Patent Publication Number: US-2019189491-A1

Title: Wafer mounting table

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wafer mounting table. 
     2. Description of the Related Art 
     As a wafer mounting table for a semiconductor manufacturing apparatus, there has been known one formed by joining a ceramic plate having a built-in electrostatic electrode and a metal plate for cooling the ceramic plate. For example, in PTL 1, a resin adhesive layer capable of absorbing the difference in thermal expansion between a ceramic plate and a metal plate is used when joining the ceramic plate and the metal plate. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP 2014-132560 
     SUMMARY OF THE INVENTION 
     However, when a resin adhesive layer is used, there is a problem that use in a high-temperature range is limited or corrosion is caused by process gas. On the other hand, although it is conceivable to fasten the ceramic plate and the metal plate together directly with screws, there is a risk that cracks may be generated in the ceramic plate due to the fastening force at the time of fastening or the stress caused by the difference in thermal expansion. 
     The present invention has been made to solve such problems, and its main object is to provide a wafer mounting table that can withstand use in a high-temperature range. 
     According to the present invention, there is provided a wafer mounting table including: 
     a ceramic plate having a wafer mounting surface and having at least one of an electrostatic electrode and a heater electrode built therein; 
     a metal plate arranged on a surface of the ceramic plate opposite to the wafer mounting surface; 
     a threaded terminal made of a low thermal expansion coefficient metal and joined to a recess provided in the surface of the ceramic plate opposite to the wafer mounting surface by a bonding layer including ceramic fine particles and a hard solder; and 
     a screw member inserted into a through hole penetrating the metal plate and screwed to the threaded terminal to fasten the ceramic plate and the metal plate together, 
     wherein in a state in which the threaded terminal and the screw member are screwed together, a play is provided in a direction in which the metal plate is displaced relative to the ceramic plate due to the difference in thermal expansion. 
     In this wafer mounting table, a threaded terminal joined to a recess provided in the surface of the ceramic plate opposite to the wafer mounting surface and a screw member inserted into a through hole having a step penetrating the metal plate are screwed together, and the ceramic plate and the metal plate are fastened together. Since the threaded terminal is made of a metal having a low thermal expansion coefficient, the thermal expansion coefficient thereof is close to that of the ceramic plate. Therefore, even in the case of repeated use at a high temperature and a low temperature, the ceramic plate and the threaded terminal are less liable to suffer cracking or the like due to thermal stress caused by the difference in thermal expansion coefficient. If a thread that can be screwed with the screw member is directly provided in the recess of the ceramic plate, the ceramic plate may be broken when screwed with the screw member. However, in this case, since the screw member is screwed to the threaded terminal joined to the ceramic plate, there is no such risk. Furthermore, since the threaded terminal is joined to the recess of the ceramic plate by the bonding layer including ceramic fine particles and a hard solder, the bonding strength between the threaded terminal and the ceramic plate is sufficiently high. Further, in a state in which the threaded terminal and the screw member are screwed together, a play p is provided in a direction in which the metal plate is displaced relative to the ceramic plate due to the difference in thermal expansion. Therefore, even in the case of repeated use at a high temperature and a low temperature, thermal stress caused by the difference in thermal expansion between the metal plate and the ceramic plate can be absorbed by this play. As described above, the wafer mounting table of the present invention can withstand use in a high-temperature range. 
     In this description, low thermal expansion coefficient means that the coefficient of linear thermal expansion (CTE) is c×10 −6 /K (c is 3 or more and less than 10) at 0 to 300° C. 
     The wafer mounting table of the present invention may include a non-adhesive heat conductive sheet between the ceramic plate and the metal plate. In the wafer mounting table of the present invention, since the ceramic plate and the metal plate are fastened together by screwing the threaded terminal and the screw member together, the heat conductive sheet between the ceramic plate and the metal plate is not required to have adhesiveness. Therefore, the degree of freedom in selecting the heat conductive sheet is increased. For example, a high thermal conductivity sheet may be employed to enhance the heat removal performance from the ceramic plate to the metal plate, and a low thermal conductivity sheet may be employed to suppress the heat removal performance. 
     In the wafer mounting table of the present invention, the ceramic fine particles may be fine particles whose surfaces are coated with a metal, and the hard solder may contain Au, Ag, Cu, Pd, Al or Ni as a base metal. This makes it easy for the molten hard solder to uniformly spread on the surfaces coated with the metal of the ceramic fine particles when the bonding layer is formed. Therefore, the bonding strength between the threaded terminal and the ceramic plate becomes higher. 
     In the wafer mounting table of the present invention, the ceramic plate is preferably made of AlN or Al 2 O 3 . The metal plate is preferably made of Al or Al alloy. The low thermal expansion coefficient metal is preferably one kind selected from the group consisting of Mo, W, Ta, Nb and Ti, an alloy containing the one kind of metal (for example, W—Cu or Mo—Cu), or Kovar (FeNiCo alloy). 
     In the wafer mounting table of the present invention, the coefficient of linear thermal expansion of the threaded terminal is preferably within a range of ±25% of the coefficient of linear thermal expansion of the ceramic plate. This makes it easier to withstand use in a high-temperature range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view schematically showing the configuration of a plasma processing apparatus  10 . 
         FIG. 2  is a sectional view of an electrostatic chuck heater  20 . 
         FIG. 3  is an enlarged view of a part surrounded by a circle in two-dot chain line of  FIG. 2 . 
         FIGS. 4A and 4B  are explanatory views showing the step of joining a recess  28  and a female threaded terminal  30 . 
         FIG. 5  is a bottom view of the electrostatic chuck heater  20 . 
         FIG. 6  is a partially enlarged view of another embodiment. 
         FIG. 7  is a partially enlarged view of another embodiment. 
         FIG. 8  is a plan view of a heat conductive sheet  36  having a trimming region  36   b.    
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An electrostatic chuck heater  20  that is a preferred embodiment of the wafer mounting table of the present invention will now be described.  FIG. 1  is an explanatory view schematically showing the configuration of a plasma processing apparatus  10  including the electrostatic chuck heater  20 ,  FIG. 2  is a sectional view of the electrostatic chuck heater  20 ,  FIG. 3  is an enlarged view of a part surrounded by a circle in two-dot chain line of  FIG. 2 ,  FIGS. 4A and 4B  are explanatory views showing the step of joining a recess  28  and a female threaded terminal  30  together, and  FIG. 5  is a bottom view of the electrostatic chuck heater  20 . The vertical relationships in  FIGS. 4A and 4B  are opposite to that in  FIG. 2 . 
     As shown in  FIG. 1 , the plasma processing apparatus  10  includes a metal (for example, aluminum alloy) vacuum chamber  12 , the internal pressure of which can be controlled, and an electrostatic chuck heater  20  and an upper electrode  60  for generating plasma that are arranged in the vacuum chamber  12 . Numerous small holes for supplying reactant gas to the wafer mounting surface are formed in a surface of the upper electrode  60  that faces the electrostatic chuck heater  20 . The vacuum chamber  12  is configured so that reactant gas can be introduced into the upper electrode  60  through a reactant gas introduction path  14 , and the internal pressure of the vacuum chamber  12  can be reduced to a predetermined degree of vacuum using a vacuum pump connected to an evacuation path  16 . 
     The electrostatic chuck heater  20  includes an electrostatic chuck  22  capable of sucking a wafer W to be subjected to plasma processing onto a wafer mounting surface  22   a , and a cooling plate  40  arranged on the lower surface of the electrostatic chuck  22 . Numerous protrusions (not shown) having a height of several gm are formed over the entire surface of the wafer mounting surface  22   a . The wafer W mounted on the wafer mounting surface  22   a  is supported on the upper surfaces of these protrusions. He gas is introduced to several of flat parts of the wafer mounting surface  22   a  where no protrusions are provided. 
     The electrostatic chuck  22  is a ceramic plate (for example, made of AlN or Al 2 O 3 ) having an outer diameter smaller than the outer diameter of the wafer W. As shown in  FIG. 2 , an electrostatic electrode  24  and a heater electrode  26  are buried in the electrostatic chuck  22 . The electrostatic electrode  24  is a planar electrode to which a DC voltage can be applied. When a DC voltage is applied to the electrostatic electrode  24 , the wafer W is sucked and fixed to the wafer mounting surface  22   a  by a Coulomb force or a Johnsen-Rahbek force. When the application of the DC voltage is stopped, the wafer W is released from being sucked and fixed to the wafer mounting surface  22   a . The heater electrode  26  is a resistance wire patterned over the entire surface in a single stroke manner. When a voltage is applied to the heater electrode  26 , the heater electrode  26  generates heat and heats the entire surface of the wafer mounting surface  22   a . The heater electrode  26  has a coil shape, a ribbon shape, a mesh shape, a plate shape or a film shape, and is formed of, for example, W, WC, Mo, or the like. Voltage can be applied to the electrostatic electrode  24  and the heater electrode  26  by a power supply member (not shown) inserted into the cooling plate  40  and the electrostatic chuck  22 . 
     Recesses  28  are provided in a surface of the electrostatic chuck  22  opposite to the wafer mounting surface  22   a . The recesses  28  are, for example, non-through holes. Female threaded terminals  30  are inserted into the recesses  28 . As shown in  FIG. 3 , the female threaded terminal  30  and the recess  28  are joined by a bonding layer  34 . The female threaded terminal  30  is a bottomed cylindrical member made of a low thermal expansion coefficient metal, and the cylindrical part is provided with a female thread  32 . Low thermal expansion coefficient means that the coefficient of linear thermal expansion (CTE) is c×10 −6 /K (c is 3 or more and less than 10, preferably 5 or more and 7 or less) at 0 to 300° C. Examples of the low thermal expansion coefficient metal include high melting point metals such as Mo, W, Ta, Nb, and Ti, alloys whose main component is one of these high melting point metals (for example, W—Cu or Mo—Cu), and Kovar (FeNiCo alloy). The CTE of the low thermal expansion coefficient metal is preferably the same as the CTE of the ceramic used for the electrostatic chuck  22 , and preferably within the range of ±25% of the CTE of the ceramic. This makes it easier to withstand use in a high-temperature range. For example, when the ceramic used for the electrostatic chuck  22  is AlN (4.6×10 −6 /K (40 to 400° C.)), Mo or W is preferably selected as the low thermal expansion coefficient metal. When the ceramic used for the electrostatic chuck  22  is Al 2 O 3  (7.2×10 −6 /K (40 to 400° C.)), Mo is preferably selected as the low thermal expansion coefficient metal. 
     The bonding layer  34  includes ceramic fine particles and a hard solder. Examples of the ceramic fine particles include Al 2 O 3  fine particles and AlN fine particles. The surfaces of the ceramic fine particles are preferably coated with a metal (for example, Ni) by plating or sputtering. The average particle size of the ceramic fine particles is not particularly limited, but is, for example, from 10 μm to 500 μm, preferably from 20 μm to 100 μm. When the average particle size is smaller than the lower limit, it is not preferable because the adhesion of the bonding layer  34  may not be sufficiently obtained. When the average particle size exceeds the upper limit, it is not preferable because the inhomogeneity becomes significant and the heat resistance characteristics, etc. may be deteriorated. Examples of hard solders include solders based on metals such as Au, Ag, Cu, Pd, Al, and Ni. When the ambient operating temperature of the electrostatic chuck heater  20  is 500° C. or less, an Al-based solder such as BA4004 (Al-10Si-1.5Mg) is preferably used as the hard solder. When the ambient operating temperature of the electrostatic chuck heater  20  is 500° C. or more, Au, BAu-4 (Au-18Ni), and BAg-8 (Ag-28Cu) are preferably used as the hard solder. The packing density of the ceramic fine particles in the hard solder is preferably from 30 to 90%, more preferably from 40 to 70% by volume. Increasing the packing density of the ceramic fine particles is advantageous in lowering the coefficient of linear thermal expansion of the bonding layer  34 , but increasing the packing density too high is not preferable because it may cause deterioration of the bonding strength. If the packing density of the ceramic fine particles is made too low, the coefficient of linear thermal expansion of the bonding layer  34  may not be sufficiently lowered, and care should be taken in this respect. Since the ceramic fine particles are coated with metal, the ceramic fine particles have good wettability with the hard solder. As a method of coating ceramic fine particles with metal, sputtering or plating can be used. 
     As an example of a method of inserting and joining the female threaded terminal  30  to the recess  28  of the electrostatic chuck  22 , first, as shown in  FIG. 4A , ceramic fine particles  34   a  are spread almost evenly on the surface of the recess  28 , a hard solder  34   b  in the form of a plate or a powder is placed so as to cover at least a part of the layer of the ceramic fine particles  34   a , and thereafter the female threaded terminal  30  is inserted. Next, in a state in which the female threaded terminal  30  is pressed against the recess  28 , heating to a predetermined temperature is performed to cause the hard solder  34   b  to melt and penetrate into the layer of the ceramic fine particles  34   a . When ceramic fine particles  34   a  whose surfaces are coated with a metal are used, the molten hard solder  34   b  easily uniformly spreads on the surfaces coated with the metal of the ceramic fine particles  34   a , and therefore easily penetrate into the layer of the ceramic fine particles  34   a . Since it is necessary for the hard solder  34   b  used to melt and penetrate into the layer of the ceramic fine particles  34   a , a temperature 10 to 150° C. higher than the melting point of the hard solder  34   b , and preferably 10 to 50° C. higher than the melting point of the hard solder  34   b , is suitable as a temperature for melting the hard solder  34   b . Thereafter, cooling is performed. The cooling time may be set appropriately, for example, in the range of 1 hour to 10 hours. In this way, as shown in  FIG. 4B , the recess  28  of the electrostatic chuck  22  and the female threaded terminal  30  are firmly bonded via the bonding layer  34 . 
     The cooling plate  40  is a member made of metal (for example, Al or Al alloy). The cooling plate  40  has a cooling medium path through which a cooling medium (for example, water) cooled by an external cooling unit (not shown) circulates. Through holes  42  each having a step  42   c  are provided at positions of the cooling plate  40  facing the recesses  28  of the electrostatic chuck  22 . As shown in  FIG. 5 , when the circular cooling plate  40  is viewed from the lower surface, the through holes  42  include a plurality of (here four) through holes provided at equal intervals along a small circle and a plurality of (here 12) through holes provided at equal intervals along a large circle. The through hole  42  has a large diameter portion  42   a  on the side opposite to the electrostatic chuck  22  and a small diameter portion  42   b  on the side of the electrostatic chuck  22  with the step  42   c  as a boundary. A male screw  44  is inserted into the through hole  42 . The male screw  44  may be made of, for example, stainless steel. The screw shank  44   b  of the male screw  44  is screwed to the female thread  32  of the female threaded terminal  30  with the screw head  44   a  in contact with the step  42   c  of the through hole  42 . That is, the male screw  44  is screwed to the female thread  32  of the female threaded terminal  30  such that the distance between the step  42   c  of the cooling plate  40  and the female threaded terminal  30  of the electrostatic chuck  22  decreases. In this manner, the electrostatic chuck  22  and the cooling plate  40  are fastened together by the female threaded terminals  30  and the male screws  44 . The diameter of the screw head  44   a  is smaller than that of the large diameter portion of the through hole  42 , and the diameter of the screw shank  44   b  is smaller than that of the small diameter portion of the through hole  42 . Therefore, in a state in which the female threaded terminal  30  and the male screw  44  are screwed together, a play p (horizontal gap in  FIG. 3 ) is provided in a direction in which the cooling plate  40  is displaced relative to the electrostatic chuck  22  due to the difference in thermal expansion. 
     The heat conductive sheet  36  is a layer made of a resin having heat resistance and insulation properties, is disposed between the electrostatic chuck  22  and the cooling plate  40 , and serves to transfer the heat of the electrostatic chuck  22  to the cooling plate  40 . The heat conductive sheet  36  does not have adhesiveness. Through holes  36   a  are formed at positions of the heat conductive sheet  36  facing the recesses  28  of the electrostatic chuck  22 . When it is desired to efficiently remove heat from the electrostatic chuck  22  to the cooling plate  40 , a sheet having a high thermal conductivity is used as the heat conductive sheet  36 . On the other hand, when it is desired to suppress heat removal from the electrostatic chuck  22  to the cooling plate  40 , a sheet having a low thermal conductivity is used as the heat conduction sheet  36 . Examples of the heat conductive sheet  36  include a polyimide sheet (for example, a Kapton sheet (Kapton is a registered trademark) or a Vespel sheet (Vespel is a registered trademark)) and a PEEK sheet. Since such a resin sheet having high heat resistance is usually hard, when the resin sheet is used as a layer for bonding the electrostatic chuck  22  and the cooling plate  40 , there is a possibility that the sheet may be peeled off or damaged due to the difference in thermal expansion between the electrostatic chuck  22  and the cooling plate  40 . In the present embodiment, since such a sheet is used as the heat conductive sheet  36  in the non-bonded state, there is no possibility that such a problem will occur. 
     Next, an example of the use of the plasma processing apparatus  10  thus configured will be described. First, in a state in which the electrostatic chuck heater  20  is installed in the vacuum chamber  12 , a wafer W is mounted on the wafer mounting surface  22   a  of the electrostatic chuck  22 . Then, the vacuum chamber  12  is reduced in pressure by a vacuum pump and adjusted to a predetermined degree of vacuum, and a DC voltage is applied to the electrostatic electrode  24  of the electrostatic chuck  22  to generate a Coulomb force or a Johnsen-Rahbek force, and the wafer W is sucked and fixed to the wafer mounting surface  22   a  of the electrostatic chuck  22 . He gas is introduced between the wafer W supported by protrusions (not shown) on the wafer mounting surface  22   a  and the wafer mounting surface  22   a . Next, the inside of the vacuum chamber  12  is set to a reactant gas atmosphere at a predetermined pressure (for example, several tens to several hundreds Pa), and in this state, a high-frequency voltage is applied between the upper electrode  60  and the electrostatic electrode  24  of the electrostatic chuck  22  in the vacuum chamber  12  to generate a plasma. Although both a DC voltage for generating an electrostatic force and a high-frequency voltage are applied to the electrostatic electrode  24 , the high-frequency voltage may be applied to the cooling plate  40  instead of the electrostatic electrode  24 . Then, the surface of the wafer W is etched by the generated plasma. The temperature of the wafer W is controlled to be a predetermined target temperature. 
     Here, the relationship between the components of the present embodiment and the components of the present invention will be clarified. The electrostatic chuck heater  20  of the present embodiment corresponds to the wafer mounting table of the present invention, the electrostatic chuck  22  corresponds to the ceramic plate, the cooling plate  40  corresponds to the metal plate, the female threaded terminal  30  corresponds to the threaded terminal, and the male screw  44  corresponds to the screw member. 
     In the above-described electrostatic chuck heater  20 , since the female threaded terminal  30  is made of a low thermal expansion coefficient metal, the thermal expansion coefficient thereof is close to that of ceramic used in the electrostatic chuck  22 . Therefore, even in the case of repeated use at a high temperature and a low temperature, the electrostatic chuck  22  and the female threaded terminal  30  are less liable to suffer cracking or the like due to thermal stress caused by the difference in thermal expansion coefficient. If a female thread that can be screwed with the male screw  44  is directly provided in the recess  28  of the electrostatic chuck  22 , the electrostatic chuck  22  may be broken when screwed with the male screw  44 . However, in this case, since the male screw  44  is screwed to the female threaded terminal  30  joined to the electrostatic chuck  22 , there is no such risk. Furthermore, since the female threaded terminal  30  is joined to the recess  28  of the electrostatic chuck  22  by the bonding layer  34  including ceramic fine particles and a hard solder, the bonding between the female threaded terminal  30  and the electrostatic chuck  22  is as sufficiently high as 100 kgf or more in terms of tensile strength (for this kind of bonding layer  34 , see Japanese Patent No. 3315919, Japanese Patent No. 3792440 and Japanese Patent No. 3967278). Further, in a state in which the female threaded terminal  30  and the male screw  44  are screwed together, a play p is provided in a direction in which the cooling plate  40  is displaced relative to the electrostatic chuck  22  due to the difference in thermal expansion. Therefore, even in the case of repeated use at a high temperature and a low temperature, displacement due to the difference in thermal expansion between the cooling plate  40  and the electrostatic chuck  22  can be absorbed by this play p. For example, the one-dot chain line in  FIG. 3  shows a state where the cooling plate  40  has expanded relative to the electrostatic chuck  22  due to the difference in thermal expansion. When the cooling plate  40  expands and contracts relative to the electrostatic chuck  22 , the screw head  44   a  can slide on the surface of the step  42   c , and the screw shank  44   b  can move in the small diameter portion  42   b  of the through hole  42  in the left-right direction in  FIG. 3 , so that the electrostatic chuck  22  is not easily broken. As described above, the electrostatic chuck heater  20  can withstand use in a high-temperature range. Further, by joining the female threaded terminal  30  into the recess  28 , it is possible to prevent the male screw  44  from being exposed to the process atmosphere and being corroded. 
     The electrostatic chuck heater  20  includes a non-adhesive heat conductive sheet  36  between the electrostatic chuck  22  and the cooling plate  40 . In this embodiment, since the electrostatic chuck  22  and the cooling plate  40  are fastened together by screwing the female threaded terminal  30  and the male screw  44  together, the heat conductive sheet  36  is not required to have adhesiveness. Therefore, the degree of freedom in selecting the heat conductive sheet  36  is increased. For example, a high thermal conductivity sheet may be employed to enhance the heat removal performance from the electrostatic chuck  22  to the cooling plate  40 , and a low thermal conductivity sheet may be employed to suppress the heat removal performance. The heat conductive sheet  36  also serves to prevent the female threaded terminals  30  and male screws  44  from being exposed to the process atmosphere (plasma or the like). 
     Further, the ceramic fine particles constituting the bonding layer  34  are fine particles whose surfaces are coated with a metal, and the hard solder contains Au, Ag, Cu, Pd, Al or Ni as a base metal. Therefore, the bonding strength between the female threaded terminal  30  and the electrostatic chuck  22  becomes higher. 
     It should be noted that the present invention is not limited to the above-described embodiment at all, and it is needless to say that the present invention can be implemented in various embodiments without departing from the technical scope of the present invention. 
     For example, in the above-described embodiment, the female threaded terminal  30  and the male screw  44  are exemplified, but the present invention is not particularly limited thereto. For example, as shown in  FIG. 6 , a male threaded terminal  130  may be joined to the recess  28  of the electrostatic chuck  22  via the bonding layer  34 , and fastened with a nut (female screw)  144  such that the distance between the male threaded terminal  130  and the step  42   c  of the cooling plate  40  decreases. In this case, the diameter of the nut  144  is smaller than that of the large diameter portion  42   a  of the through hole  42 , and the diameter of the male threaded portion  130   a  of the male threaded terminal  130  is smaller than that of the small diameter portion  42   b  of the through hole  42 . Therefore, in a state in which the male threaded terminal  130  and the nut  144  are screwed together, a play is provided in a direction in which the cooling plate  40  is displaced relative to the electrostatic chuck  22  due to the difference in thermal expansion. Therefore, according to the configuration of  FIG. 6 , the same effect as in the above-described embodiment can be obtained. 
     In the above-described embodiment, the through hole  42  of the cooling plate  40  has a step  42   c , but the present invention is not particularly limited thereto. For example, as shown in  FIG. 7 , a through hole  142  having a straight shape and having no step may be provided, and when the screw shank  44   b  of the male screw  44  is screwed to the female threaded terminal  30  of the electrostatic chuck  22 , the screw head  44   a  may be in contact with the lower surface of the cooling plate  40 . When the cooling plate  40  expands and contracts relative to the electrostatic chuck  22 , the screw head  44   a  can slide on the lower surface of the cooling plate  40 , and the screw shank  44   b  can move in the through hole  142  in the left-right direction in  FIG. 7 , so that the electrostatic chuck  22  is not broken. Therefore, according to the configuration of  FIG. 7 , the same effect as in the above-described embodiment can be obtained. 
     In the above-described embodiment, a washer or a spring may be interposed between the screw head  44   a  and the step  42   c . This prevents the screwed state between the female threaded terminal  30  and the male screw  44  from loosening. Similarly, a washer or a spring may be interposed between the nut  144  and the step  42   c  in  FIG. 6  or between the screw head  44   a  and the lower surface of the cooling plate  40  in  FIG. 7 . 
     In the above-described embodiment, the heat conductive sheet  36  does not have adhesiveness, but may have adhesiveness as needed. In that case, it is preferable that the heat conductive sheet  36  have such elasticity that it is not peeled off or broken by the thermal stress caused by the difference in thermal expansion between the electrostatic chuck  22  and the cooling plate  40 . 
     In the above-described embodiment, the electrostatic chuck  22  includes both the electrostatic electrode  24  and the heater electrode  26 , but it may include either of them. 
     In the above-described embodiment, the heat conductive sheet  36  may be partially trimmed.  FIG. 8  is a plan view of a heat conductive sheet  36  having a trimming region  36   b . A plurality of small holes are provided in the trimming region  36   b . This makes it possible to locally control heat removal from the electrostatic chuck  22  (ceramic plate) and to easily adjust the heat uniformity according to the actual use environment. Therefore, it is possible to realize a highly uniform temperature electrostatic chuck heater  20 . 
     In the above-described embodiment, an O-ring or a metal seal may be disposed on the outermost periphery of the heat conductive sheet  36  in order to ensure the sealing characteristics under a high vacuum environment and to prevent corrosion of the heat conductive sheet. 
     This application claims the priority of Japanese Patent Application No. 2016-166086, filed on Aug. 26, 2016, the entire contents of which are incorporated herein by reference in their entirety.