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Gas mixture composition and partial pressures

Relating MFC flow settings to mole fractions and partial pressures in a non-reactive gas mixture.

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

For non-reactive ideal-gas mixtures, Dalton's law gives ptotal = Σ pi, with pi = yi · ptotal. In MFC-controlled systems, mole fractions are approximated from volumetric flow ratios at consistent STP: yi ≈ Fi / ΣFj.

Why partial pressure matters

Reaction kinetics depend on reactant partial pressures, not MFC percentages alone. In dilute O₂/Ar plasmas, a 5% O₂ flow fraction does not always equal 5% O₂ partial pressure if H₂ trace impurities, pump oil backstreaming, or conductance bias alter composition at the wafer.

Worked example — PECVD dilute silane environment (Gas Mixture Partial Pressure Calculator)

Flows: SiH₄ 25 sccm, N₂ 975 sccm. Chamber total pressure 600 mTorr (0.6 Torr). Non-reactive mixture approximation.

  1. Enter gas rows: SiH₄ 25 sccm (M = 32), N₂ 975 sccm (M = 28). Total P = 0.6 Torr.
  2. Mole fractions: ySiH₄ = 25/1000 = 2.5%, yN₂ = 97.5%.
  3. Partial pressures: pSiH₄ = 15 mTorr, pN₂ = 585 mTorr.
  4. Enable molecular-flow √M scaling: light species weight F/√M shifts effective composition—useful qualitative check if long molecular-flow lines precede the chamber.

Worked example — Low-pressure etch oxidant control

Flows: Ar 180 sccm, O₂ 12 sccm, CF₄ 8 sccm. Total P = 25 mTorr.

  1. Total flow = 200 sccm. pO₂ = 12/200 × 25 = 1.5 mTorr. pCF₄ = 1.0 mTorr.
  2. If O₂ MFC drifts to 15 sccm (same Ar, CF₄), rerun: pO₂ → 1.875 mTorr (+25% oxidant)—profile angle may shift.
  3. Convert individual MFC flows to mass balance via MFC Flow Converter for abatement loading estimates.