Gas mixture composition and partial pressures

For an ideal, non-reactive gas mixture, Dalton's law states that the total pressure p_total is the sum of the partial pressures p_i of each component: p_total = Σ p_i. Each partial pressure is p_i = y_i p_total, where y_i is the mole fraction of species i in the mixture.

In semiconductor tools, steady-state composition is commonly specified in terms of MFC flows in sccm. Under a consistent STP definition, volumetric flows in sccm are proportional to molar flows, so the mole fraction can be approximated by y_i ≈ F_i / ΣF_j, where F_i is the sccm setpoint for gas i. Once y_i is known, the corresponding partial pressure is p_i = y_i p_total.

This perspective is particularly useful when reasoning about oxidant partial pressure (e.g. O₂), dilution ratios (e.g. Ar carrier gas), or reactive gas mixtures in etch processes. Instead of thinking only in terms of individual MFC values, one can think in terms of partial pressures that determine reaction kinetics and film properties.

At high vacuum in molecular-flow lines, conductance differences between gases can bias the composition at the chamber relative to the MFC manifold. In molecular flow, conductance scales roughly with 1/√M, where M is molar mass. As a crude indicator, flows weighted by 1/√M (F_i / √M_i) can be used to visualise how light gases (H₂, He) might be overrepresented at the chamber compared with heavier gases for the same MFC setpoints.

Our Gas Mixture Partial Pressure Calculator uses MFC flows and total pressure to compute simple mole fractions and partial pressures, and optionally provides a "scaled" composition using F_i / √M_i as a qualitative indicator of molecular-flow conductance effects.