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Precautions for operation of cryogenic pump vacuum system:

(1) In the cryogenic pump system, it is sometimes necessary to cool the chamber wall with liquid nitrogen in order to reduce the surface outgassing. However, attention should be paid to the interpretation of ionization gauge readings. Even if the chamber wall is not cooled, the location of the gauge in the cryogenic system is very important, especially if it is directly facing the pump. Because the ionization gauge measures the gas density rather than the pressure, if the gas temperature in the gauge is T1 and the temperature in the system is T2, then the pressure reading of the ionization gauge is pi = P (T1 / T2) 1 / 2, where p is the pressure in the system.

(2) Special attention should be paid when baking the vacuum system of low-temperature pump. Because the cryogenic pump is afraid of thermal radiation, when the system is baked to 450 ℃, the temperature of the pump may rise beyond the working range. At this time, a water-cooled baffle can be inserted between the pump and the system as a thermal radiation shield, but this will seriously reduce the pumping speed. The baffle can be made into a structure similar to butterfly valve, which can be rotated to the shielding position during baking and then rotated back. When the baffle is in the shielding position, the pumping speed of the system is reduced by 75%.

(3) During the operation of the pumping system, the operator of the system should ensure that the thermal load of the cryogenic pump does not exceed the rated index, especially the thermal load on the first stage adsorption plate of the pump. In order to ensure that the thermal load of cryogenic pump does not exceed the target (especially the thermal load on the first stage), some kind of baffle should be used to reduce the radiation amount emitted to the first stage. In addition to 300 K radiation near the wall of the vacuum chamber, the first stage also receives thermal radiation from sources such as the coating chamber, heated filament or sputtering discharge. In many processes, the thermal load can be as high as 100 to 150 W, which easily exceeds the capacity of an expander with a power of 35 to 40 W. Therefore, some kind of baffle is needed to reduce the radiation flux to the first stage. The easiest way to do this is to use a reflective non cooled baffle. If it is not enough, a herringbone baffle with cooling is needed. The baffle can be cooled with water or liquid nitrogen.

(4) During the continuous operation of the cryogenic pump, especially when the adsorbent is saturated with helium, special attention should be paid to prevent instantaneous power failure, because even if the power is cut off for a short time, helium will be released from the adsorbent, and a large amount of heat on the vacuum wall will be transferred to the surface of the vacuum. At this time, even if the system has been roughly pumped to 20 PA, it can not solve the problem, because it is not enough to prevent continuous heat conduction.

If a short time power failure occurs, that is, after the helium is burst, the pump can no longer continue to work and needs to be regenerated. If the power failure time is long, water vapor may be released from the first stage and deposited on the second stage to saturate the adsorbent. At this time, it is necessary to completely regenerate.

(5) During operation, the overpressure safety valve of the cryogenic pump should be checked regularly to prevent the failure of the safety valve and harm to the operator and the pump.

(6) It is not allowed to use low temperature pump to remove toxic or explosive and dangerous gases which are easy to produce chemical reaction. Because the low-temperature surface can condense all kinds of steam, so it will accumulate a lot of sediment. When the pump is heated, some deposits condensed on the low-temperature surface may react with each other or with the atmosphere. For example, silane reacts with water vapor at 77 K, creating an explosion hazard. If there is explosive gas condensation in the pump, when the pump suddenly heats up and a large amount of gas is released, they will flow to the evacuated system. At this time, if the ionization gauge is working, serious problems will occur.

(7) The pumping capacity of cryogenic pump for all gases is not the same. The ability of helium and hydrogen extraction of cryogenic pump is much worse than that of other gases, which should be considered in the composition and operation of the system.

(8) The compressor needs to be filled with high purity helium (99.999%). The most common impurity in the neon grade is condensation on the helium seal.