Mechanisms for the unintentional formation of CF4 during Semiconductor Manufacturing
PROFESSOR MICHAEL R. CZERNIAK
(Edwards and University of Bristol)
Carbon tetrafluoride (CF4) is a widely-used PFC (perfluorocarbon) gas in the semiconductor industry, primarily as a convenient source of fluorine for the purpose of etching silicon-containing layers. The rate of increase of its emission into the atmosphere has been purposefully restricted since the mid 1990ís by proactive industry initiatives, especially under the auspices of the WSC (World Semiconductor Council), due to the realisation of the significant climatic impacts of this gas; not only does it have a high Global Warming Potential (GWP), it also has an extremely long atmospheric lifetime (50,000 years) due to the strength of the carbon-fluorine bonds. Measures to limit emissions of this gas have included optimisation of process steps, the introduction of exhaust gas abatement systems and the replacement of PFC CVD (Chemical Vapour Deposition) chamber cleaning gases by NF3, associated with the roll-out of 300mm processes around the turn of the millennium. Significant benefits of this change were that NF3 has a much lower GWP, shorter atmospheric lifetime, and is efficiently converted into nitrogen and fluorine in a plasma. However the resulting reductions in emissions may not be quite as impactful as at first anticipated. This paper considers 2 mechanisms whereby CF4 can be generated from fluorine (derived from NF3); (a) in a chamber coated with carbon-containing materials (e.g. when using organic precursors) and (b) when the exhaust gas comes into direct contact with a hydrocarbon fuel (e.g. methane), and how this can be minimised/eliminated by suitable set-up of the equipment.