Patent Publication Number: US-9905449-B2

Title: Electrostatic chuck

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/663,526, filed Mar. 20, 2015, which is based upon and claims the benefit of priorities from Japanese Patent Application No. 2014-066667, filed on Mar. 27, 2014 and Japanese Patent Application No. 2014-262592, filed on Dec. 25, 2014. The entire contents of these prior applications are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments of the invention relate generally to an electrostatic chuck. 
     BACKGROUND 
     In a substrate treatment apparatus for performing etching, chemical vapor deposition (CVD), sputtering, ion implantation, ashing, exposure, inspection, or the like, as means for adsorbing and holding an object to be adsorbed (a treatment object) such as a semiconductor wafer or a glass substrate, an electrostatic chuck is used. 
     The electrostatic chuck is fabricated by inserting an electrode in a ceramic dielectric substrate such as alumina and performing firing. The electrostatic chuck is for applying power for electrostatic adsorption to the built-in electrode, thereby adsorbing a substrate such as a silicon wafer by an electrostatic force. 
     In such a substrate treatment apparatus, for higher throughput, an increase in output of a plasma process and an increase in temperature of the plasma process are progressing. For the higher throughput, a cooling function of the object to be adsorbed is one of the main points. Further, realizing the higher throughput leads to an increase in the amount of heat which is input to the substrate treatment apparatus. For this reason, a material of a member which can be used in the electrostatic chuck is limited to a highly thermally-resistant material. 
     For example, for an adhesive to bond a ceramic dielectric substrate to a metal plate which supports the ceramic dielectric substrate, bonding strength between ceramic and metal at a high temperature, heat transference from the ceramic to the metal, flexibility capable of coping with shear stress due to a difference in thermal expansion, electrical insulation properties, and the like are required. While there is an adhesive having relatively high thermal conductivity or an adhesive having relatively excellent heat resistance and plasma resistance, as compared to ceramic, metal, or the like, the plasma resistance of the adhesive in the plasma process is the lowest among members which are used for the electrostatic chuck. For this reason, the life of the adhesive becomes the life of the electrostatic chuck. 
     If the adhesive is damaged in a process such as etching, a ceramic filler component which improves heat conduction or an elastomer component which cannot be gasified sometimes becomes a particle source. Further, if the adhesive is damaged, the thermal conductivity of the adhesive is reduced, and thus a function of heat conduction and a function of uniformly maintaining the temperature of the object to be adsorbed are not sometimes fulfilled. Therefore, an electrostatic chuck is desired in which it is possible to reduce damage to which the adhesive is subjected. 
     SUMMARY 
     According to an aspect of the invention, there is provided an electrostatic chuck including: a ceramic dielectric substrate having a first major surface on which an object to be adsorbed is placed, a second major surface on an opposite side to the first major surface, and a through-hole provided over from the second major surface to the first major surface; a metallic base plate which supports the ceramic dielectric substrate and has a gas introduction path that communicates with the through-hole; and a bonding layer which is provided between the ceramic dielectric substrate and the base plate and includes a resin material, the bonding layer having a space which is provided between an opening of the through-hole in the second major surface and the gas introduction path and is larger than the opening in a horizontal direction, and a first area in which an end face of the bonding layer on a side of the space intersects with the second major surface being recessed from the opening further than another second area of the end face which is different from the first area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating the configuration of an electrostatic chuck according to the embodiment; 
         FIGS. 2A and 2B  are schematic enlarged views showing the vicinity of a bonding layer of the embodiment; 
         FIG. 3  is a schematic enlarged cross-sectional view of a variation of a portion A shown in  FIG. 1 ; 
         FIGS. 4A and 4B  are graphs illustrating the relationship between a distance d and a temperature difference, and a graph illustrating the relationship between the distance d and conductance; 
         FIG. 5  is a schematic enlarged view showing the vicinity of another bonding layer of the embodiment; 
         FIG. 6  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment; 
         FIG. 7  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment; 
         FIG. 8  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment; 
         FIG. 9  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment; 
         FIG. 10  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment; 
         FIG. 11  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment; 
         FIG. 12  is a schematic cross-sectional view showing the conditions of the simulation; 
         FIGS. 13A to 13C  are schematic perspective views illustrating an example of the results of the simulation; and 
         FIGS. 14A and 14B  are schematic perspective views illustrating an example of the results of the simulation. 
     
    
    
     DETAILED DESCRIPTION 
     According to a first invention, there is provided an electrostatic chuck including: a ceramic dielectric substrate having a first major surface on which an object to be adsorbed is placed, a second major surface on an opposite side to the first major surface, and a through-hole provided over from the second major surface to the first major surface; a metallic base plate which supports the ceramic dielectric substrate and has a gas introduction path that communicates with the through-hole; and a bonding layer which is provided between the ceramic dielectric substrate and the base plate and includes a resin material, the bonding layer having a space which is provided between an opening of the through-hole in the second major surface and the gas introduction path and is larger than the opening in a horizontal direction, and a first area in which an end face of the bonding layer on a side of the space intersects with the second major surface being recessed from the opening further than another second area of the end face which is different from the first area. 
     According to the electrostatic chuck, regardless of the durability of an adhesive itself, it is possible to reduce damage to which the bonding layer is subjected. Even if the bonding layer is damaged, it is possible to reduce the scattering of particles. 
     According to a second invention, in the electrostatic chuck according to the first invention, in the first area when viewed in a direction perpendicular to a normal to the second major surface, an angle between the second major surface and the end face becomes larger toward the second major surface. 
     According to the electrostatic chuck, regardless of the durability of an adhesive itself, it is possible to reduce damage to which the bonding layer is subjected. Even if the bonding layer is damaged, it is possible to reduce the scattering of particles. 
     According to a third invention, in the electrostatic chuck according to the second invention, a third area in which an angle between the second major surface and the end face becomes smaller with distance from the second major surface in a direction of the normal is provided. 
     According to the electrostatic chuck, regardless of the durability of an adhesive itself, it is possible to reduce damage to which the bonding layer is subjected. Even if the bonding layer is damaged, it is possible to reduce the scattering of particles. 
     According to a fourth invention, in the electrostatic chuck according to the first invention, a distance between the end faces facing each other becomes shorter with distance from the second major surface in a direction of the normal. 
     According to the electrostatic chuck, regardless of the durability of an adhesive itself, it is possible to reduce damage to which the bonding layer is subjected. Even if the bonding layer is damaged, it is possible to reduce the scattering of particles. 
     According to a fifth invention, in the electrostatic chuck according to the first invention, in a distance d between the end face in the first area and a center of the through-hole and a distance D between the end faces facing each other in the second area, a relational expression of 2d≧D is established. 
     In a case where a cross-sectional structure of the end face is asymmetric, the distance d is set to be a distance of the maximum value among the distances between the end face in the first area and the center of the through-hole. According to the electrostatic chuck, it is possible to form a pocket in which particles can be deposited. 
     According to a sixth invention, in the electrostatic chuck according to the fifth invention, the distance d is 0.1 millimeters or more and 5.0 millimeters or less. 
     According to the electrostatic chuck, it is possible to attain both a reduction in the amount of damage to which an adhesive is subjected and uniform temperature distribution of the object. 
     According to a seventh invention, in the electrostatic chuck according to the first invention, the bonding layer has a bonding portion which bonds the second major surface and the base plate together, and an end portion which has the end face and forms the space, and a material of the bonding portion is different from a material of the end portion. 
     According to the electrostatic chuck, the end portion is made so as not to contain fillers improving thermal conductivity, and thus it is possible to reduce occurrence of particles. Further, in a case where a silicone adhesive is used as the bonding portion, a material having more excellent plasma resistance than the silicone adhesive can be used for the end portion. 
     According to an eighth invention, in the electrostatic chuck according to the first invention, the bonding layer has a bonding portion which bonds the second major surface and the base plate together, and an end portion which has the end face and forms the space, and a material of the bonding portion is the same as a material of the end portion. 
     According to the electrostatic chuck, it is possible to further enhance an adhesive force between the bonding portion and the end portion. 
     According to a ninth invention, in the electrostatic chuck according to the seventh invention, thermal conductivity of an adhesive which is used in the bonding portion is 0.1 watts/meter·kelvin or more, dielectric breakdown strength of an adhesive which is used in the bonding portion is 1 kilovolt/millimeter or more, and a heat resistance temperature of an adhesive which is used in the bonding portion is 40° C. or more. 
     According to the electrostatic chuck, it is possible to use an adhesive which can maintain insulation while maintaining good heat transfer even if the electrostatic chuck is used in a high-temperature process. Further, it is possible to have elasticity capable of alleviating a difference between the thermal expansion of the ceramic dielectric substrate and the thermal expansion of the base plate. 
     According to a tenth invention, in the electrostatic chuck according to the fifth invention, the electrostatic chuck further includes a porous body provided in the gas introduction path, wherein in the distance d and a radius R of the porous body, a relational expression of d&gt;R is established. 
     According to the electrostatic chuck, a pocket in which particles can be deposited is formed, and thus the convection of transfer gas can be created in the space such that particles are easily deposited in the pocket. That is, the convection of the transfer gas which selectively deposits particles in the pocket can be controlled in the space. For this reason, even if particles are generated, it is possible to reduce the scattering of the particles. Further, the porous body is provided, whereby it is possible to have high voltage resistance in the through-hole and the gas introduction path. 
     According to an eleventh invention, in the electrostatic chuck according to the fifth invention, the distance d is larger than a radius of an opening of the through-hole on a side of the first major surface. 
     According to the electrostatic chuck, regardless of the durability of an adhesive itself, it is possible to reduce damage to which the bonding layer is subjected. Even if the bonding layer is damaged, it is possible to reduce the scattering of particles. 
     According to a twelfth invention, in the electrostatic chuck according to the first invention, a length in the horizontal direction of the space is longer than a thickness of the bonding layer. 
     According to the electrostatic chuck, regardless of the durability of an adhesive itself, it is possible to reduce damage to which the bonding layer is subjected. Even if the bonding layer is damaged, it is possible to reduce the scattering of particles. 
     According to a thirteenth invention, in the electrostatic chuck according to the seventh invention, the end portion comes into contact with each of the second major surface and the base plate in a plane, and a length in the horizontal direction of the plane in which the end portion comes into contact with each of the second major surface and the base plate is longer than a thickness of the bonding layer. 
     According to the electrostatic chuck, unlike a case where, instead of the end portion, an O-ring is provided, the end portion of the bonding layer can contribute to the bonding between the ceramic dielectric substrate and the base plate. 
     According to a fourteenth invention, in the electrostatic chuck according to the thirteenth invention, an outer peripheral portion of the end portion, the outer peripheral portion being on an opposite side to the space when viewed from the end portion, is filled with the resin material. 
     According to the electrostatic chuck, unlike a case where, instead of the end portion, an O-ring is provided, it is possible to prevent occurrence of a space in the bonding layer. The end portion of the bonding layer can contribute to the bonding between the ceramic dielectric substrate and the base plate, and thus, it is possible to more solidly bond the ceramic dielectric substrate and the base plate to each other. 
     According to a fifteenth invention, in the electrostatic chuck according to the thirteenth invention, a plane in which the second major surface comes into contact with the end portion is on a same plane as a plane in which the second major surface is bonded by the bonding layer, and a plane in which the base plate comes into contact with the end portion is on a same plane as a plane in which the base plate is bonded by the bonding layer. 
     According to the electrostatic chuck, unlike a case where, instead of the end portion, an O-ring is provided, the end portion of the bonding layer can contribute to the bonding between the ceramic dielectric substrate and the base plate. 
     According to a sixteenth invention, in the electrostatic chuck according to the thirteenth invention, curvature of the end face in the first area is larger than curvature of the end face in the second area. 
     According to the electrostatic chuck, unlike a case where, instead of the end portion, an O-ring is provided, the end portion of the bonding layer can contribute to the bonding between the ceramic dielectric substrate and the base plate. 
     According to a seventeenth invention, in the electrostatic chuck according to the first invention, the ceramic dielectric substrate includes a Coulomb material having volume resistivity of 1×10 14  ohm·centimeter or more. 
     According to the electrostatic chuck, regardless of the durability of an adhesive itself, it is possible to reduce damage to which the bonding layer is subjected. Even if the bonding layer is damaged, it is possible to reduce the scattering of particles. 
     Hereinafter, an embodiment of the invention will be described with reference to the drawings. In addition, in each drawing, the same constituent elements are denoted by the same reference numerals and detailed description is appropriately omitted. 
     In addition, the drawings are schematic or conceptual, and the relationship between a thickness and a width of each portion, the ratio between the sizes of portions, and the like are not necessarily the same as those of reality. Further, even in a case of showing the same portion, there is also a case where the respective dimensions or ratios are shown differently according to the drawings. 
       FIG. 1  is a schematic cross-sectional view illustrating the configuration of an electrostatic chuck according to the embodiment. 
       FIGS. 2A and 2B  are schematic enlarged views showing the vicinity of a bonding layer of the embodiment. 
       FIG. 3  is a schematic enlarged cross-sectional view of a variation of a portion A shown in  FIG. 1 . 
       FIGS. 4A and 4B  are a graph illustrating the relationship between a distance d and a temperature difference, and a graph illustrating the relationship between the distance d and conductance. 
     In addition,  FIGS. 2A and 2B  are schematic cross-sectional views in a cutting surface passing through the center of a gas introducing path of a base plate. The following schematic cross-sectional views are those in the cutting surface. 
       FIG. 2A  is a schematic enlarged view of the portion A shown in  FIG. 1 .  FIG. 2B  is a schematic enlarged view of a portion B shown in  FIG. 2A . For convenience of description, in  FIG. 2A , an electrode  12  is omitted. The omission of the electrode  12  is the same in  FIGS. 3, 5, and 6 . 
       FIG. 4A  is a graph illustrating the relationship between a distance d between an end face  64  in an area A 1  shown in  FIG. 2A  and a center C 1  of a through-hole  15  and the conductance of transfer gas.  FIG. 4B  is a graph illustrating the relationship between the distance d between the end face  64  in the area A 1  shown in  FIG. 2A  and the center C 1  of the through-hole  15  and a temperature difference in a plane of an object W. The horizontal axes of the graphs shown in  FIGS. 4A and 4B  show the distance d (mm) between the end face  64  in the area A 1  and the center C 1  of the through-hole  15 . The vertical axis of the graph shown in  FIG. 4A  shows the conductance (sccm: standard cc/min) of the transfer gas. The vertical axis of the graph shown in  FIG. 4B  shows the temperature difference (° C.) in a plane of the object W. 
     As shown in  FIGS. 1 and 2A , an electrostatic chuck  110  according to the embodiment is provided with a ceramic dielectric substrate  11 , a base plate  50 , and a bonding layer  60 . 
     The ceramic dielectric substrate  11  is a flat plate-shaped base material made of, for example, sintered ceramic, and has a first major surface  11   a  on which the object W of adsorption such as a semiconductor substrate such as a silicon wafer is placed, and a second major surface  11   b  on the side opposite to the first major surface  11   a.    
     In the ceramic dielectric substrate  11 , the electrode  12  is provided. The electrode  12  is interposed between the first major surface  11   a  and the second major surface  11   b  of the ceramic dielectric substrate  11 . That is, the electrode  12  is formed so as to be inserted into the ceramic dielectric substrate  11 . The electrostatic chuck  110  generates electric charge on the first major surface  11   a  side of the electrode  12  by applying voltage for adsorption holding  80  to the electrode  12  and adsorbs and holds the object W by an electrostatic force. 
     Here, in the description of the embodiment, a direction (a first direction) connecting the first major surface  11   a  and the second major surface  11   b  shall be referred to as a Z-direction, one (a second direction) of directions orthogonal to the Z-direction shall be referred to as a Y-direction, and a direction (a third direction) orthogonal to the Z-direction and the Y-direction shall be referred to as an X-direction. 
     The electrode  12  is provided in the form of a thin film along the first major surface  11   a  and the second major surface  11   b  of the ceramic dielectric substrate  11 . The electrode  12  is an adsorption electrode for adsorbing and holding the object W. The electrode  12  may be a unipolar type or may also be a bipolar type. The electrode  12  shown in  FIG. 1  is a bipolar type, and the two-pole electrode  12  is provided on the same plane. 
     At the electrode  12 , a connection portion  20  extending to the second major surface  11   b  side of the ceramic dielectric substrate  11  is provided. The connection portion  20  is made by connecting a via (a solid type), a via hole (a hollow type), or a metal terminal which is electrically connected to the electrode  12 , by an appropriate method such as brazing. 
     The base plate  50  is a member which supports the ceramic dielectric substrate  11 . The ceramic dielectric substrate  11  is fixed onto the base plate  50  through the bonding layer  60  shown in  FIG. 2A . That is, the bonding layer  60  is provided between the ceramic dielectric substrate  11  and the base plate  50 . 
     The bonding layer  60  has a bonding portion  61  and an end portion  63 . The bonding portion  61  bonds the second major surface  11   b  of the ceramic dielectric substrate  11  and the base plate  50  together. The bonding layer  60  includes a resin material. The bonding layer  60  includes a polymer material which is, for example, a silicone-based, acrylic, modified silicone-based, or epoxy-based polymer material and contains at least one of carbon (C), hydrogen (H), nitrogen (N), silicon (Si), oxygen (O), and sulfur (S) as its main component. As for the bonding portion  61 , for example, a silicone adhesive, a silicone-based heat conduction material having excellent electrical insulation properties, or the like is used. The end portion  63  has, for example, a ring-like shape. The details of the bonding layer  60  will be described later. 
     The base plate  50  is divided into an upper portion  50   a  and a lower portion  50   b  made of, for example, aluminum, and a communication path  55  is provided between the upper portion  50   a  and the lower portion  50   b . The communication path  55  is connected, on the one end side, to an input path  51  and connected, on the other end side, to an output path  52 . 
     The base plate  50  also plays a role of performing temperature adjustment of the electrostatic chuck  110 . For example, in a case of cooling the electrostatic chuck  110 , a cooling medium flows in from the input path  51 , passes through the communication path  55 , and then flows out from the output path  52 . In this way, the heat of the base plate  50  is absorbed by the cooling medium, and thus the electrostatic chuck  110  mounted thereon is cooled. On the other hand, in a case of keeping the electrostatic chuck  110  warm, it is also possible to put a heat maintaining medium in the communication path  55 . Or, it is also possible to make the electrostatic chuck  110  or the base plate  50  have a built-in heating element. In this manner, if the temperature of the electrostatic chuck  110  is adjusted through the base plate  50 , it is possible to adjust the temperature of the object W which is adsorbed and held by the electrostatic chuck  110 . 
     Further, on the first major surface  11   a  side of the ceramic dielectric substrate  11 , projections  13  are provided as necessary, and a groove  14  is provided between the projections  13 . The groove  14  is in communication with the outside, and thus a space is formed between the back surface of the object W placed on the electrostatic chuck  110  and the groove  14 . 
     The through-hole  15  provided in the ceramic dielectric substrate  11  is connected to the groove  14 . The through-hole  15  is provided to penetrate the ceramic dielectric substrate  11  over a range from the second major surface  11   b  to the first major surface  11   a  of the ceramic dielectric substrate  11 . The through-holes  15  may be provided at a plurality of places in the ceramic dielectric substrate  11 . 
     In addition, as shown in  FIG. 3 , the through-hole  15  may have a portion in which an axis of a hole extends in a horizontal direction (the X-direction). The through-hole  15  shown in  FIG. 3  has a first hole portion  15   a , a second hole portion  15   b , and a third hole portion  15   c . One end of the first hole portion  15   a  is connected to the second major surface  11   b  of the ceramic dielectric substrate  11 . One end of the third hole portion  15   c  is connected to the groove  14 . The second hole portion  15   b  is connected to the first hole portion  15   a  and the third hole portion  15   c . More specifically, one end of the second hole portion  15   b  is connected to the other end of the first hole portion  15   a . The other end of the second hole portion  15   b  is connected to the other end of the third hole portion  15   c . In this manner, the through-hole  15  has a space physically connecting the first major surface  11   a  and the second major surface  11   b  and is not limited to a straight line-shaped hole. Further, the shape of the through-hole  15  may be, for example, a spherical shape or an arc shape and is not limited to a specific shape. In a case where a plurality of through-holes  15  are provided, if at least one of the plurality of through-holes  15  satisfies the conditions of the through-hole of the embodiment, the electrostatic chuck  110  according to the embodiment is included in the scope of the invention. 
     As a material of the ceramic dielectric substrate  11 , for example, a Coulomb material is used. The volume resistivity of the Coulomb material is, for example, about 1×10 14  ohm·centimeter (Ω·cm) or more. In a case where the Coulomb material which is used for the ceramic dielectric substrate  11  has semipermeability with respect to infrared or visible light, it is possible to visually confirm an internal space from the surface of the ceramic dielectric substrate  11 . For this reason, as shown in  FIG. 3 , in a case where the through-hole  15  has a portion (the second hole portion  15   b ) in which an axis of a hole extends in the horizontal direction (the X-direction), it is possible to confirm the position of the second hole portion  15   b  from the surface of the ceramic dielectric substrate  11 , and thus it is possible to more easily perform processing. 
     By appropriately selecting the height of the projection  13  (the depth of the groove  14 ), the area ratio between the projection  13  and the groove  14 , the shapes of the projection  13  and the groove  14 , or the like, it is possible to control the temperature of the object W or particles which are stuck to the object W, to be in a favorable state. 
     On the other hand, in the base plate  50 , a gas introduction path  53  is provided. The gas introduction path  53  is provided so as to, for example, penetrate the base plate  50 . As shown in  FIG. 1 , in the gas introduction path  53 , an insulator plug  70  may be provided. The details of the insulator plug  70  will be described later. The gas introduction path  53  may be provided to branch from the middle of another gas introduction path  53  to the ceramic dielectric substrate  11  side without penetrating the base plate  50 . Further, gas introduction paths  53  may be provided at a plurality of places in the base plate  50 . 
     The gas introduction path  53  communicates with the through-hole  15 . If transfer gas such as helium (He) is introduced from the gas introduction path  53  in a state of adsorbing and holding the object W, the transfer gas flows into a space provided between the object W and the groove  14 , and thus it becomes possible to directly cool the object W by the transfer gas. 
     As shown in  FIG. 2A , a space  65  is present between the through-hole  15  and the gas introduction path  53 . More specifically, the space  65  is present between an opening  15   d  of the through-hole  15  in the second major surface  11   b  and the gas introduction path  53 . That is, the bonding layer  60  has the space  65 . The space  65  is located at a central portion of the end portion  63  having, for example, a ring shape and extends in the horizontal direction (the X-direction). The space  65  is formed by the ring-shaped end portion  63 . A dimension (a distance between the end portions  63  (or the end faces  64 ) facing each other) D 1  in the X-direction of the space  65  is larger than an opening dimension D 2  of the opening  15   d.    
     When bonding the ceramic dielectric substrate  11  and the base plate  50  together, first, the end portion  63  fabricated in advance is installed on a surface  57  of the base plate  50  or the second major surface  11   b  of the ceramic dielectric substrate  11  such that the space  65  is present between the through-hole  15  and the gas introduction path  53 . Subsequently, an adhesive (for example, a silicone adhesive) which becomes the bonding portion  61  after curing is applied while securing the space  65 . Subsequently, the ceramic dielectric substrate  11  and the base plate  50  are fitted to each other with the end portion  63  and the applied adhesive interposed therebetween. 
     After the adhesive is cured (after the bonding layer  60  is formed), a thickness (a dimension in the Z-direction) t 1  of the bonding layer  60  is, for example, about 100 micrometers (μm) or more and 1000 μm or less. More preferably, the thickness t 1  of the bonding layer  60  is, for example, about 200 μm or more and 600 μm or less. In this case, the thickness (the dimension in the Z-direction) of the end portion  63  in a state of being fabricated in advance (a state before installation) is, for example, about 200 μm or more and 600 μm or less. That is, the end portion  63  is crushed in the Z-direction in a process of fitting the ceramic dielectric substrate  11  and the base plate  50  to each other. After the adhesive is cured, the thickness of the end portion  63  is the same as the thickness t 1  of the bonding layer  60 . 
     The thickness t 1  of the bonding layer  60  is smaller than the dimension D 1  in the X-direction of the space  65 . That is, the length in the horizontal direction (the X-direction) of the space  65  is longer than the length in the vertical direction (the Z-direction) of the space  65 . In other words, the length in the horizontal direction of the space  65  is longer than the thickness t 1  of the bonding layer  60 . The space  65  having a cross-sectional shape which is longer in the horizontal direction than the vertical direction is connected to the through-hole  15  having a cross-sectional shape which is longer in the vertical direction than the horizontal direction. 
     The end face  64  of the bonding layer  60  on the space  65  side intersects with or comes into contact with the second major surface  11   b  of the ceramic dielectric substrate  11 . The area A 1  (a first area) in which the end face  64  intersects with the second major surface  11   b  is away from or is recessed from the opening  15   d  of the through-hole  15 , compared to another area (a second area) of the end face  64  which is different from the area A 1 . 
     More specifically, in the area A 1  when viewed in a direction perpendicular to a normal to the second major surface  11   b , the angle between the second major surface  11   b  and the end face  64  becomes larger toward the second major surface  11   b.    
     Here, in the specification, the “angle between the second major surface  11   b  and the end face  64 ” shall refer to the angle between the second major surface  11   b  of the ceramic dielectric substrate  11  and a plane tangent to an arbitrary point on the end face  64 , which is measured on a side of the end portion  63 . 
     As shown in  FIG. 2B , for example, an angle A 12  between the second major surface  11   b  and a plane S 2  tangent to a point  64   b  on the end face  64  is larger than an angle A 11  between the second major surface  11   b  and a plane S 1  tangent to a point  64   a  on the end face  64 . 
     On the other hand, in an area A 2  (a third area) in which the end face  64  intersects with or comes into contact with the surface  57  of the base plate  50 , when viewed in a direction perpendicular to a normal to the second major surface  11   b , the angle between the second major surface  11   b  and the end face  64  becomes smaller with distance from the second major surface  11   b  in a normal direction. As shown in  FIG. 2B , for example, an angle A 14  between the second major surface  11   b  and a plane S 4  tangent to a point  64   d  on the end face  64  is smaller than an angle A 13  between the second major surface  11   b  and a plane S 3  tangent to a point  64   c  on the end face  64 . 
     According to the embodiment, regardless of the durability of the adhesive, it is possible to reduce damage to which the bonding layer  60  is subjected. Even if the bonding layer  60  is damaged, it is possible to reduce the scattering of particles. 
     As shown in  FIG. 2A , with respect to the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  and the distance D 1  between the end portions  63  (the distance between the end faces  64 ) facing each other in another area of the end face  64  which is different from the area A 1 , the following expression is established.
 
2d≧D1  Expression (1)
 
     In the schematic cross-sectional view shown in  FIG. 2A , in a case where a cross-sectional structure of the end face  64  is asymmetric, the distance d is set to be a distance of the maximum value among the distances between the end face  64  in the area A 1  and the center C 1  of the through-hole  15 . Due to this, in the area A 1 , a pocket in which particles can be deposited is formed. 
     The diameter of the through-hole  15  (the opening dimension D 2  of the opening  15   d ) affects the conductance of the transfer gas flowing through the through-hole  15  and a temperature difference in the object W which is adsorbed (a temperature difference between the position on the object W just above the through-hole  15  and the periphery thereof). For example, if the diameter (D 2 ) of the through-hole  15  is small, the conductance is reduced, and thus the flow of the transfer gas sometimes becomes poor. In contrast, if the diameter (D 2 ) of the through-hole  15  is larger, an area in which a temperature difference in the object W which is adsorbed is large (so-called hot spot or cold spot) is sometimes generated. According to the knowledge that the inventor(s) has obtained, it is favorable that the diameter (D 2 ) of the through-hole  15  is, for example, 0.04 millimeters (mm) or more and 3 mm or less. It is more favorable that the diameter (D 2 ) of the through-hole  15  is, for example, 0.07 mm or more and 2.5 mm or less. It is further favorable that the diameter (D 2 ) of the through-hole  15  is, for example, 0.1 mm or more and 2 mm or less. 
     The longer the distance between plasma and the adhesive (the distance between the center C 1  of the through-hole  15  and the end face  64 ), the smaller the amount of damage to which the adhesive is subjected. On the other hand, as shown in  FIG. 4A , the longer the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15 , the larger the conductance of the transfer gas becomes. Further, if the distance d is smaller than the diameter (D 2 ) of the through-hole  15 , the conductance rapidly deteriorates (becomes small). Accordingly, it is favorable that the distance d is not less than the minimum value, 0.1 mm, of the diameter (D 2 ) of the through-hole  15 . 
     Further, if the distance between the plasma and the adhesive (the distance between the center C 1  of the through-hole  15  and the end face  64 ) is long, due to a difference between the thermal conductivity of the adhesive and the thermal conductivity of a space (air), a temperature difference sometimes occurs between the position on the object W just above the through-hole  15  and the periphery thereof. 
     As shown in  FIG. 4B , in a high-power condition (a heat input condition of 5000 W to the surface of the electrostatic chuck  110 ), when the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is 1.85 mm, the temperature difference becomes 5° C. Further, when the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is 4.0 mm, the temperature difference becomes 20° C. 
     Further, in a low-power condition (a heat input condition of 3000 W to the surface of the electrostatic chuck  110 ), when the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is 1.85 mm, the temperature difference becomes 3.3° C. When the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is 5.0 mm, the temperature difference becomes 20° C. 
     In a plasma process, an in-plane temperature difference is one of the important items. According to the knowledge that the inventor(s) has obtained, it is favorable that the temperature difference is suppressed to 20° C. or less. For this reason, it is favorable that the distance d is 5.0 mm or less. 
     Therefore, it is favorable that the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is 0.1 mm or more and 5.0 mm or less. It is favorable that the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is 0.2 mm or more and 4.5 mm or less. It is more favorable that the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is 0.4 mm or more and 4 mm or less. It is further favorable that the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is 0.6 mm or more and 3.7 mm or less. 
     According to this, it is possible to attain both a reduction in the amount of damage to which the adhesive is subjected and uniform temperature distribution of the object W. 
     A material of the end portion  63  may be the same as a material of the bonding portion  61  or may also be different from a material of the bonding portion  61 . 
     In a case where the material of the end portion  63  is the same as the material of the bonding portion  61 , it is possible to further enhance an adhesive force between the bonding portion and the end portion. 
     In a case where the material of the end portion  63  is different from the material of the bonding portion  61 , the end portion  63  is made so as not to contain fillers improving thermal conductivity, and thus it is possible to reduce occurrence of particles. Further, in a case where the material of the end portion  63  is different from the material of the bonding portion  61  and a silicone adhesive is used as the bonding portion  61 , a material having more excellent plasma resistance than the silicone adhesive can be used in the end portion  63 . 
     As for the material having more excellent plasma resistance than the silicone adhesive, a fluorine-based material can be given as an example. A fluorocarbon-based elastomer having “—CF 2 —” as a basic skeleton can be given as an example. Further, a fluorocarbon-based elastomer in which a basic structure of “—CF 2 —CF(CF 3 )—O—” is linked to a silicone chain can be given as an example. Further, a fluorosilicone rubber having “—SiF 2 —O—” and “Si(CH 3 ) 2 —O—” as a basic skeleton can be given as an example. In addition, polyimide, acrylic polymer material, epoxy-based polymer material, or the like can be given as an example. 
     In addition, even in a case where the material of the end portion  63  is the same as the material of the bonding portion  61 , or even in a case where the material of the end portion  63  is different from the material of the bonding portion  61 , a boundary line  66  is present between the end portion  63  and the bonding portion  61 . Due to this, in relation to the bonding between the ceramic dielectric substrate  11  and the base plate  50 , it is possible to determine whether or not the bonding layer  60  has the end portion  63  fabricated in advance. 
     The thermal conductivity of the adhesive which is used in the bonding portion  61  is, for example, 0.2 watts/meter·kelvin (W/m·K) or more. It is more favorable that the thermal conductivity of the adhesive which is used in the bonding portion  61  is 0.4 W/m·K or more. It is further favorable that the thermal conductivity of the adhesive which is used in the bonding portion  61  is 0.8 W/m·K. The thermal conductivity of the adhesive which is used in the bonding portion  61  is, for example, 4.0 W/m·K or less. It is more favorable that the thermal conductivity of the adhesive which is used in the bonding portion  61  is 3.0 W/m·K or less. The dielectric breakdown strength of the adhesive which is used in the bonding portion  61  is, for example, 1 kilovolt/millimeter (kV/mm) or more. It is more favorable that the dielectric breakdown strength of the adhesive which is used in the bonding portion  61  is 2 kV/mm or more. It is further favorable that the dielectric breakdown strength of the adhesive which is used in the bonding portion  61  is 5 kV/mm or more. The dielectric breakdown strength of the adhesive which is used in the bonding portion  61  is, for example, 50 kV/mm or less. A heat resistance temperature of the adhesive which is used in the bonding portion  61  is 60° C. or more. 
     According to this, it is possible to use an adhesive which can maintain insulation while maintaining good heat transfer even if the electrostatic chuck  110  is used in a high-temperature process. Further, it is possible to have elasticity capable of alleviating a difference between the thermal expansion of the ceramic dielectric substrate  11  and the thermal expansion of the base plate  50 . As a result, the life of the electrostatic chuck  110  is lengthened. 
     As shown in  FIG. 2A , the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  is larger than a radius ((D 4 )/2) of an opening  15   e  on the first major surface  11   a  side. Due to this, regardless of the durability of the adhesive, it is possible to reduce damage to which the bonding layer  60  is subjected. Even if the bonding layer  60  is damaged, it is possible to reduce the scattering of particles. 
     As shown in  FIGS. 2A and 2B , the end portion  63  comes into contact with the second major surface  11   b  of the ceramic dielectric substrate  11  in a plane  63   b  rather than a point and comes into contact with the surface  57  of the base plate  50  in a plane  63   c  rather than a point. In the schematic cross-sectional view shown in  FIG. 2A , a length (the length in the X-direction of the plane  63   b ) D 5  of a portion in which the end portion  63  comes into contact with the second major surface  11   b  of the ceramic dielectric substrate  11  is longer than the thickness t 1  of the bonding layer  60 . In the schematic cross-sectional view shown in  FIG. 2A , a length (the length in the X-direction of the plane  63   c ) D 6  of a portion in which the end portion  63  comes into contact with the surface  57  of the base plate  50  is longer than the thickness t 1  of the bonding layer  60 . Each of the length D 5  and the length D 6  is, for example, about 500 μm or more. 
     According to this, since the end portion  63  is in contact with each of the second major surface  11   b  and the surface  57  in a plane rather than a point, it is possible to prevent occurrence of a space in the bonding layer  60 . That is, an outer peripheral portion which is an outer peripheral portion of the end portion  63  and is on the side opposite to the space  65  when viewed from the end portion  63  is filled with a resin material. 
     As described above, the end portion  63  is crushed in the Z-direction in the process of fitting the ceramic dielectric substrate  11  and the base plate  50  to each other. The plane  63   b  in which the end portion  63  is crushed by the ceramic dielectric substrate  11  is on the same plane as a bonded surface (the second major surface  11   b ) of the ceramic dielectric substrate  11 . The plane  63   c  in which the end portion  63  is crushed by the base plate  50  is on the same plane as a bonded surface (the surface  57 ) of the base plate  50 . 
       FIG. 5  is a schematic enlarged view showing the vicinity of another bonding layer of the embodiment. 
       FIG. 5  is a schematic enlarged view of a portion equivalent to the portion A shown in  FIG. 1 . 
     A bonding layer  60   a  shown in  FIG. 5  has the bonding portion  61  and an end portion  63   a . The end portion  63   a  has, for example, a ring-like shape. The end face  64  of the bonding layer  60   a  on the space  65  side intersects with or comes into contact with the second major surface  11   b  of the ceramic dielectric substrate  11 . An area A 3  (the first area) in which the end face  64  intersects with the second major surface  11   b  is away from or is recessed from the opening  15   d  of the through-hole  15 , compared to another area (the second area) of the end face  64  which is different from the area A 3 . 
     In the bonding layer  60   a  shown in  FIG. 5 , a distance D 3  between the end portions  63  (or the end faces  64 ) facing each other becomes shorter with distance from the second major surface  11   b  in the normal direction. That is, the end face  64  has an inclination by which the distance D 3  between the end portions  63  (or the end faces  64 ) facing each other becomes shorter with distance from the second major surface  11   b  in the normal direction. In addition, other structures or a material of each member is as described above with respect to  FIGS. 1 to 3 . 
     According to the example shown in  FIG. 5 , regardless of the durability of the adhesive, it is possible to reduce damage to which the bonding layer  60   a  is subjected. Even if the bonding layer  60   a  is damaged, it is possible to reduce the scattering of particles. 
       FIG. 6  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment. 
     The electrostatic chuck  110  according to the embodiment may be provided with the insulator plug  70 . 
     The insulator plug  70  may be provided in the gas introduction path  53  provided in the base plate  50 . The insulator plug  70  is fitted into the ceramic dielectric substrate  11  side of the gas introduction path  53 . As shown in  FIG. 6 , for example, on the ceramic dielectric substrate  11  side of the gas introduction path  53 , a counterbore portion  53   a  is provided. The counterbore portion  53   a  is provided in a tubular shape. Due to appropriately designing the inner diameter of the counterbore portion  53   a , the insulator plug  70  may be fitted into the counterbore portion  53   a.    
     The insulator plug  70  has a ceramic porous body  71 . The ceramic porous body  71  is provided in a tubular shape (for example, a cylindrical shape) and fitted to the counterbore portion  53   a . The shape of the insulator plug  70  is preferably a cylindrical shape. However it is not limited to a cylindrical shape. For the ceramic porous body  71 , a material having insulation properties is used. As a material of the ceramic porous body  71 , for example, Al 2 O 3 , Y 2 O 3 , ZrO 2 , MgO, SiC, AlN, Si 3 N 4 , or glass such as SiO 2  is acceptable. Alternatively, the material of the ceramic porous body  71  may be Al 2 O 3 —TiO 2 , Al 2 O 3 —MgO, Al 2 O 3 —SiO 2 , Al 6 O 13 Si 2 , YAG, ZrSiO 4 , or the like. 
     The porosity of the ceramic porous body  71  is, for example, 30 percent (%) or more and 60% or less. The density of the ceramic porous body  71  is, for example, 1.5 grams/cubic centimeter (g/cm 3 ) or more and 3.0 g/cm 3  or less. Due to such porosity, the transfer gas such as He flowing through the gas introduction path  53  passes through a large number of pores of the ceramic porous body  71  and is sent from the through-hole  15  provided in the ceramic dielectric substrate  11  to the groove  14 . 
     As shown in  FIG. 6 , with respect to the distance d between the end face  64  in the area A 1  and the center C 1  of the through-hole  15  and a radius R of the ceramic porous body  71 , the following expression is established.
 
d&gt;R  Expression (2)
 
     In addition, other structures or a material of each member is as described above with respect to  FIGS. 1 to 3 . 
     Due to this, like an arrow A 21 , an arrow A 22 , an arrow A 23 , and an arrow A 24  shown in  FIG. 6 , the convection of the transfer gas can be created in the space  65  such that particles are easily deposited in a pocket formed in the area A 1 . That is, the convection of the transfer gas which selectively deposits particles in the pocket formed in the area A 1  can be controlled in the space  65 . For this reason, even if particles are generated, it is possible to reduce the scattering of the particles. Further, the ceramic porous body  71  is provided, whereby it is possible to have high voltage resistance in the through-hole  15  and the gas introduction path  53 . 
       FIG. 7  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment. 
     The electrostatic chuck  110  shown in  FIG. 7  is provided with the insulator plug  70 , similar to the electrostatic chuck  110  described above with respect to  FIG. 6 . The insulator plug  70  is provided in the through-hole  15  provided in the ceramic dielectric substrate  11 . The insulator plug  70  is fitted into the base plate  50  side of the through-hole  15 . As shown in  FIG. 7 , for example, the through-hole  15  has a counterbore portion  15   f  on the base plate  50  side. The counterbore portion  15   f  forms the opening  15   d  of the through-hole  15 . The counterbore portion  15   f  is provided in a tubular shape. Due to appropriately designing the inner diameter of the counterbore portion  15   f , the insulator plug  70  may be fitted into the counterbore portion  15   f.    
     The insulator plug  70  is as described above with respect to  FIG. 6 . That is, the insulator plug  70  has the ceramic porous body  71 . The transfer gas such as helium passes through the gas introduction path  53  and the space  65  and passes through the through-hole  15  via the insulator plug  70 , thereby flowing into the space provided between the object W and the groove  14 . In this manner, in the specification, in the range of the “through-hole”, a hole in which a thing having a pathway through which gas flows, like, for example, a porous body or the like, is provided in the middle and through which arbitrary gas or fluid penetrates is included. 
       FIG. 8  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment. 
     The electrostatic chuck  110  shown in  FIG. 8  is further provided with a heater  91 , compared to the electrostatic chuck  110  described above with respect to  FIG. 7 . The heater  91  is provided between the base plate  50  and the ceramic dielectric substrate  11 . The heater  91  is supplied with voltage, and thus an electric current flows therethrough, whereby the heater  91  generates heat, thereby raising or maintaining the temperature of the object W. 
     The heater  91  is fixed to the second major surface  11   b  of the ceramic dielectric substrate  11  via the bonding layer  60 . Further, the heater  91  is fixed to the surface  57  of the base plate  50  via the bonding layer  60 . That is, the bonding layer  60  is provided between the heater  91  and the ceramic dielectric substrate  11  and between the heater  91  and the base plate  50 . The bonding layer  60  provided between the ceramic dielectric substrate  11  and the heater  91  has the end portion  63 . The end portion  63  is as described above with respect to  FIGS. 1 to 4B . The bonding layer  60  provided between the base plate  50  and the heater  91  may have the end portion  63  or may not have the end portion  63 . 
     As shown in  FIG. 8 , the heater  91  is provided away from the gas introduction path  53 . The bonding layer  60  provided between the base plate  50  and the heater  91  is provided away from the gas introduction path  53 . The end portion  63  of the bonding layer  60  provided between the ceramic dielectric substrate  11  and the heater  91  is provided on the side opposite to the gas introduction path  53  when viewed from an end portion of the heater  91 . That is, a shortest distance D 8  between the bonding layer  60  provided between the ceramic dielectric substrate  11  and the heater  91  and a center C 2  of the gas introduction path  53  is longer than a shortest distance D 7  between the heater  91  and the center C 2  of the gas introduction path  53 . As described above with respect to  FIG. 2A , the space  65  having a cross-sectional shape which is longer in the horizontal direction than the vertical direction is connected to the through-hole  15  having a cross-sectional shape which is longer in the vertical direction than the horizontal direction. 
       FIG. 9  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment. 
     The electrostatic chuck  110  shown in  FIG. 9  is provided with the heater  91 , similar to the electrostatic chuck  110  described above with respect to  FIG. 8 . The bonding layer  60  provided between the ceramic dielectric substrate  11  and the heater  91  has the end portion  63 . The end portion  63  is as described above with respect to  FIGS. 1 to 4B . The bonding layer  60  provided between the base plate  50  and the heater  91  may have the end portion  63  or may not have the end portion  63 . 
     As shown in  FIG. 9 , the end portion of the heater  91  is provided on approximately the same plane as the inner surface of the gas introduction path  53 . An end portion of the bonding layer  60  provided between the base plate  50  and the heater  91  is provided on approximately the same plane as the inner surface of the gas introduction path  53 . 
     According to the electrostatic chucks  110  shown in  FIGS. 7 to 9 , regardless of the durability of the adhesive, it is possible to reduce damage to which the bonding layer  60  is subjected. Even if the bonding layer  60  is damaged, it is possible to reduce the scattering of particles. Further, the ceramic porous body  71  is provided, whereby it is possible to have high voltage resistance in the through-hole  15  and the gas introduction path  53 . 
       FIG. 10  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment. 
       FIG. 11  is a schematic enlarged view showing the vicinity of still another bonding layer of the embodiment. 
     In the electrostatic chucks  110  shown in  FIGS. 10 and 11 , the bonding layer  60  is provided as a sheet. That is, the bonding layer  60  exhibits a sheet shape. For this reason, the bonding layer  60  does not have, for example, the ring-like end portion  63  as described above with respect to  FIGS. 1 to 9 . The sheet shape exhibits a state where the bonding portion  61  bonding the second major surface  11   b  and the base plate  50  together and the end portion  63  forming the space  65  are integrated with each other with the same material. 
     The end face  64  of the bonding layer  60  shown in  FIG. 10  has the same shape as the shape of the end face  64  of the bonding layer  60  described above with respect to  FIGS. 2A and 2B . 
     The end face  64  of the bonding layer  60  shown in  FIG. 11  has the same shape as the shape of the end face  64  of the bonding layer  60  described above with respect to  FIG. 5 . 
     According to the electrostatic chucks  110  shown in  FIGS. 10 and 11 , even in a case where the bonding layer  60  is provided as a sheet, regardless of the durability of the adhesive, it is possible to reduce damage to which the bonding layer  60  is subjected. Even if the bonding layer  60  is damaged, it is possible to reduce the scattering of particles. 
     Next, a simulation of the end portion  63  of the bonding layer  60  carried out by the inventor(s) will be described with reference to the drawings. 
       FIG. 12  is a schematic cross-sectional view showing the conditions of the simulation. 
       FIGS. 13A to 13C  are schematic perspective views illustrating an example of the results of the simulation. 
       FIGS. 14A and 14B  are schematic perspective views illustrating an example of the results of the simulation. 
       FIG. 13A  is a schematic view showing the cross-sectional shape of the end portion  63  of the bonding layer  60  before it is compressed in a bonding process.  FIGS. 13B, 13C, 14A and 14B  are schematic views showing the cross-sectional shape of the end portion  63  of the bonding layer  60  after it is compressed in the bonding process. 
     As shown in  FIG. 12 , in the simulation, the end portion  63  is sandwiched between a first fixing portion  97  and a second fixing portion  98 . The first fixing portion  97  is equivalent to, for example, the base plate  50 . The second fixing portion  98  is equivalent to, for example, the ceramic dielectric substrate  11 . 
     As the end portion  63  of the bonding layer  60 , a model having a ring-like shape was made. An outer diameter D 11  of the end portion  63  before compression is 3 mm or more and 10 mm or less. An inner diameter D 12  of the end portion  63  before compression is 1 mm or more and 5 mm or less. In the simulation, the Young&#39;s modulus of a material of the end portion  63  was set to be 0.1 megapascals (MPa) or more and 20 MPa or less. Further, the Poisson&#39;s ratio of the material of the end portion  63  was set to be 0.3 or more and 0.5 or less. 
     In the simulation, compressive stress was applied to the end portion  63  by moving the second fixing portion  98  toward the first fixing portion  97 , like an arrow A 25  shown in  FIG. 12 . The results of the simulation are as shown in  FIGS. 13A to 14B . 
     That is,  FIG. 13B  shows the displacement in a radial direction of the end portion  63  when a thickness D 13  of the end portion  63  is thicker than the thickness t 1  of the bonding layer  60  after bonding.  FIG. 13C  shows the displacement in the radial direction of the end portion  63  when the end portion  63  has been compressed to the thickness t 1  of the bonding layer  60  after bonding.  FIG. 14A  shows the displacement in the radial direction of the end portion  63  when the thickness D 13  of the end portion  63  is thicker than the thickness t 1  of the bonding layer  60  after bonding.  FIG. 14B  shows the displacement in a thickness direction (the Z-direction) of the end portion  63  when the end portion  63  has been compressed to the thickness t 1  of the bonding layer  60  after bonding. In  FIGS. 13A to 14B , the magnitude of the displacement is shown by a color of shading. 
     As shown in  FIG. 14B , a circle  93  having a diameter of the same length as the thickness D 13  of the end portion  63  after compression is considered. In this case, the curvature of the end face  64  in an area A 4  (the first area) in which the end face  64  of the end portion  63  intersects with the second fixing portion  98  is larger than the curvature of the circle  93 . On the other hand, the curvature of the end face  64  in another area A 5  (the second area) of the end face  64  which is different from the area A 4  is smaller than the curvature of the circle  93 . That is, the curvature of the end face  64  in the area A 4  (the first area) in which the end face  64  of the end portion  63  intersects with the second fixing portion  98  is larger than the curvature of the end face  64  in another area A 5  (the second area) of the end face  64  which is different from the area A 4 . 
     The area A 4  is equivalent to the area A 1  described above with respect to  FIG. 2A . The other area A 5  of the end face  64  which is different from the area A 4  is equivalent to another area of the end face  64  which is the area described above with respect to  FIG. 2A  and is different from the area A 1 , and is, for example, an intermediate area between the first fixing portion  97  and the second fixing portion  98 . 
     The curvature of an end face  67  on the outside of the end portion  63  is larger than the curvature of the end face  64  on the inside of the end portion  63 . The visible outline of the end face  67  on the outside of the end portion  63  is equivalent to the boundary line  66  (refer to  FIG. 2A ) between the end portion  63  and the bonding portion  61 . 
     The embodiments of the invention have been described above. However, the invention is not limited to the above description. Those skilled in the art can appropriately modify the above embodiments, and such modifications are also encompassed within the scope of the invention as long as they include the features of the invention. For instance, the shape, dimension, material, arrangement and the like of various components in the electrostatic chucks  110 , and the installation configuration and the like of the bonding portion  61  and the end portion  63  are not limited to those illustrated, but can be modified appropriately. Furthermore, the configuration using Coulomb force is illustrated as the electrostatic chucks  110 . However, the configuration using Johnson-Rahbek force may be applicable as the electrostatics  110 . 
     Furthermore, various components in the above embodiments can be combined with each other as long as technically feasible. Such combinations are also encompassed within the scope of the invention as long as they include the features of the invention.