U.S. patent application number 15/061635 was filed with the patent office on 2016-06-30 for surface flaw modification for strengthening of glass articles.
The applicant listed for this patent is Corning Incorporated. Invention is credited to Jeanne Spadinger Cavuoti, Donald Arthur Clark, Sean Matthew Garner, Gregory Scott Glaesemann, Jun Hou, Jum Sik Kim, Toshihiko Ono, Daniel Arthur Sternquist.
Application Number | 20160190185 15/061635 |
Document ID | / |
Family ID | 47437709 |
Filed Date | 2016-06-30 |
United States Patent
Application |
20160190185 |
Kind Code |
A1 |
Cavuoti; Jeanne Spadinger ;
et al. |
June 30, 2016 |
SURFACE FLAW MODIFICATION FOR STRENGTHENING OF GLASS ARTICLES
Abstract
Disclosed are controlled chemical etching processes used to
modify the geometry of surface flaws in thin glass substrates and
glass substrate assemblies formed therefrom, and in particular
glass substrates suitable for the manufacture of active matrix
displays that are essentially free of alkali metal oxides such as
Na.sub.2O, K.sub.2O and Li.sub.2O.
Inventors: |
Cavuoti; Jeanne Spadinger;
(Corning, NY) ; Clark; Donald Arthur; (Corning,
NY) ; Garner; Sean Matthew; (Elmira, NY) ;
Glaesemann; Gregory Scott; (Corning, NY) ; Hou;
Jun; (Painted Post, NY) ; Kim; Jum Sik;
(Horseheads, NY) ; Ono; Toshihiko; (Fukuroi,
JP) ; Sternquist; Daniel Arthur; (Horseheads,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
|
|
Family ID: |
47437709 |
Appl. No.: |
15/061635 |
Filed: |
March 4, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13541206 |
Jul 3, 2012 |
9315412 |
|
|
15061635 |
|
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61505288 |
Jul 7, 2011 |
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61608982 |
Mar 9, 2012 |
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Current U.S.
Class: |
438/27 ;
216/38 |
Current CPC
Class: |
H01L 27/1262 20130101;
H01L 33/58 20130101; Y10T 428/1095 20150115; G02F 1/1333 20130101;
C09K 2323/061 20200801; H01L 21/02019 20130101; C03C 15/02
20130101 |
International
Class: |
H01L 27/12 20060101
H01L027/12; C03C 15/02 20060101 C03C015/02; H01L 21/02 20060101
H01L021/02 |
Claims
1. A method for modifying flaws on a glass article comprising:
contacting the glass article with an aqueous acidic solution for a
time from 10 seconds to less than or equal to 10 minutes, whereby
glass from the glass article is removed to a depth only from 2.5
.mu.m to less than or equal to 20 .mu.m when the time has elapsed;
rinsing the aqueous acidic solution from the glass article when the
time has elapsed.
2. The method according to claim 1, wherein the contacting is
conducted for a time from 10 seconds to less than or equal to 5
minutes.
3. The method according to claim 1, wherein the aqueous acidic
solution comprises HF and HCl, and wherein a molar concentration of
the HF is in a range from about 1.5M to about 4M and a molar
concentration of the HCl is in a range from about 4.0M to about
7.0M.
4. The method according to claim 1, wherein the aqueous acidic
solution comprises HF and H.sub.2SO.sub.4.
5. The method according to claim 1, wherein the glass article
comprises a glass substrate and a thickness of the glass substrate
is less than or equal to 500 .mu.m before contacting the glass
substrate with the aqueous acidic solution.
6. The method according to claim 5, further comprising depositing
thin film transistors on the glass substrate.
7. The method according to claim 6, wherein the glass substrate is
a first glass substrate, the method further comprising joining the
first glass substrate to a second glass substrate to form a glass
substrate assembly, there being a gap between the first and second
glass substrates.
8. The method according to claim 7, further comprising injecting a
liquid crystal material into the gap.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/541,206, filed on Jul. 3, 2012.
BACKGROUND
[0002] The present invention is in the field of information display
technology and relates more particularly to glass substrates and
glass substrate assemblies suitable for use as a transparent
substrate material for active matrix information displays,
photovoltaics, touch sensors, lighting and other flexible
electronics, and to methods for making such articles.
[0003] Thin glass substrates are used to make display panels and
touch sensors for mobile electronic devices such as cell phones,
laptops, and pad computers. They are also used in TVs, video
monitors, and other stationary display devices as well as
photovoltaic devices (e.g. solar cells) and lighting devices. Over
the past few decades the display glass panel (LCD, etc) has been
well protected from mechanical stress by strengthened glass cover
sheets, robust housings and/or other panel-protective device
designs. For that reason the glass substrates could be cut to size
by rough scribing techniques and surfaces could be exposed to
contact-induced damage during panel manufacture with little
consequence. However, recent developments in the design of mobile
electronic devices have required the development and deployment of
extremely thin glass panels as well as thinner and lighter housings
for the displays. The resulting housings and panels are less rigid
and higher stress levels have been observed on the glass substrate.
Stresses as high as 200 MPa have been observed under some
conditions of use in lightweight display devices such as e-books.
Hence, the risk of glass panel breakage in advanced information
display devices is increasing.
[0004] Active matrix displays such as TFT-LCD displays typically
require the use of glasses of the alkaline earth
boro-aluminosilicate type. These are generally glasses that are
essentially free of the alkali metal oxides Na.sub.2O, K.sub.2O and
Li.sub.2O, since active matrix displays require glass substrate
surfaces that are compatible with the deposition and activation of
thin-film transistor or and/or other electronic semiconductor
devices. Glasses free of alkali metal oxides, however, are not
chemically strengthenable by the ion-exchange methods used to
strengthen display components such as protective glass cover sheets
for information displays. Accordingly, the problem of glass panel
breakage cannot be addressed through conventional chemical
tempering methods.
[0005] Economical processes for producing display glass of high
optical and surface quality include drawing processes such as
fusion drawing (overflow down-drawing) and slot drawing. These
methods produce glass substrates with surfaces that are pristine
and essentially free of surface flaws as drawn. Unfortunately,
using presently available substrate handling methods, surface flaws
are invariably introduced as the drawn glass substrates are cut and
packaged for delivery to panel fabricators. Depending on the
number, size and shape of these flaws, significant weakening of the
sheets can occur. Such weakening leads to the probability of
mechanical failuire from these flaws on the glass substrates being
delivered to the fabricators for processing.
[0006] The problem of surface flaws is aggravated when the glass
substrates to be employed are required to be thin so as to be
flexible. Flexible glass substrates are of increasing interest for
lightweight electronic or other devices where the performance of
plastic or metal substrates does not meet device manufacturing or
device performance requirements. With the increasing interest in
flexible display electronics for such devices, there is a
correspondingly increased need for high quality flexible substrates
that are compatible with new product designs and manufacturing
processes. Flexible glass substrates are being more widely
integrated into such processes and designs due to their advantages
of hermeticity, optical transparency, surface smoothness, and
thermal and dimensional stability.
[0007] Commercially desirable forms of flexible glass substrates
include spooled or so-called "roll-to-roll" glass ribbons. To be
practical as a flexible electronics substrate material, however,
spooled flexible glass ribbon must improve to meet the chemical and
mechanical durability requirements of both customer processing and
end-use applications. Again, maintaining the required level of
glass mechanical reliability requires minimizing defects as well as
controlling stresses during the manufacture and rolling or spooling
of the glass ribbon.
[0008] The need for strengthening glass extends beyond removing
flaws from singular glass substrates or lengths of glass ribbon.
Even in the case where such flaws can be removed, assembly
practices for manufacturing display devices, for example the
substrate assemblies used to build display panels, may simply
re-introduce flaws into the glass. Thus, regardless the
high-strength pristine nature of the originating substrates or
ribbon used in forming the device, the manufacturing process
degrades the glass strength.
SUMMARY
[0009] In accordance with the present disclosure, controlled
treatment of glass surfaces with an aqueous acid solution are used
to modify the geometry of surface flaws. The flaws are not removed
but rather treated to modify their crack propagation
characteristics. The success of the method is based on the
recognition that handling-induced surface damage is overwhelmingly
sharp contact damage that results in flaws with sharp crack tips
having a high propensity to propagate. Through the use of the
presently disclosed methods, the geometry of such flaws can be
modified by an aqueous acid solution to substantially reduce the
tendency of sharp surface flaws to propagate, while not greatly
reducing the sizes of the flaws or materially changing the chemical
and optical characteristics of the glass surface. The result is a
significant increase in glass strength without significant material
removal or change in glass thickness.
[0010] The advantages of the disclosed methods are several. The
increases in glass substrate strength are general, affecting the
treated surfaces and may include edges and through-holes or other
features of the substrate. Thus, the strengthening process is
essentially independent of the shape of the treated glass, and in
some embodiments glass substrate assemblies comprising a plurality
of glass substrates may be treated. Further, since glass substrate
thickness changes are minimal (not more than a few microns), even
ultra-thin or flexible glass substrates can be successfully
strengthened. And, minimal material removal helps to lower
processing costs per part, both by minimizing the usage of the
aqueous acid solution and the need for refreshment of the aqueous
acid solution, and by enabling strengthening in a relatively short
period of time (order of minutes, or even seconds).
[0011] Due to the gentle nature of the treatments, less aggressive
aqueous acid solutions can be used, a factor that helps to preserve
the functionality of any pre-applied components of sensitive
electronic devices. Thus, more device types can be treated after
device fabrication than is typically allowed. Further, the
dimensional precision of the substrates and/or devices (e.g.
assemblies) can be maintained because these mild aqueous acid
solution treatments do not alter overall substrate dimensions after
precision cutting or shaping; the length, width, thickness, and
through-hole or other feature dimensions are not significantly
altered.
[0012] Accordingly, method of the present disclosure can be applied
to substrate assemblies comprising two or more joined glass
substrates. Such substrate assemblies may include, for example,
assemblies used in the manufacture of displays, such as liquid
crystal displays.
[0013] The methods of the present disclosure can also be adapted
for use in the strengthening of flexible glass substrates to be
employed in roll-to-roll display device fabrication. The aqueous
acid solution treatment steps required are of sufficiently short
duration, and the aqueous acid solution sufficiently mild, that the
continuous processing of long lengths of flexible glass becomes
both practical and economically advantageous. A wide variety of
roll-to-roll processing options can be implemented. Examples
include the continuous through-cutting of long glass ribbon lengths
in the long direction to continuously convert a wide glass web
(i.e. ribbon) to two narrower ribbons, as well as continuous
masking approaches that enable the selective etch-induced
strengthening of cut glass substrate edges.
[0014] Accordingly, in one aspect a method for manufacturing an
electronic device is disclosed comprising: depositing thin film
transistors on a first glass substrate; joining the first glass
substrate to a second glass substrates to form a glass substrate
assembly, there being a gap between the first and second glass
substrates; contacting the glass substrate assembly with an aqueous
acidic solution for a time not exceeding 10 minutes; injecting a
liquid crystal material into the gap; and wherein the contacting is
performed after the step of joining but before the step of
injecting. The contacting may be conducted, for example, for a time
not exceeding 5 minutes. In some embodiments, the contacting does
not exceed about 4 minutes, whereas in other embodiments the step
of contacting does not exceed about 3 minutes.
[0015] In some embodiments the contacting may be performed for a
period as short as 10 seconds. Therefore, suitable contacting times
in accordance with embodiments of the present disclosure fall
within a range from about 10 seconds to about 10 minutes, in some
examples in a range from about 10 seconds to about 5 minutes, in
some examples in some examples in a range from about 10 seconds to
about 4 minutes and in some examples in a range from about 10
seconds to about 3 minutes. The contacting may remove glass from
each of the first and second substrates to a depth equal to or less
than about 20 .mu.m, for example, the contacting may remove glass
from each of the first and second substrates to a depth equal to or
less than about 15 .mu.m and in other examples the contacting may
remove glass from each of the first and second substrates to a
depth equal to or less than about 10 .mu.m and in still other
examples the contacting may remove glass from each of the first and
second substrates to a depth equal to or less than about 5 .mu.m.
At least one of the first or second glass substrates comprises may
be an alkaline earth boro-aluminosilicate glass. For example, at
least one of the first or second glass substrates may comprise an
alkaline earth boro-aluminosilicate glass wherein a total
concentration of alkali metal oxides selected from the group
consisting of Na.sub.2O, K.sub.2O and Li.sub.2O in the first or
second glass substrate does not exceed 5% by weight.
[0016] In some examples, the substrate assembly is thinned prior to
the contacting by exposing the substrate assembly to an etchant
that removes glass from each of the first and second glass
substrates to a depth of at least about 100 .mu.m and then washing
the substrate assembly. The glass may be washed, for example, by
using a brush.
[0017] In some embodiments the method may further comprise applying
a polymer film to the glass article after the step of contacting.
The polymer film may be removed, such as prior to the application
of a polarizing material to the glass substrate assembly.
[0018] In some embodiments the glass substrate assembly may be a
large master glass substrate assembly, wherein the method further
comprises separating the master glass substrate assembly into a
plurality of individual glass substrate assemblies, and wherein the
step of contacting occurs after the step of separating.
[0019] The method according the present aspect may comprise an
aqueous acidic solution of HF and HCl such that a molar
concentration of the HF is in a range from about 0.1M to about 6M
and a molar concentration of the HCl is in a range from about 0.1M
to about 7.0M. In other examples the aqueous acidic solution
comprises HF and HCl, wherein a molar concentration of the HF is in
a range from about 0.1M to about 3.0M and a molar concentration of
the HCl is in a range from about 0.1M to about 6.0M. In still other
examples the aqueous acidic solution comprises HF and HCl wherein a
molar concentration of the HF is in a range from about 1.5M to
about 2.5M and a molar concentration of the HCl is in a range from
about 4.0M to about 5.0M. In yet other examples the aqueous acidic
solution comprises HF and H.sub.2SO.sub.4, wherein the molar
concentration of H.sub.2SO.sub.4 is in ranges equal to the ranges
for HCl above.
[0020] In some embodiments a thickness of either one or both of the
first or second glass substrate does not exceed 500 .mu.m, such as
before a thinning step.
[0021] In certain embodiments one or both of the first or second
glass substrates is substantially free of surface compression
layers and central tensional stresses.
[0022] In some embodiments the first or second glass substrate
comprises strength-limiting surface flaws and the strength-limiting
surface flaws after contacting with the aqueous acid solution
comprise flaws with rounded crack tips having at least one of a
flaw depth and a flaw diameter in excess of 5 .mu.m in size.
[0023] In another aspect a method for manufacturing an electronic
device is described comprising: depositing a thin film transistors
on a first glass substrate; joining the first glass substrate to a
second glass substrate to form a master glass substrate assembly,
there being a gap between the first and second glass substrates;
thinning the master glass substrate assembly by exposing the master
glass substrate assembly to an etching solution that removes glass
from each of the first and second substrates to a depth of at least
about 100 .mu.m; washing the substrate assembly, wherein the
washing comprising contacting the master glass substrate with a
brush; contacting the substrate assembly for a time not exceeding 5
minutes with a second aqueous acidic etching solution after the
washing. The method may further comprise applying a polymer film to
the master glass substrate after the step of contacting. In some
examples the method may further comprise separating the master
glass substrate assembly into a plurality of individual glass
substrate assemblies prior to the contacting. The polymer film may
be removed from at least one of the individual glass substrate
assemblies. For example, the polymer film may be removed prior to
applying a polarizing film to at least one of the individual glass
substrate assemblies.
[0024] In some embodiments a thickness of at least one of the first
or second glass substrates does not exceed 500 .mu.m before the
thinning with the etching solution. In some examples at least one
of the first or second glass substrates is substantially free of
surface compression layers and central tensional stresses.
[0025] In another aspect, the present disclosure provides
information display substrates wherein flaw-induced substrate
breaking stresses are a function of flaw shape factors rather than
being dictated solely by the fracture mechanics of a sharp surface
flaw. Particularly included is an electronic-device-compatible
information display substrate incorporating strength-limiting
surface flaws, wherein tensile stresses .sigma..sub.A arising at
crack tips emanating from the surface flaws as the result of a
global stress .sigma. applied to the substrate depend predominantly
upon the flaw depth, a, and crack tip radii, .rho., in accordance
with the equation:
.sigma. A = .sigma. ( 1 + a .rho. ) ##EQU00001##
[0026] Display substrates with the disclosed stress response
characteristics include those wherein the strength-limiting surface
flaws include flaws with rounded crack tips having at least one of
a flaw depth and a flaw diameter in excess of 5 .mu.m in size.
[0027] In another aspect, the present disclosure provides methods
for making strengthened glass substrate having stress response
characteristics such as above described. Included is a method for
increasing the fracture resistance of a glass substrate comprising
surface flaws, including surface flaws having at least one of a
flaw diameter and a flaw depth exceeding 5 .mu.m in size,
comprising contacting at least one surface of the glass substrate
with an aqueous acid solution for a time and at a temperature
effective to round or blunt the tip portions of cracks emanating
from the surface flaws. In these and other aspects, substrate
surfaces include both the 2 major surfaces as well as the surfaces
of the edge faces.
[0028] In yet another aspect, the present disclosure provides a
method for increasing the fracture resistance of a video display
panel component comprising an array of electronic device components
disposed on a glass substrate, the glass substrate comprising
surface flaws and said surface flaws including flaws having at
least one of a flaw diameter and a flaw depth in excess of 5 .mu.m
in size, comprising contacting at least one surface of the panel
with a chemical etching medium for a time and at a temperature
effective to round or blunt the tip portions of cracks emanating
from the surface flaws.
[0029] The above disclosed methods enable the manufacture of
display panel substrates that are compatible with semiconductor
electronic device components as well as sufficiently flexible for
use in roll-to-roll display substrate manufacturing procedures. One
such substrate is a flexible electronic-device-compatible
information display substrate comprising a flexible glass substrate
incorporating strength-limiting surface flaws, wherein tensile
stresses .sigma..sub.A arising at crack tips emanating from the
surface flaws as the result of a global stress .sigma. applied to
the substrate depend predominantly upon the flaw depth, a, and
crack tip radii, .rho., in accordance with the equation:
.sigma. A = .sigma. ( 1 + a .rho. ) ##EQU00002##
[0030] For the purposes of the present description a flexible glass
substrate, such as a sheet or and elongated glass ribbon or display
panel substrate is one that can be bent to a radius of curvature as
small as 10 cm without breakage, following treatment in accordance
with the present disclosure. A flexible substrate exhibiting the
above-disclosed stress response characteristics and flexibility can
be stored and processed as a spooled glass ribbon, i.e., as a
length of glass substrate coiled on any spool incorporating a
spindle having a diameter larger than the radius of curvature at
which stress breakage of the substrate will occur.
[0031] Spooled flexible glass ribbons or individual display panel
substrates or substrate assemblies provided in accordance with the
present disclosure incorporate strength-limiting surface flaws,
including flaws with blunted or rounded crack tips having at least
one of a flaw diameter and a flaw depth exceeding 5 .mu.m in size.
Further, at least a portion of the length of the spooled glass
ribbon or substrate is coiled on the spindle of a selected spool at
a bend radius larger than the radius at which breakage of the
flexible glass ribbon can occur.
[0032] In yet another aspect, the present disclosure provides
methods for manufacturing flexible electronic-device-compatible
information display substrates such as above disclosed. Particular
embodiments include a method for manufacturing a flexible
electronic-device-compatible information display substrate
comprising an elongated flexible glass ribbon, said flexible glass
ribbon comprising surface flaws that include flaws having at least
one of a flaw diameter and a flaw depth exceeding 5 .mu.m in size,
comprising the step of sequentially contacting the elongated glass
ribbon along its length with an aqueous acid solution. Methods
wherein the glass ribbon is continuously transported into contact
with and then separated from contact with the aqueous acid solution
are of particular commercial importance.
[0033] In still another aspect, a plurality of glass substrate
assemblies is described wherein each glass substrate assembly of
the plurality of substrate assemblies comprises: a first glass
substrate having a plurality of thin film transistors deposited
thereon; a second glass substrate sealed to the first glass
substrate with an adhesive to form the glass substrate assembly,
there being a gap between the first and second glass substrates;
and wherein the plurality of glass substrate assemblies comprises
equal to or greater than 30 glass substrate assemblies selected at
random from a population of like substrate assemblies, and wherein
when each of the plurality of glass substrate assemblies is
positioned on a rigid ring having a diameter of 42.5 mm and
subjected to a point force centered within a circumference of the
rigid ring and stressed to failure and the failure stresses for the
plurality of glass substrate assemblies is plotted as a Weibull
distribution, the Weibull parameter .sigma..sub.0 for the plurality
of substrate assemblies at a probability of 63% is greater than
about 800 N and the Weibull parameter m for the plurality of
substrate assemblies is greater than about 8.0.
[0034] In another aspect a glass substrate assembly is selected
from at least 30 glass substrate assemblies that are identical
except for manufacuturing tolerances, wherein each of the glass
substrate assemblies comprises: a first glass substrate having a
plurality of thin film transistors deposited thereon; a second
glass substrate sealed to the first glass substrate with an
adhesive to form the glass substrate assembly, there being a gap
between the first and second glass substrates; and wherein when
each of the at least 30 glass substrate assemblies is positioned on
a rigid ring having a diameter of 42.5 mm and subjected to a point
force centered within a circumference of the rigid ring and
stressed to failure, and the failure stresses for the at least 30
glass substrate assemblies is plotted as a Weibull distribution,
the Weibull parameter .sigma..sub.0 for the at least 30 glass
substrate assemblies at a probability of 63% is greater than about
800 N and the Weibull parameter m for the plurality of substrate
assemblies is greater than about 8.0. The at least 30 glass
substrate assemblies may be randomly selected from a population of
like glass substrate assemblies.
DESCRIPTION OF THE DRAWINGS
[0035] The glass products and methods of the present disclosure are
further described below with reference to the appended drawings,
wherein:
[0036] FIG. 1 is a cross sectional edge view of an exemplary glass
substrate assembly;
[0037] FIG. 2 is flow diagram of an exemplary glass substrate
assembly manufacturing process;
[0038] FIG. 3 is flow diagram of another exemplary glass substrate
assembly manufacturing process;
[0039] FIG. 4 is a top view of a master glass substrate assembly
from which smaller glass substrate assemblies may be separated;
[0040] FIG. 5 is a bar graph of failure stresses for various points
in the manufacturing process of FIG. 3;
[0041] FIG. 6 is a cross sectional side illustration of an
apparatus for performing ring-on-ring strength testing;
[0042] FIG. 7 illustrates factors affecting the failure strengths
of flawed glasses;
[0043] FIG. 8A is a photomicrograph of a flawed surface of a glass
substrate;
[0044] FIG. 8B is a photomicrograph of a treated flawed surface of
a glass substrate;
[0045] FIG. 9 presents surface-based failure probability data for
untreated and treated flawed glass substrates;
[0046] FIG. 10 presents surface-based failure probability data for
treated and untreated glass substrate assemblies;
[0047] FIG. 11 is a cross sectional side view of an apparatus for
performing ball-on-ring strength testing;
[0048] FIG. 12 presents edge-based failure probability data for
treated and untreated glass substrate assemblies tested in four
point bending;
[0049] FIG. 13 schematically illustrates a first arrangement for
treating a flexible glass ribbon with an aqueous acid solution;
[0050] FIG. 14 schematically illustrates a second arrangement for
treating a flexible glass ribbon with an aqueous acid solution;
[0051] FIG. 15 presents a cross-sectional view of a glass ribbon
supporting an acidic paste;
[0052] FIG. 16 schematically illustrates a third arrangement for
treating a flexible glass ribbon with an aqueous acid solution;
[0053] FIG. 17 schematically illustrates a fourth arrangement for
treating a flexible glass ribbon with an aqueous acid solution;
[0054] FIG. 18 is a schematic diagram illustrating a thinning
process wherein the thing is performed by sequential treatments
with an aqueous acid solution according to embodiment disclosed
herein; and
[0055] FIG. 19 is an exemplary glass substrate assembly
manufacturing process utilizing the process of FIG. 18.
DETAILED DESCRIPTION
[0056] It is known that the handling of display glass substrate
during the manufacture of display panels can result in sharp
contact damage to the surfaces of the panel. This is due to the
particular way in which glass responds to mechanical contact. The
flaws resulting from sharp contact damage can be produced by any of
the various types of equipment used to manipulate large glass
panels as they are cut from continuously drawn ribbons of glass
produced by the melting and forming equipment conventionally used
for continuously drawn glass substrate manufacture. The edges as
well as the surfaces of the individual glass panels can exhibit
defects caused in this way. However, recent experiments have shown
that the processing steps involved in the manufacture of glass
panel assemblies also plays a role in the reliability of assemblies
formed from the panel.
[0057] FIG. 1 illustrates an edge portion of a glass substrate
assembly 10 for use in the manufacture of an LCD display panel such
as an active matrix liquid crystal display panel. Substrate
assembly 10 comprises a first glass substrate 12, a second glass
substrate 14, wherein the first and second glass substrates are
sealed together by a sealing material 16 positioned between the
first and second glass substrates. Sealing material 16 may be, for
example, a UV-curable epoxy. However, in other embodiments, sealing
material 16 may be, for example, a glass frit. First glass
substrate 12 comprises first major surface 18 and second major
surface 20. First glass substrate 12 may include thin film
transistors and various transparent electrical busses necessary for
addressing individual pixels of the display panel disposed on a
first surface thereof, represented by layer 22. Second glass
substrate 14 comprises first major surface 24 and second major
surface 26. Second glass substrate 14 may further include color
filter materials 28 disposed on first major surface 20, and may
also include a transparent conductor layer 30, such as a layer of
indium tin oxide (ITO), which can function as a common electrode
for the display panel formed from the substrate assembly and which
transparent conductor layer 30 is formed over color filter layer
28. Substrate assembly 10 may further include spacers 32 disposed
between the first and second glass substrates, wherein spacers 32
serve to maintain a consistent width of gap 34 between the first
and second glass substrates.
[0058] FIG. 2 depicts an exemplary process 36 for manufacturing a
glass LCD display panel. Actual manufacturing methods may differ
based on specific manufactures. In accordance with the exemplary
process of FIG. 2, first glass substrate 12 is provided at step 38.
At step 40 the first glass substrate is washed, dried, and an array
of thin film transistors (TFTs), represented by layer 22 in FIG. 1,
and their accompanying data and select lines are deposited on first
surface 18 of first substrate 12 by chemical vapor deposition (e.g.
plasma enhanced chemical vapor deposition, PECVD) and/or sputtering
(e.g. RF sputtering), and etching using photolithography
techniques. The process complexity will depend on whether the
process is an amorphous silicon process or a poly-crystalline
silicon process.
[0059] At step 42, a polymer coating 44 may be applied to first
glass substrate 12 over the TFTs, for example a polyamide coating.
The polymer coating is cured, and then the surface of the cured
polymer surface is rubbed or polished at step 46 to create a
preferential alignment direction for liquid crystal molecules that
will occupy gap 34. Alternatively, a diamond-like carbon layer may
be applied over first surface 18 and ion beam-treated to create the
preferential alignment of liquid crystal molecules.
[0060] In a separate process, beginning at step 48, second glass
substrate 14 is provided. At step 50, color filter material layer
28 may be deposited on the second glass substrate. For example, in
the production of a red-green-blue (RGB) color filter, a black
matrix material (e.g. Cr/CrO.sub.2 or a black polymer material) is
deposited on the second glass substrate in a pattern that blocks
light in the inter-pixel areas, shields the TFT array from ambient
light, and improves contrast ratio. Then, a first color filter
material, such as a dyed or pigmented UV curable material, e.g.
red, may be applied to the glass substrate, exposed to UV light
through a mask, and developed, wherein the uncured regions of the
resin are removed. Next, the second color, e.g. green, is applied,
cured and developed. The process is repeated for the last color,
e.g. blue. Once the RGB colors have been deposited, a polymer layer
52, such as a polyamide layer, may be applied at step 54. The
polymer layer 52 is cured, and then the surface of cured polymer
layer 52 is rubbed or polished at step 56 to create a preferential
alignment direction for liquid crystal molecules that will occupy
gap 34. Alternatively, a diamond-like carbon layer may be applied
over first surface 24 of second glass substrate 14 and ion
beam-treated to create the preferential alignment of liquid crystal
molecules. Step 56 may further comprise the application of a
transparent conductive film 30, such as indium tin oxide (ITO),
which forms a common transparent electrode.
[0061] Next, spacers 32 may be sprayed (e.g. plastic spheres having
a diameter of about 4-5 .mu.m) onto first glass substrate 12 at
step 58. Spacers 32 aid in maintaining a consistent gap 34 between
the two glass substrates 12, 14 when the two glass substrates are
sealed together. Although FIG. 1 depicts spherical spacers, such
randomly dispersed spheres may end up over pixel areas and can
impact contrast ratio. Consequently, in some processes randomly
sprayed spacers are replaced with photolithography-patterned column
spacers.
[0062] At step 60, sealing material 16, such as a UV-curable epoxy,
is applied over first major surface 24 of second glass substrate
14.
[0063] At step 62, first glass substrate 12 and second glass
substrate 14 are aligned and brought together, wherein the sealing
material forms a loop around the TFT array. The sealing material is
then cured to form glass substrate assembly 10.
[0064] It should be recognized that the above process overview is a
simple representation of a complex process, and is not meant to be
limiting in any way. For example, while the preceding description
of a liquid crystal display manufacturing process is provided in
respect of a single substrate assembly for use in a single display
panel, such displays panels are often produced many at a time. That
is, as shown in FIG. 3, large glass substrates (i.e. first and
second glass substrates 12 and 14, respectively) may be used to
form a single master substrate assembly 10a, wherein multiple TFT
arrays and multiple color filter arrays are sealed between the
large glass substrates as separate and distinct display
elements.
[0065] For small display devices, such as mobile phones, notebook
computers and laptop computers, and other similar devices, there is
a commercial desire to drive the devices to be as thin as possible.
Thus, display-type glasses common in large devices, such as
televisions (TVs), takes up a considerable thickness of the device.
For example, a typical sheet (e.g. substrate) of glass used in a
television has a thickness of about 0.7 mm. The typical thickness
of a glass substrate used in a mobile phone has a thickness of
about 0.3 mm. However, while 0.3 mm glass substrates are
technically achievable and may be commercially available, they are
nevertheless difficult to process in a manufacturing environment
without damage, and so other steps may be undertaken to thin the
substrate assembly once the two substrates are sealed together. To
wit, in an optional process, the sealed substrate assembly
comprising the first (TFT) glass substrate 12 and the second (color
filter) glass substrate 14 are sealed around their edges by a
sealing material (not shown), after which the substrate assembly
may be exposed to an etching solution that removes material from
both exposed second major surfaces 20 and 26 of the substrate
assembly in optional thinning step 64 in FIG. 2. The edge seal
prevents ingress of the etching solution. In some processes a
thickness of the starting glass substrates may originally be about
0.5 mm, but then have approximately 100 .mu.m or even as much as
200 .mu.m or more of glass removed from the surfaces of each
substrate by the etching solution used for thinning.
[0066] One particular drawback of the optional thinning process is
the formation of sludge formed by the dissolved glass. Accordingly,
the surfaces of the substrate assembly should be thoroughly washed.
This may be accomplished at optional step 66 by contacting the
surfaces of the substrate assembly with a cleaning liquid and
brushing the surfaces of the substrate assembly with a brush, such
as a rotating brush.
[0067] At node 67, if the substrate assembly was a master substrate
assembly 10a, the process can proceed to step 68 where the master
panel assembly is separated into separate substrate assemblies 10,
such as by scribing the master panel assembly with a diamond scribe
along scribe lines 70 and breaking. As shown in FIG. 2, a
washing/rinsing step 72 may follow the separating step to remove
any particulate formed from the separating process that may have
adhered to the second major surfaces of the substrate assembly.
[0068] If the substrate assembly from step 62 was a single
substrate assembly 10 and does not require separation, the process
can proceed to step 76 from node 67. It should be understood that
other process steps may be involved between node 67 and step 76.
For example, a layer of ITO may be deposited on substrate 14, for
example as a layer comprising a touch-sensitive circuit.
Alternatively, an antireflection layer, or other layer, may be
added. In any event, process 36 may be modified appropriately.
[0069] Whether formed from a large master glass substrate assembly
comprising a plurality of TFT arrays and separated into smaller
individual glass substrate assemblies, or a single glass substrate
assembly comprising a single TFT array is formed, once the
individual glass substrate assembly is formed comprising the TFT
substrate, the color filter substrate and the sealing material, gap
34 is then filled with a liquid crystal material at step 74 by
vacuum injection, such as through a passage left open in the
adhesive seal. Thereafter the passage is sealed, and at step 76 a
polarizing film may be applied to one or typically both surfaces of
the panel assembly. Once the individual glass substrate assembly 10
is formed, the individual glass substrate assembly moves to
downstream display panel processes, represented by step 78 in FIG.
2
[0070] FIG. 3 depicts another exemplary process 37 for
manufacturing a glass LCD display panel. Again, actual
manufacturing methods may differ based on specific manufactures. In
accordance with the exemplary process of FIG. 3, first glass
substrate 12 is provided at step 38. At step 40 the first glass
substrate is washed, dried, and an array of TFTs, represented by
layer 22 in FIG. 1, and their accompanying data and select lines
are deposited on first surface 18 of first substrate 12 by chemical
vapor deposition (e.g. plasma enhanced chemical vapor deposition,
PECVD) and/or sputtering (e.g. RF sputtering), and etching using
photolithography techniques. The process complexity will depend on
whether the process is an amorphous silicon process or a
poly-crystalline silicon process.
[0071] At step 42, a polymer coating 44 may be applied to first
glass substrate 12 over the TFTs, for example a polyamide coating.
The polymer coating is cured, and then the surface of the cured
polymer surface is rubbed or polished at step 46 to create a
preferential alignment direction for liquid crystal molecules that
will occupy gap 34. Alternatively, a diamond-like carbon layer may
be applied over first surface 18 and ion beam-treated to create the
preferential alignment of liquid crystal molecules.
[0072] In a separate process, beginning at step 48, second glass
substrate 14 is provided. At step 50, color filter material layer
28 may be deposited on the second glass substrate. For example, in
the production of a red-green-blue (RGB) color filter, a black
matrix material (e.g. Cr/CrO.sub.2 or a black polymer material) is
deposited on the second glass substrate in a pattern that blocks
light in the inter-pixel areas, shields the TFT array from ambient
light, and improves contrast ratio. Then, a first color filter
material, such as a dyed or pigmented UV curable material, e.g.
red, may be applied to the glass substrate, exposed to UV light
through a mask, and developed, wherein the uncured regions of the
resin are removed. Next, the second color, e.g. green, is applied,
cured and developed. The process is repeated for the last color,
e.g. blue. Once the RGB colors have been deposited, a polymer layer
52, such as a polyamide layer, may be applied at step 54. The
polymer layer 52 is cured, and then the surface of cured polymer
layer 52 is rubbed or polished at step 56 to create a preferential
alignment direction for liquid crystal molecules that will occupy
gap 34. Alternatively, a diamond-like carbon layer may be applied
over first surface 24 of second glass substrate 14 and ion
beam-treated to create the preferential alignment of liquid crystal
molecules. Step 56 may further comprise the application of a
transparent conductive film 30, such as indium tin oxide (ITO),
which forms a common transparent electrode.
[0073] Next, spacers 32 may be sprayed (e.g. plastic spheres having
a diameter of about 4-5 .mu.m) onto first glass substrate 12 at
step 58. Spacers 32 aid in maintaining a consistent gap 34 between
the two glass substrates 12, 14 when the two glass substrates are
sealed together. Although FIG. 1 depicts spherical spacers, such
randomly dispersed spheres may end up over pixel areas and can
impact contrast ratio. Consequently, in some processes randomly
sprayed spacers are replaced with photolithography-patterned column
spacers.
[0074] At step 60, sealing material 16, such as a UV-curable epoxy,
is applied over first major surface 24 of second glass substrate
14, at which point the process of FIG. 3 deviates from the process
of FIG. 2. In the process of FIG. 2, the glass substrates were
brought together, after which the liquid crystal material could be
vacuum-injected into individual cells.
[0075] In accordance with the process of FIG. 3, after step 58,
liquid crystal material is deposited on the pixel area(s) of the
display. Thus, not until after the depositing of the liquid crystal
material are the first and second substrates brought together
wherein the sealing material forms a loop around the TFT array and
is completely sealed and cured at step 62 to form glass substrate
assembly 10
[0076] As with FIG. 2, it should be recognized that the preceding
overview of the process of FIG. 3 is a simple representation of a
complex process, and is not meant to be limiting in any way. For
example, while the preceding description of a liquid crystal
display manufacturing process according to FIG. 3 is provided in
respect of a single substrate assembly for use in a single display
panel, such displays panels are often produced many at a time. That
is, as shown in FIG. 4, large glass substrates (i.e. first and
second glass substrates 12 and 14, respectively) may be used to
form a single master substrate assembly 10a, wherein multiple TFT
arrays and multiple color filter arrays are sealed between the
large glass substrates as separate and distinct display
elements.
[0077] In optional process steps, the sealed substrate assembly
comprising the first (TFT) glass substrate 12 and the second (color
filter) glass substrate 14 may be sealed around their edges by a
sealing material (not shown), after which the substrate assembly
may be exposed to an etching solution that removes material from
both exposed second major surfaces 20 and 26 of the substrate
assembly in optional thinning step 64 in FIG. 3. The edge seal
prevents ingress of the etching solution. In some processes a
thickness of the starting glass substrates may originally be about
0.5 mm, but then have approximately 100 .mu.m or even as much as
200 .mu.m or more of glass removed from the surfaces of each
substrate by the etching solution used for thinning.
[0078] As described previously in the context of FIG. 2, a
particular drawback of the optional thinning process is the
formation of sludge formed by the dissolved glass. Accordingly, the
surfaces of the substrate assembly should be thoroughly washed.
This may be accomplished at optional step 66 in process 37 by
contacting the surfaces of the substrate assembly with a cleaning
liquid and brushing the surfaces of the substrate assembly with a
brush, such as a rotating brush.
[0079] At node 67, if the substrate assembly was a master substrate
assembly 10a, the process can proceed to step 68 where the master
panel assembly is separated into separate substrate assemblies 10,
such as by scribing the master panel assembly with a diamond scribe
along scribe lines 70 and breaking. As shown in FIG. 2, a
washing/rinsing step 72 may follow the separating step to remove
any particulate formed from the separating process that may have
adhered to the second major surfaces of the substrate assembly.
[0080] If the substrate assembly from step 62 was a single
substrate assembly 10 and does not require separation, the process
can proceed to step 76 from node 67. It should be understood that
other process steps may be involved between node 67 and step 76.
For example, a layer of ITO may be deposited on substrate 14, for
example as a layer comprising a touch-sensitive circuit.
Alternatively, an antireflection layer, or other layer, may be
added. In any event, process 36 may be modified appropriately.
[0081] Upon completion of step 76, the individual glass substrate
assembly 10 moves to downstream display panel processes,
represented by step 78 in FIG. 3
[0082] Newly-formed pristine glass that has not been contacted is
inherently very strong. However, subsequent contact generally
results in the formation of flaws in the surface of the glass that
reduce the strength of the glass. For example, the optional
brushing operation used to clean the surfaces of the substrate
assembly described above at step 66 of either process 36 or process
37 may significantly degrade the strength of the glass comprising
the substrate assembly. Even without a thinning process and
subsequent cleaning, processing of the first and second substrates
to form the TFTs and color filters requires handling of the first
and second glass substrates, respectively, which may also degrade
the strength of the glass substrates. The chart in FIG. 5
illustrates the strength degradation that can occur at different
points in the display manufacturing process.
[0083] Referring now to FIG. 5, bar 80 depicts the failure stress
of glass substrates used to produce a panel assembly (e.g. glass
substrate 12 or 14) in MPa, as provided from the manufacturer, i.e.
at steps 38, 48. The data show a failure stress of approximately
1100 MPa. Testing employed to develop FIG. 5 utilized a
ring-on-ring testing technique. An apparatus for conducting
ring-on-ring testing is shown in FIG. 6, showing a rigid ring (e.g.
a cylindrical container 82 having an open mouth). The article to be
tested is positioned over the rigid container/ring 82 and a plate
having an open rigid ring 84 attached thereto is positioned over
the article, indicated as glass substrate 12 in FIG. 6. A force F
is then applied to the plate at a predetermined rate, thereby
causing a deflection in the article produced by ring 84 until the
article fails. A load cell records the force F. See also, for
example, ASTM C1683-10, Standard Practice for Size Scaling of
Tensile Strengths Using Weibull Statistics for Advanced Ceramics.
Bar 86 represents the strength of the glass substrates prior to the
deposition of TFTs or color filter material, i.e. after steps 38,
48 but before steps 40 and 50, and indicates a failure stress of
about 800 MPa. Bar 88 depicts the failure stress after the
deposition of TFTs and color filter material, i.e. after steps 40
and 50 but before step 62, and indicates a failure stress of
approximately 400 MPa. Bar 90 shows the failure stress after
assembly step 62 but before separating step 68. Finally, bar 92
shows a failure stress of approximately 250 MPa after the
completion of step 72. The data show a continued decline in failure
stress (e.g. glass strength) of the glass substrates, and later the
glass panel assembly, as the glass substrates and panel assembly
progress through the display panel manufacturing process, from a
high of about 1100 MPa at the point where the individual glass
substrates are provided into the process to a low of only about 250
MPa where the panel assembly has been formed, diced (separated) and
rinsed to form individual panel assemblies, over a 400% decline in
strength as a consequence of handling.
[0084] FIG. 5 shows that there are optimal locations at which to
apply an aqueous acid solution treatment according to examples of
the present disclosure. For example, the data show that glass
strength continues to decline at least through the assembly step
62. Accordingly, a treatment with an aqueous acid solution as
described herein is more effective if applied, for example, at
point A, as shown in FIGS. 2 and 3, after assembly step 62, than
prior to step 62. Alternatively, if the glass substrate assembly is
a master glass substrate assembly (i.e. glass substrate assembly
10a) that undergoes separation at step 68 into individual glass
substrate assemblies 10), the individual glass substrate assemblies
10 may be treated with an aqueous acid solution at point B, after
the step of separating. In still another variation, the individual
glass substrate assemblies 10 may be treated with an aqueous acid
solution after the washing step, at point C. In short, optimal
prevention of glass strength degradation can be achieved if the
glass substrate assembly 10 (10a) is treated with an aqueous acid
solution at a point in manufacturing process 36 after assembly step
62 and before the liquid crystal injection step 74 of process 36,
or after assembly step 62 and before application of a polarizing
film at step 76 of process 36.
[0085] The effectiveness of present methods for strengthening
flawed glass panels can be understood in terms of fracture
mechanics principles. When a sharp crack emanating from an edge or
surface flaw is exposed to a global stress, e.g., a stress caused
by strain applied to the entire panel, the stress at the crack tip
goes to infinity, i.e., there is a stress singularity at the crack
tip. One reason why such cracks don't result in immediate failure
is that the glass host material has a certain resistance to crack
propagation, that resistance arising from a material property
called "fracture toughness".
[0086] Glass has a low fracture toughness. Furthermore, for flaws
that fall within the generally applicable fracture mechanics
framework, the strength of a flawed object is dependent on the flaw
size, with deeper flaws causing larger strength reductions. In all
cases, the introduction of sharp flaws in glass substrate results
in a significant loss of strength. Present approaches for
addressing that problem therefore rely on surface removal,
typically by grinding and polishing or chemical etching, to simply
remove the flawed surfaces from the glass substrate.
[0087] Without wishing to be limited by theory, it is suspected the
methods provided in the present disclosure relate to flaw
modification rather than complete removal of flaws. In accordance
with such modifications the crack propagation characteristics of
the surface flaws are then shifted from behaviors governed by
fracture mechanics to those more governed by stress concentration
behavior.
[0088] A comparison of such differing crack propagation behaviors
is schematically presented in FIG. 7. As shown in FIG. 7, for a
glass substrate with a surface S under an applied tensional stress
indicated by opposing arrows in the drawing, the magnitude of the
resulting stress .sigma..sub.f at sharp crack tip f of depth "a" in
glass surface S depends on the elastic modulus E and the surface
energy .gamma..sub.s of the glass in accordance with the
expression:
.sigma. f = ( 2 E .gamma. s .pi..alpha. ) 1 2 ##EQU00003##
[0089] In contrast, the resulting stress .sigma..sub.A at a rounded
or blunted crack tip T in a crack of the same depth a in the glass
surface under the same globally applied tensional stress .sigma. is
governed by the radius .rho. of the rounded crack tip in accordance
with the expression:
.sigma. A = .sigma. ( 1 + a .rho. ) ##EQU00004##
[0090] Thus, the resulting stress at blunted crack tip T is reduced
in proportion to the degree of crack tip rounding, ultimately
approaching the far-field or globally applied stress .sigma. as the
tip radius .rho. becomes very large.
[0091] The strengthening effects of surface flaw modification in
unstrengthened glass panel substrates for information display
applications using treatment with an aqueous acid solution are both
critical and unexpected. In particular embodiments, these
unstrengthened substrates are composed of alkaline earth
boro-aluminosilicate glasses wherein the total concentration of
alkali metal oxides selected from the group consisting of
Na.sub.2O, K.sub.2O and Li.sub.2O is low or limited to impurity
levels. Typically, the concentration of alkali metal oxides in the
glass sheet does not exceed 5% by weight, or in particular
embodiments does not exceed 1% or even 0.1% by weight. In contrast
to the case of chemically strengthened cover glass substrate plates
for video displays, wherein very high surface stresses (on the
order of 500 MPa or more) are present in the chemically
strengthened substrate to help combat the effects of surface
flawing, alkaline earth boro-aluminosilicate glasses treated in
accordance with the present disclosure are neither chemically
strengthened nor chemically strengthenable. Thus, these display
substrates are substantially free of surface compression layers and
central tensional stresses. In addition, the substrates are in many
cases very thin, with typical embodiments comprising substrates
having thicknesses not exceeding 500 .mu.m, or in the case of
flexible substrates, thicknesses of less than 300 .mu.m, such as in
the range of 50-200 .mu.m.
[0092] The failure strengths of unstrengthened, typically flawed
glass panels of the above described low-alkali glasses can be quite
low. Down-drawn alkaline earth boro-aluminosilicate glass sheets of
0.5 mm thickness subjected to conventional cutting and separation
processes in the course of preparing panels for use in active
matrix display fabrication can have modulus of rupture strengths as
low as 100-200 MPa when subjected to standardized ring-on-ring
flexural strength testing. Traditional fracture mechanics analyses
of these breakage data suggest the presence of flaws with depths in
the range of 15-60 .mu.m for glass sheet of this composition and
thickness failing at these strength levels.
[0093] Glass strength is most frequently expressed using a Weibull
distribution representing the probability of glass failure
(fracture) occurring due to a given stress. In the more common two
parameter Weibull distribution, the probability of failure may be
represented by the equation:
P f = 1 - exp [ - ( .sigma. max .sigma. 0 ) m ] ( 1 )
##EQU00005##
where P.sub.f is the probability of failure, .sigma..sub.max is the
maximum tensile stress in a test specimen at failure, .sigma..sub.0
is the Weibull characteristic strength corresponding to a
P.sub.f=63% and m is the Weibull modulus. The two parameters m and
.sigma..sub.0 are determined experimentally by stressing a sample
of the material until fracture (failure) occurs. The stress value
.sigma..sub.0 is the experimentally determined stress such that
fracture occurs with the probability noted above. The Weibull
modulus provides a measure of the statistical spread of the
distribution around Go. The stress at failure for a plurality of
samples is measured and the probability of failure (and the Weibull
parameters .sigma..sub.0 and m) can be determined.
[0094] The amount of treatment with an aqueous acid solution needed
to secure crack tip modifications that can provide effective glass
substrate strengthening in accordance with the present disclosure
is unexpectedly low, and can be completed within short time periods
under relatively mild etching conditions. As an illustrative
example, treatment with an aqueous acid solution sufficient to
remove approximately 2.5 um of surface glass from a commercially
available alkaline earth boro-aluminosilicate glass panel can be
achieved within a one minute exposure of the glass, to a dilute
aqueous fluoride solution, such as a solution comprising HF and HCL
in 3M and 6M concentrations, respectively. For purposes of the
present description a dilute aqueous fluoride solution is a
solution containing less than about 10% HF, or its equivalent, by
weight. Flaws of a size and configuration effective to produce
glass panels with failure strengths in the 100-200 MPa range can be
artificially introduced into the unstrengthened (non-compressed)
surfaces of low-alkali glasses such as the boro-aluminosilicates by
abrading at least one surface of the panel with a silicon carbide
abrasive. FIG. 8A is a photomicrograph of the surface of such a
panel, wherein surface flaws of a diameter or depth in the 15-60 um
range were identified. An example of a flaw comprising a sharp
crack tip is identified by the label "f" in the micrograph.
[0095] FIG. 8B is a photomicrograph of the surface of an
identically abraded glass panel after a short etching treatment to
achieve flaw crack tip modification. The treatment with the aqueous
acid solution employed was a one-minute exposure of the abraded
surface of the panel to the HF-HCL etching solution described
above, that treatment being effective to remove about 2.5 .mu.m of
surface glass from the panel. The significant improvement in
surface by the reduction in apparent surface abrasion is clearly
visible, even for so short an exposure time as 1 minute.
Nevertheless, there is no requirement to remove or substantially
reduce the numbers or sizes of the surface flaws in order to
achieve effective crack tip shape modifications in accordance with
the present disclosure.
[0096] The strengthening effects of the above disclosed flaw
modification through treatment with an aqueous acid solution are
evident from the data presented in FIG. 9 of the drawings. FIG. 9
presents Weibull plots of sample glass panel failure probabilities
over a range of applied ring-on-ring flexural forces applied to
sample glass substrates for two different groups of cut
display-type glass substrate samples. The first group of glass
substrates comprises alkaline earth boro-aluminosilicate glass
substrate samples of 0.5 mm thickness prepared via the cutting and
separation of continuously down-drawn glass ribbon in the manner
presently employed commercially to make panel assemblies for use in
active matrix display fabrication. Those samples thus comprise
surface flaws typically introduced during such processing. The
second group of glass substrates comprises similarly cut and
separated glass substrate samples of the same size, shape and
thickness, but which have been subjected to treatment with the
above-described HF-HCL solution for treatment times of 1, 3 and 5
minutes.
[0097] Analyses of the data presented in FIG. 9 indicate failure
stresses falling in the range of about 100 to 200 MPa for the
untreated panel samples. Curve 94 is a linear fit to the failure
probability data of the first group. In contrast, average failure
stresses in the range of about 400-480 MPa are calculated for the
glass substrate samples treated with the aqueous acid solution.
Curve 96 is a linear fit to the failure probability data of the
second group. The fact that the presently disclosed methods of
treating with an aqueous acid solution succeed through an effective
modification of the crack tip propagation characteristics of flawed
glass substrate, rather than by significantly reducing the
diameters, depths or numbers of such flaws, is shown by the fact
that the levels of strengthening achieved in accordance with the
above-described surface treatments are nearly independent of the
amount of surface glass removed from the flawed surfaces. Thus, the
five-minute etching treatment applied to the flawed glass
substrates characterized in FIG. 9, which removes as much as 14
.mu.m of surface glass from the treated panels, results in a level
of substrate strengthening that is only slightly higher than the
one-minute aqueous acid solution treatment used to remove only 2.5
.mu.m of surface glass. Thus, in particular embodiments, treatment
methods wherein the step of contacting the surface of a glass sheet
or panel with a selected aqueous acid solution is carried out for a
time not exceeding a time effective to remove a 5 .mu.m-thick
surface layer of glass from the panel can be sufficient to realize
the necessary improvements in flaw crack tip propagation
behavior.
[0098] The strengthening effects of flaw modification treatments
according to embodiments of the present disclosure are also evident
from the data presented in FIG. 10 of the drawings. FIG. 10 depicts
Weibull plots of failure probabilities over a range of applied
flexural forces applied to sample glass substrate assemblies for
two different groups of glass substrate assembly samples (e.g.
glass substrate assemblies 10) cut from a master glass substrate
assembly (e.g. glass substrate assembly 10a). The first group of
glass substrate assemblies 10 were separated from a master glass
substrate assembly 10a comprising two alkaline earth
boro-aluminosilicate glass substrate samples of 0.5 mm thickness
joined with a UV-curable epoxy adhesive and containing a liquid
crystal material sealed between the two glass substrates. The
overall thickness of the master glass substrate assembly was
approximately 1.05 mm. The master glass substrate assembly was then
chemically thinned to an overall thickness of about 0.63 mm and cut
into individual glass substrate assemblies 10 by conventional
mechanical scoring and breaking techniques. The second group of
glass substrate assemblies were similarly thinned, cut and
separated glass substrate assemblies 10 of the same size, shape and
thickness as the control group, but the edges were sealed with the
UV-curable epoxy to prevent liquid ingress and the individual
display panels treated by contacting the individual display panels
with an HF-HCL solution of 2.0M HF and 4.5M HCl at about 30.degree.
C. for a treatment time of 3 minutes, followed by two de-ionized
water rinses for a total of 20 minutes. Depth of material removal
was approximately 5 .mu.m from each glass substrate surface. The
sample size for both the control and the samples treated with the
aqueous acid solution was 180. Each glass substrate assembly had a
diagonal measurement of 24.6 cm.
[0099] Fracture testing was conducted using a ball on ring testing
method as illustrated by FIG. 11, wherein a sample display panel is
positioned on a rigid ring (e.g. container 82 as in FIG. 5) having
a diameter d' of 42.5 mm. A 30 mm diameter steel ball 98 (or other
hard material capable of producing a point force) centered over the
ring is forced into the surface of article to be tested, e.g. glass
substrate assembly 10, at a controlled speed of 10 mm/min, as
indicated by force F in the figure. Curve 100 is a linear fit to
the failure probability data of the first, untreated group, and
curves 102a and 102b indicate the 95% confidence intervals of the
fit. In comparison, curve 104 is a linear fit to the failure
probability data of the second, treated group and curves 106a and
106b indicate the 95% confidence interval of the fit.
[0100] Analyses of the data presented in FIG. 10 indicate a
characteristic strength .sigma..sub.0 in Newtons at 63% probability
for the post-thinned control samples not treated with the aqueous
acid solution to be 343 N with an m value of 1.4. On the other
hand, the post-thinning samples treated with the aqueous acid
solution had a characteristic strength .sigma..sub.0 in Newtons at
a 63% probability of 887 N and an m value of 9.30, showing that
treatment with an aqueous acid solution as described herein
provided a significant increase in strength.
[0101] Treatment with an aqueous acid solution described in
examples herein, wherein the surface of a glass sheet or panel is
contacted with a selected aqueous acid solution, are carried out
for a time not exceeding a time effective to remove no more than
about a 20 .mu.m-thick surface layer of glass from one or both
major surfaces of the glass sheet and/or panel, for example not
more than about 15 .mu.m, not more than 10 .mu.m, and in some
examples, not more than about 5 .mu.m, which material removal
amounts are sufficient to realize the aforementioned improvements
in flaw crack tip propagation behavior. According to embodiments of
the present disclosure, Weibull distributes can be obtained that
exhibit a characteristic strength, .sigma..sub.0, at a probability
of 63% that are equal to or greater than 800 N, greater than 820 N,
greater than 840 N, greater than 860 N, greater than 880 N, and
even greater than 900 N, with a Weibull modulus, m, greater than
8.0, greater than 8.4, greater than 8.8, and even greater than 9.2.
A comparison of glass strength after treatment with an aqueous acid
solution according to embodiments of the present disclosure to
untreated glass strength can be seen in reference to FIG. 5,
wherein bar 108 represents the failure stress of a glass substrate
assembly 10 after treatment with an aqueous acid solution. As
indicated, the glass substrate assembly treated with the aqueous
acid solution had a failure stress of approximately 1000 MPa,
nearly has high a failure stress as the individual glass substrates
provided at steps 32, 42. The dramatic increase in glass strength
after treatment with an aqueous acid solution according to examples
of the present disclosure can be observed by noting the stark
difference in the two Weibull plots shown in both FIGS. 6 and
10.
[0102] The glass substrate strengthening methods of the present
disclosure are applicable to a wide range of differing glass
substrate thicknesses and the glass substrate assemblies made
therefrom. Included are individual flexible glass substrates with
thicknesses of less than 200 .mu.m, or less than 100 .mu.m, or even
less than 50 .mu.m. Due to the configurations of electronic display
devices for which flexible glass substrates are designed, stress
levels much higher than are experienced by traditional active
matrix video display substrates are expected. Particular
embodiments of the presently disclosed substrates, e.g., substrates
having thicknesses not exceeding 200 .mu.m in combination with
sufficient strength and flexibility to resist breakage at a bend
radius of 10 cm following treatment with an aqueous acid solution
in accordance with the present disclosure, are examples of products
that will provide significant performance advantages for flexible
substrate applications. And these advantages are in addition to the
substantial processing advantages resulting from the relatively
mild treatment conditions and minimal treatment times useful for
effective glass substrate strengthening in accordance with the
present description.
[0103] Wide variations in processing conditions and aqueous acid
solutions can be resorted to for the practice of the disclosed
methods while still securing the benefits of such methods. HF and
ammonium fluoride solutions, with or without additions of mineral
acids such as HCl, HNO.sub.3 and H.sub.2SO.sub.4, are examples of
suitable media. Other example aqueous acidic etching solutions may
comprise a solution of HF and HCl such that a molar concentration
of the HF is in a range from about 0.1M to about 6M and a molar
concentration of the HCl is in a range from about 0.1M to about
7.0M. In other examples the aqueous acidic solution may comprise HF
and HCl wherein a molar concentration of the HF is in a range from
about 0.1M to about 3.0M and a molar concentration of the HCl is in
a range from about 0.1M to about 6.0M. In still other examples the
aqueous acidic solution comprises HF and HCl wherein a molar
concentration of the HF is in a range from about 1.5M to about 2.5M
and a molar concentration of the HCl is in a range from about 4.0M
to about 5.0M. In yet other examples the aqueous acidic solution
comprises HF and H.sub.2SO.sub.4, or HF and HNO.sub.3 or a
combination thereof wherein the molar concentration of
H.sub.2SO.sub.4 and/or HNO.sub.3 is equal to any of the ranges for
HCl above. Typically, the substrate glass is contacted by the
aqueous acid solution for a time not exceeding 10 minutes. However,
particular fluoride concentrations of the solution will be selected
depending primarily upon the treatment times and temperatures
desired within which to achieve effective flaw crack tip rounding,
these in any case being readily determinable through routine
experiment.
[0104] Accordingly, suitable contacting times in accordance with
embodiments of the present disclosure may fall within a range from
about 10 seconds to about 10 minutes, in some examples in a range
from about 10 seconds to about 5 minutes, in some examples in a
range from about 10 seconds to about 4 minutes and in other
examples in a range from about 10 seconds to about 3 minutes. In
other examples suitable contacting times may fall within a range
from about 10 seconds to about 2 minutes and in still other
examples suitable contacting times may be equal to or less than
about 1 minute.
[0105] The contacting may remove glass from each of the first and
second substrates to a depth equal to or less than about 20 .mu.m.
For example, the contacting may remove glass from each of the first
and second substrates to a depth equal to or less than about 15
.mu.m and in other examples the contacting may remove glass from
each of the first and second substrates to a depth equal to or less
than about 10 .mu.m. In still other examples the contacting may
remove glass from each of the first and second substrates to a
depth equal to or less than about 5 .mu.m.
[0106] In some embodiments, an optional polymer film 109 may be
applied to the surface of the treated glass substrates after
contacting the surfaces of the glass substrates with the aqueous
acid solution. A suitable polymer film is, for example, Visqueen
manufactured by British Polyethylene Industries Limited. For
example, the second major surfaces 20 and 26 of a glass substrate
assembly 10 (or 10a) may be treated with an aqueous acid solution
according to embodiments disclosed herein. The aqueous acid
solution may then be rinsed from the glass substrate assembly, and
a polymer film attached to one or both second surfaces 20 and/or
26. The film can remain attached to the second major surfaces
until, for example, a polarizing film is to be attached to one or
both of the second major surfaces.
[0107] Testing has found that glass sheets (e.g. glass substrate
assemblies) may also benefit from increased edge strength as a
result of the foregoing treatment with an aqueous acid solution.
This can be useful in the instance where a master glass substrate
assembly is later separated by scoring and breaking to produce
smaller individual glass substrate assemblies. For example, FIG. 12
depicts a Weibull distribution based on four point bending of
individual glass substrate assemblies 10 after separation of the
glass substrate assemblies from a master glass substrate assembly
10a. See for example, ASTM C1683-10, Standard Practice for Size
Scaling of Tensile Strengths Using Weibull Statistics for Advanced
Ceramics. Curve 110 represents a linear fit to the failure
probability data for a first group of glass substrate assemblies 10
that were untreated by an aqueous acid solution. Curves 112a and
112b indicate the 95% confidence intervals for the fit. On the
other hand, curve 114 represents a linear fit to the data of a
second group of individual glass substrate assemblies 10 after
separation of the glass substrate assemblies from a master glass
substrate assembly 10a, and after treatment with an aqueous acid
solution according to embodiments of the present disclosure. Curves
116a and 166b depict the 95% confidence interval of the first and
second fits, respectively. The data show a dramatic increase in
edge strength as a result of the treatment with an aqueous acid
solution. Results from strengthening the edges of sheets can also
be expected as the substrate increases in length for applications
as spooled ribbon in roll-to-roll processes.
[0108] The disclosed strengthening methods are not limited in their
application to alkaline earth boro-aluminosilicate glass
compositions alone. Other low alkali silicate glass sheets
providing surfaces suitable for active semiconductor device
deposition can be similarly treated with similarly satisfactory
results. Also, other glass compositions not suitable for active
semiconductor device deposition but optimized for other
applications such as lighting, photo-voltaics and touch sensors is
also possible. Further, it can be useful in some cases to apply
these strengthening methods in combination with the application of
surface coatings to the substrates that can offer protection for
the treated surfaces of the substrates. Color filters or other
polymeric protective layers are examples of such surface coatings.
U.S. patent publication US2012/0040146 describes an example of edge
coatings.
[0109] As noted above, there is increasing interest in the
development of mechanically durable flexible glass substrates
having sufficient strength and flexibility for use in roll-to-roll
active matrix display, photo-voltaic, lighting and touch sensor
device manufacturing processes. Although methods to etch rigid
glass substrates are known, treatment of spooled flexible glass
ribbons with an aqueous acid solution in accordance with the
present disclosure provides increased glass strength and thus
increased glass ribbon flexibility that can secure clear advantages
for such use.
[0110] Flexible glass substrates for active matrix display and
other applications should have a thickness of less than 300 .mu.m,
or in some embodiments not exceeding 200 .mu.m, with thicknesses in
the range of 50-100 .mu.m being best adapted for use in
roll-to-roll processing of a ribbon of thin glass. Suitable roll
widths for such processing range from 1 cm to over 1 m, while
ribbon lengths may range from 1 m to over 1 km. Flexible glass
ribbons of these dimensions is presently manufactured via a variety
of down-draw forming processes, including overflow down-drawing
(fusion), slot drawing, and glass sheet redrawing. As is the case
for the manufacture of conventional active matrix display panels,
the glass compositions used for the manufacture of flexible glass
ribbons are chosen for mechanical durability and compatibility with
the selected forming process as well as the ability to provide
substrates with surfaces that can stably support the semi-conductor
components needed for active matrix display element fabrication. In
particular embodiments the total concentration of alkali metal
oxides selected from the group consisting of Na.sub.2O, K.sub.2O
and Li.sub.2O in the glass forming the particular ribbon or
substrate does not exceed 5% by weight, or does not exceed 1% or
even 0.1% by weight.
[0111] Methods for processing spooled glass ribbons by sequentially
contacting the ribbon with an aqueous acid solution include
embodiments wherein the step of sequentially contacting comprises
continuously transporting the glass ribbon into contact with, and
then separating the glass ribbon from contact with the aqueous acid
solution. In particular embodiments a roller system can be used to
convey a flexible glass ribbon through a process wherein the
aqueous acid solution is sprayed onto surfaces of the glass ribbon
without the use of protective coatings or laminates. A suitable
arrangement for processing spooled glass ribbons in accordance with
these methods is schematically illustrated in FIG. 13, wherein a
surface 118 of a length of flexible glass ribbon 120 is contacted
with an aqueous acid solution from a plurality of spray nozzles, as
represented by reference numeral 122, as the ribbon is transported
in a direction indicated by arrow 124. As illustrated in FIG. 13,
this enables treatment of the complete surface and edges of the
flexible glass ribbon with an aqueous acid solution, with the
benefit of providing a flexible glass ribbon that is free from
surface and edge defects caused by glass drawing or previous
handling. In these embodiments the step of contacting the lengths
of the elongated glass ribbon with the aqueous acid solution is
carried out for a time and at a temperature effective to blunt tip
portions of cracks emanating from the surface flaws. Besides use of
a spray system, the flexible glass ribbon can contact the aqueous
acid solution in a bath meant to immerse the glass
sequentially.
[0112] At present it is customary to set the widths for spooled
glass ribbon to specific dimensions using continuous mechanical or
laser cutting approaches that can generate mechanical flaws along
the cut edges of the ribbon. In accordance with the present
disclosure these flaws are avoided by using the aqueous acid
solution to continuously separate the glass ribbon longitudinally
into spooled glass ribbons of a selected width. For that purpose
the elongated glass ribbon is provided with plural, spaced parallel
masking layers extending lengthwise in an elongation direction over
the surface of the glass ribbon, and wherein the step of
sequentially contacting is carried out for a contact time and at a
contact temperature effective to cause glass ribbon separation
between the masking layers.
[0113] FIG. 14 presents a schematic illustration of an arrangement
for longitudinally separating a length of spooled glass ribbon into
two glass ribbons. In accordance with that method two parallel
bands of a mask coating 126 that is effective to prevent contact
between the aqueous acid solution and the spooled glass ribbon are
applied to the surface of the glass ribbon 120. With this masking
in place, the aqueous acid solution applied to the exposed glass
between the masking bands can effect the separation of the glass
ribbon into two glass ribbons without introducing edge or surface
flaws to the separated glass ribbons.
[0114] An alternative method for addressing the problem of edge
flaws in spooled glass ribbon that has been separated by mechanical
or laser means involves preferentially applying an aqueous acid
solution to the edges of a spooled glass ribbon. In particular
embodiments aqueous acid paste 128 having a paste consistency is
used, and the step of sequentially contacting successive lengths of
the spooled glass ribbon comprises applying the paste to cut edges
of the traversing glass ribbon. This method is schematically
illustrated in cross-section in FIG. 15 of the drawings, wherein
the acidic paste is shown as applied to the edges of a glass ribbon
120. The acidic paste is applied along the edge face of a glass
ribbon before an edge laminate is applied. In that embodiment only
a minimal amount of acidic paste is used along the edge face, the
objective being to merely re-shape any pre-existing flaws in the
glass. The paste can be sealed to the glass with either a permanent
or temporarily bonded edge laminate 130.
[0115] Alternative methods for improving the edge strength of
spooled glass ribbon involve selectively treating ribbon edges with
an aqueous acid solution. FIG. 16 of the drawings provides a
schematic illustration of this method. As shown in FIG. 16, a glass
ribbon 120 to be treated is temporarily supported or attached to a
carrier web 132, for example a polymer film that is temporarily
attached to the glass through the use of a static charge or other
method as it is transported in a glass direction through an etching
station. Side jets direct a spray of aqueous acid solution
according to embodiments disclosed herein toward the edges of the
ribbon spaced away from the central surface of the glass ribbon, as
represented by the arrows 134. Following the application of the
aqueous acid solution, a de-ionized (DI) water rinse is used to
remove the aqueous acid solution and forced air drying is used to
remove the rinse water. An air knife or other method can be used to
contain the aqueous acid solution along the glass web edge, and
after contacting the glass ribbon with the aqueous acid solution
the carrier web may be removed and an optional edge lamination
applied to further protect the treated edge from damage.
[0116] In cases where a spooled glass ribbon is of high edge
quality but includes sharp flaws over central sections of the glass
ribbon formed during the forming or handling of the glass ribbon, a
roll-to-roll spray system can be used. FIG. 17 of the drawing
presents a schematic illustration of the use of such a system. As
illustrated in FIG. 17, glass ribbon 120 is continuously
transported in direction 124 past a spray nozzle array 122 emitting
the aqueous acid solution while an edge laminate 130 is applied to
the opposing high quality edges of the glass ribbon.
[0117] While foregoing examples relied upon treatment of glass
substrates, or substrate assemblies formed from individual glass
substrates, with an aqueous acid solution to modify flaw geometry
(e.g. the blunting of crack tips without significant material
removal from the surface of the glass substrate, it is recognized
that process variables may be controlled so as to eliminate
aggressive thinning and washing of, for example, a glass substrate
assembly by repeated treatment of the glass substrate assembly with
an aqueous acid solution described in the present disclosure.
Accordingly, aqueous acid solutions of the present disclosure may
be contained, for example, in separate containers, and wherein the
glass substrate or substrate assembly is treated in each separate
aqueous acid solution for a time sufficient to remove a
predetermined portion of the total material to be removed. The
concentration of acid or acids of each aqueous acid solution need
not be the same concentration as the aqueous acid solution used in
a subsequent treatment, and the treatment time for the substrate or
substrate assembly in each aqueous acid solution need not be equal
to another treatment time in another aqueous acid solution.
[0118] Referring to FIG. 18, a sub-process 200 for removing a
predetermined amount of material as desired is shown, such as a 200
.mu.m layer of glass, a glass article (e.g. glass substrate or
substrate assembly), generically labeled as reference numeral 202,
may be treated in a first aqueous acid solution 204 for a time
sufficient to remove a 70 .mu.m thick layer of glass. The glass
article 202 can then be moved to a second aqueous acid solution 206
and treated for a time sufficient to remove another portion of the
total material removal depth, such as an additional 70 .mu.m. The
glass article 202 may then be treated for a third time in a third
aqueous acid solution 208 sufficient to removed still more
material, such as the final 60 .mu.m of the total desired 200 .mu.m
of material removal. In accordance with the description above, the
concentration of acids in the first aqueous acid solution 204 need
not be the same concentration of acids in either the second or
third aqueous acid solutions 206, 208 respectively, and the
concentrations of acids in the second aqueous acid solution 206
need not be the same concentration of acids in the first or third
aqueous acid solutions, 204 and 208, respectively.
[0119] Removal of material in successive steps, in separate aqueous
acid solutions, either by spraying methods as previously described,
or by immersion in separate baths (e.g. separate containers of
aqueous acid solution) reduces the buildup of aqueous acid solution
treatment by-products (sludge), thereby extending the effective
longevity of the aqueous acid solution and reducing costs.
[0120] The preceding sequential material removal steps described in
reference to FIG. 18 may further include one or more rinsing steps
in a rinsing solution 210. For example, the glass substrate or
substrate assembly may be rinsed after the final treatment step as
shown in FIG. 18, or rinsed after each treatment step, or a
combination thereof. By rinsing what is meant is exposing the glass
substrate or substrate assembly to a rinsing solution, such as
de-ionized water, without brushing or other mechanical contact
(other than contact necessary to manipulate the glass article).
[0121] FIG. 19 illustrates an exemplary glass substrate assembly
manufacturing process 220 based on process 37 of FIG. 3, wherein
the thinning process comprising steps 64 and 66 has been replaced
by process 200 described supra.
[0122] While the methods and products disclosed herein have been
described above with reference to specific embodiments thereof, it
will be recognized that those examples have been presented for
purposes of illustration only, and that a wide variety of
alternative procedures and product features may be adopted for the
practice of the disclosed methods and the design of additional
products within the scope of the appended claims.
* * * * *