U.S. patent application number 11/001142 was filed with the patent office on 2005-06-23 for burner and method for the manufacture of synthetic quartz glass.
This patent application is currently assigned to Shin-Etsu Chmeical Co., Ltd.. Invention is credited to Otsuka, Hisatoshi, Shirota, Kazuo.
Application Number | 20050132749 11/001142 |
Document ID | / |
Family ID | 34464017 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050132749 |
Kind Code |
A1 |
Otsuka, Hisatoshi ; et
al. |
June 23, 2005 |
Burner and method for the manufacture of synthetic quartz glass
Abstract
A burner for use in the manufacture of synthetic quartz glass is
provided, which comprises a main burner (7) comprising a multi-tube
assembly (1) including a center tube (2) for feeding a
silica-forming compound, a first enclosure tube (3) surrounding the
center tube for feeding a combustion-supporting gas, and a second
enclosure tube (4) surrounding the first enclosure tube for feeding
a combustible gas; a tubular shell (5) surrounding the multi-tube
assembly for feeding a combustible gas; and a plurality of nozzles
(6) disposed within the tubular shell for feeding a
combustion-supporting gas. A double-tube assembly (8) is disposed
so as to surround the forward opening of the main burner (7) for
feeding a combustion-supporting gas. Synthetic quartz glass ingots
having high optical homogeneity are produced.
Inventors: |
Otsuka, Hisatoshi;
(Niigata-ken, JP) ; Shirota, Kazuo; (Niigata-ken,
JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW
SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
Shin-Etsu Chmeical Co.,
Ltd.
Tokyo
JP
|
Family ID: |
34464017 |
Appl. No.: |
11/001142 |
Filed: |
December 2, 2004 |
Current U.S.
Class: |
65/17.4 ; 65/413;
65/444 |
Current CPC
Class: |
C03B 2207/06 20130101;
C03B 2207/40 20130101; F23L 2900/07002 20130101; C03B 2207/20
20130101; C03B 2207/08 20130101; F23D 14/32 20130101; C03B 2207/12
20130101; C03B 2207/14 20130101; C03B 19/1423 20130101; C03B
2201/23 20130101; C03B 2207/36 20130101; F23D 14/22 20130101 |
Class at
Publication: |
065/017.4 ;
065/444; 065/413 |
International
Class: |
C03B 019/01 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 5, 2003 |
JP |
2003-406982 |
Claims
1. A burner for use in the manufacture of synthetic quartz glass,
comprising a main burner comprising a multi-tube assembly of a
three or more tube construction including a center tube for feeding
a silica-forming compound, a first enclosure tube surrounding the
center tube for feeding a combustion-supporting gas, and a second
enclosure tube surrounding the first enclosure tube for feeding a
combustible gas; a tubular shell surrounding the multi-tube
assembly for feeding a combustible gas; and a plurality of nozzles
disposed within the tubular shell for feeding a
combustion-supporting gas, the main burner defining a forward
opening, and a double-tube assembly surrounding at least the
forward opening of the main burner.
2. The burner of claim 1, wherein said double-tube assembly
includes an outer tube and an inner tube disposed within the outer
tube, said outer tube surrounds the forward opening of the main
burner and projects further forward, said inner tube has a forward
end which is disposed in register with or backward of the forward
opening of the main burner.
3. The burner of claim 1, wherein said double-tube assembly defines
therein a passage for a combustion-supporting gas.
4. The burner of claim 1, wherein the total cross-sectional area of
gas discharge ports of the plurality of nozzles disposed in the
tubular shell accounts for 5% to 20% of the cross-sectional area of
a gas discharge region between the multi-tube assembly and the
tubular shell.
5. A method for the manufacture of a synthetic quartz glass ingot
using a burner comprising a main burner comprising a multi-tube
assembly of a three or more tube construction including a center
tube, a first enclosure tube surrounding the center tube, and a
second enclosure tube surrounding the first enclosure tube, a
tubular shell surrounding the multi-tube assembly, and a plurality
of nozzles disposed within the tubular shell, the main burner
defining a forward opening, and a double-tube assembly surrounding
at least the forward opening of the main burner; said method
comprising the steps of: placing the burner to face a quartz glass
target mounted on a rotating support, feeding a silica-forming
compound to the center tube, a combustion-supporting gas to the
first enclosure tube and the nozzles, a combustible gas to the
second enclosure tube and the tubular shell, and a
combustion-supporting gas to the double-tube assembly, forming an
oxyhydrogen flame from the combustion-supporting gas and the
combustible gas for subjecting the silica-forming compound to vapor
phase hydrolysis or oxidative decomposition to form silica fines,
depositing the silica fines on the target, and melting and
vitrifying the deposited silica into quartz glass.
6. The method of claim 5, wherein the silica-forming compound is a
silane or siloxane, the combustion-supporting gas is oxygen, the
combustible gas is hydrogen, the silica-forming compound and oxygen
are fed to the burner such that the molar amount of the
silica-forming compound is at least 1.3 times the stoichiometry of
oxygen, and the molar ratio of the amount of actually fed oxygen to
the stoichiometry of oxygen needed for the silica-forming compound
and hydrogen fed to the burner is from 0.6 to 1.3.
7. The method of claim 5, wherein the combustion-supporting gas is
fed through the double-tube assembly at a flow velocity of 0.5 to
1.3 m/sec.
8. The method of claim 5, wherein the ingot has a diameter of at
least 150 mm.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This non-provisional application claims priority under 35
U.S.C. .sctn.119(a) on Patent Application No. 2003-406982 filed in
Japan on Dec. 5, 2003, the entire contents of which are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] This invention relates to a burner for use in the
manufacture of synthetic quartz glass ingots useful as the stock
material for excimer laser synthetic quartz glass optical members
and large-diameter synthetic quartz glass ingots useful as the
stock material for liquid crystal-related large-size photomask
substrates. More particularly, it relates to a burner for use in
the manufacture of synthetic quartz glass ingots having
optical-grade high homogeneity and a minimal change of light
transmittance and useful as optical members such as lenses, prisms,
mirrors, windows and photomask substrates in excimer laser systems,
especially ArF excimer laser systems. The invention also relates to
a method for the manufacture of synthetic quartz glass ingots.
[0004] 2. Background Art
[0005] To meet the recent trend of LSI toward higher integration,
the photolithography of defining an integrated circuit pattern on a
wafer requires an image exposure technique on the order of
submicron units. For finer line width patterning, efforts have been
made to reduce the wavelength of a light source of the exposure
system. In the lithography, a KrF excimer laser (wavelength 248 nm)
took over the prior art i-line (wavelength 365 nm) as the
mainstream light source in steppers; and the practical use of an
ArF excimer laser (wavelength 193 nm) has recently started. Then,
the lens for use in steppers is required to have homogeneity,
improved UV transmission, and resistance to UV irradiation.
[0006] In order to avoid contamination with metal impurities which
cause UV absorption, the synthesis of quartz glass is generally
carried out by introducing the vapor of a high purity silicon
compound such as silicon tetrachloride directly into an oxyhydrogen
flame. Flame hydrolysis takes place to form silica fines, which are
directly deposited on a rotating heat-resistant substrate such as
quartz glass while being melted and vitrified thereon. In this way,
transparent synthetic quartz glass is produced.
[0007] The transparent synthetic quartz glass thus produced
exhibits satisfactory light transmittance in the short-wavelength
range down to about 190 nm in the LSI field. It has been utilized
as a material capable of transmitting UV laser light, specifically
i-line and excimer laser light such as KrF (248 nm), XeCl (308 nm),
XeBr (282 nm), XeF (351 and 353 nm) and ArF (193 nm), and the
four-fold harmonic wave (250 nm) of YAG.
[0008] The absorption of light in the UV region that is newly
created by irradiating synthetic quartz glass with UV light having
great energy as emitted by an excimer laser is deemed to be due to
the paramagnetic defects formed through photo-reaction from
intrinsic defects in synthetic quartz glass. Many light absorption
bands due to such paramagnetic defects have been identified by ESR
spectroscopy, for example, as E' center (Si.) and NBOHC
(Si--O.).
[0009] The paramagnetic defects generally have an optical
absorption band. When UV light is irradiated to quartz glass, the
problematic absorption bands in the UV region due to paramagnetic
defects in quartz glass are, for example, at 215 nm due to E'
center (Si.) and 260 nm, which has not been accurately identified.
These absorption bands are relatively broad and sometimes entail
strong absorption. This is a serious problem when quartz glass is
used as a transmissive material for ArF and KrF excimer lasers.
[0010] Intrinsic defects in synthetic quartz glass which cause
paramagnetic defects arise from structures other than SiO.sub.2
such as Si--OH and Si--Cl and oxygen-depleted or enriched
structures such as Si--Si and Si--O--O--Si. As the approach for
suppressing paramagnetic defects, it is proposed in JP-A 6-199532
to use a chlorine-free alkoxysilane such as tetramethoxysilane as
the silane compound for preventing Si--Cl, one of paramagnetic
defects, from being incorporated in glass.
[0011] It is also known that if hydrogen molecules are present in
quartz glass in a concentration above a certain level, few defects
of E' center (Si.) which are oxygen defects are formed, leading to
improved durability to laser damage. Since ArF excimer laser light
causes several times serious damages to quartz glass as compared
with KrF excimer laser light, the quartz glass for ArF laser
application must have several times higher a hydrogen molecule
concentration than the quartz glass for KrF laser application.
[0012] It is proposed in JP-A 6-305736 to control the hydrogen
molecule concentration in synthetic quartz glass. Depending on the
energy using conditions of an ArF laser, the hydrogen molecular
concentration in glass is adjusted.
[0013] Now that the efforts to reduce the wavelength of light
source have reached excimer laser light having extremely greater
energy than the traditional i-line light, active research works
have been made on the laser durability of glass.
[0014] Exposure apparatus using such shorter wavelength light
include many optical parts such as lenses, windows, prisms, and
photomask-forming quartz glass substrates. With respect to
projection lens materials among these optical parts used in
exposure apparatus, the recent progress is toward a higher NA, the
diameter of lens is annually increasing, and the optical
homogeneity of lens material is required to be of higher precision.
Especially for the ArF excimer laser, it is required that the
initial transmittance of quartz glass, specifically the
transmittance at wavelength 193.4 nm over the entire surface of an
optical member be close to the theoretical value, the theoretical
value at wavelength 193.4 nm being computed to be 99.85% by taking
into account multiple reflection. Since the optical system in the
exposure apparatus is composed of several to several tens of
lenses, it is important that setting an initial transmittance of
quartz glass even a little higher restrains the absorption of
optical energy within the bulk of quartz glass, thereby minimizing
a possibility that the light energy once absorbed is converted to
thermal energy to incur a change of density and in some cases, a
change of refractive index. In addition to the essential uniformity
of refractive index, a reduction of birefringence becomes a crucial
problem.
[0015] As stated above, in order to avoid contamination with metal
impurities which cause UV absorption, the synthesis of quartz glass
is generally carried out by introducing the vapor of a high purity
organosilicon compound such as silicon tetrachloride directly into
an oxyhydrogen flame. Flame hydrolysis takes place to form silica
fines, which are directly deposited on a rotating heat-resistant
substrate such as quartz glass and melted and vitrified thereon to
form transparent synthetic quartz glass. The synthetic quartz glass
ingot thus produced is sliced perpendicular to its growth direction
whereupon a distribution of transmittance at wavelength 193.4 nm is
determined in a plane of the growth direction. Then, the slice has
an in-plane distribution, typically with the tendency that
transmittance decreases from the center to the periphery. If the
value required for the initial transmittance is at least 99.7% as
an internal transmittance, for example, an effective portion of the
synthetic quartz glass ingot that can be utilized, generally known
as percent yield, is determined by this value.
[0016] Apart from the LSI application, large-size quartz glass
substrates for photomasks are now used in the liquid crystal
display (LCD) application. It is required to form synthetic quartz
glass ingots for use as the stock material therefor to larger
diameters, particularly when the percent yield of the manufacturing
process of large-size glass substrates is considered. While the
mainstream of conventional synthetic quartz glass substrates for IC
use is of 6 inch square size, large-size glass substrates have
already been required to have one side of 1 meter or longer. In
fabricating large-size quartz glass substrates, as opposed to the
synthetic quartz glass ingot for IC use which must have a diameter
of about 100 to 140 mm, for example, an ingot which is of a
conventional ingot diameter must be increased in length in order to
insure a certain product weight. Shaping must be repeated many
times until the size is tailored to a desired profile. The
situation is detrimental in production yield and efficiency.
SUMMARY OF THE INVENTION
[0017] An object of the invention is to provide a burner for use in
the manufacture of synthetic quartz glass ingots which serve as the
stock material from which synthetic quartz glass members having
high optical homogeneity useful as optical parts such as lenses,
prisms, windows and photomask-forming quartz glass substrates in
excimer laser systems can be readily obtained or synthetic quartz
glass members for liquid crystal-related large size glass
substrates can be efficiently obtained. Another object is to
provide a method for the manufacture of synthetic quartz glass
ingots using the burner.
[0018] In the manufacture of synthetic quartz glass ingots by vapor
phase hydrolysis or oxidative decomposition of a silica-forming
compound with the aid of an oxyhydrogen flame, the burner structure
for forming a flame is important. The prior art burner is of the
structure including a central triple-tube assembly, a tubular shell
surrounding the triple-tube assembly, a plurality of nozzles
disposed between the triple-tube assembly and the tubular shell,
the foregoing components forming a main burner, and a tubular
jacket disposed around the tubular shell and at the forward end of
the main burner. Replacing the prior art burner by a burner for the
manufacture of synthetic quartz glass comprising a main burner
comprising at least a central triple-tube assembly, a tubular shell
surrounding the triple-tube assembly, and a plurality of nozzles
disposed between the triple-tube assembly and the tubular shell and
within the confine of the tubular shell, and a double-tube assembly
disposed around the main burner, the present invention has
succeeded in manufacturing synthetic quartz glass ingots from which
synthetic quartz glass having high optical homogeneity and
synthetic quartz glass members for liquid crystal-related large
size glass substrates are obtainable.
[0019] The inventors have intended to extend the effective portion
over the entire region of the synthetic quartz glass ingot. Factors
of the manufacturing process that substantially dictate the initial
transmittance of a synthetic quartz glass ingot include a burner
(structure and set conditions) which is an important constituent of
the direct flame process, as well as a starting material or silane
compound, a combustible gas (typically hydrogen) and a
combustion-supporting gas (typically oxygen) fed thereto, and a
balance of these gases. It has been found that the manufacturing
process largely depends on the structure of burner among other
factors.
[0020] In one aspect, the present invention provides a burner for
use in the manufacture of synthetic quartz glass, comprising
[0021] a main burner comprising a multi-tube assembly of a three or
more tube construction including a center tube for feeding a
silica-forming compound, a first enclosure tube surrounding the
center tube for feeding a combustion-supporting gas, and a second
enclosure tube surrounding the first enclosure tube for feeding a
combustible gas; a tubular shell surrounding the multi-tube
assembly for feeding a combustible gas; and a plurality of nozzles
disposed within the tubular shell for feeding a
combustion-supporting gas, the main burner defining a forward
opening, and
[0022] a double-tube assembly surrounding at least the forward
opening of the main burner.
[0023] In a preferred embodiment, the double-tube assembly includes
an outer tube and an inner tube disposed within the outer tube. The
outer tube surrounds the forward opening of the main burner and
projects further forward. The inner tube has a forward end which is
disposed in register with or backward of the forward opening of the
main burner.
[0024] In a preferred embodiment, the double-tube assembly defines
therein a passage for a combustion-supporting gas. Typically, the
combustion-supporting gas passage is defined between the outer and
inner tubes.
[0025] In a preferred embodiment, the total cross-sectional area of
gas discharge ports of the plurality of nozzles disposed in the
tubular shell accounts for 5% to 20% of the cross-sectional area of
a gas discharge region between the multi-tube assembly and the
tubular shell.
[0026] Another aspect of the invention provides a method for the
manufacture of a synthetic quartz glass ingot using the burner
defined above, comprising the steps of placing the burner to face a
quartz glass target mounted on a rotating support; feeding a
silica-forming compound to the center tube, a combustion-supporting
gas to the first enclosure tube and the nozzles, a combustible gas
to the second enclosure tube and the tubular shell, and a
combustion-supporting gas to the double-tube assembly; forming an
oxyhydrogen flame from the combustion-supporting gas and the
combustible gas for subjecting the silica-forming compound to vapor
phase hydrolysis or oxidative decomposition to form silica fines;
depositing the silica fines on the target; and melting and
vitrifying the deposited silica into quartz glass.
[0027] In a preferred embodiment, the silica-forming compound is a
silane or siloxane, the combustion-supporting gas is oxygen, and
the combustible gas is hydrogen. The silica-forming compound and
oxygen are fed to the burner such that the molar amount of the
silica-forming compound is at least 1.3 times the stoichiometry of
oxygen. The molar ratio of the amount of actually fed oxygen to the
stoichiometry of oxygen necessary for the silica-forming compound
and hydrogen fed to the burner is from 0.6 to 1.3. Typically, the
combustion-supporting gas is fed through the double-tube assembly
at a flow velocity of 0.5 to 1.3 m/sec.
[0028] Most often, the ingot has a diameter of at least 150 mm.
[0029] Using the burner of the invention, it becomes possible to
manufacture synthetic quartz glass ingots which serve as the stock
material from which are manufactured synthetic quartz glass members
having high optical homogeneity for use in excimer laser systems,
especially ArF excimer laser systems, optical members having high
laser resistance, and optical members associated with light sources
such as excimer lasers, and optical fibers for ultraviolet
radiation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a transverse cross-sectional view of a burner for
the manufacture of synthetic quartz glass in one embodiment of the
invention, gas discharge ports of nozzles being depicted.
[0031] FIG. 2 is an elevational view, partially in axial cross
section, of the burner of FIG. 1.
[0032] FIG. 3 schematically illustrates a prior art burner for the
manufacture of synthetic quartz glass, gas discharge ports of
nozzles being depicted in cross section.
[0033] FIG. 4 schematically illustrates an exemplary synthetic
quartz glass manufacturing system.
[0034] FIG. 5 is a graph showing the radial distribution of initial
transmittance of synthetic quartz glass ingots of Example and
Comparative Example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] The burner for use in the manufacture of synthetic quartz
glass ingots according to the invention comprises a main burner
which includes a multi-tube assembly of a three or more tube
construction, a tubular shell surrounding the multi-tube assembly,
and a plurality of nozzles disposed within the confine of the
tubular shell. A double-tube assembly is disposed around the main
burner.
[0036] Referring to FIGS. 1 and 2, a burner according to one
embodiment of the invention is illustrated. A multi-tube assembly 1
has a triple-tube construction including a center tube 2, a first
enclosure tube 3 surrounding the center tube 2 to define a second
passage, and a second enclosure tube 4 surrounding the first
enclosure tube 3 to define a third passage. The multi-tube assembly
(triple-tube assembly in the illustrated embodiment) 1 is
surrounded by a tubular shell 5, and a plurality of nozzles 6 are
disposed between the tubular shell 5 and the triple-tube assembly 1
and within the confine of the tubular shell 5. The multi-tube
assembly 1, tubular shell 5 and nozzles 6 are so combined to
constitute a main burner 7 which has a forward opening (depicted at
the top in FIG. 2).
[0037] According to the invention, a double-tube assembly 8 is
disposed so as to surround at least the forward opening of the main
burner 7. The double-tube assembly 8 includes an outer tube 9 and
an inner tube 10 disposed inside the outer tube 9 and outside the
tubular shell 5 of the main burner 7. The forward end of the outer
tube 9 of the double-tube assembly 8 surrounds the forward opening
of the main burner 7 and projects forward of the forward opening.
The outer tube 9 provides a guard so that the gas stream from the
main burner 7 may not be spread outside the tubular shell 9. The
forward end of the inner tube 10 is disposed in register with the
forward opening of the main burner 7 although in another
embodiment, the forward end of the inner tube 10 may be disposed
backward of the forward opening of the main burner 7.
[0038] It is understood that the burner has an axis, to which tubes
and shells extend generally parallel. As used herein, the terms
"outside" or "outer" and "inside" or "inner" refer to radial
positions with respect to the burner axis. Also, the terms
"forward" and "backward" refer to relative positions along the
burner axis. In the illustrated embodiment, all tubes and shells
are shaped cylindrical and arranged in a concentric fashion to
define annular spaces therebetween, though the shape is not
critical.
[0039] Through the center tube 2, a silica-forming compound is fed
and channeled, and generally oxygen gas or carrier gas is
additionally fed and channeled. Through the second passage (within
the confine of the first enclosure tube 3), a combustion-supporting
gas such as oxygen is fed and channeled. Through the third passage
(within the confine of the second enclosure tube 4), a combustible
gas such as hydrogen is fed and channeled. Through the nozzles 6
and the double-tube assembly 8 (between the outer and inner tubes 9
and 10), a combustion-supporting gas such as oxygen is fed and
channeled. Through the tubular shell 5, a combustible gas such as
hydrogen is fed and channeled to flow about the nozzles 6.
[0040] In a preferred embodiment, the total cross-sectional area of
gas discharge ports of the plurality of nozzles 6 disposed in the
tubular shell 5, that is, the total cross-sectional area of lumens
of nozzles 6, accounts for at least 5%, more preferably 5 to 20%,
and most preferably 8 to 13% of the cross-sectional area of a gas
discharge region between the triple-tube assembly 1 and the tubular
shell 5, that is, the cross-sectional area of an annular space
between the assembly 1 and the shell 5 (i.e., the cross-sectional
area of an entire annular space between the assembly 1 and the
shell 5 provided that the nozzles 6 are omitted).
[0041] The double-tube assembly 8 disposed outside the main burner
7 is typically made of quartz. It is also desirable that the
combustion-supporting gas be channeled through the double-tube
assembly 8 at a flow velocity of 0.5 to 1.3 m/sec. A flow velocity
of less than 0.5 m/sec may allow for undesirable back fire whereas
a flow velocity of more than 1.3 m/sec may disturb the flame of the
main burner.
[0042] The number of nozzles 6 may be determined in accordance with
the above conditions. The flow velocity of the
combustion-supporting gas through the double-tube assembly 8 may be
determined in accordance with the clearance between the outer and
inner tubes 9 and 10 and the desired flow rate of
combustion-supporting gas.
[0043] As compared with the prior art burner structure, the
provision of the double-tube assembly around the tubular shell of
the main burner ensures that when a synthetic quartz glass ingot is
produced by the direct flame process, the temperature distribution
in the melting face from the center to the periphery of the ingot
growing face is uniform in that the high-temperature region at the
center is expanded to the periphery. Thus the silica structure is
formed under the same conditions between the center and the
periphery during the deposition/fusion/vitrifi- cation process of
silica fines on the ingot melting/growing face. This makes it
possible to prevent the initial transmittance of the periphery of
an ingot from lowering to below that of the center and to minimize
the difference in initial transmittance between the periphery and
the center. At the same time, the molten area is spread, making it
possible to increase the diameter of a synthetic quartz glass
ingot.
[0044] The provision of the double-tube assembly around the tubular
shell of the prior art burner structure ensures that in a flame
produced by the inventive burner, the high-temperature region is
extended from inside flame to outside flame. This outside flame is
applied to a peripheral portion of the ingot melting/growing face.
The structure allowing the combustion-supporting gas to be
channeled between the tubular shell of the main burner and the
double-tube assembly ensures to increase the combustion efficiency
of a combustible gas such as hydrogen gas fed around the nozzle
group, making it possible to extend the high-temperature region
throughout the flame. This is particularly true when the proportion
of the cross-sectional area of nozzles disposed between the
multi-tube assembly and the tubular shell is at least 5%. Further,
the enclosure of the forward end of the main burner with the outer
tube of the double-tube assembly prevents the flame from being
disordered by gas streams within the furnace, concentrating the
flame power.
[0045] Now it is described how to produce a synthetic quartz glass
ingot using the inventive burner. The burner is placed to face a
quartz glass target mounted on a rotating support. A silica-forming
compound, a combustible gas such as hydrogen gas, and a
combustion-supporting gas such as oxygen gas are separately fed to
the tubes of the burner to form an oxyhydrogen flame with which the
compound undergoes vapor phase hydrolysis or oxidative
decomposition to form silica fines which deposit on the target. The
silica fines are simultaneously melted and vitrified to form a
synthetic quartz glass ingot.
[0046] The starting material, silica-forming compound used herein
is typically an organosilicon compound which is preferably selected
from silane compounds and siloxane compounds represented by the
following general formulae (1), (2) and (3).
(R.sup.1).sub.nSi(OR.sup.2).sub.4-n (1)
[0047] Herein each of R.sup.1 and R.sup.2, which may be the same or
different, is a monovalent aliphatic hydrocarbon group, hydrogen or
halogen atom and n is an integer of 0 to 4. 1
[0048] Herein R.sup.3 is hydrogen or a monovalent aliphatic
hydrocarbon group, m is an integer of at least 1, especially 1 or
2, and p is an integer of 3 to 5.
[0049] Examples of the monovalent aliphatic hydrocarbon groups
represented by R.sup.1, R.sup.2 and R.sup.3 include alkyl groups of
1 to 4 carbon atoms, such as methyl, ethyl, propyl, n-butyl and
tert-butyl, cycloalkyl groups of 3 to 6 carbon atoms such as
cyclohexyl, and alkenyl groups of 2 to 4 carbon atoms such as vinyl
and allyl.
[0050] Examples of the silane compound represented by formula (1)
include SiCl.sub.4, CH.sub.3SiCl.sub.3, Si(OCH.sub.3).sub.4,
Si(OCH.sub.2CH.sub.3).sub.4, and CH.sub.3Si(OCH.sub.3).sub.3.
Examples of the siloxane compounds represented by formulae (2) and
(3) include hexamethyldisiloxane, hexamethylcyclotrisiloxane,
octamethylcyclotetrasil- oxane, and
decamethylcyclopentasiloxane.
[0051] The silane or siloxane compound, a combustible gas (e.g.,
hydrogen, carbon monoxide, methane or propane) and a
combustion-sustaining gas (e.g., oxygen) are fed to the burner for
forming an oxyhydrogen flame.
[0052] An apparatus for producing a synthetic quartz glass ingot
using the inventive burner may be of either vertical or lateral
type.
[0053] The synthetic quartz glass ingot produced using the
inventive burner preferably has an internal transmittance at
wavelength 193.4 nm of at least 99.70%. The reason is that when the
ingot is finally used as an optical member, it is sometimes
required that the transmittance at the service wavelength which is
193.4 nm in the case of an ArF excimer laser, for example, be at
least 99.70% in internal transmittance. If the internal
transmittance is less than 99.70%, there is a possibility that when
ArF excimer laser light is transmitted by a quartz glass member,
light energy is absorbed and converted to thermal energy, which can
cause changes in the density of the glass and also alter its
refractive index. For instance, the use of a synthetic quartz glass
ingot having an internal transmittance of less than 99.70% as a
lens material for an exposure system which employs an ArF excimer
laser as the light source may give rise to undesirable effects such
as distortion of the image plane (or field curvature) due to
changes in the refractive index of the lens material.
[0054] Thus, the burner must be configured and arranged as
described above. For optimum operation of the burner, the
silica-forming compound and oxygen are fed to the burner in such a
mixing ratio that the molar amount of the silica-forming compound
is at least 1.3 times, especially 2.0 to 3.0 times, the
stoichiometry of oxygen.
[0055] Additionally, the molar ratio of the amount of actually fed
oxygen to the stoichiometry of oxygen needed for the silica-forming
compound (silane or siloxane compound) and hydrogen fed to the
burner is preferably in a range of 0.6 to 1.3, more preferably 0.7
to 0.9.
[0056] Specifically, the silica-forming compound is fed to the
center tube at a flow rate of preferably 0.3 to 0.7 Nm.sup.3/hr,
more preferably 0.4 to 0.5 Nm.sup.3/hr. It is recommended that the
combustion-supporting gas such as oxygen be fed to the center tube
at a flow rate of 2.0 to 4.0 Nm.sup.3/hr, more preferably 2.5 to
3.5 Nm.sup.3/hr. Additionally, an inert gas such as argon may be
fed.
[0057] Preferred settings for the remaining tubes are such that the
flow rate of combustion-supporting gas through the first enclosure
tube is 0.3 to 2.5 Nm.sup.3/hr; the flow rate of combustible gas
through the second enclosure tube is 12 to 15 Nm.sup.3/hr; the flow
rate of combustible gas through the shell is 20 to 25 Nm.sup.3/hr;
the total flow rate of combustion-supporting gas through the
nozzles is 10 to 16 Nm.sup.3/hr; and the flow rate of
combustion-supporting gas through the double-tube assembly is 2 to
5 Nm.sup.3/hr. The gas flow velocity through the double-tube
assembly is preferably in the range of 0.5 to 1.3 m/sec.
[0058] Once the necessary gases are fed to the burner as specified
above and burnt to form an oxyhydrogen flame, the silica-forming
compound undergoes vapor phase hydrolysis or oxidative
decomposition to form silica fines which deposit on the target. The
silica fines are simultaneously melted and vitrified. The
vitrifying temperature has a distribution on the growth face. By
setting a minimum temperature at this time to at least 1800.degree.
C., preferably at least 2000.degree. C. (with an upper limit of up
to 2500.degree. C., preferably up to 2400.degree. C.), the region
of synthetic quartz glass which has an internal transmittance at
wavelength 193.4 nm of at least 99.70% can be enlarged. The use of
the inventive burner and the setting of an optimum gas balance such
as that between oxygen and hydrogen greatly contribute to the
melting and vitrifying temperature at the growth face.
[0059] With respect to the transmittance versus the melting and
vitrifying temperature at the growth face, the inventors have
discovered that the melting face temperature exerts an influence on
the transmittance at wavelengths shorter than 200 nm, especially at
the wavelength of ArF excimer laser light (193.4 nm). Thus, a
higher melting and vitrifying temperature makes it possible to
maintain an internal transmittance of at least 99.70%. Moreover,
within this range of conditions, it is possible to maintain the
hydrogen molecule concentration in the synthetic quartz glass at a
level of at least 3.times.10.sup.18 molecules/cm.sup.3 and thus
achieve a long-term stability (sufficient to restrain the
transmittance from lowering) during excimer laser irradiation. It
also becomes possible to produce a jumbo synthetic quartz glass
ingot having a diameter of at least 150 mm, typically 150 to 500
mm, and especially 200 to 300 mm.
[0060] It is preferred for the synthetic quartz glass ingot
produced using the inventive burner to have an internal
transmittance at wavelength 193.4 nm of at least 99.70% over the
entire surface of a slice when the ingot is sliced perpendicular to
its axis. Also preferably the glass has an OH group content of 500
to 1,300 ppm, especially 800 to 900 ppm. Moreover, a hydrogen
molecule concentration of at least 3.times.10.sup.18
molecules/cm.sup.3, preferably 3.times.10.sup.18 to
6.times.10.sup.18 molecules/cm.sup.3, most preferably
3.times.10.sup.18 to 5.times.10.sup.18 molecules/cm.sup.3 is
desirable for good resistance to laser damage. Using the inventive
burner, a synthetic quartz glass ingot having a large diameter can
be produced, from which members for liquid crystal-related
large-size glass substrates are effectively produced.
[0061] After a synthetic quartz glass ingot is produced, it is
processed as by cylindrical grinding, thermoformed into a
rectangular block as a mask substrate by heat melting at a
temperature in the range of 1700 to 1800.degree. C., annealed at a
temperature in the range of 1000 to 1300.degree. C. for strain
relief, sliced and polished, completing a synthetic quartz glass
substrate. When the synthetic quartz glass ingot is used as optical
lens material, it is subjected to homogenizing treatment, obtaining
synthetic quartz glass free of striae in three directions.
Specifically, both ends of a synthetic quartz glass ingot are
welded to synthetic quartz glass supporting rods held in a lathe
and the ingot is drawn out to a diameter of 80 mm. One end of the
ingot is then strongly heated with an oxyhydrogen burner to at
least 1,700.degree. C., and preferably at least 1,800.degree. C.,
so as to form a molten zone. Then, the opposed chucks are rotated
at different speeds to apply shear stress to the molten zone,
thereby homogenizing the quartz glass ingot. At the same time, the
burner is moved from one end of the ingot to the other end so as to
homogenize the hydroxyl group concentration and hydrogen
concentration within the ingot growth face (homogenization by the
zone melting method). The resulting synthetic quartz glass is
typically shaped to the desired dimensions, and then preferably
annealed for the glass to take a uniform fictive temperature (FT).
Annealing can be carried out by a conventional method.
[0062] Synthetic quartz glass members thus obtained are useful as
optical quartz glass members including excimer laser lenses,
synthetic quartz glass substrates for photomasks, stepper
illumination system lenses, and projection optical system lenses,
windows, mirrors, beam splitters and prisms in the LSI field; and
members for large-size quartz glass substrates such as color filter
substrates, dust-proof glass substrates and facing glass substrates
in the liquid crystal field for LCD.
EXAMPLE
[0063] The following examples are provided to illustrate the
invention, and are not intended to limit the scope thereof. It is
noted that in Examples, an internal transmittance was measured by
ultraviolet spectrophotometry (Cary 400 by Varian Corp.).
Example and Comparative Example
[0064] A synthetic quartz glass ingot was produced by feeding
methyltrimethoxysilane as the starting material to an inventive
burner (FIG. 1) or a prior art burner (FIG. 3), effecting oxidative
or combustion decomposition of the silane in an oxyhydrogen flame
to form fine particles of silica, then depositing the silica
particles on a rotating quartz target while melting and vitrifying
them at the same time.
[0065] Specifically, as shown in FIG. 4, a quartz glass target 12
was mounted on a rotating heat-resistant support 11. Argon gas 15
was introduced into the methyltrimethoxysilane 14 held in a
starting material vaporizer 13. Methyltrimethoxysilane 14 vapor was
carried out of the vaporizer by the argon gas 15, and oxygen gas 16
was added to the silane-laden argon to form a gas mixture, which
was then fed to the center tube of a main burner 17. As shown in
FIGS. 1 and 3, the main burner 17 was also fed the following gases,
in outward order from the foregoing gas mixture at the center:
oxygen gas 18, hydrogen gas 19, hydrogen gas 20, oxygen gas 21, and
oxygen gas 22. The starting material, methyltrimethoxysilane 14 and
an oxyhydrogen flame 23 were discharged from the main burner 17
toward the target 12. Fine particles of silica 24 were deposited on
the target 12 and simultaneously melted and vitrified as clear
glass, forming a synthetic quartz glass ingot 25. The ingot thus
obtained had a diameter of 150 mm and a length of 500 mm. The flow
velocity of combustion-supporting gas through the double-tube
assembly was 0.6 m/sec. The parameters of the burners of Example
and Comparative Example including the cross-sectional areas of
tubes or nozzles, their ratio and gas feed rates are shown in Table
1.
1 TABLE 1 Example Comparative Example (FIG. 1) (FIG. 3)
Cross-sectional Gas Cross-sectional Gas area flow rate area flow
rate Gas (mm.sup.2) (Nm.sup.3/hr) (mm.sup.2) (Nm.sup.3/hr) Center
tube Silane 15 0.4 13 0.4 O.sub.2 3.0 2.0 Ar 0.1 0.1 1st enclosure
tube O.sub.2 30 1.0 32 1.0 2nd enclosure tube H.sub.2 50 14.0 60
15.0 Shell H.sub.2 1,700 24.0 1,800 25.0 Nozzles O.sub.2 150 12.0
80 16.0 Double-tube assembly O.sub.2 1,090 2.5 -- Cross-sectional
nozzles 8.8 4.4 area ratio (%) Note: Cross-sectional area ratio is
a percentage of the total cross-sectional area of lumens of nozzles
divided by the cross-sectional area of an annular space (prior to
nozzle arrangement) between the second enclosure tube and the
tubular shell.
[0066] Next, the synthetic quartz glass ingots produced in Example
and Comparative Example were sliced. Each slice was mirror
finished. The distribution of initial transmittance at 193.4 nm of
the slice from the center to the periphery was measured by
ultraviolet spectrophotometry (Cary 400 by Varian Corp.). The
results are shown in FIG. 5.
[0067] Japanese Patent Application No. 2003-406982 is incorporated
herein by reference.
[0068] Although some preferred embodiments have been described,
many modifications and variations may be made thereto in light of
the above teachings. It is therefore to be understood that the
invention may be practiced otherwise than as specifically described
without departing from the scope of the appended claims.
* * * * *