This article takes you to understand several vacuum pumps applied in the fields of ultra-high vacuum and extremely high vacuum
When the pressure exceeds 10-5Pa, it enters the field of ultra-high vacuum applications. At this point, the pumping function of most vacuum pumps has stopped or the pumping speed is very low. What are the vacuum pumps used to achieve ultra-high vacuum at this time? Let's take a brief look together.
The main function of a mechanical pump is to provide the necessary pre stage vacuum for the start-up of a turbomolecular pump. The commonly used mechanical pumps mainly include vortex dry pumps, diaphragm pumps, and oil sealed mechanical pumps.
Diaphragm pumps have a lower pumping speed but a smaller volume, and are generally used for small molecular pump groups.
Oil sealed mechanical pump is the most commonly used mechanical pump in the past, characterized by high pumping speed and good ultimate vacuum. The disadvantage is that there is a common situation of oil return. In ultra-high vacuum systems, electromagnetic valves (used to prevent accidental power outages from causing oil return) and molecular sieves (adsorption function) are generally required for use.
In recent years, vortex dry pumps have been widely used, which have the advantage of being relatively simple to use and not returning oil. However, their pumping speed and ultimate vacuum are slightly inferior to oil sealed mechanical pumps.
Mechanical pumps are an important source of laboratory noise and vibration. Choosing low-noise pumps and placing them in equipment rooms as much as possible is a better approach, but due to working distance limitations, the latter is usually not easy to achieve.
Turbomolecular pumps rely on high-speed rotating blades (usually around 1000 revolutions per minute) to achieve directional gas flow, and the ratio of the pump's exhaust pressure to the intake pressure is called the compression ratio. The compression ratio is related to the number of stages, speed, and type of gas in the pump. Generally, gases with larger molecular weights have higher compression ratios. The compression ratio for nitrogen is 108-109; Hydrogen ranges from 102 to 104. The ultimate vacuum of a turbomolecular pump is generally considered to be between 10-9-10-10mbar. In recent years, with the continuous advancement of molecular pump technology, the ultimate vacuum has been further improved.
Due to the fact that the advantages of turbomolecular pumps can only be reflected in the molecular flow state (the flow state where the average free path of gas molecules is much larger than the maximum size of the duct section), it is required to be equipped with a front-end vacuum pump with a working pressure of 1 to 10-2Pa. Due to the high-speed rotation of the blades, damage or destruction of the molecular pump may occur if encountering foreign objects, shaking, impact, resonance, or gas shock. For beginners, the most common cause of injury is gas shock caused by operational errors. Resonance caused by mechanical pumps can also lead to damage to molecular pumps. Although this situation is relatively rare, it is difficult to detect due to its concealment and requires special attention.
The advantages of sputtering ion pump are good ultimate vacuum, no vibration, no noise, no pollution, mature and stable process, no maintenance required. At the same pumping speed (except for inert gas), its cost is much lower than molecular pump, so it is widely used in ultra-high vacuum systems. The normal operating cycle of a sputtering ion pump is usually over 10 years.
Ion pumps generally require a pressure of 10-7mbar or higher to function properly (working under poorer vacuum conditions can significantly reduce their lifespan), therefore a molecular pump assembly is needed to provide them with a better pre stage vacuum. The common practice is to use an ion pump+TSP in the main chamber, and equip a small molecular pump group in the injection chamber. During baking, open the connected plug valve, and the small molecular pump group will provide the front stage vacuum. After baking is completed, close the plug valve, and use the ion pump in conjunction with TSP to achieve ultra-high vacuum
It should be noted that ion pumps have poor adsorption capacity for inert gases, and there is also a certain difference in maximum pumping speed compared to molecular pumps. Therefore, for situations with large gas release or a large amount of inert gases, molecular pump sets are needed. In addition, the ion pump generates electromagnetic fields during operation, which may cause interference to particularly sensitive systems.
The working principle of a titanium sublimation pump relies on the evaporation of metallic titanium to form a titanium film on the chamber wall for chemical adsorption. The advantages of titanium sublimation pump are simple structure, low cost, easy maintenance, no radiation, and no vibration noise.
The titanium sublimation pump is usually composed of three titanium filaments (to prevent burning out), which can be used in combination with a molecular pump or ion pump to achieve good hydrogen removal effect. It is the most important vacuum pump in the range of 10-9-11-11mbar and is equipped in most ultra-high vacuum chambers that require high vacuum degree.
The disadvantage of titanium sublimation pump is that it requires regular sputtering of titanium, and during the sputtering period (within a few minutes), the vacuum will decrease by about 1-2 orders of magnitude. Therefore, some chambers with specific requirements require the use of NEG. In addition, for samples/devices sensitive to titanium, attention should be paid to avoiding the position of the titanium sublimation pump.
Low temperature pump
Low temperature pumps mainly rely on low-temperature physical adsorption to obtain vacuum, with the advantages of high pumping speed, no pollution, and high ultimate vacuum. The main factors affecting the pumping speed of a cryogenic pump are temperature and the surface area of the pump. In large-scale molecular beam epitaxy systems, due to the high requirement for extreme vacuum, cryogenic pumps are widely used.
The disadvantage of cryogenic pumps is that they consume a large amount of liquid nitrogen and have high operating costs. A system with a circulating refrigeration unit may not consume liquid nitrogen, but it may bring corresponding issues such as energy consumption, vibration, and noise. Therefore, cryogenic pumps are less commonly used in conventional laboratory equipment
Absorbent pump is a type of vacuum pump that has been widely used in recent years. Its advantage is that it completely adopts chemical adsorption, does not produce evaporation and electromagnetic pollution, and is often used in conjunction with molecular pumps to replace titanium sublimation pumps and sputtering ion pumps. The disadvantage is that it has a higher cost and limited regeneration times. It is commonly used in systems that require high vacuum stability or are highly sensitive to electromagnetic fields.
In addition, due to the fact that the suction pump does not require additional power supply besides initial activation, it is often used as an auxiliary pump to increase pumping speed and vacuum degree in large systems, which can effectively simplify the system.
