Method for heat-treating a glass substrate

In each of the second and subsequent heat treatments, in a case where a glass substrate shrinks by heat treatment at the same average cooling rate as the average cooling rate in the immediately preceding heat treatment, the average cooling rate is set to be larger than that in the immediately preceding heat treatment, and inversely, in a case where the glass substrate extends, the average cooling rate is set to be smaller than that in the immediately preceding heat treatment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for heat treatment of a glass 
substrate suitable for the production of a display panel such as a plasma 
display panel or a liquid crystal display panel. 
2. Prior Art 
The production of a plasma display panel (hereinafter referred to as PDP) 
includes a step for laminating or forming a laminate such as an electrode 
material, a dielectric material, a diaphragm or a fluorescent material on 
a glass substrate by means of screen printing or photolithographic 
technique. In such a forming step, it is usually necessary to heat-treat 
each laminate at a temperature of from 500.degree. to 600.degree. C. in 
order to dry and sinter the material laminated on the substrate. Namely, 
the substrate will be subjected to heating and cooling repeatedly. 
It is generally known that glass undergoes a change in the specific volume 
by heat treatment. Due to such a dimensional change of glass, deviation of 
the laminated material from the pattern is likely to result, the operation 
efficiency for bonding the front substrate and the rear substrate tends to 
deteriorate, or a cross talk is likely to take place in the display. 
Heretofore, in order to suppress extension or shrinkage of glass during 
such heat treatment, it has been common to use a glass having high strain 
point, or to stabilize extension or shrinkage of glass by pre-heat 
treatment i.e. by preliminarily heat treating the glass in a pattern close 
to the temperature conditions for heat treatment. However, even when a 
glass having a high strain point is used, such extensibility or shrinkage 
of glass can not completely be removed. The pre-heat treatment is 
effective to some extent when the heat treatment step is only once during 
the production of a panel, but when heat treatment is repeated a plurality 
of times, it is difficult to suppress extension or shrinkage of glass in 
the second and subsequent heat treatments, since the degree of 
extensibility or shrinkage differs in every heat treatment. 
On the other hand, in recent years, along with the progress in 
sophistication of picture elements of a display, an improvement in 
precision of a film-forming technique is required, and an improvement of 
dimensional stability during heat treatment of glass has been increasingly 
important. 
The present invention is intended to solve the above-mentioned problems of 
the prior art and to present a technical means to control extensibility 
and shrinkage of glass during heat treatment of a glass substrate. 
SUMMARY OF THE INVENTION 
The present invention has been made to solve the above problems, and it is 
a method for heat treatment of a glass substrate, which comprises heat 
treating a glass substrate a plurality of times, wherein in each of the 
second and subsequent heat treatments, the cooling rate for cooling from a 
temperature maintained for a predetermined period of time for heat 
treatment is differentiated from the cooling rate in the immediately 
preceding heat treatment so as to control the absolute value for the 
degree of extensibility or shrinkage of the glass substrate after the heat 
treatment. 
One embodiment of the present invention is directed to a method for heat 
treatment of a glass substrate, which comprises heat treating a glass 
substrate a plurality of times at a temperature exceeding 400.degree. C., 
wherein in each of the second and subsequent heat treatments, the average 
cooling rate for cooling to 400.degree. C. from a temperature maintained 
for a predetermined period of time for heat treatment (hereinafter 
referred to as "the average cooling rate to 400.degree. C.") is set so 
that the absolute value for the degree of extensibility or shrinkage of 
the glass substrate becomes smaller than in a case of heat treatment at 
the same rate as the average cooling rate to 400.degree. C. in the 
immediately preceding heat treatment. 
In particular, in each of the second and subsequent heat treatments, in a 
case where the glass substrate shrinks by heat treatment at the same 
average cooling rate to 400.degree. C. as the average cooling rate to 
400.degree. C. in the immediately preceding heat treatment, the average 
cooling rate to 400.degree. C. is set to be larger than that in the 
immediately preceding heat treatment, and in a case where the glass 
substrate extends by heat treatment at the same average cooling rate to 
400.degree. C. as the average cooling rate to 400.degree. C. in the 
immediately preceding heat treatment, the average cooling rate to 
400.degree. C. is set to be smaller than that in the immediately preceding 
heat treatment. 
Further, another embodiment of the present invention is directed to a 
method for heat treatment of a glass substrate, which comprises heat 
treating a glass substrate a plurality of times at a temperature exceeding 
300.degree. C., wherein in each of the second and subsequent heat 
treatments, the average cooling rate for cooling to 300.degree. C. from a 
temperature maintained for a predetermined period of time for heat 
treatment (hereinafter referred to as "the average cooling rate to 
300.degree. C.") is set so that the absolute value for the degree of 
extensibility or shrinkage of the glass substrate becomes smaller than in 
a case of heat treatment at the same rate as the average cooling rate to 
300.degree. C. in the immediately preceding heat treatment. 
In particular, in each of the second and subsequent heat treatments, in a 
case where the glass substrate shrinks by heat treatment at the same 
average cooling rate to 300.degree. C. as the average cooling rate to 
300.degree. C. in the immediately preceding heat treatment, the average 
cooling rate to 300.degree. C. is set to be larger than that in the 
immediately preceding heat treatment, and in a case where the glass 
substrate extends by heat treatment at the same average cooling rate to 
300.degree. C. as the average cooling rate to 300.degree. C. in the 
immediately preceding heat treatment, the average cooling rate to 
300.degree. C. is set to be smaller than that in the immediately preceding 
heat treatment. 
According to the present invention, the degree of extensibility or 
shrinkage of the glass substrate is controlled by the cooling rate in the 
heat treatment (i.e. in the cycle of the temperature raising-temperature 
maintaining-cooling), whereby the degree of extensibility or shrinkage of 
the glass substrate in each heat treatment step can be suppressed to 
minimum even when heat treatment is repeated a plurality of times. 
Here, as the cooling rate, the average cooling rate for cooling to 
400.degree. C. from a temperature maintained for heat treatment (i.e. an 
average cooling rate to 400.degree. C.) is important in a case where heat 
treatment is carried out at a relatively high temperature (for example, a 
temperature exceeding 400.degree. C., particularly 500.degree. C.) as in 
the case of producing PDP or polysilicon TFT. On the other hand, the 
cooling rate at a temperature of lower than 400.degree. C. is less 
influential over the degree of extensibility or shrinkage of the glass 
substrate. 
On the other hand, in an application where it is required to minimize the 
absolute value for the degree of extensibility or shrinkage of a substrate 
during heat treatment, even if heat treatment is carried out at a 
relatively low temperature (for example, from 300.degree. to 450.degree. 
C.) as in the case of producing a liquid crystal display panel other than 
polysilicon TFT, the average cooling rate for cooling to room temperature 
from a temperature maintained for heat treatment (i.e. the average cooling 
rate to room temperature) becomes important. 
Further, in such an application where it is required to minimize the 
absolute value for the degree of extensibility or shrinkage of a glass 
substrate during required heat treatment, even if heat treatment is 
carried out at a low temperature (at most 400.degree. C.) as in the case 
of producing a liquid crystal display panel, the purpose of the present 
invention can be accomplished by carrying out an extreme operation such as 
placing the glass substrate of room temperature on an already heated hot 
plate or inversely dismounting it from a hot plate. 
In the present specification, the average cooling rate to 400.degree. C. 
and the average cooling rate to room temperature may generally be referred 
to simply as the average cooling rate. Further, the temperature maintained 
for heat treatment (the maximum temperature) will be referred to as the 
heat treating temperature. 
PREFERRED EMBODIMENTS 
As an embodiment of the present invention, a case will be described in 
which a glass substrate is heat treated twice under the same conditions 
with respect to the temperature raising rate, the heat treating 
temperature and the temperature maintaining period of time for heat 
treatment. 
Even if heat treatments are carried out while controlling the heat 
treatment temperature profile to be completely the same as in this case, 
it may often happen that, for example, while the glass substrate extends 
at a level of 5 ppm in the first heat treatment, the glass substrate 
shrinks at a level of 18 ppm in the next heat treatment. 
Therefore, in the present invention, in the second and subsequent heat 
treatments, the average cooling rate is adjusted. In this manner, it is 
possible to suppress the change in the degree of extensibility or 
shrinkage and the absolute degree of extensibility or shrinkage in one 
heat treatment. In many cases, it is possible to suppress the absolute 
value for the degree of extensibility or shrinkage by the second heat 
treatment to the same level as the absolute value for the degree of 
extensibility or shrinkage by the first heat treatment, by adjusting the 
average cooling rate in the second heat treatment to be larger than the 
average cooling rate in the first heat treatment. 
In general, in a case where the glass substrate shrinks by the second heat 
treatment conducted at the same cooling rate as in the first heat 
treatment, it is possible to suppress the degree of extensibility or 
shrinkage to an absolute value of a level equal to that in the first heat 
treatment by adjusting the average cooling rate in the second heat 
treatment to be larger than the first heat treatment, and inversely, in a 
case where the glass substrate extends by the second heat treatment at the 
same cooling rate as in the first heat treatment, it is possible to 
suppress the degree of expansibility or shrinkage to an absolute value of 
a level equal to that in the first heat treatment by adjusting the average 
cooling rate in the second heat treatment to be smaller than in the first 
heat treatment. 
The reason as to why the absolute value for the degree of extensibility or 
shrinkage of a glass substrate can be suppressed in a case where heat 
treatment is carried out a plurality of times as in the present invention, 
is considered to be as follows. The reason as to why glass shrinks 
substantially when treated in the second heat treatment with the same 
temperature profile as in the first heat treatment as in the above 
embodiment, is considered to be such that the glass has become close to an 
equilibrium state (the state where the density is highest) by the 
temperature rise during the heat treatment. Accordingly, in such a case, 
it may be cooled while maintaining the extended state of glass during the 
temperature rise to some extent. Namely, it is effective to adjust the 
average cooling rate in the second heat treatment to be larger than in the 
first heat treatment for suppressing the shrinkage of the glass. 
Inversely, the reason as to why the glass extends substantially when 
heat-treated in the second heat treatment with the same temperature 
profile as in the first heat treatment, is considered to be such that 
during the heat treatment, the glass has departed from the equilibrium 
state. In such a case, the average cooling rate in the second heat 
treatment may be made smaller than in the first heat treatment, so that 
the glass substrate will be close to the equilibrium state as far as 
possible. 
Namely, in each of the second and subsequent heat treatments, if the 
cooling rate for the heat treatment is set so that the stress in the glass 
substrate relaxed by the temperature rise and the temperature maintenance 
in the heat treatment, is balanced with the stress generated during 
cooling in the heat treatment, the degrees of extensibility or shrinkage 
of the substrate in both steps are canceled out, whereby the absolute 
value for the degree of extensibility or shrinkage of the glass substrate 
can be suppressed. 
For example, in a case where in a plurality of heat treatments, the same 
temperature profile including the temperature raising rate, the heat 
treating temperature, the temperature maintaining period of time and the 
cooling rate in each heat treating step, is employed, the glass approaches 
the equilibrium state at the time of every heat treatment. Accordingly, in 
order to make the degree of extensibility or shrinkage in each heat 
treatment constant, it is preferred to gradually increase the average 
cooling rate for every heat treatment. 
Further, in a case where the second heat treatment profile is substantially 
the same as in the first heat treatment, and only the heat treating 
temperature maintaining period of time is long, the glass will rapidly 
approach the equilibrium state. Accordingly, in order to make the degree 
of extensibility or shrinkage in each heat treatment constant, it is 
necessary to cool the glass substantially more quickly in the second heat 
treatment than in the first heat treatment. This average cooling rate will 
be larger than the average cooling rate required to control the absolute 
value for the degree of extensibility or shrinkage in a case where the 
heat treating temperature maintaining period of time is the same in the 
respective heat treatments. 
On the other hand, in a case where the heat treating temperature in the 
second heat treatment step is higher than the heat treating temperature in 
the first heat treatment step, if the same temperature raising rate and 
cooling rate are employed, by the second heat treatment, the glass will 
depart from the equilibrium state after the first heat treatment, and the 
glass may sometimes extend. The glass tends to extend more readily under 
an equilibrium condition at a high temperature than under an equilibrium 
condition at a lower temperature. In such a case, the average cooling rate 
in the second heat treatment may be made smaller than in the first heat 
treatment. 
In the present invention, the heat treating temperature and the heat 
treating temperature maintaining period of time for the heat treatment of 
a glass substrate, can optionally be determined depending upon the 
characteristics of the laminating material. Namely, after heat treatment 
under a condition suitable for the laminating material, the cooling rate 
is properly set, whereby the degree of extensibility or shrinkage of the 
glass substrate can be made small and constant in each heat treatment 
step. 
Further, the cooling rate of glass in each heat treatment is preferably 
controlled within a range of from 0.5.degree. to 10.degree. C./min. If the 
cooling rate is smaller than 0.5.degree. C./min, the production efficiency 
tends to be poor, and on the other hand, if it is larger than 10.degree. 
C./min, the possibility of heat crack or heat deformation due to the 
temperature distribution in the glass plane tends to be high. As mentioned 
above, if the temperature is lower than 400.degree. C., the influence of 
the cooling rate to the strain or the degree of extensibility or shrinkage 
of glass tends to be small. Accordingly, depending upon the degree of 
extensibility or shrinkage required, there may be a case where there will 
be no problem even if the cooling rate is outside the above-mentioned 
range. 
Further, it is very effective to subject the laminating material to 
pre-heat treatment once before lamination, in order to control the 
absolute value for the degree of extensibility or shrinkage of the glass 
substrate to be small in the first heat treatment step after lamination of 
the laminating material. Namely, the plurality of heat treatments in the 
present invention may include the pre-heat treatment prior to lamination 
of the laminating material. 
Further, in a case where it is not possible to adjust the degree of 
extensibility or shrinkage to a desired level within the range of from 
0.5.degree. to 10.degree. C./min of the average cooling rate, adjustment 
may sometimes be made by subjecting the glass substrate to heat treatment 
prior to forming the next laminating material. 
The furnace to be used in the heat treatment process of the present 
invention may, for example, be a batch-type furnace whereby a glass 
substrate is introduced into the furnace every time of heat treatment, and 
the temperature in the furnace is changed with time, or it may be a belt 
type furnace, a conveyor type furnace or a walking beam furnace of a type 
whereby a suitable temperature distribution is formed in the furnace, and 
a glass substrate is transported therein at a predetermined speed. 
Particularly preferred is a conveyor type furnace of a type wherein the 
conveyor is divided into a plurality of conveyors in the furnace, so that 
the time can independently be controlled in the temperature raising step, 
the heat treating step and the cooling step, whereby the cooling rate can 
simply be controlled. 
As the glass substrate to be used in the present invention, preferred is 
one, of which the thermal expansion coefficient agrees to a common 
laminating material to avoid heat crack or heat deformation in the heat 
treatment process. For example, in the production of PDP, preferred is one 
having a thermal expansion coefficient of from about 70.times.10.sup.-7 to 
90.times.10.sup.-7 /.degree.C. Further, for the same purpose, a glass 
having an annealing point higher than the maximum temperature during the 
heat treatment, is preferred. 
According to the present invention, at the time of heat treating a glass 
substrate a plurality of times, even if heat treatment is carried out at a 
relatively high temperature (such as a temperature exceeding 400.degree. 
C., particularly 500.degree. C.), the absolute value for the degree of 
extensibility or shrinkage of the glass substrate can be suppressed to a 
level of at most 50 ppm, particularly at most 20 ppm, in each heat 
treatment. Further, in a case where heat treatment is carried out at a 
relatively low temperature (for example, from 300.degree. to 450.degree. 
C.), the absolute value for the degree of extensibility or shrinkage of 
the glass substrate can be suppressed to a level of at most 10 ppm, 
particularly at most 5 ppm. 
Further, also the absolute value for the cumulative degree of extensibility 
or shrinkage of glass after laminating the laminating material, can be 
made to be at most 50 ppm, particularly at most 20 ppm. Further, in a case 
where the heat treatment is carried out at a relatively low temperature, 
it can be made to be at most 10 ppm, particularly at most 5 ppm. 
The process of the present invention can be applied not only to the 
production of a display panel such as PDP or a liquid crystal display 
panel, but also generally to a process wherein a glass substrate is 
required to be heat-treated a plurality of times with high precision. 
Further, the method of the present invention is preferably applied to all 
of the second and subsequent heat treatments. However, by applying it to 
at least one heat treatment, the effects of the present invention can be 
obtained in such heat treatment.

Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted by such specific Examples. 
EXAMPLES 1 to 7 
An aluminosilicate float glass having an annealing point of 620.degree. C. 
and a thermal expansion coefficient of 83.times.10.sup.-7 /.degree.C., 
which was once subjected to heat treatment, was used as a substrate glass. 
The size of this glass was preliminarily measured and then heat-treated in 
a belt-type heat treatment furnace, and the size after the heat treatment 
was measured again, whereupon the degree of extensibility or shrinkage 
(unit: ppm) was calculated by the following formula: 
EQU (size after the heat treatment-size before the heat treatment)/(size before 
the heat treatment) 
This operation was repeated four times, and every time, the degree of heat 
extensibility or shrinkage after the preceding heat treatment was 
obtained. Further, the temperature raising was carried out over 30 minutes 
from room temperature to the maximum temperature, and the retention time 
at the maximum temperature was 20 minutes, and natural cooling was adopted 
at a temperature of 400.degree. C. or lower. 
In Table 1, the heat treating temperature, the average cooling rate to 
400.degree. C. and the results of measurement of the degree of 
extensibility or shrinkage, are shown. Examples 1 to 5 represent cases 
wherein the heat treating temperature (the maximum temperature) was the 
same in the four heat treatments. 
As shown in Examples 1 and 2, by increasing the average cooling rate in 
every heat treatment, it was possible to control the absolute value for 
the degree of extensibility or shrinkage to a range of error of at most 5 
ppm in each stage. On the other hand, as shown in Examples 3 and 4, if 
cooling was carried out at the same rate, the results were the same as in 
Examples 1 and 2, respectively, in the first treatment, but thereafter, it 
was not possible to suppress the degree of shrinkage. Further, as shown in 
Example 5, if the cooling rate was increased uniformly, the degree of 
extension in the first heat treatment increased although the degree of 
shrinkage in the second and subsequent heat treatments decreased to some 
extent, and in all treatments, satisfactory results were not obtained. 
Further, Examples 6 and 7 represent cases wherein the heat treating 
temperature was gradually lowered in every heat treatment. In such a case, 
the effects of Example 6 wherein the cooling rate was adjusted, were 
remarkable as compared with Example 7 wherein the cooling rate was 
constant. Thus, it has been confirmed that the method of the present 
invention is effective also in a case where the heat treating temperature 
is changed in each heat treatment. 
Thus, by properly selecting the heat treating temperature and the average 
cooling rate suitable for the particular retention time, etc., it is 
possible to minimize non-uniformity in the degree for extensibility or 
shrinkage of glass and to control the absolute value thereof. 
TABLE 1 
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Example 1 2 3 4 5 6 7 
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Heat treating 
1st 580 560 580 560 580 600 600 
temp. (.degree.C.) 
2nd 580 560 580 560 580 580 580 
3rd 580 560 580 560 580 560 560 
4th 580 560 580 560 580 540 540 
Average 1st 1.5 2 1.5 2 4 1 1 
cooling rate 
2nd 2.5 4 1.5 2 4 2 1 
(.degree.C./min) 
3rd 4 6 1.5 2 4 5 1 
4th 6 8 1.5 2 4 10 1 
Degree of 
1st 4 5 4 5 58 -7 -7 
extensibility or 
2nd 2 4 -27 -18 -18 -7 -52 
shrinkage 
3rd 4 -3 -23 -17 -18 -9 -40 
(ppm) 4th 2 -5 -19 -15 -17 -18 -28 
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As described in the foregoing, the present invention is an extremely 
effective method whereby the degree of extensibility or shrinkage of glass 
can be controlled when heat treatment is carried out a plurality of times. 
That is, while the dimensional stability of a substrate has become 
important along with the progress in sophistication, trend for high 
density picture elements and trend for a large area, of a display such as 
PDP, it is possible to accomplish improvements in the quality and the 
production efficiency by the method of the present invention. 
Further, the present invention is applicable not only to the production of 
a display but also to other production process wherein a heat treating 
process at a high temperature is required.