Characteristics of extremely fast pump-down process in an aluminum ultrahigh vacuum system

Abstract

Characteristics of an extremely fast pump-down process were studied in an aluminum ultrahigh vacuum system, using super-dry nitrogen. The system is constituted of a roughing pump line, a main pump line, a venting gas line and a main chamber with vacuum gauges. The venting gas line has two types of water traps in series. Experiments were done after achieving 10−11 Torr range following a bake. Experimental results were the following: (1) using only the moisture trap in venting (case A), the pressure reached, from 760 Torr, was 1×10−5 Torr in 0.7 min, 1×10−8 Torr in 4.2 min, 1×10−10 Torr in 1000 min; the pressure was proportional to exp(−St/V) down to the 10−5 Torr range; here, V, S, and t are the chamber volume, the pumping speed, and the pumping time, respectively; (2) using both the moisture trap and the ‘‘MonoTorr’’ in venting (case B), the pressure reached from 760 Torr was 1×10−5 Torr in 0.7 min, 1×10−8 Torr in only 2.5 min, 1×10−10 Torr in only 170 min; the pressure was proportional to exp (−St/V) to the 10−5 Torr range. The difference of results between (1) and (2) is mainly due to the difference of the amount of water and hydrogen content in vent-gas; the moisture trap can reduce the H2O content of vent-gas to 200 ppb after 2 h of operation, on the other hand, the MonoTorr can reduce other contents (H2O, H2, CO, CO2, etc.) of vent-gas, except N2, to a few ppb after 2 h of operation.