U.S. patent application number 15/529583 was filed with the patent office on 2017-12-21 for method of making halogen doped optical element.
The applicant listed for this patent is Corning Incorporated. Invention is credited to Steven Bruce Dawes, Douglas Hull Jennings, Pushkar Tandon.
Application Number | 20170362115 15/529583 |
Document ID | / |
Family ID | 54782870 |
Filed Date | 2017-12-21 |
United States Patent
Application |
20170362115 |
Kind Code |
A1 |
Dawes; Steven Bruce ; et
al. |
December 21, 2017 |
METHOD OF MAKING HALOGEN DOPED OPTICAL ELEMENT
Abstract
A method of forming an optical element is provided. The method
includes producing silica-based soot particles using chemical vapor
deposition, the silica-based soot particles having an average
particle size of between about 0.05 .mu.m and about 0.25 .mu.m. The
method also includes forming a soot compact from the silica-based
soot particles and doping the soot compact with a halogen in a
closed system by contacting the silica-based soot compact with a
halogencontaining gas in the closed system at a temperature of less
than about 1200.degree. C.
Inventors: |
Dawes; Steven Bruce;
(Corning, NY) ; Jennings; Douglas Hull; (Corning,
NY) ; Tandon; Pushkar; (Painted Post, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
|
|
Family ID: |
54782870 |
Appl. No.: |
15/529583 |
Filed: |
November 24, 2015 |
PCT Filed: |
November 24, 2015 |
PCT NO: |
PCT/US15/62472 |
371 Date: |
May 25, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62084846 |
Nov 26, 2014 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C03B 37/01453 20130101;
C03C 3/06 20130101; C03B 19/066 20130101; C03B 37/01853 20130101;
C03B 19/1453 20130101; C03B 2201/075 20130101; C03B 37/014
20130101; C03B 2201/42 20130101; C03B 19/1461 20130101; C03B
37/01446 20130101; C03B 25/02 20130101; C03B 2201/20 20130101; G02B
6/036 20130101; C03B 37/01282 20130101; C03B 2201/12 20130101 |
International
Class: |
C03C 3/06 20060101
C03C003/06; C03B 37/014 20060101 C03B037/014; C03B 25/02 20060101
C03B025/02; C03B 19/14 20060101 C03B019/14; C03B 37/012 20060101
C03B037/012; G02B 6/036 20060101 G02B006/036; C03B 37/018 20060101
C03B037/018 |
Claims
1. A method of forming an optical element, the method comprising:
producing silica-based soot particles using chemical vapor
deposition, the silica-based soot particles having an average
particle size of between about 0.05 .mu.m and about 0.25 .mu.m;
forming a soot compact from the silica-based soot particles; and
doping the soot compact with a halogen in a closed system by
contacting the silica-based soot compact with a halogen-containing
gas in the closed system at a temperature of less than about
1200.degree. C.
2. The method of claim 1, wherein forming a soot compact comprises
depositing the silica-based soot particles onto a bait rod.
3. The method of claim 1, wherein forming a soot compact comprises
pressing the silica-based soot particles at a pressure of between
about 100 psi and about 1000 psi.
4. The method of claim 3, wherein pressing the silica-based soot
particles comprises axially pressing the silica-based soot
particles to form a disc shaped soot compact.
5. The method of claim 1, further comprising maintaining a
predetermined halogen-containing gas composition in the closed
system.
6. The method of claim 5, wherein maintaining a predetermined
halogen-containing gas composition in the closed system comprises
bleeding halogen-containing gas into the closed system.
7. The method of claim 1, wherein doping the soot compact comprises
maintaining a halogen composition in the closed system of greater
than about 90% of the total gas composition of the closed
system.
8. The method of claim 1, further comprising maintaining a
predetermined halogen-containing gas partial pressure in the closed
system.
9. The method of claim 8, wherein the predetermined
halogen-containing gas partial pressure in the closed system is
between about 0.10 atm and about 0.90 atm.
10. The method of claim 1, wherein the halogen-containing gas is a
fluorine-containing gas.
11. The method of claim 1, wherein the halogen-containing gas is a
chlorine-containing gas.
12. The method of claim 1, further comprising consolidating the
soot compact in the closed system to form a glass article.
13. The method of claim 12, wherein consolidating the soot compact
is sufficient to form a glass article comprising a variation of
halogen concentration of less than about 0.20 wt. %.
14. The method of claim 1, wherein the silica-based soot particles
comprise silica and titania.
15. The method of claim 1, wherein the silica-based soot particles
have a surface area of greater than about 10 m.sup.2/gram.
16. A method of forming an optical element, the method comprising:
producing silica-based soot particles using chemical vapor
deposition, the silica-based soot particles having an average
particle size of between about 0.05 .mu.m and about 0.25 .mu.m;
forming a soot compact from the silica-based soot particles; doping
the soot compact with a halogen in a closed system by contacting
the soot compact with a halogen-containing gas in the closed system
at a temperature of less than about 1200.degree. C.; and
consolidating the soot compact in the closed system to form a glass
article by simultaneously increasing the temperature in the closed
system and decreasing the concentration of the halogen-containing
gas in the closed system.
17. The method of claim 16, wherein forming a soot compact
comprises depositing the silica-based soot particles onto a bait
rod.
18. The method of claim 16, wherein forming a soot compact
comprises pressing the silica-based soot particles at a pressure of
between about 100 psi and about 1000 psi.
19. The method of claim 18, wherein pressing the silica-based soot
particles comprises axially pressing the silica-based soot
particles to form a disc shaped soot compact.
20. The method of claim 16, wherein decreasing the concentration of
the halogen-containing gas in the closed system comprises
decreasing the concentration of the halogen-containing gas
according to the following equation: y II = y I , dop Exp [ - 21741
( ( 1 T I , dop ) - ( 1 T II ) ) ] , ##EQU00002## wherein:
T.sub.I,dop is the temperature in the closed system when doping the
soot compact with halogen; T.sub.II is the temperature in the
closed system when consolidating the soot compact; y.sub.I,dop is
the concentration in mole fraction of the halogen-containing gas in
the closed system when doping the soot compact with halogen; and
y.sub.II is the maximum concentration in mole fraction of the
halogen-containing gas in the closed system when consolidating the
soot compact at T.sub.II.
21. The method of claim 16, further comprising annealing the glass
article by cooling the glass article in the closed system at a
temperature of between about 900.degree. C. and about 1100.degree.
C. and maintaining the temperature between about 900.degree. C. and
about 1100.degree. C. for less than about 10 hours.
22. The method of claim 21, wherein annealing the glass article is
sufficient to form a glass article having a fictive temperature of
less than about 1100.degree. C.
23. The method of claim 16, further comprising decreasing the
temperature of the closed system to between about 700.degree. C.
and about 850.degree. C. at a rate of less than about 10.degree. C.
per hour.
24. The method of claim 16, wherein doping the soot compact further
comprises maintaining a predetermined halogen-containing gas
composition in the closed system.
25. The method of claim 16, wherein doping the soot compact
comprises maintaining a halogen composition in the closed system of
greater than about 90% of the total gas composition of the closed
system.
26. The method of claim 16, wherein doping the soot compact further
comprises maintaining a predetermined halogen-containing gas
partial pressure in the closed system.
27. The method of claim 26, wherein the predetermined
halogen-containing gas partial pressure in the closed system is
between about 0.10 atm and about 0.90 atm.
28. The method of claim 16, wherein the halogen-containing gas is a
fluorine-containing gas.
29. The method of claim 16, wherein the halogen-containing gas is a
chlorine-containing gas.
30. The method of claim 16, wherein consolidating the soot compact
is sufficient to form a glass article comprising a variation of
halogen concentration of less than about 0.20 wt. %.
31. The method of claim 16, wherein the silica-based soot particles
comprise silica and titania.
32. The method of claim 16, wherein the silica-based soot particles
have a surface area of greater than about 10 m.sup.2/gram.
Description
[0001] This application claims the benefit of priority to U.S.
Provisional Application No. 62/084,846 filed on Nov. 26, 2014 the
content of which is incorporated herein by reference in its
entirety.
FIELD
[0002] The present disclosure relates generally to optical
elements, and in particular, to methods for forming optical
elements from doped silica glass articles.
BACKGROUND
[0003] Optics, particularly reflective optics, are an important
part of elements employed in Extreme Ultra-Violet (EUV)
lithography. These elements are used with extreme ultraviolet
radiation to illuminate, project, and reduce pattern images that
are utilized to form integrated circuit patterns. The use of
extreme ultraviolet radiation is beneficial in that smaller
integrated circuit features can be achieved; however, the
manipulation of the radiation in this wavelength range raises
challenges.
[0004] In these and similar applications, low thermal expansion
glass, such as silica-titania glass, is currently being used for
making projection optics. In contrast to other materials, low
thermal expansion glass provides improved polishability, improved
coefficient of thermal expansion (CTE) control, and improved
dimensional stability. However, as the development of these and
similar applications advances, the demand for improved material
characteristics grows.
[0005] Ultra-low expansion (ULE) glasses and EUV lithographic
elements have traditionally been made by chemical vapor deposition
(CVD) processes. In CVD processes, high purity precursors are
injected into flames to form fine particles that are directed
toward the surface of growing glass. In the process, the glass is
formed in layer deposits. One limitation of ULE glass made in
accordance with CVD processes is that the resulting glass contains
striae. Striae are compositional non-uniformities which adversely
affect optical transmission in elements made from the glass. Striae
result from thermal variations in the flame during the formation of
the fine particles, and are also a result of thermal variations of
the growing glass as the fine particles are deposited. Striae
result in alternating thin layers of different CTE and therefore
alternating planes of compression and tension between the layers.
Striae in ULE glass are evident in the direction parallel with the
top and bottom of the glass.
[0006] In some cases, striae have been found to impact surface
finish at an angstrom root mean square (rms) level in reflective
optical elements, which can adversely affect the polishability of
the glass. Polishing glass having striae results in unequal
material removal and unacceptable surface roughness which can
present problems for stringent applications like EUV lithography
elements. For example, it may create a mid-frequency surface
structure that may cause image degradation in mirrors used in the
projection systems for EUV lithography. While attempts have been
made to modify and control aspects of the CVD processes to reduce
striae, the fact that the method forms ULE glass in layer deposits
at least partially contributes to the formation of striae.
[0007] Conventional optical fibers typically have a silica-based
glass core region surrounded by a silica-based glass cladding. Some
optical fibers include a core region that is doped with a dopant,
such as GeO.sub.2, suitable for raising the refractive index of the
core region. Other optical fibers include a pure silica core region
and at least one cladding region that is doped with a dopant, such
as fluorine, suitable for lowering the refractive index of the
doped cladding. The index difference between the core and the doped
cladding is necessary to create a light guide wherein propagating
light is generally confined to the core region. However, the
optical loss, or attenuation, of the optical fiber having Ge-doped
glass in the core region is higher than the attenuation expected in
pure silica glass, and doping the cladding region affects the
viscosity of the cladding glass. A viscosity mismatch between the
core and cladding regions results in the region of the optical
fiber having the higher viscosity bearing more tension during the
process in which an optical fiber is drawn from an optical fiber
preform. The resulting stress may be retained within the optical
fiber as residual stress which may become "frozen" in the fiber
upon cooling from the draw temperature and may contribute to
increased attenuation of the resulting optical fiber.
[0008] Improved material characteristics and/or performance of
these optical elements may be achieved by forming the optical
elements from silica-based glass articles that include dopants
different from, or in addition to, conventional dopants. However,
limitations on doping efficiency and doping levels with these
different, or additional, dopants make it difficult to achieve such
improved material characteristics and/or performance.
SUMMARY
[0009] According to an embodiment of the present disclosure, a
method of forming an optical element is provided. The method
includes producing silica-based soot particles using chemical vapor
deposition, the silica-based soot particles having an average
particle size of between about 0.05 .mu.m and about 0.25 .mu.m. The
method also includes forming a soot compact from the silica-based
soot particles and doping the soot compact with a halogen in a
closed system by contacting the silica-based soot compact with a
halogen-containing gas in the closed system at a temperature of
less than about 1200.degree. C.
[0010] According to another embodiment of the present disclosure, a
method of forming an optical element is provided. The method
includes producing silica-based soot particles comprising silica
and titania using chemical vapor deposition, the silica-based soot
particles having an average particle size of between about 0.05
.mu.m and about 0.25 .mu.m. The method also includes forming a soot
compact from the silica-based soot particles and doping the soot
compact with a halogen in a closed system by contacting the
silica-based soot compact with a halogen-containing gas in the
closed system at a temperature of less than about 1200.degree. C.
The method further includes consolidating the soot compact in the
closed system to form a glass article by simultaneously increasing
the temperature in the closed system and decreasing the
concentration of the halogen-containing gas in the closed
system.
[0011] Additional features and advantages will be set forth in the
detailed description which follows, and in part will be readily
apparent to those skilled in the art from that description or
recognized by practicing the embodiments as described herein,
including the detailed description which follows, the claims, as
well as the appended drawings.
[0012] It is to be understood that both the foregoing general
description and the following detailed description are merely
exemplary, and are intended to provide an overview or framework to
understanding the nature and character of the claims. The
accompanying drawings are included to provide a further
understanding, and are incorporated in and constitute a part of
this specification. The drawings illustrate one or more
embodiment(s), and together with the description serve to explain
principles and operation of the various embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The disclosure will be understood more clearly from the
following description and from the accompanying figures, given
purely by way of non-limiting example, in which:
[0014] FIG. 1 is a top view illustrating a mold assembly in
accordance with embodiments of the present disclosure;
[0015] FIG. 2 is a schematic view of a closed system in accordance
with embodiments of the present disclosure;
[0016] FIG. 3 is a graph depicting dopant concentration vs.
distance from the outer surface of a glass article in accordance
with embodiments of the present disclosure;
[0017] FIG. 4 is a graph depicting dopant concentration vs.
distance from the outer surface of a glass article in accordance
with embodiments of the present disclosure;
[0018] FIG. 5 is a graph depicting dopant concentration vs.
distance from the outer surface of a glass article in accordance
with embodiments of the present disclosure; and
[0019] FIG. 6 is a schematic depiction of soot preform deposition
via an OVD process.
DETAILED DESCRIPTION
[0020] Reference will now be made in detail to the present
embodiment(s), an example(s) of which is/are illustrated in the
accompanying drawings. Whenever possible, the same reference
numerals will be used throughout the drawings to refer to the same
or like parts.
[0021] The singular forms "a," "an" and "the" include plural
referents unless the context clearly dictates otherwise. The
endpoints of all ranges reciting the same characteristic are
independently combinable and inclusive of the recited endpoint. All
references are incorporated herein by reference.
[0022] The present disclosure is described below, at first
generally, then in detail on the basis of several exemplary
embodiments. The features shown in combination with one another in
the individual exemplary embodiments do not all have to be
realized. In particular, individual features may also be omitted or
combined in some other way with other features shown of the same
exemplary embodiment or else of other exemplary embodiments.
[0023] Optical elements and methods of forming such optical
elements are provided herein. As used herein, the terms "optic" and
"optical element" denote a transparent glass article that can be
formed into a reflective or transmissive element that is intended
to be used to reflect, transmit or guide light. The methods
described herein facilitate forming large and uniform glass optical
elements. The methods also provide for efficient use of dopant
material during the formation of the optical elements.
[0024] The methods facilitate forming optical elements capable of
use in photolithography that are large, uniform and highly
polishable. Such elements may have a near-zero thermal expansion
over a wide operational temperature range, such as between about
20.degree. C. and about 30.degree. C., and may have a slope of CTE
versus temperature at 20.degree. C. of less than about 1.0
ppb/K.sup.2. A material that has near-zero thermal expansion is one
that undergoes little or no dimensional change in response to
changing temperature. As compared to optical elements capable of
use in photolithography formed using a CVD process, elements made
in accordance with the methods disclosed herein include lower
variations in titania and halogen composition compared to the local
average titania and halogen levels.
[0025] The methods described herein also facilitate forming optical
elements capable of use as optical fibers. Such elements include a
silica-based glass core region which may have a chlorine content of
greater than about 1.5 wt. %. The core region is surrounded by at
least one silica-based glass cladding region which may include pure
silica or which may include silica doped with a dopant, such as
fluorine, suitable for lowering the refractive index of the
cladding. Where the optical fiber includes more than one cladding
region, at least one of the cladding regions may include silica
doped with a dopant such as fluorine. As compared to optical fibers
which include a core region that is doped with GeO.sub.2, the
optical fibers described herein have a lower attenuation.
[0026] The method may include producing silica-based soot
particles. Silica-based soot particles as described herein may be a
by-product of high purity fused silica glass making processes which
may include, but are not limited to, conventional CVD processes for
making optical fiber preforms, such as outside vapor deposition
(OVD) and vapor axial deposition (VAD) processes. Silica-based soot
particles may also be collected from silica soot generation systems
in which the silica-based soot particles are collected in a loose
state. By "loose state" it is meant that the particles are not
contacted with a collecting surface prior to being cooled and are
not contacted with a collecting surface prior to substantially all
of the precursor material being consumed by the flame. The
silica-based soot particles may have an average particle size of
between about 0.05 .mu.m and about 0.25 .mu.m. The particulate
surface area of the silica-based soot particles may be greater than
about 10 m.sup.2/g, or greater than about 15 m.sup.2/g, or greater
than about 20 m.sup.2/g, or greater than about 50 m.sup.2/g, or
even greater than about 100 m.sup.2/g.
[0027] In addition to silica, the silica-based soot particles may
include between about 1.0 wt. % and about 14 wt. % titania, or
between about 5.0 wt. % and about 10 wt. %. titania. The
silica-based soot particles may include about 8.0 wt. % titania.
The silica-based soot particles may also include between about 1.0
wt. % and about 10 wt. % of one or more additives, or between about
1.0 wt. % and about 6.0 wt. % of one or more additives. The one or
more additives may include, but are not limited to, boron
containing compounds, fluorine containing compounds, chlorine
containing compounds, phosphorous containing compounds and mixtures
thereof. Where titania is included, the silica-based soot particles
may have a binary composition of silica and titania or may have a
ternary composition of silica, titania and an additive. The
silica-based soot particles may also have a composition of silica,
titania and a plurality of additives.
[0028] The method may also include forming a soot compact by
pressing silica-based soot particles to form the soot compact. As
is shown in FIG. 1, a mold assembly 10 having a mold cavity 12 may
be filled with the silica-based soot particles. The mold cavity 12
may be of any shape, such as, but not limited to, round or
elliptical, and may be selected based on a predetermined shape of
the soot compact. The mold assembly 10 may have the same shape as
the mold cavity 12. Alternatively, the mold assembly 10 may have a
different shape than the mold cavity 12. For example, if the mold
cavity 12 has an elliptical shape, the mold assembly 10 may also
have an elliptical shape. Or, as shown in FIG. 1, the mold assembly
10 may instead be rectangular. The mold cavity 12 may be filled
with the silica-based soot particles. The design of the mold
assembly 10 should be suitable to resist deformation in response to
pressure exerted by the silica-based soot particles as the
particles are pressed to form a soot compact. Embodiments of the
present disclosure facilitate formation of large soot compacts,
from which large optical elements, such as those used in EUV
lithography, may be formed or from which large optical fiber
preforms may be formed.
[0029] A pressing mechanism (not shown), such as a hydraulic press,
may have a pressing plate that may be brought into contact with the
silica-based soot particles in the mold cavity 12. The pressing
plate may be shaped to enter the mold cavity 12 to press the
silica-based soot particles without contacting the walls of the
mold cavity 12. To prevent contact with the walls of the mold
cavity 12, the pressing plate may be sized such that a gap exists
between the outside edge of the pressing plate and the walls of the
mold cavity 12. For example, where both the mold cavity 12 and the
pressing plate are elliptical in shape, a gap between the walls of
the mold cavity 12 and the pressing plate may exist around the
entire circumference of the pressing plate. The gap may be
approximately equal at all points around the outside edge of the
pressing plate, and may be less than about 0.10 inches, or between
about 0.005 inches and about 0.10 inches, or even between about
0.005 inches and about 0.06 inches. In addition to preventing
contact of the pressing plate with the walls of the mold cavity 12,
the gap may also serve as a passage for the escape of gas while
pressing the silica-based soot particles to form the soot
compact.
[0030] Once in contact with the silica-based soot particles, the
pressing plate may be moved at a rate of less than about 10 mm per
second. For example, the pressing plate may be moved at a rate of
between about 1.0 mm per second and about 10 mm per second, or
between about 1.0 mm per second and about 5.0 mm per second, or at
a rate of about 3.0 mm per second. The pressing mechanism may
assert a pressure on the silica-based soot particles of less than
about 1000 psi, or less that about 250 psi, or even less than about
200 psi. For example, the pressing mechanism may assert a pressure
on the silica-based soot particles of between about 100 psi and
about 1000 psi, or even between about 100 psi and about 500 psi.
Pressure may be applied in an axial direction to form a disc shaped
soot compact.
[0031] The mold cavity 12 may be of any volume, and may be large
enough to form a soot compact having a mass of greater than about
20 kg, greater than about 30 kg, or even greater than about 120 kg
or more. The volume of the mold cavity 12 may be determined based
on the size of the soot compact and the size of the intended
optical element. Depending on the predetermined size of the soot
compact, the mold cavity 12 should be large enough to accommodate
an unpressed volume of loose silica-based soot particles. For
example, in some cases, the volume of the mold cavity 12 may be
about three times to about six times the volume of the soot
compact.
[0032] Alternatively, the mold assembly 10 may further include a
subassembly detachably connected to the mold assembly 10 to provide
additional volume to accommodate loose silica-based soot particles
prior to pressing the silica-based soot particles to form the soot
compact. The subassembly may include a cavity having a top opening
and a bottom opening, the top and bottom openings having the same
shape as the mold cavity 12. When attached to the mold assembly 10,
the subassembly cavity may be aligned with the mold cavity 12 to
extend the volume of the mold cavity 12. In such a design, the mold
cavity 12 may have a volume approximately equal to the
predetermined size of the soot compact, and the combination of the
volume of the mold cavity 12 and the volume of the subassembly
cavity may be about three times to about six times the volume of
the soot compact.
[0033] The pressing mechanism may include an ultrasonic gauge used
to determine the density of the soot compact. Once the soot compact
is pressed to a predetermined density, as measured by the
ultrasonic gauge, movement of the pressing plate may be stopped.
The soot compact may be pressed to a density of between about 0.50
g/cc and about 1.20 g/cc, or between about 0.70 g/cc and about 1.10
g/cc, or even between about 0.80 g/cc and about 1.00 g/cc. Once the
soot compact is pressed to a predetermined density, the pressing
plate may then be released by moving the pressing plate out of
contact with, and away from, the soot compact. Alternatively, the
pressing plate may not be released and the soot compact may be
maintained under pressure for a period long enough to allow the
soot compact to relax into a compressed state. The period to allow
the soot compact to relax into a compressed state may be, for
example, greater than about 10 minutes, or between about 10 minutes
and about 48 hours. The period to allow the soot compact to relax
into a compressed state may be, for example, about 5.0 hours.
[0034] Pressing the silica-based soot particles to form the soot
compact may not require the addition of heat and may be performed
at room temperature. Also, pressing the silica-based soot particles
to form the soot compact may not include intentionally adding a
binder or liquid, such as water, to the silica soot particles.
[0035] Once the soot compact is formed, the soot compact may be
removed from the mold cavity 12. The soot compact may be heated to
a temperature above about 700.degree. C. to induce a small amount
of shrinkage of the soot compact, which may permit movement of the
soot compact out of the mold cavity 12. Alternatively, the mold may
be a segmented assembly, and removal of the soot compact may
include disassembling the mold.
[0036] As an alternative to pressing silica-based soot particles to
form a soot compact, the method may include forming a soot compact
using any of flame combustion methods, flame oxidation methods,
flame hydrolysis methods, OVD, IVD (inside vapor deposition), VAD,
double crucible method, rod-in-tube procedures, cane-in-soot
method, and doped deposited silica processes. In such methods,
silica-based soot particles are produced by combusting a silica
precursor in a flame and depositing the silica-based soot particles
on a rotating bait rod. The silica precursors may be, for example,
OMCTS (octamethylcyclotetrasiloxane) or SiCl.sub.4.
[0037] By way of example and not intended to be limiting, formation
of a soot preform according to an OVD method is illustrated in FIG.
6. As shown, soot preform 20 is formed by depositing silica-based
soot particles 22 onto the outer surface of a rotating and
translating bait rod 24. Bait rod 24 may be tapered. The
silica-based soot particles 22 are formed by providing a glass/soot
precursor 28 in gaseous form to a flame 30 of a burner 26 to
oxidize the precursor 28. Fuel 32, such as methane (CH.sub.4), and
combustion supporting gas 34, such as oxygen, are provided to the
burner 26 and ignited to form the flame 30. Mass flow controllers,
labeled V, meter the appropriate amounts of glass/soot precursor
28, fuel 32 and combustion supporting gas 34, all preferably in
gaseous form, to the burner 26. The glass/soot precursor 28 is a
glass former compound and is oxidized in the flame 30 to form the
generally cylindrically-shaped soot region 23, which may correspond
to the core of an optical fiber preform.
[0038] The method may also include doping the soot compact which
may include contacting the soot compact with a dopant containing
gas. The dopant containing gas may include, but is not limited to,
a halogen-containing gas such as a fluorine-containing gas, a
chlorine-containing gas or a bromine-containing gas. The
fluorine-containing gas may be, but is not limited to, F.sub.2,
C.sub.2F.sub.6, CF.sub.4, SF.sub.6 and SiF.sub.4, and combinations
thereof. The chlorine-containing gas may be, but is not limited to,
SiCl.sub.4, Cl.sub.2 and POCl.sub.3. The bromine-containing gas may
be, but is not limited to, SiBr.sub.4.
[0039] As shown in FIG. 2, the soot compact may be doped in a
closed system 100. As used herein, the term "closed system" denotes
a system which can enclose the entire soot compact and which limits
gases within the closed system from flowing out of the system, and
which limits ambient air from flowing into the system. The closed
system 100 as shown in FIG. 2 is a sealed reaction chamber 112
having an interior 114 of the sealed reaction chamber 112. The
sealed reaction chamber 112 may be, for example, a furnace in which
later heat treatment of the soot compact may be performed. The
temperature in the reaction chamber 112 during doping may be less
than about 1200.degree. C. For example, the temperature in the
reaction chamber 112 during doping may be between about 300.degree.
C. and about 1200.degree. C., or between about 850.degree. C. and
about 1100.degree. C. Doping at a temperature of less than about
1200.degree. C. is believed to promote uniform halogen doping of
the soot compact, particularly in large soot compacts such as those
used to form optical elements for EUV lithography and optical fiber
preforms. A heating device 116 partially or fully surrounds a
portion of the sealed reaction chamber 112. The heating device 116
may be, for example, an electrical coil that, in combination with a
susceptor, forms an inductive heater. Alternatively, the heating
device 116 may be an electrical resistance heater or any other
suitable heating device that can provide sufficient heat to perform
the method as described herein.
[0040] The closed system 100 may include a first gas source 128 in
fluid connection with the reaction chamber 112 through a first
inlet 120 from which a dopant containing gas may be introduced into
the reaction chamber 112. The closed system may also include an
outlet 122. A fluid control system may also be associated with the
closed system 100 and may control the flow of gas in the closed
system 100. The fluid control system may monitor the gas
composition in the closed system 100 and maintain a predetermined
dopant gas composition in the closed system 100. The predetermined
dopant gas composition in the closed system 100 may be greater than
about 90%, or greater than about 95%, or even greater than about
98% of the total gas composition of the closed system 100.
Additionally, or in the alternative, the fluid control system may
monitor the dopant gas partial pressure in the closed system 100,
and maintain a predetermined dopant gas partial pressure in the
closed system 100. The predetermined dopant gas partial pressure in
the closed system 100 may be less than about 0.90 atm, or less than
about 0.50 atm. The predetermined dopant gas partial pressure in
the closed system 100 may be between about 0.10 atm and about 0.50
atm.
[0041] The closed system 100 may include a second gas source 130 in
fluid connection with the reaction chamber 112 through a second
inlet 124. Between the second gas source 130 and the second inlet
124, a one-way valve 135 permits gas flow from the second gas
source 130 into the reaction chamber 112. Maintaining a
predetermined dopant gas composition, and/or maintaining a
predetermined dopant gas partial pressure, may include bleeding
dopant containing gas from the second gas source 130 into the
reaction chamber 112. Without being limited by any particular
theory, at least some amount of the dopant containing gas will be
consumed in the closed system 100 as the dopant is absorbed by the
soot compact 20. The fluid control system may measure the
consumption of the dopant containing gas as a decrease in gas
composition and/or as a decrease in partial pressure in the
reaction chamber 112. In order to maintain the predetermined dopant
gas composition, and/or the predetermined dopant gas partial
pressure, the fluid control system may open the one-way valve 135
to permit dopant containing gas flow into the reaction chamber 112
until the predetermined dopant gas composition, and/or the
predetermined dopant gas partial pressure is restored. Once the
predetermined dopant gas composition, and/or the predetermined
dopant gas partial pressure is restored, the fluid control system
may close the one-way valve 135.
[0042] Doping the soot compact may also include pulling a vacuum on
the closed system 100 prior to doping the soot compact and
maintaining vacuum conditions while doping the soot compact. When
under vacuum, total pressure in the reaction chamber 112 may be
greater than about 0.50 atm. For example, the total pressure in the
reaction chamber 112 may be between greater than about 0.25 atm,
such as between about 0.25 atm and about 2.0 atm, or between about
0.5 atm and about 5.0 atm, or between about 0.90 atm and 2.0 atm.
According to embodiments of the present disclosure, total pressure
in the reaction chamber 112 may be greater than 2.0 atm, such as
between about 2.0 atm and about 30 atm. Generally, where the intent
is to achieve uniform dopant content in the soot compact, doping
the soot compact may be performed at lower pressures such as
between about 0.25 atm and about 1.0 atm. However, where the intent
is to achieve high dopant content or both high dopant content and
uniform dopant content in the soot compact, doping the soot compact
may be performed at higher pressures such as greater than about
0.90 atm.
[0043] Prior to doping the soot compact, the method may also
include drying the soot compact. Drying the soot compact may
include a plurality of drying cycles which include filling the
reaction chamber 112 with a gas composition having less than about
50% of a drying gas, and evacuating the gas composition after a
predetermined drying period. Alternatively, the gas composition may
have less than about 10% of a drying gas, or even less than about
5.0% of a drying gas. The remainder of the gas composition may be
helium. The predetermined drying period may be greater than about
10 minutes and may be, for example, between about 10 minutes and
about 1.0 hour. The drying gas may be, but is not limited to,
chlorine or chlorine containing gases and carbon monoxide. Drying
the soot compact may prepare the soot compact for doping by
removing moisture and hydroxyl groups from the soot compact, and
also by removing transition metals or alkali metal components from
the soot compact. When the drying cycles are complete, the reaction
chamber 112 may be evacuated prior to introduction of a dopant
containing gas into the reaction chamber 112.
[0044] The method may also include consolidating the doped soot
compact to form a glass article. The doped soot compact may be
heated to a sintering temperature between about 1200.degree. C. and
about 1650.degree. C. and maintained at the sintering temperature
until the soot compact is consolidated into a glass article. As
explained above, the closed system 100 may be a furnace in which
the doped soot compact may be consolidated, or the doped soot
compact may be moved from the closed system 100 described above to
a furnace where the doped soot compact may be consolidated.
[0045] Consolidating the doped soot compact to form a glass article
may further include simultaneously increasing the temperature of
the closed system 100 and decreasing the concentration of the
dopant containing gas in the closed system 100. The concentration
of dopant containing gas may be decreased with the increase of
furnace temperature in accordance with the following relation:
y II = y I , dop Exp [ - 21741 ( ( 1 T I , dop ) - ( 1 T II ) ) ] (
1 ) ##EQU00001##
[0046] wherein: T.sub.I,dop is the temperature in the closed system
during doping of the soot compact with a halogen;
[0047] T.sub.II is the temperature in the closed system during
consolidation of the soot compact;
[0048] y.sub.I,dop is the mole fraction of the halogen-containing
gas in the closed system during doping of the soot compact with
halogen; and
[0049] y.sub.II is the maximum halogen-containing gas mole fraction
in the closed system during consolidation of the soot compact at
T.sub.II.
[0050] The method may also include oxygenating the soot compact by
contacting the soot compact with an oxygen-containing gas. The soot
compact may be oxygenated at any time, including prior to or during
drying of the soot compact, prior to or during doping of the soot
compact, and prior to or during consolidation of the soot compact.
Oxygenating the soot compact may be performed at a temperature of
between about 1000.degree. C. and about 1300.degree. C. The soot
compact may be oxygenated in an oxygen containing atmosphere with
oxygen diffusing into the soot compact and reacting with trivalent
titanium (Ti.sup.3+) to lower oxidation states of titanium and to
convert such titanium to tetravalent titanium (Ti.sup.4+). Such
oxygenating makes the consolidated glass article colorless and is
believed to prevent the occurrence of a bluish-black discoloration
of the consolidated glass article.
[0051] The method may also include annealing the glass article.
Following consolidation, the reaction chamber 112 may be cooled to
a holding temperature of between about 900.degree. C. and about
1100.degree. C. for a holding period of at least about 30 minutes,
for example, between about 30 minutes and about 10 hours. After
completion of the holding period, the reaction chamber 112
temperature may be decreased to a predetermined temperature,
between about 700.degree. C. and about 850.degree. C., at a rate of
less than about 10.degree. C. per hour. For example, the rate may
be between about 0.10.degree. C. per hour and about 10.degree. C.
per hour, or between about 0.10.degree. C. per hour and about
5.0.degree. C. per hour, or even between about 0.10.degree. C. per
hour and about 1.0.degree. C. per hour. Once the predetermined
temperature is reached, heat from the heat source may be removed,
and the reaction chamber 112 may be allowed to cool to ambient
temperature. After annealing, the fictive temperature of the glass
article may be less than about 1100.degree. C., or less than about
1000.degree. C., or less than about 900.degree. C., or even less
than about 800.degree. C.
[0052] The glass article may be annealed in the closed system 100,
as described above, or the glass article may be moved from the
closed system 100 and annealed in a separate vessel, such as a
furnace. Similarly, where the doped soot compact is removed from
the closed system 100 and consolidated in a separate furnace, the
glass article may be annealed in the consolidation furnace, or may
be moved from the consolidation furnace and annealed in a separate
vessel, such as a separate furnace.
[0053] The method may also include processing the glass article to
form optical elements. Once the glass article has been cooled to
ambient temperature, the glass article may be cut, cored, reflowed
into a target shape, or otherwise processed into shapes that are
suitable for making optical elements. Such processing, in addition
to cutting or coring, may include etching, additional thermal
treatments, grinding, polishing, applying selected metals to form a
mirror, reflowing, and such additional processing as may be
necessary to form the desired optical element. According to
embodiments of the present disclosure, additional silica-based soot
may be deposited onto the glass article to form at least one
optical fiber cladding region using the same method as explained
above with respect to the core of an optical fiber preform. The at
least one optical fiber cladding region may optionally be doped
with a halogen using a halogen-containing dopant gas as described
herein using the same doping steps as described herein, or using
doping steps known in the art.
[0054] The optical elements capable of use in photolithography
disclosed herein may be formed from a fluorine-doped silica-titania
glass article. The doped glass article may include between about
0.50 wt. % and about 2.0 wt. % fluorine and the variation of
fluorine concentration through the thickness of the doped glass
article may be less than about 0.20 wt. %. The doped glass article
may also include between about 1.0 wt. % and about 12 wt. %
titania, or between about 5.0 wt. % and about 10 wt. % titania. The
variation of titania concentration through the thickness of the
doped glass article may be less than about 0.10 wt. %, and the
doped glass article may be uniform and substantially free of
striae. Such uniformity renders the doped glass article polishable,
which in turn facilitates processing of the doped glass article to
form the optical elements disclosed herein. The optical elements
may have a near-zero thermal expansion over a wide operational
temperature range, such as between about 20.degree. C. and about
30.degree. C., and may also have a slope of CTE versus temperature
at 20.degree. C. of less than about 1.0 ppb/K.sup.2. The slope of
CTE versus temperature at 20.degree. C. may be less than about 0.80
ppb/K.sup.2, or even less than about 0.60 ppb/K.sup.2.
[0055] The optical elements disclosed herein may be photomask
blanks or projection optic mirror substrates employed in EUV
lithography. The doped glass article disclosed herein may also be
used to form the critical zone of large mirrors used in EUV
lithography. The doped glass article may be shaped and fusion
bonded into a cavity formed in the critical zone in a larger
undoped glass article.
[0056] Alternatively, the doped glass article disclosed herein may
be a soot blank which may be used to form the core of an optical
fiber preform, or may be an optical fiber preform that may be drawn
into an optical fiber. The optical fiber may be formed from a
chlorine-doped silica glass article as disclosed herein. Such
optical fiber may have a core region having a chlorine content of
greater than about 1.5 wt. %. For example, the core region of the
optical fiber may have a chlorine content of between about 1.5 wt.
% and about 4.75 wt. %, or between about 1.5 wt. % and about 4.5
wt. %, or between about 1.5 wt. % and about 4.0 wt. %, or between
about 1.5 wt. % and about 3.0 wt. %. The core region of the optical
fiber may have a chlorine content of between about 1.75 wt. % and
about 4.5 wt. %, or between about 1.75 wt. % and about 4.0 wt. %,
or greater than about 1.5 wt. %, or greater than about 1.75 wt. %,
or greater than about 2.0 wt. %, or greater than about 2.25 wt.
%.
[0057] Additionally, the optical fiber may be formed from a silica
glass article doped with both chlorine and fluorine. In addition to
a core region having a chlorine content such as described above,
such optical fiber may have at least one cladding region having a
fluorine content of between about 0.10 wt. % and about 0.50 wt. %.
For example, the at least one cladding region may have a fluorine
content of between about 0.15 wt. % and about 0.45 wt. %, or
between about 0.20 wt. % and about 0.40 wt. %.
[0058] The optical fiber described herein may also be described
using the relative refractive index of various regions of the
optical fiber. As used herein the term "relative refractive index"
or "relative refractive index percent" is defined as:
.DELTA.%=100.times.(n.sub.i.sup.2-n.sub.c.sup.2)/2n.sub.i.sup.2
where n.sub.c is the refractive index of undoped silica and n.sub.i
is the average refractive index at point i in the particular region
of the optical fiber.
[0059] As further used herein, the relative refractive index is
represented by .DELTA. and its values are given in units of "%",
unless otherwise specified. The terms .DELTA., %.DELTA., .DELTA.%,
delta index, percent index, percent delta index and % can be used
interchangeably herein. In cases where the refractive index of a
region is less than the refractive index of undoped silica, the
relative index percent is negative and is referred to as having a
depressed region or depressed index.
[0060] The optical fiber formed from the chlorine-doped silica
glass article as disclosed herein may have a core region having a
relative refractive index between about 0.08% and about 0.30%. For
example, the core region may have a relative refractive index of
between about 0.10% and about 0.25%, or between about 0.12% and
about 0.20%, or even between about 0.14% and about 0.18%.
[0061] Additionally, the optical fiber formed from a silica glass
article doped with both chlorine and fluorine may have at least one
cladding region having a relative refractive index between about 0%
and about 0.25%, or between about -0.05% and about -0.20%, or
between about -0.10% and about -0.20%.
[0062] Methods described herein enable more efficient use of dopant
containing gases, which in turn thus reduce the overall costs
associated with forming doped glass articles as described herein.
Conventional doping processes flow dopant containing gases over a
soot blank in an open system. The utilization efficiency of such
processes, defined as the amount (in wt. %) of the dopant provided
in the dopant containing gas divided by the amount (in wt. %) of
the dopant in the resulting doped glass article, is typically
utilize only about 10-25%. In such processes, between about 75-90%
of the dopant gas is discarded after the doping process is
complete. In contrast, performing a doping process in closed
systems according to embodiments of the present disclosure utilizes
or conserves greater than about 90% of the dopant gas. The closed
system eliminates the loss of dopant gas experienced during a
doping process in an open system and also allows for the recovery
of dopant gas once the doping process is complete.
EXAMPLES
[0063] Embodiments of the present disclosure are further described
below with respect to certain exemplary and specific embodiments
thereof, which are illustrative only and not intended to be
limiting.
Example 1
[0064] To produce a baseline dopant profile, a silica soot compact
having a diameter of about 14 cm was heat treated in a furnace
under a helium atmosphere. The soot compact was held isothermally
at a temperature of about 1145.degree. C. for a period of about 4.0
hours. For about 1.0 hour of the about 4.0 hour period, a gas flow
having about 2.0% chlorine was flowed through the furnace to dry
the soot compact of moisture and hydroxyl groups, and to remove any
transition metal contaminants. While maintaining the furnace
temperature at about 1145.degree. C., a gas flow having about 50%
silicon tetrafluoride (SiF.sub.4) was then flowed through the
furnace for a period of about 90 minutes to dope the soot compact
with fluorine. Subsequently, the furnace temperature was increased
from about 1145.degree. C. to about 1345.degree. C. over a period
of about 1.0 hour under a gas flow of about 6.0% SiF.sub.4, and
held for about 30 minutes. The furnace temperature was then
increased to about 1450.degree. C. and the soot compact was
sintered under a helium atmosphere to form a doped glass article.
As illustrated in FIG. 3, the dopant profile of this example shows
a monotonic decrease in fluorine content dropping from about 2.0%
at the surface of the glass article to less than about 1.0% near
the center of the glass article. The non-uniform dopant profile is
believed to be a result of a progressively diffusion limited
reaction. It is believed that the reaction of fluorine at
1145.degree. C. is sufficient to increase the densification rate at
the outer surface of the glass article, and thereby limit
diffusivity of the dopant beyond the outer surface of the glass
article.
Example 2
[0065] To test this belief, a soot compact having a diameter of
about 14 cm was heat treated in a furnace under a helium
atmosphere. The soot compact was held isothermally at a temperature
of about 1060.degree. C. for a period of about 4.0 hours. For about
1.0 hour of the about 4.0 hour period, a gas flow having about 9.0%
chlorine was flowed through the furnace to dry the soot compact of
moisture and hydroxyl groups, and to remove any transition metal
contaminants. While maintaining the furnace temperature at about
1060.degree. C., a gas flow having about 25% silicon tetrafluoride
(SiF.sub.4) was then flowed through the furnace for a period of
about 2.0 hours to dope the soot compact with fluorine.
Subsequently, the furnace temperature was increased from about
1060.degree. C. to about 1300.degree. C. over a period of about 30
minutes under a gas flow of about 2.0% SiF.sub.4, and held for
about 15 minutes. The soot compact was then sintered under a helium
atmosphere at a temperature of about 1190.degree. C. for about 12
hours to form a doped glass article. As illustrated in FIG. 4, the
dopant profile of this example shows uniform fluorine content of
about 0.80% from the center of the glass article to within about
7.0 mm from the outer surface of the glass article. At the outer
surface of the glass article, the fluorine content rises to about
1.0%. It is believed that the lower temperatures of this example,
as compared to Example 1, allowed for sufficient diffusivity of the
dopant to improve the uniformity doped glass article.
Example 3
[0066] Attempts were made to better control the process of Example
2 and improve the uniformity of a doped glass article. A soot
compact having a diameter of about 14 cm was heat treated in a
closed system under a helium atmosphere. In this example, the
closed system included a sealed furnace. The soot compact was held
isothermally at a temperature of about 1060.degree. C. for a period
of about 4.0 hours. The system was filled with a gas having about
50% chlorine to dry the soot compact of moisture and hydroxyl
groups, and to remove any transition metal contaminants. After
about 20 minutes, the gas was evacuated from the system. Three
cycles of filling for about 1.0 minute to about 2.0 minutes,
holding for about 20 minutes, and evacuating for about 10 minutes
were performed. Once the final cycle was completed, the system was
evacuated for about 10 minutes. While maintaining the system
temperature at about 1060.degree. C., the system was filled with a
gas having about 100% silicon tetrafluoride (SiF.sub.4) and the
system remained closed for about 2.0 hours to dope the soot
compact. Subsequently, a vacuum was pulled on the closed system and
the system temperature was increased from about 1060.degree. C. to
about 1350.degree. C. over a period of about 30 minutes and the
soot compact was held at about 1350.degree. C. for about 60 minutes
to sinter the soot compact and to form a doped glass article. As
illustrated in FIG. 5, the dopant profile of this example shows
uniform fluorine content of about 1.90% from the center of the
glass article to within about 7.0 mm. At the outer surface of the
glass article, the fluorine content decreases to about 1.10%. It is
believed that the lower temperatures used in Example 2, coupled
with the closed system of this example, allowed for sufficient
diffusivity of the dopant to improve the uniformity doped glass
article.
Example 4
[0067] A soot compact was doped with fluorine to show the
relationship of doping pressure and doping temperature to the
dopant content of the doped glass article. In the present example,
a doping temperature of about 1100.degree. C. and a doping pressure
of about 25 kPa were selected to form a doped glass article having
a fluorine content of about 1.5 wt. %.
[0068] A 140 gram soot compact was positioned in the center of a
hot zone of a closed system. The closed system was a tube furnace
formed form a 48 inch long and 3.5 inch diameter alumina tube that
was fitted with sealed end caps and a valve on each end of the tube
and which had a 24 inch hot zone. A vacuum pump was coupled to the
valve on one end of the tube furnace and a source of SiF.sub.4 was
coupled to the valve on the other end of the tube furnace. The tube
furnace was also equipped with a vacuum gauge (a Baratron.RTM.
Direct Pressure/Vacuum Capacitance Manometer commercially available
from MKS Instruments, Inc. of Andover, Mass.) and the pressure in
the system was monitored to confirm there was no leakage when the
valves were closed. With the valves closed, pressure in the tube
furnace was reduced to 2.0 kPa and maintained for a period of about
6.0 hours.
[0069] While under vacuum of less than 2.0 kPa, the temperature of
the tube furnace was raised to a drying temperature of about
1060.degree. C. A soot compact positioned in the tube furnace was
dried by flowing into the furnace a volume of SiF.sub.4 sufficient
to raise the pressure in the tube furnace to a predetermined target
doping pressure, closing the valves and maintaining the closed
system condition for a period of between about 15 minutes and about
30 minutes. The volume of SiF.sub.4 at standard temperature and
pressure (V.sub.SiF4 STP) was determined using V.sub.SiF4
STP=(P.sub.dope*V.sub.tube)*(273/T.sub.dry) where P.sub.dope is the
predetermined target doping pressure, V.sub.tube is the volume of
the tube furnace, and T.sub.dry is the drying temperature. The
SiF.sub.4 was absorbed on the surface of the soot compact and
reacted with SiOH on the surface of the soot compact to form HF as
a by-product in the closed system. Generally, the pressure in the
tube furnace dropped as the absorption of SiF.sub.4 was greater
than the formation of HF. Once the pressure stabilized, the tube
furnace was evacuated and then the valves were closed. While
maintaining the vacuum of less than 2.0 kPa, the temperature in the
hot zone of the tube furnace was raised to a temperature of about
1100.degree. C. A doping process was initiated by flowing SiF.sub.4
into the tube furnace until a doping pressure of 0.25 kPa was
reached. As expected, the pressure in the tube furnace dropped as
SiF.sub.4 was absorbed by the soot compact, and the target doping
pressure was maintained by opening the valve coupled to the source
of SiF.sub.4 and flowing more SiF.sub.4 into the tube furnace. The
pressure in the tube furnace was maintained at between about 24 kPa
and about 25 kPa throughout the doping process. The doping process
continued until the pressure in the tube furnace stabilized. Once
the pressure in the tube furnace stabilized, the temperature of the
tube furnace was raised to a sintering temperature of about
1300.degree. C. to complete the formation of a dense glass article.
The glass article was determined to have a fluorine content of
about 1.5 wt. %.
Example 5
[0070] A similar process as described in Example 4 was utilized to
illustrate that a soot compact can be doped with high levels of
chlorine with high doping efficiency. In this example, a doping
temperature of about 1060.degree. C. and a doping pressure of about
101 kPa were selected to form a doped glass article having a
chlorine content of greater than about 2.0 wt. %. The closed system
was the same as the closed system of Example 4, except that the
closed system includes a 3.5 inch diameter silica tube. The closed
system was placed in fluid communication with a vaporizer including
a stainless steel vessel containing SiCl.sub.4. The stainless steel
vessel was immersed in an oil bath set at greater than about
57.degree. C. in order to supply pure SiCl.sub.4 gas at a pressure
of about 101 kPa. A supply of either pure nitrogen or a combination
of 2.0 wt. % Cl.sub.2 with a balance of nitrogen gas was fixtured
outside the closed system.
[0071] A 140 gram silica soot compact was positioned in the center
of a hot zone of the closed system. The pressure in the tube
furnace was reduced to 2.0 kPa and, while under vacuum, the
temperature of the furnace was raised to a drying temperature of
about 1060.degree. C. The soot compact was dried by flowing about
1.0 slpm of 2.0 wt. % Cl.sub.2 in nitrogen for about 30 minutes
through the tube furnace, with both of the valves opened. After
drying, the tube furnace was evacuated and then both the valves
were closed. While maintaining the vacuum of less than 2.0 kPa, the
temperature in the hot zone of the tube furnace was raised to a
temperature of about 1060.degree. C. The doping process was
initiated by releasing the SiCl.sub.4 at about 57.degree. C. to
equilibrate into the system volume. As the closed system includes a
gas pressure of SiCl.sub.4 that is in equilibrium with the liquid
source at the normal boiling point of SiCl.sub.4, a pressure of 101
kPa was maintained during the doping process which was performed
for about 30 minutes. After the doping process was completed the
temperature of the furnace was raised to a sintering temperature of
about 1400.degree. C. to complete the formation of a dense glass
article. Using X-ray fluorescence, the glass article was determined
to have a chlorine content of be about 2.2 wt. %.
[0072] Optionally, to increase the material use efficiency of the
process, the SiCl.sub.4 remaining in the closed system may be
condensed back into the stainless steel vessel for re-use in
subsequent processes. After formation of the dense glass article,
the furnace was allowed to cool while the stainless steel vessel
was simultaneously cooled to 0.degree. C. in an ice bath. During
this cooling period, SiCl.sub.4 was transported back into the
stainless steel vessel. The furnace was then purged of any residual
low level gas using a scrubber. Performing a doping process in
closed systems such as described herein was observed to utilize or
conserve greater than about 90% of the dopant gas. While the doping
efficiency described in this example relates to a doping process
using chlorine, it should be understood that the increased doping
efficiencies described herein also relate to doping processes using
other dopant containing gases.
Example 6
[0073] A similar process as described in Example 5 was utilized to
illustrate doping a soot compact with even high levels of chlorine
while maintaining high doping efficiency. In this example, a doping
temperature of about 1060.degree. C. and a doping pressure of about
101 kPa were selected to form a doped glass article having a
chlorine content of greater than about 2.0 wt. %. The closed system
was the same as the closed system of Example 4, except that the
closed system includes a 3.5 inch diameter silica tube. The closed
system was placed in fluid communication with a vaporizer including
a stainless steel vessel containing SiCl.sub.4. The stainless steel
vessel was immersed in an oil bath set at greater than about
80.degree. C. in order to supply pure SiCl.sub.4 gas at a pressure
of about 180 kPa. A supply of either pure nitrogen or a combination
of 2.0 wt. % Cl.sub.2 with a balance of nitrogen gas was fixtured
outside the closed system.
[0074] A 140 gram silica soot compact was positioned in the center
of a hot zone of the closed system. The pressure in the tube
furnace was reduced to 2.0 kPa and, while under vacuum, the
temperature of the furnace was raised to a drying temperature of
about 1060.degree. C. The soot compact was dried by flowing about
1.0 slpm of 2.0 wt. % Cl.sub.2 in nitrogen for about 30 minutes
through the tube furnace, with both of the valves opened. After
drying, the tube furnace was evacuated and then both the valves
were closed. While maintaining the vacuum of less than 2.0 kPa, the
temperature in the hot zone of the tube furnace was raised to a
temperature of about 1060.degree. C. The doping process was
initiated by releasing the SiCl.sub.4 at about 80.degree. C. to
equilibrate into the system volume. As the closed system includes a
gas pressure of SiCl.sub.4 that is in equilibrium with the liquid
source at the normal boiling point of SiCl.sub.4, a pressure of 180
kPa was maintained during the doping process which was performed
for about 30 minutes. After the doping process was completed the
temperature of the furnace was raised to a sintering temperature of
about 1300.degree. C. to complete the formation of a dense glass
article. Using X-ray fluorescence, the glass article was determined
to have a chlorine content of be about 2.6 wt. %.
[0075] As exemplified, performing the doping process at higher
pressures enabled formation of a doped glass article with higher
dopant content. Performing the doping process in a closed system
such as described herein enabled achieving the higher dopant
content.
[0076] While the present disclosure has been described with respect
to a limited number of embodiments, those skilled in the art,
having benefit of this disclosure, will appreciate that other
embodiments can be devised which do not depart from the scope of
the invention as disclosed herein. Accordingly, the scope of the
present disclosure should be limited only by the attached
claims.
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