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Pumpdown log analysis for S/V and gas load

Using measured pumpdown logs to extract S/V, effective pumping speed, and approximate gas load.

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

Measured pumpdown curves contain diagnostic information that pump datasheets alone cannot provide. Fitting ln(p) vs t in the early regime extracts S/V; analyzing later segments with the full rate equation estimates time-varying gas load Qtotal(t).

Governing equation

V · dp/dt = Qtotal(t) − Seff · p

When Qtotal ≈ 0 early in pumpdown: p(t) ≈ p₀ exp(−(Seff/V)·t), so ln(p) vs t is linear with slope −Seff/V.

Later, when outgassing dominates: Qtotal(t) ≈ V·dp/dt + Seff·p, computable from finite-difference dp/dt on logged data.

Worked example — Extract Seff from log with Pumpdown Log Analyzer

After foreline PM, pumpdown log (time s, pressure Torr):

0    760
30   0.5
60   0.01
120  1e-4
300  5e-6
600  8e-7

Chamber volume V = 200 L. Fit window: t = 60–300 s (ln-linear region before outgassing tail).

  1. Paste log into Pumpdown Log Analyzer. Set V = 200 L, pressure unit Torr.
  2. Fit ln(p) vs t from 60–300 s → slope ≈ −0.015 s⁻¹ → S/V = 0.015 → Seff ≈ 3.0 L/s.
  3. Compare with pre-PM Seff = 45 L/s from same fit method → 15× degradation, consistent with partially closed valve or blocked trap.
  4. In Qtotal window (t = 300–600 s), with Seff = 3 L/s: estimated Q ≈ 2×10⁻⁶ Torr·L/s—outgassing + possible virtual leak, not pump replacement alone.

Validate predictions with Pump Down Time Predictor using updated Seff and Qtotal inputs.