Calculations suggest break-through from semiconductor bulk chemical distribution systems occurs in a matter of days and steady state permeation accounts for several milliliters of chemical loss each day.

Permeability (P) and diffusion (D) coefficients were measured for hydrogen chloride and ammonia gas transport through a polytetrafluoroethylene copolymer, perfluoroalkoxy (PFA), using standard manometric techniques. These data were subsequently used to estimate the performance characteristics, such as break-through times and permeation rates, of a representative chemical distribution system that might be found inside a semiconductor wafer fabrication facility. Our findings suggest that break-through occurs in a matter of days and that steady state permeation can account for the loss of several grams of hydrochloric acid or ammonium hydroxide each day. This loss rate for hydrogen chloride would be equivalent to dumping five milliliters of concentrated hydrochloric acid on to the floor of a fabrication facility each day, or more likely into the secondary containment sub-system, 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 sub-system must be purged. The necessary purge rate can be estimated using the mass transport data making this study useful to facility planners and operators.

Among melt-processable thermoplastics, tetrafluoroethylene (TFE) - perfluoroalkoxy copolymers, often abbreviated simply as PFA, have a unique combination of purity, toughness, and nearly universal chemical inertness. Therefore, PFA has been used broadly for the transport and storage of high purity chemicals in semiconductor wafer fabrication facilities or “fabs.”^1,2 Although much is known about the purity, mechanical, and thermal properties of PFA,^3-5 less information is available regarding its permeation characteristics.

Some permeation testing has been performed on PFA,^6-13 but most studies have not addressed two of the most widely used semiconductor process chemicals, hydrochloric acid and ammonium hydroxide. The active ingredient in both of these aqueous chemicals is dissolved gas, hydrogen chloride or ammonia, respectively. Thus, they are quite mobile and have caused concerns about unwanted permeation, cross-contamination, and corrosion.¹³ In this article, we explore breakthrough times and steady-state permeation rates of hydrogen chloride and ammonia for a PFA chemical distribution system.

A REPRESENTATIVE BULK CHEMICAL SYSTEM
In the following examples, we use a representative bulk chemical system that could be used for distributing hydrochloric acid or ammonium hydroxide. It is shown schematically in Figure 1. The distribution system consists of a bulk chemical container, a pump, tubing, pipe, and valves. The various PFA components are tabulated in Table 1. The valves are mounted inside containment boxes. The chemical, hydrochloric acid or ammonium hydroxide, is pumped from a chemical container through a one inch (25 mm) PFA supply line to three tee boxes that each contain two PFA drop valves. The drop valves feed six valve boxes that in turn distribute the chemical to 30 points of use via ¾ inch (19 mm) PFA tubing. Unused chemical is sent back to the storage container through a one inch PFA return line. For simplicity, we assume that all components are constructed from PFA.