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Pumpdown time prediction: pump curves, conductance, and gas load

How to estimate chamber pumpdown curves using pump speed vs pressure, piping conductance, and total gas load—before you run the recipe.

By Semiconductor Tools Editorial Team · Last updated: 2026-06-14

Pumpdown time is rarely a single exponential. Real curves reflect pressure-dependent pump speed S(p), conductance limits, and rising outgassing load Qout(t) as adsorbed water desorbs. Predicting time-to-base before opening the chamber saves PM validation time and sets realistic production restart expectations.

Simulation workflow

  1. Define chamber volume V and initial pressure p₀ (often atmospheric or vent pressure).
  2. Select pump curve (turbo, cryo, or custom CSV of log₁₀(p) vs S).
  3. Enter total molecular-flow conductance C from piping (or derive Seff from conductance + pump).
  4. Add constant or pressure-dependent Qtotal from leaks and outgassing presets.
  5. Integrate V·dp/dt = Q − Seff·p to obtain p(t) and time to target pressure.

Worked example — Post-vent pumpdown forecast (Pump Down Time Predictor)

Chamber V = 120 L, vented to atmosphere (p₀ = 760 Torr). Target 1×10⁻⁶ Torr. Generic turbo preset. Foreline conductance equivalent C = 45 L/s. Constant outgassing load Q = 5×10⁻⁶ Torr·L/s (from outgassing estimator after PM).

  1. Enter V = 120 L, p₀ = 760 Torr, ptarget = 1e-6 Torr.
  2. Pump preset: Generic Turbo. Conductance C = 45 L/s (series with pump).
  3. Gas load Q = 5e-6 Torr·L/s (constant approximation for initial estimate).
  4. Simulator shows roughing knee, transition region, and low-pressure tail. Note predicted time to 1×10⁻⁶ Torr—often dominated by outgassing tail, not turbo catalog speed.
  5. After actual run, paste log into Pumpdown Log Analyzer to refine Q and Seff for next prediction.

Estimate Qout with Outgassing Load Estimator and Seff with Conductance Calculator before simulating.