U.S. patent application number 14/661840 was filed with the patent office on 2015-07-09 for cover glass and method for producing cover glass.
This patent application is currently assigned to AvanStrate Inc.. The applicant listed for this patent is AvanStrate Inc., HOYA CORPORATION. Invention is credited to Satoshi AMI, Kazuaki HASHIMOTO, Akihiro KOYAMA, Mikiko MORISHITA, Tetsuo TAKANO.
Application Number | 20150191394 14/661840 |
Document ID | / |
Family ID | 45890313 |
Filed Date | 2015-07-09 |
United States Patent
Application |
20150191394 |
Kind Code |
A1 |
KOYAMA; Akihiro ; et
al. |
July 9, 2015 |
COVER GLASS AND METHOD FOR PRODUCING COVER GLASS
Abstract
The disclosed cover glass is produced by etching a glass
substrate that has been formed by a down-drawing process, and
chemically strengthening the glass substrate to provide the glass
substrate with a compressive-stress layer on the principal surfaces
thereof. The glass substrate contains, as components thereof, 50%
to 70% by mass of SiO.sub.2, 5% to 20% by mass of Al.sub.2O.sub.3,
6% to 30% by mass of Na.sub.2O, and 0% to less than 8% by mass of
Li.sub.2O. The glass substrate may also contain 0% to 2.6% by mass
of CaO, if necessary. The glass substrate has an etching
characteristic in which the etching rate is at least 3.7
.mu.m/minute in an etching environment having a temperature of
22.degree. C. and containing hydrogen fluoride with a concentration
of 10% by mass.
Inventors: |
KOYAMA; Akihiro;
(Takarazuka-shi, JP) ; MORISHITA; Mikiko;
(Pyeongtaek-si, KR) ; AMI; Satoshi;
(Yokkaichi-shi, JP) ; HASHIMOTO; Kazuaki; (Tokyo,
JP) ; TAKANO; Tetsuo; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AvanStrate Inc.
HOYA CORPORATION |
Mie
Tokyo |
|
JP
JP |
|
|
Assignee: |
AvanStrate Inc.
HOYA CORPORATION
|
Family ID: |
45890313 |
Appl. No.: |
14/661840 |
Filed: |
March 18, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13247627 |
Sep 28, 2011 |
|
|
|
14661840 |
|
|
|
|
Current U.S.
Class: |
65/30.14 |
Current CPC
Class: |
C03C 3/085 20130101;
C03C 15/00 20130101; C03C 15/02 20130101; C03C 3/087 20130101; C03C
21/002 20130101; C03C 3/083 20130101 |
International
Class: |
C03C 21/00 20060101
C03C021/00; C03C 3/083 20060101 C03C003/083; C03C 3/085 20060101
C03C003/085; C03C 15/02 20060101 C03C015/02; C03C 3/087 20060101
C03C003/087 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2010 |
JP |
2010-222826 |
Dec 29, 2010 |
JP |
2010-294557 |
Claims
1. A method for producing a cover glass, comprising: subjecting a
glass substrate to shape-processing by etching, the glass substrate
having been formed into a plate-like shape by a down-drawing
process and containing, as components thereof, 50% to 70% by mass
of SiO.sub.2, 5% to 20% by mass of Al.sub.2O.sub.3, 6% to 30% by
mass of Na.sub.2O, and 0% to less than 8% by mass of Li.sub.2O; and
forming a compressive-stress layer on the shape-processed glass
substrate by subjecting the glass substrate to chemical
strengthening.
2. The method for producing a cover glass according to claim 1,
wherein the glass substrate further contains, as a component
thereof, 0% to 2.6% by mass of CaO.
3. The method for producing a cover glass according to claim 1,
wherein, if the content by percentage of the SiO.sub.2 is X % by
mass and the content by percentage of the Al.sub.2O.sub.3 is Y % by
mass, X-1/2Y is at most 57.5% by mass.
4. The method for producing a cover glass according to claim 3,
wherein the X-1/2Y is at least 45% by mass.
5. The method for producing a cover glass according to claim 1,
wherein the etching is chemical etching.
6. A method for producing a cover glass, comprising: subjecting a
glass substrate to shape-processing, the glass substrate having
been formed into a plate-like shape and containing, as components
thereof, 50% to 70% by mass of SiO.sub.2, 5% to 20% by mass of
Al.sub.2O.sub.3, 6% to 30% by mass of Na.sub.2O, 0% to less than 8%
by mass of Li.sub.2O, and 0% to 2.6% by mass of CaO; processing at
least an end surface of the shape-processed glass substrate by
chemical etching; and forming a compressive-stress layer on the
processed glass substrate by subjecting the glass substrate to
chemical strengthening.
7. The method for producing a cover glass according to claim 6,
wherein, if the content by percentage of the SiO.sub.2 is X % by
mass and the content by percentage of the Al.sub.2O.sub.3 is Y % by
mass, X-1/2Y is at most 57.5% by mass.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/247,627, filed on Sep. 28, 2011, which
claims the benefit of priority of Japanese Patent Application No.
2010-222826, filed on Sep. 30, 2010 and Japanese Patent Application
No. 2010-294557, filed on Dec. 29, 2010, the entire contents of
U.S. patent application Ser. No. 13/247,627, Japanese Patent
Application No. 2010-222826 and Japanese Patent Application No.
2010-294557 are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a cover glass that is
subjected to shape-processing by etching and that is
chemically-strengthened so as to have a compressive-stress layer on
its principal surfaces, and to a method for producing the cover
glass. The cover glass produced according to an embodiment of the
present disclosure can be employed, for example, as a glass plate
used as a component for protecting the display screen or the like
of equipment such as a mobile phone, a personal digital assistant
(PDA), a digital camera, or a flat panel display (FPD).
[0004] 2. Background Art
[0005] Strengthened glasses produced by chemically strengthening
glass substrates have been conventionally used, for example, as
cover materials for protecting the liquid-crystal display screens
etc. of equipment such as mobile phones, PDAs, digital cameras, and
FPDs. The glass substrates are strengthened by an ion-exchange
process.
[0006] In recent years, mobile phones and PDAs have tended to
become thinner, more sophisticated in functionality, and more
complicated in shape. Thus, strengthened glasses used as cover
glasses in equipment such as mobile phones and PDAs are required to
have formed therein recesses and/or holes with negative curvatures.
Herein, what is meant by a negative curvature is that, if a point
that is located on and moves along the contour of a given region
keeps turning toward the right while the inner section of the
region is always located on the left-hand side of the point, then
the contour of said region is considered as having a negative
curvature. On the other hand, the contour is considered as having a
positive curvature if the point keeps turning toward the left as it
moves along the contour while the inner section of the region is
always located on the left-hand side of the point. The contour is
considered as having zero curvature if the point keeps moving
straight forward. It is, however, difficult to subject a
strengthened glass to outer-shape processing to form recesses or
holes that include sections with negative curvatures, because the
strengthened glass has a compressive-stress layer on its
surface.
[0007] Meanwhile, JP-A-2009-167086 discloses a cover glass for
mobile terminals that exhibits high strength even at a thin glass
substrate and that can thus reduce the thickness of the device on
which the cover glass is mounted.
[0008] The aforementioned cover glass is produced as follows.
First, a resist pattern is formed on the principal surfaces of a
plate-shaped glass substrate. Then, with the resist pattern serving
as a mask, the glass substrate is etched with an etchant which
consists of a mixed-acid aqueous solution containing hydrofluoric
acid and at least one type of acid selected from sulfuric acid,
nitric acid, hydrochloric acid, and hydrofluorosilicic acid, to
thereby cut the glass substrate into a desired shape. Then, the
etched glass substrate is subjected to chemical strengthening by an
ion-exchange process. With this method, it is possible to produce a
cover glass having end surfaces that have a surface roughness of 10
nm or less in arithmetic mean roughness (Ra).
[0009] According to the aforementioned method, it is possible to
produce a chemically-strengthened cover glass having complicated
shapes with negative curvatures and/or through-holes by etching the
plate-shaped glass substrate into predetermined shapes. In the
above method, the processing time required for etching takes up a
large proportion of the overall shape-processing step and has a
huge impact on the cover-glass production efficiency. It is
therefore important to shorten the processing time required for
etching.
[0010] Further, in the chemical etching process of the
aforementioned method, poorly-soluble chemical substances elute
into the etchant, which contains hydrofluoric acid, and adhere to
the glass substrate. This not only impairs the surface quality of
the etched cover glass, but also inhibits the progress of the
etching process if large amounts of chemical substances adhere to
the glass surface, which may extend the processing time and impair
the accuracy in shape.
SUMMARY
[0011] An object of the present disclosure is to provide a method
for producing cover glasses with which the cover-glass production
efficiency can be improved by increasing the etching rate, and to
provide cover glasses produced thereby. Another object of the
present disclosure is to provide a method for producing cover
glasses with which cover glasses having complicated shapes can be
produced with high shape accuracy, and to provide cover glasses
produced thereby.
[0012] An aspect of the present invention relates to a method for
producing a cover glass. The method involves:
[0013] a step of subjecting a glass substrate to shape-processing
by etching, the glass substrate having been formed into a
plate-like shape by a down-drawing process and containing, as
components thereof, 50% to 70% by mass of SiO.sub.2, 5% to 20% by
mass of Al.sub.2O.sub.3, 6% to 30% by mass of Na.sub.2O, and 0% to
less than 8% by mass of Li.sub.2O; and
[0014] a step of forming a compressive-stress layer on the
shape-processed glass substrate by subjecting the glass substrate
to chemical strengthening.
[0015] Another aspect of the present invention relates to a method
for producing a cover glass. The method involves:
[0016] a step of subjecting a glass substrate to shape-processing,
the glass substrate having been formed into a plate-like shape and
containing, as components thereof, 50% to 70% by mass of SiO.sub.2,
5% to 20% by mass of Al.sub.2O.sub.3, 6% to 30% by mass of
Na.sub.2O, 0% to less than 8% by mass of Li.sub.2O, and 0% to 2.6%
by mass of CaO;
[0017] a step of processing at least an end surface of the
shape-processed glass substrate by chemical etching; and
[0018] a step of forming a compressive-stress layer on the
processed glass substrate by subjecting the glass substrate to
chemical strengthening.
[0019] Another aspect of the present invention relates to a cover
glass. The cover glass is a plate-shaped glass substrate that has
been formed into a plate-like shape by a down-drawing process, that
has been subjected to shape-processing by etching, and that has
been chemically strengthened so as to have a compressive-stress
layer on the principal surfaces thereof.
[0020] The glass substrate contains, as components thereof, 50% to
70% by mass of SiO.sub.2, 5% to 20% by mass of Al.sub.2O.sub.3, 6%
to 30% by mass of Na.sub.2O, and 0% to less than 8% by mass of
Li.sub.2O.
[0021] The glass substrate has an etching characteristic in which
an etching rate is at least 3.7 .mu.m/minute in an etching
environment having a temperature of 22.degree. C. and containing
hydrogen fluoride with a concentration of 10% by mass.
[0022] Another aspect of the present invention relates to a cover
glass. The cover glass is a plate-shaped glass substrate that has
been subjected to shape-processing by etching and that has been
chemically strengthened so as to have a compressive-stress layer on
the principal surfaces thereof.
[0023] The glass substrate contains, as components thereof, 50% to
70% by mass of SiO.sub.2, 5% to 20% by mass of Al.sub.2O.sub.3, 6%
to 30% by mass of Na.sub.2O, 0% to less than 8% by mass of
Li.sub.2O, and 0% to 2.6% by mass of CaO.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 illustrates an example of a cover glass according to
an embodiment of the disclosure.
[0025] FIG. 2 is a cross-sectional view of the cover glass
illustrated in FIG. 1 taken along line A-A' therein.
[0026] FIG. 3 is a chart illustrating the relationship between
various glass compositions and etching rates according to the first
embodiment.
[0027] FIGS. 4A and 4B illustrate how to evaluate the processing
accuracy of glass-plate etching.
[0028] FIGS. 5A and 5B illustrate examples of observation images of
glass plates taken with an optical microscope.
[0029] FIG. 6 is a chart illustrating the relationship between
various glass compositions and etching rates according to the
second embodiment.
DESCRIPTION OF THE EMBODIMENTS
[0030] Cover glasses and methods for producing the same of the
present disclosure will be described in detail below in accordance
with first and second embodiments thereof. Note that the term
"percentage (percent; %)" (or content by percentage) as used herein
to indicate the content of each component constituting the glass
refers to percentage by mass (mass %) unless stated otherwise.
[0031] A cover glass, as described in the first and second
embodiments, is used for protecting the display screen of equipment
such as mobile phones, digital cameras, PDAs, and FPDs. However,
the cover glasses of the first and second embodiments are not
limited to the aforementioned applications, and can also be
employed, for example, as substrates for touch-panel displays,
substrates for magnetic disks, cover glasses for solar batteries,
and window panes. According to the first embodiment, the etching
rate can be increased, thus improving the cover-glass production
efficiency. The second embodiment allows cover glasses with high
shape accuracy to be produced in addition to improving the
cover-glass production efficiency, thus further improving the
cover-glass production efficiency.
Cover Glass According to First Embodiment
[0032] FIG. 1 illustrates a piece of cover glass 10 as an example
of the present embodiment. The cover glass 10 illustrated in FIG. 1
has recesses 12 formed in the right and left sides (in the figure)
of a piece of plate-shaped glass. In the cover glass 10, some
sections of each recess 12 have negative curvatures as defined
above. The cover glass 10 is also provided with a slit-form hole
14. The edge surrounding the hole 14 has sections with negative
curvatures as defined above.
[0033] FIG. 2 is a cross-sectional view of the cover glass 10 of
FIG. 1 taken in the direction of arrows A-A'. The cover glass 10
has a compressive-stress layer 16 formed on the principal surfaces
thereof and also on the end surfaces thereof. The
compressive-stress layer 16 can be formed on the end surfaces of
the cover glass 10 as well as the principal surfaces by the cover
glass 10 being subjected to a chemical strengthening process after
shape-processing.
[0034] More specifically, the cover glass 10 is prepared by first
subjecting a down-drawn glass substrate to shape-processing by
etching, and then subjecting the etched glass substrate to chemical
strengthening by an ion-exchange process. The glass substrate
contains, as its components, 50% to 70% of SiO.sub.2, 5% to 20% of
Al.sub.2O.sub.3, 6% to 30% of Na.sub.2O, and 0% to less than 8% of
Li.sub.2O. Further, the glass substrate has an etching
characteristic in which the etching rate is at least 3.7
.mu.m/minute in an etching environment having a temperature of
22.degree. C. and containing hydrogen fluoride with a concentration
of 10% by mass.
[0035] The composition of the glass substrate constituting the
cover glass 10 will be described in detail below.
Composition of Glass Substrate According to First Embodiment
[0036] The glass substrate to be used for the cover glass 10 in the
first embodiment contains SiO.sub.2, Al.sub.2O.sub.3, and
Na.sub.2O, and may also contain B.sub.2O.sub.3, Li.sub.2O,
K.sub.2O, MgO, CaO, SrO, BaO, ZnO, ZrO.sub.2, TiO.sub.2,
P.sub.2O.sub.5, SnO.sub.2, and SO.sub.3, if necessary.
[0037] SiO.sub.2:
[0038] SiO.sub.2 is an essential component that constitutes the
skeletal structure of the glass to be used for the glass substrate,
and has the effect of improving the chemical durability and heat
resistance of the glass. If the SiO.sub.2 content by percentage is
less than 50%, vitrification may become difficult and sufficient
effects in durability and heat resistance may not be obtained,
although the etching rate at the time of etching the glass
substrate to perform shape-processing thereon tends to improve. On
the other hand, if the SiO.sub.2 content by percentage exceeds 70%,
then the glass is likely to cause devitrification and the glass
materials will become hard to melt and form, and also, the
viscosity will increase and the glass will become hard to
homogenize, thereby posing difficulty in mass-producing glass
inexpensively by using down-drawing processes. Further, if the
content by percentage exceeds 70%, the coefficient of thermal
expansion will become too small and will less likely match the
coefficients of thermal expansion of peripheral materials such as
metals and organic adhesives. Furthermore, if the content by
percentage exceeds 70%, the low-temperature viscosity will increase
excessively and thus the ion-exchange rate will drop, resulting in
that sufficient strength cannot be achieved even with chemical
strengthening through ion exchange. Therefore, the content by
percentage of SiO.sub.2 is from 50% to 70%, preferably from 53% to
67%, more preferably from 53% to 65%, even more preferably from 55%
to 65%, and particularly preferably from 58% to 63%. Note that the
low-temperature viscosity refers to the temperature at 10.sup.7.6
to 10.sup.14.5 dPas, but in the present embodiment, it is defined
as indicating the temperature at 10.sup.14.5 dPas.
[0039] Al.sub.2O.sub.3:
[0040] Al.sub.2O.sub.3 is an essential component that constitutes
the skeletal structure of the glass to be used for the glass
substrate, and has the effect of improving the chemical durability
and heat resistance of the glass and of improving the ion-exchange
performance and the etching rate at the time of performing
shape-processing by etching. If the Al.sub.2O.sub.3 content by
percentage is less than 5%, the aforementioned effects cannot be
obtained sufficiently. On the other hand, if the Al.sub.2O.sub.3
content by percentage exceeds 20%, the glass will become hard to
melt and also the viscosity of the glass will increase, which will
make it hard to form. Thus, it becomes difficult to mass-produce
glass inexpensively by using down-drawing processes. Further, if
the Al.sub.2O.sub.3 content by percentage exceeds 20%, the acid
resistance will become excessively poor, which is not preferable
for a cover glass. Furthermore, if the Al.sub.2O.sub.3 content by
percentage exceeds 20%, the glass is likely to cause
devitrification and the devitrification resistance will deteriorate
as well, which will make down-draw processing inapplicable.
Therefore, the content by percentage of Al.sub.2O.sub.3 is from 5%
to 20%, preferably from 5% to 17%, more preferably from 7% to 16%,
and particularly preferably from 9% to 15%.
[0041] Note that in the present embodiment, it is preferable that,
if the content by percentage of SiO.sub.2 is X % and the content by
percentage of Al.sub.2O.sub.3 is Y %, X-1/2Y is 57.5% or less. In
cases where X-1/2Y is 57.5% or less, the etching rate of the glass
substrate can be improved effectively. The range for X-1/2Y is more
preferably 56% or less, and even more preferably 55% or less.
[0042] On the other hand, if the range for X-1/2Y is below 45%, the
devitrification temperature will rise and thus the devitrification
resistance will deteriorate, although the etching rate will reach 5
.mu.m/minute or higher. Therefore, in order to achieve both an
improvement in devitrification resistance and an improvement in
etching rate, the range for X-1/2Y is preferably 45% or greater,
more preferably 47% or greater, and particularly preferably 50% or
greater. Specifically, the range for X-1/2Y is preferably 45% to
57.5%, more preferably 47% to 56%, and even more preferably 50% to
55%.
[0043] B.sub.2O.sup.3:
[0044] B.sub.2O.sub.3 is a component that decreases the viscosity
of the glass and that promotes the melting and refining of the
glass to be used for the glass substrate. If the content by
percentage exceeds 5%, the acid resistance of the glass will
deteriorate and also the amount of volatilization will increase,
thereby making the glass hard to homogenize. Also, the increase in
the amount of volatilization will cause unevenness in the
components of the glass and will also cause unevenness in the
etching on the glass substrate. That is, the etching rate will
become uneven among the various areas of the glass, and therefore,
a glass substrate containing an excessive amount of B.sub.2O.sub.3
is not suitable for purposes such as etching with the aim of
shape-processing, which requires high accuracy. Further, if the
B.sub.2O.sub.3 content by percentage exceeds 5%, then the strain
point will be lowered, thus making the glass deform at the time of
subjecting the glass substrate to thermal processing. Therefore,
the B.sub.2O.sub.3 content by percentage is preferably from 0% to
5%, more preferably from 0% to 3%, even more preferably from 0% to
less than 2%, and particularly preferably less than 0.01% and
B.sub.2O.sub.3 should intentionally not be contained except for
impurities. By adjusting the B.sub.2O.sub.3 content by percentage
to 0% to 5%, it is possible to achieve the effect of improving the
etching rate and also prevent unevenness in etching, thereby
allowing the production of cover glasses with higher quality.
[0045] Li.sub.2O:
[0046] Li.sub.2O is one of the ion-exchange components and is a
component that reduces the viscosity of the glass to be used for
the glass substrate and that improves the meltability and
formability of the glass. Li.sub.2O is also a component that
improves the Young's modulus of the glass substrate, and among
various alkali metal oxides, Li.sub.2O is highly effective in
increasing the stress value of the compressive-stress layer.
However, if the Li.sub.2O content by percentage is too large, there
will be a disadvantage that the ion-exchange salts will deteriorate
too soon in the ion-exchange process, which is the step of
strengthening the glass substrate, thereby leading to an increase
in the production cost of the cover glass. Further, if the
Li.sub.2O content by percentage is too large, then the coefficient
of thermal expansion of the glass will become too large and the
thermal shock resistance thereof will deteriorate, and also the
coefficient of thermal expansion will less likely match the
coefficients of thermal expansion of peripheral materials such as
metals and organic adhesives. Furthermore, if the Li.sub.2O content
by percentage is too large, then not only will the heat resistance
deteriorate, but also the low-temperature viscosity will drop
excessively; this will cause stress relaxation in the heating step
after chemical strengthening and will reduce the stress value of
the compressive-stress layer, resulting in not being able to
produce a cover glass with sufficient strength. Therefore, the
Li.sub.2O content by percentage is from 0% to less than 8%,
preferably from 0% to 5%, more preferably from 0% to 2%, even more
preferably from 0% to 1%, further more preferably from 0% to 0.02%,
and desirably less than 0.01%, and it is particularly preferable
that Li.sub.2O is intentionally not contained except for
impurities.
[0047] Na.sub.2O:
[0048] Na.sub.2O is one of the ion-exchange components and is an
essential component that reduces the high-temperature viscosity of
the glass to be used for the glass substrate and that improves the
meltability and formability of the glass. Further, Na.sub.2O is a
component that improves the devitrification resistance of the
glass. If the Na.sub.2O content by percentage is less than 6%, the
meltability of the glass will deteriorate, which will increase the
cost for melting. Further, if the Na.sub.2O content by percentage
is less than 6%, then the ion-exchange performance will also
deteriorate, resulting in that sufficient strength cannot be
achieved. Furthermore, if the Na.sub.2O content by percentage is
less than 6%, then the coefficient of thermal expansion will be too
small and will less likely match the coefficients of thermal
expansion of peripheral materials such as metals and organic
adhesives. Moreover, if the Na.sub.2O content by percentage is less
than 6%, the glass is likely to cause devitrification and the
devitrification resistance will deteriorate as well, which will
make down-draw processing inapplicable and thereby pose difficulty
in mass-producing glass inexpensively. On the other hand, if the
Na.sub.2O content by percentage exceeds 30%, then the
low-temperature viscosity will drop and the impact resistance will
deteriorate, and also the coefficient of thermal expansion will be
too large and will less likely match the coefficients of thermal
expansion of peripheral materials such as metals and organic
adhesives. Further, if the Na.sub.2O content by percentage exceeds
30%, also the devitrification resistance will deteriorate due to
the loss of balance in the glass, thereby posing difficulty in
mass-producing glass inexpensively by using down-drawing processes.
Therefore, the Na.sub.2O content by percentage is from 6% to 30%,
preferably from 7% to 27%, more preferably from 10% to 20%, even
more preferably from 12% to 20%, and particularly preferably from
13% to 19%.
[0049] Further, in the present embodiment, the difference found by
subtracting the Al.sub.2O.sub.3 content by percentage from the
Na.sub.2O content by percentage ("Na.sub.2O content
percentage-Al.sub.2O.sub.3 content percentage") is preferably from
-10% to 15%. When the difference "Na.sub.2O content
percentage-Al.sub.2O.sub.3 content percentage" is within the range
of -10% to 15%, then not only can the cover-glass production
efficiency be improved, but also the meltability of the glass can
be improved while keeping the coefficient of thermal expansion and
heat resistance at a suitable level. Therefore, the glass can be
molten at lower temperatures, thereby allowing further reductions
in the cost for producing the cover glass. Note that the range for
the difference "Na.sub.2O content percentage-Al.sub.2O.sub.3
content percentage" is more preferably from -5% to 13%, even more
preferably from -5% to 10%, and further more preferably from -3% to
5%.
[0050] K.sub.2O:
[0051] K.sub.2O is one of the ion-exchange components and is a
component that can improve the ion-exchange performance of the
glass substrate by being included therein. K.sub.2O also reduces
the high-temperature viscosity of the glass, improves the
meltability and formability thereof, and also improves the
devitrification resistance. However, if the K.sub.2O content by
percentage is too large, the low-temperature viscosity will drop,
the coefficient of thermal expansion will become too large, and the
impact resistance will become poor, which is not preferable for a
cover glass. Further, if the K.sub.2O content by percentage is too
large, the coefficient of thermal expansion will less likely match
the coefficients of thermal expansion of peripheral materials such
as metals and organic adhesives. Furthermore, if the K.sub.2O
content by percentage is too large, also the devitrification
resistance will deteriorate due to the loss of balance in the
glass, thereby posing difficulty in mass-producing glass
inexpensively by using down-drawing processes. Therefore, the
K.sub.2O content by percentage is 15% or less, preferably 10% or
less, more preferably less than 5.6%, even more preferably less
than 5%, and particularly preferably less than 4%. On the other
hand, the lower limit of the K.sub.2O content by percentage is 0%
or greater, preferably 0.1% or greater, more preferably 1% or
greater, and even more preferably 2% or greater. Adjusting the
lower limit of the K.sub.2O content by percentage to be within the
aforementioned range not only improves the etching rate, but can
also shorten the time required for ion-exchange processing and
improve the cover glass productivity. Specifically, the content
percent of K.sub.2O is preferably 0% to 15%, preferably 0.1% to
10%, more preferably 1% to less than 5.6%, even more preferably 2%
to less than 5%, and particularly preferably 2% to less than
4%.
[0052] R.sup.1.sub.2O (R.sup.1 includes all the elements among Li,
Na, and K that are contained in the glass substrate):
[0053] In the present embodiment, the R.sup.1.sub.2O content by
percentage is preferably from 6% to 30%. If the percentage of
R.sup.1.sub.2O is less than 6%, ion exchange will not be performed
sufficiently and thus a sufficient strength cannot be obtained,
thereby posing difficulty in using the glass substrate for a cover
glass. On the other hand, if the percentage of R.sup.1.sub.2O
exceeds 30%, the devitrification temperature will be increased due
to the loss of balance in the glass, which will make down-draw
processing hard to employ and thereby pose difficulty in
mass-producing glass inexpensively. In order to achieve both
mechanical strength and devitrification resistance and to improve
productivity, the R.sup.1.sub.2O content by percentage is more
preferably from 10% to 28%, even more preferably from 14% to 25%,
further more preferably from 15% to 24%, and particularly
preferably from 17% to 23%.
[0054] Note that the aforementioned range for the content by
percentage of R.sup.1.sub.2O is a criterion to be satisfied in
addition to each range of content by percentage as set forth above
for the oxide of each of the elements among Li, Na, and K contained
in the glass substrate.
[0055] B.sub.2O.sub.3/R.sup.1.sub.2O (R.sup.1 includes all the
elements among Li, Na, and K that are contained in the glass
substrate):
[0056] In the present embodiment, the ratio in content by
percentage between B.sub.2O.sub.3 and R.sup.1.sub.2O
("B.sub.2O.sub.3/R.sup.1.sub.2O") is preferably from 0 to less than
0.3. B.sub.2O.sub.3 is prone to bond with an alkali metal oxide and
volatilize as an alkali borate, and particularly, Li.sup.+, which
has a small ionic radius, has high mobility in the glass melt and
is prone to volatilize from the surface of the melt, and
volatilization is likely to create a concentration gradient up to
the inner part of the glass and to give rise to striae on the
surface of the glass. In other words, an increase in the amount of
volatilization of B.sub.2O.sub.3 will make the produced glass
substrate nonuniform, and unevenness in etching will occur due to
the nonuniformity of the glass substrate when such a glass
substrate is subjected to etching. However, alkali metal oxides are
essential components to glass that is chemically strengthened by an
ion-exchange process. Therefore, the ratio
B.sub.2O.sub.3/R.sup.1.sub.2O in content by percentage (the ratio
in percentage by mass) is preferably adjusted to be within the
range of 0 to less than 0.3. In this range, the nonuniformity of
the glass and unevenness in etching can be reduced effectively.
Thus, not only is the etching rate improved, but also unevenness in
etching rate can be prevented, thereby allowing strengthened glass
of desired shape to be produced with high yield. Note that the
range for the ratio B.sub.2O.sub.3/R.sup.1.sub.2O in content by
percentage is more preferably from 0 to 0.1, even more preferably
from 0 to 0.07, further preferably from 0 to 0.03, even more
preferably from 0 to 0.005, and particularly preferably 0. Further,
in order to reduce unevenness in etching, it is most preferable
that the Li.sub.2O content by percentage is less than 0.01% and
Li.sub.2O should intentionally not be contained except for
impurities, as described above.
[0057] MgO:
[0058] MgO is a component that decreases the viscosity of the glass
to be used for the glass substrate and that promotes the melting
and refining of the glass. Also, among alkaline-earth metals, MgO
is an effective component for improving the meltability while
making the glass lightweight, because it only increases the glass
density by a small rate. MgO also improves formability and
increases the strain point and Young's modulus of the glass.
Furthermore, the rate at which crystallized products are produced
at the time of etching MgO-containing glass by using e.g.
hydrofluoric acid is relatively low, and therefore, it is
relatively less likely for the crystallized products to adhere to
the glass surface during etching. Thus, it is preferable to include
MgO in order to improve the glass meltability and to increase the
etching rate. However, if the MgO content is too large, then the
devitrification resistance will deteriorate, thus posing difficulty
in mass-producing glass inexpensively by using down-drawing
processes. Therefore, the MgO content by percentage is from 0% to
15%, preferably greater than 1% to 15%, more preferably greater
than 1% to 12%, further preferably greater than 1% to less than 7%,
even more preferably from 3% to less than 7%, and particularly
preferably greater than 4.5% to 6%. The inclusion of MgO within the
range of 0% to 15% will improve the etching rate and will also
allow the glass to be molten at lower temperatures, thereby
allowing further reductions in the cost for producing the cover
glass. Furthermore, because it is possible to improve the
ion-exchange performance and increase the strain point at the same
time, the MgO-containing glass is suitable for cover glasses that
require high mechanical strength. This is because a sufficient
compressive-stress layer can be formed on the surface of the glass
substrate, and the compressive-stress layer formed on the surface
can be prevented from causing stress relaxation, even during/after
thermal treatment.
[0059] CaO:
[0060] CaO is a component that decreases the viscosity of the glass
to be used for the glass substrate and that promotes the melting
and refining of the glass. Also, among alkaline-earth metals, CaO
is an effective component for improving the meltability while
making the glass lightweight, because it only increases the glass
density by a small rate. CaO also improves formability and
increases the strain point and Young's modulus of the glass.
However, if the CaO content is too large, then the devitrification
resistance will deteriorate, thus posing difficulty in
mass-producing glass inexpensively by using down-drawing processes.
Also, if the CaO content is too large, the ion-exchange performance
will deteriorate; thus, sufficient strength cannot be achieved, and
also productivity will be reduced. Further, the crystallized
products produced at the time of subjecting glass that contains
large amounts of CaO to wet-etching by using e.g. hydrofluoric acid
are not only insoluble in the etchant solution, but are produced at
an extremely high precipitation rate. Therefore, such crystallized
products adhere to the surface of the glass being etched, and if
the adherence amount is large, the etching reaction will be
inhibited and the glass-processing quality will be impaired. On the
other hand, the inclusion of CaO can lower the devitrification
temperature and improve the devitrification resistance and
meltability. Therefore, the CaO content by percentage is from 0% to
10%, preferably from 0% to 8%, more preferably from 0% to 6%, even
more preferably from 0% to 4%, and particularly preferably from 0%
to 2%. Note that in cases where an extremely-high etching quality
is required, it is preferable that substantially no CaO is
contained.
[0061] Further, it is even more preferable to include both MgO and
CaO in order to reduce the melt viscosity and at the same time
lower the devitrification temperature, but the amount of CaO shall
be adjusted as appropriate to fall within the range that will not
give rise to the aforementioned problems caused by the crystallized
products produced during etching.
[0062] SrO:
[0063] SrO is a component that decreases the viscosity of the glass
to be used for the glass substrate and that promotes the melting
and refining of the glass. SrO also improves formability and
increases the strain point and Young's modulus of the glass.
However, if the SrO content is too large, then the glass density
will increase, and the glass will be unsuitable for cover glasses,
which are required to be lightweight. Further, if the SrO content
is too large, then the coefficient of thermal expansion will be too
large and will less likely match the coefficients of thermal
expansion of peripheral materials such as metals and organic
adhesives. Furthermore, if the SrO content is too large, then the
ion-exchange performance will also deteriorate, making it difficult
to obtain the high mechanical strength demanded of cover glasses.
Therefore, the SrO content by percentage is preferably from 0% to
10%, more preferably from 0% to 5%, even more preferably from 0% to
2%, and further more preferably from 0% to 0.5%, and it is
particularly preferable that SrO is intentionally not contained
except for impurities.
[0064] BaO:
[0065] BaO is a component that decreases the viscosity of the glass
to be used for the glass substrate and that promotes the melting
and refining of the glass. BaO also improves formability and
increases the strain point and Young's modulus of the glass.
However, if the BaO content is too large, then the glass density
will increase, and the glass will be unsuitable for cover glasses,
which are required to be lightweight. Further, if the BaO content
is too large, then the coefficient of thermal expansion will be too
large and will less likely match the coefficients of thermal
expansion of peripheral materials such as metals and organic
adhesives. Furthermore, if the BaO content is too large, then the
ion-exchange performance will also deteriorate, making it difficult
to obtain the high mechanical strength demanded of cover glasses.
Therefore, the BaO content by percentage is preferably from 0% to
10%, more preferably from 0% to 5%, even more preferably from 0% to
2%, and further more preferably from 0% to 0.5%. Note that, because
BaO places a heavy burden on the environment, it is particularly
preferable that the BaO content is less than 0.01% and BaO should
intentionally not be contained except for impurities.
[0066] SrO+BaO:
[0067] In the present embodiment, the sum found by adding the SrO
content by percentage and the BaO content by percentage ("SrO
content percentage+BaO content percentage") is preferably less than
10%. When the sum "SrO content percentage+BaO content percentage"
is less than 10%, it is possible to effectively prevent an increase
in the glass density and a decrease in the ion-exchange rate. That
is, adjusting the sum "SrO content percentage+BaO content
percentage" to less than 10% will not only improve the etching
rate, but can also achieve the effect of making the cover glass
lightweight and the effect of improving productivity and glass
strength. Note that the range for the sum "SrO content
percentage+BaO content percentage" is preferably from 0% to 8%,
more preferably from 0% to 5%, even more preferably from 0% to 2%,
and further more preferably from 0% to 1%, and it is particularly
preferable that SrO and BaO are intentionally not contained except
for impurities.
[0068] RO (R includes all the elements among Mg, Ca, Sr, and Ba
that are contained in the glass substrate):
[0069] Herein, the RO content by percentage is preferably from 0%
to 20%. If the RO content is greater than 20%, the chemical
durability will deteriorate. On the other hand, the inclusion of RO
can improve the meltability and heat resistance of the glass.
Therefore, the RO content by percentage is preferably from 0% to
10%, more preferably from 0% to 7%, even more preferably from 2% to
7%, further preferably from 3% to 7%, and further more preferably
from 4% to 7%.
[0070] Note that the aforementioned range for the content by
percentage of RO is a criterion to be satisfied in addition to each
range of content by percentage as set forth above for the oxide of
each of the elements among Mg, Ca, Sr, and Ba contained in the
glass substrate.
[0071] Li.sub.2O/(RO+Li.sub.2O):
[0072] In the present embodiment, the ratio in content by
percentage between Li.sub.2O and the sum of RO and Li.sub.2O
("Li.sub.2O/(RO+Li.sub.2O)"; wherein R includes at least one type
of element selected from Mg, Ca, Sr, and Ba) is preferably less
than 0.3. In this way, it is possible to inhibit the deterioration
of ion-exchange salts in the ion-exchange process, which is the
step of strengthening the glass substrate, and it is thus possible
to reduce the cost for producing the strengthened glass to be used
as the cover glass. Further, if the ratio
"Li.sub.2O/(RO+Li.sub.2O)" in content by percentage is less than
0.3, the devitrification temperature can be lowered effectively,
and thus, the devitrification resistance can be improved
effectively. Further, if the ratio "Li.sub.2O/(RO+Li.sub.2O)" in
content by percentage is less than 0.3, then the strain point can
be increased effectively and also the heat resistance can be
improved. That is, adjusting the ratio "Li.sub.2O/(RO+Li.sub.2O)"
in content by percentage to less than 0.3 not only increases the
etching rate, but can also improve the heat resistance and prevent
such problems as stress relaxation during chemical strengthening
and the deformation of glass during other thermal treatments. Note
that the range for the ratio "Li.sub.2O/(RO+Li.sub.2O)" in content
by percentage is more preferably 0.08 or less, even more preferably
0.05 or less, further more preferably 0.01 or less, and
particularly preferably 0.
[0073] ZnO:
[0074] ZnO is a component that improves the ion-exchange
performance, that is highly effective particularly in improving the
compressive-stress value, and that lowers the high-temperature
viscosity of the glass without lowering the low-temperature
viscosity. However, if the ZnO content is too large, the glass will
cause phase separation and the devitrification resistance will
deteriorate. Further, if the ZnO content is too large, then the
glass density will increase, and the glass will be unsuitable for
cover glasses, which are required to be lightweight. Therefore, the
ZnO content by percentage is preferably from 0% to 6%, more
preferably from 0% to 4%, even more preferably from 0% to 1%,
further more preferably from 0% to 0.1%, and particularly
preferably less than 0.01% and ZnO should intentionally not be
contained except for impurities.
[0075] ZrO.sub.2:
[0076] ZrO.sub.2 is a component that significantly improves the
ion-exchange performance and that increases the strain point and
the viscosity near the devitrification temperature of the glass.
Further, ZrO.sub.2 improves the heat resistance of the glass.
However, if the ZrO.sub.2 content is too large, the devitrification
temperature will be increased and the devitrification resistance
will deteriorate. Therefore, in order to prevent a reduction in
devitrification resistance, the ZrO.sub.2 content by percentage is
preferably from 0% to 15%, more preferably from 0% to 10%, even
more preferably from 0% to 6% or less, and further more preferably
from 0% to 4% or less. By including ZrO.sub.2, it is possible to
effectively improve heat resistance, which is important for cover
glasses used in mobile phones and for cover glasses used in
touch-panel displays, and to effectively improve ion-exchange
performance, which relates to the reduction of time for chemically
strengthening the glass substrate and to the improvement of the
mechanical strength thereof. Therefore, the ZrO.sub.2 content by
percentage is preferably 0.1% or greater, more preferably 0.5% or
greater, even more preferably 1% or greater, and particularly
preferably 2% or greater. That is, by adjusting the ZrO.sub.2
content by percentage to 0.1% or greater, the heat resistance and
ion-exchange performance can be improved while also improving
devitrification resistance. Thus, the time required for
ion-exchange processing can be reduced, and thus productivity can
be improved. It is also possible to prevent the glass from
deforming during the chemical strengthening process and other
thermal treatments, and thus the yield of cover glasses can be
enhanced.
[0077] On the other hand, if the glass density is to be reduced,
then the ZrO.sub.2 content by percentage should preferably be less
than 0.1%, and it is particularly preferable that ZrO.sub.2 is
intentionally not contained except for impurities.
[0078] TiO.sub.2:
[0079] TiO.sub.2 is a component that improves the ion-exchange
performance and that reduces the high-temperature viscosity of the
glass. However, if the TiO.sub.2 content is too large, the
devitrification resistance will deteriorate. Further, if the
TiO.sub.2 content is too large, then the UV transmittance will
deteriorate and the glass will be stained, which is not suitable
for cover glasses or the like. Furthermore, if the TiO.sub.2
content is too large, then the UV transmittance will deteriorate,
thus causing a disadvantage that, in the case of using a UV-curable
resin, the resin cannot be cured sufficiently. Therefore, the
TiO.sub.2 content by percentage is preferably from 0% to 5%, more
preferably from 0% to less than 3%, even more preferably from 0% to
1%, and further more preferably from 0% to 0.01%, and it is
particularly preferable that TiO.sub.2 is intentionally not
contained except for impurities.
[0080] (ZrO.sub.2+TiO.sub.2)/SiO.sub.2:
[0081] In the present embodiment, the ratio in content by
percentage between the sum of ZrO.sub.2 and TiO.sub.2 to SiO.sub.2
("(ZrO.sub.2+TiO.sub.2)/SiO.sub.2") is preferably from 0% to 0.2%.
In the case of shape-processing a glass substrate by etching,
ion-exchange processing will be performed after etching. In the
ion-exchange process, deformation may occur due to the internal
stress within the glass substrate if ion exchange is carried out
excessively. In other words, excessive ion exchange gives rise to
the deformation of the glass substrate, and thus the shape that has
been processed with high accuracy by etching cannot be retained and
the glass substrate becomes unsuitable for a cover glass. So, by
adjusting the ratio "(ZrO.sub.2+TiO.sub.2)/SiO.sub.2" in content by
percentage to be within the range of 0 to 0.2 excessive ion
exchange can be inhibited effectively. Note that the range for the
ratio "(ZrO.sub.2+TiO.sub.2)/SiO.sub.2" in content by percentage is
preferably from 0 to 0.15, more preferably from 0 to 0.1, even more
preferably from 0 to 0.07, and particularly preferably from 0 to
0.01. When the ratio "(ZrO.sub.2+TiO.sub.2)/SiO.sub.2" in content
by percentage is within the range of 0 to 0.2, the devitrification
resistance as well as the heat resistance can be improved while
preventing excessive ion exchange.
[0082] P.sub.2O.sub.5:
[0083] P.sub.2O.sub.5 is a component that improves the ion-exchange
performance and that is highly effective particularly in increasing
the thickness of the compressive-stress layer. However, if the
P.sub.2O.sub.5 content is too large, the glass will cause phase
separation and the water resistance will deteriorate. Therefore,
the P.sub.2O.sub.5 content by percentage is preferably from 0% to
10%, more preferably from 0% to 4%, even more preferably from 0% to
1%, further more preferably from 0% to 0.1%, and particularly
preferably less than 0.01% and P.sub.2O.sub.5 should intentionally
not be contained except for impurities.
[0084] In addition to the aforementioned components, the glass
substrate contains refining agents as described below.
[0085] Refining Agent:
[0086] A refining agent is a component necessary for the refining
of the glass to be used for the glass substrate. No refining effect
can be obtained if the content is less than 0.001%, whereas the
content exceeding 5% may cause devitrification and/or staining.
Therefore, the total content by percentage of refining agent(s) is
preferably from 0.001% to 5%, more preferably from 0.01% to 3%,
even more preferably from 0.05% to 1%, and particularly preferably
from 0.05% to 0.5%.
[0087] The refining agents are not particularly limited as far as
they have little burden on the environment and provide the glass
with excellent clarity. Examples include one or more types of
agents selected from the group of oxides of metals including, for
example, Sn, Fe, Ce, Tb, Mo, and W.
[0088] The following ranges are preferable for the metal oxides,
the oxides being expressed as SnO.sub.2, Fe.sub.2O.sub.3, and
CeO.sub.2.
[0089] SnO.sub.2 is a component that is prone to devitrify the
glass. So, in order to prevent devitrification while improving the
clarity, it is preferable that the SnO.sub.2 content by percentage
is from 0% to 0.5%, more preferably from 0.01% to 0.5%, even more
preferably from 0.05% to 0.3%, and further more preferably from
0.1% to 0.2%.
[0090] Fe.sub.2O.sub.3 is a component that stains the glass. So, in
order to achieve a suitable transmittance while improving the
clarity, it is preferable that the Fe.sub.2O.sub.3 content by
percentage is from 0% to 0.2%, more preferably from 0.01% to 0.2%,
even more preferably from 0.05% to 0.15%, and further more
preferably from 0.05% to 0.10%. Note that, particularly in cases
where transparency and UV transmission characteristics are demanded
of the glass, it is preferable that the Fe.sub.2O.sub.3 content is
less than 0.02%, and particularly preferable that Fe.sub.2O.sub.3
is intentionally not contained except for impurities.
[0091] The CeO.sub.2 content by percentage is preferably from 0% to
1.2%, more preferably from 0.01% to 1.2%, even more preferably from
0.05% to 1.0%, and particularly preferably from 0.3% to 1.0%.
[0092] Further, in cases where particularly high transmittance is
demanded of glass, such as in a cover glass, it is desirable to
employ SO.sub.3 as the refining agent. It is preferable that the
SO.sub.3 content by percentage is from 0% to 5%, preferably from
0.001% to 5%, more preferably from 0.01% to 3%, even more
preferably from 0.05% to 1%, further more preferably from 0.05% to
0.5%, and particularly preferably from 0.05% to 0.20%. In the case
of employing SO.sub.3 as the refining agent, the combined use in
the melting step of carbon and a sulfate, such as sodium sulfate,
serving as the source of SO.sub.3 can achieve an even higher
refining effect. Note that SO.sub.3 can be used in combination with
other refining agents, as described above.
[0093] As.sub.2O.sub.3, Sb.sub.2O.sub.3, and PbO also have the
effect of refining glass by causing reactions that involve a change
in valance in molten glass. However, these compounds place a heavy
burden on the environment. Therefore, in the glass substrate of the
present embodiment, the amount of these compounds is limited so
that As.sub.2O.sub.3, Sb.sub.2O.sub.3, and PbO are substantially
not included in the glass. Note that, herein, the expression
"As.sub.2O.sub.3, Sb.sub.2O.sub.3, and PbO are substantially not
included" means that the content is less than 0.01% and that these
compounds are intentionally not included except for impurities.
[0094] Oxides of rare-earth elements, such as Nb.sub.2O.sub.5 and
La.sub.2O.sub.3, are components that increase the Young's modulus
of the glass to be used for the glass substrate. However, if the
content of these compounds is too large, the devitrification
resistance will deteriorate. Therefore, the content by percentage
of rare-earth oxides, such as Nb.sub.2O.sub.5 and La.sub.2O.sub.3,
is preferably 3% or less, more preferably 1% or less, even more
preferably 0.5% or less, and particularly preferably less than 0.1%
and these compounds should intentionally not be included except for
impurities.
[0095] Note that in the present embodiment, components that stain
the glass, such as Co and Ni, are not preferable because such
components reduce the transmittance of the glass substrate or the
strengthened glass obtained after ion-exchange processing. In the
case of touch-panel displays, for example, a reduction in the
transmittance of the glass substrate or the strengthened glass is
not preferable because the visibility of the touch-panel display
will be impaired. Therefore, the content of transition metal
elements that stain the glass, such as Co and Ni, is preferably 1%
or less, more preferably 0.5% or less, even more preferably 0.05%
or less, and particularly preferably less than 0.05% and such
compounds should intentionally not be included except for
impurities.
Method for Producing Cover Glass According to First Embodiment
[0096] The method for producing a cover glass according to the
present embodiment will be described below. The cover-glass
production method involves the following steps (1) to (4):
[0097] (1) a step of melting glass materials in which the
components have been formulated and blended so as to provide the
glass substrate with the aforementioned composition;
[0098] (2) a step of forming the molten glass, which has been
molten in the melting step, into a plate-like shape by a down-draw
process;
[0099] (3) a step of subjecting the plate-like shaped glass to
shape-processing by etching; and
[0100] (4) a step of forming a compressive-stress layer on the
surfaces of the shape-processed glass by subjecting the glass to
chemical strengthening.
[0101] The down-draw process used in Step (2) above includes such
processes as the overflow down-draw process and the slot down-draw
process. Among them, the overflow down-draw process is suitably
used.
[0102] Step (1):
[0103] Step (1) is the step of melting glass materials in which the
components have been formulated and blended so as to provide the
glass substrate with the aforementioned glass composition.
[0104] More specifically, the glass materials corresponding to the
aforementioned components are measured and blended, are placed in a
melting pot made, for example, of fire brick, platinum, or a
platinum alloy, where they are heated and molten, and then are
subjected to refining and homogenization, thereby preparing molten
glass having a desired composition.
[0105] Step (2):
[0106] Step (2) is the step of forming the molten glass having the
desired composition, which has been prepared in Step (1), into a
plate-like shape by a down-draw process. The down-draw process is a
known process disclosed, for example, in JP-A-2009-203080. In the
down-draw process, molten glass is fed into a trough which is
provided on a forming body and is made to flow over both sides of
the trough. The overflowed molten glass flows downward along both
the side surfaces of the forming body having a wedge-shaped cross
section and located below the trough, creating two flows of molten
glass which join at the lowermost end of the forming body. The
joining of the two flows results in a strip of glass, which is
drawn by drawing rollers provided below the forming body. Thus, the
molten glass is formed into a strip of glass having a predetermined
thickness.
[0107] There are various processes for forming glass into a
plate-like shape, including various down-draw processes, the float
process, the re-draw process, and the roll-out process. The present
embodiment employs the down-draw process, because glass substrates
formed by using the down-draw process are improved in etching rate,
as compared to other forming processes such as the float process.
Another reason is that the principal surfaces of a glass substrate
formed by using the down-draw process are extremely smooth, because
they are made by hot forming.
[0108] More specifically, in the case of shape-processing the
aforementioned glass substrate by etching, the glass substrate can
be etched evenly from both the principal surfaces thereof at the
time of etching the glass substrate from the principal surfaces
thereof that have resist patterns thereon serving as masks. In
other words, the uniform composition of the glass substrate
enhances the dimensional accuracy in etching and also improves the
sectional shape of the end surfaces of the cover glass, which may
be used in a mobile phone, a touch-panel display, etc.
[0109] Also, both the principal surfaces of the glass substrate
formed by using the down-draw process have a uniform composition,
and therefore, there will be no difference in the ion-exchange rate
between the principal surfaces during the later-described
ion-exchange process. Thus, the glass substrate can be prevented
from warping after ion exchange due to a difference in composition.
In other words, it becomes possible to produce homogeneous cover
glasses, to improve productivity, and to reduce costs.
[0110] Further, forming the glass by using the down-draw process
can do away with the polishing step after forming, thus further
reducing costs and improving productivity. Also, forming by using
the down-draw process can produce glass substrates with surfaces
having no microcracks, which, in turn, can improve the strength of
the glass substrates.
[0111] Step (3):
[0112] Step (3) is the step of subjecting the plate-shaped glass
substrate to shape-processing by etching, to process the glass
substrate into a desired shape.
[0113] The following explains how the cover glass is subjected to
shape-processing by etching prior to the ion-exchange processing
step.
[0114] First, both the principal surfaces of the plate-shaped glass
substrate prepared as above are coated with a resist material.
Then, the resist material is exposed via a photo mask having a
desired outer-shape pattern. The outer shape is not particularly
limited, and it may be an outer shape including, for example,
sections having negative curvatures, as illustrated in FIG. 1.
[0115] Next, the exposed resist material is developed to thus form
a resist pattern on the glass substrate in regions other than the
regions-to-be-etched, and then, the regions-to-be-etched on the
glass substrate are etched. If a wet etchant is used as the
etchant, the glass substrate will be etched isotropically. Thus,
each end surface of the glass substrate will be formed so as to
have: a central section that projects outward the most; and sloped
faces that curve gently from the central section toward the
respective principal surfaces. It is preferable that the boundaries
between the sloped faces and the respective principal surfaces and
the boundary between the two sloped faces are rounded.
[0116] The resist material to be used in the etching step is not
particularly limited, and it is possible to use a material that is
resistant to the etchant used for etching the glass while using the
resist pattern as a mask. For example, glass is, in general,
corroded by wet etching using an aqueous solution containing
hydrofluoric acid or by dry etching using a fluorine-based gas, so
resist materials having excellent resistance to hydrofluoric acid
are suitable. As for the etchant, it is possible to suitably use a
mixed acid containing at least one of hydrofluoric acid, sulfuric
acid, nitric acid, hydrochloric acid, and hydrofluorosilicic acid.
The use of hydrofluoric acid or the aforementioned mixed-acid
aqueous solution as the etchant can produce cover glasses with
desired shapes.
[0117] Further, even complicated outer shapes can be created
easily, just by adjusting the mask pattern at the time of
performing shape-processing by employing etching. Further,
performing shape-processing through etching can further improve
productivity and also cut down processing costs. As for the
stripping solution for stripping the resist material off from the
glass substrate, an alkaline solution such as KOH or NaOH may be
used. The type of resist material, etchant, and stripping solution
can be selected as appropriate depending on the material of the
glass substrate.
[0118] Note that the etching process is not limited to the process
of simply immersing the glass substrate into an etching solution,
but instead it is possible to employ, for example, spray etching in
which the etching solution is sprayed.
[0119] By subjecting the glass substrate to shape-processing by
employing etching as described above, it is possible to produce a
cover glass having end surfaces with a highly-smooth surface
roughness. More specifically, it is possible to prevent
microcracks, which are inevitably created in shape-processing that
employs machining, and to thus further improve the mechanical
strength of the cover glass.
[0120] The glass substrate of the present embodiment has an etching
characteristic in which the etching rate is at least 3.7
.mu.m/minute in an etching environment having a temperature of
22.degree. C. and containing hydrogen fluoride with a concentration
of 10% by mass. The above etching characteristic can be achieved by
adjusting the composition of the glass substrate in a manner such
that, if the content of the aforementioned SiO.sub.2 is X % and the
content of the aforementioned Al.sub.2O.sub.3 is Y %, X-1/2Y is
57.5% or less.
[0121] Step (4):
[0122] Step (4) is the step of subjecting the glass substrate
shape-processed in Step (3) to an ion-exchange process.
[0123] The cover glass according to the present embodiment is
produced by performing an ion-exchange process on the glass
substrate that has been shape-processed in Step (3) as described
above. More specifically, for example, after being rinsed, the
glass substrate is immersed for around 1 to 25 hours in a treatment
bath containing 100% of KNO.sub.3 and kept at around 350.degree. C.
to 550.degree. C., to thereby exchange the Na.sup.+ ions on the
superficial layer of the glass with K.sup.+ ions present in the
treatment bath. In this way, the glass substrate can be chemically
strengthened. Note that the temperature, the length of time, the
ion-exchange solution, etc., for the ion-exchange process can be
changed as appropriate. For example, the ion-exchange solution may
be a mixed solution containing two or more types of compounds.
[0124] The foregoing was a description on the cover glass of the
first embodiment.
[0125] Next, the cover glass according to the second embodiment
will be described below. The glass substrate to be used in the
cover glass of the second embodiment is a preferable embodiment of
the glass substrate according to the first embodiment, in that the
content by percentage of CaO has been limited. This limitation not
only improves the etching rate during etching, which is performed
as the shape-processing step, as in the first embodiment, but can
also provide the end surfaces of the glass with uniform etching
surfaces--i.e., can improve the processing accuracy.
Cover Glass According to Second Embodiment
[0126] As with the cover glass of the first embodiment, the cover
glass according to the second embodiment also has the shape and
features as illustrated in FIG. 1. As illustrated in FIG. 1, the
cover glass 100 of the second embodiment has recesses 102 formed in
the right and left sides (in the figure) of a piece of plate-shaped
glass. Some sections of each recess 102 have negative curvatures as
defined above. The cover glass 100 is also provided with a
slit-form hole 104. The edge surrounding the hole 104 has sections
with negative curvatures as defined above.
[0127] As illustrated in FIG. 2, the cover glass 100 has a
compressive-stress layer 106 formed on the principal surfaces
thereof and also on the end surfaces thereof. The
compressive-stress layer 106 can be formed on the end surfaces of
the cover glass 100 as well as the principal surfaces by the cover
glass 100 being subjected to a chemical strengthening process after
shape-processing.
[0128] More specifically, the cover glass 100 is prepared by first
subjecting a glass substrate that has been formed into a plate-like
shape by a down-draw process, to shape-processing by a processing
technique including chemical etching, and then subjecting the
shape-processed glass substrate to chemical strengthening by an
ion-exchange process. The glass substrate contains, as its
components, 50% to 70% of SiO.sub.2, 5% to 20% of Al.sub.2O.sub.3,
6% to 30% of Na.sub.2O, 0% to less than 8% of Li.sub.2O, and 0% to
2.6% of CaO. The content by percentage of CaO is preferably 1.5% or
less, more preferably 1.0% or less, even more preferably 0.2% or
less, and it is particularly preferable that substantially no CaO
is included. Herein, the expression "substantially no CaO is
included" means that the content is less than 0.01% and CaO is
intentionally not included except for impurities. Further, the
glass substrate has an etching characteristic in which the etching
rate is at least 3.7 .mu.m/minute in an etching environment having
a temperature of 22.degree. C. and containing hydrogen fluoride
with a concentration of 10% by mass.
[0129] The composition of the glass substrate constituting the
cover glass 100 will be described in detail below.
Composition of Glass Substrate According to Second Embodiment
[0130] The glass substrate to be used for the cover glass 100
contains SiO.sub.2, Al.sub.2O.sub.3, and Na.sub.2O, and may also
contain B.sub.2O.sub.3, Li.sub.2O, K.sub.2O, MgO, CaO, SrO, BaO,
ZnO, ZrO.sub.2, TiO.sub.2, P.sub.2O.sub.5, SnO.sub.2, and SO.sub.3,
if necessary. In the following, the essential components refer to
components that need to be included in the glass substrate, whereas
the optional components refer to components that do not have to be
included at all in the glass substrate.
[0131] SiO.sub.2:
[0132] SiO.sub.2 is an essential component that constitutes the
skeletal structure of the glass to be used for the glass substrate,
and has the effect of improving the chemical durability and heat
resistance of the glass. If the SiO.sub.2 content by percentage is
less than 50%, vitrification may become difficult and sufficient
effects in durability and heat resistance may not be obtained,
although the etching rate at the time of etching the glass
substrate to perform shape-processing thereon tends to improve. On
the other hand, if the SiO.sub.2 content by percentage exceeds 70%,
then the glass is likely to cause devitrification and the glass
materials will become hard to melt and form, and also, the
viscosity will increase and the glass will become hard to
homogenize, thereby posing difficulty in mass-producing glass
inexpensively by using down-drawing processes. Further, if the
content by percentage exceeds 70%, the coefficient of thermal
expansion will become too small and will less likely match the
coefficients of thermal expansion of peripheral materials such as
metals and organic adhesives. Furthermore, if the content by
percentage exceeds 70%, the low-temperature viscosity will increase
excessively and thus the ion-exchange rate will drop, resulting in
that sufficient strength cannot be achieved even with chemical
strengthening through ion exchange. Therefore, the content by
percentage of SiO.sub.2 is from 50% to 70%, preferably from 53% to
67%, more preferably from 55% to 67%, even more preferably from 58%
to 65%, and particularly preferably from 60% to 65%. Note that
low-temperature viscosity refers to the temperature at 10.sup.7.6
to 10.sup.14.5 dPas, but in the present embodiment, it is defined
as indicating the temperature at 10.sup.14.5 dPas.
[0133] Al.sub.2O.sub.3:
[0134] Al.sub.2O.sub.3 is an essential component that constitutes
the skeletal structure of the glass to be used for the glass
substrate, and has the effect of improving the chemical durability
and heat resistance of the glass and of improving the ion-exchange
performance and the etching rate at the time of performing
shape-processing by etching. If the Al.sub.2O.sub.3 content by
percentage is less than 5%, the aforementioned effects cannot be
obtained sufficiently. On the other hand, if the Al.sub.2O.sub.3
content by percentage exceeds 20%, the glass will become hard to
melt and also the viscosity of the glass will increase, which will
make it hard to form. Thus, it becomes difficult to mass-produce
glass inexpensively by using down-drawing processes. Further, if
the Al.sub.2O.sub.3 content by percentage exceeds 20%, the acid
resistance will become excessively poor, which is not preferable
for a cover glass which is used as a protective component.
Furthermore, if the Al.sub.2O.sub.3 content by percentage exceeds
20%, the glass is likely to cause devitrification and the
devitrification resistance will deteriorate as well, which will
make down-draw processing inapplicable. Therefore, the content by
percentage of Al.sub.2O.sub.3 is from 5% to 20%, preferably from 5%
to 17%, more preferably from 7% to 16%, and particularly preferably
from 9% to 15%.
[0135] Note that in the present embodiment, it is preferable that,
if the content by percentage of SiO.sub.2 is X % and the content by
percentage of Al.sub.2O.sub.3 is Y %, X-1/2Y is 57.5% or less. When
X-1/2Y is 57.5% or less, the etching rate of the glass substrate
can be improved effectively. The preferable range for X-1/2. Y is
more preferably 56% or less, and even more preferably 55% or
less.
[0136] On the other hand, if X-1/2Y is below 45%, the
devitrification temperature will rise and thus the devitrification
resistance will deteriorate, although the etching rate will reach 5
.mu.m/minute or higher. Therefore, in order to achieve both an
improvement in devitrification resistance and an improvement in
etching rate, X-1/2Y is preferably 45% or greater, more preferably
47% or greater, and particularly preferably 50% or greater.
Specifically, the range for X-1/2Y is preferably 45% to 57.5%, more
preferably 47% to 56%, and even more preferably 50% to 55%.
[0137] B.sub.2O.sub.3:
[0138] B.sub.2O.sub.3 is an optional component that decreases the
viscosity of the glass and that promotes the melting and refining
of the glass to be used for the glass substrate. If the content by
percentage exceeds 5%, the acid resistance of the glass will
deteriorate and also volatilization will increase, thereby making
the glass hard to homogenize. Also, the increase in volatilization
will cause unevenness in the glass and will also cause unevenness
in the etching on the glass substrate. That is, the etching rate
will become uneven among the various areas of the glass, and
therefore, a glass substrate containing an excessive amount of
B.sub.2O.sub.3 is not suitable for purposes such as etching with
the aim of shape-processing, which requires high accuracy.
Therefore, the B.sub.2O.sub.3 content by percentage is preferably
from 0% to 5%, more preferably from 0% to 3%, even more preferably
from 0% to less than 2%, and particularly preferably less than
0.01% and B.sub.2O.sub.3 should intentionally not be contained
except for impurities. By adjusting the B.sub.2O.sub.3 content by
percentage to 0% to 5%, it is possible to achieve the effect of
improving the etching rate and also prevent unevenness in etching,
thereby allowing the production of cover glasses with higher
quality.
[0139] Li.sub.2O:
[0140] Li.sub.2O is one of the ion-exchange components and is an
optional component that reduces the viscosity of the glass to be
used for the glass substrate and that improves the meltability and
formability of the glass. Li.sub.2O is also a component that
improves the Young's modulus of the glass substrate, and among
various alkali metal oxides, Li.sub.2O is highly effective in
increasing the depth of the compressive-stress layer. However, if
the Li.sub.2O content by percentage is too large, there will be a
disadvantage that the ion-exchange salts will deteriorate too soon
in the ion-exchange process, which is the step of strengthening the
glass substrate, thereby leading to an increase in the production
cost of the cover glass. Furthermore, if the Li.sub.2O content by
percentage is too large, then not only will the heat resistance
deteriorate excessively (the strain point and the glass transition
point drop excessively), but also the low-temperature viscosity
will drop excessively; this will cause stress relaxation in the
heating step after chemical strengthening and will reduce the
stress value of the compressive-stress layer, resulting in not
being able to produce a cover glass with sufficient strength.
Therefore, the Li.sub.2O content by percentage is from 0% to less
than 8%, preferably from 0% to 5%, more preferably from 0% to 2%,
even more preferably from 0% to 1%, further more preferably from 0%
to 0.02%, and desirably less than 0.01%, and it is particularly
preferable that Li.sub.2O is intentionally not contained except for
impurities.
[0141] Meanwhile, if the strain point and the glass transition
point become too high, it becomes necessary to increase the
temperature for the ion-exchange process optimal for ensuring a
compressive-stress layer having predetermined characteristics that
would match the glass composition. However, if the ion-exchange
processing temperature becomes too high, then the ion-exchange
processing temperature may exceed the decomposition temperature of
the ion-exchange salts. Incidentally, Li.sub.2O can effectively
reduce the molten glass transition point and the strain point, so
the ion-exchange processing temperature can be reduced by
increasing the content by percentage of Li.sub.2O. However, if the
Li.sub.2O content becomes too large, then it becomes difficult to
observe fringe patterns in the ion-exchanged strengthened glass
that are related to the refractive index, thus making it difficult
to measure the stress value and the thickness of the
compressive-stress layer with a stress meter while retaining the
shape of the strengthened glass. Therefore, if it is desired to
facilitate the measurement of the compressive-stress layer and at
the same time reduce the ion-exchange processing temperature, then
it is preferable to adjust the content by percentage of Li.sub.2O
to greater than 0%--i.e., to include Li.sub.2O in the glass
composition--and more preferably to 0.001% or greater, even more
preferably to 0.01% or greater, and further more preferably to
0.02% or greater. Also, the Li.sub.2O content is suitably 1.3% or
less, preferably less than 0.5%, more preferably less than 0.4%,
even more preferably 0.3% or less, and particularly preferably 0.2%
or less. Specifically, the content percentage of Li.sub.2O is
preferably 0.001% to 1.3%, more preferably 0.01% to less than 0.5%,
even more preferably 0.02% to less than 0.4%, further more
preferably 0.02% to 0.3%, and particularly preferably 0.02% to
0.2%.
[0142] Na.sub.2O:
[0143] Na.sub.2O is one of the ion-exchange components and is an
essential component that reduces the high-temperature viscosity of
the glass to be used for the glass substrate and that improves the
meltability and formability of the glass. Further, Na.sub.2O is a
component that improves the devitrification resistance of the
glass. If the Na.sub.2O content by percentage is less than 10%, the
meltability of the glass will deteriorate, which will increase the
cost for melting. Further, if the Na.sub.2O content by percentage
is less than 10%, then the ion-exchange performance will also
deteriorate, resulting in that sufficient strength cannot be
achieved. Furthermore, if the Na.sub.2O content by percentage is
less than 10%, then the coefficient of thermal expansion will be
too small and will less likely match the coefficients of thermal
expansion of peripheral materials such as metals and organic
adhesives. Moreover, if the Na.sub.2O content by percentage is less
than 10%, the glass is likely to cause devitrification and the
devitrification resistance will deteriorate as well, which will
make down-draw processing inapplicable and thereby pose difficulty
in mass-producing glass inexpensively. On the other hand, if the
Na.sub.2O content by percentage exceeds 25%, then the
low-temperature viscosity will drop and the impact resistance will
deteriorate, and also the coefficient of thermal expansion will be
too large and will less likely match the coefficients of thermal
expansion of peripheral materials such as metals and organic
adhesives. Therefore, the Na.sub.2O content by percentage is from
6% to 30%, preferably from 10% to 25%, more preferably from 10% to
20%, even more preferably from 12% to 20%, and particularly
preferably from 13% to 19%.
[0144] Further, in the present embodiment, the difference found by
subtracting the Al.sub.2O.sub.3 content by percentage from the
Na.sub.2O content by percentage ("Na.sub.2O content
percentage-Al.sub.2O.sub.3 content percentage") is preferably from
-10% to 15%. When the difference "Na.sub.2O content
percentage-Al.sub.2O.sub.3 content percentage" is within the range
of -10% to 15%, then not only can the objects described in the
present disclosure be achieved, but also the meltability of the
glass can be improved while keeping the coefficient of thermal
expansion and heat resistance at a suitable level. Therefore, the
glass can be molten at lower temperatures, thereby allowing further
reductions in the cost for producing the cover glass. Note that the
range for the difference "Na.sub.2O content
percentage-Al.sub.2O.sub.3 content percentage" is more preferably
from -5% to 13%, even more preferably from -5% to 10%, and further
more preferably from -3% to 5%.
[0145] K.sub.2O:
[0146] K.sub.2O is an optional component that can improve the
ion-exchange performance of the glass substrate by being included
therein. K.sub.2O also reduces the high-temperature viscosity of
the glass, improves the meltability and formability thereof, and
also improves the devitrification resistance. However, if the
K.sub.2O content is too large, the low-temperature viscosity will
drop, the coefficient of thermal expansion will become too large,
and the impact resistance will become poor, which is not preferable
for a cover glass. Further, if the K.sub.2O content by percentage
is too large, the coefficient of thermal expansion will less likely
match the coefficients of thermal expansion of peripheral materials
such as metals and organic adhesives. Therefore, the K.sub.2O
content by percentage is 15% or less, preferably 10% or less, more
preferably less than 5.6%, even more preferably less than 5%, and
particularly preferably less than 4%. On the other hand, the lower
limit of the K.sub.2O content by percentage is 0% or greater,
preferably 0.1% or greater, more preferably 1% or greater, and even
more preferably 2% or greater. Adjusting the lower limit of the
K.sub.2O content by percentage to be within the aforementioned
range not only improves the etching rate, but can also shorten the
time required for ion-exchange processing and improve the cover
glass productivity. Specifically, the content percent of K.sub.2O
is preferably 0% to 15%, more preferably 0.1% to 10%, even more
preferably 1% to less than 5.6%, further more preferably 2% to less
than 5%, and particularly preferably 2% to less than 4%.
[0147] R.sup.1.sub.2O (R.sup.1 includes all the elements among Li,
Na, and K that are contained in the glass substrate):
[0148] In the present embodiment, the R.sup.1.sub.2O content by
percentage (the total content by percentage of all the elements
among Li, Na, and K that are contained in the glass substrate) is
preferably from 6% to 30%. If the percentage of R.sup.1.sub.2O is
less than 6%, ion exchange will not be performed sufficiently and
thus a sufficient strength cannot be obtained, thereby posing
difficulty in using the glass substrate for a cover glass. On the
other hand, if the percentage of R.sup.1.sub.2O exceeds 30%, the
chemical durability of the glass will deteriorate. So, in order to
achieve both mechanical strength and devitrification resistance and
to improve chemical durability and productivity, the R.sup.1.sub.2O
content by percentage is more preferably from 10% to 28%, even more
preferably from 14% to 25%, further more preferably from 15% to
24%, and particularly preferably from 17% to 23%.
[0149] Note that the aforementioned range for the content by
percentage of R.sup.1.sub.2O is a criterion to be satisfied in
addition to each range of content by percentage as set forth above
for the oxide of each of the elements among Li, Na, and K contained
in the glass substrate.
[0150] B.sub.2O.sub.3/R.sup.1.sub.2O (R.sup.1 includes all the
elements among Li, Na, and K that are contained in the glass
substrate):
[0151] In the present embodiment, the ratio in content by
percentage between B.sub.2O.sub.3 and R.sup.1.sub.2O
("B.sub.2O.sub.3/R.sup.1.sub.2O") is preferably from 0 to less than
0.3. B.sub.2O.sub.3 is prone to bond with an alkali metal oxide and
volatilize as an alkali borate, and particularly, Li.sup.+, which
has a small ionic radius, has high mobility in the glass melt and
is prone to volatilize from the surface of the melt, and
volatilization is likely to create a concentration gradient up to
the inner part of the glass and to give rise to striae on the
surface of the glass. In other words, an increase in the amount of
volatilization of B.sub.2O.sub.3 will make the produced glass
substrate nonuniform, and unevenness in etching will occur due to
the nonuniformity of the glass substrate when such a glass
substrate is subjected to etching. However, alkali metal oxides are
essential components to glass that is chemically strengthened by an
ion-exchange process. Therefore, the ratio
B.sub.2O.sub.3/R.sup.1.sub.2O in content by percentage (the ratio
in percentage by mass) is preferably adjusted to be within the
range of 0 to less than 0.3. In this range, the nonuniformity of
the glass and unevenness in etching can be reduced effectively.
Thus, not only is the etching rate improved, but also unevenness in
etching rate can be prevented, thereby allowing strengthened glass
of desired shape to be produced with high yield. Note that the
range for the ratio B.sub.2O.sub.3/R.sup.1.sub.2O in content by
percentage is more preferably from 0 to 0.1, preferably from 0 to
0.07, more preferably from 0 to 0.03, even more preferably from 0
to 0.005, and particularly preferably 0. Further, in order to
reduce unevenness in etching, it is preferable that the Li.sub.2O
content by percentage is less than 0.01% and Li.sub.2O should
intentionally not be contained except for impurities, as described
above.
[0152] MgO:
[0153] MgO is an optional component that decreases the viscosity of
the glass to be used for the glass substrate and that promotes the
melting and refining of the glass. Also, among alkaline-earth
metals, MgO is an effective component for improving the meltability
while making the glass lightweight, because it only increases the
glass density by a small rate. MgO also improves formability and
increases the strain point and Young's modulus of the glass.
Furthermore, the precipitations that are produced at the time of
etching MgO-containing glass by using e.g. hydrofluoric acid have
large solubility and relatively low production rate, and therefore,
it is relatively less likely for the precipitations to adhere to
the glass surface during etching. Thus, it is preferable to include
MgO in order to improve the glass meltability and to increase the
etching rate. However, if the MgO content is too large, then the
devitrification resistance will deteriorate, thus posing difficulty
in mass-producing glass inexpensively by using down-drawing
processes. Therefore, the MgO content by percentage is from 0% to
15%, preferably greater than 1% to 15%, more preferably greater
than 1% to 12%, further preferably greater than 1% to less than 7%,
even more preferably from 3% to less than 7%, and particularly
preferably greater than 4.5% to 6%. The inclusion of MgO within the
range of 0% to 15% will improve the etching rate and will also
allow the glass to be molten at lower temperatures, thereby
allowing further reductions in the cost for producing the cover
glass. Furthermore, because it is possible to improve the
ion-exchange performance and increase the strain point at the same
time, the MgO-containing glass is suitable for cover glasses that
require high mechanical strength. This is because a sufficient
compressive-stress layer can be formed on the surface of the glass
substrate, and the compressive-stress layer formed on the surface
can be prevented from causing stress relaxation, even during/after
thermal treatment.
[0154] CaO:
[0155] CaO is an optional component that decreases the viscosity of
the glass to be used for the glass substrate and that promotes the
melting and refining of the glass. Also, among alkaline-earth
metals, CaO is an effective component for improving the meltability
while making the glass lightweight, because it only increases the
glass density by a small rate. CaO also improves formability and
increases the strain point and Young's modulus of the glass.
However, if the CaO content is too large, then the devitrification
resistance will deteriorate, thus posing difficulty in
mass-producing glass inexpensively by using down-drawing processes.
Also, if the CaO content is too large, the ion-exchange performance
will deteriorate; thus, sufficient strength cannot be achieved, and
also productivity will be reduced. Further, the precipitations
(chemical substances) produced at the time of subjecting glass that
contains large amounts of CaO to wet-etching by using e.g.
hydrofluoric acid are not only insoluble in the etchant solution,
but also are produced at an extremely high precipitation rate.
Therefore, such precipitations adhere to the surface of the glass
being etched, and if the adherence amount is large, the etching
reaction will be inhibited, the glass-processing productivity will
be impaired, and also the glass surface after etching will be
degraded. In other words, the inclusion of CaO will not only
degrade the surface quality of the etched cover glass, but may also
inhibit the progress of etching if large amounts of precipitations
adhere to the glass surface, which may extend the etching time and
deteriorate the shape accuracy (processing accuracy). On the other
hand, the inclusion of CaO can lower the devitrification
temperature and improve the devitrification resistance and
meltability. Therefore, the CaO content by percentage is from 0% to
2.6%, preferably from 0% to 1.5%, more preferably from 0% to 1.0%,
even more preferably from 0% to 0.6%, and further more preferably
from 0% to 0.2%. Note that in cases where an extremely-high etching
quality is required, it is preferable that substantially no CaO is
contained.
[0156] Further, it is even more preferable to include both MgO and
CaO in order to reduce the melt viscosity and at the same time
lower the devitrification temperature, but the amount of CaO shall
be adjusted as appropriate to fall within the range that will not
give rise to the aforementioned problems caused by the
precipitations formed during etching. Therefore, the upper limit of
the content by percentage of CaO is 2.6%.
[0157] SrO:
[0158] SrO is an optional component that decreases the viscosity of
the glass to be used for the glass substrate and that promotes the
melting and refining of the glass. SrO also improves formability
and increases the strain point and Young's modulus of the glass.
However, if the SrO content is too large, then the glass density
will increase, and the glass will be unsuitable for cover glasses,
which are required to be lightweight. Further, if the SrO content
is too large, then the coefficient of thermal expansion will be too
large and will less likely match the coefficients of thermal
expansion of peripheral materials such as metals and organic
adhesives. Furthermore, if the SrO content is too large, then the
ion-exchange performance will also deteriorate, making it difficult
to obtain the high mechanical strength demanded of cover glasses.
Therefore, the SrO content by percentage is preferably from 0% to
10%, more preferably from 0% to 5%, even more preferably from 0% to
2%, and further more preferably from 0% to 0.5%, and it is
particularly preferable that SrO is intentionally not contained
except for impurities.
[0159] BaO:
[0160] BaO is an optional component that decreases the viscosity of
the glass to be used for the glass substrate and that promotes the
melting and refining of the glass. BaO also improves formability
and increases the strain point and Young's modulus of the glass.
However, if the BaO content is too large, then the glass density
will increase, and the glass will be unsuitable for cover glasses,
which are required to be lightweight. Further, if the BaO content
is too large, then the coefficient of thermal expansion will be too
large and will less likely match the coefficients of thermal
expansion of peripheral materials such as metals and organic
adhesives. Furthermore, if the BaO content is too large, then the
ion-exchange performance will also deteriorate, making it difficult
to obtain the high mechanical strength demanded of cover glasses.
Therefore, the BaO content by percentage is preferably from 0% to
10%, more preferably from 0% to 5%, even more preferably from 0% to
2%, and further more preferably from 0% to 0.5%. Note that, because
BaO places a heavy burden on the environment, it is particularly
preferable that the BaO content is less than 0.01% and BaO should
intentionally not be contained except for impurities.
[0161] SrO+BaO:
[0162] In the present embodiment, the sum found by adding the SrO
content by percentage and the BaO content by percentage ("SrO
content percentage+BaO content percentage") is preferably less than
10%. When the sum "SrO content percentage+BaO content percentage"
is less than 10%, it is possible to effectively prevent an increase
in the glass density and a decrease in the ion-exchange rate. That
is, adjusting the sum "SrO content percentage+BaO content
percentage" to less than 10% will not only improve the etching
rate, but can also achieve the effect of making the cover glass
lightweight and the effect of improving productivity and glass
strength. Note that the range for the sum "SrO content
percentage+BaO content percentage" is preferably from 0% to 8%,
more preferably from 0% to 5%, even more preferably from 0% to 2%,
and further more preferably from 0% to 1%, and it is particularly
preferable that SrO and BaO are intentionally not contained except
for impurities.
[0163] RO (R includes all the elements among Mg, Ca, Sr, and Ba
that are contained in the glass substrate):
[0164] Herein, the RO content by percentage (the total content by
percentage of all the elements among Mg, Ca, Sr, and Ba that are
contained in the glass substrate) is preferably from 0% to 20%. If
the RO content is greater than 20%, the chemical durability will
deteriorate. On the other hand, the inclusion of RO can improve the
meltability and heat resistance of the glass. Therefore, the RO
content by percentage is preferably from 0% to 10%, more preferably
from 0% to 7%, even more preferably from 2% to 7%, further
preferably from 3% to 7%, and further more preferably from 4% to
7%.
[0165] Note that the aforementioned range for the content by
percentage of RO is a criterion to be satisfied in addition to each
range of content by percentage as set forth above for the oxide of
each of the elements among Mg, Ca, Sr, and Ba contained in the
glass substrate.
[0166] Li.sub.2O/(RO+Li.sub.2O):
[0167] In the present embodiment, the ratio in content by
percentage between Li.sub.2O and the sum of RO and Li.sub.2O
("Li.sub.2O/(RO+Li.sub.2O)"; wherein R includes all the elements
selected from Mg, Ca, Sr, and Ba that are contained in the glass
substrate) is preferably less than 0.3. By adjusting the ratio
Li.sub.2O/(RO+Li.sub.2O) in content by percentage to be within the
aforementioned range, it is possible to inhibit the deterioration
of ion-exchange salts in the ion-exchange process, which is the
step of strengthening the glass substrate, and it is thus possible
to reduce the cost for producing the strengthened glass to be used
as the cover glass. Further, if the ratio
"Li.sub.2O/(RO+Li.sub.2O)" in content by percentage is less than
0.3, the devitrification temperature can be lowered effectively,
and thus, the devitrification resistance can be improved
effectively. Further, if the ratio "Li.sub.2O/(RO+Li.sub.2O)" in
content by percentage is less than 0.3, then the strain point can
be increased effectively and also the heat resistance can be
improved. That is, such a content by percentage not only increases
the etching rate, but can also improve the heat resistance and
prevent such problems as stress relaxation during chemical
strengthening. Note that the range for the ratio
"Li.sub.2O/(RO+Li.sub.2O)" in content by percentage is more
preferably 0.08 or less, even more preferably 0.05 or less, further
more preferably 0.01 or less, and particularly preferably 0.
[0168] ZnO:
[0169] ZnO is an optional component that improves the ion-exchange
performance, that is highly effective particularly in improving the
compressive-stress value, and that lowers the high-temperature
viscosity of the glass without lowering the low-temperature
viscosity. However, if the ZnO content is too large, the glass will
cause phase separation and the devitrification resistance will
deteriorate. Further, if the ZnO content is too large, then the
glass density will increase, and the glass will be unsuitable for
cover glasses, which are required to be lightweight. Therefore, the
ZnO content by percentage is preferably from 0% to 6%, more
preferably from 0% to 4%, even more preferably from 0% to 1%,
further more preferably from 0% to 0.1%, and particularly
preferably less than 0.01% and ZnO should intentionally not be
contained except for impurities.
[0170] ZrO.sub.2:
[0171] ZrO.sub.2 is an optional component that significantly
improves the ion-exchange performance and that increases the strain
point and the viscosity near the devitrification temperature of the
glass. Further, ZrO.sub.2 improves the heat resistance of the
glass. However, if the ZrO.sub.2 content is too large, the
devitrification temperature will be increased and the
devitrification resistance will deteriorate. Therefore, in order to
prevent a reduction in devitrification resistance, the ZrO.sub.2
content by percentage is preferably from 0% to 10%, more preferably
from 0% to 6% or less, even more preferably from 0% to 4% or less,
and further more preferably from 0% to 2% or less. By including
ZrO.sub.2, it is possible to effectively improve heat resistance,
which is important for cover glasses used in mobile phones and for
cover glasses used in touch-panel displays, and to effectively
improve ion-exchange performance, which relates to the reduction of
time for chemically strengthening the glass substrate and to the
improvement of the mechanical strength thereof. Therefore, the
ZrO.sub.2 content by percentage is preferably 0.1% or greater, more
preferably 0.5% or greater, even more preferably 1% or greater, and
particularly preferably 2% or greater. That is, by adjusting the
ZrO.sub.2 content by percentage to 0.1% or greater, the heat
resistance and ion-exchange performance can be improved while also
improving devitrification resistance. Thus, the time required for
ion-exchange processing can be reduced, and thus productivity can
be improved.
[0172] On the other hand, if the glass density is to be reduced,
then the ZrO.sub.2 content by percentage should preferably be less
than 0.1%, and it is particularly preferable that ZrO.sub.2 is
intentionally not contained except for impurities.
[0173] TiO.sub.2:
[0174] TiO.sub.2 is an optional component that improves the
ion-exchange performance and that reduces the high-temperature
viscosity of the glass. However, if the TiO.sub.2 content is too
large, the devitrification resistance will deteriorate. Further, if
the TiO.sub.2 content is too large, then the UV transmittance will
deteriorate and the glass will be stained, which is not suitable
for cover glasses or the like. Furthermore, if the TiO.sub.2
content is too large, then the UV transmittance will deteriorate,
thus causing a disadvantage that, in the case of using a UV-curable
resin, the resin cannot be cured sufficiently. Therefore, the
TiO.sub.2 content by percentage is preferably from 0% to 5%, more
preferably from 0% to less than 3%, even more preferably from 0% to
1%, and further more preferably from 0% to 0.01%, and it is
particularly preferable that TiO.sub.2 is intentionally not
contained except for impurities.
[0175] (ZrO.sub.2+TiO.sub.2)/SiO.sub.2:
[0176] In the present embodiment, the ratio in content by
percentage between the sum of ZrO.sub.2 and TiO.sub.2 to SiO.sub.2
("(ZrO.sub.2+TiO.sub.2)/SiO.sub.2") is preferably from 0 to 0.2. In
the case of shape-processing a glass substrate by etching,
ion-exchange processing will be performed after etching. In the
ion-exchange process, deformation may occur due to the internal
stress within the glass substrate if ion exchange is carried out
excessively. In other words, excessive ion exchange gives rise to
the deformation of the glass substrate, and thus the shape that has
been processed with high accuracy by etching cannot be retained and
the glass substrate becomes unsuitable for a cover glass. So, by
adjusting the ratio "(ZrO.sub.2+TiO.sub.2)/SiO.sub.2" in content by
percentage to be within the range of 0 to 0.2, excessive ion
exchange can be inhibited effectively. Note that the range for the
ratio "(ZrO.sub.2+TiO.sub.2)/SiO.sub.2" in content by percentage is
preferably from 0 to 0.15, more preferably from 0 to 0.1, even more
preferably from 0 to 0.07, and particularly preferably from 0 to
0.01. When the ratio "(ZrO.sub.2+TiO.sub.2)/SiO.sub.2" in content
by percentage is within the range of 0 to 0.2, the devitrification
resistance as well as the heat resistance can be improved while
preventing excessive ion exchange.
[0177] P.sub.2O.sub.5:
[0178] P.sub.2O.sub.5 is an optional component that improves the
ion-exchange performance and that is highly effective particularly
in increasing the thickness of the compressive-stress layer.
However, if the P.sub.2O.sub.5 content is too large, the glass will
cause phase separation and the water resistance will deteriorate.
Therefore, the P.sub.2O.sub.5 content by percentage is preferably
from 0% to 10%, more preferably from 0% to 4%, even more preferably
from 0% to 1%, further more preferably from 0% to 0.1%, and
particularly preferably less than 0.01% and P.sub.2O.sub.5 should
intentionally not be contained except for impurities.
[0179] In addition to the aforementioned components, the glass
substrate contains refining agents as described below.
[0180] Refining Agent:
[0181] A refining agent is a component necessary for the refining
of the glass to be used for the glass substrate. No refining effect
can be obtained if the content is less than 0.001%, whereas the
content exceeding 5% may cause devitrification and/or staining.
Therefore, the total content by percentage of refining agent(s) is
preferably from 0.001% to 2%, more preferably from 0.01% to 1%,
even more preferably from 0.05% to 0.5%, and particularly
preferably from 0.05% to 0.2%.
[0182] The refining agents are not particularly limited as far as
they have little burden on the environment and provide the glass
with excellent clarity. Examples include one or more types of
agents selected from the group of oxides of metals including, for
example, Sn, Fe, Ce, Tb, Mo, and W.
[0183] The following ranges are preferable for the metal oxides,
the oxides being expressed as SnO.sub.2, Fe.sub.2O.sub.3, and
CeO.sub.2.
[0184] SnO.sub.2 is a component that is prone to devitrify the
glass. So, in order to prevent devitrification while improving the
clarity, it is preferable that the SnO.sub.2 content by percentage
is from 0% to 0.5%, more preferably from 0.01% to 0.5%, even more
preferably from 0.05% to 0.3%, and further more preferably from
0.1% to 0.2%.
[0185] Fe.sub.2O.sub.3 is a component that stains the glass. So, in
order to achieve a suitable transmittance while improving the
clarity, it is preferable that the Fe.sub.2O.sub.3 content by
percentage is from 0% to 0.2%, more preferably from 0.01% to 0.2%,
even more preferably from 0.05% to 0.15%, and further more
preferably from 0.05% to 0.10%. Note that, particularly in cases
where transparency and UV transmission characteristics are demanded
of the glass, it is preferable that the Fe.sub.2O.sub.3 content is
less than 0.02%, and particularly preferable that Fe.sub.2O.sub.3
is intentionally not contained except for impurities.
[0186] The CeO.sub.2 content by percentage is preferably from 0% to
1.2%, more preferably from 0.01% to 1.2%, even more preferably from
0.05% to 1.0%, and particularly preferably from 0.3% to 1.0%.
[0187] Further, for cover glasses that require a particularly high
transmittance, it is desirable to employ SO.sub.3 as the refining
agent. It is preferable that the SO.sub.3 content by percentage is
from 0% to 5%, preferably from 0.001% to 5%, more preferably from
0.01% to 3%, even more preferably from 0.05% to 1%, further more
preferably from 0.05% to 0.5%, and particularly preferably from
0.05% to 0.20%. In the case of employing SO.sub.3 as the refining
agent, the combined use in the melting step of carbon and a
sulfate, such as sodium sulfate, serving as the source of SO.sub.3
can achieve an even higher refining effect. Note that SO.sub.3 can
be used in combination with other refining agents, as described
above.
[0188] As.sub.2O.sub.3 and Sb.sub.2O.sub.3 also have the effect of
refining glass by causing reactions that involve a change in
valance in molten glass. However, these compounds place a heavy
burden on the environment. Therefore, in the glass substrate of the
present embodiment, the amount of these compounds is limited so
that As.sub.2O.sub.3 and Sb.sub.2O.sub.3 are substantially not
included in the glass. Note that, herein, the expression
"As.sub.2O.sub.3 and Sb.sub.2O.sub.3 are substantially not
included" means that the content is less than 0.01% and that these
compounds are intentionally not included except for impurities.
Further, PbO and F have the effect of improving the glass
meltability and refining the glass. However, these compounds place
a heavy burden on the environment. Therefore, in the cover glass of
the present embodiment, it is preferable that PbO and F are
substantially not included in the glass.
[0189] Oxides of rare-earth elements, such as Nb.sub.2O.sub.5 and
La.sub.2O.sub.3, are optional components that increase the Young's
modulus of the glass to be used for the glass substrate. However,
if the content of these compounds is too large, the devitrification
resistance will deteriorate. Therefore, the content by percentage
of rare-earth oxides, such as Nb.sub.2O.sub.5 and La.sub.2O.sub.3,
is preferably 3% or less, more preferably 1% or less, even more
preferably 0.5% or less, and particularly preferably less than 0.1%
and these compounds should intentionally not be included except for
impurities.
[0190] Note that in the present embodiment, components that stain
the glass, such as Co and Ni, are not preferable because such
components reduce the transmittance of the glass substrate or the
strengthened glass obtained after ion-exchange processing. In the
case of touch-panel displays, for example, a reduction in the
transmittance of the glass substrate or the strengthened glass is
not preferable because the visibility of the touch-panel display
will be impaired. Therefore, the content of transition metal
elements that stain the glass, such as Co and Ni, is preferably 1%
or less, more preferably 0.5% or less, even more preferably 0.05%
or less, and particularly preferably less than 0.05% and such
compounds should intentionally not be included except for
impurities.
Method for Producing Cover Glass According to Second Embodiment
[0191] The method for producing a cover glass according to the
present embodiment will be described below. The cover-glass
production method involves the following steps (1) to (4):
[0192] (1) a step of melting glass materials in which the
components have been formulated and blended so as to provide the
glass substrate with the aforementioned composition;
[0193] (2) a step of forming the molten glass, which has been
molten in the melting step, into a plate-like shape;
[0194] (3) a step of subjecting the plate-like shaped glass
substrate to shape-processing by a processing technique including
at least chemical etching; and
[0195] (4) a step of forming a compressive-stress layer on the
surfaces of the shape-processed glass substrate by subjecting the
glass to chemical strengthening.
[0196] Step (1):
[0197] Step (1) is the step of melting glass materials in which the
components have been formulated and blended so as to provide the
glass substrate with the aforementioned glass composition.
[0198] More specifically, the glass materials corresponding to the
aforementioned components are measured and blended, are placed in a
melting pot made, for example, of fire brick, platinum, or a
platinum alloy, where they are heated and molten, and then are
subjected to refining and homogenization, thereby preparing molten
glass having a desired composition.
[0199] Step (2):
[0200] Step (2) is the step of forming the molten glass having the
desired composition, which has been prepared in Step (1), into a
plate-like shape. In the present embodiment, it is preferable to
use a down-draw process for this forming step. The down-draw
process is a known process disclosed, for example, in
JP-A-2009-203080. In the down-draw process, molten glass is fed
into a trough which is provided on a forming body and is made to
flow over both sides of the trough. The overflowed molten glass
flows downward along both the side surfaces of the forming body
having a wedge-shaped cross section and located below the trough,
creating two flows of molten glass which join at the lowermost end
of the forming body. The joining of the two flows results in a
strip of glass, which is drawn by drawing rollers provided below
the forming body. Thus, the molten glass is formed into a strip of
glass having a predetermined thickness.
[0201] There are various processes for forming glass into a
plate-like shape, including various down-draw processes, the float
process, the re-draw process, and the roll-out process. Any one of
these processes can be employed, but in the present embodiment, it
is most suitable to employ the down-draw process--and particularly
the overflow down-draw process. The down-draw process is employed
because glass substrates formed by using the down-draw process are
improved in etching rate, as compared to other forming processes
such as the float process. Another reason is that the principal
surfaces of a glass substrate formed by using the down-draw process
are extremely smooth, because they are made by hot forming.
[0202] In contrast, with the known float process, a diffusion layer
of tin (Sn) is formed on the glass surface, and this gives rise to
a difference in diffusion rate of alkali ions between the front and
back principal surfaces of the glass at the time of chemical
strengthening, thus posing difficulty in forming the
compressive-stress layer stably. Meanwhile, the press-forming
process suffers in that large plate-shaped substrates cannot be
produced. With the other sheet-forming processes, the produced
glass substrate cannot be employed as a cover glass unless the
principal surfaces are polished, leading to an increase in
processing cost. The down-draw process employed in the present
embodiment has none of these disadvantages.
[0203] A glass substrate formed into a sheet by the down-draw
process can be made extremely smooth and thin. Therefore, in the
case of shape-processing the glass substrate by etching, the glass
substrate can be etched evenly from both the principal surfaces
thereof at the time of etching the glass substrate from the
principal surfaces thereof that have resist patterns thereon
serving as masks. In other words, the uniform composition of the
glass substrate enhances the dimensional accuracy in etching and
also improves the sectional shape of the end surfaces of the cover
glass, which may be used in a mobile phone, a touch-panel display,
etc.
[0204] Also, both the principal surfaces of the glass substrate
formed by using the down-draw process have a uniform composition,
and therefore, there will be no difference in the ion-exchange rate
between the principal surfaces during the later-described
ion-exchange process. Thus, the glass substrate can be prevented
from warping after ion exchange due to a difference in composition.
In other words, it becomes possible to produce homogeneous cover
glasses, to improve productivity, and to reduce costs.
[0205] Further, forming the glass into a plate-like shape by using
the down-draw process can do away with the polishing step after
forming, thus further reducing costs and improving productivity.
Also, forming by using the down-draw process can produce glass
substrates with surfaces having no microcracks, which, in turn, can
improve the strength of the glass substrates.
[0206] Step (3):
[0207] Step (3) is the shape-processing step by subjecting the
plate-shaped glass substrate to at least chemical etching, to
process the glass substrate into a desired shape. Note that
"shape-processing" refers to the forming of the shape of the
principal surfaces of the formed glass substrate, and does not
include the processing of only the end surfaces of the glass
substrate.
[0208] The following explains how the cover glass is subjected to
etching for the shape-processing thereof prior to the ion-exchange
processing step.
[0209] First, both the principal surfaces of the plate-shaped glass
substrate prepared as above are coated with a resist material.
Then, the resist material is exposed via a photo mask having a
desired outer-shape pattern. The outer shape is not particularly
limited, and it may be an outer shape including, for example,
sections having negative curvatures, as illustrated in FIG. 1.
[0210] Next, the exposed resist material is developed to thus form
a resist pattern on the glass substrate in regions other than the
regions-to-be-etched, and then, the regions-to-be-etched on the
glass substrate are etched. If a wet etchant is used as the
etchant, the glass substrate will be etched isotropically. Thus,
each end surface of the glass substrate will be formed so as to
have: a central section that projects outward the most; and sloped
faces that curve gently from the central section toward the
respective principal surfaces. It is preferable that the boundaries
between the sloped faces and the respective principal surfaces and
the boundary between the two sloped faces are rounded.
[0211] The resist material to be used in the etching step is not
particularly limited, and it is possible to use a material that is
resistant to the etchant used for etching the glass while using the
resist pattern as a mask. For example, glass is, in general,
corroded by wet etching using an aqueous solution containing
hydrofluoric acid or by dry etching using a fluorine-based gas, so
resist materials having excellent resistance to hydrofluoric acid
are suitable. As for the etchant, it is possible to suitably use
hydrofluoric acid, or a mixed acid containing hydrofluoric acid and
at least one of sulfuric acid, nitric acid, hydrochloric acid, and
hydrofluorosilicic acid. The use of hydrofluoric acid or the
aforementioned mixed-acid aqueous solution as the etchant can
produce cover glasses with desired shapes.
[0212] The etchant used for effectively dissolving the glass
contains hydrogen fluoride. In the etching process, the fluorine
(F) in the hydrogen fluoride (HF) bonds with the dissolved metal
ions contained in the glass components, and fluorine compounds
precipitate in the etchant. The fluorine compounds include calcium
fluoride, magnesium fluoride, and aluminum fluoride. If these
precipitates are produced, they will adhere to the glass surface
during etching, thereby inhibiting the progress of etching. Calcium
fluoride, in particular, has a high production rate and extremely
low solubility once it precipitates. So, in order to inhibit the
production of calcium fluoride during the etching step, it is
effective to reduce the amount of CaO introduced in the glass
components, or substantially not introduce CaO at all.
[0213] Further, even complicated outer shapes can be created
easily, just by adjusting the mask pattern at the time of
performing shape-processing by employing etching. Further,
performing shape-processing through etching can further improve
productivity and also cut down processing costs. As for the
stripping solution for stripping the resist material off from the
glass substrate, an alkaline solution such as KOH or NaOH may be
used. The type of resist material, etchant, and stripping solution
can be selected as appropriate depending on the material of the
glass substrate.
[0214] Note that the etching method is not limited to the method of
simply immersing the glass substrate into an etching solution, but
instead it is possible to employ, for example, spray etching in
which the etching solution is sprayed. The method of immersing the
glass substrate into an etching solution is preferred over the
spray-etching method because of the simplicity of the device and
etching process. The immersion method, however, is prone to cause
precipitations, such as calcium fluoride, to adhere to the glass
surface during etching, and thus there is a significant need to
improve the etching rate. By employing the glass composition of the
present embodiment, it is possible to prevent precipitations, such
as calcium fluoride, from adhering to the glass surface during
etching. In other words, by employing the glass composition of the
present embodiment, the cover glass can be shape-processed quickly,
and with high accuracy, by a simpler method.
[0215] The glass substrate of the present embodiment has an etching
characteristic in which the etching rate is at least 3.7
.mu.m/minute in an etching environment having a temperature of
22.degree. C. and containing hydrogen fluoride with a concentration
of 10% by mass. The above etching characteristic can be achieved by
adjusting the composition of the glass substrate in a manner such
that, if the content of the aforementioned SiO.sub.2 is X % and the
content of the aforementioned Al.sub.2O.sub.3 is Y %, X-1/2Y is
57.5% or less.
[0216] The above method gives an example of performing
shape-processing on the glass substrate by using only chemical
etching, but the present embodiment is not limited thereto. For
example, the shape-processing on the glass substrate may be
performed by using chemical etching and machining in combination.
For example, after shape-processing by etching, there may be a step
of grinding or polishing some sections (the end surfaces, edges,
etc.) of the glass substrate. Alternatively, prior to
shape-processing by etching, there may be a step of cutting the
glass substrate in advance by machining or a step of performing
rough shape-processing.
[0217] The glass substrate of the present embodiment can also be
suitably used in cases where an etching step is provided with the
aim of removing cracks from the end surfaces of the glass
substrate. This is because chemical substances, such as fluorine
compounds, can precipitate in the etchant and deteriorate the
etching accuracy and/or etching rate not only in cases where the
glass substrate is subjected to shape-processing, but also in an
etching step having the aim of removing cracks from the end
surfaces of the glass substrate. In other words, the glass
substrate of the present embodiment can suitably be used in cases
that involve a processing step employing at least chemical etching.
For example, the glass substrate of the present embodiment can
suitably be used in such processing methods as described in (a) to
(e) below that involve a shape-processing step and a step of
removing cracks in the end surfaces of the glass substrate.
[0218] (a) A processing method involving: shape-processing by
forming a resist film and sand-blasting thereon; then
shape-processing the outer peripheral sections with diamond and
other grindstones; and then removing remaining cracks on the
processed surfaces by etching the glass substrate.
[0219] (b) A processing method involving: bonding a protective film
on the principal surface of the glass; machining (e.g., cutting,
shape-processing, boring) the glass; and then removing microcracks
by etching the outer peripheral sections.
[0220] (c) A processing method involving: shape-processing by
grinding and polishing; and then removing remaining cracks by
etching the entire glass substrate (including the principal
surfaces and end surfaces thereof).
[0221] (d) A processing method involving: bonding two sheets of
machined glass substrates together with a hot-melt wax or a
UV-curable resin; and then removing remaining cracks by chemically
etching the exposed outer peripheral sections.
[0222] (e) A processing method involving: laminating and bonding
together a plurality of sheets of glass substrates with a hot-melt
wax or a UV-curable resin; cutting the block consisting of the
glass substrates and processing the outer periphery thereof; and
then removing remaining cracks by chemically etching the processed
surfaces (outer-peripheral end surfaces) that are exposed from the
block.
[0223] Step (4):
[0224] Step (4) is the step of subjecting the glass substrate
shape-processed in Step (3) to an ion-exchange process.
[0225] The cover glass according to the present embodiment is
produced by performing an ion-exchange process on the glass
substrate that has been shape-processed in Step (3) as described
above. More specifically, for example, after being rinsed, the
glass substrate is immersed for around 1 to 25 hours in a treatment
bath containing 100% of KNO.sub.3 and kept at around 350.degree. C.
to 550.degree. C., to thereby exchange the Na.sup.+ ions on the
superficial layer of the glass with K.sup.+ ions present in the
treatment bath. In this way, the glass substrate can be chemically
strengthened. Note that the temperature, the length of time, the
ion-exchange salt, etc., for the ion-exchange process can be
changed as appropriate. For example, the ion-exchange salt may be a
mixture containing two or more types of compounds, such as a mixed
salt of KNO.sub.3 and NaNO.sub.3.
[0226] Characteristics of Glass Substrate:
[0227] Next, the characteristics of the glass substrate to be used
for the cover glass 10, 100 of the first and second embodiments
will be described.
[0228] Etching Rate:
[0229] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has an etching rate of 3.7
.mu.m/minute or greater, preferably 4.3 .mu.m/minute or greater,
more preferably 4.5 .mu.m/minute or greater, and particularly
preferably 5 .mu.m/minute or greater, as measured according to the
method described below. By setting the etching rate within the
aforementioned range, the rate for shape-processing the glass and
the rate for processing the end surfaces by etching can be
increased, and the ability to produce the cover glass 10, 100 can
be improved. Although the ability to produce the cover glass 10,
100 is improved with the increase in etching rate, increasing the
content by percentage of Al.sub.2O.sub.3 to increase the etching
rate will also increase the devitrification temperature. So, in
order to achieve both devitrification resistance and an improvement
in etching rate, it is preferable that the glass constituting the
glass substrate of the present embodiment has an etching rate of 10
.mu.m/minute or less, more preferably 8 .mu.m/minute or less, and
even more preferably 7 .mu.m/minute or less. Specifically, the
etching rate is preferably 3.7 .mu.m/minute to 10 .mu.m/minute,
more preferably 4.3 .mu.m/minute to 10 .mu.m/minute, even more
preferably 4.5 .mu.m/minute to 8 .mu.m/minute, and particularly
preferably 5 .mu.m/minute to 7 .mu.m/minute.
[0230] The etching rate is found by measuring the etching amount
(the change in thickness) for when etching is conducted for 20
minutes in an etching environment having a temperature of
22.degree. C. and containing hydrogen fluoride with a concentration
of 10% by mass.
[0231] Density:
[0232] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has a density of preferably 2.8
g/cm.sup.3 or less, more preferably 2.7 g/cm.sup.3 or less, even
more preferably 2.6 g/cm.sup.3 or less, further more preferably
2.55 g/cm.sup.3 or less, and particularly preferably 2.5 g/cm.sup.3
or less. The smaller the density of the glass, the more lightweight
the glass can be made, and lightweight glass can suitably be used
as cover glasses, touch-panel display substrates, and the like.
[0233] Linear Thermal Expansion Coefficient:
[0234] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has a linear thermal expansion
coefficient of preferably from 50.times.10.sup.-7 to
120.times.10.sup.-7/.degree. C., more preferably from
60.times.10.sup.-7 to 120.times.10.sup.-7/.degree. C., even more
preferably from 70.times.10.sup.-7 to 110.times.10.sup.-7/.degree.
C., and particularly preferably from 80.times.10.sup.-7 to
110.times.10'.sup.7/.degree. C., within the temperature range of
100.degree. C. to 300.degree. C. By setting the linear thermal
expansion coefficient of the glass within the range of
50.times.10.sup.-7 to 120.times.10.sup.-7/.degree. C. in the
temperature range of 100.degree. C. to 300.degree. C., the
coefficient of thermal expansion will likely match the coefficients
of thermal expansion of peripheral materials, such as metals and
organic adhesives, and thus the peripheral materials can be
prevented from peeling.
[0235] Devitrification Temperature (T1):
[0236] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has a devitrification
temperature of preferably 1200.degree. C. or less, more preferably
1100.degree. C. or less, even more preferably 1000.degree. C. or
less, and particularly preferably 960.degree. C. or less. The lower
the devitrification temperature is, the more the devitrification of
glass during production can be prevented. In other words, lower
devitrification temperatures can improve devitrification
resistance, and the glass becomes more suitable for down-draw
processing and can be formed at lower temperatures, which thus
allows the reduction of glass production costs. Also, lower
devitrification temperatures can improve the formability of glass.
Note that "devitrification resistance" as used herein is a
characteristic that uses the devitrification temperature as its
index: the lower the devitrification temperature, the higher the
devitrification resistance.
[0237] Glass Transition Temperature (Tg):
[0238] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has a glass transition
temperature Tg of 500.degree. C. or higher, preferably 510.degree.
C. or higher, more preferably 530.degree. C. or higher, even more
preferably 560.degree. C. or higher, further more preferably
580.degree. C. or higher, and particularly preferably 590.degree.
C. or higher. By setting Tg to 500.degree. C. or higher, it is
possible to prevent the heat resistance from deteriorating and the
strengthening layer, which is formed on the principal surfaces and
the end surfaces of the glass substrate by ion-exchange processing,
from causing stress relaxation. Further, setting Tg to 500.degree.
C. or higher can inhibit the deformation of the glass substrate and
the chemically-strengthened glass substrate during thermal
treatments, but if Tg reaches 700.degree. C. or higher, the
ion-exchange performance will drop. Therefore, Tg is preferably
700.degree. C. or lower, more preferably 650.degree. C. or lower,
and even more preferably 620.degree. C. or lower. Specifically, Tg
is preferably 500.degree. C. to 700.degree. C., preferably
510.degree. C. to 700.degree. C., more preferably 530.degree. C. to
650.degree. C., even more preferably 560.degree. C. to 650.degree.
C., further more preferably 580.degree. C. to 650.degree. C., and
particularly preferably 590.degree. C. to 620.degree. C.
[0239] Further, the smaller the difference between the
devitrification temperature and the glass transition point is, the
more the devitrification of glass during production can be
prevented (the more the devitrification resistance can be
improved). That is, the smaller the difference between the
devitrification temperature and the glass transition point is, the
more the devitrification resistance can be improved, and the glass
becomes more suitable for down-draw processing and the glass
substrate can be formed at lower temperatures, which thus allows
the reduction of glass production costs. Therefore, in the glass
substrate to be used for the cover glass 10, 100 of the present
embodiment, the difference "T1-Tg" is preferably 500.degree. C. or
less, more preferably 450.degree. C. or less, even more preferably
400.degree. C. or less, further more preferably 380.degree. C. or
less, and particularly preferably 370.degree. C. or less. By
setting the difference "T1-Tg" to 500.degree. C. or less, the
formability of the glass substrate can be improved.
[0240] High-Temperature Viscosity:
[0241] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has a high-temperature
viscosity (temperature at 200 dPas) of preferably 1700.degree. C.
or lower, more preferably 1600.degree. C. or lower, even more
preferably 1550.degree. C. or lower, and particularly preferably
1520.degree. C. or lower. By setting the high-temperature viscosity
of the glass substrate to 1700.degree. C. or lower, it is possible
to prevent an increase in melting temperature as well as an
increase in thermal load on the glass-production facility, such as
the melting furnace. The bubble quality (the content of bubbles) of
the glass can also be improved. Thus, glass can be produced
inexpensively.
[0242] Strain Point:
[0243] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has a strain point of
preferably 460.degree. C. or higher, more preferably 470.degree. C.
or higher, even more preferably 490.degree. C. or higher, further
more preferably 520.degree. C. or higher, and particularly
preferably 560.degree. C. or higher. By setting the strain point to
460.degree. C. or higher, it is possible to prevent the heat
resistance from deteriorating and the strengthening layer, which is
formed on the principal surfaces and the end surfaces of the glass
substrate by ion-exchange processing, from causing stress
relaxation. Note that, although setting the strain point to
460.degree. C. or higher can inhibit the deformation of the glass
substrate and the chemically-strengthened glass substrate during
thermal treatments, if the strain point reaches 660.degree. C. or
higher, the ion-exchange performance will drop. Therefore, the
strain point is preferably 660.degree. C. or lower, more preferably
610.degree. C. or lower, and even more preferably 580.degree. C. or
lower. Specifically, the strain point is preferably 460.degree. C.
to 660.degree. C., more preferably 470.degree. C. to 660.degree.
C., even more preferably 490.degree. C. to 610.degree. C., further
more preferably 520.degree. C. to 610.degree. C., and particularly
preferably 560.degree. C. to 580.degree. C.
[0244] Thickness:
[0245] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has a thickness of preferably
3.0 mm or less, more preferably 2.0 mm or less, even more
preferably 1.3 mm or less, further more preferably 0.8 mm or less,
and particularly preferably 0.6 mm or less. The thinner the glass
substrate and the chemically-strengthened glass plate are, the more
the cover glasses 10, 100 can be made lightweight, which makes them
suitable for cover glasses, touch-panel display substrates, and the
like. Note that, in consideration of the flexure, rigidity,
strength, etc., of the glass substrate, the thickness is preferably
0.2 mm or greater, more preferably 0.3 mm or greater, and even more
preferably 0.4 mm or greater. Meanwhile, chemically-strengthened
glass that has been subjected to ion-exchange processing is less
prone to break even when the thickness is small. For example, by
forming the glass substrate by a down-draw process, it is possible
to produce a thin glass substrate with high mechanical strength and
excellent surface accuracy, even without polishing etc.
Specifically, the thickness of the glass substrate and the
thickness of the chemically-strengthened glass plate are preferably
0.2 mm to 2.0 mm, more preferably 0.2 mm to 1.3 mm, even more
preferably 0.4 mm to 1.3 mm, further more preferably 0.4 mm to 0.8
mm, and particularly preferably 0.4 mm to 0.6 mm.
[0246] Compressive-Stress Value:
[0247] The glass substrate to be used for the cover glass 10, 100
of the first and second embodiments has a compressive-stress layer
having a compressive-stress value of preferably 140 MPa or greater,
more preferably 300 MPa or greater, and more preferably 400 MPa or
greater, even more preferably 500 MPa or greater, and further more
preferably 600 MPa or greater. By setting the compressive-stress
value to 300 MPa or greater, the cover glasses 10, 100 can be
provided with sufficient strength to protect displays, for example.
Note that, although an increase in the compressive-stress value
will improve the glass strength, this will also increase the impact
occurring when the strengthened glass is damaged. Therefore, in
order to prevent any accidents caused by the impact, it is
preferable that the chemically-strengthened cover glass 10, 100 of
the present embodiment has a compressive-stress value of 950 MPa or
less, more preferably 800 MPa or less, more preferably 750 MPa or
less, and even more preferably 700 MPa or less. Specifically, the
compressive-stress value of the glass constituting the
chemically-strengthened glass substrate and the cover glass of the
present embodiment is preferably 300 MPa to 950 MPa, more
preferably 400 MPa to 900 MPa, even more preferably 400 MPa to 800
MPa, and even further preferably 500 MPa to 800 MPa.
[0248] Compressive Layer Depth:
[0249] The compressive layer depth of the glass constituting the
chemically-strengthened glass substrate and the cover glass of the
present embodiment is 15 .mu.m to 90 .mu.m, preferably 20 .mu.m to
85 .mu.m, more preferably 25 .mu.m to 80 .mu.m, even more
preferably 30 .mu.m to 70 .mu.m, and even further preferably 30
.mu.m to 50 .mu.m.
[0250] Note that there has been a tendency to reduce the plate
thickness of the cover glass in recent years in order to reduce the
weight, and although this is accompanied by a reduction in the
compressive layer depth, there is a demand for having a compressive
stress value that is a predetermined value or greater.
Specifically, it is preferable that the plate thickness of the
cover glass is 0.2 mm to 1.3 mm, the compressive layer depth is 20
.mu.m to 85 .mu.m, and the compressive stress value is 300 MPa to
950 MPa; it is more preferable that the plate thickness of the
cover glass is 0.4 mm to 1.3 mm, the compressive layer depth is 25
.mu.m to 80 .mu.m, and the compressive stress value is 400 MPa to
900 MPa; it is even more preferable that the plate thickness of the
cover glass is 0.4 mm to 0.8 mm, the compressive layer depth is 30
.mu.m to 70 .mu.m, and the compressive stress value is 400 MPa to
800 MPa; and it is even more preferable that the plate thickness of
the cover glass is 0.4 mm to 0.6 mm, the compressive layer depth is
30 .mu.m to 50 .mu.m, and the compressive stress value is 500 MPa
to 800 MPa.
EXAMPLES
[0251] Now, the present invention will be described in further
detail below according to Examples thereof. The present invention,
however, is not to be limited to the following Examples.
Preparation of Glass According to First Embodiment
[0252] First, glass materials (batches) were prepared by using
general glass materials, i.e., silica, alumina, sodium sulfate,
lithium carbonate, sodium carbonate, potassium carbonate, basic
magnesium carbonate, calcium carbonate, tin dioxide, and zirconium
oxide, so as to provide the glass compositions as shown in Tables 1
and 2 (Samples 1 to 28) and Tables 3 and 4 (Samples 29 to 33). Each
prepared batch was heated in an electric furnace for 4 hours at
1550.degree. C. with a platinum crucible and was made into molten
glass, and then, outside the furnace, the molten glass was spread
out onto an iron plate to cool, to thereby prepare a block of
glass. The glass block was kept in an electric furnace for 30
minutes at 600.degree. C., then the furnace was turned off, and the
glass block was gradually cooled to room temperature. The prepared
glass block was employed as the glass sample for evaluating the
physical properties of the glass.
[0253] For each glass sample prepared as above, the devitrification
temperature, the strain point, the coefficient of thermal
expansion, the glass transition temperature, the high-temperature
viscosity, and the etching rate were evaluated.
TABLE-US-00001 Glass composition (mass %) Sample SiO.sub.2
Al.sub.2O.sub.3 Li.sub.2O Na.sub.2O K.sub.2O MgO CaO ZrO.sub.2
SnO.sub.2 SO.sub.3 1 62.6 17.4 15.4 1.9 2.6 0.1 2 62.8 14.7 15.8
2.2 3.2 1.2 0.1 3 60.2 13.1 14.5 3.3 2.2 3.2 3.4 0.1 4 62.4 13.1
14.5 3.3 2.2 3.2 1.2 0.1 5 60.8 14.7 14.5 3.3 2.2 3.2 1.2 0.1 6
61.3 12.8 12.9 3.3 2.2 3.2 4.2 0.1 7 56.0 17.3 14.5 3.3 2.2 3.2 3.4
0.1 8 58.0 15.3 14.5 3.3 2.2 3.2 3.4 0.1 9 62.0 11.3 14.5 3.3 2.2
3.2 3.4 0.1 10 60.2 13.1 14.5 3.3 3.9 1.5 3.4 0.1 11 54.0 19.3 14.5
3.3 2.2 3.2 3.4 0.1 12 62.1 12.8 15.5 3.2 6.3 0.1 13 60.9 12.1 17.7
3.1 4.1 2.0 0.1 14 59.7 11.8 17.4 3.1 4.0 3.8 0.1 15 61.0 10.9 17.7
6.3 2.1 2.0 0.1 16 59.8 10.7 17.4 6.2 2.0 3.8 0.1 17 59.9 10.5 12.4
9.2 4.0 3.8 0.1 18 62.0 13.0 20.7 4.2 0.1 19 62.0 13.3 19.4 5.3 0.1
20 62.1 12.9 18.1 1.6 5.3 0.1 21 60.9 13.0 12.8 7.9 5.2 0.1 22 60.9
12.0 15.2 4.7 5.1 2.0 0.1 23 61.5 14.0 15.3 3.8 5.2 0.1 24 60.8
13.7 15.4 4.8 5.2 0.1 25 60.1 14.5 15.3 4.8 5.2 0.1 26 62.0 14.1
14.9 3.8 5.2 0.1 27 62.2 14.1 15.3 3.1 5.2 0.1 28 61.9 14.0 15.3
3.8 4.9
TABLE-US-00002 TABLE 2 Glass composition (%) SiO.sub.2-1/
B.sub.2O.sub.3/ (ZrO.sub.2 + TiO.sub.2)/ Li.sub.2O/ Sample RO
2Al.sub.2O.sub.3 R.sup.1.sub.2O SiO.sub.2 (RO +Li.sub.2O) 1 4.5
53.9 0 0 0 2 5.4 55.5 0 0.019 0 3 5.4 53.7 0 0.056 0 4 5.4 55.9 0
0.019 0 5 5.4 53.5 0 0.020 0 6 5.4 53.7 0 0.080 0 7 5.4 47.4 0
0.061 0 8 5.4 50.4 0 0.059 0 9 5.4 56.4 0 0.055 0 10 5.4 53.7 0
0.056 0 11 5.4 44.4 0 0.063 0 12 6.3 55.7 0 0 0 13 4.1 54.9 0 0.032
0 14 4.0 53.8 0 0.064 0 15 2.1 55.5 0 0.032 0 16 2.0 54.5 0 0.064 0
17 4.0 54.7 0 0.064 0 18 4.2 55.5 0 0 0 19 5.3 55.4 0 0 0 20 5.3
55.6 0 0 0 21 5.2 54.4 0 0 0 22 5.1 55.0 0 0.032 0 23 5.2 54.5 0 0
0 24 5.2 53.9 0 0 0 25 5.2 52.9 0 0 0 26 5.2 55.0 0 0 0 27 5.2 55.1
0 0 0 28 4.9 54.9 0 0 0
TABLE-US-00003 TABLE 3 Glass composition (mass %) Sample SiO.sub.2
Al.sub.2O.sub.3 Li.sub.2O Na.sub.2O K.sub.2O MgO CaO ZrO.sub.2
SnO.sub.2 SO.sub.3 29 65 8.3 14.5 3.3 2.2 3.2 3.4 0.1 30 67 6.3
14.5 3.3 2.2 3.2 3.4 0.1 31 63.2 10.1 14.5 3.3 2.2 3.2 3.4 0.1 32
63.7 9.7 14.5 3.3 2.2 3.2 3.4 0.1 33 63.5 8.0 7.8 10.2 11.2
TABLE-US-00004 TABLE 4 Glass composition (%) SiO.sub.2-1/
B.sub.2O.sub.3/ (ZrO.sub.2 + TiO.sub.2)/ Li.sub.2O/ Sample RO
2Al.sub.2O.sub.3 R.sup.1.sub.2O SiO.sub.2 (RO + Li.sub.2O) 29 5.4
60.9 0 0.052 0 30 5.4 63.9 0 0.051 0 31 5.4 58.1 0 0.054 0 32 5.4
58.9 0 0.053 0 33 0 59.5 0 0.176 1
[0254] Evaluation of Devitrification Temperature:
[0255] The glass sample was pulverized, and glass particles that
passed through a 2380-.mu.m sieve but remained on a 1000-.mu.m
sieve were immersed into ethanol, subjected to ultrasonic cleaning,
and then dried in a constant-temperature oven. Then, 25 g of the
glass particles were placed on a 12-by-200-by-10-mm platinum board
so that they assume a substantially constant thickness, and were
placed in an electric furnace having a temperature gradient from
800.degree. C. to 1200.degree. C. and kept therein for 24 hours.
Then, the particles were taken out from the furnace, and
devitrification that occurred inside the glass was observed with an
optical microscope at a magnification of 40 times. The maximum
temperature at which devitrification was observed was found as the
devitrification temperature.
[0256] Evaluation of Strain Point:
[0257] The glass sample was cut out into a 3-by-3-by-55-mm right
square prism and was ground, and the strain point (Ps) thereof was
measured with a beam bending viscometer (product of Tokyo Kogyo
Co., Ltd.). The strain point was found by calculation according to
the beam bending method (ASTM C-598).
[0258] Evaluation of Coefficient of Thermal Expansion and Glass
Transition Temperature (Tg):
[0259] The glass sample was processed into a circular cylinder 5 mm
in diameter and 20 mm long, and the coefficient of thermal
expansion and the glass transition temperature (Tg) were measured
with a differential dilatometer (Thermo Plus2 TMA8310). The average
thermal expansion coefficient in the temperature range of
100.degree. C. to 300.degree. C. was calculated from the results of
measuring the thermal expansion coefficient.
[0260] Evaluation of Density:
[0261] The density was measured according to the Archimedean
method.
[0262] Evaluation of High-Temperature Viscosity:
[0263] The glass sample was molten for 4 hours at 1600.degree. C.
and bubbles were removed therefrom, and the high-temperature
viscosity was measured with a pull-down automatic viscometer. More
specifically, the viscosity of the sample was found by suspending a
platinum ball down into the molten glass sample, pulling down the
sample and the container containing the same, and measuring the
viscous drag applied on the ball as the sample and the container
were pulled down, as the load. Tables 5 and 6 show the temperature
for when the glass viscosity was 200 dPas.
[0264] Evaluation of Etching Rate:
[0265] The glass sample was cut into a size 20 to 50 mm long, 20 to
40 mm wide, and 0.7 mm thick and was ground and polished, to
prepare a sample sheet. After being rinsed, the sample sheet was
immersed for 20 minutes in 400 mL of HF (concentration: 10% by
mass; temperature: 22.degree. C.) held in a container. After
rinsing the sample sheet with water, the thickness and mass of the
sample were measured and compared with those measured prior to the
test, to calculate the etching rate of the glass sample.
[0266] Chemical Strengthening:
[0267] The sample sheet, after being rinsed, was immersed for about
2.5 hours in a treatment bath containing 100% of KNO.sub.3 kept at
400.degree. C., to exchange the Na.sup.+ ions on the superficial
layer of the glass with K.sup.+ ions present in the treatment bath
and thereby chemically strengthen the glass sample. The
chemically-strengthened glass substrate was immersed in a rinsing
tank for rinsing and then dried, to thereby obtain a piece of
strengthened glass. As for Samples 4, 7, 9, and 10, each glass
sample was chemically strengthened by being immersed for about 5
hours in a treatment bath containing 100% of KNO.sub.3 kept at
500.degree. C.
[0268] Evaluation of Compressive-Stress Value:
[0269] The strengthened glass obtained as above was observed to
find the number of interference fringes and the interval
therebetween with a surface stress meter (Luceo FSM-6000LE), and
the compressive-stress value of the compressive-stress layer formed
in the vicinity of the surface of the glass and the thickness of
the compressive-stress layer were calculated. In the calculation,
the refractive index (nd) of each sample was regarded as 1.50, and
the stress optical coefficient was regarded as 28 ((nm/cm)/MPa). As
for samples 4, 7, 9, and 10, the refractive index (nd) of each
sample was regarded as 1.52.
[0270] Tables 5 and 6 show the characteristics of Samples 1 to 28
of the first embodiment shown in Tables 1 and 2 and those of
Samples 29 to 33 of the first embodiment shown in Tables 3 and
4.
TABLE-US-00005 Glass characteristics High- Compres- Surface temp
Avg. thermal sive compres- viscos- Strain expan- stress sive layer
Tl ity point Etching rate Tg sion coef. Tl-Tg Density value depth
Sample (.degree. C.) (.degree. C.) (.degree. C.) (.mu.m/min)
(.degree. C.) [.times.10.sup.-7/.degree. C.] (.degree. C.)
(.mu./cm.sup.3) (MPa) (.mu.m) 1 1000 563 5.1 610 89 390 2.46 2 1021
558 4.5 604 89 417 2.49 3 948 1508 563 5.2 593 95 355 2.52 659 40 4
927 532 4.3 578 97 349 2.49 5 988 545 5.3 592 97 396 2.49 6 1003
1542 582 5.2 616 90 387 2.54 768 35 7 1018 583 6.1 626 91 392 2.53
8 1003 570 6.1 614 91 389 2.53 864 41 9 968 537 4.1 586 91 382 2.51
707 36 10 1112 579 5.2 620 91 492 2.50 11 1200 587 4.8 636 92 564
12 1090 3.7 585 99.7 505 2.455 693 47 13 <840 4.0 559 106
<300 2.480 543 57 14 <840 4.6 579 97.9 <300 2.510 684 53
15 <840 3.7 511 114 <329 2.477 147 88 16 <840 4.2 527 113
<313 2.506 236 83 17 <840 4.1 572 106 <300 2.502 587 66.8
18 <840 3.7 551 102.7 <300 2.460 354 53.4 19 <840 3.8 574
99.5 <300 2.457 524 47.7 20 <840 3.7 566 102.7 <300 2.453
576 49.4 21 <840 4.2 570 105 <300 2.449 584 65 22 <840 3.9
581 102.6 <300 2.487 23 907 4.2 574 104 333 2.459 24 <840 4.5
572 103.6 <300 2.463 25 953 5.2 577 105 376 2.465 26 <840 3.9
<300 27 <840 3.9 <300 28 <840 4.0 <300
TABLE-US-00006 TABLE 6 Glass characteristics High-temp. Strain
Etching Avg. thermal Tl viscosity point rate Tg expansion Tl-Tg
Density Sample (.degree. C.) (.degree. C.) (.degree. C.)
(.mu.m/min) (.degree. C.) [.times.10.sup.-7/.degree. C. ] (.degree.
C.) (g/cm.sup.3) 29 858 513 2.3 563 90 295 2.51 30 830 504 1.7 554
90 276 2.50 31 855 530 3.4 576 90 279 2.51 32 836 527 2.8 571 90
265 2.51 33 524 3.5 570
[0271] FIG. 3 shows a distribution chart showing the etching rates
given on Tables 5 and 6 on the Y-axis and the aforementioned X-1/2Y
(wherein X is the content by percentage of SiO.sub.2 and Y is the
content by percentage of Al.sub.2O.sub.3) on the X-axis for Samples
1 to 28 in Tables 1 and 2 and Samples 29 to 33 in Tables 3 and 4.
As can be seen, the etching rate increases as the value of X-1/2Y
decreases. Meanwhile, no significant change in the etching rate can
be seen in the area where X-1/2Y is 52% or less. So, in order to
achieve an etching rate of 3.7 .mu.m/minute or greater, it is
preferable that X-1/2Y is 57.5% or less. However, even if the value
of Y is increased and X-1/2Y is adjusted to 52% or less, there will
be no further increase in the etching rate, and in addition, the
devitrification temperature will increase and thus the
devitrification resistance will deteriorate. Therefore, the lower
limit of the value of X-1/2Y is preferably 45%.
[0272] Example of Continuous Production of Glass Substrate:
[0273] Glass materials were prepared so as to provide a glass
substrate with the composition shown in Sample 4 (see Tables 1 and
2). The glass materials were molten at 1520.degree. C. by using a
continuous melting device having, for example, a fire-brick-made
melting tank, a platinum-made stirring tank and so on, were
subjected to refining at 1550.degree. C., stirred at 1350.degree.
C., and then formed into a thin plate 0.7 mm thick by down-draw
processing, to produce a glass substrate for chemical
strengthening. Etching and chemical strengthening were performed as
follows.
[0274] The glass substrate prepared as above was employed as the
sample glass substrate, and a 20-.mu.m-thick pattern made of a
phenolic heat-curable resin and having the shape of a cover glass
was formed on each of the principal surfaces of the substrate by
mesh-screen printing, and the phenolic heat-curable resin patterns
were baked for 15 minutes at 200.degree. C. With the phenolic
heat-curable resin patterns being employed as masks, the glass
sample was etched in the regions-to-be-etched from both principal
surfaces by using a mixed-acid aqueous solution (40.degree. C.)
containing hydrofluoric acid (15% by mass) and sulfuric acid (24%
by mass) as the etchant, to cut the glass sample into a
predetermined shape. Then, the phenolic heat-curable resin
remaining on the glass was dissolved by using an NaOH solution and
was removed off from the glass, and then the glass was rinsed.
[0275] Then, the rinsed sample glass substrate was immersed for
about 5 hours in a treatment bath containing 100% of KNO.sub.3 kept
at 500.degree. C., to exchange the Na ions on the superficial layer
of the glass with K ions present in the treatment bath and thereby
chemically strengthen the glass sample. The chemically-strengthened
sample glass substrate was immersed in a rinsing tank for rinsing
and then dried. The result was that it was possible to produce a
glass substrate having excellent quality and an improved etching
rate of 3.7 .mu.m/minute or higher.
Preparation of Glass according to Second Embodiment
[0276] First, glass materials (batches) were prepared by using
general glass materials, i.e., silica, alumina, sodium sulfate,
lithium carbonate, sodium carbonate, potassium carbonate, basic
magnesium carbonate, calcium carbonate, tin dioxide, and zirconium
oxide, so as to provide the glass compositions as shown in Tables 7
and 8 (Samples 34 to 62) and Tables 9 and 10 (Samples 63 to 76).
Each prepared batch was heated in an electric furnace for 4 hours
at 1550.degree. C. with a platinum crucible and was made into
molten glass, and then, outside the furnace, the molten glass was
spread out onto an iron plate to cool, to thereby prepare a block
of glass. The glass block was kept in an electric furnace for 30
minutes at 600.degree. C., then the furnace was turned off, and the
glass block was gradually cooled to room temperature. The cooled
glass block was subjected to machining, such as cutting and
polishing, and was made into a 50-by-40-mm glass sample
approximately 0.7 mm thick. For each glass substrate prepared as
above, the devitrification temperature (Tl), the etching rate, the
glass transition temperature (Tg), the average linear thermal
expansion coefficient, the density, the compressive-stress value,
the surface compressive layer depth, and the processing accuracy
were evaluated.
TABLE-US-00007 TABLE 7 Glass composition (mass %) Sample SiO.sub.2
Al.sub.2O.sub.3 Li.sub.2O Na.sub.2O K.sub.2O MgO CaO ZrO.sub.2
SnO.sub.2 SO.sub.3 34 62.6 17.4 15.4 1.9 2.6 0.1 35 60.2 13.1 14.5
3.3 3.9 1.5 3.4 0.1 36 62.1 12.8 15.5 3.2 6.3 0.1 37 60.9 12.1 17.7
3.1 4.1 2.0 0.1 38 59.7 11.8 17.4 3.1 4.0 3.8 0.1 39 61.0 10.9 17.7
6.3 2.1 2.0 0.1 40 59.8 10.7 17.4 6.2 2.0 3.8 0.1 41 59.9 10.5 12.4
9.2 4.0 3.8 0.1 42 62.0 13.0 20.7 4.2 0.1 43 62.0 13.3 19.4 5.3 0.1
44 62.1 12.9 18.1 1.6 5.3 0.1 45 60.9 13.0 12.8 7.9 5.2 0.1 46 60.9
12.0 15.2 4.7 5.1 2.0 0.1 47 61.5 14.0 15.3 3.8 5.2 0.1 48 60.8
13.7 15.4 4.8 5.2 0.1 49 60.1 14.5 15.3 4.8 5.2 0.1 50 62.0 14.1
14.9 3.8 5.2 0.1 51 62.2 14.1 15.3 3.1 5.2 0.1 52 61.9 14.0 15.3
3.8 4.9 53 62.1 12.7 19.4 1.6 4.2 0.1 54 62.2 12.2 15.5 4.8 5.3 0.1
55 62.2 12.4 14.7 4.8 5.9 0.1 56 62.1 12.7 16.0 3.2 5.9 0.1 57 61.5
14.3 15.3 3.2 5.6 0.1 58 61.5 14.0 0.001 15.4 3.8 5.2 0.1 59 61.5
14.0 0.01 15.4 3.8 5.2 0.1 60 61.5 14.0 0.04 15.4 3.8 5.2 0.1 61
61.5 14.0 0.10 15.4 3.8 5.2 0.1 62 61.3 14.0 0.4 15.4 3.8 5.2
0.1
TABLE-US-00008 TABLE 8 Glass composition SiO.sub.2-1/
B.sub.2O.sub.3/ (ZrO.sub.2 + Li.sub.2O/ Sample RO 2Al.sub.2O.sub.3
R.sup.1.sub.2O TiO.sub.2)/SiO.sub.2 (RO + Li.sub.2O) 34 4.5 53.9 0
0.000 0 35 5.4 53.7 0 0.056 0 36 6.3 55.7 0 0 0 37 4.1 54.9 0 0.032
0 38 4.0 53.8 0 0.064 0 39 2.1 55.5 0 0.032 0 40 2.0 54.5 0 0.064 0
41 4.0 54.7 0 0.064 0 42 4.2 55.5 0 0 0 43 5.3 55.4 0 0 0 44 5.3
55.6 0 0 0 45 5.2 54.4 0 0 0 46 5.1 55.0 0 0.032 0 47 5.2 54.5 0 0
0 48 5.2 53.9 0 0 0 49 5.2 52.9 0 0 0 50 5.2 55.0 0 0 0 51 5.2 55.1
0 0 0 52 4.9 54.9 0 0 0 53 4.2 55.7 0 0 0 54 5.3 56.1 0 0 0 55 5.9
56.0 0 0 0 56 5.9 55.8 0 0 0 57 5.6 54.3 0 0 0 58 5.2 54.5 0 0
0.0002 59 5.2 54.5 0 0 0.0019 60 5.2 54.5 0 0 0.0076 61 5.2 54.4 0
0 0.0189 62 5.2 54.3 0 0 0.0714
TABLE-US-00009 TABLE 9 Glass composition (mass %) Sample SiO.sub.2
Al.sub.2O.sub.3 Li.sub.2O Na.sub.2O K.sub.2O MgO CaO ZrO.sub.2
SnO.sub.2 SO.sub.3 63 65.0 8.3 14.5 3.3 2.2 3.2 3.4 0.1 64 67.0 6.3
14.5 3.3 2.2 3.2 3.4 0.1 65 63.2 10.1 14.5 3.3 2.2 3.2 3.4 0.1 66
63.7 9.7 14.5 3.3 2.2 3.2 3.4 0.1 67 63.5 8.2 8.0 10.4 11.9 68 62.8
14.7 15.8 2.2 3.2 1.2 0.1 69 60.2 13.1 14.5 3.3 2.2 3.2 3.4 0.1 70
62.4 13.1 14.5 3.3 2.2 3.2 1.2 0.1 71 60.8 14.7 14.5 3.3 2.2 3.2
1.2 0.1 72 61.3 12.8 12.9 3.3 2.2 3.2 4.2 0.1 73 56.0 17.3 14.5 3.3
2.2 3.2 3.4 0.1 74 58.0 15.3 14.5 3.3 2.2 3.2 3.4 0.1 75 62.0 11.3
14.5 3.3 2.2 3.2 3.4 0.1 76 54.0 19.3 14.5 3.3 2.2 3.2 3.4 0.1
TABLE-US-00010 Glass composition Sample RO
SiO.sub.2-1/2Al.sub.2O.sub.3 B.sub.2O.sub.3/R.sup.1.sub.2O
(ZrO.sub.2 + TiO.sub.2)/SiO.sub.2 Li.sub.2O/(RO + Li.sub.2O) 63 5.4
60.9 0 0.052 0 64 5.4 63.9 0 0.051 0 65 5.4 58.2 0 0.054 0 66 5.4
58.9 0 0.053 0 67 0 59.4 0 0.176 1 68 5.4 55.5 0 0.019 0 69 5.4
53.7 0 0.056 0 70 5.4 55.9 0 0.019 0 71 5.4 53.5 0 0.020 0 72 5.4
54.9 0 0.069 0 73 5.4 47.4 0 0.061 0 74 5.4 50.4 0 0.059 0 75 5.4
56.4 0 0.055 0 76 5.4 44.4 0 0.063 0
[0277] Evaluation of Devitrification Temperature:
[0278] The glass sample was pulverized, and glass particles that
passed through a 2380-.mu.m sieve but remained on a 1000-.mu.m
sieve were immersed into ethanol, subjected to ultrasonic cleaning,
and then dried in a constant-temperature oven. Then, 25 g of the
glass particles were placed on a 12-by-200-by-10-mm platinum board
so that they assume a substantially constant thickness, and were
placed in an electric furnace having a temperature gradient from
800.degree. C. to 1200.degree. C. and kept therein for 24 hours.
Then, the particles were taken out from the furnace, and
devitrification that occurred inside the glass was observed with an
optical microscope at a magnification of 40 times. The maximum
temperature at which devitrification was observed was found as the
devitrification temperature.
[0279] Evaluation of Etching Rate:
[0280] The glass sample, after being rinsed, was immersed for 20
minutes in 400 mL of HF (concentration: 10% by mass; temperature:
22.degree. C.) held in a container. After rinsing the sample with
water, the thickness and mass of the sample were measured and
compared with those measured prior to the test, to calculate the
etching rate of the glass substrate.
[0281] Evaluation of Accuracy in Shape-Processing including
Etching:
[0282] As illustrated in FIG. 4A, two sheets of glass 20, each of
which being a machined glass substrate prior to chemical
strengthening, were placed on top of one another, and three sheets
of dummy glass 22 were placed on the respective sides of the two
glasses 20, and the stack of glass sheets was etched. The etched
end surfaces of the glass after etching were observed with an
optical microscope.
[0283] The end surfaces were observed in two ways. Observation A
was done by visually observing the end surface on the long side of
the glass, as illustrated in FIG. 4A, at 200.times. magnification.
Observation B was done by: first splitting the glass substrate in
half at the center of the short side thereof along a line
perpendicular to the short side; and then visually observing the
end surface on the short side of the glass, as illustrated in FIG.
4B, at 200.times. magnification. The etching result was evaluated
as "Good" if both Observations A and B showed that etching was
achieved substantially evenly, was evaluated as "Fair" if only one
of Observations A and B showed that etching was achieved
substantially evenly and the other was uneven, and was evaluated as
"Poor" if both Observations A and B showed that etching was uneven.
For example, such sample images as those shown in FIG. 5A are
evaluated as "Poor", while such sample images as those shown in
FIG. 5B are evaluated as "Good". As for the sample images shown in
FIG. 5A, the end surface of the glass has projections and recesses
created by precipitations having precipitated on the end surface,
and is thus not smooth. Thus, the sample images in FIG. 5A for
Observation A, which were obtained with an optical microscope, show
an intermixture of white regions created by specular reflection of
light from the smooth areas of the glass end surface and black
regions created by diffused reflection of light from the
projections and recesses due to precipitations. Further, the sample
image in FIG. 5A for Observation B shows a concave cross-sectional
shape. In contrast, the sample images in FIG. 5B for Observations A
show clear boundaries between the white regions and the black
regions. Thus, it is considered that the samples shown in FIG. 5B
achieve high etching accuracy.
[0284] Evaluation of Coefficient of Thermal Expansion and Glass
Transition Temperature Tg:
[0285] The glass sample was processed into a circular cylinder 5 mm
in diameter and 20 mm long, and the coefficient of thermal
expansion and the glass transition temperature Tg were measured
with a differential dilatometer (Thermo Plus2 TMA8310). The average
thermal expansion coefficient in the temperature range of
100.degree. C. to 300.degree. C. was calculated from the results of
measuring the thermal expansion coefficient.
[0286] Density:
[0287] The density was measured according to the Archimedean
method.
[0288] Chemical Strengthening:
[0289] The glass sample, after being rinsed, was immersed for about
2.5 hours in a treatment bath containing 100% of KNO.sub.3 kept at
400.degree. C., to exchange the Na.sup.+ ions on the superficial
layer of the glass with K.sup.+ ions present in the treatment bath
and thereby chemically strengthen the glass sample. The
chemically-strengthened glass substrate was immersed in a rinsing
tank for rinsing and then dried, to thereby obtain a piece of
strengthened glass. As for Samples 37, 40, 42, and 52, each glass
sample was chemically strengthened by being immersed for about 5
hours in a treatment bath containing 100% of KNO.sub.3 kept at
500.degree. C.
[0290] Evaluation of Compressive-Stress Value and Thickness (Depth)
of Compressive-Stress Layer:
[0291] The strengthened glass obtained as above was observed to
find the number of interference fringes and the interval
therebetween with a surface stress meter (FSM-6000LE from Orihara
Industrial Co., Ltd.), and the compressive-stress value of the
compressive-stress layer formed in the vicinity of the surface of
the glass and the thickness (depth) of the compressive-stress layer
were calculated. The value of the refractive index (nd) of each
strengthened glass used for the calculation was measured with a
refractometer (KPR-200 from Shimadzu Device Corporation). Note that
the stress optical coefficient was regarded as 28 ((nm/cm)/MPa) in
the calculation for the compressive-stress value.
[0292] Tables 11 and 12 show the evaluation results and other
characteristics of Samples 34 to 62 of the second embodiment shown
in Tables 7 and 8 and those of Samples 53 to 76 of the second
embodiment shown in Tables 9 and 10.
TABLE-US-00011 TABLE 11 Glass Characteristics Avg. thermal Surface
Proc- Sam- Tl Etching rate Tg expansion coef. Density Compressive-
compressive essing ple (.degree. C.) (.mu.m/min) (.degree. C.)
[.times.10.sup.-7/.degree. C.] (g/cm.sup.3) stress value layer
depth accuracy 34 1000 5.1 610 89 2.46 Fair 35 1112 5.2 620 91 2.50
Fair 36 1090 3.7 585 99.7 2.455 693 47 Good 37 <840 4.0 559 106
2.480 543 57 Good 38 <840 4.6 579 97.9 2.510 684 53 Good 39
<840 3.7 511 114 2.477 147 88 Good 40 <840 4.2 527 113 2.506
236 83 Good 41 <840 4.1 572 106 2.502 587 66.8 Good 42 <840
3.7 551 102.7 2.460 354 53.4 Good 43 <840 3.8 574 99.5 2.457 524
47.7 Good 44 <840 3.7 566 102.7 2.453 576 49.4 Good 45 <840
4.2 570 105 2.449 584 65 Good 46 <840 3.9 581 102.6 2.487 Good
47 907 4.2 574 104 2.459 Good 48 <840 4.5 572 103.6 2.463 Good
49 953 5.2 577 105 2.465 Good 50 <840 3.9 Good 51 <840 3.9
Good 52 <840 4.0 Good 53 <950 3.7 544 107 2.46 350 56 Good 54
<950 3.7 558 102 2.46 529 63 Good 55 <950 3.8 574 102 2.46
616 53 Good 56 <900 3.8 580 98 2.46 591 54 Good 57 <900 3.7
586 101 2.46 705 50 Good 58 921 3.8 583 102 2.46 719 51 Good 59
<850 3.8 578 102 2.45 660 54 Good 60 879 3.8 576 100 2.45 655 54
Good 61 914 3.8 568 100 2.46 610 53 Good 62 <850 3.8 551 94 2.46
559 52 Good
TABLE-US-00012 TABLE 12 Glass characteristics Avg. thermal Proc- Tl
Etching rate Tg expansion coef. Density essing Sample (.degree. C.)
(.mu.m/min) (.degree. C.) [.times.10.sup.-7/.degree. C.]
(g/cm.sup.3) accuracy 63 858 2.3 563 90 2.51 Poor 64 830 1.7 554 90
2.50 Poor 65 855 3.4 576 90 2.51 Poor 66 836 2.8 571 90 2.51 Poor
67 3.5 570 Poor 68 1021 4.5 604 89 2.49 Poor 69 948 5.2 593 95 2.52
Poor 70 927 4.3 578 97 2.49 Poor 71 988 5.3 592 97 2.49 Poor 72
1003 5.2 616 90 2.54 Poor 73 1018 6.1 626 91 2.53 Poor 74 1003 6.1
614 91 2.53 Poor 75 968 4.1 586 91 2.51 Poor 76 1200 4.8 636 92
Poor
[0293] A comparison between Tables 11 and 12 reveals that Samples
34 and 35 containing 2.6% or less of CaO and Samples 36 to 62
substantially containing no CaO have high processing accuracies in
the etching of glass substrates. Note that the "processing
accuracy" in Tables 11 and 12 shows evaluation results for when
etching is performed on the end surfaces of glass substrates that
have been machined, but even when etching is performed with the aim
of shape-processing a glass substrate to provide it with the shape
as illustrated in FIG. 1, the evaluation result on the processing
accuracy thereof has the same tendency as the evaluation result for
when etching is performed on the end surfaces of a glass substrate
that has been machined. In other words, Samples 34 and 35
containing 2.6% or less of CaO and Samples 36 to 62 substantially
containing no CaO can be considered as having high processing
accuracies even when etching is performed with the aim of
shape-processing.
[0294] FIG. 6 shows a distribution chart showing the etching rates
given on Tables 11 and 12 on the Y-axis and the aforementioned
X-1/2Y (wherein X is the content by percentage of SiO.sub.2 and Y
is the content by percentage of Al.sub.2O.sub.3) on the X-axis for
Samples 34 to 62 in Tables 7 and 8 and Samples 63 to 76 in Tables 9
and 10. As can be seen, the etching rate increases as the value of
X-1/2Y decreases. Meanwhile, no significant change in the etching
rate can be seen in the area where X-1/2Y is 52% or less. So, in
order to achieve an etching rate of 3.7 .mu.m/minute or greater, it
is preferable that X-1/2Y is 57.5% or less. However, even if the
value of Y is increased and X-1/2Y is adjusted to 52% or less,
there will be no further increase in the etching rate, and in
addition, the devitrification temperature will increase and thus
the devitrification resistance will deteriorate. Therefore, the
lower limit of the value of X-1/2Y is preferably 45%.
[0295] Example of Continuous Production of Glass Substrate:
[0296] Glass materials were prepared so as to provide a glass
substrate with the composition shown in Sample 36 (see Tables 7 and
8). The glass materials were molten at 1520.degree. C. by using a
continuous melting device having, for example, a fire-brick-made
melting tank, a platinum-made stirring tank and so on, were
subjected to refining at 1550.degree. C., stirred at 1350.degree.
C., and then formed into a thin plate 0.7 mm thick by down-draw
processing, to produce a glass substrate. Etching and chemical
strengthening were performed as follows.
[0297] On each of the two principal surfaces of the glass substrate
prepared as above, a 20-.mu.m-thick pattern made of a phenolic
heat-curable resin and having the shape of a cover glass was formed
by mesh-screen printing, and the phenolic heat-curable resin
patterns were baked for 15 minutes at 200.degree. C. With the
phenolic heat-curable resin patterns being employed as masks, the
glass substrate was etched in the regions-to-be-etched from both
principal surfaces by using a mixed-acid aqueous solution
(40.degree. C.) containing hydrofluoric acid (15% by mass) and
sulfuric acid (24% by mass) as the etchant, to cut the glass
substrate into a predetermined shape. Then, the phenolic
heat-curable resin remaining on the glass surface was dissolved by
using an NaOH solution and was removed off from the glass
substrate, and then the substrate was rinsed.
[0298] Then, the rinsed glass substrate was immersed for about 5
hours in a treatment bath containing 100% of KNO.sub.3 kept at
500.degree. C., to exchange the Na ions on the superficial layer of
the glass with K ions present in the treatment bath and thereby
chemically strengthen the glass substrate. The
chemically-strengthened glass substrate was immersed in a rinsing
tank for rinsing and then dried.
[0299] The result was that it was possible to produce a cover glass
having excellent accuracy in shape.
[0300] As described above, with the cover-glass production method
and the cover glass of the first embodiment, the etching rate can
be improved, and thus the cover-glass production efficiency can be
improved. Further, with the cover-glass production method and the
cover glass of the second embodiment, the cover glass can be
produced with high accuracy in shape, even if the shape is
complicated, and thus the cover-glass production efficiency can be
improved.
[0301] Certain embodiments of the cover glass and the cover-glass
production method of the present invention were described in detail
above, but the present invention is not to be limited to the
foregoing embodiments and can be modified and/or improved in
various ways as far as such modifications/improvements do not
depart from the gist of the present invention.
* * * * *