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Outgassing load estimation and base pressure

Estimating outgassing load from area-based outgassing rates and predicting approximate base pressure.

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

After bulk gas is removed, outgassing from chamber walls, anodized surfaces, elastomer seals, cables, and deposited films often dominates the remaining gas load. Estimating outgassing rate qA [mbar·L/(s·cm²)] and total area A yields Qout = qA·A, which sets a floor on achievable base pressure through p ≈ Qout / Seff.

Representative qA values

  • Unbaked stainless, ~1 h pump: 10⁻⁸ mbar·L/(s·cm²) (order of magnitude)
  • Unbaked stainless, ~10 h pump: 10⁻⁹ mbar·L/(s·cm²)
  • Baked / UHV-prepared steel: 10⁻¹¹ mbar·L/(s·cm²)
  • Viton O-rings / polymers: 10⁻⁷ mbar·L/(s·cm²) or higher

qA decreases with pump time (water desorption) and increases after chamber exposure to atmosphere. Elastomer area is often underestimated—count seal grooves, feedthrough boots, and heater connector boots.

Worked example — Base pressure budget with Outgassing Load Estimator

A spherical chamber (~50 cm diameter) has internal area ≈ 15,700 cm². Includes two Viton-sealed ports equivalent to 200 cm² elastomer at higher qA. Seff at chamber = 120 L/s (from conductance analysis).

  1. Metal area ≈ 15,500 cm²: preset Stainless steel, unbaked (~10 h pump), qA = 10⁻⁹.
  2. Qmetal ≈ 1.55×10⁻⁵ mbar·L/s.
  3. Elastomer 200 cm²: preset Viton O-rings, qA = 10⁻⁷ → Qelast ≈ 2×10⁻⁵ mbar·L/s.
  4. Total Qout ≈ 3.55×10⁻⁵ mbar·L/s. Enter Seff = 120 L/s in the tool.
  5. Predicted base pressure p ≈ 3×10⁻⁷ mbar (~2.3×10⁻⁷ Torr).
  6. If measured base is 2×10⁻⁶ Torr, outgassing model is insufficient—check for virtual leaks, hydrocarbon contamination, or Seff lower than 120 L/s (partially closed valve).

Cross-check Seff using Conductance Calculator and measured pumpdown slope via Pumpdown Log Analyzer.