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Introduction • Vacuum is a volume of space essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. • Technically, vacuum refers to “The given space filled with a gas having density of molecules less then 2.5 *10^19 molecules / cubic cm at normal tempreature” • Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes. Vacuum pumps History • Before the vacuum pump, the suction pump was invented. In the city of Pompeii, dual-action suction pumps were found. • In the 13th century, an Arabic engineer also explained suction pumps. The suction pump later came up again in Europe from the 15th century. • By the 17th century, water pump designs had improved and they produced measurable vacuums, but this was not understood immediately. • It was understood by the people that suction pumps could not pull water beyond a certain height. • Vacuum pump was invented in 1650 by Otto von Guericke Defination • A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum.
Types of Vacuum Pump Pumps can be broadly categorized according to three techniques: • Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere . • Momentum transfer pumps or molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gas molecules out of the chamber . • Entrapment pumps capture gases in a solid or adsorbed state. This includes cryopumps, getters, and ion pumps . Methods of vacuum pumps There are two method to produce vacuum • Mechanical • Non Mechanical Mechanical The method in which a machine is used to reduce the momentum, number of particles or pressure is called mechanical method. It consists of three types of pumps: • Oil Rotary Pump • Diffusion Pump • Tarbomolecular Pump
Continue… Non Mechanical The method in which momentum, number of molecules or pressure through a certain region is reduced indirectly i.e to add momentum is called non mecanical method. There are three types as follow: • Sorption Pump • Ion Pump and Getter-ion Pump • Cryo Pump Mechanical Pumps • Oil Rotary Pump • This is the oldest type of vacuum pump. The basic idea is to mechanically compress the gas (as in a cylinder with a piston) and eject it into the atmosphere. The pump actually uses rotating pistons and oil is used to provide a vacuum seal. • The lowest achievable pressure is about 10^-3 torr. • Oil rotary vacuum pumps are positive displacement vacuum pumps that use oil to reduce the confidential and dead space between components such as the rotor, stator, and sliding vanes. • The main limitation of this pumping technology is that oil vapors tend to slowly contaminate the surfaces of the vacuum system. Applications Food, chucking, vacuum furnaces, leak systems, impregnation equipment, lasers, coatings, chemistry, medical treatment, central vacuum systems.
Diffusion pumps • These pumps use a jet of oil to push the gas and compress it. Hot and heavy oil molecules hit gas atoms and push them out. • The effect is similar to the downward air drift created in the shower that results in the shower curtain being sucked in. • Diffusion pumps compress the gas to about 10^-3 torr and have to be backed up with a roughing pump. • Diffusion pumps are fast and cheap, but they cause oil contamination especially if accidentally exposed to atmospheric pressure. The minimum pressure is about 10^-8 torr.
Applications • Diffusion pumps are probably the most commonly used mechanisms for creating a high vacuum in industrial vacuum processing. • It is also commonly used in mass spectrometry, analytical instrumentation, research and development, and nanotechnology.
Turbo-pumps • These pumps are basically super fans that push the gas with fast-moving blades. • To achieve efficient operation the fan blades have to move close to the thermal speed of atoms, at several hundred meters/sec. • Turbo-pumps have several hundred blades and rotate at about 70000 revolutions per minute. • Modern turbo-pumps can withstand accidental exposure to atmospheric pressure. • The minimum pressure is about 10^-9 torr. Applications Turbomolecular pumps are used in a wide range of high and ultra-high vacuum applications, covering both clean applications (e.g. in analytical instruments or R&D), and very harsh applications in Semiconductor industry where the pumps have to handle corrosive gases or critical process conditions.
Non Mechanical • Sorption Pump • These pumps do not compress and eject the gas, but absorb it inside the pump. • They typically use charcoal or similar porous material with a very high surface area. • When the charcoal is cooled with LN2 most gases stick to its surface. • The gas drawn through the inlet part inflow into a chamber with a porous adsorbent, Molecular Sieves. The molecular sieves cooled by the liquid nitrogen outside the chamber adorbs the gas in the chamber. Finally, the inside of the pump is evacuated. • This pump is fast, clean, and can achieve low pressures, but has a limited pumping capacity before it has to be regenerated by warming up the charcoal. Applications More recently, partially automated sorption pumps have been used for repetitive rough pumping of production coating systems. There have also been experimental uses of sorption pumps as fine pumps in the 10^-6 Torr range, as an air sampler, and for argon pumping in a sputtering system.
Ion Pump • Ion pumps create a plasma discharge within their volume. • Gas atoms entering the plasma are ionized and accelerated by a high voltage toward a Ti surface. • Upon collision they eject Ti atoms which then coat all surfaces. • Gas is removed by chemical reaction with Ti and by being buried under a fresh layer of Ti. • Ion pumps are robust and have no moving parts. • The lowest pressure is about 10-11 torr. • Ion pumps have a low gas holding capacity and cannot be easily regenerated. • They are only used at low pressures or for infrequent pump-downs. Applications Ion pumps are commonly used in ultra-high vacuum (UHV) systems, as they can attain ultimate pressures less than 10−11 mbar. In contrast to other common UHV pumps, such as turbomolecular pumps and diffusion pumps, ion pumps have no moving parts and use no oil.
Getter-ion Pump • Ion getter pumps (IGP) are devices able to create and maintain ultra-high-vacuum, reaching pressures as low as 10^-12 mbar. • Lower pressures are in principle achievable, but their measurement is particularly challenging and strongly depends on the outgassing of the system on which the ion pump is mounted. • With respect to other vacuum pumps, such as turbomolecular pumps or primary pumps, IGP have some characteristics that make them unique. • The ion pump is a static device. It has no moving parts, so it is vibration free and, consequently, it doesn’t need any lubricant which could be a source of contamination. • Applications • Ion getter pumps are frequently used in general UHV systems, surface analysis, and high-energy physics applications. • As well as producing UHV pressures, ion getter pumps are: Hydrocarbon-free. Operable at high temperatures.
Cryo Pump • This pump features a clean vacuum and high pumping speed. • A cryopump is an accumulating type vacuum pump. The pump condenses and adsorbs gases on a cryogenic surface installed in the pump to create a condition from high vacuum to ultra-high vacuum. Also, this pump can obtain a clean vacuum that is not contaminated by oil and has a higher pumping speed than other vacuum pumps, which attracts attention. • A cryopump is vacuum pump that traps gases and vapors by condensing them on a cold surface. For efficient evacuation under ultra-high vacuum, the vapor pressure for condensation, or the equilibrium pressure for adsorption must be less than 10-8Pa. Mechanism of Cryopump Higher vacuum with two-stage Cold stage is a two-stage type. The cooling capacity of the first stage is large, and it can cool to 80 K (Kelvin) or less. First-stage exhausts mainly the moisture and the second stage exhausts molecules such as N2, O2, Ar, and H2 by cooling even further to obtain a higher vacuum. The reason for the ultra-low temperature. • Gas molecules condense and adsorb on contact with a surface that is cooled close to 0K = absolute zero (-273.15 ° C). Applications Electronic components such as semiconductors, liquid crystals, and disks, eyeglass lenses, and extra-large space vacuum chamber.
