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dna breaks molecules
Environment


Fluorocarbons are the most common feedstock gases used in etching plasmas. However, perfluorinated gases (PFCs) have been found to be detrimental to the environment and global climate, as most of the compounds are strong greenhouse gases and ozone depleters. PFCs are consumed only partially in the etching processes and the waste gases are emitted into the atmosphere. They have an extremely high global warming potential (up to four orders of magnitude higher than CO2) and atmospheric lifetimes that are practically infinite on the timescale of human life (see table below).

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The global warming potential (GWP) depends upon infrared absorption and time horizon (hold up time) in the atmosphere. It is defined as the cumulative radiative forcing, or heating, integrated over a period of time from the emission of a unit mass of gas relative to some reference gas. Carbon dioxide (CO2) was chosen as this reference gas. The Kyoto protocol on gas emissions requires the phasing out of the use of CF4 C2F6 C3F8 and C4F8 as feedstock gases by 2010. Emission reduction strategies have to combine both optimisation of plasma equipment efficiency and search for the new plasma compounds. CF3I , C2F4 and 1-C4F8 have been proposed as new plasma reactants (Samukawa et al 1999). The first two are of particular interest from the point of view of controlling the composition of the CF2 and CF3 radicals in the plasma (Mason et al 2003).

There is very little information, however, about how CFx radicals interact with low energy electrons. Recent studies have shown that the concentration of these radicals has a significant effect on the behaviour of fluorocarbon plasmas (Chabert et al 2003) and that these radicals also occur in significant concentrations in other plasmas (Chabert et al 2001). CF3I and C2F4 have received special attention as new feedstock gases also because they both have low global warming potential and short lifetimes (see table). CFx radicals are highly reactive and experimental studies on highly reactive radicals are extremely difficult. Ab initio calculations are therefore crucial in providing data upon which the necessary models can be based (Winstead and McKoy 2000).

Species GWP100 GWP20 Lifetime
CO2 a 1 1 50 - 200
CF4 a 6500 4400 50000
C2F6 9200 6200 10000
c-C4F8 d 8700 - 3200
CF3I 0.25 < 5 b < 0.006 b
C2F4 c 0.07 0.021 0.005
GWP, calculated over 100 and 20 years time horizon, and atmospheric lifetime (years) for the molecular species used in plasma etching of silicon surfaces.

a From Gillenwater at. Al. (2002)
b From Solomon et al
c From Acerboni et al (2001)
d From IPCC (1996)


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