Hydrocarbon impurities in SiF4 and SiH4 prepared from it

Article abstract of DOI:10.1134/S0020168507040061

Using gas chromatography and high-resolution Fourier-transform IR spectroscopy, we have determined the concentrations of C₁-C ₄ hydrocarbon impurities in isotopically unmodified silicon tetrafluoride before and after fine purificatio

Full text of DOI:10.1134/S0020168507040061

ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 4, pp. 364–368. © Pleiades Publishing, Inc., 2007.
Original Russian Text © A.D. Bulanov, P.G. Sennikov, V.A. Krylov, T.G. Sorochkina, L.A. Chuprov, O.Yu. Chernova, O.Yu. Troshin, 2007, published in Neorganicheskie Materialy,
2007, Vol. 43, No. 4, pp. 427–431.
Hydrocarbon Impurities
in SiF4 and SiH4 Prepared from It
A. D. Bulanov, P. G. Sennikov, V. A. Krylov, T. G. Sorochkina,
L. A. Chuprov, O. Yu. Chernova, and O. Yu. Troshin
Institute of Chemistry of High-Purity Substances, Russian Academy of Sciences,
ul. Tropinina 49, Nizhni Novgorod, 603950 Russia
e-mail: bulanov@ihps.nnov.ru
Received July 11, 2006
Abstract—Using gas chromatography and high-resolution Fourier-transform IR spectroscopy, we have deter-
mined the concentrations of C1–C4 hydrocarbon impurities in isotopically unmodified silicon tetrafluoride
before and after fine purification and in 28Si-enriched SiF4. The concentrations of C1–C4 hydrocarbon impuri-
ties in silicon tetrafluoride for SiH4 synthesis have been shown to correlate with those in the synthesized silane.
DOI: 10.1134/S0020168507040061
INTRODUCTION
concentrations in silicon tetrafluoride and silane pre-
pared from it.
Availability of polycrystalline silicon with a con-
centration of carbon impurity below its solubility limit
(1017 cm–3) is essential for the ability to grow structur-
ally perfect silicon single crystals. The carbon content
of the polycrystalline silicon can then be reduced by
more than two orders of magnitude by zone melting.
Several processes have been proposed to date for the
preparation of silicon, in particular, isotopically pure
one, using SiF4 as a precursor [1, 2]. Devyatykh et al.
[3] described a process for the preparation of high-
purity isotopically pure 28Si, which involved the follow-
ing steps:
EXPERIMENTAL
Since the preparation of isotopically enriched sili-
con tetrafluoride involves several steps, we used several
samples of silicon tetrafluoride differing in purity.
Sample 1: isotopically unmodified SiF4 prepared
through thermal decomposition of pure-grade sodium
hexafluorosilicate, Na2SiF6, and purified by filtration
through an FP fine fiber filter (Petryanov filter).
Sample 2: the same material after further purifica-
tion by low-temperature fractional distillation in a mid-
dle-fed packed column [5].
Samples 3 and 4: overhead and bottom fractions,
enriched in lower and higher boiling hydrocarbons,
respectively, taken after 10 h of operation in withdrawal
mode.
preparation of silicon tetrafluoride through thermal
decomposition of sodium fluorosilicate;
fine purification of SiF4 by low-temperature frac-
tional distillation;
centrifugal separation of silicon isotopes;
Sample 5: silicon tetrafluoride purified by fractional
distillation and then isotopically enriched via centrifu-
gation (GAZ Experimental Design Bureau) to a 28Si
abundance of 99.7%.
Samples 6–8: silicon tetrafluoride after purification
and subsequent isotopic enrichment to a 28Si abundance
above 99.99% in a centrifuge cascade (Tsentrotekh
EKhZ Science and Technology Center).
conversion of isotopically pure silicon tetrafluoride
to silane, followed by fine purification and thermal
decomposition of the silane.
The possible sources of carbon in the final product
of this process include carbon-containing impurities in
silane, which in turn may originate from the starting,
isotopically pure silicon tetrafluoride. A literature
search revealed no data on hydrocarbon concentrations
in isotopically unmodified or isotopically pure silicon
tetrafluoride.
Hydrocarbons were determined by gas chromatog-
raphy and high-resolution Fourier-transform IR spec-
The purpose of this work was to determine the troscopy as described by Krylov et al. [6]. The detec-
hydrocarbon concentration in isotopically unmodified tion limits for hydrocarbons were in the range (2–4) ×
10–6 vol %. Concentrations exceeding the detection
silicon tetrafluoride, as-prepared and purified, and in
the resulting isotopically enriched 28Si. It was of inter- limit by an order of magnitude were determined with an
est to assess the relationship between the hydrocarbon uncertainty within 10–15%.
364

HYDROCARBON IMPURITIES IN SiF4 AND SiH4 PREPARED FROM IT
365
Table 1. Hydrocarbon impurities in samples 1 (as-prepared,
isotopically unmodified SiF4) and 2 (purified by fractional
distillation) as determined by gas chromatography
Silicon tetrafluoride was determined by gas chroma-
tography as described by Krylov and Sorochkina [7],
using a setup built around a Tswett-100 chromato-
graph. The detection limit for C1–C4 hydrocarbons was
Volume percent
Impurity
(2–6) × 10–6 vol %, and the error of determination was
10–20%.
sample 1
sample 2
IR spectroscopic determination of SiF4 was
described in detail elsewhere [8]. At typical hydrocar-
bon concentrations, the error of determination was
within 10%. The detection limit for methane was 1 ×
10–3 vol %.
The detection limits of the gas-chromatographic and
IR-spectroscopic procedures differ by about three
orders of magnitude. IR spectroscopy, though incapa-
ble of determining hydrocarbon impurities in high-
purity samples with high sensitivity, is useful for direct
CH4
(1.4 0.2) × 10–4
(3.4 0.5) × 10–4
(7.2 0.1) × 10–4
(3.1 0.6) × 10–5
(2.2 0.3) × 10–4
(1.5 0.3) × 10–5
<4 × 10–6
(1.4 0.3) × 10–5
(1.9 0.4) × 10–5
(2.0 0.4) × 10–5
<2 × 10–6
C2H6
C2H4
C3H8
C3H6
<3 × 10–6
n-C4H10
i-C4H10
<6 × 10–6
<4 × 10–6
Table 2. Hydrocarbon impurities in samples 3 (overhead) and 4 (bottom) as determined by gas chromatography (GC) and
IR spectroscopy
Volume percent
Impurity
sample 3
sample 4
GC
IR
GC
IR
CH4
(1.4 0.1) × 10–1
(1.4 0.1) × 10–2
(8.2 0.7) × 10–3
(1.0 0.1) × 10–5
(1.0 0.1) × 10–5
<6 × 10–6
(1.0 0.1) × 10–1
(2.0 0.3) × 10–5
(1.5 0.2) × 10–5
(2.0 0.2) × 10–3
(2.5 0.4) × 10–4
(1.2 0.1) × 10–3
(2.8 0.4) × 10–4
(4.0 0.4) × 10–5
<1 × 10–3
C2H6
C2H4
C3H8
C3H6
(2.0 0.2) × 10–2
<5 × 10–4
<5 × 10–3
<1 × 10–3
<5 × 10–3
<1 × 10–3
–
–
–
–
–
–
n-C4H10
i-C4H10
<4 × 10–6
Table 3. Hydrocarbon impurities in isotopically enriched 28SiF4 samples as determined by gas chromatography (GC) and IR
spectroscopy
Volume percent
Impurity
sample 5
GC
sample 6
sample 7
sample 8
GC
IR
GC
IR
GC
IR
CH4
(2.7 0.5) × (1.9 0.4) ×
10–5 10–5
<1 × 10–3
(5.1 0.2) ×
<1 × 10–3
(6.3 0.9) ×
<1 × 10–3
10–5
10–5
C2H6
(2.0 0.4) × (7.4 0.7) × (4.0 0.4) × (3.4 0.3) × (1.5 0.2) × (8.1 0.8) × (4.0 0.4) ×
10–5
(1.1 0.2) × (6.3 0.6) ×
10–5 10–3
10–3
× 10–3
10–3
10–3
10–4
10–4
C2H4
<3 × 10–3
(3.6 0.3) ×
<3 × 10–3
(9.4 0.7) ×
<3 × 10–3
10–3
10–3
C3H8
(1.6 0.3) × (2.6 0.5) ×
<1 × 10–3
(2.1 0.5) ×
<1 × 10–3
<2 × 10–6
<3 × 10–6
<6 × 10–6
<4 × 10–6
<1 × 10–3
10–5
10–5
10–5
C3H6
<3 × 10–6
(2.7 0.7) ×
–
–
–
(3.6 0.8) ×
–
–
–
–
–
–
10–5
10–5
n-C4H10
i-C4H10
(1.7 0.4) × (3.4 0.7) ×
(0.7 0.2) ×
10–5
10–5
10–5
<4 × 10–6
(2.0 0.4) ×
(1.2 0.2) ×
10–5
10–5
INORGANIC MATERIALS Vol. 43 No. 4 2007

366
BULANOV et al.
0.25
0.20
0.15
0.10
0.05
0
*
*
*
*
*
*
3020
3000
2980
2960
Wavenumber, cm–1
–1
3
–1
Fig. 1. Absorption spectrum of sample 3 in the range 2960–3020 cm (p(SiF ) = 4.8 × 10 Pa, l = 20 cm, resolution of 0.01 cm );
4
asterisks mark individual components of the rovibrational band ν of C H (0.2 vol %), overlapping with the ν band of CH .
10
2
6
3
4
determination of relatively high hydrocarbon concen- raphy and IR spectroscopy results, we evaluated the
trations in impurity-rich materials. By combining these detection limit of IR spectroscopy for ethane at 5 ×
10−4 vol %.
analytical techniques, one can accurately assess the
nature and concentration of impurities.
Table 3 presents the gas chromatography and IR
Silane was synthesized by reacting silicon tetrafluo-
ride (samples 1, 2, 5, and 6) with calcium hydride in a
flow reactor at 180°C. The diluent used was A-grade
hydrogen. The silane yield was 90–92%.
spectroscopy results for isotopically enriched SiF4
(samples 5–8). A chromatogram of sample 6 is dis-
played in Fig. 2. Figure 3 shows portions of the IR spec-
tra of samples 6–8 in the region of the strongest absorp-
tion by ethane. The ethane concentration decreases in
going from sample 6 to sample 8 and also agrees well
with gas chromatography results. The concentrations of
C1–C4 hydrocarbon impurities in the silane samples are
listed in Table 4.
EXPERIMENTAL RESULTS
Table 1 lists the hydrocarbon concentrations in sam-
ples 1 (as-prepared SiF4) and 2 (SiF4 purified by frac-
tional distillation) as determined by gas chroma-
tography.
The hydrocarbon concentrations in samples 3 and 4
are listed in Table 2. These data demonstrate that the
two analytical techniques used provide comparable
data.
Figure 1 presents a portion of the IR spectrum of
sample 3 (2960–3020 cm–1), which shows, in addition
to lines of methane, the strongest lines of ethane
(marked by asterisks). Comparing the gas chromatog-
DISCUSSION
It can be seen from Table 1 that the concentrations
of C1–C4 hydrocarbon impurities in the as-prepared sil-
icon tetrafluoride are at a level of 10–3 vol % and drop
by one to two orders of magnitude as a result of frac-
tional distillation.
The data in Table 2 indicate that, during SiF4 purifi-
cation, the C1 and C2 hydrocarbons accumulate in the
INORGANIC MATERIALS Vol. 43 No. 4 2007

HYDROCARBON IMPURITIES IN SiF4 AND SiH4 PREPARED FROM IT
367
×8
0.03
0.02
0.01
0
×16
2
3
1
2
5
1
4
6
7
17
13
9
5
1
Time, min
28
Fig. 2. Chromatogram of sample 6 ( SiF ): (1) methane,
(2) ethane, (3) ethylene, (4) propane, (5) propylene,
(6) isobutane, (7) n-butane.
4
3
top part of the fractional column (sample 3), and the C3
and C4 hydrocarbons, in the bottom part (sample 4).
3020
3000
2980
2960
Wavenumber, cm–1
From comparison of the data for samples 5 (Table 3)
and 2 (Table 1), it follows that the isotopic enrichment
of the SiF4 distillate has little or no effect on the CH4,
C2H6, and C2H4 concentrations. At the same time, the
C3H8 and n-C4H10 concentrations increase significantly,
by about one order of magnitude and by a factor of 2,
respectively. In samples 6–8, in addition to the higher
28Si abundance, the hydrocarbon concentration is nota-
bly higher (the sum C1 + C2 attains 10–2 vol %). One
possible reason for this is that we used undistilled sili-
Fig. 3. Portions of the rovibrational absorption band ν of
10
28
3
C H impurity in SiF (p(SiF ) = 66.5 × 10 Pa, l = 20 cm,
2
6
4
4
–1
–3
resolution of 0.1 cm ): (1) sample 6, 4 × 10 vol % C H ;
2
6
–3
(2) sample 7, 1.5 × 10 vol % C H ; (3) sample 8, 4 ×
2
6
−4
10 vol % C H .
2
6
trifugation, these impurities were transferred through
the cascade and accumulated in the light fraction,
con tetrafluoride in centrifugal separation. During cen- enriched in 28Si. Moreover, the concentrations of
Table 4. Hydrocarbon impurities (gas chromatography data) in SiH4 prepared from samples 1, 2, 5, and 6
Volume percent
Impurity
sample 1
sample 2
sample 5
sample 6
CH4
(1.9 0.1) × 10–3
(1.3 0.1) × 10–4
(1.4 0.1) × 10–4
(1.2 0.1) × 10–4
(1.1 0.1) × 10–4
(3.7 0.5) × 10–5
(1.4 0.2) × 10–5
(1.4 0.1) × 10–3
(9.5 0.9) × 10–5
<2 × 10–6
(4.0 0.4) × 10–5
<3 × 10–6
(3.5 0.4) × 10–3
(3.6 0.4) × 10–4
<2 × 10–6
(5.3 0.5) × 10–4
<3 × 10–6
(2.3 0.3) × 10–3
(5.3 0.5) × 10–3
(4.9 0.5) × 10–3
(1.5 0.2) × 10–4
(2.0 0.2) × 10–4
(2.1 0.3) × 10–4
(1.3 0.3) × 10–4
C2H6
C2H4
C3H8
C3H6
n-C4H10
i-C4H10
(2.0 0.2) × 10–5
<4 × 10–6
(3.3 0.4) × 10–4
(6.5 0.6) × 10–5
INORGANIC MATERIALS Vol. 43 No. 4 2007

368
BULANOV et al.
hydrocarbon impurities in isotopically enriched silicon
tetrafluoride vary from sample to sample within one
batch (samples 6, 7), which seems to be due to different
background concentrations of impurities in the con-
tainer for the 28Si-enriched gas.
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CONCLUSIONS
The qualitative and quantitative compositions of
hydrocarbon impurities in isotopically unmodified and
isotopically enriched silicon tetrafluoride were deter-
mined for the first time by gas chromatography and IR
spectroscopy, and the behavior of these impurities dur-
ing the purification and isotope separation of SiF4 was
investigated.
The results demonstrate that silicon tetrafluoride is
one of the sources of hydrocarbon impurities in the
silane prepared from it. The other source of hydrocar-
bon impurities in the silane is the calcium hydride used
to reduce the silicon tetrafluoride.
7. Krylov, V.A. and Sorochkina, T.G., Gas-Chromato-
graphic Determination of C1–C4 Hydrocarbon Microim-
purities in High-Purity Silicon Tetrafluoride, Zh. Anal.
Khim., 2005, vol. 60, no. 12, pp. 1262–1267.
8. Chuprov, L.A., Sennikov, P.G., Tokhadze, K.G., et al.,
High-Resolution Fourier-Transform IR Spectroscopic
Determination of Impurities in Silicon Tetrafluoride and
Silane Prepared from It, Neorg. Mater., 2006, vol. 42,
no. 8, pp. 1017–1024 [Inorg. Mater. (Engl. Transl.),
vol. 42, no. 8, pp. 924–931].
INORGANIC MATERIALS Vol. 43 No. 4 2007