At first sight there are several pump types that can achieve this pressure, namely:
• Cryo pumps
• Diffusion pumps
• Turbomolecular pumps
As with any high vacuum pump, a suitable forevacuum pump will also be needed.
Diffusion pumps, whilst offering pumping speeds up to 50,000 l/s, create oil contamination and are generally not suitable for the high-quality films that are demanded by the optical and semiconductor sectors. One could add baffles, but these can reduce the actual pumping speed by up to 50 per cent. Moreover, diffusion pumps have relatively high running costs in terms of power and cooling water. In addition to the forevacuum pump, an additional smaller holding pump and bypass line are needed to maintain the diffusion pump when the system is initially evacuated from atmosphere.
Whilst turbomolecular pumps are oil free, the maximum pumping speed is around 3000 l/s, which is generally too low for high-throughput processes. Moreover, these pumps are poor at pumping water vapor and light gases, crucial for many coating applications.
The lighter gases such as hydrogen would not condense at 20K, and so a charcoal coating is applied to “cryo-absorb” these species. This again allows higher pumping speeds for these gases, relative to a turbomolecular pump. These gases will eventually saturate the 2nd stage, resulting in a rise in ultimate pressure.
The pump then needs to be regenerated by heating the stages whilst pumping away the released gaseous species with a small fore vacuum pump — an oil free pump such as a scroll is recommended. This prevents contamination of the pump cryo panels. Regeneration typically takes 2 hours and can be incorporated as part of the routine maintenance. A cryo pump can meet all these demands. As the name suggests, the pump employs a refrigeration principle. Helium-4 is compressed and expanded to achieve extremely low temperatures, resulting in the condensation of gaseous species. The cryo pump is classed as an entrapment pump. The schematic below illustrates a typical cryo pump system: