Turbomolecular Pumps
Applications: Turbomolecular pumps were introduced in 1958 (Becker, 1959) and were immediately hailed as
the solution to all of the problems of the diffusion pump.Provided that recommended procedures are used, these pumps live up to the original high expectations. These are reliable, general-purpose pumps requiring
simple operating procedures and capable of maintaining clean vacuumdown to the 10-10 torr range.Pumping speeds up to 10,000 L/s are available.
Operating Principles: The pump is a multistage axial compressor, operating at rotational speeds from around
20,000 to 90,000 rpm. The drive motor is mounted inside the pump housing, avoiding the shaft seal needed with an external drive. Modern power supplies sense excessive loading of the motor, as when operating at too high an inlet pressure, and reduce the motor speed to avoid overheating and possible failure. Occasional failure of the frequency control in the supply has resulted in excessive speeds and catastrophic failure of the rotor.
At high speeds, the dominant problem is maintenance of the rotational bearings. Careful balancing of the rotor
is essential; in some models bearings can be replaced in the field, if rigorous cleanliness is assured, preferably in a clean environment such as a laminar-flow hood. In other designs, the pump must be returned to the manufacturer for bearing replacement and rotor rebalancing. This service factor should be considered in selecting a turbomolecular pump, since few facilities can keep a replacement pump on hand.
Several different types of bearings are common in turbomolecular pumps:
1. Oil Lubrication: All first-generation pumps used oil-lubricated bearings that often lasted several
years in continuous operation. These pumps were mounted horizontally with the gas inlet between
two sets of blades. The bearings were at the ends of the rotor shaft, on the forevacuum side. This
type of pump, and the magnetically levitated designs discussed below, offer minimum vibration.
Second-generation pumps are vertically mounted and single-ended. This is more compact, facilitating
easy replacement of a diffusion pump. Many of these pumps rely on gravity return of lubrication oil to the
reservoir and thus require vertical orientation.Using a wick as the oil reservoir both localizes the
liquid and allows more flexible pump orientation.
2. Grease Lubrication: A low-vapor-pressure grease lubricant was introduced to reduce transport of oil
into the vacuum chamber (Osterstrom, 1979) and to permit orientation of the pump in any direction.
Grease has lower frictional loss and allows a lowerpower drive motor, with consequent drop in operating
temperature.
3. Ceramic Ball Bearings: Most bearings now use a ceramic-balls/steel-race combination; the lighter
balls reduce centrifugal forces and the ceramic-tosteel interface minimizes galling. There appears to
be a significant improvement in bearing life for both oil and grease lubrication systems.
4. Magnetic Bearings: Magnetic suspension systems have two advantages: a non-contact bearing with a
potentially unlimited life, and very low vibration. First-generation pumps used electromagnetic suspension
with a battery backup. When nickelcadmium batteries were used, this backup was not continuously available; incomplete discharge before recharging cycles often reduces discharge capacity.A second generation using permanent magnets was more reliable and of lower cost. Some pumps now offer an improved electromagnetic suspension with better active balancing of the rotor on all axes. In some designs, the motor is used as a generator when power is interrupted, to assure safe shutdown of the magnetic suspension system. Magnetic bearing pumps use a second set of ‘‘touch-down’’ bearings for support when the pump is stationary. The bearings use a solid, low-vapor-pressure lubricant (O’Hanlon, 1989) and further protect the pump in an emergency. The life of the touch-down bearings is limited, and their replacement may be a nuisance; it is, however, preferable to replacing a shattered pump rotor and stator assembly.
5. Combination Bearings Systems: Some designs use combinations of different types of bearings. One
example uses a permanent-magnet bearing at the high-vacuum end and an oil-lubricated bearing at
the forevacuum end. A magnetic bearing does not contaminate the system and is not vulnerable to
damage by aggressive gases as is a lubricated bearing. Therefore it can be located at the very end of the
rotor shaft, while the oil-fed bearing is at the opposite forevacuum end. This geometry has the advantage
of minimizing vibration.
Problems with Pumping Reactive Gases: Very reactive gases, common in the semiconductor industry, can result in rapid bearing failure. A purge with nonreactive gas, in the viscous flow regime, can prevent the pumped gases from contacting the bearings. To permit access to the bearing for a purge, pump designs move the upper bearing below the turbine blades, which often cantilevers the center of mass of the rotor beyond the bearings. This may have been a contributing factor to premature failure seen in some pump designs.
The turbomolecular pump shares many of the performance characteristics of the diffusion pump. In the standard construction, it cannot exhaust to atmospheric pressure, and must be backed at all times by a forepump.
The critical backing pressure is generally in the 10-1 torr, or lower, region, and an oil-sealed mechanical pump is the most common choice. Failure to recognize the problem of oil contamination from this pump was a major factor in the problems with early applications of the turbomolecular pump. But, as with the diffusion pump, an operating turbomolecular pump prevents significant backstreaming from the forepump and its own bearings. A typical turbomolecular pump compression ratio for heavy oil molecules, ~10-12:1, ensures this. The key to avoiding oil contamination during evacuation is the pump reaching its operating speed as soon as is possible.
In general, turbomolecular pumps can operate continuously at pressures as high as 10-2 torr and maintain constant pumping speed to at least 10-10 torr. As the turbomolecular pump is a transfer pump, there is no accumulation of hazardous gas, and less concern with an emergency shutdown situation. The compression ratio is ~108:1 or nitrogen, but frequently below 1000:1 for hydrogen.
Some first-generation pumps managed only 50:1 for hydrogen.Fortunately, the newer compound pumps, which add an integral molecular drag backing pump, often have compression ratios for hydrogen in excess of 105:1. The large difference between hydrogen (and to a lesser extent helium) and gases such as nitrogen and oxygen leaves the residual gas in the chamber enriched in the lighter species.If a low residual hydrogen pressure is an important consideration, it may be necessary to provide supplementary pumping for this gas, such as a sublimation pump or nonevaporable getter (NEG), or to use a different class of pump.
The demand for negligible organic compound contamination has led to the compound pump, comprising a standard turbomolecular stage backed by a molecular drag stage, mounted on a common shaft. Typically, a backing pressure of only 10 torr or higher, conveniently provided by an oil-free (‘‘dry’’) diaphragm pump, is needed (see discussion of Oil-Free Pumps). In some versions, greased or oil-lubricated bearings are used (on the high-pressure side of the rotor); magnetic bearings are also available.
Compound pumps provide an extremely low risk of oil contamination and significantly higher compression ratios for light gases.
Operation of a Turbomolecular Pump System: Freedom from organic contamination demands care during both the evacuation and venting processes. However, if a pump is contaminated with oil, the cleanup requires disassembly and the use of solvents.
The following is a recommended procedure for a system in which an untrapped oil-sealed mechanical roughing/
backing pump is combined with an oil-lubricated turbomolecular pump, and an isolation valve is provided between the vacuum chamber and the turbomolecular pump.
1. Startup: Begin roughing down and turn on the pump as soon as is possible without overloading
the drive motor. Using a modern electronically controlled supply, no delay is necessary, because the
supply will adjust power to prevent overload while the pressure is high. With older power supplies,
the turbomolecular pump should be started as soon as the pressure reaches a tolerable level, as given by
the manufacturer, probably in the 10 torr region. A rapid startup ensures that the turbomolecular pump
reaches at least 50% of the operating speed while the pressure in the foreline is still in the viscous flow
regime, so that no oil backstreaming can enter the system through the turbomolecular pump.
Before opening to the turbomolecular pump, the vacuum chamber should be roughed down using a
procedure to avoid oil contamination, as was described for diffusion pump startup (see discussion
above).
2. Venting: When the entire system is to be vented to atmospheric pressure, it is essential that the venting
gas enter the turbomolecular pump at a point on the system side of any lubricated bearings in the pump.
This ensures that oil liquid or vapor is swept away from the system towards the backing system. Some
pumps have a vent midway along the turbine blades, while others have vents just above the upper, system-
side, bearings. If neither of these vent points are available, a valve must be provided on the vacuum chamber itself. Never vent the system from a point on the foreline of the turbomolecular pump; that can flush both mechanical pump oil and turbomolecular pump oil into the turbine rotor and stator blades and the vacuum chamber. Venting is best started immediately after turning off the power to the turbomolecular pump and adjusting so the chamber pressure rises into the viscous flow region within a minute or two. Too-rapid venting
exposes the turbine blades to excessive pressure in the viscous flow regime, with unnecessarily high
upward force on the bearing assembly (often called the ‘‘helicopter’’ effect). When venting frequently,
the turbomolecular pump is usually left running, isolated from the chamber, but connected to the forepump.
The major maintenance is checking the oil or grease lubrication, as recommended by the pump manufacturer,
and replacing the bearings as required. The stated life of bearings is often ~2 years continuous operation, though an actual life of ~5 years is not uncommon. In some facilities, where multiple pumps are used in production, bearings are checked by monitoring the amplitude of the vibration frequency associated with the bearings. A marked increase in amplitude indicates the approaching end of bearing life, and the pump is removed for maintenance.
Applications: Turbomolecular pumps were introduced in 1958 (Becker, 1959) and were immediately hailed as
the solution to all of the problems of the diffusion pump.Provided that recommended procedures are used, these pumps live up to the original high expectations. These are reliable, general-purpose pumps requiring
simple operating procedures and capable of maintaining clean vacuumdown to the 10-10 torr range.Pumping speeds up to 10,000 L/s are available.
Operating Principles: The pump is a multistage axial compressor, operating at rotational speeds from around
20,000 to 90,000 rpm. The drive motor is mounted inside the pump housing, avoiding the shaft seal needed with an external drive. Modern power supplies sense excessive loading of the motor, as when operating at too high an inlet pressure, and reduce the motor speed to avoid overheating and possible failure. Occasional failure of the frequency control in the supply has resulted in excessive speeds and catastrophic failure of the rotor.
At high speeds, the dominant problem is maintenance of the rotational bearings. Careful balancing of the rotor
is essential; in some models bearings can be replaced in the field, if rigorous cleanliness is assured, preferably in a clean environment such as a laminar-flow hood. In other designs, the pump must be returned to the manufacturer for bearing replacement and rotor rebalancing. This service factor should be considered in selecting a turbomolecular pump, since few facilities can keep a replacement pump on hand.
Several different types of bearings are common in turbomolecular pumps:
1. Oil Lubrication: All first-generation pumps used oil-lubricated bearings that often lasted several
years in continuous operation. These pumps were mounted horizontally with the gas inlet between
two sets of blades. The bearings were at the ends of the rotor shaft, on the forevacuum side. This
type of pump, and the magnetically levitated designs discussed below, offer minimum vibration.
Second-generation pumps are vertically mounted and single-ended. This is more compact, facilitating
easy replacement of a diffusion pump. Many of these pumps rely on gravity return of lubrication oil to the
reservoir and thus require vertical orientation.Using a wick as the oil reservoir both localizes the
liquid and allows more flexible pump orientation.
2. Grease Lubrication: A low-vapor-pressure grease lubricant was introduced to reduce transport of oil
into the vacuum chamber (Osterstrom, 1979) and to permit orientation of the pump in any direction.
Grease has lower frictional loss and allows a lowerpower drive motor, with consequent drop in operating
temperature.
3. Ceramic Ball Bearings: Most bearings now use a ceramic-balls/steel-race combination; the lighter
balls reduce centrifugal forces and the ceramic-tosteel interface minimizes galling. There appears to
be a significant improvement in bearing life for both oil and grease lubrication systems.
4. Magnetic Bearings: Magnetic suspension systems have two advantages: a non-contact bearing with a
potentially unlimited life, and very low vibration. First-generation pumps used electromagnetic suspension
with a battery backup. When nickelcadmium batteries were used, this backup was not continuously available; incomplete discharge before recharging cycles often reduces discharge capacity.A second generation using permanent magnets was more reliable and of lower cost. Some pumps now offer an improved electromagnetic suspension with better active balancing of the rotor on all axes. In some designs, the motor is used as a generator when power is interrupted, to assure safe shutdown of the magnetic suspension system. Magnetic bearing pumps use a second set of ‘‘touch-down’’ bearings for support when the pump is stationary. The bearings use a solid, low-vapor-pressure lubricant (O’Hanlon, 1989) and further protect the pump in an emergency. The life of the touch-down bearings is limited, and their replacement may be a nuisance; it is, however, preferable to replacing a shattered pump rotor and stator assembly.
5. Combination Bearings Systems: Some designs use combinations of different types of bearings. One
example uses a permanent-magnet bearing at the high-vacuum end and an oil-lubricated bearing at
the forevacuum end. A magnetic bearing does not contaminate the system and is not vulnerable to
damage by aggressive gases as is a lubricated bearing. Therefore it can be located at the very end of the
rotor shaft, while the oil-fed bearing is at the opposite forevacuum end. This geometry has the advantage
of minimizing vibration.
Problems with Pumping Reactive Gases: Very reactive gases, common in the semiconductor industry, can result in rapid bearing failure. A purge with nonreactive gas, in the viscous flow regime, can prevent the pumped gases from contacting the bearings. To permit access to the bearing for a purge, pump designs move the upper bearing below the turbine blades, which often cantilevers the center of mass of the rotor beyond the bearings. This may have been a contributing factor to premature failure seen in some pump designs.
The turbomolecular pump shares many of the performance characteristics of the diffusion pump. In the standard construction, it cannot exhaust to atmospheric pressure, and must be backed at all times by a forepump.
The critical backing pressure is generally in the 10-1 torr, or lower, region, and an oil-sealed mechanical pump is the most common choice. Failure to recognize the problem of oil contamination from this pump was a major factor in the problems with early applications of the turbomolecular pump. But, as with the diffusion pump, an operating turbomolecular pump prevents significant backstreaming from the forepump and its own bearings. A typical turbomolecular pump compression ratio for heavy oil molecules, ~10-12:1, ensures this. The key to avoiding oil contamination during evacuation is the pump reaching its operating speed as soon as is possible.
In general, turbomolecular pumps can operate continuously at pressures as high as 10-2 torr and maintain constant pumping speed to at least 10-10 torr. As the turbomolecular pump is a transfer pump, there is no accumulation of hazardous gas, and less concern with an emergency shutdown situation. The compression ratio is ~108:1 or nitrogen, but frequently below 1000:1 for hydrogen.
Some first-generation pumps managed only 50:1 for hydrogen.Fortunately, the newer compound pumps, which add an integral molecular drag backing pump, often have compression ratios for hydrogen in excess of 105:1. The large difference between hydrogen (and to a lesser extent helium) and gases such as nitrogen and oxygen leaves the residual gas in the chamber enriched in the lighter species.If a low residual hydrogen pressure is an important consideration, it may be necessary to provide supplementary pumping for this gas, such as a sublimation pump or nonevaporable getter (NEG), or to use a different class of pump.
The demand for negligible organic compound contamination has led to the compound pump, comprising a standard turbomolecular stage backed by a molecular drag stage, mounted on a common shaft. Typically, a backing pressure of only 10 torr or higher, conveniently provided by an oil-free (‘‘dry’’) diaphragm pump, is needed (see discussion of Oil-Free Pumps). In some versions, greased or oil-lubricated bearings are used (on the high-pressure side of the rotor); magnetic bearings are also available.
Compound pumps provide an extremely low risk of oil contamination and significantly higher compression ratios for light gases.
Operation of a Turbomolecular Pump System: Freedom from organic contamination demands care during both the evacuation and venting processes. However, if a pump is contaminated with oil, the cleanup requires disassembly and the use of solvents.
The following is a recommended procedure for a system in which an untrapped oil-sealed mechanical roughing/
backing pump is combined with an oil-lubricated turbomolecular pump, and an isolation valve is provided between the vacuum chamber and the turbomolecular pump.
1. Startup: Begin roughing down and turn on the pump as soon as is possible without overloading
the drive motor. Using a modern electronically controlled supply, no delay is necessary, because the
supply will adjust power to prevent overload while the pressure is high. With older power supplies,
the turbomolecular pump should be started as soon as the pressure reaches a tolerable level, as given by
the manufacturer, probably in the 10 torr region. A rapid startup ensures that the turbomolecular pump
reaches at least 50% of the operating speed while the pressure in the foreline is still in the viscous flow
regime, so that no oil backstreaming can enter the system through the turbomolecular pump.
Before opening to the turbomolecular pump, the vacuum chamber should be roughed down using a
procedure to avoid oil contamination, as was described for diffusion pump startup (see discussion
above).
2. Venting: When the entire system is to be vented to atmospheric pressure, it is essential that the venting
gas enter the turbomolecular pump at a point on the system side of any lubricated bearings in the pump.
This ensures that oil liquid or vapor is swept away from the system towards the backing system. Some
pumps have a vent midway along the turbine blades, while others have vents just above the upper, system-
side, bearings. If neither of these vent points are available, a valve must be provided on the vacuum chamber itself. Never vent the system from a point on the foreline of the turbomolecular pump; that can flush both mechanical pump oil and turbomolecular pump oil into the turbine rotor and stator blades and the vacuum chamber. Venting is best started immediately after turning off the power to the turbomolecular pump and adjusting so the chamber pressure rises into the viscous flow region within a minute or two. Too-rapid venting
exposes the turbine blades to excessive pressure in the viscous flow regime, with unnecessarily high
upward force on the bearing assembly (often called the ‘‘helicopter’’ effect). When venting frequently,
the turbomolecular pump is usually left running, isolated from the chamber, but connected to the forepump.
The major maintenance is checking the oil or grease lubrication, as recommended by the pump manufacturer,
and replacing the bearings as required. The stated life of bearings is often ~2 years continuous operation, though an actual life of ~5 years is not uncommon. In some facilities, where multiple pumps are used in production, bearings are checked by monitoring the amplitude of the vibration frequency associated with the bearings. A marked increase in amplitude indicates the approaching end of bearing life, and the pump is removed for maintenance.
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