More than ten years ago, we began to apply cryogenic pumps and dry pump oil-free systems in vacuum electronic technology-related process equipment to obtain ultra-high vacuum, with satisfactory results. This attempt is to apply the unique advantages of cryogenic pumps, which are fast, clean and reliable, to vacuum furnaces. Integrate the cryogenic pump into the vacuum system to achieve a clean ultra-high vacuum environment. This article introduces the application of cryogenic pumps in ultra-high vacuum furnaces with examples; it focuses on the design ideas of the cryogenic pump vacuum system, the main configuration and precautions of the vacuum system, the selection and calculation of the cryogenic pump, and introduces the cryogenic pump in detail using diagrams. System installation, use and maintenance. It has reference value for the research and development of similar vacuum furnaces and the application of cryogenic pumps. By strictly controlling all R&D links and carrying out cleaning, degassing and baking in accordance with ultra-high vacuum standards, the ultimate pressure of the vacuum furnace finally reached 2×10-7Pa, extending the ultimate pressure of conventional vacuum furnaces by nearly an order of magnitude. In recent years, oil-free vacuum pumps have been introduced and widely used in fields represented by the semiconductor industry. As a result, the oil-free ultra-high vacuum acquisition system represented by cryogenic pump + thousand pump has been widely promoted in the fields of semiconductor electronic technology, optical coating system and aerospace.
By reducing the temperature of the exhaust surface to below 20K through the low-temperature medium, the exhaust surface can condense gas with a lower boiling point temperature, thereby extracting a large amount of gas. This kind of pump that uses a low-temperature pumping surface to condense gas is called a cryogenic pump or a condensation pump. It uses a low-temperature medium to reduce the surface temperature and then condenses, adsorbs, or condensates + adsorbs the gas. It reduces the pressure of the pumped space and obtains a vacuum or A device that maintains a vacuum state.
As a complete air extraction system, the cryogenic pump consists of two parts, the main part is the vacuum pump body, and the second part is the compressor. Nowadays, refrigerator-type cryogenic pumps are widely used, the core of which is a cryogenic refrigerator. The basic working process is: helium is first compressed to a high-temperature and high-pressure state; then it is cooled into normal-temperature helium through a heat exchanger; the purified high-purity helium finally completes adiabatic expansion through the cylinder and becomes low-temperature helium. In this continuous cycle, the helium gas is continuously cooled down and becomes low-temperature helium gas as the refrigeration medium.
The advantages of choosing a cryogenic pump are:
1.Clean, pure oil-free, wide pumping range, and can quickly obtain an ideal ultra-high vacuum environment;
2.Compared with other vacuum pumps of the same caliber, the cryogenic pump has a greater pumping speed, especially the ability to pump out water vapor;
3.There is no choice for the pumped gas, and impurity particles will not affect the system work.
4.It can be installed at any angle, has no moving parts, does not require a backing pump, and has low operation and maintenance costs.
5.Exposure to the atmosphere has little impact on the system. The compressor can protect itself after the water is cut off, so it can be unattended.
Vacuum system design
The biggest problem when choosing a cryogenic pump system for a vacuum furnace is to solve the heat load. The heat load in the vacuum furnace mainly comes from three aspects:
1.Thermal radiation from the furnace side;
2.In the viscous flow state, gas molecules carry away heat
3.Heat conduction and radiated heat from pump inlet piping.
The cryopump works in a molecular flow state, and the heat source 2. can be ignored. Heat source 3. can be eliminated by adding water cooling structure. The design mainly considers the impact of the heat load from the furnace body on the cryogenic pump. Choose a multi-layer metal reflective screen in the vacuum furnace. The maximum vacuum heating temperature is 1300°C. A total of 6 layers of reflective screens are designed, with molybdenum screens selected for the inner 3 layers and stainless steel reflective screens for the remaining layers. Theoretically, the temperature that the heater reaches the container wall through radiation is about 200°C, and the temperature will slowly increase over time. In most cases, the cryopump can be equipped with a 90° elbow to prevent direct communication with the vacuum chamber, which further reduces the heat load transmitted to the pump. This is also the simplest and most effective way to reduce heat radiation. Under molecular flow conditions, the impact of elbows on flow conductance is negligible and is more effective than baffles for ultra-high vacuum systems. In order to further reduce the impact of heat conduction and radiation in the pump port pipeline on the cryopump, a water-cooling structure is designed on the elbow pipeline.
Conclusion
Cryogenic pumps still have great advantages in high-clean process environments, pure oil-free vacuum and ultra-high vacuum fields, and in terms of cost and lifespan, cryogenic pumps are no worse than other pumps.
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