There are several different types of dry vacuum pumps designed and produced, which have obvious differences in mechanical structure design. Therefore, the dry vacuum pumps can be classified into the following types: round split type, claw type, combined type (roots + claw), screw type, among which the first three are called multi-stage pumps. Because they work in the same way, they use multi-stage vacuum cavity to compress gas repeatedly to produce vacuum. In the process of repeated compression, the temperature and pressure of the gas also produce more complex changes, so it is easy to cause changes in the physical properties of the gas. The screw type is called a single-stage pump because it only relies on one vacuum chamber to produce vacuum. According to the different ways of gas compression, it can be divided into internal compression screw pump and external compression screw pump. The following will be these different types of pumps in the physical characteristics of some of the discussion, hoping to help you.
Semiconductor process dry vacuum pump type:
The double round split design is very similar to the most popular and widely used roots pump. In fact, some of the earliest dry pump design ideas were to stack Roots pumps. This multi-stage design makes the gas path quite complex, and each stage needs a large flow of nitrogen to play the role of dilution and isolation. At the same time, in order to achieve a good vacuum, there are very strict requirements for the gap at all levels. Of course, this design increases the internal compression ratio, which makes its power consumption relatively low.
The principle of the three lobed design is the same as that of the double lobed design, except that the gas is divided into three parts in a circle rather than two parts like the double lobed design. The two designs have the same advantages and disadvantages. In order to further reduce the power consumption, some manufacturers in the transmission part choose two DC motors, but this will also lead to reduced torque and reduced restart capacity. As with the double round split design, each stage of the three leaf round split design needs a large flow of nitrogen to play a role of dilution and isolation.
Combined (roots + claw)
In the design of combined (roots + claw), roots is used to improve the extraction efficiency under low pressure and claw is used to improve the extraction efficiency under high pressure. Its basic principle and gas path are exactly the same as the circular lobe design introduced earlier. Some manufacturers change their final stage to a star design, so that they can divide the gas into five parts in one revolution, just like the three circle split design can divide it into three parts. Similarly, in many processes, each stage of the combined design needs a large flow of nitrogen to play a role of dilution and isolation.
External compression screw type
In the external compression screw design, a pair of equidistant screws are used. This reduces internal compression to a minimum and makes the gas path the shortest and easiest. In this way, the gas stays in the pump body for the shortest time. Although the power consumption of this design is relatively high due to the reduction of internal compression ratio, it shows high stability in many complex semiconductor processes. This single-stage design makes the demand for nitrogen consumption very small and simple, so that it has good interchangeability in different processes. In many clean processes, nitrogen may not even be used.
Internal compression screw type
In addition to the use of a pair of non equidistant screws, the basic principles of internal compression screw design and external compression screw design are very similar. The reduction of the volume between the screws makes the internal compression. This design reduces the power consumption to the same level as multistage pump due to internal compression. But in many processes, this internal compression, like multi-stage pump, is very easy to cause the physical and chemical changes of gas in the pump body and produce solidification or liquefaction.
In all types of dry vacuum pumps described above, we are eager to reduce their physical size. However, this is inconsistent with the pumping rate of vacuum pump, because the pumping rate of vacuum pump is directly proportional to the volume of vacuum pump, and small physical size means small volume. Of course, there is also an important factor that affects the pumping rate of vacuum pump, that is, the rotating speed. In order to match the pumping rate of small-scale vacuum pump with that of large-scale vacuum pump, it is necessary to increase its speed. There are two ways to change the speed of vacuum pump, one is to change the frequency of power supply, that is, to use frequency converter; the other is to change the transmission ratio of gearbox. Among them, the advantage of using frequency converter is that it can provide a closed-loop control, but it is easy to cause torque loss under large load. Changing the transmission ratio of the gearbox is a very economic way, of course, it can only provide a single uncontrollable speed.
It can be seen that the dry vacuum pump has so many different designs, each of which has its own characteristics, advantages and disadvantages. All along, the use cost is an important factor that affects people’s choice of vacuum pump. More and more vacuum pump users realize that the stability of vacuum pump operation should also be considered. Because of the unexpected error of vacuum pump, the efficiency of machine will be reduced, the production and delivery time of wafer will be affected, even the scrap of processed wafer and the damage of other parts of the machine will be caused.
Selection of dry vacuum pump in semiconductor technology
As the screw type design is single-stage design, compared with the multi-stage pump such as round split type, claw type and combined type, the number of parts used is reduced by 60%. This makes it have a great advantage in stability and maintenance cost in the future.
Comprehensively, if the user thinks that the power consumption is the most important for him and the nitrogen consumption is the least important for him, then the internal compression screw type can be a good choice. But generally speaking, more and more strict requirements for tail gas treatment make it more important to reduce the nitrogen consumption in the future. If you want to use a single type of pump to simplify your inventory control without changing various semiconductor processes, the external compression screw type is your best choice.
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