Materials Edwards vFinal DR 18/12/08 12:19 Page 47
MATERIALS
47
Safe gas handling
Solar cell manufacturing is becoming increasingly important as an environmentally
friendly means of generating electrical power, and the growth of thin films of
microcrystalline silicon will form a key part of this. Dr. Michael Czerniak from
Edwards explains the exhaust gas management of the process.
T
he PECVD growth of are orders of magnitude greater, as are the process
microcrystalline silicon poses a step times.
number of significant challenges One aspect both processes have in common is
regarding the treatment of the resultant exhaust the use of high flows of silane (SiH
4
) which is
gases, in terms of removing toxic, corrosive, activated by the plasma in the process to deposit
global warming, and flammable gases safely, at silicon according to:
low cost, whilst also minimising the environmental
impact. Fortunately the means to achieve these
goals now exist.
Not all the silane will react in the process
Background chamber, in fact process chambers are designed so
Although solar cells account for a tiny fraction of that this does not happen in order to maintain
global power generation (<1%), they are acceptable levels of film thickness uniformity
experiencing a phenomenal growth rate (28%)
[1]
(technically so that the gas stream does not
as the price of solar generated electricity become depleted of reagents). However, this means
approaches parity with that of other sources; this that there will be significant flows of unreacted
is predicted to occur sometime during the next silane in the process exhaust. Not only is silane
decade (2010-2020)
[2]
. toxic (TLV = 5ppm), it is also pyrophoric
There are a number of competing technologies (combusts spontaneously with air)
[3]
and, when
for the manufacture of solar cells, the most widely incinerated, generates significant quantities of
used being silicon wafer based crystalline cells, solid silica (SiO
2
) powder, around 3 grams per
yielding up to around 22% efficiencies
[2]
. Of litre of SiH
4
. Based on the current silane gas
growing interest, however, is the competing flows used in TF silicon solar cell manufacture,
technology of thin film silicon solar cells, where this can result in tens of tons of SiO
2
solids per
microscopically thin layers of silicon are deposited chamber per year. TLV is the threshold limit value
on large area substrates to produce low cost solar of a gas, expressed as parts per million.
cells, a process similar to that employed to make Where these solar cell structures differ is that, Table 1: Abatement
the transistors in liquid crystal displays (LCDs). in order to grow µc-Si, where a much more challenges posed by
The efficiency of simple devices is significantly ordered crystalline structure is required, very high µc-Si exhaust gases
inferior to that of crystalline cells (~6-8%), but flows of hydrogen gas must be introduced into the Cells highlighted in
by combining 2 layers of differing material process mixture, and H
2
is therefore present in the gray relate to the
properties, the resulting structure is, in effect, 2 process exhaust in high quantities (100s of SLM). deposition step
cells in one with efficiencies approaching 10%
[2]
,
yet can be produced in large area format at low
cost.
New Challenges
To produce these twin cell structures, a
conventional amorphous silicon layer is combined
with a microcrystalline (µc-Si) layer; both
deposited using extremely large capacity plasma
enhanced chemical vapour deposition (PECVD)
equipment. Although the chemistry resembles that
used to make semiconductor chips, the gas flows
December 2008 / January 2009 www.euroasiasemiconductor.com
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