U.S. patent application number 10/487770 was filed with the patent office on 2007-02-08 for surface treating solution for fine processing of glass base plate having a plurality of components.
Invention is credited to Hirohisa Kikuyama, Masayuki Miyashita, Tadahiro Ohmi, Tatsuhiro Yabune.
Application Number | 20070029519 10/487770 |
Document ID | / |
Family ID | 19083362 |
Filed Date | 2007-02-08 |
United States Patent
Application |
20070029519 |
Kind Code |
A1 |
Kikuyama; Hirohisa ; et
al. |
February 8, 2007 |
Surface treating solution for fine processing of glass base plate
having a plurality of components
Abstract
A surface treatment solution for finely processing a glass
substrate containing multiple ingredients is used for the
construction of liquid crystal-based or organic
electroluminescence-based flat panel display devices without
invoking crystal precipitation and/or increasing surface roughness.
An etching solution of the invention contains, in addition to
hydrofluoric acid (HF) and ammonium fluoride (NH.sub.4F), at least
one acid whose dissociation constant is larger than that of HF. The
concentration of the acid in the solution can advantageously be
adjusted to maximize the etching rate.
Inventors: |
Kikuyama; Hirohisa; (Osaka,
JP) ; Yabune; Tatsuhiro; (Osaka, JP) ;
Miyashita; Masayuki; (Osaka, JP) ; Ohmi;
Tadahiro; (Miyagi, JP) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET
2ND FLOOR
ARLINGTON
VA
22202
US
|
Family ID: |
19083362 |
Appl. No.: |
10/487770 |
Filed: |
August 26, 2002 |
PCT Filed: |
August 26, 2002 |
PCT NO: |
PCT/JP02/08564 |
371 Date: |
September 8, 2006 |
Current U.S.
Class: |
252/79.1 |
Current CPC
Class: |
C03C 15/00 20130101 |
Class at
Publication: |
252/079.1 |
International
Class: |
C09K 13/00 20060101
C09K013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 24, 2001 |
JP |
2001-255387 |
Claims
1. (canceled)
2. A surface treatment solution for finely processing a glass
substrate containing multiple ingredients which contains as a
uniform mixture, in addition to hydrofluoric acid (HF) and ammonium
fluoride (NH.sub.4F), at least one acid whose dissociation constant
is larger than that of HF such that, if the solution is applied to
a heated silicon oxide film having a certain thickness, and the
etching rate f(x) [.ANG./min] is plotted as a function of the
concentration x [mol/kg] of the acid in the solution, and it is
found that f(x) takes a maximum f(x.sub.1) when x=x.sub.1, the
concentration x of the acid in the solution is adjusted to be
x>x.sub.1.
3. A surfaue treatment solution tor tinely processing a glass
substrate containing multiple ingredients which contains as a
uniform mixture, in addition to HF and NH.sub.4F, at least one acid
whose dissociation constant is larger than that of HF such that, if
the solution is applied to a heated silicon oxide film having a
certain thickness, and the etching rate f(x) [.ANG./min] is plotted
as a function of the concentration x (mol/kg] of the acid in the
solution, and it is found that f(x) takes a maximum f(x.sub.1) when
x=x.sub.1 and a minimum f(x.sub.2) when x=x.sub.2
(x.sub.1<x.sub.2), the concentration x of the acid in the
solution is adjusted to be in the range:
x.sub.2-(x.sub.2-x.sub.1)/2<x<x.sub.2+(x.sub.2-x.sub.1)/2.
4. A surface treatment solution as described in claim 2 or 3 for
finely processing a glass substrate containing multiple ingredients
which contains as a uniform mixture, in addition to HF and
NH.sub.4F, at least one inorganic acid either monovalent or
multivalent whose dissociation constant is larger than that of
HF.
5. A surface treatment solution as described in any one of claims 2
to 4 for finely processing a glass substrate containing multiple
ingredients wherein the acid whose dissociation constant is larger
than that of HF is one or more chosen from the group comprising
HCl, HBr, HNO.sub.3, and H.sub.2SO.sub.4.
6. A surface treatment solution as described in any one of claims 2
to 5 for finely processing a glass substrate containing multiple
ingredients which further contains a surfactant at 0.0001 to 1 wt
%.
7. A surface treatment solution as described in any one of claims 2
to 6 for finely processing a glass substrate containing multiple
ingredients, particularly a glass substrate containing, in addition
to silicates or its main ingredient, one or more elements chosen
from the group comprising Al, Ba, Ca, Mg, Sb, Sr and Zr.
8. A surface treatment solution as described in any one of claims 2
to 7 for finely processing a glass substrate containing multiple
ingredients, particularly a glass substrate used for the
construction of a flat panel display device.
9. A surface treatment solution as described in any one of claims 2
to 8 for finely processing a glass substrate containing multiple
ingredients which contains HF at 25 mol/kg or lower.
10. A surface treatment solution as described in any one of claims
2 to 9 for finely processing a glass substrate containing multiple
ingredients which contains NH.sub.4F at 0.001 to 11 mol/kg.
11. A surface treatment solution as described in any one of claims
2 to 10 for finely processing a glass substrate containing multiple
ingredients wherein: if etching is performed at an appropriate
temperature using an acid-added etching solution with the
concentration x [mol/kg] of acid being varied, and it is found that
crystal precipitation remains inhibited as long as x is below
x.sub.3 or the maximum effective concentration, the concentration x
of acid in the solution is adjusted to be x<x.sub.3.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a surface treatment
solution for finely processing the surface of a glass substrate
containing multiple ingredients. More specifically, the present
invention relates to a surface treatment solution, useful for the
fine surface processing of glass substrates, containing
cation-yielding elements and their cation-yielding oxides, which is
very profitably used for wet-etching/cleaning the surface of such
glass substrates or etching/cleaning the surface of such glass
substrates carrying finely fabricated semiconductor elements
thereon during the fabrication of semiconductor devices.
[0003] 2. Related Art
[0004] In the wet processing of glass panels for flat panel display
devices, pattern-etching/cleaning of glass substrates containing
cation-yielding elements and their cation-yielding oxides and
purification/fine-processing of pattern-etched such glass
substrates using an etching solution has been put into practice.
Yet, demand for a technique enabling the more fine processing of
glass substrates has become increasingly acute with the advent of
more highly resolved display devices. For the fine surface
processing of glass substrates which will serve as display panels,
hydrofluoric acid (HF) or a mixture (buffered hydrofluoric acid
(BHF)) of hydrofluoric acid (HF) and ammonium fluoride (NH.sub.4F)
has been used as an important and indispensable agent for
etching/cleaning glass substrates. However, demand for an etching
agent enabling finer processing of glass substrates becomes
manifest to further improve the performance and resolution of
display devices.
[0005] Glass substrates to be used for the construction of liquid
crystal (LC)-based or organic electroluminescence (EL)-based flat
panel display devices have come to have an increasingly reduced
thickness to meet the demand for more compact and power-saving
display devices. In the manufacture of such glass substrates,
however, the so-called mother glass plate has an increasingly
larger size to improve the production efficiency and reduce the
production cost. A glass substrate is obtained by thinning a mother
glass plate.
[0006] However, the mechanical thinning of a mother glass plate has
a limitation, because a mother glass plate must have strength
sufficiently high enough to withstand stresses imposed during the
thinning process. Therefore, if further thinning is required for a
mechanically thinned glass plate (coarse glass substrate), the
coarse glass substrate must be subjected to another kind of fine
processing, e.g., chemical fine processing.
[0007] However, if a coarse glass substrate containing multiple
ingredients, particularly cation-yielding elements and their
oxides, is etched/cleaned by means of a conventional etching
solution comprising HF or BHF, following two problems are
encountered which interfere with the uniform etching/cleaning.
[0008] 1) Crystals develop on the surface of a glass substrate and
adhere thereto.
[0009] 2) The surface of a glass substrate becomes significantly
roughened after the treatment.
[0010] With regard to the problem 1), analysis of crystals adhered
to the surface of a glass substrate revealed that it is composed of
fluorides of cation-yielding elements contained in the glass
substrate. Fluorides of cation-yielding elements have such a low
solubility to HF and BHF as well as to water, that they are easily
crystallized to adhere onto the surface of a glass substrate. The
present inventors succeeded in reaching this finding for the first
time.
[0011] The problem 2) was ascribed to that crystals developed on
the surface of a glass substrate and adhered thereto interfere with
etching and/or that cation-yielding elements and their oxides
contained in a glass substrate are differently susceptible to
etching, which causes the local variation in etching rate and
etching amount. The present inventors succeeded in obtaining this
knowledge for the first time.
[0012] The most important point of the technique for finely
processing glass substrates is to uniformly process or treat glass
substrates while preventing the occurrence of surface
roughness.
SUMMARY OF THE INVENTION
[0013] The present invention provides a surface treatment solution
for uniformly processing the surface of glass substrates containing
multiple ingredients such as glass substrates to be used for the
construction of LC-based or organic EL-based flat panel display
devices, with which it is possible to etch such glass substrates
without invoking crystal precipitation and surface roughness.
[0014] The present inventors had studied hard to solve the above
problems, and found that it is possible to solve the above problems
by providing a surface treatment solution which is specifically
adapted for the fine processing of glass substrates containing
multiple ingredients, the treatment solution containing, in
addition to HF and NH.sub.4F, at least one acid whose dissociation
constant is larger than that of HF. This finding led the inventors
to the present invention.
[0015] A first aspect of the present invention is to provide a
surface treatment solution for finely processing a glass substrate
containing multiple ingredients which contains as a uniform
mixture, in addition to HF and NH.sub.4F, at least one acid whose
dissociation constant is larger than that of HF.
[0016] A second aspect of the present invention is to provide a
surface treatment solution for finely processing a glass substrate
containing multiple ingredients which contains as a uniform
mixture, in addition to HF and NH.sub.4F, at least one acid whose
dissociation constant is larger than that of HF such that, if the
solution is applied to a heated silicon oxide film having a certain
thickness, and the etching rate f(x) [.ANG./min] is plotted as a
function of the concentration.times.[mol/kg] of the acid in the
solution, and it is found that f(x) takes a maximum f(x.sub.1) when
x=x.sub.1, the concentration x of the acid in the solution is
adjusted to be x>x.sub.1.
[0017] A third aspect of the present invention is to provide a
surface treatment solution for finely processing a glass substrate
containing multiple ingredients which contains as a uniform
mixture, in addition to HF and NH.sub.4F, at least one acid whose
dissociation constant is larger than that of HF such that, if the
solution is applied to a heated silicon oxide film having a certain
thickness, and the etching rate f(x) [.ANG./min] is plotted as a
function of the concentration x [mol/kg] of the acid in the
solution, and it is found that f(x) takes a maximum f(x.sub.1) when
x=x.sub.1 and a minimum f(x.sub.2) when x=x.sub.2
(x.sub.1<x.sub.2), the concentration x of the acid in the
solution is adjusted to be in the following range:
x.sub.2-(x.sub.2-x.sub.1)/2<x<x.sub.2+(x.sub.2-x.sub.1)/2.
[0018] A fourth aspect of the present invention is to provide a
surface treatment solution as described in any one of the foregoing
aspects of the invention for finely processing a glass substrate
containing multiple ingredients, which contains as a uniform
mixture, in addition to HF and NH.sub.4F, at least one inorganic
acid either monovalent or multivalent, whose dissociation constant
is larger than that of HF.
[0019] A fifth aspect of the present invention is to provide a
surface treatment solution as described in any one of the foregoing
aspects of the invention for finely processing a glass substrate
containing multiple ingredients wherein the acid whose dissociation
constant is larger than that of HF is one or more chosen from the
group comprising HCl, HBr, HNO.sub.3, and H.sub.2SO.sub.4.
[0020] A sixth aspect of the present invention is provide a surface
treatment solution as described in any one of the foregoing aspects
of the invention for finely processing a glass substrate containing
multiple ingredients which further contains a surfactant at 0.0001
to 1 wt %.
[0021] A seventh aspect of the present invention is to provide a
surface treatment solution as described in any one of the foregoing
aspects of the invention for finely processing a glass substrate
containing multiple ingredients, particularly a glass substrate
containing, in addition to silicates or its main ingredient, one or
more elements chosen from the group comprising Al, Ba, Ca, Mg, Sb,
Sr and Zr.
[0022] An eighth aspect of the present invention is to provide a
surface treatment solution as described in any one of the foregoing
aspects of the invention for finely processing a glass substrate
containing multiple ingredients, particularly a glass substrate
used for the construction of a flat panel display device.
[0023] A ninth aspect of the present invention is to provide a
surface treatment solution as described in any one of the foregoing
aspects of the invention for finely processing a glass substrate
containing multiple ingredients, which contains HF at 25 mol/kg or
lower.
[0024] A tenth aspect of the present invention is to provide a
surface treatment solution as described in any one of the foregoing
aspects of the invention for finely processing a glass substrate
containing multiple ingredients, which contains NH.sub.4F at 0.001
to 11 mol/kg.
[0025] An eleventh aspect of the present invention is to provide a
surface treatment solution as described in any one of the foregoing
aspects of the invention for finely processing a glass substrate
containing multiple ingredients wherein, if etching is performed at
an appropriate temperature using an acid-added etching solution
with the concentration x of acid being varied, and it is found that
crystal precipitation remains inhibited as long as x is below
X.sub.3 or the maximum effective concentration, the concentration x
of acid in the solution is adjusted to be x<x.sub.3
[mol/kg].
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The above-mentioned and other features and advantages of
this invention, and the manner of attaining them, will become more
apparent and the invention will be better understood by reference
to the following descriptions of the embodiments of the invention
taken in conjunction with the accompanying drawings, wherein:
[0027] FIG. 1 shows a graph relating the etching rate f(x) with the
concentration of x of the acid in the solution, when etching
solutions containing an acid whose dissociation constant is larger
than that of HF at different concentrations are applied to a heated
silicon oxide film the change of etching rate being traced;
[0028] FIG. 2 shows a graph illustrating when acid-added etching
solutions with the concentration of acid in the solution being
varied are applied to a glass substrate containing Al compounds,
with dots relating the concentration of Al ions in the solution
with the etched amount;
[0029] FIG. 3 shows a plot illustrating when acid-added etching
solutions with the concentration of acid in the solution being
varied are applied to a glass substrate containing Ba compounds,
with dots relating the concentration of Ba ions in the solution
with the etched amount;
[0030] FIG. 4 shows a graph illustrating when acid-added etching
solutions with the concentration of acid in the solution being
varied are applied to a glass substrate containing Ca compounds,
with dots relating the concentration of Ca ions in the solution
with the etched amount; and
[0031] FIG. 5 shows a plot illustrating when acid-added etching
solutions with the concentration of acid in the solution being
varied are applied to a glass substrate containing Sr compounds,
with dots relating the concentration of Sr ions in the solution
with the etched amount.
[0032] Corresponding reference characters indicate corresponding
parts through the several views. The exemplifications set out
herein illustrate preferred embodiments of the invention, in one
form, and such exemplifications are not to be construed as limiting
the scope of the invention in any manner.
DETAILED DESCRIPTION
[0033] The present invention will be detailed below. First, the
present inventors inquired about the causes responsible for the
formation of crystals and surface roughness.
[0034] As a result of etching, metal elements contained in a glass
substrate dissolve in an etching solution to turn into cations
there. Those cations derived from the etched glass substrate react
with anions derived from anion-yielding species in the etching
solution. For example, the cations react with fluorine ions
(F.sup.- ions) to produce metal salts (e.g., fluoride salts). As
such metal salts are hardly soluble to the etching solution, they
crystallize on the surface of the substrate and deposit there. This
crystal deposition on the surface of the glass substrate interferes
with surface etching which causes the etched surface to be
roughened, and the overall glass surface to look cloudy.
[0035] If etching is performed on a glass substrate containing
multiple ingredients such as cation-yielding elements and their
oxides, etching proceeds at widely different speeds according to
localities because of the ingredients having greatly different
susceptibilities to etching, which may cause the etched surface to
be roughened and irregular in profile.
[0036] To solve these problems, it is necessary to make uniform the
susceptibilities to etching of different ingredients contained in a
glass substrate, and to inhibit the development of fluorides
insoluble to an etching solution which are derived from the cations
emerging as a result of dissolution of the ingredients to the
etching solution. For this purpose, it is most important and
desirable to allow the ingredients, once they are dissolved in an
etching solution, to stably stay there as cations.
[0037] An effective measure for inhibiting the development of
fluorides which are hardly soluble to an etching solution is to
modify the reaction system in such a manner as to lower the
concentration of F.sup.- ion in the etching solution.
[0038] However, since the main ingredient of a glass substrate is
silicon oxide, in order to etch a glass substrate having such a
composition, the etching solution must include substances such as
HF or BHF that can dissolve silicon oxide.
[0039] On the other hand, HF.sup.2- ions act as dominant ions in
the etching of silicon oxide.
[0040] Accordingly, in order to uniformly etch a glass substrate
containing multiple ingredients, it is necessary not only to reduce
the concentration of F.sup.- ions in an etching solution but also
to promote the efficient generation of HF.sup.2- ions in the
solution. This purpose can be achieved by adding an appropriate
amount of acid whose dissociation constant is larger than that of
HF, to an etching/cleaning solution including at least either HF or
NH.sub.4F, because then the dissociation of HF in the solution will
become controllable. On the contrary, if an acid whose dissociation
constant is equal to or less than that of HF is added to the same
etching/cleaning solution, the efficient production of HF.sup.2-
will be hardly achievable.
[0041] In view of this, an etching solution according to the
invention must include, in addition to HF and NH.sub.4F, at least
one acid (high-ionizing acid) which has a larger dissociation
constant than HF.
[0042] The etching solution of the invention for glass substrates
containing multiple ingredients preferably contains an acid
(high-ionizing acid) whose dissociation constant is larger than
that of HF at a concentration of x [mol/kg]. Further, the x should
be x.ltoreq.x.sub.3 where x.sub.3 represents the maximum
concentration at which deposition of crystals on a glass substrate
can be prohibited at a temperature appropriate for etching. At such
a concentration, the composition of the etching solution becomes
uniform, and roughening of the etched glass surface is inhibited.
If x>x.sub.3, crystal precipitates developed in the etching
solution may adhere to the surface of a glass substrate to cause
the etched glass surface to be roughened, or crystal precipitates
may be brought into contact with a surface of the container to be
broken into fine particles which may adhere to the surface of a
glass substrate to cause it to be roughened.
[0043] The performance of an etching solution may be evaluated by
applying the solution to a heated silicon oxide film having a
certain thickness, measuring the thinning of the film over time,
and determining the etching rate (thinning per unit time). In the
same context, the performance of an etching solution containing a
high-ionizing acid may be evaluated by plotting the etching rate
f(x) [A/min] as a function of the concentration (x) of the acid.
The species of the acid is preferably chosen such that f(x) takes a
maximum f(x.sub.1) when x=x.sub.1(x.sub.1<x.sub.3), and a
minimum f(x.sub.2) when x=x.sub.2 (x.sub.1<x.sub.2) . This is
because ingredients of a glass substrate are highly soluble to an
etching solution containing an acid chosen as described above,
particularly to an etching solution whose acid satisfies the latter
requirement.
[0044] Next, the reason why the added amount of a high-ionizing
acid should be adjusted to make the concentration x of the acid in
the solution fall in the above range will be described.
[0045] In an etching solution comprising HF or BHF, HF and
NH.sub.4F dissociate into respective positive and negative ions,
and thus F.sup.- ions are produced. F.sup.- ions react with H.sup.+
ions in the solution to produce HF, a bound compound including
HF.sup.2-, or an ion. If an acid (high-ionizing acid) whose
dissociation constant is larger than that of HF is added to the
etching solution, H.sup.+ ions (newly added) react with F.sup.-
ions in the solution to produce HF or a bound compound with
HF.sup.2- ions.
[0046] Thus, free F.sup.- ions are consumed and their concentration
is reduced. As a consequence, HF.sup.2- ions become dominant with
the increased addition of the acid. If the performance of such an
acid-added etching solution is evaluated by applying the solution
to a heated silicon oxide film and by plotting the etching rate
f(x) as a function of the concentration (x) of the acid, the
etching rate f(x) increases with the increase of x, reaching a
maximum f(x.sub.1) at a certain value of x (x=x.sub.1)
[0047] However, if the concentration x is further increased, the
reaction system may undergo following reactions to maintain the
acid-base equilibrium: HF.sup.2- ions are decomposed into HF and
F.sup.- ions, and the F.sup.- ions react with newly added H.sup.+
ions to produce HF. As a consequence, the concentration of
HF.sup.2- ions is reduced, which leads to the reduction of the
etching rate. Then, the etching rate f(x) reaches a minimum
f(x.sub.2) at a certain value of x (x=x.sub.2).
[0048] With the further increase of x, the etching rate f(x) starts
to rise again.
[0049] As seen from the above description, it is possible to
control the dissociation state of ions in an etching solution,
particularly to reduce the concentration of F.sup.- ions in the
solution, by adding to the solution an acid whose dissociation
constant is larger than that of HF.
[0050] If a high-ionizing acid as described above is added to an
HF-based etching solution at a concentration higher than x.sub.1 or
the concentration that gives a maximum of f(x) or f(x.sub.1),
thereby controlling the concentration of F.sup.- ions in the
solution, it is possible to inhibit the formation of the fluorides
of cation-yielding elements contained in a glass substrate, i.e.,
fluorides hardly soluble to the solution.
[0051] The amount of a high-ionizing acid added to an etching
solution is preferably adjusted such that the concentration x of
the acid in the solution is in the following range:
x.sub.2-(x.sub.2-x.sub.1)/2<x<x.sub.2+(x.sub.2-x.sub.1)/2.
This is because then it is possible to prevent the adhesion of
crystal precipitates to the etched surface, to minimize any
increase in surface roughness, and to maintain the original
transparency of the glass substrate throughout the etching process.
The concentration in question is more preferably in the following
range:
x.sub.2-(x.sub.2-x.sub.1)/3<x<x.sub.2+(x.sub.2-x.sub.1)/3,
and most preferably in the following range:
x.sub.2-(x.sub.2-x.sub.1)/4<x<x.sub.2+(x.sub.2-x.sub.1)/4.
[0052] With regard to the concentration of x (=x.sub.2) (mol/kg] at
which f(x) takes a minimum or f(x.sub.2), if
x.sub.2+(x.sub.2-x.sub.1)/2.gtoreq.x.sub.3, the concentration x of
a high-ionizing acid is preferably adjusted to be in the range:
x.sub.2-(x.sub.2-x.sub.1)/2<x<x.sub.3. If the f(x) of an
acid-added etching solution does not exhibit any minimum even when
the concentration x [mol/kg] is varied, i.e., there is no x that
gives a minimum of f(x), the concentration x of a high-ionizing
acid is preferably adjusted to be in the range:
x.sub.1-(x.sub.3-x.sub.1)/2<x<x.sub.3.
[0053] The species of acid is not limited to any specific one, but
may include inorganic acids such as hydrochloric acid, nitric acid,
sulfuric acid, hydrobromic acid, and the like, and organic acids
such as oxalic acid, tartaric acid, iodoacetic acid, fumaric acid,
maleic acid, and the like. Preferably, the acid is a hydrophilic
acid, because it facilitates, when added to an etching solution,
the uniform dispersal of the solutes of the solution. Preferably,
the acid is an inorganic acid because it, when added to an etching
solution, can prevent the etched surface from being contaminated by
organic materials. Among inorganic acids, hydrochloric acid, nitric
acid, sulfuric acid and hydrobromic acid are most preferred because
they have a higher dissociation constant.
[0054] The acid may include multivalent acids as well as monovalent
acids. The use of a multivalent acid may be advantageous because it
gives a larger amount of H.sup.+ ions at a given concentration.
[0055] To achieve the effect according to the invention,
hydrochloric acid (HCl) is particularly preferred because it has a
most balanced action. However, the preferable species of acid
should be determined in accordance with the cation-yielding
elements and their oxides contained in a glass substrate to be
treated, together with the concentrations of those elements and
oxides.
[0056] The acid may comprise one kind of acid or multiple kinds of
acids.
[0057] Preferably, the etching solution further contains a
surfactant to enhance the uniform etching, improve the affinity of
the etched surface to a resist, and inhibit the adhesion of foreign
particles to the etched surface. The added amount of a surfactant
is preferably 0.0001 to 1 wt %.
[0058] Once the etching rate f(x) is obtained for a given
high-ionizing acid in combination with an etching solution through
a preliminary experiment performed on a heated silicon oxide film,
it is possible to readily determine the concentrations of the acid
in the solution at which f(x) takes a maximum and minimum.
[0059] If the etching rate f(x) is obtained for a given
high-ionizing acid, the concentration x of the acid in an etching
solution is preferably chosen to be in the range of x>x.sub.1,
more preferably x>x.sub.2-(x.sub.2-x.sub.1)/2, because then the
compounds derived from cations obtained as a result of dissolution
of ingredients contained in an etched glass substrate become highly
soluble to the solution.
[0060] If the etching rate f(x) is obtained for a given
high-ionizing acid, the concentration x of the acid in an etching
solution is preferably chosen to be in the range:
x.sub.2-(x.sub.2-x.sub.1)/2<x<x.sub.2+(x.sub.2-x.sub.1)/2,
because then it is possible to prevent the adhesion of crystal
precipitates to the etched surface, and to minimize any increase in
surface roughness, and to maintain the original transparency of the
glass substrate throughout the etching process.
[0061] The etching solution according to the invention, adapted for
etching a glass substrate containing multiple ingredients, contains
as indispensable components, HF and NH.sub.4F, and a high-ionizing
acid whose dissociation constant is higher than that of HF, and, as
needed, a surfactant. Other components may be added to the etching
solution, as long as their addition does not interfere with the
proper function of the etching solution.
[0062] If metal elements are allowed to exist in an etching
solution of the invention adapted for etching a glass substrate
containing multiple ingredients, the species of the metal elements
are not limited to any specific ones but may include any various
metals. However, in order to ensure the enhanced solubility of the
compounds of cations obtained as a result of dissolution of
ingredients of a glass substrate and uniform etching of the glass
substrate, their concentration is preferably limited to 1 ppb or
lower, more preferably 0.5 ppb or lower, most preferably 0.01 ppb
or lower.
[0063] A glass substrate containing multiple ingredients may
contain any metals, as long as its main ingredient is silicates.
However, the etching solution of the invention is particularly
effective for a glass substrate which contains, as metal elements,
one or more chosen from the group comprising Al, Ba, Ca, Mg, Sb, Sr
and Zr.
[0064] The glass substrate to be treated by an etching solution of
the invention preferably includes those used for the construction
of flat panel display devices.
[0065] The concentration of HF in an etching solution of the
invention is preferably 25 mol/kg or lower.
[0066] The concentration of NH.sub.4F in an etching solution of the
invention is preferably 0.001 to 11 mol/kg.
[0067] If the etching rate f(x) is obtained for a given
high-ionizing acid, the concentration x of the acid in an etching
solution is preferably chosen to satisfy x<x.sub.3 [mol/kg],
where X.sub.3 represents the maximum concentration at which
deposition of crystals on a glass substrate can be prohibited at a
temperature appropriate for etching.
[0068] The concentration of components included in an etching
solution of the invention are preferably adjusted such that
precipitation of crystals occurs at 20.degree. C. or lower, because
then it is possible to prohibit the precipitation of crystals
during the preparation of the etching solution.
EXAMPLES
[0069] The method of the invention will be described below more
specifically by means of examples. However, the present invention
is not limited to those examples.
[0070] First, as a fundamental experiment, hydrochloric acid
(HCl)-added BHF-based etching solutions were prepared with the
concentration of HCl being varied. The compositions of the etching
solutions and their features are summarized in Table 1 below.
TABLE-US-00001 TABLE 1 Rate of etching of HF Hydrochloric glass
(mol/ NH.sub.4F acid substrate kg) (mol/kg) (mol/kg) (23.degree.
C.)(.ANG./min) Features 0.5 1 0.25 1440 0.5 2260 Rate of etching of
heated oxide film maximum 1.25 3820 2.5 5000 Rate of etching of
heated oxide film minimum 3.25 6120 1 3 1 3050 2 6700 Rate of
etching of heated oxide film maximum 3 9580 4 12910 Rate of etching
of heated oxide film minimum 5 15560 3 3 0.5 4230 1.5 8090 Rate of
etching of heated oxide film maximum 2.75 13640 4 21060 Rate of
etching of heated oxide film minimum 4.5 22780 5 4 0.5 8310 1.5
14480 Rate of etching of heated oxide film maximum 2.25 18480 4
25200
[0071] The glass substrate used in the experiment included glass
substrates to be incorporated in LC displays. The test glass
substrates were submitted to EDX (energy dispersion type X-ray
analysis) to determine their ingredients. The results are shown in
Table 2. TABLE-US-00002 TABLE 2 Contents of individual Elements
Atomic weight elements (wt %) Si 28.09 30.43 O 16.00 46.65 Al 26.98
8.74 Ba 137.3 9.42 Ca 40.08 2.25 Ga 69.72 0.26 Mg 24.31 0.25 Sb
121.8 0.11 Sn 118.7 0.19 Sr 87.62 1.60 Zr 91.22 0.10
[0072] Different amounts of HCl were added to a BHF-based etching
solution that contained HF at 1 mol/kg and NH.sub.4F at 3 mol/kg as
shown in Table 1, and the etching rate f(x) of the resulting
solutions (which may be abbreviated as 1/3/x solutions) was
determined by applying the solutions to a heated silicon oxide film
at 23.degree. C., and the f(x) was plotted as a function of the
concentration x [mol/kg) of HCl in the solution as shown in FIG.
1.
[0073] In FIG. 1, x.sub.1 and x.sub.2 represent the concentrations
of HCl at which the etching rate f(x) determined on a heated
silicon oxide film was maximal and minimal respectively, and
x.sub.3 the maximum concentration of HCl at which no crystal
precipitation was present.
[0074] An etching solution of the invention having a sufficiently
large volume with respect to glass substrates to be etched was
applied to glass substrates which contain a given cation-yielding
element at different concentrations, and etching was allowed to
proceed for a certain period and then the etching solution was
stirred to ensure the uniform dispersion of the cation in the
solution. The effect of the etched amount on the concentration of
the cation in the solution was studied.
[0075] The concentration of the cation in an etching solution was
determined by ICP-MS (induction-coupled high frequency plasma mass
spectroscopy, HP-4500, Yokokawa-Hewlett-Packard).
[0076] The concentration of Al ions in an etching solution derived
from an etched glass substrate was plotted as a function of the
etched amount, and the resulting graph is shown in FIG. 2.
[0077] From FIG. 2 it is evident that generally the concentration
of Al ions in an etching solution linearly increases with the
increase of the etched amount. This suggests that Al ions are
uniformly dispersed in the etching solution.
[0078] Various etching solutions were applied to the same glass
substrate comprising Al compounds, and similar relationships were
plotted. Then, it was found that certain etching solutions gave the
relationships whose gradient is lower than the general linear
gradient. This shows that, with the use of such etching solutions,
Al compounds contained in the glass substrate dissolve in the
etching solution to produce Al salts (e.g., aluminum fluorides)
which, being hardly soluble to the solution, crystallize to
precipitate.
[0079] As seen from the above description, it is possible to
evaluate, for a given etching solution, the solubility of each
cation-yielding element contained in a glass substrate by
determining the same relationship as above for the etching
solution.
[0080] A BHF-based etching solution comprising HF at 1.0 mol/kg and
NH.sub.4F at 3.0 mol/kg was prepared, and different amounts of HCl
was added to the solution to prepare BHF-based etching solutions
with the concentration x of HCl being varied. The test etching
solutions were applied to a glass substrate containing
cation-yielding elements. For each cation derived from the glass
substrate, the relationship of its concentration in the solution
with the etched amount of the substrate was plotted as for in Al
shown above. FIGS. 3, 4 and 5 show the relationships thus obtained
for Ba, Ca and Sr, respectively.
[0081] Inspection of FIGS. 2, 3, 4 and 5 shows that, when the
BHF-based solutions comprising HF at 1.0 mol/kg and NH.sub.4F at
3.0 mol/kg, where the concentration x of HCl was varied, were
applied to the glass substrate containing Al, Ba, Ca, and Sr as
main cation-yielding elements, the solubility of the salts of those
elements to the etching solution increases when x is equal to
x.sub.1.
[0082] Further, the BHF-based solutions comprising HF at 1.0 mol/kg
and NH.sub.4F at 3.0 mol/kg, where the concentration x of HCl was
varied, were applied to a heated silicon oxide film at 23.degree.
C., and the etching rate f(x) was plotted as a function of the
concentration x [mol/kg] of HCl in the solution. As a consequence
it was found that x.sub.1 or the concentration of HCl which gave a
maximum of f(x) was x.sub.1=2 mol/kg, and x.sub.2 or the
concentration of HCl which gave a minimum of f(x) was x.sub.2=4
mol/kg. Since the concentration x of HCl is preferably in the
range: x.gtoreq.x.sub.2-(x.sub.2-x.sub.1)/2, the preferable range
in this case can be obtained by replacing x.sub.1 and x.sub.2 by 2
and 4 respectively, that is, x.gtoreq.4-(4-2)/2=3, or x.gtoreq.3.
Indeed, the graphs show that, if the concentration x of HCl is
chosen to satisfy x.ltoreq.3, solubility of the salts of the main
cations derived from the glass substrate to the etching solution is
enhanced.
[0083] Incidentally, the solubility of silicon (Si) and silicates
(SiO.sub.2) or the main ingredients of a glass substrate to the
etching solutions under study was excellent.
[0084] With regard to the metal elements contained in a glass
substrate, it has been known that their fluorides are more
insoluble to water than their other halogenated salts such as
chlorides. Thus, if it is possible to introduce a halogen atom
other than F in the present reaction system, and to replace thereby
part of the fluorides with salts of the substitutive halogen, then
the crystallization of fluorides of cations derived from a glass
substrate which are hardly soluble to water, will be effectively
avoided.
[0085] If HCl is added to an etching solution, H.sup.+ ions and
Cl.sup.- ions will be added to existent ions in the solution.
Addition of H.sup.+ ions to the reaction system disturbs the
existent acid-base equilibrium such that the concentration of
F.sup.- ions in the solution is lowered. On the other hand,
addition of Cl.sup.- ions produces, instead of fluorides of
cation-yielding elements contained in a glass substrate, the
chlorides of those cation-yielding elements which are more soluble
to the etching solution, which accounts for the reduction of
crystal precipitation in an HCl-added etching solution.
[0086] It was found, if the acidity contributed by HCl in an
etching solution is equal to that of HF, with the increase of the
ratio of chloride content to fluorine content (Cl/F ratio), the
solubility of resulting halogenated salts to the solution is more
enhanced.
[0087] The present inventors prepared HCl-added etching solutions
with the concentration x of HCl being varied whose composition was
as shown in Table 1. The etching solutions were applied to a heated
silicon oxide film at 23.degree. C., and the etching rate f(x) was
plotted as a function of the concentration x [mol/kg] of HCl. Then,
they revealed that, if f(x) takes a maximum when x=x.sub.1, and a
minimum when x=x.sub.2, an etching solution in which the
concentration x of HCl is in the range of:
x.gtoreq.x.sub.2-(x.sub.2-x.sub.1)/2 gives an environment where the
solubility of the salts of cation-yielding elements contained in a
glass substrate to the solution is enhanced with an increase of the
concentration x of HCl.
[0088] Next, the method of evaluating the micro-roughness of the
surface of an etched glass substrate will be disclosed.
[0089] A BHF-based etching solution comprising HF at 1.0 mol/kg and
NH.sub.4F at 3.0 mol/kg was prepared, to which different amounts of
HCl were added to vary its concentration x. The resulting acidified
BHF-based etching solution was applied to a glass substrate until
the surface of the glass substrate was etched away by 25, 50 or 100
.mu.m. The micro-roughness of the etched surface was evaluated by
determining the Ra value of the surface. The results are shown in
Table 3. The micro-roughness (Ra value) of an etched surface was
determined with an instrument adapted for the purpose (.alpha.-step
250, Tencor). TABLE-US-00003 TABLE 3 Acid concentration Etched
amount of glass substrate (.ANG.) (mol/kg) 25 .mu.m 50 .mu.m 100
.mu.m 1 22 520 1200 2 38 44 70 3 38 44 70 3.5 33 47 65 3.75 30 44
63 4 12 18 22 4.25 24 39 53 4.5 26 43 56 5 29 46 58
[0090] It is shown in Table 3 that, with regard to the BHF-based
etching solutions containing HF at 1.0 mol/kg and NH.sub.4F at 3.0
mol/kg with different amounts of HCl supplemented, if the
concentration x of HCl is in the range of 3<x<5, the
increment of micro-roughness with the increase of etched amount is
inhibited. The same BHF-based etching solutions containing HF at
1.0 mol/kg and NH.sub.4F at 3.0 mol/kg with the concentration x of
HCl being varied were applied to a heated silicon oxide film at
23.degree. C., and the etching rate f(x) as a function of the
concentration x (mol/kg] of HCl was determined. It was found that
the etching rate f(x) takes a maximum when x=x.sub.1 or 2 mol/kg,
and a minimum when x=x.sub.2 or 4 mol/kg. When these values are put
into the above inequalities,
x.gtoreq.x.sub.2-(x.sub.2-x.sub.1)/2=4-(4-2)/2=3 (x>3)
x.ltoreq.x.sub.2+(x.sub.2-x.sub.1)/2=4+(4-2)/2=5 (x<5). This
shows that, for the HCl-added, BHF-based etching solutions, if the
concentration x of HCl in the solutions is in the range of
3<x<5, etching by the solutions proceeds so smoothly and
uniformly that the increase of micro-roughness with the increase of
etching amount is inhibited.
[0091] If the concentration x of HCl is below the above range,
salts of cation-yielding elements derived from a glass substrate
have such a low solubility to the etching solution that they easily
crystallize and deposit on the surface of the substrate, which
causes the surface roughness to be increased with the increase of
etching amount.
[0092] To the BHF-based etching solution containing HF at 1.0
mol/kg and NH.sub.4F at 3.0, HNO.sub.3 or H.sub.2SO.sub.4 was added
instead of HCl and the same type of experiment was carried out. It
was found that those acids have the same effect as HCl.
[0093] The present inventors applied the various etching solutions,
whose composition is as shown in Table 1, to glass substrates and
examined the micro-roughness (Ra value) of etched surfaces in the
same manner as described above. In a separate experiment, the
etching solutions were applied to a heated silicon oxide film at
23.degree. C., and the etching rate f(x) as a function of the
concentration x [mol/kg] of HCl was plotted, and x.sub.1 or the
concentration of HCl at which the etching rate f(x) takes a maximum
and x.sub.2 or the concentration of HCl at which the etching rate
f(x) takes a minimum were determined. Then, it was found that, for
a given HCl-added, BHF-based etching solution, if the concentration
x of HCl in the solution is chosen to satisfy the inequality:
x.sub.2-(x.sub.2-x.sub.1)/2<x<x.sub.2+(x.sub.2-x.sub.1)/2,
the etching solution can etch glass substrates so smoothly and
uniformly that the increase of micro-roughness with the increase of
etching amount is inhibited.
[0094] To the BHF-based etching solution containing HF at 1.0
mol/kg and NH.sub.4F at 3.0, HNO.sub.3 or H.sub.2SO.sub.4 was added
instead of HCl and the same type of experiment was carried out. It
was found that those acids have the same effect as does HCl.
[0095] If a BHF-based etching solution to which HCl is added, such
that the concentration x of HCl in the solution satisfies the above
inequality, is applied to a glass substrate containing multiple
ingredients as those used for the construction of flat panel
display devices, the solubility of salts of cations derived from
the glass substrate to the solution is enhanced, and an increase of
micro-roughness with the increase of etching amount is inhibited.
Thus, with such an etching solution, it is possible to uniformly
etch a glass substrate containing multiple ingredients without
evoking crystal precipitation and increased surface roughness.
[0096] If a BHF-based etching solution is prepared, different
amounts of HCl are added to the solution, the resulting etching
solutions with the concentration x of HCl varied are applied to a
heated silicon oxide film at 23.degree. C., the etching rate f(x)
is plotted as a function of the concentration x [mol/kg] of HCl,
and x.sub.1 or the concentration of HCl at which the etching rate
f(x) takes a maximum and x.sub.2 or the concentration of HCl at
which the etching rate f(x) takes a minimum are determined, then
the same BHF-based solution to which HCl is added such that the
concentration x of HCl in the solution satisfies the following
inequality:
x.sub.2-(x.sub.2-x.sub.1)/2<x<x.sub.2+(x.sub.2-x.sub.1)/2
will ensure the uniform processing of glass substrates.
INDUSTRIAL APPLICABILITY
[0097] According to the present invention, it is possible to
process glass substrates containing multiple ingredients as those
used for the construction of flat display devices without inducing
crystal precipitation and/or an increase in surface roughness.
[0098] An etching solution according to the present invention can
also be used for cleaning filters. If a conventional etching
solution is used for etching glass substrates, the solution will
contain fluorides of cations, derived from the glass substrates, as
precipitates. The etching solution is then passed through a filter
to remove fluorides. However, after repeated use, the filter will
be clogged. If such a filter is rinsed with an etching solution of
the invention, fluorides depositing on the fiber network of the
filter will dissolve in the solution and the filter will be cleaned
sufficiently to be reused.
[0099] If an acid-added etching solution in which the concentration
of the acid is adjusted according to the present invention is
applied to a glass substrate containing multiple ingredients,
uniform etching of the surface of the substrate is achieved without
causing an increase in the surface roughness. Therefore, if the
surface of a glass substrate which has been etched by means of an
etching solution prepared according to the invention is inspected,
and it is found to exhibit considerable roughness, it is possible
to conclude that the glass substrate has a certain inherent flaw
such as entrapped air bubbles. Accordingly, it is possible to
identify glass substrates having an inherent flaw, by checking the
surface roughness of the substrates after subjecting them to the
etching by means of an etching solution prepared according to the
invention.
[0100] While this invention has been described as having a
preferred design, the present invention can be further modified
within the spirit and scope of this disclosure. This application is
therefore intended to cover any variations, uses, or adaptations of
the invention using its general principles. Further, this
application is intended to cover such departures from the present
disclosure as come within known or customary practice in the art to
which this invention pertains and which fall within the limits of
the appended claims.
* * * * *