Application example of cryogenic pump in ultra-high vacuum furnace
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More than ten years ago, it began to apply cryogenic pump and dry pump oil-free system in vacuum electronic technology related process equipment to obtain ultra-high vacuum with satisfactory effect. This attempt will be the cryogenic pump fast, clean and reliable unique advantages of the application of vacuum furnace. The cryogenic pump is incorporated into the vacuum system to achieve a clean ultra-high vacuum environment. This paper introduces the application of cryogenic pump in ultra high vacuum furnace with an example. This paper mainly introduces the design idea of cryogenic pump vacuum system, the main configuration and matters needing attention of the vacuum system, the calculation of cryogenic pump type selection, and introduces the installation, use and maintenance of the cryogenic pump system in detail with the graphical method. It is of reference value to the research and development of similar vacuum furnace and the application of cryogenic pump. Through strict control of all research and development links, cleaning, degassing and baking in accordance with ultra-high vacuum specifications, the ultimate pressure of the vacuum furnace reached 2 x 10-7pa, extending the limit pressure of the conventional vacuum furnace by nearly an order of magnitude. In recent years oil-free vacuum pumps have been introduced and widely used in semiconductor industry. Therefore, the oil-free ultra-high vacuum acquisition system represented by cryogenic pump and dry pump has been widely promoted in semiconductor electronic technology, optical coating system and aerospace fields.
By lowering the extraction surface temperature to less than 20k through low-temperature medium, the extraction surface can condense the gas with lower boiling point temperature, so as to extract a large amount of gas. The use of low-temperature pumping surface will be condensing gas pumping pump called cryogenic pump, or condensing pump. It is a device that condenses, adsorbs or condenses + adsorbs the gas after using low-temperature medium to reduce the surface temperature, so as to reduce the pressure of the pumped space and obtain vacuum or maintain vacuum state.
As a complete pumping system, the cryogenic pump is composed of two parts, the main body is the vacuum pump body, and the second part is the compressor. The cryogenic pump of refrigerating machine is widely used now, the core of which is the cryogenic refrigerating machine. The basic process is as follows: helium gas is firstly compressed to high temperature and pressure; Then it is cooled to room temperature helium through a heat exchanger. The purified high-purity helium is then adiabatically expanded by the cylinder to become low-temperature helium. And so it goes around and around, and the helium gets cooled, and it becomes a refrigerating medium -- cryogenic helium.
Advantages of choosing cryogenic pump include:
(1) clean, pure no oil, wide range of gas extraction, can quickly obtain the ideal ultra-high vacuum environment;
(2) and the same diameter of other vacuum pumps, cryogenic pump has a greater pumping speed, especially the ability to extract water vapor;
(3) there is no choice to be pumped gas, impurity particles and do not affect the system work;
(4) can be installed at any Angle, no moving parts, do not need the front stage pump, running and maintenance cost is low;
(5) Exposure to the atmosphere has a small impact on the system, after the compressor water can be self-protection, so can achieve unattended.
Design of vacuum system
Vacuum furnace chooses cryogenic pump system, the biggest problem is to solve the heat load. Heat load in vacuum furnace mainly comes from three aspects:
(1) from the side of the furnace heat radiation;
(2) viscous flow state, gas molecules carry away heat;
(3) from the pump mouth pipe heat conduction and radiation heat.
Cryogenic pump work in molecular flow state, heat source ii can be ignored. Heat source (3) can be eliminated by adding water cooling structure. The influence of heat load from furnace body on cryogenic pump is considered in the design. Multilayer metal reflector screen is selected in the vacuum furnace. Vacuum furnace heating temperature is 1300 ℃, the highest total design 6 layer of reflective screen, molybdenum dressing screen 3 layers, the rest of the layers selected stainless steel reflector screen. Theoretically the heater to the container wall via radiation temperature about 200 ℃, the temperature will slowly rise over time. Most cases, low temperature pump can include a 90 ° elbow, in order to avoid and vacuum chamber through, it is further reduced to the thermal load of the pump, which is to reduce thermal radiation is the most simple and effective method. In the molecular flow state, the influence of elbow convection guide can be ignored, and it is more effective than baffle for ultra-high vacuum system. In order to further reduce the influence of heat conduction and radiation on the cryogenic pump, a water-cooled structure was designed on the elbow pipe. The vacuum system structure is shown in figure 1.
In order to obtain the ideal ultra-high vacuum, we designed a parallel vacuum system of molecular pump unit and cryogenic pump using redundancy technology. The molecular pump unit can be used as the pre-pumping of the system, so that when the cryogenic pump cools down, it can work directly in the molecular flow state.
In addition, when the amount of air released from the vacuum chamber is relatively large, it can be timely switched to the molecular pump for air extraction, which can not only prolong the exhaust saturation time of the cryogenic pump, but also save the regeneration time. Molecular pump and cryogenic pump are connected by pneumatic ultra-high vacuum gate valve and vacuum chamber. In order to ensure that the system is clean and oil-free, and at the same time give full play to the pumping advantages of cryogenic pump, pure oil-free magnetic levitation molecular pump is selected, vortex dry pump is selected for the front stage pump, and dry pump is also used for air extraction in cryogenic pump regeneration. Schematic diagram of vacuum system is shown in FIG. 2 main configuration of vacuum system.
When the molecular pump unit and compressor are started at the same time, after about 120 min, the secondary cold head of the cryogenic pump is lowered from room temperature to below 15k (cryogenic pump valve door can be opened). At this time, the vacuum degree of the vacuum furnace system can reach 10-5pa, so that the cryogenic pump can start from high vacuum directly.
This set of equipment is a typical set of medium-sized vacuum furnace, applied to the diffusion welding process under the condition of ultra-high vacuum, the heat capacity and surface area are greater than the heat treatment furnace of the same specification, higher degree of automation, configuration of cryogenic pump can play its significant advantages. The ultimate test pressure of the vacuum furnace is 2 * 10-7 Pa(cold and empty), and the pressure rise rate is 0.002 Pa /h. The index is better than the usual high vacuum furnace. At present, domestic cryogenic pump applications are mostly dependent on imports, and the same caliber of other pumps, the price is more expensive. However, it still has great advantages in the fields of high clean process environment, pure oil-free vacuum and ultra-high vacuum, and the cryogenic pump is no worse than other pumps in terms of service cost and service life. Domestic cryogenic pump research and development is still to be mature, especially compressor technology, looking forward to the rise of domestic clean vacuum pump industry.