SOLARMATERIALS
controlled conditions, considerable amounts of
heat are generated, requiring careful thermal
management.
The final challenge comes from the need to
routinely remove the accumulated deposits of
silicon from the process chamber walls; where this
material to build up above a certain critical
thickness it would start to flake off, and the
fragments of solid silicon would seriously degrade
the quality and properties of the solar cell
structure. To avoid this situation, the entire
chamber is periodically given an in-situ cleaning,
which involves using a fluorine containing gas
such as NF3 or SF6 in a plasma to generate highly
active fluorine species (F•) which subsequently
react with the chamber wall deposits to form
volatile silicon tetrafluoride gas (SiF4), according to
the reaction:
The next question is: How to keep the combustion Fig 1: Flammability
zone from blocking with powder? There are 2 limits of H2 at 25°C
basic approaches here; either mechanical and atmospheric
“scrapers” or careful gas flow management. The pressure [4]
Thus the cleaning process adds fluorine and former may seem a simple solution, but in the
silicon tetrafluoride to the exhaust (both are toxic, harsh chemical and thermal environment of the
corrosive gases), plus any unreacted NF3 or SF6. combustor, it is difficult in practice to maintain
17
SF6 in particular is a significant global warming mechanical reliability. Careful control of gas
gas [5]. The various exhaust gas challenges are temperature, 3D flow pattern and concentration
www
summarised in table 1. profiles are therefore required to achieve efficient
.solar
gas destruction without deposition on the static
Abatement technologies Deposition step surfaces of the combustor.
-pv-management.com
The only practical solution for dealing with silane
and hydrogen, particularly in the quantities Having combusted the process gas, it now has to
encountered in mc-Si production, is combustion. be cooled as many tens of kW of heat is
Failure to effectively incinerate the hydrogen could generated. At the same time, the large quantities of
result in potential detonation of downstream silica powder have to be managed and constantly
pipework where the exhaust gas meets the removed from this region.
atmosphere and enters the flammable zone, as
Issue I 2009
illustrated in figure 1. There are 2 basic approaches, namely using water
or air cooling. Whilst water cooling is extremely
In order to attain efficient combustion of hydrogen, effective at dissipating heat energy (thanks to the
it is necessary to complete the flammability high latent heat of evaporation {vapourisation} of
triangle, i.e. fuel (H2), oxidant (air) and ignition water of 2260kJ/kg), its cost is high, disposal costs
source. Relying on the inherently pyrophoric nature are even higher, and it is difficult to handle and Table 2 Abatement
of silane to act as the ignition source is not treat large quantities of silica in suspension. A technologies
intrinsically safe; its Lower Flammability Limit is simpler and lower cost solution is to use air compared for the
around 1.4% in air [6], and if the concentration falls cooling, then transport the silica powder in a high deposition step
below this value the flame will extinguish, allowing
flammable and toxic gas to enter the exhaust
system.
Similarly, if constant dilution of the deposition
gases is relied upon as a means of gas treatment,
there will be an emission of toxic gas and the risk
of transient changes to gas flows that could cause
sudden ignition of the exhaust. Maintaining a
continuous source of ignition is therefore essential
during the deposition step, and a fuel-fired flame is
more stable than an electrical discharge.
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