Cryopumps
Applications: Cryopumping was first extensively used in the space program, where test chambers modeled the conditions encountered in outer space, notably that by which any gas molecule leaving the vehicle rarely returns.This required all inside surfaces of the chamber to function as a pump, and led to liquid-helium-cooled shrouds in the chambers on which gases condensed. This is very effective, but is not easily applicable to individual systems, given the expense and difficulty of handling liquid helium. However, the advent of reliable closed-cycle mechanical refrigeration systems, achieving temperatures in the 10 to 20 K range, allow reliable, contamination-free pumps, with a wide range of pumping speeds, and which are capable of
maintaining pressures as low as the 10-10 torr range (Welch, 1991).
Cryopumps are general purpose and available with very high pumping speeds (using internally mounted cryopanels),so they work for all chamber sizes. These are capture pumps, and, once operating, are totally isolated from the atmosphere. All pumped gas is stored in the body of the pump. They must be regenerated on a regular basis, but the quantity of gas pumped before regeneration is very large for all gases that are captured by condensation.Only helium, hydrogen, and neon are not effectively condensed. They must be captured by adsorption, for which the capacity is far smaller. Indeed, if pumping any significant quantity of helium, regeneration would have to be so frequent that another type of pump should be selected. If the refrigeration fails due to a power interruption or a mechanical failure, the pumped gas will be released within minutes. All pumps are fitted with a pressure relief valve to avoid explosion, but provision must be made for the safe disposal of any hazardous gases released.
Operating Principles: A cryopump uses a closed-cycle refrigeration system with helium as the working gas. An external compressor, incorporating a heat exchanger that is usually water-cooled, supplies helium at ~300 psi to the cold head, which is mounted on the vacuum system. The helium is cooled by passing through a pair of regenerativeheat exchangers in the cold head, and then allowed to expand, a process which cools the incoming gas, and in turn, cools the heat exchangers as the low-pressure gas returns to the compressor. Over a period of several hours, the system develops two cold zones, nominally 80 and 15 K. The ~ 80 K zone is used to cool a shroud through which gas molecules pass into its interior; water is pumped by this
shroud, and it also minimizes the heat load on the second-stage array from ambient temperature radiation. Inside the shroud is an array at ~15 K, on which most other gases are condensed. The energy available to maintain the 15 K temperature is just a few watts.The second stage should typically remain in the range 10 to 20 K, low enough to pump most common gases
to well below 10-10 torr. In order to remove helium, hydrogen, and neon the modern cryopump incorporates a bed of charcoal, having a very large surface area, cooled by the second-stage array. This bed is so positioned that most gases are first removed by condensation, leaving only these three to be physically adsorbed. As already noted, the total pumping capacity of a cryopump is very different for the gases that are condensed, as compared to those that are adsorbed. The capacity of a
pump is frequently quoted for argon, commonly used in sputtering systems. For example, a pump with a speed of ~1000 L/s will have the capability of pumping ~3 X 10 5 torr-liter of argon before requiring regeneration. This implies that a 200-L volume could be pumped down from a typical roughing pressure of 2.5 X 10¯1 torr ~6000 times.The pumping speed of a cryopump remains constant for all gases that are condensable at 20 K, down to the 10¯10 torr range, so long as the temperature of the second-stage array does not exceed 20 K. At this temperature the vapor pressure of nitrogen is ~1 X 1011 torr, and that of all
other condensable gases lies well below this figure.
The capacity for adsorption-pumped gases is not nearly so well defined. The capacity increases both with decreasing temperature and with the pressure of the adsorbing gas. The temperature of the second-stage array is controlled by the balance between the refrigeration capacity and generation of heat by both condensation and adsorption of gases. Of necessity, the heat input must be limited so that the second-stage array never exceeds 20 K, and this translates into a maximum permissible gas flow into the pump. The lowest temperature of operation is set by the pump design, nominally ~10 K. onsequently the capacity for adsorption of a gas such as hydrogen can vary by a factor of four or more when between these two temperature extremes. For a given flow of hydrogen, if this is the only gas being pumped, the heat input will be low, permitting a higher pumping capacity, but if a mixture of gases is involved, then the capacity for hydrogen will be reduced,
simply because the equilibrium operating temperature will be higher. A second factor is the pressure of hydrogen that must be maintained in a particular process. Because the adsorption capacity is determined by this pressure, a low hydrogen pressure translates into a reduced adsorptive capacity, and therefore a shorter operating time before the pump must be regenerated. The effect of these factors is very significant for helium pumping, because the adsorption capacity for this gas is so limited. A cryopump may be quite impractical for any system in which there is a deliberate and significant inlet of helium as a process
gas.
Operating Procedure: Before startup, a cryopump must first be roughed down to some recommended pressure,
often `1 X 10¯1 torr. This serves two functions. First, the vacuum vessel surrounding the cold head functions as a Dewar, thermally isolating the cold zone. Second, any gas remaining must be pumped by the cold head as it cools down; because adsorption is always effective at a much higher temperature than condensation, the gas is adsorbed in the charcoal bed of the 20 K array, partially saturating it, and limiting the capacity for subsequently adsorbing helium, hydrogen, and neon. It is essential to avoid oil contamination when roughing down, because oil vapors adsorbed on the charcoal of the second-stage array cannot be removed by regeneration and irreversibly reduce the adsorptive capacity. Once the required pressure is reached, the cryopump is isolated from the roughing line and the refrigeration system is turned on. When the temperature of the second-stage array reaches 20 K, the pump is ready for operation, and can be opened to the vacuum chamber, which has previously been roughed down to a selected cross-over pressure. This cross-over pressure can readily be calculated from the figure for the impulse gas load, specified by the manufacturer, and the volume of the chamber. The impulse load is simply
the quantity of gas to which the pump can be exposed without increasing the temperature of the second-stage array
above 20 K. When the quantity of gas that has been pumped is close to the limiting capacity, the pump must be regenerated.
This procedure involves isolation from the system, turning off the refrigeration unit, and warming the first- and second-stage arrays until all condensed and adsorbed gas has been removed. The most common method is to purge these gases using a warm (~60°C) dry gas, such as nitrogen, at atmospheric pressure. Internal heaters were deliberately avoided for many years, to avoid an ignition source in the event that explosive gas mixtures, such as hydrogen and oxygen, were released during regeneration. To the same end, the use of any pressure sensor having a hot surface was, and still is, avoided in the regeneration procedure. Current practice has changed, and many pumps now incorporate a means of independently heating each of the refrigerated surfaces. This provides the flexibility to heat the cold surfaces only to the extent that adsorbed
or condensed gases are rapidly removed, greatly reducing the time needed to cool back to the operating temperature. Consider, for example, the case where argon is the predominant gas load. At the maximum operating temperature of 20 K, its vapor pressure is well below 10¯11 torr, but warming to 90 K raises the vapor pressure to 760 torr, facilitating rapid removal.
In certain cases, the pumping of argon can cause a problem commonly referred to as argon hangup. This occurs after a high pressure of argon, e.g., >1 X 10¯3 torr, has been pumped for some time. When the argon influx stops, the argon pressure remains comparatively high instead of falling to the background level. This happens when the temperature of the pump shroud is too low. At 40 K, in contrast to 80 K, argon condenses on the outer shroud instead of being pumped by the second-stage array. Evaporation from the shroud at the argon vapor pressure of 1X10¯3 torr keeps the partial pressure high until all of the
gas has desorbed. The problem arises when the refrigeration capacity is too large, for example, when several pumps are served by a single compressor and the helium supply is improperly proportioned. An internal heater to increase the shroud temperature is an easy solution. A cryopump is an excellent general-purpose device. It can provide an extremely clean environment at base pressures in the low 10¯10 torr range. Care must be taken to ensure that the pressure-relief valve is always operable, and to ensure that any hazardous gases are safely handled in the event of an unscheduled regeneration. There is some possibility of energetic chemical reactions during regeneration. For example, ozone, which is generated in some processes, may react with combustible materials. The use of a nonreactive purge gas will minimize hazardous conditions if the flow is sufficient to dilute the gases released during regeneration. The pump has a high capital cost and fairly high running costs for power and cooling.
Maintenance of a cryopump is normally minimal. Seals in the displacer piston in the cold head must be replaced as required (at intervals of one year or more, depending on the design); an oil-adsorber cartridge in the compressor housing requires a similar replacement schedule.
Applications: Cryopumping was first extensively used in the space program, where test chambers modeled the conditions encountered in outer space, notably that by which any gas molecule leaving the vehicle rarely returns.This required all inside surfaces of the chamber to function as a pump, and led to liquid-helium-cooled shrouds in the chambers on which gases condensed. This is very effective, but is not easily applicable to individual systems, given the expense and difficulty of handling liquid helium. However, the advent of reliable closed-cycle mechanical refrigeration systems, achieving temperatures in the 10 to 20 K range, allow reliable, contamination-free pumps, with a wide range of pumping speeds, and which are capable of
maintaining pressures as low as the 10-10 torr range (Welch, 1991).
Cryopumps are general purpose and available with very high pumping speeds (using internally mounted cryopanels),so they work for all chamber sizes. These are capture pumps, and, once operating, are totally isolated from the atmosphere. All pumped gas is stored in the body of the pump. They must be regenerated on a regular basis, but the quantity of gas pumped before regeneration is very large for all gases that are captured by condensation.Only helium, hydrogen, and neon are not effectively condensed. They must be captured by adsorption, for which the capacity is far smaller. Indeed, if pumping any significant quantity of helium, regeneration would have to be so frequent that another type of pump should be selected. If the refrigeration fails due to a power interruption or a mechanical failure, the pumped gas will be released within minutes. All pumps are fitted with a pressure relief valve to avoid explosion, but provision must be made for the safe disposal of any hazardous gases released.
Operating Principles: A cryopump uses a closed-cycle refrigeration system with helium as the working gas. An external compressor, incorporating a heat exchanger that is usually water-cooled, supplies helium at ~300 psi to the cold head, which is mounted on the vacuum system. The helium is cooled by passing through a pair of regenerativeheat exchangers in the cold head, and then allowed to expand, a process which cools the incoming gas, and in turn, cools the heat exchangers as the low-pressure gas returns to the compressor. Over a period of several hours, the system develops two cold zones, nominally 80 and 15 K. The ~ 80 K zone is used to cool a shroud through which gas molecules pass into its interior; water is pumped by this
shroud, and it also minimizes the heat load on the second-stage array from ambient temperature radiation. Inside the shroud is an array at ~15 K, on which most other gases are condensed. The energy available to maintain the 15 K temperature is just a few watts.The second stage should typically remain in the range 10 to 20 K, low enough to pump most common gases
to well below 10-10 torr. In order to remove helium, hydrogen, and neon the modern cryopump incorporates a bed of charcoal, having a very large surface area, cooled by the second-stage array. This bed is so positioned that most gases are first removed by condensation, leaving only these three to be physically adsorbed. As already noted, the total pumping capacity of a cryopump is very different for the gases that are condensed, as compared to those that are adsorbed. The capacity of a
pump is frequently quoted for argon, commonly used in sputtering systems. For example, a pump with a speed of ~1000 L/s will have the capability of pumping ~3 X 10 5 torr-liter of argon before requiring regeneration. This implies that a 200-L volume could be pumped down from a typical roughing pressure of 2.5 X 10¯1 torr ~6000 times.The pumping speed of a cryopump remains constant for all gases that are condensable at 20 K, down to the 10¯10 torr range, so long as the temperature of the second-stage array does not exceed 20 K. At this temperature the vapor pressure of nitrogen is ~1 X 1011 torr, and that of all
other condensable gases lies well below this figure.
The capacity for adsorption-pumped gases is not nearly so well defined. The capacity increases both with decreasing temperature and with the pressure of the adsorbing gas. The temperature of the second-stage array is controlled by the balance between the refrigeration capacity and generation of heat by both condensation and adsorption of gases. Of necessity, the heat input must be limited so that the second-stage array never exceeds 20 K, and this translates into a maximum permissible gas flow into the pump. The lowest temperature of operation is set by the pump design, nominally ~10 K. onsequently the capacity for adsorption of a gas such as hydrogen can vary by a factor of four or more when between these two temperature extremes. For a given flow of hydrogen, if this is the only gas being pumped, the heat input will be low, permitting a higher pumping capacity, but if a mixture of gases is involved, then the capacity for hydrogen will be reduced,
simply because the equilibrium operating temperature will be higher. A second factor is the pressure of hydrogen that must be maintained in a particular process. Because the adsorption capacity is determined by this pressure, a low hydrogen pressure translates into a reduced adsorptive capacity, and therefore a shorter operating time before the pump must be regenerated. The effect of these factors is very significant for helium pumping, because the adsorption capacity for this gas is so limited. A cryopump may be quite impractical for any system in which there is a deliberate and significant inlet of helium as a process
gas.
Operating Procedure: Before startup, a cryopump must first be roughed down to some recommended pressure,
often `1 X 10¯1 torr. This serves two functions. First, the vacuum vessel surrounding the cold head functions as a Dewar, thermally isolating the cold zone. Second, any gas remaining must be pumped by the cold head as it cools down; because adsorption is always effective at a much higher temperature than condensation, the gas is adsorbed in the charcoal bed of the 20 K array, partially saturating it, and limiting the capacity for subsequently adsorbing helium, hydrogen, and neon. It is essential to avoid oil contamination when roughing down, because oil vapors adsorbed on the charcoal of the second-stage array cannot be removed by regeneration and irreversibly reduce the adsorptive capacity. Once the required pressure is reached, the cryopump is isolated from the roughing line and the refrigeration system is turned on. When the temperature of the second-stage array reaches 20 K, the pump is ready for operation, and can be opened to the vacuum chamber, which has previously been roughed down to a selected cross-over pressure. This cross-over pressure can readily be calculated from the figure for the impulse gas load, specified by the manufacturer, and the volume of the chamber. The impulse load is simply
the quantity of gas to which the pump can be exposed without increasing the temperature of the second-stage array
above 20 K. When the quantity of gas that has been pumped is close to the limiting capacity, the pump must be regenerated.
This procedure involves isolation from the system, turning off the refrigeration unit, and warming the first- and second-stage arrays until all condensed and adsorbed gas has been removed. The most common method is to purge these gases using a warm (~60°C) dry gas, such as nitrogen, at atmospheric pressure. Internal heaters were deliberately avoided for many years, to avoid an ignition source in the event that explosive gas mixtures, such as hydrogen and oxygen, were released during regeneration. To the same end, the use of any pressure sensor having a hot surface was, and still is, avoided in the regeneration procedure. Current practice has changed, and many pumps now incorporate a means of independently heating each of the refrigerated surfaces. This provides the flexibility to heat the cold surfaces only to the extent that adsorbed
or condensed gases are rapidly removed, greatly reducing the time needed to cool back to the operating temperature. Consider, for example, the case where argon is the predominant gas load. At the maximum operating temperature of 20 K, its vapor pressure is well below 10¯11 torr, but warming to 90 K raises the vapor pressure to 760 torr, facilitating rapid removal.
In certain cases, the pumping of argon can cause a problem commonly referred to as argon hangup. This occurs after a high pressure of argon, e.g., >1 X 10¯3 torr, has been pumped for some time. When the argon influx stops, the argon pressure remains comparatively high instead of falling to the background level. This happens when the temperature of the pump shroud is too low. At 40 K, in contrast to 80 K, argon condenses on the outer shroud instead of being pumped by the second-stage array. Evaporation from the shroud at the argon vapor pressure of 1X10¯3 torr keeps the partial pressure high until all of the
gas has desorbed. The problem arises when the refrigeration capacity is too large, for example, when several pumps are served by a single compressor and the helium supply is improperly proportioned. An internal heater to increase the shroud temperature is an easy solution. A cryopump is an excellent general-purpose device. It can provide an extremely clean environment at base pressures in the low 10¯10 torr range. Care must be taken to ensure that the pressure-relief valve is always operable, and to ensure that any hazardous gases are safely handled in the event of an unscheduled regeneration. There is some possibility of energetic chemical reactions during regeneration. For example, ozone, which is generated in some processes, may react with combustible materials. The use of a nonreactive purge gas will minimize hazardous conditions if the flow is sufficient to dilute the gases released during regeneration. The pump has a high capital cost and fairly high running costs for power and cooling.
Maintenance of a cryopump is normally minimal. Seals in the displacer piston in the cold head must be replaced as required (at intervals of one year or more, depending on the design); an oil-adsorber cartridge in the compressor housing requires a similar replacement schedule.
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