Diffusion pumps are a type of vacuum pump that uses a high speed jet of oil with extremely low vapor pressure to entrap gas molecules and force them downward and out through the exhaust. The oil vapor then condenses on the walls of the pump and flows back to the oil sump.
Operating in the high vacuum regime, diffusion pumps have the advantage of using no moving parts. They require only a heater to vaporize the oil, and cooling water to force condensation on the outer walls of the pump. A liquid nitrogen cooled trap can be used to prevent any pump oil from back streaming into the vacuum chamber. Vacuum chambers are a necessity in a variety of applications, particularly in the semiconductor industry, but also for metallurgy, scientific research, and thin film coating systems.
Diffusion pumps can operate over a range of pressures from 10^-3 to 10^-10 Torr, and can not exhaust directly to atmospheric pressure. Instead, diffusion pumps direct their exhaust to a mechanical displacement pump. In order for a diffusion pump to operate at such low pressures, a working fluid with a very low vapor pressure is necessary. The earliest pumps used mercury as the working fluid, but as early as 1928 silicone oil was being used. Modern pumps mostly use silicone, hydrocarbon or perfluorinated polyethers (PFPEs).
Modern oils are formulated to have very low vapor pressures, and to be as inert and non-reactive as possible. A non-reactive fluid is less likely to be degraded by exposure to other gasses in the vacuum system. The exact composition of the oil can be optimized for different applications based on the required pressures and the expected gas load.
The most widely used diffusion pump oils are synthetic silicone oils, which are fairly stable to oxidation. Hydrocarbon oils are relatively inexpensive, but can oxidize easily to form varnish-like coatings inside the pump, eventually causing maintenance problems. Perfluorinated polyethers are the most stable and can resist oxidation and many corrosive gasses. For this reason they are most often used in pumps for reactive chemical processes such as plasma etching or chemical vapor deposition.
Eventually, the oil in a diffusion pump will need to be replaced. It will collect contaminants from the gasses pumped through it, or react with those gases to form waste byproducts. The lifetime of a batch of oil will vary depending on the load placed on the pump, and the chemical nature of the gasses passing through the pump. High volumes of reactive or corrosive gasses will necessitate more frequent oil changes. Used oil can be easily collected and recycled. Manufacturers will typically reclaim used oil for a small fee. Other routine maintenance includes cleaning the vapor jet elements and checking the temperature of the oil heater.
The primary alternative to oil based diffusion pumps are turbomolecular pumps. These pumps are able to reach the same or lower pressures in a smaller form factor using a series of rotors which rotate at speeds in the range of 800 rotations per second. Modern turbomolecular pumps use magnetic levitation bearings, obviating the need for oil based lubricants. There is no oil to change, and there is no chance of oil back streaming into the vacuum chamber.
A diffusion pump will pump out lighter gasses more quickly than heavier gasses, while a turbo molecular pump will more easily remove heavier gasses. For some applications, cryopumps or ion pumps can take the place of a diffusion pump if the load is small. But due to their simplicity and reliability, diffusion pumps are still widely used both in industry and academia.