Assessment of removal efficiency of perfluorocompounds (PFCs) in a semiconductor fabrication plant by gas chromatography.

ABSTRACT

This study investigated a gas chromatographic (GC) method to assess the destruction or removal efficiency (DRE) of local scrubbers on five perfluorocompounds (PFCs), i.e., SF(6), NF(3), CF(4), C(2)F(6), and C(3)F(8), which are very potent greenhouse gases used in a semiconductor fabrication plant. Air samples taken at inlets and outlets of local scrubbers were analyzed by a self-constructed multi-column GC system equipped with thermal conductivity detection. Three packed columns were integrated into the heart-cut GC system to allow simultaneous analysis of the five target PFCs. The Porapak Q pre-column performs rough separation and cuts eluent groups to two analytical columns for optimal separation. The Molecular Sieve - 5A column separated NF(3), CF(4), and C(3)F(8) and the second Porapak Q separated SF(6) and C(2)F(6). Linearity was greater than 0.995 (R(2)) for the five PFCs, and the reproducibility was about 4% (relative standard deviation) for NF(3), and better than 0.5% for the other four PFCs. DRE for the combustion (CB) and electric-thermal types of local scrubbers was evaluated by taking into account the in-line dilution from air and fuel gases. Both flow and tracer methods were employed to deduce the dilution factors (DFs). For the tracer method, helium was employed as the tracer and injected upstream of the scrubbers and thus mixed with the exhaust gas. With this method, the DFs were determined to be in the range from 4.8 to 5.9 for the CB unit, significantly higher than the value of 3.3 based on the flow method. The DREs for the CB unit for C(3)F(8) were greater than 90% and between 40% and 50% for CF(4).