U.S. patent application number 09/736279 was filed with the patent office on 2001-04-26 for silica glass having superior durability against excimer laser beams and method for manufacturing the same.
This patent application is currently assigned to NIKON CORPORATION. Invention is credited to Fujiwara, Seishi, Jinbo, Hiroki, Komine, Norio, Yoshida, Akiko.
Application Number | 20010000508 09/736279 |
Document ID | / |
Family ID | 18302424 |
Filed Date | 2001-04-26 |
United States Patent
Application |
20010000508 |
Kind Code |
A1 |
Jinbo, Hiroki ; et
al. |
April 26, 2001 |
Silica glass having superior durability against excimer laser beams
and method for manufacturing the same
Abstract
A silica glass is provided for use in an optical system
processing an excimer laser beam. The silica glass has a molecular
hydrogen concentration of about 5.times.10.sup.18
molecules/cm.sup.3 or less and is substantially free from defects
which become precursors susceptible to an one-photon absorption
process and a two-photon absorption process upon irradiation of the
excimer laser beam to the silica glass.
Inventors: |
Jinbo, Hiroki;
(Yokohama-shi, JP) ; Komine, Norio;
(Sagamihara-shi, JP) ; Fujiwara, Seishi;
(Sagamihara-shi, JP) ; Yoshida, Akiko;
(Kawasaki-shi, JP) |
Correspondence
Address: |
MORGAN, LEWIS & BOCKIUS
1800 M STREET NW
WASHINGTON
DC
200365869
|
Assignee: |
NIKON CORPORATION
|
Family ID: |
18302424 |
Appl. No.: |
09/736279 |
Filed: |
December 15, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09736279 |
Dec 15, 2000 |
|
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09161754 |
Sep 29, 1998 |
|
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6174830 |
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Current U.S.
Class: |
501/54 |
Current CPC
Class: |
C03B 19/1453 20130101;
C03B 32/00 20130101; Y02P 40/57 20151101; C03B 2201/21 20130101;
C03C 3/06 20130101; C03C 2203/40 20130101; C03C 2201/21 20130101;
C03C 2203/52 20130101; C03C 2203/54 20130101; C03B 2207/32
20130101 |
Class at
Publication: |
501/54 |
International
Class: |
C03C 003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 1997 |
JP |
09-336755 |
Claims
What is claimed is:
1. A method for manufacturing a silica glass for use in optical
system for processing an excimer laser beam, the method comprising
the steps of: maintaining a silica glass having a molecular
hydrogen concentration of not more than about 5.times.10.sup.18
molecules/cm.sup.3 at a temperature of about 1000.degree. C. or
more for about 10 hours or more; thereafter cooling the silica
glass to a temperature less than 1000.degree.C. in a controlled
cooling manner; and thereafter leaving the silica glass in an
atmospheric condition to cool the silica glass to a room
temperature.
2. The method according to claim 1, wherein the step of cooling the
silica glass in the controlled cooling manner includes the step of
cooling the silica glass to about 500.degree. C. at a cooling rate
less than 20.degree. C./hr.
3. The method according to claim 2, wherein the cooling rate in the
step of cooling the silica glass to about 500.degree. C. is about
10.degree.C./hr or less.
4. The method according to claim 1, wherein the step of cooling
includes the steps of: cooling the silica glass to a temperature
between about 500.degree. C. and about 700.degree. C. at a cooling
rate of about 10.degree. C./hr or less; and thereafter cooling the
silica glass to about 200.degree. C. or less at a cooling rate of
about 10.degree.C./hr or more.
5. The method according to claim 1, wherein the step of maintaining
includes the step of maintaining the silica glass under at least
one of atmospheric condition and inert gas atmosphere, and wherein
the step of cooling includes the steps of: cooling the silica glass
to about 800.degree. C. or less at a cooling rate of about
10.degree.C./hr or less; changing the at least one of the
atmospheric condition and the inert gas atmosphere to hydrogen gas
atmosphere at a temperature between about 300.degree. C. and about
800.degree. C. to prevent hydrogen from being expelled from the
silica glass and to maintain uniform hydrogen profile in the silica
glass; and cooling the silica glass to about 500.degree. C. or less
in a controlled cooling manner.
6. The method according to claim 1, wherein the excimer laser beam
includes an ArF excimer laser beam.
7. The method according to claim 1, wherein the step of cooling the
silica glass in the controlled cooling manner includes the step of
cooling the silica glass to about 800.degree. C. at a cooling rate
of about 10.degree. C./hr or less.
8. The method according to claim 7, wherein the step of cooling the
silica glass in the controlled manner further includes the step of
further cooling the silica glass cooled to about 800.degree. C. to
about 200.degree.C. at a cooling rate of about 10.degree. C./hr or
more.
9. The method according to claim 7, wherein the step of cooling the
silica glass in the controlled cooling manner further includes the
step of further cooling the silica glass cooled to about
800.degree. C. to about 200.degree.C. at a cooling rate of about
10.degree. C./hr or more.
10. The method according to claim 7, wherein the cooling rate in
the step of cooling the silica glass to about 800.degree. C. is
about 1.degree. C./hr or less.
11. The method according to claim 1, wherein the step of
maintaining includes the step of maintaining the silica glass under
at least one of atmospheric condition and inert gas atmosphere, and
wherein the step of cooling includes the step of changing the at
least one of the atmospheric condition and the inert gas atmosphere
to hydrogen atmosphere at a temperature between about 300.degree.C.
and about 800.degree.C.
12. The method according to claim 1, further including the step of
synthesizing the silica glass having the molecular hydrogen
concentration of more than about 5.times.10.sup.18
molecules/cm.sup.3 before the step of maintaining.
13. The method according to claim 12, wherein the step of
synthesizing includes synthesizing the silica glass by an
oxy-hydrogen flame hydrolysis method using SiCl.sub.4 as a
material.
14. The method according to claim 12, wherein the step of
synthesizing includes synthesizing the silica glass using HMDS
(hexamethyldisiloxane) as a material.
15. The method according to claim 1, wherein the steps of
maintaining, cooling, and leaving are conducted such that the
resulting silica glass has a molecular hydrogen concentration
between about 2.times.10.sup.18 molecules/cm.sup.3 and about
5.times.10.sup.18 molecules/cm.sup.3.
16. The method according to claim 1, wherein the steps of
maintaining, cooling, and leaving are conducted such that the
resulting silica glass has a bulk absorption of about 0.2%/cm or
less.
17. The method according to claim 1, wherein the steps of
maintaining, cooling, and leaving are conducted such that the
resulting silica glass has a molecular hydrogen concentration of
about 5.times.10.sup.18 molecules/cm.sup.3 or less and is
substantially free from defects which become color centers through
a one-photon absorption process upon irradiation of the excimer
laser beam.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
1. This application is a divisional application of commonly
assigned copending application Ser. No. 09/161,754, filed on Sep.
29, 1998 entitled "SILICA GLASS HAVING SUPERIOR DURABILITY AGAINST
EXCIMER LASER BEAMS AND METHOD FOR MANUFACTURING THE SAME," which
is also incorporated herein by reference. This application claims
the benefit of Japanese Application No.09-336755, filed in Japan on
Dec. 8, 1997, which is hereby incorporated by reference. This
application also incorporates by reference Japanese Application No.
09-083152, filed in Japan on Apr. 1, 1997.
BACKGROUND OF THE INVENTION
2. 1. Field of the Invention
3. The present invention relates to silica glass for use in optical
systems, such as lenses and mirrors, for photolithography using the
wavelength ranges of 400 nm or less and of 300 nm or less, and a
method of manufacturing such a silica glass.
4. 2. Discussion of the Related Art
5. In recent years, VLSI chips have been highly integrated and
configured to have numerous functions. In the field of logic VLSI,
a so-called "system on-chip" scheme, in which a larger system is
incorporated on one chip, is becoming more and more popular.
Accordingly, finer pattern manufacture and higher integration are
required on the silicon wafer or like substrate of such a "system
on-chip" scheme. An exposure apparatus, called a stepper, or the
like has been used in the photolithography technique for exposing
and transcribing a fine pattern of integrated circuits onto a wafer
made of silicon or the like.
6. In the case of DRAMs, as technology develops from LSI to VLSI,
the capacity increases from
1M.fwdarw.4M.fwdarw.16M.fwdarw.64M.fwdarw.256M.fw- darw.1 G.
Accordingly, the minimum line width to be produced by
photolithography apparatus should be increased from 1
.mu.m.fwdarw.0.8 .mu.m.fwdarw.0.5 .mu.m.fwdarw.0.35
.mu.m.fwdarw.0.25 .mu.m.fwdarw.0.18 .mu.m.
7. To cope with such a trend, a higher resolution and a deeper
focal depth are required for the projection lens of the stepper.
The resolution and focal depth are determined by the wave-length
.lambda. of exposure light and the numerical aperture (N.A.) of the
lenses.
8. The finer the pattern, the larger the angle of the diffraction
light. Therefore, the diffraction light can not be processed unless
the lens has a large N.A. Also, the shorter the wave-length
.lambda. of exposing light, the smaller the angle of the
diffraction light. Thus, with a shorter wavelength, a relatively
smaller N.A. is acceptable.
9. The resolution and the focal depth are expressed by,
Resolution=k1.multidot..lambda./N.A.
Focal Depth=k2.multidot..lambda./(N.A.).sup.2
10. (where, k2 and k2 are proportional constants)
11. According to these formulae, in order to improve the
resolution, either N.A. needs to be increased, or .lambda. needs to
be shortened. However, as shown in the above formula, shortening
.lambda. is preferable in terms of the focal depth. Therefore, the
wavelength of exposing light has been reduced from the g-line (436
nm) to the i-line (365 nm), and further to excimer laser beams of
KrF (248 nm) and ArF (193 nm).
12. Also, the optical system installed in the stepper is
constructed of a plurality of optical members such as lenses.
Therefore, even if the transmittance loss at each lens is small,
the cumulative effects of all the lenses may lead to decrease in
light amount received at the illumination surface. Thus, very high
transmittance is required for each of the optical member.
13. Therefore, for the wavelength band of 400 nm or less, optical
glass, which is manufactured by a special method taking into
account transmittance loss arising from a combination of optical
members, is used. For the wavelength of 300 nm or less, synthesized
silica glass or single crystal fluoride, such as CaF.sub.2, is
used.
14. As described above, one of the properties of optical members
for a photolithography technique that causes deterioration in the
image contrast is a transmission loss. The transmission loss is
mainly caused by light absorption and light scattering in the
optical member.
15. The light absorption is a phenomenon caused by electron
transition due to photon energy incident on the optical member.
When the light absorption occurs in the optical member, the
absorbed energy is converted to thermal energy. As a result, the
volume of the optical member increases and the refractive index and
the surface condition change accordingly. In this case, the desired
resolution can not be obtained.
16. With regard to silica glass, in particular, the synthesized
silica glass manufactured by the oxy-hydrogen flame hydrolysis
method using SiCl.sub.4 as a material, there is very small amount
of impurity metal. Accordingly, such a glass has superior
transmittance with respect to ultraviolet light.
17. In general, the desired specification for the transmittance of
silica glass used for the optical system of precision instruments,
such as photolithography-use projection lenses and illumination
lenses, is about 0.1%/cm or less in terms of the bulk
absorption.
18. Accordingly, deterioration in the transmittance, which may
occur over a short or long period of time (referred to as
"solarization"), is required to be within about 0.1%/cm or
less.
19. In the silica glass, especially when it is irradiated by an ArF
excimer laser beam, various color centers, such as ".ident.Si."
(the E' center) and ".ident.Si--O." (NBOHC), are generated through
two-photon processes from defect precursors (.ident.Si--Si.ident.,
.ident.Si--O--O--Si.ident.) and SiO.sub.2 primary structure
(.ident.Si--O--Si.ident.). Such color centers cause deterioration
in the transmittance for the wavelength range in use. To deal with
such two-photon absorption, increasing of molecular hydrogen
concentration in the glass has been proposed in order to improve
the durability against laser irradiation of the silica glass.
20. However, even when such a conventional silica glass, which
suppresses two-photon absorption processes, is used for
constructing an exposure apparatus, sufficient focusing properties
and adequately high enough throughput have not been achieved.
SUMMARY OF THE INVENTION
21. Accordingly, the present invention is directed to a silica
glass having superior durability against ultraviolet light and a
method for manufacturing the same that substantially obviate the
problems due to limitations and disadvantages of the related
art.
22. An object of the present invention is to provide a silica glass
having sufficient focusing properties and throughput without the
disadvantages of the conventional art, and a method of
manufacturing the same.
23. Additional features and advantages of the invention will be set
forth in the description that follows, and in part will be apparent
from the description, or may be learned by practice of the
invention. The objectives and other advantages of the invention
will be realized and attained by the structure particularly pointed
out in the written description and claims hereof as well as the
appended drawings.
24. To achieve these and other advantages and in accordance with
the purpose of the present invention, as embodied and broadly
described, the present invention provides a silica glass for use in
an optical system for processing an excimer laser beam, the silica
glass having a molecular hydrogen concentration of about
5.times.10.sup.18 molecules/cm.sup.3 or less and being
substantially free from defects which become precursors susceptible
to a one-photon absorption process and a two-photon absorption
process upon irradiation of the excimer laser beam to the silica
glass.
25. In another aspect, the present invention provides a silica
glass having a molecular hydrogen concentration of about
5.times.10.sup.18 molecules/cm.sup.3 or less to substantially
suppress defects which become color centers through a one-photon
absorption process upon irradiation of an excimer laser beam, the
molecular hydrogen concentration being greater than an amount that
is necessary to substantially suppress defects which become color
centers through a two-photon absorption process upon irradiation of
the excimer laser beam.
26. In a further aspect, the present invention provides a method
for manufacturing a silica glass for use in optical system for
processing an excimer laser beam, the method including the steps of
maintaining a silica glass having a molecular hydrogen
concentration of more than about 5.times.10.sup.18
molecules/cm.sup.3 at a temperature of about 1000.degree. C. or
more for about 10 hours or more; thereafter cooling the silica
glass to a temperature less than 1000.degree.C. in a controlled
cooling manner; and thereafter leaving the silica glass in an
atmospheric condition to cool the silica glass to a room
temperature.
27. It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWING
28. The accompanying drawing, which is included to provide a
further understanding of the invention and is incorporated in and
constitute a part of this specification, illustrates embodiments of
the invention and together with the description serves to explain
the principles of the invention.
29. In the drawing:
30. FIG. 1 shows that transmittance changes due to ArF excimer
laser irradiation in the silica glass samples of preferred
embodiments of the present invention and a comparative example.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
31. The inventors of the present invention conducted diligent
research on the properties of silica glass under irradiation of ArF
excimer laser beams. The following are examples of findings of the
research.
32. (1) Silica glass manufactured under hydrogen rich conditions,
i.e., silica glass having a molecular hydrogen concentration of
more than about 5.times.10.sup.18 (molecules/cm.sup.3), is likely
to generate color centers (absorption) therein through one-photon
processes. A conventional thermal treatment can not suppress
generation of such absorption centers completely.
33. (2) It is possible to reduce or suppress the absorption (color
centers) created by the one-photon processes by conducting
annealing for a much longer time than that sufficient for the
conventional anneal process and producing a molecular hydrogen
concentration of about 5.times.10.sup.18 (molecules/cm.sup.3) or
less.
34. In other words, the present inventors have discovered that the
conventional silica glass manufactured under the hydrogen rich
conditions for the purpose of reducing the two-photon absorption
(color center formed by two-photon processes) and accordingly
having a higher molecular hydrogen concentration is likely to
generate E'-centers (.ident.Si.) therein, which are considered to
be generated by unstable .ident.Si--H structures through one-photon
processes. This means that such silica glass, as manufactured under
the hydrogen rich conditions in order to reduce defects formed
through two-photon absorption processes, instead has defect
precursors which become new color centers through one-photon
absorption; i.e., the color centers can be generated through weaker
energy optical-excitation.
35. In view of the foregoing, the present invention provide a
silica glass member having a molecular hydrogen concentration of
about 5.times.10.sup.18 (molecules/cm.sup.3) or less and being
essentially free from defects which become precursors susceptible
to one-photon and two-photon absorption processes under irradiation
with excimer laser beams.
36. The present invention also provides a method for manufacturing
such a silica glass member, including the steps of maintaining a
silica glass having molecular hydrogen concentration of
5.times.10.sup.18 (molecules/cm.sup.3) or more at 1000.degree. C.
or more for 10 hours or more, cooling the silica glass to
500.degree. C. at a cooling rate of 10.degree.C./hr or less, and
leaving the silica glass in the atmospheric condition to cool it
down.
37. First, classification between the "one-photon absorption
process" and the "two-photon absorption process," which occur when
silica glass is irradiated with excimer laser beams, are explained.
The classification is determined by the energy density dependency;
if the amount of the absorption is proportional to the energy
density of the incident laser beam, the process is a one-photon
process; if it is proportional to the square of the energy density,
then it is a two-photon process.
38. The one-photon absorption reaches its maximum effect when the
silica glass is irradiated with about 10.sup.5 of high energy
pulses each having an energy density of 50-400 mJ/cm.sup.2/pulse,
and gradually photo-bleaches (the absorption decreases with the
irradiation). Because of this, the one-photon absorption process
has not been recognized in the past, and the conventional silica
glass manufacture method using the hydrogen rich conditions has
been considered to be to sufficient for improving the durability
against excimer laser beams.
39. It may be considered that this absorption process is not
problem because they disappears by the photo-bleach effect under
high energy irradiation of 50-400 mJ/cm.sup.2/pulse. However, this
is not the case for photolithography apparatus. In actual
photolithography apparatus, the energy density of the ArF excimer
laser irradiating each projection liens is about 1
mJ/cm.sup.2/pulse or less at most parts of the lens. Accordingly,
if silica glass having a large number of the one-photon absorption
precursors is used in the apparatus, the absorption occurs (color
centers are generated) immediately after the initial operation of
the apparatus, which leads to considerable degradation in the image
focusing properties and the throughput.
40. According to the present invention, it becomes possible to
provide a silica glass having a superior and stable transmittance
at 193.4 nm for short time as well as for long time application, by
reducing defect precursors which would become color centers, such
as E'-centers, through one-photon processes upon initial
irradiation of excimer laser.
41. In the present invention, manufacture conditions and heat
treatment conditions for silica glass are optimized so as to reduce
seeds of structural defects which may be generated through
one-photon transition processes in the silica glass. The thermal
treatment preferably is conducted for a long time but not so long
as to affect the yield. For example, a silica glass is maintained
at 1000.degree. C. or more for 10 hours or more, is cooled to
500.degree. C. at a cooling rate of 10.degree.C./hr or less, and is
left in an atmospheric condition to further cool it down. Since the
temperature of the last step may affect generation of defects, an
additional cooling step from 500.degree.C. to 200.degree.C. may
preferably be provided before the last step.
42. Reference will now be made in detail to the preferred
embodiments of the present invention, examples of which are
illustrated in the accompanying drawing.
FIRST WORKING EXAMPLE (FIRST PREFERRED EMBODIMENT)
43. A silica glass was manufactured by an oxy-hydrogen flame
hydrolysis method using SiCl.sub.4 as a material with the ratio of
oxygen gas to hydrogen gas being 0.44. The dimensions of the
resulting silica glass ingot were .phi.300.times.t800 mm. A member
having the dimensions of .phi.200.times.t50 was cut out from the
ingot and maintained at 1000.degree.C. for 10 hours, was cooled to
800.degree. at a cooling rate of 1.degree.C./hr, was cooled to
200.degree. C. at a cooling rate of 10.degree.C./hr, and was left
in the atmospheric condition to further cool it down to the room
temperature.
44. The reason why the cooling rate was changed at 800.degree. C.
is that the causes of one-photon absorption can be effectively
reduced at around 800.degree.C. to 1000.degree.C. without
deteriorating other properties of the silica glass.
45. After the treatments, evaluation samples having the dimensions
of .phi.60.times.t10 mm were cut out from the member, and were
subjected to fine grinding and fine cleaning. The resulting samples
were irradiated with ArF excimer laser beams and the absorption
properties were measured.
46. Before the ArF excimer laser irradiation, the transmittance
including reflection losses at 193.4 nm was about 90.7%/cm. The
theoretical transmittance, which is a transmittance including only
reflection losses without absorption and scattering losses, was
90.87%/cm and the bulk scattering was about 0.15%/cm. Accordingly,
the bulk absorption was estimated to be about 0.02%/cm or less.
47. The molecular hydrogen concentration in the silica glass was
measured by the laser Raman spectroscopy method, which yielded
about 2.times.10.sup.18 molecules/cm.sup.3. In general, hydrogen
molecules are expelled from a thin surface portion of the samples
(a region with the thickness of about 10 mm from the surface).
Therefore, silica glass products to be annealed need to be thicker
than the target products by at least 10 mm or more in their surface
region. Also, it is possible to reduce such hydrogen gas expelling
phenomena by annealing it under a hydrogen atmosphere. However,
anneal at 800.degree. C. or more under a hydrogen atmosphere may
generate reduction seeds on the surface or promote diffusion of
impurities, such as Na. Thus, it is preferable to anneal the
products under an inert gas atmosphere at the temperature of about
800.degree. C. or more and to change to a hydrogen atmosphere at
the temperature of about 800.degree. or less. However, since
hydrogen stops diffusing into the glass due to its low diffusion
coefficient under 300.degree. C., the temperature of the hydrogen
atmosphere thermal treatment needs to be more than 300.degree.
C.
48. The above samples were irradiated with an ArF excimer laser
beam and the absorption properties were studied. The irradiation
conditions were: the energy density, 1 mJ/cm.sup.2/pulse; the
frequency, 100 Hz; and the number of pulses, up to about
5.times.10.sup.6 pulses. FIG. 1 shows measured transmittance
changes with ArF excimer laser irradiation. As shown in FIG. 1, the
absorption induced by the one-photon process due to the irradiation
was 0.05% or less, which is a sufficiently small value. Such a
silica glass is suitable for photolithography-use optical members
because the total transmittance loss of an optical system having
multiple optical members can be made sufficiently small and the
desired resolution and the throughput can be obtained. In general,
the bulk absorption of each optical member for projection and
illumination optical systems is preferably about 0.2% or less.
49. Similar results were obtained for synthesized silica glass
manufactured using HMDS (hexamethyldisiloxane) as a material. In
this case, since organic silica compound like HMDS does not include
Cl, the synthesized silica glass is essentially free from the
.ident.Si--Cl structure. Therefore, an absorption band at 215 nm
due to SiE' generated by two-photon absorption processes can be
suppressed and such a silica glass is expected to have a superior
long time durability.
SECOND WORKING EXAMPLE (SECOND PREFERRED EMBODIMENT)
50. A silica glass was manufactured by an oxy-hydrogen flame
hydrolysis method using SiCl.sub.4 as a material with the ratio of
oxygen gas to hydrogen gas being 0.6. The dimensions of the
resulting silica glass ingot were .phi.300.times.t800 mm. A member
having the dimensions of .phi.200.times.t50 was cut out from the
ingot and maintained at 1000.degree. C. for 10 hours, was cooled to
800.degree. at a cooling rate of 1.degree. C./hr, was cooled to
500.degree. C. at a cooling rate of 10.degree.C./hr, and was left
in the atmospheric condition to further cool it down to the room
temperature.
51. The reason why the cooling rate was changed at 800.degree. C.
is that the causes of one-photon absorption can be effectively
reduced at around 800.degree. C. to 1000.degree. C. without
deteriorating other properties of the silica glass.
52. After the treatments, evaluation samples having the dimensions
of .phi.60.times.t10 mm were cut out from the member, and were
subjected to fine grinding and fine cleaning. The resulting samples
were irradiated with ArF excimer laser beams and the absorption
properties were measured.
53. Before the ArF excimer laser irradiation, the transmittance
including reflection losses at 193.4 nm was about 90.7%/cm. The
theoretical transmittance, which is a transmittance including only
reflection losses without absorption and scattering losses, was
90.87%/cm and the bulk scattering was about 0.15%/cm. Accordingly,
the bulk absorption was estimated to be about 0.02%/cm or less.
54. The above samples were irradiated with an ArF excimer laser
beam and the absorption properties were studied. The irradiation
conditions were: the energy density, 1 mJ/cm.sup.2/pulse; the
frequency, 100 Hz; and the number of pulses, up to about
5.times.10.sup.6 pulses. FIG. 1 shows measured transmittance
changes with ArF excimer laser irradiation. As shown in FIG. 1, the
absorption induced by the one-photon process due to the irradiation
was 0.1% or less, which is a sufficiently small value. Such a
silica glass is suitable for photolithography-use optical members
because the total transmittance loss of an optical system having
multiple optical members can be made sufficiently small and the
desired resolution and the throughput can be obtained. In general,
the bulk absorption of each optical member for projection and
illumination optical systems is preferably about 0.2% or less.
THIRD WORKING EXAMPLE (THIRD PREFERRED EMBODIMENT)
55. A silica glass was manufactured by an oxy-hydrogen flame
hydrolysis method using SiCl.sub.4 as a material with the ratio of
oxygen gas to hydrogen gas being 0.44. The dimensions of the
resulting silica glass ingot were .phi.300.times.t800 mm. A member
having the dimensions of .phi.200.times.t50 was cut out from the
ingot and maintained at 1000.degree. C. for 10 hours, was cooled to
500.degree. at a cooling rate of 10.degree. C./hr, and was left in
the atmospheric condition to further cool it down to the room
temperature.
56. After the treatments, evaluation samples having the dimensions
of .phi.60.times.t10 mm were cut out from the member, and were
subjected to fine grinding and fine cleaning. The resulting samples
were irradiated with ArF excimer laser beams and the absorption
properties were measured.
57. Before the ArF excimer laser irradiation, the transmittance
including reflection losses at 193.4 nm was about 90.7%/cm. The
theoretical transmittance, which is a transmittance including only
reflection losses without absorption and scattering losses, was
90.87%/cm and the bulk scattering was about 0.15%/cm. Accordingly,
the bulk absorption was estimated to be about 0.02%/cm or less.
58. The above samples were irradiated with an ArF excimer laser
beam and the absorption properties were studied. The irradiation
conditions were: the energy density, 1 mJ/cm.sup.2/pulse; the
frequency, 100 Hz; and the number of pulses, up to about
5.times.10.sup.6 pulses. FIG. 1 shows measured transmittance
changes with ArF excimer laser irradiation. As shown in FIG. 1, the
absorption induced by the one-photon process due to the irradiation
was 0.1% or less, which is a sufficiently small value. Such a
silica glass is suitable for photolithography-use optical members
because the total transmittance loss of an optical system having
multiple optical members can be made sufficiently small and the
desired resolution and the throughput can be obtained. In general,
the bulk absorption of each optical member for projection and
illumination optical systems is preferably about 0.2% or less.
COMPARATIVE EXAMPLE
59. A silica glass was manufactured by an oxy-hydrogen flame
hydrolysis method using SiCl.sub.4 as a material with the ratio of
oxygen gas to hydrogen gas being 0.8. The dimensions of the
resulting silica glass ingot were .phi.300.times.t800 mm. A member
having the dimensions of .phi.200.times.t50 was cut out from the
ingot and maintained at 1000.degree.C. for 10 hours, was cooled to
500.degree. at a cooling rate of 20.degree. C./hr, and was left in
the atmospheric condition to further cool it down to the room
temperature.
60. After the treatments, evaluation samples having the dimensions
of .phi.60.times.t10 mm were cut out from the member, and were
subjected to fine grinding and fine cleaning. The resulting samples
were irradiated with ArF excimer laser beams and the absorption
properties were measured.
61. Before the ArF excimer laser irradiation, the transmittance
including reflection losses at 193.4 nm was about 90.7%/cm. The
theoretical transmittance, which is a transmittance including only
reflection losses without absorption and scattering losses, was
90.87%/cm and the bulk scattering was about 0.15%/cm. Accordingly,
the bulk absorption was estimated to be about 0.02%/cm or less.
62. The above samples were irradiated with an ArF excimer laser
beam and the absorption properties were studied. The irradiation
conditions were: the energy density, 1 mJ/cm.sup.2/pulse; the
frequency, 100 Hz; and the number of pulses, up to about
5.times.10.sup.6 pulses. FIG. 1 shows measured transmittance
changes with ArF excimer laser irradiation. As shown in FIG. 1, the
absorption induced by the one-photon process due to the irradiation
was 1% or less, which is too large. Such a silica glass is not
suitable for photolithography-use optical members because the total
transmittance loss of an optical system having multiple optical
members cannot be made sufficiently small and the desired
resolution and the throughput cannot be obtained. In general, the
bulk absorption of each optical member for projection and
illumination optical systems is preferably about 0.2% or less.
63. According to the present invention, it becomes possible to
provide lens members which are free from one-photon absorption
effects caused by ArF excimer laser irradiation and have a
sufficient and stable transmittance at 193.4 nm.
64. It will be apparent to those skilled in the art that various
modifications and variations can be made in a silica glass having
superior durability against ultraviolet light and a method for
manufacturing the same of the present invention without departing
from the spirit or scope of the invention. Thus, it is intended
that the present invention cover the modifications and variations
of this invention provided they come within the scope of the
appended claims and their equivalents.
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