Provided temperature is low enough, all gas molecules can be condensed onto a cold surface and thus creates vacuum in an enclosed chamber.
Cryopump system
As shown in the above figure (reference 1), a cryopump system consists:
1) Cryopump unit. cold surface to condense gas molecules to create vacuum.
2) Compresser. supply compressed Helium gas to cryopump where it expands to reduce temperature. The Helium gas return to compressor for cycles by cycles operation. It also includes adsorber and oil mist separator, which are the key components need to be well maintained.
3) Helium line. Its aeroquip design ensure the line is open only when the connection is established. It prevents Helium leak and ambient air contamination.
Cryopump
The cryopump design is as the above figure (reference 2). The vacuum that can be reached depends on the following saturation pressure Vs temperature of relevant gases.
Maintenace
One major problem for cryopump malfunction is helium contamination. The purity of He is 99.999% (5N); however during operation, air and oil mist can contaminate the system. One consequence of He contamination is the observation of “ratcheting” sound when the cryopump is cooling down to a low temperature, which is due to the deposition of contaminants (frozen) on the wall of pump. As a result it may possibly break the moving shaft (see the above figure). The pump should be immediately stopped for repair. Regeneration might give a false sense of correcting the problem; to fundamentally solve the problem, Decontamination Procedure (change new pure He) should be performed on that pump (reference 3).
Two major sources that contribute to helium contamination are the Adsorber and the Oil Mist Separation components. Therefore, they should be regularly maintained with the frequency suggested as the following:
