|Title of Invention||
METHOD FOR PRODUCTION OF SILICON FROM SILICON HALIDE
|Abstract||A process for the production of silicon from halosilanes characterized in that in a first step the halosilane is converted with the production of a plasma discharge to a halogenated polysilane which in a second step is then decomposed to give silicon with heating.|
|Full Text||METHOD FOR THE PRODUCTION OF SILICON FROM SILYL
 The present invention relates to a method for the production of
silicon from silyl halides.
 In the state of the art, different methods for the production of
high-purity silicon are known. In an industrially established process,
trichlorosilane HSiCI3 is used, which is thermally decomposed on a
hot substrate, in the presence of hydrogen. The decomposition
temperatures lie in the range of 800-1300° C, for example in the
method described in the patent DE 1061593, which is known as the
"Siemens process." A decisive disadvantage of this method of
procedure is the low conversion rate of the trichlorosilane used,
which makes it necessary to work with large excess amounts of
hydrogen, at 3.5 to 16 times the amount in comparison to
stoichiometric use, in order to achieve a conversion of 20-40% of the
trichlorosilane to silicon. Another disadvantage is that trichlorosilane
must be produced, in cost-intensive manner, from metallurgical low-
purity silicon and hydrogen chloride HC1 (K. Hata, S. Nakamura, S.
Yasuda, Japan. Tohoku Daigaku Senko Seiren Kenkyusho Iho 23(1)
(1967) 45-54; GB 883326) or from tetrachlorosilane SiCI4 and
hydrogen (U.S. Pat. No. 4,836,997; U.S. Pat. No. 3,933,985; U.S. Pat.
No. 4,309,259). The latter method only achieves a low yield. In
addition, the trichlorosilane introduced into the process is converted
into SiC14, to a great extent, during the precipitation of silicon, thereby
making it impossible to simply return the chiorosilane into the
 In a variant of this process as disclosed in DE 1061593, for
example, the cheaper tetrachlorosilane is used as the silicon source,
but even lower yields of silicon are obtained, with even greater
 The precipitation methods of solid silicon from silyl halides at
high temperatures have the common disadvantage that particularly
these high temperatures promote a reverse reaction of the
precipitated silicon with HC1 that is also formed during the
precipitation, forming silyl halides. A method disclosed in U.S. Pat.
No. 3,625,846 counters this circumstance by means of intensive
cooling of the product gases.
 A starting compound in the case of which no equilibrium with
by-products can occur during the precipitation of silicon is monosilane
SiH4. However, this product must be produced in cost-intensive
manner, for example from trichlorosilane (DE 2,507,864).
 According to a more recent method; which is described in DE
1982587 CI, chlorosilanes are hydrogenated with hydrogen, to form
monosilane, by means of stoichiometric amounts of alkali metal in
 The required high activation energies for the reaction of
chlorosilanes with hydrogen are furthermore made available, in the
state of the art, by means of the use of plasmas. Thus, for example, a
capacitatively coupled plasma is utilized in GB 892014, for the
decomposition of SiCVH2 mixtures, in order to precipitate silicon on
hot surfaces (several hundred ° C). Chlorosilane/hydrogen mixtures
are also converted to melted silicon, in U.S. Pat. No. 4,102,985 and
U.S. Pat. No. 4,102,764, using an electric arc discharge, at normal
 Inductively coupled plasmas are also described in the state of
the art; for example, the excitation of an SiF4/H2/ Ar gas mixture to
produce a gas discharge by means of an induction coil is described in
US 2004/0250764 Al. The resulting silicon precipitates on silicon
particles that are passed through the plasma zone.
 A method described in GB 851290 precipitates elemental
silicon by means of the action of atomic hydrogen on SiC14, in the
manner of a remote plasma source. For this purpose, the atomic
hydrogen is generated by means of an electric discharge (50 Hz-100
MHz), at a pressure of 1 Torr, and the silyl halide is subsequently
metered in through a nozzle.
 In GB 823383, Si droplets are precipitated onto the electrodes
by means of the action of an electric arc between electrodes. The
electrodes are slowly drawn apart from one another, to the extent that
the silicon is growing. Furthermore, microwave radiation for plasma
generation in the production of Si is described, whereby very energy-
rich microwave pulses of 1 MW power are used in U.S. Pat. No.
2,945,797, in order to achieve coupling-in of the radiation. There,
contamination of the reactor wall with silicon is also mentioned, and
this is supposed to be countered by means of intensive cooling.
 The precipitation of crystalline silicon under reduced pressure,
by means of microwave discharge at low power, in H2 that is mixed
with 5% SiCI4, is described in the literature (P. M. Jeffers, S. H. Bauer,
J. Non-Cryst. Solids 57 (1983) 189-193). U.S. Pat. No. 4,908,330
discloses the production of thin films of silicon at less than 1 Ton
pressure, by means of the reduction of SiF4/Si2F6 with atomic
hydrogen, which is generated by means of a microwave discharge in
a separate plasma chamber (remote plasma).
 Microwave radiation is also used in U.S. Pat. No. 4,786,477,
U.S. Pat. No. 4,416,913, and U.S. Pat. No. 5,374, 413, in order to
achieve Si precipitation. In these methods, however, the purely thermal
effect of the radiation on glowing silicon is utilized in order to heat Si
particles to a high temperature, without any plasma formation taking
 Another form of silicon that can be precipitated by means of
plasma discharges is the so-called amorphous silicon, which usually
still contains certain amounts of other elements (H, CI, F, etc.). A
method is known from the literature, in which an amorphous silicon is
precipitated by means of an electric glow discharge in a gas mixture
SiCl4/H2/He, under normal pressure, which mixture contains not only H
but also about 1% CI (0. H. Giraldo, W. S. Willis, M. Marquez, S. L.
Suib,Y. Hayashi, H. Matsumoto, Chem. Mater. 10 (1998) 366-371).
 Finally, the transition to polysilanes/polysilylenes is made with
an increasing content of halogen and hydrogen in the silicon that is
produced; in these, only two of the four possible valences are
saturated off by means of bonds to additional Si atoms, on the
average. Chlorinated polysilanes are produced in to Deted manner, for
example as described in JP 62143814, by means of the conversion of
elemental Si with chlorine, in inert organic solvents, or as disclosed in
JP 59195519, by means of reaction of silicides with chlorine. U.S.
Pat. No. 4,374,182, EP 0282037 A2. JP 1197309, and JP 1192716
disclose the formation of silicon from chlorinated polysilanes S'„C12„ 2
by means of disproportionation or reduction with H2, at elevated
 High-purity silicon is also obtained, in the state of the art, by
means of transport reactions, using subhalogenides of Si having lesser
purity. A process known in the literature converts Sil4 with Si, to
produce Sil2i at high temperatures; the latter decomposes again at low
temperatures, with dispro-portionation (T. F. Ciszek, T. H. Wang, M.
R. Page, R. E. Bauer, M. D. Landry, Conference Record of the IEEE
Photovoltaic Specialists Conference (2002), 29th 206-209). Other
subhalogenides can also be used for the transport reaction; thus, for
example, the use of SiC12 is particularly described in GB 754554, and
the use of SiF2 is particularly described in U.S. Pat. No. 4,070,444
and U.S. Pat. No. 4,138,509.
 An economic advantage within a method for the production of
high-purity silicon results from the use of inexpensive, easily
accessible starting compounds. Preferred educts should therefore be
the tetrahalogen silanes SiX4, of which SiF4 and SiC14, in particular,
can be produced in cost-advantageous manner. For example, U.S.
Pat. No. 4,382,071 describes a method for the characterization of
SiF4 from material containing HF and SiO2. SiC14 can be produced by
means of carbochlorination of material containing silicon oxide,
SiOz+2 C12+2 C-SiC14+2 Co
 (Examples: EP 0167156 B1, JP 60112610, EP 0302604 Bl),
and occurs as a by-product in various technical processes, for
example in the production of HSiC13 from silicon and HC1, or within
the Siemens process. Another advantage of SiCI4, in particular, is
that it is available with great purity, with the purification technique
developed to industrial maturity for the Siemens process, just like the
HSiCI3 that is processed nowadays.
 Furthermore, the greatest possible conversion rates of the
starting compounds that contain silicon to elemental silicon, or at least
simple re-use, are desirable. If the use of hydrogen as a reduction
agent is unavoidable, the hydrogen feed should not lie significantly
above what is necessary stoichiometrically, if possible, in order to
allow comparatively small gas volumes per silicon produced, on the
one hand, and to have to purify as little hydrogen as possible for re-
use, on the other hand.
 Methods at temperatures above 1000° C. for the
characterization of silicon should be avoided, in order to keep the
thermal stress on the apparatus low, on the one hand, and to minimize
the risk of contamination of the silicon that is produced by
contaminants contained in the reactor walls, on the other hand.
 The present invention is based on the task of making available
a method for the production of high-purity silicon, which is
characterized by a high conversion rate of the starting compound that
contains silicon, a low demand for energy, and the use of cost-
advantageous starting compounds.
 This task is accomplished, according to the invention, in the
case of a method as indicated initially, by means of the characterizing
features of claim 1.
 Compounds of the type
H SiX4_„ (X=F, CI, Br, I; n=0-3)
as well as mixtures of them are referred to as silyl halides in the sense
of the method according to the invention.
 A halogenated polysilane in the sense of the method according
to the invention is, in particular, a pure compound or a mixture of
compounds having at least one direct Si Si bond, the substituents of
which consist exclusively of halogen or of halogen and hydrogen, and in
the composition of which the atomic ratio of substituents:silicon is at
least 1:1. In contrast to the state of the art, the halogenated polysilane
suitable for the production of silicon in the method according to the
invention is not limited to compounds having pure halogen substitution
and atomic ratios of substituent: silicon close to 2.
 The method according to the invention is character ized in that a
plasma process is used for the production of the halogenated polysilane
from silyl halide. In this connection the zone in which the reaction to the
polysilane takes place is characterized by comparatively low gas
temperatures, as well as by also comparatively low wall temperatures of
the reactor. In this way, the precipitation of elemental silicon in the
reaction zone and on the reactor walls is avoided.
 A "plasma" in the sense of the method according to the
invention is a gas or gas mixture that is characterized by a variable
proportion of non-neutral gas particles, which is elevated as
compared with the proportion that occurs by means of the natural
 A "plasma discharge" in the sense of the method according to
the invention is the generation of a plasma by means of applying
suitable forms of energy to a gas or gas mixture. The plasma
discharge is not necessarily accompaniedby optical effects, such as
visible light, depending on the conditions of the plasma generation.
 The halogenated polysilane formed in the first step of the
method according to the invention occurs as a liquid, partially also a
liquid having great viscosity, and/or as a wax-like, not very compact,
colorless to yellow-brown solid, and can easily be removed from the
reactor walls and from the reaction chamber, by means of a suitable
 The method according to the invention, for the production of
halogenated polysilane, can fundamentally be implemented by means
of two different embodiments of the plasma reaction:
 A. The plasma reaction takes place in the gas that contains
 In this coection, silyl halide can be present in a mixture with
hydrogen, hydrogen halide, and/or inert gases, as well as admixtures
that promote the plasma discharge.
 Silyl halide can also be used undiluted.
 B. Silyl halide and/or hydrogen is mixed into a gas stream
(that can already contain silyl halide), after this stream has passed
through a plasma zone (remote plasma).
 In this connection, the gas stream that passes through the
plasma zone can contain hydrogen or silyl halide and/or inert gases,
hydrogen halide, as well as admixtures that promote the plasma
 In both embodiments of the method according to the invention,
the plasma generation can be additionally sup-ported by means of
suitable measures. Non-limiting examples for such supporting
measures are, for example, the injection of electrons from a glow
cathode or an electron cannon, or the generation of free charge
carriers by means of applying a high voltage or the use of ionizing
 In a variant of Embodiment A of the method according to the
invention, a plasma is generated in a mixture of silyl halide and
hydrogen, under reduced pressure, by means of the action of an
electromagnetic alternating field, for example microwave radiation.
The irradiation can take place continuously or discontinuously.
 In another variant of Embodiment A, a high voltage (direct
voltage or alternating voltage) is additionally applied between two
electrodes situated outside of the discharge zone, in order to stabilize
the plasma, by means of which voltage the discharge current
generates free charge carriers in the discharge zone. As a result,
coupling of the electromagnetic alternating field into the gas mixture is
significantly facilitated, so that the plasma generation already
succeeds at clearly lower irradiation energy than without the high
voltage being applied. Furthermore, this high-voltage-supported
variant of the method according to the invention permits the use of
higher pressures within the reaction zone, so that the amount of
polysilane produced per volume and time is increased.
 In a variant of Embodiment B, a plasma is generated in a
hydrogen gas stream, under reduced pressure, by means of an
electromagnetic alternating field, for example micro-wave radiation,
whereby the plasma generation is supported by means of applying a
high voltage between two electrodes situated outside of the plasma
zone. Following the plasma zone, gaseous silyl halide is introduced
into the hydrogen stream; it is converted to halogenated polysilane.
 The precipitation of elemental silicon in the plasma generation
zone, particularly on the walls, has a negative effect on the plasma
generation in some variants of EmbodimentA of the method according
to the invention, so that in an extreme case, it is suppressed.
 The conversion of silyl halide to halogenated polysilane,
avoiding the precipitation of elemental silicon, is therefore essential
for these variants, since the polysilane does not markedly influence
the plasma generation.
 In the two embodiments A and B of the method according to
the invention, the precipitation of the halogenated polysilane is of
significant advantage as compared with the precipitation of elemental
silicon known from the state of the art, since the polysilane can be
more easily removed from the reactor, because of its consistency.
 The temperature of the reactor preferably lies at less than 400°
C. during the precipitation of the halogenated polysilane, preferably at
less than 300° C. By means of this low temperature, a back-formation
of the polysilane formed, with the HC1 that occurs as a by-product, to
form volatile halogenated monosilanes, is suppressed. At the same
time, the undesirable spontaneous formation of elemental silicon in
the reactor, for example by means of thermal decomposition of the
polysilane formed as the primary product, is avoided.
 The reactor walls can be cooled during the precipitation
reaction, by means of suitable measures, in order to control the
temperature. Examples of suitable measures are passing an air
stream by, or the use of cooling fluids adapted to the apparatus
 The temperature of the gas that contains silyl halide when it
enters into the reaction chamber preferably amounts to between minus
67° C. and 100° C. The temperature of the gas mixture that leaves the
reactor preferably amounts to less than 400° C.
 Preferably, silyl fluorides or silyl chlorides are used as silyl
halides. A particularly preferred starting compound is SiC14.
 The volume ratio of silyl halide:hydrogen prefer-ably amounts
to 1:0 to 1:20 in the production of the halogenated polysilane, more
preferably 1:2 to 1:10.
 For the production of the halogenated polysilane, the pressure
within the reaction zone preferably amounts to 0.1-1000 hPa, more
preferably 5-100 hPa.
 The halogenated polysilane obtained in the first step is
decomposed into silicon and volatile silanes in a subsequent second
reaction zone of the apparatus, with heating in the presence of slight
HCI concentrations, or the polysilane is first removed from the reactor,
and converted to silicon in a second reactor.
 The decomposition of the polysilane can also take place in the
presence of hydrogen andlor inert gases. A gas stream can support
removal transport of the volatile decomposition products.
 The decomposition of the polysilane can be sup-ported by
means of reduced pressure during heating.
 In order to decompose the halogenated polysilane, the latter is
preferably heated to a temperature of 400° C. to 1500° C, preferably
of 450° C. to 1000° C.
 In addition to elemental silicon, halogenated monosilanes
mainly occur in the thermal decomposition of the halogenated
polysilane; these can be passed back into the first step of the method
according to the invention.
 In the following, the invention will be described using
 A mixture of H2 and SiC14 (8:1) is passed through a quartz
tube having an inside diameter of 10 mm, at a pressure of 10-20 hPa,
and a weak glow discharge (-10 W) is generated within the tube by
means of a high voltage between two electrodes. Then, pulsed
microwave radiation (2.45 GHz) is radiated onto a stretch of 4.2 cm, at
pulse energies of 500 to 1500 W, and a pulse duration of 1 ms,
followed by a pause of 9 ms, corresponding to an average power of 50-
150 W. After 11 h, the brown to colorless oily product is heated in a
tubular heater, under vacuum, to 800° C. A gray-black residue forms
(2.2 g), which was confirmed as crystalline Si by means of X-ray
powder diffractometry. In addition to SiC14, several grams of a
yellowish oil were distilled off; this can also be decomposed to Si, by
means of rapid heating.
 In a flask, 1.48 g SiCI4 were evaporated in 2 L hydrogen, at
room temperature, corresponding to a mole ratio of 1:10. This mixture
was passedthrough a quartz tube having an inside diameter of 10 mm,
at a pressure of -20 hPa. Microwave radiation (2.45 GHz) was
continuously radiated onto a stretch of 4.2 cm, at a power of 200 W.
After 30 min, the experiment was terminated and the reaction tube was
heated to -700° C. using a Bunsen burner, in order to decompose the
colorless to brown chlorinated polysilane that had precipitated. The
yield amounted to approximately 150 mg silicon, corresponding to a
yield of about 60% with reference to the SiCI4 used.
 Pure, gaseous HSiC13 (864 mMol) is passed through a quartz
tube having an inside diameter of 25 mnm, at a pressure of 5 hPa. and
a weak glow discharge (-10 W) is generated within the tube by means
of a high voltage between two electrodes. Then, pulsed microwave
radiation (2.45 GHz) is radiated onto a stretch of 4.2 cm, at pulse
energies of 500-1000 W, and a pulse duration of 1 ms, followed by a
pause of 19 ms, corresponding to an average power of 25-50 W. After
6 h, 29 g (289 mMol=33% of theory, depending on the hydrogen
content, which was not determined) of a light-yellow to dark-yellow oil
was isolated, which in part flowed out of the reaction zone on its own
during the reaction. The oily product solidifies when stored at room
temperature for an extended time, to form a glassy, transparent solid.
5.82 g of the product are pyrolized in a tubular heater, under a weak
H Z stream, at a temperature of 900° C. 0.824 g 96.7% of theory (with
reference to the maximal yield in the disproportionation of polysilanes)
of a gray-black residue of elemental silicon form.
 A 2 L balloon is filled with a mixture of H2 and SiF4 (1:1; 45
mMol). The resulting gas mixture is passed through a quartz tube
having an inside diameter of 13 mm, at a pressure of 10-20 hPa, and a
weak glow discharge (-10 W) is generated within the tube by means of
a high voltage between two electrodes. Then, pulsed microwave
radiation (2.45 GHz) is radiated onto a stretch of 4.2 cm, at a pulse
energy of 800 W, and a pulse duration of 1 ms, followed by a pause of
19 ms, corresponding to an average power of 40 W. After approxi-
mately 7 h, 0.63 g (approximately 20% of theory) of a white to brownish
solid are obtained. When heated to 800° C. in a vacuum, the material
decomposes, and silicon is formed.
 According to the invention, the decomposition to silicon can
take place within or outside of a reactor. The gas in which the plasma
discharge is generated can also contain silyl halide.
1. A process for the production of silicon from halosilanes characterized in
that in a first step the halosilane is converted with the production of a
plasma discharge to a halogenated polysilane which in a second step is
then decomposed to give silicon with heating.
2. The process as claimed in claim 1, wherein compounds of the type
HnSiX4-n (X = F, CI, Br, I, n = 0 - 3) or mixtures thereof are used as
3. The process as claimed in claim 1 or 2, wherein the plasma discharge is
produced in a gas containing or consisting of halosilane.
4. The process as claimed in claim 1 or 2, wherein the plasma discharge is
produced in a gas to which a gas containing or consisting of halosilane is
5. The process as claimed in claim 3 or 4, wherein the gas in which the
plasma discharge is produced additionally contains one or more of
hydrogen, a diluting inert gas or admixtures promoting the plasma
6. The process as claimed in one of the preceding claims, wherein the
plasma discharge is assisted by the introduction of free charge carriers
into the discharge zone.
7. The process as claimed in claim 6, wherein the free charge carriers are
produced by applying a high voltage between electrodes.
8. The process as claimed in one of the preceding claims wherein the
plasma discharge is produced by radiating an alternating electromagnetic
9. The process as claimed in one of the preceding claims, wherein
production of the halogenated polysilane is effected at a pressure 0.1 to
10. The process as claimed in one of the preceding claims, wherein the entry
temperature of the halosilane-bearing gas into a reactor in which
production of the halogenated polysilane is effected is between minus
67°C and 100°C and the temperature of the gas mixture leaving the
reactor is between minus 67°C and 400°C.
11. The process as claimed in one of the preceding claims, wherein the
temperature of the walls of a reactor in which the production of the
halogenated polysilane is effected are cooled.
12. The process as claimed in one of the preceding claims, wherein the
temperature for decomposition of the halogenated polysilane is 400°C to
13. The process as claimed in one of the preceding claims, wherein
decomposition of the halogenated polysilane is effected under a reduced
14. The process as claimed in one of the preceding claims, wherein during
decomposition of the halogenated polysilane a gas flow is passed through
the apparatus used.
15. The process as claimed in claim 14, wherein the gas flow which is passed
through the apparatus during the decomposition of the polysilane contains
16. The process as claimed in claim 14, wherein the gas flow which is passed
through the apparatus during the decomposition of the polysilane
comprises inert gases.
17. The process as claimed in one of the preceding claims, wherein
decomposition to give silicon is effected within or outside a reactor.
18. The process as claimed in one of the preceding claims, wherein the gas in
which the plasma discharge is produced contains hydrogen halide.
A process for the production of silicon from halosilanes characterized in that in a
first step the halosilane is converted with the production of a plasma discharge to
a halogenated polysilane which in a second step is then decomposed to give
silicon with heating.
|Indian Patent Application Number||4892/KOLNP/2007|
|PG Journal Number||37/2011|
|Date of Filing||17-Dec-2007|
|Name of Patentee||SPAWNT PRIVATE S.A.R.L.|
|Applicant Address||16, RUE JEAN I AVEUGLE, L-1148, LUXEMBOURG, LUXEMBOURG|
|PCT International Classification Number||C01B 33/03|
|PCT International Application Number||PCT/DE2006/000891|
|PCT International Filing date||2006-05-23|