Why actual pumpdown time differs from theoretical calculation
Practical reasons pumpdown curves deviate from simple exponential models—pump curves, conductance, outgassing, and leaks.
By Semiconductor Tools Editorial Team · Last updated: 2026-06-14
A textbook pumpdown estimate assumes constant pumping speed and negligible gas load: p(t) ≈ p₀ exp(−S/V·t). Real semiconductor chambers rarely follow this curve because S is pressure-dependent, S_eff is conductance-limited, and Q_total from outgassing and leaks grows as pressure falls.
Main deviation sources
- Pump curve roll-off: Turbo/cryo speed varies with pressure; constant-S models underestimate roughing time or overestimate UHV time.
- Conductance bottleneck: 1/S_eff = 1/S_pump + 1/C — restrictive foreline caps S_eff regardless of catalog pump speed.
- Outgassing tail: Q_out from walls, seals, and fixtures dominates at low pressure, flattening the curve.
- Leaks and virtual leaks: Constant Q_leak sets a pressure floor p ≈ Q/S_eff.
- Regime change: Viscous roughing vs molecular high-vacuum transitions are not one exponential.
Worked example — Simulate then validate with site tools
- Pump Down Time Predictor: V=120 L, p₀=760 Torr, target 1e-6 Torr, Generic Turbo, C=45 L/s, Q=5e-6 Torr·L/s → note predicted tail.
- Run actual pumpdown; paste log into Pumpdown Log Analyzer → extract real S_eff and Q_total.
- Feed revised S_eff and Q back into predictor—iterate until simulation matches measurement.
See Pumpdown time prediction guide and Pumpdown log analysis.