Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
  • F04B—POSITIVE-DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS
  • F04B37/00—Pumps having pertinent characteristics not provided for in, or of interest apart from, groups F04B25/00 - F04B35/00
  • F04B37/06—Pumps having pertinent characteristics not provided for in, or of interest apart from, groups F04B25/00 - F04B35/00 for evacuating by thermal means
  • F04B37/08—Pumps having pertinent characteristics not provided for in, or of interest apart from, groups F04B25/00 - F04B35/00 for evacuating by thermal means by condensing or freezing, e.g. cryogenic pumps

Definitions

• 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.
• 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.
• high boiling point gases such as water vapor
• 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.
• the present invention relates to a cryopump having different temperature * stages for effectively pumping gases.
• 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.
• the adsorbent surface is not effectively utilized for pumping gases because as the adsorbent is cooled to a temperature for adsorbing gases
• invention over conventional cryopumps is that internal surfaces of the pores and wells are not blocked at their entrances.
• the 5 second stage temperature must be at or below 14°K.
• the temperature of the second stage is maintained at the optimal temperature for the agses and load conditions that are present.
• the cryopump has three different temperature stages: a first temperature stage for 5 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 0 by the first two temperature stages.
• a first temperature stage for 5 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
• a third temperature stage 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 embodiment of the invention utilizes a second stage temperature control system during pump operation to obtain optimal cryosorption of the gas being pumped at the second stage_
• the second stage temperature can be adjusted to maintain the second stage temperature at the optimal level.
• the temperature at which this optimum occurs is generally between 10 and 14° depending upon the specific H- 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.
• Fig. 1 is a view illustrating a magnified partial cross sectional surface of charcoal.
• Fig. 2 is a cryopump embodying the present invention 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.
• 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 consid- erations 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.
• FIG. 1 a magnified view of the surface area of charcoal is illustrated in figure 1.
• 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.
• noncodensibles such as helium, neon, and hydrogen have critical mobility temperatures when adsorbed on charcoal.
• 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
• hydrogen has been found to have a critical mobility temperature of about 13"K.
• 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.
• an optimal cryopump can be constructed having three temperature stages: a first stage to pump gases which freeze readily at temperatures of approximately 100 , such as water; a second stage to effectively pump gases which freeze readily at temperatures of approximately 15° , 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.
• the first stage temperature is cooled to 70 to 140°K
• the second stage temperature is cooled to 10 to 1 "
• the third stage temperature is cooled to approximately 5° .
• a three temperature stage cryopump can be constructed in a variety of ways.
• a two-staged, cold finger of a closed-cycle refrigerator R extends into a housing 14 of a conventional cryopump through an opening 16.
• the refrigerator is a Gifford-MacMahon refrigerator but other refrigerators may be used.
• 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 U.S. Patent No. 3,218,815 to Chellis et al.
• the first stage 18 of the cold finger is mounted to a radiation shield 20 which is coupled to a frontal array 22.
• 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.
• the first stage of the cold finger 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.
• a low temperature adsorbent 34 is a low temperature adsorbent 34.
• 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° .
• 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.
• both the second and third temperature stage can be obtained from the second stage, the coldest stage, of the cold finger.
• a three-staged, closed-cycle refrigerator could be used to maintain the three temperature stages.
• gases from a work chamber 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.
• 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.
• 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.
• 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.
• FIG. 3 Another 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 conducane 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 .
• 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 .
• 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 U.S. Patent No. 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.
• a third 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
• a three-staged, closed-cycle refrigerator could be used to maintain the three temperature stages.
• separate refrigerators may be used to maintain the different temperature stages.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Compressors, Vaccum Pumps And Other Relevant Systems (AREA)

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• 1988
  • 1988-01-27 WO PCT/US1988/000225 patent/WO1988005500A1/en not_active Ceased
  • 1988-01-27 EP EP88902995A patent/EP0349577B1/de not_active Expired - Lifetime
  • 1988-01-27 JP JP63502773A patent/JP2597696B2/ja not_active Expired - Lifetime
  • 1988-01-27 CA CA000557461A patent/CA1315111C/en not_active Expired - Fee Related
  • 1988-01-27 DE DE3852303T patent/DE3852303T2/de not_active Expired - Fee Related

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