The time taken to form a single complete layer of gas on a surface is given by
where tm is the time to form a monolayer (in seconds), n = number of molecules per unit volume, vrms is the average velocity of the molecules and d is the diameter of a molecule. In case of air at room temperature, the expression becomes: t m = 1.86 x 10-6/P, when P is the pressure expressed in torr.
Thin film depositions are carried out in high vacuum environment for achieving quality films. Vacuum pumps are used to create vacuum in a sealed chamber where the film deposition takes place. A partial vacuum, or rough vacuum, can be created using a rotary pump. The vacuum pumps could be classified into two based on how the molecules are removed from the chamber. In case of mechanical, turbo molecular and oil diffusion pumps, the molecules are physically removed from the chamber. However in case of cryo pumps and sputter/ion pump with Ti sublimation, the molecules are adsorbed on a surface or buried in a layer. The mechanical pumps have moving parts and oil. They are used to pump down from 1 atm pressure to roughing (medium) vacuum. Examples are Piston pumps (not used much due to particle problems), Rotary vane pumps (majority of cheap applications) and Dry pumps. Turbo molecular pumps pump down from 1 atm and are clean pumps. They also have moving parts and pump speeds are low. Oil diffusion pumps have no moving parts but based on oil in vacuum. Sputter/ion as well as cryo pumps are clean with no moving parts and are used to pump down from 10-4 torr.