Cryogenic pump vacuum system operation precautions:
(1) In the cryogenic pump system, it is sometimes necessary to cool the wall of the working chamber with liquid nitrogen in order to reduce the surface outgassing. However, care should be taken when interpreting the ion gauge readings. Even if the chamber walls do not cool, the position of the gauge within the cryogenic system is important, especially if it is directly against the pump. Since the gauge measures gas density rather than pressure, if the gauge gas temperature is T1 and the system temperature is T2, then the ionization gauge pressure reading is Pi = P (T1 / T2) 1/2, where P is the value in the system pressure.
(2) Special attention should be paid when baking the cryogenic pump vacuum system. As the cryogenic pump is afraid of heat radiation, when the system is baked to 450 â„ƒ, it may cause the temperature of the pump to rise beyond the working range. At this time, Water-cooled baffles are inserted as thermal radiation shields, but this can severely reduce pumping speed. The baffle can be made to rotate to the screening position when baking, and the structure similar to the butterfly valve is rotated back when the baffle is rotated. When the baffle is in the shielding position, the pumping speed of the system is reduced by 75%.
(3) During the operation of the extraction system, the operator of the system should ensure that the thermal load of the cryogenic pump does not exceed the rated value during operation. Special attention should be paid to the thermal load on the first-stage adsorption plate of the pump. To ensure that the cryogenic pump's thermal load does not exceed the target, especially the thermal load on the first stage, some form of baffle should be used to reduce the amount of radiation emitted to the first stage. In addition to the 300 K radiation near the wall of the vacuum chamber, the first stage also receives thermal radiation from sources such as coating chambers, heated filaments or sputter discharge. In many processes the thermal load can be as high as 100 to 150 W, at which point it is easy to exceed the expander capacity of 35 to 40 W. There is therefore a need to add some form of baffle to reduce the radiant flux incident on the first stage. The easiest way is to use a reflective non-cooled baffle. If not enough, then you need to use the cooling herringbone baffle. This baffle can be cooled with water or liquid nitrogen.
(4) Cryopumps In continuous operation, especially when the adsorbents are saturated with helium, special care should be taken to prevent momentary power failures because even in the event of a short power outage, helium is released from the adsorbent and the vacuum A lot of heat on the wall spread to the suction surface. At this point, even a system that has been roughly drawn to 20 Pa can not solve the problem, because that is not enough to prevent continuous heat conduction.
If a brief power outage occurs, that is, when the helium is buffeted, the pump can not continue to work and needs to be regenerated. A longer power outage may allow water vapor to be released from the first stage and deposited on the second stage to saturate the adsorbent, requiring complete regeneration.
(5) should always check the operation of the cryogenic pump over-pressure safety valve is normal, to prevent failure of safety valve, the operator and the pump harm.
(6) Cryogenic pumps can not be used to remove hazardous gases that are toxic or explosive and liable to cause chemical reactions. Because cryogenic surfaces can condense various vapors, they accumulate large amounts of sediment, and some deposits that condense on cryogenic surfaces may react with each other or react with the atmosphere as the pump heats up. For example, silane and steam react at 77 K, creating a risk of explosion. If the pump condenses with explosive gases, they will flow to the evacuated system when the pump suddenly heats up and a significant amount of gas is released, which could cause serious problems with the ionization gauge at work.
(7) cryogenic pump for all gas pumping capacity is not the same. The ability of cryogenic pumps to pump helium and hydrogen is much less than the ability to pump other gases, which should be taken into account in the composition and operation of the system.
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