PRACTICAL ASPECTS OF VACUUM TECHNOLOGY
Vacuum Pumps
The operation of most vacuum systems can be divided into two regimes. The first involves pumping the system from atmosphere to a pressure at which a high-vacuum pump can be brought into operation. This is traditionally known as the rough vacuum regime and the pumps used are commonly referred to as roughing pumps. Clearly, a system that operates at an ultimate pressure within the capability of the roughing pump will require no additional pumps. Once the system has been roughed down, a high vacuum pump must be used to achieve lower pressures.
If the high-vacuum pump is the type known as a transfer pump, such as a diffusion or turbomolecular pump, it will require the continuous support of the roughing pump in order to maintain the pressure at the exit of the highvacuum pump at a tolerable level (in this phase of the pumping operation the function of the roughing pump has changed, and it is frequently referred to as a backing or forepump). Transfer pumps have the advantage that their capacity for continuous pumping of gas, within their operating pressure range, is limited only by their reliability.
They do not accumulate gas, an important consideration where hazardous gases are involved. Note that the
reliability of transfer pumping systems depends upon the satisfactory performance of two separate pumps. A second class of pumps, known collectively as capture pumps, require no further support from a roughing pump once they have started to pump. Examples of this class are cryogenic pumps and sputter-ion pumps. These types of pump have the advantage that the vacuum system is isolated from the atmosphere, so that system operation depends upon the reliability of only one pump. Their disadvantage is that they can provide only limited storage of pumped gas, and as that limit is reached, pumping will deteriorate.
The effect of such a limitation is quite different for the two examples cited. A cryogenic pump can be totally regenerated by a brief purging at ambient temperature, but a sputter-ion pump requires replacement of its internal components.
One aspect of the cryopump that should not be overlooked is that hazardous gases are stored, unchanged,
within the pump, so that an unexpected failure of the pump can release these accumulated gases, requiring provision for their automatic safe dispersal in such an emergency.
Roughing Pumps
Two classes of roughing pumps are in use. The first type, the oil-sealed mechanical pump, is by far the most common, but because of the enormous concern in the semiconductor industry about oil contamination, a second type, the so-called ‘‘dry’’ pump, is now frequently used. In this context, ‘‘dry’’ implies the absence of volatile organics in the part of the pump that communicates with the vacuum system.
Oil-Sealed Pumps The earliest roughing pumps used either a piston or liquid to displace the gas. The first production methods for incandescent lamps used such pumps, and the development of the oil-sealed mechanical pump by Gaede, around 1907, was driven by the need to accelerate the pumping process.
Applications :
The modern versions of this pump are the most economic and convenient for achieving pressures as
low as the 10-4 torr range. The pumps are widely used as a backing pump for both diffusion and turbomolecular pumps; in this application the backstreaming of mechanical pump oil is intercepted by the high vacuum pump, and a foreline trap is not required.
Operating Principles:
The oil-sealed pump is a positive displacement pump, of either the vane or piston type, with a compression ratio of the order of 105:1 (Dobrowolski,1979). It is available as a single or two-stage pump, capable
of reaching base pressures in the 10-2 and 10-4 torr range, respectively. The pump uses oil to maintain sealing, and to provide lubrication and heat transfer, particularly at the contact between the sliding vanes and the pump wall.Oil also serves to fill the significant dead space leading to the exhaust valve, essentially functioning as a hydraulic valve lifter and permitting the very high compression ratio.
The speed of such pumps is often quoted as the ‘‘free-air displacement,’’ which is simply the volume swept by the pump rotor. In a typical two-stage pump this speed is sustained down to ~ 1 x 10-1 torr; below this pressure the speed decreases, reaching zero in the 10-5 torr range. If a pump is to sustain pressures near the bottom of its range, the required pump size must be determined from published pumping-speed performance data. It should be noted that mechanical pumps have relatively small pumping
speed, at least when compared with typical highvacuum pumps. A typical laboratory-sized pump, powered
by a 1/3 hp motor, may have a speed of ~3.5 cubic feet per minute (cfm), or rather less than 2 L/s, as compared to the smallest turbomolecular pump, which has a rated speed of 50 L/s.
Avoiding Oil Contamination from an Oil-Sealed Mechanical Pump:
The versatility and reliability of the oil-sealed mechanical pump carries with it a serious penalty. When
used improperly, contamination of the vacuum system is inevitable. These pumps are probably the most prevalent source of oil contamination in vacuum systems. The problem arises when thay are untrapped and pump a system down to its ultimate pressure, often in the free molecular flow regime. In this regime, oil molecules flow freely into the vacuum chamber. The problem can readily be avoided by careful control of the pumping procedures, but possible system or operator malfunction, leading to contamination, must be considered. For many years, it was common practice to leave a system in the standby condition evacuated
only by an untrapped mechanical pump, making contamination inevitable.
Mechanical pump oil has a vapor pressure, at room temperature,in the low 10-5 torr range when first installed,
but this rapidly deteriorates up to two orders of magnitude as the pump is operated (Holland, 1971). A pump operates at temperatures of 60C, or higher, so the oil vapor pressure far exceeds 10-3 torr, and evaporation results in a substantial flux of oil into the roughing line.
When a system at atmospheric pressure is connected to the mechanical pump, the initial gas flow from the vacuum chamber is in the viscous flow regime, and oil molecules are driven back to the pump by collisions with the gas being exhausted (Holland, 1971; Lewin, 1985). Provided the roughing process is terminated while the gas flow is still in the viscous flow regime, no significant contamination of the vacuum chamber will occur.
The condition for viscous flow is given by the equation
PD >/ 0.5
where P is the pressure in torr and D is the internal diameter of the roughing line in centimeters.
Termination of the roughing process in the viscous flow region is entirely practical when the high-vacuum pump is either a turbomolecular or modern diffusion pump (see precautions discussed under Diffusion Pumps and Turbomolecular Pumps, below). Once these pumps are in operation, they function as an effective barrier against oil migration into the system from the forepump. Hoffman (1979) has described the use of a continuous gas purge on the foreline of a diffusion-pumped system as a means of avoiding backstreaming from the forepump.
Foreline Traps.
A foreline trap is a second approach to preventing oil backstreaming. If a liquid nitrogen-cooled trap is always in place between a forepump and the vacuum chamber, cleanliness is assured. But the operative word is ‘‘always.’’ If the trap warms to ambient temperature, oil from the trap will migrate upstream, and this is
much more serious if it occurs while the line is evacuated. A different class of trap uses an adsorbent for oil. Typical adsorbents are activated alumina, molecular sieve (a synthetic zeolite), a proprietary ceramic (Micromaze foreline traps; Kurt J. Lesker Co.), and metal wool. The metal wool traps have much less capacity than the other types, and unless there is evidence of their efficacy, they are best avoided. Published data show that activated alumina can trap 99% of the backstreaming oil molecules (Fulker, 1968). However, one must know when such traps should be reactivated. Unequivocal determination requires insertion of an oil-detection device, such as a mass spectrometer, on the foreline. The saturation time of a trap depends upon the rate of oil influx, which in turn depends upon the vapor pressure of oil in the pump and the conductance of the line between pump and trap. The only safe procedure is frequent reactivation of traps on a conservative
schedule. Reactivation may be done by venting the system, replacing the adsorbent with a new charge, or by baking the adsorbent in a stream of dry air or inert gas to a temperature of -300C for several hours. Some traps can be regenerated by heating in situ, but only using a stream of inert gas, at a pressure in the viscous flow region, flowing from the system side of the trap to the pump (D.J. Santeler, pers. comm.). The foreline is isolated from the rest of the system and the gas flow is continued throughout the heating cycle, until the trap has cooled back to ambient temperature. An adsorbent foreline trap must be optically dense, so the oil molecules have no path past the adsorbent; commercial traps do not always fulfill this basic requirement. Where regeneration of the foreline trap has been totally neglected, acceptable performance may still be achieved simply because a diffusion pump or turbomolecular pump serves as the true ‘‘trap,’’ intercepting the oil from the forepump.
Oil contamination can also result from improperly turning a pump off. If it is stopped and left under vacuum, oil
frequently leaks slowly across the exhaust valve into the pump. When it is partially filled with oil, a hydraulic
lock may prevent the pump from starting. Continued leakage will drive oil into the vacuum system itself; an interesting procedure for recovery from such a catastrophe has been described (Hoffman, 1979).
Whenever the pump is stopped, either deliberately or by power failure or other failure, automatic controls that first isolate it from the vacuum system, and then vent it to atmospheric pressure, should be used.
Most gases exhausted from a system, including oxygen and nitrogen, are readily removed from the pump oil, but some can liquify under maximum compression just before the exhaust valve opens. Such liquids mix with the oil and are more difficult to remove. They include water and solvents frequently used to clean system components. When pumping large volumes of air from a vacuum chamber, particularly during periods of high humidity (or whenever solvent residues are present), it is advantageous to use a gas-ballast feature commonly fitted to two-stage and also to some single-stage pumps. This feature admits air during the final stage of compression, raising the pressure and forcing the exhaust valve to open before the partial pressure of water has reached saturation. The ballast feature minimizes pump contamination and reduces pumpdown
time for a chamber exposed to humid air, although at the cost of about ten-times-poorer base pressure.
Vacuum Pumps
The operation of most vacuum systems can be divided into two regimes. The first involves pumping the system from atmosphere to a pressure at which a high-vacuum pump can be brought into operation. This is traditionally known as the rough vacuum regime and the pumps used are commonly referred to as roughing pumps. Clearly, a system that operates at an ultimate pressure within the capability of the roughing pump will require no additional pumps. Once the system has been roughed down, a high vacuum pump must be used to achieve lower pressures.
If the high-vacuum pump is the type known as a transfer pump, such as a diffusion or turbomolecular pump, it will require the continuous support of the roughing pump in order to maintain the pressure at the exit of the highvacuum pump at a tolerable level (in this phase of the pumping operation the function of the roughing pump has changed, and it is frequently referred to as a backing or forepump). Transfer pumps have the advantage that their capacity for continuous pumping of gas, within their operating pressure range, is limited only by their reliability.
They do not accumulate gas, an important consideration where hazardous gases are involved. Note that the
reliability of transfer pumping systems depends upon the satisfactory performance of two separate pumps. A second class of pumps, known collectively as capture pumps, require no further support from a roughing pump once they have started to pump. Examples of this class are cryogenic pumps and sputter-ion pumps. These types of pump have the advantage that the vacuum system is isolated from the atmosphere, so that system operation depends upon the reliability of only one pump. Their disadvantage is that they can provide only limited storage of pumped gas, and as that limit is reached, pumping will deteriorate.
The effect of such a limitation is quite different for the two examples cited. A cryogenic pump can be totally regenerated by a brief purging at ambient temperature, but a sputter-ion pump requires replacement of its internal components.
One aspect of the cryopump that should not be overlooked is that hazardous gases are stored, unchanged,
within the pump, so that an unexpected failure of the pump can release these accumulated gases, requiring provision for their automatic safe dispersal in such an emergency.
Roughing Pumps
Two classes of roughing pumps are in use. The first type, the oil-sealed mechanical pump, is by far the most common, but because of the enormous concern in the semiconductor industry about oil contamination, a second type, the so-called ‘‘dry’’ pump, is now frequently used. In this context, ‘‘dry’’ implies the absence of volatile organics in the part of the pump that communicates with the vacuum system.
Oil-Sealed Pumps The earliest roughing pumps used either a piston or liquid to displace the gas. The first production methods for incandescent lamps used such pumps, and the development of the oil-sealed mechanical pump by Gaede, around 1907, was driven by the need to accelerate the pumping process.
Applications :
The modern versions of this pump are the most economic and convenient for achieving pressures as
low as the 10-4 torr range. The pumps are widely used as a backing pump for both diffusion and turbomolecular pumps; in this application the backstreaming of mechanical pump oil is intercepted by the high vacuum pump, and a foreline trap is not required.
Operating Principles:
The oil-sealed pump is a positive displacement pump, of either the vane or piston type, with a compression ratio of the order of 105:1 (Dobrowolski,1979). It is available as a single or two-stage pump, capable
of reaching base pressures in the 10-2 and 10-4 torr range, respectively. The pump uses oil to maintain sealing, and to provide lubrication and heat transfer, particularly at the contact between the sliding vanes and the pump wall.Oil also serves to fill the significant dead space leading to the exhaust valve, essentially functioning as a hydraulic valve lifter and permitting the very high compression ratio.
The speed of such pumps is often quoted as the ‘‘free-air displacement,’’ which is simply the volume swept by the pump rotor. In a typical two-stage pump this speed is sustained down to ~ 1 x 10-1 torr; below this pressure the speed decreases, reaching zero in the 10-5 torr range. If a pump is to sustain pressures near the bottom of its range, the required pump size must be determined from published pumping-speed performance data. It should be noted that mechanical pumps have relatively small pumping
speed, at least when compared with typical highvacuum pumps. A typical laboratory-sized pump, powered
by a 1/3 hp motor, may have a speed of ~3.5 cubic feet per minute (cfm), or rather less than 2 L/s, as compared to the smallest turbomolecular pump, which has a rated speed of 50 L/s.
Avoiding Oil Contamination from an Oil-Sealed Mechanical Pump:
The versatility and reliability of the oil-sealed mechanical pump carries with it a serious penalty. When
used improperly, contamination of the vacuum system is inevitable. These pumps are probably the most prevalent source of oil contamination in vacuum systems. The problem arises when thay are untrapped and pump a system down to its ultimate pressure, often in the free molecular flow regime. In this regime, oil molecules flow freely into the vacuum chamber. The problem can readily be avoided by careful control of the pumping procedures, but possible system or operator malfunction, leading to contamination, must be considered. For many years, it was common practice to leave a system in the standby condition evacuated
only by an untrapped mechanical pump, making contamination inevitable.
Mechanical pump oil has a vapor pressure, at room temperature,in the low 10-5 torr range when first installed,
but this rapidly deteriorates up to two orders of magnitude as the pump is operated (Holland, 1971). A pump operates at temperatures of 60C, or higher, so the oil vapor pressure far exceeds 10-3 torr, and evaporation results in a substantial flux of oil into the roughing line.
When a system at atmospheric pressure is connected to the mechanical pump, the initial gas flow from the vacuum chamber is in the viscous flow regime, and oil molecules are driven back to the pump by collisions with the gas being exhausted (Holland, 1971; Lewin, 1985). Provided the roughing process is terminated while the gas flow is still in the viscous flow regime, no significant contamination of the vacuum chamber will occur.
The condition for viscous flow is given by the equation
PD >/ 0.5
where P is the pressure in torr and D is the internal diameter of the roughing line in centimeters.
Termination of the roughing process in the viscous flow region is entirely practical when the high-vacuum pump is either a turbomolecular or modern diffusion pump (see precautions discussed under Diffusion Pumps and Turbomolecular Pumps, below). Once these pumps are in operation, they function as an effective barrier against oil migration into the system from the forepump. Hoffman (1979) has described the use of a continuous gas purge on the foreline of a diffusion-pumped system as a means of avoiding backstreaming from the forepump.
Foreline Traps.
A foreline trap is a second approach to preventing oil backstreaming. If a liquid nitrogen-cooled trap is always in place between a forepump and the vacuum chamber, cleanliness is assured. But the operative word is ‘‘always.’’ If the trap warms to ambient temperature, oil from the trap will migrate upstream, and this is
much more serious if it occurs while the line is evacuated. A different class of trap uses an adsorbent for oil. Typical adsorbents are activated alumina, molecular sieve (a synthetic zeolite), a proprietary ceramic (Micromaze foreline traps; Kurt J. Lesker Co.), and metal wool. The metal wool traps have much less capacity than the other types, and unless there is evidence of their efficacy, they are best avoided. Published data show that activated alumina can trap 99% of the backstreaming oil molecules (Fulker, 1968). However, one must know when such traps should be reactivated. Unequivocal determination requires insertion of an oil-detection device, such as a mass spectrometer, on the foreline. The saturation time of a trap depends upon the rate of oil influx, which in turn depends upon the vapor pressure of oil in the pump and the conductance of the line between pump and trap. The only safe procedure is frequent reactivation of traps on a conservative
schedule. Reactivation may be done by venting the system, replacing the adsorbent with a new charge, or by baking the adsorbent in a stream of dry air or inert gas to a temperature of -300C for several hours. Some traps can be regenerated by heating in situ, but only using a stream of inert gas, at a pressure in the viscous flow region, flowing from the system side of the trap to the pump (D.J. Santeler, pers. comm.). The foreline is isolated from the rest of the system and the gas flow is continued throughout the heating cycle, until the trap has cooled back to ambient temperature. An adsorbent foreline trap must be optically dense, so the oil molecules have no path past the adsorbent; commercial traps do not always fulfill this basic requirement. Where regeneration of the foreline trap has been totally neglected, acceptable performance may still be achieved simply because a diffusion pump or turbomolecular pump serves as the true ‘‘trap,’’ intercepting the oil from the forepump.
Oil contamination can also result from improperly turning a pump off. If it is stopped and left under vacuum, oil
frequently leaks slowly across the exhaust valve into the pump. When it is partially filled with oil, a hydraulic
lock may prevent the pump from starting. Continued leakage will drive oil into the vacuum system itself; an interesting procedure for recovery from such a catastrophe has been described (Hoffman, 1979).
Whenever the pump is stopped, either deliberately or by power failure or other failure, automatic controls that first isolate it from the vacuum system, and then vent it to atmospheric pressure, should be used.
Most gases exhausted from a system, including oxygen and nitrogen, are readily removed from the pump oil, but some can liquify under maximum compression just before the exhaust valve opens. Such liquids mix with the oil and are more difficult to remove. They include water and solvents frequently used to clean system components. When pumping large volumes of air from a vacuum chamber, particularly during periods of high humidity (or whenever solvent residues are present), it is advantageous to use a gas-ballast feature commonly fitted to two-stage and also to some single-stage pumps. This feature admits air during the final stage of compression, raising the pressure and forcing the exhaust valve to open before the partial pressure of water has reached saturation. The ballast feature minimizes pump contamination and reduces pumpdown
time for a chamber exposed to humid air, although at the cost of about ten-times-poorer base pressure.
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