U.S. patent application number 11/902165 was filed with the patent office on 2008-02-28 for image display device and evaluating method of glass substrate for use in it.
Invention is credited to Daisuke Adachi, Keisuke Sumida, Hiroyasu Tsuji.
Application Number | 20080049305 11/902165 |
Document ID | / |
Family ID | 32462875 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080049305 |
Kind Code |
A1 |
Adachi; Daisuke ; et
al. |
February 28, 2008 |
Image display device and evaluating method of glass substrate for
use in it
Abstract
The present invention provides an image display device capable
of displaying a good image by suppressing yellowing of a glass
substrate, and an evaluating method of the glass substrate. The
image display device is formed using the glass substrate where
reflectance at wavelength of 220 nm is 5% or lower. In the
evaluating method of the glass substrate for the image display
device, Sn.sup.++ content in the glass substrate is analyzed using
reflectance at wavelength of 220 nm.
Inventors: |
Adachi; Daisuke;
(Kameoka-shi, JP) ; Tsuji; Hiroyasu; (Sennan-gun,
JP) ; Sumida; Keisuke; (Hirakata-shi, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK L.L.P.
2033 K. STREET, NW
SUITE 800
WASHINGTON
DC
20006
US
|
Family ID: |
32462875 |
Appl. No.: |
11/902165 |
Filed: |
September 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10499339 |
Jun 17, 2004 |
|
|
|
PCT/JP2003/015123 |
Nov 27, 2003 |
|
|
|
11902165 |
Sep 19, 2007 |
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Current U.S.
Class: |
359/350 |
Current CPC
Class: |
H01J 11/12 20130101;
H01J 11/44 20130101; H01J 2211/442 20130101; Y10S 359/90 20130101;
H01J 11/34 20130101 |
Class at
Publication: |
359/350 |
International
Class: |
G02B 1/00 20060101
G02B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2002 |
JP |
2002-347188 |
Claims
1-5. (canceled)
6. An image display device employing a glass substrate in which a
mean reflectance at wavelength of 200 to 250 nm is 5% or lower.
7-11. (canceled)
12. An evaluating method of a glass substrate for an image display
device, wherein Sn.sup.++ content in the glass substrate is
analyzed using a mean reflectance at wavelength of 200 to 250 nm.
Description
[0001] This application is a divisional of application Ser. No.
10/499,339, which is the National Stage of International
Application No. PCT/JP2003/015123, filed Nov. 27, 2003.
TECHNICAL FIELD
[0002] The present invention relates to an image display device
such as a plasma display panel (PDP) and an evaluating method of a
glass substrate for use in it.
BACKGROUND ART
[0003] There are various types of image display devices for
displaying a high definition television image on a large screen.
PDPs belong to one of the various types. A PDP is hereinafter
described as an example.
[0004] A PDP is formed of two glass substrates: a front-side glass
substrate for displaying an image; and a back-side glass substrate
facing the front-side glass substrate. The front-side glass
substrate has the following elements: [0005] a display electrode
that is formed on one principal surface thereof and includes a
stripe-like transparent electrode and a bus electrode; [0006] a
dielectric film that covers the display electrode and works as a
capacitor; and [0007] a MgO protective layer formed on the
dielectric film. While, the back-side glass substrate has the
following elements: [0008] a stripe-like address electrode formed
on one principal surface thereof; [0009] a dielectric film for
covering the address electrode; [0010] barrier ribs formed on the
dielectric film; and [0011] phosphor layers that are formed between
the barrier ribs and emit red light, green light, and blue light,
respectively.
[0012] As the front-side glass substrate and the back-side glass
substrate, glass substrates that are easily increased in area, have
high flatness, are inexpensive, and are manufactured by a float
method are used. These glass substrates are disclosed in Electronic
Journal, Separate Volume "2001, FPD Technology Summa" (Electronic
Journal Co. Ltd. Oct. 25, 2000, p706-p707).
[0013] The float method is a method of forming plate-shaped glass
by floating and conveying molten glass onto molten metallic tin
under reducing atmosphere. An inexpensive glass sheet having large
area can be precisely manufactured in the float method, so that the
float method is in widespread use in manufacturing of a window
glass or the like.
[0014] When an Ag electrode made of silver material is formed on a
float glass substrate manufactured by the float method, however, a
colored layer is disadvantageously formed on the surface of the
glass substrate and the glass substrate changes into yellow
(yellows).
[0015] This coloring phenomenon of the glass substrate by the Ag
electrode is caused by the following processes: [0016] a silver
colloid is generated by oxidation-reduction reaction between
reducing bivalent tin ions (Sn.sup.++) existing on the glass
substrate and silver ions (Ag.sup.+); and [0017] light absorption
therefore occurs near wavelength of 350 to 450 nm.
[0018] In other words, the glass substrate is exposed to the
reducing atmosphere containing hydrogen in a molding process in a
float furnace as a molten metallic tin bath. A reducing layer with
a thickness of several .mu.m containing tin ions (Sn.sup.++) of the
molten tin (Sn) is generated on the surface of the glass substrate.
When a bus electrode including an Ag electrode is formed on the
glass substrate having the reducing layer on its surface, silver
ions (Ag.sup.+) separate from the bus electrode, and infiltrate
into the glass due to ion exchange with alkali metal ions contained
in the glass. The infiltrating silver ions (Ag.sup.+) are reduced
by the tin ions (Sn.sup.++) existing in the reducing layer to
generate metallic silver (Ag) colloid. The metallic silver (Ag)
colloid yellows the glass substrate. The yellowing occurs also on
the front-side glass substrate having the bus electrode on the
transparent electrode.
[0019] When the glass substrate, especially the front-side glass
substrate, yellows, the yellowing is fatal in the image display
device. Due to the yellowing of the glass substrate, the panel
looks yellow, the commercial value decreases, display brightness of
blue decreases to change display chromaticity, and, color
temperature decreases to degrade picture quality especially in
displaying white.
[0020] These problems occur not only in a PDP but also in a general
image display device having a structure where an Ag electrode is
formed on a glass substrate.
[0021] The present invention addresses the problems, provides an
image display device allowing good image display by suppressing
yellowing of the glass substrate, and provides an evaluating method
of the glass substrate for use in the image display device.
DISCLOSURE OF THE INVENTION
[0022] An image display device of the present invention, for
addressing the problems, employs a glass substrate of which
reflectance at a wavelength of 220 nm is 5% or lower.
[0023] Thanks to this structure, even in the image display device
where an electrode made of Ag material is formed on the glass
substrate manufactured by the float method, the glass substrate
does not yellow and the image display quality is high.
[0024] In the evaluating method of the glass substrate for the
image display device of the present invention, content of Sn.sup.++
in the glass substrate is analyzed based on the reflectance at
wavelength of 220 nm. In providing an image display device by
forming an electrode made of Ag material on a glass substrate
manufactured by the float method, this evaluating method allows
easy and efficient selection of a glass substrate that does not
yellow and provision of a glass substrate optimum for an image
display device having high image display quality.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a sectional and perspective view showing a
schematic structure of a PDP as an image display device in
accordance with an exemplary embodiment of the present
invention.
[0026] FIG. 2 is a graph showing a relation between a surface
removing amount and a reflection spectrum of a glass substrate
manufactured by a float method.
[0027] FIG. 3 is a graph showing a relation between reflectance at
the wavelength of 220 nm and coloring degree of glass.
[0028] FIG. 4 is a graph showing difference .DELTA.R between
reflection spectrum R.sub.S(.lamda.) of the glass substrate and
reflection spectrum R.sub.B(.lamda.) in a nonexistent state of
Sn.sup.++.
[0029] FIG. 5 is a graph showing an analyzing result of the
reflection spectrum of the glass substrate.
[0030] FIG. 6 is a graph illustrating wavelength .lamda.*
maximizing difference .DELTA.R between reflection spectrum
R.sub.S(.lamda.) of the glass substrate and reflection spectrum
R.sub.B(.lamda.) in a nonexistent state of Sn.sup.++.
[0031] FIG. 7 is a schematic block diagram of a manufacturing
apparatus of the glass substrate for the image display device in
accordance with the exemplary embodiment of the present
invention.
[0032] FIG. 8 is a schematic block diagram of another manufacturing
apparatus of the glass substrate for the image display device in
accordance with the exemplary embodiment.
[0033] FIG. 9 is a schematic block diagram of still another
manufacturing apparatus of the glass substrate for the image
display device in accordance with the exemplary embodiment.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0034] An exemplary embodiment of the present invention will be
described with reference to the drawings.
[0035] A PDP is hereinafter described as an example of image
display devices. However, the present invention is useful for an
image display device having a structure where an electrode made of
Ag material is disposed on a glass substrate that is manufactured
by the float method and has Sn.sup.++ on its surface.
[0036] FIG. 1 is a sectional and perspective view showing a
schematic structure of the PDP. PDP 1 is formed of two glass
substrates: front-side glass substrate 3 for displaying an image;
and back-side glass substrate 10 facing the front-side glass
substrate.
[0037] Front substrate 2 of PDP 1 has the following elements:
[0038] display electrodes 6 that are formed on one principal
surface of front-side glass substrate 3 and include scan electrode
4 and sustain electrode 5; [0039] dielectric layer 7 for covering
display electrodes 6; and [0040] protective layer 8, made of MgO
for example, for covering dielectric layer 7. In scan electrode 4
and sustain electrode 5, for decreasing electric resistance, bus
electrodes 4b and 5b made of Ag material are laminated on
transparent electrodes 4a and 5a, respectively.
[0041] Back substrate 9 has the following elements: [0042] address
electrodes 11, made of Ag material, formed on one principal surface
of back-side glass substrate 10; [0043] dielectric layer 12 for
covering address electrodes 11; [0044] barrier ribs 13 formed on
dielectric layer 12 at positions corresponding to clearances
between address electrodes 11; and [0045] phosphor layers 14R, 14G,
and 14B between barrier ribs 13.
[0046] Front substrate 2 faces back substrate 9 with barrier ribs
13 sandwiched so that display electrodes 6 are orthogonal to
address electrodes 11, and the outer periphery of the image display
region is sealed by a sealing member. Discharge spaces 15 formed
between front substrate 2 and back substrate 9 are filled with
discharge gas such as Ne--Xe 5% at pressure of 66.5 kPa (500
Torr).
[0047] Crossing parts between display electrodes 6 and address
electrodes 11 in discharge spaces 15 work as discharge cells 16
(unit light emitting regions).
[0048] As front-side glass substrate 3 and back-side glass
substrate 10, glass substrates that are easily increased in area,
have high flatness, are inexpensive, and are manufactured by a
float method are used.
[0049] In the structure discussed above, bus electrodes 4b and 5b
on front-side glass substrate 3 are formed of Ag electrodes. If
front-side glass substrate 3 contains Sn.sup.++, the glass
substrate yellows even when each of transparent electrodes 4a and
5a is interposed between each of bus electrodes 4b and 5b and glass
substrate 3. Depending on the degree of the yellowing, an image
display characteristic of the image display device is adversely
affected.
[0050] The glass substrate used as front-side glass substrate 3 of
PDP 1 is analyzed to determine Sn.sup.++ content on the surface
thereof on which bus electrodes 4b and 5b containing Ag are to be
formed. When the appearance quality is concerned, back-side glass
substrate 10 is also analyzed to determine Sn.sup.++ content
thereof on the surface on which address electrodes 11 containing Ag
are to be formed.
[0051] Specifically, reflectance of the glass substrate at the
wavelength of 220 nm is measured, and the analysis is performed
based on the reflectance. This method is provided based on
inventors' study. The inventors found the following facts: [0052]
the reflectance near the wavelength of 220 nm increases with
increase in Sn.sup.++ content on the glass substrate; and [0053]
there is a correlation between the reflectance near the wavelength
of 220 nm and coloring of the glass substrate by silver colloid.
Here, the reflectance may be measured by a general measuring
device.
[0054] The Sn.sup.++ content on the glass substrate is determined
by a secondary ion-mass spectrometry (SIMS) or an
inductively-coupled plasma (ICP) optical emission spectrometry. An
allowance of Sn.sup.++ content is determined based on a calibration
curve derived from the relation between the Sn.sup.++ content
determined by the spectrometry and the measured reflectance. The
allowance of Sn.sup.++ content can be therefore determined from the
reflectance without breaking the glass substrate.
[0055] In other words, firstly, the surface on the non-contact side
with tin (top surface) of the glass substrate manufactured by the
float method is uniformly removed by thickness of 3, 7, 15, or 20
.mu.m, and reflection spectrum of the remaining glass substrate is
measured at wavelength of 200 to 300 nm. The measurement result is
shown in FIG. 2. FIG. 2 also shows a measurement result of a glass
substrate without removal for comparison. The reason why the top
side surface is removed is as follows. Adhesion and diffusion
amounts of tin are less and hence the yellowing degree is lower on
the top-side surface than the bottom-side surface (contact side
with tin), so that a bus electrode made of Ag material is generally
formed on the top-side surface. When the Ag electrode is formed on
the bottom side, the coloring degree is two or three times higher
than that in the case that the Ag electrode is formed on the top
side.
[0056] FIG. 2 shows that, when removed thickness is 15 .mu.m or
less, reflectance at peak A near the wavelength of 220 nm decreases
with increase of the removed thickness. When removed thickness is
15 .mu.m or more, the decrease of the reflectance stops. It is
considered that Sn.sup.++ content monotonously decreases in the
depth direction from the top side of the glass substrate. The
result shown in FIG. 2 matches with the consideration, and hence
the decrease of the reflectance at peak A is considered to match
with the decrease of Sn.sup.++ content.
[0057] Next, for clarifying a relation between the peak near the
wavelength of 220 nm appearing in the reflection spectrum and
yellowing of the glass substrate, an Ag electrode is formed on the
glass substrate and coloring degree of the glass substrate is
measured. In other words, 5 .mu.m thick silver paste as the Ag
electrode is applied onto the glass substrate by screen printing,
they are calcined at 600.degree. C., and a relation between the
coloring degree of the glass substrate and reflectance at the
wavelength of 220 nm is investigated. FIG. 3 shows the
investigation result. The coloring degree of the glass substrate is
evaluated using b* in a L*a*b* color system (JIS Z 8729). The
larger b* value is, the higher the degree of yellowing is. The
coloring degree of the glass substrate is measured from the side
having no Ag electrode. As shown in FIG. 3, the reflectance of
light at the wavelength of 220 nm on the glass substrate and the
coloring degree b* of the glass substrate are directly
proportional.
[0058] The investigation result discussed above shows that increase
of the reflectance of the glass substrate at the wavelength of 220
nm has a correlation to the Sn.sup.++ content in the glass
substrate, namely content of reducing material at least causing
yellowing. Therefore, by measuring the reflectance at the
wavelength of 220 nm, the Sn.sup.++ content in the glass substrate
on which the Ag electrode is to be formed can be analyzed based on
the calibration curve, and the degree of yellowing of the glass
substrate can be also estimated. This method is useful for
evaluating whether or not a selected glass substrate is optimum for
an image display device.
[0059] In FIG. 2, after 15 .mu.m or more thick glass substrate is
removed, the reflectance (about 2%) near the wavelength of 220 nm
is not resulted from existence of Sn.sup.++ but by bottom part of a
reflection spectrum having a peak at another wavelength. The stop
of the decrease of the reflectance at the wavelength of 220 nm is
considered to be caused by decrease of the Sn.sup.++ content in the
glass substrate. FIG. 4 shows difference
.DELTA.R(.lamda.)=R.sub.S(.lamda.)-R.sub.B(.lamda.). Here,
R.sub.S(.lamda.) is a reflection spectrum of the glass substrate
shown in FIG. 2, and R.sub.B(.lamda.) is a reflection spectrum in a
nonexistent state of Sn.sup.++, namely in a state where the removal
of 15 .mu.m or more thick glass substrate stops the decrease of the
reflectance. Reflection difference .DELTA.R is considered to
indicate the existence of Sn.sup.++.
[0060] The reflectance at the wavelength of 220 nm may be read from
a reflection spectrum distribution as shown in FIG. 2. However, for
more precisely evaluating signal strength of the reflection
spectrum having a correlation to Sn.sup.++, the following method
can be used. Reflection spectrum is measured in a wider range of
wavelength, for example 180 to 280 nm. The measured reflection
spectrum is then divided into two Gaussian peaks of a component
having a correlation to Sn.sup.++ and a component having no
correlation to Sn.sup.++ shown in FIG. 5 by a curve fitting method
using M .times. .times. 1 .times. exp .times. { - ( 1240 / .lamda.
.times. - 1240 / M .times. .times. 2 ) 2 M .times. .times. 3 2 ) +
M .times. .times. 4 .times. exp .times. { - ( 1240 / .lamda. - 1240
/ M .times. .times. 5 ) 2 M .times. .times. 6 2 } . ( Eq . .times.
1 ) ##EQU1## Where, .lamda. is a wavelength (nm), and M1 to M6 are
fitting parameters.
[0061] The lower limit of the measured wavelength range is set at
180 nm because oxygen in atmospheric air absorbs light at a
wavelength lower than 180 nm, hence vacuum or atmosphere containing
no oxygen is required for measurement, and construction of a
measuring system and measurement require much effort.
[0062] This method is also useful for evaluating whether or not a
selected glass substrate is optimum for an image display
device.
[0063] The position of the wavelength of the peak of the
reflectance caused by Sn.sup.++ can slightly change depending on
the manufacturing condition and the composition of the glass
substrate. Therefore, for increasing analysis accuracy of
Sn.sup.++, it is more effective to analyze not only the reflectance
at wavelength of 220 nm but also the bottom part of the reflectance
extending to wider wavelength range, for example 200 to 250 nm.
[0064] Specifically, wavelength .lamda.* maximizing difference
.DELTA.R(.lamda.)=R.sub.S(.lamda.)-R.sub.B(.lamda.) in a wavelength
range of 200 to 250 nm is considered to indicate the existence of
Sn.sup.++, as shown in FIG. 6. Here, R.sub.S(.lamda.) is a
reflection spectrum of the glass substrate, and R.sub.B(.lamda.) is
a reflection spectrum in a nonexistent state of Sn.sup.++. In FIG.
6, wavelength .lamda.* maximizing .DELTA.R(.lamda.) is additionally
written in FIG. 4. Sn.sup.++ content in the glass substrate is
analyzed based on the reflectance R.sub.S(.lamda.*) at wavelength
.lamda.* or reflectance difference
.DELTA.R(.lamda.*)=R.sub.S(.lamda.*)-R.sub.B(.lamda.*).
[0065] The reflectance difference
.DELTA.R(.lamda.*)=R.sub.S(.lamda.*)-R.sub.B(.lamda.*) means the
maximum value of
.DELTA.R(.lamda.)=R.sub.S(.lamda.)-R.sub.B(.lamda.). Here,
R.sub.S(.lamda.) is a reflection spectrum of the glass substrate at
wavelength of 200 to 250 nm, and R.sub.B(.lamda.) is a reflection
spectrum in a nonexistent state of Sn.sup.++.
[0066] Sn.sup.++ locally exists only in a region from the outermost
surface of the glass substrate to depth of about 15 .mu.m, as shown
in FIG. 2. Therefore, reflection spectrum on the glass substrate of
which top part having thickness of 15 .mu.m or more, preferably 20
.mu.m or more, is removed can be set as reflection spectrum
R.sub.B(.lamda.) in the nonexistent state of Sn.sup.++.
[0067] Another specific method of analyzing reflection spectrum
also including the extending bottom part of the reflection spectrum
is provided as follows. A mean reflectance is determined from area
integral of reflection spectrum at the wavelength of 200 to 250 nm,
for example, and Sn.sup.++ content is analyzed.
[0068] Either of the methods discussed above is useful for
evaluating whether or not a selected glass substrate is optimum for
an image display device.
[0069] A judgment standard for the analysis result of the Sn.sup.++
content on the surface of the glass substrate on which the Ag
electrode is to be formed is described hereinafter.
[0070] Existence of Sn.sup.++ reduces Ag.sup.+ of the Ag electrode
to generate Ag colloid, and the glass substrate yellows. The
coloring (yellowing) degree of the glass substrate is determined
based on the Sn.sup.++ content, so that an allowance of the
Sn.sup.++ content is a judgment standard when the glass substrate
is used for an image display device.
[0071] As shown in the result of FIG. 2, for preventing the
yellowing, it is preferable that the following parameter is
smaller: [0072] reflectance at a wavelength indicating existence of
Sn.sup.++, such as reflectance R.sub.S(220) at the wavelength of
220 nm; [0073] reflectance R.sub.S(.lamda.*) at wavelength .lamda.*
maximizing reflection spectrum difference
.DELTA.R(.lamda.)=R.sub.S(.lamda.)-R.sub.B(.lamda.); [0074]
reflectance difference
.DELTA.R(.lamda.*)=R.sub.S(.lamda.*)-R.sub.B(.lamda.*); or [0075]
mean reflectance R.sub.S-mean (200-250) at wavelength of 200 to 250
nm. Specifically, reflectance R.sub.S(220) is 5% or less,
reflectance R.sub.S(.lamda.*) is 5% or less, reflectance difference
.DELTA.R(.lamda.*) is 3% or less, or mean reflectance R.sub.S-mean
(200-250) is 5% or less. In this case, it is verified that the
Sn.sup.++ content is so low that the yellowing of the glass
substrate presents no problem even when an image display device is
manufactured by forming an Ag electrode on the glass substrate.
[0076] However, the low Sn.sup.++ content in the glass substrate
can be caused by weak reducing force of the atmosphere in a float
furnace. In this case, disadvantageously, metallic tin in a tin
bath continuously oxidizes and volatilizes in manufacturing the
glass substrate. Too low Sn.sup.++ content in the glass substrate
is not preferable in manufacturing the glass substrate.
[0077] It is therefore preferable that reflectance R.sub.S(220) is
between 2.5% and 5%, reflectance R.sub.S(.lamda.*) is between 2.5%
and 5%, reflectance difference .DELTA.R(.lamda.*) is between 0.5%
and 3%, or mean reflectance R.sub.S-mean (200-250) is between 2.5%
and 5%.
[0078] When a measured reflectance of the glass substrate exceeds
the range discussed above, Sn.sup.++ content exceeds an allowance
where yellowing of the glass substrate is prevented from affecting
the image display. In this case, when an image display device is
manufactured by forming an Ag electrode on the glass substrate,
yellowing producing a defect in the image display device
occurs.
[0079] When the Sn.sup.++ content is determined to exceed the
allowance, the reducing force in a float furnace is weakened in
manufacturing the glass substrate, and the Sn.sup.++ content of the
glass substrate is decreased. For weakening the reducing force in
the float furnace, specially, hydrogen concentration in the float
furnace is deceased. Mixed gas of hydrogen and nitrogen is
generally used as atmospheric gas in the float furnace. The mixed
gas contains 2 to 10 vol % of hydrogen. The reducing force is
controlled by changing hydrogen concentration in this hydrogen
concentration range in response to the allowance of the Sn.sup.++
content.
[0080] FIG. 7 shows a manufacturing apparatus of the glass
substrate. A manufacturing method of the glass substrate is
described.
[0081] Material for the glass substrate is injected into melting
furnace 21, heated to a high temperature to be molten, and then
supplied to float furnace 22. Float furnace 22 has molten tin 24 in
its lower part, and has reducing atmosphere 25 (mixed gas of
hydrogen and nitrogen) for preventing oxidation of tin in its upper
space. Molten glass is continuously moved on molten tin 24 and
molded as plate-like glass ribbon 23. Glass ribbon 23 is lifted up
from the tin bath and moved to slow cooling furnace 27 by conveying
roller 26. Distortion occurring during the molding is decreased by
slowly cooling glass ribbon 23 in slow cooling furnace 27.
[0082] After the slow cooling process, a surface analyzing process
of measuring reflectance with reflectance measuring device 32 and
analyzing the Sn.sup.++ content of the glass substrate is performed
in the manufacturing apparatus in FIG. 7. Reflectance measuring
device 32 measures the following parameter: [0083] reflectance at a
wavelength indicating existence of Sn.sup.++ in the glass
substrate, such as reflectance R.sub.S(220) at the wavelength of
220 nm; [0084] reflectance R.sub.S(.lamda.*) at wavelength .lamda.*
maximizing .DELTA.R(.lamda.)=R.sub.S(.lamda.)-R.sub.B(.lamda.);
[0085] reflectance difference
.DELTA.R(.lamda.*)=R.sub.S(.lamda.*)-R.sub.B(.lamda.*); or [0086]
mean reflectance R.sub.S-mean (200-250) at wavelength of 200 to 250
nm.
[0087] When the Sn.sup.++ content is determined to exceed the
allowance based on the measured reflectance, concentration of the
atmosphere gas is controlled to weaken the reducing force in float
furnace 22. For preventing yellowing, it is preferable that the
reflectance is as low as possible. While, when the reducing force
of atmosphere 25 in float furnace 22 is excessively weakened for
reducing the Sn.sup.++ content in the glass substrate,
disadvantageously, metallic tin contained in molten tin 24
continuously oxidizes and volatilizes in manufacturing the glass
substrate.
[0088] Therefore, when the reflectance corresponding to the
Sn.sup.++ content in the glass substrate is higher than the
allowance value discussed above, the hydrogen concentration of the
atmosphere in the float furnace is controlled to be decreased. When
the reflectance is lower than the allowance value, the hydrogen
concentration is preferably increased for preventing oxidation of
the metallic tin.
[0089] This reflectance measurement can be performed
nondestructively, in a non-contact matter, and in a short time, so
that the measurement is applicable also to a process control of a
routine manufacturing process of a glass substrate. The image
display device is especially required to be uniform on its surface,
so that the reflectance is preferably measured at a plurality of
positions for recognizing dispersion on the glass substrate.
[0090] The Sn.sup.++ content can be evaluated by the secondary
ion-mass spectrometry (SIMS) or the inductively-coupled plasma
(ICP) optical emission spectrometry. However, these methods are
destructive inspections and can hardly used for measurement on a
large area, so that the methods are inappropriate for in-line
measurement of Sn.sup.++ content in a glass substrate in the glass
substrate manufacturing process. When Sn.sup.++ content in a
predetermined sample is measured, reflectance of the sample is
measured, and a calibration curve is prepared, however, Sn.sup.++
content can be determined based on the reflectance.
[0091] When hydrogen concentration of the atmosphere in the float
furnace increases, reducing property of the atmosphere is increased
to increase the Sn.sup.++ content of the glass substrate, and the
yellowing of the glass substrate presents a problem, as discussed
above. The variation in the Sn.sup.++ content of the glass
substrate appears as difference in yellowing degree of the glass
substrate, so that this variation must be within a certain range.
When the reflectance of the glass substrate is higher than the
predetermined range discussed above, the hydrogen concentration in
the float furnace is decreased. The decreasing weakens the reducing
property of the atmosphere, so that the reflectance of the glass
substrate can be decreased.
[0092] After the surface analyzing process of measuring
reflectance, in a cutting process, glass ribbon 23 is cut into an
arbitrary size by a cutter 28 and glass substrate 100 is
produced.
[0093] Though the reducing force in float furnace 22 is controlled
to weaken, the analyzed Sn.sup.++ content of the glass substrate on
which an Ag electrode is to be formed sometimes exceeds the
allowance. In this case, as shown in FIG. 8, the Ag electrode
receiving surface of the glass substrate is partially removed until
the Sn.sup.++ content becomes within the allowance in surface
removing furnace 29 in a surface removing process. In other words,
by controlling the reducing force in float furnace 22 to weaken and
partially removing the surface of the glass substrate, the surface
of the glass substrate formed so as to have decreased Sn.sup.++
content is further partially removed. In this case, removed
thickness can be decreased comparing with the case that the surface
is partially removed without controlling the reducing force in
float furnace 22. When the reducing force in float furnace 22 is
not controlled, as shown in FIG. 2, Sn.sup.++ exists in a range
from the glass surface to depth of about 15 .mu.m. For thoroughly
removing Sn.sup.++, the glass substrate having a large area must be
uniformly removed by thickness of 15 .mu.m or more, preferably 20
.mu.m or more. This removing process requires mirror finish, and
increasing the removed thickness increases cost extremely, so that
decrease of the removed thickness is extremely economical.
[0094] The surface removing process may employ a chemical method or
a physical method. In the chemical method, the glass substrate
surface is etched by dipping glass substrate 100 into etchant 30
such as aqueous hydrofluoric acid or aqueous sodium hydroxide. The
physical method includes a buffing method or a sand blasting
method. Sufficient surface removing thickness is about 3 to 15
.mu.m, as shown by the study of the reflectance.
[0095] In a method shown in FIG. 9, surface removal is performed in
surface removing furnace 29, Sn.sup.++ content in glass substrate
100 is analyzed again by reflectance measuring device 32 in a
second surface analyzing process, and surface removal is performed
again if necessary. Thus, the surface analyzing process and the
surface removing process are repeated and the surface state of the
glass substrate is severely controlled, thereby further increasing
the advantage of the present invention.
[0096] When the Sn.sup.++ content in the glass substrate is
determined to be higher than the allowance, the glass substrate may
be used as a back-side glass substrate of an image display device.
When the Sn.sup.++ content in the glass substrate is determined to
be not higher than the allowance, the glass substrate may be used
as a front-side glass substrate of an image display device.
[0097] When a PDP is the image display device manufactured using
the glass substrate formed as discussed above, the PDP does not
generate yellowing that is so strong as to affect the image display
characteristic, and can sufficiently display an image.
[0098] An investigation result of the PDP manufactured in
accordance with the present invention is described.
[0099] The surface of a glass substrate (PD-200 manufactured by
Asahi Glass Co. Ltd.) manufactured by the float method is partially
removed so that various amount of the reducing layer remains on the
surface of the glass substrate. In other words, a maximum value of
.DELTA.R(.lamda.)=R.sub.S(.lamda.)-R.sub.B(.lamda.), namely
difference between reflection spectrum R.sub.S(.lamda.) and
reflection spectrum R.sub.B(.lamda.) in a wavelength range of 210
to 250 nm, is 0.1%, 0.8%, 2.1%, 3.3%, or 4.0%. Specifically,
surface removal is performed by dipping the glass substrate into
etchant composed of aqueous hydrofluoric acid (10%), and the
surface removing thickness is controlled using the dipping period.
When temperature of the aqueous hydrofluoric acid is set at
27.degree. C., etching speed is 2 .mu.m/min. After the dipping for
a predetermined period, the glass substrate is washed. Then,
reflection spectrum is measured.
[0100] Using these glass substrates, three kinds of PDPs having
different resolution and structure are manufactured, and a relation
between reflection spectrum difference .DELTA.R(.lamda.) and
coloring degree (b*) by yellowing of the PDPs is evaluated.
[0101] PDP111 corresponds to video graphics array (VGA)
(480.times.640 pixel), and has a transparent electrode between an
Ag electrode (bus electrode) and a glass substrate. PDP222
corresponds to extended graphics array (XGA) (768.times.1024
pixel), and has a transparent electrode between an Ag electrode and
a glass substrate. PDP333 corresponds to XGA and has no transparent
electrode between an Ag electrode and a glass substrate.
[0102] Table 1 shows a measurement result of reflection spectrum
difference .DELTA.R (.lamda.) and coloring degree (b*) by yellowing
of three kinds of PDPs. The value of b* is preferably as small as
possible, but, actually, the yellowing has no particular problem
when b* is 2 or smaller. The PDPs have no problem as an image
display device in the following conditions: [0103]
.DELTA.R(.lamda.) is about 3% or lower in PDP111 having a
transparent electrode between the Ag electrode and the glass
substrate and a wide pixel interval; [0104] .DELTA.R(.lamda.) is
about 2% or lower in PDP222 having a transparent electrode between
the Ag electrode and the glass substrate but a narrow pixel
interval; and
[0105] .DELTA.R(.lamda.) is about 1% or lower in PDP333 having no
transparent electrode. TABLE-US-00001 TABLE 1 b * .DELTA.R [%]
PDP111 PDP222 PDP333 0.1 0.4 0.4 0.5 0.8 0.8 0.6 1.3 2.1 1.2 2.3
2.2 3.3 2.0 2.8 4.2 4.0 2.4 3.4 5.5
[0106] The advantage of the present invention is useful for not
only a PDP but also an image display device having the structure
where an Ag electrode is formed on a glass substrate having
Sn.sup.++ on its surface. This glass substrate is a glass substrate
formed by the float method, for example.
INDUSTRIAL APPLICABILITY
[0107] The present invention provides an image display device that
can suppress yellowing from occurring on a glass substrate
manufactured by the float method even when an Ag electrode is
formed on the glass substrate, and has high image display quality.
The present invention provides a manufacturing method of the glass
substrate for use in the image display device.
* * * * *