EP0349577B1

EP0349577B1 - An optimally staged cryopump

Info

Publication number: EP0349577B1
Application number: EP88902995A
Authority: EP
Inventors: Philip A. Lessard; Thomas Dunn
Original assignee: Helix Technology Corp
Current assignee: Azenta Inc
Priority date: 1987-01-27
Filing date: 1988-01-27
Publication date: 1994-11-30
Anticipated expiration: 2008-01-27
Also published as: JP2597696B2; EP0349577A1; WO1988005500A1; DE3852303T2; DE3852303D1; JPH02502034A; CA1315111C

Abstract

A cryopump having at least two temperature stages (20, 32) for pumping gases at their optimal temperatures. A first embodiment has a third temperature stage (36) that is surrounded by and separated from the second temperature stage (32) which is surrounded by and separated from the first temperature stage (20). Adsorbent (34, 36) placed on the second and third stages are operated at different temperatures to prevent gases with higher critical mobility temperatures from becoming immobilized at the entrance of pores and wells along the surface of the adsorbent. Another embodiment has at least a second stage (32), a temperature sensor (46), and a heater (44) to maintain the second stage (32) temperature at the optimal level for the gas being pumped.

Description

Cryopumps are typically used in equipment for the manufacture of integrated circuits and other electronic components, as well as for the deposition of thin films in a variety of consumer and industrial products. The cryopumps are used to create a vacuum by freezing or pumping out gases in a work environment. Refrigerators employed by the cryopumps for pumping out gases may be open or closed-cycle cryogenic refrigerators. The most common refrigerator used is a two-stage cold finger, closed-cycle refrigerator.
Typically, the cold end of the second stage, which is the coldest stage of the two-stage refrigerator, is connected to a primary pumping surface. The primary pumping surface operates in a temperature range of 4 to 25° K. The first stage of the two-stage refrigerator is connected to a radiation shield which surrounds the primary pumping surface. The spacing between the primary pumping surface and the radiation shield must be sufficient to permit unobstructed flow of low-boiling temperature gases from a vacuum chamber created by the shield to the primary pumping surface. The radiation shield typically operates in a range of 70° to 140° K. Separating the evacuation chamber and the radiation shield is a frontal array, which also serves as a radiation shield for the primary pumping surface. The frontal array is typically cooled to 110 to 130° K by thermally coupling it to the radiation shield.
In operation, high boiling point gases, such as water vapor, are condensed on the frontal array. Lower boiling point gases pass through that array and into a volume within the radiation shielding, where they condense on the primary pumping surface. An adsorbent, such as charcoal, is typically placed adjacent to the primary pumping surface and is operated at a temperature of that surface to adsorb gases which have very low boiling point temperatures and are not condensed on the primary surface.
Multiple stage refrigerators have used temperature measuring and control devices on the first and second stages to prevent crossover hangup during cooling and for full or partial regeneration procedures. Such a temperature control system is described in US-A-4,679,401, where a refrigerant gas is diverted and brought into heat exchange relationship with the first and/or second stage heat sinks of a cryopump. By controlling the flow and temperature of the diverted refrigerant gas the problems of crossover hangup and or achieving more efficient regeneration can be addressed.
A further cryopump having means for periodic desorption is disclosed in the closest prior art document WO-A-8400404. The cryopump is provided with primary and secondary cryopanels, and means for selectively transferring heat to the primary cryopanel to raise the temperature of the primary cryopanel above that which is necessary to cause gas to become desorbed therefrom. The regeneration process is carried out with the cryopump isolated from the process chamber, the pumping operation of the primary pumping surface being interrupted as the surface is heated to 40°K.
GB-A-2127493 discloses a technique for regeneration whereby the first and second temperature stages are provided with a by-pass line to reverse the gas flow in order to heat the second temperature stage in a periodic defrost operation.
It is also known from US-A-4614093 to extend the operating time available for such a cryopump by preventing gas adsorption during a start-up procedure by heating of the cooling head prior to cool-down to the operating temperature.
A cryopump including temperature adjusting means is disclosed in JP 60-204981(A) which includes a power source which is controlled according to the temperature of a second cryopanel to vary the exhaust speed.
A cryopump which includes first, second and third temperature stages is disclosed in JP 60-187781(A), the third temperature stage including a third panel which has an adsorbent for adsorbing gases such as hydrogen or helium of low condensing temperatures.
A further cryopump including a third temperature stage is disclosed in European Patent 0158295A. The third temperature stage has a third surface or cold adsorbent coated panel for adsorption of Group III gases.
Another cryopump having a third refrigerating stage is disclosed in JP 58-131831A. The third stage is separated from the second stage and is maintained at a lower temperature.
The present invention differs from conventional cryopumps which provide a temperature stage having an adsorbent that is cooled as cold as possible for pumping gases which were not pumped on the first temperature stage, which is typically used for pumping water. In the conventional cryopumps, the adsorbent surface is not effectively utilized for pumping gases because as the adsorbent is cooled to a temperature for adsorbing gases having a lower critical mobility temperature, gases with higher critical mobility temperatures become immobile at the entrance of the pores and wells. As a result, a smaller amount of surface area becomes available for adsorbing gases. Thus, the advantage of the present invention over conventional cryopumps is that internal surfaces of the pores and wells are not blocked at their entrances.
It has been generally perceived that to obtain optimal pumping of hydrogen at the second stage, the second stage temperature must be at or below 14°K. However, it has been found that there is a specific temperature at which optimal pumping occurs under given load conditions and that below this temperature the rate of adsorption is substantially reduced. Thus, in accordance with the present invention, the temperature of the second stage is maintained at the optimal temperature for the gases and load conditions that are present.
According to the present invention there is provided a cryopump having a pumping operation in which gas is pumped from a work chamber and a regeneration cycle, comprising:
a first temperature stage having a pumping surface for condensing gases,
a second temperature stage having a second pumping surface with a first adsorbent thereon, which is cooled to a lower temperature than the first temperature stage, for pumping gases not pumped by the first pumping surface, and,
a temperature control system having a heating element in thermally conductive contact with the second temperature stage,
characterised by a third temperature stage having a second absorbent surface thereon; and
in that the temperature control system is adapted to heat the second temperature stage during the pumping of gases from a work chamber by the second pumping surface, so as to maintain the temperature of the second temperature stage at a sufficiently low pumping temperature to pump gases not pumped by the first pumping surface yet above a minimum temperature below which the capacity of the first adsorbent, in pumping the gas to be pumped by the first adsorbent during the pumping operation, is substantially reduced.
According to a further aspect of the present invention there is provided a method of adsorbing gases in a cryopump having a pumping operation in which gas is pumped from a work chamber and a regeneration cycle, comprising the steps of:
cooling a first temperature stage of the cryopump having a first condensing surface for condensing gases;
cooling a second temperature stage of the cryopump by adsorbing gases with a second temperature stage adsorbent mounted thereon; characterized by controlling the temperature of the second temperature stage during adsorption with a temperature control system such that the temperature of the second stage is maintained, during the pumping operation of pumping gases from a work chamber by the second stage adsorbent, at a sufficiently low pumping temperature to pump gases not pumped by the first condensing surface yet above a minimum temperature below which the capacity of the second temperature stage adsorbent for adsorbing gases not condensed on the first temperature stage is substantially reduced.
The cryopump according to the invention has three different temperature stages: a first temperature stage for pumping gases which have high boiling point temperatures, such as water; a second temperature stage for pumping gases which were not pumped by the first stage; and a third temperature stage, the coldest stage, for pumping gases having a very low boiling point and were not pumped by the first two temperature stages. Located at the second and third temperature stages are adsorbents which have pores and wells for effectively adsorbing gases with different critical mobility temperatures. The third temperature stage is surrounded by and separated from the second temperature stage, which is, in turn, surrounded by and separated from the first temperature stage. The spacing between the temperature stages permits unobstructed flow of low-boiling temperature gases from the first temperature stage to the third temperature stage.
A second stage temperature control system is used during pump operation to obtain optimal cryosorption of the gas being pumped at the second stage. Thus, the second stage temperature can be adjusted to maintain the second stage temperature at the optimal level. In the case of hydrogen, the temperature at which this optimum occurs is generally between 10 and 14°K depending upon the specific H₂ loading of the pump. This optimal temperature of hydrogen must be maintained so that the pumped molecules can move about on the adsorbent surface without clogging the pores.
A preferred embodiment of this temperature control system incorporates a temperature sensor contacting the second stage heat sink and an electrical resistance-type heater in heat conductive contact with the second stage. The wires used to conduct power to the heating filament are hermetically sealed to avoid their exposure to volatile gases within the pumping chamber.
Embodiments of the invention will be described, by way of example only, with reference to the following drawings in which like reference characters refer to the same parts throughout the different views, and in which:
Fig. 1 is a view illustrating a magnified partial cross sectional surface of charcoal.
Fig. 2 is a cryopump having three temperature stages.
Fig. 3 is a cryopump embodying the present invention with a heater system attached to the first and second stages.
Fig. 4 is an arbitrary graphical representation of the dependence of effective pumping speed versus temperature for hydrogen under specific load conditions.
It is known that the number of molecules adsorbed per unit area equals the rate at which gas impinges on the unit area of the surface times the average time which a molecule spends on the surface. Therefore, by increasing the unit of surface area, more molecules can be adsorbed by an absorbent. Of the cryogenic adsorbents available, charcoal and zeolites are the most commonly used adsorbents because they have a large number of pores and cavities along their surfaces. The large number of pores and cavities of these adsorbents provide for a large effective surface area for adsorbing molecules relative to the size of the adsorbent. Other considerations, such as temperature and time required for activation, amount of dust produced by the adsorbent, thermal conductivity, etc., also make charcoal and zeolites the best choice.
By way of an example, a magnified view of the surface area of charcoal is illustrated in figure 1. On being adsorbed, gas molecules M will migrate along the surface 11 of the charcoal and fall into a potential well 10 until such time as they receive enough thermal energy to desorb. The gas molecules M migrate along the surface 11 because during the time in which they remain on the surface 11 of the adsorbent, called the residence time, they are more likely to receive a small amount of energy from the adsorbent. If the temperature of the adsorbent is sufficiently low, the probability of the molecules M acquiring sufficient energy to escape or migrate along the surface 11 of the adsorbent becomes small. The molecules M thus become less mobile. Therefore, according to conventional theory, the amount of gas adsorbed must increase rapidly with decreasing temperature.
In the present invention, tests have indicated that noncodensibles such as helium, neon, and hydrogen have critical mobility temperatures when adsorbed on charcoal. Specifically, helium has been found to have a critical mobility temperature of below 5°K, neon has been found to have a critical mobility temperature of about 10°K, and hydrogen has been found to have a critical mobility temperature of about 13°K. Similarly, other noncondensibles have critical mobility temperatures. Below these critical temperatures, it is believed that the adsorbed noncondensibles can become immobile on the surface of the adsorbent. As a consequence, the entrance of the cavities and pores of the adsorbent can become blocked with immobile molecules because of its insufficient mobility to penetrate the less accessible internal areas. Such a situation is shown in figure 1. As a result, less effective surface area of the adsorbent is utilized to adsorb gases having a lower critical mobility temperature.
It has also been found that the rate of cryosorption of hydrogen reaches a maximum in the temperature range between 10 and 14°K and is substantially reduced below that level. The exact temperature depends upon the level of H₂ loading.
Figure 4 is an arbitrary graphical illustration that under a given H₂ load condition, the rate at which hydrogen is pumping reaches an optimum value at a temperature T_o. As indicated above, conventional theory has taught that the amount of gas adsorbed should increase with decreasing temperature. Figure 4 illustrates that the rate of adsorption drops rapidly at temperatures below T_o.
As a result, the present invention provides that an optimal cryopump can be constructed having three temperature stages: a first stage to pump gases which freeze readily at temperatures of approximately 100°K, such as water; a second stage to effectively pump gases which freeze readily at temperatures of approximately 15°K, such as nitrogen and argon, and also to provide an adsorbent to pump those noncondensibles which have a higher critical mobility temperature, such as hydrogen and neon; and a small third stage, maintained as cold as possible to effectively pump gases with very low critical mobility temperatures such as helium. Preferably, the first stage temperature is cooled to 70 to 140°K, the second stage temperature is cooled to 10 to 14°K, and the third stage temperature is cooled to approximately 5°K.
A three temperature stage cryopump can be constructed in a variety of ways. For example, in figure 2, (which is not strictly in accordance with the invention in that no temperature control system is utilised) a two-staged, cold finger of a closed-cycle refrigerator R extends into a housing 14 of a conventional cryopump through an opening 16. In this case, the refrigerator is a Gifford-MacMahon refrigerator but other refrigerators may be used. In the refrigerator, a displacer in the cold finger is driven by a motor 12. With each cycle, helium gas introduced into the cold finger under pressure through a feed line 13 is expanded and thus cooled and then exhausted through a return line 15. Such a refrigerator is disclosed in US-A-3,218,815.
The first stage 18 of the cold finger is mounted to a radiation shield 20 which is coupled to a frontal array 22. Typically, the temperature differential across the thermal path from the frontal array 22 to the first stage 18 of the cold finger is between 30°K and 50°K. Thus, in order to hold the frontal array 22 at a temperature sufficiently low to condense out water vapor, the first stage of the cold finger must operate at between 90° and 110°K. The radiation shield 20 and the frontal array serve as the first temperature stage.
The cold end 24 of the second stage 26 of the cold finger is mounted to a heat sink 28. The heat sink 28 comprises a disk 30 and a set of circular chevrons 32 mounted to the the disk 30 in a vertical array. The heat sink 28 and the vertical array of chevrons 32 form the primary pumping surface of the cryopump. Along a cylindrical surface between the chevrons of the primary pumping surface is a low temperature adsorbent 34. Preferably, the primary pumping surface forms the second temperature stage and is cooled to 10 to 14°K. The temperature of the primary pumping surface can be maintained by cooling the second stage of the cold finger to approximately 5°K and designing the heat sink 28 to use a low conductance material 30 so that the temperature differential across the heat sink 28 is approximately 9°K. The third temperature stage can be achieved by placing adsorbent 36 in thermal contact with the cold end of the second stage 26 of the cold finger. Thus, both the second and third temperature stage can be obtained from the second stage, the coldest stage, of the cold finger. Alternatively, a three-staged, closed-cycle refrigerator could be used to maintain the three temperature stages.
During operation, gases from a work chamber (not shown) enter through an opening 37 in the cryopump to the frontal array 22 where high boiling point temperature gases are condensed on the surface of the frontal array 22. Lower boiling point gases pass through that array and into a volume 38 within the radiation shield 20 where gas is condensed on the chevron surfaces 32 and adsorbed by the adsorbent 34 located on the surface between the chevrons 32. Gases having a very low boiling point, such as helium, which are not pumped by the primary pumping surface passes to the adsorbent 36 of the third temperature stage for adsorption.
In conventional cryopumps, the design of the cryopump conforms with conventional theory where it is believed that the colder the adsorbent surface the more gas that adsorbent would adsorb. In the described embodiments, the adsorbent along both the second and third temperature surfaces are operated at different temperatures. The adsorbent on the second temperature stage, the warmer of the two, allows gas which would otherwise be immobile on the third temperature stage to be adsorbed effectively along the entire surface area, including the wells of the adsorbent. As a result, more gas is adsorbed per surface area at the second temperature stage than conventional cryopumps because the pores and wells of the adsorbent are not blocked with immobilized gas molecules. Gases with very low critical mobility temperatures are instead pumped at the third temperature stage.
A preferred embodiment of the invention is illustrated in Figure 3. This embodiment utilizes a heater 40 which extends through the housing 14 and shield 20. A first heating element 42 contacts the first stage and a second heating element 44 contacts the second stage heat sink 28. The heating element 44 is used to adjust the temperature of the primary pumping surface so that there is optimal cryosorption of the gas being pumped on that surface. This embodiment uses a high conductance material such as copper for the member 30. The present embodiment uses the heating element 44 contacting the second stage heat sink to maintain the primary pumping surface at the optimal temperature T_o.
A temperature measuring device 46 such as a thermistor or thermocouple, is located on the cold end 24 to monitor the temperature of the primary pumping surface. The temperature measured by the monitor 46 can be used to automatically adjust the heating element 44 to maintain the predetermined temperature T_o. The control circuit 50 provides a signal to the heater 40 based on the sensed temperature.
The heating system 40 can also be of the type described in US-A-4,679,401 wherein refrigerant gas of the refrigerator R is diverted to heat exchangers associated with the first and second stages of a cryopump. This heating system is also used to prevent cross-over hangup and provide a more efficient regeneration procedure. Alternatively, a further embodiment of the invention utilizes three temperature stages with the temperature control system mounted on the second stage. This embodiment utilizes active control of the second stage temperature instead of a low conductance material for the member 30 to control the second stage temperature.
Also a three-staged, closed-cycle refrigerator could be used to maintain the three temperature stages, and separate refrigerators may be used to maintain the different temperature stages.

Claims

A cryopump having a pumping operation in which gas is pumped from a work chamber and a regeneration cycle, comprising:
a first temperature stage (20,22) having a pumping surface for condensing gases,
a second temperature stage (30) having a second pumping surface with a first adsorbent (34) thereon, which is cooled to a lower temperature than the first temperature stage, for pumping gases not pumped by the first pumping surface, and,
a temperature control system having a heating element (44) in thermally conductive contact with the second temperature stage (30),
characterised by a third temperature stage having a second adsorbent surface (36) thereon, and
in that the temperature control system is adapted to heat the second temperature stage during the pumping of gases from a work chamber by the second pumping surface, so as to maintain the temperature of the second temperature stage (30) at a sufficiently low pumping temperature to pump gases not pumped by the first pumping surface yet above a minimum temperature below which the capacity of the first adsorbent, in pumping the gas to be pumped by the first adsorbent during the pumping operation, is substantially reduced.
A cryopump as claimed in claim 1 further characterized in that another heating element (42) is in thermal contact with the first temperature stage (20,22).
A cryopump as claimed in claim 1 or 2 wherein:
the first temperature stage (20,22) is cooled to approximately 90-140⁰K;
the second temperature stage (30) is cooled to approximately 10-14°K; and
the third temperature stage is cooled to approximately 5°K.
A cryopump as claimed in any one of claims 1 to 3 further characterized in that the second and third temperature stages are separated by a thermal insulator to maintain the first adsorbent (34) above the minimum temperature.
A cryopump as claimed in any one of claims 1 to 4 further characterized in that it has a condensing surface on the second pumping surface for condensing lower condensation temperature gases not condensed on the first condensing surface and the first adsorbent adsorbs gases having lower condensation temperatures not condensed on the second condensing surface.
A cryopump as claimed in claim 1 further characterized in that a temperature detector (46) is provided for detecting the temperature of the second stage (30), the temperature control system responding to the detected temperature to maintain the temperature of the second stage (30) at a predetermined level during pumping by the second stage (30).
A cryopump as claimed in claim 6 characterized in that the primary gas to be pumped is hydrogen, and the predetermined level at which the second stage is maintained is above 10°K.
A cryopump as claimed in claim 7 further characterized in that the temperature control system maintains the temperature of the second stage (30) with an electric heater (40).
A cryopump as claimed in any preceding claim further comprising a two-stage cold finger refrigerator having a first stage (18) and a second stage (26) wherein the first temperature stage (20,22) is cooled by the first stage of the refrigerator and, the second and third temperature stages (30) are cooled by the second stage of the refrigerator.
A cryopump as claimed in claim 9 wherein the refrigerator is a closed-cycle refrigerator.
A cryopump as claimed in any preceding claim wherein the third temperature stage is surrounded by and spaced from the second temperature stage, which is surrounded by and spaced from the first temperature stage.
A cryopump as claimed in claim 1 characterized in that separate refrigerators cool the first and second temperature stages of the cryopump.
A method of adsorbing gases in a cryopump having a pumping operation in which gas is pumped from a work chamber and a regeneration cycle, comprising the steps of:
cooling a first temperature stage (20,22) of the cryopump having a first condensing surface for condensing gases;
cooling a second temperature stage (30) of the cryopump by adsorbing gases with a second temperature stage adsorbent (34) mounted thereon; characterized by controlling the temperature of the second temperature stage during adsorption with a temperature control system such that the temperature of the second stage is maintained, during the pumping operation of pumping gases from a work chamber by the second stage adsorbent (34), at a sufficiently low pumping temperature to pump gases not pumped by the first condensing surface yet above a minimum temperature below which the capacity of the second temperature stage adsorbent (34) for adsorbing gases not condensed on the first temperature stage is substantially reduced.
A method of adsorbing gases in a cryopump as claimed in claim 13, further characterized by cooling a third temperature stage of the cryopump having a second adsorbent surface (36) for adsorbing gases not adsorbed by the second stage (30).
A method of adsorbing gases in a cryopump as claimed in claim 14 wherein:
the first temperature stage (20,22) is cooled to approximately 90-140°K;
the second temperature stage (30) is cooled to approximately 10-14°K; and
the third temperature stage is cooled to approximately 5°K.
A method of adsorbing gases in a cryopump as claimed in claim 14 wherein the second temperature stage (30) of the cryopump has a second condensing surface provided with the adsorbent (34) for condensing and adsorbing gases of lower condensation temperature than are condensed on the first stage; and characterized by
cooling a third temperature stage of the cryopump having a second adsorbent surface (32) for adsorbing gases not adsorbed by the second stage (30).
A method of adsorbing gases in a cryopump as claimed in any one of claims 13 to 16 further characterized by detecting the temperature of the second stage (30) and responding to the detected temperature to maintain the temperature of the second stage (3) at a predetermined level during the pumping operation by the first (20,22) and second (30) stages.
A method of adsorbing gases in a cryopump as claimed in claim 17, wherein the predetermined temperature is a critical temperature below which the capacity of the adsorbent, in pumping the principal gas to be pumped by the second stage adsorbent, is substantially reduced.
A method of adsorbing gases in a cryopump as claimed in any one of claims 13 to 18 wherein the primary gas to be pumped is hydrogen, and the predetermined level at which the second stage (30) is maintained above about 10K.