U.S. patent application number 12/370693 was filed with the patent office on 2009-06-11 for deuteroxyl-doped silica glass, optical member and lithographic system comprising same and method of making same.
Invention is credited to Dana Craig Bookbinder, Richard Michael Fiacco, Ulrich Wilhelm Heinz Neukirch.
Application Number | 20090148627 12/370693 |
Document ID | / |
Family ID | 37745897 |
Filed Date | 2009-06-11 |
United States Patent
Application |
20090148627 |
Kind Code |
A1 |
Bookbinder; Dana Craig ; et
al. |
June 11, 2009 |
DEUTEROXYL-DOPED SILICA GLASS, OPTICAL MEMBER AND LITHOGRAPHIC
SYSTEM COMPRISING SAME AND METHOD OF MAKING SAME
Abstract
What is disclosed includes OD-doped synthetic silica glass
capable of being used in optical elements for use in lithography
below about 300 nm. OD-doped synthetic silica glass was found to
have significantly lower polarization-induced birefringence value
than non-OD-doped silica glass with comparable concentration of OH.
Also disclosed are processes for making OD-doped synthetic silica
glasses, optical member comprising such glasses, and lithographic
systems comprising such optical member. The glass is particularly
suitable for immersion lithographic systems due to the
exceptionally low polarization-induced birefringence values at
about 193 nm.
Inventors: |
Bookbinder; Dana Craig;
(Corning, NY) ; Fiacco; Richard Michael; (Corning,
NY) ; Neukirch; Ulrich Wilhelm Heinz; (Painted Post,
NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
37745897 |
Appl. No.: |
12/370693 |
Filed: |
February 13, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11583619 |
Oct 19, 2006 |
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12370693 |
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11348956 |
Feb 6, 2006 |
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11583619 |
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60734527 |
Nov 7, 2005 |
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Current U.S.
Class: |
427/578 ;
427/162; 501/53 |
Current CPC
Class: |
C03B 2201/22 20130101;
C03B 2201/08 20130101; C03B 2201/23 20130101; C03C 2201/22
20130101; C03C 4/0085 20130101; C03B 32/00 20130101; C03B 2201/07
20130101; C03B 2201/12 20130101; C03B 19/066 20130101; C03B 19/14
20130101; C03C 2201/11 20130101; C03C 2201/12 20130101; C03B
19/1453 20130101; C03B 19/12 20130101; C03C 3/06 20130101; C03B
20/00 20130101; C03C 2201/23 20130101; C03B 2201/32 20130101; C03B
2201/075 20130101; C03B 2201/42 20130101; C03B 19/06 20130101 |
Class at
Publication: |
427/578 ;
427/162; 501/53 |
International
Class: |
B05D 1/00 20060101
B05D001/00 |
Claims
1-44. (canceled)
45. A process for making OD-doped synthetic silica glass material
capable of being used in the light path of the lithographic
irradiation of a lithographic device operating at a wavelength
below about 300 nm, comprising the following steps: (I) providing a
plurality of particles comprising silica; (II) depositing the a
plurality of particles on a supportive deposition surface at an
elevated temperature such that the particles are consolidated into
transparent glass material in situ, wherein: either in step (I),
the a plurality of particles provided are D-containing and/or in
step (II), the deposition and consolidation are carried out in a
D-containing atmosphere, such that the obtained silica glass
comprises OD and optionally OH, and the ratio of
n(OD)/(n(OD)+n(OH)) is higher than about 2.times.10.sup.-4.
46. A process according to claim 45, wherein the obtained silica
glass comprise sodium less than about 50 ppb by weight.
47. A process according to claim 45, wherein in step (I), the
particles are generated by flame hydrolysis of at least one
Si-containing precursor compound.
48. A process according to claim 47, wherein in step (I), the
Si-containing precursor compound is selected from organosilicon
compounds and silicon halides.
49. A process according to claim 45, wherein in step (II), the
deposition is initiated on an essentially planar top surface of a
horizontally rotating table.
50. A process according to claim 45, wherein in step (II), the
deposition and consolidation are carried out in the presence of
D.sub.2O.
51. A process according to claim 45, wherein in step (II), the
deposition and consolidation are carried out in the presence of
H.sub.2O.
52. A process according to claim 51, wherein the Si-containing
precursor compound comprises D.
53. A process according to claim 47, wherein the flame is generated
by at least one reaction involving a D-containing compound.
54. A process according to claim 45, wherein in step (I), the
particles are provided via a soot dispenser.
55. A process according to claim 45, wherein in step (I), the
particles are provided via a plasma-assisted process.
56. A process according to claim 45, further comprising the
following step: (III) treating the consolidated glass obtained in
step (II) in an atmosphere comprising H.sub.2 and/or HD and/or
D2.
57. A process according to claim 56, wherein in step (III), the
treatment temperature is lower than about 600.degree. C.
58. A process according to claim 56, wherein in step (III), the
treatment temperature is higher than about 600.degree. C.
59. A process according to claim 56, wherein in step (III), the
ratio of (2n(H.sub.2)+n(HD)))/2(n(H.sub.2)+n(D.sub.2)+n(HD)) is
higher than or equal to the natural abundance of H.
60. A process according to claim 56, wherein in step (III), the
ratio of (2n(D.sub.2)+n(HD))/2(n(H.sub.2)+n(D.sub.2)+n(HD)) is
higher than or equal to the natural abundance of D.
61. A process according to claim 56, wherein in step (III), the
treatment time and temperature is chosen such that the sum total of
the concentration of H.sub.2, HD and D.sub.2 in the treated glass
is between about 0.1.times.10.sup.16 to about 5.times.10.sup.19
molecules/cm.sup.3.
62. A process according to claim 56, wherein in step (I), particles
comprising dopants are provided and mixed with the particles
comprising silica.
63. A process according to claim 62, wherein the particles
comprising dopants comprise at least one of Cl, TiO.sub.2, F and
Al.sub.2O.sub.3.
64. A process according to claim 63, wherein the particles
comprising dopants comprise fluorine.
65. A process for making OD-doped synthetic silica glass material
capable of being used in the light path of the lithographic
irradiation of a lithographic device operating at a wavelength
below about 300 nm, comprising the following steps: (A) providing a
particle preform comprising a plurality of particles comprising
silica; (B) optionally purifying and/or drying the particle
preform; (C) optionally further doping the particle preform with
dopants; (D) consolidating the particle preform at an elevated
temperature to dense glass; and (E) optionally treating the
consolidated glass obtained in step (D) in the presence of H.sub.2,
HD and/or D.sub.2, wherein in at least one of steps (A), (B), (C),
(D) and (E), OD is introduced into or formed in the glass such that
obtained silica glass comprises OD and optionally OH, and the ratio
of n(OD)/(n(OD)+n(OH)) is higher than about 2.times.10.sup.-4.
66. A process according to claim 65, wherein the obtained silica
glass comprises less than about 50 ppb by weight of sodium.
67. A process according to claim 66, wherein the soot preform
provided in step (A) comprises sodium lower than about 50 ppb by
weight.
68. A process according to claim 65, wherein: the soot preform
provided in step (A) comprises sodium higher than about 50 ppb by
weight; step (B) is carried out subsequent to step (A); and upon
completion of step (B), the soot preform comprises sodium less than
about 50 ppb by weight.
69. A process according to claim 65, wherein in at least one of
steps (A), (B), (C) and (D), OD is introduced into or formed in the
glass.
70. A process according to claim 65, wherein step (A) comprises the
following steps: (A1) providing a plurality of particles; and (A2)
depositing the particles on a rotating supporting surface to form
the particle preform.
71. A process according to claim 70, wherein in step (A1), the
particles are provided by (A1.1) flame hydrolysis of at least one
silicon-containing precursor compound, which may be
plasma-assisted; or (A1.2) a soot dispenser, which may be plasma
assisted; or (A1.3) other plasma-assisted process.
72. A process according to claim 71, wherein in step (A1), the
particles are provided by (A1.1), and the particles are essentially
not OD-doped.
73. A process according to claim 71, wherein in step (A1), the
particles are provided by (A1.1), and the particles provided are
OD-doped.
74. A process according to claim 73, wherein in step (A1), the
particles are provided by flame hydrolysis in the presence of a
D-containing compound.
75. A process according to claim 74, wherein in step (A1), the
particles are provided by flame hydrolysis in the presence of
D.sub.2O.
76. A process according to claim 70, wherein in step (A2), the
deposition involves a process selected from (A2.1) outside vapor
deposition; (A2.2) inside vapor deposition; (A2.3) vapor axial
deposition; and (A2.4) planar deposition.
77. A process according to claim 65, wherein step (A) comprises the
following steps: (A(i)) forming a sol-gel comprising silica; and
(A(ii)) forming the particle preform from the sol-gel.
78. A process according to claim 77, wherein step (A(i)) is carried
out in the presence of or from a D-containing compound.
79. A process according to claim 78, wherein step (A(i)) is carried
out in the presence of D.sub.2O.
80. A process according to claim 65, wherein step (B) is carried
out and such step is carried out in an atmosphere comprising at
least one purifying/drying agent selected from F.sub.2, Cl.sub.2,
Br.sub.2, a halogen-containing compound, CO, CO.sub.2, and
compatible mixtures thereof.
81. A process according to claim 80, wherein the halogen-containing
compound is selected from HX, COX.sub.2, SOX.sub.2, CX.sub.4 and
SX.sub.6, wherein X is selected from F, Cl, Br and combinations
thereof.
82. A process according to claim 80, wherein step (B) is carried
out in an atmosphere comprising Cl.sub.2, Br.sub.2 or mixtures
thereof, with or without containing CO.
83. A process according to claim 80, wherein immediately after step
(B), the particle preform has an [OH]+[OD] less than about 50 ppm
by weight of the total composition.
84. A process according to claim 65, wherein step (C) is carried
out, and such step is carried out in the presence of an atmosphere
comprising dopant(s).
85. A process according to claim 84, wherein step (C) is carried
out in the presence of a D-containing compound.
86. A process according to claim 84, wherein step (C) is carried
out in the presence of D.sub.2O, D.sub.2 or both.
87. A process according to claim 85, wherein in step (C) exchange
of OD for OH is carried out.
88. A process according to claim 87, wherein immediately after step
(C), the ratio of n(OD)/(n(OD)+n(OH)) in the particle preform is
higher than about 0.02.
89. A process according to claim 65, if step (B) or step (C) is
carried out, at least one of these two steps is carried out in the
presence of a reductive atmosphere.
90. A process according to claim 89, wherein in the reductive
atmosphere in which step (B) or step (C) is carried out comprises a
gas selected from H.sub.2, D.sub.2, HD, hydrocarbons, D-containing
hydrocarbons, and the like.
91. A process according to claim 89, wherein after step (B) or step
(C), if carried out, and whichever is later, an oxidation step
(C(A)) is carried out wherein the particle preform is subjected to
an oxidative atmosphere in which oxygen-deficient sites in the
particle preform can be healed.
92. A process according to claim 91, wherein step (C(A)) is at
least part of step (D).
93. A process according to claim 91, wherein the oxidative
atmosphere in step (C(A)) comprises H.sub.2O, D.sub.2O, O.sub.2
and/or O.sub.3.
94. A process according to claim 65, wherein steps (B) and (C) are
carried out at least partially simultaneously.
95. A process according to claim 65, wherein steps (C) and (D) are
carried out at least partially simultaneously.
96. A process according to claim 65, wherein step (D) is carried
out in an atmosphere comprising He.
97. A process according to claim 65, wherein step (U) is carried
out in an atmosphere comprising O.sub.2.
98. A process according to claim 65, wherein step (D) is carried
out in the presence of H.sub.2O.
99. A process according to claim 65, wherein step (D) is carried
out in the presence of D.sub.2O.
100. A process according to claim 99, wherein step (D) is carried
out in an atmosphere essentially free of H.sub.2O and HDO.
101. A process according to claim 65, wherein step (D) is carried
out in the presence of D.sub.2, HD or both.
102. A process according to claim 65, wherein step (E) is carried
out, and such step (E) is carried out in the presence of
H.sub.2.
103. A process according to claim 102, wherein step (E) is carried
out in an atmosphere essentially devoid of D.sub.2 and HD.
104. A process according to claim 103, wherein step (E) is carried
out at a temperature lower than about 600.degree. C.
105. A process according to claim 101, wherein step (E) is carried
out at a temperature below about 1000.degree. C.
106. A process according to claim 65, wherein step (E) is carried
out, and such step (F) is carried out in the presence of D.sub.2
and/or HD.
107. A process according to claim 106, wherein step (E) is carried
out in an atmosphere essentially devoid of H.sub.2.
108. A process according to claim 107, wherein step (E) is carried
out in an atmosphere essentially devoid of HD and H.sub.2.
109. A process according to claim 106, wherein step (E) is carried
out at a temperature higher than about 600.degree. C.
110. A process according to claim 65, wherein: the dense glass
resulting from step (D) comprises OH; step (F) is carried out; and
in step (E), the glass is treated in an atmosphere comprising
D.sub.2, HD and/or H.sub.2 to effect H/D exchange in the dense
glass to obtain the desired [OH] and [OD] in the glass.
111. A process according to claim 110, wherein: the dense glass
resulting from step (D) is essentially not OD-doped; and in step
(E), the glass is treated in an atmosphere comprising D.sub.2 to
effect H/D exchange in the dense glass to obtain the desired [OH]
and [OD] in the glass.
112. A process according to claim 111, wherein: in step (E), the
glass is treated in an atmosphere comprising D.sub.2 to effect H/D
exchange such that at the end of the step (E), the glass has a
ratio of n(OD)/(n(OD)+n(OH)) of at least 0.5.
113. A process according to claim 112, wherein: at the end of step
(E), the glass has a ratio of n(OD)/(n(OD)+n(OH)) of at least
0.9.
114. A process according to claim 110, wherein step (E) is carried
out at a temperature of at least 600.degree. C.
115. A process according to claim 114, wherein step (E) is carried
out at a temperature of at least 800.degree. C.
116. A process for making OD-doped synthetic silica glass capable
of being used in the light path of the lithographic irradiation of
a lithographic device operating at a wavelength below about 300 nm,
comprising the following steps: (a) providing a plurality of
OD-doped particles comprising silica; and (b) melting the particles
at an elevated temperature to obtain a transparent glass.
117. A process according to claim 116, wherein step (a) comprise
the following steps: (a1) generating a plurality of particles
comprising silica; (a2) optionally purifying and/or drying the
particles; (a3) optionally doping the particles in an atmosphere
comprising at least one D-containing compound, and (a4) optionally
treating the particles in an oxidative atmosphere to at least
partly heal oxygen-deficient sites in the particles.
118. A process according to claim 117, wherein in step (a3), the at
least one D-containing compound comprises D.sub.2O.
119. A process according to claim 116, wherein step (a) involves
flame hydrolysis of a Si-containing precursor compound.
120. A process according to claim 116, wherein step (a) involves a
sol-gel process of a Si-containing compound.
121. A process according to claim 116, wherein in step (b), the
melted glass is also homogenized.
122. A process according to claim 116, further comprising the
following step (c) after step (b): (c) treating the glass in an
atmosphere comprising H.sub.2, D.sub.2 and/or HD.
123-124. (canceled)
125. A process for making OD-doped synthetic silica glass capable
of being used in the light path of the lithographic irradiation of
a lithographic device operating at a wavelength below about 300 nm,
comprising the following steps: (a) providing at least one
consolidated OD-doped silica glass; (b) melting the OD-doped silica
glass and homogenizing it at an elevated temperature to obtain a
glass having an essentially uniformly distributed [OD] and/or [OH]
therein.
126. A process according to claim 125, wherein: in step (a), at
least two OD-doped silica glasses having differing [OD] are
provided; and in step (b), the at least two silica glasses are
mixed and homogenized.
127. A process for making OD-doped synthetic silica glass capable
of being used in the light path of the lithographic irradiation of
a lithographic device operating at a wavelength below about 300 nm,
comprising the following steps: (a) providing a consolidated silica
glass comprising OH; (b) treating the consolidated glass in an
atmosphere comprising D.sub.2, H.sub.2, and/or HD to effect H/D
exchange to the desired [OH] and [OD] in the glass.
128. A process according to claim 127, wherein: the dense glass
provided in step (a) is essentially not OD-doped; and in step (b),
the glass is treated in an atmosphere comprising D.sub.2 to effect
H/D exchange in the dense glass to obtain the desired [OH] and [OD]
in the glass.
129. A process according to claim 128, wherein: in step (b), the
glass is treated in an atmosphere comprising D.sub.2 to effect H/D
exchange such that at the end of the step (b), the glass has a
ratio of n(OD)/(n(OD)+n(OH)) of at least 0.5.
130. A process according to claim 129, wherein: at the end of step
(b), the glass has a ratio of n(OD)/(n(OD)+n(OH)) of at least
0.9.
131. A process according to claim 127, wherein step (b) is carried
out at a temperature of at least 600.degree. C.
132. A process according to claim 131, wherein step (b) is carried
out at a temperature of at least 800.degree. C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present invention is a continuation-in-part application
of U.S. patent application Ser. No. 11/348,956, entitled
"DEUTEROXYL-DOPED SILICA GLASS, OPTICAL MEMBER AND LITHOGRAPHIC
SYSTEM COMPRISING SAME AND METHOD OF MAKING SAME," filed on Feb. 6,
2006, which, in turn, claims the benefit of the earlier filing date
of U.S. Provisional Patent Application Ser. No. 60/734,527,
entitled "DEUTEROXYL-DOPED SILICA GLASS, OPTICAL MEMBER AND
LITHOGRAPHIC SYSTEM COMPRISING SAME AND METHOD OF MAKING SAME,"
filed on Nov. 7, 2005, the content of both of which are relied upon
and hereby incorporated by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to synthetic silica glass
materials, optical elements and devices comprising the same and
method of making the same. In particular, the present invention
relates to synthetic silica glass material capable of being used in
the optical elements in lithographic devices operating at a
wavelength below about 300 nm, optical elements comprising the
same, lithographic systems comprising such optical elements,
process for making such glass material, and soot preform produced
in such process. The present invention is useful, for example, in
making synthetic fused silica glass materials for optical elements
used in deep UV and vacuum UV lithographic devices, especially
those involving immersion lithography in which linearly polarized
UV light is employed.
BACKGROUND OF THE INVENTION
[0003] As practiced commercially, fused silica optical members such
as lenses, prisms, filters, photomasks, reflectors, etalon plates
and windows, have been manufactured from bulk pieces of fused
silica made in large production furnaces. Bulk pieces of fused
silica manufactured in large production furnaces are known in the
art as preforms, boules or ingots. Blanks are cut from boules or
ingots, and finished optical members are manufactured from glass
blanks, utilizing manufacturing steps that may include, but are not
limited to, cutting, polishing, and/or coating pieces of glass from
a blank. Many of these optical members are used in various
apparatus employed in environments where they are exposed to
ultraviolet light having a wavelength of about 360 nm or less, for
example, an excimer laser beam or some other ultraviolet laser
beam. The optical members are incorporated into a variety of
instruments, including lithographic laser exposure equipment for
producing highly integrated circuits, laser generation equipment,
medical equipment, nuclear fusion equipment, or some other
apparatus which uses a high-power ultraviolet laser beam.
[0004] As the photon energy, pulse energy and pulse rate of lasers
increase, the optical members which are used in conjunction with
such lasers are exposed to increased levels of energy. Fused silica
has become widely used as the material of choice for optical
members in such laser-based optical systems due to their excellent
optical properties and resistance to light-induced damage.
[0005] Laser technology has advanced into the short wavelength,
high energy ultraviolet spectral region, the effect of which is an
increase in the frequency (decrease in wavelength) of light
produced by lasers. Of particular interest are short wavelength
lasers operating in the UV and deep UV (DUV) and vacuum UV
wavelength ranges, which include, but are not limited to, lasers
operating at about 248 nm, 193 nm, 157 nm and even shorter
wavelengths. Excimer laser systems are popular in microlithography
applications, and the shortened wavelengths allow for increased
feature resolution and thus line densities in the manufacturing of
integrated circuits and microchips, which enables the manufacture
of circuits having decreased feature sizes. A direct physical
consequence of shorter wavelengths (higher frequencies) is higher
photon energies. In such optical systems, fused silica optics are
exposed to high irradiation levels for prolonged periods of time,
and this may result in the degradation of the optical properties of
the optical members.
[0006] It is known that such light-induced degradation adversely
affects the optical properties and performance of the fused silica
optics by decreasing light transmission levels, discoloring the
glass, altering the index of refraction, altering the density, and
increasing absorption levels of the glass. Over the years, many
methods have been suggested for improving the optical damage
resistance of fused silica glass. It has been generally known that
high purity fused silica prepared by such methods as flame
hydrolysis, CVD-soot remelting process, plasma CVD process,
electrical fusing of quartz crystal powder, and other methods, is
susceptible to laser damage to various degrees.
[0007] It has been reported that when silica glass is exposed to
non-polarized or circularly polarized UV laser beam, usually in the
peripheral area of the exposure light beam, additional
birefringence (induced edge birefringence) is generated due to
strain caused by laser damage, but in the center area of the light
beam, there is usually negligible induced birefringence. Recently,
a new phenomenon of laser damage to silica material has been
observed: when the silica glass is exposed to linearly polarized
deep UV laser beam, in addition to the induced edge birefringence,
additional birefringence is induced in the center of the exposed
area of the glass ("polarization-induced birefringence" or "PIB").
The induced birefringence, especially polarization-induced
birefringence, is of particular concern to immersion lithography
systems where a liquid fills the gap between the last lens element
and the wafer in order to enlarge the numerical aperture of the
lens system. In such immersion lithography systems, the
polarization state of the UV radiation needs to be controlled,
desirably linearly polarized. The induced birefringence in the
glass alters the polarization state of the UV radiation, causing
reduction of phase contrast and system resolution. Therefore, for
deep UV and vacuum UV immersion lithographic systems, it is highly
desirable that the glass material used in making the lens elements
has low induced birefringence damage, especially a low
polarization-induced birefringence, when exposed to linearly or
elliptically polarized UV radiation, in addition to low
light-induced wave-front distortion ("LIWFD") and high
transmission.
[0008] Therefore, there exists a need for a synthetic silica
material having, inter alia, a low level of polarization-induced
birefringence, a low level of light-induced wavefront distortion, a
high level of initial internal transmission, and method of making
the same.
[0009] The present invention satisfies the above described needs
for synthetic silica glass for use in lithographic
applications.
SUMMARY OF THE INVENTION
[0010] According to a first aspect of the present invention,
provided is an OD-doped synthetic silica glass material capable of
being used in the light path of the lithographic irradiation of a
lithographic device operating at a wavelength below about 300 nm,
comprising OD and optionally OH, wherein the ratio of
n(OD)/(n(OD)+n(OH)) is higher than 2.times.10.sup.-4.
[0011] In one embodiment of the first aspect of the present
invention, the glass comprises less than about 500 ppm by weight of
OH and 0.15-1400 ppm OD.
[0012] In another embodiment of the first aspect of the present
invention, the glass comprises less than about 150 ppm by weight of
OH and about 0.1-1400 ppm OD.
[0013] In yet another embodiment of the first aspect of the present
invention, the glass comprises less than about 20 ppm by weight of
OH and about 0.01-1400 ppm OD.
[0014] In still another embodiment of the first aspect of the
present invention, the glass comprises less than about 20 ppm by
weight OH and between about 0.01-300 ppm OD.
[0015] In still another embodiment of the first aspect of the
present invention, the glass comprises less than about 20 ppm by
weight OH and between about 0.01-150 ppm OD.
[0016] In yet another embodiment of the first aspect of the present
invention, the glass comprises less than about 1 ppm by weight OH
and between about 0.01-150 ppm OD.
[0017] A second aspect of the present invention is an optical
member capable of being used in the light path of the lithographic
irradiation of a lithographic device operating at a wavelength
below about 300 nm comprising the OD-doped synthetic silica glass
of the present invention described summarily above and in detail
below. In certain embodiments, the optical member is a refractive
optical member where the irradiation travels through at least part
of the body of the optical member. In certain other embodiments,
the optical member is a reflective optical member where the
irradiation is reflected upon at least part of the surface of the
optical member.
[0018] A third aspect of the present invention is a lithographic
system comprising the optical member of the present invention
described summarily above and in detail below. In certain
embodiments, the lithographic system is an immersion lithographic
system. The lithographic system may operate at about 248 nm, 193 nm
or even shorter.
[0019] A fourth aspect of the present invention is a process for
making OD-doped synthetic silica glass material capable of being
used in the light path of the lithographic irradiation of a
lithographic device operating at a wavelength below about 300 nm,
comprising the following steps:
[0020] (I) providing a plurality of particles comprising
silica;
[0021] (II) depositing the a plurality of particles on a supportive
deposition surface at an elevated temperature such that the
particles are consolidated into transparent glass material in
situ,
[0022] wherein:
[0023] either in step (I), the a plurality of particles provided
are D-containing and/or in step (II), the deposition and
consolidation are carried out in a D-containing atmosphere,
[0024] such that the obtained silica glass comprises OD and
optionally OH, and the ratio of n(OD)/(n(OD)+n(OH)) is higher than
about 2.times.10.sup.-4, in certain embodiments preferably higher
than about 0.1, in certain other embodiments higher than about 0.3,
in certain other embodiments higher than about 0.5, in certain
other embodiments higher than 0.8, in yet still other embodiments
higher than about 0.9.
[0025] A fifth aspect of the present application is a process for
making OD-doped synthetic silica glass material capable of being
used in the light path of the lithographic irradiation of a
lithographic device operating at a wavelength below about 300 nm,
comprising the following steps:
[0026] (A) providing a particle preform comprising a plurality of
particles comprising silica;
[0027] (B) optionally purifying and/or drying the particle
preform;
[0028] (C) optionally farther doping the particle preform with
dopants;
[0029] (D) consolidating the particle preform at an elevated
temperature to dense glass; and
[0030] (E) optionally treating the consolidated glass obtained in
step (D) in the presence of H.sub.2, HD and/or D.sub.2,
[0031] wherein in at least one of steps (A), (B), (C), (D) and (E),
OD is introduced into or formed in the glass.
[0032] A sixth aspect of the present invention is a process for
making OD-doped synthetic silica glass, comprising the following
steps:
[0033] (a) providing a plurality of OD-doped particles comprising
silica; and
[0034] (b) melting the particles at an elevated temperature to
obtain a transparent glass.
[0035] A seventh aspect of the present invention is a particle
preform formed during a process of the present invention generally
described above and in detail below.
[0036] An eighth aspect of the present invention is a process for
making OD-doped synthetic silica glass capable of being used in the
light path of the lithographic irradiation of a lithographic device
operating at a wavelength below about 300 nm, comprising the
following steps:
[0037] (a) providing a consolidated silica glass comprising OH;
[0038] (b) treating the consolidated glass in an atmosphere
comprising D.sub.2, H.sub.2, and/or HD to effect H/D exchange to
the desired [OH] and [OD] in the glass.
[0039] The OD-doped synthetic lithographic silica glass of the
present invention has the advantage of higher optical performance
at certain wavelength shorter than about 300 nm, such as at 193 nm,
compared to conventional silica glass which is essentially
non-OD-doped.
[0040] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from the
description or recognized by practicing the invention as described
in the written description and claims hereof, as well as the
appended drawings.
[0041] It is to be understood that the foregoing general
description and the following detailed description are merely
exemplary of the invention, and are intended to provide an overview
or framework to understanding the nature and character of the
invention as it is claimed.
[0042] The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] In the accompanying drawings,
[0044] FIG. 1 is schematic illustration of a proposed mechanism
accounting at least partly for polarization-induced birefringence
in silica glass comprising OH and/or OD moieties.
[0045] FIG. 2 is a schematic illustration of a proposed mechanism
accounting at least partly for polarization-induced birefringence
in silica glass comprising OH and/or OD moieties, and the
difference in terms of level of polarization-induced birefringence
between glasses having different n(OD)/(n(OD)+n(OH)) ratios.
[0046] FIG. 3 is a schematic illustration of a proposed mechanism
accounting at least partly for the polarization-induced
birefringence and light-induced wavefront distortion (LIWFD) in
silica glass comprising OH and/or OD moieties, and the difference
in terms of level of polarization-induced birefringence and LIWFD
between glasses having different n(OD)/(n(OD)+n(OH)) ratios.
[0047] FIG. 4 is a schematic illustration of a proposed mechanism
accounting at least partly for the induced absorption (IA) in
silica glass comprising OH and/or OD moieties, and the difference
in terms of level of induced absorption between glasses having
different n(OD)/(n(OD)+n(OH)) ratios.
[0048] FIG. 5 is a schematic illustration of a proposed mechanism
accounting at least partly for the induced absorption in silica
glass comprising OH and/or OD moieties, and the effect of doped
hydrogen molecules (H.sub.2, D.sub.2 and/or HD) in reducing induced
absorption.
[0049] FIG. 6 is a diagram showing the OH concentration ([OH]) and
OD concentration ([OD]) profiles of a consolidated silica glass
sample produced according to one embodiment of the processes of the
present invention for making OD-doped silica glass of the present
invention.
[0050] FIG. 7 is a diagram showing the [OH] and [OD] profiles of a
consolidated OD-doped silica glass prepared according to one
embodiment of the process of the present invention for making
OD-doped silica glass of the present invention.
[0051] FIG. 8 is a diagram showing polarization-induced
birefringence, measured at 633 nm, of a series of OD-doped silica
glass samples of the present invention, and a series of OH-doped
silica glass samples, having various levels of molecular H.sub.2 or
D.sub.2, as a function of N(P)F, where F is fluence, and N(P) is
number of pulses of a pulsed laser beam is having a wavelength of
about 193 nm, a fluence of about 200 .mu.Jcm.sup.-2pulse.sup.-1 and
a pulse length of approximately 25 ns and a repetition rate of
about 4 kHz to which the glass samples were exposed.
[0052] FIG. 9 is a diagram showing normalized polarization-induced
birefringence of the same samples of FIG. 8, as a function of
number of pulses of a pulsed laser beam having a wavelength of
about 193 nm, a fluence of about 200 .mu.Jcm.sup.-2pulse.sup.-1 and
a pulse length of approximately 25 ns and a repetition rate of
about 4 kHz to which the glass samples were exposed.
[0053] FIG. 10 is a diagram showing normalized LIWFD measured at
633 nm of the same series of OD-doped silica glass samples of the
present invention, and the same series of OH-doped silica glass
samples as presented in FIG. 8 above, as a function of number of
pulses of a pulsed laser beam having a wavelength of about 193 nm,
a fluence of about 200 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse
length of approximately 25 ns and a repetition rate of about 4 kHz
to which the glass samples were exposed.
[0054] FIG. 11 is a diagram showing normalized LIWFD measured at
193 nm of the same series of OD-doped silica glass samples of the
present invention, and the same series of OH-doped silica glass
samples as presented in FIG. 8 above, as a function of number of
pulses of a pulsed laser beam having a wavelength of about 193 nm,
a fluence of about 200 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse
length of approximately 25 ns and a repetition rate of about 4 kHz
to which the glass samples were exposed.
[0055] FIG. 12 is the OH concentration ([OH]) and OD concentration
([OD]) profiles of a consolidated silica glass sample produced
according to one embodiment of the processes of the present
invention for making OD-doped silica glass of the present
invention.
[0056] FIG. 13 is a diagram showing polarization-induced
birefringence, measured at 633 nm, of a series of OD-doped silica
glass samples of the present invention, and a series of OH-doped
silica glass samples, having various levels of molecular H.sub.2,
as a function of number of pulses of a pulsed laser beam having a
wavelength of about 193 nm, a fluence of 600
.mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of approximately 21
ns and a repetition rate of about 4 kHz to which the glass samples
G, H, J and K were exposed, and a fluence of 200
.mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of approximately 25
ns and a repetition rate of about 4 kHz to which the glass samples
F and L were exposed.
[0057] FIG. 14 is a diagram showing normalized polarization-induced
birefringence of a series of OD-doped silica glass samples of the
present invention, and a series of OH-doped silica glass samples,
having various levels of molecular H.sub.2, as a function of number
of pulses of a pulsed laser beam having a wavelength of about 193
nm, a fluence of 600 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse length
of approximately 21 ns and a repetition rate of about 4 kHz to
which the glass samples G, H, J and K were exposed, and a fluence
of 200 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of
approximately 25 ns and a repetition rate of about 4 kHz to which
the glass samples F and L were exposed.
[0058] FIG. 15 is a diagram showing normalized LIWFD measured at
633 nm of the same series of OD-doped silica glass samples of the
present invention, and the same series of OH-doped silica glass
samples G, H, J and K as presented in FIG. 14 above, as a function
of number of pulses of a pulsed laser beam having a wavelength of
about 193 nm, a fluence of 600 .mu.Jcm.sup.-2pulse.sup.-1 and a
pulse length of approximately 21 ns and a repetition rate of about
4 kHz to which the glass samples were exposed.
[0059] FIG. 16 is a diagram showing normalized induced absorbance,
normalized IA, measured at 193 nm of the same series of OD-doped
silica glass samples of the present invention, and the same series
of OH-doped silica glass samples G, H, J and K as presented in FIG.
14 above, as a function of number of pulses of a pulsed laser beam
having a wavelength of about 193 nm, a fluence of 600
.mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of approximately 21
ns and a repetition rate of about 4 kHz to which the glass samples
were exposed.
[0060] FIG. 17 is a diagram showing the OH concentration ([OH]) and
OD concentration ([OD]) profiles of a consolidated silica glass
sample produced according to one embodiment of the processes of the
present invention for making OD-doped silica glass of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0061] As used herein, the term "D-containing compound" means a
chemical compound or an elemental substance comprising deuterium
atom(s) (.sub.1.sup.2H or .sub.1.sup.2D, "D") and optionally
pronium atom(s) (.sub.1.sup.1H, "H"), in which the ratio of
n(D)/(n(D)+n(H)) is higher than the natural isotopic abundance of
D, where n(D) is the total number of D atoms in the molecule of the
D-containing compound, and n(H) is the total number of H atoms in
the molecule of the D-containing compound. Examples of D-containing
compound thus include, but are not limited to: D.sub.2, DH,
CD.sub.4, CDH.sub.3, D.sub.2O, DHO, and the like. As used herein,
the term "D-containing" means that an elemental substance, a
compound, a material, or an atmosphere in which the ratio of
n(D)/(n(D)+n(H)) is higher than the natural isotopic abundance of
D.
[0062] As used herein, the term "hydroxyl(s)" or OH means a moiety
or a group of moieties each consisting of an oxygen atom and a
pronium atom (H). The oxygen atom may be .sup.16O, .sup.17O or
.sup.18O, or mixtures thereof at any proportion. As used herein,
n(OH) means the total number of OH moieties in a material.
[0063] As used herein, the term "deuteroxyl(s)" or OD means a
moiety or a group of moieties each consisting of an oxygen atom and
a deuterium atom (D). The oxygen atom may be .sup.16O, .sup.17O or
.sup.18O, or mixtures thereof at any proportion. As used herein,
n(OD) means the total number of OD moieties in a material.
[0064] In the present application, the two terms "hydroxyl-doped"
and "OH-doped" are used interchangeably. A hydroxyl-doped or
OH-doped material means the material comprises OH moieties and
optionally OD moieties, and the ratio of n(OH)/(n(OD)+n(OH)) in the
material is equal to or higher than the natural isotopic abundance
of H. To that extent, a material in which all the OH moieties
originate from normal water comprising H.sub.2O and D.sub.2O at
essentially the natural isotopic abundances of H and D is regarded
as OH-doped.
[0065] In the present application, the two terms "deuteroxyl-doped"
or "OD-doped" are used interchangeably. A deuteroxyl-doped or
OD-doped material means the material comprises OD moieties and
optionally OH moieties, and the ratio of n(OD)/(n(OD)+n(OH)) in the
material is higher than the natural isotopic abundance of D.
[0066] In the present application, OY means OH or OD or if not
specified, both. Y--Y' means D.sub.2 or H.sub.2 or, if not
specified, HD or any mixture or combination of two or three of them
at any proportion.
[0067] By "F-doped" in the present application, it is meant that
the glass comprises at least 1 ppm by weight of fluorine.
[0068] By "capable of being used in the light path of the
lithographic irradiation of a lithographic device operating at a
wavelength below about 300 nm," it is meant that:
[0069] (i) The material can be used in the light path of the
lithographic irradiation while the lithographic device is being
operated during the normal use for the intended function, i.e.,
performing lithography function in, e.g., the process of making
semiconductor devices; and
[0070] (ii) The material can be used in the light path for the
purpose of re-directing or manipulating the lithographic
irradiation.
[0071] One of ordinary skill in the art of lithography understands
that for a material to be capable of being used in the light path
of the lithographic irradiation of a lithographic device operating
at a certain wavelength, the material should have the required
composition and properties, such as internal transmission, laser
induced wavefront distortion, induced absorption, and the like. One
of ordinary skill in the art of lithography also understands that
it is generally desired that the materials can be made at a
reasonably low cost to the manufacturer and to the society at large
(thus lower negative environmental impact if possible).
[0072] Typically, to be capable of being used in the light path of
the lithographic irradiation of a lithographic device operating at
a wavelength of below about 300 nm, the silica glass is desired to
have an internal transmission at about 248 nm of at least
99.00%/cm. It is highly desired, in certain applications,
especially lithographic applications for making semiconductor chips
operating at about 193 nm, the silica glass has an internal
transmission of at least 99.00%/cm at about 193 nm.
[0073] Typically, to be capable of being used in the light path of
the lithographic irradiation of a lithographic device operating at
a wavelength of below about 300 nm, the silica glass is desired to
have a sodium concentration of lower than about 100 ppm by weight,
in certain embodiments lower than about 50 ppm, in certain other
embodiments lower than about 10 ppm. To be capable of being used in
the in light path of lithographic irradiation of a lithographic
device operating at a wavelength of below about 300 nm, such as at
about 248 nm or about 193 nm, it is desired that the silica glass
has a sodium concentration of lower than about 500 ppb by weight,
in certain embodiments lower than about 100 ppb, in certain
embodiments lower than about 50 ppb, in certain other embodiments
lower than about 10 ppb.
[0074] Fictive temperature is a temperature at which a frozen-in
glass structure would be at equilibrium. The Si--O--Si bond angle
is a function of fictive temperature. The infrared absorption
wavelength, or frequency, of Si--O--Si species varies with bond
angle. Thus infrared absorption can be used to determine an
approximate fictive temperature. An empirical relation between
fictive temperature and absorption frequency is given in the prior
art such as Agarwal et al., A simple IR spectroscopic method for
determining fictive temperature of silica glasses, Journal of
Non-crystalline Solids 185 (1995) 191. Raman scattering can also be
used to determine fictive temperature using the scattering
frequency of silica defects related to strained ring structure.
[0075] As used herein, the term "polarization-induced
birefringence" means the peak measured birefringence level in the
center portion of the uniformly exposed area of the glass after a
certain time interval or laser pulses, if a pulsed laser beam is
used, less the initial birefringence of the glass before the
exposure. The polarization-induced birefringence levels as claimed
in the present application are magnitude (absolute value) thereof.
In the present application, when exposing the glass to quantify the
polarization-induced birefringence level of the silica glass, a
linearly polarized pulsed laser beam at approximately 193 mm having
about 3 mm diameter with a given fluence and pulse length is
directed to a fixed area of the glass sample. The birefringence at
the center portion of the exposed area is measured after a certain
number of pulses. The polarization-induced birefringence value is
calculated by subtracting the initial birefringence of the glass
from the measured center birefringence.
[0076] As used herein, the term "induced edge birefringence" means
the measured peak birefringence level in the peripheral portion
outside of but abutting the exposed area (i.e., the area right at
the aperture where the light intensity changes from nominal value
to zero) of the glass after a certain time interval or laser
pulses, if a pulsed laser beam is used, less the initial
birefringence of the glass before the exposure. In the present
application, the induced edge birefringence of the silica glass is
measured after a linearly polarized pulsed laser beam at
approximately 193 Dm having about 3 mm diameter with a given
fluence and pulse length has been directed to a fixed area of the
glass sample for a certain period of time or a given number of
pulses. The induced edge birefringence value is calculated by
subtracting the initial birefringence of the glass from the peak
measured birefringence at the peripheral portion.
[0077] As used herein, "low polarization-induced birefringence"
means a polarization-induced birefringence of less than or equal to
0.1 nm/cm measured at about 633 nm after being subjected to
5.times.10.sup.9 pulses of linearly polarized pulsed laser beam at
about 193 nm having a fluence of about 40
.mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of about 25 ns.
[0078] As used herein, "normalized polarization-induced
birefringence" is calculated from the measured polarization-induced
birefringence as follows:
P I B ( N ) = P I B ( M ) N 1 F .times. 14 , ##EQU00001##
[0079] where PIB(N) is normalized polarization-induced
birefringence, PIB(M) is magnitude (i.e., absolute value thereof
irrespective of sign thereof) of measured polarization-induced
birefringence in nm/cm measured at about 633 nm, N.sub.1 is number
of pulses in billion pulses, F is fluence of the linearly polarized
ArF laser to which the glass is exposed to in
mJcm.sup.-2pulse.sup.-1. For example, for a glass sample exposed to
ArF laser with a fluence of 40 .mu.Jcm.sup.-2pulse.sup.-1 for
2.times.10.sup.10 pulses having a magnitude of resultant measured
PIB(M) of 0.2 nm n/cm, its PIB(N) is calculated as follows:
P I B ( N ) = P I B ( M ) N 1 F .times. 14 = 0.2 20 .times. 0.04
.times. 14 = 3.5 . ##EQU00002##
[0080] A single sample may have differing PIB(N) when measured at
differing N.sub.1 and F. Where N.sub.1 and F are not specified, the
PIB(N) value is average value.
[0081] Light-induced wavefront distortion of the bulk glass ("bulk
LIWFD") is measured at 633 nm or 193 nm using method and apparatus
available in the prior art. Normalized LIWFD of the glass subjected
to pulsed ArF excimer laser (about 193 nm) measured at 633 nm
(L633) and 193 nm (L193) are calculated in accordance with the
following equations:
L 633 = L B 633 0.95 ( N ' F 2 .tau. ) 0.6 ##EQU00003## and
##EQU00003.2## L 193 = L B 193 1.67 ( N ' F 2 .tau. ) 0.6 ,
##EQU00003.3##
where LB633 is the bulk LIWFD measured at 633 nm in nm/cm (could
bear a "+" or "-" sign, depending on whether the glass compacts or
expands), LB193 is bulk LIWFD measured at 193 nm in nm/cm (could
bear a + or - sign, depending on whether the glass compacts or
expands), N' is number of pulse in million of the linearly
polarized ArF excimer laser to which the sample was exposed to when
the LB633 or LB193 is measured, F is the fluence of the ArF excimer
laser in mJcm.sup.-2pulse.sup.-1, and .tau. is pulse length of the
ArF excimer laser in ns. The L633 and L193 values enable direct
comparison of LIWFD performance of the silica glasses at different
N', F and .tau. values.
[0082] Induced absorption (IA) of the glass upon exposure to an
excimer laser at approximately 193 mm is reported in the present
application. Normalized induced absorption (IA(N)) of the glass is
further calculated from induced absorption. The calculation is done
as follows in the present application:
IA=log(T.sub.1/T.sub.2),
where T.sub.1 is the internal transmission of the glass in terms of
%/cm prior to laser exposure, and T.sub.2 is the internal
transmission of the glass in terms of %/cm after laser exposure;
and
I A ( N ) = I A .tau. N ' F 2 , ##EQU00004##
where N' is number of pulses in the million, F is fluence of the
ArF laser to which the glass is exposed to in
mJcm.sup.-2pulse.sup.-1, and .tau. is pulse length of the ArF
excimer laser in ns.
[0083] As used herein, the term "variation of refractive index," or
"refractive index variation," or ".DELTA.n," means the maximal
variation of refractive indices measured in a plane perpendicular
to the optical axis of the glass material or glass optical member
along a predetermined direction by using interferometry at about
633 nm (He--Ne laser) (with tilt and piston taken out, as indicated
infra). As is typically done by one skilled in the art, when
discussing refractive index variation along a certain direction,
tilt and piston are subtracted. Therefore, the refractive index
variation along a certain direction (such as the radial direction
in a sample prepared by using the OVD process) in the meaning of
the present application does not include tilt or piston. Typically,
the optical axis of a glass optical member, a glass blank, or a
piece of glass material, is selected to be perpendicular to a plane
(a cross-section) in which the measured refractive index
inhomogeneity is the smallest, in order to obtain a glass member
having large clear aperture area.
[0084] The preferred method, also the method used herein, for
determination of interstitial molecular H.sub.2 in fused silica is
Raman scattering. Raman spectrometry is obtained using a T64000
spectrometer from HORIBA Jobin Yvon Inc. with an EEV charge-coupled
device (CCD) detector. The hydrogen molecule concentration in
molecules/cm.sup.3 was obtained from the ratio of the intensity
detected from the hydrogen molecule scattering peak at 4135
cm.sup.-1 (L.sub.4135) to the intensity of the silica scattering
peak at 800 cm.sup.-1 (I.sub.800), i.e., I.sub.4135/I.sub.800, in
the laser Raman spectrum (See, V. S. Khotimchenko et al.,
Prikladnoi Spektroskopii, 46 (6), 987-997 (1986)). More
specifically, the intensities of the peaks were determined by
integrating the areas under the peaks using a linear or quadratic
fit to the background. D.sub.2 and HD concentrations in the glass
in the present application were measured using Raman spectroscopy
as well (see, e.g., B. Schrader, Infrared and Raman Spectroscopy,
Methods and Applications, VCH, Weinheim (1995), ISBN 3-527-26446-9;
H. Komine, IEEE Journal of Quantum Electronics, vol. QE-22, No. 4
(April 1986)). D.sub.2 concentration was measured at 2973 cm.sup.-1
and HD concentration was measured at 3606 cm.sup.-1.
[0085] The OH group has characteristic absorption bands near 2.72
.mu.m (3676 cm.sup.-1), 2.21 .mu.m (4525 cm.sup.-1) and 1.38 .mu.m
(7246 cm.sup.-1) in fused silica. Concentration of OH was measured
by FTIR using the peak height of either the 3676 cm.sup.-1 or the
4525 cm.sup.-1 absorption band.
[0086] The OH concentration, c, in molliter.sup.-1, is derived from
the Beers-Lambert Law
A=.epsilon.cb,
where the absorbance A=log(T.sub.ref/T.sub.OH);
T.sub.ref=Transmittance of sample at reference position, a
non-absorbing wavelength such as 4000 cm.sup.-1;
T.sub.OH=Transmittance of sample at OH absorption peak (.about.3676
cm.sup.-1 for silica); .epsilon. is the molar absorptivity in
litermol.sup.-1cm.sup.-1; c is concentration in molliter.sup.-1;
and b is the pathlength (sample thickness) in cm:
c(molliter.sup.-1)=A/(.epsilon.b).
[0087] Concentration of OH in ppm by weight was calculated from c
in molliter.sup.-1 using the density of the silica glass
(approximately 2.2 g/cm.sup.3) and molecular weight of OH
(approximate 17 g/mol). The constant .epsilon. for high purity
silica glass at a particular wavelength is available in the prior
art.
[0088] Concentration of OD in silica glass was obtained in a
similar manner, namely, starting from FTIR measurement and
calculated by using the Beers-Lambert Law:
A'=.epsilon.'c'b',
where the absorbance A'=log(T'.sub.ref/T.sub.OD);
T'.sub.ref=Transmittance of sample at reference position, a
non-absorbing wavelength such as 2780 cm.sup.-1;
T.sub.OD=Transmittance of sample at OD absorption peak (.about.2705
cm.sup.-1 for silica); .epsilon.' is the molar absorptivity in
litermol.sup.-1cm.sup.-1 (57.4 litermol.sup.-1cm.sup.-1 at 2705
cm.sup.-1); c' is concentration of OD in molliter.sup.-1; and b' is
the path length (sample thickness) in cm:
c'(molliter.sup.-1)=A'/(.epsilon.'b').
[0089] Concentration of OD in ppm by weight was calculated from c'
in molliter.sup.-1 using the density of the silica glass
(approximately 2.2 g/cm.sup.3) and molecular weight of OD
(approximately 18 g/mol). The constant .epsilon.' for high purity
silica glass at a particular wavelength is available in the prior
art.
[0090] As used herein, a "particle preform)" means an object having
a shape and comprising a plurality of solid particles. Thus a
particle preform in the present application may be, for example, a
soot preform consisting essentially of silica soot particles
obtained from flame hydrolysis processes, a green body comprising a
number of silica particles obtained from the sol-gel process, and
the like.
[0091] As used herein, the term "soot dispenser" means a device
which dispenses pre-formed soot particles by, e.g., spraying.
[0092] In search of silica glass materials with desired optical
properties, especially in terms of initial internal transmission,
LIWFD, light-induced absorption, polarization-induced
birefringence, and the like, the present inventors have
unexpectedly found that OD-doped high purity fused silica glass
exhibits comparable, and in certain important respects, superior,
performance than non-OD-doped glass with comparable OH
concentration. The present invention is based on this
discovery.
[0093] Silica glasses comprising D.sub.2 (molecular deuterium) has
been disclosed and studied in the prior art before. For example,
U.S. Pat. No. 5,325,230(A) to Yamagata et al. mentions that
D.sub.2, as well as H.sub.2, may be doped into fused silica glass.
However, this reference does not provide an example of D.sub.2
doped silica glass. Moreover, it does not mention doping silica
glass with OD. Still, it does not mention the potential impact of
doping silica glass with D.sub.2 on the optical properties of the
glass. For another example, J. E. Shelby, Molecular diffusion and
solubility of hydrogen isotopes in vitreous silica, Journal of
Applied Physics, Volume 48, No. 8 (August 1977), discloses the
diffusion and solubility of D.sub.2 in fused silica glass.
[0094] D. L. Fry et al., Hydrogen-Deuterium Exchange in Fused
Silica, Journal of The Optical Society of America, Volume 50, No.
12 (December 1960), pages 1321-22, discusses OD-doped fused silica
glass. No mention is made in this reference of the optical
properties of the OD-doped fused silica glass in this article.
Given the early publication date of this article, one of ordinary
skill in the art can reasonably believe that the glass studied in
this article did not have the required composition and optical
properties for use in modern deep UV and vacuum UV lithography.
James E. Shelby, Quantitative Determination of the Deuteroxyl
Content of Vitreous Silica, Communication of the American Ceramic
Society (January 1987), C-9 to C-10, discloses OD-doped fused
silica glass and method for characterizing such glass. J. E. Shelby
t al., Radiation-induced isotope exchange in vitreous silica,
Journal of Applied Physics, 50(8) (August 1979), pages 5533-35,
studied the formation of OD in fused silica glass from the reaction
of silica and D.sub.2 when exposed to .gamma.-radiation.
[0095] None of the above references mentions a synthetic silica
glass material capable of being used in the light path of the
lithographic irradiation of a lithographic device operating at a
wavelength below about 300 nm. None of the references above
discloses or suggests the desirability of doping synthetic silica
glass with OD or D.sub.2 for UV lithographic applications. In view
of the early publication dates of most of the above-mentioned
references, one of ordinary skill in the art has reason to believe
that the actual D.sub.2 or OD-doped fused silica glass samples
studied in the above references do not have the composition and
properties required for use in deep UV or vacuum UV lithographic
applications, especially in terms of initial internal transmission,
LIWFD, polarization-induced birefringence, induced absorption and
the like, such as at about 248 run or 193 nm.
[0096] The present invention is described mostly in the context of
microlithography at about 193 nm. However, it should be understood
that the material of the present invention may be used in and for
other applications, including but not limited to: lithography at
about 248 nm, lithography at about 157 nm, i-line, g-line
lithography, laser generators, lithographic inspection devices, and
the like.
[0097] The present inventors have prepared synthetic silica glass
materials doped with OD capable of being used in below 300 nm UV
lithographic applications. As mentioned above, the present
inventors have found that, unexpectedly, OD-doped lithographic
synthetic silica glass materials, especially those with a high
n(OD)/(n(OD)+n(OH)) ratio, tend to have better optical properties
than non-OD-doped silica glass with essentially the same level of
total concentration of OH and OD ([OH]+[OD]).
[0098] Moreover, the present inventors have discovered,
unexpectedly, that OD-doped high purity fused silica glass exhibits
improved light induced absorption (IA) over the corresponding
OH-doped high purity fused silica glass. The data in FIG. 16 shows
this improvement. The data is plotted as normalized induced
absorbance (Normalized IA, IA(N)) at 193 nm which is calculated as
described above.
[0099] Co-pending, co-assigned U.S. patent application Ser. No,
11/241,075, entitled "SYNTHETIC SILICA HAVING LOW
POLARIZATION-INDUCED BIREFRINGENCE, METHOD OF MAKING SAME AND
LITHOGRAPHIC DEVICE COMPRISING SAME" and filed on Sep. 30, 2005,
now published as United States Patent Application Publication No.
2006-0137399 A1) discloses and studies the polarization-induced
birefringence phenomenon in synthetic silica glass material, the
content of which is incorporated herein by reference in its
entirety. The silica glass materials studied in the examples of
this patent application were essentially OH-doped. It states that
"among others, OH concentration in the glass is a major factor
affecting the polarization-induced birefringence of the glass.
Generally, all other conditions remaining equal, the higher the OH
level, the higher the polarization-induced birefringence of the
glass. Thus, the present inventors have found that, to achieve a
low level of polarization-induced birefringence in the silica
glass, it is desired that the OH concentration in the glass is less
than 500 ppm by weight, preferably less than 300 ppm, more
preferably less than 100 ppm, still more preferably less than 50
ppm, most preferably less than 20 ppm."
[0100] With no intention or necessity to be bound by any particular
theory, the present inventors present the following explanation of
the mechanism of polarization-induced birefringence in fused silica
glass comprising OH and/or OD, as well as the mechanism accounting
for the lower polarization-induced birefringence in fused silica
glass having a higher n(OD)/(n(OD)+n(OH)) ratio. The explanation is
schematically illustrated in FIGS. 1-3 of the present application.
In these three figures, Y represents H or D, and hydrogen bonds are
shown as dotted lines.
[0101] A 1999 paper titled "Hydroxyl Groups in High-Purity Silica
Glass," Journal of Non-Crystalline Solids, 261 (2000), pages
186-94, describes different types of OH bonding in SiO.sub.2. The
present inventors expect that ODs are bonded in the network of
SiO.sub.2 glass structure in essentially the same ways. FIG. 1
schematically illustrates a proposed mechanism aiming to interpret
at least part of the polarization-induced birefringence generated
in OH- and/or OD-containing silica glass. Formulae (F1) and (F2)
represent the partial structures of a silica glass prior to and
after exposure to UV irradiation, respectively. It is believed that
initially, prior to the exposure to UV light, Si--OY bonds are
randomly arranged in the SiO.sub.2 network and certain hydrogen
bonds are formed. Exposure to UV light can provide enough allowing
activation energy for --OY (or --Y) bond movement (other
wavelengths may be affective if there is sufficient absorption). If
the light is linearly polarized then those bonds aligned with the
light's polarization are activated and can move, resulting in the
breakage of previously existing hydrogen bonds and/or formation of
new hydrogen bonds; thus creating a Polarization-Induced
Birefringence (PIB) damage in the sample. The more SiOY in the
sample the more PIB damage: we are predicting an approximate linear
response with ppm OY in the silica.
[0102] FIG. 2 schematically illustrates a photochemical reaction
possibly involved in the polarization-induced birefringence
phenomenon in a manner slightly different from that of FIG. 1. As
in FIG. 1, the mechanism involves the breakage of certain hydrogen
bonds pre-existing in the partial glass structure (F3) prior to
exposure and the formation of new hydrogen bonds in the partial
glass structure (F4) after exposure. The reaction rate k(Y) of the
photoreaction is k(H) and k(D), respectively, when the atom Y in
the figure is H and D, respectively. It is hypothesized by the
present inventors that, due to the significant mass difference
between D and H (approximately 2 times difference), the reaction
rate k(D) is significantly lower than k(H). Accordingly, all other
conditions, such as the total quantity of OY in the glass,
remaining equal, silica glass with higher n(OD)/(n(OD)+n(OH)) ratio
is expected to have lower polarization-induced birefringence.
[0103] Furthermore, in FIG. 3, the present inventors propose
another mechanism in an effort to account for both LIWFD and
polarization-induced birefringence as a result of exposure to
linearly or elliptically polarized UV irradiation. This proposed
mechanism is essentially a two-step reaction involving the breakage
and formation of both hydrogen bonds and covalent bonds. The first
step, a photolysis reaction having a reaction rate of k.sub.1(Y),
involves the breakage of a covalent bond b (a Si--O bond) and
possible breakage of a hydrogen bond a in the partial structure
(F5) prior to exposure. The reverse reaction of this first step has
a reaction rate of k'.sub.1(Y). The second step, having a reaction
rate of k.sub.2(Y), involves the breakage of bond c (an O--Y bond)
in an intermediate structure (F6), the formation of new bond d (a
Si--O bond) and e (a Y--O bond), and possibly the formation of a
new hydrogen bond f in the post-exposure partial structure (F7).
Because (F5) is a less open and denser structure than F(7), the
reactions result in a density change in the exposed area, hence
LIWFD. It is hypothesized that both k.sub.1(D)<k.sub.1(H) and/or
k.sub.2(D)<k.sub.2(14). Consequently, at essentially the same
level of total OY concentration, silica glass with higher
n(OD)/(n(OD)+n(OH)) ratio is expected to have lower
polarization-induced birefringence, as well as lower LIWFD. Based
on this hypothesis, silica glass doped with OY moieties (OD and/or
OR) in which the oxygen atoms have higher proportions of .sup.17O
and .sup.18O are expected to have lower level of
polarization-induced birefringence and LIWFD as well. It may be
possible in some applications to use other isotope of hydrogen,
tritium (T), atoms in the preparation of the glass, thus forming
OT-doped glass.
[0104] FIG. 4 explains at least partly the induced absorption of
OH/OD-containing silica and the differing degrees thereof at
various n(OD)/(n(OD)+n(OH)) ratios at approximately the same level
of total [OD]+[OH] in the glass. It is known that the photolysis of
the Si--O linkage in the glass due to exposure to high-energy
photons could result in the production of E' center (Si.) and
Si--O--, both believed to be absorbing in the deep UV and/or vacuum
UV. E' center has a center absorption peak at about 215 nm, and
extends to about 193 nm. According to the schematic illustration in
this figure, the E' centers and Si--O. centers created in the
photolysis reaction from structure (F8) to structure (F9) is
reversible to a certain degree. Thus, some of the absorption
centers would automatically heal by the reverse reaction from
structure (F9) to structure (F8). It is hypothesized that the
intra-network reaction from structure (F9) to structure (F10)
resulted in E' center and Si--O. that have more distance than those
in structure (F9), causing more difficulty to the combination
reaction therebetween, thus leading to a relatively more stable
structure with E' and Si--O., and consequently induced absorption.
The more structure (F10) in the glass network, the more stable
absorption centers, hence the higher the induced absorption. It is
believed that k.sub.4''(H)>k.sub.4''(D). Consequently, at a same
level of total [OH].sup.+[OD], fewer structures (F 10), which are
more stable due to, inter alia, structure relaxation and the like,
are formed in a silica glass with higher n(O)D)/(n(OD)+n(OH))
ratio. This explains why silica glass doped with higher
n(OD)/(n(OD)+n(OH)) ratio at essentially the same level of
[OH].sup.+[OD] tends to have lower level of induced absorption as
observed by the present inventors from the OD-doped silica glasses
of the present invention.
[0105] FIG. 5 is a schematic illustration of a mechanism accounting
for the effect of hydrogen (Y--Y') molecules doped in the glass on
the induced absorption of the glass. The hydrogen molecules reacts
with the E' and SiO. color centers to produce Si--OY and Si--Y'.
The OD-doped silica glass of the present invention is capable of
being used in lithography below about 300 nm. It can be used in
lithographic devices operating at longer wavelength, such as in the
I-line lithography at about 365 nm. In certain preferred
embodiments, the OD-doped silica glass of the present invention is
capable of being used as refractive lens elements in the light path
of the UV irradiation utilized in the dry lithographic devices
operating at about 248 nm. In certain preferred embodiments, the
OD-doped silica glass of the present invention has the composition
and property requirements for use as refractive lens elements in
the light path of the LTV irradiation utilized in the immersion
lithographic devices operating at about 248 nm. In certain other
preferred embodiments, the OD-doped silica glass of the present
invention is capable of being used as refractive lens elements in
the light path of the UV irradiation utilized in the dry
lithographic devices operating at about 193 nm. In certain
preferred embodiments, the OD-doped silica glass of the present
invention has the composition and property requirements for use as
refractive lens elements in the light path of the UV irradiation
utilized in the immersion lithographic devices operating at about
193 nm. One of ordinary skill in the art of lithography knows that
for silica glasses to be used as lens elements in these
applications, stringent requirements regarding optical performance
such UV transmission, UV degradation in terms of induced
absorption, light induced wavefront distortion (LIWFD), refractive
index homogeneity, fictive temperature, birefringence, light
induced birefringence, must satisfied. Ample literature has
discussed the relationship between these required optical
performance and the composition of the glass in terms of OH
concentration and distribution, halogen concentration and
distribution, alkali metal concentration and distribution,
transition metal concentration and distribution, and the like. As
discussed above, in a totally unexpected manner, the present
inventors have discovered that high purity fused silica glass doped
with OD has, inter alia, superior performance in
polarization-induced birefringence when subjected to linear
polarized irradiation. Therefore, the glass of the present
invention, especially those doped with high ratio of OD, can be
advantageously used in immersion lithographic technology. Of
course, the OD-doped silica glass may be used as the material for
lens elements in reflective lithography operating in the vacuum CV
and X-ray spectrum. These applications have special requirements on
the other physical properties of the glass.
[0106] The natural isotopic abundance of deuterium (D) is about
1.15.times.10.sup.-4 by mole. The OD-doped silica glass of the
present invention has an n(D)/(n(D)+n(H)) higher than about
2.times.10.sup.-4, thus higher than the natural isotopic abundance
of D. The synthetic silica glass material of the present invention
may be essentially devoid of OH. However, it is not ruled out that
it may contain a certain level of OH in the glass. Nonetheless, in
certain preferred embodiments of the OD-doped synthetic silica
glass of the present invention, it has an n(OD)/(n(OD)+n(OH)) ratio
of higher than about 0.05, in certain embodiments to preferably
higher than about 0.1, in certain embodiments preferably higher
than about 0.2, in certain embodiments preferably higher than about
0.3, in certain embodiments preferably higher than about 0.4, in
certain embodiments preferably higher than about 0.5, in certain
other embodiments preferably higher than about 0.8, in certain
other embodiments preferably higher than about 0.90, in certain
other preferred embodiments higher than about 0.95, in certain
other embodiments preferably higher than about 0.99. It has been
demonstrated by the present inventors that high purity synthetic
silica glass with various levels of [OD] can be obtained by using
the soot-to-glass method. High isotopic purity D.sub.2O having a
higher than 99.9% of n(D)/(n(D)+n(H)) may be employed in the
soot-to-glass method of the present invention, to be described
infra as one of the processes of the present invention, can be used
to produce synthetic silica glass with n(OD)/(n(OD)+n(OH)) higher
than 99%. When used with normal H.sub.2O at various proportions,
synthetic silica glass with various levels of n(OD)/(n(OD)+n(OH))
can be produced.
[0107] In the OD-doped synthetic silica glass of the present
invention, in the OD and optionally OH moieties, the oxygen atoms
may be .sup.16O, .sup.17O and .sup.18O at their natural isotopic
abundances. The natural isotopic abundances of these three
isotopes, by mole, are 99.757%, 0.038% and 0.205%, respectively. As
described supra, in certain preferred embodiments, the silica glass
of the present invention may comprise higher percentage of .sup.17O
and .sup.18O, especially .sup.18O (a stable isotope), than their
respective natural isotopic abundances.
[0108] In certain embodiments of the OD-doped silica glass of the
present invention, the glass has an OH concentration of lower than
about 600 ppm by weight, in certain preferred embodiments
preferably lower than about 160 ppm, in certain other preferred
embodiments lower than about 50 ppm, in certain other embodiments
preferably lower than about 20 ppm, in certain other embodiments
preferably lower than about 1 ppm, in certain other embodiments
still preferably lower than 0.1 ppm.
[0109] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass has an OD concentration
of lower than about 1400 ppm by weight, in certain preferred
embodiments lower than about 1000 ppm, in certain preferred
embodiments lower than about 800 ppm, in certain other preferred
embodiments lower than about 500 ppm, in certain other preferred
embodiments lower than about 300 ppm, in certain other preferred
embodiments lower than about 150 ppm, in certain other preferred
embodiments lower than about 50 ppm, in certain other preferred
embodiments lower than about 20 ppm, in certain other embodiments
lower than about 1 ppm, in certain embodiments between about
0.1-1400 ppm, in certain other embodiments between about 0.1-1000
ppm, in certain embodiments between about 0.1-800 ppm, in certain
other embodiments between about 0.1-500 ppm, in certain other
embodiments between about 0.01-150 ppm, in certain other
embodiments between about 0.01-50 ppm, in certain other embodiments
between about 0.01-20 ppm.
[0110] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass has less than about 500
ppm by weight of OH and 0.15-1400 ppm OD. In certain embodiments of
the OD-doped synthetic silica glass of the present invention, the
glass comprises less than about 150 ppm by weight of OH and about
0.1-1400 ppm OD. In certain other embodiments of the OD-doped
synthetic silica glass of the present invention, the glass
comprises less than about 20 ppm by weight of OH and 0.01-1400 ppm
OD. In certain other embodiments of the OD-doped synthetic silica
glass of the present invention, the glass comprises less than about
20 ppm by weight of OH and about 0.01-300 ppm OD.
[0111] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass has an essentially
constant ratio of concentration of OD ([OD]) to the concentration
of OH ([OH]) in different locations in the glass, i.e., [OD]/[OH].
By "essentially constant ratio," it is meant that the difference
between the maximal ratio (R.sub.max) and the minimal ratio
(R.sub.min) as measured has the following relationship:
2(R.sub.max-R.sub.min)/(R.sub.max+R.sub.min).ltoreq.0.1. In certain
embodiments,
2(R.sub.max-R.sub.min)/(R.sub.max+R.sub.min).ltoreq.0.05.
[0112] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass has a [OD] variation,
measured in a plane essentially perpendicular to the optical axis
of the glass, of less than about 10 ppm by weight, in certain
embodiments less than about 5 ppm, in certain other embodiments
less than about 2 ppm, in certain other embodiments less than about
1 ppm, in certain other embodiments less than about 0.1 ppm. In
certain embodiments of the OD-doped synthetic silica glass of the
present invention, the glass has, in addition to the or in absence
of the [OD] variation described in this paragraph, an [OH]
variation, measured in a plane essentially perpendicular to the
optical axis of the glass, of less than about 10 ppm by weight, in
certain embodiments less than about 5 ppm, in certain other
embodiments less than about 2 ppm, in certain other embodiments
less than about 1 ppm.
[0113] The OD-doped synthetic silica glass of the present invention
may be essentially free of dopants other than OD and OH. However,
it is not ruled that the OD-doped synthetic silica glass of the
present invention comprises dopants such as Al, F, Cl and Ti. The
Ti-containing OD-doped silica glass of the present invention may be
advantageously used in the substrates for reflective optical
elements, especially those requiring high thermal dimensional
stability, such as those used in reflective lithography technology
operating in vacuum UV and X-ray spectra. F-doped silica glass of
the present invention may comprise, for example, fluorine in the
amount of less than 1000 ppm by weight, in certain embodiments less
than about 500 ppm, in certain other embodiments less than about
300 ppm, in certain other embodiments less than about 100 ppm, in
certain embodiments less than about 50 ppm, in certain other
embodiments less than about 10 ppm. In certain embodiments of the
OD-dopes silica glass of the present invention, it comprises less
than about 150 ppm by weight of OH, about 0.1-1400 ppm by weight OD
and about 1-500 ppm by weight F. In certain other embodiments of
the OD-doped synthetic silica glass of the present invention, the
glass comprises less than about 20 ppm by weight of OH, about
0.01-1400 ppm OD and about 1-500 ppm F. In certain other
embodiments of the OD-doped synthetic silica glass of the present
invention, the glass comprises less than about 20 ppm by weight of
OH, about 0.01-300 ppm OD and about 1500 ppm F.
[0114] The OD-doped synthetic silica glass may be doped with
molecular H.sub.2, HD and/or D.sub.2. In certain preferred
embodiments, the OD-doped synthetic silica glass of the present
invention has a concentration of [H.sub.2], [HD] and [D.sub.2], in
total, of between 1.times.10.sup.15 to 1.times.10.sup.19
molecules/cm.sup.3, in certain embodiments higher than about
5.times.10.sup.15 molecules/cm.sup.3, in certain embodiments higher
than about 1.times.10.sup.16 molecules/cm.sup.3, in certain
preferred embodiments below about 5.times.10.sup.18
molecules/cm.sup.3, in certain other preferred embodiments below
about 5.times.10.sup.17 molecules/cm.sup.3, in certain other
preferred embodiments below about 1.times.10.sup.17
molecules/cm.sup.3, in certain other preferred embodiments between
about 1.times.10.sup.16 to 1.times.10.sup.17 molecules/cm.sup.3. In
certain preferred embodiments of the OD-doped synthetic silica
glass of the present invention, the ratio of
(2n(H.sub.2)+n(HD))/2(n(H.sub.2)+n(HD)+n(D.sub.2)) is higher than
0.1, in certain preferred embodiments higher than about 0.3, in
certain other preferred embodiments higher than about 0.5, in
certain other embodiment higher than about 0.7, in certain other
preferred embodiments higher than about 0.9. In certain preferred
embodiment, the ratio of
(2n(H.sub.2)+n(HD))/2(n(H.sub.2)+n(HD)+n(D.sub.2)) in the glass is
essentially the natural isotopic abundance of X by mole. In certain
other embodiments, the ratio of
(2n(D.sub.2)+n(HD))/2(n(H.sub.2)+n(HD)+n(D.sub.2)) is higher than
0.1, in certain preferred embodiments higher than about 0.3, in
certain other preferred embodiments higher than about 0.5, in
certain other embodiment higher than about 0.7, in certain other
preferred embodiments higher than about 0.9. In certain preferred
embodiment, the ratio of
(2n(D.sub.2)+n(HD))/2(n(H.sub.2)+n(HD)+n(D.sub.2)) in the glass is
essentially the natural isotopic abundance of D by mole.
[0115] As can be expected, the hydrogen molecules (H.sub.2, D.sub.2
and/or HD) in the silica glass may exchange with the OH or OD
moieties in the glass network under suitable conditions. When a
silica glass doped with --OY (where Y could be H and/or D) and
Y'.sub.2 (where Y' could be H or D or combinations thereof) is
heated to an elevated temperature, such as at above about
1000.degree. C., it is believed rate of such exchange reactions
would increase significantly than at about room temperature. The
following schematically illustrates the reaction:
##STR00001##
[0116] The result of the this reaction (1), therefore, would
include, inter alia: [0117] (i) For OH-doped silica glass doped
with D.sub.2 and/or HD, --OD can result in the glass network; and
[0118] (ii) For OD-doped silica glass doped with H.sub.2, the above
reaction could result in a higher n(OH)/(n(OD)+n(OH)) ratio in the
glass network.
[0119] It is believed that the above reaction (1) is very slow at
around room temperature when the glass is not exposed to
high-energy irradiations. Therefore, for an OD-doped glass doped
with H.sub.2 but no intentionally added D.sub.2, it is expected
that the glass would have a fairly stable n(OD)/(n(OD)+n(OH)) ratio
at around room temperature if not exposed to irradiation that could
accelerate the above reaction.
[0120] However, in a totally unexpected manner, it has been
observed that, when the glass is exposed to high-energy UV light,
such as an excimer laser at about 193 nm, the above reaction (1) is
accelerated considerably even at around room temperature. As
mentioned above, for an OD-doped silica glass doped with H.sub.2
but no intentionally added D.sub.2, the result of the accelerated
reaction is the change (decrease) of n(OD)/(n(OD)+n(OH)) ratio over
the life of the glass during which the glass is exposed to such
high energy irradiation. This could be undesirable, as detailed
herein, for the performance of the glass for some applications
where a low [OH] and/or a low n(OH)/(n(OH)+n(OD)) is highly
desired. Likewise, for applications which require a stable
n(OD)/(n(OD)+n(OH)) ratio during the life of the material, the
fluctuation of it should generally be limited to a tolerable
range.
[0121] Therefore, in certain embodiments of the OD-doped silica
glass of the present invention, it is desired that the total
concentration of [OD]+[OH] in terms of molecm.sup.-1, is
significantly higher than the total concentration of hydrogen
molecules (including H.sub.2, D.sub.2 and HD) in terms of
molecm.sup.-3. By "significantly higher," it is meant that, prior
to being irradiated to a irradiation with a wavelength of below
about 300 nm, the glass has a ratio of [OY]/[Y--Y'] of at least 10,
in certain embodiments higher than about 50, in certain embodiments
higher than about 100, in certain other embodiments higher than
about 200, in certain other embodiments higher than about 500, in
certain other embodiments higher than about 1000, where
[OY]=[OH]+[OD], [Y--Y']=[H.sub.2]+[D.sub.2]+[HD], all in
molecm.sup.-3. In these glasses, even if the exchange reaction (1)
is allowed to proceed to equilibrium, the impact on the final
[OD]/([OH]+[OD]) upon exposure would be limited.
[0122] In certain embodiments of the OD-doped silica glass of the
present invention, it is desired that the glass, prior to exposure
to an irradiation having a wavelength of below about 300 nm, is
essentially loaded only with H.sub.2 and essentially no D.sub.2. By
"essentially no D.sub.2," it is meant that the glass comprises
D.sub.2 in the amount of [D.sub.2].ltoreq.2.times.10.sup.15
molecules/cm.sup.3. In these embodiments, it is sometimes desired
that [OD]/[H.sub.2]>10, in certain embodiments
[OD]/[H.sub.2]>20, in certain embodiments [OD]/[H.sub.2]>50,
in certain embodiments [OD]/[H.sub.2]>100, in certain
embodiments [OD]/[H.sub.2]>200, in certain embodiments
[OD]/[H.sub.2]>300, in certain embodiments
[OD]/[H.sub.2]>500, in certain embodiments
[OD]/[H.sub.2]>800, in certain embodiments
[OD]/[H.sub.2]>1000, where both [OD] and [H.sub.2] are expressed
in terms of molecm.sup.-3. In these embodiments, it is sometimes
desired that the glass, prior to exposure to an irradiation having
a wavelength of below about 300 nm, has a
[H.sub.2]<2.times.10.sup.17 molecules/cm.sup.3, in certain
embodiments [H.sub.2]<1.times.10.sup.17 molecules/cm.sup.3, in
certain embodiments [H.sub.2]<0.8.times.10.sup.17
molecules/cm.sup.3, in certain embodiments
[H.sub.2]<0.5.times.10.sup.17 molecules/cm.sup.3, in certain
embodiments [H.sub.2]<0.2.times.10.sup.17 molecules/cm.sup.3 in
certain embodiments [H.sub.2]>1.times.10.sup.15
molecules/cm.sup.3, in certain embodiments
[H.sub.2]>1.times.10.sup.15 molecules/cm.sup.3, in certain
embodiments [H.sub.2]>5.times.10.sup.15 molecules/cm.sup.3; in
certain embodiments [H.sub.2]<1.times.10.sup.16
molecules/cm.sup.3.
[0123] In certain other embodiments of the OD-doped silica glass of
the present invention, it is desired that the glass, prior to
exposure to an irradiation having a wavelength of below about 300
nm, is primarily loaded with D.sub.2. By "primarily loaded with
D.sub.2," it is meant that in the glass
[D.sub.2]/([D.sub.2]+[H.sub.2]) is higher than about 0.5, in
certain embodiments higher than about 0.6, in certain embodiments
higher than about 0.7, in certain embodiments higher than about
0.8, in certain embodiments higher than about 0.9, in certain
embodiments higher than about 0.95, in certain embodiments higher
than about 0.99. Such OD-doped silica glass primarily loaded with
D.sub.2 is especially advantageous for OH-averse applications, such
as immersion lithography operating at about 193 nm. In these
embodiments, it is sometimes desired that [OD]/[H.sub.2]>10, in
certain embodiments [OD]/[H.sub.2]>20, in certain embodiments
[OD]/[H.sub.2]>50, in certain embodiments [OD]/[H.sub.2]>100,
in certain embodiments [OD]/[H.sub.2]>200, in certain
embodiments [OD]/[H.sub.2]>300, in certain embodiments
[OD]/[H.sub.2]>500, in certain embodiments
[OD]/[H.sub.2]>800, in certain embodiments
[OD]/[H.sub.2]>1000, where both [OD] and [H.sub.2] are expressed
in terms of molecm.sup.-3 . In these embodiments, it is sometimes
desired that the glass, prior to exposure to an irradiation having
a wavelength of below about 300 nm, has a
[H.sub.2]<2.times.10.sup.17 molecules/cm.sup.3, in certain
embodiments [H.sub.2]<2.times.10.sup.17 molecules/cm.sup.3, in
certain embodiments [H.sub.2]<0.8.times.10.sup.17
molecules/cm.sup.3, in certain embodiments
[H.sub.2]<0.5.times.10.sup.17 molecules/cm.sup.3, in certain
embodiments [H.sub.2]<0.2.times.10.sup.17 molecules/cm.sup.3, in
certain embodiments [H.sub.2]>1.times.10.sup.15
molecules/cm.sup.3, in certain embodiments
[H.sub.2]>5.times.10.sup.15 molecules/cm.sup.3, in certain
embodiments [H.sub.2]<1.times.10.sup.16 molecules/cm.sup.3.
[0124] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass has an essentially
constant ratio R' of [D.sub.2]/[H.sub.2] at different locations of
the glass. By "essentially constant ratio," it is meant that the
difference between the maximal ratio (R'.sub.max) and the minimal
ratio (R'.sub.min) as measured has the following relationship:
2(R'.sub.max-R'.sub.min)/(R'.sub.max+R'.sub.min).ltoreq.0.1. In
certain embodiments,
2(R'.sub.max-R'.sub.min)/(R'.sub.max+R'.sub.min).ltoreq.0.05.
[0125] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass has a concentration
variation of OH and OD ([OH]+[OD]) measured in a plane
perpendicular to at least one direction of less than about 50 ppm,
in certain embodiments preferably less than about 30 ppm, in
certain other embodiments preferably less than about 20 ppm, in
certain other embodiments less than about 10 ppm, in certain other
embodiments preferably less than about 1 ppm, in certain other
embodiments preferably less than about 0.1 ppm.
[0126] In certain embodiments of the OD-doped silica glass of the
present invention, the glass has a Cl concentration less than about
100 ppm, in certain embodiments less than about 50 ppm, in certain
other embodiments less than about 10 ppm.
[0127] It is known that alkali, alkaline earth and transition
metals can be detrimental to the transmission characteristics
silica glasses. See, for example, Schultz, P. C., Optical
Absorption of the Transition Elements in Vitreous Silica, Journal
of The American Ceramic Society, 57 (7), pp 309-313, (July 1974);
U.S. Pat. No. 6,174,509 B1 to Corning Incorporated, Pure Fused
Silica, Furnace and Method; U.S. Pat. No. 6,698,248 B2 to Corning
Incorporated, Methods and Furnaces for Fused Silica Production.
U.S. Pat. No. 6,174,509 B1 discloses an article produced by
collecting molten silica particles in a refractory furnace in which
at least a portion of the refractory has been exposed to a
halogen-containing gas to react with contaminating metal ions in
the refractory. Improvement in the zircon refractory, as disclosed
in U.S. Pat. No. 6,174,609, alleviated the affect of sodium ion
contamination in a fused silica article. However, it was then found
that other contaminants also exist in the furnace refractory in
addition to sodium. These include the alkaline earth metals, and
transition metals, such as iron, titanium and lead, aluminum,
phosphorous and sulfur. U.S. Pat. No. 6,698,248 B2 discloses
methods and apparatus for producing fused silica members having
high internal transmission. The apparatus and methods as disclosed
was capable of producing fused silica having internal transmission
of at least 99.650/cm at 193 nm. In this reference, it was stated
that: "The next generation of fused silica glass used in the
microlithography market will require ArF (193 nm) internal
transmission exceeding 99.65%/cm, and preferably exceeding
99.75%/cm. The standard manufacturing processes described above is
capable of consistently producing fused silica lens blanks with
99.5%/cm. Reduction of metal contaminants, which have a major
impact on UV transmission, has played a major role in the
production of higher transmission fused silica. The effects of
metals, such as sodium, potassium and iron, are evident at the 10's
of parts per billion level. The standard process has demonstrated
the ability to produce fused silica having transmission of
99.65%/cm, without sacrificing glass homogeneity, but not in the
quantity needed to make large production quantities of lens blanks
and not with the consistency to serve as a basis for a production
process. Accordingly, it would be desirable to provide methods and
apparatus capable of consistently manufacturing large production
quantities of fused silica having internal transmission equal to or
greater than 99.65%/cm at 193 nm, and preferably greater than
99.75%/cm." It should be noted, however, that the silica glasses
discussed in these references were all OH-containing,
non-OD-doped.
[0128] It is also known that high purity synthetic silica glass
material are required to have a very low level of alkali metals,
alkaline earth and transition metals in order to have a sufficient
transmission properties (e.g., absorption, induced absorption,
fluence-dependent-transmission, birefringence, light-induced
birefringence, LIWFD, and the like) in the wavelength of interest
in the UV, such as for use as refractive members in KrF and ArF
lithography devices. Certain metals having multiple oxidation
states can cause more absorption at one oxidation state than at
others. Thus, in certain embodiments of the OD-doped silica glass
of the present invention, the glass comprises less than 100 ppm by
weight, in certain embodiments less than about 50 ppm, in certain
embodiments less than about 10 ppm, in certain embodiments
preferably less than 1 ppm, in certain embodiments preferably less
than 500 ppb, in certain embodiments less than about 300 ppb, in
certain embodiments less than about 100 ppb, in certain embodiments
less than 50 ppb, in certain embodiments preferably less than about
20 ppb, in certain other embodiments preferably less than about 10
ppb, of any alkali metal, any alkali earth metal, and any
transition metal. Among all metals, sodium is one of the most
difficult to reduce from the glass composition because it is
virtually ubiquitous and can be introduced into the glass in the
handling process. Sodium also diffused into consolidated glass and
soot preforms extraordinarily fast at elevated temperatures,
especially at above 800.degree. C. Nonetheless, in order for the
glass to have the capability to be used as refractive optical
element in a lithographic device operating at a wavelength below
about 300 nm, such as at about 248 nm or 193 nm, it is typically
desired that the glass comprises sodium lower than about 100 ppb by
weight, in certain embodiments lower than about 50 ppb, in certain
embodiments lower than 30 ppb, in certain embodiments lower than
about 10 ppb (such as for use in lithography devices operating at
about 193 nm), and in certain embodiments lower than 5 ppb. The
present inventors have made OD-doped high purity silica glass with
such low level of sodium. In certain embodiments, the glass
comprises any transition metal at less than 2 ppb. In certain other
embodiments, the glass comprises any transition metal at less than
1 ppb. In certain other embodiments, the glass comprises any
transition metal at less than 0.5 ppb. In certain embodiments,
especially for glasses to be used as refractive optical member in
ArF laser lithography devices, it is preferred that the glass
comprises any individual element in all oxidation states of the
following in concentrations less than 2 ppb by weight, in certain
embodiments preferably less than 1 ppb, in certain other
embodiments less than 0.5 ppb, in certain other embodiments less
than 0.1 ppb: Ti (+2, +4, for example), V (+5, +4, for example), Cr
(+6, +3, for example), Mn (+6, +4, +2, for example), Fe (+3, +2,
for example), Co (+3, +2, for example), Ni (+2, for example), Cu
(+2, +1, for example), Zn (+2, for example), Ge (+4, +2, for
example), Zr (+4, for example), Ag (+1, for example), Cd (+1, for
example), Sn (+4, +2, for example), Pb (+4, +2, for example), Bi
(+5, +3, for example) and U (+6, +3, for example). Of course,
elemental metals (in 0 state) are generally detrimental to the
transmission properties of the glass. In certain embodiments of the
OD-doped synthetic silica glass of the present invention, it
comprises less than 100 ppm by weight, in certain embodiments less
than about 50 ppm, in certain embodiments less than about 10 ppm,
in certain embodiments preferably less than 1 ppm, in certain
embodiments preferably less than 500 ppb, in certain embodiments
less than about 300 ppb, in certain embodiments less than about 100
ppb, in certain embodiments less than about 50 ppb, in certain
embodiments preferably less than 30 ppb, in certain other
embodiments preferably less than 10 ppb, of any and all metals in
all oxidation states in total. Similar low levels of such elements
are also desired for OH-doped lithographic silica glass and F-doped
lithographic silica glass.
[0129] In certain preferred embodiments of the OD-doped synthetic
silica glass of the present invention, the glass exhibits a
light-induced wavefront distortion (LIWFD), measured at 633 nm, of
between -0.1 and 0.1 nm/cm, in certain preferred embodiments
between -0.5 to 0.5 nm/cm, in certain other preferred embodiments
between about 0 and 1 nm/cm, in certain other preferred embodiments
between about 0 and 0.5 nm/cm, when subjected to 10 billion pulses
of a laser beam operating at approximately 193 nm and having a
fluence of approximately 70 .mu.J/(cm.sup.2pulse) and a pulse
length of approximately 25 ns.
[0130] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass exhibits, in addition to
or in absence of the LIWFD properties described above, a normalized
wavefront distortion L633 when subjected to excimer laser pulses at
about 193 nm of less than or equal to about 20 billion pulses,
measured at about 633 nm, wherein -1.0<L633.ltoreq.1.0, in
certain embodiments -0.5.ltoreq.L633.ltoreq.1.0, in certain
embodiments -0.1.ltoreq.L633.ltoreq.1.0, in certain embodiments
0.ltoreq.L633.ltoreq.1.0, in certain preferably embodiments
0.ltoreq.L633.ltoreq.0.5, in certain other preferred embodiments
0.ltoreq.L633.ltoreq.0.4, in certain other embodiments preferably
0.ltoreq.L633.ltoreq.0.3.
[0131] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass exhibits, in addition to
or in absence of the LIWFD and L633 properties described above, a
normalized wavefront distortion L193 when subjected to excimer
laser pulses at about 193 nm of less than or equal to about 20
billion pulses, measured at about 193 nm, wherein
-1.0<L193.ltoreq.1.0, in certain embodiments
-0.5.ltoreq.L193.ltoreq.1.0, in certain embodiments
-0.1.ltoreq.L193.ltoreq.1.0, in certain embodiments
0.ltoreq.L193.ltoreq.1.0, in certain embodiments preferably
0.ltoreq.L193.ltoreq.0.5, in certain embodiments preferably
0.ltoreq.L193.ltoreq.0.4, in certain other embodiments preferably
0.ltoreq.L193.ltoreq.0.3.
[0132] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass exhibits less than about
1 nm/cm, in certain embodiments preferably less than 0.1 nm/cm, of
polarization-induced birefringence (magnitude) measured at about
633 nm after being subjected to 5.times.10.sup.9 pulses of linearly
polarized pulsed laser beam at about 193 nm having a fluence of
about 40 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of about 25
ns. In certain other embodiments of the OD-doped synthetic silica
glass of the present invention, the glass exhibits less than about
1 nm/cm, in certain embodiments preferably less than 0.1 nm/cm, of
polarization-induced birefringence (magnitude) measured at about
633 nm after being subjected to 1.times.10.sup.10 pulses of
linearly polarized pulsed laser beam at about 193 nm having a
fluence of about 40 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse length
of about 25 ns. In certain embodiments of the OD-doped synthetic
silica glass of the present invention, the glass exhibits less than
about 0.1 nm/cm of polarization-induced birefringence (magnitude)
measured at about 633 nm after being subjected to 2.times.10.sup.10
pulses of linearly polarized pulsed laser beam at about 193 nm
having a fluence of about 40 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse
length of about 25 ns. In certain embodiments of the OD-doped
synthetic silica glass of the present invention, the glass exhibits
less than about 0.04 nm/cm of polarization-induced birefringence
(magnitude) measured at about 633 nm after being subjected to
2.times.10.sup.10 pulses of linearly polarized pulsed laser beam at
about 193 nm having a fluence of about 40
.mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of about 25 ns. In
certain embodiments of the synthetic silica glass of the present
invention, the glass exhibits higher than about 0.001 nm/cm of
polarization-induced birefringence (magnitude) measured at about
633 nm after being subjected to 2.times.10.sup.10 pulses of
linearly polarized pulsed laser beam at about 193 nm having a
fluence of about 40 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse length
of about 25 ns. In certain embodiments of the OD-doped synthetic
silica glass of the present invention, the glass exhibits higher
than about 0.01 nm/cm of polarization-induced birefringence
(magnitude) measured at about 633 nm n after being subjected to
2.times.10.sup.10 pulses of linearly polarized pulsed laser beam at
about 193 nm having a fluence of about 40
.mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of about 25 ns.
[0133] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass exhibits a normalized
polarization-induced birefringence less than 10, in certain
embodiments less than 5, when subjected to linearly-polarized
excimer laser pulses at about 193 nm of less than or equal to about
20 billion pulses.
[0134] In certain embodiments of the OD-doped silica glass of the
present invention, the glass exhibits a polarization-induced
birefringence less than about 0.04 nm/cm, in certain embodiments
less than about 0.02 nm/cm of polarization-induced birefringence
measured at about 633 nm after being subjected to 2.times.10.sup.9
pulses of linearly polarized pulsed laser beam at about 193 nm
having a fluence of about 200 .mu.Jcm.sup.-2pulse.sup.-1 and a
pulse length of about 25 ns, in certain embodiments less than about
0.02 nm/cm of polarization-induced birefringence measured at about
633 nm after being subjected to 5.times.10.sup.9 pulses of linearly
polarized pulsed laser beam at about 193 nm having a fluence of
about 200 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse length of about 25
ns.
[0135] In certain embodiments of the OD-doped silica glass of the
present invention, the glass exhibits a normalized
polarization-induced birefringence less than 2, in certain
embodiments less than 1, in certain embodiments less than 0.5, when
subjected to linearly-polarized excimer laser pulses at about 193
nm of less than or equal to about 2 billion pulses. In certain
embodiments, the glass exhibits a normalized polarization-induced
birefringence less than 2, in certain embodiments less than 1, in
certain embodiments less than 0.5, when subjected to excimer laser
pulses at about 193 nm of less than or equal to about 5 billion
pulses. In certain other embodiments, the glass exhibits a
normalized polarization-induced birefringence less than 2, in
certain embodiments less than 1, in certain embodiments less than
0.5, when subjected to excimer laser pulses at about 193 nm of less
than or equal to about 8 billion pulses.
[0136] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass exhibits an initial
internal transmission at about 193 nm of at least 99.00%/cm, in
certain embodiments desirably at least 99.50%/cm, in certain
embodiments desirably at least 99.65%/cm, in certain embodiments
preferably at least 99.75%/cm, in certain other embodiments
preferably at least 99.80%/cm.
[0137] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass exhibits a fictive
temperature of lower than about 1150.degree. C. In certain other
embodiments of the OD-doped synthetic silica glass of the present
invention, the glass exhibits a fictive temperature of lower than
about 1000.degree. C. In certain embodiments of the glass of the
present invention, it exhibits a fictive temperature of higher than
about 800.degree. C.
[0138] In certain embodiments of the OD-doped synthetic silica
glass of the present invention, the glass exhibits a refractive
index variation measured in a plane perpendicular to at least one
direction of less than about 10 ppm, in certain embodiments
preferably less than about 5 ppm, in certain other embodiments
preferably less than about 2 ppm, in certain other embodiments
preferably less than about 1 ppm, in certain other embodiments
preferably less than about 0.5 ppm.
[0139] Another aspect of the present invention is an optical glass
member comprising the OD-doped synthetic silica glass material of
the present invention described in general and in detail above and
illustrated below. The optical glass member advantageously is used
in the light path of an irradiation having a wavelength of shorter
than about 300 nm, though the glass member of the present invention
may be used in the light path of irradiation having a longer
wavelength, such as in the visible spectrum, or in the infrared
spectrum. The OD-doped glass of the present invention is
particularly advantageous for use in certain infrared applications
where OH is undesirable and OD is acceptable. Non-limiting examples
of such glass member of the present invention may include, but are
not limited to, optical members for use as refractive lens
elements, sputter targets, and the like. The refractive lens
elements may be used in, e.g., lithographic scanners and steppers
machines, laser generators, laser etalons, lithographic inspection
devices, and the like. The OD-doped glass optical member of the
present invention is particularly suited for devices involving
high-fluence irradiations due to its improved laser damage
resistance.
[0140] Still another aspect of the present invention is a
lithographic system comprising at least one optical member of the
present invention. The lithographic system is advantageously an
immersion system in which at least one lens element is allowed to
contact a liquid. Emersion lithographic systems usually utilize
linearly polarized irradiation. Due to the high resistance to
polarization-induced birefringence damage, the OD-doped synthetic
silica glass member of the present invention is particularly
suitable for such lithographic systems. Due to the excellent
performance of the OD-doped glass material of the present
invention, as mentioned supra, it may be used in traditional dry
lithographic tools operating below about 300 nm, such as at about
248 nm, 193 nm and 157 nm.
[0141] The OD-doped synthetic silica glass material of the present
invention may be produced by using various methods, such as the
direct-to-glass method, the soot-to-glass methods and the sol-gel
processes, to name a few. Generally, the OD-doped silica glass of
the present invention may be produced by: (i) utilizing D-exchanged
or D-enriched starting materials to produce silica; (ii) making
silica glass in a D-rich environment; or (iii) doping silica glass
with OD.
[0142] The first method contemplated by the present inventors is a
direct-to-glass method. Broadly speaking, this process includes the
following steps:
[0143] (I) providing a plurality of particles comprising
silica;
[0144] (II) depositing the a plurality of particles on a supportive
deposition surface at an elevated temperature such that the
particles are consolidated into transparent glass material in
situ,
[0145] wherein:
[0146] in step (II), the deposition and consolidation are carried
out in a D-containing atmosphere, such that the obtained silica
glass comprises OD and optionally OH, and the ratio of
n(OD)/(n(OD)+n(OH)) is higher than about 2.times.10.sup.-4, in
certain embodiments preferably higher than about 0.05, in certain
embodiments preferably higher than about 0.1, in certain other
embodiments higher than about 0.3, in certain other embodiments
higher than about 0.5, in certain other embodiments higher than
0.8, in yet still other embodiments higher than about 0.9, in
certain other embodiments higher than about 0.95.
[0147] In step (I), the a plurality of particles comprising silica
may be provided by flame hydrolysis of at least one precursor
compound comprising silicon, such as silicon halides (such as
SiCl.sub.4) or organosilicon compounds. As non-limiting example of
organosilicon compound, mention may be made of
octamethylcyclotetrasiloxane (OMCTS). The precursor compound may
comprise D at a level higher than its natural isotopic abundance
(such as D-containing OMCTS), in which case the particles per se
are usually OD-doped when originally produced. Alternatively, the
precursor compound may comprise D at a level no more than its
natural isotopic abundance, yet the precursor compound is allowed
to undergo flame hydrolysis in an atmosphere comprising D at a
level higher than its natural isotopic abundance, such as an
atmosphere comprising added D.sub.2O or D.sub.2O produced from
burning D-containing compound as fuel, such as CD.sub.4, CDH.sub.3,
CD.sub.2H.sub.2, D.sub.2, HD, and the like. The particle comprising
silicon may be pre-fabricated or produced in situ in the same
furnace where they are deposited and consolidated in step (II).
Where they are pre-fabricated, they may be provided in step (I) via
a soot dispenser, sprayed to the supportive deposition surface and
allowed to consolidate. If the pre-fabricated particles are
D-containing, step (II) may be carried out in an environment that
is D-containing or not, depending on the level of OD in the
pre-fabricated particles and the desired level of OD in the final
consolidated glass. If the pre-fabricated particles are not
D-containing, step (II) should be carried out in a D-containing
environment (such as in the presence of D.sub.2O or D.sub.2 gas or
combinations thereof) in order to introduce OD into the
consolidated glass. This direct-to-glass method for making the
OD-doped silica glass of the present invention may be
plasma-assisted. By adjusting the ratio of n(D)/(n(D)+n(H)) in the
atmosphere in which the particles are produced, and in the
atmosphere in which step (II) is carried out, one can produce
OD-doped final glass with desired level of OD-doping.
[0148] There is abundant literature on equipment and processes for
making high-purity fused silica material by using the
direct-to-glass method, which can be adapted for making high-purity
OD-doped fused silica glass of the present invention. For example,
it is highly desired that the supportive deposition surface in step
(II) is an essentially planar deposition surface of a horizontal
rotating table. Generally, in order to obtain OD-doped fused silica
glass for use in deep UV and vacuum UV lithographic devices, the
glass should be produced by using high purity raw materials and
processing agents in a very clean environment, and care should be
taken to avoid contamination by metals detrimental to the desired
properties. Low metal impurities are obtained via high purity
starting materials and apparatus for making the soot (and
corresponding consolidated glass) and/or purifying the soot (and
apparatus used to consolidate the soot) with, e.g., Cl.sub.2 or
Cl.sub.2+CO, to remove trace metals. However, as in the case of
producing regular non-OD-doped high purity fused silica, where
desired, it is also possible to dope the OD-doped synthetic silica
glass material with various dopants, such as F, Al, Ti, and the
like, in a direct-to-glass furnace. Where the particles in step (I)
are pre-fabricated, they may have essentially the same composition
or differing compositions (e.g., certain particles comprising
dopants and particles essentially free of dopants can be mixed and
provided in step (I)).
[0149] The consolidated glass produced in step (II) may be further
subjected to the following step:
[0150] (III) treating the consolidated glass obtained in step (II)
in an atmosphere comprising H.sub.2 and/or HD and/or D.sub.2.
[0151] The purpose of step (III) is to adjust the level of
molecular hydrogen (H.sub.2, HD and/or D.sub.2) in the consolidated
glass to a desired level. Hydrogen molecules doped at desired level
in the glass can improve the optical performance of the material.
Such hydrogen-treatment is desired to be conducted below about
600.degree. C. In certain cases it may be desired to be conducted
at above about 600.degree. C. Generally, it is desired that it is
carried out at below about 1000.degree. C. Generally, it is desired
that the treatment time and temperature of step (III) is chosen
such that the sum total of the concentration of H.sub.2, HD and
D.sub.2 in the treated glass is between about 0.5.times.10.sup.15
to about 5.times.10.sup.19 molecules/cm.sup.3, in certain
embodiments preferably from about 0.5.times.10.sup.15 to about
5.times.10.sup.19 molecules/cm.sup.3, in certain other embodiments
preferably from about 1.times.10.sup.15 to about 1.times.10.sup.18
molecules/cm.sup.3 in certain embodiments preferably from about
0.5.times.10.sup.16 to about 5.times.10.sup.18 molecules/cm.sup.3,
in certain other embodiments preferably from about
1.times.10.sup.16 to about 1.times.10.sup.18 molecules/cm.sup.3. In
certain embodiments, it is desired that the atmosphere in which
step (III) is D-containing, i.e., the atmosphere has a ratio of
(2n(D.sub.2)+n(HD))/2(n(H.sub.2)+n(D.sub.2)+n(HD)) higher than the
natural isotopic abundance of D. It is also desired that after step
(III), the ratio of [D.sub.2]/[H.sub.2] at different locations in
the glass is essentially constant, i.e., the distribution profiles
of D.sub.2 and H.sub.2 are essentially the same (though maybe at
differing concentrations). However, to lower the cost, it may be
desired that in step (III), the atmosphere is essentially
non-D-containing, i.e., the atmosphere has a ratio of
(2n(H.sub.2)+n(HD))/2(n(H.sub.2)+n(D.sub.2)+n(HD)) higher than or
about equal to the natural isotopic abundance of H.
[0152] Another method of the present invention for making OD-doped
synthetic silica glass of the present invention, termed
"particle-to-glass" herein, involves the formation of a porous
particle preform. This method comprises the following steps:
[0153] (A) providing a particle preform comprising a plurality of
particles comprising silica;
[0154] (B) optionally purifying and/or drying the particle
preform;
[0155] (C) optionally further doping the particle preform with
dopants;
[0156] (D) consolidating the particle preform at an elevated
temperature to dense glass; and
[0157] (E) optionally treating the consolidated glass obtained in
step (P) in the presence of H.sub.2, HD and/or D.sub.2,
[0158] wherein in at least one of steps (A), (B), (C), (D) and (E),
OD is introduced into or formed in the glass. Generally, it is
preferred that in at least one of steps (A), (B), (C) and (D), OD
is introduced into the glass.
[0159] In one embodiment of this process, step (A) comprises the
following steps:
[0160] (A1) providing a plurality of particles; and
[0161] (A2) depositing the particles on a supporting surface to
form the particle preform. The supporting surface is preferred to
be rotating in certain embodiments.
[0162] In step (A1), the particles may be provided by (A1.1) flame
hydrolysis of at least one silicon-containing precursor compound
(such as silicon halides (e.g., SiCl.sub.4) or organosilicon
compounds. As non-limiting example of organosilicon compound,
mention may be made of octamethylcyclotetrasiloxane (OMCTS)), which
may be plasma-assisted; or (A1.2) a soot dispenser, which may be
plasma-assisted; or (A1.3) other plasma-assisted process. In the
present application, the particle-to-glass process involving step
(A1.1) is termed "soot-to-glass" process. Soot-to-glass process for
making regular non-OP-doped high-purity fused silica glass is
described in, for example, co-pending, co-assigned patent
application Ser. No. 11/148,764, entitled "HIGH REFRACTIVE INDEX
HOMOGENEITY FUSED SILICA GLASS AND METHOD OF MAKING SAME" and filed
on Jun. 8, 2005, now published as United States Patent Application
Publication No. 2006-0137398 A1, the relevant part of which is
incorporated herein by reference.
[0163] Particles provided by step (A1.1) may be OD-doped or
non-OD-doped. Where D-containing compounds are used in the flame
hydrolysis process, the particles provided are usually OD-doped. If
the atmosphere of the flame hydrolysis process of step (A1.1)
comprises D.sub.2O, the particles thus provided are usually
OD-doped.
[0164] Step (A2) can be carried out by various methods such as
(A2.1) outside vapor deposition; (A2.2) inside vapor deposition;
(A2.3) vapor axial deposition; (A2.4) planar deposition, and the
like. There is abundance literature describing these methods for
making regular non-OD-doped glass comprising silica, which can be
adapted for making the OD-doped synthetic silica glass of the
present invention.
[0165] The sol-gel process may be employed in step (A) to produce
the particle preform, which comprises the following steps:
[0166] (A(i)) forming a sol-gel comprising silica; and
[0167] (A(ii)) forming the particle preform from the sol-gel.
[0168] Step (A(i)) may be carried out in the presence of or from at
least one D-containing compound. In particular, step (A(i)) may be
carried out in the presence of D.sub.2O. For example, the sol-gel
can be produced by the hydrolysis of a silicon-containing precursor
compound (such as siloxane) in liquid D.sub.2O. Thus the particle
preform produced in step (A) comprises OD-doped silica particles.
There is abundant literature describing methods for making
non-OD-doped glass comprising silica via the sol-gel process, which
can be adapted for making the OD-doped synthetic silica glass of
the present invention. Typically, the sol-gel process includes a
step of hydrolysis of a silicon-containing precursor compound (such
as a silane, siloxane, or polysiloxane) in an aqueous media to
produce a sol-gel of silica. The sol-gel can then be cast into a
green body, which is a form of the particle preform in the meaning
of the present application. The green body may be partially dried
before further processing in subsequent steps (B)-(E).
[0169] Particle preforms produced by flame hydrolysis and sol-gel
processes may comprise undesirably high amount of OH and OD.
Particle preforms produced from sol-gel process may even comprise
substantial amounts of H.sub.2O and/or D.sub.2O. Particle preforms
(typically called soot preforms) produced by flame hydrolysis
methods mentioned above (IVD, OVD, VAD, Pa) involving the burning
of fuels comprising H and/or D (H.sub.2, D.sub.2, CH.sub.4,
CDH.sub.3, and the like, for example) and/or precursor compounds
comprising H and/or D (OMCTS, for example) typically comprise in
the soot particles OH and OD groups. For many applications, such
amounts of OH and/or OD in the preform would lead to undesirably
high level of OH and/or OD in the consolidated glass for the
intended purposes. For example, it is understood by the present
inventor that low OH/OD glass, such as those comprising a total
concentration of OH and OD of less than 500 ppm, in certain
embodiments preferably lower than 300 ppm, in certain embodiments
preferably lower than about 150 ppm, in certain embodiments
preferably lower than about 50 ppm, may be desired for high purity
silica glass for use in optical members used in UV and deep UV
lithography devices.
[0170] For those particle preforms with undesirably high level of
H.sub.2O, D.sub.2O, OH and/or OD, it is desired that before it is
further optionally doped with additional dopants, and before it is
consolidated into dense glass, it is at least dried to lower the OD
and/or OD concentration to a desirable level. In order to control
the final OH and/or OD concentration in the consolidated glass, it
is desirable in many cases that the particle preform is dried to
have a total concentration of OH and/or OD below about 50 ppm by
weight, in certain embodiments preferably below about 10 ppm, in
certain other embodiments preferably below about 1 ppm, in certain
other embodiments preferably below about 0.01 ppm. Where a particle
preform comprises below about 1 ppm by weight of total OH and/or
OD, the particle preform is considered essentially dry for the
purpose of the present application.
[0171] Drying agents such as dry inert gas, including but not
limited to He, Ar, N.sub.2, and the like, may be used to reduce the
H.sub.2O, D.sub.2O, OH and/or OD in the particle preform, at an
elevated temperature, such as higher than 500.degree. C., in
certain embodiments higher than about 800.degree. C. CO, CO.sub.2,
and the like, may be used as drying agent as well. CO may react
with silica particles to produce defects in the glass. Such defects
may be healed as described infra. Preferred drying agents are
F.sub.2, Cl.sub.2, Br.sub.2, halogen compound, CO, CO.sub.2, and
compatible mixtures thereof. The halogen compound is preferably
selected from HX, COX.sub.2, SOX.sub.2, CX.sub.4 and SX.sub.6,
where X is selected from F, Cl, Br and combination thereof. The
most preferred drying agent is Cl.sub.2 and Br.sub.2, without or
including CO and mixtures thereof.
[0172] The particle preform as provided in step (A) may contain
contaminants, especially detrimental metal ions, at unacceptably
high amounts. This is especially true if sol-gel process is used in
producing the particle preforms. Particle preforms produced by
sol-gel processes typically contain high levels of Fe, Na, and the
like, which are detrimental to the optical performance of the glass
in deep UV and vacuum NV spectra. Once the glass is consolidated
and the contaminants are incorporated into the consolidated into
the glass, their removal becomes difficult. Therefore, it is highly
desirable that prior to consolidation, where necessary, the
particle preform is subjected to purification such that contaminant
concentrations are reduced to a desired level prior to
consolidation of the preform.
[0173] Many of the drying agents for removing H.sub.2O, D.sub.2O,
OD and/or OH from the particle preform have contaminant stripping
function as well. Those drying agents, when used in the drying
process, may function to purify the particle preform
simultaneously. Therefore, drying and purifying may advantageously
be carried out at the same time, or if desired, different agents
may be used to achieve these two different functions. Preferred
purifying agents include, but are not limited to, Cl.sub.2,
F.sub.2, Br, a halogen-containing compound, CO, CO.sub.2, and the
like, and mixtures and combinations thereof. The halogen-containing
compound may be HX, COX.sub.2, SOX.sub.2, CX.sub.4 and SX.sub.6,
and the like, where X is selected from F, Cl, Br and combination
thereof. The most preferred drying agent are Cl.sub.2 and Br.sub.2,
with or without CO, and compatible mixtures thereof.
[0174] The particle preform may be further doped in step (C) prior
to consolidation in step (D). It is also generally understood that
doping dopants into consolidated glass is difficult, while doping
particle preforms can be carried out in a controlled manner. Thus,
the particle preform, with or without the drying/purifying step
(B), may be further doped to with dopants such as OD, OH, F, Cl,
and the like. Doping at elevated temperature such as higher than
500.degree. C., in certain embodiments higher than about
800.degree. C., is desirable in order to expedite the doping
process. By controlling the doping temperature, the concentration
of the dopants in the doping atmosphere, and doping time, one can
control the final concentration of the desired dopants in the
particle preform, hence the concentration of the desired dopants in
the final consolidated glass. To dope the particle preform with F,
F-containing compounds such as HF, DF, COF.sub.2, SOF.sub.2,
SiF.sub.4, CF.sub.4 and SF.sub.6 may be used. Therefore, during the
drying and/or purifying step (B), the doping of F may be carried
out. To dope the particle preform with Cl, Cl.sub.2 and
Cl-containing compounds such as HCl, COCl.sub.2, SOCl.sub.2 and
CCl.sub.4 may be used. Therefore, during the drying and/or
purifying step (B), the doping of Cl may be carried out. Thus steps
(B) and (C) may be carried out at least partially
simultaneously.
[0175] For the purpose of the present invention, controlling the
concentration of OH and/or OD in the consolidated glass is highly
desirable for many applications, as mentioned supra. This can be
desirably done in steps (B) and/or (C). For example, in step (B),
the particle preform can be dried and purified to a level
essentially free of OH and/or OD. Subsequently, in step (C), the
dried particle preform is controllably doped with OH and/or OD to a
desirable level so that the final consolidated OD-doped glass has
the desired OD and/or OH concentrations, Doping is desirably
effected at an elevated temperature such as higher than 500.degree.
C., in certain embodiments higher than about 800.degree. C. By
choosing the proper doping time, doping temperature, concentration
of dopants in the doping atmosphere, one can not only control the
final concentrations of OD and/or OH, and other dopants, but also
achieve a homogeneous distribution thereof in the consolidated
glass. To dope the particle preform with OD and/or OH,
OD-containing and/or OH-containing compounds may be used at various
partial pressures in the doping atmosphere. For example, to dope
the particle preforms with OD, the doping atmosphere may comprise
D.sub.2, HD, D.sub.2O, CH.sub.3OD, C.sub.2H.sub.5OD, CH.sub.3COOD,
and other OD-containing compounds. When D.sub.2 and/or HD are
present in the doping atmosphere, they may react with the SiO.sub.2
glass to produce Si--OD and/or Si--OH in the glass. To dope the
particle preforms with OH, the doping atmosphere may comprise
H.sub.2, HD, H.sub.2O, CH.sub.3OH, C.sub.2H.sub.5OH, CH.sub.3COOH,
and other OH-containing compounds. Similarly, when H.sub.2 and/or
HD are present in the doping atmosphere, they may react with the
SiO.sub.2 glass to produce Si--OH and/or Si--OD in the glass. It is
known that reaction between hydrogen gas (D.sub.2, DR and/or
H.sub.2) and SiO.sub.2 can lead to the formation of
oxygen-deficient sites in the silica glass. Thus, as described
infra, it is desired that the particle preform is treated in an
oxidizing atmosphere to heal the defects before or during
consolidation of the particle preform into dense glass if hydrogen
gas is used as a doping agent in the doping atmosphere. If D.sub.2O
and/or H.sub.2O are used as the doping agent in the doping
atmosphere, they may be fed as they are into the doping
environment, or formed in situ by, e.g., reactions between
D.sub.2/H.sub.2 and O.sub.2 fed into the environment separately. To
achieve the desired [OD]/[OH] ratio in the final consolidated
glass, in the doping step (C), the doping atmosphere may be
adjusted to contain the OD-containing and OH-containing compounds
having the desired partial pressures thereof. The most preferred
OD-doping agent for the particle preform is D.sub.2O. D.sub.2O at
higher than 99.9% by mole of isotopic purity is commercially
available. The most preferred OH-doping agent for the particle
preform is H.sub.2O. when doping essentially dry particle preform,
the doping atmosphere may be adjusted to have the desired D.sub.2O
and H.sub.2O partial pressures to obtain the desired [OD] and [OH]
concentration in the final glass. When doing particle preforms
comprising OH at a certain level with OD, the particle preform may
be treated in a doping atmosphere comprising a D-containing
compound, such as an OD-containing compound, such as D.sub.2O, for
a sufficient time, such that a desirable amount of OH in the
particle preform is exchanged by OD. By controlling the partial
pressure ratio of the OD-containing and OH-containing compounds in
the doping atmosphere, doping temperature and doping time, glass
with desired levels of [OD] and [OH] can be obtained in this manner
as well. It is not ruled out that the particle preform may comprise
certain amount of OD before step (C), and in step (C), it is doped
or exchanged with OH only to achieve the desired [OD] and [OH]
concentrations in the final glass. Ratio of n(OD)/(n(OD)+n(OH)) in
the particle preform higher than about 0.02, in certain embodiments
higher than about 0.05, in certain other embodiments higher than
about 0.1, in certain other embodiments higher than about 0.3, in
certain other embodiments higher than about 0.5, in certain other
embodiments higher than about 0.9, in certain other embodiments
higher than about 0.95, can be achieved.
[0176] The doping atmosphere may comprise, in addition to doping
compounds, O.sub.2, and inert gases. Since doping of OH and OD is
usually carried out at elevated temperature such as higher than
about 500.degree. C., in certain embodiments higher than about
800.degree. C., H.sub.2, D.sub.2, HD, O.sub.2, H.sub.2O and
D.sub.2O exist at amounts typically determined by chemical
equilibrium and dynamic factors. Steps (B) and/or (C) may be
carried out in the presence of other reductive gases than H.sub.2
and D.sub.2, such as hydrocarbons, D-containing hydrocarbons, and
the like.
[0177] It is known that when particle preforms comprising silica is
treated in a reductive atmosphere at an elevated temperature such
as in steps (B) and/or (C), oxygen-deficient defects in the glass
may be generated. Such defects are particularly detrimental for
transmission properties in deep UV and vacuum UV, such as at about
248 nm and 193 nm. Therefore, after steps (B) and (C), it is highly
desirable that the particle preform is treated in an oxidative
atmosphere in a step (C(A)). The oxidation agent in the oxidative
atmosphere may be, for example, O.sub.2, O.sub.3, D.sub.2O,
H.sub.2O, and the like.
[0178] In step (D) of the process of the present invention, the
particle preform is consolidated into dense silica glass. Steps (C)
and (D) may be carried out at least partially simultaneously,
meaning that, at least part of the doping is carried out while the
particle preform is consolidated into dense glass. Step (C(A))
described above and step (D) may be carried out at least partly
simultaneously, meaning that, at least in part of step (D), at
least part of the oxygen-deficient defects in the glass is oxidized
and healed. In step (D), the particle preform is heated to an
elevated temperature, desirably higher than 1000.degree. C., in
certain embodiments higher than 1200.degree. C., in certain
embodiments higher than about 1400.degree. C., where the particles
are sintered into dense glass. Temperature elevation rate during
consolidation step (D) may be controlled in a manner such that a
homogeneous distribution of dopants such as OH, OD, F and the like
is achieved. Step (D) may be conducted in a consolidation
atmosphere comprising inert gas such as He, Ar, N.sub.2, and the
like. The consolidation atmosphere may further comprise O.sub.2
and/or D.sub.2O and/or H.sub.2O at a desired level. The O.sub.2,
D.sub.2O and/or H.sub.2O can function to oxidize and heal the
oxygen-deficient sites in the glass. Where high [OD] glass is
desired, the consolidation atmosphere may be essentially devoid of
H.sub.2O and HDO. The consolidation atmosphere may further comprise
H.sub.2, D.sub.2, HD, and the like. However, as mentioned above,
reaction between such reductive gases with silica glass at elevated
temperature can lead to the formation of defects in the glass. It
is believed by the present inventors that glasses with high [OH]
and/or [OD] tend to have less defects than glasses with lower [OH]
and [OD] when treated in a reductive atmosphere at high
temperature, such as an atmosphere comprising H.sub.2, HD and/or
D.sub.2.
[0179] Step (E) of this process of the present invention involves
hydrogen doping the consolidated glass with a hydrogen doping
atmosphere comprising molecular H.sub.2, HD and/or D.sub.2. The
hydrogen doping atmosphere may comprise essentially no D.sub.2 and
HD even for glasses doped with high percentages of OD, especially
if the hydrogen loading temperature is relatively low, such as
below about 500.degree. C. In certain embodiments it is desired
that for glasses doped with high percentage of OD, the hydrogen
doping atmosphere is essentially devoid of HD and H.sub.2 where the
hydrogen doping temperature is higher than 500.degree. C.
Nonetheless, it has been found that where the silica glass is
loaded at a temperature below about 500.degree. C., the loading of
H.sub.2 or D.sub.2 does not appreciably alter the [OH] and [OD] in
the glass. The hydrogen doping may be advantageously carried out at
a temperature below about 600.degree. C. (cold loading), or to
expedite the process, at a temperature above about 600.degree. C.
(hot loading). However, it is usually conducted at a temperature
below 1000.degree. C. Due to the laws of diffusion, to reach the
same loaded hydrogen level in the glass, cold loading tends to take
longer time. Nonetheless, cold loading is preferred for the
production of certain silica glass, especially those with
relatively low water (e.g., [OD]+[OH]<100 ppm) for use in
refractive lens elements in deep UV and vacuum UV lithographic
devices, because it tends to generate less defects in the
consolidated glass.
[0180] As mentioned supra, it was stated in the copending,
co-assigned patent application Ser. No. 11/241,075 (filed on Sep.
30, 2005 and entitled "SYNTHETIC SILICA HAVING LOW
POLARIZATION-INDUCED BIREFRINGENCE, METHOD OF MAKING SAME AND
LITHOGRAPHIC DEVICE COMPRISING SAME," now published as United
States Patent Application Publication No. 2006-0137399 A1, the
relevant part thereof is incorporated herein by reference) that
non-OD-doped silica glass tends to have worse polarization-induced
birefringence performance at higher [OH]. It was also stated in
this patent application that the amount of polarization-induced
birefringence was approximately proportional to the [OH] in an
OH-doped silica glass. Therefore, for the sake of acceptable
polarization-induced birefringence performance, for non-OD-doped
synthetic silica glass, it is generally preferred that it has an
[OH] of less than 500 ppm, in certain embodiments preferably less
than 160 ppm, in certain other embodiments less than 50 ppm.
However, in a totally unexpected manner, the present inventors have
found that [OD]-doped silica glass, especially those essentially
free of OH, tends to have much lower polarization-induced
birefringence value compared to non-OD-doped silica glass with
comparable [OH]. In certain OD-doped glass samples essentially
devoid of OH, the polarization-induced birefringence was discovered
to be essentially zero when exposed to 193 nm pulses of linearly
polarized excimer laser beam at about 200
.mu.Jcm.sup.-2pulse.sup.-1 even after 8 billion pulses. Therefore,
the present inventors expect that the OD-doped high purity
synthetic silica glass of the present invention, especially those
essentially devoid of OH will exhibit very low polarization-induced
birefringence even with a high [OD] up to or exceeding 1000
ppm.
[0181] In another co-pending, co-assigned patent application Ser.
No. 11/261,005 (filed on Oct. 26, 2005 and entitled "SYNTHETIC
SILICA WITH LOW FLUENCE-DEPENDENT-TRANSMISSION AND METHOD OF MAKING
THE SAME," the relevant portion thereof is incorporated herein by
reference), it was found that for non-OD-doped high purity
synthetic silica glass, from the standpoint of
fluence-dependent-transmission ("FDT") and LIWFD, it is preferable
that for those with [OH].ltoreq.160 ppm, H.sub.2 loading should be
conducted at lower than about 600.degree. C. It was shown that hot
loading can cause deterioration in FDT and LIWFD in such OH-doped
silica glass with [OH].ltoreq.160 ppm. Yet, it was also shown that
for those with [OH].gtoreq.500 ppm, hot loading does not alter the
FDT and LIWFD performance more appreciably than cold loading.
[0182] Therefore, the present inventors expect that the OD-doped
synthetic silica glass of the present invention, especially those
with essentially no OH, even with [OD] up to or exceeding 1000 ppm,
can be hot loaded with hydrogen to result in glass with acceptable
polarization-induced birefringence performance and without
compromising the FDT and LIWFD properties. Therefore, high [OD]
silica glass, at least those essentially free of OH, may be useable
in applications where non-OD-doped silica glasses with comparable
[OH] cannot be used. These high [OD] glasses can be produced with
much higher efficiency and speed because it can be hot loaded,
compared to the non-OD-doped, low [OH] glasses, which are typically
required to be cold loaded.
[0183] Another method of making the OD-doped synthetic silica glass
of the present invention includes the following steps:
[0184] (a) providing a plurality of OD-doped particles comprising
silica; and
[0185] (b) melting the particles at an elevated temperature to
obtain a transparent glass.
[0186] Step (a) in this process may comprise the following
steps:
[0187] (a1) generating a plurality of particles comprising
silica;
[0188] (a2) optionally purifying and/or drying the particles;
[0189] (a3) optionally doping the particles in an atmosphere
comprising at least one D-containing compound, and
[0190] (a4) optionally treating the particles in an oxidative
atmosphere to at least partly heal oxygen-deficient sites in the
particles.
[0191] wherein at least in one of steps (a1), (a2), (a3) and (a4),
OD moieties are introduced into the particles.
[0192] In step (a1), the particles comprising silica may be
generated by flame hydrolysis or sol-gel processes as described
above in connection with the particle-to-glass process wherein the
particle preforms are finally consolidated instead of melted to
form the glass.
[0193] In step (a2), the purifying and/or drying may be carried out
mutatis mutandis as described above in connection with the
particle-to-glass process wherein the particle preforms are finally
consolidated instead of melted to form the glass. Low level of
metal impurities can be obtained via high purity starting materials
and apparatus for making the soot (and corresponding consolidated
glass) and/or purifying the soot (and apparatus used to consolidate
the soot) with, e.g., Cl.sub.2 or Cl.sub.2+CO, to remove trace
metals.
[0194] In step (a3), the doping may be carried out mutatis mutandis
as described above in connection with the particle-to-glass process
wherein the particle preforms are finally consolidated instead of
melted to form the glass.
[0195] In step (a4), the treatment may be carried out mutatis
mutandis as described above in connection with the
particle-to-glass process wherein the particle preforms are finally
consolidated instead of melted to form the glass.
[0196] In step (b), the glass is heated to a temperature where the
glass is melted, such as at a temperature higher than 1500.degree.
C., in certain embodiments above 1800.degree. C. in certain
embodiments about 2000.degree. C. The melted glass may be further
homogenized when melted in order to obtain a high homogeneity of
composition and property in the final glass. Where homogenization
is carried out, the glass particles melted may have essentially the
same composition or differing compositions. For example, the
particles may be an admixture of particles having differing [OH]
and [OD]. Upon homogenization, the final glass obtained has a
uniform [OH] and/or [OD].
[0197] Homogenization of consolidated glass can be carried out as
well. Thus the consolidated OD-doped synthetic silica glass of the
present invention or mixtures thereof, irrespective of the method
of making, may be heated to an elevated temperature, such as above
1500.degree. C., in certain embodiments above 1800.degree. C.,
where they are melted and homogenized to form a glass with uniform
composition and properties.
[0198] Upon homogenization, the final, cooled, consolidated glass
may be further doped with molecular hydrogen as described above in
connection with the particle-to-glass process wherein the particle
preforms are finally consolidated instead of melted to form the
glass, mutatis mutandis.
[0199] It is possible to make OD-doped silica glass material of the
present invention by MD) exchanging a consolidated, dense silica
glass comprising OH and/or OD in an atmosphere comprising H.sub.2,
D.sub.2 and/or HD at an elevated temperature, e.g., higher than
600.degree. C., in certain embodiments higher than about
800.degree. C., in certain embodiments as high as 1000.degree. C.
to achieve the desired level of n(OD)/(n(OD)+n(OH)) in the dense
glass. The dense glass may be made of direct-to-glass processes, or
may be made of soot-to-glass or sol gel processes mentioned supra.
For a dense glass comprising essentially no OD, such as an OH-doped
glass made of traditional direct-to-glass process in
non-D-containing environment starting from a non-D-containing
materials (e.g., typical Corning HPFS.RTM. glass code 7980.TM.,
made by Corning Incorporated, Corning, N.Y., which comprises about
1000 ppm by weight of OH and essentially no OD), OD-doped glass at
various n(OD)/(n(OD)+n(OH)) can be made by D.sub.2-loading the
glass at an elevated temperature, such as about 900.RTM. C., for a
sufficient period of time. Glasses with very low [OH], such as
n(OD)/(n(OD)+n(OH)) higher than 0.5, in certain embodiments higher
than about 0.8, in certain embodiments higher than about 0.9, can
be successfully made.
[0200] The synthetic silica glass material of the present invention
can be further processed into optical members for use in the light
path of lithographic irradiation of a lithographic device operating
at a wavelength of below about 300 nm, such as about 248 nm, 193 nm
and even shorter. The optical member may take various geometry and
size. The optical member may be used in low-fluence or high-fluence
irradiation paths. Thus a process for making optical member based
on the silica glass of the present invention can be a combination
of the processes of the present invention for making the glass
material and additional steps of processing the glass material of
the present invention. As mentioned supra, whereas OD-doped
synthetic silica glass were studied and disclosed previously, to
the best knowledge of the present inventors, none of the references
discloses OD-doped synthetic silica glass capable of being used in
optical members in the irradiation light path of lithographic
devices operating at below about 300 nm, much less OD-doped
synthetic silica glass having the unexpected optical performances
at wavelengths such as about 193 nm. It is believed that the
example materials disclosed in the prior art references discussed
supra do not have the optical performance of the material of the
present invention or the optical performance required for
lithographic applications below about 300 nm.
[0201] The following non-limiting examples further illustrate the
present invention.
EXAMPLES
Example 1a
[0202] In this Example, an OD-doped fused silica glass was made by
using the soot-to-glass process as described in co-pending,
co-assigned patent application Ser. No. 11/148,764, entitled "HIGH
REFRACTIVE INDEX HOMOGENEITY FUSED SILICA GLASS AND METHOD OF
MAKING SAME" and filed on Jun. 8, 2005 now published as United
States Patent Application Publication No. 2006-0137398 A1, the
relevant part of which is incorporated herein by reference. In
particular, silica soot preform was formed by depositing a
plurality of soot particles obtained by flame hydrolysis of a
Si-containing precursor compound, octamethylcyclotetrasiloxane
(OMCTS), on the rotating surface of a mandrel. The soot preform
thus prepared was OH-doped. The soot preform was subsequently D/H
exchanged and OD-doped with 99.9+% isotopic purity D.sub.2O by
placing the preform into a consolidation furnace set at
approximately II 00.degree. C. and bubbling helium containing 2.5%
oxygen through liquid D.sub.2O into the consolidation furnace for 6
hours to obtain the OD-doped soot. The OD-doped soot preform was
then sintered to a consolidated OD-doped silica glass in a helium
atmosphere containing D.sub.2O by raising the furnace temperature
approximately 1400.degree. C. Following consolidation the silica
glass was placed in a nitrogen purged holding oven at about
1100.degree. C. for about 24 hours and cooled at less than a
25.degree. C./hr to 850.degree. C. then cooled to room temperature
(this silica glass was used for Samples C, D and F in TABLE I).
Deuteroxyl-doping was successful, wherein the consolidated glass
comprised about 130 ppm by weight of OD and less than 1 ppm of OH.
[OD] and [OH] along the radial direction of the glass was measured
and presented in FIG. 6. These samples were found to have less than
10 ppb by weight of sodium, less than 10 ppb by weight in total of
all alkali metals, less than 10 ppb by weight of alkaline earth
metals, and less than 1 ppb by weight of Fe, Cr and Ni.
Example 1b
[0203] In this Example, an OD-doped fused silica glass was made by
using the soot-to-glass process as described in Example 1a. The
soot preform thus prepared was OH-doped. The soot preform was
subsequently D/H exchanged and OD-doped with 99.9+% isotopic purity
D.sub.2O by placing the preform into a consolidation furnace set at
approximately 1100.degree. C. and bubbling helium containing 2.5%
oxygen through liquid D.sub.2O into the consolidation furnace for 6
hours to obtain OD-doped soot, The OD-doped soot preform was then
sintered to a consolidated OD-doped silica glass in a helium
atmosphere containing D.sub.2O by raising the furnace temperature
approximately 1400.degree. C. Following consolidation the silica
was placed in a nitrogen purged holding oven at 1100.degree. C. for
24 hours and cooled at less than 25.degree. C./hour to 850.degree.
C. then cooled to room temperature (this silica was used for sample
H shown in TABLE I). Another sample was then placed in a nitrogen
purged holding oven ramped to 1100.degree. C. then cooled at less
than 1.degree. C./hour to 800.degree. C. then cooled at less than
25.degree. C./hour to room temperature (this silica was used for
sample G shown in TABLE I). Deuteroxyl-doping was successful,
wherein the consolidated glass comprised about 70 ppm by weight of
OD and less than 1 ppm of OH. [OD] and [OH] along the radial
direction of the glass was measured and presented in FIG. 12. The
fictive temperature of these materials was measured to be
1126.degree. C. and 1032.degree. C., respectively, for Samples H
and G. These samples were found to have less than 10 ppb by weight
of sodium, less than 10 ppb by weight in total of all alkali
metals, less than 10 ppb by weight of alkaline earth metals, less
than 1 ppb by weight of Fe, Cr and Ni.
Example 1c
[0204] In this Example, an OD-doped fused silica glass was made by
using the soot-to-glass process as described in Example 1a. The
soot preform thus prepared was OH-doped. The soot preform was
placed in a consolidation furnace set at 1100.degree. C. and
treated for 4 hours flowing helium containing 1.6% by volume
Cl.sub.2 and 0.3% by volume CO; this process was used to remove all
the OH and trace metals from the soot preform. The soot preform was
then exposed at 1100.degree. C. for 8 hours to D.sub.2O and O.sub.2
by flowing helium containing 2.5% by volume O.sub.2 through a
D.sub.2O containing bubbler; this process removed all of the
chlorine and re-oxidized any reduced silica from the previous step.
This produced OD-doped soot preform. The soot preform was then
sintered to a consolidated OD-doped silica glass in a helium
atmosphere containing D.sub.2O by raising the furnace temperature
approximately 1400.degree. C. Following consolidation the OD-doped
silica was placed in a nitrogen purged holding oven at 1100.degree.
C. for 24 hours and cooled at less than 25.degree. C./hour to
850.degree. C. then cooled to room temperature. Deuteroxyl-doping
was successful, wherein the consolidated glass comprised about 220
ppm by weight of OD and about 8 ppm of OH. [OD] and [OH] along the
radial direction of the glass was measured and presented in FIG.
17. Samples of this OD-doped silica glass were post-loaded with
H.sub.2 to about 3.times.10.sup.16 molecules/cm.sup.3 at
425.degree. C. The fictive temperature of this material was
measured to be 1085.degree. C. This sample had an internal
transmission at 193 nm of 99.66%/cm. This sample had less than 10
ppm by weight of Cl, less than 10 ppb by weight of sodium, less
than 10 ppb by weight of total alkali metals, less than 10 ppb by
weight of total alkaline earth elements, and less than 1 ppb by
weight of iron, chromium or nickel.
Example 2
[0205] In this Example, an OD-doped fused silica glass was made by
using the soot-to-glass process as described in co-pending,
co-assigned patent application Ser. No. 11/148,764 (United States
Patent Application Publication No. 2006-0137398 A1). In particular,
silica soot preform was formed by depositing a plurality of soot
particles obtained by flame hydrolysis of a Si-containing precursor
compound on the rotating surface of a mandrel. The soot preform
thus prepared was OH-doped. The soot preform was subsequently
partially D/H exchanged and OD-doped with 99.9+% isotopic purity
D.sub.2O by bubbling helium through liquid D.sub.2O into the
consolidation furnace during the consolidation process in a manner
similar to the described in Example 1a. Deuteroxyl-doping was
successful, wherein the consolidated glass comprised about 40-50
ppm by weight of OD and about 10 ppm of OH. [OD] and [OH] along the
radial direction of the glass were measured and presented in FIG.
7. It is interesting to note that at different locations in the
radial direction, both [OD] and [OH] vary, yet the ratio of
[OD]/[OH] remains essentially constant. This indicates that the OH
groups in the soot preform were exchanged to OD groups at
essentially the same proportion.
Example 3
[0206] All samples of OH-doped silica glass and OD-doped silica
glass were annealed in a manner similar to that described in
Example 1b prior to H.sub.2 or D.sub.2 loading and the
corresponding fictive temperature for each sample, measured after
H.sub.2 or D.sub.2 loading, is shown in TABLE I. Samples of the
OD-doped silica glass of Examples 1a, 1b and 1c were post-loaded
with H.sub.2 or D.sub.2 to about 4.times.10.sup.16
molecules/cm.sup.3 at 375.degree. C. OH-doped glasses prepared by
using the soot-to-glass process as described in co-pending,
co-assigned patent application Ser. No. 11/148,764 (United States
Patent Application Publication No. 2006-0137398) were loaded with
H.sub.2 or D.sub.2 to about 4.times.10.sup.16 or 6.times.10.sup.16
molecules/cm.sup.3 at 375.degree. C. FTIR results showed that the
[OH] and [OD] levels in the glasses were not changed by the H.sub.2
or D.sub.2 loading process. The glasses were exposed to linearly
polarized 193 nm ArF excimer laser having a repetition rate of 4
kHz, a fluence of 200 .mu.Jcm.sup.-2pulse.sup.-1 and a pulse length
of 25 ns for multiple millions of pulses. The exposed samples were
then characterized of polarization-induced birefringence,
normalized polarization-induced birefringence (PIB(N)), normalized
LIWFD measured at 193 mm (L193) and normalized LIWFD measured at
633 nm (L633) and normalized induced absorption (IA(N)). Data are
presented in FIGS. 8, 9, 10, 11, 12, 13, 14, 15 and 16,
respectively. Samples A, B, C, D, E, F, G, H, J, K and L were found
to comprise less than 10 ppb by weight of sodium, less than 10 ppb
by weight in total of alkali metals, less than 10 ppb by weight in
total of alkaline earth metals, and less than 1 ppb by weight of
Fe, Cr or Ni. Samples A, B, C, D, F and H had internal transmission
at 193 nm of about 99.78%/cm; Samples E, G, H, K and L were found
to have internal transmission of about 99.74%/cm; and Sample J was
found to have an internal transmission at 193 nm of about
99.70%/cm. Samples A, B, C, D, E, F, G, H, J, K and L have the
compositions as listed in TABLE I below:
TABLE-US-00001 TABLE I [OH] [OD] [H.sub.2] (.times.10.sup.17
[D.sub.2] (.times.10.sup.17 Fluence (mJ cm.sup.-2 Sample (ppm)
(ppm) molecule/cm.sup.3) molecule/cm.sup.3) T.sub.f (.degree. C.)
pulse.sup.-1) A 105 ND 0.4 ND 1056 0.2 B 105 ND ND 0.4 1056 0.2 C
ND 130 0.4 ND 1109 0.2 D ND 130 ND 0.4 1109 0.2 E 60 ND 0.6 ND 1066
0.2 F ND 130 0.4 ND 1109 0.6 G ND 70 0.8 ND 1032 0.6 H ND 69 0.7 ND
1126 0.6 J 57 ND 0.7 ND 1029 0.6 K 56 ND 0.7 ND 1101 0.6 L 60 ND
0.6 ND 1066 0.2 ND: Not detected
[0207] In FIGS. 8 and 13, the horizontal axis represents N(P)F,
where N(P) is the number of pulses in the millions, and F is the
fluence of the linearly polarized 193 m excimer laser pulses in
mJ/(cm.sup.2pulse). These figures clearly show, in a totally
surprising manner, that the OD-doped samples (Samples C, D, F, G
and H) demonstrated much less measured bulk polarization-induced
birefringence (PIB(M) at various N(P)F. Normalized
polarization-induced birefringence values provided in FIGS. 9 and
14 further corroborate the conclusion that OD-doped glass samples
have appreciably lower amount of polarization-induced birefringence
than glasses doped by OH at comparable levels. Data in these
figures clearly show that OD-doped silica glass exhibits superior
performance to OH-doped silica glass in terms of
polarization-induced birefringence. Even more surprisingly, PIB(M)
and PIB(N) data of Sample C measured at over 8 billion pulses shows
that the polarization-induced birefringence essentially did not
change over those at about 2 and 5 billion pulses, in contrast to
the significantly enlarged polarization-induced birefringence value
of the OH-doped Samples A and B.
[0208] Normalized LIWFD data of Samples A, B, C, D and E in FIGS.
10, 11 and 15 show that the LIWFD performances of the OD-doped
OH-free silica glass is unexpectedly better than that of OH-doped
glasses when the concentrations of [OD] and [OH] are comparable. In
addition, it was also found that silica glass samples having [OD]
at equivalent concentration, but lower fictive temperature
(T.sub.f) showed lower LIWFD (thus better performance in terms of
LIWFD).
[0209] As briefly mentioned supra, normalized induced absorption
data (IA(N)) shown in FIG. 16 clearly indicate that when exposed to
the same dosage of linearly-polarized radiation at 193 nm, the
OD-doped silica glasses of the present invention exhibit an
appreciably lower level of induced absorption than OH-doped glass
with [OH] comparable to the [OD] of the OD-doped glasses of the
present invention. This was totally unexpected.
Example 4
[0210] In this Example, an OD-doped fused silica glass was made by
D/H exchanging OH-doped fused silica glass. The OH-doped fused
silica glass used was Corning Incorporated glass code 7980 which
was made by using the direct-to-glass process as described U.S.
Pat. No. 6,698,248 B2. The glass tested contained approximately
1000 ppm OH and had less than 10 ppb sodium, less than 10 ppb total
alkali, less than 10 ppb total alkaline earth elements, and less
than 1 ppb iron, chromium or nickel. D/H was accomplished by
placing 10 mm.times.25 mm.times.200 mm samples in a clean quartz
muffle and heating the samples to 900.degree. C. for 30 days in
5.4% D.sub.2 in N.sub.2 bubbled through 99.9+% D.sub.2O in order to
produce an OD-doped fused silica glass containing approximately
1000 ppm by weight of OD, less than 20 ppm by weight of OH and no
additional metal contamination.
[0211] This example shows that OD-doped silica glass of the present
invention can be made by D/H exchange of OH-doped silica glass.
[0212] It will be apparent to those skilled in the art that various
modifications and alterations can be made to the present invention
without departing from the scope and spirit 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.
* * * * *