Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR FABRICATING SILICON OXYNITRIDE
FIELD OF THE INVENTION
The present invention relates to nitrogen doped silica, which may also be
called
silicon oxynitride or SiOXN~.. More particularly, the present invention
relates to
nitrogen doped silica formed by using silazane or siloxazane starting
materials.
BACKGROUND OF THE INVENTION
Silicon oxynitride is used in a variety of applications. The ability to vary
the
refractive index of silicon oxynitride over a wide range makes it an
attractive material
for optical applications. The refractive index of pure Si02 is 1.46, and the
refractive
index of Si3N4 is 2.1. Therefore, the refractive index of silica doped with
nitrogen can
be varied between 1.46 and 2.1. In addition, doping silica optical waveguides
with
nitrogen helps to prevent UV radiation damage to the waveguide which causes
1 S undesirable losses.
In optical waveguide applications, silicon oxynitride has been produced by
plasma and nonplasma CVD processes, using silane and/or ammonia gases. For
optical
applications, however, use of ammonia is undesirable because ammonia contains
hydrogen, and the resulting synthesized silicon oxynitride may contain a
substantial
proportion of hydrogen which significantly contributes to losses in the
waveguide.
In addition, silane raw materials must be handled very carefully due to the
violent reaction caused when air is int>;oduced into a closed container of
silane. Silane
is typically used in producing thin films on semiconductor substrates, which
requires
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the deposition of a film having good characteristics for semiconductor
applications. In
the manufacture of semiconductor thin films the properties of the film are
more
important than deposition rate. In the production of optical devices, however.
large
quantities of material must be produced quickly, and the deposition rates for
producing
optical devices such as optical waveguides are much faster than deposition
rates for
semiconductor thin films.
Silicon oxynitride may also be produced by the pyrolysis or hydrolysis of
organometallic halides such as silicon tetrachloride. However, use of halides
is not
favored because the pyrolysis and hydrolysis of these materials produces
chlorine or a
very strong acid by-product, hydrochloric acid (HCl). Hydrochloric acid is
detrimental
not only to many deposition substrates and to reaction equipment but also is
harmful to
the environment.
Additionally, it is difficult to produce bulk silicon oxynitride and waveguide
preforms using conventional outside vapor deposition (OVD) processes, which
expose
I S the deposited material to air. One difficulty encountered in forming
silicon oxynitride
using conventional OVD processes is that when processing occurs in a system
open to
air, oxygen atoms preferentially bond to silicon atoms over nitrogen atoms,
forming
silica instead of silicon oxynitride.
In a typical OVD process, a Garner gas is bubbled through a liquid organic
silicon containing compound. The resulting vaporous compound is transported to
a
burner via a carrier gas, wherein the vaporous gas streams are combusted in a
burner
flame fueled with natural gas and oxygen. The presence of oxygen in
conventional
OVD processes converts the vaporous reactants to their respective oxides,
exiting the
burner orifice to form a stream of volatile gases and finely-divided spherical
particles of
soot that may be deposited onto a substrate forming a porous blank or preform
of soot.
for example, silica soot.
U.S. Patent No. 5,152,819 to Blackwell et al., the disclosure of which is
incorporated by reference, describes the use halide-free silicon containing
compounds
including octamethylcyclotetrasilazane in an OVD process to produce high
purity fused
silica glass. Octamethylcyclotetrasilazane, [(CH3)2SiNH]4, hereinafter
referred to as
OMCTSZ, is a white solid at room temperature and has a boiling point of 225
°C. An
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OVD process described in U.S. Patent No. 5,152,819, which used OMCTSZ as a
feedstock for the process produced a pure silica soot with Less than 0.01 %
nitrogen
contained in the soot.
In view of the di~culties encountered in manufacturing silicon oxynitride,
there
is an explicit need for a method for manufacturing silicon oxynitride which
avoids the
aforementioned problems. Specifically, it would be desirable to provide a
method for
manufacturing silicon oxynitride which does not contain a substantial
proportion of
hydrogen. In addition, it would be desirable to provide a process which avoids
the
preferential bonding of oxygen atoms to silicon atoms, which results in the
formation of
pure silica.
SUMMARY OF INVENTION
Applicants have discovered a method for manufacturing silicon oxynitride
comprising the steps of providing a vaporous gas stream of a compound selected
from
the group consisting of siloxazanes and silazanes. As one example of
processing a
compound in accordance with the method of the present invention, solid
octamethyIcyclotetrasilazane (OMCTSZ) is heated, preferably to a temperature
of about
130 °C to about 225 °C, to provide OMCTSZ liquid, and a vaporous
gas stream may be
provided by bubbling an inert carrier gas through the OMCTSZ liquid to create
a
vaporous OMCTSZ gas stream. The vaporous silazane gas stream is delivered to
an
enclosed reaction site which is heated to a temperature of at least about 500
°C,
preferably between 700 °C and about 900 °C, where the gas stream
is converted into
particles of silicon oxynitride containing greater than 0.1 % nitrogen by
weight.
In an important aspect of the invention, the amount of oxygen present at the
reaction site is strictly limited to prevent formation of pure silica at the
reaction site and
to promote the formation of silicon oxynitride. Preferably the level of oxygen
at the
reaction site is limited to very low levels by controlling the partial
pressure of oxygen
in the enclosed reaction site. The amount of oxygen present at the reaction
site will
depend on the desired composition of the silicon oxynitride end product
produced by
the method of the present invention. The stream of vaporous silazane forms
silicon
oxynitride at the heated reaction site. In an alternative embodiment, the
stream of
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vaporous silazane gas can be combined with a vaporous gas stream of a silicon
containing compound such as octamethylcyclotetrasiloxane.
Thus. the present invention provides a method for manufacturing silicon
oxynitride which does not contain a substantial proportion of hydrogen and
provides a
method which avoids the preferential bonding of oxygen atoms to silicon atoms
encountered in OVD processes. Additional features and advantages of the
invention
will be set forth in the description which follows, and in part will be
apparent from the
description, or may be learned by practice of the invention. It is to be
understood that
both the foregoing general description and the following detailed description
are
exemplary and explanatory and are intended to provide further explanation of
the
invention as claimed.
DETAILED DESCRIPTION
Reference will now be made in detail to a preferred embodiment of the
invention. The present invention provides a method of manufacturing silicon
oxynitride using silazane or siloxazane starting materials.
The present invention provides a method of manufacturing silicon oxynitride
comprising the steps of providing a vaporous gas stream of a siloxazane or a
silazane
and delivering the vaporous gas stream to an enclosed reaction site, which is
heated to a
temperature of at least about 500 °C. As used in this application, the
term "silazane"
means an organosilicon nitrogen compound containing one or more silicon-
nitrogen
bonds, including amino silazanes, linear silazanes, and cyclosiloxanes,
wherein a
nitrogen atom and a single element or group of elements are bonded to the
silicon atom.
The term "siloxazane" as used in this application are compounds containing the
unit
[O-Si-N], including linear and cyclic siloxanes. A variety of silazanes and
siloxazane
may be used in the method of the present invention, including polysilazanes
and
polysiloxazanes.
Delivery of the vaporous silazane or siloxazane gas stream to the reaction
site
may be accomplished by using an inert carrier gas such as nitrogen, argon, or
helium.
Advantageously, the amount of oxygen present at the reaction site is strictly
limited to
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prevent the formation of silica. The amount of oxygen at the reaction site is
limited by
controlling the partial pressure of the oxygen in the enclosed reaction site.
In an exemplary embodiment, a vaporous siIazane gas stream is pro~-ided by
heating solid octamethylcyclotetrasilazane (OMCTSZ) to provide OMCTSZ liquid.
5 The solid OMCTSZ should be heated to at least about 120 °C,
preferably to about
140°C, to melt the OMCTSZ to its liquid state. The solid OMCTSZ may be
contained
in a vessel and heated with any suitable heat source such as a hot plate, an
oil bath, or
heat tape. The method may further comprise bubbling an inert carrier gas
through the
OMCTSZ liquid to create a vaporous OMCTSZ gas stream. As used in this
specification "inert gas" means a nonreactive gas, such as argon, nitrogen, or
helium.
The vaporous OMCTSZ is then delivered to an enclosed reaction site heated to
about
700°C to about 800°C, where the amount of oxygen is strictly
controlled to promote the
formation of SiOXNy. particles containing greater than 0.1 % nitrogen by
weight.
The enclosed reaction site may be, for example, a fused silica tube. The tube
may be placed in a furnace to heat the reaction site, or the tube may be
surrounded by a
heating element or a flame. By sealing the tube, the amount of oxygen inside
the tube
may be controlled. The vaporous OMCTSZ gas may be delivered by into the tube
by a
mass flow controller.
The oxygen present at the reaction site may be controlled several ways. For
example, delivering oxygen to the reaction site via a mass flow controller
enables
control of the amount of oxygen in the composition of the final SiON product.
Limiting the amount of oxygen present at the reaction site promotes the
formation
silicon oxynitride and prevents the formation of pure silica. The composition
of the
silicon oxynitride produced by the process of the present invention can be
varied
according to the desired end use of the material. The material may, for
example, be
used for optical waveguide applications, and the amount of nitrogen in the
silicon
oxynitride composition would depend on the optical properties of the waveguide
such
as the desired refractive index profile of the waveguide. For any desired
composition,
the optimum flow rate of the OMCTSZ gas stream and oxygen gas can be
determined
by experimentation.
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The amount of oxygen present at the reaction site may also be limited by
simply
enclosing the reaction site, for example in a sealed tube" and allowing the
reaction to
occur with the oxygen present in the ambient air inside the tube. Thus, to
form SiON, it
may not be necessary to supply any oxygen to the reactor tube. For example. in
one
experimental run, solid OMCTSZ was heated to 133 °C to form OMCTSZ
liquid, and
200 standard cubic centimeters per minute of nitrogen was bubbled through the
liquid
to form a vaporous OMCTSZ gas stream. The OMCTSZ gas stream was delivered to
a reaction site, which was a silica tube heated to 750 °C. No oxygen
was added to the
reaction site, and the final SiON material produced contained 25.84% oxygen,
as
determined by electron spectroscopy for chemical analysis (ESCA).
As mentioned above, the silicon oxynitride made by the method of the present
invention may be used for optical waveguides. As previously discussed, the
refractive
index of Si3N4 is higher than the refractive index of SiO,, By doping a silica
waveguide
with nitrogen to form SiOxNy, a waveguide core may be formed, over which a
silica
cladding may be added to form an optical waveguide. The amount of nitrogen
contained within the core material will depend on the desired refractive index
profile of
the waveguide.
If desired, the reaction stream of vaporous silazane or siloxazane can be
combined with the with reaction stream of another silicon containing organic
material
such as octamethylcyclotetrasiloxane, which is delivered to the reaction site
to provide
an additional silica source material. An optical waveguide preform may be
fabricated
by using the method of the present invention wherein silicon oxynitride is
deposited
inside a fused silica tube. Thus, the silicon oxynitride deposited material
forms a core
region having a higher index of refraction than the cladding region which may
comprise
the wall of the silica tube.
If the material formed according to the method of the present invention is to
be
used as a waveguide, the processes following the formation of the waveguide
blank
would follow those practiced in industry. In conventional practice, optical
waveguide
fabrication is a three-step process. Most of the processes currently used for
the
manufacture of optical waveguides involve a laydown process wherein a blank is
manufactured by a CVD process such as OVD, MCVD, AVD, or PCVD. The second
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stage of an optical fiber manufacturing process typically involves heat
treating the
blank in a helium/chlorine atmosphere to full consolidation. The third stage
the blank
is drawn into a waveguide, such as a waveguide fiber.
It will be apparent to those skilled in the art that various modifications and
S variations can be made in the of the present invention without departing
from the spirit
or scope of the invention. Thus, it is intended that the present invention
cover the
modifications and variations of this invention provided they come within the
scope of
the appended claims and their equivalents.
