U.S. patent application number 10/139358 was filed with the patent office on 2003-02-27 for fluorine-containing silica glass and its method of manufacture.
This patent application is currently assigned to NIKON CORPORATION. Invention is credited to Fujiwara, Seishi, Hiraiwa, Hiroyuki, Jinbo, Hiroki, Komine, Norio, Nakagawa, Kazuhiro, Yajima, Shouji.
Application Number | 20030037568 10/139358 |
Document ID | / |
Family ID | 27330230 |
Filed Date | 2003-02-27 |
United States Patent
Application |
20030037568 |
Kind Code |
A1 |
Fujiwara, Seishi ; et
al. |
February 27, 2003 |
Fluorine-containing silica glass and its method of manufacture
Abstract
The invention relates to fluorine-containing silica glasses, and
methods of their production. The silica glass may be used for an
ultraviolet light optical system in which light in a wavelength
region of 200 mn or less, such as an ArF (193 nm) excimer laser, is
used The invention also relates to a projection exposure apparatus
containing fluorine-containing glass of the invention.
Inventors: |
Fujiwara, Seishi;
(Sagamihara-shi, JP) ; Hiraiwa, Hiroyuki;
(Yokohama-shi, JP) ; Nakagawa, Kazuhiro;
(Hachioji-shi, JP) ; Yajima, Shouji;
(Sagamihara-shi, JP) ; Komine, Norio;
(Sagamihara-shi, JP) ; Jinbo, Hiroki;
(Kawasaki-shi, JP) |
Correspondence
Address: |
MORGAN LEWIS & BOCKIUS LLP
1111 PENNSYLVANIA AVENUE NW
WASHINGTON
DC
20004
US
|
Assignee: |
NIKON CORPORATION
|
Family ID: |
27330230 |
Appl. No.: |
10/139358 |
Filed: |
May 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10139358 |
May 7, 2002 |
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09345764 |
Jul 1, 1999 |
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09345764 |
Jul 1, 1999 |
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08915562 |
Aug 21, 1997 |
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5958809 |
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Current U.S.
Class: |
65/17.4 ;
428/428 |
Current CPC
Class: |
C03C 3/06 20130101; C03C
2201/23 20130101; C03B 2207/66 20130101; Y02P 40/57 20151101; C03B
37/0142 20130101; Y10S 501/905 20130101; C03B 19/14 20130101; C03B
2201/07 20130101; C03B 37/014 20130101; C03C 2201/21 20130101; C03C
2203/46 20130101; C03C 2203/42 20130101; C03C 4/0085 20130101; G03F
7/2004 20130101; C03C 2201/12 20130101 |
Class at
Publication: |
65/17.4 ;
428/428 |
International
Class: |
C03B 020/00; C03B
008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 1996 |
JP |
08-218991 |
Aug 22, 1996 |
JP |
08-221248 |
Aug 22, 1996 |
JP |
08-221254 |
Claims
What is claimed is:
1. An optical, fluorine-containing silica glass having a weight
ratio of fluorine to sulfur of no less than 100.
2. An optical, fluorine-containing silica glass of claim 1, wherein
the silica glass has a transmittance of 99.9% or higher for
ultraviolet light of wavelength 193 nm.
3. An optical, fluorine-containing silica glass of claim 1, wherein
the fluorine concentration contained in the silica glass is less
than 100 ppm.
4. A projection exposure apparatus, which projects and exposes a
pattern image of a mask onto a substrate using ultraviolet light,
comprising an optical, fluorine-containing silica glass of claim
1.
5. A projection exposure apparatus, which projects and exposes a
pattern image of a mask onto a substrate, comprising an
illumination optical system for illuminating the mask using
ultraviolet light as exposure light and a projection optical system
comprising the fluorine-containing silica glass of claim 1 for
forming the pattern image of the mask on a substrate.
6. A direct flame hydrolysis method of producing a
fluorine-containing silica glass comprising the steps of:
hydrolyzing a silicon-containing gas in a flame and in the presence
of a fluorine-containing gas to form a fluorine-containing, silica
glass powder, wherein the silicon-containing gas, the
fluorine-containing gas, a carrier gas, and a combustion gas are
expelled through a burner having a circular center pipe and at
least one concentric circular ring pipe surrounding the center
pipe, and wherein the silicon-containing gas is expelled through
the center pipe and the fluorine-containing gas is expelled through
the center pipe or a ring pipe adjacent to the center pipe; and
allowing the glass powder to accumulate on a target and melt to
form a fluorine-containing silica glass.
7. A method of claim 6, wherein the burner and the target are moved
relative to each other in a plane.
8. A fluorine-containing silica glass manufactured by the method of
claim 7, wherein the silica glass comprises between 50 and 1000 ppm
fluorine with the difference between the minimum fluorine
concentration and the maximum fluorine concentration in the silica
glass being no more than about 15 ppm.
9. A method of claim 6, further comprising, during the hydrolyzing
step, the step of: rocking the target relative to the vertical
direction of growth of the silica glass on the target.
10. A method of claim 6, further comprising, during the hydrolysis
step, the step of: rotating, rocking, or lowering the target with
respect to the burner.
11. A method of claim 6, wherein the burner comprises four
concentric ring pipes and the fluorine-containing gas is expelled
from the first ring pipe adjacent to the center pipe.
12. A method of claim 6, wherein the burner comprises four
concentric ring pipes and the fluorine-containing gas is expelled
from the center pipe and from the first ring pipe adjacent to the
center pipe.
13. A method of claim 12, wherein the fluorine-containing gas is
selected from a group comprising SiF.sub.4, SF.sub.6, F.sub.2 or
mixtures thereof.
14. A method of claim 6, wherein the silicon-containing gas is a
mixture of SiF.sub.4 and SiCl.sub.4.
15. A method of manufacturing of claim 14, wherein the SiCl.sub.4
is expelled at a flow rate ranging from about 5 g/min.
16. A method of manufacturing of claim 6, wherein both the
fluorine-containing gas and the silicon-containing gas are
SiF.sub.4.
17. A method of claim 16, wherein the flame is a hydrogen/oxygen
flame and the ratio of oxygen to hydrogen expelled from the burner
ranges from about 0.2 to 0.5.
18. A fluorine-containing silica glass manufactured by the method
of claim 17, wherein the hydrogen molecule concentration in the
silica glass ranges from about 2.times.10.sup.17 to
5.times.10.sup.18 molecules/cm.sup.3 and the hydroxyl group
concentration ranges from about 600 ppm to 1300 ppm.
19. A projection exposure apparatus, which projects and exposes a
pattern image of a mask onto a substrate using ultraviolet light,
comprising a fluorine-containing silica glass manufactured by the
method of claim 6.
20. A projection exposure apparatus, which projects and exposes a
pattern image of a mask onto a substrate, comprising an
illumination optical system for illuminating the mask using
ultraviolet light as exposure light and a projection optical system
comprising a fluorine-containing silica glass manufactured by the
method of claim 6.
21. A fluorine-containing silica glass manufactured by the method
of claim 6, wherein the silica glass is substantially free of
chlorine and has a structure determination temperature ranging from
900 K to 1200 K.
22. A silica glass of claim 21, wherein the fluorine concentration
ranges from 10 to 1000 ppm.
23. A silica glass of claim 21, wherein the hydroxyl concentration
glass ranges from 600 to 1300 ppm and the hydrogen molecule
concentration ranges from 2.times.10.sup.17 to 5.times.10.sup.18
molecules/cm.sup.3.
24. A direct flame hydrolysis method of producing a
fluorine-containing silica glass substantially free of chlorine,
the method comprising the steps of: hydrolyzing a
silicon-containing gas in a flame and in the presence of a
fluorine-containing gas to form a fluorine-containing, silica glass
powder, wherein the silicon-containing gas, the fluorine-containing
gas, a carrier gas, and a combustion gas are expelled through a
burner and wherein the silicon-containing gas is a silane alkoxide
or silicon tetrafluoride; and allowing the glass powder to
accumulate on a target and melt to form a fluorine-containing
silica glass.
25. A method of claim 24, wherein the silicon-containing gas, the
fluorine-containing gas, a carrier gas, and a combustion gas are
expelled through a burner having a circular center pipe and at
least one concentric circular ring pipe surrounding the center
pipe, and wherein the silicon-containing gas is expelled through
the center pipe and the fluorine-containing gas is expelled through
the center pipe or a ring pipe adjacent to the center pipe.
26. A method of claim 24, further comprising the steps of: heating
the resulting silica glass to a temperature of at least 1200 K for
a minimum of five hours; and cooling the silica glass at a rate of
10 K per hour to a temperature of 773 K or lower.
27. A projection exposure apparatus, which projects and exposes a
pattern image of a mask onto a substrate using ultraviolet light,
comprising a fluorine-containing silica glass of claim 21.
28. A projection exposure apparatus, which projects and exposes a
pattern image of a mask onto a substrate, comprising an
illumination optical system for illuminating the mask using
ultraviolet light as exposure light and a projection optical system
comprising the fluorine-containing silica glass of claim 21.
Description
[0001] This application claims benefit under 35 U.S.C. .sctn.119 of
Japanese Patent Application No. 08-218991, filed Aug. 21, 1996,
Japanese Patent Application No. 08-221248, filed Aug. 22, 1996 and
Japanese Patent No. 08-221254, filed Aug. 22, 1996. These Japanese
Patent Applications are hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The invention relates generally to fluorine-containing
silica glasses, and methods of their production. The silica glass
may be used for an ultraviolet light optical system in which light
in a wavelength region of 200 nm or less, such as an ArF (193 nm)
excimer laser, is used. The invention also relates to a projection
exposure apparatus containing fluorine-containing glass of the
invention.
BACKGROUND OF THE INVENTION
[0003] A reduction projection type exposure apparatus known as a
stepper is used in photolithographic techniques in which fine
patterns of integrated circuits are exposed and transferred onto
wafers such as those made of silicon. A stepper optical system
contains an illumination optical system and a projection optical
system. The illumination optical system illuminates light from a
light source uniformly onto a reticle on which an integrated
circuit pattern is drawn. The projection optical system projects
and transfer the integrated circuit pattern of the reticle onto the
wafer, typically by reducing the circuit patterns projection to one
fifth its original size. These stepper optical systems are widely
used in large scale integration (LSI) and very large scale
integration (VLSI) photolithography operations.
[0004] In recent years, large scale integration (LSI) and, more
recently, very large scale integration (VLSI) have rapidly become
more highly integrated and functionalized. In the field of logical
VLSI, larger systems are being required with the shift to
system-on-chip. With this progress, finer processability and higher
integration capabilities are required for a wafer, such as that
made of silicon, which constitutes a substrate for VLSI. Indeed,
the advance from LSI to VLSI has gradually increased the capacity
of DRAM from 1 KB through 256 KB, 1 MB, and 4 MB, with the
corresponding processing line width required for the stepper
becoming increasingly finer from 10 .mu.m through 2 .mu.m, 1 .mu.m,
0.8 .mu.m and 0.3 .mu.m. Accordingly, it is necessary for a
projection lens of the stepper to have a high resolution and a
great depth of focus.
[0005] The resolution and the depth of focus of an optical lens is
determined by the wavelength of the light used for exposure and the
numerical aperture (N.A.) of the lens. The resolution and the depth
of focus are expressed by the following equations:
resolution=k1.times..lambda./N.A.
depth of focus=k2.times..lambda./N.A..sup.2
[0006] where k1 and k2 are constants of proportionality. The
resolution of the transfer pattern is proportional to the number of
apertures of the projection optical lens system and is inversely
proportional to the wavelength of the light from the light source.
Thus, higher resolutions may be obtained by either increasing the
numerical aperture or making the wavelength shorter.
[0007] From a practical standpoint, it is desirable to use a
shorter wavelength of light to produce higher resolutions of
transfer patterns. When the numerical aperture of the lens is
increased, the angle of the refracted light is also increased
preventing the capture of the refracted light. In contrast, when
the exposure wavelength .lambda. becomes shorter, the angle of the
refracted light becomes smaller in the same pattern, thereby
allowing the numerical apperature to remain small. Furthermore, the
number of apertures of the lens is limited by the lens production
process, hence shortening of the wavelength is the only way to
increase the resolution.
[0008] In order to achieve higher resolution of the transfer
patterns, the wavelength of light sources used in a stepper is
becoming shorter, going from g-line (436 nm) to I-line (365 nm) and
further to KrF excimer laser beam (248 nm) and ArF excimer laser
beam (193 nm). Indeed, as mentioned above, production of VLSI such
as DRAM with storage capacity of more 4 MB requires the
line-and-space, an index for stepper resolution, to be no more than
0.3 .mu.m. This high degree of resolution, requires the use of
ultraviolet and vacuum ultraviolet wavelengths of no more than 250
nm such as those achieved with an excimer laser light source.
[0009] A stepper typically contains a combination of numerous
optical members such as lenses. Unfortunately, even when each lens
element in the stepper has a small transmission loss, such a loss
is multiplied by the number of the lens sheets used. Accordingly,
it is necessary for the optical member to have a high
transmittance. However, large light transmission losses occur when
using a wavelength which is shorter than the I-line. Indeed, light
in the wavelength region below 250 nm ceases to transmit through
most optical glass materials. Only crystalline materials or silica
glasses have proven to have sufficient light transmission
properties for use as optical members in excimer laser light source
steppers. Among these, silica glass is widely used not only for an
excimer laser stepper but also for an optical system using general
ultraviolet and vacuum ultraviolet light.
[0010] Silica glasses used as optical members in photolithography
apparatus are required to meet exacting specifications. Indeed,
high uniformity of the refractive index distribution is required in
order to reduce the amount of multiple refraction, or to reduce
inner strain (birefringence) of the optical member. For example, a
refractive index distribution is required to be of an order of no
more than 10.sup.-6 in an apperature having a diameter of 200 nm.
Furthermore, a high transmission for the silica glass is also
required. Typically, lenses having large curvatures are needed for
aberration correction in projection optical systems, often causing
the total optical path length in the projection optical system to
exceed 1000 mm. In order to maintain the throughput of such a
projection optical system at 80% or more, an internal transmittance
per 1 cm of the optical member needs to be 99.8% or higher, i.e.,
no more than 0.002 cm.sup.-1 converted in terms of inner absorption
coefficient. Moreover, such a high transmittance is required to be
maintained over the entire area of the optical member. For these
reasons, only high purity silica glasses may be used in a optical
system such as an excimer laser stepper. Thus, a need exists for
silica glasses capable of being used in these optical systems.
[0011] Synthetic silica glasses are roughly classified according to
production method into synthetic silica glass and fused silica
glass. The production of synthetic silica glass is further
classified mainly into the Vapor Phase Axial Deposition (VAD)
method, which is also known as the soot re-melting method; the
direct method, which is also known as the flame hydrolysis method;
and the plasma method. All of these synthetic methods belong to a
general manufacturing category known as a gas phase synthetic
method.
[0012] In the Vapor Phase Axial Deposition (VAD) method, a high
purity gaseous silicon compound is hydrolyzed in an oxygen/hydrogen
flame, and deposits soot on a target. This result in a soot ingot,
which is sintered at 800.degree. C. and consolidated by heating the
soot ingot at a relatively low temperature of 1600.degree. C. while
performing a dehydration process with chlorine gas. A silica glass
ingot is then obtained.
[0013] In the direct method (flame hydrolysis method), a high
purity gaseous silicon compound, such as silicon tetrachloride, is
hydrolyzed in an oxygen and hydrogen flame to form minute silica
glass particles (soot particles). The gaseous silicon compound,
oxygen and hydrogen are expelled from a burer. A silica glass ingot
is obtained by depositing to soot particles on a target, melting
the soot particles to form glass particles, in a single step, while
the target is being rotated, rocked and/or lowered in the direction
of the bumer. Using a direct method, an attempt has been made to
obtain an even more uniform silica glass by performing a secondary
heat treatment of about 2000.degree. C. on the silica glass optical
member which is obtained by the direct method. In that method, the
subsequent heat treatment is termed "secondary" as opposed to the
process of synthesizing the silica glass which is the first
process.
[0014] In the plasma method, a high purity, gaseous silicon
compound is oxidized in a high frequency plasma flame of oxygen and
argon to form the soot. A silica glass lump is obtained by
depositing the soot onto a target, melting it, and making it
transparent, all at once, while the target is being rotated and
lowered in the direction of the burner. In general, synthetic
silica glasses are obtained by using a VAD method or direct method
rather than a plasma method.
[0015] Using such gas phase methods, it is possible to obtain a
silica glass optical member with higher purity, higher light
transmittance for wavelengths below 250 mn, larger aperture
diameter, and more uniformity than obtained from fused silica
glass. Fused silica glass is obtained by electric or flame melting
of a natural quartz powder. For these reasons, a synthetic silica
glass is viewed as a promising material for a photolithography
apparatus optical system such as an excimer laser stepper.
[0016] However, the synthetic silica glasses described above are
susceptible to degradation when exposed to ultraviolet light
typically used in a stepper. When synthetic silica glass is exposed
for a long period of time to high output ultraviolet light or
excimer laser beam an absorption band of 215 nm often appears due
to a structural defect known as E'-center. In addition, an
absorption band of 260 nm caused by a structural defect known as
NBOHC (Non-Bridging Oxygen Hole Center), may also appear. The
presence of either absorption band results in a rapid transmission
loss in the ultraviolet wavelength region. An E'-center represents
a structure of .ident.Si., in which .ident. indicates bonding with
three oxygen atoms rather than a triple bond and . indicates
unpaired Was electron. An NBOHC is a structure corresponding to
.ident.Si--O. These structural defects result in transmission
losses in the ultraviolet wavelength region and render a synthetic
silica glass unsuitable for use with an ultraviolet light or
excimer laser.
[0017] Examples of precursors which generate structural defects
include a .ident.Si--Si.ident., a .ident.Si--O--O--Si.ident., and
Cl. Silica glasses produced by the plasma method or by the VAD
method are known to contain such precursors. However, absorption
measurements using vacuum ultraviolet, ultraviolet, visible light
and infra-red spectrometer have verified that an imperfect
structure due to an oxygen deficiency or an excess of oxygen
generally does not occur in synthetic silica glass produced by the
direct method. Moreover, using a direct method a synthetic silica
glass, a high degree of purity may be obtained with the
concentration of metal impurities, such as, Mg, Ca, Ti, Cr, Fe, Ni,
Cu, Zn, Co, Mn, being less than 20 ppb. Consequently, a synthetic
silica glass obtained by the direct method is generally considered
a promising optical member for use in excimer laser stepper.
Unfortunately, synthetic silica glass produced by a direct method
of the prior art often undergoes a reduction in transmittance due
to the presence of unidentified precursors in the final glass.
[0018] Attempts to prevent the reduction in transmittance of a
synthetic silica glass have been made. One such attempt is
described in Japanese Laid-Open Patent Publication 1-201664,
incorporated herein by reference, which teaches heat treating the
silica glass in a hydrogen atmosphere. Another technique is
described in Japanese Laid-Open Patent Publication 3-109233
incorporated herein by reference, which teaches hydrogen molecule
doping. However; doping silica glass with hydrogen molecules
repairs, but does not eliminate, structural defects caused by
irradiation of ultraviolet light. For example, hydrogen molecules
react with an E' center and are transformed to .ident.Si--H bonds,
helping to reduce the E' center concentration. However,
.ident.Si--H is rapidly transformed back to an E' center defect
upon further irradiation with ultraviolet light. It is important to
prevent generation of structural defects such as an E' center or
NBOHC by eliminating or reducing the amount of materials which
cause these structural defects.
[0019] Attempts have also been made to control the generation of
structural defects such as E' center or NBOHC by reducing or
excluding the presence of Cl in a synthetic silica glass. By doing
this, precursors related to Cl are reduced or excluded with a
certain degree of control against excimer laser or ultraviolet
light irradiation defects being achieved. However, although
conventional techniques, such as those described above, have shown
some degree of success, often these techniques failed to reduce
degradation of silica glass when using an excimer laser stepper.
The invention described below aims to solve the shortcoming of
conventional techniques described by providing a synthetic silica
glass optical member which controls the generation of ultraviolet
light defects and prevents a decline in transmittance even when the
subjecting silica glass optical member to ultraviolet light and
excimer laser beam of short wavelength and of high output for long
durations.
[0020] Another approach to preventing the formation of structural
defects has been to add fluorine to the silica glass via a VAD
method. In contrast to Si--Si bonds, Si--F bonds have a large
bonding energy and are not rapidly dissociated by ultraviolet
light. A method of manufacturing a silica glass containing
fluorine, hydroxyl group and hydrogen molecules-by the VAD method
is described in Japanese Laid-Open Patent Publications 8-67530 and
8-7590, incorporated herein by reference. The references disclose
that by doping the silica glass with fluorine to form Si--F bonds,
the number of Si--Si bonds, which are oxygen deficient type defects
causing an absorption of light at 163 nm, may be reduced. Indeed,
in Japanese Laid-Open Patent Publication 6-156302, incorporated
herein by reference, it was discovered that some silica glasses
formed by a VAD method in which fluorine concentration is no less
than 100 ppm were effective as vacuum ultraviolet-use optical
members. However, some silica glasses containing 100 ppm formed by
the VAD method were found to generate an emission band having a
peak wavelength of 585 nm and suffered a decreased initial
transmittance at wavelengths below 250 nm. Thus, silica glass
formed by previous VAD methods have not proven satisfactory for
photolithography techniques requiring the use of ultraviolet light
less than or equal to 250 nm.
[0021] The VAD method generally requires secondary processing such
as heat processing in a hydrogen atmosphere. Without such heat
treatment it was impossible to have fluorine, hydroxyl group and
hydrogen molecules present at the same time in the silica glass.
However, problems also arise during this heat treatment. The silica
glass may become contaminated with impurities resulting in
ultraviolet light transmission losses. Also, hydrogen diffuses into
the interior of the glass substrate. This diffusion creates a
non-uniform distribution of the hydrogen in the silica glass with a
higher hydrogen molecules concentration occurring at the edges of
the glass as compared to the center of the glass. This effect is
exacerbated as the diameter of the glass increases. Thus,
sufficient silica glass resistance to ultraviolet light degradation
is not obtained when using previous VAD methods.
[0022] Previous attempts at fluorine doping of silica glass have
been performed using the VAD method. This is because the use of the
direct method with its oxygen/hydrogen flame causes fluorine
contained in a dopant gas to react with hydrogen contained in
oxygen/hydrogen flame to form hydrofluoric acid which is expelled
out of the system. In other words, fluorine and hydroxyl groups
cannot co-exist at high temperature. This can be seen from a free
energy point of view because the Gibbs free energy sign for the
reaction of fluorine and hydroxyl group reverses around 1200 K.
Therefore, fluoride doping using a direct method performed at a
high temperature of 2000 K or more will cause hydroxyl group and
fluorine to react, preventing fluorine doping of the silica
glass.
[0023] Even with such attempts, there remains a need to produce
high quality synthetic silica glass which may be used in optical
devices, such as steppers. Moreover, there remains a need for
synthetic silica glasses which may be used with ultraviolet (UV)
light and excimer laser beam but which resist forming defects
associated with that use.
SUMMARY OF THE INVENTION
[0024] The invention answers these needs, particularly the problems
of using ultraviolet light lasers with silica glasses, by providing
fluorine-containing silica glasses which have excellent resistance
to degradation and structural defects when exposed to ultraviolet
light while providing superior ultraviolet light transmittance. The
invention also provides various cost effective methods of
manufacturing fluorine-containing silica glasses.
[0025] A first embodiment of the invention relates to an optical,
fluorine-containing silica glass preferably produced by a direct
method using a fluorine-containing gas. The optical
fluorine-containing silica glass having a weight ratio of fluorine
to sulfur of no less than 100. By having a sufficiently high weight
ratio of fluorine to sulfur in the silica glass it is possible to
achieve transmittance of 99.9% or higher for ultraviolet light of
wavelength 193 nm. The fluorine concentration in silica glasses
formed by this method is generally 100 ppm or higher. Throughout
the specification, "transmittance" represents a bulk transmittance
(transmittance of the interior of the glass) which excludes effects
of reflection and scattering at the surface of the glass.
[0026] A second embodiment of the invention relates to a direct
method of producing a fluorine-containing silica glass. The method
hydrolyzes a silicon-containing gas in a oxygen/hydrogen flame and
in the presence of a fluorine-containing gas to form a
fluorine-containing, silica glass powder, and allowing the glass
powder to accumulate on a target and melt to form a
fluorine-containing silica glass. The silicon-containing gas, the
fluorine-containing gas, a carrier gas, and a combustion gas are
expelled through a burner having a circular center pipe and at
least one concentric circular ring pipe surrounding the center
pipe. The silicon-containing gas is expelled through the center
pipe and the fluorine-containing gas is expelled through the center
pipe or a ring pipe adjacent to the center pipe. Silica glass
manufactured by this method contains fluorine, hydrogen molecules
and hydroxyl group. A preferred concentration range for the
fruorine in the silica glass ranges from about 50 to 1000 ppm.
[0027] In a third embodiment of the invention a fluorine-containing
silica glass is manufactured by direct method such that it is
substantially free of chlorine and has a structure determination
temperature ranging from 900 K to 1200 K.
[0028] The fluorine-containing silica glasses made by the above
embodiments may be used in a projection exposure apparatus, which
projects and exposes a pattern image of a mask onto a substrate
using ultraviolet light. The projection exposure apparatus contains
an illumination optical system for illuminating the mask using
ultraviolet light as exposure light and a projection optical system
for forming the pattern image of mask on a substrate. The
projection exposure apparatus of the invention includes an optical
member made of a fluorine-containing silica glass. The
fluorine-containing silica glass may be contained in either the
illumination optical system or the projection optical system or
both.
[0029] The silica glasses of the invention has higher durability as
well as higher transmittance against the ultraviolet light thanks
to the effects of fluorine, hydrogen molecules and hydroxyl group
which are contained in the silica glass. Such silica glasses have a
longer life than conventional exposure apparatus.
[0030] Other advantages and features of the invention will be
apparent from consideration of the detailed description of the
invention provided below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic diagram depicting the basic structure
of a typical exposure apparatus of the invention.
[0032] FIG. 2 is graph depicting the correlation between the
fluorine-sulfur ratio and the initial transmittance.
[0033] FIG. 3 is a simplified partial diagram of a synthetic
furnace of the invention.
[0034] FIG. 4 is a cross section of the mouth of a suitable burner
used for synthesis.
[0035] FIG. 5 is a diagram depicting a movement pattern of the
stage unit of the target.
[0036] FIG. 6 is a graph depicting a fluorine concentration
distribution and refractive index distribution obtained in Example
10.
[0037] FIG. 7 is a graph depicting a fluorine concentration
distribution and refractive index distribution obtained in Example
11.
[0038] FIG. 8 is a graph depicting a fluorine concentration
distribution and refractive index distribution obtained in Example
12.
[0039] FIG. 9 is a graph depicting a fluorine concentration
distribution and refractive index distribution obtained in Example
13.
[0040] FIG. 10 is a graph depicting the correlation between the
structure determination temperature and the absorption amount
produced by ArF excimer laser irradiation.
[0041] FIG. 11 is a graph depicting the change of 193 nm
transmittance caused by ArF excimer irradiation.
DETAILED DESCRIPTION OF THE INVENTION
[0042] The invention provides a commercially feasible method of
forming a fluorine-containing silica glass which may be used in a
photolithography apparatus. Silica glasses of the invention exhibit
a high degree of ultraviolet light transmittance and are better
able to withstand degradation when exposed to ultraviolet light of
wavelengths of less than 250 nm than conventional silica glasses.
The fluorine-containing silica glasses can be formed with large
diameters and are usable in the vacuum ultraviolet region without
absorption of ultraviolet light in the vacuum ultraviolet light
region of from 160 nm to 250 nm. Preferably, the silica glasses of
the invention may be used as optical devices in photolithography
devices such as a stepper.
[0043] High UV Transmission Fluorine-containing Silica Glass
[0044] A first embodiment of the invention provides an optical,
fluorine-containing silica glass produced by a direct method using
a fluorine-containing gas. Optical, fluorine-containing silica
glasses according to this embodiment have high UV transmittance
particularly below 230 nm and do not suffer from the defects caused
by UV light as do prior silica glasses. The optical,
fluorine-containing silica glass has a weight ratio of fluorine to
sulfur of no less than 100. Preferably, the optical,
fluorine-containing silica glass has a transmittance of 99.9% or
higher for ultraviolet light of wavelength 193 nm. FIG. 2 depicts
the correlation between the fluorine-sulfur weight ratio and
transmittance at 193 nm. An optical, fluorine-containing silica
glass of this embodiment also preferably has fluorine concentration
of no less than 100 ppm.
[0045] In general, the fluorine-containing silica glass may be made
by a direct method, and is preferably made by the direct method
described below. When using sulfur hexafluoride as the
fluorine-containing gas, a large number of sulfur atoms may be
introduced into the fluorine-containing silica glass. Sulfur atoms,
which are often used as emission elements, can cause light
emissions in conventional silica glasses causing them to be
unsuitable for use. Indeed, if the weight ratio of sulfur to
fluorine in the silica glass is below 100, a decreased initial
transmittance and an emission band with strong yellow light having
a peak wavelength of 585 nm is observed. The reason for this
phenomena is not clear, however, it is believed that the sulfur
contained is the glass may act in a manner similar to alkaline
metals, such as sodium, and may cause generation of non-bridge
oxygen NBO defect. Such a non-bridge oxygen defect possesses an
absorption around 185 nm resulting in poor transmittance of
ultraviolet light. However, the non-bridge oxygen defect generated
by the presence of the sulfur atoms is terminated by fluorine. If
the silica glass has a small fluorine concentration, the non-bridge
oxygen defect generated by the sulfur may remain.
[0046] It has been discovered that a high transmittance in the
ultraviolet region below 250 nm is obtained when the weight ratio
of fluorine to sulfur is 100:1 or above. Accordingly, an optical
fluorine-containing silica glass of the invention has a molar ratio
of fluorine to sulfur of no less than 100. When the weight ratio is
no less than 100, a transmittance of 99.9% or higher for
ultraviolet light of wavelength 193 nm may also be obtained.
Preferably, the amount of fluorine contained in an optical,
fluorine-containing silica glass of the invention is also 100 ppm
or higher. In this way, optical fluorine-containing silica glasses
of the invention have high transmittance in the ultraviolet region
below 250 nm and advantageously have properties which resist
defects caused by UV light, e.g., NBOHC. Such silica glasses are
particularly useful in precision optical equipment, such as a
stepper.
[0047] Direct Method
[0048] In a second embodiment of the invention, a
fluorine-containing silica glass is manufactured by a direct
method. The method hydrolyzes a silicon-containing gas in a flame,
preferably a hydrogen/oxygen flame, and in the presence of a
fluorine-containing gas to form a fluorine-containing, silica glass
powder, and allows the glass powder to accumulate on a target and
melt to form a fluorine-containing silica glass. In the method, the
silicon-containing gas, the fluorine-containing gas, a carrier gas,
and a combustion gas are expelled through a burner having a
circular center pipe and at least one concentric circular ring pipe
surrounding the center pipe. The silicon-containing gas is expelled
through the center pipe and the fluorine-containing gas is expelled
through the center pipe or a ring pipe adjacent to the center
pipe.
[0049] The flame hydrolysis progresses such that the
silicon-containing gas and the fluorine-containing gas are well
mixed (the silicon-containing gas and the fluorine-containing gas
may be the same gas, e.g., a gas having Si--F bonds) and the
production of the fluorine-containing silica glass in the reaction
zone is not governed by free energy considerations. Accordingly,
the invention provides a method of producing a fluorine-containing
silica glass in which the silicon-containing gas, a carrier gas,
and a combustion gas are expelled from a burner which also expels
the fluorine-containing gas.
[0050] A preferred direct method expels the flammable gas, the
carrier gas, the silicon-containing gas and the fluorine-containing
gas from a burner having four ring pipes arranged in a concentric
circle around a center pipe. This type of burner is shown in FIG. 3
and is discussed in Example 4, below. The silicon-containing gas is
expelled from the center pipe. The fluorine-containing gas may be
expelled from either the center pipe, the first ring pipe adjacent
to the center pipe, or both. Preferably, the fluorine-containing
gas is expelled from the center pipe. In a more preferred method,
the fluorine-containing gas is also expelled both from the center
pipe and from the first concentric ring pipe adjacent to the center
pipe. By doing so, large amounts of fluorine can be doped into the
silica glass. The glasses produced by these methods contain
fluorine, hydrogen molecules and hydroxide ions. The resulting
silica glasses exhibit excellent durability against degradation by
ultraviolet light and virtually no decline in transmittance when
subjected to irradiation with high output excimer laser beam or
ultraviolet light for long periods of time.
[0051] As discussed below, a direct flame hydrolysis method of the
invention, produces a fluorine-containing having a hydroxyl group
and hydrogen molecule concentration of more than 100 ppm and
1.times.10.sup.17 molecules/cm.sup.3 respectively are obtained.
Generally, the hydrogen molecule concentration in the silica glass
ranges from about 1.times.10.sup.17 to 5.times.10.sup.18,
preferably from about 2.times.10.sup.17 to 5.times.10.sup.18
molecules/cm.sup.3. The fluorine concentration in the glass is more
than or equal to 0.01 weight percent but less than or equal of 0.5
weight percent. However, if the amount of fluorine doping is too
large, the fluorine may exist in the glass as Si--F in addition to
as fluorine molecules, causing absorption by fluorine molecules in
ultraviolet region. This results in poor initial transmission loss.
Moreover, fluorine molecules are dissociated by ultraviolet light
irradiation, which may have detrimental effects on the silica glass
decreasing its resistance against ultraviolet light.
[0052] Preferably, the fluoride is evenly distributed throughout
the silica glass. An even distribution of fluoride results in a
silica glass with a large diameter and a high level of uniformity
of refractive index in the radial direction. To do this, the
temperature on the surface of the glass ingot, formed by the
deposition of minute glass particles on the target should be
constant and uniform. Additionally, to improve the uniformity of
the silica glass, it is preferred that the relative position of the
target is moved in a plane with respect to the burner. Furthermore,
it is preferred that the movement pattern creates a uniform
residence time of silicon-containing gas relative to the silica
glass ingot surface. The moving body may be either the target or
the burner, but from a maneuverability point of view, it is easier
to move the target relative to the burner. A suitable movement
pattern of the stage unit of the target is shown in FIG. 5. Such
movement patterns allow the fluorine in the gaseous mixture to be
diffused uniformly in the glass such that a high level of
uniformity is obtained for the glass. By moving the target relative
to the burner in this manner, a silica glass is formed such that
the maximum value and the minimum value of the fluorine
concentration distribution is no more than 15 ppm.
[0053] In another aspect of this method, the burner of the
production apparatus may be placed at a certain angle with respect
to the growth direction of the target. A preferred angle may range
from 10 to 20.degree., and more preferably be about 15.degree.. By
placing the burner at an angle to the growth, it becomes possible
to diffuse fluorine uniformly in the silica glass by rocking the
target in the growth direction of the ingot. Preferably, the silica
glass ingot is formed by simultaneously deposition, melting and
making the silica glass particles transparent while rotating,
rocking and lowering the target with respect to the burner.
[0054] In a direct method of the invention, a fluorine-containing
silica glass is formed using a fluorine-containing gas, a
silicon-containing gas, a carrier gas and a flammable gas. The
fluorine-containing gas is selected from a group including, but not
limited to, SiF.sub.4, SF.sub.6, F.sub.2 or mixtures thereof. As
mentioned above, if the fluorine doping amount is too large,
fluorine will exist in the glass as both Si--F and as fluorine
molecules, allowing absorption by fluorine molecules to occur in
the ultraviolet region. This results in poor initial transmission
loss. Since fluorine molecules are dissociated by ultraviolet light
irradiation, the silica glass to degrade when exposed to
ultraviolet light. Thus, the fluorine concentration in the silica
glass is typically more than about 10 ppm and less than about 5000
ppm. A preferred concentration range is from about 50 ppm to 1000
ppm, more preferably about 100 ppm to 1000 ppm.
[0055] When a silica glass is synthesized using silicon
tetrafluoride, it is desirable to make the fluorine concentration
within a range from 50 to 1000 ppm. If the fluorine concentration
is 1000 ppm or higher, it becomes difficult to control the maximum
concentration difference of fluorine distribution in the radial
direction to be less than about 10 ppm when a member with large
diameter, for example, 200 mm or more, is required In this case,
the fluorine concentration difference is equivalent to about
4.times.10.sup.-6 computed in terms of refractive index difference.
Thus, it is desirable to maintain a fluorine concentration of 1000
ppm or less in order maintain the coo uniformity of the refractive
index.
[0056] The silicon-containing gas is selected from a group
including, but not limited to, SiF.sub.4, SiCl.sub.4,
SiH.sub.nCl.sub.4-n (n=1-4), an organic silicon compound, or
mixtures of such gases. Preferably, the flow amount of SiCl.sub.4
ranges from about 0 g/m to 5 g/min. The fluorine-containing gas and
the silica-containing gas may be SiF.sub.4. Preferably the
silicon-containing gas is a mixture having of SiCl.sub.4 and
SiF.sub.4, and the fluorine-containing gas is SiF.sub.4.
Preferably, the ratio of SiCl.sub.4 to SiF.sub.4 ranges from 0 to
0.25. The mixture of SiCl.sub.4 and SiF4 may be expelled from the
circular pipe. By mixing SiF.sub.4 and SiCl.sub.4, the chlorine
contained in the flame is also able to control the etching effect
of the fluorine, contained in the flame, on the glass surface. The
use of such a mixture increases the SiO.sub.2 layer amount per unit
time and increases the probability of fluorine remaining in the
silica glass. Moreover, by mixing SiCl.sub.4, the corrosion of the
exhaust system and other parts due to hydrofluoric acid generated
by SiF.sub.4 is reduced. Moreover, SiF.sub.4 is a relatively
expensive gas, and thus, its use increases the manufacturing costs
of the silica glass. Consequently, substantially reduced
manufacturing costs are achieved by mixing SiCl.sub.4 with
SiF.sub.4.
[0057] Any carrier gas typically used in a direct flame hydrolysis
may also be used in the methods of this invention. Typical carrier
gases, include, but not limited to, He, O.sub.2, F.sub.2, SF.sub.6,
or mixtures thereof. If sulfur hexafluoride (SF.sub.6) is used as
the carrier gas, it should preferably be mixed with oxygen gas in
order to reduce the amount of sulfur contained in the glass.
Preferably, the carrier gas is He or O.sub.2, most preferably
O.sub.2 It is-desirable to expel the carrier gas from the ring pipe
adjacent to the circular pipe. This has an effect of preventing the
surrounding hydrogen gas and fluorine-containing gas from forming
hydrofluoric acid and escaping the system. Moreover, either a
mixture of oxygen gas and a gas containing fluorine atom in its
molecule or a gas containing only fluorine may be used as carrier
gas when silicon-containing gas is expelled from the circular pipe.
However, if the gas containing only fluorine is used as a carrier,
a temperature of the synthesis surface of the ingot tends to fail
slightly, causing synthesis to be difficult. Hence, the mixture of
an oxygen gas and the gas containing fluorine atom in its molecule
is preferred.
[0058] The hydroxyl group concentration in the silica glass (which
exist as Si--OH groups) ranges from about 600 ppm to 1300 ppm. The
presence of hydroxyl group provides a more stable silica glass as
it eliminates bridging over unstable bonding angles of
.ident.Si--O--Si.ident.. The desired hydroxyl group concentration
in the silica glass may be obtained by controlling the ratio of
oxygen and hydrogen in the flame to be in the range of
0.2.ltoreq.O.sub.2/H.sub.2.ltoreq.0.5. This ratio of oxygen and
hydrogen optimizes the hydroxyl group concentration and diffuses
hydrogen molecules into the silica glass. The presence of hydrogen
molecules terminates defects generated by ultraviolet light
irradiation resulting in improved silica glass durability against
ultraviolet light. By making the ratio of oxygen and hydrogen to be
less than the stoichiometric ratio of 0.5 in the oxygen and
hydrogen flame, both hydroxyl group and hydrogen molecules may be
optimized while at the same time, hydrogen molecule concentration
may be made to be 2.times.10.sup.17 molecules/cm.sup.3 or higher.
Silica glass manufactured containing several 100 ppm or more
hydroxyl groups is stable structurally.
[0059] To increase the amount of fluorine contained in the silica
glass, the weight of oxygen may be made slightly higher in the
ratio of carrier gas such as oxygen gas and combustible gas such as
hydrogen gas which are burst from the burner during synthesis. In
particular, a desirable ratio of oxygen to hydrogen in the gas to
be burst from the entire burner is 0.50-0.70. More preferably, the
oxygen gas may be expelled from the first ring pipe which is
adjacent to the circular pipe. This prevents surrounding hydrogen
gas and fluorine-containing gas from forming hydrofluoric acid and
escaping completely outside the system. Moreover, synthesis under
oxygen heavy condition reduces the water existing within the
system, enabling control of generation speed of hydrofluoric acid
which is also a decomposition generation product to be low. By
increasing the ratio of O.sub.2 to H.sub.2, it becomes possible to
increase the amount of fluorine contained in the silica glass.
[0060] Fluorine-containing Silica Glasses Having Substantially No
Chlorine
[0061] One way to enhance the fundamental anti-ultraviolet light
property of a fluorine-containing silica glass is to control the
existence of structural defects in the glass itself. This would
involve (1) stabilizing of the silica glass structure itself and
(2) optimizing the amounts of hydroxyl groups, chlorine and
fluorine contained in the silica glass. Silica glasses of this
third embodiment possess enhanced anti-ultraviolet light properties
because chlorine is substantially excluded from the silica glass
and the silica glass has a structural determination temperature
ranging from 900 to 1200 K. "Structural determination temperature
is a parameter indicating the stabilization of the silica glass
structure. Additionally, the silica glass preferably has a fluorine
concentration ranging from 10 to 1000 ppm exhibit increased
enhancement of their resistance to defects caused by ultraviolet
light. In other preferred embodiments, the hydroxyl group
concentration in the silica glass ranges from 600 to 1300 and the
hydrogen molecule concentration ranges from 2.times.10.sup.17 to
5.times.10.sup.18 molecules/cm.sup.3.
[0062] Accordingly, another embodiment of the invention involves
the manufacturing of fluorine-containing silica glasses having
reduced amounts of chlorine. In manufacturing a silica glass using
the direct method, silica tetrachloride (SiCl.sub.4) is normally
used as it is inexpensive, easy to handle and readily available.
Consequently, several dozen ppm of chlorine typically remains in
silica glasses manufactured by the direct method. As previously
described, silica glass having lower levels of chlorine exhibits
less degradation when subjected to ultraviolet light. Indeed,
silica glass containing virtually no residual chlorine exhibits
very little degradation when exposed to ultraviolet light.
[0063] A silica glass of this embodiment may be prepared by a
direct flame hydrolysis method and is substantially free of
chlorine. Preferably, the silica glass has a chlorine concentration
of preferably 50 ppm or less and more preferably 10 ppm or less. As
mentioned, the structure determination temperature of the silica
glass ranges between 900 K and 1200 K.
[0064] A direct method of this invention may be used to obtain a
silica glass, which is substantially free of chlorine. In the
direct method, a silicon-containing gas, for example, silicon
tetrafluoride (Si.sub.4), silicon tetrachloride or an alkoxy silane
such as Si(CH.sub.3O).sub.4, Si(C.sub.2H.sub.5O).sub.4 and
Si(CH.sub.3)(OCH.sub.3).sub.3 substantially, if not entirely,
replaces the SiCl.sub.4 used in conventional direct method.
However, use of the alkoxy silane as the silicon-containing gas for
synthesis may form a carbon compound which remains in the silica
glass. For this reason, it is preferable to use silicon
tetrafluoride or silicon tetrachloride as the silicon-containing
gas for forming the silica glass. In this manner, a silica glass,
which is substantially free of chlorine, having a structure
determination temperature ranging from 900 K to 1200 K, and without
any carbon related impure objects, may be formed.
[0065] Silica glasses having lower structure determination
temperatures exhibit less degradation when subjected to ultraviolet
light than those with higher structure determination temperature.
As discussed in European Patent Application EPO 720969A1 and EPO
720970A1 (the entire disclosures of which are incorporated here by
reference), the structure determination temperature indicates the
structural stability of a silica glass. The fluctuation in density
of a silica glass at room temperature, namely structural stability,
is determined by the density of the silica glass in a state of melt
at high temperature, and the density and structure of the silica
glass when the density and the structure are frozen at around the
glass transition point during cooling. That is, the thermodynamic
density and structure corresponding to the temperature at which the
density and structure are frozen are also retained at room
temperature. The temperature when the density and structure are
frozen is the structure determination temperature of the
invention.
[0066] The structure determination temperature can be determined in
the following manner. First a test piece of silica glass is
retained at a plurality of temperatures within the range of 1073 K
to 1700 K for a period of longer than the structure relaxation
time, a time required for the structure of the silica glass being
relaxed at that temperature. The glass is retained at the retention
temperature in air in a tubular oven made of silica glass, thereby
allowing the structure of the test piece to reach the structure at
the retention temperature. As a result, the test piece has a
structure which is in thermal equilibrium state at the retention
temperature. Then, the test piece is placed, not into water, but
into liquid nitrogen within 0.2 seconds to quench it. If the test
piece is placed in water, quenching is insufficient and structural
relaxation occurs such that the structure at the retention
temperature differs from the final structure of the silica glass.
Moreover, adverse effects may arise due to the reaction between
water and the silica glass. Thus, by quenching the test piece in
liquid nitrogen, the structure determination temperature coincides
with the retention temperature structure.
[0067] The test pieces having various structure determination
temperatures, equal to the retention temperature, are subjected to
measurement of Raman scattering. A 606 cm.sup.-1 line intensity is
obtained as a ratio to 800 cm.sup.-1 line intensity. From this
data, a graph can be prepared with the structure determination
temperature for the 606 cm.sup.-1 line intensity used as a
calibration curve. This is shown, for example, in FIG. 10. Thus,
the structure determination temperature of a test piece may be
calculated from the measured 606 cm.sup.-1 line intensity using the
calibration curve.
[0068] Silica glasses having lower structure determination
temperatures exhibit less degradation when subjected to ultraviolet
light than those with higher structure determination temperature.
Previously, silica glasses were formed having a structure
determination temperature of 1300 K or higher. A silica glass
formed by the direct method, i.e., hydrolysis of a silica compound,
such as silica tetrachloride, in an oxygen and hydrogen flame to
form minute glass particles, with the resulting glass particles
deposited and melted on a target to produce a silica glass ingot,
normally has a structure determination temperature of 1300 K or
higher. Also, a silica glass synthesized by the direct method,
hydrolysis of a high purity silica compound in a hydrogen and
oxygen flame to form glass particles with the particles being
deposited on a target to produce a glass lump, which is made
transparent by secondary process to obtain a silica glass lump,
normally has a structure temperature of higher than 1300 K because
the secondary transparency process is performed generally in the
temperature around 1700 K. Silica glass fibers have a structure
determination temperature of about 1400 K as the fiber drawing
process is conducted at a temperature of about 1700 K.
[0069] Superior silica glass resistance to degradation caused by
ultraviolet light is obtained for glasses having a structure
determination temperature of less than 1200 K. Preferably the
structure determination temperature ranges from abut 900 K to 1200
K. Silica grasses having the structure determination temperature of
1200 K or below may be obtained by several different methods.
[0070] One method of achieving a structure determination
temperature of below 1200 K involves using a silica glass with the
structure determination temperature 1300 K or higher which is kept
at a temperature of 1200 K or lower until the structure reaches
equilibrium at that temperature. For example, a structural
relaxation time of a silica glass containing about 1000 ppm
hydroxide ion at 1173 K is expected to be about 1900 seconds. Thus,
for such a silica glass the structure determination temperature is
made 1173 K by keeping the silica glass at that temperature for
longer than 1900 seconds.
[0071] A second method of achieving a structure temperature of 1200
K or less raises the temperature of a silica glass to about 1200 K
to 1400 K until the structure reaches equilibrium, usually about
ranging from about five hours to several dozen hours. Then, the
silica glass is annealed by lowering the temperature to 1000 K or
less, preferably to 773 K or less, annealing completion
temperature, with a temperature lowering speed of 50 K/hour or
less, preferably of 10 K/hour or less. This is referred to as the
annealing speed or cooling speed. If the annealing temperature
exceeds 1000 K, or if the annealing speed exceeds 50 K/hour, the
structure determination temperature cannot be reduced below 1200 K,
and structural distortion is not sufficiently removed. Once
properly annealed the silica glass is normally left alone and
allowed to cool to room temperature, although there is no special
restriction after the silica glass reaches the aforementioned
annealing completion temperature. Furthermore, in the production
methods above, there is no special restriction on atmosphere or
pressure. Thus, regular air may be used at atmospheric
pressure.
[0072] Optical Devices
[0073] A synthetic fluorine-containing silica glass made by the
method of the present invention may be used in any type of optical
member, particularly optical members for use with ultraviolet
light. There is no restriction on the type an optical member except
that the optical member contains synthetic silica glass made by the
above method. The silica glass maybe used in various optical
members such as lenses and prisms to be used for an exposure
apparatus such as a stepper. Moreover, the possible optical members
includes materials used to manufacture lenses, prisms and the like.
Furthermore, conventional processing methods may be used to process
the synthetic silica glass of the invention into to a desired
optical member. For example, there are no specific restrictions and
normal cutting and grinding methods may be used.
[0074] An optical member using a fluorine-containing silica glass
of the invention has a longer life than conventional optical member
because it is made of synthetic glass with a higher resistance and
higher transmittance against ultraviolet light. This is due to the
beneficial combination of properties from the fluorine, hydrogen
molecules and hydroxyl groups contained in the silica glass
prepared according to the invention.
[0075] The silica glasses of the invention may be used in any
optical device. The silica glasses can be used in many ways
including, but not limited to a lens member, a fiber, a window
member, a mirror, an etalon and a prism of an illumination optical
system or a projection optical system such as an excimer laser
lithography apparatus, a photo CVD apparatus and a laser processing
apparatus whose light source whose wavelength is 250 nm or
lower.
[0076] Preferably, the silica glasses are used in a projection
exposure apparatus, such as a stepper, for projecting an image of
patterns of reticle onto a wafer coated with a photoresist. FIG. 1
shows a basic structure of the exposure apparatus according to the
invention. As shown in FIG. 1, an exposure apparatus of the
invention comprises at least a wafer stage 3 allowing a
photosensitive wafer W to be held on a main surface 3a; an
illumination optical system 1 for emitting vacuum ultraviolet light
of a predetermined wavelength as exposure light and transferring a
predetermined pattern of a mask (reticle R) onto the wafer W; a
light source 100 for supplying the exposure light to the
illumination optical system 1; a projection optical system 5
provided between a first surface P1 (object plane) on which the
mask R is disposed; and, a second surface P2 (image plane) which
corresponds to a surface of the wafer W, for projecting an image of
the pattern of the mask R onto the wafer W. The illumination
optical system 1 includes an alignment optical system 110 for
adjusting relative positions between the mask R and the wafer W.
The mask R is disposed on a reticle stage 2 which is movable in
parallel with respect to the main surface of the wafer stage 3. A
reticle exchange system 200 trio conveys and changes a reticle,
mask R, to be set on the reticle stage 2. The reticle exchange
system 200 includes a stage driver for moving the reticle stage 2
in parallel with respect to the surface 3a of the wafer stage 3.
The projection optical system 5 has an alignment optical system
which is applied to a scanning type exposure apparatus.
[0077] The exposure apparatus of the invention contains an optical
member which comprises the silica glasses of the invention, for
example an optical lens of a silica glass made according to a
method of the invention. More specifically, the exposure apparatus
of the invention shown in FIG. 1 can include the optical lens of
the invention as an optical lenses 9 and/or an optical lens 10 in
the projection optical system 5.
[0078] The following examples illustrate the manufacture and use of
fluorine-containing silica glasses according to the invention. The
examples describe the manufacture of silica glasses which exhibit
excellent durability against degradation by ultraviolet light and
having virtually no decline in transmission rate when subjected to
irradiation with high output excimer laser bean or ultraviolet
light for long periods of time.
EXAMPLES 1-9
[0079] A silica glass synthesis furnace such as one described in
FIG. 3 was used to perform synthesis while a heat resistant target
was rotated and lowered with a similar velocity as the deposition
speed. The furnace of FIG. 3 contains a target 11 upon which an
ingot 17 is formed, a burner 12 for expelling the gases, a rotary
shaft 13 which allows for the rotation of the target 11 and the X,
Y, Z stage 16, an exhaustion opening 14 for the exhaust gases to
flow through, and a refractory 15 which forms the interior of the
furnace. The burner was of a 5-pipe structure as shown in FIG. 4
which depicts a circular pipe 21 surrounded by ring pipes 1 22,
ring pipe 2 23, ring pipe 3 24 and ring pipe 4 25. Flow amount of
oxygen, hydrogen and silicon compounds are shown in Table 1.
1 TABLE 1 Circular Ring Ring Ring Ring Oxygen:Hydrogen H.sub.2 OH
Solari- Pipe Pipe 1 Pipe 2 Pipe 3 Pipe 4 Ratio (Molecule/Cm.sup.3)
(ppm) F (ppm) Transmittance zation Ex. 1 SiF.sub.4(slm) 2.5 0 0 0 0
0.45 8.00E+17 830 1300 .gtoreq.99.9 None SiCl.sub.4(g/min) 0 0 0 0
0 O.sub.2(slm) 0.5 11.5 0 20 0 H.sub.2(slm) 0 0 26 0 45 Ex. 2
SiF.sub.4(slm) 3.5 0 0 0 0 0.45 7.00E+17 800 2500 .gtoreq.99.9 None
SiCl.sub.4(g/min) 0 0 0 0 0 O.sub.2(slm) 0.5 11.5 0 20 0
H.sub.2(slm) 0 0 26 0 45 Ex. 3 SiF.sub.4(slm) 2.5 0 0 0 0 0.45
8.00E+17 830 1500 .gtoreq.99.9 None SiCl.sub.4(g/min) 1 0 0 0 0
O.sub.2(slm) 0.5 11.5 0 20 0 H.sub.2(slm) 0 0 26 0 45 Ex. 4
SiF.sub.4(slm) 3.5 0 0 0 0 0.45 8.00E+17 830 3500 .gtoreq.99.9 None
SiCl.sub.4(g/min) 0.9 0 0 0 0 O.sub.2(slm) 0.5 11.5 0 20 0
H.sub.2(slm) 0 0 26 0 45 Ex. 5 SF.sub.6(slm) 0.5 2 0 0 0 0.45
1.00E+18 830 1200 .gtoreq.99.9 None SiCl.sub.4(g/min) 5 0 0 0 0
O.sub.2(slm) 0.5 11 0 22 0 H.sub.2(slm) 0 0 25 0 50 Ex. 6
SF.sub.6(slm) 2 8 0 0 0 0.45 6.00E+17 720 5000 .gtoreq.99.9 None
SiCl.sub.4(g/min) 5 0 0 0 0 O.sub.2(slm) 0.5 11 0 22 0 H.sub.2(slm)
0 0 25 0 50 Ex. 7 SF.sub.6(slm) 1.5 0 0 0 0 0.68 5.00E+17 920 800
.gtoreq.99.9 None SiCl.sub.4(g/min) 5 0 0 0 0 O.sub.2(slm) 0.5 14 0
40 0 H.sub.2(slm) 0 0 20 0 60 Ex. 8 SF.sub.6(slm) 0.5 0 0 0 0 0.68
7.00E+17 980 900 .gtoreq.99.9 None SiCl.sub.4(g/min) 3 0 0 0 0
O.sub.2(slm) 0.5 14 0 40 0 H.sub.2(slm) 0 0 20 0 60 Ex. 9
SF.sub.6(slm) 0.5 0 0 0 0 0.29 1.00E+18 500 580 .gtoreq.99.9 None
SiCl.sub.4(g/min) 3 0 0 0 0 C.sub.2(slm) 0.5 7 0 16 0 H.sub.2(slm)
0 0 20 0 60
[0080] A test piece of diameter 60 mm, and a thickness of 10 mm was
cut out from the center of silica glass ingot having a diameter
about 120 mm which was synthesized in a manner described above, and
each of the fluorine, hydroxyl group and hydrogen molecule
concentrations was measured.
[0081] Fluorine concentration and hydrogen concentration were
measured using a Raman scattering spectrometer (Nippon Bunko K.
K.:NR-1800). Fluorine concentration was obtained from the ratio of
the Si--F Raman scattering intensity of 940 cm.sup.-1 and Si--O
Raman scattering intensity of 800 cm.sup.-1. Hydrogen concentration
was obtained from the ratio of Raman scattering intensity of 4135
cm.sup.-1 and Raman scattering intensity of 800 cm.sup.-1. Hydroxyl
group concentration was obtained by measuring absorption of 2.73
.mu.m using an infra-red spectrometer. Moreover, a test was
conducted by irradiation of various excimer lasers (energy density
50-100 mJ/cm.sup.2) to determine whether or not the silica glass
was usable as optical device for ultraviolet lithography. As shown
in Table 1, fluorine, hydroxyl group and hydrogen molecule co-exist
in high concentration for each example. Measurement of the initial
transmittance revealed that the transmission rate was 99.9% and
above using an arbitrarily selected wavelength of 175 nm and above.
Moreover, irradiation of the sample with 100 mJ/cm.sup.2p, 100 Hz,
1.times.10.sup.6 pulse ArF excimer laser, which is a type of
ultraviolet light pulse laser, did not result in decrease in
transmittance.
EXAMPLE 10
[0082] Using a sealed synthetic furnace such as one depicted in
FIG. 3, synthesis was performed under the synthesis conditions
reported in Table 2, below. The target, while rotating, was lowered
in Z-direction at the same speed as the growth speed of the ingot.
Moreover, synthesis was performed while the center of the target
was offset by 10 mm from the extension of the center line of the
burner in X-direction by the XY stage and while the target was
moved continuously in Y-direction with the movement width of .+-.40
mm and a period of 90 seconds, having a pattern shown in FIG. 5.
From the center of the fluorine doped silica glass ingot of 200 mm
diameter, a sample having a diameter of 160 mm was cut out in the
radial direction from the center of the ingot, and the fluorine
concentration distribution was measured using Raman scattering
spectrometer. The ratio of 940 cm.sup.-1 Si--F Raman scattering
intensity and 800 cm.sup.-1 Si--O Raman scattering intensity was
used as a measurement method. Uniformity of the same sample was
measured using Fizeau interferometer. As shown in FIG. 6, the
maximum value and the minimum value of the fluorine concentration
were 1007 ppm and 996 ppm respectively, and the difference between
the maximum value and the minimum value was 11 ppm. An extremely
high uniformity of .DELTA.n(pv)=3.times.10.sup.-6 was obtained.
2 TABLE 2 Circular Ring Ring Ring Ring Oxygen:Hydrogen H.sub.2 OH
Solari- Pipe Pipe 1 Pipe 2 Pipe 3 Pipe 4 Ratio (Molecule/Cm.sup.3)
(ppm) F (ppm) Transmittance zation Ex. 10 SiF.sub.4(slm) 3 0 0 0 0
0.44 8.00E+17 850 1000 .gtoreq.99.9 None SiCl.sub.4(g/min) 5 0 0 0
0 O.sub.2(slm) 0.5 26 0 66 0 H.sub.2(slm) 0 0 60 0 150 Ex. 11
SiF.sub.4(slm) 3 0 0 0 0 0.44 7.00E+17 980 750 .gtoreq.99.9 None
SiCl.sub.4(g/min) 5 0 0 0 0 O.sub.2(slm) 0.5 26 0 66 0 H.sub.2(slm)
0 0 60 0 150 Ex. 12 SiF.sub.4(slm) 2.5 0 0 0 0 0.43 8.00E+17 840
1000 .gtoreq.99.9 None SiCl.sub.4(g/min) 2 0 0 0 0 O.sub.2(slm) 0.5
24 0 78 0 H.sub.2(slm) 0 0 60 0 180 Ex. 13 SiF.sub.4(slm) 2.5 0 0 0
0 0.43 9.00E+17 950 830 .gtoreq.99.9 None SiCl.sub.4(g/min) 2 0 0 0
0 O.sub.2(slm) 0.5 24 0 78 0 H.sub.2(slm) 0 0 60 0 180
EXAMPLE 11
[0083] A silica glass was formed using the same amount of gases as
that used in Example 10. (See Table 2). The target was rotated and
lowered in Z-direction but was not moved in XY plane. From the
center of the fluorine doped silica glass ingot having a 200 mm
diameter, a sample having a diameter of 160 mm was cut out in the
radial direction from the center of the ingot. The fluorine
concentration distribution was measured using Raman scattering
spectrometer (Nippon Bunko K.K.: NR-1800). Uniformity of the sample
was measured using Fizeau interferometer. As shown in FIG. 7, the
difference between the maximum value (1102 ppm) and the minimum
value (750 ppm) was 352 ppm, and the distribution of refractive
index was extremely poor with .zeta.n(pv)=8.5.times.10.sup.-5.
EXAMPLE 12
[0084] Synthesis was performed using a sealed synthesis furnace
such as one described in FIG. 3 and under the conditions listed in
Table 2. The burner was arranged to have a 15.degree. inclination.
Synthesis was performed while moving the target continuously in
Z-direction with a period of 90 seconds and with the width of +20
mm from the reference position which is the intersection of
extension of the central line of the burner and the center of the
synthesis surface. The reference position was lowered at the same
speed as the growth speed of the ingot. The target was rotated, but
was fixed on the reference position in both the X-direction and in
the Y-direction. From the center of the fluorine doped silica glass
ingot having a diameter of 200 mm, a sample having a diameter of
160 mm was cut out in the radial direction from the center of the
ingot, and the fluorine concentration distribution was measured
using Raman scattering spectrometer. Uniformity of the same sample
was measured using a Fizeau interferometer. As shown in FIG. 8, the
maximum value and the minimum value of the fluorine concentration
were 1003 ppm and 997 ppm respectively, and the difference between
the maximum value and the minimum value was 6 ppm. An extremely
high uniformity of .zeta.n(pv)=1.7.times.10.sup.-6 was
obtained.
EXAMPLE 13
[0085] A silica glass was formed using the same amount of gas as
that used in Example 12. (See Table 2). The target was rotated and
lowered in the Z-direction but was not moved in the XY plane. From
the center of the fluorine doped silica glass ingot having a
diameter of 200 m, a sample having a diameter of 160 mm was cut out
in radial direction from the center of the ingot. The fluorine
concentration distribution was measured using a Raman scattering
spectrometer (Nippon Bunko K. K.: NR-1800). Uniformity of the same
sample was measured using a Fizeau interferometer. As shown in FIG.
9, the difference between the maximum value (1112 ppm) and the
minimum value (825 ppm) was 287 ppm, and the distribution of
refractive index was extremely poor with
.zeta.n(pv)=9.5.times.10.sup.-5.
EXAMPLE 14
[0086] Lenses were produced using the silica glass of Examples 10
and 12. The lens was used for a projection system and an
illumination system of the semiconductor exposure apparatus with a
light source of ArF laser described in FIG. 1. The lens was found
to satisfy the desired design characteristics. The lens had a
resolution capability of about 0.18 .mu.m line width. An integrated
circuit pattern having sufficient flatness was obtained using the
exposure apparatus comprising the lens. Moreover, both lenses of
the invention which were used in the illumination optical system
and the projection optical system were found to have about a 2.5
fold longevity compared to the conventional lenses.
[0087] The invention makes it possible to have hydrogen molecules,
OH and F co-exist in a silica glass. The fluorine doped silica
glass was highly resistant to degradation caused by ultraviolet
light while allowing a high level of ultraviolet light transmission
and a high degree of uniformity. Hence, a highly functional
photolithography apparatus with long longevity which uses vacuum
ultraviolet light can be obtained by the method of the
invention.
EXAMPLES 15-16
Comparative Examples 1-2
[0088] A silica ingot was synthesized by burning mixture of oxygen
gas and hydrogen gas, in a burner of silica glass, in the presence
of a high purity (99.9% or higher purity level, 10 ppb or less Fe
concentration, 2 ppb or less Ni and Cr concentration for impure
metal contents) silicon tetrafluoride gas and carrier gas (oxygen
gas: flow amount 1.8 slm). The silicon tetrafluoride and the
carrier gas were expelled from the central pipe of the burner. By
combusting the oxygen and hydrogen gas, hydrolysis of the
silicon-containing gas occurred, generating silica glass particles
(soot). By rotating the soot with a speed of 7 revolutions per
minute, and by depositing and melting the soot on a silica glass
target plate having a diameter 200 mm which was being rocked with a
period of 90 seconds and was lowered with a speed of 3.93 mm per
hour for distance of 80 mm. The total hydrogen gas flow amount at
this time was about 500 slm an&the ratio of total oxygen gas
flow amount and the hydrogen gas flow amount was set at
O.sub.2/H.sub.2=0.4. The minimum distance from the refractory body
to the deposition point in the synthesis furnace was set at 300
mm.
[0089] The deposition point was a position where the soot being
expelled from the burner reaches the ingot surface. The refractory
body was arranged such that the refractory body becomes an inner
shape having length 600 mm.times.width 800 mm.times.height 800 mm
which surrounds the silica glass ingot in the synthesis furnace.
The refractory body was made of alumina (Al.sub.2O.sub.3) of 99%
purity level. In this method, a silica glass ingot of diameter 300
mm and length 300 mm was obtained. The uniformity of refractive
index of the silica glass ingot obtained was measured using a
Fizeau interferometer whose light source was He--Ne laser beam. The
silica glass ingot has high level of uniformity with the maximum
refractive index difference of 2.times.10.sup.-6 in the region of
diameter 200 mm.
[0090] Four test pieces, each having a shape of 30.times.20
mm.sup.2 and thickness 10 mm, were cut out from the location which
was 100 mm away from the center in the radial direction of the
silica glass ingot thus obtained. These test pieces were used for
measuring the transmittance, for dehydration treatment and for ArF
excimer laser irradiation.
[0091] Test pieces of 10.times.10.times.5 mm.sup.3 for Cl, Na and K
analysis were cut out from the location immediately below the
location where the test piece for transmission rate was cut out.
Measurements of Na and K were performed by radiation analysis using
heat neutron ray irradiation. Samples for chemical element analysis
for alkaline earth metals, transition metals and Al, and samples
for F content measurement were cut out from the location which was
adjacent to the location where test pieces for Cl, Na and K
analysis were cut out. Measurement for all elements except for F
was performed with an inductive coupling type plasma light emitting
spectrum method. The measurement of F content was performed by ion
chromatograph analysis after melting the sample with sodium
carbonate to obtain a constant amount. Finally, the hydroxyl group
concentration was determined by infrared absorption spectrum method
which measures the absorption amount of 1.38 .mu.m.
[0092] The concentration of each element, that is, of Mg and Ca in
alkaline earth metals, and Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn
and Al of transition metals of the test piece in Example 15 was
found to be 20 ppb or less. Moreover, Cl concentration was found to
be less than the lower limit of detection (0.2 ppm), Na
concentration was less than the lower limit of detection (1 ppb),
and K concentration was less than the lower limit of detection (50
ppb). Moreover, F content was found to be 900 ppm, and OH group
concentration was 950 ppm.
[0093] Next a process to remove hydrogen gas from the test pieces
was performed by heating the test pieces to 700.degree. C. for
sixty hours (vacuum annealing) followed by cooling the four test
pieces to room temperature in waterless (not containing OH group)
silica glass tubal heat treatment furnaces having an inner diameter
of 110 mm and the length 1000 mm. The hydrogen gas was removed in
order to define the silica glasses durability, i.e, resistance to
degradation, when exposed to light produced by a laser. Measurement
of the hydrogen molecule concentration in the annealed silica glass
was performed with a laser Raman spectrometer. The amount of
hydrogen remaining in each of the test piece was found to be less
than the detection limit (1.times.10.sup.16 molecules/cm.sup.3).
Moreover, the 606 cm.sup.-1 Raman line intensity was not changed as
a result of the treatment to remove hydrogen gas. It was believed
that the silica glass structure did not change as a result of the
above treatment.
[0094] Next, a process to change the structure determination
temperature for each of four test pieces obtained thus far was
conducted. Each test piece was placed in the center of the
waterless silica glass tubal furnaces which had an inner diameter
of 40 mm and length of 300 mm. The test pieces were kept at
respective temperatures of 1073, 1183, 1273 and 1373 K. Total
durations of time during which the test pieces were kept at
respective temperature were 240, 100, 24 and 20 hours respectively.
The test pieces were then introduced to a Dewar containing liquid
nitrogen for 0.2 second or less. The test pieces were labeled
Examples 15-16 and Comparative Examples 1-2. Measurements on these
test pieces for 606 cm.sup.-1 Raman line intensity revealed that
the intensity was proportional to the temperature at which the test
pieces were kept. Hence, the structure determination temperatures
of the test pieces were determined to be 1073, 1183, 1273 and 1373K
respectively.
[0095] Next, each test piece was ground precisely so that the
degree of parallelization between two facing planes was 10 seconds
or less, flatness of each plane was 3 or less newton rings,
coarseness of each surface, rins was 10 angstrom or less with a
final thickness of 10.+-.0.1 mm. Finally, the test piece was
polished with highly pure Si.sub.2 powder to remove any grinding
agent from the surface.
[0096] The inner transmittance of the test pieces was evaluated by
a spectrometer which was adjusted using the method disclosed in
Japanese Laid-Open Patent Publication 5-211217, which is herein
incorporated by reference in its entirety. The test pieces
exhibited an inner absortion coefficient at wavelength 193 nm of
0.01 cm.sup.-1 or less for each of the test pieces of the Examples
15-16 and Comparative Examples 1-2 which was excellent value of
99.9% or better per 1 cm when converted in terms of inner
transmittance. Here, the absorption coefficient was computed using
the following equation:
Absorption coefficient=-1 n (transmittance/theoretical
transmittance)/(test piece thickness)
[0097] where, the theoretical transmittance refers to a
transmittance which was determined only by reflection loss of the
sample surface having zero inner absorption loss.
[0098] The test pieces were subjected to an irradiation test with
ArF excimer laser beam of one pulse energy density: 200 mJ/cm.sup.2
and repetition: 100 Hz. FIG. 11 describes 193 nm transmittance
change as a result of irradiation for Example 15 and Comparative
Example 1 test pieces. The transmittance drops right after the
start of irradiation as described in FIG. 11, but it saturates at
certain pulse number, beyond which the transmittance does not drop.
Absorption coefficient at the minimum value of the transmittance
caused by drop in irradiation was computed from the aforementioned
equation and was graphed against the structure determination
temperature, which is shown in FIG. 10. It was verified that the
193 nm absorption amount of Examples 15-16 were smaller than that
of Comparative Examples 1-2 and that the resistance to ultraviolet
light degradation was superior. The result is compiled in Table
3.
3 TABLE 3 Ts C1 OH F H.sub.2 Absorption (k) (ppm) (ppm) (ppm)
(cm.sup.-3) Coefficient (cm.sup.-1) Ex. 15 1073 <0.02 950 900
<1E+16 0.11 Ex. 16 1183 <0.2 950 900 <1E+16 0.11 Comp. Ex.
1 1273 <0.2 950 900 <1E+16 0.16 Comp. Ex. 2 1373 <0.2 950
900 <1E+16 0.21 Ex. 17 1073 <0.2 650 <50 <1E+16 0.14
Ex. 18 1164 <0.2 650 <50 <1E+16 0.14 Comp. Ex. 3 1273
<0.2 650 <50 <1E+16 0.19 Comp. Ex. 4 1373 <0.2 650
<50 <1E+16 0.25 Comp. Ex. 5 1073 50 950 <50 <1E+16 0.29
Comp. Ex. 6 1183 50 950 <50 <1E+16 0.30 Comp. Ex. 7 1273 50
950 <50 <1E+16 0.34 Comp. Ex. 8 1373 50 950 <50 <1E+16
0.36
EXAMPLES 17-18
Comparative Examples 3-4
[0099] The same conditions as Example 18 were used except for use
of methyltrimethoxysilane (Si(CH.sub.3)(OCH.sub.3).sub.3) as the
silicon-containing gas (flow amount 20 g/min, carrier gas He 5
slm). Through this method, a silica glass ingot of diameter 150 mm
and length 300 mm was obtained. Test pieces were cut out and heat
treated in the same manner as Example 15. The structure
determination temperatures of test pieces are shown in Table 3. The
ArF excimer laser irradiation test of Example 15 was conducted for
the silica glasses of Examples 17-18 with the results summarized in
Table 3 and FIG. 10. As described in FIG. 10, the 193 nm absorption
amount of Examples 17-18 was smaller than Comparative Examples 5-6,
confirming the superiority of the silica glasses ability to resist
ArF excimer laser ultraviolet light degradation. The results are
summarized in Table 3.
Comparative Examples 5-8
[0100] The same synthesis conditions as Example 15 was repeated
except for use of silicon tetrachloride as the silicon-containing
gas (flow amount 30 g/min, carrier gas oxygen 1.8 slm). Through
this method, a silica glass ingot of diameter 300 mm and length 300
mm was obtained. Test pieces were cut out and heat treated in the
same manner as Example 15. The structure determination temperatures
of test pieces thus obtained are shown in Table 3. The ArF excimer
laser irradiation test was conducted as set forth in Example 15.
The results are summarized in Table 3 and FIG. 11. As described in
FIG. 10, the absorption amounts due to irradiation of Comparative
Examples 5-8 were substantial. It was determined that the
absorption amount may be reduced by lowering the structure
determination temperature, but the effect was small when compared
to Example 15.
EXAMPLE 19
Comparative Example 9
[0101] The test piece of Example 19 was manufactured using the same
method as described in Examples 15-16. The difference from Examples
15-16 was that the use of a rapid cooling treatment was not
performed but rather a heat treatment as described below was
performed on the silica glass ingot. A sample of diameter 250 mm
and thickness 50 mm was cut out from the ingot, The sample was
maintained under at atmospheric pressure with a 1000.degree. C.
temperature for 10 hours. The sample was then cooled gradually to
500.degree. with the temperature decreasing speed of 10.degree.
C./hour, which was then left to be cooled further. A test piece of
diameter 60 mm and thickness 10 mm was cut out from the sample
after heat treatment, for which same optical polishing was
performed as in Example 15 and was made to be Example 19.
[0102] Next, a sample of diameter 60 mm and the thickness 10 mm was
cut out from the silica glass ingot described above, for which a
rapid cooling treatment was performed under the same conditions as
Comparative Example 2 except for maintenance time was one hour.
This test piece was made to be Comparative Example 9.
[0103] The structure determination temperature (Ts), F content, OH
content, Cl content and hydrogen molecule content were measured for
the test pieces of Example 19 and Comparative Example 9. The
results for Example 19 were Ts=1183 K, F=900 ppm, OH=950 ppm,
Cl.ltoreq.0.2 ppm, H.sub.2=2.1.times.10.sup.18 cm.sup.-3.
[0104] The test pieces of Example 19 and Comparative Example 9 were
2.times.10.sup.7 pulse irradiated with an ArF excimer laser having
one pulse energy density: 200 mJ/cm.sup.2/pulse, repetition: 100
Hz. As a result, 193 mn absorption amount were determined to be
0.06 cm.sup.-1 and 0.12 cm.sup.-1, respectively, which verifies the
superiority of Example 19.
[0105] The invention provides an optical device such as a silica
glass optical member, a fiber, a window member, a mirror, an etalon
and a prism which have high throughput for ultraviolet light,
vacuum ultraviolet light of wavelength no more than 250 nm and
laser beam in the same wavelength region. The invention provides an
optical system capable of providing uniform imaging over a wide
area by improving the throughput of ultraviolet light, vacuum
ultraviolet light of wavelength no more than 250 nm and laser beams
in the same wavelength region. Moreover, the invention enables
photolithographic manufacturing having a high degree of precision
when using a light source of wavelength no more than 250 nm.
EXAMPLE 20
[0106] Silicon tetrachloride was subjected to hydrolysis and
simultaneous vitrification in an oxygen-hydrogen flame under the
conditions shown below using a burner with a quintuple tube
structure.
4 Silicon tetrachloride 15 g/min + sulfur hexafluoride carrier 1
slm Second tube oxygen 5 slm Third tube hydrogen 10 slm Fourth tube
oxygen 15 slm Fifth tube hydrogen 40 slm
[0107] Hydrolysis for 20 hours under these conditions produced a
fluorine-doped silica glass with a diameter of 90 mm. When the
transmittance of this sample at 193 nm was measured, a value
exceeding 99.9% was obtained. When the sulfur and fluorine
concentrations in this sample were determined by means of an ion
chromatograph, the sulfur concentration was 10 ppm, and the
fluorine concentration was 1500 ppm (weight ratio: 150).
Comparative Example 10
[0108] Silicon tetrachloride was subjected to hydrolysis and
simultaneous vitrification in an oxygen-hydrogen flame under the
conditions shown below using a burner with a quintuple tube
structure.
5 Silicon tetrachloride 15 g/min + sulfur hexafluoride carrier 1.5
slm Second tube oxygen 5 slm Third tube hydrogen 10 slm Fourth tube
oxygen 15 slm Fifth tube hydrogen 40 slm
[0109] Hydrolysis for 20 hours under these conditions produced a
fluorine-doped quartz glass with a diameter of 90 mm. When the
transmittance of this sample at 193 nm was measured, a value of
97.2% was obtained. When the sulfur and fluorine concentrations in
this sample were determined by means of an ion chromatograph, the
sulfur concentration was 40 ppm, and the fluorine concentration was
1800 ppm (weight ratio: 45).
EXAMPLE 21
[0110] Silicon tetrachloride was subjected to hydrolysis and
simultaneous vitrification in an oxygen-hydrogen flame under the
conditions shown below using a burner with a quintuple tube
structure.
6 Silicon tetrachloride 1.32 slm + sulfur hexafluoride carrier 1
slm Second tube oxygen 5 slm Third tube hydrogen 10 slm Fourth tube
oxygen 15 slm Fifth tube hydrogen 40 slm
[0111] Hydrolysis for 20 hours under these conditions produced a
fluorine-doped silica glass with a diameter of 90 mm. When the
transmittance of this sample at 193 nm was measured, a value
exceeding 99.9% was obtained. When the sulfur and fluorine
concentrations in this sample were determined by means of an ion
chromatograph, the sulfur concentration was 20 ppm, and the
fluorine concentration was 3700 ppm (weight ratio: 185).
Comparative Example 11
[0112] Silicon tetrachloride was subjected to hydrolysis and
simultaneous vitrification in an oxygen-hydrogen flame under the
conditions shown below using a burner with a quintuple tube
structure.
7 Silicon tetrachloride 1.32 slm + sulfur hexafluoride carrier 1.5
slm Second tube oxygen 5 slm Third tube hydrogen 10 slm Fourth tube
oxygen 15 slm Fifth tube hydrogen 40 slm
[0113] Hydrolysis for 20 hours under these conditions produced a
fluorine-doped silica glass with a diameter of 90 mm. When the
transmittance of this sample at 193 nm was measured, a value of
97.0% was obtained. When the sulfur and fluorine concentrations in
this sample were determined by means of an ion chromatograph, the
sulfur concentration was 30 ppm, and the fluorine concentration was
2500 ppm (weight ratio: 83).
* * * * *