About 2 min.
Clearly, this depends on just how fast the slow initial pumping is done, and how
low a pressure must be attained before switching over to rapid pumping through
the turbo pump system. But in any case, the pressure cycling times (especially if both
pumpdown and venting) will constitute a significant portion of the time budget available
to process each wafer. Therefore vacuum and gas flow equipment design is a critical
aspect of semiconductor manufacturing.
This exercise also emphasizes a potential advantage from the use of cluster tools,
in which multiple process steps are carried out in a multichamber tool in which the
central wafer handling chamber between the reaction chambers operates at a low pressure
(e.g., 1-10 torr). In this case, the pumpdown and venting need only cycle to a
few torr, rather than 760 torr, and the time is less at the higher pressures where
particle generation is a concern.