Technical Explanation
What is Mean Free Path?
Mean free path (λ) is the average distance a molecule travels between collisions. In vacuum and thin-film processes, it indicates whether the process is in the molecular flow regime (λ > chamber dimensions) or viscous flow regime (λ << chamber dimensions). Comparing λ with target–substrate distance helps predict scattering and film uniformity in sputtering, evaporation, and PVD.
Formula
For an ideal gas with hard-sphere collisions:
λ = kT / (√2 π d² P)
where k = Boltzmann constant (1.38×10⁻²³ J/K), T = temperature (K), d = collision diameter (m), P = pressure (Pa). Supports Ar, N2, H2, He, Kr, Xe.
Semiconductor Applications
In PVD and sputtering, λ > target–substrate distance implies less scattering and better step coverage. In high vacuum (e.g. mTorr), heavier gases (Ar, Kr, Xe) have shorter λ than lighter ones (He, H2), affecting process design and gas selection.
References & Disclaimer
Collision diameters from NIST and Handbook of Chemistry and Physics. For critical applications, verify values independently.