Why is CF4 So Difficult to Abate?
Van Gompel, Joe; Cottle, Steve
(Edwards Vacuum; Austin TX and Clevedon, UK)
Semiconductor manufacturing uses fluorine-based chemistries for both etch processes as well as for clean steps of many deposition processes because the resulting inorganic fluorides are volatile and readily removed under vacuum. Perfluorinated gases (F-gases; CF4, C2F6, SF6, etc.), decomposed in a plasma, are routinely used as in situ sources of fluorine atoms because they are relatively nontoxic and are available in sufficient purity and bulk at reasonable prices. Utilization factors for these gases in the process can be poor – as low as 15% for CF4 – so much of the gas going into the chamber exits unreacted. As very potent greenhouse gases (GHGs), these compounds must be inventoried annually and emissions reported to the EPA under the GHG Reporting Rule. Technologies to mediate emissions of F-gases have been available to the electronics industry since the mid-1990’s. Experience shows that most of the F-gases are relatively easy to destroy through combustion, plasma discharge, consumable dry bed, or catalytic dry bed methods. Of the F-gases, CF4 stands out as the most difficult gas to abate using the most common methods (combustion, plasma). CF4 is very stable, leading to an exceptionally long atmospheric lifetime (~50,000 years). This presentation discusses why F-gases are such potent GHGs and the relative ease of abatement from the perspective of both thermodynamics and chemical kinetics. Also discussed are abatement considerations of both combustion and plasma techniques, and how these properties affect the measurement and reporting of said emissions to the EPA.