Oil Diffusion pumps produce high vacuum and operate with a fast-moving stream of pump fluid in vapor form (oil vapor). All fluid-entrainment pumps have a similar pumping mechanism where the pump fluid is heated by the electrical heater vessel and the oil vapor expands after passing through the jet stack nozzles. The pumped gas molecules diffuse into the vapor stream and exit out of the pumping chamber.

The molecules of the pump fluid stream transfer by way of impact impulses to the gas molecules in the direction of the flow. Thus, the gas which is to be pumped is moved to a space with higher pressure. This technology is wear -free as it doesn’t need any moving parts. In oil diffusion pumps, corresponding vapor pressures increases during operation depending on the type of pump fluid (silicon- or mineral- oil) and the temperature as well as the design of the nozzles.

The vapor jet does not enter the process chamber since it condenses after ejecting through the nozzles on the cooled outer walls of the pump. Furthermore, the modern unique baffle design reduces the oil back streaming and oil consumption through improved condensation surface design. This simple physical mechanism makes Diffusion pump a robust technology.

The backing pressure in the pump must be low enough to allow the vapor to flow out. To ensure this, corresponding backing pumps are used, typically mechanical fore vacuum pumps from Leybold. The fore vacuum pressure also influences the vapor jet and becomes detrimental if its value exceeds a certain critical limit called maximum backing pressure or critical fore line pressure.