Handbook of Photovoltaic Science and Engineering

5.4.1 The Siemens Process
A schematic overview of the process is given in Figure 5.2.
Trichlorosilane HSiCl
3
is prepared by hydrochlorination of metallurgical grade
silicon in a fluidised bed reactor:
Si
(
s
)
+
3HCl
=
HSiCl
3
+
H
2
(
5
.
26
)
This reaction occurs at 350
◦
C normally without a catalyst. A competing reaction is
Si
(
s
)
+
4HCl
=
SiCl
4
+
2H
2
(
5
.
27
)

---

PRODUCTION OF SEMICONDUCTOR GRADE SILICON (POLYSILICON)
169
TCS synthesis by
hydrochlorination
of Si in fluidised
bed reactor at
300ºC
Separation and
purification of
TCS by distillation
Second
purification of
TCS
Pyrolysis in
Siemens reactor
(hot rod)
Gas recovery
separation and
purification
Residue:
spent mass
TET
by-product
Low boiling
impurities
TCS (85%)
TET (15%)
MG-Si: Metallurgical
Grade Silicon
TCS: Trichlorosilane
TET: Tetrachlorosilane
TCS 
+ 
TET
Dry HCI
H
2
HCI
H
2
Gas
mixture
MG-Si
Hyper-
pure
TCS
Poly
silicon
Pure
TCS
Figure 5.2
Schematic representation of the Siemens process
contributing to the formation of unsuitable tetrachlorosilane in molar proportion of 10
to 20%.
Trichlorosilane is chosen because of its high deposition rate, its low boiling point
(31.8
◦
C) and its comparatively high volatility and hence the ease of purification with
respect to boron and phosphorus down to the ppb level. The boiling point of other silanes
frequently found with trichlorosilane are as follows: SiH
4
(–112
◦
C), SiH
2
Cl
2
(8.6
◦
C) and
SiCl
4
(57.6
◦
C). The suitable trichlorosilane undergoes a double purification through frac-
tional distillation, the first step removing the heaviest components resulting from the direct
synthesis and the second step eliminating the components lighter than trichlorosilane, also
called
volatiles
.
High-purity SiHCl
3
is then vaporised, diluted with high-purity hydrogen and intro-
duced into the deposition reactors. The gas is decomposed onto the surface of heated
silicon seed rods, electrically heated to about 1100
◦
C, growing large rods of hyper-
pure silicon.
The main reactions are:
2SiHCl
3
=
SiH
2
Cl
2
+
SiCl
4
(5.28)
SiH
2
Cl
2
=
Si
+
2HCl
(5.29)
H
2
+
HSiCl
3
=
Si
+
3HCl
(5.30)
HCl
+
HSiCl
3
=
SiCl
4
+
H
2
(5.31)
The stream of reaction by-products, which leaves the reactor, contains H
2
, HCl, HSiCl
3
,
SiCl
4
and H
2
SiCl
2
.

---

170
SOLAR GRADE SILICON FEEDSTOCK
Cooling medium
External
envelope
Internal
wall
Cooling medium
Electrical
contact
to resistive
heating
Reactor
outlet
SiCl
4
(H
2
)
(SiHCl
3
)
(SiH
2
Cl
3
)
HCl
Reactor
inlet
SiHCl
3
H
2
(Poly)
silicon
rods