The rate at which the pressure decreases during pumpdown depends on several factors:

• Speed of the mechanical pump
  larger speed ==> faster pumpdown
• Conductance of the bypass line
  larger conductance ==> faster pumpdown
• Chamber pressure and volume
  higher chamber pressure and/or volumne ==> slower pumpdown
  This is because larger chamber pressure and/or volume mean more gas must be evacuated.

Fast pumpdown for manufacturing speed: In general, one would always want process steps to be as fast as possible.  Pumping down the reaction chamber doesn't add value to the semiconductor wafer (since no material change is made to the wafer), so we want to shorten pumpdown to enhance manufacturing speed.

Slow pumpdown to reduce particle generation: However, if the chamber contains condensible gases (e.g., water vapor from the air), too rapid a pumpdown cycle can cause serious problems with manufacturing yield due to intrinsic physical mechanisms.  Rapid pressure reduction causes gas cooling.  If condensible gases are present in the chamber, they can condense into particles or onto existing very small particles, making in either case larger particles.  When such particles land on the wafer surface, they can destroy the functionality of the semiconductor chips, degrading manufacturing yield.  Therefore, when pumpdown begins with condensibles in the reaction chamber (e.g., after opening the chamber to air), it is essential to arrange for slow initial pumpdown cycles, taking something like 100 sec. to reduce pressure from atmospheric to about 1-10 torr.  These effects are influenced by concentration of condensibles (e.g., relative humidity of air), physical properties of the condensibles, chamber surface-to-volume ratio, and temperatures of chamber walls and initial gas in the chamber.