Lower base pressures mean that less tramp molecules from the atmosphere are present in the chamber and the lower the possibility they will be included in the depositing films. As the pressure in a vacuum chamber decreases (i.e., as the number of molecules bouncing around in the deposition environment goes down), the mean free path, or the average distance that one molecule has to travel before it collides with another molecule, increases. For example, the mean free path at 760 Torr, or 1 atmosphere, when the molecular density is on the order of 2.4 x 1019 molecules per cm3, is about 6 x 10-6 cm. At a pressure of 5 x 10-8 Torr the mean free path increases to about 1 kilometer.
Molecules in a vacuum chamber are moving at very high speeds, generally unaffected by gravity, and moving in straight paths until they hit another molecule or a chamber wall. Hopefully, they are bouncing into the inlet of a vacuum pump where they may be exhausted from the system. Deposition processes like thermal and e-beam evaporation are very much line-of sight techniques where the evaporant can be delivered at an angle normal to the substrate if it is uninhibited by collisions with tramp molecules.
With substantially large mean free paths, (larger than the dimensions of a deposition chamber), there is some right to expect that the evaporating flux will not be impeded by intermediate collisions with atmospheric gases during their flight from source to substrate. Certainly at 5 x 10-8 Torr, when the mean free path is on the order of 1,000,000 cm, pristine thin films without inclusion of unwanted gases are possible.
Category: Vacuum Systems
Sub-Category: PVD Vacuum System