Measuring hydrochloric acid and ammonium hydroxide permeation in bulk chemical distribution

Permeability (P) and diffusion (D) coefficients were measured for hydrogen chloride and ammonia gas through a tetrafluoroethylene - perfluoroalkoxy (PFA) copolymer using standard manometric techniques. These data were subsequently used to estimate the performance characteristics, such as breakthrough times and permeation rates, of a representative chemical distribution system that may be found inside a semiconductor wafer fabrication facility. Our findings suggest that breakthrough occurs in a matter of days and that steady state permeation can account for the loss of several grams of hydrogen chloride or ammonia each day. This loss rate for hydrogen chloride would be equivalent to spilling five milliliters of concentrated hydrochloric acid on to the floor of a fab each day, or more likely into the secondary containment subsystem, and allowing the acid to dissipate over the course of a day. This equates to two liters per year. Everything else being equal, the loss rate of ammonium hydroxide is expected to be more than two times that of hydrochloric acid. To prevent accumulation of hydrogen chloride or ammonia, the secondary containment subsystem must be purged. The necessary purge rate can be estimated using the mass transport data from this study.