U.S. patent application number 14/879788 was filed with the patent office on 2016-02-25 for glass substrate.
This patent application is currently assigned to NIPPON ELECTRIC GLASS CO., LTD.. The applicant listed for this patent is Nippon Electric Glass Co., Ltd.. Invention is credited to Hisatoshi AIBA, Katsutoshi FUJIWARA, Yoshinari KATO, Takahiro KAWAGUCHI.
Application Number | 20160052819 14/879788 |
Document ID | / |
Family ID | 44673100 |
Filed Date | 2016-02-25 |
United States Patent
Application |
20160052819 |
Kind Code |
A1 |
KAWAGUCHI; Takahiro ; et
al. |
February 25, 2016 |
GLASS SUBSTRATE
Abstract
The present invention is aimed to provide a method for producing
a glass substrate with a thickness of not more than 200 .mu.m,
which is satisfied with the quality required for a substrate on
which a thin-film electric circuit is formed, and a sheet glass
substrate obtained according to this method. The present invention
is concerned with a method for producing a glass substrate having a
sheet thickness of from 10 to 200 .mu.m, including a forming step
of forming a molten glass into a ribbon shape in accordance with a
down draw method, an annealing step of annealing the glass ribbon,
and a cutting step of cutting the glass ribbon to give a glass
substrate, wherein an average cooling rate in a temperature range
of from the (annealing point +200.degree. C.) to the (annealing
point +50.degree. C.) is controlled to the range of from 300 to
2,500.degree. C./min.
Inventors: |
KAWAGUCHI; Takahiro;
(Otsu-shi, JP) ; FUJIWARA; Katsutoshi; (Otsu-shi,
JP) ; KATO; Yoshinari; (Otsu-shi, JP) ; AIBA;
Hisatoshi; (Otsu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Electric Glass Co., Ltd. |
Otsu-shi |
|
JP |
|
|
Assignee: |
NIPPON ELECTRIC GLASS CO.,
LTD.
|
Family ID: |
44673100 |
Appl. No.: |
14/879788 |
Filed: |
October 9, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13636474 |
Sep 21, 2012 |
9199869 |
|
|
PCT/JP2011/056700 |
Mar 22, 2011 |
|
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14879788 |
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Current U.S.
Class: |
428/141 ;
428/220 |
Current CPC
Class: |
C03C 3/091 20130101;
Y10T 428/24355 20150115; C03B 17/067 20130101 |
International
Class: |
C03C 3/091 20060101
C03C003/091 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 23, 2010 |
JP |
2010-065568 |
Mar 8, 2011 |
JP |
2011-049763 |
Claims
1.-4. (canceled)
5. A glass substrate having a sheet thickness of from 10 to 200
.mu.m, wherein a sheet thickness difference between a maximum sheet
thickness and a minimum sheet thickness in the substrate is not
more than 30 .mu.m.
6. The glass substrate according to claim 5, wherein a residual
stress value thereof is not more than 2.5 nm.
7. The glass substrate according to claim 5, wherein a warpage
value thereof is not more than 200 .mu.m.
8. The glass substrate according to claim 5, wherein a thermal
shrinkage ratio at the time of heating from ordinary temperature at
a rate of 5.degree. C./min, keeping at 450.degree. C. for 10 hours
and then cooling at a rate of 5.degree. C./min is less than 300
ppm.
9. The glass substrate according to claim 5, wherein an average
surface roughness Ra thereof is not more than 0.3 nm.
10. The glass substrate according to claim 5, which is composed of
a glass comprising from 50 to 70% of SiO.sub.2, from 10 to 25% of
Al.sub.2O.sub.3, from 1 to 15% of B.sub.2O.sub.3, from 0 to 10% of
MgO, from 0 to 15% of CaO, from 0 to 15% of SrO, from 0 to 15% of
BaO, and from 0 to 5% of Na.sub.2O in terms of percentage by
mass.
11. The glass substrate according to claim 5, which is used as a
substrate for forming a thin-film electric circuit.
12. The glass substrate according to claim 5, which is used as a
substrate of a flexible display.
Description
CROSS-REFERENCE TO PRIOR APPLICATION
[0001] This application is a divisional application of U.S. patent
application Ser. No. 13/636,474 (allowed), filed Sep. 21, 2012,
which is a .sctn.371 National Stage Application of PCT
International Application No. PCT/JP2011/056700, filed Mar. 22,
2011, which claims priority to JP2010-065568, filed Mar. 23, 2010
and JP 2011-049763, filed Mar. 8, 2011, which are incorporated
herein in their entirety.
TECHNICAL FIELD
[0002] The present invention relates to a glass substrate on which
a thin-film electric circuit is formed, in particular to a glass
substrate which is used for flat panel displays and flexible
displays such as liquid crystal displays, organic EL displays, and
the like.
BACKGROUND ART
[0003] Glass substrates which are used for a display application
are generally formed according to a float method, a down draw
method represented by an overflow down draw method, or the
like.
[0004] The float method is a method of casting a molten glass onto
molten tin (float bath) and stretching it in the horizontal
direction to form the glass in a sheet form. According to the
method, a glass ribbon is formed on the float bath, and the glass
ribbon is then annealed (on-line annealed) in a long annealing
furnace. Accordingly, the glass substrate formed according to the
float method is characterized by having a small thermal shrinkage
ratio.
[0005] However, the float method involves such disadvantages that
it is difficult to make the sheet thin, the glass substrate is
required to be polished to remove tin attached onto the glass
surface, and the surface quality of the substrate is lowered.
[0006] However, the float method involves such disadvantages that
it is difficult to make the sheet thin, the glass substrate is
required to be polished to remove tin attached onto the glass
surface, and the surface quality of the substrate is lowered.
[0007] On the other hand, the down draw method is a generic term
for a forming method of drawing a glass in the vertical downward
direction to form it in a sheet form, and a slot (slit) down draw
method, an overflow down draw method, and the like are known. For
example, in the overflow down draw method that is widely adopted, a
molten glass is introduced into the top of a trough-shaped
refractory (forming body) having a nearly wedge-shaped cross
section, and the glass is allowed to overflow out from the both
side thereof to flow down along the side face, and the two streams
are joined together at the lower end of the refractory and drawn
downward to form the glass in a sheet form. The down draw method is
advantageous in that a glass is easy to be formed into a thin
sheet.
[0008] Furthermore, in the case of the overflow down draw method,
since the glass surface does not come into contact with any other
than air, there is also such an advantage that a glass substrate
having high surface quality can be obtained even in an unpolished
state.
CITED REFERENCES
Patent Documents
[0009] Patent Document 1: JP-A-2008-105882
[0010] Patent Document 2: JP-A-2008-133174
SUMMARY OF THE INVENTION
Problems that the Inventions is to Solve
[0011] In recent years, from the viewpoint of space-saving,
thinning and weight reduction of flat panel displays such as liquid
crystal displays, organic EL displays, etc. are progressing, and as
an extension thereof, researches toward flexibilization of the
panels are energetically advanced. In addition, because of
expansion of electronic paper, new display applications (e.g.,
electronic book, electronic newspaper, electronic price tag,
digital signage, etc.) are being developed, and a requirement for
thin and bendable flexible displays expands.
[0012] In order to realize a flexible display, the development of a
substrate technology is indispensable. A substrate having not only
suppleness but barrier properties against oxygen and moisture, etc.
is necessary. As the substrate having these characteristics, thin
sheet glasses which are made thin as films are regarded as
promising. In particular, from the viewpoint of suppleness, glasses
which are thinner than 200 .mu.m are desired. Under these
circumstances, the development of a method for producing a thin
sheet glass by adopting the down draw method is advanced (see, for
example, Patent Documents 1 and 2).
[0013] Similar to the current flat panel displays, it is expected
that requirements, such as high precision, high fineness, etc.,
will be also increased for the flexible displays. In order to meet
these requirements, it is necessary to make a pattern of a
thin-film electric circuit finer, and it is thought that a
requirement of the surface quality for the substrate will increase
more and more. Incidentally, if a surface roughness of the
substrate (local unevenness) is large, or a sheet thickness of the
substrate (overall unevenness) is not uniform, it is difficult to
form a fine circuit pattern.
[0014] However, in the case where it is intended to form a thin
sheet glass of not more than 200 .mu.m according to the down drawn
method, it is difficult to stably draw out the glass from the
forming equipment, and it is difficult to keep the uniformity of
the sheet thickness. For that reason, there was involved such a
problem that the quality required for the substrate on which a
thin-film electric circuit is formed cannot be satisfied. In order
to make the sheet thickness uniform, it is thought to conduct an
off-line polishing treatment. However, it is very technically
difficult to polish a glass substrate of not more than 200 .mu.m,
and the production costs greatly increase.
[0015] An object of the present invention is to provide a method
for producing a glass substrate of not more than 200 .mu.m, which
is satisfied with the quality required for a substrate on which a
thin-film electric circuit is formed, and a thin sheet glass
substrate obtained according to this method.
Means for Solving the Problems
[0016] As a result of extensive and intensive investigations, the
present inventors have found that the foregoing object can be
achieved by regulating an average cooling rate of the glass in a
temperature region higher than an annealing point to 300.degree.
C./min or more and proposed it as the present invention.
[0017] That is, a method for producing a glass substrate of the
present invention is a method for producing a glass substrate
having a sheet thickness of from 10 to 200 .mu.m, including a
forming step of forming a molten glass into a ribbon shape in
accordance with a down draw method, an annealing step of annealing
the glass ribbon, and a cutting step of cutting the glass ribbon to
give a glass substrate, wherein an average cooling rate in a
temperature range of from the (annealing point +200.degree. C.) to
the (annealing point +50.degree. C.) is controlled to the range of
from 300 to 2,500.degree. C./min. Incidentally, the "annealing
point" is a temperature at which the glass has a viscosity of
10.sup.13 dPas, and this can be measured based on the method
according to ASTM C336-71. The "average cooling rate" means a rate
obtained in such a manner that the time in which a center portion
of a glass ribbon in the sheet width direction passes through a
prescribed temperature region is calculated, and a temperature
difference (here, 150.degree. C.) within this region is divided by
the time taken for the passing.
[0018] According to the foregoing constitution, a glass substrate
having a uniform sheet thickness and having small warpage and
residual stress can be obtained by regulating the average cooling
rate in a temperature region higher than the annealing point to
300.degree. C./min or more. In addition, since the glass is rapidly
cooled to the annealing point, the time (or distance) capable of
being taken for the subsequent annealing can be sufficiently
secured. As a result, nonetheless a fictive temperature is high, by
adequately regulating the subsequent annealing condition, it is
possible to produce a glass substrate having a small thermal
shrinkage ratio.
[0019] Furthermore, in the present invention, it is preferable to
regulate an average cooling rate of from the annealing point to the
(annealing point -100.degree. C.) to the range of from 10 to
300.degree. C./min.
[0020] In the case of increasing a cooling rate of the glass in a
temperature region higher than the annealing point to form a sheet
glass having a sheet thickness of not more than 200 .mu.m, the
fictive temperature of the glass is easy to become high. When the
fictive temperature of the glass becomes high, in general, the
thermal shrinkage ratio tends to become high. As a result, there is
a possibility that the quality required as a substrate for forming
a thin-film electric circuit cannot be satisfied. Even in such
case, when the foregoing constitution is adopted, nonetheless the
sheet thickness is not more than 200 .mu.m, it is possible to
obtain a glass substrate having a low thermal shrinkage ratio.
[0021] In the present invention, the down draw method is preferably
an overflow down draw method.
[0022] According to the foregoing constitution, it is possible to
produce a substrate for forming a thin-film electric circuit, in
particular a glass substrate capable of being used as a substrate
of a flexible display, in a surface state at the time of forming as
it is. Accordingly, it is possible to omit a polishing step, and
this constitution is suitable as a method for producing a thin
sheet which is difficult to be polished.
[0023] In the present invention, it is preferable to use a glass
comprising from 50 to 70% of SiO.sub.2, from 10 to 25% of
Al.sub.2O.sub.3, from 1 to 15% of B.sub.2O.sub.3, from 0 to 10% of
MgO, from 0 to 15% of CaO, from 0 to 15% of SrO, from 0 to 15% of
BaO, and from 0 to 5% of Na.sub.2O in terms of percentage by
mass.
[0024] According to the foregoing constitution, it is easy to
select a glass composition having a high strain point and having a
liquidus viscosity suitable for the overflow down draw method. In
addition, it is possible to make a glass composition which is
excellent in various characteristics required for display
substrates, such as chemical resistance, specific modulus, chemical
durability, meltability, etc.
[0025] The glass substrate of the present invention is a glass
substrate having a sheet thickness of from 10 to 200 .mu.m, and it
is characterized in that a sheet thickness difference between a
maximum sheet thickness and a minimum sheet thickness in the
substrate is not more than 30 .mu.m. Incidentally, the "sheet
thickness difference between a maximum sheet thickness and a
minimum sheet thickness in the substrate" means a value obtained by
measuring thickness variation along an arbitrary line across a
glass substrate using a laser type thickness measuring device,
determining a maximum thickness and a minimum thickness of the
glass substrate, and then subtracting a value of the minimum sheet
thickness from a value of the maximum sheet thickness.
[0026] According to the foregoing constitution, since the substrate
has flexibility, it is possible to use the substrate for an
application for a substrate of a flexible display, or the like. In
addition, the sheet thickness difference necessary for the
substrate on which a thin-film electric circuit is formed can be
satisfied.
[0027] In the present invention, a residual stress value is
preferably not more than 2.5 nm. In the present invention, the
"residual stress value" means a retardation value measured using a
stress meter according to an optical heterodyne method.
[0028] According to the foregoing constitution, the distortion
value necessary for the substrate on which a thin-film electric
circuit is formed can be satisfied.
[0029] In the present invention, a warpage value is preferably not
more than 200 .mu.m. Incidentally, in the present invention, the
"warpage value" means a value measured by a warpage measurement
system.
[0030] According to the foregoing constitution, the warpage value
necessary for the substrate on which a thin-film electric circuit
is formed can be satisfied.
[0031] In the present invention, a thermal shrinkage ratio at the
time of heating from ordinary temperature at a rate of 5.degree.
C./min, keeping at 450.degree. C. for 10 hours, and then cooling at
a rate of 5.degree. C./min is preferably less than 300 ppm.
Incidentally, in the present invention, the "thermal shrinkage
ratio" means a value obtained through the measurement in the
following manner. First of all, a strip sample of 160 mm.times.30
mm is prepared as a sample for the measurement (FIG. 2(a)).
Markings are given to the area around from 20 to 40 mm from each
end of this strip sample in the long side direction with a #1000
waterproof abrasive paper, and the sample is divided into two
pieces along the center line vertical to the markings (FIG. 2(b)).
After one of the pieces is heat treated under prescribed
conditions, the heat-treated piece and untreated piece are put in
parallel (FIG. 2(c)), displacement of the markings (AL1 and AL2)
are measured with a laser microscope, and the thermal shrinkage
ratio is calculated according to the following equation.
Thermal shrinkage ratio [ppm]=(.DELTA.L1 [.mu.m]+.DELTA.L2
[.mu.m])/160.times.10.sup.-3
[0032] According to the foregoing constitution, there is brought
such an effect that even when the heat treatment is applied in the
forming step of a thin-film circuit pattern, a pattern displacement
is hardly caused.
[0033] In the present invention, an average surface roughness Ra is
preferably not more than 0.3 nm. Incidentally, in the present
invention, the "average surface roughness Ra" means a value
measured according to a method in conformity with the "FPD Glass
Substrate Surface Roughness Measurement Method" in SEMI D7-94.
[0034] So far as the foregoing constitution can be directly
achieved by adopting the overflow down draw method or the like, it
is possible to omit the polishing step.
[0035] In the present invention, the substrate is preferably
composed of a glass comprising from 50 to 70% of SiO.sub.2, from 10
to 25% of Al.sub.2O.sub.3, from 1 to 15% of B.sub.2O.sub.3, from 0
to 10% of MgO, from 0 to 15% of CaO, from 0 to 15% of SrO, from 0
to 15% of BaO, and from 0 to 5% of Na.sub.2O in terms of percentage
by mass.
[0036] According to the foregoing constitution, since the glass has
a high strain point, and a liquidus viscosity suitable for the
overflow down draw method, a glass which is low in the thermal
shrinkage ratio and excellent in the surface quality can be
obtained without being polished.
[0037] In the present invention, it is preferable to use the
substrate as a substrate for forming a thin-film electric circuit,
in particular a substrate of a flexible display.
[0038] According to the foregoing constitution, the characteristic
features of the present invention that not only the sheet thickness
is small, but the surface quality is excellent can be made the best
use.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is an outline front view showing production equipment
for a glass substrate in carrying out the present invention.
[0040] FIGS. 2(a) to 2(c) are explanatory views showing a method
for thermal shrinkage ratio determination.
MODES FOR CARRYING OUT THE INVENTION
[0041] The method of the present invention is described in
detail.
[0042] First of all, the method of the present invention includes a
forming step of forming a molten glass into a ribbon shape in
accordance with a down draw method. In this forming step, it is
important to regulate forming conditions such that a sheet
thickness of the glass to be finally obtained is from 10 to 200
.mu.m. The sheet thickness can be regulated by controlling a flow
rate of the glass, a forming temperature, a rate of drawing the
glass (sheet drawing rate), and the like. Incidentally, as for the
forming conditions, it is preferable to regulate the sheet
thickness of the glass to be obtained to from 10 to 150 .mu.m, in
particular from 10 to 100 .mu.m.
[0043] Though the forming method is not particularly limited so far
as it is the down draw method, it is preferable to adopt the
overflow down draw method capable of producing a ribbon-shaped
glass with a favorable surface quality without conducting
polishing. The reason why when the overflow down draw method is
adopted, a ribbon-shaped glass with a favorable surface quality can
be produced resides in the matter that the side thereof serving as
a surface of the ribbon does not come into contact with any other
than air and is formed in a free-surface state. Incidentally, the
overflow down draw method is a method in which a molten glass is
allowed to overflow out from the both side of a heat-resistant
trough-shaped structure, and the overflown glasses are drawn and
formed downward while being joined together at a lower end of the
trough-shaped structure, thereby producing a ribbon-shaped glass.
The structure and material quality of the trough-shaped structure
are not particularly limited so far as the dimension or surface
precision of the ribbon-shaped glass, or the quality required for a
predetermined application can be realized. In addition, for the
downward drawing, any method for applying a force to the
ribbon-shaped glass may be adopted. For example, a method in which
the molten glass is drawn by rotating heat-resistant rollers having
a sufficiently large width in a state of being brought into contact
with the ribbon-shaped glass may be adopted; and a method in which
the molten glass is drawn while bringing plural pairs of
heat-resistant rollers into contact with only around the both end
surfaces of the ribbon-shaped glass may be adopted.
[0044] Incidentally, in the present invention, in addition to the
overflow down draw method, various down draw methods can be
adopted. For example, it is possible to adopt a slot down method, a
redraw method, or the like.
[0045] The method of the present invention comprises an annealing
step of cooling the glass formed into a ribbon shape. In this step,
in a process of cooling the high-temperature ribbon-shaped glass
immediately after forming, control of the sheet thickness, removal
of the residual stress or warpage, reduction of the thermal
shrinkage, and the like are conducted. In particular, the present
invention is characterized by controlling the cooling rate to a
specified rate in a temperature region of the annealing point or
higher at which the sheet thickness, residual stress, or warpage is
greatly influenced. Specifically, an average cooling rate in a
temperature range of from the (annealing point +200.degree. C.) to
the (annealing point +50.degree. C.) is controlled to the range of
from 300 to 2,500.degree. C./min, preferably from 300 to
2,000.degree. C./min, from 300 to 1,500.degree. C./min, from 400 to
1,000 .degree. C./min, from 500 to 900.degree. C./min, and
especially preferably from 600 to 800.degree. C./min. Incidentally,
for the sake of convenience, the temperature range of from the
(annealing point +200.degree. C.) to the (annealing point
+50.degree. C.) is hereinafter referred to as "first annealing
temperature region".
[0046] Incidentally, the temperature of the glass can be determined
by means of non-contact measurement with a pyrometer or contact
measurement utilizing a thermocouple.
[0047] When the cooling rate of the first annealing temperature
region is too low, the shape of the glass sheet is not rapidly
defined, and hence, it is difficult to make the sheet thickness
uniform. In addition, the time (or distance) capable of being taken
for the subsequent annealing becomes short, and hence, the thermal
shrinkage ratio becomes large. On the other hand, when the cooling
rate of the first annealing temperature region is too high, the
glass is rapidly cooled, and hence, a non-uniform, large residual
stress is generated, resulting in deterioration of the warpage. In
addition, the fictive temperature of the glass becomes too high,
and therefore, even by regulating the subsequent annealing
conditions, it is difficult to sufficiently decrease the thermal
shrinkage ratio.
[0048] Incidentally, the fictive temperature is a temperature of a
supercooled liquid having the same structure as a glass structure,
and this is an index of the structure of glass. Glass is low in
viscosity and liquidus at a high temperature, and in this stage,
the glass has an open structure. Then, when the glass is cooled,
the glass structure becomes dense and is frozen. This glass
structure change occurs because the glass is likely to be in the
most stable state at that temperature. However, when the cooling
rate of glass is high, the glass structure is frozen before it has
a dense structure corresponding to that temperature, so that the
glass structure is frozen in a state of a high-temperature side.
The temperature corresponding to the solidified glass structure is
said to be a fictive temperature. When the fictive temperature is
higher, the glass structure is more open and therefore, the thermal
shrinkage ratio becomes large. However, when the subsequent
annealing is adequately conducted, it is possible to make the
thermal shrinkage ratio small. In the case of carrying out the
method of the present invention, the fictive temperature of the
glass substrate is easy to fall within the range of from the
(annealing point +45.degree. C.) to the (annealing point
+100.degree. C.), in particular the range of from the (annealing
point +45.degree. C.) to the (annealing point +80.degree. C.), and
moreover the range of from the (annealing point +45.degree. C.) to
the (annealing point +60.degree. C.). In the case of the method of
the present invention, according to a fast cooling rate in the
first annealing region, the time capable of being taken for the
annealing in a temperature region of not higher than the annealing
point can be ensured long. Therefore, by adequately regulating the
annealing conditions, nonetheless the fictive temperature is high,
a glass substrate having a practically acceptable thermal shrinkage
ratio can be obtained.
[0049] The "fictive temperature" is a temperature determined as
follows. First of all, the same glass piece as that in the thermal
shrinkage determination is put into an electric furnace controlled
at the annealing point temperature, and after one hour, the glass
piece is taken out of the electric furnace and rapidly cooled on an
aluminum plate, followed by measuring a thermal shrinkage ratio
thereof. The same treatment is carried out at the (annealing point
+20.degree. C.), the (annealing point +40.degree. C.), and the
(annealing point +60.degree. C.), respectively, and a graph of a
relationship between the treatment temperature and the thermal
shrinkage ratio is prepared. A heat treatment temperature at which
the thermal shrinkage ratio is 0 ppm is determined from a linear
approximate curve of this graph, and this is defined as the fictive
temperature of glass.
[0050] Now, in the down draw method, in view of the relationship
that an annealing furnace is provided just below the forming body,
it is actually impossible to dispose a long annealing furnace like
that in the float method. Accordingly, the annealing furnace is
necessarily short. In other words, the cooling rate within the
annealing furnace is fast, and a glass is frozen in a high
temperature state, and therefore, it is difficult to obtain a glass
substrate having a small thermal shrinkage ratio.
[0051] In liquid crystal displays or organic EL displays, a
thin-film electric circuit such as a thin film transistor (TFT) is
formed on the surface of a glass substrate. In this forming
process, when the glass substrate is exposed to a high-temperature
atmosphere, structural relaxation is advanced, and its volume
shrinks (thermally shrinks). When the glass substrate thermally
shrinks in a forming step of a thin-film electric circuit, the
shape and dimension of the circuit pattern deviate from the
designed values, whereby desired electric performances are not
obtainable. For that reason, it is required that the thermal
shrinkage of the substrate on which a thin-film electric circuit is
formed is small.
[0052] Then, in the method of the present invention, it is
preferable to regulate an average cooling rate in a temperature
range of from the annealing point to the (annealing point
-100.degree. C.), which is a temperature region subsequent to the
first annealing temperature region, to the range of from 10 to
300.degree. C./min. In particular, it is desirable to regulate the
average cooling rate to the range of from 10 to 200.degree. C./min,
from 10 to 150.degree. C./min, and from 50 to 150.degree. C./min.
Incidentally, for the sake of convenience, the temperature range of
from the annealing point to the (annealing point -100.degree. C.)
is hereinafter referred to as "second annealing temperature
region". The second annealing temperature region is a temperature
region at which the thermal shrinkage ratio is greatly influenced,
and by passing through this region at the foregoing cooling rate,
nonetheless the fictive temperature is high, a glass substrate
having a practically acceptable thermal shrinkage ratio can be
produced. When the cooling rate in this range is too low, in the
case of the present invention for forming a glass according to the
down draw method, a glass melting apparatus or a forming furnace
must be set at higher sites, so that there is a concern that this
brings about limitations from the standpoint of designing the
equipment. On the other hand, when the cooling rate is too high,
the time capable of being taken for the annealing is short, and
hence, as a result, it becomes difficult to reduce the thermal
shrinkage ratio.
[0053] Incidentally, in the method of the present invention, it is
desirable that in the annealing step, an average cooling rate in a
temperature region positioning between the first annealing
temperature region and the second annealing temperature region,
namely in a temperature range of from the (annealing point
+50.degree. C.) to the annealing point, is set lower than the
cooling rate in the first annealing temperature region and higher
than the cooling rate in the second annealing temperature region.
Incidentally, for the sake of convenience, the temperature range of
from the (annealing point +50.degree. C.) to the annealing point is
hereinafter referred to as "intermediate annealing temperature
region". By setting the cooling rate in the intermediate annealing
temperature region as described above, a change of the cooling rate
from the first annealing temperature region to the second annealing
temperature region can be smoothly achieved.
[0054] The method of the present invention comprises a cutting step
of cutting the ribbon-shaped glass after completion of the
annealing into a prescribed length to form a glass substrate. The
cutting as referred to herein is not limited to the case of cutting
off the ribbon-shaped glass directly every sheet. That is, the
cutting includes the case where the ribbon-shaped glass is once
wound up in a roll form and then subjected to various processings
such as rewinding, sheet width adjustment, film coating, etc., and
thereafter, the ribbon-shaped glass is again drawn out and cut
every sheet. For the cutting, various methods such as a method for
previously making a scribed line on a glass with a cutter or a
laser light and then divided the glass, a method for fusing a glass
with a laser light, etc. can be adopted.
[0055] In the method of the present invention, it is desirable that
the surface of the obtained glass substrate is not subjected to
polishing. Namely, in a glass having a sheet thickness of from 10
to 200 .mu.m, the possibility of breakage during polishing process
is very high. Accordingly, when polishing is applied, a production
yield becomes low, and special equipment for preventing the
breakage caused during polishing process is needed, and hence, the
costs increase. Moreover, when polishing is conducted, the glass
surface is scratched, and the original strength of glass is
impaired. Incidentally, in order to obtain a glass substrate having
an excellent surface quality even without applying polishing, an
overflow down draw method may be adopted as the forming method.
Incidentally, the "surface" as referred to in this description
means a translucent surface (or a main surface) of the glass
substrate, and it is differentiated from an edge surface to which
polishing is applied for the purpose of preventing cracking,
etc.
[0056] In the method of the present invention, it is preferable to
use a glass having a liquidus viscosity of 10.sup.4.5 dPas or more.
In particular, in the case of forming a glass according to the
overflow down draw method, it is important that the liquidus
viscosity of the glass is high. Specifically, the liquidus
viscosity of the glass is preferably 10.sup.4.5 dPas or more,
10.sup.5.0 dPas or more, 10.sup.5.5 dPas or more, and 10.sup.60
dPas or more. Incidentally, the liquidus viscosity is a viscosity
at the temperature of precipitation of a crystal, and a composition
having a higher liquidus viscosity is more hardly devitrified at
the time of glass forming and is easier to be formed into a
glass.
[0057] In the method of the present invention, it is preferable to
use a glass having a strain point of 600.degree. C. or higher. The
stain point as referred to herein means a temperature at which the
glass has a viscosity of 10.sup.14.5 dPas. According to this
constitution, it is easy to produce a glass substrate having a
small thermal shrinkage ratio.
[0058] The method of the present invention can be applied to
various glasses. For example, in the case of expecting the use for
a liquid crystal display, an organic EL display, etc., a glass
comprising from 50 to 70% of SiO.sub.2, from 10 to 25% of
Al.sub.2O.sub.3, from 1 to 15% of B.sub.2O.sub.3, from 0 to 10% of
MgO, from 0 to 15% of CaO, from 0 to 15% of SrO, from 0 to 15% of
BaO, and from 0 to 5% of Na.sub.2O in terms of percentage by mass
may be used. So far as the composition falls within this range, it
is easy to design a glass composition having a high strain point
and having a liquidus viscosity suitable for down draw forming.
[0059] In the glass substrate obtained by the present invention, by
adequately regulating the first annealing temperature region, it is
possible to regulate a sheet thickness difference between a maximum
sheet thickness and a minimum sheet thickness in the substrate to
not more than 30 .mu.m, in particular not more than 25 .mu.m, and
moreover not more than 20 .mu.m. In the case where the sheet
thickness difference is too large, it is difficult to conduct
accurate patterning of an electrode, etc., and faults such as
disconnection or short circuit of a circuit electrode, etc. are
easily caused.
[0060] In the glass substrate obtained by the present invention, by
adequately regulating the first annealing temperature region, it is
possible to regulate a residual stress value to not more than 2.5
nm, in particular not more than 2.2 nm, and moreover not more than
2.0 nm. When the residual stress value is too large, there are
caused such faults that a pattern deviates at the time of cutting
the glass substrate; that in an application of liquid crystal
display substrate, a homogenous image is not obtained due to
birefringence; and the like.
[0061] In the glass substrate obtained by the present invention, by
adequately regulating the first annealing temperature region, it is
possible to regulate a warpage value to not more than 200 .mu.m, in
particular not more than 100 .mu.m, and moreover not more than 80
.mu.m. When the warpage value is too large, it is difficult to
conduct accurate patterning of an electrode, etc., and faults such
as disconnection or short circuit of a circuit electrode, etc. are
easily caused.
[0062] In the glass substrate obtained by the present invention, a
thermal shrinkage ratio at the time of heating from ordinary
temperature at a rate of 5.degree. C./min, keeping at 450.degree.
C. for 10 hours, and then cooling at a rate of 5.degree. C./min is
easy to become less than 300 ppm. Since it is preferable that the
thermal shrinkage ratio of the glass is smaller, by adequately
regulating the second annealing temperature region, it is possible
to control the thermal shrinkage ratio of glass to not more than
250 ppm, moreover not more than 200 ppm, and in particular not more
than 100 ppm. When the thermal shrinkage ratio is too large, in the
case where the glass substrate is used as a substrate for forming a
thin-film electric circuit, the circuit pattern deviates from the
expected design, and electric performances cannot be
maintained.
[0063] In the glass substrate obtained by the present invention, by
forming a glass substrate according to the overflow down draw
method and omitting the polishing step, it is possible to regulate
an average surface roughness Ra to not more than 0.3 nm, in
particular not more than 0.2 nm. Incidentally, the average surface
roughness of a glass to which polishing is applied exceeds 0.3
nm.
[0064] Next, the glass substrate of the present invention is
described.
[0065] Various characteristic features of the glass substrate of
the present invention, such as sheet thickness, sheet thickness
difference, distortion value, warpage value, thermal shrinkage
ratio, surface roughness, composition, etc., are those as already
described, and a description thereof is omitted herein. In
addition, the glass substrate of the present invention can be
produced according to the method of the present invention as
described above.
[0066] Incidentally, in the glass sheet of the present invention,
its sheet width is not particularly limited. The sheet width can be
varied by regulating the length of a slot or the like from which a
glass is drawn out in the case of the slot down draw method, or by
regulating the length of a forming body or the like in the case of
the overflow down draw method.
[0067] The glass substrate of the present invention can be used for
various applications. For example, the glass substrate of the
present invention can be used as a glass substrate on which a
thin-film electric circuit is formed. Since the glass substrate of
the present invention has a uniform sheet thickness and has a small
residual stress value or warpage value, the quality required for a
substrate on which a thin-film electric circuit is formed can be
satisfied. Furthermore, when the thermal shrinkage ratio is made
small, the substrate hardly causes thermal shrinkage by the heat
treatment in the forming step of a thin-film electric circuit, and
problems such as a displacement of the circuit pattern, etc. can be
easily avoided.
[0068] In addition, it is preferable to use the glass substrate of
the present invention as a substrate for a flexible display. In
view of the fact that the glass substrate of the present invention
has a small sheet thickness, it has flexibility, and suppleness
necessary as a flexible display substrate can be obtained.
EXAMPLES
[0069] The present invention is hereunder described in detail by
reference to the accompanying drawings.
[0070] FIG. 1 is an outline front view showing production equipment
for a glass substrate in carrying out the present invention. The
production equipment is for producing a glass substrate according
to an overflow down draw method, and it includes a forming furnace
1 having a trough-shaped forming body 11 and cooling rollers 12
therein in this order from the top thereof; an annealing furnace 2
disposed in a lower portion of the forming furnace 1 and having
heaters 21 and guide rollers 22 therein; and a cooling section 3
and a cutting section 4 provided in a lower portion of the
annealing furnace 2.
[0071] The trough-shaped forming body 11 has a nearly wedge-shaped
cross section and allows a molten glass G1 to be fed to overflow
out from the top thereof and fuse at the bottom thereof to form a
glass ribbon G2. The annealing furnace 2 anneals the glass ribbon
G2. In detail, in the inside of the annealing furnace 2, a plural
number of the panel heaters 21 are provided at the both side of the
glass ribbon G2 facing to the glass ribbon G2. The heaters 21 are
disposed in plural series and in plural rows in the conveyance
direction (vertical direction) and in the sheet width direction
(horizontal direction), and the temperature thereof can be
independently controlled. The cooling section 3 thoroughly cools
the annealed glass ribbon G2. The cutting section 4 cuts the cooled
glass ribbon G2 into a prescribed dimension. In addition, in the
cutting section 4, a conveyance route for conveying a glass
substrate G3 into a non-illustrated subsequent step (for example, a
packing step, etc.) is separately provided.
[0072] Next, the production method for a glass substrate of the
present invention using the foregoing production equipment is
described.
[0073] In this production equipment, first of all, the molten glass
G1 is fed to the top of the trough-shaped forming body 11 provided
within the forming furnace 1, and the molten glass G1 is then
allowed to overflow out from the top of the trough-shaped forming
body 11 and fuse at the bottom thereof to form the glass ribbon G2
in a sheet form. Around the trough-shaped forming body 11, a pair
of the cooling rollers 12 is provided. In view of the fact that the
glass ribbon G2 is sandwiched between the cooling rollers 12 at its
both edges, its both ends are cooled, so that the shrinkage in the
width direction is minimized.
[0074] Next, the formed glass ribbon G2 is annealed in the
annealing furnace 2 to reduce the thermal shrinkage ratio thereof
In the annealing furnace 2, plural pairs of the guide rollers 22
are disposed in the vertical direction and grasp the glass ribbon
G2 to guide it downward. In addition, the inside of the annealing
furnace 2 is sectioned into a first annealing zone 231
corresponding to the first annealing temperature region (from the
(annealing point +200.degree. C.) to the (annealing point
+50.degree. C.)), an intermediate annealing zone 232 corresponding
to the intermediate annealing temperature region (from the
(annealing point +50.degree. C.) to the annealing point), and a
second annealing zone 233 corresponding to the second annealing
temperature region (from the annealing point to the (annealing
point -100.degree. C.)), and an output of each heater 21 is
controlled such that the cooling rate in every zone differs from
each other.
[0075] In the cooling section 3 provided in a lower portion of the
annealing furnace 2, the glass ribbon G2 is cooled to substantially
room temperature by means of natural cooling.
[0076] In the cutting section 4 provided just below the cooling
section 3, the glass ribbon cooled to the vicinity of room
temperature is cut into the glass sheet G3 having a prescribed
dimension and conveyed into the subsequent step.
[0077] Using the foregoing production equipment, a glass substrate
having a composition containing 60% of SiO.sub.2, 15% of
Al.sub.2O.sub.3, 10% of B.sub.2O.sub.3, 8% of CaO, 5% of SrO, and
2% of BaO in terms of percentage by mass and having a size of 500
mm.times.650 mm.times.100 .mu.m in thickness (annealing point:
705.degree. C., strain point: 655.degree. C.) was produced under
two kinds of annealing conditions. The annealing condition (average
cooling rate), the fictive temperature, the thermal shrinkage
ratio, the average surface roughness Ra, the sheet thickness
difference, the residual stress value, and the warpage value are
shown in Table 1. Incidentally, in producing the foregoing glass,
the respective zones were set such that the first annealing
temperature region was from 905 to 755.degree. C., the intermediate
annealing temperature region was from 755 to 705.degree. C., and
the second annealing temperature region was 705 to 605.degree.
C.
[0078] Incidentally, the average cooling rate was computed based on
the temperature of the glass measured with a pyrometer.
TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Annealing
condition (.degree. C./min) First annealing zone 750 750 420
Intermediate annealing zone 150 530 380 Second annealing zone 120
250 120 Fictive temperature (.degree. C.) 760 760 750 Average
surface roughness Ra (nm) 0.2 0.2 0.2 Sheet thickness difference
(nm) 15 15 20 Residual stress value (nm) 1.5 1.5 1.3 Warpage
(.mu.m) 70 70 60 Thermal shrinkage ratio (ppm) 40 50 40
[0079] As is clear from the table, it is noted that when the
average cooling rate in the first annealing temperature region is
high, the sheet thickness difference becomes small, and when the
average cooling rate in the second annealing temperature region is
low, the thermal shrinkage ratio becomes small. In addition, in
Examples 1 and 3, 100 pm-thick glass substrates having excellent
surface quality and a thermal shrinkage ratio of 40 ppm were
obtained.
[0080] Incidentally, the strain point and the annealing point were
measured based on the method according to ASTM C336-71.
[0081] The fictive temperature was determined as follows. First of
all, the same glass piece as that in the foregoing thermal
shrinkage determination was put into an electric furnace controlled
at 705.degree. C., and after one hour, the glass piece was taken
out of the electric furnace and rapidly cooled on an aluminum
plate, followed by measuring a thermal shrinkage ratio thereof. The
same treatment was carried out at 725.degree. C., 745.degree. C.,
and 765.degree. C., respectively, and a graph of a relationship
between the treatment temperature and the thermal shrinkage ratio
was prepared. A heat treatment temperature at which the thermal
shrinkage ratio was 0 ppm was determined from a linear approximate
curve of this graph, and this was defined as the fictive
temperature of glass.
[0082] The average surface roughness Ra was measured according to a
method in conformity with the "FPD Glass Substrate Surface
Roughness Measurement Method" in SEMI D7-94.
[0083] The residual stress value was measured using a stress meter,
manufactured by Uniopt Corporation, Ltd. according to an optical
heterodyne method.
[0084] The warpage value was measured as follows. That is, a sample
having a size of 550 mm.times.650 mm, as cut out from the center
portion of the glass substrate, was measured with a glass substrate
warpage measurement system, manufactured by Toshiba
Corporation.
[0085] A value obtained by measuring thickness variation along an
arbitrary line across a glass substrate using a laser type
thickness measuring device, determining a maximum thickness and a
minimum thickness of the glass substrate, and then subtracting a
value of the minimum sheet thickness from a value of the maximum
sheet thickness was defined as the sheet thickness difference.
[0086] The thermal shrinkage ratio was determined as follows. As
shown in FIG. 2(a), linear markings were given to predetermined
sites of the glass sheet G3; and thereafter, as shown in FIG. 2(b),
this glass sheet G3 was broken vertically to markings M and divided
into two glass sheet pieces G31 and G32. Then, only one glass sheet
piece G31 was subjected to a predetermined heat treatment (heating
from ordinary temperature at a rate of 5.degree. C./min, keeping at
450.degree. C. for a holding time of 10 hours, and then cooling at
a rate of 5.degree. C./min). Thereafter, as shown in FIG. 2(c), the
heat-treated glass sheet piece G31 and the untreated glass sheet
G32 were put in parallel, the both were fixed with an adhesive tape
T, and a marking displacement was determined. The thermal shrinkage
ratio was calculated according to the following numerical formula
1.
S = .DELTA. l 1 ( m ) + .DELTA. l 2 ( m ) l 0 ( mm ) .times. 10 3 (
ppm ) Numerical Formula 1 ##EQU00001##
[0087] While the present invention has been described in detail and
with reference to specific embodiments thereof, it will be apparent
to one skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope
thereof
[0088] Incidentally, the present application is based on a Japanese
patent application filed on Mar. 23, 2010 (Japanese Patent
Application No. 2010-65568) and a Japanese patent application filed
on Mar. 8, 2011 (Japanese Patent Application No. 2011-49763), the
entire contents of which are incorporated herein by reference. All
references cited herein are incorporated in their entirety.
INDUSTRIAL APPLICABILITY
[0089] The glass sheet produced according to the method of the
present invention is suitable as a substrate for flat panel
displays which are required to achieve thinning and weight
reduction, such as liquid crystal displays, organic EL displays,
etc., and a substrate for displays which are required to have
flexibility. Furthermore, the present invention can be used for new
display applications requiring a thin-film electric circuit, such
as electronic paper, digital signage, etc.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
[0090] 1: Forming furnace
[0091] 11: Trough-shaped forming body
[0092] 12: Cooling roller
[0093] 2: Annealing furnace
[0094] 21: Heater
[0095] 22: Guide roller
[0096] 231: First annealing zone
[0097] 232: Intermediate annealing zone
[0098] 233: Second annealing zone
[0099] 3: Cooling section
[0100] 4: Cutting section
[0101] G1: Molten glass
[0102] G2: Glass ribbon
[0103] G3: Glass sheet
[0104] G31, G32: Glass sheet piece
[0105] M: Marking
[0106] T: Tape
* * * * *