A cryopump is a vacuum pump that uses a low temperature surface to condense gas, also known as a condensate pump. The cryopump can obtain a clean vacuum with the highest pumping rate and the lowest ultimate pressure, and is widely used in the research and production of semiconductors and integrated circuits, as well as molecular beam research, vacuum coating equipment, vacuum surface analysis instruments, ion implanters and space simulation devices, etc. aspect.
pumping principle
Inside the cryopump is a cold plate cooled to a very low temperature by liquid helium or a refrigerator. It condenses the gas and keeps the vapor pressure of the condensate below the ultimate pressure of the pump, so as to achieve the pumping effect. The main functions of low temperature pumping are low temperature condensation, low temperature adsorption and low temperature capture.
① Low temperature condensation: gas molecules condense on the surface of the cold plate or on the condensed gas layer, and the equilibrium pressure is basically equal to the vapor pressure of the condensate. When pumping air, the temperature of the cold plate must be lower than 25K; when pumping hydrogen, the temperature of the cold plate is lower. The thickness of the low-temperature condensation and extraction condensation layer can reach about 10 mm.
②Low temperature adsorption: The gas molecules are adsorbed on the surface of the adsorbent coated on the cold plate with a thickness of a monomolecular layer (in the order of centimeters). The equilibrium pressure for adsorption is much lower than the vapor pressure at the same temperature.
If the vapor pressure of hydrogen is equal to atmospheric pressure at 20K, the equilibrium pressure of adsorption is lower than Pa when absorbing hydrogen with 20K activated carbon. This makes it possible to perform pumping by cryogenic adsorption at higher temperatures.
③Cryogenic trapping: Gas molecules that cannot be condensed at the extraction temperature are buried and adsorbed by the growing layer of condensable gas.
Generally speaking, the ultimate pressure of the pump is the vapor pressure of the condensed gas at the temperature of the cold plate. When the temperature is 120K, the vapor pressure of water is already lower than Pa. At a temperature of 20K, except for helium, neon and hydrogen, the vapor pressures of other gases are also lower than Pa.
However, due to the different temperatures of the pumped container and the cryogenic cold plate, the ultimate pressure of the pump is higher than the vapor pressure of the condensate.
For a vessel at room temperature, with a cryopanel of 20K, the ultimate pressure of the pump is about 4 times the vapor pressure of the condensate.