Roughing a chamber through a small orifice or a partly open valve for the first part of the pump-down, then switching to the full roughing line, often somewhere between 100 and 10 mbar. The gentle start keeps particles on the walls instead of on the wafers and limits the sudden expansion cooling that can condense water into fog. Fab load locks run a soft pump and a soft vent step on every cycle.
Soft pump's set to switch at 20 torr, any faster and we see adders on the particle wafer.
Also heard as
- slow pump
- soft rough
Related terms
Rough out
To pump a vented chamber from atmosphere to crossover with the roughing pump, before the high vacuum valve opens.
Fogging
Mist that forms inside a chamber during a fast pump-down when expanding air cools and its water condenses.
Vent
To let gas back into an evacuated chamber, usually dry nitrogen, to bring it up to atmosphere.
Viscous flow
Flow regime in which molecules collide mostly with each other, so the gas moves as a continuous fluid.