Base pressure vs process pressure in vacuum systems
Distinguishing idle base pressure, crossover pressure, and recipe process pressure—and which tool to use for each.
By Semiconductor Tools Editorial Team · Last updated: 2026-06-14
Semiconductor vacuum systems operate at several distinct pressure levels. Confusing them leads to wrong pump sizing, incorrect leak specs, and misinterpreted gauge readings.
Definitions
- Base pressure: Lowest stable pressure with pump connected, MFCs closed, after sufficient pumpdown (outgassing/leak limited).
- Crossover / transition pressure: Pressure at which rough pump hands off to high-vacuum pump.
- Process pressure: Setpoint during recipe with gas flowing—often orders of magnitude higher than base (mTorr to Torr).
Worked example — Base vs process with site calculators
- Base budget: Outgassing Estimator + S_eff → predicted base p.
- Process steady state: 150 sccm total, target 12 mTorr → Pumping Speed gives required S ≈ 158 L/s.
- Pressure Converter: express 12 mTorr as 1.6 Pa or 16 mbar for vendor docs—avoid unit mix-ups in PLC limits.