In practice, industrial vacuums are produced using different kinds of vacuum pumps. Vacuum pumps can be based on a number of different principles:
In practice, within the semiconductor industry, the reader will primarily encounter mechanical pumps that operate on the compression-expansion principle, molecular drag pumps such as turbopumps, and physical sorption pumps such as cryopumps. We will therefore limit our discussion of pumping options for the creation of vacuum to these methods.
Within a turbomolecular pump (Figure 5), a number of circular discs with angled blades (referred to as rotor discs) rotate at a very high speed (up to 90,000 rpm). Between each moving disc/blade assembly is a mirror-image static disc/blade assembly known as a stator disc. The moving blades impact gas molecules within the pump and physically impart additional momentum (proportional to the blade speed) to the molecules, moving them in the direction of the stator disc. The pump is designed to ensure that the mean free path of molecules in the gas within the pump is greater than the distance between the rotor disc blades and the stator disc blades. This assures that the momentum imparted by the rotor is not lost in molecular collisions, but rather moves the gas molecules through the stator blades. Here they encounter another rotating blade that moves them towards another stator further down the pump and closer to the exhaust. The gas is thus compressed through a number of stages in the turbomolecular pump until it finally exits the turbomolecular pump and is drawn away by the backing mechanical pump. Depending on the model, turbopumps can handle gas loads of up to a few thousand liters/second and are capable of reducing chamber pressures to around 10-10 Torr.