Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a process for the germanium-
doping of optical waveguide base material based on vitreoussilica. More particularly, it relates to such a process wherein
a gas that can be oxidized to form silica is reacted with
oxygen, or with gases that release oxygen under the reaction
conditions, in the presence of germanium tetrachloride.
It is known, in optical waveguide technology, how to
increase the refractive index of vitreous silica by doping with
germanium. For this purpose, in the customary processes, a
specific amount of germanium tetrachloride is reacted,
simultaneously with the reaction of the vitreous-silica-former
silicon tetrachloride, with oxygen, by which means a vitreous
silica doped with germanium is finally produced.
During the manufacture of optical waveguide base
material, e.g., according to the so-called MCVD (modified
chemical vapor deposition~ process at a reaction temperature of
greater than 1430C, however, only approximately 20% of the
germanium tetrachloride used, in fact, reacts to form GeO2,
while approximately 80% remains unreacted.
As i~ explained in a publication of M.P. Bohrer, J.A.
Amelse, P.L. ~arasimham, B.K. Tariyal, J.M. Turnipseed and R.F.
Gill (9th European Conference on Optical Communication,
H. Melchior and A. Solberger (editors), Elsevier Science
Publishers B.V. (North-Holland), 1983, pages 365-368) and in
works cited therein, the cause lies in the unfavorable balance
of the equilibrium
GeC14 + 2 ' GeO2 + 2C12
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which is displaced toward the left in the direction of the
volatile GeC14 as a result of high chlorine concentrations,
from the reaction of the silicon tetrachloride. Furthermore,
only 30 - 40% of the GeC14 which has reacted to form GeO2
is, in fact, deposited as so-called "soot" and can therefore act
as dopant. Losses of such magnitude are economically
unacceptable in view of the high cost of GeC14.
The above-mentioned publication does indeed describe a
process according to which the major part of the unused
germanium is recovered from the exhaust flow and, after
reprocessing, is reused. This does not alter the fact that the
actual reaction of GeC14 to form GeO2 occurs with
unsatisfactory yields so that it is still necessary to supply
large amounts of expensive GeC14 in order to obtain a
relatively small amount of dopant.
This object of the present invention is therefore to
provide a process which permits effective germanium-doping of
vitreous silica.
The object is accomplished by a process in which
silicon chlorides of the formula SinC12n~2, in which 'n' is
an integer from 2 to 6 are used as gases to be oxidized to form
silica.
Russian application 88 74 63, laid open on December 7,
1981, authors W.F. Kotschubej et al., which relates to the manu-
facture of highly dispersed silica of high purity by reacting
Si2C16 with oxygen discloses the possibility of using the
resulting SiO2 for the manufacture of optical fiber. In that
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process, however, the SiO2 occurs in undoped form and, under
the conditions mentioned, cannot be effectively doped, and thus
cannot be considered for the manufacture of doped vitreous
silica.
The doplng process according to the invention can, in
principle, be used for all methods which are known to the person
skilled in the art and are customarily used for the manufacture
of germanium-doped optical waveguide base material based on
vitreous silica and in which the doping is effected by the
gaseous phase reaction of germanium tetrachloride. Such
processes are, e.g., the so-called MCVD (modified chemical vapor
deposition~ process, the IVPO (inside vapor phase oxidation)
process, the OVPO (outside vapor phase oxidation) process or the
VAD (vapor axial deposition) process.
It is also possible to manufacture optical waveguide
base material doped with different dopants by carrying out only
the germanium-doping according to the process of the invention
and using conventional processes, e.g., the reaction of silicon
tetrachloride with oxygen in the presence of the particular
doping compound selected, for the doping with other dopants,
e.g., boron or phosphorus.
When selecting suitable silicon chlorides, two
factors, especially, are to be considered. On the one hand, the
lower the amount of chlorine released during the formation of
quartz from the silicon chlorides, the lower the germanium loss
to be expected. Accordingly, it would be desirable to use
chlorosilanes that are as rich in silicon as possible, such as
g ' 4 110' si5cll2 or Si6C114. On the other
hand, it is relatively difficult to convert these halides into a
gaseous phase, and therefore their use requires extra
experimental equipment, e.g., the use of a vacuum. Such
processes are described, e.g., in DE-PS 24 44 100 for
SiC14/oxygen at 1-10 Torr and can be appropriately applied
also to ch]orosilanes that are richer in silicon.
In the process according to the invention, however, it
is preferable to use octachlorotrisilane or, especially,
hexachlorodisilane, both of which can be easily converted into
the gaseous p~ase owing to their advantageous boiling points and
therefore allow a less expensive operating procedure, even
though both form a relatively large amount of chloride when
reacted.
Silicon chlorides, SinC12n+2, in which "n" is an
integer from 2 to 6 especially hexachlorodisilane, occur as
by-products in exhaust gases, e.g., during the manufacture of
high purity silicon by the decomposition of trichlorosilane on
heated substrates or during the conversion of silicon
tetrachloride to form trichlorosilane, and they can be separated
out from the exhaust gases, e.g., by condensing out, and
subsequently be worked up and isolated by distillation. The
advantage of this process is that, in addition to using waste
products that could not be used hitherto, silicon chlorides
having a high purity are obta~ned. For this reason, it is
generally preferable to use silicon chlorides that occur in the
manufacture of high-purity silicon, rather than silicon
chlorides that can be obtained in another manner, e.g., from
silicides.
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Advantageously, the particular silicon chlorides
selected are used in the purest form possible, that is to say,
if possibLe without the addition of other silicon chlorides, in
order to make it possible to achieve an exact adjustment of the
silicon amount, i.e., silicon : germanium ratio. Special
attention should also be paid to the removal of any fractions
containing hydrogen.
During the glass-formation reaction, in which vitreous
silica having the desired doping is formed by reactinq silicon
chloride, germanium chloride and oxygen, temperatures within the
range of 1100 - 1600C are advantageously maintained. They can
be adjusted, according to the manufacturing process selected,
e.g., by using burners, plasma heating or resistance heating.
The oxygen required for the reaction can be conveyed
to the reaction zone in the form of one or more oxygen gas fLows
which, at the user's option, may already be charged with the
reactants. It is also possible to consider replacing the
oxygen, either partially or completely, by other gases that
release oxygen under the reaction conditions, e.g., nitrous
oxide, nitric monoxide or carbon dioxide. It is also possible
to add inert gases, such as nitrogen or argon, e.g., to
influence the flow and concentration ratios.
In this connection, it has proven useful to pretreat
"auxiliary gases", e.g., helium, neon, argon, nitrogen, oxygen,
nitrous oxide, nitric oxide, carbon dioxide, Freons, and halo-
gens, used primarily as carrier gases or oxidizing agents in the
manufacture of qlass fibers, that are used with D2O according
to Canadian Application No. 465,992, filed October 9, 1984, since
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in this manner vitreous silica having an especially low
OH-content is ohtained.
When manufacturing doped vitreous silica according to
the process of the invention it is possible, in principle, to
use the procedures which are known for the conventional methods
based on silicon tetrachloride. In general, it is sufficient to
feed in the selected silicon chloride, preferably hexachlorodi-
silane, in place of the SiC14. This can be carried out in a
simple manner, e.g., by loading an evaporator provided for
char~ing the auxiliary gas, e.g., oxygen, with silicon chloriae
preferably Si2C16 or Si3C18 in place of SiC14, and
thermostatically heating it to the temperature corresponding to
the desired vapor pressure of the silicon chloride selected.
Obv;ously, the proportion of germanium tetrachloride, which is
generally conveyed to the reaction zone via a separate, second,
gas flow, has to be adapted to the germanium concentration
desired in the vitreous silica.
The use according to the invention of silicon
chlorides, SinC12n+2, in which "n" is an integer from 2 to
6, especially hexachlorodisilane (n=2), in the manufacture of
germanium-doped vitreous silica makes it possible to
considerably increase the utilization of the expensive germanium
tetrachloride used as a dopant, vis-a-vis the use of silicon
tetrachloride, without great additional expenditure on apparatus.
In the following, the invention will be more fully
described in an example, but it should be understood that it is
given by way of illustration only, and not of limitation.
Example
~ exachlorodisilane, which has been separated, by
conaensation and subsequent distillation, from the exhaust gas
mixture resulting during the manufacture of high purity
elemental silicon by decomposition of trichlorosilane on heated
substrates, was placed in an evaporator. An oxygen flow,
preheated to 80C, was bubbled through the fluid, maintained at
130C. Into the resulting oxygen flow, saturated with
Si2C16, was fed an oxygen flow that had been saturated in an
analogous manner with germanium tetrachloride. To prevent
condensation of the components, the combined gas flows were
conveyed via thermostatically heated lines to the reaction
zones, where they were blown through a nozzle into a vitreous
silica tube heated to 1500C by means of a tubular furnace. The
reaction produced a vitreous silica soot doped with germanium,
which was collected in a collecting vessel.
During the course of the reaction, a total of 150 ml
of Si2C16 and 80 ml of GeC14, corresponding to a molar
ratio of 1 : 0.77, was reacted with an approximately two-fold
excess of oxygen. 126 g of a pulverulent substance, which could
be sintered to form a glass article and which contained 9.9
mole-% of GeO2 (corresponding to a mixture of 22 g of GeO2
and 104 g of SiO2), were isolated. The resulting yield of
SiO2 was 98.2%, based on the Si2C16 used, and the yield of
GeO2 was 27.9%, based on the GeC14 used.
In a comparison test, a mixture of silicon
tetrachloride and germanium tetrachloride having the same Si/Ge
ratio (molar ratio 2 : 0.77) was reacted with an approximately
two-fold excess of oxygen, under conditions which were otherwise
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identical. The resulting product had a content oE 7.2 mole-% of
GeO2. The yie].d of SiO2 was 96.0%, based on the SiC14
used, while the yield of GeO2, based on the GeC14 used, was
only 19.5%.
Thus, while only several embodiments and an example of
the present invention have been described, it will be obvious
that many changes and modifications may be made thereto, without
departing from the spirit and scope of the invention.
