CONTAMINATION OF ULTRAPURE SYSTEMS BY BACK-DIFFUSION OF GASEOUS IMPURITIES

Diffusion of impurities against the direction of gas flow can be a sig nificant source of contamination in the ultrapure gas systems. The mec hanism and the kinetics of impurity transport due to back-diffusion is studied both experimentally and theoretically. A model is developed t o simulate the process and calculate the extent of impurity transport due to back-diffusion. Experiments for measuring oxygen back-diffusion in a nitrogen carrier stream are conducted. The data and the model pr edictions are in good agreement. Back-diffusion involves both bulk and surface diffusion and is higher for lower pressures, lower flow rates , larger tube diameters, and for smaller nonadsorbing molecules.