Patent Publication Number: US-2023144107-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 
     Hitherto, there is known a wafer placement table that includes a ceramic base having a wafer placement surface and incorporating an electrode, a cooling base having a refrigerant flow channel, and a bonding layer bonding the ceramic base with the cooling base. For example, Patent Literatures 1 and 2 describe that, in such a wafer placement table, the cooling base made of a metal matrix composite material of which the coefficient of linear thermal expansion is substantially the same as that of the ceramic base is used. Patent Literatures 1 and 2 also describe that the wafer placement table has a terminal hole for allowing insertion of a power supply terminal for supplying electric power to an electrode, gas holes for supplying He gas to the back surface of a wafer, and lift pin holes for allowing insertion of lift pins to lift a wafer from the wafer placement surface. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent No. 5666748 B 
         PTL 2: Japanese Patent No. 5666749 B 
       
    
     SUMMARY OF THE INVENTION 
     However, a distance from a ceiling surface of the refrigerant flow channel to a wafer is constant from an inlet of the refrigerant flow channel to an outlet of the refrigerant flow channel, so the wafer tends to be easy to cool down near the inlet of the refrigerant flow channel and hard to cool down near the outlet. As a result, the soaking performance of a wafer can be not able to be sufficiently obtained as a result. 
     The present invention is made to solve such an inconvenience, and it is a main object to increase the soaking performance of a wafer. 
     A wafer placement table of the present invention includes a ceramic base having a wafer placement surface on its top surface where a wafer is able to be placed and incorporating an electrode, a cooling base having a refrigerant flow channel, and a bonding layer that bonds the ceramic base with the cooling base. In an area that overlaps the wafer placement surface in plan view of the refrigerant flow channel, a distance from a ceiling surface of the refrigerant flow channel to the wafer placement surface at a most downstream part of the refrigerant flow channel is shorter than the distance at a most upstream part of the refrigerant flow channel. 
     In the wafer placement table, in an area that overlaps the wafer placement surface in plan view of the refrigerant flow channel, a distance from a ceiling surface of the refrigerant flow channel to the wafer placement surface at a most downstream part of the refrigerant flow channel is shorter than the distance at a most upstream part of the refrigerant flow channel. When the wafer placement table is used, refrigerant flows from the most upstream part of the refrigerant flow channel toward the most downstream part while dissipating heat from a high-temperature wafer, so the temperature of refrigerant flowing through the refrigerant flow channel at the most downstream part is higher than the temperature of refrigerant flowing through the refrigerant flow channel at the most upstream part. On the other hand, since the distance from the ceiling surface of the refrigerant flow channel to the wafer placement surface at the most downstream part of the refrigerant flow channel is shorter than the distance at the most upstream part of the refrigerant flow channel, thermal resistance from the ceiling surface of the refrigerant flow channel to the wafer placement surface at the most downstream part is lower than thermal resistance from the ceiling surface of the refrigerant flow channel to the wafer placement surface at the most upstream part. Therefore, generally, it is possible to reduce the temperature difference between a location facing the most upstream part of the refrigerant flow channel and a location facing the most downstream part of the refrigerant flow channel in the wafer placement surface. Therefore, the soaking performance of a wafer increases. 
     In the wafer placement table of the present invention, a distance from the ceiling surface of the refrigerant flow channel to the wafer placement surface may gradually reduce from the most upstream part of the refrigerant flow channel toward the most downstream part. With this configuration, the soaking performance of a wafer increases. 
     In the wafer placement table of the present invention, a distance from the ceiling surface of the refrigerant flow channel to the wafer placement surface may be adjusted by at least one of a distance from the ceiling surface of the refrigerant flow channel to a top surface of the cooling base, a thickness of the bonding layer, and a thickness of the ceramic base. Of these, the distance from the ceiling surface of the refrigerant flow channel to the wafer placement surface is preferably adjusted by the distance from the ceiling surface of the refrigerant flow channel to the top surface the cooling base. 
     In the wafer placement table of the present invention, a distance from the ceiling surface of the refrigerant flow channel to the wafer placement surface at the most downstream part is preferably 50% to 90% of a distance from the ceiling surface of the refrigerant flow channel to the wafer placement surface at the most upstream part. When the percentage is lower than or equal to 90%, the soaking performance of a wafer W sufficiently increases. When the percentage is higher than or equal to 50%, it is possible to avoid occurrence of a crack above a most downstream part. 
     In the wafer placement table of the present invention, the cooling base may be made of a metal matrix composite material, and the bonding layer may be a metal bonding layer. With the structure that the cooling base is a metal matrix composite material and the bonding layer is a metal bonding layer, thermal resistance from the refrigerant flow channel to the wafer placement surface is small, so the wafer temperature is susceptible to the influence of the temperature gradient of refrigerant. Therefore, the significance to apply the present invention is high. Since the metal bonding layer has a high thermal conductivity, the metal bonding layer is suitable for heat dissipation. Furthermore, a difference in coefficient of thermal expansion between the ceramic base and the cooling base made of a metal matrix composite material is able to be reduced, so a trouble is less likely to occur even when the stress relaxation properties of the metal bonding layer are low. 
     The wafer placement table of the present invention may further include a hole extending through the cooling base in an up and down direction. In the refrigerant flow channel, a distance from the ceiling surface of the refrigerant flow channel to the wafer placement surface in an area around the hole may be shorter than the distance in an area outside the area around the hole. Generally, an area around just above such a hole in a wafer tends to be a hot spot; however, a distance from the ceiling surface of the refrigerant flow channel to the wafer placement surface in an area around the hole is shorter than the distance in an area outside the area around the hole. Therefore, heat dissipation of the area around the hole is promoted. Therefore, the soaking performance of a wafer increases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a vertical cross-sectional view of a wafer placement table  10  placed in a chamber  94 . 
         FIG.  2    is a sectional view of a cooling base  30  when a cross section taken along a horizontal plane passing through a refrigerant flow channel  32  is viewed from above. 
         FIGS.  3 A to  3 G  are manufacturing process charts of the wafer placement table  10 . 
         FIG.  4    is a sectional view of the cooling base  30  when a cross section taken along a horizontal plane passing through a refrigerant flow channel  82  is viewed from above. 
         FIG.  5    is a vertical cross-sectional view of an example in which a part  32   x  with a short distance d is provided in the middle of the refrigerant flow channel  32 . 
         FIG.  6    is a vertical cross-sectional view of an example using a ceramic base  20  without an FR placement surface. 
         FIG.  7    is a vertical cross-sectional view of a wafer placement table  210 . 
         FIG.  8    is a vertical cross-sectional view of a wafer placement table  310 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention will be described below with reference to the drawings.  FIG.  1    is a vertical cross-sectional view of a wafer placement table  10  (a sectional view taken along a plane including the central axis of the wafer placement table  10 ) placed in a chamber  94 , and  FIG.  2    is a sectional view of a cooling base  30  when a cross section taken along a horizontal plane passing through a refrigerant flow channel  32  is viewed from above. In  FIG.  2   , a terminal hole  51 , a power supply terminal  54 , an electrically insulating tube  55 , and the like are omitted. 
     The wafer placement table  10  is used to perform CVD, etching, or the like on a wafer W by using plasma, and is fixed to a mounting plate  96  provided inside the semiconductor process chamber  94 . The wafer placement table  10  includes a ceramic base  20 , a cooling base  30 , and a metal bonding layer  40 . 
     The ceramic base  20  includes an outer peripheral  24  part having an annular focus ring placement surface  24   a , on the outer peripheral side of a central part  22  having a circular wafer placement surface  22   a . Hereinafter, a focus ring may be abbreviated as “FR”. A 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 ceramic base  20  is made of a ceramic material, typically, alumina, aluminum nitride, or the like. The FR placement surface  24   a  is lower in level than the wafer placement surface  22   a.    
     The central part  22  of the ceramic base  20  incorporates a wafer attraction electrode  26  on the side close to the wafer placement surface  22   a . The wafer attraction electrode  26  is made of a material that contains, for example, W, Mo, WC, MoC, or the like. The wafer attraction electrode  26  is a disk-shaped or mesh-shaped single-pole electrostatic attraction electrode. A layer of the ceramic base  20  on the upper side of the wafer attraction electrode  26  functions as a dielectric layer. A wafer attraction direct current power supply  52  is connected to the wafer attraction electrode  26  via a power supply terminal  54 . The power supply terminal  54  is inserted through a terminal hole  51  provided between the bottom surface of the wafer attraction electrode  26  and the bottom surface of the cooling base  30  in the wafer placement table  10 . The power supply terminal  54  is provided so as to pass through an electrically insulating tube  55  disposed in a through-hole extending through the cooling base  30  and the metal bonding layer  40  in the up and down direction in the terminal hole  51  and reach the wafer attraction electrode  26  from the bottom surface of the ceramic base  20 . A low pass filter (LPF)  53  is provided between the wafer attraction direct current power supply  52  and the wafer attraction electrode  26 . 
     The cooling base  30  is a disk member made of a metal matrix composite material (also referred to as metal matrix composite (MMC)). The cooling base  30  has the refrigerant flow channel  32  in which refrigerant is able to circulate. The refrigerant flow channel  32  is connected to a refrigerant supply passage  36  and a refrigerant discharge passage  38 , and refrigerant discharged from the refrigerant discharge passage  38  is adjusted in temperature and then returned to the refrigerant supply passage  36  again. Examples of the MMC include a material including Si, SiC, and Ti, and a material obtained by impregnating an SiC porous body with Al and/or Si. The material including Si, SiC, and Ti is referred to as SiSiCTi, the material that impregnates an SiC porous body with Al is referred to as AlSiC, and the material that impregnates an SiC porous body with Si is referred to as SiSiC. When the ceramic base  20  is an alumina base, the MMC used for the cooling base  30  is preferably AlSiC, SiSiCTi, or the like of which the coefficient of thermal expansion is close to the coefficient of thermal expansion of alumina. The cooling base  30  is connected to an RF power supply  62  via a power supply 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  on the bottom surface side. The flange  34  is used to clamp the wafer placement table  10  to a mounting plate  96 . 
     As shown in  FIG.  2   , the refrigerant flow channel  32  is formed in a one-stroke pattern from an inlet  32   a  to an outlet  32   s  over the entire area other than the flange  34  in the cooling base  30  when the cross section of the refrigerant flow channel  32 , taken along the horizontal plane, is viewed from above. In the present embodiment, the refrigerant flow channel  32  is formed in a zigzag shape. Specifically, the refrigerant flow channel  32  is formed in a zigzag shape so as to run from the inlet  32   a  connected to the refrigerant supply passage  36  via a straight part  32   b , a folded part  32   c , a straight part  32   d , a folded part  32   e , a straight part  32   f , a folded part  32   g , a straight part  32   h , a folded part  32   i , a straight part  32   j , a folded part  32   k , a straight part  32   i , a folded part  32   m , a straight part  32   n , a folded part  32   o , a straight part  32   p , a folded part  32   q , and a straight part  32   r  to the outlet  32   s  connected to the refrigerant discharge passage  38 . Here, when a most upstream part  32 U and a most downstream part  32 L are determined in an area that overlaps the wafer placement surface  22   a  in plan view of the refrigerant flow channel  32 , the most upstream part  32 U and the most downstream part  32 L are at locations shown in  FIG.  2   . As shown in  FIG.  1   , a distance d from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  at the most downstream part  32 L is shorter than the distance d at the most upstream part  32 U. The distance d gradually reduces from the most upstream part  32 U toward the most downstream part  32 L. In the present embodiment, as shown in  FIG.  1   , the ceiling surfaces at the inlet  32   a , the outlet  32   s , and the straight parts  32   d ,  32   f ,  32   h ,  32   j ,  321 ,  32   n ,  32   p  of the refrigerant flow channel  32  are inclined downward from the outlet  32   s  side toward the inlet  32   a  side. The bottom surface of the refrigerant flow channel  32  is disposed in the same horizontal plane. The distance d at the most downstream part  32 L is preferably 50% to 90% of the distance d at the most upstream part  32 U. 
     When the relationship between the location in the refrigerant flow channel  32  and the distance d is represented by a graph, the distance d may continuously reduce or reduce in a stepwise manner from the most upstream part  32 U toward the most downstream part  32 L, and preferably continuously reduces. The case where the distance d continuously reduces from the most upstream part  32 U toward the most downstream part  32 L may be, for example, a case where the distance d continuously reduces at a constant gradient (slope), a case where the distance d reduces while drawing a downward-convex curve, or a case where the distance d reduces while drawing an upward-convex curve. 
     The metal bonding layer  40  bonds the bottom surface of the ceramic base  20  with the top surface of the cooling base  30 . The metal bonding layer  40  may be, for example, a layer made of solder or a brazing metal material. The metal bonding layer  40  is formed by, for example, TCB (thermal compression bonding). TCB is a known method of sandwiching a metal bonding material between two members to be bonded and bonding the two members in a state of being heated to a temperature lower than or equal to a solidus temperature of the metal bonding material. 
     The side surface of the outer peripheral part  24  of the ceramic base  20 , the outer periphery of the metal bonding layer  40 , and the side surface of the cooling base  30  are coated with an electrically insulating film  42 . Examples of the electrically insulating film  42  include a sprayed film made of alumina, yttria, or the like. 
     The thus configured wafer placement table  10  is attached to the mounting plate  96  inside the chamber  94  by using a clamp member  70 . The clamp member  70  is an annular member with a substantially inverted L-shaped cross section and has an inner peripheral step surface  70   a . The wafer placement table  10  and the mounting plate  96  are united by the clamp member  70 . In a state where the inner peripheral step surface  70   a  of the clamp member  70  is placed on the flange  34  of the cooling base  30  of the wafer placement table  10 , bolts  72  are inserted from the top surface of the clamp member  70  and screwed to threaded holes provided on the top surface of the mounting plate  96 . The bolts  72  are mounted at multiple locations (for example, eight locations or 12 locations) provided at equal intervals along the circumferential direction of the clamp member  70 . The clamp member  70  and the bolts  72  may be made of an electrically insulating material or may be made of an electrically conductive material (metal or the like). 
     Next, an example of manufacturing of the wafer placement table  10  will be described with reference to  FIGS.  3 A to  3 G .  FIGS.  3 A to  3 G  are a manufacturing process chart of the wafer placement table  10 . Initially, a disk-shaped ceramic sintered body  120  that is the source of the ceramic base  20  is made by firing a ceramic powder molded body by hot pressing ( FIG.  3 A ). The ceramic sintered body  120  incorporates the wafer attraction electrode  26 . Subsequently, a terminal hole upper part  151   a  is formed from the bottom surface of the ceramic sintered body  120  to the wafer attraction electrode  26  ( FIG.  3 B ). Then, the power supply terminal  54  is inserted into the terminal hole upper part  151   a , and the power supply terminal  54  and the wafer attraction electrode  26  are bonded ( FIG.  3 C ). 
     In parallel with this, two MMC disk members  131 ,  136  are made ( FIG.  3 D ). Then, holes extending through both the MMC disk members  131 ,  136  in the up and down direction are perforated, and a groove  132  that will be finally the refrigerant flow channel  32  is formed on the bottom surface of the upper-side MMC disk member  131  ( FIG.  3 E ). Specifically, a terminal hole middle part  151   b  is perforated in the upper-side MMC disk member  131 . The groove  132  is formed by machining the upper-side MMC disk member  131  so as to have a shape similar to the refrigerant flow channel  32 . A terminal hole lower part  151   c , a refrigerant supply passage through-hole  133 , and a refrigerant discharge passage through-hole  134  are perforated in the lower-side MMC disk member  136 . When the ceramic sintered body  120  is made of alumina, the MMC disk members  131 ,  136  are preferably made of SiSiCTi or AlSiC. This is because the coefficient of thermal expansion of alumina and the coefficient of thermal expansion of SiSiCTi or AlSiC are almost the same. 
     The disk member made of SiSiCTi can be made by, for example, as follows. Initially, a powder mixture is made by mixing silicon carbide, metal Si and metal Ti. After that, a disk-shaped molded body is made by uniaxial pressing of the obtained powder mixture, and the molded body is sintered by hot pressing in an inert atmosphere, with the result that the disk member made of SiSiCTi is obtained. 
     Subsequently, a metal bonding material is disposed between the bottom surface of the upper-side MMC disk member  131  and the top surface of the lower-side MMC disk member  136 , and a metal bonding material is disposed on the top surface of the upper-side MMC disk member  131 . Through-holes are provided in advance in each of the metal bonding materials at locations facing the holes. The power supply terminal  54  of the ceramic sintered body  120  is inserted into the terminal hole middle part  151   b  and the terminal hole lower part  151   c , and the ceramic sintered body  120  is placed on the metal bonding material disposed on the top surface of the MMC disk member  131 . Thus, a laminated body in which the lower-side MMC disk member  136 , the metal bonding material, the upper-side MMC disk member  131 , the metal bonding material, and the ceramic sintered body  120  are laminated in this order from the bottom is obtained. By pressurizing the laminated body while heating the laminated body (TCB), a bonded body  110  is obtained ( FIG.  3 F ). The bonded body  110  is configured such that the ceramic sintered body  120  is bonded via the metal bonding layer  40  to the top surface of the MMC block  130  that is the source of the cooling base  30 . The MMC block  130  is the one in which the upper-side MMC disk member  131  and the lower-side MMC disk member  136  are bonded via a metal bonding layer  135 . The MMC block  130  has the refrigerant flow channel  32 , the refrigerant supply passage  36 , the refrigerant discharge passage  38 , and the terminal hole  51 . The terminal hole  51  is a hole made up of the continuous terminal hole upper part  151   a , terminal hole middle part  151   b , and terminal hole lower part  151   c.    
     TCB is performed, for example, as follows. In other words, the laminated body is pressurized at a temperature lower than or equal to a solidus temperature of the metal bonding material (for example, higher than or equal to a temperature obtained by subtracting 20° C. from the solidus temperature and lower than or equal to the solidus temperature) to perform bonding, after that the temperature is returned to a room temperature. Thus, the metal bonding material becomes the metal bonding layer. An Al—Mg bonding material or an Al—Si—Mg bonding material may be used as the metal bonding material at this time. When, for example, TCB is performed by using an Al—Si—Mg bonding material, the laminated body is pressurized in a state of being heated under vacuum atmosphere. The metal bonding material with a thickness of about 100 μm is preferable. 
     Subsequently, the ceramic base  20  with the central part  22  and the outer peripheral part  24  is obtained by cutting the outer periphery of the ceramic sintered body  120  to form a step. The cooling base  30  with the flange  34  is obtained by cutting the outer periphery of the MMC block  130  to form a step. The electrically insulating tube  55  that allows insertion of the power supply terminal  54  is disposed in the terminal hole  51  from the bottom surface of the ceramic base  20  to the bottom surface of the cooling base  30 . The side surface of the outer peripheral part  24  of the ceramic base  20 , the periphery of the metal bonding layer  40 , and the side surface of the cooling base  30  are subjected to thermal spraying by using ceramic powder to form the electrically insulating film  42  ( FIG.  3 G ). Thus, the wafer placement table  10  is obtained. 
     The cooling base  30  of  FIG.  1    has been described as a single-piece product; however, as shown in  FIG.  3 G , the cooling base  30  may be configured such that two members are bonded by a metal bonding layer or may be configured such that three or more members are bonded by metal bonding layers. 
     Next, an example of the use of the wafer placement table  10  will be described with reference to  FIG.  1   . The wafer placement table  10  is fixed to the mounting plate  96  in the chamber  94  by the clamp member  70  as described above. A shower head  98  that discharges process gas from a large number of gas injection holes into the chamber  94  is disposed on the ceiling surface of the chamber  94 . 
     A focus ring  78  is placed on the FR placement surface  24   a  of the wafer placement table  10 , and a disk-shaped wafer W is placed on the wafer placement surface  22   a . The focus ring  78  has a step along the inner periphery of an upper end part so as not to interfere with the wafer W. In this state, the wafer W is attracted to the wafer placement surface  22   a  by applying a direct current voltage of the wafer attraction direct current power supply  52  to the wafer attraction electrode  26 . Then, the inside of the chamber  94  is set to a predetermined vacuum atmosphere (or decompression atmosphere), and an RF voltage from the RF power supply  62  is applied to the cooling base  30  while process gas is being supplied from the shower head  98 . As a result, plasma is generated between the wafer W and the shower head  98 . Then, the wafer W is subjected to CVD deposition or etching by using the plasma. As the wafer W is subjected to a plasma process, the focus ring  78  abrades; however, the focus ring  78  is thicker than the wafer W, replacement of the focus ring  78  is performed after processing a plurality of wafers W. 
     When a wafer W is processed with high-power plasma, it is necessary to efficiently cool the wafer W. In the wafer placement table  10 , not a resin layer with a low thermal conductivity but the metal bonding layer  40  with a high thermal conductivity is used as the bonding layer between the ceramic base  20  and the cooling base  30 . Therefore, performance to dissipate heat from a wafer W (heat dissipation performance) is high. Since a difference in thermal expansion between the ceramic base  20  and the cooling base  30  is small, a trouble is less likely to occur even when stress relaxation properties of the metal bonding layer  40  are low. Furthermore, a distance d from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  at the most downstream part  32 L of the refrigerant flow channel  32  is shorter than the distance d at the most upstream part  32 U of the refrigerant flow channel  32 . The distance d at the most downstream part  32 L is shorter than the distance d at the most upstream part  32 U. When the wafer placement table  10  is used, refrigerant flows from the most upstream part  32 U of the refrigerant flow channel  32  toward the most downstream part  32 L of the refrigerant flow channel  32  while dissipating heat from a high-temperature wafer W, so the temperature of refrigerant flowing through the refrigerant flow channel  32  at the most downstream part  32 L is higher than the temperature of refrigerant flowing through the refrigerant flow channel  32  at the most upstream part  32 U. On the other hand, since the distance d from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  at the most downstream part  32 L of the refrigerant flow channel  32  is shorter than the distance d at the most upstream part  32 U of the refrigerant flow channel  32 , thermal resistance from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  at the most downstream part  32 L is lower than thermal resistance from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  at the most upstream part  32 U. Therefore, generally, it is possible to reduce the temperature difference between a location facing the most upstream part  32 U of the refrigerant flow channel  32  and a location facing the most downstream part  32 L of the refrigerant flow channel  32  in the wafer placement surface  22   a . The flow rate of refrigerant flowing through the refrigerant flow channel  32  is preferably set to 15 L/min to 50 L/min and more preferably set to 20 L/min to 40 L/min. 
     With the wafer placement table  10  of the above-described present embodiment, the distance d from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  at the most downstream part  32 L of the refrigerant flow channel  32  is shorter than the distance d at the most upstream part  32 U of the refrigerant flow channel  32 , so the soaking performance of a wafer W increases. 
     The distance d gradually reduces from the most upstream part  32 U of the refrigerant flow channel  32  toward the most downstream part  32 L of the refrigerant flow channel  32 . Therefore, the soaking performance of a wafer W further increases. 
     Furthermore, the distance d is adjusted by a distance from the ceiling surface of the refrigerant flow channel  32  to the top surface of the cooling base  30 . Therefore, the distance d is relatively easily adjusted. 
     Furthermore, the refrigerant flow channel  32  is formed in a zigzag shape when the cooling base  30  is viewed in plan. Therefore, the refrigerant flow channel  32  is easily routed all over the cooling base  30 . 
     The distance d at the most downstream part  32 L is preferably 50% to 90% of the distance d at the most upstream part  32 U. When the percentage is lower than or equal to 90%, the soaking performance of a wafer W sufficiently increases. When the percentage is higher than or equal to 50%, it is possible to avoid occurrence of a crack above the most downstream part  32 L. 
     In addition, the cooling base  30  is made of an MMC and is bonded to the ceramic base  20  via the metal bonding layer  40 . With the structure that the cooling base  30  is an MMC and the bonding layer is the metal bonding layer  40 , thermal resistance from the refrigerant flow channel  32  to the wafer placement surface  22   a  is small, so the wafer temperature is susceptible to the influence of the temperature gradient of refrigerant. Therefore, the significance to apply the present invention is high. Since the metal bonding layer  40  has a high thermal conductivity, the metal bonding layer  40  is suitable for heat dissipation. Since a difference in thermal expansion between the ceramic base  20  and the cooling base  30  made of an MMC is able to be reduced, a trouble is less likely to occur even when the stress relaxation properties of the metal bonding layer  40  are low. 
     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 above-described embodiment, instead of the refrigerant flow channel  32  in a zigzag shape in plan view, a refrigerant flow channel  82  in a spiral shape in plan view may be adopted as shown in  FIG.  4   . The refrigerant flow channel  82  is formed in a spiral shape all over a part excluding the flange  34  of the cooling base  30  in a one-stroke pattern from an inlet  82   a  provided at the center to an outlet  82   s  provided at an outer peripheral part. In this case, when a most upstream part  82 U and a most downstream part  82 L are determined in an area that overlaps the wafer placement surface  22   a  in plan view of the refrigerant flow channel  82 , the most upstream part  82 U and the most downstream part  82 L are at locations shown in  FIG.  4   . Although not shown in the drawing, a distance d from the ceiling surface of the refrigerant flow channel  82  to the wafer placement surface  22   a  at the most downstream part  82 L is shorter than the distance d at the most upstream part  82 U. The distance d may be formed to gradually reduce from the most upstream part  82 U toward the most downstream part  82 L. Alternatively, the outer peripheral part of the refrigerant flow channel  82  may be set as an inlet, and the center may be set as an outlet. 
     In the above-described embodiment, as shown in FIG.  5 , the refrigerant flow channel  32  may have a part  32   x  at which the distance d from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  is shorter in an area around the terminal hole  51  than the distance d in an area outside the area around the terminal hole  51 . The refrigerant flow channel  32  is formed in a shape similar to that of the above-described embodiment except that the part  32   x  is provided, and the distance d at the most downstream part  32 L is shorter than the distance d at the most upstream part  32 U. The distance d gradually reduces from the most upstream part  32 U of the refrigerant flow channel  32  toward the most downstream part  32 L of the refrigerant flow channel  32  except an area around the terminal hole  51 . Generally, an area around just above such the terminal hole  51  in the wafer placement surface  22   a  tends to be a hot spot, and, here, the distance d in an area around the terminal hole  51  is shorter than the distance d in an area outside the area around the terminal hole  51 . Therefore, heat dissipation of the area around the terminal hole  51  is promoted. Therefore, the soaking performance of a wafer W increases. The distance d at the part  32   x  is preferably 50% to 90% of the distance d at the most upstream part  32 U. 
     In the above-described embodiment, as shown in  FIG.  6   , the ceramic base  20  has the wafer placement surface  22   a  but the ceramic base  20  does not need to have an FR placement surface. In this case, when the most upstream part  32 U and the most downstream part  32 L are determined in an area that overlaps the wafer placement surface  22   a  in plan view of the refrigerant flow channel  32 , the most upstream part  32 U and the most downstream part  32 L respectively coincide with the inlet  32   a  and the outlet  32   s.    
     In the above-described embodiment, the distance d from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  is adjusted by the distance from the ceiling surface of the refrigerant flow channel  32  to the top surface the cooling base  30 ; however, the configuration is not limited thereto. For example, as in the case of a wafer placement table  210  shown in  FIG.  7   , the distance d may be adjusted by the thickness of a metal bonding layer  240 . The wafer placement table  210  is formed by bonding the ceramic base  20  with a cooling base  230  by the metal bonding layer  240 . The ceramic base  20  is the same as that used in the above-described embodiment, and the thickness of the central part  22  is uniform. In the cooling base  230 , the thickness of a central part except a flange  234  is uniform, the distance between the ceiling surface of the refrigerant flow channel  232  and the top surface of the cooling base  230  is also uniform, and the height (the length from the bottom surface to the ceiling surface) of the refrigerant flow channel  232  is also uniform. The shape of the refrigerant flow channel  232  in plan view is a zigzag shape, as in the case of  FIG.  2   . The metal bonding layer  240  is provided such that the thickness gradually reduces from an inlet  232   a  of the refrigerant flow channel  232  toward an outlet  232   s  of the refrigerant flow channel  232 . With this configuration as well, the distance d between the ceiling surface of the refrigerant flow channel  232  and the wafer placement surface  22   a  at a most downstream part  232 L of the refrigerant flow channel  232  may be shorter than the distance d at a most upstream part  232 U of the refrigerant flow channel  232 . 
     Alternatively, as in the case of a wafer placement table  310  shown in  FIG.  8   , the distance d may be adjusted by the thickness of a ceramic base  320 . The wafer placement table  310  is formed by bonding the ceramic base  320  with the cooling base  230  by the metal bonding layer  340 . The cooling base  230  is the same as that used in  FIG.  7   . The ceramic base  320  is provided such that the thickness of a central part  322  gradually reduces from the inlet  232   a  of the refrigerant flow channel  232  toward the outlet  232   s  of the refrigerant flow channel  232 . The thickness of the metal bonding layer  340  is uniform. With this configuration as well, the distance d between the ceiling surface of the refrigerant flow channel  232  and a wafer placement surface  322   a  at a most downstream part  232 L of the refrigerant flow channel  232  may be shorter than the distance d at a most upstream part  232 U of the refrigerant flow channel  232 . 
     In the above-described embodiment, the ceiling surfaces at the inlet  32   a , the outlet  32   s , and the straight parts  32   d ,  32   f ,  32   h ,  32   j ,  321 ,  32   n ,  32   p  of the refrigerant flow channel  32  are inclined surfaces. Alternatively, the ceiling surfaces may be horizontal surfaces. 
     In the above-described embodiment, the distance d between the ceiling surface of the refrigerant flow channel  32  and the wafer placement surface  22   a  is set to gradually reduce from the most upstream part  32 U toward the most downstream part  32 L; however, the configuration is not limited thereto. The distance d may be configured in any shape between the most upstream part  32 U and the most downstream part  32 L as long as the distance d at the most downstream part  32 L is shorter than the distance d at the most upstream part  32 U. For example, between the most upstream part  32 U and the most downstream part  32 L, there may be a section in which the distance d is uniform, or a section in which the distance d gradually increases from the most upstream part  32 U toward the most downstream part  32 L, or a section in which the distance d irregularly changes. 
     In the above-described embodiment, on the wafer placement surface  22   a , a seal band may be formed along the outer periphery, a plurality of small projections may be formed all over the surface, and a wafer W may be supported by the top face of the seal band and the top faces of the small projections. In this case, the distance d between the ceiling surface of the refrigerant flow channel  32  and the wafer placement surface  22   a  is a distance between the ceiling surface of the refrigerant flow channel  32  and the top faces of the small projections (the top face of the seal band). 
     In the above-described embodiment, the wafer placement table  10  may have a plurality of holes that extend through the wafer placement table  10  in the up and down direction. Such holes include a plurality of gas holes that open at the wafer placement surface  22   a  and lift pin holes for allowing insertion of lift pins used to raise and lower the wafer W with respect to the wafer placement surface  22   a . The plurality of gas holes is provided at adequate locations when the wafer placement surface  22   a  is viewed in plan. Heat transfer gas, such as He gas, is supplied to the gas holes. Generally, the gas holes are provided so as to open at locations where the seal band or the small projections are not provided on the wafer placement surface  22   a  on which the seal band and the small projections are provided. When heat transfer gas is supplied to the gas holes, heat transfer gas is filled into a space on the back side of the wafer W placed on the wafer placement surface  22   a . The plurality of lift pin holes is provided at equal intervals along the concentric circle of the wafer placement surface  22   a  when the wafer placement surface  22   a  is viewed in plan. When the wafer placement table  10  has gas holes and lift pin holes, a part where the distance d from the ceiling surface of the refrigerant flow channel  32  to the wafer placement surface  22   a  is shorter in an area around each hole than in an area outside the area around each hole may be provided as in the case of the part  32   x  of  FIG.  5   . With this configuration, the soaking performance of a wafer W further increases. 
     In the above-described embodiment, the cooling base  30  is made of an MMC; however, the configuration is not limited thereto. The cooling base  30  may be made of metal (for example, aluminum, titanium, molybdenum, tungsten, and alloys of them). 
     In the above-described embodiment, the ceramic base  20  and the cooling base  30  are bonded via the metal bonding layer  40 ; however, the configuration is not limited thereto. For example, instead of the metal bonding layer  40 , a resin bonding layer may be used. 
     In the above-described embodiment, the wafer attraction electrode  26  is incorporated in the central part  22  of the ceramic base  20 . Instead of or in addition to this, an RF electrode for generating plasma may be incorporated, and a heater electrode (resistance heating element) may be incorporated. A focus ring (FR) attraction electrode may be incorporated in the outer peripheral part  24  of the ceramic base  20 , and an RF electrode or a heater electrode may be incorporated. 
     In the above-described embodiment, the ceramic sintered body  120  of  FIG.  3 A  is made by firing a ceramic powder molded body by hot pressing. The molded body at that time may be made by laminating a plurality of molded tapes, or may be made by mold casting, or may be made by compacting ceramic powder. 
     The present application claims priority from Japanese Patent Application No. 2021-183240, filed on Nov. 10, 2021, the entire contents of which are incorporated herein by reference.