In the past, oil vacuum pumps were widely used in various semiconductor processes, among which suspension and piston vacuum pumps were the main representatives. And their performance in various applications is increasingly unsatisfactory. Therefore, dry vacuum pump came into being. Over the past decade, several different types of dry vacuum pumps have been designed and manufactured, with significant structural differences in their mechanical design. Because of this difference, we can classify the dry vacuum pump into the following types: circular lobe type, claw type, combined type (Roots + claw), screw type. Today, these types are widely used by different manufacturers. Among them, the circular lobe type, claw type and combined type (Roots + claw) are called multistage pump. Because they work in roughly the same way, they use multiple vacuum chambers to repeatedly compress the gas to create a vacuum. In the process of repeated compression, the temperature and pressure of the gas also have more complex changes, so it is easier to cause the change of the physical characteristics of the gas. The screw type is called a single-stage pump because it relies on only a vacuum chamber to create a vacuum. According to the different ways of gas compression, it can be divided into internal compression type screw pump and external compression type screw pump. The following will be different types of pumps in the physical characteristics of some discussion, I hope to be helpful to you.

 

Classification of dry vacuum pumps

 

Double lobe type:

The twin-lobe design is very similar to the most popular and widely used Roots pump. In fact, some of the earliest dry pump design ideas were the roots pump superposition. This multistage design makes the gas path quite complex, and each stage requires a large flow of nitrogen for dilution and isolation. At the same time, in order to achieve a good vacuum, there are very strict requirements for the clearance of all levels. Of course, this design increases the internal compression ratio so that the power consumption is relatively low.

 

Three-leaf circular lobe type:

The three-lobe design works in exactly the same way as the double-lobe design, where the gas is divided into three parts per rotation rather than two parts as in the double-lobe design. The three - lobe and double - lobe design have the same advantages and disadvantages. In order to further reduce power consumption, some manufacturers choose to use two DC motors in the transmission part, but this will also lead to a decrease in torque and the ability to restart. As with the double-lobe design, each stage of the three-lobe design requires a large flow of nitrogen for dilution and isolation.

 

Combined type (Roots + claw) :

The combined design (Roots + claw) uses roots to improve the pumping efficiency under lower pressure, and uses claws to improve the pumping efficiency under higher pressure. The basic principle and gas path are exactly the same as the circular lobe design described above. Some manufacturers have also changed the final stage to a star design, so that the gas can be divided into five parts in one rotation, just as the three-lobe design can be divided into three parts. Similarly, in many processes, each stage of the modular design requires a larger flow of nitrogen for dilution and isolation.

 

External compression screw type:

In the design of external compression screw, a pair of equidistant screws is used. This minimizes the internal compression and makes the gas path the shortest and simplest. In this way, the gas in the pump body stay the shortest time. Although this design results in relatively high power consumption by reducing the internal compression ratio, it is highly stable in many complex semiconductor processes. This single-stage design also makes the nitrogen requirement very small and simple, which makes it very interchangeable in different processes. In many cleaning processes, nitrogen is not even used.

 

Internal compression type screw type:

The basic principles of the internal compression screw design and the external compression screw design are very similar, except for the use of a pair of non-equidistant screws. The decreasing volume between the screws causes the internal compression. This design reduces power consumption to the level of a multistage pump due to internal compression. But in many processes, this internal compression and multistage pump, very easy to cause the physical and chemical changes in the pump body gas solidification or liquefaction.

Now, you can see that there are so many different designs for dry vacuum pumps, each with its own characteristics and advantages and disadvantages. Service cost has always been an important factor influencing people's choice of vacuum pump. More and more vacuum pump users realize that the operation stability of vacuum pump should also be taken into consideration. Due to the unexpected error of the vacuum pump, the efficiency of the machine will be reduced, the production and delivery time of the wafer will be affected, and even the wafer will be scrapped and other parts of the machine will be damaged.

Because of its single-stage design, the screw reduces the number of parts used by 60 percent compared to multistage pumps such as the splitter, claw, and combination. This gives it a big advantage in terms of stability and future maintenance costs.

All in all, if the user thinks that power consumption is the most important to him and nitrogen consumption is the least important to him, then an internal compression screw is a good choice. But in general, stricter requirements for tail gas treatment make it even more important to reduce nitrogen use in the future. If you want to simplify your inventory control by using a single type of pump to handle a variety of semiconductor processes without changing, an external compression screw is your best bet.

 

The development trend

In all types of dry vacuum pumps described above, we are eager to be able to reduce their physical size. But this is in contradiction with the suction rate of vacuum pump, because the suction rate of vacuum pump is proportional to the volume of vacuum pump, small physical size means small volume. Of course, there is also an important factor affecting the suction rate of vacuum pump, that is the speed. In order to match the suction rate of a small vacuum pump with that of a large vacuum pump, it is necessary to increase its speed. There are two ways to change the speed of the vacuum pump. One is to change the frequency of the power supply, that is, the use of inverter, and the other is to change the gearbox transmission ratio. The advantage of using frequency converter is that it can provide a closed loop control, but it is easy to cause the loss of torque under heavy load. Changing the gearbox ratio is a very economical method, of course, it can only provide an uncontrollable single speed.

 

Conclusion

Depending on your particular situation, combining the best of all of these design types is certainly your best bet, but it's not likely yet. Therefore, this requires users to first know that there are so many different types of design in the dry pump design, and then according to their own process characteristics to understand the impact of different types of pump design. In this way, he can make the choice to reduce the use cost as much as possible, and obtain a long life of the stable vacuum system. Today, we have entered the era of the dry vacuum pump, and a well-trained consumer plays an important role in the selection of vacuum pumps as compared to the previous era of suspension and piston oil vacuum pumps. Remember that there are so many types of dry pumps available to you, and choosing the type that best matches your semiconductor process is key.