The mean distance a molecule covers between collisions with other molecules, inversely proportional to pressure. For air at 20 °C the product of mean free path and pressure is about 6.65×10⁻³ cm·mbar, which gives about 66 nm at atmosphere, 66 µm at 1 mbar, 6.6 cm at 10⁻³ mbar and 660 m at 10⁻⁷ mbar. Comparing it with the size of the pipes and chamber tells you the flow regime and how pumps and gauges will behave.
At 1E-5 mbar the mean free path is metres, so the sputtered atoms fly straight to the substrate without scattering.
Handy mental number for air: about 5 cm at 1 millitorr. Divide 5 cm by the pressure in millitorr and you have it.
Also heard as
- MFP
- lambda
Related terms
Knudsen number
Ratio of the mean free path to a characteristic size such as pipe diameter, used to name the flow regime.
Molecular flow
Flow regime in which molecules bounce between the walls almost without meeting each other.
Viscous flow
Flow regime in which molecules collide mostly with each other, so the gas moves as a continuous fluid.