Preparation of trichlorosilane by plasma hydrogenation of silicon tetrachloride

Article abstract of DOI:10.1134/S0020168506090172

We have studied silicon tetrachloride hydrogenation in an rf (40.68 MHz) plasma and have determined the trichlorosilane yield as a function of the molar energy input, H₂: SiCl₄ molar ratio, and pressure. The highest trichlorosilane y

Full text of DOI:10.1134/S0020168506090172

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 9, pp. 1023–1026. © Pleiades Publishing, Inc., 2006.
Original Russian Text © A.V. Gusev, R.A. Kornev, A.Yu. Sukhanov, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 9, pp. 1123–1126.
Preparation of Trichlorosilane by Plasma Hydrogenation
of Silicon Tetrachloride
A. V. Gusev, R. A. Kornev, and A. Yu. Sukhanov
Institute of Chemistry of High-Purity Substances, Russian Academy of Sciences,
ul. Tropinina 49, Nizhni Novgorod, 603950 Russia
e-mail: gusev@ihps.nnov.ru
Received April 6, 2006
Abstract—We have studied silicon tetrachloride hydrogenation in an rf (40.68 MHz) plasma and have deter-
mined the trichlorosilane yield as a function of the molar energy input, H2 : SiCl4 molar ratio, and pressure. The
highest trichlorosilane yield achieved is 60%, and the minimum energy input is 0.3 kW h per mole of SiHCl3.
DOI: 10.1134/S0020168506090172
INTRODUCTION
The power of the rf generator was 340 W, and it was
operated at a frequency of 40.68 MHz. The power sup-
plied to the discharge zone was 110–120 W, and the
Silicon tetrachloride is a major by-product in silicon
production through hydrogen reduction of trichlorosi-
lane and in silane production through trichlorosilane
disproportionation [1]. In connection with this, there is
considerable interest in the development of effective
processes for silicon tetrachloride conversion to trichlo-
rosilane with the aim of achieving waste-free high-
purity silicon production.
9
H2O
SiCl4 can be hydrogenated using highly reactive
reductants [2–5] or high-temperature and catalytic
hydrogen reduction processes [6, 7]. Grankov and
Ivanov [8] described a commercial-scale process for
the catalytic hydrogenation of silicon tetrachloride at
elevated pressures of up to 4 MPa. The trichlorosilane
yield of this process is up to 38%. Considerable prom-
ise is offered by plasma processes for silicon tetrachlo-
ride hydrogenation [9–11]. Using such processes, one
can supply energy to a local region and activate more
effective reaction mechanisms.
The effect of process parameters on the trichlorosi-
lane yield of silicon tetrachloride hydrogenation in a
hydrogen plasma has not yet been studied in sufficient
detail. Sarma and Chanley [11] investigated the effect
of the H2 : SiCl4 ratio on silicon tetrachloride conver-
sion in an rf discharge, but they did not analyze the
trichlorosilane yield as a function of pressure or energy
input.
5
SiHCl3 + SiCl4 + H2
+ HCl
8
H2O
4
SiCl
RF
4
H2
To
6
a roughing
pump
3
H2
Liquid N
7
2
The purpose of this work was to assess the trichlo-
rosilane yield of the plasma hydrogenation of SiCl4 as
a function of the main process parameters: molar
energy input, pressure, and H2 : SiCl4 molar ratio.
2
1
Fig. 1. Apparatus for plasma hydrogenation of silicon tetra-
chloride: (1) H cylinder, (2) diffusion purification unit,
EXPERIMENTAL
2
The apparatus used in our experiments is shown
schematically in Fig. 1.
(3, 8) rotameters, (4) SiCl -filled bubbler, (5) reactor, (6) rf
generator, (7) trap, (9) susceptor.
4
1023

1024
GUSEV et al.
RESULTS AND DISCUSSION
50
40
30
20
10
Figure 2 shows the plot of the trichlorosilane yield
versus molar energy input. The molar energy input P
was evaluated from the power W supplied to the dis-
charge zone from the rf generator and the molar flow
rate Q of the plasma gas. The power was maintained
constant at 115 10 W. The molar energy input was
changed by varying the molar flow rate of the plasma
gas and was determined as
P = W/Q.
The molar flow rate of silicon tetrachloride was var-
ied from 0.15 to 0.7 mol/h, and the molar energy input,
from 118 to 550 kJ/mol.
100
200
300
400
500
As seen in Fig. 2, the trichlorosilane yield is about
40%, independent of the energy input. The minimum
energy input per mole of trichlorosilane is ~0.3 kW
h/mol and is determined by the minimum power needed
to maintain a stable discharge at the maximum flow rate
of the plasma gas.
Energy input, kJ/mol
Fig. 2. Trichlorosilane yield as a function of the molar
energy input for SiCl conversion in a plasma discharge.
4
It was of interest to estimate the gas temperature in
the discharge zone. To this end, we evaluated the power
going to the heating of the gas in the discharge zone
using a heat-balance equation taking into account the
main contributions to heat exchange: the power
removed from the plasma discharge zone through the
electrodes (Wel), radiation heat loss (Wrad), and the
power going to the heating of the gas (Wgas):
molar flow rate of the plasma gas, H2 + SiCl4, was var-
ied from 0.7 to 3.42 mol/h. During the process, the
pressure was varied from 0.7 to 101.08 kPa. The plasma
reactor had the form of a quartz tube with electrodes
along its axis. Lerage and Gerard [12] used tungsten
and copper electrodes. The electrodes in our reactor
were of silicon [13]. The use of silicon electrodes offers
the possibility of reducing the contamination level in
reaction products. After initiating a discharge by apply-
ing the rf voltage from the generator to the electrodes
through a tuner, a mixture of silicon tetrachloride vapor
and hydrogen was introduced into the discharge zone.
The mixture was prepared by bubbling hydrogen
through liquid silicon tetrachloride at a constant tem-
perature. The flow rates of the reagents were monitored
with rotameters and were controlled, together with the
pressure, using inlet and outlet valves. The plasma
power was determined calorimetrically as described by
Krasheninnikov [14].
W = Wel + Wrad + Wgas.
(1)
The total discharge power was determined calori-
metrically. The power removed through the electrodes
and the radiation heat loss were calculated using a heat
equation and the Stefan–Boltzmann law. From Eq. (1),
the power going to the heating of the gas was deter-
mined to be 15 3 W. The gas temperature Tgas in the
plasma discharge zone was evaluated from the relation
Wgas = CpQ(Tgas – Tin),
(2)
where Q is the flow rate of the reagents, Cp is the heat
capacity of the gas, and Tin is the gas temperature at the
reactor inlet. Tgas was found to be ~900 K.
In known plasma hydrogenation processes [16], the
gas temperature Tgas exceeds 2000 K and is close to the
electron temperature Te. The reaction involves atomic
hydrogen and proceeds according to the scheme [16]
We measured the trichlorosilane concentration in
reaction products at varied SiCl4 hydrogenation condi-
tions. The trichlorosilane content was determined by
gas chromatography [15]. The detection limit was 1%.
The effect of the energy input on the trichlorosilane
yield was studied at an H2 : SiCl4 molar ratio of 3.5.
The effect of the H2 : SiCl4 molar ratio (2.7–11.2) on
the trichlorosilane yield was investigated at atmo-
spheric pressure.
ç2
2ç0,
SiCl4 + 2H0
SiHCl3 + HCl.
The pressure effect on the trichlorosilane yield was
studied in the range 0.7–101.08 kPa at H2 : SiCl4 = 3.5,
6.2, and 8.9. The energy input was maintained constant
at 300 kJ/mol.
In the proposed process, Tgas is far lower, indicating that
the mechanism of trichlorosilane formation differs
from those in the processes described by Sivoshinskaya
et al. [16].
INORGANIC MATERIALS Vol. 42 No. 9 2006

PREPARATION OF TRICHLOROSILANE BY PLASMA HYDROGENATION
1025
It is of key importance, for commercial applications,
α, %
to be able to perform silicon tetrachloride hydrogena-
tion at atmospheric pressure. In connection with this,
we determined conditions that were optimal in terms of
the trichlorosilane yield of SiCl4 hydrogenation at a
60
1
pressure of 105 Pa.
48
The effect of the H2 : SiCl4 ratio on the SiCl4 con-
version to trichlorosilane and silicon (Fig. 3) was stud-
ied at a constant flow rate of silicon tetrachloride
(0.7 mol/h) and constant discharge power (110 W). The
highest trichlorosilane yield (44%) was achieved at
H2 : SiCl4 = 6.9.At higher hydrogen concentrations, the
trichlorosilane yield was lower. The silicon yield was
found to rise steadily from 8 to 47% with increasing
hydrogen concentration.
36
24
12
4
2
3
Note that our results on the trichlorosilane yield
(Fig. 3) agree well with earlier data [11].
Since the reaction products contained neither
SiH2Cl2 nor SiH3Cl, the hydrogenation process can be
represented by two reactions:
2
3
4
5
6
7
9
10 11 12
H2 : SiCl4
Fig. 3. (1) Overall SiCl conversion, (2) trichlorosilane
yield, and (3) silicon yield as functions of the H : SiCl
4
2
4
molar ratio in the starting mixture; (4) data from Sarma and
Chanley [11].
SiCl4 + H2
SiHCl3 + HCl,
Si + 3HCl.
(3)
(4)
SiHCl3 + ç2
1
A similar result follows from thermodynamic anal-
ysis [17, 18].
60
Silicon tetrachloride hydrogenation with yields of
up to 50% takes place at Tgas < 1000 K. This correlates
with our estimate, Tgas ~ 900 K, and with the observed
trichlorosilane yield, ~45%.
48
2
36
Figure 4 show pressure dependences of the SiHCl3
yield. In the pressure range 0.7 to 10.1 kPa, no trichlo-
rosilane was formed; the major reaction products were
hydrogen chloride, silicon, and polychlorosilanes.
Trichlorosilane formation in the reaction products was
observed at pressures above 13.1 kPa. The highest
trichlorosilane yield (60%) was achieved at 73.1 kPa.
3
24
12
The high trichlorosilane yield at pressures above
13.1 kPa suggests that trichlorosilane formation
133 266 399 532 665 798 931
Pressure × 10–2, Pa
*
involves excited hydrogen molecules, H2 :
Fig. 4. Pressure dependences of the trichlorosilane yield at
H : SiCl = (1) 6.2, (2) 3.5, and (3) 8.9.
H2 + e
H2–
H2 + e,
(5)
(6)
*
2
4
*
SiCl4 + H2
SiHCl3 + HCl.
Since the excitation cross section of the lowest vibra-
tional level of the hydrogen molecule as a function of
electron energy exhibits resonance [21], it is reasonable
to assume that the maximum in trichlorosilane yield at
a pressure of 73.1 kPa is due to the effective excitation
of vibrations of hydrogen molecules in the discharge.
High effectiveness of the vibrational excitation of mol-
ecules was also reported for some other plasma reac-
tions [20].
The vibration quantum of the H2 molecule is
0.53 eV [19], while the heat effect of reaction (3) is
0.77 eV. Thus, the energy of the lowest vibrational level
of the hydrogen molecule may notably reduce the activa-
tion energy of silicon tetrachloride hydrogenation [20].
The proposed mechanism is supported by the sharp
maximum in SiHCl3 yield at 73.1 kPa. As shown by
Rusanov and Fridman [20], the use of the vibrational
excitation energy of reagents is highly effective in sur-
It seems likely that raising the pressure to above
mounting the energy barrier of chemical reactions. 73.1 kPa reduces the fraction of vibrationally excited
INORGANIC MATERIALS Vol. 42 No. 9 2006

1026
GUSEV et al.
7. Iya, S.K., Production of Ultra-High-Purity Polycrystal-
hydrogen because of the rise in the rate of vibrational–
translational energy exchange. This may be responsible
for the reduction in trichlorosilane yield at increased
pressures. Moreover, in the pressure range in question
the gas temperature may rise, leading to partial thermal
decomposition of trichlorosilane.
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8. Grankov, I.V. and Ivanov, L.S., Intensification of Poly-
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pp. 60–64.
9. Sarma, K.R. and Rice, M.J., Jr., US Patent 4309259,
1985.
10. Karpov, A.P., Suris, A.L., and Shorin, S.N., Thermody-
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RF Patent 2142909, 1998.
CONCLUSIONS
The present results lead us to conclude that the pres-
sure in the reaction zone plays a key role in determining
the trichlorosilane yield of silicon tetrachloride plasma
hydrogenation. The highest trichlorosilane yield (60%)
is achieved at a pressure of 73.1 Pa. The maximum in
the trichlorosilane yield as a function of pressure and
H2 : SiCl4 ratio suggests that the formation of trichlo-
rosilane involves vibrationally excited hydrogen mole-
cules.
14. Krasheninnikov, E.G., Rusanov, V.D., Sanyuk, S.V., and
Fridman, A.A., Hydrogen Sulfide Dissociation in an RF
Discharge, Zh. Tekh. Fiz., 1986, vol. 56, no. 6.
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con Tetrachloride, Zh. Anal. Khim., 2001, vol. 56, no. 9,
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conductor Silicon Production), Moscow: TsNIItsvetmet
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ACKNOWLEDGMENTS
This work was supported by the Chemistry and
Materials Science Division of the Russian Academy of
Sciences through the Basic Research Program.
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