Cutting-edge technology in vacuum pumps – screw pumps

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Screw pumps actually have a history of over a hundred years, so they can’t be considered cutting-edge technology. However, using them as vacuum pumps, especially in semiconductor manufacturing lines, presents many unique challenges. Abroad, only a few vacuum pump manufacturers, such as Busch, Ebara, Kashiyama, etc., are capable of producing decent screw vacuum pumps. In China, although some companies are engaged in research and development or small-scale production, it seems that none have yet achieved successful large-scale production. There are actually 3 types of screw pumps: single-screw, twin-screw, and triple-screw. Single-screw pumps do not generate high levels of vacuum. It is rarely used in vacuum conditions. It is generally used only in plastic extrusion processing. The three-screw type is less suitable for the vacuum industry due to its complexity, as it includes an additional driven intermediate screw. In this way, dry screw vacuum pumps specifically refer to twin-screw pumps. A screw pump features two screws that are driven synchronously by a pair of timing gears; the gap between the teeth is very small, yet they do not come into contact, which allows for the creation of a high vacuum level while also reducing the risk of wear. It can be said that, compared to other dry pumps such as vortex pumps and claw pumps, screw pumps achieve the highest vacuum level. One of the greatest advantages of twin-screw pumps is that, due to the uniform gap between the screws, they can pump gases containing water vapor, hard particles, and other substances. This is particularly important for semiconductor processing. Probably most people consider the contour shape of the screw pump to be the most important. However, in addition to the linear profile, other factors such as bearings, seals, process compatibility, vibration and noise, and automatic control are also very important. In terms of geometry, the screw pump’s unique blow hole design is the most important feature. Pores can cause internal leakage, affecting the vacuum level. Regarding profile machining, there are thread grinders and machine tools. A technical executive from a internationally renowned screw compressor manufacturer told me that only by using screw grinders for processing can technical quality be ensured for screw compressors. When I think a CNC screw machine will suffice. For example, the most famous Holroyd screw mill is more than sufficient. Generally, helices have a uniform pitch, but variable-pitch helices are also used; Kashiyama has carried out research in this area, though the cost may be high. One of the main challenges with screw pumps is control, which involves two aspects: one is the control related to the host machine it is connected to ； The other is gap control. Its compression mainly occurs in the first one or two turns on the side facing the atmosphere, resulting in significant local heating. Inadequate cooling results in poor vacuum levels and significant wear on bearings and other components. The cooling transition can sometimes cause shaft seizure. So controlling the gap is important. The sealing of screw pumps – and of course, the most important aspect is the sealing between the screws – relies primarily on manufacturing precision and the design of their profiles. The other one is bearing sealing. Since the timing gear must be lubricated, it is somewhat difficult, as with a dry pump, to keep the lubricant outside the vacuum chamber. Bearing selection design is also an important issue. Precision bearings should generally be used. The operating precision must be high; otherwise, vibration will occur. Vibration and noise are major drawbacks of screw pumps. Vortex pumps and multi-pole Roots pumps generate less vibration and noise than screw pumps. The semiconductor industry has high requirements for noise levels, which should generally be below 65 decibels. It is very difficult for screw pumps to achieve this. Noise is divided into mechanical noise and aerodynamic noise. Pneumatic noise is mainly addressed with silencers. Mechanical noise, on the other hand, requires careful analysis. First, identify the noise frequency, and then analyze its source. Process compatibility is important in the semiconductor industry. Sometimes, the process requirements dictate that the temperature along the entire vacuum path be low, which necessitates effective cooling. Other processes require higher temperatures, and in such cases, heaters are even added to the pumps. For the semiconductor industry, control is particularly important. Vacuum pumps are generally equipped with a control system based on an industrial computer. This system often also requires an internet connection. It seems very difficult from the above perspective. But if you make up your mind to understand it, the rewards are great. For example, Butsch in Germany started out by producing screw pumps, and today it has a much larger output and a greater number of employees than Leybold.