Today, I want to explain to you the role of the oil diffusion pump of the pvd coating machine. Professionals who want to enter this industry can learn more about this knowledge, and they can quickly enter the state when they are engaged in this work in the later stage, which is of great help.

 Oil diffusion pump: The ultimate vacuum of the mechanical pump is only 10-2 Pa. When it reaches 10-1 Pa, the actual pumping speed is only 1/10 of the theoretical value. If you want to obtain high vacuum, you must use an oil diffusion pump. Since the oil diffusion pump is the earliest pump used to obtain high vacuum, it is widely used due to its low cost, convenient maintenance and extensive use, so this article will focus on the discussion. The application pressure range of the oil diffusion pump is 10-1 Pa-10-7 Pa. It uses the gas diffusion phenomenon to exhaust. It has the advantages of simple structure, convenient operation and high pumping speed (the highest can reach 10+5 liters / seconds) and so on. The oil diffusion pump is mainly composed of a pump casing, a nozzle, a guide tube and a heater. The diffusion pump oil is mainly added in it (the Japanese model is D-704#). According to the number of nozzles, it can be divided into single-stage pump and multi-stage pump. Diffusion pump oil is stored in the bottom of the diffusion pump, the upper part is the air inlet, and the lower part on the right side is the air outlet. When working, the air outlet is provided with the pre-pressure by the mechanical pump, and the mechanical pump acts as the pre-pump.

When the oil of the diffusion pump is heated by the electric furnace, the oil vapor produced provides the pre-pressure, and the mechanical pump acts as the pre-pump. When the diffusion pump oil is heated by the electric furnace, the generated oil vapor is sprayed down through the umbrella nozzle along the guide tube. Because there is a vacuum of 1-10-1 Pa provided by a mechanical pump outside the nozzle, the oil vapor can be ejected for a certain distance to form a jet that moves toward the air outlet. The jet finally hits the pump wall cooled by the cooling water, condenses into a liquid, and flows back to the evaporator, that is, by the evaporation of oil - spray - condensation, and repeated cycles to achieve air extraction.
 The gas molecules entering the pump from the air inlet, once they fall into the steam flow, gain momentum of downward movement and fly downward. Since the jet has a high flow velocity (about 200 m/s) and high steam density, And the diffusion pump oil has high molecular weight (300-500), so it can effectively take away gas molecules, so in the interface of the jet, the gas molecules cannot stay for a long time, and on both sides of the jet interface, the pumped gas has a large amount of gas. Concentration difference, it is precisely because of this concentration difference that the pumped gas can continuously cross the interface, diffuse into the jet, and be taken to the outlet, where it is pumped away by a mechanical pump. The oil vapor pressure of the diffusion pump is an important factor in determining the ultimate vacuum of the pump, so try to choose pump oil with low saturated vapor pressure, and its chemical properties are better.
 Diffusion pump cannot be used for pumping alone, generally the maximum outlet pressure of the pump is required to be 40 Pa. The pumping speed of the diffusion pump is determined by the size of the annular area between the first-stage nozzle and the inlet of the pump body. The pumping speed is not a constant value, but changes with the pressure of the air inlet. When the pressure is 10- When the pressure is 2 to 10-3 Pa, the pumping speed of the diffusion pump is the fastest. When the pressure is less than 5*10-4 Pa, the pumping speed of the diffusion pump is the smallest, and there is almost no pumping ability (at this time, the air intake The pressure of the port is higher, because the density of the air is higher, so that the steam flow cannot form a high-speed jet to block the reverse diffusion of the air, so the pumping speed decreases). The diffusion pump should be cleaned before installation, and then the diffusion oil can be loaded. Before the oil is heated, the pump must be evacuated first. Before the shutdown, the diffusion pump oil must be cooled to 60-70 degrees Celsius before the front pump can be turned off. Air, and finally turn off the cooling water.
 Since the oil diffusion pump of the pvd coating machine cannot prevent the possibility of oil returning, there is no way to ensure the 100% purity of precision products, especially in the semiconductor industry, so there is a "high vacuum condensing pump + low vacuum mechanical pump". The oil-free vacuum system and the exhaust system composed of the condensate pump not only have high exhaust efficiency, but also effectively ensure the cleanliness of the vacuum chamber and the quality of the product (to avoid product contamination and enhance the adhesion between the film and the substrate), but The maintenance cost is very high and the cost is expensive, so the penetration rate is not as wide as that of oil diffusion pumps.