U.S. patent application number 12/248527 was filed with the patent office on 2009-02-12 for particle deposition system and method.
This patent application is currently assigned to ASI/SILICA MACHINERY, LLC. Invention is credited to Franklin W. Dabby, Bedros Orchanian.
Application Number | 20090038346 12/248527 |
Document ID | / |
Family ID | 32928161 |
Filed Date | 2009-02-12 |
United States Patent
Application |
20090038346 |
Kind Code |
A1 |
Dabby; Franklin W. ; et
al. |
February 12, 2009 |
Particle Deposition System and Method
Abstract
A deposition system for depositing a chemical vapor onto a
workpiece, including a deposition chamber having a plurality of
components for performing chemical vapor deposition on the
workpiece. The deposition chamber includes an inner skin made of
Hasteloy for sealing the plurality of components and the workpiece
from the air surrounding the deposition system, and an outer skin
that encloses the inner skin and is separated from the inner skin
by an air gap. The outer skin includes vents that create a
convection current in the air gap between the inner skin and outer
skin of the deposition chamber. The deposition system also has a
gas panel for regulating the flow of gases and vapors into the
deposition chamber, and a computer for controlling operation of the
gas panel and the components in the deposition chamber.
Inventors: |
Dabby; Franklin W.; (Los
Angeles, CA) ; Orchanian; Bedros; (North Hills,
CA) |
Correspondence
Address: |
HOGAN & HARTSON L.L.P.
1999 AVENUE OF THE STARS, SUITE 1400
LOS ANGELES
CA
90067
US
|
Assignee: |
ASI/SILICA MACHINERY, LLC
North Hills
CA
|
Family ID: |
32928161 |
Appl. No.: |
12/248527 |
Filed: |
October 9, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
10897784 |
Jul 23, 2004 |
7451623 |
|
|
12248527 |
|
|
|
|
09894447 |
Jun 28, 2001 |
6789401 |
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10897784 |
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Current U.S.
Class: |
65/531 |
Current CPC
Class: |
C03B 2207/66 20130101;
C03B 37/0142 20130101; C03B 2207/80 20130101; C03B 37/01486
20130101; C03B 2207/36 20130101; C03B 2207/70 20130101; C03B
37/01406 20130101 |
Class at
Publication: |
65/531 |
International
Class: |
C03B 37/018 20060101
C03B037/018 |
Claims
1. A deposition system for depositing silica particles onto a
workpiece comprising: a burner for depositing the particles onto
the workpiece; a lathe for holding the workpiece and for rotating
and translating the workpiece relative to the burner; and a
computer for controlling the translating and rotating of the
workpiece relative to the burner; wherein the lathe is for at times
translating the workpiece at a rate of greater than about 1.4
meters per minute.
2-51. (canceled)
Description
PRIORITY APPLICATION
[0001] This is a continuation of application Ser. No. 09/894,447,
filed Jun. 28, 2001, which application is hereby incorporated in
its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The field of the present invention relates to systems and
methods for depositing chemicals onto workpieces, and the products
therefrom. More particularly, the present invention relates to
systems and methods for depositing silica soot on a start rod for
fabricating optical fiber preforms, fused silica rods, and other
optical components.
[0004] 2. Background
[0005] Today's communications grade optical fiber of fused silica,
SiO.sub.2, is manufactured according to three basic steps: 1) core
preform or "start rod" fabrication, 2) core-with-cladding preform
fabrication, and 3) fiber drawing. The core and cladding of a
preform correspond in ratios and geometry to those of the ultimate
glass fiber that is drawn from the preform. The composition of the
core and cladding must be such that there is a lower index of
refraction in the cladding than in the core. The relatively higher
index of refraction of the core to a relatively lower index of
refraction of the cladding is predetermined so that when the
preform is drawn into a fiber, the fiber conducts light, either in
single mode or in multi-mode form.
[0006] The first step is to build up a start rod, forming it into a
glass that will eventually become the core of the fiber, and in
some cases, part of the cladding layer. The start rod is a glass
rod made of silica, SiO.sub.2, with the portion of the start rod
that comprises the core being doped with a small amount of a
dopant, typically Germania, GeO.sub.2. The presence of the dopant
in the core increases the refractive index of the glass material
compared to the surrounding outer (cladding) layer. In the second
step, a cladding layer is built up on the start rod. The result of
this step is a preform having a core and a cladding, which is
typically about 80 mm in diameter and about one meter long. The
third step is fiber drawing, where the preform is heated and
stretched, and typically yields about 400 km of optical fiber.
[0007] The primary raw ingredient to fabricating the glass preform
is silicon tetrachloride, SiCl.sub.4, which generally comes in a
liquid form. As noted above, however, SiO.sub.2, typically in the
form of glass soot, is deposited on the start rod. The chemical
reactions involved in the formation of the glass soot are complex,
involving, SiCl.sub.4, oxygen, O.sub.2, and the fuel gas combustion
products. In all of the techniques, the silica, SiO.sub.2,
comprises the cladding of the preform according, generally, to the
reaction:
SiCl.sub.4+O.sub.2=SiO.sub.2+2Cl.sub.2.
[0008] Generally, there are four distinct technologies for
fabricating core preforms. These technologies include Modified
Chemical Vapor Deposition (MCVD), Outside Vapor Deposition (OVD),
Vapor Axial Deposition (VAD), and Plasma Chemical Vapor Deposition
(PCVD). The resulting product for all of these technologies is
generally the same: a "start rod" that is generally on the order of
one meter long and 20 mm in diameter. The core is generally about 5
mm in diameter.
[0009] Similarly, there are generally four technologies for
performing the step of adding the cladding. These technologies
include tube sleeving (conceptually paralleling MCVD), OVD soot
overcladding (conceptually paralleling OVD), VAD soot overcladding
(conceptually paralleling VAD), and plasma (conceptually
paralleling PCVD). In this step, additional cladding layers of pure
or substantially pure fused silica are added to the start rod to
make a final preform that can be prepared for fiber drawing.
[0010] In MCVD, the step of manufacturing the start rod is
performed inside of a tube. Similarly, when the cladding step is
performed, a larger tube is sleeved onto and fused to the start
rod. Presently, the company, Heraeus, manufactures the tubes used
for producing start rods and for sleeving onto and fusing with the
start rods to make the preforms.
[0011] In OVD, when fabricating start rods, glass is deposited onto
a rotating mandrel in a "soot" deposition process. The start rod is
slowly built up by first depositing the germanium doped core, and
then the pure silica layers. When the core deposition is completed,
the mandrel is removed and then sintered into a start rod of
glass.
[0012] In the process of OVD soot overcladding, where a cladding is
deposited onto a fabricated start rod, the start rod is rotating
and traversing on a lathe such that many thin layers of soot are
deposited on the rod over a period of time. Although the SiO.sub.2
is not deposited onto the start rod as a vapor, but rather as
SiO.sub.2 particles, the process is known in the art as a "chemical
vapor deposition" process because the SiCl.sub.4, which reacts in
the stream between the burner and the start rod to form SiO.sub.2,
is input to the burner as a vapor. The porous preform that results
from the OVD soot overcladding process is then sintered in a helium
atmosphere at about 1500.degree. C., into a solid, bubble-free
glass blank. U.S. Pat. No. 4,599,098, issued to Sarkar, which is
incorporated by reference as though fully set forth herein,
provides further background on systems and techniques for OVD and
OVD soot overcladding.
[0013] VAD is a process of depositing silica soot onto the end of a
mandrel in a deposition station. Unlike the OVD process the mandrel
is not removed prior to sintering. Furthermore, like the OVD soot
overcladding technique, VAD soot overcladding is also used to
deposit silica soot on a start rod to fabricate a preform. However,
unlike OVD and OVD soot overcladding, VAD soot overcladding orients
the mandrel and start rod vertically rather than horizontally and
deposits the silica in one thick layer in one pass.
[0014] PCVD uses plasma radiation as a source of heat, and
therefore is unlike the above-described processes, which use
hydrogen or methane as the source of heat for the chemical
reaction.
[0015] For the above-described technologies, typically any one of
the core fabrication technologies may be combined with any one of
the cladding fabrication technologies to generate a preform that
may be used for drawing fiber.
[0016] In the OVD soot overcladding processes, one of the key
measures of economic viability in comparison to the other available
techniques is the deposition rate of the SiO.sub.2 on the
workpiece. For example, some companies involved in optical fiber
manufacturing opt for the most cost-effective method of performing
the step of overcladding the start rod in the fiber manufacturing
process. With respect to this step in the process, the choice is
either to purchase the cladding tubes or to perform a deposition
process to add the cladding.
[0017] In comparing the relative costs of the two approaches, the
economics often come down to whether a particular vapor deposition
system that a company is considering performs at a certain minimum
deposition rate. The deposition rate may be characterized, for
example, by the average grams/minute of silica soot that can be
deposited on the start rod until completion (i.e., an optical fiber
preform ready for sintering). Thus, above a certain threshold
deposition rate, performing the soot overcladding process is likely
to be economically more attractive to the company than purchasing
cladding tubes. Thus, companies that manufacture systems for
performing soot overcladding focus on achieving the highest
possible deposition rates without compromising the quality of the
preform that is produced for fiber drawing.
[0018] The factors that determine a deposition system's deposition
rate are the chemical vapor delivery rate and the efficiency of
chemical vapor deposition onto the workpiece. With respect to vapor
delivery, key issues generally revolve around continuously and
efficiently maintaining a high (e.g., greater than 200
grams/minute) delivery rate over a prolonged period (e.g., greater
than 2 hours). Several methods have been described in the prior art
for supplying a hydrolyzing burner with a substantially constant
flow of vaporized source material entrained in a carrier gas. For
example, in U.S. Pat. No. 4,314,837 issued to Blankenship ("the
Blankenship reference"), a system is described that includes
several enclosed reservoirs each containing liquid for the reaction
product constituent. The liquids are heated to a temperature
sufficient to maintain a predetermined vapor pressure within each
reservoir. Metering devices are coupled to each reservoir for
delivering vapors of the liquids at a controlled flow rate. The
respective vapors from each reservoir are then combined before they
are delivered to the burner.
[0019] This device, however, is inefficient for maintaining a
substantial and steady delivery rate of chemical vapor to the
burner for a prolonged chemical vapor deposition process. For a
substantial and steady delivery rate, the chemical reservoir
described in the Blankenship reference must be vast, and
significant energy expenditure is required to maintain the chemical
in the reservoir in a vapor state. On the other hand, if the
chemical reservoir described in the Blankenship reference is small
enough to be energy-efficient, then the deposition flow must be
periodically interrupted to refill the reservoir with the chemical
liquid and heat the chemical until it is in a vapor state. Because
maintaining a constant, high delivery flow rate, as noted above, is
a critical factor in the effective deposition rate, a need exists
for a system and method of chemical vapor delivery that is
energy-efficient and provides high, constant and continuous
delivery of chemical vapor.
[0020] With respect to enhancing the deposition efficiency of
SiO.sub.2 on the workpiece to improve the effective deposition
rate, studies have been performed to characterize the flow of
chemical vapor in the reaction chamber from the burners to the
surface of the workpiece. One reference directed to this issue is
Li, Tingye, Fiber Fabrication, pp. 75-77, Optical Fiber
Communications, (Academic Press, Inc. 1985). As discussed in the
above reference, because of the small size of the formed glass
particles, momentum does not cause an impaction of the particles
onto the surface of the workpiece. The small sizes of the glass
particles would tend to force them to follow the gas stream around
instead of at the preform surface. Rather, thermophoresis is the
dominant mechanism for collection on the surface of the preform. As
the hot gas stream and glass particles travel around the workpiece,
a thermal gradient is established near the surface of the preform.
Preferably, the thermal gradient is steep, effectively pulling the
glass particles by a thermophoretic force towards the preform.
[0021] Various methods have been proposed to increase deposition
efficiency based on establishing and maintaining the thermophoretic
force, including varying the distance between the burner and the
workpiece. See H. C. Tsai, R. Greif and S. Joh, "A Study of
Thermophoretic Transport In a Reacting Flow With Application To
External Chemical Vapor Deposition Processes," Int. J. Heat Mass
Transfer, v. 38, pp. 1901-1910 (1995). However, even applying these
methods, demand for even higher deposition rates has gone unmet. A
need exists therefore, for systems and methods that offer further
improvement to deposition efficiency, chemical delivery and,
thereby, the overall deposition rate of chemical vapor.
SUMMARY OF THE INVENTION
[0022] The present invention generally provides, in one aspect,
systems and methods for enhancing the effective deposition rate of
chemicals onto a workpiece, such as the deposition of SiO.sub.2
from a SiCl.sub.4 vapor onto a start rod for making a preform
usable for drawing into optical fiber.
[0023] In a second separate aspect as described herein, the present
invention comprises systems and methods for manufacturing optical
fiber preforms, optical fiber, silica rods, including fused silica
rods, and silica wafers.
[0024] In a third separate aspect as described herein, the present
invention comprises a deposition system that preferably includes a
deposition chamber having a plurality of components for performing
chemical vapor deposition on the workpiece, an inner skin made of
Hasteloy for sealing the plurality of components and the workpiece
from the air surrounding the deposition system, and an outer skin
enclosing the inner skin and preferably separated from the inner
skin by an air gap. The outer skin preferably includes vents that
create a convection current in the air gap between the inner skin
and outer skin of the deposition chamber. The deposition system
also has a gas panel for regulating the flow of gases and vapors
into the deposition chamber, and a computer for controlling
operation of the gas panel and the deposition chamber.
[0025] In a fourth separate aspect as described herein, the present
invention comprises a deposition system that preferably includes a
deposition chamber comprised of Hasteloy and including components
for performing chemical vapor deposition on the workpiece, a gas
panel for regulating a flow of gases and vapors into the deposition
chamber, and a computer for controlling operation of the gas panel
and at least one of the plurality of components.
[0026] In a fifth separate aspect as described herein, the present
invention comprises a deposition system that preferably includes a
burner for depositing particles onto the workpiece and a lathe for
holding the workpiece at its two ends and for rotating and
translating the workpiece relative to the burner. The deposition
system preferably further includes a computer for controlling the
translating and rotating of the workpiece relative to the burner.
Preferably, the computer is configured to translate the workpiece
relative to the burner at a rate of greater than about 1.4 meters
per minute during at least a portion of the deposition process.
This relative motion may be caused by translating the burner,
translating the workpiece, or translating both.
[0027] In a sixth separate aspect as described herein, the present
invention comprises an optical fiber preform formed using
deposition system such as that described above in the fourth
separate aspect of the invention. In a seventh separate aspect as
described herein, the present invention comprises a silica rod,
preferably formed of substantially pure fused silica, and formed
using deposition system such as that described above in the fourth
separate aspect of the invention.
[0028] In an eighth separate aspect as described herein, the
present invention comprises a deposition system that preferably
includes a deposition chamber including components for performing
chemical vapor deposition on the workpiece. The deposition system
preferably further includes an exhaust subsystem for exhausting
constituents from the deposition chamber, and an intake subsystem
for providing air into the deposition chamber. The intake subsystem
preferably includes a blower for actively conveying air into the
deposition chamber and a passive air intake for allowing air into
the deposition chamber based on a negative pressure differential
between the blower and the exhaust subsystem.
[0029] In a ninth separate aspect as described herein, the present
invention comprises a system of chemical vapor delivery, where the
system includes a reservoir, a preheater, and a vaporizer generally
connected in series for transferring silicon tetrachloride
(SiCl.sub.4) in liquid form from the reservoir to the preheater, in
which the SiCl.sub.4 is initially heated. The SiCl.sub.4 is then
transferred from the preheater to the vaporizer where the
SiCl.sub.4 is vaporized. Preferably, the system further includes a
valve interposed between the preheater and the vaporizer, and a
computer electrically connected to the valve for controlling flow
of the chemical into the vaporizer based on the amount of the
chemical in the vaporizer.
[0030] Further embodiments as well as modifications, variations and
enhancements of the invention are also described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1A is a schematic diagram of a perspective view of a
preferred embodiment of a chemical vapor deposition system.
[0032] FIG. 1B is a diagram of a preferred embodiment of an intake
and exhaust subsystem such as may be provided by the chemical vapor
deposition system depicted in FIG. 1A.
[0033] FIG. 2 is a function-oriented diagram of a preferred
embodiment of a chemical vapor deposition system.
[0034] FIG. 3A is a diagram of a functional representation of a
burner, represented generally in FIG. 2, as used in a chemical
vapor deposition system as depicted in FIGS. 1A and 2.
[0035] FIG. 3B is a side view of burner and a representative stream
of emissions from the burner, which is depicted generally in FIG.
2, as is preferably provided in a chemical vapor deposition system
as depicted in FIGS. 1A and 2.
[0036] FIG. 4 is a schematic diagram providing a perspective view
of a preferred embodiment of a lathe, such as is generally depicted
in FIG. 2, for holding and moving a workpiece in a chemical vapor
deposition system as is depicted in FIGS. 1A and 2.
[0037] FIG. 5 is a diagram depicting an example of a final preform
having a usable portion demonstrating a tapering effect at the ends
of the preform.
[0038] FIG. 6 is a diagram depicting a motion profile of the
translational movement of a workpiece relative to a chemical
deposition burner.
[0039] FIG. 7 is a process flow diagram illustrating a preferred
embodiment of a process of performing chemical vapor deposition
such as may be performed by the chemical vapor deposition system
illustrated in FIG. 1A.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] FIG. 1A depicts a preferred embodiment of a chemical vapor
deposition system 100. The chemical vapor deposition system 100
preferably includes a reaction or deposition chamber 102, and a
enclosure 104 for the deposition chamber 102, a computer 106 and
other electronic components, an enclosure 108 for the computer 106
and the other electronic components, a power distribution subsystem
110, an enclosure 112 for the power distribution subsystem 110, a
gas panel 114, a gas panel enclosure 116 and an intake and exhaust
subsystem (see FIG. 1B), including a main exhaust 118 and secondary
exhausts 122, 123. A door 105 is preferably provided through which
a workpiece or start rod can be inserted into and removed from the
deposition chamber 102. Removable panels 107 are also provided to
facilitate cleaning of the chamber 102. The door 105 and panels 107
preferably include moldings to seal the chamber 102 from vapor
leakage out or air leakage into the chamber 102. Overhead lights
(not shown) preferably on each side of the chamber 102 may be
provided to illuminate the interior of the chamber 102.
[0041] Generally, the deposition chamber 102 is structured to house
a process of deposition of particles (e.g., particles of silica
soot) onto a workpiece or start rod. The deposition chamber 102,
and the deposition system 100 generally, may be used to produce an
optical fiber preform that, in a subsequent drawing step, may be
used to manufacture optical fiber. The deposition system 100 may
also be used to manufacture fused silica rods, including pure fused
silica rods. For this application, the deposition system 100
generally applies silica soot to an initial start rod of
substantially pure fused silica. The product of this deposition
process, a pure fused silica preform, is then sintered to form the
pure fused silica rod and may be used to manufacture (e.g., by
drawing, slicing or otherwise reforming the pure fused silica rod)
silica wafers or substrates, multi-mode optical fiber, and other
optical components for a variety of applications.
[0042] The other subsystems and components of the deposition
chamber 102 are generally provided to support the deposition
process. In one embodiment, deposition material generally
comprising a vapor of silicon tetrachloride (SiCl.sub.4) and oxygen
(O.sub.2) is emitted from a chemical burner in a process of
fabricating optical fiber preforms in a deposition region 146 (see
FIG. 1B) of the deposition chamber 102. In the deposition region
146 are the chemical burner, a chemical stream from the burner, and
the workpiece, where the stream is directed towards the workpiece
from the burner (see FIG. 2). The burner also preferably issues and
ignites streams of hydrogen and oxygen. The resulting flame heats
the chemical constituents to temperatures generally exceeding about
1000.degree. C. A chemical reaction with the hydrogen, oxygen and
SiCl.sub.4 occurs in the stream, in which the SiCl.sub.4 in the
stream is oxidized producing particles of silicon dioxide (that are
then deposited on the workpiece) and a byproduct of hydrogen
chloride (HCl).
[0043] FIG. 1B depicts a preferred embodiment of an air intake and
exhaust subsystem 150 such as may be provided in the chemical vapor
deposition system 100 illustrated in FIG. 1A. Containment and
processing of the constituents of the chemical vapor deposition
process preferably is, at least in part, provided by the intake and
exhaust subsystem 150. The intake and exhaust subsystem 150
preferably provides a flux of air through the deposition chamber
102 to keep the deposition chamber environment, and the thereby the
workpiece, cool relative to the chemical stream that issues from
the burner.
[0044] As illustrated in FIG. 1B, the intake and exhaust subsystem
150 preferably is configured to cool the deposition chamber and the
workpiece, provide clean air and a negative pressure within the
deposition chamber 102, and provide a laminar air flow in the
deposition region 146. The intake and exhaust subsystem 150
preferably includes intake elements and exhaust elements. The
intake elements preferably include a blower 125, comprising a fan
126 and a motor 128, a prefilter 124, a Hepa filter 134 and a Hepa
filter interface 132, air diffusers 140, 142, 144, and passive air
intakes 136, 138. The exhaust elements preferably include an
exhaust hood 130, a main exhaust 118, and secondary exhausts 122,
123.
[0045] With respect to the air intake elements, air external to the
deposition system 100 enters the prefilter 124 under the active
power of the blower 125. The prefilter 124 preferably has an
efficiency of at least about 90% in removing contaminants from the
external air. Furthermore, the prefilter 124 preferably is mounted
into the intake and exhaust subsystem 150 to conveniently enable an
operator to periodically replace the prefilter 124. External air
that passes through the prefilter 124 is then preferably forced
through the Hepa filter 134, via a connector 132. The Hepa filter
134 preferably has a filtering efficiency of greater than about 95%
and, due to the presence of the prefilter 124, requires replacement
infrequently.
[0046] The forced clean air exiting the Hepa filter 134 is
preferably piped to a location beneath the chemical burner where
the air is spread laterally by a central diffuser 142 into the
deposition region 145. The forced clean air may be conveyed via
stainless steel tubing (e.g., six-inch diameter) or via other
convenient means as are known in the art. From the central diffuser
142, the air is directed upward through the deposition region 146
towards the workpiece and the exhaust hood 130, and in an upward
direction generally parallel to the chemical stream emitted from
the burner.
[0047] Due to the preferably negative pressure within the
deposition chamber 102, outside air also passively enters the
deposition chamber 102 through the passive air intakes 136, 138.
The passive air intakes 136, 138 preferably include filters similar
in design and filtering efficiency to the prefilter 124. From the
passive air intakes 136, 138, the filtered external air then enters
the diffusers 140, 144 which laterally spread the generally upward
movement of air in the deposition chamber 102. In the embodiment
shown in FIG. 1B, one passive air intake is provided on each side
of the deposition region 146. In alternative embodiments,
additional intakes may be provided on each side of the deposition
region 146. Alternatively, one passive air intake may be provided
which diffuses the air laterally to both sides of the deposition
chamber 102. In any event, passive intake of external air is
preferably balanced on each side of the deposition region 146.
[0048] A generally vertical flow of air in the deposition chamber
102 is preferably at least in part maintained by the exhaust
component of the intake and exhaust subsystem 150. The main exhaust
118 and the secondary exhausts 122, 123 encourage a generally
upward flow of air into and out of the deposition chamber 102.
Although the exhaust hood 130 may perturb the vertical flow
somewhat, the exhaust hood 130 minimizes the spread of chemicals to
interior walls 104 of the deposition chamber 102 by generally
capturing any chemical constituents immediately after they move
past the workpiece or otherwise escape the deposition region 146.
Like the passive air intakes 136, 138, a secondary exhaust 122, 123
is preferably provided on each side of the main exhaust 118 to
encourage the generally upward flow of air in the chamber 102. In
alternative embodiments, additional secondary exhausts may be
provided on each side of the main exhaust 118.
[0049] In one embodiment, the main exhaust 118 is preferably an
eight-inch diameter stainless steel duct and the secondary exhausts
122, 123 preferably comprise six-inch diameter stainless steel
ducts. However, the intakes and exhausts as described herein may be
of any convenient shape or diameter for maintaining the desired
laminar flow of air through the chamber 102. Preferably, however,
the exhaust hood 130 and the main exhaust 118 are comprised of
Hasteloy to minimize any potential for corrosion due to the exhaust
of chemicals from the chamber 102. Furthermore, optionally, the
secondary exhausts 122, 123 also are comprised of Hasteloy.
[0050] The intake and exhaust subsystem 150 preferably provides a
power differential between the intake and exhaust of air from the
deposition chamber 102 such that a greater flux of air is forced
out of the chamber 102 than is forced into the chamber 102. For
example, the intake and exhaust subsystem 150 may be configured to
exhaust 2000 cubic feet per minute (CFM) of air and deposition
chamber constituents from the deposition chamber 102, while the
blower 125 may force 500 CFM of filtered air into the deposition
chamber 102. The resulting pressure differential is preferably
addressed by the plurality of passive air intakes 136, 138 that
passively allow filtered external air into the deposition chamber
102.
[0051] In this embodiment, the exhaust component of the subsystem
150 inherently is provided an allowance for sub-optimal
performance, as long as it exhausts air and chemical constituents
at a greater rate than the active air intake component of the
subsystem 150 (i.e., the blower 125) drives air into the chamber
102. Potentially dangerous fumes from the deposition chamber 102
are therefore controlled efficiently and at low cost compared to a
subsystem that may be designed to precisely balance between intake
and exhaust. Furthermore, according to this embodiment, the
deposition system 100 may be compatible with a greater variety of
exhaust implementations, in that such exhaust implementations only
need to meet certain minimum exhaust requirements related to a
relatively low active air intake specification. Such flexibility in
permissible exhaust performance is particularly advantageous
because the intake and exhaust subsystem 150 may be only partially
design-integrated with the rest of the deposition system 100, where
an exhaust motor for the intake and exhaust subsystem 150 is
supplied externally. For example, the deposition system 100 may be
connected to the exhaust fixtures, including the exhaust motor, of
a building or other structure that houses the deposition system
100.
[0052] The intake and exhaust subsystem 150 preferably provides a
laminar flow of air in the deposition region 146. The laminar flow
provided by the intake and exhaust subsystem 150 preferably assists
in maintaining a focused stream of heat and chemical vapor from the
burner towards the workpiece. A narrow and tight stream of flame
enhances the thermophoretic effect that attracts the SiO.sub.2
particles to the workpiece because the SiO.sub.2 particles get
hotter while the surface of the workpiece remain relatively
cooler.
[0053] The enclosure 104 for the deposition chamber 102 preferably
is comprised of preferably two "skins" or sets of walls that
enclose the components that perform the deposition process. An
inner skin of the two skins encloses the deposition components and
is preferably entirely comprised of Hasteloy. Hasteloy is
preferably used for the inner skin because of its chemically
resistive properties at high temperatures. In an alternative
embodiment, the inner skin of the enclosure 104 is comprised of
stainless steel and has its interior walls lined with Hasteloy. In
yet another alternative embodiment, the inner skin of the enclosure
104 is comprised of stainless steel with an interior lining of
Teflon. Stainless steel, however, has generally been observed to
rust due to the presence of HCl in the reaction chamber.
Furthermore, the lining of Teflon generally may begin to peel from
the steel surface at temperatures above about 420.degree. C.
[0054] The outer skin of the enclosure 104 preferably is separated
from the inner skin by an air gap, and preferably completely
encases the inner skin. Alternatively, the outer skin encases a
portion of the inner skin that is most exposed to the heat inside
the deposition chamber. The outer skin is preferably formed of
stainless steel, stainless steel lined with Hasteloy, stainless
steel lined with Teflon, or another convenient material or
combination of materials. The outer skin is preferably not
substantially exposed to corrosive chemicals like the inner skin,
so any of the above alternative compositions may be conveniently
selected. The outer skin preferably includes numerous vents 120 to
cool the inner skin of the enclosure 104 and otherwise prevent the
deposition chamber 102 from overheating. In one alternative
embodiment, the air gap between the outer skin and the inner skin
is actively cooled. FIG. 1A depicts the vents 120 on a front side
of the enclosure 104, on the door 105 and on the removable panels
107 of the deposition system 100. Preferably, a similarly
configured array of vents 120 is provided on the opposing side of
the enclosure 104 of the deposition system.
[0055] Large quantities of oxygen (O.sub.2) and fuel gas, typically
in the form of hydrogen (H.sub.2) or natural gas, are passed
through the deposition chamber 102 to enable the deposition process
of converting SiCl.sub.4 into SiO.sub.2 soot that is deposited in
layers onto a workpiece. During the deposition process, even with
the continuous flow of air, the inner skin of the enclosure 104
rapidly heats up. Due to the preferably advantageous heat transfer
properties of the inner skin, heat in the deposition chamber 102 is
preferably conducted to the inner skin's outer surface. The high
temperature of the inner skin's outer surface generally causes the
air between the inner skin and the outer skin to heat. The vents
120 on the outer skin preferably enable cooler air outside the
deposition system 100 to enter one or more vents 120, and enable
the hotter air in the air gap between the two skins to exit other
vents 120.
[0056] In a preferred embodiment, the vents 120 are positioned in
at least two rows, including an upper and a lower row. External air
preferably enters the lower row of vents 120 and hot air exits the
upper row of vents 120. By funneling hot air out of the air gap
between the two skins of the enclosure 104, heat is effectively
transferred out of deposition chamber 102, through the inner skin,
and out through the vents 120 on the outer skin of the enclosure
104. Consequently, maintaining the temperature of the workpiece in
the deposition chamber 102 is furthered, so that the average
temperature of the workpiece is preferably significantly lower than
the temperature of the silica soot particles that are expelled from
the burner. Typically, the temperature of the workpiece is greater
than but follows the temperature of the deposition chamber. Thus,
an increased thermal gradient between the hot silica soot particles
and the cooler workpiece is provided than may exist without the
vents 120, enabling an increased thermophoretic force that attracts
the silica soot particles to the surface of the cooler
workpiece.
[0057] FIG. 2 is a diagram depicting a functional view of the
chemical vapor deposition system 100 generally shown in FIG. 1. As
depicted in FIG. 2, the chemical vapor deposition system 200
preferably includes a SiCl.sub.4 source 202, a nitrogen (N.sub.2)
source 204, an oxygen (O.sub.2) source 206, and an H.sub.2 source
208 as raw materials for the vapor deposition system 200.
Alternatively, the N.sub.2, O.sub.2, and H.sub.2 sources 204, 206,
208 may be piped in from an external location. The deposition
system 200 preferably further includes a computer 106, 210, a gas
panel 114, 212, a preheater 214, and a vaporizer 216 for
controlling the flow of the materials used for the deposition
process.
[0058] The deposition system 200 preferably includes a deposition
chamber or cabinet 218, enclosing one or more, and preferably two
chemical burners 220, a lathe 222 for holding a workpiece 224 and
for moving the workpiece 224 rotationally and translationally
relative to the one or more burners 220. The deposition chamber 218
preferably encloses one or more end-torches (not shown) positioned
near the ends of the workpiece 224, and which move with the
workpiece 224 (in an embodiment in which the workpiece 224 moves
and the chemical burner 220 remains stationary). The end torches
preferably direct heat to the ends of the workpiece 224 to prevent
it 224 from breaking and/or cracking. Preferably, the workpiece 224
and the end torches move so that the exhaust around the chemical
burner 220 is relatively constant. Alternatively, the chemical
burner 220 is moving and the workpiece 224 and end torches are
stationary (except for the rotation of the workpiece 224). The
deposition system 200 preferably further includes an air intake and
exhaust subsystem 150, 226 including scrubbers (not shown) and
other pollution control devices for removing and collecting the
gasses and vapors that are expelled by the deposition system
200.
[0059] The computer 106, 210 preferably includes electronic
connections to the vaporizer 216, the gas panel 212, and the
deposition cabinet 218 for automatically controlling functions of
each component. The computer 106, 210 preferably further includes a
connection to a user-input device such as a keyboard, touch screen,
knobs, buttons, switches, mouse and/or microphone for voice
activated command input for providing operational control of the
deposition system 200 to a user. Moreover, the computer 106, 210
preferably includes a user output device, such as a display monitor
or speaker for presenting a status of the system.
[0060] The raw deposition materials' sources 202, 204, 206, 208 are
preferably reservoirs, which may be commercially available
pressurized tanks for containing each constituent material. The
SiCl.sub.4 preferably is contained in a reservoir in liquid form,
preferably at room temperature. The SiCl.sub.4 source 202
preferably is connected by a pipe or line to the preheater 214,
such that SiCl.sub.4 may be conveyed as a liquid into the preheater
214. Preferably, positioned above the SiCl.sub.4 source 202 is an
exhaust port 203 to convey SiCl.sub.4 to a pollution control system
(not shown) in event of a leak of SiCl.sub.4 from its source
202.
[0061] Preferably, N.sub.2 is used to maintain a pressure of
preferably about 15 PSI on the SiCl.sub.4 source 202 to enable flow
of SiCl.sub.4 out of the SiCl.sub.4 source 202 and into the
preheater 214. Generally, N.sub.2 is used throughout the deposition
system 200 during non-operation to purge the components of the
corrosive chemicals, such as HCl and SiCl.sub.4, which are used or
produced when the deposition system 200 is in operation.
[0062] The preheater 214 preferably is a commercially available
device comprising a container for holding small volumes of
SiCl.sub.4 and may include a heating element such a heating blanket
around the container or a coil in the container for heating the
liquid form of SiCl.sub.4. Alternatively, the preheater 214 may be
a long line wrapped in heating tape that heats the flowing
SiCl.sub.4 liquid to a desired temperature before it reaches the
vaporizer 216. Preferably, the preheater 214 receives the
SiCl.sub.4 liquid at preferably room temperature and heats the
SiCl.sub.4 liquid to a temperature of preferably about 50.degree.
C. Preferably, the preheater 214 maintains a substantially constant
level of SiCl.sub.4 in its container throughout an operation of the
deposition process. The preheater 214 is connected to the vaporizer
216 for transferring the heated SiCl.sub.4 liquid out of the
preheater 214 and into the vaporizer 216.
[0063] The vaporizer 216 preferably is a commercially available
device that comprises a container for containing a substantial
volume of SiCl.sub.4, a heating element to heat the SiCl.sub.4 in
the container, and numerous valves (not shown) to regulate the flow
of materials into and out of the vaporizer 216. The vaporizer 216
preferably heats the SiCl.sub.4 to a temperature of between about
70.degree. C. and about 80.degree. C. Preferably, the SiCl.sub.4 is
heated until it is boiling in the vaporizer 216. The variation in
temperature between about 70.degree. C. and about 80.degree. C. for
the vaporized SiCl.sub.4 preferably depends on the mass flow rate
of SiCl.sub.4 vapor that is required at the burner 220 for the
deposition process. Higher temperatures generally provide higher
mass flow rates.
[0064] The vaporizer 216 is preferably electronically connected to
the computer 210. Through this electronic connection, the volume of
SiCl.sub.4 in the vaporizer 216 is preferably regulated and
maintained between a minimum and maximum level. The computer 210
preferably controls the flow of SiCl.sub.4 liquid from the
SiCl.sub.4 source 202 to the vaporizer 216 from a solenoid valve
217 preferably located at or near the pneumatic input to the
vaporizer 216.
[0065] The vaporizer 216 is also pneumatically connected by a line
to the N.sub.2 source 204. As a generally inert gas, the N.sub.2
preferably is used in the vaporizer 216 both to purge parts of the
vaporizer 216 of SiCl.sub.4 when the deposition system 100, 200 is
not in operation and to actuate deposition system's numerous
pneumatic valves. Through control from the computer 210, the
SiCl.sub.4 source 202, the preheater 214 and the vaporizer 216
preferably provide a constant, automatic and prolonged flow of
vaporized SiCl.sub.4 from the vaporizer 216 to the one or more
burners 220 in the deposition cabinet 218. This characteristic of
constant flow only ceases when and if the SiCl.sub.4 source
reservoir 202 is emptied.
[0066] A constant flow of preferably 100% SiCl.sub.4 vapor out of
the vaporizer 216 to the burner 220 is provided because the
vaporizer 216 is preferably automatically refilled from the
preheater 214 by the computer 210 without disrupting the flow of
vaporized SiCl.sub.4 from the vaporizer 216 to the burner 220. The
preheater 214 allows the vaporizer 216 to maintain the SiCl.sub.4
contained within the vaporizer 216 at a temperature necessary to
have vaporized SiCl.sub.4 output from the vaporizer 216 at all
times during operation of the deposition system 100, 200. When the
level of SiCl.sub.4 in the vaporizer 216 drops below a
predetermined minimum level, the computer 210 actuates the valve
217 controlling input of SiCl.sub.4 to allow heated SiCl.sub.4 into
the vaporizer 216 from the preheater 214. This valve 217 is then
preferably closed when a predetermined maximum level of SiCl.sub.4
is reached in the vaporizer 216. To maintain the temperature of the
SiCl.sub.4 from the vaporizer 216 to the burner 220, the line
between the vaporizer 216 and the burner 220 for transferring the
SiCl.sub.4 vapor is preferably heated, such as with heating tape,
to maintain the SiCl.sub.4 as a vapor as the SiCl.sub.4 is conveyed
to the burner 220. Because the SiCl.sub.4 is a vapor, mass flow
controllers (MFCs) preferably are used to regulate the flow as
controlled by the computer 210.
[0067] The gas sources 204, 206, 208 are preferably pneumatically
connected to the gas panel 212. The gas panel 212 includes valves
and MFCs to regulate the flow of gasses from the gas sources 204,
206, 208. Control of the valves in the gas panel 212 is provided by
the computer 210, which is electronically connected to the gas
panel 212. Lines for O.sub.2 and H.sub.2 are provided to
pneumatically connect the gas panel 212 and the burner 220 in the
deposition cabinet 218. Further, a separate line is preferably
provided to convey O.sub.2 to the line carrying the vaporized
SiCl.sub.4 to the burner 220. Thus, at a "T" fitting 219, the
vaporized SiCl.sub.4 and O.sub.2 are mixed, and continue as a
mixture in their transport to the burner 220. Preferably, the
O.sub.2 line exiting from the gas panel 212 to the "T" fitting 219
is similarly heated using any convenient means such as heating
tape. Thus, the mixture of O.sub.2 and SiCl.sub.4 from the "T"
fitting 219 to the burner 220 is maintained at a temperature such
that the SiCl.sub.4 at least remains a vapor, and preferably may
have a temperature at about or above 100.degree. C. as it
approaches the burner 220.
[0068] Thus, four separate lines are input to the burner 220: a
line conveying a mixture of vaporized SiCl.sub.4 and O.sub.2, a
line conveying H.sub.2 or another convenient fuel gas, a line
conveying O.sub.2 for the combustion of hydrogen, and a line
conveying O.sub.2 to shield the SiCl.sub.4 and O.sub.2 mixture.
Preferably, a fixed ratio of H.sub.2 to O.sub.2 is maintained, such
as two-to-one H.sub.2 by volume. In one preferred embodiment, the
fixed ratio of H.sub.2 to O.sub.2 is three-to-one, due to the
higher effective deposition rate that is observed. FIG. 3A depicts
a preferred embodiment of a burner 220, 300 for use in the
deposition system 100, 200. The burner 300 preferably receives the
four streams, and emits preferably four streams from a burner face
302, each stream being emitted from one of at least four concentric
rings 304, 306, 308, 310 of emission holes.
[0069] As illustrated in the side view of the burner 300 in FIG.
3B, the innermost ring 304 of holes is provided to emit the
chemical stream 314 of vaporized SiCl.sub.4 and O.sub.2. The second
ring 306 preferably streams O.sub.2 alone. As shown in FIG. 3B,
this inner O.sub.2 ring 306 acts as an inner shield 312 to prevent
the oxidation reaction of the SiCl.sub.4 into SiO.sub.2 too close
to the burner face 302, which would eventually cause a build-up of
glass soot at the burner face 302. The third concentric ring 308 of
holes emits a stream 316 of the fuel gas, preferably H.sub.2. The
fourth (outer) ring 310 of holes preferably emits a stream of
O.sub.2, often referred to as the fuel oxygen 318, which is used in
the combustion process and to control the shape of the flame.
[0070] As the constituents are emitted from the burner 300, the
fuel gas and the oxygen are ignited. The SiCl.sub.4 particles react
in the flame at a controlled distance away from the face of the
burner 300. The SiCl.sub.4 particles passing through the flame are
oxidized to form silica soot that continue in a directed stream
toward a workpiece 224 that may initially be in the form or a start
rod. As silica soot approaches the workpiece 224, the silica soot
has a temperature on the order of about 1100.degree. C. The
chlorine is preferably separated from the other materials and
combines with hydrogen to ultimately form hydrochloric fumes (HCl).
These reactions generally apply to the deposition process for a
cladding on an optical fiber preform. Other constituents may be
used for chemical vapor deposition for other applications applying
the different embodiments and aspects of the chemical vapor
deposition system described herein.
[0071] Referring again to FIG. 2, the silica soot is deposited in
layers on a continuously moving workpiece 224. The workpiece 224 is
mounted on the lathe 222, which preferably rotates and translates
the workpiece 224 relative to the burner 220. As shown in FIG. 4,
the lathe 222, 400 preferably includes end holders 402 into which
the ends of the workpiece 224 (e.g., the start rod) are inserted.
The lathe 400, 222 further includes at least one and preferably two
motors 404 and 406 for moving the workpiece 224 relative to the
burner 220 both rotationally and translationally. The motors 404,
406 are preferably controlled by a computer 210, such as that
depicted in FIG. 2, for controlling the speed of rotation and
translation of the workpiece 224 throughout the course of the
deposition process on the workpiece 224.
[0072] In one preferred embodiment, a particular translation
characteristic is applied to the workpiece to minimize a slow
tapering effect on the resulting workpiece 224. This tapering
effect, or "footballing," as illustrated in FIG. 5 may cause a loss
of significant portion of the useful length of the preform when the
preform is finally ready for drawing. Because of the need to
maintain the proper diameter ratios for the core and cladding in
the final preform, the tapering effect in the deposition of soot on
the workpiece 224 causes a significant length at the ends of the
final preform to be unusable, or if used, typically results in a
degraded quality of optical fiber. Generally, the unusable portion
may be more than about 20 cm at each end of the preform.
[0073] The tapering effect may be substantially reduced by
translationally moving the workpiece 224 at a maximum speed of
greater than about 1.4 meters per minute, and preferably greater
than about seven (7) meters per minute during at least a part of
the deposition process, and preferably after the first several
deposition passes have been completed. In one embodiment, the lathe
222, 400 translationally moves the workpiece 224 according to a
motion profile, an example of which is depicted in FIG. 6. In FIG.
6, the example shows a motion profile where the maximum velocity is
eight (8) meters per minute. Preferably, as the ends of the
workpiece approach the burner, the workpiece decelerates at a
constant deceleration (e.g., -250 mm/sec.sup.2) and then
accelerates with an opposite constant acceleration (e.g., 250
mm/sec.sup.2). Preferably, the maximum speed and acceleration is
limited only by the stress limitations of the workpiece caused by
such motion. Applying such a motion profile, the unusable portion
at each end of the final preform may be significantly reduced.
[0074] The positive effect on the workpiece 224 due to the increase
in its translational speed may be explained by several factors.
First, because of the change in direction of the motion at the ends
of the workpiece 224, the workpiece 224 at the ends tends to be
hotter than at other locations. Greater heat at one location tends
to cause the soot on the workpiece 224 to densify more
substantially than at other locations where the temperature is
lower. This densification results in a smaller diameter workpiece
224 at these locations. The smaller diameter translates into a
smaller target for the streaming silica soot, and because a smaller
target results in lower collecting efficiency, the deposition rate
at the ends is reduced, thus increasing the tapering affect.
[0075] Second, the additional heat at the ends of the workpiece 224
generally decreases the temperature gradient between the silica
soot particles and the workpiece 224, and thereby, decreases the
thermophoretic effect. The reduction in the thermophoretic effect
causes a reduction in the deposition efficiency at the ends of the
workpiece 224. The diameter at the ends therefore increases even
more slowly than at other locations on the workpiece 224.
[0076] By translating the workpiece at speeds above a certain
threshold, preferably the workpiece 224 at no location exceeds a
predetermined temperature, and therefore the cumulative effect of
the above factors is greatly reduced. The result of a reduced
"footballing" effect translates into a larger useful preform and a
substantially increased effective deposition rate.
[0077] With respect to the rotation of the workpiece, in one
embodiment, the workpiece is rotated at a speed of greater than
about 60 rotations per minute (RPM) to maintain a substantial
thermophoretic effect between the soot particles and the workpiece.
In a preferred embodiment, the speed of rotation is randomly varied
between about 60 RPM and 80 RPM on each translational pass over the
burner to reduce a potential for "a rippling effect" in the
layering of soot on the workpiece.
[0078] FIG. 7 depicts a preferred embodiment of a process 700 of
performing chemical vapor deposition such as may be performed by
the chemical vapor deposition system 100 illustrated in FIG. 1A.
Optionally, in a first step 702, a length for a start rod is set.
In different runs of the vapor deposition system, start rods of
various lengths, preferably between about 0.8 meters and about 2
meters, may be used. Preferably, a length of a start rod is input
at an operator terminal and transmitted to a computer. The computer
then communicates with components of the deposition system that
have functions dependent on the start rod length. Specifically, the
lathe may be programmed according the length of the start rod that
is used for a particular run of the deposition process 700. When
the lathe receives the length value from the computer, the torch at
one end of the lathe is preferably automatically repositioned to
apply heat to one end of the rod. The other torch is preferably
stationary. Furthermore, the motor controlling the translation of
the rod executes a traverse motion profile that reflects the
entered length of the start rod.
[0079] In a next step 704, a first pass of depositing silica soot
is performed with a high flow of fuel gas and oxygen from the
chemical burner relative to the flow of SiCl.sub.4. As one example,
the flow rate of H.sub.2 may be about 300 standard liters per
minute, with O.sub.2 at about 100 standard liters per minute, and
SiCl.sub.4 at about 25 grams per minute. Furthermore, on this first
pass the traverse speed is relatively low, preferably at about 0.5
meters/minute. The resulting high heat of the soot stream and of
the workpiece on this first pass hardens the initial interfacial
layers between the start rod and the cladding layers that are
subsequently deposited, preferably preventing interface defects and
slippage of the soot over the start rod.
[0080] During the first pass and throughout a run of a deposition
process 700, certain parameters are preferably fixed throughout the
run. Specifically, the end torches at each end of the start rod
provide a flame that preferably provides a source of heat. The end
torches provide heat at the ends of the workpiece to prevent the
soot from cracking and to eliminate the soot slippage over the
start rod during sintering, by keeping the ends denser and tightly
adhered to the handle glass. The rod should be hot enough to affix
the ends of the soot to a particular point on the start rod.
However, if the end burners provide too much heat, then generally
the start rod bends. Furthermore, to enhance the effective
deposition rate, a fixed distance between the torch and the
workpiece is preferably maintained throughout the run. Thus, as the
workpiece increases in diameter, the chemical burner preferably
retreats from the axial center of the workpiece in conformance with
the increase in diameter of the workpiece.
[0081] In a next step 706, the traverse speed of the workpiece is
ramped up, preferably to a maximum speed exceeding seven meters per
minute. Furthermore, the flow rates of fuel gas and oxygen from the
chemical burner are ramped down and the flow rate of SiCl.sub.4 is
slowly increased.
[0082] In a next step 708, the flow rates of the fuel gas and
oxygen are ramped up slowly. Consequently, the density of the
workpiece is slowly decreased to near a threshold density, below
which the workpiece may begin to crack or break. This threshold
density is preferably approached to maximize the effective
deposition rate. For example, in the deposition of silica soot for
optical fiber preforms, the workpiece generally cracks at densities
below about 0.3 grams/cm.sup.3. However, to maximize the effective
deposition rate, the density is preferably maintained near the
about 0.3 grams/cm.sup.3 threshold. The approach of the threshold
minimum density, however, is preferably approached gradually to
minimize the potential for bubbles trapped in the preform.
[0083] While preferred embodiments of the invention have been
described herein, and are further explained in the accompanying
materials, many variations are possible which remain within the
concept and scope of the invention. Such variations would become
clear to one of ordinary skill in the art after inspection of the
specification and the drawings. The invention therefore is not to
be restricted except within the spirit and scope of any appended
claims.
* * * * *