what is a turbomolecular pump?

  The turbomolecular pump was first invented in 1957, and companies in various countries have put their products on the market and have been developed and popularized. The application range of the turbomolecular pump is basically the same as that of the diffusion pump. The two types of pumps can be continuously exhausted into the atmosphere by the backing pump. The inlet pressure is in the range of 1 Pa~10-7 Pa, and the pumping has no choice for the molecular type. performance, from 70 l/s to 10,000 l/s pumps. There are also smaller or larger turbomolecular pumps for special purposes. Diffusion pumps have products of 60,000 l/s, and currently, there are no general-purpose turbomolecular pumps greater than 10,000 l/s.

Turbomolecular vacuum pump working principle

The gas delivered by the molecular pump should meet two necessary conditions:

a. Turbomolecular vacuum pumps must work under molecular flow conditions. Because when the pressure of the gas contained in a container of a certain volume is reduced, the mean free path of the gas molecules increases accordingly. At normal pressure, the mean free path of air molecules is only 0.06 μm, that is, on average, a gas molecule may collide with a second gas molecule as long as it moves 0.06 μm in space. At 1.3Pa, the mean free path between molecules can reach 4.4mm. If the mean free path is increased to be larger than the distance between the walls of the container, the collision chance of gas molecules with the container wall will be greater than the collision chance between gas molecules. In the molecular flow range, the mean free path length of gas molecules is much larger than the spacing between the molecular pump blades. When the wall is composed of stationary stator blades and moving rotor blades, more gas molecules will shoot toward the rotor and stator blades, laying the foundation for the directional movement of gas molecules.

b. The rotor blades of the molecular pump must have a linear velocity close to that of the gas molecules. With such a high speed, the gas molecules can collide with the moving blades and change the characteristics of random scattering to make directional movements.

The higher the rotational speed of the molecular pump, the more favorable it is to increase the pumping speed of the molecular pump. The practice has shown that the higher the velocity of gas molecules of different molecular weights, the more difficult it is to pump out. Example: H2 has a very high content in the air, but because the H2 molecule has a large moving speed (the most probable speed is 1557m/s), it is difficult for the molecular pump to pump H2. Through the analysis of the residual gas in the ultimate vacuum, it can be found that the proportion of hydrogen can reach 85%, while the molecular weight is relatively large, and the proportion of oil molecules with slow-moving speed is almost zero. This is the reason why the molecular pump has a high compression ratio for high molecular weight gases such as oil vapor, and the suction effect is good.

  The actual turbomolecular pump is composed of multi-stage blades in series, that is, they are arranged alternately in the order of moving blade, fixed blade, and moving plate,... The total compression ratio of the pump is determined by the number of stages of the blade row. In the design of the turbomolecular pump, the combination of the multi-stage impellers should be optimized and matched. Generally, the shape and size of the blade with a larger pumping speed should be selected near the inlet side of the pump, and its compression ratio can be relatively small. After several stages of compression, the gas pressure increases and the pumping speed decreases. At this time, the blade shape with a high compression ratio and low pumping speed should be selected. This design can make the pumping performance of the whole pump obtain the ideal result of high pumping speed, high compression ratio, and few stages.

Advantages and disadvantages of turbo molecular pump

  a. Advantages

  Because turbomolecular pumps perform better than cryopumps, ion pumps, and diffusion pumps in some respects. Therefore, in general, turbomolecular pumps are often used. Its advantages are:

•  Clean, no oil vapor backflow
  The turbomolecular pump can provide an extremely clean vacuum environment for the pumped container without any traps and work according to the operating procedures and does not contain any hydrocarbons. Since modern turbomolecular pumps are rarely lubricated with oil except for large pumps, grease-lubricated bearings are often used for small pumps, and air bearings are also used, but magnetic suspension bearings are used more.
  
• Easy to use
  In many applications, turbomolecular pumps can be used without high or rough vacuum valves. With a simple push of a button, the pump starts working. From atmospheric pressure down to ultimate pressure. This system can be rough-pumped by a turbomolecular pump and can be accelerated all the way to operating speed. This eliminates the need for vacuum components such as valves, pipes, traps, valve controllers, etc. At the same time, the failures caused by these components are also eliminated. Therefore, the space occupied by the turbomolecular pump system is small, and the installation direction of the turbomolecular pump is not limited and can be installed in any direction (except for the oil-lubricated pump, which can only work within the vertical range of ±5°). This feature can be used where the installation location is restricted.
  
• Strong gas delivery capability
  Most turbomolecular pumps are very capable of transporting light gases such as hydrogen and helium. Therefore, it is very suitable for process operation under ultra-high vacuum. For those hydrogen-rich processes, helium mass spectrometer leak detectors and other occasions can be used. There are turbomolecular pumps specially designed to pump out corrosive gases, suitable for etching, reactive ion etching, ion beam processing, low-pressure chemical vapor deposition, epitaxy, ion implantation, and other process operations. During these processes, the extracted gas will corrode the cryopump, ion pump, diffusion pump oil, etc. Even standard unprotected turbomolecular pumps will be destroyed. Since the turbomolecular pump is a transmission-type pump, the pumped gas can pass through the bore and not accumulate in the pump. Therefore it is suitable for processes with high gas loads. Such as sputtering, etching, etc.
  
• Suitable for ultra-high vacuum applications
  A turbomolecular pump with good sealing and degassing, matched with a two-stage rotary vane pump with good performance (or a dry backing pump with the same performance), the ultimate vacuum can generally reach 10^{- 9} ~ 10^{- 10} Torr. If a turbomolecular pump is connected with another turbomolecular pump, and the pump is sealed with metal and well degassed, the ultimate pressure is generally between 1×10-10^～1×10^-11 Torr. Unlike cryopumps or ion pumps, turbomolecular pumps operate at full pumping speed under ultra-high vacuum conditions. These properties, coupled with their good cleanliness (no hydrocarbons can be detected), make it obvious that users will choose turbomolecular pumps for high-resolution mass spectrometers, molecular beam epitaxy equipment, and ultra-high vacuum analysis equipment.
  
• Good performance under high pressure
  The inlet pressure of some turbomolecular pumps can be operated between 10^-1 to 10^-3 Torr. In this pressure range, the ion pump cannot be used, and the operation of the diffusion pump will also become unstable for the cryopump, which requires throttling of the pumping speed or frequent regeneration.
  
• Short cycle time
  For most turbomolecular pumps, especially the smaller ones, it usually takes 1 to 3 minutes to reach the normal operating speed. And can be closed immediately and can be exposed to the atmosphere. This fast-cycling feature is useful in sample input systems, especially portable helium leak detectors.
  
• Long operation 
  In some applications, the uptime of turbomolecular pumps is superior to other pumps. Because in the case of heavy gas load and valve leakage, the cryopump will be regenerated frequently or the ion pump will be repaired frequently, and the use of the turbomolecular pump can also eliminate the contamination of the vacuum chamber due to the pump oil.
  
  

  b. Disadvantages

• High cost of equipment
  The equipment investment of a turbomolecular pump with a pumping speed greater than 1000 L/s is larger than that of a diffusion pump and the cryopump. However, turbomolecular pumps are useful in special applications where diffusion pumps and cryopumps cannot be used due to process gases or other reasons. Small turbomolecular pumps are quite expensive when compared to small or medium-sized diffusion pumps. However, considering that the diffusion pump system requires valves, baffles, traps, valve controllers and pipes, etc., the total cost is calculated, and the difference between the two is not much. In some cases, turbomolecular pumps are still cheaper options.
  
• Sensitive to particulate matter or sediment
  If objects (screws, glass fragments, filaments, or silicon chips) fall into a running turbomolecular pump, the turbine will be damaged, often requiring a return to the factory for repair. In the event of an accident, the damage is serious. Repairs and replacement parts are expensive. For the sake of work safety, a filter with fine holes is installed at the pump inlet to protect the normal operation of the pump. This measure has a large loss in the effective pumping speed of the pump. Thick deposits on the blades can cause wear and blockage of the blades, and also affect the unbalance of the rotor. If some particles enter the bearing, causing wear, it can reduce the working life of the pump. Therefore, in some applications, it is necessary to install protective measures.
  
• Noise and vibration 
  From the experience of use, the pump will have vibration and noise problems, most of which are caused by bearing damage and poor balance. In normal work, the pump is in a quiet state, and the maximum amplitude is between 0.1 and 0.001 μm (ie, 100 to 1 nm). It is used in some precision equipment.
  
• Broken problem
  Some users are afraid to use the turbomolecular pump because they are afraid of the rotor blades being broken. The crushing occurs when the impeller is suddenly sucked in foreign objects or the bearings are worn out when the pump is running normally. There are usually protective measures, such as adding filters at the entrance, and fragmentation can usually be avoided.
  
• Exposure to the atmosphere can cause accidents
  Any high vacuum pump will encounter this kind of accident during operation. If the gauge is broken, problems with the pipes, valves, and seals at the inlet may suddenly expose the vacuum pump inlet to atmospheric pressure. Different types of turbomolecular pumps have different resistance to atmospheric pressure shock. Some pumps will be damaged due to the resonant bending of the blades, but some pumps will not be damaged by the impact of the atmosphere. The best way is for manufacturers to get the results through experiments. Diffusion pumps and cryopumps are also more troublesome to encounter such sudden accidents in their work and are less resistant than turbomolecular pumps. For example, the oxidation of diffusion pump oil will quickly contaminate the vacuum chamber, and cryopumps require regeneration.