Effectiveness of water-based flourine scrubber proven

08/01/1998

Effectiveness of water-based fluorine scrubbing proven

Recent development work has shown the viability of water scrubbing, with and without a non-caustic additive, for treating corrosive fluorine gas effluent and essentially eliminating the potential of producing lethal oxygen difluoride (OF2). The process has been developed and patented by ATMI-EcoSys, Danbury, CT.

As new chemical vapor deposition chamber-cleaning technologies are introduced, the semiconductor industry will need better control of F2 emissions from fab processes. The Semiconductor Industry Association has warned of the hazards of OF2 - produced when using caustic solutions to treat fluorine gas effluent from CVD processes. OF2 is potentially lethal, posing serious health and safety risks in semiconductor wafer production facilities. Established guidelines set by the American Conference of Governmental Industrial Hygienists for OF2 are a toxicity threshold limit value of 50 parts/billion.

Josep Arn?, chief technologist of ATMI-EcoSys, said, "We improved F2 abatement by combining EcoSys liquid scrubbing technology with a unique injection system that uses an inexpensive, noncaustic chemical agent - the mild reducing agent sodium thiosulfate (Na2S2O3). This technology replaces the need for using caustic solutions, such as sodium or potassium hydroxide, for treating fluorine effluents." Previous unreported tests at EcoSys revealed that treatment with caustic solutions was not a suitable option because it enhanced fluorine abatement at the expense of generating unacceptable levels of OF2.

Performance of the new process was investigated using point-of-use water-based scrubbing with changing (up to 6%, i.e., 0.5-5 slpm F2 diluted with 50 slpm N2) F2 concentrations released during a typical PFC plasma clean. Contrary to popular belief these tests showed that fluorine gas reacts quickly and efficiently with H2O; the main products of the reaction are HF, O2, and H2O2. Arn? said, "Objections to using water scrubbers to abate fluorine are mostly triggered by the lack of applicable experimental data, concerns over the formation of unwanted OF2, and thermal considerations over exothermic reactions."

Results of testing (see table) revealed impressive abatement efficiencies - fluorine elimination and minimization of water use and formation of OF2. The role of the Na2S2O3, in boosting water scrubbing efficiency, involves its injection into the scrubber during high fluorine gas challenges to improve fluorine gas removal and to eliminate the formation of OF2 as a by-product.

Arn? added, "This study confirmed that fluorine gas reacts with water quickly and effectively. All conditions investigated resulted in over 99% destruction and removal efficiencies of fluorine." Results also showed that any exothermic effects were effectively dissipated within the scrubber and no corrosion was found anywhere within the scrubber.

EcoSys is also working on "breakthrough" technology for in-situ fluoride removal from exhaust water streams and silane abatement using water scrubbing. Commenting on the latter, Arn? said, "Adding this revolutionary feature to water scrubbers could enable using a single system for the abatement of almost all processes involved." - P.B.