Patent Publication Number: US-2023154781-A1

Title: Wafer placement table

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wafer placement table. 
     2. Description of the Related Art 
     Conventionally, a wafer placement table has been known, in which a ceramic base incorporating an electrode, and a cooling base including a refrigerant flow path formed therein are bonded by an adhesive material. PTL 1 describes an example in which such a wafer placement table is placed on an installation plate and fixed by a screw. Specifically, a female thread hole is provided in the lower surface of the cooling base, and a male thread of a bolt is screwed into the female thread hole, the bolt being inserted into a screw insertion hole penetrating the installation plate vertically. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] Japanese Unexamined Patent Application Publication No. 2014-150104 
       
    
     SUMMARY OF THE INVENTION 
     However, when a female thread hole is provided in the lower surface of the cooling base, and a male thread of a bolt inserted in the installation plate is screwed into the female thread hole, if the material of the cooling base is a ductile material (for instance, aluminum), no problem arises, but if the cooling base is a brittle material (for instance, metal matrix composite material), a problem arises. Specifically, the female thread hole of the cooling base is pulled down locally by a large force acting on the bolt, thus if an inductile material is used for the cooling base, it may be broken. 
     The present invention has been made to solve such a problem, and it is a main object to make it possible to tighten the wafer placement table including a brittle cooling base to the installation plate without any problem. 
     [1] A wafer placement table of the present invention includes: an alumina base that has a wafer placement surface on its upper surface, and incorporates an electrode; a brittle cooling base bonded to a lower surface of the alumina base; and a ductile connection member stored in a storage hole, opened in a lower surface of the cooling base, in a state of restricted axial rotation and in a state of engaging with an engagement section of the storage hole, the ductile connection member having a male thread section or a female thread section. 
     In the wafer placement table, the connection member having the male thread section or the female thread section is stored in the storage hole, opened in the lower surface of the cooling base, in a state of restricted axial rotation and in a state of engaging with the engagement section of the storage hole. Since axial rotation of the connection member is restricted, it can be screwed to a to-be-connected member having the male thread section or the female thread section disposed on the lower surface side of the cooling base. In addition, even when the connection member in a state of engaging with the engagement section of the storage hole is pulled toward the installation plate by the to-be-connected member provided in the installation plate, the connection member is unlikely to break because it has ductility. Therefore, it is possible to tighten the wafer placement table including a brittle cooling base to the installation plate without any problem. 
     Note that in the present specification, the present invention may be described using up and down, right and left, and front and back; however up and down, right and left, and front and back merely indicate a relative positional relationship. Thus, when the orientation of the wafer placement table is changed, up and down may change to right and left, or right and left may change to up and down, and such a case is also included in the technical scope of the present invention. 
     [2] In the above-described wafer placement table (the wafer placement table according to [ 1 ]), the connection member may be a member having the female thread section, and being screwable to a male thread of a bolt inserted from a lower surface side of the cooling base. 
     [3] In the above-described wafer placement table (the wafer placement table according to [1] or [2]), the cooling base may be composed of a composite material of metal and ceramic or an alumina material. Because a composite material of metal and ceramic and an alumina material are brittle materials, application of the present invention has high significance. For instance, when a composite material of metal and ceramic is used, it is preferable that a composite material having the same thermal expansion coefficient as alumina be used. 
     [4] In the above-described wafer placement table (the wafer placement table according to any one of [1] to [3]), the engagement section may be a step section or an inclined section provided in an inner circumferential surface of the storage hole, and the connection member may have a to-be-engaged section which engages with the engagement section to prevent the connection member from falling from the storage hole. In this manner, the engagement section and the to-be-engaged section can be produced relatively easily. For instance, when the engagement section is a step section, the connection member may be provided with a to-be-engaged section which hooks to the step section. When the engagement section is an inclined section, the connection member may be provided with an inclined face as the to-be-engaged section, the inclined face conforming to the inclined section. 
     [5] In the above-described wafer placement table (the wafer placement table according to any one of [1] to [4]), when being attempted to be axially rotated, the connection member may come into contact with a wall of the storage hole to undergo restricted axial rotation. In this manner, axial rotation of the connection member can be restricted by a relatively simple configuration. 
     [6] In the above-described wafer placement table (the wafer placement table according to any one of [1] to [5]), the cooling base may have a refrigerant flow path internally, and the storage hole may be provided in a region of the cooling base, the region being lower than the bottom surface of the refrigerant flow path. In this manner, the storage hole does not interfere with the refrigerant flow path, thus the degree of freedom of the design of the refrigerant flow path is not reduced. 
     [7] In the above-described wafer placement table (the wafer placement table according to any one of [1] to [6]), the connection member may not be connected to the cooling base in the storage hole, and may be stored in a free state. In this manner, the connection member only has to be inserted in the storage hole, which does not take much effort. 
     [8] In the above-described wafer placement table (the wafer placement table according to any one of [1] to [7]), the connection member may engage with the engagement section with a stress buffering member interposed between the connection member and the engagement section, the stress buffering member having a lower Young&#39;s modulus lower than the connection member. In this manner, even when the connection member is pulled toward the installation plate by a to-be-connected member provided in the installation plate, the stress tends to be dispersed because the stress buffering member is interposed between the connection member and the engagement section. 
     [9] In the above-described wafer placement table (the wafer placement table according to any one of [1] to [8]), the gap between the connection member and the storage hole may be filled with a filling material. In this manner, thermal conduction becomes favorable as compared to when the gap between the connection member and the storage hole is void. Therefore, the thermal uniformity of the wafer improves. 
     [10] In the above-described wafer placement table (the wafer placement table according to any one of [1] to [9]), the storage hole may include a first storage section that stores the connection member, and a second storage section provided from the first storage section to the lower surface of the cooling base. The engagement section may be a stepped surface provided at a joint between the first storage section and the second storage section. 
     [11] In the above-described wafer placement table (the wafer placement table according to [10]), the first storage section may be opened in an upper surface of the cooling base, and an opening surface may be covered by a bonding layer that bonds the ceramic base and the cooling base. In this manner, the first storage section can be produced relatively easily as compared to when the first storage section is incorporated in the inside of the cooling base. In this structure, a refrigerant flow path (or a refrigerant flow path groove) needs to be provided skirting the storage hole, thus the thermal uniformity is likely to reduce in the vicinity immediately above the storage hole of the wafer. In order to prevent the reduction in the thermal uniformity, the gap between the connection member and the storage hole is preferably filled with a filling material. Consequently, the thermal conduction around the storage hole is favorable, thus reduction in the thermal uniformity can be prevented. 
     [12] In the above-described wafer placement table (the wafer placement table according to [10] or [11]), the width of an annular region in which the stepped surface is in direct or indirect contact with the connection member is preferably 3 mm or more. In this manner, with the width of the annular region of 3 mm or more, even when the connection member is pulled toward the installation plate by the to-be-connected member provided in the installation plate, the stress tends to be dispersed because the annular region in which the stepped surface is in direct or indirect contact with the connection member is large. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a vertical cross-sectional view of a wafer placement table  10  installed in a chamber  94 . 
         FIG.  2    is a plan view of the wafer placement table  10 . 
         FIG.  3    is an enlarged cross-sectional view illustrating the vicinity of a storage hole  36  and a female thread member  38 . 
         FIG.  4    is a cross-sectional view of the cooling base  30  as seen from above when it is cut horizontally along the ceiling surface of the storage hole  36 . 
         FIG.  5    is an enlarged cross-sectional view illustrating a refrigerant supply path  321  and a refrigerant discharge path  322 . 
         FIGS.  6 A to  6 G  are manufacturing process charts of the wafer placement table  10 . 
         FIG.  7    is an enlarged cross-sectional view illustrating another example of the storage hole  36  and the female thread member  38 . 
         FIG.  8    is an enlarged cross-sectional view illustrating another example of the storage hole  36  and the female thread member  38 . 
         FIG.  9    is an enlarged cross-sectional view when a male thread member  80  is used as a connection member. 
         FIG.  10    is a vertical cross-sectional view of a wafer placement table  510  installed in the chamber  94 . 
         FIG.  11    is an enlarged cross-sectional view illustrating the vicinity of a storage hole  536  and a female thread member  538 . 
         FIGS.  12 A to  12 G  are manufacturing process charts of the wafer placement table  510 . 
         FIG.  13    is a cross-sectional view for dimensional illustration of the storage hole  536  and the female thread member  538 . 
         FIGS.  14 A and  14 B  show explanatory diagrams when a triangular prism-shaped nut is used as the female thread member  538 . 
         FIG.  15    is a vertical cross-sectional view of the wafer placement table  510  of another example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     A preferred embodiment of the present invention will be described below with reference to the drawings.  FIG.  1    is a vertical cross-sectional view (cross-sectional view when the wafer placement table  10  is cut along a plane including the central axis thereof) of a wafer placement table  10  installed in a chamber  94 ,  FIG.  2    is a plan view of the wafer placement table  10 ,  FIG.  3    is an enlarged cross-sectional view illustrating the vicinity of a storage hole  36  and a female thread member  38 , and  FIG.  4    is a cross-sectional view of a cooling base  30  as seen from above when it is cut horizontally along the ceiling surface of the storage hole  36 . 
     The wafer placement table  10  is used for performing CVD and etching on wafer W utilizing plasma, and is fixed to an installation plate  96  provided inside the chamber  94  for semiconductor process. The wafer placement table  10  includes an alumina base  20 , a cooling base  30 , and a metal bonding layer  40 . 
     The alumina base  20  includes an outer circumferential section  24  having an annular focus ring placement surface  24   a , on an outer circumference of a central section  22  having a circular wafer placement surface  22   a . Hereinafter focus ring may be abbreviated as “FR”. The wafer W is placed on the wafer placement surface  22   a , and a focus ring  78  is placed on the FR placement surface  24   a . The FR placement surface  24   a  is lower by one step than the wafer placement surface  22   a.    
     The central section  22  of the alumina base  20  incorporates a wafer attracting electrode  26  at a position near the wafer placement surface  22   a . The wafer attracting electrode  26  is composed of a material containing, for instance, W, Mo, WC, MoC. The wafer attracting electrode  26  is a circular plate-shaped or mesh-shaped monopole electrostatic attraction electrode. Of the alumina base  20 , an upper layer over the wafer attracting electrode  26  functions as a dielectric layer. The wafer attracting electrode  26  is coupled to a wafer attracting DC power supply  52  via a power feed terminal  54 . The power feed terminal  54  is provided to reach the wafer attracting electrode  26  from the lower surface of the alumina base  20  through an insulated tube  55  disposed in a through-hole vertically penetrating the cooling base  30  and the metal bonding layer  40 . A low pass filter (LPF)  53  is provided between the wafer attracting DC power supply  52  and the wafer attracting electrode  26 . 
     The cooling base  30  is a disc member. A composite material of metal and ceramic is preferably used as the material for the cooling base  30 . As such a composite material, a metal matrix composite material (also referred to as a metal matrix composite (MMC)) and a ceramic matrix composite material (also referred to as a ceramic matrix composite (CMC)) may be mentioned. These composite materials are a kind of a brittle material. The cooling base  30  internally includes a refrigerant flow path  32  in which a refrigerant can circulate. The refrigerant flow path  32  is connected to a refrigerant supply path and a refrigerant discharge path which are not illustrated, and the refrigerant discharged through the refrigerant discharge path undergoes temperature control, then is returned to the refrigerant supply path again. The refrigerant flowing through the refrigerant flow path  32  is preferably liquid, and preferably has an electrical insulation property. As the liquid having an electrical insulation property, for instance, fluorine-based inert liquid may be mentioned. As the composite material of metal and ceramic, a material containing Si, SiC and Ti, a material obtained by impregnating SiC porous body with Al and/or Si, and a composite material of Al 2 O 3  and TiC may be mentioned. A material containing Si, SiC and Ti is called SiSiCTi, a material obtained by impregnating SiC porous body with Al is called AlSiC, and a material obtained by impregnating SiC porous body with Si is called SiSiC. As the composite material used for the cooling base  30 , AlSiC and SiSiCTi having a thermal expansion coefficient closer to that of alumina are preferable. The cooling base  30  is coupled to an RF power supply  62  via a power feed terminal  64 . A high pass filter (HPF)  63  is disposed between the cooling base  30  and the RF power supply  62 . The cooling base  30  has a flange  34 , near its lower surface, which is used to clamp the wafer placement table  10  to the installation plate  96 . 
     The cooling base  30  is provided with a plurality of storage holes  36 , in each of which a female thread member  38  (connection member) is stored. The plurality of storage holes  36  are provided in a region of the cooling base  30 , the region being lower than a bottom surface  32   a  of the refrigerant flow path  32 . The plurality of storage holes  36  are plurally provided at regular intervals (for instance, six or eight) along a concentric circle (for instance, a circle with a diameter equal to ½ or ⅓ the diameter of the wafer W) of the cooling base  30 . In other words, as illustrated in  FIG.  2   , the plurality of storage holes  36  are provided in a region near the center of the wafer placement table  10 . As illustrated in  FIG.  3   , the storage holes  36  are opened in the lower surface of the cooling base  30 . Each storage hole  36  includes a first storage section  36   a , a second storage section  36   b , and a step section  36   c . The first storage section  36   a  is a cuboid-shaped space provided at an upper portion of the storage hole  36 . The second storage section  36   b  is a cylindrical space provided at a lower portion of the storage hole  36 . The step section  36   c  is a joint part between the first storage section  36   a  and the second storage section  36   b . The storage hole  36  stores a female thread member  38 . The female thread member  38  has a cuboid-shaped head  38   a , and a cylindrical section  38   b  provided in the lower surface of the head  38   a , and a thread is formed on the inner circumferential surface of the cylindrical section  38   b . The head  38   a  of the female thread member  38  is stored in the first storage section  36   a  of the storage hole  36 . Since the lower surface of the head  38   a  engages with the step section  36   c  of the storage hole  36 , the female thread member  38  does not fall from the storage hole  36 . The cylindrical section  38   b  of the female thread member  38  is stored in the second storage section  36   b  of the storage hole  36 . As illustrated in  FIG.  4   , when the female thread member  38  is attempted to be axially rotated, the head  38   a  comes into contact with the side walls of the first storage section  36   a  to undergo restricted axial rotation. The female thread member  38  is composed of a ductile material (for instance, Ti, Mo, W). 
     The metal bonding layer  40  bonds the lower surface of the alumina base  20  and the upper surface of the cooling base  30 . The metal bonding layer  40  may be a layer composed of a solder or a metal brazing material, for instance. The metal bonding layer  40  is formed by TCB (Thermal compression bonding), for instance. TCB is a publicly known method, in which a metal bonding material is inserted between two members to be bonded, and the two members are pressure-bonded in a state of heated to a temperature lower than or equal to the solidus temperature of the metal bonding material. 
     The side surface of the outer circumferential section  24  of the alumina base  20 , the outer circumference of the metal bonding layer  40  and the side surface of the cooling base  30  are covered with an insulating film  42 . As the insulating film  42 , for instance, a thermal spray film such as alumina and yttria may be mentioned. 
     The wafer placement table  10  is mounted on the installation plate  96  provided inside the chamber  94  with a seal ring  76  interposed between the wafer placement table  10  and the installation plate  96 . The seal ring  76  is made of metal or resin, and its outer diameter is slightly smaller than the outer diameter of the cooling base  30 . The outer circumferential area of the wafer placement table  10  is attached to the installation plate  96  using a clamping member  70 . The clamping member  70  is an annular member with a substantially inverted L-shaped cross section, and has an inner circumferential stepped surface  70   a . With the inner circumferential stepped surface  70   a  of the clamping member  70  placed on the flange  34  of the cooling base  30  of the wafer placement table  10 , bolts  72  are each inserted through the upper surface of the clamping member  70  and screwed into a screw hole provided on the upper surface of the installation plate  96 . The bolts  72  are attached to multiple sites (for instance, eight sites or 12 sites) provided at regular intervals in the circumferential direction of the clamping member  70 . The clamping member  70  and the bolts  72  may be produced with an insulating material, or produced with a conductive material (such as metal). In addition, the central area of the wafer placement table  10  is attached to the installation plate  96  using bolts  98  (to-be-connected members). As illustrated in  FIG.  3   , the foot of each bolt  98  is provided with a male thread  98   a . The bolt  98  is inserted into a through-hole  97  provided at the position, opposed to the storage hole  36 , of the installation plate  96  through the lower surface of the installation plate  96 , and the male thread  98   a  is screwed into the female thread member  38  in the storage hole  36 . The through-hole  97  has a smaller diameter in the upper section and a larger diameter in the lower section, and has a step section  97   a  between the upper section and the lower section. The head of the bolt  98  hooks to the step section  97   a  of the through-hole  97 . Since the female thread member  38  is stored in the first storage section  36   a  of the storage hole  36  in a state of restricted axial rotation, the bolt  98  can be screwed into the female thread member  38 . When the bolt  98  is screwed into the female thread member  38 , the female thread member  38  is set in a state of being pulled toward the installation plate  96  with the head  38   a  engaging with the step section  36   c  of the storage hole  36 . 
     At the time of use of the wafer placement table  10 , the wafer placement surface  22   a  side of the alumina base  20  turns to vacuum, the lower surface side of the cooling base  30  turns to atmosphere, thus the wafer placement table  10  tends to project upward. When the wafer W is treated with a high-power plasma, the wafer placement surface  22   a  side of the alumina base  20  has a high temperature, and the lower surface side has a low temperature, thus the wafer placement surface  22   a  side tends to extend, and the wafer placement table  10  tends to project upward. However, in the embodiment, the central area of the wafer placement table  10  is fixed by the bolts  98 , thus, it is possible to prevent the wafer placement table  10  from projecting upward. Even if a seal ring, which is not illustrated, is disposed between the central area of the lower surface of the cooling base  30  and the upper surface of the installation plate  96 , since the central area of the wafer placement table  10  is fixed by the bolts  98 , the seal ring is maintained in a firmly crushed state. 
     For instance, as illustrated in  FIG.  5   , the refrigerant supply path  321  is formed by a first supply path  32   p  penetrating the installation plate  96 , the inside of a refrigerant supply seal ring  32   q  disposed between the installation plate  96  and the cooling base  30 , and a second supply path  32   r  from the lower surface of the cooling base  30  to the refrigerant flow path  32 . The refrigerant discharge path  322  is formed by a first discharge path  32   s  from the refrigerant flow path  32  to the lower surface of the cooling base  30 , the inside of a refrigerant discharge seal ring  32   t  disposed between the cooling base  30  and the installation plate  96 , and a second discharge path  32   u  which penetrates the installation plate  96  and reaches the lower surface of the installation plate  96 . In this case, the central area of the wafer placement table  10  is fixed by screwing the female thread member  38  in the storage hole  36  and the bolt  98  together, thus these seal rings  32   q ,  32   t  are maintained in a firmly crushed state. Thus, the seal rings  32   q ,  32   t  can secure the sealing property sufficiently. 
     Next, a manufacturing example of the wafer placement table  10  will be described using  FIGS.  6 A to  6 G .  FIGS.  6 A to  6 G  are manufacturing process charts of the wafer placement table  10 . First, a disc-shaped alumina sintered body  120 , from which the alumina base  20  is made, is produced by hot-press firing a molded body of alumina powder ( FIG.  6 A ). The alumina sintered body  120  incorporates the wafer attracting electrode  26 . Next, a hole  27  is formed from the lower surface of the alumina sintered body  120  to the wafer attracting electrode  26  ( FIG.  6 B ), and the power feed terminal  54  is inserted into the hole  27  to join the power feed terminal  54  to the wafer attracting electrode  26  ( FIG.  6 C ). 
     Concurrently, three MMC disc members  131 ,  133 ,  135  are produced ( FIG.  6 D ). Then a groove  132 , which eventually serves as the refrigerant flow path  32 , is formed in the lower surface of the upper MMC disc member  131 , a step hole  136 , which eventually serves as the storage hole  36 , is formed in the lower MMC disc member  135 , and additionally, a through-hole vertically penetrating the three MMC disc members  131 ,  133 ,  135  is formed ( FIG.  6 E ). These through-holes eventually serve as holes for inserting the power feed terminal  54 . When the alumina sintered body  120  is made of alumina, the MMC disc members  131 ,  133 ,  135  are preferably made of SiSiCTi or AlSiC. This is because the thermal expansion coefficient of alumina is approximately the same as the thermal expansion coefficients of SiSiCTi and AlSiC. 
     A disc member made of SiSiCTi can be produced as follows, for instance. First, silicon carbide, metal Si and metal Ti are mixed to produce a powder mixture. Next, a disc-shaped molded body is produced by applying uniaxial pressure molding to the obtained powder mixture, and hot-press sintering is applied to the molded body in an inert atmosphere to obtain a disc member made of SiSiCTi. 
     Next, the female thread member  38  is stored in the step hole  136  of the lower MMC disc member  135 . Then a metal bonding material is disposed between the lower surface of the upper MMC disc member  131  and the upper surface of the middle MMC disc member  133 , a metal bonding material is disposed between the lower surface of the middle MMC disc member  133  and the upper surface of the lower MMC disc member  135 , and a metal bonding material is further disposed on the upper surface of the upper MMC disc member  131 . Each metal bonding material is provided with a through-hole at position for inserting the power feed terminal  54 . Next, the power feed terminal  54  of the alumina sintered body  120  is inserted into the through-holes of the MMC disc members  131 ,  133 ,  135 , and the alumina sintered body  120  is placed on the metal bonding material disposed on the upper surface of the upper MMC disc member  131 . Thus, a laminated body is obtained, in which the MMC disc member  135 , a metal bonding material, the MMC disc member  133 , a metal bonding material, the MMC disc member  131 , a metal bonding material and the alumina sintered body  120  are laminated in that order from the bottom. A bonded body  110  is obtained ( FIG.  6 F ) by pressurizing the laminated body while heating it (TCB). The bonded body  110  is such that the alumina sintered body  120  is bonded onto the upper surface of the MMC block  130 , from which the cooling base  30  is produced, with the metal bonding layer  40  interposed between the alumina sintered body  120  and the MMC block  130 . The MMC block  130  is such that the upper MMC disc member  131  and the middle MMC disc member  133  are bonded with a metal bonding layer interposed therebetween, and the middle MMC disc member  133  and the lower MMC disc member  135  are bonded with a metal bonding layer interposed therebetween. The MMC block  130  has the refrigerant flow path  32  and the storage holes  36  internally. In addition, a female thread member  38  is stored in each storage hole  36 . 
     TCB is performed as follows, for instance. Specifically, a laminated body is pressurized and bonded at a temperature (for instance, the solidus temperature minus 20° C. or higher and the solidus temperature or lower) lower than or equal to the solidus temperature of the metal bonding material, and subsequently, the temperature is returned to the room temperature. Consequently, the metal bonding material becomes a metal bonding layer. Then an Al—Mg based bonding material and an Al—Si—Mg based bonding material can be used as the metal bonding material. For instance, when TCB is performed using the Al—Si—Mg based bonding material, the laminated body is pressurized in a state of heated in a vacuum atmosphere. A metal bonding material with a thickness of approximately 100 μm is preferably used. 
     Subsequently, the outer circumference of the alumina sintered body  120  is cut to form a step, thus the alumina base  20  including the central section  22  and the outer circumferential section  24  is produced. Also, the outer circumference of the MMC block  130  is cut to form a step, thus the cooling base  30  including the flange  34  is produced. In addition, the insulated tube  55  is disposed in the insertion hole of the power feed terminal  54  provided in the MMC block  130  and the metal bonding layer  40 . Furthermore, the insulating film  42  is formed by applying thermal spraying using alumina powder to the side surface of the outer circumferential section  24  of the alumina base  20 , the periphery of the metal bonding layer  40  and the side surface of the cooling base  30  ( FIG.  6 G ). Consequently, the wafer placement table  10  is obtained. 
     Although the cooling base  30  of  FIG.  1    has been described as an integrated component, a structure may be adopted, in which three members are bonded by metal bonding layers, or a structure may be adopted, in which two or four or more members are bonded by metal bonding layers as illustrated in  FIG.  60   . 
     Next, an example of use of the wafer placement table  10  will be described using  FIG.  1   . As described above, on the installation plate  96  of the chamber  94 , the outer circumferential area of the wafer placement table  10  is fixed by the clamping member  70 , and the central area of the wafer placement table  10  is fixed by the bolts  98 . On the ceiling surface of the chamber  94 , a shower head  95  is disposed which injects a process gas through a large number of gas injection holes to the inside of the chamber  94 . The installation plate  96  is composed of an insulating material such as alumina, for instance. 
     The focus ring  78  is placed on the FR placement surface  24   a  of the wafer placement table  10 , and a disc-shaped wafer W is placed on the wafer placement surface  22   a . The focus ring  78  includes a step along the inner circumference of the upper end so as not to interfere with the wafer W. In this state, the DC voltage of the wafer attracting DC power supply  52  is applied to the wafer attracting electrode  26  to cause the wafer placement surface  22   a  to attract the wafer W. The inside of the chamber  94  is set to have a predetermined vacuum atmosphere (or a reduced pressure atmosphere), and an RF voltage from the RF power supply  62  is applied to the cooling base  30  while supplying a process gas from the shower head  95 . Then a plasma is generated between the wafer W and the shower head  95 . The plasma is utilized to perform CVD film formation and etching on the wafer W. Although the focus ring  78  is also worn out along with plasma treatment of the wafer W, the focus ring  78  is thicker than the wafer W, thus the focus ring  78  is replaced after several wafers W are treated. 
     When the wafer W is treated with a high-power plasma, it is necessary to cool the wafer W efficiently. In the wafer placement table  10 , as the bonding layer between the alumina base  20  and the cooling base  30 , the metal bonding layer  40  having a high thermal conductivity is used rather than a resin layer having a low thermal conductivity. Thus, the capacity (heat removal capacity) to remove heat from the wafer W is high. In addition, the thermal expansion difference between the alumina base  20  and the cooling base  30  is small, thus even when the stress relaxation performance of the metal bonding layer  40  is low, a problem is unlikely to occur. 
     In the wafer placement table  10  described above, each female thread member  38  is stored in a storage hole  36 , opened in the lower surface of the cooling base  30 , in a state of restricted axial rotation and in a state of engaging with the step section  36   c  (engagement section) of the storage hole  36  so as not to fall from the storage hole  36 . Since axial rotation of the female thread member  38  is restricted, the male thread  98   a  of the bolt  98  inserted from the lower surface side of the cooling base  30  can be screwed into the female thread member  38 . In addition, even when the female thread member  38  in a state of engaging with the step section  36   c  of the storage hole  36  is pulled toward the installation plate  96  by the bolt  98  inserted in the installation plate  96 , the female thread member  38  is unlikely to break because it has ductility. Consequently, it is possible to tighten the wafer placement table  10  including the brittle cooling base  30  to the installation plate  96  without a problem. 
     The cooling base  30  is composed of the MMC. Because the MMC is a brittle material, application of the present invention has high significance. 
     Furthermore, the storage hole  36  includes the step section  36   c  as an engagement section, and the female thread member  38  includes the head  38   a  as a to-be-engaged section. Thus, the engagement section and the to-be-engaged section can be produced relatively easily. 
     In addition, when the female thread member  38  is attempted to be axially rotated, it comes into contact with the side walls of the first storage section  36   a  of the storage hole  36  to undergo restricted axial rotation. Thus, axial rotation of the female thread member  38  can be restricted by a relatively simple configuration. 
     The storage hole  36  is provided in a region of the cooling base  30 , the region being lower than the bottom surface  32   a  of the refrigerant flow path  32 . Thus, the storage hole  36  does not interfere with the refrigerant flow path  32 . Therefore, the degree of freedom of the design of the refrigerant flow path  32  is not reduced. 
     In addition, the female thread member  38  is not connected to the cooling base  30  in the storage hole  36 , and is stored in a free state. When the wafer placement table  10  is produced, the female thread member  38  only has to be inserted in the storage hole  36 , which does not take much effort. 
     Note that the present invention is not limited to the above-described embodiment, and may be, of course, implemented in various modes within the technical scope of the present invention. 
     In the first embodiment described above, each storage holes  36  is provided in a region of the cooling base  30 , the region being lower than the bottom surface  32   a  of the refrigerant flow path  32 ; however, the present invention is not limited to this. For instance, as illustrated in  FIG.  7   , a ceiling surface  36   d  of the storage hole  36  may be provided so as to be higher than the bottom surface  32   a  of the refrigerant flow path  32 . In  FIG.  7   , the same components as in the above-described first embodiment are labeled with the same symbols. In  FIG.  7   , the first storage section  36   a  of the storage hole  36  has the same size as in the above-described first embodiment; however, the vertical length of the second storage section  36   b  is longer than in the above-described first embodiment. The head  38   a  of the female thread member  38  has the same size as in the above-described first embodiment; however, the vertical length of the cylindrical section  38   b  is longer than in the above-described first embodiment. Also in this manner, substantially the same effects as in the above-described first embodiment are obtained. However, the degree of freedom of the design of the refrigerant flow path  32  is limited as compared to the above-described first embodiment. 
     In the above-described first embodiment, the inner circumferential surface of the storage hole  36  is provided with the step section  36   c ; however, the present invention is not limited to this. For instance, as illustrated in  FIG.  8   , let inclined faces  38   c  be opposed side faces of the head  38   a  of the female thread member  38 , and the inner peripheral surface of the storage hole  36  may be provided with inclined sections  36   e  which conform to the inclined faces  38   c . In this case, the inclined faces  38   c  of the female thread member  38  engage with the inclined sections  36   e  of the storage hole  36 , thus the female thread member  38  does not fall from the storage hole  36 . When the bolt  98  is screwed into the female thread member  38 , the female thread member  38  is set in a state of being pulled toward the installation plate  96  with the inclined faces  38   c  engaging with the inclined sections  36   e  of the storage hole  36 . 
     In the above-described first embodiment, the shape of the head  38   a  of the female thread member  38  is rectangular in a plan view; however, the present invention is not particularly limited to this. For instance, the shape of the head  38   a  may be a polygonal shape, such as a triangular shape and a pentagonal shape, or may be a plus (+) shape or an elliptic shape. The shape of the first storage section  36   a  of the storage hole  36  should be such that attempt to axially rotate the female thread member  38  causes the head  38   a  to collide with the side walls. This point also applies to a female thread member  538  in the second embodiment described below. 
     In the above-described first embodiment, the female thread member  38  is used as the connection member, and the bolt  98  is used as the to-be-connected member; however, as illustrated in  FIG.  9   , a male thread member  80  may be used as the connection member, and a nut  82  may be used as the to-be-connected member. In  FIG.  9   , the same components as in the above-described first embodiment are labeled with the same symbols. The male thread member  80  is composed of a ductile material. The male thread member  80  has a head  80   a  having the same shape as the head  38   a , a foot  80   b  provided at the center of the back surface of the head  80   a , and a male thread section  80   c  provided at the leading end of the foot  80   b . The head  80   a  is stored in the first storage section  36   a  of the storage hole  36 . The foot  80   b  is inserted in the second storage section  36   b  and the through-hole  97  of the installation plate  96 . The male thread section  80   c  is screwed into the nut  82 . The nut  82  is designed to hook to the step section  97   a  of the through-hole  97 . Since the lower surface of the head  80   a  engages with the step section  36   c  of the storage hole  36 , the male thread member  80  does not fall from the storage hole  36 . When the male thread member  80  is attempted to be axially rotated, the head  80   a  comes into contact with the side walls of the first storage section  36   a  to undergo restricted axial rotation. Thus, the nut  82  can be screwed into the male thread section  80   c  of the male thread member  80 . When the nut  82  is screwed into the male thread section  80   c  of the male thread member  80 , the male thread member  80  is set in a state of being pulled toward the installation plate  96  with the head  80   a  engaging with the step section  36   c  of the storage hole  36 . When the configuration of  FIG.  9    is adopted, the same effects as in the above-described first embodiment are obtained. Also, in the second embodiment described below, a male thread member may be used as the connection member instead of the female thread member  538 , and a nut may be used as the to-be-connected member instead of the bolt  98 . 
     In the above-described first embodiment, a hole may be provided which penetrates the wafer placement table  10  from the lower surface of the cooling base  30  to the wafer placement surface  22   a . As such a hole, a gas supply hole for supplying a thermally conductive gas (for instance, He gas) to the back surface of the wafer W, and a lift pin hole for inserting a lift pin to lift or lower the wafer W with respect to the wafer placement surface  22   a  may be mentioned. The thermally conductive gas is supplied to the space formed by the wafer W and a large number of small protrusions, not illustrated, (which support the wafer W) provided on the wafer placement surface  22   a . For instance, when the wafer W is supported by three lift pins, lift pin holes are provided at three sites. Seal rings (for instance, O-rings) made of resin or metal are disposed at positions between the lower surface of the cooling base  30  and the upper surface of the installation plate  96 , the positions being opposed to those holes. Since the central area of the wafer placement table  10  is fixed by the bolts  98 , these seal rings are maintained in a firmly crushed state. Thus, these seal rings can secure the sealing property sufficiently. This point also applies to the second embodiment described below. 
     In the above-described first embodiment, the cooling base  30  is produced with MMC; however, the cooling base  30  may be produced with a brittle material (for instance, alumina material) other than the MMC. This point also applies to cooling base  530  in the second embodiment described below. 
     In the above-described first embodiment, the wafer attracting electrode  26  is incorporated in the central section  22  of the alumina base  20 ; however, instead of or in addition to this, an RF electrode for plasma generation may be incorporated, or a heater electrode (resistance heating element) may be incorporated. In addition, a focus ring (FR) attracting electrode may be incorporated, or an RF electrode or a heater electrode may be incorporated in the outer circumferential section  24  of the alumina base  20 . This point also applies to the second embodiment described below. 
     In the above-described first embodiment, the alumina sintered body  120  of  FIG.  6 A  is produced by hot-press firing a molded body of alumina powder, and the molded body may be produced by laminating multiple tape molded bodies, or produced by a mold cast method, or produced by compacting alumina powder. This point also applies to the second embodiment described below. 
     In the above-described first embodiment, the alumina base  20  and the cooling base  30  are bonded by the metal bonding layer  40 ; however, a resin bonding layer may be used instead of the metal bonding layer  40 . This point also applies to the second embodiment described below. 
     In the above-described first embodiment, the gap between the female thread member  38  and the first storage section  36   a  of the storage hole  36  may be filled with a filling material. In this manner, thermal conduction becomes favorable as compared to when the gap is void. Therefore, the thermal uniformity of the wafer W improves. As the filling material, in addition to an adhesive resin and a non-adhesive resin, for instance, a material obtained by adding thermally conductive powder (such as metal powder) to these resins may be mentioned. The female thread member  38  is preferably provided with a through-hole (a through-hole from the inner space of the cylindrical section  38   b  to the top surface of the head  38   a ) that penetrates the female thread member  38  vertically. In this manner, at the stage of  FIG.  6 F , a fluid filling material can be easily injected through the through-hole into the gap between the female thread member  38  and the first storage section  36   a  of the storage hole  36 . 
     Second Embodiment 
       FIG.  10    is a vertical cross-sectional view (cross-sectional view of a wafer placement table  510  cut along a plane including the central axis thereof) of a wafer placement table  510  installed in the chamber  94 , and  FIG.  11    is an enlarged cross-sectional view illustrating the vicinity of a storage hole  536  and a female thread member  538 . 
     The wafer placement table  510  is also used for performing CVD and etching on the wafer W utilizing plasma, and is fixed to the installation plate  96  provided inside the chamber  94  for semiconductor process. Since the chamber  94  has been explained in the first embodiment, the same components are labeled with the same symbols, and a description thereof is omitted. The wafer placement table  510  includes the alumina base  20 , a cooling base  530 , and a metal bonding layer  540 . 
     Since the alumina base  20  has been explained in the first embodiment, the same components are labeled with the same symbols, and a description thereof is omitted. 
     The cooling base  530  is a disc member, and composed of the same material as for the cooling base  30 . Herein, the cooling base  530  is assumed to be a disc member made of MMC. The cooling base  530  has a refrigerant flow path groove  582 . The refrigerant flow path groove  582  is formed from one end to the other end in a one-stroke pattern, and is provided in the cooling base  530  so as to be opened in the lower surface of the cooling base  530 . In the refrigerant flow path groove  582 , the opening thereof is closed by the upper surface of the installation plate  96  of the chamber  94 , thereby forming a refrigerant flow path  532 . Thus, the refrigerant flow path groove  582  constitutes the side walls and the ceiling surface of the refrigerant flow path  532 . As in the refrigerant flow path  32  of the above-described first embodiment, the refrigerant flow path  532  is also connected to a refrigerant supply path and a refrigerant discharge path which are not illustrated, and the refrigerant discharged through the refrigerant discharge path undergoes temperature control, then is returned to the refrigerant supply path again. The thickness of the upper side of the cooling base  530  above the refrigerant flow path groove  582  is preferably 5 mm or less, more preferably 3 mm or less. The corners (the corners where the side walls and the ceiling surface intersect) of the upper side of the refrigerant flow path groove  582  are preferably R faces, and the radius of curvature of the R face is preferably 0.5 to 2 mm, for instance. The cooling base  530  is coupled to the RF power supply  62  via the power feed terminal  64 . A HPF  63  is disposed between the cooling base  530  and the RF power supply  62 . The cooling base  530  has a flange  534  used to clamp to the installation plate  96 . 
     The cooling base  530  is provided with a plurality of storage holes  536 , and a female thread member  538  (connection member) is stored in each storage hole  536 . As in the storage holes  36  of the first embodiment, the plurality of storage holes  536  are plurally provided at regular intervals along a concentric circle of the cooling base  530 . As illustrated in  FIG.  11   , each storage hole  536  includes a first storage section  536   a , a second storage section  536   b , and a step section  536   c . The first storage section  536   a  is the space provided at an upper portion of the storage hole  536 , and is opened in the upper surface of the cooling base  530 . The opening surface (the upper surface) of the first storage section  536   a  is covered by the metal bonding layer  540 . The second storage section  536   b  is a passage which is provided to be thinner than the first storage section  536   a  from the first storage section  536   a  to the lower surface of the cooling base  530 . The step section  536   c  is a stepped surface provided at a joint between the first storage section  536   a  and the second storage section  536   b . The female thread member  538  is stored in the first storage section  536   a . The female thread member  538  is a cuboid-shaped (rectangular in a plan view) nut having a thread hole (female thread) in the center. The first storage section  536   a  is also a cuboid-shaped (rectangular in a plan view) space, and stores the female thread member  538  with allowance. The gap between the female thread member  538  and the first storage section  536   a  is filled with a filling material  539 . Specifically, the gap surrounded by the upper face and the side faces of the female thread member  538 , the inner peripheral surface of the first storage section  536   a , and the metal bonding layer  540  is filled with the filling material  539 . As the filling material  539 , in addition to an adhesive resin and a non-adhesive resin, for instance, a material obtained by adding thermally conductive powder (such as metal powder) to these resins may be mentioned. The thermal conductivity of the filling material  539  is preferably, 1×10 −4  W/mm·K or higher, more preferably, 1×10 −3  W/mm·K or higher, further more preferably, 1×10 −2  W/mm·K or higher. The thermal conductivity of the filling material  539  can be adjusted, for instance, by the amount of the thermally conductive powder to be added to the resin. A width d of the gap between the female thread member  538  and the first storage section  536   a  is preferably 0.2 mm or more in consideration of injection of a fluid uncured filling material into the gap. The step section  536   c  is disposed below the ceiling surface of the refrigerant flow path groove  582 . The female thread member  538  engages with the step section  536   c  of the storage hole  536 . When the female thread member  538  is attempted to be axially rotated, it comes into contact with the side walls of the first storage section  536   a  to undergo restricted axial rotation. In the embodiment, due to the presence of the filling material  539 , axial rotation of the female thread member  538  is also restricted by the filling material  539 . The female thread member  538  is composed of a ductile material (for instance, Ti, Mo, W). 
     The metal bonding layer  540  bonds the lower surface of the alumina base  20  and the upper surface of the cooling base  530 . Since the metal bonding layer  540  is the same as the metal bonding layer  40  of the first embodiment, a description thereof is omitted. 
     The outer circumferential section  24  of the alumina base  20 , the outer circumference of the metal bonding layer  540  and the side surface of the cooling base  530  are covered with an insulating film  542 . As the insulating film  542 , for instance, a thermal spray film such as alumina and yttria may be mentioned. 
     The wafer placement table  510  is mounted on the installation plate  96  provided inside the chamber  94  with a seal ring  576  having a large diameter and seal rings  577  to  579  having a small diameter interposed between wafer placement table  510  and the installation plate  96 . The seal rings  576  to  579  are made of metal or resin. The seal ring  576  is disposed slightly inward of the outer edge of the cooling base  530  to prevent the refrigerant from leaking outwardly of the seal ring  576 . The seal ring  577  is disposed to surround the periphery of the foot of the bolt  98  to prevent the refrigerant from entering inwardly of the seal ring  577 . The seal ring  578  is disposed at the opening edge of the insulated tube  55  to prevent the refrigerant from entering inwardly of the seal ring  578 . The seal ring  579  is disposed to surround the periphery of the power feed terminal  64  to prevent the refrigerant from entering inwardly of the seal ring  579 . 
     The flange  534  provided at the outer circumference of the cooling base  530  is attached to the installation plate  96  using the clamping member  70  and the bolts  72 . Since the clamping member  70 , the bolts  72  and a clamp method have been explained in the first embodiment, a description thereof is omitted. In addition, the central area of the cooling base  530  is attached to the installation plate  96  using the bolts  98  (to-be-connected members). As illustrated in  FIG.  11   , the foot of each bolt  98  is provided with the male thread  98   a . The bolt  98  is inserted into the through-hole  97  provided at the position, opposed to the storage hole  536 , of the installation plate  96  through the lower surface of the installation plate  96 , and the male thread  98   a  is screwed into the female thread member  538  in the first storage section  536   a . The through-hole  97  has a smaller diameter in the upper section and a larger diameter in the lower section, and has the step section  97   a  between the upper section and the lower section. The head of the bolt  98  hooks to the step section  97   a  of the through-hole  97 . Since the female thread member  538  is stored in the first storage section  536   a  in a state of restricted axial rotation, the bolt  98  can be screwed into the female thread member  538 . When the bolt  98  is screwed into the female thread member  538 , the female thread member  538  in a state of engaging with the step section  536   c  of the storage hole  536  is set in a state of being pulled toward the installation plate  96 . 
     In the embodiment, the central area of the wafer placement table  510  is fixed by the bolts  98 , thus, it is possible to prevent the wafer placement table  510  from projecting upward, and to maintain the seal rings  576  to  578  in a firmly crushed state. 
     Note that refrigerant is supplied and discharged to and from the refrigerant flow path  582  by adopting the same structure as in  FIG.  5    explained in the first embodiment. 
     Next, a manufacturing example of the wafer placement table  510  will be described using  FIGS.  12 A to  12 G .  FIGS.  12 A to  12 G  are manufacturing process charts of the wafer placement table  510 . First, in the same manner as in the first embodiment, the alumina sintered body  120  including the power feed terminal  54  is produced ( FIGS.  12 A  to C). 
     Concurrently, an MMC disc member  630  is produced ( FIG.  12 D ), and the refrigerant flow path groove  582  is formed in the lower surface of the MMC disc member  630  as well as the storage hole  536  (the first storage section  536   a , the second storage section  536   b  and the step section  536   c ) which penetrates the MMC disc member  630  vertically, and a through-hole for inserting the power feed terminal  54  are formed ( FIG.  12 E ). In this case, the MMC disc member  630  is preferably made of SiSiCTi or AlSiC. This is because the thermal expansion coefficient of alumina is approximately the same as the thermal expansion coefficients of SiSiCTi and AlSiC. 
     Next, the female thread member  538  is stored in the first storage section  536   a , then a metal bonding material is disposed on the upper surface of the MMC disc member  630 . The metal bonding material is provided with a through-hole for inserting the power feed terminal  54 . Next, the alumina sintered body  120  is placed on the metal bonding material while inserting the power feed terminal  54  of the alumina sintered body  120  into the through-hole of the metal bonding material and the through-hole of the MMC disc member  630 . Thus, a laminated body is obtained, in which the MMC disc member  630 , the metal bonding material and the alumina sintered body  120  are laminated in that order from the bottom. A bonded body  610  is obtained ( FIG.  12 F ) by pressurizing the laminated body while heating it (TCB). The bonded body  610  is such that the alumina sintered body  120  and the MMC disc member  630  are bonded by the metal bonding layer  540 . The female thread member  538  is stored in the first storage section  536   a  of the bonded body  610 . Since the metal bonding material and the TCB have been explained in the first embodiment, a description thereof is omitted herein. 
     Subsequently, a fluid uncured filling material is injected into the gap between the female thread member  538  and the first storage section  536   a  through the second storage section  536   b  and the thread hole of the female thread member  538 . The gap is preferably 0.2 mm or more in consideration of easy injection of the uncured filling material. A filling material  539  is formed by curing the injected uncured filling material. Subsequently, the outer circumference of the alumina sintered body  120  is cut to form a step, thus the alumina base  20  including the central section  22  and the outer circumferential section  24  is produced. Also, the outer circumference of the MMC disc member  630  is cut to form a step, thus the cooling base  530  including the flange  534  is produced. In addition, the insulated tube  55  is disposed in the insertion hole of the power feed terminal  54 . Furthermore, the insulating film  542  is formed by applying thermal spraying using alumina powder to the side surface of the outer circumferential section  24  of the alumina base  20 , the periphery of the metal bonding layer  540  and the side surface of the cooling base  530  ( FIG.  12 G ). Consequently, the wafer placement table  510  is obtained. 
     Since an example of use of the wafer placement table  510  is the same as the example of use of the wafer placement table  10  of the above-described first embodiment, a description thereof is omitted. 
     In the wafer placement table  510  described above, each female thread member  538  is stored in a storage hole  536 , opened in the lower surface of the cooling base  530 , in a state of restricted axial rotation and in a state of engaging with the step section  536   c  (engagement section) of the storage hole  536  so as not to fall from the storage hole  536 . Axial rotation of the female thread member  538  is restricted, thus the male thread  98   a  of the bolt  98  inserted from the lower surface side of the cooling base  530  can be screwed into the female thread member  538 . In addition, even when the female thread member  538  in a state of engaging with the step section  536   c  of the storage hole  536  is pulled toward the installation plate  96  by the bolt  98  inserted in the installation plate  96 , the female thread member  538  is unlikely to break because it has ductility. Consequently, it is possible to tighten the wafer placement table  510  including the brittle cooling base  530  to the installation plate  96  without a problem. 
     The cooling base  530  is composed of the MMC. Because the MMC is a brittle material, application of the present invention has high significance. 
     Furthermore, the storage hole  536  includes the step section  536   c  as an engagement section, and the bottom surface of the female thread member  538  functions as a to-be-engaged section. Thus, the engagement section and the to-be-engaged section can be produced relatively easily. 
     Furthermore, when the female thread member  538  is attempted to be axially rotated, it comes into contact with the side walls of the first storage section  536   a  of the storage hole  536  to undergo restricted axial rotation. Thus, axial rotation of the female thread member  538  can be restricted by a relatively simple configuration. In addition, axial rotation of the female thread member  538  is also restricted by the filling material  539 . 
     Furthermore, the first storage section  536   a  is opened in the upper surface of the cooling base  530 , and the opening surface is covered with the metal bonding layer  540 . Thus, the first storage section  536   a  can be produced relatively easily as compared to the first storage section  36   a  which is incorporated in the inside of the cooling base  30  as in the first embodiment. In this structure, the refrigerant flow path  532  (the refrigerant flow path groove  582 ) needs to be provided skirting the storage hole  536 , thus the thermal uniformity is likely to reduce in the vicinity immediately above the storage hole  536  of the wafer W. In order to prevent the reduction in the thermal uniformity, the gap between the female thread member  538  and the first storage section  536   a  of the storage hole  536  is filled with the filling material  539 . Consequently, the thermal conduction around the storage hole  536  becomes favorable, thus reduction in the thermal uniformity can be prevented. 
     It is a matter of course that the present invention is not limited to the above-described embodiment and can be implemented in various forms insofar as falling within the technical scope of the present invention. 
     In the above-described second embodiment, as illustrated in  FIG.  13   , the female thread member  538  may engage with the step section  536   c  of the storage hole  536  with a stress buffering member  537  interposed between the female thread member  538  and the step section  536   c , the stress buffering member  537  having a lower Young&#39;s modulus than the female thread member  538 . For instance, Ti alloy may serve as the female thread member  538 , and pure Al may serve as the stress buffering member  537 . In this manner, even when the female thread member  538  is pulled toward the installation plate  96  by the bolts  98  provided in the installation plate  96 , the stress tends to be dispersed because the stress buffering member  537  is interposed between the female thread member  538  and the step section  536   c . First, in order to reduce the stress in the first storage section  536   a , width w of the annular region where the step section  536   c  and the female thread member  538  are in indirect contact with each other is preferably 3 mm or greater, and more preferably 5 mm or greater. Second, the inner diameter x of the thread hole of the female thread member  538  is preferably 10 mm or less, and more preferably 7 mm or less. Third, when corners between the bottom face and the side faces of the first storage section  536   a  are chamfered to R faces (rounded faces), the radius r of curvature is preferably 0.3 mm or greater, and more preferably 0.5 mm or greater. The first to third conditions indicate the order of stress reduction effect, and the first condition has the highest stress reduction effect. The corners between the bottom face and the side faces of the female thread member  538  are preferably R faces or C faces. The thickness t from the step section  536   c  to the lower surface of the cooling base  530  is preferably 3 mm or greater and 10 mm or less. These numerical value ranges apply to the case where the stress buffering member  537  is not provided, and to the first embodiment. 
     In the above-described second embodiment, the lower surface of the cooling base  530  is provided with the refrigerant flow path groove  582 , the installation plate  96  (lower base) is disposed below the cooling base  530 , and the seal rings which liquid-tightly seal the refrigerant flow path groove  582  are disposed between the lower surface of the cooling base  530  and the installation plate  96 . However, the present invention is not particularly limited to this. For instance, a refrigerant flow path groove may be provided in the upper surface of the installation plate instead of the lower surface of the cooling base, and the seal rings which liquid-tightly seal the refrigerant flow path groove may be disposed between the lower surface of the cooling base and the installation plate. The cooling base is brittle MMC or alumina, and the cooling base is provided with the first storage section. 
     In the above-described second embodiment, as illustrated in  FIG.  14 A , a triangular prism-shaped (triangular in a plan view) nut having a thread hole in the center may be used as the female thread member  538 . In this case, the storage hole  536  may be formed as in  FIG.  14 B .  FIG.  14 B  is a partial enlarged view of the vicinity of the storage hole  536 , as seen from below the cooling base  530 . In  FIG.  14 B , the second storage section  536   b  is a hole through which the female thread member  538  can be passed, the hole having a triangular plan view, and the first storage section  536   a  is a space (a composite figure of a triangle and a circle in a plan view) which allows the female thread member  538  to be axially rotated only by a predetermined angle. A dashed-two dotted line indicates a state immediately after the female thread member  538  is inserted in the second storage section  536   b  and stored in the first storage section  536   a , and a dashed-dotted line indicates the manner in which the female thread member  538  stored in the first storage section  536   a  is axially rotated in the arrow direction only by a predetermined angle. At this point, the step section  536   c  is the section which is inside the outer edge of the first storage section  536   a  and outside the opening edge of the second storage section  536   b , and this section engages with the female thread member  538 . With this structure, after the alumina base  20  and the cooling base  530  are bonded, the female thread member  538  can be stored in the first storage section  536   a . For instance, first, the alumina base  20  and the cooling base  530  are bonded by the metal bonding layer  540  without storing the female thread member  538  in the first storage section  536   a . Subsequently, a fluid uncured filling material is injected into the first storage section  536   a  with the lower surface of the cooling base  30  facing upward. Subsequently, the female thread member  538  is stored in the first storage section  536   a  from the second storage section  536   b , and the female thread member  538  is axially rotated only by a predetermined angle. Consequently, the uncured filling material is evenly filled between the female thread member  538  and the first storage section  536   a . Subsequently, the uncured filling material is cured to become the filling material  539 . Note that this structure can be applied to not only a triangular prism-shaped nut, but also a polygonal prism-shaped (quadrilateral prism-shaped or hexagonal prism-shaped) nut. 
     Instead of the female thread member  538  of the above-described second embodiment, the female thread member  38  ( FIG.  3   ,  FIG.  7    or  FIG.  8   ) of the first embodiment may be used. In that case, the female thread member  38  is preferably provided with a through-hole (from the inner space of the cylindrical section  38   b  to the top surface of the head  38   a ) that penetrates the female thread member  38  vertically. This is because when the through-hole is utilized, a fluid uncured filling material is easily filled in the gap between the head  38   a  and the first storage section of the storage hole. 
     In the above-described second embodiment, as illustrated in  FIG.  15   , instead of providing the refrigerant flow path groove  582  (refrigerant flow path  532 ) in the cooling base  530 , the upper surface of the installation plate  96  may be provided with the refrigerant flow path groove  91 , and a refrigerant flow path  92  may be formed by closing the upper opening of the refrigerant flow path groove  91  with the cooling base  530 . In  FIG.  15   , the same components as in the above-described second embodiment are labeled with the same symbols. Note that also in the first embodiment, instead of providing the refrigerant flow path  32  in the cooling base  30 , the upper surface of the installation plate  96  may be provided with a refrigerant flow path groove as in  FIG.  15   , and a refrigerant flow path may be formed by closing the upper opening of the refrigerant flow path groove with the cooling base  30 . 
     The present application claims priority from Japanese Patent Application No. 2021-185369, filed on Nov. 15, 2021, and Japanese Patent Application No. 2022-108438, filed on Jul. 5, 2022, the entire contents of which are incorporated herein by reference.