Patent Publication Number: US-2023146001-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 
     A wafer placement table has been used for performing CVD or etching on a wafer by using plasma. The wafer placement table includes an electrostatic chuck for attracting and securing a wafer to a wafer placement surface and a cooling base that cools the electrostatic chuck. In some cases, a focus ring is disposed on the outer circumference of the wafer placement surface. The focus ring is placed on a focus ring placement surface lower than the wafer placement surface and has a function of stably generating plasma toward an outer circumferential edge of the wafer and a function of controlling the temperature of the outer circumferential edge of the wafer. In some cases, a central refrigerant flow path for the wafer and an outer circumferential refrigerant flow path for the focus ring are formed in the cooling base in order to separately control the temperatures of the wafer placement surface and the focus ring placement surface, and the temperatures of refrigerant that flows through the respective refrigerant flow paths are separately adjusted. As for a known wafer placement table, an electrostatic chuck is divided into a central ceramic base that has a wafer placement surface and an outer circumferential ceramic base that has a focus ring placement surface, and a single upward groove that opens from an upper surface of a cooling base is formed into an annular shape so as to be along the boundary between the central ceramic base and the outer circumferential ceramic base (for example, PTL 1 to PTL 4). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP 6080571 B 
         PTL 2: JP 6452449 B 
         PTL 3: JP 6442296 B 
         PTL 4: JP 6741461 B 
       
    
     SUMMARY OF THE INVENTION 
     As for the cooling base that has the single upward groove described above, however, only the bottom of the upward groove corresponds to a heat-transfer path in a coupler that couples an outer circumferential portion and a central portion of the cooling base with each other, and accordingly, there is a possibility that the coupler is damaged when a thermal stress is applied to the bottom of the upward groove due to a difference in temperature between the outer circumferential portion and the central portion of the cooling base. 
     The present invention has been accomplished to solve the problem, and it is a main object of the present invention to prevent a wafer placement table that has a wafer placement surface and a focus ring placement surface and that has an integrated body from being damaged due to a thermal stress. 
     A wafer placement table according to the present invention includes a central ceramic base that has an upper surface including a wafer placement surface and that contains an electrode, an outer circumferential ceramic base that has an upper surface including a focus ring placement surface, that is disposed on an outer circumference of the central ceramic base, that is separated from the central ceramic base, and that has an annular shape, and a cooling base that includes a central portion that is joined to a lower surface of the central ceramic base, an outer circumferential portion that is joined to a lower surface of the outer circumferential ceramic base, and a coupler that couples the central portion and the outer circumferential portion with each other. The cooling base has a central refrigerant flow path that is formed in the central portion and an outer circumferential refrigerant flow path that is formed in the outer circumferential portion. The coupler is formed outside an outermost edge of the central refrigerant flow path and inside an innermost edge of the outer circumferential refrigerant flow path, and has one or more upward grooves that open from an upper surface, and that have an annular shape, and has one or more downward grooves that open from a lower surface, that have a ceiling surface higher than a bottom surface of the one or more upward grooves, and that have an annular shape. 
     The wafer placement table has not only the one or more upward grooves that open from the upper surface but also the one or more downward grooves that open from the lower surface, and the ceiling surface of the one or more downward grooves is higher than the bottom surface of the one or more upward grooves. Accordingly, the length of a heat-transfer path in the coupler between the central portion and the outer circumferential portion increases by that of a path in an up-down direction. Consequently, the temperature gradient of the coupler decreases, a thermal stress that is applied to the coupler is reduced, and accordingly, damage due to a thermal stress is reduced. The wafer placement table has the integrated body and is accordingly easier to handle than in the case where the central portion and the outer circumferential portion have separated bodies. 
     In the present specification, the words “up-down”, “left-right”, and “front-rear” are used to describe the present invention in some cases, but the words “up-down”, “left-right”, and “front-rear” merely represent relative positional relationships. For this reason, the word “up-down” is changed into the word “left-right” or the word “left-right” is changed into the word “up-down” in some cases where the direction of the wafer placement table is changed. These cases are also included in the technical scope of the present invention. 
     As for the wafer placement table according to the present invention, refrigerant may be separately supplied to the central refrigerant flow path and the outer circumferential refrigerant flow path. This enables the temperatures of the wafer placement surface that is located above the central refrigerant flow path and the focus ring placement surface that is located above the outer circumferential refrigerant flow path to be separately controlled. 
     As for the wafer placement table according to the present invention, the one or more upward grooves may be formed next to the central refrigerant flow path, the outer circumferential refrigerant flow path, or both, and the bottom surface of the one or more upward grooves may be flush with or lower than a bottom surface of the central refrigerant flow path and a bottom surface of the outer circumferential refrigerant flow path. Consequently, the bottom surface of the one or more upward grooves is located at a portion that is cooled by refrigerant and that has a stable temperature, and accordingly, damage due to a thermal stress is more effectively reduced than in the case where the bottom surface of the one or more upward grooves is located at a portion higher than that. 
     As for the wafer placement table according to the present invention, the coupler may have the single upward groove and the single downward groove. Consequently, the number of the upward groove and the number of the downward groove are minimized, and accordingly, process costs can be inhibited from increasing. A portion that can have no refrigerant flow paths can be reduced, and accordingly, the degree of freedom of the arrangement of the refrigerant flow paths can be increased. 
     As for wafer placement table according to the present invention, the coupler may have two of the upward grooves and the single downward groove, and the upward grooves and the downward groove may be alternately arranged. This enables the coupler to be longer than in the case where the number of the upward groove is one, and the number of the downward groove is one, and a thermal stress that is applied to the coupler can be reduced. The upward grooves are formed next to both of the central refrigerant flow path and the outer circumferential refrigerant flow path, and accordingly, the central portion and the outer circumferential portion can be coupled with each other at a lower portion that is cooled by the refrigerant and that has a relatively stable temperature unlike the case where a downward groove is formed next to one of these or both. For this reason, damage due to a thermal stress can be reduced. 
     As for the wafer placement table according to the present invention, the cooling base may be composed of a metal matrix composite material. The wafer placement table according to the present invention can reduce damage due to a thermal stress and is accordingly particularly effective for the case where a fragile material such as a metal matrix composite material is used. The cooling base composed of a metal matrix composite material has a linear thermal expansion coefficient close to that of a ceramic material of which the ceramic base is composed. Accordingly, when the ceramic base and the cooling base are joined to each other, it is not necessary to use a layer (for example, a resin joining layer) for relieving the effect of a difference in expansion coefficient between these, and a metal joining layer can be used. The thermal conductivity of the metal joining layer is higher than that of the resin joining layer, and accordingly, the ability to remove heat required for the case where a wafer is processed by using high power plasma can be acquired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a vertical cross-sectional view of a wafer placement table  10 . 
         FIG.  2    is a plan view of the wafer placement table  10 . 
         FIG.  3    is a cross sectional view of the wafer placement table  10 . 
         FIG.  4    illustrates a heat-transfer path X in a coupler  40  between a central portion  31  and an outer circumferential portion  35 . 
         FIG.  5    is a partial sectional view of another example of the wafer placement table  10 . 
         FIG.  6    is a partial sectional view of another example of the wafer placement table  10 . 
         FIG.  7    is a partial sectional view of another example of the wafer placement table  10 . 
         FIG.  8    is a partial sectional view of another example of the wafer placement table  10 . 
         FIG.  9    is a partial sectional view of another example of the wafer placement table  10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention will hereinafter be described with reference to the drawings.  FIG.  1    is a vertical cross-sectional view (a sectional view taken along a plane containing the central axis of a wafer placement table  10 ) of the wafer placement table  10 .  FIG.  2    is a plan view of the wafer placement table  10 .  FIG.  3    is a cross sectional view (a sectional view of a section of the wafer placement table  10  taken along a horizontal plane that passes through refrigerant flow paths  32  and  37  when viewed from above) of the wafer placement table  10 .  FIG.  4    illustrates a heat-transfer path X in a coupler  40  between a central portion  31  and an outer circumferential portion  35 . 
     The wafer placement table  10  is used to perform CVD or etching on a wafer W by using plasma. The wafer placement table  10  includes a central ceramic base  20 , an outer circumferential ceramic base  25 , and a cooling base  30 . The central ceramic base  20  is joined to an upper surface of the cooling base  30  with a central joining layer  50  interposed therebetween, the outer circumferential ceramic base  25  is joined to the upper surface of the cooling base  30  with an outer circumferential joining layer  55  interposed therebetween, and the central ceramic base  20  and the outer circumferential ceramic base  25  constitute the wafer placement table  10  that has an integrated body. 
     The central ceramic base  20  is a disk-like plate composed of a ceramic material represented by, for example, alumna or aluminum nitride. The central ceramic base  20  has an upper surface including a wafer placement surface  20   a  on which the wafer W is to be placed. The central ceramic base  20  contains a central electrode  22  near the wafer placement surface  20   a . The central electrode  22  is a single-pole electrostatic attraction electrode that has a circular plate or mesh shape and is composed of, for example, a material containing W, Mo, WC, and MoC. A layer in the central ceramic base  20  above the central electrode  22  functions as a dielectric layer. A direct current power supply, not illustrated, is connected to the central electrode  22  with a power supply terminal  60  interposed therebetween. The power supply terminal  60  extends beyond an insulating tube  61  that is disposed in a through-hole that extends through the cooling base  30  and the central joining layer  50  in an up-down direction and reaches the central electrode  22  via a lower surface of the central ceramic base  20 . 
     The outer circumferential ceramic base  25  is an annular plate that is composed of a ceramic material represented by alumna or aluminum nitride. The outer circumferential ceramic base  25  has an upper surface including a focus ring placement surface  25   a  on which a focus ring FR is to be placed. The outer circumferential ceramic base  25  is disposed on the outer circumference of the central ceramic base  20 . The focus ring placement surface  25   a  is lower than the wafer placement surface  20   a . The outer circumferential ceramic base  25  contains an outer circumferential electrode  27  near the focus ring placement surface  25   a . The outer circumferential electrode  27  is a single-pole electrostatic attraction electrode that has an annular plate or mesh shape and is composed of, for example, a material containing W, Mo, WC, and MoC. A layer in the outer circumferential ceramic base  25  above the outer circumferential electrode  27  functions as a dielectric layer. A direct current power supply, not illustrated, is connected to the outer circumferential electrode  27  with a power supply terminal  65  interposed therebetween. The power supply terminal  65  extends beyond an insulating tube  66  that is disposed in a through-hole that extends through the cooling base  30  and the outer circumferential joining layer  55  in the up-down direction and reaches the outer circumferential electrode  27  via a lower surface of the outer circumferential ceramic base  25 . 
     The cooling base  30  is a disk-like member composed of a metal matrix composite material (also referred to as a metal matrix composite (MMC)). The cooling base  30  contains the central refrigerant flow path  32  and the outer circumferential refrigerant flow path  37  through which refrigerant can circulate. The central refrigerant flow path  32  has a spiral shape that extends from an inlet  33  to an outlet  34  across the entire central ceramic base  20 . The inlet  33  and the outlet  34  of the central refrigerant flow path  32  are connected to a central refrigerant cooling device not illustrated. The refrigerant that is discharged via the outlet  34  returns to the inlet  33  after temperature is adjusted by the central refrigerant cooling device and is supplied to the central refrigerant flow path  32 . The outer circumferential refrigerant flow path  37  has a spiral shape that extends from an inlet  38  to an outlet  39  across the entire outer circumferential ceramic base  25 . The inlet  38  and the outlet  39  of the outer circumferential refrigerant flow path  37  are connected to an outer circumferential refrigerant cooling device that differs from the central refrigerant cooling device and that is not illustrated. The refrigerant that is discharged via the outlet  39  returns to the inlet  38  after temperature is adjusted by the outer circumferential refrigerant cooling device and is supplied to the outer circumferential refrigerant flow path  37 . The central refrigerant flow path  32  and the outer circumferential refrigerant flow path  37  are thus connected to the respective different refrigerant cooling devices, and the refrigerant is separately supplied. Consequently, the temperatures of the central portion  31  and the outer circumferential portion  35  of the cooling base  30  are separately controlled, and the temperatures of the wafer placement surface  20   a  and the focus ring placement surface  25   a  are separately controlled. The cooling base  30  also functions as a radio frequency (RF) electrode for generating plasma and is connected to a RF power supply, not illustrated, with a power supply terminal  70  interposed therebetween. The power supply terminal  70  is joined to a lower surface of the cooling base  30 . 
     The cooling base  30  includes the coupler  40  outside an outermost edge of the central refrigerant flow path  32  and inside an innermost edge of the outer circumferential refrigerant flow path  37 . The coupler  40  has an upward groove  42  that opens from an upper surface and a downward groove  44  that opens from a lower surface. The upward groove  42  is formed at a position adjacent to the central refrigerant flow path  32  (that is, inside the downward groove  44 ) and has an annular shape that extends from the upper surface of the cooling base  30  to a bottom surface  42   b  that is flush with or lower than a bottom surface  32   b  of the central refrigerant flow path  32  and a bottom surface  37   b  of the outer circumferential refrigerant flow path  37 . The downward groove  44  is formed at a position adjacent to the outer circumferential refrigerant flow path  37  (that is, outside the upward groove  42 ) and has an annular shape that extends from the lower surface of the cooling base  30  to a ceiling surface  44   c  that is flush with or higher than a ceiling surface  32   c  of the central refrigerant flow path  32  and a ceiling surface  37   c  of the outer circumferential refrigerant flow path  37 . According to the present embodiment, the central portion  31  is a portion of the cooling base  30  inside an inner edge of the upward groove  42 , the outer circumferential portion  35  is a portion outside an outer edge of the downward groove  44 , and the coupler  40  is a portion therebetween. 
     The MMC that is used for the cooling base  30  preferably has a thermal expansion coefficient close to those of a ceramic material that is used for the central ceramic base  20  and a ceramic material that is used for the outer circumferential ceramic base  25 . Examples of the MMC include a material containing Si, SiC and Ti and a material obtained by immersing Al and/or Si in a SiC porous body. The material containing Si, SiC and Ti is referred to as SiSiCTi. A material obtained by immersing Al in a SiC porous body is referred to as AlSiC. A material obtained by immersing Si in a SiC porous body is referred to as SiSiC. In the case where the central ceramic base  20  and the outer circumferential ceramic base  25  are alumna bases, the MMC that is used for the cooling base  30  is preferably AlSiC or SiSiCTi. In the case where the central ceramic base  20  and the outer circumferential ceramic base  25  are aluminum nitride bases, the MMC that is used for the cooling base  30  is preferably AlSiC or SiSiC. 
     The central joining layer  50  is a metal joining layer that joins the lower surface of the central ceramic base  20  and an upper surface of the central portion  31  of the cooling base  30  to each other. The central joining layer  50  may be a layer composed of, for example, a solder or brazing metal material. The central joining layer  50  may be a layer composed of, for example, an Al—Mg joining material or an Al—Si—Mg joining material. The thickness of the central joining layer  50  is preferably, for example, about 100 μm. The central joining layer  50  is formed by, for example, TCB (Thermal compression bonding). TCB is a known method in which a metal joining material is put between two members to be joined, and the two members are compressed and joined with the two members heated to a temperature equal to or lower than the solidus temperature of the metal joining material. 
     The outer circumferential joining layer  55  is a metal joining layer that joins the lower surface of the outer circumferential ceramic base  25  and an upper surface of the outer circumferential portion  35  of the cooling base  30  to each other. The outer circumferential joining layer  55  may be a layer composed of, for example, a solder or brazing metal material. The outer circumferential joining layer  55  may be a layer composed of, for example, an Al—Mg joining material or an Al—Si—Mg joining material. The thickness of the outer circumferential joining layer  55  is preferably, for example, about 100 μm. The outer circumferential joining layer  55  is formed by, for example, TCB. 
     An example of the use of the wafer placement table  10  will now be described. The wafer W is first placed on the wafer placement surface  20   a  with the wafer placement table  10  installed in a chamber not illustrated, and the focus ring FR is placed on the focus ring placement surface  25   a . The focus ring FR has a step along the inner circumference of an upper end portion so as not to interfere with the wafer W. In this state, a direct voltage is applied to the central electrode  22  and the outer circumferential electrode  27 , the wafer W is attracted to the wafer placement surface  20   a , and the focus ring FR is attracted to the focus ring placement surface  25   a . Settings are adjusted such that the interior of the chamber becomes a vacuum atmosphere (or a decompression atmosphere), a RF voltage is applied to the cooling base  30  while process gas is supplied from a shower head that is disposed on a ceiling portion in the chamber and that is not illustrated. Plasma is then generated between the wafer W and the shower head. A CVD film is formed or etching is performed on the wafer W by using the plasma. 
     While the wafer W is processed by using the plasma, refrigerant is separately supplied to the central refrigerant flow path  32  from the central refrigerant cooling device and to the outer circumferential refrigerant flow path  37  from the outer circumferential refrigerant cooling device. Consequently, the temperatures of the wafer placement surface  20   a  that is located above the central refrigerant flow path  32  and the focus ring placement surface  25   a  that is located above the outer circumferential refrigerant flow path  37  are separately controlled (cooling). At this time, as for the cooling base  30 , the central portion  31  that has the central refrigerant flow path  32  and the outer circumferential portion  35  that has the outer circumferential refrigerant flow path  37  are coupled with each other by using the coupler  40 , and accordingly, heat is transferred between the central portion  31  and the outer circumferential portion  35  via the coupler  40 .  FIG.  4    conceptually illustrates the heat-transfer path X in the coupler  40  (a portion that is surrounded by a one-dot chain line) between the central portion  31  and the outer circumferential portion  35  (see a dashed line). The heat-transfer path X includes a path that extends in the horizontal direction from a lower outer edge portion of the central portion  31  along the bottom (a portion below the bottom surface  42   b ) of the upward groove  42 , a path that extends in the up-down direction along a wall between the upward groove  42  and the downward groove  44 , a path that extends in the horizontal direction along the ceiling (a portion above the ceiling surface  44   c ) of the downward groove  44 , and a path that extends to an upper inner edge portion of the outer circumferential portion  35  (the directions may be opposite directions). If the downward groove  44  is omitted, then the heat-transfer path in the coupler  40  is only the path that extends in the horizontal direction along the bottom of the upward groove  42  and is short. If a difference in temperature between the central portion  31  and the outer circumferential portion  35  increases in this state (for example, if a difference in temperature between the refrigerant that is supplied to the central refrigerant flow path  32  and the refrigerant that is supplied to the outer circumferential refrigerant flow path  37  increases), then the temperature gradient of the coupler  40  increases, and there is a possibility that the coupler  40  is damaged due to a thermal stress. As for the wafer placement table  10  according to the present embodiment, however, the heat-transfer path X in the coupler  40  includes not only the paths that extend in the horizontal direction but also the path that extends in the up-down direction as described above and is long. Accordingly, the temperature gradient of the coupler  40  does not become too large even when the difference in temperature between the central portion  31  and the outer circumferential portion  35  increases, and damage due to a thermal stress is unlikely to occur. 
     The wafer placement table  10  described above has not only the upward groove  42  but also the downward groove  44 , the ceiling surface  44   c  of the downward groove  44  is higher than the bottom surface  42   b  of the upward groove  42 , and accordingly, damage due to a thermal stress is reduced as described above. The wafer placement table  10  has the integrated body and is accordingly easier to handle than in the case where the central portion  31  and the outer circumferential portion  35  have separated bodies. 
     The refrigerant is separately supplied to the central refrigerant flow path  32  and the outer circumferential refrigerant flow path  37 , and accordingly, the temperatures of the wafer placement surface  20   a  that is located above the central refrigerant flow path  32  and the focus ring placement surface  25   a  that is located above the outer circumferential refrigerant flow path  37  can be separately controlled. 
     In addition, the upward groove  42  is formed next to the central refrigerant flow path  32 , and the bottom surface  42   b  of the upward groove  42  is lower than the bottom surface  32   b  of the central refrigerant flow path  32  and the bottom surface  37   b  of the outer circumferential refrigerant flow path  37 . With this structure, the bottom surface  42   b  of the upward groove  42  is located at a portion that is cooled by the refrigerant and that has a stable temperature, and accordingly, damage due to a thermal stress is more effectively reduced than in the case where the bottom surface  42   b  of the upward groove  42  is located at a portion higher than that. 
     In addition, the coupler  40  has the single upward groove  42  and the single downward groove  44 , and the number of the upward groove and the number of the downward groove are minimized. Accordingly, process costs can be inhibited from increasing. A portion that cannot have the refrigerant flow paths  32  and  37  can be reduced, and accordingly, the degree of freedom of the arrangement of the refrigerant flow paths  32  and  37  can be increased. 
     The cooling base  30  is composed of the MMC that has a linear thermal expansion coefficient close to those of the ceramic material of which the central ceramic base  20  is composed and the ceramic material of which the outer circumferential ceramic base  25  is composed, and accordingly, a metal joining layer can be used instead of a resin joining layer when the central ceramic base  20  and the cooling base  30  is joined to each other, and the outer circumferential ceramic base  25  and the cooling base  30  is joined to each other. The thermal conductivity of the metal joining layer is higher than that of the resin joining layer, and accordingly, the ability to remove heat required for the case where a wafer is processed by using high power plasma can be acquired. The MMC is conductive. 
     Accordingly, the cooling base  30  can be used as a RF electrode, and it is not necessary to prepare an additional RF electrode. According to the present embodiment, the central joining layer  50  and the outer circumferential joining layer  55  are composed of metal, and accordingly, these can be used as a RF electrode. 
     It goes without saying that the present invention is not limited to the embodiment described above and can be carried out in various aspects within the technical scope of the present invention. 
     For example, according to the embodiment described above, the outer circumferential ceramic base  25  is joined also to an upper surface of the ceiling above the downward groove  44  but may not be joined to the upper surface of the ceiling above the downward groove  44 . In this case, as illustrated in  FIG.  5   , the upper surface of the ceiling above the downward groove  44  may be a lowered surface  44   a  that is lower than the upper surface of the outer circumferential portion  35 . The lowered surface  44   a  may be lower than, flush with, or higher than the ceiling surface (the ceiling surface  37   c ) of the refrigerant flow path (the outer circumferential refrigerant flow path  37 ) adjacent to the downward groove  44 . 
     According to the embodiment described above, the upward groove  42  is formed inside the downward groove  44 . As illustrated in  FIG.  6   , however, the upward groove  42  may be formed outside the downward groove  44 . In this case, the central portion  31  is a portion of the cooling base  30  inside an inner edge of the downward groove  44 , the outer circumferential portion  35  is a portion outside an outer edge of the upward groove  42 , and the coupler  40  is a portion therebetween. Also, in this way, the same effects as those according to the embodiment described above can be achieved. In the case where the upper surface of the central portion  31  is used while being heated to a temperature higher than that of the upper surface of the outer circumferential portion  35 , the upward groove  42  may be formed inside the downward groove  44 . In the case where the upper surface of the outer circumferential portion  35  is used while being heated to a temperature higher than that of the upper surface of the central portion  31 , the upward groove  42  may be formed outside the downward groove  44 . This enables a difference in temperature between both ends of the heat-transfer path in the coupler  40  to be reduced. In  FIG.  6   , the central ceramic base  20  may not be joined to the upper surface of the ceiling above the downward groove  44 . In this case, the upper surface of the ceiling above the downward groove  44  may be a lowered surface that is lower than the upper surface of the central portion  31 . The lowered surface may be lower than, flush with, or higher than the ceiling surface (the ceiling surface  32   c ) of the refrigerant flow path (the central refrigerant flow path  32 ) adjacent to the downward groove  44 . 
     According to the embodiment described above, the coupler  40  has the single upward groove and the single downward groove. However, the number of the upward groove, the number of the downward groove, or both may be increased to two or more. In this case, the one or two upward grooves and the one or two downward grooves are preferably alternately arranged. For example, as illustrated in  FIG.  7   , an upward groove  46  is added outside the downward groove  44 , and the two upward grooves (the upward grooves  42  and  46 ) and the single downward groove (the downward groove  44 ) may be formed. In this case, the central portion  31  is a portion inside an inner edge of the upward groove  42 , the outer circumferential portion  35  is a portion outside an outer edge of the upward groove  46 , and the coupler  40  is a portion therebetween. This enables the heat-transfer path in the coupler  40  to be longer than in the case where the number of the upward groove is one, and the number of the downward groove is one and enables a thermal stress that is applied to the coupler  40  to be reduced. The upward grooves are formed next to both of the central refrigerant flow path  32  and the outer circumferential refrigerant flow path  37 , and accordingly, the central portion  31  and the outer circumferential portion  35  can be coupled with each other at a lower portion that is cooled by the refrigerant and that has a relatively stable temperature unlike the case where a downward groove is formed next to one of these or both. For this reason, damage due to a thermal stress can be further reduced. When a bottom surface  46   b  of the upward groove  46  is flush with or lower than the bottom surface  32   b  of the central refrigerant flow path  32  and the bottom surface  37   b  of the outer circumferential refrigerant flow path  37 , the bottom surface  46   b  of the upward groove  46  is located at a portion that has a more stable temperature, and accordingly, damage due to a thermal stress is more unlikely to occur. As illustrated in  FIG.  8   , the upper surface of the ceiling above the downward groove  44  may be the lowered surface  44   a  that is lower than the upper surface of the central portion  31 , the upper surface of the outer circumferential portion  35 , or both. The lowered surface  44   a  may be lower than, flush with, or higher than the ceiling surface of the central refrigerant flow path  32 , the ceiling surface of the outer circumferential refrigerant flow path  37 , or both. 
     According to the embodiment described above, the upper surface of the central portion  31  and the upper surface of the outer circumferential portion  35  are flush with each other. However, the upper surface of the central portion  31  may be high or the upper surface of the outer circumferential portion  35  may be high. The bottom surface  32   b  of the central refrigerant flow path  32  and the bottom surface  37   b  of the outer circumferential refrigerant flow path  37  are flush with each other. However, the bottom surface  32   b  of the central refrigerant flow path  32  may be high, or the bottom surface  37   b  of the outer circumferential refrigerant flow path  37  may be high. The ceiling surface  32   c  of the central refrigerant flow path  32  and the ceiling surface  37   c  of the outer circumferential refrigerant flow path  37  are flush with each other. However, the ceiling surface  32   c  of the central refrigerant flow path  32  may be high, and the ceiling surface  37   c  of the outer circumferential refrigerant flow path  37  may be high. 
     According to the embodiment described above, the bottom surface  42   b  of the upward groove  42  is flush with or lower than both of the bottom surface  32   b  of the central refrigerant flow path  32  and the bottom surface  37   b  of the outer circumferential refrigerant flow path  37 . However, this may not be satisfied. Even in this case, the bottom surface  42   b  of the upward groove  42  is preferably lower than the ceiling surface of the central refrigerant flow path  32  or the outer circumferential refrigerant flow path  37  adjacent to the upward groove  42  and is preferably flush with or lower than the bottom surface of the refrigerant flow path adjacent to the upward groove  42 . The same is true for the bottom surface  46   b  of the upward groove  46 . 
     According to the embodiment described above, the ceiling surface  44   c  of the downward groove  44  is flush with or higher than both of the ceiling surface  32   c  of the central refrigerant flow path  32  and the ceiling surface  37   c  of the outer circumferential refrigerant flow path  37 . However, this may not be satisfied. Even in this case, the ceiling surface  44   c  of the downward groove  44  is preferably higher than the bottom surface of the central refrigerant flow path  32  or the outer circumferential refrigerant flow path  37  adjacent to the downward groove  44  and is preferably flush with or higher than the ceiling surface of the refrigerant flow path adjacent to the downward groove  44 . 
     According to the embodiment described above, the cooling base  30  is composed of the MMC but may be composed of, for example, a metal material such as molybdenum, tungsten, aluminum, aluminum alloy, or stainless steel (a SUS material) or may be composed of a resin material. Among these, the metal material is conductive. Accordingly, when the cooling base  30  is composed of the metal material, the cooling base  30  can be used as a RF electrode. A metal material that less thermally expands such as molybdenum or tungsten has a linear thermal expansion coefficient close to those of the ceramic material of which the central ceramic base  20  is composed and the ceramic material of which the outer circumferential ceramic base  25  is composed. 
     Accordingly, when the cooling base  30  is composed of the metal material that less thermally expands, metal joining layers can be used as the central joining layer  50  and the outer circumferential joining layer  55  instead of resin joining layers. The cooling base  30  may be composed of the metal material as described above or resin. The wafer placement table according to the present invention can reduce damage due to a thermal stress and is accordingly particularly effective for the case where a fragile material such as the MMC is used for the cooling base  30 . 
     According to the embodiment described above, the central ceramic base  20  contains the central electrode  22  for attracting the wafer. However, in addition to this or instead of this, a RF electrode for generating plasma may be contained. This is also referred to as a central RF electrode. In this case, a radio frequency power supply is connected to the central RF electrode. The central ceramic base  20  may contain a heater electrode (a resistance heating element). This is also referred to as a central heater electrode. 
     According to the embodiment described above, the outer circumferential ceramic base  25  contains the outer circumferential electrode  27  for attracting the focus ring. However, in addition to this or instead of this, a RF electrode for generating plasma may be contained. This is also referred to as an outer circumferential RF electrode. In this case, a radio frequency power supply is connected to the outer circumferential RF electrode. The outer circumferential ceramic base  25  may contain a heater electrode (a resistance heating element). This is also referred to as an outer circumferential heater electrode. The outer circumferential heater electrode may control temperature separately from the central heater electrode. This enables the temperatures of the wafer placement surface  20   a  and the focus ring placement surface  25   a  to be controlled with more precision. 
     According to the embodiment described above, the central refrigerant flow path  32  has a spiral shape that extends from the inlet  33  to the outlet  34 . However, a planer shape of the central refrigerant flow path  32  is not particularly limited. Multiple central refrigerant flow paths  32  may be provided. The central refrigerant flow path  32  has a rectangular section. However, a sectional shape of the central refrigerant flow path  32  is not particularly limited. For example, an upper corner portion in the section of the central refrigerant flow path  32  may has a R surface. This prevents a crack starting from the upper corner portion in the section of the central refrigerant flow path  32  from occurring. The same is true for the outer circumferential refrigerant flow path  37 . 
     In the description according to the embodiment described above, the cooling base  30  is a member. As illustrated in  FIG.  9   , however, an upper member  131  and a lower member  132  may be joined to each other by using a joining layer  135 . The upper member  131  and the lower member  132  have a shape obtained by cutting the cooling base  30  along a horizontal plane containing the ceiling surface of the refrigerant flow path  32  into two pieces. The joining layer  135  is preferably a metal joining layer. The cooling base  30  may have a structure that includes three or more members that are joined to each other. 
     According to the embodiment described above, a hole that extends through the wafer placement table  10  from the lower surface of the cooling base  30  to the wafer placement surface  20   a  may be formed. Example of the hole include a gas supply hole for supplying heat-transfer gas (for example, He gas) to a back surface of the wafer W and a lift pin hole through which a lift pin that lifts and lowers the wafer W with respect to the wafer placement surface  20   a  extends. The heat-transfer gas is supplied to spaces that are defined by a large number of small projections (that support the wafer W) that are disposed on the wafer placement surface  20   a  and that are not illustrated and the wafer W. For example, in the case where the wafer W is supported by three lift pins, lift pin holes are formed at three positions. 
     The present application claims priority from Japanese Patent Application No. 2021-181625, filed on Nov. 8, 2021, the entire contents of which are incorporated herein by reference.