The speed seen at the chamber port once the conductance of everything between it and the pump is counted: 1/S_eff = 1/S + 1/C. It is always lower than the rated speed, and in molecular flow it can be dramatically lower, since a long, narrow line can throw away most of a large pump. Designers bolt high vacuum pumps straight onto the chamber through the shortest and widest valve available to keep S_eff close to S.
Effective pumping speed at the wafer is maybe 40 percent of nameplate with that gate valve and baffle in the way.
Also heard as
- net pumping speed
- S_eff
Related terms
Pumping speed
Volume of gas a pump removes from its inlet per unit time, measured at the inlet pressure.
Conductance
How easily a pipe, orifice or valve lets gas through, expressed in litres per second.
Molecular flow
Flow regime in which molecules bounce between the walls almost without meeting each other.
Gate valve
Full-bore valve with a sliding plate that isolates a high vacuum pump or a load lock from the chamber.