
Li
2
O is a constituent that is exchanged in an ion exchange, and serves to increase the solubility at the time the glass is melted. If the proportion of Li
2
O were less than 5 molar %, then the surface compressive stress of the glass substrate after the ion exchange would be insufficient, the viscosity of the glass would be increased, and the glass would not easily be melted. If the proportion of Li
2
O were in excess of 20 molar %, then the chemical durability of the glass substrate would be poor. Therefore, the proportion of Li
2
O should preferably be in the range from 5 to 20 molar %.
Na
2
O is a constituent that is exchanged in an ion exchange, and serves to increase the solubility at the time the glass is melted. If the proportion of Na
2
O were in excess of 12 molar %, then the chemical durability of the glass substrate would be poor. Therefore, the proportion of Na
2
O should preferably be at most 12 molar %.
K
2
O serves to increase the solubility at the time the glass is melted. If the proportion of Na
2
O were in excess of 2 molar %, then the chemical durability of the glass substrate would be poor, and the surface compressive stress of the glass substrate after the ion exchange would be lowered. Theref
Ataka Koichi
Koyama Akihiro
Kurachi Junji
Matsuno Yoshihiro
Mitani Kazuishi
Merchant & Gould P.C.
Nippon Sheet Glass Co. Ltd.
Vincent Sean
LandOfFree
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