Application example of cryogenic pump in ultra high vacuum furnace
More than ten years ago, we 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 applies the unique advantages of the cryogenic pump, which is fast, clean and reliable, to the 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. The design idea of cryogenic pump vacuum system, the main configuration and matters needing attention of the vacuum system, the selection and calculation of cryogenic pump are introduced emphatically. It has reference value for the research and development of similar vacuum furnace and the application of cryogenic pump. By strictly controlling all research and development links, cleaning, degassing and baking according to the ultra-high vacuum specification, the ultimate pressure of the vacuum furnace reached 2 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 the field of 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 field.
By lowering the temperature of the pumping surface below 20 K through the low-temperature medium, the gas with a lower boiling point can be condensed on the pumping surface and a large amount of gas can be extracted. The use of low - temperature pumping surface will be condensed gas 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 or maintain the 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. Now is widely used cryogenic pump refrigerator, its core is cryogenic refrigerator. The basic flow is as follows: first, helium is compressed to high temperature and pressure; It is then cooled by a heat exchanger to room temperature helium. The purified high purity helium is then adiabatically expanded in the cylinder to become low temperature helium. As the cycle continues, the helium is cooled and becomes a refrigerating medium -- cryogenic helium.
Advantages of choosing cryogenic pump include:
(1) clean, pure without oil, pumping range wide, can quickly obtain the ideal ultra-high vacuum environment;
(2) compared with other vacuum pumps of the same diameter, the cryogenic pump has a greater pumping speed, especially the ability to extract water vapor;
(3) 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 pump, operation and maintenance cost is low;
Exposure to the atmosphere has a small impact on the system, after the compressor water can be self-protection, so can realize unattended.
Vacuum system design
The biggest problem of choosing cryogenic pump system for vacuum furnace is to solve the heat load. The heat load in the vacuum furnace mainly comes from three aspects:
(1) thermal radiation from furnace side;
(2) viscous flow, gas molecules carry heat;
(3) heat conduction and radiation heat from the pump mouth pipe.
Cryopump work in molecular flow state, heat source ii can be ignored. Heat source () 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 the elbow on the convection guide can be ignored, which is more effective than the baffle for the ultra high vacuum system. In order to further reduce the influence of heat conduction and radiation on cryogenic pump, a water cooling structure was designed on the elbow pipe. The vacuum system structure is shown in figure 1.
FIG. 1 vacuum system structure diagram
In order to obtain the ideal ultra high vacuum, and at the same time to use the redundancy technology, we designed the vacuum system of molecular pump unit and cryogenic pump in parallel. The molecular pump unit can be used as the system's pre-pumping, so that when the cryogenic pump is cooled, it can work directly in the molecular flow state.
In addition, when the vacuum chamber has a large amount of air, it can be timely switched to the molecular pump for air extraction, which can not only extend the pumping 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 without oil and give full play to the pumping advantages of the cryogenic pump, the pure oil-free magnetic levitation molecular pump is selected, the vortex dry pump is selected for the front pump, and the dry pump is also used for air extraction during the regeneration of the cryogenic pump. The schematic diagram of vacuum system is shown in figure 2.
FIG. 2 schematic diagram 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 15 K (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.
Table 1 main configuration conclusions of vacuum system
This set of equipment is a typical set of medium-sized vacuum furnace, which is applied to the diffusion welding process under the condition of ultra-high vacuum. The heat capacity and surface area are both larger than the heat treatment furnace of the same specification, with a higher degree of automation. The configuration of cryogenic pump can give full play to its remarkable 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 common high vacuum furnace. At present, the application of domestic cryogenic pump is mostly dependent on imports, and other pumps of the same caliber, the price is also relatively 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 the compressor technology, looking forward to the rise of domestic clean vacuum pump industry.