A cryopump is a vacuum pump that uses a low temperature surface to condense gas, also known as a condensate pump. The cryopump is the vacuum pump with the lowest ultimate pressure and the highest pumping rate to obtain a clean vacuum. It 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 simulations. devices, etc.
The pumping principle is equipped with a cold plate cooled to a very low temperature by liquid helium or a refrigerator in the cryopump. 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.
①Low temperature condensation: gas molecules are condensed 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 (10-8 cm order). The equilibrium pressure for adsorption is much lower than the vapor pressure at the same temperature. For example, the vapor pressure of hydrogen at 20K is equal to atmospheric pressure, and the adsorption equilibrium pressure is lower than 10-8 Pa when 20K activated carbon absorbs hydrogen. 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 10-8 Pa. When the temperature is 20K, except for helium, neon and hydrogen, the vapor pressure of other gases is also lower than 10-8 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.
Type cryopumps are divided into two types: injection liquid helium cryopumps and closed-circuit gas helium refrigerator cryopumps.
①Injected liquid helium cryopump: It is mainly composed of a liquid helium container, a pump body and a liquid nitrogen cavity connected to a baffle. In order to reduce the consumption of liquid helium, the outer wall of the liquid helium container adopts a double-layer thermal insulation wall and is evacuated in between.
When the pump is pre-pumped to a pressure of 10-6 Pa, liquid nitrogen and liquid helium are poured into it, and the gas condenses on the working cold plate of 4.2K. After pre-pumping, the partial pressure of helium and hydrogen is in the order of 10-12 Pa, so the pump can obtain the ultimate pressure below 10-11 Pa. If the liquid helium container is evacuated and decompressed to 6650 Pa, the temperature of the liquid helium can be reduced to 2.3K, and a lower limit pressure can be obtained.
②Cryogenic pump of closed-circuit gas helium refrigerator: It is a new type of cryogenic pump that appeared in the 1970s (pictured). This pump does not consume helium, is easy to operate, easy to maintain, and is increasingly used. The refrigeration medium of the refrigerator is gas helium, the temperature of the primary cold plate is 50-100K, which is used to condense water vapor and pre-cool other gases; the temperature of the secondary cold plate is 10-20K, which is used to condense nitrogen, oxygen and argon and other gases .
The inner surface of the secondary cold plate is coated with activated carbon. The specific surface area of activated carbon is 500-2500 m2/g, and it has strong adsorption capacity for helium, neon and hydrogen at low temperature. The cold plate is made of oxygen-free copper, and the surface is polished to a mirror level to reduce the emissivity. The ultimate pressure of the pump is 10-7 ~ 10-8 Pa, the working pressure range is 10-1 ~ 10-7 Pa, and the pre-pumping pressure is required to be 1 Pa.
The pumping rate of the finished product has reached 60,000 liters/second (1 liter = 10-3 m3). In addition, according to the characteristics of the process, the air extraction cold plate can be arranged in the pumped container, and the air extraction rate can reach more than 106 liters/second.
Low heat load The heat load of the oil pump is mainly the condensation heat of the gas and the radiant heat of the surrounding wall facing the working cold plate. The heat of condensation is related to the type of gas. For nitrogen at 80K and 133.322 Pa liters, the condensation heat on a 20K cold plate is 0.3-0.6 joules.
The radiant heat received by the working cold plate is proportional to the difference between the 4th power of the temperature of the surrounding wall panel and the temperature of the working cold plate. Therefore, 4.2K and 20K working cold plates are shielded with 50-100K cold plates to reduce the radiant heat received by the working cold plates.