Advantages of Slow High-Vacuum Pumping for Suppressing Excessive Gas Load in Dynamic Evacuation Systems

Abstract

In dynamic diffusion pump systems, such as vacuum evaporators and sputter coating systems, where the chamber is frequently evacuated from atmospheric pressure to high vacuum, the oil-vapor backstreaming problem sometimes occurs owing to the excessive gas load flowing into the high-vacuum pump just after switching the evacuation mode from low-speed roughing to high-speed, high-vacuum pumping. Two kinds of excessive gas loads exist just after crossover: (1) gas molecules in the vacuum chamber space and (2) temporarily increased outgassing from the chamber wall surface. The outgassing rate from the chamber wall surface becomes very large with the rapid reduction of pressure owing to high-speed, high-vacuum pumping, because the time constant of diffusion of gas molecules in the wall surface is much larger compared with the time constants of pumping down and the resultant reduction of sorption rate of impinging gas molecules. Slow high-vacuum pumping, followed by high-speed pumping, is very effective to suppress the temporarily increased outgassing load and the adverse effect of the space gas load, and to meet the maximum throughput capacity of the diffusion pump. Providing with a low-conductance bypass valve makes it possible to use a small-volume buffer tank and a low-speed rotary pump as a backing pump, leading to a reduction in the cost of high-vacuum evacuation systems.