Patent Publication Number: US-11644024-B2

Title: Cryopump

Description:
RELATED APPLICATIONS 
     The contents of Japanese Patent Application No. 2017-020601, and of International Patent Application No. PCT/JP2018/003572, on the basis of each of which priority benefits are claimed in an accompanying application data sheet, are in their entirety incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     Certain embodiment of the present invention relates to a cryopump. 
     Description of Related Art 
     A cryopump is a vacuum pump which condenses and adsorbs gas molecules on a cryopanel cooled to a cryogenic temperature to capture and exhaust the gas molecules. In general, the cryopump is used to realize a clean vacuum environment which is required in a semiconductor circuit manufacturing process or the like. For example, in one of applications of the cryopump like an ion implantation process, most of gases to be exhausted may be a non-condensable gas such as hydrogen. The non-condensable gas can be exhausted by being adsorbed to an adsorption region cooled to a cryogenic temperature. 
     SUMMARY 
     According to an embodiment of the present invention, there is provided a cryopump including: a cryocooler which includes a high-temperature cooling stage and a low-temperature cooling stage; a radiation shield which is thermally coupled to the high-temperature cooling stage and axially extends in a tubular shape from a cryopump intake port; and a low-temperature cryopanel section which is thermally coupled to the low-temperature cooling stage, is surrounded by the radiation shield, and includes a plurality of cryopanels and a plurality of heat transfer bodies axially arranged in columnar shape, and in which the plurality of cryopanels and the plurality of heat transfer bodies are axially stacked. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a view schematically showing a cryopump according to an embodiment. 
         FIG.  2    is a perspective view schematically showing an upper cryopanel of a second stage cryopanel assembly according to the embodiment. 
         FIG.  3    is a top view schematically showing a lower cryopanel of the second stage cryopanel assembly according to the embodiment. 
         FIG.  4    is a sectional view schematically showing an upper structure of the second stage cryopanel assembly according to the embodiment. 
         FIG.  5    is an exploded perspective view schematically showing upper structure of the second stage cryopanel assembly according to the embodiment. 
         FIG.  6    is a top view schematically showing another example of the upper cryopanel of the second stage cryopanel assembly according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     It is desirable to improve exhaust performance of a cryopump. 
     In addition, arbitrary combinations of the above-described components, or components or expression of the present invention may be replaced by each other in methods, devices, systems, or the like, and these replacements are also included in aspects of the present invention. 
     According to the present invention, it is possible to improve exhaust performance of a cryopump. 
     Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In descriptions and drawings, the same or equivalent components, members, and processes are denoted by the same reference numerals, and repeated descriptions thereof will be appropriately omitted. Scales and shapes of shown parts are set conveniently for ease of explanation, and are not to be interpreted as being limited unless otherwise noted. The embodiment is illustrative and do not limit the scope of the present invention. All features or combinations thereof described in the embodiment are not necessarily essential to the invention. 
     In general, a cryopump includes a high-temperature cryopanel section which is cooled by a high-temperature cooling stage of a cryocooler and a low-temperature cryopanel section which is cooled by a low-temperature cooling stage of the cryocooler. The high-temperature cryopanel section is provided to protect the low-temperature cryopanel section from radiant heat. The low-temperature cryopanel section includes a plurality of cryopanels, and the plurality of cryopanels are attached to the low-temperature cooling stage via an attachment structure. 
     As a result of intensive studies on a cryopump, the present inventors have come to recognize the following problems. In most cryopumps, the high-temperature cryopanel section and the low-temperature cryopanel section are designed based on the axisymmetric shapes such as a disk, a cylinder, and a cone. Nevertheless, the cryopanel attachment structure is based on non-axisymmetric shapes such as rectangles and cuboids. This cause limitation on simplification and miniaturization of the attachment structure. If the attachment structure has a complicated shape and a size thereof increases, a space for disposing the cryopanel will be cut accordingly. As a result, the cryopanel area is reduced and exhaust performance (for example, storage capacity of non-condensable gas, exhaust speed) of the cryopump decreases. Therefore, there is a room for improvement in a design of the existing cryopanel attachment structure in order to improve the exhaust performance. 
       FIG.  1    schematically shows a cryopump  10  according to an embodiment.  FIG.  2    is a perspective view schematically showing an upper cryopanel of a second stage cryopanel assembly according to the embodiment.  FIG.  3    is a top view schematically showing a lower cryopanel of the second stage cryopanel assembly according to the embodiment. 
     For example, the cryopump  10  is attached to a vacuum chamber of an ion implanter, a sputtering apparatus, vapor deposition apparatus, or other vacuum processing apparatus, and is used to increase a degree of vacuum inside the vacuum chamber to the level required for a desired vacuum process. The cryopump  10  has a cryopump intake port (hereinafter, simply referred to as an “intake port”)  12  for receiving a gas to be exhausted from the vacuum chamber. The gas enters an internal space  14  of the cryopump  10  through the intake port  12 . 
     In addition, hereinafter, terms such as an “axial direction” and a “radial direction” are used to easily indicate positional relationships of components of the cryopump  10 . The axial direction of the cryopump  10  indicates a direction (a direction along a center axis C in the drawings) passing through the intake port  12 , and the radial direction indicates a direction (a direction perpendicular to the center axis C) along the intake port  12 . For convenience, a side relatively close to the intake port  12  in the axial direction may be referred to as an “upper side”, and a side relatively far from the intake port  12  may be referred to as a “lower side”. That is, a side relatively far from a bottom section of the cryopump  10  may be referred to as the “upper side”, and a side relatively close to the bottom section may be referred to as the “lower side”. A side close to a center (the center axis C in the drawings) of the intake port  12  in the radial direction may be referred to as an “inner side”, and a side close to a peripheral edge of the intake port  12  may be referred to as an “outer side”. In addition, the above-described expressions are not related to the disposition of the cryopump  10  when the cryopump  10  is attached to the vacuum chamber. For example, the cryopump  10  may be attached to the vacuum chamber in a state where the intake port  12  is positioned downward in a vertical direction. 
     In addition, a direction surrounding the axial direction may be referred to a “circumferential direction”. The circumferential direction is a second direction along the intake port  12  and is a tangential direction orthogonal to the radial direction. 
     The cryopump  10  includes a cryocooler  16 , a first stage cryopanel  18 , a second stage cryopanel assembly  20 , and a cryopump housing  70 . The first stage cryopanel  18  may be referred to as a high-temperature cryopanel section or a 100K section. The second stage cryopanel assembly  20  may be referred to as a low-temperature cryopanel section or a 10K section. 
     For example, the cryocooler  16  is a cryocooler such as a Gifford McMahon type cryocooler (so-called GM cryocooler). The cryocooler  16  is a two-stage cryocooler. Accordingly, the cryocooler  16  includes a first cooling stage  22  and a second cooling stage  24 . The cryocooler  16  is configured so as to cool the first cooling stage  22  to a first cooling temperature and cool the second cooling stage  24  to a second cooling temperature. The second cooling temperature is lower than the first cooling temperature. For example, the first cooling stage  22  is cooled to approximately 65K to 120K, preferably, 80K to 100K, and the second cooling stage  24  is cooled to approximately 10K to 20K. 
     In addition, the cryocooler  16  includes a cryocooler structural section  21  which structurally supports the second cooling stage  24  to the first cooling stage  22  and structurally supports the first cooling stage  22  to a room-temperature section  26  of the cryocooler  16 . Accordingly, the cryocooler structural section  21  includes a first cylinder  23  and a second cylinder  25  which coaxially extend in the radial direction. The first cylinder  23  connects the room-temperature section  26  of the cryocooler  16  to the first cooling stage  22 . The second cylinder  25  connects the first cooling stage  22  to the second cooling stage  24 . The room-temperature section  26 , the first cylinder  23 , the first cooling stage  22 , the second cylinder  25 , and the second cooling stage  24  are linearly arranged in this order. 
     A first displacer (not shown) and a second displacer (not shown) are respectively disposed inside the first cylinder  23  and the second cylinder  25  so as to be reciprocated. A first regenerator and a second regenerator (not shown) are respectively incorporated into the first displacer and the second displacer. Moreover, the room-temperature section  26  includes a drive mechanism (not shown) for reciprocating the first displacer and the second displacer. The drive mechanism includes a flow path switching mechanism which switches a flow path of a working gas (for example, helium) such that the working gas is repeatedly supplied to or discharged from the inside of the cryocooler  16  periodically. 
     The cryocooler  16  is connected to a compressor (not shown) of the working gas. The cryocooler  16  expands the working gas compressed by the compressor inside the cryocooler  16  to cool the first cooling stage  22  and the second cooling stage  24 . The expanded working gas is recovered to the compressor so as to be compressed again. The cryocooler  16  repeats a thermal cycle which includes supplying and discharging of the working gas and reciprocations of the first displacer and the second displacer synchronized with the supplying and the discharging, and generates chill. 
     The shown cryopump  10  is a so-called horizontal cryopump. In general, the horizontal cryopump is a cryopump in which the cryocooler  16  is disposed to intersect (generally, to be orthogonal to) the center axis C of the cryopump  10 . 
     The first stage cryopanel  18  includes a radiation shield  30  and an inlet cryopanel  32 , and encloses the second stage cryopanel assembly  20 . The first stage cryopanel  18  provides a cryogenic surface to protect the second stage cryopanel assembly  20  from radiant heat from the outside of the cryopump  10  or the cryopump housing  70 . The first stage cryopanel  18  is thermally coupled to the first cooling stage  22 . Accordingly, the first stage cryopanel  18  is cooled to the first cooling temperature. The first stage cryopanel  18  has a gap between the first stage cryopanel  18  and the second stage cryopanel assembly  20 , and the first stage cryopanel  18  is not in contact with the second stage cryopanel assembly  20 . The first stage cryopanel  18  is not in contact with the cryopump housing  70 . 
     The radiation shield  30  is provided to protect the second stage cryopanel assembly  20  from the radiant heat of the cryopump housing  70 . The radiation shield  30  extends in a tubular shape (for example, a cylindrical shape) in the axial direction from the intake port  12 . The radiation shield  30  is positioned between the cryopump housing  70  and the second stage cryopanel assembly  20 , and surrounds the second stage cryopanel assembly  20 . The radiation shield  30  includes a shield main opening  34  for receiving a gas from the outside of the cryopump  10  to the internal space  14 . The shield main opening  34  is positioned at the intake port  12 . 
     The radiation shield  30  includes a shield front end  36  which defines the shield main opening  34 , a shield bottom section  38  which is positioned on a side opposite to the shield main opening  34 , and a shield side section  40  which connects the shield front end  36  to the shield bottom section  38 . The shield side section  40  extends from the shield front end  36  to the side opposite to the shield main opening  34  in the axial direction, and extends to surround the second cooling stage  24  in the circumferential direction. 
     The shield side section  40  includes a shield side section opening  44  through which the cryocooler structural section  21  is inserted. The second cooling stage  24  and the second cylinder  25  are inserted from the outside of the radiation shield  30  into the radiation shield  30  through the shield side section opening  44 . The shield side section opening  44  is an attachment hole which is formed on the shield side section  40 , and, for example, has a circular shape. The first cooling stage  22  is disposed outside the radiation shield  30 . 
     The shield side section  40  includes an attachment pedestal  46  of the cryocooler  16 . The attachment pedestal  46  is a flat portion for attaching the first cooling stage  22  to the radiation shield  30 , and is slightly recessed when viewed from the outside of the radiation shield  30 . The attachment pedestal  46  forms the outer periphery of the shield side section opening  44 . The first cooling stage  22  is attached to the attachment pedestal  46 . Therefore, the radiation shield  30  is thermally coupled to the first cooling stage  22 . 
     Instead of the radiation shield  30  being directly attached to the first cooling stage  22 , in an embodiment, the radiation shield  30  maybe thermally coupled to the first cooling stage  22  via an additional heat transfer member. For example, the heat transfer member may be a short hollow tube having flanges on both ends. The heat transfer member may be fixed to the attachment pedestal  46  by one end flange, and may be fixed to the first cooling stage  22  by the other end flange. The heat transfer member may surround the cryocooler structural section  21  and may extend from the first cooling stage  22  to the radiation shield  30 . The shield side section  40  may include the heat transfer member. 
     In the shown embodiment, the radiation shield  30  has an integral tubular shape. Instead of this, the radiation shield  30  may have the entire tubular shape including a plurality of parts. The plurality of parts may be disposed to have gaps to each other. For example, the radiation shield  30  may be divided into two portions in the axial direction. In this case, the upper portion of the radiation shield  30  is a tube having both open ends, and includes the shield front end  36  and a first section of the shield side section  40 . The lower portion of the radiation shield  30  also is a tube having both open ends, and includes a second section of the shield side section  40  and the shield bottom section  38 . A slit is formed, which extends in the circumferential direction between the first section and the second section of the shield side section  40 . The slit may form at least a portion of the shield side section opening  44 . Alternatively, the upper half of the shield side section opening  44  may be formed on the first section of the shield side section  40 , and the lower half thereof maybe formed on the second section of the shield side section  40 . 
     The radiation shield  30  defines a gas accommodation space  50  which surrounds the second stage cryopanel assembly  20  between the intake port  12  and the shield bottom section  38 . The gas accommodation space  50  is a portion of the internal space  14  of the cryopump  10 , and is a region adjacent to the second stage cryopanel assembly  20  in the radial direction. 
     The inlet cryopanel  32  is provided in the intake port  12  (or, the shield main opening  34 , and so on) to protect the second stage cryopanel assembly  20  from radiant heat from an external heat source (for example, a heat source in the vacuum chamber to which the cryopump  10  is attached) of the cryopump  10 . In addition, a gas (for example, water) condensed at the cooling temperature of the inlet cryopanel  32  is captured on the surface. 
     The inlet cryopanel  32  is disposed at a location corresponding to the second stage cryopanel assembly  20  in the intake port  12 . The inlet cryopanel  32  occupies the center portion of an opening area of the intake port  12  and forms an annular opening region  51  between the inlet cryopanel  32  and the radiation shield  30 . When viewed in the axial direction, a shape of the inlet cryopanel  32  is a disk shape. The inlet cryopanel  32  may occupy at most ⅓, or at most ¼ of the opening area of the intake port  12 . Accordingly, the opening region  51  may occupy at least ⅔, or at least ¾ of the opening area of the intake port  12 . The opening region  51  is positioned at a location corresponding to the gas accommodation space  50  in the intake port  12 . The opening region  51  is an inlet of the gas accommodation space  50 , and the cryopump  10  receives gas into the gas accommodation space  50  through the opening region  51 . 
     The inlet cryopanel  32  is attached to the shield front end  36  via an inlet cryopanel attachment member  33 . The inlet cryopanel attachment member  33  is a linear (or cruciform) member bridged to the shield front end  36  along a diameter of the shield main opening  34 . Thus, the inlet cryopanel  32  is fixed to the radiation shield  30  and is thermally coupled to the radiation shield  30 . The inlet cryopanel  32  is close to but not in contact with the second stage cryopanel assembly  20 . 
     The second stage cryopanel assembly  20  is provided at a center portion of the internal space  14  of the cryopump  10 . The second stage cryopanel assembly  20  includes an upper structure  20   a  and a lower structure  20   b . The second stage cryopanel assembly  20  comprises a plurality of cryopanels  60  arranged in the axial direction. The plurality of cryopanels  60  are arranged at intervals in the axial direction. 
     The upper structure  20   a  of the second stage cryopanel assembly  20  includes a plurality of upper cryopanels  60   a  and a plurality of heat transfer bodies (also referred to as heat transfer spacers)  62 . The plurality of heat transfer bodies  62  are arranged in a columnar shape in the axial direction. The plurality of upper cryopanels  60   a  and the plurality of heat transfer bodies  62  are stacked in the axial direction between the intake port  12  and the second cooling stage  24 . Accordingly, the upper structure  20   a  is disposed axially above the second cooling stage  24 . The upper structure  20   a  is fixed to the second cooling stage  24  via a heat transfer block  63  and is thermally coupled to the second cooling stage  24 . Therefore, the upper structure  20   a  is cooled to the second cooling temperature. 
     The lower structure  20   b  of the second stage cryopanel assembly  20  includes a plurality of lower cryopanels  60   b  and a second stage panel attachment member  64 . The second stage panel attachment member  64  extends axially downward from the second cooling stage  24 . The plurality of lower cryopanels  60   b  are attached to the second cooling stage  24  via the second stage panel attachment member  64 . Accordingly, the lower structure  20   b  is thermally coupled to the second cooling stage  24  and is cooled to the second cooling temperature. 
     An adsorption region  66  is formed on a surface of at least a portion of the second stage cryopanel assembly  20 . The adsorption region  66  is provided to capture a non-condensable gas (for example, hydrogen) by adsorbing. For example, the adsorption region  66  is formed by adhering an adsorption material (for example, activated carbon) to a cryopanel surface. The adsorption region  66  may be formed at a shadowed position of the cryopanel  60  adjacent above so as not to be seen from the intake port  12 . For example, the adsorption region  66  is formed on the entire region of a lower surface (rear surface) of the cryopanel  60 . The adsorption region  66  maybe formed on an upper surface and/or a lower surface of the upper cryopanel  60   a . The adsorption region  66  may be formed on an upper surface and/or a lower surface of the lower cryopanel  60   b.    
     In addition, a condensation region for capturing a condensable gas by condensation is formed on a surface of at least a portion of the second stage cryopanel assembly  20 . For example, the condensation region is a missing region of the adsorption material on the cryopanel surface, and a cryopanel substrate surface, for example, a metal surface is exposed to the condensation region. An upper surface outer peripheral section of the cryopanel  60  (for example, upper cryopanel  60   a ) may be the condensation region. 
     As shown in  FIGS.  1  and  2   , the upper cryopanel  60   a  has an inverted truncated cone shape and is disposed to be circular when viewed in the axial direction. A center of upper cryopanel  60   a  is positioned on the center axis C. The upper cryopanel  60   a  can have a mortar shape, a bowl shape, or a ball shape. The upper cryopanel  60   a  has a large dimension (that is, has a large diameter) at an upper end portion  74  and has a smaller dimension (that is, has a smaller diameter) at a lower end portion  76 . The upper cryopanel  60   a  includes an inclined region  78  which connects the upper end portion  74  and the lower end portion  76  to each other. The inclined region  78  corresponds to a side surface of the inverted truncated cone. Accordingly, the upper cryopanel  60   a  is inclined such that a normal of an upper surface of the upper cryopanel  60   a  intersects the center axis C. The upper cryopanel  60   a  has a plurality of through-holes  80  at the lower end portion  76 . The through-hole  80  is provided to attach the upper cryopanel  60   a  to the heat transfer body  62  (or heat transfer block  63 ). 
     A first upper cryopanel  60   a  has a smallest diameter. The first upper cryopanel  60   a  is positioned axially uppermost and closest to the inlet cryopanel  32 . A second upper cryopanel  60   a  has a diameter slightly larger than that of the first upper cryopanel  60   a . The same applies to third, fourth, and fifth upper cryopanels  60   a . An upper cryopanel  60   a  positioned below has a diameter slightly larger than that of the upper cryopanel  60   a  above adjacent to the upper cryopanel  60   a  positioned below. 
     The inclined regions  78  of the first and second upper cryopanels  60   a  are parallel to each other. In addition, the inclined regions  78  of the third to fifth upper cryopanels  60   a  are parallel to each other. An inclination angle of the first upper cryopanel  60   a  is smaller than an inclination angle of the third upper cryopanel  60   a . The third, fourth, and fifth upper cryopanels  60   a  are disposed in a nested manner. A lower portion of an upper cryopanel  60   a  positioned above is inserted into an upper cryopanel  60   a  below adjacent to the upper cryopanel  60   a  positioned above. 
     Further details of the upper structure  20   a  will be described later. In addition, a specific configuration of the upper structure  20   a  is not limited to the above. For example, the upper structure  20   a  may have any number of upper cryopanels  60   a . The upper cryopanel  60   a  may have a flat plate, a conical shape, or other shapes. For example, the first upper cryopanel  60   a  may be a flat plate, for example, a disk. 
     As shown in  FIG.  3   , the lower cryopanel  60   b  is a flat plate, for example, a disk. The lower cryopanel  60   b  has a diameter larger than that of the upper cryopanel  60   a . However, in order to attach the lower cryopanel  60   b  to the second stage panel attachment member  64 , in the lower cryopanel  60   b , a cut-out portion  82  is formed from a portion of an outer periphery toward a center portion. In addition, similarly to the upper cryopanel  60   a , the lower cryopanel  60   b  may have an inverted truncated cone shape, a conical shape, or other shapes. 
     Unlike the lower cryopanel  60   b , the upper cryopanel  60   a  does not have the cut-out portion  82 . Accordingly, the upper cryopanel  60   a  can take a more effective cryopanel area (that is, the adsorption region  66  and/or the condensation region). 
     In the adsorption region  66 , many activated carbon particles are adhered in an irregular arrangement in a state of being closely arranged on the surface of the cryopanel  60 . For example, the activated carbon particles are formed in a cylindrical shape. In addition, the shape of the adsorption material may not be a cylindrical shape, and for example, may be a spherical shape, other formed shapes, or an irregular shape. An arrangement on an adsorption material panel may be a regular arrangement or an irregular arrangement. 
     The cryopump housing  70  is a case of the cryopump  10  which accommodates the first stage cryopanel  18 , the second stage cryopanel assembly  20 , and the cryocooler  16 , and is a vacuum vessel which is configured so as to hold vacuum sealing of the internal space  14 . The cryopump housing  70  includes the first stage cryopanel  18  and the cryocooler structural section  21  in a non-contact manner. The cryopump housing  70  is attached to the room-temperature section  26  of the cryocooler  16 . 
     The intake port  12  is defined by a front end of the cryopump housing  70 . The cryopump housing  70  includes an intake port flange  72  which extends radially outward from the front end. The intake port flange  72  is provided over the entire periphery of the cryopump housing  70 . The cryopump  10  is attached to the vacuum chamber of an evacuation object using the intake port flange  72 . 
     Hereinafter, an operation of the cryopump  10  having the above-described configuration will be described. When the cryopump  10  is operated, first, a pressure inside the vacuum chamber is roughly set to approximately 1 Pa by other appropriate roughing pumps before the cryopump  10  is operated. Thereafter, the cryopump  10  is operated. The first cooling stage  22  and the second cooling stage  24  are respectively cooled to the first cooling temperature and the second cooling temperature by driving of the cryocooler  16 . Accordingly, the first stage cryopanel  18  and the second stage cryopanel assembly  20 , which are thermally coupled to the first cooling stage  22  and the second cooling stage  24 , are respectively cooled to the first cooling temperature and the second cooling temperature. 
     The inlet cryopanel  32  cools gas flying from the vacuum chamber toward cryopump  10 . Gas is condensed so as to have a sufficiently low vapor pressure (for example, 10 −8  Pa or less) at the first cooling temperature on the surface of the inlet cryopanel  32 . This gas may be referred to as a first kind of gas. For example, the first kind of gas is water vapor. In this way, the inlet cryopanel  32  through which the first kind of gas can be exhausted. A portion of gas having a vapor pressure which is not sufficiently low at the first cooling temperature can enter the internal space  14  from the intake port  12 . Alternatively, the other portion of the gas is reflected by the inlet cryopanel  32 , and does not enter the internal space  14 . 
     The gas entering internal space  14  is cooled by the second stage cryopanel assembly  20 . Gas having a sufficiently low vapor pressure (for example, 10 −8  Pa or less) at the second cooling temperature is condensed on the surface of the second stage cryopanel assembly  20 . This gas may be referred to as a second kind of gas. For example, the second kind of gas is argon. In this way, the second stage cryopanel assembly  20  can exhaust the second kind of gas. 
     Gas having a vapor pressure which is not sufficiently low at the second cooling temperature is adsorbed to the adsorption material of the second stage cryopanel assembly  20 . This gas maybe referred to as a third kind of gas. For example, the third kind of gas is hydrogen. In this way, the second stage cryopanel assembly  20  can exhaust the third kind of gas. Accordingly, the cryopump  10  exhausts various gas by condensation and adsorption, and a vacuum degree of the vacuum chamber can reach a desired level. 
     Next, the upper structure  20   a  of the second stage cryopanel assembly  20  according to the embodiment will be described in more detail.  FIG.  4    is a sectional view schematically showing an upper structure  20   a  of the second stage cryopanel assembly  20  according to the embodiment.  FIG.  5    is an exploded perspective view schematically showing upper structure  20   a  of the second stage cryopanel assembly  20  according to the embodiment. 
     As described above, the upper structure  20   a  of the second stage cryopanel assembly  20  includes the plurality of upper cryopanels  60   a  and the plurality of heat transfer bodies  62 . The plurality of heat transfer bodies  62  are axially arranged in a columnar shape. A second stage cryopanel support structure according to the embodiment includes the plurality of heat transfer bodies  62  and includes a cryopanel support column supporting the plurality of upper cryopanels  60   a . The upper structure  20   a  is configured in axial symmetry with respect to the center axis C. 
     The plurality of upper cryopanels  60   a  and the plurality of heat transfer bodies  62  are stacked in the axial direction. The plurality of upper cryopanels  60   a  and the plurality of heat transfer bodies  62  are stacked in the axial direction such that at least one heat transfer body  62  is positioned between two upper cryopanels  60   a  adjacent to each other. The plurality of upper cryopanels  60   a  and the plurality of heat transfer bodies  62  are alternately stacked in the axial direction. The stacked configuration has an advantage of facilitating an assembly operation. In addition, it is also to adjust the number of upper cryopanels  60   a  mounted on the cryopump  10  (only by changing the number of stacked cryopanels). 
     Each heat transfer body  62  has a columnar shape. The heat transfer body  62  has a relatively short columnar shape and an axial height of the heat transfer body  62  is smaller than a diameter of the heat transfer body  62 . 
     The plurality of heat transfer bodies  62  are arranged in a columnar shape in the axial direction, and each of the plurality of heat transfer bodies  62  has a circular end surface. Accordingly, a cross-sectional area (a cross section perpendicular to the axial direction) of the heat transfer body  62  can be made relatively large while a dimension (for example, a radius) of the heat transfer body  62  can be made relatively small. If the dimension of the heat transfer body  62  decreases, it is possible to increase an area of the adsorption region  66  (and/or the condensation region), which improve the exhaust performance of the cryopump  10 . If the cross-sectional area increases, it is possible to increase an amount of heat transfer in the axial direction. Accordingly, it is possible to decrease a cooling time of the plurality of heat transfer bodies  62  and the upper structure  20   a  of the second stage cryopanel assembly  20 . 
     An axial height of the heat transfer body  62  defines an axial distance between two adjacent upper cryopanels  60   a . By decreasing the axial height of the heat transfer body  62 , the upper cryopanel  60   a  can be densely arranged. As described above, even if the heat transfer body  62  is thinned in the axial direction, a cross-sectional area (a cross section perpendicular to the axial direction) of the heat transfer body  62  is maintained, and thus, the amount of heat transfer of the heat transfer body  62  is significantly not affected. 
     The upper cryopanel  60   a  includes a center disk (that is, lower end portion  76 ) having a size corresponding to the circular end surface of the heat transfer body  62  and a conical cryopanel surface (that is, the inclined region  78 ) which is inclined from the center disk toward the intake port  12 . The center disk of the upper cryopanel  60   a  becomes an attachment surface to the heat transfer body  62 . The conical cryopanel surface extends obliquely upward from an outline of the circular end surface of the heat transfer body  62 . Similarly to the heat transfer body  62 , a diameter of the center disk is relatively small, and thus, it is possible to relatively increase the conical cryopanel surface. In addition, compared to a circular shape having the same diameter as that of the conical cryopanel surface, it is possible to increase the cryopanel area. Accordingly, it is possible to increase the area of the adsorption region  66  (and/or condensation region) of the upper cryopanel  60   a.    
     An outer diameter of (circular end surface) of the heat transfer body  62  maybe smaller than ½, smaller than ⅓, smaller than ⅓, smaller than ¼ of an outer diameter of (upper end portion  74 ) of the upper cryopanel  60   a . The outer diameter of the heat transfer body  62  may be larger than 1/10 or larger than ⅕ of the outer diameter of the upper cryopanel  60   a.    
     The upper structure  20   a  of the second stage cryopanel assembly  20  includes an intervening layer  84  between the upper cryopanel  60   a  and the heat transfer body  62 . The intervening layer  84  is sandwiched between the upper cryopanel  60   a  and the heat transfer body  62  axially adjacent to each other to ensure an improved thermal contact. More specifically, the intervening layer  84  is interposed between the center disk of the upper cryopanel  60   a  and the circular end surface of the heat transfer body  62 . The intervening layer  84  is formed of a softer material than the upper cryopanel  60   a  and the heat transfer body  62 . For example, the intervening layer  84  is formed of an indium sheet (a sheet-like member formed of indium). A diameter of the intervening layer  84  may be slightly larger than the diameter of the heat transfer body  62  and may be smaller than the diameter of the center disk of the upper cryopanel  60   a.    
     The upper structure  20   a  of the second stage cryopanel assembly  20  includes a plurality of fastening members  86  which axially penetrate the plurality of upper cryopanels  60   a  and the plurality of heat transfer bodies  62 . The upper cryopanels  60   a , the heat transfer bodies  62 , and the intervening layers  84  are fixed to the heat transfer block  63  by the fastening members  86 . The upper structure  20   a  is fixed to the second cooling stage  24  by the fastening members  86 . In this way, the plurality of upper cryopanels  60   a  and the plurality of heat transfer bodies  62  can be collectively fastened and fixed to each other at one time, and thus, the manufacturing (assembly work) is easy. 
     In the shown example, three fastening members  86  are used. In the center disk of the upper cryopanel  60   a , six through-holes  80  are formed in the circumferential direction around the center. The through-holes  80  are arranged at equal angular intervals (every 60 degrees) at the same radial position. The through-holes are similarly formed in the heat transfer body  62  and the intervening layer  84 . The fastening members  86  are inserted into the through-holes  80 . For example, each fastening member  86  is a long screw and the through-hole  80  is a screw hole. For example, the fastening member  86  is formed of stainless steel. Six through-holes  80  are used every other one, and three fastening members  86  are disposed every 120°. Unused through-holes  80  helps to reduce weight of the heat transfer body  62 . 
     A center portion of the heat transfer body  62  is solid and there is not through-hole (that is, void). Therefore, the center portion of the heat transfer body  62  acts as a heat transfer path. This can also help to increase the amount of heat transfer of the heat transfer body  62 . 
     The plurality of upper cryopanels  60   a  are formed of a first material having a first thermal conductivity. The plurality of heat transfer bodies  62  are formed of a second material having a second thermal conductivity. The second thermal conductivity is smaller than the first thermal conductivity. The first material and/or the second material may be a metal material. The first material is copper (pure copper, for example, tough pitch copper). The second material is aluminum (for example, pure aluminum). 
     The first material has a first density, the second material has a second density, and the second density is smaller than the first density. 
     The upper cryopanel  60   a  may include a cryopanel substrate which is formed of the first material, and a coating layer (for example, a nickel layer) which is formed of a material different from the first material and coats the cryopanel substrate. Similarly, the heat transfer body  62  may include a main body which is formed of the second material and a coating layer (for example, a nickel layer) which is formed of a material different from the second material and coats the main body. 
     Typically, the cryopanel is formed of copper. In general, copper is one of highest thermal conductivity materials available. However, copper is relatively dense, and thus, the cryopanel tends to be heavy, and as a result, a heat capacity of the cryopanel also tends to increase. 
     In a case where the cryopanel and the heat transfer body  62  are formed of copper, a high thermal conductivity is realized, and thus, there is an advantage of cooling the upper cryopanel  60   a  to a lower temperature. Meanwhile, the upper structure  20   a  of the second stage cryopanel assembly  20  is heavy, the heat capacity is large, and as a result, it takes a relatively long time to cool the upper structure. However, in the present embodiment, as the material of the heat transfer body  62 , a metal material (for example, aluminum) having a relatively high thermal conductivity and a relatively small density although not having a thermal conductivity as high as copper can be adopted. The heat conductivity and weight reduction can be achieved, and thus, a cooling time of the heat transfer body  62  is shortened. In addition, the heat transfer body  62  may be formed of copper. 
     The plurality of upper cryopanels  60   a  have a first heat capacity, the plurality of heat transfer bodies  62  have a second heat capacity, and the second heat capacity is smaller than the first heat capacity. Here, the first heat capacity is a total heat capacity of the plurality of upper cryopanels  60   a , and the second heat capacity is a total heat capacity of the plurality of heat transfer bodies  62 . In this way, the heat transfer body  62  has a relatively small heat capacity, and thus, the heat transfer body  62  can be cooled at a relatively short time. 
     All of the plurality of heat transfer bodies  62  are formed of the same material (for example, the second material). However, this is not essential. At least a portion (that is, at least one heat transfer body  62 ) of the plurality of heat transfer bodies  62  is formed of the second material, and another portion (that is, the remaining heat transfer body  62 ) of the plurality of heat transfer bodies  62  is different from a material (that is, first material) different form the second material. In this way, the thermal conductivity of at least a portion of the plurality of heat transfer bodies  62  may be larger or smaller than the thermal conductivities of the other portions of the plurality of heat transfer bodies  62 . The density of at least a portion of the plurality of heat transfer bodies  62  may be greater or smaller than the densities of the other portions of the plurality of heat transfer bodies  62 . The heat capacity of at least a portion of the plurality of heat transfer bodies  62  may be larger or smaller than the heat capacities of the other portions of the plurality of heat transfer bodies  62 . 
     The material of the heat transfer body  62  maybe selected according to a location (for example, axial height) of the heat transfer body  62 . For example, in the plurality of heat transfer bodies  62 , one or more heat transfer bodies  62  which are disposed at a position relatively close to the low-temperature cooling stage may be formed of the first material, and one or more other heat transfer bodies  62  which are disposed at a position relatively far from the low-temperature cooling stage may be formed of the second material. In other words, in the plurality of heat transfer bodies  62 , the first heat transfer body  62  may be formed of the first material and the second heat transfer body  62  may be formed of the second material. The first heat transfer body  62  is disposed at a first axial height, the second heat transfer body  62  is disposed at a second axial height, and the first axial height may be closer to the low-temperature cooling stage than the second axial height. The first and second heat transfer bodies  62  are disposed between the cryopump intake port and the low-temperature cooling stage in the axial direction. 
     Moreover, the heat transfer block  63  may be formed of the first material. In addition, the heat transfer block  63  may be formed of the second material. 
     In the cryopump  10  according to the embodiment, the axial stacked configuration of the upper cryopanels  60   a  and the heat transfer bodies  62  is adopted. Accordingly, the upper structure  20   a  of the second stage cryopanel assembly  20  is configured to be axially symmetric so as to include the cryopanel attachment structure. Unlike a typical cryopump having an asymmetric attachment structure, an effective cryopanel area (that is, an adsorption region  66  and/or a condensation region) of the upper cryopanel  60   a  can be made wider. In a cryopump to which the above-described design is applied, the adsorption region  66  of the second stage cryopanel assembly  20  can increase by approximately 15%. Accordingly, a storage capacity of the non-condensable gas increases by approximately 15%. In addition, an exhaust speed of the non-condensable gas is estimated to increase by approximately 2%. Thus, the exhaust performance of the cryopump  10  is improved. 
     Hereinbefore, embodiments of the present invention are described. It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention. 
     In the above-described embodiment, at least one upper cryopanel  60   a  has an inverted truncated cone shape. However, as shown in  FIG.  6   , at least one upper cryopanel  60   a  may be a flat disk having a diameter larger than that of the circular end surface of the heat transfer body  62 . In this way, the upper cryopanel  60   a  may be a flat plate, and for example, may have a disk shape. The upper cryopanel  60   a  may include the plurality of through-holes  80 . 
     In the above-described embodiment, the upper structure  20   a  is described as an example. However, the above-described configuration can be applied to the lower structure  20   b . In this case, as context permits, the upper structure  20   a  may be read as the “lower structure  20   b ” and the upper cryopanel  60   a  may be read as the “lower cryopanel  60   b”.    
     The embodiment of the present invention can be also be expressed as follows. 
     1. A cryopump including: a cryocooler which includes a high-temperature cooling stage and a low-temperature cooling stage; a radiation shield which is thermally coupled to the high-temperature cooling stage and axially extends in a tubular shape from a cryopump intake port; and a low-temperature cryopanel section which is thermally coupled to the low-temperature cooling stage, is surrounded by the radiation shield, and includes a plurality of cryopanels and a plurality of heat transfer bodies axially arranged in a columnar shape, and in which the plurality of cryopanels and the plurality of heat transfer bodies are axially stacked. 
     2. The cryopump described in 1, wherein the plurality of cryopanels are formed of a first material having a first thermal conductivity, at least a portion of the plurality of heat transfer bodies is formed of a second material having a second thermal conductivity, and the second thermal conductivity is smaller than the first thermal conductivity. 
     3. The cryopump described in any one of 1 or 2, wherein the plurality of cryopanels have a first heat capacity, the plurality of heat transfer bodies have a second heat capacity, and the second heat capacity is smaller than the first heat capacity. 
     4. The cryopump described in any one of 1 to 3, wherein the plurality of heat transfer bodies are axially arranged in a columnar shape and each of the plurality of heat transfer bodies has a circular end surface. 
     5. The cryopump described in 4, wherein at least one cryopanel includes a center disk having a size corresponding to the circular end surface of the heat transfer body, and a conical cryopanel surface inclined from the center disk toward the cryopump intake port. 
     6. The cryopump described in 4 or 5, wherein at least one cryopanel is a flat disk having a diameter larger than that of the circular end surface of the heat transfer body. 
     7. The cryopump described in any one of 1 to 6, wherein the low-temperature cryopanel section includes a fastening member which axially penetrates the plurality of cryopanels and the plurality of heat transfer bodies. 
     8. The cryopump described in any one of 1 to 7, wherein the plurality of cryopanels and the plurality of heat transfer bodies are axially stacked between the cryopump intake port and the low-temperature cooling stage. 
     9. The cryopump described in any one of 1 to 8, wherein the low-temperature cryopanel section includes an intervening layer between the cryopanel and the heat transfer body. 
     The present invention can be used in a field of a cryopump.