Impurity back diffusion through a main UHV turbomolecular pump to a reduced-gas-pressure processing chamber has been experimentally studied. A large throughput of process gases severely degrades the capability of a UHV main pump in dragging impurities, resulting in the back diffusion of impurities to the processing chamber. Such degradation is greatly influenced by the pumping speed of the low-vacuum backing pump. However, we have found for the first time that the impurity level in the chamber can be made even lower than that for the best performance of the main pump under UHV operation when the gas flow rate is optimized for the system. It is shown that the optimized gas flow rate corresponds to the flow rate at which the effective pumping speed of a UHV main pump begins to decrease. The back diffusion levels are largely dependent upon the mass of impurity gas molecules and less back diffusion occurs for heavier gas molecules. The guidelines for designing a vacuum pumping system for use in reduced-gas-pressure processing are presented.