Materials Edwards vFinal DR 18/12/08 12:19 Page 48
48
MATERIALS
gas
[5]
. The various exhaust gas challenges are
summarised in table 1.
Abatement technologies
Deposition step
The only practical solution for dealing with silane
and hydrogen, particularly in the quantities
encountered in µc-Si production, is combustion.
Failure to effectively incinerate the hydrogen could
result in potential detonation of downstream
pipework where the exhaust gas meets the
atmosphere and enters the flammable zone, as
illustrated in figure 1.
In order to attain efficient combustion of
hydrogen, it is necessary to complete the
flammability triangle, i.e. fuel (H
2
), oxidant (air)
and ignition source. Relying on the inherently
pyrophoric nature of silane to act as the ignition
source is not intrinsically safe; its Lower
Flammability Limit is around 1.4% in air
[6]
, and
Fig 1: Flammability Hydrogen has a reputation for its flammability, if the concentration falls below this value the
limits of H2 at 25°C and the exceptionally high flame speeds that are flame will extinguish, allowing flammable and
and atmospheric attainable mean that it readily detonates under toxic gas to enter the exhaust system. Similarly, if
pressure [4] certain conditions
[4]
. Even when combusted under constant dilution of the deposition gases is relied
controlled conditions, considerable amounts of upon as a means of gas treatment, there will be an
heat are generated, requiring careful thermal emission of toxic gas and the risk of transient
management. changes to gas flows that could cause sudden
The final challenge comes from the need to ignition of the exhaust. Maintaining a strong and
routinely remove the accumulated deposits of continuous source of ignition is therefore essential
silicon from the process chamber walls; where this during the deposition step, and a fuel-fired flame
material to build up above a certain critical is more stable than an electrical discharge.
thickness it would start to flake off, and the The next question is: How to keep the
fragments of solid silicon would seriously degrade combustion zone from blocking with powder?
the quality and properties of the solar cell There are 2 basic approaches here; either
structure. To avoid this situation, the entire mechanical “scrapers” or careful gas flow
chamber is periodically given an in-situ cleaning, management. The former may seem a simple
which involves using a fluorine containing gas such solution, but in the harsh chemical and thermal
as NF
3
or SF
6
in a plasma to generate highly environment of the combustor, it is difficult in
active fluorine species (F
•
) which subsequently practice to maintain mechanical reliability. Careful
react with the chamber wall deposits to form control of gas temperature, 3D flow pattern and
volatile silicon tetrafluoride gas (SiF
4
), according concentration profiles are therefore required to
to the reaction: achieve efficient gas destruction without deposition
on the static surfaces of the combustor.
Having combusted the process gas, it now has
Table 2 Abatement Thus the cleaning process adds fluorine and silicon to be cooled as many tens of kW of heat is
technologies tetrafluoride to the exhaust (both are toxic, generated. At the same time, the large quantities
compared for the corrosive gases), plus any unreacted NF
3
or SF
6
. of silica powder have to be managed and
deposition step SF
6
in particular is a significant global warming constantly removed from this region.
There are 2 basic approaches, namely using
water or air cooling. Whilst water cooling is
extremely effective at dissipating heat energy
(thanks to the high latent heat of evaporation
{vapourisation} of water of 2260kJ/kg), its cost is
high, disposal costs are even higher, and it is
difficult to handle and treat large quantities of
silica in suspension. A simpler and lower cost
solution is to use air cooling, then transport the
silica powder in a high air flow, and collect the
solids on a dry filter bed. The different
technologies for abating the deposition gases are
compared in table 2.
www.euroasiasemiconductor.com December 2008 / January 2009
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