Patent Publication Number: US-9404486-B2

Title: Cryopump, cryopanel structure, and vacuum evacuation method

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a cryopump. 
     2. Description of Related Art 
     A cryopump is a vacuum pump that captures and pumps gas molecules by condensing or adsorbing molecules on a cryopanel cooled to an extremely low temperature. A cryopump is generally used to achieve a clean vacuum environment required in a semiconductor circuit manufacturing process or the like. One of the applications of a cryopump includes a case where, for example, a non-condensable gas such as hydrogen makes up most of a gas to be pumped, as in the case of, for example, an ion implantation step. A non-condensable gas can be pumped only after the non-condensable gas is adsorbed by an adsorption area that is cooled to an extremely-low temperature. 
     SUMMARY 
     An exemplary purpose of an embodiment of the present invention is to provide a cryopump, a cryopanel structure, and a vacuum evacuation method for high-speed evacuation of a non-condensable gas. 
     According to one embodiment of the present invention, there is provided a cryopump including: a radiation shield configured to include a shield front end that defines a shield opening, a shield bottom portion that faces the shield opening, and a shield side portion that extends from the shield front end to the shield bottom portion; and a cryopanel assembly configured to be cooled to a temperature that is lower than that of the radiation shield, including a plurality of cryopanels arranged along a direction toward the shield bottom portion from the shield opening, wherein the plurality of cryopanels includes: a first cryopanel including a first inner end portion and a first outer end portion that is directed to the shield side portion; a second cryopanel including a second inner end portion and a second outer end portion that is directed to the shield side portion, wherein a distance from the shield opening to the second inner end portion is longer than a distance from the shield opening to the first inner end portion, wherein a distance from the shield opening to the second outer end portion is longer than a distance from the shield opening to the first outer end portion, and wherein a distance from the shield opening to the second outer end portion is shorter than a distance from the shield opening to the first inner end portion. 
     According to one embodiment of the present invention, there is provided a cryopump structure including a plurality of cryosorption panels, wherein each of the plurality of cryosorption panels includes an inclined front surface that is close to a cryopump inlet on a radially outer side thereof and that is away from the inlet on a radially inner side thereof, the inclined front surface having a non-adsorption area, and wherein the plurality of cryosorption panels are arranged in a nested manner such that one cryosorption panel out of two adjacent cryosorption panels that is close to the cryopump inlet extends toward the cryopump inlet over a non-adsorption area of the other cryosorption panel that is away from the cryopump inlet. 
     According to one embodiment of the present invention, there is a cryopump structure including a plurality of cryosorption panels, wherein each of the plurality of cryosorption panels includes an inclined front surface that is close to a cryopump inlet on a radially outer side thereof and that is away from the inlet on a radially inner side thereof, the inclined front surface having an inclination angle toward a radiation shield, and wherein the plurality of cryosorption panels are arranged in a nested manner such that one cryosorption panel out of two adjacent cryosorption panels that is close to the cryopump inlet extends toward the cryopump inlet over an upper end of the other cryosorption panel that is away from the cryopump inlet. 
     According to one embodiment of the present invention, there is a vacuum evacuation method of pumping hydrogen by a cryopump including a nested array of cryopanels, including: reflecting, by a cryopanel, a hydrogen molecule incident into a clearance in the nested array of cryopanels; and adsorbing a reflected hydrogen molecule by another cryopanel. 
     Optional combinations of the aforementioned constituting elements, and implementations of the invention in the form of methods, apparatuses, and systems, may also be practiced as additional modes of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
         FIG. 1  is a cross-sectional view schematically illustrating a cryopump according to an embodiment of the present invention; 
         FIG. 2  is a lateral view schematically illustrating a low-temperature cryopanel according to an embodiment of the present invention; 
         FIG. 3  is a perspective view schematically illustrating a cryopanel according to an embodiment of the present invention; 
         FIG. 4  is a view for explaining the arrangement of the cryopanel shown in  FIG. 2 ; 
         FIG. 5  is a view for explaining the behavior of a hydrogen molecule when the hydrogen molecule collides against a cryopanel; 
         FIG. 6  is a view schematically illustrating part of a cryopanel according to an embodiment of the present invention; 
         FIG. 7  is a view for explaining a hydrogen gas vacuum evacuation method according to an embodiment of the present invention; 
         FIG. 8  is a schematic lateral view of a cryopump according to an embodiment of the present invention; and 
         FIG. 9  is a schematic top view of a cryopump according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
       FIG. 1  is a cross-sectional view schematically illustrating a cryopump  10  according to an embodiment of the present invention.  FIG. 1  illustrates a cross section including both a central axis A of an internal space  14  of the cryopump  10  and a refrigerator  16 . 
     The cryopump  10  is installed in a vacuum chamber in, for example, an ion implantation apparatus, sputtering apparatus, or the like, to be used for improving the vacuum degree of the inside of the vacuum chamber to a level required in a desired process. 
     The cryopump  10  has a cryopump inlet  12  serving as an intake port for receiving a gas. The cryopump inlet  12  may be referred to simply as an inlet  12  or a pump inlet  12  in the following. A gas to be pumped enters the internal space  14  of the cryopump  10  via the inlet  12  from the vacuum chamber in which the cryopump  10  is mounted. 
     In the following, terms “axial direction” and “radial direction” are often used to facilitate the understanding of a positional relationship of constituting elements of the cryopump  10 . The axial direction represents a direction passing through the pump inlet  12  (a direction along a dashed-dotted line A in  FIG. 1 ), and the radial direction represents a direction along the inlet  12  (a direction perpendicular to the dashed-dotted line A). For the sake of convenience, relative closeness to the pump inlet  12  in the axial direction may be referred to as “upper” and “upward,” and relative remoteness therefrom may be referred to as “lower” and “downward.” In other words, relative remoteness from the bottom of the cryopump  10  may be referred to as “upper” and “upward,” and relative closeness thereto may be referred to as “lower” and “downward,” both in the axial direction. With respect to the radial direction, relative closeness to the center of the pump inlet  12  (a central axis A in  FIG. 1 ) may be referred to as “inner” and “inside,” and relative closeness to the circumference of the inlet  12  may be referred to as “outer” and “outside.” It should be noted that these expressions are not related to a position of the cryopump  10  as mounted on a vacuum chamber. For example, the cryopump  10  may be mounted on a vacuum chamber in such a manner that the pump inlet  12  faces downward in the vertical direction. 
     The cryopump  10  is provided with a refrigerator  16 . The refrigerator  16  is, for example, a cryogenic refrigerator such as a Gifford-McMahon refrigerator (so-called GM refrigerator). The refrigerator  16  is a two-stage refrigerator provided with a first stage  22  and a second stage  24 . The refrigerator  16  is configured to cool the first stage  22  to a first temperature level and cool the second stage  24  to a second temperature level. The second temperature level is lower than the first temperature level. For example, the first stage  22  is cooled to approximately 65 K to 120 K and preferably to 80 K to 100 K, and the second stage  24  is cooled to approximately 10 K to 20 K. 
     The cryopump  10  illustrated in  FIG. 1  is a so-called horizontal-type cryopump. In general, a horizontal-type cryopump is a cryopump arranged such that the refrigerator  16  intersects (orthogonally in general) with the central axis A of the internal space  14  of the cryopump  10 . The present invention is also applicable to a so-called vertical-type cryopump in a similar manner. A vertical-type cryopump is a cryopump with a refrigerator arranged along the axial direction of the cryopump. 
     The cryopump  10  is provided with a high-temperature cryopanel  18  and a low-temperature cryopanel  20 . The high-temperature cryopanel  18  is mainly a cryopanel that is provided to protect the low-temperature cryopanel  20  from radiant heat emitted from a cryopump housing  38 . The high-temperature cryopanel  18  includes a radiation shield  30  and an inlet cryopanel  32  and surrounds the low-temperature cryopanel  20 . The high-temperature cryopanel  18  is thermally connected to the first stage  22 . Therefore, the high-temperature cryopanel  18  is cooled to the first temperature level. 
     The radiation shield  30  is located between the cryopump housing  38  and the low-temperature cryopanel  20  and surrounds the low-temperature cryopanel  20 . The radiation shield  30  includes a shield front end  28  that defines a shield opening  26 , a shield bottom portion  34  that faces the shield opening  26 , and a shield side portion  36  that extends from the shield front end  28  to the shield bottom portion  34 . 
     The radiation shield  30  has an open upper end in the axial direction and is provided with a shield opening  26  at the pump inlet  12 . The pump inlet  12  is defined by a front end  40  of the cryopump housing  38 . The radiation shield  30  has a tubular shape (e.g., cylindrical) where the shield bottom portion  34  is closed and is formed into a cup-like shape. The shield side portion  36  has a hole for mounting the refrigerator  16 , and the second stage  24  is inserted inside the radiation shield  30  via the hole. The first stage  22  is fixed to the outer surface of the radiation shield  30  at the outer circumferential portion of the mounting hole. As described, the radiation shield  30  is thermally connected to the first stage  22 . 
     The inlet cryopanel  32  is arranged such that the inlet cryopanel  32  occupies the central part of the opening area of the pump inlet  12  and forms an annular open area between the radiation shield  30  and the inlet cryopanel  32 . The inlet cryopanel  32  is mounted to the shield front end  28  via a panel mounting structure  158  (see  FIG. 9 ). As described, the inlet cryopanel  32  is fixed to the radiation shield  30  and is thermally connected to the radiation shield  30 . The inlet cryopanel  32  may be, for example, a disc-shaped baffle. Alternatively, the inlet cryopanel  32  may have a louver shape where the inlet cryopanel  32  is formed concentrically or may have a chevron shape. Although the inlet cryopanel  32  is located close to the low-temperature cryopanel  20 , the inlet cryopanel  32  is not in contact with the low-temperature cryopanel  20 . 
     A gas (for example, moisture) that condenses at a cooling temperature of the inlet cryopanel  32  is trapped on the surface thereof. The inlet cryopanel  32  is provided also to protect the low-temperature cryopanel  20  from radiant heat emitted from a heat source outside the cryopump  10  (for example, a heat source inside a vacuum chamber on which the cryopump  10  is mounted). 
     The low-temperature cryopanel  20  is arranged in a center portion of the internal space  14  of the cryopump  10 . For example, the low-temperature cryopanel  20  is arranged in a layout where the low-temperature cryopanel  20  surrounds the central axis A of the radiation shield  30 .  FIG. 1  shows, by a broken line, an approximate area in which the low-temperature cryopanel  20  is installed. Details of the low-temperature cryopanel  20  will be described later. The low-temperature cryopanel  20  is mounted to the second stage  24  via a panel mounting member  112  (see  FIG. 2 ). The low-temperature cryopanel  20  is thermally connected to the second stage  24  in this way. Thus, the low-temperature cryopanel  20  is cooled to the second temperature level. 
     An adsorption area is formed on at least part of the surface of the low-temperature cryopanel  20 . A detailed explanation thereof will be described later. An adsorption area is provided to capture a non-condensable gas (e.g., hydrogen) by adsorption. The adsorption area is formed by, for example, attaching an adsorbent (e.g., activated charcoal) to the cryopanel surface. A condensation area for capturing a condensable gas by condensing the condensable gas is formed on at least part of the low-temperature cryopanel  20 . The condensation area is, for example, a section where the absorbent is absent on a cryopanel surface, exposing the surface (e.g., metal surface) of a cryopanel substrate. Thus, a condensation area can be also called a non-adsorption area. Therefore, the low-temperature cryopanel  20  can be considered as an adsorption panel or a cryosorption panel that has a condensation area (also referred to as non-adsorption area) on part thereof. Also, the low-temperature cryopanel  20  can be considered as a condensation panel or a cryocondensation panel that has an adsorption area on part thereof. 
       FIG. 2  is a lateral view schematically illustrating a low-temperature cryopanel  20  according to an embodiment of the present invention. The illustration of a refrigerator  16  is omitted in  FIG. 2  for the purpose of simplifying the figure. The low-temperature cryopanel  20  is configured as a cryopanel assembly  100  provided with a plurality of cryopanels  102 . The plurality of cryopanels  102  are arranged along a direction directed toward a shield bottom portion  34  from a shield opening  26  (i.e., along a central axis A). 
     In the embodiment shown in  FIG. 2 , an individual cryopanel  102  has a cryopanel surface that surrounds the central axis A on the outside of the central axis A. The cryopanel assembly  100  is provided with a plurality of inclined cryopanels where the normal of the front surface of a cryopanel  102  extends obliquely upward in a radially inward direction toward the central axis A. The cryopanel assembly  100  has fourteen cryopanels  102 . 
       FIG. 3  is a perspective view schematically illustrating a cryopanel  102  according to an embodiment of the present invention. The cryopanel  102  has the shape of an inverted truncated cone. The cryopanel  102  can be also said to have a mortar shape, a basinal shape, or a ball shape. The cryopanel  102  has a large dimension at an upper end portion  104  (i.e., has a large diameter) and has a small dimension at a lower end portion  106  (i.e., has a small diameter). 
     The cryopanel  102  is provided with an inclined area  108  connecting the upper end portion  104  and the lower end portion  106 . The inclined area  108  represents the side surface of the inverted truncated cone. Therefore, the cryopanel  102  have an inclination such that the normal of the front surface of the cryopanel  102  intersects the central axis A. The inclined area  108  occupies substantially the whole of a width D of the cryopanel in the radial direction. 
     As shown in  FIG. 3 , the cryopanel  102  may be provided with a mounting portion  110  at the lower end portion  106 . The mounting portion  110  is a flat area. The mounting portion  110  is a flange for mounting the cryopanel  102  on a panel mounting member  112  (see  FIG. 2  and  FIG. 4 ). The panel mounting member  112  is provided for mechanically fixing a cryopanel  102  to the second stage  24  of the refrigerator  16  (see  FIG. 1 ) and thermally connecting the cryopanel  102  to the second stage  24 . The cryopanel  102  can be easily mounted on the panel mounting member  112  by providing such a flat mounting flange. 
     The shape of a cryopanel  102  is not limited to an inverted truncated cone shape. Alternatively, a cryopanel  102  may have another arbitrary shape, for example, an inverted frustum shape. The inclined area  108  may occupy at least a half of the width D of the cryopanel in the radial direction from the central axis of the cryopanel  102 . The inclined area  108  may be provided at the outer circumferential portion of the cryopanel  102 . In this case, parts other than the inclined area  108  of the cryopanel  102  (e.g., inner circumferential portion) may extend horizontally along the radial direction. The mounting portion  110  for mounting the cryopanel  102  on a panel mounting member  112  (see  FIG. 3 ) is not limited to a flat portion that extends horizontally on a surface perpendicular to the central axis of the cryopanel  102 . The mounting portion  110  may be, for example, an arbitrary non-inclined area that includes a flat portion extending in a vertical direction along the central axis of the cryopanel  102 . 
     A cutout or an opening (not shown) for insertion of the refrigerator  16  may be formed in the cryopanel  102 . 
     The plurality of cryopanels  102  are arranged coaxially with the central axis A of the radiation shield  30 , as illustrated in  FIG. 2 . Therefore, the inclined area  108  of each of the plurality of cryopanels  102  is away from the shield opening  26  at the lower end portion  106 , which is close to the central axis A (see  FIG. 3 ), and is inclined to be close to the shield opening  26  at the upper end portion  104 , which is far from the central axis A. The inclined area  108  occupies substantially the whole of the width of the cryopanel  102  from the central axis A in the radial direction. A cryopanel  102  that is close to the pump inlet  12  is smaller than a cryopanel  102  that is far from the pump inlet  12 . Of two cryopanels  102  that are adjacent to each other, the upper cryopanel has a smaller diameter than that of the lower cryopanel  102 . In this way, a clearance for receiving a hydrogen gas is formed between the upper cryopanel and the lower cryopanel. 
       FIG. 4  is a view for explaining the arrangement of the cryopanel shown in  FIG. 2 .  FIG. 4  shows, by a broken line, the internal structure of the cryopanel assembly  100  shown in  FIG. 2 . 
     In the cryopanel assembly  100 , a plurality of cryopanels  102  are arranged in a nested manner. An explanation is given in the following regarding this cryopanel arrangement using, for example, three cryopanels  114 ,  116 , and  118  that are adjacent to one another as examples. The upper cryopanel  114  close to the pump inlet  12  is referred to as a first cryopanel  114 . Of the three cryopanels, the intermediate cryopanel  116  is referred to as a second cryopanel  116 , and the lower cryopanel  118  far from the pump inlet  12  is referred to as a third cryopanel  118 . In  FIG. 2 , the first cryopanel  114  is the fourth cryopanel from the bottom, the second cryopanel  116  is the third cryopanel from the bottom, and the third cryopanel  118  is the second cryopanel from the bottom. 
     In the following, an explanation is given regarding a positional relationship using the three cryopanels  114 ,  116 , and  118 . It should be understood that other cryopanels also have a similar relationship, as shown in the figure. 
     A first line of sight  120  and a second line of sight  122  from the shield front end  28  are illustrated by broken lines in  FIG. 4  for explanation. The first line of sight  120  is a line of sight from the shield front end  28  to the exterior end of the first cryopanel  114 . The second line of sight  122  is a line of sight from the shield front end  28  to the exterior end of the second cryopanel  116 . 
     The trajectory of the first line of sight  120  on the front surface of the second cryopanel  116  provides a boundary between the adsorption area  124  and the condensation area  126  on the front surface of the second cryopanel  116 . The trajectory of the second line of sight  122  on the front surface of the third cryopanel  118  provides a boundary between the adsorption area  124  and the condensation area  126  on the front surface of the third cryopanel  118 . In the same way, a boundary between a adsorption area  124  and a condensation area  126  can be determined for the rest of the cryopanels  102 . 
     Therefore, in a cryopanel  102  that is far from the pump inlet  12 , the area ratio of the adsorption area  124  on the front surface of the cryopanel is large. On the other hand, in a cryopanel  102  that is close to the pump inlet  12 , the adsorption area  124  either have a small area ratio on the front surface of the cryopanel or does not exist, leaving the entire area of the front surface to be a condensation area  126 . In particular, in a top cryopanel  137 , which is the closest to the pump inlet  12 , the entire area of the front surface is a condensation area  126 . The entire area of the respective front surfaces of a few or several cryopanels that are the closest to the pump inlet  12  may be a condensation area  126 . 
       FIG. 2  is referred back. The cryopanel assembly  100  is divided into an upper structure  128  and a lower structure  130 . The upper structure  128  includes at least one cryopanel  102 , and said at least one cryopanel  102  is provided with an inclined area  108  that has an inclination angle toward the shield front end  28  (see  FIG. 3 ). The cryopanel  102  having such an inclination may be referred to as an upper cryopanel in the following. The inclination angle of a cryopanel is an angle between a plane perpendicular to the central axis A and the surface of a cryopanel  102 . 
     The upper cryopanel  102  has an inclination angle that is adjusted such that a back surface  132  thereof is not visible from the outside of the cryopump  10 . In other words, the inclination angle of the back surface  132  (i.e., inclined area  108 ) is determined such that the line of sight from the shield front end  28  does not intersect the back surface  132 . Therefore, the respective exterior ends of the upper cryopanels  102  are directed to a point slightly below the shield front end  28 , as shown by a broken-line arrow  134  in  FIG. 2 . Therefore, each upper cryopanel  102  has a different inclination angle, and an inclination angle becomes smaller toward the pump inlet  12 . There can be a situation where a line of sight from the front end  40  of the cryopump housing  38 , instead of the shield front end  28 , needs to be taken into consideration so that the back surface  132  of an upper cryopanel  102  is not visible from the outside of the cryopump  10 . 
     The lower structure  130  of the cryopanel assembly  100  includes at least one cryopanel  102 . Said at least one cryopanel  102  is provided with an inclined area  108  (see  FIG. 3 ) that is inclined toward the shield side portion  36 , as shown by a broken-line arrow  136  in  FIG. 2 . The cryopanel  102  having such an inclination may be referred to as a lower cryopanel in the following. In other words, since the lower cryopanel  102  has an inclination angle toward the shield side portion  36 , a back surface  138  thereof is not visible from the outside of the cryopump  10 . All lower cryopanels  102  have the same inclination angle. 
     The adsorbent is provided on the entire area of the back surface  132  of the upper cryopanel  102 . The adsorbent is also provided on the entire area of the back surface  138  of the lower cryopanel  102 . In this way, each of the plurality of cryopanels  102  is provided with the adsorption area  124  at a site that is invisible from the outside of the cryopump  10 . Thus, the cryopanel assembly  100  is configured such that the adsorption area  124  is completely invisible from the outside of the cryopump  10 . 
     A gas accumulated in a cryopump is normally discharged substantially completely by a regeneration process. When the regeneration process is completed, the cryopump is recovered to have pumping performance according to the specifications. However, some constituents of an accumulated gas are relatively more likely to remain in the adsorbent even after the regeneration process. 
     For example, it has been observed in a cryopump installed for vacuum evacuation of an ion implantation apparatus that adhesive materials attach to activated charcoal that serves as the adsorbent. It has been difficult to completely remove these adhesive materials even by the regeneration process. These adhesive materials are considered to result from an organic outgas that is discharged from a photoresist coating on a substrate to be processed. It is also possible that these adhesive materials result from a poisonous gas used as a dopant gas, i.e., a source gas during an ion implantation process. There is also a possibility that the adhesive materials result from other byproduct gases in the ion implantation process. It is also possible the adhesive materials are created due to the complex interaction of these gases. 
     Most of the gas pumped by a cryopump can be hydrogen gas in the ion implantation process. The hydrogen gas is substantially completely discharged to the outside by the regeneration. If there is only a tiny amount of a hard-to-regenerate gas, an insignificant effect on the pumping performance of the cryopump will be found after a single cryopumping process. However, it is possible that the hard-to-regenerate gas is gradually accumulated in the adsorbent through the repetition of cryopumping and regeneration processes, thereby lowering the pumping performance. When the pumping performance drops below an acceptable range, maintenance work including, for example, as an exchange of either an adsorbent or a cryopanel along with the adsorbent, or a chemical process of removing a hard-to-regenerate gas performed on the adsorbent, will be required. 
     Almost without exception, the hard-to-regenerate gas is a condensable gas. Molecules of the condensable gas that fly toward the cryopump  10  from the outside reach the radiation shield  30  or the condensation area  126  at the outer circumference of the cryopanel assembly  100  in a straight route through the open area around the inlet cryopanel  32  and are captured on the surfaces thereof. By avoiding the exposure of the adsorption area to the pump inlet  12 , the adsorption area is protected from the hard-to-regenerate gas contained in a gas entering the cryopump  10 . The hard-to-regenerate gas is accumulated in the condensation area. In this way, both the protection of the adsorption area from the hard-to-regenerate gas and the high-speed pumping of a non-condensable gas can be achieved. Prevention of the exposure of the adsorption area is also useful in protecting the adsorption area from moisture. 
     As described above, the cryopanels  102  are arranged in a nested manner. Each cryopanel  102  is provided with a condensation area  126  at the outer end portion of the inclined area  108  on the front surface thereof. The upper end portion  104  of the first cryopanel  114  extends toward the pump inlet  12  (more properly, obliquely upward) over the condensation area  126  of the second cryopanel  116  (i.e., upper end portion  104 ). The second cryopanel  116 , which is far from the pump inlet  12 , surrounds a large part of the inclined area  108  and the lower end portion  106  of the first cryopanel  114 , which is near the pump inlet  12 . In this way, the plurality of cryopanels  102  is densely arranged overlapped with each other in the axial direction. 
     As shown in  FIG. 2  and  FIG. 4 , of the plurality of cryopanels  102 , the top cryopanel  137 , which is the closest to the inlet cryopanel  32 , does not overlap in the axial direction with an upper cryopanel  139 , which is the second closest to the inlet cryopanel  32 . As described, the upper structure  128  of the cryopanel assembly  100  may include at least one cryopanel that is distantly arranged in the axial direction. 
     In an embodiment, at least some or all cryopanels  102  of the upper structure  128  may be arranged in parallel as in the case of the cryopanels  102  of the lower structure  130 . Manufacturing is easy when all the cryopanels are arranged in parallel. In this case, a distal end of the top panel  137  may be directed to (slightly downward of) the front end of the cryopump, and the cryopanels that are below the top panel  137  may be directed to the shield side portion  36 . 
     As shown in  FIG. 4 , the first cryopanel  114  has a first inner end portion  140 , a first outer end portion  141 , and a first inclined portion  142  connecting the first inner end portion  140  and the first outer end portion  141 . A second cryopanel  116  has a second inner end portion  143 , a second outer end portion  144 , and a second inclined portion  145  connecting the second inner end portion  143  and the second outer end portion  144 . A third cryopanel  118  has a third inner end portion  146 , a third outer end portion  147 , and a third inclined portion  148  connecting the third inner end portion  146  and the third outer end portion  147 . 
     These cryopanels  114 ,  116 , and  118  are arranged in a nested manner in the axial direction as described above. The inner end portions  140 ,  143 , and  146  corresponds to the lower end portion  106  (see  FIG. 3 ), and the outer end portions  141 ,  144 , and  147  corresponds to the upper end portion  104  (see  FIG. 3 ). The inner end portions  140 ,  143 , and  146  are mounted on the panel mounting member  112 , and thus the respective bottom portions of the cryopanels  114 ,  116 , and  118  are closed. The outer end portions  141 ,  144 , and  147  define the respective inlet openings of the cryopanels  114 ,  116 , and  118 , which are open toward the pump inlet  12 . The outer end portions  141 ,  144 , and  147  are directed toward the shield side portion  36 . 
     The inclined portions  142 ,  145 , and  148  corresponds to the inclined area  108  (see  FIG. 3 ) and extend from the inner end portions  140 ,  143 , and  146  toward the outer end portions  141 ,  144 , and  147  in a linear manner, respectively. The inclined portions  142 ,  145 , and  148  extend radially outward from the central axis A toward the shield opening  26  from the shield bottom portion  34 . Therefore, there is a first clearance  149  extending obliquely upward in a radially outward direction in a linear manner from the proximity of the central axis A between the first cryopanel  114  and the second cryopanel  116 . There is a second clearance  150  extending obliquely upward in a radially outward direction in a linear manner from the proximity of the central axis A between the second cryopanel  116  and the third cryopanel  118 . In this way, the cryopanels  114 ,  116 , and  118  are arranged such that the cryopanels  114 ,  116 , and  118  reflect, toward the central axis A, gas molecules that have entered the clearances  149  and  150  between the cryopanels  114 ,  116 , and  118  by the respective upper inclined surfaces of the inclined portions  142 ,  145 , and  148 , respectively. 
     Arranged closer to the pump inlet  12  are the first cryopanel  114 , the second cryopanel, and the third cryopanel in said order. Therefore, a distance to the second inner end portion  143 , a distance to the second inclined portion  145 , and a distance to the second outer end portion  144  all from the shield opening  26  are longer than a distance to the first inner end portion  140 , a distance to the first inclined portion  142 , and a distance to the first outer end portion  141  all from the shield opening  26 , respectively. In the same way, a distance to the third inner end portion  146 , a distance to the third inclined portion  148 , and a distance to the third outer end portion  147  all from the shield opening  26  are longer than the distance to the second inner end portion  143 , the distance to the second inclined portion  145 , and the distance to the second outer end portion  144  all from the shield opening  26 , respectively. 
     Also, a distance F to the second outer end portion  144  from the shield opening  26  is shorter than a distance E to the first inner end portion  140  from the shield opening  26 . Further, a distance G to the third outer end portion  147  from the shield opening  26  is shorter than the distance E to the first inner end portion  140  from the shield opening  26 . As described, compared to the inner end portion of one given topside cryopanel, the respective outer end portions of some cryopanels located below the topside cryopanel are closer to the pump inlet  12 . In other words, the inclined portion of one given bottom-side cryopanel extends obliquely upward over the respective inner end portions of some cryopanels located above the bottom-side cryopanel. As described, a plurality of cryopanels  102  are arranged in a nested manner. 
     Such a positional relationship among cryopanels also applies to some cryopanels in the upper structure  128  as well as the lower structure  130 . This positional relationship is prominent in the lower structure  130 . For example, the outer end portion of the bottommost cryopanel  151  is closer to the pump inlet  12  than the respective inner end portions of six cryopanels that are located just above the bottommost cryopanel  151 . 
     In this way, the deep and narrow clearances  149  and  150  are formed between the cryopanels  114 ,  116 , and  118 . These clearances  149  and  150  extend deeply toward the inner end portions  140 ,  143 , and  146  from the respective clearance inlets of the outer end portions  141 ,  144 , and  147 , respectively. The respective depths of the clearances are larger than the respective widths of the clearance inlets. The depth of a clearance represents a distance to the inner end portion from the outer end portion or a length of the inclined portion from the outer side to the inner side in the radial direction. Having such a deep clearance structure, the cryopanel assembly  100  can increase the rate of capturing the hydrogen gas. In other words, the cryopanel assembly  100  can capture hydrogen molecules that have once entered the clearances  149  and  150  without letting the hydrogen molecules escape to the outside as possible. 
       FIGS. 5 and 6  are views for explaining the behavior of a hydrogen molecule when the hydrogen molecule collides against a cryopanel. In a cryopanel arrangement shown in  FIG. 5 , a first cryopanel  114  and a second cryopanel  116 , which are flat panels, are arranged in parallel. The first cryopanel  114  and the second cryopanel  116  extend along a surface that is perpendicular to a cryopump central axis. A first outer end portion  141  is arranged immediately above a second outer end portion  144 . 
     The behavior of a hydrogen molecule  152  (or other gas molecules) on a cryopanel surface at the time of collision can be basically considered just like the reflection of light. The hydrogen molecule  152 , however, is not simply reflected specularly on the cryopanel surface. The hydrogen molecule  152  is once captured momentarily on the cryopanel surface and is then released again from the cryopanel surface immediately after that. Accordingly, the direction in which the hydrogen molecule  152  is released is probabilistic and is not constant. The hydrogen molecule  152  can be considered to be released at almost an equal probability in all directions. Therefore, the reflection of the hydrogen molecule  152  is similar to diffused reflection of light. In  FIGS. 5 and 6 , the trajectory of an incoming hydrogen molecule  152  is illustrated by a solid arrow, and the trajectory of a reflected hydrogen molecule  152  is illustrated by a broken-line arrow. 
     In a cryopanel arrangement shown in  FIG. 5 , an angular range covered by the first cryopanel  114  when the first cryopanel  114  is viewed from the second outer end portion  144  is equal to exactly 90 degrees. Therefore, a hydrogen molecule  152  reflected from the second outer end portion  144  is directed to the back surface of the first cryopanel  114  with a probability of approximately 1/2 and is directed in a direction away from the first cryopanel  114  with a probability of approximately 1/2. 
     On the other hand, in a cryopanel arrangement shown in  FIG. 6 , a first cryopanel  114  and a second cryopanel  116  are inclined with respect to a cryopump central axis such that respective outer end portions  141  and  144  are directed obliquely upward. The first outer end portion  141  is arranged immediately above the second outer end portion  144 . As shown in  FIGS. 2 and 4 , the second outer end portion  144  may be located radially outward of the first outer end portion  141 . 
     In the cryopanel arrangement shown in  FIG. 6 , an angular range a covered by the first cryopanel  114  when the first cryopanel  114  is viewed from the second outer end portion  144  exceeds 90 degrees. Therefore, a hydrogen molecule  152  reflected from the second outer end portion  144  is directed to the back surface of the first cryopanel  114  with a probability larger than 1/2. The probability of a hydrogen molecule  152  being directed to the first cryopanel  114  from the second outer end portion  144  is determined by an angle α. A proportion of the angle α in the entire possible reflection range (e.g., 180 degrees) of a hydrogen molecule  152  provides this probability. In this way, more hydrogen molecules  152  can be reflected toward the adjacent cryopanel. 
       FIG. 4  is referred back again. An interval L between the first outer end portion  141  of the first cryopanel  114  and the second outer end portion  144  of the second cryopanel  116  is narrower than an interval K between the first inner end portion  140  of the first cryopanel  114  and the second inner end portion  143  of the second cryopanel  116 . In other words, a clearance inlet L between the cryopanels is narrower than a cryopanel mounting interval K. In this way, the clearance inlet L between the cryopanels can be brought close to the pump inlet  12 . 
     The cryopanel assembly  100  is provided close to the inlet cryopanel  32 . Therefore, more cryopanels can be arranged in the axial direction. In a case where a reduction in heat entering the cryopanel assembly  100  is emphasized, a space between the cryopanel assembly  100  and the inlet cryopanel  32  may be enlarged. 
       FIG. 7  is a view for explaining a method for vacuum pumping of hydrogen gas according to an embodiment of the present invention. As described above, a cryopump  10  is provided with a nested array of cryopanels  102 . This vacuum evacuation method includes reflecting a hydrogen molecule incident into a clearance in the nested array of cryopanels from a cryopanel  102  of the nested array of cryopanels, and adsorbing the reflected hydrogen molecule by another cryopanel  102  of the nested array of cryopanels. 
     For example, as shown by an arrow P in  FIG. 7 , a hydrogen molecule entering the cryopump  10  can be received in a deep and narrow clearance between cryopanels  102 . The hydrogen molecule that has entered the clearance is introduced deep into the clearance by reflection on the cryopanel surfaces. As shown by an arrow Q, a hydrogen molecule that has hit the front surface of an upper cryopanel is reflected toward the back surface of a cryopanel right above the upper cryopanel. Also, as shown by an arrow R, a hydrogen molecule reflected by a radiation shield can be also received in a deep and narrow clearance between cryopanels  102 . 
     As described, the cryopanel assembly  100  is configured such that a hydrogen molecule that has entered the cryopump  10  is introduced toward the central part of a cryopanel structure. An adsorption area is formed in the central part of the cryopanel structure. Therefore, the hydrogen molecule can be efficiently adsorbed, and high-speed pumping of hydrogen gas can be achieved. 
     The cryopump suggested earlier by the present applicant is also provided with a unique cryopanel structure that achieves both the high-speed pumping of hydrogen and the protection of the adsorbent. In this cryopanel structure, individual cryopanels extend toward a radiation shield along a plane that is perpendicular to the central axis of a cryopump. Such a cryopanel structure is illustrated in  FIG. 5 . Such a cryopump is disclosed in, for example, Japanese Patent Application No. 2011-107669, Japanese Patent Application No. 2011-107670, U.S. patent application Ser. Nos. 13/458,699, and 13/458,751, which are incorporated herein in their entirety by reference. 
     It has been confirmed that, in comparison with such a cryopump having horizontal cryopanels, the speed of pumping a hydrogen gas is 20 to 30 percent better in a cryopanel having inclined cryopanels according to the present embodiments. 
     A number of cryopumps are often installed in some vacuum systems. By using a cryopump according to the present embodiment, the number of cryopumps that are installed can be reduced. In other words, equivalent pumping speed can be achieved by a small number of cryopumps. For example, when three cryopumps are substituted for four cryopumps, the cost required for a cryopump system is reduced to approximately ¾. Therefore, the total cost for configuring a vacuum system can be greatly reduced. 
     Described above is an explanation based on the exemplary embodiments of the present invention. The invention is not limited to the above-mentioned embodiments, and various design modifications may be added. It will be obvious to those skilled in the art that such modifications are also within the scope of the present invention. 
       FIG. 8  is a schematic lateral view of a cryopump  10  according to an embodiment of the present invention. The cryopump  10  is provided with a cryopanel assembly  100 . The cryopanel assembly  100  is provided with an upper structure  128  and a lower structure  130 . The lower structure  130  is configured in the same way as in the above-described embodiment explained in reference to  FIG. 2 . In  FIG. 8 , the illustration of the upper central part of the lower structure  130  is omitted for the purpose of illustrating the entire upper structure  128 . 
     The upper structure  128  includes a cryopanel  103  shaped such that the cryopanel  102  having an inverted frustum shape is arranged upside down. In other words, a cryopanel  103  of the upper structure  128  has a frustum shape (e.g., truncated cone shape). A cryopanel  103  may be a flat plate. The size of a cryopanel  103  becomes bigger (larger diameter) as the cryopanel becomes closer to the pump inlet  12 . However, even the cryopanel  103  that is the closest to the pump inlet  12  is smaller than the inlet cryopanel  32  and is also smaller than any cryopanel  102  of the lower structure  130 . The cryopanel  103  of the upper structure  128  has an adsorption area on the back surface thereof. The cryopanel  103  of the upper structure  128  is capable of adsorbing a hydrogen molecule reflected from a cryopanel  102  of the lower structure  130 . 
     Therefore, the cryopanel assembly  100  includes at least one adsorption panel  103  provided between the shield opening  26  and the cryopanels  102 . At least one adsorption panel  103  extends toward the shield side portion  36 . At least one adsorption panel  103  is provided with an adsorption area for adsorbing a gas molecule reflected from the cryopanels  102  on the back surface thereof. As described, the upper structure  128  of the cryopanel assembly  100  may be configured as a cryopanel dedicated for adsorption. 
       FIG. 9  is a schematic top view of a cryopump  10  according to an embodiment of the present invention. Only one cryopanel  102  out of a plurality of cryopanels  102  is illustrated in  FIG. 9  for the purpose of simplifying the figure. 
     A cryopanel  102  is divided into a plurality of (e.g., three or more) panel pieces  154 , as shown in  FIG. 9 . In  FIG. 9 , the cryopanel  102  is divided into six panel pieces  154 , and an individual panel piece  154  has a triangular shape. Therefore, a cryopanel  102  has an inverted hexagonal pyramid shape. A panel piece  154  may be formed in any shape, for example, a square shape. The surface of a panel piece  154  may be flat or curved. 
     A slit  156  is formed between panel pieces  154 . A gas molecule can pass through the slit  156  and reach a cryopanel located deep inside thereof. Such a slit  156  may be provided on the cryopanel  102  shown in  FIG. 2  or on the cryopanel  102  shown in  FIG. 8 . 
     In general, most hydrogen molecules are adsorbed at the outer periphery portion of an adsorption area of the cryopanel assembly  100 . By providing the slit  156  on the cryopanel  102 , a hydrogen molecule can be introduced closer to the central part of the cryopanel assembly  100  or further deep inside the cryopanel assembly  100 . Therefore, uneven distribution of adsorbed hydrogen molecules can be decreased. Since adsorption areas in the central part or the deep part can be utilized, the storage amount of hydrogen can be increased. 
     The slits  156  may be arranged such that there are more slits  156  on the upper side of the cryopanel assembly  100  and less slits  156  on the lower side thereof. In other words, in the cryopanel assembly  100 , the panel pieces  154  may be arranged sparsely in the upper side and densely in the lower side. The slits  156  may not be provided on the lowest cryopanel  102 . The slits  156  of a cryopanel  102  may be provided such that the respective positions thereof are shifted from the slits  156  of its adjacent cryopanel  102 . For example, the slits  156  may be provided such that the slits  156  are shifted in a spiral manner from the upper side to the lower side in the axial direction. 
     A plurality of panel pieces  154  that form one given cryopanel  102  are mounted on the panel mounting member  112  at a specific mounting height in the same way as in a single cryopanel  102  that is not divided. Therefore, a mounting plane including a mounting position of an individual panel piece can be considered. This mounting plane is a plane that perpendicular to the central axis A. The plurality of panel pieces may be mounted having a torsion angle with respect to the mounting plane. In this way, a cryopanel  102  may be configured such that a hydrogen molecule reflected on the front surface of a given panel piece  154  of the cryopanel  102  is directed to the back surface of an adjacent panel piece  154  of the same cryopanel  102 . 
     In a preferred embodiment, a cryopanel assembly  100  may be provided with an upper structure  128  including a plurality of adsorption panels  103  (see  FIG. 8 ) and a lower structure  130  including a plurality of cryopanels  102  each having a plurality of slits  156  (see  FIG. 9 ). The slits  156  may not be provided on the lowest cryopanel  102 . Such a cryopanel structure can be also referred to as a pineapple type. It has been also confirmed for a pineapple-type cryopanel structure by simulation based on the Monte Carlo method that hydrogen pumping speed can be achieved that is equivalent to that of a cryopanel structure of a mortar shape described above. 
       FIG. 9  illustrates the inlet cryopanel  32  by a broken line. In addition,  FIG. 9  illustrates a cross-shaped panel mounting structure  158  for mounting the inlet cryopanel  32  on the radiation shield  30  by a broken line. 
     The embodiments of the present invention can be also expressed as follows. 
     1. A cryopump including: 
     a radiation shield including a shield front end that defines a shield opening, a shield bottom portion that faces the shield opening, and a shield side portion that extends from the shield front end to the shield bottom portion; and 
     a cryopanel assembly cooled to a temperature that is lower than that of the radiation shield, including a plurality of cryopanels arranged along a direction toward the shield bottom portion from the shield opening, 
     wherein the plurality of cryopanels includes: 
     a first cryopanel including a first inner end portion and a first outer end portion that is directed to the shield side portion; and 
     a second cryopanel including a second inner end portion and a second outer end portion that is directed to the shield side portion, 
     wherein a distance from the shield opening to the second inner end portion is longer than a distance from the shield opening to the first inner end portion, 
     wherein a distance from the shield opening to the second outer end portion is longer than a distance from the shield opening to the first outer end portion, and 
     wherein a distance from the shield opening to the second outer end portion is shorter than a distance from the shield opening to the first inner end portion. 
     According to this embodiment, the two cryopanels are arranged such that, although the second cryopanel is located behind the first cryopanel, the outer side of the second cryopanel is closer to the shield opening than the inner side of the first cryopanel. Therefore, a clearance between the two cryopanels extends obliquely upward from the respective inner end portions to the respective outer end portions of these cryopanels. By receiving a hydrogen gas in such a deep and narrow clearance, the hydrogen gas can be introduced to deep inside the clearance. Thus, the hydrogen gas can be captured efficiently. 
     2. The cryopump according to embodiment 1, 
     wherein the plurality of cryopanels further include a third cryopanel including a third inner end portion and a third outer end portion that is directed to the shield side portion, 
     wherein a distance from the shield opening to the third inner end portion is longer than a distance from the shield opening to the second inner end portion, 
     wherein a distance from the shield opening to the third outer end portion is longer than a distance from the shield opening to the second outer end portion, and 
     wherein a distance from the shield opening to the third outer end portion is shorter than a distance from the shield opening to the first inner end portion. 
     3. The cryopump according to embodiment 1 or 2, 
     wherein the first cryopanel is arranged with respect to the second outer end portion such that an angular range covered by the first cryopanel when the first cryopanel is viewed from the second outer end portion exceeds 90 degrees. 
     4. The cryopump according to any one of embodiments 1 through 3, 
     wherein each of the plurality of cryopanels includes an inclined area that is inclined such that the inclined area is away from the shield opening at a site close to a central axis of the radiation shield and is close to the shield opening at a site far from the central axis, and 
     wherein at least half of a width of the cryopanel in a radial direction from the central axis corresponds to the inclined area. 
     5. The cryopump according to embodiment 4, 
     wherein substantially the whole of the width corresponds to the inclined area. 
     6. The cryopump according to embodiment 4 or 5, 
     wherein the cryopanel assembly includes a support member configured to support the plurality of cryopanels, and 
     wherein each of the plurality of cryopanels includes a non-inclined area configured to mount the cryopanel on the support member. 
     7. The cryopump according to any one of embodiments 1 to 6, 
     Wherein each of the plurality of cryopanels has an inverted frustum shape. 
     8. The cryopump according to any one of embodiments 1 to 7, 
     wherein the plurality of cryopanels include an adsorption area at a site that is invisible from outside of the cryopump. 
     9. The cryopump according to any one of embodiments 1 to 8, 
     wherein the cryopanel assembly further includes at least one cryopanel provided between the shield opening and the plurality of cryopanels, and 
     wherein said at least one cryopanel is inclined toward the shield front end or a cryopump housing front end. 
     10. The cryopump according to embodiment 9, 
     wherein said at least one cryopanel has an inclination angle adjusted such that a back surface thereof is invisible from outside of the cryopump. 
     11. The cryopump according to any one of embodiments 1 to 10, 
     wherein the cryopanel assembly further includes at least one adsorption panel provided between the shield opening and the plurality of cryopanels, 
     wherein said at least one adsorption panel extends toward the shield side portion, and 
     wherein said at least one adsorption panel includes an adsorption area on a back surface thereof, the adsorption area configured to adsorb a gas molecule reflected from the plurality of cryopanels. 
     12. The cryopump according to any one of embodiments 1 to 11, 
     wherein a slit is formed on at least one of the plurality of cryopanels in order to allow a gas molecule to pass through said at least one of the plurality of cryopanels. 
     13. The cryopump according to any one of embodiments 1 to 12, 
     wherein a depth of a clearance formed between the first cryopanel and the second cryopanel is larger than a width of an inlet of the clearance. 
     14. A cryopump structure including a plurality of cryosorption panels, 
     wherein each of the plurality of cryosorption panels includes an inclined front surface that is close to a cryopump inlet on a radially outer side thereof and that is away from the inlet on a radially inner side thereof, the inclined front surface having a non-adsorption area, and 
     wherein the plurality of cryosorption panels are arranged in a nested manner such that one cryosorption panel out of two adjacent cryosorption panels that is close to the cryopump inlet extends toward the cryopump inlet over a non-adsorption area of the other cryosorption panel that is away from the cryopump inlet. 
     According to this embodiment, the two adjacent cryosorption panels are arranged in the nested manner. By receiving a hydrogen gas in such a clearance in the nested arrangement, the hydrogen gas can be introduced to deep inside the clearance. Thus, the hydrogen gas can be captured efficiently. 
     15. The cryopanel structure according to embodiment 14, 
     wherein each of the plurality of cryosorption panels has an inverted frustum shape having a large dimension at a side close to the cryopump inlet and having a small dimension at a side far from the cryopump inlet, and 
     wherein the plurality of cryosorption panels are arranged such that said other cryosorption panel surrounds said one cryosorption panel. 
     16. The cryopump according to embodiment 14 or 15, 
     wherein the non-adsorption area is formed on an outer circumferential portion of the plurality of cryosorption panels that is visually recognized through the cryopump inlet. 
     17. A cryopump structure including a plurality of cryosorption panels, 
     wherein each of the plurality of cryosorption panels includes an inclined front surface that is close to a cryopump inlet on a radially outer side thereof and that is away from the inlet on a radially inner side thereof, the inclined front surface having an inclination angle toward a radiation shield, and 
     wherein the plurality of cryosorption panels are arranged in a nested manner such that one cryosorption panel out of two adjacent cryosorption panels that is close to the cryopump inlet extends toward the cryopump inlet over an upper end of the other cryosorption panel that is away from the cryopump inlet. 
     18. A vacuum evacuation method of pumping hydrogen by a cryopump that includes a nested array of cryopanels, including: 
     reflecting, by a cryopanel, a hydrogen molecule incident into a clearance in the nested array of cryopanels; and 
     adsorbing a reflected hydrogen molecule by another cryopanel. 
     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. 
     Priority is claimed to Japanese Patent Application No. 2012-249001, filed on Nov. 13, 2012, the entire content of which is incorporated herein by reference.