U.S. patent application number 10/194787 was filed with the patent office on 2003-04-03 for glass substrate for information recording medium, manufacturing method thereof, and information recording medium.
This patent application is currently assigned to Nippon Sheet Glass Co., Ltd.. Invention is credited to Ataka, Koichi, Koyama, Akihiro, Kurachi, Junji, Matsuno, Yoshihiro, Mitani, Kazuishi, Saito, Yasuhiro, Yamamoto, Nobuyuki.
Application Number | 20030064196 10/194787 |
Document ID | / |
Family ID | 26466958 |
Filed Date | 2003-04-03 |
United States Patent
Application |
20030064196 |
Kind Code |
A1 |
Kurachi, Junji ; et
al. |
April 3, 2003 |
Glass substrate for information recording medium, manufacturing
method thereof, and information recording medium
Abstract
A glass substrate for an information recording medium has 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.degree. C. and a concentration of 0.1 weight %. The glass
substrate has a recording surface having an average surface
roughness Ra smaller than 0.3 nm.
Inventors: |
Kurachi, Junji; (Osaka,
JP) ; Koyama, Akihiro; (Osaka, JP) ; Yamamoto,
Nobuyuki; (Osaka, JP) ; Saito, Yasuhiro;
(Osaka, JP) ; Mitani, Kazuishi; (Osaka, JP)
; Ataka, Koichi; (Osaka, JP) ; Matsuno,
Yoshihiro; (Osaka, JP) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
Nippon Sheet Glass Co.,
Ltd.
Osaka
JP
|
Family ID: |
26466958 |
Appl. No.: |
10/194787 |
Filed: |
July 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10194787 |
Jul 12, 2002 |
|
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09569754 |
May 12, 2000 |
|
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6440531 |
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Current U.S.
Class: |
428/141 ;
G9B/5.288 |
Current CPC
Class: |
G11B 5/73921 20190501;
C03C 2204/08 20130101; Y10T 428/8305 20150401; C03C 15/02 20130101;
Y10T 428/24355 20150115; C03C 21/002 20130101; C03C 3/087 20130101;
C03C 15/00 20130101 |
Class at
Publication: |
428/141 |
International
Class: |
B32B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 13, 1999 |
JP |
11-132371 |
May 12, 2000 |
JP |
2000-139675 |
Claims
What is claimed is:
1. 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.degree. C. and a concentration of 0.1 weight %,
at least the recording surface having an average surface roughness
Ra smaller than 0.3 nm.
2. 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
30 nm/min. upon contact with a hydrofluoric acid having a
temperature of 50.degree. C. and a concentration of 0.1 weight %,
at least the recording surface having an average surface roughness
Ra smaller than 0.3 nm.
3. A glass substrate according to claim 1 or 2, wherein the number
of projections having a height of at least 3 nm in an arbitrary
area of 50 .mu.m.times.50 .mu.m in the recording surface is at most
500.
4. A glass substrate according to any one of claims 1 through 3,
wherein the glass substrate contains SiO.sub.2 and Al.sub.2O.sub.3,
and the difference (SiO.sub.2-Al.sub.2O.sub.3) between molar
fractions of SiO.sub.2 and Al.sub.2O.sub.3 is at least 56.5 mol
%.
5. A glass substrate according to claim 4, wherein the glass
substrate contains SiO.sub.2 and Al.sub.2O.sub.3, and the
difference (SiO.sub.2-Al.sub.2O.sub.3) between molar fractions of
SiO.sub.2 and Al.sub.2O.sub.3 is at least 58.5 mol %.
6. A glass substrate according to claim 4 or 5, wherein the glass
substrate contains components whose molar fractions are given as
follows: SiO.sub.2: 63-70 mol %, Al.sub.2O.sub.3: 4-11 mol %,
Li.sub.2O: 5-11 mol %, Na.sub.2O: 6-14 mol %, K.sub.2O: 0-2 mol %,
MgO: 0-6 mol %, CaO: 1-9 mol %, SrO: 0-3 mol %, BaO: 0-2 mol %,
TiO.sub.2: 0-5 mol %, ZrO.sub.2: 0-2.5 mol %, and RO: 2-15 mol %
where RO=MgO+CaO+SrO+BaO.
7. A glass substrate according to claim 4 or 5, wherein the glass
substrate contains components whose molar fractions are given as
follows: SiO.sub.2: 63-70 mol %, Al.sub.2O.sub.3: 6-11 mol %,
Li.sub.2O: 5-11 mol %, Na.sub.2O: 6-14 mol %, K.sub.2O: 0-2 mol %,
TiO.sub.2: 0-5 mol %, ZrO.sub.2: 0-2.5 mol %, RO: 2-12 mol % where
RO=MgO+CaO+SrO+BaO, MgO: 0-4.5 mol %, CaO: 2-7.5 mol %, SrO: 0-3
mol %, BaO: 0-2 mol %, and other components: 0-3 mol %.
8. A glass substrate according to claim 4 or 5, wherein the glass
substrate contains components whose molar fractions are given as
follows: SiO.sub.2: 63-70 mol %, Al.sub.2O.sub.3: 6-11 mol %,
Li.sub.2O: 5-11 mol %, Na.sub.2O: 6-14 mol %, K.sub.2O: 0-2 mol %,
TiO.sub.2: 0-3 mol %, ZrO.sub.2: 0-2 mol %, RO: 2-12 mol % where
RO=MgO+CaO+SrO+BaO, MgO: 0-4.5 mol %, CaO: 2-7.5 mol %, SrO: 0-3
mol %, BaO: 0-2 mol %, and other components: 0-3 mol %.
9. A glass substrate according to any one of claims 1 through 8,
wherein the surface thereof is chemically strengthened by an ion
exchange.
10. A glass substrate according to any one of claims 1 through 9,
wherein the glass substrate is treated with an acid to remove
foreign matter from the surface thereof, and then a layer produced
on the surface by the acid is removed by being treated with an
alkali.
11. A manufacturing method of a glass substrate for an information
recording medium comprising the steps of: melting components for a
glass having an etching rate of at most 45 nm/min. upon contact
with a hydrofluoric acid having a temperature of 50.degree. C. and
a concentration of 0.1 weight %; forming said melted components to
a desired substrate shape; polishing the surface of said substrate
with a polishing compound; removing the polishing compound which
remain attached to the surface of said substrate by cleaning with
an acid; and holding the surface smoothness (average surface
roughness) Ra of said substrate in the range of Ra<0.3 nm by
cleaning with an alkali.
12. A manufacturing method of a glass substrate for an information
recording medium comprising the steps of: melting components for a
glass having an etching rate of at most 30 nm/min. upon contact
with a hydrofluoric acid having a temperature of 50.degree. C. and
a concentration of 6.1 weight %; forming said melted components to
a desired substrate shape; polishing the surface of said substrate
with a polishing compound; removing the polishing compound which
remain attached to the surface of said substrate by cleaning with
an acid; and holding the surface smoothness (average surface
roughness) Ra of said substrate in the range of Ra<0.3 nm by
cleaning with an alkali.
13. A manufacturing method of a glass substrate for an information
recording medium according to claim 11 or 12, wherein a sulfuric
acid having a concentration in the range from 0.01 to 5 weight % is
used for said cleaning with an acid.
14. A manufacturing method of a glass substrate for an information
recording medium according to claim 11 or 12, a hydrofluoric acid
having a concentration in the range from 0.002 to 1 weight % is
used for said cleaning with an acid.
15. An information recording medium according to any one of claims
1 through 10, wherein a recording layer comprising a magnetic layer
is disposed on the surface of said substrate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Description of the Related Art
[0004] 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.
[0005] 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.
[0006] 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.sub.2O, 5 to 25 weight % of Na.sub.2O, 5 to 25
weight % of Al.sub.2O.sub.3+ZrO.sub.2, and SiO.sub.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.
[0007] 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.sub.2, 5 to 15 weight
% of Al.sub.2O.sub.3, 4 to 10 weight % of Li.sub.2O, 4 to 12 weight
% of Na.sub.2O, and 5.5 to 15 weight % of ZrO.sub.2, with the
weight ratio of Na.sub.2O/ZrO.sub.2 being in the range from 0.5 to
2.0 and the weight ratio of Al.sub.2O.sub.3/ZrO.sub.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.sub.2, ZrO.sub.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.
[0008] Japanese laid-open patent publication No. 62-187140 reveals
still another chemically strengthened glass composed of 64 to 70
weight % of SiO.sub.2, 14 to 20 weight % of Al.sub.2O.sub.3, 4 to 6
weight % of Li.sub.2O, 7 to 10 weight % of Na.sub.2O, 0 to 4 weight
% MgO, and 0 to 1.5 weight % of ZrO.sub.2. However, the publication
also fails to show a causal dependence of the acid resistance on
surface irregularities.
[0009] 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
[0010] 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.
[0011] 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.degree. C. and a
concentration of 0.1 weight %, at least the recording surface
having an average surface roughness Ra smaller than 0.3 nm.
[0012] 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.degree. C., as an
indication of acid resistance, needs to be 45 nm/min. or less.
[0013] 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.
[0014] 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.degree. C. should preferably be 45 nm/min. or
less.
[0015] 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, such as hydrofluoric acid. If the
concentration of the hydrofluoric acid used to remove the polishing
compound were less than 0.002 weight %, then the removing ability
would be insufficient, and if the concentration of the hydrofluoric
acid exceeded 1 weight %, then fine defects of the glass substrate
would appear on the surface. Therefore, the concentration of the
hydrofluoric acid should preferably in the range from 0.002 to 1
weight %. In order to require the glass to have a high level of
acid resistance and keep a desired level of surface smoothness, the
etching rate of the substrate glass with an aqueous solution of 0.1
weight % of hydrofluoric acid at a temperature of 50.degree. C.
should preferably be 30 nm/min. or less.
[0016] The composition of the glass substrate which meets the
requirement for the etching rate of 45 nm/min. or less with the
aqueous solution of 0.1 weight % of hydrofluoric acid is preferably
as follows from the standpoint of glass solubility and substrate
weather resistance: In terms of molar fractions, the difference
between SiO.sub.2 and Al.sub.2O.sub.3 (SiO.sub.2-Al.sub.2O.sub.3):
56.5%, SiO.sub.2: 63-70%, Al.sub.2O.sub.3: 4-11%, Li.sub.2O: 5-11%,
Na.sub.2O: 6-14%, K.sub.2O: 0-2%, TiO.sub.2: 0-5%, ZrO.sub.2:
0-2.5%, RO: 2-15% (RO=MgO+CaO+SrO+BaO, MgO: 0-6%, CaO: 1-9%, SrO:
0-3%, BaO: 0-2%), and other components: 3% or less. The glass
composition will hereinafter be expressed in terms of molar
fractions unless otherwise specified.
[0017] The composition of the glass substrate which meets the
requirement for the etching rate of 30 nm/min. or less with the
aqueous solution, of 0.1 weight % of hydrofluoric acid is
preferably substantially the same as the above composition except
that the difference SiO.sub.2-Al.sub.2O.sub.- 3 is 58.5%, from the
standpoint of glass solubility and substrate weather
resistance.
[0018] From the standpoint of solubility, a glass composition
similar to the above composition except that TiO.sub.2: 0-3% and
ZrO.sub.2: 0-2% is more advantageous as the devitrification
temperature is lower.
[0019] The reasons for the limitations of the glass composition are
as follows:
[0020] The etching rate based on the aqueous solution of 0.1 weight
% of hydrofluoric acid depends strongly on the difference
SiO.sub.2-Al.sub.2O.sub.3 as expressed by a molar fraction. If the
difference SiO.sub.2-Al.sub.2O.sub.3 were less than 56.5%, then the
alkaline component would be required to be greatly reduced in order
to make the etching rate equal to or less than 45 nm/min. through
adjustments of other components. The resultant composition would
have a high dissolving temperature, and would not easily be
chemically strengthened by an ion exchange.
[0021] The same problem would arise if the etching rate were 30
nm/min. or less with the difference SiO.sub.2-Al.sub.2O.sub.3 being
58.5% or less. Therefore, in order to make the etching rate equal
to or less than 45 nm/min. or 30 nm/min., the difference
SiO.sub.2-Al.sub.2O.sub.3 should preferably be 56.5% or 58.5% or
higher.
[0022] SiO.sub.2 is a major component of the glass. If the
proportion of SiO.sub.2 were less than 63%, then the chemical
durability of the glass would be lowered. If the proportion of
SiO.sub.2 were in excess of 70%, then the viscosity would increase
to the extent that it would be difficult to melt the glass.
Therefore, the proportion of SiO.sub.2 should preferably range from
63% to 70%.
[0023] Al.sub.2O.sub.3 is a component for increasing the depth of a
compressive stress layer due to an ion exchange and also increasing
the water resistance of the glass. If the proportion of
Al.sub.2O.sub.3 were less than 4%, the above effects would not
sufficiently be developed. If the proportion of Al.sub.2O.sub.3
were greater than 11%, the viscosity would increase and the liquid
phase temperature would increase more than the viscosity, resulting
in a reduction in solubility. Therefore, the proportion of
Al.sub.2O.sub.3 should preferably range from 4% to 11%, and more
preferably range from 6% to 11%.
[0024] Li.sub.2O is a component to be exchanged in an ion exchange
and to increase the solubility. If the ratio of Li.sub.2O were less
than 5%, the compressive stress after the ion exchange would become
insufficient and the viscosity would increase to the extent that it
would be difficult to melt the glass. If the ratio of Li.sub.2O
exceeded 11%, then the weather resistance and acid resistance of
the substrate would be lowered. Therefore, the proportion of
Li.sub.2O should preferably range from 5% to 11%.
[0025] Na.sub.2O is a component to be exchanged in an ion exchange
and to increase the solubility. If the ratio of Na.sub.2O were less
than 6%, the compressive stress after the ion exchange would become
insufficient and the viscosity would increase to the extent that it
would be difficult to melt the glass. If the ratio of Na.sub.2O
exceeded 14%, then the weather resistance and acid resistance of
the substrate would be lowered. Therefore, the proportion of
Na.sub.2O should preferably be equal to or less than 14%.
[0026] K.sub.2O is a component to increase the solubility. If the
proportion of K.sub.2O exceeded 2%, then the weather resistance
would be lowered, and the surface compressive stress after the ion
exchange would be reduced. Therefore, the proportion of K.sub.20
should preferably be equal to or less than 2%.
[0027] TiO.sub.2 is a component to increase the weather resistance
of the glass. If the proportion of TiO.sub.2 exceeded 5%, then the
liquid phase temperature of the glass would rise and the
devitrification resistance would be lowered. Therefore, the
proportion of TiO.sub.2 should preferably be equal to or less than
5%, and more preferably be equal to or less than 3%.
[0028] ZrO.sub.2 is a component to increase the weather resistance
of the glass. If the proportion of ZrO.sub.2 were in excess of
2.5%, the possibility for ZrO.sub.2 to be separated out as fine
crystals when melted would be increased. Therefore, the proportion
of ZrO.sub.2 should preferably be equal to or less than 2.5%, and
more preferably be equal to or less than 2%.
[0029] MgO is a component to increase the solubility of the glass.
If the proportion of MgO were in excess of 6%, then the liquid
phase temperature of the glass would rise and the devitrification
resistance would be lowered. Therefore, the proportion of MgO
should preferably be equal to or less than 6%, and more preferably
be equal to or less than 4.5%.
[0030] CaO is a component to increase the solubility and the
thermal expansion coefficient of the glass. If the proportion of
CaO were less than 2%, then the effect to increase the solubility
and the thermal expansion coefficient of the glass would be small.
If CaO existed together with either of SrO or BaO, or both of SrO
and BaO, then the effect would be sufficiently produced in the
proportion equal to or more than 1%. If the proportion of CaO were
in excess of 7.5%, then the liquid phase temperature of the glass
would tend to rise. If the proportion of CaO were in excess of 9%,
then such tendency would be remarkable. Therefore, the proportion
of CaO should preferably be in the range from 1 to 9%, more
preferably be in the range from 2 to 7.5%.
[0031] SrO is a component to increase the solubility and the
thermal expansion coefficient of the glass. The effect of SrO to
increase the thermal expansion coefficient is higher than that of
CaO. However, if the glass contained a large amount of SrO, then
the specific gravity of the glass would increase. The proportion of
SrO should preferably be equal to or less than 3%.
[0032] BaO is a component to increase the solubility and the
thermal expansion coefficient of the glass. The effect of BaO to
increase the thermal expansion coefficient is higher than that of
CaO and SrO. However, if the glass contained a large amount of BaO,
then the specific gravity of the glass would increase. The
proportion of BaO should preferably be equal to or less than
2%.
[0033] If the proportion of the sum of MgO+CaO+SrO+BaO (RO) were
smaller than 2%, then the solubility would be insufficient. If the
proportion of the sum would exceed 15%, then the liquid phase
temperature of the glass would rise and the devitrification
resistance would be lowered. Therefore, the proportion of the sum
(RO) should preferably range from 2 to 15%, and more preferably
range from 2 to 12%.
[0034] In addition to the above components, other components
including As.sub.2O.sub.3, Sb.sub.2O.sub.3, SO.sub.3, SnO.sub.2,
Fe.sub.2O.sub.3, CoO, Cl, F, etc., for example, may be added in a
total proportion up to 3% for the purpose of cleaning the glass
when melted.
[0035] Since the glass substrate contains lithium, an ion exchange
may be performed in a molten salt including ions of potassium,
sodium, or both for imparting compressive stresses to the glass
surface to increase the fracture strength.
[0036] An information recording medium which is fabricated using
the above glass substrate has a high level of surface smoothness
and is free of surface projections due to foreign matter.
Therefore, the height of a magnetic head which is lifted from the
surface of the information recording medium can be reduced for
recording information at a high density on the information
recording medium.
[0037] The treatment with the acid described above can be carried
out anytime after a process of polishing the glass substrate to a
mirror finish and immediately before the growth of a recording
layer on the glass sub- strate.
[0038] The above and other objects, features, and advantages of the
present invention will become more apparent from the following
description when taken in conjunction with the accompanying
drawings in which preferred embodiments of the present invention
are shown by way of illustrative example.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a graph showing the relationship between etching
rates with a hydrofluoric acid having a concentration of 0.1 weight
% at a temperature of 50.degree. C. and surface roughnesses after
being treated with an acid and an alkali; and
[0040] FIG. 2 is a graph showing the relationship between the
difference SiO.sub.2-Al.sub.2O.sub.3 and etching rates with a
hydrofluoric acid a concentration of 0.1 weight % at a temperature
of 50.degree. C.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] Tables 1 through 4 show details of 31 Inventive Examples and
3 Comparative Examples.
1 TABLE 1 Inventive Examples 1 2 3 4 5 6 7 8 9 10 Composition mol %
SiO.sub.2 67.78 67.78 67.32 66.31 67.50 68.00 66.62 66.00 66.50
66.84 Al.sub.2O.sub.3 7.18 7.18 7.11 7.07 8.50 9.00 8.60 8.50 9.50
9.87 Li.sub.2O 5.37 5.37 5.31 5.21 8.00 8.00 6.43 8.00 9.00 7.38
Na.sub.2O 11.04 11.04 10.93 12.27 8.54 7.54 10.85 8.54 12.00 8.56
K.sub.2O 0.19 0.19 0.18 0.17 0.00 0.00 0.18 0.00 0.00 0.18 MgO 2.60
3.41 1.87 2.81 2.98 2.98 2.96 2.98 0.00 2.88 CaO 3.90 5.04 7.27
4.14 4.48 4.48 4.37 4.48 3.00 4.25 SrO 0 0 0 1.98 0 0 0 0 0 0 BaO
1.95 0 0 0 0 0 0 0 0 0 TiO.sub.2 0 0 0 0 0 0 0 1.5 0 0 ZrO.sub.2 0
0 0 0 0 0 0 0 0 0 SiO.sub.2--Al.sub.2O.sub.3 60.60 60.60 60.21
59.24 59.00 59.00 58.02 57.50 57.00 56.97 R.sub.2O 16.60 16.60
16.48 17.65 16.54 15.54 17.46 16.54 21.06 16.12 RO 8.45 8.45 8.94
8.93 7.46 7.46 7.33 7.46 3.00 7.13 Melting Temperature Tm (.degree.
C.) 1516 1535 1501 1477 1563 1580 1543 1535 1530 1579 Working
Temperature Tw (.degree. C.) 1035 1047 1029 1002 1039 1069 1050
1046 1009 1068 Liquid Phase Temperature Tl (.degree. C.) Unrecog-
1020 1023 Unrecog- 1004 1041 1004 1003 Unrecog- 1038 nizable
nizable nizable T.sub.w - T.sub.l (.degree. C.) 105 or 27 6 72 or
35 28 46 43 79 or 30 greater greater greater Density (g/cm3) 2.56
2.47 2.49 2.56 2.47 2.46 2.47 2.49 2.47 2.47 Average Thermal
Expansion Coefficient, 89 93 94 93 89 86 92 88 94 87
.times.10.sup.-7.degree. C..sup.-1, 50.about.350.degree. C. Etching
Rate (nm/min), 50.degree. C., 0.1% 13 12 14 18 21 21 31 32 38 42
Hydrofluoric acid Surface Roughness Sulfuric acid 1% 0.26 0.26 0.27
0.27 0.28 0.26 0.27 0.26 0.28 0.29 after cleaning Ra (nm)
Hydrofluoric acid 0.26 0.26 0.27 0.27 0.28 0.27 0.34 0.36 0.48 0.59
0.005% Number of Projections, Sulfuric acid 1% 15 10 13 18 15 13 17
14 14 18 3 nm or higher, Hydrofluoric acid 18 12 14 19 13 10 19 18
50 366 Projections/50 .times. 50 .mu.m 0.005
[0042]
2 TABLE 2 Inventive Examples 11 12 13 14 15 16 17 18 19 20
Composition mol % SiO.sub.2 67.51 67.22 66.62 64.84 64.84 66.84
68.84 65.42 68.42 67.51 Al.sub.2O.sub.3 5.93 6.87 6.87 5.87 5.87
7.87 5.87 6.87 7.01 5.93 Li.sub.2O 5.43 7.38 7.38 7.38 7.38 7.38
7.38 8.88 5.49 9.47 Na.sub.2O 12.69 12.06 12.06 11.56 10.56 10.56
10.56 10.56 9.75 8.65 K.sub.2O 0.18 0.00 0.00 0.18 0.18 0.18 0.18
0.00 2.00 0.18 MgO 0.89 1.88 4.28 2.88 5.38 1.38 2.88 1.88 2.94
0.89 CaO 7.32 4.55 2.75 7.25 5.75 1.75 4.25 6.35 4.34 7.32 SrO 0 0
0 0 0 0 0 0 0 0 BaO 0 0 0 0 0 4 0 0 0 0 TiO.sub.2 0 0 0 0 0 0 0 0 0
0 ZrO.sub.2 0 0 0 0 0 0 0 0 0 0 SiO.sub.2--Al.sub.2O.sub.3 61.58
60.35 59.75 58.97 58.97 58.97 62.97 58.55 61.41 61.58 R.sub.2O
18.29 19.43 19.43 19.11 18.11 18.11 18.11 19.43 17.24 18.29 RO 8.21
6.43 7.03 10.13 11.13 7.13 7.13 8.23 7.28 8.21 Melting Temperature
Tm (.degree. C.) -- 1503 1532 -- -- 1511 1539 1491 1551 -- Working
Temperature Tw (.degree. C.) -- 1006 1012 -- -- 1008 1008 1002 1064
-- Liquid Phase Temperature Tll (.degree. C.) 1017 945 924 1006
1036 925 923 961 996 1013 T.sub.w - T.sub.l (.degree. C.) -- 61 88
-- -- 83 85 41 68 -- Density (g/cm3) 2.50 2.47 2.47 2.51 2.50 2.61
2.46 2.49 2.46 2.48 Average Thermal Expansion Coefficient, 98 98 96
96 96 96 96 96 94 93 .times.10.sup.-7.degree. C..sup.-1,
50.about.350.degree. C. Etching Rate (nm/min), 50.degree. C., 0.1%
13 16 18 26 26 33 6 23 20 8 Hydrofluoric acid Surface Roughness
Sulfuric acid 1% 0.28 0.28 0.29 0.27 0.29 0.29 0.27 0.28 0.25 0.29
after cleaning Ra (nm) Hydrofluoric acid 0.005% 0.26 0.26 0.26 0.28
0.29 0.29 0.26 0.29 0.26 0.26 Number of Projections, Sulfuric acid
1% 10 12 16 22 22 22 9 21 8 14 3 nm or higher, Hydrofluoric acid
0.005% 12 36 43 12 16 69 35 14 132 36 Projections/50 .times. 50
.mu.m
[0043]
3 TABLE 3 Inventive Examples 21 22 23 24 25 26 27 28 29 30 31
Composition mol % SiO.sub.2 67.82 67.84 66.84 67.18 65.52 66.84
66.84 66.84 66.84 67.84 67.51 Al.sub.2O.sub.3 6.87 6.87 7.87 7.01
8.87 7.87 7.87 7.87 7.87 6.87 5.93 Li.sub.2O 8.88 7.38 7.38 9.57
7.38 7.38 6.38 7.38 7.38 7.38 5.43 Na.sub.2O 10.56 10.56 10.56 8.73
10.56 10.06 11.56 10.56 10.06 10.56 12.69 K.sub.2O 0.00 0.18 0.18
0.18 0.00 0.68 0.18 0.18 0.68 0.18 0.18 MgO 3.08 0.88 2.88 2.94
3.08 2.38 2.88 1.88 2.38 2.88 5.94 CaO 2.75 6.25 4.25 4.34 4.55
3.25 4.25 5.25 3.25 4.25 2.27 SrO 0 0 0 0 0 1.5 0 0 0 0 0 BaO 0 0 0
0 0 0 0 0 0 0 0 TiO.sub.2 0 0 0 0 0 0 0 0 1.5 0 0 ZrO.sub.2 0 0 0 0
0 0 0 0 0 0 0 SiO.sub.2--Al.sub.2O.sub.3 60.95 60.97 58.97 60.17
56.65 58.97 58.97 58.97 58.97 60.97 61.58 R.sub.2O 19.43 18.11
18.11 18.48 17.93 18.11 18.11 18.11 18.11 18.11 18.29 RO 5.83 7.13
7.13 7.28 7.63 7.13 7.13 7.13 5.63 7.13 8.21 Melting Temperature Tm
(.degree. C.) 1546 -- 1540 1551 1536 1504 1531 1527 1516 1540 1540
Working Temperature Tw (.degree. C.) 1005 -- 1024 1005 1030 1005
1037 1016 1050 1014 1043 Liquid Phase Temperature Tl (.degree. C.)
910 1006 990 968 995 938 982 971 923 963 971 T.sub.w - T.sub.l
(.degree. C.) 95 -- 34 37 35 67 55 45 127 51 72 Density (g/cm3)
2.46 2.48 2.47 2.46 2.48 2.49 2.47 2.48 2.47 2.47 2.47 Average
Thermal Expansion Coefficient, 93 92 92 91 90 94 92 94 90 93 93
.times.10.sup.-7.degree. C..sup.-1, 50.about.350.degree. C. Etching
Rate (nm/min), 50.degree. C., 0.1% 14 13 23 13 35 20 24 22 18 12 10
Hydrofluoric acid Surface Roughness Sulfuric acid 1% 0.29 0.28 0.28
0.29 0.26 0.28 0.27 0.28 0.25 0.28 0.27 after cleaning Ra (nm)
Hydrofluoric acid 0.26 0.26 0.28 0.26 0.62 0.30 0.28 0.28 0.28 0.26
0.26 0.005% Number of Projections, Sulfuric acid 1% 11 10 15 16 23
14 14 15 9 11 11 3 nm or higher, Hydrofluoric acid 54 20 9 35 189
95 10 9 18 33 27 Projections/50 .times. 50 .mu.m 0.005%
[0044]
4 TABLE 4 Comparative Examples 1 2 3 Composition mol % SiO.sub.2
65.87 66.00 64.20 Al.sub.2O.sub.3 9.84 11.00 11.30 Li.sub.2O 7.38
8.00 6.20 Na.sub.2O 9.56 9.05 8.30 K.sub.2O 0.18 0.00 2.00 MgO 2.88
2.38 2.00 CaO 4.25 3.57 6.00 SrO 0 0 0 BaO 0 0 0 TiO.sub.2 0 0 0
ZrO.sub.2 0 0 0 SiO.sub.2--Al.sub.2O.sub.3 56.03 55.00 52.90
R.sub.2O 17.12 17.05 16.50 RO 7.13 5.95 8.00 Melting Temperature Tm
(.degree. C.) 1556 1590 1579 Working Temperature Tw (.degree. C.)
1055 1075 1093 Liquid Phase Temperature Tl (.degree. C.) 1016 987
1075 T.sub.w - T.sub.l (.degree. C.) 39 88 18 Density (g/cm3) 2.47
2.47 2.49 Average Thermal Expansion Coefficient, 89 87 91
.times.10.sup.-7.degree. C..sup.-1, 50.about.350.degree. C. Etching
Rate (nm/min), 50.degree. C., 0.1% 57 113 174 Hydrofluoric acid
Surface Roughness Sulfuric acid 1% 0.41 0.49 0.58 after cleaning Ra
(nm) Hydrofluoric acid 0.26 0.26 1.1 0.005% Number of Projections,
Sulfuric acid 1% 26 39 70 3 nm or higher, Hydrofluoric acid 583 835
1621 Projections/50 .times. 50 .mu.m 0.005%
[0045] The base material of the glass substrate was fabricated as
follows:
[0046] Batches were prepared of silica, alumina, lithium carbonate,
sodium carbonate, basic magnesium carbonate, calcium carbonate,
potassium carbonate, strontium carbonate, barium carbonate,
titania, and zirconia, which are ordinary glass materials, in order
to achieve the compositions shown in above Tables. The prepared
batches were kept at 1550.degree. C. for 4 hours in a platinum
crucible, and then flowed onto an iron plate. The glasses were kept
at 550.degree. C. for 2 hours, and thereafter gradually lowered in
temperature to 450.degree. C. in 10 hours, after which the power
supply was turned off to let the glasses be cooled to room
temperature, thus producing glass specimens.
[0047] Characteristics of the glass specimens, i.e., a melting
temperature Tm (temperature of log .eta.=2), a working temperature
Tw (temperature of log .eta.=4), a liquid phase temperature Tl, a
difference Tw-Tl between the working and liquid phase temperatures,
and an average thermal expansion coefficient at a temperature in
the range from 50.degree. C. to 350.degree. C. were measured and
shown in Tables 1 through 4.
[0048] The viscosity of each of the glass specimens in a high
temperature range was measured by an automatic viscosity measuring
device with a pull-up platinum ball.
[0049] The liquid phase temperature of each of the glass specimens
was measured as follows: The glass specimen was crushed and passed
through a sieve of 2380 .mu.m. Glass particles remaining on a sieve
of 1000 .mu.m were dipped in ethanol and ultrasonically cleaned,
and thereafter dried in a high-temperature tank. 25 g of the glass
particles were placed to a uniform thickness in a platinum boat
having a width of 12 mm, a length of 200 mm, and a depth of 10 mm,
and held for 2 hours in a furnace with a temperature gradient from
930 to 1180.degree. C. Thereafter, the glass was removed from the
furnace and observed for devitrification developed therein by an
optical microscope at a magnification of 40. The highest
temperature at which devitrification was observed was regarded as
the liquid phase temperature.
[0050] The thermal expansion coefficient was determined by
measuring the glass specimen, which was grinded into a cylindrical
shape having a diameter of 5 mm and a length of 15 mm, with a
thermal expansion rate measuring device on the market.
[0051] The glass substrates according to all Inventive Examples had
excellent solubility and moldability and were of high quality free
of foreign matter and warpage because Tm<1580.degree. C.,
Tw<1100.degree. C., and Tl<Tw.
[0052] Etching rates per minute when the glass substrates were
etched with a hydrofluoric acid having a concentration of 0.1
weight % at a temperature of 50.degree. C. are shown in Tables.
Each of the etching rates was measured as follows:
[0053] The glass specimen was cut into a doughnut shape having an
outside diameter of 65 mm and an inside diameter of 20 mm, polished
to a mirror finish (Ra=0.25 nm or less) to achieve a thickness of
0.635 mm, after which the glass specimen was cleaned with pure
water, thereby producing a sample substrate. The sample substrate
was then partly masked, dipped in a hydrofluoric acid having a
concentration of 0.1 weight % at a temperature of 50.degree. C. for
2.5 minutes. After the sample substrate was cleaned with water, the
mask was removed, and the step between the masked area and the
etched area was measured by a surface irregularity meter
(.alpha.-STEP500 manufactured by TENCORINSTRUMENT). The etching
rate was calculated based on the measured step.
[0054] The surface roughness Ra of the glass substrate of each of
the compositions, as treated under two different conditions, is
shown in Tables. The surface roughness Ra of each glass substrate
was measured as follows:
[0055] A substrate prepared in the same manner as with the
substrate measured for the etching rates was dipped in acid
solutions at 50.degree. C. for 2.5 minutes, and then dipped in an
alkaline solution (aqueous solution of KOH with pH 11) at
50.degree. C. for 2.5 minutes. After the substrate was cleaned in
pure water and dried, it was observed in a field of 5 .mu.m.times.5
.mu.m by a scanning probe microscope (SPM SPI3700 manufactured by
SII), and an average surface roughness Ra thereof was calculated.
The average surface roughness Ra used herein represents a
three-dimensional roughness applied to a surface based on the
central line average roughness defined according to JIS B0601, and
was calculated according to the following equation:
Ra=(1/n).sub.i=1.about.nabs (Zi-Zo)
[0056] where n: the number of data points of the microscope,
[0057] abs: absolute value,
[0058] Zi: the height of the ith data point, and
[0059] Zo: the average value of all Zi.
[0060] FIG. 1 is a graph showing the relationship between etching
rates with a hydrofluoric acid having a concentration of 0.1 weight
% at a temperature of 50.degree. C. and surface roughnesses after
cleaning, among the specimens shown in Tables. In FIG. 1,
.tangle-solidup. represents glass substrates treated with a
hydrofluoric acid having a concentration of 0.005%, and
.largecircle. represents glass substrates treated with a sulfuric
acid having a concentration of 1%. It can be seen from FIG. 1 that
the surface roughness of the cleaned glass substrate changes
greatly in an etching rate range from 30 to 60 nm/min. If the glass
substrates whose etching rates exceed 30 nm/min. and 45 nm/min. are
treated with 0.005% of hydrofluoric acid and 1% of sulfuric acid,
then the surface roughness Ra of the treated glass substrates
cannot be held below a desired surface irregularity level of Ra=0.3
nm.
[0061] FIG. 2 is a graph showing the relationship between the
difference SiO.sub.2-Al.sub.2O.sub.3 and etching rates with a
hydrofluoric acid a concentration of 0.1 weight % at a temperature
of 50.degree. C., among the specimens shown in Tables. A review of
FIG. 2 indicates that the etching rate and the difference
SiO.sub.2-Al.sub.2O.sub.3 are correlated to each other very
strongly. Values of the difference SiO.sub.2-Al.sub.2O.sub.3 at the
respective etching rates of 45 nm/min. and 30 nm/min. are 56.5% and
58.5%, respectively.
[0062] The glass substrates according to Comparative Examples have
etching rates in excess of 60 nm/min. with a hydrofluoric acid
having a concentration of 0.1 weight %, and surface roughnesses Ra
in excess of 0.3 nm after being treated with acids. According to
Comparative Examples, it is virtually impossible to remove foreign
matter with an acid treatment and keep a desired level of surface
smoothness after cleaning.
[0063] According to Inventive Examples 1-31, the glass substrates
have etching rates equal to or less than 45 nm/min. with a
hydrofluoric acid having a concentration of 0.1 weight %. The
surface roughness of these glass substrates after the removal of
foreign matter from their surface using 1% or less of sulfuric acid
can be held to Ra=0.3 nm or less.
[0064] According to Inventive Examples 1-6, the glass substrates
have etching rates equal to or less than 30 nm/min. with a
hydrofluoric acid having a concentration of 0.1 weight %. The
surface roughness of these glass substrates after the removal of
foreign matter from their surface using 0.005% of hydrofluoric acid
can be held to Ra=0.3 nm or less.
[0065] With the glass substrates according to Inventive Examples
1-31 and Comparative Examples 1-3, the substrate prepared in the
same manner as measuring a surface roughness mentioned above was
dipped in a hydrofluoric acid having a concentration of 0.005% at
50.degree. C. for 2.5 minutes, and then dipped in an alkaline
solution (aqueous solution of KOH with pH 11). It was observed in a
field of 50 nm.times.50 nm by a scanning probe microscope, and a
number-of projections having a height of 3 nm or higher on the
surface was counted. The result is shown in Tables 1 through 4.
Since the substrates according to Inventive Examples 1-31 are high
in acid resistance, surface irregularities are not produced by the
acid and the number of such projections was at most 500. Next, with
the glass substrate according to Inventive Examples 10, the
substrate prepared was dipped only in an alkaline solution (aqueous
solution of KOH with pH 11) without dipping in a hydrofluoric acid.
A number of projections having a height of 3 nm or higher on the
surface was counted in the same manner mentioned above. The number
of such projections was 4100/50 .mu.m.sup.2. This result shows that
a number of projections having a height of at least 3 nm in an
arbitrary area of 50 .mu.m.times.50 .mu.m can be at most 500 by
cleaning with an acid, compared with the result in Inventive
Example 10 where the glass,substrate was dipped in a hydrofluoric
acid. Further, in Inventive Example 10, the amount of cerium
attached to the surface of the substrate was measured before
cleaning and after cleaning. Before cleaning it was 32 .mu.g and
after cleaning it was 0.6 .mu.g. The removal rate of cerium by
cleaning with an acid was 98%. This shows that almost the
projections are caused by a polishing compound which is left on the
surface of the substrate and it can efficiently be removed by
cleaning with an acid.
[0066] The glass substrates according to Inventive Examples were
chemically strengthened by being dipped in a mixed molten salt of
KNO.sub.3 and NaNO.sub.3 (KNO.sub.3: NaNO.sub.3=80: 20), heated to
380.degree. C., for one hour, and then observed for the thickness
of a compressive stress layer by a polarization microscope. All of
the glass substrates had a compressive stress layer having a
thickness of 50 .mu.m or more, and a composition suitable for
chemical strengthening.
[0067] An actual application of a glass substrate according to the
present invention to an information recording medium will be
described below.
[0068] A disk having an outside diameter of 65 mm, an inside
diameter of 20 mm, a thickness of 0.635 mm, and a surface roughness
Ra of 0.25 nm was prepared in the same manner as the samples
measured for their etching rates.
[0069] The disk was cleaned with a hydrofluoric acid having a
concentration of 0.005% at a temperature of 50.degree. C. to remove
foreign matter from the surface thereof. The disk was then cleaned
successively with an aqueous solution of KOH with pH 11 at a
temperature of 50.degree. C. and pure water to produce a clean
smooth surface.
[0070] The disk was then chemically strengthened by being dipped in
a mixed molten salt of KNO.sub.3 and NaNO.sub.3 (KNO.sub.3:
NaNO.sub.3=80:20), heated to 380.degree. C., for one hour. The disk
thus chemically strengthened was used as a glass substrate for an
information recording medium.
[0071] An information recording medium was then prepared using the
glass substrate thus produced.
[0072] A base layer of Cr, a recording layer of Co--Cr--Ta, and a
protective layer of Cc were deposited on the glass substrate by
sputtering. Then, a lubricating layer was deposited on the glass
substrate, thus producing an information recording medium.
[0073] The information recording medium thus produced was loaded in
a ramp-loading-type disk drive, and continuously operated at a
rotational speed of 4,500 rpm. As a result, any error and head
crash which would otherwise be caused by a collision between
foreign matter on the medium and the head did not occur. It was
confirmed that the information recording medium was suitable for
use in a magnetic disk drive having a low glide height.
[0074] As described above, a glass substrate for an information
recording medium according to the present invention has an etching
rate of at most 45 nm/min. upon contact with a hydrofluoric acid
having a temperature of 50.degree. C. and a concentration of 0.1
weight %. Therefore, foreign matter attached to the surface of the
glass substrate can be removed by an acid treatment. By treating
the glass substrate with an alkali in combination with the acid
treatment, the recording surface of the glass substrate can have an
average surface roughness Ra smaller than 0.3 nm. As a consequence,
the glass substrate makes it possible to provide a high-density
information recording device with a low error rate.
[0075] Although certain preferred embodiments of the present
invention have been shown and described in detail, it should be
understood that various changes and modifications may be made
therein without departing from the scope of the appended
claims.
* * * * *