Patent Publication Number: US-2021190057-A1

Title: Cryopump and cryopanel

Description:
RELATED APPLICATIONS 
     The contents of Japanese Patent Application No. 2018-167177, and of International Patent Application No. PCT/JP2019/030302, 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 embodiments of the present invention relate to a cryopump and a cryopanel. 
     Description of Related Art 
     A cryopump is a vacuum pump which captures gas molecules on a cryopanel cooled to a cryogenic temperature by condensation or adsorption to pump the gas molecules. The cryopump is generally used to realize a clean vacuum environment which is required for a semiconductor circuit manufacturing process or the like. Since the cryopump is a so-called gas accumulation type vacuum pump, regeneration to periodically discharge the captured gas to the outside is required. 
     SUMMARY 
     According to an embodiment of the present invention, there is provided a cryopump including: a cryopanel assembly which includes an exposed area that a gas to be pumped can linearly reach through a cryopump intake port and a non-exposed area that the gas to be pumped cannot linearly reach through the cryopump intake port. The non-exposed area has an adsorption area capable of adsorbing a non-condensable gas, and the exposed area is covered with a removable protective surface. 
     According to another embodiment of the present invention, there is provided a cryopanel including: a cryopanel base material; and a removable protective surface that covers at least a part of the cryopanel base material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a cryopump according to a certain embodiment. 
         FIG. 2  is a schematic perspective view of an exemplary cryopanel that can be used in the cryopump shown in  FIG. 1 . 
         FIG. 3  is a schematic perspective view of another exemplary cryopanel that can be used in the cryopump shown in  FIG. 1 . 
         FIG. 4  is a schematic top view of still another exemplary cryopanel that can be used in the cryopump shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Depending on a use of the cryopump, during an evacuation operation, a certain kind of gas that is not easily discharged even if regeneration is performed flows into the cryopump and condenses on and adheres to a cryopanel, and thus the cryopanel can be contaminated with such deposits. The contaminated cryopanel may need to be disassembled from the cryopump and washed during maintenance of the cryopump. The washed cryopanel is reassembled and used in a case where it is reusable. In a case where it cannot be reused, it is discarded and replaced with a new cryopanel. In any case, such maintenance is troublesome. 
     It is desirable to facilitate maintenance of a cryopump. 
     Any combination of the constituent elements described above, or replacement of constituent elements or expressions of the present invention with each other between methods, apparatuses, systems, or the like is also valid as an aspect of the present invention. 
     Hereinafter, modes for carrying out the present invention will be described in detail with reference to the drawings. In the description and the drawings, identical or equivalent constituent elements, members, and processing are denoted by the same reference numerals, and overlapping description is omitted appropriately. The scales or shapes of the respective parts shown in the drawings are set for convenience in order to facilitate description and are not interpreted to a limited extent unless otherwise specified. Embodiments are exemplification and do not limit the scope of the present invention. All features described in the embodiments or combinations thereof are not necessarily essential to the invention. 
       FIG. 1  schematically shows a cryopump  10  according to a certain embodiment. 
     The cryopump  10  is mounted to a vacuum chamber of, for example, an ion implanter, a sputtering apparatus, a vapor deposition apparatus, or other vacuum process equipment and is used to increase the degree of vacuum in the interior of the vacuum chamber to a level which is required for a desired vacuum process. The cryopump  10  has a cryopump intake port (hereinafter, also simply referred to as an “intake port”)  12  for receiving a gas to be pumped, from the vacuum chamber. The gas enters an internal space  14  of the cryopump  10  through the intake port  12 . 
     In the following, there is a case where the terms “axial direction” and “radial direction” are used in order to express the positional relationship between constituent elements of the cryopump  10  in an easily understandable manner. The axial direction of the cryopump  10  represents a direction passing through the intake port  12  (that is, a direction along a central axis C in the drawing), and the radial direction represents a direction along the intake port  12  (a first direction in the plane perpendicular to the central axis C). For convenience, with respect to the axial direction, there is a case where the side relatively close to the intake port  12  is referred to as an “upper side” and the side relatively distant from the intake port  12  is referred to as a “lower side”. That is, there is a case where the side relatively distant from the bottom of the cryopump  10  is referred to as an “upper side” and the side relatively close to the bottom of the cryopump  10  is referred to as a “lower side”. With respect to the radial direction, there is a case where the side close to the center (in the drawing, the central axis C) of the intake port  12  is referred to as an “inner side” and the side close to the peripheral edge of the intake port  12  is referred to as an “outer side”. Such expressions are not related to the disposition when the cryopump  10  is mounted to the vacuum chamber. For example, the cryopump  10  may be mounted to the vacuum chamber with the intake port  12  facing downward in the vertical direction. 
     Further, there is a case where a direction surrounding the axial direction is referred to as a “circumferential direction”. The circumferential direction is a second direction along the intake port  12  (a second direction in the plane perpendicular to the central axis C) 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 part or a 100 K part. The second-stage cryopanel assembly  20  may be referred to as a low-temperature cryopanel part or a 10 K part. 
     The cryocooler  16  is a cryocooler such as a Gifford McMahon type cryocooler (a so-called GM cryocooler), for example. The cryocooler  16  is a two-stage cryocooler. Therefore, the cryocooler  16  includes a first cooling stage  22  and a second cooling stage  24 . The cryocooler  16  is configured 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 a temperature in a range of about 65 K to 120 K, preferably, in a range of 80 K to 100 K, and the second cooling stage  24  is cooled to a temperature in a range of about 10 K to 20 K. The first cooling stage  22  and the second cooling stage  24  may be referred to as a high-temperature cooling stage and a low-temperature cooling stage, respectively. 
     Further, the cryocooler  16  includes a cryocooler structure part  21  that structurally supports the second cooling stage  24  on the first cooling stage  22  and structurally supports the first cooling stage  22  on a room temperature part  26  of the cryocooler  16 . Therefore, the cryocooler structure part  21  includes a first cylinder  23  and a second cylinder  25  that extend coaxially along the radial direction. The first cylinder  23  connects the room temperature part  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 part  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 and a second displacer (not shown) are reciprocally disposed in the interiors of the first cylinder  23  and the second cylinder  25 , respectively. A first regenerator and a second regenerator (not shown) are respectively incorporated into the first displacer and the second displacer. Further, the room temperature part  26  has a drive mechanism (not shown) for reciprocating the first displacer and the second displacer. The drive mechanism includes a flow path switching mechanism that switches a flow path of a working gas (for example, helium) so as to periodically repeat the supply and discharge of the working gas to and from the interior of the cryocooler  16 . 
     The cryocooler  16  is connected to a compressor (not shown) for the working gas. The cryocooler  16  cools the first cooling stage  22  and the second cooling stage  24  by expanding the working gas pressurized by the compressor in the interior thereof. The expanded working gas is recovered to the compressor and pressurized again. The cryocooler  16  generates cold by repeating a thermodynamic cycle (for example, a refrigeration cycle such as a GM cycle) including the supply and discharge of the working gas and the reciprocation of the first displacer and the second displacer in synchronization with the supply and discharge of the working gas. 
     The cryopump  10  which is shown in the drawing is a so-called horizontal cryopump. The horizontal cryopump is generally a cryopump in which the cryocooler  16  is disposed so as to intersect (usually, be orthogonal to) the central axis C of the cryopump  10 . 
     The first-stage cryopanel  18  includes a radiation shield  30  and an inlet cryopanel  32  and surrounds the second-stage cryopanel assembly  20 . The first-stage cryopanel  18  provides a cryogenic surface for protecting the second-stage cryopanel assembly  20  from radiant heat outside the cryopump  10  or from 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 itself 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 also 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 located between the cryopump housing  70  and the second-stage cryopanel assembly  20  and surrounds the second-stage cryopanel assembly  20 . The radiation shield  30  has a shield main opening  34  for receiving gas from the outside of the cryopump  10  into the internal space  14 . The shield main opening  34  is located at the intake port  12 . 
     The radiation shield  30  is provided with a shield front end  36  defining the shield main opening  34 , a shield bottom portion  38  which is located on the side opposite to the shield main opening  34 , and a shield side portion  40  connecting the shield front end  36  to the shield bottom portion  38 . The shield side portion  40  extends in the axial direction from the shield front end  36  to the side opposite to the shield main opening  34 , and extends so as to surround the second cooling stage  24  in the circumferential direction. 
     The shield side portion  40  has a shield side portion opening  44  into which the cryocooler structure part  21  is inserted. The second cooling stage  24  and the second cylinder  25  are inserted into the radiation shield  30  from outside the radiation shield  30  through the shield side portion opening  44 . The shield side portion opening  44  is a mounting hole formed in the shield side portion  40  and is, for example, circular. The first cooling stage  22  is disposed outside the radiation shield  30 . 
     The shield side portion  40  is provided with a mounting seat  46  for the cryocooler  16 . The mounting seat  46  is a flat portion for mounting the first cooling stage  22  to the radiation shield  30 , and is slightly depressed when viewed from outside the radiation shield  30 . The mounting seat  46  forms the outer periphery of the shield side portion opening  44 . The first cooling stage  22  is mounted to the mounting seat  46 , whereby the radiation shield  30  is thermally coupled to the first cooling stage  22 . 
     Instead of directly mounting the radiation shield  30  to the first cooling stage  22  in this manner, in an embodiment, the radiation shield  30  may be thermally coupled to the first cooling stage  22  through an additional heat transfer member. The heat transfer member may be, for example, a hollow short cylinder having flanges at both ends. The heat transfer member may be fixed to the mounting seat  46  by the flange at one end and fixed to the first cooling stage  22  by the flange at the other end. The heat transfer member may extend from the first cooling stage  22  to the radiation shield  30  to surround the cryocooler structure part  21 . The shield side portion  40  may include such a heat transfer member. 
     In the illustrated embodiment, the radiation shield  30  is configured in an integral tubular shape. Instead, the radiation shield  30  may be configured to have a tubular shape as a whole by a plurality of parts. The plurality of parts may be disposed with a gap therebetween. For example, the radiation shield  30  may be divided into two parts in the axial direction. 
     The inlet cryopanel  32  is provided at the intake port  12  (or the shield main opening  34 , the same applies hereinafter) in order to protect the second-stage cryopanel assembly  20  from the radiant heat from a heat source outside the cryopump  10  (for example, a heat source in the vacuum chamber to which the cryopump  10  is mounted). Further, gas (for example, moisture) condensing at the cooling temperature of the inlet cryopanel  32  is captured on the surface thereof. 
     The inlet cryopanel  32  is disposed at a location corresponding to the second-stage cryopanel assembly  20  at the intake port  12 . The inlet cryopanel  32  occupies the central portion of the opening area of the intake port  12 , and forms an annular (for example, circular ring-shaped) open area  51  between itself and the radiation shield  30 . The shape of the inlet cryopanel  32  when viewed in the axial direction is, for example, a disk shape. The diameter of the inlet cryopanel  32  is relatively small and is smaller than the diameter of the second-stage cryopanel assembly  20 , for example. The inlet cryopanel  32  may occupy at most ⅓ or at most ¼ of the opening area of the intake port  12 . In this way, the open area  51  may occupy at least ⅔ or at least ¾ of the opening area of the intake port  12 . 
     The inlet cryopanel  32  is mounted to the shield front end  36  through an inlet cryopanel mounting member  33 . As shown in  FIG. 1 , the inlet cryopanel mounting member  33  is a linear member that extends over the shield front end  36  along the diameter of the shield main opening  34 . In this manner, 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 adjacent to, but not in contact with, the second-stage cryopanel assembly  20 . Further, the inlet cryopanel mounting member  33  divides the open area  51  in the circumferential direction. The open area  51  is composed of a plurality of (for example, two) arc-shaped areas. The inlet cryopanel mounting member  33  may have a cross shape or another shape. 
     The inlet cryopanel  32  is disposed at the central portion of the intake port  12 . The center of the inlet cryopanel  32  is located on the central axis C. However, the center of the inlet cryopanel  32  may be located somewhat off the central axis C, and even in that case, the inlet cryopanel  32  can be regarded as being disposed at the central portion of the intake port  12 . The inlet cryopanel  32  is disposed perpendicular to the central axis C. Further, with respect to the axial direction, the inlet cryopanel  32  may be disposed slightly above the shield front end  36 . 
     Alternatively, the inlet cryopanel  32  may be disposed at substantially the same height as the shield front end  36  in the axial direction, or slightly below the shield front end  36  in the axial direction. 
     The second-stage cryopanel assembly  20  is provided at the central 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  includes a plurality of adsorption cryopanels  60  arranged in the axial direction. The plurality of adsorption 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 upper cryopanels  60   a  are disposed between the inlet cryopanel  32  and the second cooling stage  24  in the axial direction. 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 alternately stacked in the axial direction between the intake port  12  and the second cooling stage  24 . The centers of the upper cryopanel  60   a  and the heat transfer body  62  are located together on the central axis C. In this way, the upper structure  20   a  is disposed above the second cooling stage  24  in the axial direction. The upper structure  20   a  is fixed to the second cooling stage  24  through a heat transfer block  63  formed of a high heat conductive metal material such as copper (for example, pure copper), 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 cryopanel mounting member  64 . The plurality of lower cryopanels  60   b  are disposed between the second cooling stage  24  and the shield bottom portion  38  in the axial direction. The second-stage cryopanel mounting member  64  extends downward in the axial direction from the second cooling stage  24 . The plurality of lower cryopanels  60   b  are mounted to the second cooling stage  24  through the second-stage cryopanel mounting members  64 . In this way, the lower structure  20   b  is thermally coupled to the second cooling stage  24  and is cooled to the second cooling temperature. 
     In the second-stage cryopanel assembly  20 , an adsorption area  66  is formed on at least a part of the surface. The adsorption area  66  is provided, for capturing a non-condensable gas (for example, hydrogen) by adsorption. The adsorption area  66  is formed, for example, by bonding an adsorbent (for example, activated carbon) to the surface of the cryopanel. 
     At least one of the plurality of adsorption cryopanels  60  (for example, each of the plurality of upper cryopanels  60   a  and/or at least one of the plurality of lower cryopanels  60   b ) includes an exposed area  68  and a non-exposed area  69 . With respect to a certain cryopanel, the exposed area  68  refers to the place on the cryopanel that a gas to be pumped can linearly reach through the intake port  12 , and the non-exposed area  69  refers to the place on the cryopanel that the gas to be pumped cannot linearly reach through the intake port  12 . Therefore, the front surface of the cryopanel, which faces the intake port  12 , can be divided into the exposed area  68  and the non-exposed area  69 . The back surface of the cryopanel, which faces the side opposite to the intake port  12 , that is, the shield bottom portion  38 , becomes the non-exposed area  69 . 
     The boundary between the exposed area  68  and the non-exposed area  69  on the front surface of a certain cryopanel may be determined in consideration of a line of sight which is directed from the inner peripheral edge of the shield front end  36  (which may be the inner peripheral edge of an intake port flange  72 ) to the outer peripheral edge of the cryopanel directly above the cryopanel. When the line of sight is extended, the line of sight forms an intersection on the front surface of the cryopanel. When the line of sight is scanned over the entire circumference of the shield front end  36 , the intersection draws a locus on the front surface of the cryopanel. The area inside the locus is behind the cryopanel directly above and is not visible from the outside of the cryopump  10  through the intake port  12 . The area outside the locus is visible from the outside of the cryopump  10  through the intake port  12 . In this manner, the boundary between the exposed area  68  and the non-exposed area  69  can be determined by using the line of sight. 
     As an example, in  FIG. 1 , a first line of sight  74   a  and a second line of sight  74   b  are shown with broken lines. The first line of sight  74   a  is drawn from the shield front end  36  to the outer peripheral edge of the second upper cryopanel  60   a  from below and intersects the lowermost upper cryopanel  60   a . Therefore, on the front surface of the lowermost upper cryopanel  60   a , the area radially outside the first line of sight  74   a  becomes the exposed area  68 , and the area radially inside the first line of sight  74   a  becomes the non-exposed area  69 . The second line of sight  74   b  is drawn from the shield front end  36  to the outer peripheral edge of the lowermost upper cryopanel  60   a  and intersects the uppermost lower cryopanel  60   b . Therefore, on the front surface of the uppermost lower cryopanel  60   b , the area radially outside the second line of sight  74   b  becomes the exposed area  68 , and the area radially inside the second line of sight  74   b  becomes the non-exposed area  69 . 
     As an example, one or a plurality of upper cryopanels  60   a  that are closest to the inlet cryopanel  32  in the axial direction, among the plurality of upper cryopanels  60   a , are flat plates (for example, disk-shaped) and are disposed perpendicular to the central axis C. The remaining upper cryopanels  60   a  have an inverted truncated cone shape, and a circular bottom surface is disposed perpendicular to the central axis C. 
     The upper cryopanel  60   a  closest to the inlet cryopanel  32  (that is, the upper cryopanel  60   a  located directly below the inlet cryopanel  32  in the axial direction, also referred to as a top cryopanel  61 ), among the upper cryopanels  60   a , has a diameter larger than that of the inlet cryopanel  32 . However, the diameter of the top cryopanel  61  may be equal to or smaller than the diameter of the inlet cryopanel  32 . The top cryopanel  61  directly faces the inlet cryopanel  32 , and no other cryopanel exists between the top cryopanel  61  and the inlet cryopanel  32 . 
     The diameters of the plurality of upper cryopanels  60   a  gradually increase toward the lower side in the axial direction. Further, the inverted truncated cone-shaped upper cryopanel  60   a  is disposed in a nested manner. The lower part of the upper cryopanel  60   a  on the higher side enters the inverted truncated conical space in the upper cryopanel  60   a  adjacent thereunder. 
     Each heat transfer body  62  has a columnar shape. The heat transfer body  62  may have a relatively short columnar shape and may have an axial height smaller than the diameter of the heat transfer body  62 . The cryopanel such as the adsorption cryopanel  60  is generally formed of a high heat conductive metal material such as copper (for example, pure copper), and as necessary, the surface thereof is coated with a metal layer such as nickel. In contrast, the heat transfer body  62  may be formed of a material different from that of the cryopanel. The heat transfer body  62  may be formed of a metal material, such as aluminum or an aluminum alloy, for example, having a lower density although it has a lower thermal conductivity than the adsorption cryopanel  60 . In this way, both the thermal conductivity and the reduction in weight of the heat transfer body  62  can be achieved to some extent, which is helpful to reduce the cooling time of the second-stage cryopanel assembly  20 . 
     The lower cryopanel  60   b  is a flat plate, for example, in a disk shape. The lower cryopanel  60   b  has a larger diameter than the upper cryopanel  60   a . However, a cutout portion (for example, a cutout portion  82  shown in  FIG. 4 ) extending from a portion of the outer periphery to the central portion may be formed in the lower cryopanel  60   b  for mounting the lower cryopanel  60   b  to the second-stage cryopanel mounting member  64 . 
     The specific configuration of the second-stage cryopanel assembly  20  is not limited to the configuration described above. The upper structure  20   a  may have any number of upper cryopanels  60   a . The upper cryopanel  60   a  may have a flat plate shape, a conical shape, or other shapes. Similarly, the lower structure  20   b  may have any number of lower cryopanels  60   b . The lower cryopanel  60   b  may have a flat plate shape, a conical shape, or other shapes. 
     The adsorption area  66  may be formed in a place that is hidden behind the adsorption cryopanel  60  adjacent to the upper side so as not to be seen from the intake port  12 . That is, the adsorption area  66  is disposed in the non-exposed area  69 . For example, the adsorption area  66  is formed on the entire lower surface of the adsorption cryopanel  60 . The adsorption area  66  may be formed on the upper surface of the lower cryopanel  60   b . Further, although not shown in  FIG. 1  for the sake of simplification, the adsorption area  66  is also formed on the lower surface (back surface) of the upper cryopanel  60   a . As necessary, the adsorption area  66  may be formed on the upper surface of the upper cryopanel  60   a.    
     In the adsorption area  66 , a large number of activated carbon particles are bonded in an irregular arrangement in a state of being densely arranged on the surface of the adsorption cryopanel  60 . The activated carbon particles are molded, for example, in a columnar shape. The shape of the adsorbent may not be a columnar shape and may be, for example, a spherical shape, another molded shape, or an irregular shape. The arrangement of the adsorbents on the panel may be a regular arrangement or an irregular arrangement. 
     Further, a condensation area for capturing a condensable gas by condensation is formed on at least a part of the surface of the second-stage cryopanel assembly  20 . The exposed area  68  can serve as a condensation area. The condensation area is, for example, a section where the adsorbent is missing on the surface of the cryopanel, and the surface of the cryopanel base material, for example, the metal surface is exposed. The upper surface, the outer peripheral portion of the upper surface, or the outer peripheral portion of the lower surface of the adsorption cryopanel  60  (for example, the upper cryopanel  60   a ) may be a condensation area. 
     Both the upper and lower surfaces of the top cryopanel  61  may be condensation areas. That is, the top cryopanel  61  may not have the adsorption area  66 . In this manner, in the second-stage cryopanel assembly  20 , the cryopanel which does not have the adsorption area  66  may be referred to as a condensation cryopanel. For example, the upper structure  20   a  may be provided with at least one condensation cryopanel (for example, the top cryopanel  61 ). 
     The cryopump housing  70  is a casing 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 container configured to maintain the vacuum tightness of the internal space  14 . The cryopump housing  70  includes the first-stage cryopanel  18  and the cryocooler structure part  21  in a non-contact manner. The cryopump housing  70  is mounted to the room temperature part  26  of the cryocooler  16 . 
     The intake port  12  is defined by a front end of the cryopump housing  70 . The cryopump housing  70  has the intake port flange  72  extending radially outward from the front end thereof. The intake port flange  72  is provided over the entire circumference of the cryopump housing  70 . The cryopump  10  is mounted to the vacuum chamber to be evacuated by using the intake port flange  72 . 
     As described above, the second-stage cryopanel assembly  20  has a large number of adsorption cryopanels  60  (that is, the plurality of upper cryopanels  60   a  and lower cryopanels  60   b ), and therefore, it has high pumping performance for a non-condensable gas. For example, the second-stage cryopanel assembly  20  can pump hydrogen gas at a high pumping speed. 
     Each of the plurality of adsorption cryopanels  60  includes the adsorption area  66  at a portion which is not visible from the outside from the cryopump  10 . Therefore, the second-stage cryopanel assembly  20  is configured such that all or most of the adsorption areas  66  are completely invisible from the outside of the cryopump  10 . The cryopump  10  can also be called an adsorbent non-exposure type cryopump. 
     Incidentally, the gas accumulated in the cryopump is usually discharged substantially completely by regeneration treatment, and when the regeneration is completed, the cryopump is restored to the pumping performance in the specification. However, in an adsorbent exposure type cryopump in which the adsorbent is disposed so as to be visible from the outside of the cryopump, the percentage of components of the accumulated gas, which remain in the adsorbent even after the regeneration treatment, is relatively high. 
     For example, in a cryopump installed for evacuation of an ion implanter, it was observed that a sticky substance adheres to activated carbon as an adsorbent. It was difficult to completely remove the sticky substance even after regeneration treatment. It is considered that the sticky substance is caused by an organic outgas which is discharged from photoresist coated on a substrate to be treated. Alternatively, there is also a possibility that it may be caused by a dopant gas, that is, a toxic gas which is used as a raw material gas in ion implantation treatment. A possibility that it may be caused by other by-produced gases in the ion implantation treatment is also considered. There is also a possibility that these gases may be related to each other in a complex manner to form a sticky substance. 
     In the ion implantation treatment, most of the gases which are pumped by the cryopump can be hydrogen gas. Hydrogen gas is discharged substantially completely to the outside by regeneration. When the amount of poorly regenerated gas is very small, the influence of the poorly regenerated gas on the pumping performance of the cryopump in single cryopumping treatment is minor. However, in the adsorbent exposure type cryopump, the poorly regenerated gas may be gradually accumulated on the adsorbent as the cryopumping treatment and the regeneration treatment are repeated, and thus there is a possibility that the pumping performance may be reduced. When the pumping performance falls below an allowable range, maintenance work including, for example, replacement of the adsorbent or the cryopanel together with it, or chemical removal treatment of the poorly regenerated gas on the adsorbent is required. 
     The poorly regenerated gas is a condensable gas almost without exception. The molecules of the condensable gas which comes flying from the outside toward the cryopump  10  pass through an open area around the inlet cryopanel  32 , then reach the condensation area on the outer periphery of the radiation shield  30  or the second-stage cryopanel assembly  20  in a linear path, and are captured on the surface thereof. The poorly regenerated gas is deposited on the condensation area. As described above, since the cryopump  10  is an adsorbent non-exposure type and the adsorption area  66  is disposed in the non-exposed area  69 , the adsorption area  66  is protected from the poorly regenerated gas. 
     On the other hand, the exposed area  68  can be contaminated with the poorly regenerated gas. The contaminated adsorption cryopanel  60  may need to be disassembled from the cryopump  10  and washed during the maintenance of the cryopump  10 . Since the adsorbent such as activated carbon provided in the adsorption area  66  is not contaminated with the poorly regenerated gas, it is considered that it can be reused. The washed cryopanel is reassembled and used in a case where it is reusable. However, depending on a washing method, the adsorption function of the adsorption area  66  may be lost. In that case, the adsorption cryopanel  60  after washing cannot be reused, and therefore, it has to be discarded. 
     Therefore, the exposed area  68  is covered with a removable protective surface  76 . The removable protective surface  76  is provided in the exposed area  68  of at least one adsorption cryopanel  60 . The removable protective surface  76  may be provided on each of the plurality of adsorption cryopanels  60 . The removable protective surface  76  may have various exemplary configurations, which will be described below. 
       FIG. 2  is a schematic perspective view of an exemplary cryopanel that can be used in the cryopump  10  shown in  FIG. 1 . The cryopanel shown in the drawing is a cryopanel that can be used in the second-stage cryopanel assembly  20 , and is the top cryopanel  61 . However, the cryopanel shown in the drawing may be another adsorption cryopanel  60  which is used in the second-stage cryopanel assembly  20 . 
     The top cryopanel  61  includes a first cryopanel base material  78   a  and a second cryopanel base material  78   b . The cryopanel base materials  78   a  and  78   b  are formed of the same material (for example, a metal material) and have the same shape. The cryopanel base materials  78   a  and  78   b  are formed of, for example, a high heat conductive metal material such as copper (for example, pure copper), and as necessary, the surface is coated with a metal layer such as nickel. Therefore, the cryopanel base materials  78   a  and  78   b  themselves cannot adsorb the non-condensable gas. Although not shown in the drawing, in order to make the top cryopanel  61  be capable of adsorbing the non-condensable gas, the first cryopanel base material  78   a  may have an adsorbent provided on the back surface (lower surface) thereof. Alternatively, the first cryopanel base material  78   a  may not be provided with an adsorbent, and in that case, the top cryopanel  61  does not adsorb the non-condensable gas. The cryopanel base materials  78   a  and  78   b  have, for example, a disk shape. The cryopanel base materials  78   a  and  78   b  may have a conical shape or other shapes. 
     The second cryopanel base material  78   b  is removably mounted on the first cryopanel base material  78   a  so as to provide the removable protective surface  76 . The second cryopanel base material  78   b  is removably mounted on the first cryopanel base material  78   a  such that the back surface thereof is in contact with the front surface of the first cryopanel base material  78   a  and covers the entire front surface of the first cryopanel base material  78   a . The front surface of the second cryopanel base material  78   b  is used as the protective surface  76 . 
     Further, the second cryopanel base material  78   b  is thermally coupled to the first cryopanel base material  78   a  and is cooled together with the first cryopanel base material  78   a . The second cryopanel base material  78   b  is mounted on the first cryopanel base material  78   a  by an appropriate removable mounting method such as a removable fastening member such as a bolt or a peelable adhesive such that there is good thermal contact between the cryopanel base materials  78   a  and  78   b.    
     The first cryopanel base material  78   a  corresponds to a cryopanel that is typically used. In the embodiment shown in  FIG. 2 , the second cryopanel base material  78   b  is superimposed on the first cryopanel base material  78   a . The second cryopanel base material  78   b  added in this way provides the removable protective surface  76 . 
     The second cryopanel base material  78   b  does not have an adsorption area, that is, an adsorbent, because it is made be unable to adsorb a non-condensable gas. Therefore, in the manufacturing process, a process of attaching an adsorbent to the cryopanel base material is not required. On the other hand, the adsorption cryopanel  60  which requires such an adsorbent attachment process is costly to manufacture. Therefore, the second cryopanel base material  78   b  can be provided at a relatively low cost. 
     Further, since the second cryopanel base material  78   b  is designed to be equivalent to the first cryopanel base material  78   a  which is typically used for the cryopanel, the thermal performance, mechanical strength, and other necessary conditions which are required for use in the cryopump  10  are satisfied. Therefore, the second cryopanel base material  78   b  can be easily used by a designer of the cryopump  10 . 
     Since the second cryopanel base material  78   b  is cooled to the second cooling temperature in the same manner as the first cryopanel base material  78   a , the poorly regenerated gas condenses on the protective surface  76  on the second cryopanel base material  78   b  and can contaminate the protective surface  76 . However, with respect to the first cryopanel base material  78   a , contamination is prevented or mitigated by the protective surface  76 . In a case where there is no contamination or the degree of contamination is light, it is possible to reuse the top cryopanel  61  without performing complicated work such as disassembling or washing during the maintenance of the cryopump  10 . Since the second cryopanel base material  78   b  does not have an adsorbent, it can be reused if it is washed. Alternatively, as described above, since the second cryopanel base material  78   b  is relatively inexpensive, even if the used cryopanel base material  78   b  is discarded and replaced with a new cryopanel base material  78   b , the influence in terms of a cost is small. 
     After the used cryopanel base material  78   b  is removed, a new cryopanel base material  78   b  may not be mounted on the first cryopanel base material  78   a . In this case, since the protective surface  76  is not provided on the first cryopanel base material  78   a , the front surface of the first cryopanel base material  78   a  may be contaminated during the subsequent operation of the cryopump  10 . The first cryopanel base material  78   a  may have to be replaced with a new first cryopanel base material at the next maintenance. However, since the adsorbent on the first cryopanel base material  78   a  also has a limited life, it is eventually necessary to replace the first cryopanel base material  78   a  together with the adsorbent regardless of the presence or absence of contamination of the first cryopanel base material  78   a . Therefore, whether or not to mount a new cryopanel base material  78   b  may be determined in consideration of the cost of the cryopanel base material  78   b  or the life of the adsorbent. 
       FIG. 3  is a schematic perspective view of another exemplary cryopanel that can be used in the cryopump  10  shown in  FIG. 1 . The cryopanel shown in the drawing is a cryopanel that can be used in the second-stage cryopanel assembly  20 , and is the upper cryopanel  60   a . However, the cryopanel shown in the drawing may be another adsorption cryopanel  60  which is used in the second-stage cryopanel assembly  20 . 
     The upper cryopanel  60   a  has, for example, an inverted conical shape, as described with reference to  FIG. 1 . The front surface of the upper cryopanel  60   a  has the exposed area  68  at the outer peripheral portion and has the non-exposed area  69  inside the exposed area  68 . An adsorbent may be provided in the non-exposed area  69 . However, for the sake of simplification of the illustration, the illustration thereof is omitted in  FIG. 3 . 
     The upper cryopanel  60   a  (or the adsorption cryopanel  60 ) includes a protective layer  80  that covers the exposed area  68  so as to provide the removable protective surface  76 . The non-exposed area  69  is not provided with the protective layer  80 . The surface of the protective layer  80  that functions as the protective surface  76  may be formed of a material having corrosion resistance against the poorly regenerated gas, for example, fluororesin such as polytetrafluoroethylene or another resin, or metal such as aluminum or copper. Accordingly, the protective layer  80  may be an adhesive tape having a surface made of such a synthetic resin material or metal material, or a peelably bonded protective film. The protective layer  80  is bonded to the cryopanel base material of the upper cryopanel  60   a , thereby being thermally coupled thereto and cooled to the same cooling temperature. 
     Since the protective layer  80  is installed in the exposed area  68  and cooled to the second cooling temperature, the poorly regenerated gas condenses on the protective surface  76  and can contaminate the protective surface  76 . Since the protective layer  80  is peelably bonded to the upper cryopanel  60   a , it is possible to remove contaminants from the upper cryopanel  60   a  by peeling off the protective layer  80  during the maintenance of the cryopump  10 . The upper cryopanel  60   a  can be reused without performing complicated work such as disassembly or washing during the maintenance. 
       FIG. 4  is a schematic top view of still another exemplary cryopanel that can be used in the cryopump  10  shown in  FIG. 1 . The cryopanel shown in the drawing is a cryopanel that can be used in the second-stage cryopanel assembly  20 , and is the lower cryopanel  60   b . However, the cryopanel shown in the drawing may be another adsorption cryopanel  60  which is used in the second-stage cryopanel assembly  20 . 
     The lower cryopanel  60   b  has, for example, a disk-like shape, as described with reference to  FIG. 1 . However, the cutout portion  82  extending from a portion of the outer periphery to the center portion is formed in the lower cryopanel  60   b  for mounting of the lower cryopanel  60   b  to the second-stage cryopanel mounting member  64 . The front surface of the lower cryopanel  60   b  has the exposed area  68  at the outer peripheral portion and has the non-exposed area  69  inside the exposed area  68 . Granular activated carbon  84  as an adsorbent is attached to the non-exposed area  69 . 
     The lower cryopanel  60   b  (or the adsorption cryopanel  60 ) includes the protective layer  80  made of synthetic resin or metal and peelably bonded to the exposed area  68  so as to provide the removable protective surface  76 . The protective layer  80  is bonded to the cryopanel base material of the lower cryopanel  60   b , thereby being thermally coupled thereto and cooled to the same cooling temperature. 
     Since the protective layer  80  is installed in the exposed area  68  and cooled to the second cooling temperature, the poorly regenerated gas condenses on the protective surface  76  and can contaminate the protective surface  76 . Since the protective layer  80  is peelably bonded to the lower cryopanel  60   b , it is possible to remove contaminants from the lower cryopanel  60   b  by peeling off the protective layer  80  during the maintenance of the cryopump  10 . The lower cryopanel  60   b  can be reused without performing complicated work such as disassembly or washing during the maintenance. 
     After the used protective layer  80  is peeled off, a new protective layer  80  may or may not be attached to the adsorption cryopanel  60 . Whether or not to attach the new protective layer  80  may be determined in consideration of the cost of the protective layer  80  or the life of the adsorbent on the adsorption cryopanel  60 . 
     Alternatively, a plurality of protective layers  80  may be layered on the exposed area  68 . In this way, when the used protective layer  80  is peeled off, a new protective layer  80  directly below it is exposed and can be used. 
     The operation of the cryopump  10  having the above configuration will be described below. When the cryopump  10  is operated, first, the interior of the vacuum chamber is roughed to about 1 Pa with another appropriate roughing pump before the operation. 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 the driving of the cryocooler  16 . Accordingly, the first-stage cryopanel  18  and the second-stage cryopanel assembly  20  thermally coupled to these are also respectively cooled to the first cooling temperature and the second cooling temperature. 
     The inlet cryopanel  32  cools the gas which comes flying from the vacuum chamber toward the cryopump  10 . A gas having a sufficiently low vapor pressure (for example, 10 −8  Pa or less) at the first cooling temperature condenses on the surface of the inlet cryopanel  32 . This gas may be referred to as a type-1 gas. The type-1 gas is, for example, water vapor. In this way, the inlet cryopanel  32  can pump the type-1 gas. A part of a gas in which vapor pressure is not sufficiently low at the first cooling temperature enters the internal space  14  from the intake port  12 . Alternatively, the other part of the gas is reflected by the inlet cryopanel  32  and does not enter the internal space  14 . 
     The gas that has entered the internal space  14  is cooled by the second-stage cryopanel assembly  20 . A gas having a sufficiently low vapor pressure (for example, 10 −8  Pa or less) at the second cooling temperature condenses on the surface of the condensation area of the adsorption cryopanel  60 . This gas may be referred to as a type-2 gas. The type-2 gas is, for example, nitrogen (N 2 ) or argon (Ar). In this way, the second-stage cryopanel assembly  20  can pump the type-2 gas. 
     A gas in which vapor pressure is not sufficiently low at the second cooling temperature is adsorbed by the adsorption area  66  of the adsorption cryopanel  60 . This gas may be referred to as a type-3 gas. The type-3 gas is, for example, hydrogen (H 2 ). In this way, the second-stage cryopanel assembly  20  can pump the type-3 gas. Therefore, the cryopump  10  can pump various gases by condensation or adsorption and can make the degree of vacuum of the vacuum chamber reach a desired level. 
     According to the cryopump  10  according to the embodiment, the exposed area  68  is covered with the removable protective surface  76 . Since it is cooled to the second cooling temperature in the same manner as the second-stage cryopanel assembly  20 , the poorly regenerated gas is condensed on the protective surface  76 . The poorly regenerated gas can adhere to the protective surface  76  to contaminate it. However, the protective surface  76  can be removed. The protective surface  76  is removed, whereby the clean surface which has been covered with the protective surface  76  is exposed. Alternatively, the exposed area  68  is protected again by attaching a new protective surface  76 . Therefore, the cryopump  10  does not need to disassemble and wash the second-stage cryopanel assembly  20  in order to remove deposits such as the poorly regenerated gas during the maintenance. The maintenance of the cryopump  10  can be easily performed as compared with a cryopump which is not provided with such a removable protective surface  76 . 
     In particular, as described above, since the cryopump  10  is an adsorbent non-exposure type and the adsorption area  66  is disposed in the non-exposed area  69 , the adsorption area  66  is protected from the poorly regenerated gas. Therefore, in a case where the poorly regenerated gas is removed by removing or replacing the protective surface  76 , the second-stage cryopanel assembly  20  can be reused. In this manner, in a case where the cryopump  10  is an adsorbent non-exposure type, in particular, the maintenance of the cryopump  10  can be easily performed. 
     The present invention has been described above based on the examples. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, various design changes can be made, various modification examples can be made, and such modification examples are also within the scope of the present invention. 
     In the embodiments described above, the case where the protective layer  80  is not provided in the non-exposed area  69  has been described as an example. However, this is not essential to the present invention. In a certain embodiment, at least a part of the non-exposed area  69  (for example, the portion outside the adsorption area  66  in the non-exposed area  69 ) may be covered with the removable protective surface  76 . For example, in the non-exposed area  69 , the protective layer  80  may be peelably bonded to an area to which an adsorbent such as activated carbon is not attached. 
     In the above description, the horizontal cryopump has been exemplified. However, the present invention is also applicable to other vertical cryopumps. The vertical cryopump refers to a cryopump in which the cryocooler  16  is disposed along the central axis C of the cryopump  10 . Further, the internal configuration of the cryopump, such as the arrangement, the shape, the number, or the like of a cryopanel, is not limited to the specific embodiment described above. Various known configurations can be appropriately adopted. 
     The present invention can be used in the field of cryopumps and cryopanels. 
     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.